The existence of nanoscale ductility during the fracture of silicate glasses remains controversial. Here, based on molecular dynamics simulations coupled with topological constraint theory, we show that nanoductility arises from the spatial heterogeneity of the atomic network's rigidity. Specifically, we report that localized floppy modes of deformation in underconstrained regions of the glass enable plastic deformations of the network, resulting in permanent change in bond configurations. Ultimately, these heterogeneous plastic events percolate, thereby resulting in a nonbrittle mode of fracture. This suggests that nanoductility is intrinsic to multicomponent silicate glasses having nanoscale heterogeneities.

We report on the influence of disorder on an exciton-polariton condensate in a ZnO-based bulk planar microcavity and compare experimental results with a theoretical model for a nonequilibrium condensate. Experimentally, we detect intensity fluctuations within the far-field emission pattern even at high condensate densities, which indicates a significant impact of disorder. We show that these effects rely on the driven dissipative nature of the condensate and argue that they can be accounted for by spatial phase inhomogeneities induced by disorder, which occur even for increasing condensate densities realized in the regime of high excitation power. Thus, nonequilibrium effects strongly suppress the stabilization of the condensate against disorder, contrary to what is expected for equilibrium condensates in the high-density limit. Numerical simulations based on our theoretical model reproduce the experimental data.

Spin transfer torque and bias field effects on the magnetization reversal time of a nanoscale ferromagnet are investigated in the very-low-damping regime via the energy-controlled diffusion equation. That equation is rooted in a generalization of the Kramers escape rate theory for point Brownian particles in a potential to the magnetic relaxation of a macrospin. Using the mean first-passage method, the reversal time is then evaluated in closed integral form for a nanomagnet with the free-energy density given in the standard form of superimposed easy-plane and in-plane easy-axis anisotropies with the dc bias field along the easy axis. The results completely agree with those yielded by independent numerical methods.

Superconductivity is observed with critical temperatures near 9 K in the tetragonal compound ${\mathrm{Mo}}_5{\mathrm{PB}}_2$. This material adopts the ${\mathrm{Cr}}_5{\mathrm{B}}_3$ structure type common to superconducting ${\mathrm{Nb}}_5{\mathrm{Si}}_{3-x}{\mathrm{B}}_x,{\mathrm{Mo}}_5{\mathrm{SiB}}_2$, and ${\mathrm{W}}_5{\mathrm{SiB}}_2$, which have critical temperatures of $5.8\text{–}7.8$ K. We have synthesized polycrystalline samples of the compound, made measurements of electrical resistivity, magnetic susceptibility, and heat capacity, and performed first-principles electronic structure calculations. The highest $T_c$ value (9.2 K) occurs in slightly phosphorus rich samples, with composition near ${\mathrm{Mo}}_5{\mathrm{P}}_{1.1}{\mathrm{B}}_{1.9}$, and the upper critical field ${\mathrm{H}}_{c2}$ at $T=0$ is estimated to be $\approx 17$ kOe. Together, the measurements and band-structure calculations indicate intermediate coupling $\left(\lambda =1.0\right)$, phonon mediated superconductivity. The temperature dependence of the heat capacity and upper critical field $H_{c2}$ below $T_c$ suggest multiple superconducting gaps may be present.

We predict the thermoelectric properties of layered ${\left[\mathrm{GeTe}\right]}_m{\left[\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\right]}_n$ (GBT) compounds $(1\le m\le 8,1\le n\le 3)$, using first-principles-Boltzmann transport calculations of the homogeneous ($\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ and GeTe) data. The lattice strain and the quantum-confinement effects of compounds evolve the band-gap structures, resulting in asymmetric and large Seebeck coefficient, at high GeTe content. Using semiempirical calculations of electron scattering rate $1/{\tau }_e$, dominated by electron-acoustic phonon scattering, we reproduce reported TE properties of GBT compounds. We predict that, due to small Seebeck coefficient, the GBT compounds with high n- and $p$-type doping $\left(\sim {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}\right)$, do not have high $ZT$ near room temperature. However, we predict that the moderately doped $(\sim {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}),p$-type GBT compounds have enhanced $ZT$≈1.4 near room temperature.

