Spin-polarized Surface States Research Articles

Ambipolar field effect in the ternary topological insulator (BixSb1–x)2Te3 by composition tuning

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping and electrical gating, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (Bi(x)Sb(1-x))2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi(x)Sb(1-x))2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.

Effect of the external electric field on surface states: Anab initiostudy

Here we present an ab initio study of surface states exposed to an external electric field. We demonstrate that the external electric field affects both the vacuum decay rates and the dispersion relations of surface states. In particular, the Cu(111) surface-state band bottom is shifted by means of the external electric field. Spin-selective screening of spin-polarized surface states on a Co bilayer on Cu(111) leads to the possibility of reversing the sign of the surface-state spin polarization.

Passage from Spin-Polarized Surface States to Unpolarized Quantum Well States in Topologically Nontrivial Sb Films

Topological materials have unusual surface spin properties including a net surface spin current protected by the bulk symmetry properties. When such materials are reduced to thin films, their gapless spin-polarized surface states must connect, by analytic continuation, to bulk-derived quantum-well states, which are spin-unpolarized in centrosymmetric systems. The nature of this passage in a model system, Sb films, is investigated. Angle-resolved photoemission shows a smooth transition, while calculations elucidate the correlated evolution of the spin and charge distributions in real space.

Direct observation of spin-polarized surface states in the parent compound of a topological insulator using spin- and angle-resolved photoemission spectroscopy in a Mott-polarimetry mode

We report high-resolution spin-resolved photoemission spectroscopy (spin-ARPES) measurements on the parent compound Sb of the recently discovered three-dimensional topological insulator Bi1−xSbx (Hsieh et al 2008 Nature452 970, Hsieh et al 2009 Science 323 919). By modulating the incident photon energy, we are able to map both the bulk and the (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin–orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a k-splitting of its spin-polarized electronic channels. These results complement our previously published works on this class of materials and re-confirm our discovery of topological insulator states in the Bi1−xSbx series.

Spin-polarized semiconductor surface states localized in subsurface layers

A pair of different surface-state and surface-resonance bands has been identified on $\text{Bi}/\text{Ge}(111)\text{\ensuremath{-}}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ by a combined experimental and computational study. The wave functions of the states have negligible amplitude at Bi atoms and are extended over more than 20 subsurface layers. These bands exhibit characteristic spin structure, which is ascribed to the combined Rashba and atomic spin-orbit interaction (SOI). Unlike previously known surface Rashba systems, the spin polarization is induced by SOI of a light element (Ge) with negligible contribution of a heavier one (Bi).

PbBi共吸着Ge(111)表面のスピン偏極した金属的表面電子状態

PbBi共吸着Ge(111)表面のスピン偏極した金属的表面電子状態

Asymmetric Spin Gap Opening of Graphene on Cubic Boron Nitride (111) Substrate

Graphene grown on cubic BN (111) is studied using the first-principles method based on density functional theory. Results of our calculations reveal that spin polarized surface states break the equivalence of the A and B sublattices and lead to spin polarized graphene bands with finite energy gaps. As a result, spin polarized carriers can be generated by shifting the Fermi-level inside the band gap of minority (majority) spin but inside the conduction (valence) band of majority (minority) spin. The spin state of the injected electrons can be manipulated by an external electric field alone.

Spin Polarized Surface State of fcc Co/Cu(001) Studied by Spin- and Angle-resolved Photoelectron Spectroscopy

We have newly developed a retarding-type Mott spin detector for spin- and angle-resolved photoelectron (SARPE) spectroscopy. Detection of scattered electrons at the Au target in an appropriate energy-loss window with a large opening angle has helped us to increase the efficiency of the spin polarimeter. The SARPE spectroscopy equipped with the developed spin detector has been applied to the study of surface and bulk electronic structures of fcc Co thin films grown on Cu(001) surface. The improved efficiency has enabled us to obtain the spin-resolved spectra while the surface is kept clean. As a result, we have successfully resolved a negatively spin-polarized surface resonance state of fcc Co/Cu(001) at Γ− of the surface Brillouin zone. It has also been clarified that the surface resonance state is markedly influenced by a spin-orbit coupling.

