Grain-boundary Josephson junctions formed by epitaxial growth of high-temperature ($T_{\mathrm{c}}$) superconductor films on bicrystal substrates have been employed to determine the pairing symmetry of high-$T_{\mathrm{c}}$ superconductivity. Since high-$T_{\mathrm{c}}$ superconductivity predominately takes place within a single unit cell along the $c$ axis and superconducting coherence length is typically in the nanometer scale, the microstructure of grain boundaries of the bicrystal substrates is crucial for fabricating junctions required for Josephson tunneling study. In this study, we develop a two-step method to prepare insulating $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ bicrystal substrates with atomically uniform and coalesced grain boundaries. Using in situ scanning tunneling microscopy, we demonstrate that FeSe films grown on such substrates exhibit single-facet grain boundaries, leading to ideal Josephson junctions for investigating the pairing symmetry of FeSe superconductors.

In the last decade, hybrid organic-inorganic halide perovskites have emerged as a new type of semiconductor for photovoltaics and other optoelectronic applications. Unlike standard, tetrahedrally bonded semiconductors (e.g., Si and GaAs), the ionic thermal fluctuations in the halide perovskites (i.e., structural dynamics) are strongly coupled to the electronic dynamics. Therefore, it is crucial to obtain accurate and detailed knowledge about the nature of the atomic motions within the crystal. This has proved to be challenging due to low thermal stability and the complex, temperature-dependent structural phase sequence of the halide perovskites. Here, these challenges are overcome and a detailed analysis of the low-frequency lattice mode symmetries is provided in the low-temperature orthorhombic phase of methylammonium-lead iodide. Raman measurements using linearly and circularly polarized light at 1.16 eV excitation are combined with density functional perturbation theory (DFPT). By performing an iterative analysis of Raman polarization-orientation dependence and DFPT mode analysis, the crystal orientation is determined. Subsequently, accounting for birefringence effects detected using circularly polarized light excitation, the symmetries of all of the observed Raman-active modes at 10 K are assigned.

Symmetry plays a major role in all disciplines of physics. Within the field of topological materials there is a great interest in understanding how the mechanics of crystalline and internal symmetries protect crossings between the conduction and valence bands. Additionally, exploring this direction can lead to a deeper understanding on the topological properties of crystals hosting a variety of symmetries. We report the experimental observation of topological surface states in the nodal-loop semimetal ${\mathrm{HfP}}_2$ using angle-resolved photoemission spectroscopy which is supported by our first-principles calculations. Our paper shows termination-dependent surface states in this compound. Our experimental data reveal surface states linked to three unique nodal loops confirmed by theoretical calculation to be topologically nontrivial. This paper demonstrates that transition-metal dipnictides provide a good platform to study nontrivial topological states protected by nonsymmorphic symmetry.

The quantum critical point (QCP) in the archetypical heavy-fermion compound ${\mathrm{CeCu}}_6$ doped by Au is described, accounting for the localized $4f$ electron of Ce, using realistic electronic structure calculations combined with dynamical mean-field theory. Magnetism trends in $\mathrm{Ce}{\left({\mathrm{Cu}}_{1-\epsilon }{\mathrm{Au}}_{\epsilon }\right)}_6$ ($0<\epsilon \ll 1$) are compared with those in Co-doped ${\mathrm{CeCu}}_5$, which resides on the nonferromagnetic side of the composition space of one of the earliest rare-earth permanent magnet compounds, $\mathrm{Ce}{(\mathrm{Co},\mathrm{Cu})}_5$. The construction of a realistic Doniach phase diagram shows that the system crosses over a magnetic quantum critical point in the Kondo lattice in $0.2<x<0.4$ of $\mathrm{Ce}{\left({\mathrm{Cu}}_{1-x}{\mathrm{Co}}_x\right)}_5$. Comparison between Au-doped ${\mathrm{CeCu}}_6$ and Co-doped ${\mathrm{CeCu}}_5$ reveals that the swept region in the vicinity of QCP for the latter thoroughly covers that of the former. The implications of these trends on the coercivity of the bulk rare-earth permanent magnets are discussed.

