Magnetic properties of NiO have been studied in the multimegabar pressure range by nuclear forward scattering of synchrotron radiation using the 67.4 keV Mössbauer transition of ${}^{61}\mathrm{Ni}$. The observed magnetic hyperfine splitting confirms the antiferromagnetic state of NiO up to 280 GPa, the highest pressure where magnetism has been observed so far, in any material. Remarkably, the hyperfine field increases from 8.47 T at ambient pressure to $\sim 24$ T at the highest pressure, ruling out the possibility of a magnetic collapse. A joint x-ray diffraction and extended x-ray-absorption fine structure investigation reveals that NiO remains in a distorted sodium chloride structure in the entire studied pressure range. Ab initio calculations support the experimental observations, and further indicate a complete absence of Mott transition in NiO up to at least 280 GPa.

We quantitatively illustrate the fundamental limit that exciton-exciton annihilation (EEA) may impose on the light emission of monolayer transition metal dichalcogenide (TMDC) materials. The EEA in TMDC monolayers shows a dependence on the interaction with substrates as its rate increases from $0.1\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$ ($0.05\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$) to $0.3\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$ ($0.1\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$) with the substrates removed for $\mathrm{W}{\mathrm{S}}_2$ ($\mathrm{Mo}{\mathrm{S}}_2$) monolayers. It turns to be the major pathway of exciton decay and dominates the luminescence efficiency when the exciton density is beyond ${10}^{10}\mathrm{c}{\mathrm{m}}^{-2}$ in suspended monolayers or ${10}^{11}\mathrm{c}{\mathrm{m}}^{-2}$ in supported monolayers. This sets an upper limit on the density of injected charges in light-emission devices for the realization of optimal luminescence efficiency. The strong EEA rate also dictates the pumping threshold for population inversion in the monolayers to be $12–18\mathrm{MW}/\mathrm{c}{\mathrm{m}}^2$ (optically) or $2.5–4\times {10}^5\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ (electrically).

Using angle-resolved photoemission spectroscopy (ARPES), we studied the effect of the impurity potential on the electronic structure of ${\mathrm{FeTe}}_{0.5}{\mathrm{Se}}_{0.5}$ superconductor by substituting 10% of Ni for Fe, which leads to an electron doping of the system. We could resolve three hole pockets near the zone center and an electron pocket near the zone corner in the case of ${\mathrm{FeTe}}_{0.5}{\mathrm{Se}}_{0.5}$, whereas only two hole pockets near the zone center and an electron pocket near the zone corner are resolved in the case of ${\mathrm{Fe}}_{0.9}{\mathrm{Ni}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$, suggesting that the hole pocket having predominantly the $xy$ orbital character is very sensitive to the impurity scattering. Upon electron doping, the size of the hole pockets decreases and the size of the electron pockets increases as compared to the host compound. However, the observed changes in the size of the electron and hole pockets are not consistent with the rigid-band model. Moreover, the effective mass of the hole pockets is reduced near the zone center and of the electron pockets is increased near the zone corner in the doped ${\mathrm{Fe}}_{0.9}{\mathrm{Ni}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ as compared to ${\mathrm{FeTe}}_{0.5}{\mathrm{Se}}_{0.5}$. We refer these observations to the changes of the spectral function due to the effect of the impurity potential of the dopants.

We investigate the ground state properties of the gaseous mixture of a single species of bosons and fermions at zero temperature, where bosons are major in population over fermions, and form the Bose-Einstein condensate (BEC). The boson-boson and boson-fermion interactions are assumed to be weakly repulsive and attractive, respectively, while the fermion-fermion interaction is absent due to the Pauli exclusion for the low energy $s$-wave scattering. We treat fermions as a gas of polarons dressed with Bogoliubov phonons, which is an elementary excitation of the BEC, and evaluate the ground state properties with the method developed by Lemmens, Devreese, and Brosens (LDB) originally for the electron polaron gas, and also with a general extension of the Lee-Low-Pines theory for many-body systems (eLLP), which incorporates the phonon drag effects as in the original LLP theory. The formulation of eLLP is developed and discussed in the present paper. The binding (interaction) energy of the polaron gas is calculated in these methods and shown to be finite (negative) for the dilute gas of heavy fermions with attractive boson-fermion interactions, though the suppression by the many-body effects exists.

