[en] While the superconducting transition temperature T_c of Sr₂RuO₄ is 1.5 K, its onset T_c is enhanced as high as 3 K in the Sr₂RuO₄-Ru eutectic system, which is often referred to as the 3-K phase. We have investigated effects of uniaxial pressure on the non-bulk superconductivity in the 3-K phase. While T_c of pure Sr₂RuO₄ is known to be suppressed by hydrostatic pressure, a large enhancement of the superconducting volume fraction of the 3-K phase was observed for both out-of-plane and in-plane uniaxial pressures. Especially, under the in-plane pressure, the shielding fraction at 1.8 K of only less than 0.5% at 0 GPa exceeds 40% at 0.4 GPa. Such a large shielding fraction suggests that under the uniaxial pressure interfacial 3-K phase superconductivity penetrates deep into the bulk of Sr₂RuO₄. The present finding provides a significant implication to the unresolved origin of the enhancement of T_c to 3 K in the Sr₂RuO₄-Ru eutectic system. (author)

[en] The gap structure of Sr₂RuO₄, which is a longstanding candidate for a chiral p-wave superconductor, has been investigated from the perspective of the dependence of its specific heat on magnetic field angles at temperatures as low as 0.06 K (∼ 0.04T_c). Except near H_c2, its fourfold specific-heat oscillation under an in-plane rotating magnetic field is unlikely to change its sign down to the lowest temperature of 0.06 K. This feature is qualitatively different from nodal quasiparticle excitations of a quasi-two-dimensional superconductor possessing vertical lines of gap minima. The overall specific-heat behavior of Sr₂RuO₄ can be explained by Doppler-shifted quasiparticles around horizontal line nodes on the Fermi surface, whose in-plane Fermi velocity is highly anisotropic, along with the occurrence of the Pauli-paramagnetic effect. These findings, in particular, the presence of horizontal line nodes in the gap, call for a reconsideration of the order parameter of Sr₂RuO₄. (author)

[en] We have investigated effects of uniaxial pressure on superconductivity in the 3-K phase in the eutectic system Sr₂RuO₄-Ru. We have found a large enhancement of 3-K phase superconductivity particularly in volume fraction for both in-plane pressure P||ab and the c-axis pressure P||c in contrast to expectations for the 1.5-K phase in pure Sr₂RuO₄. Surprisingly, the effect of the in-plane pressure is even greater than that of the c-axis pressure. This could be related to the origin of the 3-K phase.

[en] Field-angle-resolved specific-heat measurements provide crucial hints for resolving the gap symmetry of unconventional superconductors. Recent studies on heavy-fermion superconductors such as Ce115, URu_2Si_2, CeCu_2Si_2, UBe_1_3, and UPt_3 have been reviewed. (author)

[en] Ferro-quadrupole (FQ) order in the non-Kramers Γ₃ doublet system PrTi₂Al₂₀ has been investigated via angle-resolved measurements of the specific heat, rotational magnetocaloric effect, and entropy, under a rotating magnetic field within the (11-bar0) plane. The FQ transition occurring at 2 K is robust when the magnetic field B is applied precisely along the [111] direction or in low fields below 1 T. By contrast, the magnetic field of larger than 1 T tilted away from the [111] direction sensitively changes the FQ transition to a crossover. The energy gap between the ground and first-excited states in the FQ order increases remarkably with the magnetic field in B∥[001], but hardly depends on the magnetic-field strength, at least up to 5 T, in the field orientation between the [111] and [110] axes. These features can be reproduced by using a phenomenological model for FQ order assuming an anisotropic field-dependent interaction between quadrupoles, which has been recently proposed to explain the field-induced first-order phase transition in PrTi₂Al₂₀. The present study demonstrates the great potential of the field-angle-resolved measurements for evaluating possible scenarios for multipole orders. (author)

[en] Owing to a strong Coulomb repulsion, heavy electron superconductors mostly have anisotropic gap functions which have nodes for certain directions in the momentum space. Since the nodal structure is closely related to the pairing mechanism, its experimental determination is of primary importance. This article discusses the experimental methods of the gap determination by bulk heat capacity measurements in a rotating magnetic field. The basic idea is based on the fact that the quasiparticle density of states in the vortex state of nodal superconductors is field and direction dependent. We present our recent experimental results of the field-orientation dependence of the heat capacity in heavy fermion superconductors CeTIn₅ (T   =  Co, Ir), UPt₃, CeCu₂Si₂, and UBe₁₃ and discuss their gap structures. (report on progress)

