Experimental evidence for ferromagnetic spin-pairing superconductivity emerging inUGe2: AGe73

Abstract

We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe2 via 73 Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure (P ). Measurements of the nuclear spin-lattice relaxation rate (1/T1) have revealed an unconventional nature of superconductivity such that the up-spin band is gapped with line nodes, but the down-spin band remains gapless at the Fermi level. This result is consistent with that of a ferromagnetic spin-pairing model in which Cooper pairs are formed among ferromagnetically polarized electrons. The present experiment has shed new light on a possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.The coexistence of magnetism and superconductivity (SC) has recently become an important topic in condensed-matter physics. The recent discovery of SC in ferromagnets UGe 2 [1, 2] and URhGe [3] has been a great surprise because the Cooper pairs are influenced by a non-vanishing internal field due to the onset of ferromagnetism (FM), which is believed to prevent the onset of SC. In the ferromagnet UGe 2 with a Curie temperature T Curie = 52 K at ambient pressure (P = 0), the emergence of P -induced SC has observed in the P range of 1.0 -1.6 GPa [1,2]. It is noteworthy that the SC in UGe 2 disappears above P c ∼ 1.6 GPa, beyond which FM is suppressed. The SC and FM in this compound have been shown to be cooperative phenomena [4]. The superconducting transition temperature (T sc ) is the highest at P x ∼ 1.2 GPa, where a first-order transition occurs from FM2 to FM1 as P increases. Here, it should be noted that ferromagnetic moments are increased in the first-order transition from FM1 to FM2 as functions of temperature and pressure, as shown in Fig. 1(a) [5,6,7]. The P -induced SC in UGe 2 coexists with FM1 and FM2 exhibiting the large magnetization of an order 1 µ B /U even for the case of T Curie ∼ 30 K [6]. Therefore, the onset of SC is proposed to be suitable for the formation of a spin-triplet pairing state rather than a spin-singlet pairing state [2]. However, there are few reports that address the type of order parameter is realized in FM1 and FM2. In a previous study, an unconventional nature of the SC has been suggested from the measurement of the 73 Ge-NQR nuclear spin-lattice relaxation rate 1/T 1 [6]. However, it has not been well understood whether the presence of the residual density of states (RDOS) at the Fermi level in the SC state is intrinsic or not, suggesting the occurrence of a possible extrinsic effect due to the presence of any impurity and/or imperfection in the sample [6]. In particular, it is unclear why SC emerges with the highest T sc when the first-order transition occurs from FM1 to FM2 at P x ∼ 1.2 GPa. In order to gain insight into this issue, further experiments are required for understanding a P -induced evolution in the FM states and a novel order-parameter symmetry emer...

Evolution of an Unconventional Superconducting State inside the Antiferromagnetic Phase of CeNiGe3 under Pressure: A 73Ge-Nuclear-Quadrupole-Resonance Study

“…Notably, this longitudinal spin-density fluctuation was also indicated at the end point of the quantum first-order transition from FM2 to FM1 in the ferromagnetic superconductor UGe 2 . 20, 21 We suggest that the longitudinal spin-density fluctuation may be the another possible mechanism for enhancing T sc in magnetic state.…”

Theory of Ferromagnetic Unconventional Superconductors with Spin-Triplet Electron Pairing

“…The experimental data for UGe 2 indicate T f 0 = 52 K, P c = 1.6 GPa (≡ 16 kbar), T m = 0.75 K, P m ≈ 1.15 GPa, and P 0c ≈ 1.05 GPa [2][3][4][5]. Using again the variant n = 1f o rT f (P) and the above values for T m and P 0c we obtainγ ≈ 0.0984 andγ 1 ≈ 0.1678.…”

“…The coexistence of superconductivity and ferromagnetism as a result of collective behavior of f -band electrons has been found experimentally for some Uranium-based intermetallic compounds as, UGe 2 [2][3][4][5], URhGe [6][7][8], UCoGe [9,10], and UIr [11,12]. At low temperature (T ∼ 1K ) all these compounds exhibit thermodynamically stable phase of coexistence of spin-triplet superconductivity and itinerant ( f -band) electron ferromagnetism (in short, FS phase).…”

“…[27][28][29]), in these ferromagnetic compounds the phase transition temperature T F to the ferromagnetic state is much higher than the phase transition temperature T FS from ferromagnetic to a (mixed) state of coexistence of ferromagnetism and superconductivity. For example, in UGe 2 we have T FS ∼ 0.8 K versus maximal T F = 52 K [2][3][4][5]. Another important difference between the ternary rare earth compounds and the intermetallic compounds (UGe 2 , UCoGe, etc.…”

Novel metallic states at low temperatures

“…6 [50] which is expected to be similar to the Fermi surface of ZrZn 2 in the paramagnetic phase [26,51]. These features could lead to the existence of two or more quasi-critical fields and to a [25], (ii) an example of the coexistence of superconductivity and metallic ferromagnetism (UGe 2 [53][54][55]), (iii) quantum tri-criticality in Ni 3 Ga [56], (iv) spin textured phase in MnSi [31,32,36], (v) spin-triplet superconductivity on the border of ferromagnetism (M 2 RuO 4 , where M stands for Sr [57,58] or potentially Ca at high pressures [59]) and (vi) an electronic nematic phase in Sr 3 Ru 2 O 7 [60,61] near to a quantum critical end point in high magnetic fields. [25].…”