arXiv subjects

Andrew T. Boothroyd

Publications and source records attributed to Andrew T. Boothroyd.

At least 19 recordsLinked to original sources

arXiv2026

Resolving the Magnetic Ground State and Field-Induced Transitions in Magnetic Dirac Semimetal Candidate EuMnSb$_2$

The magnetic structure of a magnetic topological semimetal EuMnSb$_2$ is investigated in fields up to 30 T using polarized and unpolarized neutron diffraction, pulsed-field x-ray magnetic circular dichroism and pulsed-field magnetometry. We determine the zero-field magnetic structures of the Eu and Mn sublattices, and find that magnetic transitions induced by applied fields below 2 T correspond to changes in the magnetic order of the Eu spins alone without detectable perturbation to the order of the Mn spins. An additional magnetic transition is observed at fields close to the saturation field for the Eu spins. We present a mean-field model which describes key features of the magnetic behavior and allows us to estimate the dominant Eu--Eu and Eu--Mn exchange interactions responsible for the coupling between magnetism and electronic topology.

cond-mat.str-el↗

arXiv2026

Higher-order Weyl nodes driven by helical magnetic order in EuAgAs

Magnetic topological semimetals provide a fertile ground for exploring how long-range magnetic order can alter electronic band structures and generate novel quasiparticles such as Weyl fermions. Here, we investigate the coupled magnetic and electronic structure of single-crystalline EuAgAs, a hexagonal pnictide whose magnetic ground state has remained elusive. Using neutron diffraction and resonant elastic X-ray scattering, we identify an unusual magnetic ordering sequence with two successive phase transitions at $T_\mathrm{N1} = 12$ K and $T_\mathrm{N2} = 8$ K. We observe two slightly different magnetic propagation vectors, one associated with $T_\mathrm{N1}$ and the other with $T_\mathrm{N2}$. Spherical neutron polarimetry reveals that the magnetic structure is a transverse helix aligned along the $c$ axis with a period that is approximately twice the $c$ lattice parameter. First-principles calculations for the helical phase predict subtle band folding effects and the emergence of effective higher-order Weyl nodes. These topological features appear near the calculated Fermi energy $E_{\mathrm{F}}$ which, however, lies above the position of $E_{\mathrm{F}}$ obtained from angle-resolved photoemission spectroscopy so could not be probed in this study.

cond-mat.str-el↗

arXiv2025

Ground state magnetic structure of Mn3Sn

We use spherical neutron polarimetry to determine the ground state magnetic structure of Mn3Sn. We find that Mn3Sn adopts an inverse triangular structure with spins parallel to <100> (Type III) rather than spins parallel to <110> (Type IV). Density functional theory calculations reveal no energy difference between these two structures, suggesting that the selection is caused by subtle effects such as sixth-order anisotropy. Partial control of the magnetic domain population through a moderate magnetic field is key to distinguish between the two models. We find that three of the six domains are approximately equally populated, while the others have negligible population. Upon entering the low temperature incommensurate phase, the domain structure is lost. The domains decouple from the magnetic field, and can therefore not be controlled by any known method.

cond-mat.mtrl-sci↗

arXiv2025

Long-range magnetic interactions in Nd$_2$PdSi$_3$ and the formation of skyrmion phases in centrosymmetric metals

We present an extensive X-ray and neutron scattering study of the structure and magnetic excitations of Nd$_2$PdSi$_3$, a sister compound of Gd$_2$PdSi$_3$ which was recently found to host a skyrmion lattice phase despite its centrosymmetric crystal structure. Dispersive magnetic excitations were measured throughout the Brillouin zone and modeled to determine the magnetic interactions between Nd ions. Our analysis reveals that the magnetic interactions in this system extend over large distances and are significantly affected by a crystallographic superstructure formed by ordering of the Pd and Si atoms. The results suggest that the mechanism for the skyrmion phase formation in this family of materials, specifically Gd$_2$PdSi$_3$, is through the long-range RKKY interactions rather than short-range triangular-lattice frustration.

