Superconducters

Continued by John Blount (Fall 2024), previous authors are listed below:

Ian C. Hamlett Amethyst Massaquoi, Ian Sabastian, Savannah Lee

What are Superconductors

Superconductors are special materials that can conduct electricity with no potential energy loss, as they exhibit no electrical resistance when cooled below a certain temperature. This phenomenon, known as superconductivity, comes from unique formations called "Cooper Pairs," which cause electrons to pair up and move in sync with the material's natural lattice vibrations.

Superconductors have a wide range of practical applications such as creating magnetic fields for Magnetic Resonance Imaging (MRI) machines, particle accelerators, and even long-range power transmission. As research continues into the field of superconductors, scientists are driving innovations in industrial and scientific fields to improve our daily lives.

Introduction to Resistance

Electric resistance is a property of all metals and conductors except superconductors. It is the reason an electrical device heats up after prolonged use and why they deteriorate over prolonged use. This heat is caused by the resistance in the wires as the electrical current flows through them. Broadly speaking, resistance measures how difficult it is for electrons to pass through a conductor. Resistance dissipates energy as heat, preventing a system from perfect energy efficiency.

Electrical resistance can be thought of as similar to water flowing through a pipe and encountering obstacles. Just as larger pipes allow water to flow more easily, a thicker wire can carry more current. However, eliminating electrical resistance is for more complex. In a conductor, electrons flow through a lattice of metal ions on their way to their "endpoint." The ease in which these electrons flow is determined by the conductor's resistance, which depends on how well the ions "hold on" to the electrons. Different metallic lattice structures interact with electrons in unique ways, causing variations in the resistance of various materials. In nature, electrons lose energy due to collisions with these metal ions, however, in a superconductor, these collisions are completely eliminated. In a lab setting, this phenomenon typically occurs around temperatures close to absolute zero.

Check out this interactive link to understand how temperature can change the resistance of a copper wire.

https://trinket.io/embed/glowscript/db69409a3f79?outputOnly=true&start=result"

Resistance and Quasiparticles

To understand Quasiparticles, consider a lattice of Silicon atoms. Silicon has 4 valence electrons in its outermost shell, which allows it to form covalent with other silicon atoms, creating a stable, silicon lattice. In this lattice, electrons are tightly bound and typically remain in place. Now, imagine a photon strikes the lattice with sufficient energy and knocks an electron free. This creates a "hole" where this electron used to be. As electrons move to fill this "hole" in the lattice, the "hole" would appear to move like a bubble through the lattice structure. This "movement" would be referred to as a Quasi-electron.

In some cases, atoms within a material's lattice vibrate. For example, when a soundwave passes through an object, it causes the atoms to oscillate in a predictable manner. These oscillations can also be described as Quasiparticles referred to as Phonons. Phonons behave similarly to photons as they both travel at the speed of the force they materialize (i.e. Photons travel at the speed of light, via electromagnetic force). Another way of viewing phonons is as a representation of heat. The random motions of atoms within a material, which we perceive as "heat", are also just randomized oscillations of atoms through a material. Phonons are closely related to thermal properties of a material, as they manifest the transfer of heat through vibrational energy.

Photo credit: https://www2.physics.ox.ac.uk/sites/default/files/page/2020/07/13/karenonowskauniq2-2020-47125.pdf

Metals create lattices similar to silicon, however metals do not use all their valence electrons for bonding. This leaves room for electrons to move freely through the lattice, allowing metals to act as conductors. However, the naturally occurring random vibrations of atoms in these lattices, also known as phonons, disrupt how freely these electrons can flow, resulting in resistance. Interestingly, phonons do not always act as obstacles. under extremely cold conditions, they facilitate a quasi-force which can bind electrons together, creating what is known as a "Cooper pair". Cooper pairs all move collectively and occupy the lowest energy quantum state. Because they do not have sufficient energy to excite new protons, they do not experience scattering as in typical conductors. This mechanism allows Cooper pairs to stream through the lattice with zero resistance, also known as superconductivity. Through this phenomenon, electrons achieve a perfectly efficient energy flow, unimpeded by resistance.

