arXiv subjects

P. A. McClarty

Publications and source records attributed to P. A. McClarty.

18 recordsLinked to original sources

arXiv2024

Observing Altermagnetism using Polarized Neutrons

Altermagnets are colinear compensated magnets whose magnetic symmetries at zero spin-orbit coupling break spin degeneracy leading to spin-split electronic and magnonic bands that reflect an underlying multipolar order. When there is an approximate $U(1)$ symmetry the magnons in altermagnets are split into equal and opposite chiral pairs. We show that in altermagnets polarized neutrons provide a means to detect the population of time-reversed domains and allow direct measurement of the magnon chirality anisotropy in momentum space -- the central signature of the altermagnetic phase. We demonstrate this response to polarized neutrons in two candidate materials MnF$_2$ and MnTe and show that the presence of these chiralities is stable to small perturbations that break spin-rotation symmetry. This provides a magnonic analogue of spin polarized ARPES that has been used to discern altermagnetism in the electronic band structures of various candidate materials.

cond-mat.str-el↗

arXiv2024

The landscape of symmetry enhancement in tight-binding models

Band structures are ubiquitous in condensed matter physics and their symmetries constrain possible degeneracies, topology and response functions across a broad range of different systems. Here we address the question: given a parent crystal, what is the symmetry of hopping models on that lattice at a given shell number? We find that the parent structure does not, in general, determine the symmetry of the tight-binding model. Instead, the symmetry is dependent on the hopping range. The key to symmetry breakdown on the lattice is the existence of different {\it bond equivalence classes} whose number is related to group-subgroup indices for a broad classes of cases. We find all bond equivalence classes for $s$-wave hopping out to 20th neighbor across the different space groups and Wyckoff positions and the symmetries of the associated tight-binding models. These observations naturally lead to the definition of a {\it bond complex} $-$ the possible classes of networks of bonds to which symmetries may be enhanced from a given parent structure.

cond-mat.str-el↗

arXiv2021

Order-by-Disorder from Bond-Dependent Exchange and Intensity Signature of Nodal Quasiparticles in a Honeycomb Cobaltate

Recent theoretical proposals have argued that cobaltates with edge-sharing octahedral coordination can have significant bond-dependent exchange couplings thus offering a platform in 3$d$ ions for such physics beyond the much-explored realizations in 4$d$ and 5$d$ materials. Here we present high-resolution inelastic neutron scattering data within the magnetically ordered phase of the stacked honeycomb magnet CoTiO$_3$ revealing the presence of a finite energy gap and demonstrate that this implies the presence of bond-dependent anisotropic couplings. We also show through an extensive theoretical analysis that the gap further implies the existence of a quantum order-by-disorder mechanism that, in this material, crucially involves virtual crystal field fluctuations. Our data also provide an experimental observation of a universal winding of the scattering intensity in angular scans around linear band-touching points for both magnons and dispersive spin-orbit excitons, which is directly related to the non-trivial topology of the quasiparticle wavefunction in momentum space near nodal points.

cond-mat.str-el↗

arXiv2021

Spin-Space Groups and Magnon Band Topology

Band topology is both constrained and enriched by the presence of symmetry. The importance of anti-unitary symmetries such as time reversal was recognized early on leading to the classification of topological band structures based on the ten-fold way. Since then, lattice point group and non-symmorphic symmetries have been seen to lead to a vast range of possible topologically nontrivial band structures many of which are realized in materials. In this paper we show that band topology is further enriched in many physically realizable instances where magnetic and lattice degrees of freedom are wholly or partially decoupled. The appropriate symmetry groups to describe general magnetic systems are the spin-space groups. Here we describe cases where spin-space groups are essential to understand the band topology in magnetic materials. We then focus on magnon band topology where the theory of spin-space groups has its simplest realization. We consider magnetic Hamiltonians with various types of coupling including Heisenberg and Kitaev couplings revealing a hierarchy of enhanced magnetic symmetry groups depending on the nature of the lattice and the couplings. We describe, in detail, the associated representation theory and compatibility relations thus characterizing symmetry-enforced constraints on the magnon bands revealing a proliferation of nodal points, lines, planes and volumes.

cond-mat.str-el↗

arXiv2020

Non-collinear Order and Spin-Orbit Coupling in Sr$_3$ZnIrO$_6$

Sr$_{3}$ZnIrO$_{6}$ is an effective spin one-half Mott insulating iridate belonging to a family of magnets which includes a number of quasi-one dimensional systems as well as materials exhibiting three dimensional order. Here we present the results of an extensive investigation into the magnetism including heat capacity, a.c. susceptibility, muon spin rotation ($μ$SR), neutron diffraction and inelastic neutron scattering on the same sample. It is established that the material exhibits a transition at about $17$ K into a three-dimensional antiferromagnetic structure with propagation vector $\boldsymbol{k}=(0,\frac{1}{2},1)$ in the hexagonal setting of R$\bar{3}$c and non-collinear moments of $0.87$$μ_B$ on Ir$^{4+}$ ions. Further we have observed a well defined powder averaged spin wave spectrum with zone boundary energy of $\sim 5$ meV at $5$ K. We stress that a theoretical analysis shows that the observed non-collinear magnetic structure arises from anisotropic inter- and intra- chain exchange which has its origin in significant spin-orbit coupling. The model can satisfactorily explain the observed spin wave excitations.

