Doping molecular semiconductors
Jun Takeya
Department of Advanced Materials and Science, School of Frontier Sciences, The Univ. of Tokyo
Email: takeya@k.u-tokyo.ac.jp

Recent development of organic semiconductors presents significant progress both in small-molecular
crystals and semi-crystalline -conjugated polymers, achieving high carrier mobility based on coherent
electronic transport. Extremely thin crystal films of Cn-DNBDT small molecules are successfully grown to afew molecular layers with the thickness of only 10 nm, so that very high carrier mobility more than 10 cm 2/Vs isachieved to the dimension of 30 cm x 30 cm. Current technologies for printed integrated circuits provides amanufacturable process for low-cost platforms for RFID tags, sensing circuitries, and large-area light-weightdisplay sheets. Single crystal field-effect transistors of such materials display coherent band transport near room
temperature as the result of Hall-effect measurement. In addition, mobility exceeding 1 cm2/Vs are achieved forsemi-crystalline -conjugated polymers such as PBTTT, in which Hall-effect measurement indicate charge
coherence over a-few-molecular distance. The presentation focuses recently developed methods of carrier
doping without carrier mobility, which are essential for semiconductor functions but have been challenging due
to difficulty in preserving crystalline order.
Employing a molecule composed of an elongated -electron core and -electron units connected to bothedges of the core, a quantum well is formed as the result of two-dimensional crystallization. We attach aelectron-accepting layer on the surface of the ultrathin molecular crystals without modifying the crystal structure.
Upon doping with an electric double-layer gate of an ionic liquid, the 2D crystal layer of the -electron core iswell protected from invasion of the anions, so that high carrier mobility exceeding 20 cm 2/Vs is preserved with
1/4 holes per molecule, resulting in the first observation of metallic phase in organic semiconductors [1]. At thehigh carrier concentration about 1014 cm-2, which approximately fills 1/8 of the HOMO band, a pseudo-gap
phase appears as the result of strong electron correlation.
In addition to the above surface doping, small dopant molecules and semi-crystalline polymers can locallyform co-crystallized structures, so that the doping level to a fraction of density of repeating -conjugated unit isachieved preserving high carrier mobility exceeding 1 cm2/Vs. Furthermore, we recently developed a method to
employ an ion-exchange reaction, so that stability of the highly doped conductive state is much improved asstable ions are replaced to redox active molecules which was necessary only in the doping reaction [2]. Veryrecently we also found that such ion-exchange process is useful even in aqueous solution. Proton-coupledelectron transfer turned out to be extremely useful to precisely control the doping level, just like in living
cells[3].
[1] N. Kasuya, S. Watanabe, J. Takeya et.al., Nature Mater. 20, 1401 (2021).
[2] Y. Yamashita, J. Takeya, S. Watanabe et.al., Nature 572, 634 (2019).
[3] M. Ishii, Y. Yamashita, S. Watanabe, J. Takeya et.al., Nature 622, 285 (2023).

Fermi level tuning in transition metal dichalcogenides
Patrick Amsalem
Humboldt-Universität zu Berlin, Institut für Physik & IRIS Adlershof, Berlin, Germany
Transition metal dichalcogenides (TMDCs) are layered materials with unique optoelectronic
properties owing to their low dimension, making them potential candidates for the next-generation
optoelectronic devices. In particular, MoS2, MoSe2, WS2 and WSe2 have attracted significant
attention due to their ease of (large scale) synthesis, high carrier mobility, direct bandgap, and large
exciton binding energy in their single-layer form. However, because of the low dimension of thesematerials, tuning their Fermi level may require different methods than those traditionally used, suchas impurity doping, for conventional semiconductors. In this presentation, we use angle-resolved
photoemission (ARPES) to explore the feasibility of Fermi level control of MoS2 monolayers by tuningof the substrate work function and by molecular doping. We will present the energy level alignment
at interfaces between MoS2 and various substrates and show that Fermi level pinning occurs for
substrate work functions below 4.5 eV while vacuum level alignment occurs above this value. In
addition, we show that the charge injection barriers do not follow the expected Schottky-Mott ruledue to the role of dielectric screening and subsequent bandgap renormalization. We will thenexamine the mechanisms of Fermi level tuning of TMDCs by adsorption of strong molecular
acceptors and demonstrate the role of temperature in such process.
Park et al., 2D Materials 5, 025003 (2018)
Park et al., ACS Nano 15, 14794 (2021)
Park et al., Commun Phys 2, 109 (2019)
Park et al., Commun Phys 2, 68 (2019)
Park et al. Adv. Mater. 33, 2008677 (2021)

Figure: (left) schematic of the charge transfer mechanism at a molecular acceptor (F6TCNNQ) / MoS2
/ HOPG heterojunction and (right) corresponding temperature-dependent photoemission spectra of
the Fermi level region.

Transport properties of an organic topological insulator candidate
α-(BETS)2I3
Tetsuya Nomoto
Center for Emergent Matter Science, RIKEN, Wako-shi, Saitama, 351-0198, Japan
A topological insulator is an exotic material in which the surface is metallic while the bulk is
an insulator. Owing to the unique properties of the surface electrons, topological insulators are
expected to serve as materials for next-generation electronics and spintronics. In particular, anorganic topological insulator (OTI) is considered a dream material, as it is expected to possesscharacteristics that inorganic topological insulators lack, such as flexibility and lower
environmental impact. Despite various theoretical proposals [1, 2], there have been no
experimental observations to date.
α-(BETS)2I3 is a molecular charge-transfer complex that has recently been proposed as apromising candidate for an OTI. Although the first-principles calculations suggest it is a 2D
topological insulator [3], experimental verification is still lacking. To understand the electronic
state of this material in detail, we perform transport measurements, including electrical resistanceand magnetic response. We observe a negative magnetoresistance proportional to B² at low
temperatures, which can be attributed to the chiral magnetic effect, suggesting the presence of 3D
Dirac electrons [4]. Furthermore, the temperature dependence of in-plane and out-of-plane
resistance implies the emergence of surface metallic states below 20 K. These results suggest thatthe low-temperature electronic state of α-(BETS)2I3 is a 3D topological insulator candidate with asmall inversion gap. In my talk, I will discuss the possible electronic state of this OTI candidate
material and the origin of the unique transport phenomena.
[1] Wang, Z. F., Liu, Z. & Liu, F., Nat. Commun. 4, 1471 (2013).
[2] Wang, Z. F., Su, N. & Liu, F., Nano Lett. 13, 2842–2845 (2013).
[3] Kitou, S. et al., Phys. Rev. B 103, 035135 (2021).
[4] Nomoto, T. et al., Nat. Commun. 14, 2130 (2023).

