arXiv subjects

Clemens B. Winkelmann

Publications and source records attributed to Clemens B. Winkelmann.

16 recordsLinked to original sources

arXiv2026

Nanoparticle manipulation with a carbon fiber tip in an electron microscope for $μ$-SQUID magnetometry

We report a carbon-fiber-tip based nanomanipulation system integrated into a scanning electron microscope for individual nanoparticle (NP) manipulation on a surface. Electrochemically etched amorphous carbon fiber tips with excellent mechanical rigidity and sub-100 nm apex radii effectively reduce the van der Waals adhesion and enable reliable positioning of about 100 nm size NPs with about 100 nm precision. This system combines a piezoelectric bimorph for vertical tip motion, a four-quadrant piezo-tube for two-dimensional fine tip control and a two-dimensional piezoelectric walker for coarse lateral translation. Using this setup, we successfully position single Fe$_3$O$_4$ magnetic NPs on micron sized superconducting quantum interference devices for optimal magnetic coupling between them and probe a NP's magnetism.

cond-mat.mes-hall↗

arXiv2025

Chiral-helical junctions in screened graphene

Reproducibility and quantization in quantum spin Hall platforms is a persisting challenge, limiting their use in hybrid realizations of topological superconductivity. We report robust and reproducible quantized transport in a graphene quantum Hall topological insulator, stabilized at low magnetic fields by screening long-range Coulomb interactions with a metallic Bi$_2$Se$_3$ back gate. Beyond quantized resistance plateaus, we demonstrate mode-resolved control via gate-defined chiral-helical junctions that selectively transmit or backscatter a single helical channel, a capability inaccessible in time-reversal symmetric quantum spin Hall systems. Targeted experiments and simulations identify contact-induced doping, effectively creating unintended chiral-helical interfaces, as a generic mechanism for quantization breakdown, which is mitigated by large area contacts that enhance edge-channel equilibration. Our findings establish metal screened graphene as a gate-tunable, interaction-driven helical system with quantized transport, spatially separable helical channels, and compatibility with superconducting proximity for topological devices.

cond-mat.mes-hall↗

arXiv2025

From Shapiro steps to photon-assisted tunneling in microwave-driven atomic-scale Josephson junctions with a single (magnetic) adatom

Ultra-small Josephson junctions are strongly influenced by noise and damping due to energy dissipation into the environment, which are expected to suppress phase coherence. Here, we investigate the coherence properties of atomic-scale Josephson junctions in a scanning tunneling microscope under microwave excitation. Plain Pb-Pb junctions exhibit hysteretic Shapiro steps as signature of a coherent resonant state. With increasing AC amplitude, phase coherence is reduced due to an increase of thermal fluctuations. In the presence of magnetic adatoms the Josephson coupling energy is reduced and quasi-particle tunneling is enhanced. With AC driving we observe a rapid suppression of coherence that we ascribe to photon-assisted quasi-particle tunneling through Yu-Shiba-Rusinov states. Our results highlight the presence of phase coherence and shed light on the origin of the transition to incoherent transport, thereby revealing the importance of controlling dissipation in nanoscale superconducting devices.

cond-mat.mes-hall↗

arXiv2025

Tunable exchange bias in Y$_3$Fe$_5$O$_{12}$ film on Gd$_3$Ga$_5$O$_{12}$

Ferrimagnetic Y$_3$Fe$_5$O$_{12}$ grown on the (001) surface of paramagnetic Gd$_3$Ga$_5$O$_{12}$ experiences an exchange bias field, which has been attributed to the magnetism of an interface layer between the two materials. We report here that when grown using sputtering and with lower post-annealing temperatures than in previous works, the blocking temperature of the interface magnetic layer is lowered to about 7 K, while still displaying a strong exchange bias. This exchange bias is then found to be tunable between its two extreme values by carefully varying the field cooling protocol. This is attributed to a slow and complex dynamics of the spins of the interface-layer when it is warmed up close to its blocking (or melting) temperature, which is reminiscent of a spin glass.

cond-mat.mtrl-sci↗

arXiv2024

Magnetointerferometry of multiterminal Josephson junctions

We report a theoretical study of multiterminal Josephson junctions under the influence of a magnetic field $B$. We consider a ballistic rectangular two-dimensional metal $N_0$ connected by the edges to the left, right, top and bottom superconductors $S_L$, $S_R$, $S_T$ and $S_B$, respectively. We numerically calculate in the large-gap approximation the critical current $I_c$ versus $B$ between the left and right $S_L$ and $S_R$ for various aspect ratios, with the top and bottom $S_T$ and $S_B$ playing the role of superconducting mirrors. We find the critical current $I_c$ to be enhanced by orders of magnitude, especially at long distance, due to the phase rigidity provided by the mirrors. We obtain superconducting quantum interference device-like magnetic oscillations. With symmetric couplings, the self-consistent superconducting phase variables of the top and bottom mirrors take the values $0$ or $π$, as for emerging Ising degrees of freedom. We propose a simple effective Josephson junction circuit model that is compatible with these microscopic numerical calculations. From the $I_c(B)$ patterns we infer where the supercurrent flows in various device geometries. In particular in the elongated geometry, we show that the supercurrent flows between all pairs of contacts, which allows exploring the full phase space of the relevant phase differences.

