## Featured in EPL

Most recent highlights from EPL:

**Black-hole entropy from complex Ashtekar variables**^{1}* by E. Frodden et al.*

In loop quantum gravity, the number N_{Γ}(A,γ) of microstates of a black hole for a given discrete geometry Γ depends on the so-called Barbero-Immirzi parameter γ. Using a suitable analytic continuation of γ to complex values, we show that the number N_{Γ}(A,±1) of microstates behaves as exp(A/(4l_{Pl}²)) for a large area A in the large spin semiclassical limit. Such a correspondence with the semiclassical Bekenstein-Hawking entropy law points towards an unanticipated and remarkable feature of the original complex Ashtekar variables for quantum gravity.

**From the sine-Gordon field theory to the Kardar-Parisi-Zhang growth equation**^{2}* by Pasquale Calabrese et al.*

We unveil a remarkable connection between the sine-Gordon quantum field theory and the Kardar-Parisi-Zhang (KPZ) growth equation. We find that the non-relativistic limit of the two-point correlation function of the sine-Gordon theory is related to the generating function of the height distribution of the KPZ field with droplet initial conditions, i.e. the directed polymer free energy with two endpoints fixed. As shown recently, the latter can be expressed as a Fredholm determinant which in the large-time separation limit converges to the GUE Tracy-Widom cumulative distribution. Possible applications and extensions are discussed.

**Evidence for a Bose-Einstein condensate of excitons**^{3}* by Mathieu Alloing et al.*

We report compelling evidence for a “gray” condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas (∼10^{10} cm^{-2}) that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the “gray” condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.

**Subharmonic oscillations of collective molecular motors**^{4}* by D. Oriola et al.*

We study a generic two-state model for an assembly of molecular motors which is described by means of a pair of integro-partial differential equations and leads to oscillatory motion in the presence of an elastic coupling to its environment. We discuss a reduction of the system to a minimal set of three ordinary differential equations that successfully capture the complex nonlinear dynamics of the full system. In the limit of high mobility and large elastic modulus, we report on the emergence of subharmonics in the power spectrum of the oscillations. This provides a rationale for the unexplained observation of secondary peaks in a minimal actomyosin system in vitro (Plaçais P.-Y. et al., Phys. Rev. Lett. 103, (2009) 158102), showing that the phenomenon is robust and genuine.

**Uncovering temporal transitions and self-organization during slow aging of dense granular media in the absence of shear bands**^{5}* by David M. Walker et al.*

We present a method for discovering temporal transitions in the macroscopic response of two granular assemblies of photoelastic disks, subject to prolonged symmetric cyclic pure shear —under uniform deformation. A distance-matrix–based analysis is used in a reconstructed state space formed from the macroscopic stress ratio time series, with the technique of quadrant scans applied to extract transition times from the recurrence plots. Macroscopic measures of pressure and shear stress exhibit limit cycle behavior with respect to the applied cyclic strain. The contact network and the strong force filamentary network, however, show a gradual change across shear cycles. A quantitative characterization of the self-organization process at the mesoscale reveals that the observed transition in system dynamics at the macroscale is consistent with the process of aging. A distinct and consistent pattern of self-organization with respect to the contact topology and the structural stability of force chains is uncovered. The contact topology evolves to a more densely connected and stable truss-laced lattice, embodying force chain columns endowed with higher levels of triangular and rectangular bracing. This results in an increase in the structural stability of force chains, consistent with the prevailing conjecture on the structural mechanism behind the observed increase in shear strength and shear stiffness in an aging sand.

- E. Frodden et al. 2014. Black-hole entropy from complex Ashtekar variables. EPL, 107, 10005. DOI: 10.1209/0295-5075/107/10005 [↩]
- Pasquale Calabrese et al. 2014. From the sine-Gordon field theory to the Kardar-Parisi-Zhang growth equation. EPL, 107, 10011. DOI: 10.1209/0295-5075/107/10011 [↩]
- Mathieu Alloing et al. 2014. Evidence for a Bose-Einstein condensate of excitons. EPL, 107, 10012. DOI: 10.1209/0295-5075/107/10012 [↩]
- D. Oriola et al. 2014. Subharmonic oscillations of collective molecular motors. EPL, 107, 18002. DOI: 10.1209/0295-5075/107/18002 [↩]
- David M. Walker et al. 2014. Uncovering temporal transitions and self-organization during slow aging of dense granular media in the absence of shear bands. EPL, 107, 18005. DOI: 10.1209/0295-5075/107/18005 [↩]