To elucidate charge and orbital order below the Verwey transition temperature $T_{\mathrm{V}}\sim 125$ K, a thin layer of magnetite partially detwined by growth on the stepped MgO(001) substrate has been studied by means of soft x-ray diffraction at the Fe $L_{2,3}$ resonance. The azimuth angle, incident photon polarization, and energy dependence of the ${\left(00\frac{1}{2}\right)}_c$ and ${\left(001\right)}_c$ reflection intensities have been measured, and analyzed using a configuration-interaction FeO${}_6$ cluster model. The azimuth dependence of the ${\left(00\frac{1}{2}\right)}_c$ reflection intensities directly represents the space-group symmetry of the orbital order in the initial state rather than indirectly through the intermediate-state level shifts caused by the order-induced lattice distortions. From the analysis of the ${\left(00\frac{1}{2}\right)}_c$ reflection intensities, the orbital order in the $t_{2g}$ orbitals of $B$ sites below $T_{\mathrm{V}}$ is proved to have a large monoclinic deformation with the value of $\mathrm{Re}\left[F_{xy}\right]/\mathrm{Re}\left[F_{yz}\right]\sim 2$. This finding contradicts the majority of theories on the Verwey transition so far proposed. We show that the experimentally observed resonance spectra cannot be explained by orbital and charge orders obtained with recent LDA+$U$ and GGA+$U$ band structure calculations but by a complex-number orbital order with excellent agreement.

The electronic excitations of Ca-doped picene films have been investigated using electron energy-loss spectroscopy in transmission. We demonstrate that a composition of Ca${}_3$picene can be achieved by evaporation of Ca onto pristine picene films under ultrahigh vacuum conditions. The core level excitations indicate a charge transfer of one electron per added Ca to the picene molecules and a hybridization of Ca and picene derived electronic states. Upon doping a new spectral feature is observed at about 2 eV in the electronic excitation spectrum. This new feature does not disperse which signals its localized character. The data analysis using a Kramers-Kronig transformation suggests that Ca${}_3$picene has a nonmetallic ground state.

The concepts of many optical devices are based on fundamental physical phenomena such as resonances. One of the commonly used devices is an electromagnetic antenna that converts localized energy into freely propagating radiation and vise versa, offering unique capabilities for controlling electromagnetic radiation. Here we propose a concept for controlling the intensity and directionality of electromagnetic wave scattering in radio-frequency and optical antennas based on the physics of Fano resonances. We develop an analytical theory of spatial Fano resonances in antennas that describes switching of the radiation pattern between the forward and backward directions, and we confirm our theory with both numerical calculations and microwave experiments. Our approach bridges the concepts of conventional radio antennas and photonic nanoantennas, and it provides a paradigm for the design of wireless optical devices with various functionalities and architectures.

Topological insulators are currently of considerable interest due to peculiar electronic properties originating from helical states on their surfaces. Here we demonstrate that the sound excited by helical particles on surfaces of topological insulators has several exotic properties fundamentally different from sound propagating in nonhelical or even isotropic helical systems. Specifically, the sound may have strictly forward propagation absent for isotropic helical states. Its dependence on the anisotropy of the realistic surface states is of distinguished behavior which may be used as an alternative experimental tool to measure the anisotropy strength. Fascinating from the fundamental point of view backward, or anomalous, Cherenkov sound is excited above the critical angle $\pi /2$ when the anisotropy exceeds a critical value. Strikingly, at strong anisotropy the sound localizes into a few forward and backward beams propagating along specific directions.

We demonstrate a heterogeneous photonic metastructure involving monolayer graphene achieving a substantial Faraday rotation angle and nearly perfect transmittance simultaneously. This breaks the longstanding tradeoff between the Faraday rotation angle and the transmission coefficient in conventional magneto-optical devices. Faraday rotation is enhanced across the interface between two photonic crystals due to the presence of an interface mode, i.e. optical Tamm state, which presents a strong electromagnetic field confinement at the location of the graphene sheet. This interface mode also helps to maintain a nearly perfect optical transmission through the structure thanks to a resonant tunneling mechanism. This exciting result was achieved particularly in the quantum regime with low Fermi level.

The low-temperature universal properties in disordered and amorphous solids are considered. We introduce a model that includes two types of two-level systems (TLSs), which, based on their local symmetry, interact weakly or strongly with the phonon field. This accounts well for the experimental results and addresses some long-standing questions: the nature of the TLSs, the smallness and universality of the phonon attenuation, and the energy scale of 3 K below which universality is observed. Our model describes disordered lattices; we also discuss its application to amorphous solids.

Below $T_C$, the single-crystal study of CaBaCo${}_4$O${}_7$, a noncollinear ferrimagnet ($T_C=64$ K) with a polar orthorhombic space group (Pbn2${}_1$) between 4 and 293 K, shows a large change in the electric polarization along its $\vec{c}$ axis reaching $\Delta P=17$ mC/m${}^2$ at 10 K. At 62.5 K, a magnetic-field-driven giant variation in polarization $P$(9 T) − $P$(0 T) $=$ 8 mC/m${}^2$ is observed. Moreover, the present magnetoelectric measurements are fully consistent with the $m$′$m$2′ magnetic point group. This ferrimagnetic oxide, which belongs to the “114” structural family, opens an avenue for the search for new magnetoelectrics.

