We investigate the effect of electron-electron interactions in $ABC$-stacked graphene trilayers. In the gapless regime, we show that the self-energy corrections lead to the renormalization of the dynamical exponent $z=3+{\alpha }_1/N$, with ${\alpha }_1\approx 0.52$ and $N$ is the number of fermionic species. Although the quasiparticle residue is suppressed near the neutrality point, the lifetime has a sublinear scaling with the energy and the quasiparticles are well defined even at zero energy. We calculate the renormalization of a variety of physical observables, which can be directly measured in experiments.

The Kibble-Zurek (KZ) mechanism has been applied to a variety of systems ranging from low-temperature Bose-Einstein condensations to grand unification scales in particle physics and cosmology and from classical phase transitions to quantum phase transitions. Here, we show that finite-time scaling (FTS) provides a detailed improved understanding of the mechanism. In particular, the finite time scale, which is introduced by the external driving (or quenching) and results in FTS, is the origin of the division of the adiabatic regimes from the impulse regime in the KZ mechanism. The origin of the KZ scaling for the defect density, generated during the driving through a critical point, is not that the correlation length ceases growing in the nonadiabatic impulse regime, but rather, is that it is taken over by the effective finite length scale corresponding to the finite time scale. We also show that FTS accounts well for and improves the scaling ansatz proposed recently by Liu, Polkovnikov, and Sandvik, [Phys. Rev. B 89, 054307 (2014)]. Further, we show that their universal power-law scaling form applies only to some observables in cooling but not to heating. Even in cooling, it is invalid either when an appropriate external field is present. However, this finite-time-finite-size scaling calls for caution in application of FTS. Detailed scaling behaviors of the FTS and finite-size scaling, along with their crossover, are explicitly demonstrated, with the dynamic critical exponent $z$ being estimated for two- and three-dimensional Ising models under the usual Metropolis dynamics. These values of $z$ are found to give rise to better data collapses than the extant values do in most cases but take on different values in heating and cooling in both two- and three-dimensional spaces.

We investigate the Seebeck effect in $R\mathrm{FeAsO}$ ($R=\mathrm{rare}$ earth) compounds as a function of temperature and magnetic field up to 30 T. The Seebeck curves are characterized by a broad negative bump around 50 K, which is sample dependent and strongly enhanced by the application of a magnetic field. A model for the temperature and field dependence of the magnon drag contribution to the Seebeck effect by antiferromagnetic (AFM) spin fluctuation is developed. It accounts for the magnitude and scaling properties of such bump feature in our experimental data in LaFeAsO. This analysis accounts for the apparent inconsistency of literature Seebeck effect data on these compounds and has the potential to extract precious information on the coupling between electrons and AFM spin fluctuations in these parent compound systems, with implications on the pairing mechanism of the related superconducting compounds.

We perform a systematic study on the evolution of the magnetic spin moment ($m_s$) of epitaxial [100]- and [111]-magnetite films of increasing thickness. The ultrathin films are characterized by low-energy electron diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). By employing sum rules on the XMCD spectra we obtain $m_s=3.6$ ${\mu }_B$/f.u. for samples of around 35 Å. This is considered a bulk value and has been reported only for films more than 10 times thicker. Moreover, we show that even 10-Å-thick magnetite already presents a significant magnetic moment. For both grown directions the moment increases similarly with the thickness. The ferromagnetic behavior for each iron ion site (${\mathrm{Fe}}_{\mathrm{octa}}^{2+}$, ${\mathrm{Fe}}_{\mathrm{octa}}^{3+}$, ${\mathrm{Fe}}_{\mathrm{tetra}}^{3+}$) of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ is measured by monitoring XMCD peaks. The deduced hysteresis curves (per ion, per site) exhibit a coercive field of 300 Oe. Our results show that both the ferrimagnetic order and the bulk moment value are preserved at room temperature around the thickness of 2 unit cells.

Superconductivity with $T_c=53.5$ K has been induced in a large ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_6$ (La-2126) single crystal by annealing in a high partial pressure of oxygen at 1200 °C. Using transmission electron microscopy techniques, we show that a secondary Ca-doped ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (La-214) phase, not present in the as-grown crystal, appears as a coherent intergrowthlike defect as a consequence of the annealing. A corresponding secondary superconducting transition near 13 K is evident in the magnetization measurement. Electron energy-loss spectroscopy reveals a pre-edge peak at the O-$K$ edge in the superconducting La-2126 phase, which is absent in the as-grown crystal, confirming the hole doping by interstitial oxygen.

