Szemináriumok

Topological fine structure of an energy band

Időpont: 
2025. 02. 28. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
János Asbóth (BME/Wigner)
A szeminárium részletei: 
https://physics.bme.hu/node/5763
A band with a nonzero Chern number cannot be fully localized by weak disorder: There must remain at least one extended state, which “carries the Chern number.” Here [1] we show that a trivial band can behave in a similar way. Instead of fully localizing, arbitrarily weak disorder leads to the emergence of two sets of extended states, positioned at two different energy intervals, which carry opposite Chern numbers. Thus, a single trivial band can show the same behavior as two separate Chern bands. We show that this property is predicted by a topological invariant, the “localizer index.” Even though the band as a whole is trivial as far as the Chern number is concerned, the localizer index allows access to a topological fine structure. This index changes as a function of energy within the bandwidth of the trivial band, causing nontrivial extended states to appear as soon as disorder is introduced. Our work points to a previously overlooked manifestation of topology, which impacts the response of systems to impurities beyond the information included in conventional topological invariants.
 
[1]: Hui Liu, Cosma Fulga, Emil J. Bergholtz, Janos Asboth: Topological fine structure of an energy band. arXiv:2312.08436

Neuromorfikus számítástechnika, avagy hogyan váltsuk aprópénzre a 2024-es fizikai Nobel-díjat?

Időpont: 
2025. 02. 28. 16:00
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Halbritter András (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5765
Szeretettel hívunk meg minden érdeklődőt a BME TTK ScienceCampus tudománynépszerűsítő előadássorozat következő előadására:
 
Halbritter András (BME TTK Fizika Tanszék):
Neuromorfikus számítástechnika, avagy hogyan váltsuk aprópénzre a 2024-es fizikai Nobel-díjat?
 
február 28. péntek 16:00-17:15
BME TTK FIII 213-as terem
 
Tudtad, hogy öt év múlva a világ teljes elektromosenergia-fogyasztásának akár 20%-át a mesterséges intelligencián alapuló információs technológia fogja igényelni? Ahhoz, hogy az adatközpontok mégse emésszék fel a világ áramtermelését, újfajta energiatakarékos hardvereszközökre van szükség, melyek a biológiai idegrendszer felépítéséből nyernek inspirációt. Ezen kutatás-fejlesztési területbe, az ún. neuromorfikus számítástechnikába adunk betekintést a 2024-es fizikai Nobel-díj szemszögéből.
 
További információ és megközelítés: https://felvi.ttk.bme.hu/hu/sciencecampus
 
Az érdeklődőket kérjük, lehetőleg regisztráljanak előre, itt: https://lu.ma/wpeg7p3w 
 
Az előadásokkal elsősorban a természettudományok iránt érdeklődő középiskolás korosztályt célozzuk meg, de természetesen minden érdeklődőt szeretettel várunk!
 
Asbóth János
BME TTK Fizikai Intézet, Science Campus koordinátor
 

Phase transition dynamics: from cosmology to quantum computing

Időpont: 
2025. 03. 04. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Adolfo del Campo (Luxemburg)
A szeminárium részletei: 
https://physics.bme.hu/node/5764

The dynamics across a phase transition in a cosmological setting was studied by Sir Thomas W. B. Kibble, who predicted the formation of topological defects such as cosmic strings. LANL Fellow Wojciech H. Zurek recognized that such cosmological principles could be tested in the laboratory. Decades later, the study of topological defects has resurfaced in a different context: quantum computing. Quantum annealing devices and quantum optimization algorithms solve computational problems by finding low-energy configurations of complex spin systems. In this context,  computational errors result in excitations and topological defects, as demonstrated in D-Wave annealing devices and IBM digital quantum computers. Cutting-edge research focuses on reducing such errors by harnessing shortcuts to adiabaticity, an approach that has led to a new family of counterdiabatic quantum algorithms.

Entanglement, topological order and glassiness in finite temperature quantum phases

Időpont: 
2025. 03. 07. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Tibor Rakovszky (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5766
In recent decades, one of the central goals of condensed matter physics has been the classification and characterization of quantum phases of matter at zero temperature. This has led to the discovery of many exotic phases, notable those exhibiting topological order, characterized by long-range quantum entanglement and a ground state degeneracy that depends on the underlying topology. Much less is known about the situation at finite, non-zero temperatures; for example no examples of finite temperature topological order exist in spatial dimensions below 4. 
 
In this talk, I will discuss a series of results [1-3] relevant to the problem of quantum phases at nonzero temperatures. First, I will discuss a theorem that provides a "dynamical" perspective on finite temperature phases, relating them to open system dynamics. As an application of this perspective, I will discuss a family of spatially non-local models that exhibit a new kind of "topological quantum spin glass", combining quantum topological order, with features of classical mean-field spin glasses. Finally, I will discuss a model that realizes quantum topological order in 3 spatial dimensions for the first time. 
 
