Forthcoming Seminars at F-1

Seminars Archive

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11 Dec 2018
Lucas Hackl (MPQ Garching)Low energy spectra from variational tangent planes
Variational methods are extensively used to study properties of ground states and low excited states. By focusing on a suitable sub-manifold of states, we can overcome the exponential growth of the Hilbert space with the system size. In this talk, I will introduce a geometric framework to approximate the low lying energy spectrum by studying the linearized Hamiltonian flow on the tangent plane at the ground state. The spectrum is then given by the eigenvalues of the covariant derivative on the variational manifold. Using the paradigmatic Bose-Hubbard model as an example, I will discuss advantages compared to methods based on directly diagonalizing the Hamiltonian restricted to the tangent plane.

Seminar room for physics (JSI main building)

4 Dec 2018
Jan Skolimowski (JSI)DC-transport properties of correlated electrons in presence of binary disorder
In this talk I will present the recent results on dc-transport properties of a hole-doped Hubbard model with binary disorder. I will begin by discussing the current state of knowledge about this model, with a special emphasis on the metal-insulator transition (MIT) at non-integer filling. This transition takes place in the system above a critical disorder strength, when the electron filling (n) is equal to the disorder concentration (x). The aim of my study was to understand the consequences that this peculiar MIT has on the transport. Therefore the result for n == x were compared against the results for n != x. To describe the dc-transport properties the longitudinal resistivity (LR), cotangent of Hall angle (COHA) and thermopower (S) were used. For n == x the system is susceptible to form a gap at the Fermi level. As a result, both LR and COHA exhibit a similar non-monotonic behaviour characteristic for Mott physics. Depending on the interaction strength LR and COHA have either a jump, in case of MIT, or smooth peak for the crossover. The difference between these two scenarios will be discussed in details. For n != x system remains metallic and the similarity between the LR and COHA is absent. Instead, the two quantities display a typical behaviour for doped Mott insulators. At high temperatures LR and COHA for all studied electron fillings display a similar behaviour, highlighting the decreasing role of disorder in the system. In contrast to LR and COHA the results for thermopower shows no qualitative difference between the two considered electron fillings. In both cases S exhibits two zero crossing: one is connected to decaying width of the QP resonance and the other is connected to the increasing number of thermal excitations. At high temperatures thermopower saturates as in a pristine hole-doped Hubbard model.

Seminar room for physics (JSI main building)

6 Nov 2018
Tadej Mežnaršič (F-5, JSI)Cesium solitons
When a non-interacting Bose-Einstein condensate (BEC) is confined to a quasi one-dimensional channel it will spread due to dispersion as dictated by the Schrödinger equation. The spreading rate can be affected by changing the interaction between the atoms via the Feshbach resonance. If the interaction is set to just the right value, the attraction between atoms exactly compensates the dispersion. In this case the BEC does not spread and we get a bright matter-wave soliton. The maximum number of atoms in a soliton is limited by the frequency of the channel and the interaction between atoms. By setting the inter-atom interaction to different attractive values we are able to create soliton trains with different number of solitons from a single BEC. Soliton trains form due to modulational instability, meaning that a small variation in density, caused by some broadband perturbation, is exponentially amplified and causes the BEC to split into several solitons.

Seminar room for physics (JSI main building)

23 Oct 2018
Friedrich KrienThe smothered phase separation: Microscopic Fermi liquid theory of the Mott transition
The compressibility of the Mott insulating phase is vanishingly small, the doped Mott insulator however often exhibits a phase separation (divergence of the compressibility), which is a Pomeranchuk instability of the Fermi liquid in the charge sector. In order to better understand the counter-intuitive proximity of the two phases we take a look at the two-particle level of the Fermi liquid theory within the DMFT approximation. Our analysis shows that the vanishing compressibility of the Mott insulator in fact requires the divergence of the charge forward scattering vertex at the Mott transition, which for finite quasi-particle weight would lead to a divergence of the compressibility. The Mott transition is thus accompanied by a smothered phase separation. We also take a look at the fermion-boson response function, which allows to understand the role of the response of incoherent states in the Fermi liquid phase and at the Mott transition.

