arXiv subjects

S. E. Nagler

Publications and source records attributed to S. E. Nagler.

At least 19 recordsLinked to original sources

arXiv2026

Spin sensitive transport in RuCl_3/Pt heterostructures: revealing a field-transverse spin anisotropy in a spin liquid candidate

Alpha-phase (a-) RuCl_3 has emerged as a possible candidate for a quantum spin liquid (QSL) that promises exotic quasi-particles relevant for fault-tolerant quantum computation. Here, we report spin-sensitive transport measurements using a proximal spin Hall metal -- platinum (Pt) -- to probe magnetic moments in the insulator a-RuCl_3. We observe a spin Hall magnetoresistance (SMR) where both the transverse and longitudinal resistivities of a-RuCl_3/Pt exhibit oscillations as functions of the angle between the in-plane magnetic field and the current, arising from the interplay between the spin Hall effect in Pt and local magnetic moments in a-RuCl3. The oscillations are observed from 1.5 T to 18 T, spanning the range of magnetic fields where zig-zag antiferromagnetic phase, the putative QSL phase, and the partially field-polarized states are reported in a-RuCl_3. The phase of the SMR oscillations suggest that the local moments, regardless of static or fluctuating, in a-RuCl_3 develop an anisotropy with an in-plane quantization axis that aligns largely transverse to the magnetic field for all fields above 1.5 T. Temperature dependance of the SMR revealed that the spin anisotropy has a similar energy scale to reported QSL signatures in a-RuCl_3. The coupling between the spin states within a-RuCl_3 and Pt demonstrated in our experiment opens a transport route to exploring exotic spin phases and device functionalities of QSL materials.

cond-mat.str-el↗

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.

cond-mat.str-el↗

arXiv2021

Evidence of a Phonon Hall Effect in the Kitaev Spin Liquid Candidate $α$-RuCl$_3$

The material $α$-RuCl$_3$ has been the subject of intense scrutiny as a potential Kitaev quantum spin liquid, predicted to display Majorana fermions as low energy excitations. In practice, $α$-RuCl$_3$ undergoes a transition to a state with antiferromagnetic order below a temperature $T_{\rm N}$ $\approx$ 7 K, but this order can be suppressed by applying an external in-plane magnetic field of $H_\parallel$ = 7 T. Whether a quantum spin liquid phase exists just above that field is still an open question, but the reported observation of a quantized thermal Hall conductivity at $H_\parallel$ $>$ 7 T by Kasahara and co-workers $\big[$Kasahara ${\it et \ al}$., Nature ${\bf 559}$, 227 (2018)$\big]$ has been interpreted as evidence of itinerant Majorana fermions in the Kitaev quantum spin liquid state. In this study, we re-examine the origin of the thermal Hall conductivity $κ_{\rm xy}$ in $α$-RuCl$_3$. Our measurements of $κ_{\rm xy}$($T$) on several different crystals yield a temperature dependence very similar to that of the phonon-dominated longitudinal thermal conductivity $κ_{\rm xx}$($T$), for which the natural explanation is that $κ_{\rm xy}$ is also mostly carried by phonons. Upon cooling, $κ_{\rm xx}$ peaks at $T \simeq$ 20 K, then drops until $T_{\rm N}$, whereupon it suddenly increases again. The abrupt increase below $T_{\rm N}$ is attributed to a sudden reduction in the scattering of phonons by low-energy spin fluctuations as these become partially gapped when the system orders. The fact that $κ_{\rm xy}$ also increases suddenly below $T_{\rm N}$ is strong evidence that the thermal Hall effect in $α$-RuCl$_3$ is also carried predominantly by phonons. This implies that any quantized signal from Majorana edge modes would have to come on top of a sizable -- and sample-dependent -- phonon background.

cond-mat.str-el↗

arXiv2021

Field-induced intermediate phase in $α$-RuCl$_3$: Non-coplanar order, phase diagram, and proximate spin liquid

