Publications


Persistent coherence of quantum superpositions in an optimally doped cuprate revealed by 2D spectroscopy

Science Advances American Association for the Advancement of Science (AAAS) 6 (2020) eaaw9932-eaaw9932

D Prabhakaran, JA Davis, JO Tollerud, F Novelli

<jats:p>Quantum materials displaying intriguing magnetic and electronic properties could be key to the development of future technologies. However, it is poorly understood how the macroscopic behavior emerges in complex materials with strong electronic correlations. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. Here, we apply multidimensional coherent spectroscopy to a prototypical cuprate and report unprecedented coherent dynamics persisting for ~500 fs, originating directly from the quantum superposition of optically excited states separated by 20 to 60 meV. These results reveal that the states in this energy range are correlated with the optically excited states at ~1.5 eV and point to nontrivial interactions between quantum many-body states on the different energy scales. In revealing these dynamics and correlations, we demonstrate that multidimensional coherent spectroscopy can interrogate complex quantum materials in unprecedented ways.</jats:p>


Evidence for a J(eff)=0 ground state and defect-induced spin glass behavior in the pyrochlore osmate Y2Os2O7

PHYSICAL REVIEW B 99 (2019) ARTN 174442

NR Davies, CV Topping, H Jacobsen, AJ Princep, FKK Kirschner, MC Rahn, M Bristow, JG Vale, I da Silva, PJ Baker, CJ Sahle, Y-F Guo, D-Y Yan, Y-G Shi, SJ Blundell, DF McMorrow, AT Boothroyd


Magnetic structure and excitations of the topological semimetal YbMnBi2

PHYSICAL REVIEW B 100 (2019) ARTN 144431

J-R Soh, H Jacobsen, B Ouladdiaf, A Ivanov, A Piovano, T Tejsner, Z Feng, H Wang, H Su, Y Guo, Y Shi, AT Boothroyd


Photocatalytic water splitting by N-TiO2 on MgO(111) with exceptional quantum efficiencies at elevated temperature

Nature Communications Springer Nature 10 (2019) 4421

Y Li, Y-K Peng, L Hu, J Zheng, R Taylor, D Prabhakaran, S Wu, TJ Puchtler, M Li, K-Y Wong, SCE Tsang

Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In the past decades, various systems have been developed but most of them suffer from low activities, a narrow range of absorption and poor quantum efficiencies (Q.E.) due to fast recombination of charge carriers. Here we report a dramatic suppression of electron-hole pair recombination on the surface of N-doped TiO2 based nanocatalysts under enhanced concentrations of H+ and OH−, and local electric field polarization of a MgO (111) support during photolysis of water at elevated temperatures. Thus, a broad optical absorption is seen, producing O2 and H2 in a 1:2 molar ratio with a H2 evolution rate of over 11,000 μmol g−1 h−1 without any sacrificial reagents at 270 °C. An exceptional range of Q.E. from 81.8% at 437 nm to 3.2% at 1000 nm is also reported.


Wave Vector Difference of Magnetic Bragg Reflections and Low Energy Magnetic Excitations in Charge-stripe Ordered La2NiO4.11.

Scientific reports 9 (2019) 14468-

PG Freeman, SR Giblin, M Skoulatos, RA Mole, D Prabhakaran

We report on the magnetism of charge-stripe ordered La2NiO4.11±0.01 by neutron scattering and μSR. On going towards zero energy transfer there is an observed wave vector offset in the centring of the magnetic excitations and magnetic Bragg reflections, meaning the excitations cannot be described as Goldstone modes of the magnetic order. Weak transverse field μSR measurements determine the magnetically order volume fraction is 87% from the two stripe twins, and the temperature evolution of the magnetic excitations is consistent with the low energy excitations coming from the magnetically ordered volume of the material. We will discuss how these results contrast with the proposed origin of a similar wave vector offset recently observed in a La-based cuprate, and possible origins of this effect in La2NiO4.11.


