Showing 1–19 of 19 results for author: Schwarz, T

Vacuum Dealloyed Brass as Li-Metal Battery Current Collector: Effect of Zinc and Porosity

Authors: Eric V Woods, Xinren Chen, Shaolou Wei, Yuwei Zhang, Alisson Kwiatkowski da Silva, Ayman A El-Zoka, J Manoj Prabhakar, Tim M Schwarz, Yongqiang Kang, Leonardo S Aota, Mahander P Singh, Katja Angenendt, Ozge Ozgun, Matic Jovivcevic-Klug, Patricia Jovivcevic-Klug, Christian Bross, Jian Liu, Rene de Kloe, Gerhard Dehm, Stefan Zaefferer, Yug Joshi, Baptiste Gault

Abstract: "Anode-free" lithium-metal batteries promise significantly higher energy density than conventional graphite-based lithium-ion batteries; however, lithium dendrite growth can lead to internal short circuits with associated safety risks. While porous current collectors can suppress dendrite growth, optimal porosity and composition remain unknown. Here, we show that the temperature during vapor phase… ▽ More "Anode-free" lithium-metal batteries promise significantly higher energy density than conventional graphite-based lithium-ion batteries; however, lithium dendrite growth can lead to internal short circuits with associated safety risks. While porous current collectors can suppress dendrite growth, optimal porosity and composition remain unknown. Here, we show that the temperature during vapor phase dealloying (VPD) of alpha-brass (Cu63Zn37) controls the surface Zn concentration, decreasing from 8 percent to below 1 percent from 500 to 800 degrees C. The surface composition is controlled by the temperature-dependent diffusion. A battery cell maintains greater than 90 percent Coulombic efficiency (CE) over 100 cycles when the Zn content is the lowest, whereas the higher-Zn samples degraded to approximately 70 percent CE. The difference in surface composition has hence dramatic effects on battery performance, and our results demonstrate how precise compositional control enables stable lithium-metal battery operation, establishing about 1 atomic percent surface Zn as optimal for preventing capacity fading and uniform lithium plating, while establishing predictive relationships between processing temperature and surface composition. This work provides design rules for multifunctional current collectors and demonstrates scalable VPD production for next-generation batteries. △ Less

Submitted 12 August, 2025; v1 submitted 8 August, 2025; originally announced August 2025.

Comments: 58 pages, 6 main figures, 20 SI figures; main text and supplementary information included in a single PDF

Unraveling the thermodynamics and mechanism behind the lowering of reduction temperatures in oxide mixtures

Authors: Shiv Shankar, Barak Ratzker, Alisson Kwiatkowski da Silva, Tim M. Schwarz, Hans Brouwer, Baptiste Gault, Yan Ma, Dierk Raabe

Abstract: Hydrogen-based direct reduction offers a sustainable pathway to decarbonize the metal production industry. However, stable metal oxides, like Cr$_2$O$_3$, are notoriously difficult to reduce, requiring extremely high temperatures (above 1300 $^\circ$C). Herein, we show how reducing mixed oxides can be leveraged to lower hydrogen-based reduction temperatures of stable oxides and produce alloys in a… ▽ More Hydrogen-based direct reduction offers a sustainable pathway to decarbonize the metal production industry. However, stable metal oxides, like Cr$_2$O$_3$, are notoriously difficult to reduce, requiring extremely high temperatures (above 1300 $^\circ$C). Herein, we show how reducing mixed oxides can be leveraged to lower hydrogen-based reduction temperatures of stable oxides and produce alloys in a single process. Using a newly developed thermodynamic framework, we predict the precise conditions (oxygen partial pressure, temperature, and oxide composition) needed for co-reduction. We showcase this approach by reducing Cr$_2$O$_3$ mixed with Fe$_2$O$_3$ at 1100 $^\circ$C, significantly lowering reduction temperatures (by $\geq$200 $^\circ$C). Our model and post-reduction atom probe tomography analysis elucidate that the temperature-lowering effect is driven by the lower chemical activity of Cr in the metallic phase. This strategy achieves low-temperature co-reduction of mixed oxides, dramatically reducing energy consumption and CO$_2$ emissions, while unlocking transformative pathways toward sustainable alloy design. △ Less

Submitted 17 April, 2025; originally announced April 2025.

