We report the behavior of the charge carrier plasmon of $2H$-transition metal dichalcogenides (TMDs) as a function of intercalation with alkali metals. Intercalation and concurrent doping of the TMD layers have a substantial impact on plasmon energy and dispersion. While the plasmon energy shifts are related to the intercalation level as expected within a simple homogeneous electron gas picture, the plasmon dispersion changes in a peculiar manner independent of the intercalant and the TMD materials. Starting from a negative dispersion, the slope of the plasmon dispersion changes sign and grows monotonously upon doping. Quantitatively, the increase of this slope depends on the orbital character $(4d$ or $5d)$ of the conduction bands, which indicates a decisive role of band structure effects on the plasmon behavior.

The yellow luminescence band was studied in undoped and Si-doped GaN samples by steady-state and time-resolved photoluminescence. At low temperature (18 K), the zero-phonon line (ZPL) for the yellow band is observed at 2.57 eV and attributed to electron transitions from a shallow donor to a deep-level defect. At higher temperatures, the ZPL at 2.59 eV emerges, which is attributed to electron transitions from the conduction band to the same defect. In addition to the ZPL, a set of phonon replicas is observed, which is caused by the emission of phonons with energies of 39.5 meV and 91.5 meV. The defect is called the YL1 center. The possible identity of the YL1 center is discussed. The results indicate that the same defect is responsible for the strong YL1 band in undoped and Si-doped GaN samples.

Very initial stages of the homoepitaxy on the $\mathrm{Si}\left(111\right)7\times 7$ surface are studied at 310, 356, and $366{}^{\circ }\mathrm{C}$ by in situ continuous observation using high-temperature scanning tunneling microscopy (HTSTM) at the atomic scale during Si deposition under a slow rate of 0.02 bilayer $\left(\mathrm{BL}\right){min}^{-1}$. The substrate surface is reconstructed to the well-established dimer-adatom-stacking fault (DAS) structure with the $7\times 7$ unit cell consisting of two triangular half unit cells (HUCs): a stacking faulted (F)-HUC and a normally stacked one. It is expected that the complex, large unit cell compels the initial homoepitaxy to proceed in a quite different manner from that on unreconstructed surfaces. Formation and growth of various adsorbed clusters are pursued by the continuous observation of the same narrow areas during the deposition, avoiding the tip-shadow effect. The most anomalous finding is a quasiliquid cluster (QLC) spreading to plural HUCs (spread QLC) on the $7\times 7$ DAS substrate. This appears as a result of the difficulty of the F-HUC to be transformed into the normally stacked BL, being essential for the homoepitaxy. After the transformation, the spread QLC undergoes the following structural changes with an increase of the Si deposition: crystallization to a small epitaxial BL → surface reconstruction to the DAS structure. Validity and reasons of the transitional formation of the spread QLC are discussed. The spread QLC mediated homoepitaxy mechanism is concluded and is a new mode in the crystal growth. Real in situ HTSTM observations of the same areas on the nanoscale during Si deposition are indispensable to explore for the dynamic atomistic mechanism of the homoepitaxial growth on the $\mathrm{Si}\left(111\right)7\times 7$.

We have observed secondary electron emission (SEE) from nanographene by applying time-dependent density functional theory simulations in real-time and real-space to electron scattering on target graphene-flakes. We obtained the incident-electron energy dependence and bilayer effect on the amount of secondary electron (SE). The dynamics of SEE and collective density oscillations, which are electronic excitations induced by electron impact, were demonstrated numerically, and elucidated by the time-dependent occupation numbers of the Kohn-Sham electronic levels. The SE yields from graphene flakes are found to be $\sim 0.1$. The highest energy of SE is $\sim 20$ eV, which is compatible with the characteristics observed in SEE experiments.

The possibility of injecting a single electron into ballistic conductors is at the basis of the new field of electron quantum optics. Here, we consider a single electron injection into the helical edge channels of a topological insulator. Their counterpropagating nature and the unavoidable presence of electron-electron interactions dramatically affect the time evolution of the single wave packet. Modeling the injection process from a mesoscopic capacitor in the presence of nonlocal tunneling, we focus on the time-resolved charge and energy packet dynamics. Both quantities split up into counterpropagating contributions whose profiles are strongly affected by the interaction strength. In addition, stronger signatures are found for the injected energy, which is also affected by the finite width of the tunneling region, in contrast to what happens for the charge. Indeed, the energy flow can be controlled by tuning the injection parameters, and we demonstrate that, in the presence of nonlocal tunneling, it is possible to achieve a situation in which charge and energy flow in opposite directions.

