Coulomb blockade of the persistent current in a one-dimensional ballistic Luttinger liquid ring

The effect of an electrostatic energy (in the geometrical capacitance approach) on a persistent current is considered. It is shown that at high temperatures the current amplitude shows periodic dips as a function of the potential difference between a ring and a reservoir. These dips correspond to a lift of the Coulomb blockade. In a minimum of a dips a current is periodic in a magnetic flux with a period at any temperatures. Received: 6 April 1998 / Revised: 5 June 1998 / Accepted: 21 July 1998     

Comparison of tunneling through molecules with Mott-Hubbard and with dimerization gaps

In order to study the tunneling of electrons through an interacting, 1D, dimerized molecule connected to leads, we consider the persistent current in a ring embedding this molecule. We find numerically that, for spinless fermions, a molecule with a gap mostly due to interactions, i.e. a Mott-Hubbard gap, gives rise to a larger persistent current than a molecule with the same gap, but due only to the dimerization. In both cases, the tunneling current decreases exponentially with the size of the molecule, but more slowly in the interacting case. Implications for molecular electronic are briefly discussed. Received: 17 November 1997 / Revised: 16 January 1998 / Accepted: 16 January 1998     

Interaction versus dimerization in one-dimensional Fermi systems

In order to study the effect of interaction and lattice distortion on quantum coherence in one-dimensional Fermi systems, we calculate the ground state energy and the phase sensitivity of a ring of interacting spinless fermions on a dimerized lattice. Our numerical DMRG studies, in which we keep up to 1000 states for systems of about 100 sites, are supplemented by analytical considerations using bosonization techniques. We find a delocalized phase for an attractive interaction, which differs from that obtained for random lattice distortions. The extension of this delocalized phase depends strongly on the dimerization induced modification of the interaction. Taking into account the harmonic lattice energy, we find a dimerized ground state for a repulsive interaction only. The dimerization is suppressed at half filling, when the correlation gap becomes large. Received: 11 February 1998 / Revised: 1st April 1998 / Accepted: 30 April 1998     

An experimental test of the critical behaviour of fracture precursors

The statistical properties and the localization of fracture precursors on heteregeneous materials is studied by recording their acoustic emission as a function of the applied load. It's found that the microcrack cluster together as the load increases and the instantaneous acoustic energy has an invariant power law distribution. The integrated acoustic energy presents a critical divergency close to the failure load for the sample. These result support the idea that fracture can be viewed as a critical phenomenon. Finally a measure of the concentration of microcraks, which allows us to predict the failure load, is introduced. These properties are studied also when a periodic load is applied to the sample. It's found that the Kaiser effect is not strictly satisfied in heteregeneous materials. Received: 23 January 1998 / Revised: 20 April 1998 / Accepted: 17 June 1998     

Wavenumber dependence of structural relaxation in a molecular liquid

Structural relaxation in liquid orthoterphenyl is studied by means of coherent neutron time-of-flight and backscattering spectroscopy over a large temperature range. Not only amplitude and relaxation time but also the spectral line shape show a significant variation with wavenumber. These variations are correlated with the static structure factor, in qualitative accord with mode-coupling results for simple liquids. Even far above the melting point, relaxation remains non-exponential. Received: 5 January 1998 / Revised: 3 April 1998 / Accepted: 19 June 1998     

The topological structure of vortex in BEC

We find that there exists an elementary topological current in Bose-Einstein condensation. Based on the -mapping topological current theory, the implicit function theorem and the Taylor expansion, the topological structure of vortex lines is detailed in the neighborhoods of the bifurcation points of the condensate wave function. Received: 9 April 1998 / Revised: 28 August 1998 / Accepted: 31 August 1998     

First and second order transition in frustrated XY systems

The nature of the phase transition for the XY stacked triangular antiferromagnet (STA) is a controversial subject at present. The field theoretical renormalization group (RG) in three dimensions predicts a first order transition. This prediction disagrees with Monte-Carlo (MC) simulations which favor a new universality class or a tricritical transition. We simulate by the Monte-Carlo method two models derived from the STA by imposing the constraint of local rigidity which should have the same critical behavior as the original model. A strong first order transition is found. Following Zumbach we analyze the second order transition observed in MC studies as due to a fixed point in the complex plane. We review the experimental results in order to clarify the critical behavior observed. Received: 18 February 1998 / Revised: 24 April 1998 / Accepted: 30 April 1998     

Persistent current and transmission probability in the Aharonov-Bohm ring with an embedded quantum dot

Persistent current and transmission probability in the Aharonov-Bohm (AB) ring with an embedded quantum dot (QD) are studied using the technique of the scattering matrix. For the first time, we find that the persistent current can arise in the absence of magnetic flux in the ring with an embedded QD. The persistent current and the transmission probability are sensitive to the lead-ring coupling and the short-range potential barrier. It is shown that increasing the lead-ring coupling or the short-range potential barrier causes the suppression of the persistent current and the increasing resonance width of the transmission probability. The effect of the potential barrier on the number of the transmission peaks is also investigated. The dependence of the persistent current and the transmission probability on the magnetic flux exhibits a periodic property with period of the flux quantum.     

