Session J66: The Superconductor-Insulator Transition: Beyond Universal Scaling

Collapse of the Cooper pair phase coherence length at a superconductor-to-insulator transition

The superconductor-to-insulator transition (SIT) in quasi-2D thin films now serves as a canonical quantum phase transition (QPT). However, variation in its features between different material systems continues to raise perplexing questions about universality and universality classes in QPTs. A key to understanding the different manifestations of SITs has been through experimental probes of microscopic transport mechanisms. In this talk, I will present transport experiments on two morphologies of amorphous Bi quench-condensed films patterned with a nanohoneycomb array of holes, where the holes provide an embedded probe of local phase coherence. Films with smooth, nano-scale undulations in thickness exhibit a bosonic insulating phase that includes a giant magnetoresistance peak. In contrast, films of uniform thickness exhibit a fermionic insulating phase. The distinct phases are supported by the presence or absence of magneto-resistance oscillations that occur when Cooper pairs are phase coherent over a scale similar to the spacing of the nanohoneycomb array. These data provide a microscopic view of locally phase coherent Cooper pairs, confirm the existence of the bose insulator, and distinguish two classes of disorder-tuned SITs.   

Experimentally observed magnetic-field driven quantum phase transition in superconducting nanowires and its striking agreement with critical theory.

We have discovered that a magnetic-field driven quantum phase transition (QPT) in MoGe superconducting nanowires can be fully explained by the pair-breaking critical theory with exponents v≈1 and z≈ 2. We find that in the quantum critical regime, the electrical conductivity is in agreement with a theoretically predicted scaling function and, moreover, that the theory quantitatively describes the nonuniversal dependence of conductivity on the critical temperature, field magnitude and orientation, nanowire cross-sectional area, and microscopic parameters of the nanowire material. Our data analysis is very different from what was used in the past for QPT in superconducting films: (i) we have subtracted contribution of normal electrons, both Drude and quantum corrections, and found that QPT occurs only in the superconducting part of the system, (ii) we also have kept the critical exponents fixed to their theoretical values and not varied them in the scaling procedure. In the second part of the talk, we will briefly comment on reliability of the finite-size scaling analysis and present our work-in-progress on QPT in MoGe films. In these films we have not observed the bosonic “strange metal” phase. However, we have found some evidences of a pair-breaking QPT that occurs at about 0.1 of quantum conductance of Cooper pairs.   

Theory of the pair-breaking quantum phase transition in superconducting nanowires

We present a theoretical description of a zero temperature phase transition between superconducting and diffusive metallic states in ultra thin wires induced by Cooper pair-breaking perturbations. Fluctuation corrections to BCS theory motivate a dissipative field theory that can be used to compute the universal scaling of the electrical and thermal conductivity in the quantum critical regime. In the large-N limit, we obtain predictions for transport in striking agreement with recent experimental measurements of the fluctuation conductivity of metallic nanowires in parallel and transverse magnetic fields. The ability to quantitatively describe the experiment goes beyond the usual scaling approach and validates the underlying pair-breaking mechanisms near the quantum phase transition. Extensions of the theory including the effects of disorder and higher dimensions are considered, with implications for the interpretation of transport measurements in superconducting films.   

Low-temperature anomaly in disordered superconductors near Bc2

Strongly disordered superconductors in a magnetic field display many characteristic properties of type-II superconductivity --except at low temperatures where an anomalous linear T-dependence of the resistive critical field B_c2 is routinely observed. This behavior violates the conventional theory of superconductivity, and its origin remains a long-standing puzzle. In this talk I will present systematic measurements of the critical magnetic field and current on disordered superconducting films of various levels of disorder. Surprisingly, our measurements show that the B_c2 anomaly near zero-temperature is accompanied by a clear mean-field like scaling behavior of the critical current. Our experimental findings together with theoretical considerations on the inherent vortex-glass state and its thermal fluctuations enable to explain the linear-T anomaly to occur in films as well as bulk superconductors with a slope that depends on the normal-state sheet resistance, in full agreement with the data.   

Quasiparticle screening near a bosonic superconductor-insulator transition revealed by magnetic impurity doping

While bosonic superconductor-insulator transitions (SIT) have been clearly observed in a number of thin film systems, the mechanisms driving the Cooper pair localization remain to be established. These SITs feature thermally activated Cooper pair transport in the insulating phase with an activation energy T₀ that grows continuously from zero at the critical point. Some models attribute this behavior to disorder effects that give rise to Anderson localization of Cooper pairs while others invoke Coulomb interaction effects that drive a Mott transition. I will describe experiments on ultrathin, nanoporous a-Bi films that focus on how T₀ depends on the pairbreaking effects induced by magnetic impurity doping. I will discuss how the data provide strong evidence that the bosonic SIT in thin films is a Mott transition driven by Coulomb interactions that are screened by virtual quasi-particle excitations[1]. The dependence of these SITs on on underlying fermionic degrees of freedom distinguishes them from those occuring in micro-fabricated Josephson Junction Arrays, cold atom systems, and likely in high temperature superconductors with nodes in their quasiparticle density of states.
Work done in collaboration with: Xue Zhang, James Joy, Wu Chunshu, and Jimmy Xu