


Forthcoming Seminars at F1
Seminars Archive
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 6 Dec 2019 11:15  Bosiljka Tadić (JSI and Complexity Science Hub Vienna)  Hidden Topology of Brain Networks 

Recent interdisciplinary studies comprising the science of the brain aim to understand the brain’s functional principles and unravel the structure of its complex pathways that underlie entire human activity, psychology and behaviour. In this context, a massive amount of experimental data are constantly generating within different worldwide research projects. The physics research of brain structure and dynamics is involved in all stages, from brain imaging to the mapping and interpretation. Further motivation is to study higherorder connectivity (and interactions) at work, for which the brain as a most sophisticated system in nature represents an inspiring example. In this seminar, we first briefly review different imaging techniques and the related data that can lead to a wide variety of functional brain networks. We then focus [1] on the Human Connectome as a wholebrain network, which we generated on the Budapest Conenctoem Server [2] from the data of Human Connectome Project. We then show how the appropriate mathematical methods of Graph theory and Algebraic topology of graphs enable us to determine the structural properties of the brain graphs and its higherorder structures. Based on the empirical data that allow separate analysis of female/male brain imaging, we show how these mathematical measures tell the sexrelated differences, which are subject of debate in the literature.
[1] B. Tadić, M. Andjelković, R. Melnik, Functional geometry of human connectomes, Scientific Reports 9:12060 (2019)
[2] B. Szalkai, C. Kerepesi, B. Varga, V. Grolmusz, Parameterizable Consensus Connectomes from the Human Connectome Project: The Budapest Reference Connectome Server v3.0, Cognitive Neurodynamics (2016)
F1 tea room (C building, 2nd floor) 
19 Nov 2019 11:15  Neven Barišić (TU Vienna and U Zagreb)  HighTc cuprates  story of two electronic subsystems 

The highTc cuprates are amongst the most intensively studied correlated materials. Nevertheless, pivotal questions regarding their principal phases and regimes, as well as the transitions between them, remain unanswered. This is, largely, due to the complexity of these materials that renders the extraction of intrinsic properties difficult. We have performed a thorough experimental study of HgBa2CuO(4+delta), which, in many respects, is a model cuprate compound. From the comparison of our measurements with data for other cuprates, we are able to separate universal behavior from compoundspecific features. This exercise leads to a series of remarkable findings, the most important of which are that the effective mass and the scattering rate remain essentially unchanged across the phase diagram [1,2], and that the scattering rate is dominated by an umklapp process [3]. These novel insights enabled an accurate count of charges across the phase diagram. The electronic system is thus found to consist of 1+p charges, where p corresponds to doping. At low dopings, within the pseudogap regime, exactly one hole is localized per planar copperoxygen unit. Upon increasing doping and temperature, the hole is gradually delocalized and becomes itinerant [4]. The overall behaviors are consistent with a gradual extension of Fermi arcs to a full Fermi surface, without an essential change of the underlying Fermi surface that encloses 1+p states. Finally, we have established that the itinerant Fermiliquid holes become superconducting while the localized hole provides the glue.
[1] N. Barišić et al, New J. Phys. 21, 113007 (2019)
[2] Li et al, Phys. Rev. Lett. 117, 197001 (2016)
[3] N. Barišić et al, Proc. Natl. Acad. Sci. 110, 12235 (2013); S. I. Mirzaei et al, Proc. Natl. Acad. Sci. 110, 5774 (2013); M. Chan et al, Phys. Rev. Lett. 113, 177005 (2014); P. Popčević et al, Quantum Mater. 3, 42 (2018)
[4] D. Pelc et al, Sci. Adv. 5, eaau4538 (2019)
Seminar room for physics (JSI main building) 
12 Nov 2019 11:15  Patrycja Lydzba (Wroclaw University of S&T, Poland)  Numerical methods of quantifying correlations in lowdimensional systems 