Functionalization of the MXene ${\mathrm{Sc}}_2\mathrm{C}$, which has the rare property to realize semiconducting states for various functionalizations including O, F, and OH, is studied with respect to the electronic and thermal behavior. The lowest lattice thermal conductivity is obtained for OH functionalization and an additional 30% decrease can be achieved by confining the phonon mean free path to 100 nm. Despite a relatively low Seebeck coefficient, ${\mathrm{Sc}}_2\mathrm{C}({\mathrm{OH})}_2$ is a candidate for intermediate-temperature thermoelectric applications due to compensation by a high electrical conductivity and very low lattice thermal conductivity.

The development of active and passive plasmonic devices is challenging due to the high level of dissipation in normal metals. One possible solution to this problem is using alternative materials. Graphene is a good candidate for plasmonics in the near-infrared region. In this paper, we develop a quantum theory of a graphene plasmon generator. We account for quantum correlations and dissipation effects, thus we are able to describe such regimes of a quantum plasmonic amplifier as a surface plasmon emitting diode and a surface plasmon amplifier using stimulated radiation emission. Switching between these generation types is possible in situ with a variance of the graphene Fermi level. We provide explicit expressions for dissipation and interaction constants through material parameters, and we identify the generation spectrum and the second-order correlation function, which predicts the laser statistics.

Many experiments have shown the detection of spin currents driven by radio-frequency spin pumping from yttrium iron garnet (YIG), by making use of the inverse spin-Hall effect, which is present in materials with strong spin-orbit coupling, such as Pt. Here we show that it is also possible to directly detect the resonance-driven spin current using Au|permalloy (Py, ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) devices, where Py is used as a detector for the spins pumped across a YIG|Au interface. This detection mechanism is equivalent to the spin-current detection in metallic nonlocal spin-valve devices. By finite element modeling we compare the pumped spin current from a reference Pt strip with the detected signals from the Au|Py devices. We find that for one series of Au|Py devices the calculated spin pumping signals mostly match the measurements, within 20%, whereas for a second series of devices additional signals are present which are up to a factor 10 higher than the calculated signals from spin pumping. We also identify contributions from thermoelectric effects caused by the resonant (spin-related) and nonresonant heating of the YIG. Thermocouples are used to investigate the presence of these thermal effects and to quantify the magnitude of the spin-(dependent-)Seebeck effect. Several additional features are observed, which are also discussed.

In topological insulators (TIs), metallic surface conductance saturates the insulating bulk resistance with decreasing temperature, resulting in resistivity plateau at low temperatures as a transport signature originating from metallic surface modes protected by time reversal symmetry (TRS). Such a characteristic has been found in several materials including ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, ${\mathrm{SmB}}_6$ etc. Recently, similar behavior has been observed in metallic compound LaSb, accompanying an extremely large magnetoresistance (XMR). Shubnikov-de Hass (SdH) oscillation at low temperatures further confirms the metallic behavior of the plateau region under magnetic fields. LaSb [Tafti et al., Nat. Phys. 12, 272 (2015)] has been proposed by the authors as a possible topological semimetal (TSM), while negative magnetoresistance is absent at this moment. Here, high quality single crystals of ${\mathrm{NbAs}}_2$/${\mathrm{TaAs}}_2$ with inversion symmetry have been grown, and the resistivity under magnetic field is systematically investigated. Both of them exhibit metallic behavior under zero magnetic field, and a metal-to-insulator transition occurs when a nonzero magnetic field is applied, resulting in XMR ($1.0\times {10}^5\%$ for ${\mathrm{NbAs}}_2$ and $7.3\times {10}^5\%$ for ${\mathrm{TaAs}}_2$ at 2.5 K and 14 T). With temperature decreased, a resistivity plateau emerges after the insulatorlike regime, and SdH oscillation has also been observed in ${\mathrm{NbAs}}_2$ and ${\mathrm{TaAs}}_2$.

We describe the critical behavior of the electric field-driven (dynamic) Mott insulator-to-metal transitions in dissipative Fermi and Bose systems in terms of non-Hermitian Hamiltonians invariant under simultaneous parity $\left(\mathcal{P}\right)$ and time-reversal $\left(\mathcal{T}\right)$ operations. The dynamic Mott transition is identified as a $\mathcal{PT}$ symmetry-breaking phase transition, with the Mott insulating state corresponding to the regime of unbroken $\mathcal{PT}$ symmetry with a real energy spectrum. We establish that the imaginary part of the Hamiltonian arises from the combined effects of the driving field and inherent dissipation. We derive the renormalization and collapse of the Mott gap at the dielectric breakdown and describe the resulting critical behavior of transport characteristics. The obtained critical exponent is in an excellent agreement with experimental findings.

