Fu-Cho Pu

Mott-Hubbard transition in infinite dimensions

Grazing incidence x-ray reflectivity spectra from copper/permalloy multilayers have been recorded as a function of incident x-ray energy under conditions of fixed scattering vector. From the smoothly varying part of the spectrum, it is possible separately to determine the density and composition as the energy is scanned through the K edges of the constituent elements. From the fit to model layer structures, we determine that there exists a strained layer of permalloy at the permalloy/copper interface that relaxes towards the unstrained density of permalloy on annealing. No such layer exists at the copper/permalloy interface. Cobalt doping results in the formation of a layer of mixed CoNiFe composition, rather than a single Co layer. Oscillations in the spectra above the absorption edges are analyzed in a similar manner to that for diffraction anomalous fine structure and enable the short range order within the layers to be determined. No changes are observed in the Cu or Co environments, but a reduction in the in-plane first shell distance around the Ni atoms is found on annealing.

Exact boundary critical exponents and tunneling effects in integrable models for quantum wires.

Using the principles of the conformal quantum-field theory and the finite size corrections of the energy of the ground and various excited states, we calculate the boundary critical exponents of single- and multicomponent Bethe-Ansatz soluble models. The boundary critical exponents are given in terms of the dressed-charge matrix which has the same form as that of systems with periodic boundary conditions and is uniquely determined by the Bethe-ansatz equations. A Luttinger liquid with open boundaries is the effective low-energy theory of these models. As applications of the theory, the Friedel-oscillations due to the boundaries and the tunneling conductance through a barrier are also calculated. The tunneling conductance is determined by a nonuniversal boundary exponent which governs its power law dependence on temperature and frequency.

Spin-polarized resonant tunneling and quantum-size effect in ferromagnetic tunnel junctions with double barriers subjected to an electric field

Abstract Based on the two-band model, we present a transfer-matrix treatment of tunnel magnetoresistance (TMR) for tunneling through double ferromagnetic tunnel junctions with any magnetization angle of the middle ferromagnetic layer subjected to an electric field. Our results show that the TMR changes non-monotonically with the magnetization angle of the middle ferromagnetic layer in double junctions, which is different from that in a single junction. Owing to the comprehensive role of the quantum-size effect and the spin-polarized resonant tunneling, the TMR oscillates with the thickness of the middle ferromagnetic layer and can reach very large values under suitable conditions. Furthermore, the quantum-size effect can also give rise to a positive TMR (inverse spin-valve effect).

THEORY OF SPIN-POLARIZED TUNNELING BETWEEN FERROMAGNETIC FILMS

Abstract We consider the spin-flip effect and present a theoretical analysis of tunnel conductance and magnetoresistance for a ferromagnetic tunnel junction subjected to a dc bias by using the nonequilibrium Green function method. Our results are in qualitative agreement with the experimental measurements. The spin-flip tunneling is found to diminish the magnitude of tunnel magnetoresistance while improving the value of tunnel conductance. In certain cases, by considering the spin-flip effect, quantitative agreement is obtained between our theoretical results and recent experimental data.

Quantum tunneling for the asymmetric double-well potential at finite energy

Abstract A bouncelike solution with nonzero energy is used for the explicit calculation of the decay rate of an excited state of the asymmetric double-well potential. The imaginary part of the energy, which results only from the symmetry of the bounce itself, is calculated with the help of the path-integral method.

Quantum–classical crossover of the escape rate of a biaxial spin system with an applied magnetic field

Employing the method of mapping the spin problem onto a particle one developed by Ulyanov and Zaslavskii, we derive the effective Hamiltonian for a biaxial spin system with an applied magnetic field along the medium axis and find an analytical form of the phase boundary line between first and second order transitions, from which a complete phase diagram can be obtained. We also derive an analytical form of the crossover temperature as a function of the scaled anisotropy constant at the phase boundary.

Giant tunneling magnetoresistance in ferromagnet/insulator (semiconductor) coupling double-tunnel-junction subjected to electric field

Based on the two-band model, a transfer-matrix treatment of the tunnel conductance and magnetoresistance is presented for tunneling through ferromagnet/insulator (semiconductor) double-junction subjected to dc bias. There exist the spin polarized resonant tunneling and the giant tunnel magnetoresistance. The highest value of the magnetoresistance in double-junction can reach 90%. It is expected that these results can cause the interest in experimental efforts in designing spin polarized resonant tunneling devices. Our theories can also be extended to the single-junction and the superlattice easily. For the single-junction, our results are qualitatively in agreement with the experimental measurements.

Phase transition in quantum tunneling for a parameterized double-well potential

Abstract The transition from the quantum tunneling regime to the classical crossover regime for a parameterized double-well potential is investigated. Using the different methods, the critical value of the potential parameter at which the first-order transition occurs is obtained.

Calculation of tunnel splitting in a biaxial spin particle without instanton technique

Abstract The level splitting formulae of excited states as well as ground state for a biaxial spin particle are derived from those of eigenvalues of Schrodinger equation (with periodic boundary conditions) which is obtained in terms of a quantum treatment of spin, i.e., the potential field description of quantum spin systems, developed by Ulyanov and Zaslavskii. The results obtained are compared with those previously derived by complicated pseudoparticle methods.

Integrability of spin chain with boundary impurities

Abstract A general approach to construct the integrable spin chain model with boundary impurities is presented in the framework of the quantum inverse scattering method. By selecting the impurity vertex matrix appropriately, the corresponding Hamiltonians for the XYZ , XXZ , XXX chains are derived.