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Previous experimental and theoretical work has given evidence of the existence of doubly charged exciton states in strongly screened bilayers of transition metal dichalcogenide (TMD) layers. These complexes are important because they are performed electron pairs that can, in principle, undergo Bose-Einstein condensation (BEC), in which case they would also form a new type of superconductor, consisting of stable bosons with net charges. In this paper, we present key electrostatic and magnetic measurements that definitively confirm the existence of these charged bosons. These measurements include 1) continuous control of the doping density with both positive and negative carriers, showing the expected population dependencies on the free carrier density, and 2) measurement of the dependence on the magnetic field, showing that this new bound state is a spin triplet. These results imply that it is promising to look for BEC and superconductivity in this system., Comment: 16 pages, 10 figures

We identify and classify topologically protected singularities for the reflection coefficient of transdimensional plasmonic systems. Originating from nonlocal electromagnetic response due to vertical electron confinement in the system, such singularities lead to lateral (angular) Goos-H\"anchen shifts on the millimeter (milliradian) scale in the visible range, greatly exceeding those reported previously for artificially designed metasurfaces, offering new opportunities for quantum material development., Comment: Main text of 7 pages (two-column), 7 figures, 45 references + Supplementary info of 14 pages (single column), 10 figures, 14 references

Plasmon resonance in metals represents the collective oscillation of the free electron gas density and enables enhanced light-matter interactions in nanoscale dimensions. Traditionally, the classical Drude model describes the plasmonic excitation, wherein the plasma frequency exhibits no spatial dispersion. Here, we show conclusive experimental evidence of the breakdown of the plasmon resonance and a consequent photonic metal-insulator transition in an ultrathin archetypal refractory plasmonic material, hafnium nitride (HfN). Epitaxial HfN thick films exhibit a low-loss and high-quality Drude-like plasmon resonance in the visible spectral range. However, as the film thickness is reduced to nanoscale dimensions, the Coulomb interaction among electrons increases due to the electron confinement, leading to the spatial dispersion of the plasma frequency. Importantly, with the further decrease in thickness, electrons lose their ability to shield the incident electric field, turning the medium into a dielectric. The breakdown of the plasmon resonance in epitaxial ultrathin metals could be useful for fundamental physics studies in transdimensional regimes and novel photonic device applications.

We present a theoretical study of the directionality effects in spontaneous emission and resonance fluorescence of a quantum two-level dipole emitter near an ultrathin closely packed periodically aligned single-wall carbon nanotube film. Such films present an example of highly anisotropic flexible metasurfaces that are now available experimentally. The nanotube alignment is shown to provide an extra measure for quantum control of dipolar spontaneous emission and resonance fluorescence in such systems, in addition to film thickness and composition parameters such as tube diameter, chirality and translational period. The processes studied are shown to be highly anisotropic, being enhanced by orders of magnitude in the direction perpendicular to the alignment and metasurface plane, contrasting with the commonly believed viewpoint of their uncontrollably random directionality., Comment: 12 pages, 4 figures, 95 references

The Casimir interaction and torque are related phenomena originating from the exchange of electromagnetic excitations between objects. While the Casimir force exists between any types of objects, the materials or geometrical anisotropy drives the emergence of the Casimir torque. Here both phenomena are studied theoretically between dielectric films with immersed parallel single wall carbon nanotubes in the dilute limit with their chirality and collective electronic and optical response properties taken into account. It is found that the Casimir interaction is dominated by thermal fluctuations at sub-micron separations, while the torque is primarily determined by quantum mechanical effects. This peculiar quantum vs. thermal separation is attributed to the strong influence of reduced dimensionality and inherent anisotropy of the materials. Our study suggests that nanostructured anisotropic materials can serve as novel platforms to uncover new functionalities in ubiquitous Casimir phenomena., Comment: 9 pages, 4 figures, submitted to Phys. Rev. B

We study within the framework of the Lifshitz theory the long-range Casimir force for in-plane isotropic and anisotropic free-standing transdimensional material slabs. In the former case, we show that the confinement-induced nonlocality not only weakens the attraction of ultrathin slabs but also changes the distance dependence of the material-dependent correction to the Casimir force to go as $\sim\!1/\!\sqrt{l}$ contrary to the $\sim\!1/l$ dependence of that of the local Lifshitz force. In the latter case, we use closely packed array of parallel aligned single-wall carbon nanotubes in a dielectric layer of finite thickness to demonstrate strong orientational anisotropy and crossover behavior for the inter-slab attractive force in addition to its reduction with decreasing slab thickness. We give physical insight as to why such a pair of ultrathin slabs prefers to stick together in the perpendicularly oriented manner, rather than in the parallel relative orientation as one would customarily expect., Comment: 20 pages, 4 figures, 52 references

