We present here results of high-resolution synchrotron x-ray diffraction (SXRD) and dielectric studies in conjunction with Landau theory considerations on (Pb${}_{0.94}$Sr${}_{0.06}$)(Zr${}_x$Ti${}_{1-x}$)O${}_3$ compositions in the vicinity of the morphotropic phase boundary (MPB) to find evidence for the flattening of the free-energy surface at the MPB proposed in recent ab initio studies on strongly piezoelectric ceramics. SXRD results reveal that the tetragonal and pseudorhombohedral monoclinic compositions with $x$ $=$ 0.515 and 0.550 transform directly into the cubic paraelectric phase, whereas for 0.520 ≤ $x$ ≤ 0.545, the pseudotetragonal and pseudorhombohedral monoclinic compositions transform first to the tetragonal phase and then to the cubic phase. Our results reveal the existence of a triple point at $x$ $\simeq$ 0.550. It is shown that the tetragonal-to-cubic transition, irrespective of the composition, up to $x$ $\simeq$ 0.545 is accompanied with a discontinuous change in the unit cell volume and thermal hysteresis, confirming the first-order nature of this transition. However, the pseudorhombohedral monoclinic composition for $x$ $=$ 0.550 transforms directly into the cubic phase in a second-order manner. Our experimental results thus reveal a crossover from a first-order to a second-order phase transition through a tricritical point around $x$ $=$ 0.550. Landau theory calculations also confirm gradual flattening of the free-energy surface on approaching the tricritical composition $x$ $=$ 0.550. We conclude that the triple point in the Pb(Zr${}_x$Ti${}_{1-x}$)O${}_3$ phase diagram is a tricritical point.

Antiferromagnetic semiconductors are new alternative materials for spintronic applications and spin valves. In this work, we report a detailed investigation of two antiferromagnetic semiconductors $A$MnAs ($A=\mathrm{Li}$, LaO), which are isostructural to the well-known LiFeAs and LaOFeAs superconductors. Here we present a comparison between the structural, magnetic, and electronic properties of LiMnAs, LaOMnAs, and related materials. Interestingly, both LiMnAs and LaOMnAs show a variation in resistivity with more than five orders of magnitude, making them particularly suitable for use in future electronic devices. Neutron and x-ray diffraction measurements on LiMnAs show a magnetic phase transition corresponding to the Néel temperature of 373.8 K, and a structural transition from the tetragonal to the cubic phase at 768 K. These experimental results are supported by density functional theory calculations.

The idea that a quantum magnet could act like a liquid crystal, breaking spin-rotation symmetry without breaking time-reversal symmetry, holds an abiding fascination. However, the very fact that spin nematic states do not break time-reversal symmetry renders them “invisible” to the most common probes of magnetism—they do not exhibit magnetic Bragg peaks, a static splitting of lines in NMR spectra, or oscillations in $\mu$SR. Nonetheless, as a consequence of breaking spin-rotation symmetry, spin-nematic states $\mathit{do}$ possess a characteristic spectrum of dispersing excitations which could be observed in experiment. With this in mind, we develop a symmetry-based description of long-wavelength excitations in a spin-nematic state, based on an SU(3) generalization of the quantum nonlinear $\sigma$ model. We use this field theory to make explicit predictions for inelastic neutron scattering, and argue that the wavelike excitations it predicts could be used to identify the symmetries broken by the otherwise unseen spin-nematic order.

We have succeeded in synthesizing single crystals of an organic radical $p$-Br-V [3-(4-bromophenyl)-1,5-diphenylverdazyl]. The dominant intermolecular interactions forming a one-dimensional spin chain are deduced from the molecular packing in the crystal, and the ab initio molecular orbital calculation also indicates such an exchange network. We analyzed the magnetic susceptibility, magnetization curve, and magnetic specific heat through the quantum Monte Carlo method and successfully explained them as being attributed to an $S=\frac{1}{2}$ Heisenberg antiferromagnetic chain. Furthermore, we have found an unexpected successive phase transition in the specific heats, which indicates the formation of a noncollinear magnetic structure induced by frustration.

In the first paper of this series [DiStasio, Jr., Marcotte, Car, Stillinger, and Torquato, Phys. Rev. B 88, 134104 (2013)], we applied inverse statistical-mechanical techniques to study the extent to which targeted spin configurations on the square lattice can be ground states of finite-ranged radial spin-spin interactions. In this sequel, we enumerate all of the spin configurations within a unit cell on the one-dimensional integer lattice and the two-dimensional square lattice up to some modest size under periodic boundary conditions. We then classify these spin configurations into those that can or cannot be unique classical ground states of the aforementioned radial pair spin interactions and found the relative occurrences of these ground-state solution classes for different system sizes. As a result, we also determined the minimal radial extent of the spin-spin interaction potentials required to stabilize those configurations whose ground states are either unique or degenerate (i.e., those sharing the same radial spin-spin correlation function). This enumeration study has established that unique ground states are not limited to simple target configurations. However, we also found that many simple target spin configurations cannot be unique ground states.

