Materials Science (cond-mat.mtrl-sci)

Abstract: Bilayer (BL) molybdenum disulfide (MoS$_2$) is one of the most important electronic structures not only in valleytronics but also in realizing twistronic systems on the basis of the topological mosaics in Moir\'e superlattices. In this work, BL MoS$_2$ on sapphire substrate with 2$H$-stacking structure is fabricated. We apply the terahertz (THz) time-domain spectroscopy (TDS) for examining the basic optoelectronic properties of this kind of BL MoS$_2$. The optical conductivity of BL MoS$_2$ is obtained in temperature regime from 80 to 280 K. Through fitting the experimental data with the theoretical formula, the key sample parameters of BL MoS$_2$ can be determined, such as the electron density, the electronic relaxation time and the electronic localization factor. The temperature dependence of these parameters is examined and analyzed. We find that, similar to monolayer (ML) MoS$_2$, BL MoS$_2$ with 2$H$-stacking can respond strongly to THz radiation field and show semiconductor-like optoelectronic features. The theoretical calculations using density functional theory (DFT) can help us to further understand why the THz optoelectronic properties of BL MoS$_2$ differ from those observed for ML MoS$_2$. The results obtained from this study indicate that the THz TDS can be applied suitably to study the optoelectronic properties of BL MoS$_2$ based twistronic systems for novel applications as optical and optoelectronic materials and devices.

Abstract: We study the complex ferroelastic/ferroelectric domain structure in the prototypical ferroelectric PbTiO3 epitaxially strained on (110)o-oriented DyScO3 substrates, using a combination of atomic force microscopy, laboratory and synchrotron x-ray diffraction and high resolution scanning transmission electron microscopy. We observe that the anisotropic strain imposed by the orthorhombic substrate creates a large asymmetry in the domain configuration, with domain walls macroscopically aligned along one of the two in-plane directions. We show that the periodicity as a function of film thickness deviates from the Kittel law. As the ferroelectric film thickness increases, we find that the domain configuration evolves from flux-closure to a/c-phase, with a larger scale arrangement of domains into superdomains.

Abstract: Double perovskites R2NiMnO6 (R= Rare earth element) (RNMO) are a significant class of materials owing to their Multifunctional properties with structural modifications. In particular, multifunctional double perovskite oxides La2NiMnO6 (LNMO) which possess both electric and magnetic orderings, chemical flexibility, versatility, and indispensable properties like high ferromagnetic curie temperature, high absorption rates, dielectrics, etc. have drawn a lot of attention due their rich physics and diverse applications in various technology. This justifies the intense research in this class of materials, and the keen interest they are subject to both the fundamental and practical side. In view of the demands of this material in lead-free perovskite solar cells, photocatalytic degradation of organic dyes, clean hydrogen production, electric tuneable devices, Fuel cells, gas sensing, and Biomedical applications, there is a need for an overview of all the literature so far, the ongoing research and the future prospective. This review summarised all the Physical and Structural Properties of LNMO such as electric, magnetic, catalytic, and dielectric properties with their underlying mechanisms. This review article provides insight into the scope of studies in LNMO material for exploring unexposed properties in new material research and to identify areas of future investigation of the materials in the double perovskite family.

Abstract: Group-IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group-IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of gamma-GeSe, a recently identified polymorph of GeSe. gamma-GeSe exhibits high electrical conductivity (~106 S/m) and a relatively low Seebeck coefficient (9.4 uV/K at room temperature) owing to its high p-doping level (5x1021 cm-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak-antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that gamma-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.

Abstract: It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is for example of interest for applications in waste heat management in integrated circuits. Here, we show by atomic scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials including C and BN. In all three materials introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moir\'e structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4 degree depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.

Abstract: Reactive single-step hot-pressing at 1473 K and 35 MPa for 4 h produces dense, bulk, near single-phase, low-cost and low-criticality Fe$_2$Al$_{1.15}$B$_2$ and Fe$_2$Al$_{1.1}$B$_2$Ge$_{0.05}$Ga$_{0.05}$ MAB samples, showing a second-order magnetic phase transition with favorable magnetocaloric properties around room temperature. The magnetic as well as magnetocaloric properties can be tailored upon Ge and Ga doping, leading to an increase of Curie temperature $T_C$ and spontaneous magnetization $m_S$. The maximum isothermal entropy change $\Delta s_{T,max}$ of hot-pressed Fe$_2$Al$_{1.15}$B$_2$ in magnetic field changes of 2 and 5 T amounts to 2.5 and 5 J(kgK)$^{-1}$ at 287.5 K and increases by Ge and Ga addition to 3.1 and 6.2 J(kgK)$^{-1}$ at 306.5 K, respectively. The directly measured maximum adiabatic temperature change $\Delta T_{ad,max}$ is improved by the composition modification from 0.9 to 1.1 K in magnetic field changes of 1.93 T. Overall, we demonstrate that hot-pressing provides a much faster, more scalable and processing cost reducing alternative compared to conventional synthesis routes to produce heat exchangers for magnetic cooling devices. Therefore, our criticality assessment shows that hot-pressed Fe-based MAB phases provide a promising compromise of material and processing cost, criticality and magnetocaloric performance, demonstrating the potential for low-cost and low-criticality magnetocaloric applications around room temperature.

