Spin-resolved Local Density Of States Research Articles

Competing ferromagnetic superconducting states in europium-based iron pnictides

In europium-based iron pnictides superconducting Fe-planes can be influenced by a Zeeman field originated from the neighboring Eu-planes. The field tends to induce spin-density waves with a ferromagnetic average which coexists with the superconducting order by forming complementary patterns of the superconducting and magnetic order parameters in a Fulde-Ferrell-Larkin-Ovchinnikov phase and a two-dimensional textured-superconducting phase. The hard gap around the Fermi energy disappears in these fragile inhomogeneous superconducting states, which features, instead, V-shaped spin-resolved local density of states. The inhomogeneous states are also competing with either a homogeneous superconducting or a homogeneous ferromagnetic state, manifesting the intertwining influences of the magnetic orders in Fe and Eu planes, the spin-density wave band structure, and the superconducting pairing order.

DensityTool: A post-processing tool for space- and spin-resolved density of states from VASP

The knowledge of the local electronic structure of heterogeneous solid materials is crucial for understanding their electronic, magnetic, transport, optical, and other properties. VASP, one of the mostly used packages for density-functional calculations, provides local electronic structure either by projecting the electronic wave functions on atomic spheres, or as a band-decomposed partial charge density. Here, we present a simple tool which takes the partial charge density and the energy eigenvalues calculated by VASP as input and constructs local charge and spin densities. The new data provides a much better spatial understanding than the projection on the atomic spheres. It can be visualized directly in the real space e.g. with Vesta, or averaged along planes spanned by two of the lattice vectors of the periodic unit cell. The plane-averaged local (spin) density of states can be easily plotted e.g. as color-coded data using almost any plotting program. DensityTool can be applied to visualize and understand the local electronic structure of any system calculated with VASP. We expect it to be useful especially for researchers concerned with inhomogeneous systems, such as interfaces, defects, surfaces, adsorbed molecules, or hybrid inorganic-organic composites. Program summaryProgram Title: DensityToolCPC Library link to program files:https://doi.org/10.17632/26mcg4jwvd.1Developer's repository link:https://github.com/llodeiro/DensityToolLicensing provisions: MITProgramming language: FORTRANSupplementary material: A complementary User Manual and examples can be found in the developer's repository link.Nature of problem: Total and local density of states (LDOS) are widely-used quantities for understanding the electronic structure of condensed matter. In VASP, one of the mostly used packages for numerically solving the Kohn-Sham equations of density-functional theory, one does not directly obtain the LDOS. Usually, one works with partial density of states (PDOS), where the wave function is projected onto atomic orbitals. Alternatively, it is possible to calculate the band- and wavevector-dependent charge densities. Together with the band structure, LDOS can be obtained using post-processing tools.Solution method: DensityTool combines the band structure with band- and wavevector-dependent charge densities calculated by VASP to construct LDOS. In addition, spin-resolved LDOS (LSDOS) can be calculated for magnetic systems. Thus obtained LDOS and LSDOS can be further processed with DensityTool. In particular, it can be averaged along chosen spatial directions, which is convenient for example to demonstrate the spatially-resolved electronic structure of inhomogeneous systems.DensityTool can handle all three types of approaches implemented in VASP (non-magnetic, collinear magnetic, non-collinear magnetic including spin-orbit coupling), as well as all types of self-consistent band calculations as DFT (e.g., LDA, GGA, mGGA, BJ, and hybrid exchange-correlation functionals) and HF.For convenience, the output of DensityTool follows the format of VASP (the CHGCAR file), which can be easily used in other programs for visualization. We provide a sample python script to plot averaged L(S)DOS from the calculated data.

Effect of Antiferromagnetic Order on Mixed States in Electron-Doped High-Tc Superconductors

The vortex states in electron-doped high-Tc superconductors are studied theoretically at the mean-field approach and based on the Bogoliubov-de Gennes equations. The local density of states is calculated, through which the in-gap bound states are revealed. They are robust at the sites around the vortex core. In the underdoped region where the antiferromagnetic order and the superconducting order coexist, the in-gap peaks are lying symmetric with respect to the Fermi energy, while the peak intensities at the negative energy are much stronger. A stripe-type modulation is revealed for the spin-resolved local density of states. As the doping density increases to the optimal or over-doped region, the long-range antiferromagnetic order disappears, and the two in-gap peaks merge to one single peak. The peak position is near the Fermi energy. Our results indicate that the vortex states may be used to detect the signature of the antiferromagnetic order in electron-doped high-Tc superconductors.

