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We investigate the local nanoscale changes of the magnetic anisotropy of a Ni film subject to an inverse magnetostrictive effect by proximity to a V2O3 layer. Using temperature-dependent photoemission electron microscopy (PEEM) combined with X-ray magnetic circular dichroism (XMCD), direct images of the Ni spin alignment across the first-order structural phase transition (SPT) of V2O3 were obtained. We find an abrupt temperature-driven reorientation of the Ni magnetic domains across the SPT, which is associated with a large increase of the coercive field. Moreover, angular dependent ferromagnetic resonance (FMR) shows a remarkable change in the magnetic anisotropy of the Ni film across the SPT of V2O3. Micromagnetic simulations based on these results are in quantitative agreement with the PEEM data. Direct measurements of the lateral correlation length of the Ni domains from XMCD images show an increase of almost one order of magnitude at the SPT compared to room temperature, as well as a broad spatial distribution of the local transition temperatures, thus corroborating the phase coexistence of Ni anisotropies caused by the V2O3 SPT. We show that the rearrangement of the Ni domains is due to strain induced by the oxide layers’ structural domains across the SPT. Our results illustrate the use of alternative hybrid systems to manipulate magnetic domains at the nanoscale, which allows for engineering of coercive fields for novel data storage architectures.

Metasurfaces are two-dimensional nanostructures that allow unprecedented control of light through engineering the amplitude, phase, and polarization across meta-atom resonators. Adding tunability to metasurface components would boost their potential and unlock a vast array of new application possibilities such as dynamic beam steering, tunable metalenses, and reconfigurable meta-holograms, to name a few. We present here high-index meta-atoms, resonators, and metasurfaces reconfigured by thermal effects, across the near to mid-infrared spectral ranges. We study thermal tunability in group IV and group IV–VI semiconductors, as well as in phase-transition materials, and demonstrate large dynamic resonance frequency shifts accompanied by significant amplitude and phase modulation in metasurfaces and resonators. We highlight the importance of high-Q resonances along with peak performance of thermal and thermo-optic effects, for efficient and practical reconfigurable devices. This paper paves the way to efficient high-Q reconfigurable and active infrared metadevices.

Metasurfaces manipulate light through engineering the amplitude, phase and polarization across arrays of meta-atom antenna resonators. Adding tunability and active functionality to metasurface components would boost their potential and unlock a vast array of new application possibilities such as dynamic beam steering, LIDAR, tunable metalenses, reconfigurable meta-holograms and many more. We present here high-index reconfigurable meta-atoms, resonators and metasurfaces that can dynamically and continuously tune their frequency, amplitude and phase, across the infrared spectral ranges. We utilize narrow linewidth resonances along with peak performance of tunable mechanisms for efficient and practical reconfigurable devices.

Dielectric-resonator-based nanophotonic devices show promise owing to their low intrinsic losses, support of multipolar resonances, and efficient operation in both reflection and transmission confi...

The properties of oxides are critically controlled by the oxygen stoichiometry. Minimal variations in oxygen content can lead to vast changes in their properties. The addition of oxygen during synthesis may not be a precise enough knob for tuning the oxygen stoichiometry when the material has several stable and close oxidation states. We use sputtered V2O3 films as an example to show that rapid transfer of the sample away from the heating element after growth causes a temperature decrease (quenching) quick enough to freeze the correct oxygen stoichiometry in the sample. This procedure has allowed us to improve dramatically the V2O3 electronic properties without any adverse measurable effects on the structural properties. In this fashion, the metal–insulator transition resistance change was increased by two orders of magnitude, while the transition width was decreased by 20 K.

