The functionalization of spin crossover materials as nanoscopic devices is a big challenge to overcome. It will entail a detailed exploration of why and how size reduction affects spin crossover behavior. We investigated the thermal spin transition of a nanoparticle in a core-shell structure as a function of particle size, the intensity of interactions between molecules, and also the thickness of the shell and its stiffness. The analysis was performed using an electroelastic model based on Monte Carlo methods on a distortable two-dimensional lattice, the sites of which are occupied by high-spin or low-spin atoms. Such analysis reflects the crucial influence of size as well as of the surrounding environment on the behavior of the spin transition, and it provides reliable explanations based on the elastic properties of the system.

Using density-functional theory calculations, we investigated the electronic structure and magnetic exchange interactions of the ordered 3$d$-5$d$ double perovskite ${\mathrm{Sr}}_2$${\mathrm{FeOsO}}_6$, which has recently drawn attention for interesting antiferromagnetic transitions. Our study reveals the vital role played by long-range magnetic exchange interactions in this compound. The competition between the ferromagnetic nearest-neighbor Os-O-Fe interaction and antiferromagnetic next-nearest-neighbor Os-O-Fe-O-Os interaction induces strong frustration in this system, which explains the lattice distortion and magnetic phase transitions observed in experiments.

We investigate the topological defects in phenomenological models describing mixtures of charged condensates with commensurate electric charges. Such situations are expected to appear for example in liquid metallic deuterium. This is modeled by a multicomponent Ginzburg-Landau theory where the condensates are coupled to the same gauge field by different coupling constants whose ratio is a rational number. We also briefly discuss the case where electric charges are incommensurate. Flux quantization and finiteness of the energy per unit length dictate that the different condensates have different winding and thus different number of (fractional) vortices. Competing attractive and repulsive interactions lead to molecule-like bound states between fractional vortices. Such bound states have finite energy and carry integer flux quanta. These can be characterized by the $\mathbb{C}{P}^1$ topological invariant that motivates their denomination as skyrmions.

The anomalous Hall effect (AHE) in perpendicularly magnetized $L{1}_0$-${\mathrm{Mn}}_{1.5}$Ga epitaxial films is investigated as a function of the degree of long-range chemical ordering at different temperatures. Our results provide conclusive evidence that the only correct AHE scaling is ${\rho }_{\mathrm{AH}}=a_0{\rho }_{xx0}+b{\rho }_{xx}^2$. Based on this scaling, we unambiguously identify the pronounced influence of the chemical ordering on both the intrinsic and extrinsic contributions to the AHE, and significant suppression of the extrinsic anomalous Hall conductivity by phonon and magnon scatterings. Our results help clarify the controversy over the AHE scaling and its microscopic mechanisms in this material. The experiments also demonstrate an approach for the study of the AHE by altering the long-range chemical ordering of magnetic materials.

We report on a comprehensive de Haas–van Alphen (dHvA) study of the iron pnictide ${\text{LaFe}}_2$${\text{P}}_2$. Our extensive density-functional band-structure calculations can well explain the measured angular-dependent dHvA frequencies. As salient feature, we observe only one quasi-two-dimensional Fermi-surface sheet; i.e., a hole-like Fermi-surface cylinder around $\Gamma$, essential for $s_{\pm }$ pairing, is missing. In spite of considerable mass enhancements due to many-body effects, ${\text{LaFe}}_2$${\text{P}}_2$ shows no superconductivity. This is likely caused by the absence of any nesting between electron and hole bands.

The electronic structure of the iron chalcogenide superconductor ${\text{FeSe}}_{1-x}$ was investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of ${\text{FeSe}}_{1-x}$, DFT calculations, and existing data on ${\text{FeSe}}_x{\text{Te}}_{1-x}$. The bands undergo a pronounced orbital-dependent renormalization, different from what was observed for ${\text{FeSe}}_x{\text{Te}}_{1-x}$ and any other pnictides.

