The recent renaissances in the field of ferroelectric oxides are owing to various photoinduced phenomena. However, the microscopic correlations among various photoinduced physical characteristics are essential for a detailed understanding of the dynamics associated with these phenomena. In this work, the photoinduced dielectric, pyroelectric, and ferroelectric properties of a lead-free $\mathrm{B}{\mathrm{a}}_{0.875}{\left(\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{L}{\mathrm{i}}_{0.5}\right)}_{0.125}\mathrm{Ti}{\mathrm{O}}_3$ ferroelectric oxide are carried out in out-of-plane and in-plane geometry. The dielectric studies under 405-nm light illumination revealed a significant change in dielectric permittivity along with a decrease in Curie temperature. The observed effects are attributed to the change in internal field distribution inflicted by the photogenerated nonequilibrium charge carriers and trapped charges. The sample exhibits a photoinduced significant change in pyroelectric current and the associated polarization. These effects demonstrate that light could be used as an additional degree of freedom to influence the ferroelectric order parameter. The observed photoinduced dielectric and ferroelectric characteristics give the experimental evidence for the theoretical formalism on the photoinduced ferroelectric phenomena. In addition, the Rayleigh analysis performed on this system highlights the strong correlation between the observed photoferroelectric characteristics with the Rayleigh parameters. These findings could widen the understanding of the photoferroelectric characteristics to extend its applications toward advanced optoelectronic devices.

An in-depth structural study of a 23-nm-thick ${\mathrm{BiFeO}}_3$ film grown on orthorhombic ${\mathrm{NdScO}}_3{\left(110\right)}_O$ substrates demonstrates the presence of a mixed phases. Atomic resolution scanning transmission electron microscopy measurements reveal an out-of-plane stripe domain structure typical of rhombohedral ${\mathrm{BiFeO}}_3$ films but with a polarization component along pseudocubic ${〈100〉}_{PC}$ or canted from the ${〈111〉}_{PC}$ towards the in-plane direction. Photovoltaic measurements display an anomalous modulation of the open circuit voltage as temperature is decreased that is attributed to a structural change associated with a transition to a single structural phase.

$\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ is a promising material for light-emitting-diode applications. However, the microscopic origin of the broad yellow photoluminescence in $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ remains unknown. Here, defect properties and defect-related optical transitions are investigated based on the hybrid functional calculation. Our results show that the nitrogen vacancy introduces multiple localized defect states within the band gap, which play a dominant role in luminescent properties of $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$. Common impurities like hydrogen, oxygen, and carbon and their complexes with native defects are also studied. Compared with isolated ${\mathrm{V}}_{\mathrm{Mg}}, {\mathrm{V}}_{\mathrm{Mg}}$-H and ${\mathrm{V}}_{\mathrm{Mg}}-{\mathrm{O}}_{\mathrm{N}}$ complexes have shallower transition levels. Our calculated optical excitation and emission peaks associated with isolated ${\mathrm{V}}_{\mathrm{N}}$ are in good agreement with those observed in experiments. In addition, the impurity ${\mathrm{C}}_{\mathrm{N}}$ is also a potential source for the observed yellow emission in $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$.

We apply a hybrid density functional theory approach, based on a tuned and screened range-separated hybrid (SRSH) exchange-correlation functional, to describe the optoelectronic properties of defective gallium nitride (GaN). SRSH and time-dependent SRSH (TDSRSH) are tuned to produce accurate energetics for the pristine material and applied to the study of a series of point defects in bulk GaN, a blue light-emitting material that degrades in the presence of defects. We first establish the accuracy of the method by comparing the predicted quasiparticle gap and low-energy excitation spectra of (TD)SRSH and many-body perturbation theory for both pristine GaN and GaN containing a single nitrogen vacancy. Aided by the reduced computational cost of (TD)SRSH, we then report on three additional technologically relevant point defects and defect complexes in GaN: the gallium vacancy, the carbon interstitial, and the carbon-silicon complex. We compute the low-energy optical absorption spectra for these defects and show the presence of defect-centered transitions. Furthermore, by estimating the Stokes shift, we predict, in agreement with previous studies, that the carbon substitutional defect is a candidate for the detrimental yellow luminescence in GaN. This study indicates that TDSRSH is a promising and computationally feasible approach for quantitatively accurate, first-principles modeling of defective semiconductors.