cond-mat.str-el↗

arXiv2025

Probing spectral features of quantum many-body systems with quantum simulators

The efficient probing of spectral features is important for characterising and understanding the structure and dynamics of quantum materials. In this work, we establish a framework for probing the excitation spectrum of quantum many-body systems with quantum simulators. Our approach effectively realises a spectral detector by processing the dynamics of observables with time intervals drawn from a defined probability distribution, which only requires native time evolution governed by the Hamiltonian without ancilla. The critical element of our method is the engineered emergence of frequency resonance such that the excitation spectrum can be probed. We show that the time complexity for transition energy estimation has a logarithmic dependence on simulation accuracy and how such observation can be guaranteed in certain many-body systems. We discuss the noise robustness of our spectroscopic method and show that the total running time maintains polynomial dependence on accuracy in the presence of device noise. We further numerically test the error dependence and the scalability of our method for lattice models. We present simulation results for the spectral features of typical quantum systems, either gapped or gapless, including quantum spins, fermions and bosons. We demonstrate how excitation spectra of spin-lattice models can be probed experimentally with IBM quantum devices.

quant-ph↗

arXiv2025

Goldstone mode of the broken helix in U(1) magnet EuIn2As2

Goldstone modes acquire a frequency gap in the presence of perturbations that break the underlying continuous symmetry. Here, we study the response of a spin-based Goldstone mode to strain and magnetic field in the broken helix, a multi-$\textbf{Q}$ phase of EuIn$_2$As$_2$. Optical polarimetry with spatial and temporal resolution allows us to access information about both the structure and frequency of optically excited spin-wave modes under different strain conditions. We observe nearly uniform spin precession characteristic of a Goldstone mode only when magnetic field dominates over strain. In this regime, the frequency depends linearly on the applied field. A symmetry analysis for predicting the mode frequency near zero field demonstrates that the observed scaling is of the lowest allowed order. This work thus demonstrates the connections between magnetic symmetries and the frequency dependence of the Goldstone mode in an external field, and illustrates the power of our technique for studying the dynamics of complex magnets.

cond-mat.mtrl-sci↗

arXiv2023

Topological electronic bands in crystalline solids

Topology is now securely established as a means to explore and classify electronic states in crystalline solids. This review provides a gentle but firm introduction to topological electronic band structure suitable for new researchers in the field. I begin by outlining the relevant concepts from topology, then give a summary of the theory of non-interacting electrons in periodic potentials. Next, I explain the concepts of the Berry phase and Berry curvature, and derive key formulae. The remainder of the article deals with how these ideas are applied to classify crystalline solids according to the topology of the electronic states, and the implications for observable properties. Among the topics covered are the role of symmetry in determining band degeneracies in momentum space, the Chern number and Z2 topological invariants, surface electronic states, two- and three-dimensional topological insulators, and Weyl and Dirac semimetals

cond-mat.str-el↗

arXiv2023

Weyl metallic state induced by helical magnetic order

In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetals have been realized in ferro- or ferrimagnetically ordered compounds, but a disadvantage of these materials for practical use is their stray magnetic field which limits the minimum size of devices. Here we show that Weyl nodes can be induced by a helical spin configuration, in which the magnetization is fully compensated. Using a combination of neutron diffraction and resonant elastic x-ray scattering, we find that EuCuAs develops a planar helical structure below $T_\textrm{N}$ = 14.5 K which induces Weyl nodes along the $Γ$--A high symmetry line in the Brillouin zone.

cond-mat.str-el↗

arXiv2023

Magnetic excitations in the topological semimetal $\mathrm{YbMnSb}_2$

We report neutron scattering measurements on YbMnSb$_2$ which shed new light on the nature of the magnetic moments and their interaction with Dirac fermions. Using half-polarized neutron diffraction we measured the field-induced magnetization distribution in the paramagnetic phase and found that the magnetic moments are well localised on the Mn atoms. Using triple-axis neutron scattering we measured the magnon spectrum throughout the Brillouin zone in the antiferromagnetically ordered phase, and we determined the dominant exchange interactions from linear spin-wave theory. The analysis shows that the interlayer exchange is five times larger than in several related compounds containing Bi instead of Sb. We argue that the coupling between the Mn local magnetic moments and the topological band states is more important in YbMnSb$_2$ than in the Bi compounds.

