"k-resolved electronic structure by soft-X-ray ARPES: From 3D systems to buried interfaces and impurities"

Who: V. Strocov (Swiss Light Source and Paul Scherrer Institute)

Place: Donostia International Physics Center

Date: Friday, 26 June 2015, 12:00

k-resolved electronic structure by soft-X-ray ARPES:
From 3D systems to buried interfaces and impurities

Vladimir N. STROCOV

Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland

ARPES is the unique tool to explore electronic structure of solid-state
systems resolved in electron momentum k. Pushing this technique into the
soft-X-ray energy range (SX-ARPES) extends its applications from surface
physics towards 3D crystal systems, buried interfaces and impurity systems.
These spectroscopic abilities result from enhancement of the photoelectron
escape depth and a possibility of resonant photoexcitation delivering the
elemental and chemical state specificity. In this talk, I unfold a diversity
of new scientific highlights achieved with SX-ARPES [1].

3D materials. The applications to 3D systems are based on sharp definition
of surface-perpendicular momentum k resulting from the enhanced photoelectron
delocalization. An example is the 3D perovskite La(1-x)Sr(x)MnO3 exhibiting
CMR properties due to interplay of the double-exchange itineracy and polaronic
self-localization. The experimental 3D Fermi surface (FS) reveals characteristic
"shadow" contours, resembling those in cuprates, which manifest the rhombohedral
structural distortion reducing the CMR critical temperature. Other applications
include VSe2 with charge-density waves resulting from 3D nesting of its FS, bulk
Rashba splitting in non-centrosymmetric topological insulator BiTeI, conventional
superconductor MgB2, Fermi states in quasicrystalline AlNiCo, etc.

Buried heterostructures. The LaAlO3/SrTiO3 interface of two transition metal
oxides embeds mobile 2D electron gas. Its signal can be accentuated using resonant
SX-ARPES at the interface Ti3+ ions, which exposes the dxy-, dyz- and dxz-derived
subbands localized in the interface quantum well. Their intensity variations in
k-space reveal the Fourier composition of their wavefunctions. The peak-dip-hump
spectral function manifests strong polaronic coupling of interface electrons as
the fundamental limit of their temperature-dependent mobility. Oxygen vacancies
increase the electron concentration, as expressed by the Luttinger count of the
FS, and reduce the polaronic coupling. These findings extend to other oxide
heterostructure systems.

Buried impurities. Resonant SX-ARPES applied to the paradigm diluted magnetic
semiconductor GaMnAs has identified the ferromagnetic Mn impurity band, and
established its energy alignment and mechanism of hybridization with the host
GaAs bands. Combining the previous p-d exchange and double-exchange models,
these results suggest a microscopic picture of the GaMnAs ferromagnetism based
on the Anderson impurity model. Another example is InFeAs showing the ferromagnetism
induced by doped highly mobile electron carriers.

[1] V.N. Strocov et al, Synchr. Rad. News 27, N2 (2014) 31

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