Sound propagation has been investigated at megahertz and gigahertz frequencies in ${\mathrm{B}}_2{\mathrm{O}}_3$ solids which have been pressure quenched to go through different glassy phases having growing density until the crystalline state. Over the temperature range between 10 and 300 K, the attenuation and velocity of sound in glasses are regulated by classical activation over potential barriers of structural defects and vibrational anharmonicity. The defect states are absent in the crystalline phase whose acoustic behavior is determined only by the anharmonic coupling between sound waves and thermal vibrations. Our study leads to identification of the specific superstructural units subjected to thermally activated motions in vitreous ${\mathrm{B}}_2{\mathrm{O}}_3$. These molecular groups are the boroxol rings which are formed due to the poor atomic packing of glassy networks and are missing in the crystal.

The magnetic penetration depth $\left(\lambda \right)$ as a function of applied magnetic field and temperature in ${\mathrm{SrPt}}_3\mathrm{P}$ ($T_c\simeq 8.4$ K) was studied by means of muon-spin rotation $\left(\mu \mathrm{SR}\right)$. The dependence of ${\lambda }^{-2}$ on temperature suggests the existence of a single $s$-wave energy gap with the zero-temperature value ${\Delta }_0=1.55\left(2\right)$ meV. At the same time $\lambda$ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multiband systems. The multiband nature of the superconducting state is further suggested by the upward curvature of the upper critical field. This apparent contradiction is resolved by ${\mathrm{SrPt}}_3\mathrm{P}$ being a particular two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets.

The presence of superclusters based on Cu-centered icosahedra was studied via classical molecular dynamics simulations in ${\mathrm{Cu}}_{64}{\mathrm{Zr}}_{36}$ metallic glass. Medium-range order was identified by determining the nearest-neighbor histogram and bond-angle distribution of superclusters. A heterogeneous distribution of icosahedra was observed while other common cluster types are distributed homogeneously. The degree of superclustering, as quantified by the number of cap-sharing bonds per icosahedron, was found to depend only on the icosahedron fraction, irrespective of thermal history. This is further supported by a long-time annealing in the supercooled liquid regime, where the number of icosahedra significantly increases and the formation of superclusters is consequently enhanced. While distorted icosahedra are not found in the vicinity of full icosahedra, the Zr-centered ⟨0 0 12 4⟩ clusters show a high spatial correlation to Cu-centered icosahedra. This is indicative of early stages of crystallization into a ${\mathrm{Cu}}_2\mathrm{Zr}$ Laves phase.

The specific heat and thermodynamics of ${\mathrm{Fe}}_2\mathrm{P}$ single crystals around the first-order paramagnetic to ferromagnetic (FM) phase transition at $T_{\mathrm{C}}\simeq 217\mathrm{K}$ are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived $H\text{−}T$ phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the $c$ axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields the phase transition is split into two sections, with an intermediate FM phase where there is no spontaneous magnetization along the $c$ axis. The zero-field transition displays a textbook example of a first-order transition with different phase stability limits on heating and cooling. The results have special significance since ${\mathrm{Fe}}_2\mathrm{P}$ is the parent material to a family of compounds with outstanding magnetocaloric properties.

Films of Fe-doped ${\mathrm{In}}_2{\mathrm{O}}_3$ that were deliberately fabricated so they contained ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles were deposited on sapphire substrates by pulsed laser deposition at low oxygen pressure. The concentration of Fe was varied between 1% and 5%, and the effect of including 5% of Sn and vacuum annealing were also investigated. Structural analysis indicated a high concentration of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles that caused substantial values of the coercive field at room temperature. Transport measurements indicated that the films were metallic, and an anomalous Hall effect was observed for the sample with 5% of Fe. The concentration of nanoparticles was reduced dramatically by the inclusion of 5% of Sn. Magnetic circular dichroism spectra taken in field and at remanence were analyzed to show that the samples had a magnetically polarized defect band located below the conduction band as well as magnetic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles. The signal from the defect states near the band edge was enhanced by increasing the number of carriers by either including Sn or by annealing in vacuum.

An SDW antiferromagnetic (SDW-AF) low-temperature phase transition is generally observed and the AF spin fluctuations are considered to play an important role for the superconductivity pairing mechanism in FeAs superconductors. However, a similar magnetic phase transition is not observed in FeSe superconductors, which has caused considerable discussion. We report on the intrinsic electronic states of FeSe as elucidated by electric transport measurements under magnetic fields using a high quality single crystal. A mobility spectrum analysis, an ab initio method that does not make assumptions on the transport parameters in a multicarrier system, provides very important and clear evidence that another hidden order, most likely the symmetry broken from the tetragonal C4 symmetry to the C2 symmetry nematicity associated with the selective $d$-orbital splitting, exists in the case of superconducting FeSe other than the AF magnetic order spin fluctuations. The intrinsic low-temperature phase in FeSe is in the almost compensated semimetallic states but is additionally accompanied by Dirac cone-like ultrafast electrons $\sim {10}^4{\mathrm{cm}}^2{\left(\mathrm{VS}\right)}^{-1}$ as minority carriers.

