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

1. 我分别用自通量法和蒸气输运法成功地生长了FeTe1-xSex单晶和FeSe单晶。我还在te气氛中退火了FeTe1-xSex晶体，以去除晶体中多余的Fe。得到的最佳晶体表现为Tc ~ 14.5 K，急剧转变宽度小于1 K。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，用Bean模型从磁滞回线（mls）测量中计算出的Jc在10 K时达到了~ 3×10^4 a /cm2的值。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造了宽约680 nm的窄桥。为了避免损坏晶体，我们在3秒内对样品施加了一个矩形100毫安脉冲电流。温度依赖性在10 K时表现为~ 1.3×10^6 a /cm2。该值比从MHLs计算得到的值大40倍以上。这样大的输运Jc在离流密度的情况下进行了讨论。通过输运测量，包括电阻率、Hc2和输运Jc的温度依赖性，研究了不同过量铁量的FeTe0.6Se0.4单晶的各向异性。在超导状态下，各向异性小于2，且与过量铁的量无关。而正常状态各向异性则与铁的过量量有关，其范围为6 ~ 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.