Possible spin triplet superconductivity in NaxCoO2.yH2O

Symmetry-based considerations are combined with inputs from available experimental results to make the case that a novel spin-triplet superconductivity triggered by antiferromagnetic fluctuations may be realized in the newly discovered layered cobaltide NaxCoO2.yH(2)O. In the proposed picture, inaccessible via resonating-valence-bond physics extrapolated from half-filling, the pairing process is similar to that advanced for Sr2RuO4, but enjoys a further advantage coming from the hexagonal structure of the Fermi surface which gives a stronger pairing tendency.     

Nesting, spin fluctuations, and odd-gap superconductivity in NaxCoO2.yH2O

We calculated the one-electron susceptibility of hydrated NaxCoO2 and find strong nesting, involving about 70% of all electrons at the Fermi level and nearly commensurate with a 2 x 2 superstructure. This nesting creates a tendency to a charge density wave compatible with the charge order often seen at x approximately 0.5 and usually ascribed to electrostatic repulsion of Na ions. In the spin channel, it leads to strong spin fluctuations, which should be important for superconductivity. The state most compatible with this nesting structure is an odd-gap triplet s-wave state.     

Anomalous electronic Raman scattering in NaxCoO2.yH2O

Raman scattering experiments on NaxCoO2.yH2O single crystals show a broad electronic continuum with a pronounced peak around 100 cm(-1) and a cutoff at approximately 560 cm(-1) over a wide range of doping levels. The electronic Raman spectra in superconducting and nonsuperconducting samples are similar at room temperature, but evolve in markedly different ways with decreasing temperature. For superconducting samples, the low-energy spectral weight is depleted upon cooling below T* approximately 150 K, indicating the opening of a pseudogap that is not present in nonsuperconducting materials. Weak additional phonon modes observed below T* suggest that the pseudogap is associated with charge ordering.     

Ferromagnetic fluctuation and possible triplet superconductivity in NaxCoO2.yH2O: fluctuation-exchange study of the multiorbital Hubbard model

Spin and charge fluctuations and superconductivity in NaxCoO2.yH(2)O are studied based on a multiorbital Hubbard model. Tight-binding parameters are determined to reproduce the results of band calculations. By applying the fluctuation-exchange approximation, we show that the Hund's-rule coupling between the Co t(2g) orbitals causes ferromagnetic (FM) spin fluctuation. Triplet fy((y(2)-3x(2)))-wave and p-wave pairings are favored by this FM fluctuation on the hole-pocket band. We propose that, in NaxCoO2.yH(2)O, the Co t(2g) orbitals and interorbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor in which the orbital degrees of freedom play substantial roles.     

Spin-singlet superconductivity and antiferromagnetic correlations for the field-aligned powder of the triangular lattice NaxCoO2 · yH2O

Nuclear magnetic resonance and relaxation measurements of the ⁵⁹Co nuclei for the well-defined field-aligned powder of the triangular lattice superconductor Na_xCoO₂ · yH₂O with a nearly optimal composition for the transition temperature T_c have been performed. Detailed analyses indicate that the Knight shifts for the directions parallel and perpendicular to the CoO₂ plane and the spin-lattice relaxation rates, taken by making an inevitable deterioration of specimen minimal, are significantly smaller than most of the data reported to date, and that this compound is classified into one of the unconventional spin-singlet superconductors as suggested by recent works for single crystals with a lower T_c. A small enhancement of two-dimensional antiferromagnetic spin correlations at some nonzero wave vector may emerge near T_c.     

17O NMR studies of a triangular-lattice superconductor NaxCoO2 x yH(2)O

We report the first 17O NMR studies of a triangular-lattice superconductor Na(1/3)CoO2 x 4/3H(2)O and the host material Na(x)CoO2 (x=0.35 and 0.72). Knight shift measurements reveal that p-d hybridization induces sizable spin polarization in the O triangular-lattice layers. Water intercalation makes CoO2 planes homogeneous and enhances low frequency spin fluctuations near T(c)=4.5 K at some finite wave vectors different from both the ferromagnetic and "120 degree" modes.     