[en] We developed a new technique for measuring the thermodynamic entropy as a function of the magnetic field angle. This technique enables high-resolution angle-resolved measurements of the entropy in an unprecedentedly short measuring time. When the magnetic field is rotated under adiabatic conditions, the sample temperature changes owing to the field-angle variation of its entropy, which is referred to as the rotational magnetocaloric effect. By investigating this effect along with the specific heat, the field-angle dependence of the entropy can be determined. To demonstrate this technique, we chose the spin-ice compound Dy₂Ti₂O₇ as a benchmark and showed good agreement between the measured and theoretical entropies as a function of the field angle. This development provides a new approach to studying condensed-matter physics, in which multiple degrees of freedom play an important role. (author)

[en] Low-temperature thermodynamic investigation for UBe₁₃ and UPd₂Al₃ were performed in order to gain insight into their unusual ground states of $5f$ electrons. Our heat-capacity data for the cubic UBe₁₃ strongly suggest that nodal quasiparticles are absent and its superconducting (SC) gap is fully open over the Fermi surface. Moreover, two unusual thermodynamic anomalies are also observed in UBe₁₃ at ∼ 3 T and ∼ 9 T; the lower-field anomaly is seen only in the SC mixed state by dc magnetization $M(H)$ as well as heat-capacity $C(H)$, while the higher-field anomaly appears for $C(H)$ in the normal phase above the upper critical field. On the other hand, field-orientation dependence of the heat capacity in the hexagonal UPd₂Al₃ shows a significantly anisotropic behavior of $C(H)\propto H^{1/2}$, reflecting the nodal gap structure of this system. Our result strongly suggests the presence of a horizontal line node on the Fermi surface with heavy effective mass in UPd₂Al₃.

[en] In this study, we measure the magnetic field and temperature variations of the magnetization M(H,T) of the cubic chiral compound EuPtSi and extensively investigate the magnetic phase transitions in a wide temperature range down to 60 mK. The incommensurate-commensurate transition at T_N^∗≃2.5 K in the helical state, which has not been observed in transport or thermodynamic measurements, is exclusively detected in M(T) for H || <111>. T_N^∗ shows a rapid decrease in H. For H || [100] and [111], M(H) at T < 100 mK showed that the helical q vectors are depinned from the preferred orientations at H≳10 kOe and gradually rotate toward the field directions as H increases. The skyrmion lattice phase (A-phase) field-induced in H || <111> only above 0.4 K in equilibrium conditions can be created at 60 mK under field cooling, with a sharp tilted plateau structure developing in M(H) of the A-phase. A similar metastable state is also created below 100 mK for the A′ phase induced by H || <100> at T≳0.3 K in equilibrium conditions. The magnetic phase diagrams established for the three directions [111], [110], and [100] are highly anisotropic and discussed herein on the basis of the q vectors rather strongly pinned to the lattice and transform under the lattice symmetry operations. (author)

[en] We have investigated the field-angle variation of the specific heat C(H,φ,θ) of the heavy-fermion superconductor UPt₃ at low temperatures T down to 50 mK, where φ and θ denote the azimuthal and polar angles of the magnetic field H, respectively. For T=88 mK, C(H,θ=90°) increases proportionally to √H up to nearly the upper critical field H_c2, indicating the presence of line nodes. By contrast, C(H,θ=0°) deviates upward from the √H dependence for √H/H_c2 ≳0.5. This behavior can be related to the suppression of H_c2 along the c direction, whose origin has not been resolved yet. Our data show that the unusual H_c2 limit becomes marked only when θ is smaller than 30°. In order to explore the possible vertical line nodes in the gap structure, we measured the φ dependence of C in wide T and H ranges. However, we did not observe any in-plane angular oscillation of C within the accuracy of ΔC/C∼0.5%. This result implies that field-induced excitations of the heavy quasiparticles occur isotropically with respect to φ, which is apparently contrary to the recent finding of a twofold thermal-conductivity κ(φ) oscillation. (author)