In 1995, international migrant remittances exceeded US$70 billion. How have these remittances shaped development in migrant sending areas? Pessimistic views on migration and development pervade the literature. In contrast, the new economics of labour migration (NELM) argues that migration may set in motion a development dynamic, lessening production and investment constraints faced by households in imperfect market environments and creating income growth linkages. This article assesses the development potential of remittances from a NELM perspective and cites empirical evidence that remittances may be a positive factor in economic development. Governments in migrant origin countries may increase the development potential of remittances through a variety of economic policies. Creating a fertile ground for remittances to contribute to broad based income growth in migrant sending areas is a key to promoting development from migration.

The crossover from weak coupling Bardeen-Cooper-Schrieffer (BCS) pairing to a Bose-Einstein condensate (BEC) of tightly bound pairs, as a function of the attractive interaction in Fermi systems, has long been of interest to theoretical physicists. The past decade has seen a series of remarkable experimental developments in ultracold Fermi gases that has realized the BCS-BEC crossover in the laboratory, bringing with it fresh new insights into the very strongly interacting unitary regime in the middle of this crossover. In this review, we start with a pedagogical introduction to the crossover and then focus on recent progress in the strongly interacting regime. While our focus is on new theoretical developments, we also describe three key experiments that probe the thermodynamics, transport and spectroscopy of the unitary Fermi gas. We discuss connections between the unitary regime and other areas of physics -quark-gluon plasmas, gauge-gravity duality and high temperature superconductivity -and conclude with open questions about strongly interacting Fermi gases.

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The viscosity of strongly interacting systems is a topic of great interest in diverse fields. We focus here on the bulk and shear viscosities of nonrelativistic quantum fluids, with particular emphasis on strongly interacting ultracold Fermi gases. We use Kubo formulas for the bulk and shear viscosity spectral functions, ζ (ω) and η(ω), respectively, to derive exact, nonperturbative results. Our results include a microscopic connection between the shear viscosity η and the normal-fluid density ρ n ; sum rules for ζ (ω) and η(ω) and their evolution through the BCS-BEC crossover (where BEC denotes Bose-Einstein condensate); and universal high-frequency tails for η(ω) and the dynamic structure factor S(q,ω). We use our sum rules to show that, at unitarity, ζ (ω) is identically zero and thus relate η(ω) to density-density correlations. We predict that frequency-dependent shear viscosity η(ω) of the unitary Fermi gas can be experimentally measured using Bragg spectroscopy.

We identify an intrinsic Hall effect in multiband chiral superconductors in the absence of a magnetic field (i.e., an anomalous Hall effect). This effect arises from interband transitions involving time-reversal symmetry-breaking chiral Cooper pairs. We discuss the implications of this effect for the putative chiral p-wave superconductor, Sr2RuO4, and show that it can contribute significantly to Kerr rotation experiments. Since the magnitude of the effect depends on the structure of the order parameter across the bands, this result may be used to distinguish between different models proposed for the superconducting state of Sr2RuO4.PACS numbers: 73.43. Cd, 74.70.Pq, 74.20.Rp Chiral superconducting states have attracted an enormous amount of interest in recent years due in large part to their potential for quantum information processing. They break both parity and time-reversal symmetries and have been predicted to harbor Majorana fermions in vortex cores and along their edges (see, e.g., [1][2][3]). The non-Abelian statistics exhibited by these quasiparticles-they are their own antiparticles-endows them with a topological robustness, making them an ideal resource for quantum computation [4]. To date, one of the most promising candidate chiral superconductors is Sr 2 RuO 4 [5]. However, unambiguous evidence of chiral superconductivity is lacking and there is a pressing need to better understand experimental signatures of potential chiral superconductors. The anomalous Hall effect, or the closely related Kerr effect [2], is arguably the most direct signature of chiral superconductivity. However, this effect vanishes in models of clean chiral superconductors studied to date [2,[7][8][9][10].In this Letter, we show that an intrinsic anomalous Hall effect (IAHE) will arise in multiband chiral superconductors provided there is interband pairing with a relative phase (defined below) that differs from that of one (or more) of the intraband order parameters and particle-hole symmetry is broken. Neither condition is very restrictive and one generally expects any multiband chiral superconductor to satisfy both. In this case, interband transitions in response to an applied electric field are sensitive to the relative phase of the Cooper pairs, giving rise to a transverse Hall current response. Using a two-band model of chiral superconductivity, we derive expressions for the frequency dependent Hall conductivity that show this physics explicitly.In general, the orbital part of a 2D chiral order parameter has the formwhere ∆ ′ and ∆ ′′ are real. (The global U (1) phase is set to zero). In a multiband system, there will be multiple order parameters, α = 1, 2, ..., possibly arising from both intraband and interband pairing. ∆ α is complex and breaks time-reversal symmetry and parity if the real and imaginary parts have different momentum dependencies, such that the Cooper pair electrons have nonzero relative angular momenta. The momentum-dependent) plays a central role in characterizing chiral superconductors. Responsible fo...

