Host Superconductor Research Articles

Superconductivity investigation of (Bi, Pb)-2223 added with different nanoferrites and their composites

In this study, the impact of different types of nanoferrites additions of Zn0.5Ni0.5Fe2O4 and Ba0.4Sr0.4Ca0.2Fe12O19 nanoparticles and their nanocomposite of (Zn0.5Ni0.5Fe2O4)0.5 (Ba0.4Sr0.4Ca0.2Fe12O19)0.5 on the structural and normal state resistivity of Bi1.6Pb0.4Sr2Ca2Cu3O10, ((Bi,Pb)-2223) superconducting phase were systematically investigated. The samples were synthesised using high purity oxide powders via a solid-state reaction approach. Based on the Rietveld analysis of x-ray diffraction (XRD) data, the tetragonal structure of (Bi,Pb)-2223 was established to be the predominant phase for all composite samples of the three nano-additions’ contents. However, a little change to the lattice parameters ( a and c ) was obtained. Furthermore, the small amounts (0.04 wt%) of Ba0.4Sr0.4Ca0.2Fe12O19 and (Zn0.5Ni0.5Fe2O4)0.5(Ba0.4Sr0.4Ca0.2Fe12O19)0.5 improved the development of the (Bi,Pb)-2223 phase. The volume fraction of the (Bi,Pb)-2223 phase increased from 86.47% to 88.73% and 88.69%, respectively. Then, it declined to 74.25% and 71.48% upon rising x to 0.40 wt%, respectively, as well as for Zn0.5Ni0.5Fe2O4 nanoparticles up to 0.40 wt%. SEM images verify the host superconducting matrix’s granular structure for all three nanoadditions up to 0.40 wt%. The existence of three nano additions in the host superconductor matrix is verified by employing energy dispersive x-ray (EDX) and x-ray photoelectron spectroscopy (XPS) analysis. The measurements for the resistivity dependency of temperature presented that, as compared to the pure sample (Tc = 108.67 K), the nano-(Zn0.5Ni0.5Fe2O4) x added in the (Bi,Pb)-2223 phase exhibits more negative impact than it is with the other two additions. In contrast, the nano-(Ba0.4Sr0.4Ca0.2Fe12O19) x has the highest value of Tc with a value of 110.95 K at x = 0.04 wt%. Furthermore, the superconducting transition width (∆Tc) improved for all composite samples except for the sample at x = 0.04 wt%, for Ba0.4Sr0.4Ca0.2Fe12O19 and (Zn0.5Ni0.5Fe2O4)0.5 (Ba0.4Sr0.4 Ca0.2Fe12O19)0.5 which showed the sharpest transition width. This suggests that their addition is anticipated to act as artificial pinning centers and strengthen the coupling between the grains in the (Bi,Pb)-2223 ceramic.

Fate of magnetic impurity induced states in a non-Hermitian s -wave superconductor

Recently, the non-Hermitian system has attracted great attention. However, the interplay between the non-Hermiticity and impurities in superconductors remains to be explored. In this work, we investigate the magnetic impurity chain on a three-dimensional non-Hermitian $s$-wave superconductor with the imbalanced pairing term. For the Yu-Shiba-Rusinov (YSR)-type state, we find that their fate is determined by the generalized in-gap nature of these states: These in-gap states and related topological phases survive only when the effective superconducting gap exists. With the increasing strength of the non-Hermitian part of the pairing term, the host superconductor is lead to a non-Hermitian gapless phase with exceptional surfaces, while these in-gap states and topological phases disappear. Our work generalizes the in-gap state nature of the YSR state to the non-Hermitian superconductor and reveals the relation between the exceptional surfaces and the YSR-type state, which provides a minimal starting point to explore the interplay between the non-Hermiticity and magnetic impurities in the $s$-wave superconductor.

