The performance of advanced magnetic tunnel junctions built of ferromagnetic (FM) electrodes and MgO as an insulating barrier depends decisively on the properties of the FM/insulator interface. Here we investigate interface formation between the Co-based Heusler compound ${\mathrm{Co}}_2\mathrm{MnSi}$ (CMS) and MgO by means of Auger electron spectroscopy, low-energy electron diffraction, and low-energy photoemission. The studies are performed for different annealing temperatures ($T_A$) and MgO layer coverages (4, 6, 10, 20, and 50 ML). Thin MgO top layers ($t_{\mathrm{MgO}}\le 10$ ML) show distinct surface crystalline distortions, which can only be partly healed out by annealing and, furthermore, lead to distinct adsorption of carbon species after the MgO surface is exposed to air. For $t_{\mathrm{MgO}}>10$ ML, the MgO layer surface exhibits clearly improved crystalline structure and hence only marginal amounts of adsorbates. We attribute these findings to MgO misfit dislocations occurring at the interface, inducing further defects throughout the MgO layer for up to at least 10 ML. Furthermore, spin-polarized photoemission spectra of the CMS/MgO interface are obtained for MgO coverages up to 20 ML, showing a clear positive spin polarization near the Fermi energy ($E_F$) in all cases.

The fabrication of high-quality thin superconducting films is essential for single-photon detectors. Their device performance is crucially affected by their material parameters, thus requiring reliable and nondestructive characterization methods after the fabrication and patterning processes. Important material parameters to know are the resistivity, superconducting transition temperature, relaxation time of quasiparticles, and uniformity of patterned wires. In this work, we characterize micropatterned thin NbN films by using transport measurements in magnetic fields. We show that from the instability of vortex motion at high currents in the flux-flow state of the $IV$ characteristic, the inelastic lifetime of quasiparticles can be determined to be about 2 ns. Additionally, from the depinning transition of vortices at low currents, as a function of magnetic field, the size distribution of grains can be extracted. This size distribution is found to be in agreement with the film morphology obtained from scanning electron microscopy and high-resolution transmission electron microscopy images.

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr${}_{1-x}$Mg${}_x$O${}_2$ ($0\le x\le 0.03$) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation $+U$ ($\mathrm{LDA}+U$) scheme. Cr/Cu 3$p$-$3d$ resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr $3d$ character with a minor contribution from the Cu $3d$ through Cu $3d-$O $2p-$Cr $3d$ hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr $3d$ character. Cr $2p$ PES and $L$-edge XAS spectra exhibit typical Cr${}^{3+}$ features for all $x$, while the Cu $L$-edge XAS spectra exhibited a systematic change with $x$. This indicates now that the Cu valence is monovalent at $x=0$ and the doped hole should have Cu $3d$ character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu${}^{+}$ ($3d^{10}$) and Cu${}^{2+}$ ($3d^9$) like initial states in Cu $2p$-$3d$ resonant PES spectrum of at $x=0$, while Cu $2p$ PES spectra with no doubt shows the Cu${}^{+}$ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu $4s$ state, but also finite Cu $3d,4s-$Cr $3d$ charge transfer via O $2p$ states in the ground-state electronic configuration.

The plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene. All edge site binding energies were found to be substantially higher than the adsorption energies for all metals. The adatom migration activation barriers for the lowest energy migration paths on pristine monolayer, bilayer, and trilayer graphene were then calculated and found to be smaller than or within an order of magnitude of $k_{B}T$ at room temperature, implying very high mobility for all adatoms studied. This suggests that metal atoms evaporated onto graphene samples quickly migrate across the lattice and bind to the energetically favorable edge sites before being characterized in the microscope. We then prove this notion for Al and Au on graphene with scanning transmission electron microscopy (STEM) images showing that these atoms are observed exclusively at edge sites, and also hydrocarbon-contaminated regions, where the pristine regions of the lattice are completely devoid of adatoms. Additionally, we review the issue of fixing selected atomic positions during geometry optimization calculations for graphene/adatom systems and suggest a guiding principle for future studies.

