Exotic correlated phases in oxide two-dimensional electron systems at ultra-low temperature

• Applicant: Dr. Alexander Boris, since 6/2020
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 391068461

We outline a study of correlation and spin physics of electrons which are confined to two dimensions at a high quality MgZnO/ZnO crystalline interface. This system has attracted attention recently as its quality now rivals that of the best semiconductor materials, and the fractional quantum Hall physics it displays may be beneficial for quantum computation concepts based on topological properties of carriers. We aim to study its characteristics at ultra-low temperatures (T < 10mK); a regime which remains unexplored. Some possible manifestations of correlation physics include ferromagnetism, where electron spins align spontaneously, nematicity, where particles arrange into spatially non-uniform patterns and superconductivity, where current may pass without energy dissipation. These phenomena have been studied in other correlated systems, for example high temperature superconductors or heavy Fermion systems. Their observation in high quality two-dimensional electron systems however remains elusive and may aid in resolving outstanding problems.Previous studies of the MgZnO/ZnO system have been limited to temperatures available in standard commercial cryogenic equipment (T > 20 mK). Here we propose significant modification of such infrastructure as to achieve ultra-low temperatures of T < 10 mK. For this, we plan to develop and implement an enclosure which contains liquefied pure 3He at a temperature of ~ 5 mK, in which the sample is immersed. Further deployment of heat exchangers connected to the sample enables access to the ultra-low temperature regime. The goal is to suppress thermal fluctuations which mask the fragile correlation effects which are present in the system. At ultra-low temperatures we plan to explore the electrical characteristics of these samples through sensitive transport techniques. The resistance will be studied as a function of temperature, magnetic field, charge density and crystal direction. All these experimental degrees of freedom are essential knobs for revealing the nature of the electronic ground state. A key aspect of transport measurements is exploring the spin polarization of the samples while changing the charge density with the electric field effect. We can control this spin polarization by rotating the sample in a magnetic field, allowing its accurate quantification. We will build on previous work which identified an increase at low densities, indicating the approach to a quantum critical state where interaction effects dominate. Finally, we envisage complementary means of probing the electron interaction effects through non-equilibrium resonance methods. This will involve illuminating the sample with radiation in the microwave region of the electromagnetic spectrum, which provides photons of energies in resonance with typical scales encountered in semiconductors. Exploring the frequency dependence of these resonances will provide quantitative spectroscopic results to support observations made in transport.

• DFG Programme: Research Grants
• International Connection: Japan, USA
• Cooperation Partners: Professor Dr. Masashi Kawasaki; Dr. Jurgen H. Smet; Professor Dr. Neil Sullivan
• Ehemaliger Antragsteller: Professor Dr. Joseph Falson, until 6/2020