Masafumi Kyogaku

NMR and NQR Studies of Magnetism and Superconductivity in the Antiferromagnetic Heavy Fermion Superconductors UM2Al3 (M=Ni and Pd)

Both magnetic and superconducting characteristics of the new heavy fermion superconductors (HFS), UNi 2 Al 3 and UPd 2 Al 3 have been investigated extensively by means of 27 Al NMR and NQR. Systematic studies of the nuclear-spin-lattice relaxation time T 1 , the Knight shift and the spectrum of 27 Al have revealed different types of magnetic fluctuations and spin structure between these two compounds. UNi 2 Al 3 shows a band-type itinerant antiferromagnetic behavior, while UPd 2 Al 3 belongs to a heavy fermion antiferromagnet with an ordinary size of U-derived moment. In the superconducting state, the relaxation behavior in UPd 2 Al 3 is similar to those observed in the other HFS reported so far, providing an evidence for an unconventional anisotropic superconductivity with line of gap zeros on the Fermi surface as well. Knight shift has been found to decrease considerably below T c , showing that the pseudo spin is well defined, not affected by the impurity scattering.

NMR Investigation of Energy Gap Formation in the Valence Fluctuating Compound CeNiSn

119 Sn NMR study has been made in a gap-type valence fluctuating compound CeNiSn. From the measurements of Knight shift and the nuclear spin lattice relaxation time T 1 of 119 Sn, it is proved that no magnetic ordering occurs down to 0.38 K. The relaxation rate, 1/ T 1 markedly decreases below 30 K down to 0.40 K. This behavior can be well interpreted by a quasiparticle band model which possesses a band width of 140 K and a low energy excitation inside an energy gap of 14 K at the Fermi level. Then it is clarified from the microscopic point of view that the pseudo-gap formed at the Fermi level has a character that the quasiparticle density of state is linearly proportional to the energy near the Fermi level at low temperatures.

Nuclear Relaxation Study in Strong Coupling Superconductors –A Comparison with High-T c Superconductors–

The nuclear relaxation behavior in s - wave superconductors has been investigated as for Chevrel-phase superconductors, TlMo 6 Se 7.5 and Sn 1.1 Mo 6 Se 7.5 with T c =12.2 K and 4.2 K, respectively. The nuclear-spin-lattice-relaxation rate, 1/ T 1 , of 119 Sn in Sn 1.1 Mo 6 Se 7.5 reveals a distinct coherence peak just below T c , followed by an exponential decrease with the isotropic energy gap of 2Δ=3.6 k B T c in a weak coupling regime. By contrast, 1/ T 1 of 205 Tl in TlMo 6 Se 7.5 possesses no coherence peak, although the exponential decrease of 1/ T 1 has also been observed with a somewhat larger value of 2Δ=4.5 k B T c in a strong coupling regime. The stronger electron-phonon coupling enhances the superconducting transition temperature and at the same time, opens a decay channel to cause an intense damping of quasiparticles, resulting in the depression of the coherence peak. The comparison with high- T c superconductors will be discussed.

Superconductivity and magnetism of the new heavy fermion superconductors UM2Al3(M=Ni,Pd)-27Al NMR study-

The magnetic and superconducting properties of the new heavy fermion superconductors (HFS), UNi 2 Al 3 and UPd 2 Al 3 have been investigated by 27 Al NMR. The magnetic ordering is unambiguously identified by a significant increase of 27 Al linewidth below T N =4.6 K and an enhancement of the nuclear-spin-lattice relaxation rate 27 (1/ T 1 ) around T N in UNi 2 Al 3 and by a gradual increase of the linewidth and a distinct drop of 27 (1/ T 1 ) below T N =14 K in UPd 2 Al 3 . Below T c =0.9 K in a magnetic field of 2.8 kOe, no anomalies of 1/ T 1 and Knight shift have been observed in UNi 2 Al 3 , whereas the superconductivity in UPd 2 Al 3 is well evidenced by a significant decrease of both 1/ T 1 and Knight shift below T c =1.8 K in 4.9 kOe. The behavior of 1/ T 1 in UPd 2 Al 3 , with no coherence peak just below T c , is well described down to 0.65 K by a model of gap zeros on lines at the Fermi surface, being quite similar to the behavior in HFS reported thus far.

