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C. P. Vlahacos

Publications and source records attributed to C. P. Vlahacos.

14 recordsLinked to original sources
arXiv2011

Atoms Talking to SQUIDs

We present a scheme to couple trapped $^{87}$Rb atoms to a superconducting flux qubit through a magnetic dipole transition. We plan to trap atoms on the evanescent wave outside an ultrathin fiber to bring the atoms to less than 10 $μ$m above the surface of the superconductor. This hybrid setup lends itself to probing sources of decoherence in superconducting qubits. Our current plan has the intermediate goal of coupling the atoms to a superconducting LC resonator.

quant-ph↗
arXiv2011

Thin-film superconducting resonator tunable to the ground-state hyperfine splitting of $^{87}$Rb

We describe a thin-film superconducting Nb microwave resonator, tunable to within 0.3 ppm of the hyperfine splitting of $^{87}$Rb at $f_{Rb}=6.834683$ GHz. We coarsely tuned the resonator using electron-beam lithography, decreasing the resonance frequency from 6.8637 GHz to 6.8278 GHz. For \emph{in situ} fine tuning at 15 mK, the resonator inductance was varied using a piezoelectric stage to move a superconducting pin above the resonator. We found a maximum frequency shift of about 8.7 kHz per 60-nm piezoelectric step and a tuning range of 18 MHz.

quant-ph↗
arXiv2000

Quantitative imaging of dielectric permittivity and tunability with a near-field scanning microwave microscope

We describe the use of a near-field scanning microwave microscope to image the permittivity and tunability of bulk and thin film dielectric samples on a length scale of about 1 micron. The microscope is sensitive to the linear permittivity, as well as to nonlinear dielectric terms, which can be measured as a function of an applied electric field. We introduce a versatile finite element model for the system, which allows quantitative results to be obtained. We demonstrate use of the microscope at 7.2 GHz with a 370 nm thick barium strontium titanate thin film on a lanthanum aluminate substrate. This technique is nondestructive and has broadband (0.1-50 GHz) capability. The sensitivity of the microscope to changes in relative permittivity is 2 at permittivity = 500, while the nonlinear dielectric tunability sensitivity is 10^-3 cm/kV.

cond-mat.mtrl-sci↗
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↗
arXiv1999

Imaging of Microwave Permittivity, Tunability, and Damage Recovery in (Ba,Sr)TiO3 Thin Films

We describe the use of a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 micron. We demonstrate this technique with permittivity images and local hysteresis loops of a 370 nm thick barium strontium titanate thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity as small as 2 at 500, and changes in dielectric tunability as small as 0.03 V$^{-1}$.

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

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

Surface Resistance Imaging with a Scanning Near-Field Microwave Microscope

We describe near-field imaging of sample sheet resistance via frequency shifts in a resonant coaxial scanning microwave microscope. The frequency shifts are related to local sample properties, such as surface resistance and dielectric constant. We use a feedback circuit to track a given resonant frequency, allowing measurements with a sensitivity to frequency shifts as small as one parts in 50000 for a 30 ms sampling time. The frequency shifts can be converted to sheet resistance based on a simple model of the system.

cond-mat.mtrl-sci↗