Possible spin-tripletf-wave pairing due to disconnected fermi surfaces in NaxCoO2.yH2O

We propose that the spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [Phys. Rev. B 63, 174507 (2001)]] may be at work in a recently discovered superconductor NaxCoO2.yH2O. We introduce a single band effective model that takes into account the pocketlike Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying the fluctuation exchange method and solving the linearized Eliashberg equation, the most dominant pairing is found to have spin-triplet f-wave symmetry, where the nodes of the gap function do not intersect the pocket Fermi surfaces. The presence of finite Tc is suggested in sharp contrast to cases when the gap nodes intersect the Fermi surface.     

Spin dynamics in the carrier-doped S=1/2 triangular lattice of NaxCoO2.yH2O

We probed the local electronic properties of the mixed-valent Co+4-x triangular lattice in NaxCoO2.yH(2)O by 59Co NMR. We observed two distinct types of Co sites for x > or =1/2, but the valence seems averaged out for x approximately 1/3. Local spin fluctuations exhibit qualitatively the same trend down to approximately 100 K regardless of the carrier concentration x, and hence the nature of the electronic ground state. A canonical Fermi-liquid behavior emerges below approximately 100 K only for x approximately 1/3.     

Vortex state in NaxCoO2.yH2O: px+/-ipy-wave versus d(x2-y2)+/-id xy-wave pairing

Based on an effective Hamiltonian specified in the triangular lattice with possible p(x)+/-ip(y)- or dx(2)(-y(2))+/-id(xy)-wave pairing, which has close relevance to the newly discovered Na0.35CoO2.yH(2)O, the electronic structure of the vortex state is studied by solving the Bogoliubov-de Gennes equations. It is found that p(x)+/-ip(y) wave is favored for the electron doping as the hopping integral t<0. The lowest-lying vortex bound states are found to have, respectively, zero and positive energies for p(x)+/-ip(y)- and dx(2)(-y(2))+/-id(xy)-wave superconductors, whose vortex structures exhibit the intriguing sixfold symmetry. In the presence of strong on-site repulsion, the antiferromagnetic order and local ferromagnetic moment are induced around the vortex cores for the former and the latter, respectively, both of which cause the splitting of the local density of states peaks due to the lifting of spin degeneracy.     

Unconventional superconductivity and electron correlations in the cobalt oxyhydrate Na0.35CoO2.yH2O from nuclear quadrupole resonance

We report a careful 59Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na0.35CoO2.yH(2)O superconductor from T=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T(1) follows a Curie-Weiss type temperature (T) variation, 1/T(1)T=C/(T-theta), with theta=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T(c)=3.9 K, 1/T(1) decreases with no coherence peak and follows a T(n) dependence with n approximately 2.2 down to approximately 2.0 K but crosses over to a 1/T(1) proportional to T variation below T=1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.     

Unified origin for the 3D magnetism and superconductivity in NaxCoO2

We analyze the origin of the three-dimensional (3D) magnetism observed in nonhydrated Na-rich Na(x)CoO2 within an itinerant spin picture using a 3D Hubbard model. The origin is identified as the 3D nesting between the inner and outer portions of the Fermi surface, which arise due to the local minimum structure of the a(1g) band at the Gamma-A line. The calculated spin wave dispersion strikingly resembles the neutron scattering result. We argue that this 3D magnetism and the spin fluctuations responsible for superconductivity in the hydrated systems share essentially the same origin.     

Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals

The recently discovered (Li_1-xFe_x)OHFeSe superconductor with T_c about 40 K provides a good platform for investigating the magnetization and electrical transport properties of FeSe-based superconductors. By using a hydrothermal ion-exchange method, we have successfully grown crystals of (Li_1-xFe_x)OHFeSe. X-ray diffraction on the sample shows the single crystalline PbO-type structure with the c-axis preferential orientation. Magnetic susceptibility and resistive measurements show an onset superconducting transition at around T_c=38.3 K. Using the magnetization hysteresis loops and Bean critical state model, a large critical current J_s is observed in low temperature region. The critical current density is suppressed exponentially with increasing magnetic field. Temperature dependencies of resistivity under various currents and fields are measured, revealing a robust superconducting current density and bulk superconductivity.     

