Among transition metal dichalcogenides (TMD), ${\mathrm{NbS}}_2$ is often considered as the archetypal compound that does not have a charge density wave (CDW) in any of its polytypes. By comparison, close isoelectronic compounds such as ${\mathrm{NbSe}}_2$, ${\mathrm{TaS}}_2$, and ${\mathrm{TaSe}}_2$ all have CDW in at least one polytype. Here we report traces of CDW in the 2H polytype of ${\mathrm{NbS}}_2$, using diffuse x-ray scattering measurements at 77 K and room temperature. We observe 12 extremely weak satellite peaks located at $\pm 13.9^{\circ }$ from ${\vec{a}}^{*}$ and ${\vec{b}}^{*}$ around each Bragg peak in the $(h,k,0)$ plane. These satellite peaks are commensurate with the lattice via $3\vec{q}-\vec{q}{}^{\prime }={\vec{a}}^{*}$, where $\vec{q}{}^{\prime }$ is the ${120}^{\circ }$ rotation of $\vec{q}$, and define two $\sqrt{13}a\times \sqrt{13}a$ superlattices in real space. These commensurate wave vectors and tilt angle are identical to those of the CDW observed in the 1T polytype of ${\mathrm{TaS}}_2$ and ${\mathrm{TaSe}}_2$. To understand this similarity and the faintness of the peaks, we discuss possible sources of local 1T polytype environment in bulk 2H-${\mathrm{NbS}}_2$ crystals.

Thermal properties of graphene display peculiar characteristics associated to the two-dimensional nature of this crystalline membrane. These properties can be changed and tuned in the presence of applied stresses, both tensile and compressive. Here, we study graphene monolayers under tensile stress by using path-integral molecular dynamics (PIMD) simulations, which allows one to take into account quantization of vibrational modes and analyze the effect of anharmonicity on physical observables. The influence of the elastic energy due to strain in the crystalline membrane is studied for increasing tensile stress and for rising temperature (thermal expansion). We analyze the internal energy, enthalpy, and specific heat of graphene, and compare the results obtained from PIMD simulations with those given by a harmonic approximation for the vibrational modes. This approximation turns out to be precise at low temperatures, and deteriorates as temperature and pressure are increased. At low temperature, the specific heat changes as $c_p\sim T$ for stress-free graphene, and evolves to a dependence $c_p\sim T^2$ as the tensile stress is increased. Structural and thermodynamic properties display non-negligible quantum effects, even at temperatures higher than 300 K. Moreover, differences in the behavior of the in-plane and real areas of graphene are discussed, along with their associated properties. These differences show up clearly in the corresponding compressibility and thermal expansion coefficient.

We study the topology of two-dimensional open systems in terms of the Green's function. The Ishikawa-Matsuyama formula for the integer topological invariant is applied in open systems, which indicates the number difference of gapless edge bands arising from the poles and zeros of the Green's function. Meanwhile, we define another topological invariant via the single-particle density matrix, which works for general gapped systems and is equivalent to the former for the case of weak coupling to an environment. We also discuss two applications. For time-reversal-invariant insulators, the $Z_2$ index can be expressed by the invariant of each spin subsystem. As a second application, we consider the proximity effect when an ordinary insulator is coupled to a topological insulator.

Although plasmon modes exist in doped graphene, the limited range of doping achieved by gating restricts the plasmon frequencies to a range that does not include the visible and infrared. Here we show, through the use of first-principles calculations, that the high levels of doping achieved by lithium intercalation in bilayer and trilayer graphene shift the plasmon frequencies into the visible range. To obtain physically meaningful results, we introduce a correction of the effect of plasmon interaction across the vacuum separating periodic images of the doped graphene layers, consisting of transparent boundary conditions in the direction perpendicular to the layers; this represents a significant improvement over the exact Coulomb cutoff technique employed in earlier works. The resulting plasmon modes are due to local field effects and the nonlocal response of the material to external electromagnetic fields, requiring a fully quantum mechanical treatment. We describe the features of these quantum plasmons, including the dispersion relation, losses, and field localization. Our findings point to a strategy for fine-tuning the plasmon frequencies in graphene and other two-dimensional materials.

