Investigation of the Upper Critical Magnetic Field and Activation Energy in MgB2 Thin Film

MgB₂ film with a thickness of about 600?nm was deposited on the MgO (100) single crystal substrate using a ??two-step?? synthesis technique. First, deposition of boron thin film was carried out by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850?°C in magnesium vapor. The upper critical field H _c2 has been estimated from temperature dependences of resistivity curves in both directions of the magnetic fields perpendicular and parallel to the c-axis. Resistivity measurements of the film were performed using a standard four-probe method under different magnetic fields up to 70?kOe in zero fields cooling regime. The upper critical magnetic field H _c2(0) at T=0?K for 90?% of R _n was calculated by the extrapolation H _c2(T) to the temperature T=0?K. The results showed that H _c2??ab(0) and H _c2??c(0) was found to be around 22?T and 18?T, respectively. Using extracted data, the zero-temperature coherence lengths and field anisotropy ratio were calculated. In order to determine the activation energy of thermally activated flux flow of the film, Arrhenius law was taken into account.     

Exchange Bias, Memory and Freezing Effects in NiFe2O4 Nanoparticles

Single-phase NiFe₂O₄ nanoparticles embedded in SiO₂ matrix have been synthesized by sol-gel method. Average particle size lies in the range 8?C12 nm. Magnetic measurements are taken by SQUID-magnetometer with a maximum applied field of ??7 T and temperature down to 4.2 K. An exchange bias effect in nanoparticles is due to the existence of strong core-shell interactions and it vanishes as the particle size decreases (<4?nm). Spin disorder and frustration appear at the core-shell interface due to broken bonds on the surface. We have observed the exchange bias effect via hysteresis loop shift, when the sample is cooled in an applied field of 5?T. In both AC and DC fields, our system exhibit memory effects at the halted temperatures. Furthermore, a sharp increase of coercivity at low temperatures (<50 K) is observed, which is attributed to increased surface anisotropy at low temperatures. For saturation magnetization vs. temperature data, Bloch??s T ^3/2 law (M(T)=M(0)?(1?BT ^b )) is fitted well and yields: B=4??10^?05 K^?3/2 and b=1.53. All these measurements prove the presence of exchange bias (core-shell interactions), memory effects, validation of Bloch??s T ^3/2 law and freezing effects in nickel ferrite nanoparticles dispersed in SiO₂ matrix.     

Position Dependent Response of Superconducting MgB2 Neutron Detectors Studied by Pulsed Laser Irradiation

The MgB₂ detector consists of 200?nm thick MgB₂ thin film meander structure of 3?μm line width protected by 300?nm thick SiO layer. The electrical connections were provided by 150?nm thick Nb. The devices were placed in a 4?K refrigerator to investigate the behavior at temperatures below T _c. The position-dependent response of the MgB₂ detector can be investigated by scanning a laser spot with the aid of the XYZ piezo-driven stage and an optical fiber with a focusing lens. We found that the inhomogeneity in a detector is very critical to specify the detection efficiency since the detector has the sensitivity only in a narrow temperature regime near T _c.     

Observation of Re-entrant Resistance in NbN/NbO/Co Trilayer

Transport measurements on a niobium nitride (NbN) film covered with a ferromagnetic cobalt layer are reported here. The sample shows a superconducting transition (T _SC) at 6.5?K. In the superconducting state, a dip in resistance is observed at a temperature referred to as T _MIN. Below T _MIN, resistance reappears, with a magnitude of about 1% of the normal-state resistance. The observed resistance is found to decrease on increasing the applied current?(I). A?possible reason for re-entrant resistance might be the vortex dynamics in NbN superconducting layer due to the stray fields created by the Co layer. Further confirmation of the observed behavior is obtained from the plot of resistance versus probe current measured at various temperatures in close vicinity of?T _SC.     

The Dynamics of Vortex Generation in Superfluid 3He-B

A profound change occurs in the stability of quantized vortices in externally applied flow of superfluid ³He-B at temperatures ?0.6?T _c, owing to the rapidly decreasing damping in vortex motion with decreasing temperature. At low damping an evolving vortex may become unstable and generate a new independent vortex loop. This single-vortex instability is the generic precursor of turbulence. We investigate the instability with non-invasive NMR measurements on a rotating cylindrical sample in the intermediate temperature regime (0.3–0.6)?T _c. From comparisons with numerical calculations we interpret that the instability occurs at the container wall, when the vortex end moves along the wall in applied flow.     

