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arXiv2009

Classical Correlations and Quantum Interference in Ballistic Conductors

We illustrate how classical chaotic dynamics influences the quantum properties at mesoscopic scales. As a model case we study semiclassically coherent transport through ballistic mesoscopic systems within the Landauer formalism beyond the so-called diagonal approximation, i.e. by incorporating classical action correlations. In this context we review and explain the two main trajectory-based methods developed for calculating quantum corrections: the configuration space approach and the phase space approach that can be regarded as less illustrative but more general than the first one.

cond-mat.mes-hall↗

arXiv2010

Edge-to-bulk transition of the IQHE at cleaved edge overgrown samples: a screening theory based experimental proposal

In this work, we exploit the findings of the screening theory of the integer quantized Hall effect (QHE) based on the formation of the incompressible strips and its essential influence on the global resistances and propose certain experimental conditions to observe the bulk to edge transition of the QHE in a phenomenological model. We propose a Hall bar design on a cleaved edge overgrown wafer, which allows us to manipulate the edge potential profile from smooth to extremely sharp. For a particular sample design and by the help of a side gate perpendicular to the two dimensional electron system (2DES), it is shown that the plateau widths can be changed and even made to vanish when changing the edge potential profile. Such a control of the edge potential implies peculiar transport results when considering the screening theory, which includes direct Coulomb interaction explicitly. We think that, these experiments will shed new light on the understanding of the QHE.

cond-mat.mes-hall↗

arXiv2009

On the critical behavior of two-dimensional liquid crystals

The Lebwohl-Lasher (LL) model is the traditional model used to describe the nematic-isotropic transition of real liquid crystals. In this paper, we develop a numerical study of the temperature behaviour and of finite-size scaling of the two-dimensional (2D) LL-model. We discuss two possible scenarios. In the first one, the 2D LL-model presents a phase transition similar to the topological transition appearing in the 2D XY-model. In the second one, the 2D LL-model does not exhibit any critical transition, but its low temperature behaviour is rather characterized by a crossover from a disordered phase to an ordered phase at zero temperature. We realize and discuss various comparisons with the 2D XY-model and the 2D Heisenberg model. Adding to previous studies of finite-size scaling behaviour of the order parameter and conformal mapping of order parameter profile, we analyze the critical scaling of the probability distribution function, hyperscaling relations and stiffness order parameter and conclude that the second scenario (no critical transition) is the most plausible.

cond-mat.stat-mech↗

arXiv2010

Negative and Positive Magnetoresistance in Bilayer Graphene: Effects of Weak Localization and Charge Inhomogeneity

We report measurements of magnetoresistance in bilayer graphene as a function of gate voltage (carrier density) and temperature. We examine multiple contributions to the magnetoresistance, including those of weak localization (WL), universal conductance fluctuations (UCF), and inhomogeneous charge transport. A clear WL signal is evident at all measured gate voltages (in the hole doped regime) and temperature ranges (from 0.25 K to 4.3 K), and the phase coherence length extracted from WL data does not saturate at low temperatures. The WL data is fit to demonstrate that electron-electron Nyquist scattering is the major source of phase decoherence. A decrease in UCF amplitude with increasing gate voltage and temperature is shown to be consistent with a corresponding decrease in the phase coherence length. In addition, a weak positive magnetoresistance at higher magnetic fields is observed, and attributed to inhomogeneous charge transport.

cond-mat.mtrl-sci↗

arXiv2010

Mapping universal conductance fluctuations

Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. To understand the coherent flow of electrons through a graphene device, we employ a nanoscale probe that can access the relevant length scales - the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below, creating a movable scatterer for electron waves. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). By scanning the tip over a device, we map these conductance fluctuations \textit{vs.} scatterer position. We find that the conductance is highly sensitive to the tip position, producing $δG \sim e^2/h$ fluctuations when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment, and demonstrate the value of a cooled SPM for probing coherent transport in graphene.

cond-mat.mes-hall↗

arXiv2009

Singlet-triplet dephasing in asymmetric quantum dot molecules

We discuss pure dephasing of singlet-triplet superpositions in two-electron double quantum dots due to elastic phonon scattering. We generalize our previous results to a system built of two non-identical dots. We show that the asymmetry must be very strong in order to considerably affect the dephasing rate.

