arXiv subjects

Steven M. Anlage

Publications and source records attributed to Steven M. Anlage.

At least 127 records · Page 7Linked to original sources

arXiv2005

Effect of tip-geometry on contrast and spatial-resolution of the Near-Field Microwave Microscope

The Near-Field Scanning Microwave Microscope (NSMM) can quantitatively image materials properties at length scales far shorter than the free space wavelength (λ). Here we report a study of the effect of tip-geometry on the NSMM signals. This particular NSMM utilizes scanning tunneling microscopy (STM) for distance-following control. We systematically examined many commercially available STM tips, and find them to have a conical structure on the macroscopic scale, with an embedded sphere (of radius rsphere) at the apex of the tip. The rsphere values used in the study ranged from 0.1 μm to 12.6 μm. Tips with larger rsphere show good signal contrast (as measured by the frequency shift (δf) signal between tunneling height and 2 μm away from the sample) with NSMM. For example, the tips with rsphere = 8 μm give signal contrast of 1000 kHz compared to 85 kHz with a tip of rsphere = 0.55 μm. However, large rsphere tips distort the topographic features acquired through STM. A theoretical model is used to understand the tip-to-sample interaction. The model quantitatively explains the measured change in quality factor (Q) as a function of height over bulk Copper and Silicon samples.

cond-mat.mtrl-sci↗

arXiv2005

Aspects of the Scattering and Impedance Properties of Chaotic Microwave Cavities

We consider the statistics of the impedance of a chaotic microwave cavity coupled to a single port. We remove the non-universal effects of the coupling from the experimental data using the radiation impedance obtained directly from the experiments. We thus obtain the normalized impedance whose Probability Density Function (PDF) is predicted to be universal in that it depends only on the loss (quality factor) of the cavity. We find that impedance fluctuations decrease with increasing loss. The results apply to scattering measurements on any wave chaotic system.

nlin.CD↗

arXiv2005

Superconducting Metamaterials

Evanescent wave amplification has been predicted under the ideal condition n = -1 + i 0 precisely, in the absence of retardation effects, but is difficult to observe in practice because current metamaterial designs suffer from high losses. We present results on a new metamaterial design that employs superconducting Niobium metals and low-loss dielectric materials to reduce the losses. We present transmission data on a wire medium, a split-ring resonator medium, and a combination of the two at temperatures between 4.2 K and room temperature. Evidence of negative effective permittivity, permeability, and a negative effective index passband are seen in the superconducting state between 50 MHz and 18 GHz. We find a dielectric loss of ε_{eff,2} = 3 x 10^{-3} in a superconducting wire array at 10.75 GHz.

cond-mat.supr-con↗

arXiv2005

Universal Statistics of the Scattering Coefficient of Chaotic Microwave Cavities

We consider the statistics of the scattering coefficient S of a chaotic microwave cavity coupled to a single port. We remove the non-universal effects of the coupling from the experimental S data using the radiation impedance obtained directly from the experiments. We thus obtain the normalized, complex scattering coefficient whose Probability Density Function (PDF) is predicted to be universal in that it depends only on the loss (quality factor) of the cavity. We compare experimental PDFs of the normalized scattering coefficients with those obtained from Random Matrix Theory (RMT), and find excellent agreement. The results apply to scattering measurements on any wave chaotic system.

nlin.CD↗

arXiv2004

Doping-Dependent Nonlinear Meissner Effect and Spontaneous Currents in High-Tc Superconductors

We measure the local harmonic generation from superconducting thin films at microwave frequencies to investigate the intrinsic nonlinear Meissner effect near Tc in zero magnetic field. Both second and third harmonic generation are measured to identify time-reversal symmetry breaking (TRSB) and time-reversal symmetric (TRS) nonlinearities. We perform a systematic doping-dependent study of the nonlinear response and find that the TRS characteristic nonlinearity current density scale follows the doping dependence of the de-pairing critical current density. We also extract a spontaneous TRSB characteristic current density scale that onsets at Tc, grows with decreasing temperature, and systematically decreases in magnitude (at fixed T/Tc) with under-doping. The origin of this current scale could be Josephson circulating currents or the spontaneous magnetization associated with a TRSB order parameter.