In BaFe${}_2$As${}_2$, structural and magnetic planar defects begin to proliferate below the structural phase transition, affecting descriptions of magnetism and superconductivity. We study, using density-functional theory, the stability and magnetic properties of competing antiphase and domain boundaries, twins and isolated $nano$twins (twin nuclei), and spin excitations proposed and/or observed. These nanoscale defects have a very low surface energy (22–210 m Jm${}^{-2}$), with twins favorable to the mesoscale. Defects exhibit smaller moments confined near their boundaries—making a uniform-moment picture inappropriate for long-range magnetic order in real samples. Nanotwins explain features in measured pair distribution functions so should be considered when analyzing scattering data. All these defects can be weakly mobile and/or can have fluctuations that lower assessed “ordered” moments from longer spatial and/or time averaging and should be considered directly.

We report Raman scattering experiments on the strongly correlated electron system Na${}_x$CoO${}_2$ with $x$ $=$ 0.71 and ordered Na vacancies. This phase exhibits unusual transport and thermodynamic properties. Furthermore, hydrated samples with the same Co valency are superconducting. Our Raman scattering data reveal pronounced low-energy fluctuations that diverge in intensity at low temperatures. There is a related decrease of the electronic scattering rate Γ from 50 to 3 cm${}^{-1}$. Simultaneous with the evolution of the scattering rate, phonon anomalies point to an increasing out-of-plane coherence of the lattice with decreasing temperature. These observations may indicate the condensation of spin polarons into an unusual, highly dynamic ground state.

By solving the many-body Bethe-Salpeter equation at finite momentum transfer, we characterize the exciton dispersion in two prototypical molecular crystals, picene and pentacene, in which localized Frenkel excitons compete with delocalized charge-transfer excitons. We explain the exciton dispersion on the basis of the interplay between electron and hole hopping and electron-hole exchange interaction, unraveling a simple microscopic description to distinguish Frenkel and charge-transfer excitons. This analysis is general and can be applied to other systems in which the electron wave functions are strongly localized, as in strongly correlated insulators.

We study doping properties and charge carrier trap distributions in pentacene Schottky diodes doped by the fluorinated fullerene derivate ${\text{C}}_{60}{\text{F}}_{36}$ and 2,2'$-$(perdiylidene)dimalononitrile (${\text{F}}_6\text{-TCNNQ}$) upon small signal excitation. We show that the charge carrier depletion zones present in these Schottky diodes are tunable by the applied bias and temperature. Mott-Schottky evaluations yield reduced doping efficiencies and dopant activation energies between 19 and 54 meV. In the low-frequency regime, we resolve additional capacitive contributions from inherent charge carrier traps. A Gaussian distributed trap center 0.6 eV above the hole transport level with a density in the range of ${10}^{16}$ cm${}^{-3}$ depending on the material purity is found to be an intrinsic feature of the pentacene matrix. Upon doping, the deep Gaussian trap center saturates in density and broad exponentially tailing trap distributions arise. Subsequent ultraviolet photoelectron spectroscopy measurements are conducted to inspect for energetic broadening due to doping.

We report the observation and interpretation of new Raman peaks in few-layer tungsten diselenide (WSe${}_2$), induced by the reduction of symmetry going from three-dimensional (3D) to two-dimensional (2D). In general, Raman frequencies in 2D materials follow quite closely the frequencies of corresponding eigenmodes in the bulk. However, while the modes that are Raman active in the bulk are also Raman active in the thin films, the reverse is not always true due to the reduced symmetry in thin films. Here, we predict from group theory and density functional calculations that two intralayer vibrational modes, which are Raman inactive in bulk WSe${}_2$ in our experimental configuration become Raman active in thin film WSe${}_2$, due to reduced symmetry in thin films. This phenomenon explains the Raman peaks we observe experimentally at ∼310 and 176 cm${}^{-1}$ in thin film WSe${}_2$. Interestingly, the bulk $B_{2g}^1$ mode at ∼310 cm${}^{-1}$ that is Raman inactive can, in fact, be detected in Raman measurements under specific wavelengths of irradiation, suggesting that in this case, crystal symmetry selection rules may be broken due to resonant scattering. Both theory and experiment indicate that the $E_{2g}^1$ and $B_{2g}^1$ modes blueshift with decreasing thickness, which we attribute to surface effects. Our results shed light on a general understanding of the Raman/infrared activities of the phonon modes in layered transition metal dichalcogenide materials and their evolution behavior from 3D to 2D.

