We present results obtained from a combination of dielectric and x-ray diffraction measurements for compositional design of $(1-x)\mathrm{NaNb}{\mathrm{O}}_3-x\mathrm{BaTi}{\mathrm{O}}_3\left(\mathrm{NNBT}x\right)$, which can induce interferroelectric phase transitions. Anomalies are observed in dielectric measurements performed for various compositions at 300 K, as well as at different temperatures for NNBT03. We observed the appearance(disappearance) of the superlattice reflections along with change in the intensities of the main perovskite peaks in the powder x-ray diffraction data, which provide clear evidences for structural phase transitions with composition and temperature. We found that increasing the concentration of $\mathrm{BaTi}{\mathrm{O}}_3$ leads to the suppression of out-of-phase rotation of octahedra and an increment in tetragonality ($c/a$ ratio), which promotes the polar mode at room temperature. The temperature-dependent powder diffraction study shows that the ferroelectric rhombohedral phase of pure sodium niobate gets suppressed for the composition $x=0.03$, and the monoclinic phase $Cc$ gets stabilized at low temperature. The monoclinic phase is believed to provide for a flexible polarization rotation and is considered to be directly linked to the high-performance piezoelectricity in materials due to presence of more easy axes for spontaneous polarizations than the rhombohedral phase.

Electron energy-loss magnetic chiral dichroism (EMCD) is a novel technique that allows magnetic information determination down to the nanoscale. However, constrained by the predefined diffraction geometry in regular EMCD experiments, it has not yet been feasible to obtain EMCD signals from amorphous materials, due to the lack of long-range ordering. Here we propose a protocol for EMCD detection in amorphous materials utilizing a single-crystalline overlayer acting as a two-beam splitter. Phase locking of the EMCD signals is observed and explained by two conceivable scenarios. Both experimental results and theoretical calculations demonstrate significant EMCD signals of amorphous materials.

We introduce a real-space approach to understand the relationship between optical absorption and crystal structure. We apply this approach to alternative phases of silicon, with a focus on the ${\mathrm{Si}}_{20}$ crystal phase as a case study. We find that about 83% of the changes in the calculated low-energy absorption in ${\mathrm{Si}}_{20}$ as compared to Si in the diamond structure can be attributed to reducing the differences between the on-site energies of the bonding and antibonding orbitals as well as increasing the hopping integrals for specific Si-Si bonds.

Despite significant efforts, a complete mechanistic understanding of thermally activated delayed fluorescence (TADF) materials has not yet been fully uncovered. Part of the complexity arises from the apparent dichotomy between the need for close energy resonance and for a significant spin-orbit coupling between alike charge-transfer singlet and triplet excitations. Here we show, in the case of reference carbazole derivatives, that this dichotomy can be resolved in a fully atomistic model accounting for thermal fluctuations of the molecular conformations and microscopic electronic polarization effects in amorphous films. These effects yield electronic excitations with a dynamically mixed charge-transfer and localized character, resulting in thermally averaged singlet-triplet energy differences and interconversion rates in excellent agreement with careful spectroscopic studies.

Germanium telluride has attracted great research interest, primarily because of its phase-change properties. We have developed a general scheme, based on the ab initio random structure searching (AIRSS) method, for predicting the structures of encapsulated nanowires, and using this we predict a number of thermodynamically stable structures of GeTe nanowires encapsulated inside carbon nanotubes of radii under $9\AA$. We construct the phase diagram of encapsulated GeTe, which provides quantitative predictions about the energetic favorability of different filling structures as a function of the nanotube radius, such as the formation of a quasi-one-dimensional rock-salt-like phase inside nanotubes of radii between 5.4 and $7.9\AA$. Simulated TEM images of our structures show excellent agreement between our results and experimental TEM imagery. We show that, for some nanotubes, the nanowires undergo temperature-induced phase transitions from one crystalline structure to another due to vibrational contributions to the free energy, which is a first step toward nano-phase-change memory devices.

Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and photoelectron spectroscopy have been used to study an ordered structure formed by Ge atoms deposited onto the Au(111) surface. Based on a careful analysis of STM images and LEED patterns, we propose a $\left(\begin{array}{cc}5& 0\\ 8& -14\end{array}\right)$ unit cell for the atomic structure of the Ge layer. Core-level data indicate that some Ge atoms diffuse into the Au(111) crystal during annealing after deposition at room temperature. This is further corroborated by angle-resolved photoelectron spectroscopy (ARPES) measured for different amounts of Ge remaining after sputtering and annealing. The results of the ARPES study clearly disprove an earlier assignment of a parabolic band, centered at normal emission, as a part of a Dirac cone of germanene.

