J. Cmaidalka

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.

Thermodynamic properties in the normal and superconducting states of NaxCoO2.yH2O powder measured by heat capacity experiments

The heat capacity of superconducting Na(x)CoO(2)*yH(2)O was measured and the data are discussed based on three different models: The thermodynamic Ginzburg-Landau model, the BCS theory, and a model including the effects of line nodes in the superconducting gap function. The electronic heat capacity is separated from the lattice contribution in a thermodynamically consistent way maintaining the entropy balance of superconducting and normal states at the critical temperature. It is shown that for a fully gapped superconductor the data can only be explained by a reduced (about 55 %) superconducting volume fraction. The data are compatible with 100 % superconductivity in the case where line nodes are present in the superconducting gap function.

Study of intermetallic compounds isostructural to MgB/sub 2/

The small intergrain effect of MgB/sub 2/ on supercurrent makes it one of the most promising candidates for superconducting conductors due to its easier processing and the associated lower manufacturing cost. Unfortunately, the superconducting transition temperature T/sub c/ is only 40 K. However, band-structure calculations predict that a higher T/sub c/ than that of MgB/sub 2/ is possible for isostructural and isovalent intermetallic compounds with greater lattice parameters or greater unit cell volumes. The prediction appears to be consistent with the negative pressure effect on T/sub c/ observed. The substitution of the larger Ca-ions for the smaller Mg-ions has thus been suggested to raise T/sub c/, but not yet realized. Alternatively, we have synthesized and studied a series of binary and pseudobinary intermetallic compounds, AGa/sub 2/, AGa/sub 2-x/Si/sub x/, and AAl/sub 2-x/Si/sub x/, where A=Ca, Sr, or Ba, which are isostructural to MgB/sub 2/ and have greater lattice parameters than MgB/sub 2/. In spite of the greater lattice parameters, AGa/sub 2/ are not superconducting. However, all pseudobinary compounds AGa/sub 2-x/Si/sub x/ and AAl/sub 2-x/Si/sub x/ for 0.6<x<1.5 are superconducting. Superconductivity is reported for the first time in SrAlSi, with T/sub c/ of 5.1 K. Although T/sub c/ varies with x in a similar fashion for all members of the series, no specific correlation between T/sub c/ and lattice parameters or ionic mass is observed. The maximum T/sub c/ of these compound series with different As varies between 5.5 and 7.8 K, much lower than that of MgB/sub 2/. The results strongly suggest the unique role of B in the superconductivity of C32 intermetallic compounds. They also demonstrate that factors additional to the lattice parameters and densities of states must play an important role, and that the rigid-band model is not sufficient to account for the observations.

Absence of Meissner state in RuSr2GdCu2O8

Abstract The magnetization of single-phased Ru-1212, which possesses a ferromagnetic transition at 133 K and a superconducting transition around 30 K, was measured. No bulk Meissner effect can be detected down to 2 K and 0.2 Oe, although both a diamagnetic susceptibility −4 πχ ≥ 1 in the zero-field-cooled mode and a small M-drop in the field-cooled mode appear. Almost the same reversible magnetizations in Ru-1212 and Zn-doped Ru-1212 further suggest that the superconducting state of Ru-1212 is severely modified by the coexistence of ferromagnetism.

Crypto-superconductivity in RuSr2GdCu2O8

Abstract The susceptibility χ = χ ′ + iχ ″ of RuSr 2 GdCu 2 O 8 powders at 2 K has been deduced from ac and dc magnetizations with their particle size d from 800 to 10 μm and grain size 1–5 μm. It is observed that the loss part, χ″, scales with d over a field range of 0.0001 to 100 Oe, suggesting that the supercurrent is homogeneously distributed over particles as small as 10 μm. However, the inductance part, χ′, decreases with d systematically and drastically. An effective penetration depth as large as 30–50 μm is needed, which suggests a very small super-carrier concentration. Crypto-superconductivity is proposed to accomodate the data.

Pressure dependence of the superconducting transition and electron correlations in Na x Co O 2 ∙ 1.3 H 2 O

We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the decrease of T_c is accompanied by a weakening of the spin correlations at a finite wave vector and a reduction of the density of states (DOS) at the Fermi level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly due to a reduction of the DOS. These results indicate that the electronic/magnetic state of Co is primarily responsible for the superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling evidence for the presence of line nodes in the superconducting gap function.

NMR and NQR studies of the pairing symmetry in the superconducting state of NaxCoO2·1.3H2ONaxCoO2·1.3H2O

We report intensive studies on the cobalt oxide superconductors NaxCoO2·1.3H2ONaxCoO2·1.3H2O via 59Co NMR and NQR measurements. In the sample with x=0.26x=0.26 and Tc=4.6Tc=4.6, we find that the spin–lattice relaxation rate 1/T11/T1 shows a T3T3 variation below TcTc and down to very low temperatures, which indicates the presence of line nodes in the superconducting (SC) gap function. We have also studied the spin susceptibility below TcTc via the Knight shift measurement. The spin part of the Knight shift decreases below TcTc in both the aa- and cc-directions indicating singlet spin state of the Cooper pairs. Our results strongly suggest that the superconducting state is a d-wave state.

NMR and NQR studies of the pairing symmetry in the superconducting state of

We report intensive studies on the cobalt oxide superconductors NaxCoO2·1.3H2ONaxCoO2·1.3H2O via 59Co NMR and NQR measurements. In the sample with x=0.26x=0.26 and Tc=4.6Tc=4.6, we find that the spin–lattice relaxation rate 1/T11/T1 shows a T3T3 variation below TcTc and down to very low temperatures, which indicates the presence of line nodes in the superconducting (SC) gap function. We have also studied the spin susceptibility below TcTc via the Knight shift measurement. The spin part of the Knight shift decreases below TcTc in both the aa- and cc-directions indicating singlet spin state of the Cooper pairs. Our results strongly suggest that the superconducting state is a d-wave state.

Unconventional Superconductivity and Electron Correlations in the Cobalt Oxyhydrate [Formula presented] 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.

Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate Na

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.