cond-mat.mes-hall↗

arXiv2009

Quantum circuits under magnetic field

We consider a quantum $LC$ circuit under a constant magnetic flux $f$, and derive a discretized form of the Schrödinger equation, which is equivalent to introducing a {\em potential} $V(ϕ,f)$ in the pseudo-flux $ϕ$-representation, which is different from that found previously by Li and Chen \cite{LI-CHEN}. Firts, we discuss the physical assumptions leading to these different results, and then study the energy spectrum of the $LC$ quantum circuit as a function of a constant external magnetic flux, using a direct numerical approach. The results are compared with the spectrum obtained using the Li-Chen potential. Our results indicate that the energy spectra from both models are quite different numerically, and as a function of $f$, so that they may be clearly distinguished under appropiate experimental conditions.

cond-mat.mes-hall↗

arXiv2010

Above-Bandgap Magneto-optical Kerr Effect in Ferromagnetic GaMnAs

We have performed a systematic magneto-optical Kerr spectroscopy study of GaMnAs with varying Mn densities as a function of temperature, magnetic field, and photon energy. Unlike previous studies, the magnetization easy axis was perpendicular to the sample surface, allowing us to take remanent polar Kerr spectra in the absence of an external magnetic field. The remanent Kerr angle strongly depended on the photon energy, exhibiting a large positive peak at $\sim1.7$ eV. This peak increased in intensity and blue-shifted with Mn doping and further blue-shifted with annealing. Using a 30-band ${\bf k\cdot p}$ model with antiferromagnetic $s,p$-$d$ exchange interaction, we calculated the dielectric tensor of GaMnAs in the interband transition region, assuming that our samples are in the metallic regime and the impurity band has merged with the valence band. We successfully reproduced the observed spectra without \emph{ad hoc} introduction of the optical transitions originated from impurity states in the band gap. These results lead us to conclude that above-bandgap magneto-optical Kerr rotation in ferromagnetic GaMnAs is predominantly determined by interband transitions between the conduction and valence bands.

cond-mat.mtrl-sci↗

arXiv2009

Surface electromagnetic phenomena in pristine and atomically doped carbon nanotubes

The article reviews recent progress in the theoretical understanding of near-field surface electromagnetic phenomena in pristine and atomically doped carbon nanotubes. The phenomena involving strong coupling effects are outlined. They are the optical absorption by single-walled carbon nanotubes doped with single atoms or ions in the frequency range close to the atomic transition frequency, the entanglement of the pair of atomic qubits strongly coupled to a common high-finesse surface photonic mode of the nanotube, and the optical response of the strongly coupled surface exciton-plasmon excitations in pristine semiconducting carbon nanotubes. The phenomena reviewed have a great potential to be exploited for the future development of the nanotube based tunable optoelectronic device applications in areas such as nanophotonics, nanoplasmonics, cavity quantum electrodynamics, and quantum information science.

cond-mat.mes-hall↗

arXiv2009

Exact eigenfunctions of FQHE systems at fractional filling factors 1/q. I. Formal results

Eigenstates of the FQHE hamiltonian problem after to be projected on the LLL are determined for filling factors 1/q, with q an odd number. The solutions are found for an infinite class of finite samples in which the Coulomb potential is periodically extended. Therefore, a thermodynamic limit solution is also identified. The results suggest the presence of integrability properties in FQHE systems. The many particle states are simple Slater determinants constructed with special single particle states. These orbitals are defined as powers of order q of "composite fermion" like wavefunctions associated to a reduced magnetic field B/q. At the same time, those "composite fermion" states were obtained by factorizing and canceling fixed position (quasi-momentum independent) zeros in previously derived exact Hartree-Fock orbitals. A formula for the energy per particle of the FQHE states is given for finite samples as well as for the thermodynamic limit state. As a side result, the same "composite fermions" like orbitals are employed to construct variational wavefunctions of the system, showing zeros of order q as two electrons approach each other, as Laughlin states do. The long range spatial correlation associated to the starting HF solutions may further reduce the energy of these states.

cond-mat.mes-hall↗

arXiv2009

Maintaining the local temperature below the critical value in thermally out of equilibrium superconducting wires

A generalized theory of open quantum systems combined with mean-field theory is used to study a superconducting wire in contact with thermal baths at different temperatures. It is shown that, depending on the temperature of the colder bath, the temperature of the hotter bath can greatly exceed the equilibrium critical temperature, and still the local temperature in the wire is maintained below the critical temperature and hence the wire remains in the superconducting state. The effects of contact areas and disorder are studied. Finally, an experimental setup is suggested to test our predictions.