We investigate the effect of strain-induced gauge fields on statistical distribution of energy levels of triangular graphene nanoflakes with zigzag edges. In the absence of strain fields but in the presence of weak potential disorder such systems were found by Rycerz [Phys. Rev. B 85, 245424 (2012)] to display the spectral statistics of the Gaussian unitary ensemble (GUE) due to the effective time-reversal (symplectic) symmetry breaking. Here we show that in the absence of disorder, strain fields may solely lead to spectral fluctuations of GUE providing a nanoflake is deformed such that all its geometric symmetries are broken. In a particular case when a single mirror symmetry is preserved the spectral statistics follow the Gaussian orthogonal ensemble (GOE) rather then GUE. The corresponding transitions to quantum chaos are rationalized by means of additive random-matrix models and the analogy between strain-induced gauge fields and real magnetic fields is discussed.

We show that the functional renormalization group is a numerically cheap method to obtain the low-energy behavior of the Anderson impurity model describing a localized impurity level coupled to a bath of conduction electrons. Our approach uses an external magnetic field as the flow parameter, partial bosonization of the transverse spin fluctuations, and frequency-independent interaction vertices determined by Ward identities. The magnetic field serves also as a regulator for the bosonized spin fluctuations, which are suppressed at large field. We calculate the quasiparticle residue and spin susceptibility in the particle-hole symmetric case and obtain excellent agreement with the Bethe ansatz results for arbitrary coupling.

We use scanning tunneling microscopy to determine the surface structure and dopant distribution in Pr${}_x$Ca${}_{1-x}$Fe${}_2$As${}_2$, the highest-$T_c$ member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-$T_c$ superconducting phase is unlikely to originate from Pr inhomogeneity.

Recently demonstrated solid-state single-electron sources generate different quantum states depending on their operation condition. For adiabatic and nonadiabatic sources, we determine the Glauber correlation function in terms of the Floquet scattering matrix of the source. The correlation function provides full information on the shape of the state and on its time-dependent amplitude and phase. The coherence properties of single-electron states are therefore essential for the production of quantum multiparticle states.

We present first-principles calculations of the quasiparticle band structures of four isostructural semiconducting metal chalcogenides $A_2{B}_3$ (with $A$ = Sb, Bi and $B$ = S, Se) of the stibnite family within the $G_0{W}_0$ approach. We perform extensive convergence tests and identify a sensitivity of the quasiparticle corrections to the structural parameters and to the semicore $d$ electrons. Our calculations indicate that all four chalcogenides exhibit direct band gaps, if we exclude some indirect transitions marginally below the direct gap. Relativistic spin-orbit effects are evaluated for the Kohn-Sham band structures, and included as scissor corrections in the quasiparticle band gaps. Our calculated band gaps are 1.5 eV (Sb${}_2$S${}_3$), 1.3 eV (Sb${}_2$Se${}_3$), 1.4 eV (Bi${}_2$S${}_3$), and 0.9 eV (Bi${}_2$Se${}_3$). By comparing our calculated gaps with the ideal Shockley-Queisser value we find that all four chalcogenides are promising as light sensitizers for nanostructured photovoltaics.

We analyze the characteristics of polarity in unsupported nanoribbons with zigzag edges, by a combination of analytic models, semiempirical Hartree-Fock simulations, and first-principles approach. We consider two materials with widely different ionic-covalent character, MgO and MoS${}_2$, and two polarity healing mechanisms: the so-called electronic compensation in ribbons with pristine edges, and ionic compensation in ribbons with an adequately chosen density of missing edge ions. The general expression of compensating charges, the edge metallization and spin polarization in the electronic mechanism, and the efficiency of the ionic mechanism are similar to those known in thin films and at polar surfaces. Differences, however, exist and are related to the low dimensionality, the atomic structure, and the strong undercoordination of edge atoms in nanoribbons. Polarity signatures are specified and a discussion of the possible origins of metallic edge states in these low dimensional objects is provided.

