Motivated by the recent experimental and theoretical progress of the magnetic properties in iron-based superconductors, we provide a comprehensive analysis of the extended spin-1 bilinear-biquadratic (BBQ) model on the square lattice. Using a variational approach at the mean-field level, we identify the existence of various magnetic phases, including conventional spin dipolar orders (ferro- and antiferromagnet), novel quadrupolar orders (spin nematic), and mixed dipolar-quadrupolar orders. In contrast to the regular Heisenberg model, the elementary excitations of the spin-1 BBQ model are described by the SU(3) flavor-wave theory. By fitting the experimental spin-wave dispersion, we determine the refined exchange couplings corresponding to the collinear antiferromagnetic iron pnictides. We also present the dynamic structure factors of both spin dipolar and quadrupolar components with connections to the future experiments.

While the interaction potential between two dipoles residing in a single plane is repulsive, in a system of two vertically adjacent layers of dipoles it changes from repulsive interaction in the long range to attractive interaction in the short range. Here we show that for dipolar excitons in semiconductor heterostructures, such a potential may give rise to bound states if two such excitons are excited in two separate layers, leading to the formation of vertically coupled dipolar exciton molecules. Our calculations prove the existence of such bound states and predict their binding energy as a function of the layers separation as well as their thermal distributions. We show that these molecules should be observed in realistic systems such as semiconductor coupled quantum well structures and the more recent van der Waals bound heterostructures. Formation of such molecules can lead to new effects such as a collective dipolar drag between layers and new forms of multiparticle correlations, as well as to the study of dipolar molecular dynamics in a controlled system.

Accumulation layers with very large concentrations of electrons where many subbands are filled became recently available due to ionic liquid and other new methods of gating. The low-temperature mobility in such layers is limited by the surface roughness scattering. However, theories of roughness scattering so far dealt only with the small-density single subband two-dimensional electron gas (2DEG). Here we develop a theory of roughness-scattering limited mobility for the multisubband large concentration case. We show that with growing 2D electron concentration $n$ the surface dimensionless conductivity $\sigma /(2e^2/h)$ first decreases as $\propto n^{-6/5}$ and then saturates as $\sim (d{a}_B/{\mathrm{\Delta }}^2)\gg 1$, where $d$ and $\mathrm{\Delta }$ are the characteristic length and height of the surface roughness and $a_B$ is the effective Bohr radius. This means that in spite of the shrinkage of the 2DEG thickness and the related increase of the scattering rate the 2DEG remains a good metal.

J. Y. Park et al. [Science 309, 1354 (2005)] have reported eight times greater atomic-scale friction in the periodic than in the quasiperiodic direction on the twofold face of a decagonal Al-Ni-Co quasicrystal. We present results of molecular-dynamics simulations intended to elucidate mechanisms behind this giant frictional anisotropy. Simulations of a bare atomic-force-microscope tip on several model substrates and under a variety of conditions failed to reproduce experimental results. On the other hand, including the experimental passivation of the tip with chains of hexadecane thiol, we reproduce qualitatively the experimental anisotropy in friction, finding evidence for entrainment of the organic chains in surface furrows parallel to the periodic direction.

Theoretical studies of the optical bistability in two-dimensional nonlinear composites, where Kerr type dielectric/nonlocal metal core-shell cylinders are randomly embedded in the host medium, have been carried out. Within the quasistatic approximation, we demonstrate the tunable Fano resonances in the scattering spectra of the single core-shell cylinder based on different degrees of nonlocality in the linear case. It is found that nonlocality of the metallic shell would enhance the Fano resonance peak and the near-field strength, thus it can further boost the nonlinear response of the core-shell cylinder in both near field and far field, indicating the nonlocality-enhanced optical bistability. Furthermore, we show that nonlocality can efficiently reduce the bistable switching threshold of the nonlinear composite, and these self-tunable optical resonant scatters can be used as all-optical switches and nanomemories.

