Nonlinear Response of HTSC Thin Film Microwave Resonators in an Applied DC Magnetic Field

The non-linear microwave surface impedance of patterned YBCO thin films, was measured using a suspended line resonator in the presence of a perpendicular DC magnetic field of magnitude comparable to that of the microwave field. Signature of the virgin state was found to be absent even for relatively low microwave power levels. The microwave loss was initially found to decrease for small applied DC field before increasing again. Also, non-linearities inherent in the sample were found to be substantially suppressed at low powers at these applied fields. These two features together can lead to significant improvement in device performance.Comment: 4 pages, LaTeX type, Uses IEEE style files, 600 dpi PostScript file with color figures available at http://sagar.physics.neu.edu/preprints.html Submitted to IEEE Transactions on Applied Superconductivit

Comments on "Vortex Glass and Lattice Melting Transitions in a YNi_2B_2C Single Crystal"

Recently, Mun et.al. (Phys. Rev. Lett., 76, 2790 (1996)) have published their results on single crystal YNi_2B_2C, claiming that their experimental observations can be explained in terms of formation of Vortex Glass and Lattice melting. Our experiments, carried out on samples obtained from the SAME source, reveal a much richer phase diagram and span wider regions of experimental parameter space than Mun et. al. that encompasses most of their observations. We speculate that this material has anomalous intrinsic properties and the results cannot be explained by simple models about the flux lattice.Comment: 1 page, LaTeX type, 1 PostScript figure, Uses PRABIB.STY file, 600 dpi PS file available at http://sagar.physics.neu.edu/preprints.html To appear in Physical Review Letter

New features in the microwave response of YBCO crystals: Evidence for a multi-component order parameter

New features are reported in precision measurements of the complex microwave conductivity of high quality $YBa_{2}Cu_{3}O_{6.95}$ crystals grown in $BaZrO_{3}$ crucibles. A third peak in the normal conductivity, $\sigma_{1}(T)$, at around $80K$, and enhanced pair conductivity $\sigma_{2}(T)$ below $\sim 65K$ are observed. The data are inconsistent with a single order parameter, and instead are indicative of multi-component superconductivity. Overall, these results point to the presence of multiple pairing interactions in $YBa_{2}Cu_{3}O_{6.95}$ and also provide a natural explanation to account for the low temperature $35K$ conductivity peak observed in all $YBa_{2}Cu_{3}O_{6.95}$ crystals.Comment: 4 pages 4 figures, Revtex, multicols [to appear in Phys. Rev. B (Rapid Comm)]. Our preprints also available at http://sagar.physics.neu.edu/preprints.htm

Critical State Flux Penetration and Linear Microwave Vortex Response in YBa_2Cu_3O_{7-x} Films

The vortex contribution to the dc field (H) dependent microwave surface impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using suspended patterned resonators. Z_s(H) is shown to be a direct measure of the flux density B(H) enabling a very precise test of models of flux penetration. Three regimes of field-dependent behavior were observed: (1) Initial flux penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate fields the flux penetration into the virgin state is in excellent agreement with calculations based upon the field-induced Bean critical state for thin film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very high fields H >>H_s, the flux density is uniform and the measurements enable direct determination of vortex parameters such as pinning force constants \alpha_p and vortex viscosity \eta. However hysteresis loops are in disagreement with the thin film Bean model, and instead are governed by the low field scale H_i, rather than by H_s. Geometric barriers are insufficient to account for the observed results.Comment: 20 pages, LaTeX type, Uses REVTeX style files, Submitted to Physical Review B, 600 dpi PostScript file with high resolution figures available at http://sagar.physics.neu.edu/preprints.htm

Microwave properties of borocarbide superconductors RNi<SUB>2</SUB>B<SUB>2</SUB>C (R=Y,Er,Tm,Ho)

We report measurements of the microwave surface impedance of the borocarbide family of superconductors RNi2B2C (R=Y,Er,Tm,Ho). The experiments enable direct measurements of the superfluid density, and are particularly sensitive to the influence of magnetic pair breaking. In HoNi2B2C the antiferromagnetic transition is clearly observed at zero field, and leads to a drastic reduction of the superfluid density, which recovers at lower temperatures. In ErNi2B2C the antiferromagnetic transition is not seen in zero-field data. Magnetic effects are responsible for anomalies in the low-temperature surface impedance below approximately 4 K in HoNi2B2C and TmNi2B2C. The temperature dependence of the microwave impedance disagrees with BCS calculations

Nonlinear Microwave Switching Response Of Bscco Single Crystals

this paper was motivated by a chance observation during the study of the microwave response of high quality Bi 2 Sr 2 Ca 1 Cu 2 O8+ffi (BSCCO) single crystals. While measuring the surface impedance, Z s (T ), of these crystals, an unusual step-like switching feature was observed in the cavity resonances. (See Fig. 1) These effects are only observed in the presence of microwave currents induced along the c axis, and seem qualitatively consistent with the underdamped response of intrinsic Josephson junctions (JJs) in these samples. The measurements are performed in a Nb cavit

Nonlinear Response of HTSC Thin Film Microwave Resonators in an Applied DC Magnetic Field

The non-linear microwave surface impedance, Zs = Rs + iXs , of patterned YBCO thin films, was measured using a suspended line resonator in the presence of a perpendicular DC magnetic field, HDC , of magnitude comparable to that of the microwave field, Hrf . Signature of the virgin state was found to be absent even for relatively low microwave power levels. The microwave loss was initially found to decrease for small applied HDC before increasing again. Also, non-linearities inherent in the sample were found to be substantially suppressed at low powers at these applied fields. These two features together can lead to significant improvement in device performance. I. Introduction The microwave response of high-T c superconductors (HTSC) is important both from the point of view of microwave applications of HTSC[1] and fundamental physics[2]. An understanding of the loss mechanisms, field and current profiles and nature of non-linearities can lead to improvement in fabricated devices tha..