Ferroic domain walls could play an important role in microelectronics, given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate $\left(\mathrm{CaTi}{\mathrm{O}}_3\right)$. Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential.

The mechanism of widely observed intermixing of wurtzite and zinc-blende crystal structures in InAs nanowire (NW) grown by selective area epitaxy (SAE) is studied. We demonstrate that the crystal structure in InAs NW grown by SAE can be controlled using basic growth parameters, and wurtzitelike InAs NWs are achieved. We link the polytypic InAs NWs SAE to the reconstruction of the growth front (111)B surface. Surface reconstruction study of InAs (111) substrate and the following homoepitaxy experiment suggest that (111) planar defect nucleation is related to the (1 × 1) reconstruction of InAs (111)B surface. In order to reveal it more clearly, a model is presented to correlate growth temperature and arsenic partial pressure with InAs NW crystal structure. This model considers the transition between (1 × 1) and (2 × 2) surface reconstructions in the frame of adatom atoms adsorption/desorption, and the polytypism is thus linked to reconstruction quantitatively. The experimental data fit well with the model, which highly suggests that surface reconstruction plays an important role in the polytypism phenomenon in InAs NWs SAE.

The superconducting property of platinum diselenide $\left(\mathrm{PtS}{\mathrm{e}}_2\right)$, being a type-II Dirac semimetal, doped with electron and hole carriers has been investigated by first-principles calculations. It is found that the superconducting transition temperature $T_{\mathrm{c}}$ of pristine $\mathrm{PtS}{\mathrm{e}}_2$ is very low $(\le 0.002$ K), making it infeasible as a practical superconductor. The electron doping is an effective way to increases the $T_{\mathrm{c}}$ of $\mathrm{PtS}{\mathrm{e}}_2$ to 2.15 K at $0.5e^{-}$ per unit cell. We further find that the mechanism of superconducting transition is the acoustic branch vibration mode softening rather than the Fermi surface nesting. Our results provide an important clue to increase superconducting temperature in the heavy transition-metal dichalcogenides and shed light on searching for new superconducting topological semimetals.

We present a joint theoretical and experimental analysis of the dislocation distribution in graded epitaxial SiGe crystals grown on under-etched Si pillars by low-energy plasma-enhanced chemical vapor deposition. Dislocation dynamics simulations are used to investigate preferential positioning of ${60}^{\circ }$ dislocations introduced in the system to release the lattice misfit strain. Coupling to a finite-element solver is exploited to allow for the exact numerical treatment of the stress fields in the presence of a complex distribution of free surfaces. The results show that, by suitably under-etching the Si pillars, it is possible to reverse the sign of the Burgers vector of the dislocations. This helps explaining differences in the experimentally observed distribution of dislocations in SiGe crystals grown on vertical and under-etched pillars, leading to a strong reduction of defects in the latter case. The agreement between simulations and experiments is not simply qualitative: the predicted number of defects generated by multiplication processes in tall crystals is indeed fully consistent with the measured one.

Group VIII transition-metal dichalcogenides have recently been proposed to host type-II Dirac fermions. They are Lorentz-violating quasiparticles marked by a strongly tilted conic dispersion along a certain momentum direction and therefore have no analogs in the standard model. Using high-resolution angle-resolved photoemission spectroscopy, we systematically studied the electronic structure of $\mathrm{PtS}{\mathrm{e}}_2$ in the full three-dimensional Brillouin zone. As predicted, a pair of type-II Dirac crossings is experimentally confirmed along the $k_z$ axis. Interestingly, we observed conic surface states around time-reversal-invariant momenta $\overline{\mathrm{\Gamma }}$ and $\overline{M}$ points. The signatures of nontrivial topology are confirmed by the first-principles calculation, which shows an intricate parity inversion of bulk states. Our discoveries not only contribute to a better understanding of topological band structure in $\mathrm{PtS}{\mathrm{e}}_2$ but also help further explore the exotic properties, as well as potential application, of group VIII transition-metal dichalcogenides.

