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Due to various aspects of quantum mechanics, quantum materials have unusual and often extraordinary qualities.
While all materials display quantum mechanical qualities to some degree, 'quantum materials' offer distinctive qualities based on their atomic structure, such as extreme strength, as well as exhibiting quantum entanglement.
Although these qualities can often be described in a classical, macroscopic manner, there has been growing focus on describing material systems where quantum effects can be fully described through a non-classical framework.
A lot of quantum materials get their qualities from the restricted dimensionality of a system, specifically from confinement of electrons to two-dimensional layers. Also, quantum materials often have electrons, which can't be thought of as independent particles, interacting powerfully with each other and bringing about collective excitations referred to as quasiparticles.
The capability to fabricate quantum materials with atomic precision offers a new path for technology, in the direction of so-called quantum devices. Recent research has revealed that nanoscale fabrication techniques may be capable of mitigating or reducing typical sources of decoherence that negatively impact standard solid-state quantum systems.
Quantum materials include graphene, superconductors, Weyl semimetals, and quantum spin liquids.
Graphene
Probably the most famous quantum material, graphene is a form of carbon that is just one atom thick. Despite being so thin and being considered a two-dimensional material, graphene is extremely strong, lightweight, and highly conductive of both heat and electricity.
These qualities are mostly attributed to the honeycomb-like structure of carbon atoms that makes up graphene.
Potential applications of this “super material” are being discovered regularly. Its ability to conduct electricity extremely well is making graphene a lead candidate for next-generation computers. Its unique structure enables it to trap single gas molecules, which could make for a highly-selective filtration system. Researchers have even found that it has distinctive antibacterial properties.
Superconductors
Superconductors are materials that conduct electricity without resistance, unlike common conductors like copper, a superconductor can conduct a current without losing energy, or generating heat.
The electrons in superconductors flow effortlessly in pairs. However, electrons are stubbornly independent and cajoling these particles to create pairs often calls for extreme conditions, like very high pressure or extreme cold temperatures.
There is a form of superconductivity known to occur in iron selenide and other materials that is quite different. For reasons that cannot be fully described just yet, electrons in some superconductors form pairs at comparatively high temperatures. This action has been seen in tens of materials within the last few decades. And while the precise system behind this ability is a mystery, physicists have been able to calculate how unconventional superconductors behave in certain situations.
Weyl Semimetals
In 1937, physicist Hermann Weyl calculated the existence of a type of massless particle that could conduct an electrical charge at high speed. Although the existence of ‘Weyl fermions’ has been theorized, in keeping with equations that form the Standard Model of subatomic physics, these particles have never been physically observed.
In 2017, however, scientists detected a phenomenon that mimicked crucial facets of these theoretical particles, in a metal known as tantalum arsenide (TaAs). In their report, the researchers described how directing a circularly-polarized light on TaAs can generate an electrical current, notably without the use of external voltages. Furthermore, the team also found the current could be alternated by switching the light polarization.
The quantity of current produced in the trial was relatively large; 10 to 100 times greater than that seen in other materials. The results of this trial indicated that Weyl semimetals could be useful for highly-sensitive light detectors.
Quantum Spin Liquids
In a quantum spin liquid, the electron spins never line up, and continuously vary even at the lowest temperatures approaching absolute zero, the point at which spins normally become frozen.
Because quantum spin liquids have unremarkable ground states, they are difficult to identify by straightforward experimental means. The existence of a quantum spin liquid can be deduced based on an absence of alignment of electron spins, but conclusive confirmation is difficult.
In a 2016 study, scientists said they were able to use neutron spectroscopy to discover that the magnetic material ruthenium trichloride (a-RuCl3) exhibits properties that closely resemble the unique qualities of a quantum spin liquid.
Quantum liquid states might be used in the creation of quantum computers and other quantum devices that many expect will revolutionize technology throughout the rest of the century.
References and Further Reading:
https://seas.harvard.edu/
https://www.nature.com/articles/nphys4302
https://phys.org/news/2019-01-physicists-quantum-materials-tuned-superconductivity.html
https://news.mit.edu/2017/weyl-fermions-high-energy-particles-infrared-detectors-0530
https://www.iflscience.com/physics/new-state-matter-quantum-spin-liquids-explained/
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