Seminar room for physics (JSI main building)

16 Oct 2018
Adrian Feiguin (Northeastern University, USA)The spectral function of Mott-insulating Hubbard ladders: From fractional excitations to coherent quasi-particles
When electrons are confined in one spatial dimension, they may loose their identity as individual particles, and ”break” into separate excitations carrying spin (a spinon), and charge (a holon) and, as a consequence, Landau’s quasi-particle picture breaks down. Whether spin-charge separation survives in dimensions greater than d=1 has been a topic of great debate, particularly in the context of high-temperature superconductivity. A peculiar case that can shed some light on this problem consists of coupling two chains into a ladder geometry. When the Mott insulator is doped, it is expected that polaronic quasiparticles (a bound state of a spinon and a holon) will form, restoring Landau’s paradigm. We study the spectral function of two-leg Mott insulating Hubbard ladders using the time-dependent density matrix renormalization group method (tDMRG). The spectrum displays features of both spin-charge separation and coherent bound states. As the inter-leg hopping is increased, both branches merge into a single coherent quasi-particle band and the spectrum undergoes a crossover from a regime with two incommensurate minima, to one with a single minimum. At the same time, the system transitions from a Mott insulator to a band insulator. Interestingly, while the bonding sector of the spectrum realizes quasi-particles, the anti-bonding one dis- plays a broad scattering continuum, which can be associated to the lack of quasi-particles. We identify the processes leading to quasiparticle formation by studying the time evolution of charge and spin degrees of freedom in real space after a hole is created. At short times, incoherent holons and spinons are emitted but charge and spin quickly form polarons that propagate coherently. In addition, we infer the full spectrum of the 2D Hubbard model in the thermodynamic limit by coupling ladders in the perpendicular direction using cluster perturbation theory. A remarkable advantage of this approach is that unlike small clusters, the one-dimensional systems are already ”infinite”. Excellent agreement with quantum Monte Carlo and other cluster methods is obtained.

Seminar room for physics (JSI main building)

4 Sep 2018
Prof Deepak DharMultiple singularities of the equilibrium free energy in a one dimensional model of repulsive soft rods
I will discuss a counter-example to the folk-wisdom that the equilibrium free energy of one dimensional models with finite-ranged interactions, and at non-zero temperature, cannot show singularities as the coupling constants are varied. We consider hard linear rods of length b whose centers are placed on equi-spaced points along a line. The interaction between rods is a repulsive soft-core interaction, having energy U per overlap of rods. We show that the free energy per rod F(b/a,T) has an infinite number of singularities as a function of the lattice spacing a. [Reference: arXiv:1806.09841] more...

4 Jul 2018
Minjae KimSpin-orbit coupling and electronic correlations in Hund metal: Sr2RuO4
We investigate the interplay of spin-orbit coupling (SOC) and Hund rule coupling driven electronic correlations of Sr2RuO4 using the dynamical mean-field theory. We have shown that (i) SOC is not effective on the dynamical electronic correlation of Sr2RuO4, and (ii) effective SOC constants of the quasiparticle are enhanced by factor of 2 from the original value of SOC constants because of the electronic correlation. We found very similar quasiparticle renormalizations and their scattering to the ones found in a calculation that neglects the SOC. This finding validates the Hund metal picture of ruthenates even in the presence of sizable SOC. On the one hand, the enhanced SOC constants prone to energy dependent renormalization of quasiparticle. These aspects are essential to describe experiments, (i) SOC driven degeneracy lifting at Gamma point, (ii) SOC effective Fermi surface, and (iii) correct renormalization of bands [1,2,3,4]. This work elucidates the SOC effects on the correlated Hund metal which verifies previous arguments on (i) the correlation enhanced effective SOC in the quasiparticle [5], and (ii) the mechanism of ineffective SOC in the dynamical correlation in Hund metal in which having larger energy scale of the coherence energy in the orbital sector with respect to the energy scale of the original SOC [6].

[1] C. Veenstra et al., Physical Review Letters 112, 127002 (2014)
[2] M. Haverkort et al., Physical Review Letters 101, 026406 (2008)
[3] J. Mravlje et al., Physical Review Letters 106, 096401 (2011)
[4] M. Kim et al., Physical Review Letters 120, 126401 (2018)
[5] G.-Q. Liu et al., Physical Review Letters 101, 026408 (2008)
[6] A. Horvat et al., Physical Review B 96, 085122 (2017)

F1 tea room

26 Jun 2018
Milovan ŠuvakovThe three-body problem in the pairwise strong potential: periodic solution
We performed a numerical search for periodic solutions of the planar three-body problem in the pairwise strong potential 1/r^2 with equal masses and zero angular momentum. Such system is described with six dimensional phase space. Using additional constraints that periodic solutions obey, one can define a three-dimensional subspace of phase space in which up to the scaling all periodic solutions belong. Dynamical system defined in that subspace is useful as a framework for numerical search for periodic solutions. We report new orbits and study topological influence on period and action values of solutions.

