Zhibo Yan

Optimizing the thermoelectric performance of low-temperature SnSe compounds by electronic structure design

Recently, the SnSe compound was reported to have a peak thermoelectric figure-of-merit (ZT) of ∼2.62 at 923 K, but the ZT values at temperatures below 750 K are relatively low. In this work, the electronic structures of SnSe are calculated using the density functional theory, and the electro- and thermo-transport properties upon carrier density are evaluated by the semi-classic Boltzmann transport theory, revealing that the calculated ZT values along the a- and c-axes below 675 K are in agreement with reported values, but that along the b-axis can be as high as 2.57 by optimizing the carrier concentration to n ∼ 3.6 × 1019 cm−3. It is suggested that a mixed ionic–covalent bonding and heavy-light band overlapping near the valence band are the reasons for the higher thermoelectric performance.

High thermoelectric performance of superionic argyrodite compound Ag8SnSe6

A good thermoelectric material usually has a high power factor and low thermal conductivity for high figure of merit (ZT), and is also environmentally friendly and economical. Superionic compounds are heavily studied because of their ultra-low thermal conductivity even though the thermal stability remains an issue. In this work, we report a superionic argyrodite compound Ag8SnSe6 as a promising mid-temperature thermoelectric material because of its high ZT and good thermal stability up to 550 °C. It is revealed that Ag8SnSe6 exhibits a decent Seebeck coefficient (n-type) and electrical conductivity. At the same time, its thermal conductivity is lower than the glass limit with the thermal capacity CV below 3NkB at high temperature, where N is the Avogadros number and kB the Boltzmann constant. Detailed microstructural and thermodynamic characterization of this compound is performed to understand the electronic and phononic origins of the thermoelectric properties. The highly random ionic occupations in the cubic phase, leading to the molten silver sublattice and phononic mode softening, are responsible for the very low thermal conductivity and ZT ∼ 1.1 at 450 °C.

Polarization enhancement and ferroelectric switching enabled by interacting magnetic structures in DyMnO3 thin films

The mutual controls of ferroelectricity and magnetism are stepping towards practical applications proposed for quite a few promising devices in which multiferroic thin films are involved. Although ferroelectricity stemming from specific spiral spin ordering has been reported in highly distorted bulk perovskite manganites, the existence of magnetically induced ferroelectricity in the corresponding thin films remains an unresolved issue, which unfortunately halts this step. In this work, we report magnetically induced electric polarization and its remarkable response to magnetic field (an enhancement of ~800% upon a field of 2 Tesla at 2u2005K) in DyMnO3 thin films grown on Nb-SrTiO3 substrates. Accompanying with the large polarization enhancement, the ferroelectric coercivity corresponding to the magnetic chirality switching field is significantly increased. A picture based on coupled multicomponent magnetic structures is proposed to understand these features. Moreover, different magnetic anisotropy related to strain-suppressed GdFeO3-type distortion and Jahn-Teller effect is identified in the films.

Novel multiferroicity in GdMnO3 thin films with self-assembled nano-twinned domains

There have been many interests in exploring multiferroic materials with superior ferroelectric and magnetic properties for the purpose of developing multifunctional devices. Fabrication of thin films plays an important role in achieving this purpose, since the multiferroicity can be tuned via strain, dimensionality, and size effect, without varying the chemical composition. Here, we report exotic multiferroic behaviors, including high-TC (~75u2005K) ferroelectric state, a large spontaneous polarization (~4900u2005μC/m2) and relatively strong ferromagnetism emerging at ~105u2005K, in orthorhombic GdMnO3/SrTiO3 (001) thin films with self-assembled nano-scale twin-like domains. We propose a possible ab-plane spiral-spin-order phase to be responsible for the large spontaneous polarization in the films, which can only be stabilized by relatively high magnetic field H > 6 T in the bulk crystals. It is suggested that the nano-scale twin-like domain structure is essential for the high temperature ferroelectricity and ferromagnetism of the thin films.

Coupled ferroelectric polarization and magnetization in spinel FeCr2S4

One of the core issues for multiferroicity is the strongly coupled ferroelectric polarization and magnetization, while so far most multiferroics have antiferromagnetic order with nearly zero magnetization. Magnetic spinel compounds with ferrimagnetic order may be alternative candidates offering large magnetization when ferroelectricity can be activated simultaneously. In this work, we investigate the ferroelectricity and magnetism of spinel FeCr2S4 in which the Fe2+ sublattice and Cr3+ sublattice are coupled in antiparallel alignment. Well defined ferroelectric transitions below the Fe2+ orbital ordering termperature Too = 8.5u2005K are demonstrated. The ferroelectric polarization has two components. One component arises mainly from the noncollinear conical spin order associated with the spin-orbit coupling, which is thus magnetic field sensitive. The other is probably attributed to the Jahn-Teller distortion induced lattice symmetry breaking, occuring below the orbital ordering of Fe2+. Furthermore, the coupled ferroelectric polarization and magnetization in response to magnetic field are observed. The present work suggests that spinel FeCr2S4 is a multiferroic offering both ferroelectricity and ferrimagnetism with large net magnetization.

