arXiv subjects

D. A. Mayoh

Publications and source records attributed to D. A. Mayoh.

At least 19 recordsLinked to original sources

arXiv2026

Local magnetic properties of the rare-earth intermetallics $R$Mn$_2$Ge$_2$ ($R$=Ce, Pr, Nd)

We present an investigation of the rare-earth intermetallic materials, $R$Mn$_2$Ge$_2$ ($R$=Ce, Pr, Nd), reported to host a lattice of skyrmionic bubbles at room temperature. The magnetism of all three materials is characterised by the onset of local fluctuations as the magnetic state changes from a conical magnetic structure to a collinear antiferromagnetic one as temperature $T$ is increased. In this $T$ regime, where skyrmion-bubble textures have been reported, we see dynamics similar to those observed in other skyrmion-hosting materials. The level of disorder above this transition increases across the series from $R$=Ce to Nd. At low temperatures the magnetism is affected by the ordering of the rare-earth ions, resulting in distinct behaviour for the different members of the series.

cond-mat.str-el↗

arXiv2026

Magnetoelastic properties in the high-temperature magnetic phase of the skyrmion compound GdRu$_2$Si$_2$

We investigated the magnetoelastic properties of a GdRu$_2$Si$_2$ single crystal under a magnetic field applied along the crystallographic [001] and [110] directions. We report a series of strong anomalies in the sound velocity that is consistent with the complex phase diagram reported previously for this compound. In particular, in our study we focus on the recently identified magnetic phase in the high-temperature region. We show that while this phase is easily destroyed for magnetic fields applied along [001], it is rather stable for fields along [110]. Furthermore, we introduce a Landau theory and a microscopic toy model describing the elastic response at zero field. We reproduce qualitatively the observed anomalies for different acoustic modes, which allows us to propose a magnetic structure for this new high-temperature phase.

cond-mat.str-el↗

arXiv2025

Magnetism in $M_{1/3}$NbS$_2$ ($M$ = Fe, V, Mn): insight into intercalated transition-metal dichalcogenides using $μ$SR

We present the results of muon-spin relaxation ($μ$SR) measurements of the static and dynamic magnetism of $M_{1/3}$NbS$_2$ ($M$ = Fe, V, Mn), three intercalated transition-metal dichalcogenides. Transitions to long-range magnetic order are observed in all three materials and local magnetic fields at muon sites are compared to dipole field calculations. Measurements on Fe$_{1/3}$NbS$_2$ capture the evolution of two coexisting magnetic phases. In V$_{1/3}$NbS$_2$ we observe a peak in the dynamic response at $9$ K, coincident with previous reports of a possible low-temperature phase transition. The observation of high-frequency muon precession in Mn$_{1/3}$NbS$_2$ suggests the existence of an additional muon site that implies a difference in electronic energy landscape compared to the other materials in the series. Taken together, this demonstrates that the change in intercalant species drives significant variations in magnetism, highlighting the $M_{1/3}$NbS$_2$ ($M$ = Fe, V, Mn) series as an ideal group of materials for investigating a wide range of magnetic phenomena.

cond-mat.str-el↗

arXiv2025

Uniaxial Pressure Effects, Phase Diagram, and Tricritical Point in the Centrosymmetric Skyrmion Lattice Magnet GdRu$_2$Si$_2$

The magnetic phase diagram, magnetoelastic coupling, and uniaxial pressure effects of centrosymmetric magnetic skyrmion-hosting GdRu$_2$Si$_2$ are investigated by means of high-resolution capacitance dilatometry in fields up to 15\,T supported by specific heat and magnetisation studies. In addition to the previously reported phases in the $H$-$T$ phase diagram, we observe a third antiferromagnetic phase in zero magnetic field. We present the magnetic phase diagram and find two unreported phases, one of which features a comparably giant uniaxial pressure dependence. Our dilatometric measurements show magnetoelastic effects associated with the various magnetic ordering phenomena. We determine the uniaxial pressure dependencies of the various phases, in particular of the skyrmion lattice phase which is enhanced at higher fields and temperatures and also widens at a rate of 0.07~T/GPa when uniaxial pressure is applied along the $c$ axis. The relevance of fluctuations is further highlighted by the presence of tricritical point indicated by our thermodynamic data at the phase boundary separating two double-\textit{Q} magnetic configurations between which the skyrmion pocket phase evolves upon further cooling.

cond-mat.str-el↗

arXiv2025

Anisotropic skyrmion and multi-$q$ spin dynamics in centrosymmetric Gd$_2$PdSi$_3$

