arXiv subjects

Hans Hilgenkamp

Publications and source records attributed to Hans Hilgenkamp.

18 recordsLinked to original sources

arXiv2026

Nanoscale wireframe SQUID on a cantilever by corner lithography

We present the fabrication of nanoscale superconducting quantum interference devices (SQUIDs) at the apex of wireframe tips on self-aligned superconducting cantilever probes. The probes are made on silicon wafers using molding techniques in combination with corner lithography, which results in a nanowire frame tip with a tuneable apex structure. A shadow effect deposition using magnetron sputtering of Nb creates self-aligned superconducting wireframes on cantilevers with accompanying device circuitry. Superconducting weak links are realized at the apex of the wireframes with the use of focused ion beam nanopatterning. The realized SQUIDs have effective diameters ranging from several micrometers down to 100 nm and can be operated in magnetic fields up to 1 T. Furthermore, the nanowires in the wireframe can be used to flux modulate the SQUID locally. This fabrication process enables the production of wafer-scale templates for probes based on on-tip superconducting devices.

cond-mat.mes-hall↗

arXiv2024

Imaging the suppression of ferromagnetism in LaMnO$_3$ by metallic overlayers

LaMnO$_3$ (LMO) thin films epitaxially grown on SrTiO$_3$ (STO) usually exhibit ferromagnetism above a critical layer thickness. We report the use of scanning SQUID microscopy (SSM) to study the suppression of the ferromagnetism in STO/LMO/metal structures. By partially covering the LMO surface with a metallic layer, both covered and uncovered LMO regions can be studied simultaneously. While Au does not significantly influence the ferromagnetic order of the underlying LMO film, a thin Ti layer induces a strong suppression of the ferromagnetism, over tens of nanometers, which increases with time on a timescale of days. Detailed EELS analysis of the Ti-LaMnO$_3$ interface reveals \textcolor{black}{the presence of Mn$^{2+}$ and} an evolution of the Ti valence state from Ti$^0$ to Ti$^{4+}$ over approximately 5 nanometers. Furthermore, we demonstrate that by patterning Ti/Au overlayers, we can locally suppress the ferromagnetism and define ferromagnetic structures down to sub-micrometer scales.

cond-mat.mtrl-sci↗

arXiv2023

Beyond domain alignment: Revealing the effect of intrinsic magnetic order on electrochemical water splitting

To reach a long term viable green hydrogen economy, rational design of active oxygen evolution reaction (OER) catalysts is critical. An important hurdle in this reaction originates from the fact that the reactants are singlet molecules, whereas the oxygen molecule has a triplet ground state with parallel spin alignment, implying that magnetic order in the catalyst is essential. Accordingly, multiple experimentalists reported a positive effect of external magnetic fields on OER activity of ferromagnetic catalysts. However, it remains a challenge to investigate the influence of the intrinsic magnetic order on catalytic activity. Here, we tuned the intrinsic magnetic order of epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ thin film model catalysts from ferro- to paramagnetic by changing the temperature in-situ during water electrolysis. Using this strategy, we show that ferromagnetic ordering below the Curie temperature enhances OER activity. Moreover, we show a slight current density enhancement upon application of an external magnetic field and find that the dependence of magnetic field direction correlates with the magnetic anisotropy in the catalyst film. Our work thus suggests that both the intrinsic magnetic order in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ films and magnetic domain alignment increase their catalytic activity. We observe no long-range magnetic order at the catalytic surface, implying that the OER enhancement is connected to the magnetic order of the bulk catalyst. Combining the effects found with existing literature, we propose a unifying picture for the spin-polarized enhancement in magnetic oxide catalysts.

cond-mat.mtrl-sci↗

arXiv2023

Multibridge VO2-Based Resistive Switching Devices in a Two-Terminal Configuration

Vanadium dioxide (VO2) exhibits a hysteretic insulator-to-metal transition near room temperature, forming the foundation for various forms of resistive switching devices. Usually, these are realized in the form of two-terminal bridge-like structures. We show here that by incorporating multiple, parallel VO2 bridges in a single two-terminal device, a wider range of possible characteristics can be obtained, including a manifold of addressable resistance states. Different device configurations are studied, in which the number of bridges, the bridge dimensions and the interbridge distances are varied. The switching characteristics of the multibridge devices are influenced by the thermal crosstalk between the bridges. Scanning Thermal Microscopy has been used to image the current distributions at various voltage/current bias conditions. This work presents a route to realize devices exhibiting highly non-linear, multistate current-voltage characteristics, with potential applications in e.g., tunable electronic components and novel, neuromorphic information processing circuitry.

