We image simultaneously the geometric, the electronic, and the magnetic structures of a buckled iron bilayer film that exhibits chiral magnetic order. We achieve this by combining spin-polarized scanning tunneling microscopy and magnetic exchange force microscopy (SPEX) to independently characterize the geometric as well as the electronic and magnetic structures of nonflat surfaces. This new SPEX imaging technique reveals the geometric height corrugation of the reconstruction lines resulting from strong strain relaxation in the bilayer, enabling the decomposition of the real-space from the electronic structure at the atomic level and the correlation with the resultant spin-spiral ground state. By additionally utilizing adatom manipulation, we reveal the chiral magnetic ground state of portions of the unit cell that were not previously imaged with spin-polarized scanning tunneling microscopy alone. Using density functional theory, we investigate the structural and electronic properties of the reconstructed bilayer and identify the favorable stoichiometry regime in agreement with our experimental result.

A possible explanation for the long-range proximity effect observed in single-crystalline cobalt nanowires sandwiched between two tungsten superconducting electrodes [Nat. Phys. 6, 389 (2010)] is proposed. The theoretical model uses properties of a ferromagnet band structure. Specifically, to connect the exchange field with the momentum of quasiparticles the distinction between the effective masses in majority and minority spin subbands and the Fermi-surface anisotropy are considered. The derived Eilenberger-like equations allowed us to obtain a renormalized exchange interaction that is completely compensated for some crystallographic directions under certain conditions. The proposed theoretical model is compared with previous approaches.

Disordered thin films close to the superconductor-insulator phase transition (SIT) hold the key to understanding quantum phase transition in strongly correlated materials. The SIT is governed by superconducting quantum fluctuations, which can be revealed, for example, by tunneling measurements. These experiments detect a spectral gap, accompanied by suppressed coherence peaks, on both sides of the transition. Here we describe the insulating side in terms of a fluctuating superconducting field with finite-range correlations. We perform a controlled diagrammatic resummation and derive analytic expressions for the tunneling differential conductance. We find that short-range superconducting fluctuations suppress the coherence peaks even in the presence of long-range correlations. Our approach offers a quantitative description of existing measurements on disordered thin films and accounts for tunneling spectra with suppressed coherence peaks.

A short quasimonochromatic wave packet incident on a semi-infinite disordered medium gives rise to a reflected wave. The intensity of the latter decays as a power law, $1/t^{\alpha }$, in the long-time limit. Using the one-dimensional Aubry-André model, we show that in the vicinity of the critical point of Anderson localization transition, the decay slows down, and the power-law exponent $\alpha$ becomes smaller than both $\alpha =2$ found in the Anderson localization regime and $\alpha =3/2$ expected for a one-dimensional random walk of classical particles.

The spin-state-dominated frictional force and stick-slip behaviors are shown through first-principles calculations using ${\mathrm{Mn}}_2\mathrm{C}$, a two-dimensional antiferromagnetic crystal, as a prototype. While the frictional force in the spin-unpolarized state is isotropic and independent with forward and backward moving directions, the antiferromagnetic ordering not only reduces the rotational symmetries of potential energy surfaces, but also dramatically changes the shapes of energy landscapes on the energetically preferred sliding paths, leading to anisotropic and even direction-dependent frictional force. Besides, a novel stick-slip behavior with the slips occurring across a fractional number of lattice sites is observed and the transition conditions of the stick-slip behavior can be predicted by the Tomlinson model.

We study the extended Hubbard model away from half filling on a two-dimensional square lattice using cluster dynamical mean field theory on clusters of size 8. We show that the model exhibits metallic, compressible charge-ordered, and insulating charge-ordered phases. We determine the location of the charge-ordering phase transition line at finite temperature and the properties of the phases as a function of doping, temperature, local interaction, and nearest-neighbor interaction. An analysis of the energetics of the charge-order transition shows that the charge-ordering transition mainly results in a rearrangement of local and nonlocal potential energy. We show the doping evolution of the spectral function from the isotropic metal via a charge-ordered metal to a charge-ordered insulator with a big gap, and study finite-size effects of the approximation.

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we have mapped out the reconstructed electronic structure in the commensurate charge-density-wave (CDW) state of quasi-two-dimensional transition metal dichalcogenide $2H-{\mathrm{TaSe}}_2$. The observation of the fine structure near Brillouin zone (BZ) center supplements the picture of Fermi surface folding in the $3\times 3$ CDW state. In addition to the anisotropic CDW band gaps that energetically stabilize the system at the Fermi level in the first-order lock-in transition, we found band reconstruction at high binding energy, which can be well explained by the hybridization between main bands (MBs) and folded bands (FBs). Furthermore, in contrast to the perfectly nested quasi-one-dimensional system, triple-nesting-vector-induced CDW FBs increase the degeneracy of the band crossing and thus further enlarge the magnitude of band gap at certain momentum-energy positions. The visualization and modeling of CDW gaps in momentum-energy space reconciles the long-lasting controversy on the gap magnitude and suggests a weak-coupling Peierls physics in this system.