SYMMETRY AND NONLINEAR RESPONSES IN VAN DER WAALS 2D MATERIALS.
Y. Iwasa
1

RIKEN Center for Emergent Matter Science, 351-1098 Wako, JapanA noticeable advantage of nanomaterials including 2D material is that their structural symmetry canbe changed by forming nanostructures, i.e., isolating mono or few layers, or van der Waals (vdW)
heterostructures, even though they are based on chemically identical materials. This is in marked
contrast with bulk single crystals, where the structural symmetry is uniquely defined by their spacegroup. In this presentation, we discuss the bulk photovoltaic effect (BPVE) and nonreciprocaltransport in symmetry engineered van der Waals 2D semiconductors and conductors, respectively [1].
BPVE means the photovoltaic effect or zero-bias photocurrent without p-n junctions. BPVE effectis a unique property of noncentrosymmetric materials, and it is the second order nonlinear
phenomenon in terms of the electric field of light. Here we report emergence of BPVE in transition
metal dichalcodgenide (TMD) nanotubes and vdW heterostructures, in contrast to the absence of
BPVE in monolayer TMDs with trigonal symmetry [2-4].
Nonreciprocal transport is a kind of rectification behavior where the resistance for the leftward
current is distinct from that for the rightward current. This phenomenon was discovered in the
beginning of this century, and are recently understood from the microscopic viewpoint [5]. We
extended this nonreciprocal transport to noncentrosymmetric superconductors [6-7], and nowadays
superconducting diode effect [8] discovered by Ono’s group is attracting significant attention. We
will discuss about this symmetry controlled superconducting diode effect.
[1] T. Ideue and Y. Iwasa, Annu. Rev. Condens. Matter. Phys. 12, 201 (2021).
[2] Y. J. Zhang et al., Nature 570, 349 (2019).
[3] T. Akamatsu, T. Ideue et al., Science 372, 190 (2021).
[4] Y. Dong, M. Yang et al., Nat. Nanotechnol. 18, 36 (2023).
[5] T. Ideue et al., Nat. Phys. 13, 578 (2017).
[6] R. Wakatsuki, Y. Saito et al., Sci. Adv. 3, e1602390 (2017).
[7] Y. M. Itahashi et al., Nat. Comm. 13, 1659 (2022).
[8] F. Ando et al., Nature 584, 373 (2020).

Ultrathin Films of MoS2, ReS2, and BP and Their Heterojunctions in BackGated Field-Effect Transistors
Antonio Di Bartolomeo1, A. Kumar1, O. Durante1, K. Intonti1, L. Viscardi1, S. De Stefano1,
F. Giubileo2, N. Martucciello2
1 Department of Physics, University of Salerno, 84084 Fisciano, Salerno, Italy
2 CNR-SPIN Salerno, 84084 Fisciano, Salerno, Italy*Corresponding Author E-mail: adibartolomeo@unisa.it

ABSTRACT

Two-dimensional (2D) materials, along with their van der Waals heterojunctions, holdimmense promise for electronic and optoelectronic applications.
In this presentation, we delve into several critical aspects related to electrical transport andphotoconduction within 2D materials, specifically focusing on black phosphorus (BP) andtransition metal dichalcogenides like MoS2 and ReS2.
We investigate back-gate transistors with channels composed of either a single material or aheterojunction of two materials (e.g., BP/MoS2). These studies are conducted under varying
air pressures and temperatures, both in darkness and under illumination. The dominant n- or
p-type conduction and the observed rectification of the devices can be understood through anenergy band model that considers the van der Waals heterojunction together with theinterfaces that the 2D materials form with the metal contacts.
Our findings reveal that temperature and air pressure significantly influence both electricalconductivity and photoconductivity. Photocurrent transient measurements highlight thedominance of slow photobolometric and even slower photogating effects in thephotoresponse of 2D materials-based devices.
Temperature and light-induced desorption of adsorbates, such as O2 and H2O molecules, andphotogating effect due to charge trapping contribute to the occurrence of positive andnegative photoconductivity in the same device. These phenomena are gaining attention due totheir potential for fabricating multifunctional devices, that can serve as memory elements orfind applications in neuromorphic vision sensors.
In summary, this presentation sheds light on some intriguing properties of 2D materials andtheir potential for applications in the field of electronics and photonics.
References
1. A. Kumar, … A. Di Bartolomeo Memory Effect and Coexistence of Negative and Positive
Photoconductivity in Black Phosphorus Field Effect Transistor for Neuromorphic Vision Sensors,
Materials Horizons, 2024, DOI: 2024, 10.1039/d4mh00027g
2. A. Di Bartolomeo, … M. Schleberger, Temperature-dependent photoconductivity in twodimensional MoS2 transistors, Materials Today Nano, 24 (2023) 100382
3. O. Durante, … A. Di Bartolomeo, Subthreshold Current Suppression in ReS2 Nanosheet-Based
Field-Effect Transistors at High Temperatures, ACS Applied Nano Materials, 6 (2023) 21663–21670
4. K. Intonti, … Antonio Di Bartolomeo Temperature-Dependent Conduction and Photoresponse in
Few-Layer ReS2, ACS Applied Materials & Interfaces 15 (2023) 50302–50311

Biography:
Antonio DI BARTOLOMEO is a professor of Experimental Condensed Matter Physics at the
University of Salerno, Italy, where he teaches Analog and Digital Electronics Laboratory andNanoelectronics.
Antonio’s current research focuses on nanostructured materials, including 2D materials, nanotubes
and nanowires. He explores their optical and electrical properties. His investigations extend to van der
Waals heterostructures, Schottky junctions, field-effect transistors, memories, solar cells,
photodetectors and field emission devices.
Antonio earned his Ph.D. in Physics from the University of Salerno. His scientific career commenced
at CERN (Switzerland), where he collaborated on experiments related to neutrino oscillations andheavy-ion collisions. He spent several years in the industry, working as a semiconductor device
engineer for prominent companies, like ST Microelectronics, Infineon Technologies, and Intel
Corporation.
Antonio is a senior member of the IEEE (Institute of Electrical and Electronics Engineers). He also
holds the title of 2022-23 IEEE Nanotechnology Council Distinguished Lecturer.
He has been an invited speaker at over 150 international conferences. His scholarly output includes
more than 200 publications in peer-reviewed journals.

Recent applications of X-ray fluorescence holography
Koichi Hayashi
Nagoya Institute of Technology
X-ray fluoresce holography (XFH)1) is a structural analysis method, which provides 3D atomic
images of middle-range local structures around specific elements. Although XFH simply measuresanisotropies of X-ray fluorescence intensities around samples or directional dependences of incident
X-rays on X-ray fluorescence intensities, it can record both the amplitude and phase information of
scattered waves from atoms, unlike ordinary X-ray diffraction. Therefore, by applying Fouriertransformation to measured holograms, 3D atomic images around target elements can be reconstructedwithout any prior models of structures. Another distinct advantage is the elucidation of middle range
local structures up to several nanometers in radius. Owing to these features, we have used XFH todopants and sometimes found interesting nanostructures, such as defect complexes. Recently, weapplied XFH to ferroelectrics2), ferromagnetic semiconductor3) and III-V semiconductor4) andobtained remarkable achievements.
Most recent application is the investigation of local structures in K and Ca co-interrelated graphite.5)
We measured Ca–Kβ and K–Kα holograms, and reconstructed the environments around Ca and K
atoms. Since these atomic arrangements are different, it is found that K0.7Ca0.3C8 did not take the solidsolution type random distribution of Ca and K atoms; instead, a nanoscale phase separation of CaC6and KC8 was observed, which was also supported by the EXAFS results. Detailed analysis of hologram
data indicated the scale of Ca sublayer, which dispersed within the sample, is ~10 Å, and the Ca
nanolayer was offset from the center between the C sublayers. These features would be related to thesuperconductivity of this material.6)
References:1) K. Kimura, J. Phys. Soc. Jpn. 91, 091005 (2022).
2) Y. Yamamoto, K. Kawamura, H. Sugimoto, A. Gadelmawla, K. Kimura, N. Happo, H. Tajiri, K. G.
Webber, K. Kakimoto, and K. Hayashi, Appl. Phys. Lett.120, 052905 (2022).
3) H. Kizaki, K. Hayashi, Cong Lu, N. Happo, S. Hosokawa, S. Hidaka, S. Hayashi, M. Suzuki, and N.
Uchitomi, Phys. Rev. B 106, 064434 (2022).
4) A. Nagaoka, K. Kimura, A. K. R. Ang, Y. Takabayashi, K. Yoshino, Q. Sun, B. Dou, S.-H. Wei, K.
Hayashi, and K. Nishioka, J. Am. Chem. Soc. 145, 9191–9197(2023).
5) N. Happo, A. Kubota, X. Yang, R. Eguchi, H. Goto, M. Ikeda, K. Kimura, Y. Takabayashi, J. R. Stellhorn,
S. Hayakawa, K. Hayashi and Y. Kubozono, Chem. Mater. 36, 4135-4143 (2024).

