| 1. |
- Güttler, M., et al.
(author)
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Divalent EuRh 2 Si 2 as a reference for the Luttinger theorem and antiferromagnetism in trivalent heavy-fermion YbRh 2 Si 2
- 2019
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10:1
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Journal article (peer-reviewed)abstract
- Application of the Luttinger theorem to the Kondo lattice YbRh 2 Si 2 suggests that its large 4f-derived Fermi surface (FS) in the paramagnetic (PM) regime should be similar in shape and volume to that of the divalent local-moment antiferromagnet (AFM) EuRh 2 Si 2 in its PM regime. Here we show by angle-resolved photoemission spectroscopy that paramagnetic EuRh 2 Si 2 has a large FS essentially similar to the one seen in YbRh 2 Si 2 down to 1 K. In EuRh 2 Si 2 the onset of AFM order below 24.5 K induces an extensive fragmentation of the FS due to Brillouin zone folding, intersection and resulting hybridization of the Fermi-surface sheets. Our results on EuRh 2 Si 2 indicate that the formation of the AFM state in YbRh 2 Si 2 is very likely also connected with similar changes in the FS, which have to be taken into account in the controversial analysis and discussion of anomalies observed at the quantum critical point in this system.
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| 2. |
- Chikina, A., et al.
(author)
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Valence instability in the bulk and at the surface of the antiferromagnet SmRh2Si2
- 2017
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In: Physical Review B - Condensed Matter and Materials Physics. - 1098-0121. ; 95:15
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Journal article (peer-reviewed)abstract
- Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh2Si2. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4f emission strongly dominates, a small divalent 4f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RERh2Si2 (RE = rare-earth elements) compounds are discussed.
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| 3. |
- Fedorov, A. V., et al.
(author)
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Insight into the Temperature Evolution of Electronic Structure and Mechanism of Exchange Interaction in EuS
- 2021
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In: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:34, s. 8328-8334
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Journal article (peer-reviewed)abstract
- Discovered in 1962, the divalent ferromagnetic semiconductor EuS (TC = 16.5 K, Eg = 1.65 eV) has remained constantly relevant to the engineering of novel magnetically active interfaces, heterostructures, and multilayer sequences and to combination with topological materials. Because detailed information on the electronic structure of EuS and, in particular, its evolution across TC is not well-represented in the literature but is essential for the development of new functional systems, the present work aims at filling this gap. Our angle-resolved photoemission measurements complemented with first-principles calculations demonstrate how the electronic structure of EuS evolves across a paramagnetic-ferromagnetic transition. Our results emphasize the importance of the strong Eu 4f-S 3p mixing for exchange-magnetic splittings of the sulfur-derived bands as well as coupling between f and d orbitals of neighboring Eu atoms to derive the value of TC accurately. The 4f-3p mixing facilitates the coupling between 4f and 5d orbitals of neighboring Eu atoms, which mainly governs the exchange interaction in EuS.
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| 4. |
- Generalov, A., et al.
(author)
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Strong spin-orbit coupling in the noncentrosymmetric Kondo lattice
- 2018
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In: Physical Review B. - 2469-9950. ; 98:11
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Journal article (peer-reviewed)abstract
- Strong spin-orbit coupling (SOC) in combination with a lack of inversion symmetry and exchange magnetic interaction proves to be a sophisticated instrument allowing efficient control of the spin orientation, energy and trajectories of two-dimensional (2D) electrons and holes trapped at surfaces or interfaces. Exploiting Kondo-related phenomena and crystal-electric-field effects at reduced dimensionalities opens new opportunities to handle their spin-dependent properties offering novel functionalities. We consider here a 2D Kondo lattice represented by a Si-Ir-Si-Yb (SISY) surface block of the heavy-fermion material YbIr2Si2. We show that the Kondo interaction with 4f moments allows finely tuning the group velocities of the strongly spin-polarized carriers in 2D itinerant states of this noncentrosymmetric system. To unveil the peculiarities of this interaction, we used angle-resolved photoemission measurements complemented by first-principles calculations. We established that the strong SOC of the Ir atoms induces spin polarization of the 2D states in SISY block, while the 2D lattice of Yb 4f moments acts as a source for coherent f-d interplay. The strong SOC and lack of inversion symmetry turn out to lead not only to the anticipated Rashba-like splitting of the 2D states, but also to spin splitting of the 4f Kramers doublets. They couple temperature-dependently to the spin-polarized 2D states and thereby guide the properties of the latter.
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| 5. |
- Patil, S., et al.