We design and analyze theoretically photometamaterials with each meta-atom containing both photodiode and light-emitting diode. Illumination of the photodiode by the light-emitting diode gives rise to an additional optical feedback within each unit cell, which strongly affects resonant properties and nonlinear response of the meta-atom. In particular, we demonstrate that inversion symmetry breaking occurs upon a certain threshold magnitude of the incident wave intensity resulting in an abrupt emergence of second-harmonic generation, which was not originally available, as well as in the reduced third-harmonic signal.

We address the shot noise in the tunneling current through a local spin, pertaining to recent experiments on magnetic adatoms and single molecular magnets. We show that both uncorrelated and spin-correlated scattering processes contribute vitally to the noise spectrum. The spin-correlated scattering processes provide an additional contribution to the Landauer-Büttiker shot noise expression, accounting for correlations between the tunneling electrons and the localized spin moment. By calculating the Fano factor, we show that both super- and sub-Poissonian shot noise can be described within our approach. Our theory provides transparent insights into noise spectroscopy, consistent with recent experiments using local probing techniques on magnetic atoms.

The theory of the optical response of a metamaterial slab which is represented by metal nanoparticles embedded in a dielectric matrix is developed. It is demonstrated that the account of the reflections from the slab boundaries essentially modifies the local field in the slab and leads to the anisotropy and spatial dispersion of its dielectric function as well as to the emergence of modes which do not exist in an infinite metamaterial. It is shown that these features introduce the existence of self-excited normal waves (polaritons) and mechanical excitons (polarization waves). These findings reveal that the metamaterial slab can be regarded as an active device (“plasmonic oscillator”) which generates sustained polaritons in the presence of dissipation. A relation of this effect with the phenomenon of a plasmonic blackbody or perfect absorber, observed in such structures, is discussed and a possible mechanism of this phenomenon is proposed.

We study the frequency dependencies of the fermion and photon dressing functions in dynamical gap generation in graphene. We use a low-energy effective QED-like description, but within this approximation, we include all frequency-dependent effects, including retardation. We obtain the critical coupling by calculating the gap using a nonperturbative Dyson-Schwinger approach. Compared to the results of our previous calculation [M. E. Carrington et al., Phys. Rev. B 94, 125102 (2016)], which used a Lindhard screening approximation instead of including a self-consistently calculated dynamical screening function, the critical coupling is substantially reduced.

By combining theoretical density functional theory (DFT) and experimental studies, structural and magnetic phase stabilities and electronic structural, elastic, and vibrational properties of the Laves-phase compound ${\mathrm{NbMn}}_2$ have been investigated for the C14, C15, and C36 crystal structures. At low temperatures C14 is the ground-state structure, with ferromagnetic and antiferromagnetic orderings being degenerate in energy. The degenerate spin configurations result in a rather large electronic density of states at Fermi energy for all magnetic cases, even for the spin-polarized DFT calculations. Based on the DFT-derived phonon dispersions and densities of states, temperature-dependent free energies were derived for the ferromagnetic and antiferromagnetic C14 phase, demonstrating that the spin-configuration degeneracy possibly exists up to finite temperatures. The heat of formation ${\mathrm{\Delta }}_{298}{H}^0=-45.05\pm 3.64\mathrm{kJ}{(\mathrm{mol}\mathrm{f}.\mathrm{u}.{\mathrm{NbMn}}_2)}^{-1}$ was extracted from drop isoperibolic calorimetry in a Ni bath. The DFT-derived enthalpy of formation of ${\mathrm{NbMn}}_2$ is in good agreement with the calorimetric measurements. Second-order elastic constants for ${\mathrm{NbMn}}_2$ as well as for related compounds were calculated.

We have measured the signatures of electronic energy scales and their doping evolution in the band structure of ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ using angle-resolved photoemission spectroscopy. While band splittings and positions corresponding to the bilayer splitting and spin-orbit coupling undergo only small changes, the Mott gap and effective mass of both the lower Hubbard band and conduction band exhibit strong variations with doping. These changes correspond to similar observations in the cuprate superconductors, and are likely connected to the changing effective Coulomb interaction upon addition of itinerant carriers.

