We investigate the existence of linear magnetism in the metal organic framework materials MOF-74-Fe, MOF-74-Co, and MOF-74-Ni, using first-principles density functional theory. MOF-74 displays regular quasilinear chains of open-shell transition-metal atoms, which are well separated. Our results show that within these chains, for all three materials, ferromagnetic coupling of significant strength occurs. In addition, the coupling in between chains is at least one order of magnitude smaller, making these materials almost perfect one-dimensional (1D) magnets at low temperature. The interchain coupling is found to be antiferromagnetic, in agreement with experiments. While some quasi-1D materials exist that exhibit linear magnetism, mostly complex oxides, polymers, and a few other rare materials, they are typically very difficult to synthesize. The significance of our finding is that MOF-74 is very easy to synthesize and it is likely the simplest realization of the 1D Ising model in nature. MOF-74 could thus be used in future experiments to study 1D magnetism at low temperature.

Room temperature Fe and Co K-edge x-ray magnetic circular dichroism, synchrotron x-ray powder diffraction, and magnetization measurements were carried out to investigate the stability of the ferrimagnetic ground state in Co${}_x$Fe${}_{3-x}$O${}_4$ ($x=0$, 1, 1.5, and 2) ferrites under pressure up to about 30 GPa using diamond anvil cells. The x-ray magnetic circular dichroism at the Fe K-edge is observed to decrease continuously up to the highest reached pressure by ∼50$\%$ in Fe${}_3$O${}_4$ (at 25 GPa) and by ∼70$\%$ in Co${}_{1.5}$Fe${}_{1.5}$O${}_4$ (at 16 GPa) and Co${}_2$FeO${}_4$ (at 21 GPa). In CoFe${}_2$O${}_4$, the suppression by ∼80$\%$ of the x-ray magnetic circular dichroic signal occurs simultaneously at both Fe and Co K-edges between 24 and 27 GPa. However, a continuous decrease of the dichroic signal with pressure is only observed at the Co K-edge, whereas the Fe K-edge dichroic intensity drops drastically. The synchrotron x-ray powder diffraction measurements indicate the occurrence of structural phase transitions at critical pressures in coincidence with the suppression of ferrimagnetism. In CoFe${}_2$O${}_{4,}$ bulk magnetization measurements up to 26 GPa confirm the disappearance of ferrimagnetism and indicate an almost linear dependence of the magnetization with the magnetic field in the high pressure phase. We thus conclude that high-pressure CoFe${}_2$O${}_4$ is either paramagnetic or antiferromagnetic.

In-plane resistivity measurements as a function of temperature, magnetic field, and its orientation with respect to the crystallographic $ab$ plane were used to study the upper critical field, $H_{c2}$, of two overdoped compositions of the iron-based superconductor Ba(Fe${}_{1-x}$Ni${}_x$)${}_2$As${}_2$, $x=0.054$ and $x=0.072$. Measurements were performed using precise alignment (with accuracy less than 0.1${}^{\circ }$) of magnetic field with respect to the Fe-As plane. The dependence of the $H_{c2}$ on angle $\theta$ between the field and the $ab$ plane was measured in isothermal conditions in a broad temperature range. We found that the shape of $H_{c2}\left(\theta \right)$ clearly deviates from the Ginzburg-Landau functional form.

In-situ spectroscopy is applied to analyze the spatial distribution of point defects ($F$-color centers) formed in the nanometric vicinities of the trajectories of different swift heavy ions (from C to U) decelerated in LiF crystals in the electronic stopping regime. The formation of these defects results from the decay of self-trapped excitons appearing due to self-trapping and further relaxation of valence holes generated during beam induced ionization processes. A Monte Carlo model is applied to estimate the initial radial distribution of valence holes in the ion tracks by the time the ionization cascades are finished. It is demonstrated that there exists a large difference between this and the radial distribution of point defects detected in the experiments. This difference is described by fast and long-range diffusion of valence holes before their self-trapping. The effect illustrates the fundamental importance of the relaxation kinetics of the valence band on structure transformations in tracks. The average diffusion coefficient of holes $D_h$ and their conversion efficiencies into color centers β in relaxing tracks of different ions in LiF are estimated ($D_h$ = 0.016–1.4 cm${}^2$/s, β = 0.01–0.04).

