Landau level mixing should influence the quantum Hall effect for all but the strongest applied magnetic fields. However, the effects of Landau level mixing have proven difficult to model theoretically. We propose a simple method for examining the effects of Landau level mixing on electron pairs for intermediate magnetic fields by incorporating multiple Landau levels into the pseudopotentials on the Haldane sphere.

In previous experiments on electron-induced two-electron emission from the ferromagnetic surface system Co/W(110), spin-orbit coupling effects have been observed, which are comparable in size to the magnetic exchange effects. The present theoretical work aims at a detailed understanding of such effects, in particular their relation to the spin-dependent electronic structure and to collision dynamics. To this end, we have developed a formalism, which is based on a Dirac equation with an effective magnetic field. Magnetic exchange and spin-orbit coupling are thus incorporated simultaneously. Typical numerical two-electron emission results are presented for W(110) and for ultrathin Co films on W(110) together with the underlying spin- and layer-resolved valence electron spectral density. More detailed insight is provided by calculations, in which SOC was selectively switched off for the valence electron and for the primary and emitted electrons. Our theoretical results are in overall agreement with the experimental data. Furthermore, we predict sizable magnetic dichroism.

A quantum kinetic study of correlated spin transfer between optically excited electrons and Mn atoms in a ZnMnSe quantum well is presented. The simulations predict genuine signatures of non-Markovian spin dynamics which are particularly pronounced for special two-color laser excitations with a zero net angular momentum where a Markovian theory predicts an almost zero total electron spin for all times. In contrast, in the quantum kinetic simulations a sizable total electron spin builds up. Subsequently, a coherent oscillatory exchange of spin between the electron and Mn subsystems is observed.

Negatively charged nitrogen-vacancy (NV${}^{-}$) color centers in diamond have generated much interest for use in quantum technology. Despite the progress made in developing their applications, many questions about the basic properties of NV${}^{-}$ centers remain unresolved. Understanding these properties can validate theoretical models of NV${}^{-}$, improve their use in applications, and support their development into competitive quantum devices. In particular, knowledge of the phonon modes of the ${}^1{A}_1$ electronic state is key for understanding the optical pumping process. Using pump-probe spectroscopy, we measured the phonon sideband of the ${}^{1}E\to {}^1{A}_1$ electronic transition in the NV${}^{-}$ center. From this we calculated the ${}^{1}E\to {}^1{A}_1$ one-phonon absorption spectrum and found it to differ from that of the ${}^{3}E\to {}^3{A}_2$ transition, a result which is not anticipated by previous group-theoretical models of the NV${}^{-}$ electronic states. We identified a high-energy 169-meV localized phonon mode of the ${}^1{A}_1$ level.

Ohmic contacts are crucial elements of electron optics that have not received a clear theoretical description yet. We propose a model of an Ohmic contact as a piece of metal of the finite capacitance $C$ attached to a quantum Hall edge. It is shown that charged quantum Hall edge states may have weak coupling to neutral excitations in an Ohmic contact. Consequently, despite being a reservoir of neutral excitations, an Ohmic contact is not able to efficiently equilibrate edge states if its temperature is smaller than $\hslash {\Omega }_c$, where ${\Omega }_c$ is the inverse $RC$ time of the contact. This energy scale for a floating contact may become as large as the single-electron charging energy $e^2/C$.

Interlayer tunneling measurements in the strongly correlated bilayer quantized Hall phase at ${\nu }_T=1$ are reported. The maximum, or critical, current for tunneling at ${\nu }_T=1$ is shown to be a well-defined global property of the coherent phase, insensitive to extrinsic circuit effects and the precise configuration used to measure it, but also exhibiting a surprising scaling behavior with temperature. Comparisons between the experimentally observed tunneling characteristics and a recent theory are favorable at high temperatures, but not at low temperatures where the tunneling closely resembles the dc Josephson effect. The zero-bias tunneling resistance becomes extremely small at low temperatures, vastly less than that observed at zero magnetic field, but nonetheless remains finite. The temperature dependence of this tunneling resistance is similar to that of the ordinary in-plane resistivity of the quantum Hall phase.