physics.app-ph↗

arXiv2023

Near-field imaging of domain switching in in-operando VO$_{2}$ devices

Experimental insight in the nanoscale dynamics underlying switching in novel memristive devices is limited owing to the scarcity of techniques that can probe the electronic structure of these devices. Scattering scanning near-field optical microscopy is a relatively novel approach to probe the optical response of materials with a spatial resolution well below the diffraction limit. We use this non-invasive tool to demonstrate that it provides detailed information on the origin and memory behaviour of ultra-thin films of vanadium dioxide. Simultaneously recorded $I(V)$ characteristics and near-field maps show that discontinuities in the I(V) characteristics arise from the sudden switching of insulating domains to metallic domains. At the threshold voltage, the domains form a continuous current path. The metallic domains persist once the bias voltage is removed, but narrow monoclinic regions appear around the domain boundaries. The key advantage of our approach is that it provides detailed information on the electronic structure at length scales raging from tens of nanometers up to tens of microns and is easily applied under \textit{in operando} conditions.

cond-mat.str-el↗

arXiv2023

Benchmarking energy consumption and latency for neuromorphic computing in condensed matter and particle physics

The massive use of artificial neural networks (ANNs), increasingly popular in many areas of scientific computing, rapidly increases the energy consumption of modern high-performance computing systems. An appealing and possibly more sustainable alternative is provided by novel neuromorphic paradigms, which directly implement ANNs in hardware. However, little is known about the actual benefits of running ANNs on neuromorphic hardware for use cases in scientific computing. Here we present a methodology for measuring the energy cost and compute time for inference tasks with ANNs on conventional hardware. In addition, we have designed an architecture for these tasks and estimate the same metrics based on a state-of-the-art analog in-memory computing (AIMC) platform, one of the key paradigms in neuromorphic computing. Both methodologies are compared for a use case in quantum many-body physics in two dimensional condensed matter systems and for anomaly detection at 40 MHz rates at the Large Hadron Collider in particle physics. We find that AIMC can achieve up to one order of magnitude shorter computation times than conventional hardware, at an energy cost that is up to three orders of magnitude smaller. This suggests great potential for faster and more sustainable scientific computing with neuromorphic hardware.

cs.ET↗

arXiv2020

Structure and magnetic properties of epitaxial CaFe2O4 thin films

CaFe2O4 is a highly anisotropic antiferromagnet reported to display two spin arrangements with up-up-down-down (phase A) and up-down-up-down (phase B) configurations. The relative stability of these phases is ruled by the competing ferromagnetic and antiferromagnetic interactions between Fe3+ spins arranged in two different environments, but a complete understanding of the magnetic structure of this material does not exist yet. In this study we investigate epitaxial CaFe2O4 thin films grown on TiO2 (110) substrates by means of Pulsed Laser Deposition (PLD). Structural characterization reveals the coexistence of two out-of-plane crystal orientations and the formation of three in-plane oriented domains. The magnetic properties of the films, investigated macroscopically as well as locally, including highly sensitive Mossbauer spectroscopy, reveal the presence of just one order parameter showing long-range ordering below T = 185 K and the critical nature of the transition. In addition, a non-zero in-plane magnetization is found, consistent with the presence of uncompensated spins at phase or domain boundaries, as proposed for bulk samples.

cond-mat.mtrl-sci↗

arXiv2019

New insights into the electron trapping mechanism in LaAlO_3 / SrTiO3 heterostructures

In LaAlO3/SrTiO3 heterostructures, a commonly observed but poorly understood phenomenon is that of electron trapping in back-gating experiments. In this work, by combining magnetotransport measurements and self-consistent Schroedinger-Poisson calculations, we obtain an empirical relation between the amount of trapped electrons and the gate voltage. We find that the trapped electrons follow an exponentially decaying spatial distribution away from the interface. However, contrary to earlier observations, we find that the Fermi level remains well within the quantum well. The enhanced trapping of electrons induced by the gate voltage can therefore not be explained by a thermal escape mechanism. Further gate sweeping experiments strengthen our conclusion that the thermal escape mechanism is not valid. We propose a new mechanism which involves the electromigration and clustering of oxygen vacancies in SrTiO3. Our work indicates that electron trapping is a universal phenomenon in SrTiO3-based two-dimensional electron systems.