By combining the time-dependent Schrödinger equation with microscopic kinetic equations, a model for femtosecond-laser-induced electron emission from a metallic surface is developed to capture the physics of nonequilibrium heating and also the multiple-energy time-dependent tunneling. Using this model, we observe an enhanced electron emission due to the tranmission resonance at a particular energy level with a barrier height that produces a resonant frequency near to the laser frequency. Contrary to both pure optical tunneling and pure multiphoton emission models, the model provides better agreement with experiments. The generalized Einstein photoelectric effect is found to be inaccurate in this regime because of the nonequilibrium electron energy distribution, the resonant enhancement, and the ultrashort laser pulse width. The role of photons in the multiphoton electron emission regime is studied in detail. A smooth transition from the multiphoton regime to the optical tunneling regime is demonstrated.

Time-, angle-, and energy-resolved two-photon photoelectron spectroscopy is used to study image-potential states of weakly bound monolayer graphene on the noble-metal surfaces Pt(111), Au/Ir(111), Au/Ni(111), and Ir(100). Binding energies are increased relative to the substrates, and lifetimes are similar for all systems under investigation. The properties of the image-potential states are mainly determined by the graphene layer with reduced influence of the substrate. Substrate-related occupied surface states close to the Fermi level are observed for graphene on fcc(111) substrates.

We report on a detailed investigation of the electronic and atomic structure of nanometer-size Co/Ge islands obtained by solid-state reactive deposition of Co atoms on a $\mathrm{Ge}\left(111\right)c(2\times 8)$ surface. Relying on scanning tunneling microscopy (STM) and spectroscopy (STS) measurements combined with density functional theory based calculations, the atomic structure of the Co/Ge(111)$\sqrt{13}\times \sqrt{13}R13.9^{\circ }$ surface reconstruction is determined. Real-space STM imaging combined with Fourier-transform analysis reveals the coexistence of two inequivalent phases of $\sqrt{13}\times \sqrt{13}R13.9^{\circ }$ reconstructed Co/Ge nanoislands that are rotated by $+13.9^{\circ }$ and $-13.9^{\circ }$ with respect to the $\left[2\overline{11}\right]$ direction. STS spectra probe a small band gap that varies within the $\sqrt{13}\times \sqrt{13}R13.9^{\circ }$ surface unit cell between 10 and $250\mathrm{meV}$, suggesting local metallic behavior. According to the proposed atomic-structure model, each Co/Ge(111)$\sqrt{13}\times \sqrt{13}R13.9^{\circ }$ surface unit cell contains one Ge adatom and six Co atoms that are located at hollow sites below the top surface Ge layer and that are stacked in the form of an equilateral triangle. The Ge adatom is located asymmetrically with respect to the Co triangle and occupies two different, yet physically equivalent, positions, giving rise to two chiral phases of Co/Ge nanoislands. The Co/Ge valence band is dominated by Co atom derived 3$d$ states, while states in the conduction band stem from Ge adatom and Ge rest-atom derived states. Analysis of the bonding properties confirms the stability of the proposed Co/Ge atomic structure and reveals significant charge transfer from Co atoms to Ge rest atoms, suggesting ionic or metallic-covalent interaction.

Semiconducting transition metal dichalcogenides (STMDC) are two-dimensional (2D) crystals characterized by electron-volt-size band gaps, spin-orbit coupling (SOC), and $d$-orbital character of its valence and conduction bands. We show that these materials carry unique exciton quasiparticles (electron-hole bound states) with energy within the gap but which can “collapse” in the strong coupling regime by merging into the band structure continuum. The exciton collapse seems to be a generic effect in these 2D crystals.

In various manganese-based II-VI diluted magnetic semiconductors and their quantum structures, the measured variations of electron spin dephasing time in an external magnetic field are conflicting with the most advanced spin relaxation theory based on quantum kinetic equations. We demonstrate, by time-resolved optical spin beat measurements performed on high-mobility $n$-doped CdMnTe quantum wells, that these contradictions are resolved if one takes into account the electron spin dephasing induced by laser heating of the manganese spins. We then provide a test of the spin relaxation theory in Mn-based quantum wells by a careful comparison with existing data, including our measurements. It turns out that the theory systematically underestimates the relaxation rates by a factor of 5.

The double-differential spectrum of coincidences between backscattered electrons and secondary electrons (SEs) emitted from a polycrystalline Al surface bombarded with 100-eV electrons was measured. For energy losses of the scattered electron in between the work function of Al and the bulk plasmon energy, a sharp peak is observed in the SE spectra, corresponding to ejection of a single electron near the Fermi edge receiving the full energy loss and momentum of the primary electron. This process predominantly takes place when the primary electron suffers a surface energy loss in vacuum, and leads to SE ejection from the very surface. At energy losses just above the bulk plasmon energy, a sharp transition is observed, corresponding to a sudden increase in the depth of ejection. The latter is a direct consequence of the complementarity of surface and bulk plasmons, the so-called Begrenzungs effect.

