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Pressure Induced Enhancement of Superconductivity in LaRu2P2.

Li B, Lu P, Liu J, Sun J, Li S, Zhu X, Wen HH - Sci Rep (2016)

Bottom Line: The ab-initio calculation shows that the superconductivity in LaRu2P2 at ambient pressure can be explained by the McMillan's theory with strong electron-phonon coupling.Detailed analysis of the pressure induced evolution of resistivity and upper critical field Hc2(T) reveals that the increase of Tc with pressure may be accompanied by the involvement of extra electron-boson interaction.This suggests that the Ru-based system has some commonality as the Fe-based superconductors.

View Article: PubMed Central - PubMed

Affiliation: National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.

ABSTRACT
To explore new superconductors beyond the copper-based and iron-based systems is very important. The Ru element locates just below the Fe in the periodic table and behaves like the Fe in many ways. One of the common thread to induce high temperature superconductivity is to introduce moderate correlation into the system. In this paper, we report the significant enhancement of superconducting transition temperature from 3.8 K to 5.8 K by using a pressure only of 1.74 ± 0.05 GPa in LaRu2P2 which has an iso-structure of the iron-based 122 superconductors. The ab-initio calculation shows that the superconductivity in LaRu2P2 at ambient pressure can be explained by the McMillan's theory with strong electron-phonon coupling. However, it is difficult to interpret the enhancement of Tc versus pressure within this picture. Detailed analysis of the pressure induced evolution of resistivity and upper critical field Hc2(T) reveals that the increase of Tc with pressure may be accompanied by the involvement of extra electron-boson interaction. This suggests that the Ru-based system has some commonality as the Fe-based superconductors.

No MeSH data available.


Theoretical calculations under different pressures.(a) Calculated values of Ru-P bond length (red line) and P-Ru-P angle (black line) in the RuP4 tetrahedron vs. pressure; An obvious enhancement of the Ru-P bond length (and reducing of the P-Ru-P angle) can be found at the pressure from 2 to 3.5 GPa. (b) The electronic band structure and DOS of LaRu2P2 (I4/mmm) with calculated lattice parameters at 2.5 GPa; The band lying on Fermi level between X and P (black) at 2.5 GPa are mainly contributed by P 3p(pz) and Ru 4d(dxz + dyz) orbit. Panel on the right shows three partial DOS which make main contributions to the total DOS. (c) Calculated electronic DOS near Fermi level vs pressure, presents an increasing of Ru 4d orbit and a dropping of P 3p orbit, corresponding to (a,d) Calculated phonon dispersions at 2 GPa; the size of the bubble represents the electron-phonon interaction magnitude; phonon DOS and the integral value of electron-phonon coefficient λ are also shown on the right panel.
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f6: Theoretical calculations under different pressures.(a) Calculated values of Ru-P bond length (red line) and P-Ru-P angle (black line) in the RuP4 tetrahedron vs. pressure; An obvious enhancement of the Ru-P bond length (and reducing of the P-Ru-P angle) can be found at the pressure from 2 to 3.5 GPa. (b) The electronic band structure and DOS of LaRu2P2 (I4/mmm) with calculated lattice parameters at 2.5 GPa; The band lying on Fermi level between X and P (black) at 2.5 GPa are mainly contributed by P 3p(pz) and Ru 4d(dxz + dyz) orbit. Panel on the right shows three partial DOS which make main contributions to the total DOS. (c) Calculated electronic DOS near Fermi level vs pressure, presents an increasing of Ru 4d orbit and a dropping of P 3p orbit, corresponding to (a,d) Calculated phonon dispersions at 2 GPa; the size of the bubble represents the electron-phonon interaction magnitude; phonon DOS and the integral value of electron-phonon coefficient λ are also shown on the right panel.

