We present observations of highly frustrated quasi two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the Seff= 1/2 compound γ-BaCo2(PO4)2 (γ-BCPO). Specific heat shows a broad peak comprised of two weak kink features at TN1 ∼ 6 K and TN2 ∼ 3.5 K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below TN1 and TN2, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length ξc = 60 ± 2 Å (TN1) and in quasi-2D helical domains with ξ h = 350 ± 11 Å (TN2). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on γ-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ J1-J2-J3 model with ferromagnetic nearest-neighbor exchange J1 are favored, both near regions of high classical degeneracy. High energy coherent excitations (∼ 10 meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above TN . These data show that γ-BCPO is a rare highly frustrated, quasi-2D Seff= 1/2 honeycomb lattice material which resists long range magnetic order and spin freezing. arXiv:1712.06208v2 [cond-mat.str-el]
We report on the crystal growth of rare-earth pyrosilicates, R 2Si 2O 7 for R = Yb and Er using the optical floating zone method. The grown crystals comprise members from the family of pyrosilicates where the rare-earth atoms form a distorted honeycomb lattice. C-Yb 2Si 2O 7 is a quantum dimer magnet with field-induced long range magnetic order, while D-Er 2Si 2O 7 is an Ising-type antiferromagnet. Both growths resulted in multi-crystal boules, with cracks forming between the different crystal orientations. The Yb 2Si 2O 7 crystals form the C-type rare-earth pyrosilicate structure with space group C 2 / m and are colorless and transparent or milky white, whereas the Er-variant is D-type, P 2 1 / b , and has a pink hue originating from Er 3 +. The crystal structures of the grown single crystals were confirmed through a Rietveld analysis of the powder X-ray diffraction patterns from pulverized crystals. The specific heat of both C-Yb 2Si 2O 7 and D-Er 2Si 2O 7 measured down to 50 mK indicated a phase transition at T N ≈ 1.8 K for D-Er 2Si 2O 7 and a broad Schottky-type feature with a sharp anomaly at 250 mK in an applied magnetic field of 0.8T along the c-axis in the case of C-Yb 2Si 2O 7 .
The quantum dimer magnet (QDM) is the canonical example of quantum magnetism. The QDM state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced triplon Bose-Einstein condensate (BEC) phase. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb2Si2O7. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet ground state at zero field with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of Hc1 ∼ 0.4 T and Hc2 ∼ 1.4 T. Using inelastic neutron scattering in an applied magnetic field we observe a Goldstone mode (gapless to within δE = 0.037 meV) that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well-known cases of this phase, the high-field (µ0H ≥ 1.2T) part of the phase diagram in Yb2Si2O7 is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.Quantum dimer magnets (QDMs) represent the simplest cases of quantum magnetism, where entanglement is a required ingredient for even a qualitative understanding of the phase. In a QDM, entangled pairs of spins form S tot = 0 dimers and result in a non-magnetic ground state. The excited states of these entangled spins can be treated as bosons, called triplons, which can undergo Bose-Einstein condensation (BEC) as their density is tuned by an applied magnetic field. This BEC state is a magnetic field-induced long range ordered phase, which occupies a symmetric "dome" in the field vs. temperature phase diagram with two temperature-dependent critical fields, H c1 (T ) and H c2 (T ). The vast majority of the previously studied QDMs are based on 3d transition metal ions with "bare" (spin-only) S = 1/2 or S = 1 angular momentum, resulting in simple Heisenberg or XXZ spin interaction Hamiltonians, and high critical fields set by the relatively high energy scale of exchange interactions [1-6].Lanthanide-based magnetic materials with spin-orbit coupled pseudo-spin 1/2 (S eff = 1/2) angular momenta can also exhibit quantum phases, and these are often directly analogous to their traditional 3d transition metal ion counterparts. However, entirely new phases are possible due to the anisotropic exchange in these materials [7][8][9][10][11][12]. In the lanthanide series, Yb 3+ has been of particular interest as it can generically host interactions leading to quantum fluctuations irrespective of the Crystal Electric Field (CEF) ground state doublet composition [13]. Indeed, various quantum phases have been discovered in Yb-based systems [14][15][...
We have studied the diffusion mechanism of lithium ions in glassy oxide-based solid state electrolytes using elastic and quasielastic neutron scattering. Samples of xLi2SO4-(1-x)(Li2O-P2O5) were prepared using conventional melt techniques. Elastic and inelastic scattering measurements were performed using the triple-axis spectrometer (TRIAX) at Missouri University Research Reactor at University of Missouri and High Flux Backscattering Spectrometer (HFBS) at NIST Center for Neutron Research, respectively. These compounds have a base glass compound of P2O5 which is modified with Li2O. Addition of Li2SO4 leads to the modification of the structure and to an increase lithium ion (Li+) conduction. We find that an increase of Li2SO4 in the compounds leads to an increase in the Lorentzian width of the fit for the quasielastic data, which corresponds to an increase in Li+ diffusion until an over-saturation point is reached (< 60% Li2SO4). We find that the hopping mechanism is best described by the vacancy mediated Chudley-Elliot model. A fundamental understanding of the diffusion process for these glassy compounds can help lead to the development of a highly efficient solid electrolyte and improve the viability of clean energy technologies.
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