Evolution of magnetic surfboards and spin glass behavior in (Fe1-pGap)2TiO5

The unusual anisotropy of the spin glass (SG) transition in the pseudobrookite system Fe2TiO5has been interpreted as arising from an induced, van der Waals-like, interaction among magnetic clusters. Here we present susceptibility (χ) and specific heat data (C) for Fe2TiO5diluted with non-magnetic Ga, (Fe1-pGap)2TiO5, for disorder parameterp= 0, 0.11, and 0.42, and elastic neutron scattering data forp= 0.20. A uniform suppression ofTgis observed upon increasingp, along with a value ofχTgthat increases asTgdecreases, i.e.dχ(Tg)/dTg<0We also observeCT∝T2in the low temperature limit. The observed behavior places (Fe1-pGap)2TiO5in the category of a strongly geometrically frustrated SG.

Magnetoquantum oscillations in the specific heat of a topological Kondo insulator

Surprisingly, magnetoquantum oscillations (MQOs) characteristic of a metal with a Fermi surface have been observed in measurements of the topological Kondo insulator SmB₆. As these MQO have only been observed in measurements of magnetic torque (dHvA) and not in measurements of magnetoresistance (SdH), a debate has arisen as to whether the MQO are an extrinsic effect arising from rare-earth impurities, defects, and/or aluminum inclusions or an intrinsic effect revealing the existence of charge-neutral excitations. We report here the first observation of MQO in the low-temperature specific heat of SmB₆. The observed frequencies and their angular dependence for these flux-grown samples are consistent with previous results based on magnetic torque for SmB₆but the inferred effective masses are significantly larger than previously reported. Such oscillations can only be observed if the MQO are of bulk thermodynamic origin; the measured magnetic-field dependent oscillation amplitude and effective mass allow us to rule out suggestions of an extrinsic, aluminum inclusion-based origin for the MQO.

Extremely Weakly Interacting ΔS_{z}=0 and ΔS_{z}=1 Excitations and Evidence for Fractional Quantization in a Magnetization Plateau: CeSb

The plateau at 1/3 of the saturation magnetization M_{s} in the metamagnet CeSb is accompanied by a state of ferromagnetic layers of spins in an up-up-down sequence. We measured M and the specific heat C in the plateau, spin wave analyses of which reveal two distinct branches of excitations. Those with ΔS_{z}=1 as measured by M, coexist with a much larger population of ΔS_{z}=0 excitations measured by C but invisible to M. The large density of ΔS_{z}=0 excitations, their energy gap, and their seeming lack of interaction with ΔS_{z}=1 excitations suggest an analogy with astrophysical dark matter. Additionally, in the middle of the plateau three sharp jumps in M(H) are seen, the size of which, 0.15%M_{s}, is consistent with fractional quantization of magnetization per site in the down-spin layers.

Evidence for undoped Weyl semimetal charge transport in Y2Ir2O7

Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature [Formula: see text] in a single particle model. Here we show that, in closed-environment-grown polycrystalline samples of Y₂Ir₂O₇, [Formula: see text] over four orders of magnitude in [Formula: see text]. While the measured prefactor, [Formula: see text], is obtained from the model using reasonable materials parameters, the [Formula: see text] behavior extends far beyond the model's range of applicability. In particular, the behavior extends into the low-temperature, high-resistivity region where the Ioffe-Regel parameter, [Formula: see text]. Strong on-site Coulomb correlations, instrumental for predicting a Weyl semimetal state in Y₂Ir₂O₇, are the possible origin of such 'bad' Weyl semimetal behavior.

Local Moment Instability of Os in Honeycomb Li2.15Os0.85O3

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li_2.15(3)Os_0.85(3)O₃ (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μ_B, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T^(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)−Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.