arXiv subjects

G. Jackeli

Publications and source records attributed to G. Jackeli.

26 records · Page 2Linked to original sources

arXiv2001

Ground state properties and excitation spectra of non-Galilean invariant interacting Bose systems

We study the ground state properties and the excitation spectrum of bosons which, in addition to a short-range repulsive two body potential, interact through the exchange of some dispersionless bosonic modes. The latter induces a time dependent (retarded) boson-boson interaction which is attractive in the static limit. Moreover the coupling with dispersionless modes introduces a reference frame for the moving boson system and hence breaks the Galilean invariance of this system. The ground state of such a system is depleted {\it linearly} in the boson density due to the zero point fluctuations driven by the retarded part of the interaction. Both quasiparticle (microscopic) and compressional (macroscopic) sound velocities of the system are studied. The microscopic sound velocity is calculated up the second order in the effective two body interaction in a perturbative treatment, similar to that of Beliaev for the dilute weakly interacting Bose gas. The hydrodynamic equations are used to obtain the macroscopic sound velocity. We show that these velocities are identical within our perturbative approach. We present analytical results for them in terms of two dimensional parameters -- an effective interaction strength and an adiabaticity parameter -- which characterize the system. We find that due the presence of several competing effects, which determine the speed of the sound of the system, three qualitatively different regimes can be in principle realized in the parameter space and discuss them on physical grounds.

cond-mat.supr-con↗

arXiv2001

Superfluidity of bosons on a deformable lattice

We study the superfluid properties of a system of interacting bosons on a lattice which, moreover, are coupled to the vibrational modes of this lattice, treated here in terms of Einstein phonon model. The ground state corresponds to two correlated condensates: that of the bosons and that of the phonons. Two competing effects determine the common collective soundwave-like mode with sound velocity $v$, arising from gauge symmetry breaking: i) The sound velocity $v_0$ (corresponding to a weakly interacting Bose system on a rigid lattice) in the lowest order approximation is reduced due to reduction of the repulsive boson-boson interaction, arising from the attractive part of phonon mediated interaction in the static limit. ii) the second order correction to the sound velocity is enhanced as compared to the one of bosons on a rigid lattice when the the boson-phonon interaction is switched on due to the retarded nature of phonon mediated interaction. The overall effect is that the sound velocity is practically unaffected by the coupling with phonons, indicating the robustness of the superfluid state. The induction of a coherent state in the phonon system, driven by the condensation of the bosons could be of experimental significance, permitting spectroscopic detections of superfluid properties of the bosons. Our results are based on an extension of the Beliaev - Popov formalism for a weakly interacting Bose gas on a rigid lattice to that on a deformable lattice with which it interacts.

cond-mat.supr-con↗

arXiv2000

Charge and magnetic order in two-orbital double-exchange model for manganites

Phase diagram of half-doped perovskite manganites is studied within the extended double-exchange model. To demonstrate the role of orbital degrees of freedom both one- and two-orbital models are examined. A rich phase diagram is obtained in the mean-field theory at zero temperature as a function of J (antiferromagnetic (AFM) superexchange interaction) and V (intersite Coulomb repulsion). For the one-orbital model a charge-ordered (CO) state appears at any value of V with different types of magnetic order which changes with increasing J from ferromagnetic (F) to AFM ones of the types A, C and G. The orbital degeneracy results in appearance of a new CE-type spin order that is favorable due to opening of the "dimerization" gap at the Fermi surface. In addition, the CO state appears only for V>V_c for F and CE states while C-type AFM state disappears and A-type AFM state is observed only at small values of V as a charge disordered one. The relevance of our results to the experimental data are dicussed.

cond-mat.str-el↗

arXiv1999

Dynamic spin susceptibility in the t-J model

Dynamic spin susceptibility is calculated for the t-J model in the paramagnetic phase by applying the memory function method in terms of the Hubbard operators. A self-consistent system of equations for the memory function is obtained within the mode coupling approximation. Both itinerant hole excitations and localized spin fluctuations give contributions to the memory function. Spin dynamics have a diffusive character in the hydrodynamic limit; spin-wave-like excitations are regained in the high-frequency region.

cond-mat.str-el↗

arXiv1999

Normal Fermi Liquid Behavior of Quasiholes in the Spin-Polaron Model for Copper Oxides

Based on the t-J model and the self-consistent Born approximation, the damping of quasiparticle hole states near the Fermi surface is calculated in a low doping regime. Renormalization of spin-wave excitations due to hole doping is taken into account. The damping is shown to be described by a familiar form $\text{Im}Σ({\bf k}^{\prime},ε)\propto (ε^{2}/ ε_{F})\ln(ε/ ε_{F})$ characteristic of the 2-dimensional Fermi liquid, in contrast with the earlier statement reported by Li and Gong [Phys. Rev. B {\bf 51}, 6343 (1995)] on the marginal Fermi liquid behavior of quasiholes.

cond-mat.str-el↗

arXiv1999

Alternating chain with Hubbard-type interactions: renormalization group analysis

The canonical transformation diagonalizing the one-particle tight binding Hamiltonian for an alternating chain with two non-equivalent sites per unit cell has been used to introduce the Hubbard-like interactions (on-site, inter-site, bond-site and intra-bond) in the corresponding two-band model. The considerations have been restricted to the particular case, suggested by the undistorted 3/4-filled $CuO_{3}$ chain, where the alternating structure comes only from non-equal on-site atomic energies and the gap between the two bands is sufficiently large. The renormalization group method has been then applied to the upper band of this model and the corresponding phase diagram has been analyzed in terms of some renormalized (density dependent) Hubbard-type couplings for arbitrary filling of the upper band. The particular case of $CuO_{3}$ chain is also discussed and the corresponding phase diagram has been drawn in terms of the original (and realistic) coupling constants.

cond-mat.str-el↗

arXiv1999

Renormalization group approach to the one-dimensional 1/4-filled Hubbard model with alternating on-site interactions

The one-dimensional Hubbard model with different on-site interactions is investigated by renormalization group technique. In the case of a 1/4-filled band the dynamical nonequivalence of sites leads to the appearance of Umklapp processes in the system and to the dynamical generation of a gap in the charge excitation spectrum for $U_{a}\not=U_{b}$, $U_{a}>0$ or $U_{b}>0$. The ground-state phase diagram is obtained in the limit of second order renormalization. Depending on the sign and relative values of the bare coupling constants, there is a gap in the spin or charge excitation spectrum and the model system tends to superconducting or antiferromagnetic order at T=0, with doubled period. The role of interaction between particles on nearest and next-nearest neighbor sites is also considered.

cond-mat.str-el↗

arXiv1999

Charge dynamics and optical conductivity of the t-J model

The dynamic charge susceptibility and the optical conductivity are calculated in the planar t-J model within the memory function method, working directly in terms of Hubbard operators. The density fluctuation spectrum consists of a damped sound-like mode for small wave vectors and a broad high energy peak ($\sim t$) for large momenta. The study of the optical conductivity shows that electron scattering from spin fluctuations leads to the Drude-frequency dependent relaxation rate which exhibits a crossover from $ω^{3/2}$ behavior at low frequencies ($ω<2|μ|$), to a linear $ω$-dependence for frequencies larger than $2|μ|$. Due to the spin-polaron nature of charge carriers, extra absorbtions arise starting at a frequency $ω\agt J$. The obtained results are in a good agreement with exact diagonalization studies.

cond-mat.str-el↗