Some degree of quenched disorder is present in nearly all solids, and can have a marked impact on their macroscopic properties. A manifestation of this effect is the Lifshitz tail of localized states that then gets attached to the energy spectrum, resulting in the nonzero density of states in the band gap. We present here a systematic approach for deriving the asymptotic behavior of the density of states and of the typical shape of the disorder potentials in the Lifshitz tail. The analysis is carried out first for the well-controlled case of noninteracting particles moving in a Gaussian random potential and then for a broad class of disordered scale-invariant models—pertinent to a variety of systems ranging from semiconductors to semimetals to quantum critical systems. For relevant Gaussian disorder, we obtain the general expression for the density of states deep in the tail, with the rate of exponential suppression governed by the dynamical exponent and spatial dimensions. For marginally relevant disorder, however, we would expect a power-law scaling. We discuss the implications of these results for understanding conduction in disordered materials.

The microscopic wave functions of the composite fermion theory can incorporate electron mass anisotropy by a trivial rescaling of the coordinates. These wave functions are very likely adiabatically connected to the actual wave functions of the anisotropic fractional quantum Hall states. We show in this paper that they possess the nice property that their energies can be analytically related to the previously calculated energies for the isotropic states through a universal scale factor, thus allowing an estimation of several observables in the thermodynamic limit for all fractional quantum Hall states as well as the composite fermion Fermi sea. The rather weak dependence of the scale factor on the anisotropy provides insight into why fractional quantum Hall effect and composite fermions are quite robust to electron mass anisotropy. We discuss how better, though still approximate, wave functions can be obtained by introducing a variational parameter, following Haldane [F. D. M. Haldane, Phys. Rev. Lett. 107, 116801 (2011)], but the resulting wave functions are not readily amenable to calculations. Our considerations are also applicable, with minimal modification, to the case where the dielectric function of the background material is anisotropic.

A resonant-state expansion (RSE) for open optical systems with a general frequency dispersion of the permittivity is presented. The RSE of dispersive systems converts Maxwell's wave equation into a linear matrix eigenvalue problem in the basis of unperturbed resonant states, in this way numerically exactly determining all relevant eigenmodes of the optical system. The dispersive RSE is verified by application to the analytically solvable system of a sphere in vacuum, with a dispersion of the permittivity described by the Drude and Drude-Lorentz models. We calculate the optical modes converting the sphere material from gold to nondispersive sand and back to gold, and evaluate the accuracy using exact solutions.

First-principles calculations are made for the primary pyroelectric coefficients of wurtzite GaN and ZnO. The pyroelectricity is attributed to the quasiharmonic thermal shifts of internal strains (internal displacements of cations and anions carrying their Born effective charges). The primary (zero-external-strain) pyroelectricity dominates at low temperatures, while the secondary pyroelectricity (the correction from external thermal strains) becomes comparable with the primary pyroelectricity at high temperatures. Contributions from the acoustic and the optical phonon modes to the primary pyroelectric coefficient are only moderately well described by the corresponding Debye and Einstein functions, respectively.

We present numerical and analytical results for the lifetime of emitters in close proximity to graphene sheets. Specifically, we analyze the contributions from different physical channels that participate in the decay processes. Our results demonstrate that measuring the emitters' decay rates provides an efficient route for sensing graphene's optoelectronic properties, notably the existence and size of a potential band gap in its electronic band structure.

We theoretically investigate gap plasmons for two silver nanocubes coupled through a molecular tunnel junction. In the absence of tunneling, the redshift of the bonding mode saturates with decreasing gap distance. Tunneling at small gap distances leads to a damping and slight blueshift of the bonding mode, but no low-energy charge transfer plasmon mode appears in the spectra. This finding is in stark contrast to recent work of Tan et al. [Science 343, 1496 (2014)].

