Abstract Body

The quantum metallic state, where the drop in temperature dependence of resistance R(T) levels off at low temperature in out-of-plane magnetic field, has been one of the topical subjects of two-dimensional (2D) superconductors [1]. While this anomalous vortex state has been widely recognized in various 2D superconductors with weak pinning potential, such as electric-field-induced superconductors [2, 3], exfoliated ultrathin films of atomic layer materials [4] and amorphous thin films [5, 6], the condition for its occurrence, as well as the physical origin, is under debate. In addition, it is also still unclear whether this state corresponds to the vortex solid or liquid phase.

In this study, we investigated the magnetotransport properties of thick exfoliated NbSe₂ single crystal flakes (superconducting transition temperature T_c =7 K) with the thickness of d = 50 - 80 nm, which maintain the condition of 2D vortex system with d comparable to the field penetration length. At low magnetic field, R(T) shows the usual transition to zero resistance. Above the characteristic field B_SM depending on thickness, on the other hand, R(T) becomes temperature-independent with finite resistance at low temperature, while the Hall resistance becomes zero in this resistive state. These combined phenomena are the common properties of the quantum metallic state reported in superconductor films in 2D limit, indicating that this anomalous state can occurs in the condition of 2D vortex state even if the cooper pairs have 3D nature. In the measurement of current-voltage (I - V) characteristics, we observed the evolution from linear to sub-linear I - V behavior intervened by strongly nonlinear regions, which can be ascribed to the transition of the quantum metallic state to the moving vortex lattice state via the plastic vortex flow state. The result suggests that the quantum metallic state consists of vortices with lattice-like correlation.

References

[1] Y. Saito, T. Nojima and Y. Iwasa, Nat. Rev. Mater. 2, 16094 (2017).
[2] Y. Saito, Y. Kasahara, J. Ye, Y. Iwasa, T. Nojima, Science 350, 409 (2015).
[3] Y. M. Itahashi et al., Phys. Rev. Research. 2, 023127 (2020).
[4] A.W. Tsen et al., Nat. Phys. 12, 208 (2016).
[5] D. Ephron et al., Phys. Rev. Lett. 76, 1529 (1996).
[6] K. Ienaga et al., Phys. Rev. Lett. 125, 257001 (2020).

Acknowledgment

This work is supported by JSPS KAKENHI Grant No. JP21H01792.