We generalize the concept of three-dimensional topological Weyl semimetals to a class of five dimensional (5D) gapless solids, where Weyl points are generalized to Weyl surfaces which are two-dimensional closed manifolds in the momentum space. Each Weyl surface is characterized by a $U\left(1\right)$ second Chern number $C_2$ defined on a four-dimensional manifold enclosing the Weyl surface, which is equal to its topological linking number with other Weyl surfaces in 5D. In analogy to the Weyl semimetals, the surface states of the 5D metal take the form of topologically protected Weyl fermion arcs, which connect the projections of the bulk Weyl surfaces. The further generalization of topological metals in $2n+1$ dimensions carrying the $n\mathrm{th}$ Chern number $C_n$ is also discussed.

We investigate single crystals of ${\mathrm{Ce}}_{1-x}{\mathrm{Nd}}_x{\mathrm{RhIn}}_5$ by means of x-ray-diffraction, microprobe, magnetic susceptibility, heat capacity, and electrical resistivity measurements. Our data reveal that the antiferromagnetic transition of ${\mathrm{CeRhIn}}_5$, at $T_N^{\mathrm{Ce}}=3.8$ K, is linearly suppressed with $x_{\mathrm{Nd}}$. We associate this effect with the presence of a “Kondo hole” created by Nd substitution. The extrapolation of $T_N^{\mathrm{Ce}}$ to zero temperature, however, occurs at $x_c\sim 0.3$, which is below the two-dimensional percolation limit found in ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{RhIn}}_5$. This result strongly suggests the presence of a crystal-field induced magnetic frustration. Near $x_{\mathrm{Nd}}\sim 0.2$, the Ising antiferromagnetic order from ${\mathrm{Nd}}^{3+}$ ions is stabilized and $T_N^{\mathrm{Nd}}$ increases up to 11 K in ${\mathrm{NdRhIn}}_5$. Our results shed light on the effects of magnetic doping in heavy-fermion antiferromagnets and stimulate the study of such systems under applied pressure.

The interaction energy $W\left[\gamma \right]$ of the Hubbard model is regarded as a functional of the single-particle density matrix $\gamma$ in the framework of lattice density-functional theory. The local character of the Hubbard interaction is exploited to express $W$ as a sum of local contributions ${\omega }_i\left[\gamma \right]$, for which a simple semilocal scaling approximation is proposed. The method is applied to the ionic Hubbard model on one- and two-dimensional lattices with homogeneous and inhomogeneous Coulomb repulsions. Results are given for the kinetic and Coulomb energies, interatomic charge transfers, local magnetic moments, and charge gaps. Goals and limitations of the functional are discussed by comparison with exact results.

We study low temperature properties of the Hubbard model for the bismuth nickelate, where degenerate orbitals in the nickel ions and a single orbital in the bismuth ions are taken into account, combining dynamical mean-field theory with the continuous-time quantum Monte Carlo method. We discuss the effect of the attractive interactions to mimic the valence skipping phenomenon in the bismuth ions. We demonstrate how the charge and magnetically ordered states are stable against thermal fluctuations. It is furthermore clarified that the ferromagnetically ordered and orbital ordered states are stabilized due to the presence of the orbital degeneracy at low temperatures. The crossover between metallic and insulating states is also discussed.

The transport dynamics of a quenched Luttinger liquid tunnel-coupled to a fermionic reservoir is investigated. In the transient dynamics, we show that for a sudden quench of the electron interaction universal power-law decay in time of the tunneling current occurs, ascribed to the presence of entangled compound excitations created by the quench. In sharp contrast to the usual nonuniversal power-law behavior of a zero-temperature nonquenched Luttinger liquid, the steady-state tunneling current is Ohmic and can be explained in terms of an effective quench-activated heating of the system. Our study unveils an unconventional dynamics for a quenched Luttinger liquid that could be identified in quenched cold Fermi gases.

Expanding the library of known inorganic materials with functional electronic or magnetic behavior is a long-standing goal in condensed matter physics and materials science. Recently, the transition metal chalcogenides including selenium and sulfur have been of interest because of their correlated-electron properties, as seen in the iron-based superconductors and the transition metal dichalcogenides. However, the chalcogenide chemical space is less explored than that of oxides, and there is an open question of whether there may be new materials heretofore undiscovered. We perform a systematic combined theoretical and experimental search over ternary phase diagrams that are empty in the Inorganic Crystal Structure Database containing cations, transition metals, and one of selenium or sulfur. In these 27 ternary systems, we use a probabilistic model to reduce the likelihood of false negative predictions, which results in a list of 24 candidate materials. We then conduct a variety of synthesis experiments to check the candidate materials for stability. While the prediction method did obtain previously unknown compositions that are predicted stable within density functional theory, none of the candidate materials formed in our experiments. We come to the conclusion that these phase diagrams are “empty” in the case of bulk synthesis, but it remains a possibility that alternate synthesis routes may produce some of these phases.

