Spin-Gap Proximity Effect Mechanism of High Temperature Superconductivity

When holes are doped into an antiferromagnetic insulator they form a slowly fluctuating array of ``topological defects'' (metallic stripes) in which the motion of the holes exhibits a self-organized quasi one-dimensional electronic character. The accompanying lateral confinement of the intervening Mott-insulating regions induces a spin gap or pseudogap in the environment of the stripes. We present a theory of underdoped high temperature superconductors and show that there is a {\it local} separation of spin and charge, and that the mobile holes on an individual stripe acquire a spin gap via pair hopping between the stripe and its environment; i.e. via a magnetic analog of the usual superconducting proximity effect. In this way a high pairing scale without a large mass renormalization is established despite the strong Coulomb repulsion between the holes. Thus the {\it mechanism} of pairing is the generation of a spin gap in spatially-confined {\it Mott-insulating} regions of the material in the proximity of the metallic stripes. At non-vanishing stripe densities, Josephson coupling between stripes produces a dimensional crossover to a state with long-range superconducting phase coherence. This picture is established by obtaining exact and well-controlled approximate solutions of a model of a one-dimensional electron gas in an active environment. An extended discussion of the experimental evidence supporting the relevance of these results to the cuprate superconductors is given.Comment: 30 pages, 2 figure

Adiabatic Transport in the Fractional Quantum Hall Effect Regime

Wetensch. publicati

Ultrasound studies of U2Zn17 and UCu5

We present here resonant ultrasound spectroscopy measurements of the elastic moduli and attenuation for U2Zn17 and UCu5 through the respective TN's of 9.7 and 15 K. For single-crystal U2Zn17, the data are the first modulus measurements for this material, and exhibit a very large softening of the sound velocity and an anomalous increase in attenuation below TN, while no precursors at all are observed above. Polycrystalline UCu5, in contrast, exhibits weak softening above TN, and a weak stiffening below. © 1994

Photocurrent-driven transient symmetry breaking in the Weyl semimetal TaAs

Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry changes a critical step in developing future technologies that rely on such control. Topological materials, like topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second harmonic generation spectroscopy as a sensitive probe of symmetry changes, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast timescales