[论文解读] Interlayer Coupling Driven High-Temperature Superconductivity in La$_3$Ni$_2$O$_7$ Under Pressure

Lu Chen, Zhiming Pan+2 more|arXiv (Cornell University)|Jul 27, 2023

Magnetic and transport properties of perovskites and related materials被引用 20

一句话总结

论文在La3Ni2O7受压条件下推导出双层t-J模型，显示层间耦合J_perp驱动从d波配对向层间s波配对转变，并可显著提高Tc，同时可能存在s+id的TRSB态。

ABSTRACT

The newly discovered high-temperature superconductivity in La$_3$Ni$_2$O$_7$ under pressure has attracted a great deal of attentions. The essential ingredient characterizing the electronic properties is the bilayer NiO$_2$ planes coupled by the interlayer bonding of $3d_{z^2}$ orbitals through the intermediate oxygen-atoms. In the strong coupling limit, the low energy physics is described by an intralayer antiferromagnetic spin-exchange interaction $J_{\parallel}$ between $3d_{x^2-y^2}$ orbitals and an interlayer one $J_{\perp}$ between $3d_{z^2}$ orbitals. Taking into account Hund's rule on each site and integrating out the $3d_{z^2}$ spin degree of freedom, the system reduces to a single-orbital bilayer $t$-$J$ model based on the $3d_{x^2-y^2}$ orbital. By employing the slave-boson approach, the self-consistent equations for the bonding and pairing order parameters are solved. Near the physically relevant $\frac{1}{4}$-filling regime (doping $δ=0.3\sim 0.5$), the interlayer coupling $J_{\perp}$ tunes the conventional single-layer $d$-wave superconducting state to the $s$-wave one. A strong $J_{\perp}$ could enhance the inter-layer superconducting order, leading to a dramatically increased $T_c$. Interestingly, there could exist a finite regime in which an $s+id$ state emerges.

研究动机与目标

• 理解La3Ni2O7在压力下高Tc超导的理解动机并识别层间耦合的作用。
• 开发一个强相关的双层模型，包含Ni 3d轨道之间的Hund规则耦合。
• 展示层间耦合可将配对对称性从d波转变为层间s波并提升Tc。
• 预测在中间参数区间可能存在TRSB的s+id相。
• 将理论参数与在压力及电子/空穴掺杂可能性下的实验情境联系起来。

提出的方法

• 为Ni基单元构建一个两轨道双层t-J-J_H模型，H = H_parallel + H_perp。
• 简并出3d_z^2自旋自由度，得到3d_x2-y2自旋之间的有效层间S=1/2交换J_perp。
• 应用 slave-boson 平均场理论将自旋子(spinon)与空洞子(holon)解耦，空洞子凝聚给出来自RVB配对的Tc。
• 引入并自洽求解层内和层间键结与配对有序参量（χ和Δ），包括Δ_z和Δ_x/y。
• 作为填充x和比值J_perp/J_parallel对基态配对的变化，分析s波、d波和s+id区域。
• 将现实的LNO参数（J_perp/J_parallel ~ 1.75）与层间s波配对的出现及Tc提升联系起来。

Figure 1: (a). Schematic diagram for the two-orbital bilayer $t-J-J_{H}$ model. The charge carriers reside on the $3d_{x^{2}-y^{2}}$ orbitals, with intra-layer hopping $t$ and spin exchange $J_{\parallel}$ . The $3d_{z^{2}}$ orbital is a singly-occupied spin, with inter-layer spin exchange $J_{\perp

实验结果

研究问题

• RQ1 层间耦合J_perp是否在受压下增强La3Ni2O7的超导性？
• RQ2 J_perp/J_parallel如何影响双层t-J模型中的配对对称性？
• RQ3 Hund规则耦合是否将层间相互作用传导到有效的层间配对通道？
• RQ4 是否存在使TRSB的s+id配对出现的参数区？
• RQ5 现实的LNO参数对压力下Tc和配对对称性有何含义？

主要发现

• 层间耦合J_perp将在相关掺杂区将单层d波配对转变为层间s波配对。
• Tc随J_perp/J_parallel的增加而显著提高，当J_perp > J_parallel时，与压力下的高Tc一致。
• 对于现实的LNO参数（J_perp/J_parallel ≈ 1.75），配对为层间s波，Δ_z主导Δ_x/y。
• 参数空间中的一个狭窄区域存在一个位于s-wave与d-wave之间的TRSB的s+id配对态。
• 预测电子掺杂将进一步提升Tc，而顶端氧空位缺陷（降低J_perp）会抑制超导性。

Figure 2: (a) Ground state phase diagram with respect to the filling $x$ and $J_{\perp}/J_{\parallel}$ with $J_{\parallel}=0.4t$ . The inset shows the pairing order parameters. At point I (0.28,1.4): $\Delta_{z}=3.1\times 10^{-3}$ , $\Delta^{(1,2)}_{x}=\Delta^{(1,2)}_{y}=-1.6\times 10^{-4}$ . At poi