Theoretical Condensed Matter Physics Research Articles

Unitary braided-enriched monoidal categories

Braided-enriched monoidal categories were introduced in the work of Morrison–Penneys, where they were characterized using braided central functors. The recent work of Kong–Yuan–Zhang–Zheng and Dell extended this characterization to an equivalence of 2-categories. Since their introduction, braided-enriched fusion categories have been used to describe certain phenomena in topologically ordered systems in theoretical condensed matter physics. While these systems are unitary, there was previously no general notion of unitarity for enriched categories in the literature. We supply the notion of unitarity for enriched categories and braided-enriched monoidal categories and extend the above 2-equivalence to the unitary setting.

Neural network enhanced hybrid quantum many-body dynamical distributions

Computing dynamical distributions in quantum many-body systems represents one of the paradigmatic open problems in theoretical condensed matter physics. Despite the existence of different techniques both in real-time and frequency space, computational limitations often dramatically constrain the physical regimes in which quantum many-body dynamics can be efficiently solved. Here we show that the combination of machine learning methods and complementary many-body tensor network techniques substantially decreases the computational cost of quantum many-body dynamics. We demonstrate that combining kernel polynomial techniques and real-time evolution, together with deep neural networks, allows to compute dynamical quantities faithfully. Focusing on many-body dynamical distributions, we show that this hybrid neural-network many-body algorithm, trained with single-particle data only, can efficiently extrapolate dynamics for many-body systems without prior knowledge. Importantly, this algorithm is shown to be substantially resilient to numerical noise, a feature of major importance when using this algorithm together with noisy many-body methods. Ultimately, our results provide a starting point towards neural-network powered algorithms to support a variety of quantum many-body dynamical methods, that could potentially solve computationally expensive many-body systems in a more efficient manner.

Thermal Vibration Correlation Function Formalism for Molecular Excited State Decay Rates

Thermal Vibration Correlation Function Formalism for Molecular Excited State Decay Rates

Applying high‐throughput computational techniques for discovering next‐generation of permanent magnets

AbstractThe uncertainty in rare‐earth market resulted in worldwide efforts to develop rare‐earth‐lean/free permanent magnets. In this paper, we discuss about this problem and analyse how advances in computational and theoretical condensed matter physics could be essential in the development of a new generation of high‐performance permanent magnets via high‐throughput computational technique for material design. Additionally, we show that an adaptive genetic algorithm based methodology could be a useful tool for finding new magnetic phases. In particular, we apply such approach to Fe0.75Sn0.25 compound recovering well‐known experimental results and also finding new low‐energy magnetic metastable structures. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Abdus Salam center cultivated science, transcended politics

The story by Jermey Matthews (Physics Today, October 2014, page 20) about the role of the Abdus Salam International Centre for Theoretical Physics (ICTP) in the developing world was interesting and informative. It also evoked memories from 20 years ago, when I was a student in the ICTP postgraduate diploma program. I was the first Albanian student accepted into its condensed-matter-physics track, in the 1993–94 school year.I and probably many other former students will attest that these ICTP programs were an excellent opportunity for students to prepare for graduate school and receive a quick and intensive immersion in frontier topics of research. The ICTP also provided a life-changing experience for many students who were not able to pursue their physics dreams at home. Following the diploma program, I moved on to obtain a PhD in theoretical condensed-matter physics and eventually took a position in academia in the US.I also want to highlight the important role of the ICTP in bridging the East–West political divisions of the time. Even its location—in Trieste, Italy, close to the Cold War border in Europe—transmitted a sense that the place rose above challenges and brought people together. The institute clearly demonstrated that science transcends politics, and it inspired many physics students and faculty throughout Eastern Europe. That closing of political divisions is an amazing side of the story of the ICTP.© 2015 American Institute of Physics.

Numerical optimization using flow equations.

We develop a method for multidimensional optimization using flow equations. This method is based on homotopy continuation in combination with a maximum entropy approach. Extrema of the optimizing functional correspond to fixed points of the flow equation. While ideas based on Bayesian inference such as the maximum entropy method always depend on a prior probability, the additional step in our approach is to perform a continuous update of the prior during the homotopy flow. The prior probability thus enters the flow equation only as an initial condition. We demonstrate the applicability of this optimization method for two paradigmatic problems in theoretical condensed matter physics: numerical analytic continuation from imaginary to real frequencies and finding (variational) ground states of frustrated (quantum) Ising models with random or long-range antiferromagnetic interactions.

