about the technique

Photoemission experiments are at present witnessing a continuous progress towards the increase in resolution and optimization. In such experiments, the modern ability to control with the highest degree of precision the state of the incident photons in terms of momentum, monochromaticity and polarization, can be combined with state-of-the art techniques to evaluate the energy, momentum and spin state of the photoemitted electron. This allows to completely characterize the initial and final states of the probe particles and hence to reconstruct with highest precision the electronic structure of the sample under investigation and its possible excitations.

Another field of exploration, however, is being opened by the realization of Free Electron Lasers, new light sources enabling to deliver X-ray pulses of femtosecond (fs) duration and extremely high brilliance. The flux of photons in such ultrashort pulses is so large that the number of emitted electrons is sufficient to perform even the statistically challenging spin resolved photoelectron spectroscopy and ARPES in single-shot mode. This will enable to study a whole new plethora of behaviours: the dynamics of relaxation processes.

Thanks to the short duration of the X-ray pulses, indeed, it will be possible to obtain all the aforementioned information on the electronic structure, and to relate it to a very specific instant in time. If a strong enough excitation can be delivered, driving the system out of equilibrium suddenly, it is possible to follow with stroboscopic measurements (pump-probe method) the relaxation processes of the material in study. 

Fig. 1 Photographic picture of the experimental chamber during the commissioning with a pulsed electron beam.

Fig. 1 Photographic picture of the experimental chamber during the commissioning with a pulsed electron beam.

This yields a high interest especially in condensed matter physics, where the high degree of correlation of the constituent particles and emergent quasiparticles allows for extremely fast relaxation times: achieving a profound comprehension of such mechanisms can drive towards the realization of novel functional devices in which every characteristic of the system is exploited to the fullest.

With the SPRINT project, we have developed a new endstation, addressed at the study of ultrafast magnetic processes in solid state physics with a vectorial Mott detector upgraded to perform single-shot spin-polarization detection.

The source

The new SPRINT laboratory has been equipped with a laser source (PHAROS), able to produce pulses at 1030 nm, with time duration of 300 fs, variable repetition rate from 50 Khz up to 1 Mhz, pulse energy of 400 J and average power up to 20 Watts. This laser will be the heart of the new high harmonic generation based beamline, ready for users from the next year. Thanks to the very high power of PHAROS, it can be used to produce harmonics (up to the fifth) with crystals based setup.
We developed two different setups: a collinear one (see Fig.2.1), to produce the 2nd and the 3rd harmonic and a crossed one (see Fig.2.2), to produce the 4th and the 5th.

 

In the collinear setup, the laser beam at 1030 nm is sent on a beta barium borate (BBO) crystals of about 2 mm thickness, which produces the 2nd harmonic at 515 nm. The two beams pass across two plates which assure that the two beams have the same polarization and are temporally overlapped before enter into the other BBO crystal, which creates the 3rd harmonic at 343 nm. The beams are then spatially separated with a prism.
In the non collinear setup, after SHG in a BBO crystal, the two beams are spatially separated with a dichroic mirror. The 2nd harmonic is sent to a SHG crystal optimized for 515nm in order to produce its 2nd harmonic (which means the 4th harmonics of 1030 nm) at 257 nm (4.8 eV). The 4th harmonics and the fundamental are then temporally and spatially coupled on a BBO crystal optimized to sum these two frequencies, allowing the production of the 5th harmonics at 206 nm (6 eV).

 

The temporal overlap is guaranteed by a delay stage, which varies the optical
path of the 2nd harmonics, maintaining the spatial alignment between the
fundamental and the 4th harmonic.
The efficiency of the spectrometer is different for different wavelengths.
So, the relative intensity between the various peaks is not real. The
conversion efficiencies for the 2nd and 3rd harmonic are more than 35 %
and 2 %, respectively. The really good quality of the spots is obtained
avoiding the focalization of the beams into the BBO crystal. It can be done
thanks to the PHAROS high power.
The measured efficiency for 4th and 5th harmonic is 3 % and 0.5 %,
respectively. Also in this case, a good beam profile is observed. As an
example, the two beam profiles at 1030 nm and 515 nm are reported in
Fig. 3.

Fig.2.1 2nd and 3rd harmonic setup in collinear configuration

Fig.2.1 2nd and 3rd harmonic setup in collinear configuration

Fig.2.2 4th and 5th harmonic setup in crossed configuration; SHG: second harmonic generation, THG: third harmonic generation.

Fig.2.2 4th and 5th harmonic setup in crossed configuration; SHG: second harmonic generation, THG: third harmonic generation.

Fig.3 3D (left panel) and 2d (right panel) profile of 1030 nm and 515 nm after the prism separation. The higher intensity peak is at 1030 nm.

Fig.3 3D (left panel) and 2d (right panel) profile of 1030 nm and 515 nm after the prism separation. The higher intensity peak is at 1030 nm.

The method

The main instrument is a Mott detector measuring the spin polarization of the secondary electrons photoemitted under the X-ray pulse excitation. The secondary electrons, that undergo several inelastic collisions when travelling in the material, have different mean free paths in a ferromagnetic solid. For this reason, a high spin polarization is observed in the extremely low energy component of the photoelectron spectrum, which is proportional to the magnetization state of the sample. In particular, such information is retrieved only from the extreme surface, as experiments of overlayer deposition proved that the polarization signal is lost after deposition of few Å of non-magnetic material.

