Poster presentations
For
each poster presentation one poster wall will be
available.
These poster walls have the following size
(don't feel obliged to
really fill it):
Width:
97cm
Height: 250cm
Posters can be put
up for the full duration of the event. Discussions during the
breaks are encouraged.
There will be a poster blitz session taking place on Monday, 28th September. Each poster is presented in 3 minutes and a maximum of 5 slides.
The poster sessions take place on Monday, 28th September, from 20:30 to 22:00 (focus on odd poster numbers), on Tuesday, 29th September, from 20:30 to 22:00 (focus on even poster numbers) and on Thursday, 1st October, from 20:00 to 22:00 (discussing all the posters).
| 1 | Bamler, Robert | Electronic transport in chiral magnets: interplay between spin-orbit interaction and smooth magnetic textures | Abstract |
| 2 | Cho, Dai-Ning | Superconductivity in a 2D Edwards model | Abstract |
| 3 | Daghofer, Maria | Unconventional magnetic patterns in hexagonal-lattice iridates | Abstract |
| 4 | Dorfner, Florian | Real-time decay of a highly excited charge carrier in the one-dimensional Holstein model | Abstract |
| 5 | Evertz, Hans Gerd | Efficient DMFT impurity solver using real-time dynamics with Matrix Product States | Abstract |
| 6 | Fehske, Holger | Topological insulators in random magnetic fields | Abstract |
| 7 | Février, Clément | Multi-orbital kinetic effects on charge ordering of frustrated electrons on the triangular lattice | Abstract |
| 8 | Franco Peñaloza, Roberto Emilio | Spin-current Seebeck effect in an interacting quantum dot: atomic approximation for the Anderson impurity model | Abstract |
| 9 | Franco Peñaloza, Roberto Emilio | X-boson cumulant approach to the topological Kondo insulators | Abstract |
| 10 | Fuji, Yohei | U(1) symmetry protected topological phase in the interacting lattice model | Abstract |
| 11 | Hauschild, Johannes | Sudden expansion of hard-core bosons: crossover from 1D to 2D | Abstract |
| 12 | Hofmann, Johannes | Interaction effects along the edge of a topological superconductor | Abstract |
| 13 | Honecker, Andreas | Finite-temperature dynamics of a highly frustrated quantum spin ladder | Abstract |
| 14 | Karnaukhov, Igor | Topological Kondo insulator. Exact results | Abstract |
| 15 | Knap, Michael | Dynamics of disordered many-body systems: from thermal transport to many-body localization | Abstract |
| 16 | Kroha, Johann | Many-Body Anderson Localization of BECs in the Bose-Hubbard Model | Abstract |
| 17 | Lechtenberg, Benedikt | Spatial and temporal propagation of Kondo correlations | Abstract |
| 18 | Maiti, Moitri | Superconducting junctions as Dirac and Majorana fermion detectors | Abstract |
| 19 | Meng, Tobias | Time reversal symmetry broken fractional topological phases at zero magnetic field | Abstract |
| 20 | Morawetz, Klaus | Asymmetric Bethe-Salpeter equation - many phases of interacting Bose and Fermi gases | Abstract |
| 21 | Müller, Patrick | Thermodynamics of the frustrated J1-J2 Heisenberg ferromagnet on the body-centred cubic lattice with arbitrary spin | Abstract |
| 22 | Nejati, Ammar | Kondo breakdown in asymmetric double quantum dots | Abstract |
| 23 | Schuricht, Dirk | Charge fluctuations in nonlinear heat transport | Abstract |
| 24 | Szalowski, Karol | Indirect magnetic coupling in graphene nanostructures from exact diagonalization of Hubbard model | Abstract |
| 25 | Tomczak, Piotr | Study of the Kosterlitz-Thouless transition in 1D Heisenberg antiferromagnet on the basis of topological properties of its ground state function | Abstract |
| 26 | Weber, Manuel | Phonon spectral function of the one-dimensional Holstein-Hubbard model | Abstract |
| 27 | Yadav, Ravi | d5 Kitaev exchange: the angle you approach it matters | Abstract |
| Electronic transport in chiral magnets: interplay between spin-orbit interaction and smooth magnetic textures Bamler, Robert (University of Cologne, Institute for Theoretical Physics, Chair for Theoretical Nanophysics, Cologne, Germany) |
Magnetic materials without inversion symmetry (chiral magnets, such as MnSi), show a rich set of phenomena that have important consequences for electronic transport. On the one hand, spin-orbit interaction leads to Berry phases in momentum space, which explain the intrinsic contribution to the anomalous Hall effect. On the other hand, due to the absence of inversion symmetry, Dzyaloshinskii-Moriya interactions lead to twisted magnetic textures, most notably the so-called skyrmion lattice phase [1]. In this phase, a regular arrangement of smooth magnetic whirls leads to Berry phases in position space, which can be described by an emergent magnetic field and which give rise to a strong additional Hall signal, the so-called topological Hall effect [2]. In our work, we study the interplay between Berry phases in both position and momentum space and their influence on electronic transport in chiral magnets. A consistent description of transport in chiral magnets requires an additional type of Berry phases that are defined in mixed position/momentum space. These mixed phase-space Berry phases manifest themselves, for example, in a non-vanishing electric charge of skyrmions [3]. Based on the Kubo-Středa formula, we employ a systematic gradient expansion to calculate the Hall conductivity of a metal with both spin-orbit interaction and a smooth magnetization texture. By comparing our results to a phenomenological semiclassical model we provide an interpretation of the terms that arise to each order in the gradient expansion. [1] S. Mühlbauer, et al., Science 323, 915 (2009) [2] A. Neubauer et al., Phys. Rev. Lett. 102, 186602 (2009) [3] F. Freimuth et al., Phys. Rev. B 88, 214409 (2013) |
