Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

Suppression of localization in two dimensionally doped
semiconductors at half-filling
Saquib Shamim 1 , Suddhasatta Mahapatra 2 , Giordano Scapucci 2 , W. M. Klesse 2 , P.B.S.
Mahapatra 1 , Michelle Y. Simmons 2 and Arindam Ghosh 1
1 Department of Physics, Indian Institute of Science, Bangalore 560 012, India
2
Centre for Quantum Computation and Communication Technology, University of New South Wales, Sydney
NSW 2052, Australia
The nature of quantum states, and transport of electrons, in a two dimensional (2D) disordered solid
change significantly in the presence of a background periodic potential, such as that arising from the
host crystal lattice. The interplay of the electron wavelength and lattice periodicity leads to a number
of exotic phenomena, in particular close to half filling, that range from collapse of scaling theory and
Anderson localization, hole-mediated (Nagaoka) ferromagnetism, to new modes of quantum
transport. Much of these phenomena remain unexplored experimentally, primarily due to the lack of a
suitable material platform. Advances in material engineering now allow the dopants, such as
phosphorus (P), confined within one atomic layer inside bulk crystals of silicon (Si) and germanium
(Ge), leading to a unique two-dimensional (2D) electron system whose Fermi energy exists naturally
at or very close to the center of band. Here we demonstrate that quantum interference effect in 2D
Si:P and Ge:P is drastically different from conventional weakly localized 2D electron systems. This
is manifested in two unexpected ways: First, the quantum correction to low temperature conductivity
shows a strong weak antilocalization (WAL), in addition to the conventional weak localization (WL)
behaviour, and second, a spontaneous factor of two suppression in the universal conductance
fluctuations at low areal density of the dopants. We attribute these observations to the emergence of
new modes, called the π-Cooperons and π-Diffusons, of quantum diffusion at half-filled bands, where
electrons propagate through ‘umklapp scattering’ in the presence of a nested of Fermi surface and the
resulting particle-hole symmetry.