We propose an initial-state-dependent quantum-dot gain without population inversion in the vicinity of a resonant metallic nanoparticle. The gain originates from the hybridization of a dark plasmon-exciton and is accompanied by efficient energy transfer from the nanoparticle to the quantum dot. This hybridization of the dark plasmon-exciton, attached to the hybridization of the bright plasmon-exciton, strengthens nonlinear light-quantum emitter interactions at the nanoscale, thus the spectral overlap between the dark and the bright plasmons enhances the gain effect. This hybrid system has potential applications in ultracompact tunable quantum devices.

Based on a structure consisting of a single graphene layer situated on periodic dielectric gratings, we show theoretically that terahertz radiation can be generated by low-energy electron bunches moving atop the graphene layer. The THz emission arises from graphene plasmons excited efficiently by the moving electrons. We find that the radiation intensity can be strongly enhanced due to the local field enhancement of graphene plasmons arising from their low losses and high confinement. Importantly, the radiation frequency can be tuned over a wide spectral range by varying the Fermi level of the graphene layer. Our results could find applications in developing tunable and miniature free-electron terahertz radiation sources.

We use a single-ion electric-field noise sensor in combination with in situ surface treatment and analysis tools, to investigate the relationship between electric-field noise from metal surfaces in vacuum and the composition of the surface. These experiments are performed in a setup that integrates ion trapping capabilities with surface analysis tools. We find that treatment of an aluminum-copper surface with energetic argon ions significantly reduces the level of room-temperature electric-field noise, but the surface does not need to be atomically clean to show noise levels comparable to those of the best cryogenic traps. The noise levels after treatment are low enough to allow fault-tolerant trapped-ion quantum information processing on a microfabricated surface trap at room temperature.

We present a refined model of the $\left\{{\mathrm{Fe}}_9\right\}$ tridiminished icosahedron magnetic molecule system. This molecule was originally modeled as being composed of two ($\left\{{\mathrm{Fe}}_3\right\}$ and $\left\{{\mathrm{Fe}}_6\right\}$) clusters, with the ${\mathrm{Fe}}^{3+}$ ions within each cluster being coupled via exchange interactions, but with no coupling between the clusters. The present inelastic neutron scattering (INS) measurements were used to probe the low-lying energy spectrum of $\left\{{\mathrm{Fe}}_9\right\}$, and these results demonstrate that the previously published model of two uncoupled clusters is incomplete. To achieve agreement between the experiment and theory, we have augmented the model with relatively small exchange coupling between the clusters. A combination of Lanczos matrix diagonalization and quantum Monte Carlo simulations have been used to achieve good agreement between the experimental data and the improved model of the full $\left\{{\mathrm{Fe}}_9\right\}$ system despite the complexity of this model (with Hilbert space dimension $>{10}^7$).

The magnetic phase transition near $T_0=8.5$ K in americium dioxide (${\mathrm{AmO}}_2$) has been investigated microscopically by means of ${}^{17}\mathrm{O}$ NMR. To avoid complexities arising from sample aging associated with the $\alpha$ decay of ${}^{243}\mathrm{Am}$, all measurements have been performed within 40 days after sample synthesis. Even during such a short period, however, a rapid change of NMR line shape has been observed at 1.5 K, suggesting that the ground state of ${\mathrm{AmO}}_2$ is very sensitive to disorder. We have also confirmed the loss of ${}^{17}\mathrm{O}$ NMR signal intensity over a wide temperature range below $T_0$, and more than half of oxygen nuclei are undetectable at 1.5 K. This behavior reveals the persistence of slow and distributed spin fluctuations down to temperatures well below $T_0$. In the paramagnetic state, strong NMR line broadening and spatially inhomogeneous spin fluctuations have been observed. The results are all indicative of short-range, spin-glass-like character for the magnetic transition in this system.

