arXiv subjects

D. Baeriswyl

Publications and source records attributed to D. Baeriswyl.

32 records · Page 2Linked to original sources

arXiv2003

Charge and spin order in one-dimensional electron systems with long-range Coulomb interactions

We study a system of electrons interacting through long--range Coulomb forces on a one--dimensional lattice, by means of a variational ansatz which is the strong--coupling counterpart of the Gutzwiller wave function. Our aim is to describe the quantum analogue of Hubbard's classical ``generalized Wigner crystal''. We first analyse charge ordering in a system of spinless fermions, with particular attention to the effects of lattice commensurability. We argue that for a general (rational) number of electrons per site $n$ there are three regimes, depending on the relative strength $V$ of the long--range Coulomb interaction (as compared to the hopping amplitude $t$). For very large $V$ the quantum ground state differs little from Hubbard's classical solution, for intermediate to large values of $V$ we recover essentially the Wigner crystal of the continuum model, and for small $V$ the charge modulation amounts to a small--amplitude charge--density wave. We then include the spin degrees of freedom and show that in the Wigner crystal regimes (i.e. for large $V$) they are coupled by an antiferromagnetic kinetic exchange $J$, which turns out to be smaller than the energy scale governing the charge degrees of freedom. Our results shed new light on the insulating phases of organic quasi--1D compounds where the long--range part of the interaction is unscreened, and magnetic and charge orderings coexist at low temperatures.

cond-mat.str-el↗

arXiv2002

Variational Wave Function for Generalized Wigner Lattices in One Dimension

We study a system of electrons on a one-dimensional lattice, interacting through the long range Coulomb forces, by means of a variational technique which is the strong coupling analog of the Gutzwiller approach. The problem is thus the quantum version of Hubbard's classical model of the generalized Wigner crystal [J. Hubbard, Phys. Rev. B 17, 494 (1978)]. The magnetic exchange energy arising from quantum fluctuations is calculated, and turns out to be smaller than the energy scale governing charge degrees of freedom. This approach could be relevant in insulating quasi-one-dimensional compounds where the long range Coulomb interactions are not screened. In these compounds charge order often appears at high temperatures and coexists with magnetic order at low temperatures.

cond-mat.str-el↗

arXiv2002

Wilson's renormalization group applied to 2D lattice electrons in the presence of van Hove singularities

The weak coupling instabilities of a two dimensional Fermi system are investigated for the case of a square lattice using a Wilson renormalization group scheme to one loop order. We focus on a situation where the Fermi surface passes through two saddle points of the single particle dispersion. In the case of perfect nesting, the dominant instability is a spin density wave but d-wave superconductivity as well as charge or spin flux phases are also obtained in certain regions in the space of coupling parameters. The low energy regime in the vicinity of these instabilities can be studied analytically. Although saddle points play a major role (through their large contribution to the single particle density of states), the presence of low energy excitations along the Fermi surface rather than at isolated points is crucial and leads to an asymptotic decoupling of the various instabilities. This suggests a more mean-field like picture of these instabilities, than the one recently established by numerical studies using discretized Fermi surfaces.

cond-mat.str-el↗

arXiv2000

Dielectric catastrophe at the Mott transition

We study the Mott transition as a function of interaction strength in the half-filled Hubbard chain with next-nearest-neighbor hopping t' by calculating the response to an external electric field using the Density Matrix Renormalization Group. The electric susceptibility chi diverges when approaching the critical point from the insulating side. We show that the correlation length xi characterizing this transition is directly proportional to fluctuations of the polarization and that chi ~ xi^2. The critical behavior shows that the transition is infinite-order for all t', whether or not a spin gap is present, and that hyperscaling holds.

cond-mat.str-el↗

arXiv1999

Comment on "Density Functional Simulation of a Breaking Nanowire"

In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)] described first principles calculations for a breaking Na nanocontact. Their system consists of a periodic one-dimensional array of supercells, each of which contains 39 Na atoms, originally forming a straight, crystalline wire with a length of 6 atoms. The system is elongated by increasing the length of the unit cell. At each step, the atomic configuration is relaxed to a new local equilibrium, and the tensile force is evaluated from the change of the total energy with elongation. Aside from a discontinuity of the force occuring at the transition from a crytalline to an amorphous configuration during the early stages of elongation, they were unable to identify any simple correlations between the force and the number of electronic modes transmitted through the contact. An important question is whether their model is realistic, i.e., whether it can be compared to experimental results obtained for a single nanocontact between two macroscopic pieces of metal. In this Comment, we demonstrate that with such a small unit cell, the interference effects between neighboring contacts are of the same size as the force oscillations in a single nanocontact.

