41 Search Results

Here, we report magnetization and neutron diffraction studies on crystal and magnetic structures of Ir- and Rh-substituted honeycomb lattice α-RuCl₃. The iridium or rhodium atoms are distributed at the Ru site with little structural modification. Both systems undergo a room-temperature monoclinic C2/m to low-temperature trigonal $$R\bar{3}$$ phase transformation with a large recoverable hysteresis. At low temperature, a zigzag spin order is observed with the same characteristic wave vector (0,0.5,1) as in the parent α-RuCl₃. Detailed magnetic structure refinement reveals an ordered moment of 0.32(5)µB/Ru and an upper boundary of canting angle of 15(4)º away from the basal plane at 5more » K for the 10% Ir-substituted α-RuCl₃, which is different from the 0.45–0.73 µB/Ru and 32°–48° canting angle reported in the parent compound α-RuCl₃. The observation of unchanged RuCl₆ local octahedral environment, reduced ordered magnetic moment size and canting angle compared to previously reported highlights the potential to study quantum spin-liquid behavior through nonmagnetic ion doping.« less

α-RuCl₃, a well-known candidate material for Kitaev quantum spin liquid, is prone to stacking disorder due to the weak van der Waals bonding between the honeycomb layers. After a decade of intensive experimental and theoretical studies, the detailed correlation between stacking degree of freedom, structure transition, magnetic, and thermal transport properties remains unresolved. In this work, we reveal the effects of a small amount of stacking disorder inherent even in high quality α-RuCl₃ crystals. This small amount of stacking disorder results in the variation of the magnetic ordering temperature, and it suppresses the structure transition and thermal conductivity. Crystals withmore » a minimal amount of stacking disorder have a T_N > 7.4 K and exhibit a well-defined structure transition around 140 K upon cooling. For those with more stacking faults and a T_N below 7 K, the structure transition occurs well below 140 K upon cooling and is incomplete, manifested by the diffuse streaks and the coexistence of both high-temperature and low-temperature phases down to the lowest measurement temperature. Both types of crystals exhibit oscillatory field-dependent thermal conductivity and a plateaulike feature in thermal Hall resistivity in the field-induced quantum spin liquid state. However, α-RuCl₃ crystals with a minimal amount of stacking disorder have a higher thermal conductivity that pushes the thermal Hall conductivity to be closer to the half-integer quantized value. Importantly, these findings demonstrate a strong correlation between layer stacking, structure transition, magnetic, and thermal transport properties, underscoring the importance of interlayer coupling in α-RuCl3 despite the weak van der Waals bonding.« less

In this study, we investigated the thermal transport properties of two α–RuCl₃ crystals with different degrees of stacking disorder to understand the origin of the previously reported oscillatory feature in the field dependence of thermal conductivity. Crystal I shows only one magnetic order around 13 K, which is near the highest T_N for α–RuCl₃ with stacking faults. Crystal II has less stacking disorder, with a dominant heat capacity at 7.6 K along with weak anomalies at 10 and 13 K. In the temperature and field dependence of thermal conductivity, no obvious anomaly was observed to be associated with the magneticmore » order around 13 K for either crystal or around 10 K for crystal II. Crystal II showed clear oscillations in the field dependence of thermal conductivity, while crystal I did not. For crystal I, an L-shaped region in the temperature-field space was observed where thermal Hall conductivity κ_xy/T is within ±20% of the half quantized thermal Hall conductivity κ_HQ/T, while for crystal II, κ_xy/T reaches κ_HQ/T only in the high field and high temperature regime with no indication of a plateau at κ_HQ/T. Our thermal conductivity data suggest the oscillatory features are inherent to the zigzag ordered phase with T_N near 7 K. Our planar thermal Hall effect measurements suggest the sensitivity of this phenomena to stacking disorder. Overall, our results highlight the importance of understanding and controlling crystallographic disorder for obtaining and interpreting intrinsic thermal transport properties in α–RuCl₃.« less

Mixed halide chemistry has recently been utilized to tune the intrinsic magnetic properties of transition-metal halides—one of the largest families of magnetic van der Waals materials. Prior studies have shown that the strength of exchange interactions, hence the critical temperature, can be tuned smoothly with halide composition for a given ground state. Here we show that the ground state itself can be altered by a small change of halide composition in FeCl_3–$$x$$Br_$$x$$. Specifically, we find a threefold jump in the Néel temperature and a sign change in the Weiss temperature at $$x$$=0.08 corresponding to only 3% bromine doping. Using neutronmore » scattering, we reveal a change of the ground state from spiral order in FeCl₃ to $$A$$-type antiferromagnetic order in FeBr₃. From first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. In conclusion, these results demonstrate how varying the halide composition can tune the competing interactions and change the ground state of a spiral spin liquid system.« less

The use of work-function-mediated charge transfer has recently emerged as a reliable route toward nanoscale electrostatic control of individual atomic layers. Using α-RuCl₃ as a 2D electron acceptor, we are able to induce emergent nano-optical behavior in hexagonal boron nitride (hBN) that arises due to interlayer charge polarization. Using scattering-type scanning near-field optical microscopy (s-SNOM), we find that a thin layer of α-RuCl₃ adjacent to an hBN slab reduces the propagation length of hBN phonon polaritons (PhPs) in significant excess of what can be attributed to intrinsic optical losses. Concomitant nano-optical spectroscopy experiments reveal a novel resonance that aligns energeticallymore » with the region of excess PhP losses. These experimental observations are elucidated by first-principles density-functional theory and near-field model calculations, which show that the formation of a large interfacial dipole suppresses out-of-plane PhP propagation. Finally, our results demonstrate the potential utility of charge-transfer heterostructures for tailoring optoelectronic properties of 2D insulators.« less

