We employ classical molecular dynamics simulations to investigate the molecular-level structure of water during the isothermal compression of hexagonal ice ($\mathrm{I}h$) and low-density amorphous (LDA) ice at low temperatures. In both cases, the system transforms to high-density amorphous ice (HDA) via a first-order-like phase transition. We employ a sensitive local order metric (LOM) [F. Martelli et al., Phys. Rev. B 97, 064105 (2018)] that can discriminate among different crystalline and noncrystalline ice structures and is based on the positions of the oxygen atoms in the first- and/or second-hydration shell. Our results confirm that LDA and HDA are indeed amorphous, i.e., they lack polydispersed ice domains. Interestingly, HDA contains a small number of domains that are reminiscent of the unit cell of ice IV, although the hydrogen-bond network (HBN) of these domains differs from the HBN of ice IV. The presence of ice-IV-like domains provides some support to the hypothesis that HDA could be the result of a detour on the HBN rearrangement along the $\mathrm{I}h$-to-ice-IV pressure-induced transformation. Both nonequilibrium LDA-to-HDA and $\mathrm{I}h$-to-HDA transformations are two-step processes where a small distortion of the HBN first occurs at low pressures and then, a sudden, extensive rearrangement of hydrogen bonds at the corresponding transformation pressure follows. Interestingly, the $\mathrm{I}h$-to-HDA and LDA-to-HDA transformations occur when LDA and $\mathrm{I}h$ have similar local order, as quantified by the site-averaged LOMs. Since $\mathrm{I}h$ has a perfect tetrahedral HBN while LDA does not, it follows that higher pressures are needed to transform $\mathrm{I}h$ into HDA than that for the conversion of LDA to HDA. In correspondence with both first-order-like phase transitions, the samples are composed of a large HDA cluster that percolates within the $\mathrm{I}h/\mathrm{LDA}$ samples. Our results shed light on the debated structural properties of amorphous ices and indicate that the kinetics of the $\mathrm{I}h$-to-HDA and LDA-to-HDA transformations require an in-depth inspection of the underlying HBN. Such investigation is currently ongoing.

Several recent experiments have demonstrated the failure of the Fourier heat equation in semiconductors at the nanoscale. We show that a generalized heat transport equation including a hydrodynamic term can explain three of these experiments on a silicon substrate: heater lines experiments both in stationary and nonstationary settings and the nonstationary response in grating experiments. To this end, we solve the hydrodynamic heat equation either numerically or analytically. The non-Fourier response observed in those experimental situations can be easily explained in terms of hydrodynamic concepts, such as friction and vorticity. For instance, the experimentally observed increase of thermal boundary resistance as size decreases can be simply understood as an increase of friction. We conclude that, contrary to common belief, hydrodynamics is a fundamental ingredient of semiconductor heat transport in the nanoscale at room temperature, providing physical insight and a unifying framework on the observed non-Fourier response. The relations between hydrodynamic heat transport with phonon momentum conservation and the Boltzmann transport equation are also provided.

We present the hydrothermal synthesis, as well as structural and chemical analysis, of single crystals of ${\mathrm{EuCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_3$, ${\mathrm{Zn}}_x{\mathrm{Cu}}_{4-x}{\left(\mathrm{OH}\right)}_6{\left({\mathrm{NO}}_3\right)}_2$ and haydeeite, and ${\mathrm{MgCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ compounds, all arising from the atacamite family. Magnetic and specific-heat measurements down to 1.8 K are carried out for these systems. ${\mathrm{EuCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_3$ has a frustrated antiferromagnetic ${\mathrm{Cu}}^{2+}$ ground state with order at 15 K, and a strong anisotropy and increased magnetization from Van Vleck paramagnetic ${\mathrm{Eu}}^{3+}$ contributions. ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\left({\mathrm{NO}}_3\right)}_2$ reveals antiferromagnetic order at 9 K and measurements on haydeeite single crystals confirm the ferromagnetic order at 4.2 K with the easy axis within the kagome plane. These results prove that the atacamite family presents a broad class of materials with interesting magnetic ground states.