cond-mat.mes-hall↗

arXiv2009

Anomalous magnetoresistance on the topological surface

We investigate charge transport in two-dimensional ferromagnet/feromagnet junction on a topological insulator. The conductance across the interface depends sensitively on the directions of the magnetizations of the two ferromagnets, showing anomalous behaviors compared with the conventional spin-valve. It is found that the conductance depends strongly on the in-plane direction of the magnetization. Moreover, in sharp contrast to the conventional magnetoresistance effect, in the p-n junction, the conductance at the parallel configuration is much smaller than that at the antiparallel configuration. This stems from the way how the wavefunctions connect between both sides.

cond-mat.mes-hall↗

arXiv2010

Magnetic Nanoparticle Assemblies

This chapter provides an introduction to the fundamental physical ideas and models relevant to the phenomenon of magnetic hysteresis in nanoparticle assemblies. The concepts of single-domain particles and superparamagnetism are discussed. The mechanisms of magnetization by coherent rotation and the role of temperature in the gradual decay of magnetization are analyzed in the framework of simple analytical models. Modern numerical techniques (Monte Carlo simulations, Magnetization Dynamics) used to study dense nanoparticle assemblies are presented. An overview of the most common experimental techniques used to measure the magnetic hysteresis effect in nanoparticle assemblies are presented and the underlying principles are exposed.

cond-mat.mes-hall↗

arXiv2009

Collective luminescence and phonon-induced processes in double quantum dots

We study the evolution of a quantum state of a double quantum dot system interacting with the electromagnetic environment and with the lattice modes, in the presence of a coupling between the two dots. We propose a unified approach to the simulation of the system evolution under joint impact of the two reservoirs. We discuss the sub- and superradiant radiative decay of the system, the phonon-induced decay of entanglement between the dots, and the transfer of excitation between them.

cond-mat.mes-hall↗

arXiv2009

Phosphorous alloying: controlling the magnetic anisotropy in ferromagnetic (Ga,Mn)(As,P) Layers

Phosphorous alloying of GaMnAs thin films has been used for the manipulation of the magnetic anisotropies in ferromagnetic Ga0.93Mn0.07As1-yPy layers. We have determined the anisotropy constants as a function of temperature for phosphorous alloying levels between 0 and 8.8 at % for a Mn doping level of ~ 7at%. We show that it is possible to obtain layers with robust ferromagnetism and either in-plane or out-of plane easy axes with small barriers for magnetization reorientation by phosphorous alloying with y< 6at% or y> 6at%. The critical temperatures are not significantly increased by the P alloying.

cond-mat.mtrl-sci↗

arXiv2009

Fractal dimension and threshold properties in a spatially correlated percolation model

We consider the effects of spatial correlations in a two-dimensional site percolation model. By generalizing the Newman-Ziff Monte Carlo algorithm to include spatial correlations, percolation thresholds and fractal dimensions of percolation clusters are obtained. For a wide range of spatial correlations, the percolation threshold differs little from the uncorrelated result. In contrast, the fractal dimension differs sharply from the uncorrelated result for almost all types of correlation studied. We interpret these results in the framework of long-range correlated percolation.

cond-mat.dis-nn↗

arXiv2009

Drude-Interband Coupling, Screening, and the Optical Conductivity of Doped Bilayer Graphene

We present a theory of the influence of band renormalization and excitonic electron-electron interaction effects on the optical conductivity $σ(ω)$ of doped bilayer graphene. Using the Keldysh formalism, we derive a kinetic equation from which we extract numerical and approximate analytic results for $σ(ω)$. Our calculations reveal a previously unrecognized mechanism which couples the Drude and interband response and renormalizes the plasmon frequency, and suggest that screening must play an essential role in explaining the weakly renormalized conductivity seen in recent experiments.

cond-mat.mes-hall↗

arXiv2009

Thermodynamics of small systems by nanocalorimetry: from physical to biological nano-objects

Membrane based nanocalorimeters have been developed for ac calorimetry experiments. It has allowed highly sensitive measurements of heat capacity from solid state physics to complex systems like polymers and proteins. In this article we review what has been developed in ac calorimetry toward the measurement of very small systems. Firstly, at low temperature ac calorimetry using silicon membrane permits the measurement of superconducting sample having geometry down to the nanometer scale. New phase transitions have been found in these nanosystems illustrated by heat capacity jumps versus the applied magnetic field. Secondly, a sensor based on ultra-thin polymer membrane will be presented. It has been devoted to thermal measurements of nanomagnetic systems at intermediate temperature (20K to 300K). Thirdly, three specific polyimide membrane based sensors have been designed for room temperature measurements. One is devoted to phase transitions detection in polymer, the second one to protein folding/unfolding studies and the third one will be used for the study of heat release in living cells. The possibility of measuring systems out of equilibrium will be emphasized.

cond-mat.mes-hall↗