cond-mat.supr-con↗

arXiv2004

Microwave Nonlinearities of an Isolated Long YBa_2Cu_3O_(7-d) Bi-crystal Grain Boundary

We measure the local harmonic generation from an YBa_2Cu_3O_(7-d)(YBCO) bi-crystal grain boundary to examine the local Josephson nonlinearities. Spatially resolved images of second and third harmonic signals generated by the grain boundary are shown. The harmonic generation and the vortex dynamics along the grain boundary are modeled with the Extended Resistively Shunted Josephson (ERSJ) array model, which shows reasonable agreement with the experimental data. The model also gives qualitative insight into the vortex dynamics induced in the junction by the probing current distribution. A characteristic nonlinearity scaling current density JNL ~ 1.5 x 10^5 A/cm^2 for the Josephson nonlinearity is also extracted.

cond-mat.supr-con↗

arXiv2004

Microwave Current Imaging in Passive HTS Components by Low-Temperature Laser Scanning Microscopy (LTLSM)

We have used the LTLSM technique for a spatially resolved investigation of the microwave transport properties, nonlinearities and material inhomogeneities in an operating coplanar waveguide YBa_2Cu_3O_{7-δ} (YBCO) microwave resonator on an LaAlO_3 (LAO) substrate. The influence of twin-domain blocks, in-plane rotated grains, and micro-cracks in the YBCO film on the nonuniform rf current distribution were measured with a micrometer-scale spatial resolution. The impact of the peaked edge currents and rf field penetration into weak links on the linear device performance were studied as well. The LTLSM capabilities and its future potential for non-destructive characterization of the microwave properties of superconducting circuits are discussed.

cond-mat.supr-con↗

arXiv2004

Evidence for power-law frequency dependence of intrinsic dielectric response in the CaCu$_{3}$Ti$_{4}$O$_{12}$

We investigated the dielectric response of CaCu$_3$Ti$_4$O$_{12}$ (CCTO) thin films grown epitaxially on LaAlO$_3$ (001) substrates by Pulsed Laser Deposition (PLD). The dielectric response of the films was found to be strongly dominated by a power-law in frequency, typical of materials with localized hopping charge carriers, in contrast to the Debye-like response of the bulk material. The film conductivity decreases with annealing in oxygen, and it suggests that oxygen deficit is a cause of the relatively high film conductivity. With increase of the oxygen content, the room temperature frequency response of the CCTO thin films changes from the response indicating the presence of some relatively low conducting capacitive layers to purely power law, and then towards frequency independent response with a relative dielectric constant $ε'\sim10^2$. The film conductance and dielectric response decrease upon decrease of the temperature with dielectric response being dominated by the power law frequency dependence. Below $\sim$80 K, the dielectric response of the films is frequency independent with $ε'$ close to $10^2$. The results provide another piece of evidence for an extrinsic, Maxwell-Wagner type, origin of the colossal dielectric response of the bulk CCTO material, connected with electrical inhomogeneity of the bulk material.

cond-mat.mtrl-sci↗

arXiv2004

Universal Impedance Fluctuations in Wave Chaotic Systems

We experimentally investigate theoretical predictions of universal impedance fluctuations in wave chaotic systems using a microwave analog of a quantum chaotic infinite square well potential. Our approach emphasizes the use of the radiation impedance to remove the non-universal effects of the particular coupling from the outside world to the scatterer. Specific predictions that we test include the probability distribution functions (PDFs) of the real (related to the local density of states in disordered metals) and imaginary parts of the normalized cavity impedance, the equality of the variances of these PDFs, and the dependence of the universal PDFs on a single control parameter characterizing the level of loss. We find excellent agreement between the statistical data and theoretical predictions.

cond-mat.dis-nn↗

arXiv2004

Spatially Resolved Analyses of Microwave and Intermodulation Current Flow Across HTS Resonator Using Low Temperature Laser Scanning Microscopy

