CaF2 Substrates Research Articles

Co-Doped BaFe2As2 Superconducting Nanowires for Detector Applications

Nanowires of Co-doped BaFe 2 As 2 have been realized and characterized. Thin films having thickness of 20 nm were grown on CaF 2 (100) substrates by pulsed laser deposition. The films were then passivated in situ with a thin layer of MgAl 2 O 4 . Using electron beam lithography, several submicron nanowire structures were patterned and electrically contacted to gold pads. The nanostructures have a superconducting critical temperature T of 16 K, for a 500-nm-wide nanowire, and properties, in terms of normal state resistivity (0.3 mΩ·cm) and critical current (1 MA/cm 2 at 3 K), comparable with those of NbN nanowires. Moreover, they show localization effects in the normal state, while the current-voltage characteristics are dominated by flux creep.

Raman spectroscopy detects biochemical changes due to different cell culture environments in live cells in vitro.

The in vitro cell culture environment can impact on cell biochemistry and cell cycle. The manifestation of such substrate-induced changes in cell cycle in the Raman microspectroscopic profiles of cell cultures is investigated at the level of nucleolus, nucleus and cytoplasm. HeLa immortalised human cervical cells and HaCaT dermal cells were cultured on three different substrates, conventional polystyrene cell culture dishes, CaF2 slides as a commonly used Raman substrate, and glass slides coated with collagen rat tail, as a mimic of the extra-cellular matrix (ECM) environment. A cell cycle study, based on percentage DNA content, as determined using propidium iodide staining and monitored by flow cytometry, was performed on cells of both types, grown on the different substrates, confirming that the in vitro cell culture environment impacts significantly on the cell cycle. Live cell in vitro Raman spectroscopic analysis of cells on the 2D CaF2 and 3D collagen substrates was performed and data was analysed using principal component analysis (PCA). The spectroscopic analysis revealed differences in profiles which reflect the differences in cell cycle for both in vitro culture environments. In particular, the Raman spectra of cells grown on CaF2 show indicators of cell stress, which are also associated with cell cycle arrest at the G0/G1 phase. This work contributes to the field of Raman spectroscopic analysis by providing a fresh look at the significance of the effect of in vitro culture environment to cell cycle and the sensitivity of Raman spectroscopy to such differences in cell metabolism.

Watt-Level Continuous-Wave and Black Phosphorus Passive Q-Switching Operation of Ho3+,Pr 3+:LiLuF4 Bulk Laser at 2.95 μm

Efficient continuous wave (CW) and passively Q-switched Ho3+,Pr3+:LiLuF 4 (Ho,Pr:LLF) laser operating at 2.95 μ m were realized using a 1150-nm Raman fiber laser as the pump source. A CW output power as high as 1.15 W, which we believe to be the highest one ever achieved from Ho 3+-doped bulk laser emission around 3 μ m, corresponds to an optical-to-optical conversion efficiency of 14.5% and a slope efficiency of 15.5%, respectively. A high-quality saturable absorber (SA) based on multilayered black phosphorus (BP) nanosheet film deposited on a CaF2 substrate was successfully fabricated and employed. Under the absorbed pump power of 7.36 W, the shortest pulse width of 194.3 ns was obtained, which is the shortest among the two-dimensional materials as SA around 3 μ m. The results not only indicated that Ho,Pr:LLF crystal would be a promising mid-infrared (MIR) gain medium for obtaining high power output, but verified that the multilayered BP is a promising optical modulator for generating short pulses in MIR spectral range.

The grating of micro-holes as a chemosensor with rhodamine 6G molecules

Chemo-sensing of the produced 4×4 mm2 gratings of micro-holes (approx. half-million of micro-holes) on the 30-nm thick silver film on the IR-transparent CaF2 substrate was investigated in terms of their resonant EOT-enhanced IR-transmission as surface-enhanced IR-absorption (SEIRA). Selective IR absorption at 1261 cm−1 enhanced by 455 times, was demonstrated for rhodamine 6G molecules. The reference absorption lines were taken near 2900 cm−1, where the IR radiation is freely channeling through the microholes, indicating the reference substrate coverage by the dye molecules for its relative internal calibration.

