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arXiv2009

Quantum Hall Effect in Biased Bilayer Graphene

We numerically study the quantum Hall effect in biased bilayer graphene based on a tight-binding model in the presence of disorder. Integer quantum Hall plateaus with quantized conductivity $σ_{xy}=νe^2/h$ (where $ν$ is any integer) are observed around the band center due to the split of the valley degeneracy by an opposite voltage bias added to the two layers. The central ($n=0$) Dirac Landau level is also split, which leads to a pronounced $ν=0$ plateau. This is consistent with the opening of a sizable gap between the valence and conduction bands. The exact spectrum in an open system further reveals that there are no conducting edge states near zero energy, indicating an insulator state with zero conductance. Consequently, the resistivity should diverge at Dirac point. Interestingly, the $ν=0$ insulating state can be destroyed by disorder scattering with intermediate strength, where a metallic region is observed near zero energy. In the strong disorder regime, the Hall plateaus with nonzero $ν$ are destroyed due to the float-up of extended levels toward the band center and higher plateaus disappear first.

cond-mat.mes-hall↗

arXiv2009

Equilibrium Molecular Dynamics Study of Lattice Thermal Conductivity/Conductance of Au-SAM-Au Junctions

In this paper, equilibrium molecular dynamics simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions. The SAM consisted of alkanedithiol molecules. The out-of-plane (z-direction) thermal conductance and in-plane (x- and y-direction) thermal conductivities were calculated. Simulation finite size effect, gold substrate thickness effect, temperature effect, normal pressure effect, molecule chain length effect and molecule coverage effect on thermal conductivity/conductance were studied. Vibration power spectra of gold atoms in the substrate and sulfur atoms in the SAM were calculated and vibration coupling of these two parts was analyzed. The calculated thermal conductance values of Au-SAM-Au junctions are in the range of experimental data on metal-nonmetal junctions. The temperature dependence of thermal conductance has similar trend to experimental observations. It is concluded that the Au-SAM interface resistance dominates thermal energy transport across the junction, while the substrate is the dominant media in which in-plane thermal energy transport happens.

cond-mat.mes-hall↗

arXiv2009

Non-Equilibrium Molecular Dynamics Study of Thermal Energy Transport in Au-SAM-Au junctions

Non-equilibrium molecular dynamics (NEMD) simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions to study the thermal energy transport across the junctions. Thermal conductance of the Au-SAM interfaces was calculated. Temperature effects, simulated external pressure effects, SAM molecule coverage effects and Au-SAM bond strength effects on the interfacial thermal conductance were studied. It was found that the interfacial thermal conductance increased with temperature increase at temperatures lower than 250K, but it did not have large changes at temperatures from 250K to 400K. Such a trend was found to be similar to experimental observations on similar junctions. The simulated external pressure did not affect the interfacial thermal conductance. SAM molecule coverage and Au-SAM bond strength were found to significantly affect on the thermal conductance. The vibration densities of state (VDOS) were calculated to explore the mechanism of thermal energy transport. Interfacial thermal resistance was found mainly due to the limited population of low-frequency vibration modes of the SAM molecule. Ballistic energy transport inside the SAM molecules was confirmed, and the anharmonicity played an important role in energy transport across the junctions. A heat pulse was imposed on the junction substrate, and heat dissipation inside the junction was studied. Analysis of the junction response to the heat pulse showed that the Au-SAM interfacial thermal resistance was much larger than the Au substrate and SAM resistances separately. This work showed that both the Au substrate and SAM molecules transported thermal energy efficiently, and it was the Au-SAM interfaces that dominated the thermal energy transport across the Au-SAM-Au junctions.

cond-mat.mes-hall↗

arXiv2010

(De)confinement of supercurrent in Z_2 Topological Insulators

It is shown that the electric supercurrent flows in a Z_2 topological insulator with U_em(1) X U_z(1) (electromagnetic and spin) gauge symmetries. When U_z(1) is broken, a dissipationless electric current is still possible to flow locally but net charge transfer is absent, i.e., current is confined. In the Kane-Mele model for graphene, this confining-deconfining (superconducting) transition is driven by the Rashba spin-orbit interaction, which breaks U_z(1).

cond-mat.mes-hall↗

arXiv2009

Isotropic to Nematic Phase Transition in Carbon Nanotube dispersed Liquid Crystal Composites

A high-resolution dielectric and calorimetric study of the isotropic (I) to nematic (N) phase transition of carbon nanotube (CNT) dispersed liquid crystal (LC) functional composites as a function of CNT concentration is reported. The evolution of the I-N phase transition, the temperature dependence of local nematic ordering formed by dispersed CNTs in the LC media and the transition enthalpy were coherently monitored. Anisotropic CNTs induce local deformation to the nematic director of LC and form lyotropic pseudo-nematic phase in the LC media. Results clearly indicate the dramatic impact of dispersed CNTs on both the isotropic and nematic phases of the composite.

