We present an application of time-dependent density-functional theory (TDDFT) to the study of the optical properties of the $\text{Si}\left(111\right)2\times 1$ surface from the infrared to the ultraviolet. We have carried out ab initio calculations using different methods, from DFT to Bethe-Salpeter equation (BSE) and, within TDDFT, we have tested the ability of different kernels to describe the optical features in a wide range of energies. We find good agreement between TDDFT and BSE results, by using in TDDFT a long-range frequency-dependent exchange-correlation kernel derived from the many-body formalism (the MB kernel). The agreement between theory and experiment is very good in the whole frequency range studied. Excitonic effects, important in the infrared part of the spectrum, are less pronounced in the visible and UV ranges.

Exchange bias properties have been investigated in a $\left({\text{Sm}}_{1-x}{\text{Gd}}_x{\text{Al}}_2/{\text{SmAl}}_2\right)$ bilayer (for $x=0.028$) with perpendicular magnetization, an original exchange-coupled system based on the zero magnetization ferromagnet $\left({\text{Sm}}_{1-x}{\text{Gd}}_x{\text{Al}}_2\right)$ as the pinning layer. This unusual magnet exhibits a magnetic compensation temperature where it presents both a zero magnetization and a long-range spin ferromagnetic order. In the $\left({\text{Sm}}_{1-0.028}{\text{Gd}}_{0.028}{\text{Al}}_2/{\text{SmAl}}_2\right)$ bilayer, a large positive exchange bias has been observed for the ${\text{SmAl}}_2$ magnetization reversal, attesting for the exchange coupling between both compounds, even at the ${\text{Sm}}_{1-0.028}{\text{Gd}}_{0.028}{\text{Al}}_2$ magnetic compensation temperature; the magnetization of the ${\text{Sm}}_{1-0.028}{\text{Gd}}_{0.028}{\text{Al}}_2$ part of the bilayer appears to be pinned but the pinned component is obviously smaller than the one observed in a single uncovered ${\text{Sm}}_{1-0.028}{\text{Gd}}_{0.028}{\text{Al}}_2$. X-ray Magnetic Circular Dichroïsm experiments have been undertaken to investigate the magnetization reversal in both layers independently and more particularly to probe the ${\text{Sm}}_{1-0.028}{\text{Gd}}_{0.028}{\text{Al}}_2$ behavior in its zero-magnetization state. They reveal that approximately 20% of the ${\text{Sm}}_{1-0.028}{\text{Gd}}_{0.028}{\text{Al}}_2$ layer is driven to reverse by exchange coupling to ${\text{SmAl}}_2$. The presence of lateral domains in the ${\text{Sm}}_{1-0.028}{\text{Gd}}_{0.028}{\text{Al}}_2$ layer likely accounts for the relatively large proportion of rotatable moments and for the observed temperature dependence of exchange-bias field.

Very large anisotropies in transport quantities have been observed in the presence of very small in-plane structural anisotropy in many strongly correlated electron materials. By studying the two-dimensional Hubbard model with dynamical-mean-field theory for clusters, we show that such large anisotropies can be induced without static stripe order if the interaction is large enough to yield a Mott transition. Anisotropy decreases at large frequency. The maximum effect on conductivity anisotropy occurs in the underdoped regime, as observed in high-temperature superconductors.

We performed in situ time-resolved small-angle neutron scattering (SANS) measurements on high-purity Fe-Cu and Fe-Cu-B-N alloys during isothermal aging at $550°\text{C}$ in order to study the potential self-healing of deformation-induced defects by nanosized Cu precipitation. Three different samples with 0%, 8%, and 24% prestrain were used to study the influence of variable defect levels on the Cu precipitation kinetics. The time-resolved SANS measurements show the presence of two contributions corresponding to spherical precipitates and precipitation at dislocations and/or interfaces, as confirmed by complementary transmission electron microscopy experiments. For the Fe-Cu alloy, prestrain was found to accelerate the formation of spherical precipitates in initial aging stage and lead to a significant enhancement of copper precipitation at dislocations and/or interfaces. For the Fe-Cu-B-N alloy, the addition of boron and nitrogen accelerates the formation of spherical precipitates but suppresses the precipitation along dislocations in the prestrained samples.

We investigate the details of pattern formation and transitions between different modulated phases in ultrathin Fe films on Cu(001). At high temperature, the transitions between the uniform saturated state, the bubble state, and the striped state are completely reversible while at low temperature the bubble phase is avoided. The observed non-equilibrium behavior can be qualitatively explained by considering the intrinsic energy barriers appearing in the system due to the competition between the short-ranged exchange and the long-ranged dipolar interactions. Our experiments suggest that the height of these energy barriers is related to the domain size and is therefore strongly temperature dependent.

