arXiv subjects

Peter Werner

Publications and source records attributed to Peter Werner.

31 records · Page 2Linked to original sources

arXiv2019

Vision-Based Proprioceptive Sensing for Soft Inflatable Actuators

This paper presents a vision-based sensing approach for a soft linear actuator, which is equipped with an integrated camera. The proposed vision-based sensing pipeline predicts the three-dimensional position of a point of interest on the actuator. To train and evaluate the algorithm, predictions are compared to ground truth data from an external motion capture system. An off-the-shelf distance sensor is integrated in a similar actuator and its performance is used as a baseline for comparison. The resulting sensing pipeline runs at 40 Hz in real-time on a standard laptop and is additionally used for closed loop elongation control of the actuator. It is shown that the approach can achieve comparable accuracy to the distance sensor.

cs.RO↗

arXiv2019

Extremely high conductivity observed in the triple point topological metal MoP

Weyl and Dirac fermions have created much attention in condensed matter physics and materials science. Recently, several additional distinct types of fermions have been predicted. Here, we report ultra-high electrical conductivity in MoP at low temperature, which has recently been established as a triple point Fermion material. Here we show that the electrical resistivity is 6 n-ohm cm at 2 K with a large mean free path of 11 microns. de Haas-van Alphen oscillations reveal spin splitting of the Fermi surfaces. In contrast to noble metals with similar conductivity and number of carriers, the magnetoresistance in MoP does not saturate up to 9 T at 2 K. Interestingly, the momentum relaxing time of the electrons is found to be more than 15 times larger than the quantum coherence time. This difference between the scattering scales shows that momentum conserving scattering dominates in MoP at low temperatures.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the non-collinear antiferromagnet Mn$_{3}$Ir

Non-collinear antiferromagnets, with either an L1$_{2}$ cubic crystal lattice (e.g. Mn$_{3}$Ir and Mn$_{3}$Pt) or a D0$_{19}$ hexagonal structure (e.g. Mn$_{3}$Sn and Mn$_{3}$Ge), exhibit a number of novel phenomena of interest to topological spintronics. Amongst the cubic systems, for example, tetragonally distorted Mn$_{3}$Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn$_{3}$Pt only enters a non-collinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to Mn$_{3}$Ir, the material of choice for use in exchange bias heterostructures. In this paper, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic $γ$-Mn$_{3}$Ir thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (2.24 $\pm$ 0.08) $\times$ 10$^{23}$ cm$^{-3}$. On the other hand, TEM characterization demonstrates that ultrathin 3 nm films grow with significant in-plane tensile strain. This may explain a small remanent moment, observed at low temperatures, shown by XMCD spectroscopy to arise from uncompensated Mn spins. Of the order 0.02 $μ_{B}$ / atom, this dominates electrical transport behavior, leading to a small AHE and negative magnetoresistance. These results are discussed in terms of crystal microstructure and chiral domain behavior, with spatially resolved XML(C)D-PEEM supporting the conclusion that small antiferromagnetic domains, < 20 nm in size, of differing chirality account for the absence of observed Berry curvature driven magnetotransport effects.

cond-mat.mtrl-sci↗

arXiv2019

Epitaxial growth, structural characterization and exchange bias of non-collinear antiferromagnetic Mn$_{3}$Ir thin films

Antiferromagnetic materials are of great interest for spintronics. Here we present a comprehensive study of the growth, structural characterization, and resulting magnetic properties of thin films of the non-collinear antiferromagnet Mn$_{3}$Ir. Using epitaxial engineering on MgO (001) and Al$_{2}$O$_{3}$ (0001) single crystal substrates, we control the growth of cubic $γ$-Mn$_{3}$Ir in both (001) and (111) crystal orientations, and discuss the optimization of growth conditions to achieve high-quality crystal structures with low surface roughness. Exchange bias is studied in bilayers, with exchange bias fields as large as -29 mT (equivalent to a unidirectional anisotropy constant of 11.5 nJ cm$^{-2}$) measured in Mn$_{3}$Ir (111) / permalloy heterostructures at room temperature. In addition, a distinct dependence of blocking temperature on in-plane crystallographic direction in Mn$_{3}$Ir (001) / Py bilayers is observed. These findings are discussed in the context of chiral antiferromagnetic domain structures, and will inform progress towards topological antiferromagnetic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Thermoelectric signatures of the electron-phonon fluid in PtSn4

