Latest Papers in Condensed Matter Physics

Statistical Mechanics

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Probability density of the fractional Langevin equation with reflecting walls (1907.08188v1)

Thomas Vojta, Sarah Skinner, Ralf Metzler

2019-07-18

We investigate anomalous diffusion processes governed by the fractional Langevin equation and confined to a finite or semi-infinite interval by reflecting potential barriers. As the random and damping forces in the fractional Langevin equation fulfill the appropriate fluctuation-dissipation relation, the probability density on a finite interval converges for long times towards the expected uniform distribution prescribed by thermal equilibrium. In contrast, on a semi-infinite interval with a reflecting wall at the origin, the probability density shows pronounced deviations from the Gaussian behavior observed for normal diffusion. If the correlations of the random force are persistent (positive), particles accumulate at the reflecting wall while anti-persistent (negative) correlations lead to a depletion of particles near the wall. We compare and contrast these results with the strong accumulation and depletion effects recently observed for non-thermal fractional Brownian motion with reflecting walls, and we discuss broader implications.

The Ghost of a Vanishing Stripe Order in the Triangular Quantum Ising Magnet TmMgGaO (1907.08173v1)

Han Li, Yuan-Da Liao, Bin-Bin Chen, Xu-Tao Zen, Xian-Lei Sheng, Yang Qi, Zi Yang Meng, Wei Li

2019-07-18

Frustrated magnets host the promises of material realizations of new paradigm of quantum matter. However, due to their strongly correlated nature, direct comparison of unbiased model calculations with experiment results is still a challenge to the entire community. Here, we design and implement a protocol of employing quantum many-body computation methodologies -- quantum Monte Carlo and thermal tensor network methods -- to provide model exact calculation of both equilibrium and dynamical properties of a frustrated rare-earth magnet TmMgGaO (TMGO) that perfectly explains the corresponding experimental findings. Our results confirm TMGO as an ideal realization of the transverse-field triangular lattice Ising model, and there emerge in its dynamical spectrum ghost images of the vanished magnetic stripe order, i.e., rotonlike excitation modes, representing the vortex-antivortex pair excitations. We propose the TMGO material realizes at finite temperature a Kosterlitz-Thouless (KT) phase resembling that in a superfluid helium film, and further suggest experimental detections of KT physics in this triangular quantum magnet.

Inhomogeneous coherent states in small-world networks: application to the functional brain networks (1808.06214v3)

Bahruz Gadjiev, Tatiana Progulova

2018-08-19

We study the dynamics of the processes in the small-world networks with a power-law degree distribution where every node is considered to be in one of the two available statuses. We present an algorithm for generation of such network and determine analytically a temporal dependence of the network nodes degrees and using the maximum entropy principle we define a degree distribution of the network. We discuss the results of the Ising discrete model for small-world networks and in the framework of the continuous approach using the principle of least action, we derive an equation of motion for the order parameter in these networks in the form of a fractional differential equation. The obtained equation enables the description of the problem of a spontaneous symmetry breaking in the system and determination of the spatio-temporal dependencies of the order parameter in varies stable phases of the system. In the cases of one and two component order parameters with taken into account major and secondary order parameters we obtain analytical solutions of the equation of motion for the order parameters and determine solutions for various regimes of the system functioning. We apply the obtained results to the description of the processes in the brain and discuss the problems of emergence of mind.

Lectures on entanglement entropy in field theory and holography (1907.08126v1)

Matthew Headrick

2019-07-18

These notes, based on lectures given at various schools over the last few years, aim to provide an introduction to entanglement entropies in quantum field theories, including holographic ones. We explore basic properties and simple examples of entanglement entropies, mostly in two dimensions, with an emphasis on physical rather than formal aspects of the subject. In the holographic case, the focus is on how the Ryu-Takayanagi formula geometrically realizes general features of field-theory entanglement, while revealing special properties of holographic theories. In order to make the notes somewhat self-contained for readers whose background is in high-energy theory, a brief introduction to the relevant aspects of quantum information theory is included.

Replica Symmetry and Replica Symmetry Breaking for the Traveling Salesperson Problem (1806.08681v2)

Hendrik Schawe, Jitesh Kumar Jha, Alexander K. Hartmann

2018-06-22

We study the energy landscape of the Traveling Salesperson problem (TSP) using exact ground states and a novel linear programming approach to generate excited states with closely defined properties. We look at four different ensembles, notably the classic finite dimensional Euclidean TSP and the mean-field-like (1,2)-TSP, which has its origin directly in the mapping of the Hamiltonian circuit problem on the TSP. Our data supports previous conjectures that the Euclidean TSP does not show signatures of replica symmetry breaking neither in two nor in higher dimension. On the other hand the (1,2)-TSP exhibits some signature which does not exclude broken replica symmetry, making it a candidate for further studies in the future.

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