Intrinsic second-order magnon thermal Hall effect.

In this paper, we study the intrinsic contribution of nonlinear magnon thermal Hall Effect. We derive the intrinsic second-order thermal Hall conductivity of magnon by the thermal scalar potential method and the thermal vector potential method. We find that the intrinsic second-order magnon thermal Hall conductivity is related to the thermal Berry-connection polarizability. We apply our theory to the monolayer ferromagnetic Hexagonal lattice, and we find that the second-order magnon thermal Hall conductivity can be controlled by changing Dzyaloshinskii-Moriya strength and applying strain.

Symmetry dictionary on charge and spin nonlinear responses for all magnetic point groups with nontrivial topological nature.

Recently, charge or spin nonlinear transport with nontrivial topological properties in crystal materials has attracted much attention. In this paper, we perform a comprehensive symmetry analysis for all 122 magnetic point groups (MPGs) and provide a useful dictionary for charge and spin nonlinear transport from the Berry curvature dipole, Berry connection polarizability and Drude term with nontrivial topological nature. The results are obtained by conducting a full symmetry investigation of the matrix representations of six nonlinear response tensors. We further identify every MPG that can accommodate two or three of the nonlinear tensors. The present work gives a solid theoretical basis for an overall understanding of the second-order nonlinear responses in realistic materials.

Unexpected spontaneous symmetry breaking and diverse ferroicity in two-dimensional mono-metal phosphorus chalcogenides.

Mono-metal phosphorus trichalcogenides (MPX3) have attracted intensive interest due to their intriguing magnetic properties and potential applications. Generally, single-layer two-dimensional (2D) MPX3 are believed to be centrosymmetric. However, we discovered that unexpected spontaneous symmetry breaking may occur in some 2D MPX3, i.e., vertical P-P dimers move out of the plane and become tilted, leading to the structural stability being enhanced, the inversion symmetry being simultaneously broken, and ferroelectricity or ferroelasticity emerging. By systematically investigating the family (176) of 2D MPX3, we found that 34 members undergo such symmetry breaking during geometric optimization, in which ten are identified to be dynamically stable. We show that the mismatch between the triangular sublattice of P-P dimers and the hexagonal sublattice of M atoms and the variable accommodation of P lone-pair electrons in different valence states of M atoms play dominant roles in the inversion symmetry breaking and the emergence of ferroicity. We obtained a ferroic atlas of the whole 2D MPX3 family, which also includes many stable antiferromagnetic and non-ferroic members that have never been reported. Our work not only presents ferroelectricity in the 2D MPX3 family but also reveals how diverse ferroicity emerges with various spontaneous symmetry breakings, which will be helpful for further exploration of 2D ferroic materials.

High-efficient ab initio Bayesian active learning method and applications in prediction of two-dimensional functional materials.

Beyond the conventional trial-and-error method, machine learning offers a great opportunity to accelerate the discovery of functional materials, but still often suffers from difficulties such as limited materials data and the unbalanced distribution of target properties. Here, we propose the ab initio Bayesian active learning method that combines active learning and high-throughput ab initio calculations to accelerate the prediction of desired functional materials with ultrahigh efficiency and accuracy. We apply it as an instance to a large family (3119) of two-dimensional hexagonal binary compounds with unbalanced materials properties, and accurately screen out the materials with maximal electric polarization and proper photovoltaic band gaps, respectively, whereas the computational costs are significantly reduced by only calculating a few tenths of the possible candidates in comparison with a random search. This approach shows the enormous advantages for the cases with unbalanced distribution of target properties. It can be readily applied to seek a broad range of advanced materials.

Compositional Engineering Study of Lead-Free Hybrid Perovskites for Solar Cell Applications.

Hybrid organic-inorganic perovskite solar cells (HOIPs), especially CH3NH3PbI3 (MAPbI3), have received tremendous attention due to their excellent power conversion efficiency (25.2%). However, two fundamental hurdles, long-term stability and lead (Pb) toxicity, prevent HOIPs from practical applications in the solar industry. To overcome these issues, compositional engineering has been used to modify cations at A- and B-sites and anions at the X-site in the general form ABX3. In this work, we used the density functional theory (DFT) to incorporate Rb, Cs, and FA at the A-site to minimize the volatile nature of MA, while the highly stable Ca2+ and Sr2+ were mixed with the less stable Ge2+ and Sn2+ at the B-site to obtain a Pb-free perovskite. To further enhance the stability, we mixed the X-site anions (I/Br). Through this approach, we introduced 20 new perovskite species to the lead-free perovskite family and 7 to the lead-containing perovskite family. The molecular dynamic (MD) simulations, enthalpy formation, and tolerance and octahedral factor study confirm that all of the perovskite alloys we introduced here are as stable as pristine MAPbI3. All Pb-free perovskites have suitable and direct band gaps (1.42-1.77 eV) at the Γ-point, which are highly desirable for solar cell applications. Most of our Pb-free perovskites have smaller effective masses and exciton binding energies. Finally, we show that the introduced perovskites have high absorption coefficients (105 cm-1) and strong absorption efficiencies (above 90%) in a wide spectral range (300-1200 nm), reinforcing their significant potential applications. This study provides a new way of searching for stable lead-free perovskites for sustainable and green energy applications.