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| Superconductivity in a 2D Edwards model Cho, Dai-Ning (Leibniz Institut für Festkörper- und Werkstoffforschung, Institute for Theoretical Solid State Physics, Dresden, Germany) |
Understanding of the origin of superconductivity in strongly correlated electron systems has been an intensively debated problem since the discovery of high-temperature superconductivity. Here the pairing interaction is believed to be predominantly mediated by electron-electron interactions rather than by the coupling to phonons. We study the possibility of unconventional pairing in the framework of a novel two-dimensional quantum transport model, where the charge carriers are strongly affected by the correlations and fluctuations of a background medium, described by bosonic degrees of freedom. Using the projective renormalization method (PRM) we find for half filling stable superconducting solutions very close to a charge-density-wave state. Due to a strong renormalization of the original fermionic band in the superconducting state the Fermi surface splits into two disconnected parts, which are characterized by different signs of the superconducting order parameter function. The additional hole-like Fermi surface is formed nearby the center of the Brillouin zone and has the largest superconducting gap. |
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| Unconventional magnetic patterns in hexagonal-lattice iridates Daghofer, Maria (Universität Stuttgart, Institut für funktionelle Materie und Quantentechnologien, Stuttgart, Germany) |
We use classical Monte-Carlo methods to investigate potential ground states of several frustrated Kitaev-Heisenberg models in hexagonal symmetry. In addition to "conventional" ferro- and antiferro-magnetic states, we find a persistent tendency towards incommensurate and non-coplanar patterns. However, different broken symmetries lead to a number of quite distinct low-energy phases: Both on triangular and on honeycomb lattices, we find the following two types of states: (i) regular lattices of topological defects (vortices and domain walls) and (ii) nematic phases with "partial" order, where spins within chains or ladders are strongly coupled, but couple at most weakly between chains/ladders. In the case of the defect lattices, analytic considerations establish that they are unlikely to be artifacts of the Monte-Carlo algorithm, but are instead a ground-state feature of the models. In the case of the nematic phases, we discuss the different ordering temperatures associated with the emergent strongly and weakly coupled directions. |
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| Real-time decay of a highly excited charge carrier in the one-dimensional Holstein model Dorfner, Florian (Ludwig-Maximilians-Universität München, Department of Physics, München, Germany) |
We study the real-time dynamics of a highly excited charge carrier coupled to quantum phonons via a Holstein-type electron-phonon coupling. This is a prototypical example for the nonequilibrium dynamics in an interacting many-body system where excess energy is transferred from electronic to phononic degrees of freedom. We use diagonalization in a limited functional space (LFS) to study the nonequilibrium dynamics on a finite one-dimensional chain. This method agrees with exact diagonalization and the time-evolving block-decimation method, in both the relaxation regime and the long-time stationary state, and among these three methods it is the most efficient and versatile one for this problem. We perform a comprehensive analysis of the time evolution by calculating the electron, phonon and electron-phonon coupling energies, and the electronic momentum distribution function. The numerical results are compared to analytical solutions for short times, for a small hopping amplitude and for a weak electron-phonon coupling. In the latter case, the relaxation dynamics obtained from the Boltzmann equation agrees very well with the LFS data. We also study the time dependence of the eigenstates of the single-site reduced density matrix, which defines the so-called optimal phonon modes. We discuss their structure in nonequilibrium and the distribution of their weights. Our analysis shows that the structure of optimal phonon modes contains very useful information for the interpretation of the numerical data. F. Dorfner, L. Vidmar, C. Brockt, E. Jeckelmann and F. Heidrich-Meisner, PRB 2015 |
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| Efficient DMFT impurity solver using real-time dynamics with Matrix Product States Evertz, Hans Gerd (Technische Universität Graz, Theoretical and Computational Physics, Graz, Austria) |
