Spatial effects of Fano resonance in local tunneling conductivity in vicinity of impurity on semiconductor surface

We present the results of local tunneling conductivity spatial distribution detailed theoretical investigations in vicinity of impurity atom for a wide range of applied bias voltage. We observed Fano resonance in tunneling conductivity resulting from interference between resonant tunneling channel through impurity energy level and direct tunneling channel between the tunneling contact leads. We have found that interference between tunneling channels strongly modifies form of tunneling conductivity measured by the scanning tunneling microscopy/spectroscopy (STM/STS) depending on the distance value from the impurity.     

Coulomb singularity effects in the tunneling spectroscopy of individual impurities

Nonequilibrium Coulomb effects in resonant tunneling through deep impurity states are analyzed. It is shown that Coulomb vertex corrections to the tunneling transfer amplitude lead to power law singularity in the current-voltage characteristics.     

The influence of tunneling matrix element modification due to on-site Coulomb interaction on local tunneling conductivity

Interplay between changes of energy levels and tunneling amplitudes caused by localized electrons’ on-site Coulomb interaction depending on non-equilibrium electron filling numbers is analyzed. Specific features of local tunneling conductivity spectra for different positions of localized states’ energy relative to the Fermi level have been investigated by means of self-consistent mean field approximation in the presence of non-equilibrium effects. The conditions when modifications of tunneling transfer amplitude due to changes of electron filling numbers in the presence of on-site Coulomb interaction should be taken into account in tunneling conductivity spectra have been revealed.     

Intensities of the line and photoionization spectra of shallow nonhydrogenlike impurities in semiconductors

Numerical nonvariational methods are proposed for the calculation of energies and wave functions of bound states, ground state wave functions with the account of central cell corrections, and the orthonormalized wave functions of the continuous spectrum of nonhydrogenlike shallow impurities in semiconductors. A number of different spectral characteristics is calculated for donor impurities in Ge and Si and for acceptor impurities in Ge and GaAs. The theory of photofield ionization i.e. tunnel ionization of an optically excited impurity atom is presented for shallow donors with the account of the effective mass anisotropy. The possibility of the observation of the line spectrum (due to transitions to shallow Coulomb excited levels) of a deep impurity in the presence of a high magnetic field is shown.     

Dynamic transport characteristics of side-coupled double-quantum-impurity systems

A systematic study is performed on time-dependent dynamic transport characteristics of a side-coupled double-quantum-impurity system based on the hierarchical equations of motion. It is found that the transport current behaves like a single quantum dot when the coupling strength is low during tunneling or Coulomb coupling. For the case of only tunneling transition, the dynamic current oscillates due to the temporal coherence of the electron tunneling device. The oscillation frequency of the transport current is related to the step voltage applied by the lead, while temperature $T$, electron--electron interaction $U$ and the bandwidth $W$ have little influence. The amplitude of the current oscillation exists in positive correlation with $W$ and negative correlation with $U$. With the increase in coupling $t_{12}$ between impurities, the ground state of the system changes from a Kondo singlet of one impurity to a spin singlet of two impurities. Moreover, lowering the temperature could promote the Kondo effect to intensify the oscillation of the dynamic current. When only the Coulomb transition is coupled, it is found that the two split-off Hubbard peaks move upward and have different interference effects on the Kondo peak at the Fermi surface with the increase in $U_{12}$, from the dynamics point of view.     

Effect of localized charge states on the low-frequency part of the tunneling current spectrum (1/<Emphasis Type="Italic">f</Emphasis><Superscript><Emphasis Type="Italic">α</Emphasis></Superscript>)

The dependence of the low-frequency part of the tunneling current spectra (1/f ^α ) above a clean surface and above isolated impurity atoms on the InAs(110) cleaved surface has been investigated by scanning tunneling microscopy/spectroscopy in high vacuum. A theoretical model is proposed to explain the experimental results, which takes into account the many-body interaction of conduction electrons (with suddenly switched on and off Coulomb interaction on an impurity atom and on the scanning tunneling microscope tip) with the continuous-spectrum states in the tunneling contact leads.     

External field induced switching of a tunneling current in coupled quantum dots

We investigated the tunneling current peculiarities in the system of two quantum dots that are coupled by means of the external field and are weakly connected to the electrodes in the presence of Coulomb correlations. It was found that tuning of the Rabi frequency induces fast multiple tunneling current switching and leads to the negative tunneling conductivity. Special role of multielectron states was demonstrated. Moreover we revealed conditions for bistable behavior of the tunneling current in the coupled quantum dots with Coulomb correlations.     

