Atomic arrays paper in SciPost Physics


Have a look at our latest paper, published in SciPost Physics. Here we report on the investigation of atomic arrays of various shapes and sizes, built out of individual atomic vacancies. This way, we construct small artificial two-dimensional materials which show the beginnings of band formation.

SciPost is a recently initiated publication portal that is fully open-access. In contrast to most other journals, it features ‘peer-witnessed’ review. This means that the refereeing process is completely open and that anyone – so not just the invited referees – can contribute comments, resulting in a fair and transparent review process.

STM topography and corresponding local electron tunneling spectroscopy on various arrays built from atomic vacancies in the CuCl/Cu(100) surface. It can be observed that for denser lattices, the onset of the conduction band – originally at 3.5 eV – is shifted further downward.

Lecture at ‘Diligentia’


On April 3rd, Sander Otte gave a lecture at the Koninklijke Maatschappij voor Natuurkunde ‘Diligentia’ in the Diligentia Theatre in The Hague. This one hour lecture, in Dutch, provides an overview of the field and discusses several concepts on a level that is accessible to first-year college physics students.

A kilobyte rewritable atomic memory


Every day, modern society creates more than a billion gigabytes of new data. To store all this data, it is increasingly important that each single bit occupies as little space as possible. As reported in Nature Nanotechnology today, Floris Kalff and coworkers managed to bring this reduction to the ultimate limit: they built a memory of 1 kilobyte (8,000 bits), where each bit is represented by the position of one single chlorine atom.

With an areal storage density exceeding 500 Terabits per square inch, the memory outperforms existing state-of-the-art harddisk drives by three orders of magnitude. In theory, with this storage density all books ever written by mankind could be spelled out on the surface of a postage stamp.

Apart from being the largest atomically assembled architecture ever created, the memory also features the first demonstration of atomic-scale markers that allow the STM tip to navigate through the large array of bits. Markers indicate the start and end of each line, but can also state if a sector cannot be used for data storage due to contamination or a crystal defect. Such protocols are crucial for scaling-up technology beyond a few hundred bits.

The movie below demonstrates the mechanism of the atomic storage memory.


Check here for an overview of press coverage of ‘the kilobyte’.

Quantum criticality paper in Nature Physics


Phase transitions, such as the transition between a solid and a liquid, play an important role in condensed matter physics. Since the number of participating particles is huge, it is practically impossible to predict the exact behaviour of a material near a phase transition: this is what makes them so intriguing.

An extra fascinating class of phase transitions consists of those transitions in which not thermal fluctuations, but quantum fluctuations drive the change in the material. These quantum phase transitions are maintained even at zero temperature. In general it is very difficult to probe a quantum phase transition experimentally, as in practice it is often overshadowed by more mundane effects.

Ranko Toskovic and coworkers now report in Nature Physics that they have succeeded in designing and building tiny ‘materials’ consisting of only a few atoms, exactly in such a way that they display the beginnings of quantum criticality. The materials consist of magnetic atoms that prefer to align in an alternating fashion. Only when a magnetic field of 6 Tesla is applied, they collectively surrender and point in the same direction. This transition is not sudden, but consists of a number of discrete quantum jumps, which the researchers could observe in detail. Together, these jumps constitute the start of a quantum phase transition.

Scanning tunnelling spectroscopy measurements taken on each atom of chains with lengths ranging from one (left) to six atoms (right). At predicted magnetic field strengths (red dashed lines) the spectroscopic features show sudden jumps. These jumps get closer as the critical field value of 6 Tesla is reached, after which the transition to the paramagnetic phase is complete.

ERC Starting Grant for Sander Otte


Sander Otte has been awarded an ERC Starting Grant for his research proposal Spin correlations by atomic design (SPINCAD). In this project, Otte will investigate collective dynamics in artificially designed atomic spin structures. The excitations of such spin lattices, such as spinons or magnons, can be viewed as quantum-mechanical quasiparticles that propagate at tremendous speeds. The implementation of atomically crafted detectors may help to visualise the motion of these elementary particles with atomic local precision.

The ERC Grant represents a sum of 1.45 million euro’s, to be spent in the coming five years. A large part of this budget will be used to employ PhD students and postdoctoral researchers. Employment opportunities will be announced soon on our openings page.

Two-impurity Kondo paper in Nature Communications


The Kondo effect – an intricate quantum phenomenon involving the spins of many electrons surrounding a magnetic atom – is already quite intriguing by itself. But an even higher level of complexity is reached when two coupled atoms are together Kondo-screened. Depending on the competition between the exchange interaction and the screening strength, combined with an external magnetic field, a variety of different correlated phases can be realised. So far, some of these phases existed only on paper.

Today, Anna Spinelli and coworkers show in Nature Communications that the complete phase diagram of the two-impurity Kondo problem in a magnetic field can be covered experimentally. In order to achieve this, a range of different pairs of Co atoms was designed and assembled through atom manipulation. This work forms the basis for advanced experiments on e.g. Kondo chains and Kondo lattices.

Phase diagram of the two-impurity Kondo problem. Horizontal axis: exchange interaction ranging from
ferromagnetic (left) to antiferromagnetic (right). Vertical axis: applied magnetic field. The two-impurity Kondo screening phase was not reached experimentally before.

SPICE workshop talk on YouTube


Have a look at this talk by Sander Otte, presented at the SPICE workshop Magnetic adatoms as building blocks for quantum magnetism, Mainz (Germany), August 19 2015. It gives an excellent overview of recent research performed in our group. Other talks from this great workshop can be found at the workshop YouTube channel.

Controlled closure of cotunneling paths


When electrons tunnel through an atom, they may lose energy in the process. Such inelastic cotunneling events render the atom in an excited state, either with a flipped spin or with an entirely different orbital filling. In our recent paper in Nano Letters by Ben Bryant et al., we report the observation of both types of cotunneling events. In addition, we demonstrate how both inelastic cotunneling processes may be switched off entirely through a controlled modification of the immediate environment of the atom. This work provides starting points for research on engineering the slowdown of decoherence in atomic spin systems.

Tunneling spectroscopy measurements performed on individual cobalt atoms assembled into a chain. Inelastic cotunneling events are observed as steps in the spectroscopy. For atoms in the inside of the chain, complete suppression of these steps is observed.