Tensile Strain-Dependent Ultrafast Electron Transfer and Relaxation Dynamics in Flexible WSe2/MoS2 Heterostructures.

Understanding and controlling carrier dynamics in two-dimensional (2D) van der Waals heterostructures through strain are crucial for their flexible applications. Here, femtosecond transient absorption spectroscopy is employed to elucidate the interlayer electron transfer and relaxation dynamics under external tensile strains in a WSe2/MoS2 heterostructure. The results show that a modest ∼1% tensile strain can significantly alter the lifetimes of electron transfer and nonradiative electron-hole recombination by >30%. Ab initio non-adiabatic molecular dynamics simulations suggest that tensile strain weakens the electron-phonon coupling, thereby suppressing the transfer and recombination dynamics. Theoretical predictions indicate that strain-induced energy difference increases along the electron transfer path could contribute to the prolongation of the transfer lifetime. A subpicosecond decay process, related to hot-electron cooling, remains almost unaffected by strain. This study demonstrates the potential of tuning interlayer carrier dynamics through external strains, offering insights into flexible optoelectronic device design with 2D materials.

The effect of antisite defects on the mechanical and dynamic stability of Nb3Al.

The A15-type conventional superconductor Nb3Al alloys has been considered as an ideal candidate for next generation high field magnets due to its higher superconducting properties and less sensitivity to stain than that of industrialized Nb3Sn superconductor. First-principles methods are employed to study the potential point defects, vacancy and antisite defects in deviating stoichiometric Nb3Al alloys and their effect on structure and mechanical properties. Our results show that antisite defects are easier to be produced than vacancy defects, and NbAlantisite defects can keep the tetragonal structure of Nb3Al. Furthermore, the influence of antisite defects on dynamic stability of Nb3Al is investigated together with NbAldefects. With the increase of Nb antisite defect content and the formation of orderly arrangement, we found the phonon spectrum yields no more soft phonon modes, which is in contradiction with the dynamical instability of stoichiometric Nb3Al with no defects. Our calculations indicate Nb antisite defects play a crucial role on the dynamic stability of Nb3Al compounds.

Ultrafast nonequilibrium dynamic process of separate electrons and holes during exciton formation in few-layer tungsten disulfide.

Femtosecond transient absorption spectroscopy has been employed to unravel separate initial nonequilibrium dynamic processes of photo-injected electrons and holes during the formation process of the lowest excitons at the K-valley in few-layer tungsten disulfide. Charge carrier thermalization and cooling, as well as concomitant many-body effects on the exciton resonances, are distinguished. The thermalization of holes is observed to be faster than that of electrons. Both of them proceed predominantly via carrier-carrier scattering, as evidenced by the observed dependence of the thermalization time on pump fluences. The fluence dependent time constants also suggest that the subsequent cooling for electrons is probably dominated by acoustic phonons, whereas for holes it is mostly controlled by LO phonons. An extremely fast red- and blue-shift crossover followed by a slow blue-shift of exciton resonance was observed in the temporal evolution of exciton resonances by resonant exciton A excitation. The rapid red-shift could be due to the strong screening of the Coulomb interaction between quasi-free charge carriers in electron-hole plasma. The subsequent slow blue-shift is the net result of the competition among many-body effects in the hot-exciton cooling process. Our findings elucidate the carrier-selective ultrafast dynamics and their many-body effects, underpinning new possibilities for developing optoelectronic devices based on transport properties of a single type of carrier.

Erasable and recreatable two-dimensional electron gas at the heterointerface of SrTiO3 and a water-dissolvable overlayer.

While benefiting greatly from electronics, our society also faces a major problem of electronic waste, which has already caused environmental pollution and adverse human health effects. Therefore, recyclability becomes a must-have feature in future electronics. Here, we demonstrate an erasable and recreatable two-dimensional electron gas (2DEG), which can be easily created and patterned by depositing a water-dissolvable overlayer of amorphous Sr3Al2O6 (a-SAO) on SrTiO3 (STO) at room temperature. The 2DEG can be repeatedly erased or recreated by depositing the a-SAO or dissolving in water, respectively. Photoluminescence results show that the 2DEG arises from the a-SAO-induced oxygen vacancy. Furthermore, by gradually depleting the 2DEG, a transition of nonlinear to linear Hall effect is observed, demonstrating an unexpected interfacial band structure. The convenience and repeatability in the creation of the water-dissolvable 2DEG with rich physics could potentially contribute to the exploration of next generation electronics, such as environment-friendly or water-soluble electronics.

Graphene-based half-metal and spin-semiconductor for spintronic applications.

In this letter we propose a strategy to make graphene become a half-metal or spin-semiconductor by combining the magnetic proximity effects and sublattice symmetry breaking in graphone/graphene and graphone/graphene/BN heterostructures. Exchange interactions lift the spin degeneracy and sublattice symmetry breaking opens a band gap in graphene. More interestingly, the gap opening depends on the spin direction and the competition between the sublattice asymmetry and exchange field determines the system is a half-metal or a spin-semiconductor. By first-principles calculations and a low-energy effective model analysis, we elucidate the underlying physical mechanism of spin-dependent gap opening and spin degeneracy splitting. This offers an alternative practical platform for graphene-based spintronics.

First-Principles Study of the Magnetic Structure of Na2IrO3.

The iridate Na2IrO3 was proposed to be a realization of the Kitaev model with a quantum spin liquid ground state. Experiments have now established that this material hosts a zigzag antiferromagnetic order. However, the previous assignment of the ordered moment direction to the a axis is controversial. We examine the magnetic moment direction of Na2IrO3 using the local spin density approximation plus spin orbit coupling+U calculations. Our calculations reveal that the total energy is minimized when the zigzag-ordered moments are aligned along g≈a+c direction. The dependence of the total energy on moment directions can be explained by adding anisotropic interactions to the nearest-neighbor Kitaev-Heisenberg model, on which the spin-wave spectrum is also calculated. The revision of ordered moments is very important to understanding and achieving possible exotic electronic phases in this compound.

Strain tuning of magnetism in Mn doped MoS2 monolayer.

We study the strain tuning of magnetism in Mn doped MoS2 monolayer system. With the increase of the tensile strain, the magnetic ground state changes from a state with total magnetic moment Mtot =1.0 B to another state with Mtot =3.0 B for single doping in a 4 × 4 supercell. Physical mechanism is elucidated from the effects of the local bonding and geometry symmetries on orbital hybridization. In addition, we find the ferromagnetic coupling is favored for small distances between Mn atoms corresponding to the uniform doping concentration of 25%. More importantly, the ferromagnetic state is highly stable and robust to tensile strains. Therefore, diluted magnetic semiconductors can be obtained and the strain engineering should be a very promising approach to tune the magnetic moments.