We present the quantitative phase diagram of the bilayer bosonic fractional quantum Hall system on the torus geometry at total filling factor $\nu =1$ in the lowest Landau level. We consider short-range interactions within and between the two layers, as well as the interlayer tunneling. In the fully polarized regime, we provide an updated detailed numerical analysis to establish the presence of the Moore-Read phase of both even and odd numbers of particles. In the actual bilayer situation, we find that both interlayer interactions and tunneling can provide the physical mechanism necessary for the low-energy physics to be driven by the fully polarized regime, thus leading to the emergence of the Moore-Read phase. Interlayer interactions favor a ferromagnetic phase when the system is $\mathrm{SU}\left(2\right)$ symmetric, while the interlayer tunneling acts as a Zeeman field polarizing the system. Besides the Moore-Read phase, the $\left(220\right)$ Halperin state and the coupled Moore-Read state are also realized in this model. We study their stability against each other.

We demonstrate that a spin-dependent Seebeck effect can be detected in quantum wells with zinc-blend structure with equal Rashba-Dresselhaus spin-orbit couplings. This theory is based on the establishment of an itinerant antiferromagnetic state, a low total-energy configuration realized in the presence of the Coulomb interaction enabled by the $\mathbf{k}=0$ degeneracy of the opposite-spin single-particle energy spectra. Transport in this state is modeled by using the solutions of a Boltzmann equation obtained within the relaxation time approximation. Numerical estimates performed for realistic GaAs samples indicate that at low temperatures, the amplitude of the spin Seebeck coefficient can be increased by scattering on magnetic impurities.

The exposed surface of a truncated photonic crystal (PC) may or may not carry surface states. Likewise, when two PCs are joined together, there is no assurance that interface states can be found at the boundary. We show, however, that interface states must exist at the boundary between a two-dimensional dielectric PC and its “inverted” partner (the conjugate structure formed by interchanging the high/low dielectric region) as long as a common gap can be found above the lowest band and the crystal structures possess a mirror symmetry on average along the interface direction. The interface states form deterministically as a result of different geometric phases of the bulk bands across the boundary. As the existence is protected by topological principles, the interface state will persist even in the limit of extremely small common gaps. The presence of interface states is demonstrated theoretically for a variety of PCs and also experimentally.

The low temperature dynamic magnetic properties of two dense magnetic nanoparticle assemblies with similar superspin glass transition temperatures $T_g\sim 140$ K are compared. The two samples are made from batches of 6 and 8 nm monodisperse $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles, respectively. The properties of the individual particles are extracted from measurements on reference samples where the particles have been covered with a thick silica coating. The blocking temperatures of these dilute assemblies are found at 12.5 K for the 6 nm particles and at 35 K for the 8 nm particles, which implies different anisotropy energy barriers of the individual particles and vastly different temperature evolution of their relaxation times. The results of the measurements on the concentrated particle assemblies suggest a strong influence of the particle energy barrier on the details of the aging dynamics, memory behavior, and apparent superspin dimensionality of the particles.

Quantum antiferromagnets have proven to be some of the cleanest realizations available for theoretical, numerical, and experimental studies of quantum fluctuation effects. At finite temperatures, however, the additional effects of thermal fluctuations in the restricted phase space of a low-dimensional system have received much less attention, particularly the situation in frustrated quantum magnets, where the excitations may be complex collective (bound or even fractionalized) modes. We investigate this problem by studying the thermodynamic properties of the frustrated two-leg $S=\frac{1}{2}$ spin ladder, with particular emphasis on the fully frustrated case. We present numerical results for the magnetic specific heat and susceptibility, obtained from exact diagonalization and quantum Monte Carlo studies, which we show can be rendered free of the sign problem even in a strongly frustrated system and which allow us to reach unprecedented sizes of $L=200$ ladder rungs. We find that frustration effects cause an unconventional evolution of the thermodynamic response across the full parameter regime of the model. However, close to the first-order transition they cause a highly anomalous reduction in temperature scales with no concomitant changes in the gap; the specific heat shows a very narrow peak at very low energies and the susceptibility rises abruptly at extremely low temperatures. Unusually, the two quantities have different gaps over an extended region of the parameter space. We demonstrate that these results reflect the presence of large numbers of multiparticle bound-state excitations, whose energies fall below the one-triplon gap in the transition region.

