Perovskite Superconductor Research Articles

Normal-state and superconducting properties of Sr2RuO4

We discuss some of the current issues on the copper-free layered perovskite superconductor Sr2RuO4, for which a sharp transition at Tc = 1.2 K has been reproducibly obtained. The normal state is characterized as an essentially twodimensional Fermi liquid, and the coherent interlayer transport is established only at low temperatures. The cylindrical Fermi surface observed by de Haas-van Alphen experiments is consistent with other thermodynamic and transport properties. Although the specific heat jump across Tcconfirms the bulk superconductivity, the large residual T-linear term which correlates with the variation in Tcis unusual and suggestive of unconventional pairing.

Angle-resolved photoemission study of Sr2RuO4.

We present high-resolution (HR) angle-resolved photoemission spectroscopy (ARPES) measurements of the noncuprate layered perovskite superconductor ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$. ARPES spectra of the whole valence-band region obtained along two high-symmetry directions in the Brillouin zone show clear dispersion, generally similar to that of a band calculation. However, HRARPES measurements taken in the vicinity of the Fermi level (${\mathit{E}}_{\mathit{F}}$) show narrower Ru4d\ensuremath{\varepsilon}(xy,yz,zx)-O2p\ensuremath{\pi} antibonding bands than those predicted by the band calculation. More significantly, there is an extended van Hove singularity very close to ${\mathit{E}}_{\mathit{F}}$ (${\mathit{E}}_{\mathit{B}}$=11 meV) along the Ru-O bonding direction, which is known to exist in cuprate high-temperature superconductors. The Fermi-surface topology obtained by HRARPES (one electronlike Fermi surface sheet centered at the \ensuremath{\Gamma} point and two holelike sheets centered at the X point) is different from the band calculation (two electronlike sheets centered at the \ensuremath{\Gamma} point and one holelike sheet centered at the X point), although the electron count is the same in both cases. These results suggest that electron-electron correlations cause the modification of the Fermi-surface topology, and is thus necessary for understanding the electronic structure and properties of ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$. \textcopyright{} 1996 The American Physical Society.

Hall effect in the two-dimensional metal Sr2RuO4.

We report a detailed study of the Hall effect in ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$, the first layered perovskite superconductor which does not contain copper (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}1 K). The Hall coefficient (${\mathit{R}}_{\mathit{H}}$) was measured at temperatures between 20 mK and 300 K, and an unusual dependence of ${\mathit{R}}_{\mathit{H}}$ on the applied magnetic field was observed. ${\mathit{R}}_{\mathit{H}}$ has a strong temperature dependence below 25 K, but below 1 K it saturates at a value of -1.15\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{m}}^{3}$/C. The Fermi surface of ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$ is known from quantum oscillation measurements, and since it is nearly two dimensional, it is possible to derive a simple expression for ${\mathit{R}}_{\mathit{H}}$ using methods developed by Ong [Phys. Rev. B 43, 193 (1991)]. We show that if the mean free path, l, is assumed to be isotropic at low temperatures, it is possible to make an accurate quantitative calculation of ${\mathit{R}}_{\mathit{H}}$ on the basis of the known Fermi surface parameters. \textcopyright{} 1996 The American Physical Society.

Chemical Effects of Lanthanides and Actinides in Glasses Determined with Electron Energy Loss Spectroscopy

The chemical and structural environments of f-electron elements in glasses are the origin of many of the important optical, electronic, and magnetic properties of materials incorporating these elements. Thus, the oxidation state and chemical coordination of lanthanides and actinides in host materials is an important design consideration in optically active glasses, magnetic materials, perovskite superconductors, and nuclear waste materials.The energy loss spectra of the lanthanides are characterized by sharp 3d3/2→4f5/2 (M4) and 3d5/2→4f7/2 (M5) lines, the relative intensities of which are determined by the 4f-shell occupancy of the excited ion. For the light lanthanides, the dependence of these relative peak heights on 4f-shell occupancy is quite pronounced. In particular, the ratio of the M4 to M5 peak areas in the second derivative spectra is extremely sensitive to the formal valence of the multivalent elements Ce and Pr. In figure 1, the EELS spectra are presented for Ce(III) and Ce(IV) in well-characterized reference materials.

