The effects of nonequilibrium charge distribution in scanning tunneling spectroscopy of semiconductors

Results are presented from a low-temperature scanning tunneling microscopy (STM) investigation of III-V semiconductor surfaces cleaved in situ along a (110) plane. The STM topographic images reveal the presence of surface charge structures. The possibility of their observation depends on the charge state of the apex of the STM tip. Peaks are also observed in the local tunneling conductivity spectra. The energy position of these peaks and the energy position of the edges of the band gap change with distance from the defect. A theoretical model is proposed which demonstrates that the experimental scanning tunneling spectroscopy (STS) data can be explained by effects due to a nonequilibrium electron distribution in the contact area, which gives rise to localized charges. In this model the on-site Coulomb repulsion of localized charges and their interaction with semiconductor electrons are treated self-consistently. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 299–304 (25 August 1998)     

Smearing of the coulomb blockade by resonant tunneling

We study the Coulomb blockade in a grain coupled to a lead via a resonant impurity level. We show that the strong energy dependence of the transmission coefficient through the impurity level can have a dramatic effect on the quantization of the grain charge. In particular, if the resonance is sufficiently narrow, the Coulomb staircase shows very sharp steps even if the transmission through the impurity at the Fermi energy is perfect. This is in contrast to the naive expectation that perfect transmission should completely smear charging effects.     

Spatial correlations in nonequilibrium systems: The effect of diffusion

We show how the ideas of the fluctuation-dissipation theory can be used to calculate spatial correlations in interacting systems away from equilibrium. The only spatially dependent dissipative process considered is diffusion, and spatial correlations are generated by the nonlocal spatial dependence of the chemical potential. The results are the lowest order in a hierarchy of generalized hydrodynamic type calculations applicable to nonequilibrium systems. We derive equations for the density correlation functions at stationary state supported by diffusive fluxes and show that they have the local equilibrium form. The static correlation function is obtained from the fluctuation-dissipation theorem, which we show to be equivalent to the Ornstein-Zernike integral equation. At equilibrium we demonstrate that the dynamical structure factor obtained by these methods includes the expected wave-vector dependent diffusion constant. Finally we derive a necessary and sufficient condition for local equilibrium to obtain at a stationary state and show by explicit calculation that chemical processes can give rise to significant nonequilibrium correlations.     

Pressure enhancement of charge density wave formation in VSe2; The role of coulomb correlations

We have followed the pressure dependence of the onset and lock-in CDW transitions in VSe₂ by measurements of resistivity and Hall effect up to 30 kbar. (The onset transition appears as a break in the slope of the resistivity, and the lock-in as ≈ 1% temperature hysteresis). Both transition temperatures rise at 0.8 K kbar^?. We attribute this behaviour to pressure broadening of the d conduction band in the presence of strong electron Coulomb repulsion.     

Size effect in the magnetic field dependence of nonequilibrium electron tunneling

The electron bottleneck in normal metal-insulator-metal tunnel junctions due to nonequilibrium effects has been investigated in the presence of a magnetic field. The observed results are due to a new geometrical size effect and are explained in an extension of our nonequilibrium theory.     

GaMnN铁磁共振隧穿二极管自旋电流输运以及极化效应的影响

通过理论计算研究GaMnN铁磁共振隧穿二极管自旋电流输运特性.理论结果表明在电流特性曲线上出现两个明显的自旋分裂峰.该电流自旋分裂峰和相应的自旋极化随温度的升高而逐渐减小消失.当进一步考虑到GaN异质结界面极化电荷影响时,自旋向下的电流共振峰得到明显增强,同时电流的自旋极化也得到相应的提高.在一定的极化电荷条件下,可以获得较高的自旋极化电流. 关键词： GaMnN 共振隧穿 自旋电流 极化电荷     

The influence of impurities on the quasiparticle tunneling current between weakly coupled quasi-one-dimensional charge density waves

The dc and ac conductivity of a tunneling junction between two impure quasi-one-dimensional charge density wave (CDW) systems is calculated. The non-magnetic impurities are considered in the self-consistent Born approximation (SCBA). Impurities modify the density of states (DOS) of the pure CDW system for quasiparticles inside the energy region of the gap 2(T). As in the pure case, the theory predicts in addition to a tunneling current which is proportional to the product of the DOS a term proportional to the cosine of the order parameter phase difference. In the case of a normal state/CDW junction, analytical expressions are obtained forT=0 showing deviations from the pure case. The linear ac conductivity is obtained by the scaling relation between the dc and the ac response.     

The redistribution of charge in naphthalene caused by methyl substitution

The partial rate factors kⁱ for H-D exchange in α, α- and β, β-dimethylnaphthalenes were measured in CF₃COOD-HPO₂F₂-CCl₄ mixtures. The values of log kⁱ correlate well with the excess of electron charge on the carbon atoms calculated on the basis of the inductive effect of the methyl group.     

