Type-II Weyl semimetal ${\mathrm{MoTe}}_2$ exhibits a first-order structural transition at $T_s\sim 250$ K and superconducts at $T_c\sim 0.1$ K at ambient pressure. Both $T_s$ and $T_c$ can be manipulated by several tuning parameters, such as hydrostatic pressure and chemical substitution. It is often reported that suppressing $T_s$ enhances $T_c$, but our study shows a different behavior when ${\mathrm{MoTe}}_2$ is hole-doped by Ta. When $T_s$ is suppressed by Ta doping, $T_c$ is also suppressed. Our findings suggest that the suppression of $T_s$ does not necessarily enhance superconductivity in ${\mathrm{MoTe}}_2$. By connecting with the findings of electron-doped ${\mathrm{MoTe}}_2$, we argue that varying electron carrier concentration can effectively tune $T_c$. In addition, the Hall coefficient is enhanced around the doping region, where $T_s$ is completely suppressed, suggesting that the critical scattering around the structural transition may also play a role in suppressing $T_c$.

In metal-organic-framework (MOF) perovskites, both magnetic and ferroelectric orderings can be readily realized by compounding spin and charge degrees of freedom. The hydrogen bonds that bridge the magnetic framework and organic molecules have long been thought of as a key in generating multiferroic properties. However, the underlying physical mechanisms remain unclear. Here, we combine neutron diffraction, quasielastic and inelastic neutron scattering, and THz spectroscopy techniques to thoroughly investigate the dynamical properties of the multiferroic MOF candidate $\left[{\mathrm{CH}}_3{\mathrm{NH}}_3\right]{\left[\mathrm{Co}(\mathrm{HCOO}\right)}_3]$ through its multiple phase transitions. The wide range of energy resolutions reachable by these techniques enables us to scrutinize the coupling between the molecules and the framework throughout the phase transitions and interrogate a possible magnetoelectric coupling. Our results also reveal a structural change around 220 K, which may be associated with the activation of a nodding donkey mode of the methylammonium molecule due to the ordering of the ${\mathrm{CH}}_3$ groups. Upon the occurrence of the modulated phase transition around 130 K, the methylammonium molecule undergoes a freezing of its reorientational motions, which is concomitant with a change of the lattice parameters and anomalies of collective lattice vibrations. No significant change has been however observed in the lattice dynamics around the magnetic ordering, which therefore indicates the absence of a substantial magnetoelectric coupling in zero field.

Recently emerged all-$d$-metal Ni(Co)-Mn-Ti Heusler systems often exhibit multifunctional properties which are accompanied by magnetic-field-induced inverse martensitic transformation (IMT). Hence, the study of the transitional dynamics of IMT on the corresponding intriguing physical properties is of great importance. In this paper, we report the critical significance of field-induced austenite phase fraction ($\mathrm{\Delta }{f}_{\text{IA}}$) in achieving giant reversible magnetoresponsive properties of the optimizing Cu doped in ${\mathrm{Ni}}_{35}{\mathrm{Co}}_{15}{\mathrm{Mn}}_{34.5-x}{\mathrm{Cu}}_x{\mathrm{Ti}}_{15.5}$ ($x$ = 1, 2, and 3) all-$d$-metal Heusler systems. We show that the Cu substitution in the Mn site pulls the magnetic transition towards structural transitions and therefore the distance between them decreases. The evolution of $\mathrm{\Delta }{f}_{\text{IA}}$ has been investigated using the Clausius Clapeyron equation and the Landauer equation under field cycling by variation of magnetization and resistivity as a function of temperature and field for $x$ = 2 sample. A giant reversible magnetic entropy change ($\mathrm{\Delta }{\mathrm{S}}_M$) of $\sim$ 22.8 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 257 K as well as a giant effective refrigerant capacity (${\mathrm{RC}}_{\text{eff}}$) of $\sim 610\mathrm{J}{\mathrm{kg}}^{-1}$ in a field change of 7 T ($\sim 9\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at 269 K and ${\mathrm{RC}}_{\text{eff}}\sim 144\mathrm{J}{\mathrm{kg}}^{-1}$ under 2 T) is obtained, correlating with the good geometrical compatibility between two phases (${\lambda }_2$ closer to 1) and the high sensitivity of phase transition temperature to the magnetic field ($\sim 4.3\mathrm{K}/\mathrm{T}$). Moreover, a remarkably giant reversible magnetoresistance (MR) of $\sim 40\%$ over 42% of the total MR in a field change of 7 T is observed, when $\mathrm{\Delta }{f}_{\text{IA}}$ is fully induced. The observed magnitude of reversible magnetocaloric effect and MR is the highest reported value so far in the all-$d$-metal Heusler family. Our findings corroborate the generality of using the $\mathrm{\Delta }{f}_{\text{IA}}$ to enhance reversible magnetoresponsive properties in conventional all-$d$-metal Heusler system. These giant magnetoresponsive characteristics over a wide temperature window may therefore lead to the all-$d$-metal Heusler system as a suitable state-of-the-art caloric material for solid-state-based technological applications.