The scientifically fascinating question of the spatial extent and bonding of the 5$f$ orbitals of Pu and its six different phases extends to its δ-retained alloys and the mechanism by which Ga and a number of other unrelated elements stabilize its low density face-centered-cubic (fcc) structure. This issue of phase stability is also important technologically because of its significance to Science-Based Stockpile Stewardship. Answering these questions requires information on the local order and structure around the Ga and its effects on the Pu. We have addressed this by characterizing the structures of a large number of Pu-Ga and two Pu-In and one Pu-Ce δ alloys, including a set of high purity δ Pu${}_{1-x}$Ga${}_x$ materials with 1.7 ≤ $x$ ≤ 6.4 at. % Ga that span the low [Ga] portion of the δ region of the phase diagram across the ∼3.3 at. % Ga metastability boundary, with extended x-ray absorption fine structure (EXAFS) spectroscopy that probes the element specific local structure, supplemented by x-ray pair distribution function analysis that gives the total local structure to longer distances, and x-ray diffraction that gives the long-range average structure of the periodic component of the materials. Detailed analyses indicate that the alloys at and below a nominal composition of ∼3.3 at. % Ga are heterogeneous and in addition to the δ phase also contain up to ∼20% of a novel, coexisting “σ” structure for Pu that forms in nanometer scale domains that are locally depleted in Ga. The invariance of the Ga EXAFS with composition indicates that this σ structure forms in Ga-depleted domains that result from the Ga atoms in the δ phase self-organizing into a quasi-intermetallic with a stoichiometry of Pu${}_{25-35}$Ga so that δ Pu-Ga is neither a random solid solution nor the more stable Pu${}_3$Ga + α. Above this 3.3 at. % Ga nominal composition, the δ Pu-Ga alloy is homogeneous, and no σ phase is present. These results that demonstrate that collective and cooperative behavior in the interactions between the alloy elements as well as local elastic forces are crucial in determining the properties of complex materials and contradict the conventional mechanism for martensitic transformations, in this case indicating that nucleation is not the rate limiting step.

We report an anomalous tunneling conductance with a zero-bias peak in flakes of superconducting 2$H$-${\mathrm{TaS}}_2$ detached through mechanical exfoliation. To explain the observed phenomenon, we construct a minimal model for a single unit cell layer of superconducting 2$H$-${\mathrm{TaS}}_2$ with a simplified two-dimensional Fermi surface and a sign-changing Cooper-pair wave function induced by Coulomb repulsion. Superconductivity is induced in the central $\Gamma$ pocket, where it becomes nodal. We show that weak scattering at the nodal Fermi surface, produced by nonperturbative coupling between tip and sample, gives Andreev states that lead to a zero-bias peak in the tunneling conductance. We suggest that reducing dimensionality down to crystals a few atoms thick could drive a crossover from conventional to sign-changing pairing in the superconductor 2$H$-${\mathrm{TaS}}_2$.

The electronic structure of strongly correlated systems is usually calculated by using an effective model Hamiltonian with a small number of states and an effective on-site interaction. The mode, however, neglects the frequency dependence of the interaction, which emerges as a result of dynamical screening processes not included in the model. The self-energy calculated in this kind of model within dynamical mean-field theory (DMFT) is usually assumed to contain on-site components only. To study the validity of model calculations for the simulation of realistic materials, we make a detailed comparison between the downfolded self-energy in a model Hamiltonian with static and dynamic on-site interaction and the full $ab$ initio self-energy for Fe and SrVO${}_3$ within the $GW$ approximation. We find that the model $GW$ self-energy shows weaker $\mathbf{k}$ (momentum) dependence than the ab initio $GW$ self-energy, which is attributed to the lack of the long-range interaction and of contributions from other electrons not included in the models. This weak $\mathbf{k}$ dependence is found to lead to an artificial narrowing of the quasiparticle band structure. Moreover, this band narrowing is stronger for the dynamic (frequency-dependent) interaction, due to a larger renormalization of the quasiparticle states. These findings indicate a crucial role of the $\mathbf{k}$ dependence of the self-energy and dynamical screening for the electronic structure of correlated systems. We also discuss the effects beyond the $GW$ approximation for correlated systems by comparing the $GW$ and DMFT results.

A Raman study on hydrogen donors in ZnO reveals the properties of bond-centered hydrogen ($H_{\mathrm{BC}}$) and hydrogen trapped in the oxygen vacancy ($H_{\mathrm{O}}$). The donors are identified by their electronic $1s\to 2s\left(2p\right)$ transitions and their characteristic local vibrational modes. The $H_{\mathrm{O}}$ donor was detected preferentially below the sample surface, where a high oxygen vacancy concentration is generated by thermal treatment of the samples. $H_{\mathrm{BC}}$ and $H_{\mathrm{O}}$ exhibit different thermal stabilities and their concentrations depend strongly on the sample history. Molecular hydrogen is an essential part of the interplay of the two hydrogen donors.