(author)
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ARPES view on surface and bulk hybridization phenomena in the antiferromagnetic Kondo lattice CeRh2Si2
- 2016
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
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Journal article (peer-reviewed)abstract
- The hybridization between localized 4f electrons and itinerant electrons in rare-earth-based materials gives rise to their exotic properties like valence fluctuations, Kondo behaviour, heavy-fermions, or unconventional superconductivity. Here we present an angle-resolved photoemission spectroscopy (ARPES) study of the Kondo lattice antiferromagnet CeRh2Si2, where the surface and bulk Ce-4f spectral responses were clearly resolved. The pronounced 4f0 peak seen for the Ce terminated surface gets strongly suppressed in the bulk Ce-4f spectra taken from a Si-terminated crystal due to much larger f-d hybridization. Most interestingly, the bulk Ce-4f spectra reveal a fine structure near the Fermi edge reflecting the crystal electric field splitting of the bulk magnetic 4f15/2 state. This structure presents a clear dispersion upon crossing valence states, providing direct evidence of f-d hybridization. Our findings give precise insight into f-d hybridization penomena and highlight their importance in the antiferromagnetic phases of Kondo lattices.
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| 6. |
- Schulz, S., et al.
(author)
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Classical and cubic Rashba effect in the presence of in-plane 4f magnetism at the iridium silicide surface of the antiferromagnet GdIr2Si2
- 2021
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In: Physical Review B. - 2469-9950. ; 103:3
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Journal article (peer-reviewed)abstract
- We present a combined experimental and theoretical study of the two-dimensional electron states at the iridium-silicide surface of the antiferromagnet GdIr2Si2 above and below the Néel temperature. Using angle-resolved photoemission spectroscopy (ARPES) we find a significant spin-orbit splitting of the surface states in the paramagnetic phase. By means of ab initio density-functional-theory (DFT) calculations we establish that the surface electron states that reside in the projected band gap around the M¯ point exhibit very different spin structures which are governed by the conventional and the cubic Rashba effect. The latter is reflected in a triple spin winding, i.e., the surface electron spin reveals three complete rotations upon moving once around the constant energy contours. Below the Néel temperature, our ARPES measurements show an intricate photoemission intensity picture characteristic of a complex magnetic domain structure. The orientation of the domains, however, can be clarified from a comparative analysis of the ARPES data and their DFT modeling. To characterize a single magnetic domain picture, we resort to the calculations and scrutinize the interplay of the Rashba spin-orbit coupling field with the in-plane exchange field, provided by the ferromagnetically ordered 4f moments of the near-surface Gd layer.
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| 7. |
- Usachov, D. Yu, et al.
(author)
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Cubic Rashba Effect in the Surface Spin Structure of Rare-Earth Ternary Materials
- 2020
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In: Physical Review Letters. - 0031-9007. ; 124:23
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Journal article (peer-reviewed)abstract
- Spin-orbit interaction and structure inversion asymmetry in combination with magnetic ordering is a promising route to novel materials with highly mobile spin-polarized carriers at the surface. Spin-resolved measurements of the photoemission current from the Si-terminated surface of the antiferromagnet TbRh2Si2 and their analysis within an ab initio one-step theory unveil an unusual triple winding of the electron spin along the fourfold-symmetric constant energy contours of the surface states. A two-band k·p model is presented that yields the triple winding as a cubic Rashba effect. The curious in-plane spin-momentum locking is remarkably robust and remains intact across a paramagnetic-antiferromagnetic transition in spite of spin-orbit interaction on Rh atoms being considerably weaker than the out-of-plane exchange field due to the Tb 4f moments.
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| 8. |
- Generalov, A., et al.
(author)
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Insight into the temperature dependent properties of the ferromagnetic Kondo lattice YbNiSn
- 2017
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In: Physical Review B. - 2469-9950. ; 95:18
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Journal article (peer-reviewed)abstract
- Analyzing temperature dependent photoemission (PE) data of the ferromagnetic Kondo-lattice (KL) system YbNiSn in the light of the periodic Anderson model (PAM) we show that the KL behavior is not limited to temperatures below a temperature TK, defined empirically from resistivity and specific heat measurements. As characteristic for weakly hybridized Ce and Yb systems, the PE spectra reveal a 4f-derived Fermi level peak, which reflects contributions from the Kondo resonance and its crystal electric field (CEF) satellites. In YbNiSn this peak has an unusual temperature dependence: With decreasing temperature a steady linear increase of intensity is observed which extends over a large interval ranging from 100 K down to 1 K without showing any peculiarities in the region of TK∼TC=5.6 K. In the light of the single-impurity Anderson model (SIAM) this intensity variation reflects a linear increase of 4f occupancy with decreasing temperature, indicating an onset of Kondo screening at temperatures above 100 K. Within the PAM this phenomenon could be described by a non-Fermi-liquid-like T- linear damping of the self-energy which accounts phenomenologically for the feedback from the closely spaced CEF states.
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| 9. |
- Generalov, Alexander, et al.