A technically simple way of probing the formation of skyrmions is to measure the topological Hall resistivity that should occur in the presence of skyrmions as an additional contribution to the ordinary and anomalous Hall effect. This type of probing, lately intensively used for thin film samples, relies on the assumption that the topological Hall effect contribution can be extracted unambiguously from the measured total Hall resistivity. Ultrathin films and heterostructures of the $4d$ ferromagnet ${\mathrm{SrRuO}}_3$ have stirred up a lot of attention after the observation of anomalies in the Hall resistivity, which resembled a topological Hall effect contribution. These anomalies, first reported for bilayers in which the ${\mathrm{SrRuO}}_3$ was interfaced with the strong spin-orbit coupled oxide ${\mathrm{SrIrO}}_3$, were attributed to the formation of tiny Néel-type skyrmions. Here we present the investigation of heterostructures with two magnetically decoupled and electrically parallel connected ${\mathrm{SrRuO}}_3$ layers. The two ${\mathrm{SrRuO}}_3$ layers deliberately have different thicknesses, which affects the coercive field and ferromagnetic transition temperature of the two layers, and the magnitude and temperature dependence of their anomalous Hall constants. The ${\mathrm{SrRuO}}_3$ layers were separated by ultrathin layers of either the strong spin-orbit coupling oxide ${\mathrm{SrIrO}}_3$ or of the large band-gap insulator ${\mathrm{SrZrO}}_3$. Our magnetic and magnetotransport studies confirm the additivity of the anomalous Hall transverse voltages for the parallel conducting channels originating from the two ferromagnetic ${\mathrm{SrRuO}}_3$ layers as well as the possibility to tune the global anomalous Hall resistivity by magnetic field, temperature, or structural modifications at the epitaxial all-oxide interfaces. The Hall voltage loops of these two-layer heterostructures demonstrate the possibility to generate humplike structures in the Hall voltage loops of ${\mathrm{SrRuO}}_3$ heterostructures without the formation of skyrmions and emphasize that the detection of skyrmions only by Hall measurements can be misleading.

We examine physical properties of heavily boron-doped $s{p}^3$-hybridized carbon allotropes: cubic diamond, hexagonal diamond, and body centered tetragonal ${\mathrm{C}}_4$. The structural similarity between cubic diamond and hexagonal diamond leads to similar responses to substitutional boron doping. On the other hand, body centered tetragonal ${\mathrm{C}}_4$ exhibits distinct structural and electronic properties because of its characteristic structure. Our study also shows that the superconducting transition temperatures up to 60 K are possible in heavily doped carbon materials despite their various atomistic structures.

Carbonates are the major hosts of carbon on Earth's surface and their fate during subduction needs to be known to understand the deep carbon cycle. Magnesite (${\mathrm{MgCO}}_3$) is thought to be an important phase participating in deep Earth processes, but its phase stability is still a matter of debate for the conditions prevalent in the lowest part of the mantle and at the core mantle boundary. Here, we have studied the phase relations and stabilities of ${\mathrm{MgCO}}_3$ at these $P,T$ conditions, using Raman spectroscopy at high pressures ($\sim 148\mathrm{GPa}$) and after heating to high temperatures ($\sim 3600\mathrm{K}$) in laser-heated diamond anvil cell experiments. The experimental Raman experiments were supplemented by x-ray powder diffraction data, obtained at a pressure of 110 GPa. Density-functional-theory-based model calculations were used to compute Raman spectra for several ${\mathrm{MgCO}}_3$ high-pressure polymorphs, thus allowing an unambiguous assignment of Raman modes. By combining the experimental observations with the density-functional-theory results, we constrain the phase stability field of ${\mathrm{MgCO}}_3$ with respect to the high-pressure polymorph, ${\mathrm{MgCO}}_3$-II. We further confirm that Fe-free ${\mathrm{MgCO}}_3$-II is a tetracarbonate with monoclinic symmetry (space group $C2/m$), which is stable over the entire $P,T$ range of the Earth's lowermost mantle geotherm.

