Publications


Magnetic structure and spin-flop transition in the A-site columnar-ordered quadruple perovskite TmMn3O6

PHYSICAL REVIEW B 99 (2019) ARTN 104424

AM Vibhakar, DD Khalyavin, P Manuel, L Zhang, K Yamaura, PG Radaelli, AA Belik, RD Johnson


Rare Earth Doping of Topological Insulators: A Brief Review of Thin Film and Heterostructure Systems (Phys. Status Solidi A 8∕2019)

Wiley (2019)

THORSTEN Hesjedal


Enhancement of intrinsic magnetic damping in defect-free epitaxial Fe3O4 thin films

APPLIED PHYSICS LETTERS 114 (2019) ARTN 192406

X Lu, LJ Atkinson, B Kuerbanjiang, B Liu, G Li, Y Wang, J Wang, X Ruan, J Wu, RFL Evans, VK Lazarov, RW Chantrell, Y Xu


Strong spin-orbit coupling and Dirac nodal lines in the three-dimensional electronic structure of metallic rutile IrO2

PHYSICAL REVIEW B 99 (2019) ARTN 195106

X Xu, J Jiang, WJ Shi, V Suess, C Shekhar, SC Sun, YJ Chen, S-K Mo, C Felser, BH Yan, HF Yang, ZK Liu, Y Sun, LX Yang, YL Chen


Strain Engineering a Multiferroic Monodomain in Thin-Film BiFeO3

PHYSICAL REVIEW APPLIED 11 (2019) ARTN 024035

NW Price, AM Vibhakar, RD Johnson, J Schad, W Saenrang, A Bombardi, FP Chmiel, CB Eom, PG Radaelli


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.


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$_{6}$.


Spin dynamics and field-induced magnetic phase transition in the honeycomb Kitaev magnet α-Li2IrO3

Physical review B: Condensed matter and materials physics American Physical Society (0)

S Choi, S Manni, J Singleton, CV Topping, T Lancaster, SJ Blundell, DT Adroja, V Zapf, P Gegenwart, R Coldea

The layered honeycomb iridate alpha-Li2IrO3 displays an incommensurate magnetic structure with counterrotating moments on nearest-neighbor sites, proposed to be stabilized by strongly-frustrated anisotropic Kitaev interactions between spin-orbit entangled Ir4+ magnetic moments. Here we report powder inelastic neutron scattering measurements that observe sharply dispersive low-energy magnetic excitations centered at the magnetic ordering wavevector, attributed to Goldstone excitations of the incommensurate order, as well as an additional intense mode above a gap Delta~2.3 meV. Zero-field muon-spin relaxation measurements show clear oscillations in the muon polarization below the Neel temperature T_N ~ 15 K with a time-dependent profile consistent with bulk incommensurate long-range magnetism. Pulsed field magnetization measurements observe that only about half the saturation magnetization value is reached at the maximum field of 64 T. A clear anomaly near 25 T indicates a transition to a phase with reduced susceptibility. The transition field has a Zeeman energy comparable to the zero-field gapped mode, suggesting gap suppression as a possible mechanism for the field-induced transition.


The effect of substrate and surface plasmons on symmetry breaking at the substrate interface of the topological insulator Bi2Te3

Scientific Reports Nature Research 9 (2019) 6147

M Wiesner, RH Roberts, J-F Lin, D Akinwande, T Hesjedal, LB Duffy, S Wang, Y Song, J Jenczyk, S Jurga, B Mroz

A pressing challenge in engineering devices with topological insulators (TIs) is that electron transport is dominated by the bulk conductance, and so dissipationless surface states account for only a small fraction of the conductance. Enhancing the surface-to-volume ratio is a common method to enhance the relative contribution of such states. In thin films with reduced thickness, the confinement results in symmetry-breaking and is critical for the experimental observation of topologically protected surface states. We employ micro-Raman and tip-enhanced Raman spectroscopy to examine three different mechanisms of symmetry breaking in Bi2Te3 TI thin films: surface plasmon generation, charge transfer, and application of a periodic strain potential. These mechanisms are facilitated by semiconducting and insulating substrates that modify the electronic and mechanical conditions at the sample surface and alter the long-range interactions between Bi2Te3 and the substrate. We confirm the symmetry breaking in Bi2Te3 via the emergence of the Raman-forbidden ܣଵ௨ ଶ mode. Our results suggest that topological surface states can exist at the Bi2Te3/substrate interface, which is in a good agreement with previous theoretical results predicting the tunability of the vertical location of helical surface states in TI/substrate heterostructures.


