Forthcoming Seminars at F-1

Seminars Archive

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4 Jul 2019
Wojciech De Roeck (KU Leuven, Belgium)Slow heating and prethermalization in many-body systems at intermediate frequency
In last years, ergodicity-breaking phenomena in many-body 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 disorder-free 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
Dario Rosa (Korea Institute for Advanced Study) The importance of the tail: Chaotic/integrable and Hawking-Page phase transitions in SYK-like systems
We will discuss some recent results obtained by a careful study of the chaotic properties of various deformations of the Sachdev-Ye-Kitaev (SYK) model. In the first part of the talk, we will focus on the mass-deformed 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 temperature-dependent. In the second part of the talk, we will discuss the holographic implications of this phenomenon: we will study a two-site SYK-model coupled by a relevant quadratic interaction. Such a model is argued to be holographically dual to a global AdS2 geometry, which displays a Hawking-Page phase transition from a wormhole geometry to a two-black holes geometry as a function of the temperature and of the strength of the relevant coupling. We will show that the Hawking-Page phase transition is identified in the two-site SYK model with a temperature-dependent chaos/integrable transition, like the one described in the first part of the talk. Hence, we conjecture that a temperature-dependent chaos/integrable transition is dual to a Hawking-Page phase transition in the gravity side.

[1] T. Nosaka, D. Rosa and J. Yoon, JHEP 1809, 041 (2018)
[2] A. M. Garcia-Garcia, T. Nosaka, D. Rosa and J. J. M. Verbaarschot, arXiv:1901.06031

Seminar room for physics (JSI main building)

28 Jun 2019
G. Reza Jafari (CEU Budapest)Frustration in Gene-Gene Interactions of the Cancerous Cells
According to many reports, genes communicate with each other through different regulatory effects which lead to higher-level structures in the cells. We are interested in this question that

20 Jun 2019
Brijesh Kumar (New Delhi, India)Inversion and Quantum Oscillations in Kondo insulators
Conventionally, the quantum oscillations of magnetisation [the de Haas-van 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 self-consistent 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
Janez BončaSpectral Function of the Holstein Polaron at Finite Temperature
I will present the Holstein polaron spectral function on a one dimensional ring using the finite-temperature (T) Lanczos method. The problem was solved on small one-dimensional 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 high-T limit. In the strong coupling regime the main features of the low-T spectral function remain detectable up to the highest T used in our calculations. The effective polaron mass shows a non-monotonic behavior as a function of T at small phonon frequency but increases with T at larger frequencies. The self energy remains k-independent 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 finite-size effects, which were also used as a test of the numerical approach.

Seminar room for physics (JSI main building)

29 May 2019
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 sub-extensive 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 non-ergodic 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, many-body localized phase does not survive in the thermodynamic limit. However, for a finite system, there exists ergodic-many body localization (MBL) transition as a function of quenched disorder. Interestingly, the MBL phase observed for finite size systems shows the sub-extensive 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
Jan Šuntajs (JSI)Quantum chaos challenges many-body localization
Characterizing states of matter through the lens of their ergodic properties is a fascinating new direction of research. In the quantum realm, the many-body 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 many-body systems. Here we study a paradigmatic class of models that are expected to exhibit MBL, i.e., disordered spin chains with Heisenberg-like 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 many-body 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
Peter Prelovšek (JSI)Spin liquid in Heisenberg models on triangular and kagome lattices
In the seminar I will present the reduced-basis 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 spin-liquid parameter range. The central quantity characterizing the spin liquids appears to be the temperature-dependent 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
Laura Fanfarillo (SISSA, Italy)Electronic correlation in Iron-based 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 spin-density-wave 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 orbital-dependent character of electronic correlations in IBS, with the simultaneous presence of strong- and weak-correlated 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 orbital-ordered state and inducing a differentiation in the effective masses of the zx-yz 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) low-energy 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
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 first-order 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 liquid-gas transition.

Next, we address the fate of the helical edge states in interacting TIs. We show that Time-Reversal 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 semi-metal (WSM). We consider a WSM generated in a 3-dimensional 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 non-local annihilation of the Weyl cones.

Seminar room for physics (JSI main building)

9 Apr 2019
Anna Gorczyca-Goraj (University of Silesia, Poland)Zero-energy Majorana modes in condensed matter physics
Quasiparticles induced at the edges of spinless (p-wave) superconducting sample in one or two dimensions have the exotic character of zero-energy bound states. These emergent Majorana-type objects have been predicted in various systems, such as topological insulators, semiconducting nanowires, ferromagnetic chains coupled to s-wave superconducting reservoirs. The most convincing experimental evidence for the zero-energy Majorana modes has been provided so far by the tunneling measurement using the nanoscopic chains proximity coupled to the s-wave 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 s-wave 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
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 well-known; 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 non-relativistic 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
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 Gutzwiller-projected fermionic wave functions, the low-energy spectrum is constructed by considering two-spinon excitations. Benchmarks on the one-dimensional 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 spin-liquid 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 low-energy 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
Jürgen SchnackMagnetism 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 non-magnetic metallic substrates. I am also going to explain very recent developments to calculate magnetic observables for large spin systems.

Geometric spin frustration in low-dimensional 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 non-magnetic 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
Marko Žnidarič (FMF, UL)Localized and ballistic eigenstates in chaotic spin ladders and the Fermi-Hubbard 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 non-Abelian 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
Mikhail KiselevLandau-Zener Interferometry in Multilevel Systems
We propose universal approach to Landau-Zener (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 (N-1)/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) three-level crossing problem representing a minimal model for a LZ interferometer and ii) four-level 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 4-level crossing problems. The solution demonstrates oscillation patterns which radically differ from the standard patterns for the two-level Landau-Zener problem:

5 Feb 2019
Tomaž RejecTime-dependent 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 Landauer-Büttiker 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)

19 Aug, 2019 Publication:
Tadic, B., Andjelkovic,...
Scientific Reports 9...
24 Jul, 2019 Publication:
Mitrovic Dankulov M., T...
Physical Review E 100...
3 Jul, 2019 Seminar:
Jul 4: Slow heating an...
Wojciech De Roeck (KU L...
3 Jul, 2019 Seminar:
May 21: Quantum chaos c...
Jan Šuntajs (JSI)...
2 Jul, 2019 Seminar:
Jul 2: The importance...
Dario Rosa (Korea Insti...
27 Jun, 2019 Seminar:
Jun 28: Frustration in ...
G. Reza Jafari (CEU Bud...
19 Jun, 2019 Seminar:
Jun 20: Inversion and Q...
Brijesh Kumar (New Delh...
17 Jun, 2019 Seminar:
Jun 18: Spectral Functi...
Janez Bonča...
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