19 Search Results

MgCr₂O₄ is one of the best-known realizations of the pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic exchange interactions are perturbed by small further-neighbor exchanges such that this compound may in principle realize a spiral spin liquid (SSL) phase in the zero-temperature limit. However, a spin Jahn-Teller transition below T_N ≈ 13 K yields a complicated long-range magnetic order with multiple coexisting propagation vectors. We present neutron scattering and thermomagnetic measurements of MgCr₂O₄ samples under applied hydrostatic pressure up to P = 1.7 GPa demonstrating the existence of multiple close-lying nearly degenerate magnetic ground states. We show that the application of hydrostaticmore » pressure increases the ordering temperature by around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by around 0.5 meV per GPa. We also evidence a strong tendency for the preferential occupation of a subset of magnetic domains under pressure. In particular, we show that the $$k$$ = (0,0,1) magnetic phase, which is almost negligible at ambient pressure, dramatically increases in spectral weight under pressure. This modifies the spectrum of magnetic excitations, which we interpret unambiguously as spin waves from multiple magnetic domains. Moreover, we report that the application of pressure reveals a feature in the magnetic susceptibility above the magnetostructural transition. We interpret this as the onset of a short-range ordered phase associated with $$k$$ = (0,0,1), previously not observed in magnetometry measurements.« less

Abstract The search for new elementary particles is one of the most basic pursuits in physics, spanning from subatomic physics to quantum materials. Magnons are the ubiquitous elementary quasiparticle to describe the excitations of fully-ordered magnetic systems. But other possibilities exist, including fractional and multipolar excitations. Here, we demonstrate that strong quantum interactions exist between three flavors of elementary quasiparticles in the uniaxial spin-one magnet FeI ₂ . Using neutron scattering in an applied magnetic field, we observe spontaneous decay between conventional and heavy magnons and the recombination of these quasiparticles into a super-heavy bound-state. Akin to other contemporary problemsmore » in quantum materials, the microscopic origin for unusual physics in FeI ₂ is the quasi-flat nature of excitation bands and the presence of Kitaev anisotropic magnetic exchange interactions.« less

Following tremendous success of the graphene-derived fundamental and applied research, the magnetic van der Waals materials that can be cleaved into monolayer two-dimensional atomic crystals have emerged as a new platform in the studies of low-dimensional physics and in the design of artificial heterostructures with novel properties. Because of that, the family of dihalides and trihalides of the 3d transition group receive a strong renewed interest. Together with our experimental collaborators, we have investigated the novel dynamical properties of one of such promising material CoI₂, and demonstrated that spin excitations in it are prone to substantial breakdown and complex levelmore » repulsion, the important quantum effects whose theoretical understanding has been significantly advanced by our group in the last two decades. Both phenomena are dramatically revealed by experiments and verified by the theory in our joint study, published in Nature Physics. Altogether, our results pave the way toward a new research direction for the magnetic van der Waals materials.« less

We report the A -site spinel NiRh₂O₄ is the only known realization of a spin-1 diamond lattice magnet and is predicted to host unconventional magnetic phenomena driven by frustrated nearest and next-nearest neighbor exchange as well as orbital degeneracy. Previous works found no sign of magnetic order but found a gapped dispersive magnetic excitation indicating a possible valence bond magnetic ground state. However, the presence of many competing low energy degrees of freedom and limited empirical microscopic constraints complicates further analysis. Here we carry out resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy (XAS), and inelastic neutron scattering (INS) tomore » characterize the local electronic structure and lattice dynamics of NiRh₂O₄. The RIXS data can be partly described by a single-ion model for tetrahedrally coordinated Ni²⁺ and indicates a tetragonal distortion Δt₂ = 70 meV that splits the t₂ orbitals into a high energy orbital singlet and lower energy orbital doublet. We identify features of the RIXS spectra that are consistent with a Rh-Ni two-site excitation indicating strong metal-metal hybridization mediated by oxygen in NiRh₂O₄. We also identify signatures of electron-phonon coupling through the appearance of phonon sidebands that dress crystal field excitations. These results establish the key energy scales relevant to the magnetism in NiRh₂O₄ and further demonstrate that covalency and lattice dynamics play essential roles in controlling the magnetic ground states of A -site spinels.« less

Abstract The identification of a non-trivial band topology usually relies on directly probing the protected surface/edge states. But, it is difficult to achieve electronically in narrow-gap topological materials due to the small (meV) energy scales. Here, we demonstrate that band inversion, a crucial ingredient of the non-trivial band topology, can serve as an alternative, experimentally accessible indicator. We show that an inverted band can lead to a four-fold splitting of the non-zero Landau levels, contrasting the two-fold splitting (spin splitting only) in the normal band. We confirm our predictions in magneto-transport experiments on a narrow-gap strong topological insulator, zirconium pentatelluridemore » (ZrTe ₅ ), with the observation of additional splittings in the quantum oscillations and also an anomalous peak in the extreme quantum limit. Our work establishes an effective strategy for identifying the band inversion as well as the associated topological phases for future topological materials research.« less

