Quantum Phase Transitions

Superconductor-insulator transition

When a superconducting material is deposited as a thin film, it may behave as an insulator due to disorder and quantum fluctuations. The nature of this superconductor-insulator transition (SIT) is still not completely understood. According to one school of thought, the Cooper pairs break up into their constituent fermions leading to a metallic phase near the SIT. An alternative viewpoint holds that the Cooper pairs remain bound as “composite bosons”, and that the SIT is caused by phase fluctuations that destroy long-range phase coherence between superfluid patches. We have employed determinantal quantum Monte Carlo for the attractive U fermionic Hubbard model with disorder. We have further coarse grained the emergent granularity and exploited the finite single particle gap (as seen in the figure on the right) to develop an effective bosonic XY model with disorder that was also simulated using QMC techniques. These state-of-the art numerical methods give information about dynamical quantities across the disorder tuned superconductor-insulator and have addressed many of the outstanding problems in this field.

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Metal-insulator transition

It is well established that increasing disorder in a metal with non-interacting electrons drives it into a gapless localized Anderson insulator. In an interacting system the nature of the phases and the metal-insulator transition remains unresolved. Here we explore the effect of disorder without changing the carrier concentration on a strongly correlated Mott insulator by Cu intercalation of single crystals of the layered dichalcogenide 1T-TaS2. Angle resolved photoemission spectroscopy (ARPES) measurements reveal, rather unexpectedly, that increasing disorder introduces delocalized states within the Mott gap that lead to a finite conductivity, defying conventional wisdom. Our results not only provide the first experimental realization of a disorder-induced metallic state but in addition also reveal that the metal is a non-Fermi liquid with a pseudogap in the ground state that persists at finite temperatures. Detailed theoretical analysis of the disordered Hubbard model shows that interplay of strong interaction and disorder generates the novel metal.

Publications:

  • E. Lahoud, O. N. Meetei, K.B. Chaska, A. Kanigel and N. Trivedi “Emergent Novel Metallic State in a Disordered 2D Mott Insulator,” arXiv 1309:0649

Larkin-Ovchinnikov Physics in Superconducting Films in Zeeman Fields

In a superconducting film in a Zeeman field, the competition between pairing and spin polarization drives a quantum phase transition from a superconducting state to a normal state. This is usually assumed to be a first-order transition occurring at the Chandrasekhar-Clogston field hCC = 0.71 Δ0. However, the transition may actually proceed via a Larkin-Ovchinnikov state, even in the presence of disorder. A disordered Larkin-Ovchinnikov state is characterized by microscale coexistence of pairing and polarization: a sufficiently large Zeeman field induces magnetization in irregular domain walls, at which the pairing order parameter changes sign. In addition, we have found that the spectral function (density of states) contains weight within the gap due to the “emergent magnetic impurities” at the domain walls. The spectral function is important because it is closely related to the tunneling conductance, which is an important experimental observable. We propose that “dirty LO physics” may help to explain the excess zero-bias conductance in recent experiments.

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