We investigate the structural and magnetic properties of interfaces with large lattice mismatch, choosing Pt/Co and Au/Co as prototypes. For our first-principles calculations, we reduce the lattice mismatch to 0.2% by constructing Moiré supercells. Our results show that the roughness and atomic density, and thus the magnetic properties, depend strongly on the substrate and thickness of the Co slab. An increasing thickness leads to the formation of a Co transition layer at the interface, especially for Pt/Co due to strong Pt-Co interaction. A Moiré supercell with a transition layer is found to reproduce the main experimental findings and thus turns out to be the appropriate model for simulating magnetic misfit interfaces.

We investigate the fate of topological edge states in the quantum Hall (QH) regime when they experience tunneling processes through a narrow superconducting (S) strip. By computing the charge conductance flowing through a graphene-based QH/S/QH junction, the S strip is shown to act as an Andreev interferometer, giving rise to spectacular conductance oscillations as a function of the Landau level filling factor. We provide a semiclassical analysis allowing for a natural interpretation of these oscillations in terms of interferences between electron and hole trajectories propagating along the QH/S interfaces. Our results suggest that nontrivial junctions between topologically distinct phases could offer a highly tunable means of partitioning the flow of edge states.

We determine the reaction phase diagram and the transformation mechanism of (5,5) and (10,10) single-walled carbon nanotube bundles up to 20 GPa and 4000 K. We use Monte Carlo simulations, based on the state-of-the-art reactive potential LCBOPII, that incorporates both covalent and van der Waals interactions among the tubes. At low temperature, upon increasing pressure, large (10,10) nanotubes first collapse and then coalesce, yielding almost perfect graphitic structures. In contrast, small (5,5) nanotubes do not collapse, but coalesce and transform to graphite via a mixed graphite-tube structure. At high temperature (above $\sim 2000$ K), for both (10,10) and (5,5) nanotubes, coalescence dominates the transformation to graphitic structures. We argue that the $s{p}^3$-interlinking defects appearing at coalescence can act as seed and facilitate the transformation to diamond structures.

A classical model is developed for modeling ultrafast nonlinear coherent magneto-optical experiments performed on ferromagnetic thin films. Theoretical predictions of the Faraday rotation angles are compared to available experimental values and give meaningful insight into the physical mechanisms underlying the observed coherent magneto-optical phenomena. Under realistic conditions, the model successfully explains the observed trends. The crucial role played by the spin-orbit mechanism resulting from the direct interaction between the external electric field of the laser and the electron spins of the sample is underlined.

We construct the symmetry adapted low-energy effective Hamiltonian for the electronic states in the vicinity of the Fermi level in iron-based superconductors. We use Luttinger's method of invariants, expanding about $\Gamma$ and $M$ points in the Brillouin zone corresponding to a two-iron unit cell, and then matching the coefficients of the expansion to the five- and eight-band models. We then use the method of invariants to study the effects of the spin-density wave order parameters on the electronic spectrum, with and without spin-orbit coupling included. Among the results of this analysis is the finding that the nodal spin-density wave is unstable once spin-orbit coupling is included. Similar analysis is performed for the $A_{1g}$ spin singlet superconducting state. Without spin-orbit coupling there is one pairing invariant near the $\Gamma$ point but two near the $M$ point. This leads to an isotropic spectral gap at the hole Fermi surface near $\Gamma$, but anisotropic near $M$. The relative values of these three parameters determine whether the superconducting state is $s_{++}$, $s_{+-}$, or nodal. Inclusion of spin-orbit coupling leads to additional mixing of spin triplet pairing, with one additional pairing parameter near $\Gamma$ and one near $M$. This leads to an anisotropic spectral gap near both hole and electron Fermi surfaces, the latter no longer cross but rather split.