Exploring the properties of 2D materials at surfaces through synchrotron radiation.
Luca Bignardi (University of Trieste (Italy))

In recent years, the exploration of two-dimensional materials (2DMs) has sparked
considerable interdisciplinary research within condensed matter physics andmaterials science. Surface-science techniques have been pivotal in uncovering
many unique properties of 2DMs, despite the diverse experimental and theoretical
approaches employed. Among these techniques, synchrotron-based X-ray
photoelectron spectroscopy (XPS) has emerged as a crucial tool, offering detailed
and precise characterization of 2DMs and elucidating their complex attributes.
The surface sensitivity and ability of XPS to disentangle differences in core levels
related to the surrounding electronic environment make it essential for accurately
understanding the factors controlling the chemical composition, structure, oxidation
state, and reactivity of surfaces and interfaces. This extends naturally to the study of
2DMs. XPS has been instrumental in understanding the growth mechanisms ofvarious 2DMs on surfaces and identifying the key parameters that influence theseprocesses.
In my talk, I will highlight significant results we have achieved in the last years
investigating the properties of several 2D materials, such as graphene, transition
metal dichalcogenides (TMDCs), and borophene, using synchrotron-based highresolution XPS as the principal technique. Our approach, combined with structural
techniques such as low-energy electron diffraction, valence-band photoelectron
spectroscopy, X-ray photoelectron diffraction, and scanning-tunneling microscopy,
and complemented by density functional theory (DFT) calculations, has been
instrumental in shedding light on a wide range of properties of this broad class ofmaterials.

Observation of the superstructure in van der Waals ferromagnet
Fe5-xGeTe2 by X-ray fluorescence holography
Ritsuko Eguchi
Graduate School of Science, University of Hyogo, Hyogo 678-1297, Japan
A series of FenGeTe2 (3 ≤ n ≤ 5) compounds have attracted much attention because these show
ferromagnetic order with high Curie temperature (TC); TC = 230 K for Fe3GeTe2, TC = 270 K for
Fe4GeTe2, and TC = 270 ~ 310 K for Fe5-xGeTe2. Scanning tunneling microscopy (STM)
topography of the surface cleaved at van der Waals gap between Te layers of Fe5-xGeTe2 revealed√3√3 superstructure with ordered triangle trimer of Te atoms in the charge-ordered state at

low temperature [1]. The ordered arrangement of the Fe atoms located below the Te layer was
anticipated from the results.
In this study, we employed Ge K and Te K X-ray fluorescence holography (XFH) of Fe5xGeTe2 to directly investigate the local structure in the charge-ordered state, i.e., the √3√3

superstructure. The charge-ordered state has been confirmed by STM of either Te or Fe atoms,
but the holography study should provide the detailed information of arrangements of Te, Fe andGe in the crystal lattice. The XFH experiment was performed at BL47XU in SPring-8.
The reconstructed atomic images of Te were obtained from the Ge K XFH at 298, 175, and
123 K. The atomic image demonstrates that the electron density gradually grows toward the center(x = y = 0) of z = 3.3 Å with a decrease in temperature (Figure 1(a)). Such behavior corresponds
exactly to the evolution of short Te-Te distance (a)in the ab-plane at 123 K, implying the evolution(b)of the spatial area providing the Te triangle
trimer ( √3√3 superstructure). The Te−Te

distances were established to be 3.4, 3.6, 4.0,
and 4.6 Å from the four-way splitting of the line(c)profile at 113 K (Figures 1(b) and (c)). The
values are consistent with those obtained fromthe crystal structure and STM. Detailed resultsof Ge K and Te KXFH will be discussed in
my presentation.
This work was supported by JSPS Grants-inAid for Transformative Research Areas (A)“Hyper-Ordered Structures Sciences”.
[1] T. T. Ly et al., Adv. Funct. Mater. 31, 2009758(2021).

Figure 1. (a) Reconstructed atomic image of Te
at z = 3.3 Å obtained by Ge K XFH at 123 K.
(The position at z = 0 Å correspond to the Gelayer.) (b) Reconstructed atomic image of Te atz = 0.0 Å obtained by Te K XFH at 113 K.
(The position at z = 0 Å correspond to the Te
layer.) Green circles refer to the positions ofatoms predicted from crystal structure. (c) Lineprofile of atomic images along the dashed lineof (b) from the center toward the Te atom.

Tailoring the charge separation barrier at organic
semiconductor interfaces
Roberto Costantini1,2, Alberto Morgante1,2 and Martina Dell’Angela2

1

2

Dipartimento di Fisica, Università di Trieste, Via Valerio 2, I-34127 Trieste, Italy
CNR-IOM – Istituto Officina dei Materiali, Area Science Park, Strada Statale 14, Km 163.5,
Trieste I-34149, Italy

Bound electron-hole pairs formed at an interface between different materials are referred to as
charge transfer (CT) excitons [1]. Upon the absorption of a photon by an organic photovoltaic
device, CT excitons are formed at the donor/acceptor interface as an intermediate step prior to
charge separation. To increase the charge generation efficiency in such systems, an in-depthcharacterization of the CT dynamics is mandatory.
We use time-resolved two-photon photoemission (2PPE) to probe the CT excitons on thesurface of thin films of perylene diimides (PDI): in particular, we compare PDI with adiisopropylphenyl-substituted (iPr) PDI. We show that the excited states in the latter arecharacterized by a lower binding energy, which translates to a smaller energy barrier for charge
separation in a hypothetical device. This behavior is correlated to the different morphologies of
the two molecular films which influence the dielectric constant, and thus the screening
properties of the material. In fact, the influence of molecular orientation on the energy levelalignment has already been reported [2]; we show here that PDI films grow flat on silver
substrates, while the iPr functionalization makes the molecule non-flat, thus altering the selfassembly of the film. Based on our findings we suggest that molecular functionalization is a
viable pathway for controlling also the binding energy of the CT excitons, which govern the
charge separation processes at semiconductor heterojunctions.
References[1] Yang, Q.; Muntwiler, M.; Zhu, X.-Y. Phys. Rev. B 80 (2009), 115214.

[2] Duhm, S.; Heimel, et al. Nat. Mater. 7 (2008), 326–332.

Very sharp switching operation of printed organic transistors
Tatsuo Hasegawa
Department of Applied Physics, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
Tel/Fax: +81-3-5841-7757, E-mail: t-hasegawa@ap.t.u-tokyo.ac.jp
Recent studies have shown that certain asymmetric rodlike organic molecules,
characterized by extended π-electron skeletons linked with long alkyl chains, exhibit
exceptionally high layered crystallinity [1]. These compounds can form single-crystalline
organic semiconductor (OSC) ultrathin films with uniform molecular bilayer thickness overareas as large as wafer scale using simple solution coating processes [2‒7], as presented in Fig.
a. A variety of similar asymmetric rodlike molecules also demonstrate high layered crystallinity[8‒14], leading to their classification as “molecular layer materials”. Utilizing these materials,
we successfully fabricated very clean semiconductor-insulator interfaces on highly lyophobic
amorphous perfluoropolymer insulator layers through a unique extended meniscus guided
coating technique, as shown in Fig. b. Field-effect transistors (FETs) with OSCs of
Ph-BTNT-Cn, produced using this method, exhibit sharp on/off switching characteristics with anaverage subthreshold swing of 67 mV/dec, which is close to the theoretical limit, as presented inFig. c [15‒17]. In this presentation, I will discuss the origins of the high layered crystallinity,
the unique solution process involving a layered liquid-crystalline phase at liquid-air interfaces[18], and the unique features of the sharp switching molecular layer FETs.

