484 research outputs found
Thermodynamics of the Mg-B system: Implications for the deposition of MgB2 thin films
We have studied thermodynamics of the Mg-B system with the modeling technique
CALPHAD using a computerized optimization procedure. Temperature-composition,
pressure-composition, and pressure-temperature phase diagrams under different
conditions are obtained. The results provide helpful insights into appropriate
processing conditions for thin films of the superconducting phase, MgB2,
including the identification of the pressure/temperature region for
adsorption-controlled growth. Due to the high volatility of Mg, MgB2 is
thermodynamically stable only under fairly high Mg overpressures for likely
growth temperatures. This constraint places severe temperature constraints on
deposition techniques employing high vacuum conditions
Si-compatible candidates for high-K dielectrics with the Pbnm perovskite structure
We analyze both experimentally (where possible) and theoretically from
first-principles the dielectric tensor components and crystal structure of five
classes of Pbnm perovskites. All of these materials are believed to be stable
on silicon and are therefore promising candidates for high-K dielectrics. We
also analyze the structure of these materials with various simple models,
decompose the lattice contribution to the dielectric tensor into force constant
matrix eigenmode contributions, explore a peculiar correlation between
structural and dielectric anisotropies in these compounds and give phonon
frequencies and infrared activities of those modes that are infrared-active. We
find that CaZrO_3, SrZrO_3, LaHoO_3, and LaYO_3 are among the most promising
candidates for high-K dielectrics among the compounds we considered.Comment: 17 pages, 9 figures, 4 tables. Supplementary information:
http://link.aps.org/supplemental/10.1103/PhysRevB.82.064101 or
http://www.physics.rutgers.edu/~sinisa/highk/supp.pd
Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO
We apply ultrafast X-ray diffraction with femtosecond temporal resolution to
monitor the lattice dynamics in a thin film of multiferroic BiFeO after
above-bandgap photoexcitation. The sound-velocity limited evolution of the
observed lattice strains indicates a quasi-instantaneous photoinduced stress
which decays on a nanosecond time scale. This stress exhibits an inhomogeneous
spatial profile evidenced by the broadening of the Bragg peak. These new data
require substantial modification of existing models of photogenerated stresses
in BiFeO: the relevant excited charge carriers must remain localized to be
consistent with the data
Formation and observation of a quasi-two-dimensional electron liquid in epitaxially stabilized SrLaTiO thin films
We report the formation and observation of an electron liquid in
SrLaTiO, the quasi-two-dimensional counterpart of SrTiO,
through reactive molecular-beam epitaxy and {\it in situ} angle-resolved
photoemission spectroscopy. The lowest lying states are found to be comprised
of Ti 3 orbitals, analogous to the LaAlO/SrTiO interface and
exhibit unusually broad features characterized by quantized energy levels and a
reduced Luttinger volume. Using model calculations, we explain these
characteristics through an interplay of disorder and electron-phonon coupling
acting co-operatively at similar energy scales, which provides a possible
mechanism for explaining the low free carrier concentrations observed at
various oxide heterostructures such as the LaAlO/SrTiO interface
Quantifying electronic correlation strength in a complex oxide: a combined DMFT and ARPES study of LaNiO
The electronic correlation strength is a basic quantity that characterizes
the physical properties of materials such as transition metal oxides.
Determining correlation strengths requires both precise definitions and a
careful comparison between experiment and theory. In this paper we define the
correlation strength via the magnitude of the electron self-energy near the
Fermi level. For the case of LaNiO, we obtain both the experimental and
theoretical mass enhancements by considering high resolution
angle-resolved photoemission spectroscopy (ARPES) measurements and density
functional + dynamical mean field theory (DFT + DMFT) calculations. We use
valence-band photoemission data to constrain the free parameters in the theory,
and demonstrate a quantitative agreement between the experiment and theory when
both the realistic crystal structure and strong electronic correlations are
taken into account. These results provide a benchmark for the accuracy of the
DFT+DMFT theoretical approach, and can serve as a test case when considering
other complex materials. By establishing the level of accuracy of the theory,
this work also will enable better quantitative predictions when engineering new
emergent properties in nickelate heterostructures.Comment: 10 pages, 5 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
- …