We report measurements of the thermopower of atomic-size gold contacts realized by the mechanically controllable break junction (MCBJ) technique over a temperature range from 18 K to 295 K. A thermometer included in the lithographic structure close to the constriction provides a direct measurement of the temperature increase generated by heating one side of the contact with a focused laser beam. While the conductance histograms confirm the quantum nature of the transport, we observe a nonmonotonic temperature dependence of the ensemble-averaged thermopower with a minimum of -2 μVK^-1 at about 150 K. The values for the thermopower obtained at the lowest and the high temperature are compatible with values reported in the literature, but the nonmonotonic behavior in between disagrees with the expected linear dependence for quantum coherent conductors described by the Landauer formula. We develop a theoretical model based on an energy dependent transmission function that qualitatively reproduces the nonmonotonic behavior, but fails quantitatively. We therefore interpret our data as a result of phonon contributions to the thermopower beyond the Landauer model and with opposite sign than the classical phonon drag known from bulk systems. Our findings show that, firstly, the thermopower gives important insight into the transport properties of atomic-size structures and second that the linear approximation of the Landauer model has to be used with caution when studying more complex transport properties even for atomic contacts from free-electron metals.

The combination of superconductivity and magnetic textures represents a promising approach to explore unconventional superconducting phenomena, including new correlated and topological phases. Van der Waals (vdW) materials have emerged in this context as a versatile platform to explore the interplay between these two competing orders. Here, we report on individual NbSe2/NiPS3/NbSe2 vdW Josephson junctions behaving as superconducting quantum interference devices (SQUIDs), which we attribute to the interplay between the superconductivity of NbSe2 and the spin texture of the vdW antiferromagnetic insulator NiPS3. The SQUID behavior, which persists for in-plane magnetic fields of at least 6 T, is the result of interference between localized transport channels that form in two separate regions of the sample. Microscopic modeling of the antiferromagnet insulator/superconductor (AFI/S) interface reveals the formation of localized states at the edges of the junction that can lead to localized channels that dominate the transport. Our findings highlight the potential of vdW superconducting heterostructures with AFs as platforms for engineering and probing novel superconducting phenomena, and they establish a new route for lithographic-free SQUIDs that operate in high magnetic fields.

Incorporating oxygen into metallic atomic-scale junctions modifies the interatomic bonding and may even promote the formation of monoatomic chains. In the specific case of copper oxide, first-principles studies have predicted the emergence of ferromagnetic ground states, attributing certain atomic configurations with spin filtering capabilities. By means of low-temperature transport measurements, we provide a series of experimental evidence indicating the presence of local magnetism in air oxidized mechanically controllable copper break junctions. Our findings include hysteretic magnetoresistance, zero-bias anomalies attributed to the Kondo effect and anomalous shot noise behavior. We provide an extension of the Landauer description of shot noise for quantum coherent transport to include energy-dependent transmission functions. The analysis of the anomalous shot noise in the presence of strong zero-bias anomalies arising from the Kondo effect allows to determine the spin polarization of the current which may reach even full polarization, confirming the spin-filtering capability of copper oxide atomic contacts.

In conventional superconductors, Cooper pairs form in an even-parity spin-singlet state. Noncentrosymmetric superconductors, which lack inversion symmetry, exhibit antisymmetric spin-orbit coupling (ASOC) that can combine even-parity spin-singlet and odd-parity spin-triplet pairs into a mixed-parity order parameter. Spin-triplet components are highly beneficial for superspintronic devices. Whether ASOC alone - without strong electronic correlations - is sufficient to generate a measurable triplet component remains a central open question. Here, we resolve this question in Nb_18Re_82 (Nb-Re), a weakly-correlated noncentrosymmetric metal whose superconducting pairing symmetry has been actively debated. Using low-temperature scanning tunneling spectroscopy on single crystals with four distinct crystallographic orientations, find a pronounced orientation-dependent anisotropy in the local density of states. Supported by a symmetry-constrained model, we show that the complete set of tunneling spectra requires a mixed-parity order parameter with the triplet amplitude reaching up to half of the singlet component. These results reconcile the conflicting reports in the literature on Nb-Re and demonstrate that ASOC is sufficient to foster a sizable spin-triplet component even without strong electronic correlations, suggesting that mixed-parity superconducting states may be more widespread than previously assumed. Since Nb-Re can be readily fabricated in thin-film form, these findings position it as an accessible platform for superspintronic devices and establish orientation-resolved tunneling spectroscopy as a general protocol for the detection of mixed-parity order parameters.

