Multiband Character Research Articles

Fast Spin Fluctuation Viewed by Muon Spin Relaxation in Optimally Doped and Overdoped Iron-Based Oxypnictide Superconductors LaFeAsO1-xFx

Zero field and longitudinal field muon spin relaxation measurements have been performed in optimally doped and overdoped superconductors LaFeAsO$_{1-x}$F$_x$ in order to investigate the magnetic fluctuation over a wide range of temperature and longitudinal field. We have observed no sign of magnetic fluctuation against temperature in the muons' time window (10$^{-5}\sim$ 10$^{-9}$s). Considering the current results and other results, i.e., spin fluctuation observed by neutron scattering, pseudogap-like behaviors by NMR and photoemission spectroscopy, it is suggested that not only the spin fluctuation but also the multiband character with several different orbital contributions at the Fermi surface may play an important role in the superconducting pairing mechanism of LaFeAsO$_{1-x}$F$_x$.

Fermi-surface Shrinking, Interband Coupling and Multiple Gaps in Iron-based Pnictides

In this contribution we present a comprehensive explanation for the origin of the band shifts observed in dHvA and ARPES experiments. Using a four-band Eliashberg analysis, we show that they are a natural consequence of the multiband character of these systems and of the strong particle-hole asymmetry of the bands. We also show that the relative sign of such shifts provides a direct experimental evidence of a dominant interband scattering. A quantitative analysis in LaFePO yields a spin-mediated interband coupling of the order V≈0.46 eV, which corresponds to a mass enhancement Z≈1.4. We also employ such four-band model to investigate the magnitude of the superconducting gap on different Fermi sheets of Ba0.6K0.4Fe2As2, and we show that the same four-band model provides a simple explanation of the different gap values on different Fermi sheets and of the thermodynamics properties (specific heat, superfluid density, …).

Fermi surface shrinking, band shifts and interband coupling in iron-based pnictides

Band shifts are systematically observed in pnictides using de Hass-van Alphen and ARPES techniques. Most interesting, such shifts result to be band-sensitive, so that hole-like bands are shifted downwards while electron-like ones are shifted upwards. In this contribution we present a comprehensive explanation for the origin of such band shifts. Using a four band Eliashberg analysis, we show that they are a natural consequence of the multiband character of these systems and of the strong particle-hole asymmetry of the bands. We also show that the relative sign of such shifts provides a direct experimental evidence of a dominant interband scattering. A quantitative analysis in LaFePO yields a spin-mediated interband coupling of the order V≈0.46eV which corresponds to a mass enhancement Z≈1.5.

Magnetic quantum oscillations in borocarbide superconductors

We report systematic de Haas-van Alphen (dHvA) investigations in the normal and superconducting state of RNi2B2C (R=Y and Lu). The observed rich frequency spectrum of the dHvA signals results from a rather complex electronic band structure with different open and closed Fermi-surface sheets. From our data in combination with full-potential local-orbital calculations we are able to extract the angular-resolved mass-enhancement factors λ for different bands. We find a strong anisotropy and band dependence of λ, clearly reflecting the multiband character of the superconductivity in RNi2B2C. We further were able to resolve dHvA oscillations deep into the superconducting state. The observed additional damping of the dHvA amplitudes is much less than expected from most theories. This hints at a reduced or even zero superconducting gap for the detected Fermi surface.

Optical and Raman spectroscopy studies on Fe-based superconductors

A brief review of optical and Raman studies on the Fe-based superconductors is given, with special emphasis on the competing phenomenon in this system. Optical investigations on ReFeAsO (Re = rare-earth element) and AFe 2As 2 (A = alkaline-earth metal) families provide clear evidence for the gap formation in the broken symmetry states, including the partial gaps in the spin-density wave states of parent compounds, and the pairing gaps in the superconducting states for doped compounds. Especially, the superconducting gap has an s-wave pairing lineshape in hole-doped BaFe 2As 2. Optical phonons at zone center detected by Raman and infrared techniques are classified for several Fe-based compounds. Related issues, such as the electron–phonon coupling and the effect of spin-density wave and superconducting transitions on phonons, are also discussed. Meanwhile, open questions including the T-dependent mid-infrared peak at 0.6–0.7 eV, electronic correlation, and the similarities/differences between high- T c cuprates and Fe-based superconductors are also briefly discussed. Important results from other experimental probes are compared with optical data to better understand the spin-density wave properties, the superconductivity, and the multi-band character in Fe-based compounds.

Optimum nitride concentration in multiband III-N–V alloys for high efficiency ideal solar cells

III-Nx–V1−x highly mismatched alloys (HMAs) have been proposed as promising material candidates for the development of high efficiency solar cells. According to the band anticrossing model, these alloys present a multiband character with an intermediate band within the otherwise fundamental bandgap that gives them the ability of improving the efficiency by means of below-bandgap photon absorption. The efficiency of GaNxAs1−x, GaNxP1−x, and their quaternaries InyGa1−yNxAs1−x and GaNxP1−x−yAsy is estimated theoretically versus nitrogen content in this letter. Low nitrogen content in the range of 1%–3.5% in the HMAs analyzed leads to theoretical efficiencies above 60%.

Anisotropic Multiband Many-Body Interactions inLuNi2B2C

We present a comprehensive de Haas-van Alphen study on the nonmagnetic borocarbide superconductor LuNi2B2C. The analysis of the angular-dependent effective masses for different bands in combination with full-potential density functional calculations allowed us to determine the mass-enhancement factors, lambda, for the different electronic bands and their wave-vector dependences. Our data clearly show the anisotropic multiband character of the superconductivity in LuNi2B2C.