We study a weak interaction quench in a three-dimensional Fermi gas. We first show that, under some general assumptions on time-dependent perturbation theory, the perturbative expansion of the long-wavelength structure factor $S\left(\mathbf{q}\right)$ is not compatible with the hypothesis that steady-state averages correspond to thermal ones. In particular, $S\left(\mathbf{q}\right)$ does develop an analytical component $\sim \mathrm{const}+O\left(q^2\right)$ at $\mathbf{q}\to \mathbf{0}$, as implied by thermalization, but, in contrast, it maintains a nonanalytic part $\sim \left|\mathbf{q}\right|$ characteristic of a Fermi liquid at zero-temperature. In real space, this nonanalyticity corresponds to persisting power-law decaying density-density correlations, whereas thermalization would predict only an exponential decay. We next consider the case of a dilute gas, where one can obtain nonperturbative results in the interaction strength but at lowest order in the density. We find that in the steady state the momentum distribution jump at the Fermi surface remains finite, though smaller than in equilibrium, up to second order in $k_F{f}_0$, where $f_0$ is the scattering length of two particles in the vacuum. Both results question the emergence of a finite length scale in the quench dynamics as expected by thermalization.

Modeling layered intercalation compounds from first principles poses a problem, as many of their properties are determined by a subtle balance between van der Waals interactions and chemical or Madelung terms, and a good description of van der Waals interactions is often lacking. Using van der Waals density functionals we study the structures, phonons and energetics of the archetype layered intercalation compound Li-graphite. Intercalation of Li in graphite leads to stable systems with calculated intercalation energies of $-0.2$ to $-0.3$ eV/Li atom, (referred to bulk graphite and Li metal). The fully loaded stage 1 and stage 2 compounds ${\mathrm{LiC}}_6$ and ${\mathrm{Li}}_{1/2}{\mathrm{C}}_6$ are stable, corresponding to two-dimensional $\sqrt{3}\times \sqrt{3}$ lattices of Li atoms intercalated between two graphene planes. Stage $N>2$ structures are unstable compared to dilute stage 2 compounds with the same concentration. At elevated temperatures dilute stage 2 compounds easily become disordered, but the structure of ${\mathrm{Li}}_{3/16}{\mathrm{C}}_6$ is relatively stable, corresponding to a $\sqrt{7}\times \sqrt{7}$ in-plane packing of Li atoms. First-principles calculations, along with a Bethe-Peierls model of finite temperature effects, allow for a microscopic description of the observed voltage profiles.

We studied multiexciton dynamics in monolayer ${\mathrm{WSe}}_2$ using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density and long-distance exciton diffusion, reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation (EEA) model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient EEA process determined the unusual multiexciton dynamics in atomically thin layered transition metal dichalcogenides.

We report on unprecedentedly large-scale density-functional calculations that clarify atomic and electronic structures of twisted bilayer graphene (BLG). We find the existence of the critical twist angle from either the AB or the AA stacking BLG, above which the two graphene layers are essentially decoupled and below which the atomic planes are corrugated and the Dirac electrons are localized. We also find a magic angle at which the Fermi velocity of the Dirac electron vanishes. We clarify that the Moiré pattern in tBLG with a tiny twist angle generates inhomogeneity for the electron systems and thus causes the drastic modification of the electronic properties, leading to flat bands at the Fermi level. Sensitivity to the Moiré of the valence-electron density and the electron state near the Fermi level is discussed.

A new principle of manipulation of an electron phase is proposed. The phase is added not directly to an electron but to a finite-length portion of the Fermi sea associated with it, which I name a phase carrier. This phase can be read out with the help of an imbalanced interferometer, where the phase carrier interferes with the reference Fermi sea. As a result of such interference, the same value but opposite sign charge appears at the interferometer's outputs. A phase carrier can be created, for instance, with the help of an on-demand single-electron source able to produce excitations with multiple-peak density profiles.

Spin-orbit coupling is increasingly seen as a rich source of novel phenomena, as shown by the recent excitement around topological insulators and Rashba effects. We here show that the addition of ferroelectric degrees of freedom to a semiconductor featuring topologically nontrivial properties, such as SnTe, merges the intriguing field of spin-orbit-driven physics with nonvolatile functionalities appealing for spintronics. By using a variety of modeling techniques, we show that a strikingly rich sequence of phases can be induced in SnTe, when going from a room-temperature cubic phase to a low-temperature ferroelectric structure, ranging from a topological crystalline insulator to a time-reversal-invariant $Z_2$ topological insulator to a “ferroelectric Rashba semiconductor,” exhibiting a huge electrically controllable Rashba effect in the bulk band structure.