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Breakthrough in HAXPES Performance Combining Full-Field k-Imaging with Time-of-Flight Recording
Authors:
K. Medjanik,
S. V. Babenkov,
S. Chernov,
D. Vasilyev,
H. J. Elmers,
B. Schoenhense,
C. Schlueter,
A. Gloskowskii,
Yu. Matveyev,
W. Drube,
G. Schoenhense
Abstract:
We established a new approach to hard-X-ray photoelectron spectroscopy (HAXPES). The instrumental key feature is an increase of the dimensionality of the recording scheme from 2D to 3D. A high-energy momentum microscope can detect electrons with initial kinetic energies more than 6 keV with high angular resolution < 0.1°. The large k-space acceptance of the special objective lens allows for simult…
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We established a new approach to hard-X-ray photoelectron spectroscopy (HAXPES). The instrumental key feature is an increase of the dimensionality of the recording scheme from 2D to 3D. A high-energy momentum microscope can detect electrons with initial kinetic energies more than 6 keV with high angular resolution < 0.1°. The large k-space acceptance of the special objective lens allows for simultaneous full-field imaging of many Brillouin zones. Combined with time-of-flight parallel energy recording, this method yields maximum parallelization of data acquisition. In a pilot experiment at the new beamline P22 at PETRA III, Hamburg, count rates of more than $10^{6}$ counts per second in the d-band complex of transition metals established an unprecedented HAXPES recording speed. It was found that the concept of tomographic k-space mapping previously demonstrated in the soft X-ray regime works equally well in the hard X-ray range. Sharp valence band k-patterns of Re collected at an excitation energy of 6 keV correspond to direct transitions to the 28th repeated Brillouin zone. Given the high X-ray brilliance (1.1x$10^{13}$ hv/s in a spot of less than 20x15 $mu^{2}$), the 3D bulk Brillouin zone can be mapped in a few hours. X-ray photoelectron diffraction (XPD) patterns with < 0.1° resolution are recorded within minutes. Previously unobserved fine details in the diffractograms reflect the large number of scatterers, several $10^{4}$ to $10^{6}$, depending on energy. The short photoelectron wavelength (an order of magnitude smaller than the interatomic distance) amplifies phase differences and makes hard X-ray XPD with high resolution a very sensitive structural tool. The high count rates pave the way towards spin-resolved HAXPES using an imaging spin filter.
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Submitted 26 October, 2018;
originally announced October 2018.
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Raman and fluorescence contributions to resonant inelastic soft x-ray scattering on LaAlO$_3$/SrTiO$_3$ heterostructures
Authors:
F. Pfaff,
H. Fujiwara,
G. Berner,
A. Yamasaki,
H. Niwa,
H. Kiuchi,
A. Gloskovskii,
W. Drube,
O. Kirilmaz,
A. Sekiyama,
J. Miyawaki,
Y. Harada,
S. Suga,
M. Sing,
R. Claessen
Abstract:
We present a detailed study of the Ti 3$d$ carriers at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures by high-resolution resonant inelastic soft x-ray scattering (RIXS), with special focus on the roles of overlayer thickness and oxygen vacancies. Our measurements show the existence of interfacial Ti 3$d$ electrons already below the critical thickness for conductivity and an increase of the…
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We present a detailed study of the Ti 3$d$ carriers at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures by high-resolution resonant inelastic soft x-ray scattering (RIXS), with special focus on the roles of overlayer thickness and oxygen vacancies. Our measurements show the existence of interfacial Ti 3$d$ electrons already below the critical thickness for conductivity and an increase of the total interface charge up to a LaAlO$_3$ overlayer thickness of 6 unit cells before it levels out. By comparing stoichiometric and oxygen deficient samples we observe strong Ti 3$d$ charge carrier doping by oxygen vacancies. The RIXS data combined with photoelectron spectroscopy and transport measurements indicate the simultaneous presence of localized and itinerant charge carriers. However, it is demonstrated that the relative amount of localized and itinerant Ti $3d$ electrons in the ground state cannot be deduced from the relative intensities of the Raman and fluorescence peaks in excitation energy dependent RIXS measurements, in contrast to previous interpretations. Rather, we attribute the observation of either the Raman or the fluorescence signal to the spatial extension of the intermediate state reached in the RIXS excitation process.
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Submitted 29 May, 2017;
originally announced May 2017.
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Accurate determination of the valence band edge in hard x-ray photoemission spectra using GW theory
Authors:
Johannes Lischner,
Slavomir Nemsak,
Giuseppina Conti,
Andrei Gloskovskii,
Gunnar Karl Palsson,
Claus M. Schneider,
Wolfgang Drube,
Steven G. Louie,
Charles Fadley
Abstract:
We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray ph…
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We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method.
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Submitted 18 April, 2016;
originally announced April 2016.
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High photon energy spectroscopy of NiO: experiment and theory
Authors:
S. K. Panda,
Banabir Pal,
Suman Mandal,
Mihaela Gorgoi,
Shyamashis Das,
Indranil Sarkar,
Wolfgang Drube,
Weiwei Sun,
I. Di Marco,
A. Delin,
Olof Karis,
Y. O. Kvashnin,
M. van Schilfgaarde,
O. Eriksson,
D. D. Sarma
Abstract:
We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT met…
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We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab-initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO.We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.
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Submitted 13 January, 2016;
originally announced January 2016.
