We show that a specific superposition principle is valid for nonlinear Josephson plasma waves in layered superconductors, which is very unusual for nonlinear media. We study theoretically the reflection and transmission of terahertz waves through a finite-size superconducting slab placed inside a rectangular waveguide with ideal-metal walls. We assume that the superconducting layers are parallel to the waveguide axis. We show that there exist two specific mutually orthogonal polarizations for waves which, in spite of the nonlinearity, reflect and transmit through the superconductor independently. The wave of the first polarization causes a strong shielding current along the crystallographic $ab$ plane of the superconductor. Therefore, this wave reflects nearly completely from the superconductor and excites only an evanescent mode inside it. The wave of the other polarization does not contain the electric field component parallel to both the sample surface and the crystallographic $ab$ plane and excites much weaker shielding currents. Therefore, it partially reflects from and partially transmits through the sample. Moreover, this wave excites the nonlinear mode in the layered superconductor, and the transmission coefficient of the superconductor depends on the amplitude of the incident wave of this polarization. However, this transmission coefficient is independent of the amplitude of the wave with first polarization. On the basis of the discussed superposition principle, we suggest a method for solving nonlinear problems of waves interacting in layered superconductors. Namely, it is reasonable to represent incident, reflected, and transmitted waves of any polarization as superpositions of the modes with the two specific polarizations considered here and then to solve the problem separately for these modes. We apply this method to the case of nonlinear interaction and mutual transformation of the transverse-electric and transverse-magnetic modes in layered superconductors.

We propose a hybrid quantum architecture for engineering a photonic Mott insulator-superfluid phase transition in a two-dimensional (2D) square lattice of a superconducting transmission line resonator (TLR) coupled to a single nitrogen-vacancy (NV) center encircled by a persistent current qubit. The localization-delocalization transition results from the interplay between the on-site repulsion and the nonlocal tunneling. The phase boundary in the case of photon hopping with real-valued and complex-valued amplitudes can be obtained using the mean-field approach. Also, the quantum jump technique is employed to describe the phase diagram when the dissipative effects are considered. The unique feature of our architecture is the good tunability of effective on-site repulsion and photon-hopping rate, and the local statistical property of TLRs which can be analyzed readily using present microwave techniques. Our work opens new perspectives in quantum simulation of condensed-matter and many-body physics using a hybrid spin circuit-QED system. The experimental challenges are realizable using currently available technologies.

Using density functional theory we investigate the lattice instability and electronic structure of recently discovered ferroelectric metal ${\mathrm{LiOsO}}_3$. We show that the ferroelectric-like lattice instability is related to the Li-O distortion modes while the Os-O displacements change the $d-p$ hybridization as in common ferroelectric insulators. Within the manifold of the $d$ orbitals, a dual behavior emerges. In the ferroelectric transition the empty $e_g$ orbitals change their hybridization with the oxygen $p$ orbitals, while the $t_{2g}$ orbitals are responsible for the metallic response. Interestingly, these orbitals are nominally half filled by three electrons, a configuration which suffers from strong correlation effects even for moderate values of the screened Coulomb interaction.

Using extended dynamical mean-field theory and its combination with the $GW$ approximation, we compute the phase diagrams and local spectral functions of the single-band extended Hubbard model on the square and simple cubic lattices, considering long-range interactions up to the third nearest neighbors. The longer-range interactions shift the boundaries between the metallic, charge-ordered insulating, and Mott insulating phases, and lead to characteristic changes in the screening modes and local spectral functions. Momentum-dependent self-energy contributions enhance the correlation effects and thus compete with the additional screening effect from longer-range Coulomb interactions. Our results suggest that the influence of longer-range intersite interactions is significant, and that these effects deserve attention in realistic studies of correlated materials.

Given the recent success in achieving efficient organic photovoltaic solar cells based on thieno[3,4-b]thiophene/benzodithiophene polymers (PTB7) and growing efforts to further improve the power conversion efficiency of the PTB7-based devices, a detailed atomic-scale picture of the electronic structure and the excitonic properties of PTB7 crystal is highly desirable. We report electronic and optical properties of PTB7 on the basis of first-principles density functional theory and GW many-body plus Bethe-Salpeter equation (GW-BSE) calculations. It is established that the first two highest valence bands (HVBs) and the first two lowest conduction bands (LCBs) originate from the benzodithiophene and thieno[3,4-b]thiophene functional units, respectively, thus confirming the donor-acceptor nature of PTB7. A significant difference of band splitting between HVBs and LCBs is found and its origins are explained. Our results strongly suggest that the strength of the interchain $\pi \text{−}\pi$ interaction is not only a function of interchain distance, but is also highly dependent on the nature of the fused rings. The experimental optical absorption spectrum of PTB7 is well reproduced and explained by our GW-BSE calculations. Further analysis shows that the nature of the lowest singlet (triplet) excitons in polymeric crystals such as PTB7 differs from that of organic molecular crystals. A possible reason is explored by combining BSE calculations with a simple Hamiltonian model.

