Publications by Thorsten Hesjedal


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.


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 American Physical Society 120 (2018) 227202

S Zhang, G van der Laan, A Haghighirad, WW Wang, 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.


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, K Underwood, OJ Clark, I Marckovic, M McLaren, A Hunter, L Duffy, DM Burn, S Barua, G Balakrishnan, P LeFevre, F Bertran, T Kim, T Hesjedal, G van der Laan, 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 American Physical Society 97 (2018) 174427

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


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

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, G van der Laan, T Hesjedal, J Herrero-Martin, A Ernst

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.


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, T Kluyver, G Downing, R Pepper, 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, T Hesjedal, L Bawden, G van der Laan, K Volckaert, F Giustino, M Hoesch, 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 American Chemical Society 4 (2017) 2711-2718

HE Beere, L Duffy, T Hesjedal, DA Ritchie, VS Kamboj, A Singh, T Ferrus, C Barnes

Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (&lt; 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm2/V•s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×1012 cm-2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state.


Codoping of Sb 2 Te 3 thin films with V and Cr

Physical Review Materials American Physical Society 1 (2017) 064409

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

Magnetically doped topological insulators (TIs) are key to realizing the quantum anomalous Hall (QAH) effect, with the prospect of enabling dissipationless electronic devices in the future. Doping of the well-established three-dimensional TIs of the (Bi,Sb)2(Se,Te)3 family with the transition metals Cr and V is now an established approach for observing the QAH state at very low temperatures. While the magnetic transition temperatures of these materials is on the order of 10’s of K, full quantization of the QAH state is achieved below ∼100 mK, governed by the size of the magnetic gap and thus the out-of-plane magnetic moment. In an attempt to raise the size of the magnetic moment and transition temperature, we carried out a structural and magnetic investigation of codoped (V,Cr):Sb2Te3 thin films. Starting from singly doped Cr:Sb2Te3 films, free of secondary phases and with a transition temperature of ∼72 K, we introduced increasing fractions of V and found a doubling of the transition temperature, while the magnetic moment decreases. In order to separate the properties and contributions of the two transition metals in the complex doping scenario independently, we employed spectroscopic x-ray techniques. Surprisingly, already small amounts of V lead to the formation of the secondary phase Cr2Te3. No V was detectable in the Sb2Te3 matrix. Instead, it acts as a surfactant and can be found in the near-surface layers at the end of the growth. Our study highlights the importance of x-ray-based studies for the doping of van der Waals systems, for which the optimization of magnetic moment or transition temperature alone is not necessarily a good strategy.


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


Thermoelectric measurement of a single, TiO₂-catalyzed Bi₂Te₃ nanowire

Proceedings MDPI 1 (2017) 311-

HS Moosavi, D Kojda, M Kockert, P Schoenherr, T Hesjedal, SF Fischer, M Kroener, P Woias

We report on the functionality of our Thermoelectric Nanowire Characterization Platform (TNCP). As a proof of concept of our design, we present a set of experimental results obtained from the characterization of a single Bi2Te3 nanowire, allowing for the determination of the nanowire’s electrical conductivity and Seebeck coefficient.


Synthesis of superconductor-topological insulator nanoribbon heterostructures

Nano World Scientific Publishing 12 (2017) 1750095

P Schönherr, F Zhang, V Srot, T Hesjedal, P van Aken

Superconductors in proximity to topological insulators (TIs) have the potential to unlock exotic quantum phenomena, such as Majorana fermions. Quasi-one dimensional structures are particularly suited to host these quantum states. Despite the growth of TI nanostructures being relatively straightforward, the in-situ synthesis of superconductor-TI structures has been challenging. Here, we present a systematic study of the growth of the s-wave superconductor Sn on the TI Bi2Te3 by physical vapor transport. If Sn does not enter the Bi2Te3 lattice as a dopant, two types of structures are formed: Sn nanoparticles, that cover Bi2Te3 plates and belts in a cloud-like shape, and thin Sn layers on Bi2Te3 plates, that appear in puddle-like recessions. These heterostructures have potential applications as novel quantum devices.


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

Physical Review B American Physical Society 96 (2017) 094401

SL Zhang, G van der Laan, T Hesjedal

Long-wavelength spin spiral structures are ubiquitous in a large variety of magnetic materials. The detailed magnetic structure can take many variations owing to their different physical origins. Therefore, the unambiguous structural determination is crucial for understanding these spin systems, though such a task is experimentally challenging. Here we show that ordered spin spiral structures can be fully determined in a single measurement by dichroic resonant elastic x-ray scattering using circularly polarized light. It is found that at certain geometrical conditions, the circular dichroism of the diffraction vanishes completely, revealing a one-to-one correspondence with the spin structure. We demonstrate both theoretically and experimentally this experimental principle, which allows for unambiguous structure determination immediately from the measured signal, whereby no modeling- based data refinement is needed. This largely expands the capabilities of conventional magnetic characterization techniques.


Perfect quintuple layer Bi₂Te₃ nanowires: Growth and thermoelectric properties

APL Materials American Institute of Physics 5 (2017) 086110-

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

Bi2Te3 nanowires are promising candidates for thermoelectric applications. Vapor-liquid-solid growth of these nanowires is straightforward, but the traditional Au-catalyzed method is expected to lead to Au contamination and subsequently crystal defects. Here, we present a comparison of the Au-catalyzed growth method with an alternative method using TiO2. We observe that the latter approach results in perfect quintuple layer nanowires, whilst using Au leads to mixed quintuple and septuple layer structures. Despite these differences, we surprisingly find only a negligible effect on their thermoelectric properties, namely conductivity and Seebeck coefficient. This result is relevant for the further optimization and engineering of thermoelectric nanomaterials for device applications.


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.

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