Publications


Controlling spin current polarization through non-collinear antiferromagnetism.

Nature communications 11 (2020) 4671-

T Nan, CX Quintela, J Irwin, G Gurung, DF Shao, J Gibbons, N Campbell, K Song, S-Y Choi, L Guo, RD Johnson, P Manuel, RV Chopdekar, I Hallsteinsen, T Tybell, PJ Ryan, J-W Kim, Y Choi, PG Radaelli, DC Ralph, EY Tsymbal, MS Rzchowski, CB Eom

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.


Severe Dirac Mass Gap Suppression in Sb2Te3-Based Quantum Anomalous Hall Materials.

Nano letters 20 (2020) 8001-8007

YX Chong, X Liu, R Sharma, A Kostin, G Gu, K Fujita, JCS Davis, PO Sprau

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr0.08(Bi0.1Sb0.9)1.92Te3 to that of its nonmagnetic parent (Bi0.1Sb0.9)2Te3, to explore the cause. In (Bi0.1Sb0.9)2Te3, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr0.08(Bi0.1Sb0.9)1.92Te3 with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb2Te3-based FMTI materials to very low temperatures.


Kerr effect anomaly in magnetic topological insulator superlattices

Nanotechnology IOP Publishing 31 (2020) 434001

J Liu, A Singh, B Kuerbanjiang, C Barnes, T Hesjedal

We report the magneto-optical Kerr effect (MOKE) study of magnetic topological insulator superlattice films with alternating transition-metal and rare-earth doping. We observe an unexpected hump in the MOKE hysteresis loops upon magnetization reversal at low temperatures, reminiscent of the topological Hall effect(THE) reported in transport measurements. The THE is commonly associated with the existence of magnetic skyrmions, i.e., chiral spin textures originating from topological defects in real space. Here, the observation of the effect is tied to ferromagnetic ordering in the rare-earth-doped layers of the superlattice. Our study may provide a new approach for the non-invasive optical investigation of skyrmions in magnetic films, complementary to electrical transport measurements, where the topological Hall signal is often the only hint of non-trivial magnetization patterns.


Observation of a neutron spin resonance in the bilayered superconductor CsCa2Fe4As4F2.

Journal of physics. Condensed matter : an Institute of Physics journal 32 (2020) 435603-

DT Adroja, SJ Blundell, F Lang, H Luo, Z-C Wang, G-H Cao

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa2Fe4As4F2 above and below its superconducting transition temperature T c ≈ 28.9 K to investigate the presence of a neutron spin resonance. This compound crystallises in a body-centred tetragonal lattice containing asymmetric double layers of Fe2As2 separated by insulating CaF2 layers and is known to be highly anisotropic. Our INS study clearly reveals the presence of a neutron spin resonance that exhibits higher intensity at lower momentum transfer (Q) at 5 K compared to 54 K, at an energy of 15 meV. The energy E R of the observed spin resonance is broadly consistent with the relationship E R = 4.9k B T c, but is slightly enhanced compared to the values observed in other Fe-based superconductors. We discuss the nature of the electron pairing symmetry by comparing the value of E R with that deduced from the total superconducting gap value integrated over the Fermi surface.


Glide symmetry breaking and Ising criticality in the quasi-1D magnet CoNb2O6

Proceedings of the National Academy of Sciences National Academy of Sciences 117 (2020) 25219-25224

M Fava, R Coldea, S Ashok Parameswaran

We construct a microscopic spin-exchange Hamiltonian for the quasi–one-dimensional (1D) Ising magnet CoNb2O6 that captures detailed and hitherto-unexplained aspects of its dynamic spin structure factor. We perform a symmetry analysis that recalls that an individual Ising chain in this material is buckled, with two sites in each unit cell related by a glide symmetry. Combining this with numerical simulations benchmarked against neutron scattering experiments, we argue that the single-chain Hamiltonian contains a staggered spin-exchange term. We further argue that the transverse-field–tuned quantum critical point in CoNb2O6 corresponds to breaking this glide symmetry, rather than an on-site Ising symmetry as previously believed. This gives a unified microscopic explanation of the dispersion of confined states in the ordered phase and quasiparticle breakdown in the polarized phase at high transverse field.


