Quasiparticle Energy Spectrum Research Articles

Enhanced many‐body effects in one‐dimensional linear atomic chains

AbstractWe present the quasiparticle energy and optical absorption spectrum results of 1D linear H, BN, C, and Au chains, which are the lower size limit of a periodic material, by using ab initio many‐body approaches. Unprecedentedly large quasiparticle corrections and excitonic effects are revealed in these extreme systems compared with quasi‐1D, 2D, and 3D periodic systems. The binding energy of the bound exciton is up to 3.55 eV in the semiconducting BN chain and 0.5 eV in the metallic C chain, the latter of which is the largest in a metallic system.

Entropy excess in strongly correlated Fermi systems near a quantum critical point

A system of interacting, identical fermions described by standard Landau Fermi-liquid (FL) theory can experience a rearrangement of its Fermi surface if the correlations grow sufficiently strong, as occurs at a quantum critical point where the effective mass diverges. As yet, this phenomenon defies full understanding, but salient aspects of the non-Fermi-liquid (NFL) behavior observed beyond the quantum critical point are still accessible within the general framework of the Landau quasiparticle picture. Self-consistent solutions of the coupled Landau equations for the quasiparticle momentum distribution n(p) and quasiparticle energy spectrum ϵ(p) are shown to exist in two distinct classes, depending on coupling strength and on whether the quasiparticle interaction is regular or singular at zero momentum transfer. One class of solutions maintains the idempotency condition n2(p)=n(p) of standard FL theory at zero temperature T while adding pockets to the Fermi surface. The other solutions are characterized by a swelling of the Fermi surface and a flattening of the spectrum ϵ(p) over a range of momenta in which the quasiparticle occupancies lie between 0 and 1 even at T=0. The latter, non-idempotent solution is revealed by analysis of a Poincaré mapping associated with the fundamental Landau equation connecting n(p) and ϵ(p) and validated by solution of a variational condition that yields the symmetry-preserving ground state. Significantly, this extraordinary solution carries the burden of a large temperature-dependent excess entropy down to very low temperatures, threatening violation of the Nernst Theorem. It is argued that certain low-temperature phase transitions, notably those involving Cooper-pair formation, offer effective mechanisms for shedding the entropy excess. Available measurements in heavy-fermion compounds provide concrete support for such a scenario.

Thermal Casimir-Polder interaction of different atoms with graphene

The thermal correction to the energy of Casimir-Polder interaction of atoms with a suspended graphene membrane described by the Dirac model is investigated. We show that a major impact on the thermal correction is made by the size of the gap in the energy spectrum of graphene quasiparticles. Specifically, if the temperature is much smaller than the gap parameter (alternatively, larger or of the order of the gap parameter), the thermal correction is shown to be relatively small (alternatively, large). We have calculated the free energy of the thermal Casimir-Polder interaction of atoms of He, Na, Rb, and Cs with graphene described by both the hydrodynamic and Dirac models. It is shown that in exact computations using the Dirac model, one should use the polarization operator at nonzero temperature. The computational results for the Casimir-Polder free energy obtained in the framework of hydrodynamic model of graphene are several times larger than in the Dirac model within the separation region below 2$\mu$m. We conclude that the theoretical predictions following from the two models can be reliably discriminated in experiments on quantum reflection of different atoms on graphene.

Swallowtail Band Structure of the Superfluid Fermi Gas in an Optical Lattice

We investigate the energy band structure of the superfluid flow of ultracold dilute Fermi gases in a one-dimensional optical lattice along the BCS to Bose-Einstein condensate (BEC) crossover within a mean-field approach. In each side of the crossover region, a loop structure (swallowtail) appears in the Bloch energy band of the superfluid above a critical value of the interaction strength. The width of the swallowtail is largest near unitarity. Across the critical value of the interaction strength, the profiles of density and pairing field change more drastically in the BCS side than in the BEC side. It is found that along with the appearance of the swallowtail, there exists a narrow band in the quasiparticle energy spectrum close to the chemical potential, and the incompressibility of the Fermi gas consequently experiences a profound dip in the BCS side, unlike in the BEC side.

Quasiparticle continuum and resonances in the Hartree-Fock-Bogoliubov theory

The quasi-particle energy spectrum of the Hartree-Fock-Bogoliubov (HFB) equations contains discrete bound states, resonances, and non-resonant continuum states. We study the structure of the unbound quasi-particle spectrum of weakly bound nuclei within several methods that do not rely on imposing scattering or outgoing boundary conditions. Various approximations are examined to estimate resonance widths. It is shown that the stabilization method works well for all HFB resonances except for very narrow ones. The Thomas-Fermi approximation to the non-resonant continuum has been shown to be very effective, especially for coordinate-space HFB calculations in large boxes that involve huge amounts of discretized quasi-particle continuum states.

