摘要:
采用随机级数展开的量子蒙特卡罗方法研究二维硬核的玻色-赫伯德模型的热力学性质.首先通过算符变换将模型映射成为二维反铁磁准海森伯模型.变换后的模型比通常的海森伯模型多一项,该项正比于系统的格点总数N,对于大粒子数的系统,该项使模拟耗时指数增加,所以难以计算大粒子数系统.采用非局域操作循环更新后,这个困难可以得到很好的解决,可使粒子数总数增大到几千个.研究结果表明,粒子数密度在0—0.5范围内增大时,能量呈递减趋势,并趋于某一定值,随着正方晶格系统尺度增大,能量也随之增大;正方晶格系统尺度一定时,能量和磁化强度随着温度的升高而增大,化学势的变化对能量和磁化强度没有影响,能量随着正方晶格系统尺度增大而增大,磁化强度却随之减小;正方晶格系统尺度一定时,化学势的增大对比热没有影响,随着温度的升高比热出现先增大后减小的趋势,最后趋于某个值,达到平衡,而正方晶格系统尺度越大,比热曲线增大部分的趋势越大,减小部分的趋势也更明显,参照朗道超流理论,本文模拟的能量和比热曲线趋势与朗道二流体模型下HeⅡ的理论研究一致;不同正方晶格系统尺度的影响不大,均匀磁化率倒数在0—0.5（J/k_B）的低温范围内有很小的波动,J为耦合能,k_B为玻尔兹曼常数,温度在0.5—2（J/k_B）的范围内,均匀磁化率的倒数随着温度的升高而增大,且曲线的趋势显示了一种类似近藤行为.
Abstract:
In this paper,the stochastic series expansion quantum Monte Carlo method is employed to investigate the thermodynamic properties of hardcore Bose-Hubbard model in two-dimensional space.The two-dimensional hardcore Bose-Hubbard model can be mapped into the two-dimensional antiferromagnetic quasi-Heisenberg model under transform of bosonic operators.There is an additional term which is proportional to the total number of sites compared with real Heisenberg model and it is difficult for simulation.Using a nonlocal ＂operator-loop update＂,it allows one to simulate thousands of sites.Our simulation results show that,first,energy decreases with the increase of density of particles in a range from 0 to 0.5,and finally approaches to a fixed value.Moreover,with the size of square lattice increasing,energy also increases.Second,when we fix the system size,energy and magnetization increase with temperature, but not with of chemical potential.When we increase the system size,energy increases,while,the magnetization decreases. Third,specific heat is independent of chemical potential,but it dramatically increases with temperature and approaches to a peak,then decreases slowly.According to Landau theory of superfluidity,the tends of curve for energy and specific heat fit the research of He II in the Landau two-fluid model.Fourth,different square lattice linear system sizes have a little influence on tiny differences to the reciprocal of uniform susceptibility.There are small fluctuations in a range from 0 to 0.5（J/k_B）,where J is the coupling energy,k_B is the Boltzmann constant,but the reciprocal of uniform susceptibility increases with temperature increasing in a range from 0.5 to 2（J/fca）.The tends of curve are similar to those of Kondo effect.
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