(author)
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Spin Orientation of Two-Dimensional Electrons Driven by Temperature-Tunable Competition of Spin-Orbit and Exchange-Magnetic Interactions
- 2017
-
In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:2, s. 811-820
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Journal article (peer-reviewed)abstract
- Finding ways to create and control the spin-dependent properties of two-dimensional electron states (2DESs) is a major challenge for the elaboration of novel spin-based devices. Spin-orbit and exchange-magnetic interactions (SOI and EMI) are two fundamental mechanisms that enable access to the tunability of spin-dependent properties of carriers. The silicon surface of HoRh2Si2 appears to be a unique model system, where concurrent SOI and EMI can be visualized and controlled by varying the temperature. The beauty and simplicity of this system lie in the 4f moments, which act as a multiple tuning instrument on the 2DESs, as the 4f projections parallel and perpendicular to the surface order at essentially different temperatures. Here we show that the SOI locks the spins of the 2DESs exclusively in the surface plane when the 4f moments are disordered: the Rashba-Bychkov effect. When the temperature is gradually lowered and the system experiences magnetic order, the rising EMI progressively competes with the SOI leading to a fundamental change in the spin-dependent properties of the 2DESs. The spins rotate and reorient toward the out-of-plane Ho 4f moments. Our findings show that the direction of the spins and the spin-splitting of the two-dimensional electrons at the surface can be manipulated in a controlled way by using only one parameter: the temperature.
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| 10. |
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| 11. |
- Mende, Max, et al.
(author)
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Strong Rashba Effect and Different f−d Hybridization Phenomena at the Surface of the Heavy-Fermion Superconductor CeIrIn 5
- 2022
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In: Advanced Electronic Materials. - : Wiley. - 2199-160X .- 2199-160X. ; 8:3
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Journal article (peer-reviewed)abstract
- New temperature scales and remarkable differences from bulk properties have increasingly placed the surfaces of strongly correlated f materials into the focus of research activities. Applying first-principles calculations and angle-resolved photoelectron spectroscopy measurements, a strong Rashba effect and spin-split surface states at the CeIn surface of the heavy-fermion superconductor CeIrIn5 are revealed. The unveiled 4f-derived electron landscape is remarkably distinct for surface and bulk Ce implying the existence of novel temperature scales near the surface region in this material. These results show that ab initio calculations can reliably predict the unusual electronic and spin structure of surfaces of strongly correlated 4f systems where Rashba spin-orbit-coupling phenomena emerge. It is suggested that the structural blocks of such materials can be combined with magnetically active layers for engineering of novel nanostructural objects with appropriate substrates where the diversity of f-driven properties can be applied for the development of novel functionalities.
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| 12. |
- Preobrajenski, Alexei, et al.
(author)
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Molecular effects in solid NaNO3 observed by x-ray absorption and resonant Auger spectroscopy
- 2002
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 65:20
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Journal article (peer-reviewed)abstract
- X-ray absorption and resonant Auger spectroscopy were used to study the formation and decay of nitrogen and oxygen core excitations in ionic-molecular solid NaNO3. It has been shown that the most prominent features in the electronic structure of both valence and conduction bands of the NaNO3 crystal are determined by molecular states of the quasi-isolated NO3- group. In the Auger decay following the strongly localized N 1s-->2a(2)(')(pi) and O 1s-->2a(2)(')(pi) core excitations both spectator and participator signals of extremely high intensity have been found. The nuclear out-of-plane motion inside the NO3- group has been shown to be observable by resonant Auger spectroscopy as a strongly non-Raman dispersion of individual participator features upon tuning the photon energy across the N 1s-->2a(2)(')(pi) and O 1s-->2a(2)(')(pi) resonances. All results on electronic and vibrational properties of NaNO3 are compared with those of the gas-phase BF3 molecule, which is isoelectronic and isostructural to the NO3- group.
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| 13. |
- Preobrajenski, Alexei, et al.
(author)
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Resonant Auger spectroscopy in solid alkali nitrates as a probe of nuclear motion in the core-excited NO3- anion
- 2003
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In: Chemical Physics Letters. - 0009-2614. ; 368:1-2, s. 125-131
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Journal article (peer-reviewed)abstract
- We investigate the interplay between electronic and nuclear relaxation of the core-excited NO3 anion in solid LiNO3 and NaNO3 by means of the high-resolution X-ray absorption and resonant Auger spectroscopy. It is shown how the relative energy distances between the potential energy surfaces of the intermediate and final states can be mapped using the information on the dispersion of individual participator Auger features upon scanning the photon energy across the resonance. The influence of the cation nature (Li+ or Na+) on the potential surfaces is traced and found to be minimal as a consequence of the rather ionic cation-anion interaction. (C) 2002 Published by Elsevier Science B.V.
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| 14. |
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| 15. |
- Usachov, D., et al.
