Daoxin Yao

Spin waves in striped phases

(1) Dept. of Physics, Purdue University, West Lafayette, IN 47907(2) Dept. of Physics and Dept. of Electrical and Computer Engineering, Boston University, Boston, MA 02215(Dated: February 2, 2008)In many antiferromagnetic, quasi-two-dimensional materials, doping with holes leads to “stripe”phases, in which the holes congregate along antiphase domain walls in the otherwise antiferromag-netic texture. Using a suitably parametrized two-dimensional Heisenberg model on a square lattice,we study the spin wave spectra of well-ordered spin stripes, comparing bond-centered antiphasedomain walls to site-centered antiphase domain walls for a range of spacings between the stripesand for stripes both aligned with the lattice (“vertical”) and oriented along the diagonals of thelattice (“diagonal”). Our results establish that there are qualitative differences between the ex-pected neutron scattering responses for the bond-centered and site-centered cases. In particular,bond-centered stripes of odd spacing generically exhibit more elastic peaks than their site-centeredcounterparts. For inelastic scattering, we find that bond-centered stripes produce more spin wavebands than site-centered stripes of the same spacing and that bond-centered stripes produce ratherisotropic low energy spin wave cones for a large range of parameters, despite local microscopicanisotropy. We find that extra scattering intensity due to the crossing of spin wave modes (whichmay be linked to the “resonance peak” in the cuprates) is more likely for diagonal stripes, whethersite- or bond-centered, whereas spin wave bands generically repel, rather than cross, when stripesare vertical.I. INTRODUCTION

Distinguishing s(+/-) and s(++) electron pairing symmetries by neutron spin resonance in superconducting NaFe0.935Co0.045As

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechanism for superconductivity. For iron pnictide superconductors, the s(+/-)-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies (E <= 2 Delta) below T-c. On the other hand, the s(++)-pairing symmetry expects a broad spin excitation enhancement at an energy above 2 Delta below Tc. Although the resonance has been observed in iron pnictide superconductors at an energy below 2 Delta consistent with the s(+/-)-pairing symmetry, the mode has also been interpreted as arising from the s++-pairing symmetry with E <= 2 Delta due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E = 7 meV in SC NaFe0.935Co0.045As (T-c = 18 K). On warming towards Tc, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitation spectra within the s(+/-) and s++-pairing symmetries, we conclude that the ground-state resonance in NaFe0.935Co0.045As is only consistent with the s(+/-) pairing, and is inconsistent with the s(++)-pairing symmetry.

Magnetic Excitations of Stripes near a Quantum Critical Point

We calculate the dynamical spin structure factor of spin waves for weakly coupled stripes. At low energy, the spin-wave cone intensity is strongly peaked on the inner branches. As energy is increased, there is a saddlepoint followed by a square-shaped continuum rotated 45 degrees from the low energy peaks. This is reminiscent of recent high energy neutron scattering data on the cuprates. The similarity at high energy between this semiclassical treatment and quantum fluctuations in spin ladders may be attributed to the proximity of a quantum critical point with a small critical exponent eta.

Quantum phase transition in the four-spin exchange antiferromagnet

We study the S=1/2 Heisenberg antiferromagnet on a square lattice with nearest-neighbor and plaquette four- spin exchanges (introduced by A.W. Sandvik, Phys. Rev. Lett. 98, 227202 (2007).) This model undergoes a quantum phase transition from a spontaneously dimerized phase to Neel order at a critical coupling. We show that as the critical point is approached from the dimerized s ide, the system exhibits strong fluctuations in the dimer background, reflected in the presence of a low-energy s inglet mode, with a simultaneous rise in the triplet quasiparticle density. We find that both singlet and triplet modes of high density condense at the transition, signaling restoration of lattice symmetry. In our approach, which goes beyond mean-field theory in terms of the triplet excitations, the transition appears sharp; howeve r since our method breaks down near the critical point, we argue that we cannot make a definite conclusion regarding t he order of the transition.

Properties of the random-singlet phase: From the disordered Heisenberg chain to an amorphous valence-bond solid

We use a strong-disorder renormalization group (SDRG) method and ground-state quantum Monte Carlo (QMC) simulations to study

Reply to “Comment on ‘Quantum phase transition in the four-spin exchange antiferromagnet’ ”

S=1/2

Spin-wave dispersion in half-doped La3/2Sr1/2NiO4

spin chains with random couplings, calculating disorder-averaged spin and dimer correlations. The QMC simulations demonstrate logarithmic corrections to the power-law decaying correlations obtained with the SDRG scheme. The same asymptotic forms apply both for systems with standard Heisenberg exchange and for certain multispin couplings leading to spontaneous dimerization in the clean system. We show that the logarithmic corrections arise in the valence-bond (singlet pair) basis from a contribution that cannot be generated by the SDRG scheme. In the model with multispin couplings, where the clean system dimerizes spontaneously, random singlets form between spinons localized at domain walls in the presence of disorder. This amorphous valence-bond solid is asymptotically a random-singlet state and only differs from the random-exchange Heisenberg chain in its short-distance properties.

Quantum phase transitions in disordered dimerized quantum spin models and the Harris criterion

We argue that our analysis of the J-Q model, presented in Phys. Rev. B 80, 174403 (2009), and based on a field-theory description of coupled dimers, captures properly the strong quantum fluctuations tendencies, and the objections outlined by L. Isaev, G. Ortiz, and J. Dukelsky, arXiv:1003.5205, are misplaced.

Magnetic excitations of stripes and checkerboards in the cuprates

Recent neutron scattering measurements reveal spin and charge ordering in the half-doped nickelate,

Strong ferromagnetic exchange interaction under ambient pressure in BaFe2S3

{\mathrm{La}}_{3∕2}{\mathrm{Sr}}_{1∕2}{\mathrm{NiO}}_{4}