The recent theoretical discovery of the predicted high-temperature superconductivity (superconducting transition temperature $T_{\mathrm{c}}\sim 351\mathrm{K}$ at 300 GPa) in clathrate ${\mathrm{Li}}_2\mathrm{Mg}{\mathrm{H}}_{16}$ is an important advance toward room-temperature superconductors. Here we use first-principle approaches to identify a new ternary hydride of clathrate structure of $Fd\overline{3}m\text{−}{\mathrm{Rb}}_2\mathrm{Mg}{\mathrm{H}}_{16}$ by Rb-doped $\mathrm{Mg}{\mathrm{H}}_{16}$ at 300 GPa. We first verified the thermal and dynamic stability for the $Fd\overline{3}m\text{−}{\mathrm{Rb}}_2\mathrm{Mg}{\mathrm{H}}_{16}$ through the formation enthalpy and phonon calculations, respectively. Next, we showcased that $Fd\overline{3}m\text{−}{\mathrm{Rb}}_2\mathrm{Mg}{\mathrm{H}}_{16}$ has two Fermi surface (FS) sheets, which are both dominated by the H2 $s$ orbital. Using the Eliashberg formalism, the $T_{\mathrm{c}}$ of $Fd\overline{3}m\text{−}{\mathrm{Rb}}_2\mathrm{Mg}{\mathrm{H}}_{16}$ was calculated to be 130 K at 300 GPa. Furthermore, the calculations of the electronic, phonon, and superconducting properties reveal that $Fd\overline{3}m\text{−}{\mathrm{Li}}_2\mathrm{Mg}{\mathrm{H}}_{16}$ has four FS sheets with mixed H1 $s$ and H2 $s$ orbital character and reaches the $T_{\mathrm{c}}$ of 352 K at 300 GPa. Compared with $Fd\overline{3}m\text{−}{\mathrm{Rb}}_2\mathrm{Mg}{\mathrm{H}}_{16}$, $Fd\overline{3}m\text{−}{\mathrm{Li}}_2\mathrm{Mg}{\mathrm{H}}_{16}$ produces high-frequency phonon softening, leading to the enhancement of $T_{\mathrm{c}}$. Moreover, it is demonstrated that Rb substitution for Li produces the reduction of contribution of H orbitals to the FS sheets and the decrease in FS sheets in number, which tend to decrease the electron-phonon coupling strength and $T_{\mathrm{c}}$. Our observed FS sheets and their associated superconducting gap will provide key insights into the understanding of the superconductivity mechanism in these and related systems.

We present evidence for and characterization of a $\approx 4\text{−}\mathrm{nm}$-thick ${\left({\mathrm{Y}}_{1-x}{\mathrm{Gd}}_x\right)}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ layer with $x\ge 0.4$ formed at the interface between a gadolinium gallium garnet (GGG) substrate and a sputtered ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) epitaxial film with nominal thickness of 20 nm. Temperature-dependent polarized neutron reflectometry (PNR) and superconducting quantum interference device (SQUID) magnetometry show antiferromagnetic alignment of this interfacial layer with the bulk of the YIG film at low $T$ that persists to at least 3 tesla. These experiments also show that this interfacial alignment switches from antiparallel to parallel between 100 and 200 K in small applied magnetic fields. Simple modeling suggests correlation of this crossover with the ferrimagnetic compensation point of the mixed garnet that forms this interfacial layer.

Having been a ground for various topological fermionic phases, the family of ZrSiS-type 111 materials has been under experimental and theoretical investigations. Within this family of materials, the subfamily $\mathit{Ln}\mathrm{SbTe}$ $(Ln=\text{lanthanide}\text{elements})$ is gaining interest in recent times as the strong correlation effects and magnetism arising from the $4f$ electrons of the lanthanides can provide an important platform to study the link between topology, magnetism, and correlation. In this Letter, we report the systematic study of the electronic structure of $\mathrm{SmSbTe}$—a member of the $\mathit{Ln}\mathrm{SbTe}$ subfamily—by utilizing angle-resolved photoemission spectroscopy in conjunction with first-principles calculations, transport, and magnetic measurements. Our experimental results identify multiple Dirac nodes forming the nodal lines along the $\mathrm{\Gamma }\text{−}X$ and $Z\text{−}R$ directions in the bulk Brillouin zone (BZ) as predicted by our theoretical calculations. A surface Dirac-like state is also observed at the $\overline{X}$ point of the surface BZ. Our study highlights $\mathrm{SmSbTe}$ as a promising candidate to understand the topological electronic structure of $\mathit{Ln}\mathrm{SbTe}$ materials.

