Using ab initio band-structure and model calculations, we studied magnetic properties of one of the Mn${}_4$ molecular magnets [Mn${}_4$(hmp)${}_6$], where two types of the Mn ions exist: Mn${}^{3+}$ and Mn${}^{2+}$. The direct calculation of the exchange constants in the GGA$+U$ approximation shows that in contrast to a common belief, the strongest exchange coupling is not between two Mn${}^{3+}$ ions ($J_{bb}$), but along two out of four exchange paths connecting Mn${}^{3+}$ and Mn${}^{2+}$ ions ($J_{wb}$). Within the perturbation theory, we performed the microscopic analysis of different contributions to the exchange constants, which allows us to establish the mechanism for the largest ferromagnetic exchange. In the presence of the charge order, the lowest in energy virtual excitations, contributing to the superexchange, will not be those across the Hubbard gap $\sim U$, but will be those between the Mn${}^{3+}$ and Mn${}^{4+}$ ions, which cost much smaller energy $V$ ($\ll U$). Together with strong Hund's rule coupling and specific orbital order, this leads to large ferromagnetic exchange interaction for two out of four Mn${}^{2+}$-Mn${}^{3+}$ pairs.

We investigated the effect of nonstoichiometry on the half-metallic character of the Heusler alloy Co${}_2$MnSi (CMS) through the Mn composition (α) dependence of the saturation magnetization per formula unit (${\mu }_s$) of Co${}_2$Mn${}_{\alpha }$Si${}_{\beta }$ thin films and the tunneling magnetoresistance (TMR) ratio of CMS/MgO/CMS magnetic tunnel junctions (CMS MTJs) having Co${}_2$Mn${}_{\alpha }$Si${}_{\beta }$ electrodes. As a basis for understanding the effect of nonstoichiometry in CMS, we developed a generalized form of the site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defects, not vacancies, to accommodate nonstoichiometry. The α dependence of ${\mu }_s$ was well explained by density functional calculations with the coherent potential approximation based on the SSFU composition model for α up to a certain critical value (${\alpha }_c$)>1.0. The ${\mu }_s$ data for Mn-deficient films deviated from the Slater-Pauling predicted data for half-metals due to Co atoms at the nominal Mn sites (Co${}_{\mathrm{Mn}}$). The theoretical spin polarizations, obtained from only the $s$- and $p$-orbital components, $P_{\mathrm{th}}$($sp$), were found to qualitatively explain the α dependence of the TMR ratio except for α > ${\alpha }_c$. This is in contrast to the theoretical spin polarizations obtained from the $s$-, $p$-, and $d$-orbital components, $P_{\mathrm{th}}$(spd). A decrease in the TMR ratio observed for CMS MTJs having Mn-deficient electrodes was ascribed to small $s$- and $p$-orbital components of the local density of minority-spin in-gap states at the Fermi level that appeared for both antisite Co${}_{\mathrm{Mn}}$ atoms and Co atoms at the regular sites.

We have measured the drag force from quantum turbulence on a series of quartz tuning forks in superfluid helium. The tuning forks were custom made from a 75-$\mu$m-thick wafer. They have identical prong widths and prong spacings, but different lengths to give different resonant frequencies. We have used both the fundamental and overtone flexure modes to probe the turbulent drag over a broad range of frequencies $f=\omega /2\pi$ from 6.5 to 300 kHz. Optical measurements show that the velocity profiles of the flexure modes are well described by a cantilever beam model. The critical velocity for the onset of quantum turbulence at low temperatures is measured to be $v_c\approx \sqrt{0.7\kappa \omega }$ where $\kappa$ is the circulation quantum. The drag from quantum turbulence shows a small frequency dependence when plotted against the scaled velocity $v/v_c$.

