The electrical resistivity and thermoelectric power of the paramagnetic $A\mathrm{C}{\mathrm{u}}_3\mathrm{R}{\mathrm{u}}_4{\mathrm{O}}_{12}$ quadruple perovskite ruthenates with $A=\mathrm{Na},\mathrm{Ca},\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{L}{\mathrm{a}}_{0.5}$, and $\mathrm{La}$ have been measured from $T=5\mathrm{K}$ up to 900 K, with the formal Ru oxidation state changing from $3.75(A=\mathrm{L}{\mathrm{a}}^{3+})$ to $4.25(A=\mathrm{N}{\mathrm{a}}^{+})$. In this series of distorted metallic perovskites, resistivity curves present a metallic behavior, with no sign of saturation up to 900 K, typical of a bad metal. Two regimes are evidenced for the Seebeck coefficient $S$: at high $T$, a quasilinear increase of $S$ is observed up to 900 K, to a common value of $\sim 29\text{–}32\mu \mathrm{V}{\mathrm{K}}^{-1}$ at 900 K, while for $T<200\mathrm{K}$, strong differences of $S$ are observed which can be directly correlated with the evolution of the paramagnetic Pauli susceptibility χ. The comparison of these quadruple perovskites with $\mathrm{SrRu}{\mathrm{O}}_3$ and $\mathrm{CaRu}{\mathrm{O}}_3$ perovskites show that at high $T$, similar $S$ values are obtained, but the $S\left(T\right)$ dependences are completely different in the whole $T$ range. In perovskites, the constant $S$ of $\sim 25\text{–}35\mu \mathrm{V}/\mathrm{K}$ was explained by the spin degeneracy term calculated from the generalized Heikes formula, while in the quadruple perovskites the high $T$ linear $S\left(T\right)$ is typical of Mott's formula, used for metals. The origin of the similar $S$ values measured at 900 K could thus be coincidental but remains to be understood. From the comparison of $S\left(T\right)$ at low $T$ among all these ruthenates, a clear connection between the low $T$ slope of $S$ and the magnetic behavior is evidenced, with a large slope in the case of ferromagnetic $\mathrm{SrRu}{\mathrm{O}}_3$, in contrast to a very small $dS/dT$ in the case of the Pauli paramagnetic $\mathrm{NaC}{\mathrm{u}}_3\mathrm{R}{\mathrm{u}}_4{\mathrm{O}}_{12}$, which possesses the smallest susceptibility.

Large-scale atomistic simulations using the reactive empirical bond order force field approach is implemented to investigate thermal and mechanical properties of single-layer (SL) and multilayer (ML) molybdenum disulfide (${\mathrm{MoS}}_2$). The amplitude of the intrinsic ripples of SL ${\mathrm{MoS}}_2$ are found to be smaller than those exhibited by graphene (GE). Furthermore, because of the van der Waals interaction between layers, the out-of-plane thermal fluctuations of ML ${\mathrm{MoS}}_2$ decreases rapidly with increasing number of layers. This trend is confirmed by the buckling transition due to uniaxial stress which occurs for a significantly larger applied tension as compared to graphene. For SL ${\mathrm{MoS}}_2$, the melting temperature is estimated to be 3700 K which occurs through dimerization followed by the formation of small molecules consisting of two to five atoms. When different types of vacancies are inserted in the SL ${\mathrm{MoS}}_2$ it results in a decrease of both the melting temperature as well as the stiffness.

