Space-, time- and spin-resolved photoemission

Schoenhense, G.; Medjanik, K.; Elmers, H. J.

doi:10.1016/j.elspec.2015.05.016

Cited by 91 publications

(96 citation statements)

References 126 publications

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“…The maximum parallelization of data acquisition using this method relies on pulsed photon sources such as synchrotron radiation, high harmonic sources or laser radiation. In the determination of the spectral function I(E B , k x , k y ) the usable energy width of a single measurement is limited to typically 6 eV by the chromatic aberration of the electron-optical system (for further details, see [43]). Acquiring ca.…”

Section: Methodsmentioning

confidence: 99%

4D texture of circular dichroism in soft-x-ray photoemission from tungsten

et al. 2019

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Photoemission-intensity distributions I RCP/LCP (E B , k) measured for right-and left-circularly polarized soft x-rays revealed a large circular dichroism in angular distribution (CDAD) in the 4D parameter space (E B binding energy, k momentum vector). Full-field k-imaging combined with timeof-flight energy recording at a high-brilliance soft x-ray beamline allowed mapping the CDAD in the bulk Brillouin zone of tungsten and the entire d-band complex within a few hours. CDADasymmetries are very high (up to 90%), persist throughout the whole photon-energy range (300-1300 eV) and show a pronounced dependence on momentum k and binding energy E B , visualized as movies or sequences of cuts through the 4D object. One-step photoemission calculations for the same photon energies show fair agreement with the measured results. In addition to the requirement of a 'handed' experimental geometry, known from previous experiments on adsorbates and surface states, we find an anti-symmetric behavior of the CDAD with respect to two bulk mirror planes. A new symmetry condition along the perpendicular momentum k z makes CDAD a valuable tool for an unambiguous identification of high-symmetry planes in direct transitions in the periodic zone scheme. Technically, the method provides a circular polarimeter for soft, tender and hard x-rays. being sensitive to their phase-shift differences. The differential photoemission cross section is proportional to the squared total dipole matrix element that contains interference terms between different final-state partial waves. Upon reversal of the photon helicity (in case of CDAD) or switching of the electric vector between orthogonal directions (for LDAD), interference terms with odd symmetry change their signs. This is the origin of the dichroism in the photoelectron angular distribution. This phenomenon exists in the pure electric dipole approximation and thus it differs from the 'natural' CD of chiral molecules, which is described by higher-order terms in the electron-photon operator [14].In the present work we employ the technique of time-of-flight (ToF) k-microscopy for a comprehensive study of the CDAD in the soft-x-ray spectral range. This new method captures simultaneously more than one full Brillouin zone (BZ) for binding energies in a range of several eV, yielding full information from the valence bands. In the vacuum ultraviolet (VUV) range, this instrument was previously used for the study of LDAD in photoemission from Mo(110) surface states and surface resonances [15]. These states are characterized by their 2D character and their location at the surface, wherefore they fulfill the criteria discussed for adsorbate systems [13]. Earlier work on graphite [16] and Pd [17] revealed that a CDAD also exists in photoemission from crystalline samples. However, the low photon energies used essentially probe the surface character of the bands. Hence, these results cannot be considered representative for true 3D bulk states. Moreover, no information on the dependence of the CDAD on the electro...

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Section: Methodsmentioning

confidence: 99%

4D texture of circular dichroism in soft-x-ray photoemission from tungsten

et al. 2019

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“…Details of this instrument are given in Ref. [23]. For the present experiment the overall energy and the k || resolution is arXiv:1512.01363v1 [cond-mat.mtrl-sci] 4 Dec 2015…”

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confidence: 99%

Spin mapping of surface and bulk Rashba states in ferroelectric α -GeTe(111) films

