Low-Temperature Thermal Conductance in Superlattice Nanowire with Structural Defect

Using the scattering-matrix cascading method, we investigate the effect of structural defect on the acoustic phonon transmission and thermal conductance in the superlattice nanowire at low temperatures. In the present system, the phonon transmissions exhibit quite complex oscillatory behaviour. It is found that a lateral defect in an otherwise periodic structure significantly decrease the thermal conductance and completely washes away the transmission quantization. However, the appreciable transmission quantization survives in the presence of a longitudinal defect whereas a good quantization plateau of thermal conductance emerges below the universal level in a wide temperature range with the lateral defect.     

Dynamics of tilted superlattices with time-dependent impurity

The influence of time-dependent impurity on the electronic and transport properties of the tilted superlattices modeled by the tight-binding Hamiltonian is investigated. The quasienergies of the system are calculated in two methods: exact diagonalization and Floquet replica technique, the subtle effects of the impurity on the dynamics of the system are discussed in detail. Through a perturbative analysis in terms of the inverse of the time-dependent impurity frequency, we obtain the time-independent effective Hamiltonian, whose reliability and physical implication are discussed.     

Preexponential factor in variable-range hopping conduction in CuInTe2

We study the temperature dependence of the electrical resistivity in a single crystal of p-type uncompensated CuInTe₂ on the insulating side of the metal-insulator transition down to 0.4 K. We observe a crossover from Mott to Efros-Shklovskii variable-range hopping conduction. In Efros-Shklovskii-type conduction, the resistivity is best described by explicitly including a preexponential temperature dependence according to the general expression ρ=ρ₀T^αexp(T_ES/T)^1/2, with α≠0. A theory based on the resistor network model was developed to derive an explicit relation between α and the decay of the wavefunction of the localized states. A consistent correspondence between the asymptotic extension of the wavefunction and the conduction regime is proposed. The results indicate a new mechanism for a local resistivity maximum in insulators, not involving magnetic effects.     

Pressure and the quadratic temperature term in the electrical resistance of NbTi

We have measured the electrical resistance of an Nb_0.53Ti_0.47 alloy sample as a function of temperature T (4-300 K) and pressure (<20 GPa). At low temperatures, above the superconducting transition, we observe a T² term whose coefficient decreases with pressure. It is linearly dependent on the residual resistance, that also decreases with pressure, in strong agreement with a Koshino-Taylor origin, i.e. inelastic carrier scattering against impurities.     

Fano Interference versus Kondo Effect in Strongly Correlated T-Shaped Quantum Dots Embedded in an Aharonov-Bohm Ring

We theoretically investigate the properties of the ground state of the strongly correlated T-shaped double quantum dots embedded in an Aharonov-Bohm ring in the Kondo regime by means of the one-impurity Anderson Hamiltonian. It is found that in this system, the persistent current depends sensitively on the parity and size of the ring. With the increase of interdot coupling, the persistent current is suppressed due to the enhancing Fano interference weakening the Kondo effect. Moreover, when the spin of quantum dot embedded in the Aharonov- Bohm ring is screened, the persistent current peak is not affected by interdot coupling. Thus this model may be a new candidate for detecting Kondo screening cloud.     

Order-disorder transition for corrugated Au layers

Atomic-scale structure of the growth of a gold film on (1 1 2) plane of Mo single crystal was investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) up to two monolayers (ML) of gold coverage. Both LEED and STM results establish that Au grows on Mo(1 1 2) in a layer-by-layer mode, for at least the first two monolayers. A number of ordered structures are formed and both the first and second layers adopt the Mo(1 1 2) 1 × 1 surface structure upon completion. For some gold layers on Mo(1 1 2), notably the 1.66 monolayer 3 × 1 and 1.75 monolayer 4 × 1 gold overlayers, we find evidence of a phase transition associated with increasing disorder in gold layers with structural corrugation and anisotropic band structure. The signature of this phase transition, at temperatures in the range of 400-500 K, is a sharp decrease in the overlayer effective Debye temperature.     

The large anisotropy of the magnetic and transport properties in the Ba5Co5ClO13 single crystal

In this Letter, the single crystals of Ba₅Co₅ClO₁₃ were grown by the flux method successfully. Their structure, magnetic and transport properties were studied. A large anisotropy of the magnetic and transport properties has been detected in this compound. Below the TN∼108 K, the magnetic susceptibility exhibits an antiferromagnetic peak in χc and an upturn transition in χab. We suggest that this behavior is consistent with the competition of the ferromagnetic (FM) intra-blocks coupling and antiferromagnetic (AFM) inter-blocks coupling in this compound. The temperature dependence of the resistivity displays a hump in ρab and a kink in ρc around TN, suggesting the strong coupling between the transport and magnetic properties. Above and below the transition, the transport properties in ab plane follow the three-dimensional (3D) variable range hopping (VRH) mechanism.     

Spin-Triplet Andreev Reflection in Ferromagnet/Ferromangnet/s-Wave Superconductor Junctions

Four-component Bogoliubov-de Gennes equations are applied to study the tunnelling conductance spectra G（ E） of half-metallic ferromagnet/ferromagnet/s-wave superconductor tunnel junctions. It is found that only for noncollinear magnetizations, there exists nonzero G（ E） structure within the energy gap, which is a signature of appearance of the novel Andreev reflection and spin-triplet pairing correlations.     

Absence of metal-insulator transition and coherent interlayer transport in oriented graphite in parallel magnetic fields

Measurements of the magnetoresistivity of graphite with a high degree of control of the angle between the sample and magnetic field indicate that the metal-insulator transition, shown to be induced by a magnetic field applied perpendicular to the layers, does not appear in parallel field orientation. Furthermore, we show that interlayer transport is coherent in less ordered samples and high magnetic fields, whereas appears to be incoherent in less disordered samples. Our results demonstrate the two-dimensionality of the electron system in ideal graphite samples.     

