Latest Papers in Condensed Matter Physics
Mesoscale And Nanoscale Physics
A. Drinko, F. M. Andrade, D. Bazeia
2019-06-18
The control of quantum information is an issue of current interest for the construction of quantum machines. In this work, we investigate this possibility in the realm of quantum graphs. The study allows the identification of two distinct effects which are related to quantum complexity, one being the Braess paradox and the other the presence of quantum interference in some elementary arrangements of graphs. Motivated by the power of quantum graphs, we elaborate on the construction of simple devices, based on microwave and optical fibers networks, and also on quantum dots, nanowires and nanorings. The elementary devices can be used to construct composed structures with important quantum properties, which may be used to manipulate quantum information.
Nathaniel Rupprecht, Dervis Can Vural
2019-06-21
When two macromolecules come very near in a fluid, the molecules that surround them, having finite volume, are less likely to get in between. This leads to a pressure difference manifesting as an entropic attraction, called depletion force. Here we calculate the density profile of liquid molecules surrounding a disordered linear macromolecule, and analytically determine the position dependence of the depletion force between two such molecules. We then verify our formulas with realistic molecular dynamics simulations. Our result can be regarded as an extension of the classical Asakura-Oosawa formula.
Yonatan Betancur-Ocampo, François Leyvraz, Thomas Stegmann
2019-06-19
Ballistic electrons in phosphorene 
junctions show optical-like phenomena. Phosphorene is modeled by a tight-binding Hamiltonian that describes its electronic structure at low energies, where the electrons behave in the zigzag direction as massive Dirac fermions and in the orthogonal armchair direction as Schr"odinger electrons. Applying the continuum approximation, we derive the electron optics laws in phosphorene junctions, which show very particular and unusual properties. Due to the anisotropy of the electronic structure, these laws depend strongly on the orientation of the junction with respect to the sublattice. Negative and anomalous reflection are observed for tilted junctions, while the typical specular reflection is found only, if the junction is parallel to the zigzag or armchair edges. Moreover, omni-directional total reflection, called anti-super Klein tunneling, is observed if the junction is parallel to the armchair edge. Applying the nonequilibrium Green's function method on the tight-binding model, we calculate numerically the current flow. The good agreement of both approaches confirms the atypical transport properties, which can be used in nano-devices to collimate and filter the electron flow, or to switch its direction.
Dmitry A. Garanin, Reem Jaafar, Eugene M. Chudnovsky
2019-06-21
The breathing mode of a skyrmion, corresponding to coupled oscillations of its size and chirality angle is studied numerically for a conservative classical-spin system on a 
lattice. The dependence of the oscillation frequency on the magnetic field is computed. It is linear at small fields, reaches maximum on increasing the field, then sharply tends to zero as the field approaches the threshold above which the skyrmion loses stability and collapses. Physically transparent analytical model is developed that explains the results qualitatively and provides the field dependence of the oscillation frequency that is close to the one computed numerically. It is shown that a large-amplitude breathing motion in which the skyrmion chirality angle 
is rotating in one direction is strongly damped and quickly ends by the skyrmion collapse. To the contrary, smaller-amplitude breathing motion in which oscillates is undamped.
Mathieu Istas, Christoph Groth, Xavier Waintal
2019-06-21
Simulations of quantum transport in coherent conductors have evolved into mature techniques that are used in fields of physics ranging from electrical engineering to quantum nanoelectronics and material science. The most efficient general-purpose algorithms have a computational cost that scales as 
in 3D, which on the one hand is a substantial improvement over older algorithms, but on the other hand still severely restricts the size of the simulation domain, limiting the usefulness of simulations through strong finite-size effects. Here, we present a novel class of algorithms that, for certain systems, allows to directly access the thermodynamic limit. Our approach, based on the Green's function formalism for discrete models, targets systems which are mostly invariant by translation, i.e. invariant by translation up to a finite number of orbitals and/or quasi-1D electrodes and/or the presence of edges or surfaces. Our approach is based on an automatic calculation of the poles and residues of series expansions of the Green's function in momentum space. This expansion allows to integrate analytically in one momentum variable. We illustrate our algorithms with several applications: devices with graphene electrodes that consist of half an infinite sheet; Friedel oscillation calculations of infinite 2D systems in presence of an impurity; quantum spin Hall physics in presence of an edge; the surface of a Weyl semi-metal in presence of impurities and electrodes connected to the surface. In this last example, we study the conduction through the Fermi arcs of the topological material and its resilience to the presence of disorder. Our approach provides a practical route for simulating 3D bulk systems or surfaces as well as other setups that have so far remained elusive.
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