Vacancy-engineered nodal-line semimetals.

Symmetry-enforced nodal-line semimetals are immune to perturbations that preserve the underlying symmetries. This intrinsic robustness enables investigations of fundamental phenomena and applications utilizing diverse materials design techniques. The drawback of symmetry-enforced nodal-line semimetals is that the crossings of energy bands are constrained to symmetry-invariant momenta in the Brillouin zone.

Intrinsic Valley Splitting and Direct-to-Indirect Band Gap Transition in Monolayer HfZrSiCO.

Both a reasonably large valley splitting (VS) and a sufficiently long valley exciton lifetime are crucial in valleytronics device applications. Currently, no single system possesses both attributes simultaneously. Herein, we demonstrate that a Janus monolayer HfZrSiCO concurrently hosts a giant intrinsic VS and excitonic quasi-particles with long valley lifetime due to valley-sublayer coupling and built-in electric field.

A bifunctional GeC/SnSSe heterostructure for highly efficient photocatalysts and photovoltaic devices.

Alongside highly efficient photocatalysts, high photovoltaic performance is also a key element for efficiently harvesting solar energy. Developing bifunctional materials which satisfy concurrently these two demands is an appealing strategy for solving the current serious energy and environmental issues. Based on first-principles and quantum transport calculations, we designed this kind of novel bifunctional material: Janus GeC/SnSSe van der Waals heterostructure (vdWH).

Design of a noble-metal-free direct Z-scheme photocatalyst for overall water splitting based on a SnC/SnSSe van der Waals heterostructure.

Semiconductor photocatalysts, using sunlight to stimulate various photocatalytic reactions, are promising materials for solving the energy crisis and environmental problems. However, the low photocatalytic efficiency and high cost pose major challenges for their widespread application. Mimicking the natural photosynthesis system, we propose a direct Z-scheme photocatalyst based on a Janus van der Waals heterostructure (vdWH) comprising SnC and Janus SeSnS monolayers.

Hopping-Dominated Spin Transport in Unintentionally Doped Organic Semiconductors.

We report a spin diffusion theory to predict unusual pure spin current transport in unintentionally doped organic semiconductors. We demonstrate that the feasibility of pure spin current transport via polaron hopping at a low carrier density. Our theoretical prediction, 40 nm, for spin diffusion length (SDL) in dinaphtho[2,3-b:2,3-f]thieno[3,2-]thiophene (DNTT) is in very good agreement with experimental data.

Sub-bandgap photoexcited dynamics at an organic donor/acceptor photovoltaic interface.

Although sub-bandgap light absorption signals in organic donor/acceptor (D/A) photovoltaic systems have been studied extensively, the underlying origins, as well as the impacting factors, are still elusive. By theoretically constructing an organic D/A interface under a femtosecond electric pulse pumping, we obtain an insightful understanding of this issue. First, a careful comparison between the absorption spectra of the D/A interface and the individual donor (acceptor) demonstrates the existence of two weak absorption signals below the donor (acceptor) optical gap.

Robust room temperature emissions of trion in darkish WSemonolayers: effects of dark neutral and charged excitonic states.

Owing to nonzero charge and spin degrees of freedom, trions offer unprecedented tunability and open new paths for applications in devices based on 2D semiconductors. However, in monolayer WSe, the trion photoluminescence is commonly detected only at low temperatures and vanishes at room temperature, which undermines practical applications. To unveil how to overcome this obstacle, we have developed a comprehensive theory to probe the impact of different excitonic channels on the trion emission in WSemonolayers, which combinestight-binding formalism, Bethe-Salpeter equation and a set of coupled rate equations to describe valley dynamics of excitonic particles.

Spin Transport Based on Exchange Coupling in Doped Organic Polymers.

We develop a spin diffusion theory based on the exchange mechanism among polarons to understand the organic pure spin current. It is demonstrated that the exchange coupling is strong enough to induce spin transport within the organic layer with impurity concentrations higher than 10 cm. By calculating the inverse spin Hall voltage in an organic spin device, we predict that the voltage depends nonmonotonically on the impurity concentration of the organic material.

Exotic magnetism in As-doped α/β-InSe monolayers with tunable anisotropic carrier mobility.

The two-dimensional (2D) material family is expanding fast as novel metal chalcogenides are being continually fabricated and intriguingly, plenty of them are ideal candidates for future nanoscale electronic and magnetic devices. Based on first-principles calculations, we investigated the electronic and magnetic properties of α/β-InSe monolayers. We find singularities of density of states appear in the valence band and hole doping (such as a Se atom substituted by a lower valence atom) can induce various ferromagnetic phase transitions in the α/β-InSe monolayers.