Skyrmions are particle-like vortices of magnetization with non-trivial topology, which are usually stabilized by Dzyaloshinskii-Moriya interactions (DMI) in noncentrosymmetric bulk materials. Exceptions are centrosymmetric Gd- and Eu-based skyrmion-lattice (SkL) hosts with zero DMI, where both the SkL stabilization mechanisms and magnetic ground states remain controversial. We address these here by investigating both the static and dynamical spin properties of the centrosymmetric SkL host Gd$_2$PdSi$_3$ using muon spectroscopy ($μ$SR). We find that spin fluctuations in the non-coplanar SkL phase are highly anisotropic, implying that spin anisotropy plays a prominent role in stabilizing this phase. We also observe strongly-anisotropic spin dynamics in the ground-state (IC-1) incommensurate magnetic phase of the material, indicating that it hosts a meron-like multi-$q$ structure. In contrast, the higher-field, coplanar IC-2 phase is found to be single-$q$ with nearly-isotropic spin dynamics.

cond-mat.str-el↗

arXiv2025

Muon spectroscopy investigation of anomalous dynamic magnetism in NiI$_2$

We present the results of muon-spin relaxation ($μ^{+}$SR) measurements of the van der Waals magnet NiI$_2$, which probe magnetic phase transitions at $T_{\mathrm{N1}}=73$~K and $T_{\mathrm{N2}}=60$~K. Supporting density functional theory (DFT) calculations allow the determination of a single muon stopping site whose magnetic environment is consistent with the proposed ground-state magnetic structure. $μ^{+}$SR measurements of the dynamics reveal behavior consistent with spin-wave excitations below $T_{\mathrm{N2}}$. In the region $T_{\mathrm{N2}}<T<T_{\mathrm{N1}}$ the character of the dynamics changes qualitatively, resulting in an unusual region of temperature-independent fluctuations.

cond-mat.str-el↗

arXiv2025

Field-orientation-dependent magnetic phases in GdRu$_2$Si$_2$ probed with muon-spin spectroscopy

Centrosymmetric GdRu$_2$Si$_2$ exhibits a variety of multi-Q magnetic states as a function of temperature and applied magnetic field, including a square skyrmion-lattice phase. The material's behavior is strongly dependent on the direction of the applied field, with different phase diagrams resulting for fields applied parallel or perpendicular to the crystallographic $c$ axis. Here, we present the results of muon-spin relaxation ($μ^+$SR) measurements on single crystals of GdRu$_2$Si$_2$. Our analysis is based on the computation of muon stopping sites and consideration of zero-point motion effects, allowing direct comparison with the underlying spin textures in the material. The muon site is confirmed experimentally, using angle-dependent measurements of the muon Knight shift. Using transverse-field $μ^+$SR with fields applied along either the [001] or [100] crystallographic directions, we distinguish between the magnetic phases in this system via their distinct muon response, providing additional evidence for the skyrmion and meron-lattice phases, while also suggesting the existence of RKKY-driven muon hyperfine coupling. Zero-field $μ^+$SR provides clear evidence for a transition between two distinct magnetically-ordered phases at 39 K.

cond-mat.str-el↗

arXiv2025

Magnetic properties of the zigzag ladder compound SrTb2O4

We report on the properties of SrTb2O4, a frustrated zigzag ladder antiferromagnet, studied by single crystal neutron diffraction (with polarised neutrons in zero field and unpolarised neutrons in an applied magnetic field), as well as by neutron spectroscopy on a polycrystalline sample. The neutron scattering results are supported by single crystal magnetisation and heat capacity measurements. In zero field, neutron diffraction data show no transition to a magnetically ordered state down to the lowest experimentally available temperature of 35 mK, and the material remains magnetically disordered down to this temperature. Polarised neutron diffraction measurements reveal the presence of a diffuse scattering signal suggesting only very weak spin-spin correlations in the ground state. For H // c (the easy magnetisation direction), we followed the magnetisation process using neutron diffraction measurements and observed the appearance of field-induced magnetic Bragg peaks with integer h and k indices in the (hk0) scattering plane. No magnetic peaks with a non-zero propagation vector were detected. The observed in-field data fit well to a simple two-sublattice model with magnetic moments aligned along the field direction but being significantly different in magnitude for the two inequivalent Tb3+ sites in the unit cell. Overall, the collected data point to a nonmagnetic ground state in SrTb2O4 despite the presence of strong interactions.

