arXiv subjects

Stephen J. Blundell

Publications and source records attributed to Stephen J. Blundell.

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

Gapped 1/9 Magnetization Plateau in the Anisotropic Kagome Antiferromagnet Y-kapellasite

Fractional magnetization plateaus provide a sensitive probe of many-body spin states in frustrated quantum magnets, yet their microscopic origin in kagome antiferromagnets remains unresolved. This is particularly true of the mysterious $1/9$ plateau, which is predicted by theory but infrequently observed in experiment. Here, we investigate this problem in the $S = 1/2$ anisotropic kagome antiferromagnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, using pulsed-field magnetization measurements on single crystals and high-field $^{35}$Cl NMR. We identify a hierarchy of field-induced fractional features, including $1/3$ and $1/9$ plateaus, as well as a weaker low-field feature. Analysis of the NMR spectra and the magnetic susceptibility across the $1/9$ plateau demonstrate that it is accompanied by an ordered local spin configuration, a strong suppression of low-energy spin fluctuations and activated behavior, consistent with a gapped fractional state. These features differ from those in the only other material YCu$_3$(OH)$_6$Br$_2$[Br$_{1-y}$(OH)$_y$] in which this plateau is observed, implying a surprising robustness of the $1/9$ state to the details of the underlying magnetism.

cond-mat.str-el↗

arXiv2026

Spinon mediation of witness spin dynamics in herbertsmithite

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such 'impurity' atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z2 or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

cond-mat.str-el↗

arXiv2026

Discovery of Dynamical Heterogeneity in a Supercooled Magnetic Monopole Fluid

Dynamical heterogeneity, in which transitory local fluctuations occur in the conformation and dynamics of constituent particles, is widely hypothesized to be essential to evolution of supercooled liquids into the structural glass state. Yet its microscopic spatiotemporal phenomenology is challenging to detect directly in molecular glass forming liquids. Because recent theoretical advances predict that corresponding dynamical heterogeneity could occur in supercooled magnetic monopole fluids (Proc. Nat. Acad. Sci. 112, 8549 (2015)), we searched for such phenomena in Dy2Ti2O7. By measuring its microsecond-resolved spontaneous magnetization fluctuations $M(t,T)$ we discovered a sharp bifurcation in monopole noise characteristics below $T \approx 1500$ mK, with the appearance of powerful spontaneous monopole current bursts. This intense dynamics emerges upon entering the supercooled monopole fluid regime, reaches maximum strength near $T \approx 750$ mK and then collapses along with coincident loss of ergodicity approaching $T_g \approx 750$ mK. Moreover, when the four-point dynamical susceptibility $χ_4(τ, T)$ is determined directly from temperature dependence of correlations in $M(t,T)$, it evolves as predicted when dynamical heterogeneity is present, revealing its simultaneously and rapidly escalating length and time scales, $ξ(T)$. and $τ_4(T)$. This overall phenomenology 2 greatly expands our empirical knowledge of supercooled monopole fluids and, more generally, demonstrates direct detection of the time sequence, magnitude, statistics and correlations of dynamical heterogeneity, access to which may greatly accelerate fundamental vitrification studies.

cond-mat.str-el↗

arXiv2025

Vector spin polarization evolution determined in an entangled muon-fluorine system under pulsed excitation

A spin-polarized muon implanted into a fluoride forms a coupled F--$μ$--F complex in which the muon spin and neighbouring fluorine nuclear spins become entangled. Here we apply radio-frequency (RF) excitation to this coupled system and use the three-dimensional distribution of emitted positrons to reconstruct the time-dependent evolution of the muon spin polarization. This three-dimensional readout, using single spin detection, is not possible in a single NMR experiment and demonstrates significant advantages that are achieved by using RF muon techniques. We demonstrate the application of this vector-readout method to the experimental observation of a muon spin echo signal that is controlled by the dipolar coupling to fluorine, as well as to a double resonance experiment, in which we use pulses tuned to separate frequencies to address both the muon and fluorine spins. This targeted approach, in which selective RF pulses can control the muon spin and other spins to which it is coupled, provides a novel route for probing systems of entangled spins.

