Spin Andreev-like reflection in metal-Mott insulator heterostructures.

Using the time-dependent density-matrix renormalization group (tDMRG), we study the time evolution of electron wave packets in one-dimensional (1D) metal-superconductor heterostructures. The results show Andreev reflection at the interface, as expected. By combining these results with the well-known single-spin-species electron-hole transformation in the Hubbard model, we predict an analogous spin Andreev reflection in metal-Mott insulator heterostructures. This effect is numerically confirmed using 1D tDMRG, but it is expected to also be present in higher dimensions, as well as in more general Hamiltonians. We present an intuitive picture of the spin reflection, analogous to that of Andreev reflection at metal-superconductor interfaces. This allows us to discuss a novel antiferromagnetic proximity effect. Possible experimental realizations are discussed.

Orbital disorder induced by charge fluctuations in vanadium spinels.

Motivated by recent experiments on vanadium spinels, AV2O4, that show an increasing degree of electronic delocalization for smaller cation sizes, we study the evolution of orbital ordering (OO) between the strong and intermediate-coupling regimes of a multiorbital Hubbard Hamiltonian. The underlying magnetic ordering of the Mott insulating state leads to a rapid suppression of OO due to enhanced charge fluctuations along ferromagnetic bonds. Orbital double occupancy is rather low at the transition point indicating that the system is in the crossover region between strong and intermediate-coupling regimes when the orbital degrees of freedom become disordered.

Thermal transport and strong mass renormalization in NiCl2-4SC(NH2)2.

Several quantum paramagnets exhibit magnetic-field-induced quantum phase transitions to an antiferromagnetic state that exists for H c1 ≤ H ≤ H c2. For some of these compounds, there is a significant asymmetry between the low- and high-field transitions. We present specific heat and thermal conductivity measurements in NiCl2-4SC(NH2)2, together with calculations which show that the asymmetry is caused by a strong mass renormalization due to quantum fluctuations for H ≤ H c1 that are absent for H ≥ H c2. We argue that the enigmatic lack of asymmetry in thermal conductivity is due to a concomitant renormalization of the impurity scattering.

Effective low-energy model for f-electron delocalization.

We consider a periodic Anderson model (PAM) with a momentum-dependent interband hybridization that is strongly suppressed near the Fermi level. Under these conditions, we reduce the PAM to an effective low-energy Hamiltonian, H(eff), by expanding in the small parameter V0/t (V0 is the maximum interband hybridization amplitude and t is the hopping integral of the broadband). The resulting model consists of a t - J f-band coupled via the Kondo exchange to the electrons in the broadband. H(eff) allows for studying the f-electron delocalization transition. The result is a doping-induced Mott transition for the f-electron delocalization, which we demonstrate by density-matrix renormalization group calculations.

Asymmetric quintuplet condensation in the frustrated S = 1 spin dimer compound Ba3Mn2O8.

Ba_{3}Mn_{2}O_{8} is a spin-dimer compound based on pairs of S = 1, 3d;{2}, Mn;{5+} ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero field, with excited triplet and quintuplet states at higher energy. High field thermodynamic measurements are used to establish the phase diagram, revealing a substantial asymmetry of the quintuplet condensate. This striking effect, all but absent for the triplet condensate, is due to a fundamental asymmetry in quantum fluctuations of the paramagnetic phases near the various critical fields.

Critical properties at the field-induced Bose-Einstein condensation in NiCl(2)-4SC(NH2)2.

We report new magnetization measurements on the spin-gap compound NiCl(2)-4SC(NH2)2 at the low-field boundary of the magnetic field-induced ordering. The critical density of the magnetization is analyzed in terms of a Bose-Einstein condensation of bosonic quasiparticles. The analysis of the magnetization at the transition leads to the conclusion for the preservation of the U(1) symmetry, as required for Bose-Einstein condensation. The experimental data are well described by quantum Monte Carlo simulations.

Field-induced orbital antiferromagnetism in mott insulators.

We report on a new electromagnetic phenomenon that emerges in Mott insulators. The phenomenon manifests as antiferromagnetic ordering due to orbital electric currents which are spontaneously generated from the coupling between spin currents and an external homogenous magnetic field. This novel spin-charge-current effect provides the mechanism to measure the so-far elusive spin currents by means of unpolarized neutron scattering, nuclear magnetic resonance or muon spectroscopy. We illustrate this mechanism by solving a half-filled Hubbard model on a frustrated ladder.

Excitons in the one-dimensional Hubbard model: a real-time study.

We study the real-time dynamics of a hole and doubly occupied site pair, namely, a holon and a doublon, in a 1D Hubbard insulator with on-site and nearest-neighbor Coulomb repulsion. Our analysis shows that the pair is long-lived and the expected decay mechanism to underlying spin excitations is actually inefficient. For a nonzero intersite Coulomb repulsion, we observe that part of the wave function remains in a bound state. Our study also provides insight on the holon-doublon propagation in real space. Because of the one-dimensional nature of the problem, these particles move in opposite directions even in the absence of an applied electric field. The potential relevance of our results to solar cell applications is discussed.

Numerical modeling of the central spin problem using the spin-coherent-state representation.

In this work, we consider decoherence of a central spin by a spin bath. In order to study the nonperturbative decoherence regimes, we develop an efficient mean-field-based method for modeling the spin-bath decoherence, based on the representation of the central spin density matrix. The method can be applied to longitudinal and transverse relaxation at different external fields. In particular, by modeling large-size quantum systems (up to 16 000 bath spins), we make controlled predictions for the slow long-time decoherence of the central spin.

Transport properties of strongly correlated electrons in quantum dots studied with a simple circuit model.

Numerical calculations are shown to reproduce the main results of recent experiments involving nonlocal spin control in quantum dots [Craig, Science 304, 565 (2004).]. In particular, the experimentally reported zero-bias-peak splitting is clearly observed in our studies. To understand these results, a simple "circuit model" is introduced and shown to qualitatively describe the experiments. The main idea is that the splitting originates in a Fano antiresonance, which is caused by having one quantum dot side connected in relation to the current's path. This scenario provides an explanation of the results of Craig et al. that is an alternative to the RKKY proposal, also addressed here.

Electron transport through a molecular conductor with center-of-mass motion.

The linear conductance of a molecular conductor oscillating between two metallic leads is investigated numerically both for Hubbard interacting and noninteracting electrons. The molecule-leads tunneling barriers depend on the molecule displacement from its equilibrium position. The results present an interesting interference which leads to a conductance dip at the electron-hole symmetry point that could be experimentally observable. It is shown that this dip is caused by the destructive interference between the purely electronic and phonon-assisted tunneling channels, which are found to carry opposite phases. When an internal vibrational mode is also active, the electron-hole symmetry is broken but a Fano-like interference is still observed.

Prediction of ferromagnetic correlations in coupled double-level quantum dots.

Numerical results for transport properties of two coupled double-level quantum dots (QDs) strongly suggest that under appropriate conditions the dots develop a novel ferromagnetic (FM) correlation at quarter filling (one electron per dot). In the strong coupling regime (Coulomb repulsion larger than electron hopping) and with interdot tunneling larger than tunneling to the leads, an S=1 Kondo resonance develops in the density of states, leading to a peak in the conductance. A qualitative "phase diagram," incorporating the new FM phase, is presented. In addition, the necessary conditions for the FM regime are less restrictive than naively believed, leading to its possible experimental observation in real QDs.