Detection of an Unconventional Superconducting Phase in the Vicinity of the Strong First-Order Magnetic Transition in CrAs UsingAs75-Nuclear Quadrupole Resonance

Abstract

Pressure-induced superconductivity was recently discovered in the binary helimagnet CrAs. We report the results of measurements of nuclear quadrupole resonance for CrAs under pressure. In the vicinity of the critical pressure Pc between the helimagnetic (HM) and paramagnetic (PM) phases, a phase separation is observed. The large internal field remaining in the phase-separated HM state indicates that the HM phase disappears through a strong first-order transition. This indicates the absence of a quantum critical point in CrAs; however, the nuclear spin-lattice relaxation rate 1/T1 reveals that substantial magnetic fluctuations are present in the PM state. The absence of a coherence effect in 1/T1 in the superconducting state provides evidence that CrAs is the first Cr-based unconventional superconductor. In a conventional Bardeen-Cooper-Schrieffer (BCS) superconductor, electrons form Cooper pairs through negative interactions via an electron-phonon coupling. This gives a sign-uniform s-wave character to the superconducting (SC) gap symmetry. If positive interactions work electron pairs to induce superconductivity, unconventional superconductivity with a sign-changing order parameter is realized beyond the well-established simple s wave. This fascinating form of superconductivity has been discovered in specific materials, such as heavy fermion systems, high-T c cuprates, organic systems, ruthenate, and recent Fe-based superconductors. A common feature of these materials is that superconductivity appears due to the instability of some degree of freedom, which is mainly induced on the verge of a magnetically ordered phase.Very recently, Wu et al. and Kotegawa et al. independently discovered pressure-induced superconductivity in CrAs in the vicinity of the helimagnetic (HM) phase [1,2]. This is the first example of superconductivity found in a Cr-based magnetic system. CrAs has a MnPtype orthorhombic crystal structure with a P nma space group. This structure possesses a space-inversion symmetry, but it is locally missing at the Cr and As sites, and both of these atoms form a zigzag chain along the a-axis. The magnetic transition of the first order into a double-helical state occurs at T N ∼ 265 K. This transition is accompanied by a large magnetostriction, and a lowering of the crystal structure symmetry has not been reported [3,4]. The propagation vector is incommensurate with 0.354 · 2πc * , where c * is a unit vector along the c axis, and magnetic moments of ∼ 1.7µ B /Cr lie in the ab plane [5,6]. The application of pressure drastically suppresses T N , and the HM phase disappears above a critical pressure of P c ∼ 0.7 GPa [1,2,7]. Superconductivity appears together with the suppression of the HM phase, showing a maximum SC transition temperature of T c ∼ 2.2 K at ∼ 1.0 GPa, after which T c decreases gradually with increasing pressure. The pressure-temperature phase diagram of CrAs with a first-order magnetic transition is reminiscent of those of some pressure-induced Fe-based superconductors, such as SrFe 2 As 2 ...

Single crystal growth of the new pressure-induced-superconductor CrAs via chemical vapor transport

“…The P dependent phase diagram is reminiscent of doped iron based superconductors. Moreover, nuclear quadrupole resonance results indicate that it is a unconventional superconductor [14].…”

Evolution of Magnetic Double Helix and Quantum Criticality near a Dome of Superconductivity in CrAs

“…The findings were later confirmed by Kotegawa et al [17] and their subsequent 75 As-nuclear quadrupole resonance (NQR) study evidenced no coherence effect in the nuclear spin relaxation rate 1/T1, thus suggesting an unconventional nature for the observed SC in CrAs. [18] These observations make CrAs an ideal material for in-depth studies on how the coupled structural and helimagnetic orders are tuned to the critical point and how the magnetism is coupled to the unconventional SC developed nearby. In combinations of elastic and inelastic neutron scattering, resistivity, and specific-heat measurements on undoped CrAs and P-doped CrAs1-xPx single crystals and powders, we have studied systematically the evolutions of static helical order and dynamic spin correlations as well as the electronic properties of CrAs when its coupled structural/helical order is suppressed by either external pressure or the chemical substitutions.…”

“…In addition, a recent high-pressure NQR study on CrAs single crystals also found no clear signature of a change in the magnetic structure under pressure. [18] If we assume that the helical structure does not change other than  with pressure and normalize the magnetic signal by the (020) nuclear Bragg intensity, the ordered magnetic moment at 6.9 kbar, which is proportional to the square root of the magnetic intensity, is estimated to be ~ 65% of that at ambient pressure. This estimated moment is between those two polycrystalline results (~25% [26] and ~85% [27]).…”

Tripletpz-wave pairing in quasi-one-dimensionalA2Cr3As3superconductorsWu1, Yang2, Le3 et al. 2015Phys. Rev. B100101093View full textAdd to dashboardCite

“…Their superconducting mechanism shown in many experimental results [13][14][15][16] is very likely related to electron-electron interactions. Among the families of the chromium-based superconductors, the Q1D A 2 Cr 3 As 3 (A=K,Rb,Cs) superconductors family [10][11][12] , which has T c up to 6.1 K 10 at ambient pressure, is particularly intriguing.…”