Limits on superconductivity in flatland

doi:10.1126/science.abg1752

GSTDTAP > 气候变化

DOI	10.1126/science.abg1752
	Limits on superconductivity in flatland
	Mohit Randeria
	2021-04-09
发表期刊	Science
出版年	2021
英文摘要	The superconducting state, with dissipationless electrical current flow, has long been a source of fascination, with implications for fundamental questions like spontaneous symmetry breaking and the Higgs phenomena, as well as for applications ranging from magnets for magnetic resonance imaging machines to hardware for quantum computers. In the classic Bardeen-Cooper-Schrieffer (BCS) theory, electrons in a metal form pairs at a transition temperature ( Tc ) below which the system is superconducting. The low Tc of BCS superconductors is related to the small fraction of electrons that have their quantum states modified by pairing. One route to exploring a higher Tc is to find quantum materials in which a larger fraction of the electrons get involved in superconductivity. On page 190 of this issue, Nakagawa et al. ([ 1 ][1]) report experiments on superconductors in a new regime, where they come close to achieving the theoretically predicted Tc limit ([ 2 ][2]) in two dimensions (2D). Progress in understanding superconductivity beyond the BCS paradigm has been spurred in part by ultracold Fermi gases ([ 3 ][3]). Though seemingly different, the theoretical description of pairing is essentially the same for neutral Fermi atoms or charged electrons. States below the Fermi energy ( E F) in a noninteracting Fermi system are occupied, whereas those above E F are empty. BCS showed that a weak attraction between particles leads to the formation of pairs whose radius (ξ) is exponentially large compared to the interparticle spacing (1/ k F, the inverse Fermi wave vector). In the BCS limit, pairing and superconductivity occur at the same Tc , which is exponentially small compared to the temperature ( T F,) corresponding to E F. With increasing attraction, the pair size decreases, and a Bose-Einstein condensate (BEC) of tightly bound pairs forms in the extreme limit with ξ << 1/ k F. This BCS-to-BEC crossover ([ 4 ][4]) has been realized in Fermi gases, where the interaction between atoms can be precisely controlled ([ 3 ][3]). There is, however, no general way for tuning the interaction between electrons in a quantum material—hence the difficulty of observing a crossover in the solid state. Although it suffices to tune the attraction relative to E F, controlling E F in a metal is not easy either. Nakagawa et al. address this by doping the layered material ZrNCl with Li+ intercalation. Using ionic liquid gating, the authors varied the electron density by two orders of magnitude, tuned E F, and made a compelling case for driving the system to the BCS-BEC crossover regime. The authors measured the electron density, and thus k F, using the Hall effect and probed superconducting properties, including the pair size ξ (estimated from upper critical field measurements) and energy gap Δ (obtained from tunneling spectroscopy). The authors also presented clear evidence from tunneling for a pseudogap arising from preformed pairing that onsets at a temperature T * well above the superconducting Tc , a characteristic feature of the BCS-BEC crossover ([ 4 ][4]) (see the figure). ![Figure][5] THE BCS-BEC crossover in 2D The crossover regime from the BCS to BEC in a 2D Fermi gas has a Tc close to the theoretical limit ( Tc ≤ TF/8 for parabolic dispersion). The ratio of interparticle distance (1/ k F) to pair size (ξ) grows with increasing attraction between fermions. Pairs form below T * and superconductivity occurs below Tc . GRAPHIC: N. CARY/ SCIENCE BASED ON M. RANDERIA Exact upper bounds ([ 2 ][2]) on Tc were recently derived on the basis of general considerations for the Berezenskii-Kosterlitz-Thouless phase transition in 2D. For parabolic dispersion, Tc ≤ T F/8. From the known behavior of Tc in the BCS and BEC limits in 2D, the maximum Tc should be attained in the crossover regime and be close to the T F/8 bound. Nakagawa et al. show that their samples are in a 2D regime, the dispersion is parabolic, and, notably, their Tc comes close to saturating the 2D bound. The general 2D bounds ([ 2 ][2]) on Tc are also relevant for several exciting materials such as monolayer FeSe ([ 5 ][6]), magic-angle twisted bilayer ([ 6 ][7]), and trilayer graphene ([ 7 ][8]). These are all multiband superconductors with more complex electronic structure than the parabolic band of Li: ZrNCl, which makes comparison with theory less straightforward. It should be emphasized that there are no known absolute limits to the superconducting Tc . Room-temperature superconductivity was observed ([ 8 ][9]) in carbonaceous sulfur hydride at high pressures, and Tc may be ∼109 K in neutron stars. However, Tc in these systems is much smaller than T F. The highest known Tc / T F ratio is around 0.22 in the 3D BCS-BEC crossover ([ 4 ][4]) in ultracold Fermi gases. This value is higher than the 3D BEC result, which is known not to be an upper bound on Tc . Even though experiments suggest that there could be a limit on Tc / T F, establishing such a bound in 3D remains an open theoretical question ([ 2 ][2]). The experiments of Nakagawa et al. raise many other questions, such as the microscopic pairing mechanism that leads to Tc as high as 20 K in a 2D material with an electron density as low as 1020 per cubic centimeter. Determining why ZrNCl can be doped into a strongly interacting BCS-BEC crossover regime is also of clear importance. The authors' discovery opens the door to a fascinating new world of strongly interacting superconductivity in 2D materials. 1. [↵][10]1. Y. Nakagawa et al ., Science 372, 190 (2021). [OpenUrl][11][Abstract/FREE Full Text][12] 2. [↵][13]1. T. Hazra, 2. N. Verma, 3. M. Randeria , Phys. Rev. X 9, 031049 (2019). [OpenUrl][14][CrossRef][15] 3. [↵][16]1. W. Ketterle, 2. M. W. Zwierlein , Riv. Nuovo Cim. 31, 247 (2008). [OpenUrl][17] 4. [↵][18]1. M. Randeria, 2. E. Taylor , Annu. Rev. Condens. Matter Phys. 5, 209 (2014). [OpenUrl][19][CrossRef][20] 5. [↵][21]1. D. Huang, 2. J. E. Hoffman , Annu. Rev. Condens. Matter Phys. 8, 311 (2017). [OpenUrl][22] 6. [↵][23]1. Y. Cao et al ., Nature 556, 80 (2018). [OpenUrl][24][CrossRef][25][PubMed][26] 7. [↵][27]1. J. M. Park et al ., Nature 590, 249 (2021). [OpenUrl][28][CrossRef][29][PubMed][30] 8. [↵][31]1. E. Snider et al ., Nature 586, 373 (2020). [OpenUrl][32] Acknowledgments: M.R. acknowledges support from NSF Materials Research Science and Engineering Center grant DMR-2011876. 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领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/322078
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Mohit Randeria. Limits on superconductivity in flatland[J]. Science,2021.
APA	Mohit Randeria.(2021).Limits on superconductivity in flatland.Science.
MLA	Mohit Randeria."Limits on superconductivity in flatland".Science (2021).