First-principles calculations are performed to predict structural, electric, magnetic, and magnetoelectric properties of hexagonal rare-earth ferrites (RFeO${}_3$) under chemical and hydrostatic pressures. Decreasing the rare-earth ionic radius has two dramatic consequences: (i) an enhancement of the electrical polarization by a factor of 60% and (ii) a magnetic transition, which renders the systems (weakly) ferromagnetic. Moreover and unlike conventional ferroelectrics, the electrical polarization strengthens as a hydrostatic pressure is applied and increases in magnitude in any hexagonal rare-earth ferrites. Finally, applying a hydrostatic pressure in RFeO${}_3$ having small or intermediate rare-earth ionic radius results in the sudden disappearance of a weak magnetization and of the linear magnetoelectric effect above some critical pressure. Origins of these striking effects are revealed.

The vibrational properties of the wurtzstannite ${\text{Cu}}_2$${\text{ZnGeS}}_4$ are studied experimentally by polarized Raman scattering in off-resonant and resonant conditions and theoretically by ab initio lattice dynamics calculations. Twenty-nine modes from 45 Raman active theoretically predicted have been experimentally detected and identified, including polar ${\text{A}}_1$(TO), ${\text{A}}_1$(LO), and ${\text{B}}_1$(TO+LO)/${\text{B}}_2$(TO+LO) and nonpolar ${\text{A}}_2$ symmetry phonon modes from measurements on (2 1 0) and (0 0 1) crystallographic planes of ${\text{Cu}}_2$${\text{ZnGeS}}_4$ single crystals. The lattice dynamics calculations provide a full picture of the zone center phonon spectrum and allow the assignment of experimentally observed lines to first- and second-order lattice vibrations. Using resonance Raman conditions, a strong enhancement of the ${\text{A}}_1$(LO) modes with the highest longitudinal-transversal spiting is observed.

We analyze the full-counting statistics of the electric heat current flowing in a two-terminal quantum conductor whose temperature is probed by a third electrode (“probe electrode”). In particular we demonstrate that the cumulant-generating function obeys the fluctuation theorem in the presence of a constant magnetic field. The analysis is based on the scattering matrix of the three-terminal junction (comprising the two electronic terminals and the probe electrode), and a separation of time scales: it is assumed that the rapid charge transfer across the conductor and the rapid relaxation of the electrons inside the probe electrode give rise to much slower energy fluctuations in the latter. This separation allows for a stochastic treatment of the probe dynamics, and the reduction of the three-terminal setup to an effective two-terminal one. Expressions for the lowest nonlinear transport coefficients, e.g., the linear-response heat-current noise and the second nonlinear thermal conductance, are obtained and explicitly shown to preserve the symmetry of the fluctuation theorem for the two-terminal conductor. The derivation of our expressions, which is based on the transport coefficients of the three-terminal system explicitly satisfying the fluctuation theorem, requires full calculations of vertex corrections.

Density functional theory (DFT) has been immensely successful in its ability to predict physical properties and, in particular, structures of condensed matter systems. Here, however, we show that DFT qualitatively fails to predict the dimerized structural phase for a monatomic Co wire that is self-assembled on a vicinal (i.e. stepped) Cu(111) substrate. To elucidate the nature of this failure, we compute the energetics of a Co chain on a Cu surface, step, and notch and embedded in bulk. The results demonstrate that increasing Co coordination extinguishes the dimerization, indicating that the failure of DFT for Co on the Cu step arises from excessive hybridization, which both weakens the ferromagnetic correlations that drive the dimerization and increases the bonding that opposes dimerization. Additionally, we show that including local interactions via DFT + U or DFT + dynamical mean field theory (DMFT) does not restore the dimerization for the stepped-substrate-supported wire, although the Co wire does dimerize in DFT + DMFT for the isolated vacuum case. This system can serve as a benchmark for future electronic structure methods.

We report results of ab initio density functional theory including van der Waals interaction for the formation of a silicene bilayer on top of the $\mathrm{Ag}\mathrm{(111)}$ surface with significant differences to the monolayer case. We find $(2\sqrt{3}\times 2\sqrt{3})R{30}^{\circ }$ bilayer silicene on $(\sqrt{19}\times \sqrt{19})R23.4^{\circ }$ silver substrate to be the most stable. The calculated STM images, however, exhibit a $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ symmetry. This translational symmetry, the resulting lattice spacing, and the height of the topmost monolayer agree with recent experimental findings. The band structure of the complete adsorbate system shows conical linear bands near the Fermi level due to the hybridization of adsorbate and substrate states.

We measured the longitudinal sound velocity in Mo shock compressed up to 4.4 Mbars on the Hugoniot. Its sound speed increases linearly with pressure up to 2.6 Mbars; the slope then decreases up to the melting pressure of $\sim$3.8 Mbars. This suggests a decrease of shear modulus before the melt. A linear extrapolation of our data to 1 bar agrees with the ambient sound speed. The results suggest that Mo remains in the bcc phase on the Hugoniot up to the melting pressure. There is no statistically significant evidence for a previously reported bcc$\to$hcp phase transition on the Hugoniot.

