arXiv subjects

Dung-Hai Lee

Publications and source records attributed to Dung-Hai Lee.

At least 109 records · Page 6Linked to original sources

arXiv2009

Localization of Metal-Induced Gap States at the Metal-Insulator Interface:Origin of Flux Noise in SQUIDs and Superconducting Qubits

The origin of magnetic flux noise in Superconducting Quantum Interference Devices with a power spectrum scaling as $1/f$ ($f$ is frequency) has been a puzzle for over 20 years. This noise limits the decoherence time of superconducting qubits. A consensus has emerged that the noise arises from fluctuating spins of localized electrons with an areal density of $5\times10^{17}$m$^{-2}$. We show that, in the presence of potential disorder at the metal-insulator interface, some of the metal-induced gap states become localized and produce local moments. A modest level of disorder yields the observed areal density.

cond-mat.supr-con↗

arXiv2009

The surface states of topological insulators - Dirac fermion in curved two dimensional spaces

The surface of a topological insulator is a closed two dimensional manifold. The surface states are described by the Dirac Hamiltonian in curved two dimensional spaces. For a slab-like sample with a magnetic field perpendicular to its top and bottom surfaces, there are chiral states delocalized on the four side faces. These "chiral sheets" carry both charge and spin currents. In strong magnetic fields the quantized charge Hall effect ($\s_{xy}=(2n+1)e^2/h$) will coexist with spin Hall effect.

cond-mat.mes-hall↗

arXiv2009

Antiferromagnetically Driven Electronic Correlation in Iron Pnictides and Cuprates

The iron pnictides and the cuprates represent two families of materials, where strong antiferromagnetic correlation drives three other distinct ordering tendencies: (1) superconducting pairing, (2) Fermi surface distortion, and (3) orbital current order. We propose that (1)-(3) and the antiferromagnetic correlation are the hallmarks of a class of strongly correlated materials to which the cuprates and pnictides belong. In this paper we present the results of the functional renormalization group studies to support the above claim. In addition, we show that as a function of the interlayer hopping parameter, the double layer Hubbard model nicely interpolates between the cuprate and the iron pnictide physics. Finally, as a check, we will present the renormalization group study of a ladder version of the iron pnictide, and compare the results to those of the two-dimensional model.

cond-mat.supr-con↗

arXiv2009

Nodal Spin Density Wave and band topology of the FeAs based materials

The recently discovered FeAs-based materials exhibit a $(π,0)$ Spin Density Wave (SDW) in the undoped state, which gives way to superconductivity upon doping. Here we show that due to an interesting topological feature of the band structure, the SDW state cannot acquire a full gap. This is demonstrated within the SDW mean-field theory of both a simplified two band model and a more realistic 5-band model. The positions of the nodes are different in the two models and can be used to detected the validity of each model.

cond-mat.supr-con↗

arXiv2009

The quantum valley Hall effect in proximity-induced superconducting graphene: an experimental window for deconfined quantum criticality

We argue that by inducing superconductivity in graphene via the proximity effect, it is possible to observes the "quantum valley Hall effect". In the presence of magnetic field, supercurrent causes "valley pseudospin" to accumulate at the edges of the superconducting strip. This, and the structure of the superconducting vortex core, provide possibilities to experimentally observe aspects of the "deconfined quantum criticality".

cond-mat.supr-con↗

arXiv2009

Universal linear-temperature dependence of static magnetic susceptibility in iron-pnictides

A universal linear-temperature dependence of the uniform magnetic susceptibility has been observed in the nonmagnetic normal state of iron-pnictides. This non-Pauli and non-Curie-Weiss-like paramagnetic behavior cannot be understood within a pure itinerant picture. We argue that it results from the existence of a wide antiferromagnetic fluctuation window in which the local spin-density-wave correlations exist but the global directional order has not been established yet.

cond-mat.str-el↗

arXiv2008

A Numerical Renormalization Group Study of the Superconducting and Spin Density Wave Instabilities in MFeAsO$_{1-x}$F$_x$ Compounds

We apply the fermion renormalization group method, implemented numerically by Honerkamp et.al., to a two-band model of FeAs-based materials. At half filling we find the $(π,0)$ or $(0,π)$ spin density wave order and a sub-dominant superconducting pairing tendency. Due to a topological reason, the spin density wave gap has nodes on the fermi surfaces. Away from half filling we find an unconventional s-wave and a sub-dominant $d_{x^2-y^2}$ pairing instability. The former has $s$ symmetry around the hole fermi surface but exhibits $s+d_{x^2-y^2}$ symmetry around the electron pockets where the 90 degree rotation is broken. The pairing mechanism is inter-pocket pair hopping. Interestingly, the same interaction also drives the antiferromagnetism.

cond-mat.supr-con↗

arXiv2008

A direct transition between a Neel ordered Mott insulator and a $d_{x^2-y^2}$ superconductor on the square lattice

In this paper we study a bandwidth-controlled direct, continuous, phase transition from a Mott insulator, with easy plane Neel order, to a fully gapped $d_{x^2-y^2}$ superconductor with a doubled unit cell on the square lattice, a transition that is forbidden according to the Landau paradigm. This transition is made possible because the vortices of the antiferromagnet are charged and the vortices of the superconductor carry spins. These nontrivial vortex quantum numbers arise because the ordered phases are intimately related to a topological band insulator. We describe the lattice model as well as the effective field theory.

