Institut für Festkörperphysik

Projekte

• Spin Noise Spectroscopy (SNS) simulations
• Organic molecules on wide-gap insulator surfaces

Jens Hübner

Nowadays, spin noise spectroscopy (SNS) employing real-time FFT is routinely used to study the spin dynamics in many bulk and low-dimensional semiconductors. However, the technique is limited by the bandwidth of the detection system and in particular by the bandwidth of the electrical analog-to-digital (A/D) conversion via a digitizer.

Ultrafast digitizers with a corresponding bandwidth of up to 13 GHz are commercially available but show an effective resolution as low as 4 bit at their maximum frequency. We simulate realistic spin noise measurements to investigate to which extent the low bit depth of fast digitizers reduces the experimental sensitivity of SNS. For this purpose we systematically vary the bit depth of the digitizer and the voltage load at the digitizer input in order to study the influence of the A/D conversion on the experimental sensitivity of SNS. The digital data is produced by a simulated R-bit digitizer and blocks of 1024 points are Fourier transformed via the FFT algorithm.

Further we study the granular and the overload error within our simulations. The simulations prove that, at well chosen input load, fast A/D converters with few effective bits allow SNS with excellent sensitivity. The simulations pave the way toward high bandwidth SNS with commercial ultrafast digitizers exceeding frequencies of 10 GHz. The experimental bandwidth that is accordingly accessible with current technology is more than one order of magnitude larger than the highest currently demonstrated bandwidth.

Weitere Informationen:

• Institute for Solid State Physics - Department Nanostructures
• Efficient data averaging for spin noise spectroscopy in semiconductors, Appl. Phys. Lett  97, 192109 (2010).

Wei Chen

One central ingredient in a wide class of applications in surface science (such as heterogeneous catalysts and self-assembled monolayer) is the molecule-surface interaction. Notably, Gerhard Ertl's contribution to the investigation of the CO molecules adsorbed on Pt surfaces earned him a Nobel Prize of Chemistry in 2007. For the last decade, adsorption of molecules on metal surfaces and semiconductors gains a great deal of attention stimulated by their relevances in catalysis and molecular electronics. Meanwhile, we have seen considerably fewer studies of the molecule-insulator interfaces. Adsorption of the gas-phase organic molecules on wide-gap insulator surfaces is usually chemically inert and it is treated as weak physisorption as long as the surface is free of defect. For instance, the features of the frontier molecular orbitals of a pentacene molecule are preserved upon its adsorption on Cu-supported NaCl films as is resolved by scanning tunneling microscopy. Due to the inertness of the wide-gap insulator surfaces, they are excellent candidates for the supporting substrates in chemical and technical applications.

Nevertheless, an intriguing separation process in the mining industry draws back our attention to revisit the interface between organic molecules and wide-gap insulators. It has been observed that the addition of certain organic molecules can trigger the separation of various minerals such as halite (NaCl), sylvite (KCl) and kieserite (MgSO₄ . H₂O) by electrostatic forces in an inhomogeneous electrical field. The electrostatic separation process is plausibly explained by a charge transfer model. Without adsorbates, electronic excitation is practically impossible at room temperature. This situation, however, is modified by the unoccupied states brought by the adsorbate on the insulator surface, since the electrons can be effectively excited to these unoccupied molecular states provided that the excitation energy is small. When the adsorbate covered surfaces are brought into contact, electrons can hop from one side to the other. A contact voltage drop appears across the interface as a result of the net charge transfer, which is supposedly responsible for the electrostatic separation process.

A precise control of the efficiency of the separation process requires not only the accurate knowledge of the organic molecules and the host insulator, but also the microscopic picture of the molecule-insulator interaction. Although the properties of the conditioner molecules (such as benzoic acid and its phenolic derivatives) and the rocksalt are well-known, the detailed mechanism of the interaction between the organic molecules and the insulating surface is not present. Moreover, the surface of NaCl or KCl grains can never be free of defects. These defects are subject to have enormous impact on the adsorption configuration and the electronic structure of the adsorbate system.

In this project, we resort to the first-principles calculations to investigate the the adsorption of benzoic acid and its various phenolic derivatives on wide-gap insulators (e.g. NaCl and KCl surfaces), as an attempt to demystify the contact charging effect between salt mines mixed with organic molecules. The adsorption configurations, energetics, and excitation properties are obtained using a variety of ab initio methods, from density functional theory (with hybrid functionals and dispersion force corrections) to the state-of-the-art many-body Green's function perturbation theory. The important roles of the nonlocal correlations, surface defects and excitonic effect are presented in the context of the molecule-insulator interfaces.

Publications:

• W. Chen, C. Tegenkamp, H. Pfnür, and T. Bredow: "Tailoring band gaps of insulators by adsorption at surface defects", Phys. Rev. B 79, 235419 (2009)
• W. Chen, C. Tegenkamp, H. Pfnür, and T. Bredow: "The interplay of van der Waals and weak chemical forces in the adsorption of salicylic acid on NaCl(001)", Phys. Chem. Chem. Phys. 11, 9337 (2009)
• W. Chen, C. Tegenkamp, H. Pfnür, and T. Bredow: "Insight from first-principles calculations into the interaction between hydroxybenzoic acids and alkali chloride surface", J. Phys. Chem. C 114, 460 (2010)
• W. Chen, C. Tegenkamp, H. Pfnür, and T. Bredow: "Anomalous molecular orbital variation upon adsorption on a wide band gap insulator", J. Chem. Phys. 132, 214706 (2010)