Recently, significant progress has been made in increasing the figure-of-merit (ZT) of various nanostructured materials, including thin-film and quantum dot superlattice structures. Studies have focused on the size reduction and control of the surface or interface of nanostructured materials since these approaches enhance the thermopower and phonon scattering in quantum and superlattice structures. Currently, bismuth-tellurium-based semiconductor materials are widely employed for thermoelectric (TE) devices such as TE energy generators and coolers, in addition to other sensors, for use at temperatures under 400 K. However, new and promising TE materials with enhanced TE performance, including doped zinc oxide (ZnO) multilayer or superlattice thin films, are also required for designing solid-state TE power generating devices with the maximum output power density and for investigating the physics of in-plane TE generators. Herein, we report the growth of AlO/ZnO (AO/ZnO) superlattice thin films, which were prepared by atomic layer deposition (ALD), and the evaluation of their electrical and TE properties. All the in-plane TE properties, including the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ), of the AO/ZnO superlattice (with a 0.82 nm-thick AO layer) and AO/ZnO films (with a 0.13 nm-thick AO layer) were evaluated in the temperature range 40-300 K, and the measured S, σ, and κ were -62.4 and -17.5 μV K, 113 and 847 (Ω cm), and 0.96 and 1.04 W m K, respectively, at 300 K. Consequently, the in-plane TE ZT factor of AO/ZnO superlattice films was found to be ∼0.014, which is approximately two times more than that of AO/ZnO films (ZT of ∼0.007) at 300 K. Furthermore, the electrical power generation efficiency of the TE energy generator consisting of four couples of n-AO/ZnO superlattice films and p-BiSbTe (p-BST) thin-film legs on the substrate was demonstrated. Surprisingly, the output power of the 100 nm-thick n-AO/ZnO superlattice film/p-BST TE energy generator was determined to be ∼1.0 nW at a temperature difference of 80 K, corresponding to a significant improvement of ∼130% and ∼220% compared to the 100 nm-thick AO/ZnO film/p-BST and n-BT/p-BST film generators, respectively, owing to the enhancement of the TE properties, including the power factor of the superlattice film.

Recent success in experimental and theoretical works on metal thiophosphates (MTPs) paved the way to add multiple functionalities of complex oxides, such as ferroelectricity, in 2D materials. To realize multiferroicity and magnetoelectric coupling on layered van der Waals materials, incorporating magnetic ions in the ferroelectric framework is desirable. Unfortunately, replacing the metal ion with a magnetic one in MTPs results in antiferroelectricity in which spontaneous macroscopic polarization is absent. Herein, the emergence of a tunable local ferroelectric state in antiferroelectric CuCrP2S6 possessing magnetic Cr3+ ion is reported. The spontaneous macroscopic polarization is observed, which is switchable by an external poling field through controlling a defect‐dipole polarization in the quasi‐antipolar state. The observations suggest that the formation of defect dipoles, which is facilitated by an order‐disorder‐type structural transition, is likely related to a metastable Cu site within the van der Waals gap and therefore is a smoking gun of the existence of a uniaxial quadruple potential well. The interaction between the defect‐dipole polarization and dipoles in the antipolar matrix may lead to the emerging local ferroelectricity in antiferroelectric CuCrP2S6. The findings suggest a possibility of utilizing the local ferroelectricity of multiferroic MTPs for novel 2D applications.

We report p-type tin monoselenide (SnSe) single crystals, grown in double-sealed quartz ampoules using a modified Bridgman technique at 920 °C. X-ray powder diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) measurements clearly confirm that the grown SnSe consists of single-crystal SnSe. Electrical transport of multi-layer SnSe nanoflakes, which were prepared by exfoliation from bulk single crystals, was conducted using back-gated field-effect transistor (FET) structures with Au and Ti contacts on SiO2/Si substrates, revealing that multi-layer SnSe nanoflakes exhibit p-type semiconductor characteristics owing to the Sn vacancies on the surfaces of SnSe nanoflakes. In addition, a strong carrier screening effect was observed in 70−90-nm-thick SnSe nanoflake FETs. Furthermore, the effect of the metal contacts to multi-layer SnSe nanoflake-based FETs is also discussed with two different metals, such as Ti/Au and Au contacts.

Polarization properties and dielectric relaxations were investigated in single crystalline Mn-substituted YFeO3. Thermally stimulated depolarization currents (TSDC) were observed at Tm ≈ 110 K along all three orthorhombic directions but with different magnitudes. These anisotropic behaviors were also observed in temperature dependent dielectric responses. Electric field sweep polarization measurements down to 10 K showed neither hysteresis nor remanent polarization. Based on the result of the temperature- and frequency-dependent dielectric measurements and the relaxation analysis of the TSDC, we suggest that previously reported pyroelectric currents at ∼110 K are not due to a ferroelectric phase transition but due to the formation of frozen defect dipoles which are induced by the localized charge carriers.

We report on the crystal and magnetic structure of bulk hexagonal (Lu,In)FeO3. Neutron powder diffraction revealed that Lu0.2In0.8FeO3 has a single-phase P63/mmc structure down to T ≈ 3 K and shows an antiferromagnetic transition at TN ≈ 164 K. Unlike the distorted hexagonal LuFeO3 family with an A2(Γ2)-type spin configuration, undistorted hexagonal Lu0.2In0.8FeO3 shows either an A1(Γ1) or a B1(Γ3)-type spin configuration below TN, which does not produce the c-directional canted ferromagnetic moment. A significant reduction in the ordered magnetic moment was observed at 3 K without trimerization, and hints of a magnetic cluster state were observed in the paramagnetic phase near room temperature. Therefore, the system presents a rare example to study the geometrically frustrated magnetism in the undistorted hexagonal magnet that has a perfect triangular lattice below room temperature.

Magnetic multiferroics TbMn2O5 shows rich magnetic phases that are strongly coupled to multiple electrical phases. Herein, we report that Fe substitutions in TbMn2O5 can selectively and completely suppress the lower-temperature anti-polar phase, whereas the higher-temperature ferroelectric phase is almost intact down to T ≈ 10 K. Neutron diffraction on the single crystalline TbMn2-xFexO5 (x ≈ 0.06) shows that the commensurate magnetic phase persists down to T ≈ 1.7 K, indicating that the commensurability, which is responsible for the exchange-striction mechanism, plays a deterministic role on the ferroelectric polarization in TbMn2O5.

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Improper ferroelectricity and canted ferromagnetism in antiferromagnetically ordered hexagonal ferrites with A2-type spin configuration have been intensely studied due to their potential for room-temperature multiferroicity with strong magnetoelectric couplings. However, the subtle interplay between the magnetic structure and trimer structural distortion, which is a critical ingredient for ferroelectricity, has not been clearly verified in experiments due to the lack of control over trimer distortion. In this study, we report on Lu0.6In0.4FeO3, which exhibits weaker trimer distortion primarily related to the smaller tilting of the FeO5 bipyramids. The reduced vertical displacement of the equatorial oxygen of FeO5 located at the center of the trimer lowers the magnitude of the in-plane Dzyaloshinskii–Moriya vector component, resulting in the absence of canted ferromagnetism with the planar A1-type spin configuration, rather than the canted A2-type observed in other hexagonal ferrites. Our findings demonstrate that the degree of trimer distortion plays an important role in determining the spin configuration in related hexagonal systems.