Multiband Effects Research Articles

Fermionic transport and out-of-equilibrium dynamics in a homogeneous Hubbard model with ultracold atoms

Transport properties are among the defining characteristics of many important phases in condensed matter physics. In the presence of strong correlations they are difficult to predict even for model systems like the Hubbard model. In real materials they are in general obscured by additional complications including impurities, lattice defects or multi-band effects. Ultracold atoms in contrast offer the possibility to study transport and out-of-equilibrium phenomena in a clean and well-controlled environment and can therefore act as a quantum simulator for condensed matter systems. Here we studied the expansion of an initially confined fermionic quantum gas in the lowest band of a homogeneous optical lattice. While we observe ballistic transport for non-interacting atoms, even small interactions render the expansion almost bimodal with a dramatically reduced expansion velocity. The dynamics is independent of the sign of the interaction, revealing a novel, dynamic symmetry of the Hubbard model.

Pauli-limiting effects in the upper critical fields of a clean LiFeAs single crystal

We have investigated the temperature dependence of the upper critical field ${H}_{\mathrm{c}2}$($T$ ) in a LiFeAs single crystal by direct measurements of resistivity under static magnetic fields up to 36 T. We find in the case of a magnetic field $H$ along the $ab$ plane that ${H}_{\mathrm{c}2}^{ab}$(0) $=$ 30 T is clearly lower than the orbital limiting field ${H}_{\mathrm{c}2}^{\mathrm{orb},ab}$(0) $=$ 39.6 T estimated by the |${\mathit{dH}}_{\mathrm{c}2}^{ab}$/$\mathit{dT}$ |${}_{Tc}$, suggesting the presence of both Pauli- and orbital-limiting effects in the pair breaking process. The best fit of ${H}_{\mathrm{c}2}^{ab}$($T$ ) to the Werthamer--Helfand--Hohenberg formula results in the Maki parameter \ensuremath{\alpha} $=$ 0.9 and negligible spin-orbit scattering constant (${\ensuremath{\lambda}}_{\mathrm{so}}$ $=$ 0.0). On the other hand, for $H$ along the $c$ axis, ${H}_{\mathrm{c}2}^{c}$($T$ ) increases linearly down to our lowest temperature of 0.8 K, which can be explained by the multiband effects. The anisotropy ratio ${H}_{\mathrm{c}2}^{ab}$($T$ )/${H}_{\mathrm{c}2}^{c}$($T$ ) is 3 near ${T}_{\mathrm{c}}$ and systematically decreases upon lowering temperature to become 1.3 at zero temperature. A comparative overview of the behavior of ${H}_{\mathrm{c}2}^{ab}$($T$ ) in various Fe-based superconductors shows that, similar to LiFeAs, the calculated ${H}_{\mathrm{c}2}^{\mathrm{orb},ab}$(0) is generally much larger than the measured ${H}_{\mathrm{c}2}^{ab}$(0) and thus finite \ensuremath{\alpha} values ranging from \ensuremath{\sim}0.4 to 3 are necessary to describe the low temperature ${H}_{\mathrm{c}2}^{ab}$($T$ ) behaviors. Moreover, LiFeAs is found to have the smallest |${\mathit{dH}}_{\mathrm{c}2}^{ab}$/$\mathit{dT}$ |${}_{Tc}$ values, indicating that LiFeAs is one of the cleanest Fe-based superconductors with a finite Maki parameter. We also discuss the implications of multiband effects and spin-orbit scattering based on the finding that the estimated Pauli-limiting field is generally much larger than the BCS prediction in the Fe-based superconductors.

Screening of the Raman response in multiband superconductors: Application to iron pnictides

We performed model calculations of Raman responses for multiband 2D superconductors. The multiband effects of screening in the A_{1g} symmetry channel were investigated analytically and numerically for a band structure model mimicing ARPES data on iron-pnictide materials. An acceptable agreement between our model calculations and recent experimental data is demonstrated by modification of the band structure parameters.

