We predict that a metal nanofilm subjected to an ultrashort (near-single oscillation) optical pulse of a high field amplitude $\gtrsim$$3\mathrm{V}/\AA$ at normal incidence undergoes an ultrafast (at subcycle times $\lesssim$$1\mathrm{fs}$) transition to a state resembling semimetal. Its reflectivity is greatly reduced, while its transmissivity and the optical field inside the metal are greatly increased. Despite the metal being a centrosymmetric medium, the strong pulse causes net charge transfer in the direction determined by the carrier envelope phase (CEP) of the pulse, which is opposite to the direction of the maximum field.

An efficient procedure is introduced for the calculation of the optical response of individual and coupled metallic nanoparticles in the framework of the discrete-dipole approximation (DDA). We introduce a modal expansion in the basis set of discrete dipoles and show that a few suitably selected modes are sufficient to compute optical spectra with reasonable accuracy, thus reducing the required numerical effort relative to other DDA approaches. Our method offers a natural framework for the study of localized plasmon modes, including plasmon hybridization. As a proof of concept, we investigate optical extinction and electron energy-loss spectra of monomers, dimers, and quadrumers formed by flat silver squares. This method should find application to the previously prohibited simulation of complex particle arrays.

Various types of defects in MoS${}_2$ monolayers and their influence on the electronic structure and transport properties have been studied using the density-functional-based tight-binding method in conjunction with the Green's function approach. Intrinsic defects in MoS${}_2$ monolayers significantly affect their electronic properties. Even at low concentration they considerably alter the quantum conductance. While the electron transport is practically isotropic in pristine MoS${}_2$, strong anisotropy is observed in the presence of defects. Localized midgap states are observed in semiconducting MoS${}_2$ that do not contribute to the conductivity but direction-dependent scatter the current, and that the conductivity is strongly reduced across line defects and selected grain boundary models.

We study the dynamics of an electron spin in a graphene quantum dot, which is interacting with a bath of less than ten nuclear spins via the anisotropic hyperfine interaction. Due to substantial progress in the fabrication of graphene quantum dots, the consideration of such a small number of nuclear spins is experimentally relevant. This choice allows us to use exact diagonalization to calculate the long-time average of the electron spin as well as its decoherence time. We investigate the dependence of spin observables on the initial states of nuclear spins and on the position of nuclear spins in the quantum dot. Moreover, we analyze the effects of the anisotropy of the hyperfine interaction for different orientations of the spin quantization axis with respect to the graphene plane. Interestingly, we then predict remarkable long decoherence times of more than 10 ms in the limit of few nuclear spins.

We have performed ab initio density functional theory calculations to study freestanding alloy monolayers of Fe and Au in centered rectangular and hexagonal geometries, as well as Fe-Au surface alloys on Ru(0001) and Mo(110) substrates. Though Fe and Au are bulk immiscible, in all four classes of systems we obtain negative formation energies. While the properties of the two classes of freestanding monolayers are roughly similar, with small differences due to the anisotropy and longer bond lengths of the centered rectangular case, the surface alloys on the two substrates behave quite differently. The formation energies on Mo(110) are markedly smaller; we trace this to the fact that magnetism contributes significantly to mixing on Ru(0001) but not on Mo(110). On Ru(0001), there is a very stable $(\sqrt{3}\times \sqrt{3})$ FeAu${}_2$ phase, in agreement with experiments. By performing cluster expansion calculations, we show that ordering is not favored on Mo(110), again in accordance with experimental data.

In this paper we show that solid nitrogen has considerably enhanced elastic properties in the nanopores of a porous glass. Particularly, the first adsorbed molecular layer near the pore wall possesses a shear modulus more than twice as high as the bulk modulus. The interaction between pore surface and pore filling provides a mean for the enhancement of elastic properties. Our discovery shows that nanoconfinement in porous samples can be used to influence the mechanical properties of substances.

We introduce a methodology for constructing anharmonic vibrational Hamiltonians that are parametrized from first-principles electronic-structure calculations and can be used to study high-temperature properties of crystalline materials. Our method provides an accurate description of the Born-Oppenheimer potential energy surface of a crystal that can be systematically refined and is invariant to space-group symmetries of the ideal reference crystal and finite rigid-body rotations and translations. These features make it ideally suited for Monte Carlo or molecular dynamics simulations to predict finite-temperature thermodynamic properties, structural phase transitions, and thermal conductivity. We use this method to construct an anharmonic Hamiltonian for ZrH${}_2$, which exhibits a high-temperature cubic phase that undergoes a symmetry-breaking second-order transition to one of three equivalent tetragonal phases upon cooling. Although density functional theory predicts a zero-Kelvin dynamical instability of cubic ZrH${}_2$, we find via Monte Carlo simulation that the cubic phase can be anharmonically stabilized at high temperature and predict a cubic-to-tetragonal transition temperature that is in good agreement with extrapolation from experiments. We also calculate finite-temperature free energies for the cubic and tetragonal phases, finding that they are consistent with the phenomenological Landau theory of second-order phase transitions.

