The hydrogenation of C-rich Si leads to the formation of two (almost) energetically degenerate ${\mathrm{H}}_2^{*}\left(\mathrm{C}\right)$ complexes, each containing one substitutional C (${\mathrm{C}}_{\mathrm{s}}$) and two interstitial H atoms which are located at a bond-centered (bc) and an antibonding (ab) site, respectively. The two defects are trigonal: ${\mathrm{C}}_{\mathrm{s}}-{\mathrm{H}}_{\mathrm{bc}}\cdots \mathrm{Si}-{\mathrm{H}}_{\mathrm{ab}}$ and ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}\cdots {\mathrm{H}}_{\mathrm{bc}}-\mathrm{Si}$. Fourier-transform infrared (FTIR) absorption spectra of these two defects should show two ${\mathrm{C}}_{\mathrm{s}}-\mathrm{H}$ and two $\mathrm{Si}-\mathrm{H}$ stretch modes, but the ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}$ mode was absent in earlier studies. The missing line has now been observed by FTIR in especially C rich Si material. The line is unexpectedly broad, suggesting a very short vibrational lifetime. Partial D substitutions result in the formation of a ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}\cdots {\mathrm{D}}_{\mathrm{bc}}-\mathrm{Si}$ center. In this defect, the ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}$ line shifts by only $0.3{\mathrm{cm}}^{-1}$ but becomes very sharp, suggesting a long lifetime. The IR line widths show that the vibrational lifetime of the ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}$ mode in ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}\cdots {\mathrm{D}}_{\mathrm{bc}}-\mathrm{Si}$ is about 16 times longer than that of the same ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}$ mode in ${\mathrm{H}}_{\mathrm{ab}}-{\mathrm{C}}_{\mathrm{s}}\cdots {\mathrm{H}}_{\mathrm{bc}}-\mathrm{Si}$. This paper contains experimental data and first-principles calculations which explain this isotope effect.

The temperature dependence of the coherence time of hole spins confined in self-assembled (In,Ga)As/GaAs quantum dots is studied by spin-mode-locking and spin-echo techniques. Coherence times limited to about a microsecond are measured for temperatures below 8 K. For higher temperatures, a fast drop occurs down to a few nanoseconds over a 10-K range. The hole-nuclear hyperfine interaction appears too weak to account for these limitations. We suggest that spin-orbit-related interactions are the decisive sources for hole spin decoherence.

Direct comparison of tip-sample forces obtained by dynamic force spectroscopy experiments with theoretical simulations is extremely difficult, since the precise tip shape and chemical identity of the apex atoms of the force sensing tip remain unknown in most experiments. Here, we present force curves measured with a tungsten tip on a Ag(111) surface obtained in a low-temperature atomic force microscope using tips that were analyzed by field ion microscopy down to atomic levels. The resulting van der Waals and electrostatic forces were found to be in quantitative agreement with analytical models, if the tip shape parameters from the field ion microscopy analysis were used. Furthermore, our analysis shows an additional long-range force interaction at tip-sample distances above 1.3 nm. We suggest that this unexpected force is related to patch charges arising from the inhomogeneous work function distribution on the surface of highly faceted sharp tips.

We study by Raman scattering the shear and layer breathing modes in multilayer MoS${}_2$. These are identified by polarization measurements and symmetry analysis. Their positions change significantly with the number of layers, with different scaling for odd and even layers. A chain model can explain the results, with general applicability to any layered material, allowing a reliable diagnostic of their thickness.

In this article we study the ferromagnetic behavior of ABC-stacked trilayer graphene. This is done using a nearest-neighbor tight-binding model, in the presence of long-range Coulomb interactions. For a given electron-electron interaction $g$ and doping level $n$, we determine whether the total energy is minimized for a paramagnetic or ferromagnetic configuration of our variational parameters. The $g$ versus $n$ phase diagram is first calculated for the unscreened case. We then include the effects of screening using a simplified expression for the fermion bubble diagram. We show that ferromagnetism in ABC-stacked trilayer graphene is more robust than in monolayer, in bilayer, and in ABA-stacked trilayer graphene. Although the screening reduces the ferromagnetic regime in ABC-stacked trilayer graphene, the critical doping level remains one order of magnitude larger than in unscreened bilayer graphene.

