Topological materials have attracted significant interest in condensed matter physics for their unique topological properties leading to potential technological applications. Topological nodal line semimetals, a subclass of topological materials, exhibit symmetry-protected nodal lines, where band crossings occur along closed curves in the three-dimensional Brillouin zone. When the nodal lines are gapped out due to perturbation in the Hamiltonian, a large Berry curvature (BC) arises in the surrounding area of the gapped nodal line, leading to exotic anomalous transport responses. In this paper, we studied the ${\mathrm{Co}}_2\mathrm{Cr}X$ ($X$=Ga, Ge) Heusler compounds that exhibit mirror symmetry-protected nodal line states below the Fermi level. The BC calculation yields anomalous Hall conductivity (AHC) of about 292 and 217 S/cm for ${\mathrm{Co}}_2\mathrm{Cr}X$ ($X$=Ga, Ge), respectively, at the Fermi level, which increases by up to 400% at the nodal line energy level. We theoretically analyzed that 20% and 60% zinc (Zn) alloying in ${\mathrm{Co}}_2\mathrm{Cr}X$ ($X$=Ga, Ge) effectively lowers the Fermi level by 50 meV and 330 meV, respectively, aligning with the protected crossings. Consequently, we identified ${\mathrm{Co}}_2{\mathrm{CrGe}}_{0.4}{\mathrm{Zn}}_{0.6}$ and ${\mathrm{Co}}_2{\mathrm{CrGa}}_{0.8}{\mathrm{Zn}}_{0.2}$ as compositions to achieve the significant AHC of 800 and 1300 S/cm, respectively. The explicit AHC calculation for these alloyed compositions is in good agreement with our predictions. Our findings highlight that chemical alloying is an efficient way to enhance AHC in nodal line hosting materials.

We report the synthesis and characterization of thin films of the Weyl semimetal NbAs grown on GaAs (100) and As-terminated GaAs (111)B substrates. By choosing the appropriate substrate, we can stabilize the growth of NbAs in the [001] and [100] directions. We combine x-ray characterization with high-angle annular dark field scanning transmission electron microscopy to understand both the macroscopic and microscopic structure of the NbAs thin films. We show that these films are textured with domains that are tens of nanometers in size and that, on a macroscopic scale, are mostly aligned to a single crystalline direction. We describe electrical transport measurements that reveal similar behavior in films grown in both crystalline orientations, namely resistivity in the range $420–450\mu \mathrm{\Omega }\mathrm{cm}$ and carrier densities in the range $\sim {10}^{21}–{10}^{22}{\mathrm{cm}}^{-3}$ at 2 K. Finally we measure spin to charge conversion in NbAs and show that it qualitatively agrees with first principles calculations.

With increasing attention in Majorana physics for possible quantum bit applications, a large interest has been developed to understand the properties of the interface between an $s$-type superconductor and a topological insulator. Up to this point the interface analysis was mainly focused on in situ prepared Josephson junctions, which consist of two coupled single interfaces or to ex situ fabricated single interface devices. In our work we utilize a fabrication process, combining selective area growth and shadow evaporation which allows the characterization of a single in situ fabricated $\mathrm{Nb}/{\left({\mathrm{Bi}}_{0.15}{\mathrm{Sb}}_{0.85}\right)}_2{\mathrm{Te}}_3$ nanointerface. The resulting high interface transparency is apparent by a zero bias conductance increase by a factor of 1.7. Furthermore, we present a comprehensive differential conductance analysis of our single in situ interface for various magnetic fields, temperatures, and gate voltages. Additionally, density functional theory calculations of the superconductor/topological insulator interface are performed in order to explain the peaklike shape of our differential conductance spectra and the origin of the observed smearing of conductance features.