A coupled spin deformation model for spin-crossover (SC) materials, consisting of distortable 2D square-shaped lattices, whose sites may be occupied by high-spin $\left(\mathrm{HS}\right)$ or low-spin $\left(\mathrm{LS}\right)$ atoms, is studied by Monte Carlo simulations. To be consistent with the experimental studies, we have studied shell-free and core-shell nanoparticles. In the case of shell-free nanoparticles, we constrained the surface of the nanoparticle (one layer) to be in the $\mathrm{HS}$ state from the electronic and the elastic point of view because the surface atoms have weaker ligand-field energy than those of the bulk. We then investigated the size effects and found that the thermal hysteresis width $\Delta T$ and the transition temperature $T_{\mathrm{eq}}$ follow the respective universal laws, $\Delta T\propto \sqrt{\left(L-L_0\right)}$ and $T_{\mathrm{eq}}\propto \sqrt{\ln (L-L_c)}$, where $L$ is the nanoparticle size. These laws hold independently of the sweeping rate of temperature. In a second stage, we studied the effect of a soft shell on the thermal properties of the core shell nanoparticle for which we have investigated the thermodynamic properties at various sizes of the shell. We find that the thermal hysteresis shifts downwards and the corresponding width increases; a result that contrasts with that of shell-free nanoparticles. In addition, we have observed that large shell size widths hinder the domain formation upon the first-order transition, although the transition is still of first order. These behaviors originate from the elastic stress produced by the shell on the bulk of the nanoparticle, and are identified through the spatial distribution of the internal stress upon the thermal transition. Moreover, we studied the effect of the shell size on the relaxation of the photoinduced metastable HS fraction at low temperature. At this end, a preliminary optimization of the structure of the nanoparticle is performed. We then evidenced that increasing the size of the shell results in an acceleration of the relaxation process. This behavior is in excellent agreement with recent data reported on core shell nanoparticles of Prussian Blue analogs.

The long-standing issue of a possible molten-salt-like state in the equimolar liquid alloy Cs-Au is discussed on the basis of an ab initio study of its collective dynamics at $T=913$ K and three pressures. The dispersions of the longitudinal and transverse collective excitations for all pressures were calculated by a combination of ab initio simulations and the method of generalized collective modes (GCMs). A fit-free GCM analysis of contributions to the concentration dynamic structure factors and the existing gap between the long-wavelength longitudinal (LO) and transverse (TO) optical branches give evidence that the liquid alloy CsAu at ambient pressure reveals typical molten-salt-like features of its collective dynamics. Increasing pressure causes the reduction of the LO-TO gap and leads to metallization of the compressed liquid CsAu.

We study the magnetic phase diagram of the $J_1$-$J_2$ Heisenberg antiferromagnet on a honeycomb lattice at the strongly frustrated point $J_2/J_1=1/2$ using large-scale Monte Carlo simulations. At low temperatures we find three different field regimes, each characterized by different broken discrete symmetries. In low magnetic fields up to $h_{c1}/J_1\approx 2.9$ the $Z_3$ rotational lattice symmetry is spontaneously broken while a 1/2-magnetization plateau is stabilized around $h_{c2}/J_1=4$. The collinear plateau state and the coplanar state in higher fields break the $Z_4$ translational symmetry and correspond to triple-$q$ magnetic structures. The intermediate phase $h_{c1}<h<h_{c2}$ has an interesting symmetry structure, breaking simultaneously the $Z_3$ and $Z_4$ symmetries. At much lower temperatures the spatial broken discrete symmetries coexist with the quasi-long-range order of the transverse spin components.

Temperature-dependent interplane resistivity ${\rho }_c\left(T\right)$ was measured for the iron-based superconductor BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ over a broad isoelectron phosphorus substitution range from $x=0$ to $x=0.60$, from nonsuperconducting parent compound to heavily overdoped superconducting composition with $T_c\approx 10\text{K}$. The features due to structural and magnetic transitions are clearly resolved in ${\rho }_c\left(T\right)$ of the underdoped crystals. A characteristic maximum in ${\rho }_c\left(T\right)$, found in the parent BaFe${}_2$As${}_2$ at around 200 K, moves rapidly with phosphorus substitution to high temperatures. At the optimal doping, the interplane resistivity shows $T$-linear temperature dependence without any crossover anomalies, similar to the previously reported in-plane resistivity. This observation is in stark contrast with dissimilar temperature dependencies found at optimal doping in electron-doped Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. Our finding suggests that despite similar values of the resistivity and its anisotropy, the temperature-dependent transport in the normal state is very different in electron and isoelectron-doped compounds. Similar temperature dependence of both in-plane and interplane resistivities, in which the dominant contributions are coming from different parts of the Fermi surface, suggests that scattering is the same on the whole Fermi surface. Since magnetic fluctuations are expected to be much stronger on the quasinested sheets, this observation may point to the importance of the interorbital scattering between different sheets.

