In this paper we demonstrate theoretically how to use an external laser field to control the population inversion of a single quantum dot exciton qubit in a nanocavity. We consider the Jaynes-Cummings model to describe the system, and the incoherent losses were taken into account by using Lindblad operators. We have demonstrated how to prepare the initial state in a superposition of the exciton in the ground state and the cavity in a coherent state. The effects of exciton-cavity detuning, the laser-cavity detunings, the pulse area, and losses over the qubit dynamics are analyzed. We also show how to use a continuous laser pumping in resonance with the cavity mode to sustain a coherent state inside the cavity, providing some protection to the qubit against cavity loss.

The temperature-dependence of the direct band gap and thermal expansion in the metastable anti-${\mathrm{ReO}}_3$ semiconductor ${\mathrm{Cu}}_3\mathrm{N}$ are investigated between 4.2 and 300 K by Fourier-transform infrared spectroscopy and x-ray diffraction. Complementary refractive index spectra are determined by spectroscopic ellipsometry at $300\mathrm{K}$. A direct gap of $1.68\mathrm{eV}$ is associated with the absorption onset at $300\mathrm{K}$, which strengthens continuously and reaches a magnitude of $3.5\times {10}^5{\mathrm{cm}}^{-1}$ at $2.7\mathrm{eV}$, suggesting potential for photovoltaic applications. Notably, the direct gap redshifts by just $24\mathrm{meV}$ between 4.2 and $300\mathrm{K}$, giving an atypically small band-gap temperature coefficient $d{E}_{\text{g}}/dT$ of $-0.082\mathrm{meV}/\mathrm{K}$. Additionally, the band structure, dielectric function, phonon dispersion, linear expansion, and heat capacity are calculated using density functional theory; remarkable similarities between the experimental and calculated refractive index spectra support the accuracy of these calculations, which indicate beneficially low hole effective masses and potential negative thermal expansion below $50\mathrm{K}$. To assess the lattice expansion contribution to the band-gap temperature-dependence, a quasiharmonic model fit to the observed lattice contraction finds a monotonically decreasing linear expansion (descending past ${10}^{-6}{\mathrm{K}}^{-1}$ below $80\mathrm{K}$), while estimating the Debye temperature, lattice heat capacity, and Grüneisen parameter. Accounting for lattice and electron-phonon contributions to the observed band-gap evolution suggests average phonon energies that are qualitatively consistent with predicted maxima in the phonon density of states. As band-edge temperature-dependence has significant consequences for device performance, copper nitride should be well suited for applications that require a largely temperature-invariant band gap.

We study properties of a periodically driven system coupled to a thermal bath. As a nontrivial example, we consider a periodically driven metallic system coupled to a superconducting bath. The effect of the superconductor on the driven system is twofold: it (a) modifies density of states in the metal via the proximity effect and (b) acts as a thermal bath for light-excited quasiparticles. Using Keldysh formalism, we calculate, nonperturbatively in the system-bath coupling, the steady-state properties of the system and obtain nonequilibrium distribution function. The latter allows one to calculate observable quantities which can be spectroscopically measured in tunneling experiments.

Localized magnetic moments have been predicted to develop in graphene samples with vacancies or adsorbates. The interplay between such magnetic impurities and graphene's Dirac quasiparticles leads to remarkable many-body phenomena, which have, so far, proved elusive to experimental efforts. In this article we study the thermodynamic, spectral, and transport signatures of quantum criticality and Kondo physics of a dilute ensemble of atomic impurities in graphene. We consider vacancies and adatoms that either break or preserve graphene's $C_{3v}$ and inversion symmetries. In a neutral graphene sample, all cases display symmetry-dependent quantum criticality, leading to enhanced impurity scattering for asymmetric impurities, in a manner analogous to bound-state formation by nonmagnetic resonant scatterers. Kondo correlations emerge only in the presence of a back gate, with estimated Kondo temperatures well within the experimentally accessible domain for all impurity types. For symmetry-breaking impurities at charge neutrality, quantum criticality is signaled by $T^{-2}$ resistivity scaling, leading to full insulating behavior at low temperatures, while low-temperature resistivity plateaus appear both in the noncritical and Kondo regimes. By contrast, the resistivity contribution from symmetric vacancies and hollow-site adsorbates vanishes at charge neutrality and for arbitrary back-gate voltages, respectively. This implies that local probing methods are required for the detection of both Kondo and quantum critical signatures in these symmetry-preserving cases.

We investigate the structural, mechanical, and electronic properties of the two-dimensional hexagonal structure of group III-VI binary monolayers, $MX$ ($M=\text{B}$, Al, Ga, In and $X=\text{O}$, S, Se, Te) using first-principles calculations based on the density functional theory. The structural optimization calculations and phonon spectrum analysis indicate that all of the 16 possible binary compounds are thermally stable. In-plane stiffness values cover a range depending on the element types and can be as high as that of graphene, while the calculated bending rigidity is found to be an order of magnitude higher than that of graphene. The obtained electronic band structures show that $MX$ monolayers are indirect band-gap semiconductors. The calculated band gaps span a wide optical spectrum from deep ultraviolet to near infrared. The electronic structure of oxides ($M\mathrm{O}$) is different from the rest because of the high electronegativity of oxygen atoms. The dispersions of the electronic band edges and the nature of bonding between atoms can also be correlated with electronegativities of constituent elements. The unique characteristics of group III-VI binary monolayers can be suitable for high-performance device applications in nanoelectronics and optics.

