Co growth on Cu(111) was investigated at several temperatures between 120 K and 300 K by variable-temperature fast-scanning tunneling microscopy at submonolayer coverage. Islands nucleate heterogeneously at step edges and homogeneously on terraces. The height and area distribution difference between these two types of differently nucleated islands is attributed to a step edge alloy. Furthermore, the transformation from one-monolayer high islands to two-monolayer high islands is followed in time-lapsed sequences between 145 and 165 K. A surprising low-energy barrier for upward mass transport of $E_{\mathrm{upward}}\approx (0.15\pm 0.04)$ eV is determined for islands on terraces. At 120 and 150 K, the terrace islands are pure Cu; in contrast, at room temperature, terrace islands larger than $\approx 120$ ${\mathrm{nm}}^2$ alloy at their border.

Quantum-size effects are essential for understanding the terahertz conductivity of semiconductor nanocrystals, particularly at low temperatures. We derived a quantum mechanical expression for the linear terahertz response of nanocrystals; its introduction into an appropriate effective medium model provides a comprehensive microscopic approach for the analysis of terahertz conductivity spectra as a function of frequency, temperature, and excitation fluence. We performed optical pump–terahertz probe experiments in multilayer Si quantum dot networks with various degrees of percolation at 300 and 20 K and with variable pump fluence (initial carrier density) over nearly three orders of magnitude. Our theoretical approach was successfully applied to quantitatively interpret all the measured data within a single model. A careful data analysis made it possible to assess the distribution of sizes of nanocrystals participating to the photoconduction. We show and justify that such conductivity-weighted distribution may differ from the size distribution obtained by standard analysis of transmission electron microscopy images.

Incident-energy-dependent electron transmittances through single-layer graphene (SLG) and bilayer graphene (BLG) were investigated using time-dependent density functional theory. The transmittances of BLG with two kinds of stacking exhibit an unexpected crossing at a certain incident electron energy. The behavior is preserved for the BLG with reduced or increased layer distances compared to that of typical BLG. We determined the origin of the crossing by investigating transmission electron diffraction patterns for SLG.

We start from the polynomial interatomic potentials introduced by Wojdeł et al. [J. Phys.: Condens. Matter 25, 305401 (2013)] and take advantage of one of their key features—namely, the linear dependence of the energy on the potential's adjustable parameters—to devise a scheme for the construction of first-principles-based (second-principles) models for large-scale lattice-dynamical simulations. Our method presents the following convenient features. The parameters of the model are computed in a very fast and efficient way, as it is possible to recast the fit to a training set of first-principles data into a simple matrix diagonalization problem. Our method selects automatically the interactions that are most relevant to reproduce the training-set data, by choosing from a pool that includes virtually all possible coupling terms, and produces a family of models of increasing complexity and accuracy. We work with practical and convenient cross-validation criteria linked to the physical properties that will be relevant in future simulations based on the new model, and which greatly facilitate the task of identifying a potential that is simultaneously simple (thus computationally light), very accurate, and predictive. We also discuss practical ways to guarantee that our energy models are bounded from below, with a minimal impact on their accuracy. Finally, we demonstrate our scheme with an application to ferroelastic perovskite ${\mathrm{SrTiO}}_3$, which features many nontrivial lattice-dynamical features (e.g., a phase transition driven by soft phonons, competing structural instabilities, highly anharmonic dynamics) and provides a very demanding test.

We investigate the accuracy and transferability of a recently developed high-dimensional neural network (NN) method for calcium fluoride, fitted to a database of ab initio density functional theory (DFT) calculations based on the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional. We call the method charge equilibration via neural network technique (CENT). Although the fitting database contains only clusters (i.e., nonperiodic structures), the NN scheme accurately describes a variety of bulk properties. In contrast to other available empirical methods the CENT potential has a much simpler functional form, nevertheless it correctly reproduces the PBE energetics of various crystalline phases both at ambient and high pressure. Surface energies and structures as well as dynamical properties derived from phonon calculations are also in good agreement with PBE results. Overall, the difference between the values obtained by the CENT potential and the PBE reference values is less than or equal to the difference between the values of local density approximation (LDA) and Born-Mayer-Huggins (BMH) with those calculated by the PBE exchange correlation functional.

