It was discovered that at ambient temperature, graphene has a higher magnetoresistance than any other known material. This property could help develop new magnetic sensors and shed light on the physics of unconventional metals.
Twenty years after its discovery, graphene can be expected to have exhausted all its surprises. But the most conductive, strongest and thinnest material has now set another record. According to a team led by Nobel Prize-winning graphene discoverer Andre Geim of the University of Manchester in England, graphene has 100 times more magnetoresistance at room temperature than any other material.
The enormous magnetic resistance of graphene
Graphene's enormous magnetic resistivity may enable the development of new magnetic field sensors, while also providing experimental insight into unique quantum regimes of electrical conduction that may be linked to mysterious "strange metals."
Magnetic field sensors, such as those used to read data from magnetic memory, are an excellent use case for magnetoresistance, which can be found in both bulk materials and multilayer structures. The limits of this phenomenon have long been of interest to scientists, and as a result "giant", "enormous" and "extraordinary" types of magnetoresistance have been found. When bonded materials are exposed to magnetic fields with many teslas (T), their resistance can vary by up to one million percent. However, extremely low temperatures are needed to achieve the best results, which can only be achieved with liquid helium cooling systems, which is not possible.
The magnetoresistance process is what causes this temperature restriction. Magnetic fields affect the resistance of a material by changing the path taken by current-carrying electrons. Therefore, the ability of electrons to move freely from atoms in matter without being constantly scattering is essential for great effect. In other words, electrons must have a lot of "mobility" for the field to have a noticeable effect on their path. Since the mobility decreases with increasing temperature, the magnetoresistance is usually negligible at ambient temperature.
Thus, a suitable target was identified in graphene, which has the highest mobility reported for a material at ambient temperature. But according to lead author Alexey Berdyugin of the National University of Singapore, electron mobility is insufficient to achieve a high magnetoresistance. Graphene has a low magnetoresistance in most cases because it behaves like a metal in which the current is carried by a single type of carrier - the electron. It is known that the magnetoresistance in a metal rapidly saturates with the magnetic field; hence a strong magnetic field has little effect on resistance.
The conduction and valence bands "contact" in the "semimetal" state of graphene, which Berdyugin, Geim and colleagues created to prevent this saturation. The “charge neutrality point” is the temperature at which both positive charges (holes) and negative charges (electrons) can carry current in a semimetal. Resistance changes caused by a magnetic field are not balanced by two carriers of opposite polarity; instead, it continues to scale with the square of the field strength. According to Berdyugin, “We discovered that graphene can meet all conditions at room temperature.
Berdyugin, Geim and colleagues were able to position their device at the charge neutrality point, using a high-quality graphene sheet and using a voltage to adjust the position of the valence and conduction bands. The researchers reported that when applying a very weak magnetic field of 100 mT they achieved a magnetoresistance as high as 100%, an improvement over 100 times the natural magnetoresistance found in any known material.
“Graphene continues to amaze!” said Frank Koppens, an experimental physicist at the Institute of Photonic Sciences in Spain. she exclaims. He claims that this unusual behavior is both practical and fundamentally interesting. The result could produce highly sensitive magnetic sensors, according to Harvard University condensed matter researcher Philip Kim.
According to Berdyugin, graphene has a slightly lower magnetoresistance than the magnetoresistive components used in modern computers. (The magnetoresistance of these devices is an "extrinsic" property resulting from spin tunneling between various layers of material rather than an "intrinsic" material property). But unlike such technologies, he claims, graphene can continue to operate at much higher temperatures, opening the door to new uses.
When the magnetic field was further amplified, the researchers also looked at how the materials responded. They discovered that the quadratic scaling of the resistivity changes to a linear scaling when the field grows up to the 1-T scale. While the transition to quadratic linear scaling suggests a transition to an exotic quantum conduction regime, Berdyugin argues that further research is needed to establish a microscopic theory for this phenomenon.
In this regime where the orbits of the charged particles in the magnetic field are quantized, all particles simultaneously exist at the zero energy level of these quantized states.
According to Berdyugin, this "quantum semimetal" regime shares many properties with strange metals, a family of materials that defy conduction theories that are considered metallic at low temperatures and superconducting at high temperatures. Both systems have “Planckian” electron scattering, which means that the scattering time scale is restricted only by the Heisenberg uncertainty principle and has magnetoresistance that scales linearly with the applied field. According to Berdyugin, the quantum regime of graphene can be used as a model system to investigate the physics associated with unusual metals. Who agrees. Comparison with strange metals is quite reasonable.
Source: physics.aps.org/articles/v16/64
Günceleme: 19/04/2023 09:22