In developing the phonon quasiparticle picture, Peierls discovered that, in a perfect crystal, without anharmonic umklapp ($U$) events, a current-carrying distribution can never relax to a zero-current distribution. Callaway introduced a simplified approximate model version of the Peierls-Boltzmann equation, retaining its ability to deal separately with normal ($N$) and $U$ events. This paper clarifies and improves the Callaway model, and shows that Callaway underestimated the suppression of $N$ processes in relaxing thermal current. The new result should improve computations of thermal conductivity from relaxation-time studies.

We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C${}_{22}$H${}_{14}$) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the observed phonon peaks. We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a $\sim 20\%$ compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure, without any evidence of structural phase transitions. The Grüneisen parameter of the 1380 cm${}^{-1}$ $a_1$ Raman peak (${\gamma }_p=0.1$) is much lower than the effective value (${\gamma }_d=0.8$) due to K doping. Therefore, doping and pressure have very different effects and it can be argued that softening of the 1380 cm${}^{-1}$ mode is probably due to coupling with electronic states in K-doped solid picene.

We discuss a possible realization of overscreened $N$-channel Kondo model in quasi-one-dimensional magnets. We suggest that it is feasible to have arrangements for $N$ from 2 to 6 and discuss the related quantum chemistry problems.

Motivated by recent experimental and theoretical progress on the Er${}_2$Ti${}_2$O${}_7$ pyrochlore $XY$ antiferromagnet, we study the general problem of quantum order-by-disorder in pyrochlore $XY$ systems. We consider the nearest-neighbor pseudo-spin-1/2 Hamiltonian for such a system characterized by anisotropic spin-spin couplings $J_e\equiv$$\{J_{\pm },J_{\pm \pm },J_{z\pm },J_{zz}\}$ and construct zero-temperature phase diagrams. Combining symmetry arguments and spin-wave calculations, we show that the ground state phase boundaries between the two candidate ground states of the ${\Gamma }_5$ irreducible representation, the ${\psi }_2$ and ${\psi }_3$ (basis) states, are rather accurately determined by a cubic equation in $\left(J_{\pm }{J}_{\pm \pm }\right)/J_{z\pm }^2$. Depending on the value of $J_{zz}$, there can be one or three phase boundaries that separate alternating regions of ${\psi }_2$ and ${\psi }_3$ states. In particular, we find for sufficiently small $J_{zz}/J_{\pm }$ a narrow ${\psi }_2$ sliver sandwiched between two ${\psi }_3$ regions in the $J_{\pm \pm }/J_{\pm }$ vs $J_{z\pm }/J_{\pm }$ phase diagram. From our results, one would be able to predict which state (${\psi }_2$ or ${\psi }_3$) may be realized in a real material given its set of $J_e$ couplings. Our results further illustrate the very large potential sensitivity of the ground state of $XY$ pyrochlore systems to minute changes in their spin Hamiltonian. Finally, using the experimentally determined $J_e\equiv$$\{J_{\pm },J_{\pm \pm },J_{z\pm },J_{zz}\}$ and $g$-tensor values for Er${}_2$Ti${}_2$O${}_7$, we show that the heretofore neglected long-range $1/r^3$ magnetostatic dipole-dipole interactions do not change the conclusion that Er${}_2$Ti${}_2$O${}_7$ has a ${\psi }_2$ ground state induced via a quantum order-by-disorder mechanism. As an avenue of research in $XY$ pyrochlore materials distinct from the rare-earth pyrochlore oxides, we propose that the Cd${}_2$Dy${}_2$Se${}_4$ chalcogenide spinel, in which the Dy${}^{3+}$ ions form a pyrochlore lattice and may be $XY$-like, could be interesting to investigate.

Using the electron-spin-resonance technique we investigate the magnetic structure of CuCrO${}_2$, a quasi-two-dimensional antiferromagnet with a weakly distorted triangular lattice. Resonance frequencies and the excitation conditions in CuCrO${}_2$ at low temperatures are well described in the frame of cycloidal spin structure, which is defined by two susceptibilities parallel and perpendicular to the spin plane (${\chi }_{\perp }$ and ${\chi }_{\parallel }$) and by a biaxial crystal-field anisotropy. In agreement with the calculations, the character of the eigenmodes changes drastically at the spin-flop transition. The splitting of the observed modes can be well attributed to the resonances from different domains. The domain structure in CuCrO${}_2$ can be controlled by annealing of the sample in a magnetic field.

