$\mathrm{MoT}{\mathrm{e}}_2$ belongs to the semiconducting transition-metal dichalcogenide family with certain properties differing from the other well-studied members $(\mathrm{Mo},\mathrm{W}){\left(\mathrm{S},\mathrm{Se}\right)}_2$. The optical band gap is in the near-infrared region, and both monolayers and bilayers may have a direct optical band gap. We first simulate the single-particle band structure of both monolayer and bilayer $\mathrm{MoT}{\mathrm{e}}_2$ with density-functional-theory-$GW$ calculations. We find a direct (indirect) electronic band gap for the monolayer (bilayer). By solving in addition the Bethe-Salpeter equation, we find similar energies for the direct exciton transitions in monolayers and bilayers. We then study the optical properties by means of photoluminescence (PL) excitation, reflectivity, time-resolved PL, and power-dependent PL spectroscopy. With differential reflectivity, we find a similar oscillator strength for the optical transition observed in PL in both monolayers and bilayers suggesting a direct transition in both cases. We identify the same energy for the $B$-exciton state in the monolayer and the bilayer. Following circularly polarized excitation, we do not find any exciton polarization for a large range of excitation energies. At low temperatures $(T=10\mathrm{K})$, we measure similar PL decay times on the order of 4 ps for both monolayer and bilayer excitons with a slightly longer one for the bilayer. Finally, we observe a reduction of the exciton-exciton annihilation contribution to the nonradiative recombination in bilayers.

We investigated the structural and electronic properties of vacuum sublimed 7,8,15,16-tetraazaterrylene (TAT) thin films on Au(111), Ag(111), and Cu(111) substrates using inverse photoemission spectroscopy, ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and the x-ray standing wave (XSW) technique. The LEED reveals a flat adsorption geometry of the monolayer TAT on these three substrates, which is in accordance with the XSW results. The molecules are slightly distorted in monolayers on all three substrates with the nitrogen atoms having smaller averaged bonding distances than the carbon atoms. On Ag(111) and Cu(111), chemisorption with a net electron transfer from the substrate to the adsorbate takes place, as evidenced by UPS and XPS. Combining these results, we gain full insight into the correlation between electronic properties and interface geometry.

Solid Si under intense femtosecond irradiation is investigated over a wide range of fluences below the melting transition by supercontinuum light (400–800 nm) transient reflectivity. By solving a system of time-dependent equations, the fast and slow components of the energy and carriers diffusion can be disentangled from the reflectivity data, providing considerable insight into the nonequilibrium phase-change dynamics. The study of the fluence values immediately preceding the dramatic melting transition can be useful for discriminating between the thermal and electronic origins of the disordering of the structure by looking at the modification of the solid-state properties of the Si surface.

The excitonic spectra of single-layer GeS and GeSe are predicted by ab initio GW and Bethe-Salpeter equation calculations. $G_0{W}_0$ calculations for the band structures find a fundamental band gap of 2.85 eV for GeS and 1.70 eV for GeSe monolayer. However, excitons are tightly bound, specially in GeS at the $\mathrm{\Gamma }$ point, where the quasiparticle interactions are so strong that they shift the $\mathrm{\Gamma }$ exciton peak energy into the visible range and below the $\text{off-}\mathrm{\Gamma }$ exciton peak. The lowest energy excitons in both materials are excited by light along the zigzag direction and have exciton binding energies of 1.05 and 0.4 eV, respectively; but despite the strong binding, the calculated binding energies are in agreement with a Mott-Wannier model.

The lifetime of localized surface plasmon plays an important role in many aspects of plasmonics and its applications. In small metal nanostructures, the dominant mechanism of plasmon decay is size-dependent Landau damping. We performed quantum-mechanical calculations of Landau damping for the bright surface plasmon mode in a metal nanoshell with dielectric core. In contrast to the conventional model based on the electron surface scattering, we found that the damping rate decreases as the nanoshell thickness is reduced. The origin of this behavior is traced to the spatial distribution of plasmon local field in the metal shell. We also found that, due to the interference of electron scattering amplitudes from the two nanoshell metal surfaces, the damping rate exhibits pronounced quantum beats with changing shell thickness.

By exploiting the guided resonance of a photonic hypercrystal—a periodic structure that combines the properties of hyperbolic materials and photonic crystals—we numerically demonstrate a perfect absorber consisting of a photonic hypercrystal slab, a dielectric spacer, and a back reflector. The guided resonance of the photonic hypercrystal slab creates a field enhancement and confinement within a deep subwavelength thickness; therefore, the ultrathin photonic hypercrystal slab serves as a two-dimensional resonator that can be treated accurately as a metasurface. We show that the perfect absorber is equivalent to a metasurface Salisbury screen, where the metasurface formed by the photonic hypercrystal slab provides the appropriate electric sheet conductivity required for critical coupling. We also present examples of combining the perfect absorber with graphene to make optical modulators and to improve the absorption in graphene photodetectors.

