Modifications of the electronic bands of thin FeSe films due to oxygen vacancies in the supporting ${\mathrm{SrTiO}}_3$(001) substrate—and the interplay with spin-orbit coupling, magnetism, and epitaxy—are investigated by first-principles supercell calculations. Unfolded $(k$-projected) bands show that the oxygen vacancies both provide electron doping to the interface FeSe layer and also have notable effects on the details of the bands around the Fermi level, including renormalizing the width of the $\mathrm{Fe}\text{−}3d$ band near the Fermi level by a factor of about 0.6 and causing a splitting of $\sim 40\mathrm{meV}$ at the $M$ point for the checkerboard antiferromagnetic configuration. For an FeSe bilayer, the modifications to the bands are mainly limited to the interface FeSe layer. While spin-orbit-coupling induced band splittings of $\sim 30\mathrm{meV}$ at $M$ for the ideal $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$(001) interfaces are comparable to the splitting due to oxygen vacancies, the effects are not simply additive. Calculations and comparison to our scanning tunneling microscopy images of MBE-grown FeSe films on ${\mathrm{SrTiO}}_3$(001) suggest that a common defect may be Se bound to an oxygen vacancy at the interface.

In this paper we perform a critical assessment of different known methods for the analytical continuation of bosonic functions, namely, the maximum entropy method, the non-negative least-squares method, the non-negative Tikhonov method, the Padé approximant method, and a stochastic sampling method. Four functions of different shape are investigated, corresponding to four physically relevant scenarios. They include a simple two-pole model function; two flavors of the tight-binding model on a square lattice, i.e., a single-orbital metallic system and a two-orbital insulating system; and the Hubbard dimer. The effect of numerical noise in the input data on the analytical continuation is discussed in detail. Overall, the stochastic method by A. S. Mishchenko et al. [Phys. Rev. B 62, 6317 (2000)] is shown to be the most reliable tool for input data whose numerical precision is not known. For high-precision input data, this approach is slightly outperformed by the Padé approximant method, which combines a good-resolution power with a good numerical stability. Although none of the methods retrieves all features in the spectra in the presence of noise, our analysis provides a useful guideline for obtaining reliable information of the spectral function in cases of practical interest.

The electron-nuclear hyperfine interaction shows up in a variety of phenomena including, e.g., NMR studies of correlated states and spin decoherence effects in quantum dots. Here we focus on the hyperfine coupling and the NMR spin relaxation time $T_1$ in Weyl semimetals. Since the density of states in Weyl semimetals varies with the square of the energy around the Weyl point, a naive power counting predicts a $1/T_{1}T\sim E^4$ scaling, with $E$ the maximum of temperature ($T$) and chemical potential. By carefully investigating the hyperfine interaction between nuclear spins and Weyl fermions, we find that while its spin part behaves conventionally, its orbital part diverges unusually, with the inverse of the energy around the Weyl point. Consequently, the nuclear spin relaxation rate scales in a graphenelike manner as $1/T_{1}T\sim E^{2}ln(E/{\omega }_0)$, with ${\omega }_0$ the nuclear Larmor frequency. This allows us to identify an effective hyperfine coupling constant, which is tunable by gating or doping. This is relevant for the decoherence effect in spintronics devices and double quantum dots, where hyperfine coupling is the dominant source of spin-blockade lifting.

We study the occurrence of negative differential conductance induced by resonance effects in a model for a multilayer heterostructure. In particular, we consider a system consisting of several correlated and noncorrelated monatomic layers sandwiched between two metallic leads. The geometry confines electrons in wells within the heterostructures, which are connected to each other and to the leads by tunneling processes. The nonequilibrium situation is produced by applying a bias voltage to the leads. Our results show that, for specific values of the parameters, resonance tunneling takes place. We investigate in detail its influence on the current-voltage characteristics. Our results are obtained via nonequilibrium real-space dynamical mean-field theory. As an impurity solver we use the so-called auxiliary master equation approach, which addresses the impurity problem within an auxiliary system consisting of a correlated impurity, a small number of uncorrelated bath sites, and two Markovian environments described by a generalized master equation.