Very recent experiments have discovered that localized light in strongly absorbing media displays intriguing diffusive phenomena. Here we develop a first-principles theory of light propagation in open media with arbitrary absorption strength and sample length. We show analytically that waves in localized open absorbing media exhibit highly unconventional diffusion. Specifically, wave energy transport follows the diffusion equation with the diffusion coefficient exhibiting spatial resolution. Most strikingly, despite that the system is controlled by two parameters—the ratio of the localization (absorption) length to the sample length—the spatially resolved diffusion coefficient displays novel single parameter scaling: It depends on the position in the sample via the returning probability. Our analytic predictions for this diffusion coefficient are confirmed by numerical simulations. In the strong absorption limit they agree well with the experimental results.

Long-standing challenges in cluster expansion (CE) construction include choosing how to truncate the expansion and which crystal structures to use for training. Compressive sensing (CS), which is emerging as a powerful tool for model construction in physics, provides a mathematically rigorous framework for addressing these challenges. A recently-developed Bayesian implementation of CS (BCS) provides a parameterless framework, a vast speed-up over current CE construction techniques, and error estimates on model coefficients. Here, we demonstrate the use of BCS to build cluster expansion models for several binary alloy systems. The speed of the method and the accuracy of the resulting fits are shown to be far superior than state-of-the-art evolutionary methods for all alloy systems shown. When combined with high-throughput first-principles frameworks, the implications of BCS are that hundreds of lattice models can be automatically constructed, paving the way to high-throughput thermodynamic modeling of alloys.

Adsorption of Fe on the rutile (110) surface is investigated by means of ab initio density functional theory calculations. We discuss the deposition of single Fe atoms and increasing Fe coverage, as well as the adsorption of small Fe clusters. It is shown that the different interface structures found in experiment can be understood in terms of the adsorption of the Fe atoms landing first on the rutile surface. Strong interface bonds form if single Fe atoms are deposited. The Fe-Fe bonds in deposited Fe clusters lead to a three-dimensional growth mode. Mainly ionic Fe oxide bonds are formed in both cases and the electronic band gap of the surface is reduced due to interface states. In addition to the structural and electronic properties, we discuss the influence of the interface on the magnetic properties, finding stable Fe moments and induced moments within the interface which lead to a large spin polarization of the Fe atoms at the rutile (110)/Fe interface.

We present density functional theory calculations of phosphorus dopants in bulk silicon and of several properties relating to their use as spin qubits for quantum computation. Rather than a mixed pseudopotential or a Heitler-London approach, we have used an explicit treatment for the phosphorus donor and examined the detailed electronic structure of the system as a function of the isotropic doping fraction, including lattice relaxation due to the presence of the impurity. Doping electron densities (${\rho }_{\mathrm{doped}}-{\rho }_{\mathrm{bulk}}$) and spin densities (${\rho }_{\uparrow }-{\rho }_{\downarrow }$) are examined in order to study the properties of the dopant electron as a function of the isotropic doping fraction. Doping potentials ($V_{\mathrm{doped}}-V_{\mathrm{bulk}}$) are also calculated for use in calculations of the scattering cross sections of the phosphorus dopants, which are important in the understanding of electrically detected magnetic resonance experiments. We find that the electron density around the dopant leads to nonspherical features in the doping potentials, such as trigonal lobes in the (001) plane at energy scales of $+$12 eV near the nucleus and of −700 meV extending away from the dopants. These features are generally neglected in effective mass theory and will affect the coupling between the donor electron and the phosphorus nucleus. Our density functional calculations reveal detail in the densities and potentials of the dopants which are not evident in calculations that do not include explicit treatment of the phosphorus donor atom and relaxation of the crystal lattice. These details can also be used to parametrize tight-binding models for simulation of large-scale devices.

We present an electronic structure study of the quasi-one-dimensional chalcogenide BaFe${}_2$Se${}_3$ using resonant inelastic x-ray scattering (RIXS) at the Fe $L_3$ edge. In addition to broad spectral contributions from fluorescence, sharp peaks are seen at the low-energy end of the spectra, suggesting the coexistence of both band and localized properties in this compound up to the antiferromagnetic transition temperature. The width of the fluorescence line can be accounted for by band structure calculations, while the positions of the sharp peaks are in good agreement with crystal-field multiplet calculations. At measurement temperatures below 300 K, the temperature dependence of the low-energy part of the spectra is accounted for by the changes in thermal occupation of closely lying energy states. At 300 K, however, a substantial broadening of the spectra is observed at low-energy loss, which is consistent with the closure of the antiferromagnetic band gap.

