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We use synchrotron X-ray reflectivity and grazing incidence small-angle X-ray scattering to investigate the surface assembly of the poly(ethylene glycol) (PEG)-grafted gold nanoparticles (PEG-AuNPs) induced by different salts. We find that NaCl and CsCl behave as many other electrolytes, namely, drive the PEG-AuNPs to the vapor/suspension interface to form a layer of single-particle depth and organize them into very high-quality two-dimensional (2D) hexagonal crystals. By contrast, NaI induces the migration of PEG-AuNPs to the aqueous surface at much higher surface densities than the other salts (at similar concentrations). The resulting 2D ordering at moderate NaI concentrations is very short ranged, and at a higher NaI concentration, the high-density monolayer is amorphous. Considering NaCl, CsCl and the majority of salts behave similarly, this implicates the anomaly of iodine ion (I-) in this unusual surface population. We argue that the influence of most electrolytes on the PEG corona preserves the polymer in the θ-point with sufficient flexibility to settle into a highly ordered state, whereas I- has a much more severe effect on the corona by collapsing it. The collapsed PEG renders the grafted AuNP a nonspherical shaped complex that, although packs at high density, cannot organize into a 2D ordered arrangement.

We report on the assembly of gold nanoparticle (AuNPs) superlattices at the liquid/vapor interface and in the bulk of their suspensions. Interparticle distances in the assemblies are achieved on multiple length scales by varying chain lengths of surface grafted AuNPs by polyethylene glycol (PEG) with molecular weights in the range 2000-40,000 Da. Crystal structures and lattice constants in both 2D and 3D assemblies are determined by synchrotron-based surface-sensitive and small-angle X-ray scattering. Assuming knowledge of grafting density, we show that experimentally determined interparticle distances are adequately modeled by spherical brushes close to the θ point (Flory-Huggins parameter, χ≈12) for 2D superlattices at a liquid interface and a nonsolvent (χ = ∞) for the 3D dry superlattices.

Highly surface-roughened zirconia substrates were obtained from additive zirconia powder coating by room temperature spray processing. Homogeneous and dense zirconia coatings were deposited on sintered zirconia substrates with strong bonding by a powder spray coating method. The thickness and surface roughness of the coating layers on zirconia substrates increased with increasing coating cycles, which was confirmed from atomic force microscopy (AFM) and roughness analyses. The tetragonal phase and chemical composition of the zirconia coating layers were similar to those of the raw 3Y-TZP powder used as a raw material, indicating that no phase or composition changes occurred during the spray process.

We report the development of a liquid crystal (LC)-based aptasensor for the detection of interferon-γ (IFN-γ) in human blood as well as the diagnosis of tuberculosis (TB). In this system, the binding of IFN-γ to an aptamer immobilized on a surface disrupts the orientation of LCs, inducing a transition from a homeotropic orientation to a random one. This change in the orientation of the LCs can easily be converted and observed as a shift from a dark optical LC image to a bright one under a polarized light microscope. Through this sensing mechanism, IFN-γ levels as low as 1 pM (17 pg/ml) could be detected. This LC-based aptasensor was employed for the diagnosis of TB using blood samples from patients with latent TB. With this LC-based approach, not only could IFN-γ readily be detected, but also latent TB could be diagnosed using human blood simply and effectively without any intricate processes or instrumentation. Therefore, our present research provides a promising IFN-γ sensor with applications in clinical diagnosis of various infectious diseases, and in immunological research.

To obtain Si anodes with high-performance Li-ion batteries, a nanostructured Si-based composite is synthesized via a facile one-pot solid-state reaction. The Si-based anode composed of Si, Cu, and polyvinylidene fluoride (PVDF) binder is heated at 500 °C under continuous argon flow to obtain a Si/Cu/Cu3Si/C (SCCC) anode. The Cu/Cu3Si phases and carbonized PVDF provide conductive pathways between active Si particles, improving electrical conductivity. In addition, Cu3Si phases form between Si and Cu, anchor metallic Cu in Si particles and help to maintain electronic conductivity and alleviate mechanical stress during cycling. As a result, the SCCC anode shows significantly enhanced electrochemical properties. It delivers 1,773 mAh g−1 after 300 cycles at a current density of 2 A g-1, whereas the Si anode exhibits close to zero capacity. The improvements in electrochemical performance are ascribed to reduced internal resistance and mechanical stress resulting from the Cu/Cu3Si phases and carbonized PVDF binder.

