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

Abstract: Localised emission from defect states in monolayer transition metal dichalcogenides is of great interest for optoelectronic and quantum device applications. Recent progress towards high temperature localised emission relies on the application of strain to induce highly confined excitonic states. Here we propose an alternative paradigm based on curvature, rather than in-plane stretching, achieved through free-standing wrinkles of monolayer tungsten diselenide (WSe2). We probe these nanostructures using tip-enhanced optical spectroscopy to reveal the spatial localisation of out-of-plane polarised emission from the WSe2 wrinkles. Based on the photoluminescence and Raman scattering signatures resolved with nanoscale spatial resolution, we propose the existence of a manifold of spin-forbidden excitonic states that are activated by the local curvature of the WSe2. We are able to access these dark states through the out-of-plane polarised surface plasmon polariton resulting in enhanced strongly localised emission at room temperature, which is of potential interest for quantum technologies and photonic devices.

Abstract: We examine the response of the Fermi arc in the context of quasi-particle interference (QPI) with regard to a localized surface impurity on various three-dimensional Weyl semimetals (WSMs). Our study also reveals the variation of the local density of states (LDOS), obtained by Fourier transforming the QPI profile, on the two-dimensional surface. We use the $T$-matrix formalism to numerically (analytically and numerically) capture the details of the momentum space scattering in QPI (real space decay in LDOS), considering relevant tight-binding lattice and/or low-energy continuum models modeling a range of different WSMs. In particular, we consider multi-WSM (mWSM), hosting multiple Fermi arcs between two opposite chirality Weyl nodes (WNs), where we find a universal $1/r$-decay ($r$ measuring the radial distance from the impurity core) of the impurity-induced LDOS, irrespective of the topological charge. Interestingly, the inter-Fermi arc scattering is only present for triple WSMs, where we find an additional $1/r^3$-decay as compared to double and single WSMs. The untilted single (double) [triple] WSM shows a straight-line (leaf-like) [oval-shaped] QPI profile. The above QPI profiles are canted for hybrid WSMs where type-I and type-II Weyl nodes coexist, however, hybrid single WSM demonstrates strong non-uniformity, unlike the hybrid double and triple WSMs. We also show that the chirality and the positions of the Weyl nodes imprint marked signatures in the QPI profile. This allows us to distinguish between different WSMs, including the time-reversal-broken WSMs from the time-reversal-invariant WSM, even though both of the WSMs can host two pairs of Weyl nodes. Our study can thus shed light on experimentally obtainable complex QPI profiles and help differentiate different WSMs and their surface band structures.

Abstract: Employing the original, all-optical method, we quantify the magnetic susceptibility of a two-dimensional electron gas (2DEG) confined in the MoSe$_2$ monolayer in the range of low and moderate carrier densities. The impact of electron-electron interactions on the 2DEG magnetic susceptibility is found to be particularly strong in the limit of, studied in detail, low carrier densities. Following the existing models, we derive $g_0 = 2.5 \pm 0.4$ for the bare (single particle) g-factor of the ground state electronic band in the MoSe$_2$ monolayer. The derived value of this parameter is discussed in the context of estimations from other experimental approaches. Surprisingly, the conclusions drawn differ from theoretical ab-initio studies.

Abstract: We investigate the orbital Hall effect through a disordered mesoscopic device with momentum-space orbital texture that is connected to four semi-infinite terminals embedded in the Landauer-B\"uttiker configuration for quantum transport. We present clear analytical and numerical evidence that the orbital Hall current fluctuations are universals (as with spin Hall current fluctuations). The universal orbital Hall current fluctuations (UOCF) exhibit two universal numbers of 0.36 and 0.18 for weak and strong spin-orbit coupling, respectively. The universal numbers are obtained by analytical calculation via random matrix theory and are supported by numerical calculations based on the tight-binding model. Furthermore, the UOCF lead to two universal relationships between the orbital Hall angle and conductivity. Finally, we confront the two universal relations with experimental data of the orbital Hall angle, which shows good concordance between theory and experiment.