In epitaxial polar-oxide interfaces, conductivity sets in beyond a finite number of monolayers (ML). This threshold for conductivity is explained by accumulating sufficient electric potential to initiate charge transfer to the interface. Here we experimentally and theoretically study the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (111) interface where a critical thickness $t_c$ of nine epitaxial ${\mathrm{LaAlO}}_3$ ML is required to turn the interface from insulating to conducting and even superconducting. We show that $t_c$ decreases to 3 ML when depositing a cobalt overlayer (capping) and 6 ML for platinum capping. The latter result contrasts with the (001) interface, where platinum capping increases $t_c$ beyond the bare interface. Our density functional theory calculations with a Hubbard U term confirm the observed threshold for conductivity for the bare and the metal-capped interfaces. Interestingly, conductivity appears concomitantly with superconductivity for metal/${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (111) interfaces, in contrast with the metal/${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (001) interfaces where conductivity appears without superconductivity. We attribute this dissimilarity to the different orbital polarization of $e_g^{\prime }$ for the (111) versus $d_{xy}$ for the (001) interface.

We present a method for producing high quality $\mathrm{K}{\mathrm{Co}}_2{\mathrm{As}}_2$ crystals, stable in air and suitable for a variety of measurements. X-ray diffraction, magnetic susceptibility, electrical transport, and heat capacity measurements confirm the high quality and an absence of long range magnetic order down to at least 2 K. Residual resistivity values approaching 0.25 $\mu \mathrm{\Omega }$ cm are representative of the high quality and low impurity scattering, and a Sommerfeld coefficient $\gamma =7.3 \mathrm{mJ}/\mathrm{mol}$ ${\mathrm{K}}^2$ signifies weaker correlations than the Fe-based counterparts. Together with Hall effect measurements, angle-resolved photoemission experiments reveal a Fermi surface consisting of electron pockets at the center and corner of the Brillouin zone, in line with theoretical predictions and in contrast to the mixed carrier types of other pnictides with the $\mathrm{Th}{\mathrm{Cr}}_2{\mathrm{Si}}_2$ structure. A large, linear magnetoresistance of 200% at 14 T, together with an observed linear and hyperbolic, rather than parabolic, band dispersions are unusual characteristics of this compound.

We use an atomistic approach to study the electronic properties of monolayer and bilayer black phosphorus in the vicinity of a charged defect. In particular, we combine screened defect potentials obtained from first-principles linear response theory with large-scale tight-binding simulations to calculate the wave functions and energies of bound acceptor and donor states. As a consequence of the anisotropic band structure, the defect states in these systems form distorted hydrogenic orbitals with a different ordering from that in isotropic materials. For the monolayer, we study the dependence of the binding energies of charged adsorbates on the defect height and the dielectric constant of a substrate in an experimental setup. We also compare our results with an anisotropic effective mass model and find quantitative and qualitative differences when the charged defect is close to the black phosphorus or when the screening from the substrate is weak. For the bilayer, we compare results for charged adsorbates and charged intercalants and find that intercalants induce more prominent secondary peaks in the local density of states because they interact strongly with electronic states on both layers. These insights can be directly tested in scanning tunneling spectroscopy measurements and enable a detailed understanding of the role of Coulomb impurities in electronic devices.

A wide range of unconventional transport phenomena has recently been observed in single-crystal delafossite metals. Here, we present a theoretical framework to elucidate electron transport using a combination of first-principles calculations and numerical modeling of the anisotropic Boltzmann transport equation. Using ${\mathrm{PdCoO}}_2$ as a model system, we study different microscopic electron and phonon scattering mechanisms and establish the mean free path hierarchy of quasiparticles at different temperatures. We treat the anisotropic Fermi surface explicitly to numerically obtain experimentally-accessible transport observables, which bridge between the “diffusive,” “ballistic,” and “hydrodynamic” transport regime limits. We illustrate that the distinction between the “quasiballistic” and “quasihydrodynamic” regimes is challenging and often needs to be quantitative in nature. From first-principles calculations, we populate the resulting transport regime plots and demonstrate how the Fermi surface orientation adds complexity to the observed transport signatures in micrometer-scale devices. Our work provides key insights into microscopic interaction mechanisms on open hexagonal Fermi surfaces and establishes their connection to the macroscopic electron transport in finite-size channels.