This study demonstrates a method for improving supercapacitive performance of two-dimensional nanosheet-based composite electrode. As a hybridized electrostatic double layer capacitor–electrochemical pseudocapacitor (EDLC–PC) electrode, we synthesized reduced graphene oxide–polyaniline nanofibers (rGO–PANi NFs) composite electrode. For the enhanced supercapacitive performances, insulator silver chloride nanoparticles (AgCl NPs) were intercalated into the interlayer gap of rGO. The AgCl NP intercalation (i) exfoliated rGO layers and (ii) prevented rGO-self-agglomeration that makes it difficult to utilize the high surface-to-volume ratio of ideal mono- (or few-) atomic-thick rGO layers. As a result, (iii) the specific capacitance was improved in accordance with the enlarged specific surface area of rGO. Furthermore, (iv) the well-developed rGO edges, which were opened by the AgCl intercalation, enabled formation of more bonds between PANi and rGO by selective grafting of PANi to the rGO edges. Hence, the bonds of PANi–rGO, as conducting paths, substantially reduced the total electrical resistance. Enhanced specific capacitance, ion diffusion efficiency, and reduced electrical resistance indicated the bi-functional roles of AgCl NP insertion for high performance hybridized EDLC–PC electrodes.

SeS2 is a promising cathode material that has both advantages of S and Se. In a carbonate-based electrolyte, Li-SeS2 batteries show good cycle performance but work at low discharge voltage due to slow Li-ion diffusion. Here, we report a simple solvent treatment method for synthesizing cathode composite materials to overcome the disadvantage of carbonate-based electrolytes. The solvent washing treatment can effectively remove excess SeS2 that blocks carbon pores, which act as an electrolyte pathway. Additionally, the SeS2/OMC composite (w-SeS2/OMC) maintains the SeS2 configuration, thus leading to both the advantages of S and Se. As a result, enhanced Li-ion diffusion helps to reduce polarization and elevate the discharge voltage to ~2 V. In addition, it shows excellent capacity retention (360 mAh g−1) for 2,000 cycles at a 1C rate.

In this study, we investigate the effect of diphenyl diselenide (DPDS) as a bifunctional additive on LiCoO2/graphite batteries charged to 4.4 V. In the 3.0–4.4 V potential window, a LiCoO2/graphite full cell suffers from poor cycle performance, with a capacity retention of 88.7% after 200 cycles. With the addition of 0.1 wt% DPDS, the capacity retention is increased to 95.2% after 200 cycles. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments indicate that DPDS is oxidized and reduced prior to the decomposition of the electrolyte. Density functional theory (DFT) calculations theoretically show that DPDS has higher highest occupied molecular orbital (HOMO) and lower lowest unoccupied molecular orbital (LUMO) energy levels than the electrolyte. Field-emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses demonstrate that DPDS is decomposed at the LiCoO2 and graphite surface and modifies the properties of the SEI layer. As a result, the improved battery performance enabled by diphenyl diselenide can be attributed to the SEI layers preventing collapse of the LiCoO2 crystal on the cathode and decreasing the reactions of graphite with the electrolyte on the anode.

Cs2CuCl4 is known to possess a quantum spin liquid phase with antiferromagnetic interaction below 2.8 K. We report the observation of a new metastable magnetic phase of the triangular frustrated quantum spin system Cs2CuCl4 induced by the application of hydrostatic pressure. We measured the magnetic properties of Cs2CuCl4 following the application and release of pressure after 3 days. We observed a previously unknown ordered magnetic phase with a transition temperature of 9 K. Furthermore, the recovered sample with new magnetic ground state possesses an equivalent crystal structure to the uncompressed one with antiferromagnetic quantum spin liquid phase., Comment: 9 pages, 5 figures 1 table

This paper investigates the thermal distribution of an LED headlight for vehicles based on the thermal conductivity of thermally conductive plastics (TCP). In general, heat dissipation structures used for LED headlights are made from metallic materials. However, headlight structures made from TCP have not been investigated. The headlights made from TCP having a various thermal conductivity were fabricated by injection molding with and without a metal plate insert. The temperature characteristics were compared and analyzed using thermal simulations and measurement. The inserted metal in TCP greatly reduced the temperature at solder point, indicating that the fast heat dissipation from the high power LED package to TCP though the inserted metal is essential. The measured temperature at solder points decreased as the thermal conductivity of TCP increased, which is well matched to the simulation results. The measured temperature at the solder point was lower than 150 °C when the thermal conductivity of the TCP was 10 W/mK.