We study the proximity effect in a topological nanowire tunnel coupled to an $s$-wave superconducting substrate. We use a general Green's function approach that allows us to study the evolution of the Andreev bound states in the wire into Majorana fermions. We show that the strength of the tunnel coupling induces a topological transition in which the Majorana fermionic states can be destroyed when the coupling is very strong. Moreover, we provide a phenomenological study of the effects of disorder in the superconductor on the formation of Majorana fermions. We note a nontrivial effect of a quasiparticle broadening term which can take the wire from a topological into a nontopological phase in certain ranges of parameters. Our results also have direct consequences for a nanowire coupled to an inhomogeneous superconductor.

The electronic structure of iron pnictides is topologically nontrivial, leading to the appearance of Dirac cones in the band structure for the antiferromagnetic phase. Motivated by the analogy with Dirac cones in graphene, we explore the possible existence of topologically protected surface states. Surprisingly, bands of surface states exist even in the paramagnetic state. A realistic five-orbital model predicts two such bands. In the antiferromagnetic phase, these surface bands survive but split. We obtain the bulk and surface dispersion from exact diagonalization of two- and five-orbital models in a strip geometry and discuss the results based on topology.

Transport coefficients, the elements of the so-called Onsager matrix, are essential quantities for modeling solid-state kinetics controlled by diffusion. Focusing on diffusion in binary alloys with a body-centered cubic crystal structure, we investigate the drag of solute atoms by vacancies, an effect induced by kinetic correlations. To accomplish this, an analytic method—the self-consistent mean field method—is extended to take into account interactions between the solute atom and a vacancy up to the third nearest neighbor sites. We identify kinetic effects involving one or more frequencies. Analytic results are compared with select atomic kinetic Monte Carlo simulations. We show that (1) solute drag is a more general phenomena than previously assumed, (2) it can induced by association and dissociation exchanges, and (3) we identify the mechanisms involved.

Correlation effects play an important role in the electronic structure of half-metallic (HM) magnets. In particular, they give rise to nonquasiparticle states above (or below) the Fermi energy at finite temperatures that reduce the spin polarization and, as a consequence, the efficiency of spintronics devices. Employing the constrained random-phase approximation (cRPA) within the full-potential linearized augmented-plane-wave (FLAPW) method using maximally localized Wannier functions, we calculate the strength of the effective on-site Coulomb interaction (Hubbard $U$ and Hund exchange $J$) between localized electrons in different classes of HM magnets considering: (i) $\mathit{sp}$-electron ferromagnets in rock-salt structure, (ii) zinc-blende 3$d$ binary ferromagnets, as well as (iii) ferromagnetic and ferrimagnetic semi- and full-Heusler compounds. For HM $\mathit{sp}$-electron ferromagnets, the calculated Hubbard $U$ parameters are between 2.7 and 3.9 eV, while for transition-metal-based HM compounds, they lie between 1.7 and 3.8 eV, being smallest for MnAs (Mn-3$d$ orbitals) and largest for Cr${}_2$CoGa (Co-3$d$ orbitals). For the HM full-Heusler compounds, the Hubbard $U$ parameters are comparable to the ones in elementary 3$d$ transition metals, while for semi-Heusler compounds, they are slightly smaller. We show that the increase of the Hubbard $U$ with structural complexity, i.e., from MnAs to Cr${}_2$CoGa, stems from the screening of the $p$ electrons of the nonmagnetic $\mathit{sp}$ atoms. The $p$-electron screening turns out to be more efficient for MnAs than for Cr${}_2$CoGa. The calculated Hubbard $U$ parameters for CrAs, NiMnSb, and Co${}_2$MnSi are about two times smaller than previous estimates based on the constrained local-density approximation (cLDA) method. Furthermore, the width of the correlated $d$ or $p$ bands of the studied compounds is usually smaller than the calculated Hubbard $U$ parameters. Thus these HM magnets should be classified as weakly correlated materials.

