Poster Contributions

For each poster contribution there will be one poster wall (width: 97 cm, height: 250 cm) available. Please do not feel obliged to fill the whole space. Posters can be put up for the full duration of the event.

Irradiated Weyl Nanowires go Superconducting

Bachmann, Maja

The non-trivial topology of the bulk bands in topological semi-metals protects new electronic states at the surface, such as the famous Fermi arc states. If a superconducting gap is induced in these materials, exotic electronic states are expected to appear at the interface such as zero-energy Majorana modes. These novel states provide insights into the topological aspects of electronic matter and are of interest for quantum coherent applications. Here we will present a new route to reliably fabricating superconducting microstructures from the intrinsically non-superconducting Weyl semi-metals NbAs and TaAs under ion irradiation. The large difference in the surface binding energy of Nb/Ta and As leads to a natural enrichment of Nb/Ta at the surface during ion milling, forming a superconducting surface layer ($T_c$ $\sim$ 3.5K). Being formed from the target crystal itself, the ideal contact between the superconductor and the bulk enables an effective gapping of the nodes due to the proximity effect. Simple low energy ion irradiation may thus serve as a powerful tool to fabricate topological quantum devices from mono-arsenides, even on an industrial scale.

Bulk and surface properties of SmRh$_2$Si$_2$ single crystals

Banda, Jacintha

Jacintha Banda 1 , Kristin Kliemt 2 , Alla Chikina 3 , Alexan- der Generalov 4 , Kurt Kummer 5 , Monika Güttler 3 , Victor N. Antonov 6 , Yuri Kucherenko 6 , Steffen Danzenbächer 3 , Christoph Geibel 1 , Clemens Laubschat 3 , Denis V. Vyalikh 3,7,8,9 , Cornelius Krellner 2 , and Manuel Brando 1 — 1 MPI CPfS, Dresden, Germany — 2 Goethe-University Frankfurt, Frankfurt, Germany — 3 Dresden University of Technology, Dresden, Germany — 4 MAX IV Laboratory, Lund, Sweden — 5 ESRF, Grenoble, France — 6 National Academy of Sciences of Ukraine, Kiev, Ukraine — 7 Saint Petersburg State University, Saint Petersburg, Russia — 8 Donostia International Physics Center, San Sebastian, Spain — 9 IKERBASQUE, Bilbao, Spain We present the properties of SmRh$_2$_Si$_2$ which is isostructural to YbRh$_2$Si$_2$ . By a modified Bridgman method with indium flux [1] we obtained platelet-shaped SmRh$_2$Si$_2$ single crystals. We are trying to characterize magnetic ground state using magnetization, specific-heat and electrical transport measurements. Due to their layered structure, the crystals can be cleaved easily parallel to the Sm- layers leading to well-defined atomically flat surfaces that are populated exclusively either by Sm- or Si-atoms. This property allows for performing ARPES to investigate the valence of the Sm ions in the bulk and at the surface [2] [1] C. Krellner et al., Phil. Mag. 92, 2508 (2012). [2] A. Chikina et al., to be published

Scrambling and thermalization in a diffusive quantum many-body system

Bohrdt, Annabelle

Out-of-time ordered (OTO) correlation functions describe scrambling of information in correlated quantum matter. The behavior of such correlators are of particular interest in incoherent quantum systems without well defined quasi-particles, in which transport is diffusive but information is expected to propagate ballistically. Here, we study the dynamical response of the non-integrable, one-dimensional Bose-Hubbard model in the incoherent high-temperature regime. Our system exhibits diffusive dynamics in time-ordered correlators of globally conserved quantities, whereas OTO correlators display a light-cone spreading of quantum information. The slowest process in the global thermalization of the system is thus diffusive, yet information spreading is not inhibited by such slow dynamics. We furthermore develop an experimentally feasible protocol to overcome some challenges faced by existing proposals and probe the behavior of time-ordered and OTO correlation func- tions. Our study opens new avenues for both the theoretical and experimental exploration of thermalization and information scrambling dynamics.