The junction temperature of high-power LED lighting was reduced effectively using a lens plate made from a thermally-conductive plastics (TCP). TCP has an excellent thermal conductivity, approximately 5 times that of polymethylmethacrylate (PMMA). Two sets of high-power LED lighting were designed using a multi array LED package with a lens plate for thermal simulation. The difference between two models was the materials of the lens plate. The lens plates of first and second models were fabricated by PMMA (PMMA lighting) and TCP (TCP lighting), respectively. At the lens plate, the simulated temperature of the TCP lighting was higher than that of the PMMA lighting. Near the LED package, the temperature of the TCP lighting was 2 °C lower than that of the PMMA lighting. This was well matched with the measured temperature of the fabricated lighting with TCP and PMMA.

Growth of high-quality a -plane (11–20) GaN using a double-lens structure (DLS) on r -plane sapphire is reported. The DLS consisted of a hemispherically patterned SiO 2 mask formed on an a -GaN template grown on a hemispherically patterned r -plane sapphire substrate. Our study suggests that the DLS was very effective in improving the crystal quality and optical properties in a -plane GaN growth on r -plane sapphire substrate owing to decreased defects and enhanced light extraction.

We measured the magneto-optical images (MOIs) and the magnetic hysteresis (M – H) curves of c-axis-oriented MgB 2 thin films to investigate the flux penetration in the form of dendritic avalanches. In order to understand the role of the thermal effects, we prepared Au -coated MgB 2 thin films with different thicknesses of gold. While the MOI provides a spatially resolved flux pattern, the M – H curve presents global and average information about the flux noise associated with avalanches. These two types of measurements complement each other. The upper threshold field, above which the flux noise disappears, was determined from the M-H curves while the lower threshold field was determined from both the M – H curves and the MO images. The field range where the flux penetrates via avalanches is found to be smaller for thicker gold layers. These results are important for many superconducting applications.

Newly discovered Bi-O-S compounds remain an enigma in attempts to understand their electronic properties. A recent study of Bi$_{4}$O$_{4}$S$_{3}$ has shown it to be a mixture of two phases, Bi$_{2}$OS$_{2}$ and Bi$_{3}$O$_{2}$S$_{3}$, the latter being superconducting [W. A. Phelan et al., J. Am. Chem. Soc. 135, 5372 (2013)]. Using density functional theory, we explore the electronic structure of both the phases and the effect of the introduction of stacking faults. Our results demonstrate that the S$_{2}$ layers dope the bismuth-sulphur bands. The bands at the Fermi level are of clear two-dimensional character. One band manifold is confined to the two adjacent, square-lattice bismuth-sulphur planes, a second manifold is confined to the square lattice of sulphur dimers. We show that the introduction of defects in the stacking does not influence the electronic structure. Finally, we also show that spin-orbit coupling does not have any significant effect on the states close to the Fermi level at the energy scale considered.

The Hall resistivity ( ρ xy ) and the longitudinal resistivity ( ρ xx ) in c -axis-oriented superconducting MgB 2 thin films have been investigated in extended fields up to 18 T. We have observed a scaling behavior between the Hall resistivity and the longitudinal resistivity, ρ xy = A ρ xx β , where the exponent ( β ) is observed to be independent of the temperatures and the magnetic fields. For a wide magnetic field region from 1 to 18 T and a wide temperature region from 10 to 28 K, a universal power law with β = 2.0 ± 0.1 was observed in c -axis-oriented MgB 2 thin films. These results can be well interpreted by using recent models.

We investigated the mixed-state magnetoresistance of high quality c -axis-oriented MgB 2 thin films while varying the magnetic field and the current density. The temperature dependence of the magnetoresistance was well described by a vortex-glass theory for the low dissipative part of the resistivity and by a thermal fluctuation theory near the transition temperature. Interestingly, the observed thermally activated flux-flow region was very narrow or did not exist whereas, compared to conventional superconductors, the vortex-liquid phase was relatively broad in the H – T phase diagram. From the Werthamer–Helfand–Hohenberg theory, we obtained H c2 (0)=25.4 T, which was consistent with the value measured using a direct method.

We have built a high-resolution soft X-ray spectrometer using a 3 /spl times/ 3 array of superconducting Nb-Al-AlO/sub x/-Al-Nb tunnel junctions. The detector is cooled to /spl sim/0.1 K by a two-stage adiabatic demagnetization refrigerator while being held at the end of a 40-cm-long detector snout that can be inserted into an experimental chamber. The 0.6 mm /spl times/ 0.6 mm array has an energy resolution of 10-15 eV FWHM for X-ray energies below 1 keV and total count rate capabilities above 100 000 counts/s. We demonstrate the capabilities of the instrument in a study of anisotropy in c-axis oriented MgB/sub 2/ films by near edge X-ray absorption spectroscopy. The absorption fine structure at the B K-edge differs for excitation with X-rays polarized along the c-axis and in the ab-plane of the MgB/sub 2/ film. We discuss the implications of the anisotropy for the density of states in MgB/sub 2/ at the Fermi surface.

