S o f t X - r a y P h o t o e l e c t r o n S p e c t r o s c o p y i n S i l i c o n C l a t h r a t e S u p e r c o n d u c t o r s S o f t X - r a y P h o t o e l e c t r o n S p e c t r o s c o p y i n S i l i c o n C l a t h r a t e S u p e r c o n d u c t o r s Silicon clathrate compounds consist of fullerene- like polyhedral Si-cages, which share their faces to for m a thr ee- dim ens ion al Si- sp 3 cov ale nt net wor k. Guest elements such as an alkali atom or an alkali- e a r t h o n e c a n b e e n c a p s u l a t e d i n t h e c a g e . Re ce nt ly , th es e co mp ou nd s ha ve at tr ac te d st ro ng interest due to the discovery of superconductivity in a metal-doped silicon clathrate, Ba 6 Na 2 Si 46 [1], as well as the potential for thermoelectric applications. B a r i u m e n c a p s u l a t e d t y p e - I s i l i c o n c l a t h r a t e , B a 8 S i 4 6 , h a s b e e n r e p o r t e d t o s h o w s u p e r c o n d u c t i v i t y below T c ~ 8 K [2], which is the highest T c observed i n t h e s i l i c o n c l a t h r a t e f a m i l y . A s s c h e m a t i c a l l y s h o w n i n F i g . 1 , t h e c r y s t a l s t r u c t u r e o f B a 8 S i 4 6 c o n s i s t s o f t w o t y p e s o f p o l y h e d r a l c a g e s , i . e . d o d e c a h e d r a l ( S i 20 ) a n d t e t r a k a i d e c a h e d r a l ( S i 24 ) ca ge s. In an i de al -c om po si ti on ph as e, Ba at om s o c c u p y a l l o f t h e c a g e s r e s u l t i n g i n t h e c h e m i c a l for mu la of Ba 8 Si 46 pe r cu bi c un it ce ll . In Ba 8 Si 46 , we can substitute Ag atoms for Si atoms in ‘Si (1)’ sites shown in Fig. 1 , where the chemical formula becomes Ba 8 Ag x Si 46-x (0 x 6). The solid solution o f S i a n d G e c l a t h r a t e , B a 8 G e y S i 46 -y ( 0 y 4 0 ) , and p ure ger man ium cla thr ate Ba 8 Ge 43 als o hav e b e e n s y n t h e s i z e d . I n b o t h c a s e s , t h e s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e T c h a s b e e n f o u n d t o s y s t e m a t i c a l l y d e c r e a s e w i t h increasing Ag or Ge concentration. Photoelectron spectroscopy (PES) allows us to d i r e c t l y o b s e r v e t h e e l e c t r o n i c s t r u c t u r e s i n t h e v a l e n c e b a n d i n a d d i t i o n t o t h e c o r e e l e c t r o n s i n solid substances. Recently, it has been recognized t h a t h i g h - r e s o l u t i o n P E S u s i n g r e l a t i v e l y h i g h - e n e r g y s o f t X - r a y i s q u i t e a p o w e r f u l t o o l f o r i n v e s t i g a t i n g t h e b u l k - s t a t e s o f s o l i d s . I n t h e present study [3], P ES was carried out by using a sy nc hr ot ro n ra di at io n so ft X- ra y to in ve st ig at e th e electronic structure of silicon clathrate family. The r e l a t i o n s h i p b e t w e e n t h e e l e c t r o n i c s t r u c t u r e a n d th e sy st em at ic ch an ge of su pe rc on du ct in g na tu re of silicon clathrate family is discussed. Po ly cr ys ta ll in e sa mp le s of cl at hr at es Ba 8 Ag x Si 46 -x (0 x 6), Ba 8 Ge y Si 46-y (0 y 40) and Ba 8 Ge 43 (pure germanium clathrate) were prepared by using a high-pressure synthesis technique as reported in previous works [1, 2]. Photoelectron spectra were m e a s u r e d a t b e a m l i n e B L 2 5 S U w i t h v a r i o u s photon energies h ν between ~ 240 and ~ 1200 eV. The total energy resolution was ~ 150 meV at h ν ~ 800 eV . Samples were cooled down to 20 K. T o b t a i n c l e a n s u r f a c e s , s a m p l e s w e r e f r a c t u r e d under the ultrahigh vacuum of ~ 1 × 10 -10 Torr just before the measurements were taken . T h e v a l e n c e - b a n d p h o t o e l e c t r o n s p e c t r a o f Ba 8 Si 46 ar e sh ow n in Fi g. 2 . Th e ho ri zo nt al ax is i n d i c a t e s t h e b i n d i n g e n e r g y E B , w h e r e 0 e V corresponds to the Fermi energy E F . Red and blue d o t s a r e m e a s u r e d b y u s i n g e x c i t a t i o n p h o t o n energy of h ν ~ 785 and ~ 776 eV, respectively. Th e fo rm er co rr es po nd s to th e Ba 3 d to 4 f tr an si ti on Fig. 1. Schematic illustration of crystal structure of silicon clathrate Ba 8 Si 46 . Two types of fullerene- like polyhedral cages, i.e. Si 20 and Si 24 cages, share their faces to form the sp 3 covalent network. A Ba atom is encapsulated in each cage. Si(1) Si(2) Si(3) Ba(1) Ba(2) Si 20 cage Si 24 cage 51 energy, c a lled ‘ on resonance,’ and t he latter is below the Ba 3 d to 4 f t ransition energy, c alled ‘off resonance.’ T he letters A-D indicate four prominent structures . T he overall spectral shape is similar to t he density of states (DOS) calculated by usin g local density approximation (LDA) [4]. Th e structures A-C are assigned to the s -, sp - and p -lik e bands c haracteristics to t he sp 3 bonded Si 46 network. T he extremely narrow band D , with th e width of ~ 0.3 eV , is seen beside the p -band with a gap of ~ 1.2 eV. In a previous work [5], however, ultraviolet photoelectron s pectra (UPS) s howed a broad extra peak in the gap between C and D. It was proposed t hat t he discrepancy between th e U PS and t he c alculated DOS were c aused by electron-correlation e ffects. T he U PS, however , may include surface-state components due to a Fig. 2. Valence-band photoelectron spectra of s ilicon clathrate Ba 8 Si 46 . Red and blue dots ar e measured by using excitation photon energy of h ν ~ 785 and ~ 776 eV, respectively. The forme r corresponds to the Ba 3d to 4f transition energy , called ‘on resonance,’ and the latter is below the Ba 3d to 4f transition energy, so called ‘off resonance. ’ The black dots show the difference spectrum between the ‘on resonance’ and the ‘off resonance’ spectra . short photoelectron mean free path. T he present results, which are more bulk- s ensitive s pectra, basically s upport t he LD A calculation, indicating that the electron-correlation effects are not so strong in Ba 8 Si 46 . It is noticed t hat t he Fermi level E F is located at t he band D, resulting in the high DOS at E F . T he black dots in Fig. 2 s how the difference spectrum between th e ‘on resonance’ and t he ‘off resonance’ s pectra. Generally speaking, this spectrum corresponds to t he Ba related partial DOS in t he valence band. The band D is obviously enhanced by the Ba 3 d - 4 f resonance excitation ; t herefore, it is confirmed that the Ba orbital strongly contributes to the band D. T he LDA calculation has predicted the narrow band at E F , constructed by a hybridization of Ba 5 d orbital and the conduction band of th e Si 46 network . The high DOS at E F is expected to be responsible for the relatively high T c in terms of the Bardeen- C ooper-Schrieffer (BCS) t heory for phonon- mediated s uperconductivity. Furthermore, Ba -r elated states seem to spread w i dely ov er in the valenc e band , as seen in the difference spectrum in Fig. 2 . It may originate from the hybridization between s - like state of Ba and v alence band of t he Si 46 network . T he valence-band photoelectron s pectra of Ba 8 Ag x Si 46- x , Ba 8 Ge y Si 46- y and Ba 8 Ge 43 are show n in Fig. 3 . T he intensity of the band