States of Thermal Equilibrium in β‐Rhombohedral Boron and Their Alteration by Optical Excitation

Abstract

It is shown that p-rhombohedra1 boron has different states of thermal equilibrium a t T 5 150 and 2 180 K. These states can be distinguished by their optical absorption spectra showing electronic transitions into a bound state at 0.4 eV at low temperatures, and transitions into an energy continuum a t higher temperatures. By steady state optical excitation the transition temperat,ure is lowered indicating a kind of Mott transition. This phase transition is assumed to be the reason for the discontinuity of numerous physical properties of P-rhombohedra1 boron in this range of temperature. E s wird nachgewiesen, daB P-rhomboedrisches Bor bei T 5 150 und 2 180 K verschiedene thermodynamische Gleichgewichtszustande besitzt. Diese werden anhand der optischen Absorptionsspektren unterschieden, die bei tiefen Temperaturen elektronische obergange in einen gebundenen Zustand bei 0,4 eV, bei hohen Temperaturen in ein Energiekontinuum zeigen. Durch stationare optische Erregung wird die Umwandlungstemperatur abgesenkt, was fur eine Art Mottttbergang spricht. Dieser Phasenubergang liefert eine Begrundung fur die Diskontinuitat diverser physikalischer Eigenschaften des P-rhomboedrischen Bors in diesem Temperaturbereich.

Geometrical frustration in an elemental solid: An Ising model to explain the defect structure ofβ-rhombohedral boron

“…2 and its caption͒. We also show that the rather peculiar transport properties of boron that have been reported over the past 40 years [15][16][17][18] can be rationalized by the presence of geometrical frustration.…”

VASP on a GPU: Application to exact-exchange calculations of the stability of elemental boron

“…That is, it predicts the partially occupied β ′ structure is lower in energy than α. Experimental evidence exists for structural anomalies at low temperatures based on photoabsorption [22,23], internal friction [24,25] and heat capacity [26]. These phenomena could be consistent with the predicted low temperature symmetry-breaking phase transition [19,27].…”