In this Rapid Communication, we study an effective model for the normal state of iron-based superconductors. It has separate but interacting itinerant and localized degrees of freedom, originating from the $d_{xz}$ and $d_{yz}$ and from the $d_{xy}$ iron orbitals, respectively. At low temperatures, below a mean-field phase transition, these different states condense together in an excitonic order parameter. We show that, at even lower temperatures, after another phase transition, this ordered state can spontaneously break the $C_4$ lattice symmetry and become nematic. We propose this mechanism as an explanation of the tendency towards nematicity observed in several iron-based compounds.

We report the systematic investigation of the specific heat of the noncentrosymmetric superconductor Li${}_2$(Pd${}_{1-x}$Pt${}_x$)${}_3$B as a function of $x$. There is a large deviation of the phononic specific heat from the conventional Debye specific heat for Pt-rich samples. In contrast with the fully gapped conventional behavior for small $x$, a power-law temperature dependence of the electronic specific heat is observed even at $x=0.5$. These results manifest a strongly anisotropic or nodal superconducting gap even at $x=0.5$ and a nodal superconducting gap for $x\gtrsim 0.9$.

Optical spectroscopy is performed for $c$-plane homoepitaxial aluminum nitride (AlN) films. The temperature dependence of the polarization-resolved photoluminescence spectra reveals the exciton fine structure. The experimental results demonstrate that the electron-hole exchange interaction energy ($j$) in AlN is $j=6.8\text{meV}$, which is the largest value for typical III-V and II-VI compound semiconductors. We propose the effective interatomic distance as the criterion of the electron-hole exchange interaction energy, revealing a universal rule. This study should encourage potential applications of excitonic optoelectronic devices in nitride semiconductors similar to those using II-VI compound semiconductors.

Interfaces between transition-metal oxides provide an intriguing platform for modifying the ground states of single constituent materials and creating desired functionalities for applications in oxide-based electronics. Here, we demonstrate that the metallic and ferromagnetic responses of La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ (LSMO) films with thicknesses less than 4 nm, which are insulating when grown directly on SrTiO${}_3$ substrates, can be restored through interfacing with a high-temperature cuprate superconductor EuBa${}_2$Cu${}_3$O${}_{7-\delta }$ (EBCO). We carried out scanning tunneling microscopy and spectroscopy measurements on the electronic structure of LSMO/EBCO bilayers with thickness of LSMO layer varying from 2 to 7 nm. Our results suggest that the transfer of holes from EBCO to LSMO, caused by the difference in their work functions, is responsible for driving LSMO film with thickness of only five unit cells to the metallic state.

We report the electronic structure of a prototypical valence fluctuation system, YbAl${}_2$, using angle-resolved photoemission spectroscopy. The observed band dispersions and Fermi surfaces are well described in terms of band structure calculations based on local density approximation. Strong hybridization between the conduction and 4$f$ bands is identified on the basis of the periodic Anderson model. The evaluated small mass enhancement factor and the high Kondo temperature qualitatively agree with those obtained from thermodynamic measurements. Such findings suggest that the strong hybridization suppresses band renormalization and is responsible for the valence fluctuations in YbAl${}_2$.

Motivated by recent studies which show that topological phases may emerge in strongly correlated electron systems, we theoretically study the strong electron correlation effect in a three-dimensional topological insulator, where the effective Hamiltonian can be described by the Wilson fermions. We adopt a $1/r$ long-range Coulomb interaction as the interaction between the bulk electrons. Based on the U(1) lattice gauge theory, the strong coupling expansion is applied by assuming that the effective interaction is strong. It is shown that the effect of the Coulomb interaction is equivalent to the renormalization of the bare mass of the Wilson fermions and, as a result, the topological insulator phase survives in the strong coupling limit.

For some heavy-fermion compounds, it has been suggested that a Fermi-surface-changing Lifshitz transition, which can be driven, e.g., by varying an applied magnetic field, occurs inside the heavy-fermion regime. Here we discuss, based on microscopic calculations, how the location of such a transition can be influenced by carrier doping. While doping rigidly shifts the bands in a free-electron system, strong electronic correlations drastically modify the picture. We therefore systematically study the doping-induced energetic shift of band features in heavy Fermi liquids, with the surprising result that the actual shift is determined by the interplay of heavy and additional light bands crossing the Fermi level: doped carriers tend to populate heavy and light bands equally, despite the fact that the latter contribute a small density of states of excitations only. This invalidates naive estimates of the band shift based on the low-temperature specific heat of the heavy Fermi liquid. We discuss applications of our results.