We report hard (14 keV) x-ray diffraction measurements on three compositions ($x=0.11,0.12,0.13)$ of the high-temperature superconductor ${\text{La}}_{2-x}$${\text{Sr}}_x$${\text{CuO}}_4$. All samples show charge density wave (CDW) order with onset temperatures in the range 51–80 K and ordering wave vectors close to (0.23,0,0.5). The CDW is strongest with the longest in-plane correlation length near 1/8 doping. On entering the superconducting state the CDW is suppressed, demonstrating the strong competition between the charge order and superconductivity. CDW order coexists with incommensurate magnetic order and the wave vectors of the two modulations have the simple relationship ${\mathit{\delta }}_{\mathrm{charge}}=2{\mathit{\delta }}_{\mathrm{spin}}$. The intensity of the CDW Bragg peak tracks the intensity of the low-energy (quasielastic) spin fluctuations. We present a phase diagram of ${\text{La}}_{2-x}$${\text{Sr}}_x$${\text{CuO}}_4$ including the pseudogap phase, CDW, and magnetic order.

We propose an order parameter for a general one-dimensional gapped system with an open boundary condition. The order parameter can be computed from the ground state entanglement entropy of some regions near one of the boundaries. Hence, it is well defined even in the presence of arbitrary interaction and disorder. We also show that it is invariant under a finite-depth local quantum circuit, suggesting its stability against an arbitrary local perturbation that does not close the energy gap. Further, it can unambiguously distinguish a Majorana chain from a trivial chain under global fermion parity conservation. We argue that the order parameter can be, in principle, measured in an optical lattice system.

The quasiparticle bands of diamond, a prototype covalent insulator, are herein studied by means of a wave-function electronic-structure theory, with emphasis on the nature of the correlation hole around a bare particle. Short-range correlations are in such a system conveniently described by using a real-space representation and many-body techniques from ab initio quantum chemistry. To account for long-range polarization effects, on the other hand, we adopt the approximation of a dielectric continuum. Having as “uncorrelated” reference the Hartree-Fock band structure, the post-Hartree-Fock treatment is carried out in terms of localized Wannier functions derived from the Hartree-Fock solution. The computed correlation-induced corrections to the relevant real-space matrix elements are important and give rise to a strong reduction, in the range of $50\%$, of the initial Hartree-Fock gap. While our final results for the indirect and direct gaps, 5.4 and 6.9 eV, respectively, compare very well with the experimental data, the width of the valence band comes out by 10% to $15\%$ too large as compared to experiment. This overestimation of the valence-band width appears to be related to size-consistency effects in the configuration-interaction correlation treatment.

Recent numerical studies of the $J_1$-$J_2$ model on a square lattice suggest a possible continuous phase transition between the Néel state and a gapped spin-liquid state with $Z_2$ topological order. We show that such a phase transition can be realized through two steps: First bring the Néel state to the U(1) deconfined quantum critical point, which has been studied in the context of Néel–valence bond solid (VBS) state phase transition. Then condense the spinon pair–skyrmion/antiskyrmion bound state, which carries both gauge charge and flux of the U(1) gauge field emerging at the deconfined quantum critical point. We also propose a Schwinger boson projective wave function to realize such a $Z_2$ spin liquid state and find that it has a relatively low variational energy ($-0.4893J_1$/site) for the $J_1$-$J_2$ model at $J_2=0.5J_1$. The spin liquid state we obtain breaks the fourfold rotational symmetry of the square lattice and therefore is a nematic spin liquid state. This direct continuous phase transition from the Néel state to a spin liquid state may be realized in the $J_1$-$J_2$ model, or the anisotropic $J_{1x}$-$J_{1y}$-$J_2$ model.

We theoretically investigate the optical absorption of a hybrid system consisting of an organic molecular film on top of a semiconductor substrate. The electronic states of the isolated spatially separated constituents couple due to the Coulomb interaction of the optically induced charge carriers across the film-substrate interface. Focussing on the coupling of optical active molecular transitions to semiconductor continuum states, we find that the nonradiative dipole-dipole energy transfer causes the formation of coupled excitations, effectively reducing the excitation energy of the optical resonance in the molecular film and inducing a broadening of the associated absorption peak. In the framework of the Heisenberg equation of motion technique, we derive the Bloch equations for these hybrid systems. The input parameters for our model system of ladder-type quarterphenyl (L4P) molecules on the ZnO$\left(10\overline{1}0\right)$ surface are taken from density functional theory calculations.

The Berry curvature in Chern insulators appears to be a non-gauge-invariant quantity and does not immediately allow local length characterization. However, in two examples of two- and three-band models that we discuss, we find high-symmetry points in the Brillouin zone that have the Berry curvature invariant under diagonal gauge transformations, and may serve as expansion points of geometrical description. On the basis of the geometrical description, in the case of the Dirac based two-band Chern insulators, we conclude that the characteristic length based on the value of the Berry curvature at the expansion point plays the role of the magnetic length in the expression for the Hall viscosity. In the case of two-band models, the characteristic “cyclotron” spin is equal to 1/2, while in the three-band kagome case this spin is likely nonquantized and nonuniversal.

