Excitons with nonzero momenta and their energy dependence are important for time dependent phenomena such as transport properties or the coupling to external or internal fields, for example in electron energy loss spectroscopy. In this paper we calculate the momentum dependent energy landscape of excitons in rubrene single crystals. We show that singlet excitons exhibit a dispersion that is qualitatively similar to the electronic valence and conduction bands, namely a relatively large bandwidth along $\mathrm{\Gamma }$–Y and much flatter bands along $\mathrm{\Gamma }$–X and $\mathrm{\Gamma }$–Z. However, the absolute value of the bandwidths is significantly weaker than for both electronic bands. Triplet excitons, on the other hand, show much less dispersion and the exciton bands are much flatter than their singlet counterparts.

In this paper, we report insights into the local atomic and electronic structure of ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ epitaxial thin films with controlled properties on $\mathrm{Mg}{\mathrm{Al}}_2{\mathrm{O}}_4\left(001\right)$ substrates using pulsed laser deposition. Films grown at low temperatures ($<400{}^{\circ }\mathrm{C}$) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the ${\mathrm{Ni}}^{3+}$ valence state promoted at low growth temperature introduces delocalized $\mathrm{Ni}3d$-derived states at the Fermi level ($E_F$), responsible for the metallic state in ${\mathrm{NiCo}}_2{\mathrm{O}}_4$, while the $\mathrm{Co}3d$-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and $\sim +2$. Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the ${\mathrm{Ni}}^{2+}$ valence state in ${\mathrm{NiCo}}_2{\mathrm{O}}_4$. The $\mathrm{Ni}3d$-related state becomes localized away from $E_F$, opening a band gap for a semiconducting state. The band gap of the semiconducting ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ is estimated to be $<0.8\mathrm{eV}$, which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is $d\text{−}d$ dipole forbidden, and therefore, the semiconducting ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ still shows reasonable transparency in the infrared-visible light region. The present insights into the role of ${\mathrm{Ni}}^{3+}$ in determining the electronic structure and defect chemistry of ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ provide important guidance for use of ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ in electrocatalysis and opto-electronics.

$\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ is a charged ordered Peierls-like perovskite well known for its superconducting properties upon K or Pb doping. We present a study on the transport and electronic properties of $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ perovskite with strong Bi deficiency. We show that it is possible to synthesize $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ thin layers with Bi vacancies above 8–10% by depositing an yttrium-stabilized zirconia capping layer. By combining transport measurements with ab initio calculations we propose a scenario where the Bi vacancies give rise to the formation of polarons and suggest that the electrical transport is dominated by the migration of these polarons trapped at ${\mathrm{Bi}}^{3+}$ sites. Our paper shows that cation vacancies engineering, hardly explored to date, appears as a promising pathway to tune the electronic and functional properties of perovskites.

In Weyl semimetals the location of linear band crossings, the Weyl cones, is not bound to any high-symmetry point of the Brillouin zone, unlike the Dirac nodes in graphene. This flexibility is advantageous for valleytronics, where information is encoded in the valleys of the band structure when intervalley scattering is weak. However, if numerous Weyl cones coexist the encoded information can decohere rapidly because of band mixing. Here, we investigate how the helical isospin texture of Weyl cones affects valleytronics in heterojunctions of Weyl materials, and show how the chirality of this isospin texture can serve to encode information.

The GaAs/InGaAs quantum wells with a ferromagnetic $\delta \langle \mathrm{Mn}\rangle$ layer in GaAs barrier demonstrate a set of interesting spin-related phenomena originating from Mn-hole interaction. One of such phenomena is a spin-memory effect which consists of Mn spin polarization induced by interaction with vicinity spin-polarized holes generated under the exposure by short circularly polarized light pulses. Here long Mn spin relaxation time (∼5 ns) allows preserving the spin polarization of the entire system. In the present paper the spin-memory effect investigation was carried out by analyzing the polarization kinetics of quantum-well photoluminescence in the pump-probe technique. It was shown that the photoluminescence circular polarization degree is strongly affected by the magnetic interaction of holes with Mn atoms prepolarized by the pump pulse. In the case of antiparallel Mn and hole polarizations, magnetic interaction leads to decrease of circular polarization degree as compared with single-pulse excitation (so called Δ$P$ effect). Interestingly, the amplitude of hole-mediated Δ$P$ effect is strongly affected by the concentration of resident electrons in the quantum well. The latter was shown to be caused by the specific compliance with selection rules for optical transitions with the participation of unpolarized resident electrons and spin-polarized holes affected by Mn-hole interaction.

