The enthalpy of formation is an important thermodynamic property. Developing fast and accurate methods for its prediction is of practical interest in a variety of applications. Material informatics techniques based on machine learning have recently been introduced in the literature as an inexpensive means of exploiting materials data, and can be used to examine a variety of thermodynamics properties. We investigate the use of such machine learning tools for predicting the formation enthalpies of binary intermetallic compounds that contain at least one transition metal. We consider certain easily available properties of the constituting elements complemented by some basic properties of the compounds, to predict the formation enthalpies. We show how choosing these properties (input features) based on a literature study (using prior physics knowledge) seems to outperform machine learning based feature selection methods such as sensitivity analysis and LASSO (least absolute shrinkage and selection operator) based methods. A nonlinear kernel based support vector regression method is employed to perform the predictions. The predictive ability of our model is illustrated via several experiments on a dataset containing 648 binary alloys. We train and validate the model using the formation enthalpies calculated using a model by Miedema, which is a popular semiempirical model used for the prediction of formation enthalpies of metal alloys.

We show that electron-magnon interaction delocalizes the particle in a system with strong charge disorder. The analysis is based on results obtained for a single hole in the one-dimensional $t\text{−}J$ model. Unless there exists a mechanism that localizes spin excitations, the charge carrier remains delocalized even for a very strong charge disorder and shows subdiffusive motion up to the longest accessible times. Moreover, upon inspection of the propagation times between neighboring sites as well as a careful finite-size scaling, we conjecture that the anomalous subdiffusive transport may be transient and should eventually evolve into a normal diffusive motion.

We investigate how the quantum control of a two-level system (TLS) coupled to photons can modify and tune the TLS's photon absorption spectrum. Tuning and controlling the emission and the absorption are of great interest, e.g., for the development of efficient interfaces between stationary and flying qubits in modern architectures for quantum computation and quantum communication. We consider periodic pulse control, where the TLS is subjected to a periodic sequence of the near-resonant Rabi driving pulses, each pulse implementing a ${180}^{\circ }$ rotation. For small interpulse delays, the absorption spectrum features a pronounced peak of stimulated emission at the pulse frequency, similar satellite peaks with smaller spectral weights, and the net absorption peaks on the sides. As long as the detuning between the carrier frequency of the driving and the TLS transition frequency remains moderate, this spectral shape shows little change. Therefore, the pulse control allows shifting the absorption peak to a desired position and locks the overall absorption spectrum to the carrier frequency of the driving pulses. A detailed description of the spectrum and its evolution as a function time, the interpulse spacing, and the detuning is presented.

We use general hydrodynamic equations to determine the long-wavelength spin excitations in isotropic antiferromagnets in the presence of a homogeneous magnetization. The latter may be induced, such as in antiferromagnets in an external magnetic field, or spontaneous, such as in ferrimagnetic or canted phases that are characterized by the coexistence of antiferromagnetic and ferromagnetic order. Depending on the physical situation, we find propagating spin waves that are gapped in some cases and gapless in others, diffusive modes, or relaxational modes. The excitation spectra turn out to be qualitatively different depending on whether or not the homogeneous magnetization is a conserved quantity. The results lay the foundation for a description of a variety of quantum phase transitions, including the transition from a ferromagnetic metal to an antiferromagnetic one, and the spin-flop transitions that are observed in some antiferromagnets. They also are crucial for incorporating weak localization and Altshuler-Aronov effects into the descriptions of quantum phases in both clean and disordered magnetic metals.

We perform ab initio molecular dynamics simulations of liquid Zn near the melting point in order to study the longitudinal and transverse dynamic properties of the system. We find two propagating excitations in both of them in a wide range of wave vectors. This is in agreement with some experimental observations of the dynamic structure factor in the region around half the position of the main peak. Moreover, the two-mode structure in the transverse and longitudinal current correlation functions had also been previously observed in high pressure liquid metallic systems. We perform a theoretical analysis in order to investigate the possible origin of such two components by resorting to mode-coupling theories. They are found to describe qualitatively the appearance of two modes in the dynamics, but their relative intensities are not quantitatively reproduced. We suggest some possible improvements of the theory through the analysis of the structure of the memory functions. We also analyze the single-particle dynamics embedded in the velocity autocorrelation function, and explain its characteristics through mode-coupling concepts.