cond-mat.str-el↗

arXiv2025

A Magnon Band Analysis of GdRu2Si2 in the Field-Polarized State

Understanding the formation of skyrmions in centrosymmetric materials is a problem of fundamental and technological interest. GdRu2Si2 is one such candidate material which has been shown to host a variety of multi-Q magnetic structures, including in zero-field. Here, inelastic neutron scattering is used to measure the spin excitations in the field-polarized phase of GdRu2Si2. Linear spin wave theory and a method of interaction invariant path analysis are used to derive a Hamiltonian accounting for the observed spectra, and comparisons to \textit{ab initio} calculations are made. No evidence for anisotropic or higher order-exchange terms beyond bilinear Heisenberg exchange is found. This is discussed in the context of the multi-Q states existing at lower fields, for which these types of terms have previously been conceived to be significant in the formation of multi-Q ground states.

cond-mat.str-el↗

arXiv2023

The Double-$Q$ Ground State with Topological Charge Stripes in the Skyrmion Candidate $\text{GdRu}_{\text{2}}\text{Si}_{\text{2}}$

$\text{GdRu}_{\text{2}}\text{Si}_{\text{2}}$ is a centrosymmetric magnet in which a skyrmion lattice has recently been discovered. Here, we investigate the magnetic structure of the zero field ground state using neutron diffraction on single crystal and polycrystalline $^{\text{160}}\text{GdRu}_{\text{2}}\text{Si}_{\text{2}}$. In addition to observing the principal propagation vectors $\mathbf{q}_{1}$ and $\mathbf{q}_{2}$, we discover higher order magnetic satellites, notably $\mathbf{q}_{1} + 2\mathbf{q}_{2}$. The appearance of these satellites are explained within the framework of a new double-$Q$ constant-moment solution. Using powder diffraction we implement a quantitative refinement of this model. This structure, which contains vortexlike motifs, is shown to have a one-dimensional topological charge density.

cond-mat.str-el↗

arXiv2022

Giant topological and planar Hall effect in Cr$_{1/3}$NbS$_{2}$

Cr$_{1/3}$NbS$_{2}$ is a transition metal dichalcogenide that has been of significant interest due to its ability to host a magnetic chiral soliton lattice. Conventional and planar Hall measurements provide valuable insight into the detection of exotic spin structures in chiral magnets. We show that the presence of a giant planar Hall effect can be attributed to a tilted soliton lattice in Cr$_{1/3}$NbS$_{2}$. Our detailed angular dependent study shows the planar Hall effect and anisotropic magnetoresistance are intrinsically linked in complex non-coplanar magnets. From the conventional Hall signal we show the presence of a giant unconventional, likely topological Hall component, that is the fingerprint of non-coplanar spin textures.

cond-mat.str-el↗

arXiv2022

Effects of Fe deficiency and Co substitution in polycrystalline and single crystals of Fe$_{3}$GeTe$_{2}$

Fe$_{3}$GeTe$_{2}$ is a two-dimensional van der Waals material with a ferromagnetic ground state and a maximum transition temperature $T_{\mathrm{c}}\sim225$ K. However, when Fe$_{3}$GeTe$_{2}$ is synthesized lower values of $T_{\mathrm{c}}$ are often reported. This is attributed to a deficiency in the Fe at the 2c site in the crystal structure. Here we investigate the effect of Fe deficiency and the substitution of Co for Fe on the magnetic properties of this system. We have synthesized both polycrystalline material and single crystals by chemical vapor transport and the flux method, with the largest crystals obtained using the flux method. Cobalt substitution at the Fe site is found to significantly reduce the magnetic transition temperature. Crystals of Fe$_{3}$GeTe$_{2}$ grown by chemical vapor transport with $\sim 8\%$ excess Fe in the starting materials display an optimum Fe content and magnetic transition temperature.

cond-mat.mtrl-sci↗

arXiv2022

Quantum muon diffusion and the preservation of time-reversal symmetry in the superconducting state of type-I rhenium

Elemental rhenium exhibiting type-II superconductivity has been previously reported to break time-reversal symmetry in the superconducting state. We have investigated an arc-melted sample of rhenium exhibiting type-I superconductivity. Low temperature zero-field muon-spin relaxation measurements indicate that time-reversal symmetry is preserved in the superconducting state. Muon diffusion is observed, which is due to quantum mechanical tunneling between interstitial sites. The normal state behavior is characterized by the conduction electrons screening the muons and thermal broadening, and is typical for a metal. Energy asymmetries between muon trapping sites and the superconducting energy gap also characterize the superconducting state behavior.

cond-mat.supr-con↗

arXiv2022

Anisotropic superconductivity and unusually robust electronic critical field in single crystal La$_{7}$Ir$_{3}$

