Physicists investigate onset of effective mass

May 21, 2014 by Lisa Zyga feature

( —Although mass may seem to be a fairly straightforward concept, from a physics perspective it can be much more complex than weighing an object and reading off a number in grams. For instance, an object's mass can be modified by putting it in a different medium.

The familiar type of mass, also called the "inertial mass" or "bare mass," is a of an object that is determined by an object's resistance to acceleration when subject to a force, for example gravity.

When an object's mass is modified by putting it in a different medium, the modified mass is called the "effective mass." Effective mass is distinctly different than bare mass, as it is not a fundamental property of an object, but changes depending on the material it is in. For practical purposes, effective mass plays an important role in electrical conductivity and .

"The effective mass description is incredibly powerful because it allows one to sweep away a lot of the complex physics, which describes the interactions between the electron and the medium, replacing it with an intuitive 'classical' description, while burying the details within a modified mass," physicist Rockson Chang, currently at the Institut d'Optique at the Université Paris-Sud, told

"Effective mass behavior is well established, both in fundamental science and modern electronic engineering," he said. "An example of the latter is in the use of the semiconductor gallium arsenide (GaAs). Electrons in GaAs turn out to have a very small effective mass, just 0.067 times that of the bare electron mass. This very small mass means that when the electrons are subjected to a potential difference, they move significantly faster than outside this medium, making them ideal for high-speed electronic devices. "

While working at the University of Toronto, Chang and his coauthors investigated what happens to effective mass when an external force is applied very abruptly. In the simplified theory of effective mass, which is often used in research, the assumption is that the forces are applied slowly. The researchers wanted to find out what happens when this assumption is not met.

To do this, the researchers used a Bose-Einstein condensate (BEC)—a large number of atoms cooled to near absolute zero temperature. These atoms were trapped in an artificial crystal made of light called an . Although this system contains atoms rather than electrons, it exhibits many of the same fundamental properties as a semiconductor. However, because BECs exist on larger length and time scales, they have the advantage of being much more easily accessed experimentally than semiconductors. As the researchers explained, this idea of using one well-controlled quantum system to "simulate" the properties of another is called "quantum simulation."

After trapping the BEC in an optical lattice, the researchers abruptly (in about 20 µs) applied a force on the atoms with an external magnetic field. This force results in an oscillation of an atom's motion in the crystal, and from this motion the researchers were able to extract the particle's mass.

The researchers' most important finding was that the initial response of the atoms to this applied force is characterized not by the effective mass as expected, but by the bare mass. Shortly after this initial time period, the atoms response undergo rapid oscillations and the onset of effective mass occurs.

"For over 50 years, the observed effective mass behavior has been well described by theory," Chang said. "However, the popular and intuitive description is based on some fundamental assumptions, in particular that the external force is applied relatively slowly. In our work we probe what happens when this assumption is invalidated. Our work shows that if one were to apply a force sufficiently quickly, the effective mass description falls apart. Interestingly, the initial response of the particle is that of a particle with its bare mass—or in other words, the particle responds as if the medium wasn't there at all. Only over longer time does the medium have a chance to catch up, again modifying the behavior of the particle and gradually returning to the usual effective mass response."

To give a loose analogy, Chang explains that the onset of effective mass is somewhat like walking through a muddy field. Because of the mud, you tend to walk slower than you would otherwise. This is like having an effective mass larger than a bare mass. However, if you started off by running into the mud, you would at first move very quickly. At this point, effective mass is still low. But as you sink deeper and deeper into the mud, it becomes more and more difficult to maintain that speed, and you eventually slow down. Likewise, it takes some time for the effective mass behavior to arise when subjected to a .

"It's not that our work is the first observation of the effective mass," Chang said. "Rather, our experiment demonstrates that the simple and intuitive picture of effective mass can actually break down, while revealing a conceptually very satisfying transition between two different behaviors, from a response in free space (described by the bare mass) to in a medium (described by an effective mass). Our work explores a new regime of dynamics, fast compared to the characteristic timescale of the material-particle interaction."

By showing how effective mass develops on short time scales, the results shed light on some of the most fundamental properties of solid state systems.

"This regime is increasingly of interest for researchers looking to continually push the speed of electronic devices," Chang said. "For example, there has been recent interest in using ultrafast laser pulses (femto-second and shorter) for fast electronic switches. Our results show that the speed and effectiveness of this switching may be affected since the effective mass description may no longer be valid."

Explore further: Metamaterials experts show a way to reduce electrons' effective mass to nearly zero

More information: Rockson Chang, et al. "Observing the Onset of Effective Mass." Physical Review Letters. DOI: 10.1103/PhysRevLett.112.170404

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not rated yet May 21, 2014
Seems like an analogy might be the phase difference between voltage and current in capacitive and inductive circuits…

It would have to be:
mass_bare = mass_eff – (electro-weak interactions)

Wouldn't it?
May 21, 2014
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not rated yet May 22, 2014
Sounds like the 19th century theory of retarded potential applied to mass.
not rated yet May 22, 2014
By that effect one could characterize a "mean-free" space dependent on element/molecule type and density of the surrounding medium.

This is pure speculation (and I think it's wrong): if this effect always happens - and isn't just a statistical average - then one could 'stutter' an electron through a medium with its bare mass inertia instead of its effective mass one.
1 / 5 (1) May 22, 2014
Quite evident, when the slow excitation allowing the surounding collective reponses which change the mass, becomes fast, we recover the real bare mass, the surrounding not able to follow so fast !!!
Some, for explaining the strange cold fusion, forget this evidence for the very small effective charge to allow the penetration of protons inside the nucleus at low energy, starting cold fusion.
If cold fusion exist, it cannot be explained by these models of low collective effective charges.
1 / 5 (1) May 22, 2014
The same can be applied to the bare mass given by the Higgs boson, but it must be so infinitively fast to reach energies above the Higgs boson mass 120GeV !!!
5 / 5 (1) May 22, 2014
This is pure speculation (and I think it's wrong): if this effect always happens - and isn't just a statistical average - then one could 'stutter' an electron through a medium with its bare mass inertia instead of its effective mass one.

Pretty good speculation, AA, and I think it's necessarily correct if the motion of an electron 'stutters' say, on graphene, or on the surface of a topological insulator.
Whydening Gyre
5 / 5 (1) May 22, 2014
Essentially, it's just a way of defining "friction" in atomic/quantum scale...
May 22, 2014
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1 / 5 (1) May 24, 2014
Pejico thinking :
"it worked.. in plasma... but was very costly in the amount of electricity required".
high RF currents for the small water and salt molecules are not very different from slower 60Hz alternating current, which gives a mixture of reactive H2, O2 and HCLO and plasma.
Quite more convincing, laboratories have observed the formation of Fe iron atoms with such strong discharges with only Carbon electrodes and mass spectroscopy, and, with very simple discharge setup in water at home, you can smell the smell of this iron with your nose ( the same odor as for train iron braking ), This cold fusion is very disturbing with no coherent explanation, may be at the solid nano level, there is sometimes collective particles nano accelerators to a lot of ten KV?
Ball lighning is more strange!!
not rated yet May 26, 2014
I agree with the last comment by Chang. The speed of the actuating force will affect the result. It fits in well with electronic models of the atom, i.e. the Planck electronic model of the atom and my own new model of the atom which gets around the Bohr problem.

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