Figure. Schematics for (a) the solution processing of
ultrathin OSC ﬁlms and (b) BGBC-type organic FET
composed of Ph-BTNT-Cn OSC and CYTOP gate
dielectric. (c) The obtained transfer characteristic.
Reference[1] T. Hasegawa and S. Inoue, JSAP Review; https://doi.org/10.11470/jsaprev.220206. [2] S.
Inoue, et al., Chem. Mater. 27, 3809 (2015). [3] H. Minemawari, et al., Chem. Mater. 29, 1245
(2017). [4] T. Hamai, et al., Phys. Rev. Appl. 8, 054011 (2017). [5] T. Hamai, et al., Phys. Rev.
Mater. 4, 074601 (2020). [6] S. Arai, et al., Adv. Mater. 30, 1707256 (2018). [7] S. Arai, et al.,Adv. Funct. Mater. 30, 1906406 (2020). [8] S. Inoue, et al., Chem. Mater. 30, 5050 (2018). [9]S. Inoue, et al., Chem. Sci. 11, 12493 (2020). [10] T. Higashino, et al., Chem. Mater. 33, 7379
(2021). [11] S. Inoue, et al., Chem. Mater. 34, 72 (2022). [12] K. Nikaido, et al., Adv. Mater. Int.
9, 2201789 (2022). [13] T. Higashino, Chem. Mater. 36, 848 (2024). [14] S. Inoue, Adv. Sci.
11, 2308270 (2024). [15] G. Kitahara, et al., Sci. Adv. 6, eabc8847 (2020). [16] G. Kitahara, etal., Adv. Funct. Mater. 31, 105933 (2021). [17] K. Murata, et al., Phys. Rev. Appl. 21, 024005(2024). [18] M. Yoneya, et al., J. Phys. Chem. C 121, 8796.

Exploring the role of Indirect Excitons in the Spectroscopic Properties of 2D
Materials: Insight from First Principle Many-Body Perturbation Theory
Daniele Varsano
S3 Centre, Istituto Nanoscienze, CNR, Via Campi 213/a, Modena, Italy
Many-Body Perturbation Theory (MBPT) is a powerful approach for predicting thespectroscopic properties of materials, including quasiparticle energies and optical spectra.
The Bethe-Salpeter equation, in particular, has been instrumental in elucidating excitonic
features within the optical absorption spectra of semiconductors. In this talk, we introduce
a recent implementation in the Yambo code [1], specifically tailored for characterizing
finite-momentum indirect excitons, excited states where electrons and holes occupy
distinct regions of the Brillouin zone. Our research showcases recent outcomes,
highlighting the essential role of electron-hole interactions in achieving Electron Energy
Loss Spectroscopy (EELS) spectra that are in qualitative and quantitative agreement withhigh-resolution experiments conducted on free-standing graphene [2,3]. Furthermore, I will
discuss the role of indirect excitons in the exciton instability in monolayer WTe2 below a
critical temperature, providing insights into the realisation of the long-sought excitonic
insulator (EI) phase [4,5,6,7]. This phase is a permanent Bose-Einstein condensate of
excitons forming in the absence of optical excitation. The ab-initio solution of the BetheSalpeter equation reveals that the exciton binding energy exceeds 100 meV and the radius
is as small as 4 nm, which explains the observed formation of excitons at high temperaturesand doping levels [7]. A multi-valley mean-field calculation of the EI chemical potential asa function of doping shows quantitative agreement with the experimental results.

Calculated and experimental onset dispersion of EELS
experiment in free standing graphene with high energy andmomentum resolution.

WTe2: Conductance versus gate-induced density ng at a series oftemperatures T on the same device. Inset: temperature dependence
at positive values of doping.

[1] D. Sangalli et al., Journal of Physics: Condensed Matter 31, 325902 (2019); A. Marini, C. Hogan, M.
Grüning and D. Varsano, Comp. Phys. Comm., 180, 1392 (2009)[2] A. Guandalini et al., Nano Letters, 23, 11835 (2023 (2023)[3] A. Guandalini, D. A. Leon, P. D'Amico, C. Cardoso, A. Ferretti, and D. Varsano, Phys. Rev. B 109,
075120 (2024)[4] D. Varsano et al., Nature Communications, 8, 1461 (2017)[5] D. Varsano et al., Nature Nanotechnology, 15, 367 (2020)[6] S. S. Ataei, D. Varsano, E. Molinari and M. Rontani, Proc. Natl. Acad. Sci. 118 , 13 (2021)[7] B. Sun et al,. Nature Physics 18 94 (2022)

Charge transport in 2D materials thin films: role of surface chemistry, morphology andassembly
Andrea Candini a*, Alessandro Kovtun a, Alex Boschi a, Andrea Liscio b, Massimiliano Cavallini c, Vincenzo
Palermo aa) National Research Council of Italy, Institute for Organic Synthesis and Photoreactivity (CNR - ISOF). Via
Gobetti 101, 40129 Bologna, Italy.
b) National Research Council of Italy, Institute for Microelectronic and Microsystems (CNR - IMM). Via
Fosso del Cavaliere 100, 00133 Roma, Italyc) National Research Council of Italy, Institute for the Study of Nanostructured Materials (CNR - ISMN). Via
Gobetti 101, 40129 Bologna, Italy.
* andrea.candini@isof.cnr.it
Realistic devices made of 2-Dimensional Materials (2DMs) are often made by networks of randomly stacked
nanosheets, where interfaces play a fundamental role in determining the charge transport mechanisms. Here
we address the fundamental problem of how charges move across disordered multilayers of 2DMs through acomprehensive study of a prototypical system made of reduced graphene oxide (rGO), where we are able totune the chemical, morphological and geometrical properties across several orders of magnitude, ranging fromindividual to billions of sheets [1]. By the analysis of the charge localization length ξ, we show how the various
parameters modify the charge transport characteristics and we finally propose a microscopic model for the
charge transport, taking into account the degree of overlap of sp2 aromatic domains belonging to different
nanosheets, leading to quasi-one-dimensional charge transport paths with an extension of up to several
microns. Our observations, reproducibly found for all geometries and defects of the nanosheets, suggest ageneralized description of 2DMs thin films, further corroborated by applying a similar approach to another2DM such as electrochemically exfoliated graphene nanoplatelets (EGO) [2].
In the final part of the presentation we show how defect engineering can be used to tune the surface electronic
properties of 2DMs. Using MoS2 thin films as a proof-of-principle case, we report the fabrication of sulfur
atomic vacancies using electrochemical nanolithography (ECL). ECL combines high resolution down to thenanometric scale with the possibility to fabricate large area devices in a time-effective procedure. By analyzing
the material characteristics after the creation of sulfur vacancies, we demonstrate changes in the optical andelectronic properties, as well as the enhancement of the electrocatalytic activity for the hydrogen evolution
reaction. Surface defect nanopatterning can be regarded as a new and promising way to tune and enhance
2DMs properties for nano-electronic and electrochemical applications.
References[1] A. Kovtun et al. ACS Nano 2021, 15, 2654−2667[2] A. Boschi et al. Small 2023, 19, 2303238[3] D. Gentili et al. under review 2024

Figure 1: scheme of the charge transport mechanism in rGO single layers and thin films. While in single layers thetransport is semi-metallic only within the aromatic domains and the presence of defects induces hopping, in the case of
stacked sheets charges can move in three dimensions due to π−π stacking, allowing to circumvent the defects.

