Recently, the research team of Argonne National Laboratory reported a new two-dimensional superconducting system with oxide interface. Liu Changjiang, a postdoctoral researcher, found that the two-dimensional electron gas formed on the surface of single crystal oxide KTaO3(KTO) was superconducting around 2.2 K. The critical temperature of superconductivity was 10 times higher than that of interface system based on strontium titanate (STO). This temperature range can be achieved without diluting the refrigeration equipment, which provides great convenience for the experimental research and development and utilization of two-dimensional superconducting electronic devices in the future. At the same time, it is observed that KTO interface superconductivity has some unusual strange properties, which provides a new platform for the study of two-dimensional superconductivity mechanism. 202 1, 65438+ 10, 2 1, and the research work was first published in the online magazine Science, entitled "Two-dimensional superconductivity and animal transport at the interface of ktao 3 (1 1)".
Figure 1, superconductivity is observed in EUO/KTO (11) samples.
There are many similarities between KTO and STO, two classical perovskite systems. They are all band gap insulators, and the dielectric constant will increase a lot with the decrease of temperature. Ta is a 5d group element in KTO, which has great spin-orbit coupling (one order of magnitude larger than STO), so it has greater spin splitting in the energy band of KTO. Figure 1 is the superconducting transition measured at the KTO interface in the experiment. Four EuO/KTO samples have different carrier concentrations and corresponding superconducting critical temperatures.
Fig. 2 shows that superconductivity is also observed on the surface of Lao/KTO (11).
It is found that the formation of superconducting state has nothing to do with the type of oxide film grown on KTO surface. In this study, EuO and LaAlO3 (LAO) oxide coatings were used to generate two-dimensional electron gas. It is found that superconducting phase transition can also occur in two-dimensional electron gas by using LAO oxide layer (Figure 2). This result has brought great convenience to experimental research. Researchers can choose suitable coating materials according to the film growth equipment.
Fig. 3, schematic diagram of two-dimensional superconductivity
By measuring the in-plane and out-of-plane critical magnetic field of superconducting state and the BKT phase transition under critical current, the researchers confirmed that the superconducting state at KTO interface is completely two-dimensional. Fig. 3 shows an image of two-dimensional superconductivity. In the experiment, the thickness of superconducting state obtained by a sample is about 5 nm, which is less than its superconducting coherence length 13 nm.
Figure 4. Characterization of atomic structure near EuO/KTO interface
Further research shows that the superconductivity of KTO interface has some unusual characteristics. Firstly, the formation of superconducting state is very dependent on the crystallographic orientation of KTO surface. It is found that the electron gas on KTO (11) plane can undergo superconducting phase transition, but when two-dimensional electron gas is prepared on (00 1) plane, (the carrier concentration is the same as (11). This crystallographic orientation dependence phenomenon is very different from that of Lao /STO system. In Lauder /STO system, the appearance of interfacial superconductivity has nothing to do with the crystal plane orientation of STO. Researchers used scanning transmission electron microscopy (Figure 4) and Electron Energy Loss Spectroscopy (EELS) to characterize the structure and chemical composition of different KTO interfaces. The results show that oxygen vacancies are formed in several atomic layers of KTO interface, and some Eu or La elements appear at the K atom position. These doping processes promote the formation of cruise electrons at the interface. When comparing (11) and (00 1) oriented samples, the researchers found that their interface elements were similar. Therefore, the appearance of superconductivity has little to do with the interface chemical elements.
Structurally, Ta atoms on KTO (11) are arranged in a hexagonal honeycomb, similar to graphene. The electron gas on the surface of KTO (11) has a unique fermi surface with six symmetries, which was discovered by ARPES. Perhaps the superconductivity found on KTO (11) plane is related to this special electronic band structure. For example, the superconductivity of 20 18 in corner double-layer graphene is closely related to the energy band structure at the magic corner. At present, the mechanism of KTO interface superconductivity needs further theoretical research and demonstration.
Fig. 5, spontaneous symmetry breaking of in-plane transport.
Another strange phenomenon is the spontaneous in-plane symmetry breaking of the transport properties of EUO/KTO (11) samples. This phenomenon is especially obvious in samples with low carrier concentration. Fig. 5 shows that when the temperature drops from high temperature to about 2.2 K, the resistances in the in-plane directions [1-1 0] and [1-2] are very different. Specifically, the resistance (red data) measured along the [1-1 0] crystal direction current suddenly increases, while the resistance along the [1 1 -2] direction spontaneously decreases. In a temperature range (green background), the system has great anisotropy. Phenomenologically, this phenomenon can be understood as a stripe phase modulation structure formed spontaneously by electronic states on the (11) plane. There is a great difference in resistance between parallel stripes and vertical stripes. When the temperature continues to decrease, the resistance along the two crystal directions eventually drops to zero and enters a completely superconducting state.
Fig. 6, schematic diagram of anisotropic superconductivity
It is found that the fringe phase will eventually be suppressed with the increase of external magnetic field and an isotropic normal state will be obtained. At present, the mechanism of spontaneous symmetry breaking in this plane is not clear. Fig. 6 depicts a possible physical image. The superconducting coherence intensity on KTO (11) plane may be anisotropic. Along the side of armchair ([1 1 -2]) (vertical direction in the figure), superconductivity has stronger coherence. It is indicated by light blue stripes in the figure. In this way, before the system forms a global superconducting state, there will be very different transport properties along the stripes and vertical stripes. Researchers believe that the mechanism of stripe phase needs more experimental research, such as using local electronic structure and magnetic detection technology (such as STM, nano-ARPES or scanning SQUID) to observe whether there is a modulated superconducting energy gap or magnetic moment distribution in superconducting electron gas in space. In addition, it is worth noting that recent theoretical calculations have predicted that the KTO (11) plane is a good platform for realizing a possible topological superconducting state. The follow-up related research work is very worthy of attention.
This work was completed by some scientists from research groups of several research institutions. Including Dr. Liu Changjiang (first correspondent), Dr. Yan Xi (same work), Dr. Dillon Fong and Dr. Anand Bhattacharya (correspondent) from the Department of Materials Science of Argonne National Laboratory. In addition, Dr. Advanced Synchrotron Radiation Source (APS) in the United States, Dr. Jin, Dr. Wen Jianguo, Dr. Zhou and Dr. Lin Yulin from the National Center for Nanomaterials, Professor Zuo Jianmin from the University of Illinois at Urbana-Champaign, researcher Sun from the Institute of Physics of the Chinese Academy of Sciences and professor from the Peking University Center for Quantum Materials also participated in the project.
Paper link:
https://science . science mag . org/content/early/202 1/0 1/2 1/science . ABA 55 1 1？ rss= 1