Science & Technology

Controlling the Spin Polarization of Electrons in Three Dimensions

The inside bulk of a topological insulator is certainly an insulator, however electrons (spheres) transfer swiftly on the floor as if by a steel. They’re spin-polarized, nevertheless, with their momenta (directional ribbons) and spins (arrows) locked collectively. Berkeley Lab researchers have found that the spin polarization of photoelectrons (arrowed sphere at higher proper) emitted when the materials is struck with high-energy photons (blue-green waves from left) is totally decided by the polarization of this incident mild. (Picture Chris Jozwiak, Zina Deretsky, and Berkeley Lab Artistic Providers Workplace)

Scientists from the Berkeley Lab found that when topological insulators are hit with a laser beam, the spin polarization of the electrons they emit may be fully managed in three dimensions.

Plain-looking however inherently unusual crystalline supplies known as 3D topological insulators (TIs) are all the rage in supplies science. Even at room temperature, a single chunk of TI is an effective insulator in the bulk, but behaves like a steel on its floor.

Researchers discover TIs thrilling partly as a result of the electrons that move swiftly throughout their surfaces are “spin polarized”: the electron’s spin is locked to its momentum, perpendicular to the path of journey. These attention-grabbing digital states promise many makes use of – some unique, like observing never-before-seen elementary particles, however many sensible, together with constructing extra versatile and environment friendly high-tech devices, or, additional into the future, platforms for quantum computing.

A crew of researchers from the U.S. Division of Power’s Lawrence Berkeley Nationwide Laboratory (Berkeley Lab) and the College of California at Berkeley has simply widened the vista of prospects with an sudden discovery about TIs: when hit with a laser beam, the spin polarization of the electrons they emit (in a course of known as photoemission) may be fully managed in three dimensions, just by tuning the polarization of the incident mild.

“The primary time I noticed this it was a shock; it was such a big impact and was counter to what most researchers had assumed about photoemission from topological insulators, or some other materials,” says Chris Jozwiak of Berkeley Lab’s Superior Gentle Supply (ALS), who labored on the experiment. “With the ability to management the interplay of polarized mild and photoelectron spin opens a playground of prospects.”

The Berkeley Lab-UC Berkeley crew was led by Alessandra Lanzara of Berkeley Lab’s Supplies Sciences Division (MSD) and UC Berkeley’s Division of Physics, working in collaboration with Jozwiak and Zahid Hussain of the ALS; Robert Birgeneau, Dung-Hai Lee, and Steve Louie of MSD and UC Berkeley; and Cheol-Hwan Park of UC Berkeley and Seoul Nationwide College. They and their colleagues report their findings in Nature Physics.

Unusual digital states and the way to measure them

In diagrams of what physicists name momentum house, a TI’s digital states look eerily like the identical sorts of diagrams for graphene, the single sheet of carbon atoms that, earlier than topological insulators got here alongside, was the hottest subject in the supplies science world.

In energy-momentum diagrams of graphene and TIs, the conduction bands (the place energetic electrons transfer freely) and valence bands (the place lower-energy electrons are confined to atoms) don’t overlap as they do in metals, neither is there an power hole between the bands, as in insulators and semiconductors. As an alternative the “bands” seem as cones that meet at some extent, known as the Dirac level, throughout which power varies constantly.

The experimental method that immediately maps these states is ARPES, angle-resolved photoemission spectroscopy. When energetic photons from a synchrotron mild supply or laser strike a fabric, it emits electrons whose personal power and momentum are decided by the materials’s distribution of digital states. Steered by the spectrometer onto a detector, these photoelectrons present an image of the momentum-space diagram of the materials’s digital construction.

The diagram at proper reveals the digital states of bismuth selenide in momentum house. ARPES, at left, can immediately create such maps with photoelectrons. A slice by the conduction cone at the Fermi power maps the topological insulator’s floor as a circle (higher left); right here electron spins and momenta are locked collectively. Preliminary ARPES measurements in this experiment had been made with p-polarized incident mild in the areas indicated by the inexperienced circle and line, the place the spin polarization of the photoelectrons is in step with the intrinsic spin polarization of the floor.

Comparable as their Dirac-cone diagrams might seem, the digital states on the floor of TIs and in graphene are basically completely different: these in graphene are usually not spin polarized, whereas these of TIs are fully spin polarized, and in a peculiar method.

A slice by the Dirac-cone diagram produces a round contour. In TIs, spin orientation modifications constantly round the circle, from as much as down and again once more, and the locked-in spin of floor electrons is decided by the place they lie on the circle. Scientists name this relation of momentum and spin the “helical spin texture” of a TI’s floor electrons. (Electron spin isn’t like that of a spinning prime, nevertheless; it’s a quantum quantity representing an intrinsic quantity of angular momentum.)

