Real-Space Observation of Skyrmionium in a Ferromagnet-Magnetic Topological Insulator Heterostructure.

Nano letters ACS 18 (2018) 1057-1063

S Zhang, F Kronast, G van der Laan, T Hesjedal

The combination of topological insulators, i.e., bulk insulators with gapless, topologically protected surface states, with magnetic order is a love-hate relationship that can unlock new quantum states and exotic physical phenomena, such as the quantum anomalous Hall effect and axion electrodynamics. Moreover, the unusual coupling between topological insulators and ferromagnets can also result in the formation of topological spin textures in the ferromagnetic layer. Skyrmions are topologically-protected magnetization swirls that are promising candidates for spintronics memory carriers. Here, we report on the observation of skyrmionium in thin ferromagnetic films coupled to a magnetic topological insulator. The occurrence of skyrmionium, which appears as a soliton composed of two skyrmions with opposite winding numbers, is tied to the ferromagnetic state of the topological insulator. Our work presents a new combination of two important classes of topological materials and may open the door to new topologically inspired information-storage concepts in the future.

Direct Observation of Twisted Surface skyrmions in Bulk Crystals.

Physical review letters 120 (2018) 227202-

SL Zhang, G van der Laan, WW Wang, AA Haghighirad, T Hesjedal

Magnetic skyrmions in noncentrosymmetric helimagnets with D_{n} 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 an 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 Cu_{2}OSeO_{3}, 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.

Electronic Structure and Enhanced Charge-Density Wave Order of Monolayer VSe2.

Nano letters (2018)

J Feng, D Biswas, A Rajan, MD Watson, F Mazzola, OJ Clark, K Underwood, I Marković, M McLaren, A Hunter, DM Burn, LB Duffy, S Barua, G Balakrishnan, F Bertran, P Le Fèvre, TK 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.

Microscopic effects of Dy doping in the topological insulator Bi2Te3

PHYSICAL REVIEW B 97 (2018) ARTN 174427

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

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

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

S Zhang, G van der Laan, J Müller, 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


LB Duffy, A Frisk, DM Burn, N-J Steinke, J Herrero-Martin, A Ernst, G van der Laan, T Hesjedal

Manipulation of skyrmion motion by magnetic field gradients.

Nature communications 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.

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

Physical review B: Condensed matter and materials physics American Physical Society 97 (2018) 075101

MR Scholz, 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

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

Nature communications 9 (2018) 2305-

JM Riley, F Caruso, C Verdi, LB Duffy, MD Watson, 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.

Probing the Topological Surface State in Bi2Se3 Thin Films Using Temperature-Dependent Terahertz Spectroscopy

ACS PHOTONICS 4 (2017) 2711-2718

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

Codoping of Sb2Te3 thin films with V and Cr


LB Duffy, AI Figueroa, G van der Laan, T Hesjedal

Correction to Step-Flow Growth of Bi 2 Te 3 Nanobelts

Crystal Growth & Design 17 (2017) 1438-1438

P Schönherr, T Tilbury, H Wang, AA Haghighirad, V Srot, PA van Aken, T Hesjedal

Ultrahigh magnetic field spectroscopy reveals the band structure of the three-dimensional topological insulator Bi2Se3

PHYSICAL REVIEW B 96 (2017) ARTN 121111

A Miyata, Z Yang, A Surrente, O Drachenko, DK Maude, O Portugall, LB Duffy, T Hesjedal, P Plochocka, RJ Nicholas

Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations


A Baker, M Beg, G Ashton, M Albert, D Chernyshenko, W Wang, S Zhang, M-A Bisotti, M Franchin, CL Hu, R Stamps, T Hesjedal, H Fangohr

Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations

Journal of Magnetism and Magnetic Materials 421 (2017) 428

AA Baker, M Beg, G Ashton, M Albert, D Chernyshenko, W Wang, S Zhang, M-A Bisotti, M Franchin, CL Hu, R Stamps, T Hesjedal, H Fangohr

Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with OOMMF and Nmag simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general.

X-ray magnetic circular dichroism study of Dy-doped Bi2Te3 topological insulator thin films


AI Figueroa, AA Baker, SE Harrison, K Kummer, G van der Laan, T Hesjedal

Direct experimental determination of spiral spin structures via the dichroism extinction effect in resonant elastic soft x-ray scattering

PHYSICAL REVIEW B 96 (2017) ARTN 094401

SL Zhang, G van der Laan, T Hesjedal

Perfect quintuple layer Bi2Te3 nanowires: Growth and thermoelectric properties

APL MATERIALS 5 (2017) ARTN 086110

P Schonherr, D Kojda, V Srot, SF Fischer, PA van Aken, T Hesjedal

Emergence of Dirac-like bands in the monolayer limit of epitaxial Ge films on Au(1 1 1)

2D Materials 4 (2017) 031005

NBM Schröter, MD Watson, LB Duffy, M Hoesch, Y Chen, T Hesjedal, TK Kim

After the discovery of Dirac fermions in graphene, it has become a natural question to ask whether it is possible to realize Dirac fermions in other two-dimensional (2D) materials as well. In this work, we report the discovery of multiple Dirac-like electronic bands in ultrathin Ge flms grown on Au(1 1 1) by angle-resolved photoelectron spectroscopy. By tuning the thickness of the flms, we are able to observe the evolution of their electronic structure when passing through the monolayer limit. Our discovery may signify the synthesis of germanene, a 2D honeycomb structure made of Ge, which is a promising platform for exploring exotic topological phenomena and enabling potential applications.