Capacitance-voltage spectroscopy has proved to be a very powerful experimental technique towards the investigation of carrier-carrier interactions both qualitatively and quantitatively in complex coupled nanostructures. Here, we exploit this method to observe indirect exciton complexes in a quantum dot molecule and to quantify the electron-hole interactions between two dots in a quantum dot molecule, formed by vertical stacking of self-assembled quantum dot layers. While frequency-dependent measurements distinguish between the $s$- and $p$-charging behavior, under perpendicular magnetic fields, reordering of the quantized states charging sequence is observed along with the formation of a Landau fan in the wetting layer that is used to reconstruct the Fermi energy level.

The role of hydrogen and other defects in the stabilization of polar oxide interfaces is a matter of significant fundamental and practical interest. Using experimental (scanning tunneling microscopy, x-ray photoelectron spectroscopy) and theoretical (density functional theory) surface science techniques, we find that the polar Zn-terminated ZnO(0001) surface becomes excessively Zn deficient during high-temperature annealing (780 K) in ultrahigh vacuum (UHV). The Zn vacancies align themselves into rows parallel to the $\left[10\overline{1}0\right]$ direction, and the remaining surface Zn ions alternately occupy wurtzite (hcp) and zinc-blende (fcc) lattice positions, giving a characteristic “striped” $c(\sqrt{12}\times \sqrt{12})\mathrm{R}30$° surface morphology with three types of rows: wurtzite Zn, zinc-blende Zn, and Zn vacancies. Interstitial H plays a central role in such a reconstruction, as it helps to compensate the excessive Zn deficiency. We propose a model in which hydrogen occupies positions in half of the vacancy rows to form hydroxide ions that can participate in hydrogen bonds in the O subsurface layer as a result of the mixed wurtzite/zinc-blende stacking.

We theoretically investigate excitons in ${\mathrm{MoS}}_2$ monolayers in an applied in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations predict a quadratic Stark shift, of the order of a few meV for fields of 10 V/$\mu \mathrm{m}$, in the linear absorption spectra. The spectral weight of the main exciton peaks decreases by a few percent with an increasing electric field due to the exciton field ionization into free carriers as reflected in the exciton wave functions. Subpicosecond exciton decay lifetimes at fields of a few tens of V/$\mu \mathrm{m}$ could be utilized in solar energy harvesting and photodetection. We find simple scaling relations of the exciton binding, radius, and oscillator strength with the dielectric environment and an electric field, which provides a path to engineering the ${\mathrm{MoS}}_2$ electro-optical response.

Recent experiments involving tilted graphene samples have shown evidence of a continuous phase transition in the $\nu =0$ quantum Hall bulk state. We present here a simple model that supports such a transition. In addition to a long-range SU(4) symmetric Coulomb interaction, we include Hubbard on-site and nearest neighbor interactions with tunable coupling strengths, and perform a self-consistent Hartree-Fock analysis. A large sea of negative energy Landau levels is retained, and is shown to have important qualitative and quantitative effects. Phase diagrams are constructed within the space of physically relevant parameters, yielding results consistent with experimental observation.

Two kinds of phase diagrams can be observed in doped ferroic materials. A glass phase diagram is formed by doping a nontransforming end into a ferroic matrix, while doping a transforming end forms phase diagrams with a phase boundary separating two different ferroic phases. Here we report a phase diagram in which a strain glass state is sandwiched between two distinct ferroelastic phases. This type of phase diagram in doped ferroelastic materials bridges the one with a glass state and the one with a phase boundary. We thus establish a 3D phase diagram of ${\mathrm{Ti}}_{50-y}{\mathrm{Ni}}_{50+y-x}{\mathrm{Pd}}_x$ ternary alloys, in which the evolution of these different kinds of phase diagrams can be observed. An understanding from the Landau free energy landscape suggests that the transforming doping end plays three roles in influencing the ferroic matrix: (1) to destabilize the ferroic matrix phase, (2) to stabilize another ferroic phase different from the matrix one, and (3) to create random local fields. The competition between these effects determines various phase diagrams in doped ferroic materials. Thus our work may provide an experimental foundation for a unified mechanism to all three kinds of phase diagrams.

