arXiv subjects

Helen C. Walker

Publications and source records attributed to Helen C. Walker.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of hidden order in the Shastry-Sutherland magnet Nd2Be2GeO7

Hidden order typically manifests as a thermodynamic phase transition without a conventional order parameter, leaving its true nature concealed even at the lowest temperatures. In the frustrated Shastry-Sutherland magnet Nd$_2$Be$_2$GeO$_7$, we observe a related yet fundamentally distinct phenomenon. A sharp specific-heat anomaly appears at 250 mK, but zero-field neutron diffraction and muon spin relaxation detect no static magnetism down to 100 and 30 mK, respectively, pointing to a hidden-order state. Remarkably, this hidden order does not emerge under an applied magnetic field, but instead reveals itself only after the field is applied and subsequently removed where magnetic Bragg peaks appear, albeit with strongly suppressed moments. A glassy state is ruled out by ac susceptibility and specific heat measurements. Complementary $\mu$SR measurements reveal coherent spin fluctuations at a rate on the order of gigahertz. Taken together, these results suggest that the system lies in close proximity to the quantum spin liquid and long-range magnetic order state such that a small perturbation can effectively drive the system towards distinct ground states. These findings also distinguish Nd$_2$Be$_2$GeO$_7$ from known frustrated systems, establishing it as a unique platform where the synergistic interplay among the spin-orbit coupling, crystal field, and magnetic frustration leads to unexpected quantum states.

cond-mat.str-el↗

arXiv2025

Anisotropic Band-Split Magnetism in Magnetostrictive CoFe$_2$O$_4$

Single crystal spinel CoFe$_2$O$_4$ exhibits the largest room-temperature saturation magnetostriction among non-rare-earth compounds and a high Curie temperature ($T_c \sim 780$ K), properties that are critical to a wide range of industrial and medical applications. Neutron spectroscopy reveals a large band splitting ($\sim$ 60 meV) between two ferrimagnetic magnon branches, which is driven by site mixing between Co$^{2+}$ and Fe$^{3+}$ cations, and a significantly weaker magnetocrystalline anisotropy ($\sim$ 3 meV). Central to this behavior is the competition between extremely large mismatched molecular fields on the tetrahedral $A$-site and octahedral $B$-site sublattices and the single-ion anisotropy on the $B$-site. This creates a strong energetic anisotropy that locks the magnetic moment within each structural domain in place. As a result of these differing energy scales, switching structural domains is energetically favored over a global spin reorientation under applied magnetic fields, and this is what amplifies the magnetostrictive nature of CoFe$_2$O$_4$.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic ground state and persistent spin fluctuations in triangular-lattice antiferromagnet NdZnAl$_{11}$O$_{19}$

Rare-earth triangular-lattice magnets serve as an excellent platform for investigating exotic quantum magnetic phenomena. Recently, the hexaaluminate \cmao\ has been proposed to host a $U(1)$ Dirac quantum spin liquid state with dominant Ising anisotropy. Here, we report a systematic study of its analogue, \nzao, employing ac susceptibility, inelastic neutron scattering, and muon spin relaxation measurements. Inelastic neutron scattering measurements establish a well-defined $J_\mathrm{eff}$ = 1/2 ground state with moderate Ising anisotropy ($g_c$ = 4.54, $g_\mathrm{ab}$ = 1.42). Muon spin relaxation measurements reveal persistent fluctuations emerging below $\sim$15\,K, and extending down to at least 0.28 K. AC susceptibility data further indicate an absence of magnetic ordering or spin freezing down to 50\,mK, despite an overall antiferromagnetic interaction with the Curie-Weiss temprature of $-0.42$\,K. These results suggest that \nzao\ is a good candidate material for realizing a quantum spin liquid state.

cond-mat.str-el↗

arXiv2024

Magnetic diffuse scattering of the $S$ = 5/2 fcc antiferromagnets Ba$_2$MnTeO$_6$ and Ba$_2$MnWO$_6$

We have investigated the magnetic diffuse scattering of isostructural $S$ = 5/2 fcc antiferromagnets Ba$_2$MnTeO$_6$ and Ba$_2$MnWO$_6$ using polarized neutrons. Both materials display short-range correlated magnetism above their respective magnetic ordering temperatures of 20 K and 8 K. The spin correlations were analysed using a Reverse Monte Carlo approach. For Ba$_2$MnTeO$_6$, we find antiferromagnetic nearest-neighbor correlations along with ferromagnetic next-nearest neighbor correlations directly linked to the Type I order below $T_N$. For Ba$_2$MnWO$_6$, both the nearest-neighbor and next-nearest-neighbor spin correlations are antiferromagnetic in the paramagnetic state. The short-range spin correlations persist up to $T$ = 7$T_N$. The magnetic diffuse scattering was also fitted using Onsager reaction-field theory allowing us to evaluate the magnetic interactions in these materials. We obtained $J_1$ = -3.25(3) K and $J_2$ = 0.41(2) K for Ba$_2$MnTeO$_6$ and $J_1$ = -1.08(1) K and $J_2$ = -0.88(1) K for Ba$_2$MnWO$_6$. These interactions are comparable to previous results from inelastic neutron scattering experiments below $T_N$, which highlights the potential of the Onsager approach for the analysis of magnetic interactions.

