List of talks

Magnetic anisotropy is a fundamental property of magnetic materials that governs the stability and directionality of their magnetization. At the atomic level, magnetic anisotropy originates from anisotropy in the orbital angular momentum (L) and the spin-orbit coupling that connects the spin moment of a magnetic atom to the spatial symmetry of its ligand field environment. Generally, the ligand field, that is necessary for the anisotropy, also quenches the orbital moment and reduces the total magnetic moment of the atom to its spin component. However, careful design of the coordination geometry of a single atom can restore the orbital moment while inducing uniaxial anisotropy, as we present here for single iron atoms deposited on top of a thin MgO film. Scanning tunneling spectroscopy and x-ray absorption spectroscopy measurements show a large magnetic anisotropy of 19 meV, as well as relaxation times of many milliseconds. These results offer a strategy, based on sy
 mmetry arguments and careful tailoring of the interaction with the environment, for the rational design of nanoscopic permanent magnets and single atom magnets.

I will present new developments in the theory of topological metals and topological superconductors. A Weyl semimetal phase has be predicted by us  to exist in TaAs and related families of compounds and has recently been discovered experimentally. I will also show that topogical one dimensional Superconductivity can exist in a new platform - of magnetic chains on the surface of a conventional Pb Superconductor.

The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a concomitant spin density wave transition both being in close proximity to the superconducting phase. A key question is the microscopic mechanism of high-Tc  superconductivity and its relation to orbital ordering and magnetism. Here we study the NaFeCoAs superconductor using polarization resolved Raman spectroscopy. The Raman susceptibility shows critical non-symmetric charge fluctuations across the entire phase diagram associated with a hidden ordered state. The charge fluctuations are interpreted in terms of plasma waves of quadrupole intra-orbital excitations in which the electron and hole Fermi surfaces breath in-phase. Below , these plasmons undergo a metamorphosis into a coherent ingap mode of extraordinary strength and at the same time serve as a glue for non-conventional superconducting pairing. 
http://arxiv.org/abs/1410.6456 

The spin–momentum locking of topological states offers an ideal platform to explore novel magnetoelectric effects. These intimately depend on the ability to manipulate the spin texture in a controlled way. Here we combine scanning tunnelling microscopy with single-atom and -molecule deposition to detect and map the evolution of topological states under the influence of different magnetic perturbations. We obtain signatures of Dirac fermion-mediated magnetic order for extremely dilute adatom concentrations. This striking observation is found to critically depend on the single adatoms’ magnetic anisotropy and the position of the Fermi level. Our findings open new perspectives in spin engineering topological states at the atomic scale.

Analysis of excitations in materials is of wide spread interest due to the coupling of electronic and bosonic degrees of freedom, in particular for high temperature super-conductors. Typically the spectrum and dispersion of excitations is investigated by e. g. inelastic scattering and angle-resolved photoemission spectroscopy (ARPES). Here we report on femtosecond time-resolved ARPES results on the cuprates and the Fe-pnictides which were obtained by 1.5 eV pump and 6 eV probe photon energies with typically 100 fs time resolution. We discuss how such excitations are probed in tr-ARPES. On the cuprates we have identified a weakening of the well known kink in the electronic structure E(k) near 70 meV below the Fermi level E_F, which represents a pump-induced reduction of the electron-boson coupling strength [1]. Coupling of electrons to that mode is also evident at an energy of 70 meV above E_F from a step in the energy-dependent electron relaxation times tau(E). A pronounced decrease of the step height in tau(E) with increasing pump fluence reflects that weakening of coupling also above E_F. Experiments on Fe-pnictides exhibit a similar step in the energy dependent electron relaxation times, although the effect is weaker and occurs at higher energies in agreement with e. g. inelastic neutron scattering experiments. Furthermore, electron redistribution upon pump laser excitation modifies the Fermi momentum k_F [1], which allows (a) to transiently change the effective doping level and (b) suggests a new way to probe the dynamic response of the Fermi surface of complex materials.

