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Optical properties of MX chain materials: an extended Peierls-Hubbard model
Synthetic Metals, 49-50 (1992) 109-115
109
Optical properties of MX chain materials: an extended Pei e r l s- H u b b a rd model* A. R. Bishop, I. Batisti6, J. T i n k a G a m m e l a n d A. S a x e n a Theoretical Division a n d Centers f o r N o n l i n e a r Studies a n d Materials Science, Los Alamos N a t i o n a l Laboratory, Los Alamos, N M 87545 (USA)
Abstract We describe theoretical modeling of both pure (MX) and mixed-halide (MX~X'~_z) halogen
(X)-bridged transition metal (M) linear chain complexes in terms of an extended Peierls-Hubbard, tight-binding Hamiltonian with 3/4-filling of two bands. Both inter- and intra-site electron-phonon coupling are included. Electronic (optical absorption), lattice dynamic (IR, Raman) and spin (ESR) signatures are obtained for the ground states, localized excited states produced by impurities, doping or photoexcitation (excitons, polarons, bipolarons, solitons) and the edge states (which occur in mixed-halide crystals, e.g. PtClxBr~_x). Adiabatic molecular dynamics is used to explore photodecay channels in pure and impure systems for ground states as well as in the presence of pre-existing polaronic states: 71.38 + i, 78.30 - j , 71.10 + x.
Introduction H a l o g e n - b r i d g e d t r a n s i t i o n m e t a l c o m p l e x e s ( o r MX chains), as well as b e i n g i m p o r t a n t in t h e i r o w n right, p r o v i d e a n i m p o r t a n t t e s t c a s e f o r t h e o r e t i c a l t e c h n i q u e s a n d issues in l o w - d i m e n s i o n a l m a t e r i a l s with s t r o n g elect r o n - e l e c t r o n ( e - e ) a n d e l e c t r o n - p h o n o n ( e - p ) i n t e r a c t i o n s [ 1 - 3 ] . This is p a r t i c u l a r l y t r u e b e c a u s e o f the e x t r e m e r a n g e o f b r o k e n - s y m m e t r y g r o u n d s t a t e s t h a t are a c h i e v e d b y v a r y i n g M (Pt, Pd, Ni) a n d X (C1, Br, I, etc.) as well as ligands, p r e s s u r e , m a g n e t i c field -- g r o u n d s t a t e s r a n g i n g f r o m a s t r o n g l y d i s p r o p o r t i o n a l C D W (e.g. PtC1) to a w e a k C D W (e.g. PtI) to S D W or s p i n - P e i e r l s ( a n d o t h e r m i x e d C D W / S D W [4]) p h a s e s (e.g. NiBr, NiC1), in addition to l o n g - p e r i o d ' s u p e r l a t t i c e ' s t r u c t u r e s [5]. Theoretically, we h a v e u s e d a H a r t r e e - F o c k ( H - F ) spatially i n h o m o g e n e o u s m e a n - f i e l d a p p r o x i m a t i o n (MFA) to s t u d y the e l e c t r o n i c s t r u c t u r e [3] a n d a d i r e c t - s p a c e r a n d o m p h a s e a p p r o x i m a t i o n (RPA) to i n v e s t i g a t e p h o n o n s [6] ( a n d a s s o c i a t e d i n f r a r e d a n d R a m a n s p e c t r a ) in a p p r o p r i a t e many-body Hamiltonians. With p a r a m e t e r input from local-density functional [ 7 [ a n d a b i n i t i o q u a n t u m c h e m i s t r y c a l c u l a t i o n s [8 ], a n d c o m p a r i s o n s with e x p e r i m e n t s [1, 9], w e h a v e e m p l o y e d m a n y - b o d y t e c h n i q u e s including H - F , e x a c t d i a g o n a l i z a t i o n a n d a d i a b a t i c m o l e c u l a r d y n a m i c s t o e x p l o r e the g r o u n d *Invited paper.
