196 research outputs found

### New quantum critical points of $j=3/2$ Dirac electrons in antiperovskite topological crystalline insulators

We study the effect of the long-range Coulomb interaction in $j=3/2$ Dirac
electrons in cubic crystals with the $O_h$ symmetry, which serves as an
effective model for antiperovskite topological crystalline insulators. The
renormalization group analysis reveals three fixed points that are Lorentz
invariant, rotationally invariant, and $O_h$ invariant. Among them, the
Lorentz- and $O_h$-invariant fixed points are stable in the low-energy limit
while the rotationally invariant fixed point is unstable. The existence of a
stable $O_h$-invariant fixed point of Dirac fermions with finite velocity
anisotropy presents an interesting counterexample to emergent Lorentz
invariance in solids.Comment: 5 pages, 3 figure

### Theory of interacting topological crystalline insulators

We study the effect of electron interactions in topological crystalline
insulators (TCIs) protected by mirror symmetry, which are realized in the SnTe
material class and host multi-valley Dirac fermion surface states. We find that
interactions reduce the integer classification of noninteracting TCIs in three
dimensions, indexed by the mirror Chern number, to a finite group $Z_8$. In
particular, we explicitly construct a microscopic interaction Hamiltonian to
gap 8 flavors of Dirac fermions on the TCI surface, while preserving the mirror
symmetry. Our construction builds on interacting edge states of $U(1)\times
Z_2$ symmetry-protected topological (SPT) phases of fermions in two dimensions,
which we classify. Our work reveals a deep connection between 3D topological
phases protected by spatial symmetries and 2D topological phases protected by
internal symmetries.Comment: v2. 10 pages, 3 figures. Added new materials and improved
presentatio

### Unconventional Superconductivity and Density Waves in Twisted Bilayer Graphene

We study electronic ordering instabilities of twisted bilayer graphene with
$n=2$ electrons per supercell, where correlated insulator state and
superconductivity are recently observed. Motivated by the Fermi surface nesting
and the proximity to Van Hove singularity, we introduce a hot-spot model to
study the effect of various electron interactions systematically. Using
renormalization group method, we find $d$/$p$-wave superconductivity and
charge/spin density wave emerge as the two types of leading instabilities
driven by Coulomb repulsion. The density wave state has a gapped energy
spectrum at $n=2$ and yields a single doubly-degenerate pocket upon doping to
$n>2$. The intertwinement of density wave and superconductivity and the
quasiparticle spectrum in the density wave state are consistent with
experimental observations.Comment: 15 pages, 12 figures; updated discussion and analysis on density wave
state

### Anomalous Hall effect from a non-Hermitian viewpoint

Non-Hermitian descriptions often model open or driven systems away from the
equilibrium. Nonetheless, in equilibrium electronic systems, a non-Hermitian
nature of an effective Hamiltonian manifests itself as unconventional
observables such as a bulk Fermi arc and skin effects. We theoretically reveal
that spin-dependent quasiparticle lifetimes, which signify the non-Hermiticity
of an effective model in the equilibrium, induce the anomalous Hall effect,
namely the Hall effect without an external magnetic field. We first examine the
effect of nonmagnetic and magnetic impurities and obtain a non-Hermitian
effective model. Then, we calculate the Kubo formula from the microscopic model
to ascertain a non-Hermitian interpretation of the longitudinal and Hall
conductivities. Our results elucidate the vital role of the non-Hermitian
equilibrium nature in the quantum transport phenomena.Comment: 8+18 pages, 3+6 figure

### Interlayer Pairing Symmetry of Composite Fermions in Quantum Hall Bilayers

We study the pairing symmetry of the interlayer paired state of composite fermions in quantum Hall bilayers. Based on the Halperin-Lee-Read (HLR) theory, the effect of the long-range Coulomb interaction and the internal Chern-Simons gauge fluctuation is analyzed with the random-phase approximation beyond the leading order contribution in small momentum expansion, and we observe that the interlayer paired states with a relative angular momentum l=+1 are energetically favored for filling Î½=1/2+1/2 and 1/4+1/4. The degeneracy between states with Â±l is lifted by the interlayer density-current interaction arising from the interplay of the long-range Coulomb interaction and the Chern-Simons term in the HLR theory.United States. Department of Energy (DE-SC0010526

### Toroidal Scattering and Nonreciprocal Transport by Magnetic Impurities

We propose the second-order response of metals in an electric field induced
by magnetic impurities which locally break inversion symmetry. The impurities
with toroidal moments scatter conduction electrons in the presence of the
spin-orbit coupling, leading to nonreciprocal response. This mechanism is
ubiquitous when a magnetic impurity is placed off an inversion center such as
an interstitial site and a surface of a two-dimensional system.Comment: 2 pages, 1 figur

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