This work was conducted in collaboration with I. Avigo, S. Freutel, M. Ligges, L. Rettig, M. Sandhofer, J. D. Rameau, P. D. Johnson, P. Zhou, G. D. Gu, H. Eisaki, T. Wolf, P. Gegenwart, H. S. Jeevan, A. F. Kemper, and M. Sentef.
Funding by the priority program SPP 1458 of DFG, by the Mercator Research Center Ruhr, and the EU within the FP 7 under GO FAST is gratefully acknowledged.

[1] J. D. Rameau et al., Phys. Rev. B 89, 115115 (2014).

Electromagnetic excitations called surface plasmon polaritons (SPP) can be found at the interface between a metal and a dielectric. The definition refers to the coupling between a collective motion of charges in the metal, the surface plasmon, and the radiated electromagnetic field, the polariton, which is evanescently confined in the direction perpendicular to the surface. The term polariton is used to define a field that is strongly coupled to a dipolar excitation, which in this case is provided by the distribution of electrons in the metal. SPPs can be induced at the surface of metallic nanostructures, guiding the electromagnetic field in bends, corners or any arbitrary shape thanks to their ability to spatially confine it. In novel materials which exhibit exotic electronic properties due to dimensional confinement, SPPs have been reported to be very sensitive to applied electric or magnetic fields, to have low loss propagation and an unusually high refractive index. Such phenomena are widely investigated because of their potential application in photonic circuits, where sub-wavelength guiding of the electromagnetic field is crucial to miniaturization.
Here, we show that a plasmonic standing wave can be induced on an isolated metallic nanowire with an intense fs laser pulse, and demonstrate the possibility to control its spatial interference pattern by tuning the properties of light excitation, such as its polarization. A fs snapshot of the interaction between the imaging electrons and the SPPs is taken by a new ultrafast-imaging methodology that uses an electron imaging filter to form a 2D projection of one spatial coordinate and energy. These space- energy resolved images simultaneously show the quantization of the photo-induced SPP field and an interference pattern in its spatial distribution, providing an interesting perspective on the quantum properties of plasmonic fields.

TBD

Underdoped cuprates display a complex set of anomalous properties whose organizing principle has yet to be found. Translational and rotational symmetries are broken, the Fermi surface is reconstructed, and superconductivity is suppressed. Here we disentangle these intertwined phenomena by mapping their onset temperatures in YBa2Cu3Oy, detected in transport measurements across the doping phase diagram. We show that transport anisotropies can be used to define the onset temperature of nematicity and explore its interplay with charge-density-wave order, superconductivity and the pseudogap phase. The nematic phase emerges as a central feature of the cuprate phase diagram. We argue that nematicity is linked to the pseudogap and present compelling evidence for a pseudogap phase with nematic character that shapes the domes of both superconductivity and charge order.

S. D. Edkinsa,b, M. H. Hamidianb, K. Fujitac, C. –K. Kimc, A.P. Mackenziea,d,
S. Uchidae, H. Eisakif, M. J. Lawlerb,g, E.-A. Kimb, S. Sachdevh,i, J. C. Davisa,b,c
aUniversity of St. Andrews, St. Andrews, Scotland
bCornell University, Ithaca, USA
cBrookhaven National Laboratory, Upton, USA
dMax-Plank Institue for Chemical Physics of Solids, Dresden, Germany
eInstitute of Advanced Industrial Science and Technology, Tsukuba, Japan
fUniversity of Tokyo, Tokyo, Japan
gBinghampton University, Binghampton, USA
hHarvard University, Cambridge, USA
iPerimeter Institute for Theoretical Physics, Waterloo, Canada

The emergence of density wave order within the under-doped cuprates is now well
established. However the nature of the density wave, the mechanism of its formation
and its relationship to both the pseudo-gap and high temperature d-wave
superconductivity remain unclear. Visualising the electronic structure within each CuO2 unit-cell, we found that this density wave exhibits a predominantly d-symmetry form factor [1]; a long predicted but hitherto unobserved unconventional density wave state.

Now we report [2] that the cuprate density wave state maintains its predominantly dsymmetry form factor throughout the whole pseudo-gap region of the phase diagram, that its wave-functions are particle–hole anti-symmetric in the sense that a phase difference of π exists between spatial modulations of the filled states and the equivalent empty states, and that the characteristic energy of the density wave modulations in the electronic spectrum is actually the pseudo-gap energy.

