We investigate the role of electron correlation and disorder on the electronic structure of layered nickelate ${\left({\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\right)}_2{\mathrm{NiO}}_4$ using core level and valence band photoemission spectroscopy in conjunction with density functional theory (DFT) and dynamical mean-field theory (DMFT) calculations. Sr $3d$ and La $4d$ core level spectra exhibit multiple features associated with photoemission final-state effects. An increase of unscreened features in the Sr $3d$ and La $4d$ core level spectra with lowering temperature suggests the reduction in density of states (DOS) at the Fermi level, $E_F$. Valence band spectra collected using different photon energies reveal finite intensity at $E_F$ and overall spectra are well captured by $\mathrm{DFT}+\mathrm{DMFT}$. Strong renormalization of partially filled $e_g$ bands in $\mathrm{DFT}+\mathrm{DMFT}$ result indicates strong correlation in this system. Mass enhancement factor, $m^{*}/m_{\text{DFT}}\sim$ 3, agrees well with values obtained from specific heat measurements. High-resolution spectra in the vicinity of $E_F$ show monotonically decreasing spectral intensity with lowering temperature, which evolves to exhibit a Fermi cutoff at low temperatures indicating metallic character in contrast to insulating transport, suggesting Anderson insulating state. $|E-E_F{|}^{1/2}$ dependence of the spectral DOS and square-root temperature dependence of spectral DOS at $E_F$ evidences the role of disorder in the electronic structure of ${\left({\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\right)}_2{\mathrm{NiO}}_4$.

The atomic structure of surfaces and interfaces plays a vital role in the electronic quality and properties of quantum devices. The interplay between the surface and confined bulk subband states in terms of their susceptibility has been investigated in relation to crystal defects on an InAs(111)A-$(2\times 2)$ reconstructed surface, using low-temperature scanning tunneling microscopy and spectroscopy. We measure the two-dimensional quantized subband states arising from the confined potential imposed by downward bending of the conduction band edge. Furthermore, we show evidence of the existence of surface Bloch states within the confined bulk band gap projected on the surface spectrum which have originated from the surface reconstruction. As expected, larger confined bulk band gaps at the surface and conduction band offset are measured to be 0.58 and 0.31 eV, respectively. We further show the scattering of these quantum states at different surface defects and demonstrate that surface states are more susceptible to the defect potential when compared with the corresponding subband states. This apparent contrast follows from the length scale at which these defect potentials actively interact on or near the surface. Our observed experimental results are supported by empirical tight-binding simulations for the subband states and first-principles density functional theory simulations for the surface states present on the surface.

Using a real-space high-order finite-difference approach, we investigate the electronic structure of large spherical silicon nanoclusters. Within Kohn-Sham density functional theory and using pseudopotentials, we report the self-consistent field convergence of a system with over 100 000 atoms: a ${\mathrm{Si}}_{107,641}{\mathrm{H}}_{9,084}$ nanocluster with a diameter of 16 nm. Our approach uses Chebyshev-filtered subspace iteration to speed up the convergence of the eigenspace, and blockwise Hilbert space-filling curves to speed up sparse matrix-vector multiplications, all of which are implemented in the parsec code. For the largest system, we utilized 2048 nodes (114 688 cores) on the Frontera machine in the Texas Advanced Computing Center. Our quantitative analysis of the electronic structure shows how it gradually approaches its bulk counterpart as a function of nanocluster size. The band gap is enlarged due to quantum confinement in nanoclusters, but decreases as the system size increases, as expected. Our work serves as a proof of concept for the capacity of the real-space approach in efficiently parallelizing very large calculations using high-performance computer platforms, which can straightforwardly be replicated in other systems with more than ${10}^5$ atoms.

