Modern Physics Letters B

A two-dimensional problem of tunneling of a particle is studied to propose an experiment to measure tunneling time. We consider a 2D rectangular barrier in which the particle undergoes both tunneling and free lateral motion at the same time. The two processes are coupled by the same tunneling time, which leads to a simple relation between the tunneling time and the corresponding lateral shift such that L = vτ. Since the lateral speed v is constant the tunneling time can be obtained by measuring the lateral shift. The shifted length can be controlled by an initial lateral speed and become over a hundred nanometers when the particle is provided with sufficient initial lateral speed. The present model may also be used for examining the relationships between characteristic tunneling times suggested in previous articles. We also demonstrate physical differences between the 2D problem of a particle tunneling and the frustrated total internal reflection of electromagnetic waves.

We show that the vibration modes of some atom chain models (including diatomic chain) with extra strings can be derived directly by using the newly-developed invariant eigen-operator method. Especially, the chain model which takes different second-neighbor interactions into account is solved.

The entanglement in a Hubbard chain of hardcore bosons is investigated. The analytic expression of the global entanglement in the ground state is derived. The divergence of the derivative of the global entanglement shows the quantum criticality of the ground state. For the thermal equilibrium state, the bipartite and the multipartite entanglement are evaluated. The entanglement decreases to zero at a certain temperature. The thermal entanglement is rapidly decreasing with the increase of the number of sites in the lattice. The bipartite thermal entanglement approaches a constant value at a certain number of sites while the multipartite entanglement eventually vanishes.

Glasses from xGd₂O₃ · (100-x)[B₂O₃ · TeO₂] system with 0 ≤ x ≤ 10 mol% have been prepared and studied by means of FT-IR absorption and Raman scattering. Being complementary spectroscopic techniques, both types of measurements FT-IR and Raman revealed that the network structure of the studied glasses is based on BO₃ and BO₄ units placed in different structure groups and also TeO₄, TeO₃ structural units. No absorption characteristic for Gd₂O₃ was evidenced but some changes in the structure of B₂O₃ · TeO₂ glass matrix are due to its presence.

We have performed density-functional perturbation calculation for zinc-blende AlN using the pseudopotential plane-wave method. The results obtained using both the local-density approximate (LDA) and the generalized-gradient approximate (GGA) for exchange-correlation functional are compared. The ground state properties and response function properties for zinc-blende AlN, including the electronic band structure, charge density, Born effective charge, dielectric constant and vibrational properties are reported. Our results are basically in agreement with experimental data and theoretical values available, but the bandgap is underestimated and the first optical mode in the phonon band structure is overestimated. This can be attributed to the underestimation of the lattice parameter and selection of the pseudopotential.

We calculate the transmission coefficient through an Aharonov–Bohm ring with a multi-level quantum dot side-coupled to one arm. Whenever a quantum dot level is tuned across the Fermi energy, the coupled arm is blocked for electron tunneling, namely anti-resonance. This leads to the suppression of the Aharonov–Bohm oscillations, accompanied by a phase jump of π in the oscillations. The Aharonov–Bohm phase also changes sharply by π in between neighboring anti-resonances due to the multi-level effect. The transmission has an expression of the generalized Fano form with a complex Fano parameter q in the presence of the Aharonov–Bohm flux.

In the resistivity–temperature curves of La_0.7Ca_0.3MnO₃ and La_0.75Ca_0.25MnO₃, five distinct regions have been identified. For example, in La_0.7Ca_0.3MnO₃, the regions are:

(1) from the lowest temperature of measurement to 200 K where resistivity is directly proportional to temperature with a small slope,

(2) the reciprocal of resistivity follows Bloch's T^3/2 law,

(3) resistivity is directly proportional to temperature with a large slope,

(4) the cusp region, where there are FM clusters of reduced dimensions and varying degrees of correlated polarons or bipolarons and small polarons,

(5) the semiconducting region having small polarons.

The regions 1, 2 and 3 together appear to replicate the spontaneous magnetization curves of the materials. The effect of magnetic field on the resistivity–temperature curves are as follows:

(a) resistivity is reduced. In particular, the resistivity peak with magnetic field occurs at a temperature μH/K downward from the T_c of the curve without magnetic field;

(b) resistivity peak is displaced by μ · μH/K towards the higher temperature side compared to the resistivity peak without magnetic field;

(c) cusp region is broadened;

(d) magnetoresistance is appreciable only within 10% of T_p on both sides.

All these observations suggest that temperature profile of resistivity of manganites is controlled by magnetic effects.