${\mathrm{BaMn}}_2{\mathrm{As}}_2$ is an antiferromagnetic insulator where a metal-insulator transition occurs with hole doping via the substitution of Ba with K. The metal-insulator transition causes only a small suppression of the Néel temperature ($T_{\mathrm{N}}$) and the ordered moment, suggesting that doped holes interact weakly with the Mn spin system. Powder inelastic neutron scattering measurements were performed on three different samples of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Mn}}_2{\mathrm{As}}_2$ with $x=0$, 0.125, and 0.25 to study the effect of hole doping and metallization on the spin dynamics. We compare the neutron intensities to a linear spin-wave theory approximation to the $J_1-J_2-J_c$ Heisenberg model. Hole doping is found to introduce only minor modifications to the exchange energies and spin gap. The changes observed in the exchange constants are consistent with the small drop of $T_{\mathrm{N}}$ with doping.

The origin of perpendicular magnetic anisotropy (PMA) in amorphous ${\mathrm{Nd}}_x{\mathrm{Co}}_{1-x}$ thin films is investigated using x-ray magnetic circular dichroism (XMCD) spectroscopy at the Co $L_{2,3}$ and Nd $M_{4,5}$ edges. The magnetic orbital and spin moments of the $3d$ cobalt and $4f$ neodymium electrons were measured as a function of the magnetic field orientation, neodymium concentration, and temperature. In all the studied samples, the magnetic anisotropy of the neodymium subnetwork is always oriented perpendicular to the plane, whereas the anisotropy of the orbital moment of cobalt is in the basal plane. The ratio $L_z/S_z$ of the neodymium $4f$ orbitals changes with the sample orientation angle, being higher and closer to the atomic expected value at normal orientation and smaller at grazing angles. This result is well explained by assuming that the $4f$ orbital is distorted by the effect of an anisotropic crystal field when it is magnetized along its hard axis, clearly indicating that the $4f$ states are not rotationally invariant. The magnetic anisotropy energy associated to the neodymium subnetwork should be proportional to this distortion, which we demonstrate is accessible by applying the XMCD sum rules for the spin and intensity at the Nd $M_{4,5}$ edges. The analysis unveils a significant portion of neodymium atoms magnetically uncoupled to cobalt, i.e., paramagnetic, confirming the inhomogeneity of the films and the presence of a highly disordered neodymium rich phase already detected by extended x-ray-absorption fine structure (EXAFS) spectroscopy. The presence of these inhomogeneities is inherent to the evaporation preparation method when the chosen concentration in the alloy is far from its eutectic concentrations. An interesting consequence of the particular way in which cobalt and neodymium segregates in this system is the enhancement of the cobalt spin moment which reaches 1.95 ${\mu }_B$ in the sample with the largest segregation.

Topological states of matter have been a subject of intensive studies in recent years because of their exotic properties such as the topologically protected edge and surface states. The initial studies were exclusively for electron systems. It is now known that topological states can also exist for other particles. Indeed, topologically protected edge states have already been found for phonons and photons. In spite of active searching for topological states in many fields, the studies in magnetism are relatively rare although topological states are apparently important and useful in magnonics. Here we show that the pyrochlore ferromagnets with the Dzyaloshinskii-Moriya interaction are intrinsic magnonic Weyl semimetals. Similar to the electronic Weyl semimetals, the magnon bands in a magnonic Weyl semimetal are nontrivially crossing in pairs at special points (called Weyl nodes) in momentum space. The equal energy contour around the Weyl node energy is made up of the magnon arcs on sample surfaces due to the topologically protected surface states between each pair of Weyl nodes. Additional Weyl nodes and magnon arcs can be generated in lower energy magnon bands when an anisotropic exchange interaction is introduced. Finally, magnonic chiral anomaly is realized under mutually perpendicular inhomogeneous electric and magnetic fields. The magnonic chiral anomaly results in linear electric spin and heat conductances that are experimentally detectable.

The nature of ordering of B and $\mathrm{B}{}^{\prime }$ transition metal ions in double perovskite compounds of general composition ${\mathrm{A}}_2\mathrm{BB}{}^{\prime }{\mathrm{O}}_6$ is an important topic, since the physical properties crucially depend on it. In the present study, considering the specific cases of ${\mathrm{La}}_2{\mathrm{CuSnO}}_6$ and ${\mathrm{La}}_2{\mathrm{CuIrO}}_6$, we carry out first-principles calculations with an aim to obtain microscopic understanding on this issue. Our study reveals the presence of Jahn-Teller distorted B ion, like ${\mathrm{Cu}}^{2+}$ helps in band energy stabilization of the layered ordering over the rocksalt ordering. However, introduction of magnetism may reverse this trend, especially in the presence of a second magnetic ion at $\mathrm{B}{}^{\prime }$ site, which may introduce a strong superexchange path involving B-O-$\mathrm{B}{}^{\prime }$, as found in the case of ${\mathrm{La}}_2{\mathrm{CuIrO}}_6$. We further find the spin-orbit coupling at Ir site drives the ${\mathrm{La}}_2{\mathrm{CuIrO}}_6$ compound to be a spin-orbit assisted Mott insulator.