We report a strategy to induce superconductivity in the BiS${}_2$-based compound LaOBiS${}_2$. Instead of substituting F for O, we increase the charge-carrier density (electron dope) via substitution of tetravalent Th${}^{+4}$, Hf${}^{+4}$, Zr${}^{+4}$, and Ti${}^{+4}$ for trivalent La${}^{+3}$. It is found that both the LaOBiS${}_2$ and ThOBiS${}_2$ parent compounds are bad metals and that superconductivity is induced by electron doping with T${}_c$ values of up to 2.85 K. The superconducting and normal states were characterized by electrical resistivity, magnetic susceptibility, and heat capacity measurements. We also demonstrate that reducing the charge-carrier density (hole doping) via substitution of divalent Sr${}^{+2}$ for La${}^{+3}$ does not induce superconductivity.

We study quantum phase-slip (QPS) processes in a superconducting ring containing $N$ Josephson junctions and threaded by an external static magnetic flux ${\Phi }_B$. In such a system, a QPS consists of a quantum tunneling event connecting two distinct classical states of the phases with different persistent currents [Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)]. When the Josephson coupling energy $E_J$ of the junctions is larger than the charging energy $E_C=e^2/2C$, where $C$ is the junction capacitance, the quantum amplitude for the QPS process is exponentially small in the ratio $E_J/E_C$. At given magnetic flux, each QPS can be described as the tunneling of the phase difference of a single junction of almost $2\pi$, accompanied by a small harmonic displacement of the phase difference of the other $N-1$ junctions. As a consequence, the total QPS amplitude ${\nu }_{\mathrm{ring}}$ is a global property of the ring. Here, we study the dependence of ${\nu }_{\mathrm{ring}}$ on the ring size $N$, taking into account the effect of a finite capacitance $C_0$ to ground, which leads to the appearance of low-frequency dispersive modes. Josephson and charging effects compete and lead to a nonmonotonic dependence of the ring's critical current on $N$. For $N\to \infty$, the system converges either towards a superconducting or an insulating state, depending on the ratio between the charging energy $E_0=e^2/2C_0$ and the Josephson coupling energy $E_J$.

We study the effects of spin-orbit interaction (SOI) on the current-induced motion of a magnetic (Bloch) domain wall in ultrathin ferromagnetic nanowires. The conspiracy of spin relaxation and SOI is shown to generate a strong nonequilibrium Rashba field, which can dominate even for weak SOI. This field causes intricate spin precession and a transition from translatory to oscillatory wall dynamics with increasing SOI. We show that current pulses of different lengths can be used to efficiently control the domain wall motion.

The weak ferromagnetic moment in Ni-doped CuB${}_2$O${}_4$ can be rotated by applying an electric field. While this implies spin-driven ferroelectricity, no direct evidence of electric polarization has been reported to date. Here we report the induction and control of polarization in the borate in the presence of an external magnetic field. Applying a magnetic field along the $\left[110\right]$ or $\left[1\overline{1}0\right]$ axis induces electric polarization along the $\left[001\right]$ axis. The polarization is reversed by switching the magnetic-field direction between the $\left[110\right]$ and $\left[1\overline{1}0\right]$ axes. The mechanism by which electric polarization emerges can be well explained in the framework of spin-dependent metal-ligand hybridization.

The strength of perpendicular anisotropy is known to drive the spin reorientation in thin magnetic films. Here, we consider the effect different order anisotropies have on two phase transitions: the spin reorientation and the orientational order transitions. We find that the relative magnitude of different order anisotropies can significantly enhance or suppress the degree to which the system reorients. Specifically, Monte Carlo simulations reveal significant changes in the cone angle and planar magnetization. In order to facilitate rapid computation, we have developed a stream processing technique, suitable for use on graphics processing unit (GPU) systems, for computing the transition probabilities in two-dimensional systems with dipole interactions.

