X. X. Xi

Temple University, Philadelphia, Pennsylvania, United States

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Publications (361)896.69 Total impact

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    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.
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    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. · 2.76 Impact Factor
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    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
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    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.
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    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. · 2.76 Impact Factor
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    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.
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    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. · 1.57 Impact Factor
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    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 01/2014; 115(18):184102-184102-7. · 2.21 Impact Factor
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    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). · 3.52 Impact Factor
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    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; · 38.60 Impact Factor
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    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). · 2.21 Impact Factor
  • Daniel Cunnane, Ke Chen, X. X. Xi
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    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). · 3.52 Impact Factor
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    ABSTRACT: Thin film $hbox{MgB}_{2}$ is a promising material for technical improvements in superconducting radio frequency (SRF) technology and applications. At present, bulk niobium SRF accelerating cavities suffer from a fundamental upper limit in maximally sustained accelerating gradients; however, a scheme involving multilayered superstructures consisting of superconducting-insulating-superconducting (SIS) layers has been proposed to overcome this fundamental material limit of 50 MV/m. The SIS multilayer paradigm is reliant upon implementing a thin shielding material with a suitably high HC1, which may prevent early field penetration in a bulk material layer and consequently delay the high field breakdown. It has been predicted that for thin superconducting films—thickness less than the London penetration depth ( $sim$ 120 nm in the case of $hbox{MgB}_{2}$ )—the lower critical field $H_{{rm C}1}$ can be enhanced with decreasing thickness. Thus, $hbox{MgB}_{2}$ , with a high $T_{rm C}$ and relatively low $H_{{rm C}1}$ value, as compared with Nb, is a prime candidate for such SIS structures. Here we present our study on the microstructure, surface morphology, and superconducting properties on a thickness series of $hbox{MgB}_{2}$ thin films and correlate the effects of film thickness and surface morphology on $H_{{rm C}1}$ .
    IEEE Transactions on Applied Superconductivity 06/2013; 23:7500604-7500604. · 1.20 Impact Factor
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    Applied Physics Letters 03/2013; 102(10). · 3.52 Impact Factor
  • Daniel Cunnane, Ke Chen, X. X. Xi
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    ABSTRACT: High-speed superconducting circuits may benefit from the high Tc and large superconducting gap of MgB2. Nb remains the state of the art for superconducting electronics partly because of its small penetration depth and its isotropic nature. A microscopic theory on the penetration depth of multiband superconductors states that a clean MgB2 sample is nearly isotropic while a sample in the dirty limit is anisotropic. We have made and measured DC SQUIDs using MgB2 Josephson junctions to determine the inductance of an MgB2 microstrip. The penetration depth along the c-axis, λc, was calculated using the inductance value and dimensions of the microstrip. We have previously reported the absolute value of the penetration depth of our MgB2 films to be around 40 nm. Now we have made devices with film ranging from the clean limit to the dirty limit by adding defects during the deposition. The absolute value of λc at low temperature is compared to the cleanliness of the film. The temperature dependence was also measured which is non-trivial due to the two-gap nature of MgB2. These results are compared with theory that confirmed our previous results.
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    ABSTRACT: We have grown epitaxial, optimally-doped superconducting Ba(Fe0.92Co0.08)2As2 films on SrTiO3, (La, Sr)(Al, Ta)O3 and LaAlO3 substrates, which have a range of lattice mismatch, and studied the strain effect on the structural and transport properties of the films. We found that the superconducting transition temperature increased as the c lattice constant decreased and a lattice constant increased. The thickness dependence of the superconducting transition temperature was studied, which was related to the strain and strain relaxation. A zero-resistance Tc of 21.7 K was obtained in the 120 nm-thick Ba(Fe0.92Co0.08)2As2 film on SrTiO3 substrate.
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    ABSTRACT: Josephson tunnel junctions made of multi-gap and single-gap superconducting electrodes provide a useful system for understanding multiple gap superconductivity. Peaks in the differential conductance curve have been used to characterize the energy gaps of such multi-gap materials [e.g. Chen, K. et al., Nat. Commun. 3:619 (2012)]. Superconducting-to-normal switching data can also provide useful insights. While ramping the current from zero to the critical current, the washboard potential is tilted, thereby adjusting the resonant frequency of the potential well, and altering the energy level spacing. By exciting the junction with microwaves, resonant modes may be explored. We report results of conductance and switching experiments on MgB2/I/Pb and MgB2/I/Sn junctions, with and without microwaves, in a helium dilution refrigerator with a base temperature 20mK. These results exhibit tunneling modes and resonances not observed in single-gap/single-gap junctions, including a peak in the escape rate that may be consistent with coupling to the Leggett mode.
  • Wenqing Dai, Ke Chen, X. X. Xi, Qi Li
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    ABSTRACT: We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB2 by measuring planar tunnel junctions made from epitaxial MgB2 thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the π (∼1.8 meV) and σ (∼7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The σ gap was enhanced (∼7.9 meV) in MgB2 thin films on SiC substrates which had Tc values over 40 K due to epitaxial tensile strain. As the MgB2 film thickness decreased from 100 nm to 33 nm, the π gap distribution range was narrowed from ∼1.7 meV to ∼1.4 meV, and the fine peak structures disappeared due to the reduced electron mean free path limited by the film thickness. The fine peak structures were also gradually smeared out when the junction resistance increased. The results show that the electron scattering, either from the MgB2 film or the junction interface or barrier, can smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB2 film surface was also found to cause an increase in the π gap value, likely due to an enhancement of interband scattering.
    Journal of Applied Physics 02/2013; 113(8). · 2.21 Impact Factor
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    ABSTRACT: The penetration depth of MgB2 was measured using two methods of different mechanisms. The first method used MgB2 Josephson junctions and the magnetic field dependence of the junction critical current. The second method deduced the penetration depth from the inductance of a MgB2 microstrip used to modulate the voltage of a MgB2 DC SQUID. The two methods showed a consistent value of the low-temperature penetration depth for MgB2 to be about 40 nm. Both the small penetration depth value and its temperature dependence are in agreement with a microscopic theory for MgB2 in the clean limit.
    Applied Physics Letters 02/2013; 102(7). · 3.52 Impact Factor

