Non-cuprate Superconductor Research Articles

Supersolid Phases of a Doped Valence-Bond Quantum Antiferromagnet: Evidence for a Coexisting Superconducting Order Parameter

Motivated by numerical evidence of the valence-bond ground state of the two-dimensional Heisenberg pyrochlore lattice, we argue using a t-J model that it evolves under doping into novel phases characterized by superconductivity coexisting with the underlying valence-bond solid order. A fermionic mean-field theory supplemented by exact diagonalization results provide strong arguments in favor of the stability of such supersolid phases. The resemblance with modulated superconducting patterns in high-Tc cuprates as well as possible relevance to frustrated noncuprate superconductors such as spinels and pyrochlores is discussed.

Very high upper critical fields inMgB2 produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical fieldHc2 of differentMgB2 samplesalloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivitiesρ, we show a cleartrend of an increase in Hc2 as ρ increases. One particular high resistivity film had a zero-temperatureHc2(0) wellabove the Hc2 values of competing non-cuprate superconductors such asNb3Sn and Nb–Ti. Our high-field transport measurements give record values T and T for high resistivity films and T for untextured bulk polycrystals. The highestHc2 film also exhibits a significant upward curvature ofHc2(T) and a temperature dependence of the anisotropy parameter opposite to that of single crystals:γ(T) decreases as the temperature decreases, from to . This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivityin MgB2, which offers special opportunities for furtherHc2 increases by tuning of the impurity scattering by selective alloying onMg and B sites. Our experimental results can be explained by a theoryof two-gap superconductivity in the dirty limit. The very high values ofHc2(T) observedsuggest that MgB2 can be made into a versatile, competitive high-field superconductor.

Tunneling spectroscopy of the electron-doped layered superconductor Li0.48(THF)0.3HfNCl

Tunneling measurements on the electron-doped layered superconductor Li0.48(THF)0.3HfNCl(THF;C4H8O) with Tc=25.5 K have been carried out. Since the surface of this compound is very reactive, we have employed in situ break junction to obtain unaffected interface. The result shows the gap value of 2Δ=9–12 meV, which leads to the strong-coupling gap ratio 2Δ/kBTc≃5. This is the largest value among non-cuprate superconductors.

Localized High-Tc Superconductivity on the Surface of Na-Doped WO3

WO3 crystals were doped with Na on the surface to ∼7% nominal atomic concentration. Scanning tunneling microscopy and spectroscopy were employed, together with magnetization measurements, in the study of these crystals. Tunneling experiments reveal superconducting islands, 20–150 nm in size, covering about 10% of the surface, the rest of which is insulating. Magnetization measurements show that the superconducting phase formed at the surface has a critical temperature of 91 K, while tunneling spectroscopy yields superconducting gaps having values up to 2Δ/kT c ∼ 4. Presumably most of the sodium is concentrated in these islands and therefore they are metallic in nature, above 30% in atomic concentration. This material is therefore a noncuprate superconductor with a high critical temperature.

Symmetry Analysis of the Fermi Surface States of Sr2RuO4by Display-Type Photoelectron Spectroscopy

The Fermi surface of the noncuprate superconductor Sr 2 RuO 4 has been studied in detail by two-dimensional angle resolved photoelectron spectroscopy (2D-ARPES) using a display-type electron spectrometer and a linearly polarized synchrotron radiation. The existence of the large Fermi surfaces agrees with previous band calculations and conventional ARPES measurements. From the comparison of the observed photoelectron intensity distribution of the Fermi surface with that of the simulation for atomic orbitals, the symmetry of the atomic orbitals composing the Fermi surfaces is revealed to be the d e ; d x y , d y z , d z x -like in contrast to the d x 2 - y 2 -like orbital in the cuprate high- T c super conductor. The possibility of the existence of the extended van-Hove singularity just below the Fermi level, which was suggested by recent conventional ARPES measurements, is also discussed.