We study the ground-state phase diagram of the $S=1$ bilinear-biquadratic model (BLBQ) on the star lattice with the state-of-art tensor network algorithms. The system has four phases: the ferromagnetic, antiferromagnetic, ferroquadrupolar, and spin-liquid phases. The phases and their phase boundaries are determined by examining various local observables, correlation functions, and transfer matrices exhaustively. The spin-liquid phase, which is the first quantum disordered phase found in the two-dimensional BLBQ model, is gapped and devoid of any conventional long-range order. It is also characterized by fixed-parity virtual bonds in the tensor network formalism, analogous to the Haldane phase, while the parity varies depending on the location of the bond.

We develop a theoretical frame to investigate the spectral dependence of the brightness of a single-photon source made of a solid-state nanoemitter embedded in a high-quality factor microcavity. This study encompasses the cases of localized excitons embedded in a one-, two-, or three-dimensional matrix. The population evolution is calculated based on a spin-boson model, using the noninteracting blip approximation. We find that the spectral dependence of the single-photon source brightness (hereafter called spectral efficiency) can be expressed analytically through the free-space emission and absorption spectra of the emitter, the vacuum Rabi splitting, and the loss rates of the system. In other words, the free-space spectrum of the emitter encodes all the relevant information on the interaction between the exciton and the phonon bath to obtain the dynamics of the cavity-coupled system. We compute numerically the spectral efficiency for several types of localized emitters differing by the phonon bath dimensionality. In particular, in low-dimensional systems where this interaction is enhanced, a pronounced asymmetric energy exchange between the emitter and the cavity on the phonon sidebands yields a considerable extension of the tuning range of the source through phonon-assisted cavity feeding, possibly surpassing that of a purely resonant system.

Within the past few years, tin selenide (SnSe) has attracted intense interest due to its remarkable thermoelectric potential for both $n$- and $p$-type crystals. In this work, the intrinsic phonon-limited electron/hole mobilities of SnSe are investigated using a Boltzmann transport equation based on first-principles calculated electron-phonon interactions. We find that the electrons have much larger mobilities than the holes. At room temperature, the mobilities of electrons along the $a$, $b$, and $c$ axes are 325, 801, and 623 ${\mathrm{cm}}^2$/V s, respectively, whereas those of holes are 100, 299, and 291 ${\mathrm{cm}}^2$/V s, respectively. The anisotropy of mobilities is consistent with the reciprocal effective mass at band edges. The mode-specific analysis shows that the highest longitudinal optical phonons, rather than previously assumed acoustic phonons, dominate the scattering processes and consequently the mobilities in SnSe. The room-temperature largest mean free paths of electrons and holes in SnSe are about 21 and 13 nm, respectively.

We report on a fast, bandwidth-tunable single-photon source based on an epitaxial GaAs quantum dot. Exploiting spontaneous spin-flip Raman transitions, single photons at 780 nm are generated on demand with tailored temporal profiles of durations exceeding the intrinsic quantum dot lifetime by up to three orders of magnitude. Second-order correlation measurements show a low multiphoton emission probability $[g^2\left(0\right)\sim 0.10\text{–}0.15]$ at a generation rate up to 10 MHz. We observe Raman photons with linewidths as low as 200 MHz, which is narrow compared to the 1.1-GHz linewidth measured in resonance fluorescence. The generation of such narrow-band single photons with controlled temporal shapes at the rubidium wavelength is a crucial step towards the development of an optimized hybrid semiconductor-atom interface.

We study theoretically the optical response of a hybrid spherical cluster containing quantum emitters and metallic nanoparticles. The quantum emitters are modeled as two-level quantum systems whose dielectric function is obtained via a density matrix approach wherein the modified spontaneous emission decay rate at the position of each quantum emitter is calculated via the electromagnetic Green's tensor. The problem of light scattering off the hybrid cluster is solved by employing the coupled-dipole method. We find, in particular, that the presence of the quantum emitters in the cluster, even in small fractions, can significantly alter the absorption and extinction spectra of the sole cluster of the metallic nanoparticles, where the corresponding electromagnetic modes can have a weak plexcitonic character under suitable conditions.

