Coexistence of ferromagnetism and superconductivity in EuFe(AsP) 

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Sample manipulation in UHV

The second step is the introduction of the sample into the UHV system. The two used systems, one in LPEM-Paris and the other in MIPT-Dolgoprudny (103), are very similar: they are slightly modified low temperature Scanning Probe Microscopes produced by SPECST M (104) (fig. 2.2). The systems are composed by three interconnected UHV chambers: fast sample entry load-lock chamber, preparation-characterization chamber and SPM chamber (105). The position of the chambers are marked by yellow dashed ellipses on fig.2.3.
The fast entry load-lock chamber is connected to the preparation chamber via the manipulator M1. Before the load-lock chamber is filled with dry nitrogen gas, it is isolated from the rest of the UHV system by a gate valve. When the nitrogen pressure in the load-lock chamber attends the ambient one, the top flange is opened and the sample holder with the substrate is put inside, and fixed at the manipulator M1 end. To fix the sample on manipulators, three kinds of fixations are used: a screw-driver like one (not shown), the tweezers-like one (shown at the right panel in fig.2.1) and a flat drawer-like receptacle (shown in fig.2.4). All systems enable a secure fixation of the sample holder to the manipulators.
Once the sample is fixed onto manipulator M1, the top flange is locked, and a tur-bomolecular pump followed by a spiral primary pump removes the nitrogen from the chamber. In 3-4 hours the pressure in the load-lock chamber attends ∼ 10−7÷8 mbar. At this pressure, the gate valve separating the load-lock from the preparation chamber is opened, and the sample is moved into the preparation chamber with the manipulator M1.

In-UHV Si(111) surface preparation

The third step is the Si-substrate preparation. The goal of this step is to achieve atomically flat defect-less 7×7 reconstructed Si(111) surface by a series of heating pro-cesses in UHV. This is a crucial step that strongly influences the quality of the sample.
When the sample holder is transmitted from the load-lock chamber to the preparation chamber (base pressure and is ∼ 10−10 mbar), it is positioned and secured on the heating stage (see fig. 2.4). The latter is fixed on the manipulator M2 moving along the main axis of the preparation chamber (fig. 2.3). The heating stage is equipped with a metallic flag – a contact for the direct current heating of the sample, a resistive heating cartridge below, and a thermocouple for the temperature control. In addition to the thermocouple, the temperature is measured from outside by a pyrometer through a special viewport facing the sample.

Structure of Pb/(Au)/Si(111) samples: LEED and STM

In the first three parts of this section we recall the basic principles of the experi-mental approaches – Low Energy Electron Diffraction, Scanning Tunneling Microscopy and Spectroscopy and Gundlach oscillations spectroscopy, which were used for struc-tural and electronic characterizations of the samples. The other parts are devoted to the presentation and discussion of the obtained characterization results.

Surface Analysis: Low Energy Electron Diffraction

The low energy electron diffraction (LEED) analysis is a method for studying the atomic organisation at material’s surface. The method based on the analysis of the diffraction patterns produced by low-energy electrons (E∼30-200 eV) elastically back-scattered from the studied surface. It consists of sending the electron beam onto the surface, and discriminate the back-scattered electrons in energy and angle (107; 108). The working principle of LEED is based one following assumptions:
1. The de Broglie wavelength of electrons λdB in the incoming electron beam is comparable with the inter-atomic distances in the studied surface. As a result, the electrons efficiently diffract on the atomic surface structure. For this, the typical electron acceleration energy of Ee=30–200 eV is chosen, corresponding to the √ electron wavelength λdB = h/ 2mEe=∼ 0.09 – 0.2 nm. This is indeed compa-rable with the inter-atomic distance in solids.
2. The elastic depth for low-energy electrons doesn’t exceed a few atomic layers. For this reason, most elastic scatterings occurs in the one-two topmost atomic layers of the sample surface. Therefore, mainly these layers contribute to the diffraction patterns.
3. The electron beam does not alter the atomic ordering at surfaces. This assumption is valid for most of inorganic materials.
Basically, the LEED units consist of (see fig.2.5(a)): an electron gun which gen-erates a beam of low-energy electrons that is directed to the sample surface. Usually, beam comes from a hot electron emitting cathode biased with respect to the crystal kept « grounded »; a goniometer on which the studied crystal is positioned; several retard-ing grids to discriminate the back-scattered electrons in energy; a fluorescent screen to convert the electron energy in light – to visualize the diffraction patterns of elastically scattered electrons, and to take images with a camera through a viewport.

