Stick slip—the sequence of mechanical instabilities through which a slider advances on a solid substrate—is pervasive throughout sliding friction, from nanoscales to geological scales. Here we suggest that trapped cold ions in an optical lattice can also be of help in understanding stick-slip friction, and also the way friction changes when one of the sliders undergoes structural transitions. For that scope, we simulated the dynamical properties of a 101-ion chain, driven to slide back and forth by a slowly oscillating electric field in an incommensurate periodic “corrugation” potential of increasing magnitude $U_0$. We found the chain sliding to switch, as $U_0$ increases and before the Aubry transition, from a smooth-sliding regime with low dissipation to a stick-slip regime with high dissipation. In the stick-slip regime the onset of overall sliding is preceded by precursor events consisting of partial slips of a few ions only, leading to partial depinning of the chain, a nutshell remnant of precursor events at the onset of motion also observed in macroscopic sliders. Seeking to identify the possible effects on friction of a structural transition, we reduced the trapping potential aspect ratio until the ion chain shape turned from linear to zigzag. Dynamic friction was found to rise at the transition, reflecting the opening of other dissipation channels.

We describe the interaction between a transition-metal atom and a noble-metal surface by using an ionic model in which the first Hund's rule determines the filling of the atom's $d$ levels, and spin fluctuations occur due to the electron exchange between the metal band and the atom states. We apply our model to the case of adsorbed Ti atoms on noble-metal surfaces (Ag and Au) in which conductance measurements in scanning tunneling microscope experiments suggest a mixed-valence regime according to the position and width of the atomic resonance. By introducing, in our calculation, these two parameters as extracted from the experiment, we satisfactorily reproduce the experimental results in both cases. We find, in the Ag(100) surface, that the conductance spectrum reflects electronic characteristics of the metal surface modified by the presence of the magnetic atom; whereas, in the Au(111) case, only the projected density of states on the Ti atom determines the conductance spectrum shape.

In this paper, we discuss the validity of the band structure concept in silicon nanocrystals a few nanometers in size. We introduce a general method which allows reconstruction of a fuzzy electronic band structure of nanocrystals from ordinary real-space electronic structure calculations. A comprehensive study of the fuzzy band structure of a realistic nanocrystal is given including full geometric and electronic relaxation with the surface passivating groups. In particular, we combine this method with large-scale density functional theory calculations to obtain insight into the luminescence properties of silicon nanocrystals up to 3 nm in size depending on the surface passivation and geometric distortion. We conclude that the band-structure concept is applicable to silicon nanocrystals with a diameter larger than $\approx$2 nm with certain limitations. We also show how perturbations due to polarized surface groups or geometric distortion can lead to considerable moderation of momentum space selection rules.

We describe the possibility of drastically boosting the spontaneous emission of a collection of two-level quantum emitters by embedding them in an epsilon-near-zero (ENZ) environment, consisting of a plasmonic waveguide operated at cutoff. This phenomenon relies on the combination of Purcell enhancement and Dicke superradiance effects, exploiting the large and uniform local density of states in ENZ channels, which is shown to significantly extend the spatial extent of these quantum effects. We envision exciting applications in optical sensing, molecular detection, and low-threshold lasing.

We discuss the possibility of realizing utlrabroadband omnidirectional absorbers and angularly selective coherent thermal emitters based on properly patterned plasmonic metastructures. Instead of relying on resonant concentration effects that inherently limit the bandwidth, we base our design on the combination of two inherently nonresonant effects: plasmonic Brewster funneling and adiabatic plasmonic focusing. Using this approach, we propose compact, broadband absorption and emission spanning terahertz, infrared, and optical frequencies, ideal for various energy and defense applications.

The excited states, ${}^4{T}_{2g}$ and ${}^2{E}_g$, of a Cr impurity in Al${}_2$O${}_3$ were treated by constraint density functional theory by imposing a density matrix constraint (constraint field) to control the electron occupation numbers of the $d$ orbitals. The calculated excitation energies, directly calculated from the self-consistent total energies of the ${}^4{A}_{2g}$ ground states and the various excited states, correctly reproduce the experimental ordering. In addition, we find that there is no stationary solution for the excited ${}^4{T}_{2g}$ state corresponding to the crystal-field transition state in the usual Kohn-Sham equation, i.e., with no constraint field. By contrast, the excited ${}^2{E}_g$ state of the spin-flip transition state is a (meta-) stable stationary solution, and may be responsible for the long radiative decay lifetime observed in experiments on ruby.

