| |
 |
Robert
Markiewicz
Professor
PhD University of California at Berkeley, 1975
(617)373-2916
markiewic@neu.edu
|
|
Research
Summary:
The
new high-temperature cuprate superconductors offer a rich
field for Professor Markiewicz's mixture of experimental
and theoretical research. His experimental studies, carried
on in close collaboration with B. C. Giessen (Department
of Chemistry and Barnett Institute), have included synthesis
of new materials (including the first four copper layer
superconductor, TlBa2Ca3Cu4Ox,
Tc=122K) and studies of flux-lattice melting
in strong magnetic fields.
Presently, Professor Markiewicz is working on the critical
current problem in these materials. The critical currents
in bulk samples of the cuprates are found to be extremely
low. This is due in part to simple misalignment of crystal
axes across a grain boundary, which leads to a weak link
connecton across the ensuing dislocation array. He and
Professor Giessen have patented a technique of aligning
two orthogonal axes of the individual grains in a powder
prior to sintering and they are now exploring how biaxial
alignment can be combined with partial-melt-regrowth procedures
for improving critical currents.
In theory, Professor Markiewicz is currently exploring
the possibility that high-temperature superconductivity
is associated with peaks in the density of states, due
to the proximity of a saddle point van Hove singularity
to the Fermi level in these cuprates, which has recently
been confirmed by photoemission measurements. An interesting
outcome of this research is to show that there is an extremely
large electron-phonon coupling associated with the saddle
points, and hence there should be a competition between
superconductivity and structural disorder. The strong
electron-phonon coupling can easily be understood in terms
of the Jahn-Teller effect, with the electronic degeneracy
due to the presence of two van Hove singularities. In
the La2-xAxCuO4 (A=Ba,
Sr) superconductors, low-temperature tetragonal and orthorhombic
phases may be related to static or dynamic Jahn-Teller
phases. This structural instability may be the origin
of the pseudogap found when these materials are underdoped.
This research is a natural extension of Professor Markiewicz's
earlier research on anomalous electronic states in predominantly
two-dimensional systems. This work included studies of
electron-hole droplets in germanium; localization, interaction,
and quenching of superconductivity in ultrathin metal
films; and the crossover to two-dimensional electron gases
in intercalated graphites as a function of magnetic field.
Recent Publications :
- R.S. Markiewicz, and A. Bansil, "Dispersion anomalies induced by the low-energy plasmon in the cuprates", Phys. Rev. B 75, 020508 (2007). http://dx.doi.org/10.1103/PhysRevB.75.020508
-
Tanmoy Das, R.S. Markiewicz, and A. Bansil, "Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-δ" Phys. Rev. Lett. 98, 197004 (2007). http://dx.doi.org/10.1103/PhysRevLett.98.197004
- H. Lin, S. Sahrakorpi, R.S. Markiewicz, and A. Bansil, "Raising Bi-O Bands above the Fermi Energy Level of Hole-Doped Bi2Sr2CaCu2O8+δ and Other Cuprate Superconductors", Phys. Rev. Lett. 96, 097001 (2006). http://dx.doi.org/10.1103/PhysRevLett.96.097001
- R.S. Markiewicz and A. Bansil, "Collapse of the Magnetic Gap of Cuprate Superconductors within a Three-Band Model of Resonant Inelastic X-Ray Scattering", Phys. Rev. Lett. 96, 107005. (2006).http://dx.doi.org/10.1103/PhysRevLett.96.107005
- S. Sahrakorpi, M. Lindroos, A. Bansil, and R.S. Markiewicz, "Evolution of Mid-gap States and Residual Three Dimensionality in La2-xSrxCuO4", Phys. Rev. Lett. 95, 157601 (2005). http://dx.doi.org/10.1103/PhysRevLett.95.157601
|
