Imaging of Order Parameter InducedπPhase Shifts in Cuprate Superconductors by Low-Temperature Scanning Electron Microscopy

Abstract

Low-temperature scanning electron microscopy (LTSEM) has been used to image the supercurrent distribution in ramp-type Josephson junctions between Nb and either the electron-doped cuprate Nd 2−x Ce x CuO 4−y or the hole-doped cuprate YBa 2 Cu 3 O 7 . For zigzag-shaped devices in the short junction limit the critical current is strongly suppressed at zero applied magnetic field. The LTSEM images show, that this is due to the Josephson current counterflow in neighboring 0 and π facets, which is induced by the d x 2 −y 2 order parameter in the cuprates. Thus, LTSEM provides imaging of the sign change of the superconducting order parameter, which can also be applied to other types of Josephson junctions. One of the most controversial topics on high-T c cuprate superconductors has been the determination of their order parameter symmetry (OPS). A myriad of experiments have been performed, indicating a predominant d x 2 −y 2 OPS, which implies important consequences for the microscopic mechanism of Cooper pairing in these materials. Obviously, it was quite difficult to identify an unambiguous experiment for the determination of the cuprate OPS. Among the most convincing experiments is the observation of half-integer magnetic flux quanta in tricrystal grain boundary Josephson junctions (JJs) by scanning SQUID microscopy [1]. These experiments, and related integral measurements of critical current I c vs applied magnetic field B, rely on the difference π of the phase of the order parameter between orthogonal directions in (k x , k y )-space, which can be detected by interferometertype configurations, such as corner junctions [2], tricrystal rings and long JJs [3,4,5,6,7,8], and dc π SQUIDs [9,10,11,12], or by the angular dependence of I c in biepitaxial JJs [13]. High-quality hybrid ramp-type JJs, combining an s-wave superconductor (Nb) with either the hole-doped cuprate YBa 2 Cu 3 O 7−δ (YBCO) [14,15,16] or the electrondoped cuprate Nd 2−x Ce x CuO 4−y (NCCO) [17] have also been realized. Arranging such JJs in a zigzag geometry with the facets oriented along the a-and b-axis of the cuprate, one obtains alternating facets of 0 and π JJs [15,17]. π JJs [18] have negative I c , i. e., j s = − j c sin φ = j c sin(φ + π), instead of j s = j c sin φ, where j s is the supercurrent density; j c > 0 is the maximum supercurrent density, and φ is the Josephson phase. Realizations include JJs with magnetic barriers [19,20,21,22,23] JJs containing both, 0-and π-parts have also been realized using ferromagnetic barriers [28,29,30] or current injectors [31].A striking property of s-d-wave zigzag JJs in the long JJ limit (facet length a by scanning SQUID microscopy. In the short JJ limit (neglecting selffield effects), for a JJ with N facets, the supercurrent density in the n th facet can be described as [15] Here,x is the coordinate along the zigzag (withx = 0 at the JJ edge), and Φ f is the magnetic flux per facet. As the prefactor (−1) n changes sign at every corner of the zigzag, as a direct consequence of the d-wave OPS, I c (B) i...

Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 andπsegments

“…(18) and (5). It is interesting that such a shape of I c (B) was also measured for d-wave/s-wave zigzag shaped ramp junctions [32,33,34].…”

“…A junction consisting of various 0 and π segments can, thus, be formed by selectively etching the F-layer to produce two thicknesses d [31]. In the cuprate/Nb zigzag junctions [30,32,33,34] the facets should be oriented along the crystallographic a and b axes of the cuprate electrode, imposing certain topological limitations to the 0-π boundary. In contrast, the SIFS technology allows almost any 2D…”

“…), making conclusions ambiguous. We thus put a strong focus on LTSEM which allows direct imaging of the supercurrent density distribution in the junctions (including counterflow areas induced by the 0-π-segments), close to I c [34].…”

Local Electrical Imaging of Tetragonal Domains and Field-Induced Ferroelectric Twin Walls in ConductingSrTiO3

“…Recently, an influence of the local tetragonal domain structure on the conductivity of the 2DEG at the LAO/ STO interface was demonstrated by probing the magnetic field [25] or electric potential [26] induced by the 2DEG, rather than mapping the electronic properties directly, which is difficult with a probe such as scanning tunneling microscopy because the 2DEG is embedded between two insulating oxides. A method that can map the local electric transport properties right at the interface is low-temperature scanning electron microscopy (LTSEM) [27][28][29][30][31], which uses a periodically blanked focused electron beam (e beam) scanned across the sample [29][30][31]. It locally perturbs its electric conductivity with micron-scale spatial resolution and induces a voltage signal ΔV across the sample.…”

dc Josephson Current Between an Isotropic and a d-Wave or Extended s-Wave Partially Gapped Charge Density Wave Superconductor

“…It is also natural that, in the case of d-wave pairing and the absence of tunneling directionality (θ 0 = 90 • ), the Josephson current disappears due to the exactly mutually compensating contributions from superconducting order parameter lobes with different signs [119,138,143]. Intermediate θ 0 's correspond to non-zero Josephson tunnel current of either sign (conventional 0-and π-junctions [120,122,253]) except at the tilt angle γ = 45 • ,w h e n i c = 0. In this connection, one should recognize that the energy minimum for non-conventional anisotropic superconductors can occur, in principle, at any value of the order parameter phase [254].…”