3,142 research outputs found

### 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

### Spin excitations in layered antiferromagnetic metals and superconductors

The proximity of antiferromagnetic order in high-temperature superconducting
materials is considered a possible clue to the electronic excitations which
form superconducting pairs. Here we study the transverse and longitudinal spin
excitation spectrum in a one-band model in the pure spin density wave (SDW)
state and in the coexistence state of SDW and the superconductivity. We start
from a Stoner insulator and study the evolution of the spectrum with doping,
including distinct situations with only hole pockets, with only electron
pockets and with pockets of both types. In addition to the usual spin-wave
modes, in the partially gapped cases we find significant weight of low-energy
particle-hole excitations. We discuss the implications of our findings for
neutron scattering experiments and for theories of Cooper-pairing in the
metallic SDW state.Comment: (14 pages, 6 figures

### 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 $s^{+-}$ symmetry of
the order parameter. The effect of possible gap minima or nodes is discussed.Comment: 19 pages, 7 figure

### 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 $Z_2$ gauge field. In
addition to a broad continuum from spin-fractionalization, we find sharp
($\delta$-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

### Dynamics of a two-dimensional quantum spin liquid: signatures of emergent Majorana fermions and fluxes

Topological states of matter present a wide variety of striking new
phenomena. Prominent among these is the fractionalisation of electrons into
unusual particles: Majorana fermions [1], Laughlin quasiparticles [2] or
magnetic monopoles [3]. Their detection, however, is fundamentally complicated
by the lack of any local order, such as, for example, the magnetisation in a
ferromagnet. While there are now several instances of candidate topological
spin liquids [4], their identification remains challenging [5]. Here, we
provide a complete and exact theoretical study of the dynamical structure
factor of a two-dimensional quantum spin liquid in gapless and gapped phases.
We show that there are direct signatures - qualitative and quantitative - of
the Majorana fermions and gauge fluxes emerging in Kitaev's honeycomb model.
These include counterintuitive manifestations of quantum number
fractionalisation, such as a neutron scattering response with a gap even in the
presence of gapless excitations, and a sharp component despite the
fractionalisation of electron spin. Our analysis identifies new varieties of
the venerable X-ray edge problem and explores connections to the physics of
quantum quenches.Comment: 7 pages, 3 figure

### Raman scattering in correlated thin films as a probe of chargeless surface states

Several powerful techniques exist to detect topologically protected surface
states of weakly-interacting electronic systems. In contrast, surface modes of
strongly interacting systems which do not carry electric charge are much harder
to detect. We propose resonant light scattering as a means of probing the
chargeless surface modes of interacting quantum spin systems, and illustrate
its efficacy by a concrete calculation for the 3D hyperhoneycomb Kitaev quantum
spin liquid phase. We show that resonant scattering is required to efficiently
couple to this model's sublattice polarized surface modes, comprised of
emergent Majorana fermions that result from spin fractionalization. We
demonstrate that the low-energy response is dominated by the surface
contribution for thin films, allowing identification and characterization of
emergent topological band structures.Comment: 7 pages, 4 figures; added supplemental materia

### Theory of Raman response in three-dimensional Kitaev spin liquids: application to $\beta-$ and $\gamma-$Li$_2$IrO$_3$ compounds

We calculate the Raman response for the Kitaev spin model on the
$\mathcal{H}$-$0$, $\mathcal{H}$-$1$, and $\mathcal{H}$-$\infty$ harmonic
honeycomb lattices. We identify several quantitative features in the Raman
spectrum that are characteristic of the spin liquid phase. Unlike the dynamical
structure factor, which probes both the Majorana spinons and flux excitations
that emerge from spin fractionalization, the Raman spectrum in the Kitaev
models directly probes a density of states of pairs of fractional, dispersing
Majorana spinons. As a consequence, the Raman spectrum in all these models is
gapless for sufficiently isotropic couplings, with a low-energy power law that
results from the Fermi lines (or points) of the dispersing Majorana spinons. We
show that the polarization dependence of the Raman spectrum contains crucial
information about the symmetry of the ground state. We also discuss to what
extent the features of the Raman response that we find reflect generic
properties of the spin liquid phase, and comment on their possible relevance to
$\alpha-$, $\beta-$ and $\gamma-$Li$_2$IrO$_3$ compounds.Comment: 19 pages, 10 figures. VERSION 2: Corrected Figure 5 and fixed
inconsistencies between A and B chain-labelings. Also- a few typos and two
new ref

### Neutron scattering signatures of the 3D hyper-honeycomb Kitaev quantum spin-liquid

Motivated by recent synthesis of the hyper-honeycomb material
$\beta$-$\mathrm{Li_2 Ir O_3}$, we study the dynamical structure factor (DSF)
of the corresponding 3D Kitaev quantum spin-liquid (QSL), whose fractionalised
degrees of freedom are Majorana fermions and emergent flux-loops. Properties of
this 3D model are known to differ in important ways from those of its 2D
counterpart -- it has finite-temperature phase transition, as well as distinct
features in Raman response. We show, however, that the qualitative behaviour of
the DSF is broadly dimension-independent. Characteristics of the 3D DSF include
a response gap even in the gapless QSL phase and an energy dependence deriving
from the Majorana fermion density of states. Since the majority of the response
is from states containing a single Majorana excitation, our results suggest
inelastic neutron scattering as the spectroscopy of choice to illuminate the
physics of Majorana fermions in Kitaev QSLs.Comment: 5 pages, 5 figure

### Resonant Raman scattering theory for Kitaev models and their Majorana fermion boundary modes

We study the inelastic light scattering response in two- (2D) and
three-dimensional (3D) Kitaev spin-liquid models with \ms band structures in
the symmetry classes BDI and D leading to protected gapless surface modes. We
present a detailed calculation of the resonant Raman/Brillouin scattering
vertex relevant to iridate and ruthenate compounds whose low-energy physics is
believed to be proximate to these spin-liquid phases. In the symmetry class
BDI, we find that while the resonant scattering on thin films can detect the
gapless boundary modes of spin liquids, the non-resonant processes do not
couple to them. For the symmetry class D, however, we find that the coupling
between both types of light-scattering processes and the low-energy surface
states is strongly suppressed. Additionally, we describe the effect of weak
time-reversal symmetry breaking perturbations on the bulk Raman response of
these systems.Comment: 23 pages, 20 figures, 4 appendices, 2 ancillary file

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