Critical issues in ceramic microstructures

As the number and variety of ceramic materials have grown so rapidly in the last few decades, ranging from silicon nitride structural ceramics to the perovskite superconductors to the ferroelectric oxides to semiconducting sensors, the number of scientific and technical issues has also grown rapidly. Many of the basic questions relate to the role the microstructures play in determining the observed physical behavior but increasingly it is not the geometric properties of the microstructure that are of central concern but rather compositional variations and associated electrical characteristics. These require the continued development of microscopy techniques to complement the tremendous advances in microstructural understanding that have already been made possible by microscopy in the past.Since the role of microscopy is such a broad one, only a few of the most generic problems in microstructure characterization will be described in this talk. The topics selected include the characterization of intergranular films in liquid-phase sintered ceramics, the charge distribution at interfaces and the associated space charge, the epitaxial growth of oxides on oxide substrates, and the use of fluorescence imaging to identify phases and non-destructively measure local strains.

Anisotropic Flux Pinning in a Layered Superconductor Sr2RuO4

We found peculiar flux-pinning in single crystals of Sr 2 RuO 4 , the only known layered-perovskite superconductor without copper. Measurements on ac susceptibility under dc magnetic fields indicate that a second dissipation peak prominently appears only when the direction of the flux lines, as well as of their induced motions, is parallel to the a b planes. The anisotropic nature of this peak effect is clearly different from that reported in high- T c cuprates, the other class of layered perovskite superconductors. The peak effect in Sr 2 RuO 4 is attributed to the synchronization of the flux lines to the pinning centers as the flux-line lattice softens with increasing magnetic field and temperature.

Fermi surface and extended van Hove singularity in the noncuprate superconductor Sr2RuO4.

We mapped the Fermi surface of the first copper-free layered perovskite superconductor, S${\mathrm{r}}_{2}$Ru${\mathrm{O}}_{4}$, by high-resolution $(\ensuremath{\approx}22\mathrm{meV})$ angle-resolved photoemission. Three bands cross the Fermi energy, consistent with band structure calculations; one around $\overline{\ensuremath{\gamma}}$ and two around $\overline{X}$. The highlight is the observation of an extended van Hove singularity located 17 meV below the Fermi level. It extends around $\overline{M}$ for $\ensuremath{\approx}0.2{\AA{}}^{\ensuremath{-}1}$ along $\overline{\ensuremath{\gamma}}\ensuremath{-}\overline{M}\ensuremath{-}\overline{\ensuremath{\gamma}}$ and $\overline{X}\ensuremath{-}\overline{M}\ensuremath{-}\overline{X}$ in the projected Brillouin zone. This raises important questions related to the possible role of a van Hove singularity for oxide superconductivity.

Thermopower of a Layered Perovskite Superconductor,Sr2RuO4

Thermopower of a layered perovskite superconductor, Sr 2 RuO 4 , is studied. The magnitude of the thermopower in the conducting plane is about 29 µ V· K -1 at 300 K. Above 4.2 K thermopower increases almost linearly with temperature, and it tends to saturate at high temperature. Thermopower is positive in the measured temperature range. The temperature derivative of thermopower shows a sharp change at about 20 K where no anomaly has been reported for other physical properties of Sr 2 RuO 4 .

The spin susceptibility of singlet correlated oxygen band in La2?x Sr x CuO4

We have shown that the unconventional temperature dependence of the static susceptibilityϰ(T) of the perovskite high-Tc superconductors above the superconducting transition temperatureTc can be explained in terms of two relevant band models containing the singlet-correlated oxygen band and the copper character band. The usual copper-oxygen Hamiltonian containing hopping and Coulomb repulsion terms has been reduced to an effective Hubbard-liket-t′-t″-Ueff model to describe the low-energy properties. The unusual behaviour of the susceptibility is due to thermally activated oxygen holes coming into the hybridization singularity peak in the density of states. A possible physical origin ofTmax in the temperature dependence of the susceptibility is discussed.

