We study and analyze the growth of submonolayers of Au on Si(111) by a complementary set of surface techniques. Specifically, we focus on the $5\times 2$ and the $\alpha \sqrt{3}\times \sqrt{3}$ structures. We determine the gold coverage of these structures as a function of temperature by means of low energy electron diffraction (LEED) and low energy electron microscopy (LEEM). These results are independently calibrated by ex-situ ion scattering experiments. This allows us to present a phase diagram for this system. Remarkably, for all temperatures considered (820–1040 K), we find a coverage for the $5\times 2$ phase that is significantly ($\approx 10\%$) higher than the value of 0.6 monolayers which is assumed in the latest structural models. Therefore, a further refinement of the present picture of the quasi-one-dimensional $5\times 2$ structure is required.

The carbon isotope ${}^{13}$C, in contrast to ${}^{12}$C, possesses a nuclear magnetic moment and can induce electron spin dephasing in graphene. This effect is usually neglected due to the low abundance of ${}^{13}$C in natural carbon allotropes ($\sim$1%). Chemical vapor deposition (CVD) allows for artificial synthesis of graphene solely from a ${}^{13}$C precursor, potentially amplifying the influence of the nuclear magnetic moments. In this work we study the effect of hyperfine interactions in pure ${}^{13}$C-graphene on its spin transport properties. Using Hanle precession measurements we determine the spin relaxation time and observe a weak increase of ${\tau }_s$ with doping and a weak change of ${\tau }_s$ with temperature, as in natural graphene. For comparison we study spin transport in pure ${}^{12}$C-graphene, also synthesized by CVD, and observe similar spin relaxation properties. As the signatures of hyperfine effects can be better resolved in oblique spin-valve and Hanle configurations, we use finite-element modeling to emulate oblique signals in the presence of a hyperfine magnetic field for typical graphene properties. Unlike in the case of GaAs, hyperfine interactions with ${}^{13}$C nuclei influence electron spin transport only very weakly, even for a fully polarized nuclear system. Also, in the measurements of the oblique spin-valve and Hanle effects no hyperfine features could be resolved. This work experimentally confirms the weak character of hyperfine interactions and the negligible role of ${}^{13}$C atoms in the spin dephasing processes in graphene.

The typical bulk model describing 2D topological insulators (TI) consists of two types of spin-orbit terms, the so-called Dirac term, which induces out-of-plane spin polarization, and the Rashba term, which induces in-plane spin polarization. We show that for some parameters of the Fermi energy, the beam splitter device built on 2D TIs can achieve higher in-plane spin polarization than the one built on materials described by the Rashba model itself. Further, due to high tunability of the electron density and the asymmetry of the quantum well, spin polarization in different directions can be obtained. While in the normal (topologically trivial) regime the in-plane spin polarization would dominate, in the inverted regime, the out-of-plane polarization is more significant not only in the band gap but also for small Fermi energies above the gap. Further, we suggest a double beam splitter scheme to measure in-plane spin current all-electrically. Although we consider here as an example HgTe/CdTe quantum wells, this scheme could be also promising for InAs/GaSb QWs where the in- and out-of-plane polarization could be achieved in a single device.

The magneto-optical properties of a hybrid metal-dielectric structure consisting of a one-dimensional gold grating on top of a magnetic waveguide layer are studied experimentally and theoretically. It is demonstrated that a magnetic field applied in the longitudinal configuration (in the plane of the magnetic film and perpendicular to the slits in the gold grating) to the metal-dielectric structure modifies the field distribution of the optical modes and thus changes the mode excitation conditions. In the optical far field, this manifests in the alteration of the optical transmittance or reflectance when the structure becomes magnetized. This magneto-optical effect is shown to represent a novel class of effects related to the magnetic-field-induced modification of the Bloch modes of the periodic hybrid structure. That is why we define this effect as “longitudinal magnetophotonic intensity effect” (LMPIE). The LMPIE has two contributions, odd and even in magnetization. While the even LMPIE is maximal for the light polarized perpendicular to the grating slits (TM) and minimal for the orthogonal polarization (TE), the odd LMPIE takes maximum values at some intermediate polarization and vanishes for pure TM and TE polarizations. Two principal modes of the magnetic layer—TM and TE—acquire in the longitudinal magnetic field additional field components and thus turn into quasi-TM and quasi-TE modes, respectively. The largest LMPIE is observed for excitation of the antisymmetrical quasi-TE mode by TM-polarized light. The value of the LMPIE measured for the plasmonic structure with a magnetic film of Bi${}_2$Dy${}_1$Fe${}_4$Ga${}_1$O${}_{12}$ composition is about 1% for the even effect and 2% for the odd one. However, the plasmonic structure with a magnetic film with a higher concentration of bismuth (Bi${}_{2.97}$Er${}_{0.03}$Fe${}_4$Al${}_{0.5}$Ga${}_{0.5}$O${}_{12}$) gives significantly larger LMPIE: even LMPIE reaches 24% and odd LMPIE is 9%. Enhancement of the magneto-optical figure of merit (defined as the ratio of the specific Faraday angle of a magnetic film to its absorption coefficient) of the magnetic films potentially causes the even LMPIE to exceed 100% as is predicted by calculations. Thus, the nanostructured material described here may be considered as an ultrafast magnetophotonic light valve.

