Dook van Mechelen

Postdoctoral Fellow, University of Geneva

Education

Ph.D. 2010 Physics
University of Geneva

M.Sc. 2005 cum laude and B.Sc. 2003 Physics
Free University Amsterdam

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Research Summary

My current research is focussing on the electron and spin dynamics of transition metal oxides and rare-earth oxides. These phenomena are studied using standard optical spectroscopy and a novel technique of time-domain ellipsometry in the GHz and THz range and can be summarized as follows:

1. I.The technique of GHz time-domain ellipsometry has been developed to study the spintronics in pristine EuTiO3. It turns out that the very localized 4f electrons can be brought into resonance with a small dc magnetic field and a small GHz magnetic field.
   

2. II.The coupling of free electrons to phonons has been studied in Nb doped SrTiO3 in an unusually broad energy range from 0.3 meV up to 7 eV with visible light ellipsometry, FTIR spectroscopy and THz time-domain spectroscopy. We observe the presence of an unusually narrow (< 2 meV) Drude peak due to the free carriers. It turns out that at low temperatures these carriers have mass enhancement of at least a factor two.  No evidence has been found for a particularly large electron-phonon coupling that would result in small polaron formation. These observations can be understood with a many-polaron model based on the Fröhlich interaction. This provides an interpretation for the essential characteristics of the observed optical conductivity spectra of SrTi1-xNbxO3 without any adjustment of material parameters.
   

Other activities

Conference organization

Local Organizer of the Annual Meeting of the
Swiss Physical Society 2008
Uni-Mail, Geneva, March 26-27

Student supervision

2008  Iris Crassee, M.Sc., University of Geneva

2007  Diemer Cohen Stuart, Delft University, The Netherlands

Teaching

2008-2010 Animator of Physics Shows that aim at vulgarizing Physics among high school students (Swiss Cycle and Collège)
VIP sessions were hold e.g. for the secretary generals of all Dutch universities

2004-2008 Practical classes for 1st year Biology, Pharmacy, Geology and Physics students (website in French)

Recent publications               

PhD thesis

“Charge and Spin electrodynamics

of SrTiO3 and EuTiO3 studied

by optical spectroscopy” (2010)

advisor : Prof. Dirk van der Marel

Electron-phonon interaction and
charge carrier mass enhancement in SrTiO3
Physical Review Letters 100, 226403 (2008)

J.L.M. van Mechelen, D. van der Marel, C. Grimaldi,
N.P. Armitage, A.B. Kuzmenko, N. Reyren, H. Hagemann and I.I. Mazin

We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO3 as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow ( < 2 meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the  spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation.

Many-body large polaron optical conductivity in SrTi1-xNbxO3

                       Phys. Rev. B in press (2010)

J.T. Devreese, S.N. Klimin, J.L.M. van Mechelen, D. van der Marel

Recent experimental data on the optical conductivity of niobium doped SrTiO3 are interpreted in terms of a gas of large polarons with effective coupling constant α≃2. The theoretical approach takes into account many-body effects, the electron-phonon interaction with multiple LO-phonon branches, and the degeneracy and the anisotropy of the Ti t2g conduction band. Based on the Fröhlich interaction, the many-body large-polaron theory provides an interpretation for the essential characteristics, except − interestingly − for the unexpectedly large intensity of a peak at ~130 meV, of the observed optical conductivity spectra of SrTi1-xNbxO3 without any adjustment of material parameters.