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

1.我分别用自熔剂法和气相输运法成功地生长了FeTe_（1-x）Sex和FeSe单晶。我还在Te气氛中退火FeTe 1-xSex晶体，以去除晶体中多余的Fe。获得的最佳晶体表现出Tc ~ 14.5 K，具有小于1 K的急剧过渡宽度。研究了高质量FeTe_（0.6）Se_（0.4）单晶的输运临界电流密度。我们以前的报告表明，在10 K时，由Bean模型从磁滞回线（MHLs）测量计算的Jc达到约3×10^4 A/cm 2。如此大的Jc意味着我们不能直接对块状单晶进行电流-电压测量，因为需要极大的电流。我们利用聚焦离子束在单晶上制作了宽度约为680 nm的窄桥。为了避免损坏晶体，我们在3秒内施加矩形100 us脉冲电流通过样品。在10 K时，Jc的温度依赖性表现为一个很大的值，约为1.3×10^6 A/cm 2。该值比MHL计算值大40倍以上。这种大的运输Jc讨论的制度的离去电流密度。我研究了FeTe0.6Se0.4单晶体的各向异性与不同数量的过剩铁的运输测量，包括电阻率的温度依赖性，Hc 2，和运输Jc。在超导状态下，各向异性小于2，并且与过量Fe的量无关。而正常态各向异性取决于过量Fe的量，并且范围从6到50。

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.