Within a tight binding approach we investigate the energy levels of hexagonal and triangular bilayer graphene (BLG) quantum dots (QDs) with zigzag and armchair edges. We study AA- and AB- (Bernal) stacked BLG QDs and obtain the energy levels in both the absence and the presence of a perpendicular electric field (i.e., biased BLG QDs). Our results show that the size dependence of the energy levels is different from that of monolayer graphene QDs. The energy spectrum of AB-stacked BLG QDs with zigzag edges exhibits edge states which spread out into the opened energy gap in the presence of a perpendicular electric field. We found that the behavior of these edges states is different for the hexagonal and triangular geometries. In the case of AA-stacked BLG QDs, the electron and hole energy levels cross each other in both cases of armchair and zigzag edges as the dot size or the applied bias increases.

We report many-body perturbation theory calculations of excited-state properties of distorted $1T$ diamond-chain monolayer rhenium disulfide (${\mathrm{ReS}}_2$) and diselenide (${\mathrm{ReSe}}_2$). Electronic self-energy substantially enhances their quasiparticle band gaps and, surprisingly, converts monolayer ${\mathrm{ReSe}}_2$ to a direct-gap semiconductor, which was, however, regarded to be an indirect one by density-functional-theory calculations. Their optical absorption spectra are dictated by strongly bound excitons. Unlike hexagonal structures, the lowest-energy bright exciton of distorted $1T{\mathrm{ReS}}_2$ exhibits a perfect figure-eight shape polarization dependence but those of ${\mathrm{ReSe}}_2$ only exhibit a partial polarization dependence, which results from two nearly degenerated bright excitons whose polarization preferences are not aligned. Our first-principles calculations are in excellent agreement with experiments and pave the way for optoelectronic applications.

The topological insulator has an intriguing electronic structure in that it has nontrivial topology enforcing the helical Dirac fermionic states at interfaces to the band insulators. Protected by the time-reversal symmetry and finite band gaps in the bulk, the topology is immune to external nonmagnetic perturbations. One essential question is whether elementary excitations in solids like phonons can trigger a transition in the topological property of the electronic structures. Here we investigate the development of topological insulating phases in IV-VI compounds under dynamic lattice deformations using first-principles calculations. Unlike the static state of topological phases at equilibrium conditions, we show that nontrivial topological phases are induced in the compounds by the dynamic lattice deformations from selective phonon modes. Calculations of the time-reversal polarization show that the $Z_2$ invariant of the compounds is flipped by the selective phonon modes and that the compounds exhibit oscillating topological phases upon dynamic lattice deformations.

We theoretically investigate a quantum dot coupled to fermionic (electronic) leads and show how zero-point quantum fluctuations stemming from bosonic environments permit the rectification of the current. The bosonic baths are either external impedances modeled as tunable transmission lines or LC resonators (single-mode cavities). Voltage fluctuations stemming from the external impedances at zero temperature are described through harmonic oscillators (photonlike excitations) producing the quantum vacuum fluctuations. The differing sizes of the zero-point fluctuations of the quantum vacuum break the spatial symmetry of the system if the quantum dot is coupled to two reservoirs or two junctions with different bosonic environments. We consider current rectification and power production when the system is operated as a heat engine in both nonresonant and resonant sequential tunneling cases.

We model the quantum dynamics of two, three, or four quantum dots (QDs) in proximity to a plasmonic system such as a metal nanoparticle or an array of metal nanoparticles. For all systems, an initial state with only one QD in its excited state evolves spontaneously into a state with entanglement between all pairs of QDs. The entanglement arises from the couplings of the QDs to the dissipative, plasmonic environment. Moreover, we predict that similarly entangled states can be generated in systems with appropriate geometries, starting in their ground states, by exciting the entire system with a single, ultrafast laser pulse. By using a series of repeated pulses, the system can also be prepared in an entangled state at an arbitrary time.

Bilayer graphene is normally a semimetal with parabolic dispersion, but a tunable band gap up to few hundreds meV can be opened by breaking the symmetry between the layers through an external potential. Ab initio calculations show that the optical response around the band gap is strongly dominated by bound excitons, whose characteristics and selection rules differ from the usual excitons found in semiconductor quantum wells. In this work, we study the physics of those excitons resonantly coupled to a photonic microcavity, assessing the possibility to reach the strong and the ultrastrong coupling regimes of light-matter interaction. We discover that both regimes are experimentally accessible, thus opening the way for a most promising technological platform, combining midinfrared quantum polaritonics with the tunability and electronic features of graphene bilayers.