Seminar room for physics (JSI main building)

22 Jun 2018
Yicheng ZhangInformation measures for a local quantum phase transition:
Lattice fermions in a one-dimensional harmonic trap
We use quantum information measures to study the local quantum phase transition that occurs for trapped spinless fermions in one-dimensional lattices. We focus on the case of a harmonic confinement~[1]. The transition occurs upon increasing the characteristic density and results in the formation of a band-insulating domain in the center of the trap. We show that the ground-state bipartite entanglement entropy can be used as an order parameter to characterize this local quantum phase transition.We also study excited eigenstates by calculating the average von Neumann and second Renyi eigenstate entanglement entropies, and compare the results with the thermodynamic entropy and the mutual information of thermal states at the same energy density. While at low temperatures we observe a linear increase of the thermodynamic entropy with temperature at all characteristic densities, the average eigenstate entanglement entropies exhibit a strikingly different behavior as functions of temperature below and above the transition. They are linear in temperature below the transition but exhibit activated behavior above it. Hence, at nonvanishing energy densities above the ground state, the average eigenstate entanglement entropies carry fingerprints of the local quantum phase transition.

[1] Zhang, Vidmar and Rigol, Phys. Rev. A 97, 023605 (2018)

F1 tea room.

20 Jun 2018
Tilen ČadežLocalization in inhomogeneous Floquet systems
We study the localization aspects of kicked one-dimensional (1D) non-interacting quantum systems subject to either time-periodic or non-periodic pulses. These are reflected as sudden changes of the on-site energies in the lattice with different modulations in real space. When the modulation of the kick is incommensurate with the lattice spacing, and the kicks are periodic in time, a well known dynamical localization is recovered for large kick amplitudes and frequencies. We show [1] that the scaling properties at the quasi-disorder induced metal-insulator (Anderson) transition in the time periodic case are the same as in the static case, up to intermediate time periods. Moreover, the aperiodicity in time leads to the breakdown of localization and diffusion sets in. By adding superconducting pairing into the system we argue on the existence of a whole region of critical states. Further exploring the phase diagram [2] in different driving regimes, we claim on the relevance of the studied system and its possible experimental realization in cold atomic systems.

[1] T. Čadež, R. Mondaini and P. D. Sacramento, Phys. Rev. B 96, 144301 (2017)
[2] T. Čadež, R. Mondaini and P. D. Sacramento, In preparation

F1 tea room

12 Jun 2018
Lara UlčakarSlow quenches in two-dimensional time-reversal symmetric Z2 topological insulators
We study the topological properties and transport in the Bernevig-Hughes-Zhang (BHZ) model undergoing a slow quench between different topological regimes [1]. Due to the closing of the band gap during the quench, the system ends up in an excited state. We prove that for quenches that preserve the time-reversal symmetry, the Z2 invariant remains equal to the one evaluated in the initial state. On the other hand, the bulk spin Hall conductivity does change and its time average approaches that of the ground state of the final Hamiltonian. The deviations from the ground-state spin Hall conductivity as a function of the quench time follow the Kibble-Zurek scaling. We also consider the breaking of the time-reversal symmetry, which restores the correspondence between the bulk invariant and the transport properties after the quench.

[1] Ulčakar, Mravlje, Ramšak and Rejec, Phys. Rev. B 97, 195127 (2018)

F1 tea room

5 Jun 2018
Robin SteinigewegValidity of dynamical linear response for arbitrarily strong perturbations
Linear response theory is one of the main approaches to the dynamics of quantum many-body systems. However, this approach has limitations and requires, e.g., that the initial state is (i) mixed and (ii) close to equilibrium. In this situation, we take a fresh perspective and unveils that these requirements can be substantially weakened by constructing two different classes of pure initial states. While the first class agrees with linear response theory in the usual close-to-equilibrium limit, the second class yields a correlation function in the far-from-equilibrium limit. These independent statements apply to any observable but can be connected for so-called binary operators. For such operators and high temperatures, we prove that the dynamics is generated by a single correlation function in the entire regime between the cases close to and far from equilibrium. In numerical simulations, we illustrate our analytical results and present evidence that these results can also hold true for non-binary operators.

