The fluorinated oxobenzo-bridged bisdithiazolyl radical FBBO was recently observed to undergo a pressure-induced Mott insulator-to-metal transition, suggesting a novel organic system for studying Mott physics. This report describes the electronic structure of this material in relation to the observed magnetic response at low pressures. Through analysis of antiferromagnetic resonance measurements, we identify a layered antiferromagnetic ordered phase below $T_N=13$ K at ambient pressure, which requires strong ferromagnetic coupling between nearest neighbours. The origin of such coupling is elucidated from both molecular and solid-state electronic-structure calculations, which suggest a minimal two-orbital model with strong Hund's-rule coupling. This layered phase is partially frustrated by a second-nearest-neighbor antiferromagnetic coupling, which drives a magnetic phase transition at elevated pressure. On the basis of the two-orbital model, we suggest the pressure-induced Mott transition to proceed via rehybridization of the frontier molecular orbitals, resulting in a half-filled insulator to quarter-filled metal crossover.

In the framework of density functional theory, scaling and the virial theorem are essential tools for deriving the exact properties of density functionals. Preexisting mathematical difficulties in deriving the virial theorem via scaling for periodic systems are resolved via a particular scaling technique. This methodology is employed to derive universal properties of the exchange-correlation energy functional for periodic systems.

We classify discrete-rotation symmetric topological crystalline superconductors (TCS) in two dimensions and provide the criteria for a zero-energy Majorana bound state (MBS) to be present at composite defects made from magnetic flux, dislocations, and disclinations. In addition to the Chern number that encodes chirality, discrete rotation symmetry further divides TCS into distinct stable topological classes according to the rotation eigenspectrum of Bogoliubov-de Gennes quasiparticles. Conical crystalline defects are shown to be able to accommodate robust MBS when a certain combination of these bulk topological invariants is nontrivial as dictated by the index theorems proved within. The number parity of MBS is counted by a ${\mathbb{Z}}_2$-valued index that solely depends on the disclination and the topological class of the TCS. We also discuss the implications for corner-bound Majorana modes on the boundary of topological crystalline superconductors.

We demonstrate a new band engineering strategy for the design of semiconductor perovskite ferroelectrics for photovoltaic and other applications from first principles. We study six ferroelectric solid solutions created by partially substituting ${\mathrm{Zn}}^{2+}$ for ${\mathrm{Nb}}^{5+}$ into the parent ${\mathrm{KNbO}}_3$ material, combined with charge compensation at the $A$ sites with different combinations of higher valence cations. Our first-principles calculations with the HSE06 functional yield a low band gap of only 2.1 eV for the 75%${\mathrm{KNbO}}_3$-25$\%$(${\mathrm{Sr}}_{1/2}{\mathrm{La}}_{1/2}$)(${\mathrm{Zn}}_{1/2}{\mathrm{Nb}}_{1/2}$)${\mathrm{O}}_3$ solid solution, and this can be lowered further by 0.6 eV under strain through polarization rotation. The large polarization, especially under strain, of these materials provides a charge separation route by the bulk photovoltaic effect that could potentially allow power conversion efficiency beyond the Shockley-Queisser limit. This band engineering strategy is applicable to other perovskites and should be realizable by standard solid-state synthesis and thin film growth methods.

We report a density functional theory investigation of $B$-site doped ${\mathrm{CaFeO}}_3$, a prototypical charge ordered perovskite. At 290 K, ${\mathrm{CaFeO}}_3$ undergoes a metal-insulator transition and a charge disproportionation reaction 2${\mathrm{Fe}}^{4+}\to {\mathrm{Fe}}^{5+}$+${\mathrm{Fe}}^{3+}$. We observe that when Zr dopants occupy a (001) layer, the band gap of the resulting solid solution increases to 0.93 eV due to a two-dimensional Jahn-Teller-type distortion, where ${\mathrm{FeO}}_6$ cages on the $xy$ plane elongate along $x$ and $y$ alternatively between neighboring Fe sites. Furthermore, we show that the rock-salt ordering of the ${\mathrm{Fe}}^{5+}$ and ${\mathrm{Fe}}^{3+}$ cations can be enhanced when the $B$-site dopants are arranged in a (111) plane due to a collective steric effect that facilitates the size discrepancy between the ${\mathrm{Fe}}^{5+}{\mathrm{O}}_6$ and ${\mathrm{Fe}}^{3+}{\mathrm{O}}_6$ octahedra and therefore gives rise to a larger band gap. The enhanced charge disproportionation in these solid solutions is verified by rigorously calculating the oxidation states of the Fe cations with different octahedral cage sizes. We therefore predict that the corresponding transition temperature will increase due to the enhanced charge ordering and larger band gap. The compositional, structural, and electrical relationships exploited in this paper can be extended to a variety of perovskites and nonperovskite oxides, providing guidance in the structural manipulation of electrical properties of functional materials.