We propose to calculate spectral functions of quantum impurity models using the Time Evolving Block Decimation (TEBD) for Matrix Product States. The resolution of the spectral function is improved by a so-called linear prediction approach. We apply the method as an impurity solver within the Dynamical Mean Field Theory (DMFT) for the single- and two-band Hubbard model on the Bethe lattice. For the single-band model we observe sharp features at the inner edges of the Hubbard bands. A finite size scaling shows that they remain present in the thermodynamic limit. We analyze the real time-dependence of the double occupation after adding a single electron and observe oscillations at the same energy as the sharp feature in the Hubbard band, indicating that they correspond to a long-lived, coherent superposition of eigenstates with different occupations. For a two-band Hubbard model we observe an even richer structure in the Hubbard bands, whereas there is no multiplet structure of the impurity. In addition there are again sharp excitations at the band edges of a type similar to the single-band case. |
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| Topological insulators in random magnetic fields Fehske, Holger (Ernst-Moritz-Arndt University Greifswald, Institute of Physics, Greifswald, Germany) |
We study the effect of random magnetic fields on the two-dimensional surface state of three-dimensional topological insulators. To this end we investigate a minimal four band model (describing weak, and strong topological insulators, as well as conventional band insulators), which is completed by terms that allow for the breaking of inversion symmetry by magnetic fields and the Zeeman effect. Calculating the local density of states, the single-particle spectral function, and the conductance for an (contacted) slab geometry by exact numerical techniques based on the kernel polynomial method, we analyse how field-randomness affects the ground-state, spectral and transport properties of the resulting insulating massive Dirac fermion state. We establish a relationship to experimental angle-resolved photoemission results for magnetically doped topological insulators. The effects of (nonmagnetic) surface disorder will be discussed too. |
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| Multi-orbital kinetic effects on charge ordering of frustrated electrons on the triangular lattice Février, Clément (Institut NEEL, Condensed Matter - Low Temperatures, Grenoble, France) |
Strongly correlated electron systems are often characterized by complex phase diagrams, reflecting an intricate interplay between magnetic, charge and orbital degrees of freedom. The role of the multi-orbital effects on the emergence of electronic orders on the frustrated triangular lattice at half-filling is investigated though a spinless extended Hubbard model. The latter is expected to describe layered triangular compounds, such as AgNiO2 which exhibits a threefold charge order[1], or transition metal dichalcogenides where a variety of charge ordered phases have been observed[2]. Using complementary approaches, exact diagonalizations and unrestricted Hartree-Fock calculations, we unravel a very rich phase diagram with a competition between homogeneous, charge ordered and orbitally ordered phases, controlled by the strength of both local and short-range Coulomb interactions and by the hopping integrals of eg orbitals. Among the unconventional electronic states, we will focus this presentation on the charge ordered phases. We find in particular, an original pinball liquid, which is a metallic charge ordered state where localized (pins) and itinerant electrons (balls) coexist, that was first devised in the context of quarter-filled organic conductors[3]. By changing the anisotropic ratio t'/t of the hopping integrals, an inverse pinball liquid occurs, where the role of charge-rich and charge-poor sites are interchanged, an insulating large unit cell √12 × √12 droplet phase can be stabilized[4], with a periodicity close to the √13 × √13 David Star charge order presents in 1T-TaS2. These electronic states are generic for the half-filled triangular lattice and are also found in the isotropic limit, which corresponds to the single-band spinful extended Hubbard model. [1] G. L. Pascut, et al., Phys. Rev. Lett., 106, 157206 (2011). [2] B. Sipos, et al., Nat. Mater. 7, 960 (2008). [3] L. Cano-Cortés, A. Ralko, C. Février, J. Merino, and S. Fratini, Phys. Rev. B, 84, 155115 (2011). [4] C. Février, S. Fratini, A. Ralko, arXiv:1503.05714. |
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| Spin-current Seebeck effect in an interacting quantum dot: Atomic approximation for the Anderson impurity model Franco Peñaloza, Roberto Emilio (National University of Colombia, Physics Department (UNAL-Bogotá) and Physics Institute (USP-Sao Carlos), Physics Department, Bogotá, Colombia) |
We study the spin-current Seebeck effect through an immersed gate defined quantum dot, employing the U -finite atomic method for the single impurity Anderson model. Our description qualitatively confirms some of the results obtained by an earlier Hartree-Fock work, but as our calculation includes the Kondo effect, it will appear some new features in the spin-current Seebeck effect S, which as a function of the gate voltage present an oscillatory shape. At intermediate temperatures, our results show a three zero structure and at low temperatures, our results are governed by the emergence of the Kondo peak in the transmittance, which defines the behavior of the shape of the S coefficient as a function of the parameters of the model. The oscillatory behavior obtained by the Hartree-Fock approximation reproduces the shape obtained by us in a non interacting system (U = 0). The S sign is sensitive to different polarization of the quantum dot, and as a consequence the device could be employed to experimentally detect the polarization states of the system. Our results also confirm, that the large increase of S upon increasing U, obtained by the mean field approximation, is correct only for low temperatures. We also discuss the role of the Kondo peak in defining the behavior of the spin thermopower at low temperatures. |
