Spectrum Of Elementary Excitations Research Articles

Signatures of multigap superconductivity in tunneling spectroscopy

We considered a two-band superconductor with a nonzero interband quasiparticle coupling and numerically generated partial elementary excitation spectra for each band. These show deviations from the conventional Bardeen, Cooper, and Schrieffer form, resulting in characteristic signatures in the partial tunneling spectra. The total (measurable) tunneling spectra are calculated considering the $\mathbf{k}$ selection in the tunneling process. Due to the thermal smearing, the relevant spectral signatures may not be resolved in superconductor-insulator-normal-metal tunneling while they are clearly revealed in superconductor-insulator-superconductor (SIS) geometry. As an example, the excitation spectrum of $2{\text{H-NbSe}}_{2}$ is considered in the framework of the developed tunneling model. A remarkable agreement obtained with the experimental SIS data suggests the material to be a two-band superconductor rather than an anisotropic one.

Charged-boson fluid at zero-temperature in the fractional dimensional space

In the fractional dimensional space, the ground state properties of the charged-boson fluid are studied in a theory which goes beyond the random phase approximation by including correlations through a static local-field-factor. The structure factor and static local field-factor are obtained in the self-consistent method. Qualitative agreement with the Monte Carlo results on static screening is achieved by imposing self-consistency on the compressibility of the fluid in addition to self-consistency on the structure factor. Using the structure factors obtained in these methods, several properties of the charged boson system such as the energy spectrum of elementary excitations, the pair-correlation function, the ground state energy, the pressure, the chemical potential and the compressibility are obtained in several dimensions including both integer and fractional values. Results obtained in the later method in two and three dimensional systems are close to the Monte Carlo and hypernetted-chain methods. The Wigner crystallisation in the lower dimensional system is found at higher density of bosons. However, it vanishes in any lower dimensional system whose dimension lies below 2.

Dynamical equation for an electron-hole pair condensate in a system of two graphene layers

In bilayer graphene n-p systems the electron-hole pairs with spatially separated components at low temperatures transition into a superfluid state. A microscopic derivation of the dynamical equation for the wave function of the electron-hole pair condensate in a strong perpendicular magnetic field is given for low pair density. The temperature of the transition into a superfluid state is calculated in a wide interval of interlayer distances and magnetic fields on the basis of the elementary excitation spectrum.

Properties of a two-dimensional semimetal in a strong magnetic field

A system of two-dimensional electrons and holes ha s been investigated in a strong magnetic field, when it is sufficient to take into account only the ground Landau level. It has been shown that the interaction of electrons and holes can lead to an ordered state. In this problem, the exchange interaction in electron and hole subsystems is significant. The following two cases have been considered: (a) there are one electron and one hole valleys, and at some magnetic field strength, there exists an ordered state, as in an excitonic insulator; and (b) there exist one electron and two equivalent hole valleys (as in the experiment performed by Kvon et al. [1]), and the hole system has an ordered state of the Stoner ferromagnetic type in a specific range of magnetic field strengths. The spectra of elementary excitations of the Bose and Fermi types have been obtained. The Fermi excitations have a gap in the energy spectrum, whereas the Bose excitations in the ordered states begin with zero (to these excitations there corresponds an electric dipole moment). The self-consistent field approximation has been used, which is exact when the numbers of electrons and holes are equal to each other.

Competition between superconducting pair potential and Rashba spin–orbit interaction in a hybrid mesoscopic ring

We numerically investigate an isolated hybrid ring consisting of two segments, one is an s-wave superconductor and the other is two dimensional electron gas with Rashba spin–orbit interaction. Considering an Aharonov–Bohm flux through the hole of the ring, we calculate the elementary excitation spectrum and the persistent current at zero temperature and finite temperature. It is found that there is a competition between superconducting pair potential and Rashba spin–orbit interaction. At zero temperature the competition is mainly determined by the length of the superconducting segment d S . When the superconducting segment is short, quasi-particles is easy to tunnel through the superconducting segment and Rashba spin–orbit interaction effect is dominant which results in the elementary excitation spectrum splits and the persistent current obtains an additional peak. On the contrary, when the superconducting segment is long enough, the quasi-particles are difficult to tunnel through the superconducting segment and they enter the superconducting segment via Andreev reflection process. In this case Rashba spin–orbit interaction does not cause the split of the elementary excitation spectrum inside the energy gap, and thus the persistent current has no additional peak. At finite temperature, when the superconducting segment is short, although the spin split effect still exists, the peak of the persistent current resulting from Rashba spin–orbit interaction gradually disappears as the temperature rises.

