Yoshihiko Ihara

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...

Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)∼2.5  K and T(SC)∼0.6  K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.

Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies

Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were carried out for the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transition...

Anisotropic Magnetic Fluctuations in the Ferromagnetic Superconductor UCoGe Studied by Direction-Dependent ^{59}Co NMR Measurements

We have carried out direction-dependent 59Co NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight-shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.

Unconventional superconductivity induced by quantum critical fluctuations in hydrate cobaltate Nax(H3O)zCoO2 ·yH2O - : Relationship between magnetic fluctuations and superconductivity revealed by co nuclear quadrupole resonance

Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na x (H 3 O) z CoO 2 · y H 2 O with different values of the superconducting and magnetic-ordering temperatures, T c and T M , respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis ν z z rather than the asymmetry of EFG within the a b -plane. In addition, it is considered that the change of ν z z is gaverned mainly by the trigonal distortion of the CoO 2 block layers along the c axis, from the relationships between ν z z and the various physical parameters. We found the tendency that samples with ν z z larger than 4.2 MHz show magnetic ordering, whereas samples with lower ν z z show superconductivity. We measured the nuclear spin–lattice relaxation rate 1/ T 1 in these samples, and found that magnetic fluctuations depend on samples. The higher-ν z...Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na_x(H_3O)_zCoO_2\cdot yH_2O with different values of the superconducting and magnetic-ordering temperatures, T_c and T_M, respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis \nu_{zz} rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of \nu_{zz} is gaverned mainly by the trigonal distortion of the CoO_2 block layers along the c axis, from the relationships between \nu_{zz} and the various physical parameters. We found the tendency that samples with \nu_{zz} larger than 4.2 MHz show magnetic ordering, whereas samples with lower \nu_{zz} show superconductivity. We measured the nuclear spin-lattice relaxation rate 1/T_1 in these samples, and found that magnetic fluctuations depend on samples. The higher-\nu_{zz} sample has stronger magnetic fluctuations at T_c. From the relationship between \nu_{zz} and T_c or T_M, we suggest that the NQR frequency can be regarded as a tuning parameter to determine the ground state of the system, and develop the phase diagram using \nu_{zz}. This phase diagram shows that the highest-T_c sample is located at the point where T_M is considered to be zero, which suggests that the superconductivity is induced by quantum critical fluctuations. We strongly advocate that the hydrate cobaltate superconductor presents an example of the magnetic-fluctuation-mediated superconductivity argued in the heavy-fermion compounds. The coexistence of superconductivity and magnetism observed in the sample with the highest \nu_{zz} is also discussed on the basis of the results of our experiments.

Correlation between Superconducting Transition Temperature T_c and Increase of Nuclear Spin-Lattice Relaxation Rate Devided by Temperature 1/T_1T at T_c in the Hydrate Cobaltate Na_xCoO_2・yH_2O(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

We have performed Co-nuclear quadrupole resonance (NQR) studies on Na x CoO 2 · y H 2 O compounds with different Na ( x ) and hydrate ( y ) contents. Two samples with different Na contents but near...

17O NMR Measurements on Superconducting Na0.35CoO2·yH2O

An 17 O NMR measurement was performed on nonoriented polycrystalline Na 0.35 CoO 2 · y H 2 O with superconducting (SC) transition temperature T c = 4.6 K. A weak temperature dependence was observed in the Knight shift at the O site ( 17 K ). The spin part of 17 K ( 17 K spin ) is estimated from the plot of 17 K against bulk susceptibility χ. The 17 K spin decreases in the SC state, indicative of the decrease in the in-plane component of the spin susceptibility. The nuclear spin–lattice relaxation rate 1/ T 1 at the O site 17 (1/ T 1 ) shows a good scaling with 1/ T 1 at the Co site 59 (1/ T 1 ). This indicates that the spin fluctuations at the O site originate from the Co spin dynamics. The relationships between 17 (1/ T 1 T ) and 17 K spin and between 17 (1/ T 1 T ) and 59 (1/ T 1 T ) show the development of incommensurate fluctuations at q ∼ 0 other than q = 0 below 30 K. A clear indication of ferromagnetic correlations at q = 0 was not observed from the present 17 O NMR studies.