Recent progress in the fabrication of materials has made it possible to create arbitrary nonperiodic two-dimensional structures in the quantum plasmon regime. This paves the way for exploring the quantum plasmonic properties of electron gases in complex geometries. In this work we study systems with a fractal dimension. We calculate the full dielectric functions of two prototypical fractals with different ramification numbers, namely the Sierpinski carpet and gasket. We show that the Sierpinski carpet has a dispersion comparable to a square lattice, but the Sierpinski gasket features highly localized plasmon modes with a flat dispersion. This strong plasmon confinement in finitely ramified fractals can provide a novel setting for manipulating light at the quantum level.

We derive an integral equation describing surface-plasmon polaritons in graphene deposited on a substrate with a planar surface and a dielectric protrusion in the opposite surface of the dielectric slab. We show that the problem is mathematically equivalent to the solution of a Fredholm equation, which we solve exactly. In addition, we show that the dispersion relation of the channel surface plasmons is determined by the geometric parameters of the protrusion alone. We also show that such a system supports both even and odd modes. We give the electrostatic potential and the intensity plot of the electrostatic field, which clearly show the transverse localized nature of the surface plasmons in a continuous and flat graphene sheet.

Zhu et al. [Phys. Rev. B 94, 121401(R) (2016)] recently reported an experimental work that claims that epitaxial Bi(111) films grown on Si(111) are (1) topologically nontrivial, and (2) metallic only at the surface and insulating in the interior bulk, consistent with the old prediction of a semimetal-to-semiconductor (SMSC) transition. Here we point out that although Bi can be nontrivial, the SMSC transition does not happen at the same time. Through the comparison of the thickness and temperature dependence of the film conductivity of Bi with ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$ performed in situ, we find no evidence of the insulating bulk states of Bi when the thickness is less than 240 $\AA$, which is in accordance with our angle-resolved photoemission spectroscopy measurements and density functional theory calculations.

In Hirahara and Hasegawa's Comment [Hirahawa and Hasegawa, Phys. Rev. B 97, 207401 (2018)], they question our results of the unusual properties of epitaxial Bi(111) thin films. In this Reply, we show that their Bi samples, with resistivity accuracy three orders of magnitude poorer, may be different from ours. They also lack the most essential results of the Aharonov-Bohm (AB) effect for the quantum transport conclusion.

We perform first-principles calculations to investigate the phase stability of boron carbide, concentrating on the recently proposed alternative structural models composed not only of the regularly studied ${\mathrm{B}}_{11}{\mathrm{C}}^p$(CBC) and ${\mathrm{B}}_{12}$(CBC), but also of ${\mathrm{B}}_{12}$(CBCB) and ${\mathrm{B}}_{12}({\mathrm{B}}_4$). We find that a combination of the four structural motifs can result in low-energy electron precise configurations of boron carbide. Among several considered configurations within the composition range of ${\mathrm{B}}_{10.5}\mathrm{C}$ and ${\mathrm{B}}_4\mathrm{C}$, we identify in addition to the regularly studied ${\mathrm{B}}_{11}{\mathrm{C}}^p$(CBC) at the composition of ${\mathrm{B}}_4\mathrm{C}$ two low-energy configurations, resulting in a new view of the B-C convex hull. Those are $[{\mathrm{B}}_{12}$(CBC)${\mathrm{]}}_{0.67}{\left[{\mathrm{B}}_{12}\left({\mathrm{B}}_4\right)\right]}_{0.33}$ and $[{\mathrm{B}}_{12}$(CBC)${\mathrm{]}}_{0.67}[{\mathrm{B}}_{12}$(CBCB)${\mathrm{]}}_{0.33}$, corresponding to compositions of ${\mathrm{B}}_{10.5}\mathrm{C}$ and ${\mathrm{B}}_{6.67}\mathrm{C}$, respectively. As a consequence, ${\mathrm{B}}_{12}$(CBC) at the composition of ${\mathrm{B}}_{6.5}\mathrm{C}$, previously suggested in the literature as a stable configuration of boron carbide, is no longer part of the $\mathrm{B}-\mathrm{C}$ convex hull. By inspecting the electronic density of states as well as the elastic moduli, we find that the alternative models of boron carbide can provide a reasonably good description for electronic and elastic properties of the material in comparison with the experiments, highlighting the importance of considering ${\mathrm{B}}_{12}$(CBCB) and ${\mathrm{B}}_{12}({\mathrm{B}}_4$), together with the previously proposed ${\mathrm{B}}_{11}{\mathrm{C}}^p$(CBC) and ${\mathrm{B}}_{12}$(CBC), as the crucial ingredients for modeling boron carbide with compositions throughout the single-phase region.

