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

Abstract: The lowest energy states in transition metal dichalcogenide (TMD) monolayers follow valley selection rules, which have attracted vast interest due to the possibility of encoding and processing of quantum information. However, these quantum states are strongly affected by the temperature-dependent intervalley scattering causing complete valley depolarization, which is hampering any practical applications of TMD monolayers at room temperature. Therefore, for achieving clear and robust valley polarization in TMD monolayers one needs to suppress parasitic depolarization processes, which is the central challenge in the growing field of valleytronics. Here, in electron-doping experiments on TMD monolayers, we demonstrate that strong doping levels beyond $10^{13}$~cm$^{-2}$ can induce 61\% and 37\% valley contrast at room temperature in tungsten diselenide and molybdenum diselenide monolayers, respectively. Our results indicate that charged excitons in TMD monolayers can be utilized as quantum units in designing of practical valleytronic devices operating at 300 K.

Abstract: The discovery of all-optical ultra-fast deterministic magnetization switching has opened up new possibilities for manipulating magnetization in devices using femtosecond laser pulses. Previous studies on single pulse all-optical helicity-independent switching (AO-HIS) have mainly focused on perpendicularly magnetized thin films. This work presents a comprehensive study on AO-HIS for in-plane magnetized GdxCo100-x thin films. Deterministic single femtosecond laser pulse toggle magnetization switching is demonstrated in a wider concentration range (x=10% to 25%) compared to the perpendicularly magnetized counterparts with GdCo thicknesses up to 30 nm. The switching time strongly depends on the GdxCo100-x concentration, with lower Gd concentration exhibiting shorter switching times (less than 500 fs). Our findings in this geometry provide insights into the underlying mechanisms governing single pulse AO-HIS, which challenge existing theoretical predictions. Moreover, in-plane magnetized GdxCo100-x thin films offer extended potential for opto-spintronic applications compared to their perpendicular magnetized counterparts.

Abstract: We investigate the superconducting properties of inversion-symmetric twisted trilayer graphene by considering different parent states, including spin-singlet, triplet, and SO(4) degenerate states, with or without nodal points. By placing transition metal dichalcogenide layers above and below twisted trilayer graphene, spin-orbit coupling is induced in TTLG and, due to inversion symmetry, the spin-orbit coupling does not spin-split the bands. The application of a displacement field ($D_0$) breaks the inversion symmetry and creates spin-splitting. We analyze the evolution of the superconducting order parameters in response to the combined spin-orbit coupling and $D_0$-induced spin-splitting. Utilizing symmetry analysis combined with both a direct numerical evaluation and a complementary analytical study of the gap equation, we provide a comprehensive understanding of the influence of spin-orbit coupling and $D_0$ on superconductivity. These results contribute to a better understanding of the superconducting order in twisted trilayer graphene.

Abstract: For a smooth surface, the chemical composition can be readily evaluated by a variety of spectroscopy techniques; a prominent example is X-ray photoelectron spectroscopy (XPS), where the relative proportions of the elements are mainly determined by the intensity ratio of the element-specific photoelectrons. This deduction, however, is more intricate for a nanorough surface, such as black silicon, since the steep slopes of the geometry mimic local variations of the local emission angle. Here, we explicitly quantify this effect via an integral geometric analysis, by using so-called Minkowski tensors. Thus, we match the chemical information from XPS with topographical information from atomic force microscopy (AFM). Our method provides reliable estimates of layer thicknesses for nanorough surfaces. For our black silicon samples, we found that the oxide layer thickness is on average comparable to that of a native oxide layer. Our study highlights the impact of complex geometries at the nanoscale on the analysis of chemical properties with implications for a broad class of spectroscopy techniques.

Abstract: In this Reply we respond to the comment by Antipov et al. from Microsoft Quantum on Hess et al., PRL 130, 207001 (2023). Antipov et al. reported only a single simulation and claimed it did not pass the Microsoft Quantum topological gap protocol (TGP). They have provided no parameters or data for this simulation (despite request). Regardless, in this reply we demonstrate that the trivial bulk gap reopening mechanism outlined in Hess et al., in combination with trivial ZBPs, passes the TGP and therefore can result in TGP false positives.

Abstract: III-nitride wide bandgap semiconductors exhibit large exciton binding energies, preserving strong excitonic effects at room temperature. On the other hand, semiconducting two-dimensional (2D) materials, including MoS$_2$, also exhibit strong excitonic effects, attributed to enhanced Coulomb interactions. This study investigates excitonic interactions between surface GaN quantum well (QW) and 2D MoS$_2$ in van der Waals heterostructures by varying the spacing between these two excitonic systems. Optical property investigation first demonstrates the effective passivation of defect states at the GaN surface through MoS$_2$ coating. Furthermore, a strong interplay is observed between MoS$_2$ monolayers and GaN QW excitonic transitions. This highlights the interest of the 2D material/III-nitride QW system to study near-field interactions, such as F\"orster resonance energy transfer, which could open up novel optoelectronic devices based on such hybrid excitonic structures.

Abstract: Strongly enhanced electron-electron interaction in semiconducting moir\'e superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers has led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type-II aligned TMDC heterobilayer moir\'e superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and strongly interact with each other, promising for realizing tunable correlated bosonic quasiparticles with valley degree of freedom. We employ photoluminescence spectroscopy to investigate the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moir\'e superlattice and combine with theoretical calculations to reveal the spatial extent of interlayer excitons and the band hierarchy of correlated states. We further find that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moir\'e cell, evidenced by emerging photoluminescence peaks under increased optical excitation power. Double occupancy of excitons in one unit cell requires overcoming the energy cost of exciton-exciton repulsion of about 30-40 meV, depending on the stacking configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates the WS2/WSe2 moir\'e superlattice as a promising platform for engineering and exploring new correlated states of fermion, bosons, and a mixture of both.