We performed relativistic ultrafast electron diffraction (UED) measurements of the structural dynamics of photoexcited gold nanofilms and developed an atomistic model, based on the two-temperature molecular dynamics (2T-MD) method, which allows us to make a direct comparison of the time evolutions of measured and calculated Bragg peaks. The quantitative agreement between the temporal evolutions of the experimental and theoretical Bragg peaks at all fluences suggests that the 2T-MD method provides a faithful atomistic representation of the structural evolution of photoexcited gold films. The results reveal the transition between slow heterogeneous melting at low absorbed photon fluence to rapid homogeneous melting at higher fluence and nonthermally driven melting at very high fluence. At high laser fluence, the time evolution of Bragg peaks calculated using the conventional 2T-MD model disagrees with experiment. We show that using an interatomic potential that directly depends on the electron temperature delivers a much better agreement with UED data. Finally, our ab initio calculations of phonon spectra suggest electronic bond softening, if the nanofilms can expand freely under electronic pressure, and bond hardening, if they are constrained in all three dimensions.

Significant enhancement of the magnetocrystalline anisotropy (MCA) of an Fe(001) surface capped by 4$d$ and 5$d$ transition metal monolayers is presented in this study using first principles density functional calculations. In particular, an extremely large perpendicular MCA of $+$10 meV/Ir was found in Ir-capped Fe(001), which originates not from the Fe but from the large spin-orbit coupling of the Ir atoms. From the spin-channel decomposition of the MCA matrix and electronic structure analyses, we find that strong 3$d$–5$d$ band hybridization in the minority spin state is responsible for the sign changes of the MCA from parallel to perpendicular.

In an ideal bulk topological insulator (TI) conducting surface states protected by time-reversal symmetry enfold an insulating crystal. However, the archetypical TI, Bi${}_2$Se${}_3$, is actually never insulating; it is in fact a relatively good metal. Nevertheless, it is the most studied system among all the TIs, mainly due to its simple band structure and large spin-orbit gap. Recently, it was shown that copper intercalated Bi${}_2$Se${}_3$ becomes superconducting and it was suggested as a realization of a topological superconductor. Here we use a combination of techniques that are sensitive to the shape of the Fermi surface (FS): the Shubnikov-de Haas effect and angle-resolved photoemission spectroscopy to study the evolution of the FS shape with carrier concentration, $n$. We find that as $n$ increases, the FS becomes two-dimensional-like. These results are of crucial importance for understanding the superconducting properties of Cu${}_x$Bi${}_2$Se${}_3$.

The physics of nickel perovskites is rich with various competing electronic phases that can be tuned by chemical or external degrees of freedom. As such, nickelates show strong potential for oxide electronics devices based on strongly correlated systems. However, their complexity has hitherto challenged a detailed understanding of classical material engineering effects using, e.g., epitaxial strain. Here we investigate this important pending issue by comparing experimental data with results from first-principles calculations using the Heyd-Scuseria-Ernzerhof hybrid exchange-correlation functional. The theory properly describes the magnetic ground state as well as the preferred orbital occupation observed by x-ray linear dichroism. It also shows that the strain-induced modulation of the metal-to-insulator transition temperature is likely driven by changes in the bandwidth, rather than by the charge-transfer energy.

The excitation spectrum and the collective modes of graphene antidot lattices (GALs) are studied in the context of a $\pi$-band tight-binding model. The dynamical polarizability and dielectric function are calculated within the random-phase approximation. The effect of different kinds of disorder, such as geometric and chemical disorder, are included in our calculations. We highlight the main differences of GALs with respect to single-layer graphene (SLG). Our results show that, in addition to the well-understood bulk plasmon in doped samples, interband plasmons appear in GALs. We further show that the static screening properties of undoped and doped GALs quantitatively differ from SLG.

We analyze whether and how the neutron resonance mode in unconventional superconductors is affected by higher-order corrections in the coupling between spin excitations and fermionic quasiparticles and find that in general such corrections can not be ignored. In particular, we show that in two spatial dimensions ($d=2$) the corrections are of the same order as the leading, one-loop contributions, demonstrating that the neutron resonance mode in unconventional superconductors is a strong coupling phenomenon. The origin of this behavior lies in the quantum-critical nature of the low-energy spin dynamics in the superconducting state and the feedback of the resonance mode onto the fermionic excitations. While quantum-critical fluctuations occur in any dimensionality $d⩽3$, they can be analyzed in a controlled fashion by means of the $\varepsilon$ expansion ($\varepsilon =3-d$), such that the leading corrections to the resonance mode position are small. Regardless of the strong coupling nature of the resonance mode, we show that it emerges only if the phase of the superconducting gap function varies on the Fermi surface, making it a powerful tool to investigate the microscopic structure of the pair condensate.