We performed time-resolved nanoscale studies on the resistive switching of perovskite manganite thin films by means of conductive atomic force microscopy. Creep and recovery features have been observed in the evolution of metallic domains via pulse-train experiments and current map sequences. Local $I\left(t\right)$ curves show $1/f^{\alpha }$ noise signatures during the switching process, a phenomenon which occurs in various physical processes consisting of discrete jumps of different sizes, such as the Barkhausen effect. Our results imply that the resistive switching falls into this universal class of effects with dynamics determined by the pinning and depinning of structural domain walls.

Accurate electronic structures of the technologically important lanthanide/rare-earth sesquioxides (Ln${}_2$O${}_3$, with $\text{Ln}=\text{La},\cdots ,$Lu) and CeO${}_2$ have been calculated using hybrid density functionals HSE03, HSE06, and screened exchange (sX-LDA). We find that these density functional methods describe the strongly correlated Ln $f$ electrons as well as the recent $G_0{W}_0$@LDA$+U$ results, generally yielding the correct band gaps and trends across the Ln period. For HSE, the band gap between O 2$p$ states and lanthanide 5$d$ states is nearly independent of the lanthanide, while the minimum gap varies as filled or empty Ln 4$f$ states come into this gap. sX-LDA predicts the unoccupied 4$f$ levels at higher energies, which leads to a better agreement with experiments for Sm${}_2$O${}_3$, Eu${}_2$O${}_3$, and Yb${}_2$O${}_3$.

Dense interband electronic excitations on the order of 0.2 electron-hole pairs per nm${}^3$ are encountered in a number of circumstances of fundamental and practical significance. We report measurements of the kinetic order and rate constants of nonlinear quenching in pure and doped materials with band gaps in the range from 6 eV down to 1.4 eV. The principal method used can be described as interband $Z$-scan luminescence yield with subpicosecond pulse excitations. A clear delineation of second-order and third-order quenching kinetics is found between oxide and iodide insulating crystals. This delineation suggests that the hot-electron thermalization rate mediated by LO phonon frequencies governs whether free carriers can pair as excitons within the time period of nonlinear quenching.

We show that a generalized asymmetric resonant line shape derived elsewhere from rigorous electromagnetic calculations [Gallinet and Martin, Phys. Rev. B 83, 235427 (2011)] and from the two-oscillators model [Joe et al., Phys. Scr. 74, 259 (2006)] can also be obtained using a very general assumption that the spectral dependence of the scattering amplitudes is given by the transfer function of a linear system. We reformulate the line shape equation and show that in the case of a first-order transfer function all possible line shapes can be presented by a weighted sum of the original Fano and Lorentzian line shapes. We propose a new two-parameter classification scheme for asymmetric resonances with one parameter δ being the asymmetry factor of the Fano component and the other parameter η quantifying the relative weight of the Fano and Lorentzian components of the line shape. The proposed formula is used to fit experimental spectra of a silicon photonic crystal cavity nanobeam interrogated using a fiber taper probe.

Carrier trapping in colloidal nanocrystals represents a major energy loss mechanism for excitonic states crucial to devices. Surprisingly little is known about the influence of the spin degree of freedom on the nature of these intrinsic trap centers or the types of coupling that these states experience. Here, a pulsed microwave optically detected magnetic resonance study is presented that aims to probe the interaction pathways existing between shallow band-edge trap states and the deep-level emissive chemical defect states responsible for the broad, low-energy emission common to CdS nanocrystals. Due to long spin coherence times ($T_2$) of these states, Rabi flopping detected in the luminescence under magnetic resonance provides access to information regarding the modes of coupling of shallow-trapped electron-hole pairs, both of isolated species and of those in proximity to the emissive defect. Corresponding optically detected spin-echo experiments expose an extraordinarily long intrinsic spin coherence time ($T_2\approx 1.6$ μs) for colloidal nanocrystals, and an electron spin-echo envelope modulation indicative of local spin interactions. This effect provides opportunities for gaining the detailed chemical and structural information needed in order to eliminate energy loss mechanisms during the synthetic process.

Using the measured optical response and surface roughness topography as inputs, we perform realistic calculations of the combined effect of Casimir and electrostatic forces on the actuation dynamics of microelectromechanical systems (MEMS). In contrast with the expectations, roughness can influence MEMS dynamics, even at distances between bodies significantly larger than the root-mean-square roughness. This effect is associated with statistically rare high asperities that can be locally close to the point of contact. It is found that even though surface roughness appears to have a detrimental effect on the availability of stable equilibria, it ensures that those equilibria can be reached more easily than in the case of flat surfaces. Hence our findings play a principal role for the stability of microdevices such as vibration sensors, switches, and other related MEM architectures operating at distances below 100 nm.

