Publications


Mode-resolved detection of magnetization dynamics using x-ray diffractive ferromagnetic resonance

Nano Letters American Chemical Society (2019)

D Burn, S Zhang, Y Chai, K Zhai, Y Sun, G Van Der Laan, T Hesjedal

Collective spin excitations of ordered magnetic structures o er great potential for the development of novel spintronic devices. The present approach is to rely on micromagnetic models to explain the origins of dynamic modes observed by ferromagnetic resonance (FMR) studies, since experimental tools to directly reveal the origins of the complex dynamic behavior are lacking. Here we demonstrate a new approach which combines resonant magnetic x-ray diffraction with FMR, thereby allowing for a reconstruction of the real-space spin dynamics of the system. This new diffractive FMR (DFMR) technique builds on x-ray detected FMR (XFMR) that allows for element-selective dynamic studies, giving unique access to specific wave components of static and dynamic coupling in magnetic heterostructures. In combination with diffraction, FMR is elevated to the level of a modal spectroscopy technique, potentially opening new pathways for the development of spintronic devices.


Robust perpendicular skyrmions and their surface-confinement

Nano Letters American Chemical Society (2020)

S Zhang, D Burn, N Jaouen, J-Y Chauleau, A Haghighirad, Y Wang, W Wang, G Van Der Laan, T Hesjedal

Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension, and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic x-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field we find that a state of perpendicularly ordered skyrmions forms - in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined in a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface-guiding makes the robust state particular interesting for racetrack-like devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.


Quenched nematic criticality and two superconducting domes in an iron-based superconductor under pressure

Nature Physics Nature Research (2019)

P Reiss, D Graf, AA Haghighirad, W Knafo, L Drigo, M Bristow, AJ Schofield, AI Coldea

The nematic electronic state and its associated critical fluctuations have emerged as a potential candidate for the superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with magnetically ordered phases poses a significant challenge in determining their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we access a nematic quantum phase transition isolated from any other competing magnetic phases. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure and detect a Lifshitz transition that separates two distinct superconducting regions. One emerges from the nematic phase with a small Fermi surface and strong electronic correlations, while the other one has a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically ordered phase at high pressures. The absence of mass divergence at the nematic quantum phase transition suggests that the nematic fluctuations could be quenched by the strong coupling to the lattice or local strain effects. A direct consequence is the weakening of superconductivity at the nematic quantum phase transition in the absence of magnetically driven fluctuations.


Optimization of superconducting properties of the stoichiometric CaKFe4As4

Supercond. Sci. Technol. 33 (2020) 025003 IOP Publishing (2019)

SJ Singh, SJ Cassidy, M Bristow, S Blundell, SJ Clarke, AI Coldea


Magnetic Weyl semimetal phase in a Kagomé crystal

Science American Association for the Advancement of Science 365 (2019) 1282-1285

DF Liu, AJ Liang, EK Liu, QN Xu, Y Li, C Chen, D Pei, WJ Shi, SK Mo, P Dudin, T Kim, C Cacho, G Li, Y Sun, LX Yang, ZK Liu, SSP Parkin, C Felser, Y Chen

Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.


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


Probing magnetic order and disorder in the one-dimensional molecular spin chains CuF2(pyz) and [Ln(hfac)3(boaDTDA)] n (Ln  =  Sm, La) using implanted muons.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 394002-394002

T Lancaster, BM Huddart, RC Williams, F Xiao, KJA Franke, PJ Baker, FL Pratt, SJ Blundell, JA Schlueter, MB Mills, AC Maahs, KE Preuss

We present the results of muon-spin relaxation ([Formula: see text]SR) measurements on antiferromagnetic and ferromagnetic spin chains. In antiferromagnetic CuF2(pyz) we identify a transition to long range magnetic order taking place at [Formula: see text] K, allowing us to estimate a ratio with the intrachain exchange of [Formula: see text] and the ratio of interchain to intrachain exchange coupling as [Formula: see text]. The ferromagnetic chain [Sm(hfac)3(boaDTDA)] n undergoes an ordering transition at [Formula: see text] K, seen via a broad freezing of dynamic fluctuations on the muon (microsecond) timescale and implying [Formula: see text]. The ordered radical moment continues to fluctuate on this timescale down to 0.3 K, while the Sm moments remain disordered. In contrast, the radical spins in [La(hfac)3(boaDTDA)] n remain magnetically disordered down to T  =  0.1 K suggesting [Formula: see text].


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.


