In a charge-tunable device, we investigate the fine-structure splitting of neutral excitons in single long-wavelength (1.1 < $\lambda$ < 1.3 $\mu$m) InGaAs quantum dots as a function of external uniaxial strain. Nominal fine-structure splittings between 16 and 136 $\mu$eV are measured and manipulated. We observe varied responses of the splitting to the external strain, including positive and negative tuning slopes, different tuning ranges, and linear and parabolic dependencies, indicating that these physical parameters depend strongly on the unique microscopic structure of the individual quantum dot. To better understand the experimental results, we apply a phenomenological model describing the exciton polarization and fine-structure splitting under uniaxial strain. The model predicts that, with an increased experimental strain tuning range, the fine structure can be effectively canceled for select telecom-wavelength dots using uniaxial strain. These results are promising for the generation of on-demand entangled photon pairs at telecom wavelengths.

The excitation density dependence of photoluminescence (PL) blinking of single CdSe/ZnS quantum dots was studied by means of single-dot PL spectroscopy. As the excitation density increases, an intermediate state appears in addition to the highly emissive (ON) and nonemissive (OFF) states. The systematic study of the excitation density dependence of PL blinking behavior (PL intensity, probabilities of finding these states, time probability distributions, and PL spectra and lifetimes) suggests that the intermediate state can be attributed to a charged exciton (trion) state generated through biexciton generation and charging (ionization) of a quantum dot under a high-density excitation regime. Our results indicate that the biexciton generation is a precursor of the trion state and not of the OFF state.

Field emission resonances (FERs) on Cu(100) surface are investigated by means of tunneling regime simulations performed with a plane-wave based transport calculation method. FERs are located near the surface and decay into the vacuum, and their accurate simulation requires a faithful description of vacuum states. This type of simulations is thus not possible using the popular transport methods based on atom-centered localized basis sets and the use of plane waves becomes important. We introduce a procedure to treat self-consistently (SC) the finite bias nonequilibrium problem in tunneling regime. Image potential effects are included in a semiempirical way within the SC calculation. Tunneling through FERs is studied following a practical strategy to approximate the inelastic transmission for states lying in the band gap of the surface. As our approach permits the use of any tip geometry, tip effects on the energy and wave functions of FERs are explored. The method reported here provides an ideal tool for the simulation of FERs aimed at the understanding of experimental STS (scanning tunneling spectroscopy) observations.

We theoretically study subgap states appearing at the interface between two three-dimensional topological insulators which have different configurations in the spin-orbit interactions from each other. The coupling of spin $\mathit{\sigma }$ with momenta $\mathit{p}$ is configured by a material dependent $3\times 3$ matrix $\mathit{\Lambda }$ as ${\sigma }^{\mu }{\Lambda }_{\mu }^{\nu }{p}_{\nu }$. The spectra of the interface subgap states depend on the relative choices of $\mathit{\Lambda }$'s in the two topological insulators where the two $\mathit{\Lambda }$ are connected by the unitary transformation including the inversion and the rotation in momentum space. The gapless states appear at the interface when the transformation includes the inversion. The two topological insulators can be distinguished by using the topological numbers defined in the two- or one-dimensional partial Brillouin zone, which explains the presence of the gapless interface states. We also discuss the robustness of such gapless states under perturbations breaking the time-reversal symmetry or the band-inversion symmetry.

