arXiv subjects

Remko Fermin

Publications and source records attributed to Remko Fermin.

10 recordsLinked to original sources

arXiv2025

Tapping-mode SQUID-on-tip Microscopy with Proximity Josephson Junctions

Studying nanoscale dynamics is essential for understanding quantum materials and advancing quantum chip manufacturing. Still, it remains a major challenge to measure non-equilibrium properties such as current and dissipation, and their relation to structure. Scanning nanoprobes utilizing superconducting quantum interference devices (SQUIDs) are uniquely suited here, due to their unparalleled magnetic and thermal sensitivity. Here, we introduce tapping-mode SQUID-on-tip, which combines atomic force microscopy (AFM) with nanoSQUID sensing. Our probes minimize nanoSQUID-sample distance, provide in-plane magnetic sensitivity, and operate without lasers. Frequency multiplexing enables simultaneous imaging of currents, magnetism, dissipation and topography. The large voltage output of our proximity-junction nanoSQUIDs allows us to resolve nanoscale currents as small as 100 nA using a simple four-probe electronic readout without cryogenic amplification. By capturing local magnetic, thermal, and electronic response without external radiation, our technique offers a powerful non-invasive route to study dynamic phenomena in exotic materials and delicate quantum circuits.

cond-mat.mes-hall↗

arXiv2025

Time-reversal symmetry breaking in microscopic single-crystal Sr$_2$RuO$_4$ devices

Time-reversal symmetry breaking superconductivity is a quintessential unconventional quantum state. In Josephson junctions, time-reversal symmetry breaking manifests itself in the supercurrent interference pattern as the invariance of the critical current under the reversal of both transport and magnetic field directions, i.e., $I_\text{c+}(H) = I_\text{c-}(-H)$. So far, such systems have been realized in devices where superconductivity is injected into a deliberately constructed weak link medium, usually carefully tuned by external magnetic fields and electrostatic gating. In this work, we report time-reversal symmetry breaking in spontaneously emerging Josephson junctions without intentionally constructed weak links. This is realized in ultra-pure single-crystal microstructures of Sr$_2$RuO$_4$, an unconventional superconductor with a multi-component order parameter. Here, the Josephson effect emerges intrinsically at the superconducting domain wall, where the degenerate states partially overlap. In addition to violating $I_\text{c+}(H) = I_\text{c-}(-H)$, we find a rich variety of exotic transport phenomena, including a supercurrent diode effect present in the entire interference pattern, two-channel critical current oscillations with a period that deviates from $\Phi_0$, fractional Shapiro steps, and current-switchable bistable states with highly asymmetric critical currents. Our findings provide direct evidence of TRSB in unstrained Sr$_2$RuO$_4$ and reveal the potential of domain wall Josephson junctions, which can emerge in any superconductor where the pairing symmetry is described by a multi-component order parameter.

cond-mat.supr-con↗

arXiv2024

Recombination of localized quasiparticles in disordered superconductors

Disordered superconductors offer new impedance regimes for quantum circuits, enable a pathway to protected qubits, and can improve superconducting detectors due to their high kinetic inductance and sheet resistance. The performance of these devices can be limited, however, by quasiparticles - the fundamental excitations of a superconductor. While experiments have shown that disorder affects the relaxation of quasiparticles drastically, the microscopic mechanisms are still not understood. We address this issue by measuring quasiparticle relaxation in a disordered $\beta$-Ta film, which we pattern as the inductor of a microwave resonator. We observe that quasiparticle recombination is governed by the phonon scattering time, which is faster than conventional recombination in ordered superconductors. We interpret the results as recombination of localized quasiparticles, induced by disorder, which first delocalize via phonon absorption. We analyze quasiparticle relaxation measurements on superconductors with different degrees of disorder and conclude that this phenomenon is inherent to disordered superconductors.

cond-mat.supr-con↗

arXiv2023

Triplet supercurrents in lateral Josephson junctions with a half-metallic ferromagnet