Elmers¹,

Wallauer²,

Liebmann³

et al. 2016

Phys. Rev. B

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A comprehensive mapping of the spin polarization of the electronic bands in ferroelectric α-GeTe(111) films has been performed using a time-of-flight momentum microscope equipped with an imaging spin filter that enables a simultaneous measurement of more than 10.000 data points (voxels). A Rashba type splitting of both surface and bulk bands with opposite spin helicity of the inner and outer Rashba bands is found revealing a complex spin texture at the Fermi energy. The switchable inner electric field of GeTe implies new functionalities for spintronic devices.The strong coupling of electron momentum and spin in low-dimensional structures allows an electrically controlled spin manipulation in spintronic devices [1][2][3][4], e.g. via the Rashba effect [5].The Rashba effect has first been experimentally demonstrated in semiconductor heterostructures, where an electrical field perpendicular to the layered structure, i.e. perpendicular to the electron momentum, determines the electron spin orientation relative to its momentum [6][7][8]. An asymmetric interface structure causes the necessary inversion symmetry breaking and accounts for the special spin-splitting of electron states, the Rashba effect [5], the size of which can be tuned by the strength of the electrical field.For most semiconducting materials the Rashba effect causes only a quite small splitting of the order of 10 −2Å−1 and thus requires experiments at very low temperatures [9][10][11] and also implies large lateral dimensions for potential spintronic applications. A considerably larger splitting has been predicted theoretically [12] and was recently found experimentally for the surface states of GeTe(111) [13,14].GeTe is a ferroelectric semiconductor with a Curie temperature of 700 K. Thus, besides the interface induced Rashba splitting, the ferroelectric properties also imply a broken inversion symmetry within the bulk and thus would allow for the electrical tuning of the bulk Rashba splitting via switching the ferroelectric polarization [12,15,16]. This effect is of great interest for nonvolatile spin orbitronics [10].For GeTe a bulk Rashba splitting of 0.19Å −1 has been predicted theoretically [12]. Experimentally, bulkRashba bands are rare and have only been found in the layered polar semiconductors BiTeCl and BiTeI [17][18][19][20] that, however, are not switchable.A characterization of the ferroelectric properties and a measurement of the spin polarization of the surface states of GeTe(111) at selected k-points has been performed previously by force microscopy [21,22] and spinresolved angular resolved photoemission spectroscopy, respectively [13]. A recent experimental and theoretical study revealed that at the Fermi level the hybridization of surface and bulk states causes surface-bulk resonant states resulting in unconventional spin topologies with chiral symmetry [14].Here, we demonstrate the spin structure of surface and bulk bands of the GeTe(111) surface using the novel photoemission technique of spin-resolved time-of-flight momentum micro...

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“…From the literature data, it seemed that the direct transitions are strongly masked by quasi‐elastic phonon scattering leading to a diffuse background. Previously, we found excellent k ‐images in the soft X‐ray range . However, it was not clear whether the k z ‐resolution stays sufficiently high in the several‐keV range, characterized by true bulk sensitivity.…”

Section: Photoelectron Momentum Microscopymentioning

confidence: 78%

Electron‐ and X‐Ray Spectroscopies of Organic Charge‐Transfer Complexes

Medjanik

Elmers

Schönhense

et al. 2019

Physica Status Solidi (b)

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Recent work aiming at a deeper understanding of the electronic properties of classical and novel charge‐transfer (CT) complexes is reviewed herein. From a variety of studied CT salts, the prototypical examples TTF‐TCNQ, (TMTTF)2SbF6, (TMTSF)2PF6, and TMP/HMP‐TCNQ have been selected. Three different types of electron and X‐ray spectroscopies were applied, namely ultraviolet photoelectron spectroscopy (UPS), hard X‐ray photoelectron spectroscopy (HAXPES), and near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy. Using UPS, the development of the valence band structure during co‐deposition of the donor and acceptor moieties TMP/HMP and TCNQ has been studied. The spectra reveal characteristic level shifts and a strong reduction of the HOMO–LUMO gap upon formation of the complex. HAXPES probes the core levels and reveals characteristic shifts of the binding energies in (TMTTF)2SbF6 at the phase transition to the low‐temperature charge‐ordered state. NEXAFS is a local, element‐specific probe for the unoccupied bands above the Fermi level and reveals changes in the population of specific orbitals when the CT complex is formed. As an outlook, the novel method of photoelectron momentum microscopy is presented, giving access to the valence bands and core‐levels and in particular to high‐resolution X‐ray photoelectron diffraction patterns.

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Space-, time- and spin-resolved photoemission

Cited by 91 publications

References 126 publications

4D texture of circular dichroism in soft-x-ray photoemission from tungsten

4D texture of circular dichroism in soft-x-ray photoemission from tungsten

Spin mapping of surface and bulk Rashba states in ferroelectric α -GeTe(111) films

Electron‐ and X‐Ray Spectroscopies of Organic Charge‐Transfer Complexes

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