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Stress and magnetism in chiral surfaces

Final Report Summary - CHIRAMAG (Stress and magnetism in chiral surfaces)

The concept of chirality appears frequently in chemistry. A chiral molecule is such that it cannot be superimposed onto its mirror image. Two molecules related by a mirror operation are called enantiomers. When interacting with chiral systems, a chiral molecule will have different functions, depending on its handedness. Therefore, the separation and the promotion of one enantiomer over the other during synthesis.

The project CHIRAMAG is devoted to the study of physical properties of chiral surfaces, i.e. surfaces that devoid of mirror symmetry. Traditionally, the study of chirality at surfaces has been related to chemical applications, especially those related to the aforementioned enantiomer selectivity, following the applications of solid surfaces in chemical catalysis.

On clean surfaces, chirality can be achieved by cleaving a crystal through a highly asymmetrical crystallographic direction. Surface chirality can also be induced by adsorption. The lack of symmetry on the surface is transferred to its physical properties, that will present enhanced anisotropy features. CHIRAMAG is focused in the study of surface stress and magnetism using mainly density functional theory (DFT) methods. Both are intimately related to the electronic structure of the surface, and therefore DFT is well suited for this project.

The project was carried out in the Department of Chemistry of the University of Cambridge (United Kingdom (UK)). A collaboration was established with the DIPC and CFM at Donostia-San Sebastian (Spain). Since the beginning of the project in January 2009, the following activities have been conducted:
- Analysis of the intrinsic surface stress on clean transition metal surfaces, both chiral and achiral, with emphasis on the electronic structure origin of the surface stress.
- Studies the surface stress induced by chiral modifiers on an achiral substrate, taking alanine on Cu(110).
- Development of a computer code to (i) generate surface geometries of any given cleavage orientation from exact analytical methods and (ii) use that information to generate projected electron bandstructures on surfaces.
- Cooperation with experimentalists from Cambridge (UK) in order to determine the structure of intrinsically chiral Cu(531) surface and alanine on Cu(311).
- Analysis of spin polarised surface states on ferromagnetic Fe at different orientations, including chiral Fe(321).
- Analysis of the magnetic structure of Co surfaces and nanoislands, in collaboration with experimentalists from Donostia-San Sebastian (Spain).

The three main scientific accomplishments of the project are summarised below:
1. We have ascertained quantitatively the nature of the connection between magnitude and anisotropy in the surface stress tensor in transition metal surfaces. These features are not dictated by symmetry, but by the structural and electronic features of the surface. Their interplay with the chiral character of the surfaces in question. We find that the surface stress depends strongly on the atomic species (i.e. on the the electronic structure through d-band filling) rather than on the surface structure details; e.g. stress anisotropy in Fe(321) is more pronounced than that of W(321). This work reflects the risks of predicting surface properties only from geometry-based models.

M. Blanco-Rey, S.J. Pratt and S.J. Jenkins, 'Surface stress of stepped chiral metal surfaces', Phys. Rev. Lett. 102 (2009) 026102
M. Blanco-Rey and S.J. Jenkins, 'Surface stress in d-band metal surfaces', J. Phys.: Condens. Matter 22 (2010) 135007

2. The system alaninate/Cu(110) under several coverage regimes was used to study the degree of symmetry breaking in the surface stress induced by adsorption of a chiral species in an achiral substrate. We find that the asymmetry depends on the chirality of the adsorption footprint, i.e. the distribution of chemical bonds linking metal and molecule atoms, rather than on the chirality of the molecule itself. Asymmetry is weaker at higher coverage as a consequence inter-molecular weak bonding (a hydrogen-bond network in this particular case).

M. Blanco-Rey and G. Jones, 'Asymmetric relief of surface stress induced by a chiral adsorbate: Alaninate adsorption on Cu(110)', Phys. Rev. B 81 (2010) 205428

3. Experimentalists in Donostia-San Sebastian determined that high densities of Co nanoislands can be achieved on GdAu2 alloys. However, it is still unclear how the magnetic structure of Co is affected by an environment of reduced dimensionality and symmetry.

We have found, by applying the full potential augmented plane waves (FLAPW) formalism within the DFT to model Co surfaces, that antiferromagnetic coupling with the substrate may be behind an apparent demagnetisation of the nanoislands at low coverage.

Although the research carried out in CHIRAMAG is of a fundamental nature, we foresee an impact or a possible application to chiral discrimination techniques, e.g. using mechanical sensors with sensitivity to the surface stress induced by chemical analytes. Our quantitative analyses may help to predict the magnitudes of stress effects under working conditions.

From a theoretical point of view, intrinsic surface stress is a recurring theme in surface physics. We have contributed with a detailed description of symmetry related effects. We have also progressed in the field of surface magnetism by adding the new perspective of chirality to the study of surface states.

Finally, our studies on Co systems have helped to interpret experiments in an area of technological interest, with future applications to magnetic storage media.