Stacking graphene sheets forms graphite. Two in-plane vibrational modes of graphite, $E_{1u}$ and $E_{2g}^{\left(2\right)}$, are derived from the graphene $E_{2g}$ mode, the shifts of which under compression are considered as results of the in-plane bond shortening. Values of the Grüneisen parameter have been reported to quantify such a relation. However, the reason why the shift rates of these three modes with pressure differ is unclear. In this work, we introduce new parameters ${\gamma }_{E_{2g}}^{\prime }=-0.0131$ and ${\gamma }_{E_{1u}}^{\prime }=0.0585$ to quantify the contribution of out-of-plane strain to the shift of the in-plane vibrational frequencies, suggesting that the compression of the $\pi$- electrons plays a non-negligible part in both graphite and graphene under high pressure.

The magnetic properties of arrays of nanoparticles are determined by the interplay between the individual particle properties and the dipolar interactions between them. Here we present a study of arrays of elliptical Fe(001) particles of thickness 10–50 nm. The aspect ratios of the ellipses are 1:3, their short axes $a=50$, 100, or 150 nm, and the periodicity of the rectangular arrays is either two or four times the corresponding axes of the ellipses. Magnetic measurements together with numerical and micromagnetic calculations yield a consistent picture of the arrays, comprising single-domain nanoparticles. We show that the magnetization reversal, occurring in the range 100–400 mT for fields applied along the long axis, is mainly determined by the properties of the corresponding single Fe ellipses. The interaction fields of the order of tens of mT can be tuned by the array configurations. For the actual arrays the interactions promote switching. For film thicknesses below the Bloch wall width parameter of Fe, ${\mathit{l}}_{\text{w}}=22$ nm, magnetization reversal occurs without formation of domain walls or vortices. Within this range arrays may be tuned to obtain a well-defined switching field. Two general conclusions are drawn from the calculations: the character of the interaction, whether it promotes or delays magnetization reversal, is determined by the aspect ratio of the array grid, and the interaction strength saturates as the size of the array increases.

Metasurfaces characterized by a transverse gradient of local impedance have recently opened exciting directions for light manipulation at the subwavelength scale. Here we add a temporal gradient to the picture, showing that spatiotemporal variations over a surface may greatly extend the degree of wave manipulation in metasurfaces, and break several of their constraints associated with symmetries. As an example, we synthesize a nonreciprocal classical analog to electromagnetically induced transparency, opening a narrow window of one-way efficient transmission in an otherwise opaque surface. These properties pave the way to magnetic-free, planarized, nonreciprocal ultrathin surfaces for free-space isolation.

We combine magnetization, pressure-dependent electrical resistivity, and heat capacity measurements to investigate the physical properties of the novel compound ${\mathrm{UAuBi}}_2$. Our single crystals, grown by the self-flux method, share the same tetragonal ${\mathrm{HfCuSi}}_2$-type structure as their Ce-based counterparts. ${\mathrm{UAuBi}}_2$ shows ferromagnetic ordering at $T_c=22.5$ K, in contrast with the antiferromagnetic transition found in ${\mathrm{CeAuBi}}_2$ $(T_N=12\text{K})$ but closely related to ${\mathrm{UAuSb}}_2$ $(T_c=31\text{K})$. Despite the differences, all compounds display an easy axis of magnetization along the $c$ axis and a large magnetocrystalline anisotropy. The heat capacity and pressure-dependent resistivity suggest that ${\mathrm{UAuBi}}_2$ exhibits moderately heavy-fermion behavior $\left(\gamma \sim 100\mathrm{mJ}/\mathrm{mol}·{\mathrm{K}}^2\right)$ with strongly localized $5f$ electrons. An intricate competition between crystalline electric field (CEF) effects and two anisotropic exchange interactions $\left(J_{\mathrm{RKKY}}\right)$ persists in the $5f$ system, which leads to the striking difference between ground states. A systematic analysis of our macroscopic data using a mean-field model including anisotropic $J_{\mathrm{RKKY}}$ interactions and the tetragonal CEF Hamiltonian allows us to extract the CEF scheme and the values of $J_{\mathrm{RKKY}}$. Our results suggest a general trend in this family of compounds and shed light on the similarities and differences between $4f$ and $5f$ members.

