At interfaces between insulating oxides ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$, a two dimensional electron gas (2DEG) has been observed and well studied, while the predicted hole gas (2DHG) has not been realized due to the strong tendency of holes in oxygen $2p$ orbitals to localize. Here we propose, via ab initio calculations, an unexplored class of materials for the realization of parallel two dimensional (2D), two carrier (electron+hole) gases: nitride-oxide heterostructures, with (111)-oriented ScN and MgO as the specific example. Beyond a critical thickness of five ScN layers, this interface hosts spatially separated conducting $\mathrm{Sc}\text{−}3d$ electrons and $\mathrm{N}\text{−}2p$ holes, each confined to $\sim$ two atomic layers—the transition metal nitride provides both gases. A guiding concept is that the ${\mathrm{N}}^{3-}$ anion should promote robust two carrier 2D hole conduction compared to that of ${\mathrm{O}}^{2-}$; metal mononitrides are mostly metallic and even superconducting while most metal monoxides are insulating. A second positive factor is that the density of transition metal ions, hence of a resulting 2DG, is about three times larger for a rocksalt (111) interface than for a perovskite (001) interface. Our results, including calculation of Hall coefficient and thermopower for each conducting layer separately, provide guidance for new exploration, both experimental and theoretical, on nitride-based conducting gases that should promote study of long sought exotic states viz. new excitonic phases and distinct, nanoscale parallel superconducting nanolayers.

Iron orbitals in orthorhombic iron selenide (FeSe) can produce chargelike multipoles that are polar (parity-odd). Orbitals in question include $\mathrm{Fe}\left(3d\right)$, $\mathrm{Fe}\left(4p\right)$, and $p$-type ligands that participate in transport properties and bonding. The polar multipoles may contribute weak, space-group forbidden Bragg spots to diffraction patterns collected with x rays tuned in energy to a Fe atomic resonance (Templeton & Templeton scattering). Ordering of conventional, axial magnetic dipoles does not accompany the tetragonal-orthorhombic structural phase transition in FeSe, unlike other known iron-based superconductors. We initiate a new line of inquiry for this puzzling property of orthorhombic FeSe, using a hidden magnetic order that belongs to the $m^{\prime }{m}^{\prime }{m}^{\prime }$ magnetic crystal class. It is epitomized by the absence of ferromagnetism and axial magnetic dipoles and the appearance of magnetic monopoles and magnetoelectric quadrupoles. A similar magnetic order occurs in cuprate superconductors, yttrium barium copper oxide and Hg1201, where it was unveiled with the Kerr effect and in Bragg diffraction patterns revealed by polarized neutrons.

We present the momentum-resolved photoemission spectroscopy of both the valence and the conduction band region in the quasi-two-dimensional van der Waals-layered indirect band gap semiconductor $\mathrm{Sn}{\mathrm{S}}_2$. Using a combination of angle-resolved ultraviolet photoemission and angle-resolved two-photon photoemission (AR-2PPE) spectroscopy, we characterize the band structure of bulk $\mathrm{Sn}{\mathrm{S}}_2$. Comparison with density functional theory calculations shows excellent quantitative agreement in the valence band region and reveals several localized bands that likely originate from defects such as sulfur vacancies. Evidence for a moderate density of defects is also observed by AR-2PPE in the conduction band region, leading to localized bands not present in the computational results. The energetic structure and dispersion of the conduction bands is captured well by the computational treatment, with some quantitative discrepancies remaining. Our results provide a broader understanding of the electronic structure of $\mathrm{Sn}{\mathrm{S}}_2$ in particular and van der Waals-layered semiconductors in general.

We use molecular dynamics simulations for a first-principles-based effective Hamiltonian to calculate two important quantities characterizing the electrocaloric effect in ${\mathrm{BaTiO}}_3$, the adiabatic temperature change $\mathrm{\Delta }T$ and the isothermal entropy change $\mathrm{\Delta }S$, for different electric field strengths. We compare direct and indirect methods to obtain $\mathrm{\Delta }T$ and $\mathrm{\Delta }S$, and we confirm that both methods indeed lead to an identical result provided that the system does not actually undergo a first order phase transition. We also show that a large electrocaloric response is obtained for electric fields beyond the critical field strength for the first order phase transition. Furthermore, our work fills several gaps regarding the application of the first-principles-based effective Hamiltonian approach, which represents a very attractive and powerful method for the quantitative prediction of electrocaloric properties. In particular, we consider the full temperature and field dependence of the calculated specific heat for the indirect calculation of $\mathrm{\Delta }T$, and we discuss the importance of maintaining thermal equilibrium during the field ramping when calculating $\mathrm{\Delta }T$ using the direct method within a molecular dynamics approach.