Volkov-Pankratov states in topological superconductors

We study the in-gap states that appear at the boundaries of both 1D and 2D topological superconductors. While the massless Majorana quasiparticles are guaranteed to arise by the bulk-edge correspondence, we find that they could be accompanied by massive Volkov-Pankratov (VP) states which are present only when the interface is sufficiently smooth. These predictions can be tested in an s-wave superconductor with Rashba spin-orbit coupling placed on top of a magnetic domain wall. We calculate the spin-resolved local density of states of the VP states about the band inversion generated by a magnetic domain wall and find that they are oppositely spin-polarized on either side of the topological phase boundary. We also demonstrate that the spatial position, energy-level spacing, and spin polarization of the VP states can be modified by the introduction of in-plane electric fields.

Local density of states in two-dimensional topological superconductors under a magnetic field: Signature of an exterior Majorana bound state

We study quasiparticle states on a surface of a topological insulator (TI) with proximity-induced superconductivity under an external magnetic field. An applied magnetic field creates two Majorana bound states: a vortex Majorana state localized inside a vortex core and an exterior Majorana state localized along a circle centered at the vortex core. We calculate the spin-resolved local density of states (LDOS) and demonstrate that the shrinking of the radius of the exterior Majorana state, predicted in Ref. [R. S. Akzyanov et al., Phys. Rev. B 94, 125428 (2016)], under a strong magnetic field can be seen in LDOS without smeared out by non-zero-energy states. The spin-resolved LDOS further reveals that the spin of the exterior Majorana state is strongly polarized. Accordingly, the induced odd-frequency spin-triplet pairs are found to be spin-polarized as well. In order to detect the exterior Majorana states, however, the Fermi energy should be closed to the Dirac point to avoid contributions from continuum levels. We also study a different two-dimensional topological-superconducting system where a two-dimensional electron gas with the spin-orbit coupling is sandwiched between an s-wave superconductor and a ferromagnetic insulator. We show that the radius of an exterior Majorana state can be tuned by an applied magnetic field. However, on the contrary to the results at a TI surface, neither the exterior Majorana state nor the induced odd-frequency spin-triplet pairs are spin-polarized. We conclude that the spin-polarization of the Majorana state is attributed to the spin-polarized Landau level which is characteristic for systems with the Dirac-like dispersion.

Perfect spin filtering by symmetry in molecular junctions

Obtaining highly spin-polarized currents in molecular junctions is crucial and desirable for nanoscale spintronics devices. Motivated by our recent symmetry-based theoretical argument for complete blocking of one spin conductance channel in atomic-scale junctions [A. Smogunov and Y. J. Dappe, Nano Lett. 15, 3552 (2015)], we explore the generality of the proposed mechanism and the degree of achieved spin-polarized current for various ferromagnetic electrodes (Ni, Co, Fe) and two different molecules, quaterthiophene and p-quaterphenyl. A simple analysis of the spin-resolved local density of states of a free electrode allowed us to identify the Fe(110) as the most optimal electrode, providing perfect spin filtering and high conductance at the same time. These results are confirmed by $ab$ $initio$ quantum transport calculations and are similar to those reported previously for model junctions. It is found, moreover, that the distortion of the p-quaterphenyl molecule plays an important role, reducing significantly the overall conductance.

Spin-filter and negative differential resistance effect in zigzag-edged bilayer graphene nanoribbon devices

By performing first-principle quantum transport calculation, the spin-dependent transport properties of zigzag-edged bilayer graphene nanoribbon based devices are investigated. There are four kinds of structures with different stacking sequences and treatment of dangling bonds considered in our work. It is shown that the devices are perfect spin-filters with extremely large spin polarization as well as substantial negative differential resistance effects, which are affected by the stacking sequences and edge structures. All these phenomena can be explained by the spin-resolved local density of states and the tranmission spectra.

