A theory for the nonlinear excitonic optical response of semiconductors is developed. By adopting the length gauge, intraband effects are rigorously taken into account. We show that the second-order nonlinear response mixing intra- and interband transitions can be expressed in terms of generalized derivatives of the exciton Green's function. The theory is applied to hexagonal boron-nitride monolayers. For both the linear and nonlinear response, a dramatic influence of excitons is found. Hence, new discrete resonances appear as well as pronounced changes in the continuum spectrum.

We present a study of the rate-limiting spin-dependent charge-transfer processes in different polymer/fullerene bulk-heterojunction solar cells at $10\mathrm{K}$. Observing central spin-locking signals in pulsed electrically detected magnetic resonance and an inversion of Rabi oscillations in multifrequency electron-double-resonance spectroscopy, we find that the spin response of both spin-coated and printed P3HT/PCBM and spin-coated PCDTBT/PCBM solar cells at low temperatures is governed by bipolar polaron pair recombination and quantitatively determine the polaron-polaron coupling strength with double electron-electron resonance experiments. Furthermore spin Hahn echo decay and inversion recovery measurements are performed to measure spin coherence and recombination times of the polaron pairs, respectively.

Thermal transport in solids changes its nature from phonon propagation that suffers from perturbative scattering to thermally activated hops between localized vibrational modes as the level of disorder increases. Models have been proposed to understand these two distinct extremes that predict opposite temperature dependence of the thermal conductivity but not for the transition or the intermediate regime. Here we explore thermal transport in two-dimensional silica with varying levels of disorder $\alpha$ by performing atomistic simulations, as well as analysis based on the kinetic and Allen-Feldman theories. We demonstrate a crossover between the crystalline and amorphous regimes at $\alpha \sim 0.3$, which is characterized by a turnover of the temperature dependence in thermal conductivity and explained by the dominance of thermal hopping processes. This critical disorder level is also identified in the analysis of the participation ratio of localized vibrational modes and the spatial localization of heat flux. These factors serve as key indicators in quantitatively characterizing the mechanism of heat transport in the transitional regime.

Water confined between two graphene layers with a separation of a few $\AA$ forms a layered two-dimensional ice structure. Using large scale molecular dynamics simulations with the adoptable ReaxFF interatomic potential we found that flat monolayer ice with a rhombic-square structure nucleates between the graphene layers which is nonpolar and nonferroelectric. We provide different energetic considerations and H-bonding results that explain the interlayer and intralayer properties of two-dimensional ice. The controversial AA stacking found experimentally [Algara-Siller et al., Nature (London) 519, 443 (2015)] is consistent with our minimum-energy crystal structure of bilayer ice. Furthermore, we predict that an odd number of layers of ice has the same lattice structure as monolayer ice, while an even number of ice layers exhibits the square ice AA stacking of bilayer ice.

The characteristic structural off-centering of ${\mathrm{Pb}}^{2+}$ in oxides, associated with its $6s^2$ lone pair, allows it to play a dominant role in polar materials, and makes it a somewhat ubiquitous component of ferroelectrics. In this work, we examine the compounds ${\mathrm{Sr}}_{0.9}{\mathrm{Sn}}_{0.1}{\mathrm{TiO}}_3$ and ${\mathrm{Ba}}_{0.79}{\mathrm{Ca}}_{0.16}{\mathrm{Sn}}_{0.05}{\mathrm{TiO}}_3$ using neutron total scattering techniques with data acquired at different temperatures. In these compounds, previously reported as ferroelectrics, ${\mathrm{Sn}}^{2+}$ appears to display some of the characteristics of ${\mathrm{Pb}}^{2+}$. We compare the local and long-range structures of the ${\mathrm{Sn}}^{2+}$-substituted compositions to the unsubstituted parent compounds ${\mathrm{SrTiO}}_3$ and ${\mathrm{BaTiO}}_3$. We find that even at these small substitution levels, the ${\mathrm{Sn}}^{2+}$ lone pairs drive the local ordering behavior, with the local structure of both compounds more similar to the structure of ${\mathrm{PbTiO}}_3$ rather than the parent compounds.