We determine the viability of four-layered Aurivillius phases to exhibit long-range magnetic order above room temperature. We use Monte Carlo simulations to calculate transition temperatures for an effective Heisenberg model containing a minimal set of required couplings. The magnitude of the corresponding coupling constants has been determined previously from electronic structure calculations for ${\mathrm{Bi}}_5{\mathrm{FeTi}}_3{\mathrm{O}}_{15}$, for which we obtain a transition temperature far below room temperature. We analyze the role of further neighbor interactions within our Heisenberg model, in particular that of the second-nearest-neighbor coupling within the perovskitelike layers of the Aurivillius structure, as well as that of the weak interlayer coupling, in order to identify the main bottleneck for achieving higher magnetic transition temperatures. Based on our findings, we show that the most promising strategy to obtain magnetic order at higher temperatures is to increase the concentration of magnetic cations within the perovskitelike layers, and we propose candidate compounds where magnetic order could be achieved above room temperature.

The magnetic-field-temperature phase diagram of solid oxygen is investigated by the adiabatic magnetocaloric effect (MCE) measurement with pulsed magnetic fields. Relatively large temperature decrease with hysteresis is observed at just below the $\beta -\gamma$ and $\alpha -\beta$ phase-transition temperatures owing to the field-induced transitions. The magnetic field dependence of these phase boundaries are obtained as $T_{\beta \gamma }\left(H\right)=43.8-1.55\times {10}^{-3}{H}^2$ K and $T_{\alpha \beta }\left(H\right)=23.9-0.73\times {10}^{-3}{H}^2$ K. The magnetic Clausius-Clapeyron equation quantitatively explains the $H$ dependence of $T_{\beta \gamma }$, but does not $T_{\alpha \beta }$. The MCE curve at $T_{\beta \gamma }$ is of typical first order, while the curve at $T_{\alpha \beta }$ seems to have both characteristics of first- and second-order transitions. We discuss the order of the $\alpha -\beta$ phase transition and propose possible reasons for the unusual behaviors.

The behavior of a uniformly magnetized domain of ellipsoidal shape subject to a static external field and oscillatory external driving field is analyzed near bifurcation events. The analysis includes the effects of both linear and circularly polarized driving fields and is performed using numerical simulations of the Landau-Lifshitz-Gilbert (LLG) equation. Under a linearly polarized driving field, the LLG equation is a nonautonomous differential equation which can lead to complex magnetization motions, such as bistability, multiperiodic orbits, quasiperiodicity, and chaos. Under a circularly polarized driving field, the LLG equation can be written in autonomous form by transforming to the frame rotating with the driving field. The autonomous nature allows one to perform a fixed-point analysis of the system for select demagnetization factors. Similarities and differences between the driven systems are highlighted through bifurcation diagrams, phase portraits, basins of attraction, and Lyapunov exponents. Magnetization switching, prolonged transients, quasiperiodicity, and chaos are observed with both linearly and circularly polarized driving fields in the magnetic systems investigated.

We report on the crossover from the thermal to the athermal regime of an artificial spin ice formed from a square array of magnetic islands whose lateral size, 30 nm $\times$ 70 nm, is small enough that they are dynamic at room temperature. We used resonant magnetic soft x-ray photon correlation spectroscopy as a method to observe the time-time correlations of the fluctuating magnetic configurations of spin ice during cooling, which are found to slow abruptly as a freezing temperature of $T_0=178\pm 5$ K is approached. This slowing is well described by a Vogel-Fulcher-Tammann law, implying that the frozen state is glassy, with the freezing temperature being commensurate with the strength of magnetostatic interaction energies in the array. The activation temperature, $T_{\mathrm{A}}=40\pm 10$ K, is much less than that expected from a Stoner-Wohlfarth coherent rotation model. Zero-field-cooled/field-cooled magnetometry reveals a freeing up of fluctuations of states within islands above this temperature, caused by variation in the local anisotropy axes at the oxidised edges. This Vogel-Fulcher-Tammann behavior implies that the system enters a glassy state upon freezing, which is unexpected for a system with a well-defined ground state.