Non-Zeeman splitting for a spin-resolved STM with a Kondo adatom in a spin-polarized two-dimensional electron gas

We theoretically investigate the spin-resolved local density of states (SR-LDOS) of a spin-polarized two-dimensional electron gas in the presence of a Kondo adatom and a STM probe. Using Green function formalism and the atomic approach in the limit of infinite Coulomb correlation, it is found an analytical SR-LDOS expression in the low temperature regime of the system. This formal result is given in terms of phase shifts originated by the adatom scattering and Fano interference. The SR-LDOS is investigated as a function of the probe position and different Fano factors. Our findings provide an alternative way to spin-split the Kondo resonance without the use of huge magnetic fields, typically necessary in adatom systems characterized by large Kondo temperatures. We observe a non-Zeeman spin-splitting of the Kondo resonance in the total LDOS, with one spin-component pinned around the host Fermi level. Interestingly, this result is in accordance to recent experimental data reported in Phys. Rev. B 82, 020406(R) (2010).

Probing crossover from analogous weak antilocalization to localization by an Aharonov-Bohm interferometer on topological insulator surface

We propose a scanning tunneling microscopy Aharonov-Bohm (AB) interferometer on the surface of a topological insulator (TI) to probe the crossover from analogous weak antilocalization to weak localization phenomenon via the AB oscillations in spin-resolved local density of states (LDOS). Based on our analytical and numerical results, we show that with increasing the energy gap of TI surface states, the Φ0/2=hc/2e periodic AB oscillations in spin-resolved LDOS gradually transit into the Φ0 periodic oscillations.

Spin-Resolved Local Density of States for an Anderson Adatom in a Ferromagnetic Island

In this work, we investigate theoretically the spin-resolved local density of states (SR-LDOS) of a ferromagnetic (FM) island hybridized with an adatom, which is described by the Single Impurity Anderson Model (SIAM). Our results are comparable with Scanning Tunneling Microscope (STM) experimental data.

Spatial Modulation of Spin Polarization within a Single Co Island

When electrons are confined to nanostructures, quantum interference takes place due to their wave-particle duality. Using spin-polarized scanning tunneling microscopy, we studied the influence of quantum interference of electrons on the spatial distribution of the spin polarization within a single magnetic nanostructure. We find changes in both the magnitude and sign of the spin polarization on a subnanometer scale. We compare our experimental results with ab initio calculations of the spin-resolved local density of states (LDOS). We find that the modulation of the spin-polarization at a given energy can be ascribed to the different magnitudes of spatially modulated majority states and non-modulated minority states contributing to the total LDOS.

Probing buried magnetic nanostructures by spin-resolved local density of states at the surface: A density functional study

We demonstrate that it is possible to resolve magnetic properties of small clusters buried up to 1.5 nm deep beneath a metallic surface. The possibility of deducing magnetic properties of buried nanostructures by studying polarization maps of the surface is discussed. Moreover, we show that there is a way to determine the coupling of buried structures to each other by studying the polarization of electrons in vacuum space above the system.

スピン偏極走査トンネル顕微分光(SP-STM-STS)の原理とMn(001)のスピン依存局所状態密度

Development of spin-polarized scanning tunnel microscopy/spectroscopy (SP-STM/STS) is reviewed and the working principle of SP-STM/STS is discussed with paying special attention to the obtention of the spin-resolved local density of states (LDOS). We discuss the case of bct-Mn epitaxial films grown on Fe(001) whisker. The bct-Mn(001) layers couple antiferromagnetically with the adjacent layers and there is a spin-dependent LDOS peak above the Fermi level, to which highly-polarized two dz2 surface states contribute. The highest lateral magnetic resolution obtained in this system is 0.4 nm. Performing a detailed analysis of the SP-STS measurements, we conclude that the bct-Mn(001) surface has a 60±16% polarization at the energy position of the spin-dependent LDOS peak.

Theory of metastable deexcitation spectroscopy on simple metals

In this work we investigate the Auger deexcitation process of a slow metastable helium atom $({\mathrm{He}}^{*})$ in its triplet state on a Na surface. The electronic properties and the Auger rates of the interacting atom-metal system are calculated self-consistently at any distance of ${\mathrm{He}}^{*}$ from the surface in the framework of the local spin-density approximation using the embedding Green's function method. The energy distribution of the emitted electrons is obtained by integrating the Auger rates along the classical trajectory in the adiabatic approximation. The analysis of the spin-resolved local density of states shows that the excited atomic states broaden into resonances already at large atom-surface distances. The calculated atom deexcitation rates are in good agreement with experimental results.