Spinel compounds AB₂X₄ consist of both tetrahedral (AX₄) and octahedral (BX₆₎ environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn₂O₄ has been reported to exhibit one structural transition and two successive magnetic transitions. In this work, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn₂O₄ by employing the high-resolution neutron powder diffraction. The cation distribution is found to be ($$Mn_{0.9}^{2+}Fe_{0.1}^{3+}$$)$$_{A}$$($$Mn^{3+}Fe_{0.9}^{3+}Mn_{0.1}^{2+}$$)$$_{B}$$O₄. While large trigonal distortion is found evenmore » in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at T_S ≈ 750 K, driven by the Jahn–Teller effect of the orbital active B-site Mn³⁺ cation. Strong magnetoelastic coupling is unveiled at T_N1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to T_N2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn³⁺/Fe³⁺ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ~25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O–B–O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn₂O₄. Our study indicates that FeMn₂O₄ is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.« less

Abstract Traditional spectroscopy, by its very nature, characterizes physical system properties in the momentum and frequency domains. However, the most interesting and potentially practically useful quantum many-body effects emerge from local, short-time correlations. Here, using inelastic neutron scattering and methods of integrability, we experimentally observe and theoretically describe a local, coherent, long-lived, quasiperiodically oscillating magnetic state emerging out of the distillation of propagating excitations following a local quantum quench in a Heisenberg antiferromagnetic chain. This “quantum wake” displays similarities to Floquet states, discrete time crystals and nonlinear Luttinger liquids. We also show how this technique reveals the non-commutativity of spinmore » operators, and is thus a model-agnostic measure of a magnetic system’s “quantumness.”« less

Single-crystal inelastic neutron-scattering (INS) data contain rich information about the structure and dynamics of a material. Yet the challenge of matching sophisticated theoretical models with large data volumes is compounded by computational complexity and the ill-posed nature of the inverse scattering problem. Here we utilize a novel machine-learning (ML)-assisted framework featuring multiple neural network architectures to address this via high-dimensional modeling and numerical methods. A comprehensive data set of diffraction and INS measured on the Kitaev material α-RuCl₃ is processed to extract its Hamiltonian. Semiclassical Landau-Lifshitz dynamics and Monte-Carlo simulations were employed to explore the parameter space of an extendedmore » Kitaev-Heisenberg Hamiltonian. A ML-assisted iterative algorithm was developed to map the uncertainty manifold to match experimental data, a nonlinear autoencoder was used to undertake information compression, and radial basis networks were utilized as fast surrogates for diffraction and dynamics simulations to predict potential spin Hamiltonians with uncertainty. Exact diagonalization calculations were employed to assess the impact of quantum fluctuations on the selected parameters around the best prediction.« less

The honeycomb magnet α-RuCl₃ has attracted considerable interest because it is proximate to the Kitaev Hamiltonian whose excitations are Majoranas and vortices. The thermal Hall conductivity κ_xy of Majorana fermions is predicted to be half-quantized. Half-quantization of κxy/T (T, temperature) was recently reported, but this observation has proven difficult to reproduce. Here, we report detailed measurements of κ_xy on α-RuCl3 with the magnetic field B ∥ a (zigzag axis). In our experiment, κ_xy/T is observed to be strongly temperature dependent between 0.5 and 10 K. We show that its temperature profile matches the distinct form expected for topological bosonic modesmore » in a Chern-insulator-like model. Our analysis yields magnon band energies in agreement with spectroscopic experiments. At high B, the spin excitations evolve into magnon-like modes with a Chern number of ~1. In conclusion, the bosonic character is incompatible with half-quantization of κ_xy/T.« less

Tmore » he material $\alpha \text{-}{\mathrm{RuCl}}_3$ 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, $\alpha \text{-}{\mathrm{RuCl}}_3$ undergoes a transition to a state with antiferromagnetic order below a temperature ${}_N\approx 7\mathrm{K}$, but this order can be suppressed by applying an external in-plane magnetic field of $H_{\parallel }=7\mathrm{}$. 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 $H_{\parallel }>7\mathrm{}$ by Kasahara and co-workers [Nature (London) 559, 227 (2018)] has been interpreted as evidence of itinerant Majorana fermions in the Kitaev quantum spin liquid state. In this study, we reexamine the origin of the thermal Hall conductivity ${\kappa }_{xy}$ in $\alpha \text{-}{\mathrm{RuCl}}_3$. Our measurements of ${\kappa }_{xy}\left(\right)$ on several different crystals yield a temperature dependence very similar to that of the phonon-dominated longitudinal thermal conductivity ${\kappa }_{xx}\left(\right)$, for which the natural explanation is that ${\kappa }_{xy}$ is also mostly carried by phonons. Upon cooling, ${\kappa }_{xx}$ peaks at $\simeq 20\mathrm{K}$, then drops until ${}_N$, whereupon it suddenly increases again. he abrupt increase below ${}_N$ 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. he fact that ${\kappa }_{xy}$ also increases suddenly below ${}_N$ is strong evidence that the thermal Hall effect in $\alpha \text{-}{\mathrm{RuCl}}_3$ is also carried predominantly by phonons. his implies that any quantized signal from Majorana edge modes would have to come on top of a sizable—and sample-dependent—phonon background.« less