Some Results

Some Results*

* numbers in brackets refer to corresponding numbers in the publication list 

 

Most of the results were obtained in collaboration with PHD students, guests and colleagues. They often contributed most of the work.

Superconductivity

  • A Together with Ferrell a new inhomogeneous superconducting state was suggested to appear in a strong external magnetic or exchange field, i.e., in systems with imbalanced populations of particles to be paired (named Fulde-Ferrell-Larkin-Ovchinnikov state as it was independently suggested by A. Larkin and Y. Ovchinnikov). This state was apparently observed in CeCoIn5, in k-(BEDT-TTF)2 Cu(NCS)2 and probably also in ? -(BETS)2 GaCl4, ? -(BETS)2FeCl4 and (TMTSF)2ClO4. Furthermore, a FFLO like state forms the basis of ? -junctions consisting of Nb-Cu/Ni-Nb sandwiches (Bulaevskii, Buzdin, Ryazanov et al.). Those junctions are important for superconducting circuitry (PHD thesis). [2]
  • With Meservey and Tedrow the Zeeman splitting of quasiparticles in superconductors was discovered. This prompted numerous experiments on spin-polarized tunneling, by different groups among them the demonstration of phonon-induced electronic spin flips. [28, 40, 202]
  • Development of the multi-pairbreaker theory (with K. Maki), which has been used by experimentalists to interprete their experiments. [7-9, 12]
  • Strong coupling theory of antiferromagnetic superconductors (with G. Zwicknagl, P. Thalmeier and A. Amici). A number of experiments were explained. [79, 260]
  • The interplay of superconductivity and crystal field excitations was pointed out (with A. Luther, L. Hirst, J. Keller). An explanation of several experiments was provided.
  • It was shown with Chang, Eremin, McHale and Thalmeier that in PrOs4Sb12 and UPd2Al3 intra-atomic excitations contribute to a large extent to Cooper pair formation. [330, 296]
  • A theory for the magntic spin resonance in UPd2Al3, CeCu2Si2 and CeCoIn5 was developed with Eremin, Chang, Thalmeier and Zwicknagl. [322, 335]

Crystal-field effects in rare-earth systems

Development of a theory for the description of crystal-field excitation and their interactions with electrons and phonons.

Interactions with electrons

  • A theory of the anormalous thermopower in a system with crystal-field split ions was derived (with I. Peschel, H. Takayama) and experimentally verified (with J. Sierro, E. Umlauf).

  • An explanation of the strong magnetic field dependence of the effective mass in Pr metal was given (with R. White and J. Jensen). [80, 89]

  • A theory of the line width of crystal-field excitations was worked out (with K. Becker, J. Keller). [61]

Interactions with phonons

  • A microscopic derivation of the magnetoeleastic interactions in rare-earth systems was given (with V. Dohm). Predictions on the importance of rotational interactions were verified experimentally by B. Lüthi. [51]

  • A prediction of a Faraday rotation of phonons and of a phononic Cotton-Mouton effect in rare-earth systems was made (with P. Thalmeier). [63, 68] These effects were verified experimentally. 

  • The existence of bound states between phonons and crystal-field excitations was shown (with P. Thalmeier). This explained neutron scattering experiments by M. Loewenhaupt on CeAl2  and later experiments on certain cuprates. [88]


Solid Electrolytes

Theoretical models for superionic conductors were developed with W. Dietrich, I. Peschel and S. Strässler et al. were developed. [52, 76]


Glasses at low Temperatures

  • The theory of metallic glasses was extended to the superconducting state (with J. Black). Subsequent experiments were well explained by the theory. [73]
  • A special state of glasses at ultralow temperatures in a magnetic field was proposed (with S. Kettemann and P. Strehlow). [256]

Structural Phase transitions

  • A magnetic field dependence of the martensitic phase transition in A15 compounds was predicted (with W. Dieterich) and later found by E. Bucher. [33]
  • A microscopic theory of the structural phase transition of a-NaV2O5 was derived which explained the most important experimental observations (with A. Bernert, P. Thalmeier, A. Yaresko et al.). [269, 275]

Correlated electrons

Two lines of research were pursued. One consists in determining many-electron wavefunctions for the ground state of solids. The achieved accuracy is comparable with the one obtained for small molecules when quantum chemical methods are applied. For that purpose a theoretical framework had to be created and applied.

The second line of research dealt with strongly correlated electrons and is of more phenomenological nature. Different origins of heavy fermion behavior were identified and methods were developed to calculate the excitation spectrum of strongly correlated electrons.

A substantial part of that research has been described in a book on “Electron Correlations in Molecules and Solids” (3rd edition, Springer Verlag 1995).


Wavefunction methods in electronic structure theory of solids

  • A theoretical frame was developed based on the use of cumulants and projection techniques which allows for the application of quantum chemical methods to infinite systems, i.e., solids. This was done in collaboration with H. Stoll, G. Stollhoff, W. Becker, K. Kladko, T. Schork et al. The many-body ground-state wavefunction, binding energy etc. was calculated for numerous semiconductors, oxydes, van der Waals crystals etc. (with K. Doll, B. Paulus, M. Dolg, S. Kalvoda, J. Gräfenstein, K. Rosciszewski and others) [82, 93, 144, 159, 188, 191, 203, 211, 226, 230, 244]
  • The theory has been extended to the computation of energy bands in order to understand in detail the effects of correlations on them (with H. Stoll, P. Horsch, J. Gräfenstein, L. Hozoi and U. Birkenheuer). [90, 93, 119, 150, 177, 327]

Strongly correlated electrons

  • A number of microscopic processes were identified which lead to heavy-fermion behavior in strongly correlated electron systems: charge ordering like in Yb4As3, Zeeman effect like in       Nd2-xCexCuO4, partial localization of 5f electrons like in UPt3, magnetic frustration like in LiV2O4 (with G. Zwicknagl, V. Zevin, P. Thalmeier, B. Schmidt, A. Yaresko, H. Eschrig et al.). [173, 186, 190, 270, 273, 277]
  • The concept of renormalized band-structure theory was worked out (with J. Keller, H. Razafimandimby, N. d’Ambrumenil). The concept was expanded by E. Runge and G. Zwicknagl and developed into a powerful technique by G. Zwicknagl. [91, 100]
  • The theory of interaction of heavy quasiparticles with phonons, a hydrodynamic theory for heavy fermion systems, a scaling theory and other results for those systems were developed and described in a review which appeared in the Ehrenreich-Turnbull Series “Solid State Physics” with J. Keller and G. Zwicknagl. [129]
  • In a series of papers with J. Igarashi the dynamics of holes in quantum antiferromagnets was worked out and applied to the high-temperature superconductors in order to explain experiments on those materials. [161, 163, 171, 178]
  • A theory based on projection techniques was developed which allows for the computation of spectral densities of strongly correlated electrons. This way the satellite structure appearing in Ni could be explained satisfactorily. Earlier attempt were only partially successful (with Y. Kakehashi, J. Igarashi, P. Unger).
  • It was demonstrated that in molecules like Ce(C8H8)2 the valency of Ce is 3+ and not 4+ as suggested in textbooks. This resolved some puzzles of the material (with C.-S. Neumann, M. Dolg, W. Liu et al.). [156, 218]