Little is so far known about the magnetism of the $A_2{B}_2{\mathrm{O}}_7$ monoclinic layered perovskites that replace the spin-ice supporting pyrochlore structure for $r_A/r_B>1.78$. We show that high quality monoclinic ${\mathrm{Pr}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ single crystals with a three-dimensional network of non-Kramers ${\mathrm{Pr}}^{3+}$ ions that interact through edge-sharing superexchange interactions, form a singlet ground-state quantum paramagnet that does not undergo any magnetic phase transitions down to, at least, 1.8 K. The chemical phase stability, structure, and magnetic properties of the layered perovskite ${\mathrm{Pr}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ were investigated using x-ray diffraction, transmission electron microscopy, and magnetization measurements. Synthesis of polycrystalline samples with the nominal compositions of ${\mathrm{Pr}}_2{\mathrm{Ti}}_{2+x}{\mathrm{O}}_7$ ($-0.16\le x\le 0.16$) showed that deviations from the ${\mathrm{Pr}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ stoichiometry lead to secondary phases of related structures including the perovskite phase ${\mathrm{Pr}}_{2/3}{\mathrm{TiO}}_3$ and the orthorhombic phases ${\mathrm{Pr}}_4{\mathrm{Ti}}_9{\mathrm{O}}_{24}$ and ${\mathrm{Pr}}_2{\mathrm{TiO}}_5$. No indications of site disordering (stuffing and antistuffing) or vacancy defects were observed in the ${\mathrm{Pr}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ majority phase. A procedure for growth of high-structural-quality stoichiometric single crystals of ${\mathrm{Pr}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ by the traveling solvent floating zone method is reported. Thermomagnetic measurements of single-crystalline ${\mathrm{Pr}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ reveal an isolated singlet ground state that we associate with the low-symmetry crystal electric-field environments that split the ($2J+1=9$)-fold degenerate spin-orbital multiplets of the four differently coordinated ${\mathrm{Pr}}^{3+}$ ions into 36 isolated singlets resulting in an anisotropic temperature-independent van Vleck susceptibility at low $T$. A small isotropic Curie term is associated with 0.96(2)% noninteracting ${\mathrm{Pr}}^{4+}$ impurities.

${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3$-based (NBT-based) ceramics offer a viable option to replace lead-based materials for high-power applications as they are characterized by a stable mechanical quality factor with increasing vibration velocity in comparison to lead-based piezoceramics. Recently, the minor and stable extrinsic contributions were revealed as the origin for the stability of the mechanical quality factor with increasing vibration velocity. This work identifies the very unusual high poling degree as cause for the small extrinsic contributions. To this end, complete pole figure densities have been quantified and correlated to the piezoelectric coefficient and electromechanical quality factor. This hypothesis is further strengthened by correlating the piezoelectric constant (sum of intrinsic and extrinsic contributions) with the remanent polarization (correlates to remanent texturing degree). In order to assess a full picture of NBT-based piezoceramics, $0.94{\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3\text{−}0.06\mathrm{BaTi}{\mathrm{O}}_3$ has been considered with and without Zn doping and with quenching. It is compared to $0.79{\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3\text{−}0.21{\mathrm{K}}_{1/2}{\mathrm{Bi}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3$ with and without Mg doping. Finally, a contrast to soft $\mathrm{Pb}\left({\mathrm{Zr}}_{1/2}{\mathrm{Ti}}_{1/2}\right){\mathrm{O}}_3$ (PZT) flushes out the impact of domain wall motion on the piezoelectric coefficient and the electromechanical quality factor. Whereas a PZT-based reference material exhibits a linear increase in the piezoelectric constant with increasing remanent polarization, the NBT-based materials deviate from the linear trend, indicating a decrease in extrinsic contributions.

The surface area of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surfacelike. Weak antilocalization was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics.

Migration of point defects can result in various performance instabilities in organometal halide perovskite devices. In particular, the segregation of charged defects can influence the electron transport, but the correlation between ionic and electronic transports remains elusive. In this work, we use a graphene/methylammonium lead iodide capacitor to probe the effect of ion segregation on the charge injection behavior at the graphene/perovskite junction. In our samples, the ion segregation in the perovskite film is induced by a controllable electric field applied through an electrostatic gate. At the same time, the charge injection from graphene to the perovskite is probed by monitoring changes in the graphene conductivity. It is found that a positive (negative) defect accumulation layer near the graphene/perovskite interface can facilitate electron (hole) injection from the graphene electrode to the perovskite. Because the polarity of the defect accumulation layer is switchable by changing the direction of the applied electric field, the graphene/perovskite interface behaves like a Schottky junction with a tunable charge injection barrier height. This behavior should be generally found in other metal/halide perovskite interfaces as well.

