Cooperative Phenomenon of Ferromagnetism and Unconventional Superconductivity in UGe₂: A ⁷³Ge-NQR Study under Pressure

Abstract

We report on a cooperative phenomenon of ferromagnetism and unconventional superconductivity (SC) in UGe2 through the measurements of 73 Ge nuclear-quadrupole-resonance (NQR) under pressure (P ). The NQR spectra evidenced phase separation into ferromagnetic and paramagnetic phases in the vicinity of Pc ∼ 1.5 GPa, pointing to a first-order transition. The measurements of nuclear-spin-lattice-relaxation-rate 1/T1 revealed that SC emerges under the background of ferromagnetism, but not of the paramagnetic phase.KEYWORDS: itinerant ferromagnetism, unconventional superconductivity, UGe2, NQR under pressure, first-order phase transitionRecently, superconductivity (SC) has been observed under a background of ferromagnetism (FM) in UGe 2 1, 2 and URhGe.3 Since FM and SC have been generally believed to be exclusive from each other, the coexsistence of FM and SC in these compounds is a great surprise. Figure 1(a) shows the pressure (P ) versus temperature (T ) phase diagram of UGe 2 established from various measurements.1, 2, 4, 5, 7, 8 The ferromagnetic transition temperature T Curie at ambient pressure (P = 0) decreases monotonically from T Curie = 52 K with increasing P and seems to suddenly vanish around P c ∼ 1.5 GPa. SC was discovered in the range of P = 1 to 1.5 GPa, exhibiting the highest transition temperature T sc ∼ 0.7 K at P x ∼ 1.2 GPa. Upon cooling below T x , ferromagnetic moments increase markedly.1, 2, 5 The high-P -T and the low-P -T phases are denoted as FM1 and FM2 respectively, as indicated in the phase diagram. At P x , the first-order transition from FM2 to FM1 emerges as a function of P and at T x as a function of T in the case of P < P x . Here, P x is a terminal point of the first-order transition. 8, 11By contrast, it should be noted that both SC and FM1 are simultaneously suppressed at a ferromagnetic critical pressure P c . This suggests that SC and FM in UGe 2 are cooperative phenomena. Indeed, the 73 Ge-NQR measurements have revealed that both FM1 and FM2 coexist uniformly with the unconventional SC with a line nodegap in the vicinity of P x .9-11 It is surprising that the 5f electrons of uranium contribute to the onset of the uniform coexistent phase of FM and SC. This phenomenon is difficult to understand in terms of the spin-singlet pairing framework for the Cooper pairs. Therefore, this fact * Present address: Department of Physics, Faculty of Science, Okayama University, Okayama 700-8530, Japan suggests that SC in UGe 2 may be in a spin-triplet pairing state under the background of FM. In order to address the intimate interplay between FM and SC in UGe 2 near P c ∼ 1.5 GPa where FM seems to collapse, we here report on the microscopic characteristics of FM and SC near and over P c revealed by the measurements of nuclearquadrupole-resonance (NQR) of enriched 73 Ge under P at zero magnetic field (H = 0) . A polycrystalline sample enriched with 73 Ge was prepared and crushed into powder to allow a maximal penetration of oscillating magnetic field into the sample. The NQR experiment was per...

Coexistence of CDW with staggered superconductivity in a ferromagnetic material

“…The above results are in surprising qualitative agreement with findings in UGe 2 [18]. In particular, recent NQR results indicate that the T * phase coexists indeed with SC over a portion of the SC dome [19] and our results are the first to provide a theoretical picture for it.…”

On the phase diagram of UGe2

“…Thus at high pressures and low temperatures the ferromagnetic and paramagnetic states are divided by the first order type transition whereas at higher temperatures and lower pressures this transition is of the second order. Such type of behavior is typical for MnSi [1][2][3][4], itinerant ferromagnet-superconductor UGe 2 [5][6][7][8][9][10], ZrZn 2 [11]. The same behavior has been established in the ferromagnetic compounds Co(Si 1−x Se x ) 2 [12] and (Sr 1−x Ca x )RuO 3 [4] where the role of governing parameter plays the concentration of Se and Ca correspondingly.…”

Stable phase separation and heterogeneity away from the coexistence curve

“…Phase separation is observed near phase transitions that are known or suspected to be first order in a wide variety of materials with an equally wide variety of types of order; examples were given in the introduction. In addition to transitions from an ordered phase to a disordered one it also is observed near first-order transitions from one ordered phase to another; an example is UGe 2 , where it is observed both near the FM1-PM transition 5 and near the FM1-FM2 transition within the FM phase.…”