An understanding of the heterostructural implications on alloying in the aluminum nitride-scandium nitride system (${\mathrm{Al}}_{1-x}{\mathrm{Sc}}_x\mathrm{N}$) can highlight opportunities and design principles for enhancing desired material properties by leveraging nonequilibrium states. The fundamental thermodynamics, and therefore composition- and structure-dependent mechanisms, underlying property evolution in this system have not been fully described, despite significant recent efforts driven by interest in enhanced piezoelectric performance. Practical realization of these enhanced properties, however, is hindered by the strong driving thermodynamic driving force for phase separation in the system, highlighting the need for increased study into the role of heterostructural alloying on the thermodynamics and composition-structure-property relationships in this system. With this need in mind, ab initio computed alloy thermodynamics and properties are compared to combinatorial thin-film synthesis and characterization to develop a more complete picture of the structure and property evolution across the ${\mathrm{Al}}_{1-x}{\mathrm{Sc}}_x\mathrm{N}$ composition space. The combination of structural frustration and a flattened free-energy landscape lead to substantial increases in electromechanical response. The energy scale of alloy metastability is found to be much larger than previously reported, helping to explain difficulties in achieving homogeneous materials with high scandium concentration. Scandium substitution for aluminum softens the wurtzite crystal lattice, and energetic proximity to the competing hexagonal boron-nitride structure enhances the piezoelectric stress coefficient. Overall, this work provides insight into the understanding of the structure-processing-property relationships in the ${\mathrm{Al}}_{1-x}{\mathrm{Sc}}_x\mathrm{N}$ system, suggests material design strategies for even greater property enhancements, and demonstrates the increased property tunability and underexplored nature of nonequilibrium heterostructural alloys.

Magnetic properties and the magnetocaloric effect have been studied in a ferromagnetic compound ${\mathrm{EuTi}}_{0.9}{\mathrm{V}}_{0.1}{\mathrm{O}}_3$ by magnetization and heat capacity measurements. With the application of a magnetic field, the spin entropy is strongly suppressed and a large magnetic entropy change is observed near the Curie temperature $T_{\mathrm{C}}=4.54\mathrm{K}$. The values of entropy change $\mathrm{\Delta }{S}_{\mathrm{m}}$, adiabatic temperature change, and magnetic cooling capacity are 41.4 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$, 17.4 K, and 577 J ${\mathrm{kg}}^{-1}$, respectively, for a field change of 0–$7\mathrm{T}$. This compound also shows a large magnetocaloric effect at low magnetic fields. In particular, $-\mathrm{\Delta }{S}_{\mathrm{m}}$ reaches as high as 11 and 20 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for field changes of 0–1 and 0–2 T, respectively. These values of magnetocaloric parameters are significantly larger than that reported for several potential low-temperature magnetic refrigerants. The low-field large magnetocaloric effect, together with the absence of thermal and field hysteresis, makes the insulating ${\mathrm{EuTi}}_{0.9}{\mathrm{V}}_{0.1}{\mathrm{O}}_3$ compound a promising candidate for refrigeration below $30\mathrm{K}$.

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. Here, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic countercation. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by $35\pm 5\mathrm{meV}$, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Our work highlights the complexity of excitonic properties in this class of nanostructured materials.

In the absence of external magnetization, a finite Berry curvature $\left[\mathrm{\Omega }\left(k\right)\right]$ in the role of a momentum-dependent magnetic field can lead to a Hall-type thermal behavior such as the anomalous Nernst effect. The traditional and nonanomalous variants require a real-space magnetic field. In this work, we report coefficients for the anomalous Nernst effect $\left(\text{ANE}\right)$ and its spin analog, a variant of the more well-established spin Nernst effect $\left(\text{SNE}\right)$ in lead chalcogenide (PbX; $X$ = S, Se, Te) films. The narrow gapped PbX films with a large spin-orbit coupling (soc) offer a significant Rashba interaction that gives rise to $\mathrm{\Omega }\left(k\right)$ and the attendant anomalous thermal behavior. In the presence of a temperature gradient, the $\text{ANE}$ and $\text{SNE}$ establish a thermal and spin current and are characterized by their respective coefficients which acquire higher values for a stronger Rashba interaction. We further show that an extrinsic soc generated by an in-plane electric field offers a gate-like mechanism to control (and turn-off) the anomalous thermal currents. Finally, we conclude by deriving the efficiency of an $\text{ANE}$-driven low-temperature Carnot heat engine and demonstrate that it can be gainfully optimized in systems with a robust intrinsic soc resulting in low carrier effective masses. Cooling techniques in microdevices via $\mathrm{\Omega }\left(k\right)$ regulated anomalous thermal conductivity contributions are briefly discussed.

