Materials Science (cond-mat.mtrl-sci)

Abstract: Most perovskite oxides belong to the Pbnm space group, composed by an anisotropic unit cell, A-site antipolar displacements and oxygen octahedral tilts. Mapping the orientation of the orthorhombic unit cell in epitaxial heterostructures that consist of at least one Pbnm compound is often required to understand and control the different degrees of coupling established at their coherent interfaces and, therefore, their resulting physical properties. However, retrieving this information from the strain maps generated with high-resolution scanning transmission electron microscopy can be challenging, because the three pseudocubic lattice parameters are very similar in these systems. Here, we present a novel methodology for mapping the crystallographic orientation in Pbnm systems. It makes use of the geometrical phase analysis algorithm, as applied to aberration-corrected scanning transition electron microscopy images, but in an unconventional way. The method is fast and robust, giving real-space maps of the lattice orientations in Pbnm systems, from both cross-sectional and plan-view geometries and across large fields of view. As an example, we apply our methodology to rare-earth nickelate heterostructures, in order to investigate how the crystallographic orientation of these films depends on various structural constraints that are imposed by the underlying single crystal substrates. We observe that the resulting domain distributions and associated defect landscapes mainly depend on a competition between the epitaxial compressive/tensile and shear strains, together with the matching of atomic displacements at the substrate/film interface. The results point towards strategies for controlling these characteristics by appropriate substrate choice.

Abstract: The solid diffusive phase transformation involving the nucleation and growth of one nucleus is universal and frequently employed but has not yet been fully understood at the atomic level. Here, our first-principles calculations reveal a structural formation pathway of a series of topologically close-packed (TCP) phases within the hexagonally close-packed (hcp) matrix. The results show that the nucleation follows a nonclassical nucleation process, and the whole structural transformation is completely accomplished by the shuffle-based displacements, with a specific 3-layer hcp-ordering as the basic structural transformation unit. The thickening of plate-like TCP phases relies on forming these hcp-orderings at their coherent TCP/matrix interface to nucleate ledge, but the ledge lacks the dislocation characteristics considered in the conventional view. Furthermore, the atomic structure of the critical nucleus for the Mg2Ca and MgZn2 Laves phases was predicted in terms of Classical Nucleation Theory (CNT), and the formation of polytypes and off-stoichiometry in TCP precipitates is found to be related to the nonclassical nucleation behavior. Based on the insights gained, we also employed high-throughput screening to explore several common hcp-metallic (including hcp-Mg, Ti, Zr, and Zn) systems that may undergo hcp-to-TCP phase transformations. These insights can deepen our understanding of solid diffusive transformations at the atomic level, and constitute a foundation for exploring other technologically important solid diffusive transformations.

Abstract: Materials that can host macroscopic persistent current are important because they are useful for energy storage. However, there are very few examples of such materials in nature. Superconductors are known as an example in which flow of supercurrent can persist up to 100,000 years. The chiral magnetic current is possibly the second example predicted by the chiral magnetic effect. It was proposed to be realized in recently discovered Weyl semimetals. However, a no-go theorem negates the chiral magnetic effect and shows that the chiral magnetic current is generally absent in any equilibrium condensed-matter system. Here we show how to break the no-go theorem by resorting to dynamical transitions in time-frequency space. By driving an insulator using a time-periodic potential and coupling it to a phonon heat bath that provides suitable dissipation, we show that a Floquet-Weyl semi-metallic phase with Fermi-Dirac-like distribution emerges. Furthermore, we show that even in the presence of a static magnetic field, the resulting steady Floquet-Weyl semimetal supports non-vanishing chiral magnetic current. Our dynamical model provides a systematic way to fully realize the chiral magnetic effect in condensed matter systems.

Abstract: Two-dimensional organic porous networks (2DOPNs) have opened new vistas for tailoring the physicochemical characteristics of metallic surfaces. These typically chemically bound nanoporous structures act as periodical quantum wells leading to the 2D confinements of surface electron gases, adatoms and molecular guests. Here we propose a new type of porous network with weakly interacting 2,4,6-triphenyl-1,3,5-triazine (TPT) molecules on a Cu(111) surface, in which a temperature-driven (T-driven) phase transition can reversibly alter the supramolecular structures from a close-packed (CP-TPT) phase to a porous-network (PN-TPT) phase. Crucially, only the low-temperature PN-TPT exhibits subnano-scale cavities that can confine the surface state electrons and metal adatoms. The confined surface electrons undergo a significant electronic band renormalization. To activate the spin degree of freedom, the T-driven PN-TPT structure can additionally trap Co atoms within the cavities, forming highly ordered quantum dots. Our theoretical simulation reveals a complex spin carrier transfer from the confined Co cluster to the neighbouring TPT molecules via the underlying substrate. Our results demonstrate that weakly interacting 2DOPN offers a unique quantum switch capable of steering and controlling electrons and spin at surfaces via tailored quantum confinements.

