Although the supercell method has been widely used for surface calculations, it only works well with short-ranged potentials, but meets difficulty when the potential decays very slowly into the vacuum. Unfortunately, the exact exchange-correlation potential of the density functional theory is asymptotically long ranged, and therefore is not easily handled by use of supercells. This paper illustrates that the authentic slab geometry, another technique for surface calculations, is not affected by this issue: It works equally well with both short- and long-ranged potentials, with the computational cost and the convergence speed being essentially the same. Using the asymptotically long-ranged Becke-Roussel'89 exchange potential as an example, we have calculated six surfaces of various types. We found that accurate potential values can be obtained even in extremely low density regions of more than $100\text{Å}$ away from the surface. This high performance allows us to explore the asymptotic region, and prove with clean numerical evidence that the Becke-Roussel'89 potential satisfies the correct asymptotic behavior for slab surfaces, as it does for finite systems. Our finding further implies that the Slater component of the exact exchange optimized effective potential is responsible for the asymptotic behavior, not only for jellium slabs, but for slabs of any type. The Becke-Roussel'89 potential may therefore be used to build asymptotically correct model exchange potentials applicable to both finite systems and slab surfaces.

The interfacial atomic arrangement, which is different from that in the bulk form of the heterojunction, can induce a reconstruction of electrostatic field at the interface. For conventional semiconductor heterointerfaces, it is known that such reconstruction results in band bending, creating a quantum well in which the two-dimensional electron gas (2DEG) is formed. In this article, we show that this mechanism still works in a multivalent oxide heterojunction: for $\left(110\right)\mathrm{LaAl}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ (LAO/STO) heterojunctions, the coexistence of La and Ti in $\mathrm{AB}{\mathrm{O}}_3$ perovskite unit cells at the interface reduces the valence of Ti, generating a local field leading to band bending in the interfacial STO layers. The extra free electrons are trapped in this bent conduction band forming a 2DEG. It unifies two independent mechanisms for 2DEG at LAO/STO interfaces, the “polar catastrophe” model and the “$\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Ti}{\mathrm{O}}_3$” layers model, and is expected to end the decade-old controversy. This study opens insight into atomic-scale band engineering to control the behavior of complex oxide heterojunctions.

We report a high-resolution angle-resolved photoemission spectroscopy study of the evolution of the electronic structure of ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ single crystals. Isovalent S substitution onto the Se site constitutes a chemical pressure which subtly modifies the electronic structure of FeSe at high temperatures and induces a suppression of the tetragonal-symmetry-breaking structural transition temperature from 87 to 58 K for $x=0.15$. With increasing S substitution, we find smaller splitting between bands with $d_{yz}$ and $d_{xz}$ orbital character and weaker anisotropic distortions of the low-temperature Fermi surfaces. These effects evolve systematically as a function of both S substitution and temperature, providing strong evidence that an orbital ordering is the underlying order parameter of the structural transition in ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$. Finally, we detect the small inner hole pocket for $x=0.12$, which is pushed below the Fermi level in the orbitally ordered low-temperature Fermi surface of FeSe.

The Majorana fermions propagating along the edge of a topological superconductor with $p_x+i{p}_y$ pairing deliver a shot noise power of $\frac{1}{2}\times e^2/h$ per eV of voltage bias. We calculate the full counting statistics of the transferred charge and find that it becomes trinomial in the low-temperature limit, distinct from the binomial statistics of charge-$e$ transfer in a single-mode nanowire or charge-$2e$ transfer through a normal-superconductor interface. All even-order correlators of current fluctuations have a universal quantized value, insensitive to disorder and decoherence. These electrical signatures are experimentally accessible, because they persist for temperatures and voltages large compared to the Thouless energy.

The theoretical description for the reflectivity properties of dielectric, metal, and semiconductor plates coated with graphene is developed in the framework of the Dirac model. Graphene is described by the polarization tensor allowing the analytic continuation to the real frequency axis. The plate materials are described by the frequency-dependent dielectric permittivities. The general formulas for the reflection coefficients and reflectivities of the graphene-coated plates, as well as their asymptotic expressions at high and low frequencies, are derived. The developed theory is applied to the graphene-coated dielectric (fused silica), metal (Au and Ni), and semiconductor (Si with various charge carrier concentrations) plates. In all these cases, the impact of graphene coating on the plate reflectivity properties is calculated over a wide frequency range. The obtained results can be used in many applications exploiting the graphene coatings, such as the optical detectors, transparent conductors, antireflection surfaces, etc.