Temperature dependent stacking fault energies in fcc Fe and the ${\mathrm{Fe}}_{75}{\mathrm{Mn}}_{25}$ random alloy are calculated within density functional theory. The high temperature paramagnetic state of Fe is modeled by the spin wave (SW) method within a Hamiltonian formalism and by the disordered local moment (DLM) approach in the Green's function technique using the coherent potential approximation (CPA). To determine the stacking fault energy, the supercell approach is used in the case of the SW method, while the axial Ising model is used in both the SW method and CPA-DLM calculations. The SW and CPA-DLM results are in very good agreement with each other, and they also accurately reproduce the existing experimental data. In both cases, fcc Fe and the ${\mathrm{Fe}}_{75}{\mathrm{Mn}}_{25}$ alloy, the SFE increases with temperature. This increase is almost entirely due to thermal lattice expansion, in contrast to earlier claims connecting such a dependence with magnetic entropy. Additionally, we check the convergence of the SW method with respect to the number of spin waves in the calculations of the phonon spectrum and the vacancy formation energy of paramagnetic fcc Fe.

We apply variational principles from statistical physics and the Landau theory of phase transitions to multicomponent alloys using the multiple-scattering theory of Korringa-Kohn-Rostoker (KKR) and the coherent potential approximation (CPA). This theory is a multicomponent generalization of the $S^{\left(2\right)}$ theory of binary alloys developed by G. M. Stocks, J. B. Staunton, D. D. Johnson, and others. It is highly relevant to the chemical phase stability of high-entropy alloys as it predicts the kind and size of finite-temperature chemical fluctuations. In doing so, it includes effects of rearranging charge and other electronics due to changing site occupancies. When chemical fluctuations grow without bound, an absolute instability occurs and a second-order order-disorder phase transition may be inferred. The $S^{\left(2\right)}$ theory is predicated on the fluctuation-dissipation theorem; thus we derive the linear response of the CPA medium to perturbations in site-dependent chemical potentials in great detail. The theory lends itself to a natural interpretation in terms of competing effects: entropy driving disorder and favorable pair interactions driving atomic ordering. To further clarify interpretation, we present results for representative ternary alloys CuAgAu, NiPdPt, RhPdAg, and CoNiCu within a frozen charge (or band-only) approximation. These results include the so-called Onsager mean-field correction that extends the temperature range for which the theory is valid.

Exact and rigorous solutions of the ground-state problem for the classical Heisenberg model with nearest-neighbor interactions on two- and three-dimensional lattices composed of zigzag (triangular) ladders are obtained in a very simple way, with the use of a cluster method. It is shown how the geometrical frustration due to the presence of triangles as structural units leads to the emergence of incommensurate spiral orderings and their frustrated collinear limits which become nonfrustrated if the interaction along ladder legs changes the sign (becoming ferromagnetic). Interestingly, these orderings are determined by a single angle (along with the signs of the interactions between neighboring spins); therefore, the term “single-angle spiral ordering” is proposed. A hypothesis about the origin of a puzzling spin ordering in a zigzag ladder compound $\beta \text{−}{\mathrm{CaCr}}_2{\mathrm{O}}_4$ is made.

The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a spin-density-wave transition both being in close proximity to the superconducting phase. A key question is the nature of high-$T_c$ superconductivity and its relation to orbital ordering and magnetism. Here we study the ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ superconductor using polarization-resolved Raman spectroscopy. The Raman susceptibility displays critical enhancement of nonsymmetric charge fluctuations across the entire phase diagram, which are precursors to a $d$-wave Pomeranchuk instability at temperature $\theta \left(x\right)$. The charge fluctuations are interpreted in terms of quadrupole interorbital excitations in which the electron and hole Fermi surfaces breathe in-phase. Below $T_c$, the critical fluctuations acquire coherence and undergo a metamorphosis into a coherent in-gap mode of extraordinary strength.