About Superconductors

When you lower the temperature of a metal, its resistance will decrease. You could demonstrate this by taking a basic circuit and freezing it- because of ohm's law, the bulb would start to glow brighter since there is a lot more current flowing through until it heated up and the effect was nullified. (V=IR, lowering R will raise I which will raise brightness). For most materials, taking them to absolute zero (or really close) will cause the resistance to decrease to almost zero, or at least improve from where it was, but not quite zero. However, some materials, superconductors, lose all resistance to current. The difference between "almost zero" and "actually zero" is enough to give rise to some cool properties of superconductors. What is happening on the molecular level is that the atoms are not vibrating quickly enough to be attracted to electrons any more, and attractions are minimized to the point where the electrons can flow through with no resistance.

Imagine running through a tunnel where everyone wants to high-five you. You are the electron and the people trying to high-five you are atoms in the wire. If it's too cold, nobody will want to high-five you and you will run faster.

Superconductors can only maintain superconductivity within a certain range beneath their critical range. In the case of electric superconductors, this range is determined by temperature and critical. In the case of magnetic superconductors, this range is determined by a surrounding magnetic field. If it is warmer than a specified value or depending on the range of a magnetic field, superconductors behave with increased or decreased potential.

Critical Temperature- often labeled as Tc, or critical temperature Is the temperature at which the superconductor needs to be beneath in order for it to exhibit "superconductive" behaviors. The usefulness or potential of a superconductors are dependent on the material properties and chemical makeup and their critical ranges

To see a table of these, click here. 

Critical Magnetic Field- often labeled as Hc. A superconductor won't exhibit any of its properties if a magnetic field is greater than a certain value or range determined by the critical magnetic field.

Meisner effect- A principle that is unique to superconductors, this is the ability to cancel out all external magnetic fields on the inside of the superconductor below the critical magnetic field. A superconductor can create small currents on its surface below a critical magnetic field to eliminate an external electric field and make its net electric field zero.

You could never be asked to do a hall effect problem with a superconductor!

Types of Superconductors

There are two main types of superconductors, Type I and Type II. Their classification is based on how they break down once their critical magnetic field is reached. Under the critical magnetic field and critical temperature, they all behave similarly.

Type I superconductors describe the increase in magnetization in response to magnetic flux. This increase in magnetization is linear until a critical field is reached. At this point, the magnetization level of a superconductor immediately returns to zero and the properties of said conductor will maintain the magnetization of a normal conductor.

Type II superconductors also exhibit a linear increase in magnetization in response to magnetic flux until a critical point is reached. However, Type 2 conductors have two critical points and the decrease in magnetization is instead a gradual decrease until magnetization is returned to that of a normal conductor. The first point is reached once the superconductor reaches a Vortex State. Vortex State superconductors exist as the vortices of electrons at its surface sink inwards creating thread-like streaks in which magnetic fields are able to permeate through the material. This causes the material to be stuck in one fixed orientation while maintaining remained in levitation.

Types of Superconductors by Amethyst Massaquoi

Known Super Conductors

There are hundreds of materials that are known to become superconducting at low temperatures. Most known superconductors are alloys or compounds. However, Twenty-seven are chemical elements typically found in their usual crystallographic forms at low temperatures and low atmospheric pressure, all of which are metals. The most common of these include lead, tin, aluminum, and mercury. Less commonly used are metals such as lanthanum, rhenium, and protactinium.

Eleven chemical elements become superconducting at low temperatures and high pressures Among these are silicon, cerium, selenium, and uranium.

Six elements, including bismuth, become superconducting in a highly disordered form which are stable at extremely low temperatures. However, none of the magnetic elements such as chromium, manganese, iron, cobalt, or nickel can exhibit superconductivity.

It is possible for a superconductive compound to be comprised of non-superconductive constituents. Two such examples are disilver fluoride (Ag2F) and a compound of potassium and carbon (C8K).

History of Superconductors

1911: Superconductors were first discovered by Heike Kamerlingh Onnes, a Dutch Physicist. He experimented with mercury, a type 1 superconductor, cooling it to below 4.2 kelvin. This is about the temperature of deep space. All of the resistance in the mercury had vanished establishing the first occurrence of a superconductor. Onnes would also discover superfluidity while cooling helium on the way to making it a superconductor.

1933: Robert Ochsenfeld and Walther Meissner first discovered superconductors can float above magnets. This would be called the Meissner-Ochsenfeld Effect.

1935: Type II superconductors were first discovered by Leb Shubnikov.

1950: Lev Lendau and Vitaly Ginzburg were the first to theorize about why type II superconductors existed. Their equation establishes the Ginzburg-Landau paramter, κ = λ/ξ, which describes type I and II superconductors. If 0 < k < 1/√2, the material is considered a type I superconductor. If k > 1/√2, the material is a type II superconductor.

