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arXiv1995

Spectral Correlation and Response functions in Quantum Dots

We derive a general relation between correlators of density of states fluctuations and density response functions. It applies equally to quantum chaotic systems of pure symmetry (unitary, orthogonal, and symplectic) as well as to the crossover region between the universality classes. This relation is much more robust than Wigner-Dyson statistics; its validity extends to disordered metals with finite conductance and even to the Anderson insulators with large localization length.

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arXiv1996

Vortex Dynamics in Superfluid Systems: Cyclotron Type Motion

Vortex dynamics in superfluids is investigated in the framework of the nonlinear Schrödinger equation. The natural motion of the vortex is of cyclotron type, whose frequency is found to be on the order of phonon velocity divided by the coherence length, and may be heavily damped due to phonon radiation. Trapping foreign particles into the vortex core can reduce the cyclotron frequency and make the cyclotron motion underdamped. The density fluctuations can follow the vortex motion adiabatically within the phonon wave length at the cyclotron frequency, which results in a further downward renormalization of the cyclotron frequency. We have also discussed applications on the dynamics of vortices in superconducting films.

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arXiv1996

Ground states of a one-dimensional lattice-gas model with an infinite range nonconvex interaction. A numerical study

We consider a lattice-gas model with an infinite range pairwise noncovex interaction. It might be relevant, for example, for adsorption of alkaline elements on W(112) and Mo(112). We study a competition between the effective dipole-dipole and indirect interactions. The resulting ground state phase diagrams are analysed (numerically) in detail. We have found that for some model parameters the phase diagrams contain a region dominated by several phases only with periods up to nine lattice constants. The remaining phase diagrams reveal a complex structure of usually long periodic phases. We also discuss a possible role of surace states in phase transitions.

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arXiv1995

An exchange-correlation energy for a two-dimensional electron gas in a magnetic field

We present the results of a variational Monte Carlo calculation of the exchange-correlation energy for a spin-polarized two-dimensional electron gas in a perpendicular magnetic field. These energies are a necessary input to the recently developed current-density functional theory. Landau-level mixing is included in a variational manner, which gives the energy at finite density at finite field, in contrast to previous approaches. Results are presented for the exchange-correlation energy and excited-state gap at $ν=$ 1/7, 1/5, 1/3, 1, and 2. We parameterize the results as a function of $r_s$ and $ν$ in a form convenient for current-density functional calculations.

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arXiv1996

Variational Study of the Spin-Gap Phase of the One-Dimensional t-J Model

We propose a correlated spin-singlet-pairs wave function to describe the spin-gap phase of the one-dimensional $t-J$ model at low density. Adding a Jastrow factor with a variational parameter, $ν$, first introduced by Hellberg and Mele, is shown to correctly describe the long-range behavior expected for the Luther-Emery phase. Using the variational Monte Carlo method we establish a relation between $ν$ and the Luttinger exponent $K_ρ$, $K_ρ=\frac{1}{2ν}$.

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arXiv1996

Nonequilibrium orbital magnetization of strongly localized electrons

The magnetic response of strongly localized electrons to a time-dependent vector potential is considered. The orbital magnetic moment of the system, away from steady-state conditions, is obtained. The expression involves the tunneling and phonon-assisted hopping currents between localized states. The frequency and temperature dependence of the orbital magnetization is analyzed as function of the admittances connecting localized levels. It is shown that quantum interference of the localized wave functions contributes to the moment a term which follows adiabatically the time-dependent perturbation.

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arXiv1996

Reflection Symmetric Ballistic Microstructures: Quantum Transport Properties

We show that reflection symmetry has a strong influence on quantum transport properties. Using a random S-matrix theory approach, we derive the weak-localization correction, the magnitude of the conductance fluctuations, and the distribution of the conductance for three classes of reflection symmetry relevant for experimental ballistic microstructures. The S-matrix ensembles used fall within the general classification scheme introduced by Dyson, but because the conductance couples blocks of the S-matrix of different parity, the resulting conductance properties are highly non-trivial.

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arXiv1996

Long-lived charged multiple-exciton complexes in strong magnetic fields

We consider the charged exciton complexes of an ideal two-dimensional electron-hole system in the limit of strong magnetic fields. A series of charged multiple-exciton states is identified and variational and finite-size exact diagonalization calculations are used to estimate their binding energies. We find that, because of a hidden symmetry, bound states of excitons and an additional electron cannot be created by direct optical absorption and, once created, have an infinite optical recombination lifetime. We also estimate the optical recombination rates when electron and hole layers are displaced and the hidden symmetry is violated.

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arXiv1996

Dynamics in the dimerised and high field incommensurate phase of CuGeO$_3$

Temperature ($2.3<T<40$\ K) and magnetic field ($0<B<17$\ T) dependent far infrared absorption spectroscopy on the spin-Peierls coumpound CuGeO$_3$\ has revealed several new absorption processes in both the dimerised and high field phase of CuGeO$_3$. These results are discussed in terms of the modulation of the CuGeO$_3$\ structure. At low fields this is the well known spin-Peierls dimerisation. At high fields the data strongly suggests a field dependent incommensurate modulation of the lattice as well as of the spin structure.

cond-mat↗

arXiv1996

Tunneling through a barrier in Tomonaga-Luttinger liquid connected to reservoirs

The transport through a barrier is studied for a Tomonaga-Luttinger liquid of finite length connected to reservoirs. An effective action for the phase variable at the barrier is derived for spatially varying electric field. In the d.c. limit only the total voltage drop between the left and right reservoirs appears in the action. We discuss crossovers of the renormalization group flow and hence of the temperature and wire length dependence of the conductance, taking into account the location of the barrier.