Accessing fundamental structure-property correlations in ternary semiconductor nanowires is a challenging endeavor often limited by large compositional inhomogeneities. Here, we investigate strongly periodic In${}_{1-x}$Ga${}_x$As nanowire arrays grown on Si with very high compositional uniformity to identify the role of microstructural features (crystal phase and stacking order) on the phonon and optical properties via transmission electron microscopy, Raman, and photoluminescence spectroscopy. Depending on Ga content and growth kinetics, we show that the nanowire microstructure changes consistently from a wurtzite-dominated stacking to a disordered layer stacking, with broadened Raman modes, upon tuning the composition from In-rich to more Ga-rich In${}_{1-x}$Ga${}_x$As. Consequently, higher-quality wurtzite stacking leads to enhanced photoluminescence emission intensities and, most strikingly, a blue-shift in the emission energy of ∼30–40 meV with respect to the bulk In${}_{1-x}$Ga${}_x$As phase. This finding supports the theoretically predicted larger band-gap energy of composition-tuned wurtzite-type In${}_{1-x}$Ga${}_x$As.

The filling ${\nu }_{tot}=1$ quantum Hall state under charge imbalance is investigated through both transport and thermodynamic measurements on a high-mobility low-density GaAs bilayer sample with negligible single particle tunneling. The ${\nu }_{tot}=1$ state demonstrates its robustness against imbalance by evolving continuously from the single layer regime (${\nu }_{upper}=1$, ${\nu }_{lower}=0$) to the bilayer regime with fillings ${\nu }_{upper}=1/3$ and ${\nu }_{lower}=2/3$ for the separate layers. The energy gap of the ${\nu }_{tot}=1$ state obtained from compressibility measurements using single electron transistors depends on position, i.e., the local disorder potential. Nevertheless, compressibility and thermal activation measurements yield comparable values for the energy gap under imbalance.

Electron spin relaxation due to the D’yakonov-Perel’ mechanism is investigated in bilayer graphene with only the lowest conduction band being relevant. The spin-orbit coupling is constructed from the symmetry group analysis with the parameters obtained by fitting to the numerical calculation according to the latest report by Konschuh et al. [Phys. Rev. B 85, 115423 (2012)] from first principles. In contrast to single-layer graphene, the leading term of the out-of-plane component of the spin-orbit coupling in bilayer graphene shows a Zeeman-like term with opposite effective magnetic fields in the two valleys. This Zeeman-like term opens a spin relaxation channel in the presence of intervalley scattering. It is shown that the intervalley electron-phonon scattering, which has not been reported in the previous literature, strongly suppresses the in-plane spin relaxation time at high temperature whereas the intervalley short-range scattering plays an important role in the in-plane spin relaxation especially at low temperature. A marked nonmonotonic dependence of the in-plane spin relaxation time on temperature with a minimum of several hundred picoseconds is predicted in the absence of the short-range scatterers. This minimum is comparable to the experimental data. Moreover, a peak in the electron density dependence of the in-plane spin relaxation time at low temperature, which is very different from the one in semiconductors, is predicted. We also find a rapid decrease in the in-plane spin relaxation time with increasing initial spin polarization at low temperature, which is opposite to the situation in both semiconductors and single-layer graphene. A strong anisotropy between the out-of- and in-plane spin relaxations at high temperature is also revealed with the out-of-plane spin relaxation time being about two orders of magnitude larger than the in-plane one. Detailed comparisons of the temperature and electron density dependencies of the spin relaxation with the experiments of Yang et al. [Phys. Rev. Lett. 107, 047206 (2011)], Han and Kawakami [Phys. Rev. Lett. 107, 047207 (2011)], and Avsar et al. [Nano Lett. 11, 2363 (2011)] are reported.

For electronic transport at the nanoscale, coherent scattering at defects plays an important role. Therefore, the capability of visualizing the influence of defects on the conductivity of single atomic junctions may benefit the development of future nano-electronics. Here, we report imaging the coherent scattering from a defect with well-controlled geometry by quantum point contact microscopy recently developed by us. An ∼10$\%$ modulation in transport conductance of a single atomic junction is observed, with a phase shift of nearly π compared to the tunneling conductance. With the well-defined scattering geometry, we performed a theoretical calculation of the conductance and found the result consistent with the experiment.