Variational Ensembles with Maximal Entropies

Cebe, Asli

The Gibbs ensemble, describing thermal equilibrium states, can be formulated variationally as the one that maximizes von Neumann entropy, given a certain energy density. One could instead construct other ensembles that maximize different entropic quantities and respect the same constraints. One particular case, which could be of special interest for the numerics, is the ensemble maximizing 2-Renyi entropy. We construct such ensembles and study their properties.

Phase-space localization in absorbing quantum maps

Clauss, Konstantin

In absorbing quantum systems a fundamental question concerns the phase-space localization of resonance states. The semi-classical description of a chaotic resonance state is given by a classical conditionally invariant measure with the same decay rate. We present a construction of such measures that generalizes the classical natural decay to arbitrary decay rates. We investigate (i) quantum mechanical deviations, (ii) the case of partial absorption, and (iii) the applicability to a mixed phase space.

Scattering of a quantum walker from a chain of memory qubits

Danacı, Burçin

Bur ̧cin Danacı1, ∗ and A. Levent Suba ̧sı1 1Department of Physics, Faculty of Science and Letters, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey In a quantum walk, the state of the particle becomes a superposi- tion of different states in position space during time evolution. Due to interference effects, the walker has the potential of spreading faster than its classical counterpart. In a model proposed by Camilleri et al. the history of the walker’s previous locations is recorded by memory qubits assigned to every site. A different coin operator may be ap- plied depending on the local memory state affecting the direction of the walker [1]. Similar to disordered quantum walks this causes local- ization for some parameters. We study the scattering of the walker through a region of memory qubits and analyse effects of the localized states on reflection and transmission rates. We employ matrix product states [2–4] for numerical simulations to cope with the growth of the Hilbert space. ∗ [1] E. Camilleri, P. P. Rohde, and J. Twamley, Sci Rep., 4 (2014) [2] M. A. Martin-Delgado, G. Sierra, and R. M. Noack, Journal of Physics A: Mathematical and General, 32, 33 (1999) [3] R. Orus, Annals of Physics, 349 (2014) [4] U. Schollwo ̈ck, Annals of Physics, 326, 1 (2011)

Two-orbital interactions in ytterbium-173

Darkwah Oppong, Nelson

Authors: Darkwah Oppong, Nelson; Riegger, Luis; Höfer, Moritz; Bloch, Immanuel; Fölling, Simon Ytterbium as an alkaline-earth-like atom features a metastable excited state, the so-called clock state, that can be directly adressed from the ground state with an ultra-narrow laser. The metastable clock state opens up the possibility of probing interacting two-orbital many-body systems. We can tune the interaction strength between atoms in the ground and clock state using the recently observed inter-orbital Feshbach resonance of the isotope ytterbium-173. Since the ground and clock state have, in general, distinct atomic polarizabilities, the confinement and mobility can be tuned in state-dependent optical lattices. In our implementation, atoms in the clock state are pinned on individual lattice sites whereas ground-state atoms remain mobile. Utilizing this state-dependent mobility and the inter-orbital spin-exchange interaction of fermionic ytterbium, Kondo-like models can be realized. We spectroscopically probe the interactions of ytterbium-173 atoms in an one-dimensional state-dependent lattice.

Spectroscopic Imaging STM study on Dirac line node material ZrSiS

He, Qingyu

Dirac semimetals have attracted lots of interest due to their exotic physical properties such as ultrahigh mobility and large transverse magneto-resistance. Dirac line node semimetals, where the conduction and valence bands touch on a line, have been synthesized recently and open a new area to explore. Recent studies show the non-symmorphic material ZrSiS to be an example of such a Dirac line node semimetal with a large energy range of linear dispersion. [1] We conducted spectroscopic imaging STM measurement on ZrSiS to study the band structure and surface states of this compound. ZrSiS shows an atomically flat cleavage surface. The high quality of the sample allows us to study both the bulk and surface states using quasi-particle interference (QPI). For the bulk states, we observed ultra-long quasi-1D QPI which is related to the quasi-particle life-time. The surface state of ZrSiS, which is related to the breaking of the non-symmorphic symmetry is studied. We find that the surface state merges to the bulk band at high energies. [1] Schoop, L. M. et al. Dirac cone protected by non-symmorphic symmetry and three-dimensional Dirac line node in ZrSiS. Nat. Commun. 7:11696

