Useful for spintronics: Big surprises in a thin surface region

Milan Radović and Nicholas Plumb at the X09LA beamline at the SLS (Photo: P
Milan Radović and Nicholas Plumb at the X09LA beamline at the SLS (Photo: Paul Scherrer Institut / Markus Fischer)

The need for ever faster and more efficient electronic devices is growing rapidly, and thus the demand for new materials with new properties. Oxides, especially ones based on strontium titanate (SrTiO3), play an important role here. Researchers recently discovered that SrTiO3, although actually an insulator, can form a metallic layer on its surface, in which electric current can flow. A collaborative project headed by scientists from the Paul Scherrer Institute (PSI) has now revealed that the surface’s electrons come in two basic forms: electrons of a 2D character, which can move in two dimensions parallel to the surface, and electrons of a 3D character, which penetrate deeper into the material. What’s more, the 2D electrons can move in two different ways corresponding to two sub-bands. For the scientists, the large amount of energy required to move the electrons from one band to the other came as a surprise. This property makes strontium titanate an important base material for applications in spintronics. The scientists published their results in the journal Physical Review Letters and today .

Conventional electronics is based on semiconductor materials such as silicon, germanium or gallium arsenide, which are increasingly reaching their performance limits. One key approach to make these electronic devices faster and more efficient is so-called spintronics. In addition to the electron’s charge, it makes use of its spin to transmit and store information. The spin can be regarded as a rotation of the electron around its own axis, which is connected to a magnetic field. Depending on the direction of rotation, the north pole of the tiny magnets points in different directions. Therefore, opposing spins are a good way to represent two clearly distinguishable states, “0” and “1”, a principle that is already used for data storage devices.

An insulator that conducts electricity

New materials are needed for this new electronics concept. For some years, scientists have had their sights on oxides such as strontium titanate (SrTiO3) as an interesting alternative to the well-established semiconductors. These compounds have very complex structures, but promise a variety of new properties in return. “Actually, strontium titanate is an insulator. But back in 2004 scientists discovered that a current of electrons can flow in such materials if different oxides are layered on top of each other. The current then flows along the interface between the layers – even though there shouldn’t really be any current flow in oxides,” explains Milan Radovic, a scientist at PSI. In 2011, it was observed that a thin conductive layer even forms on the pure surface of strontium titanate – a so-called two-dimensional electron gas (2DEG), where electrons can virtually move freely, like gas particles.

Multi-lane electron motorway

Now, in an international research collaboration, Radović and his colleague Nicholas Plumb have measured the properties of the electrons in this 2DEG, providing the clearest description of the electronic structure of the metallic surface state on SrTiO3 to date. It is characterized by a so-called band structure, which can be imagined as a multi-lane motorway for electrons. On each lane, the electrons possess certain properties, such as a specific spin direction or certain energy levels.

Microscope for electron states

Photoemission spectroscopy is a suitable method to determine the form of the band structure and the properties of the electrons at the surface. It involves shining light with a very precisely defined wavelength onto a sample. The light’s energy induces the electrons to leave the solid. If the electrons’ energy and flight direction out of the material are measured, their behaviour while they were still in the sample can be determined. “Only because we have synchrotron light at our disposal at the Swiss Light Source (SLS) which has a very high intensity and quality and the wavelength of which can be varied continuously can we reveal the structures that determine the behaviour of the electrons with such high clarity,” says Radovic. “Together with the excellent analysis instruments, we thus have a kind of ’microscope’ for the states of the electrons.”

In the first part of their studies, the PSI scientists and their colleagues demonstrated that there are electrons with a 2D and 3D character. 2D electrons move in two dimensions parallel to the surface and only in the uppermost layer of the material. The 3D electrons can – albeit also near the surface – move in all three dimensions.

Lane change requires a lot of energy

To study the spin of the 2D electrons in more detail, spin and angular-resolution photoemission measurements (SARPES) were performed at the SLS (X09 LA beamline). Thus, Radović and Plumb joined forces with colleagues from EPFL Switzerland and CSNSM, Université Paris-Sud, France and discovered amazing new properties of these 2D electrons: they are located in two sub-bands. In both bands, the majority of the electron spins are aligned parallel to the surface. In one band, however, their orientation rotates clockwise, in the other anti-clockwise. While the researchers had expected this helical spin structure, they were surprised to find two separate sub-bands with spins oriented in opposing directions and especially that a relatively large amount of energy (100 milli-electron volts) is required to allow the electron transition from one band to the other – to switch from one lane to the other, as it were. The researchers refer to a sizeable band gap, which is around ten times larger than in other known systems up to now. “The fact that this effect is so great lends itself to applications in spintronics, where it is important to separate the two spin orientations,” stresses Radovic. “If you want to develop spin-control nano-components such as switches or transistors, for instance.” So far, however, this effect has only been observed under ultra-high vacuum conditions. Whether it can be achieved on the same scale under practical conditions remains to be seen in future studies.

The scientists are currently planning to explore the exact cause of the giant spin-splitting. While the classical, well-known Rashba effect can explain the splitting of the band and the chirality of the spins, it does not take into account the band gap. Different effects come into question. On the one hand, there might be a certain unevenness on the surface, which causes huge intrinsic electrical fields. On the other hand, magnetic effects have to be considered. “There has to be an additional magnetic order on the surface,” Radovic is convinced. Experiments are currently underway to understand this better. The results published reveal that electrons confined on the surface of oxides have complex properties which are both challenging from a theoretical perspective and promising for technological applications.

Text: Uta Deffke. The annual budget amounts to approximately CHF 350 million.