cond-mat.str-el↗

arXiv2024

Neutron Scattering Studies on the High-$T_c$ Superconductor La$_3$Ni$_2$O$_{7-\delta}$ at Ambient Pressure

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctuations have been widely believed to play a crucial role in forming the superconducting Cooper pairs. The recent discovery of high-temperature superconductivity near 80 K in the bilayer nickelate La$_3$Ni$_2$O$_7$ under pressure provides a new platform to elucidate the origins of high-temperature superconductivity. We perform elastic and inelastic neutron scattering studies on a polycrystalline sample of La$_3$Ni$_2$O$_{7-\delta}$ at ambient pressure. No magnetic order can be identified down to 10 K. The absence of long-range magnetic order in neutron diffraction measurements may be ascribed to the smallness of the magnetic moment. However, we observe a weak flat spin-fluctuation signal at $\sim$ 45 meV in the inelastic scattering spectra. The observed spin excitations could be interpreted as a result of strong interlayer and weak intralayer magnetic couplings for stripe-type antiferromagnetic orders. Our results provide crucial information on the spin dynamics and are thus important for understanding the superconductivity in La$_3$Ni$_2$O$_7$.

cond-mat.supr-con↗

arXiv2024

Nematic quantum disordered state in FeSe

The unusual quantum-disordered magnetic ground state intertwined with superconductivity and electronic nematicity in FeSe has been a research focus in iron-based superconductors. However, the intrinsic spin excitations across the entire Brillouin zone in detwinned FeSe, which forms the basis for a microscopic understanding of the magnetic state and superconductivity, remain to be determined. Here, we use inelastic neutron scattering to map out the spin excitations of FeSe dewtinned with a uniaxial-strain device. We find that the stripe spin excitations (Q=(1, 0)/(0, 1)) exhibit the $C_2$ symmetry up to $E\approx120$ meV, while the N{\'e}el spin excitations (Q=(1, 1)) retain their $C_4$ symmetry in the nematic state. The temperature dependence of the difference in the spin excitations at Q=(1, 0) and (0, 1) for temperatures above the structural phase transition unambiguously shows the establishment of the nematic quantum disordered state. The similarity of the N\'eel excitations in FeSe and NaFeAs suggests that the N\'eel excitations are driven by the enhanced electron correlations in the $3d_{xy}$ orbital. By determining the key features of the stripe excitations and fitting their dispersions using a Heisenberg Hamiltonian with biquadratic interaction ($J_1$-$K$-$J_2$), we establish a spin-interaction phase diagram and conclude that FeSe is close to a crossover region between the antiferroquadrupolar, N\'eel, and stripe ordering regimes. The results provide an experimental basis for establishing a microscopic theoretical model to describe the origin and intertwining of the emergent orders in iron-based superconductors.

cond-mat.supr-con↗

arXiv2023

Structure, spin correlations and magnetism of the $S = 1/2$ square-lattice antiferromagnet Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ ($0 \leq x \leq 1$)

Quantum spin liquids are highly entangled magnetic states with exotic properties. The $S = 1/2$ square-lattice Heisenberg model is one of the foundational models in frustrated magnetism with a predicted, but never observed, quantum spin liquid state. Isostructural double perovskites Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are physical realizations of this model, but have distinctly different types magnetic order and interactions due to a $d^{10}/d^0$ effect. Long-range magnetic order is suppressed in the solid solution Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ in a wide region of $x = 0.05-0.6$, where the ground state has been proposed to be a disorder-induced spin liquid. Here we show that the spin-liquid-like $x = 0.2$ and $x = 0.5$ samples have distinctly different local spin correlations, which suggests they have different ground states. Furthermore, the previously ignored interlayer coupling between the square-planes is likely to play a role in the suppression of magnetic order on the W-rich side at $x \approx 0.6$. These results highlight the complex magnetism of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ and hint at a new quantum critical point at $x \approx 0.3$.

cond-mat.str-el↗

arXiv2023

On (not) deriving the entropy of barocaloric phase transitions from crystallography and neutron spectroscopy

We review well-known signatures of disorder in crystallographic and inelastic neutron scattering data. We show that these can arise from different types of disorder, corresponding to different values of the system entropy. Correlating the entropy of a material with its atomistic structure and dynamics is in general a difficult problem that requires correlating information between multiple experimental techniques including crystallography, spectroscopy, and calorimetry. These comments are illustrated with particular reference to barocalorics, but are relevant to a broad range of calorics and other disordered crystalline materials.