We created a computational workflow to analyze the potential energy surface (PES) of materials using machine-learned interatomic potentials in conjunction with the minima hopping algorithm. We demonstrate this method by producing a versatile machine-learned interatomic potential for iron hydride via a neural network using an iterative training process to explore its energy landscape under different pressures. To evaluate the accuracy and comprehend the intricacies of the PES, we conducted comprehensive crystal structure predictions using our neural network-based potential paired with the minima hopping approach. The predictions spanned pressures ranging from ambient to 100 GPa. Our results reproduce the experimentally verified global minimum structures such as dhcp, hcp, and fcc, corroborating previous findings. Furthermore, our in-depth exploration of the iron hydride PES at different pressures has revealed complex alterations and stacking faults in these phases, leading to the identification of several different low-enthalpy structures. This investigation has not only confirmed the presence of regions of established FeH configurations but has also highlighted the efficacy of using data-driven, extensive structure prediction methods to uncover the multifaceted PES of materials.

$\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{N}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ (BNT)-based systems are potential candidates for actuators due to their outstanding strain performances. Despite various doping components having been utilized to regulate their strain performances, the systematical route for designing giant-strain BNT-based materials remains elusive. Herein, we decode the potential correlation between strain performances and phase boundary characteristics and then propose a complete guideline for designing BNT-based materials with the giant strain. Since the polarization rotation is closely correlated to the phase transition process, the morphology phase boundary (MPB) frame of initial BNT matrix can maintain its partial properties for the polarization rotation, when introducing bits of doping components to tailor the transition temperature. The screened K6LT matrix with the optimized MPB frame exhibits a $d_{33}$ of 260 pC/N, and the ($\mathrm{Z}{\mathrm{r}}_{0.5}\mathrm{N}{\mathrm{b}}_{0.5}$)-modified K6LT matrix (K6LT-ZN1) exhibits a low driving field $E_{\mathrm{pol}}\sim 38.4$ kV/cm and high large-signal piezoelectric coefficient $d{{}_{33@60}^{*}}_{}\sim 727$ pm/V. This proposed design guideline reveals the significance for BNT matrix screening and could be a feasible method for constructing BNT-based systems with excellent strain performances.

Desorption of deposited species plays a role in determining the evolution of surface morphology during crystal growth when the desorption time constant is short compared with the time to diffuse to a defect site, step edge, or kink. However, experiments to directly test the predictions of these effects are lacking. Novel techniques such as in situ coherent x-ray scattering can provide significant new information. Herein we present x-ray photon correlation spectroscopy (XPCS) measurements during diindenoperylene (DIP) vapor deposition on thermally oxidized silicon surfaces. DIP forms a nearly complete two-dimensional first layer over the range of temperatures studied (40–$120{}^{\circ }\text{C}$), followed by mounded growth during subsequent deposition. Local step flow within mounds was observed, and we find that there was a terrace-length-dependent behavior of the step edge dynamics. This led to unstable growth with rapid roughening ($\beta >0.5$) and deviation from a symmetric error-function-like height profile. At high temperatures, the grooves between the mounds tend to close up leading to nearly flat polycrystalline films. Numerical analysis based on a $(1+1)$-dimensional model suggests that terrace-length dependent desorption of deposited ad-molecules is an essential cause of the step dynamics, and it influences the morphology evolution.

The valley splitting (VS) of a silicon quantum dot plays an important role for the performance and scalability of silicon spin qubits. In this paper we investigate the VS of a SiGe/Si/SiGe heterostructure as a function of the size and location of the silicon quantum dot. We use the effective mass approach to describe a realistic system, which takes into account concentration fluctuations at the Si/SiGe interfaces and also the interface roughness. We predict that the size of the quantum dot is an important parameter for the enhancement of the VS and it can also induce a transition between the disorder-dominated to deterministic-enhanced regimes. Analyzing how the VS changes when we move the quantum dot in a specific direction, we obtain that the size of the quantum dot can be used to reduce the variability of the VS, which is relevant for charge/spin shuttling.

