[en] The two-electron (2e) doped Ca_1-xCe_xMnO₃ (0≤x≤0.2) system was prepared by sol-gel method and investigated by x-ray absorption spectroscopy (XAS), X-ray diffraction, and temperature and field dependent magnetic and transport measurements. The results are compared in detail to the one-electron (1e)-doped La_1-yCa_yMnO₃ system, facilitated by the analogous y=1-2x parametrization as Ca_(1+y)/2Ce_(1-y)/2MnO₃ (0.6≤y≤1.0). The XAS results indicate: that the formal valence of cerium is Ce⁴⁺, thereby validating the charge transfer of 2e per doped Ce to Mn. The XAS also shows separate Mn³⁺ and Mn⁴⁺ spectral components, in dramatic contrast to results on 1e-doped systems. Low Ce doping (x=0.025 and 0.05) rapidly stabilizes a robust ferromagnetic component in addition to a persistent antiferromagnetic component in the ordered state. Higher Ce doping (0.075≤x≤0.20) induces a charge/orbital ordering transition that increases with composition to near 255 K. In different composition ranges the magnetoresistance manifests inter-grain-tunneling and a field coupling of the local charge/orbital order parameter mechanisms. The latter mechanism appears to induce a large magnetoresistance, as high as -72%, in Ca_0.925Ce_0.075MnO₃

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No abstract available

[en] We present experiments that determine the temperature increase in a type-II superconductor due to Joule heating induced by vortex motion. The effect of Joule heating is detected by comparing the response of the vortex lattice to fixed amplitude current steps of short (10 μs) and long (4 s) duration, where the Joule heating is negligible and saturates, respectively. The thermometry is based on the temperature dependence of the voltage response of the vortex lattice to a driving current. By monitoring the temperature increase in NbSe₂ samples adhered on a sapphire substrate with GE varnish we obtain the effective heat transfer coefficient between the sample and the bath and show that the heating is primarily due to the power dissipated by the vortex motion

[en] We report on experiments investigating the depinning dynamics of a strongly pinned vortex lattice in 2H-NbSe₂ . We find that the depinning process starts at currents that are well below the critical current of the entire lattice and that it is governed by the formation of contiguous channels of mobile vortices connecting the sample edges. We obtain the formation time of the first channel by monitoring the delayed voltage response to a driving current step and by measuring the ramping rate dependence of the critical current. The subsequent increase in the number of moving vortices is determined from the temporal evolution of the voltage response and the critical current

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No abstract available

[en] We report on the observation of two types of current driven transitions in metastable vortex lattices. The metastable states, which are missed in usual slow transport measurements, are detected with a fast transport technique in the vortex lattice of undoped 2H-NbSe₂ . The transitions are seen by following the evolution of these states when driven by a current. At low currents we observe an equilibration transition from a metastable to a stable state, followed by a dynamic crystallization transition at high currents

[en] Pulsed transport measurements were used to study the dynamics of vortices in NbSe₂. The time dependence of vortex response defines a dynamic phase diagram that includes a moving Bragg glass regime. In this state the response time exhibits a universal power law dependence on the saturation velocity

[en] The synthesis, electron diffraction (ED), synchrotron X-ray and neutron structure, X-ray absorption spectroscopy (XAS) and magnetic property studies of La2MnVO6 double perovskite are described. Analysis of the synchrotron powder X-ray diffraction data for La2MnVO6 indicates a disordered arrangement of Mn and V at the B-site of the perovskite structure. Absence of super-lattice reflections in the ED patterns for La2MnVO6 supports the disordered cation arrangement. Room temperature time-of-flight (TOF) neutron powder diffraction (NPD) data show no evidence of cation ordering, in corroboration with the ED and synchrotron studies (orthorhombic Pnma, a = 5.6097(3), b = 7.8837(5) and c = 5.5668(3); 295 K, NPD). A comparison of XAS analyses of La2TVO6 with T = Ni and Co shows T2+ formal oxidation state while the T = Mn material evidences a Mn3+ admixture into a dominantly Mn2+ ground state. V-K edge measurements manifest a mirror image behavior with a V4+ state for T = Ni and Co with a V3+ admixture arising in the T = Mn material. The magnetic susceptibility data for La2MnVO6 show ferromagnetic correlations; the observed effective moment, μeff (5.72 μB) is much smaller than the calculated moment (6.16 μμB) based on the spin-only formula for Mn2+ (d5, HS)/V4+ (d1), supportive of the partly oxidized Mn and reduced V scenario (Mn3+/V3+).