In the area of superconducting spintronics, spin triplet supercurrents in half-metallic ferromagnets (HMFs) could yield dissipationless spin transport over large distances, and high current density. Promising among the HMFs is the perovskite oxide La0.7Sr0.3MnO3 (LSMO), and recent studies in combination with the high-Tc superconductor YBa2Cu3O7, or the conventional superconductor NbTi, showed long range effects. Here we focus on two issues that as yet received less attention: the value of the critical current in the HMF in the limit of a very small electrode distance (20 nm), and the nature of the spin triplet generator. We use lateral junctions shaped as bar, square, and disk, and find high supercurrent densities, of order 10^11 A/m2, pointing to an efficient triplet generation mechanism. This is surprising in the sense that no magnetic inhomogeneity is purposely built in, as is done in conventional metal triplet junctions. Furthermore, from the magnetic field dependence of the critical current interference patterns we find a uniform supercurrent distribution in bar-shaped devices, but one more constricted to the rim in disk devices, which is an expected consequence of the geometry. We also analyze the temperature dependence of the critical current and find the quadratic dependence that was predicted in the limit of small junction lengths. From studying the NbTi/LSMO interface with scanning electron transmission microscopy, we conclude that the magnetic inhomogeneity required for triplet generation resides in the LSMO layer adjacent to the interface.

cond-mat.supr-con↗

arXiv2023

The unusual distribution of spin-triplet supercurrents in disk-shaped Josephson junctions

The phenomenon of s-wave spin triplet Cooper pairs induced in ferromagnetic metals has been researched now for more than a decade, and its main aspects are well understood. Crucial in converting s-wave singlet pairs in the superconductor to s-wave triplets in the ferromagnet is the engineering of well-defined magnetic inhomogeneity (the 'generator') at the interface with the superconductor. Vertical layer stacks are typically used as such, where two separate thin ferromagnetic layers with homogeneous but non-collinear magnetizations, provide the inhomogeneity. Alternatively, magnetic textures, like ferromagnetic domain walls and vortices, are possible triplet generators, although they are far less studied. In this paper we review our experiments on lateral disk-shaped Josephson junctions where a ferromagnetic bottom layer provides a weak link with a vortex magnetization imposed by the shape of the disk. We present three different junction configurations, exhibiting their own generator mechanism. In the first, we utilize the non-collinearity with a second ferromagnetic layer to produce the triplet correlations. The second configuration consists of only the bottom ferromagnet and the superconducting contacts; it relies on the vortex magnetization itself to generate the spin-polarized supercurrents. In the third case we exploit an intrinsic generator by combining a conventional superconductor (NbTi) and a half-metallic ferromagnetic oxide (La$_{0.7}$Sr$_{0.3}$MnO$_3$). We find strong supercurrents in all cases. A particularly interesting finding is that the supercurrents are strongly confined at the rims of the device, independent of the generating mechanism, but directly related to their triplet nature. What causes these rim currents remains an open question.

cond-mat.supr-con↗

arXiv2022

Beyond the effective length: How to analyze magnetic interference patterns of thin-film planar Josephson junctions with finite lateral dimensions

The magnetic field dependent critical current $I_{\text{c}}(B)$ of a Josephson junction is determined by the screening currents in its electrodes. In macroscopic junctions, a local vector potential drives the currents, however, in thin film planar junctions, with electrodes of finite size and various shapes, they are governed by non-local electrodynamics. This complicates the extraction of parameters such as the geometry of the effective junction area, the effective junction length and, the critical current density distribution from the $I_{\text{c}}(B)$ interference patterns. Here we provide a method to tackle this problem by simulating the phase differences that drive the shielding currents and use those to find $I_{\text{c}}(B)$. To this end, we extend the technique proposed by John Clem [Phys. Rev. B, \textbf{81}, 144515 (2010)] to find $I_{\text{c}}(B)$ for Josephson junctions separating a superconducting strip of length $L$ and width $W$ with rectangular, ellipsoid and rhomboid geometries. We find the periodicity of the interference pattern ($\Delta B$) to have geometry independent limits for $L \gg W$ and $L \ll W$. By fabricating elliptically shaped S$-$N$-$S junctions with various aspect ratios, we experimentally verify the $L/W$ dependence of $\Delta B$. Finally, we incorporate these results to correctly extract the distribution of critical currents in the junction by the Fourier analysis of $I_{\text{c}}(B)$, which makes these results essential for the correct analysis of topological channels in thin film planar Josephson junctions.