Microwave resonators made from high-temperature superconducting (HTS) films are becoming increasingly commercially competitive in the market of modern communication technologies. However, unlike normal metals, the HTS device performance suffers from intrinsically nonlinear electrodynamics, resulting in harmonic generation and intermodulation (IM) distortion at all levels of circulating power. The dominant sources of IM response are believed to be inductive and localized in only small regions of the HTS film having the highest current densities J_RF(x,y). In our previous papers, examining microwave resonators of simple microstrip geometry, we have demonstrated that local nonlinearity imaging is possible with the technique of low temperature laser scanning microscopy (LTLSM) adapted to microwave experiments. Unique contrast due to IM current density J_IM(x,y) was generated by the IM imaging method. It was applied to display the regions showing large nonlinear LTLSM photoresponse (PR) in HTS devices with a few-micrometer spatial resolution. Direct local correlation between J_IM(x,y) and high J_RF(x,y) at the edges of the strips, typical defects like twindomain blocks, in-plane rotated grains and micro-cracks was shown. Here we want to indicate that the geometrical features in HTS circuit layout for microwave devices of more complex geometry also radically affect the linear device performance.

cond-mat.supr-con↗

arXiv2003

Doping dependent time-reversal symmetric nonlinearity of YBa_2Cu_3O_7-d thin films

We have measured the temperature dependent third harmonic response from a series of under-doped YBa_2Cu_3O_7-d thin films to address the mechanism of nonlinearity in these samples. We find that the intrinsic Ginzburg-Landau nonlinearity near Tc is doping dependent, with the sample becoming more nonlinear as it is under-doped. The results are consistent with the doping dependence of the condensation energy of YBa_2Cu_3O_7-d.

cond-mat.supr-con↗

arXiv2003

Unified Model, and Novel Reverse Recovery Nonlinearities, of the Driven Diode Resonator

We study the origins of period doubling and chaos in the driven series resistor-inductor-varactor diode (RLD) nonlinear resonant circuit. We find that resonators driven at frequencies much higher than the diode reverse recovery rate do not show period doubling, and that models of chaos based on the nonlinear capacitance of the varactor diode display a reverse-recovery-like effect, and this effect strongly resembles reverse recovery of real diodes. We find for the first time that in addition to the known dependence of the reverse recovery time on past current maxima, there are also important nonlinear dependencies on pulse frequency, duty-cycle, and DC voltage bias. Similar nonlinearities are present in the nonlinear capacitance models of these diodes. We conclude that a history-dependent and nonlinear reverse recovery time is an essential ingredient for chaotic behavior of this circuit, and demonstrate for the first time that all major competing models have this effect, either explicitly or implicitly. Besides unifying the two major models of RLD chaos, our work reveals that the nonlinearities of the reverse recovery time must be included for a complete understanding of period doubling and chaos in this circuit.

nlin.CD↗

arXiv2002

Imaging Local Sources of Intermodulation in Superconducting Microwave Devices

This work presents new experimental results on low-temperature (LT) characterization of local rf properties of passive superconducting (SC) microwave devices using a novel Laser Scanning Microscope (LSM). In this technique, a modulated laser beam is focused onto and scanned over the surface of a resonant SC device to probe the spatial distribution of rf current. The highly localized photo-induced change of the kinetic inductance of the SC device produces both a shift of the resonant frequency f_0 and change of the quality factor Q. An image of these changes is recorded as the laser spot is scanned over the device. We present the first measurements of spatially resolved intermodulation response in a High Temperature Superconducting (HTS) co-planar waveguide resonator, opening up a new window into the local origins of nonlinearity in the HTS materials.