The transition-metal dichalcogenide tantalum disulphide ($1\mathit{T}\text{−}\mathrm{TaS}{}_2$) hosts a commensurate charge density wave (CCDW) at temperatures below 165 K where it also becomes insulating. The low temperature CCDW phase can be driven into a metastable “mosaic” phase by means of either laser or voltage pulses, which shows a large density of CDW domain walls as well as a closing of the electronic band gap. The exact origins of this pulse-induced metallic mosaic are not yet fully understood. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we observe the occurrence of such a metallic mosaic phase on the surface of $\mathrm{TaS}{}_2$ without prior pulse excitation over continuous areas larger than $100\times 100\mathrm{nm}{}^2$ and macroscopic areas on the millimeter scale. We attribute the appearance of the mosaic phase to the presence of surface defects which cause the formation of the characteristic dense domain wall network. Based on our STM measurements, we further argue how the appearance of the metallic behavior in the mosaic phase could be explained by local stacking differences of the top layer. Thus we provide a potential avenue to explain the origin of the pulse-induced mosaic phase.

EuCuP and EuCuAs are members of a family of materials with candidates for realizing magnetically tuned electronic topology. In this work, the magnetic phase transitions and magnetoelastic coupling of EuCuP and EuCuAs have been investigated. Around the Curie temperature, the thermal expansion coefficient and specific-heat capacity of EuCuP are found to have two closely spaced features, at approximately 30 and 31.3 K, which may indicate a cascading magnetic transition or perhaps an electronic transition closely coupled to the magnetic ordering. The zero-field magnetoelastic coupling appears to be stronger in EuCuP than in EuCuAs, and field-dependent measurements suggest this is due to the dominant ferromagnetic interaction in EuCuP as opposed to antiferromagnetic order in EuCuAs below $T_{\mathrm{N}}=13.5\mathrm{K}$. Thermal expansion is anisotropic around $T_{\mathrm{N}}$ in EuCuAs, with a maximum expansion along [001] occurring above $T_{\mathrm{N}}$ prior to a stiffening of this $c$-axis component on cooling through $T_{\mathrm{N}}$; the expansion of the basal plane has a lambda-like peak centered slightly above $T_{\mathrm{N}}$. Matching these behaviors, the ac susceptibility data for EuCuAs suggest the presence of strong ferromagnetic correlations above $T_{\mathrm{N}}$ in the vicinity where the $c$-axis expansion peaks. Both compounds possess similar electrical and thermoelectric transport behaviors, with short-range magnetic order likely playing an important role above $T_{\mathrm{C}}$ or $T_{\mathrm{N}}$. Transport properties suggest these are predominantly hole doped due to intrinsic defects, and narrow-gap semiconducting or semimetallic behavior may be achievable if the underlying defects can be tuned.

Current phonon transport theory based on ground-state calculations has been successful in predicting thermal conductivity at room and medium temperatures but may misrepresent behavior at high temperatures. In this work, we predict the thermal conductivity ($\kappa$) of ZrC including electronic and phonon contributions from 300 to 3500 K, by including high-order phonon scattering; lattice expansion; temperature-dependent (TD) second-, third-, and fourth-order force constants (2FC, 3FC, and 4FC); and interband phonon conduction by using first principles. For the phonon transport, we find that four-phonon scattering (4ph) significantly reduces the phonon thermal conductivity (${\kappa }_{\mathrm{ph}}$), by as much as $\sim 75\%$ at 3500 K. After including 4ph scattering and all other factors, ${\kappa }_{\mathrm{ph}}$ shows a $\sim T^{-1.5}$ rather than $\sim T^{-1}$ dependence. TD 2FC decreases three-phonon scattering rates but increases 4ph rates by decreasing and increasing the scattering phase spaces, respectively. For 4ph phase space, the TD 2FC flattens phonon bands, and allows more redistribution-4ph processes $(1+2\to 3+4)$ to happen. The combination effect of TD 2FC and TD 4FC reduces 4ph rates of acoustic modes but increases those of optical modes. The TD 3FC and 4FC decrease the phonon scattering cross section and increase the ${\kappa }_{\mathrm{ph}}$ significantly (by 52% at 3500 K). The contribution from interband (Wigner) phonon conduction is small, even at ultrahigh temperatures. For electronic thermal transport, we find that it is sensitive to and can be changed by 20% by the TD lattice constants. The Lorenz number varies from 1.6 to $3.3\times {10}^{-8}\mathrm{W}\mathrm{\Omega }{\mathrm{K}}^{-2}$ at different temperatures. The theoretical prediction in the literature overpredicts ${\kappa }_{\mathrm{ph}}$ (e.g., $\sim 28\%$) and underpredicts the ${\kappa }_{\mathrm{el}}$ (e.g., $\sim 38\%$), resulting in an overall underprediction of $\kappa$ ($\sim 26\%$ at 1500 K). The impacts of grain size and defects are found to be strong, leading to the lower observed thermal conductivity in experiments.

