Latest Research Papers In Condensed Matter Physics | (Cond-Mat.Stat-Mech) 2019-03-15

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Latest Papers in Condensed Matter Physics

Statistical Mechanics

Subadditivity in models of interacting lattice clusters (1603.08553v3)

EJ Janse van Rensburg

2016-03-28

Subadditivity is an important tool for proving the existence of thermodynamic limits in the statistical mechanics of models of lattice clusters (such as the self-avoiding walk and percolation clusters). The partition functions of these models may satisfy a variety of super- or submultiplicative inequalities, and these may be reduced to a subadditive relation by taking logarithms, and changing signs, if necessary. In this manuscript, the function into , with , is assumed to satisfy the supermultiplicative inequality . Inequalities of these types are frequently encountered in models of interacting lattice clusters; see for example reference [10]. Several results on in reference [9] are reviewed here, and in particular, a proof is given that the limit exists and is a concave function of under very mild assumptions. In addition, it is shown that for a convergent sequence in , such that , the limit exists. This corrects an unrecoverable flaw in the proof of theorem 3.6 in reference [9].

model with antinematic interaction (1903.05586v1)

Milan Žukovič

2019-03-13

We consider the model with ferromagnetic (FM) and antinematic (AN) nearest-neighbor interactions on a square lattice for a varying interaction strength ratio. Besides the expected FM and AN quasi-long-range order (QLRO) phases we identify at low temperatures another peculiar canted ferromagnetic (CFM) QLRO phase, resulting from the competition between the collinear FM and non-collinear AN ordering tendencies. In the CFM phase neighboring spins that belong to different sublattices are canted by a non-universal (dependent on the interaction strength ratio) angle and the ordering is characterized by a fast-decaying power-law intra-sublattice correlation function. Compared to the FM phase, in the CFM phase correlations are significantly diminished by the presence of zero-energy domain walls due to the inherent degeneracy caused by the AN interactions. We present the phase diagram as a function of the interaction strength ratio and discuss the character of the respective phases as well as the transitions between them.

Entanglement transition from variable-strength weak measurements (1903.05452v1)

Marcin Szyniszewski, Alessandro Romito, Henning Schomerus

2019-03-13

We show that weak measurements can induce a quantum phase transition of interacting many-body systems from an ergodic thermal phase with a large entropy to a nonergodic localized phase with a small entropy, but only if the measurement strength exceeds a critical value. We demonstrate this effect for a one-dimensional quantum circuit evolving under random unitary transformations and generic positive operator-valued measurements of variable strength. As opposed to projective measurements describing a restricted class of open systems, the measuring device is modeled as a continuous Gaussian probe, capturing a large class of environments. By employing data collapse and studying the enhanced fluctuations at the transition, we obtain a consistent phase boundary in the space of the measurement strength and the measurement probability, clearly demonstrating a critical value of the measurement strength below which the system is always ergodic, irrespective of the measurement probability. These findings provide guidance for quantum engineering of many-body systems by controlling their environment.

How to probe the microscopic onset of irreversibility with ultracold atoms (1903.04834v2)

Ralf Bürkle, Amichay Vardi, Doron Cohen, James R. Anglin

2019-03-12

The microscopic onset of irreversibility is finally becoming an experimental subject. Recent experiments on microscopic open and even isolated systems have measured statistical properties associated with entropy production, and hysteresis-like phenomena have been seen in cold atom systems with dissipation (i.e. effectively open systems coupled to macroscopic reservoirs). Here we show how experiments on isolated systems of ultracold atoms can show dramatic irreversibility like cooking an egg. In our proposed experiments, a slow forward-and-back parameter sweep will sometimes fail to return the system close to its initial state. This probabilistic hysteresis is due to the same non-adiabatic spreading and ergodic mixing in phase space that explains macroscopic irreversibility, but realized \textit{without} dynamical chaos; moreover this fundamental mechanism quantitatively determines the probability of return to the initial state as a function of tunable parameters in the proposed experiments. Matching the predicted curve of return probability will be a conclusive experimental demonstration of the microscopic onset of irreversibility.

Quantum kinetic perturbation theory for near-integrable spin chains with weak long-range interactions (1903.05401v1)

Clément Duval, Michael Kastner

2019-03-13

For a transverse-field Ising chain with weak long-range interactions we develop a perturbative scheme, based on quantum kinetic equations, around the integrable nearest-neighbour model. We introduce, discuss, and benchmark several truncations of the time evolution equations up to eighth order in the Jordan-Wigner fermionic operators. The resulting set of differential equations can be solved for lattices with sites and facilitates the computation of spin expectation values and correlation functions to high accuracy, at least for moderate timescales. We use this scheme to study the relaxation dynamics of the model, involving prethermalisation and thermalisation. The techniques developed here can be generalised to other spin models with weak integrability-breaking terms.