Xenes, graphenelike two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular-lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principles density functional theory calculations. The flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line in the absence of spin-orbit coupling (SOC). The introduction of an out-of-plane buckling creates a glide mirror, protecting an anisotropic Dirac nodal loop. Both flat and buckled Pb monolayers become topologically trivial after including SOC. A large buckling will make the Pb sheet a 2D semiconductor with symmetry-protected Dirac points below the Fermi level. The electronic structures of other group-IV triangular lattices such as Ge and Sn demonstrate strong similarity to Pb. We further design a quasi-3D crystal $\mathrm{Pb}\mathrm{Hf}{\mathrm{O}}_2$ by alternately stacking Pb and 1T-$\mathrm{Hf}{\mathrm{O}}_2$ monolayers. The new compound $\mathrm{Pb}\mathrm{Hf}{\mathrm{O}}_2$ is dynamically stable and retains the properties of Pb monolayer. By applying epitaxial strains to $\mathrm{Pb}\mathrm{Hf}{\mathrm{O}}_2$, it is possible to drive an insulator-to-metal transition coupled with an antiferroelectric-to-paraelectric phase transition. Our results suggest the potential of the 2D triangular lattice as a complementary platform to design a new type of broadly defined xenes.

We present an experimental investigation of the magnetic and magnetocaloric properties of sputtered epitaxial and amorphous Tb thin films. We grew epitaxial Nb (50-nm)/Tb (100-nm) bilayer thin films on ${\mathrm{Al}}_2{\mathrm{O}}_3(11\overline{2}0$) substrate using DC magnetron sputtering at high temperature, with excellent crystalline quality of the hcp Tb(0001) layer. While the amorphous Tb thin film exhibits more isotropic magnetocaloric properties, we show that the epitaxial Tb thin film displays large anisotropic magnetocaloric properties, with a large maximum magnetic entropy change of $6.27\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at the Néel temperature as well as a large relative cooling power of $225\mathrm{J}{\mathrm{kg}}^{-1}$ with a magnetic field change $\mathrm{\Delta }H=20\mathrm{kOe}$ applied along the in-plane direction. These large and anisotropic magnetocaloric properties are much larger than those measured for the amorphous Tb thin film, or previously reported for Tb-based thin-film structures. Our findings highlight the opportunities for growing epitaxial rare-earth thin films using sputtering techniques and demonstrate the importance of crystallographic control on the magnetocaloric effect in Tb thin films.

In this paper we use density functional theory combined with dynamical mean-field theory ($\mathrm{DFT}+\mathrm{DMFT}$) to study interface effects between the correlated metal $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_3$ and the two typical substrate materials $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ and $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$. We find that the $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ interface has only a marginal influence on the $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_3$ thin film, with the dominant effect being the (bulklike) epitaxial strain imposed by the large lattice mismatch, rendering the $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_3$ film insulating due to the enhanced orbital polarization related to the strong level splitting between the $t_{2g}$ orbitals. In contrast, at the polar $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_3/\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ interface, the presence of the interface can have a huge effect on the physical properties, depending both on the specific interface termination and on the specific boundary conditions imposed by the multilayer geometry. We compare three approaches to modeling the $\mathrm{Ca}\mathrm{V}{\mathrm{O}}_3/\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ interface, all of which impose a different set of (electrostatic) boundary conditions. Our results demonstrate that different substrates, interface terminations, and electrostatic boundary conditions can drastically affect the properties of thin-film heterostructures, indicating the potential tunability of the interfacial properties via multilayer engineering.

The selective area growth technique, confined epitaxial lateral overgrowth (CELO), enables the growth of lateral III-V heterojunctions integrated on mismatched substrates. In CELO, effective control of facet shapes, as well as defect-free growths are essential to fabricating high-quality nanostructures with custom geometries. Here, the effects of growth temperature, V/III ratio, template alignments, and substrate orientations on the observed facets and defect densities in CELO grown InP and related materials on InP substrates are investigated. The nanostructure facets and defects are determined using a combination of plan-view and cross-sectional transmission electron microscopy. For homoepitaxial CELO growth on InP (100) substrates, growth temperatures below $575{}^{\circ }\mathrm{C}$, and high V/III ratios of 450 aid in increasing the surface areas of the {111}B facets, while reducing defect densities. Further, by changing template alignments, the effective areas of overgrowth can be tuned, and defects can be lowered, with templates aligned along $\left[0\overline{1}0\right]$ yielding the largest defect-free areas. By aligning templates in the $\left[\overline{1}10\right]$ orientation on a (110) InP substrate, near defect-free overgrowth with perfectly flat perpendicular single ($1\overline{1}$ 0) facets can be achieved. This is an essential feature to enable the growth of lateral III-V heterojunctions, as is demonstrated by growing InP/InGaAs CELO heterojunctions with {110} facets.

