Electronic collective excitations in compressed lithium fromab initiocalculations: Importance and anisotropy of local-field effects at large momenta

Abstract

According to our ab initio calculations, the recently reported undamped plasmon in compressed fcc lithium emerges not only due to the fact that the dielectric matrix determinant vanishes at long wavelengths at the plasmon energy, i.e., satisfying the ideal condition for an undamped plasmon in a crystal but also extends at large momenta specially along the ⌫L direction. Different from the case of simple metals, the local-field effects dominate, leading to the striking periodicity exhibited by the low-energy plasmon for momentum transfers beyond the first Brillouin zone. Remarkably, pressure-induced electronic anisotropy and localization increase the impact of local-field effects on the electronic response even in the compact fcc structure. Interestingly, the importance of local-field effects is similar in lithium under pressure and in materials with confined geometries such as MgB 2 .

Optical properties of calcium under pressure from first-principles calculations

“…The presence of this type of plasmons strongly modifies the reflectivity, leading it to an almost vanishing value close to the plasmon energy. This anomalous optical behavior has been already observed in Na 22 and has been predicted in Li [23][24][25] and AlH 3 . 36 Thus, calcium seems to be another example of this behavior, providing a good example of how pressure-induced complexity is also reflected in the optical properties.…”

“…The inclusion of CLFEs does not significantly modify the plasmon energy position, similar to the interband plasmon found in fcc Li at small momenta. 25 Besides the remarkable low-energy plasmon, higher energy plasmons are also present in the sc phase of Ca. Although at 35 GPa we find two prominent peaks above 10 eV, one at 10.9 eV and the second at 13.1 eV, at the other pressures the second peak is not observed, and only a regular intraband plasmon is present.…”

“…22 Theoretical calculations describe a similar mitigation of the reflectivity for fcc lithium, 23 which is directly associated to the interband low-energy plasmon already predicted in previous ab initio calculations. [23][24][25] Indeed, in systems such as Ag, 26,27 graphite, 28 single-wall carbon nanotubes, 29 or MgB 2 , 30,31 interband transitions in the band structure shape the dielectric function of metals and induce the emergence of several plasmons that cannot be understood within the free-electron model. If the origin of a plasmon resides in these interband transitions, it is sometimes referred to in the literature as an interband plasmon.…”

Anharmonicity and the isotope effect in superconducting lithium at high pressures: A first-principles approach

“…Experimental evidence [3][4][5][6][7]9 shows that in the pressure and temperature ranges where the anomalous isotope effect was measured (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) GPa and below 30 K) lithium presents a fcc structure. At around 40 GPa, it transforms to the rhombohedral hR1 phase, which is just a distortion of the fcc phase along the c axis if one switches to a hexagonal representation.…”

Dynamical stability of face centered cubic lithium at 25 GPa

“…Although it could be expected to evolve to an even more free-electron like system with increasing pressure, it has been shown that pressure not only induces several structural transformations [2,3,4], but also gives rise to a plethora of fascinating physical properties [5] normally induced by nonlocality in the core pseudopotential with pressure [6,7]. For instance, lithium becomes a semiconductor near 80 GPa [8], it melts below ambient temperature (190 K) at around 50 GPa [2] and displays a periodic undamped plasmon according to theoretical calculations [9]. Moreover, it presents one of the highest superconducting critical temperatures (Tc) for an element, reaching values as high as 15 K at around 30 GPa [10][11][12][13].…”