Alexander Menzel

Electronic structure of a catalyst poison: Br/Pt(110)

The system Br/Pt(110) is studied by scanning tunneling microscopy, low energy electron diVraction, angle resolved photoemission and self-consistent ab initio calculations. Bromine is molecularly adsorbed at 300 K, but dissociates at T>400 K. The Br atoms adsorb substitutionally and form quasi-one-dimensional (1D) PtMBrMPt chains. The 1D character of the chains gives rise to a charge density wave ground state, which can be triggered by surface doping. A Fermi surface yielding an appropriate nesting vector is identified by photoemission. Furthermore, a Br-induced lowering of Pt d-bands is observed, which accounts in part for the halogen-induced poisoning. The PtMBr bond is shown to be essentially covalent, ruling out electrostatic poisoning mechanisms.

Phase transitions driven by competing interactions in low-dimensional systems

Variable-temperature scanning tunnelling microscopy is used to study an order-order phase transition in a virtually defect-free quasi–one-dimensional surface system. The phase transition is driven by competing electronic interactions. The phase diagram is captured by a modified Landau formalism containing a coupling term between two different subsystems. The extra term has the effect of a spontaneously generated field which drives the phase transition. The proposed formalism applies to a variety of problems, where competing interactions produce sometimes counter-intuitive ordering phenomena.

Correlation in low-dimensional electronic states on metal surfaces

We investigate quasi-one-dimensional (quasi-1D) surface states on metals as a well-defined model system for the study of correlation effects by angle-resolved photoemission. Both dimensionally constrained Shockley and Tamm states are examined, the former on the striped O/Cu(1 1 0) phase, the latter on Pt(1 1 0) with and without adsorbates. We observe an unusual change in photoemission intensity of quasi-particle peaks as a function of temperature or adsorbate coverage, which is very similar to ARUPS results on layered systems, Kondo systems, Mott-insulator systems and high-Tc superconductors. The intensity change of the quasi-particle peak is interpreted in terms of a coherent–incoherent transition of the quasi-1D states. For the Tamm states on Pt(1 1 0), we also find other typical fingerprints of correlation such as a kink in the dispersion and a significant mass renormalization close to EF. A saddle point at the Fermi level provides a large density of states. Therefore, it is reasonable to expect that this quasi-1D surface resonance is involved in surface phase transitions. The results support our previous report about a surface charge-density-wave-induced phase transition on Br/Pt(1 1 0).

Structure tuning by Fermi-surface shifts in low-dimensional systems

In low-dimensional systems, electron-electron correlation as well as electron-phonon coupling is enhanced. This tends to link the geometry to the Fermi-surface position, for instance via formation of charge-density waves. The quasi-one-dimensional c(2×2)-Br/Pt(110) surface system is used here to explore this effect. NO adsorption gives rise to a considerable Fermi-surface shift and causes the system to go through a succession of complex phases comprising commensurate, long-range-ordered structures, as well as periodic and chaotic soliton lattices. The structural evolution is in qualitative agreement with the predictions of the discrete Frenkel-Kontorova model.

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.

Surface resonances on transition metals as low-dimensional model systems

Finding and investigating low-dimensional model systems is essential to improve the understanding of metals with strong electron correlation. Here, we show that suitably chosen transition metal surfaces can provide such model systems. Comparing the band structure from density functional theory (DFT) with angular resolved photoemission (ARPES) for Pt(110), we give evidence for a low-dimensional surface resonance. Details of the band topology and fingerprints of low-dimensional behaviour are presented.

Fluctuating Charge Order: A Universal Phenomenon in Unconventional Superconductivity?

Unconventional superconductors are characterized by various competing ordering phenomena in the normal state, such as antiferromagnetism, charge order, orbital order or nematicity. According to a widespread view, antiferromagnetic fluctuations are the dominant ordering phenomenon in cuprates and Fe based superconductors and are responsible for electron pairing. In contrast, charge order is believed to be subdominant and compete with superconductivity. Here, we argue that fluctuating charge order in the (0,π) direction is a feature shared by the cuprates and the Fe based superconductors alike. Recent data and theoretical models suggest that superconductivity is brought about by charge order excitations independently from spin fluctuations. Thus, quantum fluctuations of charge order may provide an alternative to spin fluctuations as a mechanism of electron pairing in unconventional superconductors.