Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite

Abstract

Irradiation-induced increase in the low-temperature specific heat has been measured in the temperature range of 1.9-43 K in graphite neutron-irradiated to 1.4ϫ 10 20 n / cm 2 ͑E Ͼ 1 MeV͒ around 333 K. The increase of the lattice specific heat is interpreted as due to the hindered rotation of interstitial C 2 molecules in the periodic potential of V͑͒ = ͑V 0 / 2͒͑1 + cos 2͒, where is the angle of rotation and V 0 is 0.040 eV. The first-excited rotational level is 0.0058 eV above the ground state and the rotational frequency is 1.39ϫ 10 12 s −1 . This result shows that C 2 molecules do not form covalent bonds with atoms in the surrounding graphite layers. It is in marked contrast with the result of recent first-principles theoretical calculations that interstitial atoms form strong covalent bonds with atoms in the graphite layers. The concentration of C 2 molecules is estimated to be f = 1.16%. The hole concentration, deduced from the electronic specific heat and the SWMcC band model, suggests that one single vacancy creates one hole and the single vacancy concentration is 2f. Since the measurement of the lattice specific heat gave the concentration of the defects directly, we could evaluate some physical properties for a unit concentration of the defects. The volume changes by a single interstitial atom, an interstitial C 2 molecule, and a single vacancy are deduced to be 2.6Ϯ 0.3, 9.3Ϯ 1.4, and −0.46Ϯ 0.07 atomic volumes, respectively. The formation energy of a Frenkel pair is estimated to be 12.6Ϯ 2.5 eV. The phonon scattering with a reciprocal relaxation time proportional to 1.5 , where is the angular frequency of phonons, is attributed to the scattering of phonons by the disklike strain around interstitial C 2 molecule clusters. A broad dip in the a-axis thermal conductivity observed below room temperature is attributed to the resonance scattering of phonons by the hindered rotation of interstitial C 2 molecules as well as by the vibration of these molecules as rigid units. The positron lifetime of 350 ps is suggested to be the lifetime of positrons trapped in an open space on the periphery of the interstitial clusters of C 2 molecules. The well-known Wigner energy is stored mainly as interstitial C 2 molecules and single vacancies.

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

“…Therefore, the structure will certainly have complex very low-frequency behaviour; however, it is not possible to quantify this at present. Nevertheless, it is likely that modes with very low frequency from this defect contribute to an anomaly observed for the specific heat of radiation damaged graphite at low temperatures [123], and that the earlier explanation for this phenomenon is probably incorrect [51].…”

Modeling chemisorption of carbon dimer at (8, 0) nanotube

“…According to the total-energy theory, in 3-D graphite [9,10], both h-and b-graftings are possible, as having lower energy than independent substrate and dimer, that is also true for SWNT (8,0). But which of them has lower total energy?…”

“…The 3-dimensional-graphite problem of radiation defects, related to the interstitial dimer [8], also required models for dimer grafting. "Grafted interlayer bridge" di-interstitial [9], later marked I 2 (7557) [10], is similar to ISW in graphene.…”

Bell states and entanglement of two-dimensional polar molecules in electric fields

“…Two-dimensional rotational manner has been observed in the experiments of hydrogen on a surface [18][19][20]. The planar rotation in diverse configurations * yyliao@nuk.edu.tw further leads to interesting consequences for energy spectrum [21,22], molecular orientation [23], and specific heat [24,25]. In this work we study the entanglement in a system of two polar molecules confined to a plane.…”