The contact between a molecule and a metallic electrode contributes to or even determines the characteristics of organic devices, such as their electronic properties. This is partly due to the charge transfer that takes place when two materials with different chemical potentials are put together. In the case of magnetic electrodes, the transfer can be accompanied by the transmission of a net spin polarization or spin doping. In nanocarbon systems, hybridization and spin doping can suppress the moment of a transition metal ferromagnet through the loss of majority spin electrons to the organic. Here, ${\mathrm{C}}_{60}$ is shown to become ferromagnetic as a result of spin doping from cobalt with an induced moment of 1.2 ${\mu }_{\mathrm{B}}$ per cage while suppressing the moment of the ferromagnet by up to 21%. Polarized neutron reflectivity and x-ray magnetic circular dichroism reveal the presence of an antiferromagnetic coupling of the interfacial layers of cobalt and ${\mathrm{C}}_{60}$, and weakly coupled induced magnetism propagating into the bulk organic. Thus, it is shown that the deposition of molecules with high electron affinity can be used to induce zero-voltage spin injection.

The theoretical prediction that gray tin represents a strong topological insulator under strain [L. Fu and C.L. Kane, Phys. Rev. B 76, 045302 (2007)] is proven for biaxially strained $\alpha \text{−}\mathrm{Sn}$ layers with varying thickness by means of a generalized density functional theory with a nonlocal exchange-correlation potential that widely simulates quasiparticle bands and a tight-binding method including intra- and interatomic spin-orbit interaction. Hydrogen-passivated surfaces are modeled by symmetric slabs. In contrast to the conventional picture of a topological insulator, we find topological gapless surface states below the strain-induced bulk gap, in agreement with photoemission studies. The topological surfaces states do not emerge out of the lower ${\Gamma }_8^{+}$ states but are strongly influenced by the inverted bulk ${\Gamma }_7^{-}$ band, i.e., lie within the negative bulk $sp$ gap. We show that the position of the Dirac point and the dispersion of the surrounding bands depend on the thickness of the $\alpha \text{−}\mathrm{Sn}$ layer but are less influenced by strain. The resulting Fermi velocities agree well with available photoemission data.

We present an analytical investigation of the quasi-Coulomb impurity states in a narrow gapped armchair graphene nanoribbon (GNR) in the presence of a uniform external electric field directed parallel to the ribbon axis. The effect of the ribbon confinement is taken to be much greater than that of the impurity electric field, which in turn considerably exceeds the external electric field. Under these conditions we employ the adiabatic approximation assuming that the motion parallel (“slow”) and perpendicular (“fast”) to the ribbon axis are separated adiabatically. In the approximation of the isolated size-quantized subbands induced by the “fast” motion, the complex energies of the impurity electron are calculated in explicit form. The real and imaginary parts of these energies determine the binding energy and width of the quasidiscrete state, respectively. The energy width increases with increasing the electric field and ribbon width. The latter forms the background of the mechanism of dimensional ionization. The $S$ matrix—the basic tool of study of the transport problems—can be trivially derived from the phases of the wave functions of the continuous spectrum presented in explicit form. In the double-subband approximation, we calculate the complete widths of the impurity states caused by the combined effect of the electric field and the Fano resonant coupling between the impurity states of the discrete and continuous spectra associated with the ground and first excited size-quantized subbands. Our analytical results are shown to be in agreement with those obtained by other theoretical approaches. Estimates of the expected experimental values for the typically employed GNRs show that for weak electric field the impurity quasidiscrete states remain sufficiently stable to be observed in the corresponding experiment, while a relatively strong field unlocks the captured electrons to further restore their contribution to the transport.

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the $\mathrm{Mg}K{L}_{23}{L}_{23}$ Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atom incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between ${\mathrm{O}}^{2-}$ ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

