Study of the band structure evolution with the increase of Tc in ion-gated FeSe

1.分别采用自熔剂法和气相传输法生长了FeTe_(1-x)Sex和FeSe单晶。我还在Te气氛中对FeTe1-xSex晶体进行了退火，以去除晶体中多余的铁。最好的晶体表现为T_c~14.5K，跃迁峰宽度小于1K。研究了高质量FeTe0.6Se0.4单晶的输运临界电流密度。我们之前的报告表明，根据Bean模型由磁滞回线(MHLS)测量得到的Jc在10K时达到~3×10^4A/cm2。如此大的Jc意味着我们不能直接对大块单晶进行电流-电压测量，因为需要非常大的电流。我们利用聚焦离子束在单晶上制作了宽度为680 nm的窄桥。为了避免对晶体的损伤，我们用100us的矩形脉冲电流通过S的样品，在10K时，JC的温度依赖性很大，约为1.3×10~(-6)A/cm~2，比MHLS的计算值大40多倍。在分离电流密度范围内讨论了如此大的输运JC。通过输运测量研究了不同过量Fe含量的FeTe0.6Se0.4单晶的各向异性，包括电阻率、Hc2和输运Jc随温度的变化。在超导态，各向异性小于2，且与过量Fe的含量无关。而正常态的各向异性依赖于过量Fe的含量，范围在6~50之间。

1. I successfully grew FeTe1-xSex and FeSe single crystals by the self-flux method, and vapor transport method, respectively. I also annealed the FeTe1-xSex crystals in the Te-atmosphere to remove the excess Fe in the crystal. The best obtained crystal manifests Tc ~ 14.5 K with a sharp transition width less than 1 K.2. I studied the transport critical current density in a high-quality FeTe0.6Se0.4 singe crystal. Our previous report shows that Jc calculated by the Bean Model from the magnetic hysteresis loops (MHLs) measurements reaches a value ~ 3×10^4 A/cm2 at 10 K. Such large Jc means that we cannot directly perform the current-voltage measurements on the bulk single crystal since an extremely large current is required. We used the focused ion beam to fabricate a narrow bridge on the single crystal to the width ~680 nm. To avoid damage to the crystal, we applied a rectangular 100 us pulse current through the sample in 3 s. The temperature dependence of the Jc manifests a large value of ~ 1.3×10^6 A/cm2 at 10 K. The value is over 40 times larger than that calculated from the MHLs. Such large transport Jc is discussed in the regime of the departing current density.3. I studied the anisotropy of FeTe0.6Se0.4 single crystals with different amount of excess Fe by transport measurements including temperature dependences of resistivity, the Hc2, and the transport Jc. In the superconducting state, the anisotropy is smaller than 2, and independent of the amount of excess Fe. While, normal state anisotropy depends on the amount of excess Fe, and ranging from 6 to 50.