Summary
The band structure of graphite and of the layer compounds GaS and GaSe is computed by using the tight-binding approach in a semi-empirical way. The band structure is related to the basic properties of these compounds and some features of the optical excitation spectrum are explained. We show that the reason why graphite is a semi-metal and GaS and GaSe are semiconductors can be understood in the two-dimensional approximation and is due to the existence of the inversion symmetry in the former case. The absorption edge in GaS corresponds to indirect transitions between the statesΓ +1 andP +1 , while in GaSe it corresponds to direct transitions between the statesΓ +1 andΓ +3 . Sharp peaks inε 2(0,ω) are attributed to saddle point singularities in the joint density of states. The effect on the optical properties produced by a change in the polarization of light is discussed.
Riassunto
La struttura a bande della grafite e dei composti lamellari GaS e GaSe è ottenuta usando l’approssimazione dell’elettrone fortemente legato in forma semi-empirica. La struttura a bande è messa in relazione con le proprietà fondamentali di tali composti ed alcune caratteristiche dello spettro di eccitazione ottica vengono spiegate. Si è mostrato che la ragione per la quale la grafite è un semimetallo, mentre GaS e GaSe sono semiconduttori, può essere capita nella approssimazione bidimensionale; essa è dovuta alla presenza, nel caso della grafite, della simmetria di inversione. Lo spigolo di assorbimento del GaS corrisponde a transizioni indirette tra gli statiΓ +1 eP +1 , mentre nel GaSe esso corrisponde a transizioni dirette tra gli statiΓ +1 eΓ +3 . Picchi nella funzione dielettricaɛ 2(0,ω) sono attribuiti a singolarità nella densità di stati in corrispondenza a punti di sella. L’effetto di un cambiamento della direzione di polarizzazione della luce sulle proprietà ottiche viene discusso.
Реэюме
Вычисляется эонная структура графита слоистых соединений GaS и GaSe, причем, полузмпирически испольэуется приближение плотной свяэи. Зонная структура свяэывается с основными свойствами зтих соединений, и общясняются некоторые особенности спектра оптических воэбуждений. Мы отмечаем, что причину того, почему графит является полуметаллом, а GaS и GaSe являются полупрово-дниками, можно понять иэ двумерного приближения, и зто обусловлено сушество-ванием обратной симметрии в первом случае. Край поглошения в GaS соответствует непрямым переходам между состояниямиΓ +1 иP +1 , тогда как в GaSe край поглошения соответствует прямым переходам между состояниямиΓ +1 иP +3 . Острые пики вɛ 2 (0,ω) относятся эа счет сингулярностей седловых точек в сочленной плотности состояний, обсуждается зффект для оптических свойств, который обусловлен иэменением поляриэации света.
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Based on work performed with the assistance and co-operation of the «Istituto di Fisica Nucleare Sezione Siciliana».
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Bassani, F., Parravicini, G.P. Band structure and optical properties of graphite and of the layer compounds GaS and GaSe. Nuovo Cimento B (1965-1970) 50, 95–128 (1967). https://doi.org/10.1007/BF02710685
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DOI: https://doi.org/10.1007/BF02710685