We study the temperature evolution of the quasiparticle bands of the FeSe monolayer on the ${\mathrm{SrTiO}}_3$ (STO) substrate from 10 to 300 K by applying the anisotropic, multiband, and full-bandwidth Eliashberg theory. To achieve this, we extend this theory by self-consistently coupling the chemical potential to the full set of Eliashberg equations. In this way, the electron filling can accurately be kept at a constant level at any temperature. Solving the coupled equations self-consistently, and with focus on the interfacial electron-phonon coupling, we compute a nearly constant Fermi surface with respect to temperature and predict a nontrivial temperature evolution of the global chemical potential. This evolution includes a total shift of 5 meV when increasing temperature from 10 to 300 K and a humplike dependence followed by a kink at the critical temperature $T_c$. We argue that the latter behavior indicates that superconductivity in $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ is near to the BCS-BEC crossover regime. Calculating the temperature-dependent angle -resolved photoemission spectroscopy (ARPES) spectra, we suggest a new route to determine the energy scale of the interfacial phonon mode by measuring the energy position of second-order replica bands. Further, we reexamine the often used symmetrization procedure applied to such ARPES curves and demonstrate substantial asymmetric deviations. Lastly, our results reveal important aspects for the experimental determination of the momentum anisotropy of the supercon- ducting gap.

The simultaneous presence of an Fe-related spin-density wave and the antiferromagnetic order of ${\mathrm{Eu}}^{2+}$ moments ranks ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ among the most interesting parent compounds of iron-based pnictide superconductors. Here we explore the consequences of the dilution of ${\mathrm{Eu}}^{2+}$ magnetic lattice through on-site Ca substitution. By employing macro- and microscopic techniques, including electrical transport and magnetometry, as well as muon-spin spectroscopy, we study the evolution of Eu magnetic order in both the weak and strong dilution regimes, achieved for Ca concentration $x\left(\mathrm{Ca}\right)=0.12$ and 0.43, respectively. We demonstrate the localized character of the Eu antiferromagnetism mediated via RKKY interactions, in contrast with the largely itinerant nature of Fe magnetic interactions. Our results suggest a weak coupling between the Fe and Eu magnetic sublattices and a rapid decrease of the Eu magnetic interaction strength upon Ca substitution. The latter is confirmed both by the depression of the ordering temperature of the ${\mathrm{Eu}}^{2+}$ moments $T_{\mathrm{N}}$, and the decrease of magnetic volume fraction with increasing $x\left(\mathrm{Ca}\right)$. We establish that, similarly to the ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ parent compound, the investigated Ca-doped compounds have a twinned structure and undergo a permanent detwinning upon applying an external magnetic field.

The equation for the gap parameter represents the main equation of the pairing theory of superconductivity. Although it is formally defined through a single-particle property, physically, it reflects the pairing correlations between opposite-spin fermions. Here, we exploit this physical connection and cast the gap equation in an alternative form which explicitly highlights these two-particle correlations by showing that it is equivalent to a Hugenholtz-Pines condition for fermion pairs. At a formal level, a direct connection is established in this way between the treatment of the condensate fraction in condensate systems of fermions and bosons. At a practical level, the use of this alternative form of the gap equation is expected to make easier the inclusion of pairing fluctuations beyond mean field. As a proof-of-concept of the new method, we apply the modified form of the gap equation to the long-pending problem about the inclusion of the Gorkov-Melik-Barkhudarov correction across the whole BCS-BEC crossover, from the BCS limit of strongly overlapping Cooper pairs to the BEC limit of dilute composite bosons, and for all temperatures in the superfluid phase. Our numerical calculations yield excellent agreement with the recently determined experimental values of the gap parameter for an ultracold Fermi gas in the intermediate regime between BCS and BEC, as well as with the available quantum Monte Carlo data in the same regime.

A recent article by Zhang et al. [Phys. Rev. B 97, 064107 (2018)] claimed the high-pressure synthesis and metallization of ${\mathrm{H}}_3\mathrm{Se}$, or selenium trihydride, at 23 GPa and 203 K. Here, we demonstrate that the synthesised compound is not ${\mathrm{H}}_3\mathrm{Se}$ but the previously reported, and already well characterized, ${\left({\mathrm{H}}_2\mathrm{Se}\right)}_2{\mathrm{H}}_2$. The claim of metallization is unsubstantiated by the experimental evidence and can be explained by the decomposition of ${\left({\mathrm{H}}_2\mathrm{Se}\right)}_2{\mathrm{H}}_2$ into metallic Se and molecular ${\mathrm{H}}_2$.

