I05-ARPES
|
S. K.
Mahatha
,
A.
Kar
,
J.
Corral-Sertal
,
J.
Diego
,
A.
Korshunov
,
C.-Y.
Lim
,
F. K.
Diekmann
,
D.
Subires
,
J.
Phillips
,
T. K.
Kim
,
D.
Ishikawa
,
G.
Marini
,
I.
Vobornik
,
I.
Errea
,
S.
Rohlf
,
M.
Kalläne
,
V.
Bellini
,
A. Q. R.
Baron
,
Adolfo O.
Fumega
,
A.
Bosak
,
V.
Pardo
,
K.
Rossnagel
,
S.
Blanco-Canosa
Diamond Proposal Number(s):
[36505]
Abstract: First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector 𝐪*=(0,
1
4
+𝛿,
1
2
) in the 1T' phase of TaTe2, which competes with fluctuating high-temperature Ta trimer bonding states at 𝐪CO=(0,
1
3
,0). The precursor state follows the temperature dependence of the hidden incommensurability of the quasi-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at 𝐪CO, characterized by a charge disproportionation of the inequivalent Ta sites, appears to be driven by local chemical bonding. Dynamical lattice calculations identify an imaginary optical mode at 𝐪*, involving an in-plane vibration of the Ta atoms forming a chainlike structure that renormalizes below 𝑇CO. Our experimental and theoretical observations suggest that the controversial first-order phase transition, as captured by phenomenological Ginzburg-Landau theory, results from the competition between two order parameters: one involving Fermi surface nesting and the other involving local chemical bonding.
|
Jul 2026
|
|
I09-Surface and Interface Structural Analysis
|
O.
Tkach
,
S.
Fragkos
,
Deepnarayan
Biswas
,
J.
Liu
,
O.
Fedchenko
,
Y.
Lytvynenko
,
S.
Babenkov
,
D.
Zimmer
,
Q. L.
Nguyen
,
S.
Chernov
,
D.
Kutnyakhov
,
M.
Scholz
,
N.
Wind
,
A.
Gloskovskii
,
F.
Pressacco
,
J.
Dilling
,
L.
Bruckmeier
,
M.
Heber
,
L.
Wenthaus
,
G.
Brenner
,
D.
Puntel
,
P. E.
Majchrzak
,
D.
Liu
,
F.
Scholz
,
J. A.
Sobota
,
J. D.
Koralek
,
G.
Dakovski
,
A.
Mehta
,
N.
Sirica
,
M.
Hoesch
,
C.
Schlueter
,
L. V.
Odnodvorets
,
Y.
Mairesse
,
T.-L.
Lee
,
A.
Kunin
,
K.
Rossnagel
,
Z. X.
Shen
,
H. J.
Elmers
,
S.
Beaulieu
,
G.
Schönhense
Abstract: A new type of objective lens has recently been proposed for use in x-ray photoemission electron microscopes (XPEEMs) and momentum microscopes. Adding a ring electrode concentric with the extractor allows the field in the gap between the sample and the extractor to be shaped. Forming a lens field in this gap reduces the field strength at the sample by up to an order of magnitude. This mitigates the risk of field emission, particularly for cleaved samples with sharp edges. A retarding field can redirect all slow electrons, thus eliminating the primary contribution to the space-charge interaction. Here, we present the first experimental investigation of the new lens, examining its performance at photon energies ranging from the extreme ultraviolet (XUV) produced by a high-harmonic generation-based source to soft and hard x rays at two synchrotron facilities. The gap lens in a region without electrodes enables large working distances up to 23 mm. Reduced aberrations allow for larger fields of view in both k-space and real-space imaging, with resolutions comparable to those of conventional cathode lenses. However, field strengths are an order of magnitude smaller. The zero-field mode enables the study of 3D structured objects and is, therefore, beneficial for small cleaved samples as well as for operando devices involving top electrodes. The repeller mode reduces space-charge effects but results in a smaller k-field diameter. This reduction ranges from 10% at hard x-ray energies to 50% in the XUV range. The usable energy interval is also reduced by a factor of two. In time-of-flight XPEEM mode, the raw data show a resolution of 250 nm, which can be improved to better than 100 nm through data processing.
|
Mar 2026
|
|
I09-Surface and Interface Structural Analysis
|
C.-H.
Min
,
M.
Scholz
,
T.-L.
Lee
,
C.
Schlueter
,
A.
Gloskovskii
,
E. D. L.
Rienks
,
V.
Hinkov
,
H.
Bentmann
,
Y. S.
Kwon
,
F.
Reinert
,
H.-D.
Kim
,
K.
Rossnagel
,
S.
Müller
,
W. J.
Choi
,
V.
Zabolotnyy
,
M.
Heber
,
J. D.
Denlinger
,
C.-J.
Kang
,
M.
Kalläne
,
N.
Wind
,
L.