Time response demonstration of in situ lattice deformation under an applied electric field by synchrotron-based time-resolved X-ray diffraction in polar-axis-oriented epitaxial Pb(Zr,Ti)O3 film

The dependence of applied rectangular pulses with various widths on the crystal structure change was investigated by time-resolved synchrotron-based X-ray diffraction measurement. A (001)-oriented epitaxial Pb(Zr0.5Ti0.5)O3 film of 2.1 µm thickness grown on a (100)cSrRuO3//(100)LaNiO3//(100)CaF2 substrate by metal organic chemical vapor deposition was investigated. The crystal lattice increased almost linearly with increasing applied electric field up to 230 kV/cm in the case of a 0.3-µs-width pulse. This elongation with the application of an electric field was ascertained to be almost independent of the pulse width from 0.3 to 2000 µs at 190 kV/cm. These values were almost consistent with the macroscopic measurements obtained at 5 and 1000 Hz by piezoelectric force macroscopy. The present results show that the time-resolved XRD measurement is very useful for analyzing the frequency dependence of the piezoelectric response in view of the crystal structure change because the crystal structure change under an applied electric field can be systematically investigated by changing the applied pulse width.

Analyzing the effect of graphene's chemical potential on the performance of a plasmonic sensor in infrared

SPR sensor based on ‘glass substrate-Ag film-insulating layer-graphene’ plasmonic structure is proposed in the NIR region. The influence of graphene chemical potential (μ) on the sensor's performance, measured in terms of figure-of-merit (FOM), has been analyzed in detail. The analysis suggests that the variation of chemical potential does not affect much the detection sensitivity and detection limit but it affects the FWHM of SPR curve. The variation of FOM with μ follows a certain pattern. In summary, the maximum and almost constant sensing performance may be achieved for 0.9 < μ < 1 eV for graphene monolayer. Also, CaF2 substrate provides much better FOM compared with 2S2G and Si substrates at any value of μ.

Angular dependence of vortex instability in a layered superconductor: the case study of Fe(Se,Te) material

Anisotropy effects on flux pinning and flux flow are strongly effective in cuprate as well as iron-based superconductors due to their intrinsically layered crystallographic structure. However Fe(Se,Te) thin films grown on CaF2 substrate result less anisotropic with respect to all the other iron based superconductors. We present the first study on the angular dependence of the flux flow instability, which occurs in the flux flow regime as a current driven transition to the normal state at the instability point (I*, V*) in the current-voltage characteristics. The voltage jumps are systematically investigated as a function of the temperature, the external magnetic field, and the angle between the field and the Fe(Se,Te) film. The scaling procedure based on the anisotropic Ginzburg-Landau approach is successfully applied to the observed angular dependence of the critical voltage V*. Anyway, we find out that Fe(Se,Te) represents the case study of a layered material characterized by a weak anisotropy of its static superconducting properties, but with an increased anisotropy in its vortex dynamics due to the predominant perpendicular component of the external applied magnetic field. Indeed, I* shows less sensitivity to angle variations, thus being promising for high field applications.

Tunable critical temperature for superconductivity in FeSe thin films by pulsed laser deposition

Stabilized FeSe thin films in ambient pressure with tunable superconducting critical temperature would be a promising candidate for superconducting electronic devices. By carefully controlling the depositions on twelve kinds of substrates using a pulsed laser deposition technique single crystalline FeSe thin films were fabricated. The high quality of the thin films was confirmed by X-ray diffraction with a full width at half maximum of 0.515° in the rocking curve and clear four-fold symmetry in φ-scan. The films have a maximum Tc ~ 15 K on the CaF2 substrate and were stable in ambient conditions air for more than half a year. Slightly tuning the stoichiometry of the FeSe targets, the superconducting critical temperature becomes adjustable below 15 K with quite narrow transition width less than 2 K. These FeSe thin films deposited on different substrates are optimized respectively. The Tc of these optimized films show a relation with the out-of-plane (c-axis) lattice parameter of the FeSe films.