The ${122}^{☆}$ series of iron chalcogenide superconductors, for example ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$, only possesses electron Fermi pockets. Their distinctive electronic structure challenges the picture built upon iron pnictide superconductors, where both electron and hole Fermi pockets coexist. However, partly due to the intrinsic phase separation in this family of compounds, many aspects of their behavior remain elusive. In particular, the evolution of the ${122}^{☆}$ series of iron chalcogenides with chemical substitution still lacks a microscopic and unified interpretation. Using angle-resolved photoemission spectroscopy, we studied a major fraction of ${122}^{☆}$ iron chalcogenides, including the isovalently “doped” ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}{\mathrm{S}}_z,{\mathrm{Rb}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}{\mathrm{Te}}_z$, and ${(\text{Tl},\mathrm{K})}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}{\mathrm{S}}_z$. We found that the bandwidths of the low energy Fe $3d$ bands in these materials depend on doping; and more crucially, as the bandwidth decreases, the ground state evolves from a metal to a superconductor, and eventually to an insulator, yet the Fermi surface in the metallic phases is unaffected by the isovalent dopants. Moreover, the correlation-driven insulator found here with small band filling may be a novel insulating phase. Our study shows that almost all the known ${122}^{☆}\text{-series}$ iron chalcogenides can be understood via one unifying phase diagram which implies that moderate correlation strength is beneficial for the superconductivity.

The emergent behavior of spin liquids that are born out of geometrical frustration makes them an intriguing state of matter. We show that in the quantum kagome antiferromagnet ${\mathrm{ZnCu}}_3({\mathrm{OH})}_6{\mathrm{SO}}_4$ several different correlated yet fluctuating states exist. By combining complementary local-probe techniques with neutron scattering, we discover a crossover from a critical regime into a gapless spin-liquid phase with decreasing temperature. An additional unconventional instability of the latter phase leads to a second, distinct spin-liquid state that is stabilized at the lowest temperatures. We advance such complex behavior as a feature common to different frustrated quantum magnets.

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical end points and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.

The paper describes lamellar heterostructures, observed in $\left(1-x\right)\text{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3\text{−}x{\mathrm{PbTiO}}_3$ (PMN-$x\text{PT}$) single crystals with $x=0.32$. These lamellas are naturally formed in single crystals cooled under bias electric field applied along $[001{\mathrm{]}}_{\mathrm{pc}}$ and then zero-field heated to the vicinity of the so-called depoling temperature $T_{\mathrm{RT}}$, but similar structures were also encountered at ambient conditions. The polarization dependence of the Raman scattering intensities was employed to demonstrate that these lamellar structures are composed of tetragonal-like and rhombohedral-like layers extending over macroscopic (mm) lengths. The properties and interest of these macroscopic heterophase interfaces are discussed.

The rotational instability of the electric polarization $\mathbf{P}$ during phase transformations between ferroelectric phases is of great practical interest, since it may be accompanied by extremely large values of the piezoelectric coefficient, and a divergence of the coupled shear compliance contributes to such enhancements. In the literature, this has been explicitly calculated in the framework of the Landau theory and discussed with specific numerical simulations involving tetragonal, orthorhombic, and rhombohedral ferroelectric phases. When monoclinic phases are involved, such an approach is practically impossible, and an approximated treatment has been proposed, based on the observation that in those cases there are shear strains almost linearly coupled to the transverse component of $\mathbf{P}$, implying a divergence of the Curie-Weiss type in the associated compliances. Here the argument is extended to the general case of transitions whose major effect is a rotation of the polarization, and the limits of its validity are discussed. As experimental verification, the elastic response of ${\mathrm{BaTiO}}_3$ is measured and analyzed, together with those of other ferroelectric perovskites available in the literature, such as KNN.