High upper-critical field superconducting alloys are required for superconducting device applications. In this paper, we extensively characterized the structure and superconducting properties of alloys ${\mathrm{Ta}}_x{\mathrm{Hf}}_{1-x}$ ($x$ = 0.2, 0.4, 0.5, 0.6 and 0.8). The substitution of Hf ($T_C$ = 0.12 K, type-I superconductor) with Ta ($T_C$ = 4.4 K, type-I superconductor) shows an anomalous enhancement of $T_C$ with a variation of composition. Interestingly, all compositions exhibited strongly coupled bulk type-II superconductivity with a high upper critical field.

We report temperature evolution of the crystal structure and electrical properties, and their interconnection in the promising two-dimensional (2D) layered battery materials $A_2{\mathrm{Ni}}_2\mathrm{Te}{\mathrm{O}}_6\left(A=\mathrm{Na}\text{/}\mathrm{Li}\right)$ by comprehensive neutron diffraction, Raman spectroscopy, and impedance spectroscopy studies. In this paper, we reveal that the crystal structures of Na- and Li-based compounds, with space group $P{6}_3/mcm$ and Cmca, respectively, have identical global crystal structures with alternating $\left(\mathrm{Ni}\text{/}\mathrm{Te}\right){\mathrm{O}}_6$ and alkali metal layers but with different internal structures of the alkali metal layers. Our soft bond valence sum analysis of the neutron diffraction patterns illustrates 2D conduction pathways within the $ab$ planes for both compounds. Despite the Li ion having smaller size and lighter weight than that of the Na ion, our results reveal that the Na compound has higher ionic conductivity than the Li compound. By detailed crystal structural analysis, we divulge that the differences in the internal structures of the alkali metal ion layers, such as distance between the alkali metal ions, local crystal structural environment, and the relative site occupancies of the alkali metal ions, result in differences in the ionic conductivity. Additionally, in this paper, we demonstrate a correlation between crystal structure and electrical properties, where anomalies are found for both temperature-dependent electrical and local crystal structural parameters at $\sim 475\text{–}575\mathrm{K}$ and $\sim 200\text{–}300\mathrm{K}$ for Na- and Li-based compounds, respectively. In this comprehensive paper, we thus facilitate the understanding of the microscopic crystal structure, electrical properties, and their intercorrelations which are essential for the design and fabrication of highly efficient layered battery materials.

The pursuit of high-performance thermoelectricity has generated an endless stream of suitable materials, but those having intrinsically low thermal conductivity are highly desirable for production. Here, we demonstrated intrinsically low lattice thermal conductivity (${\kappa }_l$) for the orthorhombic phase $\beta \text{−}{\mathrm{SrZrS}}_3$, a chalcogenide perovskite. The weak interatomic interactions are noticed for ${\mathrm{SrS}}_8$ bicapped trigonal prism geometry, leading to the bond heterogeneity and thus an effective anharmonicity in the system. This anharmonicity facilitates the low ${\kappa }_l$. The average ${\kappa }_l$ value at 300 K is found to be $\sim 1.26\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, while at 800 K it further reduces to a lower value of $\sim 0.48\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ due to enhanced phonon-phonon scattering. The presence of several valleys within a small energy offset from the primary conduction-band edge and a sort of pudding-mold-kind appearance make the band structure more complex. These band features in the conduction band facilitate the system with a high-power factor with $n$-type doping. Avoiding bipolar conduction is also confirmed via a large-band-gap value of ∼2.04 eV for $\beta \text{−}{\mathrm{SrZrS}}_3$. Furthermore, significant phonon-phonon scattering is caused by the high anharmonic interaction resulting from the coupling of the acoustic and low optical frequency domains, substantiating the low ${\kappa }_l$. Additionally, the electron's lifetime at 300 K is ∼34 fs and its mobility is $\sim {10}^2\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Thus, an impressive figure of merit $>2$ is achieved for the intermediate temperature (600–800 K) at an experimentally feasible electron doping of $\sim {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$, indicating that $\beta \text{−}{\mathrm{SrZrS}}_3$ could be an excellent wide-band-gap thermoelectric material.

