T. Kohara

Nuclear magnetic relaxation in the heavy electron superconductor CeCu2Si2

Nuclear magnetic relaxation rate 1/T1 of 63Cu in the superconducting state of CeCu2Si2 has been measured down to 50 mK by both PQR and NMR techniques. 1/T1 does not show the enhancement just below Tc and varies approximately as T3 in external field for 0.1 K < T < 0.6 K. The result shows that the superconductivity in CeCu2Si2 is consistent with in the anisotropic energy gap model.

Unusual behavior of nuclear relaxation in CeCu2Si2 “possible evidence for triplet superconductivity”

Abstract Nuclear relaxation of 63 Cu in the superconducting state of the Kondo-lattice system CeCu 2 Si 2 has been studied with the use of the 63 Cu nuclear quadrupole resonance technique under zero field and down to 65mK. The nuclear spin-lattice relaxation rate (1/T 1 ) decreases drastically just below T c =0.67 K down to 0.5T c without the apparent enchanced behavior and then is found to be almost temperature independent below 0.3T c . These results suggest that the superconductivity in CeCu 2 Si 2 is not in the usual BCS regime. The analysis based upon the existing triplet pairing model with an anisotropic energy gap describes well the behavior from T c down to 0.5T c , while the temperature independence below 0.3T c remains unexplained.

Nuclear magnetic relaxation in β-Mn alloys

Abstract Nuclear spin-lattice relaxation time, T 1 , of Mn 55 in β-Mn metal and alloys with Ge was measured in a temperature range from 1.2–130 K. 1/ T 1 is nearly proportional to √ T in the region of T ⪢ T N . The result shows the validity of the recent spin fluctuation theory.

29Si knight shift in the heavy-Fermion superconductor CeCu2Si2

We present a study of 29 Si and 63 Cu Knight shifts in the superconducting CeCu 2 Si 2 . The shifts K ∥ , parallel and K ⊥ perpendicular to the tetragonal c -axis were obtained from the anisotropic 29 Si NMR sectrum. K ⊥ shows a rapid decrease below T c ( H )=0.66 K, while the change of K ∥ is not well resolved. K ∥ at 0.2 K decreases to 40%of the value at T c ( H ). Furthermore, the distinct decrease of K ∥ and K ⊥ has also been observed for 63 Cu below T c . The strong reduction of the shift, which is independent of the direction of the crystallographic axis, supports the model of even-parity pairing without significant spin-orbit scattering by impurities, although the energy gap has an anisotropic nature, as is demonstrated by the nuclear relaxation study.

The appearance of homogeneous antiferromagnetism in URu2Si2 under high pressure: a 29Si nuclear magnetic resonance study

We have investigated the low-temperature phase appearing below To = 17.5 K in URu2Si2 by means of 29Si nuclear magnetic resonance (NMR) in a pressure range from 0 to 17.5 kbar across the pressure-induced phase transition at Pc = 15 kbar. At pressures below Pc, we have observed the 29Si NMR lines arising from antiferromagnetic (AF) and paramagnetic (PM) regions in the sample, giving evidence for a phase-separated AF ordering below To. The AF region increases in volume fraction with increasing pressure up to Pc. In the PM region, the temperature-dependence of the nuclear spin–lattice relaxation rate at Si sites shows a rapid decrease below To, strongly suggesting the occurrence of a phase transition driven by a hidden order parameter. As applied pressure exceeds Pc, the AF ordering appears uniformly at To throughout the sample. In the pressure range from 0 to 17.5 kbar, the magnitude of the internal field at Si sites in the AF region remains constant (~ 920 Oe), which indicates that the AF ordered moment is unchanged by pressure across Pc.

