arXiv subjects

Dung-Hai Lee

Publications and source records attributed to Dung-Hai Lee.

At least 73 records · Page 4Linked to original sources

arXiv2014

Spin quantum Hall effects in a spin-1 topological paramagnet

AKLT state (or Haldane phase) in a spin-1 chain represents a large class of gapped topological paramagnets, which hosts symmetry-protected gapless excitations on the boundary. In this work we show how to realize this type of featureless spin-1 states on a generic two-dimensional lattice. These states have a gapped spectrum in the bulk but supports gapless edge states protected by spin rotational symmetry along a certain direction, and are featured by spin quantum Hall effect. Using fermion representation of integer-spins we show a concrete example of such spin-1 topological paramagnets on kagome lattice, and suggest a microscopic spin-1 Hamiltonian which may realize it.

cond-mat.str-el↗

arXiv2014

Quantum phase transitions between bosonic symmetry protected topological phases in two dimensions: emergent $QED_3$ and anyon superfluid

Inspired by Chern-Simons effective theory description of symmetry protected topological (SPT) phases in two dimensions, we present a projective construction for many-body wavefunctions of SPT phases. Using this projective construction we can systematically write down trial wavefunctions of SPT phases on a lattice. An explicit example of SPT phase with $U(1)$ symmetry is constructed for two types of bosons with filling $ν_{b_1}=ν_{b_2}=\frac12$ per site on square lattice. We study continuous phase transitions between different $U(1)$-SPT phases based on projective construction. The effective theory around the critical point is emergent $QED_3$ with fermion number $N_f=2$. Such a continuous phase transition however needs fine tuning, and in general there are intermediate phases between different $U(1)$-SPT phases. We show that such an intermediate phase has the same response as an anyon superconductor, and hence dub it "anyon superfluid". A schematic phase diagram of interacting bosons with $U(1)$ symmetry is depicted.

cond-mat.str-el↗

arXiv2014

Gapped symmetric edges of symmetry protected topological phases

Symmetry protected topological (SPT) phases are gapped quantum phases which host symmetry-protected gapless edge excitations. On the other hand, the edge states can be gapped by spontaneously breaking symmetry. We show that topological defects on the symmetry-broken edge cannot proliferate due to their fractional statistics. A gapped symmetric boundary, however, can be achieved between an SPT phase and certain fractionalized phases by condensing the bound state of a topological defect and an anyon. We demonstrate this by two examples in two dimensions: an exactly solvable model for the boundary between topological Ising paramagnet and double semion model, and a fermionic example about the quantum spin Hall edge. Such a hybrid structure containing both SPT phase and fractionalized phase generally support ground state degeneracy on torus.

cond-mat.str-el↗

arXiv2014

Time- and Momentum-resolved Gap Dynamics in Bi2Sr2CaCu2O8+delta

We use time- and angle-resolved photoemission spectroscopy to characterize the dynamics of the energy gap in superconducting Bi2Sr2CaCu2O8+delta (Bi2212). Photoexcitation drives the system into a nonequilibrium pseudogap state: Near the Brillouin zone diagonal (inside the normal-state Fermi arc), the gap completely closes for a pump fluence beyond F = 15 μJ/cm^2; toward the Brillouin zone face (outside the Fermi arc), it remains open to at least 24 μJ/cm^2. This strongly anisotropic gap response may indicate multiple competing ordering tendencies in Bi2212. Despite these contrasts, the gap recovers with relatively momentum-independent dynamics at all probed momenta, which shows the persistent influence of superconductivity both inside and outside the Fermi arc.

cond-mat.str-el↗

arXiv2014

Inversion symmetry protected topological insulators and superconductors

Three dimensional topological insulator represents a class of novel quantum phases hosting robust gapless boundary excitations, which is protected by global symmetries such as time reversal, charge conservation and spin rotational symmetry. In this work we systematically study another class of topological phases of weakly interacting electrons protected by spatial inversion symmetry, which generally don't support stable gapless boundary states. We classify these inversion-symmetric topological insulators and superconductors in the framework of K-theory, and construct their lattice models. We also discuss quantized response functions of these inversion-protected topological phases, which serve as their experimental signatures.

cond-mat.mes-hall↗

arXiv2014

The emergence of topologically protected surface states in epitaxial Bi(111) thin films

Quantum transport measurements including the Altshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) effects, universal conductance fluctuations (UCF), and weak anti-localization (WAL) have been carried out on epitaxial Bi thin films ($10-70$ bilayers) on Si(111). The results show that while the film interior is insulating all six surfaces of the Bi thin films are robustly metallic. We propose that these properties are the manifestation of a novel phenomenon, namely, a topologically trivial bulk system can become topologically non-trivial when it is made into a thin film. We stress that what's observed here is entirely different from the predicted 2D topological insulating state in a single bilayer Bi where only the four side surfaces should possess topologically protected gapless states.