Elsevier Sequoia
110 and excited states. A g r e e m e n t b e t w e e n p r e d i c t e d optical, ESR, infrared (IR) and r e s o n a n c e R a m a n (RR) s p e c t r a with r e c e n t e x p e r i m e n t s [1, 9, 10] on both the p u r e and mixed-halide MX chains has b e e n quantitatively achieved. W e have m o d e l e d effective isolated single chains o f p u r e MX materials in t e r m s of a 3/4-filled, t w o - b a n d Peierls e x t e n d e d - H u b b a r d model. In the case o f an isolated mixed-halide chain [ 11 ] we r e p l a c e a segment, containing m X atoms, b y X' a t o m s w h e r e X, X' =C1, Br, I. F o c u s i n g on the metal dz~ and h a l o g e n (X, X') Pz orbitals and including only the n e a r e s t - n e i g h b o r interactions, we c o n s t r u c t the following tight-binding m a n y - b o d y Hamiltonian [2, 3]: H = ~ { ( - to + aAt)(c~,,ct+ 1.~+ c[+ ,,,cz,~) + [e~- fit(A, + A,_ ,)]c~.,ct,~} l,a
1
1
+ ~ U t n l r n ~ , + ~ ~ K t A ~ + ~ KMM~(Aet+ Aet+ 1)2 l
(1)
l
w h e r e c ~ (ct,,) d e n o t e s the c r e a t i o n (annihilation) o p e r a t o r for the e l e c t r o n i c orbitals at the lth a t o m with spin (r. M and X (or X') o c c u p y even and odd sites, respectively. At = ~)t+ ~ -~),, w h e r e ~)~ are the d i s p l a c e m e n t s f r o m u n i f o r m lattice spacing o f the a t o m s at site I. E q u a t i o n (1) includes as p a r a m e t e r s the on-site e n e r g y et (eet=eM=eo; e2t+l=ex = --eo in the X region and e2t+l = ex, = e o - 2e~) in the X' region, ei being the e l e c t r o n affinity of the ith ion), e l e c t r o n h o p p i n g (to, t~)), on-site (fiM, fiX, fl~) and inter-site (a, a ' ) e - p coupling, on-site e - e r e p u l s i o n (UM, Ux, U~), and finally effective M - X (K) or M - X ' (K') and M - M (KMM)springs to m o d e l the e l e m e n t s of the s t r u c t u r e not explicitly included. In particular, KMM a c c o u n t s for the rigidity of the metal sublattice c o n n e c t e d into a t h r e e - d i m e n s i o n a l n e t w o r k via ligands. Long-range C o u l o m b fields are also included w h e n n e c e s s a r y (as in s t r o n g valence localized cases). Note that the metal (M = Pt) energies e2t are the same in the s e g m e n t and the h o s t MX chain. At stoichiometry, t h e r e are six e l e c t r o n s p e r M2X~ (or MeX~) unit, or 3/4 band-filling. A c o m b i n a t i o n o f g r o u n d state e x p e r i m e n t a l data (the X-sublattice distortion amplitude A0, the a(X) --* do'* a b s o r p t i o n for the oxidized m o n o m e r , and the inter-valence c h a r g e t r a n s f e r (IVCT) b a n d edge, Eg) and q u a n t u m chemical calculations [8] have led us to the effective p a r a m e t e r sets for the Hamiltonian o f eqn. (1) listed in Table 1. In the following we p r e s e n t selected results f r o m o u r c o m p r e h e n s i v e study for b o t h the p u r e [3] and mixed-halide [11] MX chains. TABLE 1 Parameters for the PtX materials in eqn. (1). fix = --0.5tim MX
to (eV)
a e0 (eV//~) ( e V )
/~ KMX KMM UM (eV//~) (eV//~2) (eV//~2) (eV)
Ux (eV)
Zi (/~)
Eg (eV)
PtC1 PtBr
1.02 1.26
0.5 0.7
2.7 1.8
1.3 0.0
0.38 0.24
2.50 1.56
2.12 0.60
6.800 6.125
0.0 0.7
1.9 0.0
111
Results
F i g u r e (1) depicts calculated ESR s p e c t r a f o r p o l a r o n s o n a PtBr chain: (a) shows an e l e c t r o n p o l a r o n c e n t e r e d o n an oxidized metal a t o m (Pt 3+~) while (b) shows a hole p o l a r o n c e n t e r e d o n a r e d u c e d m e t a l a t o m (Pt3-~). Since P t B r is a w e a k l y localized CDW system, e l e c t r o n spin density is s p r e a d o v e r several sites. Multiple p e a k s in the ESR s p e c t r a are a r e s u l t of hyperfine splitting due to the i n t e r a c t i o n b e t w e e n e l e c t r o n and n u c l e a r spin. In the p r e s e n t illustrative calculations we have t a k e n the m a t r i x e l e m e n t for e l e c t r o n - n u c l e a r spin interaction to be the s a m e for b o t h Pt a n d Br. H o w e v e r , o n c e we include the a p p r o p r i a t e m a t r