# Publications

## Antidamping torques from simultaneous resonances in ferromagnet-topological insulator-ferromagnet heterostructures

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS **473** (2019) 470-476

## Skyrmions in anisotropic magnetic fields: strain and defect driven dynamics

MRS Advances Cambridge University Press (CUP) (2019)

Magnetic skyrmions are particle-like, topologically protected magnetization entities that are promising candidates for information carriers in racetrack-memory schemes. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Recently, we demonstrated experimentally that chiral skyrmions in Cu2OSeO3 can be effectively manipulated by a magnetic field gradient, leading to a collective rotation of the skyrmion lattice with well-defined dynamics in a radial field gradient. Here, we employ a skyrmion particle model to numerically study the effects of resultant shear forces on the structure of the skyrmion lattice. We demonstrate that anisotropic peak broadening in experimentally observed diffraction patterns can be attributed to extended linear regions in the magnetic field profile. We show that topological (5-7) defects emerge to protect the six-fold symmetry of the lattice under the application of local shear forces, further enhancing the stability of proposed magnetic field driven devices.

## A.C. susceptibility as a probe of low-frequency magnetic dynamics.

Journal of physics. Condensed matter : an Institute of Physics journal **31** (2018) 013001-

The experimental technique of a.c. susceptibility can be used as a probe of magnetic dynamics in a wide variety of systems. Its use is restricted to the low-frequency regime and thus is sensitive to relatively slow processes. Rather than measuring the dynamics of single spins, a.c. susceptibility can be used to probe the dynamics of collective objects, such as domain walls in ferromagnets or vortex matter in superconductors. In some frustrated systems, such as spin glasses, the complex interactions lead to substantial spectral weight of fluctuations in the low-frequency regime, and thus a.c. susceptibility can play a unique role. We review the theory underlying the technique and magnetic dynamics more generally and give applications of a.c. susceptibility to a wide variety of experimental situations.

## Evolution of the low-temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition

Nature npj Quantum Materials Springer Nature **4** (2019) 2

## Measurement of the bulk and surface bands in Dirac line-node semimetal ZrSiS

CHINESE PHYSICS B **27** (2018) ARTN 017105

## Magnetic X-ray spectroscopy of two-dimensional CrI3 layers

MATERIALS LETTERS **232** (2018) 5-7

## Probing α-RuCl_{3} Beyond Magnetic Order: Effects of Temperature and Magnetic Field.

Physical review letters **120** (2018) 077203-

Recent studies have brought α-RuCl_{3} to the forefront of experimental searches for materials realizing Kitaev spin-liquid physics. This material exhibits strongly anisotropic exchange interactions afforded by the spin-orbit coupling of the 4d Ru centers. We investigate the dynamical response at finite temperature and magnetic field for a realistic model of the magnetic interactions in α-RuCl_{3}. These regimes are thought to host unconventional paramagnetic states that emerge from the suppression of magnetic order. Using exact diagonalization calculations of the quantum model complemented by semiclassical analysis, we find a very rich evolution of the spin dynamics as the applied field suppresses the zigzag order and stabilizes a quantum paramagnetic state that is adiabatically connected to the fully polarized state at high fields. At finite temperature, we observe large redistributions of spectral weight that can be attributed to the anisotropic frustration of the model. These results are compared to recent experiments and provide a road map for further studies of these regimes.