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| X-boson cumulant approach to the topological Kondo insulators Franco Peñaloza, Roberto Emilio (National University of Colombia, Physics Department (UNAL-Bogotá) and Physics Institute (USP-Sao Carlos), Physics Department, Bogotá, Colombia) |
In this work we study a novel class of intermetallic 4f and 5f orbitals materials: the topological Kondo insulators; in which strong interactions between itinerant and predominately localized degrees of freedom give rise to a bulk insulating state at low temperatures, while the surface remains metallic. This effect arises due to inversion of even parity conduction bands and odd parity very narrow f electron bands. For an odd number of band inversions, the metallic surface states are chiral and therefore remain robust against disorder and time reversal invariant perturbations. The topological Kondo insulators have been studied employing the slave boson mean field theory (SBMFT) [1], both in the limit of the Coulomb repulsion U → ∞ [2, 3], and for finite correlation U [4]. This approximation is attractive because with a small numerical effort, it is capable of qualitatively describe the Kondo regime, but as the temperature is increased or when (μ >> Ef,τ = Ef ) at low temperatures, and also for all parameters at intermediate temperatures, the SBMFT presents an unphysical second order phase transition with the conduction and the localized electrons decoupling from each other. In the impurity case this transition occurs when T > TK, and defines the Kondo temperature. [1] Coleman P 1984 Phys. Rev. B 29 3035 [2] Dzero M, Sun K, Galitski V and Coleman P, 2010 Phys. Rev. Lett. 104 106408; Dzero M, Sun K, Coleman P, and Galitski V, 2012 Phys. Rev. B 85 045130; Alexandrov V, Dzero M, and Coleman P, 2013 Phys. Rev. Lett. 111 226403 [3] Tran M T, Takimoto T and Kim K S 2012 Phys. Rev. B 85 125128 [4] Legner M, Rueg A and Sigrist M Phys. Rev. B 89 085110 |
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| U(1) symmetry protected topological phase in the interacting lattice model Fuji, Yohei (Max Planck Institute for the Physics of Complex Systems (MPIPKS), Condensed Matter, Dresden, Germany) |
The newly discovered quantum states of matter, symmetry protected topological (SPT) phases, has fundamentally changed our ideas about quantum ordered phases in condensed matter systems. Despite recent theoretical achievements, the very important question of realizing interacting SPT phases in realistic physical systems has remained unexplored till date. Here we will report a simple model that realizes an SPT phase protected by the U(1) symmetry. The model is defined on a honeycomb lattice with correlated hopping boson. We use the density matrix renormalization group method (DMRG) to establish the existence of the U(1) SPT phase by its quantized Hall conductance and anomalous gapless edge modes. We also analyze the model by the bosonization approach in the quasi-1D limit and discuss the phase transition between the SPT phases and a possible fractional quantum Hall phase. Ref: Y.-C. He, S. Bhattacharjee, F. Pollmann, and R. Moessner, Phys. Rev. Lett. 115, 116803 (2015). |
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| Sudden expansion of hard-core bosons: Crossover from 1D to 2D Hauschild, Johannes (Max-Planck-Institut für Physik komplexer Systeme, Dresden, Germany) |
We numerically investigate the expansion of a cloud of hard core bosons in a 2D lattice system using a matrix-product state based method [1]. As the anisotropy of the hopping amplitudes Jx and Jy in different spatial directions are tuned, we observe a dimensional crossover from a ballistic expansion in the 1D limit (Jx >> Jy) to a diffusive one in the isotropic two-dimensional limit (Jx = Jy). We compare our result to recent experiments [2,3] and further contrast the 2D expansion to the melting of a domain wall. [1] Zaletel et al., PRB 91, 165112 (2015) [2] Ronzheimer et al., PRL 110, 205301 (2013) [3] Vidmar et al arXiv:1505.05150 (2015) |
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| Interaction effects along the edge of a topological superconductor Hofmann, Johannes (Julius-Maximilians-Universität Würzburg, Institut für Theoretische Physik und Astrophysik, Lehrstuhl theoretische Physik 1, Würzburg, Germany) |
Topological nodal superconductors, such as dxy-wave and nodal non-centrosymmetric superconductors, exhibit protected zero-energy flat-band edge states. These zero-energy edge modes are protected by time-reversal and translation symmetry and their stability is guaranteed by the conservation of a quantized topological invariant. Here, we study the fate of these flat-band edge states in the presence of interactions. We find that Hubbard interactions lead to spontaneous breaking of time-reversal or translation symmetry at the edge of the system. For the dxy-wave superconductor in the presence of attractive Hubbard interactions we find that the flat-band states become unstable towards the formation of a charge-density wave state or a state with s-wave type pairing correlations. Repulsive Hubbard interactions, on the other hand, induce ferromagnetic order at the edge of the dxy-wave superconductor. |
