high-Tc Superconductor YBa2Cu3O7 Research Articles

Memristive effects in YBa2Cu3O7-x devices with transistor-like structure

Cuprate superconductors are highly sensitive to disorder and oxygen stoichiometry; even minute variations of those parameters drastically change their electronic properties. In this work, we exploit this characteristic to engineer a memristive device based on the high-TC superconductor YBa2Cu3O7−x (YBCO), in which local changes of the oxygen content and induced disorder are harnessed to produce memory effects. These effects are electrically triggered in a three-terminal device whose structure is reminiscent of a transistor, consisting of a YBCO channel and an Al gate. The Al/YBCO interface, which controls the gate conductance, displays a large, bipolar, reversible switching across a continuum of non-volatile conductance states spanning over two decades. This phenomenon is controlled by the gate voltage magnitude and is caused by the oxygen exchange between YBCO and Al. Concomitantly, the channel shows a gradual, irreversible superconductor-to-insulator transition that retains a memory of the power dissipated in the device and can be explained by induced bulk disorder. The observed effects, and the understanding of the interplay between the underlying mechanisms, constitute interesting ingredients for the design and realization of novel memristors and switches for superconducting electronics.

Evolution of the Structure and Magnetic Properties of the High-Tc Superconductor YBa2Cu3O6.92 in the Sol–Gel Synthesis Process

Evolution of the Structure and Magnetic Properties of the High-Tc Superconductor YBa2Cu3O6.92 in the Sol–Gel Synthesis Process

Memory-functionality superconductor/ferromagnet/superconductor junctions based on the high- Tc cuprate superconductors YBa2Cu3O7−x and the colossal magnetoresistive manganite ferromagnets La2/3X1/3MnO3+δ(X=Ca,Sr)

Complex oxides exhibit a variety of unusual physical properties, which can be used for designing novel electronic devices. Here we fabricate and study experimentally nano-scale Superconductor/ Ferromagnet/Superconductor junctions with the high-Tc cuprate superconductor YBa2Cu3O7 and the colossal magnetoresistive (CMR) manganite ferromagnets LaXMnO3 (X: Ca or Sr). We demonstrate that in a broad temperature range the magnetization of a manganite nanoparticle, forming the junction interface, switches abruptly in a mono-domain manner. The CMR phenomenon translates the magnetization loop into a hysteretic magnetoresistance loop. The latter facilitates a memory functionality of such a junction with just a single CMR ferromagnetic layer. The orientation of the magnetization (stored information) can be read out by simply measuring the junction resistance in an applied magnetic field. The CMR facilitates a large read-out signal in a small applied field. We argue that such a simple single layer CMR junction can operate as a memory cell both in the superconducting state at cryogenic temperatures and in the normal state up to room temperature.

Broken rotational symmetry on the Fermi surface of a high-Tc superconductor

Broken fourfold rotational (C4) symmetry is observed in the experimental properties of several classes of unconventional superconductors. It has been proposed that this symmetry breaking is important for superconducting pairing in these materials, but in the high-Tc cuprates this broken symmetry has never been observed on the Fermi surface. Here we report a pronounced anisotropy in the angle dependence of the interlayer magnetoresistance of the underdoped high transition temperature (high-Tc) superconductor YBa2Cu3O6.58, directly revealing broken C4 symmetry on the Fermi surface. Moreover, we demonstrate that this Fermi surface has C2 symmetry of the type produced by a uniaxial or anisotropic density-wave phase. This establishes the central role of C4 symmetry breaking in the Fermi surface reconstruction of YBa2Cu3O6+δ, and suggests a striking degree of universality among unconventional superconductors.

Effect of magnetic field on the photovoltaic properties of YBa2Cu3O6.96/Ag heterojunction

The obvious photovoltaic effect (Voc ∼ 30 μV) induced by purple-laser illumination at high Tc superconductor YBa2Cu3O6.96/Ag (YBCO/Ag) heterojunction has been observed, revealing that there exists an electrical field across the YBCO/Ag interface. It has been found that magnetic field can dramatically change the photo-induced voltage in the vicinity of superconducting transition. With increasing magnetic fields up to 3 T, the photovoltage at 74 K and 30 mW/mm2 is reduced from 15 μV to zero and then reaches −15 μV. The polarity of the voltage can be switched by applying an external magnetic field, as well as by varying the laser intensity. Our results can be understood in terms of the magnetic vortex penetrating in high Tc superconductors and provide strong evidence for the existence of an interface electrical field in the superconductor/metal heterojunction.

