G. Yu

Transport properties of AlGaAs/GaAs parabolic quantum wells

We report on the magnetotransport properties of 100 nm wide parabolic quantum wells and observe an enhancement of the Hall resistance in one sample but not the other. This phenomenon is likely related to the effective thickness of the electronic slab. We also observe a parabolic negative magnetoresistance originating from electron-electron interactions when only one subband is occupied in one of the samples. The interaction correction to the Drude conductivity is extracted using two methods. We find that the extracted interaction correction increases with increasing tilted angle, for which two possible explanations are given.

Field sensing in MgO double barrier magnetic tunnel junctions with a superparamagnetic Co50Fe50 free layer

Linear response and low frequency noise have been investigated in MgO double barrier magnetic tunnel junctions with a superparamagnetic Co50Fe50 free layer. Linear and hysteresis-free switching was observed for the Co50Fe50 thickness tu2009≤u20091u2009nm. A tunneling magnetoresistance ratio of up to 108% and large magnetic field sensitivity value of 61%/mT were obtained at room temperature when tu2009=u20091.0u2009nm. The angular dependence of magnetoresistance suggests that weak coupling between superparamagnetic islands in a 1.0u2009nm free layer permits continuous rotation of magnetization, whereas the islands in a 0.8u2009nm layer switch rather independently. The frequency dependence of noise power spectrum density and field dependence of Hooge parameter (α) also behave differently for junctions with 0.8 and 1.0u2009nm free layers. The noise sensitivity of 1.0u2009nm free layer junctions is independent of bias, and it is estimated to reach 400 pT/Hz0.5 at 500u2009kHz.

Weak antilocalization and beating pattern in high electron mobility AlxGa1−xN/GaN two-dimensional electron gas with strong Rashba spin-orbit coupling

The weak antilocalization (WAL) effects of the two-dimensional electron gas (2DEG) in high mobility AlxGa1−xN/GaN heterostructure as well as beating patterns in the Shubnikov–de Haas (SdH) oscillatory magnetoresistance have been investigated by means of magnetotransport measurements before and after illumination. The zero-field spin splitting mainly arising from the Rashba spin-orbit coupling effect is studied using the weak antilocalization and beating patterns analysis, respectively. The Rashba spin-orbit coupling constant α deduced using the weak antilocalization analysis showed a good agreement with that estimated from the analysis of the beating patterns for the sample before and after illumination. For our sample, the electron motion in the high mobility system is in the ballistic regime, the experimental WAL curves were fitted by a simulated quantum conductance correction according to a model proposed by [Golub [Phys. Rev. B 71, 235310 (2005)].

Insulator-quantum Hall conductor transition in high electron density gated InGaAs/InAlAs quantum wells

We study the insulator-quantum Hall conductor transition in two high-density gated InGaAs/InAlAs quantum well samples. We observe a well-defined critical magnetic field and verify this marks a genuine phase transition by investigating the scaling behavior of the longitudinal resistivity with field and temperature at fixed electron density. Consistent with prevailing experimental results the critical field decreases with increasing electron density in one sample (QW0710). In the other sample (QW0715), with higher delta doping density, however, we unexpectedly find that the critical field increases with increasing electron density. This unexpected behavior may be the result of the system entering the classical percolation regime.

Tunneling processes in asymmetric double barrier magnetic tunnel junctions with a thin top MgO layer

Dynamic conductance dI/dV and inelastic electron tunneling spectroscopy (IETS) d 2I/dV 2 have been measured at different temperatures for double barrier magnetic tunnel junctions with a thin top MgO layer. The resistance in the antiparallel state exhibits a normal tunnel-like behavior, while the resistance in the parallel state shows metallic-like transport, indicating the presence of pinholes in the thin top MgO layer. Three IETS peaks are the zero-bias anomaly, interface magnons, and barrier phonons in both the parallel and antiparallel states. The zero-bias anomaly is the strongest peak in the parallel state and its intensity decreases with temperature. The magnon has the largest intensity in the antiparallel state and its intensity also decreases with temperature. The origins of the dips and peaks in the dI/dV-V curve are also discussed.

Influence of the illumination on weak antilocalization in an AlxGa1−xN∕GaN heterostructure with strong spin-orbit coupling

The weak antilocalization effects of the two-dimensional electron gas in a high mobility AlxGa1−xN∕GaN heterostructure have been investigated by means of magnetotransport measurements before and after illumination. The zero-field spin splitting mainly arising from the Rashba spin-orbit coupling effect as a function of electron concentration as well as a function of temperature is studied using the weak antilocalization analysis. The Rashba spin-orbit coupling constant α deduced using the weak antilocalization analysis shows a rapid decrease with the increase of the measured electron concentration.

Spin-dependent tunneling spectroscopy in MgO-based double-barrier magnetic tunnel junctions

We investigated the dynamic conductance and inelastic electron tunneling spectroscopy in MgO-based double barrier magnetic tunnel junctions with Co50Fe50/Co40Fe40B20 hybrid free layers. The tunneling is coherent through the MgO (001) barriers but nonresonant, and the highest tunneling magnetoresistance reaches 260% at 2u2009K. Based on the detailed discussion of the tunneling mechanisms, the double-barrier junctions investigated here can be considered as two single-barrier junctions in series.

Transport properties of a spin-split two-dimensional electron gas in an In0.53Ga0.47As∕InP quantum well structure

We study the magnetotransport properties of a gated In0.53Ga0.47As∕InP quantum well structure in the presence of spin splitting when only one electronic subband is occupied. We develop an analytical method to extract the quantum mobilities for the two spin subbands. Ionized impurity scattering and alloy disorder scattering are determined to be important in this system. Larger quantum mobility is found for the higher-energy spin subband. We also demonstrate that the difference between the quantum mobilities for the two spin subbands can be altered with the gate.

The nonlinear Rashba effect in Hg0.77Cd0.23Te inversion layers probed by weak antilocalization analysis

The Rashba spin-orbit interaction of the two-dimensional electron gas with high mobility in the inversion layer of p-type Hg0.77Cd0.23Te is investigated by magnetotransport measurements. Both the Rashba spin splitting and Rashba coefficient are extracted by analysis of the weak anti-localization effect using the Golub model. It is found that both the splitting and coefficient increase with increasing electron density (∼3.0–6.0u2009×u20091015u2009m−2), i.e., with the gate voltage. A self-consistent Schrodinger-Poisson calculation is performed and suggests that the nonlinear Rashba effect caused by the weakening of interband coupling, especially at high electron density, dominates this system.

Experimental approaches to zero-field spin splitting in a gated high-mobility In0.53Ga0.47As/InP quantum well structure: Weak antilocalization and beating pattern

We report on the strong spin-orbit (SO) interaction in a gated high-mobility In0.53Ga0.47As/InP quantum well two-dimensional electron gas. We establish that the SO interaction is dominated by the Rashba mechanism. The Rashba coupling parameters determined from analysis of both weak antilocalization and the beating pattern in the Shubnikov–de Haas oscillations are in reasonable agreement, and the small difference between them was explained by a magnetic-field-dependent effective g factor. The zero-field spin splitting shows nonmonotonic behavior with a maximum as the electron density is varied with the applied gate voltage. This is related to strong Rashba SO coupling in our sample.