i x e l e m e n t s for Pt and Br w e e x p e c t the a m p l i t u d e o f the hyperfine s t r u c t u r e in Fig. (1) to be r e d u c e d . Note the e l e c t r o n a n d hole a s y m m e t r y in the ESR s p e c t r a which is c o n s i s t e n t with e x p e r i m e n t a l data [1, 10] and is a c o n s e q u e n c e of the t w o - b a n d n a t u r e of o u r m o d e l (i.e., explicit inclusion of the f o u r a t o m s o f the unit cell). W e h a v e also investigated the p h o t o d e c a y c h a n n e l s u b s e q u e n t to p h o t o e x c i t a t i o n in the g r o u n d state as well as in the p r e s e n c e of n o n l i n e a r e x c i t a t i o n s a n d impurities using adiabatic m o l e c u l a r d y n a m i c s [3]. P h o t o e x c i t a t i o n was simulated n u m e r i c a l l y by manually r e m o v i n g an e l e c t r o n f r o m an o c c u p i e d state and i n s t a n t a n e o u s l y placing it in a n u n o c c u p i e d state. The s y s t e m was allowed to evolve adiabatically with n o f u r t h e r c h a n g e s in e l e c t r o n i c occupations. F i g u r e 2 shows the evolution o f P t B r after a single e l e c t r o n is p h o t o e x c i t e d a c r o s s the Peierls gap, with the addition of the gap e n e r g y (Eg) to the system. As is clear f r o m Fig. 2(a), an e x c i t o n is initially f o r m e d within a p h o n o n period. H o w e v e r , this e x c i t o n is unstable and evolves into a slowly s e p a r a t i n g k i n k - a n t i k i n k pair. The e l e c t r o n i c o c c u p a n c i e s of the e x c i t e d state n e c e s s i t a t e that b o t h of t h e s e kinks are neutral. Since the
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c r e a t i o n e n e r g y o f t h e k i n k - a n t i k i n k p a i r is s m a l l e r t h a n t h e g a p e n e r g y , the remaining energy goes partly into the kinetic energy of the kinks, partly i n t o t h e a c o u s t i c p h o n o n s a n d p a r t l y in t h e f o r m o f a s m a l l a m p l i t u d e l o c a l i z e d 'breather' ( p h o n o n b o u n d s t a t e ) b e t w e e n t h e k i n k s . S u c h a b r e a t h e r , a
113 temporally and spatially coher e nt state of optical phonons, is clearly visible in Fig. 2(a). Figure 2(b) shows the time evolution of associated energy levels, in particular the characteristic gap states of a kink pair: within ~ 200 fs two continuum states are pulled into one (almost) degenerate mid-gap state indicating that the initial bound e l e c t r o n - h o l e pair (exciton) quickly evolves into a kink-antikink pair. At the same time a breather level oscillates about the conduction band edge into the gap and persists with a time period larger than the p h o n o n period. Investigations of the influence of impurities on this p h o t o d e c a y channel are in progress. Next we illustrate a few salient features of mixed-halide chains [9, 11] by way of predicted optical, infrared (IR) and resonance Raman (RR) spectra. In Fig. (3) a PtC1 chain of length N = 48 atoms is considered in which a segment containing eight C1 atoms is replaced by Br atoms. The interface (edge) between the PtCI and PtBr segments is on a reduced metal site (Pt 3- ~). Figure 3(a) depicts the optical absorption for such a mixed-halide chain. Note that in addition to the two peaks at 1.5 and 2.5 eV, which correspond to the inter-valence charge transfer (IVCT) energy for PtBr and PtCl, respectively, there are absorption peaks between the two IVCTs (intragap) as well as several peaks beyond 2.5 eV (ultragap). These extra peaks arise from the strong mixing of the electronic bands of the two kinds of chains in the electronic s p ectr um (not shown). We have identified local p h o n o n modes associated with the edge in the mixed-halide chains that correlate directly with the measured RR spectra [9] and the electronic resonance energies of Fig. 3(a). Figure 3(b) shows the calculated IR spectra for the mixed-halide (PtC10.67Br0.33) chain. The peak at 349 cm-~ co r re s ponds to the PtC1 IR m o de while the