Frustrated magnets with strong spin-orbit coupling promise to host topological states of matter, with fractionalized excitations and emergent gauge fields. Kitaev's proposal for a honeycomb-lattice Majorana spin liquid has triggered an intense search for experimental realizations, with bond-dependent Ising interaction being the essential building block. A prime candidate is $α$-RuCl$_3$ whose phase diagram in a magnetic field is, however, not understood to date. Here we present conclusive experimental evidence for a novel field-induced ordered phase in $α$-RuCl$_3$, sandwiched between the zigzag and quantum disordered phases at low and high fields, respectively. We provide a detailed theoretical study of the relevant effective spin model which we show to display a field-induced intermediate phase as well. We fully characterize the intermediate phase within this model, including its complex spin structure, and pinpoint the parameters relevant to $α$-RuCl$_3$ based on the experimentally observed critical fields. Most importantly, our study connects the physics of $α$-RuCl$_3$ to that of the Kitaev-$Γ$ model, which displays a quantum spin liquid phase in zero field, and hence reveals the spin liquid whose signatures have been detected in a variety of dynamical probes of $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2021

Witnessing entanglement in quantum magnets using neutron scattering

We demonstrate how quantum entanglement can be directly witnessed in the quasi-1D Heisenberg antiferromagnet KCuF$_3$. We apply three entanglement witnesses --- one-tangle, two-tangle, and quantum Fisher information --- to its inelastic neutron spectrum, and compare with spectra simulated by finite-temperature density matrix renormalization group (DMRG) and classical Monte Carlo methods. We find that each witness provides direct access to entanglement. Of these, quantum Fisher information is the most robust experimentally, and indicates the presence of at least bipartite entanglement up to at least 50 K, corresponding to around 10% of the spinon zone-boundary energy. We apply quantum Fisher information to higher spin-S Heisenberg chains, and show theoretically that the witnessable entanglement gets suppressed to lower temperatures as the quantum number increases. Finally, we outline how these results can be applied to higher dimensional quantum materials to witness and quantify entanglement.

cond-mat.str-el↗

arXiv2020

Detection of Kardar-Parisi-Zhang hydrodynamics in a quantum Heisenberg spin-$1/2$ chain

Classical hydrodynamics is a remarkably versatile description of the coarse-grained behavior of many-particle systems once local equilibrium has been established. The form of the hydrodynamical equations is determined primarily by the conserved quantities present in a system. Some quantum spin chains are known to possess, even in the simplest cases, a greatly expanded set of conservation laws, and recent work suggests that these laws strongly modify collective spin dynamics even at high temperature. Here, by probing the dynamical exponent of the one-dimensional Heisenberg antiferromagnet KCuF$_3$ with neutron scattering, we find evidence that the spin dynamics are well described by the dynamical exponent $z=3/2$, which is consistent with the recent theoretical conjecture that the dynamics of this quantum system are described by the Kardar-Parisi-Zhang universality class. This observation shows that low-energy inelastic neutron scattering at moderate temperatures can reveal the details of emergent quantum fluid properties like those arising in non-Fermi liquids in higher dimensions.

cond-mat.str-el↗

arXiv2020

Nature of Magnetic Excitations in the High-Field Phase of $α$-RuCl$_3$

We present comprehensive electron spin resonance (ESR) studies of in-plane oriented single crystals of $α$-RuCl$_3$, a quasi-two-dimensional material with honeycomb structure, focusing on its high-field spin dynamics. The measurements were performed in magnetic fields up to 16 T, applied along the [110] and [100] directions. Several ESR modes were detected. Combining our findings with recent inelastic neutron- and Raman-scattering data, we identify most of the observed excitations. Most importantly, we show that the low-temperature ESR response beyond the boundary of the magnetically ordered region is dominated by single- and two-particle processes with magnons as elementary excitations. The peculiarities of the excitation spectrum in the vicinity of the critical field are discussed.

cond-mat.str-el↗

arXiv2020

Field-induced phase transitions of the Kitaev material $α$-RuCl$_3$ probed by thermal expansion and magnetostriction

High-resolution thermal expansion and magnetostriction measurements were performed on single crystals of $α$-RuCl$_3$ in magnetic fields applied parallel to the Ru-Ru bonds. The length changes were measured in the direction perpendicular to the honeycomb planes. Our data show clear thermodynamic characteristics for the field-induced phase transition at the critical field $μ_0H_{c1} = 7.8(2)$ T where the antiferromagnetic zigzag order is suppressed. At higher fields, a kink in the magnetostriction coefficient signals an additional phase transition around $μ_0H_{c2} \approx 11$ T. The extracted Grüneisen parameter shows typical hallmarks for quantum criticality near $H_{c1}$, but also displays anomalous behavior above $H_{c1}$. We compare our experimental data with linear spin-wave calculations employing a minimal Kitaev-Heisenberg model in the semiclassical limit. Most of the salient features are in agreement with each other, however, the peculiar features in the high-field region above $H_{c1}$ cannot be accounted for in our modelling and hence suggest a genuine quantum nature. We construct a phase diagram for $α$-RuCl$_3$ showing two low-temperature transitions induced by an in-plane field along the Ru-Ru bonds.

