Anharmonic Effects at the (100) and (110) Surfaces of Ni
- PDF / 401,240 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 49 Downloads / 226 Views
ANHARMONIC EFFECTS AT THE (100) AND (110) SURFACES OF Ni Yvon Beaudet,* Laurent J. Lewis,* and Mats Persson** *D6partement de physique et Groupe de recherche en physique et technologie des couches minces, Universit6 de Montreal, C.P. 6128, Succ. A, Montrdal, Qu6bec, Canada
H3C 3J7
"*Departmentof Applied Physics, Chalmers University of Technology, S-412 96, G6teborg, Sweden
ABSTRACT We present the results of a detailed molecular-dynamics investigation of anharmonic effects at the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom potentials. We find both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration. For the (100) surface, our results for the surface expansion can be directly compared to the low-energy electron diffraction (LEED) measurements of Cao and Conrad;1 the agreement is excellent. The anomalies appear at a temperature of about 900 K for the (100) surface, well below the onset of disordering, thereby confirming that the observed anomalies indeed arise from anharmonicities. Our simulated LEED intensities are found to reproduce well the observed intensities. The phonon spectra in the anharmonic regime will be discussed, and a detailed comparison between the two surfaces will be established.
INTRODUCTION The dependence on temperature of the relaxation of metallic surfaces is an area of active investigation. A common observation from low-energy electron diffraction (LEED) [1, 2], elastic He scattering [3, 4, 5], medium-energy ion scattering [6], and electron energy loss spectroscopy [7] experiments is that such quantities as surface thermal expansion, mean-square displacements and phonon lineshapes often exhibit a behaviour which is "anomalous" when compared to the bulk. It is more and more becoming evident that these effects are due to enhanced anharmonicity at the surface rather than disordering. A well-documented consequence of anharmonicity is thermal expansion which, because of reduced coordination, is expected to be much larger at the surface than in the bulk. Recently, low-energy electron diffraction (LEED) experiments have been reported [2], indeed revealing that the (100) surface of Ni suffers a large outward relaxation at high temperature, the thermal expansion reaching a value considerably larger than in the bulk; an anomalously high thermal expansion coefficient has also been reported in Pb(110) (6]. Indirect evidence of increased anharmonicity is provided by the temperature dependence of diffracted-beam intensities, interpreted as Debye-Waller factors with anomalously large mean-square amplitudes of vibration [1, 3, 4, 5]. We present here the results of a molecular-dynamics (MD) investigation of anhaxmonic effects at the (100) and (110) surfaces of Ni, emphasizing the differences between the two surfaces. The interactions are described in terms of the semi-empirical embeddedatom method (EAM) potentials [8, 9]. The dynamics of metal surfaces has been the object
Data Loading...
