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Dielectric Spectroscopy of Electronic Conductors


In electronically conducting materials, a variety of phenomena can be investigated by dielectric spectroscopy:


Drude Conductivity

The dynamics of free charge carriers, e.g. metallic electrons or electrons in the conduction band of semiconductors can lead to a characteristic frequency dependence of the complex conductivity. This is mainly investigated in the submillimeter and infrared region. Example...

 

Localized Electrons

In electronic conductors, electrons (or holes) can localize due to disorder. Disorder may arise from an amorphous structure (e.g. amorphous Si), from doping (substitutional disorder), or occur even in nominally pure crystals due to slight deviations from stoichiometry or lattice imperfections. Charge transport of localized charge carriers takes place via hopping conduction, which leads to a characteristic signature in the frequency dependence of the complex conductivity. We are especially interested in the role of charge-carrier localization in the so-called Mott-Hubbard insulators and in metal-insulator transitions, occurring in various materials depending on doping, temperature, or magnetic field. In addition, information on the nature of the charge carriers (e.g. polarons) and transport mechanisms (termally activated, tunneling,...) can be gained by dielectric spectroscopy. Some examples of publications from our group in this field are:

  1. AC Conductivity in La2CuO4
    P. Lunkenheimer, M. Resch, A. Loidl, and Y. Hidaka, Phys. Rev. Lett. 69, 498 (1992).
  2. Charge carrier localization in La1-xSrxMnO3 investigated by AC conductivity measurements
    A. Seeger, P. Lunkenheimer, J. Hemberger, A.A. Mukhin, V. Yu. Ivanov, A.M. Balbashov, and A. Loidl, J. Phys.: Cond. Matter 11, 3273 (1999).
  3. Dielectric properties and dynamical conductivity of LaTiO3: From dc to optical frequencies
    P. Lunkenheimer, T. Rudolf, J. Hemberger, A. Pimenov, S. Tachos, F. Lichtenberg, and A. Loidl, Phys. Rev. B 68, 245108 (2003).
 

Relaxation Processes

Known mainly from glassforming Liquids, typical relaxation feature can also occur in the dielectric spectra of electronic conductors:

Polarons
It is known that local motions of polarons or other localized charge carriers can lead to relaxational behavior.

Orbital Glass
Electronic Orbitals can freeze in a glass-like fashion, which via electron-lattice coupling can lead to relaxation features in the dielectric spectra. More...

  1. Orbital freezing and orbital glass state in FeCr2S4
    R. Fichtl, V. Tsurkan, P. Lunkenheimer, J. Hemberger, V. Fritsch, H.-A. Krug von Nidda, E.-W. Scheidt, and A. Loidl, Phys. Rev. Lett. 94, 027601 (2005).
  2. Orbitale Gläser
    A. Loidl and P. Lunkenheimer
    Physik in unserer Zeit 36, 112 (2005).

Maxwell-Wagner Relaxation
Also non-intrinsic effects can give rise to relaxation-like features in electronic conductors, e.g., the long-known Maxwell-Wagner polarization arising from a charge accumulation at the interface between the metallic contacts and the sample. Those are often mistaken for intrinsic contributions and have to be excluded making suitable experiments. More...

  1. Origin of apparent colossal dielectric constants
    P. Lunkenheimer, V. Bobnar, A.V. Pronin, A.I. Ritus, A.A. Volkov, and A. Loidl, Phys. Rev. B 66, 052105 (2002).
  2. Non-intrinsic origin of the colossal dielectric constants in CaCu3Ti4O12
    P. Lunkenheimer, R. Fichtl, S.G. Ebbinghaus, and A. Loidl, Phys. Rev. B 70, 172102 (2004).
  3. Broadband dielectric spectroscopy on single-crystalline and ceramic CaCu3Ti4O12
    S. Krohns, P. Lunkenheimer, S.G. Ebbinghaus, and A. Loidl, Appl. Phys. Lett. 91, 022910 (2007).
 

Universalities

All non-metallic disordered matter - including doped semiconductors - seems to show a universal dielectric behavior, i.e. the frequency dependence of the conductivity and dielectric constant of very different materials behaves qualitatively astonishingly similar. More...

  1. Response of disordered matter to electromagnetic fields
    P. Lunkenheimer and A. Loidl, Phys. Rev. Lett. 91, 207601 (2003).
 

Charge-Density-Waves

In highly anisotropic, approximately one-dimensional materials a charge density wave (CDW) can form. Here the charge density is a periodic function of position and its period can be incommensurate with the crystal lattice. The dielectric behavior of CDW systems shows two characteristic features: A harmonic oscillator mode at GHz frequencies caused by the CDW being pinned at defects and a huge relaxation mode at kHz-MHz involving extremely large values of the dielectric constant, whose true origin is not completely clarified. More...


For further information please contact:
peter.lunkenheimer@physik.uni-augsburg.de