Browsing by Subject "Superconductivity"

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Open Access
1/f Noise characteristics of SEJ Y-Ba-Cu-O Rf-SQUIDs on LaAlO3 substrate and the step structure, film, and temperature dependence
(IEEE, 2001-03) Fardmanesh, M.; Schubert, J.; Akram, R.; Bick, M.; Zhang, Y.; Banzet, M.; Zander, W.; Krause, H. J.; Burkhart, H.; Schilling, M.
Step edge junction (SEJ) rf-SQUID magnetometers and gradiometers were fabricated using PLD Y-Ea-Cu-0 films on LaA10,(100) and SrTi0,(100) substrates. Effects of different step structure and the film properties on the yield, optimal operating temperature, and the l/f noise of the SQUIDs were investigated. The step structure was controlled using various IBE processes. The devices on LaAIO, showed higher sensitivity to the step structure compared to those on SrTiO,. This was due to re-deposition of substrate material at the steps prepared using the conventional IBE process resulting in a very low yield of unstable SQUIDs. High yield of low l/f noise stable SQUIDs was obtained on LaAIO, substrates with sharp steps prepared using an optimized IBE process. A typical l/f noise corner frequency of about lOHz at 77K with two major temperature dependencies was obtained. The temperature dependencies of the l/f noise could be correlated to the junction and the film of washer area of the SQUIDS. The white noise of our devices showed a dependency mainly on the amplitude of the flux to voltage transfer function signal. The operating temperature range of the SQUIDs could be controlled by the step structure and narrowed when the optimal operating temperature range was increased. All the measured junctions of our devices on the modified steps showed RSJ type behavior with a moderate decrease of the R, versus temperature.
Open Access
Angular dependence of upper critical field in two-band Ginzburg-Landau theory
(Elsevier B.V., 2007) Askerzade, I. N.; Tanatar, Bilal
Generalization of two-band Ginzburg-Landau (GL) theory to the case of anisotropic mass is presented. The temperature dependence of the anisotropy parameter of upper critical field γc 2 (T) = Hc 2∥ (T) / Hc 2⊥ (T) and angular dependence of Hc2(θ, T) are calculated using anisotropic mass two-band Ginzburg-Landau theory of superconductors. It is shown that, with decreasing temperature anisotropy parameter γc2(T) is increased. Results of our calculations are in agreement with experimental data for single crystal MgB2. © 2007.
Open Access
Anomalous splitting of the first penetration peak in the local magnetization of Bi2Sr2CaCu2O8+y single crystals
(Institutul National de Cercetare-Dezvoltare pentru Optoelectronica, 2005) Mihalache, V.
A scanning Hall probe microscope (SHPM) with an effective spatial resolution of ∼1 μm has been used to study the local induction in high quality superconducting Bi2Sr2CaCu2O 8+δ single crystals at high temperatures and low magnetic fields. We observed, for the first time to our knowledge, an anomalous splitting of the peak of first full penetration of magnetic field. We discuss the observed splitting, which is connected to the effects of surface and geometrical barriers on the vortex lattice.
Open Access
Bound electron pairs in strongly correlated models of high-temperature superconductivity
(Natsional'na Akademiya Nauk Ukrainy, 1998) Boyaci, H.; Kulik, I. O.
The ground-state energy of two electrons on a ring is calculated for the one-dimensional Hubbard model with positive and negative on-site interaction and for the contraction model with additive and multiplicative interaction terms. The hc/2e periodicity of the ground-state energy with respect to a flux Φ threading the loop is derived. The periodicity may serve as an indication of superconductivity. The results are shown to be consistent with the Lieb-Wu solution for Φ=0 limit. In addition, the new states that were missing in the Lieb-Wu solution are derived.
Open Access
Collective modes in flux line liquids
(IOP, 2000) Tanatar, Bilal; Oral, A.
We study the collective modes of flux line liquids such as occur in the type-II superconductors of recent interest composed of two-dimensional layered structures. Starting from the vortex-vortex interaction and employing the dielectric formalism within the random-phase approximation, we find propagating sound modes in the long-wavelength limit.