 

The spin-polarization of the photoemitted electrons is measured through the classical Mott scattering asymmetry:

Electrons accelerated to 40keV hit a gold target, and are then detected by two detectors at ±120° in the scattering plane. Spin-orbit interaction with gold nuclei results in an asymmetry dependent on the polarization of the incoming beam. Such asymmetry is then proportional to the polarization of the incoming electron beam along the direction perpendicular to the scattering plane:

 

A vectorial Mott detector features two orthogonal scattering chambers in which eight detectors are mounted, featuring a total of four scattering planes, of which three are orthogonal and one is redundant and used to cross calibrate the signals. An electrostatic switch allows to change from one target to the other very rapidly, realizing a quasi-simultaneous measurement of the three independent components of t94D71DED816DB48E5FB8E28CE13E96AErface magnetization.

The instrument has now been mounted a dedicated UHV chamber, where a suitable sample environment has been designed to meet the demanding criteria of such kind of measurements. The complex signal readout was upgraded with a high performance electronics able to operate both in the pulsed, high peak flux regime (for pump-probe experiment with high brilliance sources) and in the continuous steady flux mode (the standard mode for synchrotron or laboratory sources).

i
i
Fig. 4 Hysteresis measured on the horizontal polarization channel in continuous mode on a test thin film Fe/Si (001) sample. The curve was acquired under a continuous excitation and completed in less than 15 minutes. The reduced signal-noise ratio at larger fields is due to the drop of intensity produced by the Lorentz deflection by the external field of the magnet

Fig. 4 Hysteresis measured on the horizontal polarization channel in continuous mode on a test thin film Fe/Si (001) sample. The curve was acquired under a continuous excitation and completed in less than 15 minutes. The reduced signal-noise ratio at larger fields is due to the drop of intensity produced by the Lorentz deflection by the external field of the magnet

Fig. 5 Image of the endstation under commissioning. The two UHV chambers are independent and can operate simultaneously. Here, thin films were being grown in the preparation chamber while other samples were measured in the main chamber.

Fig. 5 Image of the endstation under commissioning. The two UHV chambers are independent and can operate simultaneously. Here, thin films were being grown in the preparation chamber while other samples were measured in the main chamber.

The apparatus

Photoemission experiments are at present witnessing a continuous progress towards the increase in resolution and optimization. In such experiments, the modern ability to control with the highest degree of precision the state of the incident photons in terms of momentum, monochromaticity and polarization, can be combined with state-of-the art techniques to evaluate the energy, momentum and spin state of the photoemitted electron. This allows to completely characterize the initial and final states of the probe particles and hence to reconstruct with highest precision the electronic structure of the sample under investigation and its possible excitations.

Another field of exploration, however, is being opened by the realization of Free Electron Lasers, new light sources enabling to deliver X-ray pulses of femtosecond (fs) duration and extremely high brillhis allows to completely charactch ultrashort pulses is so large that the number of emitted electrons is sufficient to perform even the statistically challenging spin resolved photoelectron spectroscopy and ARPES in single-shot mode. This will enable to study a whole new plethora of behaviours: the dynamics of relaxation processes.

Thanks to the short duration of the X-ray pulses, indeed, it will be possible to obtain all the aforementioned information on the electronic structure, and to relate it to a very specific instant in time. If a strong enough excitation can be delivered, driving the system out of equilibrium suddenly, it is possible to follow with stroboscopic measurements (pump-probe method) the relaxation processes of the material in study. 

Fig. 6 A 3D rendering of the endstation. A sagittal section of the main chamber exposes the electrostatic lenses that transport the photoemitted electrons to the detectors and the two orthogonal scattering chambers (yellow).

Fig. 6 A 3D rendering of the endstation. A sagittal section of the main chamber exposes the electrostatic lenses that transport the photoemitted electrons to the detectors and the two orthogonal scattering chambers (yellow).

Technical Specifications

UHV base pressure 1×10 -10 mbar
Residual magnetic field Below 2 × 10 -7 T
On sample magnetic field ± 1000 A/m
On sample maximum voltage ± 2 kV
Cryogenic cooling 300 K (room temperature) to 4 K
Annealing stages 300 K to 800 K on manipulator 300 K to 1300K on high temperature stage

Contributions

The realization and the commissioning of the Ultraspin   apparatus has been possible thanks to the expert   contribution of Prof. Vladimir N. Petrov, Marie Curie   visiting scientists at IOM-CNR in 2013-2015 (under project   no 326641 “EXSTASY”).

The realization and the commissioning of the Ultraspin apparatus has been possible thanks to the expert contribution of Prof. Vladimir N. Petrov, Marie Curie visiting scientists at IOM-CNR in 2013-2015 (under project no 326641 “EXSTASY”).

The map of techniques

at MM building at Q2 building at Elettra experimental hall at CNR-IOM cloud at Fermi-T-Rex laboratory Surface & Nano Science Lab, STM/STS PLD XPS & ambient pressure XAS ARPES & Spin ARPES MOKE & Masked deposition system XPS MBE Oxides SPRINT laboratory SEM XRD PVD data repository open data data analysis
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