Scientists from the Moscow Institute of Physics and Technology (MIPT) and the Institute for Theoretical and Applied Electrodynamics (ITAE) of the Russian Academy of Sciences (RAS) have worked in cooperation with a collaborator from RIKEN (Institute for Physical and Chemical Research in Japan) to provide theoretical proof of the presence of an innovative class of materials.
Such systems were proposed to be referred as “spin-valley half-metals.” The paper has been published in the Physical Review Letters journal. The findings of the study will be applied in implantable electronics, apart from devices developed using nanotubes, graphene and various other prospective materials.
The microscopic mechanism put forward by the team is very different from the typical half-metal model that is dependent on a strong electron-electron interaction. This may lead Researchers to look for “nonmetallic” half-metals, or the ones that do not include atoms of transition metals (e.g. manganese, nickel and lanthanum). These may be very useful in implantable systems and devices. The Authors employ the terminology “spin-valley-tronics” to refer to the prospective substitute to conventional electronics.
From the time the electron tube was invented, majority of the probabilities in the field of electronics have been exhausted. It will be very difficult to keep increasing the transistor numbers or microprocessor clock rate. This is the reason that Scientists across the world are investigating for new probabilities. One such probability is spintronics in which spins of electrons is used. This has already found certain significant practical uses. At the start of the 21st century, the application of giant magnetoresistance materials for magnetic field sensors, that are used to read data from hard disk drives (HDDs), has allowed storing considerably huge amounts of data on HDDs.
Researchers around the globe are performing research to develop innovative spintronic devices, and half-metals are considered to have immense capability. Existence of these was first speculated by using computer simulations, and then their existence was proved experimentally. In contrast to a typical metal, in the case of a half-metallic material, electrons with only one spin orientation (e.g. spin up) take part in the electric current. The energy of spin-down electrons is very high, and hence charge current cannot be carried by these electrons, indicating that passing of current through a half-metal causes the generation of a spin-polarized current. However, spin-valley-tronics mandates the regulation of spin-polarized population of electrons in the current as well as the valley index.
The expression “valley” is taken from semiconductor physics. In mathematical terms, the excitation energy of a solid can be expressed as E (k, n). Here k is the electron’s momentum and n is the zone index, or a discrete quantum property of the electron’s state. This function might seem to be very odd, and when a number of minimums that have comparable excitation energies exist, we can consider the existence of multiple “valleys.” One can say that electrons with states corresponding to one valley do not interact with electrons whose states correspond to another valley. These electrons not only carry spin and charge but also have a unique value, that is, the valley index.
The valley index can be applied to convey information by using valley currents—due to this factor, the valley index is very similar to spin. Research in this direction is currently being carried out by various research teams, denoting that the concepts put forward in the published paper are not pure abstractions. The concept of applying the valley index is for sure not new. However, the Researchers actually theoretically proved the presence of an innovative class of materials that can be applied in spin-valley-tronics.
So what’s new?
All the half-metals at the Scientists’ disposal include atoms of transition metals such as nickel, manganese, lanthanum and so on. The research team, including Physicists from Russia and Japan, has established a theoretical mechanism for accomplishing half-metallicity that does not need transition metal atoms. This probability might find various practical uses, for example, in implantable devices.
The Physicists suggest that “nonmetallic half-metals” be obtained from a special class of dielectric materials known as charge or spin density wave insulators. The term charge or spin density wave indicates a state including periodic microscopic regions in which average charge (spin) in the material is non-zero. Theorists indicate such systems to be a quantum condensate of electron-hole pairs. Formation of such a pair requires two valleys, where one supplies the electrons and the other supplies holes. The spin-valley half-metallicity occurs due to the existence of two valleys in the original system.
In the field of semiconductor physics, a “hole” is a quasiparticle proposed to have a positive charge.
If a material including a density wave has to be transformed into a half-metal, then specific treatment known as doping has to be performed, by which holes or electrons are incorporated into the insulator. Alexander Rozhkov, a Researcher at MIPT’s Department of Problems of Physics and Energetics and a Co-author of the paper, explained that doping of a system can be performed by exposing it to an external electric field or by performing chemical modifications of surface or bulk, “For each system, a suitable type of doping atoms—such as nitrogen, phosphorus, or some other element—needs to be selected. By replacing atoms of the host system with impurities donating or accepting conduction electrons, a change in the properties of the original material is induced.”
The probability of doping materials that have density waves has been discussed in literature for a long time. Consequently, the recently published paper has cited articles as old as those published in the 1970s. The systems approached by the Researchers have been demonstrated to include different phases such as spatially inhomogeneous phases, namely, states including electronic phase separation and the phases that have domain walls, often termed as “stripes”. At present, two new phases, namely, spin-valley half-metallicity and regular, have been found in an unanticipated manner.
In a way, our discovery proved to be a surprise even to ourselves. The physical model that, we found, has a spin-valley half-metallic phase is a classical one: It has been studied for decades. It would appear that Lenin, the revolutionary, was right all along when he wrote that the electron is as inexhaustible as the atom. It is now up to the experimenters: There are plenty of materials adequately described by the model we dealt with. I am therefore convinced that the phase we predicted will eventually be discovered, either in a material that is available today or in one that is yet to be synthesized.
Artem Sboychakov, a Senior Researcher at ITAE RAS and one of the Authors of the Paper
The publishing of the paper in the Physical Review Letters journal, which is one of the most often referenced physical journals, shows the significance of the concept proposed by the Physicists from MIPT, ITAE RAS, and RIKEN. Yet, Kliment Kugel, a Chief Research Scientist at ITAE RAS, stated that more is to be done, “At a recent conference in Moscow, I discussed this idea with Sang-Wook Cheong, who is the director of the Center for Advanced Materials at Rutgers University, U.S. He is a prominent experimental physicist with an unmatched citation count; suffice to say that he authored about a hundred articles cited about a hundred times each. He liked our paper, saying that he rather enjoys pursuing specific materials that could realize properties predicted by theoretical physicists. Most of the time, he ends up finding them. Let us hope our case will be no exception.”