Charge and spin configurations in the coupled quantum dots with Coulomb correlations induced by tunneling current

We investigated the peculiarities of non-equilibrium charge states and spin configurations in the system of two strongly coupled quantum dots (QDs) weakly connected to the electrodes in the presence of Coulomb correlations. We analyzed the modification of non-equilibrium charge states and different spin configurations of the system in a wide range of applied bias voltage and revealed well pronounced ranges of system parameters where negative tunneling conductivity appears due to the Coulomb correlations.     

The effect of the surface enhanced polariton field on the tunneling current of a STM

The influence of surface polaritons (SPs), excited on the external (fast mode) and internal (slow mode) surfaces of a gold film, on the tunneling current of a STM is investigated. The role of the surface polaritons is directly demonstrated by the angular excitation spectra of the induced tunneling current. Possible mechanisms which might give rise to the SP induced change in the tunneling current are discussed. An electromagnetic field of SPs within a tunneling gap is estimated to be surface enhanced in order to explain the value of the induced tunneling current. Images of the polariton induced signal distribution over the sample surface taken by means of different SP modes are compared in order to obtain information on the structure of the internal surface of the film.     

Spontaneous emission in the optically excited single-electron-tunneling device tuned by the tunneling

The spontaneous emission spectrum in the optically excited single-electron-tunneling device coupled to the side dot is analytically found to be determined by the 12 exciton-complex optical channels with different optical weight functions. The electronic tunneling channels and optical transition channels co-determine the emission, where the competition between the electron-like and hole-like channels leads to the tail effect and the competition between the lower and higher energy resonant optical channels leads to the blueshift（redshift） of the emission signal.     

Magnetic semiconductors controlled by intra-atomic coulomb correlations: The example of HgCr2Se4

We report transport, magnetic and optical properties of the ferromagnetic semiconductor HgCr₂Se₄, as a function of temperature in the range 4.2 K < T < 300 K. The most outstanding properties are a 40% decrease of the effective mass of the conduction band upon cooling, a monotonic decrease of the electron mobility as a function of temperature, and a continuous transition from a metal-like behavior to a semiconductor-like behavior characterized by an activated regime of the resistivity, as a function of temperature, for any donor concentration in the range N_D > 10¹⁶ cm^-3. These experimental data are found incompatible with the predictions of the usual s-f model of the exchange interaction. It is argued that these properties are due to the intra-atomic Coulomb correlations between d-electrons. A theoretical model is presented, on the basis of the Hubbard Hamiltonian. A good agreement is found between this theory and the experimental data.     

On the positivity of correlations in nonequilibrium spin systems

We consider Ising spin systems, equivalently lattice gases evolving under discrete- or continuous-time Markov processes, i.e., stochastic cellular automata or interacting particle systems. We show that for certain spin-flip probabilities or rates and suitable initial states the expectation values of products of spin variables taken at equal or different times are nonnegative; they satisfy the same inequalities as the equal-time correlations of ferromagnetic systems in equilibrium (first Griffiths inequality). Extensions of FKG inequalities to time-displaced correlations are also discussed.     

Velocity correlations and the structure of nonequilibrium hard-core fluids

A model for the pair-distribution function of nonequilibrium hard-core fluids is proposed based on a model for the effect of velocity correlations on the structure. Good agreement is found with molecular dynamics simulations of granular fluids and of sheared elastic hard spheres. It is argued that the incorporation of velocity correlations are crucial to correctly modeling atomic scale structure in nonequilibrium fluids.     

Effect of dynamical coulomb correlations on the fermi surface of Na0.3CoO2

The t(2g) quasiparticle spectra of Na(0.3)CoO(2) are calculated within the dynamical mean field theory. It is shown that as a result of dynamical Coulomb correlations charge is transferred from the nearly filled e(g(')) subbands to the a(1g) band, thereby reducing orbital polarization among Co t(2g) states. Dynamical correlations therefore stabilize the small e(g(')) Fermi surface pockets, in contrast to angle-resolved photoemission data, which do not reveal these pockets.     

The influence of thermally assisted tunneling on the performance of charge trapping memory

We evaluate the influence of the thermally assisted tunneling (TAT) mechanism on charge trapping memory (CTM) cell performance by numerical simulation, and comprehensively analyse the effects of the temperature, trap depth, distribution of trapped charge, gate voltage and parameters of TAT on erasing/programming speed and retention performance. TAT is an indispensable mechanism in CTM that can increase the detrapping probability of trapped charge. Our results reveal that the TAT effect causes the sensitivity of cell performance to temperature and it could affect the operational speed, especially for the erasing operation. The results show that the retention performance degrades compared with when the TAT mechanism is ignored.