We present theoretical and experimental study of resonant Brillouin scattering of excitonic polaritons in one-dimensional multiple-quantum-well structure. We obtain general analytical results for the scattering light spectra, valid for arbitrary quantum-well arrangement. Application of our theory to the specific case of short-period superlattice shows a perfect quantitative agreement with experimental results for the height, width, and position of the Brillouin scattering peaks and allows us to determine the energy, radiative, and nonradiative decay rates of quantum-well excitons. We reveal the signatures of excitonic polariton formation in the scattering spectra and show that the spectral width and height are strongly sensitive to the number of wells in the sample.

Low-disorder and high-mobility two-dimensional (2D) electron (or hole) systems confined in semiconductor heterostructures undergo an apparent metal-insulator transition (MIT) at low temperatures as the carrier density ($n$) is varied. In some situations, the 2D MIT can be caused at a fixed low carrier density by changing an externally applied in-plane magnetic field parallel to the 2D layer. The goal of the current work is to obtain the critical density ($n_c$) for the 2D MIT with the system being an effective metal (Anderson insulator) for density $n$ above (below) $n_c$. We study the 2D MIT phenomenon theoretically as a possible strong localization induced crossover process controlled by the Ioffe-Regel criterion, $k_{F}l=1$, where $k_F\left(n\right)$ is the 2D Fermi wave vector and $l\left(n\right)$ is the disorder-limited quantum mean free path on the metallic side. Calculating the quantum mean free path in the effective metallic phase from a realistic Boltzmann transport theory including disorder scattering effects, we solve the integral equation (with $l$ depending on $n$ through multidimensional integrals) defined by the Ioffe-Regel criterion to obtain the nonuniversal critical density $n_c$ as a function of the applicable physical experimental parameters including disorder strength, in-plane magnetic field, spin and valley degeneracy, background dielectric constant and carrier effective mass, and temperature. The key physics underlying the nonuniversal parameter dependence of the critical density is the density dependence of the screened Coulomb disorder. Our calculated results for the crossover critical density $n_c$ appear to be in qualitative and semiquantitative agreement with the available experimental data in different 2D semiconductor systems lending credence to the possibility that the apparent 2D MIT signals the onset of the strong localization crossover in disordered 2D systems. We also compare the calculated critical density obtained from the Ioffe-Regel criterion with that obtained from a classical percolation theory, concluding that experiments support the Ioffe-Regel criterion for the 2D MIT crossover phenomena.

Results of positronium (Ps) emission into vacuum from self-assembled metal-organic frameworks (MOFs) are presented and discussed in detail. Four different MOF crystals are considered, namely, MOF-5, IRMOF-8, ${\mathrm{ZnO}}_4$(FMA)${}_3$, and IRMOF-20. The measurements reveal that a fraction of the Ps is emitted into vacuum with a distinctly smaller energy than what one would expect for Ps localized in the MOFs' cells. Only calculations considering the Ps delocalized in a Bloch state can reproduce the measured Ps emission energy providing a robust demonstration of wave function delocalization in quantum mechanics. We show how the Bloch state population can be controlled by tuning the initial positron beam energy. Therefore, Ps in MOFs can be used both to simulate the dynamics of delocalized excitations in materials and to probe the MOFs for their advanced characterization.

The interaction between phonons and photons is investigated theoretically in a phoxonic cavity inside a corrugated nanobeam waveguide presenting band gaps for both electromagnetic and elastic waves. The structure is made by drilling periodic holes on a silicon nanobeam with lateral periodic stubs and the tapered cavity is constructed by changing gradually the geometrical parameters of both the holes and stubs. We show that this kind of cavity displays localized phonons and photons inside the gaps, which can enhance their interaction and also promotes the presence of many optical confined modes with high quality factor. Using the finite-element method, we demonstrate that with appropriate design of the tapering construction, one can control the cavity modes frequency without altering significantly the quality factor of the photonic modes. By changing the tapering rates over the lattice constants, we establish the possibility of shifting the phononic cavity modes frequency to place them inside the desired gap, which enhances their confinement and increases the mechanical quality factor while keeping the strength of the optomechanic coupling high. In our calculations, we take account of both mechanisms that contribute to the acousto-optic interaction, namely photoelastic and interface motion effects. We show that in our case, these two effects can contribute additively to give high coupling strength between phononic and photonic cavity modes. The calculations of the coupling coefficient which gives the phonon-photon coupling strength give values as high as 2 MHz while photonic cavity modes display quality factor of 10${}^5$ and even values up to 3.4 MHz but with smaller photonic quality factors.

