# Publications by Thorsten Hesjedal

## Field and temperature dependence of the skyrmion lattice phase in chiral magnet membranes

Physical review B: Condensed matter and materials physics American Physical Society (0)

D Burn, S Wang, W Wang, G Van Der Laan, S Zhang, H Du, T Hesjedal

Magnetic skyrmions are nanosized magnetization whirls that exhibit topological robustness and nontrivial magnetoelectrical properties, such as emergent electromagnetism and intriguing spin dynamics in the microwave-frequency region. In chiral magnets, skyrmions are usually found at a pocket in the phase diagram in the vicinity of the ordering temperature, wherein they order in the form of a hexagonal skyrmion lattice (SkL). It is generally believed that this equilibrium SkL phase is a uniform, long-range-ordered magnetic structure with a well-defined lattice constant. Here, using high-resolution small angle resonant elastic x-ray scattering, we study the field- and temperature-dependence of the skyrmion lattice in FeGe and membranes. Indeed, shows the expected rigid skyrmion lattice, known from bulk samples, that is unaffected by tuning field and temperature within the phase pocket. In stark contrast, the lattice constant and skyrmion size in FeGe membranes undergo a continuous evolution within the skyrmion phase pocket, whereby the lattice constant changes by up to 15% and the magnetic scattering intensity varies significantly. Using micromagnetic modeling, it is found that for FeGe the competing energy terms contributing to the formation of the skyrmion lattice fully explain this breathing behavior. In contrast, for this stabilizing energy balance is less affected by the smaller field variation across the skyrmion pocket, leading to the observed rigid lattice structure.

## Magnetic skyrmion interactions in the micromagnetic framework

arxiv (0)

GVD Laan, R Brearton, T Hesjedal

Magnetic skyrmions are localized swirls of magnetization with a non-trivial topological winding number. This winding increases their robustness to superparamagnetism and gives rise to a myriad of novel dynamical properties, making them attractive as next-generation information carriers. Recently the equation of motion for a skyrmion was derived using the approach pioneered by Thiele, allowing for macroscopic skyrmion systems to be modeled efficiently. This powerful technique suffers from the prerequisite that one must have a priori knowledge of the functional form of the interaction between a skyrmion and all other magnetic structures in its environment. Here we attempt to alleviate this problem by providing a simple analytic expression which can generate arbitrary repulsive interaction potentials from the micromagnetic Hamiltonian. We also discuss a toy model of the radial profile of a skyrmion which is accurate for a wide range of material parameters.

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

arxiv (0)

JA Krieger, A Pertsova, SR Giblin, M Döbeli, T Prokscha, CW Schneider, A Suter, T Hesjedal, AV 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, Bi$_2$Se$_3$, 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 Bi$_2$Se$_3$ 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.

## Diameter-independent skyrmion Hall angle observed in chiral magnetic multilayers

Nature Communications Nature Research (part of Springer Nature) (0)

K Zeissler, S Finizio, C Barton, A Huxtable, J Massey, J Raabe, A Sadovnikov, S Nikitov, R Brearton, T Hesjedal, G van der Laan, M Rosamond, E Linfield, G Burnell, C Marrows

## Coherent transfer of spin angular momentum by evanescent spin waves within antiferromagnetic NiO

arxiv (0)

DG Newman, M Dabrowski, A Frisk, T Nakano, ZQ Qiu, THORSTEN Hesjedal, E Arenholz, RJ Hicken, P Shafer, GVD Laan, DM Burn, C Klewe, M Yang, Q Li

Insulating antiferromagnets are efficient and robust conductors of spin current. To realise the full potential of these materials within spintronics, the outstanding challenges are to demonstrate scalability down to nanometric lengthscales and the transmission of coherent spin currents. Here, we report the coherent transfer of spin angular momentum by excitation of evanescent spin waves of GHz frequency within antiferromagnetic NiO at room temperature. Using element-specific and phase-resolved x-ray ferromagnetic resonance, we probe the injection and transmission of ac spin current, and demonstrate that insertion of a few nanometre thick epitaxial NiO(001) layer between a ferromagnet and non-magnet can even enhance the flow of spin current. Our results pave the way towards coherent control of the phase and amplitude of spin currents at the nanoscale, and enable the realization of spin-logic devices and spin current amplifiers that operate at GHz and THz frequencies.

