Chinese team makes new breakthrough in high-temperature superconductivity!
Recently, Professor Jian Wang's group at the Center for Quantum Materials Science, School of Physics, Peking University, in collaboration with Professor Ziqiang Wang at Boston College and Associate Researcher Liao Zhang at Shanghai University, discovered intrinsic paired density waves in two-dimensional iron-based high-temperature superconductor Fe(Te,Se) films.
This work is the first time that intrinsic pairing density waves are observed in iron-based superconductors and provides a new platform for studying pairing density waves in unconventional superconductors, unconventional high-temperature superconductivity, and topological electronic states in low-dimensional high-temperature superconductivity.
Superconductors are materials that exhibit zero resistance when cooled to temperatures of only a few Kelvin.
As a macroscopic quantum state, superconductors possess unique physical properties such as zero resistance and complete antimagnetism, and have important applications in many fields such as medicine, information, transportation, energy, and quantum science and technology, and have received extensive attention from academia and industry for more than a hundred years.
-- However, to exploit the extraordinary properties of superconductors for practical applications (e.g., in energy transmission and electronics), materials that are superconducting at higher temperatures are required.
The BCS (Bardeen-Cooper-Schrieffer) microscopic theory of superconductivity states that superconductivity originates from the coherent condensation of zero-mass-momentum Cooper pairs (Cooper pairs) formed by the union of two electrons with opposite spin and momentum. However, when the time-reversal symmetry is broken (e.g., by applying a strong magnetic field), two electrons with opposite spins in a superconductor can theoretically form Cooper pairs with non-zero center-of-mass momentum, and the corresponding superconducting order parameters exhibit periodic modulation (fluctuations) in real space - the well-known FFLO (Fulde-Ferrell-Larkin-Orbitals). Ferrell-Larkin-Ovchinnikov) state.
Although the FFLO state was proposed as early as 1964, the realization of this particular superconducting state has very demanding material and experimental conditions. Until today, direct evidence of the existence of the FFLO state (e.g., periodic fluctuations of the superconducting order parameter in real space) is still difficult to obtain experimentally.
Figure 1
Further energy-dependent experiments show that the periodic modulation of the local-domain density of states is mainly present in the superconducting energy gap, suggesting a correlation between this charge order and the superconducting states of the film.
Subsequently, the group further carried out systematic measurements of the possible paired density waves at the domain boundaries. As shown in Fig. 2, the group observed the spatial modulation of the superconducting coherence peak height (Fig. 2a-c) and the superconducting energy gap (Fig. 2d-g) at the domain boundaries with the same period of 3.6 aFe. These two physical quantities are directly related to the superconducting order parameter, thus providing direct experimental evidence of paired density waves in two-dimensional iron-based high-temperature superconductors.
Figure 2
After confirming the existence of paired density waves at the domain boundary of the monolayer Fe(Te,Se) film, the group further obtained evidence of charge density waves with a period of 1.8aFe (half of the period of the paired density waves) at the domain boundary. Through in-depth analysis of the spatial phase distribution of the paired density wave (with a period of 3.6aFe) and the charge density wave (with a period of 1.8aFe), the π-phase variation position in the spatial phase distribution of the paired density wave (shown by the arrow in Fig. 3f) and the vortex position in the phase distribution of the charge density wave (shown by the black dots in Figs. 3e and 3f) are confirmed to have an obvious correspondence in real space. This correspondence is consistent with the features of the theoretically predicted intrinsic paired density wave sequence (Figs. 3a-b) and its induced second-order charge density wave sequence (Figs. 3c-d), confirming that the observed is an intrinsic paired density wave.
Figure 3
In order to explain the microscopic mechanism of the paired density waves at the domain boundaries, Wang and Zhang proposed the equal-spin pairing model based on the experimental results. Due to the existence of Rashba-Dresselhaus spin-orbit coupling caused by the spatial inversion symmetry breaking in the region where the domain boundary is located, the electrons of the same spin at the domain boundary can form a non-zero momentum Cooper pair, and then generate the intrinsic pairing density wave.
The results of the simulations based on this theoretical model are in good agreement with the experimental data, indicating that the paired density waves at the domain boundaries may have the topological superconductivity characteristics of spin triplet states.
The periodic modulation of the electron density is known as the pair density wave.
More than this study, four research groups have reported in Nature that pair density waves are actually more prevalent than previously thought: they observed these waves in three different materials.
Paired density waves in different superconductors
A powerful tool for finding paired density waves is called scanning tunneling microscopy (STM): a technique for visualizing quantum states in materials with atomic resolution. There are different ways to find pair density waves using STM:
- One approach is to look for superconducting features at low temperatures while observing another phase called a charge density wave, in which the concentration of charge changes periodically in the material. This is because the transition to the charge density wave phase is expected, which can persist at high temperatures.
- Another approach is to detect periodic changes in the "superconducting gap", which is a gap in the allowed energy of electrons in a material and is directly related to the density of Cooper pairs.
STM experiments have detected paired density waves in cuprates (materials composed of layers of copper oxide). However, it has been unclear to the academic community whether paired density waves are prevalent in most superconducting materials or only in a few of these specific materials. Theory suggests that they should be present in other materials so researchers continue to use STM to look for pair density waves in other superconductors.
One promising candidate material to host paired density waves is a class of superconductors made from iron-containing compounds. These materials are similar to cuprates in that they exhibit a variety of electronic states, including nematic (liquid crystalline) order and some type of streak-like pattern of magnetic order.
Based on this similarity, a result published in Nature at the same time studied the iron-based superconductor EuRbFe4As4, which contains europium (Eu), rubidium (Rb) and arsenic (As). This material becomes a superconductor when cooled to a temperature of about 37 Kelvin and also exhibits magnetic properties; the authors detected paired density wave order in this material.
Iron-based superconductors are layered materials, and some superconductors have higher temperatures when they exist as a single layer than when they exist as multiple layers. This result by Jian Wang and Ziqiang Wang investigates the appearance of paired density waves in a monolayer film containing iron and selenium or tellurium grown on strontium titanate. This film provides a two-dimensional platform to study interactions between states generated through interactions between electrons, as well as unconventional Cooper pairing in high-temperature superconductors.
Another result investigated uranium ditelluride. Researchers in this group had previously used STM to show a particular kind of superconductivity in this material, namely chiral superconductivity. Building on this finding, the authors revealed an unusual charge density wave order that propagates in three directions, is closely related to superconductivity, and is sensitive to magnetic fields.
These investigations represent a crucial step forward in the study of density waves, but they also raise several questions:
At the technical level, these observations rely heavily on atomically resolved imaging techniques at ultra-low temperatures. However, this approach only provides information about the structure and charge distribution of the material, making it challenging to distinguish between p-density waves and other types of density waves. Therefore, techniques other than STM are essential for further exploration of pair density waves in superconductors.
Finally, it is noteworthy that experimental studies of density waves are still at an early stage. The characteristics of such waves have been observed in various superconductors, but the ordered states that emerge from these waves, and the way they interact with other states in superconductors, remain largely unexplored. Developing an effective theoretical description of density waves has also proved challenging, and further work is needed in this direction.
The above studies provide ample motivation for this endeavor and reveal the various systems in which these fascinating waves may occur.
Reference links:
[1] https://www.nature.com/articles/d41586-023-01996-w
[2]https://www.nature.com/articles/s41586-023-06072-x
[3]https://www.nature.com/articles/s41586-023-06103-7#author-information
[4]https://www.nature.com/articles/s41586-023-06005-8
[5]https://mp.weixin.qq.com/s/sjK31bN0eacLEmww5-39Cw