We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the $\gamma$-ZnPc polymorph thereafter (thickness $\approx 1.0$ nm). As the growth of the bulk $\gamma$-ZnPc progresses, a substantial out-of-plane lattice strain ($\approx 15\%$ relative to $\gamma$-ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of $\approx 13$ nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.

We theoretically probe the emergence of strong coupling in a system consisting of a topological insulator (TI) and a III-V heterostructure using a numerical approach based on the scattering matrix formalism. Specifically, we investigate the interactions between terahertz excitations in a structure composed of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and GaAs materials. We find that the interaction between the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layer and AlGaAs/GaAs quantum wells with intersubband transitions (ISBTs) in the terahertz frequency regime creates new hybrid modes, namely Dirac plasmon-phonon-ISBT polaritons. The formation of these hybrid modes results in anticrossings (spectral mode splitting) whose magnitude is an indication of the strength of the coupling. By varying the structural parameters of the constituent materials, our numerical calculations reveal that the magnitude of splitting depends strongly on the doping level and the scattering rate in the AlGaAs/GaAs quantum wells, as well as on the thickness of the GaAs spacer layer that separates the quantum-well structure from the TI layer. Our results reveal the material and device parameters required to obtain experimentally observable signatures of strong coupling. Our model includes the contribution of an extra two-dimensional hole gas (2DHG) that is predicted to arise at the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/GaAs interface, based on density functional theory (DFT) calculations that explicitly account for details of the atomic terminations at the interface. The presence of this massive 2DHG at the TI/III-V interface shifts the dispersion of the Dirac plasmon-ISBT polaritons to higher frequencies. The damping rate at this interface, in contrast, compensates the effect of the 2DHG. Finally, we observe that the phonon resonances in the TI layer are crucial to the coupling between the THz excitations in the TI and III-V materials.

Ribbons of two-dimensional lattices have properties depending sensitively on the morphology of the two edges. For regular ribbons with two parallel straight edges, the atomic chains terminating the two edges may have more than one choice for a general edge orientation. We enumerate the possible choices for zigzag dice lattice ribbons, which are regular ribbons of the dice lattice with edges parallel to a zigzag direction, and explore the relation between the edge morphologies and their electronic spectra. A formula is introduced to count the number of distinct edge termination morphologies for the regular ribbons, which gives 18 distinct edge termination morphologies for the zigzag dice lattice ribbons. For the pure dice model, because the equivalence of the two rim sublattices, the numerical spectra of the zigzag ribbons show qualitative degeneracies among the different edge termination morphologies. For the symmetrically biased dice model, we see a one-to-one correspondence between the 18 edge termination morphologies and their electronic spectra, when both the zero-energy flat bands and the dispersive or nonzero-energy in-gap states are considered. We analytically study several interesting features in the electronic spectra, including the number and wave functions of the zero-energy flat bands, and the analytical spectrum of unique in-gap states. The in-gap states of the zigzag dice lattice ribbons both exhibit interesting similarities and show salient differences when compared to the spectra of the zigzag ribbons of the honeycomb lattice.

We study theoretically the excitonic nonlinearity in hybrid organic-inorganic Ruddlesden-Popper perovskite thin films. The composite layered structure of these materials allows for flexible modulation of their excitonic response between the limiting cases of a single atomic layer and a wide quasi-two-dimensional quantum well. In particular, we demonstrate that transverse electric field leads to the spatial separation of charge carriers within the inorganic layer, giving rise to strongly interacting excitons possessing built-in dipole moment. Combined with exciton binding energy of the order of hundreds of meVs, this makes hybrid perovskites an optimal platform for tailoring of nonlinear optical response at reduced dimensionality.