Nanophotonic resonators offer the ability to design nanoscale optical elements and engineered materials with unconventional properties. Dielectric-based resonators intrinsically support a complete multipolar resonant response with low absorption, while metallic resonators provide extreme light confinement and enhanced photon–electron interactions. Here, we construct resonators out of a prototypical metal–insulator transition material, vanadium dioxide (VO2), and demonstrate switching between dielectric and plasmonic resonances. We first characterize the temperature-dependent infrared optical constants of VO2 single crystals and thin-films. We then fabricate VO2 wire arrays and disk arrays. We show that wire resonators support dielectric resonances at low temperatures, a damped scattering response at intermediate temperatures, and plasmonic resonances at high temperatures. In disk resonators, however, upon heating, there is a pronounced enhancement of scattering at intermediate temperatures and a substanti...

We present a coordinated study of the paramagnetic-to-antiferromagnetic, rhombohedral-to-monoclinic, and metal-to-insulator transitions in thin-film specimens of the classic Mott insulator ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ using low-energy muon spin relaxation, x-ray diffraction, and nanoscale-resolved near-field infrared spectroscopic techniques. The measurements provide a detailed characterization of the thermal evolution of the magnetic, structural, and electronic phase transitions occurring in a wide temperature range, including quantitative measurements of the high- and low-temperature phase fractions for each transition. The results reveal a stable coexistence of the high- and low-temperature phases over a broad temperature range throughout the transition. Careful comparison of temperature dependence of the different measurements, calibrated by the resistance of the sample, demonstrates that the electronic, magnetic, and structural degrees of freedom remain tightly coupled to each other during the transition process. We also find evidence for antiferromagnetic fluctuations in the vicinity of the phase transition, highlighting the important role of the magnetic degree of freedom in the metal-insulator transition.

Metasurfaces allow unprecedented control of light through engineering the amplitude, phase and polarization across arrays of meta-atom resonators. Adding dynamic tunability to metasurface components would boost their potential and unlock a vast array of new application possibilities such as dynamic beam steering, LIDAR, tunable metalenses and reconfigurable meta-holograms, to name a few. We present here high-index reconfigurable metaatoms, resonators and metasurfaces that can dynamically and continuously tune their frequency, amplitude and phase, across the near to mid-infrared spectral ranges. We highlight the importance of narrow linewidth resonances along with peak performance of tunable mechanisms for efficient and practical reconfigurable devices.

We report the existence of two competing mechanisms in the current-driven electrical breakdown of vanadium sesquioxide (${\mathrm{V}}_{2}{\mathrm{O}}_{3}$) and vanadium dioxide ($\mathrm{V}{\mathrm{O}}_{2}$) nanodevices. Our experiments and simulations show that the competition between a purely electronic (PE) mechanism and an electrothermal (ET) mechanism, suppressed in nanoscale devices, explains the current-driven insulator-to-metal phase transition (IMT). We find that the relative contribution of PE and ET effects is dictated by thermal coupling and resistivity, a discovery which disambiguates a long-standing controversy surrounding the physical nature of the current-driven IMT in vanadium oxides. Furthermore, we show that the electrothermally driven IMT occurs through a nanoscopic surface-confined filament. This nanoconfined filament has a very large thermal gradient, thus generating a large Seebeck-effect electric field.

Here, we photoinduce and directly observe with x-ray scattering an ultrafast enhancement of the structural long-range order in the archetypal Mott system V_{2}O_{3}. Despite the ultrafast increase in crystal symmetry, the change of unit cell volume occurs an order of magnitude slower and coincides with the insulator-to-metal transition. The decoupling between the two structural responses in the time domain highlights the existence of a transient photoinduced precursor phase, which is distinct from the two structural phases present in equilibrium. X-ray nanoscopy reveals that acoustic phonons trapped in nanoscale twin domains govern the dynamics of the ultrafast transition into the precursor phase, while nucleation and growth of metallic domains dictate the duration of the slower transition into the metallic phase. The enhancement of the long-range order before completion of the electronic transition demonstrates the critical role the nonequilibrium structural phases play during electronic phase transitions in correlated electrons systems.