Many-Body Fluctuations and the Dynamics of Liquid 4He

It has been firmly established that dynamic pair fluctuations make important contributions to medium- and short-wavelength behavior of the dynamic structure function of superfluid 4He. These fluctuations have, so far, been treated in the so-called “uniform limit approximation.” In the first part of this work we show that the full solution of the equation of motion for pair fluctuations does not significantly change the results of the time-honored uniform limit approximation. However, pair fluctuations still fall significantly short of fully accounting for the medium-wavelength structure of the elementary excitation spectrum, i.e. the roton energy, and the energy of the short wavelength plateau seen in the dynamic structure function as measured by neutron scattering. Here we show how the inclusion of dynamic triplet fluctuations produces very satisfactory agreement with experiment. Insight into these results is obtained by examining them in terms of correlated Brillouin-Wigner perturbation theory just as it did for the physics of the contributions of pair fluctuations in previous work.

Collective properties and combined quantum transitions of two‐dimensional magnetoexcitons

AbstractThe article consists of two parts describing two associated properties of two‐dimensional magnetoexcitons. In the first part, the theory of combined exciton‐cyclotron resonance in the quantum well structures is developed for a strong magnetic field. In the first part, we calculate the absorption band structure for optical quantum transitions with circularly polarized radiation creating a two‐dimensional exciton and simultaneously exciting one of the resident electrons from the lowest to the first excited Landau level. In the second part, we discuss the collective elementary excitations of the system of two‐dimensional magnetoexcitons in the ground state of Bose–Einstein condensation on the arbitrary wave vector $\mathop K\limits^ \to$ ≠ 0 in the Hartree–Fock approximation. The breaking of the gauge symmetry of the initial Hamiltonian was introduced following the idea proposed by Bogoliubov in his theory of quasi‐averages. The energy spectrum of the collective elementary excitations is characterized by the interconnection of the exciton and plasmon branches. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Effect of the spectrum of elementary excitations on spinodal decomposition of semiconductor alloys

A new theoretical approach to the determination of the critical temperature of spinodal decomposition of alloys is suggested. The approach is based on the inclusion of the dependences of elementary excitations existing in the system on the alloy composition. It is shown that the most substantial effect on the critical temperature is produced by equilibrium plasma oscillations. An analytical expression for the critical temperature in terms of the band -structure parameters of the alloy is derived. The role of other excitations existing in the system is analyzed. The calculated critical temperatures are correlated with the currently available theoretical and experimental results.

Collective Elementary Excitations of Two-Dimensional Magnetoexcitons in the Bose-Einstein Condensation State

The collective elementary excitations of a system of two-dimensional magnetoexcitons in a state of Bose-Einstein condensation (BEC) with arbitrary wave vector was investigated in Hartree-Fock-Bogoliubov approximation. The breaking of the gauge symmetry of the Hamiltonian was introduced following the idea proposed by Bogoliubov in his theory of quasi-averages. The equations of motion were written in the frame of the starting electron and hole creation and annihilation operators. The chains of equations of motion for a set of Green's functions describing the exciton-type excitations as well as the plasmon-type excitations were deduced. Their disconnections were introduced using the perturbation theory with a small parameter of the theory proportional to the filling factor multiplied by the phase space filling factor. The energy spectrum of the collective elementary excitations is characterized by the interconnection of the exciton and plasmon branches, because the plasmon-type elementary excitations are gapless and are lying in the same spectral interval as the exciton-type elementary excitations.

EXACT SOLUTIONS IN A SPATIALLY ANISOTROPIC QUANTUM SPIN LADDER MODEL HAVING TWO- AND FOUR-SPIN EXCHANGE INTERACTIONS

We consider a two-leg S=1/2 quantum spin ladder model with two-spin (intra-rung) and four-spin (inter-rung) Heisenberg exchange interactions and a uniform magnetic field. Exact mappings are derived connecting the partition function and correlations in the three-parameter quantum model to those known in a two-parameter Ising chain. The quantum phase diagram of the ladder magnet is determined. Static correlations provide pertinent correlation lengths and underlie spatial fluctuation behaviors at arbitrary temperatures, including quantum fluctuations at absolute zero. Dynamic correlations in zero field are used to obtain an exact solution for the inelastic neutron scattering function Sxx(q, ω) at all temperatures, explicitly yielding the elementary excitation spectrum.

Probing the excitation spectrum of polariton condensates

We propose a four-wave mixing experiment to probe the elementary excitation spectrum of a nonequilibrium Bose-Einstein condensate of exciton-polaritons under nonresonant pumping. Analytical calculations based on mean-field theory show that this method is able to reveal the characteristic negative energy feature of the Bogoliubov dispersion. Numerical simulations including the finite spatial profile of the excitation laser spot and a weak disorder confirm the practical utility of the method for realistic condensates.

Topology of the superconducting order under pairing repulsion

The superconducting state with a large momentum of a pair in a doped insulator is controlled by the features of Coulomb pairing associated with the suppression of small momentum transfers during scattering owing to electron-phonon interaction and of large transfers due to redistribution of the spectral weight among the superconducting or the insulating branches of the elementary excitation spectrum. Superconductivity emerges when the characteristic energy of the pairing interaction has an upper bound. The solutions obtained for a self-consistent equation with a simple structure of the degenerate kernel are antisymmetric and symmetric relative to inversion of the momentum of relative motion of a pair. The orbital symmetry of the singlet superconducting order is analyzed.

