The magnetic properties attributed to the hydroxide layer of ${\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\left(\mathrm{OH}\right){\mathrm{Fe}}_{1-y}\mathrm{Se}$ have been elucidated by the study of superconducting and nonsuperconducting members of this family of compounds. Both ac magnetometry and muon spin relaxation measurements of nonsuperconductors find a magnetic state existing below $\approx 10$ K which exhibits slow relaxation of magnetization. This magnetic state is accompanied by a low-temperature heat capacity anomaly present in both superconducting and nonsuperconducting variants suggesting that the magnetism persists into the superconducting state. The estimated value of magnetic moment present within the hydroxide layer supports a picture of a glassy magnetic state, probably comprising clusters of iron ions of varying cluster sizes distributed within the lithium hydroxide layer.

The insulating pyrochlore compound ${\mathrm{Nd}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ has been shown to undergo a second order magnetic phase transition at $T_{\mathrm{c}}\approx 0.91$ K to a noncoplanar all-in–all-out magnetic structure of the ${\mathrm{Nd}}^{3+}$ magnetic moments. An anomalously slow paramagnetic spin dynamics has been evidenced from neutron backscattering and muon spin relaxation $\left(\mu \mathrm{SR}\right)$. In the case of $\mu \mathrm{SR}$ this has been revealed through the strong effect of a 50 mT longitudinal field on the spin-lattice relaxation rate. Here, motivated by a recent successful work performed for ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Yb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, analyzing the shape of the $\mu \mathrm{SR}$ longitudinal polarization function, we substantiate the existence of extremely slow paramagnetic spin dynamics in the microsecond time range for ${\mathrm{Nd}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. Between 1.7 and 7 K, this time scale is temperature independent. This suggests a double spin-flip tunneling relaxation mechanism to be at play, probably involving spin substructures such as tetrahedra. Unexpectedly, the standard deviation of the field distribution at the muon site increases as the system is cooled. This exotic spin dynamics is in sharp contrast with the dynamics above 100 K which is driven by the Orbach relaxation mechanism involving single ${\mathrm{Nd}}^{3+}$ magnetic moments.

Aiming to analyze relevant aspects of interacting magnetic nanoparticle systems (frequently called interacting superparamagnets), a model is built from magnetic dipolar interaction and demagnetizing mean-field concepts. By making reasonable simplifying approximations, a simple and useful expression for effective demagnetizing factors is achieved, which allows the analysis of uniform and nonuniform spatial distributions of nanoparticles, in particular the occurrence of clustering. This expression is a function of demagnetizing factors associated with specimen shape and clusters shape, and of the mean distances between near neighbor nanoparticles and between clusters, relative to the characteristic sizes of each of these two types of objects, respectively. The model explains effects of magnetic dipolar interactions, such as the observation of apparent nanoparticle magnetic moments smaller than real ones and approaching to zero as temperature decreases. It is shown that by performing a minimum set of experimental determinations along principal directions of geometrically well-defined specimens, model application allows retrieval of nanoparticle intrinsic properties, like mean volume, magnetic moment, and susceptibility in the absence of interactions. It also permits the estimation of mean interparticle and intercluster relative distances, as well as mean values of demagnetizing factors associated with clusters shape. An expression for average magnetic dipolar energy per nanoparticle is also derived, which is a function of specimen effective demagnetizing factor and magnetization. Experimental test of the model was performed by analysis of results reported in the literature and of original results reported here. The first case corresponds to oleic-acid-coated 8-nm magnetite particles dispersed in PEGDA-600 polymer, and the second one to polyacrilic-acid-coated 13-nm magnetite particles dispersed in PVA solutions from which ferrogels were later produced by a physical cross-linking route. In both cases, several specimens were studied covering a range of nanoparticle volume fractions between 0.002 and 0.046. Magnetic response is clearly different when prism-shaped specimens are measured along different principal directions. These results remark the importance of reporting complete information on measurement geometry when communicating magnetic measurement results of interacting magnetic nanoparticles. Intrinsic nanoparticle properties as well as structural information on particles spatial distribution were retrieved from our analysis in addition to, and in excellent agreement with, analysis previously performed by other authors and/or information obtained from FESEM images. In the studied samples, nanoparticles were found to be in close contact to each other within almost randomly oriented clusters. Intercluster mean distance, relative to cluster size, was found to vary between 2.2 and 7.5, depending on particles volume fraction.

