The local structure and 4$f$ orbital occupancy have been investigated in Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ via Yb $L_{\mathrm{III}}$-edge extended x-ray absorption fine structure (EXAFS), Ce and Yb $L_{\mathrm{III}}$-edge x-ray absorption near-edge structure (XANES), and angle-resolved photoemission spectroscopy (ARPES) measurements. Yb(III) (4$f^{13}$) is the hole analog of Ce(III) (4$f^1$). Yb is found to be strongly intermediate-valent in Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ throughout the entire doping range, including pure YbCoIn${}_5$, with an $f$-hole occupancy for Yb of $n_f\simeq 0.3$ (i.e., Yb${}^{2.3+}$), independent of Yb concentration and independent of temperature down to $T=20$ K. In contrast, the $f$-electron orbital occupancy for Ce remains close to 1 for all Yb concentrations, suggesting that there is no mutual influence on $n_f$ between neighboring Ce and Yb sites. Likewise, ARPES measurements at 12 K have found that the electronic structure along $\Gamma -X$ is not sensitive to the Yb substitution, suggesting that the Kondo hybridization of Ce $f$ electrons with the conduction band is not affected by the presence of Yb impurities in the lattice. The emerging picture is that in Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ there are two networks, interlaced but independent, that couple to the conduction band: one network of Ce ions in the heavy-fermion limit, and one network of Yb ions in the strongly intermediate-valent limit. The robustness of the local and electronic structure to doping suggests the absence of charge transfer between the Ce and Yb ions, and may explain the relative robustness of superconductivity for this Ce-site substitution as compared to the In-site substitution.

We analyze the full counting statistics of the multiorbital Kondo effect in a quantum dot with SU($N$) symmetry in the framework of renormalized perturbation theory. The current probability distribution function is calculated for an arbitrary dot-site Coulomb repulsion $U$ in the particle-hole-symmetric case. The resulting cumulant up to the leading nonlinear term of applied bias voltages indicates two types of electron transfer, respectively carrying charge $e$ and $2e$, with different $N$ dependences. The cross correlation between different orbital currents shows exponential enhancement with respect to $U$, which directly addresses the formation of the orbital-singlet state.

Based on recent advances in first-principles theory, we develop a general model of the band offset at metal/ferroelectric interfaces. We show that, depending on the polarization of the film, a pathological regime might occur where the metallic carriers populate the energy bands of the insulator, making it metallic. As the most common approximations of density functional theory are affected by a systematic underestimation of the fundamental band gap of insulators, this scenario is likely to be an artifact of the simulation. We provide a number of rigorous criteria, together with extensive practical examples, to systematically identify this problematic situation in the calculated electronic and structural properties of ferroelectric systems. We discuss our findings in the context of earlier literature studies, where the issues described in this work have often been overlooked. We also discuss formal analogies to the physics of polarity compensation at LaAlO${}_3$/SrTiO${}_3$ interfaces, and suggest promising avenues for future research.

We present momentum-resolved single-particle spectra for the three-dimensional Hubbard model for the paramagnetic and antiferromagnetically ordered phase obtained within the dynamical cluster approximation. The effective cluster problem is solved by continuous-time quantum Monte Carlo simulations. The absence of a time discretization error and the ability to perform Monte Carlo measurements directly in Matsubara frequencies enable us to analytically continue the self-energies by maximum entropy, which is essential to obtaining momentum-resolved spectral functions for the Néel state. We investigate the dependence on temperature and interaction strength and the effect of magnetic frustration introduced by a next-nearest-neighbor hopping. One particular question we address here is the influence of the frustrating interaction on the metal-insulator transition of the three-dimensional Hubbard model.

Atomically resolved structural and electronic properties of FeTe${}_{1-x}$Se${}_x$ ($x$ $=$ 0 and 0.45) have been studied with scanning tunneling microscopy/spectroscopy (STM/STS). In contrast to the extreme flatness of the Te-terminated FeTe surface, nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe${}_{0.55}$Se${}_{0.45}$. A statistical counting of the two kinds of atoms has the same ratio as that in the bulk. Remarkably, there is no electronic phase separation seen in the tunneling spectroscopy. This indicates that the optimally doped superconductor is chemically inhomogeneous but electronically homogeneous, in contrast to many correlated electron materials.

Carrier and phonon dynamics in Bi${}_2$Se${}_3$ crystals are studied by a spatially resolved ultrafast pump-probe technique. Pronounced oscillations in differential reflection are observed with two distinct frequencies and are attributed to coherent optical and acoustic phonons, respectively. The rising time of the signal indicates that the thermalization and energy relaxation of hot carriers are both sub-ps in this material. We found that the thermalization and relaxation time decreases with the carrier density. The expansion of the differential reflection profile allows us to estimate an ambipolar carrier diffusion coefficient on the order of 500 cm${}^2$/s. A long-term slow expansion of the profile shows a thermal diffusion coefficient of 1.2 cm${}^2$/s.