We report on the thermal conductivities of alkyl- and indene-group functionalized fullerene derivative thin films as measured via time domain thermoreflectance and steady state thermoreflectance. The thermal conductivities vary from $0.064\pm 0.007$ W ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$ for [6,6]-phenyl ${\mathrm{C}}_{61}$-butyric acid methyl ester (PCBM) to $0.15\pm 0.017$ W ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$ for indene-${\mathrm{C}}_{60}$ bisadduct at room temperature and do not exhibit significant temperature dependence from 300 to 375 K. In comparison to the thermal conductivity of PCBM, increasing the length of the alkyl chain, as in the case of [6,6]-phenyl ${\mathrm{C}}_{61}$-butyric acid butyl ester, and [6,6]-phenyl ${\mathrm{C}}_{61}$-butyric acid octyl ester leads to higher thermal conductivities. Likewise, increasing the number of alkyl chains attached to the fullerenes as in the case of bisadduct PCBM leads to a higher thermal conductivity compared to that of PCBM. We present atomistic insights into the role of chemical functionalization on the overall heat transfer in these fullerene derivatives by conducting molecular dynamics simulations and lattice dynamics calculations. The thermal conductivities predicted via our atomistic simulations qualitatively agree with the experimental trends for our fullerene derivatives. Lattice dynamics calculations reveal that one of the main factors dictating the ultralow thermal conductivities in fullerene derivatives is the large reduction in modal diffusivities in the molecular crystals as calculated from the Allen-Feldman model, thus providing an explanation for their largely reduced thermal conductivities as compared to that of ${\mathrm{C}}_{60}$ crystals. The low diffusivities result from high degrees of localization of Einstein-like vibrations in fullerene derivatives due to the molecular side chains, providing the ability to dial-in the properties of these low thermal conductivity solids via molecular engineering.

Quantitative predictions of the Li intercalation voltage and of the electronic properties of rechargeable battery cathode materials are a substantial challenge for first-principles theory due to the possibility of (1) strong correlations associated with localized transition metal $d$ electrons and (2) significant van der Waals (vdW) interactions in layered systems, both of which are not accurately captured by standard approximations to density functional theory (DFT). Here, we perform a systematic benchmark of electronic structure methods based on the widely used generalized-gradient approximation of Perdew, Burke, and Ernzerhof (PBE) and the new strongly constrained and appropriately normed (SCAN) meta-generalized-gradient approximation for battery cathode materials. Studying layered ${\mathrm{Li}}_x{\mathrm{TiS}}_2, {\mathrm{Li}}_x{\mathrm{NiO}}_2$, and ${\mathrm{Li}}_x{\mathrm{CoO}}_2$, olivine ${\mathrm{Li}}_x{\mathrm{FePO}}_4$, and spinel ${\mathrm{Li}}_x{\mathrm{Mn}}_2{\mathrm{O}}_4$, we compute the voltage, crystal structure, and electronic structure with and without extensions to incorporate onsite Hubbard interactions and vdW interactions. Within pure DFT (i.e., without corrections for onsite Hubbard interactions), SCAN is a significant improvement over PBE for describing cathode materials, decreasing the mean absolute voltage error by more than 50%. Although explicit vdW interactions are not critical and in cases even detrimental when applied in conjunction with SCAN, Hubbard-$U$ corrections are still in general necessary to achieve reasonable agreement with experiment. We show that no single method considered here can accurately describe the voltage and overall structural, electronic, and magnetic properties (i.e., errors no more than 5% for voltage, volume, band gap, and magnetic moments) of battery cathode materials, motivating a strong need for improved electronic structure approaches for such systems.

