commonly exhibit high bulk conductivities, hindering the characterization of the surface state charge transport. The optimally doped topological insulators Bi 2 Te 2 Se and Bi 2-x Sb x Te 2 S, however, allow for such characterizations to be made. Here we report the first experimental comparison of the topological surface states and bulk conductances of Bi 2 Te 2 Se and Bi 1.1 Sb 0.9 Te 2 S, based on temperature-dependent high-pressure measurements. We find that the surface state conductance at low temperatures remains constant in the face of orders of magnitude increase in the bulk state conductance, revealing in a straightforward way that the topological surface states and bulk states are decoupled at low temperatures, consistent with theoretical models, and confirming topological insulators to be an excellent venue for studying charge transport in 2D Dirac electron systems.Topological insulators (TIs) are, theoretically, quantum materials that display a bulk band gap like an ordinary insulator, but a conducting surface state that is topologically protected due to a combination of spin-orbit interactions and time-reversal symmetry 1-6 . The topologically non-trivial nature of the spin-helical Dirac fermion surface states in TIs has attracted wide interest in the research community because it results in rich new physics and materials that have potential applications in quantum technology 7,8 .An ideal TI should have topologically-protected metallic surface states (TSS), but also an electrically insulating bulk. Unfortunately, early generations of bulk TIs, materials such as the M 2 X 3 (M=Bi and Sb, X=S, Se and Te) tetradymites, are not typically insulating 2,[9][10][11][12][13] , which prevented the early characterization of many of the charge transport properties of the surface states. Recently, however, insulating bulk crystals of the tetradymite TI variants Bi 2 Te 2 Se (BTS) 14-16 and Bi 1.1 Sb 0.9 Te 2 S (BSTS) 17,18 have been grown; these two materials are high quality topological insulators with very low bulk carrier concentrations 16,[18][19][20] . The bulk bandgaps of BTS and the BSTS are ~310 meV 14,19,20 and ~350 meV 18 , and, in the latter material, the Dirac crossing in the TSS band is well isolated from the bulk states. These compounds provide an ideal platform for studying the evolution of the TSS conductance and its connections with the conductance of the bulk state.Although it is widely accepted that topological surface states and bulk electronic states should act independently, the degree to which they might or might not intermix has not been addressed in a straightforward experiment. We address that shortcoming
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 (T C ) increases to ~19.1 K from ~9.6 K, and the critical magnetic field (H C2 ) at 1.8 K has been enhanced to T from 15.4 T. These results set new records for both of the T C and the H C2among 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.
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