Effects of periodic potentials on the critical velocity of superfluid Fermi gases in the BCS-BEC crossover

We study the effects of an external periodic potential on the critical velocity of a superfluid Fermi gas in the crossover between the Bardeen-Cooper-Schrieffer (BCS) phase and Bose-Einstein condensation (BEC). We numerically solve the Bogoliubov--de Gennes equations to model a three-dimensional gas of ultracold atoms in the superfluid phase flowing through a one-dimensional optical lattice. We find that when the recoil energy is comparable to the Fermi energy, the presence of the periodic potential reduces the effect of pair-breaking excitations. This behavior is a consequence of the peculiar band structure of the quasiparticle energy spectrum in the lattice. When the lattice height is much larger than the Fermi energy, the periodic potential makes pairs of atoms to be strongly bound, even in the BCS regime, and pair-breaking excitations are further suppressed. We have also found that when the recoil energy is comparable to or larger than the Fermi energy, the critical velocity due to long-wavelength phonon excitations shows a nonmonotonic behavior along the BCS-BEC crossover.

Quasiparticle energies and uniaxial pressure effects on the properties of SnO2

We calculate the quasiparticle energy spectrum of SnO2 within the GW approximation, properly taking into account the contribution of core levels to the energy corrections. The calculated fundamental gap is of 3.85 eV. We propose that the difference with respect to the experimental optical gap (3.6 eV) is due to excitonic effects in the latter. We further consider the effect applied on uniaxial pressure along the c-axis. Compared to GW, the effect of pressure on the quasiparticle energies and band gap is underestimated by the local-density approximation. The quasiparticle effective masses, however, appear to be well described by the latter.

Effect of coexisting order of various form and wave vector on low-temperature thermal conductivity ind-wave superconductors

In light of recent experimental evidence of density wave order in the cuprates, we consider a phenomenological model of a $d$-wave superconductor with coexisting charge-, spin-, or pair-density wave order of various form and wave vector. We study the evolution of the nodal structure of the quasiparticle energy spectrum as a function of the amplitude of the coexisting order and perform diagrammatic linear-response calculations of the low-temperature (universal-limit) thermal conductivity. The work described herein expands upon our past studies, which focused on a particular unit-cell-doubling charge-density wave, generalizing our techniques to a wider class of coexisting order. We find that the question of whether the nodes of the $d$-wave superconductor survive amidst a reasonable level of coexisting order is sensitive to the form and wave vector of the order. However, in cases where the nodes do become gapped, we identify a signature of the approach to this nodal transition, in the low-temperature thermal conductivity, that appears to be quite general. The amplitude of this signature is found to be disorder dependent, which suggests a connection between the presence of coexisting order in the underdoped cuprates and recent observations of deviations from universal (disorder-independent) thermal conductivity in the underdoped regime.

Low-temperature quasiparticle transport in ad-wave superconductor with coexisting charge order

In light of the evidence that charge order coexists with d-wave superconductivity in the underdoped cuprate superconductors, we investigate the manner in which such charge order will influence the quasiparticle excitations of the system and, in particular, the low-temperature transport of heat by those quasiparticles. We consider a d-wave superconductor in which the superconductivity coexists with charge density wave order of wave vector (\pi/a,0). While the nodes of the quasiparticle energy spectrum survive the onset of charge order, there exists a critical value of the charge density wave order parameter beyond which the quasiparticle spectrum becomes fully gapped. We perform a linear response Kubo formula calculation of thermal conductivity in the low temperature (universal) limit. Results reveal the dependence of thermal transport on increasing charge order up to the critical value at which the quasiparticle spectrum becomes fully gapped and thermal conductivity vanishes. In addition to numerical results, closed-form expressions are obtained in the clean limit for the special case of isotropic Dirac nodes. Signatures of the influence of charge order on low-temperature thermal transport are identified.

Construction of a multielectron basis for Mott insulators with strong electron correlations, spin-orbit interaction, and covalence

We propose that the apparatus of quantum mechanics of a free atom (in particular, the theory of nj symbols and Rakah-Wigner genealogic coefficients generalized to the case of point groups and widely used in crystal field theory) be used for constructing multielectron bases with allowance for covalence and spinorbit interaction. This allows us to take into account the electron-electron interaction for 3d ions the most comprehensively. The basis constructed in this way can be used in the generalized strong coupling method for the multiband p-d model in describing the structure of the quasiparticle energy spectrum and physical properties of systems with strong electron correlations. The procedure of construction and computation is demonstrated for the 5T2g term in the d6 configuration of the transition metal atom in an octahedral field. The mechanism for the emergence of magnetic anisotropy in S ions (Fe3+ and Mn2+) due to covalent mixing of d6L configurations with a nonzero orbital angular momentum (L is a hole in ligands) is demonstrated.

