Temperature-dependence of the chirality-induced spin selectivity effect-Experiments and theory.

The temperature-dependence of the chirality-induced spin selectivity (CISS) effect can be used to discriminate between different theoretical proposals for the mechanism of the CISS effect. Here, we briefly review key experimental results and discuss the effect of temperature in different models for the CISS effect. We then focus on the recently suggested spinterface mechanism and describe the different possible effects temperature can have within this model. Finally, we analyze in detail recent experimental results presented in the work of Qian et al. [Nature 606, 902-908 (2022)] and demonstrate that, contrary to the original interpretation by the authors, these data actually indicate that the CISS effect increases with decreasing temperature. Finally, we show how the spinterface model can accurately reproduce these experimental results.

A streptavidin-biotin system combined with magnetic actuators for remote neuronal guidance.

The ability to control neuronal mobility and organization is of great importance in developing neuronal interfaces and novel therapeutic approaches. An emerging promising method is the manipulation of neuronal cells from afar via magnetic forces. Nevertheless, using magnetic iron oxide nanoparticles as internal actuators may lead to biotoxicity, adverse influence on intracellular processes, and thus requires prerequisite considerations for therapeutic approaches. Magnetizing the cells via the incorporation of magnetic particles that can be applied extracellularly is advantageous. Herein, we have developed a magnetic system based on streptavidin-biotin interaction to decorate cellular membrane with magnetic elements. In this model, superparamagnetic microparticles, coated with streptavidin, were specifically bound to biotinylated PC12 cells. We demonstrated that cell movement can be directed remotely by the forces produced by pre-designed magnetic fields. First, using time lapse imaging, we analyzed the kinetics of cell migration towards the higher flux zone. Next, to form organized networks of cells we designed and fabricated micro-patterned magnetic devices. The fabricated devices were composed of a variety of ferromagnetic shapes, sputter-deposited onto glass substrates. Cells that were conjugated to the magnetic particles were plated atop the micro-patterned substrates, attracted to the magnetic actuators and became fixed onto the magnetic patterns. In all, our study presents a novel system based on a well-known molecular technology combined with nanotechnology that may well lead to the expansion of implantable magnetic actuators to organize and direct cellular growth.

Controlled synthesis of multifunctional dome-shaped micro- and nano-structures via a robust physical route for biological applications.

Micro- and nano-particles are elemental for many current and developing technologies. Specifically, these particles are being used extensively in biological studies and applications, which include imaging, drug delivery and therapeutics. Recent advances have led to the development of multifunctional particles, which have the potential to further enhance their effectiveness, enabling novel applications. Therefore, many efforts have been devoted to producing well-defined particles for specific needs. However, the conventional fabrication methodologies used to develop particles are time consuming, making it extremely challenging to fine-tune the properties of the particles for multifunctional applications. Herein, we present a simple and facile method to fabricate dome-shaped micron- and nano-sized particles via a robust physical route. The presented method enables particles to be designed using a vast range of materials, with different sizes and compositions. The versatility of this method enables the engineering of multifunctional particles with pre-defined properties that can be adjusted to a specific biological application. We demonstrate the fabrication of dome-shaped particles using physical vapor deposition (PVD) and a polystyrene-bead-monolayer-based mechanical mask. We show domes from several materials and coatings; in particular, we demonstrate the development process for biocompatible magnetic iron oxide domes. We find that our magnetic domes exhibit an Fe3O4 structure with a high magnetization saturation. In addition, we examine the biocompatibility of the magnetic domes by performing viability tests and morphological analysis. The ability to design and fabricate micro- and nano-particles upon request in a simple and relatively high-throughput manner opens possibilities for the development of new smart multifunctional particles for both therapeutic and diagnostic applications.

Fabrication of Magnetic Platforms for Micron-Scale Organization of Interconnected Neurons.

