We report the observation of strong third-harmonic generation from a macroscopic array of aligned ultralong single-wall carbon nanotubes (SWCNTs) with intense midinfrared radiation. Through power-dependent experiments, we determined the absolute value of the third-order nonlinear optical susceptibility ${\chi }^{\left(3\right)}$ of our SWCNT film to be $5.53\times {10}^{-12}$ esu, three orders of magnitude larger than that of the fused silica reference we used. Taking account of the filling factor of 8.75% for our SWCNT film, we estimate a ${\chi }^{\left(3\right)}$ of $6.32\times {10}^{-11}$ esu for a fully dense film. Furthermore, through polarization-dependent experiments, we extracted all the nonzero elements of the ${\chi }^{\left(3\right)}$ tensor, determining the magnitude of the weaker tensor elements to be $\sim$1/6 of that of the dominant ${\chi }_{zzzz}^{\left(3\right)}$ component.

Tungsten trioxide (WO${}_3$) is an Earth abundant material of potential use as a light absorber for solar energy conversion processes. We carried out ab initio calculations of the band structure and absorption spectrum of WO${}_3$ using many-body perturbation theory and we present a detailed comparison of our results with photoemission and absorption data. We show that it is necessary to take into account multiple effects, including spin-orbit and electron-phonon interactions and exciton binding in order to correctly predict the measured optical gap. The absorption spectrum obtained by solving the Bethe-Salpeter equation compares well with experiments over a wide energy range, and our calculations correctly account for the redshift observed experimentally upon N${}_2$ intercalation in WO${}_3$.

We calculated the formation energies of all simple point defects in cubic fluorite structured ${\mathrm{CeO}}_2$ using density functional theory within the GGA+$U$ approximation. All possible defect charge states were considered, and also polarons ${\mathrm{Ce}}_{\mathrm{Ce}}^{\prime }$ and associates of polarons with oxygen vacancies: ${({\mathrm{V}}_{\mathrm{O}}^{··}-{\mathrm{Ce}}_{\mathrm{Ce}}^{\prime })}^{·}$ and ${({\mathrm{Ce}}_{\mathrm{Ce}}^{\prime }-{\mathrm{V}}_{\mathrm{O}}^{··}-{\mathrm{Ce}}_{\mathrm{Ce}}^{\prime })}^{\times }$. From the individual defect energies, we extracted Schottky, Frenkel, and anti-Frenkel energies: we find that anti-Frenkel disorder has the lowest energy in ceria. Energies for the reduction and the hydration of ceria are also computed, and the results are in good agreement with experiment. Finally, point-defect concentrations and conductivities are predicted for undoped and donor-doped systems as a function of oxygen partial pressure and temperature. The characteristic slopes found in experiment are reproduced.

A procedure to derive the Ginzburg-Landau (GL) theory from the multiband BCS Hamiltonian is developed in a general case with an arbitrary number of bands and arbitrary interaction matrix. It combines the standard Gor'kov truncation and a subsequent reconstruction in order to match accuracies of the obtained terms. This reconstruction recovers the phenomenological GL theory as obtained from the Landau model of phase transitions but offers explicit microscopic expressions for the relevant parameters. Detailed calculations are presented for a three-band system treated as a prototype multiband superconductor. It is demonstrated that the symmetry in the coupling matrix may lead to the chiral ground state with the phase frustration, typical for systems with broken time-reversal symmetry.

A material whose electrons are correlated can affect electron dynamics across the interface with another material. Such a “proximity effect” can have several manifestations, from order parameter leakage to generated effective interactions. The resulting combination of induced electron correlations and their intrinsic dynamics at the surface of the affected material can give rise to qualitatively new quantum states. For example, the leaking of a superconducting order parameter into certain Rashba spin-orbit-coupled materials has been recently identified as a path to creating “topological superconductors” that can host Majorana particles of use in quantum computing. Here we analyze the other aspects of the superconducting proximity effect. The proximity-induced interactions are a promising path to incompressible quantum liquids with non-Abelian fractional quasiparticles in topological insulator quantum wells, which could also find applications in topological quantum computing. We discuss the operational and design principles of a heterostructure device that could realize such states. We apply field-theoretical methods to characterize the properties of induced interactions via the electron-phonon coupling and Cooper pair tunneling across the interface. We argue that bound-state Cooper pairs can be stabilized by the interaction proximity effect inside a topological insulator quantum well at experimentally observable energy scales. The condensation of spinful triplet pairs, enabled by the Rashba spin-orbit coupling and tunable by gate voltage, would lead to novel superconducting states and fractional topological insulators.

