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arXiv 2021年

作者： Kapanowski, Andrzej Dawidowicz, S. Faculty of Physics Astronomy and Applied Computer Science Jagiellonian University ulica Lojasiewicza 11 Kraków30-348 Poland

A system of optimal biaxial molecules placed at the sites of a cubic lattice is studied in an extended Lebwohl-Lasher model. Molecules interact only with their nearest neighbors through the pair potential that depends on the molecule orientations. It is known that in the homogeneous system there is a direct second-order transition from the isotropic to the biaxial nematic phase, but properties of confined systems are less known. In the present paper the lattice has periodic boundary conditions in the X and Y directions and it has two walls with planar anchoring, perpendicular to the Z direction. We have investigated the model using Monte Carlo simulations on Nx×Ny×Nz lattices, Nx = Ny = 10, 16, Nz from 3 to 19, with and without assuming mirror symmetry. This study is complementary to the statistical description of hard spheroplatelets near a hard wall by Kapanowski and Abram [Phys. Rev. E 89, 062503 (2014)]. The temperature dependence of the order-parameter profiles between walls is calculated for many wall separations. For large wall separations there are the surface layers with biaxial ordering at both walls (4-5 lattice constants wide) and beyond the surface layers the order parameters have values as in the homogeneous system. For small wall separations the isotropic-biaxial transition is shifted and the surface layers are thinner. Above the isotropic-biaxial transition the preferable orientations in both surface layers can be different. It is interesting that planar anchoring for biaxial molecules leads to the uniaxial interactions at the wall. As a result we get the planar Lebwohl-Lasher model with additional (biaxial) interactions with the neighbors from the second layer, where the Kosterlitz-Thouless transition is present. Copyright © 2021, The Authors. All rights reserved.

关键词： Monte Carlo methods

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Entanglement duality in spin-spin interactions

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Physical Review A 2022年第3期106卷 032407-032407页

作者： Vahid Azimi-Mousolou Anders Bergman Anna Delin Olle Eriksson Manuel Pereiro Danny Thonig Erik Sjöqvist Department of Applied Mathematics and Computer Science Faculty of Mathematics and Statistics University of Isfahan Isfahan 81746-73441 Iran Department of Physics and Astronomy Uppsala University Box 516 SE-751 20 Uppsala Sweden Department of Applied Physics School of Engineering Sciences KTH Royal Institute of Technology AlbaNova University Center SE-10691 Stockholm Sweden Swedish e-Science Research Center (SeRC) KTH Royal Institute of Technology SE-10044 Stockholm Sweden School of Science and Technology Örebro University SE-701 82 Örebro Sweden

We examine entanglement of thermal states for spin-1/2 dimers in external magnetic fields. Entanglement transition in the temperature-magnetic-field plane demonstrates a duality in spin-spin interactions. This identifies a pair of dual categories of symmetric and antisymmetric dimers with each category classified into toric entanglement classes. The entanglement transition line is preserved from each toric entanglement class to its dual toric class. The toric classification is an indication of the topological signature of the entanglement, which bring about topological stability that could be relevant for quantum information processing.

关键词： Quantum correlations in quantum information Quantum entanglement Quantum information theory Spin dynamics

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Coexisting magnetic structures and spin reorientation in Er0.5Dy0.5FeO3: Bulk magnetization, neutron scattering, specific heat, and density functional theory studies

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Physical Review B 2022年第21期105卷 214436-214436页

作者： Sarita Rajput Padmanabhan Balasubramanian Ankita Singh Françoise Damay C. M. N. Kumar W. Tabis T. Maitra V. K. Malik Department of Physics Indian Institute of Technology Roorkee Roorkee 247 667 India Department of Physics Graphic Era University Dehradun Uttarakhand 248 002 India Laboratoire Léon Brillouin CEA-CNRS CEA/Saclay 91191 Gif-sur-Yvette France Institute of Solid State Physics Vienna University of Technology Wiedner Hauptstrasse 8-10 1040 Vienna Austria AGH University of Science and Technology Faculty of Physics and Applied Computer Science 30-059 Kraków Poland

