Sharp increase of the effective mass near the critical density in a metallic 2D electron system

We find that at intermediate temperatures, the metallic temperature dependence of the conductivity \sigma(T) of 2D electrons in silicon is described well by a recent interaction-based theory of Zala et al. (Phys. Rev. B 64, 214204 (2001)). The tendency of the slope d\sigma/dT to diverge near the critical electron density is in agreement with the previously suggested ferromagnetic instability in this electron system. Unexpectedly, it is found to originate from the sharp enhancement of the effective mass, while the effective Lande g factor remains nearly constant and close to its value in bulk silicon

Role of a parallel magnetic field in two dimensional disordered clusters containing a few correlated electrons

An ensemble of 2d disordered clusters with a few electrons is studied as a function of the Coulomb energy to kinetic energy ratio r_s. Between the Fermi system (small r_s) and the Wigner molecule (large r_s), an interaction induced delocalization of the ground state takes place which is suppressed when the spins are aligned by a parallel magnetic field. Our results confirm the existence of an intermediate regime where the Wigner antiferromagnetism defavors the Stoner ferromagnetism and where the enhancement of the Lande g factor observed in dilute electron systems is reproduced.Comment: 4 pages, 3 figure

Hybridization of electron subbands in a double quantum well at quantizing magnetic field

We employ magnetocapacitance and far-infrared spectroscopy techniques to study the spectrum of the double-layer electron system in a parabolic quantum well with a narrow tunnel barrier in the centre. For gate-bias-controlled asymmetric electron density distributions in this soft two-subband system we observe both individual subband gaps and double layer gaps at integer filling factor $\nu$. The bilayer gaps are shown to be either trivial common for two subbands or caused by hybridization of electron subbands in magnetic field. We describe the observed hybrid gaps at $\nu=1$ and $\nu=2$ within a simple model for the modified bilayer spectrum.Comment: REVTeX, 24 pages, 9 figures included. Submitted to Phys. Rev.

Quantum phase transition in ultrahigh mobility SiGe/Si/SiGe two-dimensional electron system

The metal-insulator transition (MIT) is an exceptional test bed for studying strong electron correlations in two dimensions in the presence of disorder. In the present study, it is found that in contrast to previous experiments on lower-mobility samples, in ultra-high mobility SiGe/Si/SiGe quantum wells the critical electron density, $n_{\text{c}}$, of the MIT becomes smaller than the density, $n_{\text{m}}$, where the effective mass at the Fermi level tends to diverge. Near the topological phase transition expected at $n_{\text{m}}$, the metallic temperature dependence of the resistance should be strengthened, which is consistent with the experimental observation of more than an order of magnitude resistance drop with decreasing temperature below $\sim1$ K.Comment: Misprints corrected. As publishe

Sharply increasing effective mass: a precursor of the spontaneous spin polarization in a dilute two-dimensional electron system

We have measured the effective mass, m, and Lande g-factor in very dilute two-dimensional electron systems in silicon. Two independent methods have been used: (i) measurements of the magnetic field required to fully polarize the electrons' spins and (ii) analysis of the Shubnikov-de Haas oscillations. We have observed a sharp increase of the effective mass with decreasing electron density while the g-factor remains nearly constant and close to its value in bulk silicon. The corresponding strong rise of the spin susceptibility may be a precursor of a spontaneous spin polarization; unlike in the Stoner scenario, it originates from the enhancement of the effective mass rather than the increase of g-factor. Furthermore, using tilted magnetic fields, we have found that the enhanced effective mass is independent of the degree of spin polarization and, therefore, its increase is not related to spin exchange effects, in contradiction with existing theories. Our results show that the dilute 2D electron system in silicon behaves well beyond a weakly interacting Fermi liquid.Comment: This paper summarizes results reported in our recent publications on the subjec

Canted antiferromagnetic phase in a double quantum well in a tilted quantizing magnetic field

We investigate the double-layer electron system in a parabolic quantum well at filling factor $\nu=2$ in a tilted magnetic field using capacitance spectroscopy. The competition between two ground states is found at the Zeeman splitting appreciably smaller than the symmetric-antisymmetric splitting. Although at the transition point the system breaks up into domains of the two competing states, the activation energy turns out to be finite, signaling the occurrence of a new insulator-insulator quantum phase transition. We interpret the obtained results in terms of a predicted canted antiferromagnetic phase.Comment: 4 pages, 3 figures included, accepted to PR