非均匀外磁场下海森堡自旋链的热纠缠

研究了两量子比特海森堡XXX自旋链处于x方向的非均匀磁场时系统的纠缠特性，并用负度N来度量.得到N的解析表达式，并在此基础上进行数值计算.仔细讨论了均匀磁场B、非均匀磁场b、温度T和自旋耦合系数J对纠缠度N的影响.结果表明:N会随着■和T的增大而减小，但会随着J的增大而增大.同时，增大的J和b还会使临界磁场■和临界温度T_th变大，从而使系统中热纠缠存在的磁场范围和温度范围都变大.这一点在较大磁场和较高温度下需要纠缠具有实际意义.由此，我们可以通过调节B、b、T和J来控制热纠缠，这对固态系统中通过构建和选择参数调整系统的纠缠度具有一定的作用和意义.

Thermal Entanglement in a Heisenberg Spin Chain with Nonuniform External Magnetic Field

The thermal entanglement of two-qubit Heisenberg XXX spin chain in x direction nonuniform external magnetic field is investigated by negativity measure of entanglement N. The influences of the uniform external magnetic field B, the nonuniform external magnetic field b, the temperature T and the spin coupling coefficient J on entanglement N are discussed in detail. The results show that the entanglement N decreases with the increase of |B| and T, but increases with the increasing J. Simultaneously, the critical magnetic field |Bc| and critical temperature Tth also increase when J and b increase. As a result, the thermal entanglement can be controlled by adjusting the values of B, b, T and J in various terrible environments, such as in strong external magnetic field, or high temperature environment, which will be useful in the research of quantum information in solid systems.