cond-mat.mes-hall↗

arXiv2014

Doping dependence of the anisotropic quasiparticle interference in NaFe1-xCoxAs iron-based superconductors

We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in NaFe1-xCoxAs iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where four-fold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always four-fold symmetric. We argue this implies small nematic susceptibility and hence insignificant nematic fluctuation in optimally doped iron pnictides. Since Tc is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.

cond-mat.supr-con↗

arXiv2013

Signatures of superconductivity and pseudogap formation in non-equilibrium nodal quasiparticles revealed by ultrafast angle-resolved photoemission

We use time- and angle-resolved photoemission to measure the nodal non-equilibrium electronic states in various dopings of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We find that the initial pump-induced transient signal of these ungapped states is strongly affected by the onset of the superconducting gap at $T_c$, superconducting pairing fluctuations at $T_p$, and the pseudogap at $T^*$. Moreover, $T_p$ marks a suggestive threshold in the fluence-dependent transient signal, with the appearance of a critical fluence below $T_p$ that corresponds to the energy required to break apart all Cooper pairs. These results challenge the notion of a nodal-antinodal dichotomy in cuprate superconductors by establishing a new link between nodal quasiparticles and the cuprate phase diagram.

cond-mat.supr-con↗

arXiv2013

Can deeply underdoped superconducting cuprates be topological superconductors?

The nodal $d_{x^2-y^2}$ superconducting gap is a hallmark of the cuprate high T$_c$ superconductors. Surprisingly recent angle-resolved photoemission spectroscopy of deeply underdoped cuprates revealed a nodeless energy gap which is adhered to the Fermi surface. Importantly this phenomenon is observed for compounds across several different cuprate families. In this letter we propose an exciting possibility, namely the fully gapped state is a topological superconductor.

cond-mat.supr-con↗

arXiv2013

Bulk-Defect Correspondence in Particle-Hole Symmetric Insulators and Semimetals

Lattices with a basis can host crystallographic defects which share the same topological charge (e.g.~the Burgers vector $\vec b$ of a dislocation) but differ in their microscopic structure of the core. We demonstrate that in insulators with particle-hole symmetry and an odd number of orbitals per site, the microscopic details drastically affect the electronic structure: modifications can create or annihilate non-trivial bound states with an associated fractional charge. We show that this observation is related to the behavior of end modes of a dimerized chain and discuss how the end or defect states are predicted from topological invariants in these more complicated cases. Furthermore, using explicit examples on the honeycomb lattice, we explain how bound states in vacancies, dislocations and disclinations are related to each other and to edge modes and how similar features arise in nodal semimetals such as graphene.

cond-mat.mes-hall↗

arXiv2013

Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5

We have performed resistivity and inelastic neutron scattering measurements on three samples of Fe0.98-zCuzTe0.5Se0.5 with z = 0, 0.02, and 0.1. It is found that with increasing Cu doping the sample's resistivity deviates progressively from that of a metal. However, in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations, the low-energy (no larger than 12 meV) magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations. In either case, the localization of the conduction states induced by the Cu doping should play a critical role. Our results are not applicable to models that treat 3d transition metal dopants simply as effective electron donors.

cond-mat.supr-con↗

arXiv2013

Concepts relating magnetic interactions, intertwined electronic orders and strongly correlated superconductivity

Unconventional superconductivity (SC) is said to occur when Cooper pair formation is dominated by repulsive electron-electron interactions, so that the symmetry of the pair wavefunction is other than isotropic s-wave. The strong, on-site, repulsive electron-electron interactions that are the proximate cause of such superconductivity are more typically drivers of commensurate magnetism. Indeed, it is the suppression of commensurate antiferromagnetism (AF) that usually allows this type of unconventional superconductivity to emerge. Importantly, however, intervening between these AF and SC phases, intertwined electronic ordered phases of an unexpected nature are frequently discovered. For this reason, it has been extremely difficult to distinguish the microscopic essence of the correlated superconductivity from the often spectacular phenomenology of the intertwined phases. Here we introduce a model conceptual framework within which to understand the relationship between antiferromagnetic electron-electron interactions, intertwined ordered phases and correlated superconductivity. We demonstrate its effectiveness in simultaneously explaining the consequences of antiferromagnetic interactions for the copper-based, iron-based and heavy-fermion superconductors, as well as for their quite distinct intertwined phases.

cond-mat.supr-con↗

arXiv2013

Fermiology, Orbital Order, Orbital Fluctuation and Cooper Pairing in Iron-based Superconductors

We address two important issues that arise in recent studies of iron-based superconductivity. (1) Why are the T_c of A_xFe_{2-y}Se_2 and the single unit cell FeSe on SrTiO$_3$ so high despite both only have electron pockets? (2) What (if any) are the effects of orbital order and orbital fluctuation on the Cooper pairing. Our conclusions are summarized in the third paragraph of the paper.