The complex magnetic structures, spin reorientation, and associated exchange interactions have been investigated in Er0.5Dy0.5FeO3 using bulk magnetization, neutron diffraction, specific heat measurements, and density functional theory calculations. The Fe3+ spins order as G-type antiferromagnet structure depicted by Γ4(Gx, Ay, Fz) irreducible representation below 700 K, similar to its end compounds. The bulk magnetization data indicate occurrence of the spin-reorientation and rare-earth magnetic moments' polarization below ∼75 K and 10 K, respectively. The neutron diffraction studies confirm an “incomplete” Γ4→ Γ2(Fx, Cy, Gz) spin-reorientation initiated ≤75 K. Although the relative volume fraction of the two magnetic structures varies with decreasing temperature, both coexist even at 1.5 K. Below 10 K, the polarization of Er3+/Dy3+ moments in a cyR arrangement develops, which gradually increases with decreasing temperature. At 2 K, magnetic structure associated with czR arrangement of Er3+/Dy3+ moments also appears. At 1.5 K, while the rare-earth magnetic moments show a cyR+czR-type arrangement, the Fe3+ spins are represented by a combination of a Γ2+Γ4 (Gz,Gx) arrangement. A clear signature of the magnetic structure with Γ1(Gy) representation, symmetrically compatible with the czR-type arrangement of rare-earth moments, is not confirmed from the refinement of the neutron diffraction data. The observed Schottky anomaly at 2.5 K suggests that the “rare-earth ordering” is induced by polarization due to Fe3+ spins. The Er3+−Fe3+ and Er3+−Dy3+ exchange interactions, obtained from first principle calculations, indicate that these interactions primarily cause the complicated spin reorientation and cyR rare-earth ordering in the system, respectively, while the dipolar interactions between rare-earth moments result in the czR type rare-earth ordering at 2 K.

关键词： Antiferromagnetism Exchange interaction Magnetic interactions Magnetic order Magnetism Density functional calculations Magnetization measurements Neutron diffraction Specific heat measurements X-ray diffraction

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Optimizing Thermalization

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Physical Review Letters 2022年第4期129卷 040602-040602页

作者： Kamil Korzekwa Matteo Lostaglio Faculty of Physics Astronomy and Applied Computer Science Jagiellonian University 30-348 Kraków Poland Korteweg-de Vries Institute for Mathematics and QuSoft University of Amsterdam Science Park 105-107 1098 XG Amsterdam Netherlands QuTech Delft University of Technology P.O. Box 5046 2600 GA Delft Netherlands

We present a rigorous approach, based on the concept of continuous thermomajorization, to algorithmically characterize the full set of energy occupations of a quantum system accessible from a given initial state through weak interactions with a heat bath. The algorithm can be deployed to solve complex optimization problems in out-of-equilibrium setups and it returns explicit elementary control sequences realizing optimal transformations. We illustrate this by finding optimal protocols in the context of cooling, work extraction, and catalysis. The same tools also allow one to quantitatively assess the role played by memory effects in the performance of thermodynamic protocols. We obtained exhaustive solutions on a laptop machine for systems with dimension d≤7, but with heuristic methods one could access much higher d.

关键词： Open quantum systems Quantum thermodynamics Resource theories Quantum master equation

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Approximation algorithms for steiner tree based on star contractions: A unified view 15

Approximation algorithms for steiner tree based on star cont...

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15th International Symposium on Parameterized and Exact Computation, IPEC 2020

作者： Hušek, Radek Knop, Dušan Masařík, Tomáš Computer Science Institute of Charles University Faculty of Mathematics and Physics Charles University Prague Czech Republic Department of Theoretical Computer Science Faculty of Information Technology Czech Technical University in Prague Czech Republic Faculty of Mathematics Informatics and Mechanics University of Warsaw Poland Department of Applied Mathematics Faculty of Mathematics and Physics Charles University Czech Republic

ISBN: (纸本)9783959771726

In the Steiner Tree problem, we are given an edge-weighted undirected graph G = (V, E) and a set of terminals R ⊆ V . The task is to find a connected subgraph of G containing R and minimizing the sum of weights of its edges. Steiner Tree is well known to be NP-complete and is undoubtedly one of the most studied problems in (applied) computer science. We observe that many approximation algorithms for Steiner Tree follow a similar scheme (meta-algorithm) and perform (exhaustively) a similar routine which we call star contraction. Here, by a star contraction, we mean finding a star-like subgraph in (the metric closure of) the input graph minimizing the ratio of its weight to the number of contained terminals minus one;and contract. It is not hard to see that the well-known MST-approximation seeks the best star to contract among those containing two terminals only. Zelikovsky’s approximation algorithm follows a similar workflow, finding the best star among those containing three terminals. We perform an empirical study of star contractions with the relaxed condition on the number of terminals in each star contraction motivated by a recent result of Dvořák et al. [Parameterized Approximation Schemes for Steiner Trees with Small Number of Steiner Vertices, STACS 2018]. Furthermore, we propose two improvements of Zelikovsky’s 11/6-approximation algorithm and we empirically confirm that the quality of the solution returned by any of these is better than the one returned by the former algorithm. However, such an improvement is exchanged for a slower running time (up to a multiplicative factor of the number of terminals). © Radek Hušek, Dušan Knop, and Tomáš Masařík;licensed under Creative Commons License CC-BY

