Publications


Chemical tuning between triangular and honeycomb structures in a 5d spin-orbit Mott insulator

PHYSICAL REVIEW B 100 (2019) ARTN 214113

RD Johnson, I Broeders, K Mehlawat, Y Li, Y Singh, R Valenti, R Coldea


Weyl-like points from band inversions of spin-polarised surface states in NbGeSb

Nature Communications Springer Science and Business Media LLC 10 (2019) 5485

I Marković, CA Hooley, OJ Clark, F Mazzola, MD Watson, JM Riley, K Volckaert, K Underwood, MS Dyer, PAE Murgatroyd, KJ Murphy, PL Fèvre, F Bertran, J Fujii, I Vobornik, S Wu, T Okuda, J Alaria, PDC King


Modular thermal Hall effect measurement setup for fast-turnaround screening of materials over wide temperature range using capacitive thermometry

Review of Scientific Instruments AIP Publishing 90 (2019) 103904-103904

H-L Kim, MJ Coak, JC Baglo, K Murphy, RW Hill, M Sutherland, MC Hatnean, G Balakrishnan, J-G Park


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

Physical review letters 120 (2018) 077203-

SM Winter, K Riedl, D Kaib, R Coldea, R Valentí

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.


Spin dynamics of coupled spin ladders near quantum criticality in Ba2CuTeO6

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

D Macdougal, AS Gibbs, T Ying, S Wessel, HC Walker, D Voneshen, F Mila, H Takagi, R Coldea

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.


Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore Yb_{2}Ti_{2}O_{7} in a Magnetic Field.

Physical review letters 119 (2017) 057203-

JD Thompson, PA McClarty, D Prabhakaran, I Cabrera, T Guidi, R Coldea

The frustrated pyrochlore magnet Yb_{2}Ti_{2}O_{7} has the remarkable property that it orders magnetically but has no propagating magnons over wide regions of the Brillouin zone. Here we use inelastic neutron scattering to follow how the spectrum evolves in cubic-axis magnetic fields. At high fields we observe, in addition to dispersive magnons, a two-magnon continuum, which grows in intensity upon reducing the field and overlaps with the one-magnon states at intermediate fields leading to strong renormalization of the dispersion relations, and magnon decays. Using heat capacity measurements we find that the low- and high-field regions are smoothly connected with no sharp phase transition, with the spin gap increasing monotonically in field. Through fits to an extensive data set of dispersion relations combined with magnetization measurements, we reevaluate the spin Hamiltonian, finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong fluctuations and quasiparticle breakdown effects observed at low fields.


Topological triplon modes and bound states in a Shastry-Sutherland magnet

NATURE PHYSICS 13 (2017) 736-+

PA McClarty, F Krueger, T Guidi, SF Parker, K Refson, AW Parker, D Prabhakaran, R Coldea


Magnetic Field Dependence of Excitations Near Spin-Orbital Quantum Criticality.

Physical review letters 118 (2017) 067205-

A Biffin, C Rüegg, J Embs, T Guidi, D Cheptiakov, A Loidl, V Tsurkan, R Coldea

The spinel FeSc_{2}S_{4} has been proposed to realize a near-critical spin-orbital singlet (SOS) state, where entangled spin and orbital moments fluctuate in a global singlet state on the verge of spin and orbital order. Here we report powder inelastic neutron scattering measurements that observe the full bandwidth of magnetic excitations and we find that spin-orbital triplon excitations of an SOS state can capture well key aspects of the spectrum in both zero and applied magnetic fields up to 8.5 T. The observed shift of low-energy spectral weight to higher energies upon increasing applied field is naturally explained by the entangled spin-orbital character of the magnetic states, a behavior that is in strong contrast to spin-only singlet ground state systems, where the spin gap decreases upon increasing applied field.


Single crystal growth from separated educts and its application to lithium transition-metal oxides.

Scientific reports 6 (2016) 35362-

F Freund, SC Williams, RD Johnson, R Coldea, P Gegenwart, A Jesche

Thorough mixing of the starting materials is the first step of a crystal growth procedure. This holds true for almost any standard technique, whereas the intentional separation of educts is considered to be restricted to a very limited number of cases. Here we show that single crystals of α-Li2IrO3 can be grown from separated educts in an open crucible in air. Elemental lithium and iridium are oxidized and transported over a distance of typically one centimeter. In contrast to classical vapor transport, the process is essentially isothermal and a temperature gradient of minor importance. Single crystals grow from an exposed condensation point placed in between the educts. The method has also been applied to the growth of Li2RuO3, Li2PtO3 and β-Li2IrO3. A successful use of this simple and low cost technique for various other materials is anticipated.