Scalable synthesis, integration and processing of graphene for high-performing electronic andphotonic devices
Vaidotas Mišeikis, Camilla Coletti
CNI@NEST, Istituto italiano di tecnologia, P.zza San Silvestro, 12, Pisa, Italy
Graphene Labs, Istituto italiano di tecnologia, Via Morego, 30, Genova, Italy

Graphene has emerged as an ideal platform for optoelectronics with high potential for applications in nextgeneration telecommunications, but its commercialisation has been limited so far due to challenges in scalable
synthesis and processing of high-quality material. For similar reasons, fundamental studies have also mostly
relied on exfoliated ﬂakes despite continuous progress in CVD synthesis of graphene.
We present our approaches for the synthesis and processing of high-quality single-layer graphene (SLG) with focus
on wafer-scale integration [1] (Fig. 1a). Seeded growth [2] and deterministic transfer allows tailored integration of
monocrystalline graphene for optoelectronic applications, matching the geometry of the target photonic circuits(Fig.1b) for the fabrication of high-performing graphene modulators [1] and detectors [3] (Fig. 1c, d). Ultraclean
processing is used to prepare scalable arrays of graphene Hall bars on Si/SiO2 substrates with record magnetic
sensitivity >5000 V A-1 T-1, thanks to the high carrier mobility >10 000 cm2 V-1 s-1[4].
By encapsulating our CVD graphene with ﬂakes of hexagonal boron nitride (hBN), we can preserve its ultrahighintrinsic quality, comparable to that of exfoliated graphene. The properties of such heterostructures are studied in
the high mobility regime, particularly, in the ﬁeld of twistronics. As-grown twisted bilayer graphene (TBG) crystals
with a 30° twist angle show decoupled transport [5], while moiré e\ects are observed [6] in manually assembled
small-angle (~2.5°) TBG, exploiting the crystallographic alignment of seeded graphene crystals (Fig. 1e).
hBN/SLG/hBN heterostructures are integrated onto photonic wafers to fabricate opto-electronic mixers (Fig. 1f),
enabling wireless data transmission at sub-THz frequencies [7].
This work has received funding from “GRAPH-X” funded by the European Commission through the HORIZON.2.4– Digital, Industry and Space programme (GA 101070482).

Figure 1. a) Dry stamping of graphene coupons on an 8” wafer. b) Graphene crystal array on photonic waveguides.
(c) Eye diagram and (d) histogram of signal amplitude of graphene photodetector at 120 gb/s. e) Electrical transportmoiré signatures in small-angle TBG CVD graphene. f) Optoelectronic mixer based on high-mobility graphene
heterostructure.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]

M. A. Giambra et al., ACS Nano. 15, 3171–3187 (2021).
V. Miseikis et al., 2D Materials. 4, 021004 (2017).
S. Marconi et al., Nature Communications. 12, 806 (2021).
A. Tyagi, et al., ACS Appl. Nano Mater., in press, doi:10.1021/acsanm.3c03920.
G. Piccinini et al., Physical Review B. 104 (2021), doi:10.1103/PhysRevB.104.L241410.
G. Piccinini et al., Nano Letters. 22, 5252–5259 (2022).
A. Montanaro et al., Nat Commun. 14, 6471 (2023).

Emerging oxidized and defective phases in low- dimensional CrCl3
Luca Ottaviano, Department of Physical and Chemical Sciences, Università dell’Aquila, (Italy)
Two-dimensional (2D) magnets like chromium trihalides CrX3 (X = I, Br, Cl) represent a frontier
for spintronics applications and, in particular, CrCl3 has attracted research interest for its
relative stability in ambient conditions without rapid degradation, as opposed to CrI 3. Herein,mechanically exfoliated CrCl3 flakes are characterized at the atomic scale and the electronic
structures of pure, oxidized, and defective monolayer CrCl3 phases are investigated employing
density functional theory (DFT) calculations, scanning tunneling spectroscopy (STS), corelevels X-ray photoemission spectroscopy (XPS), and valence band XPS and ultraviolet
photoemission spectroscopy (UPS). As revealed by atomically resolved transmission electron
microscopy (TEM) and energy dispersive X-ray (EDX) analysis, the CrCl3 flakes showspontaneous surface oxidation upon air exposure with extrinsic long-range ordered oxidized OCrCl3 structure and amorphous chromium oxide formation on edges flakes. XPS proves that
CrCl3 is thermally stable up to 200 ◦C having intrinsically Cl vacancy-defects whose
concentration is tunable via thermal annealing up to 400 ◦C. DFT calculations, supported byexperimental valence band analysis, indicate that pure monolayer (ML) CrCl3 is an insulator
with a band gap of 2.6 eV, while the electronic structure of oxidized and Cl defective phases of
ML CrCl3, extrinsically emerging in exfoliated CrCl3 flakes, show in-gap spin-polarized states
and relevant modifications of the electronic band structures [1-5].

1.
2.
3.
4.

5.

“Polaronic and Mott insulating phase of layered magnetic vanadium trihalide VCl3” D. Mastrippolito, L. Camerano, H. Swiatek,
Bretislav Šmíd , T. Klimczuk, L. Ottaviano,1,5 and G. Profeta, Phys. Rev. B 108, 045126 (2023). DOI:
10.1103/PhysRevB.108.045126“Unconventional ferrimagnetism and enhanced magnetic ordering temperature in monolayer CrCl3 by introducing O impurities
and Cl vacancies“ D. Mastrippolito, J. Wang, G. Profeta, and L. Ottaviano, J. Phys. Mater. 5, 014004 (2022). DOI: 10.1088/25157639/ac5dcd“Emerging oxidized and defective phases in low-dimensional CrCl3 “ D. Mastrippolito, L. Ottaviano, G. Profeta et. al Nanoscale
Advances 3, 4756 (2021). DOI: 10.1039/d1na00401h“Enhancement of the Magnetic Coupling in Exfoliated CrCl3 Crystals Observed by Low Temperature Magnetic Force Microscopy
and X-Ray Circular Dichroism.” M. Serri, G. Cucinotta, L. Poggini, G. Serrano, P. Sainctavit, J. Strychalska-Nowak, A. Politano, F.
Bonaccorso, A. Caneschi, R. J. Cava, R. Sessoli, L. Ottaviano, T. Klimczuk, V. Pellegrini, and M. Mannini, Adv. Materials 2000566(2020) DOI: 10.1002/adma.202000566“Mechanical exfoliation and layer number identification of single crystal monoclinic CrCl3” S. Kazim, M. Ali, S. Palleschi, G.
D'Olimpio, D. Mastrippolito, A. Politano, R. Gunnella, A. Di Cicco, M. Renzelli, G. Moccia, O. A. Cacioppo, R. Alfonsetti, J.
Strychalska-Nowak, T. Klimczuk, R. Cava, and L. Ottaviano, Nanotechology 31, 395706 (2020). DOI: 10.1088/1361-6528/ab7de6

Unconventional superconductivity in topological kagome metals
The recently discovered kagome lattice superconductors AV3Sb5 (A = K, Rb, Cs) [1]possess a unique band structure with van Hove singularities and Dirac dispersions, inwhich unusual charge-density-wave (CDW) orders with time-reversal and rotational
symmetry breaking have been reported. One of the most crucial unresolved issues is
identifying the symmetry of the superconductivity that develops inside the CDW phase.
Recent theory predicts a variety of unconventional superconducting symmetries,
including a chiral order parameter with sign change. Experimentally, however, the phase
information on the superconducting gap in AV3Sb5 is still lacking.
Here we report the impurity effects in CsV3Sb5 using electron irradiation as a phasesensitive probe of superconductivity [2]. Our magnetic penetration depth measurements
reveal that with increasing impurities, a highly anisotropic fully-gapped state changes
gradually to an isotropic full-gap state without passing through a nodal state. Furthermore,transport measurements under high pressure show that the double superconducting
dome in the pressure-temperature phase diagram survives against sufficient impurities.
These results support that CsV3Sb5 is a non-chiral, anisotropic s-wave superconductor
with no sign change both at ambient and high pressure, which provides a clue tounderstanding the relationship between CDW and superconductivity in kagome
superconductors.
[1] B. R. Ortiz et al., Phys. Rev. Lett. 125, 247002 (2020).
[2] M. Roppongi, KH et al., Nat. Commun. 14, 667 (2023).