D systematically decreases with increasing Ag c oncentration in Ba 8 Ag x Si 46- x , as s een in t he lower half of Fig. 3 . Simultaneously, new states appear between the C and D bands at E B = 0.5 ~ 2 eV, whose intensit y increases with t he Ag c oncentration. Simila r phenomena are also observed in the solid solution of Si and Ge clathrat e , as shown in the upper hal f of Fig. 3 . In Fig. 4 , the superconducting transitio n temperature T c of clathrates is plotted as a function of the integrated intensity of the band D. It is clea r that the sample with higher T c has a band D wit h higher intensit y. T he decrease in the intensity of t he band D m eans t he decrease in DOS at E F . This must be responsible for the decrease in T c in A B C D Ba 8 Si 46 T = 20 K Ba 3 d - 4 f 12 10 8 6 4 2 0 -2 Binding Energy E B (eV) Intensity (arb. units) 52 References [1] H. Kawaji et al. , Phys. Rev. Lett. 74 (1995) 1427. [2] S. Yamanak a et al . , Inorg. C hem . 39 (2000) 56. [3] T. Nakano, N. Kamakura, Y. Ikemoto, K. K obayashi, T. Muro, Y. Nozue, H. Fukuoka, S. Yamanaka and S. Shin - in preparation. [4 ] K. Moriguchi et al . , Phys. Rev. B 61 (2000) 9859. [5] T. Yokoya et al . , Phys. Rev. B 64 (2001) 172504. Takehito Nakano and Keisuk e Kobayashi SPring-8 / JASR I E-mail: nakano @s pring8.or.jp t he clathrates , Ba 8 Ag x Si 46- x and Ba 8 Ge y Si 46- y , in terms of the BCS theory . T he core level spectra of Si 2 p , Ag 3 d and Ba 4 d s how systematic c hange with increasing x in Ba 8 Ag x Si 46- x . F rom t he analysis of t he c hemica l shift of core level spectr a, i t is found that there is systematic electron transfer from Ba atom s to th e Ag + Si network with increasing x. This may weaken the hybridization between Ba and the Si network , resulting in the decrease of the band D. T he new state at E B = 0.5 ~ 2 eV may be c onstructed by a ccept or states originating from t he Ag and Si hybridization in Ba 8 Ag x Si 46- x . On the other hand, in Ba 8 Ge y Si 46- y and Ba 8 Ge 43 , the core level spectra of Ge 3 d and Ba 4 d s how systematic c hange with increasing the Ge concentration. T he Si 2 p s pectra, however, does not s how any c hange. T he new states at E B = 0.5 ~ 2 eV may be c onstructe d by t he Ge and Ba hybridization, in Ba 8 Ge y Si 46- y and Ba 8 Ge 43 . C onsequently, t he w eakened hybridization between Ba and Si network result s in the decrease in the band D. In conclusion, we have investigated the electronic structure of s ilicon clathrate s uperconductors by using the bulk-sensitive PES. We have clarified th e relationship between t he electronic structure and the systematic change of superconducting nature of the silicon clathrate family . Fig. 3. Valence-band photoelectron spectra of s ilicon clathrates Ba 8 Ag x Si 46- x , Ba 8 Ge y Si 46- y and Ba 8 Ge 43 m easured by using excitation photon energy of h ν ~ 776 eV . Fig. 4. Relationship between the s uperconducin g transition temperature T c and the intensity of the band D (see Fig. 2 and 3) in valence-band photoelectro n spectra of silicon clathrat e s Ba 8 Ag x Si 46- x , Ba 8 Ge y Si 46-y and Ba 8 Ge 43 . –y T C (K ) Ba 8 Si 46 Ba 8 Ag x Si 46 – x Ba 8 Ge y Si 46 Ba 8 Ge 43 0. 0 0 .2 0. 4 0 .6 0. 8 1 .0 0 2 4 6 8 y = 3 y = 6 x = 1 y = 12 y = 15 x = 3 x = 6 Inte g r ated Intensit y of D-band (arb. units ) Intensity (arb. units) Binding Energy E B (eV) 3 2 1 0 -1 Ge C D Ag Ba 8 Ge 43 Ba 8 Ge 15 Si 31 Ba 8 Ge 6 Si 40 Ba 8 Ag 1 Si 45 Ba 8 Ag 3 Si 43 Ba 8 Ag 6 Si 40 Ba 8 Si 46 53
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