Andreev levels in a graphene–superconductor surface

In order to consider the Dirac-like spectrum of graphene we employ the Bogoliubov de Gennes–Dirac formalism to determine the quasiparticle Andreev levels in an NS surface (normal–superconductor). The normal region is characterized by a width L while the superconducting region is semi-infinite and both regions are made of doped graphene. The quasiparticle energy spectrum is originated by the Andreev reflections that occur in the NS interface. It is shown that this spectrum depends on the width of the normal region and the Fermi energy in each region. When the Fermi energy in the normal metal is lower than the gap of the superconductor region, the spectrum is affected by specular Andreev reflections. The equation that is obtained to find the spectrum is very general and we solve it for some particular cases. We find that the energy spectrum oscillates when the Fermi energy in graphene is changed. Finally we obtain under some approximations an equation for the energy spectrum which is similar in structure as those obtained for an INS conventional junction.

Semiconducting gap of Nd1.85Ce0.15CuO4 revealed by break-junction tunnelling spectroscopy

The quasiparticle energy spectrum of the semiconducting Nd1.85Ce0.15CuO4 was probed by a break-junction technique. The tunnelling conductance exhibits well-defined gap-edge peaks at 4 K with peak-to-peak bias separations 2–2.8 eV, showing the V-shaped gap feature. The gap value 2Δ (4K) = 1–1.4 eV is inferred in view of the very break-junction nature. As temperature, T, increases, the gap gradually disappears to merge with the background at T ∼ 200 K. Possible origins of the gap are discussed on the basis of the break-junction and complementary data obtained using scanning-tunnelling spectroscopy.

Electron correlations in narrow energy bands: modified polar model approach

The electron correlations in narrow energy bands are examined within the framework of the modied form of polar model. This model permits to analyze the effect of strong Coulomb correlation, inter-atomic exchange and correlated hopping of electrons and explain some peculiarities of the properties of narrow-band materials, namely the metal-insulator transition with an increase of temperature, nonlinear concentration dependence of Curie temperature and peculiarities of transport properties of electronic subsystem. Using a variant of generalized Hartree-Fock approximation, the single-electron Green’s function and quasi-particle energy spectrum of the model are calculated. Metal-insulator transition with the change of temperature is investigated in a system with correlated hopping. Processes of ferromagnetic ordering stabilization in the system with various forms of electronic DOS are studied. The static conductivity and effective spin-dependent masses of current carriers are calculated as a function of electron concentration at various DOS forms. The correlated hopping is shown to cause the electron-hole asymmetry of transport and ferromagnetic properties of narrow band materials.

Nodal Structures of Heavy Fermion Superconductors Probed by the Specific-Heat Measurements in Magnetic Fields

Heavy electron superconductors mostly have anisotropic gap function which vanishes (has nodes) for certain directions in the momentum space. Since the nodal structure is closely related to the pairing mechanism, its experimental determination is very important. In anisotropic superconductors, low energy spectrum of the quasiparticles in the vortex state much depends on the nodal structure. In particular, the electronic specific heat has been demonstrated to exhibit the characteristic dependence on the angle between the field and the nodal direction, thereby the nodal structure can be probed experimentally. In this contribution, we present our recent experimental results on the field-orientation dependence of the specific heat on f electron superconductors CeRu 2 , CeCoIn 5 , PrOs 4 Sb 12 , and URu 2 Si 2 , and discuss their gap structures.

Electron energy spectrum in core-shell elliptic quantum wire

The electron energy spectrum in core-shell elliptic quantum wire and elliptic semiconductor nanotubes are investigated within the effective mass approximation. The solution of Schrodinger equation based on the Mathieu functions is obtained in elliptic coordinates. The dependences of the electron size quantization spectrum on the size and shape of the core-shell nanowire and nanotube are calculated. It is shown that the ellipticity of a quantum wire leads to the break of degeneration of quasiparticle energy spectrum. The dependences of the energy of odd and even electron states on the ratio between semiaxes are of a nonmonotonous character. The anticrosing effects are observed at the dependences of electron energy spectrum on the transversal size of the core-shell nanowire.

Quasiparticle energy spectrum and Josephson current in superconducting SNINS junctions

Through the solution of the Bogoliubov de Gennes equations we analyze the effect of different symmetries of the pair potential on the quasiparticle energy spectrum in SNINS junctions (S: superconductor, N: normal metal and I: Insulator). We find that the energy levels are strongly affected by the symmetry of the pair potential, the width of each normal metal (a and b) and the strength of the insulating barrier. The energy levels equation generalizes previous results in SNS and SIS junctions. The energy dispersion relation depends on the phase difference of the pair potential. The Josephson current is related to the Andreev levels; when a = b ~ x0 and T << Tc this current is approximately 1/2 of the Josephson current transported in an SIS junctions. In general, we find that for dxy symmetries there is always a zero energy state independent of the value of Z, a and b.