[1] J. Richter, R. Steinigeweg, arXiv:1711.00672 (2017)

Seminar room for physics (JSI main building)

30 May 2018
Bosiljka TadićPhysics of Social Systems: Emergent Behavior in Knowledge-Sharing Dynamics
Recently, the vast amount of data collected on various Websites has provided a unique opportunity for the quantitative study of social phenomena in analogy with natural sciences. In this respect, the methods of Statistical Physics for studying complex systems in the physics laboratory, in particular, the interacting nonlinear systems which are driven far from equilibrium, play a crucial role in understanding the mechanisms that enable the emergence of collective social phenomena on the Web. In this lecture, we discuss some of the possibilities and challenges of physics approaches to social dynamics and illustrate them by considering a specific type of data from knowledge creation endeavours (Q&A site Mathematics, IR Chats Ubuntu), and agent-based modelling. The emphasis is on the occurrence of self-organised criticality in these knowledge-sharing processes, while at the same time we deal with the structure of the co-evolving networks on which these processes take place. Learn more:

[1] B. Tadić, M. M. Dankulov, R. Melnik, Mechanisms of self-organized criticality in social processes of knowledge creation, Physical Review E 96(3), 032307 (2017)
[2] M. Andjelković, B. Tadić, M. M. Dankulov, M. Rajković, R. Melnik, Topology of innovation spaces in the knowledge networks emerging through questions-and-answers, PLOS One 11(5), e0154655 (2016)
[3] M. M. Dankulov, R. Melnik, B. Tadić, The dynamics of meaningful social interactions and the emergence of collective knowledge, Scientific Reports 5, 12197 (2015)
[4] B. Tadić, V. Gligorijević, M. Mitrović, M. Šuvakov, Co-Evolutionary Mechanisms of Emotional Bursts in Online Social Dynamics and Networks, Entropy 15(12), 5084-5120 (2014)

29 May 2018
Lev VidmarEntanglement Entropy and Quantum Chaotic Hamiltonians
It is well known that typical pure states in the Hilbert space are (nearly) maximally entangled. In my talk I will discuss, from the perspective of bipartite entanglement entropy, how different are typical eigenstates of physical Hamiltonians from typical states in the Hilbert space.

I will focus on eigenstates of quantum chaotic many-body Hamiltonians [1]. I will study (a) eigenstates of quanum spin chains in nonintegrable regime, and (b) random pure states. I will prove that, in a system that is away from half filling and divided in two equal halves, an upper bound for the average entanglement entropy of random pure states with a fixed particle number and normally distributed real coefficients exhibits a deviation from the maximal value that grows with the square root of the volume of the system. Exact numerical results for highly excited eigenstates of a particle number conserving quantum spin chain model indicate that the bound is saturated with increasing system volume.

[1] Vidmar and Rigol, Phys. Rev. Lett. 119, 220603 (2017)

Seminar room for physics (JSI main building)

22 May 2018
Janez BončaCharge vs. Spin Disorder in a Correlated Electron System
In the first part [1] I will show that electron-magnon interaction delocalizes the particle in a system with strong charge disorder. The analysis is based on results obtained for a single hole in the one–dimensional t-J model. Unless there exists a mechanism that localizes spin excitations, the charge carrier remains delocalized even for a very strong charge disorder and shows subdiffusive motion up to the longest accessible times.
In the second part [2] I will present a study of dynamics of a single hole in one dimensional t-J model subject to a random magnetic field. Strong disorder that couples only to the spin sector localizes both spin and charge degrees of freedom. While we cannot precisely pinpoint the threshold disorder, we conjecture that there are two distinct transitions. Weaker disorder first causes localization in the spin sector. Carriers become localized for somewhat stronger disorder, when the spin localization length is of the order of a single lattice spacing. I will also discuss finite doping.

[1] J. Bonča and M. Mierzejewski, Phys. Rev. B 95, 214201 (2017)
[2] G. Lemut, M. Mierzejewski, and J. Bonča, Phys. Rev. Lett. 119, 246601 (2017)

Seminar room for physics (JSI main building)

18 May 2018
Peter PrelovšekMany-body localization in spin and Hubbard chains
I will describe concepts of many-body localization (MBL) on the example of the prototype MBL model - Heisenberg chain with random local magnetic fields. In particular, I will focus on the MBL criteria of nonergodic behavior of correlation functions and the absense of d.c. transport. On the other hand, analogous studies of the Hubbard chain with random potential show that even in the case of large disorder the charge degrees can be nonergodic, while spins behave as ergodic, but with subdiffusive dynamics. Results will be explained via an effective spin model, where subdiffusion emerges as the consequence of random distribution of exchange interactions.