We present optical transmission measurements that reveal a charge gap of 0.86 eV in the local-moment antiferromagnetic insulator ${\mathrm{BaMn}}_2{\mathrm{As}}_2$, an order of magnitude larger than previously reported. Density functional theory plus dynamical mean-field theory ($\text{DFT}+\text{DMFT}$) calculations correctly reproduce this charge gap only when a strong Hund's coupling is considered. Thus, ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ is a member of a wider class of Mn pnictide compounds that are Mott–Hund insulators. We also present optical reflectance for metallic 2%-K-doped ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ that we use to extract the optical conductivity at different temperatures. The optical conductivity ${\sigma }_1\left(\omega \right)$ exhibits a metallic response that is well described by a simple Drude term. Both $\sigma (\omega \to 0,T)$ and $\rho \left(T\right)$ exhibit Fermi-liquid temperature dependencies. From these measurements, we argue that a more strongly correlated Hund-metal version of the parent compounds of the iron pnictide superconductors has not yet been realized by doping this class of Hund insulators.

We present a dynamical mean-field theory study of the charge and orbital correlations in finite-size ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (LCMO) nanoclusters. Upon nanostructuring LCMO to clusters of 3 nm diameter, the size reduction induces an insulator-to-metal transition in the high-temperature paramagnetic phase. This is ascribed to the reduction in charge disproportionation between Mn sites with different nominal valence [H. Das et al., Phys. Rev. Lett. 107, 197202 (2011)]. Here we show that upon further reducing the system size to nanoclusters of a few atoms, quantum confinement effects come into play. These lead to the opposite effect: the nanocluster turns insulating again and the charge disproportionation between Mn sites and the orbital polarization are enhanced. Electron doping by means of external gate voltage on few-atom nanoclusters is found to trigger a site- and orbital-selective Mott transition. Our results suggest that LCMO nanoclusters could be employed for the realization of technological devices, exploiting the proximity to the Mott transition and its control by size and gate voltage.

The efficiency of silicon (Si)-based solar cells is strongly affected by crystal defects and impurities. Metallic impurities, in particular interstitial iron (Fe) atoms, cause large electric losses because they act as recombination centers for photogenerated charge carriers. Here, we present a systematic first-principles density functional theory (DFT) study focusing on the influence of hydrostatic, uniaxial, and shear strains on the thermodynamic stability and the diffusivity of Fe impurities in crystalline Si. Our calculations show that the formation energy of neutral Fe interstitials in tetrahedral interstitial sites is almost unaffected by uniform deformations of the Si crystal up to strains of 5%. In contrast, the migration barrier varies significantly with strain, especially for hydrostatic deformation. In order to determine effective diffusion coefficients for different strain states, a kinetic Monte Carlo (kMC) model was set up based on the activation energy barriers and frequency factors obtained from the DFT simulations. By using the strain dependence of the migration barrier, we examined the migration of Fe interstitials in the vicinity of perfect $1/2\langle 110\rangle$ screw and ${60}^{\circ }$ mixed dislocations, and $1/6\langle 112\rangle {90}^{\circ }$ and ${30}^{\circ }$ partial dislocations. While the strain field of the perfect screw dislocation always enhances the local Fe diffusion, the existence of tensile and compressive regions around the ${60}^{\circ }$ mixed dislocation results in a strong anisotropic diffusion profile with significantly faster and slower diffusivities on its tensile and compressive sides. The influences of the partial dislocations are qualitatively similar to that of the ${60}^{\circ }$ mixed dislocation.

We show with systematic photoemission spectroscopy and scanning tunneling spectroscopy data that a spin helical surface state appears on the (110) surface of noncentrosymmetric, three-dimensional HgTe. The topological surface state in HgTe exhibits sharp, linear dispersion without $k_z$ variation, as well as clear, left-right imbalanced spin polarization and circular dichroism. Chemical gating by alkali metal deposition on the surface causes the unexpected opening and/or increase of a surface insulating gap without changing its topological property. Such an unusual behavior we uncover in three-dimensional HgTe sheds light on a convenient control of the Fermi surface and quantum transport in a topological insulator.