The grain boundary (GB) microstructure influences and is influenced by the development of residual stresses during synthesis of polycrystalline thin films. Recent studies have shown that the frustration between the preferred growth direction and rotations of abutting crystals to local cusps in GB energies leads to internal stresses localized within nanoscopic surface layers around the valleys and ridges that form at emergent boundaries (eGBs). Using a combination of continuum frameworks, numerical analyses, and all-atom simulations of bicrystal $\langle 111\rangle$ copper films, we show that eGBs tune their surface morphology and rotation extent in response to external strains. Compression favors rotation to and growth of low-energy GB phases (complexions) at eGB valleys while tension favors the transitions at eGB ridges, a reflection of the stress-induced mass efflux/influx that changes the energetic balance between interfacial and deformation energies. Molecular dynamics simulations of strained and growing bicrystal films reveal that the eGB phase transition is coupled to island formation at the surface triple junctions, providing a direct link between eGB phases and surface step flow. The interplay between eGB structure, morphology, and mechanics emerges as a crucial ingredient for predictive understanding of stress and morphological evolution during film growth, with broad implications for multifunctional response of polycrystalline surfaces in a diverse range of surface phenomena such as surface-mediated deformation, interfacial embrittlement, thermal grooving, stress corrosion, surface catalysis, and topological conduction.

Millimeter-sized ${\mathrm{Ce}}_2{\mathrm{Rh}}_{3+\delta }{\mathrm{Sb}}_4$ ($\delta \approx 1/8$) single crystals were synthesized by a Bi-flux method, and their physical properties were studied by a combination of electrical transport, magnetic, and thermodynamic measurements. The resistivity anisotropy ${\rho }_{a,b}/{\rho }_c\sim 2$, manifesting a quasi-one-dimensional electronic character. Magnetic susceptibility measurements confirm $\mathbf{ab}$ as the magnetic easy plane. A long-range antiferromagnetic transition occurs at $T_N=1.4$ K, while clear short-range ordering can be detected well above $T_N$. The low ordering temperature is ascribed to the large Ce-Ce distance as well as the geometric frustration. Kondo scale is estimated to be about 2.4 K, comparable to the strength of magnetic exchange. ${\mathrm{Ce}}_2{\mathrm{Rh}}_{3+\delta }{\mathrm{Sb}}_4$, therefore, represents a rare example of a dense Kondo lattice whose Ruderman-Kittel-Kasuya-Yosida exchange and Kondo coupling are both weak but competing.

Charge redistribution across heterojunctions has long been utilized to induce functional response in materials systems. Here we examine how the composition of the terminating surface affects charge transfer across a heterojunction consisting of Si and the crystalline complex oxide $\mathrm{SrTi}{\mathrm{O}}_3$. Itinerant electrons in Si migrate across the interface toward the surface of $\mathrm{SrTi}{\mathrm{O}}_3$ due to surface depletion. The electron transfer in turn creates an electric field across the interface that modifies the interfacial dipole associated with bonding between $\mathrm{SrTi}{\mathrm{O}}_3$ and Si. The modification in the dipole leads to a change in band alignment, in which the conduction band of $\mathrm{SrTi}{\mathrm{O}}_3$ moves from being above the valence band of Si in energy, to below it. By capping the $\mathrm{SrTi}{\mathrm{O}}_3$ surface with ultrathin ($\le 1$ nm) layers of BaO, SrO, or $\mathrm{Ti}{\mathrm{O}}_2$, charge transfer across the interface can be weakened or inhibited. Ab initio modeling implicates the adsorption of oxygen associated with exposure to ambient conditions as driving the surface depletion in $\mathrm{SrTi}{\mathrm{O}}_3$. The electronic coupling between the surface and buried interface expands the functionality of semiconductor–crystalline-oxide heterojunctions.

Color centers in diamond have attracted much attention as potential quantum photodevices. In contrast to nitrogen vacancy and group-IV-vacancy centers, the optical properties of other thermally stable vacancies have not been thoroughly studied. This work focuses on a divacancy in diamond, and its optical properties were studied by applying first-principles simulation schemes. A structure with two vacancies in the nearest-neighbor sites was chosen as a test case, and calculations of dipole matrix elements by density functional theory found two possible optical excitation paths. According to the electron-ion dynamics calculated by time-dependent density functional theory, one excitation path showed nonradiative decay, suggesting that it would not emit light. The other excitation path seemed to hold its excited state for 500 fs, but the light emission intensity appeared to be weakened. The effect of hydrogen termination of the divacancy was also studied. The current results provide a fundamental understanding of the optical properties of divacancies and should trigger future research into their influence on the properties of color centers in diamond.

