arXiv subjects

Hong Yao

Publications and source records attributed to Hong Yao.

At least 163 records · Page 9Linked to original sources

arXiv2011

Fermi-surface reconstruction in a smectic phase of a high temperature superconductor

It is shown that, in the presence of a moderately strong C_4 symmetry breaking (which could be produced either by lattice orthorhombicity or the presence of an electron nematic phase), a weak, period 4, unidirectional charge density wave ("charge stripe") order can reconstruct the Fermi surface of a typical hole-doped cuprate to produce a small electron pocket. This form of charge density wave order is consistent with that adduced from recent high field NMR experiments in YBCO. The Fermi pocket has an area and effective mass which is a rough caricature of those seen in recent high field quantum oscillation experiments.

cond-mat.str-el↗

arXiv2011

Fermionic magnons, non-Abelian spinons, and spin quantum Hall effect from an exactly solvable Kitaev Hamiltonian with SU(2) symmetry

We introduce an exactly solvable SU(2) symmetric Kitaev model with exotic spin excitations. With time reversal symmetry (TRS), the ground state is a spin liquid with gapless or gapped spin-1 but fermionic excitations. When TRS is broken, the resulting spin liquid exhibits deconfined vortex excitations which carry spin-1/2 quantum number and obey non-Abelian statistics. We show that this SU(2) invariant non-Abelian spin liquid exhibits spin quantum Hall effect with quantized spin Hall conductivity σ^s_{xy}=\hbar/2π, and that the spin response is effectively described the SO(3) level-1 Chern-Simons theory at low energy. We further propose that a SU(2) level-2 Chern-Simons theory is the effective field theory describing the topological structure of the non-Abelian SU(2) invariant spin liquid.

cond-mat.str-el↗

arXiv2011

Hidden itinerant-spin phase in heavily-overdoped La2-xSrxCuO4 revealed by dilute Fe doping: A combined neutron scattering and angle-resolved photoemission study

We demonstrated experimentally a direct way to probe a hidden propensity to the formation of spin density wave (SDW) in a non-magnetic metal with strong Fermi surface nesting. Substituting Fe for a tiny amount of Cu (1%) induced an incommensurate magnetic order below 20 K in heavily-overdoped La2-xSrxCuO4 (LSCO). Elastic neutron scattering suggested that this order cannot be ascribed to the localized spins on Cu or doped Fe. Angle-resolved photoemission spectroscopy (ARPES), combined with numerical calculations, revealed a strong Fermi surface nesting inherent in the pristine LSCO that likely drives this order. The heavily-overdoped Fe-doped LSCO thus represents the first plausible example of the long-sought "itinerant-spin extreme" of cuprates, where the spins of itinerant doped holes define the magnetic ordering ground state. This finding complements the current picture of cuprate spin physics that highlights the predominant role of localized spins at lower dopings. The demonstrated set of methods could potentially apply to studying hidden density-wave instabilities of other "nested" materials on the verge of density wave ordering.

cond-mat.supr-con↗

arXiv2011

Topological insulators and topological non-linear sigma models

In this paper we link the physics of topological nonlinear σ models with that of Chern-Simons insulators. We show that corresponding to every 2n-dimensional Chern-Simons insulator there is a (n-1)-dimensional topological nonlinear σ model with the Wess-Zumino-Witten term. Breaking internal symmetry in these nonlinear σ models leads to nonlinear σ models with the θ term. [This is analogous to the dimension reduction leading from 2n-dimensional Chern-Simons insulators to (2n-1) and (2n-2)-dimensional topological insulators protected by discrete symmetries.] The correspondence described in this paper allows one to derive the topological term in a theory involving fermions and order parameters (we shall referred to them as "fermion-σ models") when the conventional gradient-expansion method fails. We also discuss the quantum number of solitons in topological nonlinear σ model and the electromagnetic action of the (2n-1)-dimensional topological insulators. Throughout the paper we use a simple model to illustrate how things work.

cond-mat.str-el↗

arXiv2011

Exact Chiral Spin Liquid with Stable Spin Fermi Surface on the Kagome Lattice

We study an exactly solvable quantum spin model of Kitaev type on the kagome lattice. We find a rich phase diagram which includes a topological (gapped) chiral spin liquid with gapless chiral edge states, and a gapless chiral spin liquid phase with a spin Fermi surface. The ground state of the current model contains an odd number of electrons per unit cell which qualitatively distinguishes it from previously studied exactly solvable models with a spin Fermi surface. Moreover, we show that the spin Fermi surface is stable against weak perturbations.

cond-mat.str-el↗

arXiv2011

The spin excitations of the block-antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_2$

