Tuning the electronic properties of SrTiO3 with ionic liquid gating

钛酸锶(SrTiO_3)可以从电子绝缘状态掺杂到半导体态，掺杂浓度相对较低。随后冷却到足够低的温度时，钛酸锶表现出零电阻超导状态，即使是非常稀薄的掺杂样品也会保持这种状态，这与目前公认的超导产生机制不一致。在以某种晶体结构制备的钛酸锶界面处，可以看到一种称为二维电子气(2DEG)的高迁移率电子通道，这种结构在体相的钛酸锶中是看不到的。许多常用的掺杂钛酸锶的方法在低浓度和高浓度电子在这个界面上积累的程度上都面临着局限性。该项目使用一种称为离子液体(IL)门的替代掺杂方法，以高度可调谐的方式将电子聚集到低浓度和高浓度的极端位置，从而超越其他掺杂方法。这种程度的可调性对于开发一种称为Mottronics的传统计算机微处理器的替代方案是有希望的，其中，根据Mott晶体管是处于金属导通状态还是处于绝缘状态，电流信号被阻挡或通过微处理器传递。IL门可以用来在相对较小的施加电压下积累非常大的电子浓度，这使得在相对较低的开启电压下通过计算机芯片的大电流流动，从而减少了功耗和过热。从基础物理学的角度来看，IL门可以非常精确地调节超导状态消失的临界点的掺杂增量，这可能会进一步深入了解SrTiO_3超导产生的机制。以前已经观察到了奇异的电子现象，因为IL门控的SrTiO3中的电子浓度进一步降低，这可能是唯一能够在低温下提供如此低浓度的方法来验证理论预测的玻色爱因斯坦凝聚体存在于掺锆钛酸锶中。这个项目将使用专门合成的IL来增加电子在未掺杂和掺Nb和Zr掺杂的SrTiO3异质结器件的界面2DEG处的电子积累，以研究电子迁移率的改善，识别超导态出现前后较低电子浓度的奇异电子现象，并寻找玻色爱因斯坦行为的特征。净室制造将被用来制造钛酸锶异质结构器件，电子迁移率的改善将通过在IL和衬底之间放置一层薄的2D六方氮化硼作为屏障来研究。制造完成后，将在低温恒温器中进行传输特性测量，以评估器件在低温下的性能，并将施加高栅极电压以最大限度地积累每个IL。然后，将在稀释式制冷机中进行磁场测量，以确定电子沿通道积累的程度，研究向超导状态转变温度的变化，并识别在磁场影响下出现的奇异电子现象，如近藤效应和反常霍尔行为。本项目还将探索其他材料的电子性质，包括KTaO_3、UO_2和硫系2D材料在SrTiO_3上的电子性质，看看项目第一阶段中开发的电子掺杂方法是否可以应用于其他材料以诱导奇异的电子性质。

Strontium titanate (SrTiO3) can be doped from an electronically insulating to semi-conducting state for a relatively low concentration of dopants. Subsequent cooling to sufficiently low temperatures sees SrTiO3 exhibit a zero resistance superconducting state, which persists even for very dilute doped samples and is at odds with currently accepted mechanisms of how superconductivity emerges. A channel of highly mobile electrons called a two dimensional electron gas (2DEG) is seen at SrTiO3 interfaces prepared with a certain crystal structure, called a heterostructure, which is otherwise unseen in bulk SrTiO3. Many common methods to dope SrTiO3 face limitations in the extent to which low and high concentrations of electrons can be accumulated at this interface. This project uses an alternative doping approach called ionic liquid (IL) gating to accumulate electrons to extremes of low and high concentration with a large degree of tuneability, thereby surpassing other doping methods. This degree of tuneability is promising with regards to developing an alternative to conventional computer microprocessors called 'Mottronics', wherein current signals are either impeded or relayed through the microprocessor depending on whether the Mott transistor is in an 'on' metallic or an 'off' insulating state. IL gating can be used to accumulate very large concentrations of electrons for relatively small applied voltages, which enables a large current flow through computer chips for relatively low turn-on voltages, thereby reducing power consumption and excess heating.From a fundamental physics perspective, IL gating can very precisely tune the increments of doping at the critical points where the superconducting state disappears, which may offer further insight into the mechanism behind which superconductivity emerges in SrTiO3. Exotic electronic phenomena has previously been observed as the concentration of electrons is further lowered in IL gated SrTiO3, and may be the only method that can provide such low concentrations at low temperatures to verify theoretical predictions of a Bose Einstein condensate existing in zirconium-doped SrTiO3.This project will use IL's specifically synthesised to increase the accumulation of electrons at the interfacial 2DEG in both undoped and niobium- and zirconium- doped SrTiO3 heterostructure devices to investigate improvements in the mobility of electrons, to identify exotic electronic phenomena around and below the emergence of the superconducting state for lower electron concentrations, and to look for signatures of Bose Einstein behaviour. Cleanroom fabrication will be used to fabricate SrTiO3 heterostructure devices, and improvements in the electron mobility will be investigated by placing a thin, 2D layer of hexagonal boron nitride as a barrier between the IL and the substrate. Following fabrication, transfer characteristic measurements will be conducted in a cryostat to assess device performance at low temperatures, and high gate voltages will be applied to maximise accumulation for each IL. Magnetic field measurements in a dilution refrigerator will then be conducted to determine the extent of electron accumulation along the channel, to investigate changes in the transition temperature to superconducting state, and to identify the emergence of exotic electronic phenomena such as the Kondo effect and anomalous Hall behaviour under the influence of a magnetic field. This project will also explore the electronic properties of other materials including KTaO3, UO2, and chalcogenide 2D materials on SrTiO3, to see if the methods of electronic doping developed in the first stage of the project can then be applied to other materials to induce exotic electronic properties.