Enhancement of superconductivity by pressure-induced critical ferromagnetic fluctuations in UCoGe

Abstract

A 59 Co nuclear quadrupole resonance (NQR) was performed on a single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The FM phase vanished at a critical pressure P c , and the NQR spectrum just below P c showed phase separation of the FM and paramagnetic (PM) phases below Curie temperature T Curie , suggesting first-order FM quantum phase transition (QPT). We found that the internal field was absent above P c , but the superconductivity is almost unchanged. This result suggests the existence of the nonunitary to unitary transition of the superconductivity around P c . Nuclear spin-lattice relaxation rate 1/T 1 showed the FM critical fluctuations around P c , which persist above P c and are clearly related to superconductivity in the PM phase. This FM QPT is understood to be a weak first order with critical fluctuations. 1/T 1 sharply decreased in the superconducting (SC) state above P c with a single component, in contrast to the two-component 1/T 1 in the FM SC state, indicating that the inhomogeneous SC state is a characteristic feature of the FM SC state in UCoGe.

Spin-triplet superconductivity in the paramagnetic UCoGe under pressure studied by Co59 NMR

“…If the d-vector rotation occurs for a field much smaller than 2 T, the Knight shift becomes (almost) constant below T SC . The existence of a residual DOS in the present sample may cause the reduction of the decreasing χ spin as observed in 1/T 1 without a field 22 . We also note that the value of K orb is assumed to be unchanged by pressure when the spin part of the Knight shift is estimated.…”

“…A single-crystalline sample was used for 59 Co NMR measurements, and its FM and SC transition temperatures at ambient pressure are T Curie = 2.5 K and T SC = 0.46 K, respec- tively. This sample is the same one used in a recent nuclear quadrupole resonance (NQR) measurement under pressure 22 . Hydrostatic pressure was applied using a piston-cylinder-type cell with Daphne oil 7373 as a pressure medium, and the applied pressure was calibrated using an SC transition temperature of a Pb sample inside the cell.…”

“…The quadrupole parameters are ν Q ≡ ω Q /(2π) = 2.85 MHz and η = 0.52 at ambient pressure 25 , and these values slightly change as pressure increases. We used the values ν Q = 2.795 MHz and η = 0.535 determined at 1.09 GPa 22 . If the Zeeman Hamiltonian is a dominant term in the above Hamiltonian, seven lines will be observed, and they arise from m ↔ m − 1 transitions (m = 7/2, 5/2, .…”

Pairing interaction in superconducting UCoGe tunable by magnetic field

“…The other case is the pressure (P ) induced FM criticality. We studied the relationship between P -induced FM criticality and superconductivity by 59 measurements in a zero magnetic field [30,31]. The superconductivity is also enhanced by the P -induced FM critical region, which has been reported previously [32][33][34].…”

“…axis [30,31]. It should be noted that the 1/T 1 T measured by NQR is determined by the c-axis fluctuations, because the principal axis of the electric quadrupole interaction is almost parallel to the a axis.…”