The two-dimensional square lattice hard-core boson Hubbard model with near-neighbor interactions has a “checkerboard” charge density wave insulating phase at half filling and sufficiently large intersite repulsion. When doped, rather than forming a supersolid phase in which long-range charge density wave correlations coexist with a condensation of superfluid defects, the system instead phase separates. However, it is known that there are other lattice geometries and interaction patterns for which such coexistence takes place. In this paper we explore the possibility that anisotropic hopping or anisotropic near-neighbor repulsion might similarly stabilize the square lattice supersolid. By considering the charge density wave structure factor and superfluid density for different ratios of interaction strength and hybridization in the $\widehat{x}$ and $\widehat{y}$ directions, we conclude that phase separation still occurs.

We investigated the electronic properties of a single crystal of metallic pyrochlore iridate Bi${}_2$Ir${}_2$O${}_7$ by means of infrared spectroscopy. Our optical conductivity data show the splitting of $t_{2g}$ bands into $J_{\mathrm{eff}}$ ones due to strong spin-orbit coupling. We observed a sizable midinfrared absorption near 0.2 eV which can be attributed to the optical transition within the $J_{\mathrm{eff},1/2}$ bands. More interestingly, we found an abrupt suppression of optical conductivity in the very far-infrared region. Our results suggest that the electronic structure of Bi${}_2$Ir${}_2$O${}_7$ is governed by the strong spin-orbit coupling and correlation effects, which are a prerequisite for theoretically proposed nontrivial topological phases in pyrochlore iridates.

We resolve an apparent contradiction between numeric and analytic results for one-dimensional disordered systems with power-law spectral correlations. The conflict arises when considering rigorous results that constrain the set of correlation functions yielding metallic states to those with nonzero values in the thermodynamic limit. By analyzing the scaling law for a model correlated disorder that produces a mobility edge, we show that no contradiction exists as the correlation function exhibits strong anticorrelations in the thermodynamic limit. Moreover, the associated scaling function reveals a size-dependent correlation with a smoothening of disorder amplitudes as the system size increases.

In this work, we demonstrate broadband extraordinary transmission and antireflection in two-dimensional periodic metallic cuboids at optical frequencies. These phenomena originate from nonresonant excitations of surface plasmons, and represent high antireflection simultaneously for a broad spectral band and a wide angular range of incidence with polarization insensitivity. Based on this principle, we further introduce such metallic cuboids arrays into silicon solar cells. It is shown that high performance of light trapping in the cells can be achieved with a significant enhancement of the ultimate quantum efficiency. This study shows promising applications of plasmonic nanostructures to high-efficiency photovoltaic devices.

The electron transport of different conical valleys is investigated in graphene with extended line defects. Intriguingly, the electron with a definite incident angle can be completely modulated into one conical valley by a resonator which consists of several paralleling line defects. The related incident angle can be controlled easily by tuning the parameters of the resonator. Therefore, a controllable $100\%$ valley polarization, as well as the detection of the valley polarization, can be realized conveniently by tuning the number of line defects and the distance between two nearest-neighboring line defects. This fascinating finding opens a way to realize the valley polarization by line defects. With the advancement of experimental technologies, this resonator is promising to be realized and thus plays a key role in graphene valleytronics.

The paper presents time-resolved incoherent anti-Stokes Raman spectroscopy (TRIARS) measurements of the population dynamics of G-mode phonons in semiconducting single-walled carbon nanotubes (SWCNTs), in which the G${}^{+}$ and G${}^{-}$ dynamics are separated. The measurements were performed as a function of temperature over the full range $4-600$ K, and at two laser excitation energies, $1.51$ and $1.63$ eV, which give TRIARS signals from SWCNTs in two different diameter distributions, $0.99<d<1.05$ nm and $0.88<d<0.97$ nm, respectively. The population lifetimes determined from the experiments were, to within experimental error, independent of mode type and diameter. In all cases the temperature dependence of the population lifetimes was well fitted by a model that assumes that two-phonon anharmonic decay is the dominant process and that the lower-energy daughter phonon has an energy of approximately $400\pm 100$ cm${}^{-1}$.