Origin and magnetotransport properties of the multifunctional 2DES at the
LaAlO3/EuTiO3/SrTiO3(001) interface
Maria D’Antuono 1,2, Yu Chen 2, Roberta Caruso1,2, Benoit Jouault 3, Marco Salluzzo 2,
Daniela Stornaiuolo 1,2*
1Department of Physics, University of Naples Federico II, via Cinthia, 80126 Naples, Italy.
2CNR-SPIN, via Cinthia, 80126 Naples, Italy.
3Laboratoire Charles Coulomb, UMR 5221, CNRS,Université de Montpellier, 34095 Montpellier, France.
*email: daniela.stornaiuolo@unina.it

Two-dimensional electron systems (2DES) developing in SrTiO3-based heterostructurespossess a wide range of properties which are largely tunable thanks to the system band
structure and carrier density [1]. A joint experimental and theoretical approach is becoming
essential to gain detailed understanding of these novel heterostructures showing
exceptional properties, and as a guide for future design of materials for advanced
applications.
Recently, it was demonstrated that by introducing a thin layer of a magnetic oxide between
LaAlO3 and SrTiO3, as in LaAlO3 /EuTiO3/ SrTiO3 (LAO/ETO/STO), a superconducting
spin polarized 2DES can be engineered [2]. We use a combination of transport andspectroscopy measurements and DFT calculations to resolve the nature of this 2DES andto understand the mechanisms leading to spin-polarization of its carriers [3]. In particular,
we highlight the role of the carriers belonging to the Ti 3dxz,yz states in establishing aferromagnetic coupling, thanks to hybridization with specific Eu states.
We show also how the properties of the 2DES can be modulated by using electric field
effect gating in combination with visible light irradiation [4]. The latter enhances thestabilization of ferromagnetic correlations in the 2DES. Indeed, above the carrier density
threshold corresponding to the Lifshitz transition, we do see evidence of the appearance ofcarriers with highest mobility, i.e. 3d xz,yz. The mobility vs. carrier density analysis suggests
a larger enhancement of the mobility by light irradiation compared to back-gating and, atthe same time, an increase of the anomalous Hall resistance. This effect could be related
to light induced excitation of spin-polarized carriers at the interface, which distribute overthe 2DES thickness well inside the STO.

[1] Christensen, D. V. et al. Stimulating Oxide Heterostructures: A review on Controlling SrTiO3-Based
Heterointerfaces with External Stimuli. Adv. Mater. Interfaces 6, 1900772 (2019)[2] Stornaiuolo, D. et al. Tunable spin polarization and superconductivity in engineered oxide interfaces. Nat.
Mater. 15, 278–283 (2016).
[3] Di Capua, R. et al. Orbital selective switching of ferromagnetism in an oxide quasi two-dimensional
electron gas. npj Quant. Mater. 7, 1–10 (2022)[4] M.D’Antuono, et al. Tuning of the magnetotransport properties of a spin-polarized 2D electron systemusing visible light. Scientific Reports, 2023, 13.1: 10050.

Electronic structure in topological kagome metals
Kosuke Nakayama
Department of Physics, Graduate School of Science, Tohoku University
k.nakayama@arpes.phys.tohoku.ac.jp

The kagome lattice, consisting of a two-dimensional network of corner-sharing triangles, serves
as an excellent platform for exploring novel quantum phenomena originating from geometricalfrustration in real space and nontrivial band topology in reciprocal space. Recently, significant
attention has been directed to non-magnetic kagome metal AV3Sb5, in which various electronic
phenomena have been unveiled. These phenomena include charge-density wave (CDW),
superconductivity, nematic ordering, time-reversal symmetry breaking, pair density wave, and thegiant anomalous Hall effect. To elucidate the electronic states responsible for these exotic phases,
we have conducted high-resolution angle-resolved photoemission spectroscopy (ARPES) of
AV3Sb5. In this presentation, we present our recent achievements, with a particular emphasis on
the CDW-induced electronic reconstructions [1-6].

References:[1] K. Nakayama et al., Phys. Rev. B 104, L161112 (2021).
[2] K. Nakayama et al., Phys. Rev. X 12, 011001 (2022).
[3] T. Kato et al., Phys. Rev. Lett. 129, 206402 (2022).
[4] T. Kato et al., Phys. Rev. B 106, L121112 (2022).
[5] T. Kato et al., Commun. Mater. 3, 30 (2022).
[6] T. Kato et al., Phys. Rev. B 107, 245143 (2023).

Quantum properties of atomic layer films studied byin situ ARPES and transport measurements
Dept. Phys., Tokyo Tech., Toru Hirahara
Recently, thin films with thicknesses of a few atomic layers (atomic layer
and two-dimensional materials) have been fabricated in high quality andin large areas, and the realization of novel physical properties of suchmaterials that differ from those of three-dimensional bulk materials andthe elucidation of their mechanisms have attracted attention from both
fundamental and applied perspectives. In this talk, I would like to introduce
our recent fundamental studies on such atomic layer materials utilizing insitu ARPES and transport measurements, correlating the electronic
structure with their macroscopic properties.
The first topic is the discovery of a new method to introduce magnetism
into topological insulators. By using these systems such as MnBi₂Se₄ or
Mn₄Bi₂Te₇, the relation between electronic states and magnetization
properties measured with XMCD and in situ Hall measurements areclarified [1].
The second topic concerns the superconducting transition temperature
Tc determined by gap opening and transport measurements in atomic
layer high-temperature superconductors with a higher Tc than the bulk. In
general, the Tc of superconductors usually decreases by making themthinner and thinner, but contrary to this conventional wisdom, a singlelayer thin film was reported to have a higher Tc in monolayer FeSe/STO.
Combined with in situ STM/STS measurements, we uncover the role ofthe substrate surface on this system as well as on the cousin compound
FeTe/STO [2].
[1] T. Hirahara et al., Nano Lett. 17, 3493 (2017), Nat. Comm. 11, 4821
(2020). R. Fukushima et al., submitted (2024).[2] T. Tanaka et al., Phys. Rev.
B 98, 121410(R) (2018), A. Pedersen et al., Phys. Rev. Lett. 124, 227002(2020), K. Ide et al., Phys. Rev. Mat. 6, 124801 (2022). W. Si et al.,submitted (2024).

Artificial Multiferroic oxide 2D-electron gas
Marco Salluzzo1, Julien Bréhin2, Yu Chen1, Maria D’Antuono3, Sara Varotto2, Daniela Stornaiuolo3, Cinthia
Piamonteze 4, Benoit Jouault5, Daniele Preziosi6, R. Pentcheva7, Martando Rath1, Julien Varignon8, Manish
Verma7, Manuel Bibes2
1

CNR-SPIN, Complesso Monte S. Angelo - Via Cinthia, Napoli, Italy
Unité Mixte de Physique, CNRS, Thales, Université Paris Saclay, Palaiseau, France
3
University of Naples ‘Federico II’, Complesso Monte S. Angelo - Via Cinthia, Napoli, Italy
4
Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland
5
Laboratoire Charles Coulomb, UMR 5221, CNRS, Université de Montpellier, F-34095 Montpellier, France.
6
Université de Strasbourg, CNRS, IPCMS UMR 7504, 67034 Strasbourg, France
7
Department of Physics and Center for Nanointegration, University Duisburg-Essen Lotharstr. 1, D-47057
Duisburg, Germany
8
CRISMAT, CNRS UMR 6508, ENSICAEN, Normandie Université, Caen, France
2