The Comment of Pace et al. [Phys. Rev. B 98, 106101 (2018)] claims that structural analysis and nomenclature of Zhang et al. [Zhang, Xu, Wang, Jiang, Gorelli, Greenberg, Prakapenka, and Goncharov, Phys. Rev. B 97, 064107 (2018)] are incorrect, that this compound is not metallic at high pressures and 200 K, and that the compound instead decomposes. In this Reply we argue that there are no experimental data that can discriminate between theoretically predicted Cccm ${\mathrm{H}}_3\mathrm{Se}$ and $I$4/mcm ${\left({\mathrm{H}}_2\mathrm{Se}\right)}_2{\mathrm{H}}_2$ advocated by Pace et al. The difference in nomenclature is due to different naming conventions. We find the name “${\mathrm{H}}_3\mathrm{Se}$” more convenient to apply in the limit of high pressure. We also substantiate the initial claims of the stability up to 40 GPa at 170 K of the ${\mathrm{H}}_3\mathrm{Se}$ compound after synthesis at 4.6 GPa and argue that the pressure induced metallization above 23 GPa is a plausible explanation of the reported visual observations and Raman spectroscopy results.

We present a general formula for the tight-binding representation of momentum matrix elements needed for calculating the conductivity based on the Kubo-Greenwood formula using atomic orbitals, which are in general not orthogonal to other orbitals at different sites. In particular, the position matrix element is demonstrated to be important for delivering the exact momentum matrix element. This general formula, applicable to both orthonormal and nonorthonormal bases, solely needs the information of the position matrix elements and the ingredients that have already contained in the tight-binding representation. We then study the anomalous Hall conductivity in the standard example, ferromagnetic bcc Fe, by a first-principles tight-binding Hamiltonian. By assuming the commutation relation $\widehat{\vec{p}}=(i{m}_e/\hslash )[\widehat{H},\widehat{\vec{r}}]$, the obtained frequency-dependent Hall conductivity is found to be in good agreement with existing theoretical and experimental results. Better agreement with experiments can be reached by introducing a reasonable bandwidth renormalization, evidencing the strong correlation among $3d$ orbitals in bcc Fe. Since a tight-binding Hamiltonian can be straightforwardly obtained after finishing a first-principles calculation using atomic basis functions that are generated before the self-consistent calculation, the derived formula is particularly useful for those first-principles calculations.

The $5d$ transition-metal oxides offer further opportunities to test our understanding of the interplay of correlation effects and spin-orbit interactions in materials in the absence of a single dominant interaction. The subtle balance between solid-state interactions can result in mechanisms that minimize the interaction energy, and in material properties of potential use for applications. We focus here on the $5d$ transition-metal oxide $\mathrm{NaOs}{\mathrm{O}}_3$, a strong candidate for the realization of a magnetically driven transition from a metallic to an insulating state exploiting the so-called Slater mechanism. Experimental results are derived from nonresonant and resonant x-ray single-crystal diffraction at the Os $L$ edges. A change in the crystallographic symmetry does not accompany the metal-insulator transition in the Slater mechanism and, indeed, we find no evidence of such a change in $\mathrm{NaOs}{\mathrm{O}}_3$. An equally important experimental observation is the emergence of the (300) Bragg peak in the resonant condition with the onset of magnetic order. The intensity of this space-group-forbidden Bragg peak continuously increases with decreasing temperature in line with the square of intensity observed for an allowed magnetic Bragg peak. Our main experimental results, the absence of crystal symmetry breaking, and the emergence of a space-group-forbidden Bragg peak with developing magnetic order, support the use of the Slater mechanism to interpret the metal-insulator transition in $\mathrm{NaOs}{\mathrm{O}}_3$. We successfully describe our experimental results with simulations of the electronic structure and with an atomic model based on the established symmetry of the crystal and magnetic structure.

