A comprehensive study of the total energy of manganese-rich Heusler compounds using density functional theory is presented. Starting from a large set of cubic parent systems, the response to tetragonal distortions is studied in detail. We single out the systems that remain cubic from those that most likely become tetragonal. The driving force of the tetragonal distortion and its effect on the magnetic properties, especially where they deviate from the Slater-Pauling rule, as well as the trends in the Curie temperatures, are highlighted. By means of partial densities of states, the electronic structural changes reveal the microscopic origin of the observed trends. We focus our attention on the magnetocrystalline anisotropy and find astonishingly high values for tetragonal Heusler compounds containing heavy transition metals accompanied by low magnetic moments, which indicates that these materials are promising candidates for spin-transfer-torque magnetization-switching applications.

We provide evidence for the mapping of critical spin-1 chains, in particular the $\mathrm{SU}\left(3\right)$ symmetric bilinear-biquadratic model with additional interactions, to free boson theories using exact diagonalization and the density-matrix renormalization-group algorithm. Using the correspondence with a conformal field theory with central charge $c=2$, we determine the analytic formulas for the scaling dimensions in terms of four Tomonaga-Luttinger liquid parameters. By matching the lowest scaling dimensions, we numerically calculate these field-theoretic parameters and track their evolution as a function of the parameters of the lattice model.

We investigate the quantum phase transitions of the extended Hubbard model at half-filling with periodic boundary conditions employing the entanglement of particles, as opposed to the more traditional entanglement of modes. Our results show that the entanglement has either discontinuities or local minima at the critical points. We associate the discontinuities to first-order transitions, and the minima to second-order ones. Thus we show that the entanglement of particles can be used to derive the phase diagram, except for the subtle transitions between the spin-density wave (SDW) and bond-order wave (BOW) phases and the triplet superconducting (TS) and singlet superconducting (SS) phases.

Two-dimensional stanene is a promising candidate material for realizing a room-temperature quantum spin Hall (QSH) effect. Monolayer stanene has recently been fabricated by molecular beam epitaxy, but shows metallic features on a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111) substrate, which motivates us to study the important influence of the substrate. Based on first-principles calculations, we find that varying substrate conditions considerably tunes the electronic properties of stanene. The supported stanene gives either trivial or QSH states, with significant Rashba splitting induced by inversion asymmetry. More importantly, large-gap (up to 0.3 eV) QSH states are realizable when growing stanene on various substrates, like the anion-terminated (111) surfaces of SrTe, PbTe, BaSe, and BaTe. These findings provide significant guidance for future research of stanene and large-gap QSH states.

Optical-pump terahertz-probe spectroscopy is used to investigate ultrafast far-infrared conductivity dynamics during the insulator-to-metal transition in vanadium sesquioxide $\left({\mathrm{V}}_2{\mathrm{O}}_3\right)$. The resultant conductivity increase occurs on a tens of picosecond time scale, exhibiting a strong dependence on the initial temperature and fluence. We have identified a scaling of the conductivity dynamics upon renormalizing the time axis with a simple power law $\left(\alpha \simeq 1/2\right)$ that depends solely on the initial, final, and conductivity onset temperatures. Qualitative and quantitative considerations indicate that the dynamics arise from nucleation and growth of the metallic phase which can be described by the Avrami model. We show that the temporal scaling arises from spatial scaling of the growth of the metallic volume fraction, highlighting the self-similar nature of the dynamics. Our results illustrate the important role played by mesoscopic effects in phase transition dynamics.

We investigate the entanglement properties of a standard circuit-QED setup that consists of a Cooper pair box coupled to a long chain of Josephson junctions. We calculate the static charge polarization at finite distances along the device. Our calculations reveal a deep connection to the Kondo screening cloud, together with robust correlations that are difficult to measure in a condensed matter context. We also find weak sensitivity of these Kondo signatures to the actual parameters and design of the device, demonstrating the universality of the Josephson entanglement cloud.

We investigate the interplay of the electron-electron and electron-phonon interactions in the electronic structure of an exotic insulating state in the layered dichalcogenide $1T\text{−}{\mathrm{TaS}}_2$, where the charge-density-wave (CDW) order coexists with a Mott correlation gap. Scanning tunneling microscopy and spectroscopy measurements with high spatial and energy resolution determine unambiguously the CDW and the Mott gap as 0.20–0.24 eV and 0.32 eV, respectively, through the real space electron phases measured across the multiply formed energy gaps. An unusual local reduction of the Mott gap is observed on the defect site, which indicates the renormalization of the on-site Coulomb interaction by the electron-phonon coupling as predicted by the Hubbard-Holstein model. The Mott-gap renormalization provides insight into the disorder-induced quasimetallic phases of $1T\text{−}{\mathrm{TaS}}_2$.