We present experimental evidence for (a) multiphase superconductivity and (b) coexistence of magnetism and superconductivity in a single structural phase of lithiated iron selenide hydroxide $[(\mathrm{L}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{F}{\mathrm{e}}_{x})\text{OH]FeSe}$. Magnetic field modulated microwave spectroscopy data confirms superconductivity with at least two distinct transition temperatures attributed to well-defined superconducting phases at ${T}_{\mathrm{SC}1}=40\ifmmode\pm\else\textpm\fi{}2$ K and ${T}_{\mathrm{SC}2}=35\ifmmode\pm\else\textpm\fi{}2$ K. Magnetometry data for the upper critical fields reveal a change in the magnetic order $({T}_{M}=12$ K) below ${T}_{\mathrm{SC}1}$ and ${T}_{\mathrm{SC}2}$ that is consistent with ferromagnetism. This occurs because the superconducting coherence length is much smaller than the structural coherence length, allowing for several different electronic and magnetic states on a single crystallite. The results give insight into the physics of complex multinary materials, where several phenomena governed by different characteristic length scales coexist.

Here, we photoinduce and directly observe with x-ray scattering an ultrafast enhancement of the structural long-range order in the archetypal Mott system V_{2}O_{3}. Despite the ultrafast increase in crystal symmetry, the change of unit cell volume occurs an order of magnitude slower and coincides with the insulator-to-metal transition. The decoupling between the two structural responses in the time domain highlights the existence of a transient photoinduced precursor phase, which is distinct from the two structural phases present in equilibrium. X-ray nanoscopy reveals that acoustic phonons trapped in nanoscale twin domains govern the dynamics of the ultrafast transition into the precursor phase, while nucleation and growth of metallic domains dictate the duration of the slower transition into the metallic phase. The enhancement of the long-range order before completion of the electronic transition demonstrates the critical role the nonequilibrium structural phases play during electronic phase transitions in correlated electrons systems.

We found atypical pressure dependence in the transport measurements of the metal to insulator transition (MIT) in epitaxial thin films of vanadium sesquioxide (${\mathrm{V}}_{2}{\mathrm{O}}_{3}$). Three different crystallographic orientations and four thicknesses, ranging from 40 to 500 nm, were examined under hydrostatic pressures $({\mathrm{P}}_{h})$ of up to 1.5 GPa. All of the films at transition exhibited a four order of magnitude resistance change, with transition temperatures ranging from 140 to 165 K, depending on the orientation. This allowed us to build pressure-temperature phase diagrams of several orientations and film thicknesses. Interestingly, for pressures below 500 MPa, all samples deviate from bulk behavior and show a weak transition temperature $({T}_{c})$ pressure dependence $(d{T}_{c}/d{P}_{h}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.16em}{0ex}}0.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.28em}{0ex}}\mathrm{K}/\mathrm{MPa}),$ which recovers to bulklike behavior $(3.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.16em}{0ex}}0.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.28em}{0ex}}\mathrm{K}/\mathrm{MPa})$ at higher pressures. Furthermore, we found that pressurization leads to morphological but not structural changes in the films. This indicates that the difference in the thin film and bulk pressure-temperature phase diagrams is most probably due to pressure-induced grain boundary relaxation, as well as both plastic and elastic deformations in the film microstructure. These results highlight the difference between bulk and thin films behaviors.

Phone: þ34 943 574012, Fax: 34 943 574001We present performance characteristics of nanoscaled cobaltphthalocyanine (CoPc)-based organic field-effect transistors(OFETs) as a function of channel length. We found a channellength range which maximizes the field effect mobility in atrade-off between the decrease in the number of organic grainboundaries and the increase of the electrode–organic contactregion. Further reduction of channel length is limited by fringecurrents, which lead to an increased off current and to adegradation of the sub-threshold slope. From this, we definean optimal channel length of 280nm to 1mm for applicationsin submicrometric CoPc-based OFETs. Our results are particu-larly relevant for the miniaturization of chemical sensingOFETs, where metal phthalocyanines have proven to beexcellentcandidatesforthefabricationofthetransistorchannel.