We report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to a hexagonal surface and a monoclinic distorted CoO 3 nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe double layer. The exchange bias shift is preserved up to the Néel antiferromagnetic ordering temperature of $T_N=293$ K. This unique example of selfsame value for blocking and ordering temperatures, yet identical to the bulk ordering temperature, is likely related to the original strain-induced distortion and strengthened interaction between the two well-ordered spin layers.

We have performed angle-resolved photoemission spectroscopy (ARPES) of KFeAgTe${}_2$, and revealed the absence of band crossing at the Fermi level ($E_{\mathrm{F}}$) indicative of the unconventional insulating nature of this material. Comparison of the ARPES-derived band dispersions with the first-principles calculations based on local density approximation and the inclusion of electron correlation $U$ demonstrated that the ground state of KFeAgTe${}_2$ is not a simple band insulator. And also, our fitting result on the ARPES experimental density of states near $E_{\mathrm{F}}$ plausibly excludes the possibility of Anderson insulator. We suggest that KFeAgTe${}_2$ is most likely a Mott insulator or a Hund insulator, providing a deep insight into the insulating ground state.

We study the Renyi entropy in the finite-temperature crossover regime of a Hubbard chain using quantum Monte Carlo. The ground-state entropy has characteristic features such as a logarithmic divergence with block size and $2k_{\mathrm{F}}$ oscillations that are a hallmark of its Luttinger liquid nature. The interplay between the (extensive) thermal entropy and the ground-state features is studied and we analyze the temperature-induced decay of the amplitude of the oscillations as well as the scaling of the purity. Furthermore, we show how the spin and charge velocities can be extracted from the temperature dependence of the Renyi entropy, bridging our findings to recent experimental proposals on how to implement the measurement of Renyi entropies in the cold atom system. Studying the Renyi mutual information, we also demonstrate how constraints such as particle number conservation can induce persistent correlations visible in the mutual information even at high temperature.

We report on the systematic characterization of photoluminescence (PL) lifetimes in NV${}^{-}$ and NV${}^0$ centers in 2-MeV H${}^{+}$-implanted type Ib diamond samples by means of a time-correlated single-photon counting (TCSPC) microscopy technique. A dipole-dipole resonant energy transfer model was applied to interpret the experimental results, allowing a quantitative correlation of the concentration of both native (single substitutional nitrogen atoms) and ion-induced (isolated vacancies) PL-quenching defects with the measured PL lifetimes. The TCSPC measurements were carried out in both frontal (i.e., laser beam probing the main sample surface along the same normal direction of the previously implanted ions) and lateral (i.e., laser beam probing the lateral sample surface orthogonally with respect to the same ion implantation direction) geometries. In particular, the latter geometry allowed a direct probing of the centers lifetime along the strongly nonuniform damage profiles of MeV ions in the crystal. The extrapolation of empirical quasiexponential decay parameters allowed the systematic estimation of the mean quantum efficiency of the centers as a function of intrinsic and ion-induced defect concentration, which is of direct relevance for the current studies on the use of diamond color centers for photonic applications.

We study the ground-state phase diagram of two-component fermions loaded in a ladderlike lattice at half filling in the presence of spin-orbit coupling. For repulsive fermions with unidirectional spin-orbit coupling along the legs we identify a Néel state which is separated from rung-singlet and ferromagnetic states by Ising phase transition lines. These lines cross for maximal spin-orbit coupling and a direct Gaussian phase transition between rung-singlet and ferro phases is realized. For the case of Rashba-like spin-orbit coupling, besides the rung-singlet phases two distinct striped ferromagnetic phases are formed. In the case of attractive fermions with spin-orbit coupling at half filling for decoupled chains we identify a dimerized state that separates a singlet superconductor and a ferromagnetic state.

We consider spin rotations in single- and two-electron quantum dots that are driven by an external ac electric field with two mechanisms that couple the electron spatial motion and the spin degree of freedom: the spin-orbit interaction and a random fluctuation of the Overhauser field due to nuclear spin bath. We perform a systematic numerical simulation of the driven system using a finite-difference approach with an exact account taken for the electron-electron correlation. The simulation demonstrates that the electron oscillation in fluctuating nuclear field is translated into an effective magnetic field during the electron wave packet motion. The effective magnetic field drives the spin transitions according to the electric-dipole spin resonance mechanism. We find distinct signatures of selection rules for direct and higher order transitions in terms of the spin-orbital symmetries of the wave functions. The selection rules are violated by the random fluctuation of the Overhauser field.