The electronic, structural, and hyperfine properties of pure and Cd-doped lanthanide (Ln) sesquioxides with the cubic bixbyite structure (Ln${}_2$O${}_3$, Ln ranging from La to Lu) have been studied using the full-potential augmented plane wave plus local orbital (APW+lo) method within the local spin density approximation (LSDA) and the Coulomb-corrected LSDA+$U$. In the case of the pure systems, our calculations show that LSDA+$U$ gives a better representation of the band structure compared to LSDA. The predicted equilibrium structures and the electric field gradient (EFG) tensor at Ln sites were calculated and compared with those obtained by means of hyperfine techniques and with theoretical results obtained in In${}_2$O${}_3$, Sc${}_2$O${}_3$, and Lu${}_2$O${}_3$ reported in the literature. The origin of the EFG at Ln sites and the role played by the 4$f$ electrons on this quantity are discussed. In the case of the Cd-doped systems, the APW+lo method (also within LSDA and LSDA+$U$) was applied to treat the electronic structure of the doped system. The role of the Ln 4$f$ electrons on the EFG at Cd impurity sites, and other variables like structural distortions induced by the Cd impurity, were investigated in detail and are discussed and compared with available experimental results. An excellent agreement between the experimental and calculated EFGs was found for all Cd-doped systems.

The deposition of a high-$\kappa$ oxide overlayer is known to significantly enhance the room-temperature electron mobility in single-layer MoS${}_2$ (SLM) but not in single-layer graphene. We give a quantitative account of how this mobility enhancement is due to the nondegeneracy of the two-dimensional electron gas system in SLM at accessible temperatures. Using our charged impurity scattering model [Ong and Fischetti, Phys. Rev. B 86, 121409 (2012)] and temperature-dependent polarizability, we calculate the charged impurity-limited mobility (${\mu }_{\mathrm{imp}}$) in SLM with and without a high-$\kappa$ (HfO${}_2$) top-gate oxide at different electron densities and temperatures. We find that the mobility enhancement is larger at low electron densities and high temperatures because of finite-temperature screening, thus explaining the enhancement of the mobility observed at room temperature. ${\mu }_{\mathrm{imp}}$ is shown to decrease significantly with increasing temperature, suggesting that the strong temperature dependence of measured mobilities should not be interpreted as being solely due to inelastic scattering with phonons. We also reproduce the recently seen experimental trend in which the temperature scaling exponent ($\gamma$) of ${\mu }_{\mathrm{imp}}\propto T^{-\gamma }$ is smaller in top-gated SLM than in bare SLM. Finally, we show that $\sim 37\%$ mobility enhancement can be achieved by reducing the HfO${}_2$ thickness from 20 to 2 nm.

The field of charged impurities in narrow band-gap semiconductors and Weyl semimetals can create electron-hole pairs when the total charge $Ze$ of the impurity exceeds a value $Z_{c}e.$ The particles of one charge escape to infinity, leaving a screening space charge. The result is that the observable dimensionless impurity charge $Q_{\infty }$ is less than $Z$ but greater than $Z_c$. There is a corresponding effect for nuclei with $Z>Z_c\approx 170$, however, in the condensed matter setting we find $Z_c\simeq 10$. Thomas-Fermi theory indicates that $Q_{\infty }=0$ for the Weyl semimetal, but we argue that this is a defect of the theory. For the case of a highly-charged recombination center in a narrow band-gap semiconductor (or of a supercharged nucleus), the observable charge takes on a nearly universal value. In Weyl semimetals, the observable charge takes on the universal value $Q_{\infty }=Z_c$ set by the reciprocal of material's fine structure constant.

The Casimir and electromagnetic interactions between objects at short separations are strongly influenced by the local geometry near the point of closest approach. In this paper we demonstrate that the assumptions underlying common statistical analyses of roughness may not hold in experiments using microspheres as interacting objects. Based on an extensive experimental and numerical analysis of the surface topology of the widely used colloidal particle types 4310A and 4320A, we show that the actual variation in the local surface curvature may give rise to large uncertainties in the comparison of experimental data to theories.

We will consider the role played by electron-vibration and electron-electron interactions, through Jahn-Teller (JT) and Coulomb interactions, respectively, in icosahedral systems in which triplet electronic states are coupled to $h_g$-type vibrations. Starting from the electronic terms that arise from consideration of Coulomb interactions, we introduce JT couplings both within the terms and between nondegenerate terms. We show how the symmetry of the JT distortion can change when extra electrons are added, and give the conditions under which JT distortions can be suppressed entirely when the Coulomb interactions are sufficiently large. The relevance of our results to anions of the fullerene molecule C${}_{60}$ are briefly discussed, and existing experimental measurements are used to estimate values for the quadratic JT coupling constants for these anions.

