Effect of Mn substitution by Ga on the optical properties of a metallic manganite

In a metallic manganite like La(2/3)Sr(1/3)MnO(3), the substitution of Mn(+3) by Ga(+3) dilutes the ferromagnetic order and locally cancels the Jahn-Teller distortion, without heavily affecting the crystal structure. One can thus follow the changes in the charge dynamics induced by Ga, until the ferro-metallic manganite is turned into an insulator. Here this phenomenon is studied in detail through the infrared reflectivity of five samples of La(2/3)Sr(1/3)Mn(1-x)Ga(x)O(3), with x increasing from 0 to 0.30 and for 50 < T < 320 K. A simple model which links the measured optical parameters to the magnetization M(x, T) well describes the behavior of the plasma frequency, the scattering rate, and the mid-infrared absorption along the metal-to-insulator transition.Comment: 8 pages including 7 figure

Optical Properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder

We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices (SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic mid-infrared band. At n = 5 that softening is incomplete, while at the largest-period n=8 compound the MIR band is independent of T and the SL remains insulating. One can thus first observe the IMT in a manganite system in the absence of the disorder due to chemical doping. Unsuccessful reconstruction of the SL optical properties from those of the original bulk materials suggests that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic state.Comment: Published Online in Nano Letters, November 8, 2010; http://pubs.acs.org/doi/abs/10.1021/nl1022628; 5 pages, 3 figure

Magnetic imaging of pearl vortices in artificially layered (Ba 0.9Nd0.1CuO2+x)m/(CaCuO 2)n systems

We have used scanning SQUID magnetometry to image vortices in ultrathin (Ba0.9Nd0.1CuO2+x)(m)/(CaCuO2)(n) high temperature superconductor samples, with as few as three superconducting CuO2 planes. The Pearl lengths (Lambda=2lambda(L)(2)/d, lambda(L) the London penetration depth, d the superconducting film thickness) in these samples, as determined by fits to the vortex images, agree with those by local susceptibility measurements, and can be as long as 1 mm. The in-plane penetration depths lambda(ab) inferred from the Pearl lengths are longer than many bulk cuprates with comparable critical temperatures. We speculate on the causes of the long penetration depths, and on the possibility of exploiting the unique properties of these superconductors for basic experiments

Role of interband scattering in neutron irradiated MgB2_2 thin films by Scanning Tunneling Spectroscopy measurements

A series of MgB$_2$ thin films systematically disordered by neutron irradiation have been studied by Scanning Tunneling Spectroscopy. The c-axis orientation of the films allowed a reliable determination of local density of state of the $\pi$ band. With increasing disorder, the conductance peak moves towards higher voltages and becomes lower and broader, indicating a monotonic increase of the $\pi$ gap and of the broadening parameter. These results are discussed in the frame of two-band superconductivity.Comment: The text will be submitted in Latex format, and the corresponding pdf file should take 6 pages. There are 5 figures (eps files submitted) and 1 tabl

Dissipation in ultra-thin current-carrying superconducting bridges; evidence for quantum tunneling of Pearl vortices

We have made current-voltage (IV) measurements of artificially layered high-$T_c$ thin-film bridges. Scanning SQUID microscopy of these films provides values for the Pearl lengths $\Lambda$ that exceed the bridge width, and shows that the current distributions are uniform across the bridges. At high temperatures and high currents the voltages follow the power law $V \propto I^n$, with $n=\Phi_0^2/8\pi^2\Lambda k_B T+1$, and at high temperatures and low-currents the resistance is exponential in temperature, in good agreement with the predictions for thermally activated vortex motion. At low temperatures, the IV's are better fit by $\ln V$ linear in $I^{-2}$. This is expected if the low temperature dissipation is dominated by quantum tunneling of Pearl vortices.Comment: 5 pages, 7 fig

Evolution of magnetic phases and orbital occupation in (SrMnO3)n/(LaMnO3)2n superlattices

