# Publications

## The magnetic structures of rare-earth quadruple perovskite manganites RMn7O12

Physical review B: Condensed matter and materials physics American Physical Society 98 (2018) 104423-

R Johnson, D Khalyavin, P Manuel, L Zhang, K Yamaura, AA Belik

We report a neutron powder diffraction study of RMn7O12 quadruple perovskite manganites with R = La, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the A and B manganese sublattices occurs on cooling below the N´eel temperature. The respective magnetic structures are collinear, with one uncompensated Mn3+ moment per formula unit as observed in bulk magnetisation measurements. We show that both LaMn7O12 and NdMn7O12 undergo a second magnetic phase transition at low temperature, which introduces a canting of the B site sublattice moments that is commensurate in LaMn7O12 and incommensurate in NdMn7O12. This spin canting is consistent with a magnetic instability originating in the B site orbital order. Furthermore, NdMn7O12 displays a third magnetic phase transition at which long range ordering of the Nd sublattice modifies the periodicity of the incommensurate spin canting. Our results demonstrate a rich interplay between transition metal magnetism, orbital order, and the crystal lattice, which may be fine tuned by cation substitution and rare earth magnetism.

## Magnetic X-ray spectroscopy of two-dimensional CrI3 layers

Materials Letters Elsevier 232 (2018) 5-7

A Frisk, LB Duffy, S Zhang, G Van Der Laan, T Hesjedal

The recently confirmed monolayer ferromagnet CrI3 is a frisky example of a two-dimensional ferromagnetic material with great application potential in van der Waals heterostructures. Here we present a soft X-ray absorption spectroscopy study of the magnetic bulk properties of CrI3, giving insight into the magnetic coupling scenario which is relevant for understanding its thickness-dependent magnetic properties. The experimental Cr X-ray magnetic circular dichroism spectra show a good agreement with calculated spectra for a hybridized ground state. In this high-spin Cr ground state the Cr–I bonds show a strongly covalent character. This is responsible for the strong superexchange interaction and increased spin-orbit coupling, resulting in the large magnetic anisotropy of the two-dimensionally layered CrI3 crystal.

## Magneto-orbital texture in the perovskite modification of Mn2O3

PHYSICAL REVIEW B 98 (2018) ARTN 014426

DD Khalyavin, RD Johnson, P Manuel, AA Tsirlin, AM Abakumov, DP Kozlenko, Y Sun, L Dubrovinsky, SV Ovsyannikov

## Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure

Bulletin of the American Physical Society American Physical Society (2018)

F Chmiel, N Price, R Johnson, A Lamirand, J Schad, GVD Laan, DT Harris, J Irwin, C-B Eom, P Radaelli

Vortices are among the simplest topological structures, and occur whenever a flow field `whirls' around a one-dimensional core. They are ubiquitous to many branches of physics, from fluid dynamics to superconductivity and superfluidity, and are even predicted by some unified theories of particle interactions, where they might explain some of the largest-scale structures seen in today's Universe. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are only observed when the effects of the dipole-dipole interaction is modified by confinement at the nanoscale, or when the parameter associated with the vorticity does not couple directly with strain. Here, we present the discovery of a novel form of vortices in antiferromagnetic (AFM) hematite ($\alpha$-Fe$_2$O$_3$) epitaxial films, in which the primary whirling parameter is the staggered magnetisation. Remarkably, ferromagnetic (FM) topological objects with the same vorticity and winding number of the $\alpha$-Fe$_2$O$_3$ vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of-plane core magnetisation), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, H$_{\parallel}$, giving rise to large-scale vortex-antivortex annihilation.