We use a Metropolis algorithm to calculate the finite temperature spectral weight of three related models that have identical quasiparticles at $T=0$, if the exchange favors the appearance of a ferromagnetic background. The low-energy behavior of two of the models remains equivalent at finite temperature, however that of the third does not because its low-energy behavior is controlled by rare events due to thermal fluctuations, which transfer spectral weight well below the $T=0$ quasiparticle peaks and generate a pseudogaplike phenomenology. Our results demonstrate that having $T=0$ spectra with similar quasiparticles is not a sufficient condition to ensure that two models are equivalent, i.e., that their low-energy properties are similar. We also argue that the pseudogaplike phenomenology is quite generic for models of $t-J$ type, appearing in any dimension and for carriers injected into both ferromagnetic and antiferromagnetic backgrounds.

We examine the general features of the noncommutativity of the magnetization operator and Hamiltonian for small quantum spin clusters. The source of this noncommutativity can be a difference in the Landé $g$ factors for different spins in the cluster, $XY$ anisotropy in the exchange interaction, and the presence of the Dzyaloshinskii-Moriya term in a direction different from the direction of the magnetic field. As a result, zero-temperature magnetization curves for small spin clusters mimic those for the macroscopic systems with the band(s) of magnetic excitations, i.e., for the given eigenstate of the spin cluster the corresponding magnetic moment can be an explicit function of the external magnetic field yielding the nonconstant (nonplateau) form of the magnetization curve within the given eigenstate. In addition, the $XY$ anisotropy makes the saturated magnetization (the eigenstate when all spins in cluster are aligned along the magnetic field) inaccessible for finite magnetic field magnitude (asymptotical saturation). We demonstrate all these features on three examples: a spin-1/2 dimer, mixed spin-(1/2,1) dimer, and a spin-1/2 ring trimer. We consider also the simplest Ising-Heisenberg chain, the Ising-$XYZ$ diamond chain, with four different $g$ factors. In the chain model the magnetization curve has a more complicated and nontrivial structure than that for clusters.

The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization $M_S$, Gilbert damping $\alpha$, and exchange stiffness $A$. $M_S$ and $A$ are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for $M_S$ and $\alpha$. The calculated $A$ shows the same trend as the experimental data but has a larger magnitude. In addition to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices.

With the aid of the Keldysh effective action technique we develop a microscopic theory describing Andreev level spectroscopy experiments in nontunnel superconducting contacts. We derive an effective impedance of such contacts which accounts for the presence of Andreev levels in the system. At subgap bias voltages and low temperatures, inelastic Cooper pair tunneling is accompanied by transitions between these levels resulting in a set of sharp current peaks. We evaluate the intensities of such peaks, establish their dependence on the external magnetic flux piercing the structure and estimate the thermal broadening of these peaks. We also specifically address the effect of capacitance renormalization in a nontunnel superconducting contact and its impact on both the positions and heights of the current peaks. At overgap bias voltages, the $I-V$ curve is determined by quasiparticle tunneling and contains current steps related to the presence of discrete Andreev states in our system.

Compressed hydrides of the heavy group-15 elements Bi and Sb are investigated using ab initio methods. While the hydrides of Bi and Sb are known to be quite unstable at one atmosphere, our calculations predict that they can be stabilized at high pressures. Thus, at the composition of ${X\mathrm{H}}_3$ ($X=\text{Bi}$ or Sb), possible Bi hydrides are ${\mathrm{BiH}}_2\left(Pnma\right)$ + H beyond 105 GPa and ${\mathrm{BiH}}_3\left(I{4}_1/amd\right)$ beyond 250 GPa; for Sb hydrides, ${\mathrm{SbH}}_2$ + H hardly appears, and ${\mathrm{SbH}}_3\left(Pnma\right)$ is stabilized beyond 150 GPa. All of these hydrides are metallic with very dispersive electronic structures, this being in accordance with the predictions of the Goldhammer-Herzfeld criterion. Superconducting transition temperatures have also been estimated from the extended McMillan equation, and they turn out to be 39 K for ${\mathrm{BiH}}_2$ at 125 GPa, 65 K for ${\mathrm{BiH}}_3$ at 270 GPa, and 68 K for ${\mathrm{SbH}}_3$ at 170 GPa.

