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Search Results: 1 - 10 of 539297 matches for " A. V. Berenov "
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Growth of Strongly Biaxially Aligned MgB2 Thin Films on Sapphire by Post-annealing of Amorphous Precursors
A. Berenov,Z. Lockman,X. Qi,Y. Bugoslavsky,L. F. Cohen,M. -H. Jo,N. A. Stelmashenko,V. N. Tsaneva,M. Kambara,N. Hari Babu,D. A. Cardwell,M. G. Blamire,J. L. MacManus-Driscoll
Physics , 2001, DOI: 10.1063/1.1424070
Abstract: MgB2 thin films were cold-grown on sapphire substrates by pulsed laser deposition (PLD), followed by post-annealing in mixed, reducing gas, Mg-rich, Zr gettered, environments. The films had Tcs in the range 29 K to 34 K, Jcs (20K, H=0) in the range 30 kA/cm2 to 300 kA/cm2, and irreversibility fields at 20 K of 4 T to 6.2 T. An inverse correlation was found between Tc and irreversibility field. The films had grain sizes of 0.1-1 micron and a strong biaxial alignment was observed in the 950C annealed film.
The structure of the superconducting gap in MgB2 from point-contact spectroscopy
Y. Bugoslavsky,Y. Miyoshi,G. K. Perkins,A. V. Berenov,Z. Lockman,J. L. MacManus-Driscoll,L. F. Cohen,A. D. Caplin,H. Y. Zhai,M. P. Paranthaman,H. M. Christen,M. Blamire
Physics , 2001, DOI: 10.1088/0953-2048/15/4/308
Abstract: We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 +- 0.3 K and resistivity of about 20 microOhm cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, DELTA1=2.3+-0.3 meV and DELTA2=6.2+-0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder-Tinkham-Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic co-existence of two values of the gap are discussed.
Microstructural control and tuning of thermal conductivity in La0.67Ca0.33MnO3+/-δ
J. A. Turcaud,K. Morrison,A. Berenov,N. McN. Alford,K. G. Sandeman,L. F. Cohen
Physics , 2012, DOI: 10.1016/j.scriptamat.2012.11.036
Abstract: Manganites are one of only a small number of material families currently being trialled as room temperature magnetic refrigerants. Here we examine the dependence of the thermal conductivity, K, of La0.67Ca0.33MnO3+/-{\delta} as a function of density, grain size and silver impregnation around room temperature. We use a simple effective medium model to extract relevant trends in the data and demonstrate a threefold increase in thermal conductivity by silver impregnation.
Microwave Surface Resistance in MgB2
A. A. Zhukov,K. Yates,G. K Perkins,Y. Bugoslavsky,M. Polichetti,A. Berenov,J. Driscoll,A. D. Caplin,L. F. Cohen,Ling Hao,J. Gallop
Physics , 2001,
Abstract: Measurements of the temperature dependence of the surface resistance at 3 GHz of 100 micron size grains of MgB_2 separated powder are presented and discussed. The microwave surface resistance data is compared to experimental results of Nb, Bi_2Sr_2CaCu_2O_{8+\delta} (BSCCO) and theoretical predictions of s-wave weak coupling electron-phonon theory (BCS).
Influence of short time milling in R5(Si,Ge)4, R =Gd and Tb, magnetocaloric materials
A. L. Pires,J. H. Belo,J. Turcaud,G. N. P. Oliveira,J. P. Araújo,A. Berenov,L. F. Cohen,A. M. L. Lopes,A. M. Pereira
Physics , 2015,
Abstract: The effect of the short milling times on R5(Si,Ge)4 R =Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5h), a reduction of the Gd5Si1.3Ge2.7 and Tb5Si2Ge2 particles size was achieved down to approximately 3.5 DSm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd5Si1.3Ge2.7 case, a second-order phase transition emerges at high temperatures as a result of ball milling. Consequently, there is a decrease in the magnetocaloric effect of 35% after 150 minutes of milling. Interestingly, an opposite effect is observed in Tb5Si2Ge2 where a 23% increase of the magnetocaloric effect was achieved, driven by the enhancement of the coupling between magnetic and structural transitions arising from internal strain promoted by the milling process.