Superconductivity of the filled skuterrudite [formula omitted]: Phase diagram and characterisations

We present a precise ( H – T ) phase diagram of the double superconducting transition in the specific heat of the heavy fermion superconductor PrOs 4 Sb 12 , down to 350 mK, on a sample which exhibits two sharp distinct anomalies at T c 1 = 1.89 K and T c 2 = 1.72 K . Comparison with an existing phase diagram based on transport measurements will be reviewed. The intrinsic or extrinsic nature of this double transition is discussed in light of some characterisations of the superconducting transition of different samples and with respect to the historical case of UPt 3 . Finally, we give a detailed analysis of H c 2 which shows the importance of the multiband character of the electronic structure of PrOs 4 Sb 12 and gives strong support to an even parity order parameter.

Evolution of normal-state magnetic fluctuations by Ca and Ti substitutions inSr2RuO4:87Sr−NMRstudy

We report magnetic properties of $({\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}){\mathrm{RuO}}_{4}$ and ${\mathrm{Sr}}_{2}({\mathrm{Ru}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}){\mathrm{O}}_{4}$ investigated by ${}^{87}\mathrm{Sr}$ nuclear magnetic resonance NMR to shed light on the effect of multibands on the magnetic character in the spin-triplet superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ In $({\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}){\mathrm{RuO}}_{4},$ the Knight shift ${(}^{87}K)$ at 1.4 K increases progressively up to $x=1.5.$ The Korringa relation between K and the nuclear-spin-lattice-relaxation rate ${1/T}_{1}$ divided by temperature ${(1/T}_{1}T)$ reveals that ferromagnetic (FM) fluctuations develop with the increasing Ca content. In ${\mathrm{Sr}}_{2}({\mathrm{Ru}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}){\mathrm{O}}_{4},$ in contrast, it is found that anisotropic incommensurate (IC) antiferromagnetic (AFM) fluctuations present in the $y=0$ material are enhanced by a small amount of Ti substitution: ${1/T}_{1}T$ is enhanced with the magnetic field parallel to the ${\mathrm{RuO}}_{2}$ plane, whereas it is unchanged for the field along the c axis in 3% Ti-doped ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ We demonstrate that Ca substitution affects the two-dimensional (2D) $\ensuremath{\gamma}$ band by enhancing the FM fluctuations, and that the Ti-substitution affects the 1D $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ bands by enhancing the IC-AFM fluctuations. The unique substitution effects would relate with the multiband character in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$

Mechanism of radiative recombination in acceptor-doped bulk GaN crystals

Optical and electrical properties of acceptor-doped bulk GaN crystals are discussed. Though introducing Zn and Ca to bulk GaN does not significantly change electron concentration, it results in the appearance of a blue photoluminescence band accompanying the relatively strong yellow band usually present. Highly resistive GaN:Mg crystals are obtained when high amount of Mg is introduced to the Ga melt during high-pressure synthesis. Change of electrical properties of Mg-doped bulk crystals is accompanied by the appearance of a strong blue emission of GaN similar to that in Ca- and Zn-doped crystals. Optically detected magnetic resonance investigations indicate a multi-band character of this blue emission and suggest possible mechanism of compensation in acceptor-doped bulk GaN.

Superconductivity and disorder in Y xLu 1− xNi 2B 2C

The superconducting transition temperature T c, the temperature dependence of the upper critical field H c2( T), and the lattice structure of polycrystalline Y x Lu 1− x Ni 2B 2C samples have been studied by susceptibility, resistivity, and X-ray measurements, respectively. All samples exhibit the same LuNi 2B 2C-type structure and the lattice parameters depend linearly on the yttrium content. The residual resistivity ratio (RRR) decreases steeply first for slight deviations from both pure stoichiometric limiting contents and then saturates at a low level in a broad plateau in between 0.1< x<0.8, whereas T c( x) and H c2( x) at fixed T change nearly parabolically. The H c2( T)-curves show a positive curvature for all samples. This dependence can be described within a broad temperature region 0.3 T c< T≤0.95 T c by a simple and convenient empirical expression H c2( T)∝(1− T/ T c) α . The parameter α>1 describing the positive curvature of H c2( T) characterizes the sample quality and decreases with increasing disorder in the Y x Lu 1− x Ni 2B 2C samples. However, the case of complete disorder (dirty limit) where α=1 is not reached yet in the investigated samples. The nonmonotonous dependence of T c and H c2 upon x is attributed to partial disorder within the Y–Lu basis layers. Together with the RRR-saturation they reflect the complex multiband character of the materials under consideration.

Motion of an Electron in a Perturbed Periodic Potential

Wannier's theorem concerning the motion of an electron in a perturbed periodic potential is generalized so as to take into account transitions between energy bands. The generalized form of the theory is placed on a rigorous quantum-mechanical foundation and the basic approximation of Wannier's form of the theorem elucidated. A compact matrix formulation is developed which permits the multiband character of the formalism to be handled expeditiously. It is shown that corrections to Wannier's approximate equation of motion arising from variations of the potential within a unit cell require a multiband formalism, since these corrections affect the motion primarily by causing transitions between bands. The correction terms are expressed in such a form that they may be conveniently treated as perturbations. A generalized form of Wannier's theorem is presented in which the perturbing potential may depend upon both the coordinate and momentum of the electron. The special case that the perturbative potential is due to slowly varying combined electric and magnetic fields is discussed in detail. Applicability to the problem of the electron-phonon interaction is discussed briefly.