The generalized Langevin equation (GLE) method, as developed previously [L. Stella et al., Phys. Rev. B 89, 134303 (2014)], is used to calculate the dissipative dynamics of systems described at the atomic level. The GLE scheme goes beyond the commonly used bilinear coupling between the central system and the bath, and permits us to have a realistic description of both the dissipative central system and its surrounding bath. We show how to obtain the vibrational properties of a realistic bath and how to convey such properties into an extended Langevin dynamics by the use of the mapping of the bath vibrational properties onto a set of auxiliary variables. Our calculations for a model of a Lennard-Jones solid show that our GLE scheme provides a stable dynamics, with the dissipative/relaxation processes properly described. The total kinetic energy of the central system always thermalizes toward the expected bath temperature, with appropriate fluctuation around the mean value. More importantly, we obtain a velocity distribution for the individual atoms in the central system which follows the expected canonical distribution at the corresponding temperature. This confirms that both our GLE scheme and our mapping procedure onto an extended Langevin dynamics provide the correct thermostat. We also examined the velocity autocorrelation functions and compare our results with more conventional Langevin dynamics.

We examine the Dyakonov and Perel theory of the Spin Hall effect (SHE) from the viewpoint of irreversible thermodynamics, which is more constraining than the symmetry arguments of pure phenomenology. As thermodynamic driving forces we include the thermal gradient, the gradient of the electrochemical potential (rather than the potential gradient and density gradient separately), and the “internal” magnetic field that is thermodynamically conjugate to the magnetization. In turn, we obtain the form of bulk transport coefficients relating the fluxes to the thermodynamic forces. Relative to Dyakonov and Perel, in addition to the new terms due to thermal gradients, the Onsager relations require three new (nonlinear) terms in the current density, and minor revisions in the current density and spin current density. For a longitudinal current along a strip, the center-to-edge transverse voltage difference, due both to the $-\beta \vec{P}\times \vec{E}$ term of of the number current density $\vec{q}$ and to one of the new current density terms, is calculated. An ac capacitative probe likely would not significantly disturb this effect. An appendix explicitly relates the anomalous Hall Effect to the term in the (vector) number flux that is Onsager related to the SHE term in the (tensor) spin flux.

Multiferroic materials are mostly antiferromagnets, often containing incommensurate magnetic arrangements stemming from the magnetoelectric interaction. Using soft x-ray resonant magnetic scattering, we show that these long-range structures induce a magnetization wriggle in cobalt layers deposited on top of ${\mathrm{BiFeO}}_3$ single crystals. This is understood using a simple interface exchange interactions model. It leads to the appearance of a magnetic anisotropy axis, which, in the particular ${\mathrm{BiFeO}}_3$/Co system, can be manipulated using an electric field. More generally, it is demonstrated here that through interfacial magnetic exchange, antiferromagnets can leave an imprint revealing some of their hidden properties, thus providing much richer effects than mere exchange bias.

Orthorhombic ${\mathrm{TbMn}}_2{\mathrm{O}}_5$ ($o\text{-TMO}$) is a well-known multiferroic manganite with the remarkable property of polarization switching at 3 K under a bias magnetic (H) field along the $a$ axis of $Pb{2}_{1}m$. To theoretically account for this outstanding observation, we have proposed a modulated spin structure under the saturated bias $H$ field by considering the relative strength of the three relevant exchange parameters in $o\text{-TMO}$. The proposed modulated structure based on density-functional theory (DFT) calculations is described in terms of the spin angle $\varphi$ between the neighboring $\mathrm{M}{\mathrm{n}}^{4+}\text{-M}{\mathrm{n}}^{3+}$ spin moments on the a-b plane. We have shown that the computed DFT polarization plotted as a function of $\varphi$ satisfactorily accounts for the observed $H$-field-induced polarization switching. We have further theoretically shown that the square of the critical field strength ($H_c$) needed for the polarization switching is inversely proportional to the degree of the extrinsic magnetoelectric coupling. The computed partial charge density demonstrates that the $H$-field-induced polarization switching also accompanies with the switching in the sign of the excess valence-electron density.

