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Effects of proton irradiation and ageing on the superconducting properties of single crystalline and polycrystalline MgB2  [PDF]
G. K. Perkins,Y. Bugoslavsky,A . D. Caplin,J. Moore,T. J. Tate,R. Gwilliam,J. Jun,S. M. Kazakov,J. Karpinski,L. F. Cohen
Physics , 2003, DOI: 10.1088/0953-2048/17/1/039
Abstract: We present magnetisation data on crystalline and polycrystalline MgB2 before and after irradiation with ~1MeV protons. In the virgin crystal the critical current density is below our noise floor of 103A/cm2 over most of the field range. However, after irradiation a peak occurs in the current density as a function of applied magnetic field as the upper critical field Hc2 is approached. After subsequent ageing over a time period of three months, the peak effect is greatly enhanced, exhibiting much stronger pinning over a wide field range, and the upper critical field is approximately doubled, accompanied by a 2K reduction in transition temperature. Similar studies were made on polycrystalline fragments, where irradiation leads to an increased irreversibility field (Jc is enhanced at high fields but decreased at low fields) and a suppression in transition temperature. However, after two years of ageing both parameters returned towards those of the virgin sample.
Enhancement of critical current by microwave irradiation in wide superconducting films  [PDF]
V. M. Dmitriev,I. V. Zolochevskii,E. V. Bezuglyi
Physics , 2006, DOI: 10.1088/0953-2048/19/8/034
Abstract: The temperature dependences of the enhanced critical current in wide and thin Sn films exposed to the microwave field have been investigated experimentally and analyzed. It was found that the microwave field stabilizes the current state of a wide film with respect to the entry of Abrikosov vortices. The stabilizing effect of irradiation increases with frequency. Using similarity between the effects of microwave enhancement of superconductivity observed for homogeneous (narrow films) and inhomogeneous (wide films) distributions of the superconducting current over the film width, we have succeeded in partial extension of the Eliashberg theory to the case of wide films.
Neutron Irradiation of Mg11B2 : From the Enhancement to the Suppression of Superconducting Properties  [PDF]
M. Putti,V. Braccini,C. Ferdeghini,F. Gatti,P. Manfrinetti,D. Marre',A. Palenzona,I. Pallecchi,C. Tarantini,I. Sheikin,H. U. Aebersold,E. Lehmann
Physics , 2004, DOI: 10.1063/1.1880450
Abstract: In this letter we present the effect of neutron irradiation up to fluences of 3.9 1019 n/cm2 on the superconducting properties of MgB2. In order to obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 the critical temperature is slightly diminished (36 K) and the superconducting properties are significantly improved; the upper critical field is increased from 13.5 T to 20.3 T at 12 K and the irreversibility field is doubled at 5 K. For larger neutron fluences the critical temperature is suppressed down to 12 K and the superconducting properties come out strongly degraded.
Enhancement of the superconducting transition temperature of MgB2 by proximity effect of d0 ferromagnet  [PDF]
Takashi Uchino,Kazuyuki Takahashi,Yuki Uenaka,Haruka Soma,Yu Kato,Mitsunobu Okai,Kanako Kodama,Katsuya Ueno,Takafumi Nakagawa,Kumihiro Genji,Takahiro Sakurai,Hitoshi Ohta
Physics , 2013,
Abstract: Heterostructures of superconducting and ferromagnetic materials are of fundamental interest because of the mutual interaction of antagonistic kinds of ordering at the S-F interface. Normally, the superconducting transition temperature Tc should be strongly suppressed at the S-F interface owing to the penetration of Cooper pairs into the ferromagnetic side. Nevertheless, constructive interactions between S and F orders have been suggested to occur via the modification of ferromagnetic order by the superconducting state. This may induce an inhomogeneous magnetic state, often called a cryptoferromagnetic state, and the relevant domain wall effect, which will lead to a local decrease of the pair-breaking parameter. However, the domain wall effect, even if it exists, is quite subtle from the experimental view point and is normally difficult to observe. Here we show that the defect-related d0 ferromagnetism in MgO and the superconductivity in MgB2 do not antagonize, but rather enhance the superconducting transition temperature Tc to any significant degree. We found in superconducting MgB2-d0 ferromagnetic MgO composites that the superconducting transition proceeds in two steps. The first at the S-F interface, between 110-120 K, then in the rest of the bulk at 39 K, which is the Tc of single phase MgB2 superconductor. Moreover, the additional transition emerges at 60 K at the S-F interface especially in the ferromagnetic side, showing a spin-glass-like magnetic state. Our findings reveal that the proximity effect in the superconductor-d0 ferromagnet heterostructures will provide the knowledge and basis to enhance the Tc value of the existing superconductors.
