To determine the symmetry of the order parameter in superconducting YBazCu&07 z (YBCO), we use a scanning SQUID microscope at 4.2 K to perform independent experiments on six YBCO-Ag-Pb SQUIDs. We find completely unambiguous evidence for a time-reversal-invariant order parameter with a phase shift of m-between the a and b directions of YBCO. Our results are inconsistent with purely s-wave symmetric pairing and are strongly suggestive of d 2 -y2 symmetric pairing in YBCO. PACS numbers: 74.50.+r, 74.20.Mn, 74.72.Bk At present, there is enormous controversy concerning the symmetry of the order parameter in the high transition temperature (T,) superconductor YBa2Cu307 s (YBCO) [1]. The work we report here greatly extends the pioneering experiment of Wollman et al. [2], who first reported results on YBCO-Pb SQUIDs. Their conclusion of d .~2 symmetric pairing has been supported by some recent work on Josephson junctions and SQUIDs [3 -5], but not by other work [6,7]. In our own work, we have found that common effects produced by trapped vortices, magnetic field gradients, measuring currents, or asymmetries in the SQUIDs can mimic those produced by d-wave superconductivity.In light of this, the disagreements in prior work are neither surprising nor reassuring.In this Letter, we describe experiments which use a scanning SQUID microscope and a time-reversal-invariance test to provide consistent unambiguous evidence that YBCO has a time-reversal-invariant order parameter with a~shift between the crystal a and b directions.To understand how a SQUID can be used to find the pairing symmetry of a superconductor, consider Fig. 1(a), which shows a schematic of our type a bSQUID. O-ne half of the SQUID is made from the s-wave superconductor Pb and the other half from YBCO [2]. The two Josephson junctions allow tunneling of pairs between the superconductors, with one junction oriented normal to the YBCO a axis and the other normal to the b axis. If no magnetic field 8 is present and YBCO has a d 2 -y2 symmetry, then pairs tunneling through the a-axis junction have a phase shift of m with respect to pairs tunneling through the b-axis junction. Thus, a pair which travels once around the SQUID loop acquires an intrinsic phase shift of m. Such a~shift produces a current J circulating around the loop. If P = 2LIo/4O» 1, then a vr shift generates LJ = 4'o/2 of flux in the loop, where = h/2e is the flux quantum, L is the SQUID loop inductance, and Io is the average critical current of the junctions at 8 = 0. No such intrinsic phase shift, circulating current, or half quantized Aux would be produced at B = 0 if YBCO had s-wave symmetry or if the SQUID had a type a-a geometry, i.e. , both junctions oriented normal to the a axis [see Fig. 1(b)].In principle, the above ideas can be used to determine the pairing symmetry.However, in practice, great care is required because such a circulating current can arise from any small magnetic field. Such fields can be created by the measuring apparatus, by vortices in the superconducting films, or...

We have developed a magnetic flux microscope which uses a liquid nitrogen cooled thin-film YBa2Cu3O7 dc superconducting quantum interference device (SQUID) to produce two-dimensional images of magnetic fields. The instrument operates as a scanning probe microscope with a spatial resolution of about 80 μm and a field of view of about 100 mm2. The equivalent magnetic field noise in the SQUID ranges from 80 pT Hz−1/2 at 1 Hz to 20 pT Hz−1/2 at 1 kHz, yielding a field resolution of about 200 pT in our images. We have used the microscope to image magnetic domains in ferromagnetic samples, small currents flowing in fine wires, and trapped flux and diamagnetic susceptibility in superconducting thin films.

We have designed and operated a magnetic flux microscope which measures the magnetic field above a sample surface by scanning a 4.2 K thin-film dc superconducting quantum interference device (SQUID) along one direction. With the SQUID and sample separated by about 160 μm, the system can image features with a spatial resolution of about 220 μm and a magnetic field resolution of 1.8×10−13 THz−1/2. We have used the system to view trapped magnetic flux, to image thin-film strips of superconducting Pb in field strengths of 0–750 nT, and to monitor the position of a sample with a resolution of approximately 0.5 nm Hz−1/2 at a frequency of 4 kHz.

Using a 4.2K Nb-PbIn dc SQUID with a 60 pm inner hole side length, we have constructed a novel 1-D magnetic flux microscope with an unprecedented combination of spatial and magnetic field resolutions. During imaging, the sample is moved past the SQUID at a separation of about 38 pm and the output from the SQUID is recorded as a function of the sample position. The system achieves a spatial resolution of about 66 pm and a magnetic field resolution of about 5.2 pTHz-lD at a frequency of 6 kHz. We have used the microscope to obtain susceptibility images of patterned superconducting samples in low fields and have devised a simple method for measuring static magnetic fields. by no more than a distance diamagnetism causes mag feedback and field coils. 1051-8223/93$03.00 0 1993 IEEE This work was supported by the State of Maryland through the Center for Superconductivity Research. We thank E. Knouse for loan of the stepper motor and controller and J. Clarke for the design of the feedback electronics. Manuscript received August 24,1992.

No abstract