We investigate the dynamics of a strongly driven microwave-dressed donor-bound electron spin qubit in silicon. A resonant oscillating magnetic field $B_1$ is used to dress the electron spin and create a new quantum system with a level splitting proportional to $B_1$. The dressed two-level system can then be driven by modulating the detuning $\mathrm{\Delta }\nu$ between the microwave source frequency ${\nu }_{\mathrm{MW}}$ and the electron spin transition frequency ${\nu }_e$ at the frequency of the level splitting. The resulting dressed qubit Rabi frequency ${\mathrm{\Omega }}_{R\rho }$ is defined by the modulation amplitude, which can be made comparable to the level splitting using frequency modulation on the microwave source. This allows us to investigate the regime where the rotating wave approximation breaks down without requiring microwave power levels that would be incompatible with a cryogenic environment. We observe clear deviations from normal Rabi oscillations and can numerically simulate the time evolution of the states in excellent agreement with the experimental data.

We report in this Rapid Communication an antilevitation behavior of Landau levels in vanishing magnetic fields in a high quality heterojunction insulated-gated field-effect transistor. We found, in the Landau fan diagram of electron density versus magnetic field, the positions of the magnetoresistance minima at Landau-level fillings $\nu =4-6$ move below the “traditional” Landau-level line to lower electron densities. Moreover, the even and odd filling factors show quantitatively different behaviors in antilevitation, suggesting that the exchange interactions may be important.

A metamaterial composed of stacked plasmonic waveguides which support backward propagation along the layers has been shown to exhibit a nearly spherical equifrequency contour (EFC) in which the Floquet-Bloch wave vector ${\mathbf{k}}_{\mathrm{FB}}$ and Poynting vector $\mathbf{S}$ point in opposite directions everywhere on this surface. Experiments performed on this structure have also shown that polarized light beams incident from free space refract to the same side of normal over a wide range of incidence angles. Together, these observations have led researchers to describe this structure as a homogeneous medium with three-dimensionally isotropic negative refractive index; however, a close inspection of the fields throughout the structure as provided in this paper would suggest otherwise. Here, we rigorously analyze the relationship between phase and power flow within the structure by introducing a method to calculate the power flow of all Floquet-Bloch harmonics, information which cannot be obtained from either conventional analysis of EFCs or effective medium theory. Access to power flow of all harmonics enables us to demonstrate the origin of backward power (defined with respect to the direction of ${\mathbf{k}}_{\mathrm{FB}})$, and in doing so, verify the validity of the claimed three-dimensionally isotropic left-handed response and the validity of describing the medium by a simple negative effective index of refraction $n=-1$. Knowledge regarding the distribution of power flow across the harmonics can also be used to design highly efficient methods to couple light into and out of these structures. As an example, we show that tailored wave excitation can achieve coupling efficiencies of up to 96%, over 5 times greater than that achieved by normal-incidence plane-wave excitation.

A Boltzmann machine is a stochastic neural network that has been extensively used in the layers of deep architectures for modern machine learning applications. In this paper, we develop a Boltzmann machine that is capable of modeling thermodynamic observables for physical systems in thermal equilibrium. Through unsupervised learning, we train the Boltzmann machine on data sets constructed with spin configurations importance sampled from the partition function of an Ising Hamiltonian at different temperatures using Monte Carlo (MC) methods. The trained Boltzmann machine is then used to generate spin states, for which we compare thermodynamic observables to those computed by direct MC sampling. We demonstrate that the Boltzmann machine can faithfully reproduce the observables of the physical system. Further, we observe that the number of neurons required to obtain accurate results increases as the system is brought close to criticality.

We study a restricted class of correlator product states (CPS) for a spin-half chain in which each spin is contained in just two overlapping plaquettes. This class is also a restriction upon matrix product states (MPS) with local dimension $2^n$ ($n$ being the size of the overlapping regions of plaquettes) equal to the bond dimension. We investigate the trade-off between gains in efficiency due to this restriction against losses in fidelity. The time-dependent variational principle formulated for these states is numerically very stable. Moreover, it shows significant gains in efficiency compared to the naively related matrix product states—the evolution or optimization scales as $2^{3n}$ for the correlator product states versus $2^{4n}$ for the unrestricted matrix product state. However, much of this advantage is offset by a significant reduction in fidelity. Correlator product states break the local Hilbert space symmetry by the explicit selection of a local basis. We investigate this dependence in detail and formulate the broad principles under which correlator product states may be a useful tool. In particular, we find that scaling with overlap/bond order may be more stable with correlator product states allowing a more efficient extraction of critical exponents—we present an example in which the use of correlator product states is several orders of magnitude quicker than matrix product states.