The full braid group and its onedimensional unitary representation determine the quantum statistic
of particles. The assumption that they can be modified by a magnetic field in the presence of repulsive
interactions is the foundation of the cyclotron subgroup model of the quantum Hall effect (QHE). This
topological description provides manybody ground states for all fillings of the lowest Landau Level
(LLL). Furthermore, the corresponding expectation values (i.e., the potential energy, onebody density
and twobody correlation function) are consistent with predictions of exact diagonalizations.
Unfortunately, the agreement is established for many models of QHE, and it is hardly possible to
determine the most promising one. Nevertheless, the differences between models should manifest in
manybody correlations. In this presentation, we demonstrate how they can be quantified in twodimensional systems in a disc geometry. We employ the wellknown simulated annealing and a new
artificial probability density algorithms. Among other things, we determine modifications of
compositefermion states introduced by the projection onto LLL.
Footprints of Cooperlike pairs can be found in the stateoftheart experiments in ultracold quantum
gases confined in onedimensional harmonic traps. We establish correlations between momenta of
atoms in massimbalanced mixtures and demonstrate that they can support the unconventional
pairing with a nonzero centerofmass momentum.
Seminar room for physics (JSI main building) 
15 Oct 2019 11:15  Zala Lenarčič (UC Berkeley, USA)  Critical behavior near the manybody localization transition in driven open systems 

In a manybody localized (MBL) system, the coupling to an external bath typically breaks local integrals of motion. Thus the system relaxes to a unique thermal steady state. When the bath is nonthermal or when the system is weakly driven out of equilibrium, local conservation laws can be excited far from any thermal equilibrium value. I will show how this property can be used to study the MBL phase transition in weakly open systems. Here, the strength of the coupling to the nonthermal bath plays a similar role as a finite temperature in a T=0 quantum phase transition. By tuning this parameter, we can detect key features of the MBL transition: the divergence of dynamical exponent due to Griffiths effects and the critical disorder strength.
We propose a new order parameter, based on the fluctuations in local temperatures. For vanishing strength of coupling to the bath, fluctuations vanish on the ergodic side, while they are large on the MBL side. By increasing the coupling strength, fluctuations grow with a fractional exponent related to the inverse dynamical exponent on the ergodic side, while they decrease monotonically on the MBL side. This paves the way for studies of the MBL transition with new numerical approaches and, importantly, also with solidstate experiments.
[1] Z. Lenarčič, E. Altman, and A. Rosch, Phys. Rev. Lett 121, 267603 (2018).
[2] Z. Lenarčič, O. Alberton, A. Rosch, and E. Altman, arXiv:1910.01548 (2019).
Seminar room for physics (JSI main building) 
3 Oct 2019 15:00  Gal Lemut (Leiden University, Nederlands)  Effect of charge renormalization on the transport along the vortex lattice of a Weyl superconductor 

It was recently shown that a Weyl superconductor in a magnetic field can support chiral Landau level motion along the vortex lines [1]. In our work we have tried to find the signatures of this chiral motion by investigating the transport properties of such a Weyl superconductor out of equilibrium [2]. We have shown that it carries an electric current proportional to the square of the renormalized charge of the Weyl fermions in the superconducting Landau level. Since the charge renormalization is energy dependent, a nonzero thermoelectric coefficient appears even in the absence of energydependent scattering processes. This gives rise to a completely new mechanism for thermoelectric properties. In the end we will also discuss a new and robust realization of the Weyl fermions that arises from the Kramers degeneracy.
[1] M. J. Pacholski, I. Adagideli, C. W. J. Beenakker. Phys. Rev. Lett. 121, 037701 (2018)
[2] G. Lemut, M. J. Pacholski, I. Adagideli, C. W. J. Beenakker. Phys. Rev. B. 100, 035417 (2019)
F1 tea room (C building, 2nd floor) 
3 Oct 2019 14:15  Michal Pacholski (Leiden University, Nederlands)  Landau levels in Weyl superconductors 

Weyl semimetals have drawn a lot of attention in both theoretical and experimental communities due to their remarkable properties: their energy spectra feature gapless bulk, as well as surface states, both of which are topologically protected. This seems to contradict the main paradigm of topological insulators, which is that a gapped bulk spectrum is necessary to define a topological invariant. Another interesting feature of a Weyl semimetal is that in the presence of magnetic field a gapless chiral Landau level is developed, which is also protected due to Atiyah index theorem [1]. Such properties have motivated us to ask the following question: will this Landau level persist if the semimetal is subjected to induced superconductivity? A similar question asked in the context of gapless Dirac fermions in dwave superconductors by Schrieffer and Gorkov [2], and independently Anderson [3] in 1998, but was found to have a negative answer: the vortices in the superconducting order parameter spoil the chiral Landau level. We have shown that in the case of Weyl superconductor this is no longer true, and the Landau level survives [4]. This new gapless state, impossible to obtain in any other class of materials, gives access to a direct observation of chiral anomaly  an apparent violation of particle number conservation.
[1] Y. Aharonov and A. Casher, Phys. Rev. A 19, 2461 (1979)
[2] L. P. Gorkov and J. R. Schrieffer, Phys. Rev. Lett. 80, 3360 (1998)
[3] P. W. Anderson, arXiv:condmat/9812063
[4] M. J. Pacholski, C. W. J. Beenakker, and I. Adagideli, Phys. Rev. Lett. 121, 037701 (2018)
F1 tea room (C building, 2nd floor) 
2 Oct 2019 11:15  Marko Djordjevic (University of Belgrade, Serbia)  A biophysical approach to understanding gene expression regulation in bacteria 