peak at 239 cm -1 c o r r e s p o n d s to the PtBr IR mode. Figure 3(c) shows the predicted RR spectra for three different excitation energies. At an excitation energy 1.5 eV, which is about the same as the PtBr IVCT, the PtBr Raman mode at 172 cm -1 is prominent. Similarly, when the excitation energy is 2.55 eV, which is approximately equal to the PtCl IVCT, the PtCl Raman mode becom es resonant at 308 cm -I. However, at an intermediate excitation energy (2.0 eV) several other localized modes becom e resonant. Figure 3(c) shows that apart from the pure PtC1 and PtBr m odes at 308 and 172 cm -1, a mode at 225 cm -~ b e c o m e s very prominent. This m ode arises from the interface between the two differing segments. We label it as the edge mode [9]. The peaks at low f r e q u e n c y arise from the acoustic branch of the RPA phonon spectrum (not shown). We emphasize that different local modes b e c o m e Raman active at different excitation energies and their relative intensities also vary. The excitation profile for various Raman active m odes (not shown) is consistent with the optical absorption shown in Fig. 3(a). Conclusions
We have given a brief overview of the important optical probes in both the pure and mixed-halide MX chain solids. We have also studied the isotope
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115
effect in terms of IR and RR signatures in these systems. Furthermore, we are also beginning to explore mixed-metal (MxM'~_~X) and bimetallic (MMX) systems, as well as effects of magnetic fields (especially on the weak CDW/ SDW ground state materials [4]). While, on one hand, we believe that the MX class of compounds are nearly uniquely important as a testing ground for many-body modeling and materials design strategy in strongly correlated low-dimensional materials, we are on the threshold of investigating their technologically important applications on the other. Indeed, doping and photoexcitation studies of mixed-halide systems [12] indicate that electron and hole defects preferentially locate on differing chain segments (i.e. holes in the PtC1 segment, electrons in the PtBr segment). This points to photovoltaic device applications of the mixed-halide single crystals. Since the electronic and spectroscopic properties of MX crystals depend very sensitively on the presence and nature of a variety of impurities, counterions and ligands, they are very good prospects for chemical sensors (or so-called electronic noses).
Acknowledgements We acknowledge important discussions with R. Donohoe, X. Z. Huang, B. Swanson and L. Worl. This work was supported by the US DOE.
References 1 Proc. Conf. Optical Probes of Conjugated Polymers, Snowbird, UT, USA, Aug. 22-25, 1991 (these proceedings), Synth. Met., 49-50 (1992) and Proc. Int. Conf. Synthetic Metals (ICSM '90), Tiibingen, FRG, Sept. 2-7, 1990, Synth. Met., 41-43 (1991). 2 J. T. Gammel, R. J. Donohoe, A. R. Bishop and B. I. Swanson, Phys. Rev. B, 42 (1990) 10 566. 3 J. T. Gammel, A. Saxena, I. Batisti6, A. R. Bishop and S. R. Phillpot, Phys. Rev. B, 45 (1992) 6408. 4 H. R6der, J. T. Gammel and A. R. Bishop, in preparation. 5 I. Batisti6, J. T. Gammel and A. R. Bishop, Phys. Rev. B, 44 (1991) 13 228. 6 I. Batisti6 and A. R. Bishop, Phys. Rev. B, 45 (1992) 5282. 7 R. C. Albers, Synth. Met., 29 (1989) F169; R. C. Albers, M. Alouani, J. M. Wills and M. Springborg, Synth. Met., 41-43 (1991) 2739; M. Alouani and R. C. Albers, unpublished results. 8 C. Boyle, R. L. Martin and P. J. Hay, unpublished results. 9 B. I. Swanson, R. J. Donohoe, L. A. Worl, J. T. Gammel, A. Saxena, I. Batisti4 and A. R. Bishop, Synth. Met., 41 (1991) 2733; Mol. Cryst. Liq. Cryst., 194 (1991) 43. 10 S. Kurita and M. Haruki, Synth. Met., 29 (1989) F129; M. Tanaka and S. Kurita, J. Phys. C: Solid State Phys., 19 (1986) 3019. 11 A. Saxena, X. Z. Huang, I. Batisti6, A. R. Bishop, L. A. Worl and B. I. Swanson, Phys. Rev. B, in preparation. 12 L. A. Worl, S. C. Huckett, B. I. Swanson, A. Saxena, A. R. Bishop and J. T. Gammel, J. Phys.: Condens. Matter, (1992) in press.