Toward Bloch Surface Wave-Assisted Spectroscopy in the Mid-Infrared Region

We present the first observation of Bloch surface waves in the mid-infrared spectral range. Bloch surface waves are electromagnetic states that can propagate at the interface between a continuous dielectric medium and a periodic stack of materials. We designed a mid-infrared multilayer optical element specifically for this purpose. The structure is made of successive layers of ZnSe and YbF3 deposited on a CaF2 substrate. Surface waves can be coupled into this structure from 6.6 to 10.6 μm. We characterized the optical properties of the fabricated element using a tunable quantum cascade laser centered around 7.85 μm. We believe that Bloch surface wave based platforms are very promising components for surface sensing and spectroscopic applications in the mid-infrared.

Transport properties of ultrathin BaFe1.84Co0.16As2 superconducting nanowires

Superconducting nanowire single-photon detectors (SNSPDs) have an absolute advantage over other types of single-photon detectors, except for the low operating temperature. Therefore, much effort has been devoted to finding high-temperature superconducting materials that are suitable for preparing SNSPDs. Copper-based and MgB2 ultrathin superconducting nanowires have already been reported. However, the transport properties of iron-based ultrathin superconducting nanowires have not been studied. In this work, a 10 nm thick × 200 nm wide × 30 μm long high-quality superconducting nanowire was fabricated from ultrathin BaFe1.84Co0.16As2 films by a lift-off process. The precursor BaFe1.84Co0.16As2 film with a thickness of 10 nm and root-mean-square roughness of 1 nm was grown on CaF2 substrates by pulsed laser deposition. The nanowire shows a high superconducting critical temperature = 20 K with a narrow transition width of ΔT = 2.5 K and exhibits a high critical current density Jc of 1.8 × 107 A cm−2 at 10 K. These results of ultrathin BaFe1.84Co0.16As2 nanowire will attract interest in electronic applications, including SNSPDs.

Milligram-per-second femtosecond laser production of Se nanoparticle inks and ink-jet printing of nanophotonic 2D-patterns

Milligram-per-second production of selenium nanoparticles in water sols was realized through 7-W, 2 MHz-rate femtosecond laser ablation of a crystalline trigonal selenium pellet. High-yield particle formation mechanism and ultimate mass-removal yield were elucidated by optical profilometry and scanning electron microscopy characterization of the corresponding crater depths and topographies. Deposited selenium particles were inspected by scanning and transmission electron microscopy, while their hydrosols (nanoinks) were characterized by optical transmission, Raman and dynamic light scattering spectroscopy. 2D patterns and coatings were ink-jet printed on thin supported silver films and their bare silica glass substrates, as well as on IR-transparent CaF2 substrates, and characterized by electron microscopy, energy-dispersive x-ray spectroscopy, and broadband (vis-mid IR) transmission spectroscopy, exhibiting crystalline selenium nanoparticles with high refractive index as promising all-dielectric sensing building nanoblocks in nanophotonics.

Deposition and properties of Fe(Se,Te) thin films on vicinal CaF2 substrates

We report on the growth of epitaxial Fe1+δSe0.5Te0.5 thin films on 0°, 5°, 10°, 15° and 20° vicinal cut CaF2 single crystals by pulsed laser deposition. In situ electron and ex situ x-ray diffraction studies reveal a tilted growth of the Fe1+δSe0.5Te0.5 films, whereby under optimized deposition conditions the c-axis alignment coincides with the substrate [001] tilted axis up to a vicinal angle of 10°. Atomic force microscopy shows a flat island growth for all films. From resistivity measurements in longitudinal and transversal directions, the ab- and c-axis components of resistivity are derived and the mass anisotropy parameter is determined. Analysis of the critical current density indicates that no effective c-axis correlated defects are generated by vicinal growth, and pinning by normal point core defects dominates. However, for H∣∣ab the effective pinning centers change from surface defects to point core defects near the superconducting transition due to the vicinal cut. Furthermore, we show in angular-dependent critical current density data a shift of the ab-planes maxima position with the magnetic field strength.