We develop a theoretical model for a tunable coherent terahertz radiation source based on the long-lived Bose condensate of photons. In the device we propose, the original photon pumping is performed incoherently by a blackbody radiation emitter. The photons thus produced Bose-condense by the inelastic relaxation on a two-dimensional electron gas in a perpendicular magnetostatic field. The process involves neither population inversion nor light wave amplification the standard laser sources are built on. The coherence and tunability of the light emitted by such a photon condensate are provided and supported by the discrete spectrum of the electron gas in the quantizing magnetic field. The device is a compact-size semiconductor crystal. We propose the design and perform the realistic calculations of the physical properties and limiting factors for the terahertz photon Bose-condensate resonator. We show that our terahertz source can deliver the highly coherent light emission in the frequency range of 3-30 THz for the magnetic field induction of the order of 2 T, with the upper emission frequency limit adjustable by the strength of the magnetic field applied., Comment: The results of this article have been totally reconsidered and require too many corrections to be implemented to be publishable

We compare the confinement-induced nonlocal electromagnetic response model to the standard local Drude model routinely used in plasmonics. Both of them are applied to study the heat transfer for transdimensional plasmonic film systems. The former provides greater Woltersdorff length in the far-field and larger film thicknesses at which heat transfer is dominated by surface plasmons, leading to enhanced near-field heat currents. Our results show that the nonlocal response model is capable of making a significant impact on the understanding of the radiative heat transfer in ultrathin films.

We study theoretically the properties of a two-level quantum dipole emitter near an ultrathin transdimensional plasmonic film. Our model system mimics a solid-state single-photon source device. Using realistic experimental parameters, we compute the spontaneous and stimulated emission intensity profiles as functions of the excitation frequency and film thickness, followed by the analysis of the second-order photon correlations to explore the photon antibunching effect. We show that ultrathin transdimensional plasmonic films can greatly improve photon antibunching with thickness reduction, which allows one to control quantum properties of light and make them more pronounced. Knowledge of these features is advantageous for solid-state single-photon source device engineering and overall for the development of the new integrated quantum photonics material platform based on the transdimensional plasmonic films., Comment: 19 pages, 3 figures, 68 references

We develop the theory of the magnetic-field-induced Wigner crystallization effect for charged interlayer excitons (CIE) discovered recently in transition-metal-dichalcogenide (TMD) heterobilayers. We derive the ratio of the average potential interaction energy to the average kinetic energy for the many-particle CIE system subjected to the perpendicular magnetic field of an arbitrary strength, analyze the weak and strong field regimes, and discuss the 'cold' crystallization phase transition for the CIE system in the strong field regime. We also generalize the effective g-factor concept previously formulated for interlayer excitons, to include the formation of CIEs in electrostatically doped TMD heterobilayers. We show that magnetic-field-induced Wigner crystallization and melting of CIEs can be observed in strong-field magneto-photoluminescence experiments with TMD heterobilayes of systematically varied electron-hole doping concentrations. Our results advance the capabilities of this new family of transdimensional quantum materials., Comment: 26 pages, 5 figures, 65 references

We present a theoretical study of the collective quasiparticle excitations responsible for the electromagnetic response of ultrathin plane-parallel homogeneous periodic single-wall carbon nanotube arrays and weakly inhomogeneous single-wall carbon nanotube films. We show that in addition to varying film composition, the collective response can be controlled by varying the film thickness. For single-type nanotube arrays, the real part of the dielectric response shows a broad negative refraction band near a quantum interband transition of the constituent nanotube, whereby the system behaves as a hyperbolic metamaterial at higher frequencies than those classical plasma oscillations have to offer. By decreasing nanotube diameters it is possible to push this negative refraction into the visible region, and using weakly inhomogeneous multi-type nanotube films broadens its bandwidth., Comment: 16 pages, 5 figures, 5 sections, 2 appendices

We study theoretically the in-plane electromagnetic response and the exciton-plasmon interactions for an experimentally feasible carbon nanotube (CN) film systems composed of parallel aligned periodic semiconducting CN arrays embedded in an ultrathin finite-thickness dielectric. For homogeneous single-CN films, the intertube coupling and thermal broadening bring the exciton and interband plasmon resonances closer together. They can even overlap due to the inhomogeneous broadening for films composed of array mixtures with a slight CN diameter distribution. In such systems the real part of the response function is negative for a broad range of energies (negative refraction band), and the CN film behaves as a hyperbolic metamaterial. We also show that for a properly fabricated two-component CN film, by varying the relative weights of the two constituent CN array components one can tune the optical absorption profile to make the film transmit or absorb light in the neighborhood of an exciton absorption resonance on-demand., Comment: 6 two-column pages, 5 figures

We report experimental evidence for charged boson states in a solid without Cooper pairing, based on attaching two free carriers to an exciton in a semiconducting system. Theoretical calculations show that this type of complex is stable in bilayer systems next to a parallel metal layer. Our experimental measurements on structures made using two different materials show a new spectral line at the predicted energy, if and only if all the required conditions for this complex are fulfilled, including a parallel metal layer that significantly screens the repulsive interaction between the like-charge carriers, and with the predicted dependence on the distance to the metal layer. This suggests a new path for pursuing room temperature superconductivity without Cooper pairing.

