U（1）对称性弱破缺的三重子的玻色-爱因斯坦凝聚
Bose - Einstein condensation of triplons with a weakly broken U(1) symmetry
摘要：
某些量子磁体的低温性质可以用磁准粒子（三重子）的玻色-爱因斯坦凝聚（BEC）来描述。一些基于标准巨正则系综的平均场方法（MFA）没有考虑反常密度，导致内部不一致，正如霍恩伯格和马丁所证明的那样，因此可能产生非物理结果。此外，如在TlCuCl 3中观察到的U（1）对称性的明确破缺使得MFA的应用更加复杂。在目前的工作中，我们开发了一个自洽的MFA方法，类似于Hartree-Fock-Bogolyubov近似的代表性统计系综的概念，包括弱破缺的U（1）对称性的影响。我们将我们的结果应用于量子磁体TlCuCl 3的实验数据，并表明磁化曲线和能量色散可以很好地描述在这个近似假设的BEC方案仍然有效。我们预测，即使当各向异性参数γ很小时，由于有限交换各向异性，临界温度Tc的移动是相当大的，例如，H = 6 T时，ΔT_c \approx为Tc的10%，γ \approx为4 μeV。
英文摘要：
The low-temperature properties of certain quantum magnets can be described in terms of a Bose-Einstein condensation (BEC) of magnetic quasiparticles (triplons). Some mean-field approaches (MFA) to describe these systems, based on the standard grand canonical ensemble, do not take the anomalous density into account and leads to an internal inconsistency, as it has been shown by Hohenberg and Martin, and may therefore produce unphysical results. Moreover, an explicit breaking of the U(1) symmetry as observed, for example, in TlCuCl3 makes the application of MFA more complicated. In the present work, we develop a self-consistent MFA approach, similar to the Hartree-Fock-Bogolyubov approximation in the notion of representative statistical ensembles, including the effect of a weakly broken U(1) symmetry. We apply our results on experimental data of the quantum magnet TlCuCl3 and show that magnetization curves and the energy dispersion can be well described within this approximation assuming that the BEC scenario is still valid. We predict that the shift of the critical temperature Tc due to a finite exchange anisotropy is rather substantial even when the anisotropy parameter γis small, e.g., ΔT_c \approx 10%$ of Tc in H = 6T and for γ\approx 4 μeV.
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