arXiv subjects

Fengcheng Wu

Publications and source records attributed to Fengcheng Wu.

98 records · Page 6Linked to original sources

arXiv2016

Observation of a Nematic Quantum Hall Liquid on the Surface of Bismuth

Nematic quantum fluids with wavefunctions that break the underlying crystalline symmetry can form in interacting electronic systems. We examine the quantum Hall states that arise in high magnetic fields from anisotropic hole pockets on the Bi(111) surface. Spectroscopy performed with a scanning tunneling microscope shows that a combination of local strain and many-body Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form three valley-polarized quantum Hall states. We image the resulting anisotropic LL wavefunctions and show that they have a different orientation for each broken-symmetry state. The wavefunctions correspond precisely to those expected from pairs of hole valleys and provide a direct spatial signature of a nematic electronic phase.

cond-mat.mes-hall↗

arXiv2016

Microscopic Theory of Equilibrium Polariton Condensates

We present a microscopic theory of the equilibrium polariton condensate state of a semiconductor quantum well in a planar optical cavity. The theory accounts for the adjustment of matter excitations to the presence of a coherent photon field, predicts effective polariton-polariton interaction strengths that are weaker and condensate exciton fractions that are smaller than in the commonly employed exciton-photon model, and yields effective Rabi coupling strengths that depend on the detuning of the cavity photon energy relative to the bare exciton energy. The dressed quasiparticle bands that appear naturally in the theory provide a mechanism for electrical manipulation of polariton condensates.

cond-mat.mes-hall↗

arXiv2015

Direct Measurement of Exciton Valley Coherence in Monolayer WSe$_2$

In crystals, energy band extrema in momentum space can be identified by their valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time, a key quantity for manipulating the valley pseudospin, has never been measured in any material. In this work, we use a sequence of laser pulses to resonantly generate a coherent superposition of excitons (Coulomb-bound electron-hole pairs) in opposite valleys of monolayer WSe$_2$. The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population decay. Our work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.

cond-mat.mes-hall↗

arXiv2015

SU(3) and SU(4) singlet quantum Hall states at $ν=2/3$

We report on an exact diagonalization study of fractional quantum Hall states at filling factor $ν=2/3$ in a system with a four-fold degenerate $n$=0 Landau level and SU(4) symmetric Coulomb interactions. Our investigation reveals previously unidentified SU(3) and SU(4) singlet ground states which appear at flux quantum shift 2 when a spherical geometry is employed, and lie outside the established composite-fermion or multicomponent Halperin state patterns. We evaluate the two-particle correlation functions of these states, and discuss quantum phase transitions in graphene between singlet states with different number of components as magnetic field strength is increased.

cond-mat.str-el↗

arXiv2015

Exciton band structure of monolayer MoS2

We address the properties of excitons in monolayer MoS$_2$ from a theoretical point of view, showing that low-energy excitonic states occur both at the Brillouin zone center and at the Brillouin-zone corners, that binding energies at the Brillouin-zone center deviate strongly from the $(n-1/2)^{-2}$ pattern of the two-dimensional hydrogenic model, and that the valley-degenerate exciton doublet at the Brillouin-zone center splits at finite momentum into an upper mode with non-analytic linear dispersion and a lower mode with quadratic dispersion. Although monolayer MoS$_2$ is a direct-gap semiconductor when classified by its quasiparticle band structure, it may well be an indirect gap material when classified by its excitation spectra.

cond-mat.mes-hall↗

arXiv2014

SO(5) symmetry in the quantum Hall effect in graphene

Electrons in graphene have four flavors associated with low-energy spin and valley degrees of freedom. The fractional quantum Hall effect in graphene is dominated by long-range Coulomb interactions which are invariant under rotations in spin-valley space. This SU(4) symmetry is spontaneously broken at most filling factors, and also weakly broken by atomic scale valley-dependent and valley-exchange interactions with coupling constants $g_{z}$ and $g_{\perp}$. In this paper we demonstrate that when $g_{z}=-g_{\perp}$ an exact SO(5) symmetry survives which unifies the Néel spin order parameter of the antiferromagnetic state and the $XY$ valley order parameter of the Kekulé distortion state into a single five-component order parameter. The proximity of the highly insulating quantum Hall state observed in graphene at $ν=0$ to an ideal SO(5) symmetric quantum Hall state remains an open experimental question. We illustrate the physics associated with this SO(5) symmetry by studying the multiplet structure and collective dynamics of filling factor $ν=0$ quantum Hall states based on exact-diagonalization and low-energy effective theory approaches. This allows to illustrate how manifestations of the SO(5) symmetry would survive even when it is weakly broken.

cond-mat.str-el↗

arXiv2012

Quantum Efficiency of Intermediate-Band Solar Cells Based on Non-Compensated n-p Codoped TiO2

As an appealing concept for developing next-generation solar cells, intermediate-band solar cells (IBSCs) promise to drastically increase the quantum efficiency of photovoltaic conversion. Yet to date, a standing challenge lies in the lack of materials suitable for developing IBSCs. Recently, a new doping approach, termed non-compensated n-p codoping, has been proposed to construct intermediate bands (IBs) in the intrinsic energy band gaps of oxide semiconductors such as TiO$_2$. We explore theoretically the optimal quantum efficiency of IBSCs based on non-compensated n-p codoped TiO$_2$ under two different design schemes. The first preserves the ideal condition that no electrical current be extracted from the IB. The corresponding maximum quantum efficiency for the codoped TiO$_2$ can reach 52.7%. In the second scheme, current is also extracted from the IB, resulting in a further enhancement in the maximum efficiency to 56.7%. Our findings also relax the stringent requirement that the IB location be close to the optimum value, making it more feasible to realize IBSCs with high quantum efficiencies.

cond-mat.mtrl-sci↗

arXiv2012

Phase diagram of the toric code model in a parallel magnetic field

Ground-state phase diagram of the toric code model in a parallel magnetic field has three distinct phases: topological, charge-condensed, and vortex-condensed states. To study it we consider an implicit local order parameter characterizing the transition between the topological and charge-condensed phases, and sample it using continuous-time Monte Carlo simulations. The corresponding second-order transition line is obtained by finite-size scaling analysis of this order parameter. Symmetry breaking between charges and vortices along the first-order transition line is also observed, and our numerical result shows that the end point of the first-order transition line is located at $h_x^{(c)}=h_z^{(c)}=0.418(2)$.

cond-mat.stat-mech↗