Spin-split Band Research Articles

Nanoscale studies of electric field effects on monolayer 1T\u2032-WTe2

Monolayer 1 T′-WTe2 is a quantum spin Hall insulator with a gapped 2D-bulk and gapless helical edge states persisting to temperatures ~100 K. Despite the far-ranging interest, the magnitude of the bulk gap, the effect of gating on the 2D-band structure, as well the role interactions are not established. In this work we use STM spectroscopy to measure the intrinsic bulk gap of monolayer 1 T′-WTe2 and show that gate induced electric fields cause large changes of the gap magnitude. Our first-principles DFT-derived tight-binding model reveal that a combination of spatial localization of the conduction and valance bands and Rashba-like spin-orbit coupling leads to a gating induced spin-splitting of the 2D-bulk bands in the tens of meV, thereby reducing the band gap. Our work explains the large sensitivity of the band structure to electric fields and suggests a new avenue for realizing proximity induced non-trivial superconductivity in monolayer 1 T′-WTe2.

Quasiparticle interference observation of the topologically nontrivial drumhead surface state in ZrSiTe

Drumhead surface states that link together loops of nodal lines arise in Dirac nodal-line semimetals as a consequence of the topologically non-trivial band crossings. We used low-temperature scanning tunneling microscopy and Fourier-transformed scanning tunneling spectroscopy to investigate the quasiparticle interference (QPI) properties of ZrSiTe. Our results show two scattering signals across the drumhead state resolving the energy-momentum relationship through the occupied and unoccupied energy ranges it is predicted to span. Observation of this drumhead state is in contrast to previous studies on ZrSiS and ZrSiSe, where the QPI was dominated by topologically trivial bulk bands and surface states. Furthermore, we observe a near $\mathbf{k} \rightarrow -\mathbf{k}$ scattering process across the $\Gamma$-point, enabled by scattering between the spin-split drumhead bands in this material.

Spin-dependent band-gap formation for the L-gap surface state on the reconstructed Au(111) surface

We elucidate how the free-electron-like energy dispersion of the L-gap surface state on a Au(111)-(1 × 1) surface is modified by the experimentally observed uniaxial reconstruction of the topmost atomic layer. For this purpose, we perform a first-principles embedded Green’s function calculation for the reconstructed semi-infinite Au(111) surface. The obtained band structure unfolded into the surface Brillouin zone of the (1 × 1) surface can be understood in terms of two spin-split parabolic bands centered at the point, their umklapp-induced replicas centered at reciprocal lattice vectors of the superlattice (SL) with much weaker intensities, and mini band gaps at the crossing of two of them. More importantly, it is revealed that the band-gap size depends not only on the amplitude of the SL potential but also on mutual spin orientations of two crossing bands. Furthermore, we demonstrate that the band-gap size and the charge density distribution of the surface states are closely correlated with spatial profile of the SL potential.

Electronic states driven by the crystal field in two-dimensional materials: The case of antimonene

The search for novel two-dimensional (2D) giant Rashba semiconductors is a crucial step in the development of spintronic technology. The electronic properties and growth mode of antimonene grown by molecular beam epitaxy were investigated using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. Results reveal the semiconducting nature of antimonene and the presence of a highly mobile 2D hole gas (2DHG) near the Fermi level. The anisotropy factor and Fermi velocity of the 2DHG are sensitively dependent on the crystal structure and in-plane lattice parameters of antimonene. Antimonene with small lattice parameters exhibits a small anisotropy factor and electron effective mass. Furthermore, the perpendicular (out of plane) electric field breaks the inversion symmetry and, when combined with strong spin-orbit coupling, induces the spin splitting of the valence band of antimonene. ARPES and work function measurements shed light on the band alignment between antimonene and the substrates and confirmed the presence of a nonignorable interface dipole of approximately 0.2 eV. This dipole could be the cause of approximately 80 meV spin splitting at the valence-band maximum of antimonene, a value that is significantly greater than the thermal energy at room temperature. In addition, the spin-splitting degree depends on the interface dipole strength. The gate voltage can generate the out of plane electric field and regulate the spin state similar to an internal electric field. Our results suggest that antimonene and its van der Waals heterostructure system grown on substrates could be used to develop highly efficient spin field-effect transistors and nanospintronic devices.

