Pressure induced complexity in a lithium monolayer:Ab initiocalculations

Abstract

Light alkali metals have usually been considered as simple metals due to their monovalency and high conductivity. In these metals ionic pseudopotentials are weak and the nearly free electron model (NFE) becomes quite accurate at normal conditions. However, very recent experiments have shown that at high pressures their electronic properties deviate radically from the NFE model and even become unexpected good superconductors. In this work we present ab initio calculations to analyze the deviation from simplicity in a lithium monolayer (ML) when pressure is applied. We have seen that as a result of the increasing non-local character of the atomic pseudopotential with increasing pressure, the surprising half filling Hubbard-type nesting observed in the Fermi line can explain the interesting complex behavior in lithium ML, induced by its correlated structural, electronic and even magnetic properties.

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

“…In fact, as a consequence of the induced s-p orbital mixing, 1 pressure makes them depart from their simplicity at ambient conditions to become metals with rather complex behavior. 2 Not only superconductivity arises under pressure in lithium with a large T c , 3,4 but, even more strikingly, both lithium and sodium become semiconductors when further compressed, 5,6 confirming the predictions made by Neaton and Ashcroft. 7,8 Moreover, contradicting common sense, the melting curve of the alkalies shows a negative slope of the melting temperature as a function of pressure in a wide pressure range, [9][10][11][12][13] principally, due to the softening of a transverse phonon mode induced by compression.…”

Nesting Induced Peierls-Type Instability for Compressed Li-cI16

“…However, new theoretical and experimental results have shown that under pressure light alkalies depart radically from this simple behavior, as phase transitions to complex and low-coordinated structures emerge. [2][3][4][5] According to a recent x-ray analysis 3) lithium undergoes a bcc to fcc transition at 7.5 GPa, followed by a fcc to hR1 at 39 GPa and a hR1 to cI16 at around 40 GPa. On the other hand, despite experiments looking for superconductivity in lithium at equilibrium have failed, 8) it has been observed to superconduct at 20 K when pressure rises to 40 GPa.…”

Dynamical response function of a compressed lithium monolayer

“…Therefore, we have two types of carriers at the Fermi energy, a fact which plays a fundamental role in the pressure induced properties of the electronic collective excitations, as will be explained bellow. It is noteworthy that the Fermi line of the ML also suffers significant deviations with increasing electronic density [10], as it is shown in Fig. 2.…”

“…In addition, extending our conclusions to the bulk will also give another perspective to understand the physical origin under the experimentally observed features in compressed lithium. Previous ab initio analysis of the structural and electronic properties of a lithium ML [9,10] reveal important modifications in both its band structure and Fermi line, which will also lead to significant modifications on its dynamical response function, up to now just studied at equilibrium [11]. In this article we perform calculations of the dynamical response function of a lithium ML at different pressures, analyzing its evolution with increasing electronic density.…”