Elementary excitations of a two-component Fermi system using the atomic-orbital approach

We investigate the ground state properties of normal and superfluid phases of a mixture of Fermi atoms at zero temperature in a quasi-one-dimensional harmonic trap. Assuming pairing occurs in the Hartree-Fock single-particle states (obtained using atomic orbitals approach) we calculate the spectrum of elementary excitations of the system. We find that for strong enough attraction between different kinds of fermions the state consisting of Cooper pairs becomes energetically favorable. For even stronger attraction a discontinuity located at the Fermi surface appears in the spectrum of elementary excitations. At the same time the chemical potential changes its sign and the system goes from the gas of Cooper pairs to the condensed phase of molecular dimers realizing the BCS to Bose-Einstein condensate crossover in one-dimensional space.

Local-density-functional theory for superfluid fermionic systems: The unitary gas

The first detailed comparison between ab initio calculations of finite fermionic superfluid systems, performed recently by Chang and Bertsch [Phys. Rev. A 76, 021603(R) (2007)] and by von Stecher, Grange, and Blume [e-print arXiv:0705.0671v1] and the extension of the density-functional theory superfluid local-density approximation (SLDA) is presented. It is shown that SLDA reproduces the total energies, number density distributions in inhomogeneous systems along with the energy of the normal state in homogeneous systems. Unlike the Kohn-Sham LDA, in SLDA the effective fermion mass differs from the bare fermion mass and the spectrum of elementary excitations is also reproduced.

Excitations in a Nonequilibrium Bose-Einstein Condensate of Exciton Polaritons

We develop a mean-field theory of the dynamics of a nonequilibrium Bose-Einstein condensate of exciton polaritons in a semiconductor microcavity. The spectrum of elementary excitations around the stationary state is analytically studied by means of a generalized Gross-Pitaevskii equation. A diffusive behavior of the Goldstone mode is found in the spatially homogeneous case and new features are predicted for the Josephson effect in a two-well geometry.

Bose glass and superfluid phase transitions of exciton–polaritons in GaN microcavities

Microcavity exciton–polaritons within GaN-based structures are the object of the present work. The impact of the structural imperfections on the properties of the two-dimensional polariton gas is investigated through the calculation of its phase diagram. We demonstrate that the presence of disorder first induces a quasi-phase transition of the polariton system towards a Bose-glass phase before it becomes superfluid as its density increases. Calculations of the density of states as well as the condensate wavefunction and the related spectrum of elementary excitations in the framework of the Gross–Pitaevskii theory provide further insight into the properties of exciton–polaritons in GaN-based microcavities.

Effect of single-ion anisotropy on the phase states of 2D non-Heisenberg ferromagnets

The phase states and elementary excitation spectra of an anisotropic 2D non-Heisenberg ferromagnet are investigated. It is shown that easy-axis single-ion anisotropy has a strong effect on the formation of the quadrupole phase and on the appearance of the canted phase. The conditions for phase transitions with respect to the material constants and the magnetic field are determined. The phase diagrams of the system are constructed.

Crystals with sublattices and their symmetry in multidimensional crystal spaces

The symmetry of complex crystals composed of sublattices belonging to different Bravais typeshas been investigated. It is shown that description of this symmetry necessitates introduction of a multidimensional crystal space R 3k decomposing into the direct sum of k 3D orthogonal subspaces (S = 1, 2, …, k), where k is the number of sublattices. The symmetry of the subspaces R S 3 of the sublattices is a set of their crystallographic groups. The bases of subspaces R S 3 are related by the transformation of translation compatibility, at which the scale changes. A method for studying the origin of the spectra of elementary excitations in crystals is presented that is based on the analysis of the sublattice symmetry. Complex lattices composed of cubic sublattices are considered as an illustration.

Liquid 4He at Zero Temperature and the STLS Scheme

Within the framework of the self-consistent scheme proposed by Singwi, Tosi, Land and Sjolander (STLS) for an interacting system we study the properties of superfluid liquid 4He. By employing the Aziz potential (HFD-B) as the interaction potential between helium atoms, we have calculated the static structure factor, the pair-correlation function, the elementary excitation spectrum and the effective two-body interaction as a function of wave-vector, for different densities. Our results show considerable improvement over the Ng–Singwi’s model potential of a hard core plus an attractive tail and are comparable with experimental data. We have compared our results with experimental data and with the results of some theoretical models. Agreement between our results and the experimental data for the static structure factor for the small k values is fairly good.

Bose Glass and Superfluid Phases of Cavity Polaritons

We report the calculation of cavity exciton-polariton phase diagram including realistic structural disorder. With increasing density polaritons first undergo a quasiphase transition toward a Bose glass: the condensate is localized in at least one minimum of the disorder potential. A further increase of the density leads to a percolation process of the polariton fluid giving rise to a Kosterlitz-Thouless phase transition toward superfluidity. The spatial representation of the condensate wave function as well as the spectrum of elementary excitations are obtained from the Gross-Pitaevskii equation for all the phases.