Superconductor to Mott insulator transition in YBa2Cu3O7/LaCaMnO3 heterostructures.

The superconductor-to-insulator transition (SIT) induced by means such as external magnetic fields, disorder or spatial confinement is a vivid illustration of a quantum phase transition dramatically affecting the superconducting order parameter. In pursuit of a new realization of the SIT by interfacial charge transfer, we developed extremely thin superlattices composed of high Tc superconductor YBa2Cu3O7 (YBCO) and colossal magnetoresistance ferromagnet La0.67Ca0.33MnO3 (LCMO). By using linearly polarized resonant X-ray absorption spectroscopy and magnetic circular dichroism, combined with hard X-ray photoelectron spectroscopy, we derived a complete picture of the interfacial carrier doping in cuprate and manganite atomic layers, leading to the transition from superconducting to an unusual Mott insulating state emerging with the increase of LCMO layer thickness. In addition, contrary to the common perception that only transition metal ions may respond to the charge transfer process, we found that charge is also actively compensated by rare-earth and alkaline-earth metal ions of the interface. Such deterministic control of Tc by pure electronic doping without any hindering effects of chemical substitution is another promising route to disentangle the role of disorder on the pseudo-gap and charge density wave phases of underdoped cuprates.

Coherent acoustic phonons in YBa2Cu3O7/La1/3Ca2/3MnO3 superlattices

We investigate photo-induced coherent acoustic phonons in complex oxide superlattices consisting of high-Tc superconductor YBa2Cu3O7−x and ferromagnetic manganite La1/3Ca2/3MnO3 epitaxial layers with broadband pump-probe spectroscopy. Two oscillatory components have been observed in time-resolved differential reflectivity spectra. Based on the analysis, the slow oscillation mode with a frequency sensitive to the probe wavelength is ascribed to the stimulated Brillouin scattering due to the photon reflection by propagating train of coherent phonons. The fast oscillation mode with a probe-wavelength-insensitive frequency is attributed to the Bragg oscillations caused by specular phonon reflections at oxide interfaces or the electron-coupling induced modulation due to free carrier absorption in the metallic superlattices. Our findings suggest that oxide superlattice is an ideal system to tailor the coherent behaviors of acoustic phonons and to manipulate the thermal and acoustic properties.

Preparation of a ferromagnetic barrier in YBa2Cu3O7–δ thinner than the coherence length

Thin ferromagnetic La2∕3Ca1∕3MnO3 layers are deposited in between two layers of the high-Tc superconductor YBa2Cu3O7–δ (YBCO) by pulsed laser deposition. Using a tailored buffer layer allows the change of the orientation of the CuO2 planes inside the superconducting layers. With this technique it is possible to produce ferromagnetic barrier layers with high ferromagnetic transition temperatures that are thinner than the coherence length of the adjacent superconductors. Magnetization and electric transport measurements suggest that this geometry might be successful for the generation of ferromagnetic Josephson junctions in high temperature superconducting YBCO.

Fluctuation Hall Conductivity Beyond Linear Response in Layered Superconductors Under a Magnetic Field

We calculate fluctuation Hall conductivity of a strongly type-II superconductor in strong electric fields by using the time dependent Ginzburg–Landau approach. Thermal fluctuations, represented by the Langevin white noise, are assumed to be strong enough to melt the Abrikosov vortex lattice created by the magnetic field into a moving vortex liquid. The layered structure of the superconductor is accounted for by means of the Lawrence–Doniach model. The nonlinear interaction term in dynamics is treated within self-consistent Gaussian approximation and we go beyond the often used lowest Landau level approximation to treat arbitrary magnetic fields. The results are compared to experimental data on high-Tc superconductor YBa2Cu3O7−δ.