cond-mat.str-el↗

arXiv2019

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.

cond-mat.mes-hall↗

arXiv2018

Pauling entropy, metastability and equilibrium in Dy$_2$Ti$_2$O$_7$ spin ice

Determining the fate of the Pauling entropy in the classical spin ice material Dy$_2$Ti$_2$O$_7$ with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice - the dipolar spin ice model - predicts an ordering transition at $T\approx 0.15$ K, but recent experiments by Pomaranski $et\ al.$ suggest an entropy recovery over long time scales at temperatures as high as $0.5$ K, much too high to be compatible with theory. Using neutron scattering and specific heat measurements at low temperatures and with long time scales ($0.35$ K$/10^6$ s and $0.5$ K$/10^5$ s respectively) on several isotopically enriched samples we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Further, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez $et\ al.$ are, after all, essentially correct: the short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.

cond-mat.str-el↗

arXiv2018

Topological Magnons in Kitaev Magnets at High Fields

We study the Kitaev-Heisenberg-$Γ$-$Γ'$ model that describes the magnetism in strong spin-orbit coupled honeycomb lattice Mott insulators. In strong $[111]$ 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 $J/h$, 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.

cond-mat.str-el↗

arXiv2017

Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore Yb$_2$Ti$_2$O$_7$ in Magnetic Field

The frustrated pyrochlore magnet Yb$_2$Ti$_2$O$_7$ has the remarkable property that it orders magnetically, but has no propagating magnons over wide regions of the Brillouin zone. Here we use inelastic neutron scattering to follow how the spectrum evolves in cubic-axis magnetic fields. At high fields we observe in addition to dispersive magnons also a two-magnon continuum, which grows in intensity upon reducing the field and overlaps with the one-magnon states at intermediate fields leading to strong renormalization of the dispersion relations, and magnon decays. Using heat capacity measurements we find that the low and high field regions are smoothly connected with no sharp phase transition, with the spin gap increasing monotonically in field. Through fits to an extensive data set we re-evaluate the spin Hamiltonian finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong fluctuations and quasiparticle breakdown effects observed at low fields.

cond-mat.str-el↗

arXiv2017

Temperature induced phase transition from cycloidal to collinear antiferromagnetism in multiferroic Bi$_{0.9}$Sm$_{0.1}$FeO$_3$ driven by $f$-$d$ induced magnetic anisotropy

In multiferroic BiFeO$_3$ a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that by substituting samarium for 10% of the bismuth ions the periodicity of the room temperature cycloid is increased, and by cooling below $\sim15$ K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from $f-d$ exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order - a result of general significance regarding the stability of non-collinear magnetic structures in the presence of multiple magnetic sublattices.

cond-mat.str-el↗

arXiv2016

Topological Triplon Modes and Bound States in a Shastry-Sutherland Magnet

The twin discoveries of the quantum Hall effect, in the 1980's, and of topological band insulators, in the 2000's, were landmarks in physics that enriched our view of the electronic properties of solids. In a nutshell, these discoveries have taught us that quantum mechanical wavefunctions in crystalline solids may carry nontrivial topological invariants which have ramifications for the observable physics. One of the side effects of the recent topological insulator revolution has been that such physics is much more widespread than was appreciated ten years ago. For example, while topological insulators were originally studied in the context of electron wavefunctions, recent work has led to proposals of topological insulators in bosonic systems: in photonic crystals, in the vibrational modes of crystals, and in the excitations of ordered magnets. Here we confirm the recent proposal that, in a weak magnetic field, the dimerized quantum magnet SrCu$_{2}$(BO$_{3}$)$_2$ is a bosonic topological insulator with nonzero Chern number in the triplon bands and topologically protected chiral edge excitations.

cond-mat.str-el↗

arXiv2014

Order Induced by Dilution in Pyrochlore XY Antiferromagnets

XY pyrochlore antiferromagnets are well-known to exhibit order-by-disorder through both quantum and thermal selection. In this paper we consider the effect of substituting non-magnetic ions onto the magnetic sites in a pyrochlore XY model with generally anisotropic exchange tuned by a single parameter $J^{\pm\pm}/J^\pm$. The physics is controlled by two points in this space of parameters $J^{\pm\pm}/J^\pm=\pm 2$ at which there are line modes in the ground state and hence an $O(L^2)$ ground state degeneracy intermediate between that of a conventional magnet and a Coulomb phase. At each of these points, single vacancies seed pairs of line defects. Two line defects carrying incompatible spin configurations from different vacancies can cross leading to an effective one-dimensional description of the resulting spin texture. In the thermodynamic limit at finite density, we find that dilution selects a state "opposite" to the state selected by thermal and quantum disorder which is understood from the single vacancy limit. The latter finding hints at the possibility that Er$_{2-x}$Y$_x$Ti$_2$O$_7$ for small $x$ exhibits a second phase transition within the thermally selected $ψ_2$ state into a $ψ_3$ state selected by the quenched disorder.

