Strong correlations and angle-resolved photoemission

We will report our recent progress in studying high temperature superconductors by laser-based angle-resolved photoemission spectroscopy (ARPES). A new approach is developed to make ARPES able to measure not only the superconducting gap size but also the gap sign [1]. The pairing symmetry is determined to be of the nodal s+-(Gamma-M) form in the iron-based superconductor KFe2As2 which has only hole pockets [2]. Ubiquitous coexisting electron coupling with 70meV and 40meV modes is observed in cuprate superconductors [3]. The electronic origin of high-Tc maximization in trilayer cuprate superconductors is revealed [4]. We found orbital-dependent electron correlations in La3Ni2O7 [5]. These results provide key insights in understanding superconductivity mechanism and also pave a way to further enhance Tc in these high temperature superconductors. [1]. Qiang Gao, L. Zhao, X. J. Zhou et al., Nature Communications 15, 4538 (2024). [2]. Dingsong Wu, Lin Zhao, X. J. Zhou et al., Nature Physics 20, 571 (2024). [3]. Hongtao Yan, X. J. Zhou et al., PNAS 120, e2219491120 (2023). [4]. Xiangyu Luo, X. J. Zhou et al., Nature Physics 19, 1841 (2023). [5]. Jiangang Yang, L. Zhao, X. J. Zhou et al., Nature Communications 15, 4373 (2024).

The electronic structure of iron-based superconductors is characterized by an interplay of Coulomb interaction and Hund’s rule coupling acting on a multiband system. It is currently debated which role electronic correlations play in the complex phase diagrams that contain unconventional superconducting, nematic, and magnetic phases as well as strange metal behavior. Electronic correlation effects are particularly pronounced in the hole-doped AFe$_2$As$_2$ (A=K,Rb,Cs), which show large effective masses and strongly renormalized band dispersions. Here, we present a comprehensive overview on how electronic correlations affect the spectral function of RbFe$_2$As$_2$ using a combination of angle-resolved photoemission spectroscopy (ARPES) and dynamical mean field theory (DMFT). We will discuss how atomic excitations of the localized Fe 3d shell are screened by two distinct processes involving spin and orbital degrees of freedom. They lead to the emergence of long-lived, heavily renormalized quasiparticles at low temperatures and turn a bad metal into a heavy Fermi liquid. The quasiparticle dispersion develops two strong kinks that originate from electronic interactions. The low-energy kink is likely caused by a coupling to spin excitations, which suggests their involvement in the formation of unconventional superconductivity in RbFe$_2$As$_2$.

In a series of groundbreaking discoveries, superconductivity has recently been demonstrated in La3Ni2O7 up to 91 K under moderate pressure in bulk crystals, and up to 48 K at ambient pressure in thin films grown under compressive strain. Key questions remain open regarding the crystal structure and low-energy electronic states that support superconductivity in these compounds. Here we take advantage of the natural polymorphism between bilayer (2222) or alternating monolayer-trilayer (1313) stacking sequences that arises in bulk La3Ni2O7 crystals to identify common features in this family of materials. Employing angle-resolved photoemission spectroscopy (ARPES) in combination with an effective tight-binding model, we simulate the spectral weight distribution observed in our ARPES dichroism experiments and establish that the low-energy electronic phenomenology is dominated by oxygen-centered planar orbital.

In strongly correlated electron systems, quantum emergent phenomena, such as high-temperature superconductivity and colossal magnetoresistance, are closely linked to multiple competing and coexisting phases and orders. These phenomena are widely believed to arise from strong electron-electron interactions. It is also well established that the emergence of such quantum phenomena is often accompanied by spatial inhomogeneities in electronic states across various length scales. Understanding this self-organization is crucial for elucidating the macroscopic physical properties of these materials. In high-$T_c$ cuprate superconductors, scanning tunneling microscopy and spectroscopy (STM/STS) studies have revealed nanoscale spatial inhomogeneity in the superconducting gap [1]. However, identifying the key interactions—such as electron correlations, electron-lattice interactions, and electron-magnetic interactions—that drive this self-organization and influence macroscopic properties remains a major challenge. In this study, we investigate the electronic inhomogeneity of Bi$_2$Sr$_2$CaCu$2$O${8+\delta}$ (Bi2212) using micro-focused angle-resolved photoemission spectroscopy (micro-ARPES), which provides high energy and momentum resolution along with micrometer-scale spatial resolution. In this talk, we will present our recent findings on the spatial inhomogeneity of electronic states and many-body interactions along the nodal direction [2], as well as the spatial variation of electronic states and the superconducting gap along the antinodal direction [3]. [1] K. McElroy et al., Phys. Rev. Lett. 94, 197005 (2005). [2] H. Iwasawa et al., Phys. Rev. Res. 5, 043266 (2023). [3] Y. Miyai et al., Sci. Technol. Adv. Mater. 25, 2379238 (2024).

Dynamical mean field theory (DMFT)-based electronic structure calculations have become a reference tool for theoretically adressing spectroscopic properties of correlated materials. In this talk, I will discuss correlation effects beyond the local picture of DMFT.

TDB

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Over the past two decades, the discovery of various topological materials, including insulators, semimetals, and metals, has significantly deepened our understanding of material topology. However, precisely controlling topological properties within a single material remains a major challenge. Here, we demonstrate the experimental switching of a topological Dirac nodal loop in the LaSbxTe2−x square-net material by selectively opening and closing a gap in the nodal loop. We show that the crystal symmetry responsible for the nodal loop formation can be broken through electron doping in both the bulk—via chemical substitution—and at the surface—via chemical gating. Using angle-resolved photoemission spectroscopy (ARPES), we reveal that tuning the antimony content x from 0.9 to 1.0 induces a substantial bulk gap of up to 400 meV in the nodal loop. More strikingly, the same topological phase transition can be achieved in situ on the LaSbxTe2−x surface via chemical gating using potassium deposition, enabling reversible switching between gapped and gapless nodal loops. This transition is driven by a doping dependent Fermi surface instability and passes through a charge-ordered phase. Our results establish electron concentration as the fundamental control parameter governing both structural and topological transitions in the bulk and at the surface. This tunability paves the way for device applications that exploit topology switching through electrostatic gating. J. Bannies, M. Michiardi, H.-H. Kung, S. Godin, J. W. Simonson, M. Oudah, M. Zonno, S. Gorovikov, S. Zhdanovich, I. S. Elfimov, A. Damascelli, M. C. Aronson , Electronically-driven switching of topology in LaSbTe, under review in Nature Materials, arXiv:240708798

Chiral topological materials exhibit unique electronic, spin, and optical properties, arising from their lack of inversion and mirror symmetries. They host Kramers-Weyl fermions [1] and multifold chiral fermions [2], which lead to long Fermi arcs [3], unconventional spin textures [4], and large Chern numbers [5]. Additionally, these materials support quantized circular photogalvanic effects [6] and bulk photovoltage [7], making them promising candidates for optoelectronic applications. We employed time- and angle-resolved photoemission spectroscopy (trARPES) to investigate the interaction between light and the electronic properties of CdAs$_2$, a promising chiral semiconductor. By combining theory and experiments, we reveal the presence of helicoidal Fermi arcs spanning the entire Brillouin zone and multiple degenerate valleys at the edges of the valence and conduction bands, hosting distinct Weyl fermions [8]. More importantly, we demonstrate the formation of a metastable electronic population in the conduction band, lasting for several minutes after light exposure. This persistent photoconductivity, confirmed by bulk transport measurements, paves the way for the use of chiral topological semiconductors in optoelectronic devices, where spin and electronic transport can be optically and reversibly controlled, overcoming the conventional doping engineering. [1] G. Chang et al., Nature Materials 17, 978 (2018) [2] G. Chang et al., Physical Review Letters, 119 206401 (2017) [3] N. Schröter et al., Nature Physics 15, 759 (2019) [4] G. Gatti et al., Physical Review Letters 125, 216402 (2020) [5] N. Schröter et al., Science 369, 179 (2020) [6] F. de Juan et al., Nature Communication 8, 15995 (2017) [7] Y. Zhang et al., Physical Review B 100, 245206 (2019) [8] D. Gosalbez-Martinez et al., in preparation (2025)