(author)
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Nitrogen-Doped Graphene: Efficient Growth, Structure, and Electronic Properties
- 2011
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 11:12, s. 5401-5407
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Journal article (peer-reviewed)abstract
- A novel strategy for efficient growth of nitrogen-doped graphene (N-graphene) on a large scale from s-triazine molecules is presented. The growth process has been unveiled in situ using time-dependent photoemission. It has been established that a postannealing of N-graphene after gold intercalation causes a conversion of the N environment from pyridinic to graphitic, allowing to obtain more than 8096 of all embedded nitrogen in graphitic form, which is essential for the electron doping in graphene. A band gap, a doping level of 300 meV, and a charge-carrier concentration of similar to 8 x 10(12) electrons per cm 2, induced by 0.4 atom % of graphitic nitrogen, have been detected by angle-resolved photoeinission spectroscopy, which offers great promise for implementation of this system in next generation electronic devices.
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| 16. |
- Usachov, D., et al.
(author)
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Quasifreestanding single-layer hexagonal boron nitride as a substrate for graphene synthesis
- 2010
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 82:7
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Journal article (peer-reviewed)abstract
- We demonstrate that freeing a single-atom thick layer of hexagonal boron nitride (h-BN) from tight chemical bonding to a Ni(111) thin film grown on a W(110) substrate can be achieved by intercalation of Au atoms into the interface. This process has been systematically investigated using angle-resolved photoemission spectroscopy, x-ray photoemission, and absorption techniques. It has been demonstrated that the transition of the h-BN layer from the "rigid" into the "quasifreestanding" state is accompanied by a change in its lattice constant. Using chemical vapor deposition, graphene has been successfully synthesized on the insulating, quasifreestanding h-BN monolayer. We anticipate that the in situ synthesized weakly interacting graphene/h-BN double layered system could be further developed for technological applications and may provide perspectives for further inquiry into the unusual electronic properties of graphene.
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| 17. |
- Vinogradov, AS, et al.
(author)
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Low-lying unoccupied electronic states in 3d transition-metal fluorides probed by NEXAFS at the F 1s threshold
- 2005
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 71:4
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Journal article (peer-reviewed)abstract
- The near-edge x-ray absorption fine structure (NEXAFS) at the F 1s threshold has been studied with high-energy resolution for a series of binary fluorides, including KF, TiF4, VF4, VF3, CrF3, CrF2, MnF3, MnF2, FeF3, FeF2, CoF2, NiF2, CuF2, and ZnF2 as well as for SF6 in the gas phase, and for the PF6- and TiF62- molecular anions of the solid compounds KPF6 and K2TiF6. Most of these spectra were measured at the Russian-German beamline at BESSY II, while the spectra of KF and CuF2 were taken under comparable experimental conditions at the D1011 beamline at MAX-lab. The spectra of the solid samples were recorded via the total electron yield. The NEXAFS spectra were taken with the aim to elucidate the role of covalent bonding and its manifestation in x-ray absorption spectra as well as to gain information on the electronic structure of the conduction band along the whole series of 3d transition-metal (TM) fluorides. The spectra of these most ionic compounds of the 3d TM's have been analyzed in a comparative way considering also the F 1s NEXAFS spectrum of the molecular TiF62- anion in solid K2TiF6. In its turn, the latter spectrum has been interpreted by comparing with the F 1s NEXAFS spectrum of the molecular PF6- anion in KPF6 and that of SF6 in the gas phase. In this way, the low-lying empty electronic states of the 3d TM fluorides are shown to be formed by covalent mixing of the TM 3d with the fluorine 2p electronic states. It is further found that the number of low-lying empty electronic states with TM 3d-fluorine 2p hybridized character decreases gradually along the series of 3d TM fluorides, and is essentially zero in the case of ZnF2.
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| 18. |
- Vinogradov, Nikolay, et al.
(author)
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Controllable p-doping of graphene on Ir(111) by chlorination with FeCl(3).
- 2012
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In: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 24:31
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Journal article (peer-reviewed)abstract
- The in situ chlorination of graphene on Ir(111) has been achieved by depositing FeCl(3) followed by its thermal decomposition on the surface into FeCl(2) and Cl. This process is accompanied by an intercalation of Cl under graphene and formation of an epitaxial FeCl(2) film on top, which can be removed upon further annealing. A pronounced hole doping of graphene has been observed as a consequence of the annealing-assisted intercalation of Cl. This effect has been studied by a combination of core-level and angle-resolved photoelectron spectroscopies (CL PES and ARPES, respectively), near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and low-energy electron diffraction (LEED). The ease of preparation, the remarkable reproducibility of the doping level and the reversibility of the doping upon annealing are the key factors making chlorination with FeCl(3) a promising route for tuning the electronic properties in graphene.
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