## High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb).

Inorganic chemistry 57 (2018) 5987-5998

L Zhang, N Terada, RD Johnson, DD Khalyavin, P Manuel, Y Katsuya, M Tanaka, Y Matsushita, K Yamaura, AA Belik

A-site-ordered quadruple perovskites RMn7O12 with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at TN1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below TN2 ≈ 12 K were observed in SmMn7O12 and EuMn7O12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at TN1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn7O12 shows signs of nonstoichiometry expressed as (Tb1- xMn x)Mn7O12, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn7O12 and EuMn7O12 were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn7O12 and (Tb0.88Mn0.12)Mn7O12 were studied by synchrotron X-ray powder diffraction at 295 K.

## Direct observation of twisted surface Skyrmions in bulk crystals

Physical Review Letters American Physical Society 120 (2018) 227202

S Zhang, G van der Laan, WW Wang, A Haghighirad, T Hesjedal

Magnetic skyrmions in noncentrosymmetric helimagnets with Dn symmetry are Bloch-type magnetization swirls with a helicity angle of ±90∘. At the surface of helimagnetic thin films below a critical thickness, a twisted skyrmion state with arbitrary helicity angle has been proposed, however, its direct experimental observation has remained elusive. Here, we show that circularly polarized resonant elastic x-ray scattering is able to unambiguously measure the helicity angle of surface skyrmions, providing direct experimental evidence that a twisted skyrmion surface state also exists in bulk systems. The exact surface helicity angles of twisted skyrmions for both left- and right-handed chiral bulk Cu2OSeO3, in the single as well as in the multidomain skyrmion lattice state, are determined, revealing their detailed internal structure. Our findings suggest that a skyrmion surface reconstruction is a universal phenomenon, stemming from the breaking of translational symmetry at the interface.

## THz carrier dynamics and magnetotransport study of topological surface states in thin film Bi<inf>2</inf>Se<inf>3</inf>

Proceedings of SPIE - The International Society for Optical Engineering 10531 (2018)

VS Kamboj, A Singh, T Ferrus, HE Beere, LB Duffy, T Hesjedal, CHW Barnes, DA Ritchie

© 2018 SPIE. The surface of a topological insulator harbors exotic topological states, protected against backscattering from disorder by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomena in condensed matter physics, such as the spin Hall effect, but also lays the foundation for applications from quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly distinguish topological surface states from bulk states. Employing terahertz time-domain spectroscopy, we study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing for the deconvolution of the surface state response from the bulk. Our measurement of carrier dynamics give an optical mobility exceeding 2100 cm2/V•s at 4 K, indicative of a surface-dominated response, and a scattering lifetime of ∼0.18 ps and a carrier density of 6×1012cm-2at 4 K for the Bi2Se3film. The sample was further processed into a Hall bar device using two different etching techniques, a wet chemical etching and Ar+ion milling, which resulting in a reduced Hall mobility. Even so, the magneto-conductance transport reveals weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a single topological surface state mode.

## Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling

Physical review B: Condensed matter and materials physics American Physical Society (2018)

A theory describing how ferroic properties can emerge transiently in the ultra-fast regime by breaking symmetry with light through three-phonon coupling is presented. Particular emphasis is placed on the special case when two exactly degenerate mid-infra-red or THz phonons are resonantly pumped, since this situation can give rise to an exactly rectified ferroic response with damping envelopes of ~ 1 ps or less. Light-induced ferroelectricity and ferromagnetism are discussed in this context, and a number of candidate materials that could display these phenomena are proposed. The same analysis is also applied to the interpretation of previous femto-magnetism experiments, performed in different frequency ranges (visible and near-infrared), but sharing similar symmetry characteristics.

## Electronic structure and enhanced charge-density wave order of monolayer VSe2

Nano Letters American Chemical Society 18 (2018) 4493–4499-

J Feng, D Biswas, A Rajan, F Mazzola, OJ Clark, K Underwood, I Marckovic, M McLaren, A Hunter, DM Burn, L Duffy, S Barua, G Balakrishnan, F Bertran, P LeFevre, T Kim, G van der Laan, T Hesjedal, P Wahl, PDC King

How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe2 grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe2, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below Tc = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.