cond-mat.dis-nn↗

arXiv2011

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.

cond-mat.dis-nn↗

arXiv2011

Local Susceptibility of the Yb2Ti2O7 Rare Earth Pyrochlore Computed from a Hamiltonian with Anisotropic Exchange

The rare earth pyrochlore magnet Yb2Ti2O7 is among a handful of materials that apparently exhibit no long range order down to the lowest explored temperatures and well below the Curie-Weiss temperature. Paramagnetic neutron scattering on a single crystal sample has revealed the presence of anisotropic correlations and recent work has led to the proposal of a detailed microscopic Hamiltonian for this material involving significantly anisotropic exchange. In this article, we compute the local sublattice susceptibility of Yb2Ti2O7 from the proposed model and compare with the measurements of Cao and coworkers [Physical Review Letters, {103}, 056402 (2009)], finding quite good agreement. In contrast, a model with only isotropic exchange and long range magnetostatic dipoles gives rise to a local susceptiblity that is inconsistent with the data.

cond-mat.stat-mech↗

arXiv2010

Calculation of the Expected Zero Field Muon Relaxation Rate in the Geometrically Frustrated Rare Earth Pyrochlore Gd2Sn2O7 Antiferromagnet

The magnetic insulator Gd2Sn2O7 is one of many geometrically frustrated magnetic materials known to exhibit a nonzero muon spin polarization relaxation rate, $λ(T)$, down to the lowest temperature (T) studied. Such behaviour is typically interpreted as a significant level of persisting spin dynamics (PSD) of the host material. In the case of Gd2Sn2O7, such PSD comes as a surprise since magnetic specific heat measurements suggest conventional gapped magnons, which would naively lead to an exponentially vanishing $λ(T)$ as $T \rightarrow 0$. In contrast to most materials that display PSD, the ordered phase of Gd2Sn2O7 is well characterized and both the nature and the magnitude of the interactions have been inferred from the magnetic structure and the temperature dependence of the magnetic specific heat. Based on this understanding, the temperature dependence of the muon spin polarization relaxation through the scattering of spin waves (magnons) is calculated. The result explicitly shows that, despite the unusual extensive number of weakly dispersive (gapped)excitations characterizing Gd2Sn2O7, a remnant of the zero modes of the parent frustrated pyrochlore Heisenberg antiferromagnet, the temperature dependence of the calculated $λ(T)$ differs dramatically from the experimental one. Indeed, the calculation conforms to the naive expectation of an exponential collapse of $λ(T)$ at temperatures below ~ 0.7K. This result, for the first time, illustrates crisply and quantitatively the paradox that presents itself with the pervasive occurrence of PSD in highly frustrated magnetic systems as evinced by muon spin relaxation measurements.

cond-mat.dis-nn↗

arXiv2008

Energetic selection of ordered states in a model of the Er2Ti2O7 frustrated pyrochlore XY antiferromagnet

We consider the possibility that the discrete long-range ordered states of Er2Ti2O7 are selected energetically at the mean field level as an alternative scenario that suggests selection via thermal fluctuations. We show that nearest neighbour exchange interactions alone are not sufficient for this purpose, but that anisotropies arising from excited single ion crystal field states in Er2Ti2O7, together with appropriate anisotropic exchange interactions, can produce the required long range order. However, the effect of the single ion anisotropies is rather weak so we expect thermal or quantum fluctuations, in some guise, to be ultimately important in this material. We reproduce recent experimental results for the variation of magnetic Bragg peak intensities as a function of magnetic field.

cond-mat.stat-mech↗

arXiv2007

A study of a local Monte Carlo technique for simulating systems of charged particles

We study some aspects of a Monte Carlo method invented by Maggs and Rossetto for simulating systems of charged particles. It has the feature that the discretized electric field is updated locally when charges move. Results of simulations of the two dimensional one-component plasma are presented. Highly accurate results can be obtained very efficiently using this lattice method over a large temperature range. The method differs from global methods in having additional degrees of freedom which leads to the question of how a faster method can result. We argue that efficient sampling depends on charge mobility and find that the mobility is close to maximum for a low rate of independent plaquette updates for intermediate temperatures. We present a simple model to account for this behavior. We also report on the role of uniform electric field sampling using this method.

cond-mat.stat-mech↗

arXiv2007

Freezing effects in the two dimensional one-component plasma and in thin film type II superconductors

We present results of Monte Carlo simulations of the two dimensional one-component plasma and of the Ginzberg-Landau model in the lowest Landau level approximation, with both charges and vortices respectively confined within a disc. In both models we see that as the temperature is reduced, oscillations in the radial density develop which spread into the bulk from the edge of the disc. The amplitude of these oscillations grows as the temperature is lowered and the length scale over which the oscillations occurs is the same as the correlation length for local crystalline order at that temperature. At temperatures similar to those where earlier studies have reported a first-order fluid-crystal phase transition, the correlation length is comparable to the linear dimensions of the samples studied, which suggests that finite size effects will be affecting the accuracy of their conclusions.

cond-mat.supr-con↗