Confining electrons to two dimensions (2D) is known to enhance electronic correlations and promote non-trivial topological phases. Atomic monolayers on semiconductor substrates represent the ultimate 2D limit of such confinement and thus have recently come into focus as "third-generation 2D designer quantum materials", following the examples of graphene and monolayer transition metal dichalcogenides. Here I will focus on atomic monolayers as 2D topological insulators (2D-TIs) which host 1D metallic and spin-polarized edge states as hallmark of the quantum spin Hall (QSH) effect. My examples range from bismuthene (Bi/SiC(0001)) [1-3], the 2D-TI with the largest band gap realized to date, to indenene (In/SiC(0001)), a triangular lattice of In atoms with emergent honeycomb physics [4,5]. Using ARPES as well as STM/STS we have studied their electronic structure and especially their topological edge states, revealing interesting insights into their protection (or loss thereof) against single-particle backscattering. I will also demonstrate how circular dichroism in ARPES can serve as a tool to identify non-trivial topology in the bulk states [6]. Finally, I will discuss the stabilization of these monolayers in ambient conditions via van der Waals capping [7,8], paving the way towards ex situ experiments and the realization of transport devices. [1] Science 357, 287 (2017) [2] Nat. Phys. 16, 47 (2020) [3] Nat. Commun. 13, 3480 (2022) [4] Nat. Commun. 12, 5396 (2021) [5] arXiv:2503.11497 [6] Phys. Rev. Lett. 132, 196401 (2024) [7] Nat. Commun. 15, 1486 (2024) [8] arXiv:2502.01592

The electronic structure of a solid is determined by structural parameters such as bond angles and lengths. Therefore, electronic properties are commonly controlled by varying the chemical composition of a solid. Alternatively, structural parameters can be modified by applying pressure or strain. All of these methods, however, are slow. Resonant coherent driving of selected lattice vibrations with tailored lightwaves allows for a dynamic modulation of the crystal structure on femtosecond timescales. In this way, atomic displacements of a few percent of the bond length can be achieved, with dramatic effects on the electronic properties of the driven solid. Famous examples include light-induced superconducting-like states [1], ultrafast control of magnetism [2] and ferroelectricity [3] as well as ultrafast strain engineering in complex oxide heterostructures [4]. We have developed a setup that allows us to measure the transient band structure of periodically driven solids. For this purpose we combine narrow-band pump pulses tunable from the terahertz to the mid-infrared spectral range with a time- and angle-resolved photoemission probe. The pulse duration of our extreme ultraviolet probe pulses can be truned from 30 to 600 femtoseconds, allowing us to access the transient electronic structure on timescales that are either short or long compared to the period of the driving field. In my talk I will present recent results on the molecular solid K$_3$C$_{60}$ driven in the mid-infrared where previous experiments were interpreted in terms of light-induced superconductivity [1] and WS$_2$ driven at resonance to the E$_{1u}$ in-plane phonon mode. [1] Nature Physics 19, 1821 (2023) [2] Nature 617, 73 (2023) [3] Science 364, 1075 (2019) [4] Phys. Rev. Lett. 108, 136801 (2012)

The electronic structure of a solid is determined by structural parameters such as bond angles and lengths. Therefore, electronic properties are commonly controlled by varying the chemical composition of a solid. Alternatively, structural parameters can be modified by applying pressure or strain. All of these methods, however, are slow. Resonant coherent driving of selected lattice vibrations with tailored lightwaves allows for a dynamic modulation of the crystal structure on femtosecond timescales. In this way, atomic displacements of a few percent of the bond length can be achieved, with dramatic effects on the electronic properties of the driven solid. Famous examples include light-induced superconducting-like states [1], ultrafast control of magnetism [2] and ferroelectricity [3] as well as ultrafast strain engineering in complex oxide heterostructures [4]. We have developed a setup that allows us to measure the transient band structure of periodically driven solids. For this purpose we combine narrow-band pump pulses tunable from the terahertz to the mid-infrared spectral range with a time- and angle-resolved photoemission probe. The pulse duration of our extreme ultraviolet probe pulses can be truned from 30 to 600 femtoseconds, allowing us to access the transient electronic structure on timescales that are either short or long compared to the period of the driving field. In my talk I will present recent results on the molecular solid K$_3$C$_{60}$ driven in the mid-infrared where previous experiments were interpreted in terms of light-induced superconductivity [1] and WS$_2$ driven at resonance to the E$_{1u}$ in-plane phonon mode. [1] Nature Physics 19, 1821 (2023) [2] Nature 617, 73 (2023) [3] Science 364, 1075 (2019) [4] Phys. Rev. Lett. 108, 136801 (2012)

The electronic structure of a solid is determined by structural parameters such as bond angles and lengths. Therefore, electronic properties are commonly controlled by varying the chemical composition of a solid. Alternatively, structural parameters can be modified by applying pressure or strain. All of these methods, however, are slow. Resonant coherent driving of selected lattice vibrations with tailored lightwaves allows for a dynamic modulation of the crystal structure on femtosecond timescales. In this way, atomic displacements of a few percent of the bond length can be achieved, with dramatic effects on the electronic properties of the driven solid. Famous examples include light-induced superconducting-like states [1], ultrafast control of magnetism [2] and ferroelectricity [3] as well as ultrafast strain engineering in complex oxide heterostructures [4]. We have developed a setup that allows us to measure the transient band structure of periodically driven solids. For this purpose we combine narrow-band pump pulses tunable from the terahertz to the mid-infrared spectral range with a time- and angle-resolved photoemission probe. The pulse duration of our extreme ultraviolet probe pulses can be truned from 30 to 600 femtoseconds, allowing us to access the transient electronic structure on timescales that are either short or long compared to the period of the driving field. In my talk I will present recent results on the molecular solid K$_3$C$_{60}$ driven in the mid-infrared where previous experiments were interpreted in terms of light-induced superconductivity [1] and WS$_2$ driven at resonance to the E$_{1u}$ in-plane phonon mode. [1] Nature Physics 19, 1821 (2023) [2] Nature 617, 73 (2023) [3] Science 364, 1075 (2019) [4] Phys. Rev. Lett. 108, 136801 (2012)

The time- and angle-resolved photoemission (TR-ARPES, [1]) endstation at the Advanced Laser Light Source (ALLS) laboratory at INRS-EMT provides mid-infrared optical excitation capabilities (0.15-0.8 eV range) together with a 6 eV probe (soon to be upgraded to >10 eV extreme ultraviolet via high harmonic generation) [2]. Furthermore, I will show that by combining sample voltage bias and a hemispherical electron analyzer with next-generation deflector technology, we are able to probe a significant fraction of the momentum space of quantum materials even with low photon energy ultraviolet light (6 eV) [3]. I will illustrate the capabilities of the TR-ARPES endstation on the Bi2Sr2CaCu2O8+x (Bi2212) cuprate superconductor. In particular, I will discuss the first observation of a particle-hole asymmetric normal state upon ultrafast quenching of the long-range superconducting order [4]. Finally, I will briefly review our current efforts to develop a new ARPES-based technique named Correlation-ARPES (C-ARPES). C-ARPES is based on the coincidence detection of two electrons emitted by two photons of the same ultrashort pulse and promises unprecedented access to the two-particle correlations of quantum materials. [1] Boschini, Zonno, Damascelli, Rev. Mod. Phys. 96, 015003 (2024) [2] Longa et al., Opt. Express 32, 29549 (2024) [3] Gauthier et al. Rev. Sci. Instr. 92, 123907 (2021) [4] D. Armanno et al., to be submitted

