Acta Physica Sinica,2017,Vol 66,No. 03
【Abstract】 As one of the independent control parameters, pressure plays an important role in finding new phenomena, testing related theories and guiding the explorations for new superconductors. In this review article, we briefly review the progress achieved from high pressure studies on some main types of the iron pnictide superconductors, including 1111-type, 122-type, 111-type, 10-3-8 type and 112-type. A few typical results from high pressure studies are introduced in more detail, including the positive pressure effect on the superconducting transition temperature TC of 1111-type iron pnictide superconductors, which indicates a way to enhance the TC by using a smaller cation to replay La ion; the maximum TC of iron pnictide superconductors estimated by high-pressure studies on a series of 1111-type iron-based superconductors, etc. More importantly, high pressure studies on the parent compounds of iron pnictide superconductors clearly demonstrate that pressure can suppress the transition temperatures of magnetic order and crystal structure, and then drive a superconducting transition. Furthermore, many examples are given in this review to reveal how the magnetic order competes with superconductivity under pressure, which provides new constraints for the establishment of the theory on superconductivity. These high pressure results are expected to be helpful for the studies of high- TC superconductors and for the exploring of new superconductors.
Acta Physica Sinica,2017,Vol 66,No. 03
【Abstract】 Metallic hydrogen can be realized theoretically at high pressure, which suggests that it will be a room-temperature superconductor due to the high vibrational frequencies of hydrogen atoms. However, the metallic state of hydrogen is not observed in experiment at up to 388 GPa. Scientists have been exploring various new ways to achieve hydrogen metallization. Hydrogen-rich compounds can be metallized at much lower pressures because of chemical pre-compression. Moreover, because such materials are dominated by hydrogen atoms, some novel properties can be found after metallization, such as high Tc superconductivity. Therefore, hydrogen-rich compounds are potential high-temperature superconductors, and this method is also believed to be an effective way to metalize hydrogen, which has aroused significant interest in lots of fields, such as physics and material science. In a word, hydrogen-rich compounds are expected to become a new member of superconductor family: hydrogen-based superconductor. Very recently, the theoretical prediction and the successful experimental discovery of high-temperature superconductivity at 200 K in a sulfur hydride compound at high pressure have set a record, which inspires further efforts to study the superconductivity of hydrogen-rich compounds. The present review focuses on crystal structures, stabilities, interaction between atoms, metallization, and superconductivity of several typical hydrogen-rich compounds at high pressures. Furthermore, higher Tc superconductors can be expected to be found in hydrogen-rich compounds in the future.
Great Revolution in Maritime Transportation: Discussion on Preliminary Work and Operation Plan of Bridge/Tunnel Projects for Vacuum HTS Maglev Train at High Speed
Tunnel Construction,2018,Vol 38,No. 09
【Abstract】 The improvement in the operation speed of the transportation project means the progress and development of the construction technologies in transportation projects. In this paper, the following factors restricting the further improving of the speed of high-speed railway are analyzed: as the operation speed increases, the trains in the dense atmosphere are subject to the wind-induced resistance and various resistances caused by the friction between the wheels and the rails and by the irregularity of tracks, and the noise will also increase with a high power. Therefore, the economy and safety issues involved in the operation have become the main factors restricting the further improving of the speed of high-speed railway. A scheme of vacuum pipelines of underwater vacuum tunnel and/or subwater bridge is proposed in this paper: the pipelines and cars are sealed and vacuated to form the quasi-vacuum. The transportation system of the vacuum maglev train at ultrahigh speed with the HTS maglev technology can achieve the operation speed more than 4 times that of the existing HSR train (about 1 200 km/h). Key technologies for constructing the vacuum maglev tunnels/subwater bridges in respect of construction plans, vacuating and sealing, as well as maglev trains are introduced in this paper. The subjects to be further studied on the vacuum HTS maglev tunnels (pipelines) are analyzed from the aspects of technology, management, construction costs, operation expenses, candidate project, airtight materials for cars and evacuation in case of emergencies. It is recommended that “cross-sea maglev train in vacuum tunnel” should be developed in the eco-tourism project between the coastal cities and their neighboring islands and a series of necessary technical tests should be conducted during the trial operation, so as to obtain related experience. Based on the experience and lessons learned, the transportation system at ultrahigh speed may be implemented for strait-crossing projects in China. Finally, a brief introduction to the research on HSR trains at ultrahigh speed in countries such as China, the United States and the Netherlands is presented in this paper. The development of vacuum pipeline transportation can drive the development of China’s transportation modes in a faster, safer and more energy-efficient manner, facilitating the development of the fifthgeneration of transportation industry and its driving role in the social and economic development, and promoting the integration and progress of the economy of China or even the world at a higher speed.
Acta Physica Sinica,2018,Vol 67,No. 20
【Abstract】 Since the discovery of high-temperature superconductivity in cuprates, finding more unconventional superconductors and understanding their superconducting pairing mechanism has been an important theme in condensed matter physics. Recently, ternary Cr-based superconductors A2Cr 3As 3 ( A = K, Rb, Cs) and ACr 3As 3 ( A = K, Rb) were reported, which owned quasi-one-dimensional crystal structure, containing [(Cr 3As 3) −] ∞ linear chains. A2Cr 3As 3 belongs to P6 -m2 space group, and ACr 3As 3 crystallizes in a centrosymmetric structure with the space group P6 3/ m. Many experiments, such as nuclear magnetic resonance, London penetration depth, show that A2Cr 3As 3 is an unconventional superconductor. However, these A2Cr 3As 3 compounds are extremely unstable in air. Here, we study the superconducting gap of the air-stable RbCr 3As 3 single crystal, using ultralow-temperature thermal conductivity measurement. The resistivity of RbCr 3As 3 single crystal shows a superconducting transition temperature Tczero at 6.6 K. The normal-state resistivity data from 20 K to 8 K are fitted to p( T) = ρ0 + AT2, which gives a residual resistivity of ρ0 = 781 μΩ·cm. Then, the thermal conductivity of RbCr 3As 3 single crystal is measured at temperature down to 80 mK and in magnetic fields up to 9 T. In zero field, the residual linear term κ0/ T = 7.5 μW·K −2·cm −1 is observed, which is about 24% of its normal-state value, suggesting nodes in the superconducting gap. At low field, the κ0/ T of RbCr 3As 3 shows relatively faster field dependence than the single-gap s-wave superconductors. These results reveal that RbCr 3As 3 is likely an unconventional superconductor with superconducting gap nodes, although the exact superconducting gap symmetry and structure for this quasi-one-dimensional superconductor need further investigation.
Acta Physica Sinica,2018,Vol 67,No. 20
【Abstract】 We report on the observation of a superconducting gap of about 14–15 meV, significantly enlarged compared with the value of 2.2 meV for bulk FeSe, in monolayer FeSe film interfaced with MgO epitaxial on SrTiO 3(001) substrate by using the scanning tunneling microscopy. While the MgO exhibits the same work function as SrTiO 3 substrate, the gap magnitude is in coincidence with that of surface K-doped two-unit-cell FeSe film on SrTiO 3(001), suggesting that the interface enhanced superconductivity might be attributed to cooperation of interface charge transfer driven by band bending with interface electron-phonon coupling as discovered at FeSe/TiO 2 interfaces. On the other hand, the observation of such an enlarged superconducting gap, complementary to our previous transport observation of an onset superconducting transition temperature of 18 K in monolayer FeSe film on a bulk MgO substrate, implies that FeSe/MgO interface is likely to be a new interface high-temperature superconducting system, providing a new platform for investigating the mechanism of interface high temperature superconductivity.