Proximity effects and exchange coupling across interfaces of hybrid magnetic heterostructures present unique opportunities for functional material design. In this review, we present an overview of recent experiments on magnetic hybrid materials in which magnetism was controlled by proximity to an active material. In particular, we discuss interfacial strain coupling of ferromagnetic materials in contact with a material undergoing a structural deformation. Bilayers containing VO2 and V2O3 as active materials are shown to strongly affect the magnetization and coercivity of ferromagnetic materials due to stress anisotropy caused by a temperature-dependent structural displacement in the oxide. The possibilities of tuning the system by sample morphology and materials choice are discussed in detail. In addition, we highlight a length-scale competition between magnetic and structural domains which leads to a maximum change in the coercivity in a narrow temperature window of the vanadium oxide phase transition.

Hybrid materials allow the engineering of new material properties by creative uses of proximity effects. When two dissimilar materials are in close physical proximity the properties of each one may be radically modified or occasionally a completely new material emerges. In the area of magnetism, controlling the magnetic properties of ferromagnetic thin films without magnetic fields is an on- going challenge with multiple technological implications for low- energy consumption memory and logic devices. Interesting possibilities include ferromagnets in proximity to dissimilar materials such as antiferromagnets or oxides that undergo metal-insulator transitions. The proximity of ferromagnets to antiferromagnets has given rise to the extensively studied Exchange Bias[1]. Our recent investigations in this field have addressed crucial issues regarding the importance of the antiferromagnetic [2-3] and ferromagnetic [4] bulk for the Exchange Bias and the unusual short time dynamics [5]. In a series of recent studies, we have investigated the magnetic properties of different hybrids of ferromagnets (Ni, Co and Fe) and oxides, which undergo metal-insulator and structural phase transitions. Both the static as well as dynamical properties of the ferromagnets are drastically affected. Static properties such as the coercivity, anisotropy and magnetization [6-8] and dynamical properties such as the microwave response are clearly modified by the proximity effect and give raise to interesting perhaps useful properties. Work supported by US-AFOSR and US-DOE

The realization of a controllable metamagnetic transition from AFM to FM ordering would open the door to a plethora of new spintronics based devices that, rather than reorienting spins in a ferromagnet, harness direct control of a materials intrinsic magnetic ordering. In this study FeRh films with drastically reduced transition temperatures and a large magneto-thermal hysteresis were produced for magnetocaloric and spintronics applications. Remarkably, giant controllable magnetization changes (measured to be as high has ~25%) are realized by manipulating the strain transfer from the external lattice when subjected to two structural phase transitions of BaTiO3 (001) single crystal substrate. These magnetization changes are the largest seen to date to be controllably induced in the FeRh system. Using polarized neutron reflectometry we reveal how just a slight in plane surface strain change at ~290C results in a massive magnetic transformation in the bottom half of the film clearly demonstrating a strong lattice-spin coupling in FeRh. By means of these substrate induced strain changes we show a way to reproducibly explore the effects of temperature and strain on the relative stabilities of the FM and AFM phases in multi-domain metamagnetic systems. This study also demonstrates for the first time the depth dependent nature of a controllable magnetic order using strain in an artificial multiferroic heterostructure.

The on-line identification of labeled cells and vessels is a rate-limiting step in scanning microscopy. We use supervised learning to formulate an algorithm that rapidly and automatically tags fluorescently labeled somata in full-field images of cortex and constructs an optimized scan path through these cells. A single classifier works across multiple subjects, regions of the cortex of similar depth, and different magnification and contrast levels without the need to retrain the algorithm. Retraining only has to be performed when the morphological properties of the cells change significantly. In conjunction with two-photon laser scanning microscopy and bulk-labeling of cells in layers 2/3 of rat parietal cortex with a calcium indicator, we can automatically identify ∼50 cells within 1 min and sample them at ∼100 Hz with a signal-to-noise ratio of ∼10.