The electronic interaction in quantum many-body systems is determined by the degree of localization, electronic correlations, and screening. Together they typically mediate ultrafast relaxation processes on femtosecond timescales [1]. Impurity states are well known to be chemically distinct from sites in periodic crystals. If isolated from the environment they exhibit long lasting coherence and population, e.g., in NV centers in diamond. In periodic lattices such long electronic lifetimes are usually absent due to the strong interactions that mediate the relaxation. Here we investigate the effect of impurities in quantum materials on the electronic lifetimes. We combine photoelectron and scanning tunnelling spectroscopy, both in the time domain, and distinguish non-thermal many-body states from localized states at the impurities. For both we observe relaxation on slow timescales in the range of microseconds. Using spectroscopy we separate hole-like defect states and many body electron states. Both are localized due to different interactions that will be discussed. Funding by the DFG through FOR 5249 - QUAST and SFB 1242 is gratefully acknowledged. [1] M. Ligges, I. Avigo, D. Golez, H. U. R. Strand, Y. Beyazit, K. Hanff, F. Diekmann, L. Stojchevska, M.Kalläne, K. Rossnagel, M. Eckstein, P. Werner, U. Bovensiepen, Phys. Rev. Lett. 120, 166401 (2018)

We use our recently introduced nonequilibrium DMFT+CPA [1, 2] to investigate the nonequilibrium dynamics of a disordered interacting system, when it is subjected to an interaction quench. The method combines the capacity, on the one hand, of DMFT (dynamical mean field theory) to treat strongly correlated systems, and the other hand, of CPA (coherent potential approximation) to treat disordered systems, to effectively address the interplay of disorder and interaction for the nonequilibrium system. First, we benchmark the approach on the equilibrium density of states of a system described by the Anderson-Hubbard model with "box" and binary disorder. Next, we evaluate the dynamics and the thermalization of the system when the interaction strength is abruptly changed at a given time, from zero to a finite constant value. We observe that disorder affects the relaxation of the system in a nontrivial manner and we identify different thermalization regimes as a function of disorder and interaction strengths. [1] E. Dohner, H. Terletska, K.-M. Tam, J. Moreno, H. F. Fotso, Nonequilibrium DMFT+CPA for Correlated Disordered Systems, Phys. Rev. B 106 195156 (2022), DOI link: https://doi.org/10.1103/PhysRevB.106.195156 [2] E. Dohner, H. Terletska, H. F. Fotso, Thermalization of a Disordered Interacting System under an Interaction Quench, Phys. Rev. B 108, 144202 (2023), DOI link: https://doi.org/10.1103/PhysRevB.108.144202

Several recent publications on dichroic effects in angle resolved photoemission (ARPES) suggest that it may serve as an unbiased and powerful tool for exploring electronic states regarding their topological properties [1]. Although the pectral function of the photohole in the (N-1)-system has been the central focus for decades, it has long been recognized that photoemission matrix elements may play a dominant role in the data; even the spin polarization of the photocurrent depends on experimental geometry, light polarization, and photon energy. While matrix element effects were often seen as a complication, they also encode valuable information about orbital (and spin) textures in k-space, revealing topological structures such as chiral monopoles in the orbital angular momentum (OAM) [2,3,4] and nodal vortex lines as higher-dimensional objects [5]. Although dichroic effects can be modeled in great detail using elaborate numerical methods such as one-step photoemission calculations, our current understanding remains largely classical, based on scattering and interference of waves. In any case, a critical assessment before straightforward application seems to be necessary. This presentation aims to stimulate discussion on reliability and robustness of dichroic ARPES interpretations and the application to complex materials. REFERENCES [1] J. Sanchez-Barriga, O. J. Clark, O. Rader, Encycl. of Cond. Mat. Phys. 4, 334 (2024); arXiv:2501.00497 [2] M. Ünzelmann et al., Nat. Commun. 12, 3650 (2021) [3] S. S. Brinkmann, H. Bentmann et int. , Phys. Rev. Lett. 132, 196402 (2024) [4] Y. Yen, N. B. Schröter, et int., Nat. Phys. 20, 1912 (2024) [5] T. Figgemeier, M. Ünzelmann, P. Eck, J. Schusser, et al., accepted at PRX (2025); arXiv:2402.10031v2.

Quantum degrees of freedom in electronic states are a key facet of modern quantum materials. Recently, the orbital angular momentum (OAM) — an orbital analogue of electron spin — has attracted broad attention in condensed matter and surface physics. For instance, orbitronics has been predicted as a promising route towards new functionalities, such as low-dissipation orbital currents or magnetization switching by orbital torques. In this talk, I will present orbital-sensitive angle-resolved photoemission spectroscopy experiments that demonstrate three key features of the OAM: (i) it acts as a central mediator between lattice and spin in spin-orbit-coupled systems, such as Rashba-type and topological surface states [1,2], (ii) it might be associated with orbital currents, and (iii) its momentum texture carries topological information, giving rise to intriguing paradigms like OAM monopoles [3] or momentum-space quantum vortices [4]. [1] M. Ünzelmann et al., Phys. Rev. Lett. 124, 176401 (2021) [2] B. Geldiyev, M.Ü., et al., Phys. Rev. B 108, L121107 (2023) [3] M. Ünzelmann et al., Nat. Commun. 12, 3650 (2021) [4] T. Figgemeier, M.Ü., et al., (accepted at PRX) arXiv:2402.10031 (2024)

Moiré semiconductors emerged as tunable quantum simulators for strongly correlated phases [1]. The single-particle low-energy physics is ruled by the moiré-periodic superpotential that develops by twisting or stacking layers with different lattice parameters. Signatures of this modulation are observed in the spectral function measured by angle-resolved photoemission spectroscopy (ARPES) in the form of replicas and gaps opening at the nascent zone boundary. In moiré transition metal dichalcogenides (TMDs), flat bands are reported at the Brillouin zone center and their dispersion is associated to the effective moiré potential experienced by electronic states with large out-of-plane orbital character [2,3]. Here, we extend this analysis and present the orbital and wave vector dependence of this interaction over the whole Brillouin zone by comparing quantitatively our ARPES data on a TMD heterobilayer with a multi-orbital continuum tight-binding model. Our results set the fundaments for future spectroscopic studies of the electronic correlations in moiré systems. [1] Mak, K.F., Shan, J., Nat. Nanotechnol. 17, 686–695 (2022) [2] Gatti, G., Issing, J., Rademaker, L. et al., Phys. Rev. Lett. 131, 046401 (2023) [3] Pei, D., Wang, B., Zhou, Z. et al., Phys. Rev. X 12, 021065 (2022)