We provide a phenomenological formula that describes the low-frequency optical absorption of charge carriers in disordered systems with localization. This allows us to extract, from experimental data on the optical conductivity, the relevant microscopic parameters determining the transport properties, such as the carrier localization length and the elastic and inelastic scattering times. This general formula is tested and applied here to organic semiconductors, where dynamical molecular disorder is known to play a key role in the transport properties. The present treatment captures the basic ideas underlying the recently proposed transient localization scenario for charge transport, extending it from the dc mobility to the frequency domain. When applied to existing optical measurements in rubrene field-effect transistors, our analysis provides quantitative evidence for the transient localization phenomenon. Possible applications to other disordered electronic systems are briefly discussed.

New understanding has been reached on competing forces acting to stabilize the polar surfaces of intrinsic ZnO. To compensate an accumulating dipole moment normal to ionic planes, the Madelung electrostatic force and the bonding ability of undercoordinated Zn ions compete to deplete more Zn atoms from the (0001) face and more O atoms from the $\left(\mathbf{000}\overline{\mathbf{1}}\right)$ face. In this competition, the former mechanism wins because it provides very low energy binding sites for O ions at face-centered-cubic registries on both surfaces. On the Zn-face, a distorted tetrahedral structure is formed, while on the O-face, a vertical Y structure is formed. In both structures, O ions form the topmost atomic plane. The reconstructed polar surfaces containing these novel structures have cleavage energy of 2.36 J/${\mathrm{m}}^2$, comparable to that of nonpolar surfaces and in agreement with experimental observation. An earlier structure found on the Zn-face annealed at below 1000 K is stabilized mainly by the Madelung electrostatic force and is a metastable structure of that surface.

We study the competition of Coulomb interaction and hybridization effects in the five-orbital Anderson impurity model by means of continuous time quantum Monte Carlo, exact diagonalization, and Hartree-Fock calculations. The dependence of the electronic excitation spectra and thermodynamic ground-state properties on the hybridization strength and the form of the Coulomb interaction is systematically investigated for impurity occupation number $N\approx 6$. With increasing hybridization strength, a Kondo resonance emerges, broadens and merges with some of the upper and lower Hubbard peaks. Concomitantly, there is an increase of charge fluctuations at the impurity site. In contrast to the single-orbital model, some atomic multiplet peaks and exchange split satellites persist despite strong charge fluctuations. We find that Hund's coupling leads to a state that may be characterized as an itinerant single atom magnet. As the filling is increased, the magnetic moment decreases, but the spin freezing phenomenon persists up to $N\approx 8$. When the hybridization is weak, the positions of atomic ionization peaks are rather sensitive to shifts of the impurity on-site energies. This allows to distinguish atomic ionization peaks from quasiparticle peaks or satellites in the electronic excitation spectra. On the methodological side we show that a comparison between the spectra obtained from Monte Carlo and exact diagonalization calculations is possible if the charge fluctuations are properly matched.

In this contribution the exact exchange potential $v_{\mathrm{x}}$ of density functional theory is examined for slabs such as graphene, for which one has a Bravais lattice in the $x$-$y$ directions, while the electrons are confined to the finite region $-L\le z\le L$ in the $z$ direction. It is demonstrated analytically that the exact $v_{\mathrm{x}}$ behaves as $-e^2/z$ for $z\gg L$. This result extends the corresponding statement of Horowitz, Proetto, and Rigamonti [Phys. Rev. Lett. 97, 026802 (2006)] for jellium slabs to slabs with arbitrary periodic density distributions. Application of the exact exchange to a Si(111) slab (within the Krieger-Li-Iafrate approximation) indicates that the corrugation of the exact $v_{\mathrm{x}}$ is more pronounced than that of the local density approximation for $v_{\mathrm{x}}$.