We present magneto optical (MO) studies of the critical states in a c-axis oriented MgB/sub 2/ thin film with a critical current density (J/sub c/) of 6 MA/cm/sup 2/ at 5 K, and bulk MgB/sub 2//Mg nano-composites with J/sub c/ over 1 MA/cm/sup 2/ at 5 K. The magnetic flux penetration and trapping in the thin film are found remarkably different from that in the bulk samples. A tree-like magnetic flux pattern was observed entering the superconducting film, as well as various complex dendritic flux jumps due to the thermo-magnetic instability. These behaviors indicate the collapse of the critical state of a type II superconductor at local level. In contrast, a regular flux penetration into the critical state was observed in the bulk nano-composites, which indicates that the addition of pure Mg helps to stabilize the critical state.

From the magnetic hysteresis loops (M-H) and the current-voltage (I-V) relations, we obtained the critical current density (Jc) for c-axis-oriented MgB2 thin films with a transition temperature of 39 K. The temperature dependence of Jc obtained from the M-H and the I-V curves coincide, which indicates the strongly linked nature of the intergrains in these thin films. And, the high value of Jc was estimated to be ∼4×107 A/cm2 at 5 K and ∼1×105 A/cm2 at 37 K under a self field, indicating a promising candidate for technological applications.

We have measured the Hall resistivity, ρ xy , and the longitudinal resistivity, ρ xx , in superconducting MgB2 thin films in a mixed-state while changing the magnetic field and the current density. A Hall scaling behavior without the anomalous Hall effect was observed with a exponent β of 2.0±0.1 in ρ xy =Aρβ xx . This exponent is observed to be constant, i.e., independent of magnetic field, temperature, and current density.

We report the growth of high quality c -axis-oriented MgB 2 thin films on Al 2 O 3 substrates by using a pulsed laser deposition technique. The thin films show an onset transition temperature of 39.2 K with a sharp transition width of ∼0.15 K. X-ray diffraction patterns indicate a c -axis-oriented crystal structure perpendicular to the substrate surface. We observed high critical current densities ( J c ) of ∼16 MA/cm 2 at 15 K and under self-field, which is comparable to or exceeds those of cuprate high-temperature superconductors. The extrapolation J c at 5 K was observed to be ∼40 MA/cm 2 , which is the highest record for MgB 2 compounds. At a magnetic field of 5 T, the J c of ∼0.1 MA/cm 2 was detected at 15 K, suggesting that this compound is very promising candidate for the practical applications at high temperature with lower power consumption. As a possible explanation for the high current-carrying capability, the vortex-glass transition is considered.

Using a dielectric resonator technique the magnetic field penetration depth λ and the energy gap Δ of two MgB2 thin films were determined by measuring changes of resonance frequency with temperature. The exponential temperature dependence of λ is revealed at low temperatures (T Tc/2) the temperature dependence of λ can be well fitted by λ0/[1−(T/Tc)2]1/2. Our results strongly support the absence of nodes in the gap and a possible multigap scenario.

We used dc magnetization and transport measurement to estimate the superconducting critical current densities ( J c ) of polycrystalline MgB 2 sintered under high temperature and high pressure. We measured the current–voltage ( I–V ) characteristics and found the existence of a vortex-glass phase in the field–temperature ( H–T ) plane. This is notable in that the vortex-glass phase can be observed even in a polycrystalline specimen, which suggests that the supercurrent is not sensitive to the grain boundaries. Moreover, the transport (intergrain) J c seems to be comparable to a magnetic (intragrain) J c .

We have analyzed about 100 voltage-dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis oriented MgB$_2$ thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double-gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with a single-gap features result in more broad maximum at 3.5 meV. The double-gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps $\Delta_{S}=2.45\pm 0.15$ meV and $\Delta_{L}=7.0\pm 0.45$ meV in MgB$_2$ with gap ratio $2.85\pm 0.15$. Our observations prove further a widely discussed multi-gap scenario for MgB$_2$, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one., Comment: V1:4 pages incl. 4 figures, RevTex4. V2: version for JETP Letters, 4 pages incl 3 Figs., jetpl.cls

Under the influence of a magnetic field applied perpendicularly to its plane, a superconducting thin film develops dendritic patterns of penetrated regions that coexist with Meissner areas. For a thin film of MgB2 submitted to an alternate field of moderate amplitude, the AC susceptibility measured while cooling the sample exhibits a quite unusual behavior, reentering and fluctuating with temperature. The effect is more pronounced at frequencies around 1 kHz. Two plausible explanations for the effect are discussed.

Organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 (BEDT-TTF denotes bis(ethylenedithia)tetrathiafulvalene) with Tc≃9.5 K has a highly anisotropic nature similar to the high-Tc superconductor. Temperature dependence of magnetization was measured in the field range of 1 kOe≤H≤5 kOe perpendicular to the conducting layers. Near Tc, the reversible magnetization deviates from the mean-field behavior due to the positional distortion of vortices, which is reflected by the crossover of the magnetization curves at T≃8.75 K. Far below Tc where the fluctuation is diminished, the magnetization was analyzed using Hao and Clem's model, and the thermodynamic parameters, such as the Ginzburg–Landau parameter κ, the critical fields [Hc(0) and Hc2(0)] and the characteristic lengths [ξ(0) and λ(0)], were obtained.

We report the measurement of magnetoresistance in a c -axis-oriented MgB 2 thin film while varying the magnetic field and temperature. The scaling functions for the critical fluctuations were employed to analyze the excess conductance of these thin films. The fluctuating magnetoresistance, R ( T , H ), followed the 3D scaling suggested by Ullah and Dorsey.

We measured the superconducting critical current densities (Jc) from the magnetization hysteresis (M–H) loop for Au-coated MgB2 thin films. The purpose of this experiment was to determine whether the vortex avalanche phenomenon which suppresses the Jc for low temperature (T

Magnetic fields applied perpendicularly to superconducting thin films may produce dendritic patterns, where penetrated and Meissner regions coexist, as observed in Nb, YBCO and MgB/sub 2/. Due to large demagnetizing factors in the perpendicular geometry, small fields may be enough to drive portions of the sample into the mixed state. Lack of symmetry and local defects might then permeate the dendritic mode. This mode, in turn, can be detected by AC-susceptibility measurements, since penetrated fingers act as intergranular material, which reveals itself in a characteristic manner. Films of a few hundred nanometers thick and millimeter lateral sizes, develop dendrites even when submitted to Earth's field, what drastically limits the critical current, J/sub c/. This contribution studies how this field-induced granularity is further affected by the action of excitation AC-fields of small magnitudes and audio-frequencies.

We fabricated high-quality c axis-oriented epitaxial MgB2 thin films using a pulsed laser deposition technique. The thin films grown on (1 i 0 2) Al2O3 substrates have a transition temperature of 39 kelvin. The critical current density in zero field is approximately 6 x 10(6) amperes per cubic centimeter at 5 kelvin and approximately 3 x 10(5) amperes per cubic centimeter at 35 kelvin, which suggests that this compound has potential for electronic device applications, such as microwave devices and superconducting quantum interference devices. For the films deposited on Al2O3, x-ray diffraction patterns indicate a highly c axis-oriented crystal structure perpendicular to the substrate surface.

The scattering process responsible for connecting the bands remains one of the last open questions on the physical properties of MgB2. Through the analysis of the equilibrium and photo-induced far-infrared properties as well as electron spin resonance of MgB2 we propose a phonon mediated energy transfer process between the bands based on the coupling of quasiparticles to an E2g phonon.

The microwave surface impedance Z(s) = R(s) + jomegamu(0)lambda was measured with dielectric resonator techniques for two c-axis-oriented MgB(2) thin films. The temperature dependence of the penetration depth, measured with a sapphire resonator at 17.93 GHz can be well fitted from 5 K close to T(c) by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kT(c) to be as low as 1.13 and 1.03 for the two samples. From these fits the penetration depth at zero temperatures was determined to be 102 nm and 107 nm, respectively. The results clearly indicate the s-wave nature of the order parameter. The temperature dependence of surface resistance R(s) measured with a rutile dielectric resonator, shows an exponential behaviour below about T(c)/2 with a reduced energy gap being consistent with the one determined from the, data. The R(s) value at 4.2 K was found to be as low as 19 muOmega at 7.2 GHz, which is comparable with that of a high-quality high-temperature thin film of YBa(2)Cu(3)O(7). A higher-order mode at 17.9 GHz was employed to determine the frequency f dependence of R(s) alpha f(n(T)). Our results revealed a decrease of n with increasing temperature ranging from n = 2 below 8 K to n = 1 from 13 to 34 K.