Advancing atom probe tomography capabilities to understand bone microstructures at the near-atomic scale

Authors: Tim M. Schwarz, Maïtena Dumont, Victoria Garcia-Giner, Chanwon Jung, Alexandra E. Porter, Baptiste Gault

Abstract: Bone structure is generally hierarchically organized into organic (collagen, proteins,...), inorganic (hydroxyapatite (HAP)) components. However, many fundamental mechanisms of the biomineralization processes such as HAP formation, the influence of trace elements, the mineral-collagen arrangement, etc., are not clearly understood. This is partly due to the analytical challenge of simultaneously ch… ▽ More Bone structure is generally hierarchically organized into organic (collagen, proteins,...), inorganic (hydroxyapatite (HAP)) components. However, many fundamental mechanisms of the biomineralization processes such as HAP formation, the influence of trace elements, the mineral-collagen arrangement, etc., are not clearly understood. This is partly due to the analytical challenge of simultaneously characterizing the three-dimensional (3D) structure and chemical composition of biominerals in general at the nanometer scale, which can, in principle be achieved by atom probe tomography (APT). Yet, the hierarchical structures of bone represent a critical hurdle for APT analysis in terms of sample yield and analytical resolution, particularly for trace elements, and organic components from the collagen appear to systematically get lost from the analysis. Here, we applied in-situ metallic coating of APT specimens within the focused ion beam (FIB) used for preparing specimens, and demonstrate that the sample yield and chemical sensitivity are tremendously improved, allowing the analysis of individual collagen fibrils and trace elements such as Mg and Na. We explored a range of measurement parameters with and without coating, in terms of analytical resolution performance and determined the best practice parameters for analyzing bone samples in APT. To decipher the complex mass spectra of the bone specimens, reference spectra from pure HAP and collagen were acquired to unambiguously identify the signals, allowing us to analyze entire collagen fibrils and interfaces at the near-atomic scale. Our results open new possibilities for understanding the hierarchical structure and chemical heterogeneity of bone structures at the near-atomic level and demonstrate the potential of this new method to provide new, unexplored insights into biomineralization processes in the future. △ Less

Submitted 25 March, 2025; originally announced March 2025.

Consistent time reversal and reliable and accurate inference in the presence of memory

Authors: Tassilo Schwarz, Anatoly B. Kolomeisky, Aljaž Godec

Abstract: Thermodynamic inference from coarse observations remains a key challenge. Memory, in particular correlations between consecutively observed mesostates, blur signatures of irreversibility and must be accounted for in defining physical time-reversal, which remains an open problem. We derive an experimentally accessible k-th order estimator for the entropy production rate. Using novel measure-theoret… ▽ More Thermodynamic inference from coarse observations remains a key challenge. Memory, in particular correlations between consecutively observed mesostates, blur signatures of irreversibility and must be accounted for in defining physical time-reversal, which remains an open problem. We derive an experimentally accessible k-th order estimator for the entropy production rate. Using novel measure-theoretic techniques we prove necessary and sufficient conditions for guaranteed lower bounds on the dissipation even in the strongly non-Markovian setting. The proof reveals that estimators saturated in the order unravel the duration of memory which needs to be considered in defining physically consistent time-reversal. We show that Markovian estimators in absence of a time-scale separation lead to artifacts, which convey no physical meaning. Similarly, estimators not saturated in the order may overestimate the dissipation. The necessity of correctly accounting for memory in thermodynamic inference from strongly non-Markovian observations underscores the still underappreciated challenges and intricacies in defining and understanding irreversibility in presence of memory. Our results will hopefully stimulate experiments systematically considering thermodynamic inference on multiple scales consistently accounting for memory. △ Less

Submitted 23 December, 2025; v1 submitted 15 October, 2024; originally announced October 2024.

Towards establishing best practice in the analysis of hydrogen and deuterium by atom probe tomography

Authors: Baptiste Gault, Aparna Saksena, Xavier Sauvage, Paul Bagot, Leonardo S. Aota, Jonas Arlt, Lisa T. Belkacemi, Torben Boll, Yi-Sheng Chen, Luke Daly, Milos B. Djukic, James O. Douglas, Maria J. Duarte, Peter J. Felfer, Richard G. Forbes, Jing Fu, Hazel M. Gardner, Ryota Gemma, Stephan S. A. Gerstl, Yilun Gong, Guillaume Hachet, Severin Jakob, Benjamin M. Jenkins, Megan E. Jones, Heena Khanchandani , et al. (20 additional authors not shown)