The spin-density-wave ordering in ${\mathrm{Ca}}_{0.5}$${\mathrm{Sr}}_{1.5}$${\mathrm{RuO}}_4$ was studied by polarized and unpolarized neutron diffraction experiments. Below about 20 K ${\mathrm{Ca}}_{0.5}$${\mathrm{Sr}}_{1.5}$${\mathrm{RuO}}_4$ exhibits quasistatic correlations at exactly the incommensurate wave vector at which pure Sr${}_2$RuO${}_4$ shows strong inelastic fluctuations driven by Fermi-surface nesting. The magnetic character of the signal and the orientation of the ordered moments along the $c$ direction can be ascertained by neutron polarization analysis. The magnetic ordering at low temperature is very similar to that found upon minor Ti substitution of ${\mathrm{Sr}}_2$${\mathrm{RuO}}_4$, underlining that this incommensurate spin density wave is the dominant magnetic instability of the unconventional superconductor Sr${}_2$RuO${}_4$.

The microwave-induced resistance oscillations and “zero-resistance” states were discovered in ultraclean two-dimensional electron systems in 2001–2003 and have attracted great interest from researchers. A comprehensive theory of these phenomena was developed in 2011: It was shown that all experimentally observed dependencies can be naturally explained by the influence of the ponderomotive forces which arise in the near-contact regions of the two-dimensional electron gas under the action of microwaves. Now, we show that the same near-contact physical processes should lead to another nonlinear electrodynamic phenomenon: the second-harmonic generation. We calculate the frequency, magnetic field, mobility, and power dependencies of the second-harmonic intensity and show that it can be as large as $\gtrsim 0.5$ mW/cm${}^2$ under realistic experimental conditions. A part of this paper is devoted to a further development of the ponderomotive-force theory: we show how it explains different experimental details, including those which were not known in 2011.

Dynamical correlation functions of the toric code in a uniform magnetic field are studied inside the topological phase, in the small-field limit. Such an experimentally measurable quantity displays rich field-dependent features that can be understood via the interplay of the kinetics and the interaction of the anyonic excitations. In particular, it is sensitive to the two-quasiparticle bound states that are present in the spectrum for a wide range of magnetic fields. Interestingly, such collective modes can even constitute the lowest-energy excitations of the system.

We analyze the transport properties of a double quantum dot device with both dots coupled to perfect conducting leads and to a finite chain of N noninteracting sites connecting both of them. The interdot chain strongly influences the transport across the system and the local density of states of the dots. We study the case of a small number of sites, so that Kondo box effects are present, varying the coupling between the dots and the chain. For odd N and small coupling between the interdot chain and the dots, a state with two coexisting Kondo regimes develops: the bulk Kondo due to the quantum dots connected to leads and the one produced by the screening of the quantum dot spins by the spin in the finite chain at the Fermi level. As the coupling to the interdot chain increases, there is a crossover to a molecular Kondo effect, due to the screening of the molecule (formed by the finite chain and the quantum dots) spin by the leads. For even N the two Kondo temperatures regime does not develop and the physics is dominated by the usual competition between Kondo and antiferromagnetism between the quantum dots. We finally study how the transport properties are affected as N is increased. For the study we used exact multiconfigurational Lanczos calculations and finite-$U$ slave-boson mean-field theory at $T=0$. The results obtained with both methods describe qualitatively and also quantitatively the same physics.

Interactions in Fe-Pd were studied using diffuse x-ray scattering and first-principles calculations. Diffuse x-ray scattering was performed from single crystals of Fe–38.2 at. % Pd and Fe–81.0 at. % Pd to investigate states of thermal equilibrium at 1023 K and 1073 K, respectively. Short-range-order scattering was separated and effective pair interaction (EPI) parameters were determined using the inverse Monte Carlo method. The EPI parameters are found to strongly vary with respect to data from a previous investigation of Fe–50 at. % Pd. Electronic-structure calculations of effective cluster interaction (ECI) parameters for these states showed the importance of considering multibody interactions and applying the disordered local moment model of the magnetic state: in the ferromagnetic model, the strength of the interactions gets overestimated. Close agreement is reached for short-range order between calculations and the present scattering experiments. Using either a linear compositional variation of the EPI parameters or ECI parameters for alloys with other Pd fractions, the shape of the order-disorder transition line on the Pd-rich side could be reproduced.