Quantum Oscillations in the Layered Perovskite Superconductor Sr2RuO4

We report a comprehensive study of magneto-oscillatory phenomena in the normal state of S${\mathrm{r}}_{2}$Ru${\mathrm{O}}_{4}$, the first layered perovskite superconductor $({T}_{c}\ensuremath{\cong}1\mathrm{K})$ not based on copper. The form of the quasiparticle spectrum observed may be interpreted in terms of an almost two-dimensional Fermi liquid model which is consistent with Luttinger's theorem and successfully predicts bulk thermodynamic and transport properties at low temperatures. A study of the spectra and transport along the c axis provides insights into the different normal state and superconducting behavior of S${\mathrm{r}}_{2}$Ru${\mathrm{O}}_{4}$ and the cuprates.

Calculation of thermodynamic and transport properties of Sr 2RuO 4 at low temperatures using known fermi surface parameters

We report the results of a study of quantum oscillations in the layered perovskite superconductor Sr 2RuO 4. All three predicted sheets of the Fermi surface have been observed, and we show how the measured Fermi surface parameters and quasiparticle cyclotron masses can be used to make successful quantitative predictions of a number of physical properties that can be measured independently.

Controlling Defects in Double-Layer Cuprates by Chemical Modifications

AbstractIn-situ high temperature electrical conductivity and thermopower have been measured simultaneously on a number of ordered perovskite-like oxides containing double CUO4/2 sheets. Equilibrium measurements have been conducted as a function of oxygen partial pressure, temperature and chemical substitution in order to understand the relationships between the chemical architecture and the transport and defect properties. Data for LaBa2Cu2NbO8 and LaCa2Cu2GaO7 are presented and compared with those of known triple perovskite superconductors, Y1−xCaxSr2Cu2GaO7 and YBa2Cu3O7−δ, and several quadruple perovskites, Ln′Ln″Ba2Cu2M2O11 (Ln = Lanthanide, Y; M = Sn, Ti). These materials belong to a general family of superconductors which are constructed from similar ‘active’ layers (double perovskite blocks of square-pyramidal copper-oxygen sheets), and interleaved with fixed valence cations in perovskite-like ‘conditioning’ layers. Similarities in the transport properties of the non-superconducting and superconducting materials at elevated temperatures are illustrated, and the amount and types of defects, including carrier concentrations, are correlated with the internal chemistry and inner architecture of each material.

Nuclear spin-lattice relaxation and antiferromagnetic spin correlations in superconducting thiospinel Cu1.5Co1.5S4.

