The out-of-plane H_c2(θ) at various temperatures for K₂Cr₃As₃. The solid lines and the dashed pink curve (for the 5K data only) are the fitted data using orbit limited Hc2 including angle-dependence of Pauli limit and only orbit limit, respectively. Polar plot of the extracted Hc2(φ). The red (blue) triangles come from the rotation of the derived data withφ_r= 120◦(−120◦), respectively.

(Phys. Rev. B 119, 056601(2017))

Background

Recently, superconductivity was discovered in a Cr-based family A₂Cr₃As₃ (A=K, Rb, and Cs) which possess a quasi-one-dimensional (quasi-1D) crystal structure characterized by infinite [(Cr₃As₃)²⁻]_∞linear chains, called double-walled subnanotubes, which are separated byalkali-metal cations. Unconventional SC in K₂Cr₃As₃ or Rb₂Cr₃As₃ has been supported by accumulating experimental and theoretical results from the Sommerfeld specific-heat coefficient, a large upper critical field, the nuclear quadrapole resonance, penetration-depth measurement, band-structure calculations and also the expected Tc suppression.

What we discover?

We report measurements of the upper critical field H_c2 as functions of temperature T , polar angleθ(of the field direction with respect to the crystallographic c axis), and azimuthal angleφ(of the field direction relative to the a axis within the ab plane). We confirm that the anisotropy in H_c2(T) becomes inverse with decreasing temperature. At low temperatures, Hc2(θ) data are featured by two maxima atθ= 0 (H _ c) andπ/2 (H⊥c), which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for H⊥c are taken simultaneously into consideration. The in-plane H_c2(φ) profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristics of the H_c2(θ,φ,T ) data mostly resemble those of the heavy-fermion superconductor UPt₃, and we argue in favor of a dominant spin-triplet superconductivity with odd parity in K₂Cr₃As₃.

Why is this important?

Spin-triplet superconductivity is rare, and it contains rich and interesting physics which in turn help us to build up a full and deep understanding of superconductivity. K₂Cr₃As₃is a newly discovered superconductor featured by significant electron correlations as well as a quasi-one-dimensional and non-centrosymmetric crystal structure. Previous theoretical and experimental studies on this material show some signatures of spin-triplet superconductivity, but no decisive evidence has been given so far. The solid evidence of a triplet superconductivity in K₂Cr₃As₃shown here will certainly have significant impact in the area of superconductivity. It will also attract attention from the communities of the low-dimension system and strongly correlated electron systems.

Why did they need WHMFC?

The upper critical field of K₂Cr₃As₃is high. In order to measure the H_c2, we employ the pulsed magnetic field to suppress its superconductivity. The material is extremely air-sensitive. While the center also has a glove-box, enabling sample in inert gas during the whole experimental procedure.

Who did the research?

Zuakun Zuo,¹Jin-Ke Bao,²Yi Liu,²Jinhua Wang,¹Zhao Jin,¹Zhengcai Xia,¹Liang Li,¹Zhuan Xu,^2,3Jian Kang,⁴Zengwei Zhu^,1,*and Guang-Han Cao^{2,3, †}

1.Wuhan National High Magnetic Field Center, School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China

2. Department of Physics, Zhejiang University, Hangzhou 310027, China

3. Collaborative Innovation Centre of Advanced Microstructures, Nanjing 210093, China

4. School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA

Funding:

J.K. is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. de-sc0012336. Z.Z. is supported by the 1000 Youth Talents Plan, the National Science Foundation of China (Grant No. 11574097) and The National Key Research and Development Program of China (Grant No. 2016YFA0401704). G.-H.C. is supported by National Key R & D Program of the MOST of China (Grant No. 2016YFA0300202) and the National Science Foundation of China (Grant No. 11674281).