arXiv subjects

C. L. Sarkis

Publications and source records attributed to C. L. Sarkis.

7 recordsLinked to original sources

arXiv2026

Intermediate Field Spin(on) Dynamics in $α$-RuCl$_3$

We present comprehensive inelastic neutron spectroscopic maps of the magnetic field-induced disordered phase of the Kitaev quantum spin liquid candidate material $α$-RuCl$_3$. For fields along both in-plane high-symmetry directions we observe that the spin excitation spectrum at and above a magnetic field of 8~T is gapped. Excitation modes then sharpen for increasing field but are consistently broader than experimental resolution even at 13.5~T. The out-of-plane dispersion diminishes in the 7-10~T regime, signifying enhanced two-dimensional behavior as the in-plane liquid correlations are established. In this regime, excitations are very broad and largely flat for all accessible energy-momenta, which is kinematically at odds with a magnon-decay picture. By contrast, a continuum of fractionalized excitations naturally yields a broad continuum response, which crucially may be accompanied by sharper modes of bound states of fractionalized excitations. Their damping by the continuum accounts for the observed spectral broadening and field dependence. Our results provide strong evidence for the existence of fractionalized excitations in $α$-RuCl$_3$ in a magnetic field.

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arXiv2023

Spin wave Hamiltonian and anomalous scattering in NiPS$_3$

We report a comprehensive spin wave analysis of the semiconducting honeycomb van der Waal antiferromagnet NiPS$_3$. Using single crystal inelastic neutron scattering, we map out the full Brillouin zone and fit the observed modes to a spin wave model with rigorously defined uncertainty. We find that the third neighbor exchange $J_3$ dominates the Hamiltonian, a feature which we fully account for by ab-initio density functional theory calculations. We also quantify the degree to which the three-fold rotation symmetry is broken and account for the $Q=0$ excitations observed in other measurements, yielding a spin exchange model which is consistent across multiple experimental probes. We also identify a strongly reduced static ordered moment and reduced low-energy intensity relative to the linear spin wave calculations, signaling unexplained features in the magnetism which requires going beyond the linear spin wave approximation.

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arXiv2021

Microscopics of Quantum Annealing in the Disordered Dipolar Ising Ferromagnet LiHo$_{x}$Y$_{1-x}$F$_4$

Quantum annealing (QA) refers to an optimization process that uses quantum fluctuations to find the global minimum of a rugged energy landscape with many local minima. Conceptually, QA is often framed in the context of the disordered transverse field Ising model, in which a magnetic field applied perpendicular to the Ising axis tunes the quantum fluctuations and enables the system to tunnel through energy barriers, and hence reach the ground state more quickly. A solid state material closely related to this model, LiHo$_{0.45}$Y$_{0.55}$F$_4$, was shown to exhibit faster dynamics after a QA protocol, compared to thermal annealing (TA), but little is known about the actual process of optimization involved or the nature of the spin correlations in the state that is reached. Here, we report on the microscopics of QA in this material using diffuse magnetic neutron scattering. Comparing a QA to a TA protocol that reach the same end-point, we find very similar final diffuse scattering which consists of pinch-point scattering, largely consistent with critical scattering near a phase boundary in the dipolar Ising ferromagnetic model. However, comparing the time evolution at the end of the protocols, we find that the spin correlations evolve more significantly after TA, suggesting that QA produces a state closer to equilibrium. We also observe experimental evidence that the transverse field produces random fields, which had been previously predicted for this material and studied in other contexts. Thus, while the material does exhibit a quantum speedup under quantum annealing conditions, it is not a simple annealing problem; the energy landscape being optimized is changing as the optimization proceeds.

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arXiv2021

Low Temperature Domain Wall Freezing and Non-Equilibrium Dynamics in the Transverse-Field Ising Model Material CoNb$_2$O$_6$

CoNb$_2$O$_6$ is a rare realization of the transverse field Ising model (TFIM), making it a useful tool for studying both equilibrium and non-equilibrium many-body quantum physics. Despite a large body of work dedicated to characterizing this material, details of the ordered states in the presence of relatively weak transverse fields have not been discussed in detail. Here, we present a detailed study of CoNb$_2$O$_6$ via ac susceptibility measurements in order to further characterize its low temperature behavior in the presence of a transverse field. Specifically, we call attention to an unconventional freezing transition in zero-field below T$_F$ = 1.2~K, occurring \emph{within} the well-known commensurate antiferromagnetic (CAFM) state that onsets at $T_{N2}$ = 1.9~K. We performed a series of transverse-field quenches into this frozen state, which resulted in a slowly relaxing susceptibility, $χ^{\prime}(t)$, that followed a logarithmic decay within the time range measured. We discuss the frozen state in the context of the freezing of previously discussed "free" chains arising from domain walls between the four degenerate sublattices of the CAFM state. We also attempted to observe Kibble-Zurek scaling by quenching the transverse field into the frozen state at different rates. This produced a null result; the behavior can be fully explained by coarsening of domains over the timescale of the quenches. The absence of a clear Kibble-Zurek scaling is itself surprising, given the proposed ubiquity of the phenomenon for general second order phase transitions.