We introduce the electronic polarization originally defined in one-dimensional lattice systems to characterize two-dimensional topological insulators. The main idea is to use spiral boundary conditions which sweep all lattice sites in one-dimensional order. We find that the sign of the polarization changes at topological transition points of the two-dimensional Wilson-Dirac model (the lattice version of the Bernevig-Hughes-Zhang model) in the same way as in one-dimensional systems. Thus the polarization plays the role of “order parameter” to characterize the topological insulating state and enables us to study topological phases in different dimensions in a unified way.

The alkali halides are often used as optically-transparent pressure-transmitting media and thermal insulators in laser-heated diamond anvil cell experiments. High P–T equations of state for these materials would allow them to be used simultaneously as sensitive in situ pressure markers, making sample preparation and data analysis simpler. KCl is especially useful for this application because its high melting point, crystallographic simplicity, and low electron density are ideal for use in high-temperature x-ray diffraction experiments. However, the high-temperature static equation of state data for this material is limited in pressure to 8 GPa. As experiments routinely exceed 100 GPa in pressure and thousands of Kelvin, it is necessary for the equation of state of KCl to be determined experimentally at these conditions if it is to be used as an equation of state calibrant in the future. This paper combines new high-pressure, high-temperature data of the B2 phase of KCl (up to 167 GPa and 2400 K) with the previously published room-temperature data of Dewaele et al. [Phys. Rev. B 85, 214105 (2012)] to produce a Mie-Grüneisen-Debye thermal equation of state for this material. The room-temperature equation of state parameters are similar to those reported previously: $V_0=32.0\left(3\right) {\mathrm{cm}}^3/\mathrm{mole}, K_0=24\left(1\right)$ GPa, and $K_0^{\prime }=4.56\left(5\right)$. The thermal parameters, ${\gamma }_0$ and $q$ are 2.9(4) and 1.0(1), respectively. While a $q$ of 1 is expected, the ${\gamma }_0$ is higher than expected from previous calculations, lower pressure experimental data, and the available shockwave data. Thus, previously-reported equations of state underestimate the pressure of KCl at high temperatures.

The effects of hydrogen absorption on the effective magnetization $\left(4\pi M_{\mathrm{eff}}\right)$, gyromagnetic ratio $\left(\gamma \right)$, Gilbert damping constant $\left({\alpha }_{\mathrm{G}}\right)$, and the inhomogeneous linewidth broadening $\left(\mathrm{\Delta }{H}_0\right)$ in $\mathrm{Py}\left(x\right)/\mathrm{Y}$(16 nm)/Pd(15 nm) trilayer films ($x=2$, 3, 5, 8, 10, 20, 40 nm) were investigated with ferromagnetic resonance (FMR), transmission electron microscopy, and vibrating sample magnetometry. In the presence of a hydrogen atmosphere, the samples show a reduction of their FMR linewidth which is found to stem purely from a reduction of the inhomogeneous linewidth broadening. This is attributed to a rearrangement of atoms at the Py/Y interface in the presence of hydrogen, making the Py/Y interface more homogeneous. In addition, a reduction of $4\pi M_{\mathrm{eff}}$ was seen for all samples in the hydrogen atmosphere which is typical for an increase of the interfacial perpendicular magnetic anisotropy at the Py/Y interface.

Domain walls in superfluid ${}^3\mathrm{He}\text{−}B$ have gained renewed interest in light of experimental progress on confining helium in nanofabricated geometries. Here, we study the effect of strong-coupling corrections on domain wall width and interfacial tension by determining self-consistent solutions to spatially-dependent Ginzburg-Landau equations. We find that the formation of domain walls is generally energetically favored in strong coupling over weak coupling. Calculations were performed over a wide range of temperatures and pressures, showing decreasing interface energy with increasing temperature and pressure. This has implications for the observability of such domain walls in ${}^3\mathrm{He}\text{−}B$, which are of both fundamental interest and form the basis for spatially-modulated pair-density wave states, when stabilized by strong confinement.