## Evolution of magneto-orbital order upon B-site electron doping in Na1−xCaxMn7O12 quadruple perovskite manganites

Physical Review Letters American Physical Society 120 (2018) 257202-

R Johnson, F Mezzadri, P Manuel, DD Khalyavin, E Gilioli, PGR Radaelli

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na1-xCaxMn7O12 quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic and orbital ordering across a wide range of compounds.

## Microscopic effects of Dy doping in the topological insulator Bi2Te3

Physical Review B American Physical Society 97 (2018) 174427

LB Duffy, N-J Steinke, JA Krieger, AI Figueroa, K Kummer, T Lancaster, Giblin, FL Pratt, SJ Blundell, T Prokscha, A Suter, S Langridge, VN Strocov, Z Salman, G van der Laan, T Hesjedal

Magnetic doping with transition metal ions is the most widely used approach to break time-reversal symmetry in a topological insulator (TI)—a prerequisite for unlocking the TI’s exotic potential. Recently, we reported the doping of Bi2Te3 thin films with rare-earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy-doped Bi2Te3 remained elusive. Here, we present an x-ray magnetic circular dichroism, polarized neutron reflectometry, muon-spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous, magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition-metal-doped layers. However, the introduction of some charge carriers by the Dy dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition-metal-doped topological insulators, and Dy doping should thus allow for improved TI quantum devices.

## Spin-induced multiferroicity in the binary perovskite manganite Mn2O3

Nature Communications Nature Publishing Group 9 (2018) 2996-

J Cong, K Zhai, Y Chai, D Shang, DD Khalyavin, R Johnson, DP Kozlenko, Kichanov, AM Abakumov, AA Tsirlin, L Dubrovinsky, X Xu, Z Sheng, SV Ovsyannikov, Y Sun

The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.

## Reciprocal space tomography of 3D skyrmion lattice order in a chiral magnet

Proceedings of the National Academy of Sciences National Academy of Sciences 115 (2018) 6386-6391

S Zhang, G van der Laan, J Mueller, L Heinen, M Garst, A Bauer, H Berger, C Pfleiderer, T Hesjedal

It is commonly assumed that surfaces modify the properties of stable materials within the top few atomic layers of a bulk specimen only. Exploiting the polarization dependence of resonant elastic X-ray scattering to go beyond conventional diffraction and imaging techniques, we have determined the depth dependence of the full 3D spin structure of skyrmions—that is, topologically nontrivial whirls of the magnetization—below the surface of a bulk sample of Cu2OSeO3. We found that the skyrmions change exponentially from pure Néel- to pure Bloch-twisting over a distance of several hundred nanometers between the surface and the bulk, respectively. Though qualitatively consistent with theory, the strength of the Néel-twisting at the surface and the length scale of the variation observed experimentally exceed material-specific modeling substantially. In view of the exceptionally complete quantitative theoretical account of the magnetic rigidities and associated static and dynamic properties of skyrmions in Cu2OSeO3 and related materials, we conclude that subtle changes of the materials properties must exist at distances up to several hundred atomic layers into the bulk, which originate in the presence of the surface. This has far-reaching implications for the creation of skyrmions in surface-dominated systems and identifies, more generally, surface-induced gradual variations deep within a bulk material and their impact on tailored functionalities as an unchartered scientific territory.

## Imposing long-range ferromagnetic order in rare-earth doped magnetic topological-insulator heterostructures

Physical Review Materials American Physical Society 2 (2018) 054201

L Duffy, AJ Frisk, DM Burn, NJ Steinke, J Herrero-Martin, A Ernst, G van der Laan, T Hesjedal

