8 research outputs found
Detecting Fractional Chern Insulators in Optical Lattices through Quantized Displacement
The realization of interacting topological states of matter such as
fractional Chern insulators (FCIs) in cold atom systems has recently come
within experimental reach due to the engineering of optical lattices with
synthetic gauge fields providing the required topological band structures.
However, detecting their occurrence might prove difficult since transport
measurements akin to those in solid state systems are challenging to perform in
cold atom setups and alternatives have to be found. We show that for a FCI state realized in the lowest band of a Harper-Hofstadter model of
interacting bosons confined by a harmonic trapping potential, the fractionally
quantized Hall conductivity can be accurately determined by the
displacement of the atomic cloud under the action of a constant force which
provides a suitable experimentally measurable signal for detecting the
topological nature of the state. Using matrix-product state algorithms, we show
that, in both cylinder and square geometries, the movement of the particle
cloud in time under the application of a constant force field on top of the
confining potential is proportional to for an extended range of
field strengths.Comment: 5 pages, 6 figures, plus supplementary materia
Controlling Topology through Targeted Symmetry Manipulation in Magnetic Systems
The possibility of selecting magnetic space groups by orienting the
magnetization direction or tuning magnetic orders offers a vast playground for
engineering symmetry protected topological phases in magnetic materials. In
this work, we study how selective tuning of symmetry and magnetism can
influence and control the resulting topology in a 2D magnetic system, and
illustrate such procedure in the ferromagnetic monolayer MnPSe. Density
functional theory calculations reveals a symmetry-protected accidental
semimetalic (SM) phase for out-of-plane magnetization which becomes an
insulator when the magnetization is tilted in-plane, reaching band gap values
close to meV. We identify an order-two composite antiunitary symmetry and
threefold rotational symmetry that induce the band crossing and classify the
possible topological phases using symmetry analysis, which we support with
tight-binding and models. Breaking of inversion
symmetry opens a gap in the SM phase, giving rise to a Chern insulator. We
demonstrate this explicitly in the isostructural Janus compound
MnPSSe, which naturally exhibits Rashba spin-orbit coupling
that breaks inversion symmetry. Our results map out the phase space of
topological properties of ferromagnetic transition metal phosphorus
trichalcogenides and demonstrate the potential of the magnetization-dependent
metal-to-insulator transition as a spin switch in integrated two-dimensional
electronics
Distinct Floquet topological classifications from color-decorated frequency lattices with space-time symmetries
We consider nontrivial topological phases in Floquet systems using unitary
loops and stroboscopic evolutions under a static Floquet Hamiltonian in
the presence of dynamical space-time symmetries . While the latter has been
subject of out-of-equilibrium classifications that extend the ten-fold way and
systems with additional crystalline symmetries to periodically driven systems,
we explore the anomalous topological zero modes that arise in from the
coexistence of a dynamical space-time symmetry and antisymmetry of ,
and classify them using a frequency-domain formulation. Moreover, we provide an
interpretation of the resulting Floquet topological phases using a frequency
lattice with a decoration represented by color degrees of freedom on the
lattice vertices. These colors correspond to the coefficient of the group
extension of along the frequency lattice, given by . The distinct topological classifications that arise at different
energy gaps in its quasi-energy spectrum are described by the torsion product
of the cohomology group classifying the group extension.Comment: 4 pages + supplementary materia
Chiral Majorana hinge modes on a curved surface with magnetic impurities
Chiral Majorana one-dimensional modes have been proposed as they key
component for topological quantum computing. In this study, we explore their
potential realization as hinge modes in higher-order topological
superconductors. To create such phases, we engineer a sign-changing,
time-reversal symmetry-breaking mass term through an ensemble of magnetic
impurities on the surface of a sphere. The magnetization of this ensemble
arises from the competition between the external magnetic field and the
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction among the impurities, mediated
by the surface Majorana modes. We determine the magnetic phase diagram and
identify the optimal magnetic field to minimize orbital effects and induce a
sign changing mass term. This term opens a gap in the surface spectrum,
resulting in a gapless one-dimensional chiral Majorana mode along the nodal
line of the mass term, thereby implementing a second-order topological
superconductor
Prediction of topological phases in metastable ferromagnetic MPX monolayers
Density functional theory calculations are carried out to study the
electronic and topological properties of P ( = Mn, Fe, Co, Ni, and
= S, Se) monolayers in the ferromagnetic (FM) metastable magnetic state. We
find that FM MnPSe monolayers host topological semimetal signatures that
are gapped out when spin-orbit coupling (SOC) is included. These findings are
supported by explicit calculations of the Berry curvature and the Chern number.
The choice of the Hubbard- parameter to describe the -electrons is
thoroughly discussed, as well as the influence of using a hybrid-functional
approach. The presence of band inversions and the associated topological
features are found to be formalism-dependent. Nevertheless, routes to achieve
the topological phase via the application of external biaxial strain are
demonstrated. Within the hybrid-functional picture, topological band structures
are recovered under a pressure of 15% (17 GPa). The present work provides a
potential avenue for uncovering new topological phases in metastable
ferromagnetic phases
Linear magneto-conductivity as a DC probe of time-reversal symmetry breaking
Several optical experiments have shown that in magnetic materials the
principal axes of response tensors can rotate in a magnetic field. Here we
offer a microscopic explanation of this effect, and propose a closely related
DC transport phenomenon -- an off-diagonal \emph{symmetric} conductivity linear
in a magnetic field, which we refer to as linear magneto-conductivity (LMC).
Although LMC has the same functional dependence on magnetic field as the Hall
effect, its origin is fundamentally different: LMC requires time-reversal
symmetry to be broken even before a magnetic field is applied, and is therefore
a sensitive probe of magnetism. We demonstrate LMC in three different ways: via
a tight-binding toy model, density functional theory calculations on MnPSe,
and a semiclassical calculation. The third approach additionally identifies two
distinct mechanisms yielding LMC: momentum-dependent band magnetization and
Berry curvature. Finally, we propose an experimental geometry suitable for
detecting LMC, and demonstrate its applicability using Landauer-B\"{u}ttiker
simulations. Our results emphasize the importance of measuring the full
conductivity tensor in magnetic materials, and introduce LMC as a new transport
probe of symmetry.Comment: 6+8 pages, 4+3 figure
Soft, curved electrode systems capable of integration on the auricle as a persistent brain–computer interface
Recent advances in electrodes for noninvasive recording of electroencephalograms expand opportunities collecting such data for diagnosis of neurological disorders and brain–computer interfaces. Existing technologies, however, cannot be used effectively in continuous, uninterrupted modes for more than a few days due to irritation and irreversible degradation in the electrical and mechanical properties of the skin interface. Here we introduce a soft, foldable collection of electrodes in open, fractal mesh geometries that can mount directly and chronically on the complex surface topology of the auricle and the mastoid, to provide high-fidelity and long-term capture of electroencephalograms in ways that avoid any significant thermal, electrical, or mechanical loading of the skin. Experimental and computational studies establish the fundamental aspects of the bending and stretching mechanics that enable this type of intimate integration on the highly irregular and textured surfaces of the auricle. Cell level tests and thermal imaging studies establish the biocompatibility and wearability of such systems, with examples of high-quality measurements over periods of 2 wk with devices that remain mounted throughout daily activities including vigorous exercise, swimming, sleeping, and bathing. Demonstrations include a text speller with a steady-state visually evoked potential-based brain–computer interface and elicitation of an event-related potential (P300 wave)