The antiferromagnetic and superconducting properties of a thiospinel ${\mathrm{Cu}}_{1.5}$${\mathrm{Co}}_{1.5}$${\mathrm{S}}_{4}$ (${\mathit{T}}_{\mathit{N}}$=19.0 K, ${\mathit{T}}_{\mathit{c}}$=2.3 K) have been investigated with $^{63}\mathrm{Cu}$ NMR (75 MHz), $^{59}\mathrm{Co}$ NMR (75 MHz), and $^{59}\mathrm{Co}$ pure quadrupole resonance between T=1.23 and 150 K. The linear dependence of negative $^{63}\mathrm{Cu}$ Knight shift (-0.013% at 4.2 K) on the Curie-Weiss-type susceptibility \ensuremath{\chi}(T) and nearly independent $^{59}\mathrm{Co}$ Knight shift (+1.43%) indicate that the d hole band of Cu at the tetrahedral A site is mainly responsible for the spin paramagnetism. The spin-lattice relaxation rates (${\mathit{T}}_{1}$T${)}^{\mathrm{\ensuremath{-}}1}$ of both $^{63}\mathrm{Cu}$ and $^{59}\mathrm{Co}$ are significantly enhanced with lowering temperature below \ensuremath{\sim}100 K, similar to those observed in high-${\mathit{T}}_{\mathit{c}}$ copper oxygen perovskite superconductors, which are associated with the growth of antiferromagntic spin correlations at low temperatures. Below ${\mathit{T}}_{\mathit{c}}$, ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ of the fractional Co at the octahedral B site, which increases from \ensuremath{\sim}0% at 0.65${\mathit{T}}_{\mathit{c}}$ to \ensuremath{\sim}30% at 0.5${\mathit{T}}_{\mathit{c}}$, shows a rapid decrease, indicating a partial formation of the superconducting energy gap. On the other hand, ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ of the dominant part of Co follows a Korringa-like relation down to 0.5${\mathit{T}}_{\mathit{c}}$, suggesting it is in a gapless superconducting state, probably due to strong antiferromagnetic spin correlations.

Electronic band structure of the superconductor Sr2RuO4.

A local-density electronic-band-structure calculation was performed for a recently discovered non-copper-layered perovskite superconductor, ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$. It was found that the electronic structure near the Fermi energy is essentially described by antibonding bands of the Ru d\ensuremath{\varepsilon} and O p\ensuremath{\pi} states. Although two holes in the bands are predominantly situated in a d\ensuremath{\varepsilon}(xy)-p\ensuremath{\pi} state in the ab plane, the hole occupations in the other d\ensuremath{\varepsilon}-p\ensuremath{\pi} states vertical to the plane are not negligibly small, possibly in conjunction with the smallness of tetragonal distortion of the ${\mathrm{RuO}}_{6}$ octahedron. Associated with the antibonding d\ensuremath{\varepsilon}-p\ensuremath{\pi} bands, the density of states at the Fermi energy is relatively high (4.36 states/eVcell) but not enough to account for the observed specific-heat constant ${\ensuremath{\gamma}}_{\mathrm{exp}}$ and temperature-independent magnetic susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathrm{exp}}$. We found a large Stoner factor, which may explain most of the mass enhancement involved in ${\mathrm{\ensuremath{\chi}}}_{\mathrm{exp}}$. Certain similarities and dissimilarities in the electronic properties to the cuprate superconductors are discussed.

Synthesis and characterization of Ba3(Pb1-xBix)2O7.

The synthesis and initial characterization of a layered perovskite-based lead-bismuth oxide are reported. The phase, ${\mathrm{Ba}}_{3}$(${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Bi}}_{\mathit{x}}$${)}_{2}$${\mathrm{O}}_{7}$, for 0\ensuremath{\le}x\ensuremath{\le}0.5, is the n=2 member of the Ruddlesden-Popper series ${\mathit{A}}_{\mathit{n}+1}$${\mathit{B}}_{\mathit{n}}$${\mathrm{O}}_{3\mathit{n}+1}$. It can be synthesized only under very narrowly defined conditions. Despite the analogy to the well-known three-dimensional perovskite superconductor ${\mathrm{BaPb}}_{0.75}$${\mathrm{Bi}}_{0.25}$${\mathrm{O}}_{3}$, layered ${\mathrm{Ba}}_{3}$${\mathrm{Pb}}_{2}$${\mathrm{O}}_{7}$ does not become superconducting (down to 1.8 K) on doping with Bi.

