Paving the way for quantum computers miniature magnets that trigger the quantum anomaly Hall effect
Researchers at the University of Tsukuba in Japan have built a new device that can demonstrate the quantum anomalous Hall effect (QAHE), in which tiny discrete voltage steps are generated by an external magnetic field. They say this work may make possible very low-power electronic devices as well as future quantum computers. The paper [1] has been published in Physical Chemistry Letters.
In this study, they used polarized neutron reflection to investigate the magnetic proximity effect (MPE) at the interface of a heterostructure composed of topological crystal insulator (TCI) SnTe and Fe. The ferromagnetic ordering from the interface with Fe penetrates about 2.2 nm into the SnTe layer and persists up to room temperature. This is caused by an MPE on the TCI surface that maintains a coherent topological state without introducing disorder through doping with magnetic impurities. This will open a path for the realization of next-generation spintronics and quantum computing devices.
01Introducing magnetic order into a topological insulator (TI) system
If you take an ordinary wire with current flowing through it, by applying an external magnetic field, you can generate a new voltage perpendicular to the direction of the current. This so-called Hall effect has been used as part of a simple magnetic sensor, but the sensitivity can be low.
There is a corresponding quantum version, called the quantum anomalous Hall effect, which appears in defined increments or quanta. This raises the possibility of using the quantum anomalous Hall effect to build new highly conductive wires or even quantum computers.
Now, a research team led by the Institute of Materials Science at the University of Tsukuba in Japan has used one of the above-mentioned topological insulator (TI) materials, in which current flows at the interface but not through the body, to induce the quantum anomalous Hall effect.
By using a ferromagnetic material, iron (Fe), as the top layer of the device, the magnetic nearest neighbor effect can produce magnetic ordering without introducing disorder caused by alternative methods of doping with magnetic impurities. Professor Kuroda Shinji of the University of Tsukuba said [2], "The current generated by the quantum anomalous Hall effect can propagate along the interface of the layer without dissipation, which may be used in new energy-saving devices."
To fabricate the device, a thin film of a single-crystal heterostructure consisting of an iron layer on top of tin telluride (SnTe) was grown on a template using molecular beam epitaxy. The researchers measured the magnetization intensity of the surface using neutrons, which have a magnetic moment but no electrical charge. They found that ferromagnetic order penetrates about two nanometers into the SnTe layer from the interface with iron, and can be present even at room temperature.
Magnetization strength as a function of depth through the SnTe layer
Professor Kuroda said, "Our research points the way to the realization of the next generation of spintronics and quantum computing devices." These applications may require layers that exhibit the quantum anomalous Hall effect, which this research shows is possible and can be easily generated.
02Quantum anomalous Hall effect
The quantum anomaly Hall effect is a problem that has plagued the physics community for more than 130 years.
Today we already know that in an ordinary conductor, electrons move in a haphazard path and constantly collide, resulting in effects such as resistive heating. At this time, if an external magnetic field is added in the vertical direction, the electrons in the material will run to one side of the conductor due to the force of the magnetic field to form an accumulated charge, which will eventually reach equilibrium to form a stable Hall voltage, which is the Hall effect.
This phenomenon was discovered by the American physicist E.H. Hall (1855-1938) in 1879 when he was studying the mechanism of electrical conductivity in metals: at room temperature, a current passing through a conductor perpendicular to an external magnetic field will be deflected and an additional electric field will be generated perpendicular to the direction of the current and the magnetic field, thus creating an electric potential difference at the two ends of the conductor.
It is worth noting that Hall was only a graduate student when the Hall effect was discovered, and that the electron had not yet been discovered (by Thomson in 1897).
One hundred years after the discovery of the Hall effect, in the late 1970s, scientists discovered the quantum Hall effect by studying the Hall effect in semiconductors at very low temperatures and strong magnetic fields. When the external magnetic field is strong enough and the temperature is low enough, the motion of electrons can become highly ordered, with electrons moving at high speed along two boundaries at the boundary.
The Quantum Hall Effect
The quantum Hall effect was discovered in 1980 by German physicist von Klitzing and others, for which he was awarded the Nobel Prize in Physics in 1985. 2005, British scientists Andre Heim and Konstantin Novoselov succeeded in experimentally separating graphene from graphite and observing the quantum Hall effect at room temperature. They were awarded the Nobel Prize in 2010.
In 1982, Chinese-American physicist Qi Cui and American physicists Laughlin and Stemmer discovered the fractional quantum Hall effect under stronger magnetic fields, a discovery that furthered the understanding of quantum phenomena and for which they were awarded the Nobel Prize in Physics in 1998.
The fractional quantum Hall effect is crucial to the implementation of topological quantum computing, and scientists have now discovered that the composite fermion (a quasiparticle) inside the fractional quantum Hall state with filling factor v=5/2 does not follow both Fermi and Bosonic statistics and may be non-abelian arbitrators (used to implement topological quantum computing).
There is the Hall effect and the anomalous Hall effect. in 1880, while studying the Hall effect in magnetic metals, Hall discovered that the Hall effect could be observed even without the addition of an external magnetic field, and this Hall effect in a zero magnetic field is the anomalous Hall effect.