The microwave surface impedance Z/sub s/=R/sub s/+j/spl omega//spl mu//sub 0//spl lambda/ of MgB/sub 2/ thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth /spl lambda/ measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to T/sub c/ by the standard BCS integral expression assuming the reduced energy gap /spl Delta/(0)/kT/sub c/ to be as low as 1.0-1.1 assuming /spl lambda/(0)=100--110 nm. These results clearly indicate the s-wave nature of the order parameter. Similar good fits were achieved by an anisotropic one gap and an isotropic two-gap model. Second, the temperature dependence of surface resistance R/sub s/, as measured with a rutile puck, indicates an exponential behavior below about T/sub c//2 with a reduced energy gap being consistent with the one determined from the /spl lambda/ data. The R/sub s/ value at 4.2 K was found to be as low as 19 /spl mu//spl Omega/ at 7.2 GHz, which is comparable with that of a high-quality high temperature thin films of YBa/sub 2/Cu/sub 3/O/sub 7/. A higher-order mode at 17.9 GHz was employed to investigate the frequency f dependence of R/sub s//spl alpha/f/sup n(T)/. Our results revealed an decrease of n with increasing temperature ranging from n=2 below 8 K to n=1 close to T/sub c/. Finally, the microwave power handling of MgB/sub 2/ films was deduced and compared with values for YBa/sub 2/Cu/sub 3/O/sub 7/ films. We found that the power handling of MgB/sub 2/ is comparable or even better than that of YBa/sub 2/Cu/sub 3/O/sub 7/ films for temperature below 30 K.

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at Delta(1)=2.3 meV and Delta(2)=7.1 meV at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)]] with both gaps vanishing at the bulk T(c). The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)]].

We have measured the temperature dependence of the microwave surface impedance Z(s)=R(s)+iomegamu(0)lambda of two c-axis oriented MgB(2) films employing dielectric resonator techniques. The temperature dependence of the magnetic-field penetration depth lambda determined by a sapphire dielectric resonator at 17.9 GHz can be well fitted from 5 K close to T(c) by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kT(c) to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 102 and 107 nm were determined from the fit. Our results clearly indicate the s-wave character of the order parameter. A similar fit of the penetration depth data was obtained with an anisotropic s-wave BCS model. Within this model we had to assume a prolate order parameter, having a large gap value in the c-axis direction and a small gap within the ab plane. This is in contrast to recent fits of the anisotropic s-wave model to upper critical-field data, where an oblate order parameter had to be used, and raises interesting questions about the nature of the superconducting state in MgB(2). A rutile dielectric resonator was employed to obtain the temperature dependence of R(s) with high accuracy. Below about T(c)/2, R(s)(T)-R(s)(5 K) exhibits an exponential temperature dependence with a reduced energy gap consistent with that determined from the penetration depth data. The R(s) value at 4.2 K was found to be as low as 19 muOmega at 7.2 GHz, which is comparable with a high-temperature superconducting copper oxide thin film.

The critical current density (J c ) has been measured in c-axis oriented MgB 2 thin films as a function of applied magnetic field (H) and angle (θ) between H and the ab plane. The measurements have been carried out at various temperatures between 32 and 38 K. The field and angular dependence of J c indicates pinning by point pinning centers and is in a good agreement with the predictions of anisotropic Ginzburg-Landau model. Anisotropy parameter (γ) of 2.55 was determined from scaling behavior of J c .

The mixed-state Hall resistivity ${\ensuremath{\rho}}_{\mathrm{xy}}$ and the longitudinal resistivity ${\ensuremath{\rho}}_{\mathrm{xx}}$ in superconducting ${\mathrm{MgB}}_{2}$ thin films have been investigated as a function of the magnetic field over a wide range of current densities from ${10}^{2}$ to ${10}^{4} {\mathrm{A}/\mathrm{c}\mathrm{m}}^{2}.$ We observe a universal Hall scaling behavior with a constant exponent $\ensuremath{\beta}$ of $2.0\ifmmode\pm\else\textpm\fi{}0.1$ in ${\ensuremath{\rho}}_{\mathrm{xy}}=A{\ensuremath{\rho}}_{\mathrm{xx}}^{\ensuremath{\beta}},$ which is independent of the magnetic field, the temperature, and the current density. This result can be interpreted well within the context of recent theories.