Chain radius dependence on concentration in a 2D living polymer system

A living polymer system is used to study the effect of concentration on a broad, polydisperse two-dimensional polymer system. It is found that the mean squared end-to-end radius of a chain of L monomers does not decrease by following a simple power law of the concentration but by a function of the form . An origin for such a behaviour is proposed. Received: 21 November 1997 / Received in final form: 21 April 1998 / Accepted: 24 April 1998     

Balance equations for electron transport in an arbitrary energy band driven by an intense terahertz field. Application to superlattice miniband transport

We suggest a balance-equation approach to hot-electron transport in a single arbitrary energy band subject to an intense radiation field of terahertz (THz) frequency, including all the multiphoton emission and absorption processes and taking account of realistic scatterings due to impurities and phonons. This approach, which allows one to calculate THz-driving, time-averaging transport based on a set of time-independent equations, provides a convenient method to study the effect of an intense THz electric field on carrier transport in a nonparabolic energy band. As an example, these fully three-dimensional, acceleration- and energy-balance equations are applied to the discussion of superlattice miniband transport at lattice temperature T=77 and 300 K driven by the THz radiation field of varying strengths. It is shown that the current through a dc biased miniband superlattice is greatly reduced by the irradiation of an intense THz electric field. Received: 23 January 1998 / Revised: 31 March 1998 / Accepted: 20 April 1998     

Longitudinal Stern-Gerlach effect for slow cesium atoms

The mechanical Stern-Gerlach effect is investigated in the case of a slow atomic cloud falling through an inhomogeneous magnetic field featuring a strong longitudinal gradient. The resulting Zeeman sublevel state selection is demonstrated under various experimental conditions. Longitudinal spatial separations are in agreement with numerical simulations that take into account the gravitational acceleration and both the transverse and axial magnetic forces. Since separations greater than 20 cm are obtained, potential applications in atom optics are outlined. Received: 16 February 1998 / Received in final form: 1 April 1998 / Accepted: 6 April 1998     

Mesoscopic charge density wave in a magnetic flux

The stability of a Charge Density Wave (CDW) in a one-dimensional ring pierced by a Aharonov-Bohm flux is studied in a mean-field picture. It is found that the stability depends on the parity of the number N of electrons. When the size of the ring becomes as small as the coherence length , the CDW gap increases for even N and decreases for odd N. Then when N is even, the CDW gap decreases with flux but it increases when N is odd. The variation of the BCS ratio with size and flux is also calculated. We derive the harmonics expansion of the persistent current in a presence of a finite gap. Received: 16 September 1997 / Received in final form: 12 November 1997 / Accepted: 13 November 1997     

Persistent currents in a lattice correlated electron ring with a magnetic impurity

The behavior of charge and spin persistent currents in an integrable lattice ring of strongly correlated electrons with a magnetic impurity is exactly studied. Our results manifest that the oscillations of charge and spin persistent currents are similar to the ones, earlier obtained for integrable continuum models with a magnetic impurity. The difference is due to two (instead of one) Fermi velocities of low-lying excitations. The form of oscillations in the ground state is “saw-tooth”-like, generic for any multi-particle coherent one-dimensional models. The integrable magnetic impurity introduces net charge and spin chiralities in the generic integrable lattice system, which determine the initial phase shifts of charge and spin persistent currents. We show that the magnitude of the charge persistent current in the generic Kondo situation does not depend on the parameters of the magnetic impurity, unlike the (magneto)resistivity of transport currents. Received 30 January 2003 / Received in final form 12 March 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: zvyagin@fy.chalmers.se     

Fano Interference versus Kondo Effect in Strongly Correlated T-Shaped Quantum Dots Embedded in an Aharonov-Bohm Ring

We theoretically investigate the properties of the ground state of the strongly correlated T-shaped double quantum dots embedded in an Aharonov-Bohm ring in the Kondo regime by means of the one-impurity Anderson Hamiltonian. It is found that in this system, the persistent current depends sensitively on the parity and size of the ring. With the increase of interdot coupling, the persistent current is suppressed due to the enhancing Fano interference weakening the Kondo effect. Moreover, when the spin of quantum dot embedded in the Aharonov- Bohm ring is screened, the persistent current peak is not affected by interdot coupling. Thus this model may be a new candidate for detecting Kondo screening cloud.     