Markov chain Monte Carlo simulations, combining the sampling of the position of the particles and their chemical nature, are very useful when calculating, for example, average site occupancies at crystalline defects in alloys. Unfortunately, when the relaxations around the solutes are large, the exchange moves can be systematically rejected because of atoms overlapping. As a consequence, the simulations are often trapped in nonphysical configurations. In this paper, the “Smart Darting” method from Andricioaei et al. is adapted and extended to propose a solution to this limitation. The method is tested in a particularly demanding case: the sampling of the arrangements of delocalized vacancies and divacancies in grain boundaries, both in the fcc and the bcc structure. Beyond the methodological aspects, intergranular vacancy clusters are interesting in several contexts, such as ductile fracture, irradiation, or thin-film dewetting, and therefore several properties have been measured that can be useful for mesoscale modeling: segregation energies, effective diffusion barriers in and out of the grain boundaries, vacancy-vacancy binding energies, and elastic dipole tensors.

Hydrogen embrittlement in Zr-alloy fuel cladding is a primary safety concern for water-based nuclear reactors. Here we investigated the stabilization of planar defects within the forming hydrides by Sn, the primary alloying element of Zircaloy-4 used in the cladding. In order to explain the formation of hydrides and planar defects observed in our experiments, we performed atomic-scale ab initio calculations focusing on the solute interactions with generalized stacking faults in hcp $\alpha$-Zr and fcc zirconium hydrides. Our calculations showed that an increase in Sn concentration leads to a stabilization of stacking faults in both the $\alpha$-Zr and hydride phases. However, the solution enthalpy of Sn is lower in the $\alpha$-Zr as compared to the other hydride phases, indicative of two competing processes of Sn depletion/enrichment at the Zr hydride/matrix interface. This is corroborated by experimental findings, where Sn is less soluble in hydrides and is mostly found trapped at interfaces and planar defects, indicative of stacking faults inside the hydride phases. Our systematic investigation enables us to understand the presence and distribution of solutes in the hydride phases, which provides a deeper insight into the microstructural evolution of such alloy's properties during its service lifetime.

Phase field crystal (PFC) modeling has proved to be a versatile numerical tool in the analysis of crystalline microstructures. Most often, however, the focus is put on bulk crystal behavior, while crystal defects such as grain boundaries (GBs) are less explored. This is, in particular, the case for crystal structures beyond fcc and bcc. In this work, the possibilities and challenges in adopting PFC to diamond cubic (DC) crystal structures is investigated. Three different PFC models are considered for this purpose. One of them was published previously, and two are modifications proposed in the present work. The models are compared in terms of both DC phase stabilization and their ability to provide relevant GB structures. The models employ combinations of two- and three-point correlations, and the addition of a three-point correlation is found to be required for stabilization of the expected DC GB structures. It is concluded that although each of the models has limitations in terms of the GB structures which can be stabilized and performance in terms of phase stability, key PFC components for successful modeling of DC structures can be identified.

Thermal conductivity is a fundamental material property that plays an essential role in technology, but its accurate evaluation presents a challenge for theory. In this work, we demonstrate the application of $E\left(3\right)$-equivariant neutral network interatomic potentials within Green-Kubo formalism to determine the lattice thermal conductivity in amorphous and crystalline materials. We apply this method to study the thermal conductivity of germanium telluride (GeTe) as a prototypical phase change material. A single deep learning interatomic potential is able to describe the phase transitions between the amorphous, rhombohedral, and cubic phases, with critical temperatures in good agreement with experiments. Furthermore, this approach accurately captures the pronounced anharmonicity that is present in GeTe, enabling precise calculations of the thermal conductivity. In contrast, the Boltzmann transport equation including only three-phonon processes tends to overestimate the thermal conductivity by approximately a factor of 2 in the crystalline phases.