Direct evidence of controlling the population of magnetic domains in BiFeO${}_3$ thin films through electric field is reported using neutron diffraction. By fabricating BiFeO${}_3$ thin films on vicinal SrTiO${}_3$ substrates, we have achieved ferroelectric monodomains as confirmed by piezoresponse force microscopy. The application of an electric field between the bottom SrRuO${}_3$ and the top electrode switches the ferroelectric domain state with concomitant changes in magnetic reflections observed with neutron diffraction, indicating changes in the antiferromagnetic domain populations. The observed magnetoelectric switching behavior by neutron diffraction is compared with the electric-field effect on the magneto-optical Kerr effect measurement on patterned pads of exchange coupled Co film deposited on top of the BiFeO${}_3$ films. The present result shows possible new directions for the realization of magnetoelectric devices.

Treating both many-body polarization and dispersion interactions is now recognized as a key element in achieving the level of atomistic modeling required to reveal novel physics in complex systems. The quantum Drude oscillator (QDO), a Gaussian-based, coarse grained electronic structure model, captures both many-body polarization and dispersion and has linear scale computational complexity with system size, hence it is a leading candidate next-generation simulation method. Here, we investigate the extent to which the QDO treatment reproduces the desired long-range atomic and molecular properties. We present closed form expressions for leading order polarizabilities and dispersion coefficients and derive invariant (parameter-free) scaling relationships among multipole polarizability and many-body dispersion coefficients that arise due to the Gaussian nature of the model. We show that these “combining rules” hold to within a few percent for noble gas atoms, alkali metals, and simple (first-row hydride) molecules such as water; this is consistent with the surprising success that models with underlying Gaussian statistics often exhibit in physics. We present a diagrammatic Jastrow-type perturbation theory tailored to the QDO model that serves to illustrate the rich types of responses that the QDO approach engenders. QDO models for neon, argon, krypton, and xenon, designed to reproduce gas phase properties, are constructed and their condensed phase properties explored via linear scale diffusion Monte Carlo (DMC) and path integral molecular dynamics (PIMD) simulations. Good agreement with experimental data for structure, cohesive energy, and bulk modulus is found, demonstrating a degree of transferability that cannot be achieved using current empirical models or fully ab initio descriptions.

Magnetism in Ni-Mn-$Z$ ($Z=\text{Ga}$, In, Sn, Sb) Heusler alloys has so far been predominantly attributed to Rudermann-Kittel-Kasuya-Yoshida type interactions between Mn atoms. We investigate magnetic interactions in one such alloy, Ni${}_2$Mn${}_{1.4}$In${}_{0.6}$, and attempt to explain the origin of antiferromagnetic (AFM) interactions that coexist with ferromagnetic ones. Through the combination of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD), we find that Ni plays an important role along with Mn in the overall magnetism. A significant hybridization that develops between Mn and Ni orbitals results in a small antiferromagnetic moment at Ni sites. The shift in the XMCD hysteresis loops in the martensitic phase suggests that antiferromagnetism results from superexchange like interactions between Mn atoms mediated by Ni.

The spin crossover from high-spin (HS) to low-spin (LS) magnetic state of a single Fe(phen${)}_2$(NCS${)}_2$ molecule adsorbed onto a metallic substrate is shown to be possible by means of density functional theory. The calculation also shows that a monolayer of nitrogen on Cu(001) reduces drastically the molecule-surface adsorption energy and chemical bonding, making the molecule switchable between its two magnetic states by means of an external stimuli, as observed experimentally. To show that the spin transition is not strongly affected by the type of exchange and correlation potential, both the generalized-gradient approximation (GGA) and the so-called $\text{GGA}+U$ method have been used including weak van der Waals interactions. The calculated scanning tunneling microscopy (STM) images of the HS and LS states within the Hamann-Tersoff approximation explain correctly the experimental data.

