Density-functional theory (DFT) calculations of the magnetic shielding for nuclear magnetic resonance (NMR) in solids provide an important contribution for understanding the experimentally observed chemical shifts. It is known that the calculated NMR shielding parameters for a particular nucleus in a series of compounds correlate well with the experimentally measured chemical shifts; however, the slope of a linear fit often differs from the ideal value of 1.0. Focusing on a series of ionic compounds (fluorides, oxides, bromides, and chlorides), we show that the error is caused by the generalized gradient approximation (GGA) to the exchange-correlation functional and it is related to the well-known band-gap problem. In order to devise an ab initio approach that would correctly reproduce the variation of the shifts within a series of compounds, we test various DFT based approaches. A simple GGA+$U$ scheme with the orbital field acting on the cation $d$ states does not work in a general way. Also, the popular hybrid functionals (including the screened versions), which contain some fixed amount of exact exchange, lead to a large overestimation of the necessary slope correction. Surprisingly, the best solution to this problem is offered by a semilocal potential designed by Becke and Johnson to reproduce the optimized exact exchange potential in free atoms.

In an external magnetic field $\mathit{B}$, the spins of the electron and hole will precess in effective fields ${\mathit{b}}_e+\mathit{B}$ and ${\mathit{b}}_h+\mathit{B}$, where ${\mathit{b}}_e$ and ${\mathit{b}}_h$ are random hyperfine fields acting on the electron and hole, respectively. For sparse “soft” pairs the magnitudes of these effective fields coincide. The dynamics of precession for these pairs acquires a slow component, which leads to a slowing down of recombination. We study the effect of soft pairs on organic magnetoresistance, where slow recombination translates into blocking of the passage of current. It appears that when ${\mathit{b}}_e$ and ${\mathit{b}}_h$ have identical Gaussian distributions the contribution of soft pairs to the current does not depend on $\mathit{B}$. Amazingly, small inequivalence in the rms values of ${\mathit{b}}_e$ and ${\mathit{b}}_h$ gives rise to a magnetic field response, and it becomes progressively stronger as the inequivalence increases. We find the expression for this response by performing the averaging over ${\mathit{b}}_e$, ${\mathit{b}}_h$ analytically. Another source of magnetic field response in the regime when current is dominated by soft pairs is inequivalence of the $g$ factors of the pair partners. Our analytical calculation indicates that for this mechanism the response has an opposite sign.

First-principles calculations of N${}_2$ in ZnO located on the Zn site show that this defect is a shallow double acceptor and can be identified with a N-related shallow level observed experimentally by donor-acceptor-pair recombination. When the shallow acceptor level is occupied with a single electron, it can also explain a N${}_2$-related electron paramagnetic resonance signal observed in ZnO. We show that the $g$ factor and hyperfine structure observed for this center are closer to those of a N${}_2{}^{+}$ radical than to a N${}_2^{-}$ radical as previously proposed in literature and hence consistent with the Zn location.

We numerically study light scattering and absorption in self-similar aggregates of dielectric nanoparticles, as generated by simulated ballistic deposition upon a surface starting from a single seed particle. The resulting structure exhibits a complex treelike shape, intended to mimic the morphologic properties of building blocks of real nanostructured thin films produced by means of fine controlled physical deposition processes employed in nanotechnology. The relationship of scattering and absorption cross sections to morphology is investigated within a computational scheme which thoroughly takes into account both multiple scattering and near-field effects. Numerical results are compared with a preexisting single-scattering limited analytical treatment of light scattering in fractal aggregates of small dielectric particles.

We present a comparison between experimental and theoretical values of transport ${\tau }_t$ and quantum ${\tau }_q$ scattering lifetimes in high-quality undoped Al${}_{0.34}$Ga${}_{0.66}$As/GaAs heterostructures. We obtain excellent agreement between the experimental and modeled scattering times using three scattering processes: background impurity, interface roughness, and remote ionized impurity scattering from surface states. We show that despite the high mobility (${\mu }_{\text{peak}}=5.6\times {10}^6{\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$), the quantum lifetime ${\tau }_q$ is significantly reduced by small-angle scattering from remote surface charge. We further show that in shallow devices scattering from surface charges will be a limiting factor for both transport and quantum lifetimes.