Abstract: As hydrogen is touted as a key player in the decarbonization of modern society, it is critical to enable quantitative H analysis at high spatial resolution, if possible at the atomic scale. Indeed, H has a known deleterious impact on the mechanical properties (strength, ductility, toughness) of most materials that can hinder their use as part of the infrastructure of a hydrogen-based economy. Enab… ▽ More As hydrogen is touted as a key player in the decarbonization of modern society, it is critical to enable quantitative H analysis at high spatial resolution, if possible at the atomic scale. Indeed, H has a known deleterious impact on the mechanical properties (strength, ductility, toughness) of most materials that can hinder their use as part of the infrastructure of a hydrogen-based economy. Enabling H mapping, including local hydrogen concentration analyses at specific microstructural features, is essential for understanding the multiple ways that H affect the properties of materials, including for instance embrittlement mechanisms and their synergies, but also spatial mapping and quantification of hydrogen isotopes is essential to accurately predict tritium inventory of future fusion power plants, ensuring their safe and efficient operation for example. Atom probe tomography (APT) has the intrinsic capabilities for detecting hydrogen (H), and deuterium (D), and in principle the capacity for performing quantitative mapping of H within a material's microstructure. Yet the accuracy and precision of H analysis by APT remain affected by the influence of residual hydrogen from the ultra-high vacuum chamber that can obscure the signal of H from within the material, along with a complex field evaporation behavior. The present article reports the essence of discussions at a focused workshop held at the Max-Planck Institute for Sustainable Materials in April 2024. The workshop was organized to pave the way to establishing best practices in reporting APT data for the analysis of H. We first summarize the key aspects of the intricacies of H analysis by APT and propose a path for better reporting of the relevant data to support interpretation of APT-based H analysis in materials. △ Less

Submitted 21 May, 2024; originally announced May 2024.

Nanoporous gold thin films as substrates to analyze liquids by cryo-atom probe tomography

Authors: E. V. Woods, A. Saksena, A. A. El-Zoka, L. T. Stephenson, T. M. Schwarz, M. P. Singh, L. S. Aota, S. -H. Kim, J. Schneider, B. Gault

Abstract: Cryogenic atom probe tomography (cryo-APT) is being developed to enable nanoscale compositional analyses of frozen liquids. Yet, the availability of readily available substrates that allow for the fixation of liquids while providing sufficient strength to their interface, is still an issue. Here we propose the use of 1-2 microns thick binary alloy film of gold-silver (AuAg) sputtered onto flat sil… ▽ More Cryogenic atom probe tomography (cryo-APT) is being developed to enable nanoscale compositional analyses of frozen liquids. Yet, the availability of readily available substrates that allow for the fixation of liquids while providing sufficient strength to their interface, is still an issue. Here we propose the use of 1-2 microns thick binary alloy film of gold-silver (AuAg) sputtered onto flat silicon, with sufficient adhesion without an additional layer. Through chemical dealloying, we successfully fabricate a nanoporous substrate, with open-pore structure, which is mounted on a microarray of Si posts by lift out in the focused-ion beam, allowing for cryogenic fixation of liquids. We present cryo-APT results obtained after cryogenic sharpening, vacuum cryo-transfer and analysis of pure water on top and inside the nanoporous film. We demonstrate that this new substrate has the requisite characteristics for facilitating cryo-APT of frozen liquids, with a relatively lower volume of precious metals. This complete workflow represents an improved approach for frozen liquid analysis, from preparation of the films to the successful fixation of the liquid in the porous network, to cryo-atom probe tomography. △ Less

Submitted 22 November, 2023; originally announced November 2023.

In-situ metallic coating of atom probe specimen for enhanced yield, performance, and increased field-of-view

Authors: Tim M. Schwarz, Eric Woods, Mahander P. Singh, Chanwon Jung, Leonardo S. Aota, Kyuseon Jang, Mathias Krämer, Se-Ho Kim, Ingrid McCarroll, Baptiste Gault

