Entropy-driven inductance on the surface of a topological insulator
Sofie Kölling is a PhD student in the Department of Interfaces and Correlated Electron Systems. Promotors are prof.dr.ir. A. Brinkman from the Faculty of Science & Technology and prof.dr. I. Adagideli from the Sabanci University.
New opportunities for energy harvesting can be found in a century-old thought experiment known as Maxwell's demon, which suggests that energy can be extracted from information. Although systems resembling Maxwell's demon have been experimentally realized, scalability remains a challenge. A scalable solution can be found in the interfacial states of topological insulators, which could resemble Maxwell's demon due to the interaction between spin-momentum locked electrons and nuclear spins. In these systems, information - nuclear spin polarization - can be directly used to store and harvest energy. In this thesis, we focused on thin films of the three-dimensional topological insulator (Bi1-xSbx)2Te3 (BST), which can act as an ‘information engine’ to extract electrical work.
Theoretical predictions reveal that the interaction between surface states and nuclear spins in BST leads to an inductive effect, directly coupled to the entropy of the system. This ‘entropic inductance’ has potential applications in microelectronics. Experiments with Hall bar devices fabricated from molecular beam epitaxy (MBE)-grown BST were conducted to investigate this phenomenon under various source-drain bias voltages and temperatures. These experiments showed that high biases can influence the transport properties by raising the electron temperature, and via interplay between the bias and gate voltages. Working around the parasitic effects, in-plane magnetoresistance measurements indicate current-induced nuclear spin polarization in BST via an offset magnetic field dependence. However, the predicted inductive effect was small.
To enhance the inductive signal, the study explored magnetic doping using vanadium-doped BST (VBST). Despite the model predicting an enhanced inductance, no measurable inductive effect was detected due to noise limitations. Another possible method, is to reduce diffusive losses by utilizing the quantum spin Hall effect. In ultrathin BST, depending on film thickness the quantum spin Hall phase or an insulating phase can occur as a result of surface hybridization. We studied BST of thickness ~6 nm and attributed the measured insulating phase to hybridization, which is an important step in advancing the engineering of quantum spin Hall devices.
Despite challenges with high current densities and Joule heating, the study demonstrated finite current-induced nuclear polarization in BST and proposed improvements for future research. Although the surface states of BST might not be the most efficient platform for realizing an information engine, a similar principle can be applied to other material systems such as quantum spin Hall states or Fermi arc surface states, and future developments in electronic materials could therefore provide the key to a scalable information engine.