We investigate the effects of coupling between the two zigzag edges of graphene nanoribbons on the Wigner crystallization of electrons and holes using a combination of tight-binding, mean-field Hubbard and many-body configuration interaction methods. We show that the thickness of the nanoribbon plays a crucial role in the formation of Wigner crystal. For ribbon widths smaller than 16 Å, increased kinetic energy overcomes the long-range Coulomb repulsion and suppresses the Wigner crystallization. For wider ribbons up to 38 Å wide, strong Wigner localization is observed for an even number of electrons, revealing an even-odd effect also found in the Coulomb-blockade addition spectrum. Interedge correlations are found to be strong enough to allow simultaneous crystallization on both edges, although an applied electric field can decouple the two edges. Finally, we show that Wigner crystallization can also occur for holes, albeit weaker than for electrons.

Vortices are widely studied in fields ranging from nonlinear optics to magnetic systems and superconductors. A vortex carries binary information corresponding to its topological charge, “plus” or “minus”, that can be used for information storage and processing. In spatially extended optical and condensed many-particle systems, achieving full control over vortex formation and its charge is particularly difficult and is not easily extended to systems of multiple vortices. Here we demonstrate the optical creation of multiplets of phase-locked vortices in polariton condensates using off-resonant excitation with ring-shaped pump beams. We find that the vorticity of one vortex can be controlled solely using the phase locking with other nearby vortices. Using this mechanism, we demonstrate how an existing vortex with a specific topological charge can be inverted to the oppositely charged state and how the charge state of one reference vortex can be copied to a neighboring vortex. This way we can optically encode any set of binary information onto a chain of vortices. We further show that this information can be modified later by using the possibility to address and manipulate each vortex in the chain individually.

We show that the existing theory does not give correct in-plane spectrum of superlattices at small in-plane momentum. Magnetoabsorption experiments demonstrate that the energy range of the parabolic region of the spectrum near the electron sub-band bottom is lower by an order of magnitude than the value predicted by the traditional approach. We developed a modified theory according to which the energy range of the parabolic region and carrier in-plane effective masses are determined by the effective band gap of the superlattice rather than by the bulk band gaps of the superlattice layers. The results of this theory are consistent with the experiment.

The recent experimental realization of high-quality ${\mathrm{WSe}}_2$ leads to the possibility of an efficient manipulation of its spin and valley degrees of freedom. Its electronic properties comprise a huge spin-orbit coupling, a direct band gap, and a strong anisotropic lifting of the degeneracy of the valley degree of freedom in a magnetic field. We evaluate its band structure and study ballistic electron transport through single and double junctions (or barriers) on monolayer ${\mathrm{WSe}}_2$ in the presence of spin $M_s$ and valley $M_v$ Zeeman fields and of an electric potential $U$. The conductance versus the field $M_s$ or $M_v$ decreases in a fluctuating manner. For a single junction, the spin $P_s$ and valley $P_v$ polarizations rise with $M=M_v=2M_s$, reach a value of more than $55\%$, and become perfect above $U\approx 45$ meV while for a double junction this change can occur for $U\ge 50$ meV and $M\ge 5$ meV. In certain regions of the $(M,U)$ plane $P_v$ becomes perfect. The conductance $g_c$, its spin-up and spin-down components, and both polarizations oscillate with the barrier width $d$. The ability to isolate various carrier degrees of freedom in ${\mathrm{WSe}}_2$ may render it a promising candidate for new spintronic and valleytronic devices.