Chemical modification using only small amounts of elements such as Zn, In, Sb, or Ni has proven to be an effective means to control the desirable crystal structure of hexagonal $\eta \text{−}\mathrm{C}{\mathrm{u}}_6\mathrm{S}{\mathrm{n}}_5$ over a wide thermally operating window, typically found in Pb-free Sn-based soldering or Li-ion battery anode applications. Though appealing, the underlying mechanisms on the role of these dopants remain incomplete and their atomic arrangements within the $\eta \text{−}\mathrm{C}{\mathrm{u}}_6\mathrm{S}{\mathrm{n}}_5$ lattices have not yet been experimentally determined. In the current study, we directly reveal the atomic positions of Zn, In, and Sb at the Sn sites of $\eta \text{−}\mathrm{C}{\mathrm{u}}_6\mathrm{S}{\mathrm{n}}_5$ via atomic-scale x-ray energy dispersive spectroscopy (XEDS) maps utilizing advanced Cs-corrected scanning transmission electron microscopy. The use of advanced statistical algorithms including Poisson non-local principal component analysis and lattice averaging enables the fine resolution of weak XEDS maps from trace dopant elements. Our first-principles calculations further identify the influence of dopants at these atomic sites on the overall energetics, electronic structures, as well as local bonding environments, leading to the most favorable situations for $\eta \text{−}\mathrm{C}{\mathrm{u}}_6\mathrm{S}{\mathrm{n}}_5$ stabilization.

We report a growth mechanism that produces in-plane $\left[1\overline{1}0\right]$ oriented ErSb nanowires formed during codeposition of $\mathrm{E}{\mathrm{r}}_{0.3}\mathrm{G}{\mathrm{a}}_{0.7}\mathrm{Sb}$ via molecular beam epitaxy (MBE). Nanowires are characterized by in situ scanning tunneling microscopy (STM), as well as ex situ transmission electron microscopy (TEM) and electron channeling contrast imaging. We show that complexes of macrosteps with step heights on the order of 7 nm form during nanowire growth. The macrosteps are shown to be part of the in-plane nanowire growth process and are directly responsible for the observed stratified distribution of in-plane nanowires. TEM indicates that initial growth results in out-of-plane nanowires transitioning to in-plane nanowires after a critical film thickness. A surface energy model is put forward that shows the critical thickness is due to minimization of the $\mathrm{GaSb}\left\{110\right\}$ surfaces formed during out-of-plane nanowire growth. Kinetics of the transition are discussed with respect to observed features in STM, along with the material parameters needed to achieve in-plane nanowire growth.

The temperature variation of the defect densities in a crystal depends on vibrational entropy. This contribution to the system thermodynamics remains computationally challenging as it requires a diagonalization of the system's Hessian which scales as $O\left(N^3\right)$ for a crystal made of $N$ atoms. Here, to circumvent such a heavy computational task and make it feasible even for systems containing millions of atoms, the harmonic vibrational entropy of point defects is estimated directly from the relaxed atomic positions through a linear-in-descriptor machine learning approach of order $O\left(N\right)$. With a size-independent descriptor dimension and fixed model parameters, an excellent predictive power is demonstrated on a wide range of defect configurations, supercell sizes, and external deformations well outside the training database. In particular, formation entropies in a range of $250k_B$ are predicted with less than $1.6k_B$ error from a training database whose formation entropies span only $25k_B$ (training error less than $1.0k_B$). This exceptional transferability is found to hold even when the training is limited to a low-energy superbasin in the phase space while the tests are performed for a different liquid-like superbasin at higher energies.

The electronic structure of the ($3\times 3$)-ordered phase of a silicon (Si) layer on Al(111) has been studied by angle-resolved photoemission spectroscopy using synchrotron radiation and modeled by a trial atomic model. A closed Fermi surface originating from a linearly dispersing band is identified. A band structure calculation of a trial atomic model of the honeycomb silicene on Al(111) implies that the metallic band originates from the Al-Si hybrid state that has the Dirac-cone-like dispersion curve. The Si layer on Al(111) can be a model system of Xenes to realize the massless electronic system through the overlayer-substrate interaction.