Energy bands and Fermi surface ofSr2RuO4

A method combining the extended H\"uckel theory and the tight-binding approximation is proposed to study the electronic properties of the layered noncuprate superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}.$ The band structure close to the Fermi level is obtained by integrating out all low-lying oxygen degrees of freedom allowing the determination of the energy spectra, the Fermi surface, and the total density of states. The results reproduces the main features of local-density approximation calculations near the Fermi energy. As an application of the approach here proposed, we have calculated the temperature dependence of the in-plane Hall coefficient considering the effect of the curvature of the Fermi surface.

Magnetostriction of the high-temperature non-cuprate superconductor BaBiKO

Measurements of the magnetostriction of the isotropic high-temperature superconductor Ba0.66K0.34BiO3 revealed an effect of the order of 10−6 which exceeds that for conventional superconductors but is not as high as the giant magnetostriction of cuprate high-temperature superconductors. A thermodynamic analysis of the results enables comparison with the results of numerical calculations of effects induced by magnetic flux pinning.

Fermi surface of Sr 2RuO 4 studied by two-dimensional angle resolved photoelectron spectroscopy

The shape of the Fermi surface of the Sr 2RuO 4, the first noncuprate superconductor, has been directly observed by two-dimensional angle resolved photoelectron spectroscopy (2D-ARPES) technique using a display-type hemispherical mirror analyzer. The gross shape of the Fermi surface agrees with previous band calculations. The photoelectron angular distribution pattern of the Fermi level shows that the Fermi surface is composed of orbitals with d xy, d yz , and d zx -like symmetries which agree well with those predicted by previous band calculations and are in contrast to d x 2− y 2 symmetry of thehigh- T C superconductors.

Angle-resolved photoemission study of Sr 2RuO 4; An extended van-Hove singularity in non-cuprate superconductor

We have performed high-resolution angle-resolved photoemission spectroscopy of a non-cuprate layered superconductor Sr 2RuO 4 to study the electronic structure near the Fermi level ( E F). The result shows an extended van Hove singularity (vHs) along the RuO bonding direction just like the cuprate, regardless of the character of the electronic states at E F. This suggests that the extended vHs is a general electronic structure of a two dimensional correlated d-electron metal. We also have found that the observed Fermi surface topology (one electron-like Fermi surface sheet centered at Γ point and two hole-like ones centered at X point) is qualitatively different from the calculated Fermi surface topology (two electron-like Fermi sheets centered at Γ point and one hole-like one centered at X point) in the consequence of the existence of the vHs.

SOME CHEMICAL AND STRUCTURAL EFFECTS ON THE PROPERTIES OF HIGH-Tc SUPERCONDUCTORS

▪ Abstract The history of superconductivity, including superconductivity in the high-Tc cuprates, is reviewed very briefly, and the differences between conventional superconductors and the high-Tc cuprates are summarized. The basic crystal structures of the major series of high-Tc cuprates are described and compared. The relation of structures to superconducting properties is reviewed with an emphasis on the orthorhombic-tetragonal transition in (La2−x Srx)CuO4; the corresponding transition, and also the transition to the low-temperature tetragonal phase in (La2−xBax)CuO4; and the effects of oxygen vacancies, oxygen-vacancy ordering, frozen-in disorder, and occupation of the off-chain O(5) sites in YBa2Cu3O7−δ. The effects of chemical substitutions of lanthanide elements on the La/Sr sites in (La2−xSrx)CuO4 and on the Y sites in YBa2Cu3O7−δ, and of 3d elements and sp elements on the Cu sites in both systems are reviewed. The difference between the effects of the lanthanide substitutions, particularly Pr, in the two systems are considered. Major properties of the Bi-, Tl-, and Hg-cuprates, are described briefly. A comparison of the high-Tc cuprates with noncuprate oxide superconductors is given.

Nature of the electronic states in the layered perovskite noncuprate superconductor Sr2RuO4.