The energy-time uncertainty is an intrinsic limit for time-resolved experiments imposing a tradeoff between the duration of the light pulses used in experiments and their frequency content. In standard time-resolved photoemission, this limitation maps directly onto a tradeoff between the time resolution of the experiment and the energy resolution that can be achieved on the electronic spectral function. Here we propose a protocol to disentangle the energy and time resolutions in photoemission. We demonstrate that dynamical information on all time scales can be retrieved from time-resolved photoemission experiments using suitably shaped light pulses of quantum or classical nature. As a paradigmatic example, we study the dynamical buildup of the Kondo peak, a narrow feature in the electronic response function arising from the screening of a magnetic impurity by the conduction electrons. After a quench, the electronic screening builds up on timescales shorter than the inverse width of the Kondo peak and we demonstrate that the proposed experimental scheme could be used to measure the intrinsic time scales of such electronic screening. The proposed approach provides an experimental framework to access the nonequilibrium response of collective electronic properties beyond the spectral uncertainty limit and will enable the direct measurement of phenomena such as excited Higgs modes and, possibly, the retarded interactions in superconducting systems.

Monolayer transition metal dichalcogenides are promising materials for “valleytronics.” They have band gaps at energy-degenerate $K$ and $K^{\prime }$ valleys with opposite spins. Due to the lack of inversion symmetry, electron-hole pairs can be selectively created at $K$ or $K^{\prime }$ valleys by circularly polarized photons. In addition, linearly polarized light excitation creates the coherent superposition of exciton valley states, referred to as the generation of valley coherence. In this study we performed polarization resolved photoluminescence and resonant Raman spectroscopy of CVD-grown monolayer $\mathrm{Mo}{\mathrm{S}}_2$. We found that the lowest exciton photoluminescence becomes polarized, indicating the effective generation of valley polarization and valley coherence due to the resonant effect, accompanied by a drastic change of the polarization selection rule of Raman scattering. These results were theoretically explained from the viewpoint of the selection rules of resonant Raman scattering. We conclude that the Raman-like resonant second-order optical process should be the main mechanism of valley coherence.

Nanowires with a GaSb core and an InAs shell (and the inverted structure) are interesting for studies of electron-hole hybridization and interaction effects due to the bulk broken band-gap alignment at the material interface. We have used eight-band $\mathbf{k}·\mathbf{p}$ theory together with the envelope function approximation to calculate the band structure of such nanowires. For a fixed core radius, as a function of shell thickness the band structure changes from metallic (for a thick shell) to semiconducting (for a thin shell) with a gap induced by quantum confinement. For intermediate shell thickness, a different gapped band structure can appear, where the gap is induced by hybridization between the valence band in GaSb and the conduction band in InAs. To establish a relationship between the nanowire band structures and signatures in thermoelectrical measurements, we use the calculated energy dispersions as input to the Boltzmann equation and to ballistic transport equations to study the diffusive limit and the ballistic limit, respectively. Our theoretical results provide a guide for experiments, showing how thermoelectric measurements in a gated setup can be used to distinguish between different types of band gaps, or tune the system into a regime with few electrons and few holes, which can be of interest for studies of exciton physics.

Conducting polymer samples of polyaniline (PANI) exhibit a dramatic change in their conductivity as a function of protonation level, analogous with the changes in the transport properties of semiconductors upon doping. In this paper, PANI samples were prepared by protonating with varying concentrations of hydrochloric acid. The complex terahertz frequency-dependent conductivity and the dc conductivity of these samples were measured and analyzed in the framework of the disorder-driven, metal-insulator quantum phase transition. The samples were determined to all fall in the insulating phase of this phase transition. The frequency dependence of both the real and imaginary terahertz conductivity are found to be consistent with theories that include electronic correlation effects.