Spatio-temporal patterns in the thermoconvection of a planar nematic layer: I. Weakly nonlinear models

We study theoretically the formation of convection patterns in a laterally extended planar nematic layer heated from below, in the linear and weakly nonlinear regimes. By reformulating the viscous coupling terms of the basic nematohydrodynamic equations, a simple interpretation of the flow effects on the director dynamics can be proposed. A detailed linear analysis of the problem is presented. A systematic method to investigate nonlinear mechanisms is developed, and exemplified by the study of the nonlinear saturation in rolls. The extension of the roll amplitude equation with the envelope formalism is used to characterize the dynamics of the roll modulations near threshold. Coupled envelope equations are shown to describe the structure of the point defects in zig-zags observed experimentally. Finally the bifurcation to the bimodal varicose is studied. The secondary wavevector in the bimodal appears to be selected by a rotation of the director in the horizontal plane. Quantitative predictions concerning the amplitude of this rotation are given. Received: 1st December 1997 / Revised: 25 May 1998 / Accepted: 2 June 1998     

Comparative study of the critical behavior in one-dimensional random and aperiodic environments

We consider cooperative processes (quantum spin chains and random walks) in one-dimensional fluctuating random and aperiodic environments characterized by fluctuating exponents . At the critical point the random and aperiodic systems scale essentially anisotropically in a similar fashion: length (L) and time (t) scales are related as . Also some critical exponents, characterizing the singularities of average quantities, are found to be universal functions of , whereas some others do depend on details of the distribution of the disorder. In the off-critical region there is an important difference between the two types of environments: in aperiodic systems there are no extra (Griffiths)-singularities. Received: 5 February 1998 / Accepted: 17 April 1998     

Fringe contrast in three grating Mach-Zehnder atomic interferometers

Several three-grating Mach-Zehnder atomic interferometers have been built and operated in recent years but no general theory of the contrast of the fringes produced by these apparatus is available. The purpose of this paper is to develop this theory, based on the Fresnel-Kirchoff approximate treatment of diffraction. Such a theory has been developed by Turchette et al. [JOSA B 9, 1601 (1992)] but because the necessary multiple integrals were evaluated in a purely numerical way, this treatment was not fully general. We show here how to reduce the computation by analytic means and we are thus able to calculate the contrast with a modest numerical effort. Moreover, we get a simple insight of the contrast reduction related to several defects of a real apparatus. We apply our calculations to existing interferometers as well as to an apparatus working with lithium which is under construction in our laboratory. Received: 24 April 1998 / Revised: 25 October 1998 / Accepted: 11 December 1998     

Magnetic quantum coherence in trigonal and hexagonal systems

The tunneling behaviors of the magnetization vector are studied in ferromagnetic systems with trigonal and hexagonal crystal symmetries, respectively. The Euclidean transition amplitudes between the energetically degenerate easy directions are evaluated with the help of the dilute instanton-gas approximation. By using the effective Hamiltonian method, the ground-state tunneling level splittings are clearly shown for each kind of symmetry and are found to depend on the parity of the total spin of the ferromagnetic particle. The effective Hamiltonian method is demonstrated to be equivalent to the dilute instanton-gas approximation. Possible relevance to experiments is discussed. Received: 18 November 1997 / Revised: 18 March 1998 / Accepted: 6 April 1998     

A microscopic model for the intrinsic Josephson tunneling in high- superconductors

A quantitative analysis of a microscopic model for the intrinsic Josephson effect in high-temperature superconductors based on interlayer tunneling is presented both within a mean-field BCS evaluation and a numerically essentially exact Quantum Monte-Carlo study. The pairing correlations in the CuO₂-planes are modelled by a 2D Hubbard model with attractive interaction, a model which accounts well for some of the observed features such as the short planar coherence length. The stack of Hubbard planes is arranged on a torus, which is threaded by a magnetic flux. The current perpendicular to the planes is calculated as a function of applied flux (i.e. the phase), and - after careful elimination of finite-size effects due to single-particle tunneling - found to display a sinusoidal field dependence in accordance with interlayer Josephson tunneling. Studies of the temperature dependence of the supercurrent reveal at best a mild elevation of the Josephson transition temperature compared to the planar Kosterlitz-Thouless temperature. These and other results on the dependence of the model parameters are compared with a standard BCS evaluation. Received: 24 February 1998 / Revised: 28 April 1998 / Accepted: 23 June 1998