The ability to direct neurons into organized neural networks has great implications for regenerative medicine, tissue engineering, and bio-interfacing. Many studies have aimed at directing neurons using chemical and topographical cues. However, reports of organizational control on a micron-scale over large areas are scarce. Here, an effective method has been described for placing neurons in preset sites and guiding neuronal outgrowth with micron-scale resolution, using magnetic platforms embedded with micro-patterned, magnetic elements. It has been demonstrated that loading neurons with magnetic nanoparticles (MNPs) converts them into sensitive magnetic units that can be influenced by magnetic gradients. Following this approach, a unique magnetic platform has been fabricated on which PC12 cells, a common neuron-like model, were plated and loaded with superparamagnetic nanoparticles. Thin films of ferromagnetic (FM) multilayers with stable perpendicular magnetization were deposited to provide effective attraction forces toward the magnetic patterns. These MNP-loaded PC12 cells, plated and differentiated atop the magnetic platforms, were preferentially attached to the magnetic patterns, and the neurite outgrowth was well aligned with the pattern shape, forming oriented networks. Quantitative characterization methods of the magnetic properties, cellular MNP uptake, cell viability, and statistical analysis of the results are presented. This approach enables the control of neural network formation and improves neuron-to-electrode interface through the manipulation of magnetic forces, which can be an effective tool for in vitro studies of networks and may offer novel therapeutic biointerfacing directions.

Comparing Transcriptome Profiles of Neurons Interfacing Adjacent Cells and Nanopatterned Substrates Reveals Fundamental Neuronal Interactions.

Developing neuronal axons are directed by chemical and physical signals toward a myriad of target cells. According to current dogma, the resulting network architecture is critically shaped by electrical interconnections, the synapses; however, key mechanisms translating neuronal interactions into neuronal growth behavior during network formation are still unresolved. To elucidate these mechanisms, we examined neurons interfacing nanopatterned substrates and compared them to natural interneuron interactions. We grew similar neuronal populations under three connectivity conditions, (1) the neurons are isolated, (2) the neurons are interconnected, and (3) the neurons are connected only to artificial substrates, then quantitatively compared both the cell morphologies and the transcriptome-expression profiles. Our analysis shows that whereas axon-guidance signaling pathways in isolated neurons are predominant, in isolated neurons interfacing nanotopography, these pathways are downregulated, similar to the interconnected neurons. Moreover, in nanotopography, interfacing neuron genes related to synaptogenesis and synaptic regulation are highly expressed, that is, again resembling the behavior of interconnected neurons. These molecular findings demonstrate that interactions with nanotopographies, although not leading to electrical coupling, play a comparable functional role in two major routes, neuronal guidance and network formation, with high relevance to the design of regenerative interfaces.

Current-Induced Crossover of Flux Periodicity from h/2 e to h/e in a Superconducting Nb Nano-Ring.

Magnetoresistance measurements in a granular Nb nanoring reveal current-induced crossover between two distinct quantum coherence effects. At low bias currents, Cooper-pair coherence is manifested by Little-Parks oscillations with flux periodicity of h/2 e. At high bias currents, magnetoresistance oscillations with flux periods of h/ e are observed and interpreted as Aharonov-Bohm oscillations, reflecting the phase coherence of individual quasi-particles. The model explaining these data views the ring as a chain of superconducting grains weakly coupled by tunnel junctions. Low bias currents allow coherent tunneling of Cooper pairs between the grains. Increasing the current above the critical current of all the junctions creates a quasi-particles conduction channel along the ring, allowing for quantum interference of quasi-particles.

Magnetic Targeting of Growth Factors Using Iron Oxide Nanoparticles.

Growth factors play an important role in nerve regeneration and repair. An attractive drug delivery strategy, termed "magnetic targeting", aims to enhance therapeutic efficiency by directing magnetic drug carriers specifically to selected cell populations that are suitable for the nervous tissues. Here, we covalently conjugated nerve growth factor to iron oxide nanoparticles (NGF-MNPs) and used controlled magnetic fields to deliver the NGF⁻MNP complexes to target sites. In order to actuate the magnetic fields a modular magnetic device was designed and fabricated. PC12 cells that were plated homogenously in culture were differentiated selectively only in targeted sites out of the entire dish, restricted to areas above the magnetic "hot spots". To examine the ability to guide the NGF-MNPs towards specific targets in vivo, we examined two model systems. First, we injected and directed magnetic carriers within the sciatic nerve. Second, we injected the MNPs intravenously and showed a significant accumulation of MNPs in mouse retina while using an external magnet that was placed next to one of the eyes. We propose a novel approach to deliver drugs selectively to injured sites, thus, to promote an effective repair with minimal systemic side effects, overcoming current challenges in regenerative therapeutics.