Abstract: Two-dimensional metal-organic porous networks (2D-MOPNs) are highly ordered quantum boxes for exploring surface confinements. In this context, the electron confinement from occupied Shockley-type surface states (SS) has been vigorously studied in 2D-MOPNs. In contrast, the confinement of excited surface states, such as image potential states (IPSs), remains elusive. In this work, we apply two-photon photoemission to investigate the confinement exemplarily for the first image state in a Cu-coordinated T4PT porous network (Cu-T4PT). Due to the lateral potential confinement in the Cu-T4PT, periodic replicas of the IPS as well as the SS are present in a momentum map. Surprisingly, the first IPS transforms into a nearly flat band with a substantial increase of the effective mass (> 150 %), while the band dispersion of the SS is almost unchanged. The giant confinement effect of the excited electrons can be attributed to the wavefunction location of the first IPS perpendicular to the surface, where the majority probability density mainly resides at the same height as repulsive potentials formed by the Cu-T4PT network. This coincidence leads to a more effective scattering barrier to the IPS electrons, which is not observed in the SS. Our findings demonstrate that the vertical potential landscape in a porous architecture also plays a crucial role in surface electron confinement.

Abstract: Two-dimensional metal-organic porous networks (2D-MOPNs) have been identified as versatile nanoarchitectures to tailor surface electronic and magnetic properties on noble metals. In this context, we propose a protocol to redecorate a ferromagnetic surface potential landscape using a 2D-MOPN. Ultrathin cobalt (Co) films grown on Au(111) exhibit a well-ordered surface triangular reconstruction. On the ferromagnetic surface, the adsorbed 2,4,6-tris(4-pyridyl)-1,3,5triazine (T4PT) molecules can coordinate with the native Co atoms to form a large-scale Co-T4PT porous network. The Co-T4PT network with periodic nanocavities serves as a templating layer to reshape the ferromagnetic surface potential. The subsequently deposited C60 molecules are steered by the network porous potential and the neighboring C60 interactions. The prototype of the ferromagnetic-supported 2D-MOPN is a promising template for the tailoring of molecular electronic and spin properties.

Abstract: Technologies that function at room temperature often require magnets with a high Curie temperature, $T_\mathrm{C}$, and can be improved with better materials. Discovering magnetic materials with a substantial $T_\mathrm{C}$ is challenging because of the large number of candidates and the cost of fabricating and testing them. Using the two largest known data sets of experimental Curie temperatures, we develop machine-learning models to make rapid $T_\mathrm{C}$ predictions solely based on the chemical composition of a material. We train a random forest model and a $k$-NN one and predict on an initial dataset of over 2,500 materials and then validate the model on a new dataset containing over 3,000 entries. The accuracy is compared for multiple compounds' representations ("descriptors") and regression approaches. A random forest model provides the most accurate predictions and is not improved by dimensionality reduction or by using more complex descriptors based on atomic properties. A random forest model trained on a combination of both datasets shows that cobalt-rich and iron-rich materials have the highest Curie temperatures for all binary and ternary compounds. An analysis of the model reveals systematic error that causes the model to over-predict low-$T_\mathrm{C}$ materials and under-predict high-$T_\mathrm{C}$ materials. For exhaustive searches to find new high-$T_\mathrm{C}$ materials, analysis of the learning rate suggests either that much more data is needed or that more efficient descriptors are necessary.

Abstract: Magnetic multilayers with interlayer exchange coupling have been widely studied for both static and dynamic regimes. Their dynamical responses depend on the exchange coupling strength and magnetic properties of individual layers. Magnetic resonance spectra in such systems are conveniently discussed in terms of coupling of acoustic and optical modes. At a certain value of applied magnetic field, the two modes come close to being degenerate and the spectral gap indicates the strength of mode hybridisation. In this work, we theoretically and experimentally study the mode hybridisation of interlayer-exchange-coupled moments with dissimilar magnetisation and thickness of two ferromagnetic layers. In agreement with symmetry analysis for eigenmodes, our low-symmetry multilayers exhibit sizable spectral gaps for all experimental conditions. The spectra agree well with the predictions from the Landau-Lifshitz-Gilbert equation at the macrospin limit whose parameters are independently fixed by static measurements.