cond-mat.supr-con↗

arXiv2024

Heat transport at the nanoscale and ultralow temperatures -- implications for quantum technologies

In this perspective, we discuss thermal imbalance and the associated electron-mediated thermal transport in quantum electronic devices at very low temperatures. We first present the theoretical approaches describing heat transport in nanoscale conductors at low temperatures, in which quantum confinement and interactions play an important role. We then discuss the experimental techniques for generating and measuring heat currents and temperature gradients on the nanoscale. Eventually we review the most important quantum effects on heat transport, and discuss implications for quantum technologies and future directions in the field.

cond-mat.mes-hall↗

arXiv2023

Proposal for detecting the $π-$shifted Cooper quartet supercurrent

The multiterminal Josephson effect aroused considerable interest recently, in connection with theoretical and experimental evidence for correlations among Cooper pairs, that is, the so-called Cooper quartets. It was further predicted that the spectrum of Andreev bound states in such devices could host Weyl-point singularities. However, the relative phase between the Cooper pair and quartet supercurrents has not yet been addressed experimentally. Here, we propose an experiment involving four-terminal Josephson junctions with two independent orthogonal supercurrents, and calculate the critical current contours (CCCs) from a multiterminal Josephson junction circuit theory. We predict a generically $π$-shifted contribution of both the local or nonlocal second-order Josephson harmonics. Furthermore, we show that these lead to marked nonconvex shapes for the CCCs in zero magnetic field, where the dissipative state reenters into the superconducting one. Eventually, we discuss distinctive features of the non-local Josephson processes in the CCCs. The experimental observation of the latter could allow providing firm evidence of the $π$-shifted Cooper quartet current-phase relation.

cond-mat.supr-con↗

arXiv2023

Interplay of Andreev reflection and Coulomb blockade in hybrid superconducting single electron transistors

We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor-superconductor-normal metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that those processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a master equation approach in combination with the full counting statistics of multiple Andreev reflection.

cond-mat.supr-con↗

arXiv2022

Diode effect in Josephson junctions with a single magnetic atom

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes and known as non-reciprocal charge transport. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems. Probing the ultimate limits of miniaturization, we have created atomic-scale Pb--Pb Josephson junctions in a scanning tunneling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behavior, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing via Yu-Shiba-Rusinov (YSR) states inside the superconducting energy gap. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation.

cond-mat.supr-con↗

arXiv2022

An epitaxial graphene platform for zero-energy edge state nanoelectronics

Graphene's original promise to succeed silicon faltered due to pervasive edge disorder in lithographically patterned deposited graphene and the lack of a new electronics paradigm. Here we demonstrate that the annealed edges in conventionally patterned graphene epitaxially grown on a silicon carbide substrate (epigraphene) are stabilized by the substrate and support a protected edge state. The edge state has a mean free path that is greater than 50 microns, 5000 times greater that the bulk states and involves a theoretically unexpected Majorana-like zero-energy non-degenerate quasiparticle that does not produce a Hall voltage. In seamless integrated structures, the edge state forms a zero-energy one-dimensional ballistic network with essentially dissipationless nodes at ribbon-ribbon junctions. Seamless device structures offer a variety of switching possibilities including quantum coherent devices at low temperatures. This makes epigraphene a technologically viable graphene nanoelectronics platform that has the potential to succeed silicon nanoelectronics.

cond-mat.mes-hall↗

arXiv2022

Imaging tunable quantum Hall broken-symmetry orders in graphene

When electrons populate a flat band their kinetic energy becomes negligible, forcing them to organize in exotic many-body states to minimize their Coulomb energy. The zeroth Landau level of graphene under magnetic field is a particularly interesting strongly interacting flat band because inter-electron interactions are predicted to induce a rich variety of broken-symmetry states with distinct topological and lattice-scale orders. Evidence for these stems mostly from indirect transport experiments that suggest that broken-symmetry states are tunable by boosting the Zeeman energy or by dielectric screening of the Coulomb interaction. However, confirming the existence of these ground states requires a direct visualization of their lattice-scale orders. Here, we image three distinct broken-symmetry phases in graphene using scanning tunneling spectroscopy. We explore the phase diagram by tuning the screening of the Coulomb interaction by a low or high dielectric constant environment, and with a magnetic field. In the unscreened case, we unveil a Kekulé bond order, consistent with observations of an insulating state undergoing a magnetic-field driven Kosterlitz-Thouless transition. Under dielectric screening, a sublattice-unpolarized ground state emerges at low magnetic fields, and transits to a charge-density-wave order with partial sublattice polarization at higher magnetic fields. The Kekulé and charge-density-wave orders furthermore coexist with additional, secondary lattice-scale orders that enrich the phase diagram beyond current theory predictions. This screening-induced tunability of broken-symmetry orders may prove valuable to uncover correlated phases of matter in other quantum materials.