The dislocation in Dirac semimetal carries an emergent magnetic flux parallel to the dislocation axis. We show that due to the emergent magnetic field, the dislocation accommodates a single fermion massless mode of the corresponding low-energy one-particle Hamiltonian. The mode is propagating along the dislocation with its spin directed parallel to the dislocation axis. In agreement with the chiral anomaly observed in Dirac semimetals, an external electric field results in the spectral flow of the one-particle Hamiltonian, in pumping of the fermionic quasiparticles out from vacuum, and in creating a nonzero axial (chiral) charge in the vicinity of the dislocation.

We study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_4$. The maximum energy of the magnetic excitation remains robust up to $x=0.77$, with a gap opening at low dopings and increasing to over 150 meV at $x=0.77$. At these higher Ru concentrations, the dispersive magnetic excitations in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ are rendered essentially momentum independent. Up to a Ru concentration of $x=0.77$, both experiments and first-principles calculations show the Ir $J_{\text{eff}}=1/2$ state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder. Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.

We demonstrate a scheme for in-plane initialization of a single exciton spin in an InGaAs quantum dot (QD) coupled to a GaAs nanobeam waveguide. The chiral coupling of the QD and the optical mode of the nanobeam enables spin initialization fidelity approaching unity in magnetic field $B=1$ T and $>0.9$ without the field. We further show that this in-plane excitation scheme is independent of the incident excitation laser polarization and depends solely on the excitation direction. This scheme provides a robust in-plane spin excitation basis for a photon-mediated spin network for quantum information applications.

The debate about whether the insulating phases of vanadium dioxide ($\mathrm{V}{\mathrm{O}}_2$) can be described by band theory or whether it requires a theory of strong electron correlations remains unresolved even after decades of research. Energy-band calculations using hybrid exchange functionals or including self-energy corrections account for the insulating or metallic nature of different phases but have not yet successfully accounted for the observed magnetic orderings. Strongly correlated theories have had limited quantitative success. Here we report that by using hard pseudopotentials and an optimized hybrid exchange functional, the energy gaps and magnetic orderings of both monoclinic $\mathrm{V}{\mathrm{O}}_2$ phases and the metallic nature of the high-temperature rutile phase are consistent with available experimental data, obviating an explicit role for strong correlations. We also identify a potential candidate for the newly found metallic monoclinic phase.

In this work we provide for a description of the low-energy physics of interacting multispecies fermions in terms of the bound states that are stabilized in these systems when a spin gap opens. We argue that at energies much smaller than the spin gap, these systems are described by a Luttinger liquid of bound states that depends, on top of the charge stiffness $\nu$ and the charge velocity $u$, on a “Fermi” momentum $P_F$ satisfying $q{P}_F=N{k}_F$ where $q$ is the charge of the bound state, $N$ is the number of species, and $k_F$ is the Fermi momentum in the noninteracting limit. We further argue that for generic interactions, generic bound states are likely to be stabilized. They are associated with emergent, in general nonlocal, symmetries and are in the number of five. The first two consist of either a charge $q=N$ local $\mathrm{SU}\left(N\right)$ singlet or a charge $q=N$ bound state made of two local $\mathrm{SU}\left(p\right)$ and $\mathrm{SU}(N-p)$ singlets. In this case the Fermi momentum $P_F=k_F$ is preserved. The three others have an enhanced Fermi vector $P_F$. The latter are either charge $q=2$ bosonic $p$-wave and $s$-wave pairs with $\mathrm{SO}\left(N\right)$ and $\mathrm{SP}\left(N\right)$ symmetry and $P_F=N{k}_F/2$ or a composite fermion of charge $q=1$ with $P_F=N{k}_F$. The instabilities of these Luttinger liquid states towards incompressible phases and their possible topological nature are also discussed.

We present spatially and spectrally resolved photoluminescence measurements of indirect excitons in high magnetic fields. Long indirect exciton lifetimes give the opportunity to measure magnetoexciton transport by optical imaging. Indirect excitons formed from electrons and holes at zeroth Landau levels ($0_{\mathrm{e}}-0_{\mathrm{h}}$ indirect magnetoexcitons) travel over large distances and form a ring emission pattern around the excitation spot. In contrast, the spatial profiles of $1_{\mathrm{e}}-1_{\mathrm{h}}$ and $2_{\mathrm{e}}-2_{\mathrm{h}}$ indirect magnetoexciton emission closely follow the laser excitation profile. The $0_{\mathrm{e}}-0_{\mathrm{h}}$ indirect magnetoexciton transport distance reduces with increasing magnetic field. These effects are explained in terms of magnetoexciton energy relaxation and effective mass enhancement.

