arXiv subjects

Dung-Hai Lee

Publications and source records attributed to Dung-Hai Lee.

At least 91 records · Page 5Linked to original sources

arXiv2012

Fractionalized topological insulators from frustrated spin models in three dimensions

We present a theory of three dimensional fractionalized topological insulators in the form of U(1) spin liquids with gapped fermionic spinons in the bulk and topologically protected gapless spinon surface states. Starting from a spin-1/2 model on a pyrochlore lattice, with frustrated antiferromagnetic and ferromagnetic exchange interactions, we show that decomposition of the latter interactions, within slave-fermion representation of the spins, can naturally give rise to an emergent spin-orbit coupling for the spinons. This stabilizes a fractionalized topological insulators which also have bulk bond spin-nematic order. Finally, we describe the low energy properties of these states.

cond-mat.str-el↗

arXiv2012

Tracking Cooper Pairs in a Cuprate Superconductor by Ultrafast Angle-Resolved Photoemission

In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We use a femtosecond laser pump pulse to perturb superconducting Bi2Sr2CaCu2O8+δ, and study subsequent dynamics using time- and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics reveal marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap is sensitive to the pump intensity and Cooper pairs recombine slowly. Far from the nodes pumping affects the gap only weakly and recombination processes are faster. These results demonstrate a new window into the dynamical processes that govern quasiparticle recombination and gap formation in cuprates.

cond-mat.supr-con↗

arXiv2012

High $T_c$ superconductivity at the FeSe/SrTiO$_3$ Interface

In a recent experiment the superconducting gap of a single unit cell thick FeSe film on SrTiO$_3$ substrate is observed by scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. The value of the superconducting gap is much larger than that of the bulk FeSe under ambient pressure. In this paper we study the effects of screening due to the ferroelectric phonons on Cooper pairing. We conclude it can significantly enhance the energy scale of Cooper pairing and even change the pairing symmetry. Our results also raise some concerns on whether phonons can be completely ignored for bulk iron-based superconductors.

cond-mat.supr-con↗

arXiv2012

Functional renormalization group and variational Monte Carlo studies of the electronic instabilities in graphene near 1/4 doping

We study the electronic instabilities of near 1/4 electron doped graphene using the functional renormalization group (FRG) and variational Monte-Carlo method. A modified FRG implementation is utilized to improve the treatment of the von Hove singularity. At 1/4 doping the system is a chiral spin density wave state exhibiting the anomalous quantized Hall effect, or equivalently a Chern insulator. When the doping deviates from 1/4, the $d_{x^2-y^2}+i d_{xy}$ Cooper pairing becomes the leading instability. Our results suggest near 1/4 electron or hole doped graphene is a fertile playground for the search of Chern insulators and superconductors.

cond-mat.supr-con↗

arXiv2012

Time reversal invariant topological superconductivity in correlated non-centrosymmetric systems

Using functional renormalization group method, we study the favorable condition for electronic correlation driven time reversal invariant topological superconductivity in symmetry class DIII. For non-centrosymmetric systems we argue that the proximity to ferromagnetic (or small wavevector magnetic) instability can be used as a guideline for the search of this type of superconductivity. This is analogous to the appearance of singlet unconventional superconductivity in the neighborhood of antiferromagnetic instability. We show three concrete examples where ferromagnetic-like fluctuation leads to topological pairing.

cond-mat.supr-con↗

arXiv2011

Widespread spin polarization effects in photoemission from topological insulators

High resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi$_2$Se$_3$ using a recently developed high-efficiency spectrometer. The topological surface state's helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.

cond-mat.mtrl-sci↗

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

Topological insulators on a Mobius Strip

We study the two dimensional Chern insulator and spin Hall insulator on a non-orientable Riemann surface, the Mobius strip, where the usual bandstructure topological invariant is not defined. We show that while the flow pattern of edge currents can detect the twist of the Mobius strip in the case of Chern insulator, it can not do so in spin Hall insulator.

