Physicists demonstrate behavior of insulating material that transports energy
These experiments expand our knowledge of quantum physics and pave the way for applications in computing and energy-efficient sensors
Laboratory experiments have confirmed that quantum topological insulating materials can conduct electric current and store information without energy dissipation – Photo: Kahvilokki / Wikimédia Commons
“Although the general behavior of electrons is not yet fully understood, recent discoveries show that electrons exhibit properties beyond mere particle behavior”, explained professor Julio Larrea, who coordinated the research, in an interview with Jornal da USP. “In the quantum regime, electrons exhibit not only particle-like behavior, but also an energy manifestation known as low-energy excitations resulting from quantum fluctuations. This is a striking difference from classical physics, where electrons are described as particles with mass and electric charge that are influenced solely by thermal fluctuations.”
Larrea continued, “Another notable difference is spin [the electron’s angular momentum], which represents a purely quantum manifestation that causes the electron to adopt two additional quantum states, spin-up and spin-down, compared to classical physics.”
New quantum states can arise from the coupling of low-energy excitations and spin through interactions between them or influence by specific symmetries. “This can produce very robust quantum states protected by peculiar symmetries, leading to a new type of material: quantum topological materials.”
In a three-dimensional material, a topological insulator is expected to behave as an insulator within its volume but as a conductor on its surface. This is unlike a conventional (non-topological) insulator, which is solely insulating. Furthermore, he emphasized that “in a topological insulator, transport at the surface involves exotic, low-energy quasiparticles moving in the same direction or spin. Both are massless, allowing for the transport and storage of energy and information while significantly reducing dissipative processes.”
The results of the experiments are described in an article published in the journal Nature Communications.
Energy transport
Larrea stated that the manifestation of low-energy excitations and spin plays a fundamental role in the physical properties that define quantum materials, a new branch of physics. “In the classical model, a good conductor, such as copper, is defined as a medium that transports electrons under the influence of an electric field or potential difference”, he explained.
“Since the particles have mass, the drag velocity of the electrons causes them to collide, leading to a dissipative process of energy loss during transport. Consequently, the conductor heats up, representing a loss of efficiency in electrical current transport, a phenomenon known as the material’s electrical resistivity.”
On the other hand, he explained that at the quantum limit, the transport situation can unfold quite differently, reducing dissipative processes. Transport can occur not only via charge but also via other quantum degrees of freedom, such as electron spin, or via low-energy excitations that can form different types of quasiparticles. “In both cases, since these transport agents have no mass, transport reduces losses due to dissipative processes. Therefore, transport can expand its functionality beyond the transport of electric current to encompass the transport of information or energy”, the professor pointed out.
According to Larrea, the Landau levels provide information on how electrons behave at the quantum level. These are orbits where electrons are confined by the presence of a magnetic field. The electrons exhibit circular or spiral motion, also called cyclotronic motion, with discrete energies that result from the interaction between the charge and the magnetic field at the quantum limit. “Although there is a strong desire to access the Landau levels to reveal the manifestation of quantum phenomena associated with electrons, their experimental measurement remains challenging”, he reported.
This is because measurements of physical properties are performed on innovative platforms known as home-built setups. To reach the quantum limit, these setups must operate under extreme conditions of extremely low temperatures—close to absolute zero—and very high magnetic fields. “One of the few available experiments for this purpose measures quantum oscillations in electrical resistance as a function of the magnetic field, also known as magnetoresistance, in Shubnikov-de Haas (SdH) experiments.”
To uncover experimental evidence of the quantum topological insulating state, SdH experiments were conducted on the ZrTe5 crystal (zirconium pentatelluride) under extreme conditions of very low temperatures (down to 0.5 K) and high pulsed magnetic fields (up to 65 T) at the High Magnetic Field Laboratory (HMFL) in Los Alamos, New Mexico. “These experiments are extremely complex and can only be conducted in specialized, state-of-the-art laboratories known as Big User Facility Labs, such as the HMF”, the professor noted. “SdH measurements at different temperatures reveal oscillations in the material’s electrical resistance that are inversely proportional to the magnetic field. Additionally, to obtain information on the amplitudes of these oscillations, SdH measurements were taken at different crystal orientations relative to the magnetic field.”
