Crossover from S=1/2 to S=1 Haldane state in the ferromagnetic and antiferromagnetic alternating Heisenberg chain system (CH 3 ) 2 CHNH 3 CuCl 3 observed with EPR at 24 GHz
Crossover from S=1/2 to S=1 Haldane state in the ferromagnetic and antiferromagnetic alternating Heisenberg chain system (CH 3 ) 2 CHNH 3 CuCl 3 observed with EPR at 24 GHz
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在 24 GHz 下用 EPR 观察到铁磁和反铁磁交替海森堡链系统 (CH 3 ) 2 CHNH 3 CuCl 3 中从 S=1/2 到 S=1 Haldane 态的交叉
DOI：
10.1103/physrevb.62.14279
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发表时间：
2000
影响因子：
3.7
通讯作者：
I. Yamada
中科院分区：
文献类型：
--
作者：
H. Manaka;I. Yamada
化合物 $({\mathrm{CH}}_{3}{)}_{2}{\mathrm{CHNH}}_{3}{\mathrm{CuCl}}_{3},$ 由铁磁主导的铁磁和反铁磁交替海森堡链组成，$S=1/2,$ 在低温下被视为 $S=1$ 的 Haldane 系统，因为一对铁磁耦合自旋表现为 $S=1。$因此，当温度降低时，自旋态应该从 $S=1/2$ 交叉到 $S=1$。由于期望交叉会导致自旋动力学发生巨大变化，因此在 24 GHz 下对该化合物的单晶在 1.4\char21{}295 K 范围内进行了电子顺磁共振（EPR）实验。发现在 10 K 以下观察到的 EPR 光谱显示出与 10 K 以上观察到的明显不同的特征。也就是说，发现在 10 K 以上观察到的单条吸收线在 10 K 以下分裂成两条线，并且在 10 K 以下出现了一条额外的弱线。对应于两条线的平均共振场的一半的位置。两条线的谐振场随外场H的方向而变化；它们的角度依赖性为 $a\ifmmode\pm\else\textpm\fi{}b(1\ensuremath{-}3{\mathrm{cos}}^{2}\ensuremath{\theta}),$ 其中 a 和 b 是常数，$\ensuremath{\theta}$ 是 H 方向与正交晶体表面法线之一之间的角度。出现在半场位置的弱线的共振场相对于 H 的方向几乎是恒定的。当人们认为由铁磁耦合的两个自旋对引起的 $S=1$ 状态取代单个自旋的 $S=1/2$ 状态时，可以解释这些在 10 K 以下观察到的实验结果。然后，铁磁耦合的两个自旋之间的偶极\char21{}偶极相互作用 $({\mathcal{H}}_{\mathrm{DD}}^{\ensuremath{'}})$ 和各向异性交换相互作用 $({\mathcal{H}}_{\mathrm{AE}}^{\ensuremath{'}})$ 充当虚构的单离子各向异性，并消除三重$S=1,$ 的三重态简并，即 ${E}_{{S,S}_{z}}{=E}_{1,1}{,E}_{1,0},$ 和 ${E}_{1,\ensuremath{-}1}.$ 因此，$\ensuremath{\Delta}{S}_{z}=\ifmmode\pm\else\textpm\fi{}1$跃迁，即 ${E}_{1,\ensuremath{-}1}$ 和 ${E}_{1,0},$ 之间以及 ${E}_{1,0}$ 和 ${E}_{1,1}$ 之间的跃迁会产生两条吸收线。这就是为什么两条线出现在 10 K 以下的原因。弱半场线是由于 $\ensuremath{\Delta}{S}_{z}=\ifmmode\pm\else\textpm\fi{}2$ 转变造成的，这也是由 ${\mathcal{H}}_{\mathrm{DD}}^{\ensuremath{'}}$ 和铁磁耦合两个自旋之间的${\mathcal{H}}_{\mathrm{AE}}^{\ensuremath{'}}$，因为$|1,1〉$和$|1,\ensuremath{-}1〉$之间的非对角元素不为0。
The compound $({\mathrm{CH}}_{3}{)}_{2}{\mathrm{CHNH}}_{3}{\mathrm{CuCl}}_{3},$ which consists of ferromagnetic-dominant ferromagnetic and antiferromagnetic alternating Heisenberg chains with $S=1/2,$ is regarded as a Haldane system with $S=1$ at low temperatures because a pair of ferromagnetically coupled spins behaves as $S=1.$ There should therefore be a crossover of the spin state from $S=1/2$ to $S=1$ when the temperature is decreased. With the expectation that the crossover causes a drastic change in spin dynamics, electron paramagnetic resonance (EPR) experiments were performed at 24 GHz on single crystals of this compound over the region of 1.4\char21{}295 K. The EPR spectra observed below 10 K were found to show characteristics clearly distinct from those above 10 K. That is, a single absorption line observed above 10 K was found to split into two lines below 10 K, and an additional weak line appeared at a position corresponding to half of the averaged resonance fields of the two lines. The resonance fields of the two lines vary with the direction of the external field H; their angular dependence is $a\ifmmode\pm\else\textpm\fi{}b(1\ensuremath{-}3{\mathrm{cos}}^{2}\ensuremath{\theta}),$ where a and b are constants, and $\ensuremath{\theta}$ is the angle between the direction of H and one of the normals of the orthogonal crystal surfaces. The resonance field of the weak line that appeared at the half-field position was almost constant with respect to the direction of H. These experimental results observed below 10 K are explained when one considers that the $S=1$ state caused by pairs of ferromagnetically coupled two spins supersedes the $S=1/2$ state of the individual spins. Then the dipole\char21{}dipole interaction $({\mathcal{H}}_{\mathrm{DD}}^{\ensuremath{'}})$ and the anisotropic exchange interaction $({\mathcal{H}}_{\mathrm{AE}}^{\ensuremath{'}})$ between ferromagnetically coupled two spins act as a fictitious single ion anisotropy, and remove the threefold degeneracy of the triplet state of $S=1,$ i.e., ${E}_{{S,S}_{z}}{=E}_{1,1}{,E}_{1,0},$ and ${E}_{1,\ensuremath{-}1}.$ As a result, the $\ensuremath{\Delta}{S}_{z}=\ifmmode\pm\else\textpm\fi{}1$ transitions, i.e., the transitions between ${E}_{1,\ensuremath{-}1}$ and ${E}_{1,0},$ and between ${E}_{1,0}$ and ${E}_{1,1}$ bring about two absorption lines. That is why the two lines appear below 10 K. The weak half-field line is due to the $\ensuremath{\Delta}{S}_{z}=\ifmmode\pm\else\textpm\fi{}2$ transition, which is also caused by ${\mathcal{H}}_{\mathrm{DD}}^{\ensuremath{'}}$ and ${\mathcal{H}}_{\mathrm{AE}}^{\ensuremath{'}}$ between ferromagnetically coupled two spins because nondiagonal elements between $|1,1〉$ and $|1,\ensuremath{-}1〉$ are not 0.