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. On the other end are accidental nodal-line semimetals whose band crossings, not being enforced by symmetry, are easily destroyed by perturbations. Some accidental nodal-line semimetals have, however, the advantage that their band crossings can occur in generic locations in the Brillouin zone, and thus can be repositioned to tailor material properties. We show that lattice engineering with periodic distributions of vacancies yields a hybrid type of nodal-line semimetals which possess symmetry-enforced nodal lines and accidental nodal lines, with the latter endowed with an enhanced robustness to perturbations. Both types of nodal lines are explained by a symmetry analysis of an effective model which captures the relevant characteristics of the proposed materials, and are verified by first-principles calculations of vacancy-engineered borophene polymorphs. Our findings offer an alternative path to relying on complicated compounds to design robust nodal-line semimetals; one can instead remove atoms from a common monoatomic material.

Intrinsic Valley Splitting and Direct-to-Indirect Band Gap Transition in Monolayer HfZrSiCO2.

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 HfZrSiCO2 concurrently hosts a giant intrinsic VS and excitonic quasi-particles with long valley lifetime due to valley-sublayer coupling and built-in electric field. In addition, the band structure of the monolayer HfZrSiCO2 can be continuously manipulated by either an external electric field or a biaxial strain, giving rise to a tunable VS and driving a direct-to-indirect band gap transition. Moreover, the system exhibits valley-contrasting linear dichroism in exciton absorption. These results suggest that the Janus monolayer HfZrSiCO2 is a promising candidate for information applications.

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). We demonstrate that it is a highly efficient direct Z-scheme photocatalyst. However, unlike traditional direct Z-scheme photocatalysts, the GeC/SnSSe vdWH possesses a small energy separation between the low conduction band located in SnSSe and the high valence band residing in the GeC layer, which significantly fosters the interlayer charge transfer. Hence, its solar-to-hydrogen conversion efficiency reaches as high as 68.37%. Moreover, we also find that tensile strain promotes an astonishing increase in photovoltaic performance, e.g., 4% tensile strain leads to an increase of the photocurrent by 40%.

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. From first-principles calculations, the intrinsic built-in electric field of Janus SeSnS and the charge transfer from the SnC to the SeSnS layer give rise to a type-II band alignment. Such a band alignment benefits the formation of spatially separated reductive and oxidative active sites and the reduction of the global bandgap of the Janus vdWH. The proposed material increases the solar-to-hydrogen conversion efficiency to 60.8%. Besides, we also find that the light absorption coefficient is stacking configuration controllable and strain-tunable, e.g., the tensile strain promotes photocatalytic efficiency. Moreover, because Sn, S, and Se are environmentally benign and inexpensive elements, SnC/SeSnS vdWH is a promising noble-metal-free direct Z-scheme photocatalyst.

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-b]thiophene (DNTT) is in very good agreement with experimental data. Interestingly, SDL can be prolonged by restraining molecular geometry structure disorder and reducing the reorganization energy. In comparison with anisotropic organic materials, the SDL in isotropic ones increases up to 60%. Our results open up a new avenue to design organic spintronics devices with long SDL and low carrier density.

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. Furthermore, we clarify that the lower-energy signal originates from "cold" charge transfer (CT) absorption, while the higher-energy signal is from "hot" CT absorption. Finally, effects of several key factors, such as the interface structure and the photoexciting condition, on CT absorptions are discussed. These findings should be of vital importance both to understand the sub-bandgap excited states and to recognize their roles in organic photovoltaic devices.

Robust room temperature emissions of trion in darkish WSe2 monolayers: 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 WSe2, 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 WSe2 monolayers, which combines ab initio tight-binding formalism, Bethe-Salpeter equation and a set of coupled rate equations to describe valley dynamics of excitonic particles. Through a systematic study in which new scattering channels are progressively included, we found that, besides the low electron density, strong many-body correlations between bright and dark excitonic states quenches the trion emission in WSe2. Therefore, the reduction of scatterings from bright to dark states is required to achieve trion emission at room temperature for experimentally accessible carrier concentrations.

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 1018 cm-3. 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. By tuning the doping concentration, one can achieve a maximum inverse spin Hall voltage. Our results not only explain some recent experimental data but also inspire further experimental investigation on pure spin current in organic devices with variable impurity doping.