We also shed light [3] on the relationship between the d-symmetry form factor density wave (dFF-DW) and d-wave superconductivity. By measuring the magnetic field induced changes in the electronic structure with intra unit cell resolution we find an enhancement of the density wave amplitude proximal to superconducting vortex cores: a direct demonstration of the intertwined nature of the dFF-DW and superconductivity.
We find that the states dominating this enhancement are those at the boundary between superconducting and pseudo-gap states in momentum space.

Taken together the results imply that the strange electronic structure of the pseudo-gap is actually parent to the dFF-DW. Moreover, this state competes directly for spectral weight with the d-symmetry Cooper-paired superconductor at the boundary between superconductivity and pseudo-gap in momentum-space.
[1] K. Fujita, M. H. Hamidian, S. D. Edkins et al PNAS 111, E3026 (2014)
[2] M. H. Hamidian*, S. D. Edkins* et al (2015)
[3] M. H. Hamidian*, S. D. Edkins* et al (2015) 

We use a real-space renormalization group procedure to determine the ‘flux noise' spectrum of random-bond Heisenberg spin models in 1d and quasi 1d geometries. Our approach accounts for both the renormalization of the system Hamiltonian and a generic probe that measures this noise. We demonstrate that the structure factor, at both high and low temperatures, exhibits a piece-wise high- and low-frequency power-law behavior. We discuss implications of our results for the explanation of 1/f flux noise in SQUIDs.

We generalize the theory of Cooper-pairing by spin excitations in the metallic antiferromagnetic state to include situations with electron and/or hole pockets.Weshow that Cooper-pairing arises from
transverse spin waves and from gapped longitudinal spin fluctuations of comparable strength. However, each of these interactions, projected on a particular symmetry of the superconducting gap, acts
primarily within one type of pocket. We find a nodeless dx2−y2-wave state is supported primarily by the longitudinal fluctuations on the electron pockets, and both transverse and longitudinal fluctuations
support nodal dx2−y2-wave symmetry on the hole pockets. Our results may be relevant to the asymmetry of the AF/SC coexistence state in the cuprate phase diagram.

Geometrical frustration describes situations where interactions are incompatible with the lattice geometry and stabilizes exotic phases such as spin liquids. Whether geometrical frustration of magnetic interactions in metals can induce unconventional quantum critical points is an active area of research. We focus on the hexagonal heavy-fermion metal CeRhSn where the Kondo ions are located on distorted Kagome planes stacked along the c-axis. Low-temperature specific heat, thermal expansion and magnetic Grüneisen parameter measurements prove a zero-field quantum critical point. The linear thermal expansion, which measures the initial uniaxial pressure derivative of the entropy, displays a striking anisotropy. Critical and non-critical behavior along and perpendicular to the Kagome planes, respectively, proves that quantum criticality is driven be geometrical frustration. We also discovered a spin-flop-type metamagnetic crossover. This excludes an itinerant scenario and suggests quantum criticality is related to local moments in a spin-liquid like state. If time allows, we will also discuss our related results on quantum criticality in pyrochlore Pr2Ir2O7 and Pr2Zr2O7.

One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive as electronic correlations are expected to greatly accelerate the electron–boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and frequency range of transient reflectivity measurements up to an unprecedented level, enabling us to directly observe the ∼16 fs build-up of the effective electron–boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t–J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations

A useful phase diagram of a complex chemical or electronic system may not follow from the simplest chemical parameter. Cuprate superconductors are such systems and we show that the charge content of the bonding orbitals of copper as well as oxygen, which can be determined quantitatively with nuclear magnetic resonance and sum up to the actual stoichiometry, gives a very different perspective on the cuprate physics. For example, it explains the famous Uemura relation and unifies electron and hole doped systems. 

In this talk, I will present results on the unconventional superconductor LiFeAs obtained by scanning tunneling micrsopcopy/spectroscopy (STM/STS). This method on the one hand provides the possibility to directly measure the spatial variation of the order parameter in the presence of a magnetic vortex lattice, and thus the coherence length. On the other hand, the quasiparticle interference (QPI) which arises from quasiparticle scattering off impurities, and which sensitively depends on the superconducting wave function can be probed directly. LiFeAs belongs to the class of iron arsenide superconductors which have been discovered in 2008. This material possesses very clean and  charge neutral cleaved surfaces without a surface state and is thus perfectly suited for STM/STS. Our QPI data are perfectly consistent with band structure data derived from angular reolved photoemission spectroscopy [1]. However, the further analysis yields incompatibility with elementary s- or d-wave order parameters, suggestive of a more complex scenario [2]. I will discuss these findings in the context of further experimental data and theoretical results.