We compute the phase diagram of a one-dimensional model of spinless fermions with pair hopping and nearest-neighbor interaction, first introduced by Ruhman and Altman, using the density-matrix renormalization group combined with various analytical approaches. Although the main phases are a Luttinger liquid of fermions and a Luttinger liquid of pairs, we also find remarkable phases in which only a fraction of the fermions are paired. In such a case, two situations arise: either fermions and pairs coexist spatially in a two-fluid mixture, or they are spatially segregated leading to phase separation. These results are supported by several analytical models that describe in an accurate way various relevant cuts of the phase diagram. Last, we identify relevant microscopic observables that capture the presence of these two fluids: while originally introduced in a phenomenological way, they support a wider application of two-fluid models for describing pairing phenomena.

We report x-ray diffraction, magnetic susceptibility, heat capacity, ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR), and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $\alpha \text{−}{\mathrm{HCrO}}_2$ (trigonal, space group: $R\overline{3}m$). This compound undergoes a clear magnetic transition at $T_{\mathrm{N}}\simeq 22.5$ K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat capacity. Local probes (NMR and $\mu \mathrm{SR}$) reveal a broad regime with slow fluctuations down to $0.7T_{\mathrm{N}}$, this temperature corresponding to the maximum in the $\mu \mathrm{SR}$ relaxation rate and in the NMR wipe-out. From the comparison with ${\mathrm{NaCrO}}_2$ and $\alpha \text{−}{\mathrm{KCrO}}_2$, the fluctuating regime and slow dynamics below $T_{\mathrm{N}}$ appear to be hallmarks of the TLHAF with $ABC$ stacking. We discuss the role of interlayer frustration, which may have impacted recent spin-liquid candidates with triangular geometry.

To gain insight into the spin-glass state of diluted magnetic semiconductors, we have examined the magnetic and electronic properties of ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ using density-functional theory as well as performed magnetization measurements on the $x=0.43$ and 0.55 systems to demonstrate a clear spin-glass transition consistent with previous literature. Using a generalized gradient approximation, we investigate the electronic and magnetic properties for $x=0$, 0.075, 0.15, 0.25, and 0.50 doping levels using the magnetic moment of ${\mathrm{Mn}}^{2+}$ as guide for the dependence of the Hubbard onsite potential on the electronic structure. Simulations on both ferromagnetic (FM) and antiferromagnetic (AFM) configurations yield a distinct AFM ground-state preference, which is consistent with a zero-magnetic-moment spin-glass state. Here an onsite potential of up to 8 eV on the Mn $3d$ orbitals is needed to harden the magnetic moment toward $S=5/2$. From our analysis of the electronic structure evolution with doping and onsite potential, we confirm the semiconducting state of the Mn-doped ZnTe as well as show that the presence of Mn incorporated into the ZnTe matrix at the Zn lattice site produces magnetic interactions through the Te ions with a distinct Te-Mn $pd$-orbital hybridization. Furthermore, we show that this hybridization is activated with the Mn doping above 0.25 concentration, which corresponds to the doping level in which the spin-glass transition begins to rise. Therefore, it is likely that the coupling of $pd$-orbital hybridization of the Mn and Te $p$ orbitals is a precursor to the enhancement of the spin-glass transition temperature.

Higher-order topological phases host robust boundary states at the boundary of the boundary, which can be interpreted from their boundary topology. In this work, we show that based on the surface states of a weak topological insulator, the interplay between superconductors and magnetic fields can lead to the various helical or chiral Majorana hinge modes and even corner modes. Particularly, the obtained higher-order topological superconductors (TSCs) can be attributed to their certain boundaries, surfaces, or hinges, which naturally behave as a TSC in DIII or D symmetry class. Correspondingly, these higher-order TSCs can be characterized by the boundary topological invariants, such as surface Chern numbers or surface $Z_2$ topological invariants for surface TSCs. Remarkably, some chiral hinge states naturally form the Majorana interference loop without complicated heterostructure, revealing the exceptional property of higher-order TSCs. Our models not only can be realized in iron-based superconductors with desired inverted band structures and superconducting pairings, but also pave the way to the implementation of quantum interference device from the higher-order topological matters.

The engineering of the optical response of materials is a paradigm that demands microscopic-level accuracy and reliable predictive theoretical tools. Here we compare and contrast the dispersive permittivity tensor, using both a low-energy effective model and density functional theory (DFT). As a representative material, phosphorene subject to strain is considered. Employing a low-energy model Hamiltonian with a Green's function current-current correlation function, we compute the dynamical optical conductivity and its associated permittivity tensor. For the DFT approach, first-principles calculations make use of the first-order random phase approximation. Our results reveal that although the two models are generally in agreement within the low-strain and low-frequency regime, the intricate features associated with the fundamental physical properties of the system and optoelectronics devices implementation such as band gap, Drude absorption response, vanishing real part, absorptivity, and sign of permittivity over the frequency range show significant discrepancies. Our results suggest that the random phase approximation employed in widely used DFT packages should be revisited and improved to be able to predict these fundamental electronic characteristics of a given material with confidence. Furthermore, employing the permittivity results from both models, we uncover the pivotal role that phosphorene can play in optoelectronics devices to facilitate highly programable perfect absorption of electromagnetic waves by manipulating the chemical potential and exerting strain and illustrate how reliable predictions for the dielectric response of a given material are crucial to precise device design.