We study theoretically the Raman spectrum of the rotated double-layer graphene, consisting of two graphene layers rotated with respect to each other by an arbitrary angle $\theta$. We find a relatively simple dependence of the Raman $G$ peak intensity on the angle $\theta$. On the other hand, the Raman $2D$ peak position, intensity, and width show a much more complicated dependence on the angle $\theta$. We account for all of these effects, including dependence on the incoming photon energy, in good agreement with the experimental data. We find that it is sufficient to include the interaction between the graphene layers on the electronic degrees of freedom (resulting in the occurrence of Van Hove singularities in the density of states). We assume that the phonon degrees of freedom are unaffected by the interaction between the layers. Furthermore, we decompose the Raman $2D$ peak into two components having very different linewidths; these widths are almost independent of the angle $\theta$. The change in the intensity and the peak position of one of these two components gives insight into the dependence of the overall Raman $2D$ peak features as a function of the angle $\theta$. Furthermore, we study the influence of the coherence on the Raman signal, and we separately study the influence of the interaction between the layers on the electron wave functions and energies. Additionally, we show regions in the phonon spectrum giving rise to the Raman $2D$ peak signal. This work provides an insight into the interplay between the mechanical degree of freedom (angle $\theta$) and the electronic degrees of freedom (singularities in the density of states) in rotated double-layer graphene. Additionally, this work provides a way to establish experimentally the value of the rotation angle $\theta$ using Raman spectroscopy measurement. This procedure becomes even more robust if one repeats the Raman spectroscopy measurement with a different incoming photon energy.

We study the influence of defects (neutral Li${}^0$ and F${}^0$) on the extreme ultraviolet (XUV) laser-induced ablation of 10-nm-thick LiF films. Our method combines a molecular-dynamics scheme for LiF and a set of equations describing the temporal evolution of the conduction-electron density and temperature. We find that defects may decrease the ablation threshold by a factor of around 4. This is caused by the lattice destabilization and the tensile pressure the defects induce upon creation. Metallic colloids form due to the high mobility of Li${}^0$ in the heated crystal. Inhomogeneous defect distributions are shown to be even more effective for crack formation and ablation. In the extreme case, when the thermal heating induced by the laser is negligible and only the effect of defect formation is considered (“cold ablation”), LiF spalls in the solid state.

In magnetically ordered systems, the breaking of SU(2) symmetry in the thermodynamic limit is associated with the appearance of a special type of low-lying excitations in finite-size energy spectra, the so-called tower of states (TOS). In the present work, we numerically demonstrate that there is a correspondence between the SU(2) tower of states and the lower part of the ground-state entanglement spectrum (ES). Using state-of-the-art density matrix renormalization group (DMRG) calculations, we examine the ES of the 2D antiferromagnetic $J_1$-$J_2$ Heisenberg model on both the triangular and kagome lattice. At large ferromagnetic $J_2$, the model exhibits a magnetically ordered ground state. Correspondingly, its ES contains a family of low-lying levels that are reminiscent of the energy tower of states. Their behavior (level counting, finite-size scaling in the thermodynamic limit) sharply reflects TOS features, and is characterized in terms of an effective entanglement Hamiltonian that we provide. At large system sizes, TOS levels are divided from the rest by an entanglement gap. Our analysis suggests that (TOS) entanglement spectroscopy provides an alternative tool for detecting and characterizing SU(2)-broken phases using DMRG.

LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which find a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin fluctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase diagram is indeed observed in the isostructural NaFeAs compound. Previous angle-resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas–van Alphen (dHvA) measurements which detected a few, electron-like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results reconcile the apparent discrepancy between ARPES and the previous dHvA results. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor an $s$-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP suggest that the nodes in the gap function of LiFeP might be located on the hole pockets. This would be difficult to reconcile with the current understanding of the $s\pm$ scenario.