References:
[1]: Bottlenecks in quantum channels and finite temperature phases of matter, 
[2]: Topological Quantum Spin Glass Order and its realization in qLDPC codes, 
[3]: Finite-temperature quantum topological order in three dimensions, 

Theory of the Novel Phase of Fractional Quantum Hall States in the Second Landau Level

Időpont: 
2025. 03. 14. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Sudipto Das (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5771
The fractional quantum Hall states in the second Landau level exhibits exotic physics; some states have even been experimentally found to host non-Abelian quasi-particles, which could be devised as qubits for fault-tolerant topological quantum computation. The Coulomb interaction is the principal interaction in the fractional quantum Hall effect, prompting the development of several theories and models to elucidate these states over the years. However, an unresolved debate persists between theoretical predictions and experimental observations. In contrast to the lowest Landau level, the influence of Landau-level mixing becomes notably pronounced at the second Landau level, which arises in a comparatively lower magnetic field. At a moderate range of Landau level mixing strength, consistent with GaAs-GaAlAs systems, we find a reentrant Anomalous phase (A-phase) that is quantized and exhibits a substantial gap in the thermodynamic limit, being topologically distinct from the phase associated with pure Coulomb interaction. We propose a ground state wave function for the 5/2 state in the second Landau level and generalize it to all experimentally observed states in the second Landau level. These wave functions have remarkably high overlap with the corresponding exact ground states in the A-phase and can support non-Abelian quasiparticle excitation. We believe our proposed wave functions for all the second Landau level states in this A-phase should possibly corroborate with the experimentally observed states.
 
References:
[1] S. Das, S. Das, and S. S. Mandal, Phys. Rev. Lett. 131, 056202 (2023).
[2] S. Das, S. Das, and S. S. Mandal, Phys. Rev. Lett. 132, 029602 (2024).
[3] S. Das, S. Das, and S. S. Mandal, Phys. Rev. Lett. 132, 106501 (2024).
[4] S. Das, S. Das, and S. S. Mandal, Phys. Rev. B 110, L121303 (2024).

Role of interface hybridization on induced superconductivity in 2D heterostructures

Időpont: 
2025. 03. 21. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Anirban Das (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5772
Heterostructures between two-dimensional quantum spin Hall insulators (QSHIs) and superconducting materials can allow for the presence of Majorana fermions at their conducting edge states. Although a strong interface hybridization helps induce a reasonable superconducting gap on the topological material, the hybridization can modify the material’s electronic structure. In this work, we utilize a realistic low-energy model with tunable interlayer hybridization to study the edge-state physics in a heterostructure between monolayer quantum spin Hall insulator 1T ′-WTe2 and s-wave superconductor 2H-NbSe2. We find that even in the presence of strong interlayer hybridization that renders the surface to become conducting, the edge state shows a significantly enhanced local density of states and induced superconductivity compared to the surface. We provide an alternate heterostructure geometry that can utilize the strong interlayer hybridization and realize a spatial interface between a regime with a clean QSHI gap and a topological conducting edge state.
 
References:
A. Das, B. Weber, & S. Mukherjee (2023). Role of interface hybridization on induced superconductivity in 1 T′-WTe 2 and 2 H-NbSe2 heterostructures. Physical Review B, 108(7), 075410.
Tao,  et al. "Multiband superconductivity in strongly hybridized 1T′-WTe 2/NbSe 2 heterostructures." Physical Review B 105.9 (2022):094512.

Logarithmic corrections to particle lifetimes in 1D lattice fermion systems at finite temperature

Időpont: 
2025. 03. 28. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Curt von Keyserlingk (KCL, London)
A szeminárium részletei: 
https://physics.bme.hu/node/5773
A naive application of Fermi's Golden rule (FGR)  suggests that weakly interacting lattice fermions should have a lifetime ~ 1/Delta^2 in the limit of small interaction strength Delta, and at finite temperature T>0. Based on dissipation-assisted-operator-evolution (DAOE) numerical results, and two rather different non-perturbative diagrammatic re-summations, we predict that the quasiparticle lifetime is suppressed by an additional logarithmic factor so that lifetime ~ 1/[(Delta^2 \log(1/|\Delta|)] in 1 spatial dimension.
 
Authors: Thomas Young, Jerome Lloyd, Curt von Keyserlingk
[This work is not yet published.]

