Nonsuperconducting Regions Research Articles

Growth of twin-free b-oriented YBa2Cu3O7 − x films

YBa2Cu3O7 - x films of b orientation have been grown on the surface of (100)SrLaGaO4 crystal. It is demonstrated that the films pass to the superconducting state at a temperature of 89 K. In the nonsuper-conducting region, the film resistance decreases linearly with a decrease in temperature at a rate of 30 Omega per 100A degrees C. According to the X-ray diffraction data on the crystallographic structure of the YBa2Cu3O7 - x films, they are single-crystal and b-oriented. It is shown that the films contain no domains of other crystallographic orientations, including a-oriented domains. Therefore, the films are free of structural twins.

Paramagnetic moments and time effects in melt-textured NdBaCuO system with Nd422 inclusions

We have performed magnetic measurements in two melt-textured NdBa2Cu3O7-δ samples with Nd422 inclusions under magnetic fields from 0.05 up to 14 T, applied parallel to the ab planes. The measurements were made with a superconducting quantum interference device (SQUID) and a vibrating sample magnetometer (VSM). Paramagnetic moments could be observed during FCC (field-cooled cooling) and FCW (field-cooled warming) experiments. This effect, known as Paramagnetic Meissner Effect (PME), persisted up to 14 T and strong irreversibilities were observed among FCC and FCW experiments, revealing the presence of time effects. These time effects were confirmed by specific magnetic relaxation experiments in different cooling rates and temperatures, showing an anomalous and curious paramagnetic behavior. We explain our results based on the flux-compressed state generated within nonsuperconducting regions of the sample, such as the Nd422 inclusions dispersed into the superconducting matrix. These inclusions may produce a strong vortex pinning that stabilize the paramagnetic state, allowing the admission of extra vortices into the sample responsible for the positive moments during the relaxation experiments.

Activation energy and excess conductivity analysis of (Ag)x/CuTl-1223 nano-superconductor composites

Silver (Ag) nanoparticles were added into (Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (CuTl-1223) high Tc superconducting matrix to get (Ag)x/CuTl-1223, x = 0, 0.5, 1.0, 2.0, and 4 wt. %, nano-superconductor composites. The activation energy {U (eV)} and zero resistivity critical temperature {Tc (0)} were increased with increasing contents of Ag nanoparticles in (CuTl-1223) phase up to x = 2.0 wt. %. The increase of activation energy is most probably due to interaction of carriers with the metallic Ag nanoparticles present at grain boundaries of the host CuTl-1223 superconducting matrix. The systematic increase in Tc (0) and gradual decrease in normal state resistivity {ρ300 K (Ω cm)} may be due to improved inter-grains coupling by filling up the voids and pores with the inclusion of metallic Ag nanoparticles at the grain-boundaries. There are two possible mechanisms associated with the inclusion of Ag nanoparticles, one is the formation of non-superconducting regions causing the increase of activation energy and other (dominating) is the improved inter-grains connectivity promoting Tc (0). The microscopic parameters (i.e., zero temperature coherence length along c-axis {ξc (0)}, inter-layer coupling (J), inter-grain coupling (α), etc.) deduced from the fluctuation induced conductivity analysis reasonably explained the experimental findings.

Inhomogeneous superconductivity in organic conductors: the role of disorder and magnetic field

Several experimental studies have shown the presence of spatially inhomogeneous phase coexistence of superconducting and non-superconducting domains in low dimensional organic superconductors. The superconducting properties of these systems are found to be strongly dependent on the amount of disorder introduced in the sample regardless of its origin. The suppression of the superconducting transition temperature Tc shows a clear discrepancy with the result expected from the Abrikosov–Gor’kov law giving the behavior of Tc with impurities. On the basis of the time dependent Ginzburg–Landau theory, we derive a model to account for this striking feature of Tc in organic superconductors for different types of disorder by considering the segregated texture of the system. We show that the calculated Tc quantitatively agrees with experiments. We also focus on the effect of superconducting fluctuations on the upper critical fields Hc2 of layered superconductors showing slab structure where superconducting domains are sandwiched by non-superconducting regions. We found that Hc2 may be strongly enhanced by such fluctuations.

Levitation Force Density on Different Thicknesses of YBa2Cu3O7−x Bulk Superconductor Fabricated by FQMG Process

A precursor YBCO in the stoichiometric ratio of 1:2:3 was prepared by the flame-quench-melt-growth (FQMG) method. Both the superconducting and nonsuperconducting regions were decided by measuring the magnetic levitation force per unit volume as a function of thickness of the superconducting sample. It was determined that superconducting forming ratio depends on where it forms in the sample, and thus a region near the top surface of the sample contains more superconducting forming ratio than the other regions. The levitation force density measured at different thicknesses of the sample ranging from 6.54 to 1.58 mm firstly increased and then separately reached a saturation point (maximum value) for both repulsive and attractive levitation force density. It was found that the maximum values of the repulsive and attractive levitation forces were respectively 125 mN/cm3 and −23 mN/cm3. The considerably improved FL−z performance in FQMG sample can be translated to large-scale Y123 blocks intended for real superconducting levitation applications.