Strongly enhanced luminous efficiency of organic light emitting diodes in molecular heterojunctions.

We report a comprehensive theory based on the extended Su-Schrieffer-Heeger (SSH) model to study the interconversion from the dark triplet exciton state to a bright singlet one in molecular heterojunctions, containing both intrachain and interchain excitons. By studying the spin mixing and the projection of excitons onto the pure singlet and triplet excitons, unlike usual methods, we found that the internal electroluminescent quantum efficiency, which is largely determined by the singlet fraction, can be widely tuned by the spin-orbit coupling strength, the intensity of hyperfine interaction, electron-phonon coupling and the site energy offset of the two chains constituting the molecular heterojunctions. In addition, the interchain excitons possess a higher fraction of singlet states in comparison with the intrachain excitons.

Dark-exciton valley dynamics in transition metal dichalcogenide alloy monolayers.

We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer MoWSe alloys. To probe the impact of different excitonic channels, including bright and dark excitons, intravalley biexcitons, intervalley scattering between bright excitons, as well as bright biexcitons, we have performed a systematic study from the simplest system to the most complex one. In contrast to the binary WSe monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe, that presents high PL intensity, but low valley polarization, our results demonstrate that it is possible to set up a ternary alloy with intermediate W-concentration that holds simultaneously a considerably robust light emission and an efficient optical orientation of the valley pseudospin.

Optically dark excitonic states mediated exciton and biexciton valley dynamics in monolayer WSe.

We present a theory to address the photoluminescence (PL) intensity and valley polarization (VP) dynamics in monolayer WSe, under the impact of excitonic dark states of both excitons and biexcitons. We find that the PL intensity of all excitonic channels including intravalley exciton (X), intravalley biexciton (XX) and intervalley biexciton (XX[Formula: see text]) in particular for the XX PL is enhanced by laser excitation fluence. In addition, our results indicate the anomalous temperature dependence of PL, i.

Tunable spin and valley dependent magneto-optical absorption in molybdenum disulfide quantum dots.

Photonic quantum computer, quantum communication, quantum metrology and quantum optical technologies rely on the single-photon source (SPS). However, the SPS with valley-polarization remains elusive and the tunability of magneto-optical transition frequency and emission/absorption intensity is restricted, in spite of being highly in demand for valleytronic applications. Here we report a new class of SPSs based on carriers spatially localized in two-dimensional monolayer transition metal dichalcogenide quantum dots (QDs).

Robust effective Zeeman energy in monolayer MoS2 quantum dots.

We report a theoretical investigation on the energy spectrum and the effective Zeeman energy (EZE) in monolayer MoS2 circular quantum dots, subjected to an out-of-plane magnetic field. Interestingly, we observe the emergence of energy-locked modes, depending on the competition between the dot confinement and the applied magnetic field, for either the highest K-valley valence band or the lowest [Formula: see text]-valley conduction band. Moreover, an unusual dot-size-independent EZE behavior of the highest valence and the lowest conduction bands is found.

Second-order-like cluster-monomer transition within magnetic fluids and its impact upon the magnetic susceptibility.

The low-field (below 5 Oe) ac and dc magnetic response of a magnetic fluid [MF] sample in the range of 305 to 360 K and 410 to 455 K was experimentally and theoretically investigated. We found a systematic deviation of Curie's law, which predicts a linear temperature dependence of inverse initial susceptibility in the range of our investigation. This finding, as we hypothesized, is due to the onset of a second-order-like cluster-to-monomer transition with a critical exponent which is equal to 0.

Theory of electron mediated Mn-Mn interactions in quantum dots.

We present a theory of interaction of magnetic Mn ions depending strongly on the number (Ne) of electrons in a quantum dot. For closed electronic shells, we derive the RKKY interaction and its dependence on magnetic ion positions, quantum dot energy quantization omega0, and the number of filled shells Ns. For partially filled shells, the many-electron magnetopolaron effect leads to effective carrier mediated ferromagnetic Mn-Mn interactions.

Magnetic exchange interactions in quantum dots containing electrons and magnetic ions.

We present a theory of magnetic exchange interactions in quantum dots containing electrons and magnetic ions. We find the interaction between the electron and Mn ion to depend strongly on the number of electrons. It can be switched off for closed shell configurations and maximized for partially filled shells.