The threading screw dislocation in AlN and InN, two technologically important nitride-based materials, has been studied using first-principles calculations. By performing a comprehensive search for the most stable core configuration, several original core structures have been obtained depending on experimental conditions. In Al-rich and In-rich conditions, screw dislocation cores exhibit a metalliclike character, predicted to provide pathways to leakage currents in III-N based electronic devices. Specifical to AlN, most stable core geometries in N-rich conditions are shown to contain nitrogen atoms organized like nitrogen molecules. Finally, our investigations revealed that in InN dislocation cores are associated to partial or complete closure of the gap and to the injection of electrons into the bulk conduction band.

Interest in the bulk transition metal dichalcogenides for their electronic, photovoltaic, and optical properties has grown and led to their use in many technological applications. We present a systematic investigation of their interlinked vibrational and dielectric properties using density functional theory and density functional perturbation theory, studying the effects of the spin-orbit interaction and of the long-range $e^{-}-e^{-}$ correlation as part of our investigation. This study confirms that the spin-orbit interaction plays a small role in these physical properties, while the direct contribution of dispersion corrections is of crucial importance in the description of the interatomic force constants. Here, our analysis of the structural and vibrational properties, including the Raman spectra, compare well to experimental measurement. Three materials with different point groups are showcased, and data trends on the full set of 15 existing hexagonal, trigonal, and triclinic materials are demonstrated. This overall picture will enable the modeling of devices composed of these materials for novel applications.

Examples of single-crystal epitaxial thin films of a high entropy perovskite oxide are synthesized. Pulsed laser deposition is used to grow the configurationally disordered $AB{\mathrm{O}}_3$ perovskite $\mathrm{Ba}\left(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{S}{\mathrm{n}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.2}\mathrm{H}{\mathrm{f}}_{0.2}\mathrm{N}{\mathrm{b}}_{0.2}\right){\mathrm{O}}_3$ epitaxially on $\mathrm{SrTi}{\mathrm{O}}_3$ and MgO substrates. X-ray diffraction and scanning transmission electron microscopy demonstrate that the films are single phase with excellent crystallinity and atomically abrupt interfaces to the underlying substrates. Atomically resolved electron-energy-loss spectroscopy mapping shows a uniform and random distribution of all $B$-site cations. The ability to stabilize perovskites with this level of configurational disorder offers new possibilities for designing materials from a much broader combinatorial cation pallet while providing a fresh avenue for fundamental studies in strongly correlated quantum materials where local disorder can play a critical role in determining macroscopic properties.

Materials that exhibit extreme hardness, low mass density, and high thermal/chemical stability can often be modeled as simple modifications to the $\alpha$-rhombohedral phase of boron. To facilitate the development and discovery of these multipurpose, structural ceramics, the current work reveals fundamental physics on the bonding and deformation of $p$-block hexaborides, an important and representative subclass of icosahedral solids. Icosahedral separation and localization of equatorial bonding are identified as predictors of both elastic moduli and strength. This work also explores nanotwinning, an advanced nanostructure responsible for dramatic records in mechanical performance of both metals and ceramics. To help develop the first model of nanotwinning in ceramics, this work demonstrates (1) that susceptibility to nanotwinning relies on key bonding traits and (2) that nanotwinning minimally affects elasticity and high-periodicity inelasticity. Overall, this work helps rationalize both the mechanical performance of a major class of materials and the cutting-edge mechanism of nanotwinning through fundamental, physics-based approaches.

In this work we evaluate the defects and the associated distortions present in tensile and compressive-strained chemical solution deposition–derived $\mathrm{NdNi}{\mathrm{O}}_3$ (NNO) and $\mathrm{LaNi}{\mathrm{O}}_3$ (LNO) thin films by means of aberration corrected scanning transmission electron microscopy. We elucidate a fundamental link between strain and the most common defect observed in nickelate films, the Ruddlesden-Popper fault (RPF), which will ultimately impinge on the electrical properties of the films. Overall, the concentration of RPF defects increases with the lattice mismatch. More specifically, LNO films are always metallic, although transitioning from compressive to tensile strain results in the appearance of RPFs and an increase of the resistivity. On the other hand, NNO films always behave as insulators under tensile strain, whereas under compressive strain the increase of the thickness makes the onset of the metal-to-insulator transition shift to higher temperatures.

