Charge neutral bilayer graphene has a gapped ground state, as transport experiments have demonstrated. One plausible ground state is the layered antiferromagnetic spin density wave (LAF) state, where the spins in the top and bottom layers have the same magnitude with opposite directions. We propose that lightly charged bilayer graphene in an electric field perpendicular to the graphene plane may be a half metal. We show this explicitly by using a mean-field theory on a two-layer Hubbard model.

We report on measurements of nonresonant inelastic x-ray scattering (NIXS) to unravel the effective symmetry of Cu $3d$ orbitals in the ground state of CuO. A clear feature of energy loss at about 2 eV exists in the NIXS spectrum, arising from $dd$ excitations; the intensities of these excitations display a pronounced anisotropy. The comparison between the measured angular distributions of scattering and those from theoretical predictions by the tesseral harmonics indicates that, in terms of a hole picture, the lowest-energy $dd$ excitation is the orbital transition $x^2-y^2$ $\to$ $xy$. In addition, the transition $x^2-y^2$ $\to$ $3z^2-r^2$ has an energy higher than $x^2-y^2$ $\to$ $yz/zx$, in contrast to a previous interpretation. Our results imply a large Jahn-Teller-like splitting between $x^2-y^2$ and $3z^2-r^2$ orbitals. The theory assuming a $C_{4h}$ symmetry explains the angular dependence of the NIXS spectra fairly well, implying that this symmetry is a reasonable approximation. This demonstrates that NIXS can provide important information for modeling of the electronic structure of $d$ ions embedded in a complicated crystal field.

The electronic structures, lattice dynamics, and magnetic properties of crystal $\beta$-FeSe under hydrostatic pressure have been studied by using the first-principles electronic structure calculations with van der Waals corrections. With applied pressure, the energy bands around the Fermi energy level consisting mainly of Fe-$3d$ orbitals show obvious energy shifts and occupation variations, and meanwhile the frequencies of all optical phonon modes increase. Among these phonon modes, the $A_{1g}$ mode, which relates to the Se height from the Fe-Fe plane, shows a clear frequency jump in a pressure range between 5 and 6 GPa. This is also the pressure range within which the highest superconducting transition temperature $T_c$ of FeSe is reached in experiments. In comparison with other phonon modes, the atomic displacement due to the zero-point vibration of the $A_{1g}$ mode induces the strongest fluctuation of local magnetic moment on Fe under pressures from 0 to 9 GPa, and the induced fluctuation reaches a maximum around 5 GPa. These results suggest that the effect of phonon via spin-phonon coupling could not be completely omitted when exploring the superconducting mechanism in iron-based superconductors.

We present experimental results on the band-gap energies of homoepitaxial diamond films with isotopic compositions ranging from nearly pure carbon-12 (${}^{12}$C) to nearly pure carbon-13 (${}^{13}$C). Diamond crystals were grown by microwave plasma-assisted chemical vapor deposition, which controls the isotope composition and minimizes the density of impurities and defects. We find that the isotope substitution of ${}^{12}$C by ${}^{13}$C increases the band-gap energy in diamond by up to 15.4 meV at 79 K. The increase at room temperature is estimated from the temperature dependence of the band-gap renormalization due to electron-phonon interaction and is found to be even larger than that at low temperatures. These results unambiguously demonstrate the possibility of band-gap engineering of diamond via control of the isotopic composition.

Recent experiments have demonstrated that tiny amounts of Mo impurities give rise to drastic changes in the adsorption characteristic of a wide-gap CaO(001) film. In this scanning tunneling microscopy (STM) and density functional theory paper, we elucidate the underlying mechanism by analyzing the energy levels of the Mo dopants as a function of their oxidation state and depth below the surface. We show that Mo${}^{2+}$ ions in CaO subsurface layers can be reversibly charged and discharged by inducing local band-bending effects with the STM tip. A similar charge switching is not possible for Mo species in a higher oxidation state, as their highest-occupied molecular orbitals are located well below the onset of the CaO conduction band. The easiness of charge switching in Mo${}^{2+}$ ions explains the remarkable chemical properties of doped CaO films, as it renders the material a strong electron donor to adsorbates bound to the oxide surface.

