The nature of the stress and electric field driven structural and microstructural transformations in the morphotropic phase boundary (MPB) compositions of the high Curie point piezoelectric system $\mathrm{BiSc}{\mathrm{O}}_3\text{−}\mathrm{PbTi}{\mathrm{O}}_3$ has been examined by ex situ based techniques. Using a powder poling technique, which is based on the concept of exploiting the irreversible structural change that occurs after the application of a strong electric field and stress independently, it was possible to ascertain that both moderate stress and electric field induce identical structural transformation—a fraction of the monoclinic phase transforms irreversibly to the tetragonal phase. Moreover, analysis of the dielectric response before and after poling revealed a counterintuitive phenomenon of poling induced decrease in the spatial coherence of polarization for compositions around the MPB and not so for compositions far away from the MPB range. Exploiting the greater sensitivity of this technique, we demonstrate that the criticality associated with the interferroelectric transition spans a wider composition range than what is conventionally reported in the literature based on bulk x-ray/neutron powder diffraction techniques.

We develop a mean-field theory of the metallic phase near the many-body localization (MBL) transition, using the observation that a system near the MBL transition should become an increasingly slow heat bath for its constituent parts. As a first step, we consider the properties of a many-body localized system coupled to a generic ergodic bath whose characteristic dynamical time scales are much slower than those of the system. As we discuss, a wide range of experimentally relevant systems fall into this class; we argue that relaxation in these systems is dominated by collective many-particle rearrangements, and compute the associated time scales and spectral broadening. We then use the observation that the self-consistent environment of any region in a nearly localized metal can itself be modeled as a slowly fluctuating bath to outline a self-consistent mean-field description of the nearly localized metal and the localization transition. In the nearly localized regime, the spectra of local operators are highly inhomogeneous and the typical local spectral linewidth is narrow. The local spectral linewidth is proportional to the dc conductivity, which is small in the nearly localized regime. This typical linewidth and the dc conductivity go to zero as the localized phase is approached, with a scaling that we calculate, and which appears to be in good agreement with recent experimental results.

Magnetic susceptibility and heat capacity measurements are used to infer information about the short-range magnetic order above the Néel temperature $T_N$ and the antiferromagnetically ordered states below $T_N$ of quasi-one-dimensional (quasi-1D) ${\mathrm{CuSb}}_2{\mathrm{O}}_6,{\mathrm{NiTa}}_2{\mathrm{O}}_6$, and ${\mathrm{CoSb}}_2{\mathrm{O}}_6$. It is shown that two antiferromagnetic sublattices, oriented at $90{}^{\circ }$ to one another, are likely present in ${\mathrm{NiTa}}_2{\mathrm{O}}_6$ and ${\mathrm{CoSb}}_2{\mathrm{O}}_6$. Application of magnet field parallel to the quasi-1D chains of one sublattice is perpendicular to the chains of the other sublattice. This results in two antiferromagnetic transitions when the magnetic field $H\ge 2$ T ($\sim 0.2$ meV). The anisotropic influence of magnetic field on the antiferromagnetic state leads to a magnetocaloric effect that is fully investigated in this work. The effect is associated with competition among Néel, dimer, and spin-liquid states that are all present at $T_N$.

We report an experimental investigation of the magnetoelastic effects of ultrathin antiferromagnets, performed by comparing the characteristic behavior of the induced spin-reorientation transition (SRT) of Co films in two types of epitaxially grown bilayers: face-centered-cubic (fcc)–like Mn/Co (fcc-Mn: $a=3.75\AA ,c=3.76\AA )$ and face-centered-tetragonal (fct)–Mn $\left(a=3.61\AA ,c=3.78\AA \right)$. Magnetic hysteresis loops and magnetic domain images indicate that both fcc-Mn and fct-Mn films can produce a $\langle 110\rangle$ to $\langle 100\rangle$ SRT in adjacent Co films when the thickness of the Mn layer is greater than a temperature-dependent critical value. Detailed analysis of the critical thickness of Mn films and the evolution of the Co domain structure upon SRT indicate that the fct-Mn film had a higher antiferromagnetic ordering temperature and stronger lateral Mn-Mn exchange coupling compared with the fcc-Mn film. The enhanced long-range antiferromagnetic ordering emerging concurrently with the in-plane lattice variation of the fcc-like Mn film in Mn/Co bilayers clearly showed the magnetoelastic effect of ultrathin antiferromagnets.

