771 research outputs found
Maxwell electromagnetism as an emergent phenomenon in condensed matter
The formulation of a complete theory of classical electromagnetism by Maxwell
is one of the milestones of science. The capacity of many-body systems to
provide emergent mini-universes with vacua quite distinct from the one we
inhabit was only recognised much later. Here, we provide an account of how
simple systems of localised spins manage to emulate Maxwell electromagnetism in
their low-energy behaviour. They are much less constrained by symmetry
considerations than the relativistically invariant electromagnetic vacuum, as
their substrate provides a non-relativistic background with even translational
invariance broken. They can exhibit rich behaviour not encountered in
conventional electromagnetism. This includes the existence of magnetic monopole
excitations arising from fractionalisation of magnetic dipoles; as well as the
capacity of disorder, by generating defects on the lattice scale, to produce
novel physics, as exemplified by topological spin glassiness or random Coulomb
magnetism.Comment: Talk at Royal Society Symposium, "Unifying Physics and Technology in
the Light of Maxwell's Equations", November 201
Ground state and low-lying excitations of the spin-1/2 XXZ model on the kagome lattice at magnetization 1/3
We study the ground state and low-lying excitations of the S=1/2 XXZ
antiferromagnet on the kagome lattice at magnetization one third of the
saturation. An exponential number of non-magnetic states is found below a
magnetic gap. The non-magnetic excitations also have a gap above the ground
state, but it is much smaller than the magnetic gap. This ground state
corresponds to an ordered pattern with resonances in one third of the hexagons.
The spin-spin correlation function is short ranged, but there is long-range
order of valence-bond crystal type.Comment: 2 pages, 1 figure included, to appear in Physica B (proceedings of
SCES'04
Multiorbital Spin Susceptibility in a Magnetically Ordered State - Orbital versus Excitonic Spin Density Wave Scenario
We present a general theory of multiorbital spin waves in magnetically
ordered metallic systems. Motivated by the itinerant magnetism of iron-based
superconductors, we compare the magnetic excitations for two different
scenarios: when the magnetic order either sets in on the on-site orbital level;
or when it appears as an electron-hole pairing between different bands of
electron and hole character. As an example we treat the two-orbital model for
iron-based superconductors. For small magnetic moments the spin excitations
look similar in both scenarios. Going to larger interactions and larger
magnetic moments, the difference between both scenarios becomes striking. While
in the excitonic scenario the spin waves form a closed structure over the
entire Brillouin zone and the particle-hole continuum is gapped, the spin
excitations in the orbital scenario can be treated as spin waves only in a
close vicinity to the ordering momenta. The origin of this is a gapless
electronic structure with Dirac cones which is a source of large damping. We
analyze our results in connection with recent neutron scattering measurements
and show that certain features of the orbital scenario with multiple order
parameters can be observed experimentally.Comment: 12 pages, 7 figure
Disorder in a quantum spin liquid: flux binding and local moment formation
We study the consequences of disorder in the Kitaev honeycomb model,
considering both site dilution and exchange randomness. We show that a single
vacancy binds a flux and induces a local moment. This moment is polarised by an
applied field : in the gapless phase, for small the local susceptibility
diverges as ; for a pair of nearby vacancies on the same
sublattice, this even increases to . By
contrast, weak exchange randomness does not qualitatively alter the
susceptibility but has its signature in the heat capacity, which in the gapless
phase is power law in temperature with an exponent dependent on disorder
strength.Comment: 4 pages, 2 figure
Symmetry Breaking on the Three-Dimensional Hyperkagome Lattice of Na_4Ir_3O_8
We study the antiferromagnetic spin-1/2 Heisenberg model on the highly
frustrated, three-dimensional, hyperkagome lattice of Na_4Ir_3O_8 using a
series expansion method. We propose a valence bond crystal with a 72 site unit
cell as a ground state that supports many, very low lying, singlet excitations.
Low energy spinons and triplons are confined to emergent lower-dimensional
motifs. Here, and for analogous kagome and pyrochlore states, we suggest finite
temperature signatures, including an Ising transition, in the magnetic specific
heat due to a multistep breaking of discrete symmetries.Comment: 4 pages, 3 figure
Quasiparticle interference in iron-based superconductors
We systematically calculate quasiparticle interference (QPI) signatures for
the whole phase diagram of iron-based superconductors. Impurities inherent in
the sample together with ordered phases lead to distinct features in the QPI
images that are believed to be measured in spectroscopic imaging-scanning
tunneling microscopy (SI-STM). In the spin-density wave phase the rotational
symmetry of the electronic structure is broken, signatures of which are also
seen in the coexistence regime with both superconducting and magnetic order. In
the superconducting regime we show how the different scattering behavior for
magnetic and non-magnetic impurities allows to verify the symmetry of
the order parameter. The effect of possible gap minima or nodes is discussed.Comment: 19 pages, 7 figure
Frustrated magnetism and resonating valence bond physics in two-dimensional kagome-like magnets
We explore the phase diagram and the low-energy physics of three Heisenberg
antiferromagnets which, like the kagome lattice, are networks of corner-sharing
triangles but contain two sets of inequivalent short-distance resonance loops.
We use a combination of exact diagonalization, analytical strong-coupling
theories, and resonating valence bond approaches, and scan through the ratio of
the two inequivalent exchange couplings. In one limit, the lattices effectively
become bipartite, while at the opposite limit heavily frustrated nets emerge.
In between, competing tunneling processes result in short-ranged spin
correlations, a manifold of low-lying singlets (which can be understood as
localized bound states of magnetic excitations), and the stabilization of
valence bond crystals with resonating building blocks.Comment: Published versio
Dynamics of Fractionalization in Quantum Spin Liquids
We present the theory of dynamical spin-response for the Kitaev honeycomb
model, obtaining exact results for the structure factor (SF) in gapped and
gapless, Abelian and non-Abelian quantum spin-liquid (QSL) phases. We also
describe the advances in methodology necessary to compute these results. The
structure factor shows signatures of spin-fractionalization into emergent
quasiparticles -- Majorana fermions and fluxes of gauge field. In
addition to a broad continuum from spin-fractionalization, we find sharp
(-function) features in the response. These arise in two distinct ways:
from excited states containing only (static) fluxes and no (mobile) fermions;
and from excited states in which fermions are bound to fluxes. The SF is
markedly different in Abelian and non-Abelian QSLs, and bound fermion-flux
composites appear only in the non-Abelian phase.Comment: 21 pages, 14 figure
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