Enhancement of surface magnetization in antiferromagnetic nanoparticles

We report enhancement of magnetization below the antiferromagnetic ordering temperature T_N in nanoparticles of two antiferromagnets, viz CoRh₂O₄ and Cr₂O₃. The enhancement of magnetization below T_N is systematic, being larger for sample with smaller particle size. Scaling analysis showed that such enhancement of magnetization in CoRh₂O₄ nanoparticles is due to the superparamagnetic type contribution of surface (shell) spins. The present work shows that similar analysis can also be applied in Cr₂O₃ nanoparticles.     

Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.     

Vortex Pinning due to Dynamic Spin-Vortex Interaction in aSuperconductor/Ferromagnet Multilayer

We investigate the mutual interaction between superconductivity and ferromagnetism in a Nb/Ni81 Ee19 multilayer by ac susceptibility measurements. Compared with a pure superconducting Nb film, the critical current density of the multilayer is apparently enhanced in a low magnetic field region but remains nearly the same in high magnetic fields, which indicates that a continuous ferromagnetic layer with in-plane magnetization can produce strong vortex pinning in a low field region. We interpret this unusual vortex-pinning phenomenon as a consequence of dynamic spin vortex interaction which induces a spin rotation following vortex movement. In addition, we propose that this dynamic interaction could be used for spin manipulation via a superconductor.     

The dependence of barrier height on temperature for Pd Schottky contacts on ZnO

Temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements have been performed on Pd/ZnO Schottky barrier diodes in the range 60-300 K. The room temperature values for the zero bias barrier height from the I-V measurements (Φ_I-V) was found to be 0.52 eV and from the C-V measurements (Φ_C-V) as 3.83 eV. From the temperature dependence of forward bias I-V, the barrier height was observed to increase with temperature, a trend that disagrees with the negative temperature coefficient for semiconductor material. The C-V barrier height decreases with temperature, a trend that is in agreement with the negative temperature coefficient of semiconductor material. This has enabled us to fit two curves in two regions (60-120 K and 140-300 K). We have attributed this behaviour to a defect observed by DLTS with energy level 0.31 eV below the conduction band and defect concentration of between 4×10¹⁶ and 6×10¹⁶ cm⁻³ that traps carriers, influencing the determination of the barrier height.     

Low frequency, low temperature properties of the spin-boson problem

Low temperature and low frequency properties of a spin-boson model are investigated within a super operator and Liouville space formulation. The leading contributions are identified with the help of projection operators projecting onto the equilibrium state. The quantities of interest are expressed in terms of weighted bath propagators and static linear and nonlinear susceptibilities. In particular the generalized Shiba relation and Wilson ratio are recovered. Received 7 July 2000 and Received in final form 8 August 2000     

Coherent thermal emission by microstructured waveguides

Following Greffet et al. [Coherent emission of light by thermal sources. Nature 2002;416:61-4], we study in this article, the possibility to engineer thermal coherent sources with waveguides. The idea is to rule a grating on a waveguide made of a slice of germanium deposited on a participating media such as glass. The guided waves, thermally excited are coupled to the far field by the grating and increase the system emissivity in certain directions and wavelengths. We numerically compute the diffraction of a plane wave by the grating by a rigorous coupled waves algorithm (RCWA). The reflected, transmitted and absorbed energy calculated allows to obtain the system emissivity by means of the Kirchhoff law.     

Phase modulation of charge density wave around a short-range potential by the Aharonov-Bohm effect

The analytic result of the Aharonov-Bohm effect for its influence on the oscillation of a two-dimensional charge density around a short range potential is given. Without losing generality, the cases of δ-function and hard disc potentials were examined. Numerical calculations show that the interferences among quantum particles are greatly influenced by the nonlocal effect, which leads to the modulation of phase and amplitude of the oscillation of charge density. Since the presence of a nonlocal influence of the Aharonov-Bohm effect on charged particles is universal, the results in the specific potentials examined are expected to appear also in other general systems which may be beneficial to the study of nanotechnology.     

Dynamics of two interacting dipoles

We study the deterministic spin dynamic of two interacting magnetic moments with anisotropy and dipolar interaction under the presence of an applied magnetic field, by using the Landau–Lifshitz equation with and without a damping term. Due to different kinds of interactions, different time scales appear: a long time scale associated with the dipolar interaction and a short time scale associated with the Zeeman interaction. We found that the total magnetization is not conserved; furthermore, for the non-dissipative case it is a fluctuating function of time, with a strong dependence on the strength of the dipolar term. In the dissipative case there is a transient time before the total magnetization reaches its constant value. We examine this critical time as a function of the distance between the magnetic moments and the phenomenological damping coefficient, and found that it strongly depends on these control parameters.     

Surface acoustic waves in interaction with a dislocation

A surface acoustic wave can interact with dislocations that are close to the surface. We characterize this interaction and its manifestations as scattered surface acoustic waves for different orientations with respect to the surface of an edge dislocation. For dislocations that are parallel or perpendicular to the free surface, we present an analytical result for short dislocations with respect to the wave-length that reproduce qualitatively the main features observed for dislocations of various sizes.     

A canonical formulation of dissipative mechanics using complex-valued hamiltonians

We show that a large class of dissipative systems can be brought to a canonical form by introducing complex co-ordinates in phase space and a complex-valued hamiltonian. A naive canonical quantization of these systems lead to non-hermitean hamiltonian operators. The excited states are unstable and decay to the ground state. We also compute the tunneling amplitude across a potential barrier by solving the dissipative version of the Schrödinger equation. We then generalize the formalism to cases where the configuration space is a curved Riemannian manifold.