# Publications

## 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$_4$.

## 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.

## Controlling spin current polarization through non-collinear antiferromagnetism.

Nat Commun 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.

## 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

## Phase-sensitive determination of nodal d-wave order parameter in single-band and multiband superconductors

PHYSICAL REVIEW B 101 (2020) ARTN 214505

J Boeker, MA Sulangi, A Akbari, JCS Davis, PJ Hirschfeld, IM Eremin

## Information and Decoherence in a Muon-Fluorine Coupled System

PHYSICAL REVIEW LETTERS 125 (2020) 87201

J Wilkinson, S Blundell

## 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

## Proximity-induced odd-frequency superconductivity in a topological insulator

Physical Review Letters American Physical Society (2020)

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 depth-resolved 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 an odd-frequency component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer (BCS) 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.

## 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.

## Atomic-scale electronic structure of the cuprate pair density wave state coexisting with superconductivity.

Proceedings of the National Academy of Sciences of the United States of America 117 (2020) 14805-14811

P Choubey, SH Joo, K Fujita, Z Du, SD Edkins, MH Hamidian, H Eisaki, S Uchida, AP Mackenzie, J Lee, JCS Davis, PJ Hirschfeld

The defining characteristic of hole-doped cuprates is d-wave high temperature superconductivity. However, intense theoretical interest is now focused on whether a pair density wave state (PDW) could coexist with cuprate superconductivity [D. F. Agterberg et al., Annu. Rev. Condens. Matter Phys. 11, 231 (2020)]. Here, we use a strong-coupling mean-field theory of cuprates, to model the atomic-scale electronic structure of an eight-unit-cell periodic, d-symmetry form factor, pair density wave (PDW) state coexisting with d-wave superconductivity (DSC). From this PDW + DSC model, the atomically resolved density of Bogoliubov quasiparticle states [Formula: see text] is predicted at the terminal BiO surface of Bi2Sr2CaCu2O8 and compared with high-precision electronic visualization experiments using spectroscopic imaging scanning tunneling microscopy (STM). The PDW + DSC model predictions include the intraunit-cell structure and periodic modulations of [Formula: see text], the modulations of the coherence peak energy [Formula: see text] and the characteristics of Bogoliubov quasiparticle interference in scattering-wavevector space [Formula: see text] Consistency between all these predictions and the corresponding experiments indicates that lightly hole-doped Bi2Sr2CaCu2O8 does contain a PDW + DSC state. Moreover, in the model the PDW + DSC state becomes unstable to a pure DSC state at a critical hole density p*, with empirically equivalent phenomena occurring in the experiments. All these results are consistent with a picture in which the cuprate translational symmetry-breaking state is a PDW, the observed charge modulations are its consequence, the antinodal pseudogap is that of the PDW state, and the cuprate critical point at p* ≈ 19% occurs due to disappearance of this PDW.

## 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

## 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

## Electron beam lithography of magnetic skyrmions

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.

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

NATURE PHYSICS (2020)

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

## Kerr effect anomaly in magnetic topological insulator superlattices

Nanotechnology IOP Publishing (2020)

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.

## Exchange Bias in Magnetic Topological Insulator Superlattices

Nano Letters: a journal dedicated to nanoscience and nanotechnology American Chemical Society (2020)

T Hesjedal

Magnetic doping and proximity coupling can open a band gap in a topological insulator (TI) and give rise to dissipationless quantum conduction phenomena. Here, by combining these two approaches, we demonstrate a novel TI superlattice structure which is alternately doped with transition and rare earth elements. An unexpected exchange bias effect is unambiguously confirmed in the superlattice with a large exchange bias field using magneto-transport and magneto-optical techniques. Further, the Curie temperature of the Cr-doped layers in the superlattice is found to increase by 60 K compared to a Cr-doped single-layer film. This result is supported by density-functional-theory calculations, which indicate the presence of antiferromagnetic ordering in Dy:Bi2Te3 induced by proximity coupling to Cr:Sb2Te3 at the interface. This work provides a new pathway to realizing the quantum anomalous Hall effect at elevated temperatures and axion insulator state at zero magnetic field by interface engineering in TI heterostructures.

## Ultraviolet Photodetector Based on Mg0.67Ni0.33O Thin Film on SrTiO3

PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS (2020) ARTN 2000175

F Sarcan, S Orchard, B Kuerbanjiang, A Skeparovski, VK Lazarov, A Erol

## Coherent transfer of spin angular momentum by evanescent spin #Waves within Antiferromagnetic NiO

Physical Review Letters American Physical Society 124 (2020) 217201

M Dąbrowski, N Takafumi, D Burn, A Frisk, D Newman, C Klewe, Q Li, M Yang, P Shafer, E Arenholz, T Hesjedal, G van der Laan, Z Qiu, R Hicken

Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.

## 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.

## 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