Anomalous resonances in properly shaped plasmonic nanostructures can in principle lead to infinite absorption/gain efficiencies over broad bandwidths of operation. By developing a closed-form analytical solution for the fields scattered by conjoined hemicylinders, we outline the fundamental physics behind these phenomena, associated with broadband adiabatic focusing of surface plasmons at the nanoscale. Over a continuous frequency range, our proposed composite nanostructure shows finite amount of absorption/amplification even in the limit of infinitesimally small intrinsic material loss/gain. Detailed physical insights are provided to justify the nature of this apparent paradox, and its counterintuitive behavior is discussed for potential applications in nonlinear optics, spasing, sensing, and energy-harvesting devices.

We analyze a spin-$1/2$ chain with two-spin interactions which are shown to be exactly solvable by Lieb, Schultz, and Mattis [Ann. Phys. 16, 407 (1961)]. We show that the model can be viewed as a generalized Kitaev model that is analytically solvable for all defect sectors. We present an alternate proof that the defect-free sector is the ground state, which is valid for a larger parameter range. We show that the defect sectors have degenerate ground states corresponding to unpaired Majorana fermion modes and that the degeneracy is topologically protected against disorder in the spin-spin couplings. The unpaired Majorana fermions can be manipulated by tuning the model parameters and can hence be used for topological quantum computation.

Thin ${}^{57}$Fe layers evaporated onto an MgO(100) single-crystal substrate and covered by an evaporated MgO layer were studied by low-temperature conversion electron Mössbauer spectroscopy. The temperature dependence of the spectra indicates superparamagnetic behavior below 8 ML nominal thickness of the Fe layer signaling a cluster-type growth mode. The low-temperature hyperfine fields are consistent with a model that defines two types of metallic Fe atoms: bulklike and interfacial ones. Formation of FeO or (Fe,Mg)O at the interface layer is not observed. The sample with a 4-ML Fe layer when grown over a cleaved MgO substrate shows almost perfect perpendicular magnetization, as locally probed at 15 K by the hyperfine magnetic field, while random magnetization orientation and lower blocking temperature is observed in the case of a polished substrate. The perpendicular anisotropy observed at low temperature is attributed to mechanical stresses arising from the epitaxial relation and the different temperature dilatation of the subsequent layers.

Starting from the orthorhombic magnetically ordered phase, we investigate the effects of uniaxial tensile and compressive stresses along a, b, and the diagonal a+b directions in BaFe${}_2$As${}_2$ and CaFe${}_2$As${}_2$ in the framework of ab initio density functional theory (DFT) and a phenomonological Ginzburg-Landau model. While—contrary to the application of hydrostatic or $c$-axis uniaxial pressure—both systems remain in the orthorhombic phase with a pressure-dependent nonzero magnetic moment, we observe a sign-changing jump in the orthorhombicity at a critical uniaxial pressure, accompanied by a reversal of the orbital occupancy and a switch between the ferromagnetic and antiferromagnetic directions. Our Ginzburg-Landau analysis reveals that this behavior is a direct consequence of the competition between the intrinsic magneto-elastic coupling of the system and the applied compressive stress, which helps the system to overcome the energy barrier between the two possible magneto-elastic ground states. Our results shed light on the mechanisms involved in the detwinning process of an orthorhombic iron-pnictide crystal and on the changes in the magnetic properties of a system under uniaxial stress.

Kinetically frustrated bosons at half filling in the presence of a competing nearest-neighbor repulsion support a wide supersolid regime on the two-dimensional triangular lattice. We study this model on a two-leg ladder using the finite-size density-matrix renormalization-group method, obtaining a phase diagram which contains three phases: a uniform superfluid (SF), an insulating charge density wave (CDW) crystal, and a bond ordered insulator (BO). We show that the transitions from SF to CDW and SF to BO are continuous in nature, with critical exponents varying continuously along the phase boundaries, while the transition from CDW to BO is found to be first order. The phase diagram is also found to contain an exactly solvable Majumdar Ghosh point, and reentrant SF to CDW phase transitions.

The one-dimensional transverse field Ising model is solved by continuous unitary transformations in the high-field limit. A high accuracy is reached due to the closure of the relevant algebra of operators, which we call string operators. The closure is related to the possibility to map the model by Jordan-Wigner transformation to noninteracting fermions, but it is proven without referring to this mapping. The effective model derived by the continuous unitary transformations is used to compute the contributions of one, two, and three elementary excitations to the diagonal dynamic structure factors. The three-particle contributions have, so far, not been addressed analytically, except close to the quantum critical point.