The outstanding performance of cerium oxide $\left(\mathrm{Ce}{\mathrm{O}}_2\right)$ as ion conductor or catalyst strongly depends on the ease of $\mathrm{C}{\mathrm{e}}^{4+}\leftrightarrow \mathrm{C}{\mathrm{e}}^{3+}$ conversion and oxygen vacancy formation. An accurate description of $\mathrm{C}{\mathrm{e}}^{3+}$ and oxygen vacancy is therefore essential to further progress in this area. Using the HSE06 hybrid functional, we investigate the formation and migration of small polarons $\left(\mathrm{C}{\mathrm{e}}^{3+}\right)$ and their interaction with oxygen vacancies in $\mathrm{Ce}{\mathrm{O}}_2$, considering the small polaron and vacancy as independent entities. Oxygen vacancies are double donors and can bind up to two small polarons, forming a positively charged or neutral complex. We compute the electron self-trapping energy (i.e., energy gain when forming a small polaron), the small-polaron migration barrier, vacancy formation and migration energies, and vacancy-polaron binding energies. We find that small polarons weakly bind to oxygen vacancies, yet this interaction significantly contributes to the activation energy for hopping electronic conductivity. The results are compared with previous calculations and discussed in the light of available experimental data.

Diffusion of dopant in rutile is the fundamental process that determines the performance of many devices in which rutile is used. The diffusion behavior is known to be highly sample-dependent, but the reasons for this are less well understood. Here, rutile is studied by using first-principles calculations, in order to unravel the microscopic origins of the diverse diffusion behaviors for different doping elements. Anomalous diffusion behavior in the open channel along the [001] direction is found: larger atoms including Sc and Zr have lower energy barrier for diffusion via interstitial mechanism, apparently contradicting their known slow diffusion rate. To resolve this, we present an alternate model for the overall diffusion rate of the large-size dopants in rutile, showing that parallel to the [001] channel, it is limited by the formation of the interstitial states, whereas in the direction perpendicular to [001], it proceeds via a kick-out mechanism. By contrast, Co and Ni prefer to stay in the interstitial site of rutile, and have conventional diffusion with a very small migration barrier in the [001] channel. This leads to highly anisotropic and fast diffusion. The diffusion mechanisms found in the present study can explain the diffusion data measured by experiments.

We employ the nudged elastic band and the dimer methods within the standard density functional theory (DFT) formalism to study the migration of the oxygen and gallium vacancies in the monoclinic structure of $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$. We identify all the first nearest neighbor paths and calculate the migration barriers for the diffusion of the oxygen and gallium vacancies. We also identify the metastable sites of the gallium vacancies which are critical for the diffusion of the gallium atoms. The migration barriers for the diffusion of the gallium vacancies are lower than the migration barriers for oxygen vacancies by 1 eV on average, suggesting that the gallium vacancies are mobile at lower temperatures. Using the calculated migration barriers we estimate the annealing temperature of these defects within the harmonic transition state theory formalism, finding excellent agreement with the observed experimental annealing temperatures. Finally, we suggest the existence of percolation paths which enable the migration of the species without utilizing all the migration paths of the crystal.

Magnesium-doped nickel oxide (${\mathrm{Mg}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$) thin films are transparent over a wide ultraviolet-visible spectral range and over a wide Mg content range. However, the influence of the Mg dopant on the structure and properties of NiO films is poorly understood. In this work, the lattice distortion and the electronic structure of ${\mathrm{Mg}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$ ($0\le x\le 0.52$) thin films deposited on ultrasmooth sapphire substrates were investigated using synchrotron x rays. Films with higher Mg content had lower values of Debye temperature and atomic order parameter. The nearest Ni-O distance and the in-plane nearest Ni-Ni distance both expanded with increasing Mg content. The Ni $2p$ core-level spectra and the valence band spectra of the ${\mathrm{Mg}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$ thin films showed complex multiplet structures that were caused by the strong electron correlation in the Ni $3d$ states, where the spectral features are strongly dependent on both the distortion of the ${\mathrm{NiO}}_6$ octahedra and the Mg content. We found that the electronic structures are mainly a result of hybridization of Ni $3d$ and O $2p$ in the ${\mathrm{NiO}}_6$ octahedra and the reduction of the Zhang-Rice bound state following Mg doping. Finally, the flexibility of the band gap tuning in ${\mathrm{Mg}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$ thin films is explained.

We show that harmonic driving of either the magnitude or the phase of the nearest-neighbor hopping amplitude in a $p$-wave superconducting wire can generate modes localized near the ends of the wire. The Floquet eigenvalues of these modes can either be equal to $\pm 1$ (which is known to occur in other models) or can lie near other values in complex conjugate pairs, which is unusual; we call the latter anomalous end modes. All the end modes have equal probabilities of particles and holes. If the amplitude of driving is small, we observe an interesting bulk-boundary correspondence for the anomalous end modes: the Floquet eigenvalues and the peaks of the Fourier transform of these end modes lie close to the Floquet eigenvalues and momenta at which the Floquet eigenvalues of the bulk system have extrema.