We have measured the longitudinal resistivity and the Hall resistivity in the ah plane of highly c-axis-oriented MgB 2 thin films. In the normal state, the Hall coefficient (R H ) behaves as R H ∼T with increasing temperature (T) up to 130 K and then deviates from that linear T dependence at higher temperatures. The T 2 dependence of the cotangent of the Hall angle is only observed above 130 K. The mixed-state Hall effect reveals no sign anomaly over a wide range of current densities from 10 2 to 10 4 A/cm 2 and for magnetic fields up to 5 T.

We have reported far-infrared transmission measurements on a c-axis-oriented superconducting ${\mathrm{MgB}}_{2}$ thin film in the frequency range $30--250 {\mathrm{cm}}^{\ensuremath{-}1}.$ We found that these measurements were sensitive to the scattering rate $1/\ensuremath{\tau}$ and the superconducting gap 2\ensuremath{\Delta}. By fitting the experimental transmission spectra at 40 K and below, we obtained $1/\ensuremath{\tau}$ and $2\ensuremath{\Delta}(0)$ to be $(700--1000) {\mathrm{cm}}^{\ensuremath{-}1}$ and $42 {\mathrm{cm}}^{\ensuremath{-}1},$ respectively. These two quantities suggest that the ${\mathrm{MgB}}_{2}$ film belongs to the dirty limit.

We report the effect of annealing on the superconductivity of MgB2 thin films as functions of the postannealing temperature in the range from 700 C to 950 C and of the postannealing time in the range from 30 min to 120 min. On annealing at 900 C for 30 min, we obtained the best-quality MgB2 films with a transition temperature of 39 K and a critical current density of ~ 10^7 A/cm^2. Using the scanning electron microscopy, we also investigated the film growth mechanism. The samples annealed at higher temperatures showed the larger grain sizes, well-aligned crystal structures with preferential orientations along the c-axis, and smooth surface morphologies. However, a longer annealing time prevented the alignment of grains and reduced the superconductivity, indicating a strong interfacial reaction between the substrate and the MgB2 film., Comment: 7 pages, 4 figures included

The magnetic penetration depth, $\lambda^{-2}(T)$, in the basal plane of a magnesium diboride (MgB$_2$) film was measured using a two-coil mutual inductance technique at 50 kHz. This film has $T_c\simeq 38$ K, $\Delta T_c \leq 1$ K, and $\lambda(0)\sim 1500$ \AA. At low temperatures, $\lambda^{-2}(T)$ shows a clear exponential temperature dependence, indicating s-wave superconducting order parameter symmetry. However, the data are not quantitatively well described by theory assuming a single gap. From the data fit by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: $\Delta_S(0)=2.61\pm0.41$ meV and $\Delta_L(0)=6.50\pm0.33$ meV. The contributions of the small and the large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we consider the effect of gap anisotropy on the penetration depth measurements, and find that the anisotropy does not play a significant role in determining the temperature dependence of the penetration depth., Comment: 8 pages, 5 figures

The tunneling splitting in biaxial ferrimagnetic particles at excited states with an explicit calculation of the prefactor of exponent is obtained in terms of periodic instantons that are responsible for tunneling at excited states and is shown as a function of magnetic field applied along an arbitrary direction in the plane of hard and medium axes. Using complex time path integral we demonstrate the oscillation of tunnel splitting with respect to the magnitude and the direction of the magnetic field due to the quantum phase interference of two tunneling paths of opposite windings. The oscillation is gradually smeared and in the end the tunnel splitting monotonously increases with the magnitude of the magnetic field when the direction of the magnetic field tends to the medium axis. The oscillation behavior is similar to the recent experimental observation with Fe-8 molecular clusters. A candidate of possible experiments to observe the effect of quantum phase interference in the ferrimagnetic particles is proposed.

In high-quality $c$-axis-oriented ${\mathrm{MgB}}_{2}$ thin films, we observed high critical current densities ( ${J}_{c}$) of $\ensuremath{\sim}16\mathrm{MA}/{\mathrm{cm}}^{2}$ at 15 K under self-fields comparable to those of cuprate high-temperature superconductors. The extrapolated value of ${J}_{c}$ at 5 K was estimated to be $\ensuremath{\sim}40\mathrm{MA}/{\mathrm{cm}}^{2}$. For a magnetic field of 5 T, a ${J}_{c}$ of $\ensuremath{\sim}0.1\mathrm{MA}/{\mathrm{cm}}^{2}$ was detected at 15 K, suggesting that this compound would be a very promising candidate for practical applications at high temperature and lower power consumption. The vortex-glass phase is considered to be a possible explanation for the observed high current-carrying capability.

