You did not read up to the end (Graphite with a strong directional dependence of the conductivity was an example, maybe it was not lucky to place it in front) a little later you find: "In some cases you have to check whether there might be an overlapp of the valence and and conduction bands at different k-points, resulting in a semimetallic or zero-bandgap type behavior." and that is (for a semi-metal) the situation you find as C in Wikipedia, isn't it ?
By the way, the definition of semi-metals changed with time one just need to follow the textbooks in chemistry and physics see also about metalloids (https://en.wikipedia.org/wiki/Metalloid) . Ciao Gerhard DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy: "I think the problem, to be quite honest with you, is that you have never actually known what the question is." ==================================== Dr. Gerhard H. Fecher Institut of Inorganic and Analytical Chemistry Johannes Gutenberg - University 55099 Mainz and Max Planck Institute for Chemical Physics of Solids 01187 Dresden ________________________________________ Von: Wien [[email protected]] im Auftrag von Karel Vyborny [[email protected]] Gesendet: Mittwoch, 30. Januar 2019 09:08 An: A Mailing list for WIEN2k users Betreff: Re: [Wien] Metal or semimetal A quick question to add my bit into this conversation: where does the definition of semimetal as "insulating (semiconducting) in one direction and conducting in another one" come from? I have never heard of this; it may the lack of my knowledge but I hold that it's the overlaping valence and conduction bands that define semimetals (and indeed, bismuth has always been the prime example for me). The strong anisotropy in conductivity may be a concomitant feature, however, it is not necessary. Cheers, Karel --- x --- dr. Karel Vyborny Fyzikalni ustav AV CR, v.v.i. Cukrovarnicka 10 Praha 6, CZ-16253 tel: +420220318459 On Tue, 29 Jan 2019, Fecher, Gerhard wrote: > Thank you for the Link, but I don't understand your remarks > Wikipedia tells: > - Schematic > C) a semimetal (like tin (Sn) or graphite and the alkaline earth metals). > and further > - Classic semimetals > The classic semimetallic elements are arsenic, antimony, bismuth, ?-tin (gray > tin) and graphite, an allotrope of carbon. > > on the page https://en.wikipedia.org/wiki/Graphite one finds > Graphite has a layered, planar structure. The individual layers are called > graphene. ... > Atoms in the plane are bonded covalently, with only three of the four > potential bonding sites satisfied. The fourth electron is free to migrate in > the plane, making graphite electrically conductive. > However, it does not conduct in a direction at right angles to the plane. > > Maybe check the band structures of Graphite and Bi to find out what is common > and what is different. > > You did not understand the remark on the integration of the density of > states, please read it correctly. > The initial question was on the Fermi energy beeing slightly below the top of > the valence band > and this might be caused by a bad integration which depends on the number of > k-points (indeed among others) > and is used to find the Fermi energy. > > PS.: Please check the definition of the density of states and you see why > flat (say better narrow) bands result in a high density of states and steep > ones in a low density of states. > (that a band is horizontal at a certain point of the Brillouin zone does not > mean that the complete band is flat, this situation you have always at the > bottom of a parabola) . > > Ciao > Gerhard > > DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy: > "I think the problem, to be quite honest with you, > is that you have never actually known what the question is." > > ==================================== > Dr. Gerhard H. Fecher > Institut of Inorganic and Analytical Chemistry > Johannes Gutenberg - University > 55099 Mainz > and > Max Planck Institute for Chemical Physics of Solids > 01187 Dresden > ________________________________________ > Von: Wien [[email protected]] im Auftrag von delamora > [[email protected]] > Gesendet: Dienstag, 29. Januar 2019 19:43 > An: A Mailing list for WIEN2k users > Betreff: [Wien] Metal or semimetal > > I strongly disagree > > Wikipedia gives a very different definition > > https://en.wikipedia.org/wiki/Semimetal > > Bi would be a good example; it has a very low DOS at Ef (with the wikipedia > definition of semimetal) > > Graphite is conductor in the ab plane and a poor conductor (not insulating) > in the c direction > > As I said, the bands are flat near the band edge, so it would be a bad > conductor (does this fall into the definition of semimetal???) > Flat bands give high DOS? Not always, for example NaCl at the bottom of the > conduction band, the band is flat and DOS is very low > And I do not think I need a high density of k points > > On the other hand the f orbitals have flat bands and DO HAVE high DOS > > ________________________________ > De: Wien <[email protected]> en nombre de Fecher, > Gerhard <[email protected]> > Enviado: martes, 29 de enero de 2019 03:49 a. m. > Para: A Mailing list for WIEN2k users > Asunto: Re: [Wien] Metal or semimetal > > I strongly disagree, > a semi-metal (not to be confused with a half-metall) is a material that is > insulating (semiconducting) in one direction and conducting in another one, a > typical example is graphite. > (this has nothing to do whether the bands are flat or not; > and just to mention, the density of states is HIGH when the bands are FLAT.) > > The question is: What is slightly below ? > > If you have one (or more) partially filled band(s) that is(are) crossing the > Fermi energy, then you have a metal. > (You find the occupation of the bands e.g.: in case.scf2) > > If you have not enough k-points (or some other bad conditions), then the > integration of the density of states might be bad > and the Fermi energy may fall into the valence or conduction band (probably > few meV or less) even though the material is an insulator, > this can be healed in most cases by increasing the number of k-points. > > In some cases you have to check whether there might be an overlapp of the > valence and and conduction bands at different k-points, resulting in a > semimetallic or zero-bandgap type behavior. > For example you may have a large gap at Gamma with EF at the top of the > valence band and a large gap at another k-point, say X, with EF at the bottom > of the conduction band. > > PS.: To complete; in a half-metal one spin channel (e.g. minority) is > insulating (semiconducting) and the other spin channel is metallic (e.g.: > majority) > > > > Ciao > Gerhard > > DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy: > "I think the problem, to be quite honest with you, > is that you have never actually known what the question is." > > ==================================== > Dr. Gerhard H. Fecher > Institut of Inorganic and Analytical Chemistry > Johannes Gutenberg - University > 55099 Mainz > and > Max Planck Institute for Chemical Physics of Solids > 01187 Dresden > ________________________________________ > Von: Wien [[email protected]] im Auftrag von delamora > [[email protected]] > Gesendet: Freitag, 25. Januar 2019 01:53 > An: A Mailing list for WIEN2k users > Betreff: Re: [Wien] Metal or semimetal > > If Ef is near the edge of a band, close to the bandgap then it would be a bad > conductor, since the bands would be quite flat (and the velocity of the > electrons is proportional to the slope of the band) and the DOS would be low, > so I would call it a semimetal > > > ________________________________ > > Dear wien2k users: > > I have a question that does not have any relation with wien2k but I would be > grateful if you can answer me or send me a document: > > When the fermi level passes slightly below the top of the valance band with > the presence of a wide gap, this indicates a metal or semimetalic behavior.? > > Thank you in advance > _______________________________________________ > Wien mailing list > [email protected] > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > SEARCH the MAILING-LIST at: > http://www.mail-archive.com/[email protected]/index.html > _______________________________________________ > Wien mailing list > [email protected] > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > SEARCH the MAILING-LIST at: > http://www.mail-archive.com/[email protected]/index.html _______________________________________________ Wien mailing list [email protected] http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/[email protected]/index.html _______________________________________________ Wien mailing list [email protected] http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/[email protected]/index.html