Dudy
Diamond Proposal Number(s):
[22630]
Abstract: Exotic quasiparticle states have been proposed in mixed-valent compounds exhibiting valence transitions. However, clear spectroscopic evidence identifying these states has remained elusive. Using synchrotron-based hard x-ray and extreme ultraviolet photoemission spectroscopy, we have probed the Tm 3𝑑 and 4𝑓 emissions in TmSe1−𝑥Te𝑥, where a Te concentration-dependent semimetal–insulator transition occurs alongside the valence transition. Our photoemission results, which are characteristic of the bulk, track this combined transition across the critical concentration (𝑥𝑐 =0.29). Notably, our results reveal a noninteger valence for the insulating phase and a novel quasiparticle excitation in the semimetallic phase: a Holstein polaron that extends beyond the standard periodic Anderson model.
|
Oct 2025
|
|
|
|
Le Phuong
Hoang
,
David
Pesquera
,
Gerard N.
Hinsley
,
Robert
Carley
,
Laurent
Mercadier
,
Martin
Teichmann
,
Elena Martina
Unterleutner
,
Daniel
Knez
,
Martina
Dienstleder
,
Saptam
Ganguly
,
Teguh Citra
Asmara
,
Giacomo
Merzoni
,
Sergii
Parchenko
,
Justine
Schlappa
,
Zhong
Yin
,
José Manuel
Caicedo Roque
,
José
Santiso
,
Irena
Spasojevic
,
Cammille
Carinan
,
Tien-Lin
Lee
,
Kai
Rossnagel
,
Jorg
Zegenhagen
,
Gustau
Catalan
,
Ivan A.
Vartanyants
,
Andreas
Scherz
,
Giuseppe
Mercurio
Open Access
Abstract: A fundamental understanding of the interplay between lattice structure, polarization and electrons is pivotal to the optical control of ferroelectrics. The interaction between light and matter enables the remote and wireless control of the ferroelectric polarization on the picosecond timescale, while inducing strain, i.e., lattice deformation. At equilibrium, the ferroelectric polarization is proportional to the strain, and is typically assumed to be so also out of equilibrium. Decoupling the polarization from the strain would remove the constraint of sample design and provide an effective knob to manipulate the polarization by light. Here, upon above-bandgap laser excitation of the prototypical ferroelectric BaTiO3, we induce and measure an ultrafast decoupling between polarization and strain that begins within 350 fs, by softening Ti-O bonds via charge transfer, and lasts for several tens of picoseconds. We show that the ferroelectric polarization out of equilibrium is mainly determined by photoexcited electrons, instead of the strain.
|
Aug 2025
|
|
I09-Surface and Interface Structural Analysis
|
H. J.
Elmers
,
O.
Tkach
,
Y.
Lytvynenko
,
P.
Yogi
,
M.
Schmitt
,
D.
Biswas
,
J.
Liu
,
S. V.
Chernov
,
Quynh
Nguyen
,
M.
Hoesch
,
D.
Kutnyakhov
,
N.
Wind
,
L.
Wenthaus
,
M.
Scholz
,
K.
Rossnagel
,
A.
Gloskovskii
,
C.
Schlueter
,
A.
Winkelmann
,
A. A.
Haghighirad
,
T.-L.
Lee
,
M.
Sing
,
R.
Claessen
,
M.
Le Tacon
,
J.
Demsar
,
G.
Schönhense
,
O.
Fedchenko
Diamond Proposal Number(s):
[33765]
Abstract: Using x-ray photoelectron diffraction (XPD) and angle-resolved photoemission spectroscopy, we study photoemission intensity changes related to changes in the geometric and electronic structure in the kagome metal CsV3Sb5 upon transition to an unconventional charge density wave (CDW) state. The XPD patterns reveal the presence of a chiral atomic structure in the CDW phase. Furthermore, using circularly polarized x-rays, we have found a pronounced nontrivial circular dichroism in the angular distribution of the valence band photoemission in the CDW phase, indicating a chirality of the electronic structure. This observation is consistent with the proposed orbital loop current order. In view of a negligible spontaneous Kerr signal in recent magneto-optical studies, the results suggest an antiferromagnetic coupling of the orbital magnetic moments along the 𝑐 axis. While the inherent structural chirality may also induce circular dichroism, the observed asymmetry values seem to be too large in the case of the weak structural distortions caused by the CDW.
|
Mar 2025
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[27468]
Open Access
Abstract: Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, we demonstrate that the X-ray standing wave technique allows the surface polarization profile of a ferroelectric thin film, as opposed to the average film polarity, to be probed directly. The X-ray standing wave technique provides the average Ti and Ba atomic positions, along the out-of-plane direction, near the surface of three differently strained
thin films. This technique gives direct access to the local ferroelectric polarization at and below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface is obtained. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to different type, amount and spatial distribution of the oxygen-containing adsorbates.
|
Oct 2024
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[27468]
Abstract: Ferroelectric materials consist of noncentrosymmetric unit cells with permanent electric
dipole moments leading to a macroscopic spontaneous electric polarization. Uncompensated
charge at the surface of a ferroelectric material is responsible for its chemical reactivity and
can influence the polarization itself. Therefore, revealing the polarization at (and near) the
surface of a ferroelectric material is important not only from a fundamental point of view,
but also to obtain a deeper insight into its surface chemistry and learn how to exploit it for
technologically relevant catalytic reactions [1].
