Tunable Kondo screening in a quantum dot device

We consider electron transport along a single-mode channel which is in contact, via tunnel junctions in its walls, with two quantum dots. Electron tunneling to and from the dots contributes to the electron backscattering, and thus modifies the channel conductance. If the dots carry spin, the channel conductance becomes temperature dependent due to the Kondo effect. The two-dot device geometry allows for a formation of S=1 localized spin due to the indirect exchange interaction, called Ruderman-Kittel-Kasuya-Yosida interaction. This device offers a possibility to study the crossover between fully screened and under-screened Kondo impurity. We investigate the manifestation of such crossover in the channel conductance

From semiconductors to superconductors: a simple model for pseudogaps

We consider a two dimensional semiconductor with a local attraction among the carriers. We study the ground state of this system as a function of the semiconductor gap. We find a direct transition from a superconducting to an insulating phase for no doping at a critical value, the single particle excitations being always gapped. For finite doping we find a smooth crossover. We calculate the critical temperature due to both the particle excitations and the Berezinkii-Kosterlitz-Thouless transition.Comment: 14 pages. Accepted for publication on Eur. Phys. Jour.

Ground state fragmentation of repulsive BEC in double-trap potentials

The fragmentation of the ground state of a repulsive condensate immersed into a double-trap potential is found to be a general and critical phenomenon. It takes place for a given number of bosons if their scattering length is larger than some critical value or for a given value of the scattering length if the number of bosons is above some critical number. We demonstrate that the geometry of the inner trap determines these critical parameters while the number of the fragments and the fraction of bosons in the various fragments can be manipulated by the outer trap. There is also a maximal number of bosons for which the ground state is fragmented. If this number is exceeded, the fragmented state becomes a very low-lying excited state of the condensate. This maximal number of bosons can be substantially manipulated by varying the inner and outer traps. To study three-fold fragmentation we have chosen a potential well with two barriers as the inner trap and embedded by two types of outer ones. A many-fold fragmentation is also addressed.Comment: 18 pages + 9 figure

Steps and facets at the surface of soft crystals

We consider the shape of crystals which are soft in the sense that their elastic modulus $\mu$ is small compared to their surface tension $\gamma$, more precisely $\mu a < \gamma$ where $a$ is the lattice spacing. We show that their surface steps penetrate inside the crystal as edge dislocations. As a consequence, these steps are broad with a small energy which we calculate. We also calculate the elastic interaction between steps a distance $d$ apart, which is a $1/d^2$ repulsion. We finally calculate the roughening temperatures of successive facets in order to compare with the remarkable shapes of lyotropic crystals recently observed by P. Pieranski et al. Good agreement is found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.

Magnetically Tunable Kondo - Aharonov-Bohm Effect in a Triangular Quantum Dot

The role of discrete orbital symmetry in mesoscopic physics is manifested in a system consisting of three identical quantum dots forming an equilateral triangle. Under a perpendicular magnetic field, this system demonstrates a unique combination of Kondo and Aharonov-Bohm features due to an interplay between continuous [spin-rotation SU(2)] and discrete (permutation C3v) symmetries, as well as U(1) gauge invariance. The conductance as a function of magnetic flux displays sharp enhancement or complete suppression depending on contact setups.Comment: 4 pages, 3 .eps figure

The generalized multi-channel Kondo model: Thermodynamics and fusion equations

The SU(N) generalization of the multi-channel Kondo model with arbitrary rectangular impurity representations is considered by means of the Bethe Ansatz. The thermodynamics of the model is analyzed by introducing modified fusion equations for the impurity, leading to a simple description of the different IR fixed points of the theory. The entropy at zero temperature is discussed; in particular the overscreened case is explained in terms of quantum group representation.Comment: 41 pages, 8 figures, harvma

General variational many-body theory with complete self-consistency for trapped bosonic systems

In this work we develop a complete variational many-body theory for a system of $N$ trapped bosons interacting via a general two-body potential. In this theory both the many-body basis functions {\em and} the respective expansion coefficients are treated as variational parameters. The optimal variational parameters are obtained {\em self-consistently} by solving a coupled system of non-eigenvalue -- generally integro-differential -- equations to get the one-particle functions and by diagonalizing the secular matrix problem to find the expansion coefficients. We call this theory multi-configurational Hartree for bosons or MCHB(M), where M specifies explicitly the number of one-particle functions used to construct the configurations. General rules for evaluating the matrix elements of one- and two-particle operators are derived and applied to construct the secular Hamiltonian matrix. We discuss properties of the derived equations. It is demonstrated that for any practical computation where the configurational space is restricted, the description of trapped bosonic systems strongly depends on the choice of the many-body basis set used, i.e., self-consistency is of great relevance. As illustrative examples we consider bosonic systems trapped in one- and two-dimensional symmetric and asymmetric double-well potentials. We demonstrate that self-consistency has great impact on the predicted physical properties of the ground and excited states and show that the lack of self-consistency may lead to physically wrong predictions. The convergence of the general MCHB(M) scheme with a growing number M is validated in a specific case of two bosons trapped in a symmetric double-well.Comment: 53 pages, 8 figure

Understanding the Heavy Fermion Phenomenology from Microscopic Model

We solve the 3D periodic Anderson model via two impurity DMFT. We obtain the temperature v.s. hybridization phase diagram. In approaching the quantum critical point (QCP) both the Neel and lattice Kondo temperatures decrease and they do not cross at the lowest temperature we reached. While strong ferromagnetic spin fluctuation on the Kondo side is observed, our result indicates the critical static spin susceptibility is local in space at the QCP. We observe in the crossover region logarithmic temperature dependence in the specific heat coefficient and spin susceptibility

Integer filling metal insulator transitions in the degenerate Hubbard model

We obtain exact numerical solutions of the degenerate Hubbard model in the limit of large dimensions (or large lattice connectivity). Successive Mott-Hubbard metal insulator transitions at integer fillings occur at intermediate values of the interaction and low enough temperature in the paramagnetic phase. The results are relevant for transition metal oxides with partially filled narrow degenerate bands.Comment: 4 pages + 4 figures (in 5 ps-files), revte

Spin waves in quasi-equilibrium spin systems

Using the Landau Fermi liquid theory we have discovered a new regime for the propagation of spin waves in a quasi-equilibrium spin systems. We have determined the dispersion relation for the transverse spin waves and found that one of the modes is gapless. The gapless mode corresponds to the precessional mode of the magnetization in a paramagnetic system in the absence of an external magnetic field. One of the other modes is gapped which is associated with the precession of the spin current around the internal field. The gapless mode has a quadratic dispersion leading to some interesting thermodynamic properties including a $T^{3/2}$ contribution to the specific heat. We also show that these modes make significant contributions to the dynamic structure function.Comment: 4 pages, 3 figure