A commonly believed picture of colossal magnetoresistance (CMR) effect is related to a first-order phase transition and electronic phase separation with coexisting ferromagnetic metallic and antiferromagnetic insulating phases. However, the underlying mechanism, i.e., the characteristic energy scale of the interacting phases and their spatial extent, is still under debate. Here we present experimental evidence on the existence of an effective antiferromagnetic coupling between the ferromagnetic nanodomains in epitaxial thin films of a classical CMR material $\left({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y\right){}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ with Pr doping, $y$ = 0.375 and 0.4. This coupling yields to peculiar low-field CMR behavior with magnetic hysteresis and slow resistance relaxation, both induced by the magnetization reversal. The coercive field obeys a square-root temperature dependence for $T\ll T_{\mathrm{C}}$ and increases anomalously close to the phase transition. We modeled the magnetic structure within the phase-separation scenario as an assembly of single-domain ferromagnetic nanoparticles, antiferromagnetically coupled (pinned) by correlated Jahn-Teller polarons. The concentration of polarons increases drastically close to phase transition as indicated by the third harmonic of the electrical conductivity as well as Raman spectroscopy.

Here we present a detailed muon spin relaxation ($\mu$SR), neutron diffraction, and ac-susceptibility study of Yb${}_2$Sn${}_2$O${}_7$ that shows that despite the onset of ferromagnetic long-range order at $T_c\approx 0.140$ K, the low-temperature phase of this material remains highly disordered with glassy dynamics that span various frequency decades. We suggest that the observed disordered ground state might be made of infinite, percolating clusters coexisting with smaller ones. This mesoscopic cluster glass state would be the result of the enhanced effect of even a small amount of disorder, capable of disrupting the “clean” ordered state near a quantum critical point.

Below room temperature, the apparent thermal conductivity of the two-leg spin ladder, ${\text{Ca}}_9$${\text{La}}_5$${\text{Cu}}_{24}$${\text{O}}_{41}$, as measured by time-domain thermoreflectance (TDTR), depends on the frequency of the applied surface heating. This occurs because the thermal penetration depth in the TDTR experiment is comparable to the length scale for the equilibration of the magnons that are the dominant channel for heat conduction and the phonons that dominate the heat capacity. We apply a two-temperature model to analyze the TDTR data and extract an effective volumetric magnon-phonon coupling parameter $g$ for ${\text{Ca}}_9$${\text{La}}_5$${\text{Cu}}_{24}$${\text{O}}_{41}$ at temperatures from 75 K to 300 K; $g$ varies by approximately two orders of magnitude over this range of temperature and has the value $g={10}^{15}{\mathrm{W}\mathrm{m}}^{-3}{\mathrm{K}}^{-1}$ near the peak of the thermal conductivity at $T\approx 180$ K.

The motion of domain walls is influenced by their internal structure and their structural changes during motion. The macroscopic motion is well understood but the internal dynamics during motion have not been experimentally studied in detail. We study vortex domain walls excited by nanosecond long magnetic field pulses in a pinning potential and above the pinning threshold. The dynamic response is imaged by transmission x-ray microscopy and the structural changes are analyzed. From the directly observed inertial behavior of the wall the domain-wall mass equivalent is estimated. During motion, oscillations of both the vortex core around the center of the domain wall and the domain-wall width are observed. The wall shows a fully elastic behavior by compression and overexpansion when being pushed by the field.

Magnetoelectric coupling in helical multiferroics allows us to steer spin order with electric fields. Here we show theoretically that in a helical multiferroic chain quantum information processing as well as quantum phases are highly sensitive to electric ($E$) field. Applying $E$ field, the quantum state transfer fidelity can be increased and made directionally dependent. We also show that $E$ field transforms the spin-density-wave/nematic or multipolar phases of a frustrated ferromagnetic spin$\text{−}\frac{1}{2}$ chain in chiral phase with a strong magnetoelectric coupling. We find sharp reorganization of the entanglement spectrum as well as a large enhancement of fidelity susceptibility at Ising quantum phase transition from nematic to chiral states driven by electric field. These findings point to a tool for quantum information with low power consumption.

Hole-doped cuprates show a superconducting critical temperature $T_c$ which follows a universal dome-shaped behavior as a function of doping. It is believed that the origin of superconductivity in cuprates is entangled with the physics of the pseudogap phase. An open discussion is whether the source of superconductivity is the same effect that causes the pseudogap properties. The $t$-$J$ model treated in large-N expansion shows $d$-wave superconductivity triggered by nonretarded interactions, and an instability of the paramagnetic state to a flux phase or $d$-wave charge-density wave ($d$-CDW) state. In this paper we show that self-energy effects near the $d$-CDW instability may lead to a dome-shaped behavior of $T_c$. In addition, it is also shown that these self-energy contributions may describe several properties observed in the pseudogap phase. In this picture, although fluctuations responsible for the pseudogap properties lead to a dome-shaped behavior, they are not involved in pairing, which is mainly nonretarded.