On a rutile ${\mathrm{TiO}}_2\left(110\right)$ surface, vacancies at the bridging oxygen sites are thought to be major surface point defects and strongly influence the surface phenomena. Using systematic density functional theory (DFT) calculations, however, we show that vacancies at the basal oxygen sites have comparable formation energies to the bridging oxygen vacancies. The correction of self-interaction error for localized $\mathrm{Ti}3d$ states in DFT plays an important role for correctly describing the relative stability of the oxygen vacancies as well as the electronic structures. This new type of stable surface oxygen vacancy can be formed with a similar amount of the bridging oxygen vacancy in a relatively dilute condition, which may affect atom deposition and chemical reactions on the surface.

We present a paper based on combining four-wave mixing and strong fields in the terahertz frequency range to monitor the time evolution of a disturbed excitonic polarization in a multiple quantum well system. Our findings not only confirm a lower field-dependent ionization threshold for higher excitonic states, but furthermore provide experimental evidence for intraexcitonic Rabi flopping in the time domain. These measurements correspond to the picture of a reversible and irreversible transfer as previously predicted by a microscopic theory.

As one emerging plasmonic material, graphene can support surface plasmons at infrared and terahertz frequencies with unprecedented properties due to the strong interactions between graphene and low-frequency photons. Since graphene surface plasmons exist in the infrared and terahertz regime, they can be thermally pumped (excited) by the infrared evanescent waves emitted from an object. Here we show that thermal graphene plasmons can be efficiently excited and have monochromatic and tunable spectra, thus paving a way to harness thermal energy for graphene plasmonic devices. We further demonstrate that “thermal information communication” via graphene surface plasmons can be potentially realized by effectively harnessing thermal energy from various heat sources, e.g., the waste heat dissipated from nanoelectronic devices. These findings open up an avenue of thermal plasmonics based on graphene for different applications ranging from infrared emission control, to information processing and communication, to energy harvesting.

Inspired by the acoustic Helmholtz resonator, we propose a slit-box electromagnetic nanoantenna able to concentrate the energy of an incident beam into surfaces a thousand times smaller than with a classical lens. This design produces a giant electric field enhancement throughout the slit. The intensity enhancement reaches ${10}^4$ in the visible range up to ${10}^8$ in the THz range even with focused beams, thanks to an omnidirectional reception. These properties could target applications requiring extreme light concentration, such as surface-enhanced infrared absorption, nonlinear optics, and biophotonics.

We report the pressure dependence of the superconducting transition temperature $T_c$ in ${\mathrm{TlNi}}_2{\mathrm{Se}}_{2-x}{\mathrm{S}}_x$ detected via the ac susceptibility method. The pressure-temperature phase diagram constructed for ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$, ${\mathrm{TlNi}}_2{\mathrm{S}}_2$, and ${\mathrm{TlNi}}_2\mathrm{SeS}$ exhibits two unexpected features: (a) a sudden collapse of the superconducting state at moderate pressure for all three compositions and (b) a dome-shaped pressure dependence of $T_c$ for ${\mathrm{TlNi}}_2\mathrm{SeS}$. These results point to the nontrivial role of S substitution and its subtle interplay with applied pressure, as well as interesting superconducting properties of the ${\mathrm{TlNi}}_2{\mathrm{Se}}_{2-x}{\mathrm{S}}_x$ system.

Recently, a Dirac surface state (SS) was observed in Rashba semiconductor BiTeCl by angle-resolved photoemission spectroscopy (ARPES), which suggested strong inversion symmetry breaking therein, despite the absence of such symmetry breaking in existing first-principles calculations. To clarify the aforementioned conflict as well as understand the nature of such emergent phenomenon, we employ both high-field Shubnikov–de Haas (SdH) oscillation and Hall measurements to study BiTeCl single crystals. Both techniques yield consistent observation of a three-dimensional Fermi surface from a bulk state, while Dirac surface state contribution appears absent. Finally, we propose that various gating techniques could be used to explore the novel topological nature of this material.

We report on spatially resolved low-temperature photoluminescence (PL) measurements of excitons in ultrapure bulk GaAs. At moderate excitation densities we observe butterfly-shaped luminescence images in the wavelength–radial distance plane with a pronounced quench of the exciton PL intensity at the excitation center. The shapes of the PL images show a delicate dependence on excitation wavelength and pump power. We present a model that quantitatively explains the PL intensity quench by a localized overheating of the exciton ensemble due to nonresonant optical excitation. Our model allows us to extract absolute exciton temperatures and to trace the influence of excitation excess energy on the spatial dependence of the exciton energy relaxation. We observe temperature gradients in the exciton system which persist over distances $\ge 10\mu \mathrm{m}$ away from the excitation spot.

