X. X. Xi

Temple University, Filadelfia, Pennsylvania, United States

Are you X. X. Xi?

Claim your profile

Publications (373)921.77 Total impact

  • Elias Galan · Daniel Cunnane · Ke Chen · X. X. Xi
    [Show abstract] [Hide abstract]
    ABSTRACT: We have fabricated and tested rapid single flux quantum toggle flip-flop circuits using self-shunted MgB2/ MgO/MgB2 Josephson junctions with a single ground layer. The MgB2 films were deposited using HPCVD, and the MgO barrier was deposited using DC reactive sputtering. We highlight the circuit's voltage divider operation dependence on current bias and temperature which show operation from 7 to 30 K. The highest attained operating speed is 335 GHz at 7 K. These results demonstrate the versatile temperature range and speed of MgB2 digital circuits.
    IEEE Transactions on Applied Superconductivity 06/2015; 25(3):1-4. DOI:10.1109/TASC.2014.2366912 · 1.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Hot-Electron Bolometer (HEB) mixers have proven to be the best tool for high-resolution spectroscopy at the Terahertz frequencies. However, the current state of the art NbN mixers suffer from a small intermediate frequency (IF) bandwidth as well as a low operating temperature. MgB2 is a promising material for HEB mixer technology in view of its high critical temperature and fast thermal relaxation allowing for a large IF bandwidth. In this work, we have fabricated and characterized thin-film (~15 nm) MgB2-based spiral antenna-coupled HEB mixers on SiC substrate. We achieved the IF bandwidth greater than 8 GHz at 25 K and the device noise temperature <; 4000 K at 9 K using a 600 GHz source. Using temperature dependencies of the radiation power dissipated in the device we have identified the optical loss in the integrated microantenna responsible as a cause of the limited sensitivity of the current mixer devices. From the analysis of the current-voltage (IV) characteristics, we have derived the effective thermal conductance of the mixer device and estimated the required local oscillator power in an optimized device to be ~4 μW.
    IEEE Transactions on Applied Superconductivity 06/2015; 25(3):1-6. DOI:10.1109/TASC.2014.2369353 · 1.24 Impact Factor
  • T. Melbourne · D. Cunnane · E. Galan · X. X. Xi · Ke Chen
    [Show abstract] [Hide abstract]
    ABSTRACT: Sandwich-type MgB2/MgO/MgB2 Josephson junctions with sizes ranging from 0.5 μm × 0.5 μm to 10 μm × 10 μm were fabricated using electron beam lithography. The junctions show critical current densities as high as 130 kA/cm2. The smallest of the junctions operated free of hysteresis below 4.2 K. They may be ideal for use in rapid single flux quantum circuits with no requirement for external shunt resistors. The 10-μm junctions show good agreement with the expected Fraunhofer-like modulation of critical current with magnetic field; however, the smaller junctions show a sinusoidal modulation, indicating an anomalous current distribution that needs to be further investigated. Series arrays of Josephson junctions were made to quantitatively study the uniformity of junction parameters, which shows a 54% spread in critical current of a 100 junction array of 4-μm junctions.
    IEEE Transactions on Applied Superconductivity 06/2015; 25(3):1-4. DOI:10.1109/TASC.2014.2366076 · 1.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We fabricated and characterized the optical response of MgB2 nanowires with critical temperature TC = 33 K. The devices showed optical response at 4 K and sub-nanosecond relaxation time. The detectors responded to the simultaneous absorption of three photons, but not to single photons.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: For potential applications in superconducting RF cavities, we have investigated the properties of polycrystalline MgB2 films, including the thickness dependence of the lower critical field H c1. MgB2 thin films were fabricated by hybrid physical-chemical vapor deposition on (0001) SiC substrate either directly (for epitaxial films) or with a MgO buffer layer (for polycrystalline films). When the film thickness decreased from 300 nm to 100 nm, H c1 at 5 K increased from around 600 Oe to 1880 Oe in epitaxial films and to 1520 Oe in polycrystalline films. The result is promising for using MgB2/MgO multilayers to enhance the vortex penetration field.
    APL Materials 04/2015; 3(4):041101. DOI:10.1063/1.4916696 · 2.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report the influence of two-band superconductivity on the flux creep and the critical current densities of a MgB2 thin film. The small magnetic penetration depth of λ=50±10 nm at T=4 K is related to a clean π-band. We find a high self-field critical current density Jc, which is strongly reduced with applied magnetic field, and attribute this to suppression of the superconductivity in the π-band. The temperature dependence of the creep rate S (T) at low magnetic field can be explained by a simple Anderson–Kim mechanism. The system shows high pinning energies at low field that are strongly suppressed by high field.
    Solid State Communications 02/2015; 204. DOI:10.1016/j.ssc.2014.11.015 · 1.90 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Investigations of MgB2 and Fe-based superconductors in recent years have revealed many unusual effects of multiband superconductivity but manifestations of anisotropic multiband effects in the critical current density Jc have not been addressed experimentally, mostly because of the difficulties to measure Jc along the c-axis. To investigate the effect of very different intrinsic anisotropies of sigma and pi electron bands in MgB2 on current transport, we grew epitaxial films with tilted c-axis (THETA ~ 19.5{\deg}), which enabled us to measure the components of Jc both along the ab-plane and the c-axis using magneto-optical and transport techniques. These measurements were combined with scanning and transmission electron microscopy, which revealed terraced steps on the surface of the c-axis tilted films. The measured field and temperature dependencies of the anisotropic Jc(H) show that Jc,L parallel to the terraced steps is higher than Jc,T perpendicular to the terraced steps, and Jc of thinner films (50 nm) obtained from transport experiments at 0.1 T reaches ~10% of the depairing current density Jd in the ab plane, while magneto-optical imaging revealed much higher Jc at lower fields. To analyze the experimental data we developed a model of anisotropic vortex pinning which accounts for the observed behavior of Jc in the c-axis tilted films and suggests that the apparent anisotropy of Jc is affected by current pairbreaking effects in the weaker {\pi} band. Our results indicate that the out-of-plane current transport mediated by the {\pi} band could set the ultimate limit of Jc in MgB2 polycrystals.
