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Category Archives: quasiparticles

Creating Higgs-like excitations using ultracold atoms

Visualization of an optical lattice. When the "pockets" are deep, the system is an insulator: the atoms can't move from site to site. When the pockets are shallow, the atoms can move freely around, creating a superfluid.
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Systems of cold atoms can sometimes give rise to behavior surprisingly like free particles moving close to the speed of light. However, unlike the kind of physics you see in experiments such as the Large Hadron Collider (LHC), the "particles" are actually collective phenomena, arising out of strong interactions among the components of the system. By manipulating the properties of the material, researchers can produce behavior analogous to many interesting systems in high energy physics—only at very low temperatures and with a "speed of light" dictated by the material's characteristics.

A new experiment by Manuel Endres and colleagues has achieved a Higgs-like excitation in a system composed of ultracold rubidium atoms. By pushing the atoms to a quantum critical point, where they change from an insulator to a superfluid, they were able to generate a transition that was analogous to the break in symmetry that gives rise to the Higgs field.

One of the cornerstones of quantum field theory is that each particle's properties depends on its interactions. This is true whether the particle is on its own, in an atom, or part of a larger material. The Higgs field is just one of a number of these interactions.

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Splitting up the indivisible: quasiparticles separate an electron’s spin, charge, and orbit

Free electrons moving through space are fundamental and indivisible: they are not built up of smaller particles, in contrast with protons and neutrons. However, within materials, interactions among electrons and atoms can give rise to quasiparticles, quantum states in which groups of electrons behave as new, particle-like excitations.

Physicists have now successfully created quasiparticles that split the electron's orbital characteristics from its spin. To accomplish this, Justine Schlappa et al. studied a special material in which electrons are confined to one-dimensional interactions at low temperatures, so that electron-electron interactions are dominant. Using resonant inelastic X-ray scattering (RIXS) at the Swiss Light Source facility, they determined that the electron orbital states propagated through the material independently of the spin.

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Researchers mimic relativity and the Higgs field in graphene-like material

The behavior of electrons and other particles depends on their environment. In particular, the interactions inside materials can alter the collective properties of the material's electrons, producing what are effectively new "particles"—known as quasiparticles—with correspondingly new behaviors. The surfaces of solids are fertile ground for quasiparticles, since they are two-dimensional; as we've seen in a number of other experiments, the loss of the third dimension can lead to exciting new physics.

A new experiment involving a graphene-like material has shown that it's possible to perform some spectacular manipulations of the properties of these quasiparticles. The work is described in a Nature letter by Kenjiro Gomes, Warren Mar, Wonhee Ko, Francisco Guinea, and Hari C. Manoharan. The team arranged carbon monoxide molecules to form the same hexagonal pattern found in graphene, except that they could change the spacing slightly. 

This produced an environment where the material's electrons behave remarkably like relativistic particles, with a "speed of light" that they can adjust. Additionally, the researchers could change the spacing between molecules in a way that the masses of the quasiparticles changed, or cause them to behave as though they are interacting with electric and magnetic fields—without actually applying those fields to the material. This setup will potentially help us explore new physics that may arise in these environments. 

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Breaking up the indivisible to observe the implausible—particles with a fractional charge

It was 1909 when Robert Millikan and Harvey Fletcher carried out their famous oil drop experiment in which they determined that the smallest unit of charge possible was  1.592x10-19 Coulombs, a value we now refer to as e, the fundamental charge (the modern accepted value is 1.602176565(35)x10-19 C).  It is the magnitude of the negative charge carried by the electron, as well as the positive charge of a proton. It is also the smallest unit of charge that any stable, independent particle can possibly have—no particles can have -3/4e charge, nor can they carry +2.8e of charge—barring technicalities. A paper published in this week's edition of Science examines in detail one of the technical loopholes to the preceding statement.

We have spent a large amount of time breaking up hadrons to our heart's content, resulting in a spew of quarks, bosons, and other fundamental particles. But no particle collider could ever hope to split an electron (or other lepton) into smaller pieces, so we have no way of looking at something that is, say, one half of an electron. 

But there may be a way to split up something that looks a lot like an electron. Quasiparticles are collections of fundamental particles that have an emergent behavior similar to that of a single fundamental particle. But they are not bound by the rules that govern stable individual particles.

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