15 research outputs found
Jahn-Teller polarons and their superconductivity in a molecular conductor
We present a theoretical study of a possibility of superconductivity in a
three dimensional molecular conductor in which the interaction between
electrons in doubly degenerate molecular orbitals and an {\em intra}molecular
vibration mode is large enough to lead to the formation of
Jahn-Teller small polarons. We argue that the effective polaron-polaron
interaction can be attractive for material parameters realizable in molecular
conductors. This interaction is the source of superconductivity in our model.
On analyzing superconducting instability in the weak and strong coupling
regimes of this attractive interaction, we find that superconducting transition
temperatures up to 100 K are achievable in molecular conductors within this
mechanism. We also find, for two particles per molecular site, a novel Mott
insulating state in which a polaron singlet occupies one of the doubly
degenerate orbitals on each site. Relevance of this study in the search for new
molecular superconductors is pointed out.Comment: Submitted to Phys. Rev.
Exact asymptotic form of the exchange interactions between shallow centers in doped semiconductors
The method developed in [L. P. Gor'kov and L. P. Pitaevskii, Sov. Phys. Dokl.
8, 788 (1964); C. Herring and M. Flicker, Phys. Rev. 134, A362 (1964)] to
calculate the asymptotic form of exchange interactions between hydrogen atoms
in the ground state is extended to excited states. The approach is then applied
to shallow centers in semiconductors. The problem of the asymptotic dependence
of the exchange interactions in semiconductors is complicated by the multiple
degeneracy of the ground state of an impurity (donor or acceptor) center in
valley or band indices, crystalline anisotropy and strong spin-orbital
interactions, especially for acceptor centers in III-V and II-VI groups
semiconductors. Properties of two coupled centers in the dilute limit can be
accessed experimentally, and the knowledge of the exact asymptotic expressions,
in addition to being of fundamental interest, must be very helpful for
numerical calculations and for interpolation of exchange forces in the case of
intermediate concentrations. Our main conclusion concerns the sign of the
magnetic interaction -- the ground state of a pair is always non-magnetic.
Behavior of the exchange interactions in applied magnetic fields is also
discussed
Donor states in modulation-doped Si/SiGe heterostructures
We present a unified approach for calculating the properties of shallow
donors inside or outside heterostructure quantum wells. The method allows us to
obtain not only the binding energies of all localized states of any symmetry,
but also the energy width of the resonant states which may appear when a
localized state becomes degenerate with the continuous quantum well subbands.
The approach is non-variational, and we are therefore also able to evaluate the
wave functions. This is used to calculate the optical absorption spectrum,
which is strongly non-isotropic due to the selection rules. The results
obtained from calculations for Si/SiGe quantum wells allow us to
present the general behavior of the impurity states, as the donor position is
varied from the center of the well to deep inside the barrier. The influence on
the donor ground state from both the central-cell effect and the strain arising
from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure
Zeeman splitting of excited boron states in p-Ge
The photothermal spectrum of shallow acceptors in p-Ge has been investigated at various magnetic field strengths up to 5.6 T at a temperature of 7.5 K by FIR-Fourier-spectroscopy. From the observed Zeeman splittings of the excited states of the boron acceptor the coefficients of the linear and quadratic field dependence have been evaluated andg-factors of theD-,C- and theG-transitions have been determined based on a standard group theoretical approach