The discovery of gapless linear energy dispersion in low-dimensional carbon-based nanostructures had a tremendous impact on conventional condensed-matter physics by imposing relativistic physics in the electronic properties of these nanosystems. Indeed, the electrons in graphene [two-dimensional (2D)] and carbon nanotubes [one-dimensional (1D)] behave like pseudorelativistic massless particles (Dirac fermions) as described by the crossing of linearly dispersive electronic bands, also called the Dirac cone. The presence of Dirac cones is not restricted to 1D or 2D nanostructures and has recently been observed in several more complex 3D materials. Here, using density functional theory and tight-binding approaches, we predict the presence of a Dirac cone in a 3D carbon-based material. Indeed, our simulations reveal a linear band crossing merging in a point forming a Dirac (hyper)cone for a large gyroidal schwarzite structure. Such a specific linear dispersion relation as reported in this 3D negatively curved $s{p}^2$-bonded carbon allotrope is believed to be a direct consequence of the Dirac cone present in 2D graphene. The corresponding charge carriers are thus expected to behave as 3D massless Dirac fermions. Therefore, we expect this prediction to stimulate the experimental synthesis of such fascinating 3D bulk carbon allotropes, which are a remarkable playground to investigate relativistic physics of these exotic fermions.

We investigate the consequences of strong electronic coupling at the organic semiconductor/metal interface in both the ground and excited state manifold for the case of chloro-boron subphthalocyanine on Cu(111). Using a combination of low-temperature scanning tunneling microscopy and ultraviolet photoelectron spectroscopy and angle-resolved two-photon photoemission, we are able to connect local electronic interactions at the interface with thin film structure despite complex growth in the submonolayer regime. We show that strong coupling leads to charge transfer from the surface to the molecule, and we are able to correlate this observation with the specific molecular adsorption geometry. Strong coupling further results in molecular excited state anion resonances and is responsible for autoionization of highly excited image potential states, relating to the heterogeneous electronic environment in these thin films. This study provides a step towards disentangling interfacial electronic interactions at complex organic/metal interfaces.

The high-frequency (ac) conductivity of a high quality modulation doped GeSi/Ge/GeSi single quantum well structure with hole density $p=6\times {10}^{11}{\mathrm{cm}}^{-2}$ was measured by the surface acoustic wave (SAW) technique at frequencies of 30 and 85 MHz and magnetic fields $B$ of up to 18 T in the temperature range of 0.3–5.8 K. The acoustic effects were also measured as a function of the tilt angle of the magnetic field with respect to the normal of the two-dimensional channel at $T=0.3$ K. It is shown that at the minima of the conductivity oscillations holes are localized on the Fermi level, and that there is a temperature domain in which the high-frequency conductivity in the bulk of the quantum well is of the activation nature. The analysis of the temperature dependence of the conductivity at odd filling factors enables us to determine the effective $g_z$ factor. It is shown that the in-plane component of the magnetic field leads to an increase of the cyclotron mass and to a reduction of the $g_z$ factor. We developed a microscopic theory of these effects for the heavy-hole states of the complex valence band in quantum wells which describes well the experimental findings.

The surface diffusion of compact Cu tetramers on Cu(111) has been studied at the atomic scale by means of molecular dynamics simulations using embedded atom interatomic potentials. The Cu clusters diffuse by several different mechanisms; all of them have the common trait of involving concerted displacements of at least some of the atoms forming the island. The anisotropy in the rhombus shape of the Cu tetramers and the different activation energies of the translational and rotational jumps result in highly anisotropic diffusion, with a transition from practically one-dimensional motion at low temperature to nearly isotropical two-dimensional random walk at high temperature. The use of molecular dynamics allows us to also determine the preexponential factors for each jump mechanism, from which analysis we can obtain some insight into their basic nature and the relationships between them.

We theoretically demonstrate that 100% valley-polarized transport in monolayers of ${\mathrm{MoS}}_2$ and other group-VI dichalcogenides can be obtained using off-resonant circularly polarized light. By tuning the intensity of the off-resonant light the intrinsic band gap in one valley is reduced, while it is enhanced in the other valley, enabling single valley quantum transport. As a consequence, we predict (i) enhancement of the longitudinal electrical conductivity, accompanied by an increase in the spin polarization of the flowing electrons, (ii) enhancement of the intrinsic spin Hall effect, together with a reduction of the intrinsic valley Hall effect, and (iii) enhancement of the orbital magnetic moment and orbital magnetization. These mechanisms provide appealing opportunities to the design of nanoelectronics based on dichalcogenides.

