Joerg Debus

Spin‐dark excitons in semiconductor nanostructures are inherently optically silent: angular‐momentum selection rules forbid their radiative recombination, granting long lifetimes attractive for quantum information storage and excitonic condensation, but placing their energy structure and bright–dark coupling beyond standard optical spectroscopy. Established access routes rely on symmetry‐lowering perturbations, dedicated single‐emitter device fabrication, or near‐field methods, each of which perturbs the states under study or excludes ensembles. Here, we show that a Fano‐type quantum interference between two acoustic–phonon scattering pathways, consisting of a resonant spin‐flip process coupling bright and dark excitons and a non‐resonant inelastic process sharing the same acoustic phonon mode, opens an optical window onto the dark exciton in unperturbed ensembles of (In,Ga)As/GaAs quantum dots. The interference manifests as antiresonance and bipolar spectral profiles whose shape and position encode the bright–dark exciton coupling, are tunable through magnetic‐field orientation, optical pumping intensity, and temperature, and persist to about 40 K, with a characteristic thermal scale of about 5 meV. The same lineshape analysis reveals a sign reversal of the exciton exchange energy at excited quantum‐dot shells. Relying only on exchange‐split exciton states and acoustic phonons, the mechanism should transfer to two‐dimensional transition metal dichalcogenides and perovskite nanocrystals, where dark excitons play a central role.

Understanding the response of diamond abrasive grains to mechanical loading remains crucial for optimizing their performance in precision manufacturing. In that context, the role of the initial residual stress and crystallographic orientation of the grains is poorly understood. We investigate synthetic diamond grains in two different grit sizes (D126, D252) subjected to directional loading up to 100 N using x-ray diffraction and spatially resolved Raman spectroscopy. Small grains with preferential (111) orientation show an unexpected stress evolution under contact pressures up to 16 GPa, with Raman shifts increasing from 1331.3 to 1333.0 cm − 1 , indicating an enhanced local compressive stress state at the probed surface regions. Conversely, large (311)-oriented grains exhibit a heterogeneous stress development with Raman shifts varying from 1331.5 to 1332.6 cm − 1 , including regions that become more tensile (or less compressive) relative to their initial state. The relationship between the relative Raman shift and a spatially weighted contact pressure follows an empirical power law δ E ∝ p c ′ α with opposite, orientation-dependent exponents: α ≈ + 0.25 for (111) grains showing progressive compression enhancement and α ≈ − 0.35 for (311) grains exhibiting a trend toward tensile stress components with increasing load. Molecular dynamics simulations reveal that the Schmid factor governs this orientation-dependent response: The low Schmid factor (0.27) for [111] loading restricts dislocation glide, leading to stress retention with high elastic recovery, while the high Schmid factor (0.45) for [311] loading facilitates plastic flow and stress relaxation despite lower peak stress. These findings demonstrate that stress evolution in diamond under directional loading is controlled by the geometric relationship between loading direction and slip systems, providing mechanistic insights for diamond tool design.

We report an anomalous valley Zeeman effect in a monolayer with induced by in‐plane magnetic fields. Polarization‐resolved photoluminescence at K reveals linear valley splittings with effective ‐factors of (trion), (intravalley A exciton), and (intervalley exciton) in Voigt geometry. These values exceed both the out‐of‐plane response () and the expected zero splitting for in‐plane fields in binary transition‐metal dichalcogenides. Crucially, resonant inelastic laser‐light scattering yields much smaller ‐factors of 1.16–1.67, consistent with single‐particle Zeeman splitting. The factor‐of‐four ‐factor discrepancy, with near‐constant enhancement across excitonic complexes, indicates that the anomalous valley splitting arises from many‐body interactions. We attribute this effect to anisotropic tensor exchange coupling between excitons and the spin‐polarized Fermi sea. The bright exciton carries valley‐antisymmetric total spin, while the in‐plane field induces Pauli paramagnetic polarization in the Fermi sea. Isotropic exchange cannot couple these orthogonal spin components, but spin‐orbit‐induced anisotropic exchange allows for the coupling and produces valley‐dependent energy shifts. The effect is enhanced because the conduction‐band spin‐orbit splitting nearly vanishes at , unlocking the Fermi sea's in‐plane susceptibility. Our findings establish TMDC alloys near the spin‐orbit crossover as a platform for many‐body‐enhanced valley manipulation using in‐plane magnetic fields.

Gross-slip fretting corrosion occurs frequently inside taper joints of endoprosthetic implants and is expected to be accompanied by the formation of a metal-organic tribomaterial, which may influence the friction and wear behavior of implants. While it is hypothesized that it contains compounds of the surrounding human body fluid and worn particles of the implant materials, its structure, composition, formation mechanism, and distribution inside the wear area remain elusive. In a multiscale structural-chemical study using Raman scattering spectroscopy and transmission electron microscopy, we reveal the tribological formation of nanocrystalline cobalt sulfide for a CoCrMo/TiAlV couple that was subjected to in vitro gross-slip fretting in bovine calf serum. We demonstrate that sulfur atoms released from the mechanochemical decomposition of cysteine/cystine disulfide bonds in serum proteins ─ as evidenced by the complete absence of characteristic S-S Raman modes and concurrent protein unfolding from α-helix to β-sheet structures ─ react with cobalt ions released tribocorrosively from the alloy to form the CoS_x tribomaterial. The resulting tribofilm is substoichiometric (x < 2) with a primary cubic structure partially mixed by a hexagonal phase. It perfectly adheres to the Co alloy through mechanical mixing, thus exhibiting the structural features of extreme-pressure antiwear additives. The CoS_x tribofilm covers 12.2% of the fretting track in the CoCrMo alloy, while maintaining a consistent thickness of approximately 15 nm. Multiple mechanisms driving this transformation are discussed: mechanical protein unfolding exposes disulfide bonds to forces that reduce their cleavage activation energy, local temperatures allow for thermal decomposition, and the acidic crevice environment facilitates chemical cleavage. The observations on the fretting-generated CoS_x tribomaterial provide the first comprehensive structural-chemical insight into tribologically beneficial features of transition metal sulfides formed in medical alloys through protein-derived sulfur. Understanding this mechanism may enable strategies to deliberately promote such protective film formation to improve the longevity of taper junctions in medical applications.

Atomic defects associated with vacancies in two-dimensional transition metal dichalcogenide monolayers efficiently trap charged carriers and strongly localize excitons. Defects in semiconducting monolayers are seldomly utilized for enhancing optical phenomena, although they may provide resonant intermediate states within the energy band gap for applications with multiphoton excitations, like highly efficient and thermally robust photon upconversion. In an MoS₂ monolayer encapsulated by hBN with high defect and resident electron densities, we observe an upconversion of localized exciton (X_L) emission with a huge energy gain of up to 290 meV. The upconverted X_L emission is robust up to temperatures of about 120 K and exhibits a sublinear or a nearly linear laser power dependence for the energy gain of about 100 meV and above 200 meV, respectively. The upconversion mechanism is explained by a cooperative energy transfer process between the photocreated and resident electrons, in which hybridized pairs of single sulfur vacancies likely act as real intermediate states. Additionally, we find a weak upconversion of the neutral exciton photoluminescence with an energy gain of about 350 meV for quasi-resonant excitation of the X_L exciton. It is attributed to a two-step, two-photon absorption.