Evolution from non-Fermi- to Fermi-liquid transport via isovalent doping inBaFe2(As1−xPx)2superconductors

The normal-state charge transport is studied systematically in high-quality single crystals of ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ $(0\ensuremath{\le}x\ensuremath{\le}0.71)$. By substituting isovalent P for As, the spin-density-wave (SDW) state is suppressed and the dome-shaped superconducting phase $({T}_{c}\ensuremath{\lesssim}31\text{ }\text{K})$ appears. Near the SDW end point $(x\ensuremath{\approx}0.3)$, we observe striking linear temperature $(T)$ dependence of resistivity in a wide $T$ range, and remarkable low-$T$ enhancement of Hall-coefficient magnitude from the carrier number estimates. We also find that the magnetoresistance apparently violates the Kohler's rule and is well scaled by the Hall angle ${\ensuremath{\Theta}}_{H}$ as $\ensuremath{\Delta}{\ensuremath{\rho}}_{xx}/{\ensuremath{\rho}}_{xx}\ensuremath{\propto}{\text{tan}}^{2}\text{ }{\ensuremath{\Theta}}_{H}$. These non-Fermi-liquid transport anomalies cannot be attributed to the simple multiband effects. These results capture universal features of correlated electron systems in the presence of strong antiferromagnetic fluctuations.

Anomalous Hall effect

We present a review of experimental and theoretical studies of the anomalous Hall effect (AHE), focusing on recent developments that have provided a more complete framework for understanding this subtle phenomenon and have, in many instances, replaced controversy by clarity. Synergy between experimental and theoretical work, both playing a crucial role, has been at the heart of these advances. On the theoretical front, the adoption of Berry-phase concepts has established a link between the AHE and the topological nature of the Hall currents which originate from spin-orbit coupling. On the experimental front, new experimental studies of the AHE in transition metals, transition-metal oxides, spinels, pyrochlores, and metallic dilute magnetic semiconductors, have more clearly established systematic trends. These two developments in concert with first-principles electronic structure calculations, strongly favor the dominance of an intrinsic Berry-phase-related AHE mechanism in metallic ferromagnets with moderate conductivity. The intrinsic AHE can be expressed in terms of Berry-phase curvatures and it is therefore an intrinsic quantum mechanical property of a perfect cyrstal. An extrinsic mechanism, skew scattering from disorder, tends to dominate the AHE in highly conductive ferromagnets. We review the full modern semiclassical treatment of the AHE together with the more rigorous quantum-mechanical treatments based on the Kubo and Keldysh formalisms, taking into account multiband effects, and demonstrate the equivalence of all three linear response theories in the metallic regime. Finally we discuss outstanding issues and avenues for future investigation.

Transition from band insulator to Bose-Einstein-condensate superfluid and Mott state of cold Fermi gases with multiband effects in optical lattices

We study two models realized by two-component Fermi gases loaded in optical lattices. We clarify that multi-band effects inevitably caused by the optical lattices generate a rich structure, when the systems crossover from the region of weakly bound molecular bosons to the region of strongly bound atomic bosons. Here the crossover can be controlled by attractive fermion interaction. One of the present models is a case with attractive fermion interaction, where an insulator-superfluid transition takes place. The transition is characterized as the transition between a band insulator and a Bose-Einstein condensate (BEC) superfluid state. Differing from the conventional BCS superfluid transition, this transition shows unconventional properties. In contrast to the one particle excitation gap scaled by the superfluid order parameter in the conventional BCS transition, because of the multi-band effects, a large gap of one-particle density of states is retained all through the transition although the superfluid order grows continuously from zero. A reentrant transition with lowering temperature is another unconventionality. The other model is the case with coexisting attractive and repulsive interactions. Within a mean field treatment, we find a new insulating state, an orbital ordered insulator. This insulator is one candidate for the Mott insulator of molecular bosons and is the first example that the orbital internal degrees of freedom of molecular bosons appears explicitly. Besides the emergence of a new phase, a coexisting phase also appears where superfluidity and an orbital order coexist just by doping holes or particles. The insulating and superfluid particles show differentiation in momentum space as in the high-Tc cuprate superconductors.

Excitation spectrum of Mott shells in optical lattices

We theoretically study the excitation spectrum of confined macroscopic optical lattices in the Mott-insulating limit. For large systems, a fast numerical method is proposed to calculate the ground-state filling and excitation energies. We introduce many-particle on-site energies capturing multi-band effects and discuss tunnelling on a perturbative level using an effectively restricted Hilbert space. Results for small one-dimensional lattices obtained by this method are in good agreement with the exact multi-band diagonalization of the Hamiltonian. Spectral properties associated with the formation of regions with constant filling, so-called Mott shells, are investigated and interfaces between the shells with strong particle fluctuations are characterized by gapless local excitations.

