Theoretical Study of Friction

我们研究了在周期势除杂质外的二维模型中的摩擦现象。该模型也适用于薄膜超导体中的磁通晶格。结果表明，杂质可以减小静摩擦力，即使在存在杂质的情况下，运动晶格中也存在一种准长程有序等现象。对于任意大小的速度，这个顺序都存在于横向，并导致横向钉扎，即即使在横向上施加外力，运动方向也不变。典型的长度尺度随着速度的增加而增大，并导致动摩擦力和横向钉扎力的特殊行为。由于非公度结构的改变，后者的速度依赖关系变得非单调。实验中观察到了宏观量子隧穿的表现形式--CDW非线性电导。实验采用双探针法。为了用这种方法观察隧道电流，电极附近的CDW和体内的CDW都必须通过量子隧道来移动。我们把涡环的形成看作是电极附近的隧穿过程，把扭结-反扭结对的形成看作是块体中的隧穿过程，对实验进行了解释。我们计算了两种量子棘轮模型的流动和功，表明这两种体系的流动是与热浴相互作用产生的新的量子力学现象的表现。我们讨论了公度CDW的量子隧穿，表明杂质可以协助隧穿。

We have studied the frictional phenomena of a 2-dimensional model in the presence of the periodic potential in addition to impurities. This model can be applied also to the magnetic flux lattice in film superconductors. It turned out that impurity can diminishes the static frictional force, there is a kind of quasi long range order in the moving lattice even in the presence of impurities and so on. This order exists in the transverse direction for any magnitude of the velocity and cause the transverse pinning, that is, the direction of motion does not change even though we apply external force in the transverse direction. Typical length scale of the order grows with increasing velocity and cause a peculiar behavior of the kinetic frictional force and the transverse pinning force. The velocity dependence of the latter becomes nonmonotonic, which is caused by the change of the discommensurate structure.CDW nonlinear electric conduction which is the manifestation of the macroscopic quantum tunneling is observed in experiments. Two-probe method was employed in the experiments. In order to observe the tunneling currents by this method both CDW's near the electrode and in the bulk must move by quantum tunneling. We have explained the experiments by considering both vortex ring formation as the tunneling process near the electrode and the kink-antikink pair formation as that in the bulk.We have calculated the flow and the work of two kinds of quantum ratchet model and shown that the flow of these systems is the manifestation of the new quantum mechanical phenomena resulting from the interaction with the heat bath.We have discussed the quantum tunneling of the commensurate CDW and shown that the impurity can assist the tunneling.