In this work, we report the static and dynamic magnetic properties of LaSrCrO₄, a seemingly canonical spin-3/2 square-lattice antiferromagnet that exhibits frustration between magnetic layers—owing to their AB stacking—and offers a rare testbed to investigate accidental-degeneracy lifting in magnetism. Neutron diffraction experiments on single-crystal samples uncover a remarkable anticollinear magnetic order below T_N = 170 K characterized by a Néel arrangement of the spins within each layer and an orthogonal arrangement between adjacent layers. To understand the origin of this unusual magnetic structure, we analyze the spin-wave excitation spectrum by means of inelastic neutron scattering and bulk measurements. A spectralmore » gap of 0.5 meV, along with a spin-flop transition at 3.2 T, reflect the energy scale associated with the degeneracy-lifting. A minimal model to explain these observations requires both a positive biquadratic interlayer exchange and dipolar interactions, both of which are on the order of 10^–4 meV, only a few parts per million of the dominant exchange interaction J₁≈11 meV. These results provide direct evidence for the selection of a noncollinear magnetic structure by the combined effect of two distinct degeneracy lifting interactions.« less

CHESS, chopper spectrometer examining small samples, is a planned direct geometry neutron chopper spectrometer designed to detect and analyze weak signals intrinsic to small cross sections (e.g., small mass, small magnetic moments, or neutron absorbing materials) in powders, liquids, and crystals. CHESS is optimized to enable transformative investigations of quantum materials, spin liquids, thermoelectrics, battery materials, and liquids. The broad dynamic range of the instrument is also well suited to study relaxation processes and excitations in soft and biological matter. The 15 Hz repetition rate of the Second Target Station at the Spallation Neutron Source enables the use of multiple incidentmore » energies within a single source pulse, greatly expanding the information gained in a single measurement. Furthermore, the high flux grants an enhanced capability for polarization analysis. This enables the separation of nuclear from magnetic scattering or coherent from incoherent scattering in hydrogenous materials over a large range of energy and momentum transfer. This paper presents optimizations and technical solutions to address the key requirements envisioned in the science case and the anticipated uses of this instrument.« less

The magnetic ground state and the crystalline electric field level scheme of the triangular lattice antiferromagnet KCeO₂ are investigated here. Below $$T_N$$ = 300 mK, KCeO₂ develops signatures of magnetic order in specific heat measurements and low energy inelastic neutron scattering data. Trivalent Ce³⁺ ions in the $$D_{3d}$$ local environment of this compound exhibit large splittings among the lowest three 4$f^1$ Kramers doublets defining for the free ion the $$\textit{J}$$ = 5/2 sextet and a ground state doublet with dipole character, consistent with recent theoretical predictions by Eldeeb et al. [Phys. Rev. Materials 4, 124001 (2020)]. An unexplained, additional localmore » mode appears, and potential origins of this anomalous mode are discussed.« less

Thermomagnetic and inelastic neutron-scattering measurements on Na₂PrO₃ are reported. This material is an antiferromagnetic honeycomb magnet based on the tetravalent lanthanide Pr⁴⁺ and has been proposed to host dominant antiferromagnetic Kitaev interactions. These measurements reveal magnetic fluctuations in Na₂PrO₃ below an energy of 2 meV as well as crystal-field excitations around 230 meV. Furthermore, the latter energy is comparable to the scale of the spin-orbit interaction and explains both the very small effective moment of around 1.0μB per Pr⁴⁺ and the difficulty to uncover any static magnetic scattering below the ordering transition at T_N= 4.6 K. By comparing the low-energymore » magnetic excitations in Na₂PrO₃ to those of the isostructural spin-only compound, Na₂TbO₃, a microscopic model of exchange interactions is developed that implicates dominant and surprisingly large Heisenberg exchange interactions J≈1.1(1) meV. Although antiferromagnetic Kitaev interactions with K≤0.2J cannot be excluded, the inelastic neutron-scattering data of Na₂PrO₃ is best explained with a Δ=1.22 easy-axis XXZ exchange anisotropy.« less

The transverse-field Ising model on the triangular lattice is expected to host an intermediate finite-temperature Kosterlitz-Thouless (KT) phase through a mapping of the spins on each triangular unit to a complex order parameter. TmMgGaO4 is a candidate material to realize such physics due to the non-Kramers nature of the Tm3+ ion and the resulting two-singlet single-ion ground state. Using inelastic neutron scattering, we confirm this picture by determining the leading parameters of the low-energy effective Hamiltonian of TmMgGaO4. Subsequently, we track the predicted KT phase and related transitions by inspecting the field and temperature dependence of the ac susceptibility. Wemore » further probe the spin correlations in both reciprocal space and real space via single-crystal neutron diffraction and magnetic total scattering techniques, respectively. Magnetic pair distribution function analysis provides evidence for the formation of vortex-antivortex pairs that characterize the proposed KT phase around 5 K. Although structural disorder influences the field-induced behavior of TmMgGaO4, the magnetism in zero field appears relatively free from these effects. These results position TmMgGaO4 as a strong candidate for a solid-state realization of KT physics in a dense spin system.« less