Dynamic Hubbard models are extensions of the conventional Hubbard model that take into account the fact that atomic orbitals expand upon double occupancy. It is shown here that systems described by dynamic Hubbard models have a tendency to expel negative charge from their interior to the surface and to develop charge inhomogeneity and even phase separation in the bulk. These effects are associated with a lowering of the electronic kinetic energy. We propose that these models may explain the charge inhomogeneity and negatively charged grain boundaries observed in cuprate oxides and other materials.

We report numerical and experimental investigations into modulation instability in a nonlinear magnetoinductive waveguide. By numerical simulation we find that modulation instability occurs in an electrical circuit model of a magnetoinductive waveguide with third-order nonlinearity. We fabricate the nonlinear magnetoinductive waveguide for microwaves using varactor-loaded split-ring resonators and observe the generation of modulation instability in the waveguide. The condition for generating modulation instability in the experiment roughly agrees with that in the numerical analysis.

We develop a nonlocal dielectric-response theory to describe the temperature dependence of exciton lifetime in metal-semiconductor heterostructures. Coupling between excitons and surface plasmons results in a strongly nonmonotonous behavior of exciton decay rate versus temperature, affected by surface plasmons on both sides of the metal layer. Unlike the photons, surface plasmons are not able to escape the sample; hence, the additional decay channel for excitons appears solely due to the plasmon scattering in metal and metal-dielectric interfaces. Tuning the plasmon frequency, one can control the exciton lifetime and under certain conditions emission efficiency.

Site-controlled quantum dots formed during the deposition of (Al)GaAs layers by metal-organic vapor-phase epitaxy on GaAs(111)$B$ substrates patterned with inverted pyramids result in geometric and compositional self-ordering along the vertical axis of the template. We describe a theoretical scheme that reproduces the experimentally observed time-dependent behavior of this process, including the evolution of the recess and the increase of Ga incorporation along the base of the template to stationary values determined by alloy composition and other growth parameters. Our work clarifies the interplay between kinetics and geometry for the development of self-ordered nanostructures on patterned surfaces, which is essential for the reliable on-demand design of confined systems for applications to quantum optics.

When magnetic nanoparticles (MNPs) are single domain and magnetically independent, their magnetic properties and the conditions to optimize their efficiency in magnetic hyperthermia applications are now well understood. However, the influence of magnetic interactions on magnetic hyperthermia properties is still unclear. Here, we report hyperthermia and high-frequency hysteresis loop measurements on a model system consisting of MNPs with the same size but a varying anisotropy, which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the MNP anisotropy and the squareness of their hysteresis loop in colloidal solution is observed: the larger the anisotropy, the smaller the squareness. Since low anisotropy MNPs display a squareness higher than the one of magnetically independent nanoparticles, magnetic interactions enhance their heating power in this case. Hysteresis loop calculations of independent and coupled MNPs are compared to experimental results. It is shown that the observed features are a natural consequence of the formation of chains and columns of MNPs during hyperthermia experiments: in these structures, when the MNP magnetocristalline anisotropy is small enough to be dominated by magnetic interactions, the hysteresis loop shape tends to be rectangular, which enhances their efficiency. On the contrary, when MNPs do not form chains and columns, magnetic interactions reduce the hysteresis loop squareness and the efficiency of MNPs compared to independent ones. Our finding can thus explain contradictory results in the literature on the influence of magnetic interactions on magnetic hyperthermia. It also provides an alternate explanation to some experiments where an enhanced specific absorption rate for MNPs in liquids has been found compared to the one of MNPs in gels, usually interpreted with some contribution of the brownian motion. The present work should improve the understanding and interpretation of magnetic hyperthermia experiments.