Au-hyperdoped Si, synthesized by ion implantation and pulsed laser melting, is known to exhibit a strong sub-band gap photoresponse that scales monotonically with the Au concentration. However, there is thought to be a limit to this behavior since ultrahigh Au concentrations ($>1\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$) are expected to induce cellular breakdown during the rapid resolidification of Si, a process that is associated with significant lateral impurity precipitation. This work shows that the cellular morphology observed in Au-hyperdoped Si differs from that in conventional, steady-state cellular breakdown. In particular, Rutherford backscattering spectrometry combined with channeling and transmission electron microscopy revealed an inhomogeneous Au distribution and a subsurface network of Au-rich filaments, within which the Au impurities largely reside on substitutional positions in the crystalline Si lattice, at concentrations as high as $\sim 3$ at. %. The measured substitutional Au dose, regardless of the presence of Au-rich filaments, correlates strongly with the sub-band gap optical absorptance. Upon subsequent thermal treatment, the supersaturated Au forms precipitates, while the Au substitutionality and the sub-band gap optical absorption both decrease. These results offer insight into a metastable filamentary regime in Au-hyperdoped Si that has important implications for Si-based infrared optoelectronics.

We report on the selective fabrication of high-quality ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{SrIrO}}_3$ epitaxial thin films from a single polycrystalline ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ target by pulsed laser deposition. Using a combination of x-ray diffraction and photoemission spectroscopy characterizations, we discover that within a relatively narrow range of substrate temperature, the oxygen partial pressure plays a critical role in the cation stoichiometric ratio of the films, and triggers the stabilization of different Ruddlesden-Popper (RP) phases. Resonant x-ray absorption spectroscopy measurements taken at the Ir $L$ edge and the O $K$ edge demonstrate the presence of strong spin-orbit coupling, and reveal the electronic and orbital structures of both compounds. These results suggest that in addition to the conventional thermodynamics consideration, higher members of the ${\mathrm{Sr}}_{n+1}{\mathrm{Ir}}_n{\mathrm{O}}_{3n+1}$ series can possibly be achieved by kinetic control away from the thermodynamic limit. These findings offer an approach to the synthesis of ultrathin films of the RP series of iridates and can be extended to other complex oxides with layered structure.

Multiferroic tunnel junction (MFTJ) based on a ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ barrier and Heusler alloy electrode was designed using first-principles calculations. It is found that the MFTJ possesses giant tunneling electroresistance (TER) and tunneling magnetoresistance (TMR) simultaneously. Our results indicate that the ferroelectric polarization of ${\mathrm{BaTiO}}_3$ makes the adjacent ${\mathrm{SrTiO}}_3$ ferroelectric polarized as well. This changes the electrostatic potential and electronic properties of the barrier in the MFTJ and gives rise to a giant TER. On the other hand, the adoption of a highly spin-polarized Heusler alloy electrode produces a giant TMR. This study opens an avenue for the design of MFTJ, which may find applications in future nonvolatile memories.

Exploring magnetic anisotropy (MA) in single-atom-doped two-dimensional materials provides a viable ground for realizing information storage and processing at ultimate length scales. Herein, the MA of $5d$ transition-metal doped monolayer ${\mathrm{WSe}}_2$ is investigated by first-principles calculations. Large MA energy (MAE) is achieved in several doping systems. The direction of MA is determined by the dopant in-plane $d$ states in the vicinity of the Fermi level in line with previous studies. An occupation rule that the parity of the occupation number of the in-plane $d$ orbital of the dopant determines the preference between in-plane and out-of-plane anisotropy is found in this $5d$-doped system. Furthermore, this rule is understood by second-order perturbation theory and proved by charge-doping analysis. Considering relatively little research on two-dimensional MA and not sufficiently large MAE, suitable contact medium dopant pairs with large MAE and tunable MA pave the way to novel data storage paradigms.

The ultrafast relaxation dynamics of a nonequilibrium phonon gas towards thermal equilibrium involves many-body collisions that cannot be properly described by perturbative approaches. Here, we develop a nonperturbative method to elucidate the microscopic mechanisms underlying the decay of laser-excited coherent phonons in the presence of electron-hole pairs, which so far are not fully understood. Our theory relies on ab initio molecular dynamics simulations on laser-excited potential-energy surfaces. Those simulations are compared with runs in which the laser-excited coherent phonon is artificially deoccupied. We apply this method to antimony and show that the decay of the $A_{1g}$ phonon mode at low laser fluences can be accounted mainly to three-body down-conversion processes of an $A_{1g}$ phonon into acoustic phonons. For higher excitation strengths, however, we see a crossover to a four-phonon process, in which two $A_{1g}$ phonons decay into two optical phonons.