关键词： Stars

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Simplistic Approach for Determining Center Shift of Mössbauer Spectrum: Exemplified on Two Cases

SSRN

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SSRN 2022年

作者： Dubiel, Stanislaw M. Żukrowski, Jan Faculty of Physics and Applied Computer Science KrakówPL-30-059 Poland Academic Center for Materials and Nanotechnology KrakówPL-30-059 Poland AGH University of Science and Technology KrakówPL-30-059 Poland

A very simple method for determining the center (isomer) shift, CS, of a Mössbauer spectrum is outlined. Its applicability is demonstrated on two examples viz. pyrite and a ternary sigma-phase Fe-Cr-Ni compound. Sets of the spectra recorded in the temperature interval of 78-295 K for the former, and 5-293 K for the latter were analyzed with the simple and a commonly used ***(T)-values obtained with both ways of the fitting procedures were analyzed in terms of the Debye model. The determined therefrom values of the Debye temperatures agree within the error limit with each other proving thereby that this very simple method gives correct values of CS. © 2022, The Authors. All rights reserved.

关键词： Ternary alloys

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Influence of In-induced resonant level on the normal-state and superconducting properties of Sn1.03Te

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Physical Review B 2022年第7期106卷 075205-075205页

作者： Shantanu Misra Bartlomiej Wiendlocha Janusz Tobola Petr Levinský Jiří Hejtmánek Sylvie Migot Jaafar Ghanbaja Anne Dauscher Bertrand Lenoir Christophe Candolfi Institut Jean Lamour UMR 7198 CNRS – Université de Lorraine 2 allée André Guinier-Campus ARTEM BP 50840 54011 Nancy Cedex France Faculty of Physics and Applied Computer Science AGH University of Science and Technology Aleja Mickiewicza 30 30-059 Krakow Poland Institute of Physics Czech Academy of Sciences Cukrovarnická 10 162 00 Praha 6 Czech Republic

Normal-state transport properties (2–300 K) of the polycrystalline series Sn1.03−δ−xInxTe (0≤x≤0.07; δ≤0.0025) were investigated by means of electrical resistivity, thermopower, Hall effect, and thermal conductivity measurements. The distortion of the valence-band structure by the In-induced resonant level (RL) has a profound influence on the evolution of the normal-state properties with x and on the emergence of superconductivity evidenced by specific-heat measurements down to 0.35 K. In addition to a nearly 40-fold increase in the residual electrical resistivity ρ0 on going from x=0.0 to 0.05, the thermopower α shows a nonlinear, complex behavior as a function of both temperature and x. While Hall measurements indicate a dominant holelike response across the entire composition and temperature ranges, α changes sign below about 100 K and remains negative down to 5 K for 0.0015≤x≤0.0045. Additional measurements under magnetic fields μ0H of up to 14 T further shows that α(μ0H) gradually shifts towards positive values, suggestive of a dominant holelike contribution to α. Superconductivity emerges for x=0.02 at a critical temperature Tc=0.67 K, with Tc increasing with x to reach 1.73 K for x=0.07. The variations in the superconducting parameters with x, notably the specific-heat jump at Tc, confirm the results reported in prior studies and suggests a nontrivial role of the RL on the electron-phonon coupling strength. The striking similarities between this series and the canonical resonant system Pb1−xTlxTe provide an excellent experimental opportunity to gain a deeper understanding of the close interplay between resonant level, anomalous transport properties, and superconductivity.

关键词： Electronic structure Thermoelectric effects Energy materials Narrow band gap systems Polycrystalline materials Korringa-Kohn-Rostoker method Methods in transport Transport techniques

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On decreasing the orders of (k, g) -graphs

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Journal of Combinatorial Optimization 2023年第4期46卷 26-26页

作者： Exoo, Geoffrey Jajcay, Robert Raiman, Tom Department of Mathematics and Computer Science Indiana State University Terre HauteIN47809 United States Department of Algebra and Geometry Faculty of Mathematics Physics and Informatics Comenius University Bratislava Slovakia Department of Applied Mathematics VŠB - Technical University of Ostrava 17. Listopadu 15/2172 Ostrava-Poruba708 33 Czech Republic

