arXiv subjects
Publications and source records attributed to A. A. Shashkin.
Thermodynamic measurements reveal that the Pauli spin susceptibility of strongly correlated two-dimensional electrons in silicon grows critically at low electron densities - behavior that is characteristic of the existence of a phase transition.
We study the interaction-enhanced spin gaps in the two-dimensional electron gas confined in GaAs/AlGaAs single heterojunctions subjected to weak magnetic fields. The values are obtained from the chemical potential jumps measured by magnetocapacitance. The gap increase with parallel magnetic field indicates that the lowest-lying charged excitations are accompanied with a single spin flip at the odd-integer filling factor nu=1 and nu=3, in disagreement with the concept of skyrmions.
Experimental results on the metal-insulator transitions and the anomalous properties of strongly interacting two-dimensional electron systems are reviewed and critically analyzed. Special attention is given to recent results for the strongly enhanced spin susceptibility and effective mass in low-disordered silicon MOSFETs.
We show that the Comment by Reznikov and Sivan (cond-mat/0410409) is erroneous because the authors do not distinguish between Pauli and Curie spin susceptibility.
We show that the comparison between theory and experiment, performed by Pudalov et al. in PRL 91, 126403 (2003), is not valid.
The small, about 30% magnetoresistance at the onset of full spin polarization in the 2D electron system in a modulation-doped Si/SiGe quantum well gives evidence that it is the remote doping that determines the transport scattering time. Measurements of the mobility in this strongly-interacting electron system with remote-doping scattering allow us to arrive at a conclusion that the Hubbard form underestimates the local field corrections by about a factor of 2.
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.
Using magnetocapacitance data in tilted magnetic fields, we directly determine the chemical potential jump in a strongly correlated two-dimensional electron system in silicon when the filling factor traverses the spin and the cyclotron gaps. The data yield an effective g-factor that is close to its value in bulk silicon and does not depend on filling factor. The cyclotron splitting corresponds to the effective mass that is strongly enhanced at low electron densities.
We have accurately measured the effective mass in a dilute two-dimensional electron system in silicon by analyzing temperature dependence of the Shubnikov-de Haas oscillations in the low-temperature limit. A sharp increase of the effective mass with decreasing electron density has been observed. Using tilted magnetic fields, we have found that the enhanced effective mass is independent of the degree of spin polarization, which points to a spin-independent origin of the mass enhancement and is in contradiction with existing theories.
Using magnetocapacitance data, we directly determine the chemical potential jump in a strongly correlated 2D electron system in silicon when the filling factor traverses the valley gap at ν=1 and ν=3. The data yield a valley gap that is strongly enhanced compared to the single-particle value and increases LINEARLY with magnetic field. This result has not been explained by existing theories.
In a recent Letter (PRL 88, 196404 (2002)), the effective g-factor and the effective mass have been studied by Pudalov et al. in a dilute 2D electron system in silicon. The authors contrast their data with the data obtained by our group (Shashkin et al., PRL 87, 086801 (2001)) and claim that the spin susceptibility "increases gradually with decreasing density", which "does not support the occurrence of spontaneous spin polarization and divergence of gm..." The purpose of this Comment is to show that all available experimental data, including those of Pudalov et al., are consistent with each other and are in favor of a spontaneous spin polarization and divergence of gm at a finite electron density in this 2D system.
We study the plateaux of the integer quantum Hall resistance in a bilayer electron system in tilted magnetic fields. In a narrow range of tilt angles and at certain magnetic fields, the plateau level deviates appreciably from the quantized value with no dissipative transport emerging. A qualitative account of the effect is given in terms of decoupling of the edge states corresponding to different electron layers/Landau levels.
The critical electron density for the metal-insulator transition in a two-dimensional electron gas can be determined by two distinct methods: (i) a sign change of the temperature derivative of the resistance, and (ii) vanishing activation energy and vanishing nonlinearity of current-voltage characteristics as extrapolated from the insulating side. We find that in zero magnetic field (but not in the presence of a parallel magnetic field), both methods give equivalent results, adding support to the existence of a true zero-field metal-insulator transition.
We find that at intermediate temperatures, the metallic temperature dependence of the conductivity σ(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 dσ/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.
The magnetic field B_c, in which the electrons become fully spin-polarized, is found to be proportional to the deviation of the electron density from the zero-field metal-insulator transition in a two-dimensional electron system in silicon. The tendency of B_c to vanish at a finite electron density suggests a ferromagnetic instability in this strongly correlated electron system.
We measure the Hall conductivity, $σ_{xy}$, on a Corbino geometry sample of a high-mobility AlGaAs/GaAs heterostructure in a pulsed magnetic field. At a bath temperature about 80 mK, we observe well expressed plateaux in $σ_{xy}$ at integer filling factors. In the pulsed magnetic field, the Laughlin condition of the phase coherence of the electron wave functions is strongly violated and, hence, is not crucial for $σ_{xy}$ quantization.
We study the transport properties of the two-dimensional electron gas in AlGaAs/GaAs heterostructures in parallel to the interface magnetic fields at low temperatures. The magnetoresistance in the metallic phase is found to be positive and weakly anisotropic with respect to the orientation of the in-plane magnetic field and the current through the sample. At low electron densities ($n_s< 5\times 10^{10}$ cm$^{-2}$) the experimental data can be described adequately within spin-related approach while at high $n_s$ the magnetoresistance mechanism changes as inferred from $n_s$-independence of the normalized magnetoresistance.
We have studied Shubnikov-de Haas oscillations in a two-dimensional electron system in silicon at low electron densities. Near the metal-insulator transition, only "spin" minima of the resistance at Landau-level filling factors 2, 6, 10, and 14 are seen, while the "cyclotron" minima at filling factors 4, 8, and 12 disappear. A simple explanation of the observed behavior requires a giant enhancement of the spin splitting near the metal-insulator transition.