1957: The basic theory of superconductivity, known as BCS theory of superconductivity, was published by John Bardeen, Leon Cooper, and John Schrieffer. They would later go on to win a Nobel Prize for this discovery.

1986: Karl Muller and Johannes Bednorz realized that superconductors didn't have to be at absolute zero, and found a way to create one that operated at 40 degrees kelvin.

2015: We achieved the greatest record of superconductor temperature at 203 degrees kelvin, but under high pressure. We've been using pressure to cheat the temperature requirements for a while. This was the work of A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, and S. I. Shylin.

2023: A team led by Lee Sukbae at Korea University identified a grey-black, polycrystalline compound known as LK-99. Being a copper-doped lead-oxyapatite, much controversy surrounds the material as to whether it is superconducting. Regardless of its status, it has further enhanced the search for room-temperature superconductors.

Applications of Superconductors

Large Hadron Supercollider- The Large Hadron Supercollider runs between France and Switzerland, and is used to experiment with fundamental particles It uses superconductors to accelerate particles to super high speeds so that they can be observed. For more information about the LHC, click here.

Photo curtesy of http://www.forbes.com/sites/bridaineparnell/2015/03/25/short-circuit-stalls-large-hadron-colliders-restart/

Futuristic Technology like Hover boards- through a phenomenon called Quantum Levitation, superconductors can be used to create things that levitate- the superconductor will float easily above a magnet. This could be potentially used for all sorts of levitation devices like cars, hoverboards, or stuff we can't even imagine yet. To read more about Quantum Levitation, click here.

Photo courtesy of http://www.blastr.com/2015-5-22/watch-guy-break-world-record-longest-hoverboard-flight-ever

Cool cell phones- current never disappears in a superconducting loop of metal, so technology with superconducting metal would never run out of power, but continue to be charged forever. This could cut down on electricity usage worldwide, allowing for some serious energy savings.

medicine- superconductors have used in MRI machines and NMR machines, both of which can be used in modern medicine to help diagnose various medical conditions. Superconducting magnets are used to form a strong magnetic field around the person, switching on and off to create the thumping sound of the machine. The machines do require lots of liquid helium to keep temperatures well below the Tc for a given metal in use.

Other Applications

Radiation detection, Power generation, mine hunting, Electromagnetic Radiation radar,

Scintillators

Scintillators are a type of superconductor with the capacity to turn waves of radiation into photons of visible light. Scintillators can be found in nature or produced in labs as transparent crystals. The most common form of scintillator produced by mechanical involvement is constructed with the use of Thallium-doped Sodium Iodide. Thallium-doped Sodium Iodide glows, or visible emits light when exposed to gamma radiation.

The most common use of inorganic scintillators is for detecting radiation in medical diagnostics.

Inorganic scintillators are found in spectrometers which are used for analyzing geophysical changes after large amounts of radiation is released into populated environments or in environments that maintain higher levels of gamma radiation exposure.

Scintillator gif. by Amethyst Massaquoi

Super Conductor Optimization Research: Inorganic Scintillators

Scintillators can be found in nature or manufactured and can be enhanced through the use of photocathodes. A significant portion of light emitted from an inorganic scintillator is lost due to total internal reflection that is a result of the large difference in refractive index between the scintillator and the photocathode.

Research and optimization of inorganic scintillators are done by enhancing the ability of a photocathode to collect light particles using photonic crystal structures. This can be done by coating scintillators with materials that maintain a high refractive index.

Photocathode with Inorganic Scintillator by Amethyst Massaquoi

Further Reading

Superconductors Type 1

Superconductors Type 2

Futuristic Technology and Superconductors

List of corresponding Superconductors and their Tc and Hc constants

Why this matters to me (Samantha Lee)

I'm a chemical engineer and love the idea of coming up with solutions that will help out our future when it comes to life on earth. Superconductors could be the solutions to lots of problems having to deal with energy, which really excites me. I also love chemistry and know a lot about superconductors from a chemistry perspective, but wanted to add some physics knowledge to my collection.

Why this matters to me (Amethyst Massaquoi)

I'm in NROTC and I plan on becoming a submariner after graduation. In partnership with the U.S Navy and the Defense Threat Detection Agency, I interned at PENN State's Applied Research Lab in the Summer of 2022 and worked to enhance inorganic scintillators over a ten-week period. I had a great experience and gained a solid understanding of the importance of radiation detection for military applications, so I thought my limited background in superconductor technology would be a nice addition to this page.