We present a quantum Monte Carlo study for Heisenberg spin-$\frac{1}{2}$ two-leg ladder systems doped with nonmagnetic impurities. The simulations are applied to the doped spin-ladder compound Sr(Cu${}_{1-x}$Zn${}_x$)${}_2$O${}_3$, where a large broadening of the ${}^{65}$Cu NMR lines has been observed in experiment at low temperatures but far above the Néel temperature. We find that interladder couplings with a sizable coupling in the stacking direction are required to describe the line broadening, which cannot be explained by considering a single ladder only. Around a single impurity, spin correlations cause an exponentially decaying antiferromagnetic local magnetization in a magnetic field. We develop an effective model for the local magnetization of systems with many randomly distributed impurities, with few parameters which can be extracted out of quantum Monte Carlo calculations with a single impurity. The broadening arises from a drag effect, where the magnetization around an impurity works as an effective field for spins on the neighboring ladders, causing a nonexponentially decaying magnetization cloud around the impurity. Our results show that even for impurity concentrations as small as $x=0.001$ and $x=0.0025$, the broadening effect is large, in good quantitative agreement with experiment. We also develop a simple model for the effective interaction of two impurity spins.

We report the results from magnetic, resistivity, and Hall effect measurements on a ferromagnetically ordered 5% Gd low energy implanted ZnO single crystal. Temperature-dependent magnetization measurements show that the Gd ions do not contribute to the magnetic order; hence, the magnetic order is intrinsic. The electronic transport in the Gd-implanted region is inhomogeneous, and there is a nonlinear Hall resistance. The nonlinear Hall resistance is likely to be a consequence of the inhomogeneous transport and not due to an anomalous Hall effect.

Soft x-ray resonant magnetic scattering is an element-sensitive technique that enables the characterization of the magnetic properties of a wide variety of systems. Here we apply this technique to study lithographically produced artificial spin ice, a particular class of magnetically frustrated systems comprising arrays of nanomagnets. Using a CCD detector we can access a large fraction of the reciprocal space at once, allowing us to easily distinguish the signatures of the magnetic ground-state ordering. Comparing the dichroic signal at the position of the Bragg peaks with model calculations based on the kinematical theory of x-ray diffraction, we are able to determine the number of reversed moments as a function of applied magnetic field for each of the two sublattices. This study demonstrates the benefit of having direct access to a significant fraction of the reciprocal space, and opens the way towards more sophisticated x-ray based experiments on artificial spin ice such as scattering of coherent x-ray beams to explore the dynamics of thermally activated systems.

The paper derives the transverse forces (the Magnus and the Lorentz forces) in the lattice models of superfluids in the continuous approximation. The continuous approximation restores translational invariance absent in the original lattice model, but the theory is not Galilean invariant. As a result, calculation of the two transverse forces on the vortex, Magnus force and Lorentz force, requires the analysis of two balances, for the true momentum of particles in the lattice (Magnus force) and for the quasimomentum (Lorentz force) known from the Bloch theory of particles in the periodic potential. While the developed theory yields the same Lorentz force, which was well known before, a new general expression for the Magnus force was obtained. The theory demonstrates how a small Magnus force emerges in the Josephson-junction array if the particle-hole symmetry is broken. The continuous approximation for the Bose-Hubbard model close to the superfluid-insulator transition was developed, which was used for calculation of the Magnus force. The theory shows that there is an area in the phase diagram for the Bose-Hubbard model, where the Magnus force has an inverse sign with respect to that which is expected from the sign of velocity circulation.

Topological superconductors are gapped superconductors with protected Majorana surface/edge states on the boundary. In this paper, we study the Josephson coupling between time-reversal-invariant topological superconductors and $s$-wave superconductors. The Majorana edge/surface states of time-reversal-invariant topological superconductors in all physical dimensions 1, 2, and 3 have a generic topological property which we name the time-reversal anomaly. Due to the time-reversal anomaly, the Josephson coupling prefers a nonzero phase difference between topological and trivial superconductors. The nontrivial Josephson coupling leads to a current-flux relation with a half period in a superconducting quantum interference device geometry, and also a half-period Fraunhofer effect in dimensions higher than 1. We also show that an in-plane magnetic field restores the ordinary Josephson coupling, as a sharp signature that the proposed effect is a consequence of the unique time-reversal property of the topological edge/surface states. Our proposal provides a simple and general approach to experimentally verify whether a time-reversal-invariant superconductor is topological.