Vortex Formation and Annihilation in Rotating Superfluid 3He-B at Low Temperatures

In superfluid ³He-B the damping of vortex motion varies many orders of magnitude in the currently accessible temperature range from T _c to below 0.2?T _c. The large variation in the dynamics switches on sequentially different processes of vortex formation as a function of temperature. These can be examined in a smooth-walled rotating cylinder in the absence of surface pinning. We provide an overview of vortex formation in spin-up and of annihilation in spin-down measurements at temperatures below the hydrodynamic transition at 0.6?T _c where turbulence in the bulk volume becomes possible. Of central current interest are the very lowest temperatures at 0.2?T _c where the dynamic processes reflect on the properties of the T→0 limit.     

The effects of the substrate temperature on sputter-deposited niobium films

The superconducting transition temperatures and the structural properties of Nb films prepared by sputtering have been studied over a deposition temperature range of ?130 to 700°C. Niobium films show a preferred orientation in the <110> direction perpendicular to the substrate surface. The transition temperatures of Nb films deposited onto cryogenic substrates were greatly reduced. A close correspondence between thed spacingd ₁₁₀ and superconductingT _c has been found. TheT _c of Nb films decreases rapidly with increasing spacingd ₁₁₀.     

Intergrain Effects in the AC Susceptibility of?Polycrystalline LaFeAsO0.94F0.06

The AC susceptibility, ??, at zero DC magnetic field of a polycrystalline sample of LaFeAsO_0.94F_0.06 (T _c ??24?K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range H _ac =0.003?Oe??4?Oe) and the frequency (in the range f=10?kHz??100?kHz). The ??(T) curve exhibits the typical two-step transition arising from the combined response of superconducting grains and intergranular weak-coupled medium. The intergranular part of ?? strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T _c down to T=4.2?K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.     

Electron-Boson Coupling Constant of Oxygen-Deficient Y0.7Ca0.3Ba2Cu3O7?�� Measured by Ultrafast Spectroscopy

The ultrafast dynamics in an (001) oxygen-deficient Y_0.7Ca_0.3Ba₂Cu₃O_7??? thin film were measured by using the ultrafast spectroscopy. According to the model of time-dependent distributions of electrons in the case of electron-phonon relaxation, the electron-boson coupling constant ?? could be directly obtained from the relaxation time of the transient reflectivity change (??R/R). For the overdoped case with T _c=61.3 K and the underdoped case with T _c=65.5 K, the relaxation time of ??R/R is around 4.32 ps and 3.66 ps, respectively, which correspond to ????0.06. However, the ?? become larger (??0.15) in the optimal doped region with T _c=82.1 K while the relaxation time of ??R/R shrinks to ??1.51 ps. These results indicate that the ?? is strongly hole-doping dependent, which may govern the T _c of cuprate superconductors.     

Dependence of Superconductivity and Its Weakly Linked Behavior in Bulk LaO1?x F x FeAs on F Doping

Samples of oxypnictide compound LaO_1?x F _x FeAs, with x=0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ??(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2?C300?K and applied fields B=0?C8?T. With change in?x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The ??magnetic anomaly?? at T??130?K, observed in M(T) for x=0, instead of getting totally suppressed shows a tendency to reappear in x=0.2 sample. Both samples typically show ??(300?K)>2.8×10^?3????cm and critical current density J _c(5?K, 1?T)<2×10⁷?A/m². The superconducting transitions as measured by ??(T) at B=0 are found broad for both x=0.15 and 0.2 samples with transition widths ??2.5 and 6?K, respectively. The slope |dB _c2/dT| (where B _c2 is upper critical field) determined by resistive onsets, for the x=0.15 and 0.2 samples, has values ??7.5 and 3.5?T/K, respectively. The superconducting state characteristics as reflected by ??(T,B), M(T), magnetic J _c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x=0.15 sample show the presence of high angle (??43^°) grain boundaries, which are well known to limit the J _c in cuprate-based high T _c bulk materials.     

Impact of Gd Doping on Morphology and Superconductivity of NbN Sputtered Thin Films

We report the effect of Gd inclusion in the NbN superconductor thin films. The films are deposited on single crystalline Silicon (100) by DC reactive sputtering technique, i.e., deposition of Nb and Gd in presence of reactive N₂ gas. The fabricated relatively thick films (400 nm) are crystallized in cubic structure. These films are characterized for their morphology, elemental analysis, and roughness by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDAX), and Atomic Force Microscopy (AFM) respectively. The optimized film (maximum T _c ) is achieved with gas ratio of Ar:N₂ (80:20) for both pristine and Gd-doped films. The optimized NbN film possesses T _c (R=0) in zero and 140 kOe fields are at 14.8 K and 8.8 K, respectively. The Gd-doped NbN film showed T _c (R=0) in zero and 130 kOe fields at 11.2 K and 6.8 K, respectively. The upper critical field H _c2(0) of the studied superconducting films is calculated from the magneto-transport [R(T)H] measurements using GL equations. It is found that Gd doping deteriorated the superconducting performance of NbN.     

