We present a perspective of simple models of nonequilibrium directed transport described in terms of a Langevin equation formalism. We consider a Brownian particle under various circumstances and driven by thermal (equilibrium) and non-thermal (active) fluctuations. Three examples of startling behavior are unveiled: giant transport, multiple current reversal and negative mobility.
The temperature dependence of photoconductivity in p-GaSe crystals with different initial (having at 77 K) dark resistivities (ρ77 = 2·103 ÷ 7·106 Ω·cm) was experimentally studied in the temperature range of 77 ÷ 300 K. It has been established that in crystals with ρ77 < 104 Ω cm, only the value of the photocurrent changes depending on temperature. At T ≤ 250 K, in the higher-resistivity crystal, the spectral distribution, lux-ampere characteristic, as well as photoconductivity kinetics also change noticeably with a change in temperature. The obtained experimental results are explained on the basis of a model of crystalline semiconductor with random macroscopic defects.
Temperature dependence of photoconductivity in low- (1) and high-resistance (2) p-GaSe crystals
This work investigates the emergence of structural super-diffusion in finite multiplex networks, focusing on situations where long-range interactions (LRIs) are present in at least one of the layers. Employing the Lévy random walk model, we explore how the likelihood of observing LRIs and the strength of the coupling among layers, specifically through the inter-layer diffusion coefficient, affect the relaxation time of the entire multiplex network. Our aim is to determine if this collective relaxation time is shorter compared to that in isolated layer configurations. We quantify the relaxation times through algebraic connectivity, the second-smallest eigenvalue of the network’s (Supra-)Laplacian matrix. The formalism is adapted to scenarios where long-range jumps may exist by considering Mellin or Laplace transforms. We study samples of multiplexes whose layers are obtained from the models by Erdös-Rény, Watts-Strogatz and Barabási-Albert, and discuss in great detail the results for multiplexes with two layers. Besides the rather common enhancement of multiplex diffusion in comparison to the diffusion observed in their isolated layers, our results show that the timescale of layers with LRIs may be still faster than that due only to the usual multiplex network, speeding-up the whole system’s diffusion. In addition, we also provide enough evidences that a novel linear diffusion regime emerges with strong inter-layer coupling and the presence of LRIs in one or more layers.
Graphical Abstract highlights the main focus of the work, the occurrence of superdiffusion in a multiplex with (M=2) layers. The connectivity ratio (eta = Lambda _2/textrm{max}(lambda ^1_2,lambda ^2_2)) (red line) crosses the dashed line (eta =1) at a well defined value of (mathrm {D_x}), and remains in the super-diffusion region highlighted in green
Thermal heating process has been used, under a pressure of 10−7 mbar, to deposit thin films of cobalt onto monocrystalline silicon substrate. The incident beam strikes the substrates under normal incidence. The thickness ranges from 50 to 400 nm. To investigate the static magnetic properties, the hysteresis loops are performed by means of alternating gradient field magnetometer device. The zero-field magnetic structure has been investigated by magnetic force microscopy (MFM) technique, using a Veeco 3100 apparatus. MFM images reveal well-defined stripe patterns, mainly for the thickest films, which infer the dominance of the magnetocrystalline anisotropy. The easy magnetization axis lies within the plane of the film with a small component out-of-the plane for the thicker films. Coercivity evolution versus thickness follows a t−n Néel law, characteristic of Bloch domain wall motion. In addition, it was found that coercivity depends plainly on the surface roughness of the films.
AFM (left) and MFM (right) images of Co thin films, with different thickness, (a) 100 nm, (b) 300 nm, (c) 400 nm. Scan area is 20 µm × 20 µm
We consider a free-fermion chain undergoing dephasing, described by two different random-measurement protocols (unravelings): a quantum-state-diffusion and a quantum-jump one. Both protocols keep the state in a Slater-determinant form, allowing to address quite large system sizes. We find a bifurcation in the distribution of the measurement operators along the quantum trajectories, that’s to say, there is a point where the shape of this distribution changes from unimodal to bimodal. The value of the measurement strength where this phenomenon occurs is similar for the two unravelings, but the distributions and the transition have different properties reflecting the symmetries of the two measurement protocols. We also consider the scaling with the system size of the inverse participation ratio of the Slater-determinant components and find a power-law scaling that marks a multifractal behaviour, in both unravelings and for any nonvanishing measurement strength.