27Al NMR study of the new heavy-fermion superconductors UM2Al3 (M = Ni, Pd)

Abstract We report the nuclear spin-lattice relaxation time T1 and the Knight shift of 27Al of the new heavy-fermion superconductors (HFS), UNi2Al3 and UPd2Al3. The magnetism of these compounds is quite different. In UPd2Al3, the relaxation behavior below Tc is reminiscent of other HFS reported thus far, providing evidence for unconventional superconductivity. The Knight shift has been found to be reduced below Tc, irrespective of the direction of the crystal axes, pointing to the singlet nature of the pairing. In UNi2Al3, however, 1 T 1 and the Knight shift exhibit no indication of anomalies at the superconducting transition.

Magnetic Instability of the Gap State in CeNiSn at Very Low Temperature

The low-temperature magnetic property of the gap-type Kondo lattice compound CeNiSn has been investigated down to 0.08 K by the 119 Sn nuclear magnetic resonance (NMR) technique. The temperature dependence of 119 Sn nuclear-spin-lattice relaxation rate (1/ T 1 ) down to 0.4 K was well interpreted by a model that the quasi-particle density of states has a V-shaped structure of the pseudogap proportional to the energy in the vicinity of the Fermi level. Below 0.13 K, however, (1/ T 1 ) is found to decrease exponentially as exp(-Δ/ k B T ) due to the appearance of the energy gap of Δ=0.25 K. The NMR spectrum starts to broaden below 0.13 K, signifying that there develops a static magnetic correlation with a magnetic moment estimated to be as small as 10 -3 µ B / Ce atom. It is suggested that the pseudogap state is changed into the “ spin gap ” state induced by the development of the static magnetic correlation.

NMR investigations of the ground state in the gap-type Kondo lattice compound CeNiSn and the low-carrier system Yb4As3

Abstract The low-temperature magnetic properties of the gap-type (semiconducting) Kondo lattice compound CeNiSn and the low-carrier system (semimetallic) Yb4As3 have been investigated by means of 119Sn NMR and 75As NQR techniques, respectively. NMR and NQR studies have shown that the magnetic correlation develops below around 0.13 and 10 K for CeNiSn and Yb4As3, respectively. In CeNiSn, alloying depresses the V-shaped pseudogap, while it stabilizes the magnetic correlation toward a static magnetic ordering. In Yb4As3, a spatial distribution of the magnetic correlation emerges, but the magnetic ordered state has not yet been identified down to 0.45 K.

NMR study of Pt in U(Pt1−xPdx)3

Zero field NMR signal of 195 Pt was observed in the antiferromagnetic U(Pt 0.95 Pd 0.05 ) 3 . The internal magnetic field is distributed around 22–38 kOe. The transferred hyperfine field from U moments aligned antiferromagnetically does not cancel at the Pt site. On the other hand we could not observe the internal field at Pt site associated with the antiferromagnetic order in UPt 3 at 1.4 K within the experimental error of 30 Oe.

NMR study of weak magnetism and superconductivity in heavy-fermion systems

Abstract Using NMR techniques, we have investigated the superconducting and magnetic properties of the heavy-fermion superconductors (HFS), CeCu2Si2, and the recently discovered UM2AL3 (M = Ni and Pd). The NMR studies have deduced that the nature of the magnetism in HFS is varied, whereas the superconducting property possesses a rather common feature. Namely, the nuclear spin-lattice relaxation rate 1/T1 is well described in terms of an anisotropic energy gap model in which the gap vanishes on lines at the Fermi surface with values of 2Δ/kBTc = 5.0 and 5.5 for CeCu2Si2 and UPd2Al3, respectively. The unusual superconductivity coexists with the magnetism, regardless of whether it is in a static or a dynamical regime. In contrast, the NMR experiment detected no signature of an anomaly at the superconducting transition temperature in UNi2Al3. The interplay between magnetism and superconductivity is discussed from a comparison with other heavy-fermion superconductors.

Magnetic phase transition in CeTSn (T = Pd, Pt, Ni)

The magnetic properties of a series of CeTSn compounds have been investigated by the NMR technique. CePtSn is found to be antiferromagnetic with the same Neel temperature of 7 K as CePdSn. The magnetic property of CePtSn is shown to be explained by the localized moment picture. In contrast, CeNiSn, which experiences a stronger c-f hybridization than CePtSn, is identified to be anomalous valence-fluctuating compound having no magnetic ordering and instead a pseudo-energy gap at the Fermi level.