cond-mat.mtrl-sci↗

arXiv2023

Spin Fluctuations in Sr$_{1.8}$La$_{0.2}$RuO$_4$

We use inelastic neutron scattering to study spin fluctuations in Sr$_{1.8}$La$_{0.2}$RuO$_4$, where Lanthanum doping triggers a Lifshitz transition by pushing the van Hove singularity in the $γ$ band to the Fermi energy. Strong spin fluctuations emerge at an incommensurate wave vector $\mathbf{Q}_{ic} = (0.3,0.3)$, corresponding to the nesting vector between $α$ and $β$ Fermi sheets. The incommensurate antiferromagnetic fluctuations shift toward $(0.25,0.25)$ with increasing energy up to ${\sim}110$ meV. By contrast, scatterings near the ferromagnetic wave vectors $\mathbf{Q} = (1,0)$ and $(1,1)$ remain featureless at all energies. This contradicts the weak-coupling perspective that suggests a sharp enhancement of ferromagnetic susceptibility due to the divergence of density of states in the associated $γ$ band. Our findings imply that ferromagnetic fluctuations in Sr$_2$RuO$_4$ and related materials do not fit into the weak-coupling paradigm, but instead are quasi-local fluctuations induced by Hund's coupling. This imposes significant constraints for the pairing mechanism involving spin fluctuations.

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

Structure and dynamics of the negative thermal expansion material Cd(CN)$_2$ under pressure

We use a combination of variable-temperature / variable-pressure neutron powder diffraction, variable-pressure inelastic neutron scattering, and quantum chemical calculations to interrogate the behaviour of the negative thermal expansion (NTE) material $^{114}$Cd(CN)$_2$ under hydrostatic pressure. We determine the equation of state of the ambient-pressure phase, and discover the so-called `warm hardening' effect whereby the material becomes elastically stiffer as it is heated. We also identify a number of high-pressure phases, and map out the phase behaviour of Cd(CN)$_2$ over the range $0\leq p\leq0.5$\,GPa, $100\leq T\leq300$\,K. As expected for an NTE material, the low-energy phonon frequencies are found to soften under pressure, and we determine an effective Gr{ü}neisen parameter for these modes. Finally, we show that the elastic behaviour of Cd(CN)$_2$ is sensitive to the local Cd coordination environment, which suggests an interplay between short- (phononic) and long-timescale (cyanide flips) fluctuations in Cd(CN)$_2$.

cond-mat.mtrl-sci↗

arXiv2022

Dynamics in the ordered and disordered phases of barocaloric adamantane

High-entropy order-disorder phase transitions can be used for efficient and eco-friendly barocaloric solid-state cooling. Here the barocaloric effect is reported in an archetypal plastic crystal, adamantane. Adamantane has a colossal isothermally reversible entropy change of 106 J K-1 kg-1 . Extremely low hysteresis means that this can be accessed at pressure differences less than 200 bar. Configurational entropy can only account for about 40% of the total entropy change; the remainder is due to vibrational effects. Using neutron spectroscopy and supercell lattice dynamics calculations, it is found that this vibrational entropy change is mainly caused by softening in the high-entropy phase of acoustic modes that correspond to molecular rotations. We attribute this behaviour to the contrast between an 'interlocked' state in the low-entropy phase and sphere-like behaviour in the high-entropy phase. Although adamantane is a simple van der Waals solid with near-spherical molecules, this approach can be leveraged for the design of more complex barocaloric molecular crystals. Moreover, this study shows that supercell lattice dynamics calculations can accurately map the effect of orientational disorder on the phonon spectrum, paving the way for studying the vibrational entropy, thermal conductivity, and other thermodynamic effects in more complex materials.

cond-mat.mtrl-sci↗

arXiv2022

Gapless triangular-lattice spin-liquid candidate in PrZnAl$_{11}$O$_{19}$

A quantum spin liquid (QSL) is an exotic state in which electron spins are highly entangled, yet keep fluctuating even at zero temperature. Experimental realization of model QSLs has been challenging due to imperfections, such as antisite disorder, strain, and extra or a lack of interactions in real materials compared to the model Hamiltonian. Here we report the magnetic susceptibility, thermodynamic, inelastic neutron scattering (INS), and muon-spin relaxation studies on a polycrystalline sample of PrZnAl$_{11}$O$_{19}$, where the Pr$^{3+}$ ions form an ideal two-dimensional triangular lattice. Our results demonstrate that this system does not order nor freeze, but keep fluctuating down to 50 mK despite large antiferromagnetic couplings ($\sim$ -10 K). Furthermore, the INS and specific-heat data suggest that PrZnAl$_{11}$O$_{19}$ is best described as a gapless QSL.

