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APE is a facility for advanced experiments on solid surfaces and nanostructured matter. The all-UHV interconnected system includes state-of-the-art surface preparation and survey, atomic resolution scanning tunneling microscope, and two beamlines delivering monochromatic, polarized synchrotron radiation to a high-
energy/angle resolution spectrometer and to a microscope allowing for soft-X ray absorption and magnetic dichroism, photoemission, spin polarization mapping and time-resolved measurements.

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The beamlines use the synchrotron light from the Elettra storage ring. Photons with chosen polarization are emitted simultaneously by two non-collinear insertion devices and delivered in two distinct beamlines (Fig. 1). The line dedicated to high-resolution photoelectron spectroscopy uses low energy photons (hn=10-100eV) (APE-LE, LE = Low photon Energy). The second line exploits photons in the range hn=140-1500eV (APE-HE, HE = High photon Energy). The photon energy resolution after the APE-LE monochromator is better than E/DE=15000. The first results on APE-HE indicate E/DE=10000.

Current tests confirm negligible amount of cross talk between the two undulator sources. Samples can be loaded with two differentially pumped load-locks and then transferred in UHV to any of the preparation chambers, to the STM and to the both synchrotron radiation spectrometers (Fig. 1).

APE also allows the integration of users' specialized sample growth chambers or modules, which will be connected to the main sample distribution chamber and will have full access to the off-beam and on-beam facilities. Long-term users' programs will be therefore possible, having access to beamtime based on usual assignment procedures, but having semi-permanent growth facilities and access to the off-beam diagnostics.

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beamline

The variable-polarization light is delivered from the Apple II quasi-periodic undulator. It is then deviated onto the plane grating monochromator by the spherical mirror in the first mirror chamber (Fig. 1). Three variable-spacing gratings, 700 l/mm, 1200 l/mm and 1600 l/mm, cover the energy range 10-25 eV, 25-40 eV and 40-100 eV, respectively. Currently 20-100 eV range is available since the delivery of the 700 l/mm grating is still pending. The monochromatic light is focused by a spherical mirror on the exit slit. 

The photon resolution after the exit slit is determined by measuring absorption spectra of rare gases (Fig. 2). We find E/DE = 16000 at hn~47 eV (Fig. 2a) and E/DE = 13000 at hn~63 eV photons for all polarizations (Fig. 2b and 2c). The photon flux at best resolution is 2·10¹⁰ photons/s. The stability of the photon energy is such that we find <0.5 meV shift during 7 hours of measurements. The toroidal mirror refocuses the light further into the LE end station on 50x100 mm spot.

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end station

The APE-LE end station is equipped with Scienta SES-2002 electron energy analyzer. The experimental resolution (i.e. combined light + analyzer + sample temperature) is determined from the width of the measured Fermi edge of a polycrystalline Ag sample. We consistently measure 6 meV (Fig. 3) for 25-47 eV photons. 

The angular acceptance of the analyzer is ~14°. The angular resolution of the analyzer is achieved electronically, with a special set of lens parameters, which permit that all electrons leaving the sample at a given angle are imaged to the same position at the exit slit of the analyzer. It is estimated to be <0.4°.

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Depending on the type of the experiment, two alternative manipulators are available at APE-LE. A Rial translator, with x, y, z, Q degrees of movement, hosts a homemade cryostat (University of Zürich, Osterwalder group) that reaches ~15 K on the sample surface. A VG translator contains the automated manipulator (built at the University of Zürich, Osterwalder group) with the x, y, z, Q, f degrees of movement. It is optimized for the automatic Fermi surface mapping (Figure 4). The lowest temperature for the Fermi surface mapping is ~100 K.

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This end station is also equipped with a He discharge lamp, reverse-LEED and Ar sputter gun. There is a possibility for introducing compact evaporators. More sample treatments can be done in the annex preparation chamber(s). 