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

arxiv Museu de Ciències Naturals de Barcelona (0)

MR Scholz, VA Rogalev, 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 Schäfer, R Claessen

We report on the electronic structure of the elemental topological semimetal $\alpha$-Sn on InSb(001). High-resolution angle-resolved photoemission data allow 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.

## Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

arXiv:1511.04972v1 (0)

T Lancaster, F Xiao, Z Salman, IO Thomas, SJ Blundell, FL Pratt, SJ Clark, T Prokscha, A Suter, SL Zhang, AA Baker, T Hesjedal

We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around T ≈ 40 K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.

## Direct visualization of the oscillation of Au (111) surface atoms

Applied Physics Letters 69 (1996) 354-356

T Hesjedal, E Chilla, HJ Fröhlich

A high frequency oscillating Au (111) surface was measured with atomic resolution using a modified scanning tunneling microscope. On the atomic scale propagating surface acoustic waves lead to oscillations of atoms on elliptical trajectories, with the axes being determined by the material parameters of the surface. Since those oscillation frequencies are much higher than the scan frequencies the topography contrast is reduced. This basic problem is solved by measuring a stroboscopic snapshot seeing a defined state of oscillation. The atomic resolution of the phase and the amplitude contrast is explained by the superposition of the surface topography and the oscillation trajectory. © 1996 American Institute of Physics.

## Microprobe techniques in SAW measurements

Proceedings of the XI Intern. Microwave Conf. MIKON-96 (1996) 107-111

E Chilla, T Hesjedal, HJ Froehlich

## Microprobe techniques in SAW measurements

Archives of Acoustics Polish Academy of Sciences 21 (1996) 195-200

E Chilla, T Hesjedal, HJ Froehlich

Starting from the conventional microprobe techniques, a scanning acoustic tunneling microscope (SATM) and a scanning acoustic force microscope (SAFM) have been developed to detect particle displacements at solid surfaces up to GHz frequencies. Based on the non-linear dependence of the tunneling current in SATM and of the forces in SAFM on the tip to surface distance, respectively, it is demonstrated that wave field parameters of surface acoustic waves can be measured with a lateral resolution in the submicrometer range.

## Wellenfelder in Interdigitalwandlern und ihre Abstrahlung

Fortschritte der Akustik – DAGA96 DEGA (1996) 434-435

T Hesjedal, E Chilla, HJ Fröhlich

## Determination of SAW phase velocities on the nanoscale

Proceedings of the IEEE Ultrasonics Symposium 2 (1996) 811-814

T Hesjedal, E Chilla, HJ Froehlich

This paper reports about the first determination of the phase velocity of surface acoustic waves (SAWs) on the nanometer scale. With a scanning acoustic force microscope (SAFM) a maximum lateral resolution of 19.9 nm has been achieved. This is almost two orders of magnitude better than the resolution of standard quantitative acoustic microscopy. The key of measuring the phase of high frequency signals with a slowly responding SAFM cantilever is frequency mixing at its non-linear force curve. For demonstrating its abilities SAW dispersion was studied on Au layers of different thicknesses by SAFM over a lateral distance of down to 200 nm.

## Submicron IDT wave field investigation by scanning acoustic force microscopy

Proceedings of the IEEE Ultrasonics Symposium 2 (1996) 815-818

T Hesjedal, E Chilla, HJ Froehlich

We report about a new technique for the investigation of SAW fields within SAW devices reaching submicron lateral resolution. The scanning acoustic force microscope (SAFM) is based on a standard force microscope and utilizes the nonlinear force curve in the sense of a mechanical diode. Varying wave amplitudes therefore lead to different shifts of the cantilever's rest position. With SAFM we investigated SAW devices with center frequencies above 600 MHz. We found a local effect of massloading on the standing wave amplitude within IDTs. Furthermore, we measured the dynamic behavior of the IDT's wave pattern when sweeping the frequency.