109
Optical properties of MX chain materials: an extended Pei e r l s- H u b b a rd model* A. R. Bishop, I. Batisti6, J. T i n k a G a m m e l a n d A. S a x e n a Theoretical Division a n d Centers f o r N o n l i n e a r Studies a n d Materials Science, Los Alamos N a t i o n a l Laboratory, Los Alamos, N M 87545 (USA)
Abstract We describe theoretical modeling of both pure (MX) and mixed-halide (MX~X'~_z) halogen
(X)-bridged transition metal (M) linear chain complexes in terms of an extended Peierls-Hubbard, tight-binding Hamiltonian with 3/4-filling of two bands. Both inter- and intra-site electron-phonon coupling are included. Electronic (optical absorption), lattice dynamic (IR, Raman) and spin (ESR) signatures are obtained for the ground states, localized excited states produced by impurities, doping or photoexcitation (excitons, polarons, bipolarons, solitons) and the edge states (which occur in mixed-halide crystals, e.g. PtClxBr~_x). Adiabatic molecular dynamics is used to explore photodecay channels in pure and impure systems for ground states as well as in the presence of pre-existing polaronic states: 71.38 + i, 78.30 - j , 71.10 + x.
Introduction H a l o g e n - b r i d g e d t r a n s i t i o n m e t a l c o m p l e x e s ( o r MX chains), as well as b e i n g i m p o r t a n t in t h e i r o w n right, p r o v i d e a n i m p o r t a n t t e s t c a s e f o r t h e o r e t i c a l t e c h n i q u e s a n d issues in l o w - d i m e n s i o n a l m a t e r i a l s with s t r o n g elect r o n - e l e c t r o n ( e - e ) a n d e l e c t r o n - p h o n o n ( e - p ) i n t e r a c t i o n s [ 1 - 3 ] . This is p a r t i c u l a r l y t r u e b e c a u s e o f the e x t r e m e r a n g e o f b r o k e n - s y m m e t r y g r o u n d s t a t e s t h a t are a c h i e v e d b y v a r y i n g M (Pt, Pd, Ni) a n d X (C1, Br, I, etc.) as well as ligands, p r e s s u r e , m a g n e t i c field -- g r o u n d s t a t e s r a n g i n g f r o m a s t r o n g l y d i s p r o p o r t i o n a l C D W (e.g. PtC1) to a w e a k C D W (e.g. PtI) to S D W or s p i n - P e i e r l s ( a n d o t h e r m i x e d C D W / S D W [4]) p h a s e s (e.g. NiBr, NiC1), in addition to l o n g - p e r i o d ' s u p e r l a t t i c e ' s t r u c t u r e s [5]. Theoretically, we h a v e u s e d a H a r t r e e - F o c k ( H - F ) spatially i n h o m o g e n e o u s m e a n - f i e l d a p p r o x i m a t i o n (MFA) to s t u d y the e l e c t r o n i c s t r u c t u r e [3] a n d a d i r e c t - s p a c e r a n d o m p h a s e a p p r o x i m a t i o n (RPA) to i n v e s t i g a t e p h o n o n s [6] ( a n d a s s o c i a t e d i n f r a r e d a n d R a m a n s p e c t r a ) in a p p r o p r i a t e many-body Hamiltonians. With p a r a m e t e r input from local-density functional [ 7 [ a n d a b i n i t i o q u a n t u m c h e m i s t r y c a l c u l a t i o n s [8 ], a n d c o m p a r i s o n s with e x p e r i m e n t s [1, 9], w e h a v e e m p l o y e d m a n y - b o d y t e c h n i q u e s including H - F , e x a c t d i a g o n a l i z a t i o n a n d a d i a b a t i c m o l e c u l a r d y n a m i c s t o e x p l o r e the g r o u n d *Invited paper.