Microstructure Study at Different Thicknesses of Disc-Shaped YBa2Cu3O7−x Prepared by FQMG Method

The flame-quench-melt-growth (FQMG) method was used for the preparation of bulk YBa2Cu3O7−x (Y123). Both the superconducting and nonsuperconducting regions changing with thickness of superconducting sample were determined by X-ray diffraction (XRD) patterns and also observed at micrographs taken by the polarized optical microscope. It was observed that superconducting forming ratio which was examined for different thicknesses of the sample ranging from 6.54 to 1.58 mm strongly enhanced. This means that a region near the top surface of the sample contains more superconducting phase than the other regions. These observations would be helpful in designing and manufacturing the practical applications of YBCO systems.

Direct search for a ferromagnetic phase in a heavily overdoped nonsuperconducting copper oxide.

The doping of charge carriers into the CuO(2) planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case, a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La(2-x)Sr(x)CuO(4) using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping.

Pinning effects in ceramicSmO1−xFxFeAsas revealed by microwave absorption

Modulated microwave absorption (MMA) measurements have revealed strong pinning of vortices in ceramic superconducting ${\text{SmO}}_{1\ensuremath{-}x}{\text{F}}_{x}\text{FeAs}$ compounds with $x=0.06$, 0.08, and 0.1. Different behavior of MMA in small and strong fields enables to discriminate between intergranular and intragranular effects. Irreversibility lines due to the intragranular pinning exhibit a steep ascent comparable with that of the ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7}$ ceramics which is known to possess the highest pinning strength among cuprate high-temperature superconductors. A weak dependence of the critical current density on the magnetic field in the underdoped samples $(x=0.06,0.08)$ indicates the presence of additional pinning centers. The analysis of the data together with the theoretical modeling yields a conclusion that strong pinning in ${\text{SmO}}_{1\ensuremath{-}x}{\text{F}}_{x}\text{FeAs}$ is due to nonsuperconducting regions intermixed on a nanoscale with the superconducting phase.

Distinct physical behaviors of LaFe1−yCoyAsO0.89F0.11 between the superconducting and nonsuperconducting metallic regions of y divided by y ∼ 0.05

Electrical resistivities, Hall coefficients and thermoelectric powers have been measured for polycrystalline samples of LaFe1−yMyAsO1−xFx (M=Co, Mn; x=0.11) with various values of y[1]. A clear distinction of the T dependences of these quantities between the superconducting (y<yc∼0.05) and nonsuperconducting metallic (y>yc) regions has been found. This characteristic is very similar to that found for high-Tc Cu oxides, and can be considered to be, as in the case of Cu oxides, due to effects of the existence of strong magnetic fluctuations existing in the region y<yc. Based on this finding, we can extract information on the electronic state and its y dependence of the system. The superconductivity has been found to exist only in the systems with the hole Fermi surfaces. The rate of the Tc suppression by Co impurities is very small as compared with that expected for the S±-symmetry of the order parameter.

Distinct Transport Behaviors of LaFe1-yCoyAsO1-xFx (x=0.11) between the Superconducting and Nonsuperconducting Metallic y Regions Divided by y ∼0.05

Electrical resistivities, Hall coefficients and thermoelectric powers have been measured for polycrystalline samples of LaFe1-yCoyAsO1-xFx (x=0.11) with various values of y. The results show that there exists clear distinction of these transport behaviors between the superconducting and nonsuperconducting metallic regions of y divided by the boundary value yc~0.05. We have found that the behaviors in both regions are very similar to those of high-Tc Cu oxides in the corresponding phases. If they reflect, as in the case of Cu oxides, effects of strong magnetic fluctuations, the energy scale of the fluctuations is considered to be smaller than that of the high Cu oxides by a factor of ~1/2. Arguments on the electronic nature and superconducting symmetry are presented on the basis of the observed small rate of the Tc suppression rate by the Co doping.

Chiral symmetry breaking in three dimensional QED

Over the past few years three dimensional Quantum Electrodynamics (QED3) has attracted a lot of attention, because it may be an effective theory for the underdoped and non-superconducting region of the phase diagram of high Tc cuprate compounds. We present results from lattice simulations of the non-compact version of the theory in order to address the issue of whether chiral symmetry is spontaneously broken when the number of fermion flavours Nf is less than a critical value Nfc. Our results provide strong evidence that QED3 is chirally symmetric for Nf ≥ 1.5, implying that a pseudogap phase separates the superconducting phase from the antiferromagnetic phase.

