Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Ferroic orders describe spontaneous polarization of spin, charge, and lattice degrees of freedom in materials. Materials featuring multiple ferroic orders, known as multiferroics, play important roles in multi-functional electrical and magnetic device applications. 2D materials with honeycomb lattices offer exciting opportunities to engineer unconventional multiferroicity, where the ferroic orders are driven purely by the orbital degrees of freedom but not electron spin. These include ferro-valleytricity corresponding to the electron valley and ferro-orbital-magnetism supported by quantum geometric effects. Such orbital multiferroics could offer strong valley-magnetic couplings and large responses to external fields-enabling device applications such as multiple-state memory elements, and electric control of valley and magnetic states. Here we report orbital multiferroicity in pentalayer rhombohedral graphene using low temperature magneto-transport measurements. We observed anomalous Hall signals Rxy with an exceptionally large Hall angle (tan{\Theta}H > 0.6) and orbital magnetic hysteresis at hole doping. There are four such states with different valley polarizations and orbital magnetizations, forming a valley-magnetic quartet. By sweeping the gate electric field E we observed a butterfly-shaped hysteresis of Rxy connecting the quartet. This hysteresis indicates a ferro-valleytronic order that couples to the composite field E\cdot B, but not the individual fields. Tuning E would switch each ferroic order independently, and achieve non-volatile switching of them together. Our observations demonstrate a new type of multiferroics and point to electrically tunable ultra-low power valleytronic and magnetic devices.

Abstract: We investigated the effect of Fe segregated from partially Fe-substituted MgO (MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray photoelectron spectroscopy (XPS) as well as magnetic measurements revealed that the segregated Fe was reduced to metal and exhibited ferromagnetism at the CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed more than 2-fold enhancement in perpendicular magnetic anisotropy (PMA) energy density compared with a standard CoFeB/MgO multilayer. The PMA energy density was further enhanced by inserting an ultrathin MgO layer in between CoFeB and MgFeO layers. Ferromagnetic resonance measurement also revealed a remarkable reduction of magnetic damping in the CoFeB/MgFeO multilayers.

Abstract: Destructive interference between electron wavefunctions on the two-dimensional (2D) kagome lattice induces an electronic flat band, which could host a variety of interesting many-body quantum states. Key to realize these proposals is to demonstrate the real space localization of kagome flat band electrons. In particular, the extent to which the often more complex lattice structure and orbital composition of realistic materials counteract the localizing effect of destructive interference, described by the 2D kagome lattice model, is hitherto unknown. We used scanning tunneling microscopy (STM) to visualize the non-trivial Wannier states of a kagome flat band at the surface of CoSn, a kagome metal. We find that the local density of states associated with the flat bands of CoSn is localized at the center of the kagome lattice, consistent with theoretical expectations for their corresponding Wannier states. Our results show that these states exhibit an extremely small localization length of two to three angstroms concomitant with a strongly renormalized quasiparticle velocity, which is comparable to that of moir\'e superlattices. Hence, interaction effects in the flat bands of CoSn could be much more significant than previously thought. Our findings provide fundamental insight into the electronic properties of kagome metals and are a key step for future research on emergent many-body states in transition metal based kagome materials.

Abstract: The development of multidimensional, ultrafast spectroscopy techniques calls for the development of efficient computational schemes that allow for the simulation of such experiments and thus for the interpretation of the corresponding results. In this work, we present the development of a fully first-principles scheme to compute two-dimensional electron spectroscopy maps based on real-time time-dependent density-functional theory. The interface of this approach with the Ehrenfest scheme for molecular dynamics enables the inclusion of vibronic effects in the calculations. We demonstrate the effectiveness of this method by applying it to prototypical molecules such as benzene, pyridine, and pyrene. We discuss the role of the approximations that inevitably enter the adopted theoretical framework and set the stage for further extensions of the proposed method.

Abstract: We theoretically consider a graphene ripple as a Brownian particle coupled to an energy storage circuit. When circuit and particle are at the same temperature, the second law forbids harvesting energy from the thermal motion of the Brownian particle, even if the circuit contains a rectifying diode. However, when the circuit contains a junction followed by two diodes wired in opposition, the approach to equilibrium may become ultraslow. Detailed balance is temporarily broken as current flows between the two diodes and charges storage capacitors. The energy harvested by each capacitor comes from the thermal bath of the diodes while the system obeys the first and second laws of thermodynamics.

Abstract: The interplay between magnetic and topological order can give rise to phenomena such as the quantum anomalous Hall effect. The extension of topological surface states into magnetic insulators (MIs) has been proposed as an alternative to using intrinsically magnetic topological insulators (TIs). Here, we theoretically study how this extension of surface states into a magnetic insulator are influenced both by the interface barrier potential separating a topological insulator and a magnetic insulator and by finite size effects in such structures. We find that the the gap in the surface states depends non-monotonically on the barrier strength. A small, but finite, barrier potential turns out to be advantageous as it permits the surface states to penetrate even further into the MI. Moreover, we find that due to finite size effects in thin samples, increasing the spin-splitting in the MI can actually decrease the gap of the surface states, in contrast to the usual expectation that the gap opens as the spin-splitting increases.

Abstract: Nonlocal and quantum mechanical phenomena in noble metal nanostructures become increasingly crucial when the relevant length scales in hybrid nanostructures reach the few-nanometer regime. In practice, such mesoscopic effects at metal-dielectric interfaces can be described using exemplary surface-response functions (SRFs) embodied by the Feibelman $d$-parameters. Here we show that SRFs dramatically influence quantum electrodynamic phenomena -- such as the Purcell enhancement and Lamb shift -- for quantum emitters close to a diverse range of noble metal nanostructures interfacing different homogeneous media. Dielectric environments with higher permittivities are shown to increase the magnitude of SRFs calculated within the specular-reflection model. In parallel, the role of SRFs is enhanced in nanostructures characterized by large surface-to-volume ratios, such as thin planar metallic films or shells of core-shell nanoparticles. By investigating emitter quantum dynamics close to such plasmonic architectures, we show that decreasing the width of the metal region, or increasing the permittivity of the interfacing dielectric, leads to a significant change in the Purcell enhancement, Lamb shift, and visible far-field spontaneous emission spectrum, as an immediate consequence of SRFs. We anticipate that fitting the theoretically modelled spectra to experiments could allow for experimental determination of the $d$-parameters.

Abstract: In a recent publication [Wurdack et al., Nat. Comm. 14:1026 (2023)], it was shown that in microcavities containing atomically thin semiconductors non-Hermitian quantum mechanics can lead to negative exciton polariton masses. We show that mass-sign reversal can occur generally in radiative resonances in two dimensions (without cavity) and derive conditions for it (critical dephasing threshold etc.). In monolayer transition-metal dichalcogenides, this phenomenon is not invalidated by the strong electron-hole exchange interaction, which is known to make the exciton massless.