Microwave photons can nullify the conductivity of electrons confined to the surface of liquid helium

Feb 10, 2011
At specific values of external magnetic field B, and under microwave irradiation (red arrow), the conductance of a two-dimensional electron (e-, generated by a nearby filament) gas on the surface of liquid helium falls to zero. Credit: 2011 Denis Konstantinov and Kimitoshi Kono

Trapping electrons in a flat plane prevents them from moving freely in the third dimension and opens the door to a whole range of unusual physics. These effects are harnessed, for example, in modern ultrafast transistors, which confine electrons to thin layers of high-quality semiconductor crystals such as gallium arsenide. But scattering from impurities in semiconductors can mask the deeper underlying physics of these so-called two-dimensional electron gases (2DEGs). Liquid helium may provide an alternative to semiconductors since it is largely impurity free. Using this approach, Denis Konstantinov and Kimitoshi Kono from the RIKEN Advanced Science Institute (Japan) have demonstrated a novel effect where light totally switches off the conductivity of 2DEGs.

Two-dimensional electron gases form naturally at the surface of helium because an intrinsic energy barrier prevents from penetrating any deeper into the liquid. These gases vary markedly from their three-dimensional counterparts because the electron motion in one direction becomes quantized—that is, their velocity in this direction is governed by quantum mechanics and is restricted to a range of discrete values.

Konstantinov and Kono cooled liquid helium-3 to 0.3 kelvin. They supplied electrons from a nearby hot filament, and applied voltage to a plate below the helium to control the number of electrons per unit area. Then, they fired microwave radiation at the 2DEG (Fig. 1) and measured the longitudinal conductivity— the current induced by an electric field applied along one direction—as a function of external magnetic field. They saw that the conductivity periodically fell to zero as they increased the magnetic field. When they switched off the source of microwave photons, however, this effect ceased.

This previously unidentified nullifying effect of microwave photons on is a consequence of energy-conserved scattering of the ’s electrons between different energy states—specifically, the first excited and ground sub-bands. “When the electrons stay in the ground sub-band, the effects are rather dull,” says Kono. “In our experiment, absorption of microwave photons transfers electrons to a higher energy sub-band,” Konstantinov adds. “As we change the magnetic field, the energies of states in two subbands cross, and scattering redistributes electrons between the sub-bands.”

Kono and Konstantinov believe that the result will lead to the observation of more novel phenomena in these two-dimensional systems when they are shifted out of their equilibrium state. “The study of nonequilibrium transport in the extremely clean helium system will complement studies of electron transport in semiconductors,” explains Konstantinov.

Explore further: Exploring X-ray phase tomography with synchrotron radiation

More information: Konstantinov, D. & Kono, K. Photon-induced vanishing of magnetoconductance in 2D electrons on liquid helium. Physical Review Letters 105, 226801 (2010). prl.aps.org/abstract/PRL/v105/i22/e226801

add to favorites email to friend print save as pdf

Related Stories

Short-range scattering in quantum dots

Oct 20, 2010

Chinese researchers, reporting in the Journal of Applied Physics, published by the American Institute of Physics, have described a new breakthrough in understanding the way electrons travel around quantum dots. This might ...

Explained: Bandgap

Jul 23, 2010

Why do some materials work well for making solar cells or light-emitting diodes (LEDs), while other materials don't? One key factor is having the right bandgap.

Recommended for you

Backpack physics: Smaller hikers carry heavier loads

4 hours ago

Hikers are generally advised that the weight of the packs they carry should correspond to their own size, with smaller individuals carrying lighter loads. Although petite backpackers might appreciate the ...

Extremely high-resolution magnetic resonance imaging

4 hours ago

For the first time, researchers have succeeded to detect a single hydrogen atom using magnetic resonance imaging, which signifies a huge increase in the technology's spatial resolution. In the future, single-atom ...

'Attosecond' science breakthrough

5 hours ago

Scientists from Queen's University Belfast have been involved in a groundbreaking discovery in the area of experimental physics that has implications for understanding how radiotherapy kills cancer cells, among other things.

User comments : 0