We have developed a mathematical algorithm to reconstruct the entire ICF shell surface to 10 nm accuracy. Mode 0 and 1 information is inherently missing in the Spheremapper measurements and must be retrieved systematically by minimizing the radius discrepancy at trace intersections. The R(θ,ϕ) map with 1 deg. angular resolution can be constructed by this method to give the target designer a way to model implosions, to give the process engineer feedback and to give the customer a tool for product visualization

In this study, we show the ability of soft x-ray irradiation to induce room temperature metal-insulator transitions (MITs) in VO2 thin films grown on R-plane sapphire. The ability of soft x-rays to induce MIT in VO2 thin films is confirmed by photoemission spectroscopy and soft x-ray spectroscopy measurements. When irradiation was discontinued, the systems do not return to the insulating phase. Analysis of valence band photoemission spectra revealed that the density of states (DOSs) of the V 3d band increased with irradiation time, while the DOS of the O 2p band decreased. We use these results to propose a model in which the MIT is driven by oxygen desorption from thin films during irradiation.

Phthalocyanines, a class of macrocyclic, square planar molecules, are extensively studied as semiconductor materials for chemical sensors, dye-sensitized solar cells, and other applications. In this study, we use angular dependent near-edge x-ray absorption fine structure (NEXAFS) spectroscopy as a quantitative probe of the orientation and electronic structure of H2-, Fe-, Co-, and Cu-phthalocyanine molecular thin films. NEXAFS measurements at both the carbon and nitrogen K-edges reveal that phthalocyanine films deposited on sapphire have upright molecular orientations, while films up to 50 nm thick deposited on gold substrates contain prostrate molecules. Although great similarity is observed in the carbon and nitrogen K-edge NEXAFS spectra recorded for the films composed of prostrate molecules, the H2-phthalocyanine exhibits the cleanest angular dependence due to its purely out-of-plane π* resonances at the absorption onset. In contrast, organometallic-phthalocyanine nitrogen K-edges have a small in-plane resonance superimposed on this π* region that is due to a transition into molecular orbitals interacting with the 3dx(2)-y(2) empty state. NEXAFS spectra recorded at the metal L-edges for the prostrate films reveal dramatic variations in the angular dependence of specific resonances for the Cu-phthalocyanines compared with the Fe-, and Co-phthalocyanines. The Cu L3,2 edge exhibits a strong in-plane resonance, attributed to its b1g empty state with dx(2)-y(2) character at the Cu center. Conversely, the Fe- and Co- phthalocyanine L3,2 edges have strong out-of-plane resonances; these are attributed to transitions into not only b1g (dz(2)) but also eg states with dxz and dyz character at the metal center.

We developed a bulk hybrid material consisting of a vanadium sesquioxide (V2O3) matrix with nickel (Ni) rich inclusions that exhibit a switchable two-state magnetic coercivity. The V2O3 matrix magnetoelastically couples with the Ni-rich inclusions and its structural phase transition causes two possible magnetic coercivity states. Differences of up to 13% in the temperature window of 14 K are observed, depending whether the transition occurs from rhombohedral to monoclinic or vice versa. These findings provide a pathway for the development of bulk switchable coercivity materials. We present routes to further enhance the magnetoelastic effect by increasing the oxide/ferromagnetic material coupling.

Raman micro-spectroscopy is well suited for studying a variety of properties and has been applied to wide- ranging areas. Combined with tuneable temperature, Raman spectra can offer even more insights into the properties of materials. However, previous designs of variable temperature Raman microscopes have made it extremely challenging to measure samples with low signal levels due to thermal and positional instability as well as low collection efficiencies. Thus, contemporary Raman microscope has found limited applicability to probing the subtle physics involved in phase transitions and hysteresis. This paper describes a new design of a closed-cycle, Raman microscope with full polarization rotation. High collection efficiency, thermal and mechanical stability are ensured by both deliberate optical, cryogenic, and mechanical design. Measurements on two samples, Bi2Se3 and V2O3, which are known as challenging due to low thermal conductivities, low signal levels and/or hysteretic effects, are measured with previously undemonstrated temperature resolution., 11 pages, 13 figures

We study the effect of photodiode angular response on the measurement of surface plasmon resonance (SPR) in metallic thin films using the Kretschmann-Raether configuration. The photodiode signal depends not only on the light intensity but also on the incidence angle. This implies that the photodiode sensitivity changes along the SPR curve. Consequently, the measured SPR spectrum is distorted, thus affecting fits and numerical analyses of SPR curves. We analyze the magnitude of this change, determine when it is significant, and develop a calibration method of the experimental setup which corrects for this type of spectral shape distortions.

