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Magnetic skyrmions

Professor Thorsten Hesjedal FInstP

Professor of Condensed Matter Physics

Research theme

  • Quantum materials

Sub department

  • Condensed Matter Physics

Research groups

  • Thin film quantum materials
  • Oxford Quantum Institute
Thorsten.Hesjedal@physics.ox.ac.uk
Telephone: 01865 (2)72235
  • About
  • Publications

Probing the topological surface state in Bi₂Se₃ thin films using temperature-dependent terahertz spectroscopy

ACS Photonics American Chemical Society 4:11 (2017) 2711-2718

Authors:

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

Abstract:

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

PHYSICAL REVIEW B 96:12 (2017) ARTN 121111

Authors:

A Miyata, Z Yang, A Surrente, O Drachenko, DK Maude, O Portugall, LB Duffy, T Hesjedal, P Plochocka, RJ Nicholas
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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 96:9 (2017) 094401

Authors:

SL Zhang, G van der Laan, Thorsten Hesjedal

Abstract:

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.
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Synthesis of superconductor-topological insulator nanoribbon heterostructures

Nano World Scientific Publishing 12:8 (2017) 1750095

Authors:

Piet Schönherr, Fengyu Zhang, Vesna Srot, Peter van Aken, Thorsten Hesjedal

Abstract:

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.
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Perfect quintuple layer Bi₂Te₃ nanowires: Growth and thermoelectric properties

APL Materials American Institute of Physics 5 (2017) 086110

Authors:

Piet Schoenherr, D Kojda, V Srot, SF Fischer, PA van Aken, Thorsten Hesjedal

Abstract:

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.
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