Recent decades brought a revolution to biology, driven mainly by exponentially increasing amounts of data coming from 'omics' sciences. To handle these data, bioinformatics often has to combine biologically heterogeneous signals, for which methods from statistics and engineering (e.g. machine learning) are often used. While such an approach is sometimes necessary, it effectively treats the underlying biological processes as a black box. Similarly, systems biology deals with inherently complex systems, characterized by a large number of degrees of freedom, and interactions that are highly nonlinear. To deal with this complexity, the underlying physical interactions are often (over)simplified, such as in Boolean modelling of network dynamics. Through two examples from our research, which address sequence analysis and understanding intracellular gene expression dynamics, I will argue about the utility of applying a biophysical approach in bioinformatics and systems biology.
F1 tea room (C building, 2nd floor) 
1 Oct 2019 11:15  Denis Golež (Flatiron Institute, USA)  Energy conversion in photoexcited charge transfer insulators 

Charge excitations across electronic band gaps are a key ingredient for transport in
optoelectronics and lightharvesting applications. I will start with a comparison of the photo
doped state in the Mott and chargetransfer insulator. The later is described within the three
band Emery model as relevant for copper oxides. We will employ a nonequilibrium
extension of dynamical meanfield theory taking into account changes in the screening
environment (GW+EDMFT) [1]. In contrast to Mott insulators, a strong renormalization of
the chargetransfer gap and a substantial broadening of bands is present in chargetransfer
insulators [2]. The inclusion of dynamical screening leads to an ultrafast conversion of
excess kinetic energy into plasmonic excitations. The comparison with different experimental
pumpprobe techniques, like timeresolved ARPES and optical conductivity, shows
qualitative agreement and exemplify that dynamical correlations are essential for a proper
description of the photodoped state.
In the second part, I will extend the theoretical description to nickel oxides and
compare the dynamics after the photodoping with the timeresolved photoemission
spectroscopy [3]. The short time dynamics reveals the importance of Hund physics, photo
induced ingap states, and antiferromagnetic physics. The conversion of energy between Hund
and magnetic degrees of freedom leads to longlived coherent THz oscillations whose
frequency corresponds to the superexchange coupling.
[1] D. Golež, L. Boehnke, H. U. R. Strand, M. Eckstein, P. Werner, Phys. Rev. Lett. 118, 246402 (2018)
[2] D. Golež, L. Boehnke, M. Eckstein, P. Werner, Phys. Rev. B 100, 041111 (2019)
[3] K. Gillmeister, D. Golež, C. Chiang, N. Bittner, P. Werner, Y. Pavlyukh, J. Berakdar, and W.
Widdra, arXiv:1909.00828 (2019)
Seminar room for physics (JSI main building) 
24 Sep 2019 11:15  Alexander Wietek (Flatiron Institute, USA)  Largescale exact diagonalization and thermodynamics of the ShastrySutherland model 

Exact Diagonalization (ED) is a versatile and unbiased numerical method to study quantum manybody systems. We present recent algorithmic progress for ED. The socalled sublattice coding algorithm in combination with efficient distributed memory parallelization allows us to extend the range of models that can be studied using ED. Those techniques are applied in combination with the thermal pure quantum state (TPQ) technique and infinite projected entangled pair states (iPEPS) to study the thermodynamics of the ShastrySutherland spin model. We demonstrate convergence as a function of system size in TPQ calculations and complete agreement with iPEPS results. We also find close agreement of experimental thermodynamical quantities measured for the compound SrCu2(BO3)2. We thereby assess up to which precision the material SrCu2(BO3)2 is described by the ShastrySutherland model.
[1] A. Wietek, A. M. Läuchli, Phys. Rev. E 98, 033309 (2018).
[2] A. Wietek, P. Corboz, S. Wessel, B. Normand, F. Mila, A. Honecker, arXiv:1907.00008 (2019).
Seminar room for physics (JSI main building) 
10 Sep 2019 11:15  Viktor Kabanov (JSI)  Symmetryenforced Dirac points in antiferromagnetic semiconductors 

