One of the central challenges in condensed matter physics is to comprehend systems that have strong disorder and strong interactions. Over the last 50 years, it has been established that, in strongly localized regime their subtle competition leads to glassy electrons. Conversely, in well delocalized systems such as conventional metal like copper, the screening effect significantly reduces the strength of electron-electron and electron impurity interactions. Consequently, the dynamics associated with glassy behavior, which involves the existence of multiple competing ground states, is incompatible with the behavior of metals. In-fact, glassiness tends to fade away significantly prior to the transition from insulator to metal upon doping, and experimentally, there is a lack of any evidence for glassiness in good metals. In our very recent work (Nature Communications 15, 3830 (2024), arXiv:2306.14464), we present the discovery of glassy dynamics of the conduction electrons in the oxygen deficient KTaO3, in the good metal regime. Even more astonishing is the observation that glassiness emerges in a regime where quantum fluctuations are inherently present in the system. Using a combination of diverse experimental and theoretical techniques, we provide compelling evidence for quantum fluctuation-stabilized soft-polar modes in the creation of polar nano regions around the defect dipoles, which serve as the driving force behind the observed glassy behavior. Our finding of glassy dynamics in the good metal regime not only addresses one of the most fundamental questions whether quantum fluctuations can promote glassy dynamics or not but also raises the question about the envisaged role of glassy freezing of electrons as a precursor to metal-insulator transition apart from the Anderson and Mott localization.

2D superconductors undergo a topological phase transition which belongs to the Berezinskii Kosterlitz Thouless (BKT) universality class. Below the BKT phase transition temperature (TBKT), which is less than TC, bound vortex-antivortex pairs are the bare topological excitations and above TBKT the phase of the order parameter gets disturbed due to proliferation of free vortices. When subjected to an applied current, these vortices start moving leading to dissipation within the system. While the behavior of vortices under minimal driving currents is well understood, the dynamics under large current drive, especially in close proximity to the critical current, remain unexplored. Our extensive transport measurements and in-depth analysis on KTaO3 (111) based 2DEG (Communications Physics 6, 123 (2023)), furnish several indications that near TBKT, vortices tend to collapse in way predicted long back by Larkin and Ovchinnikov, leading to large dissipation in the system. While such behavior had been reported earlier in type II superconductors in the presence of magnetic field, experimental demonstration in the absence of external magnetic field had remained elusive until now. Our findings not only furnish crucial insights into the microscopic structure of 2D superconductivity, where the phase of the superconducting order parameter assumes pivotal importance, but also underscores the potential of the KTaO3 (111) based interface as an optimal platform for unraveling dissipation mechanisms in 2D superconductors.

Also see Blog: Behind the paper - Nature Communities

Practical realization of oxide-based electronics requires in depth knowledge about different charge trapping/detrapping pathways under external electric field. In our work published in Physical Review Applied 15, 054008 (2021), we demonstrate the definite role of naturally occurring topological defects namely ferroelastic twin walls in complex oxides as a prominent source of charge trapping. For this, we utilize a conducting interface between two bulk insulating oxides γ-Al2O3 and SrTiO3 (STO) where the conducting interface is in close vicinity to the twin walls of STO. By making field effect transistors in back-gate geometry, we detect clear signature of charge trapping (detrapping) at (from) ferroelastic twin walls of STO below its structural transition temperature. More interestingly, we are also able to tune the extent of such trapping/detrapping by an external electric field. The number of trapped (detrapped) charges at (from) the twin wall is controlled by the net polarity of the wall and is a completely reversible process. The possibility of tuning such microscopic twin walls can be used to realize next generation novel functional devices and opens a path for twin wall-based electronics which we refer to “Twintronics”.

Improving data storage is becoming increasingly urgent as enormous amounts of data are generated every second. Current technology using conventional magnetic systems is reaching its limits, and researchers across the world are intently searching for alternatives. Quasiparticles called magnetic skyrmions, which have nanoscale non-coplanar spin configurations with twists, are a promising platform for ultra-dense, low-power memory applications. Other novel magnetic textures such as antiskyrmions and merons have also been observed, with different types of non-coplanar spin configurations, and are therefore equally promising. Realizing such non-coplanar spin textures in new materials and through new routes is of great interest in recent times. However, till date, this has been restricted only to magnetic materials which already possess localized magnetic moments. In our work published in Advanced Quantum Technologies 3, 2000021 (2020) & Physical Review B 103, 0851205 (2021), we successfully demonstrate a new route to realize non-coplanar spin configuration in a non-magnetic system. We achieve this by deliberately creating oxygen vacancies in a non-magnetic band insulator called KTaO3 and find that the delicate interplay between quantum fluctuation-stabilized polar nano regions and localized magnetic moments around oxygen vacancies are directly responsible for resulting non-coplanar spin configuration in present case. When an electron moves in the presence of such non-trivial spin texture, it acquires a real-space Berry phase, leading to the observation of the topological Hall effect. Notably, this is a rare occurrence where real-space topology has been observed in a paramagnetic metal phase, offering an exceptional opportunity to engineer non-trivial spin textures in otherwise non-magnetic materials.

Also see: IISc News

The origin of simultaneous electronic, structural, and magnetic transitions in bulk rare-earth nickelates (RENiO3) remains puzzling with multiple conflicting reports on the nature of these entangled phase transitions. Heterostructure engineering of these materials offers unique opportunity to decouple the metal-insulator transition (MIT) from the magnetic transition. However, the evolution of underlying electronic properties across these decoupled transitions remains largely unexplored. In order to address this, we have measured Hall effect on a series of epitaxial NdNiO3 films, spanning a variety of electronic and magnetic phases (Physical Review B 99, 235153 (2019)). We find that the MIT results in only a partially gapped Fermi surface, whereas the full insulating phase forms below the magnetic transition. In addition, we also find a systematic reduction of the Hall coefficient RH in the metallic phase of these films with epitaxial strain and also a surprising transition to a negative value at large compressive strain. The partially gapped, weakly insulating, paramagnetic phase is reminiscence of pseudogap behavior of high-Tc cuprates. The precursor metallic phase, which undergoes transition to the insulating phase, is a non-Fermi liquid with a temperature exponent n of resistivity of 1, whereas the exponent increases to 4/3 in the noninsulating samples. Such a nickelate phase diagram with sign reversal of RH , a pseudogap phase, and non-Fermi-liquid behavior is intriguingly similar to high-Tc cuprates, giving important guidelines to engineer unconventional superconductivity in oxide heterostructures.