Abstract Noise measurements provide a valuable tool for revealing spin polarization effects in the electronic transport through quantum coherent conductors. We present an extension of the Landauer description of shot noise to include energy dependent transmission functions and apply it to explore local magnetic correlations in air oxidized copper contacts, for which first-principle studies have predicted the emergence of ferromagnetic ground states, attributing certain atomic configurations with spin filtering capabilities. By means of low-temperature transport measurements, we provide comprehensive experimental evidence, including hysteretic magnetoresistance and zero-bias anomalies (ZBAs) attributed to the Kondo effect, for the presence of local magnetism. The analysis of the anomalous shot noise in the presence of ZBAs allows us to determine the spin polarization of the current which may reach even full polarization, confirming the spin filtering capability of copper oxide atomic contacts.

Van der Waals (vdW) superconductors - atomically thin crystalline materials that can be stacked into more complex heterostructures - have opened a promising avenue for superconducting electronics thanks to their properties that are otherwise difficult to obtain in other superconducting materials. These include strong resilience to high in-plane fields, electrostatic tuneability, and non-reciprocal transport rooted in inversion-symmetry breaking and strong spin-orbit coupling. In addition to highlighting the importance of these properties for superconducting electronics, this review gives an overview over the physical mechanisms that govern and influence superconductivity in vdW materials including Ising pairing, band inversion, and proximity effects at superconductor/ferromagnet interfaces that do not have an equivalent in thin-film systems. This overview then sets the basis to survey the wide range of functionalities enabled by superconducting vdW devices including gate-controlled devices, superconducting diodes, and circuit elements for readout and control of quantum bits. The review concludes with a forward look at wafer-scale growth and deterministic assembly of vdW devices, highlighting concrete pathways that can enable the transition from vdW device prototypes to deployable components for cryogenic electronics and quantum technologies.

An ultrasensitive and stable scanning thermal microscope is developed to detect phonon interference in single-molecule junctions.

The electronic and thermoelectric properties of molecular junctions formed from iron and ruthenium metal acetylide were studied by using complementary experimental techniques and quantum chemical simulations. We performed physical characterizations of single-molecule and self-assembled monolayer junctions of the same molecules that allowed meaningful comparisons between the Ru and Fe adducts. In the case of the Fe-containing junctions, two distinct oxidation states are present. These junctions exhibit one of the highest Seebeck coefficients (S similar to 130 mu V/K) reported to date for similar systems paired with broad electric conductance distribution and limited thermal conductance. As a result, the experimental thermoelectric figure of merit ZT for Fe-containing junctions reaches up to 0.4 for junctions with a relatively high conductance. This is one of the highest ZT values reported for molecular systems at room temperature.

The Blatter radical has been suggested as a building block in future molecular spintronic devices because of its radical character and expected long spin lifetime. However, whether its radical character is maintained in single-molecule junctions depends on the environment. Here, we demonstrate the ability to retain the open-shell nature of the Blatter radical in a two-terminal device by the appearance of a Kondo resonance in transport spectroscopy. Additionally, a high negative magnetoresistance is observed in junctions that do not reveal a zero-bias anomaly. By combining distance-dependent and magnetic-field-dependent measurements and accompanying quantum-chemical and quantum-transport calculations, we show that both findings, the negative magnetoresistance and the Kondo features, can be consistently explained by a singlet-triplet Kondo model. Our findings provide the possibility of using the Blatter radical in a two-terminal system under cryogenic conditions and also reveal the magnetotransport properties emerging from different configurations of the molecule inside a junction.

We demonstrate a superconducting readout method for detecting current-induced magnetization switching of a polycrystalline cobalt wire, based on the measurement of the critical current of an adjacent superconducting aluminum wire. The critical current Ic of this superconductor-superconductor/ferromagnet-superconductor junction is highly dependent on the magnetization state of the ferromagnet, reflected in its local stray-field distribution. This dependency allows for a detection of changes in the magnetization of the ferromagnet while no current passes through it during readout. Furthermore, we present micromagnetic simulations replicating our observations, thereby visualizing the microscopic switching behavior in polycrystalline Co.

