Spin wave modes confined in a ferromagnetic film by the spatially inhomogeneous magnetic field generated by a scanned micromagnetic tip of a ferromagnetic resonance force microscope (FMRFM) enable microscopic imaging of the internal fields and spin dynamics in nanoscale magnetic devices. Here we report a detailed study of spin wave modes in a thin ferromagnetic film localized by magnetic field configurations frequently encountered in FMRFM experiments, including geometries in which the probe magnetic moment is both parallel and antiparallel to the applied uniform magnetic field. We demonstrate that characteristics of the localized modes, such as resonance field and confinement radius, can be broadly tuned by controlling the orientation of the applied field relative to the film plane. Micromagnetic simulations accurately reproduce our FMRFM spectra allowing quantitative understanding of the localized modes. Our results reveal a general method of generating tightly confined spin wave modes in various geometries with excellent spatial resolution that significantly facilitates the broad application of FMRFM. This paves the way to imaging of magnetic properties and spin wave dynamics in a variety of contexts for uncovering new physics of nanoscale spin excitations.

We have studied local magnetic moment and electronic phase separation in superconducting ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ by x-ray emission and absorption spectroscopy. Detailed temperature-dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature ($T_c$) and the coexisting phases exchange spectral weights with the low-spin state, gaining intensity at the expense of the higher-spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at $T_c$ as well as superconductivity disappear. The results clearly underline that the coexistence of the low-spin and high-spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of the electronically inhomogeneous ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ system.

The pressure effect on the newly discovered charge-transferred ${\mathrm{BiS}}_2$-based superconductor, ${\mathrm{Eu}}_3{\mathrm{Bi}}_2{\mathrm{S}}_4{\mathrm{F}}_4$, with a $T_c$ of 1.5 K at ambient pressure, is investigated by transport and magnetic measurements. Accompanied with the enhancement of metallicity under pressures, the onset superconducting transition temperature increases abruptly around 1.0 GPa, reaching $\sim 10.0$ K at 2.26 GPa. Alternating current magnetic susceptibility measurements indicate that a new superconducting phase with a higher $T_c$ emerges and dominates at high pressures. In the broad pressure window of $0.68\text{GPa}\le p\le 2.00$ GPa, the high-$T_c$ phase coexists with the low-$T_c$ phase. Hall effect measurements reveal a significant difference in electronic structures between the two superconducting phases. Our work devotes the effort to establish the commonality of pressure effect on the ${\mathrm{BiS}}_2$-based superconductors, and also uncovers the importance of electron carrier density in the high-$T_c$ phase.

We present an extensive microscopic magnetic study of the quasi-one-dimensional frustrated $s=\frac{1}{2}$ spin-chain compound linarite. Angular-dependent susceptibility and ${}^1\mathrm{H}$-NMR measurements were performed at various temperatures in the paramagnetic state. All relevant NMR parameters, viz., the chemical, the dipolar, and the Fermi-contact contribution, were extracted to analytically calculate the local magnetic fields at the ${}^1\mathrm{H}$ sites. From this analysis, a significant spin transfer of $\sim 11$% from the magnetic copper ions onto the two oxygen ligands was derived. This result is compared to LSDA+$U$ calculations, which further can explain the significant reduction of the coupling constants of the compound compared to other edge-sharing CuO chains due to the presence of specific OH bonds in linarite. The magnetically ordered spin-spiral ground state for $T<2.8$ K was investigated by ${}^1\mathrm{H}$-NMR for fields ${\mu }_{0}H=2.0$ T along the three crystallographic main directions. With the derived NMR parameters in the paramagnetic regime it is possible to model the NMR spectrum of the magnetically ordered state for $H\parallel c$. A deviation observed between the NMR data and the evaluated spectrum for $H\parallel b$ is discussed in terms of a significant rearrangement of the spiral state in an applied field of ${\mu }_{0}H=2.0$ T for this direction.

