arXiv subjects

Alexander P. Zhuravel

Publications and source records attributed to Alexander P. Zhuravel.

13 recordsLinked to original sources

arXiv2019

Imaging collective behavior in an rf-SQUID metamaterial tuned by DC and RF magnetic fields

We examine the collective behavior of two-dimensional nonlinear superconducting metamaterials using a non-contact spatially resolved imaging technique. The metamaterial is made up of sub-wavelength nonlinear oscillators in a strongly coupled 27x27 planar array of radio-frequency Superconducting QUantum Interference Devices (rf SQUIDs). By using low-temperature laser scanning microscopy we image microwave currents in the driven SQUIDs while in non-radiating dark modes and identify the clustering and uniformity of like-oscillating meta-atoms. We follow the rearrangement of coherent patterns due to meta-atom resonant frequency tuning as a function of external dc and rf magnetic flux bias. We find that the rf current distribution across the SQUID array at zero dc flux and small rf flux reveals a low degree of coherence. By contrast, the spatial coherence improves dramatically upon increasing of rf flux amplitude, in agreement with simulation.

cond-mat.supr-con↗

arXiv2017

Imaging the paramagnetic nonlinear Meissner effect in nodal gap superconductor

Boundary surfaces of nodal gap superconductors can host Andreev bound states (ABS) which develop a paramagnetic response under external RF field in contrast to the bulk diamagnetic response of the bulk superconductor. At low temperature this surface paramagnetic response dominates and enhances the nonlinear RF response of the sample. With a recently developed photoresponse imaging technique, the anisotropy of this "paramagnetic" nonlinear Meissner response, and its current direction (angular) and RF power dependence has been systematically studied. A theoretical model describing the current flow in the surface paramagnetic Andreev bound state, the bulk diamagnetic Meissner state, and their response to optical illumination is proposed and it shows good agreement with the experimental results.

cond-mat.supr-con↗

arXiv2013

Imaging the Anisotropic Nonlinear Meissner Effect in Nodal YBa$_{2}$Cu$_{3}$O$_{7-δ}$ Thin-Film Superconductors

We have directly imaged the anisotropic nonlinear Meissner effect in an unconventional superconductor through the nonlinear electrodynamic response of both (bulk) gap nodes and (surface) Andreev bound states. A superconducting thin film is patterned into a compact self-resonant spiral structure, excited near resonance in the radio-frequency range, and scanned with a focused laser beam perturbation. At low temperatures, direction-dependent nonlinearities in the reactive and resistive properties of the resonator create photoresponse that maps out the directions of nodes, or of bound states associated with these nodes, on the Fermi surface of the superconductor. The method is demonstrated on the nodal superconductor YBa$_{2}$Cu$_{3}$O$_{7-δ}$ and the results are consistent with theoretical predictions for the bulk and surface contributions.

cond-mat.supr-con↗

arXiv2012

Unconventional rf photoresponse from a superconducting spiral resonator

Superconducting thin film resonators employing strip geometries show great promise in rf/microwave applications due to their low loss and compact nature. However, their functionality is limited by nonlinear effects at elevated rf/microwave powers. Here, we show that by using a planar spiral geometry carrying parallel currents in adjacent turns, this limitation can be minimized. We investigate the rf current distributions in spiral resonators implemented with Nb thin films via laser scanning microscopy. The rf current density profile along the width of the individual turns of the resonators reveals an unconventional trend: maximum current in the middle of the structure and decaying toward its edges. This unusual behavior is associated with the circular nature of the geometry and the cancellation of magnetic field between the turns, which is favorable for handling high powers since it allows the linear characteristics to persist at high rf current densities.

cond-mat.supr-con↗

arXiv2012

Switching nonlinearity in a superconductor-enhanced metamaterial

We demonstrate a nonlinear metamaterial that can be switched between low and high transmission by controlling the power level of the incident beam. The origin of this nonlinear response is the superconducting Nb thin film employed in the metamaterial structure. We show that with moderate RF power of about 22 dBm it is possible to quench the superconducting state as a result of extremely strong current densities at the corners of the metamaterial's split-ring resonators. We measure a transmission contrast of 10 dB and a change in group delay of 70 ns between the low and high power states.

cond-mat.mtrl-sci↗

arXiv2011

Microscopic examination of hot spots giving rise to nonlinearity in superconducting resonators

We investigate the microscopic origins of nonlinear rf response in superconducting electromagnetic resonators. Strong nonlinearity appearing in the transmission spectra at high input powers manifests itself through the emergence of jumplike features near the resonant frequency that evolve toward lower quality factor with higher insertion loss as the rf input power is increased. We directly relate these characteristics to the dynamics of localized normal regions (hot spots) caused by microscopic features in the superconducting material making up the resonator. A clear observation of hot-spot formation inside a Nb thin film self-resonant structure is presented by employing the microwave laser scanning microscope, and a direct link between microscopic and macroscopic manifestations of nonlinearity is established.

cond-mat.supr-con↗

arXiv2011

Classical Analogue of Electromagnetically Induced Transparency with a Metal-Superconductor Hybrid Metamaterial