With one of the highest intrinsic magnetic moments $(10.6{\mu }_B/\mathrm{atom})$ among the heavy rare-earth elements, dysprosium exhibits a rich magnetic phase diagram, including several modulated magnetic phases. Aided by the Ruderman-Kittel-Kasuya-Yosida interaction, the magnetic modulations propagate coherently over a long range. Neutron diffraction experiments were performed to determine the microscopic magnetic origin of the field induced phases in bulk Dy as a function of temperature, covering regions of the well-known ferromagnetic, helical antiferromagnetic, fan phases, and several possible new phases suggested by previous studies. A short-range ordered fan phase was identified as the intermediate state between ferromagnetism and long-range ordered fan. In a field of 1 T applied along the $a$ axis, the temperature range of a coexisting helix/fan phase was determined. The magnetic phase diagram of Dy was thus refined to include the detailed magnetic origin and the associated phase boundaries. Based on the period of the magnetic modulation and the average magnetization, the evolution of the spin arrangement upon heating was derived quantitatively for the modulated magnetic phases.

We have performed systematic resistivity and inelastic neutron scattering measurements on ${\mathrm{Fe}}_{0.98-z}{\mathrm{Ni}}_z{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ samples to study the impact of Ni substitution on the transport properties and the low-energy $(\le 12\text{meV})$ magnetic excitations. It is found that, with increasing Ni doping, both the conductivity and superconductivity are gradually suppressed; in contrast, the low-energy magnetic spectral weight changes little. Comparing with the impact of Co and Cu substitution, we find that the effects on conductivity and superconductivity for the same degree of substitution grow systematically as the atomic number of the substituent deviates from that of Fe. The impact of the substituents as scattering centers appears to be greater than any contribution to carrier concentration. The fact that low-energy magnetic spectral weight is not reduced by increased electron scattering indicates that the existence of antiferromagnetic correlations does not depend on electronic states close to the Fermi energy.

We study impurity healing effects in models of strongly correlated superconductors. We show that in general both the range and the amplitude of the spatial variations caused by nonmagnetic impurities are significantly suppressed in the superconducting as well as in the normal states. We explicitly quantify the weights of the local and the nonlocal responses to inhomogeneities and show that the former are overwhelmingly dominant over the latter. We find that the local response is characterized by a well-defined healing length scale, which is restricted to only a few lattice spacings over a significant range of dopings in the vicinity of the Mott insulating state. We demonstrate that this healing effect is ultimately due to the suppression of charge fluctuations induced by Mottness. We also define and solve analytically a simplified yet accurate model of healing, within which we obtain simple expressions for quantities of direct experimental relevance.

$\mathrm{BaTi}{\mathrm{O}}_3$ is shown to exhibit anomalous piezoelectric response, comparable to that of lead-zirconate titanate, by dilute Sn modification (1–4 mol%). Using a newly discovered powder poling technique it is shown that the mechanism associated with this anomalous strain response involves electric-field-induced switching of polarization vector from [001] towards [101] pseudocubic direction. This switchability is significantly enhanced by tuning the tetragonal-orthorhombic first-order criticality near to room temperature.

We consider whether it is possible to find ground states of frustrated spin systems by solving them locally. Using spin glass physics and Imry-Ma arguments in addition to numerical benchmarks we quantify the power of such local solution methods and show that for the average low-dimensional spin glass problem outside the spin glass phase the exact ground state can be found in polynomial time. In the second part we present a heuristic, general-purpose hierarchical approach which for spin glasses on chimera graphs and lattices in two and three dimensions outperforms, to our knowledge, any other solver currently around, with significantly better scaling performance than simulated annealing.

