Browsing by Author "Liu, Xin"
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Item Calculation of Steady-State Evaporation for an Arbitrary Matric Potential at Ground Surface(2014-12-15) Liu, XinIn the past few decades, many researchers have studied evaporation from soil columns in the presence of a water table. Several water retention functions have been developed to describe the water flow behavior in the field environment. While most studies involving these functions focus on analysis of water flow and solute transport in variably saturated porous media, there is a limited amount of research to estimate the evaporation rate at bare ground surface for an arbitrary matric potential head. In previous studies, Jury and Horton proposed a method of calculating the potential evaporation rate above a water table on the basis of the Haverkamp unsaturated hydraulic conductivity equation and an assumption that the potential evaporation rate is much less than the saturated hydraulic conductivity of the soil. In this thesis, I developed a new method to estimate the evaporation rate for an arbitrary matric potential head at bare soil surface. I also presented two programs to calculate evaporation rates for a wide range of depths and the fitting parameters of the Haverkamp equation. The results show that the evaporation rates calculated by this thesis fit well with the experimental data and can reproduce the result of potential evaporation rate calculated from previous equation under the special condition of an infinite matric potential head at bare soil surface.Item Spin Dynamics in the Presence of Spin-orbit Interactions: from the Weak to the Strong Spin-orbit Coupling Regime(2012-10-19) Liu, XinWe study the spin dynamics in a high-mobility two dimensional electron gas (2DEG) system with generic spin-orbit interactions (SOIs). We derive a set of spin dynamic equations which capture the purely exponential to the damped oscillatory spin evolution modes observed in different regimes of SOI strength. Hence we provide a full treatment of the D'yakonov-Perel's mechanism by using the microscopic linear response theory from the weak to the strong SOI limit. We show that the damped oscillatory modes appear when the electron scattering time is larger than half of the spin precession time due to the SOI, in agreement with recent observations. We propose a new way to measure the scattering time and the relative strength of Rashba and linear Dresselhaus SOIs based on these modes and optical grating experiments. We discuss the physical interpretation of each of these modes in the context of Rabi oscillation. In the finite temperature, We study the spin dynamics in the presence of impurity and electron-electron (e-e) scattering in a III-V semiconductor quantum well. Starting from the Keldysh formalism, we develop the spin-charge dynamic equation at finite temperature in the presence of inelastic scattering which provide a new approach to describe the spin relaxation from the weak to the strong spin-orbit coupling (SOC) regime. In the weak SOC regime, our theory shows that when the system is near the SU(2) symmetry point, because the spin relaxation due to DP mechanism is suppressed dramatically, the spin relaxation is dominated by the Elliott-Yafet (EY) mechanism in a wide temperature regime. The non-monotonic temperature dependence of enhanced-lifetime of spin helix mode is due to the competition between the DP and EY mechanisms. In the strong SOC regime, the our theory is consistent to the previous theoretical results at zero temperature.