cond-mat.str-el↗

arXiv2008

Spin-charge Separated Solitons in a Topological Band Insulator

In this paper we construct a simple, controllable, two dimensional model based on a topological band insulator. It has many attractive properties. (1) We obtain spin-charge separated solitons that are associated with $π$ fluxes. (2) It suggests an alternative way to classify $Z_2$ topological band insulator without resorting to the sample boundary. (3) When the $π$ fluxes are dynamical variables, as in a correlated insulator with emergent gauge fluxes, these solitons are propagating bosonic excitations and their condensation triggers a phase transition into a planar ferromagnet.

cond-mat.mes-hall↗

arXiv2007

Testable signatures of quantum non-locality in a two-dimensional chiral p-wave superconductor

A class of topological excitations -- the odd-winding-number vortices -- in a spinless 2D chiral p-wave $(p_x+ip_y)$ superconductor trap Majorana fermion states in the vortex cores. For a dilute gas of such vortices, the lowest energy \textit{fermionic} eigenstates are intrinsically non-local. We predict two testable signatures of this unusual quantum non-locality in quasiparticle tunneling experiments. We discuss why the associated teleportation-like phenomenon does not imply the violation of causality.

cond-mat.str-el↗

arXiv2007

Domain wall type defects as anyons in phase space

We discuss how the braiding properties of Laughlin quasi-particles in quantum Hall states can be understood within a one-dimensional formalism we proposed earlier. In this formalism the two-dimensional space of the Hall liquid is identified with the phase space of a one-dimensional lattice system, and localized Laughlin quasi-holes can be understood as coherent states of lattice solitons. The formalism makes comparatively little use of the detailed structure of Laughlin wavefunctions, and may offer ways to be generalized to non-abelian states.

cond-mat.mes-hall↗

arXiv2007

Edge solitons of topological insulators and fractionalized quasiparticles in two dimensions

An important characteristic of topological band insulators is the necessary presence of in-gap edge states on the sample boundary. We utilize this fact to show that when the boundary is reconnected with a twist, there are always zero-energy defect states. This provides a natural connection between novel defects in the two-dimensional $p_{x}+ip_{y}$ superconductor, the Kitaev model, the fractional quantum Hall effect, and the one-dimensional domain wall of polyacetylene.

cond-mat.str-el↗

arXiv2007

Effects of a Fluctuating Interface between a Superfluid and a Polarized Fermi Gas

Motivated by recent experiments in trapped Fermi gas with spin population imbalance, we discuss the effects of the quantum and thermal fluctuations of the interface between a fully paired superfluid core and a fully polarized Fermi gas. We demonstrate that even if there is no true partially polarized thermodynamic phase in bulk, the interface fluctuation can give rise to a partially polarized transition regime in trap. Our theory yields a definite prediction for the functional forms of the spatial profile of spin polarization and pairing gap, and we show that the spin-resolved density profiles measured by both the MIT and Rice groups obey this function form. We also show that sufficient large fluctuation will lead to a visibly unequal density even at the center of the cloud. We hope this picture can shed lights on the controversial discrepancies in recent experiments.

cond-mat.supr-con↗

arXiv2007

Phase Transition in a Two-level-cavity System in an Ohmic Environment

We propose that in the presence of an Ohmic, de-phasing type environment, a two-level-cavity system undergoes a quantum phase transition from a state with damped Rabi oscillation to a state without. We present the phase diagram and make predictions for pump and probe experiment. Such a strong coupling effect of the environment is beyond the reach of conventional perturbative treatment.

cond-mat.mtrl-sci↗

arXiv2007

New family of models for incompressible quantum liquids in $d\geq 2$

Through Haldane's construction, the fractional quantum Hall states on a two-sphere was shown to be the ground states of {\it one-dimensional} SU(2) spin Hamiltonians. In this Letter we generalize this construction to obtain a new class of SU(N) spin Hamiltonians. These Hamiltonians describes center-of-mass-position conserving pair hopping fermions in space dimension $d\ge 2$. We conjecture that the excitations of these models posses fractional fermion number.

cond-mat.str-el↗

arXiv2006

The Electronic States of Two Oppositely doped Mott Insulators Bilayers

We study the effect of Coulomb interaction between two oppositely doped low-dimensional tJ model systems. We exactly show that, in the one-dimensional case, an arbitrarily weak interaction leads to the formation of charge neutral electron-hole pairs. We then use two different mean-field theories to address the two-dimensional case, where inter-layer excitons also form and condense. We propose that this results in new features which have no analog in single layers, such as the emergence of an insulating spin liquid phase. Our simple bilayer model might have relevance to the physics of doped Mott insulator interfaces and of the new four layer Ba2CaCu4O8 compound.

cond-mat.str-el↗

arXiv2006

Photoisomerization in a Dissipative Environment

We investigate photoisomerization (PI), the shape change of a molecule upon photoabsorption, in a dissipative environment using a simple spin-boson model. We identify two classes of environment depending on whether it "entangles" with the molecule. In the absence of entanglement, the environment merely causes a blue shift of the required photon frequency and reduces the quantum efficiency of PI. With entanglement the molecule can undergo a quantum phase transition between a state that photoisomerizes to a state that does not.

cond-mat.mtrl-sci↗

arXiv2006

An Index Theorem for the Majorana Zero Modes in Chiral P-Wave Superconductors

We show that the Majorana fermion zero modes in the cores of odd winding number vortices of a 2D $p_x+ip_y$-paired superconductor is due to an index theorem. This theorem is analogous to that proven by Jackiw and Rebbi for the existence of localized Dirac fermion zero modes on the mass domain walls of a 1D Dirac theory. The important difference is that, in our case, the theorem is proven for a two component fermion theory where the first and second components are related by parity reversal and hermitian conjugation.

cond-mat.str-el↗