We determine the conditions for the existence of nontransverse factorizing magnetic fields in general spin arrays with anisotropic $XYZ$ couplings of arbitrary range. It is first shown that a uniform, maximally aligned, completely separable eigenstate can exist just for fields ${\mathit{h}}_s$ parallel to a principal plane and forming four straight lines in the field space, with the alignment direction different from that of ${\mathit{h}}_s$ and determined by the anisotropy. Such a state always becomes a nondegenerate ground state for sufficiently strong (yet finite) fields along these lines, in both ferromagnetic and antiferromagnetic-type systems. In antiferromagnetic chains, this field coexists with the nontransverse factorizing field ${\mathit{h}}_s^{\prime }$ associated with a degenerate Néel-type separable ground state, which is shown to arise at a level crossing in a finite chain. It is also demonstrated for arbitrary spin that pairwise entanglement reaches full range in the vicinity of both ${\mathit{h}}_s$ and ${\mathit{h}}_s^{\prime }$, vanishing at ${\mathit{h}}_s$ but approaching small yet finite side limits at ${\mathit{h}}_s^{\prime }$, which are analytically determined. The behavior of the block entropy and entanglement spectrum in their vicinity is also analyzed.

The magnetization dynamics of individual magnetite nanocrystals was probed by variable-temperature magnetoresistive scanning tunneling microscopy, in which a magnetoresistive junction is formed between the substrate and the magnetic particle under study. By tuning the temperature close to the magnetization blocking of a superparamagnetic particle, the slow magnetization switching of the particle caused fluctuations in the tunnel current passing through the particle, which appeared as telegraph noise in current vs time measurements. Analysis of the current fluctuations yielded estimates for the low local magnetic field sensed by the particle, its magnetic anisotropy energy, and the low limit for the spin-polarization degree of the nanocrystals, which for some particles appeared to be as high as 90%.

One of the goals in understanding any new class of superconductors is to search for commonalities with other known superconductors. The present work investigates the superconducting condensation energy, $U$, in the iron based superconductors (IBSs), and compares their $U$ with a broad range of other distinct classes of superconductor, including conventional BCS elements and compounds and the unconventional heavy fermion, $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4,\mathrm{L}{\mathrm{i}}_{0.1}\mathrm{ZrNCl},\kappa {\text{-(BEDT-TTF})}_2\mathrm{Cu}(\mathrm{NCS}{)}_2$, and optimally doped cuprate superconductors. Surprisingly, both the magnitude and $T_c$ dependence $\left(U\propto T_c^{3.4\pm 0.2}\right)$ of $U$ are—contrary to the previously observed behavior of the specific heat discontinuity at $T_c$, $\Delta C$—quite similar in the IBS and BCS materials for $T_c>1.4$ K. In contrast, the heavy fermion superconductors’ $U$ vs $T_c$ are strongly (up to a factor of 100) enhanced above the IBS/BCS while the cuprate superconductors’ $U$ are strongly (factor of 8) reduced. However, scaling of $U$ with the specific heat γ (or $\mathrm{\Delta }C$) brings all the superconductors investigated onto one universal dependence upon $T_c$. This apparent universal scaling $U/\gamma \propto T_c^2$ for all superconductor classes investigated, both weak and strong coupled and both conventional and unconventional, links together extremely disparate behaviors over almost seven orders of magnitude for $U$ and almost three orders of magnitude for $T_c$. Since $U$ has not yet been explicitly calculated beyond the weak coupling limit, the present results can help direct theoretical efforts into the medium and strong coupling regimes.