The coexistence of ferroelectricity and spin-orbit coupling in noncentrosymmetric systems may allow for a nonvolatile control of spin textures in the momentum space by tuning the ferroelectric polarization. Based on first-principles calculations, supplemented with $\vec{k}·\vec{p}$ analysis, we report the emergence of the reversible spin textures in the two-dimensional (2D) $\mathrm{Ga}XY$ ($X=\mathrm{Se}$, Te; $Y=\mathrm{Cl}$, Br, I) monolayer compounds, a new class of 2D materials exhibiting in-plane ferroelectricity. We find that due to the large in-plane ferroelectric polarization, unidirectional out-of-plane spin textures are induced in the topmost valence band having giant spin splitting. Importantly, such out-of-plane spin textures, which can host a long-lived helical spin mode known as a persistent spin helix, can be fully reversed by switching the direction of the in-plane ferroelectric polarization. We show that the application of an external in-plane electric field oriented oppositely to the in-plane ferroelectric polarization direction is an effective method to flip the orientation of the out-of-plane spin textures. Thus, our findings can open avenues for interplay between the unidirectional out-of-plane spin textures and the in-plane ferroelectricity in 2D materials, which is useful for efficient and nonvolatile spintronic devices.

The NMR spectrum of FeSe shows a dramatic broadening on cooling towards the bulk nematic phase at $T_s=90$ K, due to the formation of a quasistatic, short-range-ordered nematic domain structure. However, a quantitative understanding of the NMR broadening and its relationship to the nematic susceptibility is still lacking. Here, we show that the temperature and pressure dependence of the broadening is in quantitative agreement with the mean-field Edwards-Anderson parameter of an Ising-nematic model in the presence of random-field disorder introduced by nonmagnetic impurities. Furthermore, these results reconcile the interpretation of NMR and Raman spectroscopy data in FeSe under pressure.

In recent years, the exotic properties of topological semimetals (TSMs) have attracted great attention and significant efforts have been made in seeking new topological phases and material realization. In this work, we propose a family of TSMs which harbors an unprecedented nodal line (NL) landscape consisting of a pair of concentric intersecting coplanar ellipses (CICEs) at half-filling. Meanwhile, the CICE at half-filling guarantees the presence of a second pair of CICEs beyond half-filling. Both CICEs are linked at fourfold degenerate points at zone boundaries. In addition, we identify the generic criteria for the existence of the CICE in a time-reversal-invariant spinless fermion system or a spinful system with negligible spin-orbital coupling. Consequently, 9 out of 230 space groups (SGs) are feasible for hosting CICEs whose location centers in the first Brillouin zone (BZ) are identified. We provide a simple model with SG $Pbam$ (No. 55) which exhibits CICEs, and the exotic intertwined drumhead surface states, induced by double band inversions. Finally, we propose a series of material candidates that host butterflylike CICE NLs, such as $\text{Zr}{X}_2$ ($X=\text{P}$, As), ${\mathrm{Tl}}_2{\mathrm{GeTe}}_5, {\mathrm{CYB}}_2$, and ${\mathrm{Al}}_2{\mathrm{Y}}_3$.

We demonstrate that spin-orbit coupling (SOC) strength for electrons near the conduction band edge in few-layer $\gamma$-InSe films can be tuned over a wide range. This tunability is the result of a competition between film-thickness-dependent intrinsic and electric-field-induced SOC, potentially, allowing for electrically switchable spintronic devices. Using a hybrid $\mathbf{k}·\mathbf{p}$ tight-binding model, fully parametrized with the help of density functional theory computations, we quantify SOC strength for various geometries of InSe-based field-effect transistors. The theoretically computed SOC strengths are compared with the results of weak antilocalization measurements on dual-gated multilayer InSe films, interpreted in terms of Dyakonov-Perel spin relaxation due to SOC, showing a good agreement between theory and experiment.