Direct evidence of a charge depletion region at the interface of Van der Waals monolayers and dielectric oxides The case of superconducting FeSeSTO Khalil Zakeri1Dominik Rau1Janek Wettstein1Markus D ottling1

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Direct evidence of a charge depletion region at the interface of Van der Waals monolayers

and dielectric oxides: The case of superconducting FeSe/STO

Khalil Zakeri,1, ∗Dominik Rau,1Janek Wettstein,1Markus D¨ottling,1

Jasmin Jandke,2Fang Yang,2Wulf Wulfhekel,2, 3 and J¨org Schmalian4, 3

1Heisenberg Spin-dynamics Group, Physikalisches Institut,

Karlsruhe Institute of Technology, Wolfgang-Gaede-Str. 1, D-76131 Karlsruhe, Germany

2Physikalisches Institut, Karlsruhe Institute of Technology,

Wolfgang-Gaede-Str. 1, D-76131 Karlsruhe, Germany

3Institute for Quantum Materials and Technologies,

Karlsruhe Institute of Technology, D-76344, Eggenstein-Leopoldshafen, Germany

4Institute for Theory of Condensed Matter, Karlsruhe Institute of Technology, D-76131, Karlsruhe, Germany

Abstract

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The discovery of two dimensional Van der Waals materials has opened up several possibilities for designing

novel devices. Yet a more promising way of designing exotic heterostutures with improved physical properties is

to grow a monolayer of these materials on a substrate. For example, in the ﬁeld of superconductivity it has been

demonstrated that the superconducting transition temperature of a monolayer of FeSe grown on some oxide

substrates e.g., strontium titanate (STO) is by far higher than its bulk counterpart. Although the system has

been considered as a model system for understanding the phenomenon of high-temperature superconductivity,

the physical mechanism responsible for this high transition temperature is still highly under debate. Here using

momentum and energy resolved high-resolution electron energy-loss spectroscopy we probe the dynamic charge

response of the FeSe/STO(001) system and demonstrate that the frequency- and momentum-dependent dynamic

charge response is not compatible with a simple ﬁlm/substrate model. Our analysis reveals the existence of a

depletion region at the interface between this Van der Waals monolayer and the substrate. The presence of

the depletion layer, accompanied with a considerably large charge transfer from STO into the FeSe monolayer,

leads to a strong renormalization of the STO energy bands and a substantial band bending at the interface.

Our results shed light on the electronic complexities of the FeSe/oxide interfaces and pave the way of designing

novel low-dimensional high-temperature superconductors through interface engineering. We anticipate that the

observed phenomenon is rather general and can take place in many two dimensional Van der Waals monolayers

brought in contact with dielectric oxides or semiconducting substrates.

——————————————————————————————————————————————————–

I. INTRODUCTION

Followed by the discovery of the unique physical properties of graphene, a monolayer (ML) of carbon atoms arranged

in a honeycomb lattice, several other atomically thin two dimensional (2D) Van der Waals (VdW) materials have been

discovered many of which exhibit unprecedented electrical, optical and magnetic properties [1–3]. These materials

can even be prepared in the form of a single layer in contact with a substrate. The physical properties of such

hybrid structures can be precisely controlled by several means, opening up new opportunities for their application in

nanoscale devices in the future technologies [4].

In the ﬁeld of superconductivity the discovery of the phenomenon of high-temperature superconductivity (HTSC)

in iron-based materials has triggered a tremendous amount of innovative scientiﬁc eﬀorts [5–7]. Iron chalcogenides

are among VdW materials and are structurally the simplest high-temperature superconductors [8]. Owing to their

simple crystal structure they have been considered as model systems for understanding the phenomenon of HTSC.

The remarkable discovery of HTSC in FeSe ML grown on SrTiO3(001) [hereafter STO(001) or STO] with the highest

transition temperature among all iron-based superconductors has made this system as one of the most attractive

systems in condensed-matter physics [9–15]. Although the physical mechanism leading to HTSC in FeSe ML on

STO is not yet fully understood, it is generally believed that the superconductivity in this hybrid system is largely

∗khalil.zakeri@kit.edu

arXiv:2210.02058v1 [cond-mat.supr-con] 5 Oct 2022

enhanced by interfacial eﬀects [16–21]. It has been observed by means of several experimental techniques, including

angle-resolved photoemission spectroscopy, that FeSe ML is strongly electron doped [12,17,20,22–24]. The origin of

this large carrier density inside FeSe ML has been postulated to be due to the charge transfer from the substrate into

the ﬁlm [18,25]. However, a direct and unambiguous evidence of such a large charge transfer has not been reported

experimentally.

