Showing 1–49 of 49 results for author: Konig, M

STEM EBIC as a Quantitative Probe of Semiconductor Devices

Authors: Sebastian Schneider, Sebastian Beckert, René Hammer, Markus König, Grigore Moldovan, Darius Pohl

Abstract: Electron beam-induced current (EBIC) imaging in the scanning transmission electron microscope (STEM), STEM-EBIC, provides direct access to carrier transport at the nanoscale. While well established in bulk SEM geometries, its application to thin TEM lamellae remains largely unexplored. Here, we present a systematic STEM-EBIC study of silicon photodiode lamellae prepared by gallium and xenon focuse… ▽ More Electron beam-induced current (EBIC) imaging in the scanning transmission electron microscope (STEM), STEM-EBIC, provides direct access to carrier transport at the nanoscale. While well established in bulk SEM geometries, its application to thin TEM lamellae remains largely unexplored. Here, we present a systematic STEM-EBIC study of silicon photodiode lamellae prepared by gallium and xenon focused ion beam (FIB) milling. We directly visualize the p-n junctions in thin cross sections and extract effective diffusion lengths for electrons and holes as a function of local thickness. The values are orders of magnitude smaller than those obtained by SEM-EBIC on bulk silicon, reflecting pronounced surface recombination and FIB-induced surface modifications. Current-voltage measurements further reveal severe deviations from the expected diode-like behavior, which we attribute to ohmic metal-semiconductor contacts in the emasurement setup. Our analysis establishes STEM-EBIC as a quantitative probe of carrier transport in nanoscale devices. △ Less

Submitted 14 November, 2025; originally announced November 2025.

Anisotropy of the chiral, semiconducting phase LaRhC$_{2}$: a handedness resolved study

Authors: Volodymyr Levytskyi, Ulrich Burkhardt, Markus König, Christoph Hennig, Eteri Svanidze, Yuri Grin, Roman Gumeniuk

Abstract: Chirality in quantum materials is a topic of significant importance due to its profound effects on the electronic, magnetic, and optical properties of these systems. However, it is non-trivial to decouple the behavior of two enantiomorphs within the same material -- perhaps explaining why the influence of chirality on electrical properties has remained largely unexplored. In this work, we examine… ▽ More Chirality in quantum materials is a topic of significant importance due to its profound effects on the electronic, magnetic, and optical properties of these systems. However, it is non-trivial to decouple the behavior of two enantiomorphs within the same material -- perhaps explaining why the influence of chirality on electrical properties has remained largely unexplored. In this work, we examine the electrical conductivity, magnetoresistance, and thermal expansion coefficient of LaRhC$_{2}$ -- a compound with a chiral crystal structure (tetragonal symmetry, space groups $\textit{P}$4$_{1}$ or $\textit{P}$4$_{3}$). The identification of a suitable monochiral domain was achieved via electron backscatter diffraction, which simultaneously determines crystallographic orientation and handedness. Both enantiomorphs are confirmed by single-crystal X-ray diffraction on monochiral specimens. The analysis of electrical resistivity was made possible through the single-domain extraction of enantiopure specimens from a polycrystalline sample using focused ion beam techniques. We establish that LaRhC$_{2}$ is a semiconductor with band gaps of approximately 20 meV and 33 meV parallel and perpendicular to the fourfold screw axis of the crystal structure, respectively -- consistent with band structure calculations. A significant anisotropy is also observed in the thermal expansion, electrical resistivity as well as angular-dependent magnetoresistance parallel and perpendicular o [001] crystallographic directions. △ Less

Submitted 25 September, 2025; originally announced September 2025.

Comments: 27 pages, 6 figures, 1 table, and Supplementary materials (16 p., 6 fig., 9 tab.)

Emergent heavy-fermion physics in a new family of topological insulators RAsS (R = Y, La, and Sm)

Authors: Iñigo Robredo, Yuan Fang, Lei Chen, Nazar Zaremba, Yurii Prots, Mitja Krnel, Markus König, Thomas Doert, Jeroen van den Brink, Claudia Felser, Qimiao Si, Eteri Svanidze, Maia G. Vergniory

Abstract: Realizing topological phases in strongly correlated materials has become a major impetus in condensed matter physics. Although many compounds are now classified as topological insulators, $f$-electron systems (with their strong electron correlations) provide an especially fertile platform for emergent heavy-fermion phenomena driven by the interplay of topology and many-body effects. In this study,… ▽ More Realizing topological phases in strongly correlated materials has become a major impetus in condensed matter physics. Although many compounds are now classified as topological insulators, $f$-electron systems (with their strong electron correlations) provide an especially fertile platform for emergent heavy-fermion phenomena driven by the interplay of topology and many-body effects. In this study, we examine the crystalline topology of a new RAsS series (R = Y, La, Sm), revealing a structural variant from previous reports. We demonstrate that YAsS and SmAsS host hourglass fermions protected by glide symmetry. SmAsS notably exhibits a strong effective-mass enhancement, placing it alongside SmB${}_6$ and YbB${}_{12}$ as a material that couples topological surface states with emergent Kondo physics, yet distinguished by its crystalline symmetry constraints and $f$-$p$ orbital hybridization. To capture these features, we construct a minimal model incorporating $f$-electron degrees of freedom, which reproduces the observed topological properties and predicts that the surface states survive in the correlated regime, albeit shifted in energy. Our work thus introduces a new family of correlated topological materials and forecasts the robustness of their surface states under Kondo correlations. △ Less

Submitted 2 May, 2025; originally announced May 2025.

Comments: Main text: 11 pages, 5 figures. Supplementary material: 5 pages, 2 figures

Directional ballistic magnetotransport in the delafossite metals PdCoO$_2$ and PtCoO$_2$

Authors: Michal Moravec, Graham Baker, Maja D. Bachmann, Aaron Sharpe, Nabhanila Nandi, Arthur W. Barnard, Carsten Putzke, Seunghyun Khim, Markus König, David Goldhaber-Gordon, Philip J. W. Moll, Andrew P. Mackenzie

Abstract: Studies of electronic transport in width-restricted channels of PdCoO$_2$ have recently revealed a novel `directional ballistic' regime, in which ballistic propagation of electrons on an anisotropic Fermi surface breaks the symmetries of bulk transport. Here we introduce a magnetic field to this regime, in channels of PdCoO$_2$ and PtCoO$_2$ along two crystallographically distinct directions and o… ▽ More Studies of electronic transport in width-restricted channels of PdCoO$_2$ have recently revealed a novel `directional ballistic' regime, in which ballistic propagation of electrons on an anisotropic Fermi surface breaks the symmetries of bulk transport. Here we introduce a magnetic field to this regime, in channels of PdCoO$_2$ and PtCoO$_2$ along two crystallographically distinct directions and over a wide range of widths. We observe magnetoresistance distinct from that in the bulk, with features strongly dependent on channel orientation and becoming more pronounced the narrower the channel. Comparison to semi-classical theory establishes that magnetoresistance arises from field-induced modification of boundary scattering, and helps connect features in the data with specific electronic trajectories. However, the role of bulk scattering in our measurements is yet to be fully understood. Our results demonstrate that finite-size magnetotransport is sensitive to the anisotropy of Fermi surface properties, motivating future work to fully understand and exploit this sensitivity. △ Less

Submitted 19 March, 2026; v1 submitted 27 March, 2025; originally announced March 2025.

Slow magnetic quantum oscillations in the c-axis magnetoresistance of UTe$_2$

Authors: Freya Husstedt, Motoi Kimata, Sajal Naduvile Thadathil, Beat Valentin Schwarze, Markus König, Gerard Lapertot, Jean-Pascal Brison, Georg Knebel, Dai Aoki, J. Wosnitza, Toni Helm

Abstract: Details of the electronic band structure in unconventional superconductors are key to the understanding of their fundamental ground state. The potential spin-triplet superconductor UTe$_2$, with $T_\mathrm{c}\approx 2.1\,$K, has attracted attention recently. Its main Fermi surface consists of weakly corrugated, two-dimensional Fermi-surface cylinders that run along the crystallographic $c$ axis. I… ▽ More Details of the electronic band structure in unconventional superconductors are key to the understanding of their fundamental ground state. The potential spin-triplet superconductor UTe$_2$, with $T_\mathrm{c}\approx 2.1\,$K, has attracted attention recently. Its main Fermi surface consists of weakly corrugated, two-dimensional Fermi-surface cylinders that run along the crystallographic $c$ axis. In addition, there is evidence for the presence of an additional small three-dimensional band. This has been discussed controversially as it may be essential for the realization of superconductivity in UTe$_2$. Here, we investigate the angle-resolved magnetoresistance and Hall effect in bulk crystalline samples with current along the $c$ axis in fields up to $60\,$T. We observe low-frequency magnetic quantum oscillations with light effective masses that are most pronounced for magnetic field applied along the $a$ axis. Two distinct frequencies indicate two separate changes in the Fermi-surface topology, likely connected with Lifshitz transitions. We discuss the origin of these oscillations in terms of magnetic breakdown, quantum interference, and other potential mechanisms. △ Less

Submitted 10 July, 2025; v1 submitted 14 March, 2025; originally announced March 2025.

Journal ref: Phys. Rev. B 111, 235131 (2025)

Comment on "Comments regarding "Transonic dislocation propagation in diamond" by Katagiri, et al. (Science 382, 69-72, 2023)" by Hawreliak, et al. (arXiv:2401.04213)

Authors: Kento Katagiri, Tatiana Pikuz, Lichao Fang, Bruno Albertazzi, Shunsuke Egashira, Yuichi Inubushi, Genki Kamimura, Ryosuke Kodama, Michel Koenig, Bernard Kozioziemski, Gooru Masaoka, Kohei Miyanishi, Hirotaka Nakamura, Masato Ota, Gabriel Rigon, Youichi Sakawa, Takayoshi Sano, Frank Schoofs, Zoe J. Smith, Keiichi Sueda, Tadashi Togashi, Tommaso Vinci, Yifan Wang, Makina Yabashi, Toshinori Yabuuchi , et al. (2 additional authors not shown)

Abstract: In their comment (1), Hawreliak et al. claims that our observation of stacking fault formation and transonic dislocation propagation in diamond (2) is not valid as they interpret the observed features as cracks. In this response letter, we describe our rationale for interpreting the observed features as stacking faults. We also address other points raised in their comments, including the clarifica… ▽ More In their comment (1), Hawreliak et al. claims that our observation of stacking fault formation and transonic dislocation propagation in diamond (2) is not valid as they interpret the observed features as cracks. In this response letter, we describe our rationale for interpreting the observed features as stacking faults. We also address other points raised in their comments, including the clarifications of how the results of Makarov et al. (3) are not in conflict with our study. △ Less

Submitted 9 September, 2024; originally announced September 2024.