Cerium intermetallics are rich in fascinating phenomena. Among them CeCoIn5 and Ce3PdIn11 are the interesting examples exhibiting heavy fermion state and superconductivity below 2.3 K and 0.58 K, respectively. Moreover, two transitions to antiferromagnetic state take place in Ce3PdIn11 at temperatures of 1.68 K and 1.56 K. Fortunately, resonant photoemission can be used to study the behaviour of 4f electrons, which are crucial for the physics of these systems. Angle resolved photoemission spectroscopy (ARPES) was carried out using a Ce 4d-4f resonance photon energy (122 eV) at the temperature of 6 K. Fermi surface (FS) maps in different geometries obtained with on- and off-resonance radiation enabled to find the distribution of f-electrons. In particular, on-resonance symmetrized FS map for CeCoIn5 agrees with the theoretical distribution of f-electrons in k-space obtained from tight-binding model for Ce-In planes [1]. Hybridization effects between Ce 4f electrons and valence band form heavy bands, which are visible. The estimated heavy electron masses for CeCoIn5 amount to 30 – 130 free electron masses, depending on the location in reciprocal space. Calculated Bloch spectral functions using SPR-KKR package and one-step model calculations enabled a deeper data understanding. The most intensive dispersions in the spectra for CeCoIn5 are related to surface, which is terminated with Ce-In layer. Fermi surface of Ce3PdIn11 has similar features to the ARPES data for Ce2PdIn8, Ce2RhIn8, Ce2IrIn8 indicating the actual surface termination related to 218 systems. Ce3PdIn11 is a particularly interesting material due to two inequivalent crystallographic Ce sites and their contribution to the band structure was identified by SPR-KKR calculations. References [1]. Kurleto R., et al., Phys. Rev. B 104, 125104 (2021).

Rare-earth compounds are an intriguing class of materials which can host a variety of different phenomena, such as heavy fermion behaviour, superconductivity, Kondo physics and mixed-valence behaviour. In particular, mixed valence (MV) refers to the presence in the system of a given rare-earth element holding more than one electronic occupation for the fshell. Although MV is observed in a wide range of rare-earth systems, a full microscopic understanding of its nature and limits remains elusive. Here we present an experimental study of La-substituted SmB$_6$ aiming to track the crossover of the MV character going from a periodic f-electron lattice to a dilute f-impurity system. While SmB$_6$ is a prototypical mixed-valent system characterized by the nearly-equal presence of Sm2+ (4f6 5d0) and Sm3+ (4f5 5d1) configurations for the Sm ions, its counterpart LaB$_6$ has a metallic ground state and lacks 4f electrons. By employing trivalent La ions as substituent in the Sm$_x$La$_{1−x}$B$_6$ hexaboride series, we combine angle-resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS) to investigate the evolution of the mean Sm valence, vSm. Upon decreasing x, we observe a linear decrease of vSm to an almost complete suppression of valence fluctuations in the intermediate substitution regime with vSm~2 at x=0.2. Interestingly, a re-emergent MV character develops upon further lowering x, with vSm approaching the value of vimp~2.35 in the dilute-impurity limit. Such behaviour departs from a monotonic evolution of vSm across the whole series, as well as from the expectation of its convergence to an integer value for x→0. Our experimental results, complemented by a phenomenological model, provide evidence of the realization of a dilute-impurity MV state in the Sm$_x$La$_{1−x}$B$_6$ series, and they may stimulate further theoretical and experimental considerations on the concept of MV and its influence on the macroscopic electronic and transport properties of rare-earth compounds in the dilute-to-intermediate impurity regime [1]. Reference: [1] M. Zonno et al., Nature Communications 15, 7621 (2024)

Theoretical studies universally predict SmB$_6$ to be a topological Kondo insulator: The first example of a material with strong electron correlation that is also topologically nontrivial. Most experimental results have been interpreted as vindicating the theory, even though some dissenting opinions have been voiced (see Li et al. [1] for a balanced review). Experimental studies have so far focused on the surface states properties to settle the topological classification of SmB6. In this contribution we will take a more fundamental approach and concentrate on the three dimensional electronic structure. We find that the 3-D band structure violates a fundamental property of the topological Kondo insulator. We will discuss the implications of our findings for the search for correlated topologically nontrivial materials. [1] Lu Li et al., Nature Reviews Physics 2020, 2, 463–479

Topological insulators are simple bulk insulators with symmetry protected metallic surface states which exhibit Dirac cone like dispersions. Most of the topological systems studied are weakly correlated. The properties of these massless Dirac fermions in the presence of electron correlation is an interesting emerging area of research where electron correlation is expected to enhance the effective mass of the particles. We studied the behavior of Dirac fermions in novel Kondo lattice system employing ARPES. SmB6, known to be a Kondo insulator, exhibit unusual properties [1]. Another Sm-based binary system, SmBi exhibits signature of multiple gapped and un-gapped Dirac cones in the band structure [2,3]. Employing ultra-high-resolution ARPES, we discover destruction of a surface Fermi surface across the Neel temperature while the behavior of Dirac cones survives across the magnetic transition. HAXPES data of a non symmorphic Kondo lattice system, CeAgSb2 and CeCuSb2 exhibit unusual spectral features; while the bulk properties show Kondo behavior, the typical Kondo feature is not observed. Instead, we find a new feature in the core level spectra [4,5]. The ARPES data of CeAgSb2 show distinct Dirac cones as well as diamond-shaped nodal lines; the slope of these linear bands is unusually high, larger than that in graphene and maintains its high value in a wide energy range indicating robust high velocity of these relativistic particles [6]. The slope becomes smaller in the vicinity of strongly correlated Ce 4f bands forming a kink; a unique case due to correlation induced effects. References: [1] A. P. Sakhya and K. Maiti, Scientific Reports 10, 1262 (2020). [2] A. P. Sakhya et al. Phys. Rev. Mater. 5, 054201 (2021). [3] A. P. Sakhya et al. Phys. Rev. B 106, 085132 (2022). [4] Sawani Datta et al. PRB 105, 205128 (2022). [5] Sawani Datta et al. APL 123, 201902 (2023). [6] Sawani Datta et al. Nanoscale 16, 13861 (2024).

Already soon after the discovery of high-Tc cuprates, the concept of a spin polaron has been formulated, in order to understand the behaviour of holes in lightly doped cuprates [1]. This idea has gained quite a widespread acceptance, for it could quite well describe the angular-resolved photoemission (ARPES) spectra of Mott-insulating cuprates [2]. Yet, two issues have been raised: (i) the ARPES spectrum on quasi-1D cuprates was much better explained using the spin-charge separation picture [3] and (ii) the experimentally observed line shape of the quasiparticles was much higher than the one predicted by the spin polaron theory [4]. Here I address these two issues in detail and, based on the recent works [5], I hope to give a decisive answer on the extent to which the spin polaron concept is still a valid approach. [1] B. I. Shraiman and E. D. Siggia, Phys. Rev. Lett. 60, 740 (1988). [2] A. Damascelli, Z. Hussain, and ZX Shen, Rev. Mod. Phys. 75, 473 (2003). [3] C. Kim et al., Phys. Rev. Lett. 77, 4054 (1996). [4] K. M. Shen et al., Phys. Rev. Lett. 93, 267002 (2004). [5] P. Wrzosek and K. Wohlfeld, Phys. Rev. B 103, 035113 (2021); P. Wrzosek, A. Klosinski, Y. Wang, M. Berciu, C. E. Agrapidis, K. Wohlfeld, SciPost Phys. 17, 018 (2024); K. Wohlfeld, E. Demler, T. Tohyama [in preparation (2025)].

We use density matrix renormalization group (DMRG) and variational exact diagonalization (VED) to calculate the single-electron removal spectral weight for the Hubbard-Holstein model at low electron densities. Tuning the strength of the electron-phonon coupling and of the Hubbard repulsion allows us to contrast the results for a liquid of polarons versus a liquid of bipolarons. The former shows spectral weight up to the Fermi energy, as expected for a metal. The latter has a gap in its spectral weight, set by the bipolaron binding energy, although this is also a (strongly correlated) metal. This difference suggests that angle-resolved photoemission spectroscopy could be used to identify liquids of pre-formed pairs. Furthermore, we show that the one-dimensional liquid of incoherent bipolarons is well approximated by a "Bose sea" of bosons that are hard-core in momentum space, occupying the momenta inside the Fermi sea but otherwise non-interacting. This new proposal for a strongly-correlated many-body wavefunction opens the way for studying various other properties of incoherent (non-superconducting) liquids of pre-formed pairs in any dimension.