cond-mat.mes-hall↗

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

The effects of interaction on quantum spin Hall insulators

We study the $S_z$-conserving quantum spin Hall insulator in the presence of Hubbard $U$ from a field theory point view. The main findings are the following. (1) For arbitrarily small U the edges possess power-law correlated antiferromagnetic XY local moments. Gapless charge excitations arise from the Goldstone-Wilczek mechanism. (2) The decaying exponent of the XY correlation is $\ge 2$ hence normally space-time vortices should proliferate. (3) For thermodynamic samples, vortex proliferation is prohibited by the conservation of edge charge. (4) For a sample with finite width electron tunneling between opposite edges allows vortex proliferation hence is a strongly relevant perturbation (more relevant than in the non-interacting theory). (5) The phase transition between the antiferromagnetic and the topological insulating phases is triggered by the condensation of magnetic excitons.

cond-mat.str-el↗

arXiv2011

The electronic instabilities of the Iron-based superconductors: a variational Monte-Carlo study

We report the first variational Monte Carlo (VMC) study of the iron-based superconductors. We use realistic band structures, and the ordering instabilities/variational ansatzs are suggested by previous functional renormalization group calculations. We examine antiferromagnetism, superconducting pairing, normal state fermi surface distortion, and orbital order in the antiferromagnetic state.

cond-mat.str-el↗

arXiv2011

Random walk approach to spin dynamics in a two-dimensional electron gas with spin-orbit coupling

We introduce and solve a semi-classical random walk (RW) model that describes the dynamics of spin polarization waves in zinc-blende semiconductor quantum wells. We derive the dispersion relations for these waves, including the Rashba, linear and cubic Dresselhaus spin-orbit interactions, as well as the effects of an electric field applied parallel to the spin polarization wavevector. In agreement with fully quantum mechanical calculations [Kleinert and Bryksin, Phys. Rev. B \textbf{76}, 205326 (2007)], the RW approach predicts that spin waves acquire a phase velocity in the presence of the field that crosses zero at a nonzero wavevector, $q_0$. In addition, we show that the spin-wave decay rate is independent of field at $q_0$ but increases as $(q-q_0)^2$ for $q\neq q_0$. These predictions can be tested experimentally by suitable transient spin grating experiments.

cond-mat.mes-hall↗

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

Topological term in the non-linear $σ$ model of the SO(5) spin chains

We show that there is a topological (Berry phase) term in the non-linear $σ$ model description of the SO(5) spin chain. It distinguishes the linear and projective representations of the SO(5) symmetry group, in exact analogy to the well-known $θ$-term of the SO(3) spin chain. The presence of the topological term is due to the fact that $π_2(\frac{SO(5)}{SO(3)\times SO(2)})= \mathbb{Z}$. We discuss the implication of our results on the spectra of the SO(5) spin chain, and connect it with a recent solvable SO(5) spin model which exhibits valence bond solid ground state and edge degeneracy.

cond-mat.str-el↗

arXiv2011

Continuous quantum phase transition between two topologically distinct valence bond solid states associated with the same spin value

We propose a simple one-dimensional spin-2 Hamiltonian, which exhibits two topologically distinct valence bond solid states in different exactly solvable limits. We then construct the phase diagram and study the quantum phase transition between these states using the infinite time evolving block decimation algorithms. From the scaling relation between the entanglement entropy and correlation length, we find that the central charge for the underlying critical conformal field theory is $c=2$.

cond-mat.str-el↗

arXiv2010

Nodes in the Gap Function of LaFePO, the Gap Function of the Fe(Se,Te) Systems, and the STM Signature of the s$_{\pm}$ Pairing

We reiterate, in more details, our previous proposal of using quasi-particle interference to determine the pairing form factor in iron-based superconductors. We also present our functional renormalization group(FRG) results on LaFePO and Fe(Se,Te) superconductors. In particular we found that the leading pairing channel in LaFePO is nodal s$_{\pm}$, with nodes on electron Fermi surfaces. For Fe(Se,Te) system we found fully gapped s$_{\pm}$ pairing, with substantial gap anisotropy on electron Fermi surfaces, and large gap is concentrated in regions with dominant $xy$ orbital character. We further fit the form factor obtained by FRG to real space orbital basis pairing picture, which shows more clearly the differences between different iron-based superconductors.

cond-mat.supr-con↗

arXiv2009

Field-induced Kosterlitz-Thouless transition in the N=0 Landau level of graphene

At the charge neutral point, graphene exhibits a very unusual high resistance metallic state and a transition to a complete insulating phase in a strong magnetic field. We propose that the current carriers in this state are the charged vortices of the XY valley pseudospin order-parameter, a situation which is dual to a conventional thin superconducting film. We study energetics and the stability of this phase in the presence of disorder.

cond-mat.mes-hall↗