Masato Hedo

Anomalous peak effect in CeRu2 and 2H-NbSe2: Fracturing of a flux line lattice

CeRu 2 and 2H-NbSe 2 display remarkable similarities in their magnetic response, reflecting the manner in which the weakly pinned flux line lattice (FLL) loses spatial order in the peak-effect (PE) regime. We present evidence for discontinuous changes in the screening response near the onset of the PE in these systems, and demonstrate history-dependent effects. We attribute these features to a disorder-induced fracturing and entanglement of the FLL, as an alternative to the appearance of a spatially modulated ground state for CeRu 2 .

Superconducting Properties of CeRu2

We have measured the specific heat and the magnetization in the superconducting mixed state of CeRu 2 at low temperatures and in high magnetic fields. We have clarified a rich variety of superconducting properties such as the temperature dependence of the electronic specific heat and Maki parameters κ 1 , κ 2 and κ 3 . From these experimental results, CeRu 2 is concluded to be of the BCS-type in superconductivity. Nevertheless, interesting is the magnetic field dependence of the electronic specific heat at 0.5 K, indicating a \(\sqrt{H}\) like behavior in low fields and a H -linear dependence in high fields up to the upper critical field of 50 kOe. This characteristic specific heat is discussed on the basis of a microscopic theory by Brandt et al.

de Haas-van Alphen Oscillation in Both Normal and Superconducting Mixed States of CeRu2

We have observed the de Haas-van Alphen (dHvA) oscillation in both the normal and superconducting mixed states of CeRu 2 . The dHvA amplitude in the mixed state is extremely reduced for the large cyclotron orbit or the carrier with a large cross-sectional area of the Fermi surface compared to the small one. We have detected three dHvA branches, ranging from 6×10 5 Oe to 2.3×10 7 Oe. The origin of these branches is well explained by the 4 f -itinerant band model.

Huge Lattice Softening in CeRu2 without Structural Transition

Symmetrized elastic moduli of high-quality single-crystalline CeRu 2 have been measured between 2 and 300 K by means of an ultrasonic technique. Both transverse moduli ( C 11 - C 12 )/2 and C 44 soften markedly from 300 to ∼20 K without structural transition. In contrast, the bulk modulus C B shows a normal temperature dependence without softening. These results indicate that there exists a marked structural fluctuation corresponding to a pure transverse strain in the normal conducting state of CeRu 2 . Our analysis, taking account of the deformation potential coupling, reveals that a narrow band with relatively high density of states at E F is responsible for the huge lattice softening.

Magnetoresistance and de Haas-van Alphen oscillation in normal and superconducting CeRu2

Abstract We have succeeded in growing high-quality single crystals of CeRu2 and have measured the magnetoresistance and de Haas-van Alphen (dHvA) effect in both its normal and its superconducting mixed states. From the result of the magnetoresistance, we conclude that CeRu2 is a compensated metal. The measured dHvA frequencies are well explained by the results of band-structure calculations. The dHvA frequency does not change in magnitude between the normal and superconducting mixed states, while the cyclotron mass is reduced, and the corresponding Dingle temperature increases in the mixed state. The mass and the Dingle temperature strongly depend on the magnetic field in the mixed state.

De Haas-van Alphen Effect and Energy Band Structure in UB2

We grew a single crystal of UB 2 with the hexagonal structure and observed the de Haas-van Alphen (dHvA) oscillation. All of the dHvA branches, which are about seven in number, are detected in the limited angle regions. They are well explained by the 5 f -itinerant relativistic band theory with an exchange and correlation potential in a local density approximation. Namely, UB 2 is a compensated metal with an equal number of electron and hole. The Fermi surface consists of two closed but corrugated Fermi surfaces. The cyclotron masses are moderately heavy, ranging from 1.2 to 7.7 m 0 .

Effect of field inhomogeneity on the magnetisation measurements in the peak effect region of CeRu2 superconductor

Abstract Detailed magnetisation measurements are presented for a cubic Laves phase superconducting single crystal CeRu 2 . These measurements have been performed in the peak effect regime using Quantum Design SQUID Magnetic Property Measurement System (MPMS). To understand the influence of magnetic field inhomogeneity of this set up, on the measured magnetisation, all the data is recorded for different scan lengths. We clearly demonstrate that some of the anomalous magnetisation results reported in literature are artefacts of field inhomogeneity. We also present a model based on Critical State Model to simulate the effect of field inhomogeneity on the measured magnetisation.

A novel technique to measure magnetisation hysteresis curves in the peak-effect regime of superconductors

Magnetisation hysteresis of type II superconductors measured using SQUID-based Magnetic Property Measurement System (MPMS) made by Quantum Design is known to be affected by inhomogeneity in the external field. We present a new method, viz., half-scan technique, for measuring magnetisation hysteresis loops using MPMS. This technique circumvents the problems caused by field inhomogeneity. We have used this technique to measure magnetisation hysteresis loops in the peak-effect regime of superconducting CeRu 2 and 2H-NbSe 2 . The magnetisation hysteresis obtained using half-scan technique is found to be significantly larger than that obtained by conventional MPMS measurements. At very low fields where the extent of field inhomogeneity is not significant and in the reversible region, the results of the half-scan technique are comparable to those obtained using conventional technique. Moreover, hysteresis measurements are shown to be independent of scan length used in the measurement, thus reaffirming that the results are unaffected by the field inhomogeneity.

Field-history-dependent peak effect in the superconducting mixed state of CeRu2

Abstract Detailed magnetization measurements of CeRu2 single crystals with different purity are performed in order to clarify whether their peak effects are due to the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) transition. The onset field of the peak shifts to lower field side as the sample becomes impure. Regardless of the purity the Ginzburg–Landau parameter κ2 continues to increase on cooling from Tc, indicating little paramagnetic suppression of the superconductivity. These results disagree with the scenario of the FFLO transition.

Anomalous pinning in superconductors with strong Pauli paramagnetism

Abstract The results of the magnetization, magnetostriction and AC-susceptibility experiments are presented for both the antiferromagnetic heavy-Fermion superconductor UPd 2 Al 3 and the valence fluctuation compound CeRu 2 . For ( H > 10 kOe). These results are discussed on the basis of a generalized Fulde-Ferrell-Larkin-Ovchinnikov superconducting state.