Low-dimensional superconductors \u2014 systems whose size is comparable to or smaller than the superconducting coherence length or penetration depth \u2014 display physics profoundly different from their bulk counterparts. Quantum fluctuations, disorder, and reduced dimensionality conspire to produce emergent phases, anomalous metallic states, and exotic vortex physics. We study these effects in atomically thin van der Waals crystals, disordered thin films, and oxide interfaces.<\/p>\n\n\n\n
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Some representative results from our group:<\/p>\n\n\n\n
Few-layer NbSe\u2082 is a rich platform for studying the interplay between charge order, superconductivity, dimensionality, and spin-orbit coupling. In suspended, ultrathin 2H-NbSe\u2082 devices fabricated on piezoelectric substrates \u2014 allowing independent tuning of thickness, disorder, and strain \u2014 we observed for the first time a dynamically modulated quantum phase transition between two distinct charge density wave (CDW) phases. The conductance fluctuates between two precise values separated by a quantum of conductance, a signature that disappears in disordered or on-substrate devices and arises from a dynamical phase transition between competing CDW states driven by strain.<\/p>\n\n\n\n
Moving to the superconducting phase, we used vortex dynamics as a probe of dimensionality. By studying the vortex-solid\/glass to vortex-fluid transition in both 2D and 3D NbSe\u2082 devices through magnetoresistance and nonlinear current-voltage characteristics, we extracted the dynamical critical exponents independently from both measurements. The H-T phase diagrams of 2D and 3D devices are found to be significantly different near the critical point, while both follow universal scaling relations \u2014 establishing that critical vortex dynamics are valid across dimensionalities but with distinct universality.<\/p>\n\n\n\n
We then asked whether the exotic Ising superconductivity of monolayer NbSe\u2082 \u2014 where broken inversion symmetry locks spins out-of-plane, producing an anomalously large in-plane upper critical field \u2014 could be extended to thicker, more robust crystals. By proximity-coupling few-layer NbSe\u2082 (~15 nm) to a graphene monolayer in a van der Waals heterostack, we showed that the graphene overlay effectively re-breaks the inversion symmetry of the NbSe\u2082, restoring 2D-Ising superconductivity in a system that would otherwise behave as a 3D superconductor. Finally, using low-frequency conductance fluctuation spectroscopy on a MoS\u2082\/NbSe\u2082 van der Waals heterostructure, we showed that the superconducting transition in this system is percolative, with non-Gaussian fluctuation statistics near the transition temperature revealing long-range correlations absent in either constituent material alone.<\/p>\n\n\n\n