Re_3W的点接触安德烈夫反射谱研究
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摘要
本文通过对不同晶体结构Re_3W样品的点接触测量和对比研究,证实具有中心对称结构和非中心对称结构的Re_3W都是弱耦合Bardeen-Cooper-Schrieffer超导体,同时发现在两个相表面都可以形成很理想的点接触结,即电子通过界面时受到的非弹性散射很弱.将Re_3W样品置于大气环境近六个月后重新进行测量,仍然能够得到类似的结果,表明Re_3W具有很好的稳定性. Re_3W的这种优良特性,不仅可通过点接触实验得到的参数推算出Re_3W两个相的费米速度,而且提供了一种简单的方法,可以在点接触实验中利用Re_3W来印证针尖材料的费米速度和测量其自旋极化率等.作为尝试,本文用Re_3W/Ni点接触结测量了铁磁性金属Ni的自旋极化率,得到了与前人报道一致的结果.
Non-centrosymmetric superconductors have received considerable attention because of their possible possession of unconventional spin-triplet pairing. For this reason, the non-centrosymmetric Re_3W with α-Mn structure has been widely concerned. However, almost all the previous studies support that the non-centrosymmetric phase of Re_3W is a conventional weak-coupling s-wave superconductor. Later on, it is proved that Re_3W has two different superconducting phases, one is the non-centrosymmetric phase and the other has a centrosymmetric hexagonal structure. Thus, a comparative study of these two superconducting phases could provide more information about the effect of non-centrosymmetric structure on the pairing symmetry of Re_3W.In this paper, point-contact Andreev reflection experiments are carried out on Re_3W/Au and the data can be well fitted by isotropic s-wave Blonder-Tinkham-Klapwijk(BTK) theory. In combination with our previous researches, we find that both centrosymmetric and non-centrosymmetric phases have similar temperature dependence of superconducting gap(?) with almost the same gap ratio of ?/Tc. These results present strong evidence that both phases of Re_3W are weak coupling Bardeen-Cooper-Schrieffer superconductors.Another interesting finding is that both phases of Re_3W could easily form an ideal point-contact junction(i.e.,inelastic scatterings at the interface can be ignored) with a normal metal tip. This is manifested as an extremely small broadening factor(Γ) used in the fitting process, and indicates a clean(and possibly transparent) interface. Keeping this in mind, we can assume that the effective barrier(Z) at the interface mainly comes from the mismatch between the Fermi velocity of the superconductor and that of the normal metal, which can be estimated from the formula Z~2=(1-r)~2/4 r,where r is the ratio between those two Fermi velocities. From this formula, we can obtain the Fermi velocity of Re_3W by using the known value of Au's Fermi velocity and the fitting parameter Z for the Re_3W/Au point contacts. It is interesting to find that the chemical property of Re_3W is stable in the atmospheric environment. Even if the samples are exposed to the atmospheric environment for nearly six months, the inelastic scatterings are still very weak, and the superconducting properties are unchanged.Such an exceptional performance of Re_3W can be utilized to study the physical properties of its counter electrode in a point contact. As an attempt, we build a point contact between Re_3W and a ferromagnetic Ni tip, and measure its Andreev reflection spectra which are then fitted with a modified BTK model by considering spin polarization. The determined spin polarization of Ni is in good agreement with previously reported result. Moreover, using the Fermi velocities of Re_3W and Ni, we can calculate the effective barrier to be around 0.3in the Re_3W/Ni interface, which coincides with the fitting parameter Z. These results self-consistently demonstrate the validity of the determination of Re_3W's Fermi velocity and the cleanness/transparency of the studied point-contact interface.
Non-centrosymmetric superconductors have received considerable attention because of their possible possession of unconventional spin-triplet pairing. For this reason, the non-centrosymmetric Re_3W with α-Mn structure has been widely concerned. However, almost all the previous studies support that the non-centrosymmetric phase of Re_3W is a conventional weak-coupling s-wave superconductor. Later on, it is proved that Re_3W has two different superconducting phases, one is the non-centrosymmetric phase and the other has a centrosymmetric hexagonal structure. Thus, a comparative study of these two superconducting phases could provide more information about the effect of non-centrosymmetric structure on the pairing symmetry of Re_3W.In this paper, point-contact Andreev reflection experiments are carried out on Re_3W/Au and the data can be well fitted by isotropic s-wave Blonder-Tinkham-Klapwijk(BTK) theory. In combination with our previous researches, we find that both centrosymmetric and non-centrosymmetric phases have similar temperature dependence of superconducting gap(?) with almost the same gap ratio of ?/Tc. These results present strong evidence that both phases of Re_3W are weak coupling Bardeen-Cooper-Schrieffer superconductors.Another interesting finding is that both phases of Re_3W could easily form an ideal point-contact junction(i.e.,inelastic scatterings at the interface can be ignored) with a normal metal tip. This is manifested as an extremely small broadening factor(Γ) used in the fitting process, and indicates a clean(and possibly transparent) interface. Keeping this in mind, we can assume that the effective barrier(Z) at the interface mainly comes from the mismatch between the Fermi velocity of the superconductor and that of the normal metal, which can be estimated from the formula Z~2=(1-r)~2/4 r,where r is the ratio between those two Fermi velocities. From this formula, we can obtain the Fermi velocity of Re_3W by using the known value of Au's Fermi velocity and the fitting parameter Z for the Re_3W/Au point contacts. It is interesting to find that the chemical property of Re_3W