Elsevier Sequoia
110 and excited states. A g r e e m e n t b e t w e e n p r e d i c t e d optical, ESR, infrared (IR) and r e s o n a n c e R a m a n (RR) s p e c t r a with r e c e n t e x p e r i m e n t s [1, 9, 10] on both the p u r e and mixed-halide MX chains has b e e n quantitatively achieved. W e have m o d e l e d effective isolated single chains o f p u r e MX materials in t e r m s of a 3/4-filled, t w o - b a n d Peierls e x t e n d e d - H u b b a r d model. In the case o f an isolated mixed-halide chain [ 11 ] we r e p l a c e a segment, containing m X atoms, b y X' a t o m s w h e r e X, X' =C1, Br, I. F o c u s i n g on the metal dz~ and h a l o g e n (X, X') Pz orbitals and including only the n e a r e s t - n e i g h b o r interactions, we c o n s t r u c t the following tight-binding m a n y - b o d y Hamiltonian [2, 3]: H = ~ { ( - to + aAt)(c~,,ct+ 1.~+ c[+ ,,,cz,~) + [e~- fit(A, + A,_ ,)]c~.,ct,~} l,a
1
1
+ ~ U t n l r n ~ , + ~ ~ K t A ~ + ~ KMM~(Aet+ Aet+ 1)2 l
(1)
l
w h e r e c ~ (ct,,) d e n o t e s the c r e a t i o n (annihilation) o p e r a t o r for the e l e c t r o n i c orbitals at the lth a t o m with spin (r. M and X (or X') o c c u p y even and odd sites, respectively. At = ~)t+ ~ -~),, w h e r e ~)~ are the d i s p l a c e m e n t s f r o m u n i f o r m lattice spacing o f the a t o m s at site I. E q u a t i o n (1) includes as p a r a m e t e r s the on-site e n e r g y et (eet=eM=eo; e2t+l=ex = --eo in the X region and e2t+l = ex, = e o - 2e~) in the X' region, ei being the e l e c t r o n affinity of the ith ion), e l e c t r o n h o p p i n g (to, t~)), on-site (fiM, fiX, fl~) and inter-site (a, a ' ) e - p coupling, on-site e - e r e p u l s i o n (UM, Ux, U~), and finally effective M - X (K) or M - X ' (K') and M - M (KMM)springs to m o d e l the e l e m e n t s of the s t r u c t u r e not explicitly included. In particular, KMM a c c o u n t s for the rigidity of the metal sublattice c o n n e c t e d into a t h r e e - d i m e n s i o n a l n e t w o r k via ligands. Long-range C o u l o m b fields are also included w h e n n e c e s s a r y (as in s t r o n g valence localized cases). Note that the metal (M = Pt) energies e2t are the same in the s e g m e n t and the h o s t MX chain. At stoichiometry, t h e r e are six e l e c t r o n s p e r M2X~ (or MeX~) unit, or 3/4 band-filling. A c o m b i n a t i o n o f g r o u n d state e x p e r i m e n t a l data (the X-sublattice distortion amplitude A0, the a(X) --* do'* a b s o r p t i o n for the oxidized m o n o m e r , and the inter-valence c h a r g e t r a n s f e r (IVCT) b a n d edge, Eg) and q u a n t u m chemical calculations [8] have led us to the effective p a r a m e t e r sets for the Hamiltonian o f eqn. (1) listed in Table 1. In the following we p r e s e n t selected results f r o m o u r c o m p r e h e n s i v e study for b o t h the p u r e [3] and mixed-halide [11] MX chains. TABLE 1 Parameters for the PtX materials in eqn. (1). fix = --0.5tim MX
to (eV)
a e0 (eV//~) ( e V )
/~ KMX KMM UM (eV//~) (eV//~2) (eV//~2) (eV)
Ux (eV)
Zi (/~)
Eg (eV)
PtC1 PtBr
1.02 1.26
0.5 0.7
2.7 1.8
1.3 0.0
0.38 0.24
2.50 1.56
2.12 0.60
6.800 6.125
0.0 0.7
1.9 0.0
111
Results
F i g u r e (1) depicts calculated ESR s p e c t r a f o r p o l a r o n s o n a PtBr chain: (a) shows an e l e c t r o n p o l a r o n c e n t e r e d o n an oxidized metal a t o m (Pt 3+~) while (b) shows a hole p o l a r o n c e n t e r e d o n a r e d u c e d m e t a l a t o m (Pt3-~). Since P t B r is a w e a k l y localized CDW system, e l e c t r o n spin density is s p r e a d o v e r several sites. Multiple p e a k s in the ESR s p e c t r a are a r e s u l t of hyperfine splitting due to the i n t e r a c t i o n b e t w e e n e l e c t r o n and n u c l e a r spin. In the p r e s e n t illustrative calculations we have t a k e n the m a t r i x e l e m e n t for e l e c t r o n - n u c l e a r spin interaction to be the s a m e for b o t h Pt a n d Br. H o w e v e r , o n c e we include the a p p r o p r i a t e m a t r i x e l e m e n t