Controllability of the Jaynes-Cummings-Hubbard Model

Heinze, Margret

Controllability of quantum systems covers the two questions whether pure states can be inter-converted and whether unitary operators can be approximated by suitably tuning parts of the Hamiltonian. However, these aspects cannot be analyzed with well-established conditions for infinite-dimensional systems such as the Jaynes-Cummings-Hubbard model. This model describes coupled cavities each one containing a two-level atom. In this contribution, we study its controllability in the two cavity case by using previous symmetry arguments; it is shown that one part of the control Hamiltonians can be studied in terms of infinite-dimensional block diagonal Lie algebras while the other part breaks this symmetry. An induction on the number of cavities extends this result to the general case. Hence, individual control of the atoms and collective control of the hopping between cavities is sufficient for both pure state and strong operator controllability of the Jaynes-Cummings-Hubbard model.

Nanoscale NMR - Quantum sensors for quantum matter?

Irber, Dominik

Occupation spectrum after a global quench in the many-body localized phase

Lezama Mergold Love, Talía

Closed disordered interacting quantum systems can exhibit many-body localization (MBL). When disorder is sufficiently strong, such systems enter into a nonergodic regime known as the many-body localized phase, resulting in an ideal insulator with zero charge and thermal conductivities at finite energy densities. The emergent integrability of the MBL phase can be understood in terms of localized quasiparticles. As a result, the occupations of the one-particle density matrix (OPDM) in eigenstates show a Fermi-liquid like discontinuity. In this work, we numerically explore the dynamics of the MBL phase generated by a global quench from a charge density state, in terms of the OPDM occupation spectrum. In particular, we show that in the steady state, the occupation discontinuity is smeared to a continuous distribution in a similar way as finite temperature smears the discontinuity in a Fermi liquid, but the occupation spectrum remains highly non-thermal in the thermodynamic limit.

ARPES measurements of transition-metal dichalcogenide superconductors

Markovic, Igor

Theory of magnetic molecular rings

Matysiak, Jacek

Antiferromagnetic rings form a special subclass of molecular nanomagnets with a surprisingly rich physics. These structures contain a small amount of paramagnetic ions surrounded by organic ligands which inhibit interactions between adjacent molecules. Strong antiferromagnetic coupling between the ions leads to the characteristic ground state with a minimal value of the total spin S (for even numbers of the ions S = 0). These systems are typically described by the spin Hamiltonians containing the isotropic Heisenberg exchange and anisotropic local crystal fields interactions [1]. However, this method does not take into account an impact of itinerant electrons, which are also present in these systems. Hereby we study the antiferromagnetic rings using another approach. We use the generalized Falicov-Kimball Hamiltonian [2,3] which takes into account a) the local Coulomb repulsion between ions and electrons, b) kinetic energy of electrons, c) Hund coupling and d) tunneling between states with the same energy. Using this model, we calculate thermodynamic properties of the rings and compare them with experimental results. Our focus is mainly on chromium based rings, such as Cr8, Cr7X and CR9 (X = Cr, Cd, Ni).\\\noindent [1] A. Furrer, O. Waldmann, Rev. Mod. Phys., {\bf 85}, 1, (2013). \\\noindent [2] R. Lema\'{n}ski, Phys Rev. B {\bf 71}, 035107 (2005).\\\noindent [3] B. Brzostowski, R. Lema\'{n}ski, T. \'Slusarski, D. Tomecka and G. Kamieniarz, J. Nanopart. Res. {\bf 15}, 1528 (2013).

Luttinger theorem in strongly correlated multi-orbital systems

Mitscherling, Johannes

The Luttinger theorem stating that the volume enclosed by the Fermi surface is entirely determined by the total particle number was originally derived for a Fermi liquid in the zero temperature limit. We discuss the possible extension of the theorem for certain classes of strongly correlated systems outside the Fermi-liquid regime. Similarly to Luttinger's sum rule we present a sum rule for correlated multi-orbital systems at finite temperature. We identify an exact relation between the Fermi volume of a given band index and its occupation. This relation involves additional contributions which come into play in this generalized setting while they vanish for a Fermi liquid in the zero temperature limit. The study of these contributions enables us to identify conditions under which the Luttinger theorem is valid beyond its traditional scope. As a first application, we apply the extended Luttinger sum rule to the two-site Hubbard model. As a second step, the extended Luttinger theorem is used to study the temperature evolution of the three Fermi sheets of $Sr_2RuO_4$.