In most materials, transport can be described by the motion of distinct species of quasiparticles, such as electrons and phonons. Strong interactions between quasiparticles, however, can lead to collective behaviour, including the possibility of viscous hydrodynamic flow. In the case of electrons and phonons, an electron-phonon fluid is expected to exhibit strong phonon-drag transport signatures and an anomalously low thermal conductivity. The Dirac semi-metal PtSn4 has a very low resistivity at low temperatures and shows a pronounced phonon drag peak in the low temperature thermopower; it is therefore an excellent candidate for hosting a hydrodynamic electron-phonon fluid. Here we report measurements of the temperature and magnetic field dependence of the longitudinal and Hall electrical resistivities, the thermopower and the thermal conductivity of PtSn4. We confirm a phonon drag peak in the thermopower near 14 K and observe a concurrent breakdown of the Lorenz ratio below the Sommerfeld value. Both of these facts are expected for an electron-phonon fluid with a quasi-conserved total momentum. A hierarchy between momentum-conserving and momentum-relaxing scattering timescales is corroborated through measurements of the magnetic field dependence of the electrical and Hall resistivity and of the thermal conductivity. These results show that PtSn4 exhibits key features of hydrodynamic transport.

cond-mat.mtrl-sci↗

arXiv2017

Discovery of Magnetic Antiskyrmions Beyond Room Temperature in Tetragonal Heusler Materials

Skyrmions, topologically stable spin textures, are of great interest for new generations of spintronic devices. In general, the stabilization of skyrmions has been achieved in systems with broken inversion symmetry, where the asymmetric Dzyaloshinskii-Moriya interaction (DMI) modifies the uniform magnetic state to a swirling state. Depending on the crystal symmetries, two distinct types of skyrmions, Bloch and Néel types, have been observed experimentally. Here, we present, the experimental manifestation of a special type of spin-swirling, namely antiskyrmions, in a family of acentric tetragonal Heusler compounds with D2d crystal symmetry. A spiral magnetic ground-state, which propagates in the tetragonal basal plane, is transformed into a skyrmion lattice-state under magnetic fields applied along the tetragonal axis over a wide temperature interval. Direct imaging by Lorentz Transmission Electron Microscopy (LTEM) shows field stabilized antiskyrmion lattices and isolated antiskyrmions between 100 K and 400 K, and zero-field metastable antiskyrmions at low temperatures.

cond-mat.mtrl-sci↗

arXiv2017

Pressure-induced superconductivity and topological quantum phase transitions in a quasi-one-dimensional topological insulator: Bi4I4

Superconductivity and topological quantum states are two frontier fields of research in modern condensed matter physics. The realization of superconductivity in topological materials is highly desired, however, superconductivity in such materials is typically limited to two- or three-dimensional materials and is far from being thoroughly investigated. In this work, we boost the electronic properties of the quasi-one-dimensional topological insulator bismuth iodide \b{eta}-Bi4I4 by applying high pressure. Superconductivity is observed in \b{eta}-Bi4I4 for pressures where the temperature dependence of the resistivity changes from a semiconducting-like behavior to that of a normal metal. The superconducting transition temperature Tc increases with applied pressure and reaches a maximum value of 6 K at 23 GPa, followed by a slow decrease. Our theoretical calculations suggest the presence of multiple pressure-induced topological quantum phase transitions as well as a structural-electronic instability.

cond-mat.supr-con↗

arXiv2017

Antiferromagnetic structure and electronic properties of BaCr2As2 and BaCrFeAs2

The chromium arsenides BaCr2As2 and BaCrFeAs2 with ThCr2Si2 type structure (space group I4/mmm; also adopted by '122' iron arsenide superconductors) have been suggested as mother compounds for possible new superconductors. DFT-based calculations of the electronic structure evidence metallic antiferromagnetic ground states for both compounds. By powder neutron diffraction we confirm for BaCr2As2 a robust ordering in the antiferromagnetic G-type structure at T_N = 580 K with mu_Cr = 1.9 mu_B at T = 2K. Anomalies in the lattice parameters point to magneto-structural coupling effects. In BaCrFeAs2 the Cr and Fe atoms randomly occupy the transition-metal site and G-type order is found below 265 K with mu_Cr/Fe = 1.1 mu_B. 57Fe Moessbauer spectroscopy demonstrates that only a small ordered moment is associated with the Fe atoms, in agreement with electronic structure calculations with mu_Fe ~ 0. The temperature dependence of the hyperfine field does not follow that of the total moments. Both compounds are metallic but show large enhancements of the linear specific heat coefficient gamma with respect to the band structure values. The metallic state and the electrical transport in BaCrFeAs2 is dominated by the atomic disorder of Cr and Fe and partial magnetic disorder of Fe. Our results indicate that Neel-type order is unfavorable for the Fe moments and thus it is destabilized with increasing iron content.

cond-mat.supr-con↗

arXiv2017

Separate magnetization switching of hexagonal Co/BN/Co junctions grown epitaxially on c-sapphire