Defects are crucial in determining a variety of material properties especially in low dimensions. In this work, we study point defects in monolayer α-phase Ruthenium (III) chloride $(\alpha -\mathrm{Ru}{\mathrm{Cl}}_3)$, a promising candidate to realize quantum spin liquid with nearly degenerate magnetic states. Our first-principles simulations reveal that Cl vacancies, Ru vacancies, and oxygen substitutional defects are the most energetically stable point defects. Besides, these point defects break the magnetic degeneracy: Cl vacancies and oxygen substitutional defects energetically favor the zigzag-antiferromagnetic configuration while Ru vacancies favor the ferromagnetic configuration, shedding light on understanding the observed magnetic structures and further defect engineering of magnetism in monolayer $\alpha -\mathrm{Ru}{\mathrm{Cl}}_3$. We further calculated their electronic structures and optical absorption spectra. The polarization symmetry of optical responses provides a convenient signature to identify the point defect types and long-range magnetic orders.

The electronic properties relevant to superconductivity are reported for bulk Ta-Hf and Ta-Zr body centered cubic alloys, in a large part to determine whether their properties are suitable for potential use in superconducting qbits. The body centered cubic unit cell sizes increase with increasing alloying. The results of magnetic susceptibility, electrical resistivity, and heat capacity characterization are reported. While elemental Ta is a type I superconductor, the alloys are type II strong coupling superconductors. Although decreasing the electron count per atom is expected to increase the density of electronic states at the Fermi level and thus the superconducting transition temperature ($T_c$) in these systems, we find that this is not sufficient to explain the significant increases in the superconducting $T_c$'s observed.

Nominally identical materials exchange net electric charge during contact through a mechanism that is still debated. “Mosaic models,” in which surfaces are presumed to consist of a random patchwork of microscopic donor/acceptor sites, offer an appealing explanation for this phenomenon. However, recent experiments have shown that global differences persist even between same-material samples, which the standard mosaic framework does not account for. Here, we expand the mosaic framework by incorporating global differences in the densities of donor/acceptor sites. We develop an analytical model, backed by numerical simulations, that smoothly connects the global and deterministic charge transfer of different materials to the local and stochastic mosaic picture normally associated with identical materials. Going further, we extend our model to explain the effect of contact asymmetries during sliding, providing a plausible explanation for the reversal of charging sign that has been observed experimentally.

${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT), a ferromagnetic van der Waals topological nodal line semimetal, has recently been studied. Using first-principles calculations and symmetry analysis, we investigate the effect of a uniaxial tensile strain on the nodal line and the resultant intrinsic anomalous Hall effect (AHE). Our results reveal their robustness to the in-plane strain. Moreover, the intrinsic AHE remains robust even for artificial adjustment of the atomic positions introduced to break the crystalline symmetries of FGT. When spin-orbit coupling is absent, the nodal line degeneracy remains intact if the inversion symmetry or the twofold screw symmetry is maintained, which reveals that the nodal line may emerge much more easily than previously predicted. This strong robustness is surprising and disagrees with the previous experimental report [Wang et al., Adv. Mater. 32, 2004533 (2020)], which shows that a uniaxial strain of <1% of the in-plane lattice constant can double the anomalous Hall resistance. This discrepancy implies that the present understanding of the AHE in FGT is incomplete. The possible origins of this discrepancy are discussed.

${\mathrm{EuPd}}_2{\mathrm{Si}}_2$ is a valence-fluctuating system undergoing a temperature-induced valence crossover at $T_V^{\prime }\approx 160\mathrm{K}$. We present the successful single-crystal growth using the Czochralski method for the substitution series ${\mathrm{EuPd}}_2{\left({\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x\right)}_2$, with substitution levels $x\le 0.15$. A careful determination of the germanium content revealed that only half of the nominal concentration is built into the crystal structure. From thermodynamic measurements it is established that $T_V^{\prime }$ is strongly suppressed for small substitution levels and antiferromagnetic order from stable divalent europium emerges for $x\gtrsim 0.10$. The valence transition is accompanied by a pronounced change of the lattice parameter $a$ of order 1.8%. In the antiferromagnetically ordered state below $T_N=47\mathrm{K}$, we find sizable magnetic anisotropy with an easy plane perpendicular to the crystallographic $c$ direction. An entropy analysis revealed that no valence fluctuations are present for the magnetically ordered materials. Combining the obtained thermodynamic and structural data, we construct a concentration-temperature phase diagram demonstrating a rather abrupt change from a valence-fluctuating to a magnetically ordered state in ${\mathrm{EuPd}}_2{\left({\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x\right)}_2$.

