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Register a new userDisorder induced multifractal superconductivity in monolayer niobium dichalcogenides
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The interplay between disorder and superconductivity is a subtle and fascinating phenomenon in quantum many body physics. The conventional superconductors are insensitive to dilute nonmagnetic impurities, known as the Andersons theorem. Destruction of superconductivity and even superconductor-insulator transitions occur in the regime of strong disorder. Hence disorder-enhanced superconductivity is rare and has only been observed in some alloys or granular states. Because of the entanglement of various effects, the mechanism of enhancement is still under debate. Here we report well-controlled disorder effect in the recently discovered monolayer NbSe$_2$ superconductor. The superconducting transition temperatures of NbSe$_2$ monolayers are substantially increased by disorder. Realistic theoretical modeling shows that the unusual enhancement possibly arises from the multifractality of electron wave functions. This work provides the first experimental evidence of the multifractal superconducting state.
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NbSe$_{2}$ and NbS$_{2}$ are isostructural two-dimensional materials that exhibit contrasting superconducting properties when reduced to the single monolayer limit. Monolayer NbSe$_{2}$ is an Ising superconductor, while there have been no reports of superconductivity in monolayer NbS$_{2}$. NbS$_{x}$Se$_{2-x}$ alloys exhibit an intriguing non-monotonic dependence of the superconducting transition temperature with sulfur content, which has been interpreted as a manifestation of fractal superconductivity. However, several key questions about this result are not known: (1) Does the electronic structure of the alloy differ from the parent compounds, (2) Are spin fluctuations which have been shown to be prominent in monolayer NbSe$_{2}$ also present in the alloys? Using first-principles calculations, we show that the density of states at the Fermi level and the proximity to magnetism in NbS$_{x}$Se$_{2-x}$ alloys are both reduced compared to the parent compound; the former would decrease the transition temperature while the latter would increase it. We also show that Se vacancies, which are likely magnetic pair-breaking defects, may form in large concentrations in NbSe$_{2}$. Based on our results, we suggest an alternative explanation of the non-monotonic behavior the superconducting transition temperature in NbS$_{x}$Se$_{2-x}$ alloys, which does not require the conjecture of multifractality.
Eigenstate multifractality is a distinctive feature of non-interacting disordered metals close to a metal-insulator transition, whose properties are expected to extend to superconductivity. While multifractality in three dimensions (3D) only develops near the critical point for specific strong-disorder strengths, multifractality in 2D systems is expected to be observable even for weak disorder. Here we provide evidence for multifractal features in the superconducting state of an intrinsic weakly disordered single-layer NbSe$_2$ by means of low-temperature scanning tunneling microscopy/spectroscopy. The superconducting gap, characterized by its width, depth and coherence peaks amplitude, shows a characteristic spatial modulation coincident with the periodicity of the quasiparticle interference pattern. Spatial inhomogeneity of the superconducting gap width, proportional to the local order parameter in the weak-disorder regime, follows a log-normal statistical distribution as well as a power-law decay of the two-point correlation function, in agreement with our theoretical model. Furthermore, the experimental singularity spectrum f($alpha$) shows anomalous scaling behavior typical from 2D weakly disordered systems.
The niobium rich selenide compound Nb5Se4 was synthesized at ambient pressure by high-temperature solid-state reaction in a sealed Ta tube. Resistivity and heat capacity measurements reveal that this compound is superconducting, with a T_c = 1.85K. The electronic contribution to the specific heat {gamma} and the Debye temperature are found to be 18.1 mJ/mol/K^2 and 298 K respectively. The calculated electron-phonon coupling constant {lambda}_ep = 0.5 and the {Delta}C_p/{gamma}Tc = 1.42 ratio imply that Nb5Se4 is a weak coupling BCS superconductor. The upper critical field and coherence length are found to be 1.44 T and 15.1 nm, respectively.
Here we report the observation of extraordinary superconductivity in a pressurized commercial niobium-titanium alloy. We find that its zero-resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record high pressure up to which a known superconducting state can continuously survives. Remarkably, at such an ultra-high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (TC) increases to ~19.1 K from ~9.6 K, and the critical magnetic field (HC2) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both of the TC and the HC2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high pressure superconducting properties observed in the NbTi alloy not only expand our knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.
In this work, we review the results of several recent works on the experimental and theoretical studies of monolayer superconducting transition metal dichalcogenides (TMD) such as superconducting MoS2 and NbSe2. We show how the strong Ising spin-orbit coupling (SOC), a special type of SOC which pins electron spins to out-of-plane directions, can affect the superconducting properties of the materials. Particularly, we discuss how the in-plane upper critical fields of the materials can be strongly enhanced by Ising SOC and how TMD materials can be used to engineer topological superconductors and nodal topological superconductors which support Majorana fermions.
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