Most iron-based superconductors undergo a transition to a magnetically ordered state characterized by staggered stripes of parallel spins. With ordering vectors $(\pi ,0)$ or $(0,\pi )$, this magnetic state breaks the high-temperature tetragonal symmetry of the system, which is manifested by a splitting of the lattice Bragg peaks. Remarkably, recent experiments in hole-doped iron arsenides reported an ordered state that displays magnetic Bragg peaks at $(\pi ,0)$ and $(0,\pi )$ but remains tetragonal. Despite being inconsistent with a magnetic stripe configuration, this unusual magnetic phase can be described in terms of a double-$\mathbf{Q}$ magnetic structure consisting of an equal-weight superposition of the ordering vectors $(\pi ,0)$ and $(0,\pi )$. Here we show that a noncollinear double-$\mathbf{Q}$ magnetic configuration, dubbed orthomagnetic, arises naturally within an itinerant three-band microscopic model for the iron pnictides. In particular, we find that strong deviations from perfect nesting and residual interactions between the electron pockets favor the orthomagnetic over the stripe magnetic state. Using an effective low-energy model, we also calculate the spin-wave spectrum of the orthomagnetic state. In contrast to the stripe state, there are three Goldstone modes, manifested in all diagonal and one off-diagonal components of the spin-spin correlation function. The total magnetic structure factor displays two anisotropic gapless spin-wave branches emerging from both $(\pi ,0)$ and $(0,\pi )$ momenta, in contrast to the case of domains of stripe order, where only one gapless spin-wave branch emerges from each momentum. We propose that these unique features of the orthomagnetic state can be used to unambiguously distinguish it from the stripe state via neutron scattering experiments, and discuss the implications of its existence for the nature of the magnetism of the iron arsenides.

We have studied the spin transport and the spin Hall effect as a function of temperature for platinum (Pt) and gold (Au) in lateral spin valve structures. First, by using the spin absorption technique, we extract the spin diffusion length of Pt and Au. Secondly, using the same devices, we have measured the spin Hall conductivity and analyzed its evolution with temperature to identify the dominant scattering mechanisms behind the spin Hall effect. This analysis confirms that the intrinsic mechanism dominates in Pt whereas extrinsic effects are more relevant in Au. Moreover, we identify and quantify the phonon-induced skew scattering. We show that this contribution to skew scattering becomes relevant in metals such as Au, with a low residual resistivity.

${\mathrm{La}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single-crystal x-ray and neutron diffraction. ${\mathrm{La}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at $T_s=125$ K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the $a$ direction with an ordered moment of 0.7(1) ${\mu }_{\mathrm{B}}$ at $T=5$ K. The low-temperature stripe antiferromagnetic structure is the same as that in other $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A$ = Ca, Sr, Ba) compounds. ${\mathrm{La}}_{0.5-x}{\mathrm{Na}}_{0.5+x}{\mathrm{Fe}}_2{\mathrm{As}}_2$ provides a material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying the La/Na ratio.

We investigate the electronic structure of the skutterudite-related ${\mathrm{Ce}}_3{M}_4{\mathrm{Sn}}_{13}$ compounds, where metal $M$ is Co, Ru, or Rh, using x-ray photoemission spectroscopy (XPS) and band structure calculations, both within local density approximation scheme and within scheme including additional Coulomb correlations (LDA+$U$). The $d$-electron states located near the Fermi level slightly change the intensity with applying the correlation energy $U$. We note the large impact of $d$-electron correlation, which is clearly observed in the change of Ce $5p$ states located about 18 eV in the valence band. Namely, for ${\mathrm{Ce}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ and ${\mathrm{Ce}}_3{\mathrm{Ru}}_4{\mathrm{Sn}}_{13}$ the $d$-electron correlations significantly shift the $5p$ states towards higher binding energies, while for ${\mathrm{Ce}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ the energy of the Ce $5p$ states almost does not depend on $U$. The influence of the $d$-electron correlations is also manifested by the positive magnetoresistivity (MR) of ${\mathrm{Ce}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ and the ${\mathrm{Ce}}_{3-x}{\mathrm{La}}_x{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ alloys that reaches 40% at the highest magnetic field ($B=9$ T) for the sample $x=0.6$, whereas only a small positive $\text{MR}\sim 0.5\%$ is observed in ${\mathrm{Ce}}_3{\mathrm{Ru}}_4{\mathrm{Sn}}_{13}$. The magnetoresistivity of ${\mathrm{Ce}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ exhibits quite different behavior and is negative.