NMR in pulsed high-field magnets and application to high-TC superconductors

This article deals with the implementation of Nuclear Magnetic Resonance (NMR) experiments in pulsed magnetic fields at the pulsed-field facility of the Laboratoire National des Champs Magnétiques Intenses and its application to the high-TC superconductor YBa2Cu3O6.51. The experimental setup is described in detail, including a low-temperature probe head adapted for pulsed fields. An entire paragraph is dedicated to the discussion of NMR in pulsed field and the introduction of an advanced deconvolution technique making use of the induction voltage in an additional pick-up coil. The 63Cu/65Cu NMR experiments on an YBa2Cu3O6.51 single crystal were performed at 2.5K during a field pulse of 46.8-T-amplitude. In the recorded spectrum the 63Cu center line and high-frequency satellites as well as the 65Cu center line are identified and are compared with results in literature.

Ac fluctuation resistivity in high-Tc superconductor

We use the time-dependent Ginzburg–Landau to calculate ac fluctuation conductivity and resistivity in type-II superconductor in 3D model under magnetic field. Thermal fluctuations are assumed to be strong enough to melt the Abrikosov vortex lattice created by the magnetic field into a vibrating vortex liquid and marginalize the effects of the vortex pinning by inhomogeneities. The nonlinear interaction term in the time-dependent Ginzburg–Landau is treated within self-consistent Gaussian approximation. We obtain expressions the complex conductivity and resistivity summing all Landau levels which are applicable essentially to whole liquid phase and are compared to experimental data on high-Tc superconductor YBa2Cu3 O7−δ.

Band Structure and Quantum Oscillations in YBa2Cu3O7: A Local Spin Density Approximation with the On-Site Coulomb Interaction Study

First-principles calculations have been performed on the Bravais lattice, the density of states (DOS), the band structure (BS) and the Fermi Surface (FS) topology for high-Tc superconductor YBa2Cu3O7. We used the method of the Full-Potential non-orthogonal Local-Orbital Minimum-Basis Band-structure Scheme (FPLO) [in the local spin density approximation with the on-site Coulomb interaction (LSDA+U) and the coherent potential approximation (CPA)] coupled with the XFSF program. Our FPLO Fermi surface consists of four large quasi-2D cylinders centered on the Brillouin zone corners with mostly CuO2 plane character and two quasi-1D sheets with mostly Cu–O apex (or apical O) chain character. The main feature of this Fermi surface is that each CuO chain-sheet shows up in its center a small tubular pocket with a funnel-like shape (Fermi pockets) with mainly Ba–O apex character which could give rise to the quantum oscillations observed by Doiron-Leyraud et al. (Nature 447: 565, 2007). Our FPLO band structure (BS) shows that three flat bands cross the Fermi level (FL) which suggests that the two Fermi pockets could be produced by the conjunction near the Fermi level of two flat bands arising from the apex oxygen (BaO layer). This is consistent with the density of states (DOS) where the Fermi level is dominated by the two O apex (BaO) peaks (∼90 %). Moreover, the strong narrow peak exhibited in the DOS just below the Fermi level (FL) corresponds to the van Hove singularity (VHS) and originates from the hybridization of Cu 3d and apex O2p (BaO) orbitals. In other hand, the calculated value of the bare band-structure Sommerfeld constant γ0band=10.60 mJ/(mol K2) corresponds to a very high Fermi level density of states N(EF)=4.72 states/(eV×cell) and the electron–phonon coupling constant λ is found to be about 1.64 which implies that YBa2Cu3O7 compound is a strong-coupling superconductor and the cyclotron effective mass m∗ is enhanced in comparison with the band mass mb [m∗=(1+λ)mb]. Also, the discrepancy between our FPLO calculated Debye temperature (274.15 K) and that obtained experimentally (437 K) may originate from a phonon anisotropy likely generated by the antiferromagnetic spin fluctuations. Our results therefore provide evidence of changes in the topology of the Fermi surface materialized by the Ba–O apex small pockets formed in the center of the Cu–O apex Fermi surface sheets.

Reverse Monte Carlo study of Cu—O bond distortions in YBa2Cu3O6.9

A combination of neutron total scattering measäurement and reverse Monte Carlo (RMC) refinement is applied to the study of apical Cu—O bond distortions in the high-Tc superconductor YBa2Cu3O6.93. We show that the average structure is not consistent with a split-site model for the corresponding Cu and O positions, but that the local structure nevertheless reveals the existence of two separate apical Cu—O bond lengths. Using G(r) data obtained from a variety of Qmax values we show that this result is independent of the data treatment methodology. We also find that the resulting ‘short’ and ‘long’ Cu—O bond lengths agree well with the results of previous äEXAFS studies. The existence of bimodal apical Cu—O bond distributions in the context of a single-site average structure model is interpreted in terms of correlated displacements of the Cu and O atoms. We investigate the possibility of clustering of short apical Cu—O bonds within our RMC configurations.

