Single crystals of the Kitaev spin-liquid candidate $\alpha -{\mathrm{RuCl}}_3$ have been studied to determine the low-temperature bulk properties, the structure, and the magnetic ground state. Refinements of x-ray diffraction data show that the low-temperature crystal structure is described by space group $C2/m$ with a nearly perfect honeycomb lattice exhibiting less than 0.2% in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at $T_N=7$ K. The magnetic order below this temperature exhibits a propagation vector of $k=(0,1,1/3)$, which coincides with a three-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14 K can be introduced by deformations of the crystal that result in regions in the crystal with a two-layer stacking sequence. The best-fit symmetry-allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$ plane, with a zigzag configuration in each honeycomb layer. The three-layer repeat out-of-plane structure can be refined as a ${120}^{\circ }$ spiral order or a collinear structure with a spin direction of ${35}^{\circ }$ away from the $a$ axis. The collinear spin configuration yields a slightly better fit and also is physically preferred. The average ordered moment in either structure is less than 0.45(5) ${\mu }_B$ per ${\mathrm{Ru}}^{3+}$ ion.

An interface between two media is a topologically stable two-dimensional object where 3D-symmetry breaks which allows for the existence of many exotic excitations. A direct way to explore surface excitations is to investigate their interaction with the surface waves, such as very well known capillary-gravity waves and crystallization waves. Helium remains liquid down to absolute zero where bulk excitations are frozen out and do not mask the interaction of the waves with the surface states. Here we show the possibility of the massless wave which can propagate along the surface between two different superfluids phases of ${}^3\mathrm{He}$. The displacement of the surface in this wave occurs due to the transition of helium atoms from one phase to another, so that there is no flow of particles as densities of phases are equal. We calculate the dispersion of the wave in which the inertia is provided by spin supercurrents, and the restoring force is magnetic field gradient. We calculate the dissipation of the wave and show the preferable conditions to observe it.

One of the unique features of atomically thin two-dimensional materials is strong Coulomb interactions due to the reduced dielectric screening effect; this feature enables the study of many-body phenomena such as excitons, trions, and biexcitons. However, identification of biexcitons remains unresolved owing to their broad peak feature at room temperature. Here, we investigate multiexcitons in monolayer $\mathrm{Mo}{\mathrm{S}}_2$ using both electrical and optical doping and identify the transition energies for each exciton. The binding energy of the assigned biexciton is twice that of the trion, in quantitative agreement with theoretical predictions. The biexciton population is predominant under optical doping but negligible under electrical doping. The biexciton population is quadratically proportional to the exciton population, obeying the mass-action theory. Our results illustrate the stable formation of not only trions but also biexcitons due to strong Coulomb interaction even at room temperature; therefore, these results provide a deeper understanding of the complex excitonic behaviors in two-dimensional semiconductors.

A new ground state of textbook compound strontium titanate $\left(\mathrm{SrTi}{\mathrm{O}}_3\right)$ is obtained by inducing a specific core-shell structure of the particles. Using a combination of high energy synchrotron and neutron diffraction, we demonstrate a lowering of the ferroelastic ground state towards a new antiferrodistortive phase, accompanied with strong shifts of the critical temperature. This new phase is discussed within the Landau theory and compared with the situation in thin films and during pressure experiments. The crucial competition between particle shape anisotropy, surface tension, and shear strain is analyzed. Inducing a specific core-shell structure is therefore an easy way to tailor structural properties and to stabilize new phases that cannot exist in bulk material, just like film deposition on a substrate.

We present an extension of the canonical coupled-mode theory of electromagnetic waves to the case of pulses and spatiotemporal perturbations in complex media. Unlike previous attempts to derive such a model, our approach involves no approximation, and it does not impose any restriction on the spatiotemporal profile. Moreover, the effect of modal dispersion on mode evolution and on the coupling to other modes is fully taken into account. Thus, our approach can yield any required accuracy by retaining as many terms in the expansion as needed. It also avoids various artifacts of previous derivations by introducing the correct form of the solution. We then validate the coupled-mode equations with exact numerical simulations, and we demonstrate the wide range of possibilities enabled by spatiotemporal perturbations of pulses, including pulse shortening or broadening or more complex shaping. Our formulation is valid across the electromagnetic spectrum, and it can be applied directly also to other wave systems.

