Artificial nets from superconducting nanogranules

We show that a large transport current can flow through superconducting nets composed of nano-clusters. Although thermal and quantum fluctuations lead to a finite value of dissipation, this value can be very small in one- and two-dimensional systems for realistic parameters of the nanoclusters and distances between them. The value of the action for vortex tunneling at zero temperature can be made sufficiently large to make the dissipation negligibly small. We estimate the temperature T ₀ of the transition from the thermal activation to quantum tunneling.     

The influence of interband tunneling on leakage current in manganite/titanate heterojunction

The behavior of leakage current at reverse bias in p-La_0.9Sr_0.1MnO₃/n-SrNb_0.01Ti_0.99O₃ heterojunction has been theoretically studied by calculating interband tunneling current with various doping densities and temperatures. Our results reveal that the reduction of leakage current with decrease of doping density and increase of temperature originates from properties of interband tunneling.     

Structures of energy spectrum and persistent currents in mesoscopic rings with radial potential barrier in magnetic field

The energy spectrum and the persistent currents are calculated for finite-width mesoscopic annular structures with radial potential barrier in the presence of a magnetic field. The introduction of the tunneling barrier leads to the creation of extra edge states around the barrier and the occurrence of oscillatory structures superimposed on the bulk Landau level plateaus in the energy spectrum. We found that the Fermi energy E _F increases with the number of electrons N emerging many kinks. The single eigenstate persistent current exhibits complicated structures with vortex-like texture, “bifurcation”, and multiple “furcation” patterns as N is increased. The total currents versus N display wild fluctuations.     

The effect of metastable states of the impurities on quantized acoustoelectric current

We report a detailed experimental study of the fluctuations of acoustoelectric (AE) current induced by surface acoustic waves in a shallow-etched quantum point contact defined in a GaAs/Al_xGa_1?xAs heterostructure at 1.7 K. We observe that current fluctuations between two AE current plateaus are more obvious than those at the plateaus. Switching the metastable states of the impurities is considered as the physical origin of the current fluctuations.     

Fluctuations of electroluminescence intensity of single CdSe nanocrystals excited by scanning tunneling microscope current

Electroluminescence from single CdSe nanocrystals (NCs) excited by tunneling current of scanning tunneling microscope (STM) has been measured. Two types of samples with low and high concentration of CdSe NCs deposited on the gold substrate have been prepared. Both types of samples had no plasmon emission. It enabled one to detect pure electroluminescence from single CdSe NCs. Samples with low concentration of NCs exhibit an intensive short-term luminescence of NCs for several seconds. Samples with high concentration of NCs exhibit a weak fluctuating long-term luminescence for thousand seconds. Fluctuations of NC electroluminescence differ considerably from those detected recently in photoluminescence of CdSe NCs embedded in polymer films. The difference in fluctuations results from the difference in physical conditions existing in electro- and photoluminescence. The distribution of photons w(N, T) emitted in time interval T has been found from statistical treating of fluctuating luminescence. Due to weakness of the pure signal, we paid a special attention to allowing for photomultiplier tube noise while treating these fluctuations. The photon distribution in pure signal is one of super-Poisson type, i.e. it is broader than Poisson distribution. A dynamical model for an absorber–emitter excited by tunneling current of STM has been offered. The model takes into account the thermal drift of STM tip.     

A model of enhanced STM current due to semiconductor optical absorption

We present a simple model for the change in tunneling current between a semiconductor surface and a metal tip under spectroscopic illumination in a scanning tunneling microscope. This model predicts a sharp increase in the tunneling current due to the increase in the conduction band carrier density when the photon energy exceeds the optical band gap. The tunneling current for a large diffusion length has a more pronounced onset than for a small length. Our model should provide, when combined with experiments, a method of determining localized effective stoichiometry, and therefore provides a localized alternative to the use of optical absorption measurements. Our theoretical tunneling current versus photon energy curves are in good qualitative agreement with the existing experimentally measured curves for Si, GaAs, and InP obtained by Qian and Wessels. In addition, we have examined the effects of temperature, surface recombination velocity, and degeneracy on our theoretical results for the Hg_1−xCd_xTe, Hg_1−xZn_xTe, and Hg_1−xZn_xSe ternary narrow gap semiconductor systems.     

Magnetic field effects in the onset of superconductivity

Using the time dependent Ginzburg-Landau theory the influence of a finite and even strong magnetic field on the fluctuations in superconductors aboveT _c is studied. We calculate the dynamical conductivity, the Hall angle, and the static magnetisation from the fluctuations of the charge current associated with the fluctuations of the order parameter. It is found that the magnetic field generally enhances the singular contributions of the fluctuations to the conductivity and the susceptibility. Associated with this enhancement is a reduction of the characteristic frequency scale close toT _c .     