Mechanism of antisymmetric spin polarization in centrosymmetric multiple- Q magnets based on effective chiral bilinear and biquadratic spin cross products

We investigate how to engineer an antisymmetric spin-split band structure under spin density waves with finite ordering wave vectors in centrosymmetric systems without the relativistic spin-orbit coupling. On the basis of a perturbative analysis for the spin-charge coupled model in centrosymmetric itinerant magnets, we show that nonzero chiral-type bilinear and biquadratic spin cross products in momentum space under the magnetic orderings are related to an antisymmetric spin polarization in the electronic band structure. We apply the derived formula to the single-$Q$ cycloidal spiral and double-$Q$ noncoplanar states including the meron-antimeron and skyrmion crystals. Our results present a clue to realize a giant antisymmetric spin splitting driven by magnetic phase transitions in the centrosymmetric lattice structures without the spin-orbit coupling.

Spin splitting of dopant edge state in magnetic zigzag graphene nanoribbons.

Spin-ordered electronic states in hydrogen-terminated zigzag nanographene give rise to magnetic quantum phenomena1,2 that have sparked renewed interest in carbon-based spintronics3,4. Zigzag graphene nanoribbons (ZGNRs)-quasi one-dimensional semiconducting strips of graphene bounded by parallel zigzag edges-host intrinsic electronic edge states that are ferromagnetically ordered along the edges of the ribbon and antiferromagnetically coupled across its width1,2,5. Despite recent advances in the bottom-up synthesis of GNRs featuring symmetry protected topological phases6-8 and even metallic zero mode bands9, the unique magnetic edgestructure of ZGNRs has long been obscured from direct observation by a strong hybridization of the zigzag edge states with the surface states of the underlying support10-15. Here, we present a general technique to thermodynamically stabilize and electronically decouple the highly reactive spin-polarized edge states by introducing a superlattice of substitutional N-atom dopants along the edges of a ZGNR. First-principles GW calculations and scanning tunnelling spectroscopy reveal a giant spin splitting of low-lying nitrogen lone-pair flat bands by an exchange field (~850tesla) induced by the ferromagnetically ordered edge states of ZGNRs. Our findings directly corroborate the nature of the predicted emergent magnetic order in ZGNRs and provide a robust platform for their exploration and functional integration into nanoscale sensing and logic devices15-21.

Observation of Spin-Dependent Dual Ferromagnetism in Perovskite Ruthenates

We performed insitu angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES (SARPES) experiments to investigate the relationship between electronic band structures and ferromagnetism in SrRuO_{3} (SRO) thin films. Our high quality ARPES and SARPES results show clear spin-lifted band structures. The spin polarization is strongly dependent on momentum around the Fermi level, whereas it becomes less dependent at high-binding energies. This experimental observation matches our dynamical mean-field theory results very well. As temperature increases from low to the Curie temperature, spin-splitting gap decreases and band dispersions become incoherent. Based on the ARPES study and theoretical calculation results, we found that SRO possesses spin-dependent electron correlations in which majority and minority spins are localized and itinerant, respectively. Our finding explains how ferromagnetism and electronic structure are connected, which has been under debate for decades in SRO.

Topological phase transition in a magnetic Weyl semimetal

Topological Weyl semimetals (TWSs) are exotic crystals possessing emergent relativistic Weyl fermions connected by unique surface Fermi-arcs (SFAs) in their electronic structures. To realize the TWS state, certain symmetry (such as the inversion or time reversal symmetry) must be broken, leading to a topological phase transition (TPT). Despite the great importance in understanding the formation of TWSs and their unusual properties, direct observation of such a TPT has been challenging. Here, using a recently discovered magnetic TWS Co3Sn2S2, we were able to systematically study its TPT with detailed temperature dependence of the electronic structures by angle-resolved photoemission spectroscopy. The TPT with drastic band structures evolution was clearly observed across the Curie temperature (TC = 177 K), including the disappearance of the characteristic SFAs and the recombination of the spin-split bands that leads to the annihilation of Weyl points with opposite chirality. These results not only reveal important insights on the interplay between the magnetism and band topology in TWSs, but also provide a new method to control their exotic physical properties.