We evaluate the van der Waals (vdW) interaction energy at zero temperature between two undoped strained graphene layers separated by a finite distance. We consider the following three models for the anisotropic case: (a) where one of the two layers is uniaxially strained, (b) the two layers are strained in the same direction, and (c) one of the layers is strained in the perpendicular direction with respect to the other. We find that for all three models and given value of the electron-electron interaction coupling, the vdW interaction energy increases with increasing anisotropy. The effect is most striking for the case when both layers are strained in the same direction where we observe up to an order of magnitude increase in the strained relative to the unstrained case. We also investigate the effect of electron-electron interaction renormalization in the region of large separation between the strained graphene layers. We find that the many-body renormalization contributions to the correlation energy are non-negligible and the vdW interaction energy decreases as a function of increasing distance between the layers due to renormalization of the Fermi velocity, the anisotropy, and the effective interaction. Our analysis can be useful in designing graphene-based vdW heterostructures which, in recent times, has seen an upsurge in research activity.

We demonstrate experimentally the possibility to manipulate the coupling strength in an asymmetric pair of electronically coupled InGaAs quantum dots by using externally induced strain fields. The coupling strength of holes confined in the dots increases linearly with increasing tensile strain. A model based on $\mathit{k}·\mathit{p}$ theory explains the effect in terms of modified weight of the light hole component mediating the coupling in the barrier. Our results are relevant to the creation and control of entangled states in optically active quantum dots.

We report on Landau level spectroscopy studies of two HgTe quantum wells (QWs) near or at the critical well thickness, where the band gap vanishes. In magnetic fields up to $B=16$ T, oriented perpendicular to the QW plane, we observe a $\sqrt{B}$ dependence for the energy of the dominant cyclotron resonance (CR) transition characteristic of two-dimensional Dirac fermions. The dominant CR line exhibits either a single or double absorption line shape for the gapless or gapped QW. Using an effective Dirac model, we deduce the band velocity of single-valley Dirac fermions in gapless HgTe quantum wells, $v_F=6.4\times {10}^5$ m/s, and interpret the double absorption of the gapped QW as resulting from the addition of a small relativistic mass.

We theoretically study spatially periodic exciton-polariton patterns in a semiconductor microcavity coherently driven by an optical pump. Patterns grow spontaneously in a uniformly pumped cavity above a certain threshold (Turing instability). By means of a perturbation analysis in the vicinity of this bifurcation point, we determine the conditions for pattern formation and predict their properties. The shape of those patterns depends strongly on the detuning of the pump frequency from the system's resonances. For instance, near the bottom of the lower branch of the polariton dispersion relation, a hexagonal pattern is the most favorable periodic solution similar to pattern formation in optical cavities endowed with a defocusing Kerr nonlinearity. Towards the excitonic resonance, the influence of the upper polariton branch becomes important giving rise to a diversity of other solutions, such as roll, labyrinthine, and honeycomb patterns. Our numerical simulations reveal patterns with point and line defects and the formation of disordered patterns of a soliton gas.

Earlier studies of ${\text{C}}_{60}$ adsorption on Au(111) reported many interesting and complex features. We have performed coordinated low-energy electron diffraction, scanning tunneling microscopy (STM), and density functional theory studies to elucidate some of the details of the monolayer commensurate (2√3 × 2√3)R30° phase. We have identified the adsorption geometries of the two states that image as dim and bright in STM. These consist of a ${\text{C}}_{60}$ molecule with a hexagon side down in a vacancy (hex-vac) and a ${\text{C}}_{60}$ molecule with a carbon-carbon 6:6 bond down on a top site (6:6-top), respectively. We have studied the detailed geometries of these states and find that there is little distortion of the ${\text{C}}_{60}$ molecules, but there is a rearrangement of the substrate near the ${\text{C}}_{60}$ molecules. The two types of molecules differ in height, by about 0.7 Å, which accounts for most of the difference in their contrast in the STM images. The monolayer displays dynamical behavior, in which the molecules flip from bright to dim, and vice versa. We interpret this flipping as the result of the diffusion of vacancies in the surface layers of the substrate. Our measurements of the dynamics of this flipping from one state to the other indicate that the activation energy is 0.66 ± 0.03 eV for flips that involve nearest-neighbor ${\text{C}}_{60}$ molecules, and 0.93 ± 0.03 for more distant flips. Based on calculated activation energies for vacancies diffusing in Au, we interpret these to be a result of surface vacancy diffusion and bulk vacancy diffusion. These results are compared to the similar system of Ag(111)-(2√3 × 2√3)R30°-${\text{C}}_{60}$. In both systems, the formation of the commensurate ${\text{C}}_{60}$ monolayer produces a large number of vacancies in the top substrate layer that are highly mobile, effectively melting the interfacial metal layer at temperatures well below their normal melting temperatures.