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with $H\parallel c$ on an underdoped ${\mathrm{Ba}}_{0.75}{\mathrm{K}}_{0.25}{\mathrm{Fe}}_2{\mathrm{As}}_2$ pnictide single crystal, with superconducting transition temperature $T_c=28$ K. The second magnetization peak (SMP) has been observed at temperatures below $T_c/2$ and vanished at higher temperatures. The observed behavior of the SMP has been studied by measuring the magnetic field dependence of relaxation rate $R\left(H\right)$ and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting $H\text{−}T$ phase diagram is obtained. The critical current density $\left(J_c\right)$ was estimated using Bean's model and found to be $\sim 3.4\times {10}^9$ A/${\mathrm{m}}^2$ at 10 K in the SMP region, which is comparable to an optimally doped Ba-${\mathrm{KFe}}_2{\mathrm{As}}_2$ superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual $\delta l$ and $\delta T_c$ pinning models, which suggest the intrinsic nature of pinning in the compound.

MoAlB is the first and, so far, the only transition-metal boride that forms alumina when heated in air and is thus potentially useful for high-temperature applications. Herein, the thermal stability in argon and vacuum atmospheres and the thermodynamic parameters of bulk polycrystalline MoAlB were investigated experimentally. At temperatures above 1708 K, in vacuum and inert atmospheres, this compound incongruently melts into the binary MoB and liquid aluminum metal as confirmed by differential thermal analysis, quenching experiments, x-ray diffraction, and scanning electron microscopy. Making use of that information together with heat-capacity measurements in the 4–1000-K temperature range—successfully modeled as the sum of lattice, electronic, and dilation contributions—the standard enthalpy, entropy, and free energy of formation are computed and reported for the full temperature range. The standard enthalpy of formation of MoAlB at 298 K was found to be $-132\pm 3.2$ kJ/mol. Lastly, the thermal conductivity values are computed and modeled using a variation of the Slack model in the 300–1600-K temperature range.

Graphene has emerged as a promising material for subwavelength photonics, among other reasons because of its large tunability, extreme confinement of plasmonic modes, and limited losses. Here we use the interplay of time modulation and plasmonic resonances in graphene gratings to efficiently generate frequency combs. By using rigorous simulations and a coupled-mode theory model, we show that the combs can be strongly tailored via both the grating and time modulation properties, and can range from the mid-infrared to far-infrared. The grating properties, via the resonance lifetime, strongly influence the conversion efficiency, while the temporal modulation defines the comb's frequency spacing and shape. We examine in detail the dynamics of this interplay between cavity and modulation. With the grating mechanism, a similar frequency comb generation is obtained with a modulation amplitude that is three orders of magnitude smaller than in the planar case.

The ground-state properties of spin-1/2 ladders are studied, emphasizing the role of frustration and ring exchange coupling. We present a unified field theory for ladders with general coupling constants and geometry. Rich phase diagrams can be deduced by using a renormalization group calculation for ladders with in-chain next-nearest-neighbor interactions and plaquette ring exchange coupling. In addition to established phases such as Haldane, rung singlet, and dimerized phases, we also observe a surprising instability towards an incommensurate phase for weak interchain couplings, which is characterized by an exotic coexistence of self-consistent ferromagnetic and antiferromagnetic order parameters.

The switching field distribution within arrays of single-domain ferromagnetic islands incorporates both island-island interactions and quenched disorder in island geometry. Separating these two contributions is important for disentangling the effects of disorder and interactions in the magnetization dynamics of island arrays. Using submicron, spatially resolved Kerr imaging in an external magnetic field for islands with perpendicular magnetic anisotropy, we map out the evolution of island arrays during hysteresis loops. Resolving and tracking individual islands across four different lattice types and a range of interisland spacings, we can extract the individual switching fields of every island and thereby quantitatively determine the contributions of interactions and quenched disorder in the arrays. The width of the switching field distribution is found to be well fitted by a simple model comprising the sum of an array-independent contribution (interpreted as disorder induced) and a term proportional to the maximum field the entire rest of the array could exert on a single island, i.e., in a fully polarized state. This supports the claim that disorder in these arrays is primarily a single-island property and provides a methodology by which to quantify such disorder.