Single-crystal x-ray diffraction, magnetic susceptibility, magnetization, heat capacity, and electrical resistivity measurements are reported for the new tetragonal compound ${\mathrm{Ce}}_2{\mathrm{PdGe}}_3$, which forms in the space group $P{4}_2/mmc$ (No. 131)—a relative of the $\alpha -{\mathrm{ThSi}}_2$-type structure. Measurements reveal a two-part antiferromagnetic phase transition at $T_{\mathrm{N},1}=10.7$ K and $T_{\mathrm{N},2}=9.6$ K and subsequent ferromagnetlike ordering near $T_{\mathrm{C}}\approx 2.25$ K. The ordered ground state emerges from a lattice of Ce ions that are hybridized with the conduction electrons, as revealed by the enhanced electronic coefficient of the specific heat $\gamma \approx 50$ ${\mathrm{mJ/mol-Ce-K}}^2$ (extrapolated to $T=0$ for $T$ $<$ $T_{\mathrm{C}}$). Electronic structure calculations reveal a Fermi surface that includes sheets with distinct nesting vectors. Chemical/structural disorder also plays an important role, as evidenced by results from single-crystal x-ray diffraction, the width of the peaks in the heat capacity at $T_{\mathrm{N}}$ and $T_{\mathrm{C}}$, and the small residual resistivity ratio $\text{RRR}={\rho }_{300\mathrm{K}}/{\rho }_0=1.8$.

We report neutron inelastic-scattering measurements and analysis of the spectrum of magnons propagating within the ${\mathrm{Fe}}_2{\mathrm{O}}_4$ bilayers of ${\mathrm{LuFe}}_2{\mathrm{O}}_4$. The observed spectrum is consistent with six magnetic modes and a single prominent gap, which is compatible with a single bilayer magnetic unit cell containing six spins. We model the magnon dispersion by linear spin-wave theory and find very good agreement with the domain-averaged spectrum of a spin-charge bilayer superstructure comprising one ${\mathrm{Fe}}^{3+}$-rich monolayer and one ${\mathrm{Fe}}^{2+}$-rich monolayer. These findings indicate the existence of polar bilayers in ${\mathrm{LuFe}}_2{\mathrm{O}}_4$. We also discuss a model of charge-segregated nonpolar bilayer order which we argue is not consistent with our data but which we cannot exclude definitively. Weak scattering observed below the magnon gap suggests that a fraction of the bilayers contain other combinations of charged monolayers not included in the model. Refined values for the dominant exchange interactions are reported.

This paper provides evidence for a direct dark mode excitation mechanism in a metasurface structure. The dark mode excitation mechanism is entirely determined by structures’ symmetry and does not depend on near-field coupling between elements. In our examples, we consider single-element based metasurface composed of two V antennas connected in an antisymmetric arrangement. Both experimental and modeling results show an efficient excitation of magnetic dipolar mode in such structures. The direct dark mode excitation mechanism provides a design that is more robust with respect to technology imperfections. The considered approach opens promising perspectives for new types of nanostructure designs and greatly relaxes fabrication constraints for the optical domain.

We analyze the accuracy of the atomic force within the all-electron full-potential linearized augmented plane-wave (FLAPW) method using the force formalism of Yu et al. [Phys. Rev. B 43, 6411 (1991)]. A refinement of this formalism is presented that explicitly takes into account the tail of high-lying core states leaking out of the muffin-tin sphere and considers the small discontinuities of LAPW wave function, density, and potential at the muffin-tin sphere boundaries. For MgO and ${\mathrm{EuTiO}}_3$ it is demonstrated that these amendments substantially improve the acoustic sum rule and the symmetry of the force constant matrix. Sum rule and symmetry are realized with an accuracy of $\mu \mathrm{Htr}/a_{\mathrm{B}}$.