The orthorhombic compound ${\mathrm{NdFe}}_2{\mathrm{Al}}_{10}$ has been studied by powder and single-crystal neutron diffraction. Below $T_N=3.9$ K, the ${\mathrm{Nd}}^{3+}$ magnetic moments order in a double-$\mathit{k}$ [${\mathit{k}}_1=(0,\frac{3}{4},0)$, ${\mathit{k}}_2=(0,\frac{1}{4},0)$] collinear magnetic structure, whose unit cell consists of four orthorhombic units in the $b$ direction. The refinements show that this structure consists of (0 1 0) ferromagnetic planes stacked along $b$, in which the moments are oriented parallel to $a$ (the easy anisotropy axis according to bulk magnetization measurements) and nearly equal in magnitude ($\approx 1.7–1.9{\mu }_{\text{B}}$). The alternating eight-plane sequence providing the best agreement to the data turns out to be that which yields the lowest exchange energy if one assumes antiferromagnetic near-neighbor exchange interactions with $J_1\gg J_2,J_3$. With increasing temperature, the single-crystal measurements indicate the suppression of the ${\mathit{k}}_2$ component at $T=2.7$ K, supporting the idea that the anomalies previously observed around 2–2.5 K result from a squaring transition. In a magnetic field applied along the $a$ axis, the magnetic Bragg satellites disappear at $H_c=2.45$ T, in agreement with earlier measurements. Comparisons are made with related magnetic orders occurring in $\mathrm{Ce}{T}_2{\mathrm{Al}}_{10}$ ($T=$ Ru, Os) and ${\mathrm{TbFe}}_2{\mathrm{Al}}_{10}$.

Our combined experimental and theoretical study reveals unusually large cobalt-oxygen covalency in ${\mathrm{CoO}}_4$ tetrahedral unit of a barium cobalt oxychloride compound. This drives significant charge redistribution, resulting into large hole density on tetrahedral oxygens, which effectively behave as “positively charged” anions. These positively charged oxygens form local dipoles with dopant chloride anions, situated in the same atomic plane, which gets manifested in associated structural distortions. The spatial freezing of these local dipoles below certain temperature is found to produce concomitant effects on dielectric and magnetic responses, coupled via exchange-striction driven spin-phonon interaction. Our study should form the basis for designing new functional oxide materials using the concept of covalency-driven charge redistribution.

Three-dimensional strong topological insulators (TIs) guarantee the existence of a two-dimensional (2-D) conducting surface state which completely covers the surface of the TI. The TI surface state necessarily wraps around the TI's top, bottom, and two sidewalls, and is therefore topologically distinct from ordinary 2-D electron gases (2-DEGs) which are planar. This has several consequences for the magnetoconductivity $\Delta \sigma$, a frequently studied measure of weak antilocalization which is sensitive to the quantum coherence time ${\tau }_{\varphi }$ and to temperature. We show that conduction on the TI sidewalls systematically reduces $\Delta \sigma$, multiplying it by a factor which is always less than one and decreases in thicker samples. In addition, we present both an analytical formula and numerical results for the tilted-field magnetoconductivity which has been measured in several experiments. Lastly, we predict that as the temperature is reduced $\Delta \sigma$ will enter a wrapped regime where it is sensitive to diffusion processes which make one or more circuits around the TI. In this wrapped regime the magnetoconductivity's dependence on temperature, typically $1/T^2$ in 2-DEGs, disappears. We present numerical and analytical predictions for the wrapped regime at both small and large field strengths. The wrapped regime and topological signatures discussed here should be visible in the same samples and at the same temperatures where the Altshuler-Aronov-Spivak (AAS) effect has already been observed, when the measurements are repeated with the magnetic field pointed perpendicularly to the TI's top face.

High-energy collective electronic excitations (plasmons) in freestanding multilayer graphene are studied by momentum-resolved electron energy-loss spectroscopy (EELS). For normal incidence, only the high-energy plasmon band is excited and we measure a blueshift of the $\pi$-plasmon dispersion with increasing thickness. The observed transition between two-dimensional and three-dimensional behavior is explained using a layered-electron-gas (LEG) model. We propose a method to measure all individual plasmon bands by tilting the sample with respect to the electron beam. As a proof of concept, EELS experiments for three-layer graphene are compared with predictions from the LEG model.