Spinons and holons: a class of simple models

In this paper, the authors discuss a class of quasi-one-dimensional models containing both spins and charge, which share the same generic solutions. In particular, they concentrate on a Heisenberg model of a topologically frustrated antiferromagnet, and a d-p model for oxygen hole hopping in a subgeometry of the copper-oxygen plane of the perovskite superconductors. The outstanding feature of these models lies in the simplicity of their solutions: their ground states are exact and contain uncorrelated, short-ranged singlet pairs. Excitations in these models fall into two distinct classes, namely, spin-1/2 chargeless domain-wall excitations or 'spinons', together with their conjugate excitations, which take the form of spin-1/2 chargeless 'antispinon' domain walls in the Heisenberg model, and spinless charge+e hole excitations or 'holons' in the d-p model. In the case of the Heisenberg model, the authors have successfully constructed a sufficiently simple representation for the 'spinon' excitation which allows them to calculate its dispersion using elementary methods. This is therefore a concrete example of a 'spinon' excitation for which explicit representation has been possible.

Parallel superconductor code on the iPSC/860

Researchers at Oak Ridge National Laboratory have developed an application code for calculating the electronic properties and energetics of disordered materials. The same source code has been compiled and run on workstations, Crays, and the Intel iPSC/860. This electronic structures code is capable of running at over 2 gigaflops on both an 8-processor CRAY Y-MP and a 128-processor Intel iPSC/860. Using this new KKR-CPA code, we executed density-of-state computations of a perovskite superconductor at a rate of 2527 megaflops on the Intel iPSC/860. This corresponds to a price/performance rate of 842 megaflops per $1 million based on the list price of this computer. Similar but smaller computations done on a network of ten IBM RS/6000 workstations executed at a price/performance rate of 1.3 gigaflops per $1 million.

The X-J model: perovskite superconductors and heavy fermions

The X-J model is a strong-coupling limit of both the natural tight banding model of perovskite superconductors and the Anderson lattice model of heavy fermions. The application of the model to perovskite superconductivity is straightforward, but its use to describe heavy fermions is more speculative. The straight-line motion of charge carriers in the model is sympathetic to antiferromagnetic correlations along the path traversed, although the motion destroys the long-range antiferromagnetic order by exchanging the two sublattices in passing. Antiferromagnetism is destroyed in both the square lattice geometry relevant to a CuO2 plane and the triangular geometry relevant to an isolated layer of CeAl3. A paramagnetic phase with shorter range correlations than suggested by the Heisenberg model seems preferred by the charge-carrier motion in these two-dimensional examples.

Evaluation of flux-based logic schemes for high-T/sub c/ applications

Three digital logic families that can be made using nonhysteretic Josephson junctions (potentially the only kind of Josephson device realizable with superconductors having high transition temperatures) are analyzed. These logic families utilize magnetic flux-transfer, and are characterized by very low power dissipation. Rapid single flux quantum (RSFQ) and phase-mode logic are both based on pulse propagation. The quantum flux parametron (QFP) logic family is based on current latching. Simulations of RSFQ, Phase-Mode, and QFP logic families using high-T/sub c/ junction parameters are presented to demonstrate the compatibility of these logic families with the perovskite superconductors. The operation of these logic families is analyzed, and the advantages and disadvantages of each are discussed.

Low spin correlations between copper spins in oxygen hole superconductors

The author compares three basic physical mechanisms which induce correlations between the copper spins of a single strong-coupling plane of copper oxide in the perovskite superconductors. Motions of holes by virtual Cu3+ excitations induces ferromagnetic correlations, hole motion by Cu+ excitations induces paramagnetism, and Heisenberg interactions induce antiferromagnetism. They study the smallest clusters which exhibit the phenomena and examine the competition between these effects by exact diagonalisation. The paramagnetism and antiferromagnetism are quite similar and readily coexist locally. The ferromagnetic correlations are rapidly destroyed by increasing the Heisenberg interactions which stabilise the paramagnetic state locally. The mechanism which leads to paramagnetism via hole motion by virtual Cu+ excitations is investigated by solving a one-dimensional chain topology using short-range valence bonds. The paramagnetic correlations seem the only phenomenon which is likely to be relevant in the experimental systems. The inclusion of the Heisenberg interactions may be considered a minor perturbation to the paramagnetic correlations induced by the hole motion, and so may be neglected to a first approximation for this limit.