The paper examines weak localization (WL) of surface states with a quadratic band crossing in topological crystalline insulators. It is shown that the topology of the quadratic band crossing point dictates the negative sign of the WL conductivity correction. For the surface states with broken time-reversal symmetry, an explicit dependence of the WL conductivity on the band Berry flux is obtained and analyzed for different carrier-density regimes and types of the band structure (normal or inverted). These results suggest a way to detect the band Berry flux through WL measurements.

A charge compensation mechanism is proposed for cation vacancy defects in complex oxides based on positron lifetime measurements, infrared spectroscopy, and composition analysis. Defects were characterized in samples of yttrium aluminum garnet grown in O${}_2$ or Ar. However, no positron trapping was detected in samples grown in H${}_2$. This is attributed to decoration of cation vacancies with hydrogen, thereby passivating charges of vacancies that otherwise function as positron traps. Infrared spectroscopy gave direct evidence of the presence of hydrogen. Passivation of cation vacancies with hydrogen is proposed as an important mechanism for charge compensation in the defect physics of oxides.

The structure of glassy GeS${}_2$ is studied in the framework of density functional theory, by using a fully self-consistent first-principles molecular dynamics (FPMD) scheme. A comparative analysis is performed with previous molecular dynamics data obtained within the Harris functional (HFMD) total energy approach. The calculated total neutron structure factor exhibits an unprecedented agreement with the experimental counterpart. In particular, the height of the first sharp diffraction peak (FSDP) improves considerably upon the HFMD results. Both the Ge and the S subnetworks are affected by a consistent number of miscoordinations, coexisting with the main tetrahedral structural motif. Glassy GeS${}_2$ features a short-range order quite similar to the one found in glassy GeSe${}_2$, a notable exception being the larger number of edge-sharing connections. An electronic structure localization analysis, based on the Wannier functions formalism, provides evidence of a more enhanced ionic character in glassy GeS${}_2$ when compared to glassy GeSe${}_2$.

We establish new scaling properties for the universality class of Model C, which describes relaxational critical dynamics of a nonconserved order parameter coupled to a conserved scalar density. We find an anomalous diffusion phase, which satisfies weak dynamic scaling while the conserved density diffuses only asymptotically. The properties of the phase diagram for the dynamic critical behavior include a significantly extended weak scaling region, together with a strong and a decoupled scaling regime. These calculations are done directly in $2\le d\le 4$ space dimensions within the framework of the nonperturbative functional renormalization group. The scaling exponents characterizing the different phases are determined along with subleading indices featuring the stability properties.

We have performed electron spin resonance (ESR) studies of K${}_4^{3+}$ and (K${}_3$Rb)${}^{3+}$ nanoclusters incorporated in powder specimens of aluminosilicate sodalite at several microwave frequencies between 9 and 34 GHz. The K${}_4^{3+}$ and (K${}_3$Rb)${}^{3+}$ clusters are arrayed in a bcc structure and are known to show antiferromagnetic ordering below the N$\acute{\mathrm{e}}$el temperatures of $T_{\mathrm{N}}$ $\simeq 72$ and $\simeq$80 K, respectively, due to the exchange coupling between $s$ electrons confined in the clusters. We have found sudden broadenings of ESR spectra in both samples below $T_{\mathrm{N}}$. The line shape of the spectra below $T_{\mathrm{N}}$ is analyzed by powder pattern simulations of antiferromagnetic resonance (AFMR) spectra. The calculated line shapes well reproduce the experimental ones at all the frequencies by assuming a biaxial magnetic anisotropy. We have evaluated extremely small anisotropy fields of approximately 1 Oe indicating that these materials are ideal Heisenberg antiferromagnets. We have also found that the magnetic anisotropy changes from easy-plane type to uniaxial type by changing into a heavier alkali-metal cluster and that the $g$ value shifts to a large value beyond two below $T_{\mathrm{N}}$ for K${}_4^{3+}$ and (K${}_3$Rb)${}^{3+}$ nanoclusters. These novel features of K${}_4^{3+}$ and (K${}_3$Rb)${}^{3+}$ nanoclusters incorporated in sodalite are discussed.

Bloch points are magnetic topological defects. The discrete nature of a magnetic lattice creates a periodic potential that can pin a Bloch point. The pinning force is of the order of the exchange constant, a few piconewtons in a typical ferromagnet (permalloy). A domain wall containing a Bloch point can have a sizable depinning field in the tens of oersteds.

We have investigated the magnetization properties and flux dynamics of superconducting Cu${}_x$TiSe${}_2$ single crystals within a wide range of copper concentrations. We find that the superconducting anisotropy is low and independent on copper concentration ($\gamma \sim 1.7$), except in the case of strongly underdoped samples ($x\le 0.06$) that show a gradual increase in anisotropy to $\gamma \sim 1.9$. The vortex phase diagram in this material is characterized by a broad region of vortex liquid phase that is unusual for such a low-$T_c$ superconductor with low anisotropy. Below the irreversibility line the vortex solid state supports relatively low critical current densities as compared to the depairing current limit ($J_c/J_0\sim {10}^{-7}$). All this points out that local fluctuations in copper concentration have little effect on bulk pinning properties in this system.