Publication Stats

4k Citations
896.69 Total Impact Points


  • 2011–2014
    • Temple University
      • Department of Physics
      Philadelphia, Pennsylvania, United States
  • 1998–2012
    • Pennsylvania State University
      • • Department of Physics
      • • Department of Materials Science and Engineering
      • • Materials Research Institute
      University Park, MD, United States
  • 2008–2010
    • Centro Atómico Bariloche
      San Carlos de Bariloche, Río Negro, Argentina
    • Dalton State College
      Georgia, United States
  • 1989–2008
    • Peking University
      • State Key Laboratory for Artificial Microstructure and Mesoscopic Physics
      Beijing, Beijing Shi, China
  • 2007
    • Università degli Studi di Genova
      • Department of Physics
      Genova, Liguria, Italy
    • University of Pennsylvania
      Philadelphia, Pennsylvania, United States
  • 1991–2006
    • University of Maryland, College Park
      • • Department of Physics
      • • Department of Electrical & Computer Engineering
      College Park, MD, United States
      Mississippi, United States
  • 2005
    • Universiteit Twente
      Enschede, Overijssel, Netherlands
    • University of Naples Federico II
      • Department of Physical Sciences
      Napoli, Campania, Italy
    • Brookhaven National Laboratory
      New York City, New York, United States
  • 2003
    • IPMC
      Akra, Greater Accra, Ghana
    • Arizona State University
      • Department of Chemical Engineering
      Mesa, AZ, United States
  • 1995
    • National Institute of Standards and Technology
      Maryland, United States
  • 1994
    • The Ohio State University
      • Department of Physics
      Columbus, Ohio, United States
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 1990–1991
    • Rutgers, The State University of New Jersey
      • Department of Physics
      New Brunswick, NJ, United States
    • Los Alamos National Laboratory
      Los Alamos, California, United States
  • 1988
    • Peking University People's Hospital
      Peping, Beijing, China