Takao Kohara

Superconductivity of Y1Ba2(Cu1-xFex)3Oy

A systematic study on the superconductivity of Y1Ba2(Cu1-xFex)3Oy has been made down to 30 mK for various x values. The critical temperature decreases linearly with x. The complete Meissner effect is retained down to 30 mK, up to about x=0.10. The crystal structure of the superconducting phase was found to transform from the orthorhombic to the tetragonal between x=0.02 and 0.05. It is concluded that a branching effect of a one-dimensional chain of Cu1-O through Fe impurities is essential for this structure transition. The supercurrent is considered to flow on the two-dimensional sheets of Cu2.

Competition of Magnetic Order and Superconductivity in (La1-xBax)2CuO4 Systems

Both the magnetic and superconducting properties of (La 1- x Ba x ) 2 CuO 4 systems have been investigated in terms of 139 La nuclear quadrupole resonance (NQR). For La 2 CuO 4 , it has clearly been demonstrated that the internal field H n is applied on the La site by magnetic ordering below ∼240 K. The observed H n is not consistently explained by the spin structure proposed in the neutron diffraction study. For the doped compound with x =0.01, the internal field at 1.3 K was almost the same as that for La 2 CuO 4 , while for x =0.025, it decreased and was significantly distributed. Furthermore, the superconductivity was found to set in around 2 K. From a microscopic point of view, it is concluded that the magnetic ordering and superconductivity compete around x =0.025.

29Si NMR study of antiferromagnetic superconductor URu2Si2

We present 29 Si NMR spectra, nuclear spin-lattice relaxation time, T 1 , and nuclear spin echo decay time, T 2 , measurements in URu 2 Si 2 over the temperature range from 0.15 K to 300 K. 1/ T 1 obtained with c -axis perpendicular to the external field H ext , 1/ T 1⊥ , is much larger than that obtained with c -axis parallel to H ext , 1/ T 1// . Existence of large anisotropy of 1/ T 1 together with the nearly isotropic 29 Si hyperfine field imply that the spin fluctuations exist predominantly in the direction of the tetragonal c -axis. The temperature, T , dependence of 1/ T 1⊥,// is nearly T -independent above 60 K and is proportional to T at low temperatures, which indicates the crossover from the localized state at high temperatures to the Fermi liquid state below about 60 K occurs in the electronic state of 5f electrons. 1/ T 1 and 1/ T 2 decrease markedly below T N =17.5 K associated with the antiferromagnetic order. Below T C , 1/ T 1 has no coherence peak and decreases proportionally to T 3 at l...

195Pt Knight Shift in the Heavy Fermion Superconductor UPt3

We observed 195 Pt NMR in UPt 3 in both the normal and superconducting states, The spectrum has a large negative shift and is quite anisotropic. In 5 kOe, no changes in the negative shift were observed down to the temperature of 60 mK, where the system was in the superconducting state. This means that the conduction electron spin susceptibility does not decrease in the superconducting state. The result does not contradict the possibility of anisotropic p-wave superconductivity of UPt 3 .

Nuclear magnetic resonance study of the heavy-fermion system URu2Si2

Heavy-fermion systems are a subject of recent interest. These systems, which contain periodically arranged f-electrons, have many anomalous properties in comparison with conventional metals and compounds. The mechanisms of superconductivity in heavy-fermion superconductors (HFS) have also attracted much attention. The p-wave superconductivity has been proposed as a possible explanation of the anomalous behavior in these substances by many works.

Two Crystal Phases in the Superconducting Ba-Y-Cu-O System and Their Reactivity to Water

Crystals with a stoichiometry either Ba2YCu3-xO7-y or Ba4YCu2O8-y are a high tc superconductor. The former is well known. With respect to the latter the crystal structure was determined to be tetragonal with lattice parameters a=5.78 A and c=7.99 A. It was found that both of them reacted with water. The former was slowly decomposed by warmed water at about 35°C, while the latter was destroyed at room temperature in an instant.

Existence of Line Nodes in the Superconducting Energy Gap of Antiferromagnetic Superconductor URu2Si2. 101Ru NQR Study.

We have carried out 101 Ru NQR measurements of URu 2 Si 2 in zero magnetic field in the temperature, T , range from 0.16 K to 26 K. From the NQR spectrum, the electric quadrupole interaction parameters were determined to be ν Q =5.72 MHz and η =0. The nuclear spin-lattice relaxation rate, 1/ T 1 , decreases drastically below 17.5 K. This decrease is associated with antiferromagnetic ordering. In the superconducting state, 1/ T 1 has no coherence peak just below T c =1.2 K, and decreases in proportion to T 3 with no deviation down to 0.2 K, which indicates the existence of line nodes in the superconducting energy gap.

Nuclear Magnetic Relaxation in the Heavy-Fermion Superconductor UPt3

The nuclear magnetic relaxation rate, 1/ T 1 , of 195 Pt in a normal and superconducting state was measured. In the normal state, 1/ T 1 is proportional to T , indicating the Fermi liquid nature of the system. In the superconducting state, 1/ T 1 has no enhancement just below the transition temperature and varies in proportion to T 3 at low temperatures. These results indicate the existence of an anisotropic superconducting energy gap, which vanishes at lines on the Fermi surface.

NMR Study of Antiferromagnetic β-Mn Alloys

NMR study has been made on various antiferromagnetic β-Mn alloys at liquid helium temperatures. The alloys used are β-Mn with Ti, V, Cr, Fe, Co, Ni, Ru, Rh, Pd, Ir, Al, Si, Zn, Ga, Ge, In and Sb impurities. The interatomic distances of these alloys have been also measured by X-ray diffraction. Several β-Mn alloys become antiferromagnetic at 4.2 K by an addition of small amount of impurity metal. The magnetic moment on Mn is assumed to be less than 0.5 µ B . In β-MnCo alloys, the magnetic moment of Co is estimated to be about 1.0 µ B . The magnetic ordering of β-Mn alloys with non-transition metal impurities is attributed to the lattice expansion. In the case of transition metal impurities, the lattice expansion, the change of the number of d-electrons and the induction of the magnetic moments of Mn by the localized moment on impurity atom must be considered.

Observation of 55Mn NMR and NQR Signals from Site II in β Mn Metal

NMR and NQR signals of 55 Mn in site II were observed. The electric quadrupole coupling constant and the asymmetry parameter were estimated to be e 2 q Q / h =30 MHz and η∼0, respectively. The nuclear spin lattice relaxation rate, 1/ T 1 , is proportional to \(\sqrt {T}\), which indicates β Mn to be an itinerant antiferromagnetic system with T N ∼0 K. The value of 1/ T 1 at site II is about 20 times larger than the value at site I. The electronic state of Mn at site II is more magnetic than that at site I.