Towards a new regime of electrical conductivity: hydrodynamic electron flow

Nandi, Nabhanila

The electrical resistance is conventionally determined by the momentum-relaxing scattering of electrons by the host solid and its excitations. Hydrodynamic fluid flow through channels, in contrast, is determined by geometrical factors, boundary scattering and the viscosity of the fluid, which is governed by momentum-conserving internal collisions. In almost all known materials, however, the signatures of viscosity in electron flow cannot be resolved, because the rate of momentum-relaxing collisions dominates that of the momentum-conserving ones that give the viscous term. In previously published work, we reported experimental evidence that there is a regime in restricted channels of the ultra-pure two-dimensional metal delafossite PdCoO_{2}, where the resistance has a large viscous contribution which comes from the momentum conserving scattering between electrons. In this poster I will present on our current work on magneto-hydrodynamics, discussing data both from PdCoO_{2} and a second delafossite metal, PtCoO_{2}.

Quasi-one dimensional spin chains and unconventional magnetic excitations in YbAlO$_3$

Nikitin, Stanislav

One-dimensional (1D) quantum magnets have attracted great interest in condensed matter physics because of strong quantum fluctuations and a number of novel quantum phenomena therein. In particular, emergent fractional excitation, characterized as a broad continuum of the inelastic neutron scattering (INS) spectrum, is hosted in 1D magnets. Here we present the INS studies of low-energy spin dynamics of Yb$^{3+}$ in YbAlO$_3$. We observe that Yb$^{3+}$ moments form weakly coupled spin chains running along $c$-axis without any significant magnetic interactions at the $ab$ plane despite the three-dimensional perovskite structure. Excitation spectrum of YbAlO$_3$ above ordering temperature $T_N=0.9~K$, precisely represents a gapless spinon continuum as was predicted for $S=\frac{1}{2}$ Heisenberg chain. Below $T_N$ gap $\Delta=0.3~meV$ appears and the spectrum splits into the single particle mode and two particle continuum. Our results represent a intriguing case of quantum spin dynamics at 4f based systems and provide an experimental basis for a future studies of low dimensional magnets.

Hall conductivity of strongly interacting bosons in optical lattice

Patucha, Konrad

We study Hall conductivity of ultra-cold bosonic atoms in optical lattice described by the Bose-Hubbard model. We use the quantum rotor approximation, which allows to correctly describe lattices with non-zero Chern numbers. As examples of such systems we consider square lattice in a synthetic magnetic field as well as the Haldane model. We derive the formula for Hall conductivity, which strongly resembles the famous TKNN formula. We investigate the behavior of conductivity depending on temperature and model parameters. It appears that bosonic systems substantially differ from fermionic ones, e.g. the presence of non-zero Hall conductivity is not directly related to non-zero value of Chern number.

Spectroscopic imaging STM study on Weyl Fermion material CeSbTe

Que, Xinglu

Dirac line node materials represent a new state of quantum matter, coming into focus due to properties such as unconventional magneto-transport and the potential to host topologically nontrivial phases [1]. Non-symmorphic crystals MSiS (M=Hf, Zr) with non-trivial topology near the critical point between topological phase transitions, have been proposed that extends the bulk linear touching from discrete points to 1D lines, hosting a nodal line and an unconventional surface state [2]. When inversion symmetry or time reversal symmetry of the crystal is broken, for example, by a magnetic field, a Dirac node will split into two chiral Weyl nodes, resulting in a Weyl semimetal. In CeSbTe studied here, ungapped Dirac band crossings protected by non-symmorphic symmetry are expected to emerge. Ordered magnetism introduced by f-electrons in Ce3+-ions, however, breaks Time Reversal (TR-) symmetry and results in the separation of Dirac node into Weyl nodes. Therefore, this material is a promising candidate for investigation of Weyl-semimetals [3]. A particularly appealing aspect is the possibility to utilize the magnetic field tunability to control the emergence of topologically protected Fermi arcs on the surface. Spectroscopic imaging scanning tunneling microscopy (SI-STM) has been a highly valuable technique to study the electronic structure with high energy resolution. In combination and comparison with calculation results, we make use of SI-STM for the investigation of CeSbTe by at the atomic scale, and Physical Property Measurement System (PPMS) for study on transport properties and magnetic characters. Our result provides crucial insights into the Weyl semimetals represented by CeSbTe. [1] Chen, C., et al. "Dirac Line-nodes and Effect of Spin-orbit Coupling in Non-symmorphic Critical Semimetal MSiS (M= Hf, Zr)." arXiv preprint arXiv:1701.06888. [2] Schoop, L. M., et al. "Dirac cone protected by non-symmorphic symmetry and three-dimensional Dirac line node in ZrSiS." Nat. Commun. 7:11696. [3] Lippmann, J. M., "Materials in the PbFCl structure type with extraordinary electronic and magnetic properties."