Unlike the local density approximation (LDA) and the generalized gradient approximation (GGA), calculations with meta-generalized gradient approximations (meta-GGA) are usually done according to the generalized Kohn-Sham (gKS) formalism. The exchange-correlation potential of the gKS equation is nonmultiplicative, which prevents systematic comparison of meta-GGA band structures to those of the LDA and the GGA. We implement the optimized effective potential (OEP) of the meta-GGA for periodic systems, which allows us to carry out meta-GGA calculations in the same KS manner as for the LDA and the GGA. We apply the OEP to several meta-GGAs, including the new SCAN functional [Phys. Rev. Lett. 115, 036402 (2015)]. We find that the KS gaps and KS band structures of meta-GGAs are close to those of GGAs. They are smaller than the more realistic gKS gaps of meta-GGAs, but probably close to the less-realistic gaps in the band structure of the exact KS potential, as can be seen by comparing with the gaps of the EXX+RPA OEP potential. The well-known grid sensitivity of meta-GGAs is much more severe in OEP calculations.

Recently, West and Fong [New J. Phys. 14, 083002 (2012)] introduced an echo scheme for an exchange-only qubit, which relies entirely on the exchange interaction. Here, we compare two different exchange-based sequences and two optimization strategies, Uhrig dynamical decoupling and optimized filter function dynamical decoupling, which were introduced for a single spin qubit and are applied in this paper to the three spin exchange-only qubit. The calculation shows that the adaption of the optimization concepts can be achieved by straightforward calculation. We consider two types of noise spectra, Lorentzian and Ohmic noise. For both spectra, the results reveal a slight dependence of the performance on the choice of the echo sequence.

Using Floquet theory, we illustrate that Floquet Weyl fermions can be created in three-dimensional stacked graphene systems irradiated by circularly polarized light. One or two semi-Dirac points can be formed due to overlapping of Floquet subbands. Each pair of Weyl points have a two-component semi-Dirac point parent, instead of a four-component Dirac point parent. Decreasing the light frequency will make the Weyl points move in the momentum space, and the Weyl points can approach to the Dirac points when the frequency becomes very small. The frequency-amplitude phase diagram is worked out. It is shown that there exist Fermi arcs in the surface Brillouin zones in semi-infinitely stacked and finitely multilayered graphene systems irradiated by circularly polarized light. The Floquet Weyl points emerging due to the overlap of Floquet subbands provide a new platform to study Weyl fermions.

Accurate theoretical calculations of the nonlinear elastic response of strong solids (e.g., diamond) constitute a fundamental and important scientific need for understanding the response of such materials and for exploring the potential synthesis and design of novel solids. However, without corresponding experimental data, it is difficult to select between predictions from different theoretical methods. Recently the complete set of third-order elastic constants (TOECs) for diamond was determined experimentally, and the validity of various theoretical approaches to calculate the same may now be assessed. We report on the use of density functional theory (DFT) methods to calculate the six third-order elastic constants of diamond. Two different approaches based on homogeneous deformations were used: (1) an energy-strain fitting approach using a prescribed set of deformations, and (2) a longitudinal stress-strain fitting approach using uniaxial compressive strains along the [100], [110], and [111] directions, together with calculated pressure derivatives of the second-order elastic constants. The latter approach provides a direct comparison to the experimental results. The TOECs calculated using the energy-strain approach differ significantly from the measured TOECs. In contrast, calculations using the longitudinal stress-uniaxial strain approach show good agreement with the measured TOECs and match the experimental values significantly better than the TOECs reported in previous theoretical studies. Our results on diamond have demonstrated that, with proper analysis procedures, first-principles calculations can indeed be used to accurately calculate the TOECs of strong solids.

We present a model that reveals an interesting possibility for narrowing the size distribution of nanostructures when the deterministic growth rate changes its sign from positive to negative at a certain stationary size. Such a behavior occurs in self-catalyzed one-dimensional III-V nanowires and more generally whenever a negative “adsorption-desorption” term in the growth rate is compensated by a positive “diffusion flux.” By asymptotically solving the Fokker-Planck equation, we derive an explicit representation for the size distribution that describes either Poissonian broadening or self-regulated narrowing depending on the parameters. We show how the fluctuation-induced spreading of the size distribution can be completely suppressed in systems with size self-stabilization. These results can be used for obtaining size-uniform ensembles of different nanostructures.