The widely spread oxygen vacancies $\left(V_{\mathrm{O}}\right)$ in multiferroic materials can strongly affect their physical properties. However, their exact influence has rarely been identified in hexagonal manganites. Here, with the combined use of transmission electron microscopy (TEM) and first-principles calculations, we have systematically studied the electronic and crystal structure modifications induced by $V_{\mathrm{O}}$ located at the same Mn atomic plane (in-plane $V_{\mathrm{O}})$. Our TEM experiments reveal that the easily formed in-plane $V_{\mathrm{O}}$ not only influence the electronic structure of $\mathrm{YMn}{\mathrm{O}}_3$ but alter the in-plane Wyckoff positions of Mn ions, which may subsequently affect the intraplane and interplane exchange interaction of Mn ions. The ferroelectricity is also impaired due to the introduction of $V_{\mathrm{O}}$. Further calculations confirm these electronic and structural changes and modifications. Our results indicate that the electronic and crystal structure of $\mathrm{YMn}{\mathrm{O}}_3$ can be manipulated by the creation of $V_{\mathrm{O}}$.

The optical properties of LiFeAs with $T_c\simeq 18$ K have been determined in the normal and superconducting states. The superposition of two Drude components yields a good description of the low-frequency optical response in the normal state. Below $T_c$, the optical conductivity reveals two isotropic superconducting gaps with ${\mathrm{\Delta }}_1\simeq 2.9\pm 0.2$ meV and ${\mathrm{\Delta }}_2\simeq 5.5\pm 0.4$ meV. A comparison between the superconducting-state Mattis-Bardeen and the normal-state Drude components, in combination with a spectral weight analysis, indicates that the spectral weight associated with a band which has a very small scattering rate is fully transferred to the superfluid weight upon the superconducting condensate. These observations provide clear evidence for the coexistence of clean- and dirty-limit superconductivity in LiFeAs.

We use a line of miniature Hall sensors to study the influence of the magnetic domain distribution on the flux dynamics in superconductor/ferromagnet bilayers. Two bilayers are built of a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy and a superconducting Nb layer, with the insulating layer in-between to avoid proximity effect. The magnetic domain patterns of various geometries are reversibly predefined in the Co/Pt multilayers using the appropriate magnetization procedure. The Pt thickness is different in the two bilayers, resulting in different width and length of the domains, which profoundly affects vortex dynamics. We show that narrow short domains lead to strong confinement of vortices at the sample edge, while narrow elongated domains of uniform width induce smaller confinement and easy vortex entry. Large enhancement of flux pinning and critical current density, by a factor of more than 7, is observed in the last case, while the former results in smaller enhancement. When domains are wide, the disorder in the domain widths becomes beneficial for larger enhancement of pinning, while more uniform distribution of domain widths results in a precipitous drop of the enhancement. The analysis of these results suggests that with increasing domain width, a transition occurs from vortex chains pinned by narrow domains to disordered triangular vortex lattice pinned by a maze of multiply interconnected magnetic domains.

The role of van der Waals (vdW) forces in the description of scattering processes of noble gases from metal surfaces is currently under debate. Although features of the potential energy surface such as anticorrugation or adsorption energies are sometimes found to be well described by standard density functional theory (DFT), the performance of DFT to describe diffraction spectra may rely on the accuracy of the vdW functionals used. To analyze the precise role of these vdW forces in noble gas diffraction by metal surfaces, we have thoroughly studied the case of Ne/Ru(0001), for which accurate experimental results are available. We have carried out classical and quantum dynamics calculations by using DFT-based potentials that account for the effect of vdW interactions at different levels of accuracy. From the comparison of our results with experimental data, we conclude that the inclusion of vdW effects is crucial to properly describe diffraction of noble gases from metal surfaces. We show that among the vdW-DFT functionals available in the literature, not all of them can be used to accurately describe this process.