The standard two-temperature equations for electron-phonon coupled thermal transport across metal/nonmetal interfaces are modified to include the possible coupling between metal electrons with substrate phonons. The previous two-temperature molecular dynamics (TT-MD) approach is then extended to solve these equations numerically at the atomic scale, and the method is demonstrated using Cu/Si interface as an example. A key parameter in TT-MD is the nonlocal coupling distance of metal electrons and nonmetal phonons, and here we use two different approximations. The first is based on Overhauser's “joint-modes” concept, while we use an interfacial reconstruction region as the length scale of joint region rather than the phonon mean-free path as in Overhauser's original model. In this region, the metal electrons can couple to the joint phonon modes. The second approximation is the “phonon wavelength” concept where electrons couple to phonons nonlocally within the range of one phonon wavelength. Compared with the original TT-MD, including the cross-interface electron-phonon coupling can slightly reduce the total thermal boundary resistance. Whether the electron-phonon coupling within the metal block is nonlocal or not does not make an obvious difference in the heat transfer process. Based on the temperature profiles from TT-MD, we construct a new mixed series-parallel thermal circuit. We show that such a thermal circuit is essential for understanding metal/nonmetal interfacial transport, while calculating a single resistance without solving temperature profiles as done in most previous studies is generally incomplete. As a comparison, the simple series circuit that neglects the cross-interface electron-phonon coupling could overestimate the interfacial resistance, while the simple parallel circuit in the original Overhauser's model underestimates the total interfacial resistance.

We report a resonant x-ray magnetic scattering study of multiferroic ${\mathrm{SmMn}}_2{\mathrm{O}}_5$, for which the microscopic magnetic properties have not yet been revealed. Dielectric measurements indicated that ferroelectric phase transitions occur around $T_{\mathrm{C}1}\simeq 34$ K and $T_{\mathrm{C}2}\simeq 28$ K. Resonant x-ray magnetic scattering was observed at both the Sm $L{}_{\text{I}\text{I}\text{I}}$ and Mn $K$ edges in the ferroelectric phase below $T_{\mathrm{C}2}$. The Sm and Mn magnetic moments align antiferromagnetically with a magnetic propagation wave vector ${\mathrm{q}}_{\mathrm{M}}=(1/2,0,0)$, and the ferroelectric transition coincides with the antiferromagnetic transition. The azimuthal dependence of the resonant magnetic signals confirmed that the Sm and Mn magnetic moments have a dominant $c$-axis component, which is quite unique because Mn moments have a dominant $a$-axis component in other $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ $(R=\text{rare earth},\text{Y},\text{Bi})$ compounds. We propose a magnetic structure with the magnetic space group $P_{2a}b{2}_{1}m$, which allows electric polarization along the $b$ axis in the ferroelectric phase.

We consider magnetization configurations at chiral magnet (CM)/ferromagnet (FM) heterostructures. In the CM, magnetic skyrmions and spin helices emerge due to the Dzyaloshinskii-Moriya interaction, which then penetrate into the adjacent FM. However, because the nonuniform magnetization structures are energetically unfavorable in the FM, the penetrated magnetization structures are deformed, resulting in exotic three-dimensional configurations, such as skyrmion cones, sideways skyrmions, and twisted helices and skyrmions. We discuss the stability of possible magnetization configurations at the CM/FM and CM/FM/CM hybrid structures within the framework of the variational method, and find that various magnetization configurations appear in the ground state, some of which cause nontrivial emergent magnetic field.

Uranium-based intermetallic compounds often display very strong magnetic anisotropies, the energy of which is usually not directly accessible by common experimental methods. Here, we report on static- and pulsed-field studies of ${\mathrm{U}}_3\mathrm{C}{\mathrm{u}}_4\mathrm{G}{\mathrm{e}}_4$. This material orders ferromagnetically at $T_{\mathrm{C}}=73\mathrm{K}$ with the easy magnetization direction along the $a$ axis and a strong $bc$-plane anisotropy. The magnetization measured for fields along the hard $b$ direction displays a first-order magnetization process that can be described well by use of a phenomenological theory yielding anisotropy constants up to the sixth order. This phenomenological description, working excellently for ${\mathrm{U}}_3\mathrm{C}{\mathrm{u}}_4\mathrm{G}{\mathrm{e}}_4$, may also be applied for other uranium-based compounds.

We analyze the three-point vertex function that describes the coupling of fermionic particle-hole pairs in a metal to spin or charge fluctuations at nonzero momentum. We consider Ward identities, which connect two-particle vertex functions to the self-energy, in the framework of a Hubbard model. These are derived using conservation laws following from local symmetries. The generators considered are the spin density and particle density. It is shown that at certain antiferromagnetic critical points, where the quasiparticle effective mass is diverging, the vertex function describing the coupling of particle-hole pairs to the spin density Fourier component at the antiferromagnetic wave vector is also divergent. Then we give an explicit calculation of the irreducible vertex function for the case of three-dimensional antiferromagnetic fluctuations, and show that it is proportional to the diverging quasiparticle effective mass.