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MgB2 Josephson devices
We have been studying superconductor tunneling in the novel two-band superconductor magnesium diboride (MgB2), with a view to explore MgB2 by spin-polarized tunneling and develop MgB2 Josephson devices. Besides quasiparticle tunneling into MgB2, we are actively developing MgB2/I/MgB2 Josephson tunneling devices. We showed unequivocally the presence of two superconducting energy gaps by planar S-I-S tunneling. Our preliminary studies have shown the Josephson effect in all MgB2 tunnel junctions. Currently micro- and nano-scale Josephson junctions and superconducting circuits based on Josephson junctions are under investigation.
When MgB2 showed superconductivity (SC) below 39K this field of superconductor got a major boost in fundamental physics as well application possibilities. MgB2 has the application potential for high speed superconductive electronics (SCE) with its larger coherence length compared to high TC superconductors and its higher operating temperature and device speed than Nb-based electronics. MgB2 superconductors can uniquely fill the void, and enjoy the advantages of conventional superconductors, those that follow Bardeen-Cooper-Shrieffer theory. Our results described below show clean tunneling characteristics for in situ prepared on Si (111) wafers MgB2 junctions with the measured superconducting (SC) energy gap and TC values in good agreement with theory and demonstrate the feasibility of the technology as well.
In the clean limit, two different SC order parameters have been predicted for MgB2 by Liu et al.: on two different parts of the Fermi surface leading to one SC energy gap for the two quasi two-dimensional sigma bands and another for the pair of three-dimensional pi bands. This unconventional SC behavior is due to the unusual character of the band structure in this material. The calculation by Choi et al. shows that the electronic states are dominated by orbitals in the boron plane coupling strongly to specific phonon modes giving rise to multiple gaps. These authors predicted SC energy gaps from 6.4 to 7.2 meV on the sigma band and from 1.2 to 3.7 meV on the pi band whereas Liu et al. showed for corresponding bands gap values of 7.2 and 2.4 meV.
The multiband picture of superconductivity in MgB2 has been studied to determine the SC energy gaps by various surface sensitive techniques such as probe tunneling, point contact Andreev reflection, high resolution angle resolved photoelectron spectroscopy, etc. In these experiments the measured SC energy gaps in MgB2 have varied. Ivarone using a scanning tunneling microscope (STM) observed two gaps, one at 2.3 meV (pi) and another at 7.1 meV (sigma) at 4.2K in polycrystalline bulk MgB2. The temperature dependence of both gaps followed BCS behavior, with reduced BCS parameter 2Delta/kTc of 1.4 and 4.3 for pi and sigma bands, respectively; superconductivity in MgB2 is strongly coupled in the sigma band and weakly coupled for the pi band. In recent photoemission experiments, in addition to the two SC energy gaps, there is also a surface state reported. Based on this data they concluded that it is a weakly coupled superconductor in both bands, contrary to the tunneling results. There have also been some reports on planar junctions showing MgB2 superconducting energy gap features. Leakage free planar tunneling junctions can provide the most reliable SC gap information in conventional superconductors, and this information is absent in MgB2 studies.
Our all-MgB2 thin film planar junctions are prepared in-situ using a molecular beam epitaxy (MBE) system with a base pressure better than 10-11 Torr. On the etched Si(111) substrate 5 nm of MgO seed layer was grown by e-beam evaporation at a substrate temperature of 563 K. This process was followed by the co-evaporation of Mg (Knudsen cell) and B (e-beam) to obtain MgB2 films with thickness of 56 nm. As it was determined by our previous studies, growth temperature (Ts), evaporation rate and Mg/B flux ratio (monitored by two independent quartz crystal sensors) are crucial keys to obtain high quality MgB2 films. After depositing the bottom MgB2 layer, the substrate cooled down to lower temperature (depends on the tunnel barrier) and 1-5 nm thick of tunnel barrier (MgO, Al2O3 and native oxide) was deposited on the MgB2 bottom layer. Subsequently, the substrate temperature was raised to 563 K for depositing the top MgB2 layer and the growth condition was kept the same as that of the bottom MgB2 layer. Finally, a 5 nm thick Au layer was deposited at room temperature to protect our all- MgB2 planar junctions. Following this process, we prepared small area of 10×20–64×64 mµ2 junctions by photolithography and Ar ion milling. For the defined junctions, the current versus voltage (I-V) measurements were taken using the four-terminal method in a temperature range from 4.2 to 1 K in a liquid helium bath.Our junctions are among the highest quality planar junctions yet reported. Further studies, in particular aimed at forming epitaxial tunnel barriers (e.g., MgO) and fully epitaxial tunnel junctions (e.g., MgB2/MgO/ MgB2, MgB2/MgO/Fe) are currently being carried out and our primarily results are promising.
Contact person: Dr. Helia Jalili
Tuning superconductivity with spin polarized current
Ferromagnetism is known to suppress the conventional s-wave superconductivity through the proximity effect. This is because the ferromagnetic exchange splitting energy, which prefers spin parallel alignment, is typically ~three orders of magnitude larger than the Cooper pairing energy, which prefers spin antiparallel alignment. We show that the superconducting state is tunable by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator) [5]. The superconducting transition temperature (TC) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. For an opaque ballistic interface, the supercurrent is significantly reduced, and the TC shift is attributed to the leakage of nonequilibrium spin carriers from the ferromagnets into the superconductors. For a clean interface, the Cooper pair mediated proximity effect also prevails. Superconductivity in a fully epitaxial bcc-Fe/V/Fe hybrid spin valve structures is influenced by the spin currents and supercurrents as well as band symmetry [6]. The transition temperature is spin orientation dependent in the presence of the proximity effect. A unique feature in this system is the band symmetry filtering taking place at the Fe/V interface. The absence of Δ2 Bloch states at the Fermi level in the Fe spin majority channel leads to spin selectivity and reduced transparency at the interface. Infinite magnetoresistance with clear remanence states is obtained, and implies the potential for spintronic applications.
[5] Miao, Yoon, Santos, Moodera, PRL 98, 267001 (2007).
[6] Miao, Ramos, Moodera, PRL 101, 137001 (2008).
(a) Superconducting spin valve effect in the structure (in nm): Si(100) / 10MgO / 6Fe / 40V / 6Fe / CoO. (b) Superconducting transition of the same sample in its spin P and AP configurations. Inset shows the thickness dependence of the SC spin valve effect, and an example of the MR loop with 50 nm V is also shown.