The 1D Anderson model possesses a completely localized spectrum of eigenstates for all values of the disorder. We consider the effect of projecting the Hamiltonian to a truncated Hilbert space, destroying time-reversal symmetry. We analyze the ensuing eigenstates using different measures such as inverse participation ratio and sample-averaged moments of the position operator. In addition, we examine amplitude fluctuations in detail to detect the possibility of multifractal behavior (characteristic of mobility edges) that may arise as a result of the truncation procedure.

The compound ${\mathrm{IrTe}}_2$ is known to exhibit a transition to a modulated state featuring Ir-Ir dimers, with large associated atomic displacements. Partial substitution of Pt or Rh for Ir destabilizes the modulated structure and induces superconductivity. It has been proposed that quantum critical dimer fluctuations might be associated with the superconductivity. Here we test for such local dimer correlations and demonstrate their absence. X-ray pair distribution function approach reveals that the local structure of ${\mathrm{Ir}}_{0:95}{\mathrm{Pt}}_{0:05}{\mathrm{Te}}_2$ and ${\mathrm{Ir}}_{0:8}{\mathrm{Rh}}_{0:2}{\mathrm{Te}}_2$ dichalcogenide superconductors with compositions just past the dimer/superconductor boundary is explained well by a dimer-free model down to 10 K, ruling out the possibility of there being nanoscale dimer fluctuations in this regime. This is inconsistent with the proposed quantum-critical-point-like interplay of the dimer state and superconductivity, and precludes scenarios for dimer fluctuations mediated superconducting pairing.

Topological elastic waves have recently emerged as an exciting interdisciplinary field which is still mainly focused on low-dimensional systems. Here we propose phononic topological node lines emerging from three-dimensional mechanical metacrystals. The resultant ribbon- or drumhead-like phononic edge states exhibit unusual frequency stability against perturbations, which can be exploited for robust elastic wave trapping for various applications. Our design leads to symmetry-guaranteed phononic topological node lines which are protected by both nonsymmorphic crystalline symmetry and fundamental conservation laws. The band topology, characterized by the nontrivial Zak phase, is also revealed through the reflection phase of the band gap.

Stable compounds in the Cr-H system have been synthesized through a direct reaction of chromium and hydrogen in a laser-heated diamond-anvil cell and investigated using synchrotron x-ray diffraction up to $120\mathrm{GPa}$. The sequence of hydrides CrH, ${\mathrm{Cr}}_2{\mathrm{H}}_3$, and ${\mathrm{CrH}}_2$ has been observed by increasing pressure. The known $\epsilon -hcp$-CrH hydride is formed above $3\mathrm{GPa}$. A ${\mathrm{Cr}}_2{\mathrm{H}}_3$ hydride with a $C2/m$ structure appears spontaneously above $19\mathrm{GPa}$, as a result of the filling of the tetrahedral sites of $\epsilon$-CrH. YAG laser heating helps dissolve more hydrogen inside the $hcp$ chromium structure to synthesize a ${\mathrm{CrH}}_2$ compound with a $Pnma$ structure from $30\mathrm{GPa}$ on. The volume expansion per hydrogen atom in octahedral and tetrahedral sites is measured up to the 100-GPa pressure range. The formation pressures and structures of these chromium interstitial hydrides are in very good agreement with DFT calculations. However, despite multiple heating attempts up to $100\mathrm{GPa}$, no evidence of the stability of the predicted ${\mathrm{CrH}}_3$ compound could be found.

We demonstrate a fully electric control of the heat flux, which can be continuously modulated by an externally applied electric field in ${\mathrm{PbTiO}}_3$, a prototypical ferroelectric perovskite, revealing the mechanisms by which experimentally accessible fields can be used to tune the thermal conductivity by as much as 50% at room temperature.