Increased Tc in Electrolyte‐Gated Cuprates

Field-effect transistors built using the high superconducting transition temperature (high-Tc) superconductor YBa2Cu3O7–x as the active conductor and electrolyte as the gate dielectric (see figure) show substantial and relatively robust modulation of conductivity with large increases in Tc of up to 38 K.

Anomalous magnetic field dependence ofY Ba2Cu3O7 − δ: effect of orthorhombic distortion

In this work an attempt has been made to ascertain the originof the mixed symmetry state of the order parameters in the high-Tc superconductorY Ba2Cu3O7 − δ, with the contesting candidates being the orthorhombic distortion associated with theCuO2 planesand an s-wave order parameter component originating from the CuO chains present in thesystem. Due to its applicability over the entire range of applied magnetic fieldHc1 < H < Hc2 the present theoretical model has been employed to study various unusualproperties which are signatures of the unconventional pairing state symmetryof this material. The results presented in this paper and their agreement withthe relevant experimental data suggest that the origin of the mixed symmetrystate of pairing and the various experimentally observed unusual properties ofY Ba2Cu3O7 − δ includingthe anomalous magnetic field dependence of the vortex lattice structure, unusual upward curvature ofthe Hc2(T) plot, etc, can be attributed to the orthorhombic distortion of theCuO2 planes present in this material. However, the effect discussed in the present work pertainsto the orthorhombic distortion arising only through a second-order coupling; the distortionarising from other possible sources, namely anisotropic mixed gradient coupling, has notbeen taken into account here. Hence this discussion is restricted to a qualitativeexplanation of the effect due to orthorhombic distortion arising from the second-ordercoupling only.

The comparative study of magnetic properties for YBCO coated conductors based on RABiTS and IBAD templates

The magnetic properties of high Tc superconductor (HTS) YBa2Cu3O7−δ (YBCO) coated conductors (CCs) which are based on RABiTS (rolling-assisted biaxially textured substrate) and IBAD (ion-beam-assisted deposition) templates were investigated. The measurement of isothermal magnetizations M(H) for the prepared YBCO-CCs was carried out at 77K and 100K in magnetic fields up to 3000G. The magnetic properties for the applied magnetic fields parallel to the ab-plane are much smaller than that perpendicular to the ab-plane. Large magnetic property in the YBCO-CC based on the RABiTS template comes from the strong ferromagnetism of the textured Ni–W alloy substrate.

Difficulty in the Fermi-Liquid-Type Theory for the In-Plane Magnetic Anisotropy of UntwinnedYBa2Cu3O6.85Superconductors

Recently, Eremin and Manske [1] presented a oneband Fermi-liquid theory for the in-plane magnetic anisotropy in untwinned high-Tc superconductor YBa2Cu3O6:85 (YBCO). They claimed that they found good agreement with inelastic neutron scattering (INS) spectra. In this Comment, we point out that their conclusion on this important problem may be questionable due to an error in logic about the orthorhombicity delta_0 characterizing the lattice structure of YBCO. In Ref. [1], a single band at delta_0>0 is proved to be in accordance with the angle resolved photoemission spectroscopy (ARPES) on untwinned YBCO. But in their Erratum in PRL[3], they admit that delta_0= -0.03 was used to fit the INS data. Hence publications [1,3] contain errors that we believe invalidate their approach.

Substrate Contribution to the Surface Impedance of HTS Films on Si

We study the effects of semiconductor substrates on the surface impedance of high-T c Superconductor (HTS) YBa2Cu3O7−δ (YBCO) films. The characteristic impedance of silicon (Si) (for different doping levels and for different charge carrier scattering times) is evaluated. In particular, the most relevant features of Si electrodynamics are highlighted by the introduction of suitable normalized quantities. The effective surface impedance of the YBCO films on Si substrates is then calculated and discussed for different temperatures and frequencies in the microwave range, comparing the obtained results to their limiting expressions for bulk and thin-film HTS. Our analysis shows how the widely used thin-film approximation for the surface impedance can fail, critically highlighting the conditions it requires to be correctly used. We show that substrate contributions can heavily influence the overall response.

