Our group investigates new physical phenomena the emerge in nanoscale systems, at surfaces and interfaces of different materials. Our overarching goal is to develop fundamental understanding of the effects of confinement, interfaces, the resulting emerging interactions, and strongly nonequilibrium physical states that become possible to achieve only at nanoscale, and use this knowledge to develop nanoscale devices with new functionalities. Our laboratory is located in rooms W104, W106, and E108 of the Math and Science Building on the beautiful campus of Emory University.

Research highlights

“Electronic properties of the mean-field resonance valence band model of cuprates; arXiv:2106.09924 . We theoretically address the electronic properties of high temperature cuprate superconductors. This topic has been extensively researched by the condensed matter community for 36 years, but a widely accepted theory has not yet emerged. We demonstrate that a simple mean-field resonance valence bond model of cuprates proposed in 1987 (35 years ago) by P.W. Anderson and co-workers explains the main puzzling properties of cuprates, including the "d-wave symmetry" of pairing, the "strange metal" behaviors in the normal state, the pseudo-gap observed in the normal state, and the common charge density modulations. These features stem from the interplay between spin excitations (spinons) and singlet pairs of electrons localized at the intersection between the Brillouin zone boundary and the nodal lines of the spinon spectrum. Our results may provide a pathway for room-temperature superconductivity at ambient pressure.

distribution of anisotropy and torque “Exchange bias without directional anisotropy”, PRB 2021. This paper addresses the 60+ year-old problem of exchange bias (EB) - directional anisotropy of magnetic hysteresis loop observed in ferromagnet/antiferromagnet (F/AF) bilayers. Traditionally, this effect is explained by the unidirectional anisotropy of F due to the uncompensated AF spins at the interface. We utilize a combination of several experimental techniques and simulations to demonstrate, for a classic F/AF system formed by AF=CoO and F=Permalloy, that the unidirectional anisotropy is vanishingly small. We show the EB is likely caused by the hotspots of twisted local magnetization, which serve as nucleation centers biasing the magnetization reversal. The Figure on the left shows the results of the simulations of torques exerted on the magnetization of F with two different thicknesses, as well as the simulated distribution of unidirectional anisotropy. Suprisingly, the torquess and the anisotropies strongly depend on the thickness of F, which can be explained by the 2d-to-3d dimensional crossover of its response to random interfacial fields, as demonstrated in our previous work. Our findings facilitate the efforts to develop memristive memories taking advantage of multistable magnetization configuraitons in F/AF-based heterostructures.

nonlocal resistance vs current “Transport and relaxation of nonequilibrium phonons generated by current”, PRB (2022). Current-generated Joule heat is a major roadblock for the miniaturization and the increase of speed of electronic nanodevices. The present understanding of this phenomenon and the approaches to its mitigation are based on the assumption that the phonons generated by current form a thermal distribution. We perform nonlocal electronic measurements utilizing an electrically-biased metallic nanowire as a phonon source, and a separate nanowire serving as the phonon detector, to demonstrate that contrary to the 150 year-old paradigm, the distribution of phonons generated by current is highly non-thermal. We analyze the dependence on the thickness of the spacer separating the nanowires, to show that these non-equilibrium phonons relax via strongly anharmonic processes that cannot be described in terms of the usual few-phonon scattering. Our findings provide insight into the mechanisms of current-driven phonon generation, transport, and relaxation at nanoscale, which will likely facilitate new approaches to efficient Joule heat dissipation in nanodevices.

Quantum vs classical ST in AF “Non-classical spin transfer effects in an antiferromagnet”, PRL 2021. We utilize simulations of electron scattering by a chain of quantum spins, to demonstrate large contributions to the spin transfer effect in antiferromagnets (AFs) that cannot be accounted for by the existing semiclassical models of magnetism. AFs are attractive for the development of novel nanomagnetic devices, since their vanishing magnetization and high characteristic dynamical frequencies may enable fast and low-power device operation. Spin transfer effect the transfer of angular momentum from the conduction electrons to the magnetic system plays a central role in such devices, enabling electronically-driven rotation of magnetic order. Our results demonstrate the possibility to achieve a substantial enhancement of spin transfer efficiency by taking advantage of non-classical effects, and suggest the possibility to utilize spin transfer to control purely quantum magnetic systems such as spin liquids.

<< Nanolab research highlight archive