cond-mat.str-el↗

arXiv2025

Magnetic-field-induced ordering in a spin-1/2 chiral chain

We present neutron diffraction, muon spin rotation and pulsed-field magnetometry measurements on the Heisenberg quantum chiral chain [Cu(pym)(H2O)4]SiF6.H2O, which displays a four-fold-periodic rotation of the local environment around the Cu(II) S = 1/2 ions from site to site along the chain. Previous measurements on this material have shown the absence of magnetic order down to surprisingly low temperatures >= 20 mK, as well as the presence of an energy gap for magnetic excitations that grows linearly with magnetic field. Here we find evidence at dilution refrigerator temperatures for a field-induced transition to long-range magnetic order above an applied magnetic field of 3 T. From the polarization of magnetic moments observed in applied fields we can identify the static magnetic structure that best accounts for the data. The proposed model is supported microscopically by the presence of an alternating component of the g tensor, which produces an internal two-fold staggered field that dictates both the direction of the ordered moments and the effective coupling between adjacent chains. The observed magnetic structure is contrary to previous proposals for the departure of the magnitude and field dependence of the energy gap from the predictions of the sine-Gordon model.

cond-mat.str-el↗

arXiv2025

Robust Magnetic Polaron Percolation in the Antiferromagnetic CMR System EuCd$_2$P$_2$

Antiferromagnetic EuCd$_2$P$_2$ has attracted considerable attention due to its unconventional (magneto)transport properties. At a temperature $T_{\rm peak}$ significantly above the magnetic ordering temperature $T_\textrm{N} = 11\,$K a large peak in resistivity is observed which gets strongly suppressed in magnetic field, resulting in a colossal magnetoresistance (CMR), for which magnetic fluctuations and the formation of ferromagnetic clusters have been proposed as underlying mechanisms. Employing a selection of sensitive probes including fluctuation spectroscopy and third-harmonic resistance, Hall effect, AC susceptibility and $μ$SR measurements, allows for a direct comparison of electronic and magnetic properties on multiple time scales. We find compelling evidence for the formation and percolation of magnetic polarons, which explains the CMR of the system. Large peaks in the weakly-nonlinear transport and the resistance noise power spectral density at zero magnetic field signify an inhomogeneous, percolating electronic system below $T^\ast \approx 2\,T_\textrm{N}$ with a percolation threshold at $T_{\rm peak}$. In magnetic fields, the onset of large negative MR in the paramagnetic regime occurs at a universal critical magnetization similar to ferromagnetic CMR materials. The size of the magnetic polarons at the percolation threshold is estimated to $\sim 1 - 2\,$nm. The mechanism of magntic cluster formation and percolation in EuCd$_2$P$_2$ appears to be rather robust despite large variations in carrier concentration and likely is relevant for other Eu-based antiferromagnetic CMR systems.

cond-mat.str-el↗

arXiv2025

Spin dynamics in the Dirac $U(1)$ spin liquid YbZn$_2$GaO$_5$

YbZn$_2$GaO$_5$ is a promising candidate for realizing a quantum spin liquid (QSL) state, particularly owing to its lack of significant site disorder. Pulsed-field magnetometry at 0.5 K shows magnetization saturating near 15 T, with a corrected saturation moment of 2.1(1) $μ_\mathrm{B}$ after subtracting the van Vleck contribution. Our zero-field $μ$SR measurements down to milliKelvin temperatures provide evidence for a dynamic ground state and the absence of magnetic order. To probe fluctuations in the local magnetic field at the muon site, we performed longitudinal field $μ$SR experiments. These results provide evidence for spin dynamics with a field dependence that is consistent with a U1A01 Dirac QSL as a plausible description of the ground state.

cond-mat.str-el↗

arXiv2025

Elastic softness of low-symmetry frustrated $A$Ti$_2$O$_5$ ($A$ = Co, Fe)

Orthorhombic pseudobrookites CoTi$_2$O$_5$ and FeTi$_2$O$_5$ have a low-symmetry crystal structure comprising magnetic Co$^{2+}$/Fe$^{2+}$ ions and nonmagnetic Ti$^{4+}$ ions, where the orbital-nondegenerate Co$^{2+}$/Fe$^{2+}$ ions form one-dimensional chains running along the orthorhombic $a$ axis. These compounds undergo an antiferromagnetic phase transition at $T_N \sim$ 26 K for CoTi$_2$O$_5$ and $T_N \sim$ 40 K for FeTi$_2$O$_5$. Ultrasound velocity measurements on single crystals of CoTi$_2$O$_5$ and FeTi$_2$O$_5$ reveal that CoTi$_2$O$_5$ exhibits unusual elastic softness above $T_N$ in the symmetry-lowering elastic mode of $ac$-plane shear elastic modulus, inconsistent with the structural symmetry breaking caused by antiferromagnetic ordering at $T_N$. This suggests the presence of two distinct types of magnetostructural fluctuations above $T_N$ that should be a precursor to the symmetry-lowering lattice distortion at $T_N$. In contrast, FeTi$_2$O$_5$ exhibits either negligible or smaller elastic softness, indicating weaker spin-lattice coupling. These findings highlight CoTi$_2$O$_5$ and FeTi$_2$O$_5$ as unique spin-latticed-coupled frustrated systems with low crystal symmetry, where, while the exchange interactions are quasi-one-dimensional in nature, the frustration is released by further lowering the crystal symmetry through three-dimensional spin-lattice coupling, which is stronger in CoTi$_2$O$_5$ than in FeTi$_2$O$_5$.