    Physical review. B, Condensed matter 12/2014; 90:214509. DOI:10.1103/PhysRevB.90.214509 · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A comprehensive microstructural study was conducted on optimally-doped epitaxial Ba(Fe1−xCox)2As2 thin films grown by pulsed laser deposition on various substrates of a wide range of lattice constants: SrTiO3, LaAlO3, (La,Sr)(Al,Ta)O3, MgO, CaF2, and BaF2. We found that epitaxial strain directly affects the superconductivity in the film, with the transition temperature decreasing linearly with increasing in-plane lattice constant of the film. However, the strain is not determined by the lattice mismatch between the film and substrate. Instead, the mosaic spread of the grain orientation in the film and the thermal expansion coefficient of the substrate were found to correlate well with the in-plane lattice constant of the film. The result confirms the importance of structural distortions to the superconductivity in the Ba(Fe1−xCox)2As2 films.
    Superconductor Science and Technology 10/2014; 27(11):115010. DOI:10.1088/0953-2048/27/11/115010 · 2.33 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Terahertz high-resolution spectroscopy of interstellar molecular clouds greatly relies on hot-electron superconducting bolometric (HEB) mixers. Current state-of-the-art receivers use mixer devices made from ultrathin (~ 3-5 nm) films of NbN with critical temperature ~ 9-11 K. Such mixers have been deployed on a number of groundbased, suborbital, and orbital platforms including the HIFI instrument on the Hershel Space Observatory. Despite its good sensitivity and well-established fabrication process, the NbN HEB mixer suffers from the narrow intermediate frequency (IF) bandwidth ~ 2-3 GHz and is limited to operation at liquid Helium temperature. As the heterodyne receivers are now trending towards “high THz” frequencies, the need in a larger IF bandwidth becomes more pressing since the same velocity resolution for a Doppler shifted line at 5 THz requires a 5-times greater IF bandwidth than at 1 THz. Our work is focusing on the realization of practical HEB mixers using ultrathin (10-20 nm) MgB2 films. They are prepared using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process yielding ultrathin films with critical temperature ~ 37-39 K. The expectation is that the combination of small thickness, high acoustic phonon transparency at the interface with the substrate, and very short electron-phonon relaxation time may lead to IF bandwidth ~ 10 GHz or even higher. SiC continues to be the most favorable substrate for MgB2 growth and as a result, a study has been conducted on the transparency of SiC at THz frequencies. FTIR measurements show that semi-insulating SiC substrates are at least as transparent as Si up to 2.5 THz. Currently films are passivated using a thin (10 nm) SiO2 layer which is deposited ex-situ via RF magnetron sputtering. Micron-sized spiral antenna-coupled HEB mixers have been fabricated using MgB2 films as thin as 10 nm. Fabrication was done using contact UV lithography and Ar Ion milling, with E-beam evaporated Au films deposited for the antenna. Measurements have been carried out on these devices in the DC, Microwave, and THz regimes. The devices are capable of mixing signals above 20 K indicating that operation may be possible using a cryogen-free cooling system. We will report the results of all measurements taken to indicate the local oscillator power requirements and the IF bandwidth of MgB2 HEB mixers.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A localized measurement of the RF critical field on superconducting radio frequency (SRF) cavity materials is a key step to identify specific defects that produce quenches of SRF cavities. Two measurements are performed to demonstrate these capabilities with a near-field scanning probe microwave microscope. The first is a third harmonic nonlinear measurement on a high Residual-Resistance-Ratio bulk Nb sample showing strong localized nonlinear response, with surface RF magnetic field B_{surface} ~102 mT. The second is a raster scanned harmonic response image on a MgB_{2} thin film demonstrating a uniform nonlinear response over large areas.
  • Source
  • [Show abstract] [Hide abstract]
    ABSTRACT: Polarized infrared reflectance spectra of a BaTiO3/SrTiO3 superlattice deposited on the (110) cut of SmScO3 substrate and bare substrate have been studied in the broad temperature range from 10 to 650 K. Strong infrared anisotropy of the SmScO3 substrate was observed and explained as a projection of B1u and B2u + B3u phonons into the (110) plane. In the polarization parallel to the c-axis below 200 cm−1, an anomaly in the temperature dependence of the spectra was observed. Fitting the superlattice and substrate spectra, the parameters of optical phonons and their temperature dependence were determined. The superlattice phonon frequencies show only classical behavior, it means frequency hardening on cooling. Anisotropy was also found in the superlattice phonon frequencies and their contributions to static permittivity.