Ni-Fe-Ga foam with hierarchical pore architecture was created by replication casting. The annealed foam had an open porosity 53% and reversible superelasticity with recoverable strain 4.9%. An adiabatic temperature change of 4.1 K with low hysteresis and coefficient of performance 22 were obtained under an external stress 70 MPa. The foam showed improved cyclic stability ($\sim 2.8\mathrm{K}$ in 194 cycles) because of low hysteresis loss under small stress and high crack initiation/propagation resistance in small size nodes and struts, which demonstrated a feasible strategy to enhance the mechanical and functional stabilities of ferromagnetic shape memory alloys via tailoring the material architectures. Nevertheless, the achieved elastocaloric cooling stability still cannot satisfy (${10}^7$ cycles) the commercial application, therefore, it should be further explored to become a promising candidate for elastocaloric material.

X-ray diffraction, magnetization, ac susceptibility, and specific heat have been measured on high-quality single crystals of ${\mathrm{Co}}_3{\mathrm{Mn}}_3{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$ ludwigite. Different from previously studied ludwigites, this compound is characterized by the existence two low-dimensional subunits each containing a unique ion. The subsystem formed by ions at sites 3-1-3, known as a three-legged ladder (3LL), contains only divalent Co ions, while the 3LL formed by ions at sites 4-2-4 contains both divalent and trivalent Mn ions. Although there is only a weak disorder caused by a very small amount of Mn at sites 1, the experimental results evidence the existence of a low-temperature spin-glass phase. A dynamic scaling analysis of ac susceptibility data according to conventional critical slowing down results in a spin-glass phase-transition temperature $T_g=31.9$ K and a dynamic exponent $z\nu =7.05$. The temperature dependence of heat capacity has a linear contribution with the coefficient $\gamma =21.9$ mJ/(mol ${\mathrm{K}}^2$), which shows that this compound has the highest degree of magnetic disorder among ludwigites, corroborating the spin-glass state at low temperatures. The origin of this state is discussed, taking into account the particular structure of each subunit and the competition between the different exchange interactions involved.

The atomistic mechanisms during lithiation and delithiation of amorphous Si nanowires ($a$-SiNW) have been investigated over cycles by molecular dynamics simulations. First, the modified embedded atom method potential from Cui et al. [J. Power Sources 207, 150 (2012).] has been further optimized on static (${\mathrm{Li}}_x\mathrm{Si}$ alloy phases and point-defect energies) and on dynamic properties (Li diffusion) to reproduce the lithiation of small crystalline Si nanowires calculated at the ab initio level. The lithiation of $a$-SiNW reveals a two-phase process of lithiation with a larger diffusion interface compared to crystalline Si lithiation. Compressive axial stresses are observed in the amorphous ${\mathrm{Si}}_x\mathrm{Li}$ alloy outer shell. They are easily released thanks to the soft glassy behavior of the amorphous alloy. Conversely, in crystalline SiNW, the larger stress in the narrow crystalline lithiated interface is hardly released and requires a phase transformation to amorphous to operate, which delays the lithiation. The history of the charge-discharge cycles as well as the temperature appear as driving forces for phase transformation from amorphous ${\mathrm{Li}}_x\mathrm{Si}$ alloy to the more stable crystalline phase counterpart. Our work suggests that a SiNW anode with an enhanced plastic behavior could help release the internal stress, while a full delithiation could heal the cracks that appear with time and thus increase the life cycles of Li-ion batteries using such anode materials.