cond-mat.str-el↗

arXiv2024

Spiral Spin Liquid Noise

An emerging concept for identification of different types of spin liquids is through the use of spontaneous spin noise. Here we develop spin noise spectroscopy for spin liquid studies by considering Ca$_{10}$Cr$_7$O$_{28}$, a material hypothesized to be either a quantum or a spiral spin liquid. By enhancing techniques introduced for magnetic monopole noise studies we measure the time and temperature dependence of spontaneous flux $\varPhi(t, T)$ and thus magnetization $M(t, T)$ of Ca$_{10}$Cr$_7$O$_{28}$ samples. The resulting power spectral density of magnetization noise $S_M(ω,T)$ reveals intense spin fluctuations with $S_M(ω,T) \propto ω^{-α(T)}$ and 0.84 < $α(T)$ < 1.04 . Both the variance $σ_M^2(T)$ and the correlation function $C_M(t,T)$ of this spin noise undergo crossovers at a temperature $T^* \approx$ 450 mK. While predictions for quantum spin liquids are inconsistent with this phenomenology, those from Monte-Carlo simulations of a 2D spiral spin liquid state in Ca$_{10}$Cr$_7$O$_{28}$ yield overall quantitative correspondence with the measured frequency and temperature dependences of $S_M(ω,T), C_M(t,T)$ and $σ_M^2(T)$, thus indicating that Ca$_{10}$Cr$_7$O$_{28}$ is a spiral spin liquid.

cond-mat.str-el↗

arXiv2024

Low-temperature spin dynamics and absence of magnetic order in layered $α$-RuI$_3$

The triangular-lattice system $α$-RuI$_3$ is isostructural to the widely-studied $α$-RuCl$_3$ compound which was identified as a potential Kitaev system but exhibits, instead of spin liquid behaviour, a magnetically ordered zig-zag ground state which sets in below 14~K. Here we show experimentally that, in contrast, the spins in $α$-RuI$_3$ remain dynamic down to at least 50~mK. We study the spin dynamics using muon-spin relaxation methods and determine the presence of low-frequency fluctuations which are characteristic of a two-dimensional system.

cond-mat.str-el↗

arXiv2024

Dichotomous Dynamics of Magnetic Monopole Fluids

A recent advance in the study of emergent magnetic monopoles was the discovery that monopole motion is restricted to dynamical fractal trajectories (J. Hallén et al, Science 378, 1218 (2022)) thus explaining the characteristics of magnetic monopole noise spectra (Dusad, R. et al. Nature 571, 234 (2019); Samarakoon, A. M. et al. Proc. Natl. Acad. Sci. 119, e2117453119 (2022)). Here we apply this new theory to explore the dynamics of field-driven monopole currents, finding them comprised of two quite distinct transport processes: initially swift fractal rearrangements of local monopole configurations followed by conventional monopole diffusion. This theory also predicts a characteristic frequency dependence of the dissipative loss-angle for AC-field-driven currents. To explore these novel perspectives on monopole transport, we introduce simultaneous monopole current control and measurement techniques using SQUID-based monopole current sensors. For the canonical material Dy2Ti2O7, we measure $Φ(t)$, the time-dependence of magnetic flux threading the sample when a net monopole current $J(t) = \dotΦ(t)/μ_0$ is generated by applying an external magnetic field $B_0(t)$. These experiments find a sharp dichotomy of monopole currents, separated by their distinct relaxation time-constants before and after $t \approx 600 μs$ from monopole current initiation. Application of sinusoidal magnetic fields $B_0(t) = Bcos(ωt)$ generates oscillating monopole currents whose loss angle $θ(f)$ exhibits a characteristic transition at frequency $f \approx 1.8$ kHz over the same temperature range. Finally, the magnetic noise power is also dichotomic, diminishing sharply after $t \approx 600 μs$. This complex phenomenology represents a new form of heterogeneous dynamics generated by the interplay of fractionalization and local spin configurational symmetry.

