Fluctuating and single particles conductivity channels in TSF-TCNQ

We measured the conductivity of TSF-TCNQ in the pressure domain surrounding the commensurability point. We extracted a temperature dependent quantity related to the fluctuating part of the conductivity which appears to follow the 2k_F distortion development, except below 100 K where pinning due to 2-D couplings appears.We suggest a decomposition of the conductivity which shows that the fluctuating channel plays an important but, in contrast with TTF-TCNQ, non-dominant role in the conductivity peak of TSF-TCNQ.     

D = 5 Einstein-Maxwell-Chern-Simons black holes

Five-dimensional Einstein-Maxwell-Chern-Simons theory with a Chern-Simons coefficient lambda = 1 has supersymmetric black holes with a vanishing horizon angular velocity but finite angular momentum. Here supersymmetry is associated with a borderline between stability and instability, since for lambda > 1 a rotational instability arises, where counterrotating black holes appear, whose horizon rotates in the opposite sense to the angular momentum. For lambda > 2 black holes are no longer uniquely characterized by their global charges, and rotating black holes with vanishing angular momentum appear.     

Electron transport in dirty multi-channel systems

We study the temperature dependence of the conductance and of the conductivity of a doped two-chain ladder system in the presence of a barrier or of a low impurity concentration, respectively, focusing on the effects of electronelectron interaction. Like in the purely 1-D case, the conductance vanishes at low temperature when the Luttinger-liquid exponent Kρ+ (for even-charge modes) is smaller than unity, despite the presence of dominant superconducting correlations. However, there is a region of repulsive interaction where perfect transmission across the barrier occurs. This is due to a renormalization of Kρ+ produced by the difference in Fermi velocities of the bonding and antibonding bands.     

Noncritical holographic QCD in external electric field

We investigate behavior of a noncritical model in external electric field and explore its phase structure in the quenched approximation Nf?Nc. We compute the conductivity of QCD plasma in this model and compare it with the predictions of Sakai-Sugimoto model, D3-D7 system and lattice simulations. We find that, while the behavior of conductivity in noncritical model as a function of temperature and baryon density is similar to those of D3-D7 system, the phase diagram of noncritical model resembles the phase diagram of Sakai-Sugimoto model.     

Nonequilibrium Steady States of Some Simple 1-D Mechanical Chains

We study nonequilibrium steady states of some 1-D mechanical models with N moving particles on a line segment connected to unequal heat baths. For a system in which particles move freely, exchanging energy as they collide with one another, we prove that the mean energy along the chain is constant and equal to \(\frac{1}{2} \sqrt{T_{L}T_{R}}\) where T _L and T _R are the temperatures of the two baths. We then consider systems in which particles are trapped, i.e., each confined to its designated interval in the phase space, but these intervals overlap to permit interaction of neighbors. For these systems, we show numerically that the system has well defined local temperatures and obeys Fourier’s Law (with energy-dependent conductivity) provided we vary the masses randomly to enable the repartitioning of energy. Dynamical systems issues that arise in this study are discussed though their resolution is beyond reach.     

On the conductivity of composites with two-dimensional periodic structure

A method is suggested of successive solution of the problem on the conductivity of two-dimensional periodic systems with inclusions of arbitrary shape. The complex potential outside of the inclusions is expressed in terms of the Weierstrass zeta function and its derivatives. The field induced on a separate inclusion is described using the matrix of multipole polarizabilities. The “joining” of potentials is performed at a distance such that R < ρ < a, where R is the characteristic dimension (maximum “radius”) of the inclusion and a is the half-period of the lattice. The approach suggested enables one to find exact virial expansions for the conductivity of other effective characteristics of similar systems as well.     

Anomaly in the lattice partition function

The lattice partition function Z(T)=σ_(Si) exp(−H/k_BT), where H, k_B and T are the Hamiltonian of the system, the Boltzmann constant and the absolute temperature, respectively, leads to a vanishing spontaneous magnetisation for all temperatures, independently of the lattice considered. This feature is related to the symmetry breaking in these systems. In comparing this relation to the well-known partition function Z(T)=σ_n exp(−E_n/k_BT) where E_n is energy, we observe an incompatibility which could be the reason that this partition function leads to a vanishing spontaneous magnetisation.     