cond-mat.mes-hall↗

arXiv1999

Cohesion and conductance of disordered metallic point contacts

The cohesion and conductance of a point contact in a two-dimensional metallic nanowire are investigated in an independent-electron model with hard-wall boundary conditions. All properties of the nanowire are related to the Green's function of the electronic scattering problem, which is solved exactly via a modified recursive Green's function algorithm. Our results confirm the validity of a previous approach based on the WKB approximation for a long constriction, but find an enhancement of cohesion for shorter constrictions. Surprisingly, the cohesion persists even after the last conductance channel has been closed. For disordered nanowires, a statistical analysis yields well-defined peaks in the conductance histograms even when individual conductance traces do not show well-defined plateaus. The shifts of the peaks below integer multiples of $2e^2/h$, as well as the peak heights and widths, are found to be in excellent agreement with predictions based on random matrix theory, and are similar to those observed experimentally. Thus abrupt changes in the wire geometry are not necessary for reproducing the observed conductance histograms. The effect of disorder on cohesion is found to be quite strong and very sensitive to the particular configuration of impurities at the center of the constriction.

cond-mat.mes-hall↗

arXiv1998

Optical absorption edge in one-dimensional conductors

The frequency-dependent conductivity is studied for both the one-dimensional Hubbard model and a model of spinless fermions, using a selection rule, the Bethe ansatz energy eigenstates, and conformal invariance. For densities where the system is metallic the absorption spectrum has two contributions, a Drude peak at ω= 0 separated by a pseudo-gap from a broad absorption band whose lower edge is characterized by a non-classical critical exponent. Our findings are expected to shed new light on the ``far infrared puzzle'' of metallic organic chain compounds.

cond-mat↗

arXiv1998

Solution of the infinite range t-J model

The t-J model with constant t and J between any pair of sites is studied by exploiting the symmetry of the Hamiltonian with respect to site permutations. For a given number of electrons and a given total spin the exchange term simply yields an additive constant. Therefore the real problem is to diagonalize the "t- model", or equivalently the infinite U Hubbard Hamiltonian. Using extensively the properties of the permutation group, we are able to find explicitly both the energy eigenvalues and eigenstates, labeled according to spin quantum numbers and Young diagrams. As a corollary we also obtain the degenerate ground states of the finite $U$ Hubbard model with infinite range hopping -t>0.

cond-mat.str-el↗

arXiv1997

Solution of the X-ray edge problem for 2D electrons in a magnetic field

The absorption and emission spectra of transitions between a localized level and a two-dimensional electron gas, subjected to a weak magnetic field, are calculated analytically. Adopting the Landau level bosonization technique developed in previous papers, we find an exact expression for the relative intensities of spectral lines. Their envelope function, governed by the interaction between the electron gas and the core hole, is reminescent of the famous Fermi edge singularity, which is recovered in the limit of a vanishing magnetic field.

cond-mat.mes-hall↗

arXiv1997

Jellium model of metallic nanocohesion

A unified treatment of the cohesive and conducting properties of metallic nanostructures in terms of the electronic scattering matrix is developed. A simple picture of metallic nanocohesion in which conductance channels act as delocalized chemical bonds is derived in the jellium approximation. Universal force oscillations of order epsilon_F/lambda_F are predicted when a metallic quantum wire is stretched to the breaking point, which are synchronized with quantized jumps in the conductance.

cond-mat.mes-hall↗

arXiv1997

Cooper pair delocalization in disordered media

We discuss the effect of disorder on the coherent propagation of the bound state of two attracting particles. It is shown that a result analogous to the Anderson theorem for dirty superconductors is also valid for the Cooper problem, namely, that the pair wave function is extended beyond the single-particle localization length if the latter is large. A physical justification is given in terms of the Thouless block-scaling picture of localization. These arguments are supplemented by numerical simulations. With increasing disorder we find a transition from a regime in which the interaction delocalizes the pair to a regime in which the interaction enhances localization.

cond-mat.supr-con↗

arXiv1996

Variationnal study of ferromagnetism in the t1-t2 Hubbard chain

A one-dimensional Hubbard model with nearest and (negative) next-nearest neighbour hopping is studied variationally. This allows to exclude saturated ferromagnetism for $U < U_c$. The variational boundary $U_c (n)$ has a minimum at a ``critical density'' $n_c$ and diverges for $n \rightarrow 1$.

cond-mat↗

arXiv1996

Friedel transition in a modified XY model

Weakly coupled superconducting layers are described by the three-dimensional XY model with strong coupling in two directions and weak coupling in the third direction. For the usual Josephson-type interplane coupling the coherence between the layers is lost at the same temperature as that within the layers. Thus a low-temperature layer decoupling due to a proliferation of fluxons between planes, as proposed by Friedel, does not occur in this case. However, for a modified interplane coupling there are two phase transitions, one of a Kosterlitz-Thouless type from a disordered high-temperature phase to an intermediate phase with phase coherence only parallel to the layers, the second from this effectively two-dimensional phase to a three-dimensional phase with coherence in all directions and a finite "n-state" order parameter. Thus we do find a "Friedel transition" for this special class of models.

cond-mat↗

arXiv1996

Low density ferromagnetism in the Hubbard model

A single-band Hubbard model with nearest and next-nearest neighbour hopping is studied for $d=1$, 2, 3, using both analytical and numerical techniques. In one dimension, saturated ferromagnetism is found above a critical value of $U$ for a band structure with two minima and for small and intermediate densities. This is an extension of a scenario recently proposed by Müller--Hartmann. For three dimensions and non-pathological band structures, it is proven that such a scenario does not work.

cond-mat↗