DSpace Collection:
https://dspace.lboro.ac.uk/2134/1680
2017-06-24T17:09:48ZAnomalous in-plane magnetoresistance of electron-doped cuprate La2−xCexCuO4±δ
https://dspace.lboro.ac.uk/2134/25299
Title: Anomalous in-plane magnetoresistance of electron-doped cuprate La2−xCexCuO4±δ
Authors: Yu, H.S.; He, G.; Jia, Yanli; Zhang, X.; Yuan, J.; Zhu, Beiyi; Kusmartseva, Anna F.; Kusmartsev, F.V.; Jin, K.
Abstract: We report systematic in-plane magnetoresistance measurements on the electron-doped cuprate La2−xCexCuO4±δ thin films as a function of Ce doping and oxygen content in the magnetic field up to 14T. A crossover from negative to positive magnetoresistance occurs between the doping level x = 0.07 and 0.08. Above x = 0.08, the positive magnetoresistance effect appears, and is almost
indiscernible at x = 0.15. By tuning the oxygen content, the as-grown samples show negative magnetoresistance effect, whereas the optimally annealed ones display positive magnetoresistance effect at the doping level x = 0.15. Intriguingly, a linear-field dependence of in-plane magnetoresistance is observed at the underdoping level x = 0.06, the optimal doping level x = 0.1 and slightly overdoping level x = 0.11. These anomalies of in-plane magnetoresistance may be related to the intrinsic inhomogeneity in the cuprates, which is discussed in the framework of network model.
Description: This paper is closed access until 19th May 2018.2017-01-01T00:00:00ZBogoliubov-Born-Green-Kirkwood-Yvon chain and kinetic equations for the level dynamics in an externally perturbed quantum system
https://dspace.lboro.ac.uk/2134/25257
Title: Bogoliubov-Born-Green-Kirkwood-Yvon chain and kinetic equations for the level dynamics in an externally perturbed quantum system
Authors: Qureshi, Mumnuna A.; Zhong, Johnny; Betouras, Joseph J.; Zagoskin, Alexandre M.
Abstract: Theoretical description and simulation of large quantum coherent systems out of equilibrium remains a daunting task. Here we are developing a new approach to it based on the Pechukas-Yukawa formalism, which is especially convenient in case of an adiabatically slow external perturbation. In this formalism the dynamics of energy levels in an externally perturbed quantum system as a function of the perturbation parameter is mapped on that of a fictitious one-dimensional classical gas of particles with cubic repulsion. Equilibrium statistical mechanics of this Pechukas gas allows to reproduce the random matrix theory of energy levels. In the present work, we develop the nonequilibrium statistical mechanics of the Pechukas gas, starting with the derivation of the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) chain of equations for the appropriate generalized distribution functions. Sets of approximate kinetic equations can be consistently obtained by breaking this chain at a particular point (i.e. approximating all higher-order distribution functions by the products of the lower-order ones). When complemented by the equations for the level occupation numbers and inter-level transition amplitudes, they allow to describe the nonequilibrium evolution of the quantum state of the system, which can describe better a large quantum coherent system than the currently used approaches. In particular, we find that corrections to the factorized approximation of the distribution function scale as 1/N, where N is the number of the "Pechukas gas particles" (i.e. energy levels in the system).
Description: This paper is in closed access.2017-01-01T00:00:00ZEntanglement scaling and spatial correlations of the transverse field Ising model with perturbations
https://dspace.lboro.ac.uk/2134/25256
Title: Entanglement scaling and spatial correlations of the transverse field Ising model with perturbations
Authors: Cole, Richard M.; Pollmann, Frank; Betouras, Joseph J.
Abstract: We study numerically the entanglement entropy and spatial correlations of the one dimensional transverse field Ising model with three different perturbations. First, we focus on the out of equilibrium, steady state with an energy current passing through the system. By employing a variety of matrix-product state based methods, we confirm the phase diagram and compute the entanglement entropy. Second, we consider a small perturbation that takes the system away from integrability and calculate the correlations, the central charge and the entanglement entropy. Third, we consider periodically weakened bonds, exploring the phase diagram and entanglement properties first in the situation when the weak and strong bonds alternate (period two-bonds) and then the general situation of a period of n bonds. In the latter case we find a critical weak bond that scales with the transverse field as $J'_c/J$ = $(h/J)^n$, where $J$ is the strength of the strong bond, $J'$ of the weak bond and $h$ the transverse field. We explicitly show that the energy current is not a conserved quantity in this case.
Description: This paper is in closed access until it is published.2017-01-01T00:00:00ZEpitaxial Graphene morphological imperfections: from a hindrance to the generation of new photo-responses in the visible domain
https://dspace.lboro.ac.uk/2134/25059
Title: Epitaxial Graphene morphological imperfections: from a hindrance to the generation of new photo-responses in the visible domain
Authors: Ben Gouider Trabelsi, A.; Kusmartsev, F.V.; Gaifullin, Marat; Forrester, Michael; Kusmartseva, Anna F.; Oueslati, M.
Abstract: We report the discovery of remarkable photo-physical phenomena with characteristics unique to epitaxial graphene grown on 6H-SiC (000-1). Surprisingly, the graphene electrical resistance increases under light illumination in contrast to conventional materials where it normally decreases. The resistance shows logarithmic temperature dependences which may be attributed to an Altshuler-Aronov effect. We show that the photoresistance depends on the frequency of the irradiating light, with three lasers (red, green, and violet) used to demonstrate the phenomenon. The counterintuitive rise of the positive photoresistance may be attributed to a creation of trapped charges upon irradiation. We argue that the origin of the photoresistance is related to the texture formed by graphene flakes. The photovoltage also exists and increases with light intensity. However, its value saturates quickly with irradiation and does not change in time. The saturation of the photovoltage may be associated with the formation of a quasi-equilibrium state of the excited electrons and holes associated with a charge redistribution between the graphene and SiC substrate. The obtained physical picture is in agreement with the photoresistance measurements: X-Ray photoelectron spectrometry "XPS", atomic force microscopy "AFM", Raman spectroscopy and the magnetic dependence of photo resistance decay measurements. We also observed non-decaying photoresistance and linear magnetoresistance in magnetic fields up to 1 T. We argue that this is due to topological phases, spontaneously induced by persistent current formation within graphene flake edges by magnetic fields.
Description: This paper is closed access. Supplementary data for this article is available in the Loughborough Data Repository at doi:10.17028/rd.lboro.4986233.2017-01-01T00:00:00Z