We used the coherent grazing incidence small angle x-ray scattering technique to study the average kinetics and nanoscale dynamics during early-stage a-${\mathrm{WSi}}_2$ sputter deposition. The kinetic and dynamic properties are examined as a function of pressure, which is known to be a critical factor in determining final surface roughness. Surface growth kinetics and dynamics are characterized by time parameters extracted from the height-height structure factor and correlation functions. The roughness at a given length scale reaches a maximum before relaxing down to a steady state. The lateral length scale dependence and pressure dependence are then compared among the measured kinetics and dynamics time parameters. Surfaces grown at lower pressures are smoother, leading to longer correlation times. The time parameters to reach a dynamic steady state and a kinetic steady state show contrasting pressure dependence. A dynamic steady state is reached earlier than the kinetic steady state at high pressure. A more random deposition direction and lower kinetic energy at higher pressures can explain these phenomena, along with the hypothesis that larger nanoclusters form in vapor before arriving at the surface. A continuum model is applied to simulate the overall behavior with mixed success.

The anomalous Hall conductivity (AHC) and anomalous Nernst conductivity (ANC) are two prominent transport phenomena in ferromagnetic materials of a topological nature. Based on first-principles calculations, we evaluate the AHC and ANC of 35 cubic ferromagnetic antiperovskites (APVs) and observe giant AHC and ANC as large as 1128 S/cm and 6.31 ${\mathrm{AK}}^{-1}{\mathrm{m}}^{-1}$ for ${\mathrm{Co}}_3\mathrm{LiN}$ and ${\mathrm{Co}}_3\mathrm{PtN}$, respectively. Detailed analysis reveals that the origin of giant ANC can be attributed to the occurrence of Weyl nodes near the Fermi energy, as demonstrated for ${\mathrm{Co}}_3\mathrm{PtN}$. Interestingly, both the magnitude and sign of AHC and ANC can be tuned by changing the magnetization $\left(\mathbf{M}\right)$ directions, which could be applied to realize spin-caloritronics devices.

To better control the properties of emerging ferroelectric materials, it is important to understand the microscopic origins of their phase transitions. Here, we focus on ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ (CIPS), which is an emerging layered material that exhibits ferrielectric ordering well above room temperature (Curie temperature $T_{\mathrm{C}}$ $\sim 315\mathrm{K}$). When synthesized with Cu deficiencies, CIPS spontaneously segregates into ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ and ${\mathrm{In}}_{4/3}{\mathrm{P}}_2{\mathrm{S}}_6$ domains (CIPS-IPS), which form self-assembled heterostructures within the individual lamellae. This restructuring raises the Curie temperature up to $\sim 340\mathrm{K}$ for the highest Cu deficiency. In both CIPS and CIPS-IPS, the loss of polarization through the ferrielectric-paraelectric transition is driven by the movement of Cu ions within the lattice. Here, we studied the phase transitions in pure CIPS and CIPS-IPS $\left({\mathrm{Cu}}_{0.4}{\mathrm{In}}_{1.2}{\mathrm{P}}_2{\mathrm{S}}_6\right)$ through temperature-dependent Raman spectroscopy and x-ray diffraction (XRD). We measured the frequencies and linewidths of various cation and anion phonon modes and compared them with the extracted atomic positions from the refinement of XRD data. Our analysis shows that, in addition to the Cu cation movement, the anion octahedral cages experience significant strains as they deform to accommodate the redistribution of Cu ions upon heating. This results in several discontinuities in peak frequencies and linewidths close to 315 K in CIPS. In the CIPS-IPS heterostructure, this process begins $\sim 315\mathrm{K}$ and ends $\sim 330\mathrm{K}$.

The search for valley-dependent exotic properties could enable quantum device applications and therefore attracts rapidly increasing attention. Here, using first principles, we predict existence of the intrinsic valley-related multiple Hall effect in two-dimensional (2D) ${\mathrm{Cr}}_2\mathrm{COF}$ MXene. ${\mathrm{Cr}}_2\mathrm{COF}$ MXene is a ferromagnetic semiconductor with a direct bandgap locating at the edges of the hexagonal Brillouin zone, endowing it with valley physics. Protected by time-reversal symmetry breaking and out-of-plane magnetization, the valleys are polarized spontaneously. Moreover, the valley polarization is sizeable in both the valence and conduction bands, benefiting the observation of the anomalous valley Hall effect. More remarkably, due to strong spin-orbit interaction, the valley-dependent band inversion occurs naturally, and thus, the valley-polarized quantum anomalous Hall effect can be realized simultaneously in ${\mathrm{Cr}}_2\mathrm{COF}$ MXene, giving rise to the intriguing intrinsic valley-related multiple Hall effect. In addition, this multiple Hall effect can be effectively engineered under strain. Our findings greatly enrich the research on valley-dependent physical properties in 2D systems.

