Forthcoming Seminars at F-1

Seminars Archive

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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)

9 May, 2019 Seminar:
May 14: Spin liquid in ...
Peter Prelovšek (JSI)...
20 Apr, 2019 Publication:
Tadic B., Mijatovic S.,...
Nature:Scientific Reports
17 Apr, 2019 Position:
Ph.D. (Young Resear...
Contact Jernej F. Kamen...
16 Apr, 2019 Seminar:
Apr 17: Electronic corr...
Laura Fanfarillo (SISSA...
16 Apr, 2019 Seminar:
Apr 17: Strong correlat...
Adriano Amaricci (SISSA...
5 Apr, 2019 Seminar:
Apr 9: Zero-energy Maj...
Anna Gorczyca-Goraj (Un...
2 Apr, 2019 Seminar:
Apr 5: Relativistic mo...
Charles Creffield (Univ...
29 Mar, 2019 Seminar:
Apr 2: Dynamical struc...
Federico Becca (Univers...
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