Many-body tunneling and nonequilibrium dynamics in double quantum dots with capacitive coupling.

Double quantum dots (DQDs) systems may be the minimal setups for realization of QD-based qubits and quantum computation. Pauli spin blockade (PSB) and a kind of novel many-body tunneling (MBT) are identified to play important roles in these systems, and dominate the quantum tunneling at moderate and weak interdot coupling t, respectively. On the other hand, inter-dot Coulomb interaction U' and related inter-dot Coulomb blockade (IDCB) is inevitable in DQDs. However, what would happen on the effect of U' in DQDs has not been touched, in particular for PSB and MBT. Here, we study the tunneling processes and transport properties with various U' in series-coupled DQDs, and find MBT process is rather robust against U' within U'/U < 0.1, where U is the intra-dot Coulomb interaction. Meanwhile, the linearity relationship between the carrier doublon number and MBT current remains valid. These findings enrich the understanding of the many-body tunneling in the DQDs and may shed light on the manipulation of the QD-based qubits.

Identification of spin effects in the anomalous Righi-Leduc effect in ferromagnetic metals.

The emerging of spin caloritronics leads to a series of new spin-thermal related effects, such as spin Seebeck effect (SSE), spin Nernst effect (SNE) and their corresponding inverse effects. Anomalous Righi-Leduc effect (ARLE) describes that a transverse temperature gradient can be induced by a longitudinal heat flow in ferromagnets. The driving force and the response of the ARLE are all involved with heat. It is curious if spin effects mediate the heat transport and provide extra influence. In this work, we investigate the ARLE and the interplay between the heat current, charge current, and spin current via linear response theory. We identified that spin effects do have clear roles in heat transport, which can be confirmed by phase shifts of voltage output varying with the direction of magnetization. Our formulas fit the experimental data very well. Moreover, we discuss more configuration of magnetization which is expected to be tested in the future. It should be emphasized that the present formalism including spin effects is out of the theory based on magnon transport, which may be conspicuous in the devices within the spin diffusion length.

Synthesis of Co-Doped MoS2 Monolayers with Enhanced Valley Splitting.

Internal magnetic moments induced by magnetic dopants in MoS2 monolayers are shown to serve as a new means to engineer valley Zeeman splitting (VZS). Specifically, successful synthesis of monolayer MoS2 doped with the magnetic element Co is reported, and the magnitude of the valley splitting is engineered by manipulating the dopant concentration. Valley splittings of 3.9, 5.2, and 6.15 meV at 7 T in Co-doped MoS2 with Co concentrations of 0.8%, 1.7%, and 2.5%, respectively, are achieved as revealed by polarization-resolved photoluminescence (PL) spectroscopy. Atomic-resolution electron microscopy studies clearly identify the magnetic sites of Co substitution in the MoS2 lattice, forming two distinct types of configurations, namely isolated single dopants and tridopant clusters. Density functional theory (DFT) and model calculations reveal that the observed enhanced VZS arises from an internal magnetic field induced by the tridopant clusters, which couples to the spin, atomic orbital, and valley magnetic moment of carriers from the conduction and valence bands. The present study demonstrates a new method to control the valley pseudospin via magnetic dopants in layered semiconducting materials, paving the way toward magneto-optical and spintronic devices.

Many-body Tunneling and Nonequilibrium Dynamics of Doublons in Strongly Correlated Quantum Dots.

Quantum tunneling dominates coherent transport at low temperatures in many systems of great interest. In this work we report a many-body tunneling (MBT), by nonperturbatively solving the Anderson multi-impurity model, and identify it a fundamental tunneling process on top of the well-acknowledged sequential tunneling and cotunneling. We show that the MBT involves the dynamics of doublons in strongly correlated systems. Proportional to the numbers of dynamical doublons, the MBT can dominate the off-resonant transport in the strongly correlated regime. A T 3/2-dependence of the MBT current on temperature is uncovered and can be identified as a fingerprint of the MBT in experiments. We also prove that the MBT can support the coherent long-range tunneling of doublons, which is well consistent with recent experiments on ultracold atoms. As a fundamental physical process, the MBT is expected to play important roles in general quantum systems.

Achieving High-Performance Ternary Organic Solar Cells through Tuning Acceptor Alloy.

