Paul Goddard

Abstract:

We investigate the properties of CuVOF 4 ( H 2 O ) 6 · H 2 O , in which two different spin species, Cu(II) and V(IV), form antiferromagnetic spin-1/2 dimers with weak interdimer coupling provided via hydrogen bonding. Using radio-frequency susceptometry and electron-spin resonance (ESR), we show how the temperature-magnetic field spin-dimer phase diagram evolves as a function of applied hydrostatic pressure and correlate this with pressure-induced changes to the crystal structure. These results, coupled with pressure-tuned DFT calculations, confirm the prior prediction that the primary exchange interaction is mediated via an unusual mechanism in which the V(IV) ions provide considerable spin density to the oxygen that joins the two spins in each dimer and which lies along the Jahn-Teller axis of the Cu(II) ion. In addition, the dissimilarity in the spins that make up each dimer unit leads to a nonlinear field dependence of the electronic energy levels as detected in the ESR measurements.

Abstract:

We report a systematic study of the structural and magnetic evolution in CeAlSi_1-xGe_x, a series of materials which provide a tunable platform for exploring magnetism in noncentrosymmetric Ce-based intermetallics. Polycrystalline samples and single crystals were synthesized using arc melting and flux growth techniques. Structural characterization by X-ray diffraction shows a continuous increase in the unit-cell parameters with increasing Ge content, with no evidence of a structural phase transition across the series. Magnetization measurements reveal a suppression of the ferromagnetic ordering temperature of CeAlSi with increasing Ge substitution, indicating a crossover toward antiferromagnetic behaviour in Ge-rich compositions. Neutron diffraction measurements performed on selected compositions show that weak magnetic intensity appears on some structural Bragg peaks below the magnetic ordering temperature. These results elucidate the relationship between chemical substitution, crystal structure, and magnetic ground states in CeAlSi_1-xGe_x, and establish this system as a model platform for studying compositionally tuned magnetic order in noncentrosymmetric materials.

Abstract:

<jats:p>Iridium-containing conducting materials are widely investigated for their strong spin-orbit coupling and potential topological properties. Recently the commonly used electrode material iridium dioxide was found to host a large spin-Hall conductivity and was shown to support Dirac nodal lines. Here we present quantum-oscillation experiments on high-quality <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>IrO</a:mi><a:mn>2</a:mn></a:msub></a:math> single crystals using the de Haas-van Alphen effect measured using torque magnetometry with a piezoresistive microcantilever as well as density functional theory-based band-structure calculations. The angle, temperature, and field dependencies of the oscillations and the calculated band dispersion provide valuable information on the properties of the charge carriers, including the Fermi-surface geometry and electronic correlations. Comparison of experimental results to calculations allows us to assigns the observed de Haas-van Alphen frequencies to the calculated Fermi surface topology. We find that the effective masses of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>IrO</b:mi><b:mn>2</b:mn></b:msub></b:math> are enhanced compared to the rest electron mass <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>m</c:mi><c:mi>e</c:mi></c:msub></c:math>, ranging from 1.9 to 3.0 <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>m</d:mi><d:mi>e</d:mi></d:msub></d:math>, whereas the scattering times indicate excellent sample quality. We discuss our results in context with recent ARPES and band-structure calculation results that found Dirac nodal lines in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>IrO</e:mi><e:mn>2</e:mn></e:msub></e:math> and compare the effective masses and other electronic properties to those of similar materials like the nodal chain metal <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>ReO</f:mi><f:mn>2</f:mn></f:msub></f:math> in which Dirac electrons with very light effective masses have been observed.</jats:p>

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 $\ge 20$  mK, 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 3 T. 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 supported 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.

Abstract:

${\text{YbZn}}_2{\mathrm{GaO}}_5$ is a promising candidate for realizing a quantum spin liquid (QSL) state, particularly owing to its lack of significant site disorder. Pulsed-field magnetometry at 0.5 K shows magnetization saturating near 15 T, with a corrected saturation moment of $2.1(1){\mu }_{\mathrm{B}}$ after subtracting the van Vleck contribution. Our zero-field $\mathrm{\mu }\mathrm{SR}$ measurements down to milliKelvin temperatures provide evidence for a dynamic ground state and the absence of magnetic order. To probe fluctuations in the local magnetic field at the muon site, we performed longitudinal field $\mathrm{\mu }\mathrm{SR}$ experiments. These results provide evidence for spin dynamics with a field dependence that is consistent with a U1A01 Dirac quantum spin liquid as a plausible description of the ground state.