ISS2011 Development of iron-based superconducting devices (original) (raw)
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Physica C: Superconductivity, 2012
To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS') thin film junctions with BaFe 1.8 Co 0.2 As 2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. Temperature dependent measurements of the electrical conductivity were strongly influenced by the properties of the electrodes. We developed a junction structure that allows us to characterize the electrodes, too, including the behavior of their normal state resistance in order to correct their influences on the conduction spectra. The corrected conductivity of the junction was described within an extended BTK-model and shows a behavior dominated by Andreev reflexion.
Theory of Andreev reflection in junctions with iron-based high-Tc superconductors
Physical Review B, 2009
We construct a theory for low-energy quantum transport in normal|superconductor junctions involving the recently discovered iron-based high-Tc superconductors. We properly take into account both Andreev bound surface states and the complex Fermi surface topology in our approach, and investigate the signatures of the possible order parameter symmetries for the FeAs-lattice. Our results could be helpful in determining the symmetry of the superconducting state in the iron-pnicitide superconductors. PACS numbers: 74.20.Rp, 74.50.+r, 74.70.Dd Introduction. Very recently, a family of iron-based superconductors with high transition temperatures was discovered, with a concomitant avalanche of both experimental and theoretical activity. . The highest T c measured so far in this class of materials is 55 K, and many experimental reports indicate signatures of unconventional superconducting pairing. However, it remains to be clarified what the exact symmetry is for both the orbital-and spin-part of the Cooper pair wavefunction -there has for instance been reports of both nodal 11 and fully gapped 10 order parameters (OPs) in the literature up to now.
Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes
Journal of Superconductivity and Novel Magnetism, 2014
We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2 As 2 (Ba-122) regarding its superconducting properties and possible applications. Therefore Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and ab-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I-V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I-V characteristics and I c R n -T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I c R n product of the interface-engineered barrier junction was 12 µV and the planar junction with Au barrier showed 18 µV, we could increase the I c R n to 90 µV for planar TiO x barrier junctions.
Observation of the Josephson Effect in Pb/Ba1-xKxFe2As2 Single Crystal Junctions
Physical Review Letters, 2009
We have fabricated c-axis Josephson junctions on single crystals of Ba 1-x K x Fe 2 As 2 by using Pb as the counter electrode in two geometries, planar and point contact. Junctions in both geometries show resistively shunted junction I-V curves below the T C of the counter electrode. Microwave induced steps were observed in the I-V curves, and the critical currents are suppressed with an in-plane magnetic field with well-defined modulation periods indicating that the Josephson current is flowing in a manner consistent with the small to intermediate sized junction limit. I C R N products of up to 0.3 mV have been observed in these junctions at 4.2 K. The observation of Josephson coupling along the c-axis between Ba 1-x K x Fe 2 As 2 and a conventional 2 superconductor suggests the existence of a s-wave symmetry in this class of iron pnictide superconductors.
Physical Review Letters, 2010
Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1−xCox)2As2 single crystals (Tc=24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The PCAR spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Ω b (T ) is obtained, very similar to the spinresonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s± Eliashberg model by using an electron-boson spectral function peaked at Ω0 = 12 meV ≃ Ω b (0). PACS numbers: 74.50.+r , 74.70.Dd, 74.45.+c The discovery of the first class of non-cuprate, Febased high-temperature superconductors in 2008 brought great excitement in the scientific community [1]. The phase diagram of these compounds (although still imperfectly known) looks similar to that of copper-oxide superconductors [2] and, as in cuprates, superconductivity emerges "in the vicinity" of a magnetic parent compound. The electron-phonon interaction seems not to be sufficient [3] to explain their high T c (up to 55 K [4]) even by considering a magnetic ground state . A spinfluctuation-mediated pairing mechanism has been early proposed instead, which predicts the occurrence of a sign change of the order parameter on different sheets of the Fermi surface (s±-symmetry) . This picture is naturally based on the proximity of the superconducting phase to a magnetic one, on the existence of disconnected Fermi surface (FS) sheets, and on the multiband character of superconductivity in these compounds, which are nowadays almost universally accepted . The s± model itself is strongly supported by various experimental results [8] which indicate the existence of multiple nodeless gaps on different sheets of the FS, although the possible emergence of gap nodes in some systems, along certain directions or in particular conditions [9, 10] is still debated. The role of spin fluctuations (SF) in the pairing has also found support in neutron scattering experiments that have revealed a spin resonance energy which scales linearly with T c [2]. Finally, it has been recently shown that a multiband s± Eliashberg model can reproduce several experimental quantities (such as gaps, T c , kinks in the band dispersion and effective