Diffusionless isothermal omega ($\mathrm{DI}\text{−}\omega$) transformation, which was recently defined in Phys. Rev. Materials 3, 043604 (2019), is classified into a third category of the $\omega$ transformation modes, other than the well-known isothermal and athermal modes. This work reveals the characteristic features of the $\mathrm{DI}\text{−}\omega$ transformation in the $\beta$-titanium vanadium alloy system, specifically, focusing on variations in the microstructure and mechanical properties with the proceeding of the $\mathrm{DI}\text{−}\omega$ transformation. After quenching alloys of Ti-15at%V, Ti-21at%V, and Ti-27at%V down to below room temperature from the $\beta$-stable temperature, in addition to the occurrence of the athermal $\omega$ transformation for Ti-15at%V, all of the alloys gradually undergo the $\mathrm{DI}\text{−}\omega$ transformation upon aging at a temperature as low as 373 K, leading to a moderate increase in the hardness. The degree of the hardness increase in these alloys can be successfully explained by a local instability concept based on quenched-in thermal concentration fluctuations. It is also shown that internal friction (Ti-21at%V) diminishes after the low-temperature aging, which indicates the annihilation of such unstable regions showing a dynamic collapse of ${\left\{111\right\}}_{\beta }$ pairs to form a transient $\mathrm{DI}\text{−}\omega$ phase. Comparison of inelastic x-ray scattering and ultrasound measurements can see a trail of the $\mathrm{DI}\text{−}\omega$ transformation remaining even in the Ti-27at%V alloy, which has no obvious athermal omega transformation temperature. Based on the results, the difference between athermal $\omega$ and $\mathrm{DI}\text{−}\omega$ transformations is finally discussed.

The combination of materials with dissimilar symmetries can induce a large amount of stress at the interfacial layer of films, thereby promoting the appearance of novel properties in related devices. The study of the interfacial state is critical for determining the inner mechanism. In this work, the misfit heterostructure between cubic NiO films and wurtzite ZnO is investigated. A NiO film grown using molecular beam epitaxy on a ZnO substrate shows a highly (100)-oriented texture featuring three domains with a rotation angle of 30 °, which is in agreement with first principles calculations. Misfit-induced dislocations and lattice distortions within the interfacial layers of the NiO film give rise to interfacial electronic states, which are different from those in bulk; these electronic states are analyzed by in situ synchrotron-based x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, x-ray absorption spectroscopy, and ex situ electron energy loss spectroscopy. Additionally, the origin of these interfacial states is discussed. This work aims to provide insights for the integration of semiconducting hexagonal ZnO with other functional materials that have a cubic symmetry. Additionally, we investigate the integration of photon and electron-based techniques to explore the interfacial states of complex interfaces, which is an important aspect of material science.

The emergence of multifunctional cement-based materials in the construction industry has the potential to shift the paradigm from strength-only performance to new functionalities enabled by electron conducting capabilities in one of the most material- and energy-intensive industry sectors worldwide. To enable such developments, we present results of a hybrid experimental-theoretical investigation of the electrical conductivity and resistive (Joule) heating of highly heterogeneous nanocarbon (nCB)–cement-based composites (pastes and mortars). By analogy with diffusivity, we find that electrical conductivity is determined by the electric tortuosity of a “volumetric wiring” permeating a highly heterogeneous matrix from percolation to saturation. From a combination of electrical conductivity and Joule heat rate measurements, we show that the electric energy dissipation at the origin of the Joule heating originates from spatial electric-field fluctuations, reminiscent of the fluctuation-dissipation theorem. We report that these fluctuations—in first order—are well captured by functional relations of the electric tortuosity of the composite material, and synthesize these observations into a first-order dissipation-tortuosity model. We suggest that harvesting the benefits of electron conducting cement-based materials, such as resistive heating, electromagnetic shielding, and energy storage, will ultimately focus on settling the competition between conductor concentration and electric tortuosity. Given the global environmental footprint of concrete, the results open venues for the sustainable development of concrete for existing and emerging green technology applications.