Universal scaling terms occurring in Rényi entanglement entropies have the potential to bring new understanding to quantum critical points in free and interacting systems. Quantitative comparisons between analytical continuum theories and numerical calculations on lattice models play a crucial role in advancing such studies. In this paper, we exactly calculate the universal two-cylinder shape dependence of entanglement entropies for free bosons on finite-size square lattices, and compare to approximate functions derived in the continuum using several different Ansätze. Although none of these Ansätze are exact in the thermodynamic limit, we find that numerical fits are in good agreement with continuum functions derived using the anti–de Sitter/conformal field theory correspondence, an extensive mutual information model, and a quantum Lifshitz model. We use fits of our lattice data to these functions to calculate universal scalars defined in the thin-cylinder limit, and compare to values previously obtained for the free boson field theory in the continuum.

Electrical resistivity ($\rho$), thermopower, and specific heat measurements have been performed on the novel Kondo semiconductor $\mathrm{Ce}{\left({\mathrm{Ru}}_{1-x}{\mathrm{Rh}}_x\right)}_2{\mathrm{Al}}_{10}$ ($x=0$, 0.02, 0.03, and 0.05), which has been attracting a great deal of interest due to an unusual antiferromagnetic (AFM) order below $T_0$, in order to clarify the Rh doping effect on the anisotropy of the electronic properties in the ordered state. In ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}, \rho$ shows an anisotropic increase below $T_0$ independently of the electric current direction. We propose the existence of two different mechanisms to explain the anisotropic increase of $\rho$. One is an isotropic charge gap which enhances $\rho$ below $T_0$ isotropically, although its origin is not known at present. The other is an anisotropic suppression of $\rho$ which originates from the anisotropic c-f hybridization and is largest along the orthorhombic $a$ axis. By the Rh doping, the anisotropic temperature dependence of $\rho$ below $T_0$ is drastically changed. For $I\parallel b$, the increase is almost completely suppressed and a metallic-like behavior is observed, whereas it is small and isotropic for $I\parallel a$ and $c$. From these results, we propose that as a result of the destruction of the spin-gap excitation by the Rh doping, a metallic-like electronic state is formed along the $b$ axis and the small isotropic charge gap is opened in the $ac$ plane. By taking into account the present results and the still high $T_0$ even in $x=0.05$, we conclude that the AFM order in the Rh-doped ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ should be viewed as unusual as the AFM order in ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ although the localized character of the Ce-$4f$ electron is apparently enhanced by the Rh doping. We have also examined the evolution of the AFM ordered state from $x=0$ to $x=0.05$, where the AFM ordered moment ($m_{\mathrm{AF}}$) is aligned along the $c$ axis in $x=0$ and $a$ axis in $x=0.05$. From the results of those experiments in magnetic field, we have revealed that the spin reorientation from $m_{\mathrm{AF}}\parallel c$ to $m_{\mathrm{AF}}\parallel a$ takes place quite abruptly just at $x_c\sim 0.03$, which could be related to the topology change of the Fermi surfaces.

In a recent proposal of the half-filled Landau level, the composite fermions are taken to be Dirac particles and particle-hole symmetric. Cooper pairing of these composite fermions in different angular momentum channels, $\ell$, can give rise to different kinds of Pfaffian states. In addition to the well-known Moore-Read Pfaffian and anti-Pfaffian states, a new putative particle-hole symmetric Pfaffian state, corresponding to the $s$-wave pairing channel, was also proposed. However, the possible underlying pairing mechanism is not clear at all. In this work we provide a specific pairing mechanism for realizing some of these Pfaffian states. We show that there can be nonzero pairing in angular momentum channels $\left|\ell \right|\ge 1$ depending on the magnitude of a coupling constant. There is a quantum phase transition from the Dirac composite Fermi-liquid state to Cooper pairing states in angular momentum channels $\left|\ell \right|\ge 1$ as the coupling constant is tuned across its critical point value. Surprisingly the particle-hole symmetric $\ell =0$ channel pairing turns out to be impossible irrespective of the size of the coupling constant.

Many-component electron-hole plasma is considered in the coupled quantum wells. The electrons are assumed to be localized in one quantum well (QW) while the holes are localized in the other QW. It is found that the homogeneous charge distribution within the QWs is unstable if the carrier density is sufficiently small. The instability results in the breakdown of the homogeneous charge distribution into two coexisting phases—a low-density phase and a high-density phase, which is electron-hole liquid. In turn, the homogeneous state of the electron-hole liquid is stable if the distance between the quantum wells $\ell$ is sufficiently small. However, as the distance $\ell$ increases and reaches a certain critical value ${\ell }_{\text{cr}}$, the plasmon spectrum of the electron-hole liquid becomes unstable. Hereupon, a quantum phase transition occurs, resulting in the appearance of charge-density waves of finite amplitude in both quantum wells. Strong mass renormalization and the strong $Z$-factor renormalization are found for the electron-hole liquid as the quantum phase transition occurs.