Abstract: Atom probe tomography requires needle-shaped specimens with a diameter typically below 100 nm, making them both very fragile and reactive, and defects (notches at grain boundaries or precipitates) are known to affect the yield and data quality. The use of a conformal coating directly on the sharpened specimen has been proposed to increase yield and reduce background. However, to date, these coatin… ▽ More Atom probe tomography requires needle-shaped specimens with a diameter typically below 100 nm, making them both very fragile and reactive, and defects (notches at grain boundaries or precipitates) are known to affect the yield and data quality. The use of a conformal coating directly on the sharpened specimen has been proposed to increase yield and reduce background. However, to date, these coatings have been applied ex-situ and mostly are not uniformly. Here, we report on the controlled focused ion beam in-situ deposition of a thin metal film on specimens immediately after specimen preparation. Different metallic targets e.g. Cr were attached to a micromanipulator via a conventional lift-out method and sputtered using the Ga or Xe ions. We showcase the many advantages of coating specimens from metallic to non-metallic materials. We have identified an increase in data quality and yield, an improvement of the mass resolution, as well as an increase in the effective field-of-view enabling visualization of the entire original specimen, including the complete surface oxide layer. The ease of implementation of the approach makes it very attractive for generalizing its use across a very wide range of atom probe analyses. △ Less

Submitted 17 January, 2024; v1 submitted 14 September, 2023; originally announced September 2023.

A versatile and reproducible cryo-sample preparation methodology for atom probe studies

Authors: Eric V. Woods, Mahander P. Singh, Se-Ho Kim, Tim M. Schwarz, James O. Douglas, Ayman El-Zoka, Finn Giulani, Baptiste Gault

Abstract: Repeatable and reliable site-specific preparation of specimens for atom probe tomography (APT) at cryogenic temperatures has proven challenging. A generalized workflow is required for cryogenic-specimen preparation including lift-out via focused-ion beam and in-situ deposition of capping layers, to strengthen specimens that will be exposed to high electric field and stresses during field evaporati… ▽ More Repeatable and reliable site-specific preparation of specimens for atom probe tomography (APT) at cryogenic temperatures has proven challenging. A generalized workflow is required for cryogenic-specimen preparation including lift-out via focused-ion beam and in-situ deposition of capping layers, to strengthen specimens that will be exposed to high electric field and stresses during field evaporation in APT, and protect them from environment during transfer into the atom probe. Here, we build on existing protocols, and showcase preparation and analysis of a variety of metals, oxides and supported frozen liquids and battery materials. We demonstrate reliable in-situ deposition of a metallic capping layer that significantly improve the atom probe data quality for challenging material systems, particularly battery cathode materials which are subjected to delithiation during the atom probe analysis itself. Our workflow designed is versatile and transferable widely to other instruments. △ Less

Submitted 31 March, 2023; originally announced March 2023.

The modular Atom Probe Concept

Authors: Patrick Stender, Helena Solodenko, Andreas Weigel, Irdi Balla, Tim Maximilian Schwarz, Jonas Ott, Manuel Roussell, Rüya Duran, Sebastian Eich, Mohammad Al-Shakran, Timo Jacob, Guido Schmitz

Abstract: Atomic probe tomography (APT), based on the work of Erwin Mueller, is able to generate three-dimensional chemical maps in atomic resolution. The required instruments for APT have evolved over the last 20 years from an experimental to an established method of materials analysis. Here, we describe the realization of a new instrument concept that allows the direct attachment of APT to a dual beam SEM… ▽ More Atomic probe tomography (APT), based on the work of Erwin Mueller, is able to generate three-dimensional chemical maps in atomic resolution. The required instruments for APT have evolved over the last 20 years from an experimental to an established method of materials analysis. Here, we describe the realization of a new instrument concept that allows the direct attachment of APT to a dual beam SEM microscope with the main achievement of fast and direct sample transfer. New operational modes are enabled regarding sample geometry, alignment of tips and microelectrode. The instrument is optimized to handle cryo-samples at all stages of preparation and storage. The instrument comes with its own software for evaluation and reconstruction. The performance in terms of mass resolution, aperture angle, and detection efficiency is demonstrated with a few application examples. △ Less

Submitted 7 May, 2021; originally announced May 2021.