    Journal of Applied Physics 05/2014; 115(18):184102-184102-7. DOI:10.1063/1.4875877 · 2.18 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present results of sandwich-type MgB2/TiB2/MgB2 superconductor–normal metal–superconductor Josephson junctions with MgB2 films grown by hybrid physical–chemical vapor deposition and TiB2 barriers, between 5 and 10 nm in thickness, grown by DC magnetron sputtering. Most junctions with an 8 nm or thicker barrier of TiB2 showed little excess current and no subgap features and can be described by the resistively-shunted-junction model. Meanwhile prominent multiple Andreev reflection characteristics such as excess currents and subgap peaks were observed in some junctions with thinner barriers. DC and AC Josephson effects were observed in agreement with the theory. The results demonstrate the possibility of leakage-free sandwich-type all-MgB2 junctions using conducting barrier materials
    Superconductor Science and Technology 05/2014; 27(6):065015. DOI:10.1088/0953-2048/27/6/065015 · 2.33 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Several theoretical analyses of the two superconducting energy gaps of magnesium diboride, $\Delta_\pi$ and $\Delta_\sigma$, predict substructures within each energy gap, rather than two pure numbers. Recent experiments have revealed similar structures. We report tunneling conductance data providing additional experimental evidence for these features. The absence of these features in c-axis tunneling, and a sharp peak in the subgap (associated with the counterelectrode material), support the conclusion that these features are intrinsic to MgB2. By demonstrating the inadequacy of a simple two-gap model in fitting the data, we illustrate that some distinctions between theoretical models of energy gap substructures are experimentally accessible.
    Superconductor Science and Technology 03/2014; 28(5). DOI:10.1088/0953-2048/28/5/055015 · 2.33 Impact Factor
  • Source
    M. A. Wolak · T. Tan · A. Krick · E. Johnson · M. Hambe · Ke Chen · X. X. Xi
    [Show abstract] [Hide abstract]
    ABSTRACT: We have investigated the coating of an inner surface of superconducting radio frequency cavities with a magnesium diboride thin film by hybrid physical-chemical vapor deposition (HPCVD). To simulate a 6 GHz rf cavity, a straight stainless steel tube of 1.5-inch inner diameter and a dummy stainless steel cavity were employed, on which small sapphire and metal substrates were mounted at different locations. The MgB2 films on these substrates showed uniformly good superconducting properties including Tc of 37–40 K, residual resistivity ratio of up to 14, and root-mean-square roughness Rq of 20–30 nm. This work demonstrates the feasibility of coating the interior of cylindrical and curved objects with MgB2 by the HPCVD technique, an important step towards superconducting rf cavities with MgB2 coating.
    Physical Review Special Topics - Accelerators and Beams 01/2014; 17(1):012001. DOI:10.1103/PhysRevSTAB.17.012001 · 1.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A localized measurement of the RF critical field on superconducting radio frequency (SRF) cavity materials is a key step to identify specific defects that produce quenches of SRF cavities. Two new measurements are performed to demonstrate these capabilities with a novel near-field scanning probe microwave microscope. The first is a third harmonic nonlinear measurement on a high Residual- Resistance-Ratio bulk Nb sample showing strong localized nonlinear response for the first time, with surface RF magnetic field $B_{surface} \sim 10^{2}$ $mT$. The second is a raster scanned harmonic response image on a high quality $MgB_{2}$ thin film demonstrating a quench defect-free surface over large areas.
    Applied Physics Letters 12/2013; 104(23). DOI:10.1063/1.4881880 · 3.30 Impact Factor
  • Daniel Cunnane · Elias Galan · Ke Chen · X. X. Xi
    [Show abstract] [Hide abstract]
    ABSTRACT: We report planar-type MgB2 DC SQUIDs fabricated using a multilayer process. The SQUIDs employed sandwich-type MgB2/MgO/MgB2 Josephson junctions and a MgB2 wiring layer. The SQUIDs showed large voltage modulation above 500 μV at 15 K and operated in a broad temperature range from 15 to 37 K with a transfer function as high as 2.3 mV/Φ0. The minimum SQUID noise was measured at 26 K in a cryocooler, yielding a white noise around 10 μΦ0/Hz1/2 and 1/f corner frequency near 10 Hz. The result demonstrates great promise of MgB2 SQUIDs for practical applications.
    Applied Physics Letters 11/2013; 103(21-21):212603. DOI:10.1063/1.4833022 · 3.30 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The miniaturization and integration of frequency-agile microwave circuits-relevant to electronically tunable filters, antennas, resonators and phase shifters-with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss-Srn+1TinO3n+1 phases-in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics-doping or strain-in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.
    Nature 10/2013; 502(7472). DOI:10.1038/nature12582 · 41.46 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have studied the stoichiometry of epitaxial LaAlO3 thin films on SrTiO3 substrate grown by pulsed laser deposition as a function of laser energy density and oxygen pressure during the film growth. Both x-ray diffraction (θ-2θ scan and reciprocal space mapping) and transmission electron microscopy (geometric phase analysis) revealed a change of lattice constant in the film with the distance from the substrate. Combined with composition analysis using x-ray fluorescence we found that the nominal unit-cell volume expanded when the LaAlO3 film was La-rich, but remained near the bulk value when the film was La-poor or stoichiometric. La excess was found in all the films deposited in oxygen pressures lower than 10−2 Torr. We conclude that the discussion of LaAlO3/SrTiO3 interfacial properties should include the effects of cation off-stoichiometry in the LaAlO3 films when the deposition is conducted under low oxygen pressures.
    Journal of Applied Physics 07/2013; 114(2). DOI:10.1063/1.4811821 · 2.18 Impact Factor
  • Daniel Cunnane · Ke Chen · X. X. Xi
    [Show abstract] [Hide abstract]
    ABSTRACT: Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.
    Applied Physics Letters 06/2013; 102(22). DOI:10.1063/1.4809587 · 3.30 Impact Factor

