Aoki Dai

Unconventional Heavy-Fermion Superconductivity of a New Transuranium Compound NpPd5Al2

We succeeded in growing a new transuranium compound NpPd 5 Al 2 with the tetragonal crystal structure by the Pb-flux method, and measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization. NpPd 5 Al 2 is found to be a paramagnetic heavy-fermion superconductor with strong coupling Δ C /γ T c = 2.33 [ T c =4.9 K and γ= 200 mJ/(K 2 ·mol)]. The upper critical field H c2 at 0 K is large and highly anisotropic: H c2 (0) =37 kOe for H ∥[100] and H c2 (0) =143 kOe for H ∥[001]. H c2 is found to be strongly suppressed by the magnetic field, and the magnetization curve indicates a step-like increase at H c2 , which are due to the paramagnetic effect. The low-temperature electronic specific heat approximately follows the T 3 -dependence, indicating the existence of point nodes in the energy gap. The d -wave superconductivity with point nodes is most likely realized in this compound.

The Quantum Critical Point in CeRhIn5: A Resistivity Study

The pressure–temperature phase diagram of CeRhIn 5 has been studied under high magnetic field by resistivity measurements. Clear signatures of a quantum critical point has been found at a critical pressure of p c ≈2.5 GPa. The field induced magnetic state in the superconducting state is stable up to the highest measured field. At p c the antiferromagnetic ground-state under high magnetic field collapses very rapidly. Clear signatures of p c are the strong enhancement of the resistivity in the normal state and of the inelastic scattering term. No clear T 2 temperature dependence could be found for pressures above T c . From the analysis of the upper critical field within a strong coupling model we present the pressure dependence of the coupling parameter λ and the gyromagnetic ratio g . No signatures of a spatially modulated order parameter could be evidenced. A detailed comparison with the magnetic field–temperature phase diagram of CeCoIn 5 is given. The comparison between CeRhIn 5 and CeCoIn 5 points ou...

Ferromagnetic Quantum Critical Endpoint in UCoAl

Resistivity and magnetostriction measurements were performed at high magnetic fields and under pressure on UCoAl. At ambient pressure, the 1st order metamagnetic transition at H_m ~ 0.7 T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature T_0 ~11 K. With increasing pressure, H_m linearly increases, while T_0 decreases and is suppressed at the quantum critical endpoint (QCEP, P_QCEP ~ 1.5 GPa, H_m ~ 7 T). At higher pressure, the value of H_m identified as a crossover continuously increases, while a new anomaly appears above P_QCEP at higher field H* in resistivity measurements. The field dependence of the effective mass (m*) obtained by resistivity and specific heat measurements exhibits a step-like drop at H_m at ambient pressure. With increasing pressure, it gradually changes into a peak structure and a sharp enhancement of m* is observed near the QCEP. Above P_QCEP, the enhancement of m* is reduced, and a broad plateau is found between H_m and H*. We compare our results on UCoAl with those of the ferromagnetic superconductor UGe2 and the itinerant metamagnetic ruthenate Sr3Ru2O7.

Magnetic and Superconducting Properties of CeTX3 (T: Transition Metal and X: Si and Ge) with Non-centrosymmetric Crystal Structure

We grew single crystals of non-centrosymmetric compounds CeTSi 3 and CeTGe 3 (T: transition metal), and studied the magnetic properties by measuring the electrical resistivity, magnetic susceptibility and magnetization. We also studied the effect of pressure on the electronic states in antiferromagnets CeTGe 3 (T: Co, Rh, Ir) by measuring the resistivity under pressure. No noticeable change of the Neel temperature was observed up to 8 GPa in CeRhGe 3 and CeIrGe 3 , which are far from the magnetic quantum critical point. On the other hand, the Neel temperature in CeCoGe 3 was strongly decreased as a function of pressure, and pressure-induced superconductivity was observed in the pressure region from 5.4 GPa to about 7.5 GPa. The slope of upper critical field H c2 at 6.5 GPa is found to be extremely large, with an upturn curvature of H c2 with decreasing temperature: -d H c2 /d T = 200 kOe/K at the superconducting transition temperature T sc = 0.69 K, revealing unconventional superconductivity.

