We apply grazing-incidence femtosecond x-ray diffraction to investigate the details of the atomic motion connected with a displacively excited coherent optical phonon. We concentrate on the low frequency phonon associated with the charge and orbital order in the mixed valence manganite La${}_{0.25}$Pr${}_{0.375}$Ca${}_{0.375}$MnO${}_3$ for $T\lesssim 210$ K. We measure the response of three superlattice reflections that feature different sensitivities to the motion of the unit cell constituents. The results support the assignment to a translational mode of the Mn${}^{4+}$ atoms together with the oxygen atoms connecting adjacent Mn${}^{4+}$ sites.

Theoretical research has predicted that a ripple of graphene generates an effective gauge field on its low-energy electronic structure and could lead to Landau quantization. Here, we demonstrate using a combination of an experimental method (scanning tunneling microscopy) and a theoretical approach (tight-binding approximation) that Landau levels will form when the effective pseudomagnetic flux per ripple $\Phi \sim (h^2/la){\Phi }_0$ is larger than the flux quantum ${\Phi }_0$ (here, $h$ is the height, $l$ is the width of the ripple, and $a$ is the nearest C-C bond length). The strain-induced gauge field in the ripple only results in one-dimensional (1D) Landau-level quantization along the ripple. Such 1D Landau quantization does not exist in two-dimensional systems in an external magnetic field. Its existence offers a unique opportunity to realize interesting electronic properties in strained graphene.

We study the dynamics of a quantum-coherent thermally isolated Luttinger liquid with noisy Luttinger parameter. To characterize the fluctuations of the absorbed energy in generic noise-driven systems, we first identify two types of energy moments, which can help tease apart the effects of classical (sample-to-sample) and quantum sources of fluctuations. One type of moment captures the total fluctuations due to both sources, while the other one captures the effect of the classical source only. We then demonstrate that, in the Luttinger liquid case, the two types of moments agree in the thermodynamic limit, indicating that the classical source dominates. In contrast to equilibrium thermodynamics, in this driven system the relative fluctuations of energy do not decay with the system size. Additionally, we study the deviations of equal-time correlation functions from their ground-state value, and find a simple scaling behavior.

Based on a linearized Landau-Lifshitz equation, we show that two-dimensional periodic allay of ferromagnetic particles coupled with magnetic dipole-dipole interactions supports chiral spin-wave edge modes, when subjected under the magnetic field applied perpendicular to the plane. The mode propagates along a one-dimensional boundary of the system in a unidirectional way and it always has a chiral dispersion within a band gap for spin-wave volume modes. Contrary to the well-known Damon-Eshbach surface mode, the sense of the rotation depends not only on the direction of the field but also on the strength of the field; its chiral direction is generally determined by the sum of the so-called Chern integers defined for spin-wave volume modes below the band gap. Using simple tight-binding descriptions, we explain how the magnetic dipolar interaction endows spin-wave volume modes with nonzero Chern integers and how their values will be changed by the field.

The CoFeB/MgO interface is important not only because of its ability to act as a good spin filter, but also because it exhibits significant magnetic interface anisotropy. However, there is experimental evidence that the mineral kotoite (Mg${}_3$B${}_2$O${}_6$) forms at this interface, possibly as a result of B diffusion into the oxide. Recent calculations have demonstrated that kotoite is a surprisingly good spin filter material, despite the lack of cubic symmetry. It is not a priori clear how the perpendicular anisotropy is affected if kotoite forms at the interface. Hereby, we have performed first-principles density functional calculations to investigate the magnetic anisotropy of CoFe/kotoite interfaces. We find that the Co/Fe atoms bind strongly to O atoms of kotoite, and the CoFe/kotoite interfaces exhibit strong perpendicular magnetic anisotropy (PMA) comparable to that of CoFe/MgO interfaces. Analysis of the electronic structure shows that CoFe states having $d_{xz}$ and $d_{yz}$ character are strongly affected by the magnetization direction and play an important role in determining the magnetic anisotropy of both CoFe/kotoite and CoFe/MgO interfaces. Our observations are consistent with previous experimental observations of PMA. We demonstrate that the interface properties are only weakly affected by the formation of kotoite compared to MgO.

The change in the electronic structure of layered Cu${}_x$IrTe${}_2$ has been characterized by transport and spectroscopic measurements, combined with first-principles calculations. The Cu intercalation suppresses the monoclinic distortion, giving rise to the stabilization of the trigonal phase with superconductivity. Thermopower and Hall resistivity measurements suggest the multiband nature with hole and electron carriers for this system, which is masked by the predominance of the hole carriers enhanced by the interlayer hybridization in the trigonal phase. Rather than the instability of the Ir $d$ band, a subtle balance between the interlayer and intralayer Te-Te hybridizations is proposed as a main factor dominating the structural transition and the superconductivity.