We consider a SU(3) ring-exchange model on a triangular lattice. Unlike the SU(2) case, under perturbation expansion of the SU(3) Hubbard model, the three-site ring exchange is present as well as the usual four-site ring exchange. Interestingly, the three-site ring exchange differs from the usual two-site and four-site exchanges by a minus sign and is ferromagnetic. We first present numerical site-factorized state studies on this model which shows a three-sublatticed order phase and a ferromagnetic phase. We further study the model using slave-fermion mean field in which we rewrite the exchange operators in terms of three flavors of fermions. We find the main competing trial states are the trimer state (triangular plaquette state) and the gapless U(1) spin liquid states with parton Fermi surfaces which include both the uniform zero-flux spin liquid state, and the uniform $\pi$-flux spin liquid state. Furthermore, we find there are possible pairing instabilities of the zero-flux (Fermi surface) spin liquid state toward a $f$-wave gapless (nodal) spin liquid state and the $\pi$-flux (Fermi surface) spin liquid state toward an interesting exotic $s$-wave “gapless” spin liquid state with two flavors of fermions paired up while one flavor of fermions remains gapless.

Combining tight-binding models and first-principles calculations, we investigate the quantum anomalous Hall (QAH) effect induced by intrinsic spin-orbit coupling (SOC) in buckled honeycomb lattice with $sp$ orbitals in an external exchange field. Detailed analysis reveals that nontrivial topological properties can arise utilizing not only spin but also orbital degrees of freedom in the strong SOC limit, when the bands acquire nonzero Chern numbers upon undergoing the so-called orbital purification. As a prototype of a buckled honeycomb lattice with strong SOC we choose the Bi(111) bilayer, analyzing its topological properties in detail. In particular, we show the emergence of several QAH phases upon spin exchange of the Chern numbers as a function of SOC strength and magnitude of the exchange field. Interestingly, we observe that in one of such phases, namely, in the quantum spin Chern insulator phase, the quantized charge and spin Hall conductivities coexist. We consider the possibility of tuning the SOC strength in the Bi bilayer via alloying with isoelectronic Sb, and speculate that exotic properties could be expected in such an alloyed system owing to the competition of the topological properties of its constituents. Finally, we demonstrate that $3d$ dopants can be used to induce a sizable exchange field in the Bi(111) bilayer, resulting in nontrivial Chern insulator properties.

In semiconductors, defects often assist nonradiative relaxation. However, Tl doping can significantly suppress the nonradiative relaxation in alkali halides to increase scintillation efficiency. Without the Tl, it is known that the creation of Frenkel pairs at self-trapped excitons, assisted by excited electron and hole relaxations, is the reason for the nonradiative relaxation. Here we show by first-principles calculation that Tl doping introduces Tl $p$ states inside the band gap to trap the excited electrons. The trapping is highly effective to within several picoseconds, as revealed by time-dependent density functional theory calculations. It alters the nonradiative relaxation process to result in a noticeable increase in the relaxation barrier from 0.3 to 0.63 eV, which reduces the nonradiative relaxation by roughly a factor of 10${}^5$ at room temperature.

We present a magnetophotoluminescence study on neutral and charged excitons confined to InAs/GaAs quantum dots. Our investigation relies on a confocal microscope that allows arbitrary tuning of the angle between the applied magnetic field and the sample growth axis. First, from experiments on neutral excitons and trions, we extract the in-plane and on-axis components of the Landé tensor for electrons and holes in the $s$ shell. Then, based on the doubly negatively charged exciton magnetophotoluminescence, we show that the $p$-electron wave function spreads significantly into the GaAs barriers. We also demonstrate that the $p$-electron $g$ factor depends on the presence of a hole in the $s$ shell. The magnetic field dependence of triply negatively charged excitons photoluminescence exhibits several anticrossings, as a result of coupling between the quantum dot electronic states and the wetting layer. Finally, we discuss how the system evolves from a Kondo-Anderson exciton description to the artificial atom model when the orientation of the magnetic field goes from Faraday to Voigt geometry.