We present simultaneous measurements of acoustic emission and magnetic Barkhausen noise during the thermally induced martensitic transition in a Ni-Mn-Ga single crystal. The range where structural acoustic emission avalanches are detected extends for more than 50 K for both cooling and heating ramps, with a hysteresis of $\sim$10 K. The magnetic activity occurs during the structural transition, exhibiting similar hysteresis, but concentrated in the lower half of the temperature range. Statistical analysis of individual signals allows characterization of the broad distributions of acoustic emission and Barkhausen amplitudes. By studying the times of arrival of the avalanche events we detect the existence of correlations between the two kinds of signals, with a number of acoustic emission signals occurring shortly after a Barkhausen signal. The order of magnitude of the observed delays is compatible with the time needed for the propagation of ultrasound through the sample, showing correlation of some of the signals.

We experimentally demonstrate that large amplitude magnetic vortex gyrations can be parametrically excited by the injection of radio-frequency (rf) current at twice the natural frequency of the gyrotropic vortex-core motion. The mechanism of excitation is based on the parallel pumping of vortex motion by the rf orthoradial field generated by the injected current. Theoretical analysis shows that experimental results can be interpreted as the manifestation of parametric amplification when the rf current is small, and of parametric instability when the rf current is above a certain threshold. By taking into account the energy nonlinearities, we succeed to describe the amplitude saturation of vortex oscillations as well as the coexistence of stable regimes.

Field-induced magnetic ordering in the structurally disordered quantum magnets Ni(Cl${}_{1-x}$Br${}_x$)${}_2$$·$4SC(NH${}_2$)${}_2$, $x=8$% and 13%, is studied by means of neutron diffraction. The order-parameter critical exponent is found to be very close to its value $\beta =0.5$ expected for magnetic Bose-Einstein condensation in the absence of disorder. This result applies to temperatures down to 40 mK, and a 1 T range in magnetic field. The crossover exponent is found to be $\varphi \sim 0.4$ for temperature ranges as small as 300 mK. Possible reasons for a discrepancy with recent numerical simulations and bulk measurements are discussed.

We report on the magnetic and superconducting properties of LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ by means of zero- and transverse-field (ZF/TF) muon-spin spectroscopy measurements ($\mu$SR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-$\mu$SR data give access to the superfluid density, which shows a marked two-dimensional character with a dominant $s$-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.

We study the effects of lattice deformations on the kagome spin ice, with Ising spins coupled by nearest-neighbor exchange and long-range dipolar interactions, in the presence of in-plane magnetic fields. We describe the lattice energy according to the Einstein model, where each site distortion is treated independently. Upon integration of lattice degrees of freedom, effective quadratic spin interactions arise. Classical Monte Carlo simulations are performed on the resulting model, retaining up to third neighbor interactions, under different directions of the magnetic field. We find that as the effect of the deformation is increased, a rich plateau structure appears in the magnetization curves.

We study the modification of micromagnetic configurations in nanostructures, due to the Dzyaloshinskii-Moriya interaction (DMI), that appear at the interface of an ultrathin film. We show that this interaction leads to micromagnetic boundary conditions that bend the magnetization at the edges. We explore several cases of ultrathin film nanostructures that allow analytical calculations (one-dimensional systems, domain walls, cycloids, and skyrmions), compare with fully numerical calculations, and show that a good physical understanding of this type of micromagnetics can be reached. We particularly focus on skyrmions confined in circular nanodots and show that edges allow for the isolation of single skyrmions for a large range of the DMI parameter.

The Berezinskii-Kosterlitz-Thouless (BKT) transition is a generic transition describing the loss of coherence in two-dimensional systems and has been invoked, for example, to describe the superconductor-insulator transition in thin films. However, recent experiments have shown that the BKT transition, driven by phase fluctuations, is not sufficient to describe the loss of superconducting order, and amplitude fluctuations must also be taken into account. The standard models that are extensively used to model two-dimensional superconductors are the Hubbard and $\mathit{XY}$ models. Whereas the $\mathit{XY}$ model allows only phase fluctuations, the Hubbard model has an extra degree of freedom: amplitude fluctuations. In this paper we compare two Hubbard models with the same critical temperature but with different interactions and deduce the role of the amplitude fluctuations in the superconducting transition. For this purpose, a novel approximation is presented and used. We derive an effective phase-only ($\mathit{XY}$) Hamiltonian, incorporating amplitude fluctuations in an explicit temperature dependence of the phase rigidity. We study the relation between amplitude fluctuations and coupling strength. Our results support existing claims about the suppression of phase rigidity due to amplitude fluctuations not present in the $\mathit{XY}$ model.