Currently, it is understood that the carrier recombination rate in semiconductors can be modified by metals due to pure electrodynamic interactions through surface plasmons. We propose here an electrostatic mechanism for carrier-metallic nanoparticle interaction comparable in effect to plasmonic interactions. Arising from Coulomb attraction of electrons and holes to their images in metal, this mechanism produces large carrier concentrations near metallic nanoparticles. Increased concentration results in increased quantum efficiency and enhances the rate of $e$-$h$ recombination. This manifests as emission enhancement in InGaN quantum wells radiating in the near-UV region. The proposed fundamental mechanism provides a new perspective for improving the efficiency of broadband light emitters.

The lifting of the fourfold degeneracy of the zeroth Landau level in graphene under high magnetic fields has been the subject of numerous experimental studies, and attributed to various mechanisms such as pure spin splitting, spin splitting combined with subsequent valley splitting, or the formation of a quantum Hall insulator. Unexplored, however, is the influence of an energy gap on the quantum Hall effect (QHE) states in graphene. Here we demonstrate, using measurements of the magnetoresistance of graphene antidot lattices (GALs) in magnetic fields up to 30 T and temperatures between 2 and 100 K, that gap opening in these samples is accompanied by valley polarization and a change from linear to parabolic band structure at low carrier energies. The emergence of a massive character of the carriers profoundly alters the transport characteristics of the zeroth Landau level, which manifests itself in a linear increase of the activated gap with magnetic field. Furthermore, samples of the highest quality display spin splitting on top of the valley splitting, albeit of significantly smaller magnitude.

We theoretically study the electromagnetic interaction in Dirac systems with $N$ nodes by using the renormalization group, which is relevant to the quantum critical phenomena of topological phase transition ($N=1$) and Weyl semimetals ($N=4$ or $N=12$). Compared with the previous work for $N=1$ [H. Isobe and N. Nagaosa, Phys. Rev. B 86, 165127 (2012)], we obtained the analytic solution for the large $N$ limit, which differs qualitatively for the scaling of the speed of light $c$ and that of electron $v$; i.e., $v$ does not change while $c$ is reduced to $v$. We also found a reasonably accurate approximate analytic solution for generic $N$, which well interpolates between $N=1$ and large $N$ limit, and it concludes that $c^2{v}^N$ is almost unrenormalized. The temperature dependence of the physical properties, the dielectric constant, magnetic susceptibility, spectral function, DC conductivity, and mass gap are discussed based on these results.

A method to calculate the crystal field parameters is proposed and applied to trivalent rare-earth impurities in yttrium aluminate and to Tb${}^{3+}$ ion in TbAlO${}_3$. To determine crystal field parameters local Hamiltonian expressed in the basis of Wannier functions is expanded in a series of spherical tensor operators. Wannier functions are obtained by transforming the Bloch functions calculated using the density functional theory based program. The results show that the crystal field is continuously decreasing as the number of $4f$ electrons increases and that the hybridization of $4f$ states with the states of oxygen ligands is important. The method contains a single adjustable parameter characterizing the $4f$-ligand charge transfer. Theory is confronted with experiment for Nd${}^{3+}$ and Er${}^{3+}$ ions in the YAlO${}_3$ matrix and for the Tb${}^{3+}$ ion in TbAlO${}_3$, and a good agreement within a few meV is found.

We study the Anderson transition with interactions in one dimension from the perspective of quantum entanglement. Extensive numerical calculations of the entanglement entropy (EE) of the systems are carried out through the density matrix renormalization group algorithm. We demonstrate that the EE can be used for the finite-size scaling (FSS) to characterize the Anderson transition in both noninteracting and interacting systems. From the FSS analysis we can obtain a precise estimate of the critical parameters of the transition. The method can be applied to various one-dimensional models, either interacting or noninteracting, to quantitatively characterize the Anderson transitions.

A technique is described for modeling the low-temperature thermal behavior of phononic components patterned into amorphous dielectric membranes, such as Si${}_x$N${}_y$. Ballistic, elastic diffusive, localized, and inelastic diffusive transport are included. The model is well suited to studying components where heat is carried by a discrete set of large-scale acoustic modes. The scheme gives not only the average fluxes of components having statistically characterized microstructure, but also the spread in behavior of notionally identical devices. It is applicable to a wide range of geometries. The method is illustrated by simulating the behavior of a number of simple structures and studying the interactions between the different transport mechanisms. A single-stage Fabry-Perot resonator is used to identify likely operating characteristics of phononic filters.

