| AVS 61st International Symposium & Exhibition | |
| Surface Science | Monday Sessions |
| Session SS+AS+EN-MoM |
| Session: | Mechanistic Insights into Surface Reactions: Catalysis, ALD, etc. |
| Presenter: | Erminald Bertel, University of Innsbruck, Austria |
| Authors: | M. Cordin, University of Innsbruck, Austria B.A.J. Lechner, University of Innsbruck, Austria S. Duerrbeck, University of Innsbruck, Austria A. Menzel, University of Innsbruck, Austria E. Bertel, University of Innsbruck, Austria J. Redinger, Vienna University of Technology, Austria C. Franchini, University of Vienna, Austria |
| Correspondent: | Click to Email |
First order phase transitions exhibit a nucleation barrier. Normally, the barrier is lowered by heterogeneous nucleation at interfaces or extrinsic defects. Homogeneous nucleation, in contrast, is strongly activated and results in a significant hysteresis. Molecular dynamics calculations have suggested that an important step in homogeneous nucleation involves the formation and subsequent separation of defect pairs1-2.
Here we report a (2x1)→c(2x2) order-order phase transition in a two-dimensional (2D) adsorbate system (Br/Pt(110) at 0.5 monolayer coverage) as a function of temperature3. Although an order-order phase transition is first-order according to Landau rules, the present system exhibits strong fluctuations within a wide temperature range (50K-250K). At 50 K the fluctuations are sluggish enough to allow atomically resolved observation of the phase conversion mechanism by temperature-variable Scanning Tunneling Microscopy. The transition is heralded by local density fluctuations in the adsorbate. The density variation consists of a compression (soliton) and a dilution (anti-soliton). At the transition temperature the two defect moieties are able to separate and travel independently through the system, thereby converting one phase into the other. Away from the transition temperature, separation of the defect pair would create the “wrong” phase, thus increasing the free energy of the system. This is equivalent to an effective attractive interaction between soliton and anti-soliton, in close analogy to the string interaction in spin systems4. The one-to-one correspondence can be made transparent by introducing a pseudo-spin variable, i.e. an occupation number +/-1 assigned to every bonding site. The defect-pair separation mechanism partially circumvents the nucleation barrier and thus promotes fluctuations, particularly in low-dimensional systems.
Defect-pair separation as a key step in phase transitions is a concept which arose from the analysis of molecular dynamics calculations. To our knowledge, the present study represents the first direct experimental observation with atomic resolution of such a mechanism. Furthermore, it illustrates the important role of the string interaction in suppressing fluctuations, which is very efficient in 3D systems, weaker in 2D systems and totally absent in 1D.
1 K. Mochizuki, M. Matsumoto, and I. Ohmine, Nature 498, 350 (2013).
2 M. Forsblom and G. Grimvall, Nat Mater 4, 388 (2005).
3 M. Cordin, B. A. J. Lechner, S. Duerrbeck, et al., Sci. Rep. 4 (2014).
4 T. Giamarchi, Quantum Physics in One Dimension (Oxford University Press, New York, 2004).