We study oxygen $K$-edge x-ray absorption spectroscopy (XAS) and investigate the validity of the Zhang-Rice singlet (ZRS) picture in overdoped cuprate superconductors. Using large-scale exact diagonalization of the three-orbital Hubbard model, we observe the effect of strong correlations manifesting in a dynamical spectral weight transfer from the upper Hubbard band to the ZRS band. The quantitative agreement between theory and experiment highlights an additional spectral weight reshuffling due to core-hole interaction. Our results confirm the important correlated nature of the cuprates and elucidate the changing orbital character of the low-energy quasiparticles, but also demonstrate the continued relevance of the ZRS even in the overdoped region.

We study a model of spinless fermions with infinite nearest-neighbor repulsion on the square ladder, which has microscopic supersymmetry. It has been conjectured that in the continuum, the model is described by the superconformal minimal model with central charge $c=3/2$. Thus far, it has not been possible to confirm this conjecture due to strong finite-size corrections in numerical data. We trace the origin of these corrections to the presence of unusual marginal operators that break Lorentz invariance but preserve part of the supersymmetry. By relying mostly on entanglement entropy calculations with the density-matrix renormalization group, we are able to reduce finite-size effects significantly. This allows us to unambiguously determine the continuum theory of the model. We also study perturbations of the model and establish that the supersymmetric model is a multicritical point. Our work underlines the power of entanglement entropy as a probe of the phases of quantum many-body systems.

The longitudinal optical phonon of metallic nanotubes shifts by 23 cm${}^{-1}$ to lower energies when the nanotubes are deposited from a solution onto a substrate. The linewidth increases by 13 cm${}^{-1}$. The changes are explained in terms of shifts in the Fermi energy that influence the Kohn anomaly in the longitudinal optical phonon branch in metallic nanotubes. Using in situ electrochemical Raman measurements we show that the Fermi energy is 0.16 eV below its intrinsic value in metallic nanotubes in solution. Our results impact the application of Raman spectroscopy to distinguish between metallic and semiconducting tubes by examining the high-energy mode line shape.

We present a benchmark study for the adsorption of a large $\pi$-conjugated organic molecule on different noble metal surfaces, which is based on x-ray standing wave (XSW) measurements and density functional theory calculations with van der Waals (vdW) interactions. The bonding distances of diindenoperylene on Cu(111), Ag(111), and Au(111) surfaces (2.51, 3.01, and 3.10 Å, respectively) determined with the normal-incidence XSW technique are compared with calculations. Excellent agreement with the experimental data, i.e., deviations less than 0.1 Å, is achieved using the Perdew-Burke-Ernzerhof (PBE) functional with vdW interactions that include the collective response of substrate electrons (the $\mathrm{PBE}+{\mathrm{vdW}}^{\mathrm{surf}}$ method). It is noteworthy that the calculations show that the vdW contribution to the adsorption energy increases in the order $\text{Au}\left(111\right)<\text{Ag}\left(111\right)<\text{Cu}\left(111\right)$.

Two dynamic transitions or crossovers, one at a low temperature (T${}^{*}$ ≈ 1006 K) and the other at a high temperature (T${}^0$ ≈ 1384 K), are shown to emerge in supercooled liquid silicon using molecular dynamics simulations. The high-temperature transition (T${}^0$) marks the decoupling of stress, density, and energy relaxation mechanisms. At the low-temperature transition (T${}^{*}$), depending on the cooling rate, supercooled silicon can either undergo a high-density-liquid to low-density-liquid (HDL-LDL) phase transition or experience an HDL-HDL crossover. Dynamically heterogeneous domains that emerge with supercooling become prominent across the HDL-HDL transition at 1006 K, with well-separated mobile and immobile regions. Interestingly, across the HDL-LDL transition, the most mobile atoms form large prominent aggregates while the least mobile atoms get spatially dispersed akin to that in a crystalline state. The attendant partial return to spatial uniformity with the HDL-LDL phase transition indicates a dynamic mechanism for relieving the frustration in supercooled states.

Molecules like water have vibrational modes with a zero-point energy well above room temperature. As a consequence, classical molecular dynamics simulations of their liquids largely underestimate the energy of modes with a higher zero-point temperature, which translates into an underestimation of covalent interatomic distances due to anharmonic effects. Zero-point effects can be recovered using path integral molecular dynamics simulations, but these are computationally expensive, making their combination with ab initio molecular dynamics simulations a challenge. As an alternative to path integral methods, from a computationally simple perspective, one would envision the design of a thermostat capable of equilibrating and maintaining the different vibrational modes at their corresponding zero-point temperatures. Recently, Ceriotti et al. [Phys. Rev. Lett. 102, 020601 (2009)] introduced a framework to use a custom-tailored Langevin equation with correlated noise that can be used to include quantum fluctuations in classical molecular dynamics simulations. Here we show that it is possible to use the generalized Langevin equation with suppressed noise in combination with Nose-Hoover thermostats to efficiently impose a zero-point temperature on independent modes in liquid water. Using our simple and inexpensive method, we achieve excellent agreement for all atomic pair correlation functions compared to the path integral molecular dynamics simulation.