Recent experiments on Zn-substituted 122-type iron-based superconductors (FeSCs) at electron- and hole-doped regions provide us with a testing ground for understanding the effect of Zn impurities in these systems. Our first-principles calculations of the electronic structure reveal that the Zn $3d$ orbitals are far below the Fermi level and are chemically inactive, while the Zn $4s$ orbital is partially occupied and its wave function overlaps with the $3d$ orbitals of neighboring Fe ions. This suggests that the impurity effect originates in the Zn $4s$ orbital, not its $3d$ orbitals. Employing a phenomenological two-orbital lattice model for 122-FeSCs and the self-consistent Bogoliubov–de Gennes equations, we study how the Zn impurities suppress the superconductivity in electron- and hole-doped compounds. Our obtained results qualitatively agree with the experimental measurements.

We study the low-energy spectral properties of positive center-of-mass conserving two-body Hamiltonians as they arise in models of fractional quantum Hall states. Starting from the observation that positive many-body Hamiltonians must have ground-state energies that increase monotonously in particle number, we explore what general additional constraints can be obtained for two-body interactions with “center-of-mass conservation” symmetry, both in the presence and absence of particle-hole symmetry. We find general bounds that constrain the evolution of the ground-state energy with particle number, and in particular, constrain the chemical potential at $T=0$. Special attention is given to Hamiltonians with zero modes, in which case similar bounds on the first excited state are also obtained, using a duality property. In this case, in particular, an upper bound on the charge gap is also obtained. We further comment on center of mass and relative decomposition in disk geometry within the framework of second quantization.

A bilayer system comprising a Kondo insulator coupled to a simple metal (KI-M) is considered. Employing the framework of dynamical mean-field theory, the model system is shown to exhibit a surface of quantum critical points (QCPs) that separates a Kondo screened, Fermi liquid phase from a local moment, Mott insulating phase. The quantum critical nature of these Mott transitions is characterized by the vanishing of (a) the coherence scale on the Fermi liquid side, and (b) the Mott gap on the MI side. In contrast to the usual “large-to-small” Fermi surface (FS) QCPs in heavy-fermion systems, the bilayer KI-M system exhibits a complete FS destruction.

We introduce an intuitive and semianalytical polaron master equation approach to model pulse-driven population inversion and emitted single photons from a quantum dot exciton. The master equation theory allows one to identify important phonon-induced scattering rates analytically and fully includes the role of the time-dependent pump field. As an application of the theory, we first study a quantum dot driven by a time-varying laser pulse on and off resonance, showing the population inversion caused by acoustic phonon emission in direct agreement with recent experiments of Quilter et al. [Phys. Rev. Lett. 114, 137401 (2015)]. We then model quantum dots in weakly coupled cavities and show the difference in population response between exciton-driven and cavity-driven systems. Finally, we assess the nonresonant phonon-assisted loading scheme with a quantum dot resonantly coupled to a cavity as a deterministic single-photon source. We also compare and contrast the important single photon figures of merit with direct Rabi oscillation of the population using a resonant $\pi$ pulse, and show that the resonant scheme is much more efficient.

Ion-beam sputtering (IBS) is a cost-effective technique able to produce ordered nanopatterns on the surfaces of different materials. To date, most theoretical studies of this process have focused on systems which become amorphous under irradiation, e.g., semiconductors at room temperature. Thus, in spite of the large amount of experimental work on metals, or more recently on semiconductors at high temperatures, such experimental contexts have received relatively little theoretical attention. These systems are characterized by transport mechanisms, e.g., surface diffusion, which are anisotropic as a reflection of the crystalline structure not being overruled by the irradiation. Here, we generalize a previous continuum theory of IBS at normal incidence, in order to account for anisotropic surface diffusion. We explore systematically our generalized model in order to understand the role of anisotropy in the space-ordering properties of the resulting patterns. In particular, we derive a height equation which predicts morphological transitions among hexagonal and rectangular patterns as a function of system parameters and employ an angular correlation function to assess these pattern symmetries. By suitably choosing experimental conditions, it is found that one might be able to experimentally control the type of order displayed by the patterns produced.

We study the topological structure of matter-light excitations, so-called polaritons, in a quantum spin Hall insulator coupled to photonic cavity modes. We identify a topological invariant in the presence of time reversal (TR) symmetry, and demonstrate the existence of a TR-invariant topological phase. We find protected helical edge states with energies below the lower polariton branch and characteristic uncoupled excitonic states, both detectable by optical techniques. Applying a Zeeman field allows us to relate the topological index to the double coverage of the Bloch sphere by the polaritonic pseudospin.