The interaction between electrons and phonons in materials governs critical physical properties, including electrical conductivity and superconductivity. In this study, we explore the formation of Holstein polarons and Rashba-like spin splitting in electron-doped MoSe$_2$, a prototypical transition metal dichalcogenide (TMDC). Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we identify discrete polaronic spectral features at both $\Sigma$ and ${\rm K}$ valleys and demonstrate their evolution with increasing doping. The induced Rashba-like spin splitting arises from an inversion-symmetry-breaking electric field at the doped surface, providing direct experimental evidence for spin-polarized quasiparticles in TMDCs. Furthermore, we observe a crossover from phonon-mediated Holstein polarons to plasmon-like quasiparticles with higher doping levels, revealing the rich doping-dependent electronic structure of MoSe$_2$. These findings substantiate the role of electron-phonon coupling in mediating superconductivity in TMDCs and offer critical insights into the unconventional Ising superconductivity supported by the unique spin-orbit coupling and multi-valley Fermi surface topology in these materials. Our work highlights the potential of electron-doped MoSe$_2$ as a model system for studying complex quasiparticle behaviour and advancing the understanding of superconductivity in low-dimensional systems.

Hexagonal boron nitride (hBN), a polar wide gap insulator, is the gate dielectric of choice in the field of 2D materials. However, surprisingly little is known about how hBN affects the electronic properties of 2D materials of interest. Here, we use nano-ARPES to study the prototypical systems of monolayer transition metal dichalcogenide semiconductors on hBN. Our data show two replica bands of the semiconductor  valley at energies close to hBN phonon modes. This is the fingerprint of long-range electron-phonon interaction across the interface. Our data is well reproduced by a generic model describing the propagation of a charged particle above a polar substrate, suggesting that interfacial electron-phonon coupling is universal for 2D materials encapsulated in hBN. Consistent with this interpretation, control experiments on non-polar graphite substrates do not show the replica bands observed on hBN.

The question of whether electron-phonon coupling leads to superconducting pairing has been a topic of considerable debate in condensed matter physics. Various t-J models have been proposed to explain the phenomenon of high temperature superconductivity without addressing the electron phonon interaction mechanisms. In this study, we decided to the superconductivity in layered square-lattice bilayer system and to revise the problem of possible superconductivity in such a prototype system by considering the intralayer electron-phonon coupling mechanism. The Hamiltonian in our model consists of two parts: the non-interacting part, that includes the free phonon Hamiltonian, nearest-neighbors and next-nearest neighbors hopping terms, and interlayer hopping term; and an interacting part encompassing local intralayer and interlayer Hubbard interactions, intralayer electron-phonon coupling, and coupling with an external interlayer electric field potential. Our starting point is the discretization of elementary single particle excitations as a product of two simultaneous quasiparticle excitations with the same energy. Additionally, we considered possible excitonic states between the layers of the metallic bilayer. By applying functional field integration techniques, we derive the effective electron-phonon interaction action and obtain a set of self-consistent equations for the spin-triplet superconducting gap, excitonic order parameter, chemical potential, and average charge-density imbalance between the layers. We numerically solve this system using finite difference approximation techniques employing the Newton’s fast convergence algorithm. The temperature dependence of aforementioned physical quantities is investigated across different electron doping regimes. Furthermore, we plot the superconducting critical phase transition diagram as a function of electron doping. In our case, the undoped system corresponds to the Anderson localization limit with fully occupied bands. Moreover, the superconducting gap is plotted for different limits of the electron-phonon interaction parameter within the half-filling regime. We demonstrate the coexistence of superconducting and excitonic states over a wide range of temperatures and doping levels. At the fixed temperature, we calculated numerically the excitonic and superconducting order parameters as a function of electron and hole doping in the system. Notably, at hole doping the excitonic order parameter enhances, whereas for electron doping, the superconducting order parameter is much larger and persists over a broader interval of doping. We also show that for both electron and hole doping, charge imbalance is maximized at the half-filling regime, while charge neutrality is achieved in the Anderson localization limit and at very high doping levels. In the limit of low electron-phonon coupling, the superconducting transition temperature is on order of $T_C$=25.76 K, while in the strong coupling limit, we obtain a transition temperature as high as room temperature, on the order of $T_C$=287.3 K. Moreover, we identify a range of values for the next-nearest neighbor hopping parameter, within which the superconducting gap is sustained. The proposed model may serve as a new mechanism for superconductivity in high-$T_C$ cuprate superconductors due to intralayer electron-phonon coupling.

Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like BKBO; however, conclusive experimental evidence for a (bi)polaron metallic state in this material remains elusive. Here, we combine Angle-resolved photoemission, resonant inelastic X-ray, and neutron total scattering techniques with advanced modeling to study the local lattice distortions, electronic structure, and electron-phonon coupling in BKBO as a function of doping. Data for the parent compound x=0 indicates that the electronic gap opens in predominantly oxygen-derived states strongly coupled to a long-range ordered breathing distortion of the oxygen sublattice. Upon doping, short-range breathing distortions and sizable e-ph coupling persist into the superconducting regime x=0.4. Comparisons with exact diagonalization and determinant quantum Monte Carlo calculations further support this conclusion. Our results provide compelling evidence that the BKBO metallic phase hosts a liquid of small (bi)polarons derived from local breathing distortions of the lattice, with implications for understanding the low-temperature superconducting instability.

A major discovery in condensed matter physics is the discovery of High-Temperature Superconductivity in cuprates (copper oxides), which still hold the record for the highest critical temperature at ambient pressure. They feature layers of CuO2 planes, believed to be responsible for their electronic properties, involving strong electronic correlations. Doping results in a complex phase diagram from antiferromagnetic insulator to pseudo-gap, superconductivity, strange metal phase, and coexisting and/or competing orders of charge and spin. However, a comprehensive theoretical explanation of the Cooper pairing mechanism in these materials remains a debated subject, in which magnetic excitations (paramagnons) are promising candidates [1]. The hole-doped Ca2CuO2Cl2 copper oxychloride serves as an excellent compound for investigating all these phases on common ground [2-3]. Its stable and simple I4/mmm 1-layer structure and strong 2D character make it very suitable for theoretical calculations, allowing direct comparison with experimental work. In this talk, I focus on magnetic excitations investigated by Resonant Inelastic X-ray Scattering (RIXS) and Angle-resolved Photo-Emission Spectroscopy (ARPES). In RIXS, the paramagnon exhibits a similar dispersion with doping along the (h,0) direction up to the optimal doping, similar to all cuprates, and a softening along the (h,h) direction, as observed in other cuprates. The paramagnon bandwidth has the same energy as the waterfall feature in the electronic bands, as measured in ARPES, suggesting a connection between the two phenomena. This is indeed supported by cluster-DMFT calculations [4], which suggest a spin-polaron band emerges at this energy scale. [1] M. R. Norman, Science 332, 196 (2011). [2] Z. Hiroi et al., Nature 371, 139 (1994). [3] I.Yamada et al., PRB 72, 224503 (2005). [4] B. Bacq-Labreuil, C. Fawaz et al., PRL 134, 016502 (2025)

Strong electron correlations and topological classification are two major research frontiers of condensed-matter theory. While much work has been done for noninteracting systems and for correlated one-dimensional models, much less is known for correlated systems in two and higher dimensions. Here, we discuss a generic model of a Chern insulator supplemented by a Hubbard interaction in arbitrary even dimension D and demonstrate that the model remains well defined and nontrivial in the infinite-D limit. Dynamical mean-field theory is applicable and predicts a phase diagram with a continuum of topologically different phases separating a correlated Mott insulator from the trivial band insulator. This provides us with exact results for a topologically nontrivial and strongly correlated system. We discuss various features, such as the elusive distinction between insulating and semimetal states, which are unconventional already in the noninteracting case. Topological phases are characterized by a nonquantized Chern density replacing the Chern number in the infinite-D limit.