Throughout the years, strongly correlated coherent states of excitons have been the subject of intense theoretical and experimental studies. This topic has recently boomed due to new emerging quantum materials such as van der Waals (vdW) bound atomically thin layers of transition metal dichalcogenides (TMDs). We analyze the collective properties of charged interlayer excitons observed recently in bilayer TMD heterostructures. We predict new strongly correlated phases - crystal and Wigner crystal - that can be selectively realized with TMD bilayers of properly chosen electron-hole effective masses by just varying their interlayer separation distance. Our results open up new avenues for nonlinear coherent control, charge transport and spinoptronics applications with quantum vdW heterostuctures., Comment: 34 pages, 8 figures, 57 references

We use quantum electrodynamics and the confinement-induced nonlocal dielectric response model based on the Keldysh-Rytova electron interaction potential to study the epsilon-near-zero modes of metallic films in the transdimensional regime. New peculiar effects are revealed such as the plasmon mode degeneracy lifting and the dipole emitter coupling to the split epsilon-near-zero modes, leading to thickness-controlled spontaneous decay with up to three-orders-of-magnitude increased rates., Comment: Expanded version: 7 two-column pages, 3 figures, 45 references

Finite-thickness effects are analyzed theoretically for the plasma frequency and associated dielectric response function of plasmonic films formed by periodically aligned, infinitely thin, identical metallic cylinders. The plasma frequency of the system is shown to have the unidirectional square-root-of-momentum and quasilinear momentum spatial dispersion for the thick and ultrathin films, respectively. This spatial dispersion and the unidirectional dielectric response nonlocality associated with it can be adjusted not only by the film material composition but also by varying the film thickness, the cylinder length, the cylinder-radius-to-film-thickness ratio, and by choosing the substrates and superstrates of the film appropriately. Application of the theory developed to the finite-thickness periodically aligned carbon nanotube films is discussed., Comment: 14 pages, 3 figures

We discuss the optical response peculiarities for ultrathin plasmonic films of finite lateral size. Due to their plasma frequency spatial dispersion caused by the spatial confinement of the electron motion, the film dielectric permittivity tensor is spatially dispersive as well and so nonlocal. Such a confinement induced nonlocality can result in peculiar magneto-optical effects. For example, the frequency dependence of the magnetic permeability of the film exhibits a sharp resonance structure shifting to the red as the film aspect ratio increases. The properly tuned ultrathin plasmonic films of finite lateral size can feature the negative refraction effect in the IR frequency range. We discuss how to control these magneto-optical properties and show that they can be tuned by adjusting the film chemical composition, plasmonic material quality, the aspect ratio, and the surroundings of the film., Comment: 8 pages, 2 figures, 25 references

The product of the electronic width of the $J/\psi$ meson and the branching fractions of its decay to hadrons and electrons has been measured using the KEDR detector at the VEPP-4M $e^+e^-$ collider. The obtained values are: $\Gamma_{ee}(J/\psi) = 5.550 \pm 0.056 \pm 0.089 \, \text{keV}, $ $\Gamma_{ee}(J/\psi) \cdot \mathcal{B}_\text{hadrons}(J/\psi) = 4.884 \pm 0.048 \pm 0.078 \, \text{keV}, $ $\Gamma_{ee}(J/\psi) \cdot \mathcal{B}_{ee}(J/\psi) = 0.3331 \pm 0.0066 \pm 0.0040 \, \text{keV}.$ The uncertainties shown are statistical and systematic, respectively. Using the result presented and the world-average value of the electronic branching fraction, one obtains the total width of the $J/\psi$ meson: $\Gamma = 92.94 \pm 1.83 \, \text{keV}.$ These results are consistent with the previous experiments., Comment: 19 pages, 13 figures