Ultrafast Dynamics in Optimally Doped YBa2Cu3O7??? Observed with White Light Pump-probe Spectroscopy

The quasiparticle relaxation dynamics in optimally doped (001) YBa₂Cu₃O_7??? thin films were measured at various temperatures by white light pump-probe technique. The 2D ultrafast spectroscopy with broadband (??200?nm) and high time-resolution (??9 fs) was performed by the non-collinear optical parametric amplifier (NOPA). The relaxation time and the amplitude of the transient transmissivity changes (??T/T) are wavelength-dependent. The maximum ??T/T appears at around 625 nm (??1.98 eV) which corresponds to the transition between the Fermi level and the UHB (upper Hubbard band). Furthermore, the oscillations with ??150, ??320, ??500 cm^?1 have been clearly observed in the Fourier transformed graph of the 2D ultrafast spectroscopy, which are consistent with the phonon modes in Raman-scattering spectra.     

Angular-Dependent Vortex Pinning Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>/Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Quasi-Multilayers

A series of quasi-multilayers of YBa₂Cu₃O_7?δ (YBCO)/Y₂O₃ specifically 70 × (m YBCO/n Y₂O₃) were prepared on SrTiO₃ single crystal using pulsed-laser deposition (PLD) with a controlled deposition pulses of m = 40 and n = 2, 5, and 10 for YBCO and Y₂O₃, respectively. The x-ray diffraction patterns indicate that all the present quasi-multilayers exhibit good c-axis orientation. The angular dependence of critical current density (J _c ) on applied magnetic field directions are systemically measured to study the anisotropic vortex pinning performances for those quasi-multilayers. It is revealed that compared with the pure YBCO films, the quasi-multilayers with n = 2, i.e., a proper constituent pulse of Y₂O₃, exhibits the enhanced vortex pinning abilities in all angles between c-axis orientation and the applied magnetic field direction. As well, such a quasi-multilayer film (n = 2) shows the higher lift factor J _c (Θ)/ J _c (90°) and much better vortex pinning properties at high fields and high temperatures, showing promising potential for coated conductor application.     

Dependence of the Structural, Electrical and Magnetic Properties of YBa2Cu3O7?δ Bulk Superconductor on the Ag Doping

The effects of Ag dopants on the superconducting properties of YBCO bulk samples, prepared under solid-state reaction method, have been studied by resistivity?Ctemperature (???CT), X-ray diffraction (XRD), scanning electron microscope (SEM), bulk density, AC susceptibility, DC magnetization measurements, and theoretical analysis. Small Ag substitutions (x??0.10) do not effect T _c of pure YBa₂Cu_3?x Ag _x O_7??? , while we observed small decreases of 1.5?K for larger doping levels (x??0.15). AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of DC bias field. The critical current densities (J _c ) have been estimated as a function of magnetic field from the magnetization data employing Bean??s critical state model. The increase in Ag amount (x) in YBa₂Cu_3?x Ag _x O_7??? (x??0.10) system effectively decreases the intragrain critical current density, which is attributed to the reduced connectivity between the grain boundaries. In the case of small additions (x=0.05), Ag atoms fill partly the pores, leading better crystallization of the grains.     

Two-phase structure of ultra-thin La-Sr-MnO films

The structural, electrical and magnetic properties of ultra-thin La_0.83Sr_0.17MnO₃ (LSMO) films, deposited on NdGaO₃ substrate by using the MOCVD technique, were studied. The film thickness d varied in the range from 4 to 140 nm. X-ray and RHEED measurements demonstrated that the films had a two-phase structure. One phase had an orthorhombic face centred structure (a = 0.406 nm and c = 0.46 nm), while the other one had a cubic perovskite-like structure with a = 0.388 nm. Low field dc resistance and magnetization vs. temperature dependences were investigated in the temperature range from 5 to 300 K using a conventional four-probe method and a SQUID magnetometer. It was found that the temperature of the resistivity maximum, T_m, increases with increasing film thickness and that the value of the Curie temperature T_C estimated from the temperature dependence of magnetization is very close to T_m. Modelling of the remanent magnetization vs. temperature dependence based on a two-phase model was in agreement with experimental results. This model also explains the T_m shift to lower temperatures with decreasing film thickness.     