Transport properties of Fibonacci heterostructures: a nonparabolic approach

A fourth order hamiltonian is used to explore transport properties of semiconductor Fibonacci heterostructures. The tunneling current and time delay are obtained for different Fibonacci sequences constructed withGaAsandAl_xGa_1 − xAs. Energy minibands are calculated to study the fractal dimension and critical electronic states in quasi-periodic arrays. Results show that nonparabolic corrections produce changes in the tunneling current, time delay and fractal dimension, and a low voltage shift of the current peaks compared with the parabolic theory. The electronic states preserve their critical nature in the presence of nonparabolic effects.     

The sign of Josephson's cos θ-term

Within the tunneling Hamiltonian approximation Kramers-Kronig relations between the amplitudes j_1,2 of the sine and cosine term in the Josephson current are derived. The spectral function ωj₂(ω) determines the supercurrents dissipative response to external a.c.-voltages but has no definite sign. A generla proof is given, that nevertheless the total dissipation always remains positive.     

Design and analysis of Si-based arch-shaped gate-all-around (GAA) tunneling field-effect transistor (TFET)

In this work, a Si-based arch-shaped gate-all-around (GAA) tunneling field-effect transistor (TFET) has been designed and analyzed. Various studies on III–V compound semiconductor materials for applications in TFET devices have been made and we adopt one of them to perform a physical design for boosting the tunneling probability. The GAA structure has a partially open region for extending the tunneling area and the channel is under the GAA region, which makes it an arch-shaped GAA structure. We have performed the design optimization with variables of epitaxy channel thickness (t_epi) and height of source region (H_source) in the Si-based TFET. The designed arch-shaped GAA TFET based on Si platform demonstrates excellent performances for low-power (LP) applications including on-state current (I_on) of 694 μA/μm, subthreshold swing (S) of 7.8 mV/dec, threshold voltage (V_t) of 0.1 V, current gain cut-off frequency (f_T) of 12 GHz, and maximum oscillation frequency (f_max) of 283 GHz.     

Mesoscopic fluctuations of the Josephson current of an S-I-S junction with weak structural disorder

It is shown that in the range of tunneling resonance energies of an S-I-S (superconductor-insulator-superconductor) junction with weak (low impurity concentrations) structural disorder in the I-layer, the average critical current and the magnitude of its mesoscopic fluctuations are determined by tunneling along quantum resonance-percolation trajectories. For a “small” junction situated in a parallel magnetic field at temperature T = 0 conditions for smallness of the mesoscopic fluctuations are obtained and an estimate is made of the range of parameters in which the resonance mechanism for supercurrent propagation predominates.     

Analysis of the R0A product in n+-p Hg1−xCdxTe photodiodes

The influence of different junction current components (diffusion current for radiative and Auger 7 recombination mechanisms, tunneling and depletion layer currents) on the R₀A product of n⁺-p -Hg_1−xCd_xTe photodiodes is considered. The considerations are carried out for the 77–300 K temperature region and 1–15 μm cutoff wavelength. Optimum doping concentrations in the p-type region of n⁺-p abrupt junctions are determined, taking into account the influence of the tunneling current and of a fixed surface charge density of the junction passivation layer. Results of calculations are compared with experimental data reported by many authors. An attempt is made to explain the discrepancy between theoretical calculations and experimental data.     

Fluctuations in high field superconductors

We present calculations for the influence of fluctuations in high field superconductors where the critical field is limited by Pauli paramagnetism. Due to the fact that the critical field at the second order phase transition point as function of temperature may have a maximum atT≠0 the additional conductivity due to fluctuations may have a nonmonotonic temperature dependence. This way we can account for recent experimental findings by Tedrow, Meservey and Schwartz. We also calculate the additional tunneling density of states due to fluctuations. Under proper conditions it exhibits a maximum at zero frequency like in the gapless regime. Finally we show that our findings of a nonmonotonic resistivity should also apply to superconductors containing magnetic impurities such as La_3-x Gd _x In in an external field.     

Analysis of dark current contributions in mercury cadmium telluride junction diodes

An analytical approach to analyze the dark current–voltage (I–V) and dynamic impedance vs reverse bias voltage (R_d–V) characteristics of an HgCdTe junction diode is presented in this paper. Application to the experimental data is discussed to illustrate the approach. It is shown that the relative contributions of the various dark current contributing mechanisms viz. diffusion, generation–recombination, thermal trap assisted tunneling, band-to-band tunneling, avalanche multiplication and ohmic current component can all be isolated, if present.     

Quantum dynamics of superconducting point contacts: chiral anomaly,Landau–Zener transitions,and all that

Quantum effects in the dynamics of the Josephson phase difference in Josephson junctions with large electron transparency D are studied in the adiabatic regime, when the characteristic charging energyE_C of the junction is much smaller than the superconducting energy gap Δ. In isolated junctions, quantum phase fluctuations are large and manifest themselves as Coulomb blockade of Cooper pair tunneling. The amplitude of the Coulomb blockade oscillations is calculated for single-mode junctions with arbitrary D. In particular, it is shown that the chiral anomaly completely suppresses Coulomb blockade in ballistic junctions with D =  1, and the suppression process at D →  1 can be described as the Landau–Zener transition in imaginary time. In the regime when quantum phase fluctuations are small, they lead to quantum decay of supercurrent states due to macroscopic quantum tunneling of phase through the Josephson potential barrier. The decay rate is found in the nearly-ballistic junctions.     

