The effects of the temperature on the structure of a single layer of graphene on Ir(111) have been investigated in situ in the growth chamber by grazing incidence x-ray diffraction between 10 and 1300 K. In addition, the effect of two growth temperatures has been studied. The graphene lattice parameter of the main, nonrotated phase displays a characteristic hysteresis whose lower and upper branches correspond to phases in which integer numbers ($m$,$n$)${}_{\mathrm{Ir}}$ of the Ir lattice parameters match integer numbers ($p$,$q$)${}_{\mathrm{Gr}}$ of the graphene lattice parameters, i.e., commensurate phases, (19,0)${}_{\mathrm{Ir}}$×(21,0)${}_{\mathrm{Gr}}$ and (9,0)${}_{\mathrm{Ir}}$×(10,0)${}_{\mathrm{Gr}}$, respectively. With a higher growth temperature, graphene presents, in addition to a main nonrotated, incommensurate phase, domains with a small rotation (2.37°) relative to the substrate, corresponding to a (7,2)${}_{\mathrm{Ir}}$×(8,2)${}_{\mathrm{Gr}}$ commensurate phase. Despite the weak interaction between graphene and iridium, the thermal expansion coefficient of graphene is positive at all temperatures, even at liquid helium temperature contrary to free-standing graphene. It is also close to the iridium thermal expansion because of the tendency to form commensurate phases, which creates a strong link between the graphene and substrate lattices and decreases the elastic energy.

We explore the derivation of interatomic exchange interactions in ferromagnets within density-functional theory (DFT) and the mapping of DFT results onto a spin Hamiltonian. We delve into the problem of systems comprising atoms with strong spontaneous moments together with atoms with weak induced moments. All moments are considered as degrees of freedom, with the strong moments thermally fluctuating only in angle and the weak moments thermally fluctuating in angle and magnitude. We argue that a quadratic dependence of the energy on the weak local moments magnitude, which is a good approximation in many cases, allows for an elimination of the weak-moment degrees of freedom from the thermodynamic expressions in favor of a renormalization of the Heisenberg interactions among the strong moments. We show that the renormalization is valid at all temperatures accounting for the thermal fluctuations and resulting in temperature-independent renormalized interactions. These are shown to be the ones directly derived from total-energy DFT calculations by constraining the strong-moment directions, as is done, e.g., in spin-spiral methods. We furthermore prove that within this framework the thermodynamics of the weak-moment subsystem, and in particular all correlation functions, can be derived as polynomials of the correlation functions of the strong-moment subsystem with coefficients that depend on the spin susceptibility and that can be calculated within DFT. These conclusions are rigorous under certain physical assumptions on the measure in the magnetic phase space. We implement the scheme in the full-potential linearized augmented plane wave method using the concept of spin-spiral states, considering applicable symmetry relations and the use of the magnetic force theorem. Our analytical results are corroborated by numerical calculations employing DFT and a Monte Carlo method.

We reformulate the full quantum dynamics of spin systems using a phase-space representation based on SU(2) coherent states which generates an exact mapping of the dynamics of any spin system onto a set of stochastic differential equations. This representation is superior in practice to an earlier phase-space approach based on Schwinger bosons, with the numerical effort scaling only linearly with system size. By also implementing extrapolation techniques from quasiclassical equations to the full quantum limit, we are able to extend useful simulation times severalfold. This approach is applicable in any dimension including cases where frustration is present in the spin system. The method is demonstrated by simulating quenches in the transverse-field Ising model in one and two dimensions.

Single crystalline Sn${}_2$Co${}_3$S${}_2$ with the shandite-type structure was investigated by magnetization, magnetoresistance, Hall effect, and heat capacity measurements and by ${}^{119}$Sn Mößbauer spectroscopy. Sn${}_2$Co${}_3$S${}_2$ orders ferromagnetically at 172 K with an easy-axis magnetization of $\approx$1 ${\mu }_B$ along the hexagonal $c$ axis. The half-metallic ferromagnetic state is investigated by detailed band-structure calculations by density functional theory (DFT) methods. The magnetoresistance and the Hall effect as well as the DFT results show that ferromagnetic Sn${}_2$Co${}_3$S${}_2$ is a compensated metal. The ${}^{119}$Sn Mößbauer spectroscopic data confirm these findings. Large transferred hyperfine fields $B_{\mathrm{hf}}$ up to 34.2 T are observed.