The influence of the magnetic configuration on the charge current generated by the temperature gradient in the double planar ferromagnetic tunnel junctions

The charge current generated by the temperature gradient applied to the double planar tunnel junctions with ferromagnetic external electrodes and central layer is investigated in the free-electron model. The situation when the bias voltage is applied to the junction is also considered. It has been shown that the analyzed current may depend on the orientation of magnetic moments in the ferromagnetic external electrodes and the central layer. The influence of the magnetic configuration on the tunnel current can be enhanced due to the resonant tunneling by the resonant states. Especially strong modifications of the current flowing through the junction can be observed as a result of changing the orientation of the magnetic moments in the central layer in junctions where the tunneling by the resonant states located below the bottom for the spin-down electrons in the source electrode is observed. The studied effects are strongly depended on the thickness of the central layer, spin splitting of the electron bands in the external electrodes and in the central layer, and the bias voltage applied to the junctions, but are also sensitive to the average temperature of the junction and the height and width of the barriers, whereas they are less sensitive to the temperature difference between the electrodes.

Magnetization dependent tunneling conductance of ferromagnetic barriers

Recent experiments on van der Waals antiferromagnets have shown that measuring the temperature (T) and magnetic field (H) dependence of the conductance allows their magnetic phase diagram to be mapped. Similarly, experiments on ferromagnetic CrBr3 barriers enabled the Curie temperature to be determined at H = 0, but a precise interpretation of the magnetoconductance data at H ≠ 0 is conceptually more complex, because at finite H there is no well-defined phase boundary. Here we perform systematic transport measurements on CrBr3 barriers and show that the tunneling magnetoconductance depends on H and T exclusively through the magnetization M(H, T) over the entire temperature range investigated. The phenomenon is reproduced by the spin-dependent Fowler–Nordheim model for tunneling, and is a direct manifestation of the spin splitting of the CrBr3 conduction band. Our analysis unveils a new approach to probe quantitatively different properties of atomically thin ferromagnetic insulators related to their magnetization by performing simple conductance measurements.

Pb/NiSi[formula omitted] atomic sandwich on Si(111)

Atomic sandwich consisting of single atomic layers of Pb and NiSi2 has been grown on Si(111) and its structural, electronic and transport properties have been elucidated using a set of experimental techniques, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, in situtransport and magnetotransport measurements, and density functional theory calculations. The Pb layers incorporate around 1.1 monolayers of Pb and display a melt-like 1 × 1 structure at room temperature which crystallizes into the 7 × 7-R38.2∘ (i.e., (83−35)) and “21×39” (i.e., (4−157)) structures upon cooling to low temperatures. Electronic band structure of the Pb/NiSi2/Si(111) sandwich contains two bands associated with NiSi2 and two spin-split bands associated with Pb with minor hybridization of the electrons belonging to different layers. Concerning the transport properties at low temperatures, the sandwich demonstrates a high conductance but without clear indication on superconductivity.

Magnetoconductance in epitaxial bismuth quantum films: Beyond weak (anti)localization

