Explore the vast range of titles available.

Hidden Rashba and Dresselhaus spin splittings in centrosymmetric crystals with subunits/sectors having non-centrosymmetric symmetries (the R-2 and D-2 effects) have been predicted theoretically and then observed experimentally, but the microscopic mechanism remains unclear. Here we demonstrate that the spin splitting in the R-2 effect is enforced by specific symmetries, such as non-symmorphic symmetry in the present example, which ensures that the pertinent spin wavefunctions segregate spatially on just one of the two inversion-partner sectors and thus avoid compensation. We further show that the effective Hamiltonian for the conventional Rashba (R-1) effect is also applicable for the R-2 effect, but applying a symmetry-breaking electric field to a R-2 compound produces a different spin-splitting pattern than applying a field to a trivial, non-R-2, centrosymmetric compound. This finding establishes a common fundamental source for the R-1 effect and the R-2 effect, both originating from local sector symmetries rather than from the global crystal symmetry per se. The Dresselhaus and Rashba effects have traditionally been expected only in non-centrosymmetric systems but recent work has shown that they can exist in some centrosymmetric materials. Here the authors show that the so-called hidden Rashba effect originates from wavefunction segregation enforced by local symmetries.

Spin–orbit coupling can induce spin polarization in nonmagnetic 3D crystals when the inversion symmetry is broken, as manifested by the bulk Rashba and Dresselhaus effects. We establish that these spin-polarization effects originate fundamentally from specific atomic site asymmetries, rather than, as generally accepted, from the asymmetry of the crystal space group. This understanding leads to the recognition that a previously overlooked hidden form of spin polarization should exist in centrosymmetric crystals. Although all energy bands must be doubly degenerate in centrosymmetric materials, we find that the two components of such doubly degenerate bands could have opposite polarizations, each spatially localized on one of the two separate sectors forming the inversion partners. We demonstrate such hidden spin polarizations in particular centrosymmetric crystals by first-principles calculations. This new understanding could considerably broaden the range of currently useful spintronic materials and enable the control of spin polarization by means of operations on the atomic scale. Spin polarization due to spin–orbit coupling requires broken inversion symmetry. Now, calculations show that the effect arises from local site-asymmetry rather than global space-group asymmetry, and that a hitherto overlooked form of spin polarization should also exist in centrosymmetric structures.

Germanium (Ge) is an attractive material for Silicon (Si) compatible optoelectronics, but the nature of its indirect bandgap renders it an inefficient light emitter. Drawing inspiration from the significant expansion of Ge volume upon lithiation as a Lithium (Li) ion battery anode, here, we propose incorporating Li atoms into the Ge to cause lattice expansion to achieve the desired tensile strain for a transition from an indirect to a direct bandgap. Our first-principles calculations show that a minimal amount of 3 at.% Li can convert Ge from an indirect to a direct bandgap to possess a dipole transition matrix element comparable to that of typical direct bandgap semiconductors. To enhance compatibility with Si Complementary-Metal-Oxide-Semiconductors (CMOS) technology, we additionally suggest implanting noble gas atoms instead of Li atoms. We also demonstrate the tunability of the direct-bandgap emission wavelength through the manipulation of dopant concentration, enabling coverage of the mid-infrared to far-infrared spectrum. This Ge-based light-emitting approach presents exciting prospects for surpassing the physical limitations of Si technology in the field of photonics and calls for experimental proof-of-concept studies. The authors proposed a Silicon technology-compatible approach to convert Germanium from an indirect bandgap to a direct bandgap via doping. This is done to expand the lattice to produce tunable effective tensile strain, aiming towards the on-chip light sources.