The structural and magnetic properties of BaMn${}_2$Si${}_2$O${}_7$ have been investigated. The magnetic susceptibility and specific heat, measured using single crystals, suggest that the quasi-one-dimensional magnetism originating from the loosely coupled Mn${}^{2+}$ chain carrying $S=5/2$ is present at high temperatures, which is similar to the other quasi-one-dimensional barium silicates, Ba$M$${}_2$Si${}_2$O${}_7$ ($M$: Cu and Co). The Néel temperature ($T_{\mathrm{N}}\sim 26$ K) is high compared to the magnetic interaction along the chain ($J=-6$ K). Neutron powder diffraction study has revealed that the magnetic structure is long ranged with antiferromagnetic arrangement along the chain ($c$) direction and ferromagnetic arrangement along the $a$ and $b$ axes. Detailed structural analysis suggests that the interchain interaction via Mn-O-Mn bond along the $a$ axis is relatively large, which makes the system behave more two-dimensionally in the $ac$ plane and enhances $T_{\mathrm{N}}$.

We study a quantum version of the three-state Potts model that includes as special cases the effective models of bosons and fermions on the square lattice in the Mott-insulating limit. It can be viewed as a model of quantum permutations with amplitudes $J_{\parallel }$ and $J_{\perp }$ for identical and different colors, respectively. For $J_{\parallel }=J_{\perp }>0$ it is equivalent to the SU(3) Heisenberg model, which describes the Mott-insulating phase of 3-color fermions, while the parameter range $J_{\perp }<\min (0,-J_{\parallel })$ can be realized in the Mott insulating phase of 3-color bosonic atoms. Using linear flavor wave theory, infinite projected entangled-pair states (iPEPS), and continuous-time quantum Monte Carlo simulations, we construct the full $T=0$ phase diagram, and we explore the $T\ne 0$ properties for $J_{\perp }<0$. For dominant antiferromagnetic $J_{\parallel }$ interactions, a three-sublattice long-range ordered stripe state is selected out of the ground-state manifold of the antiferromagnetic Potts model by quantum fluctuations. Upon increasing $|J_{\perp }|$, this state is replaced by a uniform superfluid for $J_{\perp }<0$, and by an exotic three-sublattice superfluid followed by a two-sublattice superfluid for $J_{\perp }>0$. The transition out of the uniform superfluid (that can be realized with bosons) is shown to be a peculiar type of Kosterlitz-Thouless transition with three types of elementary vortices.

We present experiments to systematically study the time constants of spin-dependent recombination processes in semiconductors using pulsed electrically detected magnetic resonance (EDMR). The combination of time-programmed optical excitation and pulsed spin manipulation allows us to directly measure the recombination time constants of electrons via localized spin pairs and the time constant of spin-pair formation as a function of the optical excitation intensity. Using electron nuclear double resonance, we show that the time constant of spin-pair formation is determined by an electron capture process. Based on these time constants we devise a set of rate equations to calculate the current transient after a resonant microwave pulse and compare the results with experimental data. Finally, we critically discuss the effects of different boxcar integration time intervals typically used to analyze pulsed EDMR experiments on the determination of the time constants. The experiments are performed on phosphorus-doped silicon, where EDMR via spin pairs formed by phosphorus donors and Si/SiO${}_2$ interface dangling bond defects is detected.

We present an analytical method for quantifying exciton hopping in an energetically disordered system with quenching sites. The method is subsequently used to provide a quantitative understanding of exciton hopping in a quantum dot (QD) array. Several statistical quantities that characterize the dynamics (survival probability, average number of distinct sites visited, average hopping distance, and average hopping rate in the initial stage) are obtained experimentally by measuring time-resolved fluorescence intensities at various temperatures. The time evolution of these quantities suggests in a quantitative way that at low temperature an exciton tends to be trapped at a local low-energy site, while at room temperature, exciton hopping occurs repeatedly, leading to a large hopping distance. This method will serve to facilitate highly efficient optoelectronic devices using QDs such as photovoltaic cells and light-emitting diodes, since exciton hopping is considered to strongly influence their operational parameters. The presence of a dark QD (quenching site) that exhibits fast decay is also quantified.

We derive an analytical expression for the backaction dephasing rate, which characterizes the disturbance induced by coupling with an environment containing a quantum dot detector (QDD). In this paper, we show in an explicit form that the charge noise induces backaction dephasing. In equilibrium, this backaction dephasing induced by the charge noise can be explained as a relaxation by an inelastic electron-electron scattering in Fermi-liquid theory. Unlike the quantum point contact, the backaction dephasing rate increases or decreases with the bias at low bias and finite-temperature condition. This behavior depends on the QDD energy level with respect to the Fermi energy and the asymmetry of the QDD coupling to the reservoirs. In the high bias voltage regime, the dephasing rate becomes insensitive to the bias because of the saturation of the charge noise.