Recently discovered (CaFe${}_{1-x}$Pt${}_x$As)${}_{10}$Pt${}_3$As${}_8$ and (CaFeAs)${}_{10}$Pt${}_{4-y}$As${}_8$ superconductors are very similar materials having the same elemental composition and structurally similar superconducting FeAs slabs. Yet the maximal critical temperature achieved by changing Pt concentration is approximately twice higher in the latter. Using angle-resolved photoemission spectroscopy (ARPES) we compare the electronic structure of their optimally doped compounds and find drastic differences. Our results highlight the sensitivity of critical temperature to the details of fermiology and point to the decisive role of band-edge singularities in the mechanism of high-$T_c$ superconductivity.

The critical behavior of the $O\left(2\right)$ model on dilute Lévy graphs built on a two-dimensional square lattice is analyzed. Different qualitative cases are probed, varying the exponent $\rho$ governing the dependence on the distance of the connectivity probability distribution. The mean-field regime, as well as the long-range and short-range non-mean-field regimes, are investigated by means of high-performance parallel Monte Carlo numerical simulations running on GPUs. The relationship between the long-range $\rho$ exponent and the effective dimension of an equivalent short-range system with the same critical behavior is investigated. Evidence is provided for the effective short-range dimension to coincide with the spectral dimension of the Lévy graph for the $\mathit{XY}$ model in the mean-field regime.

We present an analytic solution for heat flow in a multilayer two-channel system for the interpretation of time-domain thermoreflectance (TDTR) experiments where nonequilibrium effects are important. The two-channel solution is used to analyze new room temperature TDTR measurements of Al/Cu and Al/Si${}_{0.99}$Ge${}_{0.01}$ systems. Cu and Si${}_{0.99}$Ge${}_{0.01}$ are examples of materials well suited for analysis with a two-channel model because thermal excitations responsible for the vast majority of the heat capacity in these solids contribute little to their thermal conductivity. Nonequilibrium effects are found to be unimportant for the interpretation of the Al/Cu TDTR data but dramatic for the Al/Si${}_{0.99}$Ge${}_{0.01}$ TDTR data. The two-channel model predicts a significant reduction in the effective thermal conductivity of Si${}_{0.99}$Ge${}_{0.01}$ in a region within 150 nm of the Al/Si${}_{0.99}$Ge${}_{0.01}$ interface. The extra thermal resistance in this region is a result of the disparate heat flux boundary conditions for low- and high-frequency phonons in combination with weak coupling between low- and high-frequency phonons. When the experimental data are analyzed with a single-channel model, both the conductance and thermal conductivity appear to depend on pump-modulation frequency, consistent with the two-channel model's predictions. Finally, we compare the results of our diffusive two-channel model to a nonlocal description for steady-state heat flow near a boundary and show they yield nearly identical results.

The interplay between phonon-isotope and phonon-phonon scattering in determining lattice thermal conductivities in semiconductors and insulators is examined using an ab initio Boltzmann transport equation approach. We identify materials with large enhancements to their thermal conductivities with isotopic purification, known as the isotope effect, and we focus in particular on results for beryllium-VI compounds and cubic germanium carbide. We find that germanium carbide and beryllium selenide have very large room temperature isotope effects of 450$\%$, far larger than in any other material. Thus, isotopic purification in these materials gives surprisingly high intrinsic room temperature thermal conductivities, over 1500 Wm${}^{-1}$ K${}^{-1}$ for germanium carbide and over 600 Wm${}^{-1}$ K${}^{-1}$ for beryllium selenide, well above those of the best metals. In compound semiconductors, a large mass ratio of the constituent atoms and large isotope mixture for the heavier atom gives enhanced isotope scattering. A frequency gap between acoustic and optic phonons (also due to a large mass ratio) and bunching of the acoustic phonon branches give weak anharmonic scattering. Combined, weak anharmonic phonon scattering and strong isotope scattering give a large isotope effect in the materials examined here. The physical insights discussed in this work will help guide the efficient manipulation of thermal transport properties of compound semiconductors through isotopic modification.

