Josephson effects in magnesium diboride based Josephson junctions

Abstract

We report on Josephson effects in Josephson junctions fabricated from magnesium diboride (MgB 2 ) thin films using a focused ion beam (FIB) milling technique. The films were deposited on SrTiO 3 (100) and Al 2 O 3 (1 102) substrates at room temperature using e-beam evaporation of MgB 2 with a post-annealing stage in Mg vapour. Conventional photolithography and Ar-ion-beam milling were used to pattern the films into 4 and 8 µm wide microbridges. A focused ion beam was used to narrow the microbridges to 2 µm. The narrowed microbridges were thinned by making 50 nm wide cuts across them. The depth of each cut was calibrated to remove 75% of the film thickness. A thin MgB 2 layer remained on the substrate after this process. The current-voltage (I -V ) characteristics of junctions made using this technique show that the junctions carry excess current. The first Shapiro step was observed when one of the junctions was irradiated with a microwave field of frequency f = 8.92 GHz. The Shapiro step appeared at a voltage value V = h f /2e = 18.445 µV.

Superconducting weak bonds at grain boundaries in MgB2

“…The third group of data, including the results obtained in [3,10,18,19], as well as our results, exhibits a satisfactory agreement between the experimental dependences I c (T) and the shape of the curve calculated for two-barrier SINIS structures in the limit of small values of the effective suppression parameter. The negative curvature of this curve is a consequence of the two-band nature of the MgB 2 compound.…”

“…5, the temperature dependences I c (T) of the critical current are compared with the available theoretical dependences calculated for tunnel SIS structures [1] and two-barrier SINIS structures [27] in the case of tunneling along the c axis. The same figure shows the experimental data obtained in structures with weak coupling at natural boundaries in MgB 2 [10] and in break junctions [4,5,8] in the structures formed by ion implantation [3], etching by a focused ion beam [2], and in tunnel junctions of MgB 2 /AlO x /MgB 2 [17] and MgB 2 /AlN/MgB 2 [18,19].…”

“…Comparison precisely with the shape of the I c (T) curve is necessitated by the following factors. The spatial distributions of the superconducting and normal current components in the structures considered here, as well as bicrystalline HTSC junctions, do not coincide as a Comparison of our temperature dependence of critical current (crosses) with the available theoretical dependences calculated for tunnel SIS structures [1] (dashed curve) and two-barrier SINIS structures [27] (solid curve) for tunneling along the c axis with the results obtained by other authors: [2], ᭝ [3], ᭡ [5], ᭺ [10], ᭹ [14,18,19], ٗ [8], ᭛ [17], and ଙ [4].…”

MgB2thin films

“…Several types of weak link junction that require only one MgB 2 layer have been investigated. One type of weak link is the nanobridges [144][145][146][147][148] formed in MgB 2 films by nanofabrication. For example, Brinkman et al used a focused ion beam to produce nanobridges with a width of about 70 nm, a height of 150 nm, and a length of 150 nm [144,148], and made SQUIDs using these weak link junctions which work well over 20 K [148].…”