Variational Path-Integral Study on Bound Polarons in Parabolic Quantum Dots and Wires

The expression of the ground-state energy of the electron coupled simultaneously with a Coulomb potential and a longitudinal-optical phonon field in parabolic quantum dots and wires is derived within the framework of Feynman variational path integral theory.We obtain a general result with arbitrary electron-phonon coupling constant,Coulomb binding parameters,and confining potential strength,which could be used for further numerical calculation of polaron properties.Moreover,it is shown that all the previous path-integral formulae for free polarons,bound polarons,and polarons confined in parabolic quantum dots and wires can be recovered in the present formalism.

Second-Order Phase Transition in the Two-Dimensional Classical Lattice Coulomb Gas of Half-Integer Charges

The second-order phase transition in the two-dimensional(2D)classical Coulomb gas of half-integer charges on a square lattice is investigated by using Monte Carlo simulations.Based on the finite-size scaling analysis,we estimate the second-order phase transition temperature Tc and the static critical exponentsβand v with a new numerical analysis method.More precise critical temperature Tc=0.1311(2)and critical exponentsβ/v=0.1152(12)and v=0.857(15)are obtained.The estimated value of v indicates that the charge lattice melting transition is different from the pure 2D Ising transition.

Dynamic Approach to the Ground State of Vortex System with Random Pinning

We study the ground state of a two-dimensional vortex system with random pinnings by cooling the vortex system to zero temperature.The peak of structure factor S(k)of vortices increases with vortex density B but decreases with pin density BФ,indicating that the vortex lattice becomes better and better ordered with increasing B.When B>BФ,the triangular vortex lattice is observed away from the pins.Repulsive vortex interactions destroy the Mott insulator phase which is predicted to occur at B=BФ.The crossover line between the strong Bose glass and the weak Bose glass is nearly independent of BФfor small BФ.

Coexistence of Ferromagnetism and Superconductivity in EuSr_(2)Ru_(1-x)Nb_(x)Cu_(2)O_(8)

The structural,magnetic and transport properties of EuSr_(2)Ru_(1-x)Nb_(x)Cu_(2)O_(8)(x=0,0.1,0.2 and 0.5)samples have been investigated by means of x-ray diffraction,magnetization and resistance measurements.It is found that the EuSr_(2)Ru_(1-x)Nb_(x)Cu_(2)O_(8) compound exhibits a ferromagnetic order of the Ru moments below the Curie temperature 130.19 K,and becomes superconducting at a much lower critical temperature of 10 K.The experimental observation that partial substitution of Nb for Ru in this system results in the decrease of the Curie temperature further supports the idea that the ferromagnetic order involves only Ru moments in this compound.We suggest that the decrease of the superconducting critical temperature Tc due to the partial substitution of Nb for Ru originates from the decrease of the carrier concentration in the CuO_(2) plane because of Nb existing always in the Nb5+valance state while Ru is in a mixted valance state between Ru4+and Ru5+.The coexistence of ferromagnetism and superconductivity is also discussed.

Anomalous Suppression of the Vortex Hall Current in Underdoped YBa_(2)Cu_(3)O_(x)

The transport properties of underdoped YBa_(2)Cu_(3)O_(x)(YBCO) crystals with x = 6.95, 6.80 and 6.66 were measured and the effect of the pseudogap on the Hall conductivity was studied. In the normal state, the Hall angle remains unperturbed at the crossover temperature of resistivity for the underdoped samples. An anomalous suppression of the vortex Hall current was observed near Tc and the contribution of the vortices to the Hall current is absent above 40 K in 60 K YBCO (x = 6.66).

Numerical Study on the Two-Dimensional Vortex System with Random Pinning

We study numerically the dynamic behavior of the two-dimensional vortex lattice, moving in a random pinning potential under the action of an external driving force. Different densities of randomly distributed pointlike pinning centers and different vortex-vortex interacting strength are considered. Then the elastic and plastic regimes are found to be dependent on the relation between the vortex-vortex and the vortex-pin interacting potentials. The critical driving force increases with increasing density of pinning centers and with decreasing of the vortex-vortex interacting strength. The simulated results are consistent with the recent experiments, and some of our results fall outside the region of validity of collective pinning theory.

Semiconductor-Like Properties of Sputtered FexCul-x Thin Films

Amorphous FexCul-x thin films with x ranging from 0.21 to 0.81 have been obtained by rf-magnetron sputtering.The structure and the electric transport properties of the films can be controlled by both the composition and annealing temperature.The resistances of the films decrease with the temperature increasing from 12 to 300K,which is the typical semiconductor characteristic.After annealing at 400°C for 5h,the amorphous FexCul-x thin film is crystallized and exhibits electric transport property an normal metal film.The dependence of resistance-temperature characteristics of the films on the composition and deposition time is described.

Path Integral Approach for Multi-dimensional Polarons in a Symmetric Quantum Dot

Within the framework of Feynman-Haken path-integral theory,the general expression of the ground-state energy for multi-dimensional polarons in symmetric quantum dots for arbitrary electron-phonon coupling constants are derived.Moreover,in the weak-coupling limit,the previous results by the second-order Rayleigh-Schrodinger perturbative theory are completely recovered.More interestingly,the extended-and localized-state solutions to the ground-state energy are obtained analytically.

Oxidation State of Cu and Superconductivity in Bilayer T1-Based Cuprates

The oxidation state of Cu in the homologous Tl_(2)Ba_(2)Ca_(n-1)Cu_(n)O_(2n+4+δ)(n=1-4)series has been analyzed in terms of the bond valence sums.It is found that the totally mobile hole concentration nH,t of the type-Ⅰ CuO_(2) planes at the optimal level increases with increasing number of CuO_(2) layers per unit cell from 1 to 3,and the largest nH,t is observed to be 0.474 when n=3,which just corresponds to the highest T_(c) in the bilayer T1-based materials,and then it decreases with the increase of n.It is indicated that the correlation between the maximum value of T_(c) and n is similar to that between n_(H,t) and n.

Formation of Large Bipolarons

A variational approach is presented to calculate the ground-state energy of large bipolarons in two and three dimensions.The values of the critical coupling constantαc above which the bipolaron is stable and the critical ratio of dielectric constantsηc below which the bipolaron is stable are approximately evaluated.It is found that the stability region of bipolarons in the(l/α,η)-plane becomes larger with decreasing dimensionality.

Investigation of Hole Distribution in Y_(1-x)Pr_(x)Ba_(2)Cu_(3)O_(7)

The electronic structure of Y_(1-x)Pr_(x)Ba_(2)Cu_(3)O_(7) is described by a tight-binding Hamiltonian in which the Pr-O hybridization and the 2p_(σ)-2p_(π) Coulomb repulsion are taken into account.The dependence of planar hole-density,localized hole-density and chemical potential of holes on Pr concentration is calculated by a coherent potential approximation.The result shows that the insensitivity of hole-density on the CuO_(3) chains with the Pr-doping is due to the 2p_(σ)-2p_(π) repulsion.Some other experimental facts can also be well interpreted.