Thermal conductivity of (VO)2P2O7 single crystals

Thermal conductivity ϰ of single-crystal (VO)₂P₂O₇ has been studied within the 4–300 K range. A break was found in the ϰ(T) relation about 200 K, in the region of the transition from diffuse antiferromagnetic ordering (200–4 K) to a classical paramagnet (T=200–300 K). In the low-temperature domain (4–200 K), one may expect an additional contribution to ϰ(T) from the magnon component of thermal conductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 2093–2094 (November 1998)     

Raman scattering in YBa2Cu3O7 single crystals: Anisotropy in normal and superconductivity states

Precise temperature and polarization dependences of Raman spectra have been investigated for fully oxygenated twin-free YBa₂Cu₃O₇ single crystals. We have found a striking superconductivity-induced x−y anisotropy in the temperature behavior of the 340 cm⁻¹ line: the magnitudes of the softening and broadening are quite different in the xx-and yy-polarizations. This anisotropy suggests a contribution of the CuO-chain superconductivity with a pairing symmetry different from that for the CuO₂ plane, or indicates that the superconducting gap amplitudes are different in the k _x and k _y directions. The d+s gap symmetry is the only realistic symmetry in the case of Δ_x≠Δ_y. Fiz. Tverd. Tela (St. Petersburg) 40, 403–412 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Sodium ion ordering and vacancy cluster formation in NaxCoO2 (x=0.71 and 0.84) single crystals by synchrotron X-Ray diffraction

In the rich phase diagram of NaxCoO2, x=0.71 enjoys special stability and is called the Curie-Weiss metal due to its anomalous properties. Similarly, x=0.84 prepared from high temperature melt is a special end point beyond which the system phase separates. Using synchrotron x-ray diffraction on single crystals, we discovered sqrt[12]a and sqrt[13]a superlattice structures which we interpret as the ordering of Na (vacancy) clusters. These results lead to a picture of coexisting local moments and itinerant carriers and form the first step towards understanding the many anomalous properties of cobaltates.     

BaT2As2 single crystals (T = Fe,Co, Ni) and superconductivity upon Co-doping

The crystal structure and physical properties of BaFe₂As₂, BaCo₂As₂, and BaNi₂As₂ single crystals are surveyed. BaFe₂As₂ gives a magnetic and structural transition at T_N = 132(1) K, BaCo₂As₂ is a paramagnetic metal, while BaNi₂As₂ has a structural phase transition at T₀ = 131 K, followed by superconductivity below T_c = 0.69 K. The bulk superconductivity in Co-doped BaFe₂As₂ below T_c = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe_1.84Co_0.16As₂ indicate the inter-band scattering due to Co is weak.     

Revised superconducting phase diagram of hole-doped Na(x)(H3O)(z)CoO2 x yH(2)O

We have studied the superconducting phase diagram of NaxCoO2.yH(2)O as a function of electronic doping, characterizing our samples both in terms of Na content x and the Co valence state. Our findings are consistent with a recent report that intercalation of H3O+ ions into NaxCoO2, together with water, acts as an additional dopant, indicating that Na substoichiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal T(C) is achieved through a Co valence range of 3.24-3.35, while T(C) decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped nonsuperconducting anhydrous samples, differing from the superconducting hydrate only in interlayer spacing.     

ESR studies of Cl2- (VK) centre trapped in irradiated Ca(ClO3)2 . 2H2O single crystals

A Cl₂- centre has been trapped in X or γ-irradiated Ca(ClO₃)₂. 2H₂O single crystals at 298 K, when the irradiated crystals were illuminated with ultra-violet light (360 nm). This centre is formed at the expense of ClO₂ centres in this crystal. This Cl₂ ^- centre is trapped at two magnetically inequivalent sites in the crystal lattice and these sites become equivalent when the static magnetic field is parallel or perpendicular to the b axis. At many orientations this centre reveals ‘super-hyperfine’ interaction with a proton (I = 1/2) of the water of crystallization. The magnetic parameters are close to those observed in alkali chlorides and the E.S.R. spectrum has been fitted to an orthorhombic spin hamiltonian. The principal g values are g_xx = 2·035, g_yy = 2·033 and g_zz = 2·000 and those of the A values are A_xx = 15·0, A_yy = 31·0 and A_zz = 109·0 G. The shfs parameters are A _‖' = 5·0 A _⊥' = 1·0 G. The V_K centre trapped in this lattice is exceptionally stable at room temperature.