Magnetic tunnel junctions (MTJ) have been grown by using molecular beam epitaxy on c-plane Al2O3 substrates. The MTJ stacks consist of two ferromagnetic hcp-Co layers separated by a thin insulating h-BN barrier. The samples have been grown in a single run revealing single crystalline epitaxial structures with sharp interfaces as observed by applying transmission electron microscopy. The in-plane magnetization experiments have revealed separate magnetization switching of a thin top Co (soft) layer and a thick bottom Co (hard) layer. At zero magnetic field the two Co layers are found in an antiparallel state.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity in Weyl Semimetal Candidate MoTe2

In recent years, layered transition-metal dichalcogenides (TMDs) have attracted considerable attention because of their rich physics; for example, these materials exhibit superconductivity, charge density waves, and the valley Hall effect. As a result, TMDs have promising potential applications in electronics, catalysis, and spintronics. Despite the fact that the majority of related research focuses on semiconducting TMDs (e.g., MoS2), the characteristics of WTe2 are provoking strong interest in semimetallic TMDs with extremely large magnetoresistance, pressure-driven superconductivity, and the predicted Weyl semimetal (WSM) state. In this work, we investigate the sister compound of WTe2, MoTe2, which is also predicted to be a WSM and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that MoTe2 exhibits superconductivity with a resistive transition temperature Tc of 0.1 K. The application of a small pressure (such as 0.4 GPa) is shown to dramatically enhance the Tc, with a maximum value of 8.2 K being obtained at 11.7 GPa (a more than 80-fold increase in Tc). This yields a dome-shaped superconducting phase diagram. Further explorations into the nature of the superconductivity in this system may provide insights into the interplay between strong correlations and topological physics.

cond-mat.mtrl-sci↗

arXiv2015

Ellipsometry studies of Si/Ge superlattices with embedded Ge dots

In this paper, we present an analysis for treating the spectroscopic ellipsometry response of Si/Ge superlattices (SL) with embedded Ge dots. Spectroscopic ellipsometry (SE) measurement at room temperature was used to investigate optical and electronic properties of Si/Ge SL which were grown on silicon (Si) wafers having <111> crystallographic orientation. The results of the SE analysis between 200 nm and 1000 nm indicate that the SL system can effectively be described using interdiffusion/intermixing model by assuming a multicrystalline Si and Si1-xGex intermixing layers. The electronic transitions deduced from analysis reveal Si, Ge and alloying related critical energy points.

cond-mat.mes-hall↗

arXiv2015

Near-IR photoluminescence from Si/Ge nanowire grown silicon wafers: effect of HF treatment

We present the room temperature (RT) near-infrared (NIR) photoluminescence (PL) properties of Si/Ge nanowire (NW) grown silicon wafers which were treated by vapor of HF:HNO3 chemical mixture. This treatment activates or enhances the PL intensity at NIR region ranging from 1000 nm to 1800 nm. The PL consists of a silicon band-edge emission and a broad composite band which was centered at around 1400-1600 nm. The treatment modifies the wafer surface particularly at defect sites particularly pits around NWs and NW surfaces by etching and oxidation of Si and Ge. This process can induce spatial confinement of carriers where band-to-band (BB) emission is the dominant property in Si capped strained Si/Ge NW grown wafers. Strong signals were observed at sub-band gap energies in Ge capped Si/Ge NW grown wafers. It was found that NIR PL is a competitive property between the Si BB transition and deep-level emission which is mainly attributable to Si related defects, Ge dots and strained Ge layers. The enhancement in BB and deep-level PL was discussed in terms of strain, oxygen related defects, dots formation and carrier confinement effects. The results demonstrate the effectiveness of this method in enhancing and tuning NIR PL properties for possible applications.

cond-mat.mes-hall↗

arXiv2015

Photoluminescence from Silicon nanoparticles embedded in ammonium silicon hexafluoride

Silicon (Si) nanoparticles (NPs) were synthesized by transforming Si wafer surface to ammonium silicon hexafluoride (ASH) or (NH4)2SiF6 under acid vapor treatment. Si-NPs are embedded within the polycrystalline (ASH) layer formed on the Si surface exhibit a strong green-orange photoluminescence (PL). Difference measurements revealed a major double component spectra consisting of a broad band associated with the ASH-Si wafer interfacial porous oxide layer and a high energy band attributable to Si-NPs embedded in the ASH. The origin of the latter emission can be explained in terms of quantum/spatial confinement effects probably mediated by oxygen related defects in or around Si-NPs. Although Si-NPs are derived from the interface they are much smaller in size than those embedded within the interfacial porous oxide layer (SiOx, 1 < x < 2). Transmission electron microscopy (TEM) combined with Raman scattering and Fourier transformed infrared (FTIR) analysis confirmed the presence of Si-NPs and Si-O bondings pointing to the role of oxygen related defects. The presence of oxygen of up to 4.5 at.% in the (NH4)2SiF6 layer was confirmed by energy dispersive spectroscopy (EDS) analysis.

cond-mat.mes-hall↗