Thermoelectric properties of correlated double quantum dot system in Coulomb blockade regime

We theoretically study thermoelectric properties of a coupled double quantum dot (DQD) system coupled to normal leads using two impurity Anderson model with intra- as well as interdot Coulomb interactions. A generic formulation, which was earlier developed to study electronic properties (zero bias maximum of differential conductance and interesting partial swapping in Fano phenomena) of DQD system within Coulomb blockade regime for a non-magnetic case, is extended to investigate thermoelectric properties i.e. electrical conductance, thermoelectric power and thermal conductance of the same system, as a function of temperature by varying interdot Coulomb interaction and interdot tunneling. Interdot Coulomb interaction is found to trigger some novel features like crossover in thermoelectric power with temperature in all the configurations (series, parallel and T-shape) and a small peak in thermal conductance toward low temperatures, T≈Γ/10, in series and T-shape configurations, which is found to be missing in case of symmetric parallel configuration. The origin of these novel features is attributed to the interplay of renormalization of energy levels caused by the interdot Coulomb interaction which is interpreted in terms of local density of states and the asymmetry effects related to dot-lead couplings/interference effects.     

Theory of 2D transport in graphene for correlated disorder

We theoretically revisit graphene transport properties as a function of carrier density, taking into account possible correlations in the spatial distribution of the Coulomb impurity disorder in the environment. We find that the charged impurity correlations give rise to a density-dependent graphene conductivity, which agrees well qualitatively with the existing experimental data. We also find, quite unexpectedly, that the conductivity could increase with increasing impurity density if there is sufficient interimpurity correlation present in the system. In particular, the linearity (sublinearity) of graphene conductivity at lower (higher) gate voltage is naturally explained as arising solely from impurity correlation effects in the Coulomb disorder.     

Donor states in tunnel-coupled quantum wells

We study the energy spectrum of the impurity states in tunnel-coupled double quantum wells for Coulomb and short-range donor potentials. We calculate the impurity contribution and the density of states and detect the transformation of a localized donor state into a resonant state when the binding energy of the donor in an isolated quantum well is less than the separation of the energy levels of the double quantum wells. In the opposite case, where the binding energy is greater than the level separation, there is tunneling repulsion between adjacent impurity levels, with the degree of degeneracy of the levels changing when there is tunneling mixing of the ground and excited impurity states from different wells. Resonant states emerge in an asymmetric double quantum well, while in a symmetric double quantum well the impurity level at the barrier’s center proves to be localized even against the background of the continuum. The calculations are based on a general expression for the impurity contribution to the density of states in terms of a 2-by-2 matrix Green’s function, i.e., only a pair of tunnel-coupled levels of the double quantum wells is taken into account. For an impurity with a short-range potential, we derive a matrix generalization of the Koster-Slater solution, while the impurity with a Coulomb potential is analyzed by using the approximation of a narrow resonance and close arrangement of the repulsive levels. Zh. éksp. Teor. Fiz. 115, 1337–1352 (April 1999)     

Impurity states and interlayer tunneling in high temperature superconductors

We argue that the scanning tunneling microscope (STM) images of resonant states generated by doping Zn or Ni impurities into Cu-O planes of BSCCO are the result of quantum interference of the impurity signal coming from several distinct paths. The impurity image seen on the surface is greatly affected by interlayer tunneling matrix elements. We find that the optimal tunneling path between the STM tip and the metal (Cu, Zn, or Ni) d(x(2)-y(2)) orbitals in the Cu-O plane involves intermediate excited states. This tunneling path leads to the fourfold nonlocal filter of the impurity state in Cu-O plane that explains the experimental impurity spectra. Applications of the tunneling filter to the Cu vacancy defects and "direct" tunneling into Cu-O planes are also discussed.     

抛物量子点中强耦合束缚极化子的性质

采用Pekar类型的变分方法研究了抛物量子点中强耦合束缚极化子的基态和激发态的性质。计算了束缚极化子的基态和激发态的能量、光学声子平均数。讨论了量子点的有效束缚强度和库仑束缚势对基态能量、激发态能量以及光学声子平均数的影响。数值计算结果表明:量子点中强耦合束缚极化子的基态和激发态能量及光学声子平均数均随量子点的有效束缚强度的增加而减小,基态、激发态能量随库仑束缚势的增加而减小,光学声子平均数随库仑束缚势的增加而增大。     

Laser dressed donor impurities in free-standing GaAs films under an electric field

The laser-field dependence of the shallow donor states in a free-standing thin GaAs film under an external static field is studied within the effective mass approximation. The laser dressing effects are considered for the confinement potential of the well as well as for the impurity Coulomb interaction distorted by the dielectric mismatch at interfaces. We found that (i) the increase of the laser intensity dramatically modifies the electron potential energy, which establishes the quantum confinement; (ii) the ground state subband energy is significantly enhanced by the electrostatic self-energy arising from the interaction between the electron and its images; (iii) the impurity binding is much larger than those of the dielectrically homogenous case and it becomes stronger sensitive to the laser intensity variation; (iv) under an electric field parallel to the growth direction, the inversion symmetry with respect to the quantum well center is broken and a red/blue-shift of the binding energy, depending on the impurity position along the field direction, occurs. Therefore, the shallow donor energy levels in the free-standing thin films can be tuned in a wide range by proper tailoring of the structure parameters (well size, impurity position) as well as by varying the external applied fields.     