Open Access
Contraction of atomic orbitals in the oxygen anion network and superconductivity in metal oxide compounds
(TÜBİTAK, 1996) Kulik, Igor Orestovich
Anion network in the CuO2 plane of metal-oxide compound is considered as an intrinsic-hole metal with holes rather than electrons comprising a Fermi liquid immersed in the background of negative O2- ions. Due to the contraction of p-orbital of oxygen as a result of occupation by a hole, hole hopping between nearest neighbor sites (i, j) is dependent upon hole occupation as tij,σ = to + Vni,-σnj,-σ = W(ni,-σ + nj,-σ). Coupling parameters W and V (additive and multiplicative "contraction interaction" terms) result in the binding of holes into singlet, on-site configuration, or into triplet, nearest-neighbor-site configuration, due to W and V respectively. In the weak coupling limit, W results in the BCS type of superconductive pairing (singlet, s-wave), whereas multiplicative contraction V provides for either singlet, d-wave, or triplet, p-wave-like pairing states. It is concluded that the latter state may result in a plausible mechanism for high-Tc superconductivity in metal oxide compounds. The superconducting p-phase is shown to be in accord with recently published symmetry tests of the order parameter in oxides.
Open Access
Disorder Induced BCS-BEC Crossover in an Ultracold Fermi Gas
(Springer, 2013-05) Khan, A.; Basu, S.; Tanatar, Bilal
We develop the formalism for BCS-BEC crossover in the presence of weak random impurity and calculate the effect of the random potentials on the basic mean-field quantities. The disorder has been included through the Nozieres and Schmitt-Rink theory of superconducting fluctuations, and we obtain the disorder induced superfluid order parameter and chemical potential through a self-consistent calculation. We also calculate the condensate fraction which reveals a distinct nonmonotonic behavior. The downturn in the latter result occurs at the crossover regime with gradual depletion on the BEC side. The non-monotonic feature in the condensate fraction data has been measured in clean systems. Motivated by the above result, we discuss the stability of a disordered fermionic superfluid in the crossover regime.
Open Access
Effect of the superconductivity transition on the response of YBCO edge transition bolometers
(2003) Bozbey, Ali; Fardmanesh, Mehdi; Askerzade, I. N.; Banzet, M.; Schubert, J.
Dependence of the phase and magnitude of the response of Y-Ba-Cu-O edge transition bolometers on the superconducting transition is studied. The responses of both large and small area devices were investigated and several anomalies are observed. The response of small area LaAlO3 devices considerably differed from that expected based on the dR/dT curve. This discrepancy is observed to be strongly dependent on the superconducting transition. Both the phase and magnitude/(dR/dT) of the response of the devices showed abrupt changes for below the Tc-onset when measured versus temperature, while the phase variation also showed strong dependence on the modulation frequency. We present the analysis and propose mechanisms responsible for the modulation frequency dependence of the response characteristics versus temperature, within the superconductivity transition region of the devices.
Open Access
Electronic structure, insulator-metal transition and superconductivity in K-ET2X salts
(1998) Ivanov V.A.; Ugolkova, E.A.; Zhuravlev, M.Ye.; Hakioǧlu, T.
The electronic structure and superconductivity of layered organic materials based on the bis(ethylenedithio)tetrathiafulvalene molecule (BEDT-TTF, hereafter ET) with essential intra-ET correlations of electrons are analysed. Taking into account the Fermi surface topology, the superconducting electronic density of states (DOS) is calculated for a realistic model of K-ET2X salts. A d-symmetry of the superconducting order parameter is obtained and a relation is found between its nodes on the Fermi surface and the superconducting phase characteristics. The results are in agreement with the measured non-activated temperature dependences of the superconducting specific heat and NMR relaxation rate of central 13C atoms in ET. © 1998 John Wiley & Sons, Ltd.
Open Access
Fabrication and characterization of SmCo5/Nb ferromagnetic/superconducting hybrid thin films grown by RF magnetron sputtering technique
(Elsevier, 2017) Ongun, E.; Kuru, M.; Serhatlıoğlu, M.; Hançer, M.; Ozmetin, A. E.
Ferromagnet/Superconductor (F/S) bilayer hybrids show exclusive states due to the mutual interaction between the superconductor and the underlying ferromagnetic substructures in micron scale. In this work, we aimed to observe the effects of the interaction between superconductivity and magnetism, especially the phenomenon involving the orientation and the size of magnetic stripes has been investigated in a coupled ferromagnetic/superconducting thin-film structure. In the proposed F/S hybrid system by this work, superconducting niobium thin-films were combined with underlying segments of ferromagnetic SmCo5 substructures. 300 nm thick magnetic films fabricated by RF magnetron sputtering techniques were topographically grown in patterns with stripes oriented either transverse to or along the direction of current flow. The elemental and microstructural analyses were conducted by EDX, SEM and GIXRD characterization tools. Low-temperature DC transport measurements were conducted by means of four point probe method in a 9T closed-cycle cryogenic refrigeration system. Transport superconducting properties, transition temperature TC(H) and second critical field HC2(T) were measured in a range of applied magnetic field between H = 0–9 kOe for the hybrid system. The results revealed that the artificial periodic modulation of applied field through preferentially-oriented magnetic stripes could introduce normal and superconducting channels or barriers for the current flow.