Here by probing the dynamic charge response of the FeSe/STO interface, by means of high-resolution spectroscopy of

slow electrons, we show that the experimentally measured frequency and momentum resolved dynamic charge response

cannot be explained by assuming a simple ﬁlm/substrate model only. Our detailed analysis unambiguously identiﬁes a

charge free depletion region in the STO substrate at the interface with FeSe ML. The existence of such a thick depletion

layer is accompanied with a considerably large charge transfer from the STO into FeSe ML. The presence of the

depletion region along with the large interfacial charge transfer leads to a substantial band bending and renormalization

of the electronic bands at the interface. In addition to the fact that our ﬁndings contribute to the understanding

of HTSC in FeSe ML, they would provide guidelines for designing new high-temperature superconductors through

interface engineering. Moreover, the presence of a depletion layer at the interface is rather general and is expected to

be observed in many combinations of VdW MLs put in contact with dielectric oxides or semiconducting substrates.

II. RESULTS AND DISCUSSIONS

The frequency and momentum resolved dynamic charge response of the epitaxial FeSe MLs grown on Nb-doped

STO(001) (hereafter Nb-STO) was probed by means of high-resolution electron energy-loss spectroscopy (HREELS)

(see Sec. IV A of Materials and Methods for details), using slow electrons. Generally, in the electron scattering

experiments from surfaces the scattering near the specular geometry is governed by the dipolar scattering mechanism

[26]. In this region, known as dipolar lobe, the incoming electron interacts with the total charge density of the sample

via dipolar interactions. This means that the scattering intensity includes information from both electrons as well as

the ions in the sample. The dipolar interaction is of Coulomb nature and hence is long range. Therefore, the scattered

electrons carry information from the charge density ﬂuctuations located not only near the surface region but also

far below the surface, depending on their kinetic energy. The lower the kinetic energy the more surface sensitivity

[27–29]. Since in our experiments we are mainly interested in the properties of FeSe ML and the interfacial region,

we use electrons with kinetic energies as low as 4 to 8 eV.

Figure 1ashows the HREEL spectra recorded on the surface of FeSe ML grown on Nb-STO(001). The spectra were

recorded at a temperature of T= 15 K, below the superconducting transition temperature of the sample and at two

diﬀerent incident beam energies, namely Ei= 4.07 eV and Ei= 7.23 eV. The spectral function S(qk, ω) measured

by HREELS directly reﬂects the dynamical response of the collective charge excitations in the system and is directly

proportional to the imaginary part of the dynamic charge susceptibility Imχ(q, ω) [30,31]. Since the electrons are

scattered by the total charge distribution of the sample, the scattering intensity must include information regarding

collective ionic excitations i.e, phonons as well as collective electronic excitations i.e., plasmons. Moreover, any type

of excitation representing a hybrid mode should also be excited within this mechanism. In the measured spectra

presented in Fig. 1aone observes several interesting features. Besides the elastic peak at the energy-loss of ~ω= 0

(zero-loss peak, ZLP) there are small peaks at ~ω= 11.8, 20.5, 24.8 and 36.7 meV. These are the phonon peaks of

FeSe ML, which match perfectly to those probed on FeSe(001) single crystals [32–34] and also those reported for the

FeSe ﬁlms on Nb-STO(001) of diﬀerent thicknesses [35]. The most prominent features are the so-called Fuchs-Kliewer

(FK) phonons of the STO substrate, which appear at the energies of ~ω= 59.3 and 94.5 meV.

In principle, electrons can also excite multiple quanta (higher order harmonics) of FK phonons. Hence one should

observe these excitations at the multiple frequencies of the principle excitations. The most interesting observation is

that unlike the bare STO surfaces [36], in the case of FeSe/STO the higher order harmonics of the FK modes are

strongly suppressed. Note that within the dipolar-scattering mechanism for a semi-inﬁnite substrate the intensities of

dipolar losses normalized to the elastic peak intensity scale in a 1/√Eimanner, where Eiis the energy of the incident

electron beam. This means that the lower the incident energy the larger the amplitude of FK modes. The intensities

of the higher order harmonics of FK modes obey a Poisson distribution [26,37]. Hence, in the spectra recorded using

such low incident energies one shall clearly observe both the FK modes and their higher order harmonics, if they are

present in the system. We will see that the strong suppression of the higher order harmonics of FK modes is due to

the presence of the free charge carriers in FeSe ML and in the deeper layers of the Nb-STO substrate.