We use density-functional theory to study the ordered states of iridium manganese, demonstrating a simple but powerful method for describing magnetic structures and the transitions among them. As an illustrative example, the coupling of magnetism and crystal structure in IrMn is examined through a rigorous exploration of strain space. We then generate order parameters for the ground-state magnetic structures, providing a comprehensive framework for understanding antiferromagnetic variants and their magnetic anisotropy. In particular, we show how the most direct path between two variant structures can be analytically determined; this technique is used to equate two seemingly contradictory prior studies.

We examine the role of adding carbon to influence the superconducting properties of hydrogen sulfide. We consider a number of unique structures employing a simulation cell containing carbon, hydrogen, and sulfur atoms with randomized atomic coordinates. We also reproduced some structures reported in the literature. In general, we find the presence of carbon atoms does not raise the superconducting transition temperature of hydrogen sulfide compounds. We discuss the origin of this trend through a detailed examination of structural and electronic properties of carbon-hydrogen-sulfur materials.

Lead-free halide perovskites have recently emerged as promising photoactive materials due to their nontoxicity, stability, and unique photophysical properties. However, similar to lead-based perovskites, ion migration plays a significant role in determining their device potential and needs an in-depth understanding. Here, we study the role of A-site cation size and its nature on the ion transport dynamics in the $A_3{\mathrm{Bi}}_2{\mathrm{I}}_9$ [A= methylamine (MA) and formamidinium (FA)] perovskite single crystals (SCs) using the temperature-dependent electrochemical impedance spectroscopy. The activation energy was estimated as 0.54 and 0.66 eV with a corresponding trap density of $13\times {10}^9$ and $6.7\times {10}^9\mathrm{c}{\mathrm{m}}^{\text{–}3}$ for $\mathrm{M}{\mathrm{A}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9$ and $\mathrm{F}{\mathrm{A}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9\mathrm{SC}$, respectively. The higher activation energy in $\mathrm{F}{\mathrm{A}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9\mathrm{SCs}$ indicates suppressed ion migration which can be attributed to its large cation size leading to lattice enlargement and reduced rotational motion. Additionally, the less acidic and less polar character of the FA cation and strong hydrogen bond plays an important role to reduce the defect states. Furthermore, current-voltage (I-V) measurements also confirm the lower degree of hysteresis in $\mathrm{F}{\mathrm{A}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9\mathrm{SC}$ than that of $\mathrm{M}{\mathrm{A}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9$ SC, demonstrating lower ionic conductivity in these materials. Our paper provides a fundamental understanding of the role of the A-cation effect on the ionic and electrical responses in these lead-free SCs, which will be beneficial for designing the perovskite devices to improving their performance and overall stability.

Ultrathin ${\mathrm{Co}}_x{\mathrm{Ni}}_{1-x}$ alloy nanowires vertically embedded in ${\mathrm{SrTiO}}_3/{\mathrm{SrTiO}}_3\left(001\right)$ thin films were grown using a self-assembly approach based on sequential pulsed laser deposition. Due to vertical epitaxial coupling of the metallic and oxide phases, a large average tensile strain of up to 4% arises within the nanowires, which is evidenced using a combination of x-ray diffraction and transmission electron microscopy. Macroscopic magnetometry experiments are used to demonstrate that this huge deformation allows us to enhance the uniaxial anisotropy of the nanowires, leading to saturation field in excess of 1 T in the hard direction, large coercive field at low temperature along the easy axis, and to a blocking temperature exceeding 600 K in the case of nanowires with a diameter of 5 nm and 78% Co content. These data are complemented with angular dependent x-ray magnetic circular dichroism measurements at the Co and Ni $L_{2,3}$ edges. The value of the magnetic moment was extracted from these measurements by applying sum rules and the anisotropy of the orbital moment was investigated.