Poly($p$-phenylene terephthalamide) (PPTA) exhibits van der Waals (vdW) bonding along its $a$ axis, hydrogen bonding along its $b$ axis, and covalent bonding along its $c$ axis. We explore the structural and mechanical properties of PPTA using density functional theory with various functionals including LDA, PBE, PBE+rVV10L, SCAN, and SCAN+rVV10, compared with available experiments. The hierarchy of nonempirical semilocal functionals LDA, PBE, and SCAN (not fitted to any multicenter bonded system) includes differing amounts of intermediate-range vdW interaction. rVV10 is the long-range vdW correction. (rVV10L differs from rVV10 only in the value of a range parameter.) Among the tested functionals, SCAN shows the best performance for the lattice parameters of PPTA along the two crystal directions involving vdW or hydrogen-bond interaction. The equilibrium lattice constants obtained by SCAN and PBE+rVV10L are closest to experimental data, while SCAN+rVV10 slightly overbinds the system. We study the mechanical response of PPTA by applying strain along three lattice directions. Due to the inclusion of vdW interaction, SCAN, PBE+rVV10L, and SCAN+rVV10 all exhibit correct bonding strain-energy curves. On the contrary, PBE strongly underestimates the vdW interaction needed to resist uniaxial stretching along the $a$ axis. The Young's modulus and yield strength of PPTA are computed and compared with previous results. The experimental values are much smaller than the computed ones, mainly due to the fact that the PPTA fiber samples used for measurements are mechanically weaker than the perfect molecular crystals considered in the simulations. Interestingly, when a compressive uniaxial stress of 25 Kbar is applied along the $b$ axis, a structural phase transition, in which the hydrogen bonds reform along one diagonal of the $ab$ rectangle, is predicted by SCAN.

The phonon density of states of silicon nanocrystals with size between 4 and 7.5 nm was measured by inelastic neutron scattering in the $5\sim 600\mathrm{K}$ temperature range. The narrow particle size distributions enable the study of size effects on phonon dynamics. Giant softening of phonon features below 30 meV, universal broadening of phonon features, and the disappearance of intermediate-energy phonons were observed with decreasing nanocrystals size. Such size effects are mostly attributed to the structure variations within the nanocrystals. The phonons below 30 meV in silicon nanocrystals show temperature dependence opposite to the bulk silicon, explained by the large anharmonicity of the under-constrained near-surface phonons. This is supported by the abnormal atomic mean-square-displacement, and low energy phonon population in small silicon nanocrystals. This work provides crucial information on the phonon dynamics in spatially confined materials.

The piezoelectric (PE) and stiffness tensors of 32-atom supercells of ZnO-based alloys have been obtained from ab initio simulations using density functional perturbation theory in the local density approximation. Low concentration for substituents to Zn, O, or both were considered in unstrained and biaxially strained supercells. The $d_{33}$ coefficient for unstrained ${\mathrm{Zn}}_{15}{\mathrm{YO}}_{15}\mathrm{N}$ and ${\mathrm{Zn}}_{15}{\mathrm{LaO}}_{15}\mathrm{N}$ alloys are, respectively, 17.5 and $18\mathrm{pC}{\mathrm{N}}^{-1}$, whereas $e_{33}$ is $1.7\mathrm{C}{\mathrm{m}}^{-2}$ for both alloys. These values are significantly improved compared to simulated values for pristine ZnO ($d_{33}=11.4\mathrm{pC}{\mathrm{N}}^{-1}$ and $e_{33}=1.3\mathrm{C}{\mathrm{m}}^{-2}$). Applying 2% tensile strain on ${\mathrm{Zn}}_{15}{\mathrm{YO}}_{15}\mathrm{N}$ results in an increase of the ${\mathrm{e}}_{33}$ coefficient to 2.$1\mathrm{C}{\mathrm{m}}^{-2}$, a 62% increase over the value calculated for pristine ZnO. We confirm for a variety of ternary and quaternary ZnO-based alloys that a linear relation is verified between the ${\mathrm{e}}_{33}$ coefficient and the cell ratio $c/a$, described by a slope $\approx -9\mathrm{C}{\mathrm{m}}^{-2}$. Our results also indicate that the PE coefficients follow the same trends with respect to changes in $c/a$ caused by variations in chemical composition or by applying biaxial strain. Based on this correlation, we propose a simple method to identify promising candidates among piezoelectric alloys in the wurtzite family, effectively reducing the intensive computational resources needed to obtain optimal PE performance for applications compatible with the many requirements of thin film growth and processing.