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| Finite-temperature dynamics of a highly frustrated quantum spin ladder Honecker, Andreas (Université de Cergy-Pontoise, Laboratoire de Physique Théorique et Modélisation, UFR sciences et techniques, Cergy-Pontoise, France) |
Highly-frustrated magnets are characterized by a (nearly) flat one-triplet excitation band at zero temperature. Little is known from theoretical studies about the temperature-dependence of this single-particle dispersion and less still concerning multi-particle dynamics at finite temperature. Experimentally, inelastic neutron scattering studies of low- dimensional frustrated systems such as SrCu2(BO3)2 require an interpretation of the thermal evolution of scattering ntensities. We investigate these issues using the example of a highly frustrated spin-1/2 ladder. We find that single- and many-particle excitations persist as sharp spectral features in the dynamic structure factor to surprisingly high and even infinite temperatures. In addition, close to a zero-temperature quantum phase transition, low-lying non-magnetic excitations give rise to an unusual low-temperature feature in the specific heat as well as a concurrent anomalously rapid transfer of spectral weight out of the single-particle band to a wide range of energies. Our results offer new insight into the finite-temperature spectral functions of SrCu2(BO3)2 and of many other highly frustrated systems. |
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| Topological Kondo insulator. Exact results Karnaukhov, Igor (The National Academy of Sciences of Ukraine, G. V. Kurdyumov Institute for Metal Physics, Department of Theory of Nonideal Crystals, Kyiv , Ukraine) |
A model of a topological Kondo lattice defined on a honeycomb lattice is studied for the case when spinless fermion bands are half filled. It is shown that the hybridization between local moments and itinerant fermions should be understood as the hybridization between corresponding Majorana fermions of the spin and charge sectors. The system is a topological insulator, single fermion and spin excitations at low energies are massive. A spin gap induces a gap in the charge channel, it leads to an appearance of a topological insulator state with chiral gapless edge modes and the Chern number one or two depending on the exchange integrals' values. The relevance of this to the traditional Kondo insulator state is discussed. |
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| Dynamics of disordered many-body systems: from thermal transport to many-body localization Knap, Michael (Technische Universität München, Physik Department, Germany) |
We provide an introduction to disordered interacting many-body systems and analyze the generic phase diagram of such systems, which consists of a thermal phase at weak disorder and a many-body localizated (MBL) phase at strong disorder. Near the phase transition, Griffiths effects become important which result in new rare-region dominated phases. Furthermore, we discuss the response of the different phases to transport probes (e.g. conductivity) as well as non-transport probes (e.g. spin-echo interferometry) and demonstrate how the peculiar properties of the MBL phase can be unveiled. |
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| Many-Body Anderson Localization of BECs in the Bose-Hubbard Model Kroha, Johann (Rheinische Friedrich-Wilhelms-Universität Bonn, Physikalisches Institut, Bonn, Germany) |
We have developed the transport theory for a Bose gas in the disordered Bose-Hubbard model in 3 dimesnions in the regime of strong interactions, i.e. in the
vicinity of the Mott lobes of vanishing Bose-Einstein condensate (BEC) amplitude. In contrast to previous approaches, we consider the Bose glass not as a state with
finite compressibility but vanishing averaged BEC amplitude, but as the phase with finite average BEC amplitude but vanishing superfluid transport due to many-body
Anderson localization of the interacting BEC wave functions and their many-body excitations. The theory is based on a calculation of the local many-body ground and excited
states within a stochastic mean-field theory, treating the on-site Hubbard interaction exactly by diagonalizing the local part of the Bose-Hubbard Hamitonian in Fock space.
Non-local effects of the interaction are neglected, analogous to Dynamical Mean-Field Theory. The transport theory for these hopping many-body states, including quantum
interference processes ("Cooperons") is formulated as a generalization of the self-consistent theory of Anderson localization.
The theory describes semiquantitatively the Mott localized phase ("Mott lobes"), the superfluid phase and the Bose glass phase as well as the respective phase transitions.
In particular, the theory obeys the theorem of inclusions which states that in a disordered system there is no direct transition from the Mott phase to the superfluid phase.
(Publication in preparation) |
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| Spatial and temporal propagation of Kondo correlations Lechtenberg, Benedikt (Technische Universität Dortmund, Theoretische Physik II, Dortmund, Germany) |
We address the fundamental question how the spatial Kondo correlations are building up in time assuming an initially decoupled impurity spin $vec{S}_mathrm{imp}$.