Hybrid perovskites have been at the forefront of condensed-matter research, particularly, in context of device applications primarily in relation to applications in the field of solar cells. In this article, we demonstrate that several new functionalities may be added to the arsenal of hybrid perovskites, in terms of external stimuli driven spin transitions as well as piezochromism. As an example, we study dimethylammonium manganese formate (DMAMnF), a hybrid perovskite investigated quite extensively experimentally. We show by employing first-principles density-functional theory $+$ $U$ calculations with the aid of ab initio molecular dynamics calculations that DMAMnF shows temperature and pressure driven spin transitions from a low spin $S=1/2$ to a high spin $S=5/2$ state. This transition is accompanied by a hysteresis, and we find that this hysteresis and the transition temperature are quite close to room temperature, which is desirable for device applications particularly in memory, display, and switching devices. The operating pressure is a few gigapascals, which is accessible in standard laboratory settings. We find that the cooperative behavior showing up as hysteresis accompanying the transition is driven primarily by elastic interactions, assisted by magnetic superexchange between Mn atoms. Last but not least we demonstrate that the spin transition is associated with piezochromism which could also be important for applications.

The high thermodynamic transition temperature (≈1080 K) of niobium dioxide (${\mathrm{NbO}}_2$) makes it advantageous in differentiating the effects of electronic excitation and heat accumulation in the photoinduced phase transition. Recently, a photoinduced nonthermal metallization of ${\mathrm{NbO}}_2$ film has been claimed [R. Rana, J. M. Klopf, J. Grenzer, H. Schneider, M. Helm, and A. Pashkin, Phys. Rev. B 99, 041102(R) (2019)], but the simultaneous lattice evolution still remains blank. In this paper, photoinduced evolution in lattice vibrations of a crystalline ${\mathrm{NbO}}_2$ film has been investigated using broadband coherent phonon spectroscopy. Signatures of two optical phonons at 155 and $185\mathrm{c}{\mathrm{m}}^{\text{–}1}$ are identified, and the corresponding lattice vibrations are found to be associated with the motions of Nb-Nb dimers. Pump fluence-dependent measurements reveal an electronic phase transition at a threshold fluence of $\approx 10\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$, while the lattice structure is preserved. A transient disordering in the lattice vibration is simultaneously observed, but lattice transformation is prevented probably by the large enthalpy barrier of the strong Peierls insulation. The accompanied drastic modulation in optical properties without atomic rearrangement also suggests the potential of ${\mathrm{NbO}}_2$ in improving the operation speed of related devices.

Hybrid improper ferroelectricity (HIF) allows the generation of an electrical polarization in the AA${}^{\prime }$BB${}^{\prime }{\mathrm{O}}_6$ double perovskite materials thanks to the combination of two nonpolar octahedral distortions. Nevertheless, for selected combination of the A/A${}^{\prime }$ cations a nonpolar incommensurate phase is observed with average symmetry $C2/m$. Thanks to a detailed crystallographic description of the incommensurate phase, based on electron, neutron, and x-ray diffraction data, we show that the incommensurate modulation is related to an abrupt change of the out-of-phase tilting along the $a$ and $c$ axis, whereas the tilting along the $b$ axis remains constant across the structure. By using group theory and symmetry analysis we show that we observe an incommensurate analog of HIF, which induces a hybrid improper dipolar density wave in ${\mathrm{NaLaCoWO}}_6$. The dipolar ordering is due also in this case to a trilinear invariant involving the commensurate and incommensurate octahedra tilting.

The physical properties of oxide heterointerfaces depend crucially on the electronic and ionic reconstructions across the interface. For the formation of a two-dimensional electron liquid (2DEL) at the interface of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (LAO/STO) systems, although it is well accepted that the conduction of amorphous-LAO/STO samples comes exclusively from the oxygen vacancies due to interfacial redox reactions, the relative contribution of ionic reconstruction and electronic reconstruction to the conduction at the polar crystalline LAO/STO interface has been a topic of debate. Herein, by resonant x-ray photoemission spectroscopy and x-ray absorption spectroscopy measurements, we investigated the evolution of the Ti $2p$ signal as well as the in-gap states at the diluted oxide interface of $\mathrm{La}{\mathrm{Al}}_{1-x}{\mathrm{Mn}}_x{\mathrm{O}}_3/\mathrm{STO}$ $(0\le x\le 0.3)$, where the doping of Mn into LAO gradually decreases the carrier density of the interfacial 2DEL but does not change the interface polarity. The spectroscopic results present direct evidence that the conduction from redox reaction at polar LAO/STO interfaces is suppressed by increasing the Mn doping level without the need of oxygen postannealing. A pure polarity discontinuity induced electronic reconstruction can be achieved by deliberately controlling the Mn doping level in LAO, which results in low-carrier, high-mobility, and spin-polarized 2DELs at $x=0.3$.