Abstract: The existence of fractionally quantized topological corner states serves as a key indicator for two-dimensional second-order topological insulators (SOTIs), yet has not been experimentally observed in realistic materials. Here, based on first-principles calculations and symmetry arguments, we propose a strategy for achieving SOTI phases with in-gap corner states in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_{m}$ films with antiferromagnetic (AFM) order. Starting from the prototypical AFM MnBi$_2$Te$_4$ bilayer, we show by an effective lattice model that such SOTI phase originate from the interplay between intrinsic spin-orbital coupling and interlayer AFM exchange interactions. Furthermore, we demonstrate that the nontrivial corner states are linked to rotation topological invariants under three-fold rotation symmetry $C_3$, resulting in $C_3$-symmetric SOTIs with corner charges fractionally quantized to $\frac{n}{3} \lvert e \rvert $ (mod $e$). Due to the great recent achievements in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_{m}$ systems, our results providing reliable material candidates for experimentally accessible AFM higher-order band topology would draw intense attentions.

Abstract: Using first-principles and model Hamiltonian approach, we explore the electronic properties of polar-polar LaVO$_3$/KTaO$_3$ (LVO/KTO, 001) hetero-interfaces of up to six and five layers of KTO and LVO, respectively. Our calculations suggest the presence of multiple Lifshitz transitions (LT) in the $t_{2g}$ bands which may show up in high thermal conductivity and Seebeck coefficient. The LT can be tuned by the number of LaVO$_3$ layers or gate voltage. The spin-orbit coupling is found to be negligible, coming only from the Ta $5d_{xy}$-derived band, 5$d_{xz}$ and 5$d_{yz}$ bands being far away from the Fermi level. The magnetic properties of the interfaces, due to Vanadium ions, turn out to be intriguing. The magnetic states are highly sensitive to the number of layers of LaVO$_3$ and KTaO$_3$: the interfaces with equal number of LVO and KTO layers always favor an antiferromagnetic (AFM) ordering. Moreover, the combination of even-even and odd-odd layers shows an AFM order for more than two LaVO$_3$ layers. The spin-polarized density of states reveals that all the interfaces with ferromagnetic (FM) ground states are \textit{half-metallic}. The small energy differences between AFM and FM configurations indicate a possible coexistence of competing AFM and FM ground states in these interfaces. In addition, the interface requires different number of LVO layers for it to be metallic: half-metallic FM for three and above, and metallic AFM for four and above.

Abstract: The development of high-activity and low-price cathodic catalysts to facilitate the electrochemical sluggish O$_2$ reduction reaction (ORR) is very important to achieve the commercial application of fuel cells. Here, we have investigated the electrocatalytic activity of two-dimensional single-layer Nb-doped zirconium diselenide (2D Nb-ZrSe$_2$) towards ORR by employing the dispersion corrected Density Functional Theory (DFT-D) method. Through our study, we computed structural properties, electronic properties, and energetics of the 2D Nb-ZrSe$_2$ and ORR intermediates to analyze the electrocatalytic performance of the 2D Nb-ZrSe$_2$. The electronic properties calculations depict that the 2D monolayer ZrSe$_2$ has a large band gap of 1.48 eV, which is not favorable for the ORR mechanism. After the doping of Nb, the electronic band gap vanishes and 2D Nb-ZrSe$_2$ acts as a conductor. We studied both the dissociative and associative pathways through which the ORR can proceed to reduce the oxygen molecule (O$_2$). Our results show that the more favorable path for O$_2$ reduction on the surface of the 2D Nb-ZrSe$_2$ is the 4e$^-$ associative path. The detailed ORR mechanisms (both associated and dissociative) have been explored by computing the changes of Gibbs free energy ({\Delta}G). All the ORR reaction intermediate steps are thermodynamically stable and energetically favorable. The free energy profile for the associative path shows the downhill behavior of the free energy vs. the reaction steps, suggesting that all ORR intermediate structures are catalytically active for the 4e$^-$ associative path and a high 4e$^-$ reduction pathway selectivity. Therefore, 2D Nb-ZrSe$_2$ is a promising catalyst for the ORR which can be used as an alternative ORR catalyst compared with expensive platinum (Pt).