cond-mat.str-el↗

arXiv2019

Polarization-resolved Raman spectroscopy of α-RuCl3 and evidence of room temperature two-dimensional magnetic scattering

Polarization-resolved Raman spectroscopy was performed and analyzed from large, high quality, mono-domain single crystal of α-RuCl3, a proximate Kitaev quantum spin liquid. Spectra were collected with laser polarizations parallel and perpendicular to the honeycomb plane. Pairs of nearly degenerate phonons were discovered and show either a 4-fold or 2-fold polarization angle dependence in their Raman intensity, thereby providing evidence to definitively assign the bulk crystal point group as C2h. The low frequency continuum that is often attributed to scattering from pairs of Majorana fermions was also examined and found to disappear when the laser excitation and scattered photon polarizations were perpendicular to the honeycomb plane. This disappearance, along with the behavior of the phonon spectrum in the same polarization configuration, strongly suggests that the scattering continuum is 2-dimensional. We argue that this scattering continuum originates from the Kitaev magnetic interactions that survives up to room temperature, a scale larger than the bare Kitaev exchange energy of approximately 50 K.

cond-mat.str-el↗

arXiv2019

Anomalous Magnetic Behavior in Ba2CoO4 with Isolated CoO4 Tetrahedra

The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) Ba2CoO4. In spite of isolated CoO4 tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to TN = 26 K. More unexpectedly, spin dynamics displays quasi-2D spin wave dispersion with an unusually large spin gap, and 1D magnetoelastic coupling. Our results indicate that Ba2CoO4 is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and novel spin states in oxides.

cond-mat.str-el↗

arXiv2018

Impact of further-range exchange and cubic anisotropy on magnetic excitations in the fcc kagome antiferromagnet IrMn3

Exchange interactions up to fourth nearest neighbor are shown within a classical local-moment Heisenberg approach to be important to model inelastic neutron scattering data on the fcc kagome antiferromagnet IrMn$_3$. Spin wave frequencies are calculated using the torque equation and the magnetic scattering function, $S({\bf Q},ω)$, is determined by a Green's function method, as an extension of our previous work, LeBlanc et al, Phys. Rev. B 90, 144403 (2014). Results are compared with intensity contour data on powder samples of ordered IrMn$_3$, where magnetic Mn ions occupy lattice sites of ABC stacked kagome planes. Values of exchange parameters taken from DFT calculations used in our model provide good agreement with the experimental results only if further-neighbor exchange is included. Estimates of the observed energy gap support the existence of strong cubic anisotropy predicted by DFT calculations.

cond-mat.str-el↗

arXiv2018

Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet alpha-RuCl3

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p=0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin liquid phase in alpha-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid.

cond-mat.str-el↗

arXiv2018

Anisotropic susceptibilities in the honeycomb Kitaev system $α$-RuCl$_{3}$

The magnetic insulator $α$-RuCl$_{3}$ is a promising candidate to realize Kitaev interactions on a quasi-2D honeycomb lattice. We perform extensive susceptibility measurements on single crystals of $α$-RuCl$_{3}$, including angle-dependence of the in-plane longitudinal and transverse susceptibilities, which reveal a unidirectional anisotropy within the honeycomb plane. By comparing the experimental results to a high-temperature expansion of a Kitaev-Heisenberg-$Γ$ spin Hamiltonian with bond anisotropy, we find excellent agreement with the observed phase shift and periodicity of the angle-resolved susceptibilities. Within this model, we show that the pronounced difference between in-plane and out-of-plane susceptibilities as well as the finite transverse susceptibility are rooted in strong symmetric off-diagonal $Γ$ spin exchange. The $Γ$ couplings and relationships between other terms in the model Hamiltonian are quantified by extracting relevant Curie-Weiss intercepts from the experimental data.

cond-mat.mtrl-sci↗

arXiv2017

Unconventional spin dynamics in the honeycomb-lattice material $α$-RuCl$_3$: high-field ESR studies