The magnetic and electronic modifications induced at the interfaces in (SrMnO$_{3}$)$_{n}$/(LaMnO$_{3}$)$_{2n}$ superlattices have been investigated by linear and circular magnetic dichroism in the Mn L$_{2,3}$ x-ray absorption spectra. Together with theoretical calculations, our data demonstrate that the charge redistribution across interfaces favors in-plane ferromagnetic (FM) order and $e_{g}(x^{2}-y^{2})$ orbital occupation, in agreement with the average strain. Far from interfaces, inside LaMnO$_3$, electron localization and local strain favor antiferromagnetism (AFM) and $e_{g}(3z^{2}-r^{2})$ orbital occupation. For $n=1$ the high density of interfacial planes ultimately leads to dominant FM order forcing the residual AFM phase to be in-plane too, while for $n \geq 5$ the FM layers are separated by AFM regions having out-of-plane spin orientation.Comment: accepted for publication as a Rapid Communication in Physical Review

Multiple double-exchange mechanism by Mn2+^{2+}-doping in manganite compounds

Double-exchange mechanisms in RE$_{1-x}$AE$_{x}$MnO$_{3}$ manganites (where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies on the strong exchange interaction between two Mn$^{3+}$ and Mn$^{4+}$ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered "silent" with respect to the DE conducting mechanisms. Here we show that a new path for DE-mechanism is indeed possible by partially replacing the RE-AE elements by Mn$^{2+}$-ions, in La-deficient La$_{x}$MnO$_{3-\delta}$ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn$^{2+}$ ions, which is unambiguously proved to be substituted at La-site by Resonant Inelastic X-ray Scattering. Mn$^{2+}$ is proved to be directly correlated to the enhanced magneto-transport properties because of an additional hopping mechanism trough interfiling Mn$^{2+}$-ions, theoretically confirmed by calculations within the effective single band model. The very idea to use Mn$^{2+}$ both as a doping element and an ions electronically involved in the conduction mechanism, has never been foreseen, revealing a new phenomena in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.Comment: 6 pages, 5 figure

Analysis of Metal-Insulator Crossover in Strained {SrRuO}3 Thin Films by X-ray Photoelectron Spectroscopy

The electronic properties of ultrathin epitaxial films of strontium ruthenate SrRuO3 perovskite oxide are modified by epitaxial strain, as determined by growing by pulsed laser deposition, on different the substrates. Electron transport measurements indicated that tensile strain deformation of the SrRuO3 unit cell reduces the metallicity of the material and reduces the metal-insulator-transition (MIT) temperatures. The shrinkage of the Ru-O-Ru buckling angle due to compressive strain is counterweighted by the increased overlap of the conduction Ru-4d orbitals with the O-2p ones due to the smaller interatomic distances resulting into an increased MIT temperature, i.e. a more conducting material. In the more metallic samples the core level x-ray photoemission spectroscopy lineshapes show the occurrence of an extra-peak at the lower binding energies of the main Ru-3d peaks that is attributed to screening, as observed in volume sensitive photoemission of the unstrained material

Intrinsic nonlinearity probed by intermodulation distortion microwave measurements on high quality MgB2 thin films

The two tone intermodulation distortion arising in MgB2 thin films synthesized by hybrid physical-chemical vapour deposition is studied in order to probe the influence of the two bands on the nonlinear response of this superconductor. The measurements are carried out by using a dielectrically loaded copper cavity operating at 7 GHz. Microwave data on samples having critical temperatures above 41 K, very low resistivity values, and residual resistivity ratio larger than 10, are shown. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power feeding the cavity and on the temperature is analyzed. At low power, the signal arising from distortion versus temperature shows the intrinsic s-wave behavior expected for this compound. Data are compared with measurements performed on Nb and YBCO thin films using the same technique.Comment: Revised version. Submitted to APL on October 7th 200