Interpretation of anomalous normal state optical conductivity of K 3 C 60 fullerides

Abstract

The observed frequency dependent optical response of alkali-metal-doped fulleride superconductors (T c ≈ 19 K) has been theoretically analysed. The calculations of the optical conductivity, σ (ω), have been made within the two-component schemes: one is the coherent Drude carriers (electrons) responsible for superconductivity and the other is incoherent motion of carriers from one atom to other atom of C 60 molecule to a pairing between Drude carriers. The approach accounts for the anomalies reported (frequency dependence of optical conductivity) in the optical measurements for the normal state. The model has only one free parameter, the relaxation rate. The frequency dependent relaxation rates are expressed in terms of memory functions. The coherent Drude carriers form a sharp peak at zero frequency and a long tail at higher frequencies, i.e. in the infrared region. However, the hopping of carriers from one atom to the other (incoherent motion of doped electrons) yields a peak value in the optical conductivity centred at mid-infrared region. It is found that both the Drude and hopping carriers will contribute to the optical process of conduction in the K 3 C 60 and shows similar results on optical conductivity in the mid-infrared as well as infrared frequency regions as those revealed from experiments.

High‐Temperature Creep Behavior and Damage Mechanism of an Advanced Powder Metallurgy Ni‐Based Superalloy

“…Norton's power law describing the relationship between temperature, stress, and steady-state creep rate in superalloys at high temperatures is widely accepted, [26][27][28] as shown in Equation (6).…”

“…However, when it is dominated by microstructural degradation, the value of λ exceeds 5. [6,10] There may be many factors that affect the creep damage mechanism of the experimental alloy at different creep conditions. It is necessary to observe the microstructure of the alloy if the cause of creep damage is to be ascertained.…”

“…MGR or MMGR describes creep damage behavior, and the growth of damage is considered to start from the tertiary creep stage representing the increase of creep rate, which is recognized by many researchers. [6,8,12,19,34] Most of the creep damages are accumulated in the tertiary creep stage. In particular, when the time exceeds t MGD , the creep deformation enters the sharply accelerated creep stage, and the severe influence of damage on the creep rate begins to be fully revealed.…”

“…[1,[3][4][5] The growth of creep damage is reflected in the increase of the creep rate and leads to the fracture of the alloy. [6] The relationship between steady-state creep rate εs (or minimum creep rate εm ) and creep life t r has been described by the Monkman-Grant relation (MGR) in 1956. [7] The generality of this empirical relationship is verified by data for many alloys, [7][8][9][10] which is as follows:…”

“…The coupling between creep rate and damage not clarified by the MGR is described by the continuum creep damage mechanics (CDM) proposed by Kachanov and Rabotnov. [6] Based on the CDM, DOI: 10.1002/adem.202400230…”