The effects of phonon anharmonicity, phonon-magnon, and electron-phonon interactions on the temperature dependence of Raman optical phonon modes are theoretically investigated. Besides the Klemens result for the phonon width due to anharmonicity, the corresponding line shift is derived. We argue that the phonon decay into two magnons has very low intensity in ferromagnets with low Curie temperatures. Therefore, the electron interband transitions accompanied with the ferromagnetic ordering are included in considerations to get a good quantitative agreement with experiments.

We have studied the temperature-pressure phase diagram of CeCoSi by electrical-resistivity experiments under pressure. Our measurements revealed a very unusual phase diagram. While at low pressures no dramatic changes and only a slight shift of the Neél temperature $T_N$ ($\approx$10 K) are observed, at about 1.45 GPa a sharp and large anomaly, indicative of the opening of a spin-density wave gap, appears at a comparatively high temperature $T_S\approx 38$ K. With further increasing pressure, $T_S$ shifts rapidly to low temperatures and disappears at about 2.15 GPa, likely continuously in a quantum critical point, but without evidence for superconductivity. Even more surprisingly, we observed a clear shift of $T_S$ to higher temperatures upon applying a magnetic field. We discuss two possible origins for $T_S$: magnetic ordering of Co and a metaorbital type of transition of Ce.

The rational design of materials for organic electronics and spintronics has been a long-standing challenge. Here, we show by theoretical means how to achieve Fermi level engineering in an organic semiconductor like for instance a metal phthalocyanine. It turns out that substitution of the peripheral hydrogen atoms by either fluorine atoms or methoxy groups allows to tune the Fermi level by about 1.5 eV. Finally, we show how such tuning could be exploited to achieve a systematic way to manufacture charge and spin transfer materials by studying the system (MeO)${}_8$CoPc/F${}_{16}$CoPc.

Focusing on the efficient probe and manipulation of single-particle spin states, we investigate the coupled spin and orbital dynamics of a spin-1/2 particle in a harmonic potential subject to ultrastrong spin-orbit interaction and external magnetic field. The advantage of these systems is the clear visualization of the strong spin-orbit coupling in the orbital dynamics. We also investigate the effect of a time-dependent coupling: Its nonadiabatic change causes an interesting interplay of spin and orbital motion which is related to the direction and magnitude of the applied magnetic field. This result suggests that orbital state manipulation can be realized through ultrastrong spin-orbit interactions, becoming a useful tool for handling entangled spin and orbital degrees of freedom to produce, for example, spin-desirable polarizations in time that are interesting for spintronics implementations.

We study zigzag interfaces between insulating compounds that are isostructural to graphene, specifically II-VI, III-V, and IV-IV two-dimensional honeycomb insulators. We show that these one-dimensional interfaces are polar, with a net density of excess charge that can be simply determined by using the ideal (integer) formal valence charges, regardless of the predominant covalent character of the bonding in these materials. We justify this finding on fundamental physical grounds by analyzing the topology of the formal polarization lattice in the parent bulk materials. First-principles calculations elucidate an electronic compensation mechanism not dissimilar to oxide interfaces, which is triggered by a Zener-like charge transfer between interfaces of opposite polarity. In particular, we predict the emergence of one-dimensional electron and hole gases, which in some cases are ferromagnetic half metallic.

We demonstrate that the spin decoherence of nitrogen vacancy (NV) centers in diamond can be suppressed by a transverse magnetic field if the electron spin bath is the primary decoherence source. The NV spin coherence, created in “a decoherence-free subspace,” is protected by the transverse component of the zero-field splitting, increasing the spin-coherence time about twofold. The decoherence due to the electron spin bath is also suppressed at magnetic fields stronger than ∼25 G when applied parallel to the NV symmetry axis. Our method can be used to extend the spin-coherence time of similar spin systems for applications in quantum computing, field sensing, and other metrologies.