Publications


Correction to Step-Flow Growth of Bi 2 Te 3 Nanobelts

Crystal Growth & Design 17 (2017) 1438-1438

P Schönherr, T Tilbury, H Wang, AA Haghighirad, V Srot, PA van Aken, T Hesjedal


Layer-by-layer epitaxial thin films of the pyrochlore Tb2Ti2O7.

Nanotechnology 28 (2017) 055708-

L Bovo, CM Rouleau, D Prabhakaran, ST Bramwell

Layer-by-layer epitaxial growth of the pyrochlore magnet Tb2Ti2O7 on the isostructural substrate Y2Ti2O7 results in high-quality single crystal films of up to 60 nm thickness. Substrate-induced strain is shown to act as a strong and controlled perturbation to the exotic magnetism of Tb2Ti2O7, opening up the general prospect of strain-engineering the diverse magnetic and electrical properties of pyrochlore oxides.


Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 421 (2017) 428-439

A Baker, M Beg, G Ashton, M Albert, D Chernyshenko, W Wang, S Zhang, M-A Bisotti, M Franchin, CL Hu, R Stamps, T Hesjedal, H Fangohr


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.


Crystal growth of pyrochlore rare-earth stannates

JOURNAL OF CRYSTAL GROWTH 468 (2017) 335-339

D Prabhakaran, S Wang, AT Boothroyd


Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures.

Nature Communications 8 (2017) 1583-

W Saenrang, BA Davidson, F Maccherozzi, JP Podkaminer, J Irwin, RD Johnson, JW Freeland, J Íñiguez, JL Schad, K Reierson, JC Frederick, CAF Vaz, L Howald, TH Kim, S Ryu, MV Veenendaal, PG Radaelli, SS Dhesi, MS Rzchowski, CB Eom

Exploiting multiferroic BiFeO3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO3, as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO3 antiferromagnetic order and Co magnetization is observed, with ~ 90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. This has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO3.


Coexistence of magnetism and superconductivity in separate layers of the iron-based superconductor Li1-xFex(OH)Fe1-ySe

PHYSICAL REVIEW B 95 (2017) ARTN 134419

CV Topping, FKK Kirschner, SJ Blundell, PJ Baker, DN Woodruff, F Schild, H Sun, SJ Clarke


Crystal growth of the triangular-lattice antiferromagnet Ba3CoSb2O9

JOURNAL OF CRYSTAL GROWTH 468 (2017) 345-348

D Prabhakaran, AT Boothroyd


Broken rotational symmetry on the Fermi surface of a high-Tc superconductor

npj Quantum Materials 2 (2017)

BJ Ramshaw, N Harrison, SE Sebastian, S Ghannadzadeh, KA Modic, DA Bonn, WN Hardy, R Liang, PA Goddard


Magnetic excitations of the charge stripe electrons below half doping in La2-xSrxNiO4 (x=0.45, 0.4)

PHYSICAL REVIEW B 95 (2017) ARTN 064403

PG Freeman, SR Giblin, K Hradil, RA Mole, P Cermak, D Prabhakaran


Room-temperature helimagnetism in FeGe thin films.

Scientific reports 7 (2017) 123-

SL Zhang, I Stasinopoulos, T Lancaster, F Xiao, A Bauer, F Rucker, AA Baker, AI Figueroa, Z Salman, FL Pratt, SJ Blundell, T Prokscha, A Suter, J Waizner, M Garst, D Grundler, G van der Laan, C Pfleiderer, T Hesjedal

Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain α intr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.


Direct experimental determination of the topological winding number of skyrmions in Cu2OSeO3.

