SPring-8, the large synchrotron radiation facility

Date&Time: 13:30 - 14:30, 15 October 2004

Speaker: Harumo Morikawa
Affiliation: The University of Tokyo, School of Science, Department of Physics

Abstract
　　Low dimensional metallic systems have attracted great interests because of their exotic behaviors. Among them, the Peierls transition accompanying a Charge Density Wave(CDW) has been most widely studied from both experimental and theoretical point of view. The synthesis technology of bulky low-dimensional materials has enabled a great advance in this field. Compared with such bulk materials, solid surfaces have a large advantage in directly observing such dynamics by atomic-resolution local probe methods such as scanning tunneling microscopy/spectroscopy (STM/S). However, until recently, few candidates for CDWs in surface systems have been reported in spite of their intrinsic low dimensionality.[1-3]
　　The In/Si(111)-4x1 and Sn/Ge(111)-3^1/2x3^1/2 surfaces are known as one-dimensional and two-dimensional metallic systems respectively, and undergo phase transitions at low temperature (LT).[2-5] The purpose of the present study is to know the nature of these phase transitions and to detect the dynamics directly with STM. The former surface is formed by depositing 1 monolayer (ML) of indium on the Si(111) surface. This surface has three 1-dimensional metallic bands called m1, m2, and m3.[4] The phase transition on this surface is a 4x1 to 8x‘2’ one around 120K.[3] Due to the good nesting in the Fermi surfaces of m2 and m3 bands, the transition is considered as a CDW type. We have performed a detailed angle resolved photoelectron spectroscopy (ARPES) measurement at 300 K and 100 K, and STM observation at 6K. Firstly, we have found though the ARPES measurement a clear evidence for the metal-insulator type CDW transition on the surface, that is, m3 band folds back at low temperature. Secondly, a strong CDW-lattice locking effect has been observed by the STM observation at 6K. The charge is pinned by the lattice in two different ways. And finally, a dynamics of a soliton is shown which has been predicted for this kind of commensurate CDW system.
　　The latter surface, the Sn/Ge(111)-3^1/2x3^1/2 system, is formed by 1/3 ML of Sn on a Ge(111) surface, and the phase transition is a 3^1/2x3^1/2 -> 3x3 one which occurs around 200K.[2] The first STM observation by Carpinelli et al. showed reversed contrast between filled and empty states, meaning the 3x3 phase is due to the charge ordering, thus a 2-dimensional CDW picture was proposed[2]. However later core level (CL) photoemission study showed two Sn-4d component for both –3^1/2x3^1/2 room temperature (RT) phase and 3x3 LT phases, which challenged the flat T4 model for the RT-3^1/2x3^1/2 surface. Thus, an order-disorder transition model was proposed, in which Sn atoms fluctuates vertically between two adsorption sites at RT and freezes at LT to form the 3x3 structure, just like the Si(001) 2x1 -> c(4x2) transition.[5, 6] We have performed a surface conductivity measurement from 300K to 100 K and STS I-V measurement at 300K, 65K, and 6K on this surface. Then we have found that the 3^1/2x3^1/2 -> 3x3 transition is a metal-to-metal type, contrary to the CDWpicture. However, we have also found the electronic structure is changes by the cooling process. The order-disorder transition model is favored rather than the CDW one, but the transition is not a simple one.
　　Low dimensional metallic systems have attracted great interests because of their exotic behaviors. Among them, the Peierls transition accompanying a Charge Density Wave(CDW) has been most widely studied from both experimental and theoretical point of view. The synthesis technology of bulky low-dimensional materials has enabled a great advance in this field. Compared with such bulk materials, solid surfaces have a large advantage in directly observing such dynamics by atomic-resolution local probe methods such as scanning tunneling microscopy/spectroscopy (STM/S). However, until recently, few candidates for CDWs in surface systems have been reported in spite of their intrinsic low dimensionality.[1-3]
　　The In/Si(111)-4x1 and Sn/Ge(111)-31/2x31/2 surfaces are known as one-dimensional and two-dimensional metallic systems respectively, and undergo phase transitions at low temperature (LT).[2-5] The purpose of the present study is to know the nature of these phase transitions and to detect the dynamics directly with STM. The former surface is formed by depositing 1 monolayer (ML) of indium on the Si(111) surface. This surface has three 1-dimensional metallic bands called m1, m2, and m3.[4] The phase transition on this surface is a 4x1 to 8x‘2’ one around 120K.[3] Due to the good nesting in the Fermi surfaces of m2 and m3 bands, the transition is considered as a CDW type. We have performed a detailed angle resolved photoelectron spectroscopy (ARPES) measurement at 300 K and 100 K, and STM observation at 6K. Firstly, we have found though the ARPES measurement a clear evidence for the metal-insulator type CDW transition on the surface, that is, m3 band folds back at low temperature. Secondly, a strong CDW-lattice locking effect has been observed by the STM observation at 6K. The charge is pinned by the lattice in two different ways. And finally, a dynamics of a soliton is shown which has been predicted for this kind of commensurate CDW system.
　　The latter surface, the Sn/Ge(111)-31/2x31/2 system, is formed by 1/3 ML of Sn on a Ge(111) surface, and the phase transition is a 31/2x31/2 -> 3x3 one which occurs around 200K.[2] The first STM observation by Carpinelli et al. showed reversed contrast between filled and empty states, meaning the 3x3 phase is due to the charge ordering, thus a 2-dimensional CDW picture was proposed[2]. However later core level (CL) photoemission study showed two Sn-4d component for both –31/2x31/2 room temperature (RT) phase and 3x3 LT phases, which challenged the flat T4 model for the RT-31/2x31/2 surface. Thus, an order-disorder transition model was proposed, in which Sn atoms fluctuates vertically between two adsorption sites at RT and freezes at LT to form the 3x3 structure, just like the Si(001) 2x1 -> c(4x2) transition.[5, 6] We have performed a surface conductivity measurement from 300K to 100 K and STS I-V measurement at 300K, 65K, and 6K on this surface. Then we have found that the 31/2x31/2 -> 3x3 transition is a metal-to-metal type, contrary to the CDWpicture. However, we have also found the electronic structure is changes by the cooling process. The order-disorder transition model is favored rather than the CDW one, but the transition is not a simple one.

[1] J. M. Carpinelliet al. Nature (London) 381, 398 (1996).
[2] J. M. Carpinelliet al. Phys. Rev. Lett. 79, 2859 (1997).
[3] H. W. Yeom et al. Phys. Rev. Lett. 82,4898(1999).
[4] T. Abukawa et al. Surf. Sci. 325,33(1995).
[5] R. I. G. Uhrberg and T. Balasubramanian, Phys. Rev. Lett. 81, 2108 (1998).
[6] J. Avila et al. Phys. Rev. Lett. 82 442 (1999).