We have used ab initio density functional theory, incorporating van der Waals corrections, to study twisted bilayer graphene (TBLG) where Stone-Wales defects or monovacancies are introduced in one of the layers. We compare these results to those for defects in single-layer graphene or Bernal stacked graphene. The energetics of defect formation is not very sensitive to the stacking of the layers or the specific site at which the defect is created, suggesting a weak interlayer coupling. However, signatures of the interlayer coupling are manifested clearly in the electronic band structure. For the “$\gamma \gamma$” Stone-Wales defect in TBLG, we observe two Dirac cones that are shifted in both momentum space and energy. This up/down shift in energy results from the combined effect of a charge transfer between the two graphene layers and a chemical interaction between the layers, which mimics the effects of a transverse electric field. Charge density plots show that states near the Dirac points have significant admixture between the two layers. For Stone-Wales defects at other sites in TBLG, this basic structure is modified by the creation of minigaps at energy crossings. For a monovacancy, the Dirac cone of the pristine layer is shifted up in energy by $\sim 0.25$ eV due to a combination of the requirement of the equilibration of Fermi energy between the two layers with different numbers of electrons, charge transfer, and chemical interactions. Both kinds of defects increase the density of states at the Fermi level. The monovacancy also results in spin polarization, with magnetic moments on the defect of 1.2–1.8 ${\mu }_B$.

We numerically investigate the statistical properties of electrochemical capacitance in disordered two-dimensional mesoscopic capacitors. Based on the tight-binding Hamiltonian, the Green's function formalism is adopted to study the average electrochemical capacitance, its fluctuation as well as the distribution of capacitance of the disordered mesoscopic capacitors for three different ensembles: orthogonal (symmetry index $\beta$ = 1), unitary ($\beta$ = 2), and symplectic ($\beta$ = 4). It is found that the electrochemical capacitance in the disordered systems exhibits universal behavior. In the case of a single conducting channel, the electrochemical capacitance follows a symmetric Gaussian distribution at weak disorders as expected from the random matrix theory. In the strongly disordered regime, the distribution is found to be a sharply one-sided form with a nearly constant tail in the large capacitance region. This behavior is due to the existence of the necklace states in disordered systems, which is characterized by the multiresonance that gives rise to a large density of states. In addition, it is found that the necklace state also enhances the fluctuation of electrochemical capacitance in the case of a single conducting channel. When the number of conducting channels increases, the influence of necklace states becomes less important. For a large number of conducting channels, the electrochemical capacitance fluctuation develops a plateau region in the strongly disordered regime. The plateau value is identified as universal electrochemical capacitance fluctuation, which is independent of system parameters such as disorder strength, Fermi energy, geometric capacitance, and system size. Importantly, the universal electrochemical capacitance fluctuation is the same for all three ensembles, suggesting a superuniversal behavior.

The discovery of a novel material requires the identification of the material's composition as well as of suitable synthesis conditions. We present a data-mining approach to identify suitable substrates for the growth of two-dimensional materials and apply the method to the recently predicted two-dimensional III-V compounds. We identify several lattice-matched substrates for their epitaxial growth, stabilization, and functionalization. Density-functional calculations show that these substrates sufficiently reduce the formation energies of the metastable two-dimensional materials to make them thermodynamically stable. We show that chemical interactions of the two-dimensional materials with the substrates shift the Fermi level of these materials, resulting in doping. The large adsorption energies and strong doping indicate that these metals should provide good electrical contact to enable transport measurements and electronic applications.