I09-Surface and Interface Structural Analysis
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Dennis
Meier
,
Peter
Knecht
,
Pablo
Vezzoni Vicente
,
Fulden
Eratam
,
Hongxiang
Xu
,
Tien-Lin
Lee
,
Alexander
Generalov
,
Alexander
Riss
,
Biao
Yang
,
Francesco
Allegretti
,
Peter
Feulner
,
Joachim
Reichert
,
Johannes V.
Barth
,
Ari Paavo
Seitsonen
,
David A.
Duncan
,
Anthoula C.
Papageorgiou
Diamond Proposal Number(s):
[30095]
Open Access
Abstract: Metalloporphyrins on interfaces offer a rich playground for functional materials and hence have been subjected to intense scrutiny over the past decades. As the same porphyrin macrocycle on the same surface may exhibit vastly different physicochemical properties depending on the metal center and its substituents, it is vital to have a thorough structural and chemical characterization of such systems. Here, we explore the distinctions arising from coverage and macrocycle substituents on the closely related ruthenium octaethyl porphyrin and ruthenium tetrabenzo porphyrin on Ag(111). Our investigation employs a multitechnique approach in ultrahigh vacuum, combining scanning tunneling microscopy, low-energy electron diffraction, photoelectron spectroscopy, normal incidence X-ray standing wave, and near-edge X-ray absorption fine structure, supported by density functional theory. This methodology allows for a thorough examination of the nuanced differences in the self-assembly, substrate modification, molecular conformation and adsorption height.
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Dec 2024
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I05-ARPES
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Arindam
Pramanik
,
Ram Prakash
Pandeya
,
Denis V.
Vyalikh
,
Alexander
Generalov
,
Paolo
Moras
,
Asish K.
Kundu
,
Polina M.
Sheverdyaeva
,
Carlo
Carbone
,
Bhanu
Joshi
,
Arumugam
Thamizhavel
,
Srinivasan
Ramakrishnan
,
Kalobaran
Maiti
Diamond Proposal Number(s):
[11512]
Open Access
Abstract: BiPd is a noncentrosymmetric superconductor with Dirac-like surface states on both (010) and $\[(0\bar 10)\]$ faces. The Dirac cone on (010) surface is intense and appears at 0.66 eV binding energy. These states have drawn much attention due to contradictory reports on dimensionality and the momentum of these Dirac fermions. We have studied the properties of these Dirac fermions using varied photon energies and different experimental conditions. The behavior of the Dirac cone is found to be two-dimensional. In addition, we found few more surface states appearing at higher binding energies compared to the Dirac cone.
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Mar 2022
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I05-ARPES
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Georg
Poelchen
,
Igor P.
Rusinov
,
Susanne
Schulz
,
Monika
Guttler
,
Max
Mende
,
Alexander
Generalov
,
Dmitry Yu.
Usachov
,
Steffen
Danzenbacher
,
Johannes
Hellwig
,
Marius
Peters
,
Kristin
Kliemt
,
Yuri
Kucherenko
,
Victor N.
Antonov
,
Clemens
Laubschat
,
Evgueni V.
Chulkov
,
Arthur
Ernst
,
Kurt
Kummer
,
Cornelius
Krellner
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[24339]
Abstract: The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.
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Feb 2022
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I05-ARPES
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Arindam
Pramanik
,
Ram Prakash
Pandeya
,
Denis V.
Vyalikh
,
Alexander
Generalov
,
Paolo
Moras
,
Asish K.
Kundu
,
Polina M.
Sheverdyaeva
,
Carlo
Carbone
,
Bhanu
Joshi
,
A.
Thamizhavel
,
S.
Ramakrishnan
,
Kalobaran
Maiti
Diamond Proposal Number(s):
[11512]
Abstract: Quantum materials having Dirac fermions in conjunction with superconductivity is believed to be the candidate material to realize exotic physics as well as advanced technology. Angle-resolved photoemission spectroscopy (ARPES), a direct probe of the electronic structure, has been extensively used to study these materials. However, experiments often exhibit conflicting results on dimensionality and momentum of the Dirac fermions (e.g., Dirac states in BiPd, a novel noncentrosymmetric superconductor), which is crucial for the determination of the symmetry, time-reversal invariant momenta, and other emerging properties. Employing high-resolution ARPES at varied conditions, we demonstrated a methodology to identify the location of the Dirac node accurately and discover that the deviation from two dimensionality of the Dirac states in BiPd proposed earlier is not a material property. These results helped to reveal the topology of the anisotropy of the Dirac states accurately. We have constructed a model Hamiltonian considering higher-order spin-orbit terms and demonstrate that this model provides an excellent description of the observed anisotropy. Intriguing features of the Dirac states in a noncentrosymmetric superconductor revealed in this study are expected to have significant implications regarding the properties of topological superconductors.
