107 research outputs found
Non-Hermitian topology of spontaneous magnon decay
Spontaneous magnon decay is a generic feature of the magnetic excitations of
anisotropic magnets and isotropic magnets with non-collinear order. In this
paper, we argue that the effect of interactions on one-magnon states can, under
many circumstances, be treated in terms of an effective, energy independent,
non-Hermitian Hamiltonian for the magnons. In the vicinity of Dirac or Weyl
touching points, we show that the spectral function has a characteristic
anisotropy arising from topologically protected exceptional points or lines in
the non-Hermitian spectrum. Such features can, in principle, be detected using
inelastic neutron scattering or other spectroscopic probes. We illustrate this
physics through a concrete example: a honeycomb ferromagnet with
Dzyaloshinskii-Moriya exchange. We perform interacting spin wave calculations
of the structure factor and spectral function of this model, showing good
agreement with results from a simple effective non-Hermitian model for the
splitting of the Dirac point. Finally, we argue that the zoo of known
topological protected magnon band structures may serve as a nearly ideal
platform for realizing and exploring non-Hermitian physics in solid-state
systems.Comment: 4+epsilon page
Pseudo-Goldstone gaps and order-by-quantum-disorder in frustrated magnets
In systems with competing interactions, continuous degeneracies can appear
which are accidental, in that they are not related to any symmetry of the
Hamiltonian. Accordingly, the pseudo-Goldstone modes associated with these
degeneracies are also unprotected. Indeed, through a process known as
"order-by-quantum-disorder", quantum zero-point fluctuations can lift the
degeneracy and induce a gap for these modes. We show that this gap can be
exactly computed at leading order in in spin-wave theory from the mean
curvature of the classical and quantum zero-point energies - without the need
to consider any spin-wave interactions. We confirm this equivalence through
direct calculations of the spin-wave spectrum to in a wide variety
of theoretically and experimentally relevant quantum spin models. We prove this
equivalence through the use of an exact sum rule that provides the required
mixing of different orders of . Finally, we discuss some implications for
several leading order-by-quantum-disorder candidate materials, clarifying the
expected pseudo-Goldstone gap sizes in ErTiO and
CaFeGeO.Comment: 5 + 26 pages, 3 figures. Corrected discussion of cubic garnet,
expanded supplemental materia
Ultrasonic investigations of spin-ices DyTiO and HoTiO in and out of equilibrium
We report ultrasound studies of spin-lattice and single-ion effects in the
spin-ice materials DyTiO (DTO) and HoTiO (HTO) across a
broad field range up to 60 T, covering phase transformations, interactions with
low-energy magnetic excitations, as well as single-ion effects. In particular,
a sharp dip observed in the sound attenuation in DTO at the gas-liquid
transition of the magnetic monopoles is explained based on an approach
involving negative relaxation processes. Furthermore, quasi-periodic peaks in
the acoustic properties of DTO due to non-equilibrium processes are found to be
strongly affected by {\em macroscopic} thermal-coupling conditions: the thermal
runaway observed in previous studies in DTO can be suppressed altogether by
immersing the sample in liquid helium. Crystal-electric-field effects having
higher energy scale lead to a renormalization of the sound velocity and sound
attenuation at very high magnetic fields. We analyze our observations using an
approach based on an analysis of exchange-striction couplings and single-ion
effects
Dirac Magnons, Nodal Lines, and Nodal Plane in Elemental Gadolinium
We investigate the magnetic excitations of elemental gadolinium (Gd) using inelastic neutron scattering, showing that Gd is a Dirac magnon material with nodal lines at K and nodal planes at half integer l. We find an anisotropic intensity winding around the K-point Dirac magnon cone, which is interpreted to indicate Berry phase physics. Using linear spin wave theory calculations, we show the nodal lines have nontrivial Berry phases, and topological surface modes. We also discuss the origin of the nodal plane in terms of a screw-axis symmetry, and introduce a topological invariant characterizing its presence and effect on the scattering intensity. Together, these results indicate a highly nontrivial topology, which is generic to hexagonal close packed ferromagnets. We discuss potential implications for other such systems
Spin-exchange Hamiltonian and topological degeneracies in elemental gadolinium