Spectral properties of Andreev crystals

We present an exhaustive study of Andreev crystals (ACs) -- quasi-one-dimensional superconducting wires with a periodic distribution of magnetic regions. The exchange field in these regions is assumed to be much smaller than the Fermi energy. Hence, the transport through the magnetic region can be described within the quasiclassical approximation. In the first part of the paper, by assuming that the separation between the magnetic regions is larger than the coherence length, we derive the effective nearest-neighbour tight-binding equations for ACs with a helical magnetic configuration. The spectrum within the gap of the host superconductor shows a pair of energy-symmetric bands. By increasing the strength of the magnetic impurities in ferromagnetic ACs, these bands cross without interacting. However, in any other helical configuration, there is a value of the magnetic strength at which the bands touch each other, forming a Dirac point. Further increase of the magnetic strength leads to a system with an inverted gap. We study junctions between ACs with inverted spectrum and show that junctions between (anti)ferromagnetic ACs (always) never exhibit bound states at the interface. In the second part, we extend our analysis beyond the nearest-neighbour approximation by solving the Eilenberger equation for infinite and junctions between semi-infinite ACs with collinear magnetization. From the obtained quasiclassical Green functions, we compute the local density of states and the local spin polarization in anti- and ferromagnetic ACs. We show that these junctions may exhibit bound states at the interface and fractionalization of the surface spin polarization.

Signatures of two-dimensional superconductivity emerging within a three-dimensional host superconductor

Spatial disorder has been shown to drive two-dimensional (2D) superconductors to an insulating phase through a superconductor-insulator transition (SIT). Numerical calculations predict that with increasing disorder, emergent electronic granularity is expected in these materials-a phenomenon where superconducting (SC) domains on the scale of the material's coherence length are embedded in an insulating matrix and coherently coupled by Josephson tunneling. Here, we present spatially resolved scanning tunneling spectroscopy (STS) measurements of the three-dimensional (3D) superconductor BaPb1-x Bi x O3 (BPBO), which surprisingly demonstrate three key signatures of emergent electronic granularity, having only been previously conjectured and observed in 2D thin-film systems. These signatures include the observation of emergent SC domains on the scale of the coherence length, finite energy gap over all space, and strong enhancement of spatial anticorrelation between pairing amplitude and gap magnitude as the SIT is approached. These observations are suggestive of 2D SC behavior embedded within a conventional 3D s-wave host, an intriguing but still unexplained interdimensional phenomenon, which has been hinted at by previous experiments in which critical scaling exponents in the vicinity of a putative 3D quantum phase transition are consistent only with dimensionality d = 2.

Long-range focusing of magnetic bound states in superconducting lanthanum

Quantum mechanical systems with long-range interactions between quasiparticles provide a promising platform for coherent quantum information technology. Superconductors are a natural choice for solid-state based quantum devices, while magnetic impurities inside superconductors give rise to quasiparticle excitations of broken Cooper pairs that provide characteristic information about the host superconductor. Here, we reveal that magnetic impurities embedded below a superconducting La(0001) surface interact via quasiparticles extending to very large distances, up to several tens of nanometers. Using low-temperature scanning probe techniques, we observe the corresponding anisotropic and giant oscillations in the LDOS. Theoretical calculations indicate that the quasi-two-dimensional surface states with their strongly anisotropic Fermi surface play a crucial role for the focusing and long-range extension of the magnetic bound states. The quasiparticle focusing mechanism should facilitate the design of versatile magnetic structures with tunable and directed magnetic interactions over large distances, thereby paving the way toward the design of low-dimensional magnet–superconductor hybrid systems exhibiting topologically non-trivial quantum states as possible elements of quantum computation schemes based on Majorana quasiparticles.

Frequency and temperature dependent dielectric constant of (Ag)x/CuTl-1223 nanoparticles-superconductor composites

Polycrystalline superconducting CuTl-1223 samples were prepared by solid state reaction method. Synthesis of silver nanoparticles was done using the well known Sol-gel method. Silver nanoparticles with different concentrations i.e. x = 1, 2, 3 and 4 wt % were added to CuTl-1223 host superconductor to get (Ag)x/CuTl-1223 composites. XRD of composites shows that the added nanoparticles did not affect the crystal symmetry of CuTl-1223 phase which indicates that the nanoparticles were settled at the grain boundaries. The capacitance and conductance of the samples were measured at various frequencies (40 Hz- 100 MHz) and temperatures (77–253 K). The dielectric parameters such as real part (ԑr/) and imaginary part (ԑr//) of dielectric constant, dielectric loss (tanδ) and ac-conductivity (σac) were calculated from the measured capacitance and conductance. The parameters ԑr/, ԑr//and tanδ were reduced while σac was increased with the increasing frequency and reached saturation at higher frequencies. On the other hand ԑr/, ԑr//and tanδ were found to increase and σac suppressed with the increasing contents of Ag nanoparticles except for x = 2 wt%. The increase in the dielectric constant with the increasing x-contents may arise due to piling up of the free carriers (contributed by the conducting Ag nanoparticles) at the grain boundaries that play a key role in the space charge polarization.