Acceptor alloys based on n-type small molecular and fullerene derivatives are used to fabricate the ternary solar cell. The highest performance of optimized ternary device is 10.4%, which is the highest efficiency for one donor/two acceptors-based ternary systems. Three important parameters, JSC , VOC , and FF, of the optimized ternary device are all higher than the binary reference devices.

Highly efficient light management for perovskite solar cells.

Organic-inorganic halide perovskite solar cells have enormous potential to impact the existing photovoltaic industry. As realizing a higher conversion efficiency of the solar cell is still the most crucial task, a great number of schemes were proposed to minimize the carrier loss by optimizing the electrical properties of the perovskite solar cells. Here, we focus on another significant aspect that is to minimize the light loss by optimizing the light management to gain a high efficiency for perovskite solar cells. In our scheme, the slotted and inverted prism structured SiO2 layers are adopted to trap more light into the solar cells, and a better transparent conducting oxide layer is employed to reduce the parasitic absorption. For such an implementation, the efficiency and the serviceable angle of the perovskite solar cell can be promoted impressively. This proposal would shed new light on developing the high-performance perovskite solar cells.

Thermally Driven Pure Spin and Valley Currents via the Anomalous Nernst Effect in Monolayer Group-VI Dichalcogenides.

The spin and valley-dependent anomalous Nernst effects are analyzed for monolayer MoS_{2} and other group-VI dichalcogenides. We find that pure spin and valley currents can be generated perpendicular to the applied thermal gradient in the plane of these two-dimensional materials. This effect provides a versatile platform for applications of spin caloritronics. A spin current purity factor is introduced to quantify this effect. When time reversal symmetry is violated, e.g., two-dimensional materials on an insulating magnetic substrate, a dip-peak feature appears for the total Nernst coefficient. For the dip state it is found that carriers with only one spin and from one valley are driven by the temperature gradient.

Complete genome sequence of a malodorant-producing acetogen, Clostridium scatologenes ATCC 25775(T).

Clostridium scatologenes ATCC 25775(T) is an acetogenic anaerobic bacteria known to be capable of synthesizing volatile fatty acids and solvents from CO2 or CO on its autotrophic mode and producing 3-methylindole and 4-methylphenol on its heterotrophic mode. Here, we report the complete genome sequence of this strain, which might provide a lot of valuable information for developing metabolic engineering strategies to produce biofuels or chemicals from greenhouse gases.

Revisit to phase diagram of poly(N-isopropylacrylamide) microgel suspensions by mechanical spectroscopy.

Microgels are soft particles that can be deformed and compressed, which would induce intriguing phase behaviors at high packing fractions. Poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33 °C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature. In this work, the linear viscoelastic properties of PNIPAM microgel suspensions have been investigated using mechanical spectroscopy. A particular attention is focused on the phase behaviors at high concentrations. With increasing concentration the system undergoes a repulsive glass-to-gel transition below the LCST, while, as temperature is raised across the LCST, the system undergoes a gel-to-attractive glass transition. A mechanism of these transitions for the microgels is proposed based on the directional interaction between the particles. In moderate concentration or de-swelling microgels the interaction is isotropic leading to the glass phase, while in concentrated and deformed microgels the interaction is directional leading to the gel phase. Our results enrich the current understanding of the phase transition in microgel systems and shed new light on the phase diagram of colloidal suspensions in general.

Quantifying changes in the low-frequency dynamics of amorphous polymers by 2D correlation mechanical spectroscopy.

The longer segmental dynamics of sub-Rouse modes in polystyrene with different molecular weights has been investigated by 2D correlation mechanical spectroscopy. The sub-Rouse modes were first separated from the α relaxation and Rouse modes, and their dynamics exhibits a similar change at the same temperature, T(B) ≈ 1.2T(g), as the α relaxation. The relaxation time of sub-Rouse modes at T(B) is independent of molecular weight and has a value of about 0.1 s, indicating that solely the time scale of the relaxation determines the change in dynamics of sub-Rouse modes. According to the coupling model, the change is caused by a strong increase in intermolecular cooperativity. The present work provides direct evidence for the intermolecular coupled nature of the sub-Rouse modes and demonstrates that the properties of the sub-Rouse modes resemble those of α relaxation, which could provide a new perspective for understanding the glass transition of polymers.

Dynamics in N-isopropylacrylamide-acrylic acid copolymer aqueous solution from mechanical spectroscopy.