We present a consistent theoretical approach for calculating effective nonlinear susceptibilities of metamaterials taking into account both frequency and spatial dispersion. Employing the discrete dipole model, we demonstrate that effects of spatial dispersion become especially pronounced in the vicinity of effective permittivity resonance where nonlinear susceptibilities reach their maxima. In that case spatial dispersion may enable simultaneous generation of two harmonic signals with the same frequency and polarization but different wave vectors. We also prove that the derived expressions for nonlinear susceptibilities transform into the known form when spatial dispersion effects are negligible. In addition to revealing new physical phenomena, our results provide useful theoretical tools for analyzing resonant nonlinear metamaterials.

The strong-light field effect of ${\left(\mathrm{TMTTF}\right)}_2\mathrm{As}{\mathrm{F}}_6$ was investigated utilizing 1.5-cycle, 7-fs infrared pulses. The ultrafast $(\sim 20\mathrm{fs})$ and large $(\sim 40\%)$ response of the plasmalike reflectivity edge $(\sim 0.7\mathrm{eV})$ was analyzed by the changes in ${\omega }_p=\sqrt{n{e}^2/\left({\varepsilon }_{\infty }{\varepsilon }_{0}m\right)}$ ($n$: number of charges in the $frac14\text{−}$filled band; $m$: mass of charge; ${\varepsilon }_{\infty },{\varepsilon }_0$: dielectric constants for high frequency and vacuum; $e$: elementary charge). The 3% reduction in ${\omega }_p$ is attributed to the 6% increase in $m$. Furthermore, 20 fs oscillation of ${\omega }_p$ in the time domain indicates that the plasmalike edge is affected by the charge gap $(\sim 0.2\mathrm{eV})$ nature. Theoretical calculations suggest that the Coulomb repulsion plays an important role in the increase in $m$.

$\mathrm{HfT}{\mathrm{e}}_5$ is predicted to be a promising platform for studying topological phases. Here through an electrical transport study, we present an observation of chiral anomaly and ultrahigh mobility in $\mathrm{HfT}{\mathrm{e}}_5$ crystals. Negative magnetoresistivity in $\mathrm{HfT}{\mathrm{e}}_5$ is observed when the external magnetic and electrical fields are parallel (B//E) and quickly disappears once B deviates from the direction of E. Quantitative fitting further confirms the chiral anomaly as the underlying physics. Moreover, by analyzing the conductivity tensors of longitudinal and Hall traces, ultrahigh mobility and ultralow carrier density are revealed in $\mathrm{HfT}{\mathrm{e}}_5$, which paves the way for potential electronic applications.

The optical excitation energies of organic dye molecules are often said to depend sensitively on the polarizability of the utilized substrate. To this end, we employ differential reflectance spectroscopy (DRS) to analyze the ${\mathrm{S}}_0\to {\mathrm{S}}_1$ fundamental transition energies observed for 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as a function of coverage on various surfaces, such as ${\mathrm{sp}}^2$-bonded insulating layers [graphene and hexagonal boron nitride (h-BN)], and noble metals pre-covered by a molecular wetting layer which prevents hybridization of the second-layer molecules with the metal states. We elucidate the optical absorbance behavior of PTCDA layers grown on h-BN/Rh(111) and on h-BN/Pt(111) and characterize their structures by means of scanning tunneling microscopy. Surprisingly, although the dielectric properties of the employed substrates differ substantially, only two main transition energies are observed: (i) ${\mathrm{PTCDA}}_{\text{HE}}$ essentially mimics the behavior of isolated monomers on surfaces (particularly at submonolayer coverage), while (ii) ${\mathrm{PTCDA}}_{\text{LE}}$, red-shifted by $\approx 70\mathrm{meV} \left(\approx 560{\mathrm{cm}}^{-1}\right)$, is attributed to two-dimensional densely packed aggregates. This red-shift is in remarkable accordance with previous investigations for PTCDA on NaCl(100) and, therefore, likely arises from the same physical effects, namely the formation of two-dimensional excitonic bands and the polarizability of neighboring molecules within the monolayer. In distinction from earlier studies, we conclude that the polarizabilities of the employed substrates do not constitute the dominant contribution to the molecular ${\mathrm{S}}_0\to {\mathrm{S}}_1$ transition energies observed here.