Transition metal surfaces catalyze a broad range of thermally activated reactions involving carbon-containing species, from atomic carbon to small hydrocarbons or organic molecules, and polymers. These reactions yield well-separated phases, for instance graphene and the metal surface, or, on the contrary, alloyed phases, such as metal carbides. Here, we investigate carbon phases on a rhenium (0001) surface, where the former kind of phase can transform into the latter. We find that this transformation occurs with increasing annealing time, which is hence not suitable to increase the quality of graphene. Our scanning tunneling spectroscopy and reflection high-energy electron diffraction analysis reveal that repeated short annealing cycles are best suited to increase the lateral extension of the structurally coherent graphene domains. Using the same techniques and with the support of density functional theory calculations, we next unveil, in real space, the symmetry of the many variants (two sixfold families) of a rhenium surface carbide observed with diffraction since the 1970s, and finally propose models of the atomic details. One of these models, which nicely matches the microscopy observations, consists of parallel rows of eight aligned carbon trimers with a so-called $(7\times \sqrt{19})$ unit cell with respect to Re(0001).

A reduced pair approximation model for vacancy formation energy in multicomponent materials is proposed as an alternative to the commonly used cluster expansion method. By imposing physical constraints to the interaction coefficients, lower rank models are obtained with improved accuracy as measured by Bayesian information criterion and cross validation. Additionally, reduced models can outperform the full parametrization in high-entropy compounds with as much as 50% less training data. The results are presented for cation vacancies in the high-entropy diboride ${\mathrm{Hf}}_{0.2}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.2}{\mathrm{Ta}}_{0.2}{\mathrm{Nb}}_{0.2}{\mathrm{B}}_2$ calculated by density functional theory simulations of large cell special quasirandom structures. Further, the calculation of vacancy concentrations from a distribution of energies is considered, wherein the chemical disorder on lattice sites gives rise to non-Arrhenius temperature dependence. Preferential clustering and the possibility of short-range order in the high-entropy lattice are explored through pair affinities derived from model coefficients.

We report on multiple fundamental qualitative improvements in the growth of improper ferroelectric hexagonal ${\mathrm{YMnO}}_3$ (YMO) thin films and heterostructures by pulsed laser deposition (PLD). By a combination of pre-growth substrate annealing and low-energy-fluence PLD, we obtain a two-dimensional growth mode of the YMO films on yttria-stabilized zirconia (YSZ) with ultralow roughness and devoid of misoriented nanocrystalline inclusions. By inserting a hexagonal manganite buffer layer capped with conducting indium-tin oxide between the substrate and the final film, the latter is grown in a fully lattice-relaxed mode and, thus, without any misfit dislocations while maintaining the extraordinary flatness of the films grown directly on pre-annealed YSZ. This provides a template for the fabrication of heterostructures based on hexagonal manganites as a promising class of multiferroics with improper room-temperature ferroelectricity and the implementation of these into technologically relevant epitaxial $\mathrm{metal}|$ferroelectric-type multilayers.

Ytterbium-based delafossites with effective $\tilde{S}=1/2$ moments are investigated intensively as candidates for quantum spin-liquid ground states. While the synthesis of related cerium compounds has also been reported, many important details concerning their crystal, electronic, and magnetic structures are unclear. Here we analyze the $\tilde{S}=1/2$ system $\mathrm{K}\mathrm{Ce}{\mathrm{O}}_2$, combining complementary theoretical methods. The lattice geometry was optimized and the band structure investigated using density functional theory extended to the level of a $\mathrm{GGA}+U$ calculation in order to reproduce the correct insulating behavior. The Ce $4f^1$ states were then analyzed in more detail with the help of ab initio wave-function-based computations. Unusually large effective crystal-field splittings of up to 320 meV are predicted, which puts $\mathrm{K}\mathrm{Ce}{\mathrm{O}}_2$ in the strong field coupling regime. Our results reveal a subtle interplay between ligand-cage electrostatics and the trigonal field generated by the extended crystalline surroundings, relevant in the context of recent studies on tuning the nature of the ground-state wave function in $4f$ triangular-lattice and pyrochlore compounds. It also makes $\mathrm{K}\mathrm{Ce}{\mathrm{O}}_2$ an interesting model system in relation to the effect of large crystal-field splittings on the anisotropy of intersite exchange in spin-orbit coupled quantum magnets.