Anisotropic Behavior of Knight Shift in Superconducting State of NaxCoO2·yH2O

The Co Knight shift was measured in an aligned powder sample of Na x CoO 2 · y H 2 O, which shows superconductivity at T c ∼4.6 K. The Knight-shift components parallel ( K c ) and perpendicular to the c -axis (along the a b plane K a b ) were measured in both the normal and superconducting (SC) states. The temperature dependences of K a b and K c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3 d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K a b decreases below 5 K, whereas K c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c -axis is realized in Na x CoO 2 · y H 2 O.

Magnetic fluctuations in the metallic state of Na0.7CoO2 revealed by 23Na nuclear magnetic resonance

Nuclear magnetic resonance (NMR) studies were performed on Na 0.7 CoO 2 to investigate its magnetic properties from the microscopic viewpoint. Na 0.7 CoO 2 is a starting compound of the recently discovered superconductor Na 0.35 CoO 2 ·1.3H 2 O, in which Co atoms form a triangular structure. Although 23 Na NMR results show no magnetic anomaly down to 1.5 K, Knight shift ( 23 K ) and 23 (1/ T 1 T ) of 23 Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between 23 K and 23 (1/ T 1 T ), it is shown that ferromagnetic fluctuations are dominant below 100 K. In addition to ferromagnetic correlations, antiferromagnetic fluctuations are found develop to in a low-temperature region below 4 K. Magnetic fluctuations considered from our experiment are discussed.Nuclear-magnetic-resonance (NMR) studies were performed in Na0.7CoO2 to investigate its magnetic properties from the microscopic point of view. Na0.7CoO2 is a starting compound of the recently discovered superconductor Na0.35CoO2·1.3H2O, in which Co atoms form a tri- angular structure. Although 23 Na-NMR results show no magnetic anomaly down to 1.5 K, Knight shift ( 23 K) and 23 (1/T1T) of 23 Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between 23 K and 23 (1/T1T), it is shown that ferromagnetic fluctuations are dominant below 100 K. In addition to the ferromagnetic correlations, antifer- romagnetic fluctuations are also developed in a low temperature region below 4 K. Magnetic fluctuations suggested by our experiment are discussed. Since the superconductivity in NaxCoO2·yH2O (x � 0.35, y � 1.3) with the transition temperature Tc � 5 K was discovered by Takada et al, 1 understanding of mag- netic properties in the CoO2 plane is one of the most important topics in strongly correlated electron systems. This is because the magnetic properties in the plane might be related to the mechanism of the unconventional superconductivity in the compound. In addition, the Co atoms form a triangular lattice in the plane, which is an- ticipated to possess the geometrical frustrations. Novel types of magnetic transitions and ground states are ex- pected due to a suppression of a long-range magnetic ordering by the geometrical frustrations associated with the triangular structure. In particular, only a few com- pounds have been known to date which possess a metallic property down to the lowest temperature with the geo- metrical frustrations, e.g. (YSc)Mn2 2 and LiV2O4. 3 The NaxCoO2 (0.5 < x < 0.78) compound is considered to be one of the promising metallic compounds with the geometrical frustrations at low temperature. 4 In this paper, we report the magnetic properties in Na0.7CoO2 revealed by our 23 Na-NMR experiments. Powder samples of Na0.7CoO2 were prepared from Na2CO3 (99.99%) and Co3O4(99.9%) by solid-state re- action. Its quality was checked by X-ray powder diffrac- tion. The Na content, which is very sensitive to physical properties, is estimated to be 0.702 by the inductive- coupled plasma atomic emission spectroscopy (ICP- AES) method. Using this well-characterized powder sample, 23 Na- and 59 Co-NMR measurements were per- formed.

Ambient-pressure molecular superconductor with a superlattice containing layers of tris(oxalato)rhodate enantiomers and 18-crown-6

We report a novel multilayered organic-inorganic hybrid material, β″-(BEDT-TTF)2[(H2O)(NH4)2Rh(C2O4)3]·18-crown-6. This is the first molecular superconductor to have a superlattice with layers of both BEDT-TTF and 18-crown-6 and also the first with the anion tris(oxalato)rhodate. This is the 2D superconductor with the widest gap between conducting layers, where only a single donor packing motif is observed (β″). The strong 2D nature of this system strongly suggests that the superconducting transition is a Kosterlitz-Thouless transition. A superconducting Tc of 2.7 K at ambient pressure was found by transport measurements and 2.5 K by magnetic susceptibility measurements.