We propose an exactly solvable Hamiltonian for topological phases in $3+1$ dimensions utilizing ideas from higher lattice gauge theory, where the gauge symmetry is given by a finite 2-group. We explicitly show that the model is a Hamiltonian realization of Yetter's homotopy 2-type topological quantum field theory whereby the ground-state projector of the model defined on the manifold $M^3$ is given by the partition function of the underlying topological quantum field theory for $M^3\times [0,1]$. We show that this result holds in any dimension and illustrate it by computing the ground state degeneracy for a selection of spatial manifolds and 2-groups. As an application we show that a subset of our model is dual to a class of Abelian Walker-Wang models describing $3+1$ dimensional topological insulators.

The power conversion efficiency of ${\mathrm{Cu}}_2\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_4$ kesterite thin-film solar cells is mainly limited by a low open-circuit voltage $V_{\text{OC}}$. In the literature, this low $V_{\text{OC}}$ has been amongst others attributed to electrostatic potential fluctuations leading to fluctuating band edges. This assignment was mainly based on the observation of a shift of the photoluminescence (PL) maximum to higher photon energies as a function of excitation power, which was interpreted in terms of a screening of the electrostatic potential fluctuations. However, in this paper, we show evidence that the observed shift of the PL maximum is dominantly caused by state filling rather than screening. Our assignment is based on the investigation of the full PL line shape (not only of the PL maximum) and the PL yield as a function of excitation power and temperature. Further support of our interpretation is given by the observation of additional band-tail emission showing up as a second high-energy PL band at the highest excitation powers. The spectral position of this additional band coincides with the absorption-tail deduced from photoluminescence excitation spectroscopy (PLE).

Signatures of Majorana-fermion bound states in one-dimensional topological-insulator (TI) nanowires with proximity-effect-induced superconductivity are studied. The phase diagram and energy spectra are calculated for single-TI nanowires, and it is shown that the nanowires can be in the topologically invariant phases of winding numbers $W=0,\pm 1$, and $\pm 2$, corresponding to the cases with zero, one, and two pairs of Majorana fermions in the single-TI nanowires. It is also shown that the topological winding numbers, i.e., the numbers of pairs of Majorana fermions in the TI nanowires, can be extracted from the transport measurements of a Josephson-junction device made from two TI nanowires, while the sign in the winding numbers can be extracted using a superconducting quantum interference device setup.

We present a general formulation to calculate the dynamic interband optical conductivity, beyond the linear response regime, of any electronic system whose quasiparticle dispersion is described by a two-band model. Our formulation is based on the optical Bloch equations with phenomenological damping constants. In the nonlinear steady state regime it yields an analytic solution for the population inversion and the interband coherence, which are nonlinear in the optical field intensity, including finite doping and temperature effects. We explicitly show that the optical nonlinearities are controlled by a single dimensionless parameter which is directly proportional to the incident field strength and inversely proportional to the optical frequency. This identification leads to a unified way to study the dynamical conductivity and the differential transmission spectrum across a wide range of optical frequencies and optical field strengths. We use our formalism to analytically calculate the nonlinear interband optical conductivity of doped and gapped graphene, deriving the well known universal ac conductivity of ${\sigma }_0=e^2/4\hslash$ in the linear response regime of low optical intensities and nonlinear deviations from it which appear at high laser intensities including the impact of finite doping and band-gap opening.

At air-water interfaces, the Lifshitz interaction by itself does not promote ice growth. On the contrary, we find that the Lifshitz force promotes the growth of an ice film, up to 1–8 nm thickness, near silica-water interfaces at the triple point of water. This is achieved in a system where the combined effect of the retardation and the zero frequency mode influences the short-range interactions at low temperatures, contrary to common understanding. Cancellation between the positive and negative contributions in the Lifshitz spectral function is reversed in silica with high porosity. Our results provide a model for how water freezes on glass and other surfaces.

Plasmonic nanoarrays which support collective surface lattice resonances (SLRs) have become an exciting frontier in plasmonics. Compared with the localized surface-plasmon resonance in individual particles, these collective modes have appealing advantages such as angle-dependent dispersions and much narrower linewidths. Here, we investigate systematically how the geometry of the lattice affects the SLRs supported by metallic nanoparticles. We present a general theoretical framework from which the various SLR modes of a given geometry can be straightforwardly obtained by a simple comparison of the diffractive order vectors and orientation of the nanoparticle dipole given by the polarization of the incident field. Our experimental measurements show that while square, rectangular, hexagonal, honeycomb, and Lieb lattice arrays have similar spectra near the $\mathrm{\Gamma }$ point $\left(k=0\right)$, they have remarkably different SLR dispersions. Furthermore, their dispersions are highly dependent on the polarization. Numerical simulations are performed to elucidate the field profiles of the different modes. Our findings extend the diversity of SLRs in plasmonic nanoparticle arrays, and the theoretical framework provides a simple model for interpreting the SLRs features, and vice versa, for designing the geometrical patterns.