We report normal-state transport properties of the single-crystalline samples of the silver-lead oxide superconductor Ag${}_5$Pb${}_2$O${}_6$, including the electrical resistivity, magnetoresistance, and Hall coefficient. From the Hall coefficient measurement, we confirmed that the carrier density of this oxide is as low as $5\times {10}^{21}$ cm${}^{-3}$, one order of magnitude smaller than that for ordinary alkali metals and noble metals. The magnetoresistance behavior is well characterized by the axial symmetry of the Fermi surface and by a single relaxation time. The $T^2$ term of the resistivity is scaled with the specific-heat coefficient, based on the recent theory for the electron-electron scattering. The present results provide evidence that Ag${}_5$Pb${}_2$O${}_6$ is a low-carrier-density three-dimensional electron-gas-like system with enhanced electron-electron scatterings.

The efficiency of a two-step quantum cutting (QC) mechanism for solar downconversion via two consecutive energy transfers from Pr${}^{3+}$ to Yb${}^{3+}$ is investigated in Pr${}^{3+}$-Yb${}^{3+}$ codoped KY${}_3$F${}_{10}$ crystals. The efficiency of the QC second energy transfer involving the Pr${}^{3+}$ (${}^1$$G_4$) and Yb${}^{3+}$ (${}^2$$F_{5/2}$) levels is particularly difficult to determine in a precise manner since these two levels are coupled by reciprocal energy transfer. We present a specific rate equation modeling that accurately describes Pr${}^{3+}$ and Yb${}^{3+}$ dynamics and intensity ratios measured as a function of the Yb${}^{3+}$ concentration. The modeling enables to calculate energy transfer rates for all the processes and shows the competition between the two-step QC mechanism and parasitic processes. The first-step of the QC mechanism is, as expected, particularly efficient reaching 96.9$\%$ and is not challenged by competing processes. The second step of the QC, while reaching an intrinsic efficiency of 75.9$\%$, is plagued by backtransfers from Yb${}^{3+}$ (${}^2$$F_{5/2}$) to Pr${}^{3+}$ (${}^1$$G_4$) along with efficient concentration quenching mechanisms draining the Yb${}^{3+}$ excited-state. In addition, the investigation of intensity ratios with the Yb${}^{3+}$ concentration shows the importance of energy migration among Yb${}^{3+}$ ions that further enhances the detrimental effect of the Yb${}^{3+}$ quenching processes. As a result, the intrinsic QC efficiency (QCE) of the QC first and second energy transfers increases with the Yb${}^{3+}$ doping level up to 173$\%$ in KY${}_3$F${}_{10}$: 0.5$\%$Pr${}^{3+}$-20$\%$Yb${}^{3+}$, while the real QCE reduced by migration-assisted quenching processes is only 6.2$\%$ in this sample. These results clarify apparently conflicting reports found in the literature showing, on one hand, very high QCEs derived from spectroscopic experiments and, on the other hand, very low efficiency values obtained from integrating sphere measurements.

Exact diagonalization studies have revealed that the energy spectrum of interacting electrons in the lowest Landau level splits, nonperturbatively, into bands, which is responsible for the fascinating phenomenology of this system. The theory of nearly free composite fermions has been shown to be valid for the lowest band, and thus to capture the low-temperature physics, but it overpredicts the number of states for the excited bands. We explain the state counting of higher bands in terms of composite fermions with an infinitely strong short-range interaction between an excited composite-fermion particle and the hole it leaves behind. This interaction, the form of which we derive from the microscopic composite-fermion theory, eliminates configurations containing certain tightly bound composite-fermion excitons. With this modification, the composite-fermion theory reproduces, for all well defined excited bands seen in exact diagonalization studies, an exact counting for $\nu >1/3$, and an almost exact counting for $\nu \le 1/3$. The resulting insight clarifies that the corrections to the nearly free composite-fermion theory are not thermodynamically significant at sufficiently low temperatures, thus providing a microscopic explanation for why it has proved successful for the analysis of the various properties of the composite-fermion Fermi sea.

The photoresponse of graphene has recently received considerable attention. The main mechanisms yielding a finite dc response to an oscillating radiation field which have been investigated include responses of photovoltaic, photothermoelectric, and bolometric origin. In this article, we present a fully analytical theory of a photoresponse mechanism which is based on the excitation of plasma waves in a gated graphene sheet. By employing the theory of relativistic hydrodynamics, we demonstrate that plasma-wave photodetection is substantially influenced by the massless Dirac fermion character of carriers in graphene, and that the efficiency of photodetection can be improved with respect to that of ordinary parabolic-band electron fluids in semiconductor heterostructures.