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Seminar/Colloquim

PHYSICS/BK21 SEMINAR[09.08.20]

본문

"Two-dimensional semimetaland excitonic insulator
  
Ø  Speaker :   Dr.  Z. D. Kvon [Institute of Semiconductor Physics, Novosibirsk, Russia]    
Ø  Place : Physics Seminar Room (Science Bldg, 3-201)
Ø  Date & Time :  Aug, 20 (Thu), 15:00 ~ 16:00 pm

Abstract
  
The talk is devoted to the results of the experimental study of a principally new 2D electron system: a two-dimensional (2D) semimetal,  recently  realized for  the first time in quantum wells (QWs) on basis of zero gap semiconductor HgTe. It is shown that this 2D semimetal exists in  wide  (thickness of 18 nm – 21 nm) undoped HgTe QWs with  the  reduced  symmetry  surfaces (such as (013)  and (112)).  Because  of  an inverted band structure and respectively the peculiar properties of  size  quantization  in  these  QWs  the overlap of a conductance band (with the bottom in the centre of Brillouin zone) and valence band (with the few maximums in its side points) arises. Its value is about 10 meV. So due to this overlap we have the true 2D semimetal similar to that of classical 3D semimetals such as Bi, Sb or As and there is the opportunity to realize the simultaneous existence of an equilibrium gas or liquid of 2D electrons and 2D holes.  Moreover by means of the field effect transistor structure fabricated on the basis of QW we are  able  by  means  of simple changing of  the  gate  voltage,  firstly,  to  realize  and  study  transition  2D  electron  metal – 2D semimetal – 2D hole metal and, secondly,  2D semimetallic  state with any ratio between the densities  of 2D electrons (Ns) and 2D holes (Ps). So a lot of new interesting properties caused by their  simultaneous existence have been observed. These are: 1) sign-variable Hall effect and positive magnetoresistance; 2) electron  mobility  jump  due  to  hole  screening  of  electron  scattering  by  impurities, 3)  anomalous temperature dependence of 2D semimetal resistance because of electron scattering by heavy holes. It is likely this effect is the first clear demonstration of the  direct  influence  of  a  particle-particle  inelastic Landau scattering on metal resistance and correspondingly of Pauli exclusion  principal.  But  the  most exiting opportunity appearing due to realization of 2D semimetal is to carry out  the  direct  experimental observation of excitonic insulator (EI) state predicted almost fifty years ago by one  of  the  greatest  20 century condense matter physicists sir Nevill Mott. In 1961 he considered a semimetal and assumed that the Coulomb interaction between electrons and holes have to lead to the formation of their bound  state and a semimetal should become an insulator. It is surprising but till now this very interesting and important transition has not been directly observed. The main reason of this situation is that till the last  time  there was absent the system in which this transition could be realized and then  directly  fixed  by  resistance measurement. Conventional tree-dimensional semimetal such as Bi or Sb did  not  fit  because  of  the absence of the simple procedure to change the density of electron and holes. So in our 2D semimetal the EI state induced by magnetic field is found. It arises between electron-like and hole-like quantum Hall liquid states at the point Ns = Ps and near it. It  is  shown  that  the  transition  metal – EI  is  similar  to superconductor-normal metal transition and described by BCS (Bardeen-Coopper-Schrieffer)-scenario: up to the critical temperature Tc ≈ 1 K no temperature dependence of the  transport  and  capacitance response of the system is  observed  but just  below  this  temperature  the  strong  insulator  behavior characterizing by the gap of about few Kelvins appears.

                          
                                  Contact Person : Prof. Hu-Jong Lee(054-279-2072,  hjlee@postech.ac.kr)
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