Molecular structure of human aortic valve by µSR- FTIR microscopy

Abstract Aortic valve is a part of the heart most frequently affected by pathological processes in humans what constitute a very serious health problem. Therefore, studies of morphology and molecular microstructure of the AV are needed. µSR- FTIR spectroscopy and microscopy represent unique tools to study chemical composition of the tissue and to identify spectroscopic markers characteristic for structural and functional features. Normal AV reveals a multi-layered structure and the compositional and structural changes within particular layers may trigger degenerative processes within the valve. Thus, deep insight into the structure of the valve to understand pathological processes occurring in AV is needed. In order to identify differences between three layers of human AV, tissue sections of macroscopically normal AV were studied using µSR- FTIR spectroscopy in combination with histological and histochemical stainings. Tissue sections deposited onto CaF2 substrates were mapped and representative set of IR spectra collected from fibrosa, spongiosa and ventricularis were analysed by Principal Component Analysis (PCA) in the spectral range between 1850–1000 cm−1 and 3050–2750 cm−1. PCA revealed a layered molecular structure of the valve and it was possible to identify IR bands associated to different tissue parts. Spongiosa layer was well differentiated from other two layers mainly based on IR bands characteristic for the distribution of glycosaminoglycans (GAGs) in the tissue – like 1170 cm−1 (υas(C-O-S)) and 1380 cm−1 (acetyl amino group). Additionally, it was distinguished from fibrosa and ventricularis based on 1085 cm−1 and 1240 cm−1 bands characteristic for GAGs and for carbohydrates- ν(C-O) and ν(C-O-C) respectively and nucleic acids -νsym(PO2−) and νasym(PO2−) respectively, which were less specific for this layer. The use of µSR- FTIR spectroscopy demonstrated co-localization of GAGs and lipids in spongiosa layer what may indicate their contribution in the very early phase of aortic valve calcific degeneration.

High critical current density over 1 MA cm−2 at 13 T in BaZrO3 incorporated Ba(Fe,Co)2As2 thin film

Achieving high critical current density (Jc) under a high magnetic field in Fe based superconductors is indispensable for practical applications. Here we report that high critical current density over 1 MA cm−2 in high field (13 T) can be achieved in BaZrO3 (BZO) incorporated Ba(Fe,Co)2As2 (Co-Ba122) thin film grown on CaF2 substrate using pulsed laser deposition. The magnetization Jc of Co-Ba122 thin film incorporated with 2 mol.% BZO reaches 1.3 MA cm−2 (Hǁc, 13 T and 4.2 K), 14 times higher than that of pure Co-Ba122 thin film. Transmission electron microscopy observation revealed that BZO forms nanorods of ∼4 nm in average diameter with mean separation of 10–11 nm which corresponds to a matching field of about 17–20 T. Incorporating BZO in Co-Ba122 thin films led to a strong vortex pinning effect and a significant enhancement of Jc because a high density of BZO nanorods could form in the superconducting matrix without significant degradation of Tc, and the diameter of BZO nanorods is approximately twice the coherence length of Co-Ba122 (ξab(0) ∼ 2.5 nm) which is the optimal size as pinning centers.

Current Induced Resistive State in Fe(Se,Te) Superconducting Nanostrips

We study the current-voltage characteristics of Fe(Se,Te) thin films deposited on CaF2 substrates in form of nanostrips (width w ~ λ, λ the London penetration length). In view of a possible application of these materials to superconductive electronics and micro-electronics we focus on transport properties in small magnetic field, the one generated by the bias current. From the characteristics taken at different temperatures we derive estimates for the pinning potential U and the pinning potential range δ for the magnetic flux lines (vortices). Since the sample lines are very narrow, the classical creep flow model provides a sufficiently accurate interpretation of the data only when the attractive interaction between magnetic flux lines of opposite sign is taken into account. The observed voltages and the induced depression of the critical current of the nanostrips are compatible with the presence of a low number (lesssim 10) magnetic field lines at the equilibrium, a strongly inhomogeneous current density distribution at the two ends of the strips and a reduced Bean Livingston barrier. In particular, we argue that the sharp corners defining the bridge geometry represent points of easy magnetic flux lines injection. The results are relevant for creep flow analysis in superconducting Fe(Se,Te) nanostrips.

Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method**Project supported by the National Natural Science Foundation of China (Grant No. 11574360), the National Basic Research Program of China (Grant Nos. 2015CB921300, 2013CB921700, and 2013CB921904), the National Key Research and Development Program of China (Grant No. 2016YFA0300300), and the Strategic Priority Research Program

We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF2 substrate by pulsed laser deposition (PLD) method. Measurements on FeSe/CaF2 samples with different superconducting transition temperatures of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140–160 K, much higher than K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF2 films by PLD method with the FeSe/SrTiO3 films by molecular beam epitaxy (MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF2 is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF2 films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO3 films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.

Optimization of Deposition Conditions to Grow High-Quality Fe–Se–Te Thin Films

High-performance Fe–Se–Te (FST) films are favorable for revealing the relationships between the crystal structure and the superconducting properties of the material because of their simple structures. Therefore, it is particularly important to prepare high-quality “11” thin films. Herein, we focused on the thickness and deposition temperature dependence of $T^{\rm {zero}}_{\rm{c}}$ and found that the thickness of about 200 nm and the deposition temperature of 300 °C are the optimal parameters for growing high-quality FST films on CaF2 and SrTiO3 substrates, which exhibit $T^{\rm {zero}}_{\rm{c}}$ of 18.8 K and 17 K, respectively. Magnetic Jc values of FST films grown on CaF2 and SrTiO 3 are both larger than 2 × 106 A·cm–2 at 4.2 K and self-field, significantly higher than that of FST single crystals.

The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.

Large-Scale Suspended Graphene Used as a Transparent Substrate for Infrared Spectroscopy.

Due to weak interactions between micrometer-wavelength infrared (IR) light and nanosized samples, a high signal to noise ratio is a prerequisite in order to precisely characterize nanosized samples using IR spectroscopy. Traditional micrometer-thick window substrates, however, have considerable IR absorption which may introduce unavoidable deformations and interruptions to IR spectra of nanoscale samples. A promising alternative is the use of a suspended graphene substrate which has ultrahigh IR transmittance (>97.5%) as well as unique mechanical properties. Here, an effective method is presented for fabrication of suspended graphene over circular holes up to 150 µm in diameter to be utilized as a transparent substrate for IR spectroscopy. It is demonstrated that the suspended graphene has little impact on the measured IR spectra, an advantage which has led to the discovery of several missing vibrational modes of a 20 nm thick PEO film measured on a traditional CaF2 substrate. This can provide a better understanding of molecules' fine structures and status of hanging bands. The unique optical properties of suspended graphene are determined to be superior to those of conventional IR window materials, giving this new substrate great potential as part of a new generation of IR transparent substrates, especially for use in examining nanoscale samples.

Preparation of preferentially (111)-oriented Mg2Si thin films on (001)Al2O3 and (100)CaF2 substrates and their thermoelectric properties

Mg2Si thin films were deposited at 320 °C on (001)Al2O3 and (100)CaF2 substrates by radio-frequency magnetron sputtering. Both films showed a preferential (111) out-of-plane orientation with an in-plane random orientation irrespective of post-heat treatment. Mg2Si films on (001)Al2O3 substrates were under in-plane tensile strain, while those on (100)CaF2 substrates were under in-plane compressive strain both before and after heat treatment. Heat-treated films showed p-type conduction up to 500 °C. Their electrical conductivity and Seebeck coefficient were almost independent of the kind of substrate within the limit of the present study, from 0.22% compressive strain to 0.34% tensile strain at room temperature.