We propose a type of locally resonant structure involving arrays of structured coated inclusions. The coating consists of a structural interface with beams inclined at a certain angle. Such an elastic metamaterial supports tunable low-frequency stop bands associated with localized rotational modes that can be used in the design of filtering, reflecting, and focusing devices. Asymptotic estimates for resonant frequencies are in good agreement with finite element computations and can be used as a design tool to tune stop band changing relative inclinations, number, and cross section of the beams. Inertial resonators with inclined ligaments allow for anomalous dispersion (negative group velocity) to occur in the pressure acoustic band and this leads to the physics of negative refraction, whereby a point force located above a finite array of resonators is imaged underneath for a given polarization. We finally observe that for a periodic macrocell of the former inertial resonators with one defect in the middle, an elastic trapped mode exists within a high-frequency stop band. The latter design could be used in the enhancement of light and sound interactions in photonic crystal fiber preforms.

We investigate the excited states of the quasi-one-dimensional quantum antiferromagnets on hexagonal lattices, including the longitudinal modes based on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion anisotropy is first studied by a spin wave theory based on the one-boson method; the ground state thus obtained is employed for the study of the longitudinal modes. The full energy spectra of both the transverse modes (i.e., magnons) and the longitudinal modes are obtained as functions of the nearest-neighbor coupling and the anisotropy constants. We have found two longitudinal modes due to the noncollinear nature of the triangular antiferromagnetic order, similar to that of the phenomenological field theory approach by Affleck. The excitation energy gaps due to the anisotropy and the energy gaps of the longitudinal modes without anisotropy are then investigated. We then compare our results for the longitudinal energy gaps at the magnetic wave vectors with the experimental results for several antiferromagnetic compounds with both integer and noninteger spin quantum numbers, and we find good agreement after the higher-order contributions are included in our calculations.

We report crystal growth at high pressure, structure determination, and magnetic and transport studies of an oxypnictide superconductor Pr${}_4$Fe${}_2$As${}_2$Te${}_{1-x}$O${}_4$. Its structure resembles the known 1111 phase except for the considerably larger $c$ lattice constant and intercalated tellurium atoms and it crystallizes in a tetragonal lattice [$a$ $=$ 4.0165(2) Å, $c$ $=$ 29.8572(16) Å, and space group $I$4/mmm (no. 139)]. The electrical resistivity ρ(T) and magnetization measurement show a transition at $T_{\mathrm{c}}\approx 25$ K. The lower ($H_{\mathrm{c}1}$) and upper ($H_{\mathrm{c}2}$) critical fields are 2 mT and 6.5 T, respectively. The Ginzburg-Landau parameter of κ ≈ 80 places this compound in the family of strong type-II superconductors.

It is demonstrated by means of exact numerical methods that the ground state of nine-membered frustrated homometallic chromium-based molecular rings with a single bond defect can be unambiguously determined by the in-field local magnetization profiles, which exhibit characteristic patterns. The strength of the coupling corresponding to the defect can be determined by both total and local magnetization measurements on single crystals with the field perpendicular to the ring. This approach is illustrated with a recently synthesized chromium ring Cr${}_9$Cl${}_2$, which is experimentally characterized by low-temperature magnetic measurements and analyzed by means of the microscopic quantum model. The strength of the coupling corresponding to the defect is estimated by fitting the magnetic susceptibility of a powder sample and independently confirmed from the experimental intersection point of total magnetization profiles preserving the typical values of the remaining parameters, which are well established for known chromium rings.

Co${}_2$FeAl (CFA) thin films with thickness varying from 10 to 115 nm have been deposited on MgO(001) substrates by magnetron sputtering and then capped by a Ta or Cr layer. X-ray diffraction (XRD) revealed that the cubic [001] CFA axis is normal to the substrate and that all the CFA films exhibit full epitaxial growth. The chemical order varies from the B2 phase to the A2 phase when decreasing the thickness. Magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) measurements show that, depending on the field orientation, one- or two-step switchings occur. Moreover, the films present a quadratic MOKE signal increasing with the CFA thickness, due to the increasing chemical order. Ferromagnetic resonance (FMR), MOKE transverse bias initial inverse susceptibility and torque (TBIIST) measurements reveal that the in-plane anisotropy results from the superposition of a uniaxial and of a fourfold symmetry term. The fourfold anisotropy is in accord with the crystal structure of the samples and is correlated to the biaxial strain and to the chemical order present in the films. In addition, a large negative perpendicular uniaxial anisotropy is observed. Frequency and angular dependencies of the ferromagnetic resonance linewidth show two magnon scattering and mosaicity contributions, which depend on the CFA thickness. A Gilbert damping coefficient as low as 0.0011 is found.