KCa${}_2$Nb${}_3$O${}_{10}$ is a layered Dion–Jacobson-type perovskite important for a number of applications such as photocatalysis and as a building block for heteronanostructures. Despite this, some of its central electronic properties such as the band gap and dielectric function are not well understood. In this report we have attempted to determine the band gap and understand the electronic structure of KCa${}_2$Nb${}_3$O${}_{10}$ using density functional theory. Simultaneously, the band gap and loss function have been determined experimentally using valence electron energy loss spectroscopy. The theoretical results indicate that KCa${}_2$Nb${}_3$O${}_{10}$ is a direct band gap semiconductor with a sparse density of states close to the onset of the conduction band. The calculated band gap value of 3.1 eV is in excellent agreement with the 3.2±0.1 eV measured experimentally. The loss functions computed and experimentally determined show good agreement up to 20 eV, but the theoretical peak positions at higher energy do not agree with the experimental electron energy loss spectrum. These transitions originate from K-3$p$, Ca-3$p$, and Nb-4$p$ semicore states and their positions are not well described by Kohn-Sham eigenvalues. After a scissors shift of transitions due to these states by about 2.5 eV to higher energies we obtain good agreement with the experimental loss function and can thus explain the origin of all the features seen in the experimental electron energy loss spectrum.

We investigate the frequency and temperature dependence of the low-energy electron dynamics in a Landau Fermi liquid with a local self-energy. We show that the frequency and temperature dependencies of the optical conductivity obey universal scaling forms, for which explicit analytical expressions are obtained. For the optical conductivity and the associated memory function, we obtain a number of surprising features that differ qualitatively from the Drude model and are universal characteristics of a Fermi liquid. Different physical regimes of scaling are identified, with marked non-Drude features in the regime where $\hslash \omega \sim k_{\text{B}}T$. These analytical results for the optical conductivity are compared to numerical calculations for the doped Hubbard model within dynamical mean-field theory. For the “universal” low-energy electrodynamics, we obtain perfect agreement between numerical calculations and analytical scaling laws. Both results show that the optical conductivity displays a non-Drude “foot,” which could be easily mistaken as a signature of breakdown of the Fermi liquid, while it actually is a striking signature of its applicability. The aforementioned scaling laws provide a quantitative tool for the experimental identification and analysis of the Fermi-liquid state using optical spectroscopy, and a powerful method for the identification of alternative states of matter, when applicable.

We study how the $\mathbf{k}$ dependence in the self-energy affects the quasiparticle band structure and one-particle spectral functions. It is known that, in electron-gas-like materials, the self-energy depends significantly on $\mathbf{k}$ and there is a strong cancellation between the $\mathbf{k}$ dependence and the energy dependence of the self-energy. Analysis of the $GW$ self-energy reveals that, even in correlated materials with narrow bands, such as ${\mathrm{SrVO}}_3$, the self-energy significantly depends on $\mathbf{k}$. When the nonlocal effect is neglected, the quasiparticle band structure is over-renormalized, yielding too large mass enhancement compared to the case of k-dependent self-energy. The present result suggests that partial cancellation between the frequency dependence and the k dependence in the self-energy is important when discussing the quasiparticle band structure of correlated materials.

The $p$-type semiconductor copper oxide has three distinct phases Cu${}_2$O, CuO, and Cu${}_4$O${}_3$ with different morphologies and oxidation states of the copper ions. We investigate the structural stability and electronic band structure of all three copper oxide compounds using ab initio methods within the framework of density functional theory and consider different exchange correlation functionals. While the local density approximation (LDA) fails to describe the semiconducting states of CuO and Cu${}_4$O${}_3$, the $\mathrm{LDA}+U$ and HSE06 hybrid functional describe both compounds as indirect semiconductors. Using the HSE06 hybrid functional we calculate the electronic band structure in the full Brillouin zone for all three copper oxide compounds.