Point contact Andreev reflection spectroscopy has been utilized to determine the spin polarization of both amorphous and crystalline $\mathrm{F}{\mathrm{e}}_x\mathrm{S}{\mathrm{i}}_{1-x}$ ($0.58<x<0.68$) thin films. The amorphous materials exhibited a substantial spin polarization (generally greater than 60%), despite significant changes in magnetization and resistivity. In particular, the polarization value in the $x=0.65$ amorphous alloy is about 70%, significantly higher than most ferromagnets, including numerous Heusler compounds that are theoretically predicted to be half-metallic ferromagnets. The composition dependence of the spin polarization in the amorphous materials is proportional to (but substantially larger than) the DFT-calculated values. The polarization of a crystalline thin film with $x=0.65$, by contrast, is only 49%, similar to that of common magnetic metals. The enhanced spin polarization in the amorphous structure is attributed to the modification of the local environments. This work demonstrates that the spin polarization, as well as magnetic moment, anomalous Hall effect, and electrical resistivity, can be tuned by introducing structural disorder as an engineering tool.

The Mott transition and the vicinity of a Mott insulating phase have constantly been a fertile ground for exploring exotic quantum states, most notably the high-$T_c$ cuprates. The layered iridate represents another intriguing example. The nature of the Mott insulator phase and transition mechanism to a metallic state is still under debate. Much of the challenge originates from a series of energy scales involved in the electronics phases. Here, we report synthetization, characterization, and transport measurements on doped and undoped $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{Ir}{\mathrm{O}}_3$ films grown on $\mathrm{SrTi}{\mathrm{O}}_3$. The films are fully strained up to 70 nm thick and have a tetragonal lattice structure. For a doping level of $x=0.1$, a bad-metal state with linear temperature dependence of the resistivity beyond the Mott-Ioffe-Regel limit emerges in a wide temperature range down to a critical temperature $T_c\sim 30\mathrm{K}$, below which the system shows Mott variable range hopping (Mott VRH) conduction behavior. The ground state is confirmed to be insulating by the Mobius criterion. A strong correlation between the bad-metal state and Mott VRH localized state is found as the slope of the linear resistivity is inversely proportional to film thickness and the size of the Mott VRH activation energy $\mathrm{\Delta }$ is linearly proportional to $T_c$. We further show that upon doping the spin-orbit Mott insulator, itinerant metallic regions coexist with localized regions within a nanoscale phase-separated ground state with a small activation energy. Our results shed light on the nature of the metallic state and a crossover to a bad-metal phase for doping the spin-orbit Mott insulator.

Nanoscale topological defects, such as vortices and skyrmions in ferroic systems, have been intensively investigated due to their intriguing properties and behaviors. In addition to harnessing and tuning their functionalities, spatial manipulation and control of topological defects as functional elements are important for future nanotechnologies. Here, we report a linear alignment behavior of single-chiral vortices observed in thin transmission electron microscopy samples of multiferroic hexagonal manganites during an in situ heating experiment. In order to avoid chemical degradation during high-temperature heating in vacuum, we have utilized a rapid heating and quenching by $in$ situ electric arcing. Using atomic resolution electron microscopy and phase-field simulation, we found that vortices and antivortices formed near the phase transition temperature are pulled out in the opposite directions by mechanical strain to leave single-chiral vortices linearly aligned along an isothermal line formed during quenching. Our study provides important insight into spatial manipulation of the topological defects in oxide multiferroics by investigating far-equilibrium dynamics of symmetry-breaking continuous phase transitions.