Surface Phase Transition inH/W(110)Induced by Tuning the Fermi Surface Nesting Vector by Hydrogen Loading

At a hydrogen coverage of one monolayer, W(110) is known to exhibit a Fermi nesting in its electronic surface states with a nesting vector q{N} of 0.9 A{-1} along [001]. Here we show that additional H adsorption allows a controlled tuning of q{N}. As q{N} approaches the commensurate value of 1.0 A{-1}, its signature in inelastic He-atom scattering becomes more pronounced, finally disappearing as a surface charge density wave (CDW) develops and the surface symmetry changes from c(2 x 2) to a p(8 x 2) superstructure. The gradual change in q{N} is attributed to an energetic shift of the spin-polarized electronic surface states that eventually form the surface CDW.

Interplay of electronic, magnetic and structural properties of surface-supported clusters

We present first theoretical evidence revealing the influence of structural changes on the spin-polarized surface states of large Co nanoislands grown on Cu(111). The minority density of electronic states possesses a pronounced peak whose energetic position depends sensitively on the Co layers stacking order. Our results suggest a way to deduce the stacking order of large Co nanoislands using scanning tunnelling microscopy/spectroscopy.

Electronic confinement on stepped Cu(111) surfaces:Ab initiostudy

The state-of-the-art ab initio calculations are performed to study surface states on stepped Cu(111) surfaces with the terrace widths ranging from $12\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}21\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. In agreement with experiments of Hansmann et al. [Phys. Rev. B 67, 121409(R) (2003)], close to the Fermi energy we reveal electronic states significantly affected by repulsive potential at steps. Our calculations demonstrate that the position of such states is strongly dependent on the terrace width. By using the Kronig-Penney model and ab initio results, the strength of potential barriers at step edges is determined. It is shown that the strength of the confining barriers on Cu(111) vicinals can be significantly affected by decoration of step edges with monatomic Fe wires, similar to recent experimental findings of Shiraki et al. [Phys. Rev. Lett. 92, 096102 (2004)]. Spin-dependent scattering of surface electrons at Fe wires is shown to result in the formation of spin-polarized surface states on stepped Cu(111) surfaces. The majority states remain unaffected. A localization of the minority states at Fe wires suppresses the confinementlike features of the local density of states.

Resonant magnetoresistance in organic spin valves (invited)

We investigate theoretically the effects of surface states over the magnetoresistance of Ni-based organic spin valves. In particular, we perform ab initio electronic transport calculations for a benzene-thiolate molecule chemically attached to a Ni [001] surface and contacted either by Te to another Ni [001] surface or terminated by a thiol group and probed by a Ni scanning tunnel microscope (STM) tip. In the case of S- and Te-bonded molecules we find a large asymmetry in the spin currents as a function of the bias, although the I-V is rather symmetric. This leads to a smooth although not monotonic dependence of the magnetoresistance over the bias. In contrast, in the case of a STM-type geometry we demonstrate that the spin current and the magnetoresistance can be drastically changed with bias. This is the result of a resonance between a spin-polarized surface state of the substrate and the d-shell band edge of the tip.

Surface electronic states in Co nanoclusters onAu(111): Scanning tunneling spectroscopy measurements andab initiocalculations

Scanning tunneling spectroscopy measurements on Co nano-clusters grown on $\mathrm{Au}(111)$, revealed the presence of two peaks in the electronic states, below and above the Fermi energy. Ab initio calculations show that both peaks are due to spin-polarized surface states. The crystallographic stacking of the Co nano-clusters are shown to influence strongly the energy shift of filled surface states, providing a way to increase the low bias spin current of magnetic tunnel junctions. Due to the small lateral dimensions of the nano-clusters $(\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, edge effects dominate the amplitude of the empty states peak.

Growth and Scanning Tunneling Microscopy and Spectroscopy Studies of Ultrathin Fe Films on Cr(001)

The growth and differential conductivity (dI/dV) spectra of ultrathin Fe films on Cr(001) were studied at room temperature (RT) by scanning tunneling microscopy, together with the spin-resolved image of the Cr(001) film. It is found that 2 ML Fe films deposited at RT and postannealed at 300 °C show atomically flat terraces and a distinct peak at +300 mV in the dI/dV spectrum, which corresponds to the spin-polarized surface state of the Fe(001) surface. These features were not observed for submonolayer Fe films on Cr(001) substrates or at postannealing temperatures lower than 200 °C. The spatially resolved dI/dV signal maps of the 2 ML Fe films show a distinct contrast. This contrast may not be of magnetic origin, but a result of the Fe layer dependence of the local density of states.