Advances in growth technology of oxide materials allow single atomic layer control of heterostructures.
In particular delta doping, a key materials’ engineering tool in today’s semiconductor technology, is now
also available for oxides. Here I will present the engineering of ferromagnetic, ferroelectric, and coupledferromagnetic/ferroelectric two-dimensional electron gases (2DEGs) by combining band-insulating oxides
characterized by different functionalities [1-3]. A fully electric-ﬁeld-tunable spin-polarized andsuperconducting quasi-2D electron system (q2DES) can be artiﬁcially created by inserting a few unit cellsof delta doping EuTiO3 antiferromagnet at the interface between LaAlO3 and SrTiO3 oxides. Spinpolarization emerges at the Lifshitz transition and is due to the exchange interaction between themagnetic moments of Eu-4f and of Ti-3d electrons. A ferroelectric 2DEG is realized by simply growing
LaAlO3 on the Ca-doped (1%-2%) SrTiO3 single crystal. Finally, a ferromagnetic and ferroelectric 2DEG isfinally engineered introducing few unit cells of EuTiO3 between LaAlO3 and Ca-doped STO.
I will provide evidences of ferromagnetism, ferroelectricity and coupled ferromagnetism/ferroelectricity
from magneto-transport and Hall effect measurements, polarization vs back gate electric field, and inparticular by polarization dependent x-ray absorption spectroscopy. In particular, by performing x-ray
linear dichroism measurements across the FE-transition temperature and as function of the remanentpolarization, I will show that Polarization direction affect the orbital splitting of the t2g states in a nonvolatile manner. Moreover, I will show that also the magnetic moment of the Eu-ions, measured by x-ray
magnetic circular dichroism, is modified by the FE polarization direction, providing a direct proof ofmagneto-electric coupling in the LAO/ETO//Ca-STO. The results are compared to DFT+U calculations on
the heterostructure.
[1] D. Stornaiuolo et al., Tunable Spin Polarization and Superconductivity in Engineered Oxide Interfaces,
Nat Mater 15, 278 (2015).
[2] R. D. Capua et al., Orbital Selective Switching of Ferromagnetism in an Oxide Quasi Two-Dimensional
Electron Gas, Npj Quantum Mater 7, 41 (2022).
[3] J. Bréhin, Y. Chen, M. D’Antuono, S. Varotto, D. Stornaiuolo, C. Piamonteze, J. Varignon, M. Salluzzo,
and M. Bibes, Coexistence and Coupling of Ferroelectricity and Magnetism in an Oxide Two-Dimensional
Electron Gas, Nat Phys (2023). doi: 10.1038/s41567-023-01983-y

Chiral Spintronics and Time-Reversal Symmetry
Hiroshi M. Yamamoto1, 2, 3
1

Institute for Molecular Science
Department of Advanced Studies, SOKENDAI
3
Department of Applied Physics, The University of Tokyo
2

Chirality has been considered as just a problem of molecular shape when it is
discovered. In order to express chirality in quantum mechanical levels, however, one
needs to find an appropriate way to describe chirality by wave functions. Recently, we
have proposed to use electrical toroidal monopole G0 as a quantum mechanical definitionof chirality which has a monopole structure in reciprocal space [1]. This definition isconsistent with a classical definition of chirality proposed by Barron who also considered
time-reversal symmetry [2]. One of the interesting things in our new definition is that thismonopole in k-space can be converted to a magnetic monopole (M0) in real space once it
is integrated in the time domain [3]. This monopole structure may explain someobservations related to chirality-induced spin selectivity (CISS) effect [4], such as
enantio-separation by magnet surface and magnetoresistance larger than spin polarizationof ferromagnetic electrode. We have experimentally proved such a conversion from G0 to
M0 using chiral molecular superconductor [5]. Using this phenomenon, we have also
successfully enantio-separated helical supramolecular nanowire made of achiralmolecules by magnet [6]. The role of such an emergent chirality in conjunction with novel
spintronics will be discussed.
[1] J. Kishine, H. Kusunose, and HMY, Isr. J. Chem. 62, e202200049 (2022)[2] L. D. Barron Chem. Soc. Rev. 15, 189 (1986)[3] H. Kusunose, J. Kishine, and HMY, arXiv:2404.13636 (2024)[4] R. Naaman, Y. Paltiel, D. H. Waldeck, Nature Rev. Chem. 3, 250 (2019)[5] R. Nakajima, HMY et al., Nature 613, 479 (2023)[6] Aizawa, HMY et al., Nature Commun. 14, 4530 (2023)

2D MoS2 doping and stability in high temperature controlled atmosphere

S. Agnello1,2,3, A. Madonia1, E. Sangiorgi1, F. Migliore1, M. Cannas1, G. Buscarino1,2, F.M.
Gelardi1, F. Giannazzo3, S. E. Panasci3, E. Schiliro’3, F. Esposito4, L. Seravalli4
1

Department of Physics and Chemistry Emilio Segrè, University of Palermo, Via Archirafi 26,
90123, Palermo, Italy
2

3

4

AtenCenter, University of Palermo, Viale delle Scienze – Ed.18A, 90128, Palermo, Italy

National Research Council-Institute for Microelectronics and Microsystems (CNR-IMM), Z.I.
Strada VIII 5, 95121 Catania, Italy;

National Research Council-Institute of Materials for Electronics and Magnetism (CNR-IMEM),
Parco Area delle Scienze 37/A, 43124 Parma, Italy;

The emerging family of 2D materials comprises the transition metal dichalcogenides that are
particularly interesting due to their versatile properties for electronic applications. Among them
the MoS2 is a semiconducting material that in the monolayer 2D state is characterized by adirect bandgap of 1.8-1.9 eV and an exciton recombination photoluminescence at about 1.8 eV.
As already known, the 2D materials properties are generally affected by the interaction with
their neighborhood and in this context both the effects of interacting molecules and thephysical substrate on which the material is finally deposited are of particular concern. In details,
the doping, the strain and the electronic features can be modified or tuned properly by theinteracting neighboring species and are of current interest in the research of these materials.
In this work 2H-MoS2 monolayer (1L) and few layers have been grown by chemical vapor
deposition (CVD) or by gold assisted exfoliation (GAE) on different conducting (Au),semiconducting (GaN) and dielectric substrates (SiO2, Al2O3). The physical features of MoS2
have been studied mainly by MicroRaman spectroscopy, Microphotoluminescence, andAtomic Force Microscopy (AFM). To evaluate the stability and doping potentialities the samples
have been subjected to post-growth thermal treaments in controlled atmosphere of inert (Ar)or active (O2) gas up to 300°C.
It will be reported how the substrate affects the doping and strain features of the 1L-MoS2 andthe exciton recombination evidencing the influence of the specific interaction on the charge
carrier dynamics and material structural features. Moreover, it will be shown that the thermal
processing of the 1L-MoS2 is responsible in affecting the doping of the material and the exciton
recombination with evident effects on the Fermi level positioning and in the bandgap tuning aswell as in the charge carrier coupling in exciton and trion.

