Publications


Glide symmetry breaking and Ising criticality in the quasi-1D magnet CoNb2O6

Proceedings of the National Academy of Sciences National Academy of Sciences 117 (2020) 25219-25224

M Fava, R Coldea, S Ashok Parameswaran

We construct a microscopic spin-exchange Hamiltonian for the quasi–one-dimensional (1D) Ising magnet CoNb2O6 that captures detailed and hitherto-unexplained aspects of its dynamic spin structure factor. We perform a symmetry analysis that recalls that an individual Ising chain in this material is buckled, with two sites in each unit cell related by a glide symmetry. Combining this with numerical simulations benchmarked against neutron scattering experiments, we argue that the single-chain Hamiltonian contains a staggered spin-exchange term. We further argue that the transverse-field–tuned quantum critical point in CoNb2O6 corresponds to breaking this glide symmetry, rather than an on-site Ising symmetry as previously believed. This gives a unified microscopic explanation of the dispersion of confined states in the ordered phase and quasiparticle breakdown in the polarized phase at high transverse field.


Dynamic spin fluctuations in the frustrated A-site spinel CuAl2O4

PHYSICAL REVIEW B 102 (2020) ARTN 014439

H Cho, R Nirmala, J Jeong, PJ Baker, H Takeda, N Mera, SJ Blundell, M Takigawa, DT Adroja, J-G Park


High-resolution resonant inelastic x-ray scattering study of the electron-phonon coupling in honeycomb alpha-Li2IrO3

PHYSICAL REVIEW B 100 (2019) ARTN 224303

J Vale, C Dashwood, E Paris, L Veiga, M Garcia-Fernandez, A Nag, A Walters, K-J Zhou, I-M Pietsch, A Jesche, P Gegenwart, R Coldea, T Schmitt, D McMorrow

© 2019 American Physical Society. The excitations in honeycomb α-Li2IrO3 have been investigated with high-resolution resonant inelastic X-ray scattering (RIXS) at the O K edge. The low-energy response is dominated by a fully resolved ladder of excitations, which we interpret as being due to multiphonon processes in the presence of strong electron-phonon coupling (EPC). At higher energies, the orbital excitations are shown to be dressed by phonons. The high quality of the data permits a quantitative test of the analytical model for the RIXS cross section, which has been proposed to describe EPC in transition-metal oxides (TMOs). We find that the magnitude of the EPC is comparable to that found for a range of 3d TMOs. This indicates that EPC may be of equal importance in determining the phenomenology displayed by corresponding 5d-based systems.


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

Physical Review B American Physical Society 100 (2019) 214113

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

We report structural studies of the spin-orbit Mott insulator family K x Ir y O 2 , with triangular layers of edge-sharing IrO 6 octahedra bonded by potassium ions. The potassium content acts as a chemical tuning parameter to control the amount of charge in the Ir-O layers. Unlike the isostructural families with Ir replaced by Co or Rh ( y = 1 ), which are metallic over a range of potassium compositions x , we instead find insulating behavior with charge neutrality achieved via iridium vacancies, which order in a honeycomb supercell above a critical composition x c . By performing density functional theory calculations we attribute the observed behavior to a subtle interplay of crystal-field environment, local electronic correlations, and strong spin-orbit interaction at the Ir 4 + sites, making this structural family a candidate to display Kitaev magnetism in the experimentally unexplored regime that interpolates between triangular and honeycomb structures.


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 − RuCl3 beyond magnetic order: effects of temperature and magnetic field

Physical Review Letters American Physical Society 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 American Physical Society 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 Springer Nature 13 (2017) 736-741

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

The twin discoveries of the quantum Hall effect1, in the 1980s, and of topological band insulators2, in the 2000s, were landmarks in physics that enriched our view of the electronic properties of solids. In a nutshell, these discoveries have tau ght us that quantum mechanical wavefunctions in crystalline solids may carry nontrivial topological invariants which have ramifications for the observable physics. One of the side effects of the recent topological insulator revolution has been that such physics is much more widespread than was appreciated ten years ago. For example, while topological insulators were originally studied in the context of electron wavefunctions, recent work has initiated a hunt for topological insulators in bosonic systems: in photonic crystals3-6, in the vibrational modes of crystals7, and in the excitations of ordered magnets8. Using inelastic neutron scattering along with theoretical calculations, we demonstrate that, in a weak magnetic field, the dimerized quantum magnet SrCu 2 (BO 3 ) 2 is a bosonic topological insulator with topologically protected chiral edge modes of triplon excitations.


Magnetic field dependence of excitations near spin-orbital quantum criticality

Physical Review Letters American Physical Society 118 (2017) 067205-067205

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

<p>The spinel <b>FeSc<sub>2</sub>S<sub>4</sub></b> 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.</p>


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

Scientific Reports Nature Publishing Group 6 (2016) 35362

F Freund, SC Williams, R 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 $α$-Li$_2$IrO$_3$

Physical Review B American Physical Society 93 (2016) 195158

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

The layered honeycomb magnet α-Li2IrO3 has been theoretically proposed as a candidate to display unconventional magnetic behaviour associated with Kitaev interactions between spin-orbit entangled jeff = 1=2 magnetic moments on a honeycomb lattice. Here we report single crystal magnetic resonant x-ray diffraction combined with powder magnetic neutron diffraction to reveal an incommensurate magnetic order in the honeycomb layers with Ir magnetic moments counterrotating on nearest-neighbor sites. This unexpected type of magnetic structure for a honeycomb magnet cannot be explained by a spin Hamiltonian with dominant isotropic (Heisenberg) couplings. The magnetic structure shares many key features with the magnetic order in the structural polytypes β-and γ-Li2IrO3, understood theoretically to be stabilized by dominant Kitaev interactions between Ir moments located on the vertices of three-dimensional hyperhoneycomb and stripyhoneycomb lattices, respectively. Based on this analogy and a theoretical soft-spin analysis of magnetic ground states for candidate spin Hamiltonians, we propose that Kitaev interactions also dominate in α-Li2IrO3, indicative of universal Kitaev physics across all three members of the harmonic honeycomb family of Li2IrO3 polytypes.


Spin dynamics of counterrotating Kitaev spirals via duality

Physical Review B American Physical Society 94 (2016) 201110(R)

I Kimchi, R Coldea

Incommensurate spiral order is a common occurrence in frustrated magnetic insulators. Typically, all magnetic moments rotate uniformly, through the same wavevector. However the honeycomb iridates family Li2IrO3 shows an incommensurate order where spirals on neighboring sublattices are counter-rotating, giving each moment a different local environment. Theoretically describing its spin dynamics has remained a challenge: the Kitaev interactions proposed to stabilize this state, which arise from strong spin-orbit effects, induce magnon umklapp scattering processes in spin-wave theory. Here we propose an approach via a (Klein) duality transformation into a conventional spiral of a frustrated Heisenberg model, allowing a direct derivation of the dynamical structure factor. We analyze both Kitaev and Dzyaloshinskii-Moriya based models, both of which can stabilize counterrotating spirals, but with different spin dynamics, and we propose experimental tests to identify the origin of counterrotation.


Monoclinic crystal structure of α−RuCl3 and the zigzag antiferromagnetic ground state

Physical Review B American Physical Society 92 (2015) 235119-

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

The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.


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


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

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