In this paper, we introduce a family of two-dimensional group-V carbides with an ordered sequence of tetragons and hexagons (th-$\mathrm{C}{X}_2$, where $X=\text{P}$, As, and Sb). We demonstrate that th-$\mathrm{C}{X}_2$ monolayers exhibit robust energetic, dynamical, thermal, and mechanical stability. Our calculations show that the intrinsic structural anisotropy of the th-$\mathrm{C}{X}_2$ family induces strongly anisotropic mechanical, electronic, and optical behavior. These monolayers offer ultrahigh ultimate tensile strength, comparable with that of graphene, making them suitable for strain engineering of electronic and optical properties. They are semiconductors in nature, where th-${\mathrm{CP}}_2$, th-${\mathrm{CAs}}_2$, and th-${\mathrm{CSb}}_2$ possess quasidirect, direct, and indirect band gaps, respectively. The band gaps of th-${\mathrm{CP}}_2$ and th-${\mathrm{CAs}}_2$ are wide enough to provide the photogenerated energy required for the splitting of water. Besides, the positions of band edges are in alignment with the water oxidation and reduction potentials. For th-${\mathrm{CSb}}_2$, however, the suitable width of the band gap and the appropriate band edge positions for photocatalytic water splitting are achieved by strain engineering. Both indirect-to-direct and direct-to-indirect band gap transitions can be induced in th-$\mathrm{C}{X}_2$ compounds through strain engineering. The th-$\mathrm{C}{X}_2$ monolayers offer anisotropic high charge carrier mobility, which prolongs the average lifetime of charge carrier drift. They have good optical absorption ($\sim {10}^5$ ${\mathrm{cm}}^{-1}$) in the visible and ultraviolet regions of the light spectrum. $G{W}_0+$BSE (Bethe-Salpeter equation) calculations reveal that they exhibit strong excitonic effects where the first bright excitonic binding energy is calculated as 0.27, 0.52, and 0.22 eV for th-${\mathrm{CP}}_2$, th-${\mathrm{CAs}}_2$, and th-${\mathrm{CSb}}_2$, respectively. Having all these features in one package, the th-$\mathrm{C}{X}_2$ monolayers are among the best candidates for high-performance photocatalytic water splitting.

The term “fragility” describes the rate at which viscosity grows when a supercooled liquid approaches its putative glass transition temperature. The field of glassy materials is actively searching for a structural origin that governs this dynamical slowing down in the supercooled liquid, which occurs without any discernible change in structure. Our work shows clear evidence that growing many-body static amorphous order is intimately correlated with the kinetic fragility of glass-forming liquids. It confirms that the system's dynamical response to temperature is concealed in its microstructures. This finding may pave the way for a deeper understanding of the different temperature dependence of the relaxation time or viscosity in a wide variety of glass-forming liquids.

Materials with thermal-invariant elastic properties are of interest for resonant device frequency and dimensional stability of precision devices. Here, we demonstrate that the temperature coefficient of elasticity (TCE) of amorphous silica can be locally reduced using femtosecond laser irradiation. Notably, a self-organized laser-induced modification shows a decrease of the TCE beyond 50%. The origin of this dramatic decrease is attributed to the presence of submicron permanent densified zones within the fine multilayer structure of the laser-modified zone.

Density functional/molecular dynamics simulations have been performed to shed light on the drift of Ag atoms in an amorphous $\mathrm{Ge}{\mathrm{S}}_2$ solid-state electrolyte between Ag and Pt electrodes in the presence of a finite electric field. The system models a conductive bridging random access memory device, where the electric field induces the formation of conductive filaments across the chalcogenide. Simulations of a 1019-atom structure under an external electrostatic potential of 0.20 eV/Å at 480 and 680 K show significant atomic diffusion within 500 ps. Ag migration and the formation of percolating filaments occur in both cases. Three simulations for a smaller model (472 atoms) confirm the formation of percolating Ag strings. Significantly reduced mobility of Ag cations at 380 K means that Ag migration to the Pt electrode did not occur within 1 ns. The electronic structure analysis of selected snapshots shows that dissolved Ag atoms become markedly cationic, which changes when Ag clusters form at the Pt electrode. The electrolyte does not conduct, despite percolating single-atom Ag wire segments. Sulfur becomes anionic during the migration as a result of Ag-S bonding, and the effect is most pronounced near the active electrode. The formation of conductive filaments requires a percolating network of Ag clusters to grow from the Pt interface, and the weakest link of this network is at the Ag electrode.