Effective long-range pairing and hopping in topological nanowires weakly coupled to s -wave superconductors

In this Letter, we first formulate an effective theory, which generally captures long-range proximity effects of a surface system weakly coupled to an s-wave superconductor. The long-range proximity effects include both the emergent long-range pairing and hopping interactions in the surface system. We then model the Rashba spin-orbit-coupled nanowire in proximity with an s-wave superconductor by taking into account the emergent nonlocal effects in the weak-coupling limit. In this limit the induced superconducting pair potential is found much smaller than that of the host superconductor, which is in good agreement with recent experiments. Compared with the previously considered strong coupling limit with local proximity effects, the long-range interactions can significantly modify the topological phase diagram, and considerably lower the threshold magnetic field for the emergence of Majorana zero modes.

Field-Free Nucleation of Antivortices and Giant Vortices in Nonsuperconducting Materials.

Giant vortices with higher phase winding than 2π are usually energetically unfavorable, but geometric symmetry constraints on a superconductor in a magnetic field are known to stabilize such objects. Here, we show via microscopic calculations that giant vortices can appear in intrinsically nonsuperconducting materials, even without any applied magnetic field. The enabling mechanism is the proximity effect to a host superconductor where a current flows, and we also demonstrate that antivortices can appear in this setup. Our results open the possibility to study electrically controllable topological defects in unusual environments, which do not have to be exposed to magnetic fields or intrinsically superconducting, but instead display other types of order.

Zeroth order phase transition in a holographic superconductor with single impurity

We investigate the single normal impurity effect in a superconductor by the holographic method. When the size of impurity is much smaller than the host superconductor, we can reproduce the Anderson theorem, which states that a conventional s-wave superconductor is robust to a normal (non-magnetic) impurity with small impurity strength. However, by increasing the size of the impurity in a fixed-size host superconductor, we find a decreasing critical temperature Tc of the host superconductor, which agrees with the results in condensed matter literatures. More importantly, the phase transition at the critical impurity strength (or the critical temperature) is of zeroth order.

Strong localization of Majorana end States in chains of magnetic adatoms.

A recent experiment [Nadj-Perge etal, Science 346, 602 (2014)] gives possible evidence for Majorana bound states in chains of magnetic adatoms placed on a superconductor. While many features of the observed end states are naturally interpreted in terms of Majorana states, their strong localization remained puzzling. We consider a linear chain of Anderson impurities on a superconductor as a minimal model and treat it largely analytically within mean-field theory. We explore the phase diagram, the subgap excitation spectrum, and the Majorana wave functions. Owing to a strong velocity renormalization, the latter are localized on a scale which is parametrically small compared to the coherence length of the host superconductor.

Study of nano-sized (ZnFe2O4)y particles/CuTl-1223 superconductor composites

Zinc ferrite (ZnFe2O4)y nanoparticles/Cu0.5Tl0.5Ba2Ca2Cu3O10−δ (CuTl-1223) superconductor composites with y = 0–2 wt.% were prepared by adding ZnFe2O4 nanoparticles into CuTl-1223 superconductor matrix and characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transforms infrared (FTIR) spectroscopy and dc-resistivity (ρ) measurements. The bulk CuTl-1223 superconductor matrix was synthesized by solid-state reaction and Zinc ferrite (ZnFe2O4) nanoparticles were separately prepared by sol–gel method. XRD analysis revealed the tetragonal and spinel structure of CuTl-1223 superconductor and ZnFe2O4 nanoparticles, respectively. The XRD analysis showed that increased concentration of ZnFe2O4 nanoparticles doesn't disturb the tetragonal structure of host CuTl-1223 superconductor matrix and has no appreciable effect on its lattice parameters. The SEM images confirm the granular structure of the host superconductor matrix. The presence of ZnFe2O4 nanoparticles in host superconductor matrix is confirmed by using FTIR study. Variation of zero resistivity critical temperature {Tc (0)} depends upon the concentration of the nanoparticles in the host superconductor matrix. The overall suppression of Tc (0) and diamagnetism with increasing nanoparticles concentration is most probably due to trapping of mobile free carriers and reflection of spin charge due to presence of paramagnetic ZnFe2O4 nanoparticles. There is possibility for the incorporation of Fe and Zn in the lattice sites during the synthesis process, which may also cause the reduction of Tc (0) of the final composites.