Employing the recently developed self-consistent variational basis generation scheme, we have investigated the bipolaron-bipolaron interaction within the purview of the Holstein-Hubbard and the extended-Holstein-Hubbard (F2H) models on a discrete one-dimensional lattice. The density-matrix renormalization group method has also been used for the Holstein-Hubbard model. We have shown that there exists no bipolaron-bipolaron attraction in the Holstein-Hubbard model. In contrast, we have obtained clear-cut bipolaron-bipolaron attraction in the F2H model. Composite bipolarons are formed above a critical electron-phonon coupling strength, which can survive the finite Hubbard $U$ effect. We have constructed the phase diagram of F2H polarons and bipolarons, and discussed the phase separation in terms of the formation of composite bipolarons.

We study waveguiding of the electromagnetic energy below the diffraction limit with arrays of dielectric nanoparticles through the excitation of both electric and magnetic Mie resonances. We analyze the dispersion characteristics of such coupled-resonator optical waveguides by means of the coupled-dipole approximation and then verify the validity of the coupled-dipole model by comparing the results with direct numerical simulations. We reveal that a chain of silicon nanoparticles with realistic material losses can guide light for the distances exceeding several tens of micrometers, which is significantly better than the guiding by any plasmonic waveguide with the propagation distances less than 1 $\mu$m. We verify the main concept and our theoretical findings experimentally at microwaves for an array of ceramic particles.

We show that Rashba spin-orbit coupling may result in an energy gap in the spectrum of electrons in a two-mode quantum wire if a suitable confining potential is chosen. This leads to a dip in the conductance and a spike in the spin current at the corresponding position of the Fermi level. Therefore one may control the charge and spin currents by means of electrostatic gates without using a magnetic field or magnetic materials.

The correlated band theory implemented as a combination of the local density approximation with the exact diagonalization of the Anderson impurity model is applied to PuO${}_2$. We obtain an insulating electronic structure consistent with the experimental photoemission spectra. The calculations yield a band gap of 1.8 eV and a nonmagnetic singlet ground state that is characterized by a noninteger filling of the plutonium $f$ shell ($n_f\approx 4.5$). Due to sizable hybridization of the $f$ shell with the $p$ states of oxygen, the ground state is more complex than the four-electron Russell-Saunders ${}^5{I}_4$ manifold split by the crystal field. The inclusion of hybridization improves the agreement between the theory and experiment for the magnetic susceptibility.

The local spin susceptibility in superconducting LiFeAs was studied by polarized neutron diffraction as a function of temperature. In the superconducting phase the spin susceptibility is clearly suppressed and it can be well described by the Yosida function suggesting a singlet pairing to occur at low temperature. The spin susceptibility in the normal state and its suppression in the superconducting phase are fully comparable to observations in Co-doped BaFe${}_2$As${}_2$.

We show that the optical singularities near controllably sharp metal corners and circular tips critically depend on the relative metal permittivity $\varepsilon$ and the apex half angle ${\theta }_a$. Within a critical region of $\varepsilon$ and ${\theta }_a$, which is independent of the smoothing radius ${\rho }_0$ and different in the two- and three-dimensional cases, the light energy is strongly localized near the apex while the nonzero optical phase rapidly changes here. Outside the critical region, the energy localization is relaxed and the phase stays constant. When approaching the boundary of the region, the localization and phase characteristics obey the square-root law typical of the second-order phase transitions. The found solutions substantially generalize and extend the known ones. Finally, we establish quantitative links between the criticality of the field singularities and the plasmonic properties of nanoscale particles and surface perturbations possessing sharp corners and tips.

