Spin Polarization and Attosecond Time Delay in Photoemission from Spin Degenerate States of Solids

After photon absorption, electrons from a dispersive band of a solid require a finite time in the photoemission process before being photoemitted as free particles, in line with recent attosecond-resolved photoemission experiments. According to the Eisenbud-Wigner-Smith model, the time delay is due to a phase shift of different transitions that occur in the process. Such a phase shift is also at the origin of the angular dependent spin polarization of the photoelectron beam, observable in spin degenerate systems without angular momentum transfer by the incident photon. We propose a semiquantitative model which permits us to relate spin and time scales in photoemission from condensed matter targets and to better understand spin-and angle-resolved photoemission spectroscopy (SARPES) experiments on spin degenerate systems. We also present the first experimental determination by SARPES of this time delay in a dispersive band, which is found to be greater than 26 as for electrons emitted from the sp-bulk band of the model system Cu(111).

Spin polarization and attosecond time delay in photoemission from solids

Mauro Fanciulli

In the photoemission process electrons are emitted from a solid upon excitation with UV light. From the measurement of their energy and momentum, angle-resolved photoemission spectroscopy (ARPES) allows to reconstruct the electronic properties of the solid. In addition, in spin-resolved ARPES the spin polarization P of the electrons is measured while maintaining energy and momentum resolution of ARPES, required to probe a dispersive state of a solid. The spin information is related to the spin polarization of the state under investigation, but can be modified during the photoemission process. Indeed, even when the electrons originate from a spin-degenerate initial state, they can acquire a finite P. This effect, ultimately due to a symmetry breaking in the experiment, occurs when different channels in the matrix elements describing the transition coherently interfere. In particular, P is related to the phase shift between the complex matrix elements associated to the interfering channels. This phase shift is also closely related to another quantity: the Eisenbud-Wigner-Smith (EWS) time delay. Despite the fundamental difficulties to properly define the concept of time in quantum mechanics, the EWS time delay, introduced to describe the electron scattering process, can be adapted to study the chronoscopy of photoemission. Nowadays, thanks to advances in laser technology, time-resolved spectroscopic techniques that reach attosecond resolution allow to measure relative EWS time delays between photoelectrons from different states. In this Thesis, an alternative indirect way to probe the photoemission process in the attosecond domain without time-resolved laser techniques will be presented. The link between P from dispersive states of a solid and the EWS time delay will be introduced. An analytical model will be discussed, where the dependence of P with binding energy is related to the EWS time delay of photoemission as a scattering process and the EWS time delay between the interfering channels. The first experimental determination of EWS time delays from dispersive states by SARPES will be also presented. Synchrotron radiation-based SARPES is the main technique used in this Thesis. Results for a single crystal of Cu(111) give EWS time delays of about 26 as for the free-electron-like sp bulk-derived band. The P of the d bands and of the 3p core levels of copper is also investigated. On the other hand, experiments on the strongly correlated cuprate superconductor BSCCO2212 show EWS time delays that are at least 3 times larger than in Cu(111). A double polarization feature is observed for the dispersive states, likely related to self-energy corrections in the photoemission process. Recent results on BSCCO2212 performed with laser-based SARPES will be also discussed. The results presented in this Thesis pave the way for a qualitatively new kind of information accessible by the SARPES technique, which is complementary to attosecond-resolved spectroscopies. The model presented sheds light on the spin polarization that is obtained by one-step model photoemission calculations, where the time information becomes also available. This approach could help to advance in the understandings of the physics of materials of interests, in particular electronic correlations, but also to better interpret the spin information experimentally obtained in SARPES, as well as to describe the basics of the quantum mechanics of the photoemission process itself.

EPFL2018

Spectroscopic Studies on Semiconducting Interfaces with Giant Spin Splitting

Emmanouil Frantzeskakis

The application of an external magnetic field can lift the spin degeneracy of electronic states through its interaction with the electronic magnetic moment. A closely-related phenomenon is the Rashba-Bychkov (RB) effect where symmetry breaking at surfaces or interfaces gives rise to an electric field which is in turn seen as an effective magnetic field in the electrons' rest frame. The resulting k-dependent energy splitting of spin-polarized electronic states has been observed on various metal surfaces but the effect is much larger in artificially-grown surface alloys; such as the BiAg2 grown at the surface of Ag(111). The spin splitting magnitude observed in these systems might be very useful in spintronics applications since it could decrease the spin precession time in a spin transistor and distinguish between the extrinsic and intrinsic spin Hall effects. Nevertheless, their metallic character poses serious obstacles in the exploitation of the RB effect due to the presence of spin-degenerate electronic states at the Fermi level which would dominate transport experiments. We have used angle-resolved photoelectron spectroscopy (ARPES) to explore the RB effects on various artificially grown structures, formed on semiconducting substrates. The interplay of quantum confinement and giant RB splitting on a trilayer Si(111)-Ag-BiAg2 system reveals the formation of a complex spin-dependent structure, which can be externally tuned by varying the Ag layer thickness. This provides a means to tailor the electronic structure and spin polarization near the Fermi level, with potential applications on Si-compatible spintronic devices. Moreover, we have discovered a giant spin splitting in a true semiconducting system, namely the Si(111)-Bi trimer phase. The size of the RB parameters is comparable to those of metallic surface alloys. Using theoretical models we have identified the peculiar band topology as the origin of the giant spin splitting on the Bi/Si(111) system. All our findings are supported by relativistic first-principles calculations. Finally, a chapter of this thesis manuscript is devoted to the description of phenomeno-logical theoretical simulation, which can capture the experimental results related to the RB effect on low-dimensional systems. A parallel experimental project is discussed in a separate chapter. It has been focused on the band topology of the novel p(2 × 2) reconstruction of the Pt(111)-Ag-Bi trilayer. We investigated the symmetry properties of the interface states by varying the amount of Ag. ARPES results present the electronic signature of a strain-related structural transition.

EPFL2010

Spin polarization in photoemission from the cuprate superconductor Bi2Sr2CaCu2O8+delta

Jan Hugo Dil, Mauro Fanciulli, Stefan Peter Muff, Andrew Patton Weber

Photoelectrons produced from the excitation of spin-degenerate states in solids can have a sizable spin polarization, which is related to the phase of interfering channels in the photoemission matrix elements. Such spin polarization can be measured by spin-resolved photoemission spectroscopy to gain information about the transitions and the Wigner time delay of the process. Incorporating strongly correlated electron systems into this paradigm could yield a novel means of extracting phase information crucial to understanding the mechanism of their emergent behavior. In this work, we present, as a case study, experimental measurements of the cuprate superconductor Bi2Sr2CaCu2O8+delta by spin-resolved photoemission while maintaining full angular and energy resolution. A spin polarization of at least 10% is observed, which is related to the phase of the photoelectron wave function.