We explore self-induced parametric coupling between a driven (low frequency) and an undriven (high frequency) mode, also called internal resonances, in a membrane micromechanical system. Specifically, we focus on the formation of a limit cycle (LC) manifesting as a phononic frequency comb. As the LC formation involves a Hopf bifurcation, we developed a dedicated pump-noisy-probe technique to investigate which mechanical sidebands merge at the bifurcation. We reveal that the sideband of the driven lower mode is up-converted via a cross-Duffing nonlinearity to hybridize with the undriven high mode. When the up-conversion is initially red detuned relative to the high mode, significant squeezing and bimodality in the high mode occurs. Crucially, only when the up-converted sideband is initially blue detuned relative to the high mode, the sideband is attracted to the high mode and merges to form the Hopf bifurcation. This process delineates the microscopic origin of frequency comb formation. Our study reveals a key instability mechanism in driven nonlinear systems with implications for advanced sensing technologies and phononic metamaterials.

Superconducting electronics holds great promise for energy-efficient high-performance and quantum computing, yet no superconducting memory has matched the performance of conventional semiconductor memories - a long-standing bottleneck. Here we demonstrate a voltage-controlled, non-volatile superconducting memory that exploits two previously independent effects: gate-controlled supercurrent (GCS), the gate-voltage-induced suppression of the critical current I_c in a superconducting constriction, and charge trapping in an Al_2O_3 dielectric. Trapped charges shift the threshold gate voltage required for I_c suppression, defining two stable, well-separated I_c states that can be used to store binary information. We demonstrate reliable non-destructive readout and reversible write/erase cycling over nearly fifty consecutive cycles with the device remaining in the zero-resistance state throughout. Stored information survives thermal cycling well above the superconducting transition temperature T_c, confirming true non-volatility - a capability absent in all existing superconducting memories. We further discuss integration into a NAND architecture and show significant power-dissipation advantages over CMOS charge-trap flash memories.

Integration of graphene in silicon-based micro-/nanoelectromechanical systems (MEMS/NEMS) marries the robustness of silicon-based materials with the exceptional physical properties of graphene, drastically enhancing the system’s regulation performance which now is key for many advanced applications in nanotechnology. Here, we experimentally demonstrate and theoretically analyze a powerful on-chip integration principle consisting of a hybrid graphene/silicon nitride membrane with metallic leads on top that enables an extremely large static and dynamic parameter regulation. When a static voltage is applied to the leads of the integrated structure, a spatially confined localized electrothermomechanical (ETM) effect results in ultra-wide frequency tuning, deformation (buckling transition) and regulation of the mechanical properties. Moreover, by injecting an alternating voltage to the leads, we can excite the resonator vibrating even far beyond its linear regime without a complex and space consuming actuation system. Our results prove that the scheme provides a compact integrated system possessing mechanical robustness, high controllability, and fast response. It not only expands the limit of the application range of MEMS/NEMS devices, but also enables the further miniaturization of the device.

In this work, we present a method for characterizing nano/micro membrane resonators through the analysis of averaged interference fringes obtained from continuous light measurements. As the membrane vibrates, the interference fringes display blurring and contrast reduction, from which we establish a direct relationship between the vibration amplitude and the blurred area. This method offers a fast and straightforward approach to characterizing membrane vibrations and determining the dispersion relationship. Additionally, it enables the simultaneous extraction of multiple vibrational modes, providing mode numbers and phase differences that can be used to reconstruct dynamic vibration profiles. Its efficiency and broad frequency range make it particularly well-suited for high-frequency applications and rapid data collection.

Current-driven conductance switching in atomic-size contacts has been attributed to reversible and vibrationmediated atomic rearrangements. Here, we present a comprehensive statistical analysis on the switching properties of atomic gold, copper, and aluminium contacts fabricated by mechanically controllable break junctions. The comparative analysis shows that various bi- and multilevel switching patterns can exist with volatile as well as nonvolatile features, and with distinct material-dependent preferences among the three metals. In addition, we apply different current-biasing protocols to identify intrinsic and material-dependent switching properties under variable boundary conditions. To explore the stability of such memory states, we reveal the stochastic nature of the underlying switching mechanism and suggest a simple qualitative approach to estimate the creation and failure rate for such switching events. The approach is inspired by atomic-scale electromigration models accounting for the observed switching voltages and conductance jumps.

Logic circuits consist of devices that can be controlled between two distinct states. The recent demonstration that a superconducting current flowing in a constriction can be controlled via a gate voltage (VG)─gate-controlled supercurrent (GCS)─can lead to superconducting logic with better performance than existing logics. However, before such logic is developed, high reproducibility in the functioning of GCS devices and optimization of their performance must be achieved. Here, we report an investigation of gated Nb devices showing GCS with very high reproducibility. Based on the investigation of a statistically significant number of devices, we demonstrate that the GCS is independent of the constriction width, in contrast with previous reports, and confirm a strong correlation between the GCS and the leakage current (Ileak) induced by VG. We also achieve a voltage output in our devices larger than the typical values reported to date by at least 1 order of magnitude, which is relevant for the future interconnection of devices, and show that Ileak can be used as a tool to modulate the operational VG of devices on a SiO2 substrates. These results altogether represent an important step forward toward the optimization of reproducibility and performance of GCS devices, and the future development of a GCS-based logic.