Energy transfer from electrons to phonons is an important consideration in any Weyl- or Dirac-semimetal-based application. In this work, we analytically calculate the cooling power of acoustic phonons, i.e., the energy relaxation rate of electrons which are interacting with acoustic phonons, for Weyl and Dirac semimetals in a variety of different situations. For cold Weyl or Dirac semimetals with the Fermi energy at the nodal points, we find that the electronic temperature $T_e$ decays in time as a power law. In the heavily doped regime, $T_e$ decays linearly in time far from equilibrium. In a heavily doped system with short-range disorder we predict that the cooling power of acoustic phonons is drastically increased because of an enhanced energy transfer between electrons and phonons. When an external magnetic field is applied to an undoped system, the cooling power is linear in magnetic field strength and $T_e$ has square-root decay in time, independent of magnetic-field strength over a range of values.

We introduce transition operators that in a given basis of the single-site states of a many-body system have a single nonvanishing matrix element and introduce their correlation functions. We show that they fall into groups that decay with the same rate. The mutual information defined in terms of the von Neumann entropy between two sites is given in terms of these so-called generalized correlation functions. We confirm numerically that the long-distance decay of the mutual information follows the square of that of the most slowly decaying generalized correlation function. The main advantage of our procedure is that, in order to identify the most relevant physical processes, there is no need to know a priori the nature of the ordering in the system, i.e., no need to explicitly construct particular physical correlation functions. We explore the behavior of the mutual information and the generalized correlation functions for comformally invariant models and for the $\text{SU}\left(n\right)$ Hubbard model with $n=2,3,4$, and 5, which are, in general, not conformally invariant. In this latter case, we show that for filling $f=1/q$ and $q<n$, the ground state consists of highly entangled $q$-site units that are further entangled by single bonds. In addition, we extend the picture of the two-site mutual information and the corresponding generalized correlation functions to the $n$-site case.

Three-dimensional Weyl and Dirac semimetals can support a chiral-symmetry-breaking, fully gapped, charge-density-wave order even for sufficiently weak repulsive electron-electron interactions, when placed in strong magnetic fields. In the former systems, due to the natural momentum space separation of Weyl nodes the ordered phase lacks the translational symmetry and represents an axionic phase of matter, while that in a Dirac semimetal (neglecting the Zeeman coupling) is only a trivial insulator. We present the scaling of this spectral gap for a wide range of subcritical (weak) interactions as well as that of the diamagnetic susceptibility with the magnetic field. A similar mechanism for charge-density-wave ordering at weak coupling is shown to be operative in double- and triple-Weyl semimetals, where the dispersion is linear (quadratic and cubic, respectively) for the $z$ (planar) component(s) of the momentum. We here also address the competition between the charge-density-wave and a spin-density-wave orders, both of which breaks the chiral symmetry and leads to gapped spectrum, and show that at least in the weak coupling regime the former is energetically favored. The anomalous surface Hall conductivity, role of topological defects such as axion strings, existence of one-dimensional gapless dispersive modes along the core of such defects, and anomaly cancellation through the Callan-Harvey mechanism are discussed.

We study the charge-density-wave phase in ${\text{TiSe}}_2$ by using first-principles density functional theory calculations with the harmonic approximation for the electron-phonon coupling. We consider several local functionals and both experimental and theoretical cell parameters. The results obtained are very sensitive to the cell parameters used. However, we show that, if the experimental cell is used, harmonic calculations are able to reproduce not only the structural instability of ${\text{TiSe}}_2$ but also the effective distortion observed in the experiments, irrespective of the local functional used. If the experimental cell is used, the energy profile obtained by displacing the atoms is independent of the local functional considered too. With the semiempirical functional Grimme B97-D, aimed at describing better the van der Waals forces coupling the ${\text{TiSe}}_2$ layers, the theoretical cell is in agreement with the experimental one and the structural analysis gives results analogous to the ones obtained with the experimental cell. We also present a study of the electronic structure evolution under the charge-density-wave deformation. In particular, we apply the unfolding technique in order to compare the calculated energy bands for the distorted structure with angle-resolved photoemission spectroscopy (ARPES) data taken at low temperature. In order to obtain a better agreement between ARPES and the calculated bands, both at high and low temperature, we investigate the effect of the correlation on the electrons of the localized Ti-$3d$ orbitals by using the LDA + $U$ method. We show that within this approximation the electronic bands for both the undistorted and distorted structure are in good agreement with ARPES. On the other hand, $U$ eliminates the phonon instability of the system. A possible explanation for this counterintuitive result is proposed. Particularly, the possibility of taking into account the dependence of the parameter $U$ on the atomic positions is suggested.