Anomalous Drag in Double Bilayer Graphene Quantum-Hall Superfluids

Semiconductor double-layers in the quantum Hall regime tend to have superfluid exciton condensate ground states when the total filling factor is an odd integer, provided that the Landau orbitals at the Fermi level in the two layers have the same orbital character. Since the $N=0$ Landau level of bilayer graphene contains states with both $n=0$ and $n=1$ orbital character, the physics of double bilayers falls outside previously studied cases. We show that the superfluid phase stiffness vanishes in double bilayer graphene when $n=0$ and $n=1$ orbitals states are degenerate in one of the layers, even though the gap for charged excitations remain large, and speculate that this property is behind the recent discovery of strong anomalous drag near a $n=0/1$ degeneracy point

Electrical Reservoirs for Bilayer Excitons

The ground state of two-dimensional (2D) electron systems with equal low densities of electrons and holes in nearby layers is an exciton fluid. We show that a reservoir for excitons can be established by contacting the two layers separately and maintaining the chemical potential difference at a value less than the spatially indirect band gap. Equilibration between the exciton fluid and the contacts proceeds via a process involving virtual intermediate states in which an unpaired electron or hole occupies a free carrier state in one of the 2D layers. We derive an approximate relationship between the exciton-contact equilibration rate and the electrical conductances between the contacts and individual 2D layers when the contact chemical potentials align with the free-carrier bands, and explain how electrical measurements can be used to measure thermodynamic properties of the exciton fluid.Comment: Minor revision of v1. Accepted by PR

On the nature of the correlated insulator states in twisted bilayer graphene

We use self-consistent Hartree-Fock calculations performed in the full $\pi$-band Hilbert space to assess the nature of the recently discovered correlated insulator states in magic-angle twisted bilayer graphene (TBG). We find that gaps between the flat conduction and valence bands open at neutrality over a wide range of twist angles, sometimes without breaking the system's valley projected ${\cal C}_{2}{\cal T}$ symmetry. Broken spin/valley flavor symmetries then enable gapped states to form not only at neutrality, but also at total moir\'e band filling $n = \pm p/4$ with integer $p = 1, 2, 3$, when the twist angle is close to the magic value at which the flat bands are most narrow. Because the magic-angle flat band quasiparticles are isolated from remote band quasiparticles only for effective dielectric constants larger than $\sim 20$, the gapped states do not necessarily break \CT symmetry and as a consequence the insulating states at $n = \pm 1/4$ and $n = \pm 3/4$ need not exhibit a quantized anomalous Hall effect.Comment: 5 pages plus supplemental material. Commenst are welcome

Polariton Supercurrent Generation in Unipolar Electro-optic Devices

We describe a mechanism by which an electrical bias voltage applied across a unipolar semiconductor quantum well can drive an exciton or polariton supercurrent. The mechanism depends on the properties of electronic quasiparticles in quantum wells or two-dimensional materials that are dressed by interactions with the coherent exciton field of an exciton condensate or the coherent exciton and photon fields of a polariton condensate, and on approximate conservation laws. We propose experiments that can be performed to realize this new light-matter coupling effect, and discuss possible applications.Comment: 8 pages, 5 figures. Comments are welcome

Symmetry protected topological phases from decorated domain walls

Symmetry protected topological (SPT) phases with unusual edge excitations can emerge in strongly interacting bosonic systems and are classified in terms of the cohomology of their symmetry groups. Here we provide a physical picture that leads to an intuitive understanding and wavefunctions for several SPT phases in d=1,2,3 dimensions. We consider symmetries which include a Z_2 subgroup, that allows us to define domain walls. While the usual disordered phase is obtained by proliferating domain walls, we show that SPT phases are realized when these proliferated domain walls are `decorated', i.e. are themselves SPT phases in one lower dimension. For example a d=2 SPT phase with Z_2 and time reversal symmetry is realized when the domain walls that proliferate are themselves in a d=1 Haldane/AKLT state. Similarly, d=3 SPT phases with Z_2 * Z_2 symmetry emerges when domain walls in a d=2 SPT with Z_2 symmetry are proliferated. The resulting ground states are shown to be equivalent to that obtained from group cohomology and field theoretical techniques. The result of gauging the Z_2 symmetry in these phases is also discussed. An extension of this construction where time reversal plays the role of Z_2 symmetry allows for a discussion of several d=3 SPT phases. This construction also leads to a new perspective on some well known d=1 SPT phases, from which exactly soluble parent Hamiltonians may be derived.Comment: 17 pages, 8 figure