Nanowire quantum dots have peculiar electronic and optical properties. In this work we use atomistic tight binding to study excitonic spectra of artificial molecules formed by a double nanowire quantum dot. We demonstrate a key role of atomistic symmetry and nanowire substrate orientation rather than cylindrical shape symmetry of a nanowire and a molecule. In particular for $\left[001\right]$ nanowire orientation we observe a nonvanishing bright exciton splitting for a quasimolecule formed by two cylindrical quantum dots of different heights. This effect is due to interdot coupling that effectively reduces the overall symmetry, whereas single uncoupled $\left[001\right]$ quantum dots have zero fine structure splitting. We found that the same double quantum dot system grown on $\left[111\right]$ nanowire reveals no excitonic fine structure for all considered quantum dot distances and individual quantum dot heights. Further we demonstrate a pronounced, by several orders of magnitude, increase of the dark exciton optical activity in a quantum dot molecule as compared to a single quantum dot. For $\left[111\right]$ systems we also show spontaneous localization of single particle states in one of nominally identical quantum dots forming a molecule, which is mediated by strain and origins from the lack of the vertical inversion symmetry in $\left[111\right]$ nanostructures of overall $C_{3v}$ symmetry. Finally, we study lowering of symmetry due to alloy randomness that triggers nonzero excitonic fine structure and the dark exciton optical activity in realistic nanowire quantum dot molecules of intermixed composition.

Saturable absorption is a nonperturbative nonlinear optical phenomenon that plays a pivotal role in the generation of ultrafast light pulses. Here we show that this effect emerges in graphene at unprecedentedly low light intensities, thus opening avenues to new nonlinear physics and applications in optical technology. Specifically, we theoretically investigate saturable absorption in extended graphene by developing a semianalytical nonperturbative single-particle approach, describing electron dynamics in the atomically-thin material using the two-dimensional Dirac equation for massless Dirac fermions, which is recast in the form of generalized Bloch equations. By solving the electron dynamics nonperturbatively, we account for both interband and intraband contributions to the intensity-dependent saturated conductivity and conclude that the former dominates regardless of the intrinsic doping state of the material. We obtain results in qualitative agreement with atomistic quantum-mechanical simulations of graphene nanoribbons including electron-electron interactions, finite-size, and higher-band effects. Remarkably, such effects are found to affect mainly the linear absorption, while the predicted saturation intensities are in good quantitative agreement in the limit of extended graphene. Additionally, we find that the modulation depth of saturable absorption in graphene can be electrically manipulated through an externally applied gate voltage. Our results are relevant for the development of graphene-based optoelectronic devices, as well as for applications in mode-locking and random lasers.

An effective medium model is developed for disordered metamaterials containing a spatially random distribution of dielectric spheres. Similar to effective medium models for ordered metamaterials, this model predicts resonances in the effective permeability and permittivity arising from electric- and magnetic-dipole Mie resonances in the spheres. In addition, the model predicts a redshift of the electric resonance with increasing particle loading. Interestingly, when the particle loading exceeds the percolation threshold of 33%, the model predicts that the electric resonance overlaps with the magnetic resonance, resulting in a negative refractive index.

We introduce a Gaussian approximation potential (GAP) for atomistic simulations of liquid and amorphous elemental carbon. Based on a machine learning representation of the density-functional theory (DFT) potential-energy surface, such interatomic potentials enable materials simulations with close-to DFT accuracy but at much lower computational cost. We first determine the maximum accuracy that any finite-range potential can achieve in carbon structures; then, using a hierarchical set of two-, three-, and many-body structural descriptors, we construct a GAP model that can indeed reach the target accuracy. The potential yields accurate energetic and structural properties over a wide range of densities; it also correctly captures the structure of the liquid phases, at variance with a state-of-the-art empirical potential. Exemplary applications of the GAP model to surfaces of “diamondlike” tetrahedral amorphous carbon ($\mathit{ta}$-C) are presented, including an estimate of the amorphous material's surface energy and simulations of high-temperature surface reconstructions (“graphitization”). The presented interatomic potential appears to be promising for realistic and accurate simulations of nanoscale amorphous carbon structures.

We report intrinsic memory effect in magnetization and dielectricity for the spin-chain compound ${\mathrm{Sm}}_2{\mathrm{BaNiO}}_5$, pointing the cooperative glassy response below $\sim 8\mathrm{K}$. Signature of anomaly around 8 K is verified by the magnetization, heat capacity, dielectric permittivity, magnetostriction, and structural parameters as obtained from the synchrotron diffraction studies. Intriguingly, the memory effect is observed well below the magnetic and ferroelectric ordering temperatures, pointing to a reentrant frozen state. Ferroelectricity emerges above antiferromagnetic Néel temperature at 45 K. For 4.5 kV/cm poling field the spontaneous electric polarization attains the value of 1300 $\mu C/{\mathrm{m}}^2$, that is the highest value in the $R_2{\mathrm{BaNiO}}_5$ series. Synchrotron diffraction studies confirm that ferroelectricity emerges due to structural transition from the centrosymmetric $Immm$ to a noncentrosymmetric $Imm2$ space group. Magnetoelectric coupling is significant and scales linearly to the squared magnetization as described by the Ginzburg-Landau theory.