It is shown that the symmetryenforced Dirac points exist at some timereversal symmetric momenta in the antiferromagnetic compound GdB4. These Dirac points may be controlled by the external magnetic field or by the deformation of the crystal. Application of the external magnetic field leads to splitting of these points into Weyl points or to opening of a gap depending on the field direction. The application of the symmetrybreaking deformation also opens a gap in the spectrum. Suppression of the antiferromagnetic order leads to the formation of the nodal line instead of the Dirac points. This indicates that the symmetryenforced Dirac semimetals may be effectively used in different spintronic devices.
Seminar room for physics (JSI main building) 
2 Sep 2019 11:15  Marcos Rigol  Prethermalization and thermalization in isolated quantum systems 

Prethermalization has been extensively studied in systems close to integrability. We discuss a more general, yet conceptually simpler, setup for this phenomenon. We consider apossibly nonintegrablereference dynamics, weakly perturbed so that the perturbation breaks at least one conservation law of the reference dynamics. We argue then that the evolution of the system proceeds via intermediate (generalized) equilibrium states of the reference dynamics. The motion on the manifold of equilibrium states is governed by an autonomous equation, flowing towards global equilibrium in a time of order 1/g^2, where g is the perturbation strength. We also describe the leading correction to the timedependent reference equilibrium state, which is, in general, of order g [1]. The theory is well confirmed in numerical calculations of model Hamiltonians in the context of quantum quenches [1] and driven systems [2], for which we use numerical linked cluster expansions and full exact diagonalization. For the driven systems, we discuss the relationship between heating rates and, within the eigenstate thermalization hypothesis, the smooth function that characterizes the offdiagonal matrix elements of the drive operator in the eigenbasis of the static Hamiltonian. We show that such a function, in nonintegrable and (remarkably) integrable Hamiltonians, can be probed experimentally by studying heating rates as functions of the frequency of the drive.
[1] K. Mallayya, M. Rigol, and W. De Roeck, Prethermalization and Thermalization in Isolated Quantum Systems, Phys. Rev. X 9, 021027 (2019)
[2] K. Mallayya and M. Rigol, Heating Rates in Periodically Driven Strongly Interacting Quantum ManyBody Systems, arXiv:1907.04261 (2019)
F1 tea room (JSI, C building, 2nd floor) 
4 Jul 2019 11:00  Wojciech De Roeck (KU Leuven, Belgium)  Slow heating and prethermalization in manybody systems at intermediate frequency 

In last years, ergodicitybreaking phenomena in manybody systems have generated a lot of interest. One of the easiest understood instances is slow heating in periodically driven systems at high frequencies. Yet, slow heating has been also observed numerically at intermediate frequences (in disorderfree systems). In this talk, I will present some analytical results that aim to explain these findings. Work in progress together with my master student Victor Verreet. 
2 Jul 2019 11:15  Dario Rosa (Korea Institute for Advanced Study)  The importance of the tail: Chaotic/integrable and HawkingPage phase transitions in SYKlike systems 

We will discuss some recent results obtained by a careful study of the chaotic properties of various deformations of the SachdevYeKitaev (SYK) model.
In the first part of the talk, we will focus on the massdeformed SYK model, a variant of SYK which is deformed by a random mass term. We will see that the model displays a transition from chaotic to integrable regime while increasing the strength of the mass term. Such a transition is not homogeneous along the spectrum: the tail of the spectrum, including the ground state and the low lying modes, makes the transition for very small values of the mass deformation, while the highly excited states require strong mass deformations to migrate to the integrable regime. This behavior signals that the chaos/integrable transition is temperaturedependent. In the second part of the talk, we will discuss the holographic implications of this phenomenon: we will study a twosite SYKmodel coupled by a relevant quadratic interaction. Such a model is argued to be holographically dual to a global AdS2 geometry, which displays a HawkingPage phase transition from a wormhole geometry to a twoblack holes geometry as a function of the temperature and of the strength of the relevant coupling. We will show that the HawkingPage phase transition is identified in the twosite SYK model with a temperaturedependent chaos/integrable transition, like the one described in the first part of the talk. Hence, we conjecture that a temperaturedependent chaos/integrable transition is dual to a HawkingPage phase transition in the gravity side.
[1] T. Nosaka, D. Rosa and J. Yoon, JHEP 1809, 041 (2018)
[2] A. M. GarciaGarcia, T. Nosaka, D. Rosa and J. J. M. Verbaarschot, arXiv:1901.06031
Seminar room for physics (JSI main building) 
28 Jun 2019 11:00  G. Reza Jafari (CEU Budapest)  Frustration in GeneGene Interactions of the Cancerous Cells 