We investigate the time-dependent spin-correlation function $chi(vec{r},t) = langle vec{S}_{rm imp} vec{s}(vec{r}) rangle (t)$ in the Kondo model with antiferromagnetic
and ferromagnetic couplings where $vec{s}(vec{r})$ denotes the spin density of the conduction electrons after switching on the Kondo coupling at time t=0. We present data
obtained from a time-dependent numerical renormalisation group (TD-NRG) calculation. We gauge the accuracy of our two-band NRG by the spatial sum-rules of the equilibrium
correlation functions and the reproduction of the analytically exactly known spin-correlation function of the decoupled Fermi sea. We find a remarkable building up of
Kondo-correlation outside of the light cone defined by the Fermi velocity of the host metal. By employing a perturbative approach exact in second-order of the Kondo coupling,
we connect these surprising correlations to the intrinsic spin-density entanglement of the Fermi sea.
The thermal wave length supplies a cutoff scale at finite temperatures beyond which correlations are exponentially suppressed. We present data for the frequency dependent
retarded spin-spin susceptibility and use the results to calculate the real-time response of a weak perturbation in linear response: within the spatial resolution no response
outside of the light cone is found. |
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| Superconducting junctions as Dirac and Majorana fermion detectors Maiti, Moitri (Joint Institute for Nuclear Research, Bogoliubov Laboratory of Theoretical Physics, Dubna, Mosrow Region, Russian Federation) |
We demonstrate that the current-voltage (I-V) characteristics of resistively and capacitively shunted Josephson junctions (RCSJ) of topological superconductors hosting localized subgap Majorana states provides a phase sensitive method for their detection. We show that the I-V characteristics of such RCSJs display a novel devil staircase structure for Shapiro steps which is qualitatively different from those found in conventional superconductors. A similar study for RCSJ with graphene superconducting junctions hosting Dirac-like quasiparticles reveals that the Shapiro step width in their I-V curves oscillates with the junction barrier potential. We demonstrate that this oscillatory feature is a signature of the underlying Dirac quasiparticles. |
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| Time reversal symmetry broken fractional topological phases at zero magnetic field Meng, Tobias (Technische Universität Dresden, Institut für Theoretische Physik, Department of Physics, Dresden, Germany) |
We extend the coupled-wire construction of quantum Hall phases, and search for fractional topological insulating states in models of weakly coupled wires at zero external magnetic field. Focussing on systems beyond double copies of fractional quantum Hall states at opposite fields, we find that spin-spin interactions can stabilize a large family of fractional topological phases with broken time reversal invariance. The latter is manifest by spontaneous spin polarization, by a finite Hall conductivity, or by both. This suggests the possibility that fractional topological insulators may be unstable to spontaneous symmetry breaking. |
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| Asymmetric Bethe-Salpeter equation - many phases of interacting Bose and Fermi gases Morawetz, Klaus (Münster University of Applied Sciences, Physical Engineering, Steinfurt, Germany) |
The recently proposed asymmetric Bethe-Salpeter equation [1] to avoid unphysical repeated collisions is derived as a result of the hierarchical dependencies of correlations [2]. This form is superior over the symmetric selfconsistent one since it provides the Nambu-Gorkov equations and gap equation for fermions and the Beliaev equations for bosons while from the symmetric form no gap equation results. The corrected T-matrix becomes suitable to calculate properties above and below the critical temperature [3], without the use of anomalous functions [4]. We have identified the contribution of bound states and Cooper pairs to the self energy, leading to a gap dispersion in case a condensation of bound states or Cooper pairing occurs [5,6]. Therefore we are able to describe the BCS-BEC crossover within a unique theoretical tool. The equation of state becomes multivalued near the BEC transition which can be cured bz Maxwell construction for sufficiently weak interaction in Hartree-Fock or Popov (Hartree-Fock-Bogoliubov) approximation. However, for strong interactions there remains a multivalued region even for the corrected T-matrix approximation what is interpreted as a density hysteresis [7]. [1] Lipavský, P., Multiple scattering corrections to the -matrix approximation: Unified theory of normal and superconducting states, Phys. Rev. B 78 (2008) 214506 [2] K. Morawetz, Asymmetric Bethe-Salpeter equation for pairing and condensation, J. Stat. Phys. 143 (2011) 482-500 [3] B. Sopik, P. Lipavský, M. Männel, K. Morawetz, P. Matlock, Self-consistent T-matrix theory of superconductivity, Phys. Rev. B 84 (2011) 094529-1-13 [4] K. Morawetz, Equivalence of channel-corrected T-matrix and anomalous propagator approach, Phys. Rev. B 82 (2009) 092501-1-4 [5] M. Männel, K. Morawetz, P. Lipavský, Multiple condensed phases in attractively interacting Bose systems}, New J. Phys. 12 (2010) 033013-1-9 [6] P. Lipavský, K. Morawetz, B. Sopik, M. Männel, Stability of condensate in superconductors, Eur. Phys. J. B 87 (2014) 8 [7] M. Männel, K. Morawetz, P. Lipavský, Coexistence of phase transitions and hysteresis near the onset of Bose-Einstein condensation, Phys. Rev. A 87 (2013) 053617-1-7 |