Currently used methods for the description of thermodynamics of ferroelectric thin films (Landau theory or ab initio based Monte Carlo simulations) are based on an energy expansion in terms of internal degrees of freedom. It was shown that these methods can suffer from a substantial inaccuracy unless higher order electromechanical interactions are taken into account. The high-order electromechanical couplings strongly renormalize the sixth-power polarization terms of the thermodynamic energy expansion. In this paper, apart from the general statement, we illustrate it with an example of a temperature–misfit strain phase diagram of a ${\text{BaTiO}}_3$ thin film derived with high-order electromechanical interactions evaluated using first-principles calculations.

We analyze the effects of disorder and quasidisorder on the ground-state properties of ultracold polar bosons in optical lattices. We show that the interplay between disorder and intersite interactions leads to rich phase diagrams. A uniform disorder leads to a Haldane-insulator phase with finite parity order, whereas the density-wave phase becomes a Bose glass at very weak disorder. For quasidisorder, the Haldane insulator connects with a gapped generalized incommensurate density wave without an intermediate critical region.

Ferromagnetism in perovskites RTiO${}_3$ can be induced by a steric effect. The way in which the subtle local structural change can induce three-dimensional (3D) ferromagnetic coupling through Ti-O-Ti superexchange interactions remains controversial. A critical behavior study for the ferromagnetic phase has been made so far only on YTiO${}_3$ because the magnetization measurements are plagued by the contribution from the magnetic rare earth. Here we report critical exponents for most ferromagnetic members in the RTiO${}_3$ family by measuring the magnetocaloric effect and applying the corresponding scaling laws. Our results indicate that the ferromagnetic coupling in the RTiO${}_3$ can be well described by the 3D Heisenberg model.

We report density-functional-theory calculations of the electronic band structures and optical absorption spectra of two-dimensional crystals of Ga${}_2{X}_2$ ($X=$ S, Se, and Te). Our calculations show that all three two-dimensional materials are dynamically stable indirect-band-gap semiconductors with a sombrero dispersion of holes near the top of the valence band. We predict the existence of Lifshitz transitions—changes in the Fermi-surface topology of hole-doped Ga${}_2{X}_2$—at hole concentrations $n_S=7.96\times {10}^{13}$ cm${}^{-2}$, $n_{Se}=6.13\times {10}^{13}$ cm${}^{-2}$, and $n_{Te}=3.54\times {10}^{13}$ cm${}^{-2}$.

We investigate the structural and electrical properties of perovskite oxide superlattices composed of a ferromagnetic metal, EuMoO${}_3$, and a paramagnetic insulator, SrTiO${}_3$, grown on GdScO${}_3$ (110) substrates by pulsed laser deposition. The results of x-ray diffraction show that the superlattices are formed as designed, while considerable interface roughening is recognized by high-resolution scanning transmission electron microscopy. In the transport properties, the temperature dependence of resistivity changes from metallic to insulating behaviors on reducing the thickness of EuMoO${}_3$. An anomalous Hall effect emerges in metallic superlattices, and its amplitude clearly corresponds to conductivity. Negative magnetoresistance is observed in all superlattices and is more pronounced in the insulating superlattices. These results indicate the existence of spin-canted and low-conductivity dead layers near the heterointerfaces due to the suppressed exchange coupling between electrons and local Eu 4$f^7$ spins.

Lateral hopping and desorption of a single CO molecule on a Cu(110) surface [Bartels et al., Science 305, 648 (2004)] induced by femtosecond laser pulses are studied using an indirect heat-transfer model. In addition to a direct heating of the reaction coordinate (RC) mode [frustrated translation (FT) mode for hopping and center-of-mass (CM) mode for desorption] by laser-generated hot electrons in the substrate, we consider an indirect heating of the RC mode through intermode coupling between the frustrated rotation (FR) mode and the RC mode. We calculate the transient behavior of the effective temperature of the FT and the CM modes, and of the normalized reaction yield. The experimental result of a ratio of the hopping yield along and across a row on a Cu(110) surface is nicely calculated. Although no information is available for the attempt frequency in a form of the Arrhenius equation for thermally activated reactions, it is predicted under which condition the desorption rate becomes in the same order of magnitude as the hopping rate, although the barrier height for desorption is much higher than for hopping. The present analysis highlights the role of excitation of the FR mode in reactions of a CO molecule as has been confirmed in the real-time observation [Backus et al., Science 310, 1790 (2005)].

We theoretically study the optical absorption property of twisted bilayer graphenes with various stacking geometries and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers. We find that the absorption spectrum almost continuously evolves in changing the rotation angle, regardless of the lattice commensurability. The spectrum is characterized by series of peaks associated with the van Hove singularity, and the peak energies systematically shift with the rotation angle. We calculate the optical absorption in two frameworks: the tight-binding model and the effective continuum model based on the Dirac equation. For small rotation angles, less than ${10}^{\circ }$, the effective model well reproduces the low-energy band structure and the optical conductivity of the tight-binding model and, also, explains the optical selection rule analytically in terms of the symmetry of the effective Hamiltonian.