A new formulation of non-relativistic diffeomorphism invariance

We provide a new formulation of non-relativistic diffeomorphism invariance. It is generated by localising the usual global Galilean symmetry. The correspondence with the type of diffeomorphism invariant models currently in vogue in the theory of fractional quantum Hall effect has been discussed. Our construction is shown to open up a general approach of model building in theoretical condensed matter physics. Also, this formulation has the capacity of obtaining Newton–Cartan geometry from the gauge procedure.

Book Reviews

Sixteen papers, based on the conference on “Econophysics of Agent-Based Models” held at the Saha Institute of Nuclear Physics in November 2012, explore agent-based modeling in the field of econophysics from the perspectives of physicists, economists, mathematicians, and financial engineers. Papers discuss agent-based modeling of zapping behavior of viewers, television commercial allocation, and advertisement markets; agent-based modeling of the housing asset bubble—a simple utility function-based investigation; Urn model-based adaptive multi-arm clinical trials—a stochastic approximation approach; logistic modeling of a religious sect cult and financial features; characterizing financial crisis by means of the three states random field Ising model; themes and applications of kinetic exchange models—redux; the kinetic exchange opinion model—solution in the single parameter map limit; an overview of the new frontiers of economic complexity; Jan Tinbergen's legacy for economic networks—from the gravity model to quantum statistics; a macroscopic order of consumer demand due to heterogeneous consumer behavior on Japanese household demand tested by the random matrix theory; uncovering the network structure of the world currency market—cross-correlations in the fluctuations of daily exchange rates; systemic risk in the Japanese credit network; pricing of goods with bandwagon properties—the curse of coordination; evolution of econophysics; econophysics and sociophysics—problems and prospects; and a discussion on econophysics. Abergel and Chakraborti are with the Laboratory of Mathematics Applied to Systems at the École Centrale Paris. Aoyama is with the Department of Physics at Kyoto University. Chakrabarti is at the Saha Institute of Nuclear Physics. Ghosh is with the Theoretical Condensed Matter Physics Division at the Saha Institute of Nuclear Physics.

The beginnings of theoretical condensed matter physics in Rome: a personal remembrance

This oral history interview provides a personal view on how theoretical condensed matter physics developed in Rome starting in the sixties of the last century. It then follows along the lines of research pursued by the interviewee up to the date of the interview, in March 2006. The topics considered range from the phenomenology of superfluid helium and superconductors, critical phenomena and renormalisation group approach, quantum fluids to strongly correlated electron systems and high temperature superconductors. Within these topics, fundamental problems of condensed matter physics are touched upon, such as the microscopic derivation of scaling, the metal-insulator transition and the interaction effects on disordered electron systems beyond the Anderson localisation, and the existence of heterogeneous states in cuprates.

A fresh look at dense hydrogen under pressure. III. Two competing effects and the resulting intra-molecular H-H separation in solid hydrogen under pressure

A preliminary discussion of the general problem of localization of wave functions, and the way it is approached in theoretical condensed matter physics (Wannier functions) and theoretical chemistry (localized or fragment orbitals) is followed by an application of the ideas of Paper II in this series to the structures of hydrogen as they evolve under increasing pressure. The idea that emerges is that of simultaneously operative physical (reduction of available space by an increasingly stiff wall of neighboring molecules) and chemical (depopulation of the σ(g) bonding molecular orbital of H(2), and population of the antibonding σ(u)∗ MO) factors. The two effects work in the same direction of reducing the intermolecular separation as the pressure increases, but compete, working in opposite directions, in their effect on the intramolecular (nearest neighbor, intra-pair) distance. We examine the population of σ(g) and σ(u)∗ MOs in our numerical laboratory, as well as the total electron transfer (small), and polarization (moderate, where allowed by symmetry) of the component H(2) molecules. From a molecular model of two interacting H(2) molecules we find a linear relationship between the electron transfer from σ(g) to σ(u)∗ of a hydrogen molecular fragment and the intramolecular H-H separation, and that, in turn, allows us to estimate the expected bond lengths in H(2) under pressure if the first effect (that of simple confinement) was absent. In essence, the intramolecular H-H separations under pressure are much shorter than they would be, were there no physical/confinement effect. We then use this knowledge to understand how the separate E and PV terms contribute to hydrogen phase changes with increasing pressure.