Electrocatalytic properties of TiO$_2$ - based clusters for water decomposition

Rodríguez Hernández, Fermín

The electrocatalytic properties of TiO$_2$-based anodes are studied with focus on the oxygen evolution reaction (OER). New mechanisms for the decomposition of water molecules on TiO$_2$-based clusters are proposed. The influence of doping TiO$_2$ surfaces with transition metals (TMs) on the performance of TiO$_2$-based electrodes, is investigated within direct substitution of TMs at Ti-sites in the proposed clusters. The energetics of the OER is explored within the framework of Density Functional Theory (DFT). For each reaction path, the free energy profile is computed, at different biases, taking into account the entropic and the zero-point energy contributions. The mechanisms of the OER considered in the present work are found to be energetically more feasible than alternative reaction pathways presented in previous theoretical works based on cluster approximations as well. The representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface. From the simulation of TMs sites, spanning from Vanadium to Nickel, it is found that late TMs: Fe, Co and Ni reproduce the observed experimental trends for the overpotentials in TiO$_2$ - doped electrodes. In the case of Cr and Mn, the present study predicts an enhancement of the OER activity for the anatase-like clusters while a reduction of this activity is found for the rutile-like ones. Moreover, the vanadium-doped structures do not show relevant influence in the OER activity compared to pure TiO$_2$-based cluster models.

Simulating non-Equilibrium Systems with Matrix Product States

Roos, Julian

Understanding out of equilibrium remains a challenge for classical and quantum systems. Matrix Product States can capture the properties of non-equilibrium stationary states of many classical and quantum problems. We use these techniques for the simplest problem of particle hopping on a 1D lattice and try to understand the differences between classical and quantum non-equilibrium regimes.

Effective potentials for non-equilibrium quantum many-body systems

Roy, Sthitadhi

We construct effective potentials for non-equilibrium quantum many-body systems, analogous to thermodynamic potentials in equilibrium statistical mechanics. These potentials encode the dynamical behaviour of macroscopic observables, providing an ideal framework for the study of eigenstate-ordered phases and transitions between them. We demonstrate this for many-body localised and Ising spin glass systems.

Shaping environments for Rydberg aggregates

Schönleber, David

Excitation transport through dipole-dipole interactions plays a prominent role in molecular aggregates, assemblies of molecules that appear, for instance, in the context of photosynthesis. In these systems, the fundamentally coherent transport mechanism competes with the coupling to a complex, non-Markovian, finite-temperature environment. This poses severe challenges for both numerical investigation and clean experimental studies. Here we propose an experimental setup for quantum simulation of excitation transport with Rydberg atoms. Rydberg atoms exhibit similar dipolar state-changing interactions as found in molecules, but are considerably simpler to study. Accordingly, we mimic the coherent excitation transport in the molecular aggregate using a Rydberg aggregate, i.e., an assembly of transition-dipole interacting Rydberg atoms. A set of optically-driven ultracold atoms in turn provides a highly-tunable environment for this aggregate through which various environment effects can be introduced. Specifically, we can control the degree of decoherence as well as non-Markovianity of the transport dynamics, and can even prepare thermal states in the Rydberg aggregate.

Ultrafast Control of Superconductivity

Shabestari, Parmida

Ultrafast techniques allow for an unprecedented opportunity to investigate and control the light driven states in correlated systems. Probing these transient states will take us one step closer to understanding Superconducting phase transition in high temperature superconductors.

