8,102 research outputs found
Orbital Polarization in Strained LaNiO: Structural Distortions and Correlation Effects
Transition-metal heterostructures offer the fascinating possibility of
controlling orbital degrees of freedom via strain. Here, we investigate
theoretically the degree of orbital polarization that can be induced by
epitaxial strain in LaNiO films. Using combined electronic structure and
dynamical mean-field theory methods we take into account both structural
distortions and electron correlations and discuss their relative influence. We
confirm that Hund's rule coupling tends to decrease the polarization and point
out that this applies to both the and local
configurations of the Ni ions. Our calculations are in good agreement with
recent experiments, which revealed sizable orbital polarization under tensile
strain. We discuss why full orbital polarization is hard to achieve in this
specific system and emphasize the general limitations that must be overcome to
achieve this goal.Comment: 13 pages, 13 figure
Low-energy description of the metal-insulator transition in the rare-earth nickelates
We propose a simple theoretical description of the metal-insulator transition
of rare-earth nickelates. The theory involves only two orbitals per nickel
site, corresponding to the low-energy anti-bonding states. In the
monoclinic insulating state, bond-length disproportionation splits the manifold
of bands, corresponding to a modulation of the effective on-site energy.
We show that, when subject to a local Coulomb repulsion and Hund's coupling
, the resulting bond-disproportionated state is a paramagnetic insulator for
a wide range of interaction parameters. Furthermore, we find that when
is small or negative, a spontaneous instability to bond disproportionation
takes place for large enough . This minimal theory emphasizes that a small
or negative charge-transfer energy, a large Hund's coupling, and a strong
coupling to bond-disproportionation are the key factors underlying the
transition. Experimental consequences of this theoretical picture are
discussed.Comment: 17 pages, 10 figures; published version in the updat
Rediscounting Under Aggregate Risk with Moral Hazard
Freeman (1999) proposes a model in which discount window lending and open market operations have different effects. This is important because in most of the literature, these policies are indistinguishable. However, Freeman's argument that the central bank should absorb losses associated with default to provide risk-sharing stands in stark contrast to the concern that central banks should limit their exposure to credit risk. We extend Freeman's model by introducing moral hazard. With moral hazard, the central bank should avoid absorbing losses and Freeman's argument breaks down. However, we show that policies resembling discount window lending and open market operations can still be distinguished in this new framework. The optimal policy is for the central bank to make a restricted number of creditors compete for funds. By restricting the number of agents, the central bank can limit the moral hazard problem. By making them compete with each other, the central bank can exploit market information that reveals the state of the economy.Payment, clearing, and settlement systems; Financial markets; Central bank research
Costly banknote issuance and interest rates under the national banking system
The behavior of interest rates under the U.S. National Banking System is puzzling because of the apparent presence of persistent and large unexploited arbitrage opportunities for note issuing banks. Previous attempts to explain interest rate behavior have relied on the cost or the inelasticity of note issue. These attempts are not entirely satisfactory. Here we propose a new rationale to solve the puzzle. Inelastic note issuance arises endogenously because the marginal cost of issuing notes is an increasing function of circulation. We build a spatial separation model where some fraction of agents must move each period. Banknotes can be carried between locations; deposits cannot. Taking the model to the data on national banks, we find it matches the movements in long-term interest rates well. It also predicts movements in deposit rates during panics. However, the model displays more inelasticity of notes issuance than is in the data.Bank notes ; Interest rates ; National banks (United States)
Extended MacMahon-Schwinger's Master Theorem and Conformal Wavelets in Complex Minkowski Space
We construct the Continuous Wavelet Transform (CWT) on the homogeneous space
(Cartan domain) D_4=SO(4,2)/(SO(4)\times SO(2)) of the conformal group SO(4,2)
(locally isomorphic to SU(2,2)) in 1+3 dimensions. The manifold D_4 can be
mapped one-to-one onto the future tube domain C^4_+ of the complex Minkowski
space through a Cayley transformation, where other kind of (electromagnetic)
wavelets have already been proposed in the literature. We study the unitary
irreducible representations of the conformal group on the Hilbert spaces
L^2_h(D_4,d\nu_\lambda) and L^2_h(C^4_+,d\tilde\nu_\lambda) of square
integrable holomorphic functions with scale dimension \lambda and continuous
mass spectrum, prove the isomorphism (equivariance) between both Hilbert
spaces, admissibility and tight-frame conditions, provide reconstruction
formulas and orthonormal basis of homogeneous polynomials and discuss symmetry
properties and the Euclidean limit of the proposed conformal wavelets. For that
purpose, we firstly state and prove a \lambda-extension of Schwinger's Master
Theorem (SMT), which turns out to be a useful mathematical tool for us,
particularly as a generating function for the unitary-representation functions
of the conformal group and for the derivation of the reproducing (Bergman)
kernel of L^2_h(D_4,d\nu_\lambda). SMT is related to MacMahon's Master Theorem
(MMT) and an extension of both in terms of Louck's SU(N) solid harmonics is
also provided for completeness. Convergence conditions are also studied.Comment: LaTeX, 40 pages, three new Sections and six new references added. To
appear in ACH
Is turbulent mixing a self convolution process ?
Experimental results for the evolution of the probability distribution
function (PDF) of a scalar mixed by a turbulence flow in a channel are
presented. The sequence of PDF from an initial skewed distribution to a sharp
Gaussian is found to be non universal. The route toward homogeneization depends
on the ratio between the cross sections of the dye injector and the channel. In
link with this observation, advantages, shortcomings and applicability of
models for the PDF evolution based on a self-convolution mechanisms are
discussed.Comment: 4 page
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