Typically, reversing the direction of time twice is the same as never reversing it at all. Think of running an old-fashioned filmstrip backward, then forward (not an unusual experience for those of us um…of a certain generation): the film will look the same as though you never ran it backward. However, a particular uranium compound, URu2Si2, may break that rule. In that sense, it behaves akin to a spinor, the mathematical description of particles like electrons, protons, and so forth. (For more on spinors, read my earlier post at Galileo’s Pendulum.) This model could explain all the weird properties of the uranium compound, including its strange magnetic behaviors.
A new model may help resolve the confusion by proposing a different form of symmetry breaking. Ordinarily, if you reverse the direction of time (akin to running a movie backward), then reverse it again, everything comes back to normal. For the particular uranium-rubidium-silicon compound at issue, Premala Chandra, Piers Coleman, and Rebecca Flint argued that symmetry is broken: it will not behave normally even under double time reversal. While a literal double reversing of time isn’t possible in the lab, the broken symmetry has a measurable consequence in the distortion of electron orbits in the uranium. If confirmed, this hypothesis could resolve a thirty-year-old mystery. [Read more…]
Most accelerators, including the big ones at CERN and RHIC, use charged particles: protons, electrons, or ions (atoms with electrons removed to make them positively charged). That’s because it’s easy to accelerate that kind of particle using electric and magnetic fields. However, neutral particles like neutrons or normal atoms can’t be accelerated by those fields, even though they could be useful for particle colliders or bombarding materials for various reasons. A new multi-step method has solved that problem by accelerating ions, then restoring the electrons, leading to very energetic neutral atoms.
As we all know from elementary school physics, like charges repel. So any positively charged particle added to the plasma will experience acceleration from the plasma waves. Laser plasma accelerators are more compact than many other accelerator designs, including those used in big experiments like the Large Hadron Collider (LHC), although they haven’t yet reached the same energies. Plasma acceleration (sans lasers) is also important in many astrophysical processes. [Read more….]
Laser cooling (also known as optical cooling) is a well-established technique…but mostly for gases. The basic idea is to disperse the thermal energy of the atoms through shining light on them: the frequency of the laser is set to be slightly lower than the energy of transition between two configurations in the atoms, so that atoms’ motion provides that extra bit to lead to absorption. That converts energy of motion (kinetic energy) into light, slowing the atoms down. However, the trick doesn’t work for solids, because the kinetic energy takes the form of phonons, quasiparticles of sound. However, researchers figured out a way to annihilate the phonons using a laser, in a particular type of semiconductor. Using that trick, they cooled the material down by 40 degrees, opening the way to rapid refrigeration of some solids.
The authors of the new study used cadmium sulphide (CdS), a material known as a group-II-VI semiconductor. Commonly used in digital electronics, semiconductors are insulators under normal conditions, but can be induced to conduct electricity when impurity atoms are added. Group-II-VI semiconductors host both strong phonons, and an additional type of particle-like excitation known as an exciton. Excitons are created through interactions between electrons and “holes” that the electrons left behind. [Read more...]
A mystery: an unknown star, too faint to notice, suddenly expanded to a huge size, increasing in brightness to become one of the most luminous stars known. This star doesn’t even have a real name, just a “license plate” catalog number: V838 Monocerotis, indicating that it’s a not very important star in the constellation the Unicorn (Monoceros). However, a new paper has proposed the powerful flare could be explained by a well-accepted theory of binary star behavior, in which one star strips enough matter off the other until it suddenly grows to a huge size. These common envelope events (as they are known) could explain the V838 Monocerotis outburst, along with some other currently mysterious flares.
A new Science paper proposes that a class of violent astronomical events that we’ve observed may be due to common envelope stars, providing more direct evidence for their existence. These cataclysms are known as “red transient outbursts,” and in brightness terms, they’re somewhere between novas (flares of nuclear activity at the surfaces of white dwarfs) and supernovas, the violent deaths of stars. N. Ivanova, S. Justham, J. L. Avendado Nandez, and J. C. Lombardi Jr. identified a possible physical model for these outbursts, based on the recombination of electrons and ions in the plasma when the stars’ envelopes merge. [Read more…]
Just as a ratchet allows rotation in one direction but not the other, quantum ratchets break the symmetry of a microscopic system to facilitate preferential motion in one direction or another. Graphene is a two-dimensional hexagonal lattice of carbon atoms. As such, it’s highly symmetrical, but beneath that lurks a potentially exploitable hidden asymmetry. If you add hydrogen atoms (for example) to the top of graphene, an applied alternating current (in the form of a microwave-frequency light wave) induces electrons to flow preferentially one direction: a quantum ratchet.