Gated terahertz cyclotron resonance measurements on epitaxial Bi${}_2$Se${}_3$ thin films capped with In${}_2$Se${}_3$ enable the first spectroscopic characterization of a single topological interface state from the vicinity of the Dirac point to above the conduction band edge. A precipitous drop in the scattering rate with Fermi energy is observed, which is interpreted as the surface state decoupling from bulk states and evidence of a shift of the Dirac point towards mid-gap. Near the Dirac point, potential fluctuations of 50 meV are deduced from an observed loss of differential optical spectral weight near the Dirac point. Potential fluctuations are reduced by a factor of two at higher surface Fermi levels in the vicinity of the conduction band edge inferred from the width of the scattering rate step. The passivated topological interface state attains a high mobility of 3500 cm${}^2$/V s near the Dirac point.

The conditions under which ferromagnetism may occur in transition metal oxides with partially filled $t_{2g}$ shells such as vanadium-based perovskites are studied using a combination of density functional and single-site dynamical mean field methods. For reasonable values of the correlation strength, rotations of the VO${}_6$ octahedra play an important role in enabling ferromagnetism, with ferromagnetism typically occurring for rotations larger than a nonzero critical value. Ferromagnetism is suppressed near the Mott-insulating phase but the phase boundary is otherwise only weakly dependent on carrier concentration. Design rules are suggested for new oxide systems exhibiting ferromagnetism.

The sensitivity of the Rabi oscillations of a resonantly driven spin-$\frac{1}{2}$ system to a weak and slow modulation of the static longitudinal magnetic field $B_0$ is studied theoretically. We establish the mapping of a weakly driven two-level system with modulation onto a strongly driven system without modulation. The mapping suggests that different regimes of spin dynamics, known for a strongly driven system, can be realized under common experimental conditions of weak driving (driving field $B_1\ll B_0$) upon proper choice of the domains of modulation frequency ${\omega }_m$ and amplitude $B_2$. Fast modulation ${\omega }_m\gg {\Omega }_R$, where ${\Omega }_R$ is the Rabi frequency, emulates the regime of driving frequency much bigger than the resonant frequency. Strong modulation $B_2\gg B_1$ emulates the regime $B_1\gg B_0$. Resonant modulation ${\omega }_m\approx {\Omega }_R$ gives rise to an envelope of the Rabi oscillations. The shape of this envelope is highly sensitive to the detuning of the driving frequency from the resonance. Theoretical predictions for different domains of $B_2$ and ${\omega }_m$ were tested experimentally using NMR of protons in water, where, without modulation, the pattern of Rabi oscillations could be observed over many periods. We present experimental results which reproduce the three predicted modulation regimes, and agree with theory quantitatively.

We clarify some misunderstandings on the time-dependent current-density functional theory for open quantum systems we have recently introduced [M. Di Ventra and R. D'Agosta, Phys. Rev. Lett. 98, 226403 (2007)]. We also show that some of the recent formulations attempting to improve on this theory suffer from some inconsistencies, especially in establishing the mapping between the external potential and the quantities of interest. We offer a general argument about this mapping, which applies to any density functional theory, showing that it must fulfill certain “dimensionality” requirements.

Efficient exciton dissociation at a donor-acceptor interface is the crucial, yet not fully understood, step for obtaining high efficiency organic solar cells. Recent theoretical work suggested an influence of polymer conjugation length and of interfacial dipoles on the exciton dissociation yield. This necessitates experimental verification. To this end, we measured the dissociation yield of several polymer/C${}_{60}$ planar heterojunction solar cells up to high electric fields. The results indeed prove that the yield of exciton dissociation depends strongly on the conjugation length of the polymers. Complementary photoemission experiments were carried out to assess the importance of dipoles at the donor-acceptor interfaces. Comparison of exciton dissociation models with experimental data shows that the widely used Onsager-Braun approach is unsuitable to explain photodissociation in polymer/C${}_{60}$ cells. Better agreement can be obtained using “effective mass” models that incorporate conjugation length effects by considering a reduced effective mass of the hole on the polymer and that include dielectric screening effects by interfacial dipoles. However, successful modeling of the photocurrent field dependence over a broad field range, in particular for less efficient solar cell compounds, requires that the dissociation at localized acceptor sites is also taken into account.