The effects of various point defects and lattice distortions have been studied on the structural, electronic, and magnetic properties of the ${\mathrm{Co}}_2\mathrm{MnAl}$ alloy, considering the thermodynamically feasible range for the disorder concentrations. The ${\mathrm{Al}}_{\mathrm{Mn}}$ binary antisite-disordered structures $({\mathrm{Co}}_2{\mathrm{Mn}}_{0.9375}{\mathrm{Al}}_{1.0625} \text{and} {\mathrm{Co}}_2{\mathrm{Mn}}_{0.875}{\mathrm{Al}}_{1.125})$ and $({\mathrm{Co}}_{\mathrm{Mn}}+{\mathrm{Al}}_{\mathrm{Mn}})$-ternary antisite-disordered structure $\left({\mathrm{Co}}_{2.0625}{\mathrm{Mn}}_{0.875}{\mathrm{Al}}_{1.0625}\right)$ may be formed spontaneously during the synthesis of ${\mathrm{Co}}_2\mathrm{MnAl}$ due to their negative relative formation energies. The Al-deficient structures resulting from ${\mathrm{Co}}_{\mathrm{Al}}$ and ${\mathrm{Mn}}_{\mathrm{Al}}$ binary antisite disorder $({\mathrm{Co}}_{2.0625}{\mathrm{MnAl}}_{0.9375},$ ${\mathrm{Co}}_{2.125}{\mathrm{MnAl}}_{0.875},$ ${\mathrm{Co}}_2{\mathrm{Mn}}_{1.0625}{\mathrm{Al}}_{0.9375}, \text{and} {\mathrm{Co}}_2{\mathrm{Mn}}_{1.125}{\mathrm{Al}}_{0.875})$ and from $({\mathrm{Co}}_{\mathrm{Al}}+{\mathrm{Mn}}_{\mathrm{Al}})$-ternary antisite disorder $\left({\mathrm{Co}}_{2.0625}{\mathrm{Mn}}_{1.0625}{\mathrm{Al}}_{0.875}\right)$ exhibit perfect half-metallicity. All other antisite disorders have a marginal effect on the half-metallic properties, and the disordered ${\mathrm{Co}}_2\mathrm{MnAl}$ maintains the high spin polarization $(\ge 70\%)$ along with nearly the same magnetization as the $L{2}_1\text{-ordered}$ structure. Conversely, vacancy defects seriously affect the electronic and magnetic properties with rapid shrinking of the minority pseudogap. Also, the effect of vacancy occurrence on the structural, electronic, and magnetic properties is found to be very susceptible to their concentrations. For lattice distortions, the uniform strain $(\mathrm{\Delta }V/V_0)$ has a minimal effect on spin polarization and magnetization. Under negative strain within the range $-10\%\le \mathrm{\Delta }V/V_0\le 7\%$ (for $5.50\AA \le a\le 5.58\AA$), the strained structures showed perfect half-metallicity. For uniform strain >7%, spin polarization decreases monotonically; however, it remains high, reaching $\sim 60\%$ at $\mathrm{\Delta }V/V_0=10\%$ $(a=5.88\AA )$. Contrarily, the tetragonal distortions, except for small distortion values $(\mathrm{\Delta }c/a\le \sim \pm 0.10)$, lead to significant degradation in half-metallic behavior. Moreover, the elongation along the $c$ axis (with compression of the $ab$ plane) is found to be the favored condition for the occurrence of tetragonal distortion over compression along the $c$ axis (with elongation of the $ab$ plane). Thus, the resulting perfect half-metallicity in disordered ${\mathrm{Co}}_2\mathrm{MnAl}$ suggests that the occurrence of disorder could also be beneficial sometimes for spintronic applications.

Organic-inorganic lead halide perovskites have emerged in recent years as semiconductor materials for various optoelectronic and photovoltaic applications. The exciton binding energy is an important parameter because the formation of excitons can potentially hamper charge separation in solar cells. However, it remains a challenge to experimentally determine the exciton binding energy in hybrid bromine-based perovskites: the obtained values have a tens meV spread. Here we present in detail the near-band edge photoluminescence study in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ single crystals under different photoluminescence excitation densities with a 405 nm laser diode. We show that by using high laser excitation intensities it is possible to make a direct measurement of the exciton binding energy, which we find to be only 12 meV at low temperatures, lower than has been previously determined. In the low temperature orthorhombic phase, besides the free exciton emission at 2.25 eV, we observed the broad yellow-light emission at 2.16 eV, which exhibited a red shift with the increasing temperature and a blue shift with increasing laser excitation intensity. Based on the excitation power density and temperature dependence of the photoluminescence spectra, we interpreted yellow-light emission as a recombination of bound excitons and a donor-acceptor pair transition. The results obtained are essential for a better understanding of the electronic properties of these materials and provide a guideline for their further applications with improved performance.

