Experimental observation of defect pair separation triggering phase transitions.

First-order phase transitions typically exhibit a significant hysteresis resulting for instance in boiling retardation and supercooling. The hysteresis arises, because nucleation of the new phase is activated. The free-energy change is positive until the nucleus reaches a critical size beyond which further growth is downhill. In practice, the barrier is often circumvented by the presence of heterogeneous nucleation centres, e.g. at vessel walls or seed crystals. Recently, it has been proposed that the homogeneous melting of ice proceeds via separation of defect pairs with a substantially smaller barrier as compared to the mere aggregation of defects. Here we report the observation of an analogous mechanism catalysing a two-dimensional homogeneous phase transition. A similar process is believed to occur in spin systems. This suggests that separation of defect pairs is a common trigger for phase transitions. Partially circumventing the activation barrier it reduces the hysteresis and may promote fluctuations within a temperature range increasing with decreasing dimensionality.

Different freezing behavior of millimeter- and micrometer-scaled (NH4)2SO4/H2O droplets.

Although the freezing of aqueous solutions is important for nature and different branches of science and freeze-applications, our understanding of the freezing process is not complete. For example, numerous measurements of micrometer-scaled (NH(4))(2)SO(4)/H(2)O droplets report one freezing event below the eutectic point. However, measurements of larger millimeter-scaled droplets reveal two freezing events: the freezing out of ice and subsequent freezing of a residual freeze-concentrated solution. To resolve this apparent contradiction we performed numerous calorimetric measurements which indicate that the freezing of a residual solution of millimeter-scaled 5-38 wt% (NH(4))(2)SO(4) droplets occurs mainly between ∼ 210 and 225 K. We also find that micrometer-scaled droplets produce one freezing event which is within or in the vicinity of the ∼ 210-225 K region. This fact and the analysis of thermograms suggest that the residual solution of micrometer-scaled droplets may partly crystallize simultaneously with ice and partly transform to glass at T(g)≈172 K. Our results suggest for the first time that the size of (NH(4))(2)SO(4)/H(2)O droplets may affect the number of freezing events below the eutectic point.

The phase diagram of halogens on Pt(110): structure of the (4 × 1)-Br/Pt(110) phase.

When adsorbed on the strongly anisotropic Pt(110) surface Br forms a sequence of (n × 1) structures. In the present study we investigate the (4 × 1) structure by scanning tunneling microscopy, quantitative low-energy electron diffraction and density-functional calculations. We show that the optimal structural model contains essentially the same adsorption sites as the (3 × 1) structure, but with a different preference. The positions of the substrate atom are consistent with a frozen surface phonon of fourfold periodicity, suggesting that the phase diagram can be understood on the basis of a tunable charge density wave (Swamy et al 2001 Phys. Rev. B 86 1299). The structure could also be explained by assuming short-range interactions only, but evidence is presented that adsorbate-adsorbate interactions mediated by quasi-one-dimensional surface resonances play a major role in both cases.

Is the onset of high-temperature superconductivity associated with a dimensional crossover?

Angle-resolved ultraviolet photoemission spectroscopy (ARUPS) from high-T(c) superconductors shows an effective-mass renormalization and intense quasiparticle peaks close to the Fermi energy E(F), which change dramatically with temperature as T(c) is crossed. They are attributed to many-body effects, but their precise nature has been controversial until now. We find very similar spectral fingerprints, even with a similar temperature dependence albeit with much higher critical temperature, in a quasi-one-dimensional Br/Pt surface compound. The striking parallels support an interpretation based on spin-charge separation and are consistent with a dimensional crossover taking place at T(c).

Charge-density waves in self-assembled halogen-bridged metal chains.

Self-assembled growth of an ordered layer of Pt-Br-Pt chains on a Pt(110) surface is demonstrated. Upon slight doping with excess bromine, charge-density wave (CDW) domains separated by well-localized solutions are observed in the Br/Pt layer by scanning tunneling microscopy. Depending on annealing and adatom concentration, a global, long-range-ordered CDW ground state can be established. Angle-resolved UV photoemission data reveal the corresponding Fermi surface and its removal upon the Peierls transition. The CDW phase is stable to well above room temperature.