No abstract

Large-scale integration of vanadium dioxide (VO2) on mechanically flexible substrates is critical to the realization of flexible smart window films that can respond to environmental temperatures to modulate light transmittance. Until now, the formation of highly crystalline and stoichiometric VO2 on flexible substrate has not been demonstrated due to the high-temperature condition for VO2 growth. Here, we demonstrate a VO2-based thermochromic film with unprecedented mechanical flexibility by employing graphene as a versatile platform for VO2. The graphene effectively functions as an atomically thin, flexible, yet robust support which enables the formation of stoichiometric VO2 crystals with temperature-driven phase transition characteristics. The graphene-supported VO2 was capable of being transferred to a plastic substrate, forming a new type of flexible thermochromic film. The flexible VO2 films were then integrated into the mock-up house, exhibiting its efficient operation to reduce the in-house temperature under infrared irradiation. These results provide important progress for the fabrication of flexible thermochromic films for energy-saving windows.

We report a new class of c-axis Josephson tunneling experiments in which a conventional superconductor (Pb) is deposited across a single twin boundary of a YBa 2 Cu 3 O 72d (YBCO) crystal.We measure the critical current as a function of the magnitude and angle of a magnetic field applied in the plane of the junction. In all samples, we observe a clear experimental signature of an order parameter phase shift across the twin boundary. These results provide strong evidence for mixed d-and s-wave pairing in YBCO, with a reversal in the sign of the s-wave component across the twin boundary. [S0031-9007(97)04213-0] PACS numbers: 74.50. + r, 74.72.BkThe symmetry of the order parameter in high temperature superconductors has been a subject of theoretical and experimental debate since their original discovery [1,2]. Phase-sensitive measurements on YBa 2 Cu 3 O 72d ( YBCO) involving currents flowing within the CuO 2 planes, such as corner-junction SQUID experiments [3-5], cornerjunction flux modulation experiments [6,7], and tricrystal ring experiments [8], have indicated an order parameter D͑k͒ with primarily d x 2 2y 2 symmetry under rotations in the plane ͓D͑k͒ ϳ cos k x a 2 cos k y a͔. The observation, however, of Josephson tunneling perpendicular to the CuO 2 planes between heavily twinned YBCO and a conventional s-wave superconductor [9-13] demonstrated a significant s-wave component. In this Letter, we report results from a new c-axis Josephson tunneling experiment which resolves the apparent conflict between the two groups of experiments and makes a compelling case for mixed order parameter symmetry in YBCO, with a dominant d-wave component and a significant s-wave component.In the experiment, shown conceptually in Fig. 1(a), a c-axis Josephson tunnel junction straddles a single YBCO twin boundary. Because Pb is an s-wave superconductor, the Pb counterelectrode couples only to the s-wave component of the YBCO order parameter; the net phase difference with any d-wave component integrates to zero. If YBCO were a conventional s-wave superconductor, the critical current I c ͑B͒ would exhibit a Fraunhoferlike dependence on magnetic field strength B, with a maximum at B 0, regardless of the angle of B in the plane of the junction. If, on the other hand, YBCO were predominantly d wave, any s-wave component to the order parameter would change sign across the twin boundary [14], yielding a dramatically different angular dependence. For magnetic fields perpendicular to the boundary, I c ͑B͒ would be the same as for a conventional junction, with a maximum at B 0. Magnetic fields parallel to the boundary, however, should produce a local minimum in I c at B 0, as in the d-wave corner junction experiment [2,6].The experiments were performed at the University of California, San Diego (UCSD) and the University of California, Berkeley (UCB) on single crystals of YBa 2 Cu 3 O 72d grown at UCSD and the University of FIG. 1. (a) Schematic diagram of a Pb ͞YBCO junction (hatched region) grown across a twin boundary (dashed line). For t...

The superconducting critical temperature T c of YBa2Cu30 x single crystals (6.35^x^7) has been measured resistively as a function of pressure P (02 planes relative to the value corresponding to the fully oxygenated sample.