We explore nonanalytic quantum phase dynamics of dipolar exciton condensates formed in a system of two-dimensional quantum layers subjected to voltage quenches. We map the exciton condensate physics on to the pseudospin ferromagnet model, showing an additional oscillatory metastable phase beyond the well-known ferromagnetic phase by utilizing a time-dependent, nonperturbative theoretical model. We explain the coherent phase of the exciton condensate in quantum Hall bilayers, observed for currents equal to and slightly larger than the critical current, as a stable time-dependent phase characterized by persistent flow of charged order parameter defect in each of the individual layers with a characteristic ac Josephson frequency. As the magnitude of the voltage quench is further increased, we find that the time-dependent current oscillations associated with the charged order parameter defect flow decay, resulting in a transient pseudospin paramagnet phase characterized by partially coherent charge transfer between layers, before the state relaxes to incoherent charge transfer between the layers.

Quantum entanglement under an extensive bipartition can reveal the critical boundary theory of a topological phase in a parameter space. In this study we demonstrate that the infinite-randomness fixed point for spin-$\frac{1}{2}$ degrees of freedom can emerge from an extensive random bipartition of the spin-1 Affleck-Kennedy-Lieb-Tasaki chain. The nested entanglement entropy of the ground state of the reduced density matrix exhibits a logarithmic scaling with an effective central charge $\tilde{c}=0.72\pm 0.02\approx ln2$. We further discuss, in the language of bulk quantum entanglement, how to understand all phase boundaries and the surrounding Griffiths phases for the antiferromagnetic Heisenberg spin-1 chain with quenched disorder and dimerization.

We develop a general phenomenology describing the interplay between coherent and incoherent dynamics in ferromagnetic insulators. Using the Onsager reciprocity and Neumann's principle, we derive expressions for the local thermomagnonic torques exerted by thermal magnons on the order-parameter dynamics and the reciprocal pumping processes, which are in close analogy to the spin-transfer torque and the spin pumping at metallic interfaces. Our formalism is applicable to general long-wavelength dynamics and, although here we explicitly focus on ferromagnetic insulators possessing U(1) symmetry, our approach can be easily extended to other classes of magnetic materials. As an illustrative example, we apply our theory to investigate a domain wall floating over a spin superfluid, whose dynamics are triggered thermally at the system's edge. Our results demonstrate that the local pumping of coherent spin dynamics by a thermal magnon gas offers an alternative route—with no need for conducting components and thus devoid of ohmic losses—for the control and manipulation of topological solitons.

$\mathrm{G}{\mathrm{d}}_2\mathrm{S}{\mathrm{n}}_2{\mathrm{O}}_7$ and $\mathrm{G}{\mathrm{d}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ have been regarded as good experimental realizations of the classical Heisenberg pyrochlore antiferromagnet with dipolar interaction. The former was found to adopt the Palmer-Chalker state via a single, first-order transition at $T_{\mathrm{N}}\approx 1\mathrm{K}$, while the latter enters a distinct, partially ordered state through two successive transitions at $T_{\mathrm{N}1}\approx 1\mathrm{K}$ and $T_{\mathrm{N}2}=0.75\mathrm{K}$. To shed more light on their distinct magnetic ground states, we have synthesized two more gadolinium-based pyrochlore oxides, $\mathrm{G}{\mathrm{d}}_2\mathrm{G}{\mathrm{e}}_2{\mathrm{O}}_7$ and $\mathrm{G}{\mathrm{d}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$, under high-pressure conditions and performed detailed characterizations via x-ray powder diffraction, dc and ac magnetic susceptibility, and specific heat measurements down to 100 mK. We found that both compounds enter a long-range antiferromagnetically ordered state through a single, first-order transition at $T_{\mathrm{N}}=1.4\mathrm{K}$ for $\mathrm{G}{\mathrm{d}}_2\mathrm{G}{\mathrm{e}}_2{\mathrm{O}}_7$ and $T_{\mathrm{N}}=1.56\mathrm{K}$ for $\mathrm{G}{\mathrm{d}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$, with the specific heat anomaly similar to that of $\mathrm{G}{\mathrm{d}}_2\mathrm{S}{\mathrm{n}}_2{\mathrm{O}}_7$ rather than $\mathrm{G}{\mathrm{d}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$. Interestingly, the low-temperature magnetic specific heat values of both $\mathrm{G}{\mathrm{d}}_2\mathrm{G}{\mathrm{e}}_2{\mathrm{O}}_7$ and $\mathrm{G}{\mathrm{d}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$ were found to follow nicely the $T^3$ dependence as expected for a three-dimensional antiferromagnet with gapless spin-wave excitations. We have rationalized the enhancement of $T_{\mathrm{N}}$ in terms of the reduced Gd-Gd distances for the chemically pressurized $\mathrm{G}{\mathrm{d}}_2\mathrm{G}{\mathrm{e}}_2{\mathrm{O}}_7$ and the addition of extra superexchange pathways through the empty $\mathrm{Pt}-e_g$ orbitals for $\mathrm{G}{\mathrm{d}}_2\mathrm{P}{\mathrm{t}}_2{\mathrm{O}}_7$. Our current study has expanded the family of gadolinium-based pyrochlores and permits us to achieve a better understanding of their distinct magnetic properties in a more comprehensive perspective.

