Search Results (105 total)

We studied the intensity of resonant Raman scattering due to optical phonons in a planar II-VI-type semiconductor microcavity in the regime of strong coupling between light and matter. Two different sets of independent experiments were performed at near outgoing resonance with the middle polariton (MP) branch of the cavity. In the first, the Stokes-shifted photons were kept at exact resonance with the MP, varying the photonic or excitonic character of the polariton. In the second, only the incoming light wavelength was varied, and the resonant profile of the inelastic scattered intensity was studied when the system was tuned out of the resonant condition. Taking some matrix elements as free parameters, both independent experiments are quantitatively described by a model which incorporates lifetime effects in both excitons and photons, and the coupling of the cavity photons to the electron-hole continuum. The model is solved using a Green’s function approach which treats the exciton-photon coupling nonperturbatively.

We study the conductance through Aharonov-Bohm finite ladder rings with strongly interacting electrons, modeled by the prototypical t-J model. For a wide range of parameters we observe characteristic dips in the conductance as a function of magnetic flux, predicted so far only in chains which are a signature of spin and charge separation. These results open the possibility of observing this peculiar many-body phenomenon in anisotropic ladder systems and in real nanoscopic devices.

The physical properties including magnetic susceptibility, magnetization, specific heat, and dynamic susceptibility χ^″(E) are reported for single crystals of the cubic UM₂Zn₂₀  (M=Co,Rh) materials. Maxima in the thermodynamic data at T_max∼10 K for both compounds and a broad peak in χ^″(E) at 5 K in UCo₂Zn₂₀ of width Γ=5 meV indicate a heavy-fermion state characterized by a Kondo temperature T_K∼20–30 K arising from weak hybridization of f- and conduction-electron states. Anderson impurity model fits to the data in the Kondo limit including crystalline electric-field effects corroborate an ionic-like uranium electronic configuration in UM₂Zn₂₀.

We present a geometric characterization of the ferrotoroidic moment τ in terms of a set of Abelian Berry phases. We also introduce a fundamental complex quantity z_μν, which provides an alternative way to calculate τ and its moments and is derived from the tensor T_μν=2∑jr_j^μS_j^ν. This geometric framework defines a natural computational approach for density functional and many-body theories.

We study the conductance through a ring described by the Hubbard model (such as an array of quantum dots), threaded by a magnetic flux and subject to Rashba spin-orbit coupling (SOC). We develop a formalism that is able to describe the interference effects as well as the Kondo effect when the number of electrons in the ring is odd. In the Kondo regime, the SOC reduces the conductance from the unitary limit, and, in combination with the magnetic flux, the device acts as a spin polarizer.

A Comment on the Letter by Luis G. G. V. Dias da Silva et al., [Phys. Rev. Lett. 97, 096603 (2006)]. The authors of the Letter offer a Reply.

We determine the quantum phase diagram of the one-dimensional Hubbard model with bond-charge interaction X in addition to the usual Coulomb repulsion U>0 at half-filling. For large enough X<t the model shows three phases. For large U the system is in the spin-density wave phase as in the usual Hubbard model. As U decreases, there is first a spin transition to a spontaneously dimerized bond-ordered wave phase and then a charge transition to a novel phase in which the dominant correlations at large distances correspond to an incommensurate singlet superconductor.

We study the spectral density of electrons ρ_dσ(ω) in an interacting quantum dot (QD) with a hybridization λ to a noninteracting QD, which, in turn, is coupled to a noninteracting conduction band. The system corresponds to an impurity Anderson model in which the conduction band has a Lorentzian density of states of width Δ₂. We solved the model using perturbation theory in the Coulomb repulsion U (PTU) up to second order and a slave-boson mean-field approximation (SBMFA). The PTU works surprisingly well near the exactly solvable limit Δ₂→0. For fixed U and large enough λ or small enough Δ₂, the Kondo peak in ρ_dσ(ω) splits into two peaks. This splitting can be understood in terms of weakly interacting quasiparticles. Before the splitting takes place, the universal properties of the model in the Kondo regime are lost. Using the SBMFA, simple analytical expressions for the occurrence of split peaks are obtained. For small or moderate Δ₂, the side bands of ρ_dσ(ω) have the form of narrow resonances that were missed in previous studies using the numerical renormalization group. This technique also has shortcomings for properly describing the split Kondo peaks. As the temperature is increased, the intensity of the split Kondo peaks decreases, but it is not completely suppressed at high temperatures.