We investigate the properties of the coexistence phase of itinerant antiferromagnetism and nodal $d$-wave superconductivity $(Q$ phase) discovered in heavy-fermion ${\mathrm{CeCoIn}}_5$ under applied magnetic field. We solve the minimal model that includes $d$-wave superconductivity and underlying magnetic correlations in real space to elucidate the structure of the $Q$ phase in the presence of an externally applied magnetic field. We further focus on the role of magnetic impurities, and show that they nucleate the $Q$ phase at lower magnetic fields. Our most crucial finding is that, even at zero applied field, dilute magnetic impurities cooperate via RKKY-like exchange interactions to generate a long-range ordered coexistence state identical to the $Q$ phase. This result is in agreement with recent neutron scattering measurements [S. Raymond et al., J. Phys. Soc. Jpn. 83, 013707 (2014)].

The ground-state electron density of a one-dimensional spin-orbit coupled quantum dot with a Zeeman term and strong electron interaction is studied at the fractional helical liquid points. We show that at fractional filling factors $\nu ={(2n+1)}^{-1}$ (with $n$ a non-negative integer), the density oscillates with $N_0/\nu$ peaks. For $n\ge 1$, a number of peaks larger than the number of electrons $N_0$ suggests that a crystal of fractional charges $\nu e$ (with $e$ the electron charge) occurs. The reported effect is amenable of verification in a dot coupled to a charged atomic force microscope tip.

The spectral Matthiessen's rule is commonly used to calculate the total phonon scattering rate when multiple scattering mechanisms exist. Here we predict the spectral phonon relaxation time $\tau$ of defective bulk silicon using normal mode analysis based on molecular dynamics and show that the spectral Matthiessen's rule is not accurate due to the neglect of the coupling between anharmonic phonon-phonon scattering ${\tau }_a^{-1}$ and phonon-impurity scattering ${\tau }_i^{-1}$. As a result, the spectral Matthiessen's rule underestimates the total phonon scattering rate and hence overestimates the thermal conductivity $\kappa$ of mass-doped and Ge-doped silicon by about 20–40%. We have also directly estimated this coupling scattering rate, so-called coupled five-phonon scattering ${\tau }_{\mathrm{couple}}^{-1}$, and achieved good agreement between ${\tau }_a^{-1}+{\tau }_i^{-1}+{\tau }_{\mathrm{couple}}^{-1}$ and the total scattering rate ${\tau }_{\mathrm{tot}}^{-1}$.

The origin of the $p$-type conductivity in N-doped ZnO has been a long standing puzzle for many years because isolated ${\mathrm{N}}_{\mathrm{O}}$ is considered to be a deep acceptor. Recently, the $p$-type doping has been attributed to the shallow acceptor ${\mathrm{N}}_{\mathrm{O}}-V_{\mathrm{Zn}}$ complex that is supposedly being converted from a ${\mathrm{N}}_{\mathrm{Zn}}-V_{\mathrm{O}}$ complex at the Zn-terminated surface [L. Liu et al., Phys. Rev. Lett. 108, 215501 (2012)]. By performing first-principles calculations, we demonstrate that the $p$-type ${\mathrm{N}}_{\mathrm{O}}-V_{\mathrm{Zn}}$ pair is easy to form in bulk N-doped ZnO through the charge transfer and acceptor-acceptor level repulsion without the need of the special growth surface. More importantly, the ${\mathrm{N}}_{\mathrm{O}}-V_{\mathrm{Zn}}$ pair can be further stabilized through hydrogenation, forming the more stable shallow acceptor complexes $({\mathrm{N}}_{\mathrm{O}}-n\mathrm{H})-V_{\mathrm{Zn}}$ ($n=1-2$) with an ionization energy of less than 162 meV, in good agreement with the experiment. These new shallow acceptor complexes are proposed to be responsible for the $p$-type conductivity of the N-doped ZnO.