Tunable quantum interferometers in magic-angle twisted bilayer graphene
Shuichi Iwakiri (Weizmann Institute of Science)
Magic-Angle Twisted Bilayer Graphene (MATBG) hosts a variety of quantum states, such as
superconductivity and correlated insulating phases, which are tunable via gating. The
mechanism of these correlated phenomena is still a mystery and has been the subject of active
theoretical and experimental research.
We realize a gate-defined quantum interferometer (Fig. 1a) in magic-angle twisted bilayer
graphene (MATBG) and observe coherence effects such as the Little–Parks and the Aharonov–Bohm effect [1]. In the device architecture, we exploit the variety of tunable correlated states in
MATBG and confine a superconducting or normally conducting ring by a correlated or band
insulator (Fig. 1b).
For a superconducting ring, we observe the Little–Parks effect as oscillations in the resistanceand critical current of the ring in magnetic field. Additionally, we also show a novel probe of the
Little–Parks effect by directly measuring the oscillation of the superconducting phase diagram
as a function of carrier density and magnetic field. From the h/2e-periodicity of the oscillations,
we confirm an effective charge of 2e for the superconducting carriers.
In the normally conducting regimes, we report h/e-periodic Aharanov-Bohm oscillations anddemonstrate that the phase coherence length of moiré electrons exceeds several microns at 50
mK. Intriguingly, we identify a regime where h/e-periodic oscillations coexist withsuperconductor-like transport.
We have shown previously that gate-defined devices are excellent platforms for studying andcontrolling superconductivity in MATBG [2, 3, 4]. In this work we demonstrate that tunable
quantum interferometers enable the study of coherent phenomena of correlated quantum
states.
[1] S. Iwakiri, A. Mestre-Tora, et al. Nat Commun. 15, 390 (2024).
[2] F. K. de Vries, et al. Nat. Nanotechnol. 16, 760–763 (2021).
[3] E. Portolés, et al. Nat. Nanotechnol. 17, 1159–1164 (2022).
[4] G. Zheng, et al. Phys. Rev. Research 6, L012051 (2024).

Variable carrier mobility of graphene gated with an ionic liquid
Hidenori Goto,* Yuki Yamamoto, Lei Zhi, Ritsuko Eguchi, Yoshihiro Kubozono
Research Institute for Interdisciplinary Science, Okayama University, JapanThe field-effect transistor (FET) in double-gate structure, which has an ionic-liquid top
gate and a solid bottom gate as shown in Fig. 1, enables us to control the carrier density and theelectric field independently. The effect of the electric field on electronic properties is prominent
in two-dimensional materials whose thickness is smaller than the penetration depth. Band
structures of few-layer graphene (FLG) can be modified by applying a perpendicular electricfield 𝑬 to change the potential energy at each layer1,2. We found that the minimum conductivity
and the field-effect mobility were significantly enhanced under 𝑬, although the band structure ofmonolayer graphene (MLG) should not be modified by 𝑬. To clarify the origin, the double-gate
MLG FETs were prepared in Hall bar geometry to evaluate the carrier density and the mobility
independently.
The conductivity 𝜎 and Hall coefficient 𝑅
different setups for 𝑉 . In the setup 1, 𝑉
effect of 𝑉 . On the other hand, 𝑉

were measured by sweeping 𝑉

was also swept to hold 𝑉

in two𝑉 to minimize thewas fixed to 1.0 V in the setup 2 to add upward 𝑬. The

mobility measured in the setup 2 was about twice larger than that in the setup 1, consistent withthe previous results. Analysis of the 𝑅 𝑉

curve in the setup 2 showed that the anions weretrapped on the graphene to accumulate holes. In this case, it is energetically favorable that theanions are located on the positively charged impurities in the SiO2 dielectric. Accordingly, thecancellation of the charges of the anions and impurities is the plausible reason why theapplication of a small amount of 𝑉(= 1.0 V)

increased the mobility. The result indicates apossible induction of novel functionality ofgraphenebycontrollingtheelectrostaticinteraction of the charges at the top and bottom
sides.
[1] J. B. Oostinga et al., Nat. Mater. 7, 151 (2008).
[2] C. H. Lui et al., Nat. Phys. 7, 944 (2011).
*

Hidenori Goto: hgoto@okayama-u.ac.jp

Figure 1. Schematic side view of adouble-gate MLG FET.

Many-body band renormalization in highly doped graphene
Federico Bisti
Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, Italy.
The extreme simplicity of its electronic structure has affirmed graphene as an ideal testbed for thedemonstration of many-body renormalization effects on band structure. In this talk I will propose: 1)
an undisputable experimental demonstration of the electron-phonon origin of the kink at 170 meV;2) a demystification of the band flattening at the van-Hove singularity proximity [2].
Regarding the first case [1], I will show an energy shift towards the Fermi level of the “kink”
associated to the electron-phonon coupling once the graphene is made of only 13C isotope rather
than 12C. Such an energy shift is in excellent agreement with the expected softening of the phonon
energy distribution due to the isotope substitution. Apart from providing an indisputable
experimental proof of the electron-phonon coupling origin of this “kink”, I will discuss theexperimental accuracy achievable with a proper robust analysis framework applied to theexperimental data.
For the second case [2], I will present an alternative interpretation for the highly doped graphene
band flattening once the Fermi level approaches the van-Hove singularity. By simulating thegraphene spectral function from the density functional theory calculated bands, it is possible todemonstrate that the photoemission signal around the M point originates from the spectral function
tail of the unoccupied band above the Fermi level. Such interpretation put forward the absence of
any additional strong correlation effects at the van-Hove singularity proximity, reconciling the mean
field description of the graphene band structure even in the highly doped scenario.
[1] F. Bisti, et al., Phys. Rev. B 103, 035119 (2021).
[2] M. Jugovac, et al., Phys. Rev. B 105, L241107 (2022).

Probing Kinetic Inductance and Pairing Symmetries in Magic Angle Twisted
Bilayer Graphene
Miuko Tanaka, Joel I-J. Wang1, Thao Dinh2, Daniel Rodan-Legrain1, Sameia Zaman1,3, Max
Hays1, Kenji Watanabe4, Takashi Taniguchi5, Terry P. Orlando3, Simon Gustavsson1, Jeffrey
A. Grover1, Kyle Serniak1, Pablo Jarillo-Herrero2, William D. Oliver1,2,3
The University of Tokyo, Institute of Solid-State Physics, Japan
1Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology,

Cambridge, MA 02139, USA.
4Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-

0044, Japan.
5International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki,

Tsukuba 305-0044, JapanThe mechanism and pairing symmetry of superconductivity have always been at theforefront of the study of unconventional superconductors. The physics underlying thesuperconducting phases in magic-angle twisted bilayer graphene (MATBG) may
provide revelatory insight beyond the moiré systems and extend into other stronglycorrelated systems such as high-TC superconductors.
Here, we combine DC transport and circuit quantum electrodynamics (cQED)
techniques to characterize the kinetic inductance and, consequently the superfluid
density, within MATBG samples. The hybrid superconducting circuit grants accessto collective behaviors of the superfluid ensemble across the entire MATBG region.
We will present the temperature, DC bias current, and microwave power
dependence of the superfluid density in MATBG samples and discuss theirimplication on the gap structure in MATBG. We stress that the hybrid
superconducting circuits demonstrated can be utilized to study other atomically thin
2D superconductors in both DC and AC regimes.

Exotic superconducting state and high-field phase diagram of tetragonal FeSe1-xSx
Shigeru Kasahara
Research Institute for Interdisciplinary Science, Okayama University
The isovalently substituted FeSe1-xSx superconductors have attracted significant interest due
to various exotic properties associated with the intertwining of nematicity, magnetism, andunconventional superconductivity [1]. Of particular interest is the abrupt change in the
superconducting gap function that occurs at the nematic critical point at xc ~ 0.17 [2-6], above
which the formation of an ultra-nodal pairing state with a putative Bogolubov Fermi surface has
been proposed [5-10]. Although the emergence of the Bogoluvov Fermi surface appears toreasonably explain the huge residual density of states observed in the tetragonal FeSe 1-xSx, thenature of the exotic pairing state remains largely elusive, including the links between the ultranodal state and the colossal fluctuation-like behavior reported in the previous experiments [11].
Here, using the high-quality single crystals of FeSe 1-xSx and the precise measurements ofthermodynamic and charge transport properties, we discuss the gap formation of the ultra-nodal
pairing state, which deviates from the mean-field-like behavior. The results provide fresh insights
into our understanding of the ultra-nodal pairing state.

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