We construct an effective Hamiltonian for the motion of T_2g highly correlated states in Na_xCoO₂. We solve exactly a multiband model in a CoO₆ cluster with electronic occupation corresponding to a nominal Co valence of either +3 or +4. Using the ensuing ground states, we calculate the effective O mediated hopping t=0.10 eV between many-body T_2g states and estimate the direct hopping t^′∼0.05 eV. The trigonal splitting 3D=0.315 eV is taken from recent quantum chemistry calculations. The resulting effective Hamiltonian is solved using a generalized slave-boson mean-field approximation. The results show a significant band renormalization and a Fermi-surface topology that agrees with experiment, in contrast to predictions using the local-density approximation.

We study the differential conductance, spectral density, and magnetization for a quantum dot coupled to two conducting leads as a function of bias voltage V_ds, magnetic field B, and temperature T. The system is modeled with the Anderson model solved using a spin-dependent interpolative perturbative approximation in the Coulomb repulsion U that conserves the current. For large enough magnetic field, the differential conductance as a function of bias voltage shows split peaks. This splitting is larger than the corresponding splitting in the spectral density of states ρ_d(ω), in agreement with experiment.

We propose a simple approach to study the conductance through an array of N interacting quantum dots, weakly coupled to metallic leads. Using a mapping to an effective site which describes the low-lying excitations and a slave-boson representation in the saddle-point approximation, we calculated the conductance through the system. Explicit results are presented for N=1 and N=3: a linear array and an isosceles triangle. For N=1 in the Kondo limit, the results are in very good agreement with previous results obtained with numerical renormalization group. In the case of the linear trimer for odd N, when the parameters are such that electron-hole symmetry is induced, we obtain perfect conductance G₀=2e²∕h. The validity of the approach is discussed in detail.

In order to shed light on the electronic structure of Na_xCoO₂, and motivated by recent Co L-edge x-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a CoO₆ cluster including all interactions between 3d orbitals. We obtain the ground state for two electronic occupations in the cluster that correspond nominally to all O in the O⁻² oxidation state, and Co⁺³ or Co⁺⁴. Then, all excited states obtained by promotion of a Co 2p electron to a 3d electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large Co-O covalency and cubic crystal field effects, that result in low spin Co 3d configurations. Our results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states.

We investigate the ground-state phase diagram of the Hubbard model for the AB_N−1 chain with filling 1∕N, where N is the number of atoms per unit cell. In the strong-coupling limit, a charge transition takes place from a band insulator (BI) to a correlated insulator for increasing on-site repulsion U and positive on-site energy difference Δ (energy at A sites lower than at B sites). In the weak-coupling limit, a bosonization analysis suggests that for N>2 the physics is qualitatively similar to the case N=2 which has already been studied: an intermediate phase emerges, which corresponds to a bond-ordered ferroelectric insulator (FI) with spontaneously broken inversion symmetry. We have determined the quantum phase diagram for the cases N=3 and N=4 from the crossings of energy levels of appropriate excited states, which correspond to jumps in the charge and spin Berry phases, and from the change of sign of the localization parameter z_L^c. From these techniques we find that, quantitatively, the BI and FI phases are broader for N>2 than when N=2, in agreement with the bosonization analysis. Calculations of the Drude weight and z_L^c indicate that the system is insulating for all parameters, with the possible exception of the boundary between the BI and FI phases.

I consider a Hubbard-Anderson model which describes localized orbitals in three different atoms hybridized both among themselves and with a continuum of extended states. Using a generalized Schrieffer-Wolf transformation, I derive an effective Kondo model for the interaction between the doublet ground state of the isolated trimer and the extended states. For an isoceles trimer with distances a, l, l between the atoms, the Kondo temperature is very small for l<a and has a maximum for finite l>a when a is small. The results agree with experiments for a Cr trimer on Au(111).

