Momentum and photon energy dependence of the circular dichroic photoemission in the bulk Rashba semiconductors BiTeX (X = I, Br, Cl)

Bulk Rashba systems BiTeX (X = I, Br, Cl) are emerging as important candidates for developing spintronics devices because of the coexistence of spin-split bulk and surface states, along with the ambipolar character of the surface charge carriers. The need to study the spin texture of strongly spin-orbit-coupled materials has recently promoted circular dichroic angular resolved photoelectron spectroscopy (CD-ARPES) as an indirect tool to measure the spin and the angular degrees of freedom. Here we report a detailed photon-energy-dependent study of the CD-ARPES spectra in BiTeX (X = I, Br, Cl). Our work reveals a large variation in the magnitude and sign of the dichroism. Interestingly, we find that the dichroic signal modulates differently for the three compounds and for the different spin-split states. These findings show a momentum and photon-energy dependence for the CD-ARPES signals in the bulk Rashba semiconductor BiTeX (X = I, Br, Cl). Finally, the outcome of our experiment indicates the important relation between the modulation of the dichroism and the phase differences between the wave functions involved in the photoemission process. This phase difference can be due to initial- or final-state effects. In the former case the phase difference results in possible interference effects among the photoelectrons emitted from different atomic layers and characterized by entangled spin-orbital polarized bands. In the latter case the phase difference results from the relative phases of the expansion of the final state in different outgoing partial waves.

Spin-orbit coupling effects in the band structure of surface alloys

Luca Moreschini

At the surface of a solid, quantum mechanics allows the existence of two-dimensional Bloch waves as solutions of the Schrödinger equation. These "surface" states are interesting both for fundamental and practical reasons, and have been extensively explored, e.g., on pure metals. Conversely, relatively few studies exist of the electronic states of artificially-grown two-dimensional structures. This work is an investigation, mainly by angle-resolved photoemission, of different heterogeneous surfaces of defined stoichiometry, formed by a large-Z p metal on a noble metal substrate, which constitute what is called a "surface alloy". A breakthrough has been obtained on the Bi/Ag(111) (√3 × √3)R30° structure, which presents a band dispersion split into two spin polarized replicas in momentum space by the spin-orbit coupling. The effect is known as Rashba-Bychkov splitting in semiconductors, and it is a genuine effect of the symmetry breaking occurring at the surface, but in Bi/Ag(111) it is much larger than reported in any other compound. This sets this class of materials among the suitable candidates for spintronics applications, which have been object of rising interest in the last years. We have put our attention on other surface alloys, varying substrates (Ag(111) and Cu(111)) and adsorbates (Bi, Pb, Sb) in order to set the basis for a unifying description of the quantities involved in the determination of the size of the splitting, and in particular to discriminate between atomic and structural contributions. An analogous band structure is found in all the systems, consisting namely of two "sets" of bands with negative effective mass, originating from the hybridization between the surface states of the substrate and the p electrons of the adsorbate. We have identified a significant in-plane component of the electric potential gradient, which is not present in pure metal surfaces, as the new element most likely responsible for the large size of the splitting. At the same time, the results show that a necessary condition for it to be effective in enhancing the splitting is a strong atomic spin-orbit parameter. Our findings are confirmed by first-principle calculations, which reproduce quantitatively the band structure, and by a nearly free electron model, which qualitatively supports our interpretation of the role of the in-plane gradient. A parallel project of this thesis has concerned Yb-based Kondo systems, as studied by hard x-ray photoemission and inelastic x-ray scattering. At these high energies, photoemission probes a thicker region below the sample surface, where heavy-fermion compounds stabilize in a different electronic configuration with respect to that of the bulk. We have shown that the temperature dependence of the 4f spectral function is in line with the predictions of the Anderson impurity model, a point which has often been questioned on the basis of previous results of low-energy photoemission. On the other hand, by a comparison with inelastic scattering, we have noticed that the latter leads to a better agreement with the data from thermodynamic and transport measurements. We conclude that, despite the unique capability of photoemission of accessing directly the electron spectral function, photon-in–photon-out spectroscopies are a more trustworthy probe of the bulk electronic structure.

EPFL2009

Observation of Dirac surface states in the noncentrosymmetric superconductor BiPd

Christian Reinhard Ast, Klaus Kern, Peter Wahl

Materials with strong spin-orbit coupling (SOC) have in recent years become a subject of intense research due to their potential applications in spintronics and quantum information technology. In particular, in systems which break inversion symmetry, SOC facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Here, by combining angular resolved photoemission spectroscopy and low temperature scanning tunneling microscopy measurements with relativistic first-principles band structure calculations, we examine the role of SOC in single crystals of noncentrosymmetric BiPd. We report the detection of several Dirac surface states, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination.

Amer Physical Soc2016

Spin-orbit coupling effects in the band structure of surface alloys

Luca Moreschini

EPFL2009