Instantly measuring the electrons’ spin in addition to their power and momentum requires an addition to ARPES instrumentation. Spin polarization is tough to detect and in the previous has been established by firing high-energy electrons at gold foil and counting which method just a few of them bounce; gathering the knowledge takes a very long time.

Jozwiak, Lanzara, and Hussain collectively led the growth of a precision detector that might measure the spin of low-energy photoelectrons by measuring how they scatter from a magnetic floor. Known as a spin time-of-flight analyzer, the system is many instances extra environment friendly at knowledge assortment.

Says Hussain, “It’s the variety of undertaking that might solely be completed at a spot like Berkeley Lab, the place tight collaboration for a variety of capabilities is feasible.”

The brand new instrument was first used at the ALS to review the well-known topological insulator bismuth selenide. Whereas the outcomes confirmed that bismuth selenide’s helical spin texture persists even at room temperature, they raised a perplexing query.

Lanzara says, “In an ARPES experiment, it’s normally assumed that the spin polarization of detected photoelectrons precisely stories the spin polarization of electrons inside the materials.” She explains that “this assumption is often made when confirming the helical spin texture of a TI’s floor electrons. However in our spin-ARPES experiments, we discovered vital deviations between the spin polarizations of the floor electrons versus the photoelectrons. We knew we needed to look additional.”

Flipping photoelectron spins

Probing the TI floor electrons didn’t require the excessive photon power of a synchrotron beam, so the new examine was primarily completed in a laboratory with a laser that might produce intense ultraviolet mild succesful of stimulating photoemission, and whose polarization was readily manipulated. The experiment used high-quality samples of bismuth selenide from Birgeneau’s MSD and UC Berkeley labs.

Incident mild that’s p-polarized (higher left) produces photoelectrons in step with the normal image of spin polarization in a topological insulator’s floor, however altering the polarization of the incident mild additionally modifications the spin polarization of the photoelectrons.

In the first experiments, the incident mild was p‑polarized, which suggests the electrical half of the mild wave was parallel to a airplane that was perpendicular to the TI floor and oriented in keeping with the path of the emitted photoelectrons. Since research of topological insulators sometimes use p‑polarized mild in this geometry, certain sufficient, the spin-ARPES measurements confirmed the photoelectrons had been certainly spin polarized in instructions in step with the anticipated spin texture of the floor electrons.

“After we’d measured p‑polarization, we switched to an s‑polarized laser beam,” Jozwiak says. “It solely took a couple of minutes to gather the knowledge.” (S‑polarization means the electrical half of the mild wave is perpendicular to the identical imaginary airplane – perpendicular in German being senkrecht.)

Three minutes after he began the run, Jozwiak bought a jolt. “The experiment was fully the identical, aside from the mild polarization, however now the photoelectrons had been spin polarized in the reverse path – the reverse of what you’d count on.” His first assumption was “I will need to have completed one thing flawed.”

Repeated cautious experiments with a variety of laser polarizations confirmed, nevertheless, that the spin polarization of the photons in the laser beam managed the polarization of the emitted photoelectrons. When the laser polarization was easily assorted – and even when it was circularly polarized proper or left – the photoelectron spin polarization adopted swimsuit.

Why had no outcomes counter to the anticipated floor textures been reported earlier than? In all probability as a result of the commonest variety of spin-ARPES experiment makes just a few measurements in a typical geometry utilizing p-polarized mild. With different preparations, nevertheless, photoelectron spin polarization departs markedly from expectations.

The crew’s concept collaborators, Park, Louie, and Lee, helped clarify the uncommon theoretical outcomes once they predicted that simply such variations between photoelectron and intrinsic textures ought to happen. There are additionally strategies that the easy image of spin texture in topological insulators is extra advanced than has been assumed. Says Lanzara, “It’s an amazing motivation to maintain digging.”

The power to hit a topological insulator with a tuned laser and excite polarization-tailored electrons has nice potential for the subject of spintronics – electronics that exploit spin in addition to cost. Gadgets that optically management electron distribution and move would represent a major advance.

Optical management of TI photoemission has extra quick sensible prospects as properly. Bismuth selenide may present simply the proper of photocathode supply for experimental strategies that require electron beams whose spin polarization may be exquisitely and conveniently managed.

DOE’s Workplace of Science helps the ALS and supported this analysis.

Publication: Chris Jozwiak, et al., “Photoelectron spin-flipping and texture manipulation in a topological insulator,” Nature Physics (2013); doi:10.1038/nphys2572

Picture: Chris Jozwiak, Zina Deretsky, and Berkeley Lab Artistic Providers Workplace;
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