Experimental observations and theoretical simulations for the ferroelectric (FE) domains in multiferroic $R$MnO${}_3$ ($R$=Ho-Lu, Y, and Sc) demonstrate that annular domains can appear in the topological Kosterlitz-Thouless (KT) phase in coexistence with the “cloverleaf” vortices. Moreover, the annular domains often adopt the multilayered structures as indexed by the annular number. The double-layered FE domains, an energetically favored structure, are commonly found in $R$MnO${}_3$. Thermal annealing experiments directly show exchanges between large annular domains and cloverleaf vortices above the trimerization-structural transition temperature $T_{\mathrm{c}}$. The temperature evolution of the FE domains has been analyzed by using the six-state clock model through Monte Carlo simulations. Based on careful in situ SEM observations, the dynamic properties of annular domains and interplay among domain walls have been briefly discussed.

Recently, ScAlN alloys attracted attention for their giant piezoelectric moduli. In this study the piezoelectric response of the wurtzite group-III nitrides AlN, GaN, and InN mixed with 50 mol$\%$ of ScN or YN is investigated using ab initio calculations. We confirm that the energy flattening phenomenon gives rise to the simultaneous appearance of elastic softening and local structural instability, and explains the enhanced piezoelectricity of the alloys. Furthermore, we present a volume matching condition for an efficient search of new piezoelectric materials. It states that alloys in which the parent components show close volume matching exhibit a flatter potential-energy landscape and higher increase of piezoelectric moduli. We suggest YInN, beyond ScAlN, as a promising material for piezoelectric energy harvesting with its enhanced $\approx$400$\%$ piezoelectric moduli.

We show that voids adjacent to radiation damage cascades can be moved in their entirety by several lattice spacings. This is done using molecular dynamics cascade simulations in iron at energies of 1–5 keV. An equation describing this process is added to an an object kinetic Monte Carlo (OKMC) code to allow study of the mechanism at longer time scales. The mechanism produces an enhancement of void diffusion by two orders of magnitude from $1\times {10}^{-22}$ cm${}^2$/s to $3\times {10}^{-20}$ cm${}^2$/s. Repeating the study on He bubbles shows that the movement is damped by the presence of helium in the void.

Multifrequency electron spin resonance (ESR) transmission spectra have been measured as a function of temperature and magnetic field on single crystals of the quasi-one-dimensional spin-1 chain compound SrNi${}_2$V${}_2$O${}_8$ in the gigahertz frequency range. Magnetic resonance modes above 50 K have been observed with an effective $g$ factor of 2.24 at 100 K. Below 30 K, intratriplet excitations have been observed in the ESR spectra, which reveal the presence of single-ion anisotropy with $D=-0.29$ meV.

We have performed resonant inelastic x-ray scattering (RIXS) near the Cu-$K$ edge on cuprate superconductors La${}_{2-x}$Sr${}_x$CuO${}_4$, La${}_{2-x}$Ba${}_x$CuO${}_4$, La${}_{2-x}$Sr${}_x$Cu${}_{1-y}$Fe${}_y$O${}_4$, and Bi${}_{1.76}$Pb${}_{0.35}$Sr${}_{1.89}$CuO${}_{6+\delta }$, covering underdoped to heavily overdoped regimes and focusing on charge excitations inside the charge-transfer gap. RIXS measurements of the 214 systems with $E_i=8.993$ keV have revealed that the RIXS intensity at 1-eV energy transfer has a minimum at $(0,0)$ and maxima at $(\pm 0.4\pi ,0)$ and $(0,\pm 0.4\pi )$ for all doping points regardless of the stripe-ordered state, suggesting that the corresponding structure is not directly related to stripe order. Measurements with $E_i=9.003$ keV on metallic La${}_{1.7}$Sr${}_{0.3}$CuO${}_4$ and Bi${}_{1.76}$Pb${}_{0.35}$Sr${}_{1.89}$CuO${}_{6+\delta }$ exhibit a dispersive intraband excitation below 4 eV, similar to that observed in the electron-doped Nd${}_{1.85}$Ce${}_{0.15}$CuO${}_4$. This is the first observation of a dispersive intraband excitation in a hole-doped system, evidencing that both electron- and hole-doped systems have a similar dynamical charge correlation function.