Understanding the origin of the spin transition in ${\mathrm{LaCoO}}_3$ is one of the long-standing aims in condensed matter physics. Aside from its fundamental interest, a detailed description of this crossover will have a direct impact on the interpretation of the semiconductor-to-metal transition (SMT) and the properties of the high-temperature metallic phase in this compound, which has shown to have important applications in environmentally friendly energy production. To date, the spin transition has been investigated mainly as a function of temperature in thermal equilibrium. These results have hinted at dynamical effects. In this paper, we have investigated the SMT by means of pump-probe soft x-ray reflectivity experiments at the O $K$, Co $L$, and La $M$ edges and theoretical calculations within a ${\mathrm{DFT}}^{++}$ formalism. The results point towards a laser-induced metallization in which the optical transitions stabilize a metallic state with high-spin configuration and increased charge disproportionation.

We study the Mott insulating state of the half-filled paramagnetic Hubbard model within dynamical mean field theory using a recently formulated stochastic and nonperturbative quantum impurity solver. The method is based on calculating the impurity self energy as a sample average over a representative distribution of impurity models solved by exact diagonalization. Due to the natural parallelization of the method, millions of poles are readily generated for the self energy which allows us to work with very small pole-broadening $\eta$. Solutions at small and large $\eta$ are qualitatively different; solutions at large $\eta$ show featureless Hubbard bands whereas solutions at small $\eta \le 0.001$ (in units of half bare band width) show a band of electronic quasiparticles with very small quasiparticle weight at the inner edge of the Hubbard bands. The validity of the results are supported by agreement within statistical error ${\sigma }_{\text{QMC}}\sim {10}^{-4}$ on the imaginary frequency axis with calculations using a continuous time quantum Monte Carlo solver. Nevertheless, convergence with respect to finite size of the stochastic exact diagonalization solver remains to be rigourously established.

Resonant scattering at the atomic absorbates in graphene was investigated recently in relation with the transport and gap opening problems. Attaching an impurity atom to graphene is believed to lead to the creation of unusual zero-energy localized electron states. This paper aims to describe the behavior of the localized impurity-induced levels in graphene in a quantizing magnetic field. It is shown that in the magnetic field the impurity level effectively hybridizes with one of the $n=0$ Landau level states and splits into two opposite-energy states. The new hybridized state is doubly occupied, forming a spin singlet and reducing the polarization of a quantum Hall ferromagnet in undoped graphene. Taking into account the electron-electron interaction changes radically the spectrum of the electrons surrounding the impurity, which should be seen experimentally. While existing publications investigate graphene uniformly covered by adatoms, here we address a possibly even more experimentally relevant case of the clusterized impurity distribution. The limit of a dense bunch of the impurity atoms is considered, and it is shown how such a bunch changes the spectrum and spin polarization of a large dense electron droplet surrounding it. The droplet is encircled by an edge state carrying a persistent current.

Absence of backscattering and occurrence of weak antilocalization are two characteristic features of topological insulators. We find that the introduction of defects results in the appearance of a negative contribution to magnetoresistance (MR) in the topological insulator $\mathrm{BiSbTeS}{\mathrm{e}}_2$ at temperatures below 50 K. Our analysis shows that the negative MR originates from an increase in the density of defect states created by the introduction of disorder, which leaves the surface states unaffected. We find a decrease in the magnitude of the negative MR contribution with increasing temperature and a robustness of the topological surface states to external disorder.

We present analytical and numerical studies of the behavior of the $\alpha$-Renyi entropies in the toric code in presence of several types of perturbations aimed at studying the simulability of these perturbations to the parent Hamiltonian using local operations and classical communications (LOCC)—a property called local convertibility. In particular, the derivatives, with respect to the perturbation parameter, present different signs for different values of $\alpha$ within the topological phase. From the information-theoretic point of view, this means that such ground states cannot be continuously deformed within the topological phase by means of catalyst assisted local operations and classical communications (LOCC). Such LOCC differential convertibility is on the other hand always possible in the trivial disordered phase. The non-LOCC convertibility is remarkable because it can be computed on a system whose size is independent of correlation length. This method can therefore constitute an experimentally feasible witness of topological order.