Topology in multi-terminal superconducting structures

Időpont: 
2025. 04. 01. 14:30
Hely: 
BME building F, lecture hall 13, second floor
Előadó: 
Wolfgang Belzig (Konstanz)
A szeminárium részletei: 
https://physics.bme.hu/node/5774
Topology ultimately unveils the roots of the perfect quantization observed in complex systems. The quantum Hall effect is the celebrated archetype of a two-dimensional topological invariant.  Remarkably, topology can manifest itself even in systems defined by control parameters playing the role of synthetic dimensions. Despite the practical importance of quantized responses, non-trivial topology is a fascinating example of a hidden system property that cannot be accessed by local measurements alone.
 
In this colloquium, I introduce superconducting multi-terminal structures as a platform to investigate a host of topological phenomena that can be experimentally implemented in taylored nanostructured devices. Such systems can realize an Andreev bound state spectrum displaying a stable so-called Weyl singularity that not only shows a quantized response but can be detected spectroscopically. First experimental hints for such topological Andreev bands have recently been found. The large variability of this devices allows to go beyond more traditional topologies. On one hand, is the number of synthetic dimensions not limited to the three spatial directions and allow to simulate exotic higher-dimensional systems. On the other hand, adding non-superconducting parts can implement non-Hermitian physics that realizes yet another topological structure with unique physical properties.
 

Confinement and false vacuum decay on the Potts quantum spin chain

Időpont: 
2025. 04. 04. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Anna Krasznai (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5779
Confinement is a central concept in the theory of strong interactions, which leads to the absence of quarks (and gluons) from the spectrum of experimentally observed particles. The underlying mechanism is based on a linear potential, which can also be realised in condensed matter systems. A one-dimensional example with a great analogy to quantum chromodynamics is the mixed-field three-state Potts quantum spin chain in the ferromagnetic regime. Compared to the analogous setting for the Ising spin chain, the Potts model has a much richer phenomenology and non-equilibrium dynamics, which originates partly from baryonic excitations in the spectrum and partly from the various possible relative alignments of the initial magnetisation and the longitudinal field in a global quantum quench. In my presentation, I will discuss how we obtain the low-lying excitation spectrum by combining semi-classical approximation and exact diagonalisation, and how the results can be applied to explain the various dynamical behaviours we observe in numerical simulations. Besides recovering dynamical confinement, as well as Wannier-Stark localisation due to Bloch oscillations similar to the Ising chain, a novel feature is the presence of baryonic excitations in the quench spectroscopy. In addition, when the initial magnetisation and the longitudinal field are misaligned, both confinement and Bloch oscillations only result in partial localisation, with some correlations retaining an unsuppressed light-cone behaviour together with a corresponding growth of entanglement entropy.
 
Reference:
 
[1] O. Pomponio, A. Krasznai and G. Takács, “ Confinement and false vacuum decay on the Potts quantum spin chain,” Scipost Phys. 18 (2025) 082

Classification of Numerical SIC-POVMs in Dimensions n<8

Időpont: 
2025. 04. 11. 10:15
Hely: 
BME building F, seminar room of the Dept. of Theoretical Physics
Előadó: 
Solomon B. Samuel (BME)
A szeminárium részletei: 
https://physics.bme.hu/node/5781
The Symmetric Informationally Complete Positive Operator-Valued Measures (SIC-POVMs) are known to exist in all dimensions <= 151 and many larger dimensions as high as 39604. All known solutions with the exception of the Hoggar solutions are covariant with respect to the Weyl-Heisenberg group and in the case of dimension 3, it has been proven that all SIC-POVMs are Weyl-Heisenberg group covariant. In this work[1], we explore this by SIC-POVMs in dimensions 4-7 without the assumption of group covariance. We introduce two functions with which SIC-POVM Gram matrices can be generated without the group covariance constraint and analytically show that the SIC-POVM Gram matrices exist on critical points of the surfaces defined by the two functions on a subspace of Hermitian matrices. In dimensions 4 to 7, all known SIC-POVM Gram matrices lie in disjoint continuous sets of solution. Thus, we define an equivalent class that relates Gram matrices based on the trivial symmetries of the two functions. In dimensions 4 to 7, we generated $\{1.7\times10^6,1.1\times10^5,169,50\}$ Gram matrices, respectively. For each of the Gram matrices, we generate the symmetry group of their respective disjoint sets.  In all cases, the symmetry group was isomorphic to a subgroup of the Clifford group containing the Weyl-Heisenberg group matrices and the order-3 unitaries. In dimensions 4-6, All constructed solutions belong to a single equivalence class of Gram matrices whereas in dimension 7, we find two distinct families of SIC-POVMs. In dimensions 4 and 5, the absence of new classes of SIC-POVM strongly suggests that the functions don't have a non-trivial symmetry. Furthermore, in all the dimensions tested, the only equivalence classes found correspond to the distinct stabilizers of fiducial vectors of each dimension.
 
 
[1]: S B Samuel and Z Gedik 2024 J. Phys. A: Math. Theor. 57 295304

Oldalak