s for Pt and Br w e e x p e c t the a m p l i t u d e o f the hyperfine s t r u c t u r e in Fig. (1) to be r e d u c e d . Note the e l e c t r o n a n d hole a s y m m e t r y in the ESR s p e c t r a which is c o n s i s t e n t with e x p e r i m e n t a l data [1, 10] and is a c o n s e q u e n c e of the t w o - b a n d n a t u r e of o u r m o d e l (i.e., explicit inclusion of the f o u r a t o m s o f the unit cell). W e h a v e also investigated the p h o t o d e c a y c h a n n e l s u b s e q u e n t to p h o t o e x c i t a t i o n in the g r o u n d state as well as in the p r e s e n c e of n o n l i n e a r e x c i t a t i o n s a n d impurities using adiabatic m o l e c u l a r d y n a m i c s [3]. P h o t o e x c i t a t i o n was simulated n u m e r i c a l l y by manually r e m o v i n g an e l e c t r o n f r o m an o c c u p i e d state and i n s t a n t a n e o u s l y placing it in a n u n o c c u p i e d state. The s y s t e m was allowed to evolve adiabatically with n o f u r t h e r c h a n g e s in e l e c t r o n i c occupations. F i g u r e 2 shows the evolution o f P t B r after a single e l e c t r o n is p h o t o e x c i t e d a c r o s s the Peierls gap, with the addition of the gap e n e r g y (Eg) to the system. As is clear f r o m Fig. 2(a), an e x c i t o n is initially f o r m e d within a p h o n o n period. H o w e v e r , this e x c i t o n is unstable and evolves into a slowly s e p a r a t i n g k i n k - a n t i k i n k pair. The e l e c t r o n i c o c c u p a n c i e s of the e x c i t e d state n e c e s s i t a t e that b o t h of t h e s e kinks are neutral. Since the
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112
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c r e a t i o n e n e r g y o f t h e k i n k - a n t i k i n k p a i r is s m a l l e r t h a n t h e g a p e n e r g y , the remaining energy goes partly into the kinetic energy of the kinks, partly i n t o t h e a c o u s t i c p h o n o n s a n d p a r t l y in t h e f o r m o f a s m a l l a m p l i t u d e l o c a l i z e d 'breather' ( p h o n o n b o u n d s t a t e ) b e t w e e n t h e k i n k s . S u c h a b r e a t h e r , a
113 temporally and spatially coher e nt state of optical phonons, is clearly visible in Fig. 2(a). Figure 2(b) shows the time evolution of associated energy levels, in particular the characteristic gap states of a kink pair: within ~ 200 fs two continuum states are pulled into one (almost) degenerate mid-gap state indicating that the initial bound e l e c t r o n - h o l e pair (exciton) quickly evolves into a kink-antikink pair. At the same time a breather level oscillates about the conduction band edge into the gap and persists with a time period larger than the p h o n o n period. Investigations of the influence of impurities on this p h o t o d e c a y channel are in progress. Next we illustrate a few salient features of mixed-halide chains [9, 11] by way of predicted optical, infrared (IR) and resonance Raman (RR) spectra. In Fig. (3) a PtC1 chain of length N = 48 atoms is considered in which a segment containing eight C1 atoms is replaced by Br atoms. The interface (edge) between the PtCI and PtBr segments is on a reduced metal site (Pt 3- ~). Figure 3(a) depicts the optical absorption for such a mixed-halide chain. Note that in addition to the two peaks at 1.5 and 2.5 eV, which correspond to the inter-valence charge transfer (IVCT) energy for PtBr and PtCl, respectively, there are absorption peaks between the two IVCTs (intragap) as well as several peaks beyond 2.5 eV (ultragap). These extra peaks arise from the strong mixing of the electronic bands of the two kinds of chains in the electronic s p ectr um (not shown). We have identified local p h o n o n modes associated with the edge in the mixed-halide chains that correlate directly with the measured RR spectra [9] and the electronic resonance energies of Fig. 3(a). Figure 3(b) shows the calculated IR spectra for the mixed-halide (PtC10.67Br0.33) chain. The peak at 349 cm-~ co r re s ponds to the PtC1 IR