Magnetic properties of $d^4$ ruthenium double perovskite

Shintani, Daiki

4d transition metal oxides are expected to serve as a playground where the interplay of Coulomb interaction and spin orbit coupling leads to novel phases. In case of $d^4$ electron configuration, its ground state is supposed to be nonmagnetic J =0. However, recent studies suggest that this system can host exotic magnetic properties as a result of the interaction between excited J states [1]. Additionally, magnetic susceptibility of the $d^4$ double perovskite $La_2BRuO_6$ (B=Mg,Zn) was measured by several groups [2,3]. These materials show no clear transition down to 2K in spite of the strong antiferromagnetic interaction. In this study we discuss magnetic susceptibility and specific heat measurements on $La_2BRuO_6$, in order to investigate possible hidden order in these materials. [1] G. Khaliullin, Phys. Rev. Lett. 111, 197201 (2013). [2] R. Dass et al., Phys. Rev. B 69, 094416 (2004). [3] K. Yoshii et al., Phys. Stat. Sol. 203, 2812 (2006)

Spectroscopic Studies of Nanostructured Topological Insulator Bismuth Telluride and Impact of Surface Oxidation

Singh, Rini

Bismuth telluride (Bi$_2$Te$_3$) nanoparticles were prepared by the wet chemical method, and their surface oxidation behavior was studied by X-ray photoemission spectroscopy, Raman spectroscopy, and Soft X-ray Absorption Spectroscopy. Single crystals of Bi$_2$Te$_3$ were also studied to compare the effect of nanostructuring on surface oxidation. Besides A1g, E2g and A21g at 61.4 cm-1, 101.9 cm-1, and 143.5 cm-1 respectively vibrational modes from Bi2Te3 nanostructures, two new peaks at 93.3 cm-1 and 121.1 cm-1 were observed in Raman spectra which are assigned to α-Bi$_2$O$_3$ and TeO$_2$ respectively, confirmation of these peaks were done by comparing it to individual Bismuth and Tellurium films which are also exposed to air. Two shoulders appear in XPS spectra along with the major peaks, which are corresponding to Bi$_2$Te$_3$ while the major peaks are corresponding to TeO$_2$, this indicates that most of the surface of nanoparticles get oxidized due to high surface to volume ratio, and the surfaces of single crystals were not oxidized. The overall study concluded that in Bi$_2$Te$_3$, bismuth, and tellurium bonded with oxygen as Bi-O and Te-O instead of Bi-O-Te. Oxygen K-edge in SXAS results also supports the XPS and Raman studies. Oxidation mechanism of the nanostructures were presented. These oxidation studies on nanostructures will help to define a pathway towards nanostructures based devices

Giant tunable Rashba spin splitting in two-dimensional BiSb monolayer and BiSb/AlN heterostructures

Singh, Sobhit

Search of novel two-dimensional giant Rashba semiconductors is a crucial step in the development of the forthcoming nano-spintronics technology. Using first-principle calculations, we study a stable two-dimensional crystal phase of BiSb having buckled honeycomb lattice geometry, which is yet unexplored. The phonon, room temperature molecular dynamics and elastic constant calculations verify the dynamical and mechanical stability of the monolayer at 0~K and at room temperature. The calculated electronic bandstructure reveals the direct bandgap semiconducting nature of BiSb monolayer with presence of highly mobile two-dimensional electron gas (2DEG) near Fermi-level. Inclusion of spin-orbit coupling (SOC) yields the giant Rashba spin-splitting of 2DEG near Fermi-level. The calculated Rashba energy and Rashba splitting constant are 13 meV and 2.3 eV\AA, respectively. The strength of the Rashba splitting is amongst the largest yet known 2D Rashba semiconductors. We demonstrate that the strength of the Rashba spin-splitting can be significantly tuned by applying in-plane bi-axial strain on the BiSb monolayer. Presence of the giant Rashba spin-splitting together with the large electronic bandgap (1.6 eV) makes this system of peculiar interest for optoelectronics applications. Furthermore, we study the electronic properties of BiSb/AlN heterostructures having a lattice mismatch of 1.3\% at the interface. Our results suggest that BiSb monolayer and heterostructure systems could be potentially used to develop highly efficient spin field-effect transistors, optoelectronics and nano-spintronics devices. Thus, this comprehensive study of two-dimensional BiSb systems can expand the range of possible applications in the future spintronics technology.