Monolayers of the group called VA elements have attracted great attention with the rising of black phosphorus. Here, we derive a simple tight-binding model for monolayer grey arsenic, referred to as arsenene (ML-As), based on the first-principles calculations within the partially self-consistent $\mathrm{G}{\mathrm{W}}_0$ approach. The resulting band structure derived from the six $p$-like orbitals coincides with the quasiparticle energy from $\mathrm{G}{\mathrm{W}}_0$ calculations with a high accuracy. In the presence of a perpendicular magnetic field, ML-As exhibits two sets of Landau levels linear with respect to the magnetic field and level index. Our numerical calculation of the optical conductivity reveals that the obtained optical gap is very close to the $\mathrm{G}{\mathrm{W}}_0$ value and can be effectively tuned by external magnetic field. Thus, our proposed TB model can be used for further large-scale simulations of the electronic, optical, and transport properties of ML-As.

We report a density functional theory plus dynamical mean-field theory (DFT+DMFT) study of an oxide heterostructure of ${\mathrm{LaNiO}}_3$ (LNO) bilayers in [111] direction interleaved with four atomic monolayers of ${\mathrm{LaAlO}}_3$. DFT+$U$ optimizations yield two stable solutions: a uniform structure with equivalent ${\mathrm{NiO}}_6$ octahedra, as well as a bond-disproportionated (BD) structure featuring a breathing distortion. For both structures, we construct the low-energy models describing the Ni $e_g$ states by means of Wannier projections supplemented by the Kanamori interaction, and solve them by DMFT. Using the continuous-time quantum Monte Carlo algorithm in the hybridization expansion, we study the temperature range between 145 and 450 K. For the uniform and the BD structure, we find similar phase diagrams that comprise four phases: a ferromagnetic metal (FM), a paramagnetic metal, an antiferro-orbitally ordered insulator (AOI), as well as a paramagnetic insulator. By calculating momentum-resolved spectral functions on a torus and a cylinder, we demonstrate that the FM phase is not a Dirac metal, while both insulating phases are topologically trivial. By a comparison with available experimental data and model DMFT studies for the two-orbital Hubbard model, we suggest that LNO bilayers are in the AOI phase at room temperature.

We report a detailed and comparative study of the single-crystal ${\mathrm{CeCoInGa}}_3$ in both experiment and theory. Resistivity measurements reveal the typical behavior of a Kondo lattice with the onset temperature of coherence, $T^{*}\approx 50\mathrm{K}$. The magnetic specific heat can be well fitted using a spin-fluctuation model at low temperatures, yielding a large Sommerfeld coefficient, $\gamma \approx 172\mathrm{mJ}$/mol ${\mathrm{K}}^2$ at 6 K, suggesting that this is a heavy-fermion compound with a pronounced coherence effect. The magnetic susceptibility exhibits a broad field-independent peak at $T_{\chi }$ and shows an obvious anisotropy within the $bc$ plane, reflecting the anisotropy of the coherence effect at high temperatures. These are compared with strongly correlated calculations combining first-principles band structure calculations and dynamical mean-field theory. Our results confirm the onset of coherence at about 50 K and reveal a similar anisotropy in the hybridization gap, pointing to a close connection between the hybridization strength of the low-temperature Fermi-liquid state and the high-temperature coherence effect.

In implantation experiments, the implanted particle is shot with a certain energy into the material and comes to rest at a site which may not correspond to the final position. The rearrangements of the surrounding atoms to accommodate the particle, i.e., the reaction with the host atoms may require some time and lead to delayed formation of the final states. In the case of the implantation of positive muons, this rearrangement process can be followed on a timescale of nanoseconds to microseconds. A delay is expected if an energy barrier inhibits the prompt reaction. We note that the barrier height may change during the rearrangement of the lattice, thus giving rise to a two-dimensional potential profile for the conversion process. The barrier model describes the reaction path of the muon in analogy to the passage over a mountain with a saddle point. The passing over the saddle point corresponds to the lowest energy trajectory. As an example, we discuss the application of the barrier model to solid ${\mathrm{Lu}}_2{\mathrm{O}}_3$.

