arXiv subjects

Andrew Wildes

Publications and source records attributed to Andrew Wildes.

At least 19 recordsLinked to original sources

arXiv2022

Geometric frustration on the trillium lattice in a magnetic metal-organic framework

In the dense metal-organic framework Na[Mn(HCOO)$_3$], Mn$^{2+}$ ions ($S=\frac{5}{2}$) occupy the nodes of a `trillium' hyperkagome net. We show that this material exhibits a variety of behaviour characteristic of geometric frustration: the Néel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Néel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at $\frac{1}{3}$-saturation magnetisation. We demonstrate that a simple nearest-neighbour Heisenberg antiferromagnet model accounts quantitatively for each observation, and hence Na[Mn(HCOO)$_3$] is the first experimental realisation of this model on the trillium net. We develop a mapping between this trillium model and that on the two-dimensional Shastry-Sutherland lattice, and demonstrate how both link geometric frustration within the classical spin liquid regime to a strong magnetocaloric response at low fields.

cond-mat.str-el↗

arXiv2022

Incommensurate and multiple-$\boldsymbol{q}$ magnetic misfit order in the frustrated quantum-spin-ladder material antlerite, Cu$_3$SO$_4$(OH)$_4$

In frustrated magnetic systems, the competition amongst interactions can introduce extremely high degeneracy and prevent the system from readily selecting a unique ground state. In such cases, the magnetic order is often exquisitely sensitive to the balance among the interactions, allowing tuning among novel magnetically ordered phases. In antlerite, Cu$_3$SO$_4$(OH)$_4$, Cu$^{2+}$ ($S=1/2$) quantum spins populate three-leg zigzag ladders in a highly frustrated quasi-one-dimensional structural motif. We demonstrate that at zero applied field, in addition to its recently reported low-temperature phase of coupled ferromagnetic and antiferromagnetic spin chains, this mineral hosts an incommensurate helical+cycloidal state, an idle-spin state, and a multiple-$q$ phase which is the magnetic analog of misfit crystal structures. The antiferromagnetic order on the central leg is reentrant. The high tunability of the magnetism in antlerite makes it a particularly promising platform for pursuing exotic magnetic order.

cond-mat.str-el↗

arXiv2022

Coupled frustrated ferromagnetic and antiferromagnetic quantum spin chains in the quasi-one-dimensional mineral antlerite, Cu$_3$SO$_4$(OH)$_4$

Magnetic frustration, the competition among exchange interactions, often leads to novel magnetic ground states with unique physical properties which can hinge on details of interactions that are otherwise difficult to observe. Such states are particularly interesting when it is possible to tune the balance among the interactions to access multiple types of magnetic order. We present antlerite, Cu$_3$SO$_4$(OH)$_4$, as a potential platform for tuning frustration. Contrary to previous reports, the low-temperature magnetic state of its three-leg zigzag ladders is a quasi-one-dimensional analog of the magnetic state recently proposed to exhibit spinon-magnon mixing in botallackite. Density functional theory calculations indicate that antlerite's magnetic ground state is exquisitely sensitive to fine details of the atomic positions, with each chain independently on the cusp of a phase transition, indicating an excellent potential for tunability.

cond-mat.str-el↗

arXiv2021

Spin dynamics of the director state in frustrated hyperkagome systems

We present an experimental study of the magnetic structure and dynamics of two frustrated hyperkagome compounds, Gd3Ga5O12 and Gd3Al5O12. It has previously been shown that Gd3Ga5O12 exhibits long-range correlations of multipolar directors, that are formed from antiferromagnetic spins on loops of ten ions. Using neutron diffraction and Reverse Monte Carlo simulations we prove the existence of similar magnetic correlations in Gd3Al5O12, showing the ubiquity of these complex structures in frustrated hyperkagome materials. Using inelastic neutron scattering we shed further light on the director state and the associated low lying magnetic excitations. In addition we have measured quasielastic dynamics that show evidence of spin diffusion. Finally, we present AC susceptibility measurements on both Gd3Ga5O12 and Gd3Al5O12, revealing a large difference in the low frequency dynamics between the two otherwise similar compounds.

cond-mat.str-el↗

arXiv2021

Emergent magnetic behaviour in the frustrated Yb$_3$Ga$_5$O$_{12}$ garnet

We report neutron scattering, magnetic susceptibility and Monte Carlo theoretical analysis to verify the short range nature of the magnetic structure and spin-spin correlations in a Yb$_3$Ga$_5$O$_{12}$ single crystal. The quantum spin state of Yb$^{3+}$ in Yb$_3$Ga$_5$O$_{12}$ is verified. The quantum spins organise into a short ranged emergent director state for T $<$ 0.4 K derived from anisotropy and near neighbour exchange. We derive the magnitude of the near neighbour exchange interactions $0.6\; {\rm K} < J_1 < 0.7\; {\rm K}, J_2 = 0.12$~K and the magnitude of the dipolar exchange interaction, $D$, in the range $0.18 < D < 0.21$ K. Certain aspects of the broad experimental dataset can be modelled using a $J_1D$ model with ferromagnetic near neighbour spin-spin correlations while other aspects of the data can be accurately reproduced using a $J_1J_2D$ model with antiferromagnetic near neighbour spin-spin correlation. As such, although we do not quantify all the relevant exchange interactions we nevertheless provide a strong basis for the understanding of the complex Hamiltonian required to fully describe the magnetic state of Yb$_3$Ga$_5$O$_{12}$.