Raman scattering with strongly coupled vibron-polaritons

Strashko, Artem

Strong coupling between molecular oscillations and cavity photons can lead to formation of new half-light half-matter quasiparticles called vibron-polaritons. In a recent experiment [Shalabney et al., Angew. Chem., Int. Ed. 54, 7971 (2015)] with organic molecules in a microcavity such a coupling was reported to enhance the Raman scattering cross section by three order of magnitude. Inspired by this result, we analyze the effect of strong light-matter coupling on the Raman scattering probability of organic molecules theoretically. This problem has recently been considered by del Pino et al. [J. Phys. Chem. C 119, 29132 (2015)] numerically for a small number of molecules. In this work we derive analytic results for an arbitrary number of molecules. We predict a splitting of the Raman signal into upper and lower polariton modes, with some enhancement to the lower polariton Raman amplitude due to the mode softening under strong coupling.

Neutron scattering study of non-collinear antiferromagnets

Sukhanov, Aleksandr

Laser-induced topological superconductivity in cuprate thin films

Takasan, Kazuaki

Topological state of matter is a new paradigm in the modern condensed matter physics [1]. Topologically non-trivial superconducting state attracts particular interest because it provides a platform to host a Majorana fermion [2]. However, the topological superconducting state (TSC) has not yet been evidenced in natural superconducting systems, although a few indications have been obtained in artificial systems by state-of-the-art experiments [3, 4]. On the other hand, in recent years, tremendous developments have been achieved in the realization of topological phases by laser light applications [5, 6]. Laser-irradiated systems reach nonequilibrium steady states, which cannot be realized in equilibrium states. Thus the laser light is a new tool for controlling the topological states of matter. In this seminar, we propose a possible way to realize topological superconductivity with application of laser light to superconducting cuprate thin films [7]. Applying Floquet theory to a model of d-wave superconductors with Rashba spin-orbit coupling, we derive the effective model and discuss its topological nature. Interplay of the Rashba spin-orbit coupling and the laser light effect induces the synthetic magnetic fields, thus making the system gapped. Then the system acquires the topologically non-trivial nature. The synthetic magnetic fields do not create the vortices in superconductors, and thus the proposed scheme provides a promising way to dynamically realize a topological superconductor in cuprates. We also discuss experiments to detect the signature. References: [1] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011) [2] M. Sato and S. Fujimoto, J. Phys. Soc. Jpn. 85, 072001 (2016) [3] V. Mourik, et al. Science 336, 1003 (2012) [4] S. Nadj-Perge, et al. Science 346, 602 (2014) [5] T. Oka and H. Aoki, Phys. Rev. B 79, 081406 (2009). [6] N. H. Lindner, G. Refael, and V. Galitski, Nat. Phys. 7, 490 (2011). [7] K. Takasan, A. Daido, N. Kawakami and Y. Yanase arXiv: 1612.01596

Cooling quantum gases with entropy localization

Ünal, F. Nur

We study the dynamics of entropy in a time dependent potential and explore how disorder influences this entropy flow. We show that disorder can trap entropy at the edge of the atomic cloud enabling a novel cooling method. We demonstrate the feasibility of our cooling technique by analyzing the evolution of entropy in a one-dimensional Fermi lattice gas with a time dependent superlattice potential.