This study reports the development of ferrimagnetic cubic Heusler compound ${\mathrm{Mn}}_2{\mathrm{Fe}}_x\mathrm{Ga}$ (MFG) thin films with large perpendicular magnetic anisotropy (PMA). By growing MFG on a Cr buffer layer, a cubic ${\mathrm{X}}_{\mathrm{a}}$ phase with a tetragonal distortion $c/a\approx 1.04$ induced by the buffer layer was obtained in the range of $x=1.0–1.3$, leading to large PMA which exceeds $0.75\mathrm{MJ}{\mathrm{m}}^{-3}$ in stoichiometric ${\mathrm{Mn}}_2\mathrm{FeGa}$ $(x=1)$. Synchrotron Mössbauer spectroscopy revealed that these cubic MFG thin films show good chemical ordering close to the ${\mathrm{X}}_{\mathrm{a}}$-ordered state. First-principles calculations demonstrated that, in ${\mathrm{X}}_{\mathrm{a}}$-ordered cubic MFG, the characteristic electronic structure of Fe around the Fermi level causes a large uniaxial magnetocrystalline anisotropy under the tetragonal strain consistent with experiment. The half-metallic-like band structure of cubic MFG was also shown to be preserved under the strain. Cubic ${\mathrm{Mn}}_2\mathrm{FeGa}$ with its small saturation magnetization, large PMA, and possibility of being highly spin polarized make this material an ideal candidate for the development of magnetic random-access memory and other spintronic devices.

Hexagonal SiGe is a promising material for combining electronic and photonic technologies. In this paper, the energetic, structural, elastic, and electronic properties of the hexagonal polytypes ($2H, 4H$, and $6H$) of silicon and germanium are thoroughly analyzed under equilibrium conditions. For this purpose, we apply state-of-the-art density functional theory. The phase diagram, obtained in the framework of a generalized Ising model, shows that the diamond structure is the most stable under ambient conditions, but hexagonal modifications are close to the phase boundary, especially for Si. Our band structure calculations using the modified-Becke-Johnson–local-density-approximation (MBJLDA) and Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functionals predict significant changes in electronic states with hexagonality. While Si crystals are always semiconductors with indirect band gaps, the hexagonal Ge polytypes have direct band gaps. The branch-point energies of the Si polytypes appear in the fundamental gaps, while for the Ge crystals they are below the valence band maxima. Band alignment based on the branch-point energy leads to type-I heterocrystalline interfaces between Ge polytypes, where electrons and holes can be trapped in the layer with the higher hexagonality.

Valley-related multiple topological phase transitions have attracted significant attention because they provide significant opportunities for fundamental research and practical applications. Unfortunately, however, there is no real material as of yet that can realize valley-related multiple topological phase transitions. Here, through first-principles calculations and model analysis, we investigate the structural, magnetic, electronic, and topological properties of ${\text{VSi}X\text{N}}_4(X=\mathrm{C},\mathrm{Si},\mathrm{Ge},\mathrm{Sn},\mathrm{Pb})$ monolayers. ${\text{VSi}X\text{N}}_4$ monolayers are stable and intrinsically ferrovalley materials. Intriguingly, we found that built-in electric field and strain can induce valley-related multiple topological phase transitions in materials from valley semiconductor to valley half-semimetal, to valley quantum anomalous Hall insulator, to valley half-semimetal, and to valley semiconductor (or to valley metal). The nature of the topological phase transition is the built-in electric field and strain-induced band inversion between the $d_{xy}/d_{x^2-y^2}$ and $d_{z^2}$ orbitals at $K$ and $K^{\prime }$ valleys. Our findings not only reveal the mechanism of multiple topological phase transitions, but they also provide an ideal platform for the multifield manipulating the spin, valley, and topological physics. This will lead to alternative perspectives for spintronic, valleytronic, and topological nanoelectronic applications based on these materials.