[1] C. Hess, S. Sykora, T. Hänke, R. Schlegel, D. Baumann, V. B. Zabolotnyy, L. Harnagea, S. Wur­mehl, J. van den Brink, B. Büchner, Phys. Rev. Lett. 110, 017006 (2013)
[2] T. Hänke, S. Sykora, R. Schlegel, D. Baumann, L. Harnagea, S. Wurmehl, M. Daghofer, B. Büch­ner, J. van den Brink, C. Hess, Phys. Rev. Lett. 108, 127001 (2012) 

We have developed a method, based on piezoelectric stacks, for applying continuously tuneable compressive and tensile strains to test samples. We have also developed methods for mounting samples to ensure high strain homogeneity over the test region. The maximum achievable strain is at present ~0.5%, which is a large strain and sufficient to induce substantial changes to the Fermi surfaces of many metallic systems. It is possible, for example, to precisely investigate the response to lifting symmetries of the crystal lattice. It will also be possible to tune certain materials through density-of-states peaks and probe the response. 

So far we have investigated the layered compounds Sr2RuO4 and Sr3Ru2O7. The former is a Tc=1.5 K superconductor, under zero strain; when the tetragonal symmetry of the unstrained crystal is lifted both Tc and Hc2 increase dramatically. Sr3Ru2O7 is a metmagnetic compound with a quantum critical phase around the metamagnetic transition. Within this phase, relatively small applied strains induce large transport anisotropies: a strain of 0.1%, which by eye does not induce any dramatic changes to the Fermi surfaces, leads to an in-plane resistive anisotropy of three. 

Abstract: It has recently become possible to construct atomically precise magnetic structures atom by atom and probe their fundamental magnetic properties with atomic resolution. This has opened the possibility of fabricating a broad range of model spin systems, including spin chains and spin ladders, that are predicted to exhibit novel quantum ordering. In many situations, coupling such systems to the external environment (e.g. through exchange coupling to a nearby metal) has detrimental consequences such as decreasing the lifetime of spin excitations. However, such external coupling to the local environment can also create new opportunities for manipulating and controlling the magnetic properties and interactions that are manifested. Indeed, in recent reports such as the observation of Majorana Fermions in ferromagnetically coupled atomic chains on a superconductor surface [1], novel topological effects are only possible through such external coupling.

Using scanning tunneling microscopy and spectroscopy, we study the effects of interactions between individual magnetic atoms that are separated from an underlying metallic surface by a thin-insulating layer of copper nitride (Cu2N). For Co atoms on large Cu2N islands, we find that exchange coupling of the spin to the metallic bath can result in Kondo screening as well as dramatically shifting the energy levels of the spin and modifying its effective magnetic anisotropy [2], the property that determines the stability of its spin orientation. By controlling the exchange coupling, we can tune both the Kondo screening of the systems as well as the anisotropy energy over a broad range of values. Furthermore, this system constitutes one of the few cases in which an open quantum system’s energy levels, rather than just its excited-state lifetimes, can be controllably and observably renormalized. These results highlight the non-detrimental effects that can arise from the coupling of an open quantum system to its environment, and the importance of including these when exploring new model quantum spin systems.

[1] S. Nadj-Perge et al., Science 346, 6209 (2014)
[2] J.C. Oberg et al., Nature Nanotechnology 9, 64-68 (2014).

A two-dimensional s-wave superconductor in a magnetic field with a sufficiently strong Rashba spin-orbit coupling is a candidate system for a topological superconductor. Typically, the required magnetic field to convert the superconductor into a topologically non-trivial state is however by far larger than the upper critical field, which excludes its realization. This problem is overcome by rotating the magnetic field into the superconducting plane. The character of the superconducting state changes with the strength and the orientation of the magnetic field. A topological state indeed extends to an in-plane field orientation. Mapping the spin texture in momentum space reveals a meron-like structure. In analogy to skyrmion patterns, the momentum-space spin texture translates into an integer number which offers an alternative to reflect the topological character of the superconducting state. This number can even be followed from the superconducting into the normal conducting state. 