Electron paramagnetic resonance spectra of KTm(MoO${}_4$)${}_2$ were measured as a function of magnetic field between 3 and 11.5 cm${}^{-1}$ at $T=2$ K. We found that in addition to the absorption line caused by the electronic excitation of Tm${}^{3+}$ ions, the spectra contain sidebands. Far-infrared transmission measured with polarized light from 10 to 75 cm${}^{-1}$ revealed vibration modes at 16.7 and 25.7 cm${}^{-1}$ for polarizations $E^{\omega }\parallel a$ and $E^{\omega }\parallel c$, respectively. We show that sidebands in the spectra of paramagnetic resonance result from a parametric resonance between the electronic excitations of the Tm${}^{3+}$ ions and the acoustic vibrations of the crystal lattice.

Using the Kubo formalism we derived expressions and implemented the method for calculating the anomalous Hall conductivity (AHC) in ferromagnets with short-range Gaussian disorder directly from first-principles electronic structure of the perfect crystal. We used this method to calculate the AHC in bcc Fe, fcc Co, L1${}_0$-FePd, L1${}_0$-FePt as well as thin bcc Fe(001) films. Within our approach we can transparently decompose the conductivity into intrinsic, side jump, and intrinsic skew-scattering (ISK) contributions. The existence of ISK, which originates from asymmetric Mott scattering but is clearly distinguishable from conventional skew scattering in that it converges to a finite value in clean limit, was pointed out by Sinitsyn et al. [Phys. Rev. B 75, 045315 (2007)]. Here, we collect all contributions to the AHC in ferromagnets which result in “scattering-independent” AHE in clean limit, and analyze their relative magnitude from first-principles calculations. By comparing our results to existing experiments we show that the Gaussian disorder is well suited to model various types of disorder present in real materials, to some extent including the effect of temperature. In particular, we show that in addition to intrinsic and side-jump AHE, the intrinsic skew scattering can be a major player in determining the magnitude of the AHE in ferromagnets.

The magnetic and lattice properties of the $S=1/2$ quantum-spin-chain ferromagnet CuAs${}_2$O${}_4$, mineral name trippkeite, were investigated. The crystal structure of CuAs${}_2$O${}_4$ is characterized by the presence of corrugated CuO${}_2$ ribbon chains. Measurements of the magnetic susceptibility, heat capacity, electron paramagnetic resonance, and Raman spectroscopy were performed. Our experiments conclusively show that a ferromagnetic transition occurs at $\sim$7.4 K. Ab initio DFT calculations reveal dominant ferromagnetic nearest-neighbor and weaker antiferromagnetic next-nearest-neighbor spin exchange interactions along the ribbon chains. The ratio of $J_{\mathrm{nn}}$/$J_{\mathrm{nnn}}$ is near $-$4, placing CuAs${}_2$O${}_4$ in close proximity to a quantum critical point in the $J_{\mathrm{nn}}$-$J_{\mathrm{nnn}}$ phase diagram. TMRG simulations used to analyze the magnetic susceptibility confirm this ratio. Single-crystal magnetization measurements indicate that a magnetic anisotropy forces the Cu${}^{2+}$ spins to lie in an easy plane perpendicular to the $c$ axis. An analysis of the field- and temperature-dependent magnetization by modified Arrott plots reveals a 3d-$X$Y critical behavior. Lattice perturbations induced by quasihydrostatic pressure and temperature were mapped via magnetization and Raman spectroscopy.

We study the superfluid properties of two-dimensional spin-population-imbalanced Fermi gases to explore the interplay between the Berezinskii-Kosterlitz-Thouless (BKT) phase transition and the possible instability towards the Fulde-Ferrell (FF) state. By the mean-field approximation together with quantum fluctuations, we obtain phase diagrams as functions of temperature, chemical potential imbalance, and binding energy. We find that the fluctuations change the mean-field phase diagram significantly. We also address possible effects of the phase separation and/or the anisotropic FF phase to the BKT mechanism. The superfluid density tensor of the FF state is obtained, and its transverse component is found always vanishing. This causes divergent fluctuations and possibly precludes the existence of the FF state at any nonzero temperature.

The phase stability of vanadium metal and vanadium-chromium alloys at high temperatures and pressures is explored by means of first-principles electronic-structure calculations. Utilizing the self-consistent ab initio lattice dynamics approach in conjunction with density-functional theory, we show that pressure-induced mechanical instability of body-centered cubic vanadium metal, which results in formation of a rhombohedral phase at around 60–70 GPa at room temperature, will survive significant heating and compression. Furthermore, alloying with chromium decreases the temperature at which stabilization of the body-centered cubic phase occurs at elevated pressure.