Understanding the band alignment at oxide surfaces is of great importance for designing oxide-surface-based electronic, catalytic, and photocatalytic applications. We systematically investigate the band alignment of (001) surfaces of $A\left(\mathrm{II}\right)B\left(\mathrm{IV}\right){\mathrm{O}}_3$ perovskites ($A=\mathrm{Ca}$, Sr, Ba, Pb; $B=\mathrm{Ti}$, Zr, Hf, Ge, Sn) through first-principles calculations using semilocal and hybrid functionals. The results are discussed with an emphasis on the effects of surface reconstruction on the band alignment. Reconstructed stoichiometric surfaces are generated by an evolutionary algorithm with surface energy minimization for various surface periodicity units by taking the orthorhombic phase of $\mathrm{CaTi}{\mathrm{O}}_3$, the tetragonal phase of $\mathrm{SrTi}{\mathrm{O}}_3$, and the cubic phases of $\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{BaSn}{\mathrm{O}}_3$ as representatives. Two types of reconstruction patterns are obtained as energetically favorable configurations common to these phases, which are composed of half-$A\mathrm{O}$ and -$B{\mathrm{O}}_2$ topmost layers, respectively. These reconstructed stoichiometric surfaces have energies comparable to those of nonstoichiometric surfaces, with full $A\mathrm{O}$ or $B{\mathrm{O}}_2$ termination under specific chemical potential conditions in $\mathrm{CaTi}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$. We systematically calculate the positions of the valence-band maxima and the conduction-band minima with respect to the vacuum level, namely, the ionization potentials and the electron affinities, for the reconstructed stoichiometric and the nonstoichiometric surfaces of $A\left(\mathrm{II}\right)B\left(\mathrm{IV}\right){\mathrm{O}}_3$ perovskites. The ionization potentials and electron affinities at the reconstructed surfaces well describe the termination-plane dependencies of experimentally reported values. The surface band positions are found to show an approximately linear trend against the Goldschmidt tolerance factor, with the sign of slopes opposite to each other for the two types of reconstruction patterns. This tendency is explained by the tolerance-factor dependency of surface rumpling that significantly modifies the surface dipole, although the band positions of $A=\mathrm{Pb}$ systems exhibit a larger deviation from an expected trend due to Pb lone-pair states.

High-quality $p$-type PbTe-CdTe monocrystalline alloys containing up to 10 at.$\%$ of Cd have been obtained by self-selecting vapor transport method. Mid-infrared photoluminescence experiments have been performed in order to follow the variation of the fundamental energy gap as a function of Cd content. The Hall mobility, thermoelectric power, thermal conductivity, and thermoelectric figure-of-merit parameter $ZT$ were investigated experimentally and theoretically with special attention paid to the two-valence band structure of the material. It is shown that the heavy-hole band near the $\mathrm{\Sigma }$ point of the Brillouin zone plays an important role and is responsible for the ${\mathrm{Pb}}_{1-x}{\mathrm{Cd}}_x\mathrm{Te}$ hole transport at higher Cd content. Our data and their description can serve as monocrystalline reference for ${\mathrm{Pb}}_{1-x}{\mathrm{Cd}}_x\mathrm{Te}$ crystals with $x$ up to 0.1. It is shown that monocrystalline ${\mathrm{Pb}}_{1-x}{\mathrm{Cd}}_x\mathrm{Te}$ samples with relatively low Cd content of about 1 at.% and hole concentration of the order of ${10}^{20}{\mathrm{cm}}^{-3}$ may exhibit $ZT\approx 1.4$ at 600 K.