Spin-polarized surface states close to adatoms onCu(111)

We present a theoretical study of surface states close to 3d transition metal adatoms (Cr, Mn, Fe, Co, Ni and Cu) on a Cu(111) surface in terms of an embedding technique using the fully relativistic Korringa-Kohn-Rostoker method. For each of the adatoms we found resonances in the s-like states to be attributed to a localization of the surface states in the presence of an impurity. We studied the change of the s-like densities of states in the vicinity of the surface state band-edge due to scattering effects mediated via the adatom's d-orbitals. The obtained results show that a magnetic impurity causes spin-polarization of the surface states. In particular, the long-range oscillations of the spin-polarized s-like density of states around an Fe adatom are demonstrated.

Spin-polarized surface state of MnSb(0 0 0 1)

Knowledge of the spin-dependent electronic structure at surfaces and interfaces plays an increasingly important role when assessing possible use of novel magnetic materials for spintronic applications. It is shown that spin- and angle-resolved photoelectron spectroscopy together with ab initio electronic structure methods provides a full characterization of the surface electronic structure of ferromagnetic MnSb(0 0 0 1). Two different surface reconstructions have been compared in spin- and angle-resolved valence-band photoemission. For annealing at elevated temperatures, the (1 × 1)-structure transforms into 2 × 2 and a majority-spin peak appears at −1.7 eV inside a majority-spin bulk band gap at the surface Brillouin zone centre. Its sensitivity to oxygen supports an interpretation as magnetic compound surface state. Local spin density calculations predict at the same energy (−1.75 eV) a prominent d surface state of majority spin for (1 × 1)-Mn terminated MnSb(0 0 0 1) but no such feature for (1 × 1)-Sb termination. The calculation shows that neither the bulk nor the surface is half-metallic, in agreement with the expectation for the hexagonal NiAs structure.

Observation of Spin-Polarized Surface States on Ultrathin bct Mn(001) Films by Spin-Polarized Scanning Tunneling Spectroscopy

We report the observation of a magnetic contrast of up to 20% in the scanning tunneling spectroscopy dI/dV maps obtained with Fe-coated tips on Mn(001) layers grown on an Fe(001) whisker at 370 K. These nanometer resolution microscopy results show that the layers couple antiferromagnetically. By normalizing the dI/dV curves by tunneling probability functions, we found a spin-dependent peak on the body-centered-tetragonal (bct) Mn(001) surface at +0.8 V, whose high spin polarization gives rise to the dI/dV map contrast. Band structure calculations allow one to identify the +0.8 V peak as due to two spin-polarized d(z(2)) surface states.

Spin-polarized tunneling spectroscopy of co(0001) surface States.

Spin-polarized tunneling was studied on Co surfaces of exchange coupled Co/Cu/Co samples, using an Fe tip. A spin-polarized surface state of Co(0001) was found to exist at -0.43 eV relative to E(F), with FWHM of 0.23 eV in the spectra. The state exhibits negative magnetoresistance with an effective spin polarization of less than -23%, suggesting negatively high spin polarization of the surface state. Our first-principles calculations have supported the existence of the surface state. From the calculation, the state is identified as a minority spin Gamma-centered d(2)(z)-like surface state.

Topology-Induced Spin Frustrations at the Cr(001) Surface Studied by Spin-Polarized Scanning Tunneling Spectroscopy

The magnetic structure of the Cr(001) surface was investigated by spin-polarized scanning tunneling spectroscopy by making use of the spin-polarized surface state located close to the Fermi level. Periodic alternations of the intensity of the surface state peak in local tunneling spectra measured above different ferromagnetic terraces separated by monatomic steps confirm the topological antiferromagnetic order of the Cr(001) surface. Screw dislocations cause topology-induced spin frustration, leading to the formation of domain walls with a width of about 120 nm.

Vacuum-tunneling magnetoresistance: The role of spin-polarized surface states

We have studied magnetoresistance effects in Fe–vacuum–Gd tunnel junctions as a function of the applied bias voltage by using a scanning tunneling microscope operated under ultra-high-vacuum conditions. We found that the vacuum-tunneling magnetoresistance (VTMR) can be maximized by tunneling into highly spin-polarized surface states. By tuning the applied bias to the energetic positions of the spin-polarized surface states, a VTMR response as much as 31% at 70 K was obtained. This result is explained in terms of an enhancement caused by the spin-polarized surface state and a suppression of spin-flip tunneling processes compared to tunnel junctions with oxide barriers.