cond-mat.str-el↗

arXiv2022

The magnetic structure of the topological semimetal Co$_3$Sn$_2$S$_2$

Co$_3$Sn$_2$S$_2$ has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co$_3$Sn$_2$S$_2$ below $T_\textrm{C} = 177$ K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal $c$ axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged $(\textbf{k}=\textbf{0})$ in-plane 120$^\circ$ antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between $\sim$90 K and $T_\textrm{C}$. A discontinuous change in bulk properties and neutron polarization observed at $T = 125$ K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co$_3$Sn$_2$S$_2$ is a magnetic Weyl semimetal.

cond-mat.str-el↗

arXiv2022

Investigation of metamagnetism and crystal-field splitting in pseudo-hexagonal CeRh$_3$Si$_2$

CeRh$_3$Si$_2$ has been reported to exhibit metamagnetic transitions below 5~K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) reveal a $J_z$\,=\,1/2 groundstate for Ce when considering the crystallographic $a$ direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78\,meV for the $J$\,=\,5/2 multiplet. The neutron diffraction study in zero field reveals that on cooling from the paramagnetic state, the system first orders at $T_{\text{N}_1}=4.7$\,K in a longitudinal spin density wave with ordered Ce moments along the $b$-axis (i.e. the [0 1 0] crystal direction) and an incommensurate propagation vector $\textbf{k}=(0,0.43,0$). Below the lower-temperature transition $T_{\text{N}_2}=4.48$\,K, the propagation vector locks to the commensurate value $\textbf{k}=(0,0.5,0)$, with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field $H\parallel b$-axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at $H=3$\,kOe, followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at $H = 7$\,kOe. This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behaviour is also observed for the $H\parallel$ [0 1 1] crystal direction.

cond-mat.str-el↗

arXiv2021

A model for coupled $4f-3d$ magnetic spectra: a neutron scattering study of the Yb$-$Fe hybridisation in Yb$_3$Fe$_5$O$_{12}$

In this work, we explore experimentally and theoretically the spectrum of magnetic excitations of the Fe$^{3+}$ and Yb$^{3+}$ ions in ytterbium iron garnet (Yb$_3$Fe$_5$O$_{12}$). We present a complete description of the crystal-field splitting of the $4f$ states of Yb$^{3+}$, including the effect of the exchange field generated by the magnetically ordered Fe subsystem. We also consider a further effect of the Fe-Yb exchange interaction, which is to hybridise the Yb crystal field excitations with the Fe spin-wave modes at positions in the Brillouin zone where the two types of excitations cross. We present detailed measurements of these hybridised excitations, and propose a framework which can be used in the quantitative analysis of the coupled spectra in terms of the anisotropic $4f-3d$ exchange interaction.

cond-mat.str-el↗

arXiv2021

Magnetic and electronic structure of the topological semimetal YbMnSb$_2$

The antiferromagnetic (AFM) semimetal YbMnSb$_2$ has recently been identified as a candidate topological material, driven by time-reversal symmetry breaking. Depending on the ordered arrangement of Mn spins below the Néel temperature, $T_\mathrm{N}$ = 345 K, the electronic bands near the Fermi energy can ether have a Dirac node, a Weyl node or a nodal line. We have investigated the ground state magnetic structure of YbMnSb$_2$ using unpolarized and polarized single crystal neutron diffraction. We find that the Mn moments lie along the $c$ axis of the $P4/nmm$ space group and are arranged in a C-type AFM structure, which implies the existence of gapped Dirac nodes near the Fermi level. The results highlight how different magnetic structures can critically affect the topological nature of fermions in semimetals.

cond-mat.str-el↗

arXiv2020

Magnetically induced metal-insulator transition in Pb2CaOsO6

We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure; Pb2CaOsO6 and Pb2ZnOsO6. Upon cooling below 80 K, Pb2CaOsO6 simultaneously undergoes a metal--insulator transition and develops antiferromagnetic order. Pb2ZnOsO6, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structure. The Os atoms in Pb2ZnOsO6 are arranged on an approximately face-centred cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of Pb2CaOsO6 is heavily distorted up to at least 500 K, including antiferroelectric-like displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which in turn drives the metal--insulator transition. Our results suggest that the phase transition in Pb2CaOsO6 is spin-driven, and could be a rare example of a Slater transition.