Metamaterials are engineered materials composed of small electrical circuits producing novel interactions with electromagnetic waves. Recently, a new class of metamaterials has been created to mimic the behavior of media displaying electromagnetically induced transparency (EIT). Here we introduce a planar EIT metamaterial that creates a very large loss contrast between the dark and radiative resonators by employing a superconducting Nb film in the dark element and a normal-metal Au film in the radiative element. Below the critical temperature of Nb, the resistance contrast opens up a transparency window along with a large enhancement in group delay, enabling a significant slowdown of waves. We further demonstrate precise control of the EIT response through changes in the superfluid density. Such tunable metamaterials may be useful for telecommunication because of their large delay-bandwidth products.

cond-mat.supr-con↗

arXiv2010

Spatial and frequency dependencies of local photoresponse of HTS strip-line resonator in regime of two-tone microwave intermodulation excitation

A new phenomenological approach to spatially-resolved research of nonlinear (NL) microwave properties of operating thin-film superconducting resonators is proposed. The approach is based on frequency and spatial singularity of Laser Scanning Microscopy (LSM) images that can be extracted from a set of 2-D patterns representing x-y distribution of the LSM photoresponse, PR(x, y), at fixed third-order intermodulation (IMD) frequencies 2f1-f2 and 2f2-f1 as a result of two-tone resonator microwave excitation at equidistant frequencies f1 and f2 relative to the fundamental resonance, f0. It was shown by us earlier that the total LSM PR(x, y) originates from two independent (resistive, PRR(x, y), and inductive, PRX(x, y)) contributions which can be extracted directly from the LSM images acquired at f1 and f2 by using a method of spatially-resolved complex impedance partition [1]. Here, we show that practically the same manipulation of LSM images at 2f1-f2 and 2f2-f1 can be used to present NL components of IMD LSM PR(x, y) in terms of its independent spatial variations of (i) inductive IMD_IND(x, y) and (ii) resistive IMD_RES(x, y) contributions reflecting the origin of the local sources of microwave NL. [1] A.P. Zhuravel, S.M. Anlage, and A.V. Ustinov, Appl. Phys. Lett., vol. 88, p. 212503, 2006.

cond-mat.supr-con↗

arXiv2007

Spatial correlation of linear and nonlinear electron transport in superconducting microwave resonator: laser scanning microscopy analysis

Spatially-resolved techniques of laser scanning microscopy (LSM) have been used to image simultaneously the spatial variations of (i) rf current flow, J$_RF$(x,y), of (ii) areas of resistive dissipation and (iii) the sources of microwave nonlinearity (NL) in an operating superconducting resonator. The RF power dependent spatial evolution of these linear and NL microwave properties in the meander strip YBCO/LAO superconducting resonator have been LSM probed at different temperatures below Tc. The influence of both topologies of the twin-domain YBCO structure and of J$_RF$(x,y) peaks at the edges of superconducting strip line on its NL properties was analyzed in detail with a micron-scale spatial resolution. Result shows the resistive origin of the dominant sources of microwave NLs

cond-mat.supr-con↗

arXiv2006

Tunability of Superconducting Metamaterials

Metamaterials are artificial structures with unique electromagnetic properties, such as relative dielectric permittivity and magnetic permeability with values less than 1, or even negative. Because these properties are so sensitive to loss, we have developed metamaterials comprised of superconducting waveguides, wires, and split-ring resonators. An important requirement for applications of these metamaterials is the ability to tune the frequency at which the unique electromagnetic response occurs. In this paper we present three methods (unique to superconductors) to accomplish this tuning: temperature, dc magnetic field, and rf magnetic field. Data are shown for dc and rf magnetic field tuning of a single Nb split-ring resonator (SRR). It was found that the dc field tuning was hysteritic in the resonant frequency data, while the quality factor, $Q$, was less hystertic. The rf power tuning showed no hysteresis, but did show supression of the $Q$ at high power. Magneto-optical images reveal inhomogeneous magnetic vortex entry in the dc field tuning, and laser scanning photoresponse images for a YBa$_2$Cu$_3$O$_{7-δ}$ SRR reveals the current distribution in the rings.

cond-mat.supr-con↗

arXiv2006

Imaging of Microscopic Sources of Resistive and Reactive Nonlinearities in Superconducting Microwave Devices

The technique of low-temperature Laser Scanning Microscopy (LSM) has been applied to the investigation of local microwave properties in operating YBa2Cu3O7/LaAlO3 thin-film resonators patterned into a meandering strip transmission line. By using a modified newly developed procedure of spatially-resolved complex impedance partition, the influence of inhomogeneous current flow on the formation of nonlinear (NL) microwave response in such planar devices is analyzed in terms of the independent impact from resistive and inductive components. The modified procedure developed here is dramatically faster than our previous method. The LSM capability to probe the spatial variations of two-tone, third-order intermodulation currents on micron length scales is used to find the 2D distribution of the local sources of microwave NL. The results show that the dominant sources of microwave NL are strongly localized in the resistive domains.

cond-mat.supr-con↗

arXiv2005

Measurement of Local Reactive and Resistive Photoresponse of a Superconducting Microwave Device

We propose and demonstrate a spatial partition method for the high-frequency photo-response of superconducting devices correlated with inductive and resistive changes in microwave impedance. Using a laser scanning microscope, we show that resistive losses are mainly produced by local defects at microstrip edges and by intergrain weak links in the high-temperature superconducting material. These defects initiate nonlinear high-frequency response due to overcritical current densities and entry of vortices.

cond-mat.supr-con↗

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↗