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http://phys.org/news/2016-04-state-two-dimensional-material.html
New state of matter detected in a two-dimensional material
April 4, 2016 in Physics / Quantum Physics
New state of matter detected in a two-dimensional material
The excitation of a spin liquid on a honeycomb lattice with neutrons. Credit: Genevieve Martin, Oak Ridge National LaboratoryAn international team of researchers have found evidence of a mysterious new state of matter, first predicted 40 years ago, in a real material. This state, known as a quantum spin liquid, causes electrons – thought to be indivisible building blocks of nature – to break into pieces.
The researchers, including physicists from the University of Cambridge, measured the first signatures of these fractional particles, known as Majorana fermions, in a two-dimensional material with a structure similar to graphene. Their experimental results successfully matched with one of the main theoretical models for a quantum spin liquid, known as a Kitaev model. The results are reported in the journal Nature Materials.
Quantum spin liquids are mysterious states of matter which are thought to be hiding in certain magnetic materials, but had not been conclusively sighted in nature.
The observation of one of their most intriguing properties—electron splitting, or fractionalisation—in real materials is a breakthrough. The resulting Majorana fermions may be used as building blocks of quantum computers, which would be far faster than conventional computers and would be able to perform calculations that could not be done otherwise.
“This is a new quantum state of matter, which has been predicted but hasn’t been seen before,” said Dr Johannes Knolle of Cambridge’s Cavendish Laboratory, one of the paper’s co-authors.
In a typical magnetic material, the electrons each behave like tiny bar magnets. And when a material is cooled to a low enough temperature, the ‘magnets’ will order themselves, so that all the north magnetic poles point in the same direction, for example.
But in a material containing a spin liquid state, even if that material is cooled to absolute zero, the bar magnets would not align but form an entangled soup caused by quantum fluctuations.
“Until recently, we didn’t even know what the experimental fingerprints of a quantum spin liquid would look like,” said paper co-author Dr Dmitry Kovrizhin, also from the Theory of Condensed Matter group of the Cavendish Laboratory. “One thing we’ve done in previous work is to ask, if I were performing experiments on a possible quantum spin liquid, what would I observe?”
Knolle and Kovrizhin’s co-authors, led by the Oak Ridge National Laboratory, used neutron scattering techniques to look for experimental evidence of fractionalisation in crystals of ruthenium chloride (RuCl3). The researchers tested the magnetic properties of the RuCl3 crystals by illuminating them with neutrons, and observing the pattern of ripples that the neutrons produced on a screen.
A regular magnet would create distinct sharp spots, but it was a mystery what sort of pattern the Majorana fermions in a quantum spin liquid would make. The theoretical prediction of distinct signatures by Knolle and his collaborators in 2014 match well with what experimentalists observed on the screen, providing for the first time direct evidence of a quantum spin liquid and the fractionalisation of electrons in a two dimensional material.
“This is a new addition to a short list of known quantum states of matter,” said Knolle.
“It’s an important step for our understanding of quantum matter,” said Kovrizhin. “It’s fun to have another new quantum state that we’ve never seen before – it presents us with new possibilities to try new things.”
More information: Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet, Nature Materials, DOI: 10.1038/nmat4604
Provided by University of Cambridge
“New state of matter detected in a two-dimensional material” April 4, 2016 http://phys.org/news/2016-04-state-two-dimensional-material.html
Just as quarks form protons and neutrons, there might be something that forms electrons.
AgamemnonThis state, known as a quantum spin liquid, causes electrons – thought to be indivisible building blocks of nature – to break into pieces.
The researchers, including physicists from the University of Cambridge, measured the first signatures of these fractional particles, known as Majorana fermions, in a two-dimensional material with a structure similar to graphene…
—————-Majorana particles – Fundamentally confusing
A completely new type of particle is observed for the first time. What does that mean, fundamentally?
https://www.theguardian.com/science/life-and-physics/2014/oct/05/majorana-princeton-particles-fundamentally-confusingYou could define particle physics as the quest to discover the fundamental constituents of nature, and to understand how they interact. The so-called “Standard Model” of physics contains a list of particles which are not made of anything else, and of which everything else is made.