Using first-principles calculations, we show that the magnetic properties of a two-dimensional antiferromagnetic transition-metal surface are modified on the atomic scale by the adsorption of small organic molecules. We consider benzene (C${}_6$H${}_6$), cyclooctatetraene (C${}_8$H${}_8$), and a small transition-metal–benzene complex (BzV) adsorbed on a single atomic layer of Mn deposited on the W(110) surface—a surface which exhibits a nearly antiferromagnetic alignment of the magnetic moments in adjacent Mn rows. Due to the spin dependent hybridization of the molecular $p_z$ orbitals with the $d$ states of the Mn monolayer, there is a significant reduction of the magnetic moments in the Mn film. Furthermore, the spin polarization at this organic-antiferromagnetic interface is found to be modulated on the atomic scale, both enhanced and inverted, as a result of the molecular adsorption. We show that this effect can be resolved by spin-polarized scanning tunneling microscopy (SP-STM). Our simulated SP-STM images display a spatially dependent spin resolved vacuum charge density above an adsorbed molecule—i.e., different regions above the molecule sustain different signs of spin polarization. While states with $s$ and $p$ symmetry dominate the vacuum charge density in the vicinity of the Fermi energy for the clean magnetic surface, we demonstrate that after a molecule is adsorbed those $d$ states, which are normally suppressed due to their symmetry, can play a crucial role in the vacuum due to their interaction with the molecular orbitals. We also model the effect of small deviations from perfect antiferromagnetic ordering, induced by the slight canting of magnetic moments due to the spin spiral ground state of Mn/W(110).

We study a doped two-dimensional bosonic Hubbard model with two hard-core species using quantum Monte Carlo simulations. With doping we find five distinct phases, including a normal liquid at higher temperature, an antiferromagnetically ordered Mott insulator, a region of coexistent antiferromagnetic and superfluid phases near half filling, and farther away from half filling, a superfluid phase and a phase-separated ferromagnet. In the latter, the heavy species has Mott behavior with integer fillings, while the light species shows Mott and superfluid behaviors. The global entropy of this phase is relatively high which may provide a new avenue to obtain a polarized phase or a Mott insulator in cold-atom experiments.

Effective properties of periodic structures are investigated at all frequencies. It is shown that a simple dielectric multilayer with arbitrary low contrast can display artificial magnetoelectric coupling and magnetism. Effective parameters, as functions of the complex frequency, possess all the analytic properties required by the causality principle. The theoretical expression of the effective index and surface impedance are numerically tested; the former is shown to reduce to the mean refractive index at infinite frequency, just like the refractive index in the x-ray regime. We stress that periodic multilayers, with frequency-independent (noncausal) dielectric constants, make effective media with artificial causality.

We study the phase diagram of a Dirac semimetal in a magnetic field at a nonzero charge density. It is shown that there exists a critical value of the chemical potential at which a first-order phase transition takes place. At subcritical values of the chemical potential the ground state is a gapped state with a dynamically generated Dirac mass and a broken chiral symmetry. The supercritical phase is the normal (gapless) phase with a nontrivial chiral structure: it is a Weyl semimetal with a pair of Weyl nodes for each of the original Dirac points. The nodes are separated by a dynamically induced chiral shift. The direction of the chiral shift coincides with that of the magnetic field and its magnitude is determined by the quasiparticle charge density, the strength of the magnetic field, and the strength of the interaction. The rearrangement of the Fermi surface accompanying this phase transition is described.

Charge, spin, and lattice degrees of freedom are strongly entangled in iron superconductors. A neat consequence of this entanglement is the behavior of the $A_{1g}$ As-phonon resonance in the different polarization symmetries of Raman spectroscopy when undergoing the magnetostructural transition. In this work, we show that the observed behavior could be a direct consequence of the coupling of the phonons with the electronic excitations in the anisotropic magnetic state. We discuss this scenario within a five-orbital tight-binding model coupled to phonons via the dependence of the Slater-Koster parameters on the As position. We identify two qualitatively different channels of the electron-phonon interaction: a geometrical one related to the Fe-As-Fe angle $\alpha$ and another one associated with the modification upon As displacement of the Fe-As energy integrals $pd\sigma$ and $pd\pi$. While both mechanisms result in a finite $B_{1g}$ response, the behavior of the phonon intensity in the $A_{1g}$ and $B_{1g}$ Raman polarization geometries is qualitatively different when the coupling is driven by the angle or by the energy integral dependence. We discuss our results in view of the experimental reports.