We measured the in-plane resistivity anisotropy in the underdoped ${\text{Ca}}_{1-x}{\text{Na}}_x{\text{Fe}}_2{\text{As}}_2$ single crystals. The anisotropy (indicated by ${\rho }_{\mathrm{b}}-{\rho }_{\mathrm{a}}$) appears below a temperature well above magnetic transition temperature $T_{\mathrm{N}}$, being positive (${\rho }_{\mathrm{b}}-{\rho }_{\mathrm{a}}>0$) as $x\le 0.14$. With increasing the doping level to $x=0.19$, an intersection between ${\rho }_{\mathrm{b}}$ and ${\rho }_{\mathrm{a}}$ is observed upon cooling, with ${\rho }_{\mathrm{b}}-{\rho }_{\mathrm{a}}<0$ at low temperature deep inside a magnetically ordered state, while ${\rho }_{\mathrm{b}}-{\rho }_{\mathrm{a}}>0$ at high temperature. Subsequently, further increase of hole concentration leads to a negative anisotropy ${\rho }_{\mathrm{b}}-{\rho }_{\mathrm{a}}<0$ in the whole temperature range. These results manifest that the anisotropic behavior of resistivity in the magnetically ordered state depends strongly on the competition of the contributions from different mechanisms, and the competition between the two contributions results in a complicated evolution of the anisotropy of in-plane resistivity with doping level.

We report a quantitative experimental study of the crystallization kinetics of supercooled quantum liquid mixtures of parahydrogen (${\text{pH}}_2$) and orthodeuterium (${\text{oD}}_2$) by high spatial resolution Raman spectroscopy of liquid microjets. We show that in a wide range of compositions the crystallization rate of the isotopic mixtures is significantly reduced with respect to that of the pure substances. To clarify this behavior we have performed path-integral simulations of the nonequilibrium ${\text{pH}}_2$-${\text{oD}}_2$ liquid mixtures, revealing that differences in quantum delocalization between the two isotopic species translate into different effective particle sizes. Our results provide experimental evidence for crystallization slowdown of quantum origin, offering a benchmark for theoretical studies of quantum behavior in supercooled liquids.

We introduce and model a three-dimensional (3D) atomic-scale phononic metamaterial producing two-path phonon interference antiresonances to control the heat flux spectrum. We show that a crystal plane partially embedded with defect-atom arrays can completely reflect phonons at the frequency prescribed by masses and interaction forces. We emphasize the predominant role of the second phonon path and destructive interference in the origin of the total phonon reflection and thermal conductance reduction in comparison with the Fano-resonance concept. The random defect distribution in the plane and the anharmonicity of atom bonds do not deteriorate the antiresonance. The width of the antiresonance dip can provide a measure of the coherence length of the phonon wave packet. All our conclusions are confirmed both by analytical studies of the equivalent quasi-1D lattice models and by numerical molecular dynamics simulations of realistic 3D lattices.

Long-ranged superconductor proximity effects recently found in superconductor-ferromagnetic (S-F) systems are generally attributed to the formation of triplet-pairing correlations due to various forms of magnetic inhomogeneities at the S-F interface. In order to investigate this conjecture within a single F layer coupled to a superconductor, we performed scanning tunneling spectroscopy on bilayers of La${}_{2/3}$Ca${}_{1/3}$MnO${}_3$ (LCMO) ferromagnetic thin films grown on high-temperature superconducting films of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ or Pr${}_{1.85}$Ce${}_{0.15}$CuO${}_4$ under various magnetic fields. We find a strong correlation between the magnitude of superconductor-related spectral features measured on the LCMO layer and the degree of magnetic inhomogeneity controlled by the external magnetic field. This corroborates theoretical predictions regarding the role played by magnetic inhomogeneities in inducing triplet pairing at S-F interfaces.

Recent advances in quantum electronics have allowed us to engineer hybrid nanodevices comprising on chip a microwave electromagnetic resonator coupled to an artificial atom, a quantum dot. These systems realize novel platforms to explore nonequilibrium quantum impurity physics with light and matter. Coupling the quantum dot system to reservoir leads (source and drain) produces an electronic current as well as dissipation when applying a bias voltage across the system. Focusing on a standard model of biased quantum dot coupled to a photon mode which gives rise to an Anderson-Holstein Hamiltonian, we elucidate the signatures of the electronic correlations in the phase of the transmitted microwave signal. In addition, we illustrate the effect of the electronic degrees of freedom on the photon field, giving rise to nonlinearities, damping, and dissipation, and discuss how to control these effects by means of gate and bias voltages.

