699 research outputs found
Robust non-Abelian spin liquid and possible intermediate phase in antiferromagnetic Kitaev model with magnetic field
We investigate the non-Abelian topological chiral spin liquid phase in the
two-dimensional (2D) Kitaev honeycomb model subject to a magnetic field. By
combining density matrix renormalization group (DMRG) and exact diagonalization
(ED) we study the energy spectra, entanglement, topological degeneracy, and
expectation values of Wilson loop operators, allowing for robust
characterization. While the ferromagnetic (FM) Kitaev spin liquid is already
destroyed by a weak magnetic field with Zeeman energy , the antiferromagnetic (AFM) spin liquid remains robust up to a magnetic
field that is an order of magnitude larger, .
Interestingly, for larger fields , an
intermediate gapless phase is observed, before a second transition to the
high-field partially-polarized paramagnet. We attribute this rich phase
diagram, and the remarkable stability of the chiral topological phase in the
AFM Kitaev model, to the interplay of strong spin-orbit coupling and
frustration enhanced by the magnetic field. Our findings suggest relevance to
recent experiments on RuCl under magnetic fields.Comment: 8 pages, 8 figure
Quark Masses: An Environmental Impact Statement
We investigate worlds that lie on a slice through the parameter space of the
Standard Model over which quark masses vary. We allow as many as three quarks
to participate in nuclei, while fixing the mass of the electron and the average
mass of the lightest baryon flavor multiplet. We classify as "congenial" worlds
that satisfy the environmental constraint that the quark masses allow for
stable nuclei with charges one, six, and eight, making organic chemistry
possible. Whether a congenial world actually produces observers depends on a
multitude of historical contingencies, beginning with primordial
nucleosynthesis, which we do not explore. Such constraints may be independently
superimposed on our results. Environmental constraints such as the ones we
study may be combined with information about the a priori distribution of quark
masses over the landscape of possible universes to determine whether the
measured values of the quark masses are determined environmentally, but our
analysis is independent of such an anthropic approach.
We estimate baryon masses as functions of quark masses and nuclear masses as
functions of baryon masses. We check for the stability of nuclei against
fission, strong particle emission, and weak nucleon emission. For two light
quarks with charges 2/3 and -1/3, we find a band of congeniality roughly 29 MeV
wide in their mass difference. We also find another, less robust region of
congeniality with one light, charge -1/3 quark, and two heavier, approximately
degenerate charge -1/3 and 2/3 quarks. No other assignment of light quark
charges yields congenial worlds with two baryons participating in nuclei. We
identify and discuss the region in quark-mass space where nuclei would be made
from three or more baryon species.Comment: 40 pages, 16 figures (in color), 4 tables. See paper for a more
detailed abstract. v4: Cleaning up minor typo
Exact Chiral Spin Liquids and Mean-Field Perturbations of Gamma Matrix Models on the Ruby Lattice
We theoretically study an exactly solvable Gamma matrix generalization of the
Kitaev spin model on the ruby lattice, which is a honeycomb lattice with
"expanded" vertices and links. We find this model displays an exceptionally
rich phase diagram that includes: (i) gapless phases with stable spin fermi
surfaces, (ii) gapless phases with low-energy Dirac cones and quadratic band
touching points, and (iii) gapped phases with finite Chern numbers possessing
the values {\pm}4,{\pm}3,{\pm}2 and {\pm}1. The model is then generalized to
include Ising-like interactions that break the exact solvability of the model
in a controlled manner. When these terms are dominant, they lead to a trivial
Ising ordered phase which is shown to be adiabatically connected to a large
coupling limit of the exactly solvable phase. In the limit when these
interactions are weak, we treat them within mean-field theory and present the
resulting phase diagrams. We discuss the nature of the transitions between
various phases. Our results highlight the richness of possible ground states in
closely related magnetic systems.Comment: 9 pages, 9 figure
A new spin-anisotropic harmonic honeycomb iridate
The physics of Mott insulators underlies diverse phenomena ranging from high
temperature superconductivity to exotic magnetism. Although both the electron
spin and the structure of the local orbitals play a key role in this physics,
in most systems these are connected only indirectly --- via the Pauli exclusion
principle and the Coulomb interaction. Iridium-based oxides (iridates) open a
further dimension to this problem by introducing strong spin-orbit
interactions, such that the Mott physics has a strong orbital character. In the
layered honeycomb iridates this is thought to generate highly spin-anisotropic
interactions, coupling the spin orientation to a given spatial direction of
exchange and leading to strongly frustrated magnetism. The potential for new
physics emerging from such interactions has driven much scientific excitement,
most recently in the search for a new quantum spin liquid, first discussed by
Kitaev \cite{kitaev_anyons_2006}. Here we report a new iridate structure that
has the same local connectivity as the layered honeycomb, but in a
three-dimensional framework. The temperature dependence of the magnetic
susceptibility exhibits a striking reordering of the magnetic anisotropy,
giving evidence for highly spin-anisotropic exchange interactions. Furthermore,
the basic structural units of this material suggest the possibility of a new
family of structures, the `harmonic honeycomb' iridates. This compound thus
provides a unique and exciting glimpse into the physics of a new class of
strongly spin-orbit coupled Mott insulators.Comment: 12 pages including bibliography, 5 figure
Z topology and superconductivity from symmetry lowering of a 3D Dirac Metal AuPb
3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons
and exhibit exotic physical properties It has been suggested that structurally
distorting a DSM can create a Topological Insulator (TI), but this has not yet
been experimentally verified. Furthermore, quasiparticle excitations known as
Majorana Fermions have been theoretically proposed to exist in materials that
exhibit superconductivity and topological surface states. Here we show that the
cubic Laves phase AuPb has a bulk Dirac cone above 100 K that gaps out upon
cooling at a structural phase transition to create a topologically non trivial
phase that superconducts below 1.2 K. The nontrivial Z = -1 invariant in
the low temperature phase indicates that AuPb in its superconducting state
must have topological surface states. These characteristics make AuPb a
unique platform for studying the transition between bulk Dirac electrons and
topological surface states as well as studying the interaction of
superconductivity with topological surface states
Non-coplanar and counter-rotating incommensurate magnetic order stabilized by Kitaev interactions in -Li2IrO3
Materials that realize Kitaev spin models with bond-dependent anisotropic
interactions have long been searched for, as the resulting frustration effects
are predicted to stabilize novel forms of magnetic order or quantum spin
liquids. Here we explore the magnetism of -LiIrO, which has the
topology of a 3D Kitaev lattice of inter-connected Ir honeycombs. Using
resonant magnetic x-ray diffraction we find a complex, yet highly-symmetric
incommensurate magnetic structure with non-coplanar and counter-rotating Ir
moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally
accounts for all key features of the observed magnetic structure. Our results
provide strong evidence that -LiIrO realizes a spin Hamiltonian
with dominant Kitaev interactions.Comment: 10 pages, 7 figure
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