The local moment approach (LMA) has presented itself as a powerful semianalytical quantum impurity solver (QIS) in the context of the dynamical mean-field theory (DMFT) for the periodic Anderson model and it correctly captures the low-energy Kondo scale for the single impurity model, having excellent agreement with the Bethe ansatz and numerical renormalization group (NRG) results. However, the most common correlated lattice model, the Hubbard model, has not been explored well within the LMA+DMFT framework beyond the insulating phase. Here in our work, within the framework we complete the filling-interaction phase diagram of the single band Hubbard model at zero temperature. Our formalism is generic to any particle filling and can be extended to finite temperature. We contrast our results with another QIS, namely the iterated perturbation theory (IPT) and show that the second spectral moment sum rule improves better as the Hubbard interaction strength grows stronger in LMA, whereas it severely breaks down after the Mott transition in IPT. For the metallic case, the Fermi liquid (FL) scaling agreement with the NRG spectral density supports the fact that the FL scale emerges from the inherent Kondo physics of the impurity model. We also show that, in the metallic phase, the FL scaling of the spectral density leads to universality which extends to infinite frequency range at infinite correlation strength (strong coupling). At large interaction strength, the off half-filling spectral density forms a pseudogap near the Fermi level and filling-controlled Mott transition occurs as one approaches the half-filling. As a response property, we finally study the zero temperature optical conductivity and find universal features such as absorption peak position governed by the FL scale and a doping independent crossing point, often dubbed the isosbestic point in experiments.

We present a simple interacting tight-binding model for excitons, which is used to investigate the dependence of the exciton binding energy of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ over the disorder induced by the molecular motion at room temperature. The model is fitted to the electronic structure of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ by using data from density-functional theory and Born-Oppenheimer ab initio molecular dynamics, and it is solved in the mean-field approximation. When a finite-scale analysis is performed to extract the energetic of the excitons at experimental concentrations we find that disorder in general reduces the binding energy of about 10%. This suggests that the excitonic properties of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ largely depend on the electronic structure of the ${\mathrm{PbI}}_3$ inorganic lattice.

Silicon-vacancy (SiV) centers in diamond are promising systems for quantum information applications due to their bright single-photon emission and optically accessible spin states. Furthermore, SiV centers in low-strain diamond are insensitive to perturbations of the dielectric environment; i.e., they show very weak spectral diffusion. This property renders ensembles of SiV centers interesting for sensing applications. We here report on photoluminescence excitation (PLE) spectroscopy on an SiV ensemble in a low strain, chemical vapor deposition-grown high-quality diamond layer, where we measure the fine structure with high resolution and obtain the line widths and splittings of the SiV centers. We investigate the temperature dependence of the width and position of the fine structure peaks. Our measurements reveal line widths of about 10 GHz as compared to a lifetime limited width on the order of 0.1 GHz. This difference arises from the inhomogeneous broadening of the transitions caused by residual strain. To overcome inhomogeneous broadening we use spectral hole burning spectroscopy, which enables us to measure a nearly lifetime limited homogeneous line width of 279 MHz. Furthermore, we demonstrate evidence of coherent interaction in the system by driving a $\mathrm{\Lambda }$ scheme. Additional measurements on single emitters created by ion implantation confirm the homogeneous line widths seen in the spectral hole burning experiments and relate the ground-state splitting to the decoherence rate.

The weak interlayer binding in two-dimensional layered materials such as graphite gives rise to distinguished low-friction properties if the atomic lattices at the interface are rotated with respect to one another. The lack of crystal symmetry leads to poorly understood correlations and cancelations of the interlayer atomic forces. Here we report on a powerful tiling method based on the moiré superstructure which allows us to study the intricate interplay of the interlayer forces in a systematic manner. Based on numerical simulation data for a circular graphene flake on an infinite graphene substrate, it is shown that the sliding force is dominated by a rim area consisting of incomplete moiré tiles. This rim force, which scales with the number of atoms in a moiré tile and as the radius to the power of 0.5, is minimal whenever the sliding structure can be approximated by a hexagon composed of an integer number $N_t$ of moiré tiles. Intriguingly, the corresponding area force scales as $N_t$ to the power of 0.25, i.e., it increases with size, whereas it has been often argued that interlayer forces should add up to a zero value for large twisted systems. However, at specific twist angles the moiré structure is commensurate with the graphene lattice, leading to a perfect force correlation in the moiré tiles. Correspondingly, the area force becomes dominant and scales as $N_t$, i.e., as the radius to the power of 2.