A (k, g) -graph is a k-regular graph of girth g . Given k≥ 2 and g≥ 3 , (k, g) -graphs of infinitely many orders are known to exist and the problem of finding a (k, g)-graph of the smallest possible order is known as the Cage Problem. The aim of our paper is to develop systematic (programmable) ways for lowering the orders of existing (k, g) -graphs, while preserving their regularity and girth. Such methods, in analogy with the previously used excision, may have the potential for constructing smaller (k, g)-graphs from current smallest examples—record holders—some of which have not been improved in years. In addition, we consider constructions that preserve the regularity, the girth and the order of the considered graphs, but alter the graphs enough to possibly make them suitable for the application of our order decreasing methods. We include a detailed discussion of several specific parameter cases for which several non-isomorphic smallest examples are known to exist, and address the question of the distance between these non-isomorphic examples based on the number of changes required to move from one example to another. © 2023, The Author(s).

关键词： Graphic methods

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An Optimizing Method for Performance and ResourceUtilization in Quantum Machine Learning Circuits

Research Square

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Research Square 2022年

作者： Salehi, Tahereh Zomorodi, Mariam Plawiak, Pawel Abbaszade, Mina Salari, Vahid Department of Computer Engineering Ferdowsi University of Mashhad Mashhad Iran Department of Computer Science Faculty of Computer Science and Telecommunications Cracow University of Technology Krakow Poland Theoretical and Applied Informatics Polish Academy of Sciences Gliwice Poland Department of Physics Isfahan University of Technology Isfahan84156-83111 Iran Bilbao Spain Department of Physics and Astronomy Howard University WashingtonDC20059 United States

Quantum computing is a new and advanced topic that refers to calculations based on the principles of quantum mechanics. It makes certain kinds of problems be solved easier compared to classical computers. This advantage of quantum computing can be used to implement many existing problems in different fields incredibly effectively. One important field that quantum computing has shown great results in machine learning. Until now, many different quantum algorithms have been presented to perform different machine learning approaches. In some special cases, the execution time of these quantum algorithms will be reduced exponentially compared to the classical ones. But at the same time, with increasing data volume and computation time, taking care of systems to prevent unwanted interactions with the environment can be a daunting task and since these algorithms work on machine learning problems, which usually includes big data, their implementation is very costly in terms of quantum resources. Here, in this paper, we have proposed an approach to reduce the cost of quantum circuits and to optimize quantum machine learning circuits in particular. To reduce the number of resources used, in this paper an approach including different optimization algorithms is considered. Our approach is used to optimize quantum machine learning algorithms for big data. In this case, the optimized circuits run quantum machine learning algorithms in less time than the original ones and by preserving the original functionality. Our approach improves the number of quantum gates by 10.7% and 14.9% in different circuits and the number of time steps is reduced by three and 15 units, respectively. This is the amount of reduction for one iteration of a given sub-circuit U in the main circuit. For cases where this sub-circuit is repeated more times in the main circuit, the optimization rate is increased. Therefore, by applying the proposed method to circuits with big data, both cost and performance are

关键词： Big data

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Insight into the structural and magnetotransport properties of epitaxial heterostructures α-Fe2O3/Pt(111): Role of the reversed layer sequence

arXiv

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arXiv 2023年

作者： Koziol-Rachwal, A. Kwiatek, N. Skowroński, W. Grochot, K. Kanak, J. Madej, E. Freindl, K. Korecki, J. Spiridis, N. Faculty of Physics and Applied Computer Science AGH University of Science and Technology al. Mickiewicza 30 Kraków30-059 Poland Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences Kraków30-239 Poland Institute of Electronics AGH University of Science and Technology Al. Mickiewicza 30 Kraków30-059 Poland

We report on the chemical structure and spin Hall magnetoresistance (SMR) in epitaxial αFe2O3(hematite)(0001)/Pt(111) bilayers with hematite thicknesses of 6 nm and 15 nm grown by molecular beam epitaxy on a MgO(111) substrate. Unlike previous studies that involved Pt overlayers on hematite, the present hematite films were grown on a stable Pt buffer layer and displayed structural changes as a function of thickness. These structural differences (the presence of a ferrimagnetic phase in the thinner film) significantly affected the magnetotransport properties of the bilayers. We observed a sign change of the SMR from positive to negative when the thickness of hematite increased from 6 nm to 15 nm. For αFe2O3(15 nm)/Pt, we demonstrated room-temperature switching of the Néel order with rectangular, nondecaying switching characteristics. Such structures open the way to extending magnetotransport studies to more complex systems with double asymmetric metal/hematite/Pt interfaces. © 2023, CC BY.

关键词： Hematite

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