In this study, we investigated BaTiO3
ferroelectric thin films grown on three different
scandate substrates, with a bottom electrode (SrRuO3) in between. The interference
between the incident and Bragg-diffracted X-ray wave generates an X-ray standing wave
(XSW) [2]. The structural sensitivity of the XSW technique combined with the chemical
specificity of X-ray photoemission spectroscopy (XPS) reveals the atomic positions of Ba and
Ti atoms within the top unit cells. As a result, the off-center displacement of Ti atoms, thus
the local ferroelectric polarization, can be determined at different depths from the surface.
These results are discussed in the light of oxygen-related adsorbates measured by XPS.
|
Jul 2023
|
|
I16-Materials and Magnetism
|
Hiroki
Ueda
,
Michael
Porer
,
Jose R. L.
Mardegan
,
Sergii
Parchenko
,
Namrata
Gurung
,
Federica
Fabrizi
,
Mahesh
Ramakrishnan
,
Larissa
Boie
,
Martin Josef
Neugebauer
,
Bulat
Burganov
,
Max
Burian
,
Steven Lee
Johnson
,
Kai
Rossnagel
,
Urs
Staub
Diamond Proposal Number(s):
[15742]
Open Access
Abstract: The correlation between electronic and crystal structures of
1T − TiSe2
in the charge-density wave (CDW) state is studied by x-ray diffraction in order to clarify basic properties in the CDW state, transport properties, and chirality. Three families of reflections are used to probe atomic displacements and the orbital asymmetry in Se. Two distinct onset temperatures are found:
T
CDW
and a lower
T
∗
indicative for an onset of Se out-of-plane atomic displacements.
T
∗
coincides with a DC resistivity maximum and the onset of the proposed gyrotropic (chiral) electronic structure. However, no indication for chirality is found. The relation between the atomic displacements and the transport properties is discussed in terms of Ti
3
d
and Se
4
p
states that only weakly couple to the CDW order.
|
Apr 2021
|
|
I16-Materials and Magnetism
|
Max
Burian
,
Michael
Porer
,
Jose R. L.
Mardegan
,
Vincent
Esposito
,
Sergii
Parchenko
,
Bulat
Burganov
,
Namrata
Gurung
,
Mahesh
Ramakrishnan
,
Valerio
Scagnoli
,
Hiroki
Ueda
,
Sonia
Francoual
,
Federica
Fabrizi
,
Yoshikazu
Tanaka
,
Tadashi
Togashi
,
Yuya
Kubota
,
Makina
Yabashi
,
Kai
Rossnagel
,
Steven L.
Johnson
,
Urs
Staub
Diamond Proposal Number(s):
[15742]
Open Access
Abstract: In this work, we use ultrafast pump-probe nonresonant and resonant x-ray diffraction to track the periodic lattice distortion and the electronic charge density wave in
1
T
−
TiSe
2
upon optical excitation. We observe a fluence regime in which the periodic lattice deformation is strongly suppressed but the charge density wave related Se
4
p
orbital order remains mostly intact. Complete melting of both structural and electronic order occurs four to five times faster than expected from a purely electronic charge-screening process, strongly suggesting a structurally assisted weakening of excitonic correlations. Our experimental data provide insight on the intricate coupling between structural and electronic order in stabilizing the periodic-lattice-distortion/charge-density-wave state in
1
T
−
TiSe
2
. The results further show that electron-phonon coupling can lead to different, energy dependent phase-transition pathways in condensed matter systems, opening different possibilities in the conception of nonequilibrium phenomena at the ultrafast scale.
|
Feb 2021
|
|
I05-ARPES
|
R. C.
Vidal
,
H.
Bentmann
,
T. R. F.
Peixoto
,
A.
Zeugner
,
S.
Moser
,
C.-H.
Min
,
S.
Schatz
,
K.
Kissner
,
M.
Unzelmann
,
C. I.
Fornari
,
H. B.
Vasili
,
M.
Valvidares
,
K.
Sakamoto
,
D.
Mondal
,
J.
Fujii
,
I.
Vobornik
,
S.
Jung
,
C.
Cacho
,
T. K.
Kim
,
R. J.
Koch
,
C.
Jozwiak
,
A.
Bostwick
,
J. D.
Denlinger
,
E.
Rotenberg
,
J.
Buck
,
M.
Hoesch
,
F.
Diekmann
,
S.
Rohlf
,
M.
Kalläne
,
K.
Rossnagel
,
M. M.
Otrokov
,
E. V.
Chulkov
,
M.
Ruck
,
A.
Isaeva
,
F.
Reinert
Diamond Proposal Number(s):
[19278, 22468]
Abstract: The layered van der Waals antiferromagnet
MnBi
2
Te
4
has been predicted to combine the band ordering of archetypical topological insulators such as
Bi
2
Te
3
with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of
MnBi
2
Te
4
(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.
|
Sep 2019
|
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