Control of Si(111) 7×7 surface reconstruction

The preparation steps are described in subsections 2.2.3 and 2.2.4. After each step, successive LEED and STM experiments were provided. In this section, we present the results of these measurements.
First stage was to obtain Si of 7×7 reconstruction. When we heat silicon (111) under ultrahigh vacuum to high temperatures, the atoms on the surface form a new, more energetically favorable configuration, which is called 7×7 reconstruction. Atoms in the upper three layers occupy new places and form a much larger unit cell as compared with 1×1 silicon (111). A three-dimensional model of such a new surface is shown in fig.2.13. Different colors mean different types of atoms involved in the formation of a 7×7 reconstructed surface. The « Dimer-Adatom-Stacking fault » (DAS) model proposed by (126) shows that the distinctive features in images obtained with STM correspond to adatoms (shown in yellow in fig.2.13). The model also shows the holes in the corners, that is, the areas in the corners of unit cells of size 7×7, where there are no atoms of three upper layers. These holes are also visible on STM images in the form of dark places that are surrounded by hexagons of adatoms (fig.2.14(b)) and on a larger scale, by six triangles, where each triangle consists of six adatoms.
In the LEED experiments, the 7×7 reconstruction is revealed as a series of spots appearing 7 times more frequently on the fluorescent screen than the main spots from the bulk of the crystal (fig.2.14(a)). A clear hexagon with 6 vertex spots corresponding to 1×1 Si(111) surface is observed. It is filled with 7×7 spots. Thus, in the real space, a periodic structure increased by 7×7 times (containing 102 silicon atoms) is formed on the surface of silicon (111). The geometric parameters of such a surface over-lattice are almost independent of the method of its preparation and are a fundamental property of single-crystal silicon.

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Striped incommensurate monolayers of Pb/Si(111)

After the substrate preparation, the second step was to evaporate Pb on Si and obtain the so-called stripped incommensurate phase (SIC).
It is known that lead does not react with silicon and they are mutually insoluble (128; 129). Therefore, Pb/Si structure can form a well-defined interface just below the surface, that is perfect for studying 2D behavior of metallic Pb-system. Importantly, both lead and silicon are elements of the fourth group of the periodic table, but their bulk lattice unit cells are incommensurate. As follows from the experiments (130; 131), the Pb/Si structural phase diagram is rather complicated. The formation of a particular phase strongly depends on the number of deposited monolayers (ML), deposition temperature, and the post annealing routine. Fig.2.15(a) shows known phases realized upon the deposition of 1/6 to 4/3 ML of lead. We focused on the SIC phase, which has a Pb coverage of 4/3 ML because it has already known system and we can compare our results with it; studies of other phases are beyond the scope of this work.
In a unit cell of the SIC-Pb phase, there are four Pb atoms per three surface Si atoms. Each three of the four Pb atoms forms a covalent bond with an underlying Si atom, and the remaining Pb atom does not form a bond with Si substrate (fig.2.15(b)). Pb atoms also form metallic bonds inside the metallic upper layer, since all Pb atoms are located exactly on the same level of the atomic layer. Compared to the bulk plane of Pb (111), the SIC phase lattice is compressed by ∼ 5%.
The realization of the SIC phase is not direct. First, 1.8 ML of Pb is deposited onto 7×7 Si(111) substrate kept at room temperature. This deposition results in an amorphous ML of Pb (density of about 1.3 ML); the excess of lead forms (111)-oriented Pb-islands. After the deposition, a post annealing is made during 30 seconds at 370◦C. The Pb-islands evaporate, Pb-atoms migrate at the surface and, after cooling down to the room temperature, form a 1×1 single Pb ML-film (the corresponding LEED pattern.