A many-body quantum system on the verge of instability between competing ground states may exhibit emergent phenomena. Interacting electrons on triangular lattices are likely subjected to multiple instabilities in the charge and spin degrees of freedom. The molecular conductors are superior model systems for studying the issue because of the designability and controllability of the material parameters such as lattice geometry and bandwidth. I present various quantum manifestations that interacting electrons on triangular lattices show under variable correlation, disorder and (unfortunately, fixed) doping in molecular conductors. The topics include i) the quantum criticality of the pressure-induced Mott transition revealed by the resistivity that obeys the material-independent quantum-critical scaling, ii) the emergence of a quantum-disordered spin state from a classical magnet near the Mott-Anderson transition, and iii) a pressure-induced quantum phase transition or crossover from a doped spin-liquid (non-Fermi liquid) to a Fermi liquid and a possible BEC-to-BCS crossover in superconductivity in a doped triangular lattice. 

An empirical approach to the problems of the pseudogap and electronic ordering, their correlation and relation to superconductivity in quasi-2D superconductors is reviewed focusing on recent ARPES results [1]. The comparison between HTSC and transition metal dichalcogenides has appeared to be useful tool to disentangle the pseudogap components and to conclude about their origins: CDW, SDW, and Mott localization. The SDW and superconductivity competes for the phase space but, on the other hand, the SDW-reconstructed HTSC share with the iron based superconductors the empirical correlation between the Tc maximum and the proximity of the Fermi surface to the topological Lifshitz transition [2]. This suggests that "topological superconductivity" could be a general mechanism for high temperature 2D superconductors.

[1] A. A. Kordyuk, Pseudogap from ARPES experiment: three gaps in cuprates and topological superconductivity, arXiv:1501.04154 (2015).
[2] A. A. Kordyuk, Low Temp. Phys. 38, 888 (2012); Low Temp. Phys. 40, 286 (2014).

I will describe experimental studies of new states of magnetism on the S=1/2 kagome lattice, focusing on two materials: one with antiferromagnetic exchange and one with ferromagnetic exchange. Quantum spin liquids are new states of matter that are characterized by long-range entanglement and support exotic excitations. Herbertsmithite is a leading candidate for having a quantum spin liquid ground state. A breakthrough in crystal growth has allowed us to uncover a hallmark signature of the spin liquid state. Inelastic neutron scattering measurements reveal that the spin excitations are fractionalized. Additional measurements help identify further details of the particular spin liquid ground state. In contrast to the antiferromagnet, ferromagnetic moments on the kagome lattice are not highly frustrated. Our neutron scattering measurements on Cu(1,3-bdc) confirm that the spins order at low temperatures. However, inelastic scattering reveals an interesting flat band in the magnon dispersion relations. Moreover, each band is separated by a gap from the other bands. Here, the Dzyaloshinsky-Moriya interaction plays a key role in yielding topologically non-trivial magnon bands.

The observation of massless Dirac fermions in monolayer graphene has propelled a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials.  Using low-temperature scanning tunneling microscopy and spectroscopy, we show the emergence of Dirac fermions in a fully tunable condensed-matter system—molecular graphene—assembled via atomic manipulation of a conventional two-dimensional electron system in a surface state.  We embed, image, and tune the symmetries underlying the two-dimensional Dirac equation into these electrons by sculpting the surface potential with manipulated molecules.  By distorting the effective electron hopping parameters into a Kekulé pattern, we find that these natively massless Dirac particles can be endowed with a tunable mass engendered by the associated scalar gauge field, in analogy to the Higgs field.  With altered symmetry and texturing of the assembled lattices, the Dirac fermions can be dressed with gauge electric or magnetic fields such that the carriers believe they are in real fields and condense into the corresponding ground state, as confirmed by tunneling spectroscopy.  Using these techniques we ultimately fabricate a quantum Hall state without breaking time-reversal symmetry, in which electrons quantize in a gauge magnetic field ramped to 60 Tesla with zero applied laboratory field.  We show that these and other chiral states now possible to realize have direct analogues in topological insulators, and can be used to guide or confine charge in nontrivial ways or to synthesize new particles [1,2].