Helical magnetic ordering in thin FeGe membranes

PHYSICAL REVIEW B 100 (2019) ARTN 184403

DM Burn, SL Zhang, S Wang, HF Du, G van der Laan, T Hesjedal


Coherent spin manipulation of individual atoms on a surface

Science American Association for the Advancement of Science 366 (2019) 509-512

K Yang, S-H Phark, W Paul, P Willke, Y Bae, T Esat, T Choi, A Ardavan, A Heinrich, C Lutz

Achieving time-domain control of quantum states with atomic-scale spatial resolution in nanostructures is a long-term goal in quantum nanoscience and spintronics. Here, we demonstrate coherent spin rotations of individual atoms on a surface at the nanosecond time scale, using an all-electric scheme in a scanning tunneling microscope (STM). By modulating the atomically confined magnetic interaction between the STM tip and surface atoms, we drive quantum Rabi oscillations between spin-up and spin-down states in as little as ~20 nanoseconds. Ramsey fringes and spin echo signals allow us to understand and improve quantum coherence. We further demonstrate coherent operations on engineered atomic dimers. The coherent control of spins arranged with atomic precision provides a solid-state platform for quantum-state engineering and simulation of many-body systems.


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

Physical Review B American Physical Society 100 (2019) 214113

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


Spin-wave directional anisotropies in antiferromagnetic Ba3NbFe3Si2O14

Physical Review B American Physical Society 100 (2019) 134429

C Stock, RD Johnson, N Giles-Donovan, M Songvilay, JA Rodriguez-Rivera, N Lee, X Xu, P Radaelli, LC Chapon, A Bombardi, S Cochran, C Niedermayer, A Schneidewind, Z Husges, Z Lu, S Meng, S-W Cheong

Ba3NbFe3Si2O14 (langasite) is structurally and magnetically single-domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. Using neutron scattering, we show that the spin waves in antiferromagnetic langasite display directional anisotropy. On applying a time-reversal symmetry breaking magnetic field along the c axis, the spin-wave energies differ when the sign is reversed for either the momentum transfer ±Q- or applied magnetic field ±μ0H. When the field is applied within the crystallographic ab plane, the spin-wave dispersion is directionally isotropic and symmetric in ±μ0H. However, a directional anisotropy is observed in the spin-wave intensity. We discuss this directional anisotropy in the dispersion in langasite in terms of a field-induced precession of the dynamic unit cell staggered magnetization resulting from a broken twofold symmetry. Directional anisotropy, often referred to as nonreciprocal responses, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.


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.


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


Tailoring the topological surface state in ultrathin alpha -Sn(111) films

Physical Review B: Condensed Matter and Materials Physics American Physical Society (2019)

VA Rogalev, F Adler, F Reis, M Bauernfeind, J Erhardt, L Dudy, LB Duffy, T Hesjedal, M Hoesch, J Schaefer, G Bihlmayer, R Claessen

We report on the electronic structure of α-Sn films in the low thickness regime grown on InSb(111)A. High-resolution angle-resolved photoemission (ARPES), enhanced at low photon energies, allows for the direct observation of the linearly dispersing 2D topological surface states (TSSs) that exist between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The cross-over to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of α-Sn films where the hybridization gap in TSS coexists with the topologically non-trivial electronic structure which must result in a presence of 1D helical edge states.


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


Monitoring ultrafast metallization in LaCoO<inf>3</inf> with femtosecond soft x-ray spectroscopy

Communications Physics 2 (2019)

M Izquierdo, M Karolak, D Prabhakaran, AT Boothroyd, AO Scherz, A Lichtenstein, SL Molodtsov

© 2019, The Author(s). The study of ultrafast dynamics is a new tool to understand and control the properties of correlated oxides. By enhancing some properties and realizing new dynamically excited phrases, this tool has opened new routes for technological applications. LaCoO3 is one paradigmatic example where the strong electron, spin, and lattice coupling induced by electronic correlations results in a low-temperature spin transition and a high-temperature semiconductor-to-metal transition that is still not completely understood. Here, we monitor ultrafast metallization in LaCoO3 using time-resolved soft x-ray reflectivity experiments. While the process is entangled at the Co L3 edge, the time information of the different channels is decrypted at different resonant energies of the O K edge. Metallization is shown to occur via transient electronic, spin, and lattice separation. Our results agree with the thermodynamical model and demonstrate the potential of femtosecond soft x-ray experiments at the O K edge to understand correlated oxides.

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