Magnetic Skyrmions

MagNews (2020) 19-21

G van der Laan, T Hesjedal


Skyrmions getting an X-ray

MagNews UK Magnetics Society (2020) 22-22

S Zhang, T Hesjedal, G van der Laan


Depth-resolved magnetization dynamics revealed by x-ray reflectometry ferromagnetic resonance

Physical Review Letters American Physical Society (2020)

D Burn, S Zhang, G Yu, Y Guang, H Chen, X Qiu, G van der Laan, T Hesjedal

Magnetic multilayers offer diverse opportunities for the development of ultrafast functional devices through advanced interface and layer engineering. Nevertheless, a method for determining their dynamic properties as a function of depth throughout such stacks have remained elusive. By probing the ferromagnetic resonance (FMR) modes with element-selective soft x-ray resonant reflectivity, we gain access to the magnetization dynamics as a function of depth. Most notably, using reflectometry ferromagnetic resonance (RFMR), we find a phase lag between the coupled ferromagnetic layers in [CoFeB/MgO/Ta]4 multilayers, which is invisible to other techniques. RFMR enables the time- and layer-resolved probing of the complex magnetization dynamics of a wide range of functional magnetic heterostructures with absorption edges in the soft x-ray wavelength regime.


Photo-molecular high temperature superconductivity

Physical Review X American Physical Society 10 (2020) 031028

M Buzzi, D Nicoletti, M Fechner, N Tancogne-Dejean, MA Sentef, A Georges, T Biesner, E Uykur, M Dressel, A Henderson, T Siegrist, JA Schlueter, K Miyagawa, K Kanoda, M-S Nam, A Ardavan, J Coulthard, J Tindall, F Schlawin, D Jaksch, A Cavalleri

The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer salt κ−(BEDT−TTF)2 Cu[N(CN)2] Br induce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperature T∗≃50K, far higher than the equilibrium transition temperature TC=12.5K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.


Information and Decoherence in a Muon-Fluorine Coupled System

PHYSICAL REVIEW LETTERS 125 (2020) 87201

J Wilkinson, S Blundell


Electron beam lithography of magnetic skyrmions

Advanced Materials Wiley (2020)

Y Guang, Y Peng, Z Yan, Y Liu, J Zhang, X Zeng, S Zhang, S Zhang, DM Burn, N Jaouen, J Wei, H Xu, J Feng, C Fang, G van der Laan, T Hesjedal, B Cui, X Zhang, G Yu, X Han

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their non-trivial topology through single skyrmions, ordered, and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as the chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is key to advancing topological magnetism. Here, we present a new, yet general, approach to the &#x2018;printing&#x2019; of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, we use a focused electron beam with a graphic pattern generator to &#x2018;print&#x2019; skyrmions, which we refer to as skyrmion lithography. Our work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.


Strong in-plane anisotropy in the electronic structure of fixed-valence $β$-LuAlB$_4$

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

P Reiss, J Baglo, H Tan, X Chen, S Friedemann, K Kuga, FM Grosche, S Nakatsuji, M Sutherland

The origin of intrinsic quantum criticality in the heavy-fermion superconductor $\beta$-YbAlB$_4$ has been attributed to strong Yb valence fluctuations and its peculiar crystal structure. Here, we assess these contributions individually by studying the isostructural but fixed-valence compound $\beta$-LuAlB$_4$. Quantum oscillation measurements and DFT calculations reveal a Fermi surface markedly different from that of $\beta$-YbAlB$_4$, consistent with a `large' Fermi surface there. We also find an unexpected in-plane anisotropy of the electronic structure, in contrast to the isotropic Kondo hybridization in $\beta$-YbAlB$_4$.


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


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.


Absolute crystal and magnetic chiralities in the langasite compound Ba3NbFe3Si2O14 determined by polarized neutron and x-ray scattering

Physical Review B American Physical Society (APS) 102 (2020) 54417

N Qureshi, A Bombardi, S Picozzi, P Barone, E Lelièvre-Berna, X Xu, C Stock, D McMorrow, A Hearmon, F Fabrizi, P Radaelli, S-W Cheong, L Chapon


Emergent helical texture of electric dipoles

Science American Association for the Advancement of Science 369 (2020) 680-684

D Khalyavin, R Johnson, F Orlandi, P Radaelli, P Manuel, AA Belik


Dynamic spin fluctuations in the frustrated A-site spinel CuAl2O4

PHYSICAL REVIEW B 102 (2020) ARTN 014439

H Cho, R Nirmala, J Jeong, PJ Baker, H Takeda, N Mera, SJ Blundell, M Takigawa, DT Adroja, J-G Park


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


Proximity-induced odd-frequency superconductivity in a topological insulator

Physical Review Letters American Physical Society 125 (2020) 026802

J Krieger, A Pertsova, S Giblin, M Döbeli, T Prokscha, C Schneider, A Suter, T Hesjedal, A Balatsky, Z Salman

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an s-wave SC in a TI can develop an order parameter with a p-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi2Se3 proximity coupled to Nb. From depthresolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi2Se3 that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of such a component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting states.


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

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