Status and direction of atom probe analysis of frozen liquids

Authors: Patrick Stender, Baptiste Gault, Tim M. Schwarz, Eric V. Woods, Se-Ho Kim, Jonas Ott, Leigh T. Stephenson, Guido Schmitz, Christoph Freysoldt, Johannes Kästner, Ayman A. El-Zoka

Abstract: Imaging of liquids and cryogenic biological materials by electron microscopy has been recently enabled by innovative approaches for specimen preparation and the fast development of optimised instruments for cryo-enabled electron microscopy (cryo-EM). Yet, Cryo-EM typically lacks advanced analytical capabilities, in particular for light elements. With the development of protocols for frozen wet spe… ▽ More Imaging of liquids and cryogenic biological materials by electron microscopy has been recently enabled by innovative approaches for specimen preparation and the fast development of optimised instruments for cryo-enabled electron microscopy (cryo-EM). Yet, Cryo-EM typically lacks advanced analytical capabilities, in particular for light elements. With the development of protocols for frozen wet specimen preparation, atom probe tomography (APT) could advantageously complement insights gained by cryo-EM. Here, we report on different approaches that have been recently proposed to enable the analysis of relatively large volumes of frozen liquids from either a flat substrate or the fractured surface of a wire. Both allowed for analysing water ice layers which are several microns thick consisting of pure water, pure heavy-water and aqueous solutions. We discuss the merits of both approaches, and prospects for further developments in this area. Preliminary results raise numerous questions, in part concerning the physics underpinning field evaporation. We discuss these aspects and lay out some of the challenges regarding the APT analysis of frozen liquids. △ Less

Submitted 3 February, 2021; originally announced February 2021.

Comments: submitted for publication assocaited to the APT&M 2020 conference

Machine-learning-enhanced time-of-flight mass spectrometry analysis

Authors: Ye Wei, Rama Srinivas Varanasi, Torsten Schwarz, Leonie Gomell, Huan Zhao, David J. Larson, Binhan Sun, Geng Liu, Hao Chen, Dierk Raabe, Baptiste Gault

Abstract: Mass spectrometry is a widespread approach to work out what are the constituents of a material. Atoms and molecules are removed from the material and collected, and subsequently, a critical step is to infer their correct identities based from patterns formed in their mass-to-charge ratios and relative isotopic abundances. However, this identification step still mainly relies on individual user's e… ▽ More Mass spectrometry is a widespread approach to work out what are the constituents of a material. Atoms and molecules are removed from the material and collected, and subsequently, a critical step is to infer their correct identities based from patterns formed in their mass-to-charge ratios and relative isotopic abundances. However, this identification step still mainly relies on individual user's expertise, making its standardization challenging, and hindering efficient data processing. Here, we introduce an approach that leverages modern machine learning technique to identify peak patterns in time-of-flight mass spectra within microseconds, outperforming human users without loss of accuracy. Our approach is cross-validated on mass spectra generated from different time-of-flight mass spectrometry(ToF-MS) techniques, offering the ToF-MS community an open-source, intelligent mass spectra analysis. △ Less

Submitted 2 October, 2020; originally announced October 2020.

Comments: 20 pages, 15 figures

Index-antiguiding in narrow-ridge GaN-based laser diodes investigated by measurements of the current-dependent gain and index spectra and by self-consistent simulation

Authors: Luca Redaelli, Hans Wenzel, Joachim Piprek, Thomas Weig, Sven Einfeldt, Martin Martens, Gerrit Lükens, Ulrich T. Schwarz, Michael Kneissl

Abstract: The threshold current density of narrow (1.5 μm) ridge-waveguide InGaN multi-quantum-well laser diodes, as well as the shape of their lateral far-field patterns, strongly depend on the etch depth of the ridge waveguide. Both effects can be attributed to strong index-antiguiding. A value of the antiguiding factor R = 10 is experimentally determined near threshold by measurements of the current-depe… ▽ More The threshold current density of narrow (1.5 μm) ridge-waveguide InGaN multi-quantum-well laser diodes, as well as the shape of their lateral far-field patterns, strongly depend on the etch depth of the ridge waveguide. Both effects can be attributed to strong index-antiguiding. A value of the antiguiding factor R = 10 is experimentally determined near threshold by measurements of the current-dependent gain and refractive index spectra. The device performances are simulated self-consistently solving the Schrödinger-Poisson equations and the equations for charge transport and waveguiding. Assuming a carrier-induced index change which matches the experimentally determined antiguiding factor, both the measured high threshold current and the shape of the far-field pattern of lasers with shallow ridges can be reproduced theoretically. △ Less

Submitted 8 March, 2016; originally announced March 2016.