Features of vortex pinning and magnetic flux creep in epitaxial thin films of high-Tc superconductor YBa2Cu3O7−δ near the critical temperature

The temperature dependence of the dynamic relaxation rate in YBa2Cu3O7−δ epitaxial thin films is investigated in the temperature range from 77K to the critical temperature Tc with and without an applied dc magnetic field, and the dependence on the dc field at 77K is measured at fields up to 45mT. It is shown that the experimental results are in good agreement with the vortex lattice pinning model proposed previously which considers the main pinning centers in thin films to be threading dislocations on a network of low-angle grain boundaries. From the results of the experiment it is concluded that the influence of thermal fluctuations on the vortex pinning by threading dislocations and on the magnetic flux creep near Tc is not so crucial as in the case of pinning by extended linear defects in thick films or single crystals. Estimates are made which show that this fact can be explained by a transformation of the Abrikosov vortices into Pearl vortices when the magnetic field penetration depth becomes greater than the film thickness as the temperature approaches a critical value. Because of this, the mechanisms of pinning and flux creep in thin films are of a substantially collective character even in extremely weak magnetic fields and at temperatures very close to Tc.

Supercurrent density above 106A∕cm2 at 77K in a single-crystal film conductor of the cuprate high-Tc superconductor YBa2Cu3O7−δ—dream or reality?

Supercurrent transport phenomena in c-oriented epitaxial thin films of the the high-Tc superconducting (HTS) cuprate YBa2Cu3O7−δ (YBCO) with a high critical current density Jc(77K)⩾2×106A∕cm2 are investigated by four-probe transport measurements, low-frequency magnetic susceptibility studies, and SQUID magnetometry. The film samples are deposited on a single-crystal sapphire (r-cut) substrates with a CeO2 or LaAlO3(100) buffer layer by off-axis dc magnetron sputtering or pulsed laser ablation. A model of the mechanisms of Abrikosov vortex pinning and supercurrent limitation is developed and discussed by comparing its predictions with the results of measurements of the critical current and its dependence on applied magnetic fields of different strength and orientation and also with nanostructure data obtained by high-resolution transmission electron microscopy and electron diffraction in a backscattering geometry. It is shown that the low-angle subboundaries (LABs) formed between domains with a slight azimuthal misorientation during the epitaxial growth of the film play a key role in the phenomena observed in the transport of supercurrent. The tilt LABs form equidistant ordered rows of edge dislocations with nonsuperconducting cores about 3–4nm in diameter. The dislocation lines in the LABs are parallel to each other and perpendicular to the film plane. The average density of dislocations over the area of the film depends on the real statistics of the random system of LABs and can reach 1011cm−2. Since the diameter of the “normal” core of a dislocation is close to the diameter of the core of an Abrikosov vortex, the elementary pinning force of the vortex to the core of the dislocation is close to the maximum possible. The pinning on dislocation subboundaries has the following characteristics: 1) the achievement of high values Jc(77K)⩾2×106A∕cm2 and Jc(20K)&amp;gt;107A∕cm2 in epitaxial films and conductors; 2) the existence of a “plateau” on the Jc(H) curve, i.e., Jc(H)=const for H&amp;lt;Hm; 3) a logarithmic decline of Jc(H) for H&amp;gt;Hm, i.e., at the transition from the single-particle pinning regime to the collective pinning of the vortex lattice on the statistical ensemble of randomly distributed dislocation subboundaries; 4) the existence of a threshold field Hp that determines the limit up to which the vortices in a thin film (d⩽λ) remain rectilinear and perpendicular to the film even in a field inclined at a large angle; 5) the evolution of the angle dependence of Jc(θ) with a change of field strength is in complete agreement with the model of dominant pinning on “threading” edge dislocations. A new “peak effect”—an increase of Jc(H‖ab) with increasing longitudinal field—is observed for the first time for H&amp;gt;Hm, i.e, after the end of the “plateau” Jc(H‖ab)=const.