“In valleytronics, electrons move through the lattice of a 2D semiconductor as a wave with two energy valleys, each valley being characterized by a distinct momentum and quantum valley number,” Wang says. “This quantum valley number can be used to encode information when the electrons are in a minimum energy valley.”
Instead of relying on the electrons’ spin or their charge, valleytronics exploits their energy level in relation to their momentum.
Pioneering exploration of the valley began in earnest in 2007, when researchers first discovered that the recently developed 2-D material graphene could sort electrons and holes according to which valley they occupied. Prior to this discovery, electrons and holes had only been observed occupying different valleys at random.
The term is often used as an umbrella term to other forms of quantum manipulation of valleys in semiconductors, including quantum computation with valley-based qubits, valley blockade and other forms of quantum electronics. Several theoretical proposals and experiments were performed in a variety of systems, such as graphene, some Transition metal dichalcogenide monolayers, diamond, Bismuth, Silicon, Carbon nanotubes, Aluminium arsenide and silicene.
Faster, more efficient data storage and computer logic systems could be on the horizon thanks to a new way of tuning electronic energy levels in two-dimensional films of crystal, discovered by researchers at MIT.
Now, in a paper published today in the journal Science, researchers led by Nuh Gedik, an associate professor of physics at MIT, describe a new way of using laser light to control the electrons in both valleys independently, within atomically thin crystals of tungsten disulfide.
“The two valleys are exactly at the same energy level, which is not necessarily the best thing for applications because you want to be able to tune them, to change the energy slightly so that the electrons will move [from the higher] to the lower energy state,” Gedik says.
Although this can be achieved by applying a magnetic field, even very powerful laboratory magnets with a strength of 10 tesla are only capable of shifting the valley energy level by around 2 millielectronvolts (meV).
The researchers have previously shown that by directing an ultrafast laser pulse, tuned to a frequency very slightly below the resonance of the material, they were able to shift the energy of one of the valleys through an effect called the “optical Stark effect,” while leaving the other valley virtually unchanged. In this way they were able to achieve a shift in energy level of up to 20 meV.
“The light and the electrons inside the material form a type of hybrid state, which helps to push the energy levels around,” Gedik says.
In the latest experiment, the researchers discovered that by tuning the laser frequency to even further below resonance, and increasing its intensity, they were able to simultaneously shift the energy levels of both valleys and reveal a very rare physical phenomenon.
While one valley still shifts as a result of the optical Stark shift as before, the other valley shifts through a different mechanism, known as the “Bloch-Siegert shift,” according to MIT physics PhD student Edbert Jarvis Sie, the paper’s lead author.
Indeed, apart from so-called artificial atoms, the new mechanism has never been observed in solids until now, because the resulting shifts were too small, Sie says. The experiment performed at the Gedik Lab produced a Bloch-Siegert shift of 10 meV, which is 1,000 times larger than that seen previously.
What’s more, the two effects — the Bloch-Siegert shift and optical Stark shift — have previously tended to take place in the same optical transition, meaning researchers have had difficulty disentangling the two mechanisms, Sie says.
“In our work we can disentangle the two mechanisms very naturally, because while one valley exhibits the optical Stark shift, the other valley exhibits the Bloch-Siegert shift,” Sie says. “This can work so nicely in this material because the two mechanisms have a similar relationship with the two valleys. They are related by what is called time-reversal symmetry.”
This should allow for enhanced control over valleytronic properties in two-dimensional materials, Nuh says. “It could give you more freedom in tuning the electronic valleys,” he says.
“Gedik and his colleagues show that they can control this energy shift in a three-atom thick semiconductor,” he says. “By varying the polarization of their laser, they can use the Bloch-Siegert shift to control different electronic states.”
SOURCES- IEEE Spectrum, MIT