The thermoelectric effects in three-terminal structures with a quantum dot are considered. We propose the experimentally consistent protocol for determination of the transport coefficients in terms of the local and nonlocal conductances and thermopowers that can be measured in two steps, applying the “four-probe technique”. This proposal is compared with other approaches discussed in the literature. As an example, we study in detail the thermopower induced by the superconducting electrode in a subgap regime which might be useful for analysis of novel hybrid devices.

We investigate the temporal evolution of electronic states in strontium iridate ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. The time-resolved photoemission spectra of the intrinsic, electron-doped, and hole-doped samples are monitored in identical experimental conditions. Our data on the intrinsic and electron-doped samples, show that doublon-holon pairs relax near to the chemical potential on a time scale shorter than 70 fs. The subsequent cooling of low-energy excitations takes place in two steps: a rapid dynamics of $\cong 120$ fs is followed by a slower decay of $\cong 1$ ps. The reported time scales endorse the analogies between ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and copper oxides.

Recent angle resolved photoemission spectroscopy experiments on strongly underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ cuprates have reported an unusual gap in the nodal direction. Transport experiments on these cuprates observed variable-range hopping behavior. These cuprates have both electron and hole doping, which has led to proposals that this cuprate is analogous to partially compensated semiconductors. The nodal gap then corresponds to the Efros-Shklovskii (ES) gap in such semiconductors. We calculate the doping and temperature dependence of an ES gap model and find support for an Efros-Shklovskii model.

Using first-principles calculation and symmetry analysis, we propose that $\theta$-TaN is a topological semimetal having a new type of point nodes, i.e., triply degenerate nodal points. Each node is a band crossing between degenerate and nondegenerate bands along the high-symmetry line in the Brillouin zone, and is protected by crystalline symmetries. Such new type of nodes will always generate singular touching points between different Fermi surfaces and three-dimensional spin texture around them. Breaking the crystalline symmetry by external magnetic field or strain leads to various topological phases. By studying the Landau levels under a small field along the $c$ axis, we demonstrate that the system has a new quantum anomaly that we call “helical anomaly.'

We observe multiple electronic Raman scatterings (ERSs) in a single suspended metallic single-walled carbon nanotube. The ERS process originates from the inelastic scattering of photoexcited excitons by a continuum of low-lying electron-hole pairs. In previous work, the observed Fano factor of the G band line shape is always negative; however, in this work we find that the Fano factor can be either positive or negative depending on the relative position of the nearest ERS with respect to the G band. This supports the idea that the origin of the G band asymmetry is an interference between the discrete G band and the continuous ERS. We also report that the ERS position and intensity are sensitive to the nanotube bundling effect and the laser heating effect.

The pressure-induced changes to the structure of disordered oxide and chalcogenide network-forming materials are investigated on the length scales associated with the first three peaks in measured diffraction patterns. The density dependence of a given peak position does not yield the network dimensionality, in contrast to metallic glasses where the results indicate a fractal geometry with a local dimensionality of $\simeq 5/2$. For oxides, a common relation is found between the intermediate-range ordering, as described by the position of the first sharp diffraction peak, and the oxygen-packing fraction, a parameter that plays a key role in driving changes to the coordination number of local motifs. The first sharp diffraction peak can therefore be used to gauge when topological changes are likely to occur, events that transform network structures and their related physical properties.