We provide a tight-binding model parametrization for black phosphorus (BP) with an arbitrary number of layers. The model is derived from partially self-consistent $G{W}_0$ approach, where the screened Coulomb interaction $W_0$ is calculated within the random phase approximation on the basis of density functional theory. We thoroughly validate the model by performing a series of benchmark calculations, and determine the limits of its applicability. The application of the model to the calculations of electronic and optical properties of multilayer BP demonstrates good quantitative agreement with ab initio results in a wide energy range. We also show that the proposed model can be easily extended for the case of external fields, yielding the results consistent with those obtained from first principles. The model is expected to be suitable for a variety of realistic problems related to the electronic properties of multilayer BP including different kinds of disorder, external fields, and many-body effects.

We report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ (with $x=0.32$). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripelike domains, followed by the formation of magnetic bubbles due to a field- and temperature-dependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.

We study propagation of the Gaussian beam of spin waves and its reflection from the edge of thin yttrium-iron-garnet film with in-plane magnetization perpendicular to this edge. We have performed micromagnetic simulations supported by analytical calculations to investigate the influence of the surface magnetic anisotropy present at the film edge on the reflection, especially in the context of the Goos-Hänchen effect. We have shown the appearance of a negative lateral shift between reflected and incident spin wave beams' spots. This shift is particularly sensitive to the surface magnetic anisotropy value and is a result of the Goos-Hänchen shift which is sensitive to the magnitude of the anisotropy and of the bending of the spin wave beam. We have demonstrated that the demagnetizing field provided a graded increase of the refractive index for spin waves, which is responsible for the bending.

We present very low-temperature scanning tunneling microscopy (STM) experiments on single-crystalline samples of the superconductor $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$. We find a single superconducting gap from the zero-field tunneling conductance. However, the magnetic field dependence of the intervortex tunneling conductance is higher than the one expected in a single-gap superconductor. Such an increase in the intervortex tunneling conductance has been found in superconductors with multiple superconducting gaps. We also find that the hexagonal vortex lattice is locked to the square atomic lattice as expected in crystalline superconductors with anisotropic Fermi surfaces. Moreover, we compare the upper critical field $H_{c2}\left(T\right)$ obtained in our sample with previous measurements and find that $H_{c2}\left(T\right)$ does not increase by reducing the mean free path. We fit $H_{c2}\left(T\right)$ and show that multiband Fermi surface is needed to explain the observed behavior. We propose that $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$ is a single-gap multiband superconductor. We anticipate that single-gap superconductivity might often occur in compounds with anisotropic multiband Fermi surfaces.

The interplay of the tunneling transfer of charges and the emission and absorption of light can be investigated in a setup, where a voltage-biased Josephson junction is connected in series with a microwave cavity. We focus here on the emission processes of photons and analyze the underlying time-dependent statistics using the second-order correlation function $g^{\left(2\right)}\left(\tau \right)$ and the waiting-time distribution $w\left(\tau \right)$. Both observables highlight the crossover from a coherent light source to a single-photon source. Due to the nonlinearity of the Josephson junction, tunneling Cooper pairs can create a great variety of nonclassical states of light even at weak driving. Analytical results for the weak driving as well as the classical regime are complemented by a numerical treatment for the full nonlinear case. We also address the question of possible relations between $g^{\left(2\right)}\left(\tau \right)$ and $w\left(\tau \right)$ as well as the specific information which is provided by each of them.

We use nuclear magnetic resonance to map the complete low-temperature phase diagram of the antiferromagnetic Ising-like spin-chain system ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ as a function of the magnetic field applied along the chains. In contrast to the predicted crossover from the longitudinal incommensurate to the transverse antiferromagnetic ordered phase, we find a sequence of three magnetically ordered phases between the critical fields $3.8$ and $22.8 \mathrm{T}$. Their origin is traced to the giant magnetic field dependence of the total effective coupling between spin chains, found to vary by a factor of 24. We explain this interesting phenomenon as emerging from a combination of nontrivially coupled spin chains and incommensurate spin fluctuations in the chains treated as Tomonaga-Luttinger liquids.

We discover that the multiple degrees of freedom associated with magnetic skyrmions—size, position, and helicity—can all be used to control the Josephson effect and $0-\pi$ transitions occurring in superconductor/magnetic skyrmion/superconductor junctions. In the presence of two skyrmions, the Josephson effect depends strongly on their relative helicity and leads to the possibility of a helicity-transistor effect for the supercurrent, where the critical current is changed by several orders of magnitude simply by reversing the helicity of a magnetic skyrmion. Moreover, we demonstrate that the Fraunhofer pattern can show a local minimum at zero flux as a direct result of the skyrmion magnetic texture. These findings demonstrate the rich physics that emerges when combining topological magnetic objects with superconductors and could lead to different perspectives in superconducting spintronics.

