Large positive magnetoresistance and resistivity law at high temperatures

Abstract

This dissertation describes the measurement on the electron transport in two-dimensional GaAs electron systems. We report on the transport properties of electron gas systems of AlGaAs/GaAs heterostructure in the presence of a perpen-dicular magnetic field B.

1. Resistivity law in high-mobility two-dimensional GaAs electron systems:
There are many physical quantities that may be studied in a two-dimensional elec-tron gas (2DEG), and one of the most popular characteristics is magnetoresistance (MR). The two components of quantum Hall transport coefficient ρxx and ρxy in high-mobility 2DEG had been found to be related by ρxx=αB(dρxy /dB), called “resis-tivity law”. The work we do is to study the temperature and mobility effects on scal-ing parameter α at high temperatures in a standard 2DEG system of GaAs/AlGaAs heterostructure. The high mobility samples (sample T475-2 with a mobility μ =239 m2/Vs and sample T479-4 with a mobilityμ =512 m2/Vs at 0.3 K) we used are fab-ricated by molecular beam epitaxy (MBE). The measurement was performed in a top-loading 3He cryostat over the temperature 0.3K~80 K. Four-terminal magnetore-sistance was measured by using standard phase-sensitive lock-in techniques with Hall bar and Ohmic contact technique. We estimate the scaling factor α of the resistivity law at temperature T= 20~80 K under the magnetic field not too low (B> 0.5 T) and find it varying with temperature, consistent with the work of Simon and Halperin in 1994.

2. The explanation and the transport behavior of resistivity law in two-dimensional electron gas system:
Since we have investigated the high-temperature properties of resistivity law in high-mobility 2DEG systems, another work we do is to study resistivity law in a low-mobility 2DEG system. The low-mobility 2DEG system (sample C1335 with a mobility μ=0.804 m2/Vs at T=0.3 K) is formed at the interface of GaAs/AlGaAs heterostructure with self-assemble InAs quantum dots. Furthermore, we discuss the theoretical explanation and details of electron transport of resistivity law that also suggested by Simon and Halperin in 1994. Their argument is based on high-mobility 2DEG systems, i.e. ρxy is much larger than ρxx with percolation network. We compare our result with their conclusion.