Nuclear pairing at finite temperature and angular momentum

An approach is proposed to nuclear pairing at finite temperature and angular momentum, which includes the effects of the quasiparticle-number fluctuation and dynamic coupling to pair vibrations within the self-consistent quasiparticle random-phase approximation. The numerical calculations of pairing gaps, total energies, and heat capacities are carried out within a doubly folded multilevel model as well as several realistic nuclei. The results obtained show that, in the region of moderate and strong couplings, the sharp transition between the superconducting and normal phases is smoothed out, causing a thermal pairing gap, which does not collapse at a critical temperature predicted by the conventional Bardeen-Cooper-Schrieffer's (BCS) theory, but has a tail extended to high temperatures. The theory also predicts the appearance of a thermally assisted pairing in hot rotating nuclei.Comment: 4 pages, 1 figure, To appear in the Proceedings of the First Workshop on State of the Art in Nuclear Cluster Physics, Strasbourg 13 - 16 May, 200

Canonical and microcanonical ensemble descriptions of thermal pairing within BCS and quasiparticle RPA

We propose a description of pairing properties in finite systems within the canonical and microcanonical ensembles. The approach is derived by solving the BCS and self-consistent quasiparticle random-phase approximation with the Lipkin-Nogami particle-number projection at zero temperature. The obtained eigenvalues are embedded into the canonical and microcanonical ensembles. The results obtained are found in quite good agreement with the exact solutions of the doubly-folded equidistant multilevel pairing model as well as the experimental data for $^{56}$Fe nucleus. The merit of the present approach resides in its simplicity and its application to a wider range of particle number, where the exact solution is impracticable.Comment: 10 pages, 2 figures, accepted for publication in Phys. Rev.

Specific shear viscosity in hot rotating systems of paired fermions

The specific shear viscosity $\bar\eta$ of a classically rotating system of nucleons that interact via a monopole pairing interaction is calculated including the effects of thermal fluctuations and coupling to pair vibrations within the selfconsistent quasiparticle random-phase approximation. It is found that $\bar\eta$ increases with angular momentum $M$ at a given temperature $T$. In medium and heavy systems, $\bar\eta$ decreases with increasing $T$ at $T\geq$ 2 MeV and this feature is not affected much by angular momentum. But in lighter systems (with the mass number $A\leq$ 20), $\bar\eta$ increases with $T$ at a value of $M$ close to the maximal value $M_{max}$, which is defined as the limiting angular momentum for each system. The values of $\bar\eta$ obtained within the schematic model as well as for systems with realistic single-particle energies are always larger than the universal lower-bound conjecture $\hbar/(4\pi k_B)$ up to $T$=5 MeV.Comment: 19 pages, 7 figures, accepted for publication in Phys. Rev.

Pairing reentrance in hot rotating nuclei

The pairing gaps, heat capacities and level densities are calculated within the BCS-based quasiparticle approach including the effect of thermal fluctuations on the pairing field within the pairing model plus noncollective rotation along the z axis for $^{60}$Ni and $^{72}$Ge nuclei. The analysis of the numerical results obtained shows that, in addition to the pairing gap, the heat capacity can also serve as a good observable to detect the appearance of the pairing reentrance in hot rotating nuclei, whereas such signature in the level density is rather weak.Comment: 19 pages, 4 figures, accepted in Phys. Rev.

On The Performance Of 1-Bit ADC In Massive MIMO Communication Systems

Massive multiple-input multiple-output (MIMO) with low-resolution analog-to-digital converters is a rational solution to deal with hardware costs and accomplish optimal energy efficiency. In particular, utilizing 1-bit ADCs is one of the best choices for massive MIMO systems. This paper investigates the performance of the 1-bit ADC in the wireless coded communication systems where the robust channel coding, protograph low-density parity-check code (LDPC), is employed. The investigation reveals that the performance of the conventional 1-bit ADC with the truncation limit of 3-sigma is severely destroyed by the quantization distortion even when the number of antennas increases to 100. The optimized 1-bit ADC, though having substantial performance gain over the conventional one, is also affected by the quantization distortion at high coding rates and low MIMO configurations. Importantly, the investigation results suggest that the protograph LDPC codes should be re-designed to combat the negative effect of the quantization distortion of the 1-bit ADC