9,516 research outputs found
Black hole formation from point-like particles in three-dimensional anti-de Sitter space
We study collisions of many point-like particles in three-dimensional anti-de
Sitter space, generalizing the known result with two particles. We show how to
construct exact solutions corresponding to the formation of either a black hole
or a conical singularity from the collision of an arbitrary number of massless
particles falling in radially from the boundary. We find that when going away
from the case of equal energies and discrete rotational symmetry, this is not a
trivial generalization of the two-particle case, but requires that the excised
wedges corresponding to the particles must be chosen in a very precise way for
a consistent solution. We also explicitly take the limit when the number of
particles goes to infinity and obtain thin shell solutions that in general
break rotational invariance, corresponding to an instantaneous and
inhomogeneous perturbation at the boundary. We also compute the stress-energy
tensor of the shell using the junction formalism for null shells and obtain
agreement with the point particle picture.Comment: 42 pages, 9 figures; v2: fixed some typo
Nonmetallic impurities improve mechanical properties of vapor-deposited tungsten
Mechanical properties of vapor deposited tungsten are improved by selective incorporation of various nonmetallic impurities. Addition of trace quantities of carbon, nitrogen, or oxygen can significantly increase both low and high temperature yield strength without greatly affecting ductile-to-brittle transition temperature
Performance of the coupled cluster singles and doubles method on two-dimensional quantum dots
An implementation of the coupled-cluster single- and double excitations
(CCSD) method on two-dimensional quantum dots is presented. Advantages and
limitations are studied through comparison with other high accuracy approaches
for two to eight confined electrons. The possibility to effectively use a very
large basis set is found to be an important advantage compared to full
configuration interaction implementations. For the two to eight electron ground
states, with a confinement strength close to what is used in experiments, the
error in the energy introduced by truncating triple excitations and beyond is
shown to be on the same level or less than the differences in energy given by
two different Quantum Monte Carlo methods. Convergence of the iterative
solution of the coupled cluster equations is, for some cases, found for
surprisingly weak confinement strengths even when starting from a
non-interacting basis. The limit where the missing triple and higher
excitations become relevant is investigated through comparison with full
Configuration Interaction results.Comment: 11 pages, 1 figure, 5 table
Effects of additions of nonmetallics on the properties of vapor-deposited tungsten
Nonmetallic additive effects on properties of vapor deposited tungste
Demographic Trends and the Pension Problem in Finland
Projections of expenditures for old age pensions, survivor pensions, and disability pensions were made for the period 1985-2050 on the basis of future developments in the population structure by age, sex, and marital status. Four demographic scenarios were formulated: (i) a Benchmark scenario, with demographic rates kept constant at their 1980-84 level; (ii) a Fertility scenario, with a rise of the Total Fertility Rate (TFR) towards replacement level; (iii) a Mortality scenario, with reductions in mortality rates of 30 percent for females, and 45 percent for males; (iv) a Western scenario, which combines extreme demographic conditions of several West European countries: a TFR of 1.28, proportions never-marrying of one-third, one-third of marriages ending in divorce, and male and female life expectancies of 74 and 81 years, respectively.
The current pension system was combined with all four scenarios. Also, the impact of high female labor force participation, and a rise in the average age at retirement were investigated. The results indicate that changes in demographic conditions cannot prevent increases in and funding problems for pension expenditures in Finland. An increase in fertility has no effect on the pension system until 2030, when a larger generation will enter the labor force. Longer active periods for males and females will cause increases in pension expenditures in the future and are not long term solutions of the pension problem. Postponement of retirement age would help to balance the pension funds, but depends on the economic situation and on the labor market
Dynamically-Induced Frustration as a Route to a Quantum Spin Ice State in Tb2Ti2O7 via Virtual Crystal Field Excitations and Quantum Many-Body Effects
The TbTiO pyrochlore magnetic material is attracting much
attention for its {\em spin liquid} state, failing to develop long range order
down to 50 mK despite a Curie-Weiss temperature K.
In this paper we reinvestigate the theoretical description of this material by
considering a quantum model of independent tetrahedra to describe its low
temperature properties. The naturally-tuned proximity of this system near a
N\'eel to spin ice phase boundary allows for a resurgence of quantum
fluctuation effects that lead to an important renormalization of its effective
low energy spin Hamiltonian. As a result, TbTiO is argued to be a
{\em quantum spin ice}. We put forward an experimental test of this proposal
using neutron scattering on a single crystal.Comment: 5 pages, 3 figures. Version 2 has a modified introduction. Figure 2b
of version 1 (experimental neutron scattering has been removed. A proposal
for an experimental test is now included accompanied by a new Figure (Fig. 3
Fermionization of two-component few-fermion systems in a one-dimensional harmonic trap
The nature of strongly interacting Fermi gases and magnetism is one of the
most important and studied topics in condensed-matter physics. Still, there are
many open questions. A central issue is under what circumstances strong
short-range repulsive interactions are enough to drive magnetic correlations.
Recent progress in the field of cold atomic gases allows to address this
question in very clean systems where both particle numbers, interactions and
dimensionality can be tuned. Here we study fermionic few-body systems in a one
dimensional harmonic trap using a new rapidly converging effective-interaction
technique, plus a novel analytical approach. This allows us to calculate the
properties of a single spin-down atom interacting with a number of spin-up
particles, a case of much recent experimental interest. Our findings indicate
that, in the strongly interacting limit, spin-up and spin-down particles want
to separate in the trap, which we interpret as a microscopic precursor of
one-dimensional ferromagnetism in imbalanced systems. Our predictions are
directly addressable in current experiments on ultracold atomic few-body
systems.Comment: 12 pages, 6 figures, published version including two appendices on
our new numerical and analytical approac
Convergence of all-order many-body methods: coupled-cluster study for Li
We present and analyze results of the relativistic coupled-cluster
calculation of energies, hyperfine constants, and dipole matrix elements for
the , , and states of Li atom. The calculations are
complete through the fourth order of many-body perturbation theory for energies
and through the fifth order for matrix elements and subsume certain chains of
diagrams in all orders. A nearly complete many-body calculation allows us to
draw conclusions on the convergence pattern of the coupled-cluster method. Our
analysis suggests that the high-order many-body contributions to energies and
matrix elements scale proportionally and provides a quantitative ground for
semi-empirical fits of {\em ab inito} matrix elements to experimental energies.Comment: 4 pages, 3 figure
- …