The local order around Zr and Y atoms of nanocrystalline yttria-stabilized zirconia (YSZ) powders with different grain sizes has been investigated by x-ray absorption spectroscopies. The samples were prepared by means of mechanical alloying with or without subsequent sintering treatment and also by milling commercial YSZ. Our study is motivated by the interest in the electrical properties of grain boundaries and the controversy about the level of disorder in the intergrain regions in nanocrystalline YSZ. The x-ray absorption near edge structure (XANES) analysis indicates that the local order of all the sintered samples is independent of the grain size. This is confirmed by the analysis of the extended x-ray absorption fine structure, which points out also that, in contrast to that found in sintered samples, the local order around the cation in the samples milled without further sintering treatment extends only to the first coordination shell. Finally, the results of ab initio Zr $K$-edge XANES calculations lead us to conclude that the observed changes of the shape of the white line are not related to a phase transformation but reflects the short-range order present in the as-milled samples.

We investigate the two-magnon Raman scattering from an anisotropic $S=\frac{1}{2}$ triangular Heisenberg antiferromagnet Cs${}_2$CuCl${}_4$. We find that the Raman response is very sensitive to magnon-magnon interactions and to scattering geometries, a feature that is in remarkable contrast with the polarization-independent Raman signal from the isotropic triangular Heisenberg antiferromagnet. Since a spin-liquid ground state gives rise to a similar rotationally invariant Raman response, our results on the polarization dependence of the scattering spectrum suggest that Raman spectroscopy provides a useful probe, complementary to neutron scattering, of the ground-state properties of Cs${}_2$CuCl${}_4$, particularly whether the time-reversal symmetry is broken in the ground state.

Motivated by a search for experimental probes to access the physics of fractionalized excitations called spinons in spin liquids, we study the interaction of spinons with lattice vibrations. We consider the case of algebraic spin liquid, when spinons have fermionic statistics and a Dirac-like dispersion. We establish the general procedure for deriving spinon-phonon interactions, which is based on symmetry considerations. The procedure is illustrated for four different algebraic spin liquids: $\pi$-flux and staggered-flux phases on a square lattice, $\pi$-flux phase on a kagome lattice, and zero-flux phase on a honeycomb lattice. Although the low-energy description is similar for all these phases, different underlying symmetry groups lead to a distinct form of spinon-phonon interaction Hamiltonian. The explicit form of the spinon-phonon interaction is used to estimate the attenuation of ultrasound in an algebraic spin liquid. The prospects of the sound attenuation as a probe of spinons are discussed.

We study the $S=1/2$ Heisenberg ($J$) model on the two-dimensional (2D) square lattice in the presence of additional higher-order spin interactions ($Q$) which lead to a valence-bond-solid (VBS) ground state. Using quantum Monte Carlo simulations, we analyze the thermal VBS transition. We find continuously varying exponents, with the correlation-length exponent $\nu$ close to the Ising value for large $Q/J$ and diverging when $Q/J$ approaches the quantum-critical point (the critical temperature $T_c\to 0$). We identify the transition with a class of conformal field theories with charge $c=1$ and critical exponents varying between those of the 2D Ising model and the Kosterlitz-Thouless (KT) fixed point. We find explicit evidence for KT physics by studying the emergence of $U\left(1\right)$ symmetry of the order parameter at $T=T_c$ when $T_c\to 0$.

We report a computational theoretical investigation of electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co, and fcc Ni. The Elliott-Yafet electron-phonon spin-flip scattering is computed from first principles, employing a generalized spin-flip Eliashberg function as well as ab initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nonthermal hot electron distributions. Using the developed formalism we compute the phonon-induced spin lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated demagnetization rate is evaluated for laser-created thermalized and nonequilibrium electron distributions. Nonthermal distributions are found to lead to a stronger demagnetization rate than hot, thermalized distributions, yet their demagnetizing effect is not enough to explain the experimentally observed demagnetization occurring in the subpicosecond regime.