We investigate the emergence of topological features in the charge excitations of Mott insulators in the Chern-Hubbard model. In the strong correlation regime, treating electrons as the sum of holons and doublons excitations, we compute the topological phase diagram of Mott insulators at half-filling using composite operator formalism. The Green function zeros manifest as the tightly bound pairs of such elementary excitations of the Mott insulators. Our analysis examines the winding number associated with the occupied Hubbard bands and the band of Green's function zeros. We show that both the poles and zeros show gapless states and zeros, respectively, in line with bulk-boundary correspondence. The gapless edge states emerge in a junction geometry connecting a topological Mott band insulator and a topological Mott zeros phase. These include an edge electronic state that carries a charge and a charge-neutral gapless zero mode. Our study is relevant to several twisted materials with flat bands where interactions play a dominant role.

The translational symmetry of the ionic potential in a crystal results in a periodic electronic structure in momentum space. Analogously, the periodic electric field of an intense light pulse can create replica of electronic states. These so-called Floquet state are shifted in energy by the photon frequency [1]. During the last decade, a few examples of the generation of Floquet states have been revealed using time-resolved angle-resolved photoemission spectroscopy (ARPES) [2]. In these works, band gap opening at the crossing between the Floquet states and the original electronic states have been observed as a consequence of strong light-matter interaction. In that framework, Lindner and coworkers have proposed to use Floquet states to engineer a transient topological state with light pulses [3]. In this talk, I will present our recent study of the semiconductor SnTe, for which the occurrence of topological surface states is intimately linked to a polar structural phase transition [4]. First, I will show static ARPES data acquired as a function temperature. Our unprecedented thin film quality allows us to follow the evolution of the Rashba splitting induced by the structural distortion in the bulk bands, revealing substantial deviations from a mean-field-like transition [5]. In a second part, I will elaborate on the implications of our results for the topological nature of the surface states in SnTe. Using time-resolved ARPES data, I will then show how tailored light pulses can be used to photoinduce a transient topological state in SnTe in the absence of any significant structural change [6]. I will discuss how strong light-matter interaction involving Floquet states is the relevant mechanism. [1] S. Ito, M. Schüler, M. Meierhofer, S. Schlauderer, J. Freudenstein, J. Reimann, D. Afanasiev, K. A. Kokh, O. E. Tereshchenko, J. Güdde, M. A. Sentef, U. Höfer and R. Huber, 616, 696 (2023). [2] F. Mahmood, C. Chan, Z. Alpichshev, D. Gardner, Y. Lee, P. A. Lee and N. Gedik, Nature Physics 12, 306 (2016); S. Zhou, C. Bao, B. Fan, F. Wang, H. Zhong, H. Zhang, P. Tang, W. Duan and S. Zhou, Phys. Rev. Lett. 131, 116401(2023). [3] N. H. Lindner, G. Refael and V. Galitski, Nature Physics 7, 490 (2011). [4] E. Plekhanov, P. Barone, D. Di Sante and S. Picozzi, Phys. Rev. B 90, 161108(R) (2014). [5] F. Chassot, A. Pulkkinen, G. Kremer, T. Zakusylo, G. Krizman, M. Hajlaoui, J. H. Dil, J. Krempasky, J. Minar, G. Springholz and C. Monney, Nano Letters 24, 82 (2024). [6] F.Chassot, G. Kremer, A. Pulkkinen, C. Wang, J. Krempasky, J. Minar, G. Springholz, M. Puppin, J.H. Dil, C. Monney, condmat:2502.11967 (2025).

The search for topological electronic states in strongly correlated systems is a key frontier in modern condensed matter physics. While topological flat bands have recently been found in correlated van der Waals heterostructures, a lesser-known prediction from the early cuprate era suggested that singlet d-wave superconductors host unconventional midgap Andreev bound states on most side surfaces—except the (100) surface (1). Later, these states were identified as topological flat bands, pinned to the Fermi level by chiral symmetry and connecting surface projections of nodes with opposite nontrivial winding numbers, similar to Fermi-arc surface states in Weyl semimetals (2). These flat bands are predicted to give rise to novel correlated surface phases, such as phase crystals (3), time-reversal broken superconductivity (4), or magnetic order (5), and may serve as diagnostics to study the pseudogap. Moreover, they can be viewed as doublets of spinful Majorana fermions, potentially relevant for quantum information processing (6). However, direct experimental evidence for the topology of these flat bands has remained elusive. Edge tunnelling spectroscopy has observed a zero-bias anomaly, but the characteristic momentum dependence of the flat bands enforced by the topologically non-trivial winding numbers has not been directly observed. A major obstacle has been preparing high-quality side surfaces of cuprates for momentum-resolved spectroscopy. We addressed this using a new method to prepare (110) and (100) side surfaces of single-crystal high-Tc cuprates for ARPES. Whilst the bulk electronic bands are quite sharp, the flat band signal is blurred, most likely due to surface disorder that spreads the intensity of the flat band peak across the energy gap. Nevertheless, we are able to observe a characteristic momentum dependence in our particle-hole symmetrized spectra in the data from the (110) surface that is consistent with the presence of a topological flat band. Moreover, as the temperature is lowered, the flat band signal strengthens, indicating localization toward the surface as superconducting order parameter increases. In contrast, the (100) surface shows no such zero energy anomaly as observed on the (110) surface, which provides further evidence that the latter is of topological origin. Time-permitting, I will also present a momentum-resolved fingerprint of Mottness that allowed us to distinguish between a band- and Mott-insulating phase in the dimerized breathing Kagome lattice insulator Nb3Br8, which contains an even number of electrons per unit cell (7). This finding may help to understand the mysterious field-free Josephson diode effect that has been observed in heterostructures formed from this material. 1. C.-R. Hu, Phys. Rev. Lett. 72, 1526–1529 (1994). 2. S. Ryu, Y. Hatsugai, Phys. Rev. Lett. 89, 077002 (2002). 3. D. Chakraborty, T. Löfwander, M. Fogelström, A. M. Black-Schaffer, npj Quantum Mater. 7, 1–10 (2022). 4. C. Honerkamp, K. Wakabayashi, M. Sigrist, EPL. 50, 368 (2000). 5. A. C. Potter, P. A. Lee, Phys. Rev. Lett. 112, 117002 (2014). 6. X.-J. Liu, C. L. M. Wong, K. T. Law, Phys. Rev. X. 4, 021018 (2014). 7. M. Date et al., Nat Commun. 16, 4037 (2025).