Lattice vibrations as well as magnetic excitations are investigated using Raman scattering to understand the high-$T_c$ multiferroic properties of CuO. We observe a flat, broad spinon continuum extending to 2800 cm${}^{-1}$, being superimposed by magnon excitations with energies below 1350 cm${}^{-1}$. This allows us to estimate an exchange coupling constant of $J=108$ meV. Three Raman-active phonon modes predicted by lattice dynamical calculations are assigned and analyzed. We find a crossover temperature $T^{*}=140–150$ K through which new phonon modes are activated and phonon parameters show appreciable anomalies. This feature is discussed in terms of a competition of polar and nonpolar lattice distortions. Our study suggests the importance of competing lattice fluctuations for multiferroic compounds, which possess an intermediate multiferroic phase.

We study a Kondo impurity model with additional uniaxial anisotropy $D$ in a nonzero magnetic field $B$ using the numerical renormalization group (NRG). The ratio $g_e/g_S$ of electron and impurity $g$ factor is regarded as a free parameter and, in particular, the special cases of a “local” ($g_e=0$) and “bulk” ($g_e=g_S$) field are considered. For a bulk field, the relationship between the impurity magnetization $\mathcal{M}$ and the impurity contribution to the magnetization $M_{\text{imp}}$ is investigated and it is shown that $\mathcal{M}$ and $M_{\text{imp}}$ are proportional to each other for fixed coupling strength. Furthermore, we find that the $g$-factor ratio effectively rescales the magnetic field argument of the zero-temperature impurity magnetization. In case of an impurity with $D=0$ and $g_e=g_S$, it is demonstrated that at zero temperature $\mathcal{M}\left(B\right)$, unlike $M_{\text{imp}}\left(B\right)$, does not display universal behavior. With additional “easy-axis” anisotropy, the impurity magnetization is “stabilized” at a $D$-dependent value for $k_{B}T\ll g_S{\mu }_{B}B\ll \left|D\right|$ and, for nonzero temperature, is well described by a shifted and rescaled Brillouin function on energy scales that are small compared to $\left|D\right|$. In the case of “hard-axis” anisotropy, the magnetization curves can feature steps which are due to field-induced pseudo-spin-$\frac{1}{2}$ Kondo effects. For large hard-axis anisotropy and a local field, these screening effects are described by an exchange-anisotropic spin-$\frac{1}{2}$ Kondo model with an additional scattering term that is spin dependent (in contrast to ordinary potential scattering). In accordance with the observed step widths, this effective model predicts a decrease of the Kondo temperature with every further step that occurs upon increasing the field. Our study is motivated by the question as to how the magnetic properties of a deposited magnetic molecule are modified by the interaction with a nonmagnetic metallic surface.

We propose the existence, via analytical derivations, novel phenomenologies, and first-principles-based simulations, of a class of materials that are not only spontaneously optically active, but also for which the sense of rotation can be switched by an electric field applied to them via an induced transition between the dextrorotatory and laevorotatory forms. Such systems possess electric vortices that are coupled to a spontaneous electrical polarization. Furthermore, our atomistic simulations provide a deep microscopic insight into, and understanding of, this class of naturally optically active materials.

We employ the recently proposed scattering states numerical renormalization group (SNRG) approach to calculate $I\left(V\right)$ and the differential conductance through a single molecular level coupled to a local molecular phonon using the spinless Anderson-Holstein model. We also discuss the equilibrium physics of the model and demonstrate that the low-energy Hamiltonian is given by an effective interacting resonant level model. From the NRG level flow, we directly extract the effective charge transfer scale ${\Gamma }_{\mathrm{eff}}$ and the dynamically induced capacitive coupling $U_{\mathrm{eff}}$ between the molecular level and the lead electrons, which turns out to be proportional to the polaronic energy shift $E_p$ for the regimes investigated here. The equilibrium spectral functions for the different parameter regimes are discussed. The additional phonon peaks at multiples of the phonon frequency ${\omega }_0$ correspond to additional maxima in the differential conductance. Nonequilibrium effects, however, lead to significant deviations between a symmetric junction and a junction in the tunnel regime. The suppression of the current for particle-hole asymmetric junctions with increasing electron-phonon coupling, the hallmark of the Franck-Condon blockade, is discussed.