The single-particle state is not expected to demonstrate second-order coherence. Here I analyze the injection of electrons into a conductor and show that, at a nonzero temperature, the underlying Fermi sea causes the single-particle injected state to exhibit second-order coherence. For this purpose, I calculate the second-order correlation function, $G^{\left(2\right)}$, of electrons injected on top of the Fermi sea. At zero temperature, the function $G^{\left(2\right)}$ unambiguously demonstrates whether the injected state is a single- or a multiparticle state: $G_{}^{\left(2\right)}$ vanishes in the former case, while it does not vanish in the latter case. However, at nonzero temperatures, when the injected state is a mixed state, a purely single-particle contribution makes the function $G_{}^{\left(2\right)}$ nonvanishing even in the case of injection of one electron. The single-particle contribution puts the lower limit on the second-order correlation function and thereby limits its use as a single-particle injection test at nonzero temperatures. The existence of a single-particle contribution to $G_{}^{\left(2\right)}$ can be verified experimentally by measuring the cross-correlation electrical noise.

We study experimentally the Raman response of the undoped high-$T_c$ parent compound ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_6$, and give a unified theory of the two-magnon Raman peak and optical conductivity based on the Hubbard-Holstein model with electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively account for the experimentally observed resonance of the Raman response, but only the Hubbard-Holstein model (i) reproduces the asymmetry of the Raman spectrum, (ii) validates the experimental visibility of the two-magnon peak, and (iii) predicts the correct shape and energy of the lower edge of the charge transfer gap in optical conductivity. A comparison of experiments with the theory gives the EPC strength $\lambda =0.6$. This result convincingly indicates the vital role of EPC in high-$T_c$ cuprates, providing a clue to the mechanism of high $T_c$.

We report on the structural, magnetic, and transport properties of a set of high-pressure-synthesized compounds ${\mathrm{Mn}}_{1-x}{\mathrm{Rh}}_x\mathrm{Ge}$ ($0\le x\le 1$) with chiral magnetic ordering. The magnetic and transport properties depend substantially on the concentration of rhodium ($x$) and the pressure. The saturation magnetic moment corresponds to a known high-spin value for pristine MnGe ($x=0$) and decreases almost linearly with increasing concentration $x$. In addition, x-ray magnetic circular dichroism spectra taken at 10 K and 2 T indicate magnetic polarization of the Rh $4d$ electron states and Ge $4p$ states, which decreases with $x$, too. In rhodium-rich compounds ($x\ge 0.5$), the temperature of the magnetic ordering increases significantly with pressure, whereas in manganese-rich compounds ($x<0.5$) the temperature decreases. Three different tendencies are also found for several structural and transport properties. In the intermediate range ($0.3\le x\le 0.7$), samples are semiconducting in the paramagnetic phase, but they become metallic in the magnetically ordered state. We carried out ab initio density-functional calculations of ${\mathrm{Mn}}_{1-x}{\mathrm{Rh}}_x\mathrm{Ge}$ at various concentrations $x$ and traced the evolution of the electronic and magnetic properties. The calculation results are in good agreement with the measured magnetic moments and qualitatively explain the observed trends in transport properties.

The two-dimensional electron system in $\left(110\right){\mathrm{Al}}_2{\mathrm{O}}_{3-\delta }/{\mathrm{SrTiO}}_3$ heterostructures displays anisotropic electronic transport. Largest and lowest conductivity and electron mobility $\mu$ are observed along the [001] and $\left[1\overline{1}0\right]$ directions, respectively. The anisotropy of the sheet resistance and $\mu$ likewise leads to a distinct anisotropic normal magnetotransport (MR) for $T<30\mathrm{K}$. However, at temperatures $T<5\mathrm{K}$ and magnetic field $B<2\mathrm{T}\mathrm{MR}$ is dominated by weak antilocalization. Despite the rather strong anisotropy of the Fermi surfaces, the in-plane anisotropic magnetoresistance (AMR) displays twofold noncrystalline anisotropy. However, the AMR amplitude is found to be anisotropic with respect to the current direction, leading to a 60% larger AMR amplitude for current $I$ along the [001] direction compared to $I$ parallel to $\left[1\overline{1}0\right]$. Tight-binding calculations evidence an anisotropic Rashba-induced band splitting with dominant linear $k$ dependence. In combination with semiclassical Boltzmann theory, the noncrystalline AMR is well described, despite the anisotropic Fermi surface.