Recently, a family of halogen-based Li-rich antiperovskites were synthesized [J. Am. Chem. Soc. 134, 15042 (2012)], and the measured superionic conductivity makes these materials promising candidates as solid electrolytes for applications in Li-ion and Li-air batteries. This discovery raises several pressing issues on the fundamental physics concerning the thermodynamic and electrochemical stability of the synthesized materials and the mechanism of the observed superionic Li${}^{+}$ transport. Here, we study the reported antiperovskites Li${}_3$OCl, Li${}_3$OBr, and their mixed compounds using first-principles density functional theory and molecular dynamics simulations. Our calculations show that these materials are thermodynamically metastable. Their large electronic band gaps and chemical stability against electrodes suggest the excellent electrochemical performance, which bodes well for the use in potentially harsh working conditions in practical battery applications. The calculated low activation enthalpy for Li-ion migration well below the crystal melting temperature and superionic transport near the Li sublattice melting state explain the experimentally observed phenomena. Our study identifies mobile Li vacancies and anion disorder as the primary driving mechanisms for superionic Li${}^{+}$ conductivity in the antiperovskites. This work unveils essential working principles of the Li-rich antiperovskites, which are crucial to further exploration, development, and application of these and other charge-inverted materials with tailored properties.

We report on an investigation of optical properties of multiferroic CoCr${}_2$O${}_4$ at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr${}_2$O${}_4$ is dominated by a magnon mode, which shows a steep magnetic-field dependence. We ascribe this mode to an exchange resonance between two magnetic sublattices with different $g$ factors. In the framework of a simple two-sublattice model (the sublattices are formed by Co${}^{2+}$ and Cr${}^{3+}$ ions), we find the inter-sublattice coupling constant, $\lambda =-(18\pm 1)$ K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr${}^{3+}$ sublattice, ${\Theta }_2$ $=$ $(49\pm 2)$ K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr${}_2$O${}_4$ have been reported in literature.

We report detailed low-temperature magnetotransport and magnetization measurements in MnSi under pressures up to $\sim 12\mathrm{kbar}$. Tracking the role of sample quality, pressure transmitter, and field and temperature history allows us to link the emergence of a giant topological Hall resistivity $\sim 50\mathrm{n}\Omega \mathrm{cm}$ to the skyrmion lattice phase at ambient pressure. We show that the remarkably large size of the topological Hall resistivity in the zero-temperature limit must be generic. We discuss various mechanisms which can lead to the much smaller signal at elevated temperatures observed at ambient pressure.

Recent experimental data have indicated that magnetic vortices can be pinned to an impurity at the surface of a nanodisk. Unpinned vortices exhibit a gyrotropic precession frequency that is approximately proportional to the aspect ratio of the disk, $L/R$, where $L$ and $R$ are the disk thickness and radius, respectively. More recent experimental data have indicated that the gyrotropic frequency of pinned vortices has an approximate $R/L$ dependence for the thin disk, and the vortex is pinned at only one surface of the disk. Pinning of the vortex to one surface of the disk results in a significant contribution to the exchange energy, which is used along with the magnetostatic energy in the Thiele equation to calculate the frequency. This energy results in the $R/L$ dependence as seen in experimental thin disk measurements.

Since the presence of dopant at grain boundaries (GBs) has a big impact on physical properties of crystalline solids, GB atomic structure and dopant location have been clarified in detail in some particular cases. However, further studies are required to reveal the underlying structure-formation mechanisms, because there are huge numbers of possible structures due to a wide variety in GB geometry, such as crystallographic orientation relationships and GB planes. Although an ultimate goal of the GB-structure study is to understand all GBs with different geometries, it is quite unrealistic to investigate large numbers of GBs on the atomic scale. Alternatively, a realistic and important step toward this goal is to clarify a GB structure-geometry-chemistry relationship. In this Brief Report we report that change of GB geometry together with doping of praseodymium induces structural transformation in a zinc oxide GB. This demonstrates that geometrical and chemical effects cooperatively play a crucial role in determining the GB atomic-scale structure.

We express the superconducting gap $\Delta \left(T\right)$ in terms of thermodynamic functions in both $s$- and $d$-wave symmetries. Applying to Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ and Y${}_{0.8}$Ca${}_{0.2}$Ba${}_2$Cu${}_3$O${}_{7-\delta }$ we find that for all dopings $\Delta \left(T\right)$ persists, as a partial gap, high above $T_c$ due to strong superconducting fluctuations. Therefore in general two gaps are present above $T_c$, the superconducting gap and the pseudogap, effectively reconciling two highly polarized views concerning pseudogap physics.