UVSOR Lunch Seminar (2002-01)

日時：5月22日（水曜日）12:00〜12:35（talk:30min + discussion:5min）
場所：UVSOR棟304号室（共同利用研究室）

講演者：Dr. Joerg Sichelschmidt
所属；Max-Planck-Institut fuer Chemische Physik fester Stoffe, Dresden,
Germany

題目：Optical conductivity of the clathrate compound Ba6Ge25

要旨：最近，Geなどのケージ（かご）の中に金属を入れた系（クラスレート）
で，強い電子相関によって超伝導などの興味深い物性が現れることがわかってき
ました。そのような物質の中で，Ba6Ge25は伝導帯と局在準位との混成によって微
小なエネルギーギャップが開く近藤絶縁体と呼ばれる物質と考えられ，ドイツを
中心にして研究が行われています。講演では，この物質の光学伝導度の測定を通
してキャリアの有効質量の増大などの電子状態の変化について得られた知見につ
いて話をしていただきます。
また，講演者の所属しているマックスプランク固体化学物理研究所の概要につい
ても簡単に紹介していただく予定です。

Abstract: On the search for small-gap semiconductors/semimetals (Kondo
Insulators: KI) the new clathrate compound Ba6Ge25 may be considered as
a reference compound for a potential KI when the Ba atoms are
substituted by suitable rare earth elements with an unstable 4f shell.
Ba6Ge25 is a system with a relatively low charge carrier density which
undergoes a structural phase change near Ts=200K and gets
superconducting at about Tc=0.24K . The cubic structure consists of
large cages of germanium which encapsulate the Ba atoms. Some of the Ba
atoms anharmonically vibrate and are locked into split positions below
Ts. This behavior crucially effects the superconducting phase transition
and the properties of the charge carriers which propagate within the
framework of cages. Optical investigations of the electronic properties
of Ba6Ge25 result reflectivity spectra which are typical for systems
with low charge carrier densities. When lowering the temperature across
Ts the low energy Drude type of reflectivity is characterized by a step-
like enhancement of the effective carrier mass and an almost temperature
independent relaxation time. This behavior could indicate a formation of
polaronic quasiparticles below Ts. A reduction of a purely electronic
type of conduction is also indicated by a spectral weight transfer from
low energies to a peak at around 1 eV concomitant with the enhancement
of the effective carrier mass.