Several of the hotly studied topological materials to date are known as topological insulators. Their surfaces are expected to conduct electricity with very little resistance, somewhat akin to superconductors but without the need for incredibly chilly temperatures, while their interiors -- the so-called "bulk" of the material -- do not conduct current.
Now, a team of researchers working at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered the strongest topological conductor yet, in the form of thin crystal samples that have a spiral-staircase structure. The team's study of crystals, dubbed topological chiral crystals, is reported in the March 20 edition of the journal Nature.
The DNA-like spiraling structure, or helicoid, in the crystal sample that was the focus of the latest study exhibits a chirality or "handedness" -- as a person can be either left-handed or right-handed, and the left hand is a mirror image of the right hand. Chiral properties in some cases can be flipped, like a left-handed person becoming a right-handed person.
"In this new work we are essentially proving that this is a new state of quantum matter, which is also exhibiting nearly ideal topological surface properties that emerge as a consequence of the chirality of crystal structure," said M. Zahid Hasan, a topological materials pioneer who led the materials theory and experiments as a visiting faculty scientist in the Materials Sciences Division at Berkeley Lab. Hasan is also the Eugene Higgins Professor of Physics at Princeton University.
A property that defines topological conductivity -- which is related to the electrical conductivity of the material's surface -- was measured to be about 100 times larger than that observed in previously identified topological metals.
This property, known as the surface Fermi arc, was revealed in X-ray experiments at Berkeley Lab's Advanced Light Source (ALS) using a technique known as photoemission spectroscopy. The ALS is a synchrotron that produces intense light -- from infrared to high-energy X-rays -- for dozens of simultaneous experiments.
Topology is a well-established mathematical concept that relates to the preservation of an object's geometrical properties even if an object is stretched or deformed in other ways. Some of its experimental applications in 3D electronic materials -- such as discovering topological behaviors in materials' electronic structures -- were only realized just over a decade ago, with early and continuing contributions by Berkeley Lab.
"After more than 12 years of research in topological physics and materials, I do believe that this is only the tip of the iceberg," Hasan added. "Based on our measurements, this is the most robust, topologically protected conductor metal that anybody has discovered -- it is taking us to a new frontier."
Topologically protected means that some of the material's properties are reliably constant even if the material is not perfect. That quality also bolsters the future possibility of practical applications and manufacturability for these types of materials.
Ilya Belopolski, a Princeton researcher who participated in both the theory and experimental work, noted that a particularly interesting property of the studied crystals -- which included cobalt-silicon and rhodium-silicon crystals -- is that they can produce an electrical current of a fixed strength when you shine a light on them.
"Our previous theories showed that -- based on the material's electronic properties that we have now observed -- the current would be fixed at specific values," he said. "It doesn't matter how big the sample is, or if it's dirty. It is a universal value. That's amazing. For applications, the performance will be the same."
In previous experiments at the ALS, Hasan's team had revealed the existence of a type of massless quasiparticles known as Weyl fermions, which had only been known to exist in theory for about 85 years.
The Weyl fermions, which were observed in synthetic crystals of a semimetal called tantalum arsenide, exhibit some similar electronic properties to those found in the crystals used in the latest study, but lacked their chiral traits. Semimetals are materials that have some metal and some non-metal properties.
Now, a team of researchers working at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered the strongest topological conductor yet, in the form of thin crystal samples that have a spiral-staircase structure. The team's study of crystals, dubbed topological chiral crystals, is reported in the March 20 edition of the journal Nature.
The DNA-like spiraling structure, or helicoid, in the crystal sample that was the focus of the latest study exhibits a chirality or "handedness" -- as a person can be either left-handed or right-handed, and the left hand is a mirror image of the right hand. Chiral properties in some cases can be flipped, like a left-handed person becoming a right-handed person.
"In this new work we are essentially proving that this is a new state of quantum matter, which is also exhibiting nearly ideal topological surface properties that emerge as a consequence of the chirality of crystal structure," said M. Zahid Hasan, a topological materials pioneer who led the materials theory and experiments as a visiting faculty scientist in the Materials Sciences Division at Berkeley Lab. Hasan is also the Eugene Higgins Professor of Physics at Princeton University.
A property that defines topological conductivity -- which is related to the electrical conductivity of the material's surface -- was measured to be about 100 times larger than that observed in previously identified topological metals.
This property, known as the surface Fermi arc, was revealed in X-ray experiments at Berkeley Lab's Advanced Light Source (ALS) using a technique known as photoemission spectroscopy. The ALS is a synchrotron that produces intense light -- from infrared to high-energy X-rays -- for dozens of simultaneous experiments.
Topology is a well-established mathematical concept that relates to the preservation of an object's geometrical properties even if an object is stretched or deformed in other ways. Some of its experimental applications in 3D electronic materials -- such as discovering topological behaviors in materials' electronic structures -- were only realized just over a decade ago, with early and continuing contributions by Berkeley Lab.
"After more than 12 years of research in topological physics and materials, I do believe that this is only the tip of the iceberg," Hasan added. "Based on our measurements, this is the most robust, topologically protected conductor metal that anybody has discovered -- it is taking us to a new frontier."
Topologically protected means that some of the material's properties are reliably constant even if the material is not perfect. That quality also bolsters the future possibility of practical applications and manufacturability for these types of materials.
Ilya Belopolski, a Princeton researcher who participated in both the theory and experimental work, noted that a particularly interesting property of the studied crystals -- which included cobalt-silicon and rhodium-silicon crystals -- is that they can produce an electrical current of a fixed strength when you shine a light on them.
"Our previous theories showed that -- based on the material's electronic properties that we have now observed -- the current would be fixed at specific values," he said. "It doesn't matter how big the sample is, or if it's dirty. It is a universal value. That's amazing. For applications, the performance will be the same."
In previous experiments at the ALS, Hasan's team had revealed the existence of a type of massless quasiparticles known as Weyl fermions, which had only been known to exist in theory for about 85 years.
The Weyl fermions, which were observed in synthetic crystals of a semimetal called tantalum arsenide, exhibit some similar electronic properties to those found in the crystals used in the latest study, but lacked their chiral traits. Semimetals are materials that have some metal and some non-metal properties.