Comments: Comment on arXiv:2401.04213. 10 pages, 2 figures

Vortex motion in reconfigurable three-dimensional superconducting nanoarchitectures

Authors: Elina Zhakina, Luke Turnbull, Weijie Xu, Markus König, Paul Simon, Wilder Carrillo-Cabrera, Amalio Fernandez-Pacheco, Uri Vool, Dieter Suess, Claas Abert, Vladimir M. Fomin, Claire Donnelly

Abstract: When materials are patterned in three dimensions, there exist opportunities to tailor and create functionalities associated with an increase in complexity, the breaking of symmetries, and the introduction of curvature and non-trivial topologies. For superconducting nanostructures, the extension to the third dimension may trigger the emergence of new physical phenomena, as well as advances in techn… ▽ More When materials are patterned in three dimensions, there exist opportunities to tailor and create functionalities associated with an increase in complexity, the breaking of symmetries, and the introduction of curvature and non-trivial topologies. For superconducting nanostructures, the extension to the third dimension may trigger the emergence of new physical phenomena, as well as advances in technologies. Here, we harness three-dimensional (3D) nanopatterning to fabricate and control the emergent properties of a 3D superconducting nanostructure. Not only are we able to demonstrate the existence and motion of superconducting vortices in 3D but, with simulations, we show that the confinement leads to a well-defined bending of the vortices within the volume of the structure. Moreover, we experimentally observe a strong geometrical anisotropy of the critical field, through which we achieve the reconfigurable coexistence of superconducting and normal states in our 3D superconducting architecture, and the local definition of weak links. In this way, we uncover an intermediate regime of nanosuperconductivity, where the vortex state is truly three-dimensional and can be designed and manipulated by geometrical confinement. This insight into the influence of 3D geometries on superconducting properties offers a route to local reconfigurable control for future computing devices, sensors, and quantum technologies. △ Less

Submitted 18 April, 2024; originally announced April 2024.

Comments: 22 pages, 12 figures

Tailoring the energy landscape of a Bloch point singularity with curvature

Authors: Sandra Ruiz-Gomez, Claas Abert, Pamela Morales-Fernández, Claudia Fernandez-Gonzalez, Sabri Koraltan, Lukas Danesi, Dieter Suess, Michael Foerster, Miguel Ángel Nino, Anna Mandziak, Dorota Wilgocka-Ślęzak, Pawel Nita, Markus Koenig, Sebastian Seifert, Aurelio Hierro Rodríguez, Amalio Fernández-Pacheco, Claire Donnelly

Abstract: Topological defects, or singularities, play a key role in the statics and dynamics of complex systems. In magnetism, Bloch point singularities represent point defects that mediate the nucleation of textures such as skyrmions and hopfions. However, while the textures are typically stabilised in chiral magnets, the influence of chirality on the Bloch point singularities remains relatively unexplored… ▽ More Topological defects, or singularities, play a key role in the statics and dynamics of complex systems. In magnetism, Bloch point singularities represent point defects that mediate the nucleation of textures such as skyrmions and hopfions. However, while the textures are typically stabilised in chiral magnets, the influence of chirality on the Bloch point singularities remains relatively unexplored. Here we harness advanced three-dimensional nanofabrication to explore the influence of chirality on Bloch point singularities by introducing curvature-induced symmetry breaking in a ferromagnetic nanowire. Combining X-ray magnetic microscopy with the application of in situ magnetic fields, we demonstrate that Bloch point singularity-containing domain walls are stabilised in straight regions of the sample, and determine that curvature can be used to tune the energy landscape of the Bloch points. Not only are we able to pattern pinning points but, by controlling the gradient of curvature, we define asymmetric potential wells to realise a robust Bloch point shift-register with non-reciprocal behaviour. These insights into the influence of symmetry and chirality on singularities offers a route to the controlled nucleation and propagation of topological textures, providing opportunities for logic and computing devices. △ Less

Submitted 9 April, 2024; originally announced April 2024.

Transonic Dislocation Propagation in Diamond

Authors: Kento Katagiri, Tatiana Pikuz, Lichao Fang, Bruno Albertazzi, Shunsuke Egashira, Yuichi Inubushi, Genki Kamimura, Ryosuke Kodama, Michel Koenig, Bernard Kozioziemski, Gooru Masaoka, Kohei Miyanishi, Hirotaka Nakamura, Masato Ota, Gabriel Rigon, Youichi Sakawa, Takayoshi Sano, Frank Schoofs, Zoe J. Smith, Keiichi Sueda, Tadashi Togashi, Tommaso Vinci, Yifan Wang, Makina Yabashi, Toshinori Yabuuchi , et al. (2 additional authors not shown)

Abstract: The motion of line defects (dislocations) has been studied for over 60 years but the maximum speed at which they can move is unresolved. Recent models and atomistic simulations predict the existence of a limiting velocity of dislocation motions between the transonic and subsonic ranges at which the self-energy of dislocation diverges, though they do not deny the possibility of the transonic disloc… ▽ More The motion of line defects (dislocations) has been studied for over 60 years but the maximum speed at which they can move is unresolved. Recent models and atomistic simulations predict the existence of a limiting velocity of dislocation motions between the transonic and subsonic ranges at which the self-energy of dislocation diverges, though they do not deny the possibility of the transonic dislocations. We use femtosecond x-ray radiography to track ultrafast dislocation motion in shock-compressed single-crystal diamond. By visualizing stacking faults extending faster than the slowest sound wave speed of diamond, we show the evidence of partial dislocations at their leading edge moving transonically. Understanding the upper limit of dislocation mobility in crystals is essential to accurately model, predict, and control the mechanical properties of materials under extreme conditions. △ Less

Submitted 6 October, 2023; v1 submitted 7 March, 2023; originally announced March 2023.

Investigation of Planckian behavior in a high-conductivity oxide: PdCrO$_2$

Authors: Elina Zhakina, Ramzy Daou, Antoine Maignan, Philippa H. McGuinness, Markus König, Helge Rosner, Seo-Jin Kim, Seunghyun Khim, Romain Grasset, Marcin Konczykowski, Evyatar Tulipman, Juan Felipe Mendez-Valderrama, Debanjan Chowdhury, Erez Berg, Andrew P. Mackenzie

Abstract: The layered delafossite metal PdCrO$_2$ is a natural heterostructure of highly conductive Pd layers Kondo coupled to localized spins in the adjacent Mott insulating CrO$_2$ layers. At high temperatures $T$ it has a $T$-linear resistivity which is not seen in the isostructural but non-magnetic PdCoO$_2$. The strength of the Kondo coupling is known, as-grown crystals are extremely high purity and th… ▽ More The layered delafossite metal PdCrO$_2$ is a natural heterostructure of highly conductive Pd layers Kondo coupled to localized spins in the adjacent Mott insulating CrO$_2$ layers. At high temperatures $T$ it has a $T$-linear resistivity which is not seen in the isostructural but non-magnetic PdCoO$_2$. The strength of the Kondo coupling is known, as-grown crystals are extremely high purity and the Fermi surface is both very simple and experimentally known. It is therefore an ideal material platform in which to investigate 'Planckian metal' physics. We do this by means of controlled introduction of point disorder, measurement of the thermal conductivity and Lorenz ratio and studying the sources of its high temperature entropy. The $T$-linear resistivity is seen to be due mainly to elastic scattering and to arise from a sum of several scattering mechanisms. Remarkably, this sum leads to a scattering rate within 10$\%$ of the Planckian value of $k_BT/$$\hbar$. △ Less

Submitted 25 January, 2023; originally announced January 2023.

Microstructuring YbRh2Si2 for resistance and noise measurements down to ultra-low temperatures

Authors: Alexander Steppke, Sandra Hamann, Markus König, Andrew P. Mackenzie, Kristin Kliemt, Cornelius Krellner, Marvin Kopp, Martin Lonsky, Jens Müller, Lev V. Levitin, John Saunders, Manuel Brando

Abstract: The discovery of superconductivity in the quantum critical Kondo-lattice system YbRh2Si2 at an extremely low temperature of 2 mK has inspired efforts to perform high-resolution electrical resistivity measurements down to this temperature range in highly conductive materials. Here we show that control over the sample geometry by microstructuring using focused-ion-beam (FIB) techniques allows to rea… ▽ More The discovery of superconductivity in the quantum critical Kondo-lattice system YbRh2Si2 at an extremely low temperature of 2 mK has inspired efforts to perform high-resolution electrical resistivity measurements down to this temperature range in highly conductive materials. Here we show that control over the sample geometry by microstructuring using focused-ion-beam (FIB) techniques allows to reach ultra-low temperatures and increase signal-to-noise ratios (SNR) tenfold, without adverse effects to sample quality. In five experiments we show four-terminal sensing resistance and magnetoresistance measurements which exhibit sharp phase transitions at the Néel temperature, and Shubnikov-de-Haas (SdH) oscillations between 13 T and 18 T where we identified a new SdH frequency of 0.39 kT. The increased SNR allowed resistance fluctuation (noise) spectroscopy that would not be possible for bulk crystals, and confirmed intrinsic 1/f-type fluctuations. Under controlled strain, two thin microstructured samples exhibited a large increase of T_N from 67 mK up to 188 mK while still showing clear signatures of the phase transition and SdH oscillations. SQUID-based thermal noise spectroscopy measurements in a nuclear demagnetisation refrigerator down to 0.95 mK, show a sharp superconducting transition at T_c = 1.2 mK. These experiments demonstrate microstructuring as a powerful tool to investigate the resistance and the noise spectrum of highly conductive correlated metals over wide temperature ranges. △ Less

Submitted 10 January, 2023; originally announced January 2023.

