International Journal of Modern Physics B

We review the application of dynamical mean-field theory to Josephson junctions and study how to maximize the characteristic voltage I_cR_n which determines the width of a rapid single flux quantum pulse, and thereby the operating speed in digital electronics. We study a wide class of junctions ranging from SNS, SCmS (where Cm stands for correlated metal), SINIS (where the insulating layer is formed from a screened dipole layer), and SNSNS structures. Our review is focused on a survey of the physical results; the formalism has been developed elsewhere.

The scaling approach proposed by Hentschel and Family [Phys. Rev. Lett.66, 1982 (1991)] is generalized to the studies of the scaling of the anisotropic nonlocal Kardar–Parisi–Zhang equation with spatially correlated noise. The scaling exponents in both the weak- and strong-coupling regions are obtained, respectively. The scaling exponents obtained in the weak-coupling region can well match the results of the dynamic renormalization-group analysis.

We performed ab-initio, local density functional calculations of the electronic structure, charge transfer, and optical properties of MgB₂, using the LCAO formalism. The Fermi level of MgB₂ cuts through relatively narrow electron bands which have a dominant contribution from B(2p) states. There is a substantial charge transfer from magnesium to boron atoms. We found the ionic formula for this material to be . A clearly metallic distribution of the electronic charge density in the plane of boron atoms is interwoven with a visibly covalent one in the direction perpendicular to this plane. The calculated optical conductivities from the direct inter-band transitions exhibit a strong anisotropy between σ_xx(ω) or σ_yy(ω) and σ_zz(ω). Due to our application of the BZW procedure, major peaks in the density of states above the Fermi level are at markedly higher energies (1–1.5 eV) than the results of previously reported ones. A similar pattern is followed by our findings for optical conductivities.

The full pattern decomposition method by Le Bail was applied to the X-ray diffraction pattern of a (Bi_1.8Pb_0.46)Sr_1.88Ca_2.06(Cu_0.97Fe_0.01Ca_0.02)₃O_y superconductor presenting the same characteristics as the false 2212 reported by Masqood et al. (ICDD file 42-514). By using the Cmmm space group, AB₅ coordination in the BiO planes was evidenced, yielding an explanation for the apparent cell colapse. Orthorombic-pseudohexagonal settings for the a and b lattice constants may be derived from the group-supergroup relationships, i.e. instead of one may take a=2×3.8Å and . Indexing with these values avoids the assumption for the strange peak, and yields the refined values of a=7.664Å, b=3.356Å and c=16.172Å, space group Pmmm.

We reconsider the study of persistent currents in a disordered one-dimensional ring threaded by a magnetic flux ϕ, using he one-band tight-binding model for a ring of N-sites with random site energies. The secular equation for the eigenenergies expressed in terms of transfer matrices in the site representation is solved exactly to second order in a perturbation theory for weak disorder and fluxes differing from half-integer multiples of the flux quantum ϕ₀=hc/e. From the equilibrium currents associated with the one-electron eigenstates we derive closed analytic expressions for the disorder averaged persistent current for even and odd numbers, Ne, of electrons in the ground state. Explicit discussion for the half-filled band case confirms that the persistent current is periodic with a period ϕ₀, as in the absence of disorder, and that its amplitude is generally suppressed by the effect of the disorder. In comparison to previous results, based on an approximate analysis of the secular equation, the current suppression by disorder is strongly enhanced by a new periodic factor proportional to 1/sin²(2πϕ/ϕ₀), for ϕ≠(integer)ϕ₀/2.

The flux associated with the electron's spin is calculated on a the basis of the magnetic top (spherical top) model which can be made equivalent to a circular current loop with radius R in x–y plane in the presence of a uniform magnetic field in z-direction. It is found that the flux associated with the electron's spin is exactly equal to +Φ₀/2 for a spin down electron and -Φ₀/2 spin up one.

We derive an expression for the conditional time for the reflection of a wave from an arbitrary potential barrier using the WKB wavefunction in the barrier region. Our result indicates that the conditional times for transmission and reflection are equal for a symmetric barrier within the validity of the WKB approach.

A recently developed potential function for covalent materials (Phys. Stat. Sol.B212, 9 (1999)) is used to simulate the surface adsorption, and diffusion of Si adtom and ad-dimer on the Si(001) surface. We calculate the formation energies and diffusion activation energies of several possible binding sites. The predicted stable and metastable configurations and diffusion paths of Si ad-atom and Si ad-dimer on Si(001)-(2×1) surface are in agreement with that from the first principle calculations or experiments.

A sequence design method based on maximizing the thermodynamic occupying probability of the target structure is investigated. Some model-protein sequences are designed using the occupying-probability-maximized procedure on a 3×3×3 lattice. The thermodynamic and dynamic features of these sequences show their great improvement comparing with those of the sequences designed by an energy-minimized method. A better foldability is achieved for the occupying-probability-maximized sequences. These results suggest that the native occupying probability rather than the energy would be a better judgment for protein-like models.

The task of a classical perceptron is to classify two classes of patterns by generating a separation hyperplane. Here, we give a complete description of a quantum perceptron. The quantum algorithms for classification and learning are formulated in terms of unitary quantum gates operators. In the quantum case, the concept of separable or non-separable classes is irrelevant because the quantum perceptron can learn a superposition of patterns which are not separable by a hyperplane.