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| Thermodynamics of the frustrated J1-J2 Heisenberg ferromagnet on the body-centred cubic lattice with arbitrary spin Müller, Patrick (Otto-von-Guericke-Universität Magdeburg, Institut für Theoretische Physik (ITP), Fakultät für Naturwissenschaften (FNW), Magdeburg, Germany) |
We use the spin-rotation-invariant Green's function method as well as the high-temperature expansion to discuss the thermodynamic properties of the frustrated
spin-S J1-J2 Heisenberg magnet on the body-centered cubic lattice. We consider ferromagnetic nearest-neighbor bonds J1 < 0 and antiferromagnetic
next-nearest-neighbor bonds J2 ≥ 0 and arbitrary spin S. We find that the transition point $J_2^c$ between the ferromagnetic ground state and the antiferromagnetic
one is nearly independent of the spin S, i.e., it is very close to the classical transition point $J_2^{c,{\rm clas}}=\frac{2}{3}|J_1|$. At finite temperatures we focus on
the parameter regime $J_2 |
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| Kondo breakdown in asymmetric double quantum dots Nejati, Ammar (University of Bonn, Germany) |
Due to recent advances in semiconductor device technology, it is possible to investigate the Kondo effect in presence of magnetic correlations with fully-controllable double-quantum-dot systems which exhibit a rich variety of correlated behavior; esp. suppression of the Kondo screening due to the competition of Kondo and exchange interactions. Here we present a renormalization group (RG) method which can describe the different regimes observed in a highly-tunable semiconductor device with 2 qdots coupled to 3 leads [1]. The principal experimental observation was formation of a Kondo state for one qdot along with suppression of the Kondo resonance in the other, in presence of RKKY exchange interaction. The RG method is developed to consider local spin screening in multi-impurity Kondo systems in the absence of critical magnetic fluctuations [2]. Such a scenario can be realized well above the magnetic ordering temperature [3], in 2-impurity [4] or magnetically frustrated lattice systems. We calculate the beta-function for the magnetic coupling J between a localized spin and conduction electrons in 1-loop order, taking into account that J is modified by the RKKY coupling to neighbouring spins. Since this correction involves the local dynamical susceptibility on neighbouring Kondo sites, which is inversely proportional to T_K, it leads to a self-consistent suppression of T_K. Then T_K = T_K(y) will be a universal function only of the single-impurity Kondo scale T_K(0) and of the dimensionless RKKY coupling parameter y. Remarkably, complete Kondo screening terminates at a maximal RKKY coupling y_max which depends on the bare parametres only. [1] Tutuc, D., et al., Phys. Rev. B 83, 241308(R) (2011). [2] Nejati, A., K. Ballmann and J. Kroha, J. Phys.: Conf. Ser. 592, 012090 (2015). [3] Klein, M., et al., Phys. Rev. Lett. 101, 266404 (2008). [4] Bork, J., et al., Nature Phys. 7, 901 (2011). |
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| Charge fluctuations in nonlinear heat transport Schuricht, Dirk (Utrecht University, Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht, Netherlands) |
We show that charge fluctuation processes are crucial for the nonlinear heat conductance through an interacting nanostructure, even far from a resonance. We illustrate this for an Anderson quantum dot accounting for the first two leading orders of the tunneling in a master equation. The often made assumption that off-resonant transport proceeds entirely by virtual occupation of charge states, underlying exchange-scattering models, can fail dramatically for heat transport. The identified energy- transport resonances in the Coulomb blockade regime provide new qualitative information about relaxation processes, for instance by strong negative differential heat conductance relative to the heat current. These can go unnoticed in the charge current, making nonlinear heat-transport spectroscopy with energy-level control a promising experimental tool. |
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| Indirect magnetic coupling in graphene nanostructures from exact diagonalization of Hubbard model Szalowski, Karol (University of Lodz, Faculty of Physics and Applied Informatics, Department of Solid State Physics, Lodz, Poland) |