Rather than being free carriers or separated single-hole polarons, double holes in anatase TiO${}_2$ prefer binding with each other, to form an O-O dimer after large structural distortion. This pushes the hole states upward into the conduction band and traps the holes. Similar double-hole-induced O-O dimerization (a bipolaron) exists also in other transition metal oxides (TMOs) such as V${}_2$O${}_5$ and MoO${}_3$, which have the highest valence bands composed mainly of O 2p states, loose lattices, and short O-O distances. Since the dimerization can happen in impurity-free TMO lattices, independent of any extrinsic dopant, it acts as an intrinsic and general limit to the p-type conductivity in these TMOs.

The plasmon dispersion relations in solid and liquid Rb were determined by inelastic x-ray scattering techniques. In liquid Rb, the plasmon energy increases with the momentum transfer $q$, whereas in solid Rb, the dispersion curve exhibits a cusplike shape. We also found that the plasmon linewidth in liquid Rb is narrower than that in solid Rb near $q=0$. These features suggest that the conduction electrons in liquid Rb are more suitably described by the electron gas model than in solid Rb, since the electron gas model predicts a smooth increase in the plasmon energy with $q$ and the infinite lifetime at $q=0$. The origins of the observed variations in the plasmon energy and the linewidth upon melting are investigated with the aid of theoretical calculations.

Electrolytes with high ionic diffusivity at temperatures distinctively lower than the presently used ones are the prerequisite for the success of, e.g., solid oxide fuel cells. We have found a promising structure having an asymmetric but superior ionic mobility in the direction of the oxygen-ion current. Using a layering of zirconium and yttrium in the fluorite structure of zirconia, a high vacancy concentration and a low migration barrier in two dimensions are obtained, while the mobility in the third direction is basically sacrificed. According to our density functional theory calculations an electrolyte made of this structure could operate at a temperature reduced by $\approx$${200}^{\circ }$C. Thus a window to a different class of electrolytes has been flung open. In our structure the price paid is a more complicated manufacturing method.

In this paper, we investigate the electric field effect on epitaxial Pr${}_{0.65}$(Ca${}_{0.75}$Sr${}_{0.25}$)${}_{0.35}$MnO${}_3$ thin films in electric double-layer transistors. Different from the conventional transistors with semiconducting channels, the sub(micrometer)-scale phase separation in the manganite channels is expected to result in inhomogeneous distribution of mobile carriers and local enhancement of electric field. The field effect is much larger in the low-temperature phase separation region compared to that in the high-temperature polaron transport region. Further enhancement of electroresistance is achieved by applying a magnetic field, and a 250% modulation of resistance is observed at 80 K, equivalent to an increase of the ferromagnetic metallic phase fraction by 0.51%, as estimated by the general effective medium model. Our results illustrate the complementary nature of electric and magnetic field effects in phase-separated manganites, providing insights on such novel electronic devices based on complex oxides.

Results of magnetic field and temperature-dependent neutron-diffraction and magnetization measurements on oxy-arsenate Rb${}_2$Fe${}_2$O(AsO${}_4$)${}_2$ are reported. The crystal structure of this compound contains pseudo-one-dimensional [Fe${}_2$O${}_6$]${}^{\infty }$ sawtoothlike chains, formed by corner-sharing isosceles triangles of Fe${}^{3+}$ ions occupying two nonequivalent crystallographic sites. The chains extend infinitely along the crystallographic $b$ axis and are structurally confined from one another via diamagnetic (AsO${}_4$)${}^{3-}$ units along the $a$ axis and Rb${}^{+}$ cations along the $c$-axis direction. Neutron-diffraction measurements indicate the onset of a long-range antiferromagnetic order below approximately 25 K. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other. Within each chain, one of the two Fe sites carries a moment which lies along the $b$ axis, while the second site bears a canted moment in the opposite direction. Externally applied magnetic field induces a transition to a ferrimagnetic state, in which the coupling between the sawtooth chains becomes ferromagnetic. Magnetization measurements performed on optically aligned single crystals reveal evidence for an uncompensated magnetization at low magnetic fields that could emerge from a phase-segregated state with ferrimagnetic inclusions or from antiferromagnetic domain walls. The observed magnetic states and the competition between them are expected to arise from strongly frustrated interactions within the sawtooth chains and relatively weak coupling between them.