Vortex response and critical fields observed via RF penetration depth measurements on the superconductor YNi2B2C

Measurements of the RF penetration depth λ(T, H, θ) are used to study the superconducting order parameter, vortex dynamics in the mixed state and delineate critical fields in the borocarbide superconductor YNi2B2C. The lower critical field has an anomalous T dependence, Hc1(T) = 1.12[1 − (T/Tc)]kOe, which is, however, consistent with independent superfluid density measurements at microwave frequencies. The vortex response is dominated by viscous flux flow, indicative of extremely weak pinning, and is parameterized by a field scale Hc2, eff. The angular dependence of the vortex contribution λ(θ) is in good agreement with the Coffey-Clem model. Structure is seen in the depairing transition in the vicinity of the upper critical field, with the existence of well-defined critical fields Hc2α, Hc2βand Hc2γ, with the vortex field scale Hc2, eff closest to Hc2β. Overall the measurements indicate that YNi2B2C has a rich and unusual field dependence of its transport parameters.This is a manuscript of an article published as Oxx, S., D. P. Choudhury, Balam A. Willemsen, H. Srikanth, S. Sridhar, B. K. Cho, and P. C. Canfield. "Vortex response and critical fields observed via RF penetration depth measurements on the superconductor YNi2B2C." Physica C: Superconductivity 264, no. 1-2 (1996): 103-108. DOI: 10.1016/0921-4534(96)00230-4 Copyright 1996 Elsevier. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission

Electrostatic actuators with intrinsic stress gradient.

Electrostatic actuators based on microelectromechanical systems ͑MEMS͒ have many attractive features for use as variable capacitors in high-frequency applications. The devices consist of two electrodes, one fixed and the other movable. In this study, a curved, cantilever beam was used as the movable electrode. A novel process has been developed for fabricating an all-gold, curved beam. The cantilever beams were curved due to an intrinsic stress gradient in the metal. Electroplating and conventional lithography were used to metallize the cantilever beam electrodes. The internal stress gradient in the gold was obtained by changing the electroplating conditions during fabrication. Stiction during release and operation of the variable capacitors was alleviated by treating the gold with an alkane thiol self-assembled monolayer. The intrinsic stress gradient and the stress-induced bending moment were calculated using a generalized model for the stress gradient in the films. Compared to bimetallic, cantilever beams, the curvature of the all-gold beam was found to be independent of temperature. This implies that the operation of the single-metal variable capacitor will be more reproducible and stable with temperature than a comparable bimetallic device. Electrostatically actuated beams are one of the fundamental building blocks in microelectromechanical systems ͑MEMS͒ and find applications in a variety of fields. They are used in a number of areas such as communications, sensing, optics, microfluidics, and measurement of materials properties. 1-4 The principle of electrostatic actuation is used in the fabrication of optical switches for optical networking circuits. 5 The electrostatically actuated rf capacitors provide greater isolation and more linear response compared to solid-state varactor diodes. The MEMS rf switches and variable capacitors also have higher quality factors and provide higher on/off capacitance ratio compared to traditional p-n junction varactors. A MEMS variable capacitor with a movable electrode and a fixed electrode is studied here. A curved cantilever beam design is considered for the movable electrode. A schematic diagram of the electrostatic actuator is shown in During the operation of bimetallic actuators, temperature has to be controlled precisely, as any variation in temperature leads to a change in the deflection of the actuator, which in turn changes the capacitance of the device. In addition, the mechanical and electrical properties, such as resistive loss and the electrical conductivity of the movable electrode, can be compromised due to the presence of a different material in the bimetallic film stack. The initial curvature of the movable electrode can also be obtained by using a single material, stress gradient approach. Deposited films with intrinsic residual stresses have been used previously to create nonplanar beams. 2,4 Polysilicon beams using the Multi-User MEMS Processes ͑MUMPs͒ have been reported to have residual compressive stress and stress gradients leading to deflection of the beam. 9 The single material, stress gradient approach for fabricating the movable electrode has considerable advantages over the conventional bimetallic film approach. Precisely controlled stress gradients in the films can be used to achieve a desired level of deflection in the devices. The deflection of the movable electrode deposited with an intrinsic stress gradient is expected to be largely independent of time and temperature. The mechanical and electrical properties of the material are not compromised, as this approach uses a single material. Furthermore, the material issues such as intermetallic reaction between the two materials are also avoided. The long-term reliability and reproducibility of the device could be greatly improved using this approach. The goal of this study is to select a materials set, which is highly reproducible and stable, and to develop a simple process technique to obtain a controllable stress gradient in the movable electrode. Gold has been considered as the material for the movable electrode due to its high electrical conductivity, low loss, ease of deposition, and excellent flexibility. The fixed electrode was made of aluminum. The deposition conditions during metallization were varied to investigate the effect on deflection of the movable electrode. The devices were then subjected to time and temperature aging. Finally, using the generalized model for the stress gradient in the films, the intrinsic stress gradient and the stress-induced bending moment were calculated