We study the spin excitations of the newly discovered block-antiferromagnetic state in K$_{0.8}$Fe$_{1.6}$Se$_2$ using an effective spin Hamiltonian suggested in the literature. Interestingly in addition to the usual Goldstone mode, there exists three other "optical" spin wave branches. These spin excitations are the analog of "optical phonons" in crystals with more than one atom per unit cell. We compute the spin wave corrected ordering moment and the uniform spin susceptibilities.

cond-mat.supr-con↗

arXiv2011

Phases of the infinite U Hubbard model

We apply the density matrix renormalization group (DMRG) to study the phase diagram of the infinite U Hubbard model on 2-, 4-, and 6-leg ladders. Where the results are largely insensitive to the ladder width, we consider the results representative of the 2D square lattice model. We find a fully polarized ferromagnetic Fermi liquid phase when n, the density of electrons per site, is in the range 1>n>n_F ~ 4/5. For n=3/4 we find an unexpected commensurate insulating "checkerboard" phase with coexisting bond density order with 4 sites per unit cell and block spin antiferromagnetic order with 8 sites per unit cell. For 3/4 > n, the wider ladders have unpolarized groundstates, which is suggestive that the same is true in 2D.

cond-mat.str-el↗

arXiv2011

From a single-band metal to a high-temperature superconductor via two thermal phase transitions

The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally-doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (Tc), entangled in an energy-momentum-dependent fashion with the pre-existing pseudogap features, ushering in a ground state with coexisting orders.

cond-mat.supr-con↗

arXiv2010

Symmetry fractional quantization in two dimensions

We introduce a solvable spin-rotational and time-reversal invariant spin-1 model in two dimensions. Depending on parameters, the ground state is an equal-weight superposition of all valence loops called "resonating valence loop" (RVL) or an equal-weight superposition of all valence bonds known as "resonating valence bond" (RVB). In RVL, ends of open loops are deconfined spinons of spin-1/2 that cannot be obtained by simple combinations of spin-1 -- a phenomenon of fractionalization; while in RVB, all quasiparticles carry an integer spin, hence no fractionalization. RVL and RVB are spin liquids with identical topological order but different spin-rotational and time-reversal symmetry quantum numbers of quasiparticles. We propose that quantized symmetry quantum number gives a systematic way to (partially) classify phases with identical topological order in dimensions greater than one.

cond-mat.str-el↗

arXiv2010

Doping dependence of the $(π,π)$ shadow band in La-based cuprates studied by angle-resolved photoemission spectroscopy

The $(π,π)$ shadow band (SB) in La-based cuprate family (La214) was studied by angle-resolved photoemission spectroscopy (ARPES) over a wide doping range from $x=0.01$ to $x=0.25$. Unlike the well-studied case of the Bi-based cuprate family, an overall strong, monotonic doping dependence of the SB intensity at the Fermi level ($E_F$) was observed. In contrast to a previous report for the presence of the SB only close to $x=1/8$, we found it exists in a wide doping range, associated with a doping-independent $(π,π)$ wave vector but strongly doping-dependent intensity: It is the strongest at $x\sim 0.03$ and systematically diminishes as the doping increases until it becomes negligible in the overdoped regime. This SB with the observed doping dependence of intensity can in principle be caused by the antiferromagnetic fluctuations or a particular form of low-temperature orthorhombic lattice distortion known to persist up to $x\sim 0.21$ in the system, with both being weakened with increasing doping. However, a detailed binding energy dependent analysis of the SB at $x=0.07$ does not appear to support the former interpretation, leaving the latter as a more plausible candidate, despite a challenge in quantitatively linking the doping dependences of the SB intensity and the magnitude of the lattice distortion. Our finding highlights the necessity of a careful and global consideration of the inherent structural complications for correctly understanding the cuprate Fermiology and its microscopic implication.

cond-mat.supr-con↗

arXiv2010

Fragile Mott Insulators

We prove that there exists a class of crystalline insulators, which we call "fragile Mott insulators" which are not adiabatically connected to any sort of band insulator provided time-reversal and certain point-group symmetries are respected, but which are otherwise unspectacular in that they exhibit no topological order nor any form of fractionalized quasiparticles. Different fragile Mott insulators are characterized by different nontrivial one-dimensional representations of the crystal point group. We illustrate this new type of insulators with two examples: the d-Mott insulator discovered in the checkerboard Hubbard model at half-filling and the Affleck-Kennedy-Lieb-Tasaki insulator on the square lattice.