cond-mat.str-el↗

arXiv2020

Effects of uniaxial pressure on the spin ice Ho2Ti2O7

The spin ice materials Ho2Ti2O7 and Dy2Ti2O7 are experimental and theoretical exemplars of highly frustrated magnetic materials. However, the effects of an applied uniaxial pressure are not well studied, and here we report magnetization measurements of Ho2Ti2O7 under uniaxial pressure applied in the [001], [111] and [110] crystalline directions. The basic features are captured by an extension of the dipolar spin ice model. We find a good match between our model and measurements with pressures applied along two of the three directions, and extend the framework to discuss the influence of crystal misalignment for the third direction. The parameters determined from the magnetization measurements reproduce neutron scattering measurements we perform under uniaxial pressure applied along the [110] crystalline direction. In the detailed analysis we include the recently verified susceptibility dependence of the demagnetizing factor. Our work demonstrates the application of a moderate applied pressure to modify the magnetic interaction parameters. The knowledge can be used to predict critical pressures needed to induce new phases and transitions in frustrated materials, and in the case of Ho2Ti2O7 we expect a transition to a ferromagnetic ground state for uniaxial pressures above 3.3 GPa.

cond-mat.str-el↗

arXiv2020

Spin-waves in 2D honeycomb lattice $XXZ$-type van der Waals antiferromagnet CoPS$_3$

The magnetic excitations in CoPS$_3$, a two-dimensional van der Waals (vdW) antiferromagnet with spin $S=3/2$ on a honeycomb lattice, has been measured using powder inelastic neutron scattering. Clear dispersive spin waves are observed with a large spin gap of ~13 meV. The magnon spectra were fitted using an $XXZ$-type $J_1-J_2-J_3$ Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb layers. The best-fit parameters show ferromagnetic exchange $J_1=-2.08$ meV and $J_2=-0.26$ meV for the nearest and second-nearest neighbors and a sizeable antiferromagnetic exchange $J_3=4.21$ meV for the third-nearest neighbor with the strong easy-axis anisotropy $K=-2.06$ meV. The suitable fitting could only be achieved by the anisotropic $XXZ$-type Hamiltonian, in which the exchange interaction for the out-of-plane component is smaller than that for the in-plane one by a ratio $α=J_z/J_x=0.6$. Moreover, the absence of spin-orbit exciton around 30 meV indicates that Co$^{2+}$ ions in CoPS$_3$ have a $S=3/2$ state rather than a spin-orbital entangled $J_\rm{eff}=1/2$ ground state. Our result directly shows that CoPS$_3$ is an experimental realization of the $XXZ$ model with a honeycomb lattice in 2D vdW magnets.

cond-mat.mtrl-sci↗

arXiv2020

Spin-chain correlations in the frustrated triangular lattice material CuMnO$_2$

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO$_2$, which consists of a distorted lattice of Mn$^{3+}$ spins with single-ion anisotropy. Physical property measurements on CuMnO$_2$ are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to $\sim$25 meV above TN= 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.

cond-mat.str-el↗

arXiv2020

Magnetoelastic interaction in the two-dimensional magnetic material MnPS$_3$ studied by first principles calculations and Raman experiments

We report experimental and theoretical studies on the magnetoelastic interactions in MnPS$_3$. Raman scattering response measured as a function of temperature shows a blue shift of the Raman active modes at 120.2 and 155.1 cm$^{-1}$, when the temperature is raised across the antiferromagnetic-paramagnetic transition. Density functional theory (DFT) calculations have been performed to estimate the effective exchange interactions and calculate the Raman active phonon modes. The calculations lead to the conclusion that the peculiar behavior with temperature of the two low energy phonon modes can be explained by the symmetry of their corresponding normal coordinates which involve the virtual modification of the super-exchange angles associated with the leading antiferromagnetic (AFM) interactions.

