Publications

The magnetoresistance, R{sub xx}, at even-denominator fractional fillings, of an ultra high quality two-dimensional electron system at T {approx} 35 mK is observed to be strictly linear in magnetic field, B. While at 35 mK R{sub xx} is dominated by the integer and fractional quantum Hall states, at T {approx_equal} 1.2 K an almost perfect linear relationship between R{sub xx} and B emerges over the whole magnetic field range except for spikes at the integer quantum Hall states. This linear R{sub xx} cannot be understood within the Composite Fermion model, but can be explained through the existence of a density gradient in our sample.

We have investigated the valley splitting of two-dimensional electrons in high-quality Si/Si{sub 1-x}Ge{sub x} heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor {nu} = 3 ({Delta}{sub 3}) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear dependence of {Delta}{sub 3} on the electron density was observed, while the slope of these two configurations differs by more than a factor of 2. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of {Delta}{sub 3} before and after the coincidence.

Abstract not provided.

We have observed quantization of the diagonal resistance, Rxx, at the edges of several quantum Hall states. Each quantized Rxx value is close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, Rxy. Peaks in Rxx occur at different positions in positive and negative magnetic fields. Practically all Rxx features can be explained quantitatively by a 1%/cm electron density gradient. Therefore, Rxx is determined by Rxy and unrelated to the diagonal resistivity ρxx. Our findings throw an unexpected light on the empirical resistivity rule for 2D systems. © 2005 The American Physical Society.

Abstract not provided.

The apparent metal-insulator transition is observed in a high-quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si/Si{sub 1-x}Ge{sub x} heterostructure with mobility {mu} = 1.9 x 10{sup 5} cm{sup 2}/V s at density n = 1.45 x 10{sup 11} cm{sup -2}. The critical density, at which the thermal coefficient of low T resistivity changes sign, is -0.32 x 10{sup 11} cm{sup -2}, a very low value obtained in Si-based 2D systems. The in-plane magnetoresistivity {rho}(B{sub ip}) was measured in the density range, 0.35 x 10{sup 11} < n < 1.45 x 10{sup 11} cm{sup -2}, where the 2DES shows the metallic-like behavior. It first increases and then saturates to a finite value {rho}(B{sub c}) for B{sub ip}>B{sub c} , with B{sub c} the full spin-polarization field. Surprisingly, {rho}(B{sub c})/{rho}(0)-1.8 for all the densities, even down to n = 0.35 x 10{sup 11} cm{sup -2}, only 10% higher than n{sub c}. This is different from that in clean Si metal-oxide-semiconductor field-effect transistors, where the enhancement is strongly density dependent and {rho}(B{sub c})/{rho}(0) appears to diverge as n {yields} n{sub c}. Finally, we show that in the fully spin-polarized regime, dependent on the 2DES density, the temperature dependence of {rho}(B{sub ip}) can be either metallic-like or insulating.

Magnetotransport properties are studied in a high-mobility 2DES in the strained Si quantum well. We observe around ν = 1/2 the two-flux composite fermion (CF2) series of the FQHE states at ν = 2/3, 3/5, 4/7, and at ν = 4/9, 2/5, 1/3. Of the CF series, the ν = 3/5 state is weaker than the nearby 4/7 state and the 3/7 state is missing, resembling the observation that the ν = 3 is weaker than the ν = 4 state. Our data indicate that the CF model still applies for the multivalley Si/SiGe system when taking into account the two-fold valley degeneracy. © World Scientific Publishing Company.

The high mobility of two dimensional electron system in the second Landau level was discussed. In the second level, the larger extent of the wave function as compared to the lowest LL and its additional zero allows for a much broader range of electron correlations to be favorable. An example of electron correlations encountered in the second LL is the even-denominator v=2+1/2 fractional quantum hall effect (FQHE) state. With a varying filling factor, it was observed that quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters.

Cyclotron resonance at the microwave frequency is used to measure the band edge mass (m{sub b}) in the two-dimensional hole (2DH) system, confined in 30 nm quantum wells in the Al{sub 0.1}Ga{sub 0.9}As/GaAs/Al{sub 0.1}Ga{sub 0.9}As heterostructures. We find that for 2DH density p {le} 1.0 x 10{sup 10} cm{sup -2}, m{sub b} is nearly constant, {approx}0.35m{sub e}. It increases with increasing density, to {approx}0.5m{sub e} at p = 7.4 x 10{sup 10} cm{sup -2}.

In the second Landau level around {nu} = 5/2 filling of an extremely high quality 2D electron system and at temperatures T down to 9 mK we observe a very strong even-denominator fractional quantum Hall effect at Landau level filling {nu} = 5/2 and its energy gap is large and {Delta} {approx} 0.45 K. A clear FQHE state is seen at {nu} = 2+2/5, with well-quantized R{sub xy}. A novel, even denominator FQHE state at {nu} = 2+3/8 seems to develop, as deduced from the T-dependence of dR{sub xy}/dB. In addition, four fully developed re-entrant integral quantum Hall effect (RIQHE) states are also observed. At low temperatures, the wide RIQHE plateau around at {nu} = 2+2/7 is interrupted by a dip, indicating an additional reentrance. Finally, the tilted magnetic field experiment at an ultra-low temperature of 10 mK was carried out to examine the spin-polarization of the {nu} = 5/2 FQHE state.