Oct 2016

Oct 3-Oct 7 Max, Oct 10-Oct 14 Biao Huang, Oct 17-Oct 21 Haiyuan Zou, Oct 24-Oct 28 Ahmet Kel

Oct 21
arXiv:1610.06191 [pdf, other]
Interplay of non-symmorphic symmetry and spin-orbit coupling in hyperkagome spin liquids: Applications to Na4Ir3O8
Biao Huang, Yong Baek Kim, Yuan-Ming Lu
Comments: 27 pages, 9 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci); Superconductivity (cond-mat.supr-con)

Na4Ir3O8 provides a material platform to study three-dimensional quantum spin liquids in the geometrically frustrated hyperkagome lattice of Ir4+ ions. In this work, we consider quantum spin liquids on hyperkagome lattice for generic spin models, focusing on the effects of anisotropic spin interactions. In particular, we classify possible ℤ2 and U(1) spin liquid states, following the projective symmetry group analysis in the slave-fermion representation. There are only three distinct ℤ2 spin liquids, together with 2 different U(1)spin liquids. The non-symmorphic space group symmetry of hyperkagome lattice plays a vital role in simplifying the classification, forbidding "π-flux" or "staggered-flux" phases in contrast to symmorphic space groups. We further prove that both U(1) states and one Z2 state among all 3 are symmetry-protected gapless spin liquids, robust against any symmetry-preserving perturbations. Motivated by the "spin-freezing" behavior recently observed in Na4Ir3O8 at low temperatures, we further investigate the nearest-neighbor spin model with dominant Heisenberg interaction subject to all possible anisotropic perturbations from spin-orbit couplings. We found a U(1) spin liquid ground state with spinon fermi surfaces is energetically favored over Z2 states. Among all spin-orbit coupling terms, we show that only Dzyaloshinskii-Moriya (DM) interaction can induce spin anisotropy in the ground state when perturbing from the isotropic Heisenberg limit. Our work paves the way for a systematic study of quantum spin liquids in various materials with a hyperkagome crystal structure.

Oct 20
arXiv:1610.05810 [pdf, other]
Dynamical Chern-Simons Theory in the Brillouin Zone
Biao Lian, Cumrun Vafa, Farzan Vafa, Shou-Cheng Zhang
Comments: 13 pages, 2 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con); High Energy Physics - Theory (hep-th)

Berry connection is conventionally defined as a static gauge field in the Brillouin zone. Here we show that for three-dimensional (3d) time-reversal invariant superconductors, a generalized Berry gauge field behaves as a dynamical fluctuating field of a Chern-Simons gauge theory. The gapless nodal lines in the momentum space play the role of Wilson loop observables, while their linking and knot invariants modify the gravitational theta angle. This angle induces a topological gravitomagnetoelectric effect where a temperature gradient induces a rotational energy flow. We also show how topological strings may be realized in the 6 dimensional phase space, where the physical space defects play the role of topological D-branes.

Oct 19
arXiv:1610.05398 [pdf, other]
Many body localization in two dimensional square and triangular lattices
L. Gonzalez-Garcia, S. F. Caballero-Benitez, R. Paredes
Comments: 6 pages, 7 Figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

Ultracold interacting Bose atoms placed in disordered two dimensional optical lattices with square and triangular symmetries are found to be localized above a certain disorder strength amplitude. From a Gross-Pitaevskii mean analysis we determine the localization length as a function of the disorder strength and investigate the energy spectrum in terms of the disorder magnitude. We found that the localization length is observed to decrease faster in triangular geometries than in square ones. In the presence of a harmonic confinement localization is observed at the center of the trap. The analysis of the energy spectrum reveals that discrete energy levels acquire a finite width that is always smaller than the distance among energy levels.


Oct 18
arXiv:1610.04632 [pdf, other]
Electron Doping a Kagome Spin Liquid
Z.A. Kelly, M.J. Gallagher, T.M. McQueen
Journal-ref: Phys. Rev. X 6, 041007 13 October 2016
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Herbertsmithite, ZnCu3(OH)6Cl2, is a two dimensional kagom\'{e} lattice realization of a spin liquid, with evidence for fractionalized excitations and a gapped ground state. Such a quantum spin liquid has been proposed to underlie high temperature superconductivity and is predicted to produce a wealth of new states, including a Dirac metal at 1/3rd electron doping. Here we report the topochemical synthesis of electron-doped ZnLixCu3(OH)6Cl2 from x 0 to x 1.8 (3/5th per Cu2+). Contrary to expectations, no metallicity or superconductivity is induced. Instead, we find a systematic suppression of magnetic behavior across the phase diagram. Our results demonstrate that significant theoretical work is needed to understand and predict the role of doping in magnetically frustrated narrow band insulators, particularly the interplay between local structural disorder and tendency toward electron localization, and pave the way for future studies of doped spin liquids.

Oct 17

arXiv:1610.04300 [pdf, other]
Intertwined Order in a Frustrated 4-leg t−J Cylinder
John F. Dodaro, Hong-Chen Jiang, Steven A. Kivelson

Comments: 14 pages
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We report a density-matrix renormalization group study of the t-J model with nearest (t1 & J1) and next-nearest (t2 & J2) interactions on a 4-leg cylinder with concentration δ=1/8 of doped holes. We observe an astonishingly complex interplay between uniform d-wave superconductivity (SC) and strong spin and charge density wave ordering tendencies (SDW and CDW). Depending on parameters, the CDWs can be commensurate with period 4 or 8. By comparing the charge ordering vectors with 2kF, we rule out Fermi surface nesting-induced density wave order in our model. Magnetic frustration (i.e. J2/J1∼1/2) significantly quenches SDW correlations with little effect on the CDW. Typically, the SC order is strongly modulated at the CDW ordering vector and exhibits d-wave symmetry around the cylinder. There is no evidence of a near-degenerate tendency to pair-density wave (PDW) ordering, charge 4e SC, or orbital current order.