High Critical Current Density and Improved Irreversibility Field in Bulk MgB2 Made By A Scaleable, Nanoparticle Addition Route
J. Wang,Y. Bugoslavsky,A. Berenov,L. Cowey,A. D. Caplin,L. F. Cohen,J. L. MacManus Driscoll,L. D. Cooley,X. Song,D. C. Larbalestier
Physics , 2002, DOI: 10.1063/1.1506184
Abstract: Bulk samples of MgB2 were prepared with 5, 10, and 15% wt.% Y2O3 nanoparticles added using a simple solid-state reaction route. Transmission electron microscopy (TEM) showed a fine nanostructure consisting of ~3-5 nm YB4 nanoparticles embedded within MgB2 grains of ~400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density Jc was observed, most notably for 10% doping. At 4.2K, the lower bound Jc value was ~2x105A.cm-2 at 2T. At 20K, the corresponding value was ~8x104A.cm-2. Irreversibility fields were ~11.5 T at 4.2K and 5.5T at 20K, compared to ~4T and ~8T, respectively, for high-pressure synthesized bulk samples.
Spontaneous magnetization above $T_C$ in polycrystalline La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.7}$Ba$_{0.3}$MnO$_3$
Jeremy A. Turcaud,Andre M. Pereira,Karl G. Sandeman,Jo?o S. Amaral,Kelly Morrison,Andrey Berenov,Aziz Daoud-Aladine,Lesley F. Cohen
Physics , 2014, DOI: 10.1103/PhysRevB.90.024410
Abstract: In the present work, spontaneous magnetization is observed in the inverse magnetic susceptibility of La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.7}$Ba$_{0.3}$MnO$_3$ compounds above $T_C$ up to a temperature $T^*$. From information gathered from neutron diffraction, dilatometry, and high-field magnetization data, we suggest that $T^*$ is related to the transition temperature of the low-temperature (high magnetic field) magnetic phase. In the temperature region between $T^*$ and $T_C$, the application of a magnetic field drives the system from the high-temperature to low-temperature magnetic phases, the latter possessing a higher magnetization.
Temperature dependence of the microwave surface impedance measured on different kinds of MgB$_2$ samples
A. A. Zhukov,L. F. Cohen,A. Purnell,Y. Bugoslavsky,A. Berenov,J. L. MacManus-Driscoll,H. Y Zhai,Hans M. Christen,Mariappan P. Paranthaman,Douglas H. Lowndes,M. A. Jo,M. C. Blamire,Ling Hao,J. Gallop
Physics , 2001,
Abstract: In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB$_2$. One film has $T_c=30$ K and is not textured, the other is partially c-axis orientated with $T_c=38$ K. These samples show different types of temperature dependence of the field penetration depth: linear for the powder sample, exponential with $\Delta/kT_c<1.76$ (film with $T_c=30$ K) and strong coupling behaviour with $\Delta/kT_c\sim 2.25$ (film with $T_c=38$ K). The results are well described in terms of an anisotropic gap model or presence of a slightly deficient MgB$_2$ phase. The data set for all samples - taken as a whole cannot be fitted into a two gap scenario.
Enhancement of Critical Current Density in low level Al-doped MgB2
A. Berenov,A. Serquis,X. Z. Liao,Y. T. Zhu,D. E. Peterson,Y. Bugoslavsky,K. A. Yates,M. G. Blamire,L. F. Cohen,J. L. MacManus-Driscoll
Physics , 2004, DOI: 10.1088/0953-2048/17/10/001
Abstract: Two sets of MgB2 samples doped with up to 5 at. % of Al were prepared in different laboratories using different procedures. Decreases in the a and c lattice parameters were observed with Al doping confirming Al substitution onto the Mg site. The critical temperature (Tc) remained largely unchanged with Al doping. For 1 - 2.5 at.% doping, at 20K the in-field critical current densities (Jc's) were enhanced, particularly at lower fields. At 5K, in-field Jc was markedly improved, e.g. at 5T Jc was enhanced by a factor of 20 for a doping level of 1 at.% Al. The improved Jcs correlate with increased sample resistivity indicative of an increase in the upper critical field, Hc2, through alloying.
Microwave Power, DC Magnetic Field, Frequency and Temperature Dependence of the Surface Resistance of MgB2
A. A. Zhukov,A. Purnell,Y. Miyoshi,Y. Bugoslavsky,Z. Lockman,A. Berenov,J. L. MacManus-Driscoll,L. F. Cohen,H. Y. Zhai,H. M. Christen,M. P. Paranthaman,D. H. Lowndes,M. H. Jo,M. G. Blamire,Ling Hao,J. C. Gallop
Physics , 2001,
Abstract: The microwave power, dc magnetic field, frequency and temperature dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by a number of characteristics, the most clear being the breakdown in the omega squared law for poor quality samples. Analysis of the experimental data suggests the most attractive procedure for high quality film growth for technical applications.
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