The Fe-Cu intersite charge transfer and Fe charge disproportionation are interesting phenomena observed in some $Ln$Cu${}_3$Fe${}_4$O${}_{12}$ ($Ln$ = lanthanides) compounds containing light and heavy $Ln$ atoms, respectively. We show that a change in the spin state is responsible for the intersite charge transfer in the light $Ln$ compounds. At the high-spin state, such systems prefer an unusual Cu $d^8$ configuration, whereas at the low-spin state they retreat to the normal Cu $d^9$ configuration through a charge transfer from Fe to the Cu $3d_{xy}$ orbital. We find that the strength of the crystal-field splitting and the relative energy ordering between Cu $3d_{xy}$ and Fe $3d$ states are the key parameters determining the intersite charge transfer (charge disproportionation) in light (heavy) $Ln$ compounds. It is further proposed that the size of $Ln$ affects the on-site interaction strength of Cu 3$d$ states, leading to a strong modification of the Cu $L_3$-edge spectrum, as observed by the x-ray-absorption spectroscopy.

We study transient dynamics of hole carriers injected into a Mott insulator with antiferromagnetic long-range order. This “dynamical hole doping” contrasts with chemical hole doping. The theoretical framework for the transient carrier dynamics is presented based on the two-dimensional $t$-$J$ model. The time dependencies of the optical conductivity spectra, as well as the one-particle excitation spectra, are calculated based on the Keldysh Green's function formalism at zero temperature combined with the self-consistent Born approximation. In the early stage after dynamical hole doping, the Drude component appears, and then incoherent components originating from hole-magnon scattering start to grow. Fast oscillatory behavior owing to coherent magnon and slow relaxation dynamics are confirmed in the spectra. The time profiles are interpreted as doped bare holes being dressed by magnon clouds and relaxed into spin polaron quasiparticle states. The characteristic relaxation times for Drude and incoherent peaks strongly depend on the momentum of the dynamically doped hole and the exchange constant. Implications for recent pump-probe experiments are discussed.

We present theoretical calculations for the spectral functions and single-particle densities of states of the two-dimensional electron gas in semiconductor quantum wells at different electron densities using the $GW$ plus cumulant method. We compare our results to $GW$ only calculations and find significant differences in the description of the satellites between the two theories: While $GW$ theory predicts the existence of a plasmaron excitation, no such excitation is found in $GW$ plus cumulant theory. We compare our results to experimental tunneling spectra from semiconductor quantum wells and find good agreement for the satellite properties.

The definition of all properties of the nonequilibrium interface depends on the choice of the position of the dividing surface. However, the definition of its position has been an unsolved problem for more than a century. A missing principle to unambiguously determine the position of the Gibbs' dividing surface is found: the principle of static equivalence. A sharp interface (dividing surface) is statically equivalent to a nonequilibrium finite-width interface with distributed tensile stresses if it possesses (a) the same resultant force, equal to the interface energy, and (b) the same moment, which is zero about the interface position. Each of these conditions determines the position of a sharp interface, which may be contradictory. This principle is applied to resolve another basic problem: the development of a phase field approach to an interface motion that includes an expression for interface stresses, which are thermodynamically consistent, and consistent with a sharp-interface limit. Using an analytical solution for a curved propagating interface, it is shown that both conditions determine the same dividing surface, i.e., the theory is self-consistent. The expression for the interface energy is also consistent with the expression for the velocity of the curved sharp interface. Applications to more complex interfaces that support elastic stresses are discussed.

We propose that nonequilibrium quantum criticality in open systems under the Born-Markov approximation can be described by a master equation of the Lindblad form. This master equation is derived from a system coupling weakly to a heat bath microscopically and is suggested to provide an approach to study dynamic quantum critical behavior of the system at finite temperatures. We find that the dissipation rate in the equation representing the coupling must be included in the scaling forms as an indispensable additional scaling variable in order to correctly describe the nonequilibrium quantum critical behavior, yet the equilibrium fixed point determines the nonequilibrium critical behavior in the weak coupling limit. Through numerically solving the Lindblad equation for the quantum Ising chain, we affirm these propositions by finite-time scaling forms with the dissipation rate. Nonequilibrium dynamic critical behavior of spontaneous emissions in dissipative open systems at zero temperature near their quantum critical points is discovered and is also described well by the scaling forms.

The stability of cubic $\mathrm{Ca}\mathrm{Si}{\mathrm{O}}_3$ perovskite (CaPv) at high temperatures and pressures is investigated by vibrational normal-mode analysis. We compute power spectra of mode autocorrelation functions using a recently developed hybrid approach combining ab initio molecular dynamics with lattice dynamics. These power spectra, together with the probability distributions of atomic displacements, indicate that cubic CaPv is stabilized at $T\sim 600$ K and $P\sim$ 26 GPa. We then utilize the concept of phonon quasiparticles to characterize the vibrational properties of cubic CaPv at high temperature and obtain anharmonic phonon dispersions through the whole Brillouin zone. Such temperature-dependent phonon dispersions pave the way for more accurate calculations of free-energy, thermodynamic, and thermoelastic properties of cubic CaPv at Earth's lower mantle conditions.

