Superfluid Phase Transitions and Effects of Thermal Pairing Fluctuations in Asymmetric Nuclear Matter.

We investigate superfluid phase transitions of asymmetric nuclear matter at finite temperature (T) and density (ρ) with a low proton fraction (Yp ≤ 0.2), which is relevant to the inner crust and outer core of neutron stars. A strong-coupling theory developed for two-component atomic Fermi gases is generalized to the four-component case, and is applied to the system of spin-1/2 neutrons and protons. The phase shifts of neutron-neutron (nn), proton-proton (pp) and neutron-proton (np) interactions up to k = 2 fm-1 are described by multi-rank separable potentials. We show that the critical temperature [Formula: see text] of the neutron superfluidity at Yp = 0 agrees well with Monte Carlo data at low densities and takes a maximum value [Formula: see text]= 1.68 MeV at [Formula: see text] with ρ0 = 0.17 fm-3. Also, the critical temperature [Formula: see text] of the proton superconductivity for Yp ≤ 0.2 is substantially suppressed at low densities due to np-pairing fluctuations, and starts to dominate over [Formula: see text] only above [Formula: see text](0.77) for Yp = 0.1(0.2), and (iii) the deuteron condensation temperature [Formula: see text] is suppressed at Yp ≤ 0.2 due to a large mismatch of the two Fermi surfaces.

Non-Hermitian Phase Transition from a Polariton Bose-Einstein Condensate to a Photon Laser.

We propose a novel mechanism for a nonequilibrium phase transition in a U(1)-broken phase of an electron-hole-photon system, from a Bose-Einstein condensate of polaritons to a photon laser, induced by the non-Hermitian nature of the condensate. We show that a (uniform) steady state of the condensate can always be classified into two types, namely, arising either from lower or upper-branch polaritons. We prove (for a general model) and demonstrate (for a particular model of polaritons) that an exceptional point where the two types coalesce marks the end point of a first-order-like phase boundary between the two types, similar to a critical point in a liquid-gas phase transition. Since the phase transition found in this paper is not in general triggered by population inversion, our result implies that the second threshold observed in experiments is not necessarily a strong-to-weak-coupling transition, contrary to the widely believed understanding. Although our calculation mainly aims to clarify polariton physics, our discussion is applicable to general driven-dissipative condensates composed of two complex fields.