We present a theory of SNS junctions, where a normal metal is in contact with two superconductors, along the crossover from the BCS to the BEC regime. Such systems can be realized experimentally by gate-controlled doping of thin film superconductors. We show that Andreev levels exist even in the BEC regime, as long as there is sufficient density of states in the normal metal. We derive the Josephson current through one-dimensional Sharvin contacts. We find that the Josephson current at zero temperature has no peak along the BCS-BEC crossover at $T=0\phantom{\rule{4.pt}{0ex}}\text{K}$ unless the order paramter $\mathrm{\ensuremath{\Delta}}$ is peaked, as it is the case for superconductors with one-dimensional density of states. At finite temperature, however, we find that the Josephson current is maximal at the BCS-BEC crossover for any dimension. We identify the mechanism for this phenomenon to be a decrease in the number of Andreev levels, accompanied by an increase in excitation energy to the unoccupied levels, along the BCS-BEC crossover. This makes it less likely that these states are thermally occupied. As a consequence, the Josephson current is more strongly reduced on the BCS side of the crossover, resulting in a maximal Josephson current at the BCS-BEC crossover. Furthermore, we find that the current phase relation changes along the BCE-BEC crossover with a slope which has a pronounced maximum on the BEC-side of the BCS-BEC crossover. This maximum becomes larger and sharper as the temperature is lowered.We present a theory of SNS junctions, where a normal metal is in contact with two superconductors, along the crossover from the BCS to the BEC regime. Such systems can be realized experimentally by gate-controlled doping of thin film superconductors. We show that Andreev levels exist even in the BEC regime, as long as there is sufficient density of states in the normal metal. We derive the Josephson current through one-dimensional Sharvin contacts. We find that the Josephson current at zero temperature has no peak along the BCS-BEC crossover at $T=0\phantom{\rule{4.pt}{0ex}}\text{K}$ unless the order paramter $\mathrm{\ensuremath{\Delta}}$ is peaked, as it is the case for superconductors with one-dimensional density of states. At finite temperature, however, we find that the Josephson current is maximal at the BCS-BEC crossover for any dimension. We identify the mechanism for this phenomenon to be a decrease in the number of Andreev levels, accompanied by an increase in excitation energy to the unoccupied levels, along the BCS-BEC crossover. This makes it less likely that these states are thermally occupied. As a consequence, the Josephson current is more strongly reduced on the BCS side of the crossover, resulting in a maximal Josephson current at the BCS-BEC crossover. Furthermore, we find that the current phase relation changes along the BCE-BEC crossover with a slope which has a pronounced maximum on the BEC-side of the BCS-BEC crossover. This maximum becomes larger and sharper as the temperature is lowered.

arXiv:2411.19396Physical review. B./Physical review. BApr 25, 20251 citation

Superconductivity of the FeSe/SrTiO3 Interface in View of BCS–BEC Crossover*

Shuyuan Zhang, Guangyao Miao, Jiaqi Guan +6 more

In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen–Cooper–Schrieffer (BCS) and the Bose–Einstein condensation (BEC) limits. The concept of the BCS–BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here by analyzing the typical superconductors within the BCS–BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS–BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of single-layer FeSe on SrTiO 3 substrate, the superconductivity is relocated closer to the crossover unitary than other doped FeSe-based materials, indicating that the pairing interaction is effectively modulated. We further show that hole-doping can drive the interfacial system into the phase with possible pre-paired electrons, demonstrating its flexible tunability within the BCS–BEC crossover regime.In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen–Cooper–Schrieffer (BCS) and the Bose–Einstein condensation (BEC) limits. The concept of the BCS–BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here by analyzing the typical superconductors within the BCS–BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS–BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of single-layer FeSe on SrTiO 3 substrate, the superconductivity is relocated closer to the crossover unitary than other doped FeSe-based materials, indicating that the pairing interaction is effectively modulated. We further show that hole-doping can drive the interfacial system into the phase with possible pre-paired electrons, demonstrating its flexible tunability within the BCS–BEC crossover regime.

arXiv:1908.11126Chinese Physics LettersOct 1, 20199 citations

BCS-BEC Crossover

Peking University-World Scientific advanced physics series/Peking University-World Scientific advance physics seriesNov 1, 20180 citations

フェルミ原子ガスの新しい超流動 : BCS-BECクロスオーバーの実現(最近のトピックス)

洋士大橋

日本物理學會誌Apr 5, 20040 citations

BCS–BEC CROSSOVER IN RELATIVISTIC FERMI SYSTEMS

LIANYI HE, SHIJUN MAO, PENGFEI ZHUANG

We review the BCS–BEC crossover in relativistic Fermi systems, including the QCD matter at finite density. In the first part, we study the BCS–BEC crossover in a relativistic four-fermion interaction model and show how the relativistic effect affects the BCS–BEC crossover. In the second part, we investigate both two-color QCD at finite baryon density and pion superfluid at finite isospin density, by using an effective Nambu–Jona–Lasinio model. We will show how the model describes the weakly interacting diquark and pion condensates at low density and the BEC–BCS crossover at high density.We review the BCS–BEC crossover in relativistic Fermi systems, including the QCD matter at finite density. In the first part, we study the BCS–BEC crossover in a relativistic four-fermion interaction model and show how the relativistic effect affects the BCS–BEC crossover. In the second part, we investigate both two-color QCD at finite baryon density and pion superfluid at finite isospin density, by using an effective Nambu–Jona–Lasinio model. We will show how the model describes the weakly interacting diquark and pion condensates at low density and the BEC–BCS crossover at high density.

arXiv:1311.6704International Journal of Modern Physics ADec 27, 201322 citations

T=0\phantom{\rule{4.pt}{0ex}}\text{K}

unless the order paramter $\mathrm{\ensuremath{\Delta}}$ is peaked, as it is the case for superconductors with one-dimensional density of states. At finite temperature, however, we find that the Josephson current is maximal at the BCS-BEC crossover for any dimension. We identify the mechanism for this phenomenon to be a decrease in the number of Andreev levels, accompanied by an increase in excitation energy to the unoccupied levels, along the BCS-BEC crossover. This makes it less likely that these states are thermally occupied. As a consequence, the Josephson current is more strongly reduced on the BCS side of the crossover, resulting in a maximal Josephson current at the BCS-BEC crossover. Furthermore, we find that the current phase relation changes along the BCE-BEC crossover with a slope which has a pronounced maximum on the BEC-side of the BCS-BEC crossover. This maximum becomes larger and sharper as the temperature is lowered.