Latest Papers in Condensed Matter Physics
Statistical Mechanics
J. P. Hague, P. W. Mieczkowski, C. O'Rourke, A. J. Loughlin, J. B. Phillips
2019-02-07
Recent advances mean that samples of artificial tissue can now be grown. Hence there is a need for theoretical understanding of these tissues commensurate with the size of experimental systems. We present a minimal model for predicting cell orientations and modification of tissue shape resulting from the active forces in cells. The extracellular matrix (ECM), a biopolymer network found between cells, is represented by an elastic network. Cells in the model induce tension in this network according to their symmetries. This in turn influences nearby cells leading to nematic order and change of tissue shape. Simulated annealing solutions of the model show close agreement with experimental results for artificial neural tissue. Thus, the subtle interplay between forces generated by cells and the ECM that leads to the ordering of tissues is reproduced. Applications of the model are discussed.
Shriya Pai, Michael Pretko, Rahul M. Nandkishore
2018-07-25
We study the spreading of initially-local operators under unitary time evolution in a 1d random quantum circuit model which is constrained to conserve a 
charge and its dipole moment, motivated by the quantum dynamics of fracton phases. We discover that charge remains localized at its initial position, providing a crisp example of a non-ergodic dynamical phase of random circuit dynamics. This localization can be understood as a consequence of the return properties of low dimensional random walks, through a mechanism reminiscent of weak localization, but insensitive to dephasing. The charge dynamics is well-described by a system of coupled hydrodynamic equations, which makes several nontrivial predictions in good agreement with numerics. Importantly, these equations also predict localization in 2d fractonic circuits. Immobile fractonic charge emits non-conserved operators, whose spreading is governed by exponents distinct to non-fractonic circuits. Fractonic operators exhibit a short time linear growth of observable entanglement with saturation to an area law, as well as a subthermal volume law for operator entanglement. The entanglement spectrum follows semi-Poisson statistics, similar to eigenstates of MBL systems. The non-ergodic phenomenology persists to initial conditions containing non-zero density of dipolar or fractonic charge. Our work implies that low-dimensional fracton systems preserve forever a memory of their initial conditions in local observables under noisy quantum dynamics, thereby constituting ideal memories. It also implies that 1d and 2d fracton systems should realize true MBL under Hamiltonian dynamics, even in the absence of disorder, with the obstructions to MBL in translation invariant systems and in d>1 being evaded by the nature of the mechanism responsible for localization. We also suggest a possible route to new non-ergodic phases in high dimensions.
Eleonora Kreačić, Ginestra Bianconi
2018-10-24
The matching problem has a large variety of applications including the allocation of competitive resources and network controllability. The statistical mechanics approach based on the cavity method has shown to be exact in characterizing this combinatorial problem on locally tree-like networks. Here we use the cavity method to solve the many-to-one bipartite 
-matching problem that can be considered to be a model for the characterization of the capacity of user-server networks such as wireless communication networks. Finally we study the phase diagram of the model defined in network ensembles.
Mark T. Mitchison
2019-02-07
The inexorable miniaturisation of technologies, the relentless drive to improve efficiency and the enticing prospect of boosting performance through quantum effects are all compelling reasons to investigate microscopic machines. Thermal absorption machines are a particularly interesting class of device that operate autonomously and use only heat flows to perform a useful task. In the quantum regime, this provides a natural setting in which to quantify the thermodynamic cost of various operations such as cooling, timekeeping or entanglement generation. This article presents a pedagogical introduction to the physics of quantum absorption machines, covering refrigerators, engines and clocks in detail.
David S. Dean, Pierre Le Doussal, Satya N. Majumdar, Gregory Schehr
2019-02-07
We consider the real time dynamics of 
noninteracting fermions in 
. They evolve in a trapping potential 
, starting from the equilibrium state in a potential 
. We study the time evolution of the Wigner function 
in the phase space 
, and the associated kernel which encodes all correlation functions. At 
the Wigner function for large is uniform in phase space inside the Fermi volume, and vanishes at the Fermi surf over a scale 
being described by a universal scaling function related to the Airy function. We obtain exact solutions for the Wigner function, the density, and the correlations in the case of harmonic and inverse square potentials, for several . In the large limit, near the edges where the density vanishes, we obtain limiting kernels (of the Airy or Bessel types) that retain the form found in equilibrium, up to a time dependent rescaling. For non-harmonic traps the evolution of the Fermi volume is more complex. Nevertheless we show that, for intermediate times, the Fermi surf is still described by the same equilibrium scaling function, with a non-trivial time and space dependent width which we compute analytically. We discuss the multi-time correlations and obtain their explicit scaling forms valid near the edge for the harmonic oscillator. Finally, we address the large time limit where relaxation to the Generalized Gibbs Ensemble (GGE) was found to occur in the "classical" regime 
. Using the diagonal ensemble we compute the Wigner function in the quantum case (large , fixed 
) and show that it agrees with the GGE. We also obtain the higher order (non-local) correlations in the diagonal ensemble.
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