We have studied in detail the effect of the spin anisotropy on the electron paramagnetic resonance spectra and magnetic properties of Cu($\mathit{en}$)(H${}_2$O)${}_2$SO${}_4$, an $S$ $=$ 1/2 spatially anisotropic triangular lattice antiferromagnet. The angular and temperature dependence of the resonance fields as well as the magnetization and magnetic susceptibility reflect the exchange and $g$-factor anisotropy with $J_z/J_{x,y}<1$ and $g_z/g_{x,y}>1$, respectively. The exchange anisotropy and Dzyaloshinskii-Moriya interaction are responsible for the main broadening mechanism at higher temperatures while spin-diffusion effects prevail at helium temperatures. The ratio of the uniform susceptibilities calculated along the three crystal directions suggests an easy-axis anisotropy with the $a$ axis as the magnetic easy axis. Its impact on the physical properties of the title compound is discussed.

We develop a theory of parametric resonance in tunable superconducting cavities. The nonlinearity introduced by the superconducting quantum interference device (SQUID) attached to the cavity and damping due to connection of the cavity to a transmission line are taken into consideration. We study in detail the nonlinear classical dynamics of the cavity field below and above the parametric threshold for the degenerate parametric resonance, featuring regimes of multistability and parametric radiation. We investigate the phase-sensitive amplification of external signals on resonance, as well as amplification of detuned signals, and relate the amplifier performance to that of linear parametric amplifiers. We also discuss applications of the device for dispersive qubit readout. Beyond the classical response of the cavity, we investigate small quantum fluctuations around the amplified classical signals. We evaluate the noise power spectrum both for the internal field in the cavity and the output field. Other quantum-statistical properties of the noise are addressed such as squeezing spectra, second-order coherence, and two-mode entanglement.

We have fabricated Rb-intercalated bilayer graphene on 6H-SiC(0001) and performed low-energy electron diffraction (LEED) and high-resolution angle-resolved photoemission spectroscopy (ARPES). In LEED experiments, we observed a 2 $\times$ 2 pattern identical to that of the bulk graphite intercalation compound C${}_8$Rb. The existence of a 2 $\times$ 2 Rb layer is also confirmed by ARPES, where band folding due to the periodic potential of intercalated atoms was observed. All these results indicate that Rb atoms are intercalated between the two graphene monolayers in a regular manner similar to that of bulk C${}_8$Rb. Further, we observed a parabolic metallic band at the Brillouin-zone center, analogous to the so-called interlayer band, which is thought to play an important role for the superconductivity in graphite intercalation compounds.

When the external magnetic field applied to a ferromagnetically coupled atomic chain is reversed suddenly, the magnetization of the chain switches, due to the reversal of all the atomic magnetic moments in the chain. The quantum processes underlying the magnetization switching and the time required for the switching are analyzed for model magnetic chains adsorbed on a surface at 0 K. The sudden field reversal brings the chain into an excited state that relaxes towards the system ground state via interactions with the substrate electrons. Different mechanisms are outlined, ranging from the global stepwise rotation of the chain macrospin induced by spin-flip collisions with substrate electrons in the pure Heisenberg chain (Néel-Brown process) to a correlation-mediated direct switching process in the presence of strong magnetic anisotropies in short chains (the global spin of the chain reverses in a single electron interaction). The processes for magnetization switching induced by electrons tunneling from a scanning tunneling microscope tip are also analyzed.

We employ quantum Monte Carlo techniques to calculate the $Z_2$ topological invariant in a two-dimensional model of interacting electrons that exhibits a quantum spin Hall topological insulator phase. In particular, we consider the parity invariant for inversion-symmetric systems, which can be obtained from the bulk's imaginary-time Green's function after an appropriate continuation to zero frequency. This topological invariant is used here in order to study the trivial-band to topological-insulator transitions in an interacting system with spin-orbit coupling and an explicit bond dimerization. We discuss the accessibility and behavior of this topological invariant within quantum Monte Carlo simulations.

We study selected transition-metal-related point defects in silicon and silicon carbide semiconductors by a range-separated hybrid density functional (HSE06). We find that HSE06 does not fulfill the generalized Koopmans' theorem for every defect, which is due to the self-interaction error in the functional in such cases. Restoring the so-called generalized Koopmans' condition with a simple correction in the functional can eliminate this error and brings the calculated charge transition levels remarkably close to the experimental data as well as to the calculated quasiparticle levels from many-body perturbation theory.

Based on the nonequilibrium Green's function (NEGF) coupled with density function theory (DFT), namely, NEGF-DFT quantum transport theory, we propose an efficient formalism to calculate the transient current of molecular devices under a step-like pulse from first principles. By combining NEGF-DFT with the complex absorbing potential (CAP), the computational complexity of our formalism (NEGF-DFT-CAP) is proportional to $O\left(N\right)$ where $N$ is the number of time steps in the time-dependent transient current calculation. Compared with the state-of-the-art algorithm of first-principles time-dependent calculation that scales with at least $N^2$, this order $N$ technique drastically reduces the computational burden making it possible to tackle realistic molecular devices. We have presented a detailed discussion on how to implement this scheme numerically from first principles. To check the accuracy of our method, we carry out the benchmark calculation compared with NEGF-DFT formalism and they agree well with each other. As an application of this method, we investigate the transient current of a molecular device Al–C${}_3$–Al from first principles.