Ultrasonics have been an incisive probe of internal interfaces in a wide variety of systems ranging from stars to solids. For thin-film structures, however, ultrasound is largely ineffective because the signal is dominated by the substrate. Using confocal Raman spectromicroscopy, we show that multiple reflection of sound waves at internal interfaces of a metal-oxide superlattice generates standing waves that are insensitive to the substrate. Such modes had previously been observed only in high-quality superlattices of elemental semiconductors, and their observation in complex metal-oxide heterostructures is testimony to recent progress in this field. We use the high spatial resolution of the Raman microscope to demonstrate the high sensitivity of the mode frequency to atomic-scale thickness variations of the superlattice. Spectroscopy of acoustic standing waves can hence serve as a powerful characterization tool of thin-film structures. In analogy to ultrasound spectroscopy of bulk solids, lineshape analysis of these modes has the potential to yield detailed information about the internal structure of the interfaces as well as the coupling of sound waves to the low-frequency spin, charge, and orbital dynamics in metal-oxide superlattices.

We report on the observation of symmetry breaking and the circular photogalvanic effect in ${\mathrm{Cd}}_x{\mathrm{Hg}}_{1-x}\mathrm{Te}$ alloys. We demonstrate that irradiation of bulk epitaxial films with circularly polarized terahertz radiation leads to the circular photogalvanic effect (CPGE) yielding a photocurrent whose direction reverses upon switching the photon helicity. This effect is forbidden in bulk zinc-blende crystals by symmetry arguments; therefore, its observation indicates either the symmetry reduction of bulk material or that the photocurrent is excited in the topological surface states formed in a material with low cadmium concentration. We show that the bulk states play a crucial role because the CPGE was also clearly detected in samples with noninverted band structure. We suggest that strain is a reason for the symmetry reduction. We develop a theory of the CPGE showing that the photocurrent results from the quantum interference of different pathways contributing to the free-carrier absorption (Drude-like) of monochromatic radiation.

The magnetic Weyl semimetal $\mathrm{C}{\mathrm{o}}_3\mathrm{S}{\mathrm{n}}_2{\mathrm{S}}_2$ exhibits large anomalous Hall effect (AHE) due to its nontrivial band topology with enhanced Berry curvature. Here we investigate the pressure effect on the AHE of $\mathrm{C}{\mathrm{o}}_3\mathrm{S}{\mathrm{n}}_2{\mathrm{S}}_2$ up to 12 GPa with a palm cubic anvil cell apparatus and first-principles calculations simulation. We find that both the ferromagnetic transition temperature and the AHE are suppressed monotonically upon the application of high pressure. Data analyses revealed that in the investigated pressure range the intrinsic mechanism due to Berry curvature dominates the AHE as reflected by the validation of ${\rho }_{xy}^A\propto {\rho }_{xx}^2$. However, both the anomalous Hall conductivity and anomalous Hall angle are reduced gradually into the regime for conventional ferromagnetic metals with trivial band topology. Combined with theoretical calculations, our results indicate that the distance between Weyl points with opposite chirality in $\mathrm{C}{\mathrm{o}}_3\mathrm{S}{\mathrm{n}}_2{\mathrm{S}}_2$ is substantially reduced accompanying the suppression of ferromagnetism by pressure, thus providing an experimental route to tune the AHE of magnetic Weyl semimetals via modifying the nontrivial band topology.