According to many reports, genes communicate with each other through
different regulatory effects which lead to higherlevel structures in the cells.
We are interested in this question that 
20 Jun 2019 12:15  Brijesh Kumar (New Delhi, India)  Inversion and Quantum Oscillations in Kondo insulators 

Conventionally, the quantum oscillations of magnetisation [the de Haasvan Alphen (dHvA) effect] have come to be exclusively associated with metals. But recent observations of magnetic quantum oscillations in Kondo insulators (SmB6 and YbB12) challenge this conventional view, and call for a reexamination. We study this problem by investigating the basic models of Kondo insulators for their orbital response to uniform magnetic field. By doing a selfconsistent theory of the charge dynamics of Kondo insulators in a novel representation for electrons [1], we discover the gapped charge quasiparticles to undergo inversion upon decreasing the Kondo coupling, and establish the inversion to be the key determinant for quantum oscillations to occur as a bulk phenomenon in Kondo insulators [2,3]. The frequency of dHvA oscillations we obtain corresponds to the half of the bulk Brillouin zone, as observed experimentally [4].
[1] Brijesh Kumar, Phys. Rev. B 77, 205115 (2008)
[2] Panch Ram and Brijesh Kumar, Phys. Rev. B 96, 075115 (2017)
[3] Panch Ram and Brijesh Kumar, Phys. Rev. B 99, 235130 (2019)
[4] B. S. Tan et al, Science 349, 287 (2015)
F1 tea room (JSI, C building, 2nd floor) 
18 Jun 2019 11:15  Janez Bonča  Spectral Function of the Holstein Polaron at Finite Temperature 

I will present the Holstein polaron spectral function on a one dimensional ring using the finitetemperature (T) Lanczos method. The problem was solved on small onedimensional rings with twisted boundary conditions. With increasing T additional features in the spectral function emerge already at temperatures below the phonon frequency. We observe a substantial spread of the spectral weight towards lower frequencies and the broadening of the quasiparticle (QP) peak. In the weak coupling regime the QP peak merges with the continuum in the highT limit. In the strong coupling regime the main features of the lowT spectral function remain detectable up to the highest T used in our calculations. The effective polaron mass shows a nonmonotonic behavior as a function of T at small phonon frequency but increases with T at larger frequencies. The self energy remains kindependent even at elevated T in the frequency range corresponding to the polaron band. I will also present analytic derivation of the first few frequency moments of the spectral function that are free of finitesize effects, which were also used as a test of the numerical approach.
Seminar room for physics (JSI main building) 
29 May 2019 11:15  Ranjan Modak (SISSA, Italy)  Entanglement entropy in disordered long range hopping models 

Long range hopping plays a crucial role in several atomic, molecular and optical systems, as well as in certain condensed matter systems.
In the presence of uncorrelated disorder these models show algebraic localization in contrast to usual exponential Anderson localization.
In this talk, I will show that there exists a new subextensive scaling of entanglement entropy in this phase, while the scaling exponent seems to vary universally
with the long distance localization exponent of single particle states.
In contrast, an admixture of two species of single particle states (ergodic delocalized and nonergodic multifractal or localized), observed in presence of correlated
disorder, leads to volume law [1].
Finally, I will also show using a recently proposed real space renormalization group approach that, in the presence of interactions, manybody localized phase does not survive in the thermodynamic limit.
However, for a finite system, there exists ergodicmany body localization (MBL) transition as a function of quenched disorder.
Interestingly, the MBL phase observed for finite size systems shows the subextensive scaling of entanglement entropy as well.
[1] R. Modak and T. Nag, arXiv:1903.05099
F1 tea room (JSI, C building, 2nd floor) 
21 May 2019 11:15  Jan Šuntajs (JSI)  Quantum chaos challenges manybody localization 