The dynamical interplay between electron-electron interactions and electron-phonon coupling is investigated within the Anderson-Holstein model, a minimal model for open quantum systems that embody these effects. The influence of phonons on spectral and transport properties is explored in equilibrium, for nonequilibrium steady state and for transient dynamics after a quench. Both the particle-hole symmetric and the more generic particle-hole asymmetric cases are studied. The treatment is based on two complementary noncrossing approximations, the first of which is constructed around the weak-coupling limit and the second around the polaron limit. In general, the two methods disagree in nontrivial ways, indicating that more reliable approaches to the problem are needed. The frameworks used here can form the starting point for numerically exact methods based on bold-line continuous-time quantum Monte Carlo algorithms capable of treating open systems simultaneously coupled to multiple fermionic and bosonic baths.

We systematically investigate the spin-orbit coupling-induced band splitting originating from inversion symmetry breaking at the interface between a Co monolayer and $4d$ (Tc, Ru, Rh, Pd, and Ag) or $5d$ (Re, Os, Ir, Pt, and Au) transition metals. In spite of the complex band structure of these systems, the odd-in-$k$ spin splitting of the bands displays striking similarities with the much simpler Rashba spin-orbit coupling picture. We establish a clear connection between the overall strength of the odd-in-$k$ spin splitting of the bands and the charge transfer between the $d$ orbitals at the interface. Furthermore, we show that the spin splitting of the Fermi surface scales with the induced orbital moment, weighted by the spin-orbit coupling.

Inelastic neutron scattering study on the spin-chain compound ${\mathrm{Sr}}_3{\mathrm{NiIrO}}_6$ reveals gapped quasi-1D magnetic excitations. The observed one-magnon band between 29.5 and 39 meV consists of magnon modes of the ${\mathrm{Ni}}^{2+}$ ions. The fitting of the spin wave spectrum reveals strongly coupled Ising-like chains along the $c$ axis that are weakly coupled into a frustrated triangular lattice in the $ab$ plane. The magnetic excitations survive up to 200 K well above the magnetic ordering temperature of $T_N\sim 75$ K, also indicating a quasi-1D nature of the magnetic interactions in ${\mathrm{Sr}}_3{\mathrm{NiIrO}}_6$. Our microscopic model is in agreement with ab initio electronic structure calculations and explains the giant spin-flip field observed in bulk magnetization measurements.

We demonstrate time-resolved spectroscopy of spin-current emission from a magnetic insulator using the inverse spin Hall effect (ISHE). We measured magnetic field dependence of the spin-current emission in the time domain and found that the spectral shape of the ISHE voltage changes with time. The change in the spectral shape is due to field and power dependent temporal oscillation of the spin pumping driven by parametric magnons. The observed oscillating spin-current emission driven by dipole-exchange magnons is well reproduced by a model calculation based on the S theory. In contrast, the spin-current emission driven by short-wavelength exchange magnons cannot be reproduced with this model, illustrating an important role of higher-order nonlinear effects in the spin-current emission.

We investigate the current-induced domain wall motion in perpendicular magnetized Tb/Co wires with structure inversion asymmetry and different layered structures. We find that the critical current density to drive domain wall motion strongly depends on the layered structure. The lowest critical current density $\sim 15\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$ and the highest slope of domain wall velocity curve are obtained for the wire having thin Co sublayers and more inner Tb/Co interfaces, while the largest critical current density $\sim 26\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$ required to drive domain walls is observed in the Tb-Co alloy magnetic wire. It is found that the Co/Tb interface contributes negligibly to Dzyaloshinskii-Moriya interaction, while the effective spin-orbit torque strongly depends on the number of Tb/Co inner interfaces $\left(n\right)$. An enhancement of the antidamping torques by extrinsic spin Hall effect due to Tb rare-earth impurity-induced skew scattering is suggested to explain the high efficiency of current-induced domain wall motion.

