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Observation of new phenomenon in materials physics

A young Danish scientist succeeded in creating nothing less than a revolution in materials physics. Together with a group of American scientists he created a new method to look deep within the world of atoms, and with this new technique they observed an exotic particle that will have wide-reaching implications for finding new exciting material properties. The discovery has been published in the prestigious journal Nature Physics.

2018.01.24 | Rasmus Rørbæk

By placing layers of two dimensional materials on top of each other it was possible for a Danish lead research group to leap forward towards the development of new materials that might lead to the technology of the future. (ill: Søren Ulstrup et al)

Søren Ulstrup participated in the development of a new method that can see deep within the atomic structure of materials and has observed an elusive phenomenon – a trionic particle. The young physicist will now start building a new experiment at the particle accelerator ASTRID2 at Aarhus University, which will allow him to understand the deeper implications of trions for understanding the electronic structure of matter. (foto: Rasmus Rørbæk)

How the method works: Intense light from the Advanced Light Source is radiated on the sample from the bottom right side of this illustration. The light propagates from a slit and saturates the focusing optical element that works like a filter that only permits the ultra focused light to reach the sample. This focusing scheme sets this particular method apart from how such experiments were done before, as it allows for much better focus. Once the light reaches the sample the electrons are knocked out and measured by the spectrometer. (ill: Søren Ulstrup)

The new particle: The left image shows the “layer-cake” with h-BN (boron nitride) as the violet base of the cake and WS2 (tungsten disulfide) as the green frosting on the cake. The image behind the cake shows the actual effect where the colors relate to the energy spectrum of the electrons in the material as measured by microARPES. The series of images to the right shows the effect of electron doping on the so-called valence and conduction bands of the semiconductor. The arrows illustrate the appearance of the trion, which has been sketched in the last image on the right. (ill: Søren Ulstrup)

Our perception of the World surrounding us is highly dependent on our ability to sense our surroundings in one way or another, such as feeling the wind blow, seeing the turning of the road or smelling the freshly brewed coffee.

The same idea carries over to scientists that work with exotic ways of sensing. They search for the building blocks that surround us and define our World. For them it is not enough to feel whether the wind is cold or hot. They must find new ways of discovering the things that can be measured but are invisible to the naked eye.

The work is highly complex, but the reward is sizeable for those who succeed: New phenomena have been found pretty much every time a new method was developed to provide a higher level of detail

It is here that we meet a young physicist who recently returned home from a postdoc position in USA at the “maestro” facility at the Advanced Light Source in Berkeley. His research at “maestro” focused on finding new properties in materials that might replace those we use in current technology. Søren Ulstrup, the researcher in question, has returned home with a new way of probing exotic materials - and he also took part in a project that lead to the discovery of a new and surprising phenomenon in the world of atoms.

“The foundation in almost any physicist’s work is to look further out, deeper within and with ever higher resolution. We succeeded in doing just that at “maestro” where I was responsible for carrying out this work at the Advanced Light Source synchrotron in Berkeley, USA during my stay. This discovery has made it possible for us to explore some pretty surprising phenomena associated with the electrons in materials that we now need to investigate,” says Søren Ulstrup that now works as an assistant professor at the Department of Physics and Astronomy at Aarhus University.


Renewed focus

The method used for the research is called “ARPES” which spells out as angle-resolved photoemission spectroscopy. Without going into too much detail the method is based on radiating intense light on the material in question. This process knocks an electron out of the material, which is subsequently captured in a spectrometer that measures the energy spectrum of the electron. Think of it as picking a very small sample loose from a plate that is subsequently moved under a microscope that shows the constituents of the sample like a spectrum.

The new approach to ARPES is called microARPES. Put bluntly, this makes the old method look like the punch of a sledgehammer put up against the precision of a set of tweezers. It is possible to control the focus of the light radiated on the sample with incredible precision, enabling an improvement in focus from the millimeter scale to micrometers and further down to the current record which is 120 nanometer. This is an increase in resolution that could be compared with being able to just see the backyard to being able to focus on a single straw of grass and see how it behaves compared to the rest of the yard.

“This new method allows us to track the behavior of electrons very precisely and with a new level of detail. It also makes it possible to discover electronic structures in new samples and to test new possibilities for tuning the physics of electrons”, Søren Ulstrup explains.

Meet Trion: the new particle
The new microARPES technique has already lead to new surprising discoveries in the realm of materials physics. The same group has published a paper that not only describes the application of the method to a new material but also the observation of an elusive particle known as a “Trion”.

The so-called van der Waals approach was used to designing the materials. Here, two-dimensional materials are stacked on top of each other like a layer-cake. This is not an easy task as every layer of the cake consists of atomically thin materials. The method was developed from work with the all-carbon material graphene, but in this case it was possible to stack the insulating material “boron nitride” together with the semiconductor “tungsten disulfide” or WS2. This combination makes it possible to carry our experiments with WS2, which is a hot new type of semiconductor in basic technology research.

Søren Ulstrup and the group have used microARPES to determine the effect of filling this “layer-cake” with electrons – a process called doping – which leads to new processes in the conducting properties of WS2.

“In the experiment, we created a new exotic particle that consists of an electron bound to two holes in the material. The holes emerge when we radiate the intense light on the sample and as a consequence of the doping. This trinity consisting of an electron-hole-hole complex is called a “trion”. We directly observe using microARPES that this trion has an enormous influence on the properties of the semiconductor. This is extremely interesting for future research as these materials are highly promising for “spintronics” where information is stored in the spin of the electrons. This is one of the largest research topics in the field so it is really exciting that we found this effect,” says Søren Ulstrup

A trion has not been observed in this way before. The discovery came as a great surprise for the group. It was not expected that the material would develop into a state with such large gaps between the energy levels of the electrons. This particular aspect is really unique:

“We observe how the constituent particles of the trion interact. This realization is very interesting as it may open new ways of controlling the properties of future electronics, for example. In principle, it is possible to control these particles in similar ways as in a computer chip – here by using electron doping or not.

It all sounds good, but you could almost say that we are entering a new era of materials research and discovery with these levels of insight. Being able to probe the world of atoms with such fine detail is a very new concept, and now we can see phenomena like trions unfold,” Søren Ulstrup explains.

ASTRID2 takes over
Søren Ulstrup is now “home” at Aarhus University where he previously worked on the wonders of new materials. He is now an assistant professor with the goal of building a brand new experiment on ASTRID2 – the particle accelerator of Aarhus University. The microARPES technique will here be developed further, and this will permit new studies of the significance of trions for the understanding of electronic structure of complex materials.

The ambitions are to unlock the properties of these amazing materials and also to create entirely new materials with new forms of interactions between the electronic states.

The postdoc position at “maestro” (Microscopic And Electronic STRucture Observatory) at the Advanced Light Source in Berkeley was funded by the Danish Council for Independent Research under the Sapere Aude program.

The work at ASTRID2 takes place as a part of the Young Investigator Programme of VILLUM FONDEN, where Søren Ulstrup received 10 mio. DKK in 2017 to support the project “Electronic structure up-close with photoemission at the nanoscale”.

A part of the work will take place under the research center iMAT, which was founded in 2017.

Søren Ulstrup,
Department of Physics and Astronomy,
Aarhus Universitet,
Mobile: 2292 7702,
Email: ulstrup@phys.au.dk

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