Based on first-principles density functional calculations, we predict that nitrogen atoms can form a single-layer, buckled honeycomb structure called nitrogene, which is rigid and stable even above room temperature. This 2D crystalline phase of nitrogen, which corresponds to a local minimum in the Born-Oppenheimer surface, is a nonmagnetic insulator with saturated $\pi$ bonds. When grown on a substrate like Al(111) surface and graphene, nitrogene binds weakly to substrates and hence preserves its free-standing properties, but it can easily be pealed off. Zigzag and armchair nanoribbons of nitrogene have fundamental band gaps derived from reconstructed edge states. These band gaps are tunable with size and suitable for the emerging field of 2D electronics. Nitrogene forms not only bilayer, but also 3D graphitic multilayer structures. Single-layer nitrogene can nucleate and grow on the armchair edges of hexagonal boron nitride.

The electrochemical reactions of solid oxide fuel cells occur in the region where gas-phase species, electrode, and electrolyte coincide. When the electrode is an ionic insulator and the electrolyte is an electronic insulator, this triple phase boundary is assumed to have atomic dimensions. Here we use photoemission electron microscopy to show that the reduced surface of the electrolyte yttria-stabilized zirconia (YSZ) has a sharp electronic metal-insulator boundary near Pt negative electrodes. The electronic conductivity of the reduced YSZ allows for oxygen reduction, allowing the reduced YSZ to behave as an extended triple phase boundary. This extended triple phase boundary can be many microns in size, depending on oxygen pressure, temperature, applied voltage, and time.

We investigate cross-relaxation interactions between Tm and Al in ${\mathrm{Tm}}^{3+}:\mathrm{YAG}$ using two optical methods: spectral hole burning and stimulated echoes. These interactions lead to a substantial reduction in the hyperfine lifetime at magnetic fields that bring the Tm hyperfine transition into resonance with an Al transition. We develop models for the measured echo decay curves and hole-burning spectra near a resonance, which are used to show that the Tm-Al interaction has a resonance width of $12\pm 3$ kHz and reduces the hyperfine lifetime at resonance to $0.5\pm 0.3$ ms.

Using soft x-ray spectromicroscopy, we investigate the magnetic domain structure in embedded nanomagnets defined in $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ thin films and $\mathrm{LaFe}{\mathrm{O}}_3/\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ bilayers. We find that shape-controlled antiferromagnetic domain states give rise to a significant reduction of the switching field of the rectangular nanomagnets. This is discussed within the framework of competition between an intrinsic spin-flop coupling and shape anisotropy. The data demonstrates that shape effects in antiferromagnets may be used to control the magnetic properties in nanomagnets.

A microscopic model for the room-temperature multiferroic ${\mathrm{BiFeO}}_3$ that includes two Dzyaloshinskii-Moriya interactions and single-ion anisotropy along the ferroelectric polarization predicts both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic field. Due to simultaneously broken time-reversal and spatial-inversion symmetries, the absorption of light changes as the magnetic field or the direction of light propagation is reversed. We discuss three physical mechanisms that may contribute to this absorption asymmetry known as nonreciprocal directional dichroism: the spin current, magnetostriction, and single-ion anisotropy. We conclude that the nonreciprocal directional dichroism in ${\mathrm{BiFeO}}_3$ is dominated by the spin-current polarization and is insensitive to the magnetostriction and easy-axis anisotropy. With three independent spin-current parameters, our model accurately describes the nonreciprocal directional dichroism observed for magnetic field along $[1,-1,0]$. Since some modes are almost transparent to light traveling in one direction but opaque for light traveling in the opposite direction, ${\mathrm{BiFeO}}_3$ behaves as a room-temperature optical diode at certain frequencies in the gigahertz to terahertz range. Our work demonstrates that an analysis of the nonreciprocal directional dichroism spectra based on an effective spin model supplemented by first-principles calculations can produce a quantitative microscopic theory of the magnetoelectric couplings in multiferroic materials.

We report a positive muon spin relaxation and rotation $\left(\mu \mathrm{SR}\right)$ study of the quantum spin ice materials ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Yb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature $T_{\mathrm{c}}$ at which the specific heat displays an intense peak, namely $T_{\mathrm{c}}=0.24$ K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above $T_{\mathrm{c}}$ up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the $\mu \mathrm{SR}$ spectra recorded above $T_{\mathrm{c}}$. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ exhibits a much larger local magnetic susceptibility than ${\mathrm{Yb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ below $T_{\mathrm{c}}$. Considering previously reported ac susceptibility, neutron scattering, and $\mu \mathrm{SR}$ results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.

