International Journal of Modern Physics B

We study charge transport in a source-channel-drain system with a time-varying applied gate potential acting on the channel. We calculate both the current flowing from the source into channel and out of the channel into the drain. The current is expressed in terms of nonequilibrium Green’s functions. These nonequilibrium Green’s functions can be determined from the steady-state Green’s functions and the equilibrium Green’s functions of the free leads. We find that the application of the gate potential can induce current to flow even when there is no source-drain bias potential. However, the direction of the current from the source and the current to the drain are opposite, thereby resulting in no net current flowing within the channel. When a source-drain bias potential is present, the net current flowing to the source and drain can either be attenuated or amplified depending on the sign of the applied gate potential. We also find that the response of the system to a dynamically changing gate potential is not instantaneous, i.e., a relaxation time has to pass before the current settles into a steady value. In particular, when the gate potential is in the form of a step function, the current first overshoots to a maximum value, oscillates and then settles down to a steady-state value.

In this work, the influence of boron atom impurity is investigated on the electronic properties of a single-wall carbon nanotube superlattice which is connected by pentagon–heptagon topological defects along the circumference of the heterojunction of these superlattices. Our calculation is based on tight-binding $\pi$-electron method in nearest-neighbor approximation. The density of states (DOS) and electronic band structure in presence of boron impurity has been calculated. Results show that when boron atom impurity and nanotube atomic layers have increased, electronic band structure and the DOS have significant changes around the Fermi level.

Currently, the stability and visible light properties of Ga-2N co-doped ZnO systems have been studied extensively by experimental analysis and theoretical calculations. However, previous theoretical calculations arbitrarily assigned Ga- and 2N-doped sites in ZnO. In addition, the most stable and possible doping orientations of doped systems have not been fully and systematically considered. Therefore, in this paper, the electron structure and absorption spectra of the unit cells of doped and pure systems were calculated by first-principles plane-wave ultrasoft pseudopotential with the GGA$+$U method. Calculations were performed for pure ZnO, Ga-2N supercells heavily co-doped with Zn${}_{1-x}$Ga${}_x$O${}_{1-y}$N${}_y$ ($x=0.03125-0.0625$, $y=0.0625-0.125$) under different co-doping orientations and conditions, and the Zn${}_{16}$GaN₂O${}_{14}$ interstitial model. The results indicated that under different orientations and constant Ga-2N co-doping concentrations, the systems co-doped with Ga-N atoms vertically oriented to the $c$-axis and with another N atom located in the nearest-neighboring site exhibited higher stability over the others, thus lowering formation energy and facilitating doping. Moreover, Ga-interstitial- and 2N-co-doped ZnO systems easily formed chemical compounds. Increasing co-doping concentration while the co-doping method remained constant decreased doped system volume and lowered formation energies. Meantime, co-doped systems were more stable and doping was facilitated. The bandgap was also narrower and red shifting of the absorption spectrum was more significant. These results agreed with previously reported experimental results. In addition, the absorption spectra of Ga-interstitial- and 2N-co-doped ZnO both blue shifted in the UV region compared with that of the pure ZnO system.

For investigating the pressure dependence of thermal expansivity of materials, we have developed a formulation using the Stacey relationship between the reciprocal of pressure derivative of bulk modulus and the ratio of pressure and bulk modulus. The formulation presented here satisfies the boundary conditions both at zero pressure and also in the limit of infinite pressure at extreme compression. A physically acceptable relationship has been obtained between the volume derivative of thermal expansivity and the pressure derivatives of bulk modulus under adiabatic condition. The seismological data for the Earth lower mantle have been used to demonstrate the validity of the relationship between thermoelastic properties derived in the present study.