cond-mat.soft↗

arXiv2010

Experimental demonstration of associative memory with memristive neural networks

When someone mentions the name of a known person we immediately recall her face and possibly many other traits. This is because we possess the so-called associative memory, that is the ability to correlate different memories to the same fact or event. Associative memory is such a fundamental and encompassing human ability (and not just human) that the network of neurons in our brain must perform it quite easily. The question is then whether electronic neural networks (electronic schemes that act somewhat similarly to human brains) can be built to perform this type of function. Although the field of neural networks has developed for many years, a key element, namely the synapses between adjacent neurons, has been lacking a satisfactory electronic representation. The reason for this is that a passive circuit element able to reproduce the synapse behaviour needs to remember its past dynamical history, store a continuous set of states, and be "plastic" according to the pre-synaptic and post-synaptic neuronal activity. Here we show that all this can be accomplished by a memory-resistor (memristor for short). In particular, by using simple and inexpensive off-the-shelf components we have built a memristor emulator which realizes all required synaptic properties. Most importantly, we have demonstrated experimentally the formation of associative memory in a simple neural network consisting of three electronic neurons connected by two memristor-emulator synapses. This experimental demonstration opens up new possibilities in the understanding of neural processes using memory devices, an important step forward to reproduce complex learning, adaptive and spontaneous behaviour with electronic neural networks.

q-bio.NC↗

arXiv2009

Suppression of diamagnetism in superconducting nanorings by quantum fluctuations

Zero resistance, diamagnetism (Meissner effect) and the energy gap in the excitation spectrum are the fundamental attributes of superconductivity. In nanoparticles with characteristic size 10 nm superconductivity is suppressed by the quantization of the electron energy spectrum. In narrow quasi-1D superconducting channels thermal fluctuations enable zero resistance only at temperatures noticeably below the critical temperature Tc, while quantum fluctuations suppress the dissipationless electric current in ultra-narrow superconducting nanowires even at temperatures close to T=0. Here we demonstrate that the same essentially nanoscale phenomenon, quantum fluctuations, is responsible for quenching diamagnetism in superconducting nanorings dramatically affecting the magnitude, the period and the shape of the circulating persistent currents (PCs). The effect is of fundamental importance for our understanding of the behavior of a quantum coherent system at nanoscales; and should be taken in consideration in various nanoscience applications.

cond-mat.supr-con↗

arXiv2013

Comparison of electron and phonon transport in disordered semiconductor carbon nanotubes

Charge and thermal conductivities are the most important parameters of carbon nanomaterials as candidates for future electronics. In this paper we address the effects of Anderson type disorder in long semiconductor carbon nanotubes (CNTs) to electron charge conductivity and lattice thermal conductivity using the atomistic Green function approach. The electron and phonon transmissions are analyzed as a function of the length of the disordered nanostructures. The thermal conductance as a function of temperature is calculated for different lengths. Analysis of the transmission probabilities as a function of length of the disordered device shows that both electrons and phonons with different energies display different transport regimes, i.e. quasi-ballistic, diffusive and localization regimes coexist. In the light of the results we discuss heating of the semiconductor device in electronic applications.

cond-mat.mes-hall↗

arXiv2009

An exactly solvable model of a highly efficient thermoelectric engine

We propose a simple classical dynamical model of a thermoelectric (or thermochemical) heat engine based on a pair of ideal gas containers connected by two unequal scattering channels. The model is solved analytically and it is shown that a suitable combination of parameters can be chosen such that the engine operates at Carnot's efficiency.

cond-mat.stat-mech↗

arXiv2010

Transport Spectroscopy of Single Phosphorus Donors in a Silicon Nanoscale Transistor

We have developed nano-scale double-gated field-effect-transistors for the study of electron states and transport properties of single deliberately-implanted phosphorus donors. The devices provide a high-level of control of key parameters required for potential applications in nanoelectronics. For the donors, we resolve transitions corresponding to two charge states successively occupied by spin down and spin up electrons. The charging energies and the Lande g-factors are consistent with expectations for donors in gated nanostructures.

cond-mat.mes-hall↗

arXiv2009

Comment on "Entanglement on Demand through Time Reordering"

In a recent Letter [Phys. Rev. Lett. {\bf 100}, 120501 (2008)], Avron {\em et al.} discuss a time reordering scheme to achieve efficient "across-generation" of entangled photon pairs in a semiconductor quantum dot with a suppressed biexciton binding energy. They demonstrate that the scheme can be implemented using time delay between the generated photons. In this Comment, we derive an exact expression for the concurrence of the time delayed photons, and show that the predicted values by Avron {\em et al.} are not achievable, which stems from an approximation used in their approximate theoretical analysis.

cond-mat.mes-hall↗

arXiv2009

Manipulation of double-dot spin qubit by continuous noisy measurement

We consider evolution of a double quantum dot (DQD) two-electron spin qubit which is continuously weakly measured with a linear charge detector (quantum point contact). Since the interaction between the spins of two electrons depends on their charge state, the charge measurement affects the state of two spins, and induces non-trivial spin dynamics. We consider the regimes of strong and weak coupling to the detector, and investigate the measurement-induced spin dynamics both analytically and numerically. We observe emergence of the negative-result evolution and the system stabilization due to an analog of quantum Zeno effect. Moreover, unitary evolution between the triplet and a singlet state is induced by the negative-result measurement. We demonstrate that these effects exist for both strong and weak coupling between the detector and the DQD system.