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a $\left(2\times 2\right)$ reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications.

Based on the quantum kinetic equations for systems with SU(2) structure, regularization-free density and pseudospin currents are calculated in graphene realized as the infinite-mass limit of electrons with quadratic dispersion and a proper spin-orbit coupling. Correspondingly the currents possess no quasiparticle part but only anomalous parts. The intraband and interband conductivities are discussed with respect to magnetic fields and magnetic domain puddles. It is found that the magnetic field and mean field of domains can be represented by an effective Zeeman field. For large Zeeman fields the dynamical conductivities become independent of the density and are universal in this sense. The different limits of vanishing density, relaxation, frequency, and Zeeman field are not interchangeable. The optical conductivity agrees well with the experimental values using screened impurity scattering and an effective Zeeman field. The universal value of Hall conductivity is shown to be modified due to the Zeeman field. The pseudospin current reveals an anomaly since a quasiparticle part appears though it vanishes for particle currents. The density and pseudospin response functions to an external electric field are calculated and the dielectric function is discussed with respect to collective excitations. A frequency and wave-vector range is identified where the dielectric function changes sign and the repulsive Coulomb potential becomes effectively attractive allowing Cooper pairing.

This paper examines the thermoelectric response of a dissipative quantum-dot heat engine based on the Anderson-Holstein model in two relevant operating limits, (i) when the dot phonon modes are out of equilibrium, and (ii) when the dot phonon modes are strongly coupled to a heat bath. In the first case, a detailed analysis of the physics related to the interplay between the quantum-dot level quantization, the on-site Coulomb interaction, and the electron-phonon coupling on the thermoelectric performance reveals that an $n$-type heat engine performs better than a $p$-type heat engine. In the second case, with the aid of the dot temperature estimated by incorporating a thermometer bath, it is shown that the dot temperature deviates from the bath temperature as electron-phonon interaction in the dot becomes stronger. Consequently, it is demonstrated that the dot temperature controls the direction of phonon heat currents, thereby influencing the thermoelectric performance. Finally, the conditions on the maximum efficiency with varying phonon couplings between the dot and all the other macroscopic bodies are analyzed in order to reveal the nature of the optimum junction.

Hybrid DFT calculations of atomic structure, electronic band structure, and reflectance anisotropy (RA) are used to correlate atomic and electronic structures and optical transitions of eight structures for the $\mathrm{Si}\left(111\right)\text{−}\left(5\times 2\right)\mathrm{Au}$ phase with Au coverage and Si adatom site occupancy. The structure recently proposed by Kwon and Kang (KK) [S. G. Kwon and M. H. Kang, Phys. Rev. Lett. 113, 086101 (2014)] and strongly supported by surface x-ray diffraction [T. Shirasawa et al., Phys. Rev. Lett. 113, 165501 (2014)] is found to be the most stable in the Au coverage range from 0.6 to 0.8 ML. The band structure of the Si adatom covered $\left(5\times 4\right)$ surface has a surface state whose dispersion and position agree very well with the prominent $S_1$ surface state observed in angle-resolved photoemission experiment. Reflectance anisotropy spectra in the energy range 0.5 to 5 eV for the eight surface structures show variations with Au and Si adatom coverage which can be explained in terms of filling of this surface state. The best agreement between predicted and measured RA spectra, which are sensitive to both Au atom and Si adatom coverage, is found for coverages from 0.7 to 0.75 ML Au, when the surface has regions where Si adatoms are absent and where there are sufficient adatoms to cause surface-state band filling, in equal parts. Surface formation energy calculations favor a coverage of 0.7-ML Au. The structures of the $\left(5\times 2\right)$ KK and $\left(6\times 6\right)\mathrm{Au}$ phases on the Si(111) surface are compared and it is shown that the repeat unit of the KK phase also occurs in the $\left(6\times 6\right)$ structure.

Gap plasmonics deals with the properties of surface plasmons in the narrow region between two metallic nanoparticles forming the gap. For subnanometer gap distances, electrons can tunnel between the nanoparticles, leading to the emergence of novel charge-transfer plasmons. These are conveniently described within the quantum corrected model by introducing an artificial material with a tunnel conductivity inside the gap region. Here we develop a methodology for computing such tunnel conductivities within the first-principles framework of density functional theory and apply our approach to a jellium model representative for sodium. We show that the frequency dependence of the tunnel conductivity at infrared and optical frequencies can be significantly more complicated than previously thought.