Quantum Sound...

Credit: Getty Images

Topics: Modern Physics, Phonons, Quantum Mechanics, Theoretical Physics

Researchers have gained control of the elusive “particle” of sound, the phonon. Although phonons—the smallest units of the vibrational energy that makes up sound waves—are not matter, they can be considered particles the way photons are particles of light. Photons commonly store information in prototype quantum computers, which aim to harness quantum effects to achieve unprecedented processing power. Using sound instead may have advantages, although it would require manipulating phonons on very fine scales.

Until recently, scientists lacked this ability; just detecting an individual phonon destroyed it. Early methods involved converting phonons to electricity in quantum circuits called superconducting qubits. These circuits accept energy in specific amounts; if a phonon’s energy matches, the circuit can absorb it—destroying the phonon but giving an energy reading of its presence.

In a new study, scientists at JILA (a collaboration between the National Institute of Standards and Technology and the University of Colorado Boulder) tuned the energy units of their superconducting qubit so phonons would not be destroyed. Instead the phonons sped up the current in the circuit, thanks to a special material that created an electric field in response to vibrations. Experimenters could then detect how much change in current each phonon caused.

“There’s been a lot of recent and impressive successes using superconducting qubits to control the quantum states of light. And we were curious—what can you do with sound that you can’t with light?” says Lucas Sletten of U.C. Boulder, lead author of the study published in June in Physical Review X. One difference is speed: sound travels much slower than light. Sletten and his colleagues took advantage of this to coordinate circuit-phonon interactions that sped up the current. They trapped phonons of particular wavelengths (called modes) between two acoustic “mirrors,” which reflect sound, and the relatively long time sound takes to make a round trip allowed the precise coordination. The mirrors were a hair’s width apart—similar control of light would require mirrors separated by about 12 meters.

Trapping the Tiniest Sound, Leila Sloman, Scientific American

Hawking Radiation...

Image Source: Universe Today
Hawking radiation near an event horizon. Credit: NAU.

Topics: Black Holes, Cosmology, Einstein, General Relativity, Phonons

IN BRIEF

Scientists may have found signs that phonons, the very small packets of energy that make up sound waves, were leaking out of sonic black holes, just as Hawking’s equations predicted.

SURVIVING A BLACK HOLE

Some 42 years ago, renowned theoretical physicist Stephen Hawking proposed that not everything that comes in contact with a black hole succumbs to its unfathomable nothingness. Tiny particles of light (photons) are sometimes ejected back out, robbing the black hole of an infinitesimal amount of energy, and this gradual loss of mass over time means every black hole eventually evaporates out of existence.

Known as Hawking radiation, these escaping particles help us make sense of one of the greatest enigmas in the known Universe, but after more than four decades, no one’s been able to actually prove they exist, and Hawking’s proposal remained firmly in hypothesis territory.

But all that could be about to change, with two independent groups of researchers reporting that they’ve found evidence to back up Hawking’s claims, and it could see one of the greatest living physicists finally win a Nobel Prize.

Futurism:
Physicists Made a ‘Black Hole’ in a Lab That May Finally Prove Hawking Radiation Exists

III-V Semiconductor Superlattices...

FIG. 1.

Even between lattice-matched crystalline materials, there exist nonuniform transition layers that behave as an effective atomic-scale interface roughness with some rms height Δ. This effective interface roughness leads to phonon-momentum randomization and to interface resistance in cross-plane transport.
Citation: J. Appl. Phys. 118, 175101 (2015); http://dx.doi.org/10.1063/1.4935142

Topics: Applied Physics, Materials Science, Nanotechnology, Phonons, Semiconductor Technology, Solid State Physics, Thermodynamics

Abstract

This paper presents a semiclassical model for the anisotropic thermal transport in III-V semiconductor superlattices (SLs). An effective interface rms roughness is the only adjustable parameter. Thermal transport inside a layer is described by the Boltzmann transport equation in the relaxation time approximation and is affected by the relevant scattering mechanisms (three-phonon, mass-difference, and dopant and electron scattering of phonons), as well as by diffuse scattering from the interfaces captured via an effective interface scattering rate. The in-plane thermal conductivity is obtained from the layer conductivities connected in parallel. The cross-plane thermal conductivity is calculated from the layer thermal conductivities in series with one another and with thermal boundary resistances (TBRs) associated with each interface; the TBRs dominate cross-plane transport. The TBR of each interface is calculated from the transmission coefficient obtained by interpolating between the acoustic mismatch model (AMM) and the diffuse mismatch model (DMM), where the weight of the AMM transmission coefficient is the same wavelength-dependent specularity parameter related to the effective interface rms roughness that is commonly used to describe diffuse interface scattering. The model is applied to multiple III-arsenide superlattices, and the results are in very good agreement with experimental findings. The method is both simple and accurate, easy to implement, and applicable to complicated SL systems, such as the active regions of quantum cascade lasers. It is also valid for other SL material systems with high-quality interfaces and predominantly incoherent phonon transport.

Journal of Applied Physics:
Thermal conductivity of III-V semiconductor superlattices, S. Mei, I. Knezevic

Graphene Superconductor...

Graphene turns into a superconductor when decorated with lithium atoms. (Courtesy: Shutterstock/Inozemtsev Konstantin)

Topics: Condensed Matter Physics, Graphene, Materials Science, Nanotechnology, Phonons, Semiconductor Technology, Superconductors, Solid State Physics, Quantum Mechanics

THIS changes the game! The application to longer life batteries is the first thought that comes to mind. Within semiconductors, we could supplement the physical limitations we're butting up to at the Moore's Law limit with a neat change in the material chemistry used to build the circuitry. A step before and enhancement of carbon nanotubes when they eventually replace them. Exciting times!

The "wonder material" graphene has another significant quality to add to its impressive list of electrical and mechanical properties: superconductivity. Physicists in Canada and Germany have shown that graphene turns into a superconductor when doped with lithium atoms – a result that could lead to a new generation of superconducting nanoscale devices.

Graphene exhibits a range of remarkable properties, thanks to its special structure – a one-atom-thick hexagonal lattice of carbon atoms. It is far stronger than steel while also flexible, and is an excellent conductor of both electricity and heat. In its pristine form, however, it is not a superconductor.

Neither is pure graphite, but in 2005 physicists showed that graphite could be made to superconduct when chemically treated, so as to create bulk materials consisting of graphene alternated with one-atom-thick layers of another element. The best performing material thus created, calcium graphite (CaC₆), has a superconducting transition temperature of 11.5 K. Theorists identified the underlying mechanism for that superconductivity as electron–phonon coupling. Phonons are vibrations in a material's crystal lattice that bind electrons together into "Cooper pairs" that can travel through the lattice without resistance – one of the hallmarks of superconductivity. It was then realized that such electron–phonon coupling might occur not just in bulk graphite compounds but also by depositing atoms of a suitable element on to single layers of graphene.

Physics World: 'Decorated' graphene is a superconductor, Edwin Cartlidge