cond-mat.supr-con↗

arXiv2022

Mesoscopic superconducting memory based on bistable magnetic textures

With the ever-increasing energy need to process big data, the realization of low-power computing technologies, such as superconducting logic and memories, has become a pressing issue. Developing fast and non-volatile superconducting memory elements, however, remains a challenge. Superconductor-ferromagnet hybrid devices offer a promising solution, as they combine ultra-fast manipulation of spins with dissipationless readout. Here, we present a new type of non-volatile Josephson junction memory that utilizes the bistable magnetic texture of a single mesoscopic ferromagnet. We use micromagnetic simulations to design an ellipse-shaped planar junction structured from a Nb/Co bilayer. The ellipse can be prepared as uniformly magnetized or as a pair of vortices at zero applied field. The two states yield considerably different critical currents, enabling reliable electrical readout of the element. We describe the mechanism used to control the critical current by applying numerical calculations to quantify the local stray field from the ferromagnet, which shifts the superconducting interference pattern. By combining micromagnetic modeling with bistable spin-textured junctions, our approach presents a novel route towards realizing superconducting memory applications.

cond-mat.supr-con↗

arXiv2021

Superconducting triplet rim currents in a spin-textured ferromagnetic disk

Since the discovery of the long-range superconducting proximity effect, the interaction between spin-triplet Cooper pairs and magnetic structures such as domain walls and vortices has been the subject of intense theoretical discussions, while the relevant experiments remain scarce. We have developed nanostructured Josephson junctions with highly controllable spin texture, based on a disk-shaped Nb/Co bilayer. Here, the vortex magnetization of Co and the Cooper pairs of Nb conspire to induce long-range triplet (LRT) superconductivity in the ferromagnet. Surprisingly, the LRT correlations emerge in highly localized (sub-80 nm) channels at the rim of the ferromagnet, despite its trivial band structure. We show that these robust rim currents arise from the magnetization texture acting as an effective spin-orbit coupling, which results in spin accumulation at the bilayer-vacuum boundary. Lastly, we demonstrate that by altering the spin texture of a single ferromagnet, both $0$ and $\pi$-channels can be realized in the same device.

cond-mat.supr-con↗

arXiv2021

Universal size-dependent nonlinear charge transport in single crystals of the Mott insulator Ca$_2$RuO$_4$

The surprisingly low current density required for inducing the insulator to metal transition has made Ca$_2$RuO$_4$ an attractive candidate material for developing Mott-based electronics devices. The mechanism driving the resistive switching, however, remains a controversial topic in the field of strongly correlated electron systems. Here we probe an uncovered region of phase space by studying high-purity Ca$_2$RuO$_4$ single crystals, using the sample size as principal tuning parameter. Upon reducing the crystal size, we find a four orders of magnitude increase in the current density required for driving Ca$_2$RuO$_4$ out of the insulating state into a non-equilibrium (also called metastable) phase which is the precursor to the fully metallic phase. By integrating a microscopic platinum thermometer and performing thermal simulations, we gain insight into the local temperature during simultaneous application of current and establish that the size dependence is not a result of Joule heating. The findings suggest an inhomogeneous current distribution in the nominally homogeneous crystal. Our study calls for a reexamination of the interplay between sample size, charge current, and temperature in driving Ca$_2$RuO$_4$ towards the Mott insulator to metal transition.

cond-mat.str-el↗

arXiv2020

Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr2RuO4

The chiral p-wave order parameter in Sr$_\mathsf{2}$RuO$_\mathsf{4}$ would make it a special case amongst the unconventional superconductors. A consequence of this symmetry is the possible existence of superconducting domains of opposite chirality. At the boundary of such domains, the locally supressed condensate can produce an intrinsic Josephson junction. Here, we provide evidence of such junctions using mesoscopic rings, structured from Sr$_\mathsf{2}$RuO$_\mathsf{4}$ single crystals. Our order parameter simulations predict such rings to host stable domain walls across their arms. Thisis verified with transport experiments on loops, with a sharp transition at 1.5 K, which show distinct critical current oscillations with periodicity corresponding to the flux quantum. In contrast, loops with broadened transitions at around 3 K are void of such junctions and show standard Little-Parks oscillations. Our analysis demonstrates the junctions are of intrinsic origin and makes a compelling case for the existence of superconducting domains.

cond-mat.supr-con↗