Interband Interaction Research Articles

Electronic Compressibility of a Graphene Bilayer

We calculate the electronic compressibility arising from electron-electron interactions for a graphene bilayer within the Hartree-Fock approximation. We show that, due to the chiral nature of the particles in this system, the compressibility is rather different from those of either the two-dimensional electron gas or ordinary semiconductors. We find that an inherent competition between the contributions coming from intraband exchange interactions (dominant at low densities) and interband interactions (dominant at moderate densities) leads to a nonmonotonic behavior of the compressibility as a function of carrier density.

Gapless Fermi surfaces in superconductingCeCoIn5

According to Tanatar et al. [Phys. Rev. Lett. 95, 067002 (2005)], the low-temperature thermal conductivity in ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Co}{\mathrm{In}}_{5}$, a multiband $d$-wave superconductor, reveals unexpected dependence on the concentration of defects as if one or more Fermi surface sheets remained ungapped below superconducting transition. The interior gap superfluidity mechanism, or unbalanced pairing, recently proposed by Liu and Wilczek [Phys. Rev. Lett. 90, 047002 (2003)] has been invoked as a possible origin of gaplessness. We indicate that the Fermi surface anisotropy in the real $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$ makes this explanation highly implausible. We emphasize the fundamental difference between unbalanced pairing of different Fermi entities and the formation of superconducting gaps on Fermi surfaces belonging to different bands. We also argue that interband interactions between electrons always induce a finite order parameter on all Fermi surfaces below the temperature of a superconducting transition. We calculate specific heat and thermal conductivity in a two-band model for a $d$-wave superconductor in the presence of defects. In our simple model, superconductivity originates on one Fermi surface, inducing a smaller gap on the other one. Impurities diminish the induced gap and increase the density of states, restoring rapidly the Wiedemann-Franz law for this Fermi surface. Our calculations are in agreement with experiment.

Evolution of Two-Band Superfluidity from Weak to Strong Coupling

We analyze the evolution of two-band superfluidity from the weak to the strong coupling (Bardeen-Cooper-Schrieffer to Bose-Einstein condensation) limit. We find that population imbalances between the two bands can be created by tuning the intraband or interband (Josephson) interactions. In addition, when the Josephson interband interaction is tuned from negative to positive values, a quantum phase tran- sition occurs from a 0-phase to a π-phase state, depending on the relative phase of the two order parameters. We also find two undamped low energy collective excita- tions corresponding to in-phase phonon (Goldstone) and out-of-phase exciton (finite frequency) modes. Lastly, we derive the coupled Ginzburg-Landau equations, and show that they reduce to coupled Gross-Pitaevskii equations for two types of weakly interacting bosons (tightly bound fermions) in the BEC limit.

Cuprate superconductivity characteristics in a multiband model controlled by doping

A multiband model of cuprate superconductivity with the electron spectrum created and evolving by doping is suggested. Bare gaps between the itinerant and two components of the defect subsystem disappear with extended doping. The interband pair-transfer interaction between itinerant and defect states drives the pairing. The behaviour of T c, supercarrier density, superconducting, normal state and pseudogaps, as also the condensation energy, the second critical field, in-plane coherence length, penetration depth, paired carrier effective mass and the isotope effect exponents have been calculated on the whole doping scale. Self-consistent qualitative agreement with experimental data is obtained. Cuprates can be expected to be multi(two)-component superconductors.

Two-band superfluidity from the BCS to the BEC limit

We analyze the evolution of two-band superfluidity from the weak coupling Bardeen-Cooper-Schrieffer (BCS) to the strong coupling Bose-Einstein condensation (BEC) limit. When the interband interaction is tuned from negative to positive values, a quantum phase transition occurs from a 0-phase to a $\pi$-phase state, depending on the relative phase of two order parameters. Furthermore, population imbalances between the two bands can be created by tuning the intraband or interband interactions. We also find two undamped low energy collective excitations corresponding to in-phase and out-of-phase modes. Lastly, we derive the coupled Ginzburg-Landau equations, and show that they reduce to coupled Gross-Pitaevskii equations for two types of bosons in the BEC limit.

Two-band model of a high-temperature superconductor and the dependence of the critical temperature of transition on the parameters of the model

Within the limit of weak coupling with allowance for the level of hybridization and polarization of states, dependences of the critical temperature of the superconducting transition on the concentration of carriers and on the constant of interband interaction have been obtained in terms of the two-dimensional two-band model at a constant density of states and have been shown to be in a qualitative agreement with experiment.

Multicomponent superconductivity: the complex interplay of spin charge and lattice

High temperature superconducting copper oxides (HTSC) have been shown to be intrinsically heterogeneous. Consequently multiple ground states coexist, which are either in competition with or of support to superconductivity. Here it is shown that spin, charge and lattice are intrinsically coupled to form a composite of locally charge carrying distorted areas in an antiferromagnetic matrix on the regular lattice. The dynamical interplay between both induces level shifts together with band narrowing of the spin and charge related bands, by means of which interband interactions are possible which lead to a substantial increase in the superconducting transition temperature T c . Further consequences of this multicomponent approach as, for example, isotope effects are discussed.

