Reentrant Superconductivity Driven by Quantum Tricritical Fluctuations in URhGe: Evidence fromCo59NMR inURh0.

Abstract

Our measurements of the ^{59}Co NMR spin-spin relaxation in URh_{0.9}Co_{0.1}Ge reveal a divergence of electronic spin fluctuations in the vicinity of the field-induced quantum critical point at H_{R}≈13 T, around which reentrant superconductivity (RSC) occurs in the ferromagnetic heavy fermion compound URhGe. We map out the strength of spin fluctuations in the (H_{b},H_{c}) plane of magnetic field components and show that critical fluctuations develop in the same limited region near the field H_{R} as that where RSC is observed. This strongly suggests these quantum fluctuations as the pairing glue responsible for the RSC. The fluctuations observed are characteristic of a tricritical point, followed by a phase bifurcation toward quantum critical end points.

Wing structure in the phase diagram of the Ising ferromagnet URhGe close to its tricritical point investigated by angle-resolved magnetization measurements

“…This magnetization value is very close to the spontaneous magnetization M c at H = 0, in accordance with a simple picture of the moment reorientation from the easy c axis to the b axis at H R [9,10,14]. Just above H R , there is a small shoulder-like anomaly, which is also seen in dM /dH as a small hump.…”

“…Indeed, the zero-resistivity state of RSC at 50 mK occurs along the first-order transition line in the H b − H c plane, terminating at QWCP [13]. A possible origin of this unusual phenomena has been attributed to longitudinal ( b) magnetic fluctuations, and discussed in relation to a quantum TCP that can be expected when T TCP is very low [9,10,13]. The present results reveal, however, that there is a large disparity between T TCP > 4 K and T RSC ≈ 0.42 K; T TCP /T RSC 10, indicating that the system is not close to a quantum TCP.…”

“…The superconducting properties in the above three uranium-based superconductors are extremely unusual, such as the microscopic coexistence of superconductivity and ferromagnetism [4][5][6][7], possible occurrence of an odd-parity pairing [1][2][3], the huge enhancement of H c2 exceeding the Pauli-limiting field in UCoGe [3,8], and re-entrant superconductivity (RSC) in URhGe [9]. These anomalous behavior are observed around FM quantum phase transition, and hence magnetic quantum fluctuations are considered to be responsible for the emergence of such unusual superconducting states [4,10,11].…”

“…Previous studies by transport (T ≥ 0.5 K), magnetic torque (T ≥ 0.1 K) and magnetization (T ≥ 2 K) measurements indicate that the transition occurs at µ 0 H R ∼ 12 T in URhGe for H b and becomes first order at low temperature [9,13,14,16]. Because the critical field H R is very close to the field in which the RSC emerges, it has been argued that the magnetic fluctuations associated with the moment reorientation play an essential role of RSC [9,10,13].…”

Quantum tricriticality in antiferromagnetic Ising model with transverse field: A quantum Monte Carlo study

“…In addition, existence of the antiferromagnetic QTCP has been theoretically proposed in iron-based superconductor LaFeAsO [21]. More recently, it has been shown that the quantum tricritical fluctuations play key role in stabilizing the superconductivity under magnetic field in URh 0.9 Co 0.1 Ge [22].…”