M. L. Foo

Large enhancement of the thermopower in NaxCoO2 at high Na doping

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate NaxCoO2 exhibits several interesting electronic phases as the Na content x is varied1,2,3,4, including water-induced superconductivity4 and an insulating state3 that is destroyed by field5. Initial measurements1 showed that, in the as-grown composition, NaxCoO2 has moderately large thermopower S and conductivity σ. However, the prospects for thermoelectric cooling applications faded when the figure of merit Z was found to be small at this composition (0.60.75, S undergoes an even steeper enhancement. At the critical doping xp∼0.85, Z (at 80 K) reaches values ∼40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.

Superconductivity phase diagram of Na(x)CoO2*1.3H2O.

The microscopic origin of superconductivity in the high-transition-temperature (high-Tc) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting Tc on the degree of electronic band filling. The recent report of superconductivity near 4 K in the layered sodium cobalt oxyhydrate, Na0.35CoO2·1.3H2O, is of interest owing to both its triangular cobalt–oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting Tc of this compound displays the same kind of behaviour on chemical doping that is observed in the high-Tc copper oxides. Specifically, the optimal superconducting Tc occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that NaxCoO2·1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.

Fermi Surface and Quasiparticle Dynamics of Na0:7CoO2 Investigated by Angle-Resolved Photoemission Spectroscopy

We present the first angle-resolved photoemission study of Na0.7CoO2, the host material of the superconducting NaxCoO2.nH(2)O series. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band, a small Fermi velocity, and a large Hubbard U. The quasiparticle band crosses the Fermi level from M toward Gamma suggesting a negative sign of effective single-particle hopping t(eff) (about 10 meV) which is on the order of magnetic exchange coupling J in this system. Quasiparticles are well defined only in the T-linear resistivity (non-Fermi-liquid) regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics may have implications for understanding the phase of matter realized in this new class of a strongly interacting quantum system.

Coupling between electronic and structural degrees of freedom in the triangular lattice conductor NaxCoO2

The ambient temperature crystal structures of compounds in the NaxCoO2 family, for 0.34 , x 1.0, determined by powder neutron diffraction, are reported. The structures consist of triangular CoO 2 layers with Na ions distributed in intervening charge reservoir layers. The shapes of the CoO 6 octahedra that make up the CoO2 layers are found to be critically dependent on the electron count and on the distribution of the Na ions in the intervening layers, where two types of Na sites are available. Correlation of the shapes of cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram in NaxCoO2 is made, providing an example of how structural and electronic degrees of freedom can be coupled in electrically conducting triangular lattice systems.

Crystal structure and elementary properties of NaxCoO2 ( x=0.32 , 0.51, 0.6, 0.75, and 0.92) in the three-layer NaCoO2 family

The crystal structures of NaxCoO2 phases based on three-layer NaCoO2, with x=0.32, 0.51, 0.60, 0.75, and 0.92, determined by powder neutron diffraction, are reported. The structures have triangular CoO2 layers interleaved by sodium ions, and evolve with variation in Na content in a more complex way than has been observed in the two-layer NaxCoO2 system. The phases with highest and lowest Na content studied x=0.92 and 0.32 are trigonal, with three CoO2 layers per cell and octahedral Na ion coordination. The intermediate compositions have monoclinic structures. The x=0.75 compound has one CoO2 layer per cell, with Na in octahedral coordination and an incommensurate superlattice. The x=0.6 and 0.51 phases are also single layer, but the Na is found in trigonal prismatic coordination. The magnetic behavior of the phases is similar to that observed in the two-layer system. Both the susceptibility and the electronic contribution to the specific heat are largest for x=0.6.

Low temperature Schottky anomalies in the specific heat of LaCoO3: Defect-stabilized finite spin states

Measurement of the low temperature specific heat of LaCoO3 single crystals reveals a previously unobserved Schottky anomaly with an energy level splitting, 0.5 meV, that is associated with the first excited spin state of the Co3+ ion. These states persist well below 2 K and have a g-factor around 3.5, consistent with the high-spin spin-orbit triplet, implying the existence of a low density (approximately 0.1% of the sites) of finite-spin Co ions even in the T=0 limit. We propose that these states are trapped at defects and are consistent with the magnetic excitons observed in earlier work.

Characterization of the structural transition in Na0.75CoO2

In the Nax CoO2 chemical system, a two-phase region is found at ambient temperature near x = 0.75. The manner in which that two-phase region is formed on cooling from a uniform single phase at temperatures above 340 K is reported here. The driving force that induces the phase separation appears to be a difference in Na composition of the two phases, which, though small, is accompanied by substantial differences in the unit cell parameters. Hysteretic behaviour in the transition is observed. One of the phases exhibits negative thermal expansion in the narrow temperature region where the phases are in the process of separating.

Pressure effects in the triangular layered cobaltites Na x Co O 2

We have measured transport properties as a function of temperature and pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition temperature at 53K increases with pressure, while paradoxically the sample passes from an insulating to a metallic ground state. A similar transition is observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample transforms the sample from a metallic to an insulating state. We discuss our results in terms of interactions between band structure effects and Na+ order.

Are cobaltates conventional? An ARPES viewpoint

Recently discovered class of cobaltate superconductors (Na0.3CoO2.nH2O) is a novel realization of interacting quantum electron systems in a triangular network with low-energy degrees of freedom. We employ angle-resolved photoemission spectroscopy to uncover the nature of microscopic electron motion in the parent superconductors for the first time. Results reveal a large hole-like Fermi surface (consistent with Luttinger theorem) generated by the crossing of super-heavy quasiparticles. The measured quasiparticle parameters collectively suggest a two orders of magnitude departure from the conventional Bardeen-Cooper-Schrieffer electron dynamics paradigm and unveils cobaltates as a rather hidden class of relatively high temperature superconductors.

Superconductivity phase diagram of NaxCoO2|[middot]|1.3H2O

The microscopic origin of superconductivity in the high-transition-temperature (high-Tc) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting Tc on the degree of electronic band filling. The recent report of superconductivity near 4 K in the layered sodium cobalt oxyhydrate, Na0.35CoO2·1.3H2O, is of interest owing to both its triangular cobalt–oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting Tc of this compound displays the same kind of behaviour on chemical doping that is observed in the high-Tc copper oxides. Specifically, the optimal superconducting Tc occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that NaxCoO2·1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.