We investigate the disorder dependence of the static density, amplitude, and current correlations within the attractive Hubbard model supplemented with onsite disorder. It is found that strong disorder favors a decoupling of density and amplitude correlations due to the formation of superconducting (SC) islands. This emergent granularity also induces an enhancement of the density correlations on the SC islands whereas amplitude fluctuations are most pronounced in the “insulating” regions. While density and amplitude correlations are short ranged at strong disorder, we show that current correlations have a long-range tail due to the formation of percolative current paths in agreement with the constant behavior expected from the analysis of one-dimensional models.

We study the patterns at which the current flow stabilizes in a finite 1D superconducting wire, for various experimentally reasonable boundary conditions, for small fixed current densities and temperatures close to $T_c$. We pay special attention to the possible existence of a stationary regime. If the contacts are superconducting, truly stationary or normal regimes do not exist, but can be approached as a limit. In the case of weak superconducting contacts, a rich phase diagram is found, with several periodic regimes that involve two phase slip centers. There may be a time lag between the phase slips at each of these centers. There is a small parameter region in which this time lag is continuously tuned by the parameters. For some of these regimes, the density of Cooper pairs does not have mirror symmetry. If the contacts are normal, the stationary regime is possible. Analysis according to the Kramer–Watts-Tobin formalism leads to qualitatively the same results as the time-dependent Ginzburg-Landau model.

Owing to their strong interaction with carriers, early $3d\text{-row}$ (Ti, V, and Cr) and $4d\text{-row}$ (Zr, Nb, and Mo) transition metals (TMs) are undesired contaminants in solar- and electronic-grade Si. The increasing stringent control of contamination levels is urging an accurate picture of their electronic structure. In the present work, the electrical levels and migration energies of these TMs are determined by means of standard density-functional theory (DFT) and a rotationally invariant formulation of DFT+$U$. The latter approach improves on the treatment of electronic correlations at the TM sites and relies on on-site Hubbard Coulomb and Hund's exchange parameters $U$ and $J$, respectively. These are calculated self-consistently from linear-response theory without fitting to experimental data. The effect of correlation was found more pronounced for Ti and V, with a strong impact on the location of their electrical levels. In most cases, the agreement with the experimental data is satisfactory allowing the identification of the type and character of the levels. For Cr and Mo in particular, the results resolve longstanding controversies concerning the type and position of the levels. The obtained migration barriers display moderate charge-state and correlation dependency. High barriers were found for all metals studied, with the exception of Cr, confirming them as slow diffusers in silicon among the whole TM family.

The evolution of the electronic properties of electron-doped $({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{)}_2{\mathrm{IrO}}_4$ is experimentally explored as the doping limit of La is approached. As electrons are introduced, the electronic ground-state transitions from a spin-orbit Mott phase into an electronically phase separated state, where long-range magnetic order vanishes beyond $x=0.02$ and charge transport remains percolative up to the limit of La substitution ($x\approx 0.06$). In particular, the electronic ground state remains inhomogeneous even beyond the collapse of the parent state's long-range antiferromagnetic order, while persistent short-range magnetism survives up to the highest La-substitution levels. Furthermore, as electrons are doped into ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, we observe the appearance of a low-temperature magnetic glasslike state intermediate to the complete suppression of antiferromagnetic order. Universalities and differences in the electron-doped phase diagrams of single-layer and bilayer Ruddlesden-Popper strontium iridates are discussed.

The construction of a multichannel vector spin polarimeter requires the development of a detector type, which works as a spin polarizing mirror with high reflectivity and asymmetry properties to guarantee for a high figure of merit. Technical realizations are found by spin-polarized electron scattering from a surface at low energies. A very promising candidate for such a detector suitable material consists of an oxygen passivated iron surface, as for example a Fe(001)-$p(1\times 1)\mathrm{O}$ surface. We investigate in detail the electronic structure of this adsorbate system and calculate the corresponding spin-polarized low-energy electron scattering. Our theoretical study is based on the fully relativistic SPRKKR method in the framework of density functional theory. Furthermore, we use the local spin-density approximation in combination with dynamical mean field theory to determine the electronic structure of Fe(001)-$p(1\times 1)\mathrm{O}$ and demonstrate that a significant impact of correlation effects occurs in the calculated figure of merit.