## Extreme Sensitivity of a Topochemical Reaction to Cation Substitution: SrVO2H versus SrV1- xTi xO1.5H1.5.

Inorganic chemistry **57** (2018) 2890-2898

The anion-ordered oxide-hydride SrVO2H is an antiferromagnetic insulator due to strong correlations between vanadium d electrons. In an attempt to hole-dope SrVO2H into a metallic state, a strategy of first preparing SrV1- xTi xO3 phases and then converting them to the corresponding SrV1- xTi xO2H phases via reaction with CaH2 was followed. This revealed that the solid solution between SrVO3 and SrTiO3 is only stable at high temperature. In addition, reactions between SrV0.95Ti0.05O3 and CaH2 were observed to yield SrV0.95Ti0.05O1.5H1.5 not SrV0.95Ti0.05O2H. This dramatic change in reactivity for a very modest change in initial chemical composition is attributed to an electronic destabilization of SrVO2H on titanium substitution. Density functional theory calculations indicate that the presence of an anion-ordered, tetragonal SrMO2H phase is uniquely associated with a d2 electron count and that titanium substitution leads to an electronic destabilization of SrV1- xTi xO2H phases, which, ultimately, drives further reaction of SrV1- xTi xO2H to SrV1- xTi xO1.5H1.5. The observed sensitivity of the reaction products to the chemical composition of initial phases highlights some of the difficulties associated with electronically doping metastable materials prepared by topochemical reactions.

## Magnetic and electronic structure of the layered rare-earth pnictide EuCd2Sb2

PHYSICAL REVIEW B **98** (2018) ARTN 064419

## Comparative study of the magnetic properties of La3Ni2B ' O-9 for B ' = Nb, Ta or Sb

JOURNAL OF SOLID STATE CHEMISTRY **258** (2018) 825-834

## THz carrier dynamics and magnetotransport study of topological surface states in thin film Bi<inf>2</inf>Se<inf>3</inf>

Proceedings of SPIE - The International Society for Optical Engineering **10531** (2018)

© 2018 SPIE. The surface of a topological insulator harbors exotic topological states, protected against backscattering from disorder by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomena in condensed matter physics, such as the spin Hall effect, but also lays the foundation for applications from quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly distinguish topological surface states from bulk states. Employing terahertz time-domain spectroscopy, we study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing for the deconvolution of the surface state response from the bulk. Our measurement of carrier dynamics give an optical mobility exceeding 2100 cm2/V•s at 4 K, indicative of a surface-dominated response, and a scattering lifetime of ∼0.18 ps and a carrier density of 6×1012cm-2at 4 K for the Bi2Se3film. The sample was further processed into a Hall bar device using two different etching techniques, a wet chemical etching and Ar+ion milling, which resulting in a reduced Hall mobility. Even so, the magneto-conductance transport reveals weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a single topological surface state mode.

## Spin dynamics of coupled spin ladders near quantum criticality in Ba2CuTeO6

Physical review B: Condensed matter and materials physics American Physical Society (2018)

We report inelastic neutron scattering measurements of the magnetic excitations in Ba2CuTeO6, proposed by ab initio calculations to magnetically realize weakly coupled antiferromagnetic two-leg spin-1/2 ladders. Isolated ladders are expected to have a singlet ground state protected by a spin gap. Ba2CuTeO6 orders magnetically, but with a small Neel temperature relative to the exchange strength, suggesting that the interladder couplings are relatively small and only just able to stabilize magnetic order, placing Ba2CuTeO6 close in parameter space to the critical point separating the gapped phase and Neel order. Through comparison of the observed spin dynamics with linear spin wave theory and quantum Monte Carlo calculations, we propose values for all relevant intra- and interladder exchange parameters, which place the system on the ordered side of the phase diagram in proximity to the critical point. We provide further cross-checks of the extracted parameters by comparison with high field magnetization data.

## Observation of topological surface states and strong electron/hole imbalance in extreme magnetoresistance compound LaBi

PHYSICAL REVIEW MATERIALS **2** (2018) ARTN 024201

## Significant change in the electronic behavior associated with structural distortions in monocrystalline SrAg4As2

PHYSICAL REVIEW B **98** (2018) ARTN 235130

## Persistence of antiferromagnetic order upon La substitution in the 4d(4) Mott insulator Ca2RuO4

PHYSICAL REVIEW B **98** (2018) ARTN 014429

## Quantum magnetism in molecular spin ladders probed with muonspin spectroscopy

NEW JOURNAL OF PHYSICS **20** (2018) ARTN 103002

## Multigap superconductivity in RbCa<inf>2</inf>Fe<inf>4</inf>As<inf>4</inf>F<inf>2</inf> investigated using μsR measurements

