Charge disproportionation and spin ordering tendencies in Na x CoO 2
Charge disproportionation and spin ordering tendencies in Na x CoO 2
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DOI：
10.1103/physrevb.70.045104
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发表时间：
2004-02
影响因子：
3.7
通讯作者：
K.-W. Lee;J. Kuneš;W. E. Pickett
中科院分区：
文献类型：
--
作者：
K.-W. Lee;J. Kuneš;W. E. Pickett
用关联能带理论方法(Hubbard U的局域密度近似)计算了(水合时超导)$x保证数学1/3和‘增强’$x保证数学2/3$区域中库仑关联的强度和效应。在忽略量子涨落的情况下，我们的结果是：(1)只允许铁磁有序，存在一个临界${U}_{c}=3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$，，在该临界条件下，$x=1/3$和$x=2/3$都会发生电荷歧化；(2)当$x=1/3$时，允许反铁磁有序，${U}{c}$下降到$1\phantom{\rulem{0.3em}{0ex}}\mathm{ev}$进行歧化，(3)歧化和空隙打开同时发生。(4)在${\mathrm{Co}}^{3+}\ensuremath{-}{\mathrm{Co}}^{4+}$有序状态下，反铁磁耦合比铁磁耦合更有利，而低于{U}{c}$铁磁性则更有利。计算的费米能级密度与报道的线性比热系数的比较表明，对于$x\ensureath{\sim}0.7$，增加了5个数量级，但对于$x\ensureath{\sim}0.3$，增加可以忽略不计。这一趋势与$xg0.5$的强磁行为和局域磁矩(居里-魏斯磁化率)是一致的，而对于$xl0.5$则没有磁行为或局域磁矩的报道。我们认为，相图的特征是从有效的单带特征($U$U\ensuremath{\rightarrow}{U}_{\mathrm{eff}}\ensuremath{\leqslant}U∕\sqrt{3}\ensuremath{\sim}W$W$对应于$xg0.5$)到三带区域(对应于$xl0.5$)的交叉，其中⪢和关联效应显著降低。
The strength and effect of Coulomb correlations in the (superconducting when hydrated) $x\ensuremath{\approx}1∕3$ and ``enhanced'' $x\ensuremath{\approx}2∕3$ regimes of ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$ are evaluated using the correlated band theory $\mathrm{LDA}+\mathrm{U}$ (local density approximation of Hubbard U) method. Our results, neglecting quantum fluctuations, are (1) allowing only ferromagnetic order, there is a critical ${U}_{c}=3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, above which charge disproportionation occurs for both $x=1∕3$ and $x=2∕3$, (2) allowing antiferromagnetic order at $x=1∕3$, ${U}_{c}$ drops to $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for disproportionation, (3) disproportionation and gap opening occur simultaneously, and (4) in a ${\mathrm{Co}}^{3+}\ensuremath{-}{\mathrm{Co}}^{4+}$ ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below ${U}_{c}$ ferromagnetism is favored. A comparison of the calculated Fermi level density of states compared to reported linear specific heat coefficients indicates enhancement of the order of five for $x\ensuremath{\sim}0.7$, but negligible enhancement for $x\ensuremath{\sim}0.3$. This trend is consistent with strong magnetic behavior and local moments (Curie-Weiss susceptibility) for $xg0.5$ while there is no magnetic behavior or local moments reported for $xl0.5$. We suggest that the phase diagram is characterized by a crossover from effective single-band character with $U⪢W$ for $xg0.5$ into a three-band regime for $xl0.5$, where $U\ensuremath{\rightarrow}{U}_{\mathrm{eff}}\ensuremath{\leqslant}U∕\sqrt{3}\ensuremath{\sim}W$ and correlation effects are substantially reduced.