Co-NMR Knight Shift of NaxCoO2·yH2O Studied in Both Superconducting Regions of the Tc–νQ3 Phase Diagram Divided by the Nonsuperconducting Phase

In the temperature ( T )–ν Q3 phase diagram of Na x CoO 2 · y H 2 O, there exist two superconducting regions of ν Q3 separated by the nonsuperconducting region, where ν Q3 is usually estimated from the peak position of the 59 Co-NQR spectra of the ±5/2↔±7/2 transition and well-approximated here as \(\nu_{\text{Q3}}\cong 3\nu_{\text{Q}}\), ν Q being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the 59 Co-NMR Knight shift ( K ) for a single crystal in the higher-ν Q3 superconducting phase and found that K begins to decrease with decreasing T at T c for both magnetic field directions parallel and perpendicular to CoO 2 -planes. The result indicates together with the previous ones that the superconducting pairs are in the spin-singlet state in both phases, excluding the possibility of the spin-triplet superconductivity in this phase diagram. The superconductivity of this system spreads over the wide ν Q3 regions, but is suppre...

SIN feature observed in intrinsic Josephson junction characteristics for overdoped Bi2Sr2CaCu2O8+δ

We have fabricated mesa structures made of 5 to 10 intrinsic Josephson junctions (IJJs) from overdoped and slightly overdoped Bi2Sr2CaCu2O8+δ(BSCCO) crystals and observed in their tunneling spectra a characteristics structure or a broad peak within the superconducting gap. We propose that this structure results from the inclusion of nonsuperconducting regions in the superconducting layer of IJJs. The cause of the structure is presumed to be the tunneling current through a superconductor/insulator/normal-metal (SIN) junction and the voltage drop due to the current flow in the normal regions after the tunneling through the SIN junctions. The broad peak located at a higher than Δ/eposition reflects the current path through the normal region. All these features imply that the superconducting state is inhomogeneous. The experimental results are compared with numerical calculations based on a simple model to obtain a good agreement. From the comparison, it is concluded that the nonsuperconducting and superconducting regions coexist in overdoped BSCCO on a ~ 50 nm scale.

Spectroscopy With Nanostructured Superconducting Single Photon Detectors

Superconducting single-photon detectors (SSPDs) are nanostructured devices made from ultrathin superconducting films. They are typically operated at liquid helium temperature and exhibit high detection efficiency, in combination with very low dark counts, fast response time, and extremely low timing jitter, within a broad wavelength range from ultraviolet to mid-infrared (up to 6 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mu</i> m). SSPDs are very attractive for applications such as fiber-based telecommunication, where single-photon sensitivity and high photon-counting rates are required. We review the current state-of-the-art in the SSPD research and development, and compare the SSPD performance to the best semiconducting avalanche photodiodes and other superconducting photon detectors. Furthermore, we demonstrate that SSPDs can also be successfully implemented in photon-energy-resolving experiments. Our approach is based on the fact that the size of the hotspot, a nonsuperconducting region generated upon photon absorption, is linearly dependent on the photon energy. We introduce a statistical method, where, by measuring the SSPD system detection efficiency at different bias currents, we are able to resolve the wavelength of the incident photons with a resolution of 50 nm.

Anomalously strong doping dependence of the maximum Josephson current in intrinsic Josephson junctions and inhomogeneous superconducting state in Bi2Sr2CaCu2O8+δ

Using intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ of which the doping level p is varied in a wide range, we have measured the magnitude of the superconducting order parameter Δ, the normal tunneling resistance RN, and the maximum Josephson current density Jc. It is found that Jc decreases quite rapidly by more than two orders of magnitude as p decreases, while Δ and RN change only moderately. This implies that the superconducting state of Bi2Sr2CaCu2O8+δ is inhomogeneous and nonsuperconducting regions coexist on a fine scale when p is small. Based on the Ambegaokar–Baratoff theory, the volume fraction of the superconducting regions is estimated to be 10% or less when p∼0.1.

Strong doping dependence of the interlayer superconducting coherence in Bi2(Sr,La)2CaCu2O8+y

Microscopic electronic inhomogeneity in underdoped cuprate superconductors has been recently suggested by the scanning tunneling spectroscopy measurements. An important question is how superconducting coherence develops with doping in the bulk. We evaluate the interlayer Josephson current density jc in superconducting Bi2(Sr,La)2CaCu2O8+y from the Josephson plasma resonance experiments. We find that jc shows an exponential doping dependence in a wide doping range, which is found to be much stronger dependence than expected in the d-wave coherent tunneling theory. This result implies the inhomogeneous superconducting state in each CuO2 planes inside the bulk, where the fraction of non-superconducting regions gradually shrinks with doping and disappears in the overdoped regime.