The Bose-Hubbard Hamiltonian describes the competition between superfluidity and Mott insulating behavior at zero temperature and commensurate filling as the strength of the on-site repulsion is varied. Gapped insulating phases also occur at noninteger densities as a consequence of longer ranged repulsive interactions. In this paper we explore the formation of gapped phases in coupled chains due instead to anisotropies $t_x\ne t_y$ in the bosonic hopping, extending the work of Crepin et al. [Phys. Rev. B 84, 054517 (2011).] on two coupled chains, where a gap was shown to occur at half filling for arbitrarily small interchain hopping $t_y$. Our main result is that, unlike the two-leg chains, for three- and four-leg chains, a charge gap requires a finite nonzero critical $t_y$ to open. However, these finite values are surprisingly small, well below the analogous values required for a fermionic band gap to open.

We report the visualization and quantitative analysis of electromagnetic surface fields at solid surfaces with the potential for λ/50 resolution. To illustrate this capability, we investigate patterns in two-photon photoemission images of light-diffracting structures in waveguiding, transparent thin films. The obtained micrographs show interference patterns between incident and guided light with a remarkable sensitivity to subwavelength features. We demonstrate that photoemission rates are directly related to the surface field strengths and develop a subwavelength method to calculate the surface fields from optical properties and surface topology based on the two-dimensional Kirchhoff diffraction integral. Calculated images based on this theoretical approach compare favorably to experimental electron micrographs.

We ask which topological phases can and can not be realized by exactly soluble string-net models. We answer this question for the simplest class of topological phases, namely, those with Abelian braiding statistics. Specifically, we find that an Abelian topological phase can be realized by a string-net model if and only if (i) it has a vanishing thermal Hall conductance and (ii) it has at least one Lagrangian subgroup, a subset of quasiparticles with particular topological properties. Equivalently, we find that an Abelian topological phase is realizable if and only if it supports a gapped edge. We conjecture that the latter criterion generalizes to the non-Abelian case. We establish these results by systematically constructing all possible Abelian string-net models and analyzing the quasiparticle braiding statistics in each model. We show that the low-energy effective field theories for these models are multicomponent $\text{U}\left(1\right)$ Chern-Simons theories, and we derive the $K$-matrix description of each model. An additional feature of this work is that the models we construct are more general than the original string-net models, due to several new ingredients. First, we introduce two new objects $\gamma ,\alpha$ into the construction which are related to ${\mathbb{Z}}_2$ and ${\mathbb{Z}}_3$ Frobenius-Schur indicators. Second, we do not assume parity invariance. As a result, we can realize topological phases that were not accessible to the original construction, including phases that break time-reversal and parity symmetry.

We report the generation of undamped exciton-biexciton beats for coherent dynamics of excitons in InAs quantum dots at telecommunication wavelengths. The exciton-biexciton beat is damped rapidly in four-wave mixing measurements, but a six-wave mixing technique with three excitation pulses enables us to regenerate and control the exciton-biexciton beat perfectly. We have demonstrated the complete regeneration of the exciton-biexciton beat by adjusting the delay time between the first and second excitation pulses. This approach is different from conventional coherent control in quantum dots. Furthermore, the initial phase of the exciton-biexciton beat is controlled by using this technique. We show that the generation of undamped beats is a powerful tool for controlling quantum states.

Herein, we propose a model to describe picosecond-nanosecond charge separation and nongeminate recombination in organic semiconductors. The model is used to explain time-resolved electroabsorption (EA) measurements performed on diodes made from phenyl-C61-butyric acid methyl ester. We find that the measured shape of the EA transient is due to a combination of microscopic carrier dynamic effects such as carrier trapping, as well as macroscopic effects such as band bending caused by the nonuniform poloron generation profile across the device. We demonstrate that the initial fast phase of the EA transient is due to hot free carriers being able to move freely within the device; over time these hot free carriers cool and become trapped giving rise to the second slower phase of the transient. We further show that the commonly observed dependence of the EA signal on probe wavelength can be explained in terms of the spatial overlap of electrostatic potential within the device and the optical mode of the probe light. Finally, we discuss the implications of these results for pump-probe experiments on thin organic films.

We build up a microscopic description of the surface Bloch mode (SBM) on metallic surfaces patterned with a periodic array of subwavelength holes. The SBM also is called a spoof surface plasmon when metals approach perfect electric conductors at terahertz or microwave frequencies. In the description, the SBM is found to be composed of surface waves on flat metallic surfaces, the surface plasmon polariton (SPP), and the quasicylindrical wave (QCW) launched by every individual hole in the array, which shines new light on the design of perforated metallic surfaces as plasmonic metamaterials. We also establish explicit relations between the macroscopic SBM picture and the microscopic surface-wave picture for explaining the extraordinary optical transmission (EOT) through subwavelength metallic hole arrays. Our analysis shows that the SBM is related tightly to the EOT through a complex pole of the in-plane wave vector at which the SBM and the EOT possess similar field distributions expressed as superpositions of the SPPs and QCWs.

We studied the excitation density dependence of photoluminescence (PL) spectra of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes. We found that the PL intensity of trions exhibited a strong nonlinear saturation behavior as the excitation density increased, whereas that of excitons exhibited a weak sublinear behavior. The strong PL saturation of trions is attributed to depletion of doped holes that are captured by excitons in the formation processes. Moreover, the effective radiative lifetime of a trion was evaluated to be approximately 20 ns.