We present an experimental method to perform dual-channel lock-in magnetometry of time-dependent magnetic fields using a single spin associated with a nitrogen-vacancy color center in diamond. We incorporate multipulse quantum sensing sequences with phase estimation algorithms to achieve linearized field readout and constant, nearly decoherence-limited sensitivity over a wide dynamic range. Furthermore, we demonstrate unambiguous reconstruction of the amplitude and phase of the magnetic field. We show that our technique can be applied to measure random phase jumps in the magnetic field, as well as phase-sensitive readout of the frequency.

We study impurity bound states and impurity-induced order in the superconducting state of LiFeAs within a realistic five-band model based on the band structure and impurity potentials obtained from density functional theory. In agreement with recent experiments, we find that Co impurities are too weak to produce subgap bound states, whereas stronger impurities like Cu do. We also obtain the bound-state spectrum for magnetic impurities, such as Mn, and show how spin-resolved tunneling may determine the nature of the various defect sites in iron pnictides, a prerequisite for using impurity bound states as a probe of the ground-state pairing symmetry. Lastly, we show how impurities pin both orbital and magnetic order, providing an explanation for a growing set of experimental evidence for unusual magnetic phases in doped iron pnictides.

We have studied the intrachain magnetic interactions in chromium phthalocyanine (CrPc, spin 2) for a wide range of chain structural parameters using hybrid exchange density functional theory. A comparison of the computed interchain and intrachain exchange interactions and zero-field splitting shows that many of the chain structures including those in the $\alpha$ and $\eta$ phases are suitable one-dimensional antiferromagnetic chains of integer spins for testing Haldane's conjecture. In addition, the exchange interaction in CrPc is strongly dependent on the stacking angle and weakly on the sliding angle. The peak value of the exchange interactions is 120 K when the molecules are face on, which is much larger than those in copper phthalocynines. The prediction of the relationship between the computed magnetic properties and geometrical parameters can provide guidance to experimentalists for the applications of CrPc structures in spintronics.

We study the random field $xy$ spin model at $T=0$ numerically on lattices of up to $1000\times 1000\times 1000$ spins with emphasis on the weak random field. Our numerical method is physically equivalent to slow cooling in which the system is gradually losing energy and relaxing to an energy minimum. The system shows glass properties, the resulting spin states depending strongly on the initial conditions. Random initial condition for the spins leads to a vortex glass (VG) state with short-range spin-spin correlations defined by the average distance between vortex lines. Collinear and some other vortex-free initial conditions result in vortex-free ferromagnetic (F) states that have a lower energy. The energy difference between the F and VG states correlates with the vorticity of the VG state. The correlation functions in the F states agree with the Larkin-Imry-Ma theory at short distances. The hysteresis curves for a weak random field are dominated by topologically stable spin walls ruptured by vortex loops. We find no relaxation paths from the F, VG, or any other states to the hypothetical vortex-free state with zero magnetization.

We discuss spin-$S$ antiferromagnetic Heisenberg chains with three-spin interactions, next-nearest-neighbor interactions, and bond alternation. First, we prove rigorously that there exist parameter regions of the exact dimerized ground state in this system. This is a generalization of the Majumdar-Ghosh model to arbitrary $S$. Next, we discuss the ground-state phase diagram of the models by introducing several effective field theories and the universality classes of the transitions are described by the level-$2S$ $\mathrm{SU}\left(2\right)$ Wess-Zumino-Witten model and the Gaussian model. Finally, we determine the phase diagrams of $S=1$ and $S=3/2$ systems by using exact diagonalization and level spectroscopy.

We address the role of lateral confinement in determining the micromagnetic structure of small objects featuring perpendicular magnetic anisotropy. We have imaged the spatial distribution of magnetic domains in Co/Pd multilayered objects by x-ray holography in transmission mode, a technique based on resonant coherent scattering of polarized x rays. In addition to high spatial resolution and magnetic sensitivity, x-ray holography features chemical selectivity and probes the entire bulk of the magnetic sample. By analyzing and comparing images of several magnetic objects (squares and rectangles with 0.4–3-μm lateral sizes) that underwent the same preparation procedure and magnetic history, we highlight the influence of the object shape and of its orientation with respect to an external field in determining the remanent magnetic microstructure. The discussion of our results is backed by micromagnetic calculations.

Resistivity and Hall effect measurements of EuFe${}_2$As${}_2$ up to 3.2 GPa indicate no divergence of quasiparticle effective mass at the pressure $P_{\mathrm{c}}$ where the magnetic and structural transition disappears. This is corroborated by analysis of the temperature ($T$) dependence of the upper critical field. $T$-linear resistivity is observed at pressures slightly above $P_{\mathrm{c}}$. The scattering rates for both electrons and holes are shown to be approximately $T$-linear. When a field is applied, a $T^2$ dependence is recovered, indicating that the origin of the $T$-linear dependence is spin fluctuations.