Blood is a limited resource that supplies neurons and glia with nutriments. How is blood distributed both in a fail-safe way and in response to changing metabolic loads? We examine this issue in rodent cortex. First, in terms of topology, all-optical histology is used to reconstruct the vasculature network. Second, in terms of dynamics, the flux of blood cells in individual vessels is measured in response to neuronal activation and/or the perturbation of flow of a targeted vessel via plasma-mediated ablation. Lastly, in terms of control, optical indicators of neurovascular signaling molecules that alter vessel diameter are measured.

We present a Surface Plasmon Resonance spectroscopy study of Co-Phthalocyanine (CoPc) thin films grown on Au layers at different substrate temperatures. We demonstrate that for quantitative analysis, fitting of the resonance angle alone is insufficient and Whole Curve Analysis (WCA) needs to be performed. This is because CoPc thin film dielectric constant and thickness are strongly affected by substrate temperature, even when the total deposited mass remains fixed. Using WCA, we are able to uniquely fit both the dielectric constants and the thicknesses of the films without making a priori assumptions.

We present a software package called Microscopy Image Segmentation Tool (MIST). MIST is designed for analysis of microscopy images which contain large collections of small regions of interest (ROIs). Originally developed for analysis of porous anodic alumina scanning electron images, MIST capabilities have been expanded to allow use in a large variety of problems including analysis of biological tissue, inorganic and organic film grain structure, as well as nano- and meso-scopic structures. MIST provides a robust segmentation algorithm for the ROIs, includes many useful analysis capabilities, and is highly flexible allowing incorporation of specialized user developed analysis. We describe the unique advantages MIST has over existing analysis software. In addition, we present a number of diverse applications to scanning electron microscopy, atomic force microscopy, magnetic force microscopy, scanning tunneling microscopy, and fluorescent confocal laser scanning microscopy.

We studied the temperature dependence of coercivity and magnetization of V2O3/Ni bilayers across the Structural Phase Transition in V2O3. We found a coercivity peak that coincides with the V2O3 phase transition on top of an overall increase of the coercivity with decreasing temperature. We propose that this sharp increase arises from a length scale competition between magnetic domains of Ni and phase coexistence during the V2O3 phase transition. This model is supported by micromagnetic simulations and shows that magnetic properties of ferromagnetic films are strongly affected by a proximal first order phase transition.

We find that the ohmic conductance of Co-phthalocyanine (CoPc) vertical capacitive devices is irreversibly suppressed by orders of magnitude when they are heated above 340 K. Detailed structural and transport studies imply that the changes in the conductance are due to diffusion of the top Pd electrode into the CoPc layer. This leads to a decrease in Pd electrode effective work function, which increases the potential barrier for hole injection.

The transport properties of metallophthalocyanine thin films are important ingredients in many technological applications. Ohmic conductance of thin film (15 nm to 90 nm) Co-phthalocyanine (CoPc) capacitive devices has been investigated in the temperature range of 40 K to 300 K. For Pd and V electrodes, the electrode-film (E-F) interface and metallic micro-shorts contribute substantially to the conductance with decrease in CoPc layer thickness. A quantitative model which describes E-F interface, CoPc roughness, micro-shorts, and the exponential temperature and thickness dependence of conductance was developed. Parameters obtained from this model are in good quantitative agreement with independent measurements. The model predicts a 15-20 nm lower limit for capacitive device thickness, below which the conduction is mainly controlled by shorts. In this regime, small changes in mean CoPc thickness result in drastic variation in device conductance.