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arXiv1996

"Electro-flux" effect in superconducting hybrid Aharonov-Bohm rings

We have extended the circuit theory of Andreev conductance [Phys.~Rev.~Lett. {\bf 73}, 1420 (1994)] to diffusive superconducting hybrid structures that contain an Aharonov-Bohm ring. The electrostatic potential distribution in the system is predicted to be flux-dependent with a period of the superconducting flux quantum $Φ_{0}=h/2e$. When at least one tunnel barrier is present, the conductance of the system oscillates with the same period.

cond-mat↗

arXiv1996

Infrared response of ordered polarons in layered perovskites

We report on the infrared absorption spectra of three oxides where charged superlattices have been recently observed in diffraction experiments. In La$_{1.67}$Sr$_{0.33}$NiO$_4$, polaron localization is found to suppress the low-energy conductivity through the opening of a gap and to split the $E_{2u}$-$A_{2u}$ vibrational manifold of the oxygen octahedra. Similar effects are detected in Sr$_{1.5}$La$_{0.5}$MnO$_4$ and in La$_2$NiO$_{4+y}$, with peculiar differences related to the type of charge ordering.

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arXiv1997

Polaron transport and lattice dynamics in colossal magnetoresistance manganites

Based on the model combining the spin double exchange and the lattice polaron, we have studied the colossal magnetoresistance phenomena observed in perovskite manganites R_{1-x}A_xMnO_3. First, effects of both the double exchange and the electron-phonon interaction on the transport property are investigated. We have evaluated the temperature dependent resistance and the magnetoresistance using the Kubo formula, and examined the crossover from tunneling to hopping regime of small polarons. Second, effects of the double exchange interaction on the lattice degree of freedom are explored. It is found that both the hardening of the phonon frequency and the reduction of the phonon damping take place with decreasing the temperature.

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arXiv1996

Interband mixing between two-dimensional states localized in a surface quantum well and heavy hole states of the valence band in narrow gap semiconductor

Theoretical calculations in the framework of Kane model have been carried out in order to elucidate the role of interband mixing in forming the energy spectrum of two-dimensional carriers, localized in a surface quantum well in narrow gap semiconductor. Of interest was the mixing between the 2D states and heavy hole states in the volume of semiconductor. It has been shown that the interband mixing results in two effects: the broadening of 2D energy levels and their shift, which are mostly pronounced for semiconductors with high doping level. The interband mixing has been found to influence mostly the effective mass of 2D carriers for large their concentration, whereas it slightly changes the subband distribution in a wide concentration range.

cond-mat↗

arXiv1996

Directed polymers in a random medium: a variational approach

A disorder-dependent Gaussian variational approach is applied to the problem of a $d$ dimensional polymer chain in a random medium (or potential). Two classes of variational solutions are obtained. For $d<2$, these two classes may be interpreted as domain and domain wall. The critical exponent $ν$ describing the polymer width is $ν={1\over (4-d)}$ (domain solution) or $ν={3\over (d+4)}$ (domain wall solution). The domain wall solution is equivalent to the (full) replica symmetry breaking variational result. For $d>2$, we find $ν={1\over 2}$. No evidence of a phase transition is found for $2< d< 4$: one of the variational solutions suggests that the polymer chain breaks into Imry-Ma segments, whose probability distribution is calculated. For $d>4$, the other variational solution undergoes a phase transition, which has some similarity with B. Derrida's random energy models.

cond-mat↗

arXiv1996

Aharonov-Bohm-Type Oscillations of Thermopower in a Quantum Dot Ring Geometry

We investigate Aharonov-Bohm-type oscillations of the thermopower of a quantum dot embedded in a ring for the case when the interaction between electrons can be neglected. The thermopower is shown to be strongly flux dependent, and typically the amplitude of oscillations exceeds the background value. It is also shown to be essentially dependent on the phase of the scattering matrix which is determined by the experimental geometry and is not known in the given experiment. Two procedures to compare theory and experiment are proposed.

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arXiv2007

Voltage-probe and imaginary potential models for dephasing in a chaotic quantum dot

We compare two widely used models for dephasing in a chaotic quantum dot: The introduction of a fictitious voltage probe into the scattering matrix and the addition of an imaginary potential to the Hamiltonian. We identify the limit in which the two models are equivalent and compute the distribution of the conductance in that limit. Our analysis explains why previous treatments of dephasing gave different results. The distribution remains non-Gaussian for strong dephasing if the coupling of the quantum dot to the electron reservoirs is via ballistic single-mode point contacts, but becomes Gaussian if the coupling is via tunneling contacts.

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arXiv1996

The Magnus force in superfluids and superconductors

The forces on the vortex, transverse to its velocity, are considered. In addition to the superfluid Magnus force from the condensate (superfluid component), there are transverse forces from thermal quasiparticles and external fields violating the translational invariance. The forces between quasiparticles and the vortex originate from interference of quasiparticles with trajectories on the left and on the right from the vortex like similar forces for electrons interacting with the thin magnetic-flux tube (the Aharonov-Bohm effect). These forces are derived in the Born approximation for phonons from the equations of superfluid hydrodynamics, and for BCS quasiparticles from the Bogolyubov-de Gennes equations. The effect of external fields violating translational invariance is analyzed for vortices in the two-dimensional Josephson junction array. The symmetry analysis of the classical equations for the array shows that the total transverse force on the vortex vanishes. Therefore the Hall effect which is linear in the transverse force is absent also. This means that the Magnus force from the superfluid component exactly cancels with the transverse force from the external fields. The results of other approaches are also brought together for discussion.

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