$\mathrm{NaYb}{\mathrm{O}}_2$ has been reported as a possible host for the quantum spin-liquid state. Here, the composition-dependent polycrystalline $\mathrm{N}{\mathrm{a}}_{1\text{−}x}\mathrm{Yb}{\mathrm{O}}_2$ ($x=0$, 0.03, and 0.07) has been investigated by combining high-field magnetizations and inelastic neutron scattering techniques. For the $x=0$ sample, no signature of a magnetic order is observed down to 0.3 K. Inelastic neutron scattering measurement suggests a continuous low-energy excitation spectrum centered at momentum transfer $\left(Q\right)\sim 1.25{\AA }^{-1}$ and extending up to energy transfer ($E$) ∼ 2.0 meV. In contrast, $x=0.03$ and 0.07 samples exhibit magnetic transitions at 1.1 and 2.3 K, respectively. High-field magnetization measurements indicate similar behaviors for $x=0$ and 0.03 samples including plateaulike features at the 1/3 saturated magnetization, which implies that the spin disorder in the $x=0$ sample might be suppressed preceding the emergence of the up-up-down phase. This composition- and field-dependent study allows us to construct complete phase diagrams indicating that $\mathrm{NaYb}{\mathrm{O}}_2$ is a promising candidate for the quantum spin-liquid state in close proximity to the antiferromagnetic instability tuned by the application of magnetic fields as well as controlling the concentration of $\mathrm{N}{\mathrm{a}}^{+}$ ion vacancies.

The intersection of topology and magnetism represents a new playground to discover novel quantum phenomena and device concepts. In this work, we show that under certain synthetic conditions, a van der Waals single-crystalline compound $\mathrm{MnS}{\mathrm{b}}_{1.8}\mathrm{B}{\mathrm{i}}_{0.2}\mathrm{T}{\mathrm{e}}_4$ exhibits a net ferromagnetic state with a Curie temperature of 26 K, in contrast to the fully compensated antiferromagnetic order observed previously for other members of the $\mathrm{Mn}{\left(\mathrm{Sb},\mathrm{Bi}\right)}_2\mathrm{T}{\mathrm{e}}_4$ family. We employ magneto-transport, bulk magnetization, x-ray and neutron scattering studies to illustrate the structural, magnetic, and electrical properties of $\mathrm{MnS}{\mathrm{b}}_{1.8}\mathrm{B}{\mathrm{i}}_{0.2}\mathrm{T}{\mathrm{e}}_4$. Our structural analyses reveal considerable Mn-Sb site mixing and suggest a recently proposed mechanism, where Mn occupying the Sb site mediates a ferromagnetic coupling between Mn layers [Murakami et al., Phys. Rev. B 100, 195103 (2019)], could be at play. Close comparisons made to an antiferromagnetic compound $\mathrm{MnS}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_4$ illustrate the subtle magnetic interactions of the system and the important role played by local chemistry. The appearance of an unusual anomalous Hall effect in $\mathrm{MnS}{\mathrm{b}}_{1.8}\mathrm{B}{\mathrm{i}}_{0.2}\mathrm{T}{\mathrm{e}}_4$ at low temperatures hints at a magnetic ground state different from other members of this family. Our results are an important step in the synthesis and understanding of magnetism in materials with topological characteristics.

Biomorphs are micrometer-sized polycrystalline structures that, despite their inorganic nature, display smoothly curved, life-like shapes. The edges of substrate-bound biomorph sheets are known to trace logarithmic spirals and can be reproduced by reaction-diffusion fronts in subexcitable media where disrupted fronts shrink tangentially. While freely rotating Archimedean spirals do not exist in subexcitable systems, our numerical simulations show that pinning sites can stabilize the rotating front. This finding is confirmed experimentally for biomorph systems that, around pH 11, produce both logarithmic and Archimedean spirals. The latter are pinned to globular ${\mathrm{BaCO}}_3$ crystals and have a constant pitch in the range of 20 to $40\mu \mathrm{m}$. Akin to screw dislocations, the Archimedean spirals consist of multiple layers of constant height. Their front speeds on glass and pre-existing biomorph substrate are essentially identical. We also report the detachment of intermittently pinned fronts at highly curved features of the pinning site.

Using the $\mathrm{LDA}+U_{sc}$ method, we present calculation phase relations of iron monoxides involving five polytypes in multiple spin-state configurations. The Hubbard parameter $U$ is determined self-consistently simultaneously with the occupation matrix and structures at arbitrary pressures. The Hubbard parameter strongly depends on pressure, structure, and spin state. Comparison with experimental structural data indicates the $\mathrm{LDA}+U_{sc}$ can predict structure, compression curves, phase relations, and transition pressures very well for the insulating $B1$ and iB8 states. However, it requires additional calculations using the Mermin functional that includes the electronic entropic contribution to the free energy to obtain an nB8 metallic state and a consistent iB8 to nB8 insulator to metal transition pressure.