Seminar room for physics (JSI main building)

11 May 2018
Jan ŠuntajsIntroduction to Anderson localization and MBL
Čajna soba F1

20 Apr 2018
Jan SkolimowskiJournal club: Bad metal physics with ultracold atoms

17 Apr 2018
Rok ŽitkoRevival of the physics of sub-gap states in superconductors: from Shiba to Majorana states
Recent advances in the fabrication of nanometer scale hybrid semiconductor-superconductor devices as well as in the scanning tunneling spectroscopy of adsorbate covered surfaces of superconductors have made possible very detailed experimental studies of the old problem of paramagnetic impurities in a superconducting host using local probes with very high energy and spatial resolution. At the same time, improved theoretical tools have been devised to reliably and accurately calculate the excitation spectra of the corresponding quantum impurity problems with gapped continuum electrons. These developments have enabled very stringent tests between experiment and theory. The results demonstrate the importance to describe the magnetic impurities as quantum objects with non-trivial internal dynamics due to the coupling to their environment. I will discuss the physics of bound states induced by the exchange interaction between the magnetic impurities and the Bogoliubov quasiparticles in superconductors. They are observable as spectroscopically sharp resonances in the tunneling spectra, located well inside the superconducting gap for sufficiently strong exchange coupling. They provide a new way to study the effects of strong correlations. Particularly interesting behaviour is found for cases of high-spin impurities in the presence of magnetic anisotropy, and when nearby impurities are coupled though exchange interaction. I will also briefly comment on the relation to recent experiments aimed at detecting fractional Majorana modes localized at the ends of chains of magnetic impurities, and on the non-Fermi-liquid features found in the two-channel Kondo model with superconducting channels.

10 Apr 2018
Mikhail KiselevThermoelectric Transport through SU(N) Kondo Impurity
We investigate thermoelectric transport through a SU(N) quantum impurity in the Kondo regime. The strong coupling fixed point theory is described by the local Fermi-liquid paradigm. Using Keldysh technique we analyze the electric current through the quantum impurity at both finite bias voltage and finite temperature drop across it. The theory of a steady state at zero-current provides a complete description of the Seebeck effect. We find pronounced non-linear effects in temperature drop at low temperatures. We illustrate the significance of the non-linearities for enhancement of thermopower by two examples of SU(4) symmetric regimes characterized by a filling factor m: i) particle-hole symmetric at m=2 and ii) particle-hole non-symmetric at m=1. We analyze the effects of potential scattering and coupling asymmetry on the transport coefficients. We discuss connections between the theory and transport experiments with coupled quantum dots and carbon nanotubes. D.B. Karki and M.N. Kiselev, Thermoelectric transport through SU(N) Kondo Impurity, Phys. Rev. B 96, 121403(R) (2017)

6 Apr 2018
Jernej MravljeIntroduction to bad metals
Čajna soba F1

16 Feb 2018
Bing-Sui LuVan der Waals interaction between anisotropic topological insulator slabs
Prof. Bing-­‐Sui Lu comes from the Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore. The lecture is based on recent paper Bing-Sui Lu, Phys. Rev. B 97, 045427 (2018).
Van der Waals interactions are prevalent in Nature and account for diverse natural phenomena, such as the flocculation of colloids and the adhesion of geckos to walls. Between similar dielectric materials such interactions are typically attractive and give rise to the problem of stiction and non-contact friction in micro and nano electro-mechanical systems, thus it is of importance to find possibilities of overcoming such stiction and friction. In the seminar we consider the van der Waals interaction between topological insulators, which are materials that exhibit axion electrodynamics, for which an electric field can give rise to magnetic polarization and a magnetic field can also give rise to electric polarization. For the case of dielectrically anisotropic topological insulators, we examine how such electrodynamics can give rise to the possibility of repulsive van der Waals forces and a reduction of frictional torque.

23 Jan 2018
Lev VidmarEmergent eigenstate solution to quantum dynamics
cajna soba f1

16 Jan 2018
Kazuhiro SekiElectron-hole doping asymmetry of Fermi surface reconstructed in a simple Mott insulator
Cajna soba f1

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