Lithium-ion batteries with amorphous silicon ($a$-Si) anodes exhibit very high theoretical energy capacity, with the lithium kinetic transport having the most crucial effect on the battery performance. In this study, the lithium diffusion pathways in a series of large-scale $a$-Si models (512 atoms) with various extents of structural order are calculated using the machine learning interatomic potential. Then, the Li diffusion behavior in different atomistic environments is estimated from the transient state theory. The Li diffusion activation energy is observed to be lower (higher) in an ordered (disordered) local environment. The activation energy varies within the range of 1.21–1.46 eV, which agrees well with experimental measurements, 1.38–1.46 eV. Our simulations also show that Li diffusion is enhanced at higher Li concentration, which is consistent with experimental observations. The effects of structural disorder and Li concentration can be explained by the “trap” mechanism. Finally, we show that the sources of Li diffusion traps are dangling bonds and large voids in the $a$-Si matrix with the help of first-principles calculations. Our work provides insight into the Li diffusion mechanism, which is beneficial for improving the performance of $a$-Si anodes for lithium-ion batteries. In addition, we demonstrate the significant dependence of the ion transport behavior on the local atomic environment, which will be useful for future theoretical studies of technologically important amorphous materials beyond Si.

We describe in detail how ZnO films grow on ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ substrates by atomic layer deposition (ALD), employing a suite of in situ synchrotron x-ray techniques. Combining results from different measurements allows the distinguishment of three different growth behaviors: an initial, slow linear growth, often referred to as a growth delay (regime I), followed by a nonlinear growth (regime II), and finally, a steady, linear growth (regime III), the last of which is the self-limited growth behavior characteristic of ALD. By the end of regime I, the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ surface is covered with an ultrathin, poorly ordered Zn oxide layer. The transition from regime I to II is clearly evidenced by the appearance in the x-ray absorption spectra of characteristic features of the wurtzite structure, as well as the nucleation and growth of ZnO grains (three-dimensional) on top of the poorly ordered Zn oxide layer. Regime II ends when the growth per cycle reaches a constant level. We show that the water pressure during growth has an impact on the duration of the growth delay (regime I), unlike the substrate temperature. In the regime of steady growth, we observe that the rate of deposition obtained for all temperatures inside the ALD window is 0.17 nm ${\mathrm{cy}}^{-1}$. The deposition temperature has clear effects on the film texture and initial crystallization behavior, as well as the final crystallinity and thicknesses of the layers adjacent to the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ substrate. Based on the experimental results and earlier ab initio calculations and Monte Carlo simulations of ZnO ALD on ZnO, we suggest reaction mechanisms consistent with our findings, and we present a model of growth starting from the very earliest stages of deposition to the steady growth regime.

Twinning is an important deformation mechanism in crystalline materials. Twins nucleate, then grow, forming a discrete pattern of deformation bands with a certain thickness. The mechanism for twin growth is not completely understood. We present a simple phase field model which captures the physics of twin growth and reproduces the details observed during in situ electron backscatter diffraction experiments on $\alpha$-uranium. The interaction between residual dislocations at twin interfaces and mobile dislocations in untwinned regions increases the stress needed for twin growth. This is described by a nonlocal term in the proposed constitutive equations: the nucleation stress of a twin increases proportionally to the twin phase field in a neighborhood. Competition between slip and twinning favors the nucleation of a new twin rather than the growth of a preexisting twin. The phase field model is coupled with a crystal plasticity finite element solver that includes the plastic deformation due to twinning. Simulations are able to reproduce the number of twins and their thicknesses as a function of the strain observed during in situ electron backscatter diffraction experiments. The stress-strain curve is also reproduced. This comparison allows the values of the nucleation stress and the interaction strength between residual and mobile dislocations to be found. It also gives an estimation of the density of residual dislocations that is consistent with the observed twin thickness. This model can be applied to understand microstructural effects in materials using twinning as a strengthening mechanism.