We find evidence that superconductivity intrudes into the paramagnetic-to-magnetic transition of the Kondo lattice model if magnetic frustration is added. Specifically, we study by the variational method the model on a square lattice in the presence of both nearest-neighbor ($t$) and next-nearest-neighbor ($t^{\prime }$) hopping of the conduction electrons. We find that, when $t^{\prime }/t>0$, a $d$-wave superconducting dome emerges between the magnetic and paramagnetic metal phases and close to the compensated regime, i.e., the number of conduction electrons equals the number of localized spin-1/2 moments. Superconductivity is further strengthened by a direct antiferromagnetic exchange, $J_H$, between the localized moments, to such an extent that we observe coexistence with magnetic order.

Topological order has been proposed to go beyond Landau symmetry breaking theory for more than 20 years. But it is still a challenging problem to generally detect it in a generic many-body state. In this paper, we will introduce a systematic numerical method based on tensor network to calculate modular matrices in two-dimensional systems, which can fully identify topological order with gapped edge. Moreover, it is shown numerically that modular matrices, including $S$ and $T$ matrices, are robust characterization to describe phase transitions between topologically ordered states and trivial states, which can work as topological order parameters. This method only requires local information of one ground state in the form of a tensor network, and directly provides the universal data ($S$ and $T$ matrices), without any nonuniversal contributions. Furthermore, it is generalizable to higher dimensions. Unlike calculating topological entanglement entropy by extrapolating, in which numerical complexity is exponentially high, this method extracts a much more complete set of topological data (modular matrices) with much lower numerical cost.

We consider manifestations of topological order in time-reversal-symmetric fractional topological liquids (TRS-FTLs), defined on planar surfaces with holes. We derive a formula for the topological ground-state degeneracy of such a TRS-FTL, which applies to cases where the edge modes on each boundary are fully gapped by appropriate backscattering terms. The degeneracy is exact in the limit of infinite system size, and is given by $q^{N_{\mathrm{h}}^{}}$, where $N_{\mathrm{h}}^{}$ is the number of holes and $q$ is an integer that is determined by the topological field theory. When the degeneracy is lifted by finite-size effects, the holes realize a system of $N_{\mathrm{h}}^{}$ coupled spinlike $q$-state degrees of freedom. In particular, we provide examples where ${\mathbb{Z}}_q^{}$ quantum clock models are realized on the low-energy manifold of states. We also investigate the possibility of measuring the topological ground-state degeneracy with calorimetry, and briefly revisit the notion of topological order in $s$-wave BCS superconductors.

We demonstrate the observation of many-body lifetime effects in valence-band x-ray emission. A comparison of the N $K\alpha$ emission of crystalline ammonium nitrate to molecular-orbital calculations revealed an unexpected, extreme broadening of the NO $\sigma$ recombination—so extensively as to virtually disappear. GW calculations establish that this disappearance is due to a large imaginary component of the self-energy associated with the NO $\sigma$ orbitals. Building upon density-functional theory, we have calculated radiative transitions from the nitrogen $1s$ level of ammonium nitrate and ammonium chloride using a Bethe-Salpeter method to include electron-hole interactions. The absorption and emission spectra of both crystals evince large, orbital-dependent sensitivity to molecular dynamics. We demonstrate that many-body effects as well as thermal and zero-point motion are vital for understanding observed spectra. A computational approach using average atomic positions and uniform broadening to account for lifetime and phonon effects is unsatisfactory.

In honeycomb Dirac systems with broken inversion symmetry, orbital magnetic moments coupled to the valley degree of freedom arise due to the topology of the band structure, leading to valley-selective optical dichroism. On the other hand, in Dirac systems with prominent spin-orbit coupling, similar orbital magnetic moments emerge as well. These moments are coupled to spin, but otherwise have the same functional form as the moments stemming from spatial inversion breaking. After reviewing the basic properties of these moments, which are relevant for a whole set of newly discovered materials, such as silicene and germanene, we study the particular impact that these moments have on graphene nanoengineered barriers with artificially enhanced spin-orbit coupling. We examine transmission properties of such barriers in the presence of a magnetic field. The orbital moments are found to manifest in transport characteristics through spin-dependent transmission and conductance, making them directly accessible in experiments. Moreover, the Zeeman-type effects appear without explicitly incorporating the Zeeman term in the models, i.e., by using minimal coupling and Peierls substitution in continuum and the tight-binding methods, respectively. We find that a quasiclassical view is able to explain all the observed phenomena.

We report tight-binding and density functional theory calculations of magnetocrystalline anisotropy energy (MAE) of free Fe (body-centered-cubic) and Co (face-centered-cubic) slabs and nanocrystals. The nanocrystals are truncated square pyramids which can be grown experimentally by deposition of metal on a ${\mathrm{SrTiO}}_3$(001) substrate. For both elements our local analysis shows that the total MAE of the nanocrystals is largely dominated by the contribution of $\left(001\right)$ facets. However, while the easy axis of $\mathrm{Fe}\left(001\right)$ is out-of-plane, it is in-plane for $\mathrm{Co}\left(001\right)$. This has direct consequences on the magnetic reversal mechanism of the nanocrystals. Indeed, the very high uniaxial anisotropy of Fe nanocrystals makes them a much better potential candidate for magnetic storage devices.