High-frequency coherent phonons resonantly excited in metallic single-walled carbon nanotubes (M-SWCNTs) were investigated via spectrally resolved pump-probe spectroscopy using 7.5-fs laser pulses. In addition to first-order coherent phonons such as radial breathing mode (RBM) and $M$ and $G$ modes, we clearly observed second-order high-frequency coherent phonons of $2D$ and $2G$ modes, which can be regarded as squeezed phonons. We found that the amplitudes of the RBM, $G$ and $2D$ modes were resonantly enhanced at specific wavelengths: the maximum resides at a wavelength whose energy is smaller than that of the van Hove singularities in M-SWCNTs by an amount corresponding to the phonon energy. Furthermore, the $2D$ mode has stronger enhancement than the other first-order Raman modes. These results indicate that the enhancement originates from a Stokes-stimulated Raman-scattering process at van Hove singularities and that efficient resonance enhancement occurs for the $2D$ mode, possibly through double resonance due to the trigonal warping effect and strong electron-phonon coupling due to the Kohn anomaly.

Reconstructions of the ${\mathrm{LaAlO}}_3$ (001) surface are studied using hybrid density functional calculations. We calculate surface energies as a function of O and Al chemical potentials for both the LaO- and ${\mathrm{AlO}}_2$-terminated surfaces. We find that the $(3\times 2)$ Al adatom is the most stable reconstruction on the ${\mathrm{AlO}}_2$ termination, with the $(2\times 2)$ O vacancy slightly higher in energy. On the LaO termination, the $(3\times 2)$ La vacancy is the most stable reconstruction. Overall, the LaO termination with the $(3\times 2)$ La vacancy has the lowest surface energy, in the absence of impurities. However, our calculations also indicate that the presence of hydrogen alters the stability of the LaO versus the ${\mathrm{AlO}}_2$ termination. Under Al-rich conditions, H binds to an O atom forming a $(2\times 1)$ H-adatom reconstruction on the ${\mathrm{AlO}}_2$-terminated surface, whereas under La-rich conditions a $(2\times 1)$ OH adsorbate reconstruction forms on the LaO-terminated surface. These reconstructions are found to be the most stable terminations over a wide range of H and O chemical potentials.

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2\times 2)$ or a $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in $n$ doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moiré superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially, as found by x-ray magnetic circular dichroism. The $(2\times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.

A generalization of Büttiker's voltage probe concept for nonzero temperatures is an open third terminal of a quantum thermoelectric circuit. An explicit analytic expression for the thermoelectric correction to an ideal quantum voltage measurement in linear response is derived and interpreted in terms of local Peltier cooling/heating within the nonequilibrium system. The thermoelectric correction is found to be large (up to $\pm 24\%$ of the peak voltage) in a prototypical ballistic quantum conductor (graphene nanoribbon). The effects of measurement nonideality are also investigated. Our findings have important implications for precision local electrical measurements.

Three-dimensional Dirac semimetals are stable against weak potential disorder, but not against strong disorder. In the language of renormalization group, such stability stems from the irrelevance of weak disorder in the vicinity of the noninteracting Gaussian fixed point. However, beyond a threshold, potential disorder can take Dirac semimetals into a compressible diffusive metallic phase through a quantum phase transition (QPT), where density of states at zero energy, quasiparticle lifetime, and metallic conductivity at $T=0$ are finite. Universal behavior of such unconventional QPT is described within the framework of an $\epsilon (=d-2)$ expansion near the lower critical dimension. Various exponents near this quantum critical point are obtained after performing a two-loop perturbative expansion in the vanishing replica limit and we demonstrate that the theory is renormalizable at least to two-loop order. We argue that such QPT is always continuous in nature and shares the same university class with a similar transition driven by odd-parity disorder. The critical exponents are independent of flavor number of Dirac fermions and thus our study can be germane to disordered ${\mathrm{Cd}}_3{\mathrm{As}}_2$ and ${\mathrm{Na}}_3\mathrm{Bi}$. Scaling behaviors of various measurable quantities such as specific heat and density of states across such QPT are proposed.

We examine the photoluminescence of highly excited exciton-polariton condensates in semiconductor microcavities. Under strong pumping, exciton-polariton condensates have been observed to undergo a lasing transition where the strong coupling between the excitons and photons is lost. We discuss an alternative high-density scenario, where the strong coupling is maintained. We find that the photoluminescence smoothly transitions between the lower polariton energy to the cavity photon energy. An intuitive understanding of the change in spectral characteristics is given, as well as differences to the photoluminescence characteristics of the lasing case.

