A tunable electronic beam splitter realized with crossed graphene nanoribbons
My colleague Pedro Brandimarte performed such a thorough study of these ribbons - finite bias, different width, moving them up and down and side-to-side. Finally, we realized that the crossed nanoribbons could serve a role in manipulating electron waves coherently.
My role in this process was one of coaching and of helping tease out the story from this wealth of data.
ASE interface to SIESTA
The CalcTroll interface
Since a scientist by definition has to do new things they will never have standard applications for everything they need. As a result every materials scientist ends up writing a lot of one-off scripts to set up the calculations they need - and so did I.
After many years of doing increasingly complex scripting, and with my programming experience, my scripting eventually grew into a program, CalcTroll. I am still negotiating with the Spanish Research Council to make CalcTroll freely available.
Search for a Metallic Dangling-Bond Wire on n-Doped H-Passivated Semiconductor Surfaces
We were trying to investigate the electronic properties of molecules on the Si(001):H surface -- but no matter how hard we tried we could not find a good metallic electrode of dangling bonds. Every electrode we found seemed to fail in some way and the study grew and grew in size. Finally, this little initial step of finding a metallic electrode had grown into this article.
The Butterfly - A Well-Defined Constant-Current Topography Pattern On Si(001):H and Ge(001):H Resulting From Current-Induced Defect Fluctuation
My collaborators from Krakow are the best at producing highly detailed as well as highly mysterious STM images. When calculations and low-voltage measurements agree that the dangling-bond structure is asymmetric then why do the images appear symmetric? And if the reason for the symmetry is rapid switching then why is the image so precisely defined rather than just a blur?
Well, it turns out we had to consider how the balance between two stable asymmetric configurations was affected by the STM current. And we had to develop a simple yet versatile model for imaging bistable systems.
Diels–Alder attachment of a planar organic molecule to a dangling bond dimer on a hydrogenated semiconductor surface
This system shows a fascinating behaviour - they attach to surface defects, but can easily be detached by a voltage pulse. Since the defects can be created with atomic precision, this process really gives a lot of control over every atom in the system.
This work had its origin when I was asked by my Polish collaborators to do a follow-up study to a previous work. However, we soon realized something was wrong - that the earlier conclusions about this system didn't hold. My collaborators had to go back on earlier published results and to their credit they never once hesitated. We invited everyone from the original article and produced a staggering amount of calculations to ensure that this time we would give the definitive answer.
Interaction of a conjugated polyaromatic molecule with a single dangling bond quantum dot on a hydrogenated semiconductor
These molecules have received a scrutinyworthy of Al Capone's tax return. My Polish collaborators have investigated them, pushed them around in all kinds of configurations and thrown every available instrument after them. And on the theory side we needed to up our game too, and two theory groups were needed in collaboration. My contribution was to reasonably fast screen all the possible configurations the molecule could take - a lot. And the STM modelling done by Hiroyo Kawai was no laughing matter either.
And why do these molecules deserve this? Well, we have shown that the interaction with dangling bonds is neither too weak nor too strong. It lies somewhere in the Goldilocks regime where the specifics of the situation determines the outcome. And this is the regime where designed functionality and thereby technology lives...
Tunneling spectroscopy of close-spaced dangling-bond pairs in Si(001):H
My long-time collaborators, the Krakow STM group, presented us with some very puzzling results indeed. They had created pairs of atomically defined dangling-bond defects in a completely symmetric pattern, but the spectroscopy consistently showed a slight asymmetry between equivalent sites.
Now a huge asymmetry, that would be easy enough to understand, it could be spontaneous symmetry-breaking transferring an electron from one site to the other, but how could there be a slight asymmetry!
Well, after much head-scratching we came to the conclusion that the charge state of the site under the tip is the same during measurement, but that the charge state of the other site can vary, giving an asymmetry of exactly the right size.
GUI tools for QuantumWise A/S
When I worked as a software developer, everything was a joint effort, but this tutorial shows off several features where I was the dominant force.
Particularly the "Surface Builder", a utility to create a surface of any crystal along any direction. The user can choose an outer atom of the surface, surface cell and number of layers in an intuitive fashion.
I was also the main developer of the "Device to Bulk" tool, a utility that guesses a configuration with open boundaries based on periodicities found in a periodic configuration.
Atomic-scale model for the contact resistance of the nickel-graphene interface
During my time at QuantumWise A/S a client asked us to investigate the contact resistance between graphene and nickel and in the end they decided to let us publish the results.