Oriented Three-Dimensional Magnetic Biskyrmion in MnNiGa Bulk Crystals

Advanced Materials Wiley (2019)

XY Li, S Zhang, H Li, D Alba Venero, JS White, R Cubitt, QZ Huang, G van der Laan, WH Wang, T Hesjedal, FW Wang


Unconventional Field-Induced Spin Gap in an S=1/2 Chiral Staggered Chain

PHYSICAL REVIEW LETTERS 122 (2019) ARTN 057207

J Liu, S Kittaka, RD Johnson, T Lancaster, J Singleton, T Sakakibara, Y Kohama, J van Tol, A Ardavan, BH Williams, SJ Blundell, ZE Manson, JL Manson, PA Goddard


Anatomy of skyrmionic textures in magnetic multilayers

Advanced Materials Wiley 31 (2019) 1807683

S Zhang, M Carpentieri, W Li, G Finocchio, R Tomasello, I Bykova, J Graefe, X Zhang, J Feng, Z Yan, Y Liu, G Yu, T Hesjedal, G Van Der Laan, M Weigand, G Schuetz, Y Guang, J Wei, C Wan, X Han, C Guo, DM Burn, X Wang, H Wei, H Xu

Room temperature magnetic skyrmions in magnetic multilayers are considered as information carriers for future spintronic applications. Currently, a detailed understanding of the skyrmion stabilization mechanisms is still lacking in these systems. To gain more insight, it is first and foremost essential to determine the full real‐space spin configuration. Here, two advanced X‐ray techniques are applied, based on magnetic circular dichroism, to investigate the spin textures of skyrmions in [Ta/CoFeB/MgO] n multilayers. First, by using ptychography, a high‐resolution diffraction imaging technique, the 2D out‐of‐plane spin profile of skyrmions with a spatial resolution of 10 nm is determined. Second, by performing circular dichroism in resonant elastic X‐ray scattering, it is demonstrated that the chirality of the magnetic structure undergoes a depth‐dependent evolution. This suggests that the skyrmion structure is a complex 3D structure rather than an identical planar texture throughout the layer stack. The analyses of the spin textures confirm the theoretical predictions that the dipole–dipole interactions together with the external magnetic field play an important role in stabilizing sub‐100 nm diameter skyrmions and the hybrid structure of the skyrmion domain wall. This combined X‐ray‐based approach opens the door for in‐depth studies of magnetic skyrmion systems, which allows for precise engineering of optimized skyrmion heterostructures.


Manipulating quantum materials with quantum light (vol 99, 085116, 2019)

Physical Review B (2019)

MARTIN Kiffner, F Schlawin, A Ardavan, DIETER Jaksch

© 2019 American Physical Society. The interaction Hamiltonian (Formula Presented) Eq. (14) describing the interaction between the cavity and the electronic system was obtained by expanding the Peierls Hamiltonian in Eq. (A4) up to first order in the small parameter (Formula Presented) All results presented in the paper are consistent with this appro imate interaction Hamiltonian, leading to an effective Hamiltonian that depends quadratically on. However, it turns out that a straightforward improvement of the parameters entering the effective Hamiltonian in Eq. (26) can be obtained by including the second-order term in the Peierls Hamiltonian in Eq. (A4). This term gives rise to modifications of our results that are also of order through a renormalization of the nearest-neighbor hopping amplitude (Formula Presented) The authors would like to thank M. A. Sentef for bringing the importance of the second-order term in Eq. (A4) to our attention.


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


Manipulating quantum materials with quantum light

Physical Review B American Physical Society 99 (2019) 085116-

M Kiffner, J Coulthard, F Schlawin, A Ardavan, D Jaksch

We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold. At half filling this effect can be observed by a change in the magnetic susceptibility. At less than half filling, the cavity introduces a next-nearest-neighbor hopping and mediates a long-range electron-electron interaction between distant sites. We study the ground-state properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentum-space pairing effect for electrons.


Oriented 3D magnetic biskyrmions in MnNiGa bulk crystals

Advanced Materials Wiley 31 (2019) 1900264

X Li, S Zhang, H Li, DA Venero, JS White, R Cubitt, Q Huang, J Chen, L He, GVD Laan, W Wang, T Hesjedal, F Wang

A biskyrmion consists of two bound, topologically stable, skyrmion spin textures. These coffee‐bean‐shaped objects are observed in real space in thin plates using Lorentz transmission electron microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface‐confined objects, or, analogous to skyrmions in noncentrosymmetric helimagnets, 3D tube‐like structures in a bulk sample. Here, the biskyrmion form factor is investigated in single‐ and polycrystalline‐MnNiGa samples using small‐angle neutron scattering. It is found that biskyrmions are not long‐range ordered, not even in single crystals. Surprisingly all of the disordered biskyrmions have their in‐plane symmetry axis aligned along certain directions, governed by the magnetocrystalline anisotropy. This anisotropic nature of biskyrmions may be further exploited to encode information.


Temperature dependence of the ferromagnetic response in CrxSb2-xTe3 topological insulator thin films investigated using terahertz spectroscopy and magneto-transport

Proceedings of SPIE Society of Photo-Optical Instrumentation Engineers 10917 (2019)

VS Kamboj, A Singh, L Jakob, L Duffy, N Idros, SP Senanayak, A Ionescu, HE Beere, CHW Barnes, T Hesjedal, DA Ritchie


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.

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