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Apr 2021
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I05-ARPES
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Georg
Poelchen
,
Susanne
Schulz
,
Max
Mende
,
Monika
Guttler
,
Alexander
Generalov
,
Alexander V.
Fedorov
,
Nubia
Caroca-Canales
,
Christoph
Geibel
,
Kristin
Kliemt
,
Cornelius
Krellner
,
Steffen
Danzenbacher
,
Dmitry Y.
Usachov
,
Pavel
Dudin
,
Victor N.
Antonov
,
James W.
Allen
,
Clemens
Laubschat
,
Kurt
Kummer
,
Yuri
Kucherenko
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[17761, 24339]
Open Access
Abstract: Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due to different sets of itinerant bands to which the 4f states of surface and bulk Ce are coupled. Overall our study brings into reach the ultimate goal of quantitatively testing many-body theories that link spectroscopy and transport properties, for both the bulk and the surface, separately. It also allows for a direct insight into the broader problem of Kondo lattices with two different local-moment sublattices, providing some understanding of why the cross-talking between the two Kondo effects is weak.
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Oct 2020
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I05-ARPES
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D. y.
Usachov
,
I. a.
Nechaev
,
G.
Poelchen
,
M.
Guttler
,
E. e.
Krasovskii
,
S.
Schulz
,
A.
Generalov
,
K.
Kliemt
,
A.
Kraiker
,
C.
Krellner
,
K.
Kummer
,
S.
Danzenbächer
,
C.
Laubschat
,
A. p.
Weber
,
J.
Sánchez-Barriga
,
E. v.
Chulkov
,
A. f.
Santander-Syro
,
T.
Imai
,
K.
Miyamoto
,
T.
Okuda
,
D. V.
Vyalikh
Diamond Proposal Number(s):
[18844, 17761]
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
TbRh
2
Si
2
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
4
f
moments.
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Jun 2020
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I05-ARPES
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Susanne
Schulz
,
Ilya A.
Nechaev
,
Monika
Güttler
,
Georg
Poelchen
,
Alexander
Generalov
,
Steffen
Danzenbächer
,
Alla
Chikina
,
Silvia
Seiro
,
Kristin
Kliemt
,
Alexandra Yu.
Vyazovskaya
,
Timur K.
Kim
,
Pavel
Dudin
,
Evgueni V.
Chulkov
,
Clemens
Laubschat
,
Eugene E.
Krasovskii
,
Christoph
Geibel
,
Cornelius
Krellner
,
Kurt
Kummer
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[18844, 17761]
Open Access
Abstract: The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr2Si2, where in contrast to its non-magnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.
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Jun 2019
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I05-ARPES
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A.
Generalov
,
J.
Falke
,
I. A.
Nechaev
,
M. M.
Otrokov
,
M.
Guttler
,
A.
Chikina
,
K.
Kliemt
,
S.
Seiro
,
K.
Kummer
,
S.
Danzenbächer
,
D.
Usachov
,
T. K.
Kim
,
P.
Dudin
,
E. V.
Chulkov
,
C.
Laubschat
,
C.
Geibel
,
C.
Krellner
,
D. V.
Vyalikh
Diamond Proposal Number(s):
[14811]
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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Sep 2018
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I05-ARPES
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A.
Chikina
,
A.
Generalov
,
K.
Kummer
,
M.
Guttler
,
V. N.
Antonov
,
Yu.
Kucherenko
,
K.
Kliemt
,
C.
Krellner
,
S.
Danzenbächer
,
T.
Kim
,
P.
Dudin
,
C.
Geibel
,
C.
Laubschat
,
D. V.
Vyalikh
Diamond Proposal Number(s):
[14811]
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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Apr 2017
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I05-ARPES
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Alexander
Generalov
,
Mikhail M.
Otrokov
,
Alla
Chikina
,
Kristin
Kliemt
,
Kurt
Kummer
,
Marc
Höppner
,
Monika
Guttler
,
Silvia
Seiro
,
Alexander
Fedorov
,
Susanne
Schulz
,
Steffen
Danzenbächer
,
Evgueni V.
Chulkov
,
Christoph
Geibel
,
Clemens
Laubschat
,
Pavel
Dudin
,
Moritz
Hoesch
,
Timur
Kim
,
Milan
Radovic
,
Ming
Shi
,
Nicholas C.
Plumb
,
Cornelius
Krellner
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[11512]
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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Jan 2017
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