cond-mat.supr-con↗

arXiv2002

Study of Local Nonlinear Properties Using a Near-Field Microwave Microscope

We have developed a near-field microwave microscope to locally apply microwave frequency currents and fields to superconductors, and dielectric substrates, and measure the locally generated 2nd and 3rd harmonic responses. We measure the local nonlinear response of a Tl_2Ba_2CaCu_2O_y film grown on an MgO substrate, and observe a large response due to the enhanced current density near the edge. We also study the local nonlinear response of a YBa_2Cu_3O_7-d thin film grown on a bi-crystal SrTiO_3 (STO) substrate, and spatially identify the grain boundary through higher harmonic measurements. The spatial resolution is determined by the magnetic loop probe size. A scaling current density JNL is extracted to quantify the magnitude of the nonlinearity of the superconductor. Preliminary results on the nonlinear properties of some commonly used substrates, e.g. MgO and STO, have also been obtained

cond-mat.supr-con↗

arXiv2002

Spatially Resolved Nonlinearity Measurements of YBa$_2$Cu$_3$O$_{7-d}$ Bi-crystal Grain Boundaries

We have developed a near-field microwave microscope to locally excite a superconducting film and measure second and third harmonic responses at microwave frequencies. We study the local nonlinear response of a YBa$_2$Cu$_3$O$_{7-d}$ thin film grown on a bi-crystal SrTiO$_3$ substrate. The location of the bi-crystal grain boundary is clearly identified by the microscope through higher harmonic response, and the spatial resolution is on the order of the magnetic loop probe size, about 500$μm$. The harmonic power and spatial resolution are successfully modeled with a one-dimensional extended Josephson junction simulation. From the model, the 2nd order harmonic response is dominated by Josephson vortex generation and flow. A geometry-free nonlinear scaling current density $J_{NL}\simeq 10^4\sim 10^5 A/cm^2$ is also exstracted from the data, indicating that the grain boundary weak link is the dominant nonlinear source in this case.

cond-mat.supr-con↗

arXiv2002

Effect of LaAlO$_3$ Surface Topography on RF Current Distribution in Superconducting Microwave Devices

A laser scanning microscope with a thermal spot size of about 4 $μ$m is used to measure a quantity proportional to the rf current density in an operating superconducting co-planar waveguide microwave resonator. The twinning of the LaAlO$_3$ substrate produces a meandering of the current at the edges due to irregularities in the wet etching of the YBa$_2$Cu$_3$O$_{7-δ}$ film associated with substrate twin domain blocks, and a ($\sim$ 20%) enhancement of the rf photoresponse at these locations. These irregularities are candidates for enhanced nonlinear response from the device. The effects of substrate twinning and the resulting edge features on the superconducting film are discussed and analyzed.

cond-mat.supr-con↗

arXiv2002

Effects of RF Stimulus and Negative Feedback on Nonlinear Circuits

We investigate the combined effect of rectification and nonlinear dynamics on the behavior of several simple nonlinear circuits. We consider the classic Resistor-Inductor-Diode (RLD) circuit driven by a low frequency source when an operational amplifier with negative feedback is added to the circuit. Radio frequency signals are applied to the circuit, causing significant changes in the onset of period-doubling and chaos. Measurements indicate that this effect is associated with a DC voltage induced by rectification of the RF signal in the circuit. The combination of rectification and nonlinear circuit dynamics produce qualitatively new behavior, which opens up a new channel of RF interference in circuits.

nlin.CD↗

arXiv2002

A novel Microwave Frequency Scanning Capacitance Microscope

We report a new technique of scanning capacitance microscopy at microwave frequencies. A near field scanning microwave microscope probe is kept at a constant height of about 1nm above the samplewith the help of Scanning Tunneling Microscope (STM) feedback. The microwaves are incident onto the sample through a coaxial resonator that is terminated at one end with a sharp tip (the same tip is used to conduct STM), and capacitively coupled to a feedback circuit and microwave source at the other end. The feedback circuit keeps the source locked onto the resonance frequency of the resonator and outputs the frequency shift and quality factor change due to property variations of the sample. The spatial resolution due to capacitance variations is $\leq $ 2.5 nm. The microscope is broadband and experiments were performed from 7 GHz to 11 GHz. We develop a quantitative transmission line model that treats the tip to sample interaction as a series combination of capacitance and effective sheet resistance in the sample.

cond-mat.mtrl-sci↗