Motivated by the discovery of spiral and modulated collinear phases in several magnetic materials, we investigate the magnetic properties of Heisenberg spin $S=1/2$ antiferromagnets in two and three dimensions, with frustration arising from second-neighbor couplings in one axial direction [the axial next-nearest-neighbor Heisenberg (ANNNH) model]. Our results clearly demonstrate the presence of an incommensurate spiral phase at $T=0$ in two dimensions, extending to finite temperatures in three dimensions. The crossover between Néel and spiral order occurs at a value of the frustration parameter considerably above the classical value 0.25, a sign of substantial quantum fluctuations. We also investigate a possible modulated collinear phase with a wavelength of four lattice spacings and find that it has substantially higher energy and hence is not realized in the model.

A systematic investigation is presented for the magnetization dynamics in trilayer nanowires, consisting of two permalloy (${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) layers separated by a nonmagnetic Ru spacer layer. The width of the wires ranges from 90 to 190 nm. By varying the Ru thickness between 0.7 and 2.0 nm, the interlayer coupling can be effectively controlled, modifying the corresponding magnetic ground state and the spin-wave dynamics. By contrast with previous work on coupled trilayer nanowires with larger widths (270 nm and more), the focus here is on nanowire arrays where the strong shape anisotropy competes with the Ruderman-Kittel-Kasuya-Yosida interactions and biquadratic exchange interactions across the Ru interface, as well as dipolar interactions and Zeeman energy. As a result, the spin-wave spectrum is found to be drastically modified. Ferromagnetic resonance and hysteresis loop measurements are reported over a wide range of applied magnetic fields, showing that the overall magnetization alignment between the permalloy layers may be parallel, antiparallel, or in a spin-flop state, depending on the overall interlayer coupling. The experimental results for different stripe widths are successfully analyzed using a microscopic dipole-dipole theory and micromagnetic simulations.

We use polarized inelastic neutron scattering to study the temperature and energy dependence of spin space anisotropies in the optimally-hole-doped iron pnictide ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($T_{\mathrm{c}}=38$ K). In the superconducting state, while the high-energy part of the magnetic spectrum is nearly isotropic, the low-energy part displays a pronounced anisotropy, manifested by a $c$-axis polarized resonance. We also observe that the spin anisotropy in superconducting ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{Fe}}_2{\mathrm{As}}_2$ extends to higher energies compared with electron-doped ${\mathrm{BaFe}}_{2-x}T{M}_x{\mathrm{As}}_2$ ($TM=\mathrm{Co}$, Ni) and isovalent-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_{1.4}{\mathrm{P}}_{0.6}$, suggesting a connection between $T_{\mathrm{c}}$ and the energy scale of the spin anisotropy. In the normal state, the low-energy spin anisotropy for hole- and electron-doped iron pnictides near optimal superconductivity onset at temperatures similar to the temperatures at which the elastoresistance deviates from Curie–Weiss behavior, pointing to a possible connection between the two phenomena. Our results highlight the relevance of the spin-orbit coupling to the superconductivity of the iron pnictides.