Hexagonal boron nitride (h-BN) has received great interest in recent years as a wide band-gap analog of graphene-derived systems along with its potential in a wide range of applications, for example, as the dielectric layer for graphene devices. However, the thermal transport properties of h-BN, which can be critical for device reliability and functionality, are little studied both experimentally and theoretically. The primary challenge in the experimental measurements of the anisotropic thermal conductivity of h-BN is that typically the sample size of h-BN single crystals is too small for conventional measurement techniques, as state-of-the-art technologies synthesize h-BN single crystals with lateral sizes only up to 2.5 mm and thicknesses up to 200 μm. Recently developed time-domain thermoreflectance (TDTR) techniques are suitable to measure the anisotropic thermal conductivity of such small samples, as it only requires a small area of $50\times 50\mu {\mathrm{m}}^2$ for the measurements. Accurate atomistic modeling of thermal transport in bulk h-BN is also challenging due to the highly anisotropic layered structure. Here we conduct an integrated experimental and theoretical study on the anisotropic thermal conductivity of bulk h-BN single crystals over the temperature range of 100–500 K using TDTR measurements with multiple modulation frequencies and a full-scale numerical calculation of the phonon Boltzmann transport equation starting from first principles. Our experimental and numerical results compare favorably for both the in-plane and the through-plane thermal conductivities. We observe unusual temperature dependence and phonon-isotope scattering in the through-plane thermal conductivity of h-BN and elucidate their origins. This paper not only provides an important benchmark of the anisotropic thermal conductivity of h-BN, but also develops fundamental insight into the nature of phonon transport in this highly anisotropic layered material.

Using first-principles calculations, we show that $\mathrm{Cs}B{X}_3$ halides with $B=\text{Sn}$ or Pb undergo octahedral rotation distortions, while for $B=\text{Ge}$ and Si, they undergo a ferroelectric rhombohedral distortion accompanied by a rhombohedral stretching of the lattice. We show that these are mutually exclusive at their equilibrium volume although different distortions may occur as functions of lattice expansion. The choice between the two distortion modes is in part governed by the Goldschmidt tolerance factor. However, another factor explaining the difference between Sn and Pb compared with Ge and Si is the stronger lone-pair character of Ge and Si when forced to be divalent as is the case in these structures. The lone-pair chemistry is related to the off centering. While the Si-based compounds have not yet been synthesized, the Ge compounds have been established experimentally. As a final test of the importance of the tolerance factor we consider $\mathrm{RbGe}{X}_3$, which has smaller tolerance factor than the corresponding $\mathrm{CsGe}{X}_3$ because Rb is smaller than Cs. We find that it can lower its energy by both rotations or rhombohedral off-centering distortions but the latter lower the energy slightly more efficiently.

The defect stability in a prototypical perovskite oxide superlattice consisting of ${\mathrm{SrTiO}}_3$ and ${\mathrm{PbTiO}}_3$ (STO/PTO) is determined by using first principles density functional theory calculations. Specifically, the oxygen vacancy formation energies $E_{\mathrm{v}}$ in the paraelectric and ferroelectric phases of a superlattice with four atomic layers of STO and four layers of PTO (4STO/4PTO) are determined and compared. The effects of charge state, octahedral rotation, polarization, and interfaces on $E_{\mathrm{v}}$ are examined. The formation energies vary layer by layer in the superlattices, with $E_{\mathrm{v}}$ being higher in the ferroelectric phase than that in the paraelectric phase. The two interfaces constructed in these oxide superlattices, which are symmetrically equivalent in the paraelectric systems, exhibit very different formation energies in the ferroelectric superlattices and this can be seen to be driven by the coupling of ferroelectric and rotational modes. At equivalent lattice sites, $E_{\mathrm{v}}$ of charged vacancies is generally lower than that of neutral vacancies. Octahedral rotations $\left({\mathrm{a}}^0{\mathrm{a}}^0{\mathrm{c}}^{-}\right)$ in the superlattices have a significant effect on $E_{\mathrm{v}}$, increasing the formation energy of vacancies located near the interface but decreasing the formation energy of the oxygen vacancies located in the bulk-like regions of the STO and PTO constituent parts. The formation-energy variations among different layers are found to be primarily caused by the difference in the local relaxation at each layer. These fundamental insights into the defect stability in perovskite superlattices can be used to tune defect properties by controlling the constituent materials of superlattices and interface engineering.

This article reports on the design, numerical simulation, fabrication, and experimental test of an acoustic planar retroreflector capable of effectively reflecting sound along its incident direction for a wide operating angle range ($0^{\circ }-{70}^{\circ }$). The proposed acoustic planar retroreflector is a compound of two cascaded metasurfaces: a transmissive surface that converges the incident beam onto a second planar surface placed behind it, which serves as a reflective surface that bounces the beam back along the incident direction. Both the simulated and measured results provide evidence of the sound retroreflection effect. The structure proposed here provides a possible strategy for improving medical ultrasound, underwater communication, and illusion device design.