The Sn overlayer on the Si(111) surface has been considered as a prototypical system for exploring two-dimensional (2D) correlated physics on the triangular lattice. Most of the previous theoretical studies were based on the presumption that the surface state dominantly originates from Sn dangling-bond (DB) electrons, leading to a strongly correlated 2D electronic system. By contrast, our density-functional theory calculations show that the Sn DB state significantly hybridizes with Si substrate states to form a resonant state. The strong resonance between the $\mathrm{Sn}5p_z$ and $\mathrm{Si}3p_z$ orbitals facilitates the recently observed antiferromagnetic order through superexchange interactions, giving rise to a band-gap opening. It is thus demonstrated that the insulating ground state of Sn/Si(111) can be characterized as a Slater-type insulator via band magnetism.

We report that an external electric field applied normal to bilayers of transition-metal dichalcogenides ${TX}_2$ ($T = \mathrm{Mo}$, W, $X = \mathrm{S}$, Se) creates significant spin-orbit splittings and reduces the electronic band gap linearly with the field strength. Contrary to the ${TX}_2$ monolayers, spin-orbit splittings and valley polarization are absent in bilayers due to the presence of inversion symmetry. This symmetry can be broken by an electric field, and the spin-orbit splittings in the valence band quickly reach values similar to those in the monolayers (145 meV for ${\mathrm{MoS}}_2,...,$ 418 meV for ${\mathrm{WSe}}_2$) at saturation fields less than $500 \mathrm{mV} {\AA }^{-1}$. The band gap closure results in a semiconductor-metal transition at field strength between 1.25 ($\mathrm{W}{X}_2$) and 1.50 (${\text{Mo}X}_2$) $\mathrm{V} {\AA }^{-1}$. Thus, by using a gate voltage, the spin polarization can be switched on and off in ${TX}_2$ bilayers, thus activating them for spintronic and valleytronic applications.

We consider a class of one-dimensional compass models with antisymmetric Dzyaloshinskii-Moriya exchange interaction in an external magnetic field. Based on the exact solution derived by means of Jordan-Wigner transformation, we study the excitation gap, spin correlations, ground-state degeneracy, and critical properties at phase transitions. The phase diagram at finite electric and magnetic field consists of three phases: ferromagnetic, canted antiferromagnetic, and chiral. Dzyaloshinskii-Moriya interaction induces an electrical polarization in the ground state of the chiral phase, where the nonlocal string order and special features of entanglement spectra arise, while strong chiral correlations emerge at finite temperature in the other phases and are controlled by a gap between the nonchiral ground state and the chiral excitations. We further show that the magnetoelectric effects in all phases disappear above a typical temperature corresponding to the total bandwidth of the effective fermionic model. To this end we explore the entropy, specific heat, magnetization, electric polarization, and the magnetoelectric tensor at finite temperature. We identify rather peculiar specific-heat and polarization behavior of the compass model which follows from highly frustrated interactions.

Spinor bosons offer a conceptually simple picture of macroscopic quantum behavior of topological order-by-disorder: The paramagnetic state of two-component dipolar bosons in an orbitally degenerate zigzag lattice is unstable against infinitesimal quantum fluctuations of orbitals, $\lambda$, towards developing nonlocal hidden order. Adjacent to the topological state a locally correlated exact ground state with spontaneously a quadrupoled lattice constant is realized for the broad parameter regime. The topological order is extremely robust surviving the $\lambda \to \infty$ limit where the ground state evolves into the Majumdar-Ghosh state of a frustrated spin-$\frac{1}{2}$ chain.

We analyze symmetries and magnetic properties of Cooper pairs appearing as subdominant pairing correlations in inhomogeneous superconductors, on the basis of the quasiclassical Green-function theory. The frequency symmetry, parity, and the type of magnetic response of such subdominant correlations are opposite to those of the dominant pairing correlations in the bulk state. Our conclusion is valid even when we generalize the theory of superconductivity to recently proposed diamagnetic odd-frequency superconductors. As a consequence, Cooper pairs are classified into eight classes in terms of their symmetries and magnetic properties. Anomalous magnetic properties of subdominant components can be probed by studying the Meissner effect.

Ultrasound velocity measurements of the orbitally degenerate frustrated spinel ${\mathrm{MgV}}_2{\mathrm{O}}_4$ are performed in a high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal that two types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic-field and disorder sensitivities: huge Curie-type softening with decreasing temperature (convex temperature dependence), and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of the lattice to coexisting orbital fluctuations and spin-cluster excitations.

