We report on the fabrication and characterization of nanopatterned dc SQUIDs with grain boundary Josephson junctions based on heteroepitaxially grown YBa$_2$Cu$_3$O$_7$ (YBCO)/ SiTrO$_3$ (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 $S_Φ^{1/2}\sim 5-6\,μΦ_0/\sqrt{\rm Hz}$ at 1 Hz ($Φ_0$ 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.
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 report on nanopatterned YBa2Cu3O7-δ (YBCO) direct current superconducting quantum interference devices (SQUIDs) based on grain boundary Josephson junctions. The nanoSQUIDs are fabricated by epitaxial growth of 120 nm-thick films of the high-transition temperature cuprate superconductor YBCO via pulsed laser deposition on MgO bicrystal substrates with 24° misorientation angle, followed by sputtering of dAu = 65 nm thick Au. Nanopatterning is performed by Ga focused ion beam (FIB) milling. The SQUID performance is comparable to devices on SrTiO3 (STO), as demonstrated by electric transport and noise measurements at 4.2 K. MgO has orders of magnitude smaller dielectric permittivity than STO; i.e., one may avoid Au as a resistively shunting layer to reduce the intrinsic thermal flux noise of the nanoSQUIDs. However, we find that the Au layer is important for avoiding degradation during FIB milling. Hence, we compare devices with different dAu produced by thinning the Au layer via Ar ion milling after FIB patterning. We find that the reduction of dAu yields an increase in junction resistance, however at the expense of a reduction of the critical current and increase in SQUID inductance. This results in an estimated thermal flux noise that is almost independent of dAu. However, for two devices on MgO with 65 nm-thick Au, we find an order of magnitude lower low-frequency excess noise as compared to nanoSQUIDs on STO or those on MgO with reduced dAu. For one of those devices we obtain with bias-reversal readout ultra-low flux noise of ∼175 nΦ0 Hz-1/2 down to ∼10 Hz.
We report on the fabrication and characterization of nanopatterned superconducting quantum interference devices (SQUIDs) based on grain boundary Josephson junctions in epitaxially grown multilayer YBa2Cu3O7 (YBCO)/SiTrO3 (STO) thin films. Nanopatterning is performed by Ga+ focused ion beam milling. The evolution of the electric transport and noise properties of the YBCO nanoSQUIDs over a time span of more than one year are recorded and analyzed. We find that the multilayer YBCO/STO nanoSQUIDs show stable and high performance over time. The critical current decreases within ∼40 days by 30%–50% and then remains almost constant without obvious decline trend for nanoSQUIDs with STO layer as the interface even more than 380 days, which demonstrates the superiority of STO as the capping layer. Moreover, we find that the multilayer nanoSQUIDs have about an order of magnitude smaller low-frequency excess flux noise (compared to similar single layer devices) with root-mean-square spectral density ∼5–6 µΦ0/Hz1/2 at 1 Hz. For one device, we show that the low-frequency excess noise does not degrade within three months.
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.
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.
White noise properties of YBa2Cu3O7-δ (YBCO) grain boundary Josephson devices have been studied. Bias voltage (V) dependence of the white voltage noise spectral density (SV) were measured in detail for YBCO dc-SQUIDS and single Josephson junctions on different kinds of substrates with various misorientation angles. It was found that Sv was in good agreement with the thermal noise theory hypothesizing that the device noise was only due to Johnson noise. This implies that the quantum noise can be achieved with optimized YBCO dc-SQUIDS. We have designed a YBCO dc-SQUID with a cross section of junctions as small as 0.1 μm^2. It was predicted that the minimum intrinsic white energy resolution (E”') should be about Planck's constant (h) at 4.2 K. The actual minimum eW, value of 1.76 x 10^-33 J/Hz = 2.7 h was obtained. This result demonstrates that E” 'of YBCO dc-SQUID s can be minimized to the order of h.