arXiv subjects

T. Halloran

Publications and source records attributed to T. Halloran.

6 recordsLinked to original sources

arXiv2023

Formation of a simple cubic antiferromagnet through charge ordering in a double Dirac material

The appearance of spontaneous charge order in chemical systems is often associated with the emergence of novel, and useful, properties. Here we show through single crystal diffraction that the Eu ions in the mixed valent metal EuPd$_3$S$_4$ undergo long-range charge ordering at $T_{\mathrm{CO}} = 340 \mathrm{~K}$ resulting in simple cubic lattices of Eu$^{2+}$ ($J = 7/2$) and Eu$^{3+}$ ($J = 0$) ions. As only one of the two sublattices has a non-magnetic ground state, the charge order results in the emergence of remarkably simple G-type antiferromagnetic order at $T_{\mathrm{N}} = 2.85(6) \mathrm{~K}$, observed in magnetization, specific heat, and neutron diffraction. Application of a $0.3 \mathrm{~T}$ field is sufficient to induce a spin flop transition to a magnetically polarized, but still charge ordered, state. Density functional theory calculations show that this charge order also modifies the electronic degeneracies present in the material: without charge order, EuPd$_3$S$_4$ is an example of a double Dirac material containing 8-fold degenerate electronic states, greater than the maximum degeneracy of six possible in molecular systems. The symmetry reduction from charge order transmutes 8-fold double Dirac states into 4-fold Dirac states, a degeneracy that can be preserved even in the presence of the magnetic order. Our results show not only how charge order can be used to produce interesting magnetic lattices, but also how it can be used to engineer controlled degeneracies in electronic states.

cond-mat.str-el↗

arXiv2023

Spin-orbital order and excitons in magnetoresistive HoBi

The magnetism of the rock-salt $fcc$ rare-earth monopnictide HoBi, a candidate topological material with extreme magnetoresistance, is investigated. From the Ho$^{3+}$ non-Kramers $J$=8 spin-orbital multiplet, the cubic crystal electric field yields six nearly degenerate low-energy levels. These constitute an anisotropic magnetic moment with a Jahn-Teller-like coupling to the lattice. In the cubic phase for $T>T_N~=~5.72(1)~K$, the paramagnetic neutron scattering is centered at $\mathbf{k}=(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ and was fit to dominant antiferromagnetic interactions between Ho spins separated by $\{100\}$ and ferromagnetic interactions between spins displaced by $\{\frac{1}{2}\frac{1}{2}0\}$. For $T<T_N$, a type-II AFM long-range order with $\mathbf{k}=(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ develops along with a tetragonal lattice distortion. While neutron diffraction from a multi-domain sample cannot unambiguously determine the spin orientation within a domain, the bulk magnetization, structural distortion, and our measurements of the magnetic excitations all show the easy axis coincides with the tetragonal axis. The weakly dispersive excitons for $T<T_N$ can be accounted for by a spin Hamiltonian that includes the crystal electric field and exchange interactions within the Random Phase Approximation.

cond-mat.str-el↗

arXiv2022

A magnetic continuum observed by terahertz spectroscopy in a quantum spin liquid candidate BaCo$_2$(AsO$_4$)$_2$

Quantum spin liquids (QSLs) are topologically ordered exotic states of matter that host fractionalized excitations. Kitaev proposed a particular route towards a QSL via strongly bond-dependent interactions on the hexagonal lattice. A number of candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions, which put these systems far from the QSL regime. Using time-domain terahertz spectroscopy (TDTS) we observed a broad magnetic continuum over a wide range of temperature and field in the honeycomb cobalt-based magnet, BaCo$_2$(AsO$_4$)$_2$, which has been proposed to be more ideal versions of a Kitaev QSL. Applying a small in-plane magnetic field of $\sim$ 0.5 T suppresses the magnetic order and at at even higher fields gives rise to a spin-polarized state. With 4T magnetic field oriented principally out-of-plane, a broad magnetic continuum was observed that could be consistent with a field induced QSL. Our results indicate BaCo$_2$(AsO$_4$)$_2$ to be a promising QSL candidate.

cond-mat.str-el↗

arXiv2020

Monopolar and dipolar relaxation in spin ice Ho$_2$Ti$_2$O$_7$

When degenerate states are separated by large energy barriers, the approach to thermal equilibrium can be slow enough that physical properties are defined by the thermalization process rather than the equilibrium. The exploration of thermalization pushes experimental boundaries and provides refreshing insights into atomic scale correlations and processes that impact steady state dynamics and prospects for realizing solid state quantum entanglement. We present a comprehensive study of magnetic relaxation in Ho$_2$Ti$_2$O$_7$ based on frequency-dependent susceptibility measurements and neutron diffraction studies of the real-time atomic-scale response to field quenches. Covering nearly ten decades in time scales, these experiments uncover two distinct relaxation processes that dominate in different temperature regimes. At low temperatures (0.6K<T<1K) magnetic relaxation is associated with monopole motion along the applied field direction through the spin-ice vacuum. The increase of the relaxation time upon cooling indicates reduced monopole conductivity driven by decreasing monopole concentration and mobility as in a semiconductor. At higher temperatures (1K<T<2K) magnetic relaxation is associated with the reorientation of monopolar bound states as the system approaches the single-spin tunneling regime. Spin fractionalization is thus directly exposed in the relaxation dynamics.

cond-mat.str-el↗

arXiv2020

Low Energy Magneto-optics of Tb$_{2}$Ti$_{2}$O$_{7}$ in [111] Magnetic Field

The pyrochlore magnet Tb$_{2}$Ti$_{2}$O$_{7}$ shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform time-domain THz spectroscopy on high quality Tb$_{2}$Ti$_{2}$O$_{7}$ crystal and study the low energy excitations as a function of [111] magnetic field with high energy resolution. The low energy crystal field excitations change their energies anomalously under magnetic field. Despite several sharp field dependent changes, we show that the material's spectrum can be described not by a phase transitions, but by field dependent hybridization between the low energy crystal field levels. We highlight the strong coupling between spin and lattice degrees of freedom in Tb$_{2}$Ti$_{2}$O$_{7}$ as evidenced by the magnetic field tunable crystal field environment. Calculations based on single ion physics with field induced symmetry reduction of the crystal field environment can reproduce our data.

cond-mat.str-el↗

arXiv2018

A Geometrically Frustrated Trimer-Based Mott Insulator

The crystal structure of Ba4NbRu3O12 is based on triangular planes of elongated Ru3O12 trimers oriented perpendicular to the plane. We report that it is semiconducting, that its Weiss temperature and effective magnetic moment are -155 K and 2.59 μB/f.u. respectively, and that magnetic susceptibility and specific heat data indicate that it exhibits magnetic ordering near 4 K. The presence of a high density of low energy states is evidenced by a substantial Sommerfeld-like T-linear term (gamma = 31(2) mJ/mole-K^2) in the specific heat. Electronic structure calculations reveal that the electronic states at the Fermi Energy reside on the Ru3O12 trimers and that the calculated density of electronic states is high and continuous around the Fermi Energy - in other words density functional theory calculates the material to be a metal. Our results imply that Ba4NbRu3O12 is a geometrically frustrated trimer-based Mott insulator.

cond-mat.str-el↗