Variations of perovskite superstructure type and magnetic properties across the high-pressure ${\mathrm{Ca}}_x{\mathrm{Mn}}_{2-x}{\mathrm{FeReO}}_6$ series have been investigated by powder neutron-diffraction, x-ray magnetic circular dichroism, and magnetization measurements. Monoclinic $P{2}_1/n{A}_{2}B{B}^{\prime }{\mathrm{O}}_6$ double perovskite solid solutions with rocksalt-type ordering of $B/B^{\prime }\text{−}\mathrm{Fe}/\mathrm{Re}$ cations but no $A$-cation order are discovered to exist only close to the end members $0<x<0.17$ and $1.73<x<2$. Compositions in the range $0.74<x<\sim 1.1$ adopt a tetragonal $P{4}_2/nA{A}^{\prime }B{B}^{\prime }{\mathrm{O}}_6$ double-double perovskite phase based on ${\mathrm{CaMnFeReO}}_6$, which has both columnar $A/A^{\prime }\text{−}\mathrm{Ca}/\mathrm{Mn}$ and rocksalt $B/B^{\prime }\text{−}\mathrm{Fe}/\mathrm{Re}$ cation orders. Two-phase coexistence of double and double-double perovskites occurs over wide regions between these limits. All samples are ferrimagnetic with Curie temperatures near 500 K. Low-temperature Mn spin order is also observed in the Mn-rich double perovskites and the double-double perovskites. Spin reorientation transitions observed across all of the double perovskites are shown to result from $5d^2{\mathrm{Re}}^{5+}$ orbital ordering, but this does not occur in the double-double perovskites. The Ca-rich double perovskites show significant magnetic coercivity at low temperatures.

Molecular magnets based on heavy transition metals have recently attracted significant interest in the quest for novel magnetic properties. For systems with an odd number of valence electrons per molecule, high or low molecular spin states are typically expected in the double exchange or quasimolecular orbital limits, respectively. In this work, we use bulk characterization, muon spin relaxation, neutron diffraction, and inelastic neutron scattering to identify a rare intermediate spin-3/2 per dimer state in the 6H-perovskite ${\mathrm{Ba}}_3{\mathrm{LaRu}}_2{\mathrm{O}}_9$ that cannot be understood in a double exchange or quasimolecular orbital picture and instead arises from orbital-selective Mott insulating behavior at the molecular level. Our measurements are also indicative of collinear stripe magnetic order below $T_N=26\left(1\right)\mathrm{K}$ for these molecular spin-3/2 degrees-of-freedom, which is consistent with expectations for an ideal triangular lattice with significant in-plane next nearest-neighbor exchange. Finally, we present neutron diffraction and Raman scattering data under applied pressure that reveal low-lying structural and spin state transitions at modest pressures $P\le$ 1 GPa, which highlights the delicate balance between competing energy scales in this system.

The defect-electronic properties of {112} microfaceted surfaces of epitaxially grown ${\mathrm{CuInSe}}_2$ thin films are investigated by scanning tunneling spectroscopy and photoelectron spectroscopy techniques after various surface treatments. The intrinsic ${\mathrm{CuInSe}}_2$ surface is found to be largely passivated in terms of electronic defect levels in the band-gap region. However, surface oxidation leads to an overall high density of defect levels in conjunction with a considerable net surface dipole, which persists even after oxide removal. Yet, a subsequent annealing under vacuum restores the initial condition. Such oxidation/reduction cycles are reversible for many times providing robust control of the surface and interface properties in these materials. Based on ab initio simulations, a mechanism where oxygen dissociatively adsorbs and subsequently diffuses to a subsurface site is proposed as the initial step of the observed dipole formation. Our results emphasize the relevance of oxidation-induced dipole effects at the thin film surface and provide a comprehensive understanding toward passivation strategies of these surfaces.