October 22, 2019

Unique properties of quantum material explained for first time

WEST LAFAYETTE, Ind. — The characteristics of a new, iron-containing type of material that is thought to have future applications in nanotechnology and spintronics have been determined at Purdue University.

The native material, a topological insulator, is an unusual type of three-dimensional (3D) system that has the interesting property of not significantly changing its crystal structure when it changes electronic phases—unlike water, for example, which goes from ice to liquid to steam. More important, the material has an electrically conductive surface but a non-conducting (insulating) core.

However, once iron is introduced into the native material, during a process called doping, certain structural rearrangements and magnetic properties appear which have been found with high-performance computational methods.

"These new materials, these topological insulators, have attracted quite a bit of attention because they display new states of matter," said Jorge Rodriguez, associate professor of physics and astronomy.

“The addition of iron ions introduces new magnetic properties giving topological insulators new potential technological applications," Rodriguez said. “With the addition of magnetic dopants to topological insulators, such as iron ions, new physical phenomena is expected as a result of the combination of topological and magnetic properties.”

In 2016, three scientists received the Nobel Prize in physics for their work on related materials.

But for all of the fascination and promise of iron-containing topological insulators, the use of these materials in nanotechnology needed further understanding about how their structural, electronic and magnetic properties work together.

Rodriguez said his work uses supercomputers to explain Mössbauer spectroscopy, a technique that detects very small structural and electronic configurations, to understand what other scientists have been observing experimentally on iron systems.

"By using the laws of quantum mechanics in a computational setting, we were able to use a modeling technique called density functional theory, which solves the basic equations of quantum mechanics for this material, and we were able to fully explain the experimental results," Rodriguez said. "For the first time we were able to establish a relationship between the experimental data produced by Mössbauer spectroscopy, and the 3D structure of this material. This new understanding of the topological material will make it easier for engineers to use it in new applications."

The work was published in Physical Review B

Writer: Steve Tally, 765-494-9809, steve@purdue.edu, @sciencewriter

Source: Jorge Rodriguez, 765-496-2830, jhrodhig@purdue.edu


Mössbauer parameters of 57Fe substituents in the topological insulator  

Jorge H. Rodriguez, Purdue University, West Lafayette, Indiana 

Fe Mössbauer spectroscopy can probe several local structural, electronic, and magnetic properties of Fe-containing systems. However, to establish a direct relationship between these properties and a system's geometric structure, the experimental Mössbauer parameters need to be analyzed via electronic structure calculations. Herein, structural, electronic, and magnetic effects of iron substituents in the topological insulator Bi2Se3, as uniquely probed by 57Fe Mössbauer spectroscopy, have been determined via spin-polarized electronic structure calculations. The iron ion substituents, of nominal Fe3+(S=5/2) oxidation and spin state, are unequivocally shown to substitute Bi3+ sites in epitaxial Bi2Se3thin-films used for Mössbauer measurements. Concomitant with iron substitution, localized structural rearrangements take place whereby the longer Bi-Se bonds of the native system are replaced by significantly shorter Fe-Se counterparts in the Fe-containing system. The resulting distorted-octahedral environment about substituent iron ions gives rise to characteristic Mössbauer parameters (δFe≈0.51 mm/s, ΔEQ≈0.20 mm/s) which have been calculated to be in excellent agreement with measured values for Fe-doped Bi2Se3 thin films. Consistent with a substituent Fe3+ ion's nominal high-spin electronic configuration (t↑↑↑2ge↑↑g), an Fe-centered spin density has been established which, nevertheless, extends toward neighboring Se atoms via direct Fe-Se bonding and concomitant Fe(d)−Se(p)hybridization.

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