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Energetic, spatial and momentum character of a buried interface: the two-dimensional electron gas between two metal oxides
Authors:
S. Nemšák,
G. Conti,
A. X. Gray,
G. K. Pálsson,
C. Conlon,
D. Eiteneer,
A. Keqi,
A. Rattanachata,
A. Y. Saw,
A. Bostwick,
L. Moreschini,
V. Strocov,
M. Kobayashi,
W. Stolte,
S. Ueda,
K. Kobayashi,
A. Gloskovskii,
W. Drube,
C. Jackson,
P. Moetakef,
A. Janotti,
L. Bjaalie,
B. Himmetoglu,
C. G. Van de Walle,
S. Borek
, et al. (10 additional authors not shown)
Abstract:
The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications, and are the subject of intense study in many disciplines. We here apply novel experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consist…
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The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications, and are the subject of intense study in many disciplines. We here apply novel experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO$_3$ (STO) and GdTiO$_3$ (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells STO/3 unit cells of GTO]$_{20}$ using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been introduced as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG with unprecedented detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO, and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG, and suggesting that oxygen vacancies are not the origin of it. Similar multi-technique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.
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Submitted 9 March, 2016; v1 submitted 7 August, 2015;
originally announced August 2015.
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Interface Engineering to Create a Strong Spin Filter Contact to Silicon
Authors:
C. Caspers,
A. Gloskovskii,
M. Gorgoi,
C. Besson,
M. Luysberg,
K. Rushchanskii,
M. Ležaić,
C. S. Fadley,
W. Drube,
M. Müller
Abstract:
Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on silicon is a perfect route to enrich silicon nanotechnology with spin filter functionality.
To date, the inherent chemical reactivity between EuO and Si has prevented a heteroepitaxial integration without significant contaminations of the interface with Eu silicides and Si oxides.
We present a solution to this long-standi…
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Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on silicon is a perfect route to enrich silicon nanotechnology with spin filter functionality.
To date, the inherent chemical reactivity between EuO and Si has prevented a heteroepitaxial integration without significant contaminations of the interface with Eu silicides and Si oxides.
We present a solution to this long-standing problem by applying two complementary passivation techniques for the reactive EuO/Si interface:
($i$) an $in\:situ$ hydrogen-Si $(001)$ passivation and ($ii$) the application of oxygen-protective Eu monolayers --- without using any additional buffer layers.
By careful chemical depth profiling of the oxide-semiconductor interface via hard x-ray photoemission spectroscopy, we show how to systematically minimize both Eu silicide and Si oxide formation to the sub-monolayer regime --- and how to ultimately interface-engineer chemically clean, heteroepitaxial and ferromagnetic EuO/Si $(001)$ in order to create a strong spin filter contact to silicon.
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Submitted 17 March, 2016; v1 submitted 20 April, 2015;
originally announced April 2015.
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Bulk Nature of Layered Perovskite Iridates beyond the Mott Scenario : An Approach from Bulk Sensitive Photoemission Study
Authors:
A. Yamasaki,
S. Tachibana,
H. Fujiwara,
A. Higashiya,
A. Irizawa,
O. Kirilmaz,
F. Pfaff,
P. Scheiderer,
J. Gabel,
M. Sing,
T. Muro,
M. Yabashi,
K. Tamasaku,
H. Sato,
H. Namatame,
M. Taniguchi,
A. Hloskovskyy,
H. Yoshida,
H. Okabe,
M. Isobe,
J. Akimitsu,
W. Drube,
R. Claessen,
T. Ishikawa,
S. Imada
, et al. (2 additional authors not shown)
Abstract:
We present genuine bulk Ir 5d jeff states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s- and p-polarized lights. HAXPES spectra of Sr2IrO4 and Ba2IrO4 are well reproduced by the quasi-particle densities of states calculated by the local density approximation with dynamical mean-field theory (LDA+DMFT). It is demonstrated that the insulating nature…
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We present genuine bulk Ir 5d jeff states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s- and p-polarized lights. HAXPES spectra of Sr2IrO4 and Ba2IrO4 are well reproduced by the quasi-particle densities of states calculated by the local density approximation with dynamical mean-field theory (LDA+DMFT). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-low-energy bulk-sensitive photoemission spectroscopy reveals "bad metallic" states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Neel temperature as predicted by the LDA+DMFT.
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Submitted 2 March, 2014; v1 submitted 27 October, 2013;
originally announced October 2013.
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The Role of Oxygen at the Interface between Titanium and Carbon Nanotubes
Authors:
Alexandre Felten,
Irene Suarez-Martinez,
Xiaoxing Ke,
Gustaaf Van Tendeloo,
Jacques Ghijsen,
Jean Jacques Pireaux,
Wolfgang Drube,
Carla Bittencourt,
Christopher Ewels
Abstract:
We study the interface between carbon nanotubes (CNTs) and surface-deposited titanium using electron microscopy and photoemission spectroscopy, supported by density functional calculations. Charge transfer from the Ti atoms to the nanotube and carbide formation is observed at the interface which indicates strong interaction. Nevertheless, the presence of oxygen between the Ti and the CNTs signif…
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We study the interface between carbon nanotubes (CNTs) and surface-deposited titanium using electron microscopy and photoemission spectroscopy, supported by density functional calculations. Charge transfer from the Ti atoms to the nanotube and carbide formation is observed at the interface which indicates strong interaction. Nevertheless, the presence of oxygen between the Ti and the CNTs significantly weakens the Ti?CNT interaction. Ti atoms at the surface will preferentially bond to oxygenated sites. Potential sources of oxygen impurities are examined, namely oxygen from any residual atmosphere and pre-existing oxygen impurities on the nanotube surface, which we enhance through oxygen plasma surface pretreatment. Variation in literature data concerning Ohmic contacts between Ti and carbon nanotubes is explained via sample pre-treatment and differing vacuum levels, and we suggest improved treatment routes for reliable Schottky barrier-free Ti?nanotube contact formation.
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Submitted 13 October, 2009;
originally announced October 2009.