Basic knowledge
“The results of the SdH measurements at very low temperatures reveal anomalous quantum oscillations. This is an unexpected result compared to previous findings in ZrTe5 and other topological insulators, where periodic quantum oscillations have been reported”, Larrea noted. “Furthermore, we observed that the amplitudes of the oscillations do not decrease with increasing temperature as expected. Instead, the amplitudes reach a minimum at a certain temperature and then increase at higher temperatures.”
To interpret these unusual results, the research team developed a theoretical model that does not account for electron-electron interaction, also known as a single-particle model. “In this case, the electron’s spin plays a fundamental role because the up and down states split in the presence of a magnetic field, a phenomenon known as the Zeeman effect”, he explained. “When strong magnetic fields are applied, interaction with spin profoundly alters the electrons’ energy. As a result, Landau levels that would normally move away from the system’s relevant energy can ‘return’ and cross it again. This unusual behavior is what we call ‘re-entering Landau levels.’”
“Simply put, the energy of these levels does not evolve monotonically with the magnetic field. Instead, they can bend and cross the Fermi level again, carrying with them the nature of the spin interactions”, Larrea pointed out. As a result, this mechanism is responsible for new, non-periodic oscillations that are not predicted by conventional theory.
He stated that the research considers two coexisting quantum phenomena with different energy scales: the energy of the Landau levels, which is associated with the orbital motion of electrons in a magnetic field, and the energy of the Zeeman effect, which results from the interaction between the magnetic field and the electrons’ spin.
“Our results in ZrTe₅ demonstrate that, in materials with a strong spin-orbit interaction, these phenomena cannot be treated separately”, he said. He also added that these manifestations of electrons suggest ZrTe₅ is close to a topological transition. This opens new avenues for discovering topological phases by modifying electronic structures and symmetries under extreme conditions.
“Our experimental results and their theoretical interpretation advance our understanding of quantum topological states and resolve major controversies regarding the topological insulator state of ZrTe5. From a basic scientific perspective, ZrTe₅ is an ideal platform for investigating which electron behaviors stabilize different quantum topological phases, regardless of the interaction between quantum particles”, Larrea emphasized. “For example, by modifying the topology of the crystal structure, the local crystal symmetry, and the electron’s spin-orbit interaction, we can alter the electronic structure and lead to a new topological phase known as the Weyl semimetal.”
Applications
Regarding applications, the professor affirmed that topological materials hold great promise for developing new quantum technologies. “They are breeding grounds for quasiparticles with low-energy, massless excitations, electronic structures that are protected against small, atomic-scale deformations, and robust spin and orbital states with lossless transport, among other things”, he stated.
These properties are essential for developing new devices for quantum information and computing, high-resolution quantum sensors, and lossless, sustainable energy transport, among other applications.
Former Ph.D. student Cauê Kaufmann conducted the experiment under the guidance of professor Julio Larrea, head of the IF’s Laboratory for Quantum Matter under Extreme Conditions (LQMEC). Larrea reported, “Kaufmann developed the technical skills and expertise needed to assemble electrical contacts on very small, needle-shaped crystals that are millimeter-long and fragile to handle”. Later, Kaufmann participated in an exchange program during his Ph.D. studies to conduct SdH experiments on ZrTe5 crystals under the joint supervision of Los Alamos HMF researchers Johanna Plastrom and Sean Thomas, with funding from the São Paulo Research Foundation (Fapesp). The article Reentrant Landau levels in a Dirac topological insulator, published in Nature Communications, can be accessed at this link.
More information: email larrea@if.usp.br, with professor Julio Larrea
*Intern under the supervision of Simone Gomes
English version: Nexus Traduções, edited by Denis Pacheco
A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.