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

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

F Novelli, JO Tollerud, D Prabhakaran, JA Davis

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.

Polarizing an antiferromagnet by optical engineering of the crystal field

Nature Physics Nature Research (2020)

AS Disa, M Fechner, T Nova, B Liu, M Foerst, D Prabhakaran, P Radaelli, A Cavalleri

Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. For example, the piezomagnetic effect provides an attractive route to control magnetism with strain. In this effect, the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially appealing because, unlike magnetostriction, it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF2. Through this effect, we generate a ferrimagnetic moment of 0.2 μB per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain.

Strong quantum fluctuations from competition between magnetic phases in a pyrochlore iridate (vol 101, 104404, 2020)

PHYSICAL REVIEW B 101 (2020) ARTN 169901

H Jacobsen, CD Dashwood, E Lhotel, D Khalyavin, P Manuel, R Stewart, D Prabhakaran, DF McMorrow, AT Boothroyd

Low-temperature thermal transport measurements of oxygen-annealed Yb2Ti2O7

PHYSICAL REVIEW B 102 (2020) ARTN 014434

WH Toews, JA Reid, JD Thompson, D Prabhakaran, R Coldea, RW Hill

Strong quantum fluctuations from competition between magnetic phases in a pyrochlore iridate

PHYSICAL REVIEW B 101 (2020) ARTN 104404

H Jacobsen, CD Dashwood, E Lhotel, D Khalyavin, P Manuel, R Stewart, D Prabhakaran, DF McMorrow, AT Boothroyd

Approaching the quantum critical point in a highly correlated all-in-all-out antiferromagnet

PHYSICAL REVIEW B 101 (2020) ARTN 220404

Y Wang, TF Rosenbaum, D Prabhakaran, AT Boothroyd, Y Feng

2D photocatalysts with tuneable supports for enhanced photocatalytic water splitting

Materials Today Elsevier (2020)

Y Li, S Wu, J Zheng, Y-K Peng, D Prabhakaran, R Taylor, S Tsang

Sustainable hydrogen production is attracting increasing attention and visible-light-driven water splitting is considered as one of the most promising approaches for hydrogen evolution and solar energy storage. Different materials have been screened at mild conditions in recent decades and 2-dimensional (2D) layered materials are considered good candidates for the photocatalytic water splitting reaction. 2D single layer MoS2 has shown its potential in various catalytic systems, and has also been used in photocatalytic water splitting reaction recently. However, current studies of MoS2 monolayers give low intrinsic activity, preventing it from practical applications. This is attributed to the rapid recombination of the photo-excited charge carriers at room temperature, resulting in poor quantum efficiency (QE). Herein, a state-of-the-art strategy to prolong the exciton lifetimes is reported, which is achieved by combining the 2D MoS2 nanosheets with solid state polar-faceted supports. The charge separation process can be facilitated by the strong local polarisation introduced by the polar-faceted supports, and tuned by changing the supports with different surface polarities. Polar oxide surface is the exposure of oxygen-terminated high energetic facet, which is known to give a net dipole moment perpendicular to its surface. Such variation in the surface properties of the support to the above metal would lead to a difference in metal-support interaction(s). The resulting composite structures show great enhancement toward the visible-light-driven photocatalytic water splitting reaction, giving hydrogen and oxygen evolution in a stoichiometric 2:1 ratio at elevated temperatures from pure water. Photocatalytic performances are improved by the prolonged exciton lifetimes and exceptional hydrogen evolution activity of 2977 μmol g−1 h−1 with impressive QEs are obtained over Ru-doped MoS2 nanosheets on polar ceria support, which is among the best of the reported results of similar catalytic systems to date. More excitingly, the linear relationship between the exciton lifetimes and strength of the local polarisation is also observed, indicating that the rational design of photocatalysts can be simply achieved via engineering their local polarisation by incorporation of polar-faceted materials.

Resonant x-ray scattering study of diffuse magnetic scattering from the topological semimetals EuCd2As2 and EuCd2Sb2

PHYSICAL REVIEW B 102 (2020) ARTN 014408

J-R Soh, E Schierle, Y Yan, H Su, D Prabhakaran, E Weschke, Y-F Guo, YF Shi, AT Boothroyd

Polarizing an antiferromagnet by optical engineering of the crystal field (June, 10.1038/s41567-020-0936-3, 2020)


AS Disa, M Fechner, TF Nova, B Liu, M Foerst, D Prabhakaran, PG Radaelli, A Cavalleri

Avoided quasiparticle decay and enhanced excitation continuum in the spin-1/2 near-Heisenberg triangular antiferromagnet Ba3CoSb2O9

Physical Review B: Condensed Matter and Materials Physics American Physical Society 102 (2020) 064421

D Macdougal, S Williams, D Prabhakaran, RI Bewley, DJ Voneshen, R Coldea

We explore the magnetic excitations of the spin-1/2 triangular antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal high-resolution inelastic neutron scattering. Sharp magnons with no decay are observed throughout reciprocal space, with a strongly renormalized dispersion and multiple soft modes compared to linear spin wave theory. We propose an empirical parametrization that can quantitatively capture the complete dispersions in the three-dimensional Brillouin zone and explicitly show that the dispersion renormalizations have the direct consequence that one to two magnon decays are avoided throughout reciprocal space, whereas such decays would be allowed for the unrenormalized dispersions. At higher energies, we observe a very strong continuum of excitations with highly-structured intensity modulations extending up at least 4x the maximum one-magnon energy. The one-magnon intensities decrease much faster upon increasing energy than predicted by linear spin wave theory and the higher-energy continuum contains much more intensity than can be accounted for by a two-magnon cross-section, suggesting a significant transfer of spectral weight from the high-energy magnons into the higher-energy continuum states. We attribute the strong dispersion renormalizations and substantial transfer of spectral weight to continuum states to the effect of quantum fluctuations and interactions beyond the spin wave approximation, and make connections to theoretical approaches that might capture such effects. Finally, through measurements in a strong applied magnetic field, we find evidence for magnetic domains with opposite senses for the spin rotation in the 120 degree ordered ground state, as expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense of spin rotation is selected via spontaneous symmetry breaking.

Origin of the large ferroelectric polarization enhancement under high pressure for multiferroic DyMnO3 studied by polarized and unpolarized neutron diffraction

PHYSICAL REVIEW B 102 (2020) ARTN 085131

N Terada, N Qureshi, A Stunault, M Enderle, B Ouladdiaf, CV Colin, DD Khalyavin, P Manuel, F Orlandi, S Miyahara, D Prabhakaran, T Osakabe

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, D Prabhakaran, S Wu, TJ Puchtler, M Li, K-Y Wong, R Taylor, 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.

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.

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, RD Johnson, P Radaelli

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, D Prabhakaran, A Boothroyd

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

An ideal Weyl semimetal induced by magnetic exchange

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

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