We theoretically investigate plasmon polaritons in cubic lattices of spherical metallic nanoparticles. The nanoparticles, each supporting triply-degenerate localized surface plasmons, couple through the Coulomb dipole-dipole interaction, giving rise to collective plasmons that extend over the whole metamaterial. The latter hybridize with photons forming plasmon polaritons, which are the hybrid light-matter eigenmodes of the system. We derive general analytical expressions to evaluate both plasmon and plasmon-polariton dispersions and the corresponding eigenstates. These are obtained within a Hamiltonian formalism, which takes into account retardation effects in the dipolar interaction between the nanoparticles and considers the dielectric properties of the nanoparticles as well as their surrounding. Within this model we predict polaritonic splittings in the near-infrared to the visible range of the electromagnetic spectrum that depend on polarization, lattice symmetry, and wave-vector direction. Finally, we show that the predictions of our model are in excellent quantitative agreement with conventional finite-difference frequency-domain simulations, but with the advantages of analytical insight and significantly reduced computational cost.

Electron spin transport and dynamics are investigated in a single, high-mobility, modulation-doped, GaAs quantum well using ultrafast two-color Kerr-rotation microspectroscopy, supported by qualitative kinetic theory simulations of spin diffusion and transport. Evolution of the spins is governed by the Dresselhaus bulk and Rashba structural inversion asymmetries, which manifest as an effective magnetic field that can be extracted directly from the experimental coherent spin precession. A spin-precession length ${\lambda }_{\mathrm{SOI}}$ is defined as one complete precession in the effective magnetic field. It is observed that application of (i) an out-of-plane electric field changes the spin decay time and ${\lambda }_{\mathrm{SOI}}$ through the Rashba component of the spin-orbit coupling, (ii) an in-plane magnetic field allows for extraction of the Dresselhaus and Rashba parameters, and (iii) an in-plane electric field markedly modifies both the ${\lambda }_{\mathrm{SOI}}$ and diffusion coefficient.

Thermal decomposition of silicon carbide is a promising approach for the fabrication of graphene. However, the atomistic growth mechanism of graphene remains unclear. This paper describes the development of a new charge-transfer interatomic potential. Carbon bonds with a wide variety of characteristics can be reproduced by the proposed vectorized bond-order term. A large-scale thermal decomposition simulation enables us to observe the continuous growth process of the multiring carbon structure. The annealing simulation reveals the atomistic process by which the multiring carbon structure is transformed to flat graphene involving only six-membered rings. Also, it is found that the surface atoms of the silicon carbide substrate enhance the homogeneous graphene formation.

Many physical properties of oxides can be changed by inducing lattice distortions in the crystal through heteroepitaxial growth of thin films. The average lattice strain can often be tuned by changing the film thickness or using suitable buffer layers between film and substrate. The exploitation of the full potential of strain engineering for sample or device fabrication rests on the understanding of the fundamental mechanisms of stress generation and evolution. For this study an optical measurement of the substrate curvature is used to monitor in situ how the stress builds up and relaxes during the growth of oxide thin films by pulsed laser deposition. The relaxation behavior is correlated with the growth mode, which is monitored simultaneously with reflection high-energy electron diffraction. The stress relaxation data is fitted and compared with theoretical models for stress evolution which were established for semiconductor epitaxy. The initial stage of the growth appears to be governed by surface stress and surface energy effects, while the subsequent stress relaxation is found to be fundamentally different between films grown on single-crystal substrates and on buffer layers. The first case can be rationalized with established theoretical models, but these models fail in the attempt to describe the growth on buffer layers. This is most probably due to the larger average density of crystalline defects in the buffer layers, which leads to a two-step stress relaxation mechanism, driven first by the nucleation and later by the migration of dislocation lines.

We study the effect of the substitution of Zr by Hf on the dynamical behavior in the ${\mathrm{Zr}}_{36}{\mathrm{Ni}}_{64}$ melt. A reduced measured self-diffusion coefficient and a higher measured melt viscosity for an increased amount of Hf were observed. The ternary ${\mathrm{Hf}}_{10}{\mathrm{Zr}}_{25}{\mathrm{Ni}}_{65}$ melt, which exhibits a pronounced deviation from Arrhenius behavior over a studied temperature range of 550 K, can be accurately described by the scaling law of mode-coupling theory (MCT) with almost equal parameters for the self-diffusion and the viscosity. Although we only substitute alloy components with a nearly equal atomic size and the measured overall packing fraction remains almost unchanged, the dynamics in ${\mathrm{Hf}}_{10}{\mathrm{Zr}}_{25}{\mathrm{Ni}}_{65}$ are slower compared to ${\mathrm{Zr}}_{36}{\mathrm{Ni}}_{64}$. This corresponds also to a higher critical temperature $T_c$ and might be induced by different chemical interactions in the melts. The increased $T_c$ results in a significantly smaller difference between liquidus and critical temperature $\mathrm{\Delta }{T}_{\mathrm{LC}}=T_{\mathrm{L}}-T_c$ for the ternary melt in comparison with ${\mathrm{Zr}}_{36}{\mathrm{Ni}}_{64}$, which may favor the glass formation in the ${\mathrm{Hf}}_{10}{\mathrm{Zr}}_{25}{\mathrm{Ni}}_{65}$ melt.