Yoh Kohori

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.

Nuclear relaxation and Knight shift studies of63Cu in 90 K-and 60 K-class YBa2Cu3O7−y

Abstract The nuclear relaxation rate (1/T1) and Knight shift for Cu of YBa2Cu3O7−y were measured by using the nuclear quadrupole (NQR) and magnetic resonance (NMR) techniques. For 90 K-superconductor with y∼o, 1/T1 at the CuO2 plane site decreases markedly without the enhancement just below Tc and approximates to a T3 behavior between 40K and 10 K. Knight shift shows a distinct decreases below Tc, giving an evidence for dominant singlet pairing. For an explanation of unconventional nuclear relaxation behavior, it seems plausible to apply a d-wave model with the gap zeros of lines on the Fermi surface. For 60 K-superconductor, we found a surprising result that 1/T1 at the CuO2 plane site is by three order of magnitude suppressed as compared to that of 90 K superconductor and shows no anomalies at Tc= 60K. This marked difference of relaxation behaviors between 90K and 60K superconductors implies a strong correlation with the mechanism of high Tc superconductivity.

NMR study of heavy fermion materials

Abstract Recent NMR experiments on heavy fermion material are discussed. An antiferromagnetic type ordering is observed to appear in the ground state of several systems. The superconductivity coexists or competes with the magnetic ordering. The superconductivity is anisotropic having zero gap on lines at the Fermi surface.

195Pt NMR study of the heavy fermion superconductor UPt3

Abstract In the superconducting state, T -1 1 of 195 Pt has no enhancement just below T c and varies as T 3 at low temperatures. This indicates the existence of an anisotropic superconducting energy gap which vanishes at lines on the Fermi surface. The Knight shift data show that the conduction electron spin susceptibility does not decrease below T c .

63Cu NMR Study of Superconducting Cu/Nb Multilayer-Film

Nuclear spinlattice relaxation time T 1 of 63 Cu was measured by the field cycling method in a Cu/Nb multilayer sample, which was prepared by rf-sputtering alternately deposited in each 50 layers of Cu and Nb with thicknesses of 400 A and 245 A. As the temperature T is lowerd below the superconducting transition temperature T c =4.6 K, the spin relaxation rate T 1 -1 initially increases and then rapidly decreases. This behavior indicates the existence of superconducting energy gap 2 Δ (0)=0.96 meV=2.4 k B T c in Cu. Below 0.8 K, the rapid exponential decrease in T 1 -1 is followed by a gradual change almost proportional to T .

Heat capacities of Ba2RCu3Ox (R=Gd, Ho, Er, Dy)

Heat capacities of high- T c superconductors Ba 2 RCu 3 O x (R=Gd, No, Er, Dy) aremeasured at temperatures below the respective superconducting transition temperatures. The observed sharp peaks at 2.25 K(Gd), 0.95 K(Dy), 0.55 K(Er) and 0.1 K(Ho) are associated with the magnetic ordering of the rare-earth ions in two dimensions. A co-existence of superconductivity and magnetic ordering is also confirmed down to the measured temperature of 30 mK.

Superconducting and Magnetic Properties of High-Tc Superconductor GdBa2Cu3Ox

We report measurements of the superconducting and magnetic behaviors of GdBa2Cu3Ox. Superconductivity sets in at 95 K with large Meissner effect. This compound exhibits Curie-Weiss behavior above Tc with the free-ion effective moments of 7.94µB, and with the Weiss temperature of about - 10±5 K. Moreover, long-range magnetic ordering was observed below 2.2 K by specific-heat measurement without any distinct change of ac susceptibility. This result implies that the superconductivity coexists with the magnetic order below 2.2 K.

Nuclear Magnetic Resonance in Heavy Fermion Systems

Heavy-fermion (HF) systems discovered in rare-earth or actinide intermetallic compounds have been the intriguing subject of experimental [1,2,3] and theoretical [4] investigations. At low temperature, these systems having very large specific-heat coefficient and Pauli like susceptibility are characteristic of a strongly interacting electronic fermi liquid with heavy effective mass, which is derived from 4f- or 5f-electrons behaving as localized electrons at high temperature. It has become increasingly evident that the system has a new and unique ground state. The most outstanding and fascinating aspect is the fact that the compounds such as CeCu2Si2 [5], UBe13 [6] and UPt3 [7] exhibit a transition to the superconducting state. The superconducting properties are unusual, indicating that the heavy electrons are responsible for the superconductivity. Considerable amounts of experimental evidences, which conflict with the conventional theory, have suggested that the energy gap in these compounds is anisotropic, vanishing at points or lines on the Fermi surface [8,9,10,11]. First, these compounds have been discussed in analogy to superfluid 3He-A phase where the gap zeros of points are associated with the anisotropic triplet Cooper pairing. Subsequently, there have been many experimental and theoretical investigations to gain an insight into the nature and the mechanism of Cooper pairing of HF superconductor.

29Si NMR Study of the Heavy-Electron System URu2Si2

NMR and relaxation studies of 29Si have been carried out on the Heavy Fermion Superconductor, URu2Si2. The Knight shift measurement shows that the spin-susceptibility does not change appreciably in superconducting state through Tc down to 0.5 K. 1/T1 decreases in proportional to T3 from Tc down to 0.65 K without an enhancement just below Tc and then deviates slightly at lower temperatures. These results suggest that the pair-state of this superconductor is either singlet with spin-orbit scattering by impurities or triplet both with gap-zero on a line at the Fermi surface.

NMR study of the proximity effect in Nb-Cu multilayers

A series of measurements on the nuclear spin-lattice relaxation rate, T 1 -1 , of 63 Cu in superconducting multilayer Nb–Cu has been performed over a temperature range from 4.2 K down to 0.3 K, using the field cycling method. T 1 -1 decreases rapidly after an initial increment just below T c . At low temperatures, however, the rate of the decrease slows down, and T 1 -1 decreases in proportion to temperature. It has been concluded that such behavior of T 1 -1 originates from a distribution of the energy gap of the Cooper pairs in Cu. The electrons which are propagated nearly normal to the S–N interface have a finite energy gap predicted by McMillan, while the electrons propagated nearly parallel to the interface are gapless and behave like normal electrons.