The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena, such as the quantum anomalous Hall (QAH) effect. The size of the magnetic gap in the topological surface states, key for the robust observation of the QAH state, scales with the magnetic moment of the doped 3D topological insulator (TI). The pioneering transition-metal doped (Sb,Bi)2(Se,Te)3 thin films only allow for the observation of the QAH effect up to some 100 mK, despite the much higher magnetic ordering temperatures. On the other hand, high magnetic moment materials, such as rare-earth doped (Sb,Bi)2(Se,Te)3 thin films, show large moments but no long-range magnetic order. Proximity coupling and interfacial effects, multiplied in artificial heterostructures, allow for the engineering of the electronic and magnetic properties. Here, we show the successful growth of high-quality Dy:Bi2Te3/Cr:Sb2Te3 thin film heterostructures. Using x-ray magnetic spectroscopy we demonstrate that high transition temperature Cr:Sb2Te3 can introduce long-range magnetic order in high-moment Dy:Bi2Te3 - up to a temperature of 17 K - in excellent agreement with first-principles calculations, which reveal the origin of the long-range magnetic order in a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. Engineered magnetic TI heterostructures may be an ideal materials platform for observing the QAH effect at liquid He temperatures and above.

## Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure

Nature Materials Nature Publishing Group 17 (2018) 581–585-

F Chmiel, N Waterfield Price, R Johnson, AD Lamirand, J Schad, G van der Laan, DT Harris, C-B Eom, P Radaelli

Vortices, occurring whenever a flow field ‘whirls’ around a one-dimensional core, are among the simplest topological structures, ubiquitous to many branches of physics. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are observed only when the effects of the dipole–dipole interaction are modified by confinement at the nanoscale1,2,3, or when the parameter associated with the vorticity does not couple directly with strain4. Here, we observe an unprecedented form of vortices in antiferromagnetic haematite (α-Fe2O3) epitaxial films, in which the primary whirling parameter is the staggered magnetization. Remarkably, ferromagnetic topological objects with the same vorticity and winding number as the α-Fe2O3 vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of plane core magnetization), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, giving rise to large-scale vortex–antivortex annihilation.

## Intrinsic Triple Order in A-site Columnar-Ordered Quadruple Perovskites: Proof of Concept.

Chemphyschem : a European journal of chemical physics and physical chemistry (2018)

AA Belik, DD Khalyavin, L Zhang, Y Matsushita, Y Katsuya, M Tanaka, RD Johnson, K Yamaura

There is an emerging topic in the science of perovskite materials: A-site columnar-ordered A2 A'A''B4 O12 quadruple perovskites, which have an intrinsic triple order at the A sites. However, in many examples reported so far, A' and A'' cations are the same, and the intrinsic triple order is hidden. Here, we investigate structural properties of Dy2 CuMnMn4 O12 (1) and Ho2 MnGaMn4 O12 (2) by neutron and X-ray powder diffraction and prove the triple order at the A sites. The cation distributions determined are [Ho2 ]A [Mn]A' [Ga0.66 Mn0.34 ]A'' [Mn3.66 Ga0.34 ]B O12 and [Dy2 ]A [Cu0.73 Mn0.27 ]A' [Mn0.80 Dy0.20 ]A'' [Mn1.89 Cu0.11 ]B1 [Mn2 ]B2 O12 . There are clear signatures of Jahn-Teller distortions in 1 and 2, and the orbital pattern is combined with an original type of charge ordering in 1. Columnar-ordered quadruple perovskites represent a new playground to study complex interactions between different electronic degrees of freedom. No long-range magnetic order was found in 2 by neutron diffraction, and its magnetic properties in low fields are dominated by an impurity with negative magnetization or magnetization reversal. On the other hand, 1 shows three magnetic transitions at 21, 125, and 160 K.