Understanding the quantum dynamics of strongly interacting fermions is a problem relevant to diverse forms of matter, including high-temperature superconductors, neutron stars, and quark-gluon plasma. An appealing benchmark is offered by cold atomic gases in the unitary limit of strong interactions. Here we study the dynamics of a transversely magnetized unitary Fermi gas in an inhomogeneous magnetic field. We observe the demagnetization of the gas, caused by diffusive spin transport. At low temperatures, the diffusion constant saturates to the conjectured quantum-mechanical lower bound /m, where m is the particle mass. The development of pair correlations, indicating the transformation of the initially non-interacting gas towards a unitary spin mixture, is observed by measuring Tan's contact parameter.Short-range interactions reach their quantum-mechanical limit when the scattering length that characterizes interparticle collisions diverges. A well controlled model system that realizes this unitary regime is provided by ultracold fermionic alkali atoms tuned to a Fano-Feshbach resonance [1]. These scale-invariant gases are characterized by universal parameters relevant to diverse systems such as the crust of neutron stars at twenty-five orders of magnitude higher density [2,3]. Experiments with ultracold atoms have already greatly contributed to the understanding of equilibrium properties of unitary gases [4][5][6]. Progress has also been made in the study of unitary dynamics [7][8][9][10][11], including observations of suppressed momentum transport [7] and spin transport [8][9][10] due to strong scattering.Spin diffusion is the transport phenomenon that relaxes magnetic inhomogeneities in a many-body system. At low temperature, where Pauli blocking suppresses collision rates, one must distinguish between diffusion driven by gradients in either the magnitude or the direction of magnetization, and quantified by longitudinal spin diffusivity D . This is consistent with a dimensional argument, in which diffusivity is a typical velocity ( k F /m for a cold Fermi gas, where k F is the Fermi momentum) times the mean free path between collisions. In the absence of localization, the mean-free path in a gas cannot be smaller than the interparticle spacing ∼ 1/k F , which translates into a quantum lower bound of roughly /m [9, 14, 15]. However, D ⊥ s as low as 0.0063(8) /m was recently observed in a strongly interacting two-dimensional Fermi gas [10]. This thousand-fold range in transport coefficients remains unexplained by theory.We measure the transverse demagnetization dynamics of a three-dimensional Fermi gas that is initially fully spinpolarized. All of our measurements are carried out with samples of ultracold 40 K atoms in a harmonic trap. Each atom is prepared in an equal superposition of two resonantly interacting internal states, labeled |↑ and |↓ [16], which corresponds to a gas with full transverse magnetization M y = 1 (Fig. 1). Initially, interactions between these identical ultracold fermions is inhibited ...

The superfluid density is a fundamental quantity describing the response to a rotation as well as in two-fluid collisional hydrodynamics. We present extensive calculations of the superfluid density ρ s in the BCS-BEC crossover regime of a uniform superfluid Fermi gas at finite temperatures.We include strong-coupling or fluctuation effects on these quantities within a Gaussian approximation. We also incorporate the same fluctuation effects into the BCS single-particle excitations described by the superfluid order parameter ∆ and Fermi chemical potential µ, using the Nozières and Schmitt-Rink (NSR) approximation. This treatment is shown to be necessary for consistent treatment of ρ s over the entire BCS-BEC crossover. We also calculate the condensate fraction N c as a function of the temperature, a quantity which is quite different from the superfluid density ρ s . We show that the mean-field expression for the condensate fraction N c is a good approximation even in the strong-coupling BEC regime. Our numerical results show how ρ s and N c depend on temperature, from the weak-coupling BCS region to the BEC region of tightly-bound Cooper pair molecules. In a companion paper by the authors (cond-mat/0609187), we derive an equivalent expression for ρ s from the thermodynamic potential, which exhibits the role of the pairing fluctuations in a more explicit manner.

The edges of time reversal symmetry breaking topological superconductors support chiral Majorana bound states as well as spontaneous charge currents. The Majorana modes are a robust, topological property, but the charge currents are non-topological-and therefore sensitive to microscopic details-even if we neglect Meissner screening. We give insight into the non-topological nature of edge currents in chiral p-wave superconductors using a variety of theoretical techniques, including lattice Bogoliubov-de Gennes equations, the quasiclassical approximation, and the gradient expansion, and describe those special cases where edge currents do have a topological character. While edge currents are not quantized, they are generically large, but can be substantially reduced for a sufficiently anisotropic gap function, a scenario of possible relevance for the putative chiral p-wave superconductor Sr2RuO4.