We investigate theoretically and experimentally the statistical properties of the inhomogeneous order-parameter distribution (OPD) at the verge of the superconductor-insulator transition (SIT). We find within two prototype fermionic and bosonic models for disordered superconductors that one can identify a universal rescaling of the OPD. By performing scanning-tunneling microscopy experiments in three samples of NbN with increasing disorder we show that such a rescaling also describes the experimental data with excellent accuracy. These results can provide a breakthrough in our understanding of the SIT.

We have investigated the electrical Hanle effect with magnetic fields applied at an oblique angle (θ) to the spin direction [the oblique Hanle effect (OHE)] in CoFe/MgO/semiconductor (SC) contacts by employing a three-terminal measurement scheme. The electrical oblique Hanle signals obtained in CoFe/MgO/Si and CoFe/MgO/Ge contacts show clearly different line shapes depending on the spin lifetime of the host SC. Notably, at moderate magnetic fields, the asymptotic values of the oblique Hanle signals (in both contacts) are consistently reduced by a factor of ${\cos }^2\left(\theta \right)$ irrespective of the bias current and temperature. These results are in good agreement with predictions of the spin precession and relaxation model for the electrical OHE. At high magnetic fields where the magnetization of CoFe is significantly tilted from the film plane to the magnetic-field direction, we find that the observed angular dependence of voltage signals in the CoFe/MgO/Si and CoFe/MgO/Ge contacts is well explained by the OHE, considering the misalignment angle between the external magnetic field and the magnetization of CoFe.

Atoms or pairs of ions picked up by probe tips used in dynamic force microscopy (DFM) can be strongly displaced and even hop discontinuously upon approach to the sample surface. The energy barriers for some of those hops are of the right order of magnitude to explain the rise in energy dissipation commonly observed in DFM measurements at room temperature. The systematic computations reported here can explain the infrequent jumps and very low average energy dissipation observed low temperature in a previous DFM study on a KBr(001) sample. Close to the surface we indeed find new states separated by small energy barriers which account for those phenomena. These energy barriers strongly depend on details of the atomic arrangement in the vicinity of the tip apex.

We report an inelastic neutron-scattering investigation of the longitudinal acoustic modes of C${}_{60}$ chains confined inside single walled carbon nanotubes. We take advantage of the orientations of the chains within the plane of the pellet sample to isolate their scattering signatures in the $(Q,\omega )$ space, which we follow as a function of temperature from 260 K up to 1100 K. The results show the progressive evolution of the confined chain from a one-dimensional (1D) crystal to a linear liquid, the transition occurring within a temperature range of $\sim$150 K centered around 600 K. The comparison of the data obtained on monomer and polymer peapods allows extracting the speed of sound in the monomer crystalline chains ($v_{\mathrm{mono}}=3.5$ km s${}^{-1}$, $v_{\mathrm{poly}}/v_{\mathrm{mono}}=1.7$). We find that the sound velocity is further reduced by half in the liquid state which reveals that the melting is not only due to harmonic additive thermal fluctuations, but that anharmonic terms in the intermolecular potential play an important role at high temperatures.

The electronic tunneling properties of the two stable forms of trilayer graphene (TLG), rhombohedral ABC and Bernal ABA, are examined for $p$-$n$ and $p$-$n$-$p$ junctions as realized by using a single gate (SG) or a double gate (DG). For the rhombohedral form, due to the chirality of the electrons, the Klein paradox is found at normal incidence for SG devices, while at high-energy interband scattering between additional propagation modes can occur. The electrons in Bernal ABA TLG can have a monolayer- or bilayer-like character when incident on a SG device. Using a DG, however, both propagation modes will couple by breaking the mirror symmetry of the system, which induces intermode scattering and resonances that depend on the width of the DG $p$-$n$-$p$ junction. For ABC TLG the DG opens up a band gap which suppresses Klein tunneling. The DG induces also an unexpected asymmetry in the tunneling angle for single-valley electrons.