Magnetic Order in Ce0.95Nd0.05CoIn5: The Q-Phase at Zero Magnetic Field

We report neutron scattering experiment results revealing the nature of the magnetic order occurring in the heavy fermion superconductor Ce0.95Nd0.05CoIn5. In this system, an antiferromagnetic state is stabilized at a temperature below the superconducting transition one. We evidence that it is an incommensurate order and its propagation vector is found to be identical to that of the magnetic field induced antiferromagnetic order occurring in the stoichiometric superconductor CeCoIn5, the so-called Q-phase. The commonality between these two cases suggests that d-wave superconductivity is playing a key role for the formation of this kind of magnetic order. The proposed mechanism is the enhancement of the nesting condition by an order parameter with nodes in the nesting area.

27Al NMR Evidence for the Strong-Coupling d-Wave Superconductivity in NpPd5Al2

We present 27 Al NMR studies for a single crystal of the newly developed superconductor NpPd 5 Al 2 ( T c = 4.9 K). In the superconducting state, the 27 Al NMR spectrum shifts to lower frequencies owing to a decrease of the spin part of the Knight shift, and exhibits broadening characteristic of the local field distribution associated with a vortex lattice. We have also found that the 27 Al nuclear spin–lattice relaxation rate (1/ T 1 ) decreases just below T c with no coherence peak and shows T 3 behavior at low temperatures. The observed temperature dependences of the Knight shift and 1/ T 1 in general accord with those assuming a line-node superconducting gap with the gap value of 2Δ/ k B T c ≃6.4. Therefore, the present NMR data provides evidence for strong-coupling d -wave superconductivity in NpPd 5 Al 2 .

Electronic Band Structure and Fermi Surface of Heavy-Fermion Neptunium Superconductor NpPd5Al2

Electronic band structure calculations were carried out for a new neptunium superconductor NpPd 5 Al 2 using a relativistic linear-augmented plane wave method within a local density approximation. The Fermi energy in the band structure is found to be located just on the 5 f -bands of the j =5/2 state with the extremely narrow band width and therefore the theoretical electronic specific heat coefficient γ b is extremely large, being γ b = 74.0 mJ/(K 2 ·mol), which is compared to γ= 390 mJ/(K 2 ·mol) estimated experimentally. NpPd 5 Al 2 is an uncompensated metal. The main electron Fermi surface corresponds to a doughnut-like flat Fermi surface centered at the Γ point which is connected with cylindrical Fermi surfaces elongated along the X–W–P line.

Effect of Pressure and Magnetic Field on the Superconducting State of a Heavy Fermion Superconductor NpPd5Al2

We have carried out the electrical resistivity measurements under high pressures and high magnetic fields in NpPd 5 Al 2 in order to investigate a quantum critical behavior and the stability of the superconducting phase. A T -linear dependence of the electrical resistivity is changed into the Fermi liquid relation of ρ∼ A T 2 under the magnetic fields, and A value indicates a divergent feature around the upper critical field H c2 , indicating the existence of a field-tuned quantum critical point (QCP) around H c2 . With increasing pressure, the superconducting transition temperature T sc decreases and becomes zero above 5.7 GPa, together with a decrease of the upper critical field, indicating that the electronic state of NpPd 5 Al 2 is located in the vicinity of QCP at ambient pressure and deviates from QCP with increasing pressure. Furthermore, we have constructed the superconducting phase diagram at 3.71 GPa.

High-field magnetization of USn3 and UPb3

UX 3 (X=Si, Ge, Sn and Pb) compounds with the cubic crystal structure are interesting in relation with the lattice constant and the electronic state of 5 f electrons.We measured the high-field magnetization of USn 3 and UPb 3 grown by the self-flux method.The broad metamagnetic transition was observed in a heavy fermion paramagnetic compound USn 3 at about 30 T, which is similar to the metamagnetism of UPt 3 . On the other hand, the sharp metamagnetic transitions were observed in an antiferromagnetic compound UPb 3 .

Super Clean Sample of URu2Si2

We succeeded in growing a high-quality single crystal of URu 2 Si 2 by the Czochralski pulling method and applying the solid state electro-transport method under ultrahigh vacuum. The sample quality strongly depends on the position in the ingot. For some parts of this ingot, the electrical resistivity, specific heat and de Haas–van Alplhen effect were measured. The electrical resistivity of the surface sample clearly indicates a T -linear temperature dependence below 5 K. The mean free path in this sample was determined by the de Haas–van Alphen effect to be 11000 A.