Universal quantum Otto heat machine based on the Dicke model.

In this paper we study a quantum Otto thermal machine where the working substance is composed of N identical qubits coupled to a single mode of a bosonic field, where the atoms and the field interact with a reservoir, as described by the so-called open Dicke model. By controlling the relevant and experimentally accessible parameters of the model we show that it is possible to build a universal quantum heat machine (UQHM) that can function as an engine, refrigerator, heater, or accelerator. The heat and work exchanges are computed taking into account the growth of the number N of atoms as well as the coupling regimes characteristic of the Dicke model for several ratios of temperatures of the two thermal reservoirs. The analysis of quantum features such as entanglement and second-order correlation shows that these quantum resources do not affect either the efficiency or the performance of the UQHM based on the open Dicke model. In addition, we show that the improvement in both efficiency and coefficient of performance of our UQHM occurs for regions around the critical value of the phase transition parameter of the model.

Response of soil stoichiometric characteristics to climatic factors in temperate steppe of Longzhong region, China.

We explored the relationship between climate factors (mean annual precipitation and mean annual temperature) and the contents and stoichiometry of soil carbon (C), nitrogen (N), and phosphorus (P) at different soil depths (0-5, 5-10, 10-20, 20-30, 30-50, 50-70, and 70-100 cm) temperate steppe of Longzhong. The results showed with the increases of soil depth, soil C, N contents, C:P, and N:P gradually decreased from 21.88 g·kg-1, 1.84 g·kg-1, 33.6 and 3.1 to 7.67 g·kg-1, 0.59 g·kg-1, 12.5 and 1.0, respectively. Soil C:N showed an increasing trend from 12.2 to 13.9, while soil P content remained stable with an average of 0.61 g·kg-1. Soil C, N, C:P, and N:P were significantly positively correlated with mean annual precipitation and negatively correlated with mean annual temperature. Soil P content and C:N were not correlated with mean annual precipita-tion and mean annual temperature. With the increases of soil depth, the total explanatory power of the changes in soil C, N and P contents by mean annual precipitation and mean annual temperature decreased and then increased, and that in soil C:P, N:P and C:N did not change significantly. The changes of soil C, N and P contents on the temperature steppe were mainly influenced by mean annual precipitation. The effects and relative contributions of mean annual precipitation and mean annual temperature on the variations of soil nutrient contents and stoichiometry of C, N and P differed at different soil depths.