As a result of the high-temperature improper ferroelectric transition occurring in YMnO${}_3$ and other hexagonal manganites, a striking domain pattern was observed: an emergence of six domains of alternating polarization from one point in a vortexlike pattern. We derive the formation and distribution of the domain-vortex network from a combinatorial analysis based on order-parameter symmetry and lattice geometry. The analysis leads to stable vortexlike configurations of the six domain states in the basal plane perpendicular to the polarization and to fragmented vortices in planes parallel to the polarization. The predictions are debated in the light of existing experimental and theoretical work on the vortex domain state. The relation of the symmetry-determined geometrical defects to the topological defects proposed to describe the behavior of YMnO${}_3$ near the Curie temperature is discussed.

ZnSb and Zn${}_4$Sb${}_3$ are interesting as thermoelectric materials because of their low cost and low thermal conductivity. We introduce a model of the lattice thermal conductivity which is independent of fitting parameters and takes the full phonon dispersions into account. The model is found to give thermal conductivities with the correct relative magnitudes and in reasonable quantitative agreement with experiment for a number of semiconductor structures. The thermal conductivities of the zinc antimonides are reviewed and the relatively large effect of nanostructuring on the zinc antimonides is rationalized in terms of the mean free paths of the heat carrying phonons. The very low thermal conductivity of Zn${}_4$Sb${}_3$ is found to be intrinsic to the structure. However, the low-lying optical modes are observed in both Zn-Sb structures and involve both Zn and Sb vibrations, thereby strongly questioning dumbbell rattling. A mechanism for the very low thermal conductivity observed in Zn${}_4$Sb${}_3$ is identified. The large Grüneisen parameter of this compound is traced to the Sb atoms which coordinate only Zn atoms.

The critical behavior of the gauge-glass and the $XY$ spin-glass models in three dimensions is studied by analyzing their nonequilibrium aging dynamics. A new numerical method, which relies on the calculation of the two-time correlation and integrated response functions, is used to determine both the critical temperature and the nonequilibrium scaling exponents, both for spin and chiral degrees of freedom. First, the ferromagnetic $XY$ model is studied to validate this nonequilibirum aging method (NAM) since for this nondisordered system we can compare with known results obtained with standard equilibrium and nonequilibrium techniques. When applied to the case of the gauge-glass model, we show that the NAM allows us to obtain precise and reliable values of its critical quantities, improving previous estimates. The $XY$ spin-glass model with both Gaussian and bimodal bond distributions is analyzed in more detail. The spin and the chiral two-time correlation and integrated response functions are calculated in our simulations. The results obtained mainly for Gaussian and, to a lesser extent, for bimodal interactions support the existence of a spin-chiral decoupling scenario, where the chiral order occurs at a finite temperature while the spin degrees of freedom order at very low or zero temperature.

The propagation of magnons in temperature gradients is investigated within the framework of an atomistic spin model with the stochastic Landau-Lifshitz-Gilbert equation as underlying equation of motion. We analyze the magnon accumulation, the magnon temperature profile, as well as the propagation length of the excited magnons. The frequency distribution of the generated magnons is investigated in order to derive an expression for the influence of the anisotropy and the damping parameter on the magnon propagation length. For soft ferromagnetic insulators with low damping a propagation length in the range of some micrometers can be expected for exchange driven magnons.

We show that the magnetism of core-shell nanoparticles (made of maghemite, $\gamma$-Fe${}_2$O${}_3$, cores and transition-metal and metal-oxide shells) is altered substantially by the interface, which is a doped iron-oxide layer formed naturally during the seed-mediated synthesis process, a route used typically to produce core-shell nanoparticles. Characteristics fundamental to useful applications, such as the anisotropy and superparamagnetic blocking temperature, were altered substantially with Cu, CoO, MnO, and NiO shells. To ascertain the origin of this behavior, the prototype $\gamma$-Fe${}_2$O${}_3$/CoO core-shell nanoparticles are described in detail. We show that the magnetism originates essentially from an interfacial doped iron-oxide layer formed via migration of shell ions, e.g., Co${}^{2+}$, into octahedral site vacancies in the surface layers of the $\gamma$-Fe${}_2$O${}_3$ core. For this system, an overall Fe $m_{\mathrm{orb}}/m_{\mathrm{spin}}=0.15\pm 0.03$ is measured ($m_{\mathrm{orb}}\sim 0$ for the Fe-oxides) and an enhanced Co $m_{\mathrm{orb}}/m_{\mathrm{spin}}=0.65\pm 0.03$ elucidates the origin of the unexpectedly high overall anisotropy of the nanoparticle. This interfacial layer is responsible for the overall (e.g., bulk) magnetism and provides a perspective on how the magnetism of core-shell nanoparticles manifests from the selected core and shell materials.