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

Abstract: Magic-angle twisted bilayer graphene (MATBG) has been extensively explored both theoretically and experimentally as a suitable platform for a rich and tunable phase diagram that includes ferromagnetism, charge order, broken symmetries, and unconventional superconductivity. In this work, we investigate the intricate interplay between long-range electron-electron interactions, spin fluctuations, and superconductivity in MATBG. By employing a low-energy model for MATBG that captures the correct shape of the flat bands, we explore the effects of short- and long-range interactions on spin fluctuations and their impact on the superconducting (SC) pairing vertex in the Random Phase Approximation (RPA). We find that the SC state is notably influenced by the strength of long-range Coulomb interactions. Interestingly, our RPA calculations indicate that there is a regime where the system can traverse from a magnetic phase to the SC phase by \emph{increasing} the relative strength of long-range interactions compared to the on-site ones. These findings underscore the relevance of electron-electron interactions in shaping the intriguing properties of MATBG and offer a pathway for designing and controlling its SC phase.

Abstract: Coulomb impurity of charge $Ze$ is known to destabilize the ground state of undoped graphene with respect to creation of screening space charge if $Z$ exceeds a critical value of $1/2\alpha$ set by material's fine structure constant $\alpha$. Recent experimental advances made it possible to explore this transition in a controlled manner by tuning $Z$ across the critical point. Combined with relatively large value of $\alpha$ this opens a possibility to study graphene's screening response to a supercritical impurity $Z\alpha\gg1$ when the screening charge is large, and the Thomas-Fermi analysis, that we revisit, is adequate. The character of screening in this regime is controlled by the dimensionless screening parameter $Z\alpha^{2}$. Specifically, for circular impurity cluster most of the screening charge in the weak-screening regime $Z\alpha^{2}\ll1$ is found to reside outside the cluster. The strong-screening regime $Z\alpha^{2}\gg1$ provides a realization of the Thomson atom: most of the screening charge is inside the cluster nearly perfectly neutralizing the source charge with the exception of a transition layer near cluster's edge where the rest of the space charge is localized.

Abstract: We consider a model of two harmonically driven damped harmonic oscillators that are coupled linearly and with a cross-Kerr coupling. We show how to distinguish this combination of coupling types from the case where a coupling of optomechanical type is present. This can be useful for the characterization of various nonlinear systems, such as mechanical oscillators, qubits, and hybrid systems. We then consider a hybrid system with linear and cross-Kerr interactions and a relatively high damping in one of the modes. We derive a quantum Hamiltonian of a doubly clamped magnetic beam, showing that the cross-Kerr coupling is prominent there. We discuss, in the classical limit, measurements of its linear response as well as the specific higher-harmonic responses. These frequency-domain measurements can allow estimating the magnitude of the cross-Kerr coupling or its magnon population.

Abstract: We report resonant second harmonic generation (SHG) spectroscopy of an hBN-encapsulated monolayer of MoS$_2$. By tuning the energy of the excitation laser, we identify a dark state transition (D) that is blue detuned by +25 meV from the neutral exciton X$^0$. We observe a splitting of the SHG spectrum into two distinct peaks and a clear anticrossing between them as the SHG resonance is tuned through the energy of the dark exciton D. This observation is indicative of quantum interference arising from the strong two-photon light-matter interaction. We further probe the incoherent relaxation from the dark state to the bright excitons, including X$^0$ and localized excitons LX, by the resonant enhancement of their intensities at the SHG-D resonance. The relaxation of D to bright excitons is strongly suppressed on the bare substrate whilst enabled when the hBN/MoS$_2$/hBN heterostructure is integrated in a nanobeam cavity. The relaxation enabled by the cavity is explained by the phonon scattering enhanced by the cavity phononic effects. Our work reveals the two-photon quantum interference with long-lived dark states and enables the control through nanostructuring of the substrate. These results indicate the great potential of dark excitons in 2D-material based nonlinear quantum devices.

Abstract: We used optical laser pulses to create transient gratings (TGs) with sub-10 {\mu}m spatial periodicity in a Bismuth Germanate (310) (Bi4Ge3O12) single crystal at room temperature. The TG launches phonon modes, whose dynamics were revealed via forward diffraction of a third, time-delayed, heterodyne-detected optical pulse. Acoustic oscillations have been clearly identified in a time-frequency window not covered by previous spectroscopic studies and their characteristic dynamic parameters have been measured as a function of transferred momenta magnitude and direction.

Abstract: A pivotal challenge in present quantum technologies lies in reconciling long coherence times with efficient manipulation of the quantum states of a system. Lanthanide atoms, with their well-localized 4f electrons, emerge as a promising solution to this dilemma if provided with a rational design of the manipulation and detection schemes. Here we utilize a scanning tunneling microscope to construct tailored spin structures and perform electron spin resonance on a single lanthanide atom in such a structure. A magnetically coupled structure made of an Erbium and a Titanium atom at sub-nanometer distance enables us to both drive Erbium's 4f electron spins and indirectly probe them through the Titanium's 3d electrons. In this coupled configuration, the Erbium spin states exhibit a four-fold increase in the spin relaxation time and a two-fold increase in the driving efficiency compared to the 3d electron counterparts. Our work provides a new approach to accessing highly protected spin states, enabling us to control them in an all-electric fashion.

Abstract: Altermagnets (AM) are a recently discovered third class of collinear magnets, distinctly different from conventional ferromagnets (FM) and antiferromagnets (AF). AM have been actively researched in the last few years, but two aspects so far remain unaddressed: (1) Are there realistic 2D single-layer altermagnets? And (2) is it possible to functionalize a conventional AF into AM by external stimuli? In this paper we address both issues by demonstrating how a well-known 2D AF, MnP(S,Se)$_3$ can be functionalized into strong AM by applying out-of-plane electric field. Of particular interest is that the induced altermagnetism is of a higher even-parity wave symmetry than expected in 3D AM with similar crystal symmetries. We confirm our finding by first-principles calculations of the electronic structure and magnetooptical response. We also propose that recent observations of the time-reversal symmetry breaking in the famous Fe-based superconducting chalchogenides, either in monolayer form or in the surface layer, may be related not to an FM, as previously assumed, but to the induced 2D AM order. Finally, we show that monolayer FeSe can simultaneously exhibit unconventional altermagnetic time-reversal symmetry breaking and quantized spin Hall conductivity indicating possibility to research an intriquing interplay of 2D altermagnetism with topological and superconducting states within a common crystal-potential environment.