The opening of a spin gap in the orthorhombic compounds $\mathrm{Ce}{T}_2{\mathrm{Al}}_{10}$ ($T=\mathrm{Ru}\text{and}\mathrm{Os}$) is followed by antiferromagnetic ordering at $T_N=27$ and 28.5 K, respectively, with a small ordered moment $(0.29–0.34{\mu }_B$) along the $c$ axis, which is not an easy axis of the crystal field (CEF). In order to investigate how the moment direction and the spin gap energy change with La doping in ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{T}_2{\mathrm{Al}}_{10}$ ($T$ = Ru and Os) and also to understand the microscopic nature of the magnetic ground state, we here report on magnetic, transport, and thermal properties, neutron diffraction (ND), and inelastic neutron scattering (INS) investigations on these compounds. Our INS study reveals the persistence of spin gaps of 7 and 10 meV in the 10% La-doped $T$ = Ru and Os compounds, respectively. More interestingly our ND study shows a very small ordered moment of 0.18 ${\mu }_B$ along the $b$ axis in ${\mathrm{Ce}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$, however a moment of 0.23 ${\mu }_B$ still along the $c$ axis in ${\mathrm{Ce}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{Os}}_2{\mathrm{Al}}_{10}$. This contrasting behavior can be explained by a different degree of hybridization in ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ and ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$, being stronger in the latter than in the former. Muon spin rotation ($\mu \mathrm{SR}$) studies on ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ ($x=0$, 0.3, 0.5, and 0.7), reveal the presence of coherent frequency oscillations indicating a long-range magnetically ordered ground state for $x=0$ to 0.5, but an almost temperature independent Kubo-Toyabe response between 45 mK and 4 K for $x=0.7$. We compare the results of the present investigations with those reported on the electron and hole doping in $\mathrm{Ce}{T}_2{\mathrm{Al}}_{10}$.

By simultaneously displaying magnetism and superconductivity in a single phase, the iron-based superconductors provide a model system for the study of magnetism's role in superconductivity. The class of intercalated iron selenide superconductors is unique among these in having the additional property of phase separation and coexistence of two distinct phases—one majority phase with iron vacancy ordering and strong antiferromagnetism, and the other a poorly understood minority microscopic phase with a contested structure. Adding to the intrigue, the majority phase has never been found to show superconductivity on its own while the minority phase has never been successfully synthesized separate from the majority phase. In order to better understand this minority phase, a series of high-quality $\mathrm{C}{\mathrm{s}}_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$ single crystals with $(0.8\le x\le 1;0\le y\le 0.3)$ were grown and studied. Neutron and x-ray powder diffraction performed on ground crystals show that the average $I4/mmm$ structure of the minority phase is distinctly different from the high-temperature $I4/mmm$ parent structure. Moreover, single-crystal diffraction reveals the presence of discrete superlattice reflections that remove the degeneracy of the Cs sites in both the majority and minority phases and reduce their structural symmetries from body centered to primitive. Group theoretical analysis in conjunction with structural modeling shows that the observed superlattice reflections originate from three-dimensional Cs vacancy ordering. This model predicts a $25\%$ vacancy of the Cs site in the minority phase which is consistent with the site's refined occupancy. Magnetization measurements performed in tandem with neutron single-crystal diffraction provide evidence that the minority phase is the host of superconductivity. Our results also reveal a superconducting dome in which the superconducting transition temperature varies as a function of the nominal valence of iron.

We study the high-temperature magnetic order in a quasi-two-dimensional honeycomb compound ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ by measuring magnetization and neutron powder diffraction with both polarized and unpolarized neutrons. ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ crystallizes into the hexagonal lead antimonate (${\mathrm{PbSb}}_2{\mathrm{O}}_6$, space group $P\overline{3}1m$) structure with layers of edge-sharing ${\mathrm{RuO}}_6$ octahedra separated by ${\mathrm{Sr}}^{2+}$ ions. ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ is found to order at $T_N=565$ K with Ru moments coupled antiferromagnetically both in plane and out of plane. The magnetic moment is 1.30(2) ${\mu }_{\mathrm{B}}$/Ru at room temperature and is along the crystallographic $c$ axis in the G-type magnetic structure. We perform density functional calculations with constrained random-phase approximation (RPA) to obtain the electronic structure and effective intra- and interorbital interaction parameters. The projected density of states shows strong hybridization between Ru $4d$ and O $2p$. By downfolding to the target $t_{2g}$ bands we extract the effective magnetic Hamiltonian and perform Monte Carlo simulations to determine the transition temperature as a function of inter- and intraplane couplings. We find a weak interplane coupling, 3% of the strong intraplane coupling, permits three-dimensional magnetic order at the observed $T_N$.

Extraordinary optical transmissions through metallic films with arrays of subwavelength holes have attracted much attention in the past few years, and surface plasmons are accepted as the key factor in their origin. Further studies showed that the abilities of surface plasmons can also be extended to achieve light collimation. Here, we show that collective surface oscillation can also occur in acoustics, and different from previous reports, we demonstrate that extraordinary acoustic transmissions and sound collimation can be achieved when the wavelength is much longer than the characteristic length of the sample. The excitation and detection approaches of the acoustic surface oscillation are also presented. Our findings should open up prospects for acoustic applications, and many exotic optical phenomena related to surface plasmons can be anticipated with their counterparts in acoustics.