First-principles study of elastic, electronic and optical properties of full-Heusler Co₂V(Al, Ge, Ga and Si) compounds are calculated through density functional theory (DFT) to obtain and compare the mentioned properties. Equilibrium lattice constants of these compounds are in good agreement with other works. Electronic calculations are shown full spin polarization at Fermi level for all compounds, so in the down spin, indirect bandgap is calculated as 0.33, 0.6, 0.2 and 0.8 eV for Co₂V(Al, Ge, Ga and Si), respectively. The integer amounts of the magnetic moments are compatible with Slater–Pauling role. The optical treatment of Co₂VGa is different from three other compounds. All mentioned compounds have metallic behavior by 22 eV plasmonic frequency. The imaginary part of the dielectric function for the up spin indicates that the main optical transitions occurred in this spin mode. Moreover, the elastic results show that the Co₂VGa does not have elastic stability, but the other three compounds have fully elastic stability and the Co₂V(Al, Ge and Si) belong to the hardness of materials.

Growth of Sb/SiN${}_x$ multilayers, followed by high-temperature annealing, was shown to be an effective strategy for synthesizing Sb-doped Si quantum dots (Si-QDs). The doping concentration of Sb (from 0.32 at.% to 1.82 at.%) was controlled by varying the thickness of Sb sublayer. Moderate Sb concentrations were found to enhance the formation of Si-QDs. Photoluminescence (PL) results show that the nonradiative recombination defects caused by Sb impurities increased with the increase of Sb content, which leads to the decrease of the emission intensity. Hall measurements demonstrate that as the carrier concentration increases, the mobility of Hall decreases, and the conductivity increases at first and then decreases with the increase of Sb content. It is indicated that the excessive high Sb doping will deteriorate the conductive properties. The observed $n$-type electrical behavior and great enhancement increase in conductivity of the Sb-doped Si-QDs film suggest an effective Sb doping. A $p$–$n$ junction was formed between Sb-doped Si-QDs and a $p$-type c-Si substrate, exhibiting good rectifying properties.

Density functional theory (DFT) and generalized gradient approximation (GGA) have been employed to study origins of the intrinsic $n$-type electrical conductivity in the zinc oxide. Hubbard-like term has been introduced to provide a better description for the Zn 3$d$ electrons. Two intrinsic point defects, namely oxygen vacancy and hydrogen impurity, were taken into consideration. Results on conductivity are analyzed using density of states patterns for different configurations of defects. Microstructure and local magnetic moments are studied as well. The obtained results clearly indicate that oxygen vacancy does not and cannot be responsible for the intrinsic $n$-type electrical conductivity whereas inserted hydrogen atoms tend to lose its only valence electron, which in turn becomes a free electron contributing towards the $n$-type conductivity.

Using a model spectral function of the electron, the Hall conductivity in the normal metallic state of the Pr${}_{2-x}$Ce_xCuO₄ (PCCO) superconductor is calculated neglecting the current vertex-correction. The result is qualitatively consistent with the experiment. Consequently, the reason becomes clear why the Fermi-liquid theory fails to explain the anomaly of the Hall conductivity. The inconsistency of the fluctuation-exchange approximation also becomes clear.

This paper investigates a new asymmetric bistable model driven by correlated multiplicative colored noise and additive white noise. The mean first-passage time (MFPT) and the signal-to-noise ratio (SNR) as the indexes of evaluating the model are researched. Based on the two-state theory and the adiabatic approximation theory, the expressions of MFPT and SNR have been obtained for the asymmetric bistable system driven by a periodic signal, correlated multiplicative colored noise and additive noise. Simulation results show that it is easier to generate stochastic resonance (SR) to adjust the intensity of correlation strength $\lambda$. Meanwhile, the decrease of asymmetric coefficient $r_2$ and the increase of noise intensity are beneficial to realize the transition between the two steady states in the system. At the same time, the twice SR phenomena can be observed by adjusting additive white noise and correlation strength. The influence of asymmetry of potential function on the MFPTs in two different directions is different.

We study the transport phenomena in layered conductors with rather general electron energy spectrum placed in a high magnetic field $H$, under conditions when the distance between various sheets of the Fermi surface (FS) may become small under the external effects, such as hydrostatic pressure or impurity atom doping, and electrons can transfer from one sheet of the FS to another due to magnetic breakdown. We calculate the dependence of the in-plane electrical conductivity and magnetoresistance on magnetic field and probability of magnetic breakdown and show that the field-induced quadratic increase of the in-plane resistance in the absence of magnetic breakdown is changed by a linear dependence on $H$. With a further reduction of the energy gap between FS sheets, the in-plane resistance is saturated.