cond-mat.mes-hall↗

arXiv2024

Magnetostriction-Driven Muon Localization in an Antiferromagnetic Oxide

Magnetostriction drives a rhombohedral distortion in the cubic rock salt antiferromagnet MnO at the Néel temperature $T_{N}=118$ K. As an unexpected consequence we show that this distortion acts to localize the site of an implanted muon due to the accompanying redistribution of electron density. This lifts the degeneracy between equivalent sites, resulting in a single observed muon precession frequency. Above $T_{N}$, the muon instead becomes delocalized around a network of equivalent sites. Our first-principles simulations based on Hubbard-corrected density-functional theory and molecular dynamics are consistent with our experimental data and help to resolve a long-standing puzzle regarding muon data on MnO, as well as having wider applicability to other magnetic oxides.

cond-mat.str-el↗

arXiv2022

Low temperature magnetism of KAgF3

KAgF$_3$ is a quasi one-dimensional quantum antiferromagnet hosting a series of intriguing structural and magnetic transitions. Here we use powder neutron diffraction, $μ$SR spectroscopy, and Density Functional Theory calculations to elucidate the low temperature magnetic phases. Below $T_{N1}=29$K we find that the material orders as an A-type antiferromagnet with an ordered moment of 0.47$μ_{\rm B}$. Both neutrons and muons provide evidence for an intermediate phase at temperatures $T_{N1}<T<T_{N2}$ with $T_{N2}\approx 66$ K from a previous magnetometry study. However, the evidence is at the limit of detection and its nature remains an open problem.

cond-mat.str-el↗

arXiv2022

Muon-spin relaxation investigation of magnetic bistability in a crystalline organic radical compound

We present the results of a muon-spin relaxation ($μ^{+}$SR) investigation of the crystalline organic radical compound 4-(2-benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA), in which we demonstrate the hysteretic magnetic switching of the system that takes place at $T = 274 \pm 11\,\mathrm{K}$ caused by a structural phase transition. Muon-site analysis using electronic structure calculations suggests a range of candidate muon stopping sites. The sites are numerous and similar in energy but, significantly, differ between the two structural phases of the material. Despite the difference in the sites, the muon remains a faithful probe of the transition, revealing a dynamically-fluctuating magnetically disordered state in the low-temperature structural phase. In contrast, in the high temperature phase the relaxation is caused by static nuclear moments, with rapid electronic dynamics being motionally narrowed from the muon spectra.

cond-mat.str-el↗

arXiv2022

Beyond Single Tetrahedron Physics of Breathing Pyrochlore Compound Ba3Yb2Zn5O11

Recently a new class of quantum magnets, the so-called breathing pyrochlore spin systems, have attracted much attention due to their potential to host exotic emergent phenomena. Here, we present magnetometry, heat capacity, thermal conductivity, Muon-spin relaxation, and polarized inelastic neutron scattering measurements performed on high-quality single-crystal samples of breathing pyrochlore compound Ba3Yb2Zn5O11. We interpret these results using a simplified toy model and provide a new insight into the low-energy physics of this system beyond the single-tetrahedron physics proposed previously.

cond-mat.str-el↗

arXiv2022

Entanglement between a muon spin and $I>\frac{1}{2}$ nuclear spins

We report on the first example of quantum coherence between the spins of muons and quadrupolar nuclei. We observe this effect in vanadium intermetallic compounds which adopt the A15 crystal structure, and whose members include all technologically dominant superconductors. The entangled states are extremely sensitive to the local structural and electronic environments through the electric field gradient at the quadrupolar nuclei. This case-study demonstrates that positive muons can be used as a quantum sensing tool to probe also structural and charge related phenomena in materials, even in the absence of magnetic order.

cond-mat.mtrl-sci↗

arXiv2021

Probing the magnetic polaron state in the ferromagnetic semiconductor HgCr$_2$Se$_4$ with resistance fluctuation and muon-spin spectroscopy measurements

Combined resistance noise and muon-spin relaxation ($μ$SR) measurements of the ferromagnetic semiconductor HgCr$_2$Se$_4$ suggest a degree of magnetoelectric coupling and provide evidence for the existence of isolated magnetic polarons. These form at elevated temperatures and undergo a percolation transition with a drastic enhancement of the low-frequency 1/$f$-type charge fluctuations at the insulator-to-metal transition at $\sim 95 - 98$ K in the vicinity of the magnetic ordering temperature $T_C \sim 105 - 107$ K. Upon approaching the percolation threshold from above, the strikingly unusual dynamics of a distinct two-level fluctuator superimposed on the $1/f$ noise can be described by a slowing down of the dynamics of a nanoscale magnetic cluster, a magnetic polaron, when taking into account an effective radius of the polaron depending on the spin correlation length. Coinciding temperature scales found in $μ$SR and noise measurements suggest changes in the magnetic dynamics over a wide range of frequencies and are consistent with the existence of large polarized and domain-wall-like regions at low temperatures, that result from the freezing of spin dynamics at the magnetic polaron percolation transition.

cond-mat.str-el↗