We report the transport, thermodynamic, muon spin relaxation, and neutron study of the ${\mathrm{Ca}}_{0.74\left(1\right)}{\mathrm{La}}_{0.26\left(1\right)}$(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${\mathrm{As}}_2$ single crystals, mapping out the temperature-doping level phase diagram. Upon Co substitution on the Fe site, the structural and magnetic phase transitions in this 112 compound are suppressed and superconductivity up to 20 K occurs. Our measurements of the superconducting and magnetic volume fractions show that these two phases coexist microscopically in the underdoped region, in contrast to the related ${\mathrm{Ca}}_{10}$(${\mathrm{Pt}}_3{\mathrm{As}}_8$)((${\mathrm{Fe}}_{1-x}{\mathrm{Pt}}_x$)${}_2{\mathrm{As}}_2$)${}_5$ (10-3-8) compound, where coexistence is absent. Supported by model calculations, we discuss the differences in the phase diagrams of the 112 and 10-3-8 compounds in terms of the FeAs interlayer coupling, whose strength is affected by the character of the spacer layer, which is metallic in the 112 compound and insulating in the 10-3-8 compound.

The Josephson effect between a single-crystal ${\mathrm{UPt}}_3$ and a conventional superconductor Al has been investigated under pressure for the junction on the ${\mathrm{UPt}}_3$ surface perpendicular to the hexagonal $c\left[0001\right]$ axis. Simultaneously measured magnetization has revealed that the Meissner fraction approaches a minimum value of $\sim 1\%$ at the critical pressure of $P_{\mathrm{c}}\sim 0.4\mathrm{GPa}$. The critical temperature $T_{\mathrm{c}}$ and the temperature $T_{\mathrm{J}}$ where a measurable Josephson critical current $I_{\mathrm{c}}$ first appears decrease with increasing pressure; the decreasing rate of $|d{T}_{\mathrm{c}}/dP|$ is almost constant, whereas $|d{T}_{\mathrm{J}}/dP|$ shows an abrupt increase above $P_{\mathrm{c}}$, indicating that the Josephson effect is suppressed above $P_{\mathrm{c}}$. These results, together with the decrease in penetration depth $\lambda$ derived from the magnetic-field dependence of $I_{\mathrm{c}}$, suggest that the superconducting phase is modified above $P_{\mathrm{c}}$ by the disappearance of the symmetry-breaking antiferromagnetic order.

We theoretically demonstrate dc and ac electric voltage generation due to spin motive forces originating from domain wall motion and magnetic resonance, respectively, in two-sublattice antiferromagnets. Our theory accounts for the canting between the sublattice magnetizations, the nonadiabatic electron spin dynamics, and the Rashba spin-orbit coupling, with the intersublattice electron dynamics treated as a perturbation. This work suggests a way to observe and explore the dynamics of antiferromagnetic textures by electrical means, an important aspect in the emerging field of antiferromagnetic spintronics, where both manipulation and detection of antiferromagnets are needed.

Variation of the phase of the beam transmitted through a crystalline material as a function of the rocking angle is a well-known dynamical effect in x-ray scattering. Unfortunately, it is not so easy to directly measure these phase variations in a conventional scattering experiment. It was recently suggested that the transmitted phase can be directly measured in ptychography experiments performed on nanocrystal samples. Results of such experiment for different crystal thickness, reflections, and incoming photon energies, in principle, can be fully described in the frame of dynamical theory. However, dynamical theory does not provide a simple analytical expression for the further analysis. Here we develop a quasi-kinematical theory approach that allows one to correctly describe the phase of the transmitted beam for the crystal thickness less than extinction length that is beyond applicability of the conventional kinematical theory.

A correct description of hydrogen diffusion and trapping is the prerequisite for an understanding of the phenomenon of hydrogen embrittlement. In this study, we carried out extensive first-principles calculations based on density functional theory to investigate the interaction of H with TiC precipitates that are assumed to be efficient trapping agents mitigating HE in advanced high-strength steels. We found that there exists a large variety of possible trapping sites for H associated with different types of interfaces between the TiC particle and the Fe matrix, with misfit dislocations and other defects at these interfaces, and with carbon vacancies in TiC. The most efficient trapping by more than 1 eV occurs at carbon vacancies in the interior of TiC particles. However, these traps are difficult to populate at ambient temperatures since the energy barrier for H entering the particles is high. H trapping at the semicoherent interfaces between the TiC particles and the Fe matrix is moderate, ranging from 0.3 to 0.5 eV. However, a sufficiently large concentration of the carbide particles can significantly reduce the amount of H segregated at dislocation cores in the Fe matrix. A systematic comparison of the obtained theoretical results with available experimental observations reveals a consistent picture of hydrogen trapping at the TiC particles that is expected to be qualitatively valid also for other carbide precipitates with the rock-salt crystal structure.