10,438 research outputs found
Ab-initio procedure for effective models of correlated materials with entangled band structure
We present a first-principles method for deriving effective low-energy models
of electrons in solids having entangled band structure. The procedure starts
with dividing the Hilbert space into two subspaces, the low-energy part ("
space'') and the rest of the space (" space''). The low-energy model is
constructed for the space by eliminating the degrees of freedom of the
space. The thus derived model contains the strength of electron correlation
expressed by a partially screened Coulomb interaction, calculated in the
constrained random-phase-approximation (cRPA) where screening channels within
the space, , are subtracted. One conceptual problem of this
established downfolding method is that for entangled bands it is not clear how
to cut out the space and how to distinguish from the total
polarization. Here, we propose a simple procedure to overcome this difficulty.
In our scheme, the subspace is cut out from the Hilbert space of the Kohn
Sham eigenfunctions with the help of a procedure to construct a localized
Wannier basis. The subspace is constructed as the complementary space
orthogonal to the subspace. After this disentanglement, becomes well
defined. Using the disentangled bands, the effective parameters are uniquely
determined in the cRPA. The method is successfully applied to 3 transition
metals.Comment: 14 pages, 4 figure
Search for -cluster states in even-even Cr isotopes
The structure is investigated in even-even Cr
isotopes from the viewpoint of the local potential model. The comparison of
values for even-even Cr isotopes and even-even
isobars indicates that Cr and Cr are the most favorable even-even
Cr isotopes for -clustering. The ground state bands of the two Cr
isotopes are calculated through a local potential with
two variable parameters. The calculated spectra give a very good description of
most experimental Cr and Cr levels. The reduced -widths,
rms intercluster separations and transition rates are determined for
the ground state bands. The calculations reproduce the order of magnitude of
the available experimental values without using effective charges and
indicate that the first members of the ground state bands present a stronger
-cluster character. The volume integral per nucleon pair and rms radius
obtained for the Ti potential are consistent with those reported
previously in the analysis of elastic scattering on Ti
Realistic many-body models for Manganese Monoxide under pressure
In materials like transition metals oxides where electronic Coulomb
correlations impede a description in terms of standard band-theories, the
application of genuine many-body techniques is inevitable. Interfacing the
realism of density-functional based methods with the virtues of Hubbard-like
Hamiltonians, requires the joint ab initio construction of transfer integrals
and interaction matrix elements (like the Hubbard U) in a localized basis set.
In this work, we employ the scheme of maximally localized Wannier functions and
the constrained random phase approximation to create effective low-energy
models for Manganese monoxide, and track their evolution under external
pressure. We find that in the low pressure antiferromagnetic phase, the
compression results in an increase of the bare Coulomb interaction for specific
orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79,
235133 (2009)], this seemingly counter-intuitive behavior is a consequence of
the delocalization of the respective Wannier functions. The change of screening
processes does not alter this tendency, and thus, the screened on-site
component of the interaction - the Hubbard U of the effective low-energy system
- increases with pressure as well. The orbital anisotropy of the effects
originates from the orientation of the orbitals vis-a-vis the deformation of
the unit-cell. Within the high pressure paramagnetic phase, on the other hand,
we find the significant increase of the Hubbard U is insensitive to the orbital
orientation and almost exclusively owing to a substantial weakening of
screening channels upon compression.Comment: 13 pages, 6 figure
Circuit design tool. User's manual, revision 2
The CAM chip design was produced in a UNIX software environment using a design tool that supports definition of digital electronic modules, composition of these modules into higher level circuits, and event-driven simulation of these circuits. Our design tool provides an interface whose goals include straightforward but flexible primitive module definition and circuit composition, efficient simulation, and a debugging environment that facilitates design verification and alteration. The tool provides a set of primitive modules which can be composed into higher level circuits. Each module is a C-language subroutine that uses a set of interface protocols understood by the design tool. Primitives can be altered simply by recoding their C-code image; in addition new primitives can be added allowing higher level circuits to be described in C-code rather than as a composition of primitive modules--this feature can greatly enhance the speed of simulation
A theory of new type of heavy-electron superconductivity in PrOs_4Sb_12: quadrupolar-fluctuation mediated odd-parity pairings
It is shown that unconventional nature of superconducting state of
PrOs_4Sb_12, a Pr-based heavy electron compound with the filled-Skutterudite
structure, can be explained in a unified way by taking into account the
structure of the crystalline-electric-field (CEF) level, the shape of the Fermi
surface determined by the band structure calculation, and a picture of the
quasiparticles in f-configuration with magnetically singlet CEF ground
state. Possible types of pairing are narrowed down by consulting recent
experimental results. In particular, the chiral "p"-wave states such as
p_x+ip_y is favoured under the magnetic field due to the orbital Zeeman effect,
while the "p"-wave states with two-fold symmetery such as p_x can be stabilized
by a feedback effect without the magnetic field. It is also discussed that the
double superconducting transition without the magnetic field is possible due to
the spin-orbit coupling of the "triplet" Cooper pairs in the chiral state.Comment: 12 pages, 2 figures, submitted to J. Phys.: Condens. Matter Lette
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