Department of Physics | University of Florida

Theory of superconductivity, strongly correlated Fermi systems & electronic disorder.


The Hirschfeld group studies problems of modern many-body theory associated with quantum materials. These are condensed matter systems that cannot be described by the conventional Bloch picture of a single electron moving in a periodic potential. They exhibit remarkable collective phenomena and novel ordered phases, and are expected to be relevant to the next generation of electronic devices. A particular focus is on "unconventional" superconductors, where electron pairing is driven by repulsive Coulomb interactions.


Current group: Peter, Yifu, Mainak, Miguel, Shinibali, Chandan,Laura,Yundi. View research group alumni and collaborators. Funded by U.S. Dept. of Energy and Nat'l Science Foundation.

RESEARCH HIGHLIGHTS


  • Correlations in Fe-chalcogenides

    Fe-based superconductors ( for a recent popular review , see A. Chubukov and P.J. Hirschfeld, Phys. Today 68(6), 46 (2015)) are a second class of high-temperature superconductors with properties similar to the cuprates, except their parent compounds are metallic, indicating weaker correlations. What makes the electrons form Cooper pairs? We proposed, together with J.C. Davis group, that fluctuations of the iron spins-- differentiated by selective orbital correlations-- could explain quasiparticle interference experiments on the simplest and most intriguing iron chalcogenide, FeSe. (see Sprau et al., Science 357, 75 (2017)). More recently, the prediction of the effect of these correlations on the inelastic neutron scattering spectrum ( Kreisel et al, Phys. Rev. B98, 214518 (2018)) was confirmed by the P.Dai group (Chen et al., Nat. Mat 2019).

    FeSe gap, from Sprau et al.


  • Overdoped cuprates

    It has generally been assumed that the complex cuprate phase diagram, with many different orders intertwined with superconductivity, simplifies in the overdoped limit, where Landau Fermi liquid theory and BCS (d-wave) superconductivity should reign. This view was challenged recently by measurements from the Brookhaven group showing that in high-quality LSCO films, the superfluid density ρs ~ Tc, in contradiction to the BCS prediction for a clean system, which says ρs ~n, independent of Tc. We showed (Lee-Hone et al, PRB 96, 024501 (2017), ibid 98, 054506 , arXiv:1902.08286) that this behavior emerges from the little-studied properties of out-of-plane dopant disorder. Comparison of superfluid density vs. Tc for LSCO and Tl-2201. The latter is approximately 3 times cleaner, since the dopants are located further from the CuO2 plane.
  • Pair Density Waves

    Scanning tunneling microscopy (STM) provides spectacular real-space images of the surfaces of metals, and has been used to shed light on the superconducting and normal states of high-Tc superconductors. For many years, STM images have revealed the presence of a static density wave order in BSCCO-2212 and 2201, which has been associated with ``charge order" seen by x-rays in several cuprates. There is increasing evidence , however, for the interpretation of these phenomena in terms of ``Pair Density Waves" (PDW), a modulated pairing state that drives charge order if it coexists with uniform d-wave superconductivity. We have devloped the "BdG+Wannier" method to predict sub-unit-cell conductance maps for comparison with STM images, and used it to create images of an 8 unit cell pair density wave state coexisting with d-wave superconductivity, within a Gutzwiller approximation for the t-J model (Choubey et al, NJP 19, 013028 (2017)). Predicted conductance map at bias equal to the maximum gap, compared to experiment.

     

© 2015 Peter J. Hirschfeld. All rights reserved.
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Research sponsored in part by
DOE
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