Electron–phonon properties of pnictide superconductors

In this paper we discuss the normal and superconducting state properties of two pnictide superconductors, LaOFeAs and LaONiAs, using Migdal–Eliashberg theory and density functional perturbation theory. For pure LaOFeAs, the calculated electron–phonon coupling constant λ = 0.21 and logarithmic-averaged frequency ω ln = 206 K , give a maximum T c of 0.8 K, using the standard Migdal–Eliashberg theory. Inclusion of multi-band effects increases the T c only marginally. To reproduce the experimental T c , a 5–6 times larger coupling constant would be needed. Our results indicate that standard electron–phonon coupling is not sufficient to explain superconductivity in the whole family of Fe–As based superconductors. At the same time, the electron–phonon coupling in Ni–As based compounds is much stronger and its normal and superconducting state properties can be well described by standard Migdal–Eliashberg theory.

Multiband effects on fermionic atoms in optical lattices

The superfluid-insulator transition of two-band fermionic atom systems in optical lattices is investigated by the two-site dynamical mean-field theory. Because of the spin-flip and pair-hopping interactions, orbital fluctuations are enhanced, leading to the suppression of the Mott insulating state. The numerical results are discussed in comparison with the effective boson model.

Superfluid-insulator transition of two-band fermionic atom systems in optical lattices

We study the multiband effects on ultracold fermionic atoms in optical lattices by two-site dynamical mean-field theory. We find that the density wave state becomes stable for a small orbital splitting region. As the orbital splitting is increased, the transition from the density wave state to the superfluid state occurs. By systematically changing the orbital splitting and the attractive interaction, we obtain the phase diagram at half-filling.

Multiband Order Parameters for thePrOs4Sb12andPrRu4Sb12Skutterudite Superconductors from Thermal Conductivity Measurements

Thermal conductivity measurements were performed on single crystal samples of the superconducting filled-skutterudite compounds PrOs4Sb12 and PrRu4Sb12 both as a function of temperature and transverse magnetic field. In a zero magnetic field, the low temperature electronic thermal conductivity of PrRu4Sb12 is consistent with a fully gapped Fermi surface. For PrOs4Sb12, residual electronic conduction in the zero-temperature limit is consistent with the presence of nodes in the superconducting energy gap. The electronic thermal conductivity for both compounds shows a rapid rise at low magnetic fields. In PrRu4Sb12, this is interpreted in terms of multiband effects. In PrOs4Sb12, we consider the Doppler shift of nodal quasiparticles and multiband effects.

Multiband-driven superfluid-insulator transition of fermionic atoms in optical lattices: A dynamical mean-field-theory study

The superfluid-insulator transition of fermionic atoms in optical lattices is investigated by two-site dynamical mean-field theory. It is shown that the Mott transition occurs as a result of multiband effects. The quasiparticle weight in the superfluid state decreases significantly as the system approaches the Mott transition point. By changing the interaction and the orbital splitting, we obtain the phase diagram at half filling. The numerical results are discussed in comparison with the effective boson model.

Electric transport and magnetic properties in multilayer graphene

We discuss electric transport and orbital magnetism of multilayer graphenes in a weak-magnetic field using the matrix decomposition technique. At zero temperature, the minimum conductivity is given by that of the monolayer system multiplied by the layer number $N$, independent of the interlayer hopping $t$. When the interlayer hopping satisfies the condition $t⪢\ensuremath{\hbar}∕\ensuremath{\tau}$ with $\ensuremath{\tau}$ being collision time of impurity scattering, $[N∕2]$ kinks and $[N∕2]+1$ plateaux appear in the Fermi-energy (gate voltage) dependence of the conductivity and the Hall conductivity, respectively. These behaviors are interpreted as multiband effects. We also found that the Hall conductivity and the magnetic susceptibility take minimum value as a function of temperature, for certain value of the gate voltage. This behavior is explained by Fermi-energy dependence of these functions at zero temperature.

N‐bands Hubbard models. IV. Comparisons of electron‐ or hole‐doped quaternary oxypictides LaOMPn superconductors with cuprates