Resonant x-ray emission spectroscopy and x-ray absorption near-edge structure experiments have been performed on Am metal at the $L_3$ edge as a function of pressure. The hypothesis that the Am valence change at high pressure is associated with a mixing of the $5f^6$ and $5f^7$ configurations, hybridized with the $6d$ valence band, is not substantiated by the experiments. Neither the measured resonant x-ray emission spectroscopy nor x-ray absorption near-edge structure exhibit additional features expected for mixed valence. Only a small shift $\sim +2\text{eV}$ of the $L_3$ edge energy position and a decrease in white line intensity at high pressure is observed. The experimental results at higher pressure may be reproduced by increasing the $6d$ bandwidth and occupation and increasing the $5f$ bandwidth without any change in occupation. Further progress should be directed toward experiments at the Am $M$ edges to observe directly the $5f$ states.

Mapping the magnetic exchange interactions from model Hamiltonian to density-functional theory is a crucial step in multiscale modeling calculations. Considering the usual magnetic force theorem but with arbitrary rotational angles of the spin moments, a spurious anisotropy in the standard mapping procedure is shown to occur provided by bilinear like contributions of high-order spin interactions. The evaluation of this anisotropy gives a hint on the strength of nonbilinear terms characterizing the system under investigation.

We synthesized crystalline bulk samples of lanthanum cuprate in the metastable $T^{\prime }$ phase using cesium hydroxide flux. Its crystal structure was determined as space group $I4/mmm$, no. 139, $a=401.02\text{pm}$, $c=1252.66\text{pm}$. Muon spin rotation reveals a gradual slowing down of magnetic fluctuations below $T_{\text{N}1}\approx 200\text{K}$, and static magnetic order below $T_{\text{N}2}=115\text{K}$, in sharp contrast to ${\text{La}}_2{\text{CuO}}_4$ in the $T$ structure where $T_{\text{N}1}\approx T_{\text{N}2}\approx 300\text{K}$. Our result shows that the strikingly different magnetic behavior of the two parent compounds has its origin in the two crystal structure modifications. In addition, we find that $T^{\prime }{\text{-La}}_2{\text{CuO}}_4$ has strongly reduced magnetic interactions compared to the other $T^{\prime }$ materials ${\text{Nd}}_2{\text{CuO}}_4$ and ${\text{Pr}}_2{\text{CuO}}_4$, where ${\text{Nd}}^{3+}$ and ${\text{Pr}}^{3+}$ are magnetic ions in contrast to the nonmagnetic ${\text{La}}^{3+}$.

We have studied by means of high-pressure x-ray diffraction the structural stability of ${\text{Ni}}_2{\text{Mo}}_3\text{N}$, ${\text{Co}}_3{\text{Mo}}_3\text{N}$, and ${\text{Fe}}_3{\text{Mo}}_3\text{N}$. We also report ab initio computing modeling of the high-pressure properties of these compounds, ${\text{Pd}}_2{\text{Mo}}_3\text{N}$ and ${\text{Pt}}_2{\text{Mo}}_3\text{N}$. We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the investigated nitrides. We found that they should have a Vickers hardness similar to that of cubic spinel nitrides such as $\gamma {\text{-Si}}_3{\text{N}}_4$.

Excitonic effects of metallic single-walled carbon nanotubes are measured with temperature-dependent resonant Raman spectroscopy. By changing the temperature in the range of 300–870 K, we observe variations in the optical transition energy $E_{ii}$ as well as in the maximum Raman intensity. We find both dependences to be different for semiconducting and metallic single-walled carbon nanotubes. We suggest an interpretation in terms of excitons dissociated into free electron-hole pairs at temperatures related to the exciton binding energy. We furthermore discuss how the oscillator strength is influenced by temperature.

The dynamics of fullerene-cubane $\left({\text{C}}_{60}\cdot {\text{C}}_8{\text{H}}_8\right)$ cocrystal is studied combining experimental [x-ray diffuse scattering, quasielastic and inelastic neutron scattering (INS)] and simulation (molecular dynamics) investigations. Neutron scattering gives direct evidence of the free rotation of fullerenes and of the libration of cubanes in the high-temperature phase, validating the “rotor-stator” description of this molecular system. X-ray diffuse scattering shows that orientational disorder survives the order/disorder transition in the low-temperature phase, although the loss of fullerene isotropic rotational diffusion is featured by the appearance of a 2.2 meV mode in the INS spectra. The coupling between INS and simulations allows identifying a degeneracy lift of the cubane librations in the low temperature phase, which is used as a tool for probing the environment of cubane in this phase and for getting further insights into the phase transition mechanism.