In this article we present, as far as we know, the first numerical investigation of phonon relaxation in two-dimensional polycrystalline systems simulated with a multitimescale phase field crystal model. We first measure the phonon spectrum averaged over different polycrystalline configurations using thermal fluctuations to capture the rapid processes. We find two main peaks in the spectrum attributable, respectively, to dampening and softening of different wavelength phonons. In particular, it is shown that polycrystals have a phonon caging regime, a signature of amorphous materials. Subsequently, we report on a mechanism of grain-boundary melting resulting from the accumulation of phonon scattering. We find this behavior exhibited in both rapid temperature annealing of polycrystalline samples and from input of kinetic energy representative of rapid laser heating or hot-rolling. In the latter case, we theorize a rate relation between the maximally achieved liquid fraction as a function of the initial kinetic energy, defining a metastable activation energy that can be measured in experiments. We expect that the scattering mechanisms investigated in this work underpin grain-boundary melting and recrystallization processes encountered in rapid solidification experiments.

The rapid compaction of granular media results in localized heating that can induce chemical reactions, phase transformations, and melting. However, there are numerous mechanisms in play that can be dependent on a variety of microstructural features. Machine learning techniques such as neural networks offer a ubiquitous method to develop models for physical processes. Limiting what kind of microstructural information is used as an input and assessing normalized changes in network error, the relative importance of different mechanisms can be inferred. Here we utilize binned, initial density information as network inputs to predict local shock heating in a granular high explosive trained from large-scale molecular dynamics simulations. The spatial extent of the density field used in the network is altered to assess the importance and relevant length scales of the physical mechanisms in play, where different microstructural features result in different predictive capabilities.

Moderate systematic understanding has been established in the literature how different x-ray absorption techniques, i.e., total electron yield, partial fluorescence yield, and inverse partial fluorescence yield, compare and contrast for the $L_{2,3}$ edges of transition metal oxides. In this paper, we attempt to analyze and understand trends between different techniques and furthermore shed light on self-absorption effects and state-dependent fluorescence probabilities in fluorescence yield mode. Our experiments demonstrate strong interdependence of the manganese direct fluorescence yield and correlation effects, while the secondary $3s\to 2p$ decay as well as inverse partial fluorescence yield are unaffected. We present x-ray absorption, nonresonant x-ray emission, and resonant inelastic x-ray scattering spectra for manganese oxides at the Mn $L_{2,3}$ edges and investigate their spectral shape with multiplet ligand field calculations. The collected spectra are sensitive to valence state, local environment, and hybridization. Least-squares optimization of our parametrized ligand field model allows us to analyze the electronic structure of the studied compounds.

We have investigated the charge transport property, thermoelectric effect, and electronic state for the electron-doped narrow gap semiconductor ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Pd}}_3{\mathrm{O}}_4$ by means of the transport measurement, optical/photoemission spectroscopy, and ab initio calculation. The high-quality polycrystalline samples of ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Pd}}_3{\mathrm{O}}_4$ were synthesized by using the high-pressure synthesis technique. In the undoped system $x=0$, the optical conductivity spectra show a charge gap of about 0.1 eV, which is qualitatively consistent with the results of ab initio calculation. The electron doping causes the rapid reduction of resistivity ${\rho }_{xx}$, resulting in the metallic state at the doping level of as small as $x=0.01$. On the other hand, the magnitude of Seebeck coefficient $\left|S\right|$ moderately decreases with increasing $x$. Consequently, the power factor $S^2/{\rho }_{xx}$ reaches about $7\mathrm{\mu }\mathrm{W}/{\mathrm{K}}^2\mathrm{cm}$ at 350 K at $x=0.03$, which is much higher than that of hole-doped analog of ${\mathrm{CaPd}}_3{\mathrm{O}}_4$. Combined with the results of photoemission spectra and ab initio calculation, the large power factor in the present material likely originates from the relatively low resistivity, probably high electron mobility ($\sim 30{\mathrm{cm}}^2/\mathrm{Vs}$ at 350 K), due to the dispersive Pd $4d_{x^2-y^2}$ conduction band.