We report on the parametric excitation of magnetic precession modes in nanodisks using a parallel pumping configuration. The excitations are detected using a ferromagnetic resonance force microscopy method, and the parallel-pumped spectra reveal nonlinear characteristics including instability thresholds and multiple, narrow, sawtooth-shaped resonances. These characteristics are in accord with analytical theory and micromagnetic modeling results. Modeled mode profiles of the excitations show that higher-order standing spin-wave modes with both even and odd symmetries are excited under parallel pumping.

Electronic nematics, an electron orientational order which breaks the underlying rotational symmetry, were observed in iron pnictide superconductors several years after their discovery. However, the universality of the doping dependence of this phase and its relation to other symmetry-breaking orders (such as superconductivity) in distinct families of iron pnictides remain outstanding questions. Here we use torque magnetometry as a probe to study the rotational symmetry breaking in $\mathrm{Eu}{\mathrm{Fe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ without introducing external pressure. The nematic phase is found to proliferate well above the structural transition and to persist into the superconducting regime at optimal doping, after which it becomes absent or very weak, in sharp contrast to the behavior observed in $\mathrm{Ba}{\mathrm{Fe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$. These measurements suggest a putative quantum nematic transition near optimal doping under the superconducting dome.

The electronic structure of the ferromagnetic superconductor URhGe in the paramagnetic phase has been studied by angle-resolved photoelectron spectroscopy using soft x rays ($h\nu =595$–700 eV). Dispersive bands with large contributions from U 5$f$ states were observed in the ARPES spectra and form Fermi surfaces. The band structure in the paramagnetic phase is partly explained by the band-structure calculation treating all U 5$f$ electrons as being itinerant, suggesting that an itinerant description of U 5$f$ states is a good starting point for this compound. On the other hand, there are qualitative disagreements, especially in the band structure near the Fermi level ($E_{\mathrm{B}}\lesssim 0.5$ eV). The experimentally observed bands are less dispersive than the calculation, and the shape of the Fermi surface is different from the calculation. The changes in spectral functions due to the ferromagnetic transition were observed in bands near the Fermi level, suggesting that the ferromagnetism in this compound has an itinerant origin.

Ferromagnetism in Cr-based double perovskites is analyzed using an effective model as well as a simulation approach. Starting from a microscopic model proposed recently for this class of double perovskites, an effective spin-only model is derived in the limit of large exchange coupling at the $B$ site. Analytic expressions for the resultant exchange coupling are derived in certain limiting cases. The behavior of this exchange is used to provide a plausible explanation for the enhanced ferromagnetic tendency as well as the enigmatic increase in Curie temperature observed in Cr-based double perovskites in the series ${\mathrm{Sr}}_2\mathrm{Cr}{\mathrm{WO}}_6$, ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Re}{\mathrm{O}}_6$, and ${\mathrm{Sr}}_2\mathrm{Cr}\mathrm{Os}{\mathrm{O}}_6$. The superexchange between neighboring $B$ and $B^{\prime }$ sites is found to play a crucial role in both stabilizing ferromagnetism (especially in the latter two compounds) as well as increasing $T_c$.

We investigate the Kondo effect in nanometer-sized carbon nanotube quantum dots fabricated by electromigration. Here, we report on a pronounced dip within the Kondo peak observed at zero magnetic field. The zero-bias conductance exhibits a nonmonotonic temperature dependence with two characteristic energy scales. This behavior is ascribed to a two-stage Kondo effect occurring in an even charge state of the quantum dot. We demonstrate the universal behavior of the out-of-equilibrium conductance in this regime, and we extract the parameters of the universal scaling function.

A broad class of “local response” theories seeks to predict morphology evolution during energetic particle irradiation in terms of average surface height response to individual impacts—an approach that has been generalized by the crater function formalism of Norris et al. [J. Phys.: Condens. Matter 21, 224017 (2009); Nat. Commun. 2, 276 (2011)]. Keeping only the terms in the crater function formalism associated with the response of a flat surface has facilitated the use of molecular dynamics simulations of individual ion impacts to predict the stability or instability of a flat surface to ion bombardment. Here we report a sensitive experimental test of this truncated crater function theory that is independent of any a priori knowledge of the crater function itself. Existing measurements for 1 keV ${\mathrm{Ar}}^{+}/\mathrm{Si}$ and ${\mathrm{Kr}}^{+}/\mathrm{Ge}$ are inconsistent with the predictions of truncated crater function theory, for any conceivable crater function, at high bombardment angles. The failure of the theory suggests that the prediction of surface evolution from simulations of single-ion impacts will be more challenging than had been assumed.

We investigate the plasmonic near field of a lithographically defined split-ring resonator with angular-resolved electron energy loss spectroscopy in a scanning transmission electron microscope. By tilting the sample, different electric field components of the plasmonic modes can be probed with the electron beam. The electron energy loss spectra recorded under oblique incidence can feature plasmonic resonances that are not observable under normal incidence. Our experimental findings are supported by full numerical calculations based on the discontinuous Galerkin time-domain method.

