12,131 research outputs found
Semi-Lagrangian methods for parabolic problems in divergence form
Semi-Lagrangian methods have traditionally been developed in the framework of
hyperbolic equations, but several extensions of the Semi-Lagrangian approach to
diffusion and advection--diffusion problems have been proposed recently. These
extensions are mostly based on probabilistic arguments and share the common
feature of treating second-order operators in trace form, which makes them
unsuitable for mass conservative models like the classical formulations of
turbulent diffusion employed in computational fluid dynamics. We propose here
some basic ideas for treating second-order operators in divergence form. A
general framework for constructing consistent schemes in one space dimension is
presented, and a specific case of nonconservative discretization is discussed
in detail and analysed. Finally, an extension to (possibly nonlinear) problems
in an arbitrary number of dimensions is proposed. Although the resulting
discretization approach is only of first order in time, numerical results in a
number of test cases highlight the advantages of these methods for applications
to computational fluid dynamics and their superiority over to more standard low
order time discretization approaches
Tunneling driven tilt modes of the O octahedra in La{2-x}Sr{x}CuO{4}: strong dependence on doping
The anelastic spectrum of La{2-x}Sr{x}CuO{4} (x = 0, 0.008, 0.019, 0.032) has
been measured down to 1.5 K, in order to see the effect of doping on the
intrinsic lattice fluctuations already found in stoichiometric La{2}CuO{4}, and
identified with tunneling driven tilt modes of the O octahedra. Slight doping
with Sr causes a drastic increase of the transition rates and relaxation
strength of the tunneling systems. The influence of doping on the relaxation
rate is interpreted in terms of direct coupling between between the tilts of
the octahedra and the hole excitations. However, the observed fast dependence
of the rate on temperature cannot be explained in terms of the ususal models of
coupling between a tunneling system and the conduction electrons.Comment: LaTeX, 5 figures in a single PostScript file, submitted to Phys. Rev.
Acoustic measurement of the low-energy excitations in Nd2-xCexCuO4
The complex dynamic Young's modulus of ceramic Nd2-xCexCuO4 with x = 0, 0.05
and 0.20 has been measured from 1.5 to 100 K at frequencies of 1-10 kHz. In the
undoped sample the modulus starts decreasing below ~20 K, instead of
approaching a constant value as in a normal solid. The modulus minimum has been
interpreted in terms of paraelastic contribution from the relaxation of the
Nd^3+ 4f electrons between the levels of the ground state doublet, which is
split by the interaction with the antiferromagnetically ordered Cu sublattice.
The value of the splitting is found to be 0.34 meV, in excellent agreement with
inelastic neutron scattering, infrared and specific heat experiments. With
doping, the anomaly shifts to lower temperature and decreases in amplitude,
consistently with a reduction of the local field from the Cu sublattice.Comment: 5 pages, 2 figures, submitted to Eur. Phys. J.
One-Loop QCD Spin Chain and its Spectrum
We study the renormalization of gauge invariant operators in large Nc QCD. We
compute the complete matrix of anomalous dimensions to leading order in the 't
Hooft coupling and study its eigenvalues. Thinking of the mixing matrix as the
Hamiltonian of a generalized spin chain we find a large integrable sector
consisting of purely gluonic operators constructed with self-dual field
strengths and an arbitrary number of derivatives. This sector contains the true
ground state of the spin chain and all the gapless excitations above it. The
ground state is essentially the anti-ferromagnetic ground state of a XXX1 spin
chain and the excitations carry either a chiral spin quantum number with
relativistic dispersion relation or an anti-chiral one with non-relativistic
dispersion relation.Comment: 54 pages, v2: minor changes, references adde
Long-Term Capital Movements
International financial integration allows countries to become net creditors or net debtors with respect to the rest of the world. In this paper, we show that a small set of fundamentals--shifts in relative output levels, the stock of public debt and demographic factors--can do much to explain the evolution of net foreign asset positions. In addition, we highlight that external wealth' plays a critical role in determining the behavior of the trade balance, both through shifts in the desired net foreign asset position and the investment returns generated on the outstanding stock of net foreign assets. Finally, we provide some evidence that a portfolio balance effect exists: real interest rate differentials are inversely related to net foreign asset positions.
Anelastic relaxation process of polaronic origin in La{2-x}Sr{x}CuO{4}: interaction between the charge stripes and pinning centers
The evolution of an anelastic relaxation process occurring around 80 K in
La{2-x}Sr{x}CuO{4} at a measuring frequency of ~1 kHz has been followed from x
= 0.0075 to the overdoped region, x = 0.2, where it disappears. The dependence
of the peak intensity on doping is consistent with a polaronic mechanism,
identified with the disordered charge stripes overcoming pinning centers. A
marked decrease of the peak amplitude occurs at x > 0.045, the same doping
range where a change of the stripe order from parallel to diagonal with respect
to the Cu-O bonds has been observed by neutron diffraction. Both the energy
barrier and peak amplitude also exhibit a rise near x = 1/8.Comment: 5 pages, 4 figure
International Financial Integration
In recent decades, foreign assets and liabilities in advanced countries have grown rapidly relative to GDP, with the increase in gross cross-holdings far exceeding the size of net positions. Moreover, the portfolio equity and FDI categories have grown in importance relative to international debt stocks. In this paper, we describe the broad trends in international financial integration for a sample of industrial countries, and seek to explain the cross-country and time-series variation in the size of international balance sheets. We also examine the behaviouur of the rates of return on foreign assets and liabilities, relating them to 'market' returns.
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