Quantum phase transition of disordered Bose-Hubbard Model

Wang, Botao

We apply two different analytical methods to study the quantum phase transitions of ultracold Bose gases in disordered optical lattices, described by the so-called disordered Bose-Hubbard model. Firstly we apply the Bogoliubov theory to investigate the properties of a disordered Bose-Hubbard model in weakly interacting regime. In weak disorder regime, we find a decreased sensitivity of coherent fraction to disorder with the increase of on-site interaction strength. For strong disorder, the quantum phase boundary between superfluid phase and Bose glass phase in the disordered Bose-Hubbard system in weak interaction regime is discussed qualitatively. Our results may serve as a reference object for possible experimental investigation. In strongly interacting regime, we study the boundary of Mott insulator (MI) phase of the disordered Bose-Hubbard model within the framework of Ginzburg-Landau effective action approach. By treating MI as unperturbed ground state and performing a systematic expansion with respect to tunneling matrix element, we extend such a field-theoretic method into the disordered lattice Bose systems. To the lowest order, a second order phase transition is confirmed to happen here and the corresponding phase boundary equation coincides with the previous mean-field approximation result. Keeping all the terms second order in hopping parameter, we obtain the beyond mean-field analytic results of MI-BG phase boundary of 2D and 3D disordered Bose-Hubbard models. Our calculations are in agreement with recent numerical results.

A qualitative exploration of the Schwinger model phase diagram with MPS of small bond dimension

Wei, Zhiyuan

Matrix Product States (MPS) provide a powerful variational ansatz to explore the phase diagram of quantum many-body problems, and have proven their feasibility for high energy models. Close to criticality, large bond dimension is required to capture the ground state correlations. However, small tensors may suffice to qualitatively characterize the phases. In this work, we systematically investigated the phase transition of Schwinger model in temporal gauge with Kogut-Susskind staggered fermions using MPS with small bond dimension D ~ 20. By applying fix-volume continuum limit in both full & truncated models for the order parameter Gamma_5, we successfully located the critical point around (m/g)c = 0.389(2) for full model and (m/g)c = 0.331(9) for truncated dlink = 7 model. We also probed effects of small D limitation. Our results show that MPS with small bond dimension give good qualitative estimation for critical point, which is encouraging for PEPS exploration of higher dimensional high energy models.

Generalization of the Davydov Ansatz

Werther, Michael

The Davydov Ansatz, originally introduced to study transport of bio-logical energy in proteins by Davydov and co-workers$^{[1]}$, is an efficient numerical tool for approximate solution of the Schrödinger-equation for different Hamiltonians. In recent works the D1-Ansatz has been successfully applied to the spin boson model as well as the Holstein molecular crystal model$^{[2]}$. For strong coupling the D1-Ansatz wave function can only insufficiently describe the dynamics of the system, according to its fixed phase space width. Here we propose an extension by adding a further degree of freedom which enables squeezing in phase space. The Dirac-Frenkel variational principle is applied to establish equations of motion for the parameters. First numerical results, executed for the prototype system with one oscillator, show a considerable improvement especially for strong coupling. This allows for improved description of systems with many oscillators, as well as description of similar models like e.g. Caldeira-Legett. As an outlook we present how the Davydov Ansatz can be used to include temperature effects into these models for strong coupling. [1] A.S. Davydov, N.I. Kislukha, Phys. Status Solidi B 59 (1973) 465. [2] K.-W. Sun, M.F. Gelin, V.Y. Chernyak, Y. Zhao, J. Chem. Phys. 142 (2015) 212448

Observation of parametric resonances in 1D shaken optical lattices

Wintersperger, Karen

We study a BEC of 39K with tunable interactions in a shaken 1D optical lattice. Due to the interplay between the external drive and interactions dynamical instabilities arise [1]. The short-time dynamics can be captured by parametric resonances within Bogoliubov theory, which should lead to a fast decay of the BEC. At long times, the behavior will be dominated by collision processes described by a Fermi’s Golden rule approach that slow down the decay. Varying the shaking parameters, we observe the transition between the two heating regimes. Moreover, we can identify the onset of the parametric instabilities at short times by analysing the 2D quasimomentum distribution of the excited atoms. [1] S. Lellouch et al., arXiv:1610.02972v1 (2016)

Effect of disordered geometry on transport properties of three dimensional topological insulator nanowires

Xypakis, Emmanouil

Three dimensional topological insulator nanowires are materials which, while insulating in the bulk, have a metallic boundary described by a two dimensional Dirac Hamiltonian with antiperiodic boundary conditions. Transport properties of this system have been extensively studied in the limit where the surface manifold is conformally flat (e.g., a cylinder) in the presence of a random disordered scalar potential. In this talk I will discuss how this picture is altered when a more realistic surface manifold is chosen, such as a cylinder with a randomly fluctuating radius.