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Nanophysics

​Basics

SMALL DEVICES: We investigate the properties of small 'devices' with size in the nanometer range (our 'devices' can be as large as 10 micrometer and as small as 0.1 nm). Those devices can be quite complex, but also rather simple. The most simple ones are point contacts between two pieces of metal.

A MINIATURE ELECTRON ACCELARATOR: Such a contact represents a constriction across which electrons can be accelerated, and their scattering processes observed. Spectacular results can be achieved when one or both electrodes that form the contact are either superconducting or magnetic.

MECHANICALLY-CONTROLLABLE BREAKJUNCTIONS: We have presently no means for nano-lithography, that is to pattern very small metal structures. Such type of samples have to be obtained through cooperation with other groups. To fabricate also our own samples,  we are using a technique called 'mechanically-controllable break junctions. It is extremely simple and easy to prepare those samples, and fabricate and investigate atomic-size contacts. This can be done with ones own hands and with a sharp knive or a diamond saw, depending on the material.

Specific research topics

For a number of reasons we have no chance, at present, to compete with well-established research groups in fields which involve complicated and new type of nano-structures. However, there are many other interesting and challenging phenomena that can be investigated with means available to us. In some cases such questions arise only after looking carefully into the details.

TRANSPORT REGIMES: Our measuring plan involves the investigation of material properties (like the superconducting order parameter or the electron-phonon excitation spectrum), but also basic features of those contacts which are still unsolved. In many cases, for example, it is not clear at all what the real transport regime is. This however is required to interprete the results correctly. One of our aims is to identify the systematic behaviour in the properties of those junctions to classify them.
 
References
K.Gloos, Phys. Rev. Lett. 85 (2000) 5257.
K.Flachbart et al., Phys. Rev. B 64 (2001) 085104.

HORIZON: Another aspect is the 'horizon' of a junction. We know that in daily life we can not see - at least not with conventional means - what is going on beyond the horizon. On a macroscopic scale, the horizon also plays an important role in astrophysics. The same applies on the nanoscale to electrons that are being transferred across a point contact. How far can those electrons really 'see', how far reaches their electro-magnetic interaction with the surroundings? In some cases this restricted horizon has (or should have) pronounced affects on the experimental results. Indeed, we have strong indication that our Josephson junctions have very small horizons in the micrometer range.

References
K.Gloos and F.Anders, J. Low Temp. Phys. 116 (1999) 21.
K.Gloos and F.Anders, Physica B 284 (2000) 1854.
 
TUNNELING THROUGH QUANTUM-POINT CONTACTS: The conductance G = dV/dI of those contacts shows chacteristics steps at integer multiples of e2/2h = 1/12.9 kOhm when the width of the contact channel is increased. A closed contact (= narrow channel) has a conductance of zero. However, with a sufficiently large bias voltage a small tunneling current can still be driven through such a closed contact. so far we we can understand qualitatively the behaviour in this regime, but there are still many open questions. For example, it is not clear at all how the bias voltage drops along the contact channel when there are no electrons in the contact region. The results can be described reasonably well by a classical model, but this model is definitely wrong. The same applies for the self-gating of those contacts, that is the change of the geometrical shape of the contact channel in response to the bias voltage.

References
K. Gloos et al., Phys. Rev. B 70 (2004) 235345.
K. Gloos et al., Phys. Rev. B 73 (2006) 125326.

For students

I am looking for students who would like to work on one of those subjects as part of their Bachelor or Master's degree studies. This work can be split up into a preliminary short 'literature' and a more comprehensive 'experimental' study (both parts can also be done independently). For the experiments we have at present one dilution refrigerator for measuring resistance or conductance spectra down to 0.1 K. Below are short descriptions of these projects. If you are interested and want to have more information, you can ask me any time or send an email to kgloos@utu.fi.
 
1) Tunneling Properties of Quantum Point Contacts
In the tunneling regime part of the contact area is electronically depleted. This strongly affects the so-called self-gating. It is also unkown how the bias voltage drops along the contact. The final aim is to understand these two mechanisms (it sounds trivial, but it certainly is not!). Samples with a quantum-point contact in a two-dimensional electron gas will be supplied by collegues in Copenhagen.
 
2) Josephson Junctions of Classical Superconductors
Consider a Josephson junction between two lead electrodes. Its properties should be well known, especially since the Josephson effect was discovered and explained decades ago. However, the Josephson effect is a high-frequency phenomon. To work properly it needs a capacitance, otherwise the high-frequency displacement current can not flow and the Josephson current is suppressed. How large is this capacitance? This work involves:
a) Find out how the contact resistance changes when the contact is irradiated. The available data suggest that THz radiation (1012 Hz and above) has to be used. This is a rather difficult  frequency range for accurate measurements. However, as a start and to check the working principle a simple heater (=black-body radiator) should do it.
b) The final aim of this project is to find a way to change/control this capacitance by varying the shape of the contact (for example, is there a difference between long and short contacts, or contacts made of thick and thin samples, what is the dependence on the distance between contact and ground plane).
 
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