The complex behavior of magnetoconductance of Bi films grown epitaxially on Si(111) with a thickness of 20--100 bilayers (BL) was measured at $T$= 9 K in magnetic fields up to $B=4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, oriented in-plane parallel and perpendicular to the electric dc current $I$. Contributions to magnetoconductance (MC) by diffuse scattering, by weak localization (WL) as well as by weak antilocalization (WAL) were identified. All these components to MC turned out to be isotropic in two dimensions, i.e., no dependence on angle between $B$ and $I$ within the surface plane was found. Only for $B\phantom{\rule{0.16em}{0ex}}\ensuremath{\perp}\phantom{\rule{0.16em}{0ex}}I$ an increase of MC was detected that is, to first approximation, $\ensuremath{\propto}{B}^{2}$. It is ascribed to ballistic scattering between the Rashba-split interfaces that allow Umklapp scattering without spin flip. While MC within the surface states, dominant at small thicknesses, $d$, shows negligible diffuse scattering under the chosen geometry, their quantum corrections are characterized by WAL with $\ensuremath{\alpha}=\ensuremath{-}0.3$ and a coupling strength that decays $\ensuremath{\propto}1/d$ with layer thickness. The admixing of quantized bulk states, which dominates MC above 50 BL, not only increases diffuse scattering, it introduces WL in combination with WAL. Presumably due to hybridization with the surface states, it also modifies strongly the WAL component for $d>60$ BL. Thus our findings suggest an intriguing interplay in magnetotransport between 2D and quantized 3D states at the Fermi surface of ultrathin bismuth quantum films and provide further deep insight into the electronic transport in quantized and partly spin split bands.

Electric field induced magnetism decline in organic ferromagnets

With an Anderson-like model including electron-electron interaction and electron-lattice interaction, the effect of external electric field on the electric and magnetic properties of quasi-one-dimensional organic ferromagnets is investigated theoretically. The results demonstrate a field-induced magnetism decline of the organic ferromagnet, which manifests a continuous decrease of the total molecular spin and suppression of the spin density wave after a critical field. The analysis of energy band evolution under the field indicates that the field-induced energy levels crossover of the spin-split flat bands is responsible for the magnetism decline. A companied charge polarization and charge transfer between spin radicals and main chain in the OF are observed. Different strength of the internal interactions including electron-electron interaction, hopping integral between radicals and main chain, and electron-lattice interaction is discussed. This work is helpful to understand the property of organic ferromagnets under external field in the situation of organic-ferromagnetic devices.

Impurity-Induced Emission in Re-Doped WS2 Monolayers.

Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS2 substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe-Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge.

Triple-Point Fermions in Ferroelectric GeTe.

Ferroelectric α-GeTe is unveiled to exhibit an intriguing multiple nontrivial topology of the electronic band structure due to the existence of triple-point and type-II Weyl fermions, which goes well beyond the giant Rashba spin splitting controlled by external fields as previously reported. Using spin- and angle-resolved photoemission spectroscopy combined with abinitio density functional theory, the unique spin texture around the triple point caused by the crossing of one spin-degenerate and two spin-split bands along the ferroelectric crystal axis is derived. This consistently reveals spin winding numbers that are coupled with time-reversal symmetry and Lorentz invariance, which are found to be equal for both triple-point pairs in the Brillouin zone. The rich manifold of effects opens up promising perspectives for studying nontrivial phenomena and multicomponent fermions in condensed matter systems.

Annealing temperature effect on optical properties of Cr-implanted ZnO nanostructure

Room temperature ferromagnetism was observed in Cr-implanted ZnO nanowires annealed at 500, 600, and 700 °C. The implantation dose for Cr ions was 1 ×1016 cm−2 , while the implantation energies were 100 keV. Except for ZnO (100), (002), and (200) orientations, no ex- tra diffraction peaks from Cr-related secondary phase or im- purities were observed. With the increasing of annealing temperatures, the intensity of the peaks increased while the FWHM values decreased. The Cr 2p1/2 and 2p3/2 peaks, with a binding energy difference of 10.6 eV, appear at 586.3 and 575.7 eV, can be attributed to Cr3+ in ZnO nanowires. For the Cr-implanted ZnO nanowires without annealing, the band energy emission disappears and the defect related emission with wavelength of 500–700 nm dominates, which can be attributed to defects introduced by implantation. Cr- implanted ZnO nanowires annealed at 500 °C show a saturation magnetization value of over 11.4 × 10−5 emu and a positive coercive field of 67 Oe. The origin of ferromagnetism behavior can be explained on the basis of electrons and defects that form bound magnetic polarons, which over- lap to create a spin-split impurity band.