Exotic magnetism in As-doped α/ß-In2Se3 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/ß-In2Se3 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/ß-In2Se3 monolayers. In particular, replacement by arsenic at the anion site can enhance ferromagnetism and drive α-In2Se3 to be a robust half-metal and ß-In2Se3 to be a bipolar magnetic semiconductor. Then, we proposed spin-polarized field-effect transistors based on α-In2Se3 and a bipolar field-effect spin-filter based on ß-In2Se3. Besides, we also discussed the influences of the molecules in air on the device performance such as carrier mobility. We found that the adsorption of either O2 or H2O on α/ß-In2Se3 induced changes in hole mobility in different directions. These findings reveal a new road to electronic and magnetic modulations in 2D materials.

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. Remarkably, it can reach up to 52% by proper choice of the above-mentioned physical parameters. Our results outline a novel approach to further improve the luminous efficiency of organic light emitting diodes.

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

We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer Mo1-xWxSe2 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 WSe2 monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe2, 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. We find the critical value of W-concentration, xc, that turns alloys from bright to darkish. The dependence of the PL intensity on temperature shows three regimes: while bright monolayer alloys display a usual temperature dependence in which the intensity decreases with rising temperature, the darkish alloys exhibit the opposite behavior, and the alloys with x around xc show a non-monotonic temperature response. Remarkably, we observe that the biexciton enhances significantly the stability of the exciton emission against fluctuations of W-concentration for bright alloys. Our findings pave the way for developing high-performance valleytronic and photo-emitting devices.

Optically dark excitonic states mediated exciton and biexciton valley dynamics in monolayer WSe2.

We present a theory to address the photoluminescence (PL) intensity and valley polarization (VP) dynamics in monolayer WSe2, 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 (Xb), intravalley biexciton (XXk,k) and intervalley biexciton (XX[Formula: see text]) in particular for the XXk,k PL is enhanced by laser excitation fluence. In addition, our results indicate the anomalous temperature dependence of PL, i.e. increasing with temperature, as a result of favored phonon assisted dark-to-bright scatterings at high temperatures. Moreover, we observe that the PL is almost immune to intervalley scatterings, which trigger the exchange of excitonic states between the two valleys. As far as the valley polarization is concerned, we find that the VP of Xb shrinks as temperature increases, exhibiting opposite temperature response to PL, while the intravalley XXk,k VP is found almost independent of temperature. In contrast to both Xb and XXk,k, the intervalley XX[Formula: see text] VP identically vanishes, because of equal populations of excitons in the K and [Formula: see text] valleys bounded to form intervalley biexcitons. Notably, it is found that the Xb VP much more strongly depends on bright-dark scattering than that of XXk,k, making dark state act as a robust reservoir for valley polarization against intervalley scatterings for Xb at strong bright-dark scatterings, but not for XXk,k. Dark excitonic states enabled enhancement of VP benefits quantum technology for information processing based on the valley degree of freedom in valleytronic devices. Furthermore, the VP has strong dependence on intervalley scattering but maintains essentially constant with excitation fluence. Finally, the dependence of time evolution of PL and VP on temperature and excitation fluence is discussed.

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). We demonstrate that the photons are absorbed (or emitted) in the QDs with distinct energy but definite valley-polarization. The spin-coupled valley-polarization is invariant under either spatial or magnetic quantum quantization. However, the magneto-optical absorption peaks undergo a blue shift as the quantization is enhanced. Moreover, the absorption spectrum pattern changes considerably with a variation of Fermi energy. This together with the controllability of absorption spectrum by spatial and magnetic quantizations, offers the possibility of tuning the magneto-optical properties at will, subject to the robust spin-coupled valley polarization.

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. Although the EZEs are insensitive to the variation of quantum confinement, both of them grow linearly with the magnetic field, similar to that in the monolayer MoS2 material. The EZEs along with their 'robustness' against dot confinements open opportunities of a universal magnetic control over the valley degree of freedom, for quantum dots of all sizes.

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 0.50. The susceptibility data were well fitted by a modified Langevin function, in which cluster dissociation into monomers, at the critical temperature [T*], was included. In the ac experiments, we found that T* was reducing from 381.8 to 380.4 K as the frequency of the applied field increases from 123 to 173 Hz. In addition, our ac experiments confirm that only monomers respond for the magnetic behavior of the MF sample above T*. Furthermore, our Monte Carlo simulation and analytical results support the hypothesis of a thermal-assisted dissociation of chain-like structures.PACS: 75.75.-C; 75.30.Kz; 75.30.Cr.

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. The dependence of the magnetopolaron energy on magnetic ion positions, quantum dot energy quantization omega0, and the number of electrons Ne is predicted.

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. However, unlike the total electron spin S which is maximized for half-filled shells, we predict the exchange interaction to be independent of the filling of the electronic shell. We show how this unusual effect manifests itself in quantum dot addition and excitation spectrum.