Near-edge x-ray-absorption and photoemission spectra of the noncuprate superconductor ${\mathrm{Sr}}_{2}$Ru${\mathrm{O}}_{4}$ are presented. Holes close to the Fermi level are determined by Ru $4{d}_{\mathrm{xy}}$, $4{d}_{\mathrm{xz}}$, and $4{d}_{\mathrm{yz}}$ antibonding states. The density of the occupied states at the Fermi level derived from photoemission is higher than in the cuprates and these states have predominantly Ru $4d$ character. Angle-resolved photoemission yields a width of the occupied part of the conduction bands that is reduced by a factor of 2 in comparison to band-structure calculations. The importance of electron correlations is also evident in the appearance of screened final states in the Ru $3d$ core-level spectrum.

Extended van-Hove singularity in Sr 2RuO 4 observed by angle-resolved photoemission

We have performed low-temperature high-resolution angle-resolved photoemission spectroscopy on a non-cuprate superconductor Sr 2RuO 4 ( T c ∼ 1 K) which has the same crystal structure as La 2CuO 4 but with RuO 2 layers replacing CuO 2. We found that Sr 2RuO 4 has an extended van-Hove singularity (VHS) close to E F(20 ± 2 meV) like high- T c cuprates, regardless of the character of the electronic states at E F, suggesting that the extended VHS is a general feature of a two-dimensional d-electron metal. This demands re-examination of the VHS scenario for the superconducting properties of cuprates, while T-linear resistivity seems to be consistent with this result.

Simple tight-binding model for the Fermi surface of Sr2RuO4

The Fermi surface of Sr2RuO4 mapped out recently by Angle-Resolved Photoemission Spectroscopy [D. H. Luet al., Fermi surface and extended van Hove singularity in non-cuprate superconductor Sr2RuO4, submitted to Phys. Rev. Lett.] is fitted by a simple tight-binding model for conducting RuO2 plane. The results obtained are in agreement with LDA calculations performed by Oguchi and Singh. The Fermi surface is analytically expressed by tight-binding parameters.

The specific heat of Ba 0.6K 0.4BiO 3 below 2K

We have measured the specific heat C of a polycrystalline sample of the non-cuprate superconductor Ba 0.6K 0.4BiO 3 ( T c ≈ 27K) from T = 1.8 K down to T = 0.1 K. The data could be fitted accurately to the expression C = AT −2 + αT n + βT 3 where n = 0.53 ± 0.10. The nuclear term ( AT −2) is attributed to the presence of a crystalline electric field gradient on the Bi nuclei. The residual specific heat term ( αT n) has also been observed in the low temperature specific heat of several cuprate high- T c superconductors (0.3 ≤ n ≤ 1) suggesting a possible common origin in all of these systems.

Superconductivity Mechanism of Non-Cuprate Superconductors11project supported by National Natural Science Foundation of China.

Based on the mechanism of charge fluctuation, the effective Hamiltonian is derived to discuss the superconductivity for the non-cuprate compound . The desired versus curve is obtained. It is shown that the explanation of superconductivity for non-cuprate superconductors is the same as that of cuprate ones. Furthermore, that the non-cuprate superconductors have no ferromagnetism and anti-ferromagnetism can be explained, and the superconductivity window is due to the Cooper pairing in coordinate space.

Ultraviolet photoemission study of single-crystal BaPb1-xBixO3.

An ultraviolet photoemission study on single-crystal ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ with x=0.15 (superconductor) and x=1.0 (semiconductor) has been presented. A clear Fermi-edge structure was observed for the first time for the superconducting compound. A rigid shift of the valence band by 0.3--0.4 eV between the superconductor and the semiconductor was observed although the shift is much smaller than predicted from the band-structure calculation. The ``mysterious 9-eV peak,'' whose origin has been controversial in cuprate superconductors, was found also for degraded crystal surfaces of ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$. Its origin was discussed in connection with the difference and similarity in the high-${T}_{c}$ mechanism between cuprate and noncuprate superconductors.