Characterizing states of matter through the lens of their ergodic properties is a fascinating new direction of research. In the quantum realm, the manybody localization (MBL) was proposed to be the paradigmatic nonergodic phenomenon, which extends the concept of Anderson localization to interacting systems. At the same time, random matrix theory has established a powerful framework for characterizing the onset of quantum chaos and ergodicity (or the absence thereof) in quantum manybody systems. Here we study a paradigmatic class of models that are expected to exhibit MBL, i.e., disordered spin chains with Heisenberglike interactions. Surprisingly, we observe that exact calculations show no evidence of approaching MBL while increasing disordered strength in the ergodic regime. Moreover, a scaling analysis suggests that quantum chaotic properties survive for any disorder strength in the thermodynamic limit. Our results are based on calculations of the spectral form factor, which provides a powerful measure for the emergence of manybody quantum chaos.
[1] J. Suntajs, J. Bonca, T. Prosen and L. Vidmar, arXiv:1905.06345
Seminar room for physics (JSI main building)

14 May 2019 11:15  Peter Prelovšek (JSI)  Spin liquid in Heisenberg models on triangular and kagome lattices 

In the seminar I will present the reducedbasis approach, which allows an unified discussion and analysis of the extended Heisenberg models on triangular and kagome lattices. The numerical calculation of thermodynamical quantities indeed reveals very similar behaviour of susceptibility and entropy in both lattices in the spinliquid parameter range. The central quantity characterizing the spin liquids appears to be the temperaturedependent Wilson ratio which apparently vanishes at low temperatures, being a sensitive test of the comparison with experiments.
Seminar room for physics (JSI main building) 
17 Apr 2019 14:00  Laura Fanfarillo (SISSA, Italy)  Electronic correlation in Ironbased superconductors: nematicity and superconductivity 

Understanding the nature and strength of correlations in iron based superconductors (IBS) is key to unveil the nature of the instabilities that appear in their phase diagram. The metallic nature of the spindensitywave
characterizing the parent compound has been initially interpreted as an indication of a weak (or intermediate) regime of correlations. However, there is an increasing experimental evidence of the orbitaldependent character of
electronic correlations in IBS, with the simultaneous presence of strong and weakcorrelated electrons. In this talk I will summarize this phenomenology discussing the paramagnetic phase of IBS in terms of Hund metal. I will
then talk about the interplay between local correlations and ordered phases. Concerning the nematic instability, I will show that the local electronic interactions alone cannot drive any nematic transition, however they play a
crucial role by severely constraining the precise nature of the feasible orbitalordered state and inducing a differentiation in the effective masses of the zxyz orbitals. I will also discuss the relationship between
correlations and superconductivity. In particular, I will show that the loss of coherence of the spectral weight at the Fermi level does not imply the suppression of the superconductivity, indeed the same electronic
correlations inducing the bad metallic behavior support superconductivity as long as (incoherent) lowenergy excitations, on energy scale of the order of the Hund coupling, contribute to the pairing.
Seminar room for physics (JSI main building) 
17 Apr 2019 13:15  Adriano Amaricci (SISSA, Italy)  Strong correlation effects in topological quantum phase transitions 

Topological Quantum Phase Transitions (TQPTs) are characterized by changes in global topological invariants, beyond the conventional paradigm of local order parameters. The recent progress in identifying topological states in strongly correlated compounds and heterostructures pushed attention to the effects of the electronic interaction in topological insulators (TIs).
We discuss the effects of interaction in changing the conventional portrait of TQPT: we uncover the emergence of a firstorder character in the topological transition occurring at strong enough interaction. Our study reveals the existence of a quantum critical endpoint, associated with an orbital instability, on the transition line between a TI and a trivial insulator. We show that the conventional paradigm of continuous TQPT breaks down: The change of the topological invariants takes place without energy gap closing but preserving the symmetries protecting the topological phase. We stress the analogy of the transition line with the case of the liquidgas transition.
Next, we address the fate of the helical edge states in interacting TIs. We show that TimeReversal Symmetry (TRS), protecting the topological state, opposes to the strong interaction effects via an edge states reconstruction mechanism: The progressive penetration of the edge states into the bulk upon approaching a Mott transition.
Indeed we point out that a similar process survives also in presence of antiferromagnetic ordering. We show that, in the absence of TRS in the magnetic state, a residual U(1) rotational symmetry is enough to stabilize an inhomogeneous spin Chern insulator in the system.
Finally, we tackle the effects of interaction in a Weyl semimetal (WSM). We consider a WSM generated in a 3dimensional TI with broken TRS. In the absence of a protecting TRS the spin degeneracy of the gapless Dirac point at the TQPT is resolved resulting in two gapless Weyl nodes separated in momentum space: WSM. As for the TIs, the topological transition requires the two Weyl nodes to annihilate continuously. Yet, we show that in the presence of interaction this paradigm breaks down, giving rise to a nonlocal annihilation of the Weyl cones.
Seminar room for physics (JSI main building) 
9 Apr 2019 11:15  Anna GorczycaGoraj (University of Silesia, Poland)  Zeroenergy Majorana modes in condensed matter physics 

