The miniaturization and integration of frequency-agile microwave circuits-relevant to electronically tunable filters, antennas, resonators and phase shifters-with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss-Srn+1TinO3n+1 phases-in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics-doping or strain-in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.
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Acknowledgements We acknowledge discussions with S. Trolier-McKinstry and C. A. Randall. Research was supported by Army Research Office (ARO) grants W911NF-09-1-0415 (for C.-H.L., Y.Z., J.A.M. and D.A.M.), W911NF-12-1-0437 (for Y.N., J.Z. and D.G.S.) and W911NF-10-1-0345 (for T.B., N.A.B. and C.J.F.); by the National Science Foundation (NSF) through Materials Research Science and Engineering Centers (MRSEC) grants DMR-0820404 (for R.H., E.V., X.X.X. and V.G.) and DMR-1120296 (for Y.K., J.D.B. and L.F.K.); by the Czech Science Foundation Project no. P204/12/1163 and the Czech Ministry of Education, Youth and Sports project LD12026 (for V.G., D.N. and S.K.); and by the Spanish Government and the European Union through grants EUI-ENIAC-2011-4349 and EUI-ENIAC 2010-04252 (for E.R.). C.-H.L. acknowledges stipend support from NSF grant DMR-0820404. J.A.M. acknowledges financial support from a National Defense Science & Engineering Graduate Fellowship. The dielectric and ferroelectric measurements in Fig. 3c were conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. This work was performed in part at the Cornell NanoScale Factory, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (grant ECCS-0335765). This work made use of the electron microscopy facility of the Cornell Center for Materials Research (CCMR) with support from the NSF MRSEC programme (DMR 1120296) and NSF IMR-0417392.
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