The magnetic signatures of ${\mathrm{Ti}}^{3+}$ centers in potassium titanyl phosphate (KTP) are studied within density-functional theory (DFT). The hyperfine tensor elements are very sensitive to the structural surrounding; the paramagnetic hyperfine splittings are used to evaluate the defect models. For each of the four experimentally observed electron paramagnetic resonance (EPR) spectra, we identify a defect model that reproduces the paramagnetic signature. All of them are electron donors, whereby one specific Ti atom can be identified, whose formal oxidation number is lowered from $4+$ in the ideal crystal to $3+$. The related charge redistribution leads to a strong polarization of the corresponding ${\mathrm{Ti}}^{3+}$ center. However, in three cases a second Ti atom, connected to the first by a mutual polarized O atom, is polarized too. Positively charged O vacancies at the lattice site O(10) are unique in leading to the formation of the only ${\mathrm{Ti}}^{3+}$ center that is stable at room temperature. This defect induces a defect state within the band gap, which may be excited during second harmonic generation (SHG) applications and thus is a plausible candidate to explain the so-called gray tracking, i.e., photochromic damage.

We investigate the phase diagram of the heterostructural solid solution ${\left({\mathrm{In}}_x{\mathrm{Ga}}_{1\text{−}x}\right)}_2{\mathrm{O}}_3$ both computationally, by combining cluster expansion and density functional theory, and experimentally, by means of transmission electron microscopy (TEM) measurements of pulsed laser deposited (PLD) heteroepitaxial thin films. The shapes of the Gibbs free energy curves for the monoclinic, hexagonal, and cubic bixbyite alloy as a function of composition can be explained in terms of the preferred cation coordination environments of indium and gallium. We show by atomically resolved scanning TEM that the strong preference of indium for sixfold coordination results in ordered monoclinic and hexagonal lattices. This ordering impacts the configurational entropy in the solid solution and thereby the ${\left({\mathrm{In}}_x{\mathrm{Ga}}_{1\text{−}x}\right)}_2{\mathrm{O}}_3$ phase diagram. The resulting phase diagram is characterized by very limited solubilities of gallium and indium in the monoclinic, hexagonal, and cubic ground state phases, respectively, but exhibits wide metastable ranges at realistic growth temperatures. On the indium rich side of the phase diagram a wide miscibility gap up to temperatures higher than 1400 K is found, which results in phase separated layers. The experimentally observed indium solubilities in the PLD samples are in the range of $x=0.45$ and $x=0.55$ for monoclinic and hexagonal single-phase films, while for phase separated films we find $x=0.5$ for the monoclinic phase, $x=0.65–0.7$ for the hexagonal phase and $x\ge 0.9$ for the cubic phase. These values are consistent with the computed metastable ranges for each phase.

We investigate the $g$-factors of individual electron states in gate-defined quantum dots fabricated from ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. We consider both the case of effective positive charging energy ($U>0$) where single electrons are added upon increasing the local gate voltage, and the case of $U<0$ where electron pairing is observed. The $g$-factors are extracted from the field dependence of the quantum dot addition spectrum. Tunnel couplings and confinement are tunable by the gate voltages and in the regime of weakest coupling, we find $g$-factors close to 2 due to quenching of the orbital magnetic moment. For stronger coupling, $g$-factors are anisotropic and exhibit values up to $\sim 4.5$ in the out-of-plane orientation. We further show examples of the sequential addition of electrons with the same spin as a consequence of exchange interactions.

Equiatomic FeRh alloys undergo a fascinating first-order metamagnetic phase transition (FOMPT) just above room temperature, which has attracted reinvigorated interest for applications in spintronics. Until now, all attempts to grow nanothin FeRh alloy films have consistently shown that FeRh layers tend to grow in the Volmer-Weber growth mode. Here we show that sputter-grown sub-15-nm-thick FeRh alloy films deposited at low sputter-gas pressure, typically $\sim 0.1$ Pa, onto (001)-oriented MgO substrates, grow in a peening-induced Frank-van der Merwe growth mode for FeRh film thicknesses above 5 nm, circumventing this major drawback. The bombardment of high-energy sputtered atoms, the atom-peening effect, induces a rebalancing between adsorbate-surface and adsorbate-adsorbate interactions, leading to the formation of a smooth continuous nanothin FeRh film. Chemical order in the films increases with the FeRh thickness, $t_{\text{FeRh}}$, and varies monotonically from 0.75 up to 0.9. Specular x-ray diffraction scans around Bragg peaks show Pendellösung fringes for films with $t_{\text{FeRh}}\ge 5.2$ nm, which reflects in smooth well-ordered densified single-crystal FeRh layers. The nanothin film's roughness varies from 0.6 down to about 0.1 nm as $t_{\text{FeRh}}$ increases, and scales linearly with the integral breadth of the rocking curve, proving its microstructured origin. Magnetometry shows that the FOMPT in the nanothin films is qualitatively similar to that of the bulk alloy, except for the thinnest film of 3.7 nm.