We report inelastic neutron scattering measurements over 7–1251 K in $\mathrm{CaMn}{\mathrm{O}}_3$ covering various phase transitions, and over 6–150 K in $\mathrm{PrMn}{\mathrm{O}}_3$ covering the magnetic transition. The excitations around 20 meV in $\mathrm{CaMn}{\mathrm{O}}_3$ and at 17 meV in $\mathrm{PrMn}{\mathrm{O}}_3$ at low temperatures are found to be associated with magnetic origin. We observe coherent magnetic neutron scattering in localized regions in reciprocal space and show it to arise from long-range correlated magnetic spin-waves below the magnetic transition temperature ($T_{\mathrm{N}}$) and short-range stochastic spin-spin fluctuations above $T_{\mathrm{N}}$. In spite of the similarity of the structure of the two compounds, the neutron inelastic spectrum of $\mathrm{PrMn}{\mathrm{O}}_3$ exhibits broad features at 150 K unlike well-defined peaks in the spectrum of $\mathrm{CaMn}{\mathrm{O}}_3$. This might result from the difference in the nature of interactions in the two compounds (magnetic and Jahn-Teller distortion). Ab initio phonon calculations have been used to interpret the observed phonon spectra. The ab initio calculations at high pressures show that the variations of Mn-O distances are isotropic for $\mathrm{CaMn}{\mathrm{O}}_3$ and highly anisotropic for $\mathrm{PrMn}{\mathrm{O}}_3$. The calculation in $\mathrm{PrMn}{\mathrm{O}}_3$ shows the suppression of Jahn-Teller distortion and simultaneous insulator-to-metal transition. It appears that this transition may not be associated with the occurrence of the tetragonal phase above 20 GPa as reported in the literature, since the tetragonal phase is found to be dynamically unstable, although it is found to be energetically favored over the orthorhombic phase above 20 GPa. $\mathrm{CaMn}{\mathrm{O}}_3$ does not show any phase transition up to 60 GPa.

The muon spin rotation technique is used to study magnetic ordering in ultrapure samples of ${\mathrm{SrCu}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_2$, an archetypical $S=1/2$ antiferromagnetic Heisenberg chain system with a small number of $S=1$ defects. The ordered state in the parent compound is shown to be highly homogeneous, contrary to a previous report [M. Matsuda et al., Phys. Rev. B 55, R11953 (1997)]. Even a minute number of Ni impurities results in inhomogeneous order and a decrease of the transition temperature. At as little as $0.5$% Ni concentration, magnetic ordering is entirely suppressed. The results are compared to previous theoretical studies of weakly coupled spin chains with site defects.

Cooper pairs in two-dimensional unconventional superconductors with broken inversion symmetry are in a mixture of an even-parity spin-singlet pairing state with an odd-parity spin-triplet pairing state. We study the magnetic properties of the impurity bound states in such superconductors and find striking signatures in their spin polarization which allow one to unambiguously discriminate a nontopological superconducting phase from a topological one. Moreover, we show how these properties, which could be measured using spin-polarized scanning tunneling microscopy (STM), also enable one to determine the direction of the spin-triplet pairing vector of the host material and thus to distinguish between different types of unconventional pairing.

In this theoretical investigation we demonstrate that the adsorption of spatially extended two-dimensional (2D) $\pi$ systems such as graphene and hexagonal boron nitride on the ferromagnetic fcc Co(111) surface leads to a specific behavior of the in-plane and interlayer Co-Co magnetic exchange interactions. More specifically, for both systems the magnetic exchange coupling within the first Co layer is enhanced, while the one between the first and the second Co layer is not modified, in contrast to the magnetic interlayer softening induced by organic molecules. Importantly, the in-plane magnetic hardening effect is mainly due to the hybridization between the $p_z$ states of the 2D $\pi$ system and the $d$ states of the Co surface.

Strained ${\mathrm{NaNbO}}_3$ films of different thicknesses are epitaxially grown on (110) ${\mathrm{NdGaO}}_3$ substrates. A detailed analysis of the permittivity of these films demonstrates that strain not only leads to a modification of the permittivity and the ferroelectric transition temperature, it also results in a pronounced relaxor-type behavior and allows a direct estimation of the size and mobility of the polar nanoregions (PNRs). The compressive strain reduces the transition temperature to 125 K and enhances the corresponding permittivity up to ${\varepsilon }^{\prime }\approx 1500$ for the thinnest film. Since the strain relaxes with increasing film thickness, both effects, reduction of phase transition temperature and enhancement of ${\varepsilon }^{\prime }$, depend on the thickness of the film. The films show a characteristic frequency and electric field dependence of ${\varepsilon }^{\prime }$, which is discussed in terms of the Vogel-Fulcher equation and Rayleigh law, respectively. Using the electric field dependence of the resulting freezing temperature $T_{\mathrm{VF}}$, allows a direct estimation of the volume of the PNRs at the freezing temperature, i.e. from 70 to $270\mathrm{n}{\mathrm{m}}^3$. Assuming an idealized spherical shape of the PNRs, diameters of a few nanometers (5.2–8 nm) are determined that depend on the applied ac electric field. The irreversible part of the polarization seems to be dominated by the presence and mobility of the PNRs. It shows a characteristic peak at low temperature around $T_{\mathrm{VF}}$, vanishes at a temperature where the activation energy of the PRNs extrapolates to zero, and shows a frequency dispersion that is characteristic for relaxor-type behavior.