Mentions: In the following we try to get some insights based on our ab-initio calculations. The band structures and the Fermi surfaces at ambient pressure are presented in Fig.5a and in the supplementary information, they are consistent with the previously published calculated and experimental results2529. We relax the structure under different pressures and find a clear structural change upon pressure. As shown in Fig. 6a, the change of the Ru-P bond length within the Ru-P conducting layer exhibits an unusual step-wise feature at pressures of 2.0–3.5 GPa. This is accompanied by a slight closing of the P-Ru-P angle in the RuP4 tetrahedron and the slope for the change of the angle also varied a bit in the same pressure range. We can also find an obvious change on the electronic structures level. As we can see from Fig. 5a and Fig. 6b, four bands depicted with red, blue, pink and orange construct a complicated Fermi surface together. One band mostly contributed from Ru 4d(dxz + dyz) and P 3pz (red) moves upward with pressure and across the Fermi level at about 3.0 GPa, meanwhile, another band mainly consisting of Ru 4d(dxz + dyz) orbital (blue) moves downwards and the small wave-like feature near the N point also crosses the Fermi level at the same pressure range. This blue band opens a small tunnel in the Fermi surface after 3.0 GPa, as shown in Fig. 5b. This movement slightly reduces the slope of the bands across the Fermi level, which may be very essential to enhance the effective electron mass and induce a moderate correlation effect. From the electronic DOS in Fig. 6c, one can also see a clear variation from 2 to 3.5 GPa. The change of the Ru atom is mainly originated from its dxz + dyz orbitals while the change of the P atom is mostly coming from its 3pz orbital. Fig. 6d shows the phonon spectra, phonon linewidth, phonon density of states, Eliashberg function α2F, electron-phonon coupling (EPC) constant λ calculated for 2 GPa. It seems that a phonon mode near the N point have relatively large phonon linewidth and good contribution to the EP coupling. As shown in the Supplementary Information, this mode slightly goes soft with pressure and reach the lowest frequency at 2.5–3 GPa and goes harder afterwards. The EP coupling constant reaches a maximum value of 0.80 at about 2 GPa, which is somehow larger than that of the iron-arsenide system. As mentioned before, with all these refined structural parameters, the significant enhancement of Tc versus pressure in LaRu2P2 cannot be interpreted purely by the conventional electron-phonon coupling, extra electron-boson coupling may have been involved in the formation of superconducting pairing. In iron based superconductors, the antiferromagnetic spin fluctuations3031 have been argued to be the dominant role in inducing the pairing. Therefore the pressure can induce a sensitive change of superconducting transition temperature as well as the normal state properties. In the 1111 family of iron pnictide high-temperature superconductors REFeAsO1−xFx, the hydrostatic pressure seems to show a similarity between the trend of critical temperature vs hydrostatic pressure for different rare earth elements (RE)32. The authors of that paper argue that this may indicate the role of pressure on the competing interactions in 1111 iron pnictides. As far as we know, no investigations on successful chemical doping have been reported in the system LaRu2P2. If the extra electron-boson coupling is important to enhance the superconductivity, it would be very interesting to carry out more works with chemical doping in the present system.


Pressure Induced Enhancement of Superconductivity in LaRu2P2.

Li B, Lu P, Liu J, Sun J, Li S, Zhu X, Wen HH - Sci Rep (2016)