Position of the maxima of P n vs (gamma ) for the QSD protocol. The two maxima stem continuously and symmetrically at the bifurcation point (gamma ) QSD (sim 0.2), with a discontinuity of the derivative.
We use entanglement witnesses related to the entanglement negativity of the state to detect entanglement in the XY chain in the postquench states in the thermodynamic limit after a quench when the parameters of the Hamiltonian are changed suddenly. The entanglement negativity is related to correlations, which in the postquench stationary state are described by a generalized Gibbs ensemble, in the ideal case. If, however, integrability breaking perturbations are present, the system is expected to thermalize. Here, we compare the nearest-neighbor entanglement in the two circumstances.
Postquench states after a sudden quench protocol ((h_0, gamma ) rightarrow (h, gamma )) in the XY chain.
In the paper, we study the non-Hermitian system under dissipation and give the effective (2times 2) Hamiltonian in the k-space by reducing the (Ntimes N) Hamiltonian in the real space for them. It is discovered that the energy band shows an imaginary line gap. To describe these phenomena, we propose the theory of “non-Hermitian tearing” , in which the tearability we define reveals a continuous phase transition at the exceptional point. The non-Hermitian tearing manifests in two forms — separation of bulk state and decoupling of boundary state. In addition, we also explore the one-dimensional Su–Schrieffer–Heeger model and the Qi–Wu–Zhang model under dissipation using the theory of non-Hermitian tearing. Our results provide a theoretical approach for exploring the controlling of non-Hermitian physics on topological quantum states.
Effects of ballistic transport on the temperature profiles and thermal resistance in nanowires are studied. Computer simulations of nanowires between a heat source and a heat sink have shown that in the middle of such wires the temperature gradient is reduced compared to Fourier’s law with steep gradients close to the heat source and sink. In this work, results from molecular dynamics and phonon Monte Carlo simulations of the heat transport in nanowires are compared to a radiator model which predicts a reduced gradient with discrete jumps at the wire ends. The comparison shows that for wires longer than the typical mean free path of phonons the radiator model is able to account for ballistic transport effects. The steep gradients at the wire ends are then continuous manifestations of the discrete jumps in the model.
The second-order differential equation for the Uehling potential is derived explicitly. The right side of this differential equation is a linear combination of the two Macdonald’s functions (K_{0}(b r)) and (K_{1}(b r)). This central potential is of great interest in many QED problems, since it describes the lowest order correction for vacuum polarization in few- and many-electron atoms, ions, muonic and bi-muonic atoms/ions as well as in other similar systems.
We proposed a formally exact, probabilistic method to assess the validity of the Thomas–Fermi potential for three-dimensional condensed matter systems where electron dynamics is constrained to the Fermi surface. Our method, which relies on accurate solutions of the radial Schrödinger equation, yields the probability density function for momentum transfer. This allows for the computation of its expectation values, which can be compared with unity to confirm the validity of the Thomas–Fermi approximation. We applied this method to three n-type direct-gap III–V model semiconductors (GaAs, InAs, InSb) and found that the Thomas–Fermi approximation is certainly valid at high electron densities. In these cases, the probability density function exhibits the same profile, irrespective of the material under scrutiny. Furthermore, we show that this approximation can lead to serious errors in the computation of observables when applied to GaAs at zero temperature for most electron densities under scrutiny.
The Thomas-Fermi potential (V_textrm{ei}^textrm{TF}left( rright) ) and the the exponential cosine screened Coulomb potential (V_textrm{ei}^textrm{EC}left( rright) ) in coordinate space r, from the full interaction potential (V_textrm{ei}^textrm{RPA}left( qright) ) in the momentum space q at Random Phase approximation, through suitable Fourier transforms
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