Circular dichroism in optical second harmonic generation (CD-SHG) is studied in planar symmetrical arrays of G-shaped and mirror-G-shaped nanostructures. Anisotropic CD-SHG measurements demonstrate a strong dependence of the value and the sign of the CD effect on the angle of incidence of the fundamental radiation. We show that both dipole and higher order multipole components of the second order susceptibility are responsible for the CD response from G-shaped nanostructures.

We report on the sign inversion of the anomalous Hall effect (AHE) in EuO thin films along with photoirradiation as well as a temperature scan across ∼25 K that is well below the Curie temperature ($T_{\mathrm{C}}\sim 80$ K). The former gives an enhancement of the mobile electron density ($n$) by more than 30%, but the latter gives a negligible modification of $n$ of only 3% with a significant enhancement in mobility. It is found, in addition to the universal scaling law between longitudinal conductivity (${\sigma }_{xx}$) and anomalous Hall conductivity (${\sigma }_{\mathrm{AH}}$) as $|{\sigma }_{\mathrm{AH}}|\propto {\sigma }_{xx}^{1.6}$, that there is a critical value of about 10${}^2$ S cm${}^{-1}$ in ${\sigma }_{xx}$ that gives a boundary in the sign inversion of ${\sigma }_{\mathrm{AH}}$. If $n$ solely governs the sign of ${\sigma }_{\mathrm{AH}}$, the phenomenon could be explained by a Fermi level shift across the singularity in the band structure. However, our band calculation shows that, within any realistic adjustment of band parameters, the sign inversion of AHE never occurs. Thus, we conclude that other mechanisms of AHE are necessary to account for the AHE of EuO.

We consider the $\nu =0$ quantum Hall ferromagnetic state of bilayer graphene subject to a kinklike perpendicular electric field, which generates domain walls in the electronic state and low-energy collective modes confined to move along them. In particular, it is shown that two pairs of collective helical modes are formed at opposite sides of the kink, each pair consisting of modes with identical helicities. We derive an effective field-theoretical model of these modes in terms of two weakly coupled anisotropic quantum spin ladders, with parameters tunable through control of the electric and magnetic fields. This yields a rich phase diagram, where, due to the helical nature of the modes, distinct phases possess very different charge conduction properties. Most notably, this system can potentially exhibit a transition from a superfluid to an insulating phase.

We report room-temperature pulsed electrically detected magnetic resonance measurements of the dark conductivity of films of the fullerene derivative [6,6]-phenyl-${\mathrm{C}}_{61}$-butyric acid methyl ester (PCBM) under bipolar (electron-hole) and unipolar (electron-rich) injection conditions. Directly after material deposition, no detectable spin-dependent processes are observed, yet after storage under ambient conditions for more than a day, two distinct spin-dependent mechanisms are found under bipolar injection, suggesting the involvement of degradation-induced electronic states. Spin-Rabi beat oscillation measurements show that at least one of these processes is due to weakly spin-coupled pairs with $s=1/2$. The absence of these signals when hole injection is impeded by a barrier suggests that they are due to spin-dependent recombination. The presence of recombination confirms that fullerenes are both electron and hole acceptors, with important consequences for the design, operation, and understanding of plastic solar cells. Electron-hole recombination can occur within homogeneous domains of either the donor or the acceptor of the bulk heterojunction structure, constituting an important dissipative channel in addition to the established interfacial bimolecular recombination loss.

We revisit the theoretical description of the $F$ color center in lithium fluoride employing advanced complementary ab initio techniques. We compare the results from periodic supercell calculations involving density-functional theory (DFT) and post-DFT techniques with those from the embedded-cluster approach involving quantum-chemical many-electron wave-function techniques. These alternative approaches yield results in good agreement with each other and with the experimental data provided that correlation effects are properly taken into account.

Using the functional renormalization group we investigated possible superconductivity in doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. In the electron-doped case, a $d_{x^2-y^2}^{*}$-wave superconducting phase is found in a narrow doping region. The pairing is driven by spin fluctuations within the single conduction band. In contrast, for hole doping an $s_{\pm }^{*}$-wave phase is established, triggered by spin fluctuations within and across the two conduction bands. In all cases there are comparable singlet and triplet components in the pairing function. The Hund's rule coupling reduces (enhances) superconductivity for electron (hole) doping. Our results imply that hole doping is more promising to achieve a higher transition temperature. Experimental perspectives are discussed.

Motivated by recent development in time-resolved angle-resolved photoemission spectroscopy (trARPES) for $d$-wave superconductors, we analyze the nonequilibrium relaxation dynamics of the laser pulse excited sample within the scenario of a two-temperature model. The main features reported in the trARPES technique may be understood within this phenomenological picture. The momentum dependencies of the excited quasiparticle density and the relaxation rate are associated with the dynamics of the nodal $d$-wave superconducting gap, and the fluence dependence of the relaxation rate is related to the recombination process of quasiparticles into Cooper pairs.