Superheating and supercooling effects are characteristic kinetic processes in first-order phase transitions, and asymmetry between them is widely observed. In materials where electronic and structural degrees of freedom are coupled, a wide, asymmetric hysteresis may occur in the transition between electronic phases. Structural defects are known to seed heterogeneous nucleation of the phase transition, hence reduce the degree of superheating and supercooling. Here we show that in the metal-insulator transition of single-crystal VO${}_2$, a large kinetic asymmetry arises from the distinct spatial extension and distribution of two basic types of crystal defects: point defects and twin walls. Nanometer-thick twin walls are constantly consumed but regenerated during the transition to the metal phase, serving as dynamical heterogeneous nucleation seeds and eliminating superheating. On the other hand, the transition back to the insulator phase relies on nucleation at point defects because twinning is structurally forbidden in the metal phase, leading to a large supercooling. By controlling the formation, location, and extinction of these defects, the kinetics of the phase transition might be externally modulated, offering possible routes toward unique memory and logic device technologies.

We consider the problem of calculating the weak and strong topological indices in noncentrosymmetric time-reversal ($\mathcal{T}$) invariant insulators. In 2D we use a gauge corresponding to hybrid Wannier functions that are maximally localized in one dimension. Although this gauge is not smoothly defined on the two torus, it respects the $\mathcal{T}$ symmetry of the system and allows for a definition of the ${\mathbb{Z}}_2$ invariant in terms of time-reversal polarization. In 3D we apply the 2D approach to $\mathcal{T}$-invariant planes. We illustrate the method with first-principles calculations on GeTe and on HgTe under $[001]$ and $[111]$ strain. Our approach differs from ones used previously for noncentrosymmetric materials and should be easier to implement in ab initio code packages.

We report an experimental Raman study of few-layer graphene after chemical doping achieved by a plasma process in CHF${}_3$ gas. A systematic reduction of both the splitting and the area of the $2D$ band is observed with increasing doping level. Both effects can be ascribed to the electron-electron correlation, which, on the one hand, reduces the electron-phonon coupling strength and, on the other hand, affects the probability of the double-resonance Raman process.

Ferroelectric crystals are known to adopt low-energy, compatible domain configurations. Observations show that these configurations are commonly multirank laminate patterns. In this work, a method to classify and enumerate the laminate domain patterns that can form is presented. The criteria of exact compatibility for laminates structures in a ferroelectric single crystal are used to find all the rank-2 arrangements of domains in the polar tetragonal crystal system. Surprisingly, only eight distinct rank-2 laminate patterns that satisfy compatibility conditions at all domain walls are found. These patterns are classified and correlated with observations of domains in single crystals, showing good agreement.

Excitonic solar cells, comprised of materials such as organic semiconductors, inorganic colloidal quantum dots, and carbon nanotubes, are fundamentally different than crystalline, inorganic solar cells in that photogeneration of free charge occurs through intermediate, bound exciton states. Here, we show that the Second Law of Thermodynamics limits the maximum efficiency of excitonic solar cells below the maximum of 31% established by Shockley and Queisser [J. Appl. Phys. 32, 510 (1961)] for inorganic solar cells (whose exciton-binding energy is small). In the case of ideal heterojunction excitonic cells, the free energy for charge transfer at the interface, Δ$G$, places an additional constraint on the limiting efficiency due to a fundamental increase in the recombination rate, with typical −Δ$G$ in the range 0.3 to 0.5 eV decreasing the maximum efficiency to 27% and 22%, respectively.

We report on a detailed experimental investigation of interbranch parametric scattering processes in one-dimensional semiconductor microcavities. Band dispersion and corresponding far-field emission patterns are studied by polarization-resolved and power-dependence measurements under resonant and nonresonant excitation at normal incidence. We demonstrate the realization of optical parametric oscillation of balanced twin beams, which are degenerate in energy and split in momentum space. This achievement is shown for both the strong and the weak coupling regime, highlighting the versatility of this peculiar microcavity system.

We show that when the pseudomagnetic fields created by long-wavelength deformations are appropriately coupled with a scalar electric potential, a significant energy gap can emerge due to the formation of a Haldane state. Ramifications of this physical effect are examined through the study of various strain geometries commonly seen in experiments, such as strain superlattices and wrinkled suspended graphene. Of particular technological importance, we consider setups where this gap can be tunable through electrostatic gates, allowing for the design of electronic devices not realizable with other materials.

First-principles quasiharmonic calculations play a very important role in mineral physics because they can predict the structural and thermodynamic properties of materials at pressure and temperature conditions of the Earth's interior that are still challenging for experiments. They also enable calculations of thermal elastic properties by providing second-order derivatives of free energies with respect to strain. The latter are essential to interpret seismic tomography of the mantle in terms of temperature, composition, and mineralogy, in the context of geophysical processes. However, these are exceedingly demanding computations requiring up to ∼${10}^3$ parallel jobs running on tens or more processors each. Here we introduce an analytical and computationally simpler approach that requires only calculations of static elastic constants and phonon density of states for unstrained configurations. This approach, currently implemented for crystals with up to orthorhombic symmetry, decreases the computational effort, i.e., CPU time and human labor, by up to two orders of magnitude. Results for the major mantle phases periclase (MgO) and forsterite (α-${\mathrm{Mg}}_2{\mathrm{SiO}}_4$) show excellent agreement with previous first-principles results and experimental data.