Chiral magnets, such as MnSi, display a rich finite temperature phase diagram in an applied magnetic field. The most unusual of the phases encountered is the so-called A Phase characterized by a triangular lattice of skyrmion tubes. Its existence cannot be captured within a mean-field treatment of a Landau-Ginzburg functional, but thermal fluctuations to Gaussian order are required to stabilize it. In this paper, we go beyond Gaussian order in a fully nonperturbative study of a three-dimensional lattice spin model using classical Monte Carlo simulations. We demonstrate that the A Phase is, indeed, stabilized by thermal fluctuations, and furthermore, we reproduce the full phase diagram found in experiments. The thermodynamic signatures of the helimagnetic transition upon cooling from the paramagnet are qualitatively consistent with experimental findings and lend further support to the Brazovskiĭ scenario, which describes a fluctuation-driven first-order transition due to the abundance of soft modes.

A recently performed resonant inelastic x-ray scattering experiment (RIXS) at the copper $L_3$ edge in the quasi-one-dimensional Mott insulator Sr${}_2$CuO${}_3$ has revealed a significant dispersion of a single-orbital excitation (orbiton). This large and unexpected orbiton dispersion has been explained using the concept of spin-orbital fractionalization in which the orbiton, which is intrinsically coupled to the spinon in this material, liberates itself from the spinon due to the strictly one-dimensional nature of its motion. Here, we investigate this mechanism in detail by (i) deriving the microscopic spin-orbital superexchange model from the charge-transfer model for the CuO${}_3$ chains in Sr${}_2$CuO${}_3$, (ii) mapping the orbiton motion in the obtained spin-orbital model into a problem of a single hole moving in an effective half-filled antiferromagnetic chain $t$-$J$ model, and (iii) solving the latter model using the exact diagonalization and obtaining the orbiton spectral function. Finally, the RIXS cross section is calculated based on the obtained orbiton spectral function and compared with the RIXS experiment.

We study the strong enhancement, induced by random hopping, of the critical temperatures characterizing the transitions to superconductivity, charge-density wave, and antiferromagnetism, which can occur in bipartite lattice models at half filling, like graphene, by means of an extended Finkel'stein nonlinear $\sigma$-model renormalization group approach. We show that if Cooper channel interaction dominates, superconducting critical temperature can be enhanced at will, since superconductivity cannot be broken by entering any Anderson insulating phase. If instead, staggered interactions are relevant, antiferromagnetic order is generated by disorder at a temperature well above that expected for a clean system.

We analyze the full counting statistics of photons emitted by a double quantum dot (DQD) coupled to a high-quality microwave resonator by electric dipole interaction. We show that at the resonant condition between the energy splitting of the DQD and the photon energy in the resonator, photon statistics exhibits both a sub-Poissonian distribution and antibunching. In the ideal case, when the system decoherence stems only from photodetection, the photon noise is reduced below one-half of the noise for the Poisson distribution and is consistent with current noise. The photon distribution remains sub-Poissonian even at moderate decoherence in the DQD. We demonstrate that Josephson-junction-based photomultipliers can be used to experimentally assess the statistics of emitted photons.

We study the fluorescence light emitted from GaAs excitons in semiconductor quantum wells. The excitons are modeled as interacting bosons. By combining quantum optical methods for the excitonic emission spectrum with many particle descriptions of the transmission through the medium, we can evaluate the spectra outside the well. Comparing with experimental spectra, we get a very good agreement. The method helps to explain the main features of the observed spectra. It is demonstrated that the observed spectra show clear evidence of superfluorescent emission.

We analyze the absorption of a graphene nanoribbon placed into an optical cavity. We demonstrate the existence of strong coupling between the Van-Hove singularities in armchair graphene nanoribbons and the modes of optical cavity, and calculate the dispersion of the resulting hybrid eigenmodes of the structure. We demonstrate that the total absorption of the structure can be enhanced dramatically being tuned via the Fano resonance.

In nanoscale junctions the interaction between charge carriers and the local vibrations results in renormalization, damping, and heating of the vibrational modes. Here we formulate a nonequilibrium Green's function based theory to describe such effects. Studying a generic junction model with an off-resonant electronic level, we find a strong bias dependence of the frequency renormalization and vibrational damping accompanied by pronounced nonlinear vibrational heating in junctions with intermediate values of the coupling to the leads. Combining our theory with ab initio calculations, we furthermore show that the bias dependence of the Raman shifts and linewidths observed experimentally in an oligo(3)-phenylenevinylene (OPV3) junction [Ward et al., Nat. Nanotechnol. 6, 33 (2011)] may be explained by a combination of dynamic carrier screening and molecular charging.