Comments: This is an author-created, un-copyedited version of an article accepted for publication in the IEEE Journal of Quantum Electronics. IEEE is not responsible for any errors or omissions in this version of the manuscript or any version derived from it

Journal ref: IEEE J. Quantum Electron., Vol 51, Issue 8, Article# 2000506 (2015)

Low-Noise YBa$_2$Cu$_3$O$_7$ NanoSQUIDs for Performing Magnetization-Reversal Measurements on Magnetic Nanoparticles

Authors: Tobias Schwarz, Roman Wölbing, Christopher F. Reiche, Benedikt Müller, Maria-José Martínez-Pérez, Thomas Mühl, Bernd Büchner, Reinhold Kleiner, Dieter Koelle

Abstract: We fabricated YBa$_2$Cu$_3$O$_7$ (YBCO) direct current (dc) nano superconducting quantum interference devices (nanoSQUIDs) based on grain boundary Josephson junctions by focused ion beam patterning. Characterization of electric transport and noise properties at 4.2$\,$K in magnetically shielded environment yields a very small inductance $L$ of a few pH for an optimized device geometry. This in tur… ▽ More We fabricated YBa$_2$Cu$_3$O$_7$ (YBCO) direct current (dc) nano superconducting quantum interference devices (nanoSQUIDs) based on grain boundary Josephson junctions by focused ion beam patterning. Characterization of electric transport and noise properties at 4.2$\,$K in magnetically shielded environment yields a very small inductance $L$ of a few pH for an optimized device geometry. This in turn results in very low values of flux noise $<50\,{\rm n}Φ_0/{\rm Hz}^{1/2}$ in the thermal white noise limit, which yields spin sensitivities of a few $μ_{\rm B}/{\rm Hz}^{1/2}$ ($Φ_0$ is the magnetic flux quantum and $μ_{\rm B}$ is the Bohr magneton). We observe frequency-dependent excess noise up to 7$\,$MHz, which can only partially be eliminated by bias reversal readout. This indicates the presence of fluctuators of unknown origin, possibly related to defect-induced spins in the SrTiO$_3$ substrate. We demonstrate the potential of using YBCO nanoSQUIDs for the investigation of small spin systems, by placing a 39$\,$nm diameter Fe nanowire, encapsulated in a carbon nanotube, on top of a non-optimized YBCO nanoSQUID and by measuring the magnetization reversal of the Fe nanowire via the change of magnetic flux coupled to the nanoSQUID. The measured flux signals upon magnetization reversal of the Fe nanowire are in very good agreement with estimated values, and the determined switching fields indicate magnetization reversal of the nanowire via curling mode. △ Less

Submitted 20 March, 2015; originally announced March 2015.

Comments: 10 pages, 5 figure and supplemental material (4 pages)

Optimizing the spin sensitivity of grain boundary junction nanoSQUIDs -- towards detection of small spin systems with single-spin resolution

Authors: Roman Wölbing, Tobias Schwarz, Benedikt Müller, Joachim Nagel, Matthias Kemmler, Reinhold Kleiner, Dieter Koelle

Abstract: We present an optimization study of the spin sensitivity of nanoSQUIDs based on resistively shunted grain boundary Josephson junctions. In addition the dc SQUIDs contain a narrow constriction onto which a small magnetic particle can be placed (with its magnetic moment in the plane of the SQUID loop and perpendicular to the grain boundary) for efficient coupling of its stray magnetic field to the S… ▽ More We present an optimization study of the spin sensitivity of nanoSQUIDs based on resistively shunted grain boundary Josephson junctions. In addition the dc SQUIDs contain a narrow constriction onto which a small magnetic particle can be placed (with its magnetic moment in the plane of the SQUID loop and perpendicular to the grain boundary) for efficient coupling of its stray magnetic field to the SQUID loop. The separation of the location of optimum coupling from the junctions allows for an independent optimization of the coupling factor $φ_μ$ and junction properties. We present different methods for calculating $φ_μ$ (for a magnetic nanoparticle placed 10\,nm above the constriction) as a function of device geometry and show that those yield consistent results. Furthermore, by numerical simulations we obtain a general expression for the dependence of the SQUID inductance on geometrical parameters of our devices, which allows to estimate their impact on the spectral density of flux noise $S_Φ$ of the SQUIDs in the thermal white noise regime. Our analysis of the dependence of $S_Φ$ and $φ_μ$ on the geometric parameters of the SQUID layout yields a spin sensitivity $S_μ^{1/2}=S_Φ^{1/2}/φ_μ$ of a few $μ_{\rm{B}}/\rm{Hz^{1/2}}$ ($μ_B$ is the Bohr magneton) for optimized parameters, respecting technological constraints. However, by comparison with experimentally realized devices we find significantly larger values for the measured white flux noise, as compared to our theoretical predictions. Still, a spin sensitivity on the order of $10\,μ_{\rm B}/\rm{Hz^{1/2}}$ for optimized devices seems to be realistic. △ Less

Submitted 10 June, 2014; v1 submitted 5 June, 2014; originally announced June 2014.

