6,040 research outputs found
Thermal Hall effects due to topological spin fluctuations in YMnO_3
The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot be perfect if the triangular lattice is distorted. Here, we report that the trimerized triangular lattice of multiferroic hexagonal manganite YMnO3 produces a highly unusual thermal Hall effect under an applied magnetic field. Our theoretical calculations demonstrate that the thermal Hall conductivity is related to the splitting of the otherwise degenerate two chiralities of its 120Ëš magnetic structure. Our result is one of the most unusual cases of topological physics due to this broken Z2 symmetry of the chirality in the supposedly paramagnetic state of YMnO3, due to strong topological spin fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya interaction
Optical Spectroscopy of 3d and 4d correlated electron systems.
In the context of this work, three different materials are studied via optical spec-
troscopy methods. The three materials are La2Cu2O5, Fe3O4, and Ca2RuO4, where
the first one is investigated via Fourier spectroscopy, while the latter two are stud-
ied via spectroscopic ellipsometry
Renormalization of antiferromagnetic magnons by superconducting condensate and quasiparticles
The ability to modify and tune the spin-wave dispersion is one of the most
important requirements for engineering of magnonic networks. In this study we
demonstrate the promise of synthetic thin-film hybrids composed of an
antiferromagnetic insulator (AF) and a normal (N) or superconducting (S) metal
for tuning and modifying the spin-wave dispersion in antiferromagnetic
insulators. The key ingredient is the uncompensated magnetic moment at the
AF/S(N) interface, which induces an effective exchange field in the adjacent
metal via the interface exchange interaction. The exchange field spin polarizes
quasiparticles in the metal and induces spinful triplet Cooper pairs screening
the magnon. The quasiparticle and Cooper pair polarization renormalizes the
magnon dispersion. The renormalization results in the splitting of the
otherwise degenerate AF magnon modes with no need to apply a magnetic field. It
is also proposed that measurements of the renormalized dispersion relations can
provide the amplitude of the effective exchange field induced by the AF in the
adjacent metal
Quantum light-matter interactions in structured waveguides
We explore special features of quantum light-matter interactions inside
structured waveguides due to their finite bandwidth, band edges, and
non-trivial topological properties. We model the waveguides as either a
tight-binding (TB) chain or a Su-Schrieffer-Heeger (SSH) chain. For
unstructured waveguides with infinite bandwidth, the transmission and
reflection amplitude of a side-coupled two-level emitter (2LE) are the same as
the reflection and transmission amplitude of a direct-coupled 2LE. We show that
this analogy breaks down for structured waveguides with finite bandwidth due to
the appearance of Lamb shift only for the direct-coupled 2LE. We further
predict a robust light-emitter coupling at zero collective decay width of a
single giant 2LE (with two couplings at different points) near the band edges
of the structured waveguides where topological features can be beneficial.
Finally, we study single-photon dynamics in a heterojunction of a long TB and
short SSH waveguide connected to a 2LE at the SSH end. We show the propagation
of a photon from the excited emitter to the TB waveguide only when the SSH
waveguide is in the topological phase. Thus, the heterojunction acts as a
quantum switch or conditional propagation channel.Comment: 15 pages, 11 figure
Signatures of many-body localization of quasiparticles in a flat band superconductor
We construct a class of exact eigenstates of the Hamiltonian obtained by
projecting the Hubbard interaction term onto the flat band subspace of a
generic lattice model. These exact eigenstates are many body states in which an
arbitrary number of localized fermionic particles coexist with a sea of mobile
Cooper pairs with zero momentum. By considering the dice lattice as an example,
we provide evidence that these exact eigenstates are in fact manifestation of
local integrals of motions of the projected Hamiltonian. In particular the spin
and particle densities retain memory of the initial state for a very long time,
if localized unpaired particles are present at the beginning of the time
evolution. This shows that many-body localization of quasiparticles and
superfluidity can coexist even in generic two-dimensional lattice models with
flat bands, for which it is not known how to construct local conserved
quantities. Our results open new perspectives on the old condensed matter
problem of the interplay between superconductivity and localization.Comment: 20 Pages, 9 figure
Mirror-protected Majorana zero modes in -wave multilayer graphene superconductors
Inspired by recent experimental discoveries of superconductivity in
chirally-stacked and twisted multilayer graphene, we study models of -wave
superconductivity on the honeycomb lattice with arbitrary numbers of layers.
These models respect a mirror symmetry that allows classification of the bands
by a mirror-projected winding number . For odd numbers of layers, the
systems are topologically nontrivial with . Along each
mirror-preserving edge in armchair nanoribbons, there are two protected
Majorana zero modes. These modes are present even if the sample is finite in
both directions, such as in rectangular and hexagonal flakes. Crucially, zero
modes can also be confined to vortex cores, which can be created by a magnetic
field or localized magnetic impurities and accessed by local scanning probes.