cond-mat.mes-hall↗

arXiv2020

Cluster Frustration in the Breathing Pyrochlore Magnet LiGaCr4S8

We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr4S8. We observe an unconventional magnetic excitation spectrum with a separation of high and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parameterize the spin Hamiltonian. We find that interactions are ferromagnetic within the large and small tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic further-neighbor interactions are also essential to explain our data, in qualitative agreement with density-functional theory predictions [Ghoshet al.,npj Quantum Mater.4, 63 (2019)]. We explain the origin of geometrical frustration in LiGaCr4S8 interms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a face-centered lattice. Our results provide insight into the emergence of frustration in the presence of strong further-neighbor couplings, and a blueprint for the determination of magnetic interactions in classical spin liquids.

cond-mat.str-el↗

arXiv2019

A quantum liquid of magnetic octupoles on the pyrochlore lattice

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles$^1$. Theories typically define such states using gauge fields and deconfined quasiparticle excitations that emerge from a simple rule governing the local ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice in rare-earth pyrochlores - a state described by a lattice gauge theory of quantum electrodynamics$^{2-4}$. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders$^{5-6}$. Here we show that the correlated ground state of a Ce$^{3+}$-based pyrochlore, Ce$_2$Sn$_2$O$_7$, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with the formation of a fluid-like state of matter, but the intensity distribution is weighted to larger scattering vectors, which indicates that the correlated degrees of freedom have a more complex magnetization density than that typical of magnetic dipoles in a spin liquid. The temperature evolution of the bulk properties in the correlated regime below 1 Kelvin is well reproduced using a model of dipole-octupole doublets on a pyrochlore lattice$^{7-8}$. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule. Our work identifies Ce$_2$Sn$_2$O$_7$ as a unique example of a material where frustrated multipoles form a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

cond-mat.str-el↗

arXiv2018

Kinky DNA in solution: Small angle scattering study of a nucleosome positioning sequence

DNA is a flexible molecule, but the degree of its flexibility is subject to debate. The commonly-accepted persistence length of $l_p \approx 500\,$Å is inconsistent with recent studies on short-chain DNA that show much greater flexibility but do not probe its origin. We have performed X-ray and neutron small-angle scattering on a short DNA sequence containing a strong nucleosome positioning element, and analyzed the results using a modified Kratky-Porod model to determine possible conformations. Our results support a hypothesis from Crick and Klug in 1975 that some DNA sequences in solution can have sharp kinks, potentially resolving the discrepancy. Our conclusions are supported by measurements on a radiation-damaged sample, where single-strand breaks lead to increased flexibility and by an analysis of data from another sequence, which does not have kinks, but where our method can detect a locally enhanced flexibility due to an $AT$-domain.

physics.bio-ph↗

arXiv2018

Orientation of the intra-unit-cell magnetic moment in the high-Tc superconductor HgBa2CuO$_{4+δ}$

Polarized-neutron diffraction experiments (PND) have revealed that the pseudogap state of the cuprates exhibits unusual intra-unit-cell (IUC) magnetism. At a qualitative level, the data indicate a moment direction that is neither perpendicular nor parallel to the CuO2 layers, yet an accurate measurement of a structurally simple compound has been lacking. Here we report PND results with unprecedented accuracy for the IUC magnetic order in the simple-tetragonal single-CuO$_2$-layer compound HgBa2CuO$_{4+δ}$. At the transition temperature, we find evidence for magnetic critical scattering. Deep in the ordered state, we determine the moment direction to be 70° $\pm$ 10° away from the normal to the CuO$_2$ layers, which rules out both purely planar loop currents and high-symmetry Dirac multipoles, the two most prominent theoretical proposals for the microscopic origin of the IUC magnetism. However, the data are consistent with Dirac multipoles of lower symmetry or, alternatively, with a particular configuration of loop currents that flow on the faces of the CuO$_6$ octahedra.

cond-mat.supr-con↗

arXiv2018

Dipolar spin ice states with fast monopole hopping rate in CdEr$_2$X$_4$ (X = Se, S)

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr$_2$Se$_4$ is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three-orders of magnitude faster than the canonical spin ice Dy$_2$Ti$_2$O$_7$. In this letter we use diffuse neutron scattering to show that both CdEr$_2$Se$_4$ and CdEr$_2$S$_4$ support a dipolar spin ice state -- the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to that in Dy$_2$Ti$_2$O$_7$, i.e. dilute and uncorrelated, so cannot provide three-orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er$^{3+}$ ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr$_2$X$_4$ (X = Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr$_2$X$_4$ offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.

cond-mat.str-el↗

arXiv2015

Distinct itinerant spin-density waves and local-moment antiferromagnetism in an intermetallic ErPd$_2$Si$_2$ single crystal