Ramp-Reversal Memory and Phase-Boundary Scarring in Transition Metal Oxides.

Transition metal oxides are complex electronic systems that exhibit a multitude of collective phenomena. Two archetypal examples are VO2 and NdNiO3 , which undergo a metal-insulator phase transition (MIT), the origin of which is still under debate. Here this study reports the discovery of a memory effect in both systems, manifested through an increase of resistance at a specific temperature, which is set by reversing the temperature ramp from heating to cooling during the MIT. The characteristics of this ramp-reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron-glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, it is found that the main ingredients for the effect to arise are the spatial phase separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local transition temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal temperature leaving "scars" in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.

Current-induced SQUID behavior of superconducting Nb nano-rings.

The critical temperature in a superconducting ring changes periodically with the magnetic flux threading it, giving rise to the well-known Little-Parks magnetoresistance oscillations. Periodic changes of the critical current in a superconducting quantum interference device (SQUID), consisting of two Josephson junctions in a ring, lead to a different type of magnetoresistance oscillations utilized in detecting extremely small changes in magnetic fields. Here we demonstrate current-induced switching between Little-Parks and SQUID magnetoresistance oscillations in a superconducting nano-ring without Josephson junctions. Our measurements in Nb nano-rings show that as the bias current increases, the parabolic Little-Parks magnetoresistance oscillations become sinusoidal and eventually transform into oscillations typical of a SQUID. We associate this phenomenon with the flux-induced non-uniformity of the order parameter along a superconducting nano-ring, arising from the superconducting leads ('arms') attached to it. Current enhanced phase slip rates at the points with minimal order parameter create effective Josephson junctions in the ring, switching it into a SQUID.

Ultrathin Films of VO2 on r-Cut Sapphire Achieved by Postdeposition Etching.

The metal-insulator transition (MIT) properties of correlated oxides thin films, such as VO2, are dramatically affected by strain induced at the interface with the substrate, which usually changes with deposition thickness. For VO2 grown on r-cut sapphire, there is a minimum deposition thickness required for a significant MIT to appear, around 60 nm. We show that in these thicker films an interface layer develops, which accompanies the relaxation of film strain and enhanced electronic transition. If these interface dislocations are stable at room temperature, we conjectured, a new route opens to control thickness of VO2 films by postdeposition thinning of relaxed films, overcoming the need for thickness-dependent strain-engineered substrates. This is possible only if thinning does not alter the films' electronic properties. We find that wet etching in a dilute NaOH solution can effectively thin the VO2 films, which continue to show a significant MIT, even when etched to 10 nm, for which directly deposited films show nearly no transition. The structural and chemical composition were not modified by the etching, but the grain size and film roughness were, which modified the hysteresis width and magnitude of the MIT resistance change.

High resolution Hall measurements across the VO2 metal-insulator transition reveal impact of spatial phase separation.

Many strongly correlated transition metal oxides exhibit a metal-insulator transition (MIT), the manipulation of which is essential for their application as active device elements. However, such manipulation is hindered by lack of microscopic understanding of mechanisms involved in these transitions. A prototypical example is VO2, where previous studies indicated that the MIT resistance change correlate with changes in carrier density and mobility. We studied the MIT using Hall measurements with unprecedented resolution and accuracy, simultaneously with resistance measurements. Contrast to prior reports, we find that the MIT is not correlated with a change in mobility, but rather, is a macroscopic manifestation of the spatial phase separation which accompanies the MIT. Our results demonstrate that, surprisingly, properties of the nano-scale spatially-separated metallic and semiconducting domains actually retain their bulk properties. This study highlights the importance of taking into account local fluctuations and correlations when interpreting transport measurements in highly correlated systems.

Resolving transitions in the mesoscale domain configuration in VO2 using laser speckle pattern analysis.