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arXiv2021

Magnetic Properties of the Ising-like Rare Earth Pyrosilicate: D-Er$_{2}$Si$_{2}$O$_{7}$

Ising-like spin-1/2 magnetic materials are of interest for their ready connection to theory, particularly in the context of quantum critical behavior. In this work we report detailed studies of the magnetic properties of a member of the rare earth pyrosilicate family, D-Er$_{2}$Si$_{2}$O$_{7}$, which is known to display a highly anisotropic Ising-like g-tensor and effective spin-1/2 magnetic moments. We used powder neutron diffraction, powder inelastic neutron spectroscopy (INS), and single crystal AC susceptibility to characterize its magnetic properties. Neutron diffraction enabled us to determine the magnetic structure below the known transition temperature ($T_{N}$ = 1.9 K) in zero field, confirming that the magnetic state is a four-sublattice antiferromagnetic structure with two non-collinear Ising axes, as was previously hypothesized. Our powder INS data revealed a gapped excitation at zero field, consistent with anisotropic (possibly Ising) exchange. An applied field of 1 T produces a mode softening, which is consistent with a field-induced second order phase transition. To assess the relevance of D-Er$_{2}$Si$_{2}$O$_{7}$ to the transverse field Ising model, we performed AC susceptibility measurements on a single crystal with the magnetic field oriented in the direction transverse to the Ising axes. This revealed a transition at 2.65 T at 0.1 K, a field significantly higher than the mode-softening field observed by powder INS, showing that the field-induced phase transitions are highly field-direction dependent as expected. These measurements suggest that D-Er$_{2}$Si$_{2}$O$_{7}$ may be a candidate for further exploration related to the transverse field Ising model.

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arXiv2020

Unravelling competing microscopic interactions at a phase boundary: a single crystal study of the metastable antiferromagnetic pyrochlore Yb$_{2}$Ge$_{2}$O$_{7}$

We report inelastic neutron scattering measurements from our newly synthesized single crystals of the structurally metastable antiferromagnetic pyrochlore Yb$_{2}$Ge$_{2}$O$_{7}$. We determine the four symmetry-allowed nearest-neighbor anisotropic exchange parameters via fits to linear spin wave theory supplemented by fits of the high-temperature specific heat. The exchange parameters so-determined are strongly correlated to the values determined for the $g$-tensor components, as previously observed for the related Yb pyrochlore Yb$_{2}$Ti$_{2}$O$_{7}$. To address this issue, we directly determined the $g$-tensor from electron paramagnetic resonance of 1% Yb-doped Lu$_{2}$Ge$_{2}$O$_{7}$, thus enabling an unambiguous determination of the exchange parameters. Our results show that Yb$_{2}$Ge$_{2}$O$_{7}$ resides extremely close to the classical phase boundary between an antiferromagnetic $Γ_5$ phase and a splayed ferromagnet phase. By juxtaposing our results with recent ones on Yb$_{2}$Ti$_{2}$O$_{7}$, our work illustrates that the Yb pyrochlore oxides represent ideal systems for studying quantum magnets in close proximity to classical phase boundaries.

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arXiv2019

Partial Antiferromagnetic Helical Order in Fe$_3$PO$_4$O$_3$

Magnetic frustration in Fe$_3$PO$_4$O$_3$ has been shown to produce to an unusual magnetic state below T$_N = 163$ K, where incommensurate antiferromagnetic order is restricted to nanosized needle-like domains, as inferred from neutron powder diffraction. Here we show using single-crystal neutron diffraction that Fe$_3$PO$_4$O$_3$ does not exhibit a preferred ordering wavevector direction in the $ab$ plane despite having a well-defined ordering wavevector length. This results in the observation of continuous rings of scattering rather than satellite Bragg peaks. The lack of a preferred incommensurate ordering wavevector direction can be understood in terms of an antiferromagnetic Heisenberg model with nearest-neighbor ($J_1$) and second-neighbor ($J_2$) interactions, which produces a quasi-degenerate manifold of ordering wavevectors. This state appears to be similar to the partially ordered phase of MnSi, but in Fe$_3$PO$_4$O$_3$ arises in a frustrated antiferromagnet rather than a chiral ferromagnet.

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