We investigated the structural phase transition of ${\mathrm{BaAl}}_2{\mathrm{O}}_4$, which has a network structure with corner-sharing ${\mathrm{AlO}}_4$ tetrahedra, via synchrotron x-ray thermal diffuse scattering measurements and first-principles calculations. ${\mathrm{BaAl}}_2{\mathrm{O}}_4$ shows the structural phase transition at $T_{\mathrm{C}}=451.4$ K from the $P{6}_{3}22$ parent crystal structure to the low-temperature superstructure with a cell volume of $2a\times 2b\times c$. This phase transition is unusual, in which two energetically competing phonon modes at $M$ and $K$ points soften simultaneously. When approaching $T_{\mathrm{C}}$ from above, the $K$-point mode appears first. However, this $K$-point mode is overcome by the later-developed $M$-point mode. The thermal diffuse scattering intensities from both modes increase sharply at $T_{\mathrm{C}}$; therefore, both modes soften simultaneously. The first-principles calculations demonstrate that the $M$-point mode is electrostatically more preferable than the $K$-point mode and determines the eventual low-temperature structure, although these two modes are competing energetically. This competition is characteristic of ${\mathrm{BaAl}}_2{\mathrm{O}}_4$, which is ascribed to the structurally flexible network structure of this compound.

The local structure of ${\mathrm{IrTe}}_2$ has been studied by iridium $L_3$-edge x-ray absorption spectroscopy (XAS) measurements as a function of pressure, performed at two temperatures (100 and 295 K) across the structural phase transition at $\sim 270$ K. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra show pressure-dependent anomalies, suggesting phase transitions that are characterized by different local atomic displacements. The high-temperature phase of ${\mathrm{IrTe}}_2$ (trigonal at 295 K) reveals a clear anomaly in the Ir-Te correlations at $\sim 4$ GPa, while the low-temperature phase (at 100 K) shows a smaller change at $\sim 6$ GPa, likely to be associated with transitions in lower-symmetry phases. XANES spectra, measuring higher-order atomic correlations, also show nonlinear pressure dependence in the local geometry at the anomalous pressures. These nonlinear changes suggest that ${\mathrm{IrTe}}_2$ goes through lower local symmetry phases with increasing pressure.

We study the dephasing of an individual high-frequency tunneling two-level system (TLS) due to its interaction with an ensemble of low-frequency thermal TLSs which are described by the standard tunneling model (STM). We show that the dephasing by the bath of TLSs explains both the dependence of the Ramsey dephasing rate on an externally applied strain as well as its order of magnitude, as observed in a recent experiment [J. Lisenfeld et al., Sci. Rep. 6, 23786 (2016)]. However, the theory based on the STM predicts the Hahn-echo protocol to be much more efficient, yielding too low echo dephasing rates, as compared to the experiment. Also the strain dependence of the echo dephasing rate predicted by the STM does not agree with the measured quadratic dependence, which would fit to a high-frequency white noise environment. We suggest that few fast TLSs which are coupled much more strongly to the strain fields than the usual TLSs of the STM give rise to such a white noise. This explains the magnitude and strong fluctuations of the echo dephasing rate observed in the experiment.

We report measurement of temperature dependence of phonon spectra in quasi-one-dimensional metal cyanides $M\mathrm{CN}$ ($M=\mathrm{Cu}$, Ag, Au). Ab initio lattice dynamics calculations have been performed to interpret the phonon spectra as well as to understand the anomalous anisotropic thermal expansion behavior in these compounds. We bring out the differences in the phonon mode behavior to explain the differences in the thermal expansion behavior among the three compounds. The chain-sliding modes are found to contribute maximum to the negative thermal expansion along the “$c$” axis in the Cu and Ag compounds, while the same modes contribute to positive thermal expansion in the Au compound. Several low-energy transverse modes lead to positive thermal expansion in the $a\text{−}b$ plane in all the compounds. The calculated Born-effective charges show that AuCN has a covalent nature of bonding, which results in least distortion as well as the least number of unstable modes among the three cyanides. This result is well correlated with the fact that the coefficient of negative thermal expansion along the $c$ axis in AuCN is the smallest.