The quest for multiferroics with coexisting ferroic orders, such as ferroelectricity and (anti-)ferromagnetism, in a single phase continues to arouse interest by the promise of novel functions and technological device paradigms. However, the mutually exclusive mechanism of ferroelectricity and magnetism has limited the discovery and understanding of novel multiferroic materials. Here we demonstrate using the hybrid functional of Hartree-Fock and density-functional theories that an unusual interaction between oxygen vacancies and misfit (epitaxial) strains brings about local magnetic moments with different orders coexisting with the host ferroelectricity of nonmagnetic (NM) $\mathrm{PbTi}{\mathrm{O}}_3$ due to orbital symmetry breaking of the defect electronic states. The emerging magnetic spin moment exhibits rich local order transitions in response to switching the spontaneous polarization, i.e., magnetoelectric coupling. Individual oxygen vacancies therefore act as a source of ferroic-order parameters. The multiferroic and magnetoelectric properties exploited by engineering defects and strains provide an alternative route for multiferroics and will open a new avenue for multifunctional oxides that exhibit magnetism coexisting with host functionalities.

Accurate knowledge of the single-molecule (self-) translational dynamics of liquid para-${\mathrm{H}}_2$ is an essential requirement for the calculation of the neutron scattering properties of this important quantum liquid. We show that, by using centroid molecular dynamics (CMD) quantum simulations of the velocity autocorrelation function, calculations of the total neutron cross section (TCS) remarkably agree with experimental data at the thermal and epithermal incident neutron energies where para-${\mathrm{H}}_2$ dynamics is actually dominated by the self-contributions. This result shows that a proper account of the quantum nature of the fluid, as provided by CMD, is a necessary and very effective condition to obtain the correct absolute-scale cross section values without the need of introducing any empirically adjusted quantity. At subthermal incident energies, appropriate modeling of the para-${\mathrm{H}}_2$ intermolecular (distinct) dynamics also becomes crucial, but quantum simulations are not yet able to cope with it. Existing simple models which account for the distinct part provide an appropriate correction of self-only calculations and bring the computed results in reasonable accord with TCS experimental data available until very recently. However, if just published cross section measurements in the cold range are considered, the agreement turns out to be by far superior and very satisfactory. The possible origin of slight residual differences will be commented on and suggests further computational and experimental efforts. Nonetheless, the ability to reproduce the total cross section in the wide range between 1 and 900 meV represents an encouraging and important validation step of the CMD method and of the present simple algorithm.

We have developed a unified finite-size scaling method for quantum phase transitions that requires no prior knowledge of the dynamical exponent $z$. During a quantum Monte Carlo simulation, the temperature is automatically tuned by the Robbins-Monro stochastic approximation method, being proportional to the lowest gap of the finite-size system. The dynamical exponent is estimated in a straightforward way from the system-size dependence of the temperature. As a demonstration of our novel method, the two-dimensional $S=1/2$ quantum $XY$ model in uniform and staggered magnetic fields is investigated in the combination of the world-line quantum Monte Carlo worm algorithm. In the absence of a uniform magnetic field, we obtain the fully consistent result with the Lorentz invariance at the quantum critical point, $z=1$, i.e., the three-dimensional classical $XY$ universality class. Under a finite uniform magnetic field, on the other hand, the dynamical exponent becomes two, and the mean-field universality with effective dimension $(2+2)$ governs the quantum phase transition.

It is shown that a local bend of a nanowire is a source of pinning potential for a transversal head-to-head (tail-to-tail) domain wall. Eigenfrequency of the domain wall free oscillations at the pinning potential and the effective friction are determined as functions of the curvature and domain wall width. The pinning potential originates from the effective curvature-induced Dzyaloshinsky-like term in the exchange energy. The theoretical results are verified by means of micromagnetic simulations for the case of parabolic shape of the wire bend.

We report an experimental and computational study of the magnetic and electronic properties of the layered Ru(V) oxide $\mathrm{SrR}{\mathrm{u}}_2{\mathrm{O}}_6$ (hexagonal, $P\overline{3}1m)$, which shows antiferromagnetic order with a Néel temperature of 563(2) K, among the highest for $4d$ oxides. Magnetic order occurs both within edge-shared octahedral sheets and between layers and is accompanied by anisotropic thermal expansivity that implies strong magnetoelastic coupling of Ru(V) centers. Electrical transport measurements using focused-ion-beam–induced deposited contacts on a micron-scale crystallite as a function of temperature show $p$-type semiconductivity. The calculated electronic structure using hybrid density functional theory successfully accounts for the experimentally observed magnetic and electronic structure, and Monte Carlo simulations reveal how strong intralayer as well as weaker interlayer interactions are a defining feature of the high-temperature magnetic order in the material.