cond-mat.mes-hall↗

arXiv2019

Tailoring Vanadium Dioxide Film Orientation using Nanosheets: A Combined Microscopy, Diffraction, Transport and Soft X-ray in Transmission Study

VO2 is a much-discussed material for oxide electronics and neuromorphic computing applications. Here, heteroepitaxy of vanadium dioxide (VO2) was realized on top of oxide nanosheets that cover either the amorphous silicon dioxide surfaces of Si substrates or X-ray transparent silicon nitride membranes. The out-of-plane orientation of the VO2 thin films was controlled at will between (011)M1/(110)R and (-402)M1/(002)R by coating the bulk substrates with Ti0.87O2 and NbWO6 nanosheets, respectively, prior to VO2 growth. Temperature dependent X-ray diffraction and automated crystal orientation mapping in microprobe TEM mode (ACOM-TEM) characterized the high phase purity, the crystallographic and orientational properties of the VO2 films. Transport measurements and soft X-ray absorption in transmission are used to probe the VO2 metal-insulator transition, showing results of a quality equal to those from epitaxial films on bulk single-crystal substrates. Successful local manipulation of two different VO2 orientations on a single substrate is demonstrated using VO2 grown on lithographically-patterned lines of Ti0.87O2 and NbWO6 nanosheets investigated by electron backscatter diffraction. Finally, the excellent suitability of these nanosheet-templated VO2 films for advanced lensless imaging of the metal-insulator transition using coherent soft X-rays is discussed.

cond-mat.mtrl-sci↗

arXiv2018

Analysis of low-field isotropic vortex glass containing vortex groups in YBCO thin films visualized by scanning SQUID microscopy

The glass-like vortex distribution in pulsed laser deposited YBCO thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.

cond-mat.supr-con↗

arXiv2018

On the formation of a conducting surface channel by ionic liquid gating of an insulator

Ionic liquid gating has become a popular tool for tuning the charge carrier densities of complex oxides. Among these the band insulator SrTiO$_3$ is one of the most extensively studied materials. While experiments have succeeded in inducing (super)conductivity, the process by which ionic liquid gating turns this insulator into a conductor is still under scrutiny. Recent experiments have suggested an electrochemical rather than electrostatic origin of the induced charge carriers. Here, we report experiments probing the time evolution of conduction of SrTiO$_3$ near the glass transition temperature of the ionic liquid. By cooling down to temperatures near the glass transition of the ionic liquid the process develops slowly and can be seen to evolve in time. The experiments reveal a process characterized by waiting times that can be as long as several minutes preceding a sudden appearance of conduction. For the conditions applied in our experiments we find a consistent interpretation in terms of an electrostatic mechanism for the formation of a conducting path at the surface of SrTiO$_3$. The mechanism by which the conducting surface channel develops relies on a nearly homogeneous lowering of the surface potential until the conduction band edge of SrTiO$_3$ reaches the Fermi level of the electrodes.

cond-mat.str-el↗

arXiv2017

Scaling Universality at the Dynamic Vortex Mott Transition

The dynamic Mott insulator-to-metal transition (DMT) is key to many intriguing phenomena in condensed matter physics yet it remains nearly unexplored. The cleanest way to observe DMT, without the interference from disorder and other effects inherent to electronic and atomic systems, is to employ the vortex Mott states formed by superconducting vortices in a regular array of pinning sites. The applied electric current delocalizes vortices and drives the dynamic vortex Mott transition. Here we report the critical behavior of the vortex system as it crosses the DMT line, driven by either current or temperature. We find universal scaling with respect to both, expressed by the same scaling function and characterized by a single critical exponent coinciding with the exponent for the thermodynamic Mott transition. We develop a theory for the DMT based on the parity reflection-time reversal (PT) symmetry breaking formalism and find that the nonequilibrium-induced Mott transition has the same critical behavior as thermal Mott transition. Our findings demonstrate the existence of physical systems in which the effect of nonequilibrium drive is to generate effective temperature and hence the transition belonging in the thermal universality class. We establish PT symmetry-breaking as a universal mechanism for out-of-equilibrium phase transitions.