We study the hot-electron noise properties of two-dimensional materials with a graphene-like energy dispersion under a strong applied electric field which drives the system far from equilibrium. Calculations are based on a Boltzmann–Green-function method within a two-relaxation-time approximation that allows for both inelastic scattering coming from electron-phonon scattering and elastic scattering coming from electron-impurity scattering. The steady-state distribution function is used to calculate the average current and the low-frequency spectral density for current fluctuations (noise) in the nonequilibrium steady-state. We find that as the electric field strength increases, the noise decreases from its equilibrium thermal noise value. This is in contrast with semiconductors with a quadratic energy–wave-vector dispersion where the noise increases in a constant-relaxation-time model with the square of the electric field due to the Joule heating of the electron gas by the electric field. We have also studied these properties for an electronic dispersion with a gap introduced into the Dirac spectrum. The inclusion of the gap in the electronic dispersion causes an initial increase in the noise as a function of external electric field due to the heating of the electron gas for large gap values. At high electric fields, the noise decreases with increasing electric field as in the case of gapless dispersion at higher fields.

The ac conductance of a finite tubular two-dimensional electron gas is studied in the presence of the Rashba spin-orbit interaction. When the tube is coupled to two reservoirs, that interaction splits the steps in the dc current, introducing energy ranges with spin-polarized currents. For this setup, we calculate the current-current correlations (the noise spectrum) and show that the existence of these dc spin-polarized currents can be deduced from the shot noise. We also find that the Wigner-Smith time delay is almost unaffected by the spin-orbit interaction. When the tube is coupled to a single reservoir, we calculate the quantum capacitance and the charge-relaxation resistance, and find that they exhibit singularities near the openings of new channels.

An outstanding and fundamental problem in contemporary physics is to include and probe the many-body effect in the study of relativistic quantum manifestations of classical chaos. We address this problem using graphene systems described by the Hubbard Hamiltonian in the setting of resonant tunneling. Such a system consists of two symmetric potential wells separated by a potential barrier, and the geometric shape of the whole domain can be chosen to generate integrable or chaotic dynamics in the classical limit. Employing a standard mean-field approach to calculating a large number of eigenenergies and eigenstates, we uncover a class of localized states with near-zero tunneling in the integrable systems. These states are not the edge states typically seen in graphene systems, and as such they are the consequence of many-body interactions. The physical origin of the non-edge-state type of localized states can be understood by the one-dimensional relativistic quantum tunneling dynamics through the solutions of the Dirac equation with appropriate boundary conditions. We demonstrate that, when the geometry of the system is modified to one with chaos, the localized states are effectively removed, implying that in realistic situations where many-body interactions are present, classical chaos is capable of facilitating greatly quantum tunneling. This result, besides its fundamental importance, can be useful for the development of nanoscale devices such as graphene-based resonant-tunneling diodes.

On the basis of coupled Ginzburg-Landau equations we study nonhomogeneous states in systems with two order parameters (OPs). Superconductors with a superconducting OP $\Delta$ and a charge- or spin-density wave with amplitude $W$ are examples of such systems. When one OP, say $\Delta$, has a form of a topological defect, like, e.g., a vortex or domain wall between the domains with the phases 0 and $\pi$, the other OP $W$ is determined by the Gross-Pitaevskii equation and is localized at the center of the defect. We consider in detail the domain-wall defect for $\Delta$ and show that the shape of the associated solution for $W$ depends on temperature and doping (or on the curvature of the Fermi surface) $\mu$. It turns out that, provided the temperature or doping level is close to some discrete values $T_n$ and ${\mu }_n$, the spatial dependence of the function $W\left(x\right)$ is determined by the form of the eigenfunctions of the linearized Gross-Pitaevskii equation. The spatial dependence of $W_0$ corresponding to the ground state has the form of a soliton, while other possible solutions $W_n\left(x\right)$ have nodes. The inverse situation when $W\left(x\right)$ has the form of a topological defect and $\Delta \left(x\right)$ is localized at the center of this defect is also possible. In particular, we predict a surface or interfacial superconductivity in a system where a superconductor is in contact with a material that suppresses $W$. This superconductivity should have rather unusual temperature dependence existing only in certain intervals of temperature. Possible experimental realizations of such nonhomogeneous states of OPs are discussed.