The physical properties of modern condensed matter systems are largely determined by the details of their electronic structure. Phenomena like magnetism and superconductivity are the result of a complex interplay of charge and spin-dependent interactions on different length scales. The concepts of symmetry and topology govern the novel field of quantum materials. Only recently, comprehensive experimental access to the spin-resolved band structure of solids became feasible by spin-resolved momentum microscopy [1] and imaging spin-filters [2]. This long-awaited breakthrough of a direct spin-resolving measurement of electronic states reveals intricate effects of electron interactions, and directly affects our understanding of the complex interplay between magnetism, spin-orbit interaction, symmetry, and topology, challenging novel theoretical concepts in condensed matter physics. Our measurements give evidence that electron correlations in elemental ferromagnets like cobalt are of strongly nonlocal origin [3]. Moreover, combining strong spin-orbit interaction [4] and ferromagnetism gives rise to complex spin-orbital textures, and topological phase transitions in the Fermi surface of ultrathin iron films [5]. The intrinsic link between magnetism and topological phases adds a new level of functionality to magnetic materials, including anomalous transport properties, controllable topology and emergent states [6]. [1] C. Tusche, A. Krasyuk, J. Kirschner, Ultramicroscopy 159, p. 520 (2015) [2] C. Tusche, et al., Appl. Phys. Lett. 99, 9, 032505 (2011) [3] C. Tusche et al., Nat. Commun. 9, 3727 (2018) [4] Y.-J. Chen et al., Nat. Commun. Phys. 4, (2021) [5] Y.-J. Chen, et al., Nat. Commun. 13, 5309 (2022) [6] Y.-J. Chen, et al., Appl. Phys. Lett. 124, 093105 (2024)

Recent theoretical efforts predicted a type of unconventional antiferromagnet characterized by the crystal symmetry C (rotation or mirror), which connects antiferromagnetic sublattices in real space and simultaneously couples spin and momentum in reciprocal space. This results in a unique C-paired spin-valley locking (SVL) and corresponding novel properties such as piezomagnetism and noncollinear spin current even without spin-orbit coupling. However, the unconventional antiferromagnets reported thus far are not layered materials, limiting their potential in spintronic applications. Additionally, they do not meet the necessary symmetry requirements for nonrelativistic spin current. Here, we report the realization of C-paired SVL in a layered room-temperature antiferromagnetic compound, RbV2Te2O. Spin resolved photoemission measurements directly demonstrate the opposite spin splitting between C-paired valleys. Quasi-particle interference patterns reveal the suppression of inter-valley scattering due to the spin selection rules, as a direct consequence of C-paired SVL. All these experiments are well consistent with the results obtained from first-principles calculations. Our observations represent the first realization of layered antiferromagnets with C-paired SVL, enabling both the advantages of layered materials and possible control through crystal symmetry manipulation. These results hold significant promise and broad implications for advancements in magnetism, electronics, and information technology.

Materials with strong spin-orbit coupling and non-trivial topology are actively studied due to their novel physics and potential applications for spintronics and quantum information. To drive these investigations, direct experimental probes of spin and its dynamics are indispensable for unraveling— and manipulating— the relevant interactions at the heart of these materials. We will present spin- and time- resolved ARPES experiments on two material systems in the spotlight of spin-orbit physics. First, we use spin-resolved ARPES to examine the spin-texture of the noncentrosymmetric semiconductor BiTeCl. Our measurements reveal dramatic deviations from idealized Rashba-type behavior, including a reversal of the spin-polarization near the Fermi level. We show how this is described by higher-order contributions to the canonical Rashba model in combination with the local symmetry environment of the relevant electronic states. Next, we use time-resolved ARPES to explore whether the Rashba interaction can be optically manipulated by driving ultrafast lattice distortions which couple to the local polar environment. Second, we use spin-resolved ARPES to probe the spin-orbital texture of the magnetic topological MnBi2Te4. Despite this material’s importance for exotic quantum effects, its surface electronic structure and behavior in the presence of magnetic order are poorly understood. By mapping our results onto a model wavefunction, we quantitatively extract the spin- and orbital- angular momenta for the relevant electronic states. Our analysis reveals a substantial contribution of in-plane p orbitals, which have an antagonistic effect on the propensity of the surface states to develop a magnetic gap. These results reconcile disparate interpretations of band assignments in the literature, and shed light on the long-standing puzzle of the gapless Dirac cone in this material family.

Over the past decade the temporal characteristics of photoemission have become an important observable for studying electron motion in atoms and solids [1]. However, the relation of the experimental data of the streaking or RABBITT spectroscopies to the electronic structure of the target is still a matter of debate. As a step towards solving this problem we present a combination of the one-step photoemission theory with the Wigner delay formalism that allows to calculate from first principle the photoelectron escape time from crystals. The rigorous theory is illustrated by an exactly solvable model [2], and its ab initio implementation with the augmented-plane-wave-based scattering method is applied to the interpretation of the streaking experiment on Mg(0001). The escape time manifests a strong dependence on the initial state and rapidly varies with the photon energy due to lattice scattering. For a spectrally narrow wave packet the temporal characteristics can be inferred from the analysis of the scattering state. In particular, various fascinating phenomena are revealed in transmission through graphene multilayers: In the forbidden gaps the wave packet transit time saturates with thickness and in the allowed bands it oscillates following transmission resonances [3]. Counterintuitive negative transit time is discovered in monolayers of graphene, h-BN, and oxygen [4]. Interestingly, the Wigner time delay diverges at the scattering resonances caused by the emergence of secondary diffracted beams. These results suggest a way to control the propagation timing of the wave packet in a non-tunneling regime. REFERENCES [1] F. Siek et al., Science 357, 1274 (2017) [2] R.O. Kuzian and E.E. Krasovskii, Phys. Rev. B 102, 115116 (2020) [3] R.O. Kuzian, D.V. Efremov, and E.E. Krasovskii, Phys. Rev. Research 7, 013180 (2025) [4] E.E. Krasovskii and R.O. Kuzian, arXiv:2404.19440 (2024)

Spin-orbit coupling in 3D topological insulators and the spin-momentum locking in the topological states present unique opportunities for control of electronic states via light coupling. For example, photocurrent excitation and control with circularly polarized light has been demonstrated, as well as generation of polarized excitons linked to topological states. In this talk I will discuss how we use time-resolved ARPES to study coupling of light to topological electronic states, and in particular gain better understanding of photocurrent generation in topological insulators. Moreover, using circular polarized light in two-photon photoemission from Bi$_2$Se$_3$ allowed us to identify interference of different photoemission pathways. By analyzing the rise times of the population following the optical excitation, we gained a complete view of the occupied and unoccupied electronic states, and how they are coupled by the light. We resolve the bands involved in the interfering photoemission paths, and demonstrate control of the interference via changes in photon energy and polarization. Resolving and controlling interference pathways in photoemission paves the way to phase sensitivity in ARPES as well as coherent control of photoexcited states in solids.