We investigate the effect of large in-plane strain and vertical electric fields on the binding energies of excitonic complexes confined in single InGaAs/GaAs quantum dots (QDs) and we find that the two independently tunable perturbations modify the interaction energies among electrons and holes in a different manner. By taking advantage of this difference, we frequency-lock the QD fundamental excitation (the neutral exciton) at a predefined value, while the biexciton transition is actively tuned from a binding to an antibinding configuration. Our electrically controlled dual-knob device demonstrates unprecedented control over the electronic properties of the few-particle states in a QD and may be applied to create novel energy-tunable sources of entangled photons using the time-reordering or the time-bin scheme.

The universal scaling function $W_{+-}\left(y\right)$ of the Casimir force ($y$ is a temperature-dependent variable) away from the critical point for thermodynamic systems in the Ising universality class confined between two parallel plates with antisymmetric boundary conditions [denoted $\left(ab\right)=(+-)$] has been analyzed using the extended de Gennes–Fisher local-functional method. Results on the universal function $W_{+-}\left(y\right)$ are presented in spatial dimension $d=3$ applying the extended sine parametric model for temperatures $T>T_R$, where $T_R$ is the critical temperature of the roughening transition. The asymptotic behavior of $W_{+-}\left(y\right)$ for large values of the scaling variable $y$, $y\to \pm \infty$, is analyzed in general dimension $d$ for $T\gg T_c$, where $T_c$ is the bulk critical temperature, and in $d\ge 3$ for $T\ll T_R$.

Controlled defect creation is a prerequisite for the detailed study of disorder effects in materials. Here, we irradiate a graphene/Ir(111) interface with low-energy Ar${}^{+}$ to study the induced structural changes. Combining computer simulations and scanning-probe microscopy, we show that the resulting disorder manifests mainly in the forms of intercalated metal adatoms and vacancy-type defects in graphene. One prominent feature at higher irradiation energies (from 1 keV up) is the formation of linelike depressions, which consist of sequential graphene defects created by the ion channeling within the interface, much like a stone skipping on water. Lower energies result in simpler defects, down to 100 eV, where more than one defect in every three is a graphene single vacancy.

Recently, it has been suggested that topological superconductivity and Majorana end states can be realized in a chain of magnetic impurities on the surface of an $s$-wave superconductor when the magnetic moments form a spin helix as a result of the RKKY interaction mediated by the superconducting substrate. Here, we investigate this scenario theoretically by developing a tight-binding Bogoliubov-de Gennes description starting from the Shiba bound states induced by the individual magnetic impurities. While the resulting model Hamiltonian has similarities with the Kitaev model for one-dimensional spinless $p$-wave superconductors, there are also important differences, most notably the long-range nature of hopping and pairing as well as the complex hopping amplitudes. We use both analytical and numerical approaches to explore the consequences of these differences for the phase diagram and the localization properties of the Majorana end states when the Shiba chain is in a topological superconducting phase.

We report here an approach to tune efficiently the phase difference of light in two orthogonal directions, $\Delta \phi$, by controlling the time retardation with a microstructured surface made of L-shaped metallic patterns. The time retardation through the judicious microstructures can be adjusted on the femtosecond level, and $\Delta \phi$ can be linearly tuned accurately from $-{180}^{\circ }$ to 180${}^{\circ }$ by changing the frequency of incident light. Particularly, the amplitudes in two orthogonal directions are identical, so that the polarization state always locates on a meridian of a Poincaré sphere. Near-field measurement confirms that there is indeed time retardation between the oscillations in the orthogonal directions of the L-shaped patterns. This approach offers an alternative way to manipulate the polarization state of light.