We propose a theory of optically induced currents in dielectrics and wide gap semiconductors exposed to a nonresonant ultrashort laser pulse with a stabilized carrier-envelope phase. To describe strong-field electron dynamics, equations for the density matrix have been solved self-consistently with equations for the macroscopic electric field inside the medium, which we model by a one-dimensional potential. We provide a detailed analysis of physically important quantities (band populations, macroscopic polarization, and transferred charge), which reveals that carrier-envelope phase control of the electric current can be interpreted as a result of quantum-mechanical interference of multiphoton excitation channels. Our numerical results are in good agreement with experimental data.

We show that when following a simple cavity design metric, a quantum well exciton-microcavity photon coupling constant can be made substantially larger than the exciton binding energy in GaAs-based optical microcavities. Consequently the very strong coupling regime becomes accessible in which a strong asymmetry between upper and lower polariton branches may be observed experimentally. We further show that the corresponding polariton dissociation and saturation boundaries on the phase diagram are much extended, which suggests the possibility of constructing a room temperature, high power exciton-polariton laser without resorting to wide band-gap semiconductors.

A portion of the electromagnetic wave spectrum between $\sim 0.1$ and $\sim 10$ terahertz (THz) suffers from the lack of powerful, effective, easy-to-use, and inexpensive emitters, detectors, and mixers. We propose a multilayer graphene—boron-nitride heterostructure which is able to emit radiation in the frequency range $\sim 0.1$–30 THz with the power density up to $\sim 0.5$ W/cm${}^2$ at room temperature. The proposed device is extremely thin, light, flexible, almost invisible, and may become an important member of the family of devices operating at terahertz frequencies.

We investigate theoretically the impact of variable smoothing of corners of single metallic, two-dimensional nanoparticles (faceted nanowires) on the characteristics of their plasmonic resonances—the resonant frequencies, eigenfunctions, polarizability, and near fields—using the method of surface integral equations. This impact is very strong: Increasing sharpness of the corners leads to a strong separation of the plasmonic eigenfrequencies, a progressing localization of the surface charge at the corners, and a strong near-field enhancement of the light intensity. Dependences of the above characteristics on the apex angle and the corner curvature show striking universal features. The limit of nonsmoothed corners is found to be practically meaningless.

We experimentally demonstrate the visualization of nanoscale dislocation strain fields in a thick silicon single crystal by a coherent diffraction imaging technique called Bragg x-ray ptychography. We also propose that the x-ray microbeam carrying orbital angular momentum is selectively produced by coherent Bragg diffraction from dislocation singularities in crystals. This work not only provides us with a tool for characterizing dislocation strain fields buried within extended crystals but also opens up new scientific opportunities in femtosecond spectroscopy using x-ray free-electron lasers.

Electron paramagnetic resonance (EPR) is used to identify the intrinsic electron small polaron in TiO${}_2$ crystals having the rutile structure. These self-trapped electrons are produced at very low temperature with 442 nm laser light. The defects form when a Ti${}^{4+}$ ion at a regular lattice site traps an electron and converts to a Ti${}^{3+}$ (3$d^1$) ion. They become thermally unstable above ∼15 K. An activation energy of 24 meV describes this “release” of the electrons (either by a hopping motion or directly to the conduction band). The $g$ matrix is obtained from the angular dependence of the EPR spectrum. Principal values are 1.9807, 1.9786, and 1.9563 and principal axes are along high-symmetry directions in the crystal. The unpaired electron occupies an $|x^2-y^2\rangle$ orbital where $x$ and $y$ are in the equatorial plane of the TiO${}_6$ unit and $y$ is the [001] direction. These intrinsic small polarons serve as a prototype for many of the defect-associated Ti${}^{3+}$ ions often observed in this material. They also can be used as a computational test case to evaluate the validity of different approximations presently being employed in density-functional-theory modeling of point defects in TiO${}_2$ and other transition-metal oxides.

We study the effect of ultrastrong cavity-qubit coupling on the low-lying excitations of a chain of coupled circuit quantum electrodynamic (QED) systems. We show that, in the presence of the onsite ultrastrong coupling, the photon hopping between cavities can be mapped to the Ising interaction between the lowest two levels of individual circuit QED of the chain. Based on our mapping, we predict two nearly degenerate ground states whose wave functions involve maximal entanglement between the macroscopic quantum states of the cavities and the states of qubits and identify that they are mathematically equivalent to Majorana bound states. Further, we devise a scheme for the dispersive measurement of the ground states using an additional resonator attached to one end of the circuit QED chain. Finally, we discuss the effects of disorders and local noises on the coherence of the ground states.