Different behaviour of magnetic impurities in crystalline and amorphous states of superconductors

It has been observed 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 (KO), it is shown that as the system is getting disordered, the initial slope of the Tc depression is decreasing by a factor √ℓ/ξ0, when the mean free path ℓ becomes smaller than the BCS coherence length ξ0, which is in agreement with experimental findings. In addition, for a superconductor in a crystalline state in the presence of magnetic impurities the superconducting transition temperature Tc drops sharply from about 50% of Tc0 (for a pure system) to zero near the critical impurity concentration. This pure limit behaviour was indeed found by Roden and Zimmermeyer in crystalline Cd. Recently, Porto and Parpia have also found the same pure limit behaviour in superfluid He-3 in aerogel, which may be understood within the framework of the KO theory.

STM/STS observations of Co impurities in Bi 2.1Sr 1.8Ca(Cu 1− xCo x) 2O 8+ y single crystals

Cleaved surfaces of Bi 2.1Sr 1.8Ca(Cu 1− x Co x ) 2O 8+ y ( x=0.04) have been investigated by STM/STS at cryogenic temperatures. The obtained conductance image, which maps the local density of states (LDOS) near the Fermi level, clearly showed Co impurities present in the CuO 2 planes, indicating that the low-energy excitation spectrum is substantially affected by the impurity doping. The site-specified tunneling spectra on both the impurity site and host superconductor region at 66 K revealed a smeared gap-like structure. On the impurity sites, we have found nearly linear background extended up to a sample bias voltage of 1 V, while the background in the host superconductor region was rather flat. The present STM/STS results suggest that the scattering of electrons by magnetic impurities modifies the LDOS in an energy region scaled by J.

Vortex structure around a magnetic dot in planar superconductors.

The problem of the giant vortex state around a magnetic dot which is embedded in a superconducting film is investigated. The full non-linear, self-consistent Ginzburg-Landau equations are solved numerically in order to calculate the free energy, the order parameter of the host superconductor, the internal magnetic field due to the supercurrents, the corresponding current density, the magnetization probed in the vicinity of the dot, and the normal electron density as a function of the various parameters of the system. We find that, as we increase the magnetic moment of the dot, higher flux quanta vortex states become energetically more favorable, as they can better compete with the external magnetic field via the Meissner effect. In addition to that, they progressively become closer to each other in energy with direct experimental consequences, i.e. physical quantities like magnetization may fluctuate when measured, for example, as a function of a uniform external magnetic field.

Pressure evolution of the superconducting-normal metal phase boundary in superconductors containing high TK Kondo impurities

The pressure evolution of the superconducting-normal phase boundary is studied in the framework of the Matsuura, Ichinose and Nagaoka (MIN) theory for the Kondo effect in superconductors. The Eliashberg approach is used to obtain the normal and anomalous one-particle propagators of the host superconductor. It is found that in the high T K limit (i.e., ln( T K/ T c0) > 1 limit), the slope of pressure induced change in the T c of a superconductor containing the impurities is greater than the slope of the pressure induced change in the T c of the host superconductor, d T c0/d P and that it is possible to have a high T K alloy, normal at zero pressure, go superconducting as the pressure is increased.

Magnetic impurities in superconductors: A theory with different predictions.