Several approaches to linear-scaling density functional theory (LS-DFT) that seek to achieve accuracy equivalent to plane-wave methods do so by optimizing in situ a set of local orbitals in terms of which the density matrix can be accurately expressed. These local orbitals, which can also accurately represent the canonical Kohn-Sham orbitals, qualitatively resemble the maximally localized Wannier functions employed in band structure interpolation. As LS-DFT methods are increasingly being used in real-world applications demanding accurate band structures, it is natural to question the extent to which these optimized local orbitals can provide sufficient accuracy. In this paper, we present and compare, in principle and in practice, two methods for obtaining band structures. We apply these to a (10, 0) carbon nanotube as an example. By comparing with the results from a traditional plane-wave pseudopotential calculation, the optimized local orbitals are found to provide an excellent description of the occupied bands and some low-lying unoccupied bands, with consistent agreement across the Brillouin zone. However free-electron-like states derived from weakly bound states independent of the $\sigma$ and $\pi$ orbitals can only be found if additional local orbitals are included.

We theoretically investigate electrical transport in a two-dimensional electron system hosting bulk and edge current carrying states. Spatially varying magnetic and electric confinement creates pairs of current carrying lines that drift in the same or opposite directions depending on whether confinement is applied by a magnetic split gate or a magnetic strip gate. We study the electronic structure through calculations of the local density of states and conductivity of the channel as a function of the chirality and wave-function overlap of these states. We demonstrate a shift of the conductivity peaks to high or low magnetic field depending on the chirality of pairs of edge states and the effect of chirality on backscattering amplitude associated with collisional processes.

An extended modification of the effective medium approach (emEMA) has been developed for the thermal conductivity of anisotropic nanocomposites. This is based on extending approaches developed to treat an anisotropic particle insert and host matrix in electromagnetism of composites to anisotropic thermal interface resistance, with the inclusion of insert size and interface boundary density effects. The method has been applied to the case of spherical inclusions of the 2H dichalcodenide ${\mathrm{WS}}_2$ within a matrix of 2H ${\mathrm{MoS}}_2$, with input bulk thermal conductivities calculated using our recently developed semi-ab initio method. We find that the overall effects of anisotropy are strongest for small particles, but that as particle size increases, the surface anisotropy effects become more apparent.

We show that by utilizing an anisotropic metamaterials ring with near zero azimuthal component of the density tensor, multiple separated sound sources can overlap each other and appear like only one source. The powers emitted from the different sources within the metamaterials ring can be combined together. Analytical and numerical results show that such ideal combination performance is independent of the positions and the amount of the sources and even obstacles appearing inside the metamaterials ring cannot alter it, exhibiting great robustness. The proposed scheme is potentially useful for applications such as omnidirectional sound sources, high-resolution ultrasonography, and advanced signal modulation.

Herein, using first-principles calculations we predict magnetization reorientation and large perpendicular magnetic anisotropy (PMA) in spinel ${\mathrm{Fe}}_3{\mathrm{O}}_4$/MgO heterostructure by replacing the octahedral Fe ions with Cu. The substitutional ${\mathrm{Cu}}^{2+}$ ions prefer the octahedral site within the xy-plane layer in an inverse spinel structure, which is associated with the Jahn-Teller tetragonal and $xy$-plane twisted distortions. While magnetization of ${\mathrm{Fe}}_3{\mathrm{O}}_4$/MgO is significantly reduced in ${\mathrm{CuFe}}_2{\mathrm{O}}_4$/MgO, the presence of the substitutional ${\mathrm{Cu}}^{2+}$ ions reorients magnetization from an in-plane to perpendicular magnetic anisotropy. More remarkably, PMA further increases gradually with the film thickness of ${\mathrm{CuFe}}_2{\mathrm{O}}_4$ layers in the ${\mathrm{CuFe}}_2{\mathrm{O}}_4$/MgO heterostructure. The underlying mechanism for this large PMA is the interplay of the spin-orbit-coupled Cu $d_{xy}–d_{x^2-y^2}$ states in the center layers and the Fe $d_{z^2}$–O $p_z$ hybridization at the interface. These findings point toward the feasibility of reducing magnetization and enhancing PMA in spinel structures for spintronics applications.