Comments: 23 pages, 7 figures

Journal ref: New Journal of Physics 24 (2022) 123033

Pushing the limits of flow strength in diamond

Authors: Anirudh Hari, Kento Katagiri, Wanghui Li, Dorian P. Luccioni, Rayen Lin, Sophie E. Parsons, Zipeng Xu, Rohit Hari, Tharun Reddy, Ernest W. Cubit II, Alexis Amouretti, Jon H. Eggert, Yuichi Inubushi, Tetsuo Irifune, Sara J. Irvine, Ryosuke Kodama, Michel Koenig, Laura Madril, Takeshi Matsuoka, Kohei Miyanishi, Hirotaka Nakamura, Norimasa Nishiyama, Takuo Okuchi, Masato Ota, Toshimori Sekine , et al. (12 additional authors not shown)

Abstract: Extreme pressures and temperatures create conditions that allow even hard and brittle materials to deform plastically. Despite extensive research, the upper limits of flow strength, the resistance to plastic flow, remain uncertain, and the mechanisms driving deformation at the relevant stresses are a subject of debate. Using femtosecond in situ X-ray diffraction experiments and large-scale molecul… ▽ More Extreme pressures and temperatures create conditions that allow even hard and brittle materials to deform plastically. Despite extensive research, the upper limits of flow strength, the resistance to plastic flow, remain uncertain, and the mechanisms driving deformation at the relevant stresses are a subject of debate. Using femtosecond in situ X-ray diffraction experiments and large-scale molecular dynamics simulations, we demonstrate that stacking fault-mediated strengthening enables shock-compressed nano-polycrystalline diamond to achieve a peak flow strength of 107+-5 GPa at a stress of 227+-8 GPa. Our findings show that extreme conditions can unlock unusual strength via mechanisms that can be used as design tools in targeted applications. △ Less

Submitted 31 October, 2025; v1 submitted 5 August, 2022; originally announced August 2022.

Field-induced compensation of magnetic exchange as the possible origin of reentrant superconductivity in UTe$_2$

Authors: Toni Helm, Motoi Kimata, Kenta Sudo, Atsuhiko Miyata, Julia Stirnat, Tobias Förster, Jacob Hornung, Markus König, Ilya Sheikin, Alexandre Pourret, Gérard Lapertot, Dai Aoki, Georg Knebel, Jochen Wosnitza, Jean-Pascal Brison

Abstract: The potential spin-triplet heavy-fermion superconductor UTe$_2$ exhibits signatures of multiple distinct superconducting phases. For field aligned along the $b$ axis, a metamagnetic transition occurs at $μ_0 H_\mathrm{m}\approx35\,$T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to $H_\mathrm{m}$. Once the field is tilte… ▽ More The potential spin-triplet heavy-fermion superconductor UTe$_2$ exhibits signatures of multiple distinct superconducting phases. For field aligned along the $b$ axis, a metamagnetic transition occurs at $μ_0 H_\mathrm{m}\approx35\,$T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to $H_\mathrm{m}$. Once the field is tilted away from the $b$ towards the $c$ axis, a reentrant superconducting phase emerges just above $H_\mathrm{m}$. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of $μ_0 H_\mathrm{c2}\approx 73\,$T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above $H_\mathrm{m}$ in the angular range around $30^\circ$ caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above $H_\mathrm{m}$. △ Less

Submitted 21 December, 2023; v1 submitted 17 July, 2022; originally announced July 2022.

Comments: Main text: 29 pages, 4 figure; Supplement: 12 pages, 6 figures

Journal ref: Nat Commun 15, 37 (2024)

Martini 3 Coarse-Grained Force Field for Carbohydrates

Authors: Fabian Grünewald, Mats H. Punt, Elizabeth E. Jefferys, Petteri A. Vainikka, Valtteri Virtanen, Melanie König, Weria Pezeshkian, Maarit Karonen, Mark S. P. Sansom, Paulo C. T Souza, Siewert J. Marrink

Abstract: The Martini 3 force field is a full re-parametrization of the Martini coarse-grained model for biomolecular simulations. Due to the improved interaction balance it allows for more accurate description of condensed phase systems. In the present work we develop a consistent strategy to parametrize carbohydrate molecules accurately within the framework of Martini 3. In particular, we develop a canoni… ▽ More The Martini 3 force field is a full re-parametrization of the Martini coarse-grained model for biomolecular simulations. Due to the improved interaction balance it allows for more accurate description of condensed phase systems. In the present work we develop a consistent strategy to parametrize carbohydrate molecules accurately within the framework of Martini 3. In particular, we develop a canonical mapping scheme that decomposes arbitrarily large carbohydrates into a limited number of fragments. Bead types for these fragments have been assigned by matching physicochemical properties of mono- and disaccharides. In addition, guidelines for assigning bonds, angles, and dihedrals are developed. These guidelines enable a more accurate description of carbohydrate conformations than in the Martini 2 force field. We show that models obtained with this approach are able to accurately reproduce osmotic pressures of carbohydrate water solutions. Furthermore, we provide evidence that the model differentiates correctly the solubility of the poly-glucoses dextran (water soluble) and cellulose (water insoluble, but soluble in ionic-liquids). Finally, we demonstrate that the new building blocks can be applied to glycolipids, being able to reproduce membrane properties and to induce binding of peripheral membrane proteins. These test cases demonstrate the validity and transferability of our approach. △ Less

Submitted 12 July, 2022; originally announced July 2022.

Direct imaging of shock wave splitting in diamond at Mbar pressures

Authors: S. S. Makarov, S. A. Dyachkov, T. A. Pikuz, K. Katagiri, V. V. Zhakhovsky, N. A. Inogamov, V. A. Khokhlov, A. S. Martynenko, B. Albertazzi, G. Rigon, P. Mabey, N. Hartley, Y. Inubushi, K. Miyanishi, K. Sueda, T. Togashi, M. Yabashi, T. Yabuuchi, R. Kodama, S. A. Pikuz, M. Koenig, N. Ozaki

Abstract: The propagation of a shock wave in solids can stress them to ultra-high pressures of millions of atmospheres. Understanding the behavior of matter at these extreme pressures is essential to describe a wide range of physical phenomena, including the formation of planets, young stars and cores of super-Earths, as well as the behavior of advanced ceramic materials subjected to such stresses. Under me… ▽ More The propagation of a shock wave in solids can stress them to ultra-high pressures of millions of atmospheres. Understanding the behavior of matter at these extreme pressures is essential to describe a wide range of physical phenomena, including the formation of planets, young stars and cores of super-Earths, as well as the behavior of advanced ceramic materials subjected to such stresses. Under megabar (Mbar) pressure, even a solid with high strength exhibits plastic properties, causing the shock wave to split in two. This phenomenon is described by theoretical models, but without direct experimental measurements to confirm them, their validity is still in doubt. Here, we present the results of an experiment in which the evolution of the coupled elastic-plastic wave structure in diamond was directly observed and studied with submicron spatial resolution, using the unique capabilities of the X-ray free-electron laser. The direct measurements allowed, for the first time, the fitting and validation of a strength model for diamond in the range of several Mbar by performing continuum mechanics simulations in 2D geometry. The presented experimental approach to the study of shock waves in solids opens up new possibilities for the direct verification and construction of the equations of state of matter in the ultra-high pressure range, which are relevant for the solution of a variety of problems in high energy density physics. △ Less

Submitted 4 July, 2022; originally announced July 2022.

Comments: 14 pages, 15 figures, submitted to Nature

Antiskyrmions and their electrical footprint in crystalline mesoscale structures of Mn$_{1.4}$PtSn

Authors: Moritz Winter, Francisco J. T. Goncalves, Ivan Soldatov, Yangkun He, Belén E. Zúňiga Céspedes, Peter Milde, Kilian Lenz, Sandra Hamann, Marc Uhlarz, Praveen Vir, Markus König, Philip J. W. Moll, Richard Schlitz, Sebastian T. B. Goennenwein, Lukas M. Eng, Rudolf Schaefer, Jochen Wosnitza, Claudia Felser, Jacob Gayles, Toni Helm

Abstract: Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effe… ▽ More Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effect. However, the electrical detection of antiskyrmions, in both thin films and bulk samples has been challenging to date. Here, we apply magneto-optical microscopy combined with electrical transport to explore the antiskyrmion phase as it emerges in crystalline mesoscale structures of the Heusler magnet Mn$_{1.4}$PtSn. We reveal the Hall signature of antiskyrmions in line with our theoretical model, comprising anomalous and topological components. We examine its dependence on the vertical device thickness, field orientation, and temperature. Our atomistic simulations and experimental anisotropy studies demonstrate the link between antiskyrmions and a complex magnetism that consists of competing ferromagnetic, antiferromagnetic, and chiral exchange interactions, not captured by micromagnetic simulations. △ Less

Submitted 15 March, 2023; v1 submitted 3 November, 2021; originally announced November 2021.

