7,608 research outputs found
Metal-insulator transition through a semi-Dirac point in oxide nanostructures: VO (001) layers confined within TiO
Multilayer (TiO)/(VO) nanostructures ( -
interfaces with no polar discontinuity) show a metal-insulator transition with
respect to the VO layer thickness in first principles calculations. For
5 layers, the system becomes metallic, while being insulating for =
1 and 2. The metal-insulator transition occurs through a semi-Dirac point phase
for = 3 and 4, in which the Fermi surface is point-like and the electrons
behave as massless along the zone diagonal in k-space and as massive fermions
along the perpendicular direction. We provide an analysis of the evolution of
the electronic structure through this unprecedented insulator-to-metal
transition, and identify it as resulting from quantum confinement producing a
non-intuitive orbital ordering on the V ions, rather than being a
specific oxide interface effect. Spin-orbit coupling does not destroy the
semi-Dirac point for the calculated ground state, where the spins are aligned
along the rutile c-axis, but it does open a substantial gap if the spins lie in
the basal plane.Comment: 9 pages, 8 figure
Electronic Characteristics of Quasi-2D Metallochloronitrides: Na(x)HfNCL (T_c=25 K)
Local density functional results are presented for the electron-doped
metallochloronitrides A(x)ZrNCl and A(x)HfNCl, A = Li or Na, which superconduct
up to 25K. The alkali non-stoichiometry is treated in a virtual crystal
approximation. The electronic structure is strongly two dimensional, especially
in the conduction band region occupied by the carriers, because the states are
formed from the in-plane orbitals d_xy, d_{x^2-y^2} of the metal ion and the
p_x, p_y orbitals of the N ion. We predict a change of behavior at a doping
level of x=0.3.Comment: To appear in Proc. HTS99 Conf., Miami 1999. Four revtex pages, 5
embedded postscript figure
On the Coexistence in RuSr2GdCu2O8 of Superconductivity and Ferromagnetism
We review the reasons that make superconductivity unlikely to arise in a
ferromagnet. Then, in light of the report by Tallon and collaborators that
RuSr2GdCu2O8 becomes superconducting at approximately 35 K which is well below
the Curie temperature of 132 K, we consider whether the objections really apply
to this compound. Our considerations are supported by local spin density
calculations for this compound, which indeed indicate a ferromagnetic RuO2
layer. The Ru moment resides in t_2g orbitals but is characteristic of
itinerant magnetism (and is sensitive to choice of exchange-correlation
potential and to the atomic positions). Based on the small exchange splitting
that is induced in the Cu-O layers, the system seems capable of supporting
singlet superconductivity an FFLO-type order parameter and possibly a pi-phase
alternation between layers. If instead the pairing is triplet in the RuO2
layers, it can be distinguished by a spin-polarized supercurrent. Either type
of superconductivity seems to imply a spontaneous vortex phase if the
magnetization is rotated out of the plane.Comment: 3 revtex pages, 2 embedded figures. In press, Proc. HTS99 Conf.,
Miami, 199
Method of fan sound mode structure determination computer program user's manual: Microphone location program
A computer user's manual describing the operation and the essential features of the microphone location program is presented. The Microphone Location Program determines microphone locations that ensure accurate and stable results from the equation system used to calculate modal structures. As part of the computational procedure for the Microphone Location Program, a first-order measure of the stability of the equation system was indicated by a matrix 'conditioning' number
Method of fan sound mode structure determination computer program user's manual: Modal calculation program
A computer user's manual describing the operation and the essential features of the Modal Calculation Program is presented. The modal Calculation Program calculates the amplitude and phase of modal structures by means of acoustic pressure measurements obtained from microphones placed at selected locations within the fan inlet duct. In addition, the Modal Calculation Program also calculates the first-order errors in the modal coefficients that are due to tolerances in microphone location coordinates and inaccuracies in the acoustic pressure measurements
Electron Confinement, Orbital Ordering, and Orbital Moments in - Oxide Heterostructures
The (SrTiO)/(SrVO) multilayer system is studied
with first principles methods through the observed insulator-to-metal
transition with increasing thickness of the SrVO layer. When correlation
effects with reasonable magnitude are included, crystal field splittings from
the structural relaxations together with spin-orbit coupling (SOC) determines
the behavior of the electronic and magnetic structures. These confined slabs of
SrVO prefer =() orbital ordering of and
() orbitals within the plane, accompanied by
=(0,0) spin order (ferromagnetic alignment). The result is a
SOC-driven ferromagnetic Mott insulator. The orbital moment of 0.75
strongly compensates the spin moment on the sublattice. The
insulator-metal transition for (occurring between =4 and
=5) is reproduced. Unlike in the isoelectronic TiO/VO
(rutile structure) system and in spite of some similarities in orbital
ordering, no semi-Dirac point [{\it Phys. Rev. Lett.} {\bf 102}, 166803 (2009)]
is encountered, but the insulator-to-metal transition occurs through a
different type of unusual phase. For n=5 this system is very near (or at) a
unique semimetallic state in which the Fermi energy is topologically determined
and the Fermi surface consists of identical electron and hole Fermi circles
centered at =0. The dispersion consists of what can be regarded as a
continuum of radially-directed Dirac points, forming a "Dirac circle".Comment: 9 pages, 8 figure
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