The detail of electronic structures near the Fermi level in ${\text{URu}}_2{\text{Si}}_2$ has been investigated employing state-of-art laser angle-resolved photoemission spectroscopy. The observation of a narrow dispersive band near the Fermi level in the ordered state as well as its absence in a Rh-substituted sample strongly suggest that the emergence of the narrow band is a clear signature of the hidden-order transition. The temperature dependence of the narrow band, which appears at the onset of the hidden-order transition, invokes the occurrence of periodicity modification in the ordered state, which is shown for the first time by any spectroscopic probe. We compare our data to other previous studies and discuss possible implications.

The high-frequency dynamics of spin-polarized carriers and photons in a spin laser have been studied. The transient response of the device obtained from the rate equations is characterized by two sets of relaxation oscillations in the carrier and photon distributions corresponding to the two polarization modes. Consequently two distinct resonant peaks are observed in the small-signal modulation response. The calculated transient characteristics indicate that the best results are obtained from a spin laser when only the favored polarization mode, with lower threshold, is operational. Under this condition the small-signal modulation bandwidth is higher than that in a conventional laser, the threshold current is lower and the output polarization can be 100% with appropriate bias conditions, independent of the spin polarization of carriers in the active region. Measurements were made at 230 K on a InAs/GaAs quantum dot spin vertical cavity surface emitting laser. A time-averaged output polarization of 55% is measured with an active region spin polarization of $5–6\mathrm{\%}$. The experimental results are in good agreement with calculated data.

We present neutron inelastic-scattering results on the low-temperature magnetic state of the frustrated-hyperkagome compound ${\text{Gd}}_3{\text{Ga}}_5{\text{O}}_{12}$ gadolinium gallium garnet (GGG). Our neutron-scattering studies reveal a remarkable range of time scales. Short-range spatial correlations appear static within the instrumental resolution $\left(50\mu \text{eV}\right)$. Three distinct inelastic modes are found at 0.04(1), 0.12(2), and 0.58(3) meV at 0.06 K. The lowest and highest energy excitations show spatial dependencies indicative of dimerized short-range antiferromagnetic correlations that survive to high temperatures, comparable to the nearest-neighbor exchange interactions. Our results suggest that the ground state of a three-dimensional hyperkagome compound differs distinctly from its frustrated counterparts on a pyrochlore lattice and reveal a juxtaposition of co-operative paramagnetism and strong-dimerized coupling. These results are surprising since GGG is often classified as a strongly frustrated system with a manifold of connected states for which one would expect a continuum of gapless excitations.

The interest in low-$Z$ superconducting analogs to ${\text{MgB}}_2$ has motivated a search for new high-pressure phases in metal boride systems. Phases of LiB have electronic features intermediate between those of ${\text{MgB}}_2$ and intercalated graphites, and are expected to exhibit superconductivity. A previously unknown series of layered metal-sandwich structures that are suggested to form under pressure has been identified by ab initio data-mining techniques. We report on an experimental effort to search for structural changes in LiB related to the predicted novel behavior. We find that LiB, while becoming increasingly disordered, maintains its ambient pressure structure up to at least 70 GPa. At 5 GPa, however, we observe an abrupt change in the evolution of $c/a$ as a function of pressure, which is likely related to pressure-induced filling of Li-B bonding states.

The geometries, formation energies, and diffusion barriers of surface and subsurface intrinsic defects in graphite are calculated using spin-polarized density-functional theory and the generalized gradient approximation with a semiempirical van der Waals (vdW) correction for dispersion interactions. The calculated formation energies and diffusion barriers of subsurface interstitial (I) atoms deviate qualitatively and quantitatively from those of surface adatoms. The same trend is found also for subsurface and adatom clusters $\left({\text{I}}_2,{\text{I}}_3\right)$. In spite of the semiquantitative agreement on the optimized geometries, the formation energies and diffusion barriers of surface and subsurface vacancies (V), divacancies (VV), and intimate (I-V) Frenkel pairs differ significantly from the values for the analogous defects in the bulk of graphite. This suggests limited transferability of the bulk and subsurface defect models to the surface of graphite. These findings are rationalized in terms of the balance between the covalent and vdW interaction terms at the surface, subsurface, and bulk of graphite. Finally, pairing of individual defects (adatoms, I and V) is calculated to be energetically advantageous both on the surface and in the subsurface regions. This process is shown to either saturate residual dangling bonds or produce singlet spin states, thus contributing to the quenching of residual spin polarization from damaged graphite surfaces.