Understanding the sticking coefficient $\sigma$, i.e., the probability of an adatom sticking to a surface, is essential for controlling the stoichiometry during epitaxial film growth. However, $\sigma$ on monolayer graphene-covered surfaces and its impact on remote epitaxy are not understood. Here, using molecular-beam epitaxial growth of the magnetic shape memory alloy ${\mathrm{Ni}}_2\mathrm{MnGa}$, we show that the sticking coefficients for metals on graphene-covered MgO (001) are less than one and are temperature and element dependent, as revealed by ion backscattering spectrometry and energy-dispersive x-ray spectroscopy. This lies in stark contrast with most transition metals sticking on semiconductor and oxide substrates, for which $\sigma$ is near unity at typical growth temperatures ($T<800{}^{\circ }\mathrm{C}$). By initiating growth below $400{}^{\circ }\mathrm{C}$, where the sticking coefficients are closer to unity and wetting on the graphene surface is improved, we demonstrate the epitaxy of ${\mathrm{Ni}}_2\mathrm{MnGa}$ films with controlled stoichiometry that can be exfoliated to produce freestanding membranes. Straining these membranes tunes the magnetic coercive field. Our results provide a route to synthesize membranes with complex stoichiometries whose properties can be manipulated via strain.

Two-dimensional (2D) honeycomb ferromagnets, such as monolayer chromium trihalides, are predicted to behave as topological magnon insulators, characterized by an insulating bulk and topologically protected edge states, giving rise to a thermal magnon Hall effect. Here we report the behavior of the topological magnons in monolayer ${\mathrm{CrI}}_3$ under external stimuli, including biaxial and uniaxial strain, electric gating, as well as in-plane and out-of-plane magnetic field, revealing that one can thereby tailor the magnetic states as well as the size and the topology of the magnonic bandgap. These findings broaden the perspective of using 2D magnetic materials to design topological magnonic devices.

The anomalous Nernst effect (ANE) is a member of the extensive family of topological effects in solid state physics. It converts a heat current into electric voltage and originates from the Berry curvature of electronic bands near the Fermi level. Recent results established the ${\mathrm{Fe}}_3\mathrm{Ga}$ alloy as one of the most promising candidates for applications, due to its flat band structure consisting of a rich web of nodal lines. In this theoretical work, we study the effect of deformation of ${\mathrm{Fe}}_3\mathrm{Ga}$ on the anomalous Nernst effect, which naturally occurs in thin films. Furthermore, we demonstrate that doping, which effectively shifts the position of the Fermi level, can also significantly modify the strength of the effect. Lastly, we provide detailed analysis of the origin of ANE in the electronic structure of ${\mathrm{Fe}}_3\mathrm{Ga}$ which yields a deeper insight into the generating mechanisms, the understanding of which can lead to substantial enhancement of the effect in the future.

The band offsets at polar heterointerfaces that consist of group III-V zinc-blende semiconductors are investigated by first-principles modeling using the Heyd-Scuseria-Ernzerhof hybrid functional. Since the band alignment for such polar heterointerfaces and their constituent polar surfaces has not been fully elucidated and a reliable computational setup remains lacking, we propose in this paper the application of pseudo-H passivation in combination with the supercell approach adopting slab models. The pseudo-H passivation is shown to be a valid computational setup that quenches the macroscopic electric field generated by polar slab surfaces and stabilizes the electrostatic potential profile normal to the surfaces, and more consequentially, across the polar interface in the slab. This is essential to an accurate evaluation of the band offset at a polar interface using the supercell approach. The interfaces made of the polar (100) or (111) surfaces of AlP, AlAs, GaP, GaAs, InP, and InAs are studied. The predicted valence-band offsets are in good agreement with available experimental results. The offset values of most interfaces show weak dependence on their local chemical compositions, which is attributed to the formation of nearly complete interfacial chemical bonding.

Phase-field modeling is an elegant and versatile computation tool to predict microstructure evolution in materials in the mesoscale regime. However, these simulations require rigorous numerical solutions of differential equations, which are accurate but computationally expensive. To overcome this difficulty, we combine two popular machine-learning techniques, autoencoder and convolutional long short-term memory (ConvLSTM), to accelerate the study of microstructural evolution without compromising the resolution of the microstructural representation. After training with phase-field-generated microstructures of 10 known compositions, the model can accurately predict the microstructure for the future $n\mathrm{th}$ frames based on the previous $m$ frames for an unknown composition. Replacing $n$ phase-field steps with machine-learned microstructures can significantly accelerate the in silico study of microstructure evolution.