[1] K. K. Gomes, W. Mar, W. Ko, F. Guinea, H. C. Manoharan, “Designer Dirac Fermions and Topological Phases in Molecular Graphene,” Nature 483, 306–310 (2012).

[2] M. Polini, F. Guinea, M. Lewenstein, H. C. Manoharan, V. Pellegrini, “Artificial Honeycomb Lattices for Electrons, Atoms, and Photons,” Nature Nanotechnology 8, 625–633 (2013).

Fermi systems in the crossover regime between weakly coupled BCS and strongly coupled BEC limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting particles. The physics of this crossover has been of considerable interest both in the fields of condensed matter and ultracold atoms. Here, through the quantum oscillations, superfluid response, ARPES and quasiparticle interference by STM, we demonstrate that Ε F of FeSe (Tc=9 K) is extremely small, with the ratio Δ/ΕF ~1(~ 0.3) in the electron (hole) band, indicating that this system is in the BCS-BEC crossover regime. The highly sensitive torque magnetometry reveals the development of diamagnetic signal below T*=20 K, which is significantly enhanced from Gaussian superconducting fluctuations, providing direct evidence of preformed pairs.  The transport and thermoelectric coefficients exhibit distinct anomalies at ~T*, detecting a marked change in the quasiparticle spectrum associated with a possible pseudogap formation. These results highlight the highly unusual normal state of the superconductor in the BCS-BEC crossover regime.

We show that the surface of an bulk superconductor decorated with a one- or  two-dimensional lattice of magnetic or nonmagnetic impurities can exhibit various forms of topological superconductivity. For example, if magnetic impurities order ferromagnetically and the superconducting surface supports a sufficiently strong Rashba-type spin-orbit coupling, Shiba sub-gap states at impurity locations can hybridize into Bogoliubov bands with non-vanishing, sometimes large, Chern number C. This topological superconductor supports C chiral Majorana edge modes. We construct phase diagrams for model two-dimensional superconductors, accessing the dilute and dense magnetic impurity limits analytically and the intermediate regime numerically. To address potential experimental systems, we identify stable configurations of ferromagnetic iron atoms on the Pb (111) surface and conclude that ferromagnetic adatoms on Pb surfaces can provide a versatile platform for two-dimensional topological superconductivity.

We consider an open system near a quantum critical point that is suddenly moved towards the critical point. The bath-dominated diffusive nonequilibrium dynamics after the quench is shown to follow scaling behavior, governed by a critical exponent that emerges in addition to the known equilibrium critical exponents. We determine this exponent and show that it describes universal prethermalized coarsening dynamics of the order parameter in an intermediate time regime. Implications of this quantum critical prethermalization are: (i) a power law rise of order and correlations after an initial collapse of the equilibrium state and (ii) a crossover to thermalization that occurs arbitrarily late for sufficiently shallow quenches.

t.b.a.

It is currently not possible to predict emergent properties, such as unconventional superconductivity, on the basis of first principles calculations. Many phenomenological trends supported by calculations of model systems, however, suggest where to look. Furthermore, families of materials which share a common building block, often provide similar physics allowing one to understand trends, which hopefully transcends any one particular family of compounds. This is true for hard magnets as well as for unconventional superconductors (e.g. CuO2 planes in cuprates). Because of their high purity and small energy scales, the CeIn3 family of materials are ideal model systems through which to explore the interplay between magnetism and superconductivity, which are present in many classes of unconventional superconductors. I will review some of our recent results on these materials. In particular, a comparison of CeMIn5 with M = Co, Rh, or Ir suggests that in all 3 compounds superconductivity is mediated by AF spin fluctuations, but that details may depend on the strength of the spin-orbit coupling. The role of reduced dimensionality is investigated in CeRhIn5 and CePt2In7 both as a function of pressure as well as with applied magnetic field. Transport studies on micro patterned crystals of CeRhIn5 under high magnetic fields reveals a density wave state akin to what has been observed in cuprates, accompanied by a strong dimensional reduction. Meanwhile, neutron scattering studies of the spin waves suggests that the magnetism is frustrated along the c-axis of CeRhIn5. 