Temperature dependent optical conductivities and dc resistivity of c-axis oriented superconducting ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}39.6\mathrm{K}$) ${\mathrm{MgB}}_{2}$ films ( $\ensuremath{\sim}450\mathrm{nm}$) have been measured. The normal state ab-plane optical conductivities can be described by the Drude model with a temperature independent Drude plasma frequency of ${\ensuremath{\omega}}_{p,D}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}13600\ifmmode\pm\else\textpm\fi{}100{\mathrm{cm}}^{\ensuremath{-}1}$ or $1.68\ifmmode\pm\else\textpm\fi{}0.01\mathrm{eV}$. The normal state resistivity is fitted by the Bloch-Gr\"uneisen formula with an electron-phonon coupling constant ${\ensuremath{\lambda}}_{\mathrm{tr}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.13\ifmmode\pm\else\textpm\fi{}0.02$. The optical conductivity spectra below ${T}_{c}$ of these films suggest that ${\mathrm{MgB}}_{2}$ is a multigap superconductor.

We report the growth of high-quality c-axis-oriented epitaxial MgB2 thin films by using a pulsed laser deposition technique. The thin films grown on (1`1 0 2) Al2O3 substrates show a Tc of 39 K. The critical current density in zero field is ~ 6 x 10^6 A/cm2 at 5 K and ~ 3 x 10^5 A/cm^2 at 35 K, suggesting that this compound has great potential for electronic device applications, such as microwave devices and superconducting quantum interference devices. For the films deposited on Al2O3, X-ray diffraction patterns indicate a highly c-axis-oriented crystal structure perpendicular to the substrate surface., Comment: 3 pages and 3 figures

The longitudinal resistivity rho_{xx} and Hall coefficient R_H were measured for MgB2 sintered under high pressure. We found that R_H is positive like cuprate high-Tc superconductors, and decreases as the temperature increases for 40 K < T < 300 K. The cotangent of the Hall angle was found to follow a+bT^2 behavior from Tc to 300 K. At T = 100 K, R_H = 4.1 x 10^{-11} m^3/C from which hole carrier density was determined to be 1.5 x 10^{23}/cm^3. This carrier density is 2 - 3 orders of magnitude larger than that of Nb3Sn superconductor and that of optimally doped YBa_2Cu_3O_y., 4 pages including 3 figures

The coherence peak is observed in the temperature dependence of the real part of the microwave conductivity ( σ 1 ) of c -axis oriented MgB 2 thin film, which is extracted from microwave surface impedance ( Z s ) measurements using a sapphire resonator at 17.9 GHz. The peak appears at a temperature around 0.6 times the critical temperature ( T c ), which is greatly shifted downwards in temperature compared with conventional superconductors for which the peak occurs around 0.9 T c . Calculations based on the two-band model suggest that this peak corresponds to the smaller of the two energy gaps.

The authors measured magnetization hysteresis curves and used magneto-optical imaging to visualize the flux distributions in superconducting MgB2 films in order to study dendritic flux avalanches. The flux avalanches are found to disappear above some upper threshold field that is typically ∼1kOe, but strongly depends on the film width. If the film is made wider, this threshold field first increases and then tends to saturate. This behavior is quantitatively explained using a thermomagnetic model for the dendritic avalanches and taking into account the field dependence of Jc. The results demonstrate that patterning superconducting films into narrow strips substantially increases the range of magnetic fields for which they can be used for applications.

This study reports the effects of HF on the voltammetric responses below 3 V (vs. Li/Li + ) in LiPF 6 solutions being widely used in rechargeable lithium-ion batteries (LIBs). During the first cathodic scan on a Pt electrode, two reduction peaks are observed between 3.0 and 1.5 V in LiPF 6 solutions irrespective of the solvents used. The reduction reactions are absent in LiBETI solutions but present after the addition of trace amount of HF. Thus, the reduction reactions in LiPF 6 solutions are due to the HF, an inevitable impurity of LiPF 6 solutions. Based on the results of the rotating disk electrode and the rotating ring disk electrode experiments, two reduction peaks are assigned to the hydrogen underpotential deposition and the hydrogen evolution reaction, respectively. HF reduction reactions forms a surface layer of which the main constituent is LiF. The surface layer suppresses the other reduction reactions otherwise occurring below 3.0 V. For example, the electrodeposition of metallic Mn, which takes place below 1.0 V in LiBETI solutions, is severely hampered in LiPF 6 solutions. The passivity by HF reduction is observed not only for a Pt electrode but also for a glassy carbon electrode.