 ${\mathrm{PtBi}}_2$ is a polymorphic system with interesting electronic properties. Here we report optimized crystal growth and structural characterization of pyrite-type and trigonal modification of ${\mathrm{PtBi}}_2$. Selected area electron diffraction, x-ray powder diffraction, and further Rietveld refinement confirms that trigonal ${\mathrm{PtBi}}_2$ crystallizes in the noncentrosymmetric $P31m$ space group, and pyrite-type ${\mathrm{PtBi}}_2$ crystallizes in the $Pa\overline{3}$ space group. A series of ${\mathrm{Pt}}_{1-x}{\mathrm{Rh}}_x{\mathrm{Bi}}_2$ samples was obtained for $x=0,0.03,0.35$ in the trigonal ${\mathrm{PtBi}}_2$ structure. These ${\mathrm{Pt}}_{1-x}{\mathrm{Rh}}_x{\mathrm{Bi}}_2$ compounds become superconducting where the critical temperature increases from $T_c=600$ mK for $x=0$ up to $T_c=2.7$ K for $x=0.35$. Furthermore, we calculate the electronic band structure using the obtained structure parameters. The calculated density of states shows a minimum for the stoichiometric compound at the Fermi level. These findings warrant further research using a broader array of experimental techniques, as well as on the effect of the substitution on the nontrivial band structure.

Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate $\mathrm{Dy}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (DBCO). We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film—it is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped $\mathrm{Cu}{\mathrm{O}}_2$ plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one (at $\sim 18\mathrm{meV}$) corresponding to the superconductivity and the other one (at $\sim 90\mathrm{meV}$) to the pseudogap. While the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.

The remarkable mechanical properties of high-entropy alloys can be further improved by interstitial alloying. In this work we employ density functional theory calculations to study the solution energies of dilute carbon interstitial atoms in tetrahedral and octahedral sites in bcc HfNbTiVZr. Our results indicate that carbon interstitials in tetrahedral sites are unstable, and the preferred octahedral sites present a large spread in the energy of solution. The inclusion of carbon interstitials induces large structural relaxations with long-range effects. The effect of local chemical environment on the energy of solution is investigated by performing a local cluster expansion including studies of its correlation with the carbon atomic Voronoi volume. However, the spread in solution energetics cannot be explained with a local environment analysis only pointing towards a complex, long-range influence of interstitial carbon in this alloy.

Electric control of magnetism in magnetoelectric (ME) multiferroics is expected to have a significant impact on a wide range of technological applications. Here, we predict the modulation of magnetism in ultrathin platinum films due to the ferroelectric polarization of the ${\mathrm{BaTiO}}_3$ substrate, which along with biaxial strain changes the density of states at the Fermi energy. We demonstrate that both the magnitude and direction of the magnetization depend strongly on the polarization direction and/or strain. This leads to an unprecedented ME effect involving a giant change of magnetocrystalline anisotropy under polarization switching due to the large spin-orbit coupling of Pt. These findings pave the way of an alternative strategy for the design of nonvolatile and ultralow power spintronics and magnetic memory storage devices.

We utilized scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations to study the diffusion of ammonia $\left({\mathrm{NH}}_3\right)$ on anatase ${\mathrm{TiO}}_2\left(101\right)$. From time-lapsed STM imaging, we observed monomeric and dimeric diffusion channels, and a general tendency to higher diffusion rates with increasing ${\mathrm{NH}}_3$ coverage. In surface regions where several ${\mathrm{NH}}_3$ molecules are adsorbed within a few sites, we further observed the diffusion of ${\mathrm{NH}}_3$ molecules occurring in cascades, where the diffusion of one adsorbate triggers that of others. This eventually leads to apparent diffusion barriers that are lower than expected within a single-jump model. From the DFT calculations, we obtained mechanistic insights into the two observed ${\mathrm{NH}}_3$ diffusion channels. Within the dimeric ${\mathrm{NH}}_3$ diffusion channel, one ${\mathrm{NH}}_3$ swings around another adsorbed ${\mathrm{NH}}_3$ and experiences a reduced diffusion barrier, owing to the intermolecular bonding during the event.