Jurisdiction of three-dimensional parabolic semimetal covers a vast arena of solid state systems that includes weakly correlated gray-tin, mercury telluride as well as strongly correlated and spin-orbit coupled 227 pyroclore iridates. In this talk, first I will present the symmetry constraint on electron-electron interactions and then discuss how one can gain valuable insights into various many-body instabilities in 3D parabolic semimetals by employing a weak coupling renormalization group analysis. I will mainly focus on the competition among nematic ground state that breaks only rotational symmetry and magnetic one that, on the other hand, breaks time-reversal and rotational symmetries. I will argue that although for small number of fermion flavors (N) nematic order remains quite stable, when the electron-electron interactions are strong, for sufficiently large number N a magnetic ground state can also be realized in these systems. I will also discuss the competition  between these broken-symmetry phases with an infra-red stable non-Fermi liquid phase, resulting from long range Coulomb interaction, and various signatures of such competition on the scaling properties. Our findings can be germane, in particular, to 227 pyroclore iridates where the all-in all-out antiferromagnet order gives rise to a topological Weyl semimetal. Connections to various recent experiments will be addressed, and if time permits I will then present some recent numerical analysis on the stability of disordered Weyl semimetals and associated dirty quantum criticality in this system.

To pinpoint the microscopic mechanism for superconductivity has proven to be one of the most outstanding challenges in the physics of correlated quantum matter. Thus far, the most direct evidence for an electronic pairing mechanism is the observation of a new symmetry of the order-parameter, as done in the cuprate high-temperature superconductors. Like distinctions based on the symmetry of a locally defined order-parameter, global, topological invariants allow for a sharp discrimination between states of matter that cannot be transformed into each other adiabatically. Here we propose an unconventional pairing state for the electron fluid in two-dimensional oxide interfaces and establish a direct link to the emergence of nontrivial topological invariants. Topological superconductivity and Majorana edge states can then be used to detect the microscopic origin for superconductivity. In addition, we show that also the density wave states that compete with superconductivity sensitively depend on the nature of the pairing interaction. Our conclusion is based on the special role played by the spin-orbit coupling and the shape of the Fermi surface in SrTiO3/LaAlO3-interfaces and closely related systems.

The relationship between electronic nematicity -- i.e. tetragonal symmetry-breaking induced by electronic rather than lattice degrees of freedom -- and unconventional superconductivity remains a widely investigated topic. The prime experimental tool to probe these nematic phases in iron-based superconductors is the in-plane resistivity anisotropy Δρ = ρ xx - ρyy, which, by symmetry arguments, must be proportional to the nematic order parameter $eta$ near the nematic transition,  Δρ= Cη. In order to advance our understanding of the mechanisms and properties of the nematic phases, it is necessary however to go beyond phenomenological models and relate the proportionality constant C to the collective behavior of the nematic Fermi fluid. In this work, we employ a Boltzmann equation approach to study how anisotropic fluctuations present iron pnictides are manifested in Δρ, and, more specifically, in the proportionality constant C. We find in general two contributions to the resistivity anisotropy: one arising from the anisotropic scattering of the electrons by the actual fluctuations, which is dominant at higher temperatures, and a second one due to the anisotropic Fermi-velocity renormalization promoted by virtual fluctuations, which is dominant at lower temperatures. For the case of underdoped iron pnictides, for which anisotropic magnetic fluctuations are the lowest-energy excitations, we find that the two contributions yield generally the same sign of C, including its sign-change for sufficiently hole-doping. However, they differ in their dependence on the temperature and on the magnetic correlation length, which allows one to distinguish these two contributions experimentally. 

Using a combination of spectroscopic ellipsometry and DC transport measurements, we determine the temperature dependence of the optical conductivity of nickelate films on different substrates. The spectra show strong qualitative changes on the scale of 1 eV when the material passes through the metal-insulator transition, and additional weaker changes when it passes from the paramagnetic to the magnetically ordered phase. The spectral changes reveal the effect of bond-length disproportionation on the local electronic configuration of the nickel ions, and point toward a situation whereby the bond-disproportionated state is characterized by two types of nickel with qualitatively different electronic configurations.