Te-impurity-incorporated ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, i.e., ${\mathrm{Sb}}_2{\mathrm{Te}}_3+x\mathrm{mol}\%$ Te ($x=0$, 4, 6, and 9) composites were synthesized by solid-state reaction technique. Analysis of x-ray diffraction indicates not only Te impurity as a second phase but also doping of Te via suppression of inherent Te vacancies in the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ matrix. As a result of this doping and of the change in formation energy of different types of native defects in ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ due to synthesis in a Te-rich condition, carrier concentration $\left(n_{\mathrm{H}}\right)$ lower than the pristine sample was observed. Low $n_{\mathrm{H}}$ along with gradual convergence of valence bands due to progressive suppression of Te vacancies increases the Seebeck coefficient $\left(S\right)$ in Te-incorporated samples. Even though Te impurities increase electrical resistivity (ρ), enhanced texturing of lattice planes ensures that charge carrier mobility does not degrade due to Te addition. As a result, a maximum power factor $=17\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ at $T=480$ K for $x=6$ has been achieved. In addition, Te addition strengthens phonon scattering via an increase of phonon-phonon Umklapp scattering and point-defect-induced scattering of phonons. Due to such a strong phonon scattering, thermal conductivity $\left(\kappa \right)$ decreases, and a reduced lattice thermal conductivity $\left({\kappa }_{\mathrm{L}}\right)$, as low as $0.28\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 500 K for $x=6$, has been achieved. As a result of simultaneous increase of $S$ and decrease of κ, a high $ZT\sim 0.87$ at 480 K, almost 33% higher than that of the host material, has been achieved.

Magnetic domains play a fundamental role in magnetization processes. However, unlike in ferromagnets (FMs), the formation of domains in antiferromagnets (AFMs) is poorly understood because they are not favored by magnetostatics and are difficult to detect experimentally. In this paper, we create a synthetic planar AFM with tunable lateral coupling between neighboring FM regions to establish the role played by magnetic disorder in the formation of AFM domains. By directly imaging the synthetic AFM in real space, we observe that the AFM lattice spontaneously breaks up into domains following ac demagnetization. These AFM domains nucleate and pin at locally disordered sites that define their size and shape, which is explained with the help of a Gaussian random field Ising model. Furthermore, we can manipulate the AFM domain morphology by varying the interaction strength, which can be tuned with the geometrical parameters of the synthetic AFM.

Despite being a simple metal in which the free-electron model prevails, aluminum is known to exhibit peculiar bonding characteristics. In this work, we report a hitherto unexplored bonding behavior of Al through extensive first-principles modeling. Starting from liquid Al, we report that upon increasing pressure, electrons tend to localize between two nearest Al neighbors, forming a partitionable electronic center resembling a pseudoparticle that lies between the two Al atoms. This finding is further generalized to binary alloys containing Al under high pressure, as exemplified by an Al-Li intermetallic compound. The emergence of this bridge bonding under pressure is found to profoundly affect the physical properties of the materials, evidenced by anomalous property crossovers systematically explored in the Al-Li alloy. The finding of the anomalous electronic bonding behavior of Al described in this work will lay groundwork for interpreting and predicting physical behavior under extreme conditions.

MXenes are a novel class of two-dimensional (2D) materials whose applications in energy-storage systems have attracted substantial attention. Still, the rate performance of these materials is often diminished by sheet restacking, which is attenuated via controlled etching in ${\mathrm{H}}_2{\mathrm{SO}}_4$ solution. With this process, micropores are formed in MXene sheets that allow transport across 2D layers. As a result, the intercalation and diffusivity of ions are facilitated resulting in improved capacitance retention at high charge-discharge rates. In the present work, we used quasielastic neutron scattering to evaluate the potential changes in water dynamics as a consequence of this mechanism by assessing the behavior of weakly and strongly confined water populations in pristine and porous MXenes. First, we have found that the porous sample accommodates a noticeably higher content of both water populations. Additionally, the fraction of mobile molecules is higher either under strong or weak confinement. Interestingly, regardless of the abundance of weakly confined water in the porous sample, no considerable changes in the dynamical behavior were detected in comparison with the dynamics measured in the pristine material. For the strongly confined populations of water, our results show that water is able to permeate the micropores introduced by etching and perform unlocalized motions.

We study single-site and two-site defect structures in B2-type Fe-Al alloys by means of density functional theory supercell calculations. The defect formation energies are calculated as functions of the chemical potential, which are used to obtain the dependence of the defect concentrations on Al content at different temperatures. We also examine the converging behavior of the formation energies with respect to the supercell size to study the corresponding limit of dilute defects. The effect of magnetism is investigated by considering nonmagnetic, ferromagnetic, and paramagnetic states, calculations for the latter showing that the magnitude of the local magnetic moments strongly impacts the defect formation energies. The methodological studies are used to provide explanations for the wide spread of defect formation energies reported by experiments and other theoretical investigations. Based on these insights, the stability of the B2-FeAl structure as a function of Al concentration is obtained and discussed.