Comments: 10 pages, 5 figures, Superconductor Science and Technology (submitted). arXiv admin note: text overlap with arXiv:1301.1189 The paper has been withdrawn due to a wrong article number assignment. The article 1406.1402 was intended to be a replacement of 1301.1189

Nb nano superconducting quantum interference devices with high spin sensitivity for operation in magnetic fields up to 0.5\,T

Authors: Roman Wölbing, Joachim Nagel, Tobias Schwarz, Oliver Kieler, Thomas Weimann, Johannes Kohlmann, Alexander Zorin, Matthias Kemmler, Reinhold Kleiner, Dieter Koelle

Abstract: We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity $S_μ^{1/2}$ upon operation in intermediate magnetic fields $B$ (tens of mT), applied perpendicular to the subst… ▽ More We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity $S_μ^{1/2}$ upon operation in intermediate magnetic fields $B$ (tens of mT), applied perpendicular to the substrate plane. Our so far best SQUID can be continuously operated in fields up to $B\approx\pm50\,\rm{mT}$ with rms flux noise $S_{Φ,\rm w}^{1/2}\leq250\,\rm{nΦ_0/Hz^{1/2}}$ in the white noise regime and spin sensitivity $S_μ^{1/2}\leq29\,\rm{μ_B/Hz^{1/2}}$. Furthermore, we demonstrate operation in $B=0.5\,\rm{T}$ with high sensitivity in flux $S_{Φ,\rm w}^{1/2}\approx680\,\rm{nΦ_0/Hz^{1/2}}$ and in electron spin $S_μ^{1/2}\approx79\,\rm{μ_B/Hz^{1/2}}$. We discuss strategies to further improve the nanoSQUID performance. △ Less

Submitted 29 April, 2013; originally announced April 2013.

Comments: to appear in Apl. Phys. Lett

Superfluorescent emission in electrically pumped semiconductor laser

Authors: D. L. Boiko, X. Zeng, T. Stadelmann, S. Grossmann, A. Hoogerwerf, T. Weig, U. T. Schwarz, L. Sulmoni, J. -M. Lamy, N. Grandjean

Abstract: We report superfluorescent (SF) emission in electrically pumped InGaN/InGaN QW lasers with saturable absorber. In particular, we observe a superlinear growth of the peak power of SF pulses with increasing amplitude of injected current pulses and attribute it to cooperative pairing of electron-hole (e-h) radiative recombinations. The phase transitions from amplified spontaneous emission to superflu… ▽ More We report superfluorescent (SF) emission in electrically pumped InGaN/InGaN QW lasers with saturable absorber. In particular, we observe a superlinear growth of the peak power of SF pulses with increasing amplitude of injected current pulses and attribute it to cooperative pairing of electron-hole (e-h) radiative recombinations. The phase transitions from amplified spontaneous emission to superfluorescence and then to lasing regime is confirmed by observing (i) abrupt peak power growth accompanied by spectral broadening, (ii) spectral shape with hyperbolic secant envelope and (iii) red shift of central wavelength of SF emission pulse. The observed red shift of SF emission is shown to be caused by the pairing of e-h pairs in an indirect cooperative X-transition. △ Less

Submitted 11 February, 2013; v1 submitted 1 February, 2013; originally announced February 2013.

Comments: 18 pages, 7 figures

Optimizing the spin sensitivity of grain boundary junction nanoSQUIDs -- towards detection of small spin systems with single-spin resolution

Authors: Roman Wölbing, Tobias Schwarz, Benedikt Müller, Joachim Nagel, Matthias Kemmler, Reinhold Kleiner, Dieter Koelle