Quantum dimer models (QDMs) arise as low-energy effective models for frustrated magnets. Some of these models have proven successful in generating a scenario for exotic spin liquid phases with deconfined spinons. Doping, i.e., the introduction of mobile holes, has been considered within the QDM framework and partially studied. A fundamental issue is the possible existence of a superconducting phase in such systems and its properties. For this purpose, the question of the statistics of the mobile holes (or “holons”) shall be addressed first. Such issues are studied in detail in this paper for generic doped QDMs defined on the most common two-dimensional lattices (square, triangular, honeycomb, kagome, …) and involving general resonant loops. We prove a general “statistical transmutation” symmetry of such doped QDMs by using composite operators of dimers and holes. This exact transformation enables us to define duality equivalence classes (or families) of doped QDMs, and provides the analytic framework to analyze dynamical statistical transmutations. We discuss various possible superconducting phases of the system. In particular, the possibility of an exotic superconducting phase originating from the condensation of (bosonic) charge-$e$ holons is examined. A numerical evidence of such a superconducting phase is presented in the case of the triangular lattice, by introducing a gauge-invariant holon Green's function. We also make the connection with a Bose-Hubbard model on the kagome lattice which gives rise, as an effective model in the limit of strong interactions, to a doped QDM on the triangular lattice.

We inject a sequence of 1 ms current pulses into uniformly magnetized patterns of the itinerant ferromagnet SrRuO${}_3$ until a magnetization reversal is detected. We detect the effective temperature during the pulse and find that the cumulative pulse time $\tau$ required to induce magnetization reversal depends exponentially on $1/T$. In addition, we find that $\tau$ also depends exponentially on the current amplitude. These observations indicate current-induced magnetization reversal assisted by thermal fluctuations.

The orthorhombic perovskite NaOsO${}_3$ undergoes a continuous metal-insulator transition (MIT), accompanied by antiferromagnetic (AFM) order at $T_N=410$ K, suggested to be an example of the rare Slater (itinerant) MIT. We study this system using ab initio and related methods, focusing on the origin and nature of magnetic ordering and the MIT. The rotation and tilting of OsO${}_6$ octahedra in the GdFeO${}_3$ structure result in moderate narrowing of the bandwidth of the $t_{2g}$ manifold but sufficient to induce flattening of bands and AFM order within the local spin density approximation, where it remains metallic but with a deep pseudogap. Including on-site Coulomb repulsion $U$, at $U_c\approx 2$ eV a MIT occurs only in the AFM state. Effects of spin-orbit coupling (SOC) on the band structure seem minor, as expected for a half-filled $t_{2g}^3$ shell, but SOC doubles the critical value $U_c$ necessary to open a gap and also leads to large magnetocrystalline energy differences in spite of normal orbital moments no greater than 0.1 ${\mu }_B$. Our results are consistent with a Slater MIT driven by magnetic order, induced by a combination of structurally induced band narrowing and moderate Coulomb repulsion, with SOC necessary for a full picture. Strong $p$-$d$ hybridization reduces the moment, and when bootstrapped by the reduced Hund's rule coupling (proportional to the moment) gives a calculated moment of $\sim$1 ${\mu }_B$, consistent with the observed moment and only a third of the formal $d^3$ value. We raise and discuss one important question: Since this AFM ordering is at $q=0$ (in the 20 atom cell) where nesting is a moot issue, what is the microscopic driving force for ordering and the accompanying MIT?

The phonon dispersion and the lattice contribution to the dielectric function of PbTe in the NaCl structure are calculated ab initio. The results obtained are in agreement with the available experimental data and reproduce the main features observed in this material, such as an anharmonic LA-TO phonon coupling in the $\Gamma$-X direction, as well as high values for the dielectric constants ${\epsilon }_1\left(0\right)$ and ${\epsilon }_1\left(\infty \right)$. Calculations include the pressure dependence and indicate that the anharmonic phonon coupling is very sensitive to applied pressure. The calculations indicate that the higher values for the dielectric constant together with the anharmonic LA-TO coupling reduce the lattice thermal conductivity and increase the internal electric field, thereby enhancing the electronic conductivity in PbTe, key conditions to increase the thermoelectric figure of merit.