PHYSICS/BK21 SEMINAR[09.08.20]
관련링크
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)
"
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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