We show that the quasi-one-dimensional organic ferromagnet 2-benzimidazolyl nitronyl nitroxide (2-BIMNN) undergoes a phase transition to long-range magnetic order below $T_{\text{C}}=1.0\left(1\right)\text{K}$. The intrachain exchange interactions are $J/k_{\text{B}}=22\text{K}$ and so the low-temperature ordering demonstrates the weakness of the interchain interactions. Our electron-spin-resonance measurements provide further evidence for the one-dimensional character of the magnetic fluctuations in 2-BIMNN and, using muon-spin rotation, we are able to characterize the broad phase transition in the material. We discuss these results in the context of other spin-$\frac{1}{2}$ quasi-one-dimensional Heisenberg ferromagnets and demonstrate that 2-BIMNN is the best realization of a one-dimensional Heisenberg ferromagnet among purely organic compounds yet discovered.

Equation of state of ${}^4\text{H}\text{e}$ hcp crystals with vacancies is determined at zero temperature using the diffusion Monte Carlo technique, an exact ground-state zero-temperature method. This allows us to extract the formation enthalpy and isobaric formation energy of a single vacancy in otherwise perfect helium solid. Results are obtained for pressures up to 160 bar. The isobaric formation energy is found to reach a minimum near 57 bar where it is equal to $10.5\pm 1.2\text{K}$. At the same pressure, the vacancy formation volume exhibits a maximum and reaches the volume of the unit cell. This pressure coincides with the pressure interval over which a peak in the supersolid fraction of ${}^4\text{H}\text{e}$ was observed in a recent experiment.

The influence of lithium (Li) exposures on monolayer graphene grown on the silicon-terminated SiC(0001) surface is investigated using low-energy electron microscopy, photoelectron spectroscopy, and micro-low-energy electron diffraction. After Li deposition, islands or Li droplets are observed on the surface, and are found to coalesce together with time. Formation of a dipole layer at the interface, interpreted to originate from Li-Si bonding, is observed directly after Li deposition, and manifested by a 2 eV shift of the $\text{C}1s$ and $\text{Si}2p$ bulk SiC peaks. This indicates that Li atoms penetrate through the graphene and carbon buffer layer directly after deposition at room temperature since three $\pi$ bands are then moreover observed at the K point, instead of the single $\pi$ band for monolayer graphene. The existence of three $\pi$ bands is interpreted as a mixture of bilayer and monolayer graphene plus a difference in doping levels due to an uneven distribution of Li atoms. Li gives rise to electron doping of the graphene and results in a lowering of the Dirac point. After annealing to a few hundred degrees Celsius, a more even Li distribution and intercalation is obtained since then two distinct $\pi$ bands appear at the K point.

We report a comprehensive electronic structure investigation of the paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden order (HO) phases of ${\text{URu}}_2{\text{Si}}_2$. We have performed relativistic full-potential calculations on the basis of the density-functional theory, employing different exchange-correlation functionals to treat electron correlations within the open $5f$ shell of uranium. Specifically, we investigate—through a comparison between calculated and low-temperature experimental properties—whether the $5f$ electrons are localized or delocalized in ${\text{URu}}_2{\text{Si}}_2$. The local spin-density approximation (LSDA) and generalized gradient approximation (GGA) are adopted to explore itinerant $5f$ behavior, the GGA plus additional strong Coulomb interaction ($\text{GGA}+U$ approach) is used to approximate moderately localized $5f$ states, and the $5f$-core approximation is applied to probe potential properties of completely localized uranium $5f$ states. We also performed local-density approximation plus dynamical mean-field theory calculations (DMFT) to investigate the temperature evolution of the quasiparticle states at 100 K and above, unveiling a progressive opening of a quasiparticle gap at the chemical potential when temperature is reduced. A detailed comparison of calculated properties with known experimental data demonstrates that the LSDA and GGA approaches, in which the uranium $5f$ electrons are treated as itinerant, provide an excellent explanation of the available low-temperature experimental data of the PM and LMAF phases. We show furthermore that due to a material-specific Fermi-surface instability a large, but partial, Fermi-surface gapping of up to 750 K occurs upon antiferromagnetic symmetry breaking. The occurrence of the HO phase is explained through dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic spin fluctuations. This dynamical symmetry breaking model explains why the Fermi-surface gapping in the HO phase is similar but smaller than that in the LMAF phase and it also explains why the HO and LMAF phases have the same Fermi surfaces yet different order parameters. A suitable order parameter for the HO is proposed to be the Fermi-surface gap, and the dynamic spin-spin correlation function is further suggested as a secondary order parameter.