The human behind the beamline

The human behind the beamline

This week we are going to celebrate Felix Bloch, an inspiring scientist who lends his name to the ARPES beamline at MAX IV.

Come back to this page each day this week for a new update on the man himself, the beamline, the science and the reason why we chose this week to talk about the legendary scientist.

Happy Birthday, Felix Bloch – 23rd October 1905

Felix Bloch was born on this day (23rd October) in 1905 in Zürich, Switzerland. He got a Ph.D. in 1928 studying under Werner Heisenberg. In his thesis, he established the quantum theory of solids describing how electrons moved through crystalline materials using Bloch waves. The phenomena he described are observed today using the technique ARPES which is carried out at the Bloch beamline at MAX IV.

Bloch worked with many great scientists including Niels Bohr and Enrico Fermi, spending most of his working life in the USA. In 1934 he joined Stanford University and worked on the measurement of neutron magnetic momentum using a cyclotron. In 1952, Bloch won the Nobel Prize in Physics together with E. M. Purcell “for their development of new methods for nuclear magnetic precision measurements”.

Years later, he moved back to Europe to become the Director General of CERN.

He died at the age of 77 in 1983 in Zürich.

 

Bloch the beamline

 

Let’s talk about the Bloch beamline. The X-rays from Bloch start in the 1.5 GeV storage ring. When the electrons pass through the magnetic field of special quasi-periodic elliptically polarizing undulator (q-epu) , they oscillate and emit X-rays peaked at specific photon energies. This is the source of X-rays for Bloch. The values of these energies are dependent on the undulator gap. For a standard undulator, the peaks can be grouped into a set of precisely periodic ‘harmonics’. This periodicity makes it difficult to filter out unwanted peaks with a monochromator. Bloch uses a quasi-periodic undulator, which means that the harmonic energies are not periodic making it possible to filter them out and giving a cleaner signal at the experimental station. The filtering is performed by the Bloch monochromator  consisting of both cNIM and cPGM mode of operation. The cNIM also acts as a good polarisation control meaning that users at the experimental station have more reliable and fine grain control over the polarisation of the light reaching their samples.  Apart from the q-epu, the beamline is also equipped with a gas filter and set of solid state thin film filters that can be used to supress any further unwanted photon energies, thus enabling one to take the a clean spectra from the experimental station. For more information about this, you can visit the Bloch homepage.

This is the Bloch quasi-periodic elliptically polarizing undulator. You can see that every so often some of the magnets appear lower which creates the quasi-periodic effect and allows for effective filtering of the x-rays using the monochromator, the gas filter and the solid state filter.

 

Above is a measurement from the Bloch beamline showing the harmonic X-ray intensities for a 30mm gap. With the exception of the 3rd harmonic under vertical polarization, the periodicity of the harmonics is strongly supressed

 

Above is the measured flux of the fundamental harmonic for horizontal and vertical polarization before the final refocussing mirror for a 100 mm exit slit. The calculated and measured flux are in reasonable agreement. Commissioning of circular polarization, flux and higher photon energies are ongoing.

 

The ARPES branchline

Once the x-rays have passed through the monochromater, they can travel down one of two of the Bloch branchlines. The first is the ARPES branchline. ARPES stands for angle-resolved photoelectron spectroscopy. Photoelectron spectroscopy gives information about the energies that electrons have in a material, and is a technique performed at many beamlines at MAX IV. If we now add angle-resolved, it means that we can calculate the crystal momentum of an electron travelling through a material. So we can measure the energy and the momentum of an electron which is called electronic band structure of the material. From the information of the energy versus momentum maps, one can build constant energy surfaces. Constant energy surfaces play a very important role in enabling us to understand various material properties such as electrical and optical conductivity, superconductivity, thermo-electricity, magnetic properties etc.

The ARPES endstation at Bloch is versatile for a number of reasons. Using traditional electron analyzers and a fixed sample position, one can measure the energy versus angle or equivalently energy versus parallel component of electron momentum along one angle direction. By rotating the sample, one can then obtain energy versus angle maps for all directions (which is necessary to build constant surface maps). This works well in theory but in practice it can be difficult to get good results. Rotating small samples (or even large samples with small domains) around a single point is very difficult and can lead to complications in interpreting the results. The electron analyser at Bloch can use electronic deflectors to measure angles in all directions, thereby eliminating the need to rotate the sample. A further advantage of using a deflector based analyser is that if the sample is not rotating, then the angle of incidence of the photons and also the polarisation geometry remains constant.

 

Here you can see that the manipulator rotation and the electronic deflection give exactly the same results. This was a very important commissioning experiment for the Bloch team.

Another useful aspect of the ARPES endstation that it has two sample preparation chambers, for high and low vapour pressure evaporations. In addition to enabling a higher throughput, this reduces the risk of contamination for experiments, which demand high levels of cleanliness.

Finally, the ARPES endstation also comes equipped with a scanning tunnelling microscope (STM) for imaging samples on lengthscales from micrometers down to nanometers (see pictures below taken with STM at Bloch). Since the electronic structure measured in ARPES experiments is strongly related to the physical atomic structure, this provides a useful complement.

The Spin-ARPES Branchline

 

Photoemission spectroscopy offers an insight into the electronic bandstructure of solid materials, but in order to have a complete picture one has to measure the energy, angle and spin of the photoelectrons. The ARPES branchline at Bloch focuses on measuring only the energies and angles, but the second Spin-ARPES branchline will be capable of measuring all three. In many materials, the electron spin plays an important role in determining the properties of the material. This is the case not just for magnetic materials, but also for many non-magnetic materials containing heavy elements where spin-orbit coupling effects are strong. As an example, the last decade has seen ‘topological insulators’ attract considerable research interest and even a Nobel prize – ARPES and in particular spin-resolved ARPES have been crucial tools in understanding these materials.

 

As discussed yesterday, to determine the electronic band structure and constant energy surfaces in a crystal, the ARPES branchline employs a hemispherical electron analyser equipped with electrostatic deflectors. The spin-ARPES system works in a similar way, but after the analyser sorts the photoelectrons by energy and angle, they exit the hemispherical analyser and are directed into a spin-detector. This detector operates on the principle that when low energy electrons scatter from a certain material (in this case a magnetized Fe(001)-p(1×1)O surface), the way that they scatter depends on their spin. By counting how many electrons scatter in a specific direction, we can infer what their spin must have been.

 

Another important feature of the spin-ARPES endstation is the possibility of rapidly exchanging the manipulator that holds the sample during measurements. Manipulator design is an exercise in compromise – it is impossible to have a single sample stage that will perform well for every conceivable experiment. Since these experiments require an ultra-high vacuum environment, the process of exchanging the manipulator is typically very slow, with turnaround times of about a week. The spin-ARPES experimental chamber has been designed to facilitate rapid manipulator exchanges, reducing this time to only a day or two. This means that the users visiting Bloch will have the freedom to choose the manipulator which best caters to their specific needs. This might means going to very high or very low temperatures, or even unusual requests such as applying strain or electric fields. The overall design of the spin-ARPES branchline also permits users to bring their own preparation chambers, further increasing the flexibility of the spin-ARPES endstation.

 

With this combination of high-performance and highly flexible endstations, the Bloch team aims to not only serve their existing user community, but also expand this community by offering novel and creative experimental possibilities.