The question of how something can be made of nothing, and specifically how can a fundamental particle have mass, is both basic and deep. The answer – in the Standard Model – is that the mass of fundamental particles comes about because of the way they interact with Brout-Englert-Higgs field. The Higgs boson is the evidence that this works, and the matter particles which get their mass this way are called “Dirac fermions” because they are described by Paul Dirac’s 1928 equation. In the Standard Model, electrons, quarks and neutrinos are all Dirac fermions.
There has been small flurry of physics headlines over the last few days about the discovery, by physicists at Princeton, of a new kind of particle – a Majorana fermion. Proposed by the Italian physicist Ettore Majorana back in 1937 – a while after Dirac, but well before Brout, Englert and Higgs – so-called “Majorana fermions” get their mass via a unique and previously unobserved self-interaction, which is completely different from Dirac fermions, and nothing to do with Brout, Englert or Higgs*.
A consequence of the way this new mass mechanism works is that Majorana fermions must be their own anti-particle. Since particles and anti-particles have opposite electric charge, this can only work for neutral particles. That is to say, an electron has charge -1, so its antiparticle (the positron) has charge +1, and they are distinct from each other, and so cannot constitue a Majorana particle. The only possible candidate for a fundamental Majorana fermion in the Standard Model is the neutrino, since all the other fermions have charge. In fact, many speculative theories that extend the Standard Model – to explain some of the puzzles it doesn’t deal with – contain Majorana neutrinos. There are several highly sensitive experiments around the world searching for evidence, in rare nuclear decays, that neutrinos are Majorana fermions. It is a hot topic.
AdvertisementSo I was excited to read about the new particle, and somewhat diappointed when I did so to find out that it is not a fundamental Majorana fermion, still less a neutrino. A bit of a let-down for me and my particle-physics colleagues. Nevertheless, the result is interesting for a number reasons.
What has been seen is a quantum state in one-atom-thick wire which in a certain energy range behaves like a Majorana fermion. It is not a fundamental particle, it is a composite state, and the behaviour emerges from the interactions of atoms, electrons and photons, described by quantum electrodynamics, in which all fermions are Dirac. The fact that Majorana behaviour has been predicted, and then observed, to emerge as collective behaviour from a “more fundamental” (i.e. higher energy, shorter distance scale) theory is fascinating.
I guess that, given a mathematical principle such as the Majorana mass-mechanism, there are two distinct types of question you can ask. The one particle physicists tend to ask is “Does this appear as part of the basic structure of the universe – does it explain things we already see, answer problems we already have?”. However, one could equally ask “Can we construct a physical system in which this mechanism actually occurs, and if so what can we do with it and what can it teach us?”
The new discovery arises from addressing the second type of question. This kind of approach has yielded results before. For example, studies of emergent supersymmetry in low-energy optical systems continue even while fundamental supersymmetric particles remain elusive. It is also worth remembering that non-relativistic precursers of the Brout-Englert-Higgs mechanism were influential in the development of the Standard Model; and also that there are amazingly interesting (and useful!) phenomena such as superconductivity which arise in such tricky low-energy quantum systems.
AdvertisementI’ve talked about “fundamental particle” a lot in the above discussion, but it is important to be aware that this is just a working definition. The quantum-mechanical principles at play in the complex, composite systems at Princeton are “fundamental” to our understanding of physics, in that they form its foundations, even if the Majorana particle itself is not. When we say that an electron is a fundamental particle, we just mean that we have not been able to see any substructure inside one yet. Or to put it another way, no matter how hard we have tried (and we have tried very hard!) we haven’t broken an electron into pieces yet. It is still possible that we may eventually discover electrons, and even the Higgs boson, to be emergent phenomena of a more fundamental theory, just as these new Majorana fermions emerge from the Standard Model. Each time we look more closely, we have to be ready for surprises; ready to modify what we think of as “fundamental”.
* Nothing to do with this, either, although it does seem to crop up by mistake quite often in conference talks on the subject.
There is a pretty detailed account of the Princeton experiment and its new results here, although regretably the paper itself is not open access.
Jon Butterworth has written a book about being involved in the discovery of the Higgs boson, Smashing Physics, available here . Some interesting events where you might be able to hear him talk about it etc are listed here. Also, Twitter.
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