Gaining control over precise and predictable structures of colloidal systems and understanding the abundant dynamic behaviors remains a formidable challenge. In this study, low-frequency mechanical spectroscopy was applied to investigate the dynamics of aqueous solutions of N-isopropylacrylamide-acrylic acid (NIPAM-AA) copolymers with three different AA contents. A mechanical loss valley was found for the solution with low molar fraction of AA (f(AA)), f(AA) = 25 and 50 mol %, and a loss peak was shown for the solution with f(AA) = 75 mol %. The former is suggested to be due to the particle glass phase of repulsive micelles above the low critical solution temperature, whereas the latter is associated with the α relaxation behavior of NIPAM-AA/water mixture at high concentrations. The relaxation time of the α relaxation seems to follow a simple Arrhenius temperature dependence. The activation energy H is ∼53 kJ/mol, and the larger H value is suggested to be due to multiple strong hydrogen bonds in the copolymer solution. The present work demonstrates that by controlling the proportion of ingredients in the colloidal systems the systems can exhibit distinct dynamic behaviors, which is helpful in the design and fabrication of colloids.

Laser-assisted spin-polarized transport in graphene tunnel junctions.

The Keldysh nonequilibrium Green's function method is utilized to theoretically study spin-polarized transport through a graphene spin valve irradiated by a monochromatic laser field. It is found that the bias dependence of the differential conductance exhibits successive peaks corresponding to the resonant tunneling through the photon-assisted sidebands. The multi-photon processes originate from the combined effects of the radiation field and the graphene tunneling properties, and are shown to be substantially suppressed in a graphene spin valve which results in a decrease of the differential conductance for a high bias voltage. We also discuss the appearance of a dynamical gap around zero bias due to the radiation field. The gap width can be tuned by changing the radiation electric field strength and the frequency. This leads to a shift of the resonant peaks in the differential conductance. We also demonstrate numerically the dependences of the radiation and spin valve effects on the parameters of the external fields and those of the electrodes. We find that the combined effects of the radiation field, the graphene and the spin valve properties bring about an oscillatory behavior in the tunnel magnetoresistance, and this oscillatory amplitude can be changed by scanning the radiation field strength and/or the frequency.

Phase diagram of the Pluronic L64-H2O micellar system from mechanical spectroscopy.

The linear viscoelastic properties of aqueous Pluronic L64 solutions have been investigated at high copolymer concentrations (25-62 wt%) using our modified low-frequency mechanical spectroscopy. The concentration-temperature phase diagram of the L64/H(2)O system was constructed by studying the evolution of the loss modulus and loss tangent as temperature is increased at a fixed frequency. A particular attention was focused on the dynamics approaching the beginning and ending points (39% and 60%) of the fusiform gel region in the phase diagram. The dynamics is found to have a similar viscoelastic behavior at the low and high concentrations, where a frequency scaling expected for a static percolated network is exhibited. Moreover, with increasing temperature, the system above the critical gel concentration undergoes a transition from a viscoelastic liquid to a solid gel through a percolated particle network. Therefore, our results suggest that the formation of the gel is dominated by the percolation of the particle clusters.

Dynamic crossover of alpha' relaxation in poly(vinyl acetate) above glass transition via mechanical spectroscopy.

The molecular relaxation dynamics of poly(vinyl acetate) (PVAc) has been studied by mechanical spectroscopy above the glass transition temperature (T(g)) within a frequency range from 1 mHz to 100 Hz. The temperature-dependent mechanical spectra reveal the existence of two relaxation modes: alpha, ascribed to the glass transition, and alpha', which may be related to the softening dispersion, composed of the sub-Rouse modes and the Rouse modes. The alpha' mode has a weaker temperature dependence than the alpha mode. Furthermore, the alpha' mode from the frequency-domain spectra exhibits a similar dynamic crossover at temperature T(B) approximately 387 K as the alpha mode through the temperature dependence of relaxation time, relaxation strength, and shape parameter. However, the crossover of the alpha' mode occurs at a time of about 0.08 s, longer than 10(-6)-10(-7) s for the alpha mode by dielectric spectroscopy. According to the coupling model, the crossover is suggested to be caused by the strong increase of intermolecular cooperativity below T(B).

Electromagnetic pulse-driven spin-dependent currents in semiconductor quantum rings.

We investigate the non-equilibrium charge and spin-dependent currents in a quantum ring with a Rashba spin-orbit interaction (SOI) driven by two asymmetric picosecond electromagnetic pulses. The equilibrium persistent charge and persistent spin-dependent currents are investigated as well. It is shown that the dynamical charge and the dynamical spin-dependent currents vary smoothly with a static external magnetic flux and the SOI provides a SU(2) effective flux that changes the phases of the dynamic charge and the dynamic spin-dependent currents. The period of the oscillation of the total charge current with the delay time between the pulses is larger in a quantum ring with a larger radius. The parameters of the pulse fields control to a certain extent the total charge and the total spin-dependent currents. The calculations are applicable to nanometre rings fabricated in heterojunctions of III-V and II-VI semiconductors containing several hundreds of electrons.