The detailed optical properties have been determined for the iron-based materials $A$Fe$_2$As$_2$, where $A=\,$Ca, Sr, and Ba, for light polarized in the iron-arsenic ($a-b$) planes over a wide frequency range, above and below the magnetic and structural transitions at $T_N =$ 172, 195, and 138 K, respectively. The real and imaginary parts of the complex conductivity are fit simultaneously using two Drude terms in combination with a series of oscillators. Above $T_N$, the free-carrier response consists of a weak, narrow Drude term, and a strong, broad Drude term, both of which show only a weak temperature dependence. Below $T_N$ there is a slight decrease of the plasma frequency but a dramatic drop in the scattering rate for the narrow Drude term, and for the broad Drude term there is a significant decrease in the plasma frequency, while the decrease in the scattering rate, albeit significant, is not as severe. The small values observed for the scattering rates for the narrow Drude term for $T\ll{T_N}$ may be related to the Dirac cone-like dispersion of the electronic bands. Below $T_N$ new features emerge in the optical conductivity that are associated with the reconstruction Fermi surface and the gapping of bands at $Δ_1 \simeq$ 45 $-$ 80 meV, and $Δ_2 \simeq$ 110 $-$ 210 meV. The reduction in the spectral weight associated with the free carriers is captured by the gap structure, specifically, the spectral weight from the narrow Drude term appears to be transferred into the low-energy gap feature, while the missing weight from the broad term shifts to the high-energy gap.
Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr$_\mathbf{0.67}$Na$_\mathbf{0.33}$Fe$_\mathbf{2}$As$_\mathbf{2}$
本文报道了铁基超导体（SC）Sr $_{0.67}$ Na $_{0.33}$ Fe $_2$ As $_2$高温四元顺磁相的温度依赖性电导率和ARPES研究;在$T_{\rm N}\simeq$ 125 K下的正交自旋密度波（SDW）类相中的结构和磁性转变之下，而$T_r\simeq$在42 K时，在电荷和SDW有序同时存在的情况下，在$T_c\simeq$ 10 K时低于SC转变。自由载流子在光学电导率的分量描述了两个德鲁德贡献;一个强和广泛的，其他弱和狭窄。宽的德鲁德分量在低于$T_{\rm N}$和$T_r$时急剧减小，其大部分强度被转移到中红外的束缚激发，而窄的德鲁德分量在任一跃迁处均未显示出异常，实际上在低温下强度增加，同时急剧变窄。红外激活模式的行为表明在$T_r$下方存在带折叠。在$T_c$以下，低频光学电导率的急剧下降表明SC能隙的形成。ARPES揭示了在布里渊区（BZ）的中心处的类空穴带，在角落处具有类电子带和类空穴带。低于$T_{\rm N}$，BZ中心的空穴袋尺寸减小，与宽Drude分量的行为一致;而低于$T_r$，类电子带移动和分裂，在~20 meV处引起光学电导率的低能激发。磁状态，所产生的SDW和电荷SDW顺序，分别导致一个显着的重建的费米表面，具有深远的影响，从电子和空穴袋的运输起源，但似乎有相对较小的影响SC在这种材料。
Optical floating zone crystal growth of rare-earth disilicates, $R\mathbf{_{2}}$Si$\mathbf{_{2}}$O$\mathbf{_{7}}$ ($R=$ Er, Ho, and Tm)
在稀土二硅酸盐化合物中看到的丰富的结构相预示着同样丰富的有趣的磁性。我们报告了通过光学浮区法生长稀土二硅酸盐家族成员$R_{2}$ Si $_{2}$ O $_{7}$（含$R=$ Er、Ho和Tm）的晶体生长。通过系统的研究，我们优化了Er $_{2}$ Si $_{2}$ O $_{7}$的生长条件。我们首次用浮区法生长了Ho $_{2}$ Si $_{2}$ O $_{7}$和Tm $_{2}$ Si $_{2}$ O $_{7}$化合物的晶棒。我们发现，在多晶和单晶样品的合成中遇到的困难是由于不同的稀土硅酸盐化合物的$R$-Si-O系统的温度-组成相图中的相似的热稳定性范围。添加少量过量的SiO $_{2}$使得粉末样品中存在的杂质相的量最小化。对所生长的晶锭收集的粉末X射线衍射数据的相组成分析表明，它们是单相的，除了在二硅酸钍的情况下，其由两相组成。所有的生长都导致了多晶粒的晶锭，从中可以分离出相当大的单晶。报道了多晶和单晶$R_{2}$ Si $_{2}$ O $_{7}$材料的合成和晶体生长的最佳条件。铒和铥二硅酸盐化合物的比热测量证实了反铁磁相变低于$T_{\mathrm{N}}=$ 1.8 K的D型Er $_{2}$ Si $_{2}$ O $_{7}$和肖特基异常中心约3.5 K的C型Tm $_{2}$ Si $_{2}$ O $_{7}$，表明开始的短程磁关联…
Optical and Hall conductivities of a thermally disordered two-dimensional spin-density wave: two-particle response in the pseudogap regime of electron-doped high-$T_c$ superconductors