cond-mat.str-el↗

arXiv2010

Entanglement entropy and entanglement spectrum of the Kitaev model

In this paper, we obtain an exact formula for the entanglement entropy of the ground state and all excited states of the Kitaev model. Remarkably, the entanglement entropy can be expressed in a simple separable form S=S_G+S_F, with S_F the entanglement entropy of a free Majorana fermion system and S_G that of a Z_2 gauge field. The Z_2 gauge field part contributes to the universal "topological entanglement entropy" of the ground state while the fermion part is responsible for the non-local entanglement carried by the Z_2 vortices (visons) in the non-Abelian phase. Our result also enables the calculation of the entire entanglement spectrum and the more general Renyi entropy of the Kitaev model. Based on our results we propose a new quantity to characterize topologically ordered states--the capacity of entanglement, which can distinguish the states with and without topologically protected gapless entanglement spectrum.

cond-mat.str-el↗

arXiv2010

Spontaneous symmetry breakings in two-dimensional kagome lattice

We study spontaneous symmetry breakings for fermions (spinless and spinful) on a two-dimensional kagome lattice with nearest-neighbor repulsive interactions in weak coupling limit, and focus in particular on topological Mott insulator instability. It is found that at $\frac{1}{3}$-filling where there is a quadratic band crossing at $Γ$-point, in agreement with Ref. 1, the instabilities are infinitesimal and topological phases are dynamically generated. At $\frac{2}{3}$-filling where there are two inequivalent Dirac points, the instabilities are finite, and no topological phase is favored at this filling without breaking the lattice translational symmetry. A ferromagnetic quantum anomalous Hall state with infinitesimal instability is further proposed at half-filling of the bottom flat band.

cond-mat.mtrl-sci↗

arXiv2010

Composite Fermions and Quantum Hall Stripes on the Topological Insulator Surface

We study the problem of a single Dirac fermion in a quantizing orbital magnetic field, when the chemical potential is at the Dirac point. This can be realized on the surface of a topological insulator, such as Bi2Se3, tuned to neutrality. We study the effect of both long range Coulomb interactions (strength alpha=e^2/(epsilon hbar v_F).) and local repulsion U which capture the effect of electron correlations. Interactions resolve the degeneracy of free fermions in the zeroth Landau level at half filling, but in a manner different from that in graphene. For weak interactions, U=0 and alpha<<1, a composite Fermi liquid is expected. However, in the limit of strong local correlations (large U but alpha<<1), a charge density wave phase is predicted, which we term "axion stripe". While reminiscent of quantum Hall stripe phases, its wavelength is parametrically larger than the magnetic length, and the induced fermion mass term (axion) also oscillates with the charge density. This phase is destroyed by sufficiently strong Zeeman coupling. A phase diagram is constructed and consequences for experiments are discussed.

cond-mat.str-el↗

arXiv2009

Nature of a topological quantum phase transition in a chiral spin liquid model

We study the finite temperature nature of a quantum phase transition between an Abelian and a non-Abelian topological phase in an exactly solvable model of a chiral spin liquid. By virtue of the exact solvability, this model can serve as a testbed for developing better measures for describing topological quantum phase transitions. We characterize this phase transition in terms of the global flux and entanglement entropy, and discuss to what extent the existence of a topologically ordered ground state with non-Abelian excitations is revealed at finite temperature.

cond-mat.str-el↗

arXiv2009

Topological Insulators and Nematic Phases from Spontaneous Symmetry Breaking in 2D Fermi Systems with a Quadratic Band Crossing

We investigate the stability of a quadratic band-crossing point (QBCP) in 2D fermionic systems. At the non-interacting level, we show that a QBCP exists and is topologically stable for a Berry flux $\pm 2π$, if the point symmetry group has either fourfold or sixfold rotational symmetries. This putative topologically stable free-fermion QBCP is marginally unstable to {\em arbitrarily weak} short-range repulsive interactions. We consider both spinless and spin-1/2 fermions. Four possible ordered states result: a quantum anomalous Hall phase, a quantum spin Hall phase, a nematic phase, and a nematic-spin-nematic phase.

cond-mat.str-el↗

arXiv2008

Fe-based superconductors: unity or diversity?

Does the high temperature superconductivity observed in the newly discovered iron-pnictide materials represent another example of the same essential physics responsible for superconductivity in the cuprates, or does it embody a new mechanism?

cond-mat.str-el↗

arXiv2008

Algebraic spin liquid in an exactly solvable spin model

We have proposed an exactly solvable quantum spin-3/2 model on a square lattice. Its ground state is a quantum spin liquid with a half integer spin per unit cell. The fermionic excitations are gapless with a linear dispersion, while the topological "vison" excitations are gapped. Moreover, the massless Dirac fermions are stable. Thus, this model is, to the best of our knowledge, the first exactly solvable model of half-integer spins whose ground state is an "algebraic spin liquid."

cond-mat.str-el↗