Nature communications 8 (2017) 14619-

SL Zhang, G van der Laan, T Hesjedal

The mathematical concept of topology has brought about significant advantages that allow for a fundamental understanding of the underlying physics of a system. In magnetism, the topology of spin order manifests itself in the topological winding number which plays a pivotal role for the determination of the emergent properties of a system. However, the direct experimental determination of the topological winding number of a magnetically ordered system remains elusive. Here, we present a direct relationship between the topological winding number of the spin texture and the polarized resonant X-ray scattering process. This relationship provides a one-to-one correspondence between the measured scattering signal and the winding number. We demonstrate that the exact topological quantities of the skyrmion material Cu2OSeO3 can be directly experimentally determined this way. This technique has the potential to be applicable to a wide range of materials, allowing for a direct determination of their topological properties.


Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

PHYSICAL REVIEW B 95 (2017) ARTN 134435

J Brambleby, JL Manson, PA Goddard, MB Stone, RD Johnson, P Manuel, JA Villa, CM Brown, H Lu, S Chikara, V Zapf, SH Lapidus, R Scatena, P Macchi, Y-S Chen, L-C Wu, J Singleton


Anomalous behavior of displacement correlation function and strain in lanthanum cobalt oxide analyzed both from X-ray powder diffraction and EXAFS data

POWDER DIFFRACTION 32 (2017) S151-S154

V Efimov, V Sikolenko, IO Troyanchuk, D Karpinsky, E Efimova, SI Tiutiunnikov, BN Savenko, D Novoselov, D Prabhakaran


Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe

PHYSICAL REVIEW B 95 (2017) ARTN 081106

MD Watson, S Backes, AA Haghighirad, M Hoesch, TK Kim, AI Coldea, R Valenti


Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet.

Physical review letters 119 (2017) 147701-

M Mergenthaler, J Liu, JJ Le Roy, N Ares, AL Thompson, L Bogani, F Luis, SJ Blundell, T Lancaster, A Ardavan, GAD Briggs, PJ Leek, EA Laird

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.


Quantum-critical spin dynamics in a Tomonaga-Luttinger liquid studied with muon-spin relaxation

PHYSICAL REVIEW B 95 (2017) ARTN 020402

JS Moller, T Lancaster, SJ Blundell, FL Pratt, PJ Baker, F Xiao, RC Williams, W Hayes, MM Turnbul, CP Landee


Adiabatic physics of an exchange-coupled spin-dimer system: Magnetocaloric effect, zero-point fluctuations, and possible two-dimensional universal behavior

PHYSICAL REVIEW B 95 (2017) ARTN 024404

J Brambleby, PA Goddard, J Singleton, M Jaime, T Lancaster, L Huang, J Wosnitza, CV Topping, KE Carreiro, HE Tran, ZE Manson, JL Manson


Suppression of electronic correlations by chemical pressure from FeSe to FeS

Phys. Rev. B 96, 121103(R) (2017) American Physical Society (2017)

P Reiss, MD Watson, TK Kim, AA Haghighirad, DN Woodruff, M Bruma, SJ Clarke, AI Coldea

Iron-based chalcogenides are complex superconducting systems in which orbitally-dependent electronic correlations play an important role. Here, using high-resolution angle-resolved photoemission spectroscopy, we investigate the effect of these electronic correlations outside the nematic phase in the tetragonal phase of superconducting FeSe1-xSx (x = 0; 0:18; 1). With increasing sulfur substitution, the Fermi velocities increase significantly and the band renormalizations are suppressed towards a factor of 1.5-2 for FeS. Furthermore, the chemical pressure leads to an increase in the size of the quasi-two dimensional Fermi surface, compared with that of FeSe, however, it remains smaller than the predicted one from first principle calculations for FeS. Our results show that the isoelectronic substitution is an effective way to tune electronic correlations in FeSe1-xSx, being weakened for FeS with a lower superconducting transition temperature. This suggests indirectly that electronic correlations could help to promote higher-Tc superconductivity in FeSe.


Spin Resonance Clock Transition of the Endohedral Fullerene ^{15}N@C_{60}.

Physical review letters 119 (2017) 140801-

RT Harding, S Zhou, J Zhou, T Lindvall, WK Myers, A Ardavan, GAD Briggs, K Porfyrakis, EA Laird

The endohedral fullerene ^{15}N@C_{60} has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

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