Gauge approach to superfluid density in underdoped cuprates

We prove that a gauge approach based on a composite structure of the hole in hole-doped cuprates is able to capture analytically many features of the experimental data on superfluid density in the moderate-underdoping to nearly-optimal doping region, including critical exponent, the Uemura relation and near universality of the normalized superfluid density.Comment: 13 pages, 2 figures, accepted for publication in EP

Superfluid density in cuprates: hints on gauge compositeness of the holes

We show that several features (the three-dimensional XY universality for moderate underdoping, the almost-BCS behaviour for moderate overdoping and the critical exponent) of the superfluid density in hole-doped cuprates hint at a composite structure of the holes. This idea can be implemented in a spin-charge gauge approach to the $t - t' - J$ model and provides indeed good agreement with available experimental data.Comment: 5 pages, 1 figure, to be published in the proceedings of the International Conference in Superconductivty and Magnetism ICSM201

Spontaneous patterns in coherently driven polariton microcavities

We consider a polariton microcavity resonantly driven by two external lasers which simultaneously pump both lower and upper polariton branches at normal incidence. In this setup, we study the occurrence of instabilities of the pump-only solutions towards the spontaneous formation of patterns. Their appearance is a consequence of the spontaneous symmetry breaking of translational and rotational invariance due to interaction induced parametric scattering. We observe the evolution between diverse patterns which can be classified as single-pump, where parametric scattering occurs at the same energy as one of the pumps, and as two-pump, where scattering occurs at a different energy. For two-pump instabilities, stripe and chequerboard patterns become the dominant steady-state solutions because cubic parametric scattering processes are forbidden. This contrasts with the single-pump case, where hexagonal patterns are the most common arrangements. We study the possibility of controlling the evolution between different patterns. Our results are obtained within a linear stability analysis and are confirmed by finite size full numerical calculations.Comment: 15 pages, 9 figure

Does the Karate Kid Have a Kung Fu Dream? Hong Kong Martial Arts between Hollywood and Beijing

This analysis of the martial arts choreography in The Karate Kid (2010) examines the contradictory matrix in which action films produce meanings for global audiences. A remake of a 1984 film, this iteration of The Karate Kid begins its imaginative battle over martial arts turf with English and Chinese titles at odds with one another. For Englishspeaking audiences, the title of the film promises a remake of the popular 1984 story of a displaced Italian American teenager (Ralph Macchio) trained by a Japanese American sensei (Pat Morita) to compete against the local karate bullies. However, the 2010 version has another identity competing with the first. Its Chinese title translates as Kung Fu Dream – Japanese culture, karate, and domestic American class and racial politics out of the picture. In this version, an African American youngster (Jaden Smith) moves to Beijing from Detroit and is taken under the wing of a drunken kung fu master (Jackie Chan) to battle a group of wu shu/san da villains. With the change of choreography comes a shift in the ideological battles taken up by the film’s plot providing a different take on race, class, ethnicity, and geopolitics against the backdrop of the ‘rise’ of China. This essay explores what this shift in martial arts choreography says about the Hollywood, Hong Kong, and Mainland Chinese industries involved in its production. In addition, it explores how fans have come to grips with the depiction of karate, traditional kung fu, PRC-style wu shu, and various screen iterations of a range of martial arts in the film.published_or_final_versio

The Gendered Politics of Sex Work in Hong Kong Cinema: Herman Yau and Elsa Chan (Yeeshan)’s Whispers and Moans and True Women for Sale

The portrayal of women as prostitutes is common in Hong Kong cinema, and filmmakers have looked at sex work from various perspectives: as a social problem, as an index of women’s economic exploitation more generally, as a way to explore the relationship between economics and sexuality, as a window onto the world of sexual minorities and as erotica. Scriptwriter Elsa Chan, working in conjunction with director Herman Yau, has made two features about women in the Hong Kong sex industry—Whispers and Moans (性工作者十日談, 2007) and True Women for Sale(性工作者2 我不賣身·我賣子宮, 2008)—based on Chan’s anthropological studies of women in the sex industry in Hong Kong. Like their earlier collaboration From the Queen to the Chief Executive (等候董建華發落, 2001), these films look at the “sensational” (youth murders, prostitution) with an eye to understanding and, perhaps, remedying social injustice. In fact, the issue of “justice” and women’s role as exploited victims as well as potential agents of change link Elsa Chan’s scripts to the interests of other women filmmakers currently working in Hong Kong. Whispers and Moans and True Women for Sale offer a critical perspective on the circulation of women across borders and gesture toward a feminist intervention into the shadow economy between transnational capitalism and the socialist marketplace in China.published_or_final_versio

Plasticity in current-driven vortex lattices

We present a theoretical analysis of recent experiments on current-driven vortex dynamics in the Corbino disk geometry. This geometry introduces controlled spatial gradients in the driving force and allows the study of the onset of plasticity and tearing in clean vortex lattices. We describe plastic slip in terms of the stress-driven unbinding of dislocation pairs, which in turn contribute to the relaxation of the shear, yielding a nonlinear response. The steady state density of free dislocations induced by the applied stress is calculated as a function of the applied current and temperature. A criterion for the onset of plasticity at a radial location $r$ in the disk yields a temperature-dependent critical current that is in qualitative agreement with experiments.Comment: 11 pages, 4 figure

Beliaev damping of the Goldstone mode in atomic Fermi superfluids

Beliaev damping in a superfluid is the decay of a collective excitation into two lower frequency collective excitations; it represents the only decay mode for a bosonic collective excitation in a superfluid at T = 0. The standard treatment for this decay assumes a linear spectrum, which in turn implies that the final state momenta must be collinear to the initial state. We extend this treatment, showing that the inclusion of a gradient term in the Hamiltonian yields a realistic spectrum for the bosonic excitations; we then derive a formula for the decay rate of such excitations, and show that even moderate nonlinearities in the spectrum can yield substantial deviations from the standard result. We apply our result to an attractive Fermi gas in the BCS-BEC crossover: here the low-energy bosonic collective excitations are density oscillations driven by the phase of the pairing order field. These collective excitations, which are gapless modes as a consequence of the Goldstone mechanism, have a spectrum which is well established both theoretically and experimentally, and whose linewidth, we show, is determined at low temperatures by the Beliaev decay mechanism.Comment: 8 pages, 3 figure

Density-wave phases of dipolar fermions in a bilayer

We investigate the phase diagram of dipolar fermions with aligned dipole moments in a two-dimensional (2D) bilayer. Using a version of the Singwi-Tosi-Land-Sjolander scheme recently adapted to dipolar fermions in a single layer [M. M. Parish and F. M. Marchetti, Phys. Rev. Lett. 108, 145304 (2012)], we determine the density-wave instabilities of the bilayer system within linear response theory. We find that the bilayer geometry can stabilize the collapse of the 2D dipolar Fermi gas with intralayer attraction to form a new density wave phase that has an orientation perpendicular to the density wave expected for strong intralayer repulsion. We thus obtain a quantum phase transition between stripe phases that is driven by the interplay between strong correlations and the architecture of the low dimensional system.Comment: 5 pages, 3 figure

The Role of Algorithms in the Crisis of Democracy

The essay aims at analyzing the relationship between algorithms and democracy.Algorithms used by platforms can lead to disinformation so as to produce an impact on democracy. The essay then focuses on the possible solutions to deal with this phenomenon and assesses their limits