Publication Stats

7k Citations
921.77 Total Impact Points


  • 2011–2015
    • Temple University
      • Department of Physics
      Filadelfia, Pennsylvania, United States
  • 1998–2012
    • Pennsylvania State University
      • • Materials Research Institute
      • • Department of Materials Science and Engineering
      • • Department of Physics
      University Park, Maryland, United States
    • University of Wisconsin–Madison
      • Department of Physics
      Madison, Wisconsin, United States
  • 1999–2011
    • William Penn University
      Worcester, Massachusetts, United States
  • 2007
    • University of Pennsylvania
      Philadelphia, Pennsylvania, United States
    • Università degli Studi di Genova
      • Department of Physics
      Genova, Liguria, Italy
  • 1991–2006
    • University of Maryland, College Park
      • • Department of Physics
      • • Department of Electrical & Computer Engineering
      College Park, MD, United States
  • 2005
    • University of Naples Federico II
      • Department of Physical Sciences
      Napoli, Campania, Italy
  • 1995
    • National Institute of Standards and Technology
      Maryland, United States
  • 1993–1995
    • Loyola University Maryland
      • Department of Physics
      Baltimore, Maryland, United States
  • 1990–1991
    • Rutgers, The State University of New Jersey
      • Department of Physics
      New Brunswick, New Jersey, United States
  • 1989
    • Peking University
      Peping, Beijing, China