cond-mat.mes-hall↗

arXiv2009

The influence of the discretized Rashba spin-orbit interaction on the Harper model

The movement of the electrons under the simultaneous influence of a scalar periodic potential and of a uniform transversal magnetic field is described by the well-known second order discrete Harper equation. This equation originates from a two-dimensional energy dispersion law under the minimal substitution. Here one deals with the Harper model under the additional influence of the discretized spin orbit interaction. Converting the spin-orbit interaction in terms of discrete derivatives opens the way for analytical and numerical studies. One finds coupled equations for the spin dependent wave functions, which leads to an appreciable alteration of the nested energy subbands characterizing the self-similar structure of the usual Harper spectrum. To this aim the transfer matrix method has been applied to selected spin-up and spin-down wavefunctions. Accordingly, very manifestations of spinfiltering and of spin correlations are accounted for. Our energy-bands calculations show that the splitting effect implemented by such wavefunctions is appreciable.

cond-mat.mes-hall↗

arXiv2009

Reduced Thermal Conductivity of Nanowires and Nanoribbons with Dynamically Rough Surfaces and the "Problem of One-Dimensional Heat Conductors"

We present analytical model and molecular dynamics simulations of phonon heat transport in nanowires and nanoribbons with anharmonic lattices and dynamically rough surfaces and edges. In agreement with recent experiments on heat transport in single-crystalline silicon nanowires with rough surfaces, our model and simulations predict finite and length-independent phonon thermal conductivity in such quasi-one-dimensional systems, in contrast to anomalous phonon thermal conductivity of corresponding momentum-conserving systems with atomically smooth surfaces, divergent with the system length. Within our model, the main cause of thermal conductivity reduction is momentum-nonconserving scattering of longitudinal acoustic phonons by anharmonic side phonon leads in quasi-one-dimensional phonon waveguide with dynamically rough surface or edge layers.

cond-mat.stat-mech↗

arXiv2009

Topological Peierls Transitions in Möbius Molecular Devices

We study the topological properties of Peierls transitions in a monovalent Möbius ladder. Along the transverse and longitudinal directions of the ladder, there exist plenty Peierls phases corresponding to various dimerization patterns. Resulted from a special modulation, namely, staggered modulation along the longitudinal direction, the ladder system in the insulator phase behaves as a ``topological insulator'', which possesses charged solitons as the gapless edge states existing in the gap. Such solitary states promise the dispersionless propagation along the longitudinal direction of the ladder system. Intrinsically, these non-trivial edges states originates from the Peierls phases boundary, which arises from the non-trivial $\mathbb{Z}^{2}$ topological configuration.

cond-mat.mes-hall↗

arXiv2009

Stoner ferromagnetic phase of a graphene in the presence of an in-plane magnetic field

We study the effects of an in-plane magnetic field on the ground state properties of both gapless and gapped graphene sheets within Random Phase Approximation. The critical magnetic field which leads to a fully spin polarized phase increases by decreasing the carrier density at zero gap indicating that no spontaneous magnetic phase transition occurs. However, at large energy gap values it decreases by decreasing the density. We find a continuous quantum magnetic phase transition (Stoner phase) for Dirac fermions in a doped graphene sheet. Novel in-plane magnetic field dependence of the charge and spin susceptibilities are uncovered.

cond-mat.mes-hall↗

arXiv2011

Sum rule for a Schiff-like dipole moment

The energy-weighted sum rule for an electric dipole transition operator of a Schiff type differs from the Thomas-Reiche-Kuhn sum rule by several corrective terms which depend on the number of system components, ${\cal N}$. The deviations are evaluated by two distinct approaches. One of them is based on the charge density expansion around an uniform distribution and provides an analytical ${\cal N}$ dependence for the sum rule while, the other one hinges of the RPA approach and yields compact expressions for corrections as function of the RPA amplitudes. Although the formalism might be considered for any many body system, we applied it for illustration, to the case of $Na$ clusters. One concludes that the RPA results for $Na$ clusters obey the modified TRK sum rule.

cond-mat.mes-hall↗

arXiv2009

Fast generation of entangled photon pairs from a single quantum dot embedded in a photonic crystal cavity

We present a scheme for the fast generation of entangled photons from a single quantum dot coupled to a planar photonic crystal that support two orthogonally polarized cavity modes. We discuss ``within generation'' and ``across generation'' of entangled photons when both biexciton to exciton, and exciton to ground state transitions, are coupled through cavity modes. In the across generation, the photon entanglement is restored through a time delay between the photons. The two photon concurrence, which is a measure of entanglement, is greater than 0.7 and 0.8 using experimentally achievable parameters in across generation and within generation, respectively. We also show that the entanglement can be distilled in both cases using a simple spectral filter.

cond-mat.mes-hall↗