Open Access
High Tc superconductivity
(1989) Gedik, M. Zafer
critical summary of the recent developments in high temperature superconductivity is given. Physical properties of the new materials are summarized. The new theories proposed to explain high Tc are reviewed. In the case of a special type anisotropic gap, the currents and conductivity for single particle tunneling in high Tc superconductor junctions are calculated. Tunneling is assumed to be specular. The position of the peak in the conductivity curve, is found to be determined by the shape of the Fermi surface. Two geometries are considered: tunneling paxallel and perpendicular to CuO^ planes. The order of the peaks for these two configurations turns out to be opposite to experimental results. It is concluded that, conventional one band theory of superconductivity is not enough to explain observed tunneling spectra of high Tc superconductors.
Open Access
Impurity effects and symmetry of the order parameter in high-temperature oxide superconductors
(1997) Bayındır, Mehmet
Identification of the symmetry of the order parameter (OP) of high-l'c cuprates is important because it helps to understand possible mechanism that govern the physics of these materials. However, up to now, there is no consensus on the symmetry of the OP. On the other hand, nonmagnetic impurity substitutions would test the symmetry of OP. Present theoretical calculations overestimate suppression of the critical temperature (by a factor of 2 or more) in comparison to the experimental data. So far, differences between zinc [Zn) and nickel {Ni) substitutions have not been well understood. Considering the above arguments, effects of nonmagnetic impurities on the high-temperature cuprates are investigated by solving the Bogoliulpv-de Gennes (BdG) equations on two-dimensional square lattice. The critical impurity concentration is found to be very close to the experimental values. Possibility of extracting the symmetry of OP from the obtained results is discussed. Finiteranged impurity potentials and different potential strengths for different impurities are proposed to explain differences between Zn and Ni substitutions. Finally, it is concluded that our results are in favor of d-wave symmetry for tetragonal and s+d-wave symmetry for orthorhombic phase, and explain quite well the effects of nonmagnetic impurity substitutions in the high-7'c oxide superconductors. Beside these, the physical properties of the high-temperature oxide superconductors, the BdG equations and effects of nonmagnetic impurities on isotropic and anisotropic superconductors are reviewed.
Open Access
Impurity effects on superconductors and the electron-phonon interaction
(2000) Savran, Kerim
In this thesis effects of impurities on superconductors and electron-phonon interactions in metals are studied. The first part deals with the effect of magnetic impurities on superconductors. In particular, we focus on the experimental observation that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in the crystalline or the amorphous state. Based on the recent theory of Kim and Overhauser, it is shown that as the disorder in the system increases, the initial slope of the Tc depression decreases by a factor when the mean free path I becomes smaller than the BCS coherence length which is in agreement with experimental findings. Additionally, the transition temperature Tc for a superconductor, which is in a pure crystalline state, drops sharply from about 50% of Tco (transition temperature of a pure system) to zero near the critical impurity concentration. This pure limit behavior was found in crystalline Cd by Roden and Zimmermeyer. In the second part, the effect of weak localization on electron-phonon interactions in metals is investigated. As weak localization leads to the same correction term to both conductivity and electron-phonon coupling constant A (and Xtr), the temperature dependence of the thermal electrical resistivity is decreasing as the conductivity is decreasing due to weak localization. Consequently, the temperature coefficient of resistivity (TCR) decreases, while t he residual resistivit}' increases. As the coupling constant A approaches zero, only the residual resistivity part remains and accordingly TCR becomes negative. In other words, the Mooij rule turned out to be a manifestation of weak localization correction to the conductivity and the electron-phonon interaction.
Unknown
Investigation of the magnetic properties of BSCCO superconductors with scanning Hall probe microscope
(2002) Dede, Münir
A low temperature scanning Hall probe microscope (LT-SHPM) with low noise GaAs/AlGaAs 2DEG Hall probes is used to investigate the magnetic properties of a Ag-sheathed BSCCO-2223 tape and a single crystal BSCCO-2212 superconductor. The Hall probes are micro fabricated with 1 µm wide Hall bars which have ∼0.85 µm electrical width. The magnetization behavior, vortex structures and the vortex lattice melting are observed at 77 K. Flux penetration into the BSCCO-2212 single crystal is also observed and imaged with single vortex resolution.