First-order valence transition: Neutron diffraction, inelastic neutron scattering, and x-ray absorption investigations on the double perovskite Ba2PrRu0.9Ir0.1O6

Physical Review B American Physical Society 99 (2019) 184440

J Sannigrahi, DT Adroja, C Ritter, W Kockelmann, AD Hillier, KS Knight, A Boothroyd, M Wakeshima, Y Hinatsu, JFW Mosselmans, S Ramos

Bulk studies have revealed a first-order valence phase transition in Ba2PrRu1−xIrxO6 (0.10 ≤ x ≤ 0.25), which is absent in the parent compounds with x = 0 (Pr3+) and x = 1 (Pr4+), which exhibit antiferromagnetic order with transition temperatures TN = 120 and 72 K, respectively. In the present study, we have used magnetization, heat capacity, neutron diffraction, inelastic neutron scattering and x-ray absorption measurements to investigate the nature of the Pr ion in x = 0.1. The magnetic susceptibility and heat capacity of x = 0.1 show a clear sign of the first order valence phase transition below 175 K, where the Pr valence changes from 3+ to 4+. Neutron diffraction analysis reveals that x = 0.1 crystallizes in a monoclinic structure with space group P21/n at 300 K, but below 175 K two phases coexist, the monoclinic having the Pr ion in a 3+ valence state and a cubic one (Fm3m) having the Pr ion in a 4+ valence state. Clear evidence of an antiferromagnetic ordering of the Pr and Ru moments is found in the monoclinic phase of the x = 0.1 compound below 110 K in the neutron diffraction measurements. Meanwhile the cubic phase remains paramagnetic down to 2 K, a temperature below which heat capacity and susceptibility measurements reveal a ferromagnetic ordering. High energy inelastic neutron scattering data reveal well-defined highenergy magnetic excitations near 264 meV at temperatures below the valence transition. Low energy INS data show a broad magnetic excitation centred at 50 meV above the valence transition, but four well-defined magnetic excitations at 7 K. The high energy excitations are assigned to the Pr4+ ions in the cubic phase and the low energy excitations to the Pr3+ ions in the monoclinic phase. Further direct evidence of the Pr valence transition has been obtained from the x-ray absorption spectroscopy. The results on the x = 0.1 compound are compared with those for x = 0 and 1.


Nuclear spin assisted quantum tunnelling of magnetic monopoles in spin ice.

Nature communications 10 (2019) 1509-

C Paulsen, SR Giblin, E Lhotel, D Prabhakaran, K Matsuhira, G Balakrishnan, ST Bramwell

Extensive work on single molecule magnets has identified a fundamental mode of relaxation arising from the nuclear-spin assisted quantum tunnelling of nearly independent and quasi-classical magnetic dipoles. Here we show that nuclear-spin assisted quantum tunnelling can also control the dynamics of purely emergent excitations: magnetic monopoles in spin ice. Our low temperature experiments were conducted on canonical spin ice materials with a broad range of nuclear spin values. By measuring the magnetic relaxation, or monopole current, we demonstrate strong evidence that dynamical coupling with the hyperfine fields bring the electronic spins associated with magnetic monopoles to resonance, allowing the monopoles to hop and transport magnetic charge. Our result shows how the coupling of electronic spins with nuclear spins may be used to control the monopole current. It broadens the relevance of the assisted quantum tunnelling mechanism from single molecular spins to emergent excitations in a strongly correlated system.


Role of defects in determining the magnetic ground state of ytterbium titanate.

Nature communications 10 (2019) 637-

DF Bowman, E Cemal, T Lehner, AR Wildes, L Mangin-Thro, GJ Nilsen, MJ Gutmann, DJ Voneshen, D Prabhakaran, AT Boothroyd, DG Porter, C Castelnovo, K Refson, JP Goff

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.


FeTi$_2$O$_5$: a spin Jahn-Teller transition tuned by cation substitution

Physical Review B American Physical Society 100 (2019) 094401

F Lang, L Jowitt, D Prabhakaran, RD Johnson, SJ Blundell

We have used muon-spin rotation, heat capacity and x-ray diffraction measurements in combination with density functional theory and dipole field calculations to investigate the crystal and magnetic structure of FeTi$_2$O$_5$. We observe a long range ordered state below 41.8(5) K with indications of significant correlations existing above this temperature. We determine candidate muon stopping sites in this compound, and find that our data are consistent with the spin Jahn-Teller driven antiferromagnetic ground state with <strong>k</strong>=(1/2,1/2,0) reported for CoTi$_2$O$_5$. By comparing our data with calculated dipolar fields we can restrict the possible moment size and directions of the Fe$^{2+}$ ions.