Zeeman-Perturbed63Cu Nuclear Quadrupole Resonance Study of Vortex State of YBa2Cu3O7-δ

We report a $^{63}$Cu nuclear quadrupole resonance (NQR) study of the vortex state for an aligned polycrystalline sample of a slightly overdoped high-$T_c$ superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ ($T_{c}\sim$92 K) at a low magnetic field of 96 mT along the c axis, near a lower critical field $H_{c1}$. We observed the frequency distribution of the nuclear spin-lattice relaxation time $^{63}T_1$ in the Zeeman-perturbed $^{63}$Cu NQR spectrum below $T_c$. The characteristic behavior of 1/$^{63}T_1$, taking the minimum values with respect to temperature and frequency, indicates the significant role of antiferromagnetic spin fluctuations in the Doppler-shifted quasiparticle energy spectrum inside and outside vortex cores.

Electron energy spectra in an elliptic quantum wire and elliptic nanotube

In the effective mass approximation, the electron spectra in an elliptic quantum wire and elliptic semiconductor nanotube are investigated. An exact electron energy spectrum in the GaAs elliptic quantum wire and elliptic semiconductor nanotube with impenetrable walls is obtained and an approximate solution of the Schrodinger equation is derived for a finite potential barrier height in the GaAs/AlxGa1−xAs elliptic quantum wire. It is demonstrated that the ellipticity of the quantum wire and nanotube results in removal of degeneration of the quasiparticle energy spectrum. Dependences of energies of even and odd states on the ratio of the ellipse semiaxes are nonmonotonic in character. In the limiting case of degeneration of elliptic quantum wires and nanotubes into circular ones, the quasi-particle energy spectrum coincides with the corresponding spectrum in cylindrical nanosystems.

Magnetic field dependence of conductivity and effective mass of carriers in a model of Mott-Hubbard material

The influence of external magnetic field h on a static conductivity of MottHubbard material which is described by model with correlated hopping of electrons has been investigated. By means of canonical transformation the effective Hamiltonian which takes into account strong intra-site Coulomb repulsion and correlated hopping is obtained. Using a variant of generalized Hartree-Fock approximation the single-electron Green function and quasiparticle energy spectrum of the model have been calculated. The static conductivity σ has been calculated as a function of h, electron concentration n and temperature T . The correlated hopping is shown to cause the electron-hole asymmetry of transport properties of narrow band materials. keywordsMott-Hubbard material, conductivity, magnetic field pacs72.15-v; 72.80.Ga. The achievements of the recent years in the field of strongly correlated electron systems give us the opportunity to understand the properties of narrow-band materials, in particular those in which metal-insulator transition under the action of external influences (pressure, doping, temperature) is realized [1]. The strongly correlated electron systems demonstrate unusual transport properties [2]. For understanding of the physical mechanisms, which cause these peculiarities, the experimental and theoretical researches of the temperature dependence of conductivity are needed. The results concerning low-frequency behavior of conductivity are of the prior importance, because it gives the information about the scattering processes close to the Fermi surface. The theoretical investigation of conductivity σ(ω, T ) are mainly concentrated in the limit T = 0. Behavior of the static conductivity σ(T ) = σ(ω = 0, T ) at T > 0 has not been studied sufficiently. Theoretical investigations of the optical conductivity in the Hubbard model [3] in the frameworks of the Kubo linear response theory [4] last for many decades, we note here the investigations by analytical methods: moment method [5], in composite operators method [6], in the mean-field theory [7, 8], in the perturbative theory method [9, 10] in the limit of weak interaction (|t| ≫ U), in method of the memory function [11, 12], in the opposite limit (U ≫ |t|). The conductivity has been intensively studied in onedimensional Hubbard model [13]-[15], where the numerical results can be compared with exact ones obtained by Bethe ansatz application.

Mixed-parity superconductivity in centrosymmetric crystals

A weak-coupling formalism for superconducting states possessing both singlet (even parity) and triplet (odd parity) components of the order parameter in centrosymmetric crystals is developed. It is shown that the quasiparticle energy spectrum may be non-degenerate even if the triplet component is unitary. The superconducting gap of a mixed-parity state may have line nodes in the strong spin-orbit coupling limit. The pseudospin carried by the superconducting electrons is calculated, from which follows a prediction of a kink anomaly in the temperature dependence of muon spin relaxation rate. The anomaly occurs at the phase boundary between the bare triplet and mixed-parity states. The stability of mixed-parity states is discussed within Ginzburg-Landau theory. The results may have immediate application to the superconducting series Pr(Os,Ru)4Sb12.