71 research outputs found
Stabilizing fluctuating spin-triplet superconductivity in graphene via induced spin-orbit coupling
A recent experiment showed that proximity induced Ising spin-orbit coupling
enhances the spin-triplet superconductivity in Bernal bilayer graphene. Here,
we show that, due to the nearly perfect spin rotation symmetry of graphene, the
fluctuations of the spin orientation of the triplet order parameter suppress
the superconducting transition to nearly zero temperature. Our analysis shows
that both Ising spin-orbit coupling and in-plane magnetic field can eliminate
these low-lying fluctuations and can greatly enhance the transition
temperature, consistent with the recent experiment. Our model also suggests the
possible existence of a phase at small anisotropy and magnetic field which
exhibits quasi-long-range ordered spin-singlet charge 4e superconductivity,
even while the triplet 2e superconducting order only exhibits short-ranged
correlations. Finally, we discuss relevant experimental signatures.Comment: 6 pages, 2 figures + 8 pages, 1 figure supplementa
Cavity magnon-polaritons in cuprate parent compounds
Cavity control of quantum matter may offer new ways to study and manipulate
many-body systems. A particularly appealing idea is to use cavities to enhance
superconductivity, especially in unconventional or high- systems.
Motivated by this, we propose a scheme for coupling Terahertz resonators to the
antiferromagnetic fluctuations in a cuprate parent compound, which are believed
to provide the glue for Cooper pairs in the superconducting phase. First, we
derive the interaction between magnon excitations of the Ne\'el-order and polar
phonons associated with the planar oxygens. This mode also couples to the
cavity electric field, and in the presence of spin-orbit interactions mediates
a linear coupling between the cavity and magnons, forming hybridized
magnon-polaritons. This hybridization vanishes linearly with photon momentum,
implying the need for near-field optical methods, which we analyze within a
simple model. We then derive a higher-order coupling between the cavity and
magnons which is only present in bilayer systems, but does not rely on
spin-orbit coupling. This interaction is found to be large, but only couples to
the bimagnon operator. As a result we find a strong, but heavily damped,
bimagnon-cavity interaction which produces highly asymmetric cavity line-shapes
in the strong-coupling regime. To conclude, we outline several interesting
extensions of our theory, including applications to carrier-doped cuprates and
other strongly-correlated systems with Terahertz-scale magnetic excitations.Comment: 32 pages, 12 figure
Evaluating range-expansion models for calculating nonnative species' expansion rate
Species range shifts associated with environmental change or biological invasions are increasingly important study areas. However, quantifying range expansion rates may be heavily influenced by methodology and/or sampling bias. We compared expansion rate estimates of Roesel's bush-cricket (Metrioptera roeselii, Hagenbach 1822), a nonnative species currently expanding its range in south-central Sweden, from range statistic models based on distance measures (mean, median, 95th gamma quantile, marginal mean, maximum, and conditional maximum) and an area-based method (grid occupancy). We used sampling simulations to determine the sensitivity of the different methods to incomplete sampling across the species' range. For periods when we had comprehensive survey data, range expansion estimates clustered into two groups: (1) those calculated from range margin statistics (gamma, marginal mean, maximum, and conditional maximum: similar to 3 km/year), and (2) those calculated from the central tendency (mean and median) and the area-based method of grid occupancy (similar to 1.5 km/year). Range statistic measures differed greatly in their sensitivity to sampling effort; the proportion of sampling required to achieve an estimate within 10% of the true value ranged from 0.17 to 0.9. Grid occupancy and median were most sensitive to sampling effort, and the maximum and gamma quantile the least. If periods with incomplete sampling were included in the range expansion calculations, this generally lowered the estimates (range 16-72%), with exception of the gamma quantile that was slightly higher (6%). Care should be taken when interpreting rate expansion estimates from data sampled from only a fraction of the full distribution. Methods based on the central tendency will give rates approximately half that of methods based on the range margin. The gamma quantile method appears to be the most robust to incomplete sampling bias and should be considered as the method of choice when sampling the entire distribution is not possible
Post-Crisis Lesson for EMU Governance from the Principal-Agent Approach
This paper contributes to the ongoing debate on fiscal consolidation and the questionable effectiveness of the Stability and Growth Pact by addressing the problem of economic governance in the EMU with a game-theoretic principal-agent approach. Following the theory of delegation, we develop a principal-multi agent model where the EMU authorities act as a collective principal that designs contracts for each of two agents that reflect Europe's "South" and "North". We investigate what happens when agents face hidden-information moral hazard problem and when they are able to coordinate their actions. Bearing in mind the applicability of incentive mechanisms, we discuss the optimal contracts for the principal and each of the agents. We prove that the most efficient solution consists of tailor-made contracts, according to which highly indebted countries must be offered strong incentive mechanisms in the form of substantial penalties but also rewards (e.g., preferential loans). We also stress the importance of taking into account positive spillover effects, which could be facilitated by economic integration and fiscal policy coordination between the EMU Members
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