Lieb-Robinson Bound and Locality for General Markovian Quantum Dynamics

The Lieb-Robinson bound shows the existence of a maximum speed of signal propagation in discrete quantum mechanical systems with local interactions. This generalizes the concept of relativistic causality beyond field theory, and provides a powerful tool in theoretical condensed matter physics and quantum information science. Here, we extend the scope of this seminal result by considering general markovian quantum evolution, where we prove that an equivalent bound holds. In addition, we use the generalized bound to demonstrate that correlations in the stationary state of a Markov process decay on a length scale set by the Lieb-Robinson velocity and the system's relaxation time.

An Adventurous Physicist

Visiting a tropical forest for the first time prompted JA©r_me Chave to leave theoretical condensed matter physics to focus on tropical forests.

Electronic structures of double perovskites Ba2MnMO6 (M = W and Re) from first‐principles studies

AbstractThe cover picture of the current issue refers to the work of J. Cheng and Z. Q. Yang which is featured as Editor's Choice [1]. Double perovskites, A2B′B″O6 (A is an alkaline earth; B′B″ sites are occupied alternately by transition metals) have recently attracted considerable attention due to the discovery of large low‐field room‐temperature magnetoresistance in this kind of materials. The authors study from first principles the electronic structure of two double perovskites, Ba2MnWO6 and Ba2MnReO6, using Density Functional Theory. The ground magnetic phases for the two compounds are found to be antiferromagnetic and ferrimagnetic, respectively, in agreement with experimental results. Ba2MnWO6 was predicted to have semiconductor band structure. For Ba2MnReO6, a finite energy gap can appear only when spin–orbital coupling together with on‐site Coulomb interactions on Re ions are taken into account. The results explain the curve of resistivity versus temperature for Ba2MnReO6.The first author J. Cheng works in the field of theoretical condensed matter physics from first‐principles calculations at the Surface Physics Laboratory, Fudan University, Shanghai, China.

James Arthur Krumhansl

James Arthur Krumhansl

Special issue: Structure, Disorder and Correlations

AbstractThis special issue of physica status solidi (b) is a collection of articles covering a broad range of topics which are intensively investigated by current theoretical condensed matter physics. They include disorder and interaction, phase transitions and criticality, as well as transport properties. The contributions originate from the Workshop “Modelling and Simulation in Molecular Systems, Mesoscopic Structures, and Material Science”, held in Chemnitz, 21–23 April 2004. The workshop and this issue are dedicated to the 50th birthday of Professor Michael Schreiber, Chemnitz University of Technology, where he holds a chair in theory of disordered systems.Guest Editor of the present issue is Klaus Morawetz who is working both in Michael Schreiber's group in Chemnitz and at the Max‐Planck‐Institute for the Physics of Complex Systems in Dresden.On the cover we show the eigenfunction of the 3D Anderson model of localization at the metal‐insulator transition. The sys‐tem consists of 1113 = 1367631 sites at an energy E ∼ 0. The colours serve to enhance the legibility of spatial positions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum Physics in One Dimension

Quantum Physics in One Dimension

Stevenson receives Hess Medal

David J. Stevenson was awarded the Harry H. Hess Medal at the AGU Spring Meeting Honors Ceremony, which was held on May 27, 1998, in Boston, Massachusetts. The Harry H. Hess Medal recognizes outstanding achievements in the research of the constitution and evolution of Earth and its sister planets.A meaningful understanding of the Earth and planets requires explaining their differences. This explanation of planetary processes is difficult partly because it entails a wide range of scales—from microscale, operating at the atomic level, to macroscale, determined by boundaries thousands of kilometers apart. David Stevenson's graduate study was mainly in theoretical condensedmatter physics, but he is remarkable in his grasp of large‐scale planetary processes such as mantle convection and the dynamos. He is also remarkable in his ‘instinct to attack the jugular,’ that is to go for the most important problems and for the versatility of his approaches thereto.