## Probing of oscillating surfaces by a scanning acoustic tunneling microscope

Thin Solid Films 264 (1995) 226-229

T Hesjedal, E Chilla, HJ Fröhlich

The scanning acoustic tunneling microscope (SATM) which is based on a scanning tunneling microscope, is capable of detecting the amplitude and the phase of high-frequency surface acoustic waves (SAWs) as well as the surface topography. For our experiments the SAWs have been excited by interdigital transducers on YZ-LiNbO3 samples. A thin gold film with a thickness of about 100 nm has been deposited on its surface. The read-out of the high-frequency acoustic wave field is performed by a mixing technique. Owing to the non-linear dependence of the tunneling current on the tip-to-sample distance an additional slightly-shifted high-frequency modulation of the gap voltage leads to an increase of the d.c. tunneling current and to a low frequency signal at the difference frequency. The a.c. tunneling current contains the information on the elastic properties of the solid. Thin films deposited on the surface cause a dispersion of the phase velocity, which then reveals the elastic parameters. By measuring the phase and the amplitude of an acoustic wave field with high spatial resolution the SATM technique allows the mapping of the acoustic wave field and thereby the investigation of the elastic properties of thin films. © 1995.

## Scanning acoustic force microscope measurements on grating-like electrodes

Applied Physics A Materials Science &amp; Processing 61 (1995) 237-242

T Hesjedal, E Chilla, HJ Fröhlich

Grating-like metal electrodes supplied with high- and low-frequency voltages were investigated by a Scanning Acoustic Force Microscope (SAFM). On piezoelectric LiNbO3, these electrodes form an interdigital transducer which generates surface acoustic waves according to the charge distribution on its electrodes when excited at a transducer's passband frequency. The influence of surface deformation and of surface charges on the cantilever deflection of the SAFM in contact and non-contact mode was systematically studied. Comparing the images of the electrodes which were prepared on piezoelectric and dielectric substrates, surface deformation and charge distributions were qualitatively separated. © 1995 Springer-Verlag.

## Scanning microprobe techniques for high resolution measurements of surface acoustic waves

Proceedings of the 1995 World Congress on Ultrasonics (1995) 395-398

E Chilla, T Hesjedal, HJ Fröhlich

## Messungen an Interdigitalwandlern mit dem akustischen Kraftmikroskop

Fortschritte der Akustik - DAGA95 DEGA (1995) 731-734

T Hesjedal, E Chilla, HJ Fröhlich

## Scanning acoustic force microscope detection of SAWs

Proceedings of the IEEE Ultrasonics Symposium 1 (1994) 363-366

E Chilla, T Hesjedal, HJ Froehlich

We present a novel method for the investigation of surface acoustic wave (SAW) fields with nanometer resolution by a scanning acoustic force microscope (SAFM). The detection of ultrasound by a force microscope is connected with the nonlinear dependence of the force on the distance between the tip and the surface. Due to this nonlinearity there is an effective shift of the mean position of the tip if one modulates the gap distance by a propagating SAW. Furthermore, the surface charges influence the tip deflection. By operating the SAFM in the dynamic mode the wave amplitude as well as the charge distribution can be measured at common surface structures. The measurements were carried out by a contact and a non-contact mode scanning force microscope (SFM). The SAWs were excited by an interdigital transducer (IDT) on a YZ-LiNbO3 crystal cut at 30.75 MHz (contact) and 39.5 MHz (non-contact). The amplitude of the SAW was modulated by a low frequency (5 Hz and 50 kHz). The low frequency oscillation of the average of the tip-to-surface distance was measured by lock-in technique in dependence on the amplitude and the frequency of the SAW. The measurements of the amplitude of the SAW and the charge distribution were carried out within the IDT.

## Epitaxial Heusler Alloys on III-V Semiconductors

John Wiley & Sons, Ltd (0)

T Hesjedal, KH Ploog

## A Simple and Novel Approach to Fabricating Microfluidic Components Actuated By Termoresponsive Hydrogels

(0)

ME Harmon, MX Tang, T Hesjedal, CW Frank