It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current through the excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice and, depending on the excitation conditions, can be either attenuated or enhanced. When the phase difference between the excited evanescent modes is close to $\pi /2$, there is an optimum AFM thickness for which the output spin current reaches a maximum, which can significantly exceed the magnitude of the input spin current. The spin current transfer through the AFM depends on the ambient temperature and increases substantially when temperature approaches the Néel temperature of the AFM layer.

We study critical behavior of the diluted two-dimensional Ising model in the presence of disorder correlations which decay algebraically with distance as $\sim r^{-a}$. Mapping the problem onto two-dimensional Dirac fermions with correlated disorder we calculate the critical properties using renormalization group up to two-loop order. We show that beside the Gaussian fixed point the flow equations have a nontrivial fixed point which is stable for $0.995<a<2$ and is characterized by the correlation length exponent $\nu =2/a+O({(2-a)}^3)$. Using bosonization, we also calculate the averaged square of the spin-spin correlation function and find the corresponding critical exponent ${\eta }_2=1/2-(2-a)/4+O({(2-a)}^2)$.

We present ferromagnetic resonance (FMR) investigations of 20 nm thick permalloy (${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) elements (width $W=200$ nm, length $L=470$ nm, period $a=500$ nm) arranged in open and closed artificial kagome lattices. The measurements were done at 9.4 and 34 GHz to ensure a saturated or near-saturated magnetic state of the kagome structures. The FMR data are analyzed in the framework of an analytical macrospin model which grasps the essential features of the bulk and edge modes at these microwave frequencies and is in agreement with the results of micromagnetic simulations. Polar plots of the resonance fields versus the field angle made by the direction of the magnetic field with respect to the main symmetry directions of the kagome lattice are compared with the results of the analytical model. The measured FMR spectra with a sixfold rotational symmetry qualitatively reproduce the structure expected from the theory. Magnetic dipolar interactions between the elements of the kagome lattices result in the mixing of edge and bulklike excitations at 9.4 GHz and in a systematic deviation from the model, especially for the closed kagome lattice.

We study how the non-Fermi-liquid two-phase state reveals itself in transport properties of high-mobility Si-MOSFETs. We have found features in zero-field transport, magnetotransport, and thermodynamic spin magnetization in a 2D correlated electron system that may be directly related with the two-phase state. The features manifest above a density-dependent temperature $T^{*}$ that represents a high-energy scale, apart from the Fermi energy. More specifically, in magnetoconductivity, we found a sharp onset of the regime $\delta \sigma (B,T)\propto {(B/T)}^2$ above a density-dependent temperature $T_{\mathrm{kink}}\left(n\right)$, a high-energy behavior that “mimics” the low-temperature diffusive interaction regime. The zero-field resistivity temperature dependence exhibits an inflection point $T_{\mathrm{infl}}\left(n\right)$. In thermodynamic magnetization, the weak-field spin susceptibility per electron $\partial \chi /\partial n$ changes sign at $T_{dM/dn}\left(n\right)$. All three notable temperatures, $T_{\mathrm{kink}},T_{\mathrm{infl}}$, and $T_{dM/dn}$ behave critically $\propto (n-n_c)$, are close to each other, and are intrinsic to high-mobility samples solely; we therefore associate them with an energy scale $T^{*}$ caused by interactions in the 2DE system.