After the discovery of the quantum Hall effect, physicists went on to ask whether the anomalous Hall effect could have a corresponding quantized version like the Hall effect, which in turn became a new goal of exploration. In order to realize the quantum anomalous Hall effect, various proposals have been put forward by theoretical physicists since 1988, however, no significant progress has been made experimentally.
From the beginning, Qikun Xue's team has been working in an arena without a track, and in 2009, when Qikun Xue learned that international theories predicted that the quantum anomalous Hall effect could be found in magnetic topological insulators, he invited Prof. Yayu Wang and Prof. Ke He from the Department of Physics of Tsinghua University to join the research team.
In 2006, Shoucheng Zhang, a Chinese-American physicist, was the first to propose the concept of topological insulator, which is an insulator on the inside and a metal that conducts electricity on the surface. On this basis, Shoucheng Zhang successfully predicted the quantum spin Hall effect (consisting of two sets of edge states with opposite spin directions and running in opposite directions, and without the need for an applied magnetic field).
In 2007, a research group at the University of Wurzburg, Germany, successfully observed the quantum effect of this special edge state in a Hg Te/CdTe quantum well structure, thus experimentally confirming Shoucheng Zhang's prediction.
In 2010, Chinese theoretical physicists Fang Zhong and Dai Xi, together with Professor Shousheng Zhang, proposed that magnetically doped three-dimensional topological insulators might be the best system to realize the quantum anomalous Hall effect. At this time, Qikun Xue's team had already started to experimentally search for the quantum anomalous Hall effect.
The quantum anomalous Hall effect implies that the Hall resistance jumps to a quantum resistance value of about 25,800 Ω in a zero magnetic field. To achieve this incredible quantum phenomenon requires that the experimental sample must simultaneously satisfy four very demanding conditions.
It must be a two-dimensional system (thin film), thus having a conducting one-dimensional edge state; it needs to be in an insulating phase, thus making no contribution to the conductivity; it needs to have a ferromagnetic order, thus having an anomalous Hall effect; and it needs non-trivial topological properties, thus making the electronic energy band inverted.
This is like requiring a person with the speed of a sprinter, the height of a basketball player, the strength of a weightlifter and the dexterity of a gymnast at the same time.
After four years of efforts, Qikun Xue led an experimental team consisting of the Institute of Physics of the Chinese Academy of Sciences and Tsinghua University, using five sets of the world's highest level precision experimental instruments, growing and measuring more than 1000 samples, and finally using molecular beam epitaxy to grow high-quality Cr-doped (Bi,Sb)2Te3 topological insulator magnetic films, which were then prepared into transport devices, and under extremely low temperature environment The quantum anomalous Hall effect was successfully observed.
Qikun Xue remembers very clearly that at around 10:30 p.m. on October 12, 2012, he went home a little early that day and had just returned home when he received a text message from his students that "the initial signs of the quantum anomalous Hall effect have been found, awaiting detailed measurements."
The world problem was overcome when Qikun Xue's team found that the anomalous Hall resistance of Cr-doped (Bi,Sb)2Te3 topological insulator magnetic film in zero magnetic field reached the characteristic value of the quantum Hall effect within a certain range of applied gate voltage.
Xue Qikun could not contain his inner excitement. In scientific discovery, there is no second, only first, he confessed, "When discovering a scientific achievement, you will be very excited, after all the years of hard work done, it is patented! If the second one is made it will be a big discount."
The research result was published online by the American journal Science on March 14, 2013, causing a huge response from the academic community, with Yang praising it as a Nobel Prize-level achievement. But Academician Xue Qikun emphasized, "This is the joint result of our team's sincere cooperation and joint research, and it is a collective honor for Chinese scientists."
The Quantum Anomaly Hall Effect Research Team
Theoretically, Qikun Xue's team discovered the quantum anomalous Hall effect, which has been a world problem for more than 130 years, and the reviewer of Science commented that "this is a milestone work in condensed matter physics".
In terms of practical applications, all electronic devices invented by human beings are unable to avoid energy loss, which is caused by the disorderly motion of electrons. The quantum Hall effect can solve this problem, but its generation requires the application of a strong magnetic field, which is equivalent to the addition of 10 computer-sized magnets, and therefore, expensive and bulky factors limit its move toward practical applications.
The beauty of the quantum anomalous Hall effect is that electrons move in a fixed trajectory without applying a strong magnetic field, reducing the heat and energy loss caused by irregular collisions, and can be used to develop a new generation of low-energy transistors and electronics devices.
Using this technology to design integrated circuits and components, a supercomputer of 100 billion times is expected to be made as large as a tablet computer, and the memory of smartphones may be increased thousands of times.
Because of the significance of the discovery of the quantum anomalous Hall effect, Xue Qikun won the only first prize in the 2018 National Natural Science Awards. To date, fewer than 40 scientists have won the first prize of the National Natural Science Award. 2020, he was awarded the Fritz London Prize, the highest internationally recognized award in the field of low-temperature physics.
Reference links：
[1]https://pubs.acs.org/doi/10.1021/acs.jpclett.2c01478#
[2]https://phys.org/news/2022-09-mini-magnets-quantum-anomalous-hall-effect.html
