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Abstract:

We present the results of muon-spin relaxation $\left(\mathrm{μ}\mathrm{SR}\right)$ measurements of the static and dynamic magnetism of $M_{1/3}{\mathrm{NbS}}_2$ $(M=\mathrm{Fe},Ìý\mathrm{V},Ìý\mathrm{Mn})$ , three intercalated transition-metal dichalcogenides. Transitions to long-range magnetic order are observed in all three materials, and local magnetic fields at muon sites are compared to dipole field calculations. Measurements on ${\mathrm{Fe}}_{1/3}{\mathrm{NbS}}_2$ capture the evolution of two coexisting magnetic phases. In ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_2$ , we observe a peak in the dynamic response at 9ÌýK, coincident with previous reports of a possible low-temperature phase transition. The observation of high-frequency muon precession in ${\mathrm{Mn}}_{1/3}{\mathrm{NbS}}_2$ suggests the existence of an additional muon site that implies a difference in the electronic energy landscape compared to the other materials in the series. Taken together, this demonstrates that the change in intercalant species drives significant variations in magnetism, highlighting the $M_{1/3}{\mathrm{NbS}}_2$ series as an ideal group of materials for investigating a wide range of magnetic phenomena.

Abstract:

Magnetic heat capacity measurements of a high-quality single crystal of the dipole-octupole pyrochlore Ce2Hf2O7 down to a temperature of T=0.02 K are reported. These show a two-peaked structure, with a Schottky-like peak at T1∼0.065 K, similar to what is observed in its sister Ce pyrochlores Ce2Zr2O7 and Ce2Sn2O7. However, a second sharper peak is observed at T2∼0.025 K, signifying the entrance to the ground state. The ground state appears to have gapped excitations, as even the most abrupt extrapolation to CP=0 at T=0 K fully accounts for the Rln(2) entropy associated with the pseudospin-1/2 doublet for Ce3+ in this environment. The ground state could be conventionally ordered, although theory predicts a much larger anomaly in CP at much higher temperatures than the measured T2 for expectations from an all-in, all-out ground state of the XYZ Hamiltonian for Ce2Hf2O7. The sharp low-temperature peak could also signify a crossover from a classical spin liquid to a quantum spin liquid (QSL). For both scenarios, comparison of the measured CP with NLC calculations suggests that weak interactions beyond the nearest-neighbor XYZ Hamiltonian become relevant below T∼0.25 K. The diffuse magnetic neutron scattering observed from Ce2Hf2O7 at low temperatures between T2 and T1 resembles that observed from Ce2Zr2O7, which is well established as a π-flux quantum spin ice (QSI). Together with the peak in the heat capacity at T2, this diffuse scattering from Ce2Hf2O7 is suggestive of a classical spin liquid regime above T2 that is distinct from the zero-entropy quantum ground state below T2.

Abstract:

We present single-crystal neutron diffraction, powder muon spin rotation, and pulsed-field magnetometry measurements on the Heisenberg quantum chiral chain $\left[\mathrm{Cu}\left(\mathrm{pym}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]{\mathrm{SiF}}_6·{\mathrm{H}}_2\mathrm{O}$ (pym = pyrimidine), which displays a fourfold-periodic rotation of the local environment around the Cu() $S=1/2$ ions from site to site along the chain. Previous measurements on this material have shown the absence of magnetic order down to surprisingly low temperatures $â‰\yen 20$ ÌýmK, as well as the presence of an energy gap for magnetic excitations that grows linearly with magnetic field. Here we find evidence at dilution refrigerator temperatures for a field-induced transition to long-range magnetic order above an applied magnetic field of 3ÌýT. From the polarization of magnetic moments observed with magnetic fields applied in the $[−1,2,0]$ direction, we can identify the static magnetic structure that best accounts for the data. The proposed model is 91̽»¨ed microscopically by the presence of an alternating component of the $g$ tensor, which produces an internal twofold staggered field that dictates both the direction of the ordered moments and the effective coupling between adjacent chains. The observed magnetic structure is contrary to previous proposals for the departure of the magnitude and field dependence of the energy gap from the predictions of the sine-Gordon model.