Polycrystalline La$_{7}$Ir$_{3}$ is reported to show superconductivity breaking time-reversal symmetry while also having an isotropic $s$-wave gap. Single crystals of this noncentrosymmetric superconductor are highly desirable to understand the nature of the electron pairing mechanism in this system. Here we report the growth of high-quality single crystals of La$_{7}$Ir$_{3}$ by the Czochralski method. The structural and superconducting properties of these large crystals have been investigated using x-rays, magnetization, resistivity and heat capacity measurements. We observe a clear anisotropy in the lower and upper critical fields for magnetic fields applied parallel and perpendicular to the hexagonal $c$ axis. We also report the presence of a robust electronic critical field, that diverges from the upper critical field derived from heat capacity, which is the hallmark of surface superconductivity.

cond-mat.supr-con↗

arXiv2021

Chiral singlet superconductivity in the weakly correlated metal LaPt3P

Topological superconductors (SCs) are novel phases of matter with nontrivial bulk topology. They host at their boundaries and vortex cores zero-energy Majorana bound states, potentially useful in fault-tolerant quantum computation. Chiral SCs are particular examples of topological SCs with finite angular momentum Cooper pairs circulating around a unique chiral axis, thus spontaneously breaking time-reversal symmetry (TRS). They are rather scarce and usually feature triplet pairing: best studied examples in bulk materials are UPt3 and Sr2RuO4 proposed to be f-wave and p-wave SCs respectively, although many open questions still remain. Chiral triplet SCs are, however, topologically fragile with the gapless Majorana modes weakly protected against symmetry preserving perturbations in contrast to chiral singlet SCs. Using muon spin relaxation (muSR) measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral SC: spontaneous magnetic fields inside the superconducting state indicating broken TRS and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is chiral d-wave singlet.

cond-mat.supr-con↗

arXiv2020

Evidence for the coexistence of time-reversal symmetry breaking and Bardeen-Cooper-Schrieffer-like superconductivity in La$_{7}$Pd$_{3}$

Time-reversal symmetry breaking (TRSB) with a Bardeen-Cooper-Schrieffer (BCS) -like superconductivity occurs in a small, but growing number of noncentrosymmetric (NCS) materials, although the mechanism is poorly understood. We present heat capacity, magnetization, resistivity, and muon spin resonance/relaxation ($μ$SR) measurements on polycrystalline samples of NCS La$_{7}$Pd$_{3}$. Transverse-field $μ$SR and heat capacity data show La$_{7}$Pd$_{3}$ is a type-II superconductor with a BCS-like gap structure, while zero-field $μ$SR results provide evidence of TRSB. We discuss the implications of these results for both the La$_{7}X_{3}$ (where $X=$ Ni, Pd, Rh, Ir) group of superconductors and other CS and NCS superconductors for which TRSB has been observed.

cond-mat.supr-con↗

arXiv2019

Superconductivity and the upper critical field in the chiral noncentrosymmetric superconductor NbRh$_{2}$B$_{2}$

NbRh$_{2}$B$_{2}$ crystallises in a chiral noncentrosymmetric structure and exhibits bulk type-II superconductivity below $7.46(5)$~K. Here we show that the temperature dependence of the upper critical field deviates from the behaviour expected for both Werthamer-Helfand-Hohenberg and the Ginzburg-Landau models and that $μ_{0}H_{\mathrm{c2}}\left(0\right)\approx 18$~T exceeds the Pauli paramagnetic limit, $μ_{0}H_{\mathrm{P}}=13.9$~T. We explore the reasons for this enhancement. Transverse-field muon spectroscopy measurements suggest that the superconducting gap is either $s$-wave or $\left(s+s\right)$-wave, and the pressure dependence of $T_{\mathrm{c}}$ reveals the superconducting gap is primarily $s$-wave in character. The magnetic penetration depth $λ(0) = 595(5)$~nm. Heat capacity measurements reveal the presence of a multigap $\left(s+s\right)$-wave superconducting order parameter and moderate electron-phonon coupling.

cond-mat.supr-con↗

arXiv2018

Multigap Superconductivity in Chiral Noncentrosymmetric TaRh$_{2}$B$_{2}$

We report the first observation of multigap superconductivity in TaRh$_{2}$B$_{2}$. We show TaRh$_{2}$B$_{2}$ is a bulk type-II superconductor with a transition temperature, $T_{\mathrm{c}} = 6.00(5)$ K. We present transverse-field muon spin relaxation data where the superconducting gap can be fit using a two-gap $\left(s+s\right)$-wave model. We also report the zero-field electronic specific heat in the superconducting state that is best described by the same $\left(s+s\right)$ model providing further evidence of multiband behavior in this superconductor. Zero-field muon spin relaxation measurements show time-reversal symmetry is preserved in the superconducting state. We demonstrate that TaRh$_{2}$B$_{2}$ has an upper critical field of $15.2(1)$ T, which is significantly higher than previously reported and exceeds the Pauli limit.

cond-mat.supr-con↗