masses ) by assuming that the mediating boson has a characteristic energy similar to the spin-resonance one. In this paper we report on directional PCAR measurements on high-quality single crystals of the e-doped 122 compound BaFe 1.8 Co 0.2 As 2 . The results prove the existence of two superconducting gaps with no line nodes on the FS, and whose amplitude is almost the same in the ab plane or along the c axis. The PCAR spectra also present structures that can be related to a strong electron-boson interaction (EBI). The characteristic energy Ω b of the mediating boson extracted from the PCAR curves decreases with temperature and is very similar to the resonance energy of the spin excitation spectrum . Moreover, both the gaps and the additional EBI structures in the PCAR spectra can be reproduced within an effective three-band s± wave Eliashberg model using a boson energy Ω 0 = 12 meV ≃ Ω b (0). All these results strongly support a spin-fluctuation-mediated mechanism for superconductivity in this compound. The BaFe 1.8 Co 0.2 As 2 (10% Co) single crystals were prepared by the self-flux method under a pressure of 280 MPa at the National High Magnetic Field Laboratory in Tallahassee. The typical crystal sizes are ≈ 1 × 1 × 0.1 mm 3 . The onset of the resistive transition is T on c = 24.5 K with ∆T c (10%-90%) = 1 K (see inset to ). Instead of using the standard technique where a sharp metallic tip is pressed against the material under study, the point contacts were made by putting a small drop of Ag paste on a fresh surface exposed by breaking the crystal. Contacts made in this way are very stable and the differential conductance curves, obtained by numerical differentiation of the I-V characteristics, can be recorded up to ≈ 200 K . As an example, shows the raw conductance curves, recorded up to 180 K, of a Ag/BaFe 1.8 Co 0.2 As 2 point contact (R N = 25 Ω) with current injection along the c axis ("c-axis contact"). The clear signatures of AR in the low-T curve and the absence of heating effects or dips indicate ballistic conduction through the point contact, so that energy-resolved
Phase-Sensitive measurements on the corner junction of iron-based superconductor BaFe1.8Co0.2As2
2008
We have made a phase-sensitive measurement on the corner junction of the iron-based superconductor BaFe1.8Co0.2As2, and observed the typical Fraunhofer-like diffraction pattern. The result suggests that there is no phase shift between the a-c face and b-c face of a crystal, which indicates that the superconducting wavefunction of the iron based superconductor is different from that of a cuprate superconductor.
Conductance asymmetry in point-contacts on epitaxial thin films of Ba(Fe0.92Co0.08)2As2
Applied Physics Letters, 2010
One of the most common observations in point-contact spectra on the recently discovered ferropnictide superconductors is a large conductance asymmetry with respect to voltage across the point-contact. In this paper we show that the antisymmetric part of the point-contact spectrum between a silver tip and an epitaxial thin film of Ba(Fe0.92Co0.08)2As2 shows certain unique features that have an interesting evolution with increasing temperature up to a temperature far above the critical temperature Tc. We associate this observation with the existence of a gap above Tc that might originate from strong fluctuations of the phase of the superconducting order parameter.
Preparation of hybrid Josephson junctions on Co-doped Ba-122 single crystals
Superconductor Science and Technology, 2014
In this paper we present a method for processing a hybrid Josephson junction on Co-doped BaFe 2 As 2 (Ba-122) single crystals with a thin film Pb-counter electrode and a barrier layer of TiO x. This includes the leveling and polishing of the crystals and structuring them with thin film techniques such as photo lithography, sputtering and ion beam etching (IBE). The junctions show hysteretical resistively and capacitively shunted junction (RCSJ)-like I-V characteristics with an I c R n-product of about 800 µV.
Influence of topological edge states on the properties ofAl/Bi2Se3/Alhybrid Josephson devices
Physical Review B, 2014
In superconductor-topological insulator-superconductor hybrid junctions, the barrier edge states are expected to be protected against backscattering, to generate unconventional proximity effects, and, possibly, to signal the presence of Majorana fermions. The standards of proximity modes for these types of structures have to be settled for a neat identification of possible new entities. Through a systematic and complete set of measurements of the Josephson properties we find evidence of ballistic transport in coplanar Al-Bi2Se3-Al junctions that we attribute to a coherent transport through the topological edge state. The shunting effect of the bulk only influences the normal transport. This behavior, which can be considered to some extent universal, is fairly independent of the specific features of superconducting electrodes. A comparative study of Shubnikov-de Haas oscillations and Scanning Tunneling Spectroscopy gave an experimental signature compatible with a two dimensional electron transport channel with a Dirac dispersion relation. A reduction of the size of the Bi2Se3 flakes to the nanoscale is an unavoidable step to drive Josephson junctions in the proper regime to detect possible distinctive features of Majorana fermions.
Point-contact Andreev reflection spectroscopy of heavy-fermion-metal/superconductor junctions
Physica B-condensed Matter, 2008
Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn5 using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance. To explore their physical origins, we have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn5, CeRhIn5, and YbAl3, each with different electron mass. In contrast with Au/CeCoIn5 junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed.