Electrostatically defined perovskite oxide heterostructures, in which carriers are confined by the modulation of the A-site ion charge, offer new possibilities of tuning the magnetic properties of manganite oxides. We investigate the preferential orientation of ferromagnetic and antiferromagnetic moments in ultrathin ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ layers embedded in antiferromagnetic $\mathrm{Sr}{\mathrm{MnO}}_3$ as they undergo a metal-to-insulator transition with decreasing thickness. Our results evince the role of orbital occupation, metallicity, and competition of different magnetic phases, in absence of spurious effects occurring in thin films as a result of symmetry breaking at ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ interfaces and of incorporation of oxygen vacancies.

Double halide perovskites have attracted increasing interest due to their great potential to eliminate the toxicity and instability issues in lead halide perovskites. Unfortunately, octahedral cation antisites easily form in common double halide perovskites and induce deep defect levels. Here, using first-principles calculation methods, we report unusual defect properties in the double halide perovskite ${\mathrm{Cs}}_2{\mathrm{Au}}_2{\mathrm{I}}_6$, including the coexistence of shallow and deep defect transition energy levels of iodine vacancies, absent of octahedral cation antisites, and different dominant donor defects such as interstitials $\left({\mathrm{Au}}_{\mathrm{i}}\right)$ and I substituting Au $\left({\mathrm{I}}_{\mathrm{Au}}\right)$. We attribute these unusual defect properties to the multivalence effect of Au and strong covalent bonding between Au and I. Based on the defect properties, we propose two possible applications of ${\mathrm{Cs}}_2{\mathrm{Au}}_2{\mathrm{I}}_6$. On one hand, ${\mathrm{Cs}}_2{\mathrm{Au}}_2{\mathrm{I}}_6$ could be a useful candidate for photodetectors under I-rich growth conditions due to its relatively low carrier densities because the Fermi level is pinned near the middle of the band gap due to the strong compensation of the dominant donors ${\mathrm{Au}}_{\mathrm{i}}$ and acceptor ${\mathrm{V}}_{\mathrm{Cs}}$. On the other hand, ${\mathrm{Cs}}_2{\mathrm{Au}}_2{\mathrm{I}}_6$ can be made $n$-type if it is grown under I-poor conditions, which can be further enhanced by growing the sample at 600 K and then quenching to 300 K. Consequently, ${\mathrm{Cs}}_2{\mathrm{Au}}_2{\mathrm{I}}_6$ has potential as an $n$-type photovoltaic absorber. Our studies not only enrich the defect physics in multivalent double halide perovskites but also provide useful information for advancing ${\mathrm{Cs}}_2{\mathrm{Au}}_2{\mathrm{I}}_6$ technologies.

Motivated by recent advances in half-Heusler based thermoelectric materials, we investigated the phase stability and thermoelectric properties of compounds ZrNiSi, ZrNiGe, HfNiSi, NbCoSi, and ZrNiSb, some of which were recently reported in literature as promising half-Heuslers for thermoelectric applications using the first-principles density functional theory based calculations. Here, we show that all the named compounds actually crystallize with the orthorhombic TiNiSi structure type, which remains stable above room temperature up to at least 1100 K. In ZrNiSb, $5\%$ excess Zr is required to obtain the pure orthorhombic phase. Our first-principles electronic band structure calculations reveal that they are semimetals. In ZrNiSi, ZrNiGe, and HfNiSi, the Fermi surface consists of small electron and hole pockets with electrons as the majority charge carriers. In NbCoSi and ZrNiSb, the majority carriers are holes. A pseudogaplike feature is observed in the electronic density of states with Fermi energy ($E_F$) located either slightly below (ZrNiSi, ZrNiGe, and HfNiSi) or above the pseudogap (NbCoSi). In ZrNiSb no pseudogap is observed; however, the density of states at $E_F$ is still small. The electrical conductivity ($\sigma$) near room temperature is of the order of ${10}^3\mathrm{S}{\mathrm{cm}}^{-1}$, which is intermediate between that of the degenerate semiconductors and metallic alloys. Near room temperature the thermopower is negative for $\mathrm{ZrNi}X$ ($X$ = Si, Ge) and HfNiSi, and positive for NbCoSi and ZrNiSb as predicted theoretically. The average value of Seebeck coefficient is small, of the order of $10\mu \mathrm{V}{\mathrm{K}}^{-1}$. Despite reasonably high electrical conductivity, the thermal conductivity ($\kappa$) of these compounds is found to be generally low ($<15\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ near $300\mathrm{K}$). In ${\mathrm{Zr}}_{1.05}\mathrm{Ni}\mathrm{Sb}$, which has the highest electrical conductivity ($\approx 4000\mathrm{S}{\mathrm{cm}}^{-1}$), $\kappa$ is as low as $\approx 4\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at $300\mathrm{K}$, of which almost 70% is estimated to be due to the electronic contribution resulting in a lattice contribution which is $<1\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. This uncommon combination of high electrical conductivity and low thermal conductivity is interesting and invites further attention.

I present a fully ab initio scheme to model transient spectroscopy signals in the presence of strongly bound excitons. Using LiF as a prototype material, I show that the scheme is able to capture the exciton signature both in time-resolved angle-resolved photoemission spectroscopy and transient absorption experiments. The approach is completely general and can become the reference scheme for modeling pump and probe experiment in a wide range of materials.

${\mathrm{Li}}_2\mathrm{Sr}{\left[\mathrm{MnN}\right]}_2$ single crystals were successfully grown out of Li-rich flux. The crystal structure was determined by single-crystal x-ray diffraction and revealed almost linear $-\mathrm{N}-\mathrm{Mn}-\mathrm{N}-\mathrm{Mn}-$ chains as central structural motif. Tetragonal blocks of air and moisture-sensitive nitridomanganate were employed for electrical transport, heat capacity, and anisotropic magnetization measurements. Both the electronic and magnetic properties are most remarkable, in particular the linear increase of the magnetic susceptibility with temperature that is reminiscent of underdoped cuprate and Fe-based superconductors. Clear indications for antiferromagnetic ordering at $T_N=290$ K were obtained. Metallic transport behavior is experimentally observed in accordance with electronic band structure calculations.

High-lying Rydberg states of Mott-Wannier excitons are receiving considerable interest due to the possibility of adding long-range interactions to the physics of excitons. Here, we study Rydberg excitation in bulk synthetic cuprous oxide grown by the optical float zone technique and compare the result with natural samples. X-ray characterization confirms both materials are mostly single crystal, and midinfrared transmission spectroscopy revealed little difference between synthetic and natural material. The synthetic samples show principal quantum numbers up to $n=10$, exhibit additional absorption lines, plus enhanced spatial broadening and spatial inhomogeneity. Room temperature and cryogenic photoluminescence measurements reveal a significant quantity of copper vacancies in the synthetic material. These measurements provide a route towards achieving high-$n$ excitons in synthetic crystals, opening a route to scalable quantum devices.

Quasicrystals (QC) in condensed matter are often accompanied by competing periodic structures with large unit cells that approximate the QC. Likewise for the dodecagonal Ba-Ti-O oxide quasicrystal that forms on Pt(111), approximants with square and oblique lattices have been reported. Here we report on the identification of a hexagonal approximant for this QC. Atomically-resolved scanning tunneling microscopy shows the characteristic vertex atoms, which form triangle, square, and rhombus tiling elements, which organize into more complex motifs that are building blocks of the aperiodic QC. Low-energy electron diffraction confirms the large-cell hexagonal structure and is well reproduced by a minimal single-scattering calculation. To accommodate the misfit, the hexagonal layer is twisted $4{}^{\circ }$ against the hexagonal Pt(111) substrate, which also lowers the overall symmetry of the system.

Halide perovskites are strongly influenced by large amplitude anharmonic lattice fluctuations at room temperature. We develop a tight-binding model for dynamically disordered ${\mathrm{MAPbI}}_3$ based on density functional theory calculations to calculate electronic structure for finite temperature crystal structures at the length scale of thermal disorder and carrier localization. The model predicts individual Hamiltonian matrix elements and band structures with high accuracy, owing to the inclusion of additional matrix elements and descriptors for non-Coulombic interactions. We apply this model to electronic structure at length and timescales inaccessible to first-principles methods, finding an increase in band gap, carrier mass, and the subpicosecond fluctuations in these quantities with increasing temperature as well as the onset of carrier localization in large supercells induced by thermal disorder at 300 K. We identify the length scale $L^{*}=5$ nm as the onset of localization in the electronic structure, associated with decreasing band edge fluctuations, increasing carrier mass, and Rashba splitting approaching zero.

This work demonstrates the potential for practical scalable growth of complex garnets and evaluates the implications of a multicomponent composition in the optical quality and elemental distribution of a Czochralski-grown crystal. Our experimental approach was designed to elucidate the relation between a complex garnet composition ${\left({\mathrm{Lu}}_{1/4}{\mathrm{Yb}}_{1/4}{\mathrm{Y}}_{1/4}{\mathrm{Gd}}_{1/4}\right)}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$, crystal growth parameters, crystal structural, and elemental homogeneity. Our hypothesis is that combining multiple rare earths (REs) that will fractionally occupy the dodecahedral site in the aluminum garnet structure will result in a stable, single garnet compound that can be grown by the Czochralski method. Single-crystal and powder x-ray diffraction indicated a single garnet phase with an increasing unit cell volume from seed to tail. In addition, we propose that the pattern of elemental segregation will be based on the deviation of the ionic radius of each constituent RE from the average RE ionic radius of the multicomponent garnet. Electron probe microanalysis revealed that ions that are smaller than that average (${\mathrm{Lu}}^{3+}$ and ${\mathrm{Yb}}^{3+}$) are preferentially incorporated in the crystal, while elements that are larger than that average (${\mathrm{Gd}}^{3+}$) are rejected. The ionic radius of ${\mathrm{Y}}^{3+}$ is close to that average and yttrium segregation was minimal. The concentrations of the four REs are closer to stoichiometric on the tail end of the boule. Scanning electron microscopy and energy-dispersive x-ray spectroscopy analysis reveal Gd-rich inclusions with eutectic microstructures in the tail end of the boule.

Transition metal dichalcogenides, such as ${\mathrm{MoS}}_2$, are known to undergo a structural phase transformation as well as a change in the electronic conductivity upon Li intercalation. These properties make them candidates for charge-tunable ion-insertion materials that could be used in electrochemical devices. In this paper, we study the phase stability and electronic structure of H and T' Li-intercalated $\mathrm{Mo}{X}_2$ bilayers with X = S, Se, or Te. Using first-principles calculations in combination with classical and machine learning modeling approaches, we find that the H phase is more stable at low Li concentration for all X and the critical Li concentration at which the T'$\to \mathrm{H}$ transition occurs decreases with increasing mass of X. Furthermore, the relative free energy of the two phases becomes less sensitive to Li insertion with increasing atomic mass of the chalcogen atom X. While the electronic conductivity increases with increasing ion concentration at low concentrations, we do not observe a (positive) conductivity jump at the phase transition from H to T'.

First-principles simulations are conducted to shed light on the question of whether a two-dimensional topological insulator (2DTI) phase may be obtained at the interface between InAs and GaSb. To this end, the InAs/GaSb interface is compared and contrasted with the HgTe/CdTe interface. Density functional theory (DFT) simulations of these interfaces are performed using a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. For the HgTe/CdTe interface, our simulations show that band crossing is achieved and an inverted gap is obtained at a critical thickness of 5.1 nm of HgTe, in agreement with experiment and previous DFT calculations. In contrast, for InAs/GaSb, the gap narrows with increasing thickness of InAs; however, the gap does not close for interfaces with up to 50 layers (about 15 nm) of each material. When an external electric field is applied across the InAs/GaSb interface, the GaSb-derived valence band maximum is shifted up in energy with respect to the InAs-derived conduction band minimum until eventually the bands cross and an inverted gap opens. Our results show that it may be possible to reach the topological regime at the InAs/GaSb interface under the right conditions. However, it may be challenging to realize these conditions experimentally, which explains the difficulty of experimentally demonstrating an inverted gap in InAs/GaSb.

$\mathrm{La}{\mathrm{VO}}_3$ (LVO) has been proposed as a promising material for photovoltaics because its strongly correlated $d$ electrons can facilitate the creation of multiple electron-hole pairs per incoming photon, which would lead to increased device efficiency. In this paper, we intentionally grow off-stoichiometric LVO films by changing the growth conditions such as laser fluence. Our aim is to study how deviating La:V stoichiometries affect the electronic properties of LVO thin films. We find that the off-stoichiometry clearly alters the physical properties of the films. Structural characterization shows that both La-rich and V-rich films have different levels of structural distortion, with La-rich (V-rich) films showing a larger (smaller) out-of-plane lattice parameter compared to what one would expect from epitaxial strain effects alone. Both types of films show deviation from the behavior of bulk LVO in optical measurement, i.e., they do not show signatures of the expected long range orbital order, which can be a result of the structural distortions or the presence of structural domains. In transport measurements, La-rich films display clear signatures of electronic phase separation accompanying a temperature induced metal-insulator transition, while V-rich films behave as Mott insulators. The out-of-plane lattice parameter plays a crucial role in determining the transport properties, as the crossover from Mott-insulating to disorder-induced phase-separated behavior occurs around a lattice parameter value of 3.96 Å, quite different from what has been previously reported.

Both ${\mathrm{Li}}_3{\mathrm{BO}}_3$ and ${\mathrm{Li}}_3{\mathrm{BN}}_2$ materials have promising properties for use in all-solid-state batteries and other technologies dependent on electrolytes with significant ionic conductivity. As the first of a two-part study, this paper reports the analysis of detailed simulations of Li ion diffusion in the monoclinic forms of these materials. Using both NEB and MD methods, it is clear that Li ion migration via vacancy mechanisms provides the most efficient ion transport in each material. While the results suggest that interstitial defects in these materials do not play a direct role in Li ion migration, their relative stability seems to enhance vacancy production via the formation of Frenkel-type defects. This may partially explain why the Li ion conductivities computed from MD simulations of samples initially containing a single Li ion vacancy are in reasonable agreement with measured values of this work for ${\mathrm{Li}}_3{\mathrm{BO}}_3$ and those reported in the literature for poorly crystalline samples of both materials. The possibility of increasing vacancy concentrations by substitutional doping (F for O in ${\mathrm{Li}}_3{\mathrm{BO}}_3$ and C for B in ${\mathrm{Li}}_3{\mathrm{BN}}_2$) is also examined, finding simulated conductivities comparable to those of the ideal vacancy model.

Both ${\mathrm{Li}}_3{\mathrm{BO}}_3$ and ${\mathrm{Li}}_3{\mathrm{BN}}_2$ materials have promising properties for use in all solid-state batteries and other technologies dependent on electrolytes with significant ionic conductivity. As the second of a two-part study, the structural properties of ${\mathrm{Li}}_3{\mathrm{BO}}_3$ and three reported phases of ${\mathrm{Li}}_3{\mathrm{BN}}_2$ are investigated using first-principles modeling techniques. For $\alpha \text{−}{\mathrm{Li}}_3{\mathrm{BN}}_2$, the tetragonal $P{4}_2/mnm$ structure reported in the literature is found to be unstable as evidenced by imaginary phonon modes near the M point of its Brillouin zone. Our simulations within the harmonic approximation suggest that the real $\alpha$ phase has the orthorhombic space group symmetry $Pmmn$ formed with twice as many formula units and tiny adjustments of the equivalent lattice parameters and fractional coordinates. Extending the analysis of the $Pmmn\alpha \text{−}{\mathrm{Li}}_3{\mathrm{BN}}_2$ structure to the quasiharmonic approximation improves the agreement between the room-temperature x-ray pattern reported in the literature and the corresponding simulation results. In anticipation of the use of the monoclinic phases of ${\mathrm{Li}}_3{\mathrm{BO}}_3$ and ${\mathrm{Li}}_3{\mathrm{BN}}_2$ in Li ion conducting applications, chemical stability is investigated in terms of free-energy differences of possible decomposition and Li reaction processes, finding encouraging results. As further investigations of ${\mathrm{Li}}_3{\mathrm{BO}}_3$ and $\beta \text{−}{\mathrm{Li}}_3{\mathrm{BN}}_2$ as electrolyte or coating materials, particularly for use with Li metal anodes, idealized electrolyte/Li interfaces were investigated in terms of their geometric, energetic, and electronic properties. The results find the electrolyte/Li interfaces to be quite favorable, perhaps comparable to the pioneering LiPON/Li system.

Structures consisting of a single Si nanodot buried within an insulating nanometric ${\mathrm{SiO}}_2$ layer stacked between two Si layers show promising properties for room temperature operational single-electron transistors. Moreover, such structures are highly compatible with modern complementary metal-oxide semiconductor technologies. Metastable ${\mathrm{SiO}}_{\mathrm{x}}$ phase separates into a Si nanodot and insulating, homogeneous ${\mathrm{SiO}}_2$ during annealing, providing a solid path towards the desired structure. However, achieving the necessary amount of excessive Si, dissolved in the ${\mathrm{SiO}}_2$ for correct concentrations of ${\mathrm{SiO}}_{\mathrm{x}}$, remains a technological challenge. In this work, we investigate ion-induced atom mixing in pre-built $\mathrm{Si}/{\mathrm{SiO}}_2/\mathrm{Si}$ nanopillars, which is considered to be a technologically promising way to produce the necessary concentrations of spatially confined ${\mathrm{SiO}}_{\mathrm{x}}$ in a controlled manner. During the high-fluence ion irradiation, we notice a significant shortening of the nanopillar and preferential loss of O atoms. Both sputtering and nanoscale ion hammering are found to be the cause of the deformation. The ion-hammering effect on nanoscale is explained by multiple small displacements, strongly enhanced after the nanopillar was rendered completely amorphous. The methods presented here can be used to determine the ion-fluence threshold for sufficient atom mixing in spatially confined regions before the large structural deformations are formed.