Boron solubility and segregation in paramagnetic (PM) fcc iron have been investigated by means of DFT calculations. The results focus on the Boron site preference in both bulk and coincidence site lattice model $\mathrm{\Sigma }5$ (012) GB in fcc Fe and evaluate the validity of different model approaches for modeling the PM state. Boron and PM fcc iron are predicted to form an interstitial solid solution. The PM state model and pressure correction have been introduced into the 0 K DFT calculations to evaluate Boron solubility in fcc Fe as a function of its thermal lattice expansion within the temperature range of 0–1670 K. The relatively high segregation energy of Boron of about $-$1.57 eV to both interstitial and substitutional GB sites at 0 K is predicted to substantially decrease with thermal lattice expansion, reaching the value of about $-$0.3 eV at the lattice parameter of fcc Fe corresponding to 1670 K. The contribution of the PM state to the segregation energy was found to be of the order of 0.1 eV compared to the nonmagnetic calculations.

Diffusion in the high-temperature bcc phase of IIIB-IVB metals such as Zr, Ti, and their alloys is observed to be orders of magnitude higher than bcc metals of group VB-VIB, including Cr, Mo, and W. The underlying reason for this higher diffusion is still poorly understood. To explain this observation, we compare the first-principles-calculated parameters of monovacancy-mediated diffusion between bcc Ti, Zr, and dilute Zr- Sn alloys and bcc Cr, Mo, and W. Our results indicate that strongly anharmonic vibrations promote both the vacancy concentration and the diffusive jump rate in bcc IVB metals and can explain their markedly faster diffusion compared to bcc VIB metals. Additionally, we provide an efficient approach to calculate diffusive jump rates according to the transition state theory (TST). The use of standard harmonic TST is impractical in bcc IIIB/IVB metals due to the existence of ill-defined harmonic phonons, and most studies use classical or ab initio molecular dynamics for direct simulation of diffusive jumps. Here, instead, we use a stochastically sampled temperature-dependent phonon analysis within the transition state theory to study diffusive jumps without the need of direct molecular dynamics simulations. We validate our first-principles diffusion coefficient predictions with available experimental measurements and explain the underlying reasons for the promotion of diffusion in bcc IVB metals/alloys compared to bcc VIB metals.

Density functional theory calculations were employed to investigate the structure and energetics of vacancy defects on 84 MXene surfaces, composed of different M (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W), X (carbon or nitrogen), T (oxygen of fluorine), and atomic layer stacking (ABC or ABA) combinations. Structurally, we found that, in most cases, the effect of the presence of the vacancy on the surrounding atoms is to push them away from the vacant site due to the missing screening charge and the reinforcement of the remaining interatomic bonds. The reconstruction energy associated to this surface relaxation was found to range from a few tenths of eV to a few eV. On the bare ABC-stacked carbide MXenes, we found M and X vacancy formation energies between 2 and 3 eV. On nitrides, M vacancy formation energies are decreased, while X ones seem to increase. By terminating the MXenes with an O layer, M formation energies are significantly increased, reaching over 9 eV in some cases, in agreement with theoretical and experimental results of the literature, whereas X vacancy formation energies become smaller. The F termination was found to have the same effect as O on X vacancy formation energies with respect to clean MXenes, but to decrease M vacancy formation energies. The F termination allows more easily created M and N vacancies than the O termination. The creation of vacancies on some ABC-stacked ${\mathrm{M}}_2{\mathrm{XF}}_2$ MXenes led to extreme lattice deformation which suggests that the experimental synthesis of ${\mathrm{M}}_2{\mathrm{NF}}_2$ with metals from groups 5 and 6 of the Periodic Table, and ${\mathrm{M}}_2{\mathrm{CF}}_2$ with metals from group 6, through etching using a hydrogen fluoride aqueous solution [HF (aq)] will lead to ABA-stacked MXenes. The formation energies of the MT double vacancies were found to correlate approximately linearly with the sum of the corresponding M and T single-atom vacancy formation energies. All MXenes studied in this work fit in the same correlation line, implying that one can predict the formation energy of double vacancies on MXenes, made of any M, X, and T elements, and with atomic layers stacked in an ABC or ABA fashion, if the formation energies of the corresponding single vacancies are known. The method used by theoretical works, such as this one, to calculate formation energies has two crucial limitations: the values thus obtained depend on the chosen single-atom energy references, and kinetic effects such as energy barriers are not included. Here, we cautiously discuss these limitations, compare formation energies calculated using different energy references, and propose mechanisms for the formation of vacancies that include atomic migration barriers.