Integration of 2D materials in nanoelectromechanical systems (NEMS) marries the robustness of silicon-based materials with exceptional electrical controllability in 2D materials, drastically enhancing system performance which now is the key for many advanced applications in nanotechnology. Here, we experimentally demonstrate and theoretically analyze a powerful on-chip graphene integrated NEMS device consisting of a hybrid graphene/silicon-nitride membrane with metallic leads that enables an extremely large static and dynamic parameter regulation. When a static voltage is applied to the leads, the force induced by the thermal expansion difference between the leads and the membrane results in ultra-wide frequency tuning, deformation (post-buckling transition) and regulation of mechanical properties. Moreover, by injecting an alternating voltage to the leads, we can excite the resonator vibrating even far beyond its linear regime without a complex and space consuming actuation system. Our results prove that the device is a compact integrated system possessing mechanical robustness, high controllability, and fast response. It not only expands the limit of the application range of NEMS devices but also pushes multidimensional nanomechanical resonators into working in the nonlinear regime.

The combination of a superconductor with a magnetically inhomogeneous material has been established as an efficient mechanism for the generation of long-ranged spin-polarized (spin-triplet) Cooper pairs. Evidence for this mechanism, however, has been established based on studies done on three-dimensional systems, where the strong bonds existing at the interface between the superconductor and the magnetic material should in principle enhance proximity effects and strengthen any electronic correlations. Here, we fabricate devices based on van der Waals stacks of flakes of the two-dimensional superconductor $NbS_2$ combined with flakes of $Cr_{1/3}NbS_2$, which has a built-in magnetic inhomogeneity due to its helimagnetic spin texture at low temperatures. We find that the critical temperature of these vdW bilayers is strongly dependent on the magnetic state of $Cr_{1/3}NbS_2$, whose degree of magnetic inhomogeneity can be controlled via an applied magnetic field. Our results demonstrate evidence for the generation of long-ranged spin-triplet pairs across the $Cr_{1/3}NbS_2$/$NbS_2$ vdW interface.

A comprehensive understanding of carrier transport in photoisomeric molecular junctions is crucial for the rational design and delicate fabrication of single-molecule functional devices. It has been widely recognized that the conductance of azobenzene (a class of photoisomeric molecules) based molecular junctions is mainly determined by photoinduced conformational changes. In this study, it is demonstrated that the most probable conductance of amine-anchored azobenzene-based molecular junctions increases continuously upon UV irradiation. In contrast, the conductance of pyridyl-anchored molecular junctions with an identical azobenzene core exhibits a contrasting trend, highlighting the pivotal role that anchoring groups play, potentially overriding (even reversing) the effects of photoinduced conformational changes. It is further demonstrated that the molecule with cis-conformation cannot be fully mechanically stretched into the trans-conformation, clarifying that it is a great challenge to realize a reversible molecular switch by purely mechanical operation. Additionally, it is revealed that the coupling strength of pyridyl-anchored molecules is dramatically weakened when the UV irradiation time is prolonged, whereas it is not observed for amine-anchored molecules. The mechanisms for these observations are elucidated with the assistance of density functional theory calculations and UV-Vis spectra combined with flicker noise measurements which confirm the photoinduced conformational changes, providing insight into understanding the charge transport in photoisomeric molecular junctions and offering a routine for logical designing synchro opto-mechanical molecular switches.

The Berry phase is a fundamental concept in quantum mechanics with profound implications for understanding topological properties of quantum systems. This tutorial provides a comprehensive introduction to the Berry phase, beginning with the essential mathematical framework required to grasp its significance. We explore the intrinsic link between the emergence of a non-trivial Berry phase and the presence of topological characteristics in quantum systems, showing the connection between the Berry phase and the band structure as well as the phase’s gauge-invariant nature during cyclic evolutions. The tutorial delves into various topological effects arising from the Berry phase, such as the quantum, anomalous, and spin Hall effects, which exemplify how these quantum phases manifest in observable phenomena. We then extend our discussion to cover the transport properties of topological insulators, elucidating their unique behaviour rooted in the Berry phase physics. This tutorial aims at equipping its readers with a robust understanding of the basic theory underlying the Berry phase and of its pivotal role in the realm of topological quantum phenomena.