Unknown
Josephson effect in hybrid superconductor-normal metal structures
(2000) Çakır, Özgür
The clean SNS junction, with a phase difference x across the normal metal barrier and without Fermi level mismatch between the components, is modeled by a step like pair potential. The quasiclassical equations are obtained from the Gorkov equations by the elimination of crystal momenta and are valid in the existence of only the off-diagonal potential(pair potential). Then for the SNS junction, the quasiclassical Green functions are obtained. At zero temperature, in the long barrier limit(d ^ ^o)> the current is found to have a sawtooth dependence on the phase difference, the amplitude of which is inversely proportional to the thickness of the normal layer. At finite temperatures in the limits d and Tc T, the current is found to have exponential dependence on the thickness of the normal layer, T exp(—d/^7’) sin y, .where = vfI2ttT. An extension of single SNS structures is the periodic SNS structure which may as well exhibit Josephson effect. It is simulated by a periodic step-wise pair potential, where the phase of the pair potential changes by some constant value in subsequent superconducting islands. Bogoliubov equations in the semicla.ssical limit are employed, yielding the density of states(DOS). In the DOS, there appears some forbidden energy regions, which point out to a band structure.
Open Access
Local-pair superconductivity in very high magnetic fields
(Scientific and Technical Research Council of Turkey - TUBITAK,Turkiye Bilimsel ve Teknik Arastirma Kurumu, 1996) Gedik, Zafer
Superconductivity of narrow-band systems with local, short-range attractive interaction in very high magnetic fields is discussed. By examining the excitation spectra of both type-II superconductors with BCS like interaction and local-pair superconductors with negative-U type interaction, it is concluded that gapless single particle energy spectrum is a characteristic feature of superconductivity in very high magnetic fields.
Open Access
A new method of probing the phonon mechanism in superconductors, including MgB2
(2001) Park, M.-A.; Savran, K.; Kim, Y.-J.
Weak localization has a strong influence on both the normal and superconducting properties of metals. In particular, since weak localization leads to the decoupling of electrons and phonons, the temperature dependence of resistance (i.e. λtr) decreases with increasing disorder, as manifested by Mooij's empirical rule. In addition, Testardi's universal correlation of Tc (i.e. λ) and the resistance ratio (i.e. λtr) follows. This understanding provides a new means to probe the phonon mechanism in superconductors, including MgB2. The merits of this method are its applicability to any superconductor and its reliability because the McMillan's electron-phonon coupling constant λ and λtr change in a broad range, from finite values to zero, due to weak localization. Karkin et al's preliminary data of irradiated MgB2 show the Testardi correlation, indicating that the dominant pairing mechanism in MgB2 is a phonon-mediated interaction.
Open Access
Phonon squeezing in superconducting borocarbides
(Elsevier BV * North-Holland, 1994) Hakioglu, T.; Ivanov, V. A.; Shumovsky, A. S.; Tanatar, Bilal
The recently discovered superconductor LuNi2B2C is investigated in the context of strong electron-electron correlations modulated by a squeezed phonon mode propagating in the perpendicular direction to the layers with longitudinal polarization. The squeezed phonons arise in the anharmonic lattice expansion since the linear electron-phonon interaction vanishes due to the structure of the NiB4 tetrahedra. The observed weak isotope effect and small dependence of Tc on pressure is qualitatively understood within the framework of this model. © 1994.
Open Access
Possibility of superconductivity of two-dimensional electrons on the surface of liquid heliuM, films
(Pergamon Press, 1993) Tanatar, Bilal; Hakioǧlu T.
We consider the possibility of superconductivity in a system of two-dimensional electrons on the surface of liquid helium films. Taking into account of the interaction between electrons and the surface excitations of liquid helium films-ripplons, within the weak coupling BCS theory, we estimate the superconducting transition temperature for various interaction strengths, film thicknesses, and electron densities. The superconducting transition temperature Tc, under experimentally realizable conditions, is calculated to be a few mK's. © 1993.
Open Access
Real-time imaging of vortex-antivortex annihilation in Bi 2Sr2CaCu2O8+δ single crystals by low temperature scanning hall probe microscopy
(IOP Institute of Physics Publishing, 2006) Dede, M.; Oral, A.; Yamamoto, T.; Kadowaki, K.; Shtrikman, H.
Vortices in superconductors play an important role in operating limits and applications of the superconductors. Scanning Hall probe microscopes have proven themselves to be quantitative and non-invasive tools for investigating magnetic samples down to 50 nm scale. Penetration of vortices in high quality single crystal Bi2Sr2CaCu2O8+δ superconductor has been studied in real-time with single vortex resolution at 77 K using a low temperature scanning Hall probe microscope (LT-SHPM). Vortices have been observed to be annihilated by the antivortices in small M-H loops.