Abstract: Thermoelectricity is a next-generation solution for efficient waste heat management. Although various thermoelectric materials exist, there is still a lot of scope for advancement. Recently, two-dimensional (2D) materials, including MXenes, showed promise as thermoelectric materials. The progress of MXenes as magnificent thermoelectric materials is very well established in the form of a tailor-made review. MXenes outstanding thermoelectric activity comes from a unique band structure created from its atomically thin layers and the defective surface of the external layers of atoms. Furthermore, the variety of MXenes chemical composition and MXenes-based nanostructures facilitates the research path based on energy band engineering, optimization, carrier concentration and mobility. The thermoelectric efficiency of MXenes has been mapped over the landscape of other 2D and traditional thermoelectric materials. Meanwhile, MBenes, the latest family member of the flatland, exhibits an incredible diversity of structures with additional crystal symmetries. Owing to the orthorhombic crystal structure, an in-plane structural anisotropy, and hence, the in-plane dependent thermoelectric properties are plausible. As a future prospective, certain strategies that can enhance the thermoelectric performance of MBenes have been presented. In addition, few insights and challenges that have to be considered to overcome the limitations in the thermoelectric field have been debated.

Abstract: Arising from the extreme or saddle point in electronic bands, Van Hove singularity (VHS) strongly enhances the electronic correlation in its vicinity and leads to various new states of matter such as density wave and unconventional superconductivity. In contrast to the divergent density of states (DOS) in one and two dimensions, the VHS is generally non-divergent in three dimension (3D). Here we report the observation of divergent 3D VHS in a topological magnet EuCd2As2 by magneto-infrared spectroscopy. The divergent 3D VHS is generated by substantially tuning the energy dispersion and momentum distribution of the Weyl bands. Applying the external magnetic field allows effective control of exchange interaction between itinerant electrons and local magnetic moments. As a result, the 3D Weyl bands are found to shift continuously with the canting of magnetic moment, which leads to the gradual increase of Fermi velocity. Above the critical field Bc ~ 0.6 T, the formation of a divergent 3D VHS is evidenced by the abrupt spectral emergence of the inter-band transitions. This type of VHS allows in situ tunability by external magnetic fields with adjustable energy position, DOS and even its presence. The experimental spectrum and the emergence of 3D VHS can be quantitatively described by a two-band minimal model of Weyl semimetal with the exchange interaction. The deduced model predicts three additional optical transitions and their energy crossings, which are quantitatively verified by the magneto-near-infrared spectrum. Our results pave the way to exploring divergent VHS in the 3D systems with strong tunability and provide a platform to uncover the coordination between electronic correlation and the topological phase.

Abstract: Ferroelectric memories experienced a revival in the last decade due to the discovery of ferroelectricity in HfO$_2$-based nanometer-thick thin films. These films exhibit exceptional silicon compatibility, overcoming the scaling and integration obstacles that impeded perovskite ferroelectrics' use in high-density integrated circuits. The exact phase responsible for ferroelectricity in hafnia films remains debated with no single factor identified that could stabilize the ferroelectric phase thermodynamically. Here, supported by density functional theory (DFT) high-throughput (HT) calculations that screen a broad range of epitaxial conditions, we demonstrate conclusively that specific epitaxial conditions achievable with common substrates such as yttria-stabilized zirconia (YSZ) and SrTiO$_3$ can favor the polar Pca2$_1$ phase thermodynamically over other polar phases such as R3m and Pmn2$_1$ and nonpolar P2$_1$/c phase. The substrate's symmetry constraint-induced shear strain is crucial for the preference of Pca2$_1$. The strain-stability phase diagrams resolve experiment-theory discrepancies and can guide the improvement of ferroelectric properties of epitaxial hafnia thin films.

Abstract: While BiFeO3-based solid solutions show great promise for applications in energy conversion and storage, realizing this promise necessitates understanding the structure-property relationship in particular pertaining to the relaxor-like characteristics often exhibited by solid solutions with polar-to-non-polar morphotropic phase boundaries. To this end, we investigated the role of the compositionally-driven relaxor state in (100-x)BiFeO3-xSrTiO3 [BFO-xSTO], via in situ synchrotron X-ray diffraction under bipolar electric-field cycling. The electric-field induced changes to the crystal structure, phase fraction and domain textures were monitored via the {111}pc, {200}pc, and 1/2{311}pc Bragg peaks. The dynamics of the intensities and positions of the (111) and (11-1) reflections reveal an initial non-ergodic regime followed by long-range ferroelectric ordering after extended poling cycles. The increased degree of random multi-site occupation in BFO-42STO compared to BFO-35STO is correlated with an increase of the critical electric field needed to induce the non-ergodic-to-ferroelectric transition, and a decrease in the degree of domain reorientation. Although both compositions show an irreversible transition to a long-range ferroelectric state, our results suggest that the weaker ferroelectric response in BFO-42STO is related to an increase in ergodicity. This, in turn, serves to guide the development of BFO-based systems into promising platform for further property engineering towards specific capacitor applications.

Abstract: Volterra's definition of dislocations in crystals distinguishes edge and screw defects geometrically, according to whether the Burgers vector is perpendicular or parallel to the defect. Here, we demonstrate a distinction between screw and edge dislocations that enables a unified, purely topological means of classification. Our construction relies on the construction of real or virtual disclination-line pairs at the core of the dislocation in a smectic and can be generalized to crystals with triply-periodic order. The connection between topology and geometry is exploited.