Effect of interband interaction on isotope effect exponent of MgB2 superconductors

The exact formula of Tc’s equation and the isotope effect exponent of two-band s-wave superconductors in the weak-coupling limit are derived by considering the influence of interband interaction. In each band, our model consists of two pairing interactions: the electron-phonon interaction and non-electron-phonon interaction. We find that the isotope effect exponent of MgB2, α = 0.3 with Tc ≈ 40 K can be found in the weak coupling regime and interband interaction of electron-phonon shows more effect on the isotope effect exponent than on the interband interaction of non-phonon.

Free energy functional and critical magnetic fields of a two-gap superconductor with intra- and interband interactions

Free energy functional of a two-gap anisotropic superconductor is derived on the basis of the microscopic model where the interband pair-transfer repulsion, intraband effective attraction of electron–phonon nature, and intraband Coulomb interaction between carriers are taken into account. These interactions are active in the different regions of the momentum space. The corresponding Ginzburg–Landau (GL) equations have been obtained. By separating the soft and rigid variables the superconductivity coherence lengths and critical magnetic fields have been found. The anisotropies of the critical fields for MgB2 are calculated near Tc.

Effect of density of state on isotope effect exponent of two-band superconductors

The exact formula of T c’s equation and the isotope effect exponent of two-band s-wave superconductors in weak-coupling limit are derived by considering the influence of two kinds of density of state: constant and van Hove singularity. The paring interaction in each band consisted of two parts: the electron–phonon interaction and non-electron–phonon interaction are included in our model. We find that the interband interaction of electron–phonon show more effect on isotope exponent than the intraband interaction and the isotope effect exponent with constant density of state can fit to experimental data, MgB 2 and high- T c superconductor, better than van Hove singularity density of state.

Cyclotron resonance in rare-earth monopnictides

Recently, we have developed equipment which can be used for both cyclotron resonance (CR) and electron spin resonance (ESR) measurements for studying $f$-electron systems. Using this equipment and preparing high-quality single crystals with a residual resistivity ratio of about 500, we have successfully observed CR signals in rare-earth monopnictides $RX\phantom{\rule{0.3em}{0ex}}(R=\mathrm{La},\mathrm{Ce},\mathrm{Pr},\mathrm{Gd},X=\mathrm{Sb},\mathrm{Bi})$. The purpose of this paper is to describe the development of our measurement system in detail and to reexamine our recent results of CR measurements on the single crystals of $RX$. And then, we discuss the origin of mass enhancement in this system by comparing the case of a strongly correlated $f$-electron system CeSb with the case of a nonmagnetic simple semimetal LaSb. The CR measurements have been performed in the temperature range from $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in the frequency range from $50\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}190\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$. We have observed CR signals in LaSb, LaBi, CeSb, PrSb, and GdSb. The cyclotron effective masses ${m}_{\mathit{CR}}^{*}$ determined by the CR measurements are compared with the masses ${m}_{\mathit{QO}}^{*}$ estimated by measurements of quantum oscillations and the masses ${m}_{\mathit{BC}}^{*}$ deduced from band structure calculations. The determined ${m}_{\mathit{CR}}^{*}$ of LaSb, LaBi, and GdSb is in the range of $(0.17--0.65){m}_{0}$. These values are reasonably consistent with the values of ${m}_{\mathit{QO}}^{*}$ and ${m}_{\mathit{BC}}^{*}$. This fact shows that both ${m}_{\mathit{CR}}^{*}$ and ${m}_{\mathit{QO}}^{*}$ mainly depend on the band structure if the interaction between conduction electrons and $f$ electrons is negligible. On the other hand, in the case of a strongly correlated $f$-electron system CeSb, ${m}_{\mathit{CR}}^{*}$ is in the range of $(0.26--1.5){m}_{0}$. These values are much larger than those of ${m}_{\mathit{BC}}^{*}$. This finding shows that ${m}_{\mathit{CR}}^{*}$, as well as ${m}_{\mathit{QO}}^{*}$, is considerably enhanced in CeSb. Our results indicate that the interband interaction is important for the mass enhancement of ${m}_{\mathit{CR}}^{*}$ in CeSb.

Pairing symmetry with interband interaction

We investigate the superconductivity of highTc oxides with the dp model. It is shown that the dp pairing can dominate the superconductivity. The pairing symmetry depends on the anisotropic dp interaction in CuO2 plane. The anisotropic dp interaction can lead to dwave or swave symmetry, but the repulsive interaction in the CuO2 plane cannot lead holes to being paired. The ‘preliminary pairing' may appear in a lightly doped regime. The critical temperature cannot be found for the pairing order parameter when the pairs are localized. The delocalization of holes and evolution of pairing symmetry with increasing doping are explained.

