Hiroya Sakurai

Superconductivity in two-dimensional CoO2 layers

Since the discovery of high-transition-temperature (high-Tc) superconductivity in layered copper oxides, many researchers have searched for similar behaviour in other layered metal oxides involving 3d-transition metals, such as cobalt and nickel. Such attempts have so far failed, with the result that the copper oxide layer is thought to be essential for superconductivity. Here we report that NaxCoO2·yH2O (x ≈ 0.35, y ≈ 1.3) is a superconductor with a Tc of about 5 K. This compound consists of two-dimensional CoO2 layers separated by a thick insulating layer of Na+ ions and H2O molecules. There is a marked resemblance in superconducting properties between the present material and high-Tc copper oxides, suggesting that the two systems have similar underlying physics.

Unconventional superconductivity and nearly ferromagnetic spin fluctuations in NaxCoO2.yH2O

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor Na x CoO 2 · y H 2 O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na x CoO 2 were examined, a SC bilayer-hydrate sample with T c ∼4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/ T 1 in the SC sample, it was found that the coherence peak is absent just below T c and that 1/ T 1 is proportional to temperature far below T c . These results, which are in qualitative agreement with the previous result by Fujimoto et al. , suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/ T 1 T of the SC sample shows a gradual increase below 100 K down to T c , whereas 1/ T 1 T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/ T 1 T and χ bulk in the SC sample, the increase in 1/ T 1 T is at...

Chemical composition and crystal structure of superconducting sodium cobalt oxide bilayer-hydrate

Sodium cobalt oxide bilayer-hydrate, NaxCoO2·yH2O (x ≈ 0.35), was characterized by chemical and structure analyses. The formal valence of Co was found to be +3.4 from redox titration, which was much lower than +3.65 that had been estimated from the Na content, x. The deviation from the charge neutralization was considered to be compensated by the presence of oxonium ions in the CoO2 galleries, which was evidenced by Raman spectroscopy. Therefore, the composition of the superconducting phase was determined to be Na0.343(H3O)0.237CoO2·1.19H2O. Structural studies based on Raman spectroscopy and synchrotron X-ray powder diffraction suggested that the oxonium ions occupy the same crystallographic sites as the Na+ ions.

Unconventional upper- and lower-critical fields and normal-state magnetic susceptibility of the superconducting compound Na 0.35 CoO 2 ⋅ 1.3 H 2 O

Magnetic properties of the layered superconductor, Na 0 . 3 5 CoO 2 . 1.3H 2 O have been investigated. From the temperature dependence and field dependence of the magnetization, the superconducting transition temperature, aswell as upper- and lower-critical fields have been estimated to be T C =4.6 K, H C 2 (0)=61.0 T and H C 1 (0)=28.1 Oe, respectively. These values give quite unusual phenomenological parameters, i.e., coherent length, penetration depth and Ginzburg-Landau parameter of Β=2.32 nm, λ=5.68×10 2 nm, and κ≡λ/Β =244, respectively, suggesting an unconventional nature of superconductivity. Additionally, the magnetic susceptibility above T C and the magnetization curve up to 54 T are also reported.

Structural difference between a superconducting sodium cobalt oxide and its related phase

Abstract Monolayer hydrate (MLH) Na x CoO 2 · y ′H 2 O was obtained from superconducting bilayer hydrate (BLH) Na x CoO 2 · y H 2 O by partial extraction of H 2 O molecules between the CoO 2 layers. Magnetization measurements indicated that electron densities in the CoO 2 layer of the MLH phase remained unchanged after the water extraction. Nevertheless, superconductivity was completely suppressed in the MLH phase. This strongly suggests that the highly 2D nature in the BLH phase due to its thick insulating layers consisting of H 2 O molecules and Na + ions plays an important role for inducing superconductivity.

Phase Diagram of Superconducting NaxCoO2 .yH2O

We discovered that oxonium (H 3 O + ) ion content is a key parameter to the understanding of unique superconductivity in sodium cobalt oxyhydrate. As Na + ions are substituted by H 3 O + ions, keep...

Spin-flop driven magneto-dielectric effect in Co4Nb2O9

Co4Nb2O9 becomes antiferromagnetic (AFM) below 27.4 K with a spin-flop transition at a critical field, Hc, of 12 kOe. Room-temperature dielectric properties are dominated by finite electronic conductivity. Below 125 K, the charge carriers are frozen-out and the dielectric constant is controlled by the lattice phonons. A large (12%) spin flop-driven enhancement in dielectric constant is found in the very narrow temperature interval (Δ T = 1.6 K) in the vicinity of the AFM phase transition. Magneto-dielectric anomaly shows low-frequency dispersion; therefore, the H-induced changes in the phonon eigenfrequencies are unlikely. Other possible reasons for unusual magneto-dielectric effect in Co4Nb2O9 are discussed.

Unconventional Colossal Magnetoresistance in Sodium Chromium Oxide with a Mixed‐Valence State

ions. This material demonstrates an unusual CMReffect that is closely related to its unconventional magneticstructure caused by spin frustration. The CMR of thiscompound is unique from several aspects. First, it is observedin a chromium oxide, not in a manganese oxide. Second, it isfound in a single-phase material having an insulating groundstate. Third, the CMR is not limited to the vicinity of themagnetic phase transition but becomes progressively moreprominent with decreasing temperature down to 0 K. Thediscovery of the NaCr

Evidence of nodal superconductivity in Na 0.35 CoO 2 · 1.3 H 2 O : A specific-heat study

Comprehensive low-temperature specific-heat data

Possible unconventional superconductivity in Na x CoO 2 · y H 2 O probed by muon spin rotation and relaxation

C(T,H)