Optical conductivity of the t2gt_{2g} two-dimensional electron gas

Motivated by recent interest in perovskite surfaces and heterostructures, we present an analysis of the Kubo conductivity of a two-dimensional electron gas (2DEG) formed in the $t_{2g}$ bands of an oxide with perovskite structure. We find that when the electric field is polarized in the plane of the 2DEG, the optical conductivity is dominated by nearly independent Drude contributions from two-dimensional subband Fermi surfaces, whereas for perpendicular-to-plane polarization it has strong intersubband features. Our analysis suggests that perpendicular-to-plane optical conductivity studies may help advance understanding of the interplay between lattice distortions and electron-electron interactions in complex oxide 2DEG quantum confinement physics

Fast Adaptive Beamforming based on kernel method under Small Sample Support

It is well-known that the high computational complexity and the insufficient samples in large-scale array signal processing restrict the real-world applications of the conventional full-dimensional adaptive beamforming (sample matrix inversion) algorithms. In this paper, we propose a computationally efficient and fast adaptive beamforming algorithm under small sample support. The proposed method is implemented by formulating the adaptive weight vector as a linear combination of training samples plus a signal steering vector, on the basis of the fact that the adaptive weight vector lies in the signal-plus-interference subspace. Consequently, by using the well-known linear kernel methods with very good small-sample performance, only a low-dimension combination vector needs to be computed instead of the high-dimension adaptive weight vector itself, which remarkably reduces the degree of freedom and the computational complexity. Experimental results validate the good performance and the computational effectiveness of the proposed methods for small samples.Comment: 13 pages, 5 figure

Quantum phase transition of cold atoms in the bilayer hexagonal optical lattices

We propose a scheme to investigate the quantum phase transition of cold atoms in the bilayer hexagonal optical lattices. Using the quantum Monte Carlo method, we calculate the ground state phase diagrams which contain an antiferromagnet, a solid, a superfluid, a fully polarized state and a supersolid. We find there is a supersolid emerging in a proper parameter space, where the diagonal long range order coexists with off-diagonal long range order. We show that the bilayer optical lattices can be realized by coupling two monolayer optical lattices and give an experimental protocol to observe those novel phenomena in the real experiments.Comment: 5 pages, 6 figure

Construction of three classes of Strictly Optimal Frequency-Hopping Sequence Sets

In this paper, we construct three classes of strictly optimal frequency-hopping sequence (FHS) sets with respect to partial Hamming correlation and family size. The first class is based on a generic construction, the second and third classes are based from the trace map

Triggering star formation by both radiative and mechanical active galactic nucleus feedback

We perform two dimensional hydrodynamic numerical simulations to study the positive active galactic nucleus feedback which triggers, rather than suppresses, star formation. Recently, it was shown by Nayakshin et al. and Ishibashi et al. that star formation occurs when the cold interstellar medium is squeezed by the impact of mass outflow or radiation pressure, respectively. Mass outflow is ubiquitous in this astrophysical context, and radiation pressure is also important if the AGN is luminous. For the first time in this subject, we incorporate both mass outflow feedback and radiative feedback into our model. Consequently, the ISM is shocked into shells by the AGN feedback, and these shells soon fragment into clumps and filaments because of Rayleigh-Taylor and thermal instabilities. We have two major findings: (1) the star formation rate can indeed be very large in the clumps and filaments. However, the resultant star formation rate density is too large compared with previous works, which is mainly because we ignore the fact that most of the stars that are formed would be disrupted when they move away from the galactic center. (2) Although radiation pressure feedback has a limited effect, when mass outflow feedback is also included, they reinforce each other. Specifically, in the gas-poor case, mass outflow is always the dominant contributor to feedback.Comment: 14 pages, 5 figures, to appear in RA