Hisashi Kotegawa

Anion height dependence of Tc for the Fe-based superconductor

We have established a plot of the anion height dependence of superconducting transition temperature Tc for the typical Fe-based superconductors. The plot showed a symmetric curve with a peak around 1.38??. Both data at ambient pressure and under high pressure obeyed the unique curve. This plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and searching for the new Fe-based superconductors with higher? Tc.

Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and 77Se-NMR Study

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature ( T c ) increases sensitively under pressure, but shows a plateau between 0.5–1.5 GPa. The maximum T c , which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ∼37 K at 4.15 GPa. We also measure the nuclear spin–lattice relaxation rate 1/ T 1 under pressure using 77 Se-NMR measurement. 1/ T 1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/ T 1 T just above T c shows a plateau as well as the pressure dependence of T c , which gives clear evidence of the close relationship between 1/ T 1 T and T c . Spin fluctuations are suggested to contribute to the mechanism of superconductivity.

Pressure-induced superconductivity in UIr

Pressure-induced superconductivity is found in UIr without inversion symmetry. The pressure–temperature phase diagram has been investigated by means of the electrical resistivity and magnetization ...

Abrupt Emergence of Pressure-Induced Superconductivity of 34 K in SrFe2As2: A Resistivity Study under Pressure

We report resistivity measurement under pressure in single crystals of SrFe 2 As 2 , which is one of the parent materials of Fe-based superconductors. The structural and antiferromagnetic (AFM) transition of T 0 = 198 K at ambient pressure is suppressed under pressure, and the ordered phase disappears above P c ∼3.6–3.7 GPa. Superconductivity with a sharp transition appears accompanied by the suppression of the AFM state. T c exhibits a maximum of 34.1 K, which is realized close to the phase boundary at P c . This T c is the highest among those of the stoichiometric Fe-based superconductors.

Coexistence of superconductivity and antiferromagnetism in multilayered high-Tc superconductor HgBa2Ca4Cu 5Oy: Cu-NMR study

We report a coexistence of superconductivity and antiferromagnetism in five-layered compound HgBa 2 (Ta 4 Cu 5 O y (Hg-1245) with T c = 108 K, which is composed of two types of CuO 2 planes in a unit cell; three inner planes(IPs) and two outer planes (OPs). The Cu-NMR study has revealed that the optimally doped OP undergoes a superconducting (SC) transition at T c = 108 K, whereas the three underdoped IPs do an antiferromagnetic (AF) transition below T N ∼60 K with the Cu moments of ∼(0.3-0.4)μ B . Thus bulk superconductivity with a high value of T c =108 K and a static AF ordering at T N =60 K are realized in the alternating AF and SC layers. The AF-spin polarization at the IP is found to induce the Cu moments of ∼0.02μ B at the SC OP, which is the AF proximity effect into the SC OP.

Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe: A 77Se-NMR Study

We report the results of 77 Se-nuclear magnetic resonance (NMR) in α-FeSe, which exhibits a similar crystal structure to the LaFeAsO 1- x F x superconductor and shows superconductivity at 8 K. The nuclear-spin lattice relaxation rate 1/ T 1 shows T 3 behavior below the superconducting transition temperature T c without a coherence peak. The T 1 T = const. behavior, indicative of the Fermi liquid state, can be seen in a wide temperature range above T c . The superconductivity in α-FeSe is also an unconventional one as well as LaFeAsO 1- x F x and related materials. The FeAs layer is not essential for the occurrence of the unconventional superconductivity.

Nonmagnetic indenter-type high-pressure cell for magnetic measurements

An indenter-type high-pressure cell has been developed for electric and magnetic measurements in low-temperature and high-magnetic-field environments. The maximum pressure achieved at low temperatures is more than 4.5 GPa, which is higher than that of a conventional piston-cylinder cell. The typical sample space at maximum pressure is 1.6 mm in diameter and approximately 0.7 mm in depth, and magnetic measurements such as ac-susceptibility and nuclear magnetic resonance can be performed using a miniature coil. All the components of the indenter cell are made of nonmagnetic materials that have enough thermal conductivity for low-temperature experiments using a 3He/4He dilution refrigerator. Another indenter-type cell designed for a commercial superconducting quantum interference device magnetometer is also reported.

Superconductivity in a new iron pnictide oxide (Fe2As2)(Sr4(Mg, Ti)2O6)

We have discovered a new iron pnictide oxide superconductor (Fe2As2)(Sr4(Mg, Ti)2O6). This material is isostructural with (Fe2As2)(Sr4M2O6) (M = Sc, Cr, V), which were found in previous studies. The structure of this compound is tetragonal with a space group of P4/nmm and consists of the anti-fluorite type FeAs layer and perovskite-type block layer. The lattice constants are a = 3.935?? and c = 15.952?? for (Fe2As2)(Sr4MgTiO6). Bulk superconductivity with a Tc(onset) of ~ 26?K was observed for a partially Co-substituted sample. Moreover, Co-free and Ti-rich samples exhibited higher Tc(onset)s above 35?K, which were further enhanced by applying high pressures up to ~ 43?K.

Superconductivity of 2.2 K under Pressure in Helimagnet CrAs

We report resistivity measurements of the helimagnet CrAs under pressures. The helimagnetic transition with TN ∼ 265 K at ambient pressure is completely suppressed above a critical pressure of Pc ∼ 0.7 GPa, and superconductivity is observed at ∼2.2 K for zero resistance, which exists in a wide pressure range extending beyond 3 GPa. Both the upper critical field Hc2 and the coefficient A in the resistivity increase toward Pc, suggesting that the superconductivity of CrAs is mediated by electronic correlations enhanced in the vicinity of the helimagnetic phase.

New Superconducting and Magnetic Phases Emerge on the Magnetic Criticality in CeIn3

We report the discovery of new superconducting and novel magnetic phases in CeIn 3 on the verge of antiferromagnetism (AFM) under pressure ( P ) through the In-nuclear quadrupole resonance (NQR) measurements. We have found a P -induced phase separation of AFM and paramagnetism (PM) without any trace of a quantum phase transition in CeIn 3 . A new type of superconductivity (SC) was found, under P =2.28–2.5 GPa, to coexist with AFM. It is magnetically separated from PM where the heavy fermion SC takes place. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order magnetic phase transition might mediate attractive interaction to form Cooper pairs.