We report first-principles DFT$+U$ calculations of the paramagnetic phases of uranium dioxide (UO${}_2$) and (U,Pu) mixed oxides (MOX). We use a combination of two techniques, the first of which enables us to treat the presence of metastable states and the other the random distribution of spins (for paramagnetism) and of plutonium atoms (for MOX). The resulting method is first used to determine the ground-state properties of paramagnetic UO${}_2$ and is then applied to MOX with $12.5\%$ and $25\%$ Pu, for which both experimental and theoretical studies are lacking. We find the paramagnetic phase of fluorite UO${}_2$ to be more stable than the antiferromagnetic phase as expected from experimentation, with ground-state properties in excellent agreement with experimental data. Results on MOX show that most of the ground-state properties of UO${}_2$ are affected by addition of $12.5\%$ Pu and do not seem to be much different when the plutonium content changes, at least for Pu contents below $30\%$. For both UO${}_2$ and MOX, it is found that the true paramagnetic order has a significant effect on the calculated elastic constants, compared to the antiferromagnetic and ferromagnetic orders. By contrast, we find that regarding energetics, the antiferromagnetic order may be used as a very good approximation of the paramagnetic state.

We analyze the sharpness of crossing (“isosbestic”) points of a family of curves which are observed in many quantities described by a function $f(x,p)$, where $x$ is a variable (e.g., the frequency) and $p$ is a parameter (e.g., the temperature). We show that if a narrow crossing region is observed near $x_{*}$ for a range of parameters $p$, then $f(x,p)$ can be approximated by a perturbative expression in $p$ for a wide range of $x$. This allows us, e.g., to extract the temperature dependence of several experimentally obtained quantities, such as the Raman response of HgBa${}_2$CuO${}_{4+\delta }$, photoemission spectra of thin VO${}_2$ films, and the reflectivity of CaCu${}_3$Ti${}_4$O${}_{12}$, all of which exhibit narrow crossing regions near certain frequencies. We also explain the sharpness of isosbestic points in the optical conductivity of the Falicov-Kimball model and the spectral function of the Hubbard model.

The electronic and optical properties of zinc-blende (zb) Al${}_x$Ga${}_{1-x}$N over the whole alloy composition range are presented in a joint theoretical and experimental study. Because zb-GaN is a direct (${\Gamma }_v\to {\Gamma }_c$) semiconductor and zb-AlN shows an indirect (${\Gamma }_v\to X_c$) fundamental band gap, the ternary alloy exhibits a concentration-dependent direct-indirect band gap crossing point the position of which is highly controversial. The dielectric functions of zb-Al${}_x$Ga${}_{1-x}$N alloys are measured employing synchrotron-based ellipsometry in an energy range between 1 and 20 eV. The experimentally determined fundamental energy transitions originating from the $\Gamma$, $X$, and $L$ points are identified by comparison to theoretical band-to-band transition energies. In order to determine the direct-indirect band gap crossing point, the measured transition energies at the $X$ point have to be aligned by the calculated position of the highest valence state. Thereby density-functional theory (DFT) based approaches to the electronic structure, ranging from the standard (semi)local generalized gradient approximation (GGA), self-energy corrected local density approximation (LDA-1/2), and meta-GGA DFT (TB-mBJLDA) to hybrid functional DFT and many-body perturbation theory in the $GW$ approximation, are applied to random and special quasirandom structure models of zb-Al${}_x$Ga${}_{1-x}$N. This study provides interesting insights into the accuracy of the various numerical approaches and contains reliable ab initio data on the electronic structure and fundamental alloy band gaps of zb-Al${}_x$Ga${}_{1-x}$N. Nonlocal Heyd-Scuseria-Ernzerhof-type hybrid-functional DFT calculations or, alternatively, $GW$ quasiparticle calculations are required to reproduce prominent features of the electronic structure. The direct-indirect band gap crossing point of zb-Al${}_x$Ga${}_{1-x}$N is found in the Al rich composition range at an Al content between $x=0.64$ and $0.69$ in hybrid functional DFT, which is in good agreement with $x=0.71$ determined from the aligned experimental transition energies. Thus our study solves the long-standing debate on the nature of the fundamental zb-Al${}_x$Ga${}_{1-x}$N alloy band gap.