The proclivity of paramagnetic solutes to degrade the transition temperature of a traditional singlet-pairing superconductor is studied anew. ${\mathit{T}}_{\mathit{c}}$ degradation is proportional to the (conduction electron) spin-disorder scattering rate, 1/${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$, caused by the solute spins. Accordingly the ${\mathit{T}}_{\mathit{c}}$ loss increases with solute concentration. The initial slope (versus 1/${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$) is found to depend on the superconductor and therefore is not the universal constant proposed by Abrikosov and Gor'kov. Instead the decrease is inversely proportional to \ensuremath{\lambda}, the electron-phonon coupling constant. Consequently a weak superconductor is doubly jeopardized by paramagnetic impurities: Its superconductivity is easily suppressed not only because ${\mathit{T}}_{\mathit{c}}$ is small to begin with, but also because the initial slope is steeper. Another unexpected consequence of the theory involves potential scattering which, acting alone, does not significantly influence ${\mathit{T}}_{\mathit{c}}$ (as surmised by Anderson). Nevertheless, the ${\mathit{T}}_{\mathit{c}}$ reduction caused by exchange scattering will be partially suppressed when the overall mean free path becomes smaller than the coherence length. This compensation has been demonstrated experimentally by comparing the influence of magnetic impurities in a pure host superconductor with that in a similar host having (also) nonmagnetic solutes. Such observed recovery of ${\mathit{T}}_{\mathit{c}}$, expected from this study, contradicts prior theories for magnetic solutes.

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Ca n?1Cu n O x

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi2Sr2Ca n−1Cu n O x (n = l, 2), have been studied through polarization-resolved Raman and129I Mossbauer spectroscopy. The polarization dependence of the Raman spectra and the Mossbauer measurement confirmed the dominant species to be triiodide ions, I 3 − , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO2 planes upon intercalation, by whichT c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi2Sr2Ca n−1Cu n O x systems.

Growth of precipitates in Bi2Sr2Ca1Cu2O8

In the formation of Bi 2 Sr 2 Ca 1 Cu 2 O 8 from a Ca and Cu rich melt, the partial pressure of oxygen is a central factor controlling the phases that form as precipitates in the host superconductor. At a high oxygen pressure, a sample with a stoichiometry of Bi 2 Sr 2 Ca 2 Cu 3 O 10 will form (Sr,Ca) 2 Cu 1 O 3 as the dominant precipitate coexisting with the liquid from which Bi 2 Sr 2 Ca 1 Cu 2 O 8 forms. During the crystallization of Bi 2 Sr 2 Ca 1 Cu 2 O 8 from this liquid, part of the (Sr,Ca) 2 Cu 1 O 3 phase is consumed. By contrast, at low oxygen partial pressure a sample with the same stoichiometry will form CaO as the dominant precipitate co-existing with the liquid from which B 2 Sr 2 Ca 1 Cu 2 O 8 forms. In this case, some of the CaO is consumed and a Ca rich (Sr,Ca) 2 Cu 1 O 3 phase forms during the Bi 2 Sr 2 Ca 1 Cu 2 O 8 crystallization. Oxygen partial pressure is a key factor controlling the sequence of precipitates that form in the host superconductor.

Twin boundary flux spinning in a high- Tc superconductor

The pinning potential due to a twin boundary in an anisotropic, continuous superconductor is calculated in a simple model. The model assumes the twin boundary to constitute a thin superconducting slab, different from and embedded in the host superconductor. Flux pinning is found to originate from two sources in several instances. One is an absolute minimum of the free energy. This type of pinning occurs when the twin boundary slab penetration length is larger than that of the host (weak link). The second pinning sources is a surface barrier (local minimum in the free energy) at the interface of two superconductors. This barrier depends, in certain limits, very strongly on the twin boundary thickness and the anisotropy of the host superconductor. The model yields an estimate for the critical current parallel to the twin boundary.

FORMATION OF THE "2223" SUPERCONDUCTING PHASE IN Bi4−xPbxSr3Ca3Cu4O16 SUPERCONDUCTORS

Formation of the "2223" superconducting phase in Pb doped Bi 4 Sr 3 Ca 3 Cu 4 O 16 superconductor is studied. The X-ray diffraction patterns of the Bi 4−x Pb x Sr 3 Ca 3 Cu 4 O 16 (0.7 < x < 1.2) superconductors formed by various heat treatments indicate that they are all multiphase systems containing both the high T c and low T c superconductors, with the volume fraction of the "2223" phase increasing with increased Pb content. The high temperature component have extrapolated T c 's of around 105 K. The zero resistivity temperature decrease with lead content but are well above the reported T c (75 K) of the Pb free host superconductor.