The development of graphene-based spintronics [1,2] calls for progress in exploration of interactions between magnetic moments in graphene. In particular, graphene nanostructures attract both theoretical and experimental interest due to their potential of applications owing to non-trivial, controllable magnetic properties [3]. These facts motivate the studies of indirect magnetic (Ruderman-Kittel-Kasuya-Yosida) coupling between localized magnetic moments in graphene and its derivatives. In particular, the usefulness of external electric and magnetic field for controlling the such interactions is worthy of investigation. In the paper we present a computational study of indirect coupling in prototypical, ultrasmall graphene nanostructure (nanoflake, quantum dot). We model the system of the charge carriers using the tight-binding Hamiltonian supplemented with Hubbard term. Additional Anderson-Kondo terms are present to account for the presence of a pair of magnetic impurities. Moreover, the external magnetic field and electric field is included. We calculate the total energy of the system by performing an exact diagonalization of the Hamiltonian in question. This procedure allows to determine the indirect coupling in a fully non-perturbative way. The results are compared with the outcome of calculations based on Mean Field Approximation (MFA) applied to the same Hamiltonian. This way we extend our previous MFA-based studies performed for larger graphene nanostructures [4-8]. In particular, the effect of various factors such as, for example, charge doping and external electric and magnetic field on the indirect coupling is discussed extensively. As an example, the indirect coupling anisotropy in spin space, which arises as a consequence of the presence of external fields, is thoroughly investigated. This work has been supported by Polish Ministry of Science and Higher Education on a special purpose grant to fund the research and development activities and tasks associated with them, serving the development of young scientists and doctoral students. The computational support on Hugo cluster at Laboratory of Theoretical Aspects of Quantum Magnetism and Statistical Physics, P. J. Šafárik University in Košice is gratefully acknowledged. [1] W. Han, R.K. Kawakami, M. Gmitra, J. Fabian, Nature Nanotechnology 9 (2014) 794807. [2] Z. Bullard, E.C. Girão, J.R. Owens, W.A. Shelton, V. Meunier, Scientific Reports 5 (2015) 7634. [3] O. V. Yazyev, Reports on Progress in Physics 73 (2010) 056501. [4] K. Szalowski, Physical Review B 84 (2011) 205409. [5] K. Szalowski, Physica E 52 (2013) 46. [6] K. Szalowski, Acta Physica Polonica A 126 (2014) 236. [7] K. Szalowski, Journal of Physics: Condensed Matter 25 (2013) 166001. [8] K. Szalowski, Physical Review B 90 (2014) 085410. |
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| Study of the Kosterlitz-Thouless transition in 1D Heisenberg antiferromagnet on the basis of topological properties of its ground state function Tomczak, Piotr (Adam Mickiewicz University, Department of Physics, Poznań, Poland) |
A quantum phase transition for the Heisenberg chain with the next-nearest-neighbor interactions (λ=Jnnn/Jnn) reinvestigated once more from a different perspective: taking into account a scalar product ⟨ΨGS(λ)| ΨGS(λ)⟩ we find an unequivocal correspondence between its components and topological objects. One can classify these objects according to whether any two of them can be transformed into each other in a continuous way. A finite size scaling of the "connection" $\langle\partial_\lambda\Psi|\Psi \rangle$ with respect to chain length (16, 18, 20, 22, 24 spins) calculated for each class of above mentioned objects gives the critical value of λ with an accuracy of half percent (We do not consider a Berry phase). |
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| Phonon spectral function of the one-dimensional Holstein-Hubbard model Weber, Manuel (Universität Würzburg, Institut für Theoretische Physik und Astrophysik, Lehstuhl für Theoretische Physik 1, Würzburg, Germany) |
We use the continuous-time interaction expansion (CT-INT) quantum Monte Carlo method to calculate the phonon spectral function of the one-dimensional Holstein- Hubbard model at half-filling. Our results are consistent with a soft-mode Peierls transition in the adiabatic regime, and the existence of a central peak related to long-range order in the Peierls phase. We explain a previously observed feature at small momenta in terms of a hybridization of charge and phonon excitations. Tuning the system from a Peierls to a metallic phase with a nonzero Hubbard interaction suppresses the central peak, but a significant renormalization of the phonon dispersion remains. In contrast, the dispersion is only weakly modified in the Mott phase. |
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| d5 Kitaev exchange: the angle you approach it matters Yadav, Ravi (Leibniz Institute for Solid State and Materials Research Dresden (IFW-Dresden), ITF, Dresden, Germany) |
While electronic-structure calculations within either the wavefunction-based [1,2] or density functional theory [3] framework agree on the magnitude and the signs of the Kitaev couplings in 5d5 honeycomb iridates, much less is known on these effective exchange constants in the 4d5 analogues. We here discuss the outcome of many-body, wavefunction-based quantum chemistry computations for these interaction parameters in Li2RhO3 and RuCl3. The ab initio values for the nearest-neighbor couplings, both isotropic and anisotropic, are further fed to an extended spin Hamiltonian that includes additionally 2nd- plus 3rd-neighbor Heisenberg terms and on the basis of exact-diagonalization calculations predictions are made for the nature of the magnetic ground states in these compounds. [1] Vamshi M Katukuri et al 2014 New J. Phys. 16 013056 [2] Satoshi Nishimoto et al arXiv:1403.6698 [3] PRL 113, 107201 (2014). |
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