The metal-induced gap states (MIGS) are commonly believed to cause the strong Fermi level pinning (FLP) in the metal-semiconductor contacts. Here, we reveal that the dangling-bond-induced surface states play a crucial role, even comparable with MIGS. The first-principles calculations show that metal-germanium (Ge) and metal-silicon (Si) contacts should possess a similar FLP strength if they adopt an identical interface bonding configuration: the reconstructed bonding configuration renders Si and Ge having pinning factors of 0.16 and 0.11, respectively, and the ideal non-reconstructed bonding configuration gives them pinning factors of 0.05 and 0, respectively. We illustrate that Si favors the reconstructed bonding configuration, and Ge favors the ideal non-reconstructed bonding configuration after metal deposition. The self-passivation of the dangling bonds reduces the interface gap states to give a much weaker FLP in the metal-Si contacts than in the metal-Ge contacts. We also demonstrate that the full passivation of the interface dangling bonds can further increase the pinning factor to 0.5 by further reducing the interface gap states. These findings shed light on alleviating the FLP to lower the contact resistance for Si and emerging materials towards advanced semiconductor technology. Here authors show that dangling-bond-induced surface states are critical in governing Fermi level pinning at the metal-semiconductor interfaces. The reconstruction-induced self-passivation of dangling bonds leads to Si having a weaker pinning than Ge, which prefers non-reconstruction.

Photoinduced phase transition (PIPT) is always treated as a coherent process, but ultrafast disordering in PIPT is observed in recent experiments. Utilizing the real-time time-dependent density functional theory method, here we track the motion of individual vanadium (V) ions during PIPT in VO₂ and uncover that their coherent or disordered dynamics can be manipulated by tuning the laser fluence. We find that the photoexcited holes generate a force on each V–V dimer to drive their collective coherent motion, in competing with the thermal-induced vibrations. If the laser fluence is so weak that the photoexcited hole density is too low to drive the phase transition alone, the PIPT is a disordered process due to the interference of thermal phonons. We also reveal that the photoexcited holes populated by the V–V dimerized bonding states will become saturated if the laser fluence is too strong, limiting the timescale of photoinduced phase transition.

The insulator-to-metal transition in VO 2 has garnered extensive attention for its potential applications in ultrafast switches, neuronal network architectures, and storage technologies. However, the photoinduced insulator-to-metal transition remains controversial, especially whether a complete structural transformation from the monoclinic to rutile phase is necessary. Here we employ the real-time time-dependent density functional theory to track the dynamic evolution of atomic and electronic structures in photoexcited VO 2 , revealing the emergence of a long-lived monoclinic metal phase under low electronic excitation. The emergence of the metal phase in the monoclinic structure originates from the dissociation of the local V-V dimer, driven by the self-trapped and self-amplified dynamics of photoexcited holes, rather than by an electron-electron correction. On the other hand, the monoclinic-to-rutile phase transition does appear at higher electronic excitation. Our findings validate the existence of monoclinic metal phase and provide a comprehensive picture of the insulator-to-metal transition in photoexcited VO 2 . Vanadium dioxide exhibits an insulator-to-metal transition when exciting by a laser. Here, the authors show the transition arises from the dissociation of local V-V dimers and show the existence of the monoclinic metallic phase from ab initio simulations.

Si quantum dots (QDs) have a significant improvement in luminous efficiency compared with bulk Si, achieved by alleviating the forbiddance of no-phonon Γ-Γ radiative transition determined by the law of momentum conservation. Two divergent mechanisms have been proposed to account for the breakdown of momentum conservation in Si QDs, one is due to the space-confinement-induced spread of k-space wave functions associated with Heisenberg uncertainty principle Δr · Δk > 1/2, and the other is due to the interface-effect-induced intervalley mixing between indirect and direct bandgap states. Both mechanisms could cause a small overlap of the electron and hole wave functions in k-space and make vertical transitions allowed, which leads to the zero-phonon light emission. In this work, we unravel the hierarchical relationship between these two primary mechanisms in the process of zero-phonon light emission from indirect bandgap QDs, by performing semiempirical pseudopotential calculation including many-body interaction on the room-temperature luminescent properties of a series of Si, Ge, and Ge/Si core/shell QDs. We show that the space confinement mechanism is dominant in both Si and Ge indirect bandgap QDs, and the interface-induced intervalley coupling mechanism plays a minor role. While in Ge/Si core/shell QDs, the interface-induced intervalley coupling mechanism has a more pronounced contribution to enhanced light emission, implying one can further enhance light emission via engineering interface based on the intervalley coupling mechanism. Given this, we further engineer the Ge QD interface by bringing four motifs of Si/Ge multiple layers from previously inverse designed Si/Ge superlattices and core/shell nanowires for light emitters. We show that two out of four motifs always give rise to two orders of magnitude enhancement in light emission relative to the Ge and Si QDs. We demonstrate that the interface engineering can enhance light emission in indirect bandgap QDs substantially and regulate the intervalley coupling mechanism as the primary factor over the space confinement mechanism in breaking the momentum conservation law.

