Quantum Hall Structures Research Articles

Contactless measurement of alternating current conductance in quantum Hall structures

We report a procedure to determine the frequency-dependent conductance of quantum Hall structures in a broad frequency domain. The procedure is based on the combination of two known probeless methods—acoustic spectroscopy and microwave spectroscopy. By using the acoustic spectroscopy, we study the low-frequency attenuation and phase shift of a surface acoustic wave in a piezoelectric crystal in the vicinity of the electron (hole) layer. The electronic contribution is resolved using its dependence on a transverse magnetic field. At high frequencies, we study the attenuation of an electromagnetic wave in a coplanar waveguide. To quantitatively calibrate these data, we use the fact that in the quantum-Hall-effect regime the conductance at the maxima of its magnetic field dependence is determined by extended states. Therefore, it should be frequency independent in a broad frequency domain. The procedure is verified by studies of a well-characterized p-SiGe/Ge/SiGe heterostructure.

Photogalvanic effects originating from the violation of the Einstein relation in a 2D electron gas in high Landau levels

Recent experiment [S.I. Dorozhkin et al., Phys. Rev. Lett. 102 (2009) 036602] on quantum Hall structures with strongly asymmetric contact configuration discovered microwave-induced photocurrent and photovoltage magnetooscillations in the absence of dc driving. We show that in an irradiated sample the Landau quantization leads to violation of the Einstein relation between the dc conductivity and diffusion coefficient. Then, in the presence of a built-in electric field in a sample, the microwave illumination causes photo-galvanic signals which oscillate as a function of magnetic field with the period determined by the ratio of the microwave frequency to the cyclotron frequency, as observed in the experiment.

Theory of microwave-induced photocurrent and photovoltage magneto-oscillations in a spatially nonuniform two-dimensional electron gas

Recent experiment [S. I. Dorozhkin et al., Phys. Rev. Lett. 102, 036602 (2009)] on quantum Hall structures with strongly asymmetric contact configuration discovered microwave-induced photocurrent and photovoltage magneto-oscillations in the absence of dc driving. We show that in an irradiated sample the Landau quantization leads to violation of the Einstein relation between the dc conductivity and diffusion coefficient. Then, in the presence of a built-in electric field in a sample, the microwave illumination causes photogalvanic signals which oscillate as a function of magnetic field with the period determined by the ratio of the microwave frequency to the cyclotron frequency, as observed in the experiment.

Transport measurement of two-dimensional electron system under ultra-high magnetic fields

We have developed the RF technique for transport measurement under pulsed ultra-high magnetic fields beyond 100 T . Using this RF system and the samples with modified Corbino geometry, we have successfully measured the transverse conductivity σ xx of two-dimensional electron gas up to 100 T . In addition to the integer quantum Hall structures, we observed a structure corresponding to the 1 3 fractional quantum Hall effect (FQHE) around 80 T even at high temperatures (>4.2 K) and even in the low-mobility samples. This fact suggests that the ultra-high magnetic field stabilizes the 1 3 FQHE state.

Spin symmetry breaking in bilayer quantum Hall systems

Based on the construction of generalized Halperin wave functions, we predict the possible existence of a large class of broken spin symmetry states in bilayer quantum Hall structures, generalizing the recently suggested canted antiferromgnetic phase to many fractional fillings. We develop the appropriate Chern-Simons theory, and establish explicitly that the low-lying neutral excitation is a Goldstone mode and that the charged excitations are bimerons with continuously tunable (through the canted antiferromagnetic order parameter) electric charge on the individual merons.

Comment on “Localization and Metal-Insulator Transition in Multilayered Quantum Hall Structures”

A Comment on the Letter by Ziqiang Wang, Phys. Rev. Lett. 79, 4002 (1997). The authors of the Letter offer a Reply.