Features of the conductivity and magnetoresistance of doped two-dimensional structures near a metal-insulator transition

Theoretical and experimental studies of the conductivity and magnetoresistance of selectively doped structures of GaAs/AlGaAs quantum well structures near a metal-insulator phase transition have been reviewed. Special attention is focused on the role of the structure of impurity bands, which are narrow in the absence of intentional compensation and, in the case of doping of barriers, include the partially filled upper Hubbard band. It has been shown that the indicated structures exhibit (i) specific mixed conductivity, which can, in particular, include the contribution from delocalized states in the impurity band; (ii) the virtual Anderson transition, which is suppressed with an increase in disorder owing to compensation or with an increase in the concentration of a dopant; (iii) slow relaxations of the hopping magnetoresistance caused by the Coulomb glass effects, including, in particular, the states of the upper Hubbard band; and (iv) the suppression of the negative interference magnetoresistance owing to the spin effects.     

Berry-phase blockade in single-molecule magnets

We formulate the problem of electron transport through a single-molecule magnet (SMM) in the Coulomb blockade regime taking into account topological interference effects for the tunneling of the large spin of a SMM. The interference originates from spin Berry phases associated with different tunneling paths. We show that, in the case of incoherent spin states, it is essential to place the SMM between oppositely spin-polarized source and drain leads in order to detect the spin tunneling in the stationary current, which exhibits topological zeros as a function of the transverse magnetic field.     

Kondo tunneling through real and artificial molecules

When an asymmetric double dot is hybridized with itinerant electrons, its singlet ground state and lowly excited triplet state cross, leading to a competition between the Zhang-Rice mechanism of singlet-triplet splitting in a confined cluster and the Kondo effect (which accompanies the tunneling through quantum dot under a Coulomb blockade restriction). The rich physics of an underscreened S = 1 Kondo impurity in the presence of low-lying triplet-singlet excitations is exposed and estimates of the magnetic susceptibility and the electric conductance are presented, together with applications for molecule chemisorption on metallic substrates.     

Metallic glass electronic structure peculiarities revealed by UHV STM/STS

We present the results of ultrahigh vacuum scanning tunneling microscopy/spectroscopy investigation of metallic glass surface. The topography and electronic structure of Ni_63.5Nb_36.5 have been studied. A great number of clusters with size about 5–10 nm have been found on constant current scanning tunneling microscopy images. The tunneling spectra of normalized tunneling conductivity revealed the energy pseudogap in the vicinity of Fermi energy. For energy values above 0.1 eV the normalized tunneling conductivity changes linearly with increasing of tunneling bias. The obtained results can be understood within suggested theoretical model based on the interplay of elastic electron scattering on random defects and weak intra-cluster Coulomb interaction. The effects of the finite edges of electron spectrum of each cluster have to be taken into account to explain the experimental data. The tunneling conductivity behavior and peculiarities in current images of individual clusters can also be qualitatively analyzed in the framework of suggested model.     

Influence of inter cell resonant tunneling on the out-of-plane electronic transport behavior in layered high T<Subscript>c</Subscript> cuprates

The influence of inter unit cell resonant tunneling between the copper-oxygen planes on the c-axis electronic conductivity (σ_c) in normal state of optimal doped bilayer high T_c cuprates like Bi₂Sr₂CaCu₂O_8+x is investigated using extended Hubbard Hamiltonian including resonant tunneling term (T₁₂) between the planes in two adjoining cells. The expression for the out-of-plane (c-axis) conductivity is calculated within Kubo formalism and single particle Green's function by employing Green's function equations of motion technique within meanfield approximation. On the basis of numerical computation, it is pointed out that the renormalized c-axis conductivity increases exponentially with the increment in inter cell resonant tunneling. The effect of T₁₂ on renormalized c-axis conductivity is found to be prominent at low temperatures as compared to temperatures above room temperature (~300 °K). The Coulomb correlation suppresses the variation of renormalized c-axis conductivity with temperature, while renormalized c-axis conductivity increases on increasing carrier concentration. These theoretical results are viewed in terms of existing c-axis transport measurements.     

First-Principles Study of Intrinsic Point Defects of Monolayer GeS

The properties of six kinds of intrinsic point defects in monolayer GeS are systematically investigated using the“transfer to real state”model,based on density functional theory.We find that Ge vacancy is the dominant intrinsic acceptor defect,due to its shallow acceptor transition energy level and lowest formation energy,which is primarily responsible for the intrinsic p-type conductivity of monolayer GeS,and effectively explains the native p-type conductivity of GeS observed in experiment.The shallow acceptor transition level derives from the local structural distortion induced by Coulomb repulsion between the charged vacancy center and its surrounding anions.Furthermore,with respect to growth conditions,Ge vacancies will be compensated by fewer n-type intrinsic defects under Ge-poor growth conditions.Our results have established the physical origin of the intrinsic p-type conductivity in monolayer GeS,as well as expanding the understanding of defect properties in lowdimensional semiconductor materials.