The competing spin orders and fractional magnetization plateaus of the classical Heisenberg model on Shastry-Sutherland lattice: Consequence of long-range interactions

The competing spin orders and fractional magnetization plateaus of the classical Heisenberg model with long-range interactions on a Shastry-Sutherland lattice are investigated using Monte Carlo simulations, in order to understand the fascinating spin ordering sequence observed in TmB4 and other rare-earth tetraborides. The simulation reproduces the experimental 1/2 magnetization plateau at low temperature by considering multifold long range interactions. It is found that more local long range interactions can be satisfied in the 1/2 plateau state than those in the 1/3 plateau state, leading to the stabilization of the extended 1/2 plateau. The phase boundaries in the magnetic field at zero temperature are determined, demonstrating the simulation results. When the energies of the Neel state and the collinear state are degenerated, the former state is more likely to be stabilized due to the competitions among the local collinear spin orders. The present work provides a comprehensive proof of the phase trans...

Long Electron–Hole Diffusion Length in High‐Quality Lead‐Free Double Perovskite Films

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs2 AgBiBr6 ) solar cells using the planar structure are demonstrated. The prepared Cs2 AgBiBr6 films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO2 exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.

Resistive switching induced by charge trapping/detrapping: a unified mechanism for colossal electroresistance in certain Nb:SrTiO3-based heterojunctions

SrTiO3 remains at the core of research on oxide electronics, owing to its fascinating properties and wide applications as a commercial substrate. Heterojunctions based on Nb-doped SrTiO3 (NSTO), including both metal/NSTO Schottky junctions (MSJs) and NSTO-based ferroelectric tunnel junctions (FTJs) have received considerable attention due to the colossal electroresistance (CER) effect. However, the mechanism underpinning the CER effect is still poorly understood. Here, we conduct a comparative study on the CER effects in Au/NSTO MSJs and Au/BaTiO3/NSTO FTJs. The two types of heterojunctions show many similarities in resistive switching characteristics, including hysteretic current–voltage curves with asymmetric shapes, absence of critical switching fields, switching times on the scale of ∼1.0 μs, and resistance relaxations of the Curie–von Schweidler type. These results suggest that the CER effects in the MSJs and FTJs may have a common origin, i.e., charge trapping/detrapping, as further revealed by scanning Kelvin probe microscopy. Using temperature-dependent current–voltage, capacitance–voltage, and photo-response measurements, we demonstrate that charge trapping/detrapping could modify both the Schottky barrier profile and the tunneling process, and in turn lead to different transport mechanisms in different voltage regimes. The charge trapping/detrapping-induced CER effect can be well described by a metal–insulator–semiconductor (MIS) model, which reproduces the hysteretic current–voltage curves fairly well over a large range of voltage sweeping and thus provides a unified framework for the CER effects in certain NSTO-based heterojunctions.

Kinetics of 90° domain wall motions and high frequency mesoscopic dielectric response in strained ferroelectrics: a phase-field simulation.

The dielectric and ferroelectric behaviors of a ferroelectric are substantially determined by its domain structure and domain wall dynamics at mesoscopic level. A relationship between the domain walls and high frequency mesoscopic dielectric response is highly appreciated for high frequency applications of ferroelectrics. In this work we investigate the low electric field driven motion of 90°-domain walls and the frequency-domain spectrum of dielectric permittivity in normally strained ferroelectric lattice using the phase-field simulations. It is revealed that, the high-frequency dielectric permittivity is spatially inhomogeneous and reaches the highest value on the 90°-domain walls. A tensile strain favors the parallel domains but suppresses the kinetics of the 90° domain wall motion driven by electric field, while the compressive strain results in the opposite behaviors. The physics underlying the wall motions and thus the dielectric response is associated with the long-range elastic energy. The major contribution to the dielectric response is from the polarization fluctuations on the 90°-domain walls, which are more mobile than those inside the domains. The relevance of the simulated results wth recent experiments is discussed.

Dynamic magnetization process in the frustrated Shastry-Sutherland system TmB4

The dynamic magnetization behaviors of the classical Ising model on the Shastry-Sutherland lattice with additional long-range interactions are investigated by means of the Glauber dynamics, in order to understand the fascinating magnetization plateaus and the hysteresis loop observed in TmB4. With this algorithm, the experimental 1/n (n = 7, 9, 11) magnetization plateaus as well as the main 1/2 one can be reproduced at low temperatures. Furthermore, the hysteresis loop can also be well explained by the present theory. It is indicated that the formation of domain walls due to the non-equilibrium magnetization process may be responsible for the emergence of the fractional plateaus.