Effects associated with the change in the local density of photonic states in a periodic structure based on alternating $a-{\mathrm{Si}}_{1-x}{\mathrm{C}}_x:\mathrm{H}$ and $a-{\mathrm{SiO}}_2$ amorphous layers forming a one-dimensional (1D) photonic crystal have been analyzed. The use of $a-{\mathrm{Si}}_{1-x}{\mathrm{C}}_x:\mathrm{H}$ as the emitting material made it possible to examine the transformation of the photoluminescence spectrum contour that is comparable in width with the photonic stop-band. It was experimentally demonstrated that the emission is enhanced and suppressed in the vicinity of the stop-band. The relative intensities of the luminescence peaks at different edges of the stop-band vary with the detuning of the stop-band position and photoluminescence peak of a single $a-{\mathrm{Si}}_{1-x}{\mathrm{C}}_x:\mathrm{H}$ film. The Purcell effect in the system under consideration was theoretically described by the method in which the local density of photonic states is calculated in terms of a 1D model. It was shown that the specific part of local density of states substantially increases at the long-wavelength (low-frequency) edge of the stop-band of a 1D photonic crystal as a result of the predominant localization of the electric field of the light wave in the spatial regions of $a-{\mathrm{Si}}_{1-x}{\mathrm{C}}_x:\mathrm{H}$ which have a higher relative permittivity as compared with $a-{\mathrm{SiO}}_2$.

The study of invisibility of an infinite dielectric rod with high refractive index is based on the two-dimensional Mie scattering problem, and it suggests strong suppression of scattering due to the Fano interference between spectrally broad nonresonant waves and narrow Mie-resonant modes. However, when the dielectric rod has a finite extension, it becomes a resonator supporting the Fabry-Perot modes which introduce additional scattering and eventually destroy the invisibility. Here we reveal that for shorter rods with modest values of the aspect ratio $r/L$ (where $r$ and $L$ are the radius and length of the rod, respectively), the lowest spectral window of the scattering suppression recovers completely, so that even a finite-size resonator may become invisible. We present a direct experimental verification of the concept of switchable invisibility at microwaves using a cylindrical finite-size resonator with high refractive index.

The valley degree of freedom in atomically thin transition metal dichalcogenides (TMDCs) has generated great interest due to the possibility of using it to store and control information in analogy to the spin degree of freedom in spintronics. A signature of the valley pseudospin is the selective coupling of valley excitons to photons with defined helicity. This selectivity can have important consequences for a variety of optical phenomena associated with the valley excitons. Here we report that Raman features that seemingly violate the Raman selection rules can become prominent at valley exciton resonances in atomically thin $\mathrm{Mo}{\mathrm{S}}_2$. Specifically, the Raman selection rule requires the excitation and scattering photons to have opposite circular polarizations for the in-plane $E$′ mode phonon, but we observe an apparent $E$′ Raman peak for excitation and scattered photons with the same circular polarization at exciton resonances. We attribute this peak to a defect-assisted process that involves phonons in the transverse optical $E$′ branch slightly away from the Γ point, a process that can be enhanced by the selective coupling of valley pseudospin to photon helicity. Thus, the valley pseudospin, in addition to the crystal symmetry, may be important in understanding the Raman scattering spectra for excitations close to valley exciton resonances.

The spin-Seebeck effect refers to voltage signals induced in metals by thermally driven spin currents in adjacent magnetic systems. We present a theory of the spin-Seebeck signal in the case where the conductor that supports the voltage signal is the topologically protected two-dimensional surface-state system at the interface between a ferromagnetic insulator (FI) and a topological insulator (TI). Our theory uses a Dirac model for the TI surface states and assumes Heisenberg exchange coupling between the TI quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by the TI surface states scattering off the nonequilibrium magnon population at the surface of the semi-infinite thermally driven FI. Our theory is readily generalized to spin-Seebeck voltages in any two-dimensional conductor that is exchange-coupled to the surface of a FI. Surface-state carrier-density-dependent signal strengths calculated using ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and yttrium iron garnet material parameters are consistent with recent experiments.