Defects have been intentionally introduced in a rubrene single crystal by means of two different mechanisms: ultraviolet ozone (UVO) exposure and x-ray irradiation. A complete drift-diffusion model based on the mobility edge (ME) concept, which takes into account asymmetries and nonuniformities in the semiconductor, is used to estimate the energetic and spatial distribution of trap states. The trap distribution for pristine devices can be decomposed into two well defined regions: a shallow region ascribed to structural disorder and a deeper region ascribed to defects. UVO and x ray increase the hole trap concentration in the semiconductor with different energetic and spatial signatures. The former creates traps near the top surface in the 0.3–0.4 eV region, while the latter induces a wider distribution of traps extending from the band edge with a spatial distribution that peaks near the top and bottom interfaces. In addition to inducing hole trap states in the transport gap, both processes are shown to reduce the mobility with respect to a pristine crystal.

We investigate the effects that a tilting of the magnetic field from the parallel direction has on the states of a one-dimensional Majorana nanowire. Particularly, we focus on the conditions for the existence of Majorana zero modes, uncovering an analytical relation (the projection rule) between the field orientation relative to the wire, its magnitude, and the superconducting parameter of the material. The study is then extended to junctions of nanowires, treated as magnetically inhomogeneous straight nanowires composed of two homogeneous arms. It is shown that their spectrum can be explained in terms of the spectra of two independent arms. Finally, we investigate how the localization of the Majorana mode is transferred from the magnetic interface at the corner of the junction to the end of the nanowire when increasing the arm length.

By considering the lattice dynamics between bond strain and transverse vibration, we establish a common model for ceramic and hybrid materials that exhibit negative thermal expansion (NTE). We show that pressure-induced softening, increase in NTE with pressure, and variation of NTE with temperature all arise naturally from the same mechanical model that gives rise to the NTE itself.

Low-temperature magnetic force microscopy was used to study the threshold of nucleation of spontaneous vortex-antivortex structures in superconductor/ferromagnet (S/F) hybrid systems. We investigated S/F heterostructures composed of Py as the magnetic material and Nb as the superconductor, with different thicknesses of Py and Nb. The condition for nucleation of spontaneous vortex-antivortex structures depends on fundamental parameters such as the superconducting penetration depth and the coherence length, as well as on the thickness of the superconducting film and the magnetic domain width. We compare our experimental results with those of existing theoretical models and provide an estimate of the threshold of the local out-of-plane component of the magnetization for different Py film thicknesses.

We theoretically study topological phase transitions in four generalized versions of the Kane-Mele-Hubbard model with up to $2\times {18}^2$ sites. All models are free of the fermion-sign problem allowing numerically exact quantum Monte Carlo (QMC) calculations to be performed to extremely low temperatures. We numerically compute the ${\mathbb{Z}}_2$ invariant and spin Chern number $C_{\sigma }$ directly from the zero-frequency single-particle Green's functions, and study the topological phase transitions driven by the tight-binding parameters at different on-site interaction strengths. The ${\mathbb{Z}}_2$ invariant and spin Chern number, which are complementary to each another, characterize the topological phases and identify the critical points of topological phase transitions. Although the numerically determined phase boundaries are nearly identical for different system sizes, we find strong system-size dependence of the spin Chern number, where quantized values are only expected upon approaching the thermodynamic limit. For the Hubbard models we considered, the QMC results show that correlation effects lead to shifts in the phase boundaries relative to those in the noninteracting limit, without any spontaneously symmetry breaking. The interaction-induced shift is nonperturbative in the interactions and cannot be captured within a “simple” self-consistent calculation either, such as Hartree-Fock. Furthermore, our QMC calculations suggest that quantum fluctuations from interactions stabilize topological phases in systems where the one-body terms preserve the $D_3$ symmetry of the lattice, and destabilize topological phases when the one-body terms break the $D_3$ symmetry.