The absorption of electromagnetic waves by a thin dielectric slab backed by a metasurface has been comprehensively studied and discussed at microwave frequencies. The metasurface consists of a metallic plate decorated with a periodic distribution of coaxial- or annular-type cavities. Analytical expressions for the absorbance have been obtained by using a mode-matching method. For $P$-polarized waves it is predicted that a low-frequency peak of perfect absorption is possible by properly choosing the lossy component of the permittivity of the dielectric slab, with the position of this peak being easily tunable by the cavity length. It is also shown that non-Bravais lattices, containing several cavities in the unit cell, and the excitation of guided waves by the periodic array of resonators are complementary absorption mechanisms in the studied structures.

We present a quantum Monte Carlo method capable of sampling the full density matrix of a many-particle system at finite temperature. This allows arbitrary reduced density matrix elements and expectation values of complicated nonlocal observables to be evaluated easily. The method resembles full configuration interaction quantum Monte Carlo but works in the space of many-particle operators instead of the space of many-particle wave functions. One simulation provides the density matrix at all temperatures simultaneously, from $T=\infty$ to $T=0$, allowing the temperature dependence of expectation values to be studied. The direct sampling of the density matrix also allows the calculation of some previously inaccessible entanglement measures. We explain the theory underlying the method, describe the algorithm, and introduce an importance-sampling procedure to improve the stochastic efficiency. To demonstrate the potential of our approach, the energy and staggered magnetization of the isotropic antiferromagnetic Heisenberg model on small lattices, the concurrence of one-dimensional spin rings, and the Renyi $S_2$ entanglement entropy of various sublattices of the $6\times 6$ Heisenberg model are calculated. The nature of the sign problem in the method is also investigated.

We present a mechanism to improve the conductivity of carbon nanotube (CNT) networks by improving the conductance between CNTs of different chirality. We argue generally that a weak perturbation, e.g., from naturally present environmental impurities, can greatly improve the intertube conductance by allowing momentum-conserving tunneling. The mechanism is verified with a tight-binding model, and investigated for a network containing a range of chiralities. We conclude that improving the CNT purity by removing impurities may, counterintuitively, attenuate the overall network conductance.

The precession of electron spins in a perpendicular magnetic field, the so-called Hanle effect, provides an unique insight into spin properties of a nonmagnetic material. In practice, the spin signal is fitted to the analytic solution of the spin Bloch equation, which accounts for diffusion, relaxation, and precession effects on spin. The analytic formula, however, is derived for an infinite length of the 1D spin channel. This is usually not satisfied in the real devices. The finite size of the channel length $l_{\mathrm{dev}}$ leads to confinement of spins and increase of spin accumulation. Moreover, reflection of spins from the channel ends leads to spin interference, altering the characteristic precession line shape. In this work we study the influence of finite $l_{\mathrm{dev}}$ on the Hanle line shape and show when it can lead to a twofold discrepancy in the extracted spin coefficients. We propose the extension of the Hanle analytic formula to include the geometrical aspects of the real device and get an excellent agreement with a finite-element model of spin precession, where this geometry is explicitly set. We also demonstrate that in the limit of a channel length shorter than the spin relaxation length ${\lambda }_s$, the spin diffusion is negligible and a 0D spin transport description with the Lorentzian precession dependence applies. We provide a universal criterion for which transport description, 0D or 1D, to apply depending on the ratio $l_{\mathrm{dev}}/{\lambda }_s$ and the corresponding accuracy of such a choice.

Hexagonal $R\mathrm{Mn}{\mathrm{O}}_3$ manganites are improper ferroelectrics in which the electric polarization is a by-product of the tripling of the unit cell. In $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_3$, there is a second transition at $\sim$920 K whose nature remains unexplained. We argue that this transition can be seen as a sort of hidden order in which a residual symmetry displayed by the trimerization order parameter is spontaneously broken. This additional order gives rise to 12 structural domains instead of six, and structural domain boundaries that can be either ferroelectric or nonferroelectric domain walls. We also suggest a generic $P{6}_{3}cm\leftrightarrow P3c1\leftrightarrow P\overline{3}c1$ phase diagram, fully realized in $\mathrm{In}\mathrm{Mn}{\mathrm{O}}_3$.