Finally, we apply these models to twisted bilayer and trilayer systems, which
also feature boundary-projected and vortex-confined zero modes. Since vortices
are experimentally accessible, our study suggests that superconducting
multilayer graphene systems are promising platforms to create and manipulate
Majorana zero modes.Comment: Comments are greatly appreciated. 8 + 22 pages with 4 + 15 figure
Boundary integral representation of multipliers of fragmented affine functions and other intermediate function spaces
We develop a theory of abstract intermediate function spaces on a compact
convex set and study the behaviour of multipliers and centers of these
spaces. In particular, we provide some criteria for coincidence of the center
with the space of multipliers and a general theorem on boundary integral
representation of multipliers. We apply the general theory in several concrete
cases, among others to strongly affine Baire functions, to the space
of fragmented affine functions, to the space , the monotone
sequential closure of , to their natural subspaces formed by Borel
functions, or, in some special cases, to the space of all strongly affine
functions. In addition, we prove that the space is determined by
extreme points and provide a large number of illustrating examples and
counterexamples.Comment: 136 pages; we corrected one definition and expanded the introduction
a bi
Emergent criticality in fully frustrated quantum magnets
Phase transitions in condensed matter are often linked to exotic emergent
properties. We study the fully frustrated bilayer Heisenberg antiferromagnet to
demonstrate that an applied magnetic field creates a novel emergent
criticality. The quantum phase diagram contains four states, the DS (singlets
on every interlayer dimer bond), DTAF (all triplets with antiferromagnetic
order), TC (a singlet-triplet checkerboard) and FM (saturated ferromagnet). The
thermal phase diagram is dominated by a wall of discontinuities extending from
the zero-field DTAF-DS transition to a quantum critical endpoint where the
field drives the DTAF and TC into the FM. This first-order wall is terminated
at finite temperatures by a line of critical points, where the
Berezinskii-Kosterlitz-Thouless (BKT) transition of the DTAF and the thermal
Ising transition of the TC also terminate. We demonstrate by quantum Monte
Carlo simulations that the BKT transition does not change the Ising nature of
the DTAF-DS critical line. By contrast, the combination of symmetries merging
on the multicritical DTAF-TC line leads to a 4-state Potts universality not
contained in the microscopic Hamiltonian, which we associate with the
Ashkin-Teller model. Our results represent a systematic step in understanding
emergent phenomena in quantum magnetic materials including the
``Shastry-Sutherland compound'' SrCu(BO).Comment: 10+8 pages, 5+7 figure
The Study of Excitons in 2D Novel Materials and Their van der Waals Heterostructures in the Magnetic Field
This research focuses on the direct and indirect excitons in Rydberg states in monolayers, bilayers, and van der Waals heterostructures composed of 2D semiconductors in the presence of the external magnetic field. In our work, we report binding energies of direct and indirect excitons in Rydberg states, the energy contribution from the magnetic field to the binding energies of magnetoexcitons, and diamagnetic coefficients (DMCs) of magnetoexcitons.
We study isotropic materials: transition metal dichalcogenides, TMDCs (WSe2, WS2, MoSe2, MoS2), and Xenes (silicene, germanene, stanene), and anisotropic materials: phosphorene and transition metal trichalcogenides, TMTCs (TiSe3, TiS3, ZrSe3, ZrS3). For excitons in TMDCs, phosphorene, and TMTCs, we consider freestanding (FS) and encapsulated by hexagonal boron nitride (hBN) monolayers, FS bilayers, and van der Waals heterostructures (vdWHs) when the external magnetic field is perpendicular to the structure. In vdWHs, the top and bottom 2D semiconductor monolayers are separated by the number of hBN layers, N. For excitons in Xenes, we consider FS and hBN-encapsulated monolayers and vdWHs when the external electric and magnetic fields are perpendicular to the structure. Excitons in TMDCs and Xenes structures are studied when the external magnetic field is varied between 0 and 30 T, and for Xenes, the electric field is taken above the critical value which is unique for each material up to the value where a Xene monolayer becomes unstable. Excitons in TMTCs and phosphorene structures are studied when the magnetic field is varied between 0 and 60 T.
In our approach, within the framework of the effective mass approximation, we solve the Schrodinger equation for an interacting electron and hole. For direct excitons in a monolayer, we use the Rytova-Keldysh (RK) potential to describe interactions between an electron and hole. For indirect excitons in a bilayer and vdWH, we use the Rytova-Keldysh and Coulomb potentials to describe interactions between an electron and hole located in two different monolayers. This allows us to investigate the role of the screening in TMDCs, Xenes, TMTCs, and phosphorene.
We show that the energy contribution from the magnetic field to the binding energies and DMCs of direct and indirect magnetoexcitons can be tuned by the magnetic field. For magnetoexcitons in Xenes, the electric field is an additional degree of freedom that can be used to tune binding energies, energy contribution from the magnetic field to the binding energies, and DMCs. Moreover, in vdWHs, we show that varying the number of hBN layers is an additional degree of freedom that can be used to tailor binding energies, energy contribution from the magnetic field to the binding energies, and DMCs. The results reported in this work on excitons in TMDCs vdWHs and in TMTCs, Xenes, and phosphorene structures are the first of their kind. They can be compared with the experimental results when they become available
- …