Spin-charge-lattice coupling in quasi-one-dimensional Ising spin chain CoNb2O6

Journal of Physics: Condensed Matter IOP Publishing 31 (2019) 195802

M Nandi, D Prabhakaran, P Mandal

Magnetization, magnetostriction and dielectric constant measurements are performed on single crystals of quasi-one-dimensional Ising spin chain CoNb$_{2}$O$_{6}$ at temperatures below and above the antiferromagnetic phase transition. Field-induced magnetic transitions are clearly reflected in magnetodielectric and magnetostriction data. Sharp anomalies are observed around the critical fields of antiferromagnetic to ferrimagnetic and ferrimagnetic to saturated-paramagnetic transition in both magnetodielectric and magnetostriction experiments. Detailed analysis of temperature and field dependence of dielectric constant and magnetostriction suggests that spins are coupled with lattice as well as charges in CoNb$_{2}$O$_{6}$. Below the antiferromagnetic transition temperature, the overall resemblance in anomalies, observed in various physical parameters such as magnetization, dielectric constant, magnetostriction and magnetic entropy change gives a deeper insight about the influence of spin configuration on these parameters in CoNb$_{2}$O$_{6}$.


Topological Lifshitz transitions and Fermi arc manipulation in Weyl semimetal NbAs.

Nature communications 10 (2019) 3478-

HF Yang, LX Yang, ZK Liu, Y Sun, C Chen, H Peng, M Schmidt, D Prabhakaran, BA Bernevig, C Felser, BH Yan, YL Chen

Surface Fermi arcs (SFAs), the unique open Fermi-surfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFA-mediated quantum oscillations, chiral magnetic effects, three-dimensional quantum Hall effect, non-local voltage generation and anomalous electromagnetic wave transmission. Here, by using in-situ surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology.


Selective probing of magnetic order on Tb and Ir sites in stuffed Tb2Ir2O7  using resonant x-ray scattering.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 344001-344001

C Donnerer, MC Rahn, E Schierle, RS Perry, LSI Veiga, G Nisbet, SP Collins, D Prabhakaran, AT Boothroyd, DF McMorrow

We study the magnetic structure of the 'stuffed' (Tb-rich) pyrochlore iridate Tb2+x Ir2-x O7-y  (x ∼ 0.18), using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir L 3 and Tb M 5 edges, which provide selective sensitivity to Ir 5d and Tb 4f  magnetic moments, respectively. At the Ir L 3 edge, we found the onset of long-range [Formula: see text] magnetic order below [Formula: see text] K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of [Formula: see text] at 5 K. At the Tb M 5 edge, long-range [Formula: see text] magnetic order appeared below  ∼[Formula: see text] K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the M 5 edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb2Ir2O7, for which [Formula: see text] K and [Formula: see text], indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.


Spin-orbit excitons in CoO

PHYSICAL REVIEW B 100 (2019) ARTN 075143

PM Sarte, M Songvilay, E Pachoud, RA Ewings, CD Frost, D Prabhakaran, KH Hong, AJ Browne, Z Yamani, JP Attfield, EE Rodriguez, SD Wilson, C Stock


Magnetoelectric domains and their switching mechanism in a Y-type hexaferrite

Physical Review B American Physical Society 100 (2019) 104411

FP Chmiel, D Prabahakaran, P Steadman, J Chen, R Fan, P Radaelli, RD Johnson

By employing resonant X-ray microdiffraction, we image the magnetisation and magnetic polarity domains of the Y-type hexaferrite Ba$_{0.5}$Sr$_{1.5}$Mg$_2$Fe$_{12}$O$_{22}$. We show that the magnetic polarity domain structure can be controlled by both magnetic and electric fields, and that full inversion of these domains can be achieved simply by reversal of an applied magnetic field in the absence of an electric field bias. Furthermore, we demonstrate that the diffraction intensity measured in different X-ray polarisation channels cannot be reproduced by the accepted model for the polar magnetic structure, known as the 2-fan transverse conical (TC) model. We propose a modification to this model, which achieves good quantitative agreement with all of our data. We show that the deviations from the TC model are large, and may be the result of an internal magnetic chirality, most likely inherited from the parent helical (non-polar) phase.