Identifying the nature of magnetism, itinerant or localized, remains a major challenge in condensed-matter science. Purely localized moments appear only in magnetic insulators, whereas itinerant moments more or less co-exist with localized moments in metallic compounds such as the doped-cuprate or the iron-based superconductors, hampering a thorough understanding of the role of magnetism in phenomena like superconductivity or magnetoresistance. Here we distinguish two antiferromagnetic modulations with respective propagation wave vectors of $Q_{\pm}$ = ($H \pm 0.557(1)$, 0, $L \pm 0.150(1)$) and $Q_\text{C}$ = ($H \pm 0.564(1)$, 0, $L$), where $\left(H, L\right)$ are allowed Miller indices, in an ErPd$_2$Si$_2$ single crystal by neutron scattering and establish their respective temperature- and field-dependent phase diagrams. The modulations can co-exist but also compete depending on temperature or applied field strength. They couple differently with the underlying lattice albeit with associated moments in a common direction. The $Q_{\pm}$ modulation may be attributed to localized 4\emph{f} moments while the $Q_\text{C}$ correlates well with itinerant conduction bands, supported by our transport studies. Hence, ErPd$_2$Si$_2$ represents a new model compound that displays clearly-separated itinerant and localized moments, substantiating early theoretical predictions and providing a unique platform allowing the study of itinerant electron behavior in a localized antiferromagnetic matrix.

cond-mat.str-el↗

arXiv2014

Magnetization, crystal structure and anisotropic thermal expansion of single-crystal SrEr2O4

The magnetization, crystal structure, and thermal expansion of a nearly stoichiometric Sr$_{1.04(3)}$Er$_{2.09(6)}$O$_{4.00(1)}$ single crystal have been studied by PPMS measurements and in-house and high-resolution synchrotron X-ray powder diffraction. No evidence was detected for any structural phase transitions even up to 500 K. The average thermal expansions of lattice constants and unit-cell volume are consistent with the first-order Grüneisen approximations taking into account only the phonon contributions for an insulator, displaying an anisotropic character along the crystallographic \emph{a}, \emph{b}, and \emph{c} axes. Our magnetization measurements indicate that obvious magnetic frustration appears below $\sim$15 K, and antiferromagnetic correlations may persist up to 300 K.

cond-mat.str-el↗

arXiv2014

Incommensurate antiferromagnetic order in the manifoldly-frustrated SrTb$_2$O$_4$ with transition temperature up to 4.28 K

The N$\acute{\rm e}$el temperature of the new frustrated family of Sr\emph{RE}$_2$O$_4$ (\emph{RE} = rare earth) compounds is yet limited to $\sim$ 0.9 K, which more or less hampers a complete understanding of the relevant magnetic frustrations and spin interactions. Here we report on a new frustrated member to the family, SrTb$_2$O$_4$ with a record $T_{\rm N}$ = 4.28(2) K, and an experimental study of the magnetic interacting and frustrating mechanisms by polarized and unpolarized neutron scattering. The compound SrTb$_2$O$_4$ displays an incommensurate antiferromagnetic (AFM) order with a transverse wave vector \textbf{Q}$^{\rm 0.5 K}_{\rm AFM}$ = (0.5924(1), 0.0059(1), 0) albeit with partially-ordered moments, 1.92(6) $μ_{\rm B}$ at 0.5 K, stemming from only one of the two inequivalent Tb sites mainly by virtue of their different octahedral distortions. The localized moments are confined to the \emph{bc} plane, 11.9(66)$^\circ$ away from the \emph{b} axis probably by single-ion anisotropy. We reveal that this AFM order is dominated mainly by dipole-dipole interactions and disclose that the octahedral distortion, nearest-neighbour (NN) ferromagnetic (FM) arrangement, different next NN FM and AFM configurations, and in-plane anisotropic spin correlations are vital to the magnetic structure and associated multiple frustrations. The discovery of the thus far highest AFM transition temperature renders SrTb$_2$O$_4$ a new friendly frustrated platform in the family for exploring the nature of magnetic interactions and frustrations.

cond-mat.str-el↗

arXiv2014

Nonmagnetic ordering state of single-crystal SrTm$_2$O$_4$: A polarized and unpolarized neutron-scattering study

Our single-crystal polarized neutron scattering at 65 mK and powder unpolarized neutron diffraction at 0.5 K show no evidence for a long-range magnetic order and even detect no sign of diffuse magnetic neutron scattering in single-crystal SrTm2O4. The data refinements reveal that the two TmO6 octahedral distortion modes are the same as those of the TbO6 octahedra in SrTb2O4, i.e., one distortion is stronger than the other one especially at low temperatures, which is attributed probably to different crystal electric fields for the two inequivalent octahedra. Consequently, we conclude that SrTm2O4 has no magnetic order, neither long-ranged nor short-ranged, even down to 65 mK. Therefore, SrTm2O4 is a different compound from its brethren in the new family of frustrated SrRE2O4 (RE = Gd, Tb, Dy, Ho, Er, and Yb) magnets. We propose that crystal field anisotropy may dominate over weak dipolar spin interactions in SrTm2O4, leading to a virtually nonmagnetic ordering state.

cond-mat.str-el↗