We have performed ${}^{53}$Cr NMR measurements to investigate the local electronic state of a hollandite-type chromium oxide K${}_2$Cr${}_8$O${}_{16}$ which undergoes an unusual metal-insulator (MI) transition at $T_{\mathrm{MI}}$ $=$ 95 K in the ferromagnetic phase. In the ferromagnetic insulating phase, we observed NMR spectra with internal fields different from each other at the four Cr sites. From an analysis of the hyperfine fields, the difference in the isotropic internal field is related to the supertransferred hyperfine (STH) field. The slight difference in the electron occupation of the 3$d$ orbital at the four Cr sites is detected through the different STH field governed by the different charge-transfer passes. The presence of the STH field shows the strong hybridization of the chromium 3$d$ orbitals and oxygen 2$p$ orbital which leads to the characteristic MI transition.

We present a theoretical study based on the Anderson model of the transport properties of a Kondo impurity (atom or quantum dot) connected to ferromagnetic leads, which can sustain a nonequilibrium spin current. We analyze the case where the spin current is injected by an external source and when it is generated by the voltage bias. Due to the presence of ferromagnetic contacts, a static exchange field is produced that eventually destroys the Kondo correlations. We find that such a field can be compensated by an appropriated combination of the spin-dependent chemical potentials leading to the restoration of the Kondo resonance. In this respect, a Kondo impurity may be regarded as a very sensitive sensor for nonequilibrium spin phenomena.

The stability and electronic structure of competing silicene phases under in-plane compressive stress, either free-standing or on the ZrB${}_2$(0001) surface, has been studied by first-principles calculations. A particular ($\sqrt{3}\times \sqrt{3}$)-reconstructed structural modification was found to be stable on the ZrB${}_2$(0001) surface under epitaxial conditions. In contrast to the planar and buckled forms of free-standing silicene, in this “planar-like” phase, all but one of the Si atoms per hexagon reside in a single plane. While without substrate, for a wide range of strain, this phase is energetically less favorable than the buckled one, it is calculated to represent the ground state on the ZrB${}_2$(0001) surface. The atomic positions are found to be determined by the interactions with the nearest-neighbor Zr atoms competing with Si-Si bonding interactions provided by the constraint of the honeycomb lattice.

We study quantum dots defined by external potentials within finite flakes of bilayer graphene using the tight-binding approach. We find that in the limit of large flakes containing zigzag edges the dot-localized energy levels appear within the energy continuum formed by extended states. As a consequence no ionization threshold for the carriers contained within the dot exists. For smaller flakes with zigzag boundaries the dot-localized energy levels appear interlaced with the energy levels outside the flake, so in a charging experiment the electrons will be added alternately to the dot area and to its neighborhood. We demonstrate that for flakes with armchair boundaries only, an energy window accessible uniquely to the dot-localized states is opened. Then a number of electrons can be added to the dot before the external states start to be occupied. We also discuss coupling of the dot-localized states to the edge states in the context of the valley degeneracy lifting. Moreover, we extract smooth envelope wave functions from the tight-binding solution and discuss their spatial symmetries. The coupling of the dot localized energy levels with reconstructed zigzag edges and atomic vacancies present within the layers is also considered.

We report on large-scale nonequilibrium molecular dynamics simulations of shock wave compression in tantalum single crystals. Two new embedded atom method interatomic potentials of Ta have been developed and optimized by fitting to experimental and density functional theory data. The potentials reproduce the isothermal equation of state of Ta up to 300 GPa. We examined the nature of the plastic deformation and elastic limits as functions of crystal orientation. Shock waves along (100), (110), and (111) exhibit elastic-plastic two-wave structures. Plastic deformation in shock compression along (110) is due primarily to the formation of twins that nucleate at the shock front. The strain-rate dependence of the flow stress is found to be orientation dependent, with (110) shocks exhibiting the weaker dependence. Premelting at a temperature much below that of thermodynamic melting at the shock front is observed in all three directions for shock pressures above about 180 GPa.

In this study, we investigate the electronic and magnetic properties of Rb${}_{0.77}$Fe${}_{1.61}$Se${}_2$ ($T_{\mathrm{c}}$ $=$ 32.6 K) in normal and superconducting states by means of photoemission and $\mu$SR spectroscopies as well as band-structure calculations. We demonstrate that the unusual behavior of these materials is the result of separation into metallic ($\sim$$12\%$) and insulating ($\sim$$88\%$) phases. Only the former becomes superconducting and has a usual electronic structure of electron-doped FeSe slabs. Our results thus imply that the antiferromagnetic insulating phase is just a by-product of Rb intercalation and its magnetic properties have no direct relation to the superconductivity. Instead, we find that also in this class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a Van Hove singularity to the Fermi level.