Comments: 30 pages, 4 figures, Supplementary Information with additional 22 pages and 16 figures

Journal ref: Commun. Mater. 3, 102 (2022)

Low-symmetry non-local transport in microstructured squares of delafossite metals

Authors: Philippa H. McGuinness, Elina Zhakina, Markus König, Maja D. Bachmann, Carsten Putzke, Philip J. W. Moll, Seunghyun Khim, Andrew P. Mackenzie

Abstract: Intense work studying the ballistic regime of electron transport in two dimensional systems based on semiconductors and graphene had been thought to have established most of the key experimental facts of the field. In recent years, however, new forms of ballistic transport have become accessible in the quasi-two-dimensional delafossite metals, whose Fermi wavelength is a factor of 100 shorter than… ▽ More Intense work studying the ballistic regime of electron transport in two dimensional systems based on semiconductors and graphene had been thought to have established most of the key experimental facts of the field. In recent years, however, new forms of ballistic transport have become accessible in the quasi-two-dimensional delafossite metals, whose Fermi wavelength is a factor of 100 shorter than those typically studied in the previous work, and whose Fermi surfaces are nearly hexagonal in shape, and therefore strongly faceted. This has some profound consequences for results obtained from the classic ballistic transport experiment of studying bend and Hall resistances in mesoscopic squares fabricated from delafossite single crystals. We observe pronounced anisotropies in bend resistances and even a Hall voltage that is strongly asymmetric in magnetic field. Although some of our observations are non-intuitive at first sight, we show that they can be understood within a non-local Landauer-Büttiker analysis tailored to the symmetries of the square/hexagonal geometries of our combined device/Fermi surface system. Signatures of non-local transport can be resolved for squares of linear dimension of nearly 100 $μ$m, approximately a factor of 15 larger than the bulk mean free path of the crystal from which the device was fabricated. △ Less

Submitted 1 November, 2021; originally announced November 2021.

Comments: 23 pages, 9 figures (including supplementary information)

Journal ref: Proc. Natl Acad. Sci. USA 2021 Vol. 118 No. 47 e2113185118

Surface excitations relaxation in the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$

Authors: J. C. Souza, M. König, M. V. Ale Crivillero, M. O. Malcolms, R. R. Urbano, Z. Fisk, P. F. S. Rosa, P. G. Pagliuso, S. Wirth, J. Sichelschmidt

Abstract: The interplay between non-trivial topological states of matter and strong electronic correlations is one of the most compelling open questions in condensed matter physics. Due to experimental challenges, there is an increasing desire to find more microscopic techniques to complement the results of more traditional experiments. In this work, we locally explore the Kondo insulator Sm$_{1-x}$Gd… ▽ More The interplay between non-trivial topological states of matter and strong electronic correlations is one of the most compelling open questions in condensed matter physics. Due to experimental challenges, there is an increasing desire to find more microscopic techniques to complement the results of more traditional experiments. In this work, we locally explore the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$ by means of electron spin resonance (ESR) of Gd$^{3+}$ ions at low temperatures. Our analysis reveals that the Gd$^{3+}$ ESR line shape shows an anomalous evolution as a function of temperature, wherein for highly dilute samples (x $\approx$ 0.0002) the Gd$^{3+}$ ESR line shape changes from a localized ESR local moment character to a diffusive-like character. Upon manipulating the sample surface with a focused ion beam we demonstrate, in combination with electrical resistivity measurements, that the localized character of the Gd$^{3+}$ ESR line shape is recovered by increasing the penetration of the microwave in the sample. This provides compelling evidence for the contribution of surface or near-surface excitations to the relaxation mechanism in the Gd$^{3+}$ spin dynamics. Our work brings new insights into the importance of non-trivial surface excitations in ESR, opening new routes to be explored both theoretically and experimentally. △ Less

Submitted 9 June, 2021; originally announced June 2021.

Comments: 10 pages, 5 figures, to appear in Phys. Rev. Research

Report number: 10.1103/PhysRevResearch.3.033016

Journal ref: Phys. Rev. Research 3, 033016 (2021)

Systematic manipulation of the surface conductivity of SmB$_6$

Authors: M. Victoria Ale Crivillero, M. König, J. C. Souza, P. G. Pagliuso, J. Sichelschmidt, Priscila F. S. Rosa, Z. Fisk, S. Wirth

Abstract: We show that the resistivity plateau of SmB$_6$ at low temperature, typically taken as a hallmark of its conducting surface state, can systematically be influenced by different surface treatments. We investigate the effect of inflicting an increasing number of hand-made scratches and microscopically defined focused ion beam-cut trenches on the surfaces of flux-grown Sm$_{1-x}$Gd$_x$B$_6$ with… ▽ More We show that the resistivity plateau of SmB$_6$ at low temperature, typically taken as a hallmark of its conducting surface state, can systematically be influenced by different surface treatments. We investigate the effect of inflicting an increasing number of hand-made scratches and microscopically defined focused ion beam-cut trenches on the surfaces of flux-grown Sm$_{1-x}$Gd$_x$B$_6$ with $x =$ 0, 0.0002. Both treatments increase the resistance of the low-temperature plateau, whereas the bulk resistance at higher temperature largely remains unaffected. Notably, the temperature at which the resistance deviates from the thermally activated behavior decreases with cumulative surface damage. These features are more pronounced for the focused ion beam treated samples, with the difference likely being related to the absence of microscopic defects like subsurface cracks. Therefore, our method presents a systematic way of controlling the surface conductance. △ Less

Submitted 27 May, 2021; originally announced May 2021.

Comments: 7 pages,5 figures

Unidirectional Kondo scattering in layered NbS2

Authors: Edoardo Martino, Carsten Putzke, Markus König, Philip Moll, Helmuth Berger, David LeBoeuf, Maxime Leroux, Cyril Proust, Ana Akrap, Holm Kirmse, Christoph Koch, ShengNan Zhang, QuanSheng Wu, Oleg V. Yazyev, László Forró, Konstantin Semeniuk

Abstract: Crystalline defects can modify quantum interactions in solids, causing unintuitive, even favourable, properties such as quantum Hall effect or superconducting vortex pinning. Here we present another example of this notion - an unexpected unidirectional Kondo scattering in single crystals of 2H-NbS2. This manifests as a pronounced low-temperature enhancement in the out-of-plane resistivity and ther… ▽ More Crystalline defects can modify quantum interactions in solids, causing unintuitive, even favourable, properties such as quantum Hall effect or superconducting vortex pinning. Here we present another example of this notion - an unexpected unidirectional Kondo scattering in single crystals of 2H-NbS2. This manifests as a pronounced low-temperature enhancement in the out-of-plane resistivity and thermopower below 40 K, hidden for the in-plane charge transport. The anomaly can be suppressed by the c-axis-oriented magnetic field, but is unaffected by field applied along the planes. The magnetic moments originate from layers of 1T-NbS2, which inevitably form during the growth, undergoing a charge-density-wave reconstruction with each superlattice cell (David-star-shaped cluster of Nb atoms) hosting a localised spin. Our results demonstrate the unique and highly anisotropic response of a spontaneously formed Kondo lattice heterostructure, intercalated in a layered conductor. △ Less

Submitted 20 April, 2021; v1 submitted 19 April, 2021; originally announced April 2021.

Comments: 44 pages, including the article, 5 figures, and Supplementary Information (6 notes, 9 figures, and 1 table)

Journal ref: npj 2D Materials and Applications volume 5, Article number: 86 (2021)

Directional ballistic transport in the two-dimensional metal PdCoO2

Authors: Maja D. Bachmann, Aaron L. Sharpe, Arthur W. Barnard, Carsten Putzke, Thomas Scaffidi, Nabhanila Nandi, Seunghyun Khim, Markus Koenig, David Goldhaber- Gordon, Andrew P. Mackenzie, Philip J. W. Moll

Abstract: In an idealized infinite crystal, the material properties are constrained by the symmetries of its unit cell. Naturally, the point-group symmetry is broken by the sample shape of any finite crystal, yet this is commonly unobservable in macroscopic metals. To sense the shape-induced symmetry lowering in such metals, long-lived bulk states originating from anisotropic Fermi surfaces are needed. Here… ▽ More In an idealized infinite crystal, the material properties are constrained by the symmetries of its unit cell. Naturally, the point-group symmetry is broken by the sample shape of any finite crystal, yet this is commonly unobservable in macroscopic metals. To sense the shape-induced symmetry lowering in such metals, long-lived bulk states originating from anisotropic Fermi surfaces are needed. Here we show how strongly facetted Fermi surfaces and long quasiparticle mean free paths present in microstructures of PdCoO2 yield an in-plane resistivity anisotropy that is forbidden by symmetry on an infinite hexagonal lattice. Bar shaped transport devices narrower than the mean free path are carved from single crystals using focused ion beam (FIB) milling, such that the ballistic charge carriers at low temperatures frequently collide with both sidewalls defining a channel. Two symmetry-forbidden transport signatures appear: the in-plane resistivity anisotropy exceeds a factor of 2, and transverse voltages appear in zero magnetic field. We robustly identify the channel direction as the source of symmetry breaking via ballistic Monte- Carlo simulations and numerical solution of the Boltzmann equation. △ Less

Submitted 1 March, 2021; originally announced March 2021.

Fermi-surface reconstruction at the metamagnetic high-field transition in uranium mononitride

Authors: Sandra Hamann, Tobias Förster, Denis. I. Gorbunov, Markus König, Marc Uhlarz, Joachim Wosnitza, Toni Helm

Abstract: We report on the electronic and thermodynamic properties of the antiferromagnetic metal uranium mononitride with a Néel temperature $T_N\approx 53\,$K. The fabrication of microstructures from single crystals enables us to study the low-temperature metamagnetic transition at approximately $58\,$T by high-precision magnetotransport, Hall-effect, and magnetic-torque measurements. We confirm the evolu… ▽ More We report on the electronic and thermodynamic properties of the antiferromagnetic metal uranium mononitride with a Néel temperature $T_N\approx 53\,$K. The fabrication of microstructures from single crystals enables us to study the low-temperature metamagnetic transition at approximately $58\,$T by high-precision magnetotransport, Hall-effect, and magnetic-torque measurements. We confirm the evolution of the high-field transition from a broad and complex behavior to a sharp first-order-like step, associated with a spin flop at low temperature. In the high-field state, the magnetic contribution to the temperature dependence of the resistivity is suppressed completely. It evolves into an almost quadratic dependence at low temperatures indicative of a metallic character. Our detailed investigation of the Hall effect provides evidence for a prominent Fermi-surface reconstruction as the system is pushed into the high-field state. △ Less

Submitted 17 August, 2021; v1 submitted 15 February, 2021; originally announced February 2021.