cond-mat.supr-con↗

arXiv2013

Photoelectron spin-flipping and texture manipulation in a topological insulator

Recently discovered materials called three-dimensional topological insulators constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is the protected metallic electronic states confined to the material's surfaces. Electrons in these surface states are spin polarized with their spins governed by their direction of travel (linear momentum), resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser-based light source, we discovered another surprising property of these surface electrons which behave like Dirac fermions. We found that the spin polarization of the resulting photoelectrons can be fully manipulated in all three dimensions through selection of the light polarization. These surprising effects are due to the spin-dependent interaction of the helical Dirac fermions with light, which originates from the strong spin-orbit coupling in the material. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from the spin state of electrons in the originating initial states. The results also provide the basis for a novel source of highly spin-polarized electrons with tunable polarization in three dimensions.

cond-mat.mtrl-sci↗

arXiv2013

Critical theories of phase transition between symmetry protected topological states and their relation to the gapless boundary theories

Symmetry protected topological states (SPTs) have the same symmetry and the phase transition between them are beyond Landau's symmetry breaking formalism. In this paper we study (1) the critical theory of phase transition between trivial and non-trivial SPTs, and (2) the relation between such critical theory and the gapless boundary theory of SPTs. Based on examples of SO(3) and SU(2) SPTs, we propose that under appropriate boundary condition the critical theory contains the delocalized version of the boundary excitations. In addition, we prove that the boundary theory is the critical theory spatially confined between two SPTs. We expect these conclusions to hold in general and, in particular, for discrete symmetry groups as well.

cond-mat.str-el↗

arXiv2012

The Topological Relation Between Bulk Gap Nodes and Surface Bound States : Application to Iron-based Superconductors

In the past few years materials with protected gapless surface (edge) states have risen to the central stage of condensed matter physics. Almost all discussions centered around topological insulators and superconductors, which possess full quasiparticle gaps in the bulk. In this paper we argue systems with topological stable bulk nodes offer another class of materials with robust gapless surface states. Moreover the location of the bulk nodes determines the Miller index of the surfaces that show (or not show) such states. Measuring the spectroscopic signature of these zero modes allows a phase-sensitive determination of the nodal structures of unconventional superconductors when other phase-sensitive techniques are not applicable. We apply this idea to gapless iron based superconductors and show how to distinguish accidental from symmetry dictated nodes. We shall argue the same idea leads to a method for detecting a class of the elusive spin liquids.

cond-mat.supr-con↗

arXiv2012

Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe1-xCoxAs

Although the origin of high temperature superconductivity in the iron pnictides is still under debate, it is widely believed that magnetic interactions or fluctuations play an important role in triggering Cooper pairing. Because of the relevance of magnetism to pairing, the question of whether long range spin magnetic order can coexist with superconductivity microscopically has attracted strong interests. The available experimental methods used to answer this question are either bulk probes or local ones without control of probing position, thus the answers range from mutual exclusion to homogeneous coexistence. To definitively answer this question, here we use scanning tunneling microscopy to investigate the local electronic structure of an underdoped NaFe1-xCoxAs near the spin density wave (SDW) and superconducting (SC) phase boundary. Spatially resolved spectroscopy directly reveal both the SDW and SC gap features at the same atomic location, providing compelling evidence for the microscopic coexistence of the two phases. The strengths of the SDW and SC features are shown to anti correlate with each other, indicating the competition of the two orders. The microscopic coexistence clearly indicates that Cooper pairing occurs when portions of the Fermi surface (FS) are already gapped by the SDW order. The regime TC < T < TSDW thus show a strong resemblance to the pseudogap phase of the cuprates where growing experimental evidences suggest a FS reconstruction due to certain density wave order. In this phase of the pnictides, the residual FS has a favorable topology for magnetically mediated pairing when the ordering moment of the SDW is small.

cond-mat.supr-con↗

arXiv2012

Magnetic order tuned by Cu substitution in Fe1.1-zCuzTe

We study the effects of Cu substitution in Fe1.1Te, the non-superconducting parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at \sim 60 K without Cu, are monotonically depressed with increasing Cu content. By 10% Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of (0.5-d, 0, 0.5) with d being the incommensurability of 0.02, and correlation length of 12 angstrom along the a axis and 9 angstrom along the c axis. With 4% Cu, both transition temperatures are at 41 K, though short-range incommensurate order at (0.42, 0, 0.5) is present at 60 K. With further cooling, the incommensurability decreases linearly with temperature down to 37 K, below which there is a first order transition to a long-range almost-commensurate antiferromagnetic structure. A spin anisotropy gap of 4.5 meV is also observed in this compound. Our results show that the weakly magnetic Cu has large effects on the magnetic correlations; it is suggested that this is caused by the frustration of the exchange interactions between the coupled Fe spins.

cond-mat.supr-con↗