AbstractA lot of theoretical and experimental results are examined to obtain a unified picture of magnetism and superconductivity in strongly correlated electron systems. First principle computational results reported recently have been used to elucidate electronic structures of strongly correlated electron systems. In this series of articles theoretical efforts for the systems such as transition metal oxides have been extended to search high‐Tc superconductors (HTSC) other than cuprates in relation to possible mechanism(s) of high‐Tc superconductivity. Very recently, Kamihara et al. have found that quaternary oxypictides LaOMPn (M = V, Cr, Mn, Fe, Co, Ni, and Pn = P, As) are a new family of high‐Tc superconductors, exhibiting high‐transition temperature Tc = 26 K for the case of M = Fe and Pn = As. Since iron has been believed to be magnetic in many systems, their finding raises extremely interesting question concerning with relationships between magnetism and superconductivity. Moreover, coexistence of antiferromagnetism (AF) and superconductivity (SC) is also discovered in the recently synthesized cuprates with five CuO2 planes. This may indicate the uniformly mixed phase of AF and SC in cuprates without disorder corresponds to spin glass, instead of other complex phases resulting from random potentials in disordered cuprates. Coexistence of AF and SC phases and multiband effects are also suggested for quaternary oxypictides LaOMPn. Here, comparison between quaternary oxypictides LaOMPn and cuprates is carried out from both theoretical and experimental grounds, leading to an extension of our magnetic (J) model for LaOMPn. Implications of these analyses are discussed in relation to possible candidates of HTSC with multi (N)‐layers consisted of iso‐electronic π‐d, π‐R, and σ‐R systems, for which similar electronic phases are expected from the theoretical grounds. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Band-structure and anomalous contributions to the Hall effect of YbRh 2Si 2

We report on Hall effect measurements on YbRh 2Si 2 single crystals with different residual resistivity and on LuRh 2Si 2 single crystal. The temperature dependence of the linear-response Hall coefficient of YbRh 2Si 2 is described by the anomalous Hall effect and the normal contribution incorporating multi-band effects. Sample dependencies at low T are found and explained by slight changes in the charge-carrier concentrations.

Superconductivity in PrOs4Sb12: On the double transition and multiband effects

The superconducting state of the heavy fermion PrOs4Sb12 was studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The temperature and field dependence of the thermal conductivity confirm the recently suggested multiband superconductivity scenario and point to fully open gaps on the whole Fermi surface.

BEC-BCS crossover in an optical lattice

We present the microscopic theory for the BEC-BCS crossover of an atomic Fermi gas in an optical lattice, showing that the Feshbach resonance underlying the crossover in principle induces strong multiband effects. Nevertheless, the BEC-BCS crossover itself can be described by a single-band model since it occurs at magnetic fields that are relatively far away from the Feshbach resonance. A criterion is proposed for the latter, which is obeyed by most known Feshbach resonances in ultracold atomic gases.

Orbital-Selective Mass Enhancements in MultibandCa2−xSrxRuO4Systems Analyzed by the Extended Drude Model

We investigated optical spectra of quasi-two-dimensional multiband systems. The extended Drude model analysis on the -plane optical conductivity spectra indicates that the effective mass should be enhanced near . Based on the sum rule argument, we showed that the orbital-selective Mott-gap opening for the bands, the widely investigated picture, could not be the origin of the mass enhancement. We exploited the multiband effects in the extended Drude model analysis, and demonstrated that the intriguing heavy mass state near should come from the renormalization of the band.

Relevance of multiband Jahn-Teller effects on the electron-phonon interaction inA3C60

Assessing the effective relevance of multiband effects in fullerides is of fundamental importance in understanding the complex properties of these compounds. In this paper we investigate the role of the multiband effects on the electron-phonon (el-ph) properties of the ${t}_{1u}$ bands coupled with the Jahn-Teller intramolecular ${H}_{g}$ vibrational modes in ${\mathrm{C}}_{60}$ compounds. We show that, assuming perfect degeneracy of the electronic bands, vertex diagrams arising from the breakdown of the adiabatic hypothesis, are one order of magnitude smaller than the noncrossing terms usually retained in the Migdal-Eliashberg (ME) theory. These results permit to understand the robustness of the ME theory found by numerical calculations. The effects of the nondegeneracy of the ${t}_{1u}$ in realistic systems are also analyzed. Using a tight-binding model we show that the el-ph interaction is mainly dominated by intraband scattering within a single electronic band. Our results question the reliability of a degenerate band modeling and show the importance of these combined effects in the ${A}_{3}{\mathrm{C}}_{60}$ family.

Optimized effective potential method at finite temperature: An application to superconductivity

AbstractTo deal with superconductivity without introducing an external pairing potential we extend the effective potential methodology into that at finite‐temperature cases, starting from Green's function theory based on the Sham–Schlüter (SS) equation. This extension corresponds to a Kohn–Sham–Bogoliubov equation that includes electron exchange‐correlation effects. Then we linearize the Dyson equation with respect to the anomalous Green function and adopt a linear response approximation to the SS equation. Numerical calculation is performed with a model Hamiltonian within the exchange‐only case. We compare the results obtained by the present optimized potential method with those by the exact Hartree–Fock–Bogoliubov method and confirm a good performance of the present method. We evaluate anomalous effective exchange potentials resulting from purely positive electron interactions and observe the singlet superconducting state due to multiband effects. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005