Comments: 9 pages, 9 figures

Origin of the quasi-quantized Hall effect in ZrTe5

Authors: Stanislaw Galeski, Toni Ehmcke, Rafal Wawrzynczak, Pedro Mercado Lozano, Kyungjune Cho, Ankit Sharma, Souvik Das, Felix Kuster, Paolo Sessi, Manuel Brando, Robert Kuchler, Anastasios Markou, Markus Konig, Claudia Felser, Yasmine Sassa, Qiang Li, Genda Gu, Peter Swekis, Martin Zimmermann, Oleh Ivashko, Dennis I. Gorbunov, Sergei Zherlitsyn, Tobias Forster, Stuart Parkin, Joachim Wosnitza , et al. (2 additional authors not shown)

Abstract: The quantum Hall effect (QHE) is traditionally considered a purely two-dimensional (2D) phenomenon. Recently, a three-dimensional (3D) version of the QHE has been reported in the Dirac semimetal ZrTe5. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, thermoelectric… ▽ More The quantum Hall effect (QHE) is traditionally considered a purely two-dimensional (2D) phenomenon. Recently, a three-dimensional (3D) version of the QHE has been reported in the Dirac semimetal ZrTe5. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, thermoelectric and charge transport measurements on ZrTe5 in the quantum Hall regime. The measured thermodynamic properties: magnetization and ultrasound propagation, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response is an intrinsic property of the 3D electronic structure. Our findings render the Hall effect in ZrTe5 a truly 3D counterpart of the QHE in 2D systems. △ Less

Submitted 22 February, 2021; v1 submitted 26 May, 2020; originally announced May 2020.

Journal ref: Nature Communications 12, 3197 (2021)

Rigid platform for applying large tunable strains to mechanically delicate samples

Authors: Joonbum Park, Jack M. Bartlett, Hilary M. L. Noad, Alexander Stern, Mark E. Barber, Markus König, Suguru Hosoi, Takasada Shibauchi, Andrew P. Mackenzie, Alexander Steppke, Clifford W. Hicks

Abstract: Response to uniaxial stress has become a major probe of electronic materials. Tuneable uniaxial stress may be applied using piezoelectric actuators, and so far two methods have been developed to couple samples to actuators. In one, actuators apply force along the length of a free, beam-like sample, allowing very large strains to be achieved. In the other, samples are affixed directly to piezoelect… ▽ More Response to uniaxial stress has become a major probe of electronic materials. Tuneable uniaxial stress may be applied using piezoelectric actuators, and so far two methods have been developed to couple samples to actuators. In one, actuators apply force along the length of a free, beam-like sample, allowing very large strains to be achieved. In the other, samples are affixed directly to piezoelectric actuators, allowing study of mechanically delicate materials. Here, we describe an approach that merges the two: thin samples are affixed to a substrate, that is then pressurized uniaxially using piezoelectric actuators. Using this approach, we demonstrate application of large elastic strains to mechanically delicate samples: the van der Waals-bonded material FeSe, and a sample of CeAuSb$_2$ that was shaped with a focused ion beam. △ Less

Submitted 25 March, 2020; originally announced March 2020.

Comments: 11 pages, 9 figures

Revisiting the Possible 4f7 5d1 Ground State of Gd Impurities in SmB6 by Electron Spin Resonance

Authors: J. C. Souza, P. F. S. Rosa, U. Burkhardt, M. Konig, Z. Fisk, P. G. Pagliuso, S. Wirth, J. Sichelschmidt

Abstract: The search for topological states in strongly correlated electron systems has renewed the interest in the Kondo insulator SmB6. One of the most intriguing previous results was an anomalous electron spin resonance spectrum in Gd-doped SmB6. This spectrum was attributed to Gd2+ ions because it could be very well decribed by a model considering a change in the valence from Gd3+ to Gd2+, a dynamic Jah… ▽ More The search for topological states in strongly correlated electron systems has renewed the interest in the Kondo insulator SmB6. One of the most intriguing previous results was an anomalous electron spin resonance spectrum in Gd-doped SmB6. This spectrum was attributed to Gd2+ ions because it could be very well decribed by a model considering a change in the valence from Gd3+ to Gd2+, a dynamic Jahn-Teller effect and a 4f7 5d1 ground state in the Hamiltonian. In our work, we have revisited this scenario using electron spin resonance and energy dispersive X-ray spectroscopy measurements. Our results suggest that the resonance is produced by Gd2+ ions; however the resonance stems from an extrinsic oxide impurity phase that lies on the surface of the crystal. △ Less

Submitted 11 February, 2020; originally announced February 2020.

Comments: Accepted in JPS Conference Proceedings

Magnetic electron collimation in three-dimensional semi-metals

Authors: Xiangwei Huang, Carsten Putzke, Chunyu Guo, Jonas Diaz, Markus König, Horst Borrmann, Nityan L. Nair, James G. Analytis, Philip J. W. Moll

Abstract: While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection co… ▽ More While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection contacts, the phenomenon of "current jetting" plagues the research of longitudinal magneto-resistance which is particularly important in topological conductors. Here we demonstrate the controlled generation of tightly focused electron beams in a new class of micro-devices machined from crystals of the Dirac semi-metal Cd3As2. The current beams can be guided by tilting a magnetic field and their range tuned by the field strength. Finite element simulations quantitatively capture the voltage induced at faraway contacts when the beams are steered towards them, supporting the picture of controlled electron jets. These experiments demonstrate the first direct control over the highly nonlocal signal propagation unique to 3D semi-metals in the current jetting regime, and may lead to novel applications akin to electron optics in free space. △ Less

Submitted 9 January, 2020; originally announced January 2020.

Comments: 18 pages, 11 figures

Controlled introduction of defects to delafossite metals by electron irradiation

Authors: V. Sunko, P. H. McGuinness, C. S. Chang, E. Zhakina, S. Khim, C. E. Dreyer, M. Konczykowski, M. König, D. A. Muller, A. P. Mackenzie

Abstract: The delafossite metals PdCoO$_{2}$, PtCoO$_{2}$ and PdCrO$_{2}$ are among the highest conductivity materials known, with low temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure, or are a consequence o… ▽ More The delafossite metals PdCoO$_{2}$, PtCoO$_{2}$ and PdCrO$_{2}$ are among the highest conductivity materials known, with low temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure, or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross-section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately $0.001\%$. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths, and highlights how unusual these delafossite metals are in comparison with the vast majority of other multi-component oxides and alloys. We discuss the implications of our findings for future materials research. △ Less

Submitted 6 January, 2020; originally announced January 2020.

Journal ref: Phys. Rev. X 10, 021018 (2020)

$h/e$ Oscillations in Interlayer Transport of Delafossites

Authors: Carsten Putzke, Maja D. Bachmann, Philippa McGuinness, Elina Zhakina, Veronika Sunko, Marcin Konczykowski, Takashi Oka, Roderich Moessner, Ady Stern, Markus König, Seunghyun Khim, Andrew P. Mackenzie, Philip J. W. Moll

Abstract: Transport of electrons in a bulk metal is usually well captured by their particle-like aspects, while their wave-like nature is commonly harder to observe. Microstructures can be are fully designed to reveal the quantum phase, for example mesoscopic metal rings resembling interferometers. Here we report a new type of phase coherent oscillation of the out-of-plane magnetoresistance in the layered d… ▽ More Transport of electrons in a bulk metal is usually well captured by their particle-like aspects, while their wave-like nature is commonly harder to observe. Microstructures can be are fully designed to reveal the quantum phase, for example mesoscopic metal rings resembling interferometers. Here we report a new type of phase coherent oscillation of the out-of-plane magnetoresistance in the layered delafossites PdCoO$_2$ and PtCoO$_2$. The oscillation period is equivalent to that determined by the magnetic flux quantum, $h/e$, threading an area defined by the atomic interlayer separation and the sample width. The phase of the electron wave function in these crystals appears remarkably robust over macroscopic length scales exceeding 10$μ$m and persisting up to elevated temperatures of $T$>50K. We show that, while the experimental signal cannot be explained in a standard Aharonov-Bohm analysis, it arises due to periodic field-modulation of the out-of-plane hopping. These results demonstrate extraordinary single-particle quantum coherence lengths in the delafossites, and identify a new form of quantum interference in solids. △ Less

Submitted 7 January, 2020; v1 submitted 19 February, 2019; originally announced February 2019.

Super-geometric electron focusing on the hexagonal Fermi surface of PdCoO$_2$

Authors: Maja D. Bachmann, Aaron L. Sharpe, Arthur W. Barnard, Carsten Putzke, Markus König, Seunghyun Khim, David Goldhaber-Gordon, Andrew P. Mackenzie, Philip J. W. Moll

Abstract: Geometric electron optics may be implemented in solid state when transport is ballistic on the length scale of a device. Currently, this is realized mainly in 2D materials characterized by circular Fermi surfaces. Here we demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport. We probe this directional ballistic regime in a sing… ▽ More Geometric electron optics may be implemented in solid state when transport is ballistic on the length scale of a device. Currently, this is realized mainly in 2D materials characterized by circular Fermi surfaces. Here we demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport. We probe this directional ballistic regime in a single crystal of PdCoO2 by use of focused ion beam (FIB) micro-machining, defining crystalline ballistic circuits with features as small as 250nm. The peculiar hexagonal Fermi surface naturally leads to electron self-focusing effects in a magnetic field, well below the geometric limit associated with a circular Fermi surface. This super-geometric focusing can be quantitatively predicted for arbitrary device geometry, based on the hexagonal cyclotron orbits appearing in this material. These results suggest a novel class of ballistic electronic devices exploiting the unique transport characteristics of strongly faceted Fermi surfaces. △ Less

Submitted 11 February, 2019; originally announced February 2019.