We have determined the depth-resolved magnetization structures of a series of highly ordered ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$ (SCRO) ferrimagnetic epitaxial films via combined studies of x-ray reflectometry, polarized neutron reflectometry, and superconducting quantum interference device magnetometry. The SCRO films deposited directly on ${\left({\mathrm{LaAlO}}_3\right)}_{0.3}{\left({\mathrm{Sr}}_2{\mathrm{AlTaO}}_6\right)}_{0.7}$ or ${\mathrm{SrTiO}}_3$ substrates show reduced magnetization of similar width near the interfaces with the substrates, despite having different degrees of strain. When the SCRO film is deposited on a ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_3$ (SCNO) double perovskite buffer layer, the width of the interfacial region with reduced magnetization is decreased. However, the relative reduction of the magnetization averaged over the interfacial regions is comparable among the three samples. Interestingly, we found that the magnetization suppression region is wider than the Cr/Re antisite disorder region at the interface between SCRO and SCNO.

We present neutron diffraction analysis of ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ over a wide temperature (10 to 300 K) and compositional $\left(0.11\le x\le 0.79\right)$ range, including the normal state, the magnetically ordered state, and the superconducting state. The paramagnetic to spin-density wave and orthorhombic to tetragonal transitions are first order and coincident within the sensitivity of our measurements ($\sim 0.5$ K). Extrapolation of the orthorhombic order parameter down to zero suggests that structural quantum criticality cannot exist at compositions higher than $x=0.28$, which is much lower than values determined using other methods, but in good agreement with our observations of the actual phase stability range. The onset of spin-density wave order shows a stronger structural anomaly than the charge-doped system in the form of an enhancement of the $c/a$ ratio below the transition.

Irradiation with 2.5 MeV electrons at doses up to $5.2\times {10}^{19} \mathrm{electrons}/\mathrm{cm}{}^2$ was used to introduce pointlike defects in single crystals of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $x=0.19 \left(T_c=14\mathrm{K}\right),0.26 \left(T_c=32\mathrm{K}\right), 0.32 \left(T_c=37\mathrm{K}\right)$, and $0.34 \left(T_c=39\mathrm{K}\right)$ to study the superconducting gap structure by probing the effect of nonmagnetic scattering on electrical resistivity $\rho \left(T\right)$ and London penetration depth $\lambda \left(T\right)$. For all compositions, the irradiation suppressed the superconducting transition temperature $T_c$ and increased resistivity. The low-temperature behavior of $\lambda \left(T\right)$ is best described by the power-law function, $\Delta \lambda \left(T\right)=A{(T/T_c)}^n$. While substantial suppression of $T_c$ supports $s_{\pm }$ pairing, in samples close to the optimal doping, $x=0.26$, 0.32, and 0.34, the exponent $n$ remained high $\left(n\ge 3\right)$, indicating almost exponential attenuation and thus a robust full superconducting gap. For the $x=0.19$ composition, which exhibits coexistence of superconductivity and long-range magnetism, the suppression of $T_c$ was much more rapid, and the exponent $n$ decreased toward the $s_{\pm }$ dirty limit of $n=2$. In this sample, the irradiation also suppressed the temperature of structural/magnetic transition $T_{\mathrm{sm}}$ from 103 to 98 K, consistent with the itinerant nature of the long-range magnetic order. Our results suggest that underdoped compositions, especially in the coexisting regime, are most susceptible to nonmagnetic scattering and imply that in multiband ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ superconductors, the ratio of the interband to intraband pairing strength, as well as the related gap anisotropy, increases upon the departure from the optimal doping.

The magnetic flux distribution arising from a high-$T_c$ superconductor is detected and visualized using polarized x rays. Therefore, we introduce a sensor layer, namely, an amorphous, soft-magnetic ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ cover layer, providing a large x-ray magnetic circular dichroism (XMCD). Temperature-dependent XMCD spectroscopy on the magnetic layer has been performed. Exploiting the temperature dependence of the critical current density of the superconductor we find a quantitative correlation between the XMCD signal and the in-plane stray field of the superconductor. Magneto-optical Kerr effect experiments on the sensor layer can simulate the stray field of the superconductor and hence verify the correlation. We show that the XMCD contrast in the sensor layer corresponds to the in-plane magnetic flux distribution of the superconductor and can hence be used to image magnetic structures in superconductors.