First Mn-based superconductor discovered under high pressure
Date：10-07-2015 Print
One of the common features of unconventional superconducting systems such as the heavy-fermion, high transition-temperature cuprate and iron-pnictide superconductors is that the superconductivity emerges on the border of long-range magnetically ordered state. The close proximity of superconductivity to a magnetic instability suggests that the critical spin fluctuations would play a crucial role for mediating the Cooper pairs. In addition to doping charge carriers, the application of high pressure is an effective approach for searching novel classes of unconventional superconductors near the magnetic quantum critical point.
Recently, Prof. Jinguang Cheng, Wei Wu, Jianpin Sun, Fukun Lin, and Prof. Jinlin Luo from the Beijing National Laboratory for Condensed Matter Physics at the Institute of Physics, Chinese Academy of Sciences, in collaboration with Dr. K. Matsubayashi and Prof. Y. Uwatoko from the University of Tokyo, have discovered the first Mn-based superconductor, MnP, by suppressing its long-range magnetic order under high pressure.
At ambient pressure, MnP is an itinerant-electron helimagnet with an orthorhombic B31-type structure. In the absence of a magnetic field, MnP undergoes two successive magnetic transitions upon cooling: a transition from the paramagnetic (PM) to ferromagnetic (FM) state at TC = 291 K, and then a second transition to a double helical state at Ts ≈ 50 K. In the FM state, the Mn spins are aligned parallel to the orthorhombic b axis, and the ordered moment is about 1.3μB/Mn. In the double helical state, the Mn spins rotate in the ab plane with the propagation vector q along the c axis. By utilizing the recently developed high-pressure techniques, they have measured the resistivity and ac magnetic susceptibility of MnP in an extended pressure and temperature range, and found that (1) both Ts and TC decrease with pressure and Ts is suppressed completely around 1.4 GPa, (2) the ferromagnetic transition at TC changes to a new-type antiferromagnetic order marked as Tm above 3 GPa, and surprisingly (3) superconductivity emerges below Tsc ≈ 1 K near the critical pressure of Pc ≈ 8 GPa where the long-range magnetic order at Tm just vanishes. Since the majority of the density of states near the Fermi level for MnP is attributed to the Mn-3d states, the present discovery makes MnP the first Mn-based superconductor, and 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 quasiparticles effective mass near Pc associated with the magnetic quantum fluctuations. The present finding of the first Mn-based superconductor breaks the general wisdom about the Mn’s antagonism to superconductivity and should stimulate more work on searching for Mn- and other transition-metal-based superconductors with a higher transition temperature.
This work has been published in Physical Review Letters 114, 117001 (2015), selected as a PRL Editors’ suggestion, and highlighted in Physics 8, 24 (2015). This work was supported by the National Basic Research Program of China, National Science Foundation of China, and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, as well as the grants from Japan.
[1] J. G. Cheng, et al., Pressure induced superconductivity on the border of magnetic order in MnP, Physical Review Letters 114, 117001 (2015).
http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.114.117001
[2] Michael R. Norman, Viewpoint: Superconductivity with a Twist, Physics 8, 24 (2015). http://physics.aps.org/articles/v8/24
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| Fig. 1 (a) Resistivity ρ(T) and (b) the temperature derivative dρ/dT of the MnP single crystal under various pressures up to 10.7 GPa highlighting the variation with pressure of the magnetic transition indicated by the vertical arrows. (c) The c-axis ρc(T) data at low temperatures highlighting the evolution with pressure of the double helical transition at Ts. (d) Temperature dependence of the ac magnetic susceptibility χ'(T) measured under various pressures, which evidences the appearance of a new magnetic transition at T*. |
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| Fig. 2 Temperature dependence of (a) the resistivity ρ(T) and (b) the ac magnetic susceptibility 4πχ on MnP at low temperatures under various pressures near the critical pressure. |
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| Fig. 3 (a) Pressure dependences of the magnetic transition temperatures, TC, Tm, T*, Ts, and the superconducting transition temperature Tsc; Tsc has been scaled by a factor of 20 for clarity. (b, c) Variation with pressure of the residual resistivity ρ0 and the A coefficient extracted from a linear fitting to the ρ(T2) curves at the low-temperature limit. Inset of (a) shows the low-temperature resistivity data at 8.1 GPa in the form of ρ versus T1.5. The relatively large error bar for T* in (a) reflects the two-step feature evidenced in the ac magnetic susceptibility in Fig. 1(d). |
