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B. J. Feenstra

Publications and source records attributed to B. J. Feenstra.

14 recordsLinked to original sources
arXiv2022

Non-equilibrium Superconductivity and Quasiparticle Dynamics studied by Photo Induced Activation of Mm-Wave Absorption (PIAMA)

We present a study of non-equilibrium superconductivity in DyBa2Cu3O7-d using photo induced activation of mm-wave absorption (PIAMA). We monitor the time evolution of the thin film transmissivity at 5 cm-1 subject to pulsed infrared radiation. In addition to a positive bolometric signal we observe a second, faster, decay with a sign opposite to the bolometric signal for T>40 K. We attribute this to the unusual properties of quasi-particles residing near the nodes of an unconventional superconductor, resulting in a strong enhancement of the recombination time.

cond-mat.supr-con↗
arXiv2000

Near-Field Microwave Microscopy of Materials Properties

Near-field microwave microscopy has created the opportunity for a new class of electrodynamics experiments of materials. Freed from the constraints of traditional microwave optics, experiments can be carried out at high spatial resolution over a broad frequency range. In addition, the measurements can be done quantitatively so that images of microwave materials properties can be created. We review the five major types of near-field microwave microscopes and discuss our own form of microscopy in detail. Quantitative images of microwave sheet resistance, dielectric constant, and dielectric tunability are presented and discussed. Future prospects for near-field measurements of microwave electrodynamic properties are also presented.

cond-mat.mtrl-sci↗
arXiv1998

Superconducting Material Diagnostics using a Scanning Near-Field Microwave Microscope

We have developed scanning near-field microwave microscopes which can image electrodynamic properties of superconducting materials on length scales down to about 2 $μ$m. The microscopes are capable of quantitative imaging of sheet resistance of thin films, and surface topography. We demonstrate the utility of the microscopes through images of the sheet resistance of a YBa2Cu3O7-d thin film wafer, images of bulk Nb surfaces, and spatially resolved measurements of Tc of a YBa2Cu3O7-d thin film. We also discuss some of the limitations of the microscope and conclude with a summary of its present capabilities.

cond-mat.mtrl-sci↗
arXiv1998

Imaging Microwave Electric Fields Using a Near-Field Scanning Microwave Microscope

By scanning a fine open-ended coaxial probe above an operating microwave device, we image local electric fields generated by the device at microwave frequencies. The probe is sensitive to the electric flux normal to the face of its center conductor, allowing different components of the field to be imaged by orienting the probe appropriately. Using a simple model of the microscope, we are able to interpret the system's output and determine the magnitude of the electric field at the probe tip. We show images of electric field components above a copper microstrip transmission line driven at 8 GHz, with a spatial resolution of approximately 200 $μ$m.

cond-mat↗
arXiv1998

Frequency Following Imaging of Electric Fields from Resonant Superconducting Devices using a Scanning Near-Field Microwave Microscope

We have developed a scanning near-field microwave microscope that operates at cryogenic temperatures. Our system uses an open-ended coaxial probe with a 200 mm inner conductor diameter and operates from 77 to 300 K in the 0.01-20 GHz frequency range. In this paper, we present microwave images of the electric field distribution above a Tl2Ba2CaCu2O8 microstrip resonator at 77 K, measured at several heights. In addition, we describe the use of a frequency-following circuit to study the influence of the probe on the resonant frequency of the device.

cond-mat.supr-con↗
arXiv1998

Microwave Nonlinearities in High Tc Superconductors: The Truth Is Out There

This paper discusses some of the major experimental features of microwave nonlinearity in high temperature superconductors, both intrinsic and extrinsic. The case is made for solving the problem of extrinsic nonlinearity through the use of localized measurements of microwave surface impedance and electromagnetic fields. Along these lines, a brief introduction is given to our work on scanning near-field microwave microscopy

cond-mat.supr-con↗
arXiv1998

Low Power Superconducting Microwave Applications and Microwave Microscopy

We briefly review some non-accelerator high-frequency applications of superconductors. These include the use of high-Tc superconductors in front-end band-pass filters in cellular telephone base stations, the High Temperature Superconductor Space Experiment, and high-speed digital electronics. We also present an overview of our work on a novel form of near-field scanning microscopy at microwave frequencies. This form of microscopy can be used to investigate the microwave properties of metals and dielectrics on length scales as small as 1 mm. With this microscope we have demonstrated quantitative imaging of sheet resistance and topography at microwave frequencies. An examination of the local microwave response of the surface of a heat-treated bulk Nb sample is also presented.