Magnetic structure and excitations of the topological semimetal YbMnBi2

Physical Review B American Physical Society 100 (2019) 144431

J-R Soh, H Jacobsen, B Ouladdiaf, A Ivanov, A Piovano, T Tejsner, Z Feng, H Wang, H Su, Y Guo, Y Shi, A Boothroyd

We investigated the magnetic structure and dynamics of YbMnBi2, with elastic and inelastic neutron scattering, to shed light on the topological nature of the charge carriers in the antiferromagnetic phase. We confirm C-type antiferromagnetic ordering of the Mn spins below TN = 290 K and determine that the spins point along the c axis to within about 3◦. The observed magnon spectrum can be described very well by the same effective spin Hamiltonian that was used previously to model the magnon spectrum of CaMnBi2. Our results show conclusively that the creation of Weyl nodes in YbMnBi2 by the time-reversal symmetry-breaking mechanism can be excluded in the bulk.


Magnetic and electronic structure of Dirac semimetal candidate EuMnSb2

Physical Review B American Physical Society 100 (2019) 174406

J-R Soh, P Manuel, NMB Schroeter, CJ Yi, F Orlandi, YG Shi, A Boothroyd, D Prabhakaran

We report an experimental study of the magnetic order and electronic structure and transport of the layered pnictide EuMnSb2, performed using neutron diffraction, angle-resolved photoemission spectroscopy (ARPES), and magnetotransport measurements. We find that the Eu and Mn sublattices display antiferromagnetic (AFM) order below T EuN = 21(1) K and T MnN = 350(2) K, respectively. The former can be described by an A-type AFM structure with the Eu spins aligned along the c axis (an in-plane direction), whereas the latter has a C-type AFM structure with Mn moments along the a -axis (perpendicular to the layers). The ARPES spectra reveal Dirac-like linearly dispersing bands near the Fermi energy. Furthermore, our magnetotransport measurements show strongly anisotropic magnetoresistance and indicate that the Eu sublattice is intimately coupled to conduction electron states near the Dirac point.


Manifold of spin states and dynamical temperature effects in LaCoO3: Experimental and theoretical insights

PHYSICAL REVIEW B 100 (2019) ARTN 054306

M Feygenson, D Novoselov, S Pascarelli, R Chernikov, O Zaharko, F Porcher, D Karpinsky, A Nikitin, D Prabhakaran, A Sazonov, V Sikolenko


Tuning of the Ru4+ ground-state orbital population in the 4d(4) Mott insulator Ca2RuO4 achieved by La doping

PHYSICAL REVIEW B 99 (2019) ARTN 075125

D Pincini, LSI Veiga, CD Dashwood, F Forte, M Cuoco, RS Perry, P Bencok, AT Boothroyd, DF McMorrow


An ideal Weyl semimetal induced by magnetic exchange

Physical review B: Condensed matter and materials physics American Physical Society 100 (2019) 201102(R)

J Jiang, M Bristow, P Reiss, Y Guo, J Blandy, F De Juan, J-R Soh, N Schroeter, M Vergniory, M Rahn, S Simon, Y Chen, A Coldea, DY Yan, T Kim, Y Shi, A McCollam, A Boothroyd


Phase transitions in few-monolayer spin ice films.

Nature communications 10 (2019) 1219-

L Bovo, CM Rouleau, D Prabhakaran, ST Bramwell

Vertex models are an important class of statistical mechanical system that admit exact solutions and exotic physics. Applications include water ice, ferro- and antiferro-electrics, spin ice and artificial spin ice. Here we show that it is possible to engineer spin ice films with atomic-layer precision down to the monolayer limit. Specific heat measurements show that these films, which have a fundamentally different symmetry to bulk spin ice, realise systems close to the two-dimensional F-model, with exotic phase transitions on topologically-constrained configurational manifolds. Our results show how spin ice thin films can release the celebrated Pauling entropy of spin ice without an anomaly in the specific heat. They also significantly expand the class of vertex models available to experiment.

Pages