Physicists open the door to the first direct measurement of Berry curvature in solid matter

November 22, 2017 by Julie Cohen, University of California - Santa Barbara
This artist’s rendition features Berry curvature represented by the twisting ribbons at the top. Credit: Brian Long

Berry curvature may not be the most well-known scientific concept, but to many physicists, its direct measurement is something akin to a holy grail.

A powerful unifying principle in several branches of classical and quantum physics, Berry curvature is a strange and elusive quantum mechanical property of solids. It governs the dynamics of the motion of charges in semiconductors yet itself cannot be directly measured.

If it could be, the resulting calculation could lead to new materials for quantum computing.

Now, UC Santa Barbara physicists have opened the door to the first direct measurement of Berry curvature in solid matter. Their work, published in the journal Physical Review X, builds on a previous UCSB paper in which they describe experiments resulting in an electron-hole recollision achieved by aiming high- and low-frequency laser beams at a made of gallium arsenide.

For the new paper, scientists from UCSB's Sherwin Group collaborated with colleagues in China, at Princeton University and at the U.S. Naval Research Laboratory to improve on the previous experiment. They discovered a surprising new phenomenon using the same semiconductor subjected to extremely strong laser fields oscillating nearly 1 trillion times per second (1 terahertz). Called dynamical birefringence, this phenomenon can be used to investigate Berry curvature.

"When we originally did the experiment, we could only detect one sideband at a time and the samples were very fragile and more difficult to work with," explained corresponding author Mark Sherwin, director of UCSB's Institute for Terahertz Science and Technology and a professor in the Department of Physics.

Hunter Banks, lead author of the new paper, installed a camera that allowed the team to see all the sidebands simultaneously, which decreased the experiment's duration and increased its sensitivity. He also improved how the samples were mounted and increased the strength of the terahertz electric field that could be applied.

These enhancements revealed sidebands separated by as much as 90 times the photon energy of the terahertz—more than three times the amount in the original experiment. Sherwin noted that an increased quantity of sidebands allowed the team to learn more about the semiconductor. "As far as we know, this huge number of sidebands is the highest-order nonlinear optical process in solids," he said.

Generated by a unique laser housed in a dedicated building at UCSB, these experiments drive thin layers of semiconductor while they are illuminated by weak infrared light. The infrared light is polarized in one of two ways: either parallel or perpendicular to the terahertz field.

"The transmitted through the semiconductor exhibits a rainbowlike spectrum containing dozens of frequencies, or sidebands," Sherwin explained. "Unexpectedly, the sidebands are usually stronger when the infrared beam is polarized perpendicular to the terahertz field. Somehow the is actually defining an axis that acts like a polarizing one. We call this phenomenon dynamical birefringence, and it arises as a direct consequence of Berry curvature."

It also creates possibilities for applications in new classes of electronic and optical devices.

"We're planning to turn dynamical birefringence into a direct measurement of Berry curvature," Sherwin explained. "Once you can measure something that's a basic property of a solid, then, when you're designing new materials, you can optimize the Berry curvature for a particular device."

To navigate through an annotated 3-D rendering with virtual reality capability, click here.

Explore further: Physicists mix two lasers to create light at many frequencies

More information: Hunter B. Banks et al, Dynamical Birefringence: Electron-Hole Recollisions as Probes of Berry Curvature, Physical Review X (2017). DOI: 10.1103/PhysRevX.7.041042

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5 / 5 (1) Nov 22, 2017
yer kidding, right?
5 / 5 (3) Nov 22, 2017
The quantum phenomena are described by non-commutative operators, because the quantum fluctuations make them principally irreversible. It's not so difficult to visualize it: in classical mechanics the floaters bouncing at the surface of water always follow closed path. But in microscale the floater would always resurface at somewhat different place, because the effects of density fluctuations during floater motion through underwater cannot be neglected.

This averaged shift is called Berry phase, but its magnitude would also depend on depth of floater oscillations and their momentum. The quantity which takes account into it is called Berry curvature and it serves as a measure of irreversibility of quantum systems. For example the electrons at the surface of graphene or topological insulators are mutually compressed and the effects of the quantum fluctuations get higher there, so that their Berry curvature gets also higher than for strongly coupled electrons within metals.
5 / 5 (3) Nov 22, 2017
The above study observed the recombination of pairs and holes generated inside the GaAs semiconductor by infrared pulses and excited (i.e. separated each other) by polarized microwaves. Due to quantum fluctuations the portion of electron-hole pairs will not return into their original position during each cycle of microwave field (Berry phase), so that they will not recombine. They will contribute to birefringence of infrared light by crystal, which enables to measure the Berry curvature, i.e. the irreversibility (degree of breaking the motion symmetry) of electrons and holes.

GaAs superconductor has been used, because it allows conduction only through narrow channels between atoms within its lattice, where the motion of charge carriers gets constrained and exposed the quantum fluctuations of material. This leads into chaotic motion of electrons, typical for noise generating Gunn diodes made of GaAs.
3 / 5 (2) Nov 22, 2017
OK, I get the QM, i.e. Path selection, is subject. One may define the path as a selection of superimposed charge centers at what level or volume, structure, energy levels, the fields. Given a structure, or atom, the field is defined, the necessary and sufficient conditions to share, or move, any charge is defined by that structure. An assembly of N of these and the boundary conditions, all may be superimposed and the required fields to release a charge between a&b is defined. Build a path, to much computation? Well not so, but interesting nomenclature. Not sure the nomenclature always makes any sense. But I like the Berry curvature as an interesting point on stability and relaxation but not very definitive.
not rated yet Nov 24, 2017
is this a kind of laminar de-broglie wave effect

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