Double dosing induces magnetism while enhancing electronic quantum oscillations in a topological insulator

November 12, 2021

(Nanowerk News) A team led by Wollongong University on three FLEET nodes combined two traditional semiconductor doping methods to achieve new efficiencies in bismuth-selenide topological insulator (Bi2Se3). Two doping elements were used: samarium and iron (Physical examination B, “Massive Dirac fermions and strong Shubnikov-de Haas oscillations in single crystals of topological insulator Bi2Se3 doped with Sm and Fe “).

The resulting bismuth-selenide crystals show a clear ferromagnetic order, a broad bandgap, high electron mobility, and the openness of a surface finish interval making this system a good candidate for achieving QAHE at higher temperatures. high energy requirements for a viable and sustainable future low energy electronics.

“The combination of electronic and magnetic properties in topological systems is the keystone of new topological devices and one of the key projects of FLEET,” said project leader Prof. Xiaolin Wang (UOW). “We have proposed and successfully realized a new way to magnetize a new electronic material, a topological insulator, by adding two different magnetic ions. ”

In double-doped bismuth selenide (Sm / Fe), Sm and Fe both carry large magnetic moments (shown in red), with weaker induced moments appearing on selenium (green) atoms coordinating Fe. (© Physical Review B)

Each of the different magnetic elements used to magnetize a topological insulator has its own advantages and disadvantages. However, while in previous studies only one element was used, the UOW-Monash-RMIT team found that the combination of two elements also combined the benefits of each.

“The double doping strategy has thus proven to be viable for the growth of very high quality topological insulators with both magnetism and excellent electron mobility which are vital for low energy electronic devices,” explains the lead author of the study, Dr. Weiyao Zhao.

One dose is not enough: the limits of transition metal doping

Topological insulators (TI) are emerging materials with a unique band structure allowing the study of the quantum effect in solids, while being promising components of future high performance quantum devices.

There are the two key elements in the quantum anomalous Hall effect (QAHE) that “drives” the desirable properties in topological insulators and all associated electronic technologies: these are (a) ferromagnetism and (b) the property of topological electronic isolation.

The FLEET collaborative study, combining the expertise of UOW, Monash and RMIT, launched a ‘dual element’ doping strategy to introduce magnetism into a topological insulator, thus improving both key elements at the same time. .

The combination of the advantages of two different doping elements, iron and samarium, results in significant crystal growth, with a wide surface band gap and huge quantum transport effect.

The previous approach to achieve the quantum abnormal Hall effect (QAHE) in a topological insulator used doping with a single transition metal, such as iron (Fe), to create ferromagnetism.

The transition metal doping technique has succeeded in creating the desired magnetic order. However, this method has a significant drawback: the transition metal integrated into the grid compromises the desired high mobility of the topological insulator, which in the case of low energy electronics defeats the purpose. to use topological insulators!

Thus, QAHE was achieved via a transition metal doping strategy only at extremely low temperatures, which would require energy-intensive cooling. Again, this reduces the viability of these materials for future low-power electronics.

To increase the operating temperature of the QAHE, stronger magnetic interaction and higher mobility are desired.

Double Doping Achieves Both Desired Key Elements of QAHE

After examining the successful elements of doping using transition metals such as iron (Fe), the research team decided to further introduce a more powerful magnet, the rare earth element samarium (Sm). , in the well-known topological insulator bismuth-selenide (Bi2Se3).

The doping elements iron and samarium create the necessary ferromagnetic order in the crystals, which can open a massive deviation at the level of the Dirac cone from the surface finish. This is an essential element to achieve QAHE.

In addition, the team proved that in double magnetically doped crystals, electron mobility remains very high, confirming the presence of an ultra-strong quantum oscillation effect and a stepped Hall effect.

The mobility of topological insulators such as bismuth selenide is several times faster than in conventional semiconductors, such as silicon.

Such a crystal idealizes the two important elements of QAHE.

The resulting crystals show a clear ferromagnetic order, a wide band gap (∼44 meV) and high mobility (∼7400 cm2/ Vs at 3 K) and Hall steps in transverse resistivity, confirming the presence of QAHE.

Angular Resolution Photoelectron Spectroscopy (ARPES) shows photon-energy dependence (left) and energy distribution curves (right) Angular Resolution Photoelectron Spectroscopy (ARPES) at ALS Berkeley shows photon-energy dependence (left) and energy distribution curves (right). (© Revue Physique B)

“The double bi doped with samarium and iron2Se3 crystal will be an ideal system for achieving QAHE at higher temperatures, ”says corresponding author Dr Mark Edmonds (Monash). “This may be a new way to fill the shortage of magnetic elements. “The double doping strategy has also been shown to be positive for using topological insulators in low power electronic devices. ”

“The DFT calculation indicates that double doping results in half-metallicity and fully spin-polarized electrons in the system,” adds the corresponding author, Professor Xiaolin Wang. “This opens another avenue for potential applications in spintronics, while contributing to the diversity of the topological family of matter.”

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