The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties. However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution. We demonstrate an elegant experimental technique that bridges these local and global methods, giving access to mesoscale information on domain formation and evolution at time scales orders of magnitude faster than current spatially resolved approaches. Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

Interactions of neurons with topographic nano cues affect branching morphology mimicking neuron-neuron interactions.

We study the effect of topographic nano-cues on neuronal growth-morphology using invertebrate neurons in culture. We use photolithography to fabricate substrates with repeatable line-pattern ridges of nano-scale heights of 10-150 nm. We plate leech neurons atop the patterned-substrates and compare their growth pattern to neurons plated atop non-patterned substrates. The model system allows us the analysis of single neurite-single ridge interactions. The use of high resolution electron microscopy reveals small filopodia processes that attach to the line-pattern ridges. These fine processes, that cannot be detected in light microscopy, add anchoring sites onto the side of the ridges, thus additional physical support. These interactions of the neuronal process dominantly affect the neuronal growth direction. We analyze the response of the entire neuronal branching tree to the patterned substrates and find significant effect on the growth patterns compared to non-patterned substrates. Moreover, interactions with the nano-cues trigger a growth strategy similarly to interactions with other neuronal cells, as reflected in their morphometric parameters. The number of branches and the number of neurites originating from the soma decrease following the interaction demonstrating a tendency to a more simplified neuronal branching tree. The effect of the nano-cues on the neuronal function deserves further investigation and will strengthen our understanding of the interplay between function and form.

Topographic cues of nano-scale height direct neuronal growth pattern.

We study the role of nano-scale cues in controlling neuronal growth. We use photolithography to fabricate substrates with repeatable line-pattern ridges of nano-scale heights. We find that neuronal processes, which are of micron size, have strong interactions with ridges even as low as 10 nm. The interaction between the neuronal process and the ridge leads to a deflection of growth direction and a preferred alignment with the ridges. The interaction strength clearly depends on the ridges' height. For 25 nm ridges approximately half of the neuronal processes are modified, while at 100 nm the majority of neurites change their original growth direction post interaction. In addition, the effect on growth correlates with the incoming angle between the neuronal process and the ridge. We underline the adhesion as a key mechanism in directing neuronal growth. Our study highlights the sensitivity of growing neurites to nano-scale cues thus opens a new avenue of research for pre-designed neuronal growth and circuitry.

Multiple avalanches across the metal-insulator transition of vanadium oxide nanoscaled junctions.

The metal-insulator transition of nanoscaled VO2 devices is drastically different from the smooth transport curves generally reported. The temperature driven transition occurs through a series of resistance jumps ranging over 2 decades in magnitude, indicating that the transition is caused by avalanches. We find a power law distribution of the jump sizes, demonstrating an inherent property of the VO2 films. We report a surprising relation between jump magnitude and device size. A percolation model captures the general transport behavior, but cannot account for the statistical behavior.

Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films.

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.

Anomalous proximity effect in gold coated (110) YBa2Cu3O7-delta films: penetration of the Andreev bound states.

Scanning tunneling spectroscopy of (110)YBa(2)Cu(3)O(7-delta)/Au bilayers reveal a proximity effect markedly different from the conventional one. While proximity-induced mini-gaps rarely appear in the Au layer, the Andreev bound states clearly penetrate into the metal. Zero bias conductance peaks are measured on Au layers thinner than 7 nm with a magnitude similar to those detected on the bare superconductor films. The peaks then decay abruptly with Au thickness and disappear above 10 nm. This length is shorter than the normal coherence length and corresponds to the (ballistic) mean free path.

Proximity effect in gold-coated YBa(2)Cu(3)O(7-delta) films studied by scanning tunneling spectroscopy.

Scanning tunneling spectroscopy on gold layers overcoating c-axis YBa(2)Cu(3)O(7-delta) (YBCO) films reveals proximity-induced gap structures. The gap size reduces exponentially with the distance from a-axis facets, indicating that the proximity effect is primarily due to the (100) YBCO facets. The penetration depth of superconductivity into the gold is approximately 30 nm, in good agreement with estimations for the dirty limit. The extrapolated gap at the interface is approximately 15 meV, similar to the value of an s-wave component of the order parameter measured at the YBCO surface in recent point-contact experiments.