arXiv subjects

Ph. Brune

Publications and source records attributed to Ph. Brune.

4 recordsLinked to original sources

arXiv2003

Nature of the insulating phases in the half-filled ionic Hubbard model

We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. We identify an insulator-insulator phase transition from a band to a correlated insulator with simultaneous charge and bond-charge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed.

cond-mat.str-el↗

arXiv2000

η-superconductivity in the Hubbard chain with pair hopping

The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper pair center-of-mass momentum Q=0 (BCS-η_0 phase) and Q=π(η_π-phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator as well as an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting BCS-η_0 and η_πphases and the metallic Luttinger-liquid phase. The BCS-η_0 phase exhibits smooth crossover from a weak-coupling BCS type to a strong coupling local-pair regime. The η_πphase shows properties of the doublon (zero size Cooper pair) superconductor with Cooper pair center-of-mass momentum Q=π. The transition into the η_π- paired state corresponds to an abrupt change in the groundstate structure. After the transition the conduction band is completely destroyed and a new η_π-pair band corresponding to the strongly correlated doublon motion is created.

cond-mat.str-el↗

arXiv2000

Anisotropy of the Energy Gap in the Insulating Phase of the U-t-t' Hubbard Model

We apply a diagrammatic expansion method around the atomic limit (U >> t) for the U-t-t' Hubbard model at half filling and finite temperature by means of a continued fraction representation of the one-particle Green's function. From the analysis of the spectral function A(\vec{k},ω) we find an energy dispersion relation with a (cos k_x-cos k_y)^2 modulation of the energy gap in the insulating phase. This anisotropy is compared with experimental ARPES results on insulating cuprates.

cond-mat.str-el↗

arXiv1997

Photon-Assisted Transport Through Ultrasmall Quantum Dots: Influence of Intradot Transitions

We study transport through one or two ultrasmall quantum dots with discrete energy levels to which a time-dependent field is applied (e.g., microwaves). The AC field causes photon-assisted tunneling and also transitions between discrete energy levels of the dot. We treat the problem by introducing a generalization of the rotating-wave approximation to arbitrarily many levels. We calculate the dc-current through one dot and find satisfactory agreement with recent experiments by Oosterkamp et al. . In addition, we propose a novel electron pump consisting of two serially coupled single-level quantum dots with a time-dependent interdot barrier.

cond-mat.mes-hall↗