16 research outputs found
Isotope effect on the E2g phonon and mesoscopic phase separation near the electronic topological transition in Mg1-xAlxB2
We report the boron isotope effect on the E2g phonon mode by micro-Raman
spectroscopy on the ternary Mg1-xAlxB2 system, synthesized with pure isotopes
10B and 11B. The isotope coefficient on the phonon frequency is near 0.5 in the
full range decreasing near x = 0. The intraband electron-phonon (e-ph)
coupling, for the electrons in the sigma band, has been extracted from the E2g
line-width and frequency softening. Tuning the Fermi energy near the electronic
topological transition (ETT), where the sigma Fermi surface changes from 2D to
3D topology the E2g mode, shows the known Kohn anomaly on the 2D side of the
ETT and a splitting of the E2g phonon frequency into a hard and soft component
from x = 0 to x = 0.28. The results suggest a minor role of the intraband
phonon mediated pairing in the control of the high critical temperature in
Mg1-xAlxB2. The common physical features of diborides with the novel multigap
FeAs-based superconductors and cuprates is discussed.Comment: 19 pages, 6 figure
Crossover between liquid-like and gas-like behaviour in CH4 at 400 K
We report experimental evidence for a crossover between a liquid-like state and a gas-like state in fluid methane (CH4). This crossover is observed in all of our experiments, up to 397 K temperature; 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and X-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line - a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquid-like and gas-like properties. On the liquid-like side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gas-like side this competes with the attractive Van der Waal’s potential leading the vibration frequency to decrease as pressure is increased