We will report advances in superconducting devices and circuits made usingbi-epitaxial grain boundary Josephson junctions in YBa2Cu3O7. These bi-epitaxial grainboundary junctions are created at predetermined locations by controlling the in-planeepitaxy of YBa2Cu3O7 with a seed layer and a buffer layer on a base layer. SQUIDs madefrom these junctions show modulation up to 88 K and their white noise level is at the
We will report advances in superconducting devices and circuits made usingbi-epitaxial grain boundary Josephson junctions in YBa2Cu3O7. These bi-epitaxial grainboundary junctions are created at predetermined locations by controlling the in-planeepitaxy of YBa2Cu3O7 with a seed layer and a buffer layer on a base layer. SQUIDs madefrom these junctions show modulation up to 88 K and their white noise level is at the
We present various concepts and experimental procedures to produce biepitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ grain-boundary Josephson junctions. The device properties have an interesting phenomenology, related in part to the possible influence of $\mathrm{``}\ensuremath{\pi}$ loops.'' The performance of our junctions and superconducting quantum interference devices indicates significant improvement in the biepitaxial technique. Further, we propose methods for fabricating circuits in which 0 and $\ensuremath{\pi}$ loops are controllably located on the same chip.
YBa2Cu3O7 24∘ (30∘) bicrystal grain boundary junctions (GBJs), shunted with 60\,nm (20\,nm) thick Au, were fabricated by focused ion beam milling with widths 80nm≤w≤7.8μm. At 4.2\,K we find critical current densities jc in the 105A/cm2 range %\dkc{\#1} (without a clear dependence on w) and an increase in resistance times junction area ρ with an approximate scaling ρ∝w1/2. For the narrowest GBJs jcρ≈100μV, which is promising for the realization of sensitive nanoSQUIDs for the detection of small spin systems. We demonstrate that our fabrication process allows the realization of sensitive nanoscale dc SQUIDs; for a SQUID with w≈100\,nm wide GBJs we find an rms magnetic flux noise spectral density of SΦ1/2≈4μΦ0/Hz1/2 in the white noise limit. We also derive an expression for the spin sensitivity Sμ1/2, which depends on SΦ1/2, on the location and orientation of the magnetic moment of a magnetic particle to be detected by the SQUID, and on the SQUID geometry. For the not optimized SQUIDs presented here, we estimate Sμ1/2=390μB/Hz, which could be further improved by at least an order of magnitude.
NanoSQUIDs from YBa2Cu3O7/SrTiO3 superlattices with bicrystal grain boundary Josephson junctions
We report on the fabrication and characterization of nanopatterned dc SQUIDs with grain boundary Josephson junctions based on heteroepitaxially grown YBa2Cu3O7 (YBCO)/ SiTrO3 (STO) superlattices on STO bicrystal substrates. Nanopatterning is performed by Ga focused-ion-beam milling. The electric transport properties and thermal white flux noise of superlattice nanoSQUIDs are comparable to single layer YBCO devices on STO bicrystals. However, we find that the superlattice nanoSQUIDs have more than an order of magnitude smaller low-frequency excess flux noise, with root-mean-square spectral density SΦ1/2∼5−6μΦ0/Hz at 1 Hz (Φ0 is the magnetic flux quantum). We attribute this improvement to an improved microstructure at the grain boundaries forming the Josephson junctions in our YBCO nanoSQUDs.
We have fabricated and measured the properties of micron-size single grain boundary junctions and dc superconducting quantum interference devices (SQUIDs). The width of the junctions was varied between 0.25 and 2 μm. The junctions were formed in c-axis oriented epitaxial films with a single 24° tilt boundary. They were fabricated using electron beam lithography and ion milling. All of the junctions operate at 77 K. The noise properties of SQUIDs made of these junctions have been characterized. They show a significant 1/f component which is comparable to published values. However, the white noise level, 3×10−31 J/Hz measured at 77 K, is the lowest ever reported.
We present electric noise measurements of nanoscale biepitaxial YBa2Cu3O${}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) Josephson junctions fabricated by two different lithographic methods. The first (conventional) technique defines the junctions directly by ion milling etching through an amorphous carbon mask. The second (soft patterning) method makes use of the phase competition between the superconducting YBCO (Y123) and the insulating Y2BaCuO5 (Y211) phase at the grain boundary interface on MgO (110) substrates. The voltage noise properties of the two methods are compared in this study. For all junctions (having a thickness of 100 nm and widths of 250--500 nm), we see a significant amount of individual charge traps. We have extracted an approximate value for the effective area of the charge traps from the noise data. From the noise measurements, we infer that the soft-patterned junctions with a grain-boundary (GB) interface manifesting a large c-axis tunneling component have a uniform barrier and a superconductor-insulator-superconductor (SIS) -like behavior. The noise properties of soft-patterned junctions having a GB interface dominated by transport parallel to the ab planes are in accordance with a resonant tunneling barrier model. The conventionally patterned junctions, instead, have suppressed superconducting transport channels with an area much less than the nominal junction area. These findings are important for the implementation of nanosized Josephson junctions in quantum circuits.
We have investigated the properties of biepitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ grain boundary Josephson junctions, which appear to be characterized by low barrier transmission probabilities. Extrinsic and intrinsic d-wave effects are discussed within the framework of novel designs for $\ensuremath{\pi}$-circuitry and qubits. The absence of spontaneous magnetization and of the consequent additional noise, in a wide range of experimental conditions, seems to be encouraging for designs of ``quiet'' devices.