Journal ref: Nature Communications 10, 5081 (2019)

Non-monotonic pressure dependence of high-field nematicity and magnetism in CeRhIn$_5$

Authors: Toni Helm, Audrey D. Grockowiak, Fedor F. Balakirev, John Singleton, Jonathan B. Betts, Kent R. Shirer, Markus König, Tobias Förster, Eric D. Bauer, Filip Ronning, Stanley W. Tozer, Philip J. W. Moll

Abstract: CeRhIn$_5$ provides a textbook example of quantum criticality in a heavy fermion system: Pressure suppresses local-moment antiferromagnetic (AFM) order and induces superconductivity in a dome around the associated quantum critical point (QCP) near $p_{c} \approx 23\,$kbar. Strong magnetic fields also suppress the AFM order at a field-induced QCP at $B_{\rm c}\approx 50\,$T. In its vicinity, a nema… ▽ More CeRhIn$_5$ provides a textbook example of quantum criticality in a heavy fermion system: Pressure suppresses local-moment antiferromagnetic (AFM) order and induces superconductivity in a dome around the associated quantum critical point (QCP) near $p_{c} \approx 23\,$kbar. Strong magnetic fields also suppress the AFM order at a field-induced QCP at $B_{\rm c}\approx 50\,$T. In its vicinity, a nematic phase at $B^*\approx 28\,$T characterized by a large in-plane resistivity anisotropy emerges. Here, we directly investigate the interrelation between these phenomena via magnetoresistivity measurements under high pressure. As pressure increases, the nematic transition shifts to higher fields, until it vanishes just below $p_{\rm c}$. While pressure suppresses magnetic order in zero field as $p_{\rm c}$ is approached, we find magnetism to strengthen under strong magnetic fields due to suppression of the Kondo effect. We reveal a strongly non-mean-field-like phase diagram, much richer than the common local-moment description of CeRhIn$_5$ would suggest. △ Less

Submitted 11 December, 2020; v1 submitted 3 February, 2019; originally announced February 2019.

Comments: Main text: 13 pages, 7 figures Supplemental Material: 10 pages, 13 figures

Journal ref: Nat. Commun. 11, 3482 (2020)

Surface and bulk superconductivity at ambient pressure in the Weyl semimetal TaP

Authors: M. R. van Delft, S. Pezzini, M. König, P. Tinnemans, N. E. Hussey, S. Wiedmann

Abstract: The motivation to search for signatures of superconductivity in Weyl semi-metals and other topological phases lies in their potential for hosting exotic phenomena such as nonzero-momentum pairing or the Majorana fermion, a viable candidate for the ultimate realization of a scalable quantum computer. Until now, however, all known reports of superconductivity in Weyl semimetals have arisen through s… ▽ More The motivation to search for signatures of superconductivity in Weyl semi-metals and other topological phases lies in their potential for hosting exotic phenomena such as nonzero-momentum pairing or the Majorana fermion, a viable candidate for the ultimate realization of a scalable quantum computer. Until now, however, all known reports of superconductivity in Weyl semimetals have arisen through surface contact with a sharp tip, focused ion-beam surface treatment or the application of high pressures. Here, we demonstrate the observation of superconductivity in single crystals, even an as-grown crystal, of the Weyl semi-metal tantalum phosphide (TaP), at ambient pressure. A superconducting transition temperature, $Tc$, varying between 1.7 and 5.3 K, is observed in different samples, both as-grown and microscopic samples processed with focused ion beam (FIB) etching. Our data show that the superconductivity present in the as-grown crystal is inhomogeneous yet exists in the bulk. For samples fabricated with FIB, we observe, in addition to the bulk superconductivity, a second superconducting state that resides on the sample surface. Through measurements of the characteristic fields as a function of temperature and angle, we are able to confirm the dimensionality of the two distinct superconducting phases. △ Less

Submitted 29 August, 2018; originally announced August 2018.

Comments: 17 pages, 3 figures

Spatially modulated heavy-fermion superconductivity in CeIrIn5

Authors: Maja D. Bachmann, G. M. Ferguson, Florian Theuss, Tobias Meng, Carsten Putzke, Toni Helm, K. R. Shirer, You-Sheng Li, K. A. Modic, Michael Nicklas, Markus Koenig, D. Low, Sayak Ghosh, Andrew P. Mackenzie, Frank Arnold, Elena Hassinger, Ross D. McDonald, Laurel E. Winter, Eric D. Bauer, Filip Ronning, B. J. Ramshaw, Katja C. Nowack, Philip J. W. Moll

Abstract: The ability to spatially modulate the electronic properties of solids has led to landmark discoveries in condensed matter physics as well as new electronic applications. Although crystals of strongly correlated metals exhibit a diverse set of electronic ground states, few approaches to spatially modulating their properties exist. Here we demonstrate spatial control over the superconducting state i… ▽ More The ability to spatially modulate the electronic properties of solids has led to landmark discoveries in condensed matter physics as well as new electronic applications. Although crystals of strongly correlated metals exhibit a diverse set of electronic ground states, few approaches to spatially modulating their properties exist. Here we demonstrate spatial control over the superconducting state in mesoscale samples of the canonical heavy-fermion superconductor CeIrIn5. We use a focused ion beam (FIB) to pattern crystals on the microscale, which tailors the strain induced by differential thermal contraction into specific areas of the device. The resulting non-uniform strain fields induce complex patterns of superconductivity due to the strong dependence of the transition temperature on the strength and direction of strain. Electrical transport and magnetic imaging of devices with different geometry show that the obtained spatial modulation of superconductivity agrees with predictions based on finite element simulations. These results present a generic approach to manipulating electronic order on micrometer length scales in strongly correlated matter. △ Less

Submitted 21 September, 2018; v1 submitted 13 July, 2018; originally announced July 2018.

Comments: 29 pages, 14 figures

Journal ref: Science 366, 221 (2019)

Unconventional magneto-transport in ultrapure PdCoO2 and PtCoO2

Authors: Nabhanila Nandi, Thomas Scaffidi, Pallavi Kushwaha, Seunghyun Khim, Mark E. Barber, Veronika Sunko, Federico Mazzola, Philip D. C. King, Helge Rosner, Philip J. W. Moll, Markus König, Joel E. Moore, Sean Hartnoll, Andrew P. Mackenzie

Abstract: We have studied magneto transport in the single-band, quasi-two-dimensional metals PdCoO2 and PtCoO2, which have extremely long mean free paths. We observer a strong temperature dependence of the Hall resistivity in small applied, fields, linked to a large violation of Kohler's rule in the magnetoresistance. We discuss the extent to which these observations can be accounted for by standard transpo… ▽ More We have studied magneto transport in the single-band, quasi-two-dimensional metals PdCoO2 and PtCoO2, which have extremely long mean free paths. We observer a strong temperature dependence of the Hall resistivity in small applied, fields, linked to a large violation of Kohler's rule in the magnetoresistance. We discuss the extent to which these observations can be accounted for by standard transport theory, and describe other possible, unconventional contributions to magnetotransport in very high purity metals. △ Less

Submitted 5 April, 2018; originally announced April 2018.

Cascade of magnetic field induced Lifshitz transitions in the ferromagnetic Kondo lattice material YbNi4P2

Authors: H. Pfau, R. Daou, S. Friedemann, S. Karbassi, S. Ghannadzadeh, R. Kuechler, S. Hamann, A. Steppke, D. Sun, M. Koenig, A. P. Mackenzie, K. Kliemt, C. Krellner, M. Brando

Abstract: A ferromagnetic quantum critical point is thought not to exist in two and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally but common probes such as ARPES and quantum oscillation measurements are lacking. Here, we studi… ▽ More A ferromagnetic quantum critical point is thought not to exist in two and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally but common probes such as ARPES and quantum oscillation measurements are lacking. Here, we studied the magnetic field dependence of transport and thermodynamic properties of YbNi4P2. The Kondo effect is continuously suppressed and additionally we identify nine Lifshitz transitions between 0.4 and 18 T. We analyze the transport coefficients in detail and identify the type of Lifshitz transitions as neck or void type to gain information on the Fermi surface of YbNi4P2. The large number of Lifshitz transitions observed within this small energy window is unprecedented and results from the particular flat renormalized band structure with strong 4f-electron character shaped by the Kondo lattice effect. △ Less

Submitted 4 October, 2017; v1 submitted 19 December, 2016; originally announced December 2016.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 119, 126402 (2017)

Self-organized Growth of Graphene Nanomesh with Increased Gas Sensitivity

Authors: Matthias König, Günther Ruhl, Joerg-Martin Batke, Max C. Lemme

Abstract: A bottom-up chemical vapor deposition (CVD) process for the growth of graphene nanomesh films is demonstrated. The process relies on silicon nanospheres to block nucleation sites for graphene CVD on copper substrates. These spheres are formed in a self-organized way through silicon diffusion through a 5 $μ$m copper layer on a silicon wafer coated with 400 nm of silicon nitride. The temperature dur… ▽ More A bottom-up chemical vapor deposition (CVD) process for the growth of graphene nanomesh films is demonstrated. The process relies on silicon nanospheres to block nucleation sites for graphene CVD on copper substrates. These spheres are formed in a self-organized way through silicon diffusion through a 5 $μ$m copper layer on a silicon wafer coated with 400 nm of silicon nitride. The temperature during the growth process disintegrates the $Si_3$$N_4$ layer and silicon atoms diffuse to the copper surface, where they form the nanospheres. After graphene nanomesh growth, the Si nanospheres can be removed by a simple hydrofluoric acid etch, leaving holes in the graphene film. The nanomesh films have been successfully transferred to different substrates, including gas sensor test structures, and verified and characterized by Auger, TEM and SEM measurements. Electrical/gas-exposure measurements show a 2-fold increase in ammonia sensitivity compared to plain graphene sensors. This improvement can be explained by a higher adsorption site density (edge sites). This new method for nanopatterned graphene is scalable, inexpensive and can be carried out in standard semiconductor industry equipment. Furthermore, the substrates are reusable. △ Less

Submitted 23 August, 2016; originally announced August 2016.