Incommensurate counterrotating magnetic order stabilized by Kitaev interactions in the layered honeycomb alpha-Li2IrO3

PHYSICAL REVIEW B 93 (2016) ARTN 195158

SC Williams, RD Johnson, F Freund, S Choi, A Jesche, I Kimchi, S Manni, A Bombardi, P Manuel, P Gegenwart, R Coldea


Spin dynamics of counterrotating Kitaev spirals via duality

PHYSICAL REVIEW B 94 (2016) ARTN 201110

I Kimchi, R Coldea


Monoclinic crystal structure of alpha-RuCl3 and the zigzag antiferromagnetic ground state

PHYSICAL REVIEW B 92 (2015) ARTN 235119

RD Johnson, SC Williams, AA Haghighirad, J Singleton, V Zapf, P Manuel, II Mazin, Y Li, HO Jeschke, R Valenti, R Coldea


Unified theory of spiral magnetism in the harmonic-honeycomb iridates alpha, beta, and gamma Li2IrO3

PHYSICAL REVIEW B 91 (2015) ARTN 245134

I Kimchi, R Coldea, A Vishwanath


Unconventional magnetic order on the hyperhoneycomb Kitaev lattice in beta-Li2IrO3: Full solution via magnetic resonant x-ray diffraction

PHYSICAL REVIEW B 90 (2014) ARTN 205116

A Biffin, RD Johnson, S Choi, F Freund, S Manni, A Bombardi, P Manuel, P Gegenwart, R Coldea


Cascade of field-induced magnetic transitions in a frustrated antiferromagnetic metal

PHYSICAL REVIEW B 90 (2014) ARTN 020401

AI Coldea, L Seabra, A McCollam, A Carrington, L Malone, AF Bangura, D Vignolles, PG van Rhee, RD McDonald, T Soergel, M Jansen, N Shannon, R Coldea


Realization of a three-dimensional spin-anisotropic harmonic honeycomb iridate.

Nature communications 5 (2014) 4203-

KA Modic, TE Smidt, I Kimchi, NP Breznay, A Biffin, S Choi, RD Johnson, R Coldea, P Watkins-Curry, GT McCandless, JY Chan, F Gandara, Z Islam, A Vishwanath, A Shekhter, RD McDonald, JG Analytis

Spin and orbital quantum numbers play a key role in the physics of Mott insulators, but in most systems they are connected only indirectly--via the Pauli exclusion principle and the Coulomb interaction. Iridium-based oxides (iridates) introduce strong spin-orbit coupling directly, such that these numbers become entwined together and the Mott physics attains a strong orbital character. In the layered honeycomb iridates this is thought to generate highly spin-anisotropic magnetic interactions, coupling the spin to a given spatial direction of exchange and leading to strongly frustrated magnetism. Here we report a new iridate structure that has the same local connectivity as the layered honeycomb and exhibits striking evidence for highly spin-anisotropic exchange. The basic structural units of this material suggest that a new family of three-dimensional structures could exist, the 'harmonic honeycomb' iridates, of which the present compound is the first example.


Quasiparticle breakdown in the quasi-one-dimensional Ising ferromagnet CoNb2O6

PHYSICAL REVIEW B 90 (2014) ARTN 174406

NJ Robinson, FHL Essler, I Cabrera, R Coldea


Noncoplanar and counterrotating incommensurate magnetic order stabilized by Kitaev interactions in γ-Li(2)IrO(3).

Physical review letters 113 (2014) 197201-

A Biffin, RD Johnson, I Kimchi, R Morris, A Bombardi, JG Analytis, A Vishwanath, R Coldea

Materials that realize Kitaev spin models with bond-dependent anisotropic interactions have long been searched for, as the resulting frustration effects are predicted to stabilize novel forms of magnetic order or quantum spin liquids. Here, we explore the magnetism of γ-Li(2)IrO(3), which has the topology of a three-dimensional Kitaev lattice of interconnected Ir honeycombs. Using magnetic resonant x-ray diffraction, we find a complex, yet highly symmetric incommensurate magnetic structure with noncoplanar and counterrotating Ir moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally accounts for all key features of the observed magnetic structure. Our results provide strong evidence that γ-Li(2)IrO(3) realizes a spin Hamiltonian with dominant Kitaev interactions.


Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNb2O6 in a transverse field: Geometric frustration and quantum renormalization effects

PHYSICAL REVIEW B 90 (2014) ARTN 014418

I Cabrera, JD Thompson, R Coldea, D Prabhakaran, RI Bewley, T Guidi, JA Rodriguez-Rivera, C Stock


Spin waves and revised crystal structure of honeycomb iridate Na2IrO3

Physical Review Letters 108 (2012) 127204

SK Choi, R Coldea, AN Kolmogorov, T Lancaster, II Mazin, SJ Blundell, PG Radaelli, Y Singh, P Gegenwart, KR Choi, S-W Cheong, PJ Baker, C Stock, J Taylor

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zig-zag magnetic order. The onset of long-range magnetic order below 15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90 deg Ir-O-Ir bonds required for dominant Kitaev exchange.

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