cond-mat.mes-hall↗

arXiv2022

Magnetization Reversal Across Multiple Serial Barriers in a Single Fe$_3$O$_4$ Nanoparticle

Depinning of nanoscale magnetic textures, such as domain walls, vortices and skyrmions, is of paramount importance for magnetic storage and information processing. We measure time-resolved magnetic switching statistics of an individual, non-single-domain Fe$_3$O$_4$ nanoparticle using a micrometer-scale superconducting quantum interference device. Surprisingly, a strong narrowing of the waiting-time distributions before reaching the final state is observed as compared to the exponential distribution expected for a single barrier. The magnetization reversal across the nanostructure is thus shown to result from multiple serial barriers in the minimum energy pathway.

cond-mat.mes-hall↗

arXiv2021

Stochastic resonance in thermally bistable Josephson weak-links and micro-SQUIDs

Constriction-based Josephson weak-links display a thermal bi-stability between two states exhibiting zero and finite voltages. This manifests in experiments either as hysteresis in weak-links current voltage characteristics or as random telegraphic signal in voltage. In the latter case, a noise-driven amplification of a sinusoidal excitation of the device is observed, at frequencies matching the characteristic switching frequency in telegraphic signal, a phenomenon known as stochastic resonance. The observed behavior is understood using a two-state model of stochastic resonance and is exploited to illustrate an enhanced signal-to-noise-ratio in a micro-SQUID as a magnetic field sensor.

cond-mat.supr-con↗

arXiv2020

Photon-assisted tunneling at the atomic scale: Probing resonant Andreev reflections from Yu-Shiba-Rusinov states

Tunneling across superconducting junctions proceeds by a rich variety of processes, which transfer single electrons, Cooper pairs, or even larger numbers of electrons by multiple Andreev reflections. Photon-assisted tunneling combined with the venerable Tien-Gordon model has long been a powerful tool to identify tunneling processes between superconductors. Here, we probe superconducting tunnel junctions including an impurity-induced Yu-Shiba-Rusinov (YSR) state by exposing a scanning tunneling microscope with a superconducting tip to microwave radiation. We find that a simple Tien-Gordon description describes tunneling of single electrons and Cooper pairs into the bare substrate, but breaks down for tunneling via YSR states by resonant Andreev reflections. We develop an improved theoretical description which is in excellent agreement with the data. Our results establish photon-assisted tunneling as a powerful tool to analyze tunneling processes at the atomic scale which should be particularly informative for unconventional and topological superconductors.

cond-mat.mes-hall↗

arXiv2019

Elimination of thermal bistability in superconducting weak links by an inductive shunt

The quantum phase-coherent behavior of superconducting weak links (WL) is often quenched in the finite voltage state, due to the heat dissipation and related thermal hysteresis. The latter can be reduced by improving heat evacuation and/or by lowering the critical current, so that a phase-dynamic regime is obtained, albeit over a narrow bias-current and temperature range. Here we demonstrate that an inductive shunt with well-chosen parameters introduces unexpected nonlinear dynamics that destabilize an otherwise stable fixed point in the dissipative branch. This leads to a nonhysteretic behavior with large voltage oscillations in intrinsically hysteretic WL-based micron-size superconducting quantum interference devices. A dynamic thermal model quantitatively describes our observations and further allows us to elaborate on the optimal shunting conditions.

cond-mat.supr-con↗

arXiv2018

Josephson Coupling in the Dissipative State of a Thermally Hysteretic $μ$-SQUID

Micron-sized superconducting interference devices ($μ$-SQUIDs) based on constrictions optimized for minimizing thermal runaway are shown to exhibit voltage oscillations with applied magnetic flux despite their hysteretic behavior. We explain this remarkable feature by a significant supercurrent contribution surviving deep into the resistive state, due to efficient heat evacuation. A resistively shunted junction model, complemented by a thermal balance determining the amplitude of the critical current, describes well all experimental observations, including the flux modulation of the (dynamic) retrapping current and voltage by introducing a single dimensionless parameter. Thus hysteretic $μ$-SQUIDs can be operated in the voltage read-out mode with a faster response. The quantitative modeling of this regime incorporating both heating and phase dynamics paves the way for further optimization of $μ$-SQUIDs for nano-magnetism.

cond-mat.supr-con↗