In-gap Excitations Research Articles

Evidence for Short-Range Antiferromagnetic Fluctuations in Kondo-InsulatingYbB12

The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a high-quality single crystal. In the Kondo-insulating regime realized at low temperature, the spectra exhibit a spin-gap structure with two sharp, dispersive, in-gap excitations at E = 14.5 and approximately 20 meV. The lower mode is shown to be associated with short-range correlations near the antiferromagnetic wave vector q0 = (1/2, 1/2, 1/2). Its properties are in overall agreement with those expected for a "spin exciton'' branch in an indirect hybridization gap semiconductor.

Pairing gaps in atomic gases at the BCS–BEC crossover

Strong evidence for pairing and superfluidity has recently been found in atomic Fermi gases at the BCS–BEC crossover both in collective modes and radio frequency (RF) excitation energies. It is argued that the scale for the effective pairing gaps measured in RF experiments is set by the lowest quasiparticle in-gap excitation energies. These are calculated at the BCS–BEC crossover from semiclassical solutions to the Bogoliubov–deGennes equations. The strong damping of the radial breathing mode observed in the BCS limit occurs when the lowest quasiparticle excitation energies coincide with the radial frequency, which indicates that a coupling between them takes place.

Pairing gap and in-gap excitations in trapped fermionic superfluids.

We consider trapped atomic Fermi gases with Feshbach-resonance enhanced interactions in pseudogap and superfluid temperatures. We calculate the spectrum of radio-frequency (or laser) excitations for transitions that transfer atoms out of the superfluid state. The spectrum displays the pairing gap and also the contribution of unpaired atoms, that is, in-gap excitations. The results support the conclusion that a superfluid, in which pairing is a manybody effect, was observed in recent experiments on radio-frequency spectroscopy of the pairing gap.

Collapse of the coherence gap in Kondo semiconductors

Kondo semiconductors exhibit a gap in their low-temperature excitation spectra, and can be modeled by the Anderson lattice model at half filling which shows a hybridization gap. The mixed-valent Kondo semiconductor ${\mathrm{SmB}}_{6}$ also shows in-gap magnetic excitations that exhibit the temperature dependence and dispersion that are expected from magnetic excitons. The gaps measured in the Kondo semiconductors are extremely sensitive to doping in which the f ions are substituted by non-$f$ ions. Recent measurements on ${\mathrm{YbB}}_{12}$ shows a feature similar to that of the magnetic exciton in ${\mathrm{SmB}}_{6}.$ Furthermore, the concentration dependence of the features observed in inelastic neutron-scattering spectra of doped ${\mathrm{YbB}}_{12}$ is different from the concentration dependence found in measurements of thermodynamic and transport properties. The impurity concentration dependence of the gap and the spin-exciton features are investigated. The spectroscopic properties are calculated for the Anderson lattice model at half filling, in which the disorder is treated in the coherent potential approximation. The results are compared with the recent experiments.

Zn-doping effects on low-energy spin excitations in La1.85Sr0.15CuO4

A review is given on neutron scattering study of Zn-doping effects on low-energy spin excitations in optimally hole-doped La 2− x Sr x CuO 4 . As Zn-impurities increase, energy spectra of incommensurate spin excitations show a dramatic change from a well defined “spin gap” state to a gap-less excitation with static antiferromagnetic (AF) spin correlations. This modification of the gap structure can be understood as not simply reduction of the gap energy but a development of the Zn-induced in-gap excitation, which becomes more static with increasing Zn. An inhomogeneous mixed state of the in-gap spin excitation and the superconductivity is discussed with referring to the swiss cheese model.

Single-particle excitations and the order parameter for a trapped superfluid Fermi gas

It is found that the character of single-particle excitations of a trapped neutral-atom Fermi gas is strongly influenced by a superfluid phase transition. Below the transition temperature the presence of a spatially inhomogeneous order parameter (gap) shifts the excitation eigenenergies upward and leads to the appearance of in-gap excitations localized in the outer part of the gas sample. The eigenenergies become sensitive to the gas temperature and are no longer multiples of the trap frequencies. These features should manifest themselves in a strong change of the density oscillations induced by modulations of the trap frequencies and can be used for identifying the superfluid phase transition.

Valence-bond theory of linear Hubbard and Pariser-Parr-Pople models

The ground and low-lying states of finite quantum-cell models with one state per site are obtained exactly through a real-space basis of valence-bond (VB) diagrams that explicitly conserve the total spin. Regular and alternating Hubbard and Pariser-Parr-Pople (PPP) chains and rings with ${N}_{e}$ electrons on $N(\ensuremath{\le}12)$ sites are extrapolated to infinite arrays. The ground-state energy and optical gap of regular $U=4|t|$ Hubbard chains agree with exact results, suggesting comparable accuracy for alternating Hubbard and PPP models, but differ from mean-field results. Molecular PPP parameters describe well the excitations of finite polyenes, odd polyene ions, linear cyanine dyes, and slightly overestimate the absorption peaks in polyacetylene ${(\mathrm{CH})}_{x}$. Molecular correlations contrast sharply with uncorrelated descriptions of topological solitons, which are modeled by regular polyene radicals and their ions for both wide and narrow alternation crossovers. Neutral solitons have no midgap absorption and negative spin densities, while the intensity of the in-gap excitation of charged solitons is not enhanced. The properties of correlated states in quantum-cell models with one valence state per site are discussed in the adiabatic limit for excited-state geometries and instabilities to dimerization.