Understanding the electronic properties of CuO$_2$ planes with a low carrier concentration, close to the half-filled Mott state, is essential for elucidating the mechanism of high-$T_c$ superconductivity. The electronic phase diagram of cuprates established thus far has been predominantly based on studies of single- and bilayer cuprates. However, in these compounds, the conductive CuO$_2$ layers are adjacent to dopant layers, leading to strong disorder effects. As a result, the intrinsic nature of the electronic states, particularly in the lightly doped regime—where disorder effects are pronounced due to weak screening—may not have been fully understood. This issue has likely been a major obstacle to uncovering the superconducting mechanism in cuprates. This challenge, however, can be overcome by focusing on the inner CuO$_2$ planes in multilayer cuprates, which remain isolated from direct contact with dopant layers, thereby realizing a clean and nearly ideal electronic state ¥cite{Mukuda2012}. In this presentation, I will introduce our recent angle-resolved photoemission spectroscopy (ARPES) studies on multilayer cuprates Ba$_2$Ca$_{n-1}$Cu$_n$O$_{2n}$(F,O)$_2$ ¥cite{Kunisada2020, Kurokawa2023}. These compounds feature pristine inner CuO$_2$ layers that provide an opportunity to reveal an intrinsic phase diagram of cuprates, which has remained elusive. Most notably, we discovered small Fermi surface pockets around $(¥pi/2, ¥pi/2)$. Moreover, we observed that a $d$-wave superconducting gap opens along these pockets, indicating the coexistence of superconductivity and antiferromagnetic order within the same CuO$_2$ plane. Furthermore, by tuning the hole carrier concentration, we uncovered an unexpected phase transition from the superconducting to the metallic state. This result stands in stark contrast to the nodal liquid state with polaronic features, which has been proposed as an anomaly in heavily underdoped cuprates, possibly indicating a more intrinsic electronic state in lightly doped cuprates. ¥begin{thebibliography}{9} ¥bibitem{Mukuda2012} H. Mukuda et al., ¥textit{J. Phys. Soc. Jpn.} ¥textbf{81}, 011008 (2012). ¥bibitem{Kunisada2020} S. Kunisada et al., ¥textit{Science} ¥textbf{369}, 833 (2020). ¥bibitem{Kurokawa2023} K. Kurokawa et al., ¥textit{Nat. Commun.} ¥textbf{14}, 4064 (2023). ¥end{thebibliography}

TBA

The study of the correlation between electronic structure and collective excitations is a crucial prerequisite for understanding the emergence of electronic order in quantum materials, including unconventional superconductors, antiferromagnets, and van der Waals heterostructures. I will present first results obtained with the ToF-PAX-RIXS instrument installed and recently commissioned on the P04 beam line at DESY and which uniquely integrates resonant inelastic X-ray scattering (RIXS) and angle resolved photoemission spectroscopy (ARPES) for the combined study of electronic structure and collective excitations in quantum materials. The principle is based on the efficient detection of photoelectrons in the soft X-ray regime using time-of-flight photo-electron emission microscopy (ToF-PEEM). For the PAX-RIXS mode, RIXS photons are converted to electrons in a thin-film converter and the photon energy information is preserved in the kinetic energy distribution of the emitted electrons [1]. A brief description of the new instrument and of the first results on an antiferromagnetic cuprate will be presented. [1] Schunck et al. NJP 26, 053008 (2024)

Josephson scanning tunneling microscopy (JSTM) is a powerful probe of the local superconducting order parameter, but studies have been largely limited to cases where superconducting sample and superconducting tip both have the same gap symmetry- either s-wave or d-wave. It has been generally assumed that in an ideal s-to-d JSTM experiment the critical current would vanish everywhere, as expected for ideal c-axis planar junctions. We show here that this is not the case. Employing first-principles Wannier functions for Bi2Sr2CaCu2O8+δ, we develop a scheme to compute Josephson critical current (Ic) and quasiparticle tunneling current measured by JSTM with sub-angstrom resolution. We demonstrate that the critical current for tunneling between an s-wave tip and a superconducting cuprate sample has largest magnitude above O sites and it vanishes above Cu sites. Ic changes sign under π/2-rotation and its average over a unit cell vanishes, as a direct consequence of the d-wave gap symmetry in cuprates.We discuss similarities and differences in JSTM observables and conventional STM observables, making specific predictions that can be tested in future JSTM experiments. In the second part of the talk, I discuss a first-principles approach to conventional STM on the canonical unconventional superconductor Sr2RuO4. In many cases, researchers have reported being unable to detect the gap at all. Recently, an investigation of this issue on various local topographic structures on a Sr-terminated surface found that superconducting spectra appeared only in the region of small nanoscale canyons, corresponding to the removal of one RuO surface layer. We analyzed the electronic structure of various possible surface structures using first principles methods, and argued that bulk conditions favorable for superconductivity can be achieved when removal of the RuO layer suppresses the RuO4 octahedral rotation locally. We further propose alternative terminations to the most frequently reported Sr termination where superconductivity surfaces should be observed.

Time and angle-resolved photoemission spectroscopy (TR-ARPES) has emerged as an excellent tool to study the dynamics of charge density waves. An ultrafast optical probe pulse typically leads to a CDW melting with a subsequent recovery, both of which can be tracked by TR-ARPES. Along with the re-formation of the CDW order, several other effects might be observable in the TR-ARPES signal, for instance hot electrons, coherent phonons, a coherent CDW amplitude mode and others, and it is challenging to disentangle these. In this talk, I will discuss the application of unsupervised machine learning techniques, especially k-means, to uncover patterns in multi-dimensional TR-ARPES data that could otherwise be missed. I will discuss graphene as a simple model system, the Weyl semimetal PtBi2 [1] and finally the CDW in LaTe3. References [1] Paulina Majchrzak, Physical Review Research 7, 013025 (2025).

In the last fifteen years a few approaches were developed which establish how the strength of the electron-phonon coupling (EPC) can be determined through Resonant Inelastic X-ray Scattering (RIXS) measurements [EPL 95, 27008 (2011)]. Moreover, the theoretical studies from Refs. [PRB 51, 3840 (1995), PRB 82, 064513 (2010)] provide an analytical expression for the EPC in the high-temperature superconducting cuprates as a function of the phonon wave vectors and reproduce nicely the trends observed in previous RIXS experiments [PRR 2, 023231 (2020), PRL 123, 027001 (2019)]. However, a more systematic assessment covering other directions in the reciprocal space was still lacking.\par In this work, we carefully study the Cu L$_3$ RIXS intensity of the bond-stretching phonon modes in CaCuO$_2$, La$_2$CuO$_{4+\delta}$, La$_{1.84}$Sr$_{0.16}$CuO$_4$ and YBa$_2$Cu$_3$O$_{7–\delta}$ along the high symmetry ($\zeta$,0), ($\zeta$,$\zeta$) directions and as a function of the azimuthal angle $\phi$, at fixed modulus of the in-plane momentum \textbf{q}$_{\parallel}$ (\textit{q}). We find that RIXS signal follows the expected trend of $ g(q) \propto \mathrm{sin}^2(\pi q) $ along each of the ($\zeta$,0) and ($\zeta$,$\zeta$) directions. However, the relative values at fixed \textit{q} are opposite for experiments and well-established theories. In particular, the theory based on tight-binding calculations predicts that the EPC is stronger along ($\zeta$,$\zeta$) than ($\zeta$,0), while experimentally we find the opposite. On the contrary, with DFT the EPC matches better the experimental data. We attribute the success of DFT to more accurate modelling of the electronic band structure. We conclude that the details of the electronic structure matter for the \textbf{q}-dependent EPC, while the $\mathrm{sin}^2(\pi q) $ evolution of $g$(\textbf{q}) along the high symmetry directions is mostly determined by the phonon properties.

Strong quantum fluctuations in layered strange metals, such as cuprates and Sr2RuO4, are supposed to mediate superconductivity and strongly affect their low energy excitations. Here, we use momentum-dependent electron energy-loss spectroscopy in transmission to study the momentum dependence of collective charge excitations in the layer "strange" metal Sr2RuO4. Outside the single particle continuum, we observe well-defined optical (acoustic) plasmon due to an in-phase (out-of-phase) charge oscillation of neighbouring layers exhibiting a positive quadratic (linear) dispersion. It is possible to describe the optical plasmon excitations at high energies in a mean-field random phase approximation without mass renormalization and overdamping, i.e., without taking correlation effects into account. These observations may be reconciled with the enhancement of the acoustic plasmon velocity at low energies, due to correlation effects, by an energy dependent effective mass of the charge carriers. Due to coupling to spin excitations the mass renormalization may be large at low energies, while vanishing in the energy range of the optical plasmon well above the spin fluctuation energy.