cond-mat.supr-con↗
arXiv1998

Microwave Near-Field Imaging of Electric Fields in a Superconducting Microstrip Resonator

We describe the use of a cryogenic near-field scanning microwave microscope to image microwave electric fields from superconducting and normal-metal microstrip resonators. The microscope employs an open-ended coaxial probe and operates from 77 to 300 K in the 0.01-20 GHz frequency range with a spatial resolution of about 200 mm. We describe the operation of the system and present microwave images of Cu and Tl2Ba2CaCu2O8 microstrip resonators, showing standing wave patterns at the fundamental and second harmonic frequencies.

cond-mat.supr-con↗
arXiv1998

Quantitative topographic imaging using a near-field scanning microwave microscope

We describe a technique for extracting topographic information using a scanning near-field microwave microscope. By monitoring the shift of the system's resonant frequency, we obtain quantitative topographic images of uniformly conducting metal surfaces. At a frequency of 9.572 GHz, our technique allows for a height discrimination of about 55 nm at a separation of 30 microns. We present topographic images of uneven, conducting samples and compare the height response and sensitivity of the system with theoretical expectations.

cond-mat.mtrl-sci↗
arXiv1997

Quantitative Imaging of Sheet Resistance with a Scanning Near-Field Microwave Microscope

We describe quantitative imaging of the sheet resistance of metallic thin films by monitoring frequency shift and quality factor in a resonant scanning near-field microwave microscope. This technique allows fast acquisition of images at approximately 10 ms per pixel over a frequency range from 0.1 to 50 GHz. In its current configuration, the system can resolve changes in sheet resistance as small as 0.6 Ohms/sq for 100 Ohms/sq films. We demonstrate its use at 7.5 GHz by generating a quantitative sheet resistance image of a YBa2Cu3O7 (YBCO) thin film on a 5 cm-diameter sapphire wafer.

cond-mat.mtrl-sci↗
arXiv1997

Penetration depth and conductivity of NbN and DyBa_2Cu_3O_7 thin films measured by mm-wave transmission

Using mm-wave transmission we obtain information about both the real and imaginary part of the dielectric function of superconducting thin films. This is done by fitting the Fabry-Perot resonance spectrum for the film plus substrate over a broad frequency range (110-180 GHz) using the full Fresnel equations and a two-fluid description for the dielectric function. Depending on the thickness of the film, the transmission is mainly determined by the real or the imaginary part of the dielectric function. For NbN we find a behavior consistent with the BCS-formalism. For DyBa_2Cu_3O_7, results on films of thicknesses ranging from 20 to 80 nm are presented. We find a quadratic temperature dependence of the penetration depth at low temperatures and a strongly enhanced conductivity below T_c, indicative of a strong reduction of the quasiparticle scattering. For the 20 nm film the transmission is dominated by the real part of the conductivity, even down to 60 K.

cond-mat.supr-con↗
arXiv1996

Grazing Incidence Infrared Reflectivity of La$_{1.85}$Sr$_{0.15}$CuO$_4$ and NbN

Infrared reflectivity measurements, using p-polarized light at a grazing angle of incidence, show an increased sensitivity to the optical conductivity of highly reflecting superconducting materials. We demonstrate that when this measurement technique is applied to the conventional s-wave superconductor NbN, the results are in perfect agreement with BCS theory. For the in-plane response of a La$_{1.85}$Sr$_{0.15}$CuO$_4$ single crystal, in the superconducting state, we find a reduction of the optical conductivity in the frequency range below 20 meV. The observed frequency dependence excludes an isotropic s-wave gap, but agrees well with model calculations assuming a d-wave order parameter.

cond-mat↗
arXiv1994

Observation of the Holstein shift in high $T_c$ superconductors with thermal modulation reflectometry

We use the experimental technique of thermal modulation reflectometry to study the relatively small temperature dependence of the optical conductivity of superconductors. Due to a large cancellation of systematic errors, this technique is shown to a be very sensitive probe of small changes in reflectivity. We analyze thermal modulation reflection spectra of single crystals and epitaxially grown thin films of YBa$_2$Cu$_3$O$_{7-δ}$ and obtain the ${α_tr}^2F(ω)$ function in the normal state, as well as the superconductivity induced changes in reflectivity. We present detailed model calculations, based on the Eliashberg-Migdal extension of the BCS model, which show good qualitative and quantitative agreement with the experimental spectra. VSGD.93.12.th

cond-mat↗