Journal of the Physical Society of Japan **87** (2018)

© 2018 The Physical Society of Japan. The superconducting properties of the recently discovered high-Tc superconductor RbCa2Fe4As4F2 with double Fe2As2 layers and Tc ~ 30 K have been investigated using magnetization, heat capacity, transverse-field (TF) and zero-field (ZF) muon-spin rotation/relaxation (μSR) measurements. Low field magnetization and heat capacity (Cp) measurements reveal an onset of bulk superconductivity with Tc ~ 30.0(4) K, where there is a jump at Tc of δCP/ Tc = 94.6 (mJ/mole-K2). The temperature dependence of the magnetic penetration depth obtained from transverse-field (TF) μSR is better described by the two-gap s + s and s + d models than the single gap s- or d-wave scenarios. The presence of two superconducting gaps in RbCa2Fe4As4F2 is consistent with the multigap superconductivity observed in other Fe-based superconductors, including ACa2Fe4As4F2 (A = K and Cs). From the TF-μSR study, an in-plane magnetic penetration depth of λab(0) = 231.5(3) nm, a superconducting carrier density ns = 1.29(1) X 1027m-3, and an effective mass of the carriers of m∗ = 2.45(l)me are estimated. Zero-field μSR measurements do not reveal clear signs of time reversal symmetry breaking below Tc, but the temperature dependence of the relaxation of the asymmetry between 150 and 1.2K may indicate the presence of spin-fluctuations.

## How to probe the spin contribution to momentum relaxation in topological insulators.

Nat Commun **9** (2018) 56-

Topological insulators exhibit a metallic surface state in which the directions of the carriers' momentum and spin are locked together. This characteristic property, which lies at the heart of proposed applications of topological insulators, protects carriers in the surface state from back-scattering unless the scattering centres are time-reversal symmetry breaking (i.e. magnetic). Here, we introduce a method of probing the effect of magnetic scattering by decorating the surface of topological insulators with molecules, whose magnetic degrees of freedom can be engineered independently of their electrostatic structure. We show that this approach allows us to separate the effects of magnetic and non-magnetic scattering in the perturbative limit. We thereby confirm that the low-temperature conductivity of SmB6 is dominated by a surface state and that the momentum of quasiparticles in this state is particularly sensitive to magnetic scatterers, as expected in a topological insulator.

## Folded superstructure and degeneracy-enhanced band gap in the weak-coupling charge density wave system 2H-TaSe2

PHYSICAL REVIEW B **97** (2018) ARTN 115118

## Comparative study of the magnetic properties of La<inf>3</inf>Ni<inf>2</inf>B′O<inf>9</inf>for B′ = Nb, Taor Sb

Journal of Solid State Chemistry **258** (2018) 825-834

© 2017 Elsevier Inc. Polycrystalline samples of La 3 Ni 2 NbO 9 and La 3 Ni 2 TaO 9 have been characterised by X-ray and neutron diffraction, electron microscopy, magnetometry and muon spin relaxation (µSR); the latter technique was also applied to La 3 Ni 2 SbO 9 . On the length scale of a neutron diffraction experiment, the six-coordinate sites of the monoclinic perovskite structure are occupied in a 1:1 ordered manner by Ni and a random ⅓Ni/⅔B′ mixture. Electron microscopy demonstrated that this 1:1 ordering is maintained over microscopic distances, although diffuse scattering indicative of short-range ordering on the mixed site was observed. No magnetic Bragg scattering was observed in neutron diffraction patterns collected from La 3 Ni 2 B′O 9 (B′ = Nb or Ta) at 5 K although in each case µSR identified the presence of static spins below 30 K. Magnetometry showed that La 3 Ni 2 NbO 9 behaves as a spin glass below 29 K but significant short-range interactions are present in La 3 Ni 2 TaO 9 below 85 K. The contrasting properties of these compounds are discussed in terms of their microstructure.