Comments: 13 pages, 3 figures

Journal ref: Nanoscale, 8: 15490, 2016

Terahertz ratchet effects in graphene with a lateral superlattice

Authors: P. Olbrich, J. Kamann, M. König, J. Munzert, L. Tutsch, J. Eroms, D. Weiss, Ming-Hao Liu, L. E. Golub, E. L. Ivchenko, V. V. Popov, D. V. Fateev, K. V. Mashinsky, F. Fromm, Th. Seyller, S. D. Ganichev

Abstract: Experimental and theoretical studies on ratchet effects in graphene with a lateral superlattice excited by alternating electric fields of terahertz frequency range are presented. A lateral superlatice deposited on top of monolayer graphene is formed either by periodically repeated metal stripes having different widths and spacings or by inter-digitated comb-like dual-grating-gate (DGG) structures.… ▽ More Experimental and theoretical studies on ratchet effects in graphene with a lateral superlattice excited by alternating electric fields of terahertz frequency range are presented. A lateral superlatice deposited on top of monolayer graphene is formed either by periodically repeated metal stripes having different widths and spacings or by inter-digitated comb-like dual-grating-gate (DGG) structures. We show that the ratchet photocurrent excited by terahertz radiation and sensitive to the radiation polarization state can be efficiently controlled by the back gate driving the system through the Dirac point as well as by the lateral asymmetry varied by applying unequal voltages to the DGG subgratings. The ratchet photocurrent includes the Seebeck thermoratchet effect as well as the effects of "linear" and "circular" ratchets, sensitive to the corresponding polarization of the driving electromagnetic force. The experimental data are analyzed for the electronic and plasmonic ratchets taking into account the calculated potential profile and the near field acting on carriers in graphene. We show that the photocurrent generation is based on a combined action of a spatially periodic in-plane potential and the spatially modulated light due to the near field effects of the light diffraction. △ Less

Submitted 27 October, 2015; originally announced October 2015.

Journal ref: Phys. Rev. B 93, 075422 (2016)

Going Ballistic: Graphene Hot Electron Transistors

Authors: Sam Vaziri, Anderson D. Smith, Mikael Östling, Grzegorz Lupina, Jarek Dabrowski, Gunther Lippert, Francesco Driussi, Stefano Venica, Valerio Di Lecce, Antonio Gnudi, Matthias König, Günther Ruhl, Melkamu Belete, Max C. Lemme

Abstract: This paper reviews the experimental and theoretical state of the art in ballistic hot electron transistors that utilize two-dimensional base contacts made from graphene, i.e. graphene base transistors (GBTs). Early performance predictions that indicated potential for THz operation still hold true today, even with improved models that take non-idealities into account. Experimental results clearly d… ▽ More This paper reviews the experimental and theoretical state of the art in ballistic hot electron transistors that utilize two-dimensional base contacts made from graphene, i.e. graphene base transistors (GBTs). Early performance predictions that indicated potential for THz operation still hold true today, even with improved models that take non-idealities into account. Experimental results clearly demonstrate the basic functionality, with on/off current switching over several orders of magnitude, but further developments are required to exploit the full potential of the GBT device family. In particular, interfaces between graphene and semiconductors or dielectrics are far from perfect and thus limit experimental device integrity, reliability and performance. △ Less

Submitted 3 September, 2015; originally announced September 2015.

Journal ref: Solid State Communications 224, 64-75, 2015

Melting of iron close to Earth's inner core boundary conditions and beyond

Authors: M. Harmand, A. Ravasio, S. Mazevet, J. Bouchet, A. Denoeud, F. Dorchies, Y. Feng, C. Fourment, E . Galtier, J. Gaudin, F. Guyot, R. Kodama, M. Koenig, H. J. Lee, K. Miyanishi, G. Morard, R. Musella, B. Nagler, M. Nakatsutsumi, N. Ozaki, V. Recoules, S. Toleikis, T. Vinci, U. Zastrau, D. Zhu , et al. (1 additional authors not shown)

Abstract: Several important geophysical features such as heat flux at the Core-Mantle Boundary or geodynamo production are intimately related with the temperature profile in the Earth's core. However, measuring the melting curve of iron at conditions corresponding to the Earth inner core boundary under pressure of 330 GPa has eluded scientists for several decades. Significant discrepancies in previously rep… ▽ More Several important geophysical features such as heat flux at the Core-Mantle Boundary or geodynamo production are intimately related with the temperature profile in the Earth's core. However, measuring the melting curve of iron at conditions corresponding to the Earth inner core boundary under pressure of 330 GPa has eluded scientists for several decades. Significant discrepancies in previously reported iron melting temperatures at high pressure have called into question the validity of dynamic measurements. We report measurements made with a novel approach using X-ray absorption spectroscopy using an X-ray free electron laser source coupled to a laser shock experiment. We determine the state of iron along the shock Hugoniot up to 420 GPa (+/- 50) and 10800 K (+/- 1390) and find an upper boundary for the melting curve of iron by detecting solid iron at 130 GPa and molten at 260, 380 and 420 GPa along the shock Hugoniot. Our result establishes unambiguous agreement between dynamic measurement and recent extrapolations from static data thus resolving the long-standing controversy over the reliability of using dynamic compression to study the melting of iron at conditions close to the Earth's inner core boundary and beyond. △ Less

Submitted 7 November, 2014; originally announced November 2014.

Comments: 9 pages 3 figures

Magnetic Domain Structure of La0.7Sr0.3MnO3 thin-films probed at variable temperature with Scanning Electron Microscopy with Polarization Analysis

Authors: Robert M. Reeve, Christian Mix, Markus König, Michael Foerster, Gerhard Jakob, Mathias Kläui

Abstract: The domain configuration of 50 nm thick La0.7SrMnO3 films has been directly investigated using scanning electron microscopy with polarization analysis (SEMPA), with magnetic contrast obtained without the requirement for prior surface preparation. The large scale domain structure reflects a primarily four-fold anisotropy, with a small uniaxial component, consistent with magneto-optic Kerr effect me… ▽ More The domain configuration of 50 nm thick La0.7SrMnO3 films has been directly investigated using scanning electron microscopy with polarization analysis (SEMPA), with magnetic contrast obtained without the requirement for prior surface preparation. The large scale domain structure reflects a primarily four-fold anisotropy, with a small uniaxial component, consistent with magneto-optic Kerr effect measurements. We also determine the domain transition profile and find it to be in agreement with previous estimates of the domain wall width in this material. The temperature dependence of the image contrast is investigated and compared to superconducting-quantum interference device magnetometry data. A faster decrease in the SEMPA contrast is revealed, which can be explained by the technique's extreme surface sensitivity, allowing us to selectively probe the surface spin polarization which due to the double exchange mechanism exhibits a distinctly different temperature dependence than the bulk magnetization. △ Less

Submitted 27 February, 2013; v1 submitted 22 February, 2013; originally announced February 2013.

Unexpected edge conduction in HgTe quantum wells under broken time reversal symmetry

Authors: Eric Yue Ma, M. Reyes Calvo, Jing Wang, Biao Lian, Mathias Muhlbauer Christoph Brune, Yong-Tao Cui, Keji Lai, Worasom Kundhikanjana, Yongliang Yang, Matthias Baenninger, Markus Konig, Christopher Ames, Hartmut Buhmann, Philipp Leubner, Laurens W. Molenkamp, Shou-Cheng Zhang, David Goldhaber-Gordon, Michael K. Kelly, Zhi-Xun Shen

Abstract: The realization of quantum spin Hall (QSH) effect in HgTe quantum wells (QWs) is considered a milestone in the discovery of topological insulators. The QSH edge states are predicted to allow current to flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction of QSH theory that remains to be experimentally verified is the breakdown of the edge conduction und… ▽ More The realization of quantum spin Hall (QSH) effect in HgTe quantum wells (QWs) is considered a milestone in the discovery of topological insulators. The QSH edge states are predicted to allow current to flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction of QSH theory that remains to be experimentally verified is the breakdown of the edge conduction under broken time reversal symmetry (TRS). Here we first establish a rigorous framework for understanding the magnetic field dependence of electrostatically gated QSH devices. We then report unexpected edge conduction under broken TRS, using a unique cryogenic microwave impedance microscopy (MIM), on a 7.5 nm HgTe QW device with an inverted band structure. At zero magnetic field and low carrier densities, clear edge conduction is observed in the local conductivity profile of this device but not in the 5.5 nm control device whose band structure is trivial. Surprisingly, the edge conduction in the 7.5 nm device persists up to 9 T with little effect from the magnetic field. This indicates physics beyond simple QSH models, possibly associated with material- specific properties, other symmetry protection and/or electron-electron interactions. △ Less

Submitted 2 June, 2015; v1 submitted 27 December, 2012; originally announced December 2012.

Comments: 18 pages, 3 figures, accepted to Nature Communication

Journal ref: Nature Communications 6, Article number: 7252, 2015

Imaging currents in HgTe quantum wells in the quantum spin Hall regime

Authors: Katja C. Nowack, Eric M. Spanton, Matthias Baenninger, Markus König, John R. Kirtley, Beena Kalisky, C. Ames, Philipp Leubner, Christoph Brüne, Hartmut Buhmann, Laurens W. Molenkamp, David Goldhaber-Gordon, Kathryn A. Moler

Abstract: The quantum spin Hall (QSH) state is a genuinely new state of matter characterized by a non-trivial topology of its band structure. Its key feature is conducting edge channels whose spin polarization has potential for spintronic and quantum information applications. The QSH state was predicted and experimentally demonstrated to exist in HgTe quantum wells. The existence of the edge channels has be… ▽ More The quantum spin Hall (QSH) state is a genuinely new state of matter characterized by a non-trivial topology of its band structure. Its key feature is conducting edge channels whose spin polarization has potential for spintronic and quantum information applications. The QSH state was predicted and experimentally demonstrated to exist in HgTe quantum wells. The existence of the edge channels has been inferred from the fact that local and non-local conductance values in sufficiently small devices are close to the quantized values expected for ideal edge channels and from signatures of the spin polarization. The robustness of the edge channels in larger devices and the interplay between the edge channels and a conducting bulk are relatively unexplored experimentally, and are difficult to assess via transport measurements. Here we image the current in large Hallbars made from HgTe quantum wells by probing the magnetic field generated by the current using a scanning superconducting quantum interference device (SQUID). We observe that the current flows along the edge of the device in the QSH regime, and furthermore that an identifiable edge channel exists even in the presence of disorder and considerable bulk conduction as the device is gated or its temperature is raised. Our results represent a versatile method for the characterization of new quantum spin Hall materials systems, and confirm both the existence and the robustness of the predicted edge channels. △ Less

Submitted 13 December, 2012; v1 submitted 10 December, 2012; originally announced December 2012.

