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Towards fundamentals of oxide electronics: Polaronic nature of the LaAlO3/SrTiO3 interface

Computers and other electronic devices account for a substantial portion of worldwide energy use. With today’s technologies, it is not possible to reduce this energy consumption significantly any further; chips in the energy-saving electronics of the future will hence have to be made from novel materials. Researchers at the Paul Scherrer Institute PSI have now found important clues in the search for such materials.

They have investigated a material that intrinsically possesses the needed characteristics: It is magnetic, and it can conduct electrical current completely without resistance. The disadvantage: It exhibits these properties only at very low temperatures at which no computer can run. At realistic operating temperatures, in contrast, current in the material slows down markedly. With help from experiments at the Swiss Light Source SLS of the Paul Scherrer Institute, the researchers were able to determine the causes for the impeded current flow. Their results might now help in targeted development of new materials capable of retaining these special properties at higher temperatures, which could find application in future computers. The researchers report their findings in the journal Nature Communications.

Photo_Article Cancellieri

By Vladimir N. Strocov, Swiss Light Source, Paul Scherrer Institute – Claudia Cancellieri, Materials Theory, EMPA / ETH Zurich – Ulrich Aschauer, Chemistry and Biochemistry, University of Bern

Based on article published in Nature Communications

Press release

The conventional semiconductor based electronics is nowadays approaching its limits in terms of miniaturization, operation speed and power consumption. The severity of the latter problem can be illustrated, for example, by one of Facebook’s high-performance datacenters in northern Swedish town Lulea near the Arctic Circle, which bases on a separate 120-Megawatt hydropower plant and uses the icy outside air to keep the servers cool.

Figure 1 Cancellieri
Fig. 1. Scheme of the ARPES experiment. Incident X-rays eject photoelectrons, whose distribution in energy and emission angle directly yields the electronic band structure of the sample resolved in energy E and electron momentum k.

One of possible alternatives to the semiconductor materials is found in the wide class of complex transition metal oxides. Interplay of spin, charge, orbital and lattice degrees of freedom in these materials results in a plethora of fascinating properties, which can be exploited in new generations of electronic devices whose functionality would combine superconductivity, ferromagnetism, colossal magnetoresistance and other effects based on strong electron correlations. For example, a combination of ferromagnetic and superconducting properties on the same oxide platform offer routes towards the realization of switching devices based on magnetic Josephson junctions, which would reduce power consumption by 5 orders of magnitude compared to the nowadays electronics.

Oxide interfaces and heterostructures can offer yet more fascinating functionalities beyond any expectations based on the properties of the bulk materials. For example, certain non-magnetic oxides can become magnetic and some insulating oxides conducting at their interface. A “drosophila” example of such functional interfaces forms between the two insulators LaAlO3 and SrTiO3, where spontaneous interface conductivity develops, with charge carriers possessing highly unusual properties such as superconductivity co-existing with its conventional foe, ferromagnetism, field effect allowing the realization of oxide transistors, etc. However, one of the bottlenecks for immediate applications of this system in solid-state technology is that the electron mobility stays a factor ~3 below theoretical expectations, with its further dramatic reduction with increasing temperature.

A research team from the Swiss Light Source, supported by theoreticians from ETH Zürich and RIKEN Research Center in Japan, has explored the LaAlO3/SrTiO3 interface using Angle-Resolved Photoelectron Spectroscopy (ARPES) which gives direct information about electronic structure of solid-state systems resolved in energy E and electron momentum k (Fig. 1). Energetic soft-X-ray photons were used to penetrate through the LaAlO3 overlayer and resonantly accentuate the response of the charge carriers at the buried interface. Such experiments on buried interfaces, the core of real electronic devices, can presently be performed only at the Swiss Light Source at the ADRESS beamline which is the source of soft X-rays with highest brilliance worldwide.

The experimental results achieved at ultrahigh energy resolution and variable polarization of incident X-rays have for the first time resolved the manifold structure of energy bands in the interface quantum well (Fig. 2a).

These pioneering results open a new dimension in understanding the LaAlO3/SrTiO3 interface physics.

In particular, comparison of the experimental Fermi states with macroscopic transport properties unambiguously identifies phase separation at the interface, which is a key element in the percolative superconductivity and weak ferromagnetism of LaAlO3/SrTiO3. Most remarkably, the experimental spectral function A(E,k) (Fig. 2b) shows a pronounced peak-dip-hump structure, where the high-energy peak manifests electrons moving as free quasiparticles and the spectral hump at lower energy identifies electrons dragging behind them local lattice distortions called polarons. Resulting from strong electron-phonon interaction in SrTiO3, the polaronic nature of the interface charge carriers fundamentally limits their mobility and therefore restricts high-frequency applications of the LaAlO3/SrTiO3 system.

Figure 2 Cancellieri
Fig. 2. (a) Band structure map of the LaAlO3/SrTiO3 interface states measured with a photon energy around 460 eV; (b) the corresponding spectral function A(w,k), identifying polaronic coupling to the breathing LO3 phonon (inset); (c) Temperature dependence of A(w,k). Fading of the quasiparticle weight due to polaronic coupling to the polar TO1 phonon (inset) explains (d) the dramatic drop of mobility with temperature.

Involved in the formation of these large-radius polarons are two phonons with different energy and thermal activity. The breathing-mode LO3 phonon at ~120 meV, directly observed as the A(E,k) hump and coupled to electrons via short-range interaction, sets the low-temperature limit of the interfacial charge carrier mobility. In turn, the polar TO1 phonon, changing its frequency from ~18 to 14 meV across the antiferrodistortive phase transition in SrTiO3 and coupled to electrons via long-range interaction, causes fading of the quasiparticle weight in A(w,k) with temperature (Fig. 2c) which is the microscopic mechanism behind the dramatic mobility drop observed in transport properties (Fig. 2d). Growth of LaAlO3/SrTiO3 systems with different concentration of oxygen vacancies, which dope electrons affecting the polaronic coupling, opens ways to tune the interfacial mobility.

Researchers Claudia Cancellieri and Vladimir Strocov at the ADRESS beamline of the Swiss Light Source SLS of the Paul Scherrer Institute. Here they investigated the electrical current inside a complex material and thus gained insights that might prove useful for the development of energy-saving electronic components. (Photo: PSI/Markus Fischer)

The small energy scale of the TO1 phonon in the polaronic activity of LaAlO3/SrTiO3 suggests its potential involvement in the formation of Cooper pairs responsible for the low-temperature superconductivity in this system, tunable by oxygen vacancies. The scientists from the Swiss Light Source are presently extending such enlightening experiments to other buried interface systems including SiO2-protected EuO/Si spin injectors, multiferroic BaTiO3/La1-xSrxMnO3 interfaces, etc. This research paves a way towards new operational principles for efficient and power-saving solid-state electronics based on oxide heterostructures.

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