We discuss neutral and charged complexes (biexciton and trion) formed by indirect excitons in layered quasi-two-dimensional semiconductor heterostructures. Indirect excitons -- long-lived neutral Coulomb-bound pairs of electrons and holes of different layers -- have been known for semiconductor coupled quantum wells and are recently reported for van der Waals heterostructures such as bilayer graphene and transition metal dichalcogenides. Using the configuration space approach, we derive the analytical expressions for the trion and biexciton binding energies as functions of the interlayer distance. The method captures essential kinematics of complex formation to reveal significant binding energies, up to a few tens of meV for typical interlayer distances ~3-5 A, with the trion binding energy always being greater than that of the biexciton. Our results can contribute to the understanding of more complex many-body phenomena such as exciton Bose-Einstein condensation and Wigner-like electron-hole crystallization in layered semiconductor heterostructures., Comment: 10 pages, 4 figures, 105 references. arXiv admin note: text overlap with arXiv:1605.02348

We study theoretically confinement related effects in the optical response of thin plasmonic films of controlled variable thickness. While being constant for relatively thick films, the plasma frequency is shown to acquire spatial dispersion typical of two-dimensional materials such as graphene, gradually shifting to the red with the film thickness reduction. The dissipative loss, while decreasing at any fixed frequency, gradually goes up at the plasma frequency as it shifts to the red with the film thickness reduced. These features offer a controllable way to tune spatial dispersion and related optical properties of plasmonic films and metasurfaces on demand, by precisely controlling their thickness, material composition, and by choosing deposition substrates and coating layers appropriately., Comment: 5 pages, 4 figures

A configuration space method is developed for binding energy calculations of the lowest energy exciton complexes (trion, biexciton) in spatially confined quasi-1D semiconductor nanostructures such as nanowires and nanotubes. Quite generally, trions are shown to have greater binding energy in strongly confined structures with small reduced electron-hole masses. Biexcitons have greater binding energy in less confined structures with large reduced electron-hole masses. This results in a universal crossover behavior, whereby trions become less stable than biexcitons as the transverse size of the quasi-1D nanostructure increases. The method is also capable of evaluating binding energies for electron-hole complexes in quasi-2D semiconductors such as coupled quantum wells and bilayer van der Walls bound heterostructures with advanced optoelectronic properties., Comment: 16 pages, 10 figures. arXiv admin note: substantial text overlap with arXiv:1405.0777

We develop an electron transport theory for the hybrid system of a semiconducting carbon nanotube that encapsulates a one-atom-thick metallic wire. The theory predicts Fano resonances in electron transport through the system, whereby the interaction of electrons on the wire with nanotube plasmon generated near-fields blocks some of the wire transmission channels to open up the new coherent plasmon-mediated channel in the nanotube forbidden gap outside the wire transmission band. Such a channel makes the entire hybrid system transparent in the energy domain where neither wire, nor nanotube is indivudually transparent. This effect can be used to manipulate by the electron charge transfer in hybrid nanodevices built on metal-semiconductor nanotube systems., Comment: 16 pages, 5 figures, 1 appendix, 117 references

Charge fluctuations in nano-circuits with capacitor components are shown to give rise to a novel type of long-ranged interaction, which co-exist with the regular Casimir/van der Waals force. The developed theory distinguishes between thermal and quantum mechanical effects, and it is applied to capacitors involving graphene nanostructures. The charge fluctuations mechanism is captured via the capacitance of the system with geometrical and quantum mechanical components. The dependence on the distance separation, temperature, size, and response properties of the system shows that this type of force can have a comparable and even dominant effect to the Casimir interaction. Our results strongly indicate that fluctuations induced interactions due to various thermodynamic quantities can have important thermal and quantum mechanical contributions at the micro- and nanoscale.

Quantum electrodynamics theory of the resonance Raman scattering is developed for an atom in a close proximity to a carbon nanotube. The theory predicts a dramatic enhancement of the Raman intensity in the strong atomic coupling regime to nanotube plasmon near-fields. This resonance scattering is a manifestation of the general electromagnetic surface enhanced Raman scattering effect, and can be used in designing efficient nanotube based optical sensing substrates for single atom detection, precision spontaneous emission control, and manipulation., Comment: 12 pages, 5 figures, 39 references

A configuration space approach is developed to uncover generic stability peculiarities for the lowest energy neutral and charged exciton complexes (biexciton and trion) in quasi-one-dimensional semiconductors. Trions are shown to be more stable than biexcitons in strongly confined structures with small reduced electron-hole masses. Biexcitons are more stable in less confined structures with large reduced electron-hole masses. In semiconducting carbon nanotubes, in particular, the trion binding energy is shown to be greater than that of the biexciton by a factor ~1.4 decreasing with diameter, thus revealing the general physical principles that underlie recent experimental observations., Comment: 5 pages, 4 figures; revised: fig.1 updated, abstract and text revised, more info added