Although niobium is a well characterized material, it still shows some anomalies that are not yet understood. Therefore, we revisit its metastable phases using density functional theory. First, we systematically compare energies and ground-state volumes of chosen crystal structures and discuss possible transition paths to the bcc ground-state structure and the energy landscape for tetragonal distortions. Furthermore, we discuss their stability by means of their phonon spectra and vibronic free energies. Second we analyze the impact of tantalum impurities on phase stability. We find additional aspects of the energy landscape of the material: A local energy minimum on the bcc to $\omega$ transition path, a flat energy landscape with respect to uniaxial strain along [111], and a considerable stabilization of the $\sigma$ phase by tantalum substitution.

We present the magnetic properties of a quantum dimer magnet based on ${\mathrm{Co}}^{2+}$. The metal-organic framework compound ${\mathrm{Co}}_2{\left(\mathrm{BDC}\right)}_2{\left(\mathrm{DPTTZ}\right)}_2·\mathrm{DMF}$ features ${\mathrm{Co}}^{2+}$ dimers arranged nearly orthogonal to each other, similar to the Shastry-Sutherland lattice. Our assessment based on the magnetization and heat capacity experiments reveals that the magnetic properties at low temperatures can be described by pseudospin-1/2. The magnetic ground state of the material is found to be a singlet with a tiny spin gap. The thermodynamic properties are described by a model of isolated spin dimers with the anisotropic exchange couplings $J_{xy}\simeq 3.5$ K and $J_z\simeq 11$ K. Interestingly, no field-induced quantum phase transition is detected down to 100 mK around the critical field of gap closing, suggesting the absence of Bose-Einstein condensation of triplons and establishing isolated dimers with a negligible interdimer coupling.

Gilbert damping and saturation magnetization are of critical importance for magnetic thin films in spintronic devices. Using complementary techniques, including ferromagnetic resonance (FMR), superconducting quantum interference device (SQUID) measurements, and energy dispersive x-ray spectroscopy, we demonstrate that Gilbert damping and magnetization in ${\mathrm{Fe}}_{1-x}{\mathrm{B}}_x$ thin films can be tuned by controlling the B concentration $x$. At low B concentration, FMR and SQUID measurements show a strong reduction of the magnetic damping and the saturation magnetization, respectively. A very low magnetic damping of around $3.8\times {10}^{-3}$ is measured at $x=25\%$, which is among the lowest damping values reported so far for a conducting ferromagnet. This tunability of magnetic properties will be useful for fabricating thin films with low damping that can be used in high-performance spintronic devices.

Despite the fundamental importance of solid-solid transformations in many technologies, the microscopic mechanisms remain poorly understood. Here we explore the atomistic mechanisms at the migrating interface during solid-solid phase transformations between the topologically closed-packed A15 and body-centred cubic phase in tungsten. The high energy barriers and slow dynamics associated with this transformation require the application of enhanced molecular sampling approaches. To this end, we performed metadynamics simulations in combination with a path collective variable derived from a machine-learning classification of local structural environments, which allows the system to freely sample the complex interface structure. A disordered region of varying width forming at the migrating interface is identified as a key physical descriptor of the transformation mechanisms, facilitating the atomic shuffling and rearrangement necessary for structural transformations. Furthermore, this can directly be linked to the differences in interface mobility for distinct orientation relationships as well as the formation of interfacial ledges during the migration along low-mobility directions.

Three-dimensional lattices are fundamental to solid-state physics. The description of a lattice with an atomic basis constitutes the necessary information to predict solid phase properties and evolution. Here, we present an algorithm for systematically exploring crystallographic phase space. Coupled with ab initio techniques, such as density functional theory, this algorithm offers an approach for exploring and tuning materials behavior, with a broad range of potential applications: particularly martensitic phase transformations and materials stability.