We study the surface conductivity of a field-effect transistor (FET) made of a periodic array of spherical semiconductor nanocrystals (NCs). We show that electrons introduced to NCs by the gate voltage occupy one or two layers of the array. Computer simulations and analytical theory are used to study the array screening and corresponding evolution of electron concentrations of the first and second layers with growing gate voltage. When first-layer NCs have two electrons per NC the quantization energy gap between its $1s$ and $1p$ levels induces occupation of $1s$ levels of second-layer NCs. Only at a larger gate voltage electrons start leaving $1s$ levels of second-layer NCs and filling $1p$ levels of first-layer NCs. At substantially larger gate voltage, all electrons vacate the second layer and move to $1p$ levels of first-layer NCs. As a result of this nontrivial evolution of the two layers' concentrations, the surface conductivity of the FET nonmonotonically depends on the gate voltage. The same evolution of electron concentrations leads to nonmonotonic behavior of the differential capacitance.

Shubnikov–de Haas oscillations are observed in atomically flat hydrogen-terminated diamond surfaces with high-density hole carriers introduced by the electric field effect using an ionic liquid. The Shubnikov–de Haas oscillations depend only on the magnetic field component perpendicular to the diamond surface, thus providing evidence of two-dimensional Fermi surfaces. The effective masses estimated from the temperature dependence of the oscillations are close to the cyclotron effective masses of the valence band maxima in diamond. The estimated quantum scattering time is one order of magnitude longer than the transport scattering time and indicates that the carrier mobility is locally as high as several thousand cm${}^2$/V s at low temperature.

The equilibrium electrical conductivity of epitaxial ${\text{SrTiO}}_3$ (STO) thin films was investigated as a function of temperature, $950\le$ $T$/K $\le 1100$, and oxygen partial pressure, ${10}^{-23}\le$ $p$${\text{O}}_2$/bar $\le 1$. Compared with single-crystal STO, nanoscale thin-film STO exhibited with decreasing film thickness an increasingly enhanced electronic conductivity under highly reducing conditions, with a corresponding decrease in the activation enthalpy of conduction. This implies substantial modification of STO's point-defect thermodynamics for nanoscale film thicknesses. We argue, however, against such a finite-size effect and for an interface-proximity effect. Indeed, assuming trapping of oxygen vacancies at the STO surface and concomitant depletion of oxygen vacancies—and accumulation of electrons—in an equilibrium surface space-charge layer, we are able to predict quantitatively the conductivity as a function of temperature, oxygen partial pressure, and film thickness. Particularly complex behavior is predicted for ultrathin films that are consumed entirely by space charge.

We report on a study of the density response in doped Weyl semimetals or Weyl metals in the presence of an external magnetic field. We show that the applied field leads to a contribution to the density response, which is topological in nature and is closely related to the phenomenon of chiral anomaly. This contribution manifests in a nonanalytic nonclassical correction to the electronic compressibility and the plasmon frequency, proportional to the magnitude of the magnetic field. Such a nonanalytic correction to the electronic compressibility is a smoking-gun feature of Weyl metals, which clearly distinguishes them from ordinary ferromagnetic metals.

Transition metal dichalcogenides (TMDs) are considered as promising candidates for next generation of electronic and optoelectronic devices. To make use of these materials, for instance in field effect transistor applications, it is mandatory to know the detailed properties of contacts of such TMDs with metal electrodes. Here, we investigate the role of the contact structure on the electronic and transport properties of ${\text{metal-MoSe}}_2$ interfaces. Two different contact types, namely face and edge contacts, are studied. We consider both low (Sc) and high (Au) work function metals in order to thoroughly elucidate the role of the metal work function and the type of metal. First principles plane wave calculations and transport calculations based on nonequilibrium Green's function formalism reveal that the contact type has a large impact on the electronic and transport properties of ${\text{metal-MoSe}}_2$ interfaces. For the Sc electrode, the Schottky barrier heights are around 0.25 eV for face contact and bigger than 0.6 eV for edge contact. For the Au case, we calculate very similar barrier heights for both contact types with an average value of 0.5 eV. Furthermore, while the face contact is found to be highly advantageous as compared to the edge contact for the Sc electrode, the latter contact becomes a better choice for the Au electrode. Our findings provide guidelines for the fabrication of TMD-based devices.