In order to shed light on the origin of the observed phenomenon the measured HREEL spectra were simulated. The

simulation is based on the dipolar scattering theory (see Sec. IV B of Materials and Methods for details). Our analysis

indicates that considering ML FeSe on a semi-inﬁnite doped STO(001) cannot explain the experimental spectra. The

best model explaining the experimental spectra is considering a system composed of one ML of FeSe on 17 unit cells

of charge free insulating STO(001) on top of a semi-inﬁnite Nb-STO(001). In this model the Fe plane in FeSe ML is

0100200

0.0

0.5

1.0

Ei = 7.23 eV

Ei = 4.07 eV

)stinu .bra( ytisnetnI dezilamroN

Electron energy-loss (meV)

x30

0100200

0.0

0.1

0.2 w dep. layer

w/o dep. layer

w/o Nb in STO

w/o FeSe carriers

& w/o dep. layer

w/o FeSe carriers

& w/o Nb

)stinu .bra( ytisnetnI dezilamroN

Electron energy-loss (meV)

FeSe

abc

dFe

Figure 1.Evidence of the depletion layer in high-resolution electron energy-loss spectra. a The solid circles

represent the experimental spectra recorded at the specular geometry i.e., the wavevector of qk= 0 ˚

A−1, at the high symmetry

Γ–point and using two diﬀerent incident beam energies (Ei= 4.07 eV, orange and Ei= 7.23 eV, light-blue colour). The

simulated spectrum for Ei= 4.07 eV is represented as the solid line. bThe geometrical structure used for the simulation.

In this model the Fe plane in FeSe ML is placed in a distance dF eSe = 0.43 nm above the STO(001) surface. The thickness

of the depletion layer was varied to obtain the best ﬁt to the experimental spectra (d= 6.5 nm in this case). cSimulated

spectra for diﬀerent cases as denoted in the legend, dark-blue: considering all the required terms (the same as the one in a),

violet: without considering a depletion layer, brown: without considering the charge carriers in STO, green: without a Drude

term describing the free carriers in FeSe ML, light-red: without considering free carriers in FeSe ML and without considering

a depletion layer, orange: without considering free carriers in FeSe ML as well as in STO.

placed in a distance dF eSe = 0.43 nm above the STO(001) surface [38]. The structure is schematically sketched in Fig.

1b. Only in this way both the peak position and amplitude of the excitations associated with the FK modes agree

with those measured experimentally, as demonstrated in Fig. 1a. Similar to the experiment the higher harmonics of

the principle FK modes are strongly suppressed due to the presence of the free carriers in FeSe ML and in the interior

part of the substrate, below the depletion region.

Spectra simulated for several other conﬁgurations are presented in Fig. 1cfor a comparison. While the dark-blue

colour represents the case shown in Fig. 1a(assuming the structure shown in Fig. 1b), the violet and light-red colours

represent cases in which no depletion layer was considered. In these cases due to the presence of the charge carriers

in Nb-STO, mainly due to the Nb doping, the FK peaks are heavily damped and are blue shifted. Neither the energy

nor the peak height match the experimental spectra. Now, if one does not consider the contribution of the carriers

in Nb-STO, the FK peaks undergo a redshift and become almost identical in amplitude, as shown by the brown

spectrum in Fig. 1c. Likewise, the free carriers in FeSe ML are also essential to be considered, as demonstrated by

the green spectrum. In order to carefully investigate the inﬂuence of the depletion layer’s thickness don the spectra,

simulations were performed for various values of d. Such data are presented in Supplementary Figure 1. It turned

out that the best agreement with the experimental data can be achieved when dis assumed to be 6.5±1 nm.

Another important result of the data presented in Fig. 1cis that the higher order harmonics can only show up in the

spectra when no charge carriers are present in the system. In such a case the FK peaks are rather sharp and intense

and their higher order harmonics are clearly visible (see orange spectrum in Fig. 1c). This observation demonstrates

the important role of the free carries in the suppression of the higher order harmonics of the FK modes. This was

conﬁrmed experimentally by performing experiments on a clean STO(001) surface at T= 180 K. The experimental

spectrum recorded over a wide range of energy-loss is presented in Supplementary Figure 2. The STO(001) surface

was treated in exactly the same way as for the FeSe/STO samples, prior to the ﬁlm deposition (see Sec. IV A of

Materials and Methods). In the absence of FeSe ML one observes the higher harmonics of the principle FK modes,

indicating the role of FeSe ML’s charge carriers in the suppression of these modes. We note that the eﬀective mass

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DirectevidenceofachargedepletionregionattheinterfaceofVanderWaalsmonolayersanddielectricoxides:ThecaseofsuperconductingFeSe/STOKhalilZakeri,1,DominikRau,1JanekWettstein,1MarkusDottling,1JasminJandke,2FangYang,2WulfWulfhekel,2,3andJorgSchmalian4,31HeisenbergSpin-dynamicsGroup,PhysikalischesInstitu...

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Direct evidence of a charge depletion region at the interface of Van der Waals monolayers and dielectric oxides The case of superconducting FeSeSTO Khalil Zakeri1Dominik Rau1Janek Wettstein1Markus D ottling1

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