Highly stable two-dimensional metal-carbon monolayer with interpenetrating honeycomb structures

With the ongoing effort in proposing and realizing functional two-dimensional (2D) materials, we predict by first-principles calculations a family of 2D metal-carbon (M–C) crystals consisting of M–C trigonal lattice interpenetrated with the metal buckled honeycomb structure. We suggest by simulations that the 2D M–C crystals can be readily fabricated by a self-organizing lattice reconstruction process after placing metal atoms on hollow sites of γ-graphyne. In total, we found 12 members of the family and they exhibit a variety of electronic and magnetic properties. In this work, we highlight and focus on the Fe member of the family, 2D-Fe2C12. Each Fe in 2D-Fe2C12 has a magnetic moment of 1 μB due to the spin splitting of Fe E1 bands at Fermi surface, resulting in half metallicity and high catalytic activity with unusually high-density single-atom Fe active sites. Ab initio molecular dynamics simulations revealed that the 2D-Fe2C12 retains its structural integrity up to 700 K of simulated short duration annealing. We expect these results to stimulate experimental research for the 2D M–C crystals we proposed.

Efficient Electrical Spin Splitter Based on Nonrelativistic Collinear Antiferromagnetism.

Spin-current generation by electrical means is among the core phenomena driving the field of spintronics. Using abinitio calculations we show that a room-temperature metallic collinear antiferromagnet RuO_{2} allows for highly efficient spin-current generation, arising from anisotropically spin-split bands with conserved up and down spins along the Néel vector axis. The zero net moment antiferromagnet acts as an electrical spin splitter with a 34° propagation angle between spin-up and spin-down currents. The corresponding spin conductivity is a factor of 3 larger than the record value from a survey of 20 000 nonmagnetic spin-Hall materials. We propose a versatile spin-splitter-torque concept circumventing limitations of spin-transfer and spin-orbit torques in present magnetic memory devices.

Spin relaxation of holes in In0.53Ga0.47As/InP quantum wells

Polarization-resolved photoluminescence was used to study spin relaxation of photoexcited holes in In0.53Ga0.47As/InP quantum wells in a quantizing magnetic field as a function of temperature. At a temperature below 10 K, the circular polarization of the photoluminescence due to the spin-split valence band Landau levels was found temperature-independent. In this temperature range fast hole spin relaxation as compared to their lifetime leads to the photoluminescence circular polarization determined by the ratio of these times. Increasing temperature resulted in efficient hole spin thermalization in the Zeeman split valence band Landau levels and as a consequence, in vanishing photoluminescence polarization. Fits of the experimental data by the theory allowed a determination of the hole spin relaxation times related to different Landau levels and the corresponding hole effective g-factor. Direct measurements of the hole spin relaxation times prove the obtained results.

Effects of transition metal (Fe, Co & Ni) doping on structural, electronic and optical properties of CuO: DFT + U study

To investigate the structural, electronic and optical properties of CuO and transition metal (Ni, Fe & Co) doped CuO, first-principles calculation is performed based on standard DFT and DFT + U. The use of DFT + U calculations confirm that the growth of the CuO films is p-type in nature with an indirect bandgap of 1.58 eV which is in good agreement with the experimental results. When doped with transition metals, the band gap value decreased to be 1.51 eV with iron (Fe). While the band gap increased to be 1.6 eV with Co doping, by directly contributing in valence band maxima (VBM) formation. Shallow impurity states also formed on top of valance band when doped with Fe and Co and which in turn can reduce recombination rate acting as carrier traps leading to better quantum efficiency. Net effective magnetic moment for Co, Ni and Fe doping was measured to be 1.958 μ B , 0.972 μ B and 0.997 μ B respectively originating from spin splitting of valance and conduction bands which may improve the photocatalytic efficiency and raise the recycle rate of photocatalysts. Calculated real and imaginary part of dielectric constant also matched with experimental literatures. Significant absorptivity of CuO with low reflectivity and maximum refractive index in visible range along with excellent tunability through doping was also observed. Thus, it can be concluded that, these changes in band structure and optical properties accompanied by generation of net magnetic moment due to doping open up wide window for novel strategies of tailoring properties for desired applications.