cond-mat.supr-con↗

arXiv2014

Critical behavior at the dynamic Mott transition

We investigate magnetoresistance of a square array of superconducting islands placed on a normal metal, which offers a unique tunable laboratory for realizing and exploring quantum many-body systems and their dynamics. A vortex Mott insulator where magnetic field-induced vortices are frozen in the dimples of the egg crate potential by their strong repulsion interaction is discovered. We find an insulator-to-metal transition driven by the applied electric current and determine critical exponents that exhibit striking similarity with the common thermodynamic liquid-gas transition. A simple and straightforward quantum mechanical picture is proposed that describes both tunneling dynamics in the deep insulating state and the observed scaling behavior in the vicinity of the critical point. Our findings offer a comprehensive description of dynamic Mott critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions.

cond-mat.supr-con↗

arXiv2013

Exciton condensation in strongly correlated electron bilayers

We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems an exciton is the bound state of a double occupied and empty site. In the strong coupling limit the exciton acts as a hard-core boson. Its physics are captured by the exciton t-J model, containing an effective XXZ model describing the exciton dynamics only. Using numerical simulations and analytical mean field theory we constructed the ground state phase diagram. Three homogeneous phases can be distinguished: the antiferromagnet, the exciton checkerboard crystal and the exciton superfluid. For most model parameters, however, we predict macroscopic phase separation between these phases. The exciton superfluid exists only for large exciton hopping energy. Additionally we studied the collective modes and susceptibilities of the three phases. In the superfluid phase we find the striking feature that the bandwidth of the spin-triplet excitations, potentially detectable by resonant inelastic x-ray scattering (RIXS), is proportional to the superfluid density. The superfluid phase mode is visible in the charge susceptibility, measurable by RIXS or electron energy loss spectroscopy (EELS).

cond-mat.str-el↗

arXiv2013

Enhancement of spin propagation due to interlayer exciton condensation

We show that an interlayer exciton condensate doped into a strongly correlated Mott insulator exhibits a remarkable enhancement of the bandwidth of the magnetic excitations (triplons). This triplon is visible in the dynamical magnetic susceptibility and can be measured using resonant inelastic X-ray scattering. The bandwidth of the triplon scales with the exciton superfluid density, but only in the limit of strong correlations. As such the triplon bandwidth acts as a probe of exciton-spin interactions in the condensate.

cond-mat.str-el↗

arXiv2012

The dynamical frustration of interlayer excitons delocalizing in bilayer quantum antiferromagnets

Using the self-consistent Born approximation we study the delocalization of interlayer excitons in the bilayer Heisenberg quantum antiferromagnet. Under realistic conditions we find that the coupling between the exciton motion and the spin system is strongly enhanced as compared to the case of a single carrier, to a degree that it mimics the confinement physics of carriers in Ising spin systems. We predict that the "ladder spectrum" associated with this confinement physics should be visible in the c-axis exciton spectra of insulating bilayer cuprates such as YBa2Cu3O6. Our discovery indicates that finite density systems of such excitons should show very rich physical behavior.

cond-mat.str-el↗

arXiv2010

Prediction of the quantization of magnetic flux in double layer exciton superfluids

Currently a way is lacking to detect unambiguously the possible phase coherence of an exciton condensate in an electron-hole double layer. Here we show that despite the fact that excitons are charge-neutral, the double layer exciton superfluid exhibits a diamagnetic response. In devices with specific circular geometry the magnetic flux threading between the layers must be quantized in units of $\frac{h}{e} χ_m$ where $χ_m$ is the diamagnetic susceptibility of the device. We discuss possible experimental realizations of the predicted unconventional flux quantization.

cond-mat.str-el↗

arXiv2009

Imaging of order parameter induced $π$ phase shifts in cuprate superconductors by low-temperature scanning electron microscopy

Low-temperature scanning electron microscopy (LTSEM) has been used to image the supercurrent distribution in ramp-type Josephson junctions between Nb and either the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_{4-y}$ or the hole-doped cuprate YBa$_2$Cu$_3$O$_7$. For zigzag-shaped devices in the short junction limit the critical current is strongly suppressed at zero applied magnetic field. The LTSEM images show, that this is due to the Josephson current counterflow in neighboring 0 and $π$ facets, which is induced by the $d_{x^2-y^2}$ order parameter in the cuprates. Thus, LTSEM provides imaging of the sign change of the superconducting order parameter, which can also be applied to other types of Josephson junctions.

cond-mat.supr-con↗