Dislocations significantly influence carrier transport in semiconductors. While segments orthogonal to the channel act as scattering centers impeding conduction, electrically active dislocation cores can facilitate carrier transport. However, the mechanisms governing carrier transport along dislocation cores remain unclear. Here, we provide the first experimental evidence for the separate transport mechanisms of electrons and holes mediated by threading screw dislocations and threading edge dislocations in gallium nitride. Critically, we demonstrate that devices with a higher total dislocation density exhibit less degradation due to current collapse, owing to a larger proportion of edge dislocations mitigating electron trapping caused by screw dislocations. Screw dislocations promote electron leakage via horizontal potential barriers and vertically connected shallow states, while edge dislocations enhance hole transport through extended trap levels interacting with buffer defects. These findings clarify the long-standing debate on carrier-specific dislocation transport mechanisms and offer critical insights for defect engineering, epitaxial growth optimization, and the development of dislocation-enhanced semiconductor devices. Researchers show that two kinds of crystal dislocations in gallium nitride act as distinct path for electrons and holes. The discovery explains leakage and switching losses in GaN power devices and points to defect-guided design strategies.

Strengthening existing reinforced concrete (RC) columns using a partial wrapping strengthening technique (PWST) by fiber-reinforced polymer (FRP) strips has been widely implemented. However, compared with the confinement mechanism of confined concrete in columns strengthened with the FRP full wrapping strengthening technique (FWST), the confinement mechanism of confined concrete in FRP partially wrapped columns is less understood. This paper presents the results of an experimental investigation into the behavior of confined concrete in FRP partially wrapped square columns under axial compression. The effects of FRP strip width and thickness on stress–strain behavior were thoroughly investigated. The novel particle image velocimetry (PIV) non-contact strain sensing technique was adopted to measure the strain in the specimens. Results show that the axial strains as well as the hoop strains are generally larger at the mid-plane of adjacent FRP strips than those at the mid-plane of each FRP strip, and considerable variation in hoop strains along the height of the specimens was observed. Comparisons between the experimental results and predictions by existing design-oriented stress–strain models were carried out to examine the accuracy of the models. A new design-oriented stress–strain model is proposed for confined concrete in FRP partially wrapped square columns and the comparisons between laboratory results and predictions from the proposed model show that the proposed model is superior to the existing models.

Upon heating, almost all zinc-blende (ZB) and diamond-like semiconductors undergo volume contraction at low temperature, i.e. negative thermal expansion (NTE), instead of commonly expected expansion. Specifically, CuCl has the largest NTE among these semiconductors with a coefficient comparable with the record value of ZrW2O8. So far, underlying physical mechanism remains ambiguous. Here, we present a systematic and quantitative study of the NTE in ZB and diamond-like semiconductors using first-principles calculations. We clarified that the material ionicity, which renders the softening of the bond-angle-bending and thus, the enhancement of excitation of the transverse acoustic (TA) phonon, is responsible for the NTE of ZB and diamond-like semiconductors. With the increase in the ionicity from the groups IV, III-V, IIB-VI to IB-VII ZB semiconductors, the coefficient of the maximum NTE increases due to the weakness in bond-rotation effect, which makes the relative motion between cation and anion transverse to the direction of the bond more feasible and the mode Grüneisen parameters of the TA modes more negative. Since CuCl has the highest ionicity among all ZB and diamond-like semiconductors, it is expected to have the largest NTE, in good agreement with the experimental observation. This understanding would be beneficial for tetrahedral materials with specific applications.