Taiichiro Ohtsuka

Thermal and magnetic properties and crystal structures of CeGe2 and CeSi2

Abstract In continuation of our investigation of α-ThSi 2 type rare earth intermetallic compounds, studies on CeSi 2 and CeGe 2 are reported. The results suggest that Ce in CeSi 2 is in the intermediate valence state. The existence or non-existence of superlattice structures are also discussed.

THE SPECIFIC HEAT AND SUPERCONDUCTIVITY OF La--Y ALLOYS.

The low temperature specific heats of La-Y alloys have been measured in order to investigate the origin of superconductivity in lanthanum. The addition of yttrium to lanthanum was found to result in a large decrease of the electronic specific heat coefficient γ, a decrease in the superconducting transition temperature T c and a gradual increase of the Debye temperature \(\varTheta_{D}\). The rate of increase of \(\varTheta_{D}\) in the yttrium-rich hcp phase was observed to be appreciably larger than in the α-La phase. The effective interaction derived from the BCS formula was found to remain approximately constant in the α-La phase followed by a rapid decrease in the hcp phase. From the general trend of the measured and derived quantities with alloying, it is suggested that the superconductivity of lanthanum may be closely related to mechanism leading to the depression of the Debye temperature and the stabilization of the α-La structure. This implies that phonon processes are of importance in the enhance...

Non-Magnetic Localized State in Aluminum. : II. Specific Heat and Magnetic Susceptibility

The results of an experimental study on the residual resistance, specific heat and magnetic susceptibility of dilute solid solutions (ranging from 0.1 to 0.4 at.%) of V, Cr and Mn in aluminum are presented. The experimental data are analysed on basis of the Anderson model in order to estimate parameters which characterize the localized d -state, i. e. the mean conduction band- d -state ( s d ) admixture, the localized level width, the mean d -state occupation number and the intra-atomic d - d coulomb energy. The level width is evaluated to be about 0.2 eV. The results are combined with the change in the superconducting transition temperature measured in the same alloy system (reported in Part I) to discuss the properties of the localized d -state in light of recent theoretical studies by Ratto and Blandin, and Takanaka and Takano.

Neutron diffraction study on the hyperfine-enhanced nuclear spin order of HoVO4☆

Abstract A neutron diffraction experiment on the hyperfine enhanced nuclear spin system of HoVO4 has been carried out following adiabatic demagnetization cooling. Below 4.5 mK we observed magnetic diffraction peaks due to antiferromagnetic order of the hyperfine enhanced nuclear spin system. A spin-flop transition in an applied magnetic field of about 80 Oe was also observed. The neutron diffraction results are consistent with the magnetic structural model proposed by Bleaney on the basis of dipolar energy considerations.

SQUID NMR studies of TmPO4

The hyperfine-enhanced nuclear moment in TmPO4 was studied by SQUID NMR. A resonance frequency γx/2π = 274 MHz per tesla was obtained, leading to an enhancement factor of 77.9. The temperature dependence of the spin-lattice relaxation time T1 of 169Tm nuclear spin in TmPO4 was measured at liquid helium temperatures. The temperature dependence of T1 can be understood in terms of nuclear Orbach and direct processes.

Heat Capacity of PrNi_2,YNi_2 and Pseudobinary Pr_cLa_ Ni_2 System

Heat capacity data are presented for intermetallic compounds PrNi 2 , YNi 2 and the pseudobinary system Pr c La 1- c Ni 2 . The temperature range covered is 2 to 80 K. The magnetic heat capacity deduced under certain assumptions shows that in PrNi 2 the singlet \(\varGamma_{1}\) is the ground level and that the first excited level lies about 60 K above the ground level. An excess contribution is found around 10 K. The excess heat capacity is understood if the excited level is assumed to have appreciable broadening due to the exchange interaction acting between Pr atoms.

Magnetocaloric Effect and the Upper Critical Field of Superconducting Niobium

The reversible magnetocaloric effect observed in the mixed state of a high purity niobium single crystal is shown to yield precise values of the upper critical field H c 2 corresponding to a second order phase transition together with other properties of the mixed state. Measurements of H c 2 from about 0.3°K to T c =9.23°K are reported. Extrapolation to 0°K gives H c 2 =4035 Oe, whereas precise measurements near T c gives for the initial slope, -( d H c 2 / d T ) T = T c =531Oe/°K. Analysis of the results in terms of microscopic theory shows that reported disagreements pertaining to H c 2 (0) may be accounted for by deviations of the quantities related to the superconducting ground state from BCS laws. Brief accounts of the Ginzburg-Landau-Maki parameter κ 2 derived from magnetocaloric measurements and results obtained on a deformed crystal are given.

Impurity effect on the anisotropy of the upper critical field in Type II superconductors

The effect of the reduction in mean free path on the anisotropy of the upper critical fieldHc2 has been studied in Nb by adding Ta and Mo as impurities. It is found that impurities suppress the degree of anisotropy, in qualitative accord with the theory of Hohenberg and Werthamer, which shows that in cubic materials with a nonspherical Fermi surface anisotropy arises from nonlocal corrections to the GLAG theory. The results were analyzed by fitting the observed anisotropy curves to an expansion in Kubic harmonics. It is found that the second harmonic coefficientA2, which depends on first nonlocal corrections, decreases initially with addition of Ta or Mo according to theory, but tends to level off to an approximately constant value in the dirty limit, where it should vanish. The value of the third harmonic coefficientA3, which depends on higher-order nonlocal terms, vanishes rapidly with the addition of impurity, the rate of decrease being more rapid for Mo. It is also found that the presence of an impurity does not affect the temperature dependence ofA2 for both Nb-Ta and Nb-Mo systems andA3 for the Nb-Ta system and has an approximately similar effect as for pure Nb.

Magnetic ordering in PrNi2

Abstract A peak has been observed in the magnetic susceptibility of the singlet ground state system PrNi 2 , indicating a magnetic ordering. The transition is believed to be of a mixed nuclear-electronic type.

Antiferromagnetic ordering of enhanced nuclear spin in Cs2NaHoCl6

The field dependence of the magnetic susceptibility at low temperatures was investigated in a single crystal of Cs2NaHoCl6. The result is well explained by the second-order Zeeman splitting of the electronic ground state with a “nonmagnetic” Γ3 doublet. Enhanced nuclear cooling experiments were carried out. Antiferromagnetic ordering of the enhanced nuclear spin system was observed at about 1.5 mK.