Interplay of Crystal Field Structures withf2Configuration to Heavy Fermions

We examine a relevance between characteristic of crystal field structures and heavily renormalized quasiparticle states in the $f^0$-$f^1$-$f^2$ Anderson lattice model. Using a slave-boson mean-field approximation, we find that for $f^2$ configurations two or three quasiparticle bands are formed near the Fermi level depending on the number of the relevant $f^1$ orbitals in the $f^2$ crystal field ground state. The inter-orbital correlations characterizing the crystal field ground state closely reflect in inter-band residual interactions among quasiparticles. Particularly in the case of a singlet crystal field ground state, resulting residual antiferromagnetic exchange interactions among the quasiparticles lead to an anomalous suppression of the quasiparticle contribution of the spin susceptibility, even though the quasiparticle mass is strongly enhanced.

Mechanism of gap opening in a triple-band Peierls system: in atomic wires on Si.

One dimensional (1D) metals are unstable at low temperature undergoing a metal-insulator transition coupled with a periodic lattice distortion, a Peierls transition. Angle-resolved photoemission study for the 1D metallic chains of In on Si(111), featuring a metal-insulator transition and triple metallic bands, clarifies in detail how the multiple band gaps are formed at low temperature. In addition to the gap opening for a half-filled ideal 1D band with a proper Fermi surface nesting, two other quasi-1D metallic bands are found to merge into a single band, opening a unique but k-dependent energy gap through an interband charge transfer. This result introduces a novel gap-opening mechanism for a multiband Peierls system where the interband interaction is important.

Quartic Interband Exciton Couplings in Pump−Probe Spectroscopy of Light Harvesting Complexes

The standard Frenkel exciton Hamiltonian for molecular aggregates only contains quadratic (intraband) couplings between chromophores. However, interband interactions between chromophores whose excitation frequences are well separated are dominated by quartic couplings. We show that the latter are responsible for spectral line shifts of the Carotenoids observed in pump−probe experiments of LH2 photosynthetic antenna following excitation of the chrolophylls (Herek; et al., Biochemistry 1998, 37, 7057).

Enhancing Tc within phonon mediated coexistence of s + d wave superconductivity in cuprates

The high superconducting transition temperatures T c of oxide cuprates and their proximity to an antiferromagnetic ground state have frequently been attributed to unconventional pairing mechanisms and a d-wave superconducting order parameter. Even though effects due to the lattice have been observed experimentally, their role on superconductivity is mainly questioned. Here it will be shown that some features of hole doped superconducting cuprates can be well modeled by combining two order parameters of different symmetry, i.e. s + d, which interact through phonon induced interband scattering. A polaronic type phase (charge density wave like) precedes the superconducting state and signals the onset of the pseudogap state. Both effects, interband interactions and polaronic level shift are shown to substantially enhance T c.

Raising the superconducting transition temperature in cuprates within a two-component scenario

Abstract Two-component or two-channel superconductivity has been proposed since long to be a source of raising the superconducting transition temperature T c considerably. The first observation of two-component superconductivity was made in Nb-doped SrTiO 3 , yet without the hope for T c enhancement. An unambiguous observation of two superconducting gaps has been made in the recently discovered high- T c compound MgB 2 where the two component interband interaction amplifies T c by a factor of 1.6 as compared to a single band system. Another candidate for such a scenario has been speculated to be verified in the cuprate superconductors, but the overwhelming experimental evidence for a d-wave order parameter in the CuO 2 planes has largely suppressed this aspect. Here it is shown that the coexistence of an s-wave and d-wave order parameter in cuprates is a scenario in which the observed high T c s can be realized. The crucial quantity is here the pairwise exchange interaction of holes between the two channels caused by quantum interference. Lattice effects on superconductivity are explicitly incorporated and shown to be of major relevance to superconductivity including various exotic isotope effects.

Multigap discrete vector solitons.

We analyze nonlinear collective effects in periodic systems with multigap transmission spectra such as light in waveguide arrays or Bose-Einstein condensates in optical lattices. We reveal that the interband interactions in nonlinear periodic structures can be efficiently managed by controlling their geometry. We predict novel types of discrete vector solitons supported by nonlinear coupling between different band gaps and study their stability.

Effects of Degenerate Orbitals on the Hubbard Model

Stability of a metallic state in the two-orbital Hubbard model at half-filling is investigated. We clarify how spin and orbital fluctuations are enhanced to stabilize the formation of quasi-particles by combining dynamical mean field theory with the quantum Monte Carlo simulations. These analyses shed some light on the reason why the metallic phase is particularly stable when the intra- and inter-band Coulomb interactions are nearly equal.

Interband Coulomb Interaction and Horizontal Line Nodes in Triplet Superconductor Sr2RuO4

A possible mechanism for the horizontal line nodes in triplet superconductor, Sr$_2$RuO$_4$, is proposed. We consider the interlayer Coulomb interaction, as well as the on-site Coulomb repulsion, between electrons in different bands. In the second order perturbation in the interband interaction, the effective interaction becomes dependent on $\cos \frac{q_z}{2}$, resulting in the horizontal line nodes.