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电状态，以获得相对低浓度的掺杂剂。随后冷却到足够低的温度，SrTiO 3表现出零电阻超导状态，即使对于非常稀的掺杂样品也会持续存在，并且与目前公认的超导性如何出现的机制不一致。被称为二维电子气（2DEG）的高度移动的电子的通道在用称为异质结构的特定晶体结构制备的SrTiO 3界面处被看到，这在块状SrTiO 3中是不可见的。掺杂SrTiO 3的许多常见方法在低浓度和高浓度的电子可以在该界面处累积的程度上面临限制。该项目使用了一种称为离子液体（IL）门控的替代掺杂方法，以积累电子到极低和极高的浓度，并具有很大程度的可调谐性，从而超过了其他掺杂方法。这种调谐度对于开发一种称为“Mottronics”的传统计算机微处理器的替代品是有希望的，其中电流信号被阻止或中继通过微处理器，这取决于Mott晶体管是处于“导通”金属状态还是“关断”绝缘状态。IL门控可以用于在相对小的施加电压下积累非常大的电子浓度，这使得大电流能够在相对低的开启电压下流过计算机芯片，从而降低功耗和过度发热。从基本物理学的角度来看，IL门控可以非常精确地调整超导状态消失的临界点处的掺杂增量，这可能会提供进一步的洞察背后的超导性出现在SrTiO 3的机制。先前已经观察到奇异的电子现象，因为在IL门控SrTiO 3中电子浓度进一步降低，并且可能是唯一可以在低温下提供如此低的浓度以验证存在于锆中的玻色爱因斯坦凝聚物的理论预测的方法-掺杂SrTiO 3。该项目将使用IL的专门合成，以增加在未掺杂和铌和锆掺杂的SrTiO 3异质结构器件，以研究电子迁移率的改善，以识别较低电子浓度下超导态出现周围和下方的奇异电子现象，并寻找玻色爱因斯坦行为的特征。洁净室制造将用于制造SrTiO 3异质结构器件，并将通过放置一个薄的，2D层的六方氮化硼作为IL和衬底之间的屏障，研究电子迁移率的改善。在制造之后，将在低温恒温器中进行传输特性测量以评估低温下的器件性能，并且将施加高栅极电压以使每个IL的累积最大化。然后将在稀释制冷机中进行磁场测量，以确定电子沿通道沿着积累的程度，以研究向超导态的转变温度的变化，并确定在磁场的影响下出现的奇异电子现象，如近藤效应和异常霍尔行为。该项目还将探索其他材料的电子特性，包括KTaO 3，UO 2和SrTiO 3上的硫属化物2D材料，看看该项目第一阶段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特性。

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.