cond-mat.str-el↗

arXiv2022

Pressure dependence of atomic dynamics in barocaloric ammonium sulfate: II. Vibrations

Ammonium sulfate is a giant inverse barocaloric material that is cheaply and commercially available. Exploiting its potential for cooling applications requires an understanding of the mechanism driving the entropy change. Here we report an investigation by inelastic neutron scattering and density functional theory of the phonons under working conditions of temperature and pressure. We find excellent agreement between the experimental and calculated results. The ammonium librational modes that are crucial to the entropy change are identifiable by their negative Grüneisen parameter. Our results connect the differences in structure across the phase transition to those in the atomic dynamics, suggesting a route towards designing new caloric materials.

cond-mat.mtrl-sci↗

arXiv2022

Origin of the large entropy change in the molecular caloric and ferroelectric ammonium sulfate

The deceptively simple inorganic salt ammonium sulfate undergoes a ferroelectric phase transition associated with a very large entropy change and both electrocaloric and barocaloric functionality. While the structural origins of the electrical polarisation are now well established, those of the entropy change have been controversial for over fifty years. This question is resolved here using a combination of DFT phonon calculations with inelastic neutron scattering under variable temperature and pressure, supported by complementary total and quasielastic neutron scattering experiments. A simple model of the entropy in which each molecular ion is disordered across the mirror plane in the high symmetry phase, although widely used in the literature, proves to be untenable. Instead, the entropy arises from low-frequency librations of ammonium ions in this phase, with harmonic terms that are very small or even negative. These results suggest that, in the search for molecular materials with functionality derived from large entropy changes, vibrational entropy arising from broad energy minima is likely to be just as important as configurational entropy arising from crystallographic disorder.

cond-mat.mtrl-sci↗

arXiv2022

Frustrated magnetic interactions in FeSe

The structurally simplest high-temperature superconductor FeSe exhibits an intriguing superconducting nematic paramagnetic phase with unusual spin excitation spectra that are different from typical spin waves; thus, determining its effective magnetic exchange interactions is challenging. Here we report neutron scattering measurements of spin fluctuations of FeSe in the tetragonal paramagnetic phase. We show that the equal-time magnetic structure factor, $\mathcal{S}(\textbf{Q})$, can be effectively modeled using the self-consistent Gaussian approximation calculation with highly frustrated nearest-neighbor ($J_{1}$) and next-nearest-neighbor ($J_{2}$) exchange couplings, and very weak further neighbor exchange interaction. Our results elucidate the frustrated magnetism in FeSe, which provides a natural explanation for the highly tunable superconductivity and nematicity in FeSe and related materials.

cond-mat.supr-con↗

arXiv2022

Randomness and Frustration in a S = 1/2 Square-Lattice Heisenberg Antiferromagnet

We explore the interplay between randomness and magnetic frustration in the series of $S = 1/2$ Heisenberg square-lattice compounds Sr$_2$CuTe$_{1-x}$W$_x$O$_6$. Substituting W for Te alters the magnetic interactions dramatically, from strongly nearest-neighbor to next-nearest-neighbor antiferromagnetic coupling. We perform neutron scattering measurements to probe the magnetic ground state and excitations over a range of $x$. We propose a bond-disorder model that reproduces ground states with only short-ranged spin correlations in the mixed compounds. The calculated neutron diffraction patterns and powder spectra agree well with the measured data and allow detailed predictions for future measurements. We conclude that quenched randomness plays the major role in defining the physics of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ with frustration being less significant.

cond-mat.str-el↗

arXiv2022

Crystal-field states and defect levels in candidate quantum spin ice Ce$_{2}$Hf$_{2}$O$_{7}$

We report the synthesis of powder and single-crystal samples of the cerium pyrohafnate and their characterization using neutron diffraction, thermogravimetry and X-ray absorption spectroscopy. We evaluate the amount of non-magnetic Ce$^{4+}$ defects and use this result to interpret the spectrum of crystal-electric field transitions observed using inelastic neutron scattering. The analysis of these single-ion transitions indicates the dipole-octupole nature of the ground state doublet and a significant degree of spin-lattice coupling. The single-ion properties calculated from the crystal-electric field parameters obtained spectroscopically are in good agreement with bulk magnetic susceptibility data down to about 1 K. Below this temperature, the behavior of the magnetic susceptibility indicates a correlated regime without showing any sign of magnetic long-range order or freezing down to 0.08 K. We conclude that Ce$_2$Hf$_2$O$_{7}$ is another candidate to investigate exotic correlated states of quantum matter such as the octupolar quantum spin ice recently argued to exist in the isostructural compounds Ce$_2$Sn$_2$O$_7$ and Ce$_2$Zr$_2$O$_7$.

cond-mat.str-el↗