Abstract: We present an optimization study of the spin sensitivity of nanoSQUIDs based on resistively shunted grain boundary Josephson junctions. In addition the dc SQUIDs contain a narrow constriction onto which a small magnetic particle can be placed (with its magnetic moment in the plane of the SQUID loop and perpendicular to the grain boundary) for efficient coupling of its stray magnetic field to the S… ▽ More We present an optimization study of the spin sensitivity of nanoSQUIDs based on resistively shunted grain boundary Josephson junctions. In addition the dc SQUIDs contain a narrow constriction onto which a small magnetic particle can be placed (with its magnetic moment in the plane of the SQUID loop and perpendicular to the grain boundary) for efficient coupling of its stray magnetic field to the SQUID loop. The separation of the location of optimum coupling from the junctions allows for an independent optimization of the coupling factor $φ_μ$ and junction properties. We present different methods for calculating $φ_μ$ (for a magnetic nanoparticle placed 10\,nm above the constriction) as a function of device geometry and show that those yield consistent results. Furthermore, by numerical simulations we obtain a general expression for the dependence of the SQUID inductance on geometrical parameters of our devices, which allows to estimate their impact on the spectral density of flux noise $S_Φ$ of the SQUIDs in the thermal white noise regime. Our analysis of the dependence of $S_Φ$ and $φ_μ$ on the geometric parameters of the SQUID layout yields a spin sensitivity $S_μ^{1/2}=S_Φ^{1/2}/φ_μ$ of a few $μ_{\rm{B}}/\rm{Hz^{1/2}}$ ($μ_B$ is the Bohr magneton) for optimized parameters, respecting technological constraints. However, by comparison with experimentally realized devices we find significantly larger values for the measured white flux noise, as compared to our theoretical predictions. Still, a spin sensitivity on the order of $10\,μ_{\rm B}/\rm{Hz^{1/2}}$ for optimized devices seems to be realistic. △ Less

Submitted 10 June, 2014; v1 submitted 7 January, 2013; originally announced January 2013.

Comments: 10 pages, 5 figures, Superconductor Science and Technology (submitted)

Strong suppression of the Curie temperature in the electron-doped system La1-xCexCoO3

Authors: Dirk Fuchs, Peter Schweiss, Thorsten Schwarz, Rudolf Schneider

Abstract: We show for the system La1-xCexCoO3 (0.1 <= x <= 0.4) that it is possible to synthesize electron-doped cobaltites by the growth of epitaxial thin films. For La1-xCexCoO3, ferromagnetic order is observed within the entire doping range (with the maximum of the Curie temperature, Tc, at x ca. 0.3), resulting in a magnetic phase diagram similar to that of hole-doped lanthanum cobaltites. The measure… ▽ More We show for the system La1-xCexCoO3 (0.1 <= x <= 0.4) that it is possible to synthesize electron-doped cobaltites by the growth of epitaxial thin films. For La1-xCexCoO3, ferromagnetic order is observed within the entire doping range (with the maximum of the Curie temperature, Tc, at x ca. 0.3), resulting in a magnetic phase diagram similar to that of hole-doped lanthanum cobaltites. The measured spin values strongly suggest an intermediate-spin state of the Co ions which has been also found in the hole-doped system. In contrast to the hole-doped material, however, where Tc is well above 200 K, we observe a strong suppression of the maximum Tc to about 22 K. This is likely to be caused by a considerable decrease of the Co3d - O2p hybridization. The observed intriguing magnetic properties are in agreement with previously reported theoretical results. △ Less

Submitted 29 March, 2005; originally announced March 2005.

Experimental Generation and Observation of Intrinsic Localized Spin Wave Modes in an Antiferromagnet

Authors: U. T. Schwarz, L. Q. English, A. J. Sievers

Abstract: By driving with a microwave pulse the lowest frequency antiferromagnetic resonance of the quasi 1-D biaxial antiferromagnet (C_2 H_5 NH_3)_2 CuCl_4 into an unstable region intrinsic localized spin waves have been generated and detected in the spin wave gap. These findings are consistent with the prediction that nonlinearity plus lattice discreteness can lead to localized excitations with dimensi… ▽ More By driving with a microwave pulse the lowest frequency antiferromagnetic resonance of the quasi 1-D biaxial antiferromagnet (C_2 H_5 NH_3)_2 CuCl_4 into an unstable region intrinsic localized spin waves have been generated and detected in the spin wave gap. These findings are consistent with the prediction that nonlinearity plus lattice discreteness can lead to localized excitations with dimensions comparable to the lattice constant. △ Less

Submitted 16 May, 1999; originally announced May 1999.

Comments: 10 pages, 4 figures, accepted for publication in Physical Review Letters

Report number: SES-99-1

Journal ref: Phys. Rev. Lett. 83, 223 (1999).