Transport across an Anderson quantum dot in the intermediate coupling regime

We describe linear and nonlinear transport across a strongly interacting single impurity Anderson model quantum dot with intermediate coupling to the leads, i.e. with tunnel coupling Γ of the order of the thermal energy k _B T. The coupling is large enough that sequential tunneling processes (second order in the tunneling Hamiltonian) alone do not suffice to properly describe the transport characteristics. Upon applying a density matrix approach, the current is expressed in terms of rates obtained by considering a very small class of diagrams which dress the sequential tunneling processes by charge fluctuations. We call this the “dressed second order” (DSO) approximation. One advantage of the DSO is that, still in the Coulomb blockade regime, it can describe the crossover from thermally broadened to tunneling broadened conductance peaks. When the temperature is decreased even further (k _B T < Γ), the DSO captures Kondesque behaviours of the Anderson quantum dot qualitatively: we find a zero bias anomaly of the differential conductance versus applied bias, an enhancement of the conductance with decreasing temperature as well as universality of the shape of the conductance as function of the temperature. We can without complications address the case of a spin degenerate level split energetically by a magnetic field. In case spin dependent chemical potentials are assumed and only one of the four chemical potentials is varied, the DSO yields in principle only one resonance. This seems to be in agreement with experiments with pseudo spin [U. Wilhelm, J. Schmid, J. Weis, K.V. Klitzing, Physica E 14, 385 (2002)]. Furthermore, we get qualitative agreement with experimental data showing a cross-over from the Kondo to the empty orbital regime.     

Investigation of the electronic properties of Au nanoclusters in SiO<Subscript>2</Subscript> by combined scanning tunneling/atomic force microscopy

The tunneling of electrons through Au nanoc lusters formed by pulsed laser deposition in a SiO₂ thin film on a Si substrate has been investigated by combined scanning/atomic force microscopy (STM/AFM). Conducting Pt-coated Si cantilevers were used. The feedback was maintained via the AFM channel, and the current-voltage (I-V) characteristics of the tunnel contact between the AFM probe and the n ⁺-Si substrate through a =4-nm-thick SiO₂ film with Au nanoclusters =2 nm in diameter were measured simultaneously. The current image of the structure contained areas of increased current (tunnel-current channels) 2–15 nm in size, related to tunneling of electrons through Au nanoclusters in SiO₂. The I-V characteristics recorded in the tunnel-current channels exhibit specific features related to the Coulomb blockade of electron tunneling through Au nanoclusters.     

Influence of strain on the tunneling magnetoresistance in diluted magnetic semiconductor trilayer and double barrier structures

The influence of strain on hole tunneling in trilayer and double barrier structures made of two diluted magnetic semiconductors (DMS) (Ga, Mn)As, separated by a thin layer of non-magnetic AlAs is investigated theoretically. The strain is caused by lattice mismatch as the whole structure is grown on a (In_0.15Ga_0.85)As buffer layer. The tensile strain makes the easy axis of magnetization orient along the growth direction. We found that biaxial strain has a strong influence on the tunneling current because the spin splitting at is comparable to the Fermi energy E_F. Tensile strain decreases the tunneling magnetoresistance ratio.     

The influence of substrate temperature on electrical properties of Cu/CdS/SnO2 Schottky diode

Polycrystalline CdS samples on the SnO₂ coated glass substrate were obtained by vacuum evaporation method at low substrate temperatures (T_S=200 and 300 K) instead of the commonly used vacuum evaporation at high substrate temperatures (T_S>300 K). X-ray diffraction studies showed that the textures of the films are hexagonal with a strong (0 0 2) preferred direction. Circular Cu contacts were deposited on the upper surface of the CdS thin films at 200 K by vacuum evaporation. The effects of low substrate temperature on the current-voltage (I-V) characteristics of the Cu/CdS/SnO₂ structure were investigated in the temperature range 100-300 K. The Cu/CdS (at 300 K)/SnO₂ structure shows exponential current-voltage variations. However, I-V characteristics of the Cu/CdS (at 200 K)/SnO₂ structure deviate from exponential behavior due to high series resistance. The diodes show non-ideal I-V behavior with an ideality factor greater than unity. The results indicate that the current transport mechanism in the Cu/CdS (at 300 K)/SnO₂ structure in the whole temperature range is performed by tunneling with E₀₀=143 meV. However, the current transport mechanism in the Cu/CdS (at 200 K)/SnO₂ structure is tunneling in the range 200-300 K with E₀₀=82 meV.     

The Curie temperature of ZrZn2

The curie of temperature of ZrZn₂ is determined from the stoner theory of itinerant electron magnetism by renormalising the first coefficient of the Landau expansion thus taking spin fluctuations into account. These fluctuations are found to be saturated well below T_c so that the behaviour of ZrZn₂ around T_c is of stoner type. The calculated and measured values of T_c are in good agreement.