Competing phases in the superconducting state ofLa2−xSrxCuO4: Temperature- and magnetic-field-dependent quasiparticle relaxation measurements

We present femtosecond time-resolved measurements of the quasiparticle dynamics in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) as a function of the temperature and applied magnetic field. Temperature-dependent data correlate with independent equilibrium measurements of superfluid density and charge dynamics. At low temperature, the induced reflectivity transient is established in a two-step process. We propose that this behavior results from the coexistence of two energy scales in the superconducting state of LSCO: the superconducting (SC) gap and a partial gap at higher energy that decrease with increasing doping and vanish in overdoped material. Measurements under applied magnetic field indicate that, in the SC state, the response is remarkably sensitive to a $c$-axis field. We show that this is due to the development of nonsuperconducting regions of size $\ensuremath{\sim}130\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ surrounding vortex cores. The data show that a partial gap persists in these regions, suggesting that this gap is associated with electronic correlations competing with superconductivity.

Muon spin relaxation and magnetic susceptibility studies of the effects of nonmagnetic impurities on the Cu spin dynamics and superconductivity inLa2−xSrxCu1−yZnyO4aroundx=0.115

Zero-field muon-spin-relaxation (muSR) and magnetic-susceptibility measurements have been carried out in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with x = 0.10, 0.115 and 0.13 changing the Zn-concentration y finely up to 0.10, with the aim to clarify effects of the nonmagnetic impurity Zn on the Cu-spin dynamics and superconductivity. The muSR measurements have revealed that, in each x, a magnetic order of Cu spins is observed at y ~ 0.0075, while it disappears and Cu spins turn into a fast fluctuating state for y > 0.03. From the magnetic-susceptibility measurements, on the other hand, it has been found that the volume fraction of the superconducting state rapidly decreases through the slight doping of Zn and that its y dependence corresponds to the y dependence of the volume fraction of the fast fluctuating region of Cu spins estimated from the muSR results. Both the rapid decrease of the volume fraction of the superconducting state and the formation of the magnetic order through the slight doping of Zn can be interpreted in terms of the development of the so-called "Swiss cheese" model. That is, it is concluded that Cu spins in a non-superconducting region around Zn exhibit slowing-down of the fluctuations or form an incoherent or coherent static magnetic order. Moreover, the formation of a non-superconducting region around Zn is considered to be due to the pinning of the dynamical spin correlation or dynamical stripe correlations.

Magnetic regions inside a superconductor and its effects on the vortex matter

A superconductor with magnetic regions inside, forming an orthorhombic lattice of rigidly oriented magnetic moments, is investigated using a Ginzburg–Landau free energy expansion that naturally provides a unified view of both superconducting and non-superconducting regions. Point like magnetic moments are inside insulating regions, chosen to be either layers or spheres, of coherence length size. Thus an insulator–superconductor multi-layered system, and a superconductor with an orthorhombic lattice of spherical pinning centers are under investigation here. The rigidly oriented magnetic moments form two ground states, a ferromagnetic and a mixed one, namely, antiferromagnetic along the direction perpendicular to the moments and ferromagnetic inside the sheets containing the moments. A spontaneous vortex phase is found to exist for all systems above a critical magnetic moment value.

Core pinning by intragranular nanoprecipitates in polycrystallineMgCNi3

The nanostructure and magnetic properties of polycrystalline MgCNi_3 were studied by x-ray diffraction, electron microscopy, and vibrating sample magnetometry. While the bulk flux-pinning force curve F_p(H) indicates the expected grain-boundary pinning mechanism just below T_c = 7.2 K, a systematic change to pinning by a nanometer-scale distribution of core pinning sites is indicated by a shift of F_p(H) with decreasing temperature. The lack of scaling of F_p(H) suggests the presence of 10 to 20% of nonsuperconducting regions inside the grains, which are smaller than the diameter of fluxon cores 2xi at high temperature and become effective with decreasing temperature when xi(T) approaches the nanostructural scale. Transmission electron microscopy revealed cubic and graphite nanoprecipitates with 2 to 5 nm size, consistent with the above hypothesis since xi(0) = 6 nm. High critical current densities, more than 10^6 A/cm^2 at 1 T and 4.2 K, were obtained for grain colonies separated by carbon. Dirty-limit behavior seen in previous studies may be tied to electron scattering by the precipitates, indicating the possibility that strong core pinning might be combined with a technologically useful upper critical field if versions of MgCNi_3 with higher T_c can be found.