## Manipulation of skyrmion motion by magnetic field gradients

Nature Communications Springer Nature 9 (2018) 2115

SL Zhang, WW Wang, DM Burn, H Peng, H Berger, A Bauer, C Pfleiderer, G van der Laan, T Hesjedal

Magnetic skyrmions are particle-like, topologically protected magnetisation entities that are promising candidates as information carriers in racetrack memory. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Here, we demonstrate that chiral skyrmions in Cu2OSeO3 can be effectively manipulated under the influence of a magnetic field gradient. In a radial field gradient, skyrmions were found to rotate collectively, following a given velocity–radius relationship. As a result of this relationship, and in competition with the elastic properties of the skyrmion lattice, the rotating ensemble disintegrates into a shell-like structure of discrete circular racetracks. Upon reversing the field direction, the rotation sense reverses. Field gradients therefore offer an effective handle for the fine control of skyrmion motion, which is inherently driven by magnon currents. In this scheme, no local electric currents are needed, thus presenting a different approach to shift-register-type operations based on spin transfer torque.

## Proposal for a micromagnetic standard problem for materials with Dzyaloshinskii-Moriya interaction

arxiv (2018)

D Cortes-Ortuno, M Beg, V Nehruji, L Breth, R Pepper, T Kluyver, G Downing, T Hesjedal, P Hatton, T Lancaster, R Hertel, O Hovorka, H Fangohr

Understanding the role of the Dzyaloshinskii-Moriya interaction (DMI) for the formation of helimagnetic order, as well as the emergence of skyrmions in magnetic systems that lack inversion symmetry, has found increasing interest due to the significant potential for novel spin based technologies. Candidate materials to host skyrmions include those belonging to the B20 group such as FeGe, known for stabilising Bloch-like skyrmions, interfacial systems such as cobalt multilayers or Pd/Fe bilayers on top of Ir(111), known for stabilising Neel-like skyrmions, and, recently, alloys with a crystallographic symmetry where anti-skyrmions are stabilised. Micromagnetic simulations have become a standard approach to aid the design and optimisation of spintronic and magnetic nanodevices and are also applied to the modelling of device applications which make use of skyrmions. Several public domain micromagnetic simulation packages such as OOMMF, MuMax3 and Fidimag already offer implementations of different DMI terms. It is therefore highly desirable to propose a so-called micromagnetic standard problem that would allow one to benchmark and test the different software packages in a similar way as is done for ferromagnetic materials without DMI. Here, we provide a sequence of well-defined and increasingly complex computational problems for magnetic materials with DMI. Our test problems include 1D, 2D and 3D domains, spin wave dynamics in the presence of DMI, and validation of the analytical and numerical solutions including uniform magnetisation, edge tilting, spin waves and skyrmion formation. This set of problems can be used by developers and users of new micromagnetic simulation codes for testing and validation and hence establishing scientific credibility.

## Topological surface state of α-Sn on InSb(001) as studied by photoemission

Physical Review B American Physical Society 97 (2018) 075101

L Dudy, F Reis, F Adler, J Aulbach, LJ Collins-McIntyre, LB Duffy, HF Yang, YL Chen, T Hesjedal, ZK Liu, M Hoesch, S Muff, JH Dil, J Schaefer, R Claessen

We report on the electronic structure of the elemental topological semimetal α − Sn on InSb(001). High-resolution angle-resolved photoemission data allow us to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional p -type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We show that, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.

## Crossover from lattice to plasmonic polarons of a spin-polarised electron gas in ferromagnetic EuO

Nature Communications Springer Nature 9 (2018) 2305

JM Riley, F Caruso, C Verdi, LB Duffy, L Bawden, K Volckaert, G van der Laan, T Hesjedal, M Hoesch, F Giustino, PDC King

Strong many-body interactions in solids yield a host of fascinating and potentially useful physical properties. Here, from angle-resolved photoemission experiments and ab initio many-body calculations, we demonstrate how a strong coupling of conduction electrons with collective plasmon excitations of their own Fermi sea leads to the formation of plasmonic polarons in the doped ferromagnetic semiconductor EuO. We observe how these exhibit a significant tunability with charge carrier doping, leading to a polaronic liquid that is qualitatively distinct from its more conventional lattice-dominated analogue. Our study thus suggests powerful opportunities for tailoring quantum many-body interactions in solids via dilute charge carrier doping.