The reason for this striking change in behavior is due to what’s called a structure inversion asymmetry in graphene. In the presence of an external influence—in this case, the introduction of hydrogen atoms and a strong magnetic field—the shape of the electron orbits in the carbon atoms gets distorted in one direction. When exposed to the oscillating electric field, the electrons felt a strong resistive force in one direction (which the authors liken to friction), but increased mobility in the opposite direction. [Read more…]
Most solids compress when squeezed, though the effect isn’t very large for most technologically important materials (metals, ceramics, and so forth). A few rare materials exhibit negative compressibility: they expand in the direction the force is exerted, though again the effect is small. However, researchers figured out a way to produce extraordinarily large negative compressibility, by fabricating a material with a folding wine-rack structure. (This was a fun story to write, and I got to refer to caecilians. Which are breaking my heart. Which are shaking my confidence daily.)
No material can exhibit negative compressibility in all dimensions at once: if it expands in one direction, then it will contract in the two directions perpendicular to the expansion. The researchers’ insight was to realize that they could maximize negative compressibility in one dimension by simultaneously maximizing positive compressibility in the other two. This kind of “wine rack” configuration (their words) is similar to an example found in nature: tendon structures in the legless amphibians known as caecilians. [Read more…]
Researchers working on the next generation of photovoltaic solar cells—cells that convert sunlight directly into electrical current—are looking toward exotic materials (which are expensive) or more common substances, but use subtler methods to extract energy. A new study used a basic semiconductor material, already in use in solar cell research, but made it into a set of wires in a brushlike structure. The key was making the wires’ diameter smaller than the wavelength of light, which exploited a resonant property to extract more energy than expected from the photons. In this way, the researchers achieved efficiencies comparable to normal (planar) solar cells.
In the new study, the researchers determined that two major factors dictated optimal performance: the diameter of the wire, and the conductive properties of the InP [indium phosphide]. From theoretical predictions, wires with diameters significantly less than visible-light wavelengths achieve resonance when the light strikes, vastly increasing the amount of energy that can be absorbed. The new model suggested peak efficiency would be reached with an nanowire diameter of 180 nanometers. [Read more…]
Metamaterials are a fascinating subject, worthy of a blog post at some point (to be written in my copious spare time, of course). If a real material won’t do what you need, you design one. Instead of atoms, you place small dots (known as quantum dots or nanodots) containing semiconducting or metallic atoms in a lattice pattern, making a two-dimensional solid with special electronic properties. A recent paper described the fabrication of a metamaterial built from gold nanodots, which has very precise response to light: when photons of a specific wavelength strike the surface at a given angle, almost nothing gets reflected back, whereas shifting the angle or wavelength slightly produces measurable reflection. The researchers used this metamaterial to make a sensitive molecular sensor, one capable of sniffing out individual proteins.
V. G. Kravets and colleagues demonstrated the detection of tiny masses, on the order of a single biomolecule, using nanoscale optics. They fabricated a material that responded resonantly to light. When a tiny amount of mass was added to the surface, it caused a dramatic change in the amount of reflected light. This enabled the researchers to detect the presence of mass accumulation to the level of 10-15 grams over a millimeter patch—equivalent to detecting a single human skin cell landing on a coffee table. [Read more...]
(This was my original title for my article, but my editors evidently didn’t like it. I guess I’m too old school. Ahem. Moving right along.)
As you may know, quantum physics shows that matter has both a wavelike and particle-like character. When you combine quantum physics and special relativity, you find that a particle at rest vibrates with a frequency that depends only on its mass: the Compton frequency. For most purposes, the Compton frequency is useless: it’s huge, even for low-mass particles like electrons, and scales up proportionally for more massive particles like protons. However, researchers have figured out a way to access the Compton frequency of cesium atoms by stimulating them with lasers in a particular way. This could lead to more precise atomic clocks, enabling even more detailed measurement of the second of time—and provide a new way to measure the masses of subatomic particles.
Shau-Yu Lan and colleagues exploited advanced techniques to construct an atomic clock based on a single cesium atom, a device capable of dividing the huge natural frequencies of the atom into more manageable quantities. This provided a strong demonstration of the ability to construct clocks based on a single microscopic mass. And, because we already have excellent clocks to compare them with, this can potentially work in the opposite direction, leading to accurate mass measurements in the future. [Read more…]