Microscopic measurements in 139La-NQR of the ternary carbide superconductor LaNiC2☆

139La-NQR measurements have been carried out in the ternary carbide superconductor LaNiC2. The nuclear quadrupole frequency and the asymmetry parameter of 139La in LaNiC2 were estimated to be about 1.9 MHz and 0, respectively. In the normal state, the nuclear spin relaxation rate (1T1) in the 139La NQR signal was proportional to temperature (T) in zero external field above the superconducting transition temperature (Tc) or in an external field larger than the superconducting critical field, which means the system is in the Fermi-liquid state. In the superconducting state, on the other hand, 1T1 decreases no more linearly with T, but decreases rapidly exponentially as exp (−ΔkBT) at low T with an appreciable enhancement just below Tc. The value of the superconducting energy gap, 2Δ, was estimated to be 3.34kBTc, compared with 3.52kBTc of the BCS-value. This result strongly suggests that the superconductivity in LaNiC2 is of a conventional BCS type.

Frustration-induced valence bond crystal and its melting in Mo3Sb7.

(121/123)Sb nuclear quadrupole resonance and muon spin relaxation experiments of Mo_3Sb_7 revealed symmetry breakdown to a nonmagnetic state below the transition recently found at T_S approximately 50 K. The transition is characterized by a distinct lattice dynamics suggested from narrowing of nuclear fields. We point out that the Mo sublattice is a unique three-dimensional frustrated lattice where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions compete, and propose that tetragonal distortion to release the frustration stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal, which is thermally excited to the dynamic state with cubic symmetry.

Spin-glass like behaviour in the nanoporous Fe2O3with amorphous structure

The porous Fe2O3 was synthesized chemically. The average size of the particle was ~85.0 nm, which was observed by scanning electron microscopy. The signature of porous structure was confirmed by a N2 adsorption/desorption isotherm and intense x-ray powder diffraction peak at low angle. The x-ray diffraction pattern at high angle indicates the amorphous structure. Mossbauer investigations show that the value of the hyperfine field is ~498.0 kOe at 4.2 K which is much smaller than that of the hyperfine field of crystalline α/γ-Fe2O3 and consistent with the values of amorphous Fe2O3. The temperature dependence of zero-field cooled magnetization exhibits a peak at 18.0 K (Tf), where Tf follows the Almeida–Thouless relation as . The ageing phenomenon of the magnetic relaxation below Tf and the memory effect in the field-cooled magnetization indicate the typical features of the classical spin-glass compounds below the spin freezing temperature at Tf.

Flux pinning and flux creep in La2−xSrxCuO4 with splayed columnar defects

The effects of columnar defects with splayed configurations on flux pinning and flux creep were investigated experimentally. Parallel columnar defects and splayed columnar defects were introduced into specimens of La2−xSrxCuO4 by high-energy heavy-ion irradiation. A large enhancement of the flux pinning and the critical current density Jc due to the splayed columnar defects was observed. At temperature T<15 K and a magnetic field H=0.1 T, the effective pinning potential U0 of a specimen with splayed columns was larger than that of a specimen with parallel columns. On the other hand, at 15 K≤T≤30 K and H=0.1 T, U0 of a specimen with splayed columns was smaller than that of a specimen with parallel columns.

Magnetism and Superconductivity of CePt3Si and Ce1+xPt3+ySi1+z

We measured the dc magnetization, electrical resistivity, and ac magnetic susceptibility of a series of polycrystalline CePt 3 Si samples whose compositions vary slightly from the stoichiometric composition. The sample that showed the most distinct antiferromagnetic transition at 2.2 K was found to be Ce 1.01 Pt 3 Si annealed. This sample showed a clear bulk antiferromagnetic order at 2.2 K even in both electrical resistivity and dc magnetization measurements, although the characteristic change in dc magnetization at 2.2 K was found to be small and to be easily masked in other magnetic anomalies if they exist. Moreover, it had the largest residual resistivity ratio. We concluded that Ce 1.01 Pt 3 Si annealed has the intrinsic bulk properties of ideal CePt 3 Si. In addition, we revealed that some anomalies arise as a result of the variation in composition. One is a ferromagnetic anomaly at 3.0 K in the Pt-rich samples, and the other is an antiferromagnetic anomaly at 4.0 K in the Pt-poor samples. The two m...