Table of contents :

1 Superconductivity of ultra-thin films 
1.1 Introduction
1.1.1 Thin superconductors: Early experiments and theories
1.1.2 Berezinskii-Kosterlitz-Thouless phase transition
1.1.3 Superconductor-insulator transition
1.1.4 Interface superconductivity
1.2 Superconductivity of atomically thin materials
1.2.1 Towards ultimately thin superconductors
1.2.2 Superconductivity in atomic layers: state of the art, open questions and challenges
1.3 Choice of the samples for studies
1.3.1 Ultra-thin NbN films
1.3.2 Atomic layers Pb(Au)/Si(111)
1.3.3 EuFe2(As0.79P0.21)2) single crystals
2 Elaboration and characterization of samples 
2.1 Introduction
2.2 Fabrication of atomic layers Pb/Si(111) and Pb/Au/Si(111)
2.2.1 In-air preparation of the Si-substrate
2.2.2 Sample manipulation in UHV
2.2.3 In-UHV Si(111) surface preparation
2.2.4 Au and Pb deposition
2.3 Structure of Pb/(Au)/Si(111) samples: LEED and STM
2.3.1 Surface Analysis: Low Energy Electron Diffraction
2.3.2 Surface Analysis: Scanning Tunneling Microscopy and Spectroscopy
2.3.3 Control of Si(111) 7×7 surface reconstruction
2.3.4 Striped incommensurate monolayers of Pb/Si(111)
2.3.5 5×2 reconstructed Au/Si(111) phase
2.3.6 Structure of Pb/Au/Si(111) hybrids
2.4 Characterization of NbN films
2.5 Characterization of EuFe2As2 single crystals
2.5.1 Structure
2.5.2 Phosphorus doped EuFe2As2
3 In-UHV electron transport of ultra-thin films 
3.1 Introduction
3.2 Probe design and fabrication
3.2.1 Analysis of available developments
3.2.2 Producing the base for the probe
3.2.3 Design and manufacturing of contacts
3.2.4 Probe integration and tests in air conditions
3.2.5 Preparing the probe for in situ UHV transport measurements
3.3 Transport properties of Pb/Si(111) and Pb/Au/Si(111)
3.3.1 Description of the experiments
3.3.2 Pb/Si(111) transport measurements
3.4 Pb/Au/Si(111) and SIC-Pb/Si(111) transport measurements
3.4.1 Measurement details
3.4.2 Pb/Au/Si(111) measurements
3.4.3 SIC-Pb/Si(111) transport measurements
3.5 Transport studies of disordered NbN films
3.5.1 Experimental details
3.5.2 Properties of as-grown NbN films
3.5.3 Sputtering effect on transport properties
4 Coexistence of ferromagnetism and superconductivity in EuFe(AsP) 
4.1 Introduction
4.1.1 Predictions of the coexistence of superconductivity and ferromagnetism
4.1.2 Bulk superconductors with magnetic ordering
4.1.3 Superconductor-ferromagnet artificial systems
4.1.4 Domain structure of the simple ferromagnet
4.2 Properties of the EuFe2(As1−xPx)2
4.2.1 Mechanisms responsible for superconductivity and ferromagnetism in the sample
4.2.2 Magnetic properties of the sample
4.3 Experimental methods
4.3.1 AFM/MFM technique
4.3.2 Crystal synthesis
4.4 Probing local and global properties of EuFe2(As0.79P0.21)2
4.4.1 Transport and magnetic properties
4.4.2 Superconductivity and magnetism: coexistence
4.4.3 Variations of local magnetic properties with temperature
4.4.4 MFM experiments in a magnetic field. Hysteresis curve
5 Conclusions and Perspectives 


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