We present a comprehensive study of the magnetic exchange Hamiltonian of elemental gadolinium. We use neutron scattering to measure the magnon spectrum over the entire Brillouin zone and fit the excitations to a spin wave model to extract the first 26 nearest-neighbor magnetic exchange interactions with rigorously defined uncertainty. We find these exchange interactions to follow RKKY behavior, oscillating from ferromagnetic to antiferromagnetic as a function of distance. Finally, we discuss the topological features and degeneracies in Gd, and HCP ferromagnets in general. We show theoretically how, with asymmetric exchange, topological properties could be tuned with a magnetic field
Topological Magnons in Kitaev Magnets at High Fields
We study the Kitaev-Heisenberg-- model that describes the
magnetism in strong spin-orbit coupled honeycomb lattice Mott insulators. In
strong magnetic fields that bring the system into the fully polarized
paramagnetic phase, we find that the spin wave bands carry nontrivial Chern
numbers over large regions of the phase diagram implying the presence of chiral
magnon edge states. In contrast to other topological magnon systems, the
topological nontriviality of these systems results from the presence of magnon
number non-conserving terms in the Hamiltonian. Since the effects of
interactions are suppressed by , the validity of the single particle
picture is tunable making paramagnetic phases particularly suitable for the
exploration of this physics. Using time dependent DMRG and interacting spin
wave theory, we demonstrate the presence of the chiral edge mode and its
evolution with field.Comment: 16 pages (main text + supplementary material), 10 figure
Lack of Evidence for a Singlet Crystal Field Ground State in the Tb2Ti2O7 Magnetic Pyrochlore
We present new high resolution inelastic neutron scattering data on the
candidate spin liquid Tb2Ti2O7. We find that there is no evidence for a zero
field splitting of the ground state doublet within the 0.2 K resolution of the
instrument. This result contrasts with a pair of recent works on Tb2Ti2O7
claiming that the spin liquid behavior can be attributed to a 2 K split
singlet-singlet single-ion spectrum at low energies. We also reconsider the
entropy argument presented in Chapuis {\it et al.} as further evidence of a
singlet-singlet crystal field spectrum. We arrive at the conclusion that
estimates of the low temperature residual entropy drawn from heat capacity
measurements are a poor guide to the single ion spectrum without understanding
the nature of the correlations.Comment: 4 pages, 3 figure. Submitted to Physical Review
Proposal for a [111] Magnetization Plateau in the Spin Liquid State of Tb2Ti2O7
Despite a Curie-Weiss temperature K, the Tb2Ti2O7
pyrochlore magnetic material lacks long range magnetic order down to at least
mK. It has recently been proposed that the low temperature
collective paramagnetic or spin liquid regime of this material may be akin to a
spin ice state subject to both thermal and quantum fluctuations a {\it
quantum spin ice} (QSI) of sorts. Here we explore the effect of a magnetic
field along the direction on the QSI state. To do so, we
investigate the magnetic properties of a microscopic model of Tb2Ti2O7 in an
independent tetrahedron approximation in a finite along . Such
a model describes semi-quantitatively the collective paramagnetic regime where
nontrivial spin correlations start to develop at the shortest lengthscale, that
is over a single tetrahedron, but where no long range order is yet present. Our
results show that a magnetization plateau develops at low temperatures as the
system develops ferromagnetic spin-ice-like "two-in/two-out"
correlations at the shortest lengthscale. From these results, we are led to
propose that the observation of such a [111] magnetization plateau in Tb2Ti2O7
would provide compelling evidence for a QSI at in this material and
help guide the development of a theory for the origin of its spin liquid state.Comment: 6 pages, 3 figure
Micromagnetometry of two-dimensional ferromagnets
The study of atomically thin ferromagnetic crystals has led to the discovery
of unusual magnetic behaviour and provided insight into the magnetic properties
of bulk materials. However, the experimental techniques that have been used to
explore ferromagnetism in such materials cannot probe the magnetic field
directly. Here, we show that ballistic Hall micromagnetometry can be used to
measure the magnetization of individual two-dimensional ferromagnets. Our
devices are made by van der Waals assembly in such a way that the investigated
ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from
encapsulated graphene. We use the micromagnetometry technique to study
atomically thin chromium tribromide (CrBr3). We find that the material remains
ferromagnetic down to monolayer thickness and exhibits strong out-of-plane
anisotropy. We also find that the magnetic response of CrBr3 varies little with
the number of layers and its temperature dependence cannot be described by the
simple Ising model of two-dimensional ferromagnetism.Comment: 19 pages, 12 figure
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