Comments: 11 pages, 4 figures; v2: link to supplementary information added, see http://www.stanford.edu/group/moler/publications.html

Frontiers of the physics of dense plasmas and planetary interiors: experiments, theory, applications

Authors: J. J. Fortney, S. H. Glenzer, M. Koenig, B. Militzer, D. Saumon, D. Valencia

Abstract: Recent developments of dynamic x-ray characterization experiments of dense matter are reviewed, with particular emphasis on conditions relevant to interiors of terrestrial and gas giant planets. These studies include characterization of compressed states of matter in light elements by x-ray scattering and imaging of shocked iron by radiography. Several applications of this work are examined. The… ▽ More Recent developments of dynamic x-ray characterization experiments of dense matter are reviewed, with particular emphasis on conditions relevant to interiors of terrestrial and gas giant planets. These studies include characterization of compressed states of matter in light elements by x-ray scattering and imaging of shocked iron by radiography. Several applications of this work are examined. These include the structure of massive "Super Earth" terrestrial planets around other stars, the 40 known extrasolar gas giants with measured masses and radii, and Jupiter itself, which serves as the benchmark for giant planets. △ Less

Submitted 24 February, 2009; originally announced February 2009.

Comments: Accepted to Physics of Plasmas special issue. Review from HEDP/HEDLA-08, April 12-15, 2008

Ballistic Intrinsic Spin-Hall Effect in HgTe Nanostructures

Authors: C. Bruene, A. Roth, E. G. Novik, M. Koenig, H. Buhmann, E. M. Hankiewicz, W. Hanke, J. Sinova, L. W. Molenkamp

Abstract: We report the first electrical manipulation and detection of the mesoscopic intrinsic spin-Hall effect (ISHE) in semiconductors through non-local electrical measurement in nano-scale H-shaped structures built on high mobility HgTe/HgCdTe quantum wells. By controlling the strength of the spin-orbit splittings and the n-type to p-type transition by a top-gate, we observe a large non-local resistan… ▽ More We report the first electrical manipulation and detection of the mesoscopic intrinsic spin-Hall effect (ISHE) in semiconductors through non-local electrical measurement in nano-scale H-shaped structures built on high mobility HgTe/HgCdTe quantum wells. By controlling the strength of the spin-orbit splittings and the n-type to p-type transition by a top-gate, we observe a large non-local resistance signal due to the ISHE in the p-regime, of the order of kOhms, which is several orders of magnitude larger than in metals. In the n-regime, as predicted by theory, the signal is at least an order of magnitude smaller. We verify our experimental observation by quantum transport calculations which show quantitative agreement with the experiments. △ Less

Submitted 19 December, 2008; originally announced December 2008.

Comments: 12 pages, incl. 5 figures

The Quantum Spin Hall Effect: Theory and Experiment

Authors: Markus Koenig, Hartmut Buhmann, Laurens W. Molenkamp, Taylor L. Hughes, Chao-Xing Liu, Xiao-Liang Qi, Shou-Cheng Zhang

Abstract: The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the helical edge states give rise to the quantum sp… ▽ More The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the helical edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. Here we review a recent theory which predicts that the QSH state can be realized in HgTe/CdTe semiconductor quantum wells. By varying the thickness of the quantum well, the band structure changes from a normal to an "inverted" type at a critical thickness $d_c$. We present an analytical solution of the helical edge states and explicitly demonstrate their topological stability. We also review the recent experimental observation of the QSH state in HgTe/(Hg,Cd)Te quantum wells. We review both the fabrication of the sample and the experimental setup. For thin quantum wells with well width $d_{QW}< 6.3$ nm, the insulating regime shows the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells ($d_{QW}> 6.3$ nm), the nominally insulating regime shows a plateau of residual conductance close to $2e^2/h$. The residual conductance is independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance is destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, $d_c= 6.3$ nm, is also independently determined from the occurrence of a magnetic field induced insulator to metal transition. △ Less

Submitted 7 January, 2008; originally announced January 2008.

Comments: Invited review article for special issue of JPSJ, 32 pages. For higher resolution figures see official online version when published

Quantum Spin Hall Insulator State in HgTe Quantum Wells

Authors: Markus Koenig, Steffen Wiedmann, Christoph Bruene, Andreas Roth, Hartmut Buhmann, Laurens W. Molenkamp, Xiao-Liang Qi, Shou-Cheng Zhang

Abstract: Recent theory predicted that the Quantum Spin Hall Effect, a fundamentally novel quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We have fabricated such sample structures with low density and high mobility in which we can tune, through an external gate voltage, the carrier conduction from n-type to the p-type, passing through… ▽ More Recent theory predicted that the Quantum Spin Hall Effect, a fundamentally novel quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We have fabricated such sample structures with low density and high mobility in which we can tune, through an external gate voltage, the carrier conduction from n-type to the p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nm, the insulating regime shows the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nm), the nominally insulating regime shows a plateau of residual conductance close to 2e^2/h. The residual conductance is independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance is destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nm, is also independently determined from the magnetic field induced insulator to metal transition. These observations provide experimental evidence of the quantum spin Hall effect. △ Less

Submitted 2 October, 2007; originally announced October 2007.

Comments: 16 pages, 5 figures

Journal ref: Science, Sciencexpress 20 September 2007

Quantum dot as thermal rectifier

Authors: R. Scheibner, M. Koenig, D. Reuter, A. D. Wieck, H. Buhmann, L. W. Molenkamp

Abstract: We report the observation of thermal rectification in a semiconductor quantum dot, as inferred from the asymmetric line shape of the thermopower oscillations. The asymmetry is observed at high in-plane magnetic fields and caused by the presence of a high orbital momentum state in the dot. We report the observation of thermal rectification in a semiconductor quantum dot, as inferred from the asymmetric line shape of the thermopower oscillations. The asymmetry is observed at high in-plane magnetic fields and caused by the presence of a high orbital momentum state in the dot. △ Less

Submitted 20 March, 2007; originally announced March 2007.

Comments: 4 pages, 4 figures

Sequential and co-tunneling behavior in the temperature-dependent thermopower of few-electron quantum dots

Authors: R. Scheibner, E. G. Novik, T. Borzenko, M. Koenig, D. Reuter, A. D. Wieck, H. Buhmann, L. W. Molenkamp

Abstract: We have studied the temperature dependent thermopower of gate-defined, lateral quantum dots in the Coulomb blockade regime using an electron heating technique. The line shape of the thermopower oscillations depends strongly on the contributing tunneling processes. Between 1.5 K and 40 mK a crossover from a pure sawtooth- to an intermitted sawtooth-like line shape is observed. The latter is attri… ▽ More We have studied the temperature dependent thermopower of gate-defined, lateral quantum dots in the Coulomb blockade regime using an electron heating technique. The line shape of the thermopower oscillations depends strongly on the contributing tunneling processes. Between 1.5 K and 40 mK a crossover from a pure sawtooth- to an intermitted sawtooth-like line shape is observed. The latter is attributed to the increasing dominance of cotunneling processes in the Coulomb blockade regime at low temperatures. △ Less

Submitted 8 August, 2006; originally announced August 2006.

Comments: 4 pages, 4 figures, submitted to Phys. Rev. B

Direct observation of the Aharonov-Casher phase

Authors: M. Koenig, A. Tschetschetkin, E. M. Hankiewicz, Jairo Sinova, V. Hock, V. Daumer, M. Schaefer, C. R. Becker, H. Buhmann, L. W. Molenkamp

Abstract: Ring structures fabricated from HgTe/HgCdTe quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe non-monotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison w… ▽ More Ring structures fabricated from HgTe/HgCdTe quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe non-monotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison with numerical calculations of the magneto-conductance for a multichannel ring structure within the Landauer-Büttiker formalism. △ Less

Submitted 17 August, 2005; originally announced August 2005.

Comments: 4 pages, 4 figures

Current heating of a magnetic 2DEG in HgMnTe/HgCdTe quantum wells

Authors: Y. S. Gui, C. R. Becker, J. Liu, M. Koenig, V. Daumer, M. N. Kiselev, H. Buhmann, L. W. Molenkamp

Abstract: Heating caused by electrons with excess kinetic energy has been investigated in a magnetic two-dimensional electron gas, M2DEG, in HgMnTe/HgCdTe(001) quantum wells. The temperature of the Mn ions, T_Mn, has been determined by the node positions in the beating pattern in Shubnikov-de Haas oscillations.The experimental dependence of T_Mn on current and therefore on electron temperature, is in exce… ▽ More Heating caused by electrons with excess kinetic energy has been investigated in a magnetic two-dimensional electron gas, M2DEG, in HgMnTe/HgCdTe(001) quantum wells. The temperature of the Mn ions, T_Mn, has been determined by the node positions in the beating pattern in Shubnikov-de Haas oscillations.The experimental dependence of T_Mn on current and therefore on electron temperature, is in excellent agreement with a rate equation model. Results with this model show that the energy transfer rate from the electrons to the Mn system is proportional to the Mn concentration. △ Less

Submitted 17 December, 2003; originally announced December 2003.

Comments: 4 pages, 5 figures, 2 tables