Doping-Dependent Phase Diagram of Ca-Doped YBCO Observed by Femtosecond Spectroscopy

The quasiparticle relaxation dynamics in a single (001) Ca-doped YBCO (Y_0.7Ca_0.3Ba₂Cu₃O_7??? ) superconducting thin film was probed by using the femtosecond time-resolved spectroscopy. The (001) Y_0.7Ca_0.3Ba₂Cu₃O_7??? thin film was prepared on a (100)STO substrate by pulsed laser deposition. Through controlling the oxygen pressure and the annealing temperature within quartz tube, the doping level (0.08<p<0.22) in the phase diagram could be widely tuned from the overdoped region to the underdoped region with varying the hole concentration (p) in a single sample. In the overdoped region, two different components (positive/negative) were obviously identified in the transient reflectivity curves (??R/R). The negative component in ??R/R emerges at T<T _c and quickly defeated by the development of the positive component. The dramatic change in the positive component of ??R/R arises well below T _c. However, this anomaly change does not appear in the underdoped region. Only the positive ??R/R was clearly observed at whole temperatures. These results indicate that the scenario of high-T _c superconductivity in the overdoped region would be different from the underdoped region.     

Synthesis and Characterization of MgB2 Thin Films Prepared by?2.4?Mhz Ultrasonic Spray Pyrolysis System

Nano-sized B and Mg powders have been successfully deposited onto single crystal MgO(100) and Al₂O₃(001) substrates using 2.4 MHz ultrasonic spray pyrolysis system and an appropriate solution to obtain thin films of MgB₂ superconductors. After an in-situ heating process, ??600?C1000 nm thick superconductor films were obtained. The microstructure, electrical, and magnetic properties were characterized by means of particle size analyzer, XRD, SEM?CEDX, R?CT, and M?CH analysis. The effect of particle concentration in the solution, spraying time, and heating temperature on the quality of the MgB₂ films were discussed. The best T _c and T _zero results were obtained to be 39.5 and 37.4 K, respectively, for the film deposited on the Al₂O₃(001) substrates. Magnetic properties of the MgB₂ films were investigated at 3 different temperatures and up to 5 T. Symmetric hysteresis loops for all temperature and field cases were obtained and maximum $J_{c}^{\mathrm{mag}}$ value was calculated to be 4.0×10⁶ A?cm^?2 at 10 K for 0 T for the film deposited on the Al₂O₃(001) substrates. The results obtained were found to be highly dependant on the particle concentration in the solution, heating temperature of substrates and carrier gas flow rate during spraying.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1−x Nb x )Sr2(GdCe1.8Sr0.2)Cu2O z System

The Ru-1232 compounds have been synthesized in the (Ru_1?xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Superconductivity of Nb1−x Mg x B2: Impact of Stretched c-Parameter

We have performed a systematic study on the occurrence of superconductivity in Nb_1?x Mg _x B₂ (0.0≤x≤0.40). X-ray diffraction and magnetization measurements are carried out to determine the changes in lattice parameters, superconducting transition temperature (T _c ) and critical field (H _c1). The substitution of Mg at Nb site results in considerable stretching of c-parameter with only a slight change in a parameter. Rietveld analysis on X-ray diffraction patterns gives a=3.11 Å and c=3.26 Å for pure NbB₂ while a=3.10 Å and c=3.32 Å for Nb_0.60Mg_0.40B₂. This increased c-parameter introduces superconductivity in niobium diboride. Magnetization measurements though indicate the absence of superconductivity in NbB₂, the same shows a clear diamagnetic signal at about 10 K for Nb_0.60Mg_0.40B₂ sample. The magnetization M(H) plots exhibit weak superconductivity like hysteresis loops. The stretching of c-parameter from around 3.26 to 3.32 i.e. by 0.06 cannot be explained solely by substitution of Nb by Mg in the lattice. It seems that some Nb deficiencies are introduced in the Nb_1?x Mg _x B₂ as Mg is not substituted completely at the vacant Nb sites. This could be seen from XRD results, where one can clearly notice the presence of small amount of MgO in Nb_1?x Mg _x B₂ samples.     

Temperature and Current Dependent Matching Fields in?Superconducting NbN Film

Patterned superconducting thin films having a periodic array of submicrometric pinning centers have been of great interest due to their excellence for the studies of the vortex pinning mechanisms in the type-II superconductors. Square hole array has been fabricated over a micro-bridge 60 mm??60 mm of NbN thin film by electron beam lithography. Previous works have been carried out in Nb, Pb and Al thin films where the vortex pinning effect is assumed to be small. In this work, we study the matching pinning effect by the artificial hole array in superconducting NbN thin films. We observed the interplay between the vortex quantization and the artificial hole array. Magneto-resistance minima at integer matching fields up to five times of H ₁ (the first matching field corresponding to one vortex inside each hole) and fractional matching fields at 1/2H ₁, 3/2H ₁ and 5/2H ₁ have been observed.