One-way absorption can be obtained at terahertz frequencies in low-profile rod-type and multilayer dielectric structures with broken spatial inversion symmetry, which contain either a rod layer or an ultrathin homogeneous layer made of a polar dielectric. Perfect absorption for one of the two opposite incidence directions and perfect reflection for the other one are observed at the edge of the polaritonic gap in a wide range of the incident angle variation, when the thickness of the entire structure is of the order of the incident wavelength. Moreover, this regime appears in a wide frequency range, in which the forward-to-backward absorption contrast is strong. The exploited mechanism is connected with the parameter adjustment that enables the location of the polaritonic gap of the polar dielectric, of which the lossy part of the structure is made, inside the stop band arising due to the periodicity of the lossless part of the structure that is made of a nondispersive dielectric. It also exploits absorption enhancement in the lossy part by backing it with the highly reflecting lossless part, which has alternating stop and pass bands.

We investigate the response of passive lossless Huygens metasurfaces (HMS), designed to implement a prescribed refraction, to arbitrary plane-wave excitations. Applying a rigorous Floquet-Bloch (FB) analysis of the cotangent electric and magnetic surface reactance modulations, we discover that the scattered FB modes follow an extraordinary ray-optical model. According to this model, the HMS can be replaced by a virtual region of constant wave impedance; incident power is incoupled into this virtual Fabry-Pérot etalon, gradually leaking to successively higher-order FB modes. Analytical closed-form expressions are derived for the FB coefficients, indicating that the first FB mode in transmission is always dominant; this clarifies previous experimental findings which implied that the HMS retained its functionality over a broad angular range. Furthermore, we point out the implementation problems inherent in the cotangent function and offer a simple, realizable form of the reactive modulation which does not significantly deteriorate the HMS performance. These results provide fundamental physical insight into the working mechanism of passive lossless Huygens metasurfaces in general, revealing a new degree of freedom which enables control of the transmitted field amplitude: the virtual region wave impedance. Importantly, the implied broad acceptance angle of the HMS may be harnessed for synthesis of low-profile multifunctional devices, designed to respond to several excitations with reduced reflections.

A local structure formed by dilute In ions doped in ZnO was investigated by means of a nuclear spectroscopic technique and density functional theory (DFT) calculations. Comparative studies on a presumably isomorphous $\mathrm{CdI}{\mathrm{n}}_2{\mathrm{O}}_4$ unveiled the local structure: the impurity In ions form in ZnO a spinel $\mathrm{ZnI}{\mathrm{n}}_2{\mathrm{O}}_4$, which has been only a hypothetical binary oxide so far. The most stable structure of the spinel was determined by DFT calculations, and the hyperfine interaction parameters obtained for the structure show excellent agreement with the experimental values. The experimental synthesis and detection of the normal spinel $\mathrm{ZnI}{\mathrm{n}}_2{\mathrm{O}}_4$ are presented.

Using field effect devices with side gates, we modulate the 2-dimensional electron gas hosted at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface to study the temperature and doping evolution of the magnetotransport. The analysis of the data reveals different transport regimes depending on the interplay between the different (elastic, inelastic, and spin-orbit) scattering times and their temperature dependencies. We find that the spin-orbit interaction strongly affects the low-temperature transport in the normal state in a very large region of the phase diagram, extending beyond the superconducting dome.

Edge states are ubiquitous for many condensed matter systems with multicomponent wave functions. For example, edge states play a crucial role in transport in zigzag graphene nanoribbons. Here, we report microscopic calculations of quantum plasmonics in doped graphene nanodisks with zigzag edges. We express the nanodisk conductivity $\sigma \left(\omega \right)$ as a sum of the conventional bulk conductivity ${\sigma }_{\text{b}}\left(\omega \right)$, and a novel term ${\sigma }_{\text{e}}\left(\omega \right)$, corresponding to a coupling between the edge and bulk states. We show that the edge states give rise to a redshift and broadening of the plasmon resonance, and that they often significantly impact the absorption efficiency. We further develop simplified models, incorporating nonlocal response within a hydrodynamical approach, which allow a semiquantitative description of plasmonics in the ultrasmall size regime. Furthermore, we show that the effect of hydrodynamic and edge-conductivity corrections scale identically, approximately with the inverse of the disk radius, highlighting their equatable importance. However, the polarization dependence is only given by fully microscopic models. The approach developed here should have many applications in other systems supporting edge states.