Three-dimensional acoustic sound field reproduction based on hybrid combination of multiple parametric loudspeakers and electrodynamic subwoofer

Auditory Mixed Reality (MR) systems that reproduce Three-Dimensional (3-D) acoustic sound fields have recently become a research focus because the combination of visual and auditory MR systems can achieve a greater sense of presence than conventional visual MR systems. General auditory MR systems usually use a headphone-based system with a Head-Related Transfer Function (HRTF), which is a major system for reproducing 3-D acoustic sound fields. However, the localization accuracy of sound images with a HRTF depends on the individual. On the other hand, we have already proposed a system for reproducing a 3-D acoustic sound field with parametric loudspeakers instead of headphones. The 3-D acoustic sound field reproduced by this system has achieved a highly accurate localization of sound images. However, one problem is that it is difficult to reproduce lower frequency sounds using parametric loudspeakers, which causes a poorer sound quality. We tried to accomplish a greater sense of presence for 3-D acoustic sound fields based on a hybrid combination of an electrodynamic subwoofer and the parametric loudspeakers by improving the sound quality. Sound images were formed at the target location using the parametric loudspeakers, and a lower frequency sound was compensated for by using the electrodynamic subwoofer. Subjective evaluation experiments were conducted to verify the effectiveness of the proposed system. We confirmed the improved sound quality while maintaining a higher accuracy of sound image localization by using the proposed system. We also confirmed the optimum parameters of the proposed system to achieve a greater sense of presence.     

Optical Hall conductivity of systems with gapped spectral nodes

We calculate the optical Hall conductivity within the Kubo formalism for systems with gapped spectral nodes, where the latter have a power-law dispersion with exponent n. The optical conductivity is proportional to n and there is a characteristic logarithmic singularity as the frequency approaches the gap energy. The optical Hall conductivity is almost unaffected by thermal fluctuations and disorder for n = 1, whereas disorder has a stronger effect on transport properties if n = 2.     

Ward Identities and Vanishing of the Beta Function for d = 1 Interacting Fermi Systems

We give a self consistent and simplified proof of the (asymptotic) vanishing of the Beta function in d=1 interacting Fermi systems as a consequence of a few properties deduced from the exact solution of the Luttinger model. Moreover, since the vanishing of the Beta function is usually “proved” in the physical literature through heuristic arguments based on Ward identities, we briefly discuss here also the possibility of exploiting this idea in a rigorous approach, by using a suitable Dyson equation. We show that there are serious difficulties, related to the presence of corrections (for which we get careful bounds), which are usually neglected.     

Effect of the hexagonal warping on the dynamical conductivity of surface states in a topological insulator

We report on the effect of hexagonal warping on the dynamical conductivity of the surface states of a topological insulator in the presence of nonmagnetic impurities. It is found that the photon energy dependent conductivities are determined by a polarization-function-liked term,  Π₂ (q,ω), which contains a velocity term corresponding to the difference of group velocities between the two states due to an electron-impurity scattering. This is different from the conductivity of 2-dimentional electron systems where the conductivity depends on the inverse imaginary part of the dielectric function Im [1/κ(q,ω)]. We present both the real part and imaginary part of the polarization function with different warping strength. It is found that the warping strength can both enhance single particle excitations (SPEs) and suppress the screening effect of electrons. As a result the inverse scattering time is enhanced by up to about two orders of magnitudes. The real part of the longitudinal conductivity of the intra-band process is analog to the case with a conductivity of σ ~ μδ(ω). The broadening of the spectrum in the low energy is not only determined by chemical potential, but also dependent on the warping strength. At higher frequency, the real part of conductivity shows a jump at the threshold photon energy of μ, where the inter-band contribution takes over.     

Thermal conductivity of nonlinear waves in disordered chains

We present computational data on the thermal conductivity of nonlinear waves in disordered chains. Disorder induces Anderson localization for linear waves and results in a vanishing conductivity. Cubic nonlinearity restores normal conductivity, but with a strongly temperature-dependent conductivity κ(T). We find indications for an asymptotic low-temperature κ ∼ T ⁴ and intermediate temperature κ ∼ T ² laws. These findings are in accord with theoretical studies of wave packet spreading, where a regime of strong chaos is found to be intermediate, followed by an asymptotic regime of weak chaos (Laptyeva et al, Europhys. Lett. 91, 30001 (2010)).     

A schematic model discussion of the conductor to insulator transition for particle motion in a random potential

A schematic model for the dynamics of a particle moving in a random environment is discussed where the latter is assumed to cause normal scattering and also scattering with time inversion symmetry breaking. The second mechanism changes the exponent unity to one half for the vanishing of the conductivity near the percolation threshold. It alters also the exponent of the critical conductivity spectrum from 1/2 to 1/3. The dynamics near the transition including all cross over phenomena to the usual effective medium theory results are described by a two parameter scaling law.Dedicated to Professor W. Brenig on the occasion of his 60th birthday     

Lattice electrons on a cylinder surface in the presence of rational magnetic flux and disorder

We consider a disordered two-dimensional system of independent lattice electrons in a perpendicular magnetic field with rigid confinement in one direction and generalized periodic boundary conditions (GPBC) in the other direction. The objects investigated numerically are the orbits in the plane spanned by the energy eigenvalues and the corresponding center of mass coordinate in the confined direction, parameterized by the phase characterizing the GPBC. The Kubo Hall conductivity is expressed in terms of the winding numbers of these orbits. For vanishing disorder the spectrum of the system consists of Harper bands with energy levels corresponding to the edge states within the band gaps. Disorder leads to broadening of the bands. For sufficiently large systems localized states occur in the band tails. We find that within the mobility gaps of bulk states the Diophantine equation determines the value of the Hall conductivity as known for systems with torus geometry (PBCs in both directions). Within the spectral bands of extended states the Hall conductivity fluctuates strongly. For sufficiently large systems the generic behavior of localization-delocalization transitions characteristic for the quantum Hall effect are recovered.     

