磁热效应的研究：Stoner近似与DMFT
Investigation of magnetocaloric effect: Stoner approximation vs DMFT
摘要：
在面心立方（fcc）晶格的单带哈伯德模型下，利用平均场（Stoner）近似（MFA）和动力学平均场理论（DMFT）对金属的磁热效应（MCE）进行了比较研究。研究了从铁磁体到顺磁体的二级磁相变情况下的MCE。为了保证哈伯德模型中巡游铁磁性的存在，考虑了当费米能级$E_{\rm f}$处于带底的vinicity中时，谱参数在带底产生巨vanHove奇点的特殊情况.为了比较MCE内的MFA和DMFT的磁化强度的温度依赖性，总能量和最终熵的库仑相互作用$U$在零和有限值的磁场$h$的两种方法进行。还计算了MFA和DMFT的MCE势之一，等温熵变，作为温度的函数$ΔS (T)$。在MFA中，当$U$增长时，在居里温度$T_C$（$ΔS_{\rm max}$）下$ΔS (T)$的预期最大值相当显著地降低。对于DMFT结果，发现$ΔS_{\rm max}$类似但弱得多的降低。局域量子涨落的帐户中的$ΔS_{\rm max}$的DMFT内比在MFA内的值更大。两种方法的半峰高处$ΔS (T)$的峰宽大致相同。DMFT局域量子涨落的另一个效应是破坏了反常居里温度$T_C$对MFA中存在的$U$的依赖性，这是由巨大的范霍夫奇异性效应引起的。然而，对于相同的模型参数，DMFT和MFA中的相对冷却功率（RCP）非常接近，并且随着$U$的增加而下降。
英文摘要：
A comparative study of the magnetocaloric effect (MCE) in metals within the single-band Hubbard model on the face-centered cubic (fcc) lattice using both mean-field (Stoner) approximation (MFA) and dynamical mean-field theory (DMFT) is done. The MCE is investigated in the case of second order magnetic phase transition from ferromagnet to paramagnet. To ensure presence of itinerant ferromagnetism in the Hubbard model the special case of spectrum parameters generating giant van Hove singularity at the bottom of the band is considered, while the Fermi level $E_{\rm f}$ is in the vinicity of the band bottom. To compare MCE within MFA and DMFT temperature dependence of magnetization, total energy and finally entropy for a set of Coulomb interactions $U$ at zero and finite values of magnetic field $h$ for both methods were performed. Also one of the MCE potentials, isothermal entropy change, as a function of temperature $ΔS (T)$ for both MFA and DMFT is calculated. In the MFA, the expected maximum value of $ΔS (T)$ at the Curie temperature $T_C$ ($ΔS_{\rm max}$) quite significantly decreases while $U$ grows. Similar but much weaker decreasing of $ΔS_{\rm max}$ is found for DMFT results. The account of local quantum fluctuations results in larger values of $ΔS_{\rm max}$ within DMFT than within MFA. A peak width of $ΔS (T)$ at half height is approximately the same for both methods. Another effect of DMFT local quantum fluctuations is the destruction of anomalous Curie temperature $T_C$ dependence on $U$ present in MFA, which is invoked by an effect of giant van Hove singularity. However the relative cooling power (RCP) is very close in DMFT and MFA for the same model parameters and goes down upon $U$ increase.
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