\u201cIt\u2019s a new phase of matter, similar to how water can exist as liquid, ice or vapor,\u201d says corresponding author Luis A. Jauregui, professor of physics & astronomy at the University of California, Irvine in the US.<\/p>\n
Professor Luis Jauregui of the UC Irvine Department of Physics & Astronomy described how the new material he and his lab developed only exists in their labs. Credit: Steve Zylius \/ UC Irvine.<\/p>\n
Experiments done on a unique material, hafnium pentatelluride (HfTe5), showed that it can behave like an excitonic insulator.<\/p>\n
Insulators are materials that prevent the conduction of electricity. They do this because there is \u201cband gap\u201d separating electrons in the atom\u2019s filled outer layer from energy levels where conduction can take place.<\/p>\n
Excitonic insulators have a gap because of the emergence of excitons. Excitons<\/a> are not \u201creal\u201d particles in the sense of an electron. They are quasiparticles \u2013 objects which behave like particles \u2013 formed by a bonded pair of electrons and \u201choles\u201d.<\/p>\n Holes, too, are quasiparticles. They are the absence of an electron \u2013 an absence which can act like a positively-charged opposite of the electron in an atom or material.<\/p>\n The formation of excitons in certain materials, under extreme conditions like very low temperatures and very high magnetic fields, has been shown to form excitonic insulators.<\/p>\n Previous excitonic insulator evidence in other materials has been limited to theoretical work, 2D materials and unconfirmed tests. This is largely because the excitons have been made of electron-hole pairs where the electron and hole have the same spin direction leading to a spin-singlet exciton.<\/p>\n The new HfTe5 excitonic insulator has electrons and holes with opposite spin leading to a spin-triplet exciton.<\/p>\n \u201cUnlike conventional singlet excitonic insulators, the triplet state preserves translational symmetry and is potentially more robust under extreme conditions. This discovery paves the way for exploring emergent spin transport phenomena,\u201d the authors of the study write.<\/p>\n \u201cIt\u2019s its own new thing,\u201d says Jauregui. \u201cIf we could hold it in our hands, it would glow a bright, high-frequency light.\u201d<\/p>\n Jauregui and colleagues created the new quantum phase of matter by applying a magnetic field of about 70 Teslas in strength. A strong fridge magnet has a magnetic field of about 0.1T and the magnetic field of the Earth is just 25 to 65 millionths of a Tesla.<\/p>\n Another advantage of the HfTe5 excitonic insulator is that, while it requires intense magnetic fields, these are nothing compared to the ultrahigh magnetic fields up to 600T<\/a> required for some spin-singlet excitonic insulators.<\/p>\n Jauregui says that after the magnetic field was applied, the \u201cmaterial\u2019s ability to carry electricity suddenly drops, showing that it has transformed into this exotic state\u201d.<\/p>\n
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