Quasiparticles induced at the edges of spinless (pwave) superconducting sample in one or two dimensions have the exotic character of zeroenergy bound states. These emergent Majoranatype objects have been predicted in various systems, such as topological insulators, semiconducting nanowires, ferromagnetic chains coupled to swave superconducting reservoirs. The most convincing experimental evidence for the zeroenergy Majorana modes has been provided so far by the tunneling measurement using the nanoscopic chains proximity coupled to the swave superconducting reservoir.
In my talk I will discuss the properties of the inhomogeneous Rashba chain coupled to a superconducting substrate, hosting the Majorana quasiparticles near its edges. Despite common belief that the Majorana quasiparticles are robust to environmental influence, our results indicate that this is not truly the case and reveal that sufficiently strong disorder would be detrimental for the Majorana quasiparticles, causing a transition from the topologically nontrivial to trivial superconducting phases. On the other hand, single quantum impurities have also very unusual interplay with the Majorana quasiparticle states. Under specific conditions they can effectively induce additional pairs of the Majorana states.
Yet, the Rashba chain model has its limitations important for experimental realization. That is why we propose another model of experimental relevance, namely a chain of the localized magnetic impurities whose moments are coupled to the spins of itinerant electrons, deposited on a surface of swave bulk superconductor. With use of Monte Carlo simulations we have checked thermal stability of Majorana modes and estimated critical temperature. This evaluation should also be taken into account when considering future applications of the Majorana quasiparticles for quantum computing.
Seminar room for physics (JSI main building) 
5 Apr 2019 11:15  Charles Creffield (Universidad Compultense de Madrid, Spain)  Relativistic motion of an Airy wavepacket in a lattice: Quantum mechanics at high speed 

What happens when particles move at high speeds, comparable to the speed of light? Classically the result is wellknown; Newtonian mechanics evolves into special relativity. We can also ask the same question for a quantum mechanical system  will a quantum wavepacket pass into the relativistic regime as its speed increases?
The Airy wavepacket is a particular solution of the Schroedinger equation that appears to undergo a constant acceleration. It should thus eventually become relativistic when its velocity becomes similar to the speed of light. We can study this conveniently by confining it to move in a lattice instead of free space. The lattice provides a natural speed limit given by its maximum group velocity, which can be many orders of magnitude lower than the true speed of light. In this talk I will show that an Airy wavepacket moving in a lattice is indeed described by relativistic equations, which, rather unexpectedly, arise from evolution under the standard nonrelativistic Schroedinger equation [1].
A natural system to study this effect is in gases of cold atoms held in optical lattice potentials. I will show how these are thus excellent candidates for studying quantum systems in extreme relativistic conditions in the laboratory, and how Floquet engineering techniques can be used to control their properties.
[1] C.E. Creffield, Phys. Rev. A 98, 063609 (2018)
F1 tea room (C building, 2nd floor) 
2 Apr 2019 11:15  Federico Becca (University of Trieste)  Dynamical structure factor of frustrated spin models: a variational Monte Carlo approach 

The spin dynamical structure factor is computed within a variational framework to study frustrated Heisenberg models in one and two dimensions. Starting from Gutzwillerprojected fermionic wave functions, the lowenergy spectrum is constructed by considering twospinon excitations. Benchmarks on the onedimensional J_1  J_2 model are considered. Here, an excellent description of both the gapless and gapped (dimerized) phases is obtained, also describing the incommensurate structure for large frustrating ratios J_2/J_1>0.5 [1]. In the square [2] and triangular [3] lattices, we unveil the dynamical signatures of the
transition between the the magnetically ordered phase and the (gapless) spin liquid. In particular, for the triangular lattice, we detect a progressive softening of the magnon branch at the M points, which eventually becomes gapless within the spinliquid phase. This feature is captured by the band structure of the unprojected wave function (with 2 Dirac points for each spin component). In addition, we observe an intense signal at low energies around the K points, which cannot be understood within the unprojected picture and emerges only when the Gutzwiller projection is considered, suggesting the relevance of gauge fields for the lowenergy physics of spin liquids [3].
[1] F. Ferrari, A. Parola, S. Sorella, and F. Becca, Phys. Rev. B 97, 235103 (2018)
[2] F. Ferrari and F. Becca, Phys. Rev. B 98, 100405 (2018)
[3] F. Ferrari and F. Becca, arXiv:1903.05691
Seminar room for physics (JSI main building) 
26 Mar 2019 10:15  Jürgen Schnack  Magnetism of free and deposited magnetic molecules 