We found that the coupling between graphene and nickel so strong that the bottle-neck for transmission is the transition from free graphene to graphene on nickel.
In an interesting role-reversal, my boss did most of the actual work! I simply handed over data showing 'something interesting' and he did production runs, figures and wrote this article. Let it not be said that CEOs never get their hands dirty.
Localized edge vibrations and edge reconstruction y joule heating in graphene nanostructures
My fellow phd. student Joachim Fürst was investigating a very interesting experiment - it turns out that the edges of graphene flakes change composition when a current runs along them. Armchair edges are edged away and zigzag edges become predominant. Since vibrations - my speciality at the time - played a dominant role - I ended up leading the project.
The armchair edges display an important property - the highest vibrational frequencies of the edge is higher than those of their zigzag counterparts and bulk graphene. That means that there is nothing to cool down these vibrations if they are heated by electron current....and they are destroyed!
Atomistic theory for the damping of vibrational modes in monoatomic gold chains
This was the work that motivated my phd in vibrational transport in the first place - what we wrote in the grant proposal:)
Mono-atomic gold chains were some of the first atomic size conductors ever investigated. This was because they could be fabricated consistently again and again. This kind of situation of course attracts theorists like flies and a lot of work had been done on this and the inelastic signals were well-described by assuming a localized mode in the chain. Only one problem - why on earth should the coupling modes in the chain be weakly coupled?
After two years working on a vibrational transport code, we could answer the question with "It´s complicated".
Delta self-consistent field method to obtain potential energy surfaces of excited molecules on surfaces
This method was the subject of my master thesis and was motivated by an interesting new device, a hot-electron emitter. We set out to investigate if this device, created by the CINF center, could be used for catalysis. Particularly, we thought that the highly excited electrons created at the surface of the device might be able to break the bonds in the nitrogen molecule - the holy grail of catalysis.
So we set out to model this, and in the process we created a model of excited states. Unfortunately, our conclusion was that this device would never break a nitrogen molecule. But then Jeppe took up the mantle, changed focus to the method itself and on new molecules and finally made this nice article.
Model of vibrational self-energies
Some unpublished work from my phd thesis(section 5.3).
In this part I created a model self-energy from a massive amount of calculations. Not only does the model obey momentum-conservation - it even uses it to minimize the number of fitting parameters. Every time I see my old supervisor he always mentions that we missed a really nice opportunity in not finding the time to publish this. And sadly, I have to agree.
Vibrational conductance channels
This is a snippet from my phd thesis(section 5.2). It provides a tool for analyzing vibrational conductance. Just a thought I stumbled over during my work.
In electron transport people are always using transmission eigenchannels as an analysis tool, but this is less useful for vibrational transport. The reason is that the energy scale of vibrations is much smaller than that of electrons and often comparable to the thermal energy.
So, what can you do? Well, the important thing is figuring out what we want. Really we are not interested in the modes that transmit at a specific energy, we are interested in the modes that carry the current. For electrons at low bias this just happens to be the same modes. Not so for vibrations and I argue we should calculate conduction, not transmission, eigenchannels.
Calcium mobilization stimulates Dictyostelium discoideum shear-flow-induced cell motility
This is the work of my bachelor thesis, as a little physicist. At the time I wanted to become a biophysicist and worked as an Erasmus student in beautiful Grenoble. The new university system did not sit well with me at all, so I ended up spending more time learning French and skiing. But this cool little project, which I worked on with my fellow Dane, Christian Hansen, was an exception.
Our supervisor, Franz Bruckert, had a lot of movies of cell movement. Looking at these movies by eye you could clearly see distinct resting and movement phases of the cell. Our project was to write a program to quantify the movement of the cells. So that is what we did and eventually the guys who knew more about biology published this paper.
Measurements of scattered light on a microchip flow cytometer with integrated polymer based optical elements
This was my first real project as a very new undergraduate physics student. And it is my only real experimental work.
My supervisor posed me and a fellow student, Troels Gotsaed Hansen, an simple task: All we had to do was design, fabricate, package and characterize a lab-on-a-chip cell counter in 6 months with no experience whatsoever!
I don´t know how we did it, but we did it. Our supervisor had a prototype to continue work on - and he eventually published this paper. My fondest memory of this work was the eerie ambiance of the pitch dark room, lit only by stray beams of the turquoise laser.