Transient current calculation is essential to study the response time and capture the peak transient current for preventing meltdown of nanochips in nanoelectronics. Its calculation is known to be extremely time consuming with the best scaling $T{N}^3$ where $N$ is the dimension of the device and $T$ is the number of time steps. The dynamical response of the system is usually probed by sending a steplike pulse and monitoring its transient behavior. Here, we provide a fast algorithm to study the transient behavior due to the steplike pulse. This algorithm consists of two parts: algorithm I reduces the computational complexity to $T^0{N}^3$ for large systems as long as $T<N$; algorithm II employs the fast multipole technique and achieves scaling $T^0{N}^3$ whenever $T<N^2$ beyond which it becomes $T{log}_{2}N$ for even longer time. Hence it is of order $O\left(1\right)$ if $T<N^2$. Benchmark calculation has been done on graphene nanoribbons with $N={10}^4$ and $T={10}^8$. This algorithm allows us to tackle many large scale transient problems including magnetic tunneling junctions and ferroelectric tunneling junctions.

We studied isotopically enriched nano- and microdiamonds with optically active ${\mathrm{GeV}}^{-}$ centers synthesized at high pressures and high temperatures in nonmetallic growth systems. The influence of isotopic composition on optical properties has been thoroughly investigated by photoluminescence-excitation (PLE) and photoluminescence (PL) spectroscopy to get insight into the nature and electronic structure of this color center. We have demonstrated that the large frequency defect (difference between oscillation frequencies in the ground and excited electronic states) does bring about large discrepancy between PLE and PL spectra and comparatively high isotopic shift of the zero phonon line. Both effects seem to be rather common to split-vacancy centers (for example ${\mathrm{SiV}}^{-})$, where the frequency defect reaches record high values. Isotopic substitution of carbon atoms in the diamond lattice results in even larger shifts, which are only partially accounted for by a redistribution of electron density caused by the volume change of the diamond lattice. It was shown that the vibronic frequency in this case does not depend on the mass of carbon atoms. The greatest part of this isotopic shift is due to anharmonicity effects, which constitute a substantial part of vibronic frequency observed in this center. The exact physical mechanism, which leads to significant enhancement of anharmonicity on substitution of ${}^{12}\mathrm{C}$ to ${}^{13}\mathrm{C}$, is yet to be clarified.

We address the impact of tantalum alloying on dislocation properties of tungsten. To that aim, we calculate elastic constants, atomic-row displacement energy, dislocation core energy, and Peierls stress for different degrees of alloying within the framework of density-functional theory. We show that the elastic shear constants decrease monotonously with Ta content. Conversely, atomic-row displacement energy and, consequently, core energy and Peierls stress show a nonmonotonous behavior. These quantities peak at 25 at% Ta, indicating a tendency for embrittlement of W at such alloying concentrations. Our findings are in agreement with the experimental literature.

We investigated a system based solely on a single molecular species, $\text{Cu}({\mathrm{tmdt})}_2$, accommodating $d$ and $\pi$ orbitals within the molecule. ${}^{13}\mathrm{C}$ nuclear magnetic resonance measurements captured singlet-triplet excitations of $\pi$ spins indicating the existence of a $\pi$-electron-based spin-gapped Mott insulating subsystem, which has been hidden by the large magnetic susceptibility exhibited by the $d$ spins forming antiferromagnetic chains. The present results demonstrate a unique hybrid Mott insulator composed of antiferromagnetic and spin-singlet Mott subsystems with distinctive dimensionalities.

We study two-band one-dimensional superconducting chains of spinless fermions with inter- and intraband pairing. These bands hybridize and, depending on the relative angular momentum of their orbitals, the hybridization can be symmetric or antisymmetric. The self-consistent competition between intra- and interband superconductivity and how it is affected by the symmetry of the hybridization is investigated. In the case of antisymmetric hybridization the intra- and interband pairings do not coexist, while in the symmetric case they do coexist and the interband pairing is shown to be dominant. The topological properties of the model are obtained through the topological invariant winding number and the presence of edge states. We find the existence of a topological phase due to the interband superconductivity and induced by symmetric hybridization. In this case we find a characteristic $4\pi$-periodic Josephson current. In the case of antisymmetric hybridization we also find a $4\pi$-periodic Josephson current in the gapless interband superconducting phase, recently identified to be of Weyl type.