Theoretical calculations under different pressures.(a) Calculated values of Ru-P bond length (red line) and P-Ru-P angle (black line) in the RuP4 tetrahedron vs. pressure; An obvious enhancement of the Ru-P bond length (and reducing of the P-Ru-P angle) can be found at the pressure from 2 to 3.5 GPa. (b) The electronic band structure and DOS of LaRu2P2 (I4/mmm) with calculated lattice parameters at 2.5 GPa; The band lying on Fermi level between X and P (black) at 2.5 GPa are mainly contributed by P 3p(pz) and Ru 4d(dxz + dyz) orbit. Panel on the right shows three partial DOS which make main contributions to the total DOS. (c) Calculated electronic DOS near Fermi level vs pressure, presents an increasing of Ru 4d orbit and a dropping of P 3p orbit, corresponding to (a,d) Calculated phonon dispersions at 2 GPa; the size of the bubble represents the electron-phonon interaction magnitude; phonon DOS and the integral value of electron-phonon coefficient λ are also shown on the right panel.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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f6: Theoretical calculations under different pressures.(a) Calculated values of Ru-P bond length (red line) and P-Ru-P angle (black line) in the RuP4 tetrahedron vs. pressure; An obvious enhancement of the Ru-P bond length (and reducing of the P-Ru-P angle) can be found at the pressure from 2 to 3.5 GPa. (b) The electronic band structure and DOS of LaRu2P2 (I4/mmm) with calculated lattice parameters at 2.5 GPa; The band lying on Fermi level between X and P (black) at 2.5 GPa are mainly contributed by P 3p(pz) and Ru 4d(dxz + dyz) orbit. Panel on the right shows three partial DOS which make main contributions to the total DOS. (c) Calculated electronic DOS near Fermi level vs pressure, presents an increasing of Ru 4d orbit and a dropping of P 3p orbit, corresponding to (a,d) Calculated phonon dispersions at 2 GPa; the size of the bubble represents the electron-phonon interaction magnitude; phonon DOS and the integral value of electron-phonon coefficient λ are also shown on the right panel.
Mentions: In the following we try to get some insights based on our ab-initio calculations. The band structures and the Fermi surfaces at ambient pressure are presented in Fig.5a and in the supplementary information, they are consistent with the previously published calculated and experimental results2529. We relax the structure under different pressures and find a clear structural change upon pressure. As shown in Fig. 6a, the change of the Ru-P bond length within the Ru-P conducting layer exhibits an unusual step-wise feature at pressures of 2.0–3.5 GPa. This is accompanied by a slight closing of the P-Ru-P angle in the RuP4 tetrahedron and the slope for the change of the angle also varied a bit in the same pressure range. We can also find an obvious change on the electronic structures level. As we can see from Fig. 5a and Fig. 6b, four bands depicted with red, blue, pink and orange construct a complicated Fermi surface together. One band mostly contributed from Ru 4d(dxz + dyz) and P 3pz (red) moves upward with pressure and across the Fermi level at about 3.0 GPa, meanwhile, another band mainly consisting of Ru 4d(dxz + dyz) orbital (blue) moves downwards and the small wave-like feature near the N point also crosses the Fermi level at the same pressure range. This blue band opens a small tunnel in the Fermi surface after 3.0 GPa, as shown in Fig. 5b. This movement slightly reduces the slope of the bands across the Fermi level, which may be very essential to enhance the effective electron mass and induce a moderate correlation effect. From the electronic DOS in Fig. 6c, one can also see a clear variation from 2 to 3.5 GPa. The change of the Ru atom is mainly originated from its dxz + dyz orbitals while the change of the P atom is mostly coming from its 3pz orbital. Fig. 6d shows the phonon spectra, phonon linewidth, phonon density of states, Eliashberg function α2F, electron-phonon coupling (EPC) constant λ calculated for 2 GPa. It seems that a phonon mode near the N point have relatively large phonon linewidth and good contribution to the EP coupling. As shown in the Supplementary Information, this mode slightly goes soft with pressure and reach the lowest frequency at 2.5–3 GPa and goes harder afterwards. The EP coupling constant reaches a maximum value of 0.80 at about 2 GPa, which is somehow larger than that of the iron-arsenide system. As mentioned before, with all these refined structural parameters, the significant enhancement of Tc versus pressure in LaRu2P2 cannot be interpreted purely by the conventional electron-phonon coupling, extra electron-boson coupling may have been involved in the formation of superconducting pairing. In iron based superconductors, the antiferromagnetic spin fluctuations3031 have been argued to be the dominant role in inducing the pairing. Therefore the pressure can induce a sensitive change of superconducting transition temperature as well as the normal state properties. In the 1111 family of iron pnictide high-temperature superconductors REFeAsO1−xFx, the hydrostatic pressure seems to show a similarity between the trend of critical temperature vs hydrostatic pressure for different rare earth elements (RE)32. The authors of that paper argue that this may indicate the role of pressure on the competing interactions in 1111 iron pnictides. As far as we know, no investigations on successful chemical doping have been reported in the system LaRu2P2. If the extra electron-boson coupling is important to enhance the superconductivity, it would be very interesting to carry out more works with chemical doping in the present system.

Bottom Line: The ab-initio calculation shows that the superconductivity in LaRu2P2 at ambient pressure can be explained by the McMillan's theory with strong electron-phonon coupling.Detailed analysis of the pressure induced evolution of resistivity and upper critical field Hc2(T) reveals that the increase of Tc with pressure may be accompanied by the involvement of extra electron-boson interaction.This suggests that the Ru-based system has some commonality as the Fe-based superconductors.

View Article: PubMed Central - PubMed

Affiliation: National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.

ABSTRACT
To explore new superconductors beyond the copper-based and iron-based systems is very important. The Ru element locates just below the Fe in the periodic table and behaves like the Fe in many ways. One of the common thread to induce high temperature superconductivity is to introduce moderate correlation into the system. In this paper, we report the significant enhancement of superconducting transition temperature from 3.8 K to 5.8 K by using a pressure only of 1.74 ± 0.05 GPa in LaRu2P2 which has an iso-structure of the iron-based 122 superconductors. The ab-initio calculation shows that the superconductivity in LaRu2P2 at ambient pressure can be explained by the McMillan's theory with strong electron-phonon coupling. However, it is difficult to interpret the enhancement of Tc versus pressure within this picture. Detailed analysis of the pressure induced evolution of resistivity and upper critical field Hc2(T) reveals that the increase of Tc with pressure may be accompanied by the involvement of extra electron-boson interaction. This suggests that the Ru-based system has some commonality as the Fe-based superconductors.

No MeSH data available.