The intermediate valence compounds Yb₂M₃Ga₉ (M=Rh,Ir) exhibit an anisotropic magnetic susceptibility. We report measurements of the temperature dependence of the 4f occupation number, n_f(T), for Yb₂M₃Ga₉ as well as the magnetic inelastic neutron scattering spectrum S_mag(ΔE) at 12 and 300 K for Yb₂Rh₃Ga₉. Both n_f(T) and S_mag(ΔE) were calculated for the Anderson impurity model with crystal field terms within an approach based on the noncrossing approximation. These results corroborate the importance of crystal field effects in these materials; they also suggest that Anderson lattice effects are important to the physics of Yb₂M₃Ga₉.

Across a faceted (100)/(110) interface between two d_x²-y² superconductors the structure of the superconducting order parameter leads to an alternating sign of the local Josephson coupling. Describing the Cooper pair motion along and across the interface by a one-dimensional boson lattice model, we show that a small attractive interaction between the bosons strongly enhances their binding at the interface. As a consequence, we propose that electrons tunnel in quartets across an interface with a staggered sequence of 0- and π-junction contacts. We connect this finding to the recently observed h/4e oscillations in bicrystalline (100)/(110) SQUIDs of cuprate superconductors.

Magnetic susceptibility, heat capacity, and electrical resistivity measurements have been carried out on single crystals of the intermediate valence compounds Yb₂Rh₃Ga₉ and Yb₂Ir₃Ga₉. These measurements reveal a large anisotropy due apparently to an interplay between crystalline electric field (CEF) and Kondo effects. The temperature dependence of magnetic susceptibility can be modelled using the Anderson impurity model including CEF within an approach based on the non-crossing approximation.

We derive an effective Hamiltonian for the ionic Hubbard model at half filling, extended to include nearest-neighbor repulsion. Using a spin-particle transformation, the effective model is mapped onto simple spin-1 models in two particular cases. Using another spin-particle transformation, a slightly modified model is mapped into an SU(3) antiferromagnetic Heisenberg model whose exact ground state is known to be spontaneously dimerized. From the effective models several properties of the dimerized phase are discussed, like ferroelectricity and fractional charge excitations. Using bosonization and recent developments in the theory of macroscopic polarization, we show that the polarization is proportional to the charge of the elementary excitations.

We calculate the stopping force of charged particles traveling parallel to the axis of a cylindrical wire due to collective excitations. We used a recently developed formalism, based on a semiclassical dielectric function and the interaction of the particle with bulk and surface collective modes. The equivalence of both classical and quantum-mechanical results is demonstrated. The different contributions to the stopping force are discussed in detail on the basis of analytical and numerical results. We propose a simple way to handle the divergences due to excitations of some bulk modes.

We study the spin-dependent zero-bias conductance through a quantum wire with a single ballistic channel side coupled to a quantum dot as a function of gate voltage V_g, magnetic field B, and temperature T. The system is modeled with the Anderson model, solved using an interpolative approximation for moderate values of the Coulomb repulsion U, and within the slave-boson mean field for infinite U and T=0. Tuning V_g, for large enough B and small enough T, the system is a very efficient spin filter.

We analyze the phase transitions of an interacting electronic system weakly coupled to free-electron leads by considering its zero-bias conductance. This is expressed in terms of two effective impurity models for the cases with and without spin degeneracy. Using the half-filled ionic Hubbard ring, we demonstrate that the weight of the first conductance peak as a function of external flux or of the difference in gate voltages between even and odd sites allows one to identify the topological charge transition between a correlated insulator and a band insulator.

We start from an effective Hamiltonian for Ru ions in a square lattice, which includes the on-site interactions between t_2g orbitals derived from Coulomb repulsion, and a tetragonal crystal-field splitting. Using perturbation theory in the hopping terms, we derive effective Hamiltonians to describe the RuO₂ planes of RuSr₂(Eu,Gd)Cu₂O₈. For undoped planes (formal valence Ru⁺⁵), depending on the parameters we find three possible orderings of spin and orbitals, and construct a phase diagram. This allows us to put constraints on the parameters based on experimental data. When electron doping consistent with the hole doping of the superconducting RuO₂ planes is included, we obtain (for reasonable parameters) a double-exchange model with infinite antiferromagnetic coupling between itinerant electrons and localized spins. This model is equivalent to one used before [H. Aliaga and A. A. Aligia, Physica B 320, 34 (2002)], which consistently explains the seemingly contradictory magnetic properties of RuSr₂(Eu,Gd)Cu₂O₈.