Isamu Satoh

Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)∼2.5  K and T(SC)∼0.6  K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.

Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe

Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe ( T Curie ∼2.5 K and T SC ∼0.6 K) is reported from 59 Co nuclear quadrupole resonance (NQR). The 59 Co-NQR signal below 1 K indicates ferromagnetism throughout the sample volume, while the nuclear spin-lattice relaxation rate 1/ T 1 in the ferromagnetic (FM) phase decreases below T SC due to the opening of the superconducting (SC) gap. The SC state is found to be inhomogeneous, suggestive of a self-induced vortex state, potentially realizable in a FM superconductor. In addition, the 59 Co-NQR spectrum around T Curie shows that the FM transition in UCoGe possesses a first-order character, which is consistent with the theoretical prediction that the low-temperature FM transition in itinerant magnets is generically of first-order.Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe ( T Curie ∼2.5 K and T SC ∼0.6 K) is reported from 59 Co nuclear quadrupole resonance (NQR). The 59 Co-NQR signal below 1 K indicates ferromagnetism throughout the sample volume, while the nuclear spin-lattice relaxation rate 1/ T 1 in the ferromagnetic (FM) phase decreases below T SC due to the opening of the superconducting (SC) gap. The SC state is found to be inhomogeneous, suggestive of a self-induced vortex state, potentially realizable in a FM superconductor. In addition, the 59 Co-NQR spectrum around T Curie shows that the FM transition in UCoGe possesses a first-order character, which is consistent with the theoretical prediction that the low-temperature FM transition in itinerant magnets is generically of first-order.

Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies

Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were carried out for the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transition...

Anisotropic Magnetic Fluctuations in the Ferromagnetic Superconductor UCoGe Studied by Direction-Dependent ^{59}Co NMR Measurements

We have carried out direction-dependent 59Co NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight-shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.

Absence of Meissner State and Robust Ferromagnetism in the Superconducting State of UCoGe: Possible Evidence of Spontaneous Vortex State

We report ac magnetic susceptibility and dc magnetization measurements on the superconducting ferromagnet UCoGe (with superconducting and Curie temperatures of T SC ∼0.5 K and T Curie ∼2.5 K, respectively). In the normal, ferromagnetic state ( T SC < T < T Curie ), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below T SC , the spontaneous magnetization is unchanged, but the hysteresis is markedly enhanced. Even deeply inside the superconducting state, ferromagnetism is not completely shielded, and there is no Meissner region, a magnetic field region of H < H c1 (a lower critical field). From these results, we suggest that UCoGe is the first material in which ferromagnetism robustly survives in the superconducting state and a spontaneous vortex state without the Meissner state is realized.

AC Magnetic Susceptibility Studies of Single Crystalline CeNiGe2 under High Pressure

The magnetic susceptibility of CeNiGe 2 single crystal has been measured under high pressure up to 2.0 GPa. Although two anomalies are observed at T N1 =4.0 K and at T N2 =3.0 K at ambient pressure, T N2 disappears above 0.13 GPa, while T N1 still remains near 2.0 K up to 2.0 GPa. On the other hand, the peak height of the susceptibility curve decreases with increasing pressure, which may suggest that the correlation between conduction and f -electrons is strengthened by pressure. The Kondo temperature T K is estimated from the maximum of χ c – T curve. T K is remarkably enhanced by applying pressure.

Anomalous Enhancement of Nuclear Spin Relaxation Rates of 109Ag and 115In at Low Temperatures in Cubic Γ3 Ground-State System PrAg2In –First Observation of Octupole Fluctuations of f-Electrons–

Microscopic properties have been investigated on a cubic nonmagnetic non-Kramers Γ 3 doublet ground-state (GS) system PrAg 2 In by complementarily utilizing 115 In ( I =9/2) and 109 Ag ( I =1/2) NMR with particular emphasis on the low-frequency (low-ω) dipole and multipole (octupole and/or quadrupole) fluctuations of f -electrons as probed by the nuclear spin relaxation rates 1/ 115 T 1 and 1/ 109 T 1 . We show that 1/ 115 T 1 and 1/ 109 T 1 are anomalously enhanced respectively below \({\cong}50\) K and \({\cong}100\) K over those expected for the low-ω dipole fluctuations of the excited magnetic Γ 4 and Γ 5 states in a simple crystalline-electric-field model for a Γ 3 GS system. By comparing 1/( 115 T 1 T ) and 1/( 109 T 1 T ) and also by considering an invariant form of the hyperfine and/or quadrupole couplings of Γ 3 octupole and/or quadrupole moments with Ag/In nuclear dipole and/or quadrupole moments, we show that Γ 3 octupole fluctuations dominate 1/ 109 T 1 and quadrupole ones can possibly contrib...

A precision measurement of the hyperfine structure of87Sr

The ground state hyperfine splitting of87Sr+ was measured with a precision of 1×10−8. The experiments were performed with an RF ion trap connected to an ISOL (isotope separator on-line), where all the possible transitions between Zeeman sublevels were observed by a laser-microwave double resonance method. The magnetic dipole hyperfine constant was determined to beA=−1 000 473.673 (11) kHz.

Pressure study of magnetism in (Lu0.8Ce0.2)2Fe17 and Lu2Fe16.5Ru0.5 single crystals

The antiferromagnetic state observed in Lu2Fe17 at elevated temperatures is stabilized down to the ground state in (Lu0.8Ce0.2)2Fe17 and Lu2Fe16.5Ru0.5. The Neel temperature TN decreases from 275 K in Lu2Fe17 to 247 K and 208 K in (Lu0.8Ce0.2)2Fe17 and Lu2Fe16.5Ru0.5, respectively. Both compounds exhibit the easy-plane magnetic anisotropy and a metamagnetic transition at fields below 1 T. External pressure pushes the transition towards higher field and reduces the magnetization as well as TN to 234 K and 185 K for (Lu0.8Ce0.2)2Fe17 and Lu2Fe16.5Ru0.5, respectively, under pressure 0.8 GPa. The pressure and the substitution effects can be understood in terms of exchange interaction varying strongly with changes of the lattice parameter c.

Nuclear laser spectroscopy using a laser-microwave double-resonance method with an ion trap

With an RF-trap connected to an Isotope Separator On-Line (ISOL), we aim to study nuclear magnetic properties of nuclei far from stability through the hyperfine interaction by a laser-microwave double-resonance method. As a first step, the hyperfine splitting of the ground state of87Sr+ was measured, and the magnetic dipole hyperfine constant was precisely determined to beA=−1,000,470.8(2.4) kHz.