Chains of oscillators with negative stiffness elements

Negative stiffness is not allowed by thermodynamics and hence materials and systems whose global behaviour exhibits negative stiffness are unstable. However the stability is possible when these materials/systems are elements of a larger system sufficiently stiff to stabilise the negative stiffness elements. In order to investigate the effect of stabilisation we analyse oscillations in a chain of n linear oscillators (masses and springs connected in series) when some of the springs? stiffnesses can assume negative values. The ends of the chain are fixed. We formulated the necessary stability condition: only one spring in the chain can have negative stiffness. Furthermore, the value of negative stiffness cannot exceed a certain critical value that depends upon the (positive) stiffnesses of other springs. At the critical negative stiffness the system develops an eigenmode with vanishing frequency. In systems with viscous damping vanishing of an eigenfrequency does not yet lead to instability. Further increase in the value of negative stiffness leads to the appearance of aperiodic eigenmodes even with light damping. At the critical negative stiffness the low dissipative mode becomes non-dissipative, while for the high dissipative mode the damping coefficient becomes as twice as high as the damping coefficient of the system. A special element with controllable negative stiffness is suggested for designing hybrid materials whose stiffness and hence the dynamic behaviour is controlled by the magnitude of applied compressive force.     

Investigation of Q-S synchronization in coupled chaotic incommensurate fractional order systems

Chaos and synchronization in fractional order systems have received increasing attention in recent years. In this paper, the problem of Q-S synchronization for different dimensional incommensurate fractional order chaotic systems is investigated. Based on Laplace transform and stability theory of linear integer order differential systems, some synchronization schemes are designed to achieve Q-S synchronization between n-D and m-D incommensurate fractional order chaotic systems. Test problems and numerical simulations are used to show the effectiveness of the proposed approach.     

Hall Conductivity in the presence of repulsive magnetic impurities

The Hall conductivity of disordered magnetic systems consisting of hard-core point vortices randomly dropped on the plane with a Poissonian distribution, has a behavior analogous to the one observed experimentally by Haug, Gerhardts, Klitzling and Ploog, with repulsive scatterers [#!1!#]. We also argue that models of homogeneous magnetic field with disordered potential, have necessarily vanishing Hall conductivities when their Hilbert space is restricted to a given Landau level subspace. Received: 29 September 1998 / Accepted: 21 December 1998     

Commensurability and fluctuating conductivity in the organic conductor TSF-TCNQ

We report the pressure dependence of the phase transition in TSF-TCNQ as determined by resistivity measurements. We find a narrow pressure domain centered on 6.25 kbar where the transition temperature peaks above a monotonous pressure dependence. We suggest that there is a commensurate × 3 superlattice in this pressure regime resulting from an increase in charge transfer from 0.63 under ambient pressure to $\text{2}\text{3}$.A drop of longitudinal conductivity related to the × 3 commensurability is visible in the same temperature region where X-ray diffuse scattering data show 1-D features for the precursor fluctuations. We suggest that like TTF-TCNQ, the metallic conductivity of TSF-TCNQ is also influenced by the fluctuating Fröhlich mode.     

The enigma of the quantum Hall effect in graphene

We apply Laughlin’s gauge argument to analyze the ν=0 quantum Hall effect observed in graphene when the Fermi energy lies near the Dirac point, and conclude that this necessarily leads to divergent bulk longitudinal resistivity in the zero temperature thermodynamic limit. We further predict that in a Corbino geometry measurement, where edge transport and other mesoscopic effects are unimportant, one should find the longitudinal conductivity vanishing in all graphene samples which have an underlying ν=0 quantized Hall effect. We argue that this ν=0 graphene quantum Hall state is qualitatively similar to the high field insulating phase (also known as the Hall insulator) in the lowest Landau level of ordinary semiconductor two-dimensional electron systems. We establish the necessity of having a high magnetic field and high mobility samples for the observation of the divergent resistivity as arising from the existence of disorder-induced density inhomogeneity at the graphene Dirac point.     

Electron localization in the quantum Hall regime

The theory of the insulating state discriminates between insulators and metals by means of a localization tensor, which is finite in insulators and divergent in metals. In absence of time-reversal symmetry, this same tensor acquires an off-diagonal imaginary part, proportional to the dc transverse conductivity, leading to quantization of the latter in two-dimensional systems. I provide evidence that electron localization--in the above sense--is the common cause for both vanishing of the dc longitudinal conductivity and quantization of the transverse one in quantum Hall fluids.