Sonochemical Modification of the Superconducting Properties of MgB2  [PDF]
Tanya Prozorov,Ruslan Prozorov,Alexey Snezhko,Kenneth S. Suslick
Physics , 2003, DOI: 10.1063/1.1609248
Abstract: Ultrasonic irradiation of magnesium diboride slurries in decalin produces material with significant inter-grain fusion. Sonication in the presence of Fe(CO)5 produces magnetic Fe2O3 nanoparticles embedded in the MgB2 bulk. The resulting superconductor-ferromagnet composite exhibits considerable enhancement of the magnetic hysteresis, which implies an increase of vortex pinning strength due to embedded magnetic nanoparticles.
Doubling the Critical Current Density of High Temperature Superconducting Coated Conductors through Proton Irradiation  [PDF]
Y. Jia,M. LeRoux,D. J. Miller,J. G. Wen,W. K. Kwok,U. Welp,M. W. Rupich,X. Li,S. Sathyamurthy,S. Fleshler,A. P. Malozemoff,A. Kayani,O. Ayala-Valenzuela,L. Civale
Physics , 2013, DOI: 10.1063/1.4821440
Abstract: The in-field critical current of commercial YBa2Cu3O7 coated conductors can be substantially enhanced by post-fabrication irradiation with 4 MeV protons. Irradiation to a fluence of 8x10^16 p/cm^2 induces a near doubling of the critical current in fields of 6 T || c at a temperature of 27 K, a field and temperature range of interest for applications such as rotating machinery. A mixed pinning landscape of preexisting precipitates and twin boundaries and small, finely dispersed irradiation induced defects may account for the improved vortex pinning in high magnetic fields. Our data indicate that there is significant head-room for further enhancements.
Magnetic penetration-depth measurements of a suppressed superfluid density of superconducting Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ single crystals by proton irradiation  [PDF]
Jeehoon Kim,N. Haberkorn,M. J. Graf,I. Usov,F. Ronning,L. Civale,E. Nazaretski,G. F. Chen,W. Yu,J. D. Thompson,R. Movshovich
Physics , 2012, DOI: 10.1103/PhysRevB.86.144509
Abstract: We report on the dramatic effect of random point defects, produced by proton irradiation, on the superfluid density $\rho_{s}$ in superconducting Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ single crystals. The magnitude of the suppression is inferred from measurements of the temperature-dependent magnetic penetration depth $\lambda(T)$ using magnetic force microscopy. Our findings indicate that a radiation dose of 2$\times10^{16}$cm$^{-2}$ produced by 3 MeV protons results in a reduction of the superconducting critical temperature $T_{c}$ by approximately 10%. % with no appreciable change in the slope of the upper critical fields. In contrast, $\rho_{s}(0)$ is suppressed by approximately 60%. This break-down of the Abrikosov-Gorkov theory may be explained by the so-called "Swiss cheese model", which accounts for the spatial suppression of the order parameter near point defects similar to holes in Swiss cheese. Both the slope of the upper critical field and the penetration depth $\lambda(T/T_{c})/\lambda(0)$ exhibit similar temperature dependences before and after irradiation. This may be due to a combination of the highly disordered nature of Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ with large intraband and simultaneous interband scattering as well as the $s^\pm$-wave nature of short coherence length superconductivity.
Induced Magnetic Ordering by Proton Irradiation in Graphite  [PDF]
P. Esquinazi,D. Spemann,R. H?hne,A. Setzer,K. -H. Han,T. Butz
Physics , 2003, DOI: 10.1103/PhysRevLett.91.227201
Abstract: We provide evidence that proton irradiation of energy 2.25 MeV on highly-oriented pyrolytic graphite samples triggers ferro- or ferrimagnetism. Measurements performed with a superconducting quantum interferometer device (SQUID) and magnetic force microscopy (MFM) reveal that the magnetic ordering is stable at room temperature.
Superconducting Diamagnetic Fluctuations in MgB2  [PDF]
A. Lascialfari,T. Mishonov,A. Rigamonti,P. Tedesco,A. Varlamov
Physics , 2001, DOI: 10.1103/PhysRevB.65.180501
Abstract: The fluctuating diamagnetic magnetization Mfl at constant field H as a function of temperature and the isothermal magnetization Mfl vs H are measured in MgB2, above the superconducting transition temperature. The expressions for Mfl in randomly oriented powders are derived in the Gaussian approximation of local Ginzburg-Landau theory and used for the analysis of the data. The scaled magnetization Mfl/H^{1/2}*T is found to be field dependent. In the limit of evanescent field the behaviour for Gaussian fluctuations is obeyed while for H>~ 100 Oe the field tends to suppress the fluctuating pairs, with a field dependence of Mfl close to the one expected when short wavelength fluctuations and non-local electrodynamic effects are taken into account. Our data, besides providing the isothermal magnetization curves for T>Tc(0) in a BCS-type superconductor such as MgB2, evidence an enhancement of the fluctuating diamagnetism which is related to the occurrence in this new superconductor of an anisotropic spectrum of the superconducting fluctuations.
Growth of superconducting MgB2 thin films  [PDF]
K. Ueda,M. Naito
Physics , 2002,
Abstract: This is the first review on superconducting MgB2 thin films as far as we know.
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