The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally doped ${\mathrm{SmFeAsO}}_{0.88}{\mathrm{F}}_{0.12}$ iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents ($x$) by means of muon-spin spectroscopy ($\mu \mathrm{SR}$), both in zero and in transverse fields, to probe the magnetic and the superconducting order, respectively. The pure superconducting phase (at $x<0.03$) is replaced by a crossover region at intermediate Mn values ($0.03\le x<0.08$), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for $x=0.08$, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of $T_c$ and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors (M. N. Gastiasoro et al., Phys. Rev. Lett. 117, 257002 (2016)), which includes correlation-enhanced RKKY couplings between the impurity moments.

Wave diffraction is fundamentally difficult to overcome in the routing and interconnection of photonic signals. Although the phenomenon of reflectionless transport through sharp corners in a routing path has been realized in many previous demonstrations, wave diffraction does not allow them to transport deep-subwavelength information or sub-diffraction-limited images. Recent advances in $\varepsilon$-near-zero and anisotropic $\varepsilon$-near-infinity metamaterials have provided unique possibilities of achieving reflectionless diffraction-free electromagnetic wave routing, but their designs are fundamentally limited to narrow bandwidths, and they have not been demonstrated in reality. Here we experimentally demonstrate broadband reflectionless diffraction-free routing of electromagnetic waves through two right-angled sharp corners in a bent microwave rectangular waveguide. An image with deep-subwavelength information is transported through the bent waveguide in a broad bandwidth. This Rapid Communication supplements and extends the current studies of metamaterials with extreme permittivities and can be useful for routing and interconnection of subwavelength photonic information.

We derive general constraints on the existence of many-body localized (MBL) phases in the presence of global symmetries, and show that MBL is not possible with symmetry groups that protect multiplets (e.g., all non-Abelian symmetry groups). Based on simple representation theoretic considerations, we derive general Mermin-Wagner–type principles governing the possible alternative fates of nonequilibrium dynamics in isolated, strongly disordered quantum systems. Our results rule out the existence of MBL symmetry-protected topological phases with non-Abelian symmetry groups, as well as time-reversal symmetry-protected electronic topological insulators, and in fact all fermion topological insulators and superconductors in the 10-fold way classification. Moreover, extending our arguments to systems with intrinsic topological order, we rule out MBL phases with non-Abelian anyons as well as certain classes of symmetry-enriched topological orders.

Structures and magnetic properties of $\mathrm{F}{\mathrm{e}}_{16-x}\mathrm{C}{\mathrm{o}}_x{\mathrm{N}}_2$ are studied using adaptive genetic algorithm and first-principles calculations. We show that substituting Fe with Co in $\mathrm{F}{\mathrm{e}}_{16}{\mathrm{N}}_2$ with a Co/Fe ratio $\le 1$ can greatly improve the magnetic anisotropy of the material. The magnetocrystalline anisotropy energy from first-principles calculations reaches $3.18\mathrm{MJ}/{\mathrm{m}}^3$ (245.6 μeV per metal atom) for $\mathrm{F}{\mathrm{e}}_{12}\mathrm{C}{\mathrm{o}}_4{\mathrm{N}}_2$, much larger than that of $\mathrm{F}{\mathrm{e}}_{16}{\mathrm{N}}_2$, and is one of the largest among the reported rare-earth-free magnets. From our systematic crystal structure searches, we show that there is a structure transition from tetragonal $\mathrm{F}{\mathrm{e}}_{16}{\mathrm{N}}_2$ to cubic $\mathrm{C}{\mathrm{o}}_{16}{\mathrm{N}}_2$ in $\mathrm{F}{\mathrm{e}}_{16-x}\mathrm{C}{\mathrm{o}}_x{\mathrm{N}}_2$ as the Co concentration increases, which can be well explained by electron counting analysis. Different magnetic properties between the Fe-rich $(x\le 8)$ and Co-rich $(x>8)$ $\mathrm{F}{\mathrm{e}}_{16-x}\mathrm{C}{\mathrm{o}}_x{\mathrm{N}}_2$ is closely related to the structural transition.