We report the investigation of a strong interband photon drag effect in multilayer graphene leading to efficient emission of terahertz radiation. The obtained terahertz photoresponse of graphene layers exhibits peculiarities fundamentally predicted for free carrier transport in two-dimensional electronic systems. Owing to significant light absorption in gapless graphene, where each absorbed photon produces an electron-hole pair with the highest possible kinetic energy, the photon drag mechanism provides a possibility to achieve efficient conversion of light into broadband terahertz radiation as well as new ways towards vectorial control of the generated terahertz radiation in graphene-based materials.

Excitonic density-wave states realized by the quantum condensation of electron-hole pairs (or excitons) are studied in the two-band Hubbard model with Hund's rule coupling and the pair hopping term. Using the variational cluster approximation, we calculate the grand potential of the system and demonstrate that Hund's rule coupling always stabilizes the excitonic spin-density-wave state and destabilizes the excitonic charge-density-wave state and that the pair hopping term enhances these effects. The characteristics of these excitonic density-wave states are discussed using the calculated single-particle spectral function, density of states, condensation amplitude, and pair coherence length. Implications of our results in the materials' aspects are also discussed.

We study the quasiparticle band gap of the hybrid organic-inorganic lead halide perovskite ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, using many-body perturbation theory based on the $GW$ approximation. We perform a systematic analysis of the band gap sensitivity to relativistic spin-orbit effects, to the description of semicore Pb-$5d$ and I-$4d$ electrons, and to the starting Kohn-Sham eigenvalues. We find that the inclusion of semicore states increases the calculated band gap by 0.2 eV, and self-consistency on the quasiparticle eigenvalues using a scissor correction increases the band gap by 0.5 eV with respect to the $G_0{W}_0$ result. These findings allow us to resolve an inconsistency between previously reported $GW$ calculations for ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. Our most accurate band gap is 1.72 eV, and is in good agreement with the measured optical gap after considering a small excitonic shift as determined in experiments.

The enhanced thermopower of the correlated semiconductor FeSi is found to be robust against the sign of the relevant charge carriers. At $T\approx 70$ K, the position of both the high-temperature shoulder of the thermopower peak and the nonmagnetic-enhanced paramagnetic crossover, the Nernst coefficient $\nu$ assumes a large maximum and the Hall mobility ${\mu }_H$ diminishes to below 1 ${\mathrm{cm}}^2$/V s. These cause the dimensionless ratio $\nu /{\mu }_H$—a measure of the energy dispersion of the charge scattering time $\tau \left(\epsilon \right)$—to exceed that of classical metals and semiconductors by two orders of magnitude. Concomitantly, the resistivity exhibits a hump and the magnetoresistance changes its sign. Our observations hint at a resonant scattering of the charge carriers at the magnetic crossover, imposing strong constraints on the microscopic interpretation of the robust thermopower enhancement in FeSi.

We present a large-scale combinatorial-diagrammatic computation of high-order contributions to the strong-coupling Kato-Takahashi perturbation series for the Hubbard model in high dimensions. The ground-state energy of the Mott-insulating phase is determined exactly up to the 15th order in $1/U$. The perturbation expansion is extrapolated to infinite order and the critical behavior is determined using the Domb-Sykes method. We compare the perturbative results with two dynamical mean-field theory (DMFT) calculations using a quantum Monte Carlo method and a density-matrix renormalization group method as impurity solvers. The comparison demonstrates the excellent agreement and accuracy of both extrapolated strong-coupling perturbation theory and quantum Monte Carlo based DMFT, even close to the critical coupling where the Mott insulator becomes unstable.

We propose a general framework of nonlinear optics induced by non-Abelian Berry curvature in time-reversal-invariant (TRI) insulators. We find that the third-order response of a TRI insulator under optical and terahertz light fields is directly related to the integration of the non-Abelian Berry curvature over the Brillouin zone. We apply the result to insulators with rotational symmetry near the band edge. Under resonant excitations, the optical susceptibility is proportional to the flux of the Berry curvature through the iso-energy surface, which is equal to the Chern number of the surface times $2\pi$. For the III-V compound semiconductors, microscopic calculations based on the six-band model give a third-order susceptibility with the Chern number of the iso-energy surface equal to 3.