Remarks on the Aharonov-Casher dynamics in a CPT-odd Lorentz-violating background

The Aharonov-Casher problem in the presence of a Lorentz-violating background nonminimally coupled to a spinor and a gauge field is examined. Using an approach based on the self-adjoint extension method, an expression for the bound state energies is obtained in terms of the physics of the problem by determining the self-adjoint extension parameter.Comment: Matches published versio

Collective rearrangement at the onset of flow of a polycrystalline hexagonal columnar phase

Creep experiments on polycrystalline surfactant hexagonal columnar phases show a power law regime, followed by a drastic fluidization before reaching a final stationary flow. The scaling of the fluidization time with the shear modulus of the sample and stress applied suggests that the onset of flow involves a bulk reorganization of the material. This is confirmed by X-ray scattering under stress coupled to \textit{in situ} rheology experiments, which show a collective reorientation of all crystallites at the onset of flow. The analogy with the fracture of heterogeneous materials is discussed.Comment: to appear in Phys. Rev. Let

Nonrelativistic quantum dynamics on a cone with and without a constraining potential

In this paper we investigate the bound state problem of nonrelativistic quantum particles on a conical surface. This kind of surface appears as a topological defect in ordinary semiconductors as well as in graphene sheets. Specifically, we compare and discuss the results stemming from two different approaches. In the first one, it is assumed that the charge carriers are bound to the surface by a constraining potential, while the second one is based on the Klein-Gordon type equation on surfaces, without the constraining potential. The main difference between both theories is the presence/absence of a potential which contains the mean curvature of a given surface. This fact changes the dependence of the bound states on the angular momentum $l$. Moreover, there are bound states that are absent in the Klein-Gordon theory, which instead appear in the Schr\"{o}dinger one.Comment: Accepted for publication in Journal of Mathematical Physics, 14 pages, 1 figur

Superdiffusivity of quantum walks: A Feynman sum-over-paths description

Quantum walks constitute important tools in different applications, especially in quantum algorithms. To a great extent their usefulness is due to unusual diffusive features, allowing much faster spreading than their classical counterparts. Such behavior, although frequently credited to intrinsic quantum interference, usually is not completely characterized. Using a recently developed Green's function approach [Phys. Rev. A {\bf 84}, 042343 (2011)], here it is described -- in a rather general way -- the problem dynamics in terms of a true sum over paths history a la Feynman. It allows one to explicit identify interference effects and also to explain the emergence of superdiffusivity. The present analysis has the potential to help in designing quantum walks with distinct transport properties.Comment: 6 pages, 4 figures, Accepted in Physical Review

Prediction of the derivative discontinuity in density functional theory from an electrostatic description of the exchange and correlation potential

We propose a new approach to approximate the exchange and correlation (XC) functional in density functional theory. The XC potential is considered as an electrostatic potential, generated by a fictitious XC density, which is in turn a functional of the electronic density. We apply the approach to develop a correction scheme that fixes the asymptotic behavior of any approximated XC potential for finite systems. Additionally, the correction procedure gives the value of the derivative discontinuity; therefore it can directly predict the fundamental gap as a ground-state property.Comment: 5 pages, 4 figure

Intergovernmental grant rules, the "golden rule" of public finance and local expenditures

The Stability and Growth Pact and the process of fiscal consolidation in several European countries have enhanced the role of fiscal rules at sub-national level. This paper analyzes the combined effect of a rule to allocate capital and current block grants to local governments and the “golden rule” of public finance (surplus of current balance). We argue that the two fiscal rules introduce significant rigidities and distortions in local governments’ expenditures structure since these mimic the structure of revenues. This effect is particularly relevant in municipalities that are more dependent of intergovernmental grants, mainly rural. On the other hand, urban municipalities with greater tax revenues (current revenues) are constrained in their ability to make capital investments because they receive per capita capital grants below what economies of scale would suggest. An empirical analysis of Portuguese local governments shows that it is no longer the median voter, but fiscal rules, that command the broad pattern of expenditure (current versus capital) at a local level. This paper is a contribution to the literature on the perverse effects of fiscal rules.Intergovernmental block grants; Fiscal Rules; Local Government Expenditure; “Golden Rule”

Citizens’ Freedom to Choose Representatives: Ballot Structure, Proportionality and “Fragmented” Parliaments

The analysis of the political consequences of electoral laws has emphasized how individual characteristics of the electoral system (electoral formulas, district magnitude, ballot structure) affect the degree of parliament “fragmentation” and proportionality. This paper argues that the personal attributes of representatives are also an important consequence of electoral laws, and that they are in part determined by citizens’ freedom to choose representatives. We clarify this concept and develop an index of citizens’ freedom to choose members of parliament as a function of the ballot structure, district size and electoral formulae. Using data from twenty nine countries, we find that neither proportionality nor the effective number of parties is significantly affected by voters’ freedom of choice. This result has important normative implications for electoral reform.Ballot structure; Electoral index; Freedom to choose; Personal vote.

Defect-induced spin-glass magnetism in incommensurate spin-gap magnets

We study magnetic order induced by non-magnetic impurities in quantum paramagnets with incommensurate host spin correlations. In contrast to the well-studied commensurate case where the defect-induced magnetism is spatially disordered but non-frustrated, the present problem combines strong disorder with frustration and, consequently, leads to spin-glass order. We discuss the crossover from strong randomness in the dilute limit to more conventional glass behavior at larger doping, and numerically characterize the robust short-range order inherent to the spin-glass phase. We relate our findings to magnetic order in both BiCu2PO6 and YBa2Cu3O6.6 induced by Zn substitution.Comment: 6 pages, 5 figs, (v2) real-space RG results added; discussion extended, (v3) final version as publishe

Connecting microstructural attributes and permeability from 3D tomographic images of in situ shear-enhanced compaction bands using multiscale computations

Tomographic images taken inside and outside a compaction band in a field specimen of Aztec sandstone are analyzed by using numerical methods such as graph theory, level sets, and hybrid lattice Boltzmann/finite element techniques. The results reveal approximately an order of magnitude permeability reduction within the compaction band. This is less than the several orders of magnitude reduction measured from hydraulic experiments on compaction bands formed in laboratory experiments and about one order of magnitude less than inferences from two-dimensional images of Aztec sandstone. Geometrical analysis concludes that the elimination of connected pore space and increased tortuosities due to the porosity decrease are the major factors contributing to the permeability reduction. In addition, the multiscale flow simulations also indicate that permeability is fairly isotropic inside and outside the compaction band