We investigated the magnetic and structural properties of the quasi-one-dimensional 3$d^1$ quantum chain system TiPO${}_4$ ($J~$ 965 K) by magnetic susceptibility, heat capacity, electron spin resonance, x-ray diffraction, and nuclear magnetic resonance (NMR) measurements, and by density functional theory (DFT) calculations. TiPO${}_4$ undergoes two magnetostructural phase transitions, one at 111 K and the other at 74 K. Below 74 K, NMR detects two different ${}^{31}\mathrm{P}$ signals and the magnetic susceptibility vanishes, while DFT calculations evidence a bond alternation of the Ti-Ti distances within each chain. Thus, the 74 K phase transition is a spin-Peierls transition which evolves from an incommensurate phase existing between 111 and 74 K.

Nonlinear light propagation in a single-mode micron-size waveguide made of semiconducting excitonic material has been theoretically studied in terms of exciton polaritons by using an analysis based on macroscopic fields. When a light pulse is spectrally centered in the vicinity of the ground-state Wannier exciton resonance, it interacts with the medium nonlinearly. This optical cubic nonlinearity is caused by the repulsive exciton-exciton interactions in the semiconductor, and at resonance it is orders of magnitude larger than the Kerr nonlinearity (e.g., in silica). We demonstrate that a very strong and unconventional modulational instability takes place, which has not been previously reported. After reducing the problem to a single nonlinear Schrödinger-like equation, we also explore the formation of solitary waves both inside and outside the polaritonic gap and find evidence of spectral broadening. A realistic physical model of the excitonic waveguide structure is proposed.

A high-field study of magnetization (up to 68 T) and magnetoelastic properties (up to 60 T) of Er${}_2$Co${}_{17}$ is reported. The most significant effect, a first-order transition from the collinear ferrimagnetic to a canted state, is observed at about 40 T with H ∥ [001]. The transition is accompanied by a prominent magnetization jump as well as by step-wise anomalies of the magnetoelastic properties. Thus, the volume of the crystal reduces by about 4 per mil, while the speed of transverse sound in the [001] direction increases by as much as 5 per mil. At higher temperatures the anomalies gradually become smaller and less sharp before they finally disappear at ∼50 K. The anisotropy constants of the Er sublattice and the molecular field thereon have been determined from the magnetization curves.

A complete study of the magneto-optical (MO) and magnetoplasmonic properties of both epitaxial and polycrystalline Au/Fe/Au multilayers as a function of Fe thickness is reported. An increase for both the MO activity without surface-plasmon-polariton (SPP) excitation and the magnetic modulation of the SPP wave vector for epitaxial structures compared to the polycrystalline ones is observed. This difference is related with the larger interface roughness of the polycrystalline structures that as a consequence, exhibit a reduction in the Fe MO constants. A comparative study of the MO activity with SPP excitation for both epitaxial and polycrystalline structures is also presented, being the key factor in the deposition of the adequate layer thickness to achieve optimum SPP excitation with independence of their crystalline nature.

A thin ferromagnetic layer on a bulk equal-spin-pairing triplet superconductor is shown to mediate a Josephson coupling between the spin $\uparrow$ and $\downarrow$ condensates of the superconductor. By deriving analytic expressions for the bound states at the triplet superconductor-ferromagnet interface, we show that this spin Josephson effect establishes an effective anisotropy axis in the ferromagnetic layer. The associated Josephson spin current is predicted to cause a measurable precession of the magnetization about the vector order parameter of the triplet superconductor.

When coupling a superconductor to a normal conducting region the physical properties of the system are highly affected by the superconductor. We investigate the effect of one or two superconductors on the conductance of a ballistic chaotic quantum dot to leading order in the total channel number using trajectory-based semiclassics. The results show that the effect of one superconductor on the conductance is of the order of the number of channels and that the sign of the quantum correction from the Drude conductance depends on the particular ratios of the numbers of channels of the superconducting and normal conducting leads. In the case of two superconductors with the same chemical potential, we additionally study how the conductance and the sign of quantum corrections are affected by their phase difference. As far as random matrix theory results exist these are reproduced by our calculations. Furthermore, in the case that the chemical potential of the superconductors is the same as that of one of the two normal leads the conductance shows, under certain conditions, similar effects as a normal metal-superconductor junction. The semiclassical framework is also able to treat the thermopower of chaotic Andreev billiards consisting of one chaotic dot, two normal leads, and two superconducting islands and shows it to be antisymmetric in the phase difference of the superconductors.