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beamline

Similar to LE, the light from a periodic Apple II undulator is deviated by a spherical mirror on the plane grating monochromator which covers 140-1500 eV energy range (900, 1400, 1800 l/mm variable spacing gratings). The spherical mirror focuses the monochromatic light on the exit slit. Only a first test grating (900 l/mm) has been delivered and installed insofar. The photon resolution after the exit slit is determined by measuring the N2 and Ne absorption spectra. We find E/DE = 10000 at hn~400 eV (Fig. 5). However, E/DE = 3000 at hn~900 eV photons. This reduced resolution at higher photon energies is the consequence of the grating fabrication problems and is going to be improved with the new set of gratings whose delivery is pending. The photon flux at best resolution is 2·10¹⁰ photons/s.

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Two focusing schemes can be alternatively chosen when operating APE-HE: single reflection focussing by a toroidal mirror that produces a spot size of 25·100 mm, or deviation by a plane mirror onto a Fresnel zone plate diffractive optics. The spot size in this case is in the range 50-200 nm. The sample is moved in the wanted position by stepper motors with XY scanning capability (0.5 mm resolution). The microscopy is performed by scanning the Fresnel zone plate by piezoelectric drives.

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end station

The APE-HE end station will work as a highly focused soft-X ray spectrometer for core level photoemission, time resolved surface magnetometry and magnetic microscopy. The chamber hosts an Omicron 125 mm analyzer for photoelectron spectro(micro)scopy. A total yield Mott detector will be used for the surface magnetometry. The Mott detector, built at the Technical University of St. Petersburg, is under test with the counting electronics. Special electronics for time resolution mode (<100ps) is under development. APE HE is exploiting X-rays with variable polarization to study the magnetic properties of materials. A test experiment was lately performed on an iron thin film epitaxially grown on GaAs(001)-(6x4). A Fe_0.5Co_0.5 probing layer was deposited between two 6 ML's-thick Fe wedges to test the magnetic properties of the film as a function of distance from the interface with the semiconductor (see Fig. 6, upper part). This was done using X-Ray magnetic circular dichroism (XMCD) on the L_2,3 threshold of Co. In Fig. 6, lower part we show two spectra taken with the X-ray beam shining on the probing layer at different distances from the interface. The reducing of the dichroic intensity is a sign that the magnetization film is reduced when approaching the interface.

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ape preparation and stm chambers

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APE has two sample preparation stages. One of them was developed at the ETH Zürich (Pescia group) and transferred to Trieste in February 2000. It is equipped with LEED/Auger system, for both the chemical and structural characterization of the sample surfaces; Ar⁺ sputter gun; high temperature heating stage (2500°C); evaporators for epitaxial growth of metal thin films; room temperature scanning tunneling microscope (see Fig. 7) with in-situ tip and sample exchange. In addition, magnetic monolayers and multilayer systems can be investigated with a vectorial magneto-optical Kerr effect. The second preparation stage is located in the linear transfer chamber attached to the APE-LE end station. It is designed to host an Ar⁺ sputter gun, three evaporators and a sample heating stage.

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figure 7

atomically resolved stm image for sulfur  segregated Fe(3%Si) (100); the zigzag  lines along <110> appear on c(2x2)-S;  82x82Å (+400mV, 0.1nA)

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Samples and substrates can be loaded in the APE system via either one of the two identical load-locks connected to the STM chamber and to the switching chamber between the UFO and the linear travel chamber. The sample preparation can be done in either of the two preparation stages. Substrates or samples can be introduced also from users' sample chambers docked at one of the two dedicated flanges in the UFO chamber (100 mm CF flanges indicated in Fig. 1). Through those ports the arm of the UFO can enter and collect the sample that can be measured by any of the characterization tools and finally exposed to synchrotron radiation for experiments. All interested users can ask for a design of the UFO chamber and the environment for evaluating the possibility of using their system to APE.

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ape schedule

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APE beamline received first official users' proposals in August 2002. In the period January-June 2003 the full commissioning continues in parallel with the experiments on the basis of 20% of the available beamtime. From July 2003 APE is fully operational for users.

As the delivery of the pending elements (diffraction gratings, electron transport lenses for spin polarized measurements, electronics for time resolution pump-probe experiments, cryostats) will take place, the full deployment of the APE project will be pursued.

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ape team