A series of studies of the lowest ${}^{3}P$ state of a model of a two-electron quantum dot, that consists of a finite spherical well [see, for example, Sivakami et al., Int. J. Mod. Phys. B 24, 5561 (2010)], exhibited a range of interesting applications. However, the quantity of central interest, the expectation value of the interelectronic repulsion (that must be nonnegative) was calculated erroneously, yielding negative values. In the present note, this expectation value is evaluated with respect to the exact first-order perturbation theory wavefunction (the scheme considered in the papers cited above) as well as with respect to a minimal basis set Gaussian variational wavefunction. Reasonable agreement between the two schemes is confirmed, both yielding the expected positive values of the expectation value of the interelectronic repulsion, over the whole range of the relevant parameters.

We consider the semiclassical model of an extended tight-binding Hamiltonian comprising nearest- and next-to-nearest-neighbor interactions for a charged particle hopping in a lattice in the presence of a static arbitrary field and a rapidly oscillating uniform field. The application of Kapitza’s method yields a time-independent effective Hamiltonian with long-range hopping elements that depend on the external static and oscillating fields. Our calculations show that the semiclassical approximation is quite reliable as it yields, for a homogeneous oscillating field, the same effective hopping elements as those derived within the quantum approach. Besides, by controlling the oscillating field, we can engineer the interactions so as to suppress the otherwise dominant interactions (nearest neighbors) and leave as observable effects those due to the otherwise remanent interactions (distant neighbors).

From femtosecond spectroscopy (fs-spectroscopy) of metals, electrons and phonons reequilibrate nearly independently, which contrasts with models of heat transfer at ordinary temperatures ($T>100$ K). These electronic transfer models only agree with thermal conductivity $(k)$ data at a single temperature, but do not agree with thermal diffusivity $(D)$ data. To address the discrepancies, which are important to problems in solid state physics, we separately measured electronic (ele) and phononic (lat) components of $D$ in many metals and alloys over $\sim$290–1100 K by varying measurement duration and sample length in laser-flash experiments. These mechanisms produce distinct diffusive responses in temperature versus time acquisitions because carrier speeds $(u)$ and heat capacities $(C)$ differ greatly. Electronic transport of heat only operates for a brief time after heat is applied because $u$ is high. High $D_{\mathrm{\text{ele}}}$ is associated with moderate $T$, long lengths, low electrical resistivity, and loss of ferromagnetism. Relationships of $D_{\mathrm{\text{ele}}}$ and $D_{\mathrm{\text{lat}}}$ with physical properties support our assignments. Although $k_{\mathrm{\text{ele}}}$ reaches $\sim 20\times k_{\mathrm{\text{lat}}}$ near 470 K, it is transient. Combining previous data on $u$ with each $D$ provides mean free paths and lifetimes that are consistent with $\sim 298$ K fs-spectroscopy, and new values at high $T$. Our findings are consistent with nearly-free electrons absorbing and transmitting a small fraction of the incoming heat, whereas phonons absorb and transmit the majority. We model time-dependent, parallel heat transfer under adiabatic conditions which is one-dimensional in solids, as required by thermodynamic law. For noninteracting mechanisms, $k\cong \mathrm{\Sigma }{C}_i{k}_i\mathrm{\Sigma }{C}_i/(\mathrm{\Sigma }{C}_i^2)$. For metals, this reduces to $k=k_{\mathrm{\text{lat}}}$ above $\sim$20 K, consistent with our measurements, and shows that Meissner’s equation $(k\cong k_{\mathrm{\text{lat}}}+k_{\mathrm{\text{ele}}})$ is invalid above $\sim$20 K. For one mechanism with multiple, interacting carriers, $k\cong \mathrm{\Sigma }{C}_i{k}_i/(\mathrm{\Sigma }{C}_i)$. Thus, certain dynamic behaviors of electrons and phonons in metals have been misunderstood. Implications for theoretical models and technological advancements are briefly discussed.