Magnetic molecules possess many interesting properties. In this presentation I focus on some frustration effects such as an enhanced magnetocaloric effect as well as on the modification of magnetic properties in contact with nonmagnetic metallic substrates. I am also going to explain very recent developments to calculate magnetic observables for large spin systems.
Geometric spin frustration in lowdimensional magnetic materials such as the two dimensional antiferromagnetic kagome lattice can lead to unusual behavior. Some of these features are present in magnetic molecules and lead to a deeper understanding of extended systems. Among such findings are localized independent magnons, which are responsible for flat bands, giant magnetization jumps and an increased magnetocaloric effect. The latter can also be observed in molecules with the structure of a sawtooth chain. In addition, we could report about the experimental realization of adiabatic demagnetization experiments using heptametallic gadolinium molecules. In the cooling experiment 200 mK could be reached and a rich structure of the isentropes could be observed. When put on a nonmagnetic metallic surface, magnetic molecules may change their properties due to the coupling to the conduction electrons. Following an experimental realization of stacked molecules, which form short spin chains, we can show how strong such an interaction needs to be in order to screen the terminal spin completely.
Seminar room for physics (JSI main building) 
5 Mar 2019 11:15  Marko Žnidarič (FMF, UL)  Localized and ballistic eigenstates in chaotic spin ladders and the FermiHubbard model 

I will introduce a ladder model with or without disorder that is in general chaotic, but that includes the integrable Hubbard chain as a special case. One can analytically show the existence of atypical eigenstates of such a model that are populated by noninteracting excitations. Depending on parameters they can e.g. exhibit Anderson localization, or, surprisingly, ballistic transport at any disorder strength. These properties differ strikingly from those of typical eigenstates nearby in energy, which give rise to diffusion. Results have implications for possible localization in the presence of nonAbelian symmetries, as well as for phases of matter that are intermediate between full ergodicity and integrability.
Seminar room for physics (JSI main building) 
26 Feb 2019 11:15  Mikhail Kiselev  LandauZener Interferometry in Multilevel Systems 

We propose universal approach to LandauZener (LZ) problem in a
multilevel system. The problem is formulated in terms of generators of
SU(N) algebra and maps the Hamiltonian onto the effective anisotropic
pseudospin (N1)/2 model. The vector Bloch equation for the density matrix
describing the temporal evolution of the multilevel crossing problem is
derived and solved analytically for two generic cases: i) threelevel
crossing problem representing a minimal model for a LZ interferometer and
ii) fourlevel crossing problem corresponding to a minimal model of
coupled interferometers. It is shown that the analytic solution of the
Bloch equation is in excellent quantitative agreement with the numerical
solution of the Schroedinger equation for the 3 and 4level crossing
problems. The solution demonstrates oscillation patterns which radically
differ from the standard patterns for the twolevel LandauZener problem:

5 Feb 2019 11:15  Tomaž Rejec  Timedependent thermoelectric transport in nanosystems: reflectionless Luttinger field approach 

Recently the Luttinger field approach was proposed as a way to simulate switching on a
temperature gradient across a nanoscale device initially in thermal equilibrium. The time
dependent particle and heat currents can then be calculated by propagating the initial equilibrium state of the system in time. Unfortunately applying a uniform Luttinger field in the
whole of a lead causes a discrepancy between steady state currents in the long time limit and
those predicted by the LandauerBüttiker formulas as well as artefacts at short times. Here
we propose a modified approach where the Luttinger field gradually reaches its final value
across a transition region. If the length of the transition region is sufficient, the electrons
move through without reflecting. In this way the correct energy distribution of electrons
originating from such a lead, corresponding to the new temperature, is established in the
scattering region. Our approach is tested on a single quantum dot and a parallel double
quantum dot system.
Seminar room for physics (JSI main building) 





