13,576 research outputs found
Buckling and d-Wave Pairing in HiTc-Superconductors
We have investigated whether the electron-phonon interaction can support a
d-wave gap-anisotropy. On the basis of models derived from LDA calculations, as
well as LDA linear-response calculations we argue that this is the case, for
materials with buckled or dimpled CuO2 planes, for the so-called buckling
modes, which involve out-of-plane movements of the plane oxygens.Comment: 5pages, Latex2e, 6 Postscript figure
Third-generation muffin-tin orbitals
By the example of sp^3-bonded semiconductors, we illustrate what
3rd-generation muffin-tin orbitals (MTOs) are. We demonstrate that they can be
downfolded to smaller and smaller basis sets: sp^3d^10,sp^3, and bond orbitals.
For isolated bands, it is possible to generate Wannier functions a priori. Also
for bands, which overlap other bands, Wannier-like MTOs can be generated a
priori. Hence, MTOs have a unique capability for providing chemical
understanding.Comment: 13 pages, 8 eps figure
Cold Quark Matter, Quadratic Corrections and Gauge/String Duality
We make an estimate of the quadratic correction in the pressure of cold quark
matter using gauge/string duality.Comment: 7 pages; v.2: reference added; v.3: reference and comments added,
version to appear in PRD; v4. final version to appear in PRD; v.5: key
reference adde
Investigating 16O with the 15N(p,{\alpha})12C reaction
The 16O nucleus was investigated through the 15N(p,{\alpha})12C reaction at
excitation energies from Ex = 12 231 to 15 700 keV using proton beams from a 5
MeV Van de Graaff accelerator at beam energies of Ep = 331 to 3800 keV. Alpha
decay from resonant states in 16O was strongly observed for ten known excited
states in this region. The candidate 4-alpha cluster state at Ex = 15.1 MeV was
investigated particularly intensely in order to understand its particle decay
channels.Comment: Submitted for Proceedings of Fourth International Workshop on State
of the Art in Nuclear Cluster Physics (SOTANCP4), held from May 13 - 18, 2018
in Galveston, TX, US
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
The equilibrium structure, energy bands, phonon dispersions, and s- and
d-channel electron-phonon interactions (EPIs) are calculated for the
infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA
and the linear-response full-potential LMTO method were used. In the
equilibrium structure, oxygen is found to buckle slightly out of the plane and,
as a result, the characters of the energy bands near EF are found to be similar
to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in
accord with previous LDA calculations for YBa2Cu3O7. This supports the common
belief that the EPI mechanism alone is insufficient to explain HTSC.
Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This
is surprising and indicates that the EPI could enhance some other d-wave
pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute
significantly to the EPI, although these contributions are proportional to the
static buckling and would vanish for flat planes. These numerical results can
be understood from a generic tight-binding model originally derived from the
LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the
buckling-modes and the influence of the spin-fluctuations should be
investigated.Comment: 19 pages, 9 Postscript figures, Late
Screened Perturbation Theory to Three Loops
The thermal physics of a massless scalar field with a phi^4 interaction is
studied within screened perturbation theory (SPT). In this method the
perturbative expansion is reorganized by adding and subtracting a mass term in
the lagrangian. We consider several different mass prescriptions that
generalize the one-loop gap equation to two-loop order. We calculate the
pressure and entropy to three-loop order and the screening mass to two-loop
order. In contrast to the weak-coupling expansion, the SPT-improved
approximations appear to converge even for rather large values of the coupling
constant.Comment: 30 pages, 10 figure
First-principles scattering matrices for spin-transport
Details are presented of an efficient formalism for calculating transmission
and reflection matrices from first principles in layered materials. Within the
framework of spin density functional theory and using tight-binding muffin-tin
orbitals, scattering matrices are determined by matching the wave-functions at
the boundaries between leads which support well-defined scattering states and
the scattering region. The calculation scales linearly with the number of
principal layers N in the scattering region and as the cube of the number of
atoms H in the lateral supercell. For metallic systems for which the required
Brillouin zone sampling decreases as H increases, the final scaling goes as
H^2*N. In practice, the efficient basis set allows scattering regions for which
H^{2}*N ~ 10^6 to be handled. The method is illustrated for Co/Cu multilayers
and single interfaces using large lateral supercells (up to 20x20) to model
interface disorder. Because the scattering states are explicitly found,
``channel decomposition'' of the interface scattering for clean and disordered
interfaces can be performed.Comment: 22 pages, 13 figure
Thermodynamics of O(N) sigma models: 1/N corrections
The thermodynamics of the O(N) linear and nonlinear sigma models in 3+1
dimensions is studied. We calculate the pressure to next-to-leading order in
the 1/N expansion and show that at this order, temperature-independent
renormalization is only possible at the minimum of the effective potential. The
1/N expansion is found to be a good expansion for N as low as 4, which is the
case relevant for low-energy QCD phenomenology. We consider the cases with and
without explicit symmetry breaking. We show that previous next-to-leading order
calculations of the pressure are either breaking down in the temperatures of
interest, or based on unjustifiable high-energy approximations.Comment: 11 pages, 5 figures, revte
Augmented space recursion for partially disordered systems
Off-stoichiometric alloys exhibit partial disorder, in the sense that only
some of the sublattices of the stoichiometric ordered alloy become disordered.
This paper puts forward a generalization of the augmented space recursion (ASR)
(introduced earlier by one of us (Mookerjee et al 1997(*))) for systems with
many atoms per unit cell. In order to justify the convergence properties of ASR
we have studied the convergence of various moments of local density of states
and other physical quantities like Fermi energy and band energy. We have also
looked at the convergence of the magnetic moment of Ni, which is very sensitive
to numerical approximations towards the k-space value 0.6 with the
number of recursion steps prior to termination.Comment: Latex 2e, 21 Pages, 13 Figures, iopb style file attache
Conductance calculations for quantum wires and interfaces: mode matching and Green functions
Landauer's formula relates the conductance of a quantum wire or interface to
transmission probabilities. Total transmission probabilities are frequently
calculated using Green function techniques and an expression first derived by
Caroli. Alternatively, partial transmission probabilities can be calculated
from the scattering wave functions that are obtained by matching the wave
functions in the scattering region to the Bloch modes of ideal bulk leads. An
elegant technique for doing this, formulated originally by Ando, is here
generalized to any Hamiltonian that can be represented in tight-binding form. A
more compact expression for the transmission matrix elements is derived and it
is shown how all the Green function results can be derived from the mode
matching technique. We illustrate this for a simple model which can be studied
analytically, and for an Fe|vacuum|Fe tunnel junction which we study using
first-principles calculations.Comment: 14 pages, 5 figure
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