m o de while the peak at 239 cm -1 c o r r e s p o n d s to the PtBr IR mode. Figure 3(c) shows the predicted RR spectra for three different excitation energies. At an excitation energy 1.5 eV, which is about the same as the PtBr IVCT, the PtBr Raman mode at 172 cm -1 is prominent. Similarly, when the excitation energy is 2.55 eV, which is approximately equal to the PtCl IVCT, the PtCl Raman mode becom es resonant at 308 cm -I. However, at an intermediate excitation energy (2.0 eV) several other localized modes becom e resonant. Figure 3(c) shows that apart from the pure PtC1 and PtBr m odes at 308 and 172 cm -1, a mode at 225 cm -~ b e c o m e s very prominent. This m ode arises from the interface between the two differing segments. We label it as the edge mode [9]. The peaks at low f r e q u e n c y arise from the acoustic branch of the RPA phonon spectrum (not shown). We emphasize that different local modes b e c o m e Raman active at different excitation energies and their relative intensities also vary. The excitation profile for various Raman active m odes (not shown) is consistent with the optical absorption shown in Fig. 3(a). Conclusions
We have given a brief overview of the important optical probes in both the pure and mixed-halide MX chain solids. We have also studied the isotope
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115
effect in terms of IR and RR signatures in these systems. Furthermore, we are also beginning to explore mixed-metal (MxM'~_~X) and bimetallic (MMX) systems, as well as effects of magnetic fields (especially on the weak CDW/ SDW ground state materials [4]). While, on one hand, we believe that the MX class of compounds are nearly uniquely important as a testing ground for many-body modeling and materials design strategy in strongly correlated low-dimensional materials, we are on the threshold of investigating their technologically important applications on the other. Indeed, doping and photoexcitation studies of mixed-halide systems [12] indicate that electron and hole defects preferentially locate on differing chain segments (i.e. holes in the PtC1 segment, electrons in the PtBr segment). This points to photovoltaic device applications of the mixed-halide single crystals. Since the electronic and spectroscopic properties of MX crystals depend very sensitively on the presence and nature of a variety of impurities, counterions and ligands, they are very good prospects for chemical sensors (or so-called electronic noses).
Acknowledgements We acknowledge important discussions with R. Donohoe, X. Z. Huang, B. Swanson and L. Worl. This work was supported by the US DOE.
References 1 Proc. Conf. Optical Probes of Conjugated Polymers, Snowbird, UT, USA, Aug. 22-25, 1991 (these proceedings), Synth. Met., 49-50 (1992) and Proc. Int. Conf. Synthetic Metals (ICSM '90), Tiibingen, FRG, Sept. 2-7, 1990, Synth. Met., 41-43 (1991). 2 J. T. Gammel, R. J. Donohoe, A. R. Bishop and B. I. Swanson, Phys. Rev. B, 42 (1990) 10 566. 3 J. T. Gammel, A. Saxena, I. Batisti6, A. R. Bishop and S. R. Phillpot, Phys. Rev. B, 45 (1992) 6408. 4 H. R6der, J. T. Gammel and A. R. Bishop, in preparation. 5 I. Batisti6, J. T. Gammel and A. R. Bishop, Phys. Rev. B, 44 (1991) 13 228. 6 I. Batisti6 and A. R. Bishop, Phys. Rev. B, 45 (1992) 5282. 7 R. C. Albers, Synth. Met., 29 (1989) F169; R. C. Albers, M. Alouani, J. M. Wills and M. Springborg, Synth. Met., 41-43 (1991) 2739; M. Alouani and R. C. Albers, unpublished results. 8 C. Boyle, R. L. Martin and P. J. Hay, unpublished results. 9 B. I. Swanson, R. J. Donohoe, L. A. Worl, J. T. Gammel, A. Saxena, I. Batisti4 and A. R. Bishop, Synth. Met., 41 (1991) 2733; Mol. Cryst. Liq. Cryst., 194 (1991) 43. 10 S. Kurita and M. Haruki, Synth. Met., 29 (1989) F129; M. Tanaka and S. Kurita, J. Phys. C: Solid State Phys., 19 (1986) 3019. 11 A. Saxena, X. Z. Huang, I. Batisti6, A. R. Bishop, L. A. Worl and B. I. Swanson, Phys. Rev. B, in preparation. 12 L. A. Worl, S. C. Huckett, B. I. Swanson, A. Saxena, A. R. Bishop and J. T. Gammel, J. Phys.: Condens. Matter, (1992) in press.