25 research outputs found
Multi- hexagonal spin density waves and dynamically generated spin-orbit coupling: time-reversal invariant analog of the chiral spin density wave
We study hexagonal spin-channel ("triplet") density waves with commensurate
-point propagation vectors. We first show that the three components of
the singlet charge density and charge-current density waves can be mapped to
multi-component nonzero angular momentum order in three dimensions ()
with cubic crystal symmetry. This one-to-one correspondence is exploited to
define a symmetry classification for triplet -point density waves using the
standard classification of spin-orbit coupled electronic liquid crystal phases
of a cubic crystal. Through this classification we naturally identify a set of
non-coplanar spin density and spin-current density waves: the chiral spin
density wave and its time-reversal invariant analog. These can be thought of as
and spin-orbit coupled isotropic -phase orders. In
contrast, uniaxial spin density waves are shown to correspond to
-phases. The non-coplanar triple- spin-current density wave realizes
a novel semimetal state with three flavors of four-component spin-momentum
locked Dirac cones, protected by a crystal symmetry akin to non-symmorphic
symmetry, and sits at the boundary between a trivial and topological insulator.
In addition, we point out that a special class of classical spin states,
defined as classical spin states respecting all lattice symmetries up to global
spin rotation, are naturally obtained from the symmetry classification of
electronic triplet density waves. These symmetric classical spin states are the
classical long-range ordered limits of chiral spin liquids.Comment: 14 + 4 pages, 5 + 0 figures; published versio
Density-functional investigation of rhombohedral stacks of graphene: topological surface states, nonlinear dielectric response, and bulk limit
A DFT-based investigation of rhombohedral (ABC)-type graphene stacks in
finite static electric fields is presented. Electronic band structures and
field-induced charge densities are compared with related literature data as
well as with own results on (AB) stacks. It is found, that the undoped
AB-bilayer has a tiny Fermi line consisting of one electron pocket around the
K-point and one hole pocket on the line K-. In contrast to (AB) stacks,
the breaking of translational symmetry by the surface of finite (ABC) stacks
produces a gap in the bulk-like states for slabs up to a yet unknown critical
thickness , while ideal (ABC) bulk (-graphite)
is a semi-metal. Unlike in (AB) stacks, the ground state of (ABC) stacks is
shown to be topologically non-trivial in the absence of external electric
field. Consequently, surface states crossing the Fermi level must unavoidably
exist in the case of (ABC)-type stacking, which is not the case in (AB)-type
stacks. These surface states in conjunction with the mentioned gap in the
bulk-like states have two major implications. First, electronic transport
parallel to the slab is confined to a surface region up to the critical layer
number . Related implications are expected for stacking domain
walls and grain boundaries. Second, the electronic properties of (ABC) stacks
are highly tunable by an external electric field. In particular, the dielectric
response is found to be strongly nonlinear and can e.g. be used to discriminate
slabs with different layer numbers. Thus, (ABC) stacks rather than (AB) stacks
with more than two layers should be of potential interest for applications
relying on the tunability by an electric field.Comment: 36 pages, 17 figure
Linear and nonlinear optical responses in the chiral multifold semimetal RhSi
Chiral topological semimetals are materials that break both inversion and
mirror symmetries. They host interesting phenomena such as the quantized
circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this
work, we report a comprehensive theoretical and experimental analysis of the
linear and non-linear optical responses of the chiral topological semimetal
RhSi, which is known to host multifold fermions. We show that the
characteristic features of the optical conductivity, which display two distinct
quasi-linear regimes above and below 0.4 eV, can be linked to excitations of
different kinds of multifold fermions. The characteristic features of the CPGE,
which displays a sign change at 0.4 eV and a large non-quantized response peak
of around 160 at 0.7 eV, are explained by assuming that
the chemical potential crosses a flat hole band at the Brillouin zone center.
Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is
necessary to increase the chemical potential as well as the quasiparticle
lifetime. More broadly our methodology, especially the development of the
broadband terahertz emission spectroscopy, could be widely applied to study
photo-galvanic effects in noncentrosymmetric materials and in topological
insulators in a contact-less way and accelerate the technological development
of efficient infrared detectors based on topological semimetals.Comment: Accepted in npj Quantum Materials; Abstract update
Evidence for chiral superconductivity on a silicon surface
Sn adatoms on a Si(111) substrate with 1/3 monolayer coverage form a
two-dimensional triangular adatom lattice with one unpaired electron per site
and an antiferromagnetic Mott insulating state. The Sn layers can be modulation
hole-doped and metallized using heavily-doped -type Si(111) substrates, and
become superconducting at low temperatures. While the pairing symmetry of the
superconducting state is currently unknown, the combination of repulsive
interactions and frustration inherent to the triangular adatom lattice opens up
the possibility for a chiral order parameter. Here, we study the
superconducting state of Sn/Si(111) using scanning tunneling
microscopy/spectroscopy and quasi-particle interference imaging. We find
evidence for a doping-dependent with a fully gapped order parameter, the
presence of time-reversal symmetry breaking, and a strong enhancement of the
zero-bias conductance near the edges of the superconducting domains. While each
individual piece of evidence could have a more mundane interpretation, our
combined results suggest the tantalizing possibility that Sn/Si(111) is an
unconventional chiral d-wave superconductor
Two-photon speckle as a probe of multi-dimensional entanglement
We calculate the statistical distribution P_2(I_2) of the speckle pattern
produced by a photon pair current I_2 transmitted through a random medium, and
compare with the single-photon speckle distribution P_1(I_1). We show that the
purity Tr rho^2 of a two-photon density matrix rho can be directly extracted
from the first two moments of P_1 and P_2. A one-to-one relationship is derived
between P_1 and P_2 if the photon pair is in an M-dimensional entangled pure
state. For M>>1 the single-photon speckle disappears, while the two-photon
speckle acquires an exponential distribution. The exponential distribution
transforms into a Gaussian if the quantum entanglement is degraded to a
classical correlation of M>>1 two-photon states. Two-photon speckle can
therefore discriminate between multi-dimensional quantum and classical
correlations.Comment: 5 pages, 2 figure