48,942 research outputs found

### Environmental Law: A Reevaluation of Federal Pre-Emption and the Commerce Clause

This Comment addresses how the concern of state and local governments to regain control over environmental regulation has resulted in a marked increase in conflicts with the commerce and supremacy clauses of the Constitution. Various tests have been used by the courts to determine violations of these Constitutional provisions where environmental objectives are sought through local laws. In the field of environmental litigation, traditional tests are constantly challenged to meet the changing moral climate of the nation. This Comment weighs the desire of local legislatures for more responsive environmental regulation against the federal goal of uniform regulation and unrestrained interstate commerce, concluding that the court must decide on a policy of pre-emption in order for the nation to know whether environmental reform will be spearheaded from the states or the federal government

### Nature of the spin-glass phase in dense packings of Ising dipoles with random anisotropy axes

By Monte Carlo simulations, we study the character of the spinglass (SG) phase in dense disordered packings of magnetic nanoparticles (NPs). We focus on NPs which have large uniaxial anisotropies and can be well represented as Ising dipoles. Dipoles are placed on SC lattices and point along randomly oriented axes. From the behaviour of a SG correlation length we determine the transition temperature Tc between the paramagnetic and a SG phase. For temperatures well below Tc we find distributions of the SG overlap parameter q that are strongly sample-dependent and exhibit several spikes. We find that the average width of spikes, and the fraction of samples with spikes higher than a certain threshold does not vary appreciably with the system sizes studied. We compare these results with the ones found previously for 3D site-diluted systems of parallel Ising dipoles and with the behaviour of the Sherrington-Kirkpatrick model.We thank financial support from MINECO FIS2013-43201-P Gran

### Low-temperature spin-glass behaviour in a diluted dipolar Ising system

Using Monte Carlo simulations, we study the character of the spin-glass (SG)
state of a site-diluted dipolar Ising model. We consider systems of dipoles
randomly placed on a fraction x of all L^3 sites of a simple cubic lattice that
point up or down along a given crystalline axis. For x < 0.65 these systems are
known to exhibit an equilibrium spin-glass phase below a temperature T_sg
proportional to x. At high dilution and very low temperatures, well deep in the
SG phase, we find spiky distributions of the overlap parameter q that are
strongly sample-dependent. We focus on spikes associated with large
excitations. From cumulative distributions of q and a pair correlation function
averaged over several thousands of samples we find that, for the system sizes
studied, the average width of spikes, and the fraction of samples with spikes
higher than a certain threshold does not vary appreciably with L. This is
compared with the behaviour found for the Sherrington-Kirkpatrick model.Comment: 9 LaTeX pages, 9 pdf figures, 2 table

### Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes on a site diluted lattice

We study partially occupied lattice systems of classical magnetic dipoles
which point along randomly oriented axes. Only dipolar interactions are taken
into account. The aim of the model is to mimic collective effects in disordered
assemblies of magnetic nanoparticles. From tempered Monte Carlo simulations, we
obtain the following equilibrium results. The zero temperature entropy
approximately vanishes. Below a temperature T_c, given by k_B T_c= (0.95 +-
0.1)x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy and
x is the site occupancy rate, we find a spin glass phase. In it, (1) the mean
value , where q is the spin overlap, decreases algebraically with system
size N as N increases, and (2) D|q| = 0.5 (T/x)^1/2, independently of N,
where D|q| is the root mean square deviation of |q|.Comment: 7 LaTeX pages, 7 eps figures. Submitted to PRB on 30 December 200

### Monte Carlo study of the spin-glass phase of the site-diluted dipolar Ising model

By tempered Monte Carlo simulations, we study site-diluted Ising systems of
magnetic dipoles. All dipoles are randomly placed on a fraction x of all L^3
sites of a simple cubic lattice, and point along a given crystalline axis. For
x_c< x<=1, where x_c = 0.65, we find an antiferromagnetic phase below a
temperature which vanishes as x tends to x_c from above. At lower values of x,
we find an equilibrium spin-glass (SG) phase below a temperature given by k_B
T_{sg} = x e_d, where e_d is a nearest neighbor dipole-dipole interaction
energy. We study (a) the relative mean square deviation D_q^2 of |q|, where q
is the SG overlap parameter, and (b) xi_L/L, where xi_L is a correlation
length. From their variation with temperature and system size, we determine
T_{sg}. In the SG phase, we find (i) the mean values and decrease
algebraically with L as L increases, (ii) double peaked, but wide,
distributions of q/ appear to be independent of L, and (iii) xi_L/L rises
with L at constant T, but extrapolations to 1/L -> 0 give finite values. All of
this is consistent with quasi-long-range order in the SG phase.Comment: 15 LaTeX pages, 15 figures, 3 tables. (typos fixed in Appendix A

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