Superconductivity with a Twist

Abstract

We report the discovery of superconductivity on the border of long-range magnetic order in the itinerantelectron helimagnet MnP via the application of high pressure. Superconductivity with T sc ≈ 1 K emerges and exists merely near the critical pressure P c ≈ 8 GPa, where the long-range magnetic order just vanishes. The present finding makes MnP the first Mn-based superconductor. The close proximity of superconductivity to a magnetic instability suggests an unconventional pairing mechanism. Moreover, the detailed analysis of the normal-state transport properties evidenced non-Fermi-liquid behavior and the dramatic enhancement of the quasiparticle effective mass near P c associated with the magnetic quantum fluctuations. DOI: 10.1103/PhysRevLett.114.117001 PACS numbers: 74.70.-b, 74.20.Mn, 74.40.Kb, 74.62.Fj Extensive investigations over the last decade have uncovered the quantum criticality as a universal phenomenon connecting with many difficult problems in modern physics [1,2]. For example, the most distinguished problem of unconventional superconductivity (SC) as found in several distinct superconducting systems including the heavy-fermion, organic, cuprates, and the iron-based superconductors can be generally described in the framework of the antiferromagnetic quantum critical point (QCP) [3][4][5][6]. The close proximity of SC to a magnetic instability suggests that the critical spin fluctuations would play a crucial role for mediating the Cooper pairs [5,7]. On the other hand, to realize a magnetic QCP should provide an effective approach for searching new classes of unconventional superconductors. This is well illustrated by the recent discovery of pressure-induced SC in CrAs [8,9], the first Cr-based unconventional superconductor [10]. This discovery has left manganese (Mn) the only 3d element that does not show SC among any Mn-based compounds, even though a great effort has been devoted recently to explore the possible SC via carrier doping [11] or the application of high pressure [12]. The strong magnetism of Mn is commonly believed to be antagonistic to SC. Therefore, it is highly interesting to explore whether SC can emerge near a magnetic QCP in the Mn-based compounds.The itinerant-electron helimagnet, MnP [13], with a much reduced moment of ∼1.3μ B =Mn has attracted our attention as a good starting point to approach a magnetic instability. At ambient condition, MnP adopts an orthorhombic B31-type structure with lattice constants a ¼ 5.26, b ¼ 3.17, and c ¼ 5.92 Å, respectively [14]. In the absence of a magnetic field, MnP undergoes two successive magnetic transitions upon cooling: [13] a transition from the paramagnetic (PM) to ferromagnetic (FM) state at T C ¼ 291 K, and then a second transition to a double helical state at T s ≈ 50 K. In the FM state, the Mn spins are aligned parallel to the orthorhombic b axis, and the ordered moment is about 1.3μ B =Mn. In the double helical state, the Mn spins rotate in the ab plane with the propagation vector q along the c axis [15]. Earlier hydrostatic pr...

Pressure dependence of the magnetic ground states in MnP

“…Since superconductivity emerges in the vicinity of a new, presumably antiferromagnetic phase, it is crucial to elucidate its magnetic structure and clarify the interplay between magnetism and superconductivity in MnP. 7 CrAs and MnP adopt the same orthorhombic structure (P nma). Their difference lies mainly in the dominating magnetic interaction: antiferromagnetic in CrAs and ferromagnetic in MnP.…”

Structural and magnetic phase diagram of CrAs and its relationship with pressure-induced superconductivity

“…If these theories apply in CrAs, it would imply that the pairing state in CrAs could be wave vector/pressure dependent. It is of great theoretical importance if the pairing symmetry could be tuned in a controlled manner in a helimagnetic superconductor 8 .…”

Growth of bulk single-crystal MnP helimagnet and its structural and NMR characterization

“…Despite their apparently simple chemical formulae, due to an intrinsic structural complexity [8], these materials exhibit unusual helical magnetic structures, which are still a matter of debate [9,10]. The archetypal MnP compound has four formulas units per unit cell [Fig.…”