Using hybrid functional calculation, we identify the key intrinsic defects in Cu${}_2$ZnSnS${}_4$ (CZTS), an important earth-abundant solar-cell material. The Sn-on-Zn antisite and the defect complex having three Cu atoms occupying a Sn vacancy are found to be the main deep electron traps. This result explains the optimal growth condition for CZTS, which is Cu poor and Zn rich, as found in several recent experiments. We show that, under the growth condition that minimizes the deep traps, Cu vacancy could contribute the majority of hole carriers, while the Cu-on-Zn antisite will become the dominant acceptor if the growth condition favors its formation.

We analyze the influence of strong electron-phonon coupling and thermal fluctuations on the Hall effect in organic crystals, using a variant of the Kubo formula. The Hall effect in organic crystals cannot simply be understood based on a band picture, but it is governed by competition between intermolecular quantum coherence and dynamic lattice disorder. When the intermolecular electronic coupling strength is comparable to that of rubrene crystals, delocalized Peierls polarons exhibit the Hall effect overcoming dephasing due to thermal fluctuations. As the electronic coupling decreases, the Peierls polaron is less delocalized, and the Hall effect is diminished by the dephasing. As the temperature decreases, the dephasing is suppressed and the Hall effect is enhanced, as is observed for pentacene crystals.

The presence of edges locally breaks the inversion symmetry of heterostructures and gives rise to lateral (edge) spin-orbit coupling (SOC), which, under some conditions, can lead to the formation of helical edge states. If the edge SOC is strong enough, the helical edge states can penetrate the band gap and be energetically isolated from the bulk-like states. As a result, backward scattering is suppressed, dissipationless helical edge channels protected against time-inversion symmetric perturbations emerge, and the system behaves as a two-dimensional topological insulator (TI). However, unlike in previous work on TIs, the mechanism proposed here for the creation of protected helical edge states relies on the strong edge SOC rather than on band inversion.

The anomalous Hall effect (AHE) in epitaxial Fe films on GaAs(111) has been investigated as a function of film thickness and temperature. The intrinsic contribution from the Berry curvature is singled out from the extrinsic ones and determined to be 821 ${\Omega }^{-1}$ cm${}^{-1}$, which agrees to the theoretical prediction of 842 ${\Omega }^{-1}$ cm${}^{-1}$ and is considerably smaller than 1100 ${\Omega }^{-1}$ cm${}^{-1}$ for Fe(001). This result provides a direct experimental evidence for the anisotropy of the intrinsic AHE in single crystal Fe, reflecting its electronic band structure.

Angle-resolved photoemission spectroscopy studies of the protected surface states in the topological insulator Bi${}_2$Te${}_3$ have revealed the existence of an important hexagonal-warping term in its electronic band structure. This term distorts the shape of the Dirac cone from a circle at low energies to a snowflake shape at higher energies. We show that this implies important modifications of the interband optical transitions which no longer provide a constant universal background, as seen in graphene. Rather, the conductivity shows a quasilinear increase with a slightly concave upward bending as energy is increased. Its slope increases with increasing magnitude of the hexagonal distortion, as does the magnitude of the jump at the interband onset. The energy dependence of the density of states is also modified and deviates downward from linear behavior with increasing energy.

The reduced Rayleigh equation for the scattering of a surface plasmon polariton incident normally on a one-dimensional ridge or groove on an otherwise planar metal surface is solved by a purely numerical approach. The solution is used to calculate the reflectivity and transmissivity of the surface plasmon polariton and its conversion into volume electromagnetic waves in the vacuum above the metal surface. The results obtained are compared with those of earlier calculations of these quantities.