cond-mat.str-el↗

arXiv2019

Magnetic structure and excitations of the topological semimetal YbMnBi$_2$

We investigated the magnetic structure and dynamics of YbMnBi$_2$, with elastic and inelastic neutron scattering, to shed light on the topological nature of the charge carriers in the antiferromagnetic phase. We confirm C-type antiferromagnetic ordering of the Mn spins below $T_{\rm N} = 290$ K, and determine that the spins point along the $c$-axis to within about $3^\circ$. The observed magnon spectrum can be described very well by the same effective spin Hamiltonian as was used previously to model the magnon spectrum of CaMnBi$_2$. Our results show conclusively that the creation of Weyl nodes in YbMnBi$_2$ by the time-reversal-symmetry breaking mechanism can be excluded in the bulk.

cond-mat.str-el↗

arXiv2018

Coupling between Spin and Charge Order Driven by Magnetic Field in Triangular Ising System LuFe2O4+δ

We present a study of the magnetic-field effect on spin correlations in the charge ordered triangular Ising system LuFe2O4+δ through single crystal neutron diffraction. In the absence of a magnetic field, the strong diffuse neutron scattering observed below the Neel temperature (TN = 240 K) indicates that LuFe2O4+δ shows short-range, two-dimensional (2D) correlations in the FeO5 triangular layers, characterized by the development of a magnetic scattering rod along the 1/3 1/3 L direction, persisting down to 5 K. We also found that on top of the 2D correlations, a long range ferromagnetic component associated with the propagation vector k1 = 0 sets in at around 240 K. On the other hand, an external magnetic field applied along the c-axis effectively favours a three-dimensional (3D) spin correlation between the FeO5 bilayers evidenced by the increase of the intensity of satellite reflections with propagation vector k2 = (1/3, 1/3, 3/2). This magnetic modulation is identical to the charge ordered superstructure, highlighting the field-promoted coupling between the spin and charge degrees of freedom. Formation of the 3D spin correlations suppresses both the rod-type diffuse scattering and the k1 component. Simple symmetry-based arguments provide a natural explanation of the observed phenomenon and put forward a possible charge redistribution in the applied magnetic field.

cond-mat.str-el↗

arXiv2017

Static and Fluctuating Magnetic Moments in the Ferroelectric Metal LiOsO$_3$

LiOsO$_3$ is the first example of a new class of material called a ferroelectric metal. We performed zero-field and longitudinal-field $μ$SR, along with a combination of electronic structure and dipole field calculations, to determine the magnetic ground state of LiOsO$_3$. We find that the sample contains both static Li nuclear moments and dynamic Os electronic moments. Below $\approx 0.7\,$K, the fluctuations of the Os moments slow down, though remain dynamic down to 0.08$\,$K. We expect this could result in a frozen-out, disordered ground state at even lower temperatures.

cond-mat.str-el↗

arXiv2017

Spindynamics in the antiferromagnetic phases of the Dirac metals $A$MnBi$_2$ ($A=$ Sr, Ca)

The square Bi layers in $A$MnBi$_2$ ($A =$ Sr, Ca) host Dirac fermions which coexist with antiferromagnetic order on the Mn sublattice below $T_\mathrm{N} = 290\,$K (Sr) and $270\,$K (Ca). We have measured the spin-wave dispersion in these materials by triple-axis neutron spectroscopy. The spectra show pronounced spin gaps of 10.2(2)$\,$meV (Sr) and 8.3(8)$\,$meV (Ca) and extend to a maximum energy transfer of 61 - 63$\,$meV. The observed spectra can be accurately reproduced by linear spin-wave theory from an Heisenberg effective spin Hamiltonian. Detailed global fits of the full magnon dispersion are used to determine the in-plane and inter-layer exchange parameters as well as on the magnetocrystalline anisotropy constant. To within experimental error we find no evidence that the magnetic dynamics are influenced by the Dirac fermions.

cond-mat.str-el↗