Category Archives: 2008

Theory of spin nematic to spin-Peierls quantum phase transition in ultracold spin-1 atoms in optical lattices

We present a theory of the anisotropy tuned quantum phase transition between spin nematic and spin-Peierls phases in S = 1 systems with significant bi-quadratic exchange interactions. Based on quantum Monte Carlo studies on finite size systems, it has been proposed that this phase transition is second order with new deconfined fractional excitations that are absent in either of the two phases. The possibility of a weak first order transition, however, cannot be ruled out. To elucidate the nature of the transition, we construct a large-N SO(3N) model for this phase transition and find in the N -> ∞ limit that the transition is generically of first order. Furthermore, we find a critical point in the one-dimensional (1D) limit, where two transition lines, separating spin-nematic, ferromagnetic, and spin-Peierls phases, meet. Our study indicates that the spin-nematic phase is absent in 1D, while its correlation length diverges at the critical point. Predictions for 23Na atoms trapped in an optical lattice, where the nematic to spin-Peierls quantum phase transition naturally arises, are discussed.

Christoph M. Puetter∗  and Michael J. Lawler
Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7

Hae-Young Kee†
Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7 and
School of Physics, Korea Institute for Advanced Study, Seoul 130-722, Republic of Korea

Supercurrent in nodal superconductors

In recent years, a number of nodal superconductors have been identified; d-wave superconductors
in high Tc cuprates, CeCoIn5, and -(ET)2Cu(NCS)2, 2D f-wave superconductor in Sr2RuO4 and
hybrid s+g-wave superconductor in YNi2B2C. In this work we conduct a theoretical study of nodal
superconductors in the presence of supercurrent. For simplicity, we limit ourselves to d-wave and
2D f-wave superconductors. We compute the quasiparticle density of states and the temperature
dependence of the depairing critical current in nodal superconductors, both of which are accessible
experimentally.

Igor Khavkine1, Hae-Young Kee1 and K. Maki2
1Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada
2Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-0484

Fermi pockets and quantum oscillations of the Hall coefficient in high-temperature superconductors

Recent quantum oscillation measurements in high-temperature superconductors in high magnetic fields and low temperatures have ushered in a new era. These experiments explore the normal state from which superconductivity arises and provide evidence of a reconstructed Fermi surface consisting of electron and hole pockets in a regime in which such a possibility was previously considered to be remote. More specifically, the Hall coefficient has been found to oscillate according to the Onsager quantization condition, involving only fundamental constants and the areas of the pockets, but with a sign that is negative. Here, we explain the observations with the  theory that the alleged normal state exhibits a hidden order, the d-density wave, which breaks symmetries signifying time reversal, translation by a lattice spacing, and a rotation by an angle /2, while the product of any two symmetry operations is preserved. The success of our analysis underscores the importance of spontaneous breaking of symmetries, Fermi surface reconstruction, and conventional quasiparticles. We primarily focus on the version of the order that is commensurate with the underlying crystalline lattice, but we also touch on the consequences if the order were to incommensurate. It is shown that whereas commensurate order results in two independent oscillation frequencies as a function of the inverse of the applied magnetic field, incommensurate order leads to three independent frequencies. The oscillation amplitudes, however, are determined by the mobilities of the charge carriers comprising the Fermi pockets.

Sudip Chakravarty 1 and Hae-Young Kee 2
1 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095; and
2 Department of Physics, University of Toronto, Toronto, ON, Canada M5S 1A7