TaS2 – what changes when a strong insulator turns into a
superconductor?
Measuring the electrical properties and structural changes in TaS2 tuned from an
insulator to a superconductor.
Instructor: Amit Ribak Department of Physics
Abstract
We are used to classify different materials according to their properties. For example,
the ability to conduct electrical current helps us to distinguish between insulators and
conductors. Our lab deals with exotic materials which have unique properties, mainly their
electronic properties.
One type of such a material is a superconductor – a material which, when cooled down
below a certain temperature will become a perfect conductor with zero resistivity! Quite
surprisingly, many materials exhibit superconductivity when cooled down.
On the other range of the scale there are strong insulators. In these materials the
electrical resistivity increases significantly upon cooling down. The crystal TaS2 is a strong
insulator. When cooled down, it goes through a series of structural changes which cause an
increase in resistance.
In 2008 it was shown that when you take a pristine single crystal of TaS2 and apply
pressure on it, its resistance will decrease and it will act as a metallic conductor. Increasing
the pressure a bit more will turn it into a superconductor (see figure 1)!
In our lab we grow pristine single crystals of TaS2. By carefully controlling the crystal
growth we are able to turn the crystal into a superconductor, without any external
pressure! Even more startling is the fact that until we reach the transition temperature of
superconductivity, the crystal goes through all the structural changes like a normal
insulator.
The goal of the research is try to understand what is this new superconducting phase?
What is the mechanism which turns an insulator into a superconductor?
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Resistivity
Pressure
Fig. 1 (a) scheme of the pressure experiment. (b) The resistivity decreases sharply at
pressure larger than P1 and drops to zero at pressure larger than P2.
Student mission / Objective:
The students will get involved with the ongoing research of superconductivity in TaS2.
One challenge is to verify that the superconducting crystal still goes through all the structural
changes like the insulating one and characterize them.
In order to do so, the students will learn to perform electrical measurements as a
function of temperature. This includes preparing the crystal, connecting electrical contacts
under microscope and measure the resistance of the samples at different temperatures,
ranging from super low temperatures of -270oc up to 100oc. This will be done at the low
temperature physics laboratory at the Physics department using special equipment. The
students will learn to work in the lab and will get familiarize with the special issues
concerning low temperatures and vacuum systems.
Our lab is fully equipped with all the aids needed for the work. The data obtained will be
analyzed by students and they’ll learn how to present the data and extract meaningful
insights.
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Requirements:
The most important requirement is enthusiasm and curiosity. Solid state physics and low
temperature physics requires attention to details and involves a lot of lab work with many
different and interesting instruments.
A basic knowledge in Microsoft Excel and Word is needed in order to perform data
analysis and summarize the results.
Please read the following paper in order to get a sense of the project:
Paper name: “Squash and sandwiches” by Erio Tosatti.
Questions about the paper:
1. Think of a way to design an experiment which enables us to tell whether a sample is
a superconductor?
2. What are the different phases (different electrical conduction behavior) seen when
pressure is applied?
We will discuss the answers when we meet at the dinner in the opening ceremony.
Please fill free to contact me with questions regarding the project at: [email protected]
Recommended reading material:
1. The special structural change occurs in TaS2 is called – “charge density wave”
transition. The following link contains a short explanation about the phenomenon:
http://www.princeton.edu/~npo/SurveyTopics/CDW/ChargeDensityWave.html
2. The following website contains a lot of information about superconductors in
general. You can wonder through the site and read a little.
http://www.superconductors.org/
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