We present high-field electron spin resonance (ESR) studies of the honeycomb-lattice material $α$-RuCl$_3$, a prime candidate to exhibit Kitaev physics. Two modes of antiferromagnetic resonance were detected in the zigzag ordered phase, with magnetic field applied in the $ab$ plane. A very rich excitation spectrum was observed in the field-induced quantum paramagnetic phase. The obtained data are compared with results of recent numerical calculations, strongly suggesting a very unconventional multiparticle character of the spin dynamics in $α$-RuCl$_3$. The frequency-field diagram of the lowest-energy ESR mode is found consistent with the behavior of the field-induced energy gap, revealed by thermodynamic measurements.

cond-mat.str-el↗

arXiv2017

Evidence for the Confinement of Magnetic Monopoles in Quantum Spin Ice

Magnetic monopoles are hypothesised elementary particles connected by Dirac strings that behave like infinitely thin solenoids. Despite decades of searches, free magnetic monopoles and their Dirac strings have eluded experimental detection, although there is substantial evidence for deconfined magnetic monopole quasiparticles in spin ice materials. Here we report the detection of a hierarchy of unequally-spaced magnetic excitations \emph{via} high resolution inelastic neutron spectroscopic measurements on the quantum spin ice candidate Pr$_{2}$Sn$_{2}$O$_{7}$. These excitations are well-described by a simple model of monopole pairs bound by a linear potential with an effective tension of 0.642(8) K~$\cdot$Å$^{-1}$ at 1.65~K. The success of the linear potential model suggests that these low energy magnetic excitations are direct spectroscopic evidence for the confinement of magnetic monopole quasiparticles in the quantum spin ice candidate Pr$_{2}$Sn$_{2}$O$_{7}$.

cond-mat.str-el↗

arXiv2017

Destabilization of magnetic order in a dilute Kitaev spin liquid candidate

The insulating honeycomb magnet $α$-RuCl$_{3}$ exhibits fractionalized excitiations that signal its proximity to a Kitaev quantum spin liquid (QSL) state, however, at $T=0$, fragile long-range magnetic order arises from non-Kitaev terms in the Hamiltonian. Spin vacancies in the form of Ir$^{3+}$ substituted for Ru are found to destabilize this long-range order. Neutron diffraction and bulk characterization of Ru$_{1-x}$Ir$_{x}$Cl$_{3}$ show that the magnetic ordering temperature is suppressed with increasing $x$ and evidence of zizag magnetic order is absent for $x>0.3$. Inelastic neutron scattering demonstrates that the signature of fractionalized excitations is maintained over the full range of $x$ investigated. The depleted lattice without magnetic order thus hosts a spin-liquid-like ground state that may indicate the relevance of Kitaev physics in the magnetically dilute limit of RuCl$_{3}$.

cond-mat.str-el↗

arXiv2017

Antiferromagnetic resonance and terahertz continuum in $α-$RuCl$_3$

We report measurements of optical absorption in the zig-zag antiferromagnet $α$-RuCl$_3$ as a function of temperature, $T$, magnetic field, $B$, and photon energy, $\hbarω$ in the range $\sim$ 0.3 to 8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that 3-fold rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at $B=0$ that we identify as magnetic-dipole excitation of a zero-wavevector magnon, or antiferromagnetic resonance (AFMR). With application of $B$, the AFMR broadens and shifts to lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the $dc$ susceptibility from a magnetic excitation continuum.

cond-mat.str-el↗

arXiv2017

Comprehensive study of the dynamics of a classical Kitaev spin liquid

We study the spin-$S$ Kitaev model in the classical ($S \to \infty$) limit using Monte Carlo simulations combined with semi-classical spin dynamics. We discuss differences and similarities in the dynamical structure factors of the spin-$1/2$ and the classical Kitaev liquids. Interestingly, the low-temperature and low-energy spectrum of the classical model exhibits a finite energy peak, which is the precursor of the one produced by the Majorana modes of the $S=1/2$ model. The classical peak is spectrally narrowed compared to the quantum result and can be explained by magnon excitations within fluctuating one-dimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of magnons propagating in one-dimensional manifolds. Moreover, we show that the momentum space distribution of the low-energy spectral weight of the $S=1/2$ model follows the momentum space distribution of zero modes of the classical model.

cond-mat.str-el↗