We report resistive switching in voltage biased point contacts (PCs) based on the van der Waals transition metal tellurides (TMTs) $Me{\mathrm{Te}}_2$ ($Me=\mathrm{Mo},\mathrm{W}$) and $\mathrm{Ta}Me{\mathrm{Te}}_4$ $\left(Me=\mathrm{Ru},\mathrm{Rh},\mathrm{Ir}\right)$. The switching occurs between a low resistive “metallic-type” state, which is the ground state, and a high resistive “semiconducting-type” state by applying a certain bias voltage $\left(<1\mathrm{V}\right)$. The reverse switching takes place by applying voltage of opposite polarity. The origin of the effect can be attributed to the formation of domains with different crystal structure in the PC core due to a strong electric field $\left(>10\mathrm{kV}/\mathrm{cm}\right)$. The new functionality of the studied TMT materials, which emerges from switchable domains in submicron heterostructures, is very promising, e.g., for nonvolatile resistive random access memory engineering.

The topological flat band (TFB) has been proposed theoretically in various lattice models, to exhibit a rich spectrum of intriguing physical behaviors. However, the experimental demonstration of flat band (FB) properties has been severely hindered by the lack of materials realization. Here, by screening materials from a first-principles materials database, we identify a group of two-dimensional materials with TFBs near the Fermi level, covering some simple line-graph and generalized line-graph FB lattice models. These include the kagome sublattice of O in $\mathrm{Ti}{\mathrm{O}}_2$ yielding a spin-unpolarized TFB, and that of V in ferromagnetic ${\mathrm{V}}_3{\mathrm{F}}_8$ yielding a spin-polarized TFB. The monolayer ${\mathrm{Nb}}_3\mathrm{Te}{\mathrm{Cl}}_7$ and its counterparts from element substitution are found to be breathing-kagome-lattice crystals. The family of monolayer $\mathrm{II}{\mathrm{I}}_2\mathrm{V}{\mathrm{I}}_3$ compounds exhibit a TFB representing the coloring-triangle lattice model. $\mathrm{Re}{\mathrm{F}}_3$, $\mathrm{Mn}{\mathrm{F}}_3$, and $\mathrm{Mn}{\mathrm{Br}}_3$ are all predicted to be diatomic-kagome-lattice crystals, with TFB transitions induced by atomic substitution. Finally, $\mathrm{Hg}{\mathrm{F}}_2$, $\mathrm{Cd}{\mathrm{F}}_2$, and $\mathrm{Zn}{\mathrm{F}}_2$ are discovered to host dual TFBs in the diamond-octagon lattice. Our findings pave the way to further experimental exploration of eluding FB materials and properties.

Thermal conductivities of single- and double-wall carbon nanotubes in contact with foreign atoms on their surfaces are investigated by means of molecular dynamics simulations. Small amounts of atoms on the surfaces of single-wall nanotubes drastically reduce their thermal conductivity, while the conductivities of the inner walls of double-wall nanotubes which are only weakly coupled to the outer walls are retained even for large amounts of atoms on the surfaces. Based on the simulation results, an estimation for the conditions under which an enhancement of thermal conductivities can be expected is presented.

The so-called interlayer-sliding ferroelectricity was recently proposed as an unconventional route to pursuit electric polarity in van der Waals multilayers, which was already experimentally confirmed in a $\mathrm{W}{\mathrm{Te}}_2$ bilayer even though it is metallic. Very recently, another van der Waals system, i.e., the $\mathrm{Zr}{\mathrm{I}}_2$ bilayer, was predicted to exhibit the interlayer-sliding ferroelectricity with both in-plane and out-of-plane polarizations [Zhang et al., Phys. Rev. B 103, 165420 (2021)]. Here the $\mathrm{Zr}{\mathrm{I}}_2$ bulk is studied, which owns two competitive phases ($\alpha$ vs $\beta$), both of which are derived from the common parent $s$ phase. The $\beta -\mathrm{Zr}{\mathrm{I}}_2$ owns a considerable out-of-plane polarization ($0.39\mu \mathrm{C}/{\mathrm{cm}}^2$), while its in-plane component is fully compensated. Their proximate energies provide the opportunity to tune the ground state phase by moderate hydrostatic pressure and uniaxial strain. Furthermore, the negative longitudinal piezoelectricity in $\beta -\mathrm{Zr}{\mathrm{I}}_2$ is dominantly contributed by the enhanced dipole of $\mathrm{Zr}{\mathrm{I}}_2$ layers as a unique characteristic of interlayer-sliding ferroelectricity, which is different from many other layered ferroelectrics with negative longitudinal piezoelectricity like $\mathrm{Cu}\mathrm{In}{\mathrm{P}}_2{\mathrm{S}}_6$.