Phase transitions driven by competing interactions in low-dimensional systems (original) (raw)
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Surface Science, 2004
A reversible, temperature-driven structural surface phase transition of Pb/Si(1 1 1) nano-domains is studied with a variable-temperature scanning tunneling microscope (STM). Finite-size effects of the transition are clearly demonstrated. Most importantly, structural fluctuations in the low-temperature phase can be induced by the direct interaction between the tip atoms and the surface atoms. The structural changes reveal dynamics in the low-temperature phase. Amazingly, the largest size of the domains that can be manipulated decreases with decreasing sample temperature.
Fluctuations and Phase Separation in a Quasi-One-Dimensional System
Physical Review Letters, 2007
Phase transitions in a quasi-one-dimensional surface system on a metal substrate are investigated as a function of temperature. Upon cooling the system shows a loss of long-range order, fluctuations, and a transition into an inhomogeneous ground state due to competition of local adsorbate-adsorbate interactions with an incommensurate charge density wave. This agrees with a general phase diagram for correlated systems and high-temperature superconductors. The model surface system allows direct imaging of the fluctuations and the glassy inhomogeneous ground state by scanning tunneling microscopy.
Unveiling phase transitions in 1D systems with short-range interactions
SPIN
The statement that any phase transition is related to the appearance or disappearance of longrange spatial correlations precludes a finite transition temperature in one-dimensional (1D) systems. In this paper we demonstrate that the 1D Ising model with short-range exchange interactions exhibits a second-order phase transition at a finite temperature relying on the proper choice of the order parameter. To accomplish this, we combined analytical calculations and high-precision entropic sampling simulations and chose a slightly different order parameter, namely the module of the magnetization. Notably, we detected a phase transition with a corresponding critical temperature around 15 K, which is in excellent agreement with experimental results. Our study indicates that an inappropriate choice of the order parameter may mask phase transitions in one-dimensional systems.
Electric-field-controlled phase transition in a 2D molecular layer
Scientific reports, 2017
Self-assembly of organic molecules is a mechanism crucial for design of molecular nanodevices. We demonstrate unprecedented control over the self-assembly, which could allow switching and patterning at scales accessible by lithography techniques. We use the scanning tunneling microscope (STM) to induce a reversible 2D-gas-solid phase transition of copper phthalocyanine molecules on technologically important silicon surface functionalized by a metal monolayer. By means of ab-initio calculations we show that the charge transfer in the system results in a dipole moment carried by the molecules. The dipole moment interacts with a non-uniform electric field of the STM tip and the interaction changes the local density of molecules. To model the transition, we perform kinetic Monte Carlo simulations which reveal that the ordered molecular structures can form even without any attractive intermolecular interaction.
Physical Review E, 2005
The phase behavior of a large but finite Ising ferromagnet in the presence of competing surface magnetic fields ±Hs is studied by Monte Carlo simulations and by phenomenological theory. Specifically, the geometry of a double pyramid of height 2L is considered, such that the surface field is positive on the four upper triangular surfaces of the bi-pyramid and negative on the lower ones. It is shown that the total spontaneous magnetization vanishes (for L → ∞) at the temperature T f (H), related to the "filling transition" of a semi-infinite pyramid, which can be well below the critical temperature of the bulk. The discontinuous vanishing of the magnetization is accompanied by a susceptibility that diverges with a Curie-Weiss power law, when the transition is approached from either side. A Landau theory with size-dependent critical amplitudes is proposed to explain these observations, and confirmed by finite size scaling analysis of the simulation results. The extension of these results to other nanosystems (gas-liquid systems, binary mixtures, etc.) is briefly discussed.
Journal of Molecular Catalysis
A theoretical simulation of STM/STS has been performed for various surface systems, based on the first-principles local density functional (LDA) calculation. Cluster models made of lo-20 atoms are utilized for the tip, and slab models with several atomic layers are adopted for the sample surface. The tunnel current is almost concentrated on a single apex atom, if the other atoms on the top of the tip are not located on the same level. In such a case the STM image is normal, with atomic resolution. However, if the apex of the tip is formed by more than one atom, abnormal images tend to be formed. We verify this feature by numerical results for graphite, Si(loo), and Si (111) /Ag surfaces, and discuss the light emission from STM, based on realistic calculations of the electronic states of the tip/sample systems.
Electronically driven phase transitions in a quasi-one-dimensional adsorbate system
The European Physical Journal B, 2010
A quasi-1D system is prepared using the Pt(110) surface as a template. The electronic surface resonance structure is studied by angle-resolved photoemission spectroscopy for the clean surface as well as for different Bromine coverages. A Fermi surface mapping reveals saddle points at the Fermi level in the interior of the surface Brillouin zone. Correspondingly, a maximum in the static response function χ(q, 0) at the connecting vector q is expected. With 1/2Gx < q < 2/3Gx one observes indeed a 3-fold periodicity around defects and a 2-fold periodicity at low temperature for ΘBr = 0.5 ML. Cooling of a defect-free c(2 × 2)−Br/Pt(110) preparation counter-intuitively results in a loss of long-range order. Motivated by DFT calculations this is attributed to an anomalous order-order phase transition into the (2 × 1) phase accompanied by intense, strongly anisotropic fluctuations within a temperature range of ∼200 K. The peculiar behaviour is rationalised in terms of a competition between inter-adsorbate repulsion and an adsorbate triggered 2kF interaction in the substrate.
Surface order-disorder phase transitions and percolation
The Journal of Chemical Physics, 2006
In the present paper, the connection between surface order-disorder phase transitions and the percolating properties of the adsorbed phase has been studied. For this purpose, four lattice-gas models in the presence of repulsive interactions have been considered. Namely, monomers on honeycomb, square, and triangular lattices, and dimers (particles occupying two adjacent adsorption sites) on square substrates. By using Monte Carlo simulation and finite-size scaling analysis, we obtain the percolation threshold θc of the adlayer, which presents an interesting dependence with w∕kBT (w, kB, and T being the lateral interaction energy, the Boltzmann constant, and the temperature, respectively). For each geometry and adsorbate size, a phase diagram separating a percolating and a nonpercolating region is determined.