摘要 :
From an experimental point of view, room-temperature ferroelectricity in BiFeO_3 is raising many questions. Electric measurements made a long time ago on solid solutions of BiFeO_3 with Pb(Ti,Zr)O_3 indicate that a spontaneous ele...
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From an experimental point of view, room-temperature ferroelectricity in BiFeO_3 is raising many questions. Electric measurements made a long time ago on solid solutions of BiFeO_3 with Pb(Ti,Zr)O_3 indicate that a spontaneous electric polarization exists in BiFeO_3 below the Curie temperature T_c=1143 K. Yet in most reported works, the synthesized samples are too conductive at room temperature to get a clear polarization loop in the bulk without any effects of extrinsic physical or chemical parameters. Surprisingly, up to now there has been no report of a P(E) (polarization versus electric field) loop at room temperature on single crystals of BiFeO_3. We describe here our procedure to synthesize ceramics and to grow good quality sizeable single crystals by a flux method. We demonstrate that BiFeO_3 is indeed ferroelectric at room temperature through evidence by piezoresponse force microscopy and P(E) loops. The polarization is found to be large, around 60 μC/cm~2, a value that has only been reached in thin films. Magnetic measurements using a superconducting quantum interference device magnetometer and Mossbauer spectroscopy are also presented. The latter confirms the results of nuclear magnetic resonance measurements concerning the anisotropy of the hyperfine field attributed to the magnetic cycloidal structure.
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摘要 :
The transient response of Ba(Fe_(1_x)Co_x)_2As_2, x=0.08 was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the pr...
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The transient response of Ba(Fe_(1_x)Co_x)_2As_2, x=0.08 was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the presence of two distinct decay channels: a faster one, of less than 1 ps in the considered pump fluence range and a slower one, corresponding to lattice thermalization and lasting ≌6 ps. Our analysis indicates that the fast relaxation should be attributed to preferential scattering of the electrons with only a subset of the lattice-vibration modes with a second moment of the Eliashberg function λ<ω~2> = 64 meV~2. The simultaneous excitation of a strong fully symmetric A_(1g) optical phonon corroborates this conclusion and makes it possible to deduce the value of λ = 0.12. This small value for the electron-phonon coupling confirms that a phonon-mediated process cannot be the only mechanism leading to the formation of superconducting pairs in this family of pnictides.
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摘要 :
We report on the growth of highly epitaxial La_(2/3)Sr_(1/3)MnO_3 thin films on SrTiO_3 (001) oriented substrates by rf magnetron sputtering. It is shown that structural strain caused by lattice mismatch between film and substrate...
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We report on the growth of highly epitaxial La_(2/3)Sr_(1/3)MnO_3 thin films on SrTiO_3 (001) oriented substrates by rf magnetron sputtering. It is shown that structural strain caused by lattice mismatch between film and substrate and kinetics growing instabilities may offer unique opportunities to control the film morphologies. By increasing growth rate, the surface morphology undergoes a dramatic transformation: from very smooth surface through arrays of mounds or antidots to hatched morphology. All nano-objects form long-range ordered arrays running in the steps direction defined by the miscut angle of underlying substrate. These self-assembled structures are of high interest envisaging a new route for the fabrication of oxide-based magnetoelectronic devices by using a bottom-up approach.
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摘要 :
The understanding of the superconducting phase under pressure in the famous iron-based spin ladder BaFe_2Se_3 commands a perfect understanding of the structural properties. We present an accurate single crystal x-ray analysis whic...
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The understanding of the superconducting phase under pressure in the famous iron-based spin ladder BaFe_2Se_3 commands a perfect understanding of the structural properties. We present an accurate single crystal x-ray analysis which demonstrates that the structure admitted until now at room temperature is totally ruled out. We show that the system has a polar structure already at 300 K and is thus ferroelectric. Its structure is modified below the Neel transition and the polarization is intensified. These features unambiguously prove the multiferroic character of BaFe_2Se_3 as proposed theoretically. This system evidences a competition between multiferroicity and superconductivity.
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摘要 :
We have studied the variation of transverse magnetoresistance of underdoped YBCO_(6.6) crystals, either pure or with reduced T_c down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconductin...
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We have studied the variation of transverse magnetoresistance of underdoped YBCO_(6.6) crystals, either pure or with reduced T_c down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconducting fluctuation contribution to the conductivity is suppressed only above a threshold field H'_C{T), which is found to vanish at T'_C > T_C. In the pure YBCO_(6.6) sample, H'_c is already 50 T at T_c. We find that increasing disorder weakly depresses H'_c(0), T'_c, and T_v, the onset of the Nernst signal. Thus, these energy scales appear more characteristic of the 2D local pairing than the pseudogap temperature which is not modified by disorder.
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摘要 :
Thickness dependence of structural and electrical properties of multiferroic Mn-doped BiFeO_3 (BFMO) thin films, grown on SrTiO_3 (001) substrates with a bottom electrode of SrRuO_3, was investigated. Very good ferroelectric prope...
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Thickness dependence of structural and electrical properties of multiferroic Mn-doped BiFeO_3 (BFMO) thin films, grown on SrTiO_3 (001) substrates with a bottom electrode of SrRuO_3, was investigated. Very good ferroelectric properties were obtained in the thicker BFMO films with a typical remanent polarization of ～65 μC/cm~2, while ferroelectric polarization switching was too difficult to detect using a conventional technique in the thinnest BFMO films due to a much higher coercive field and an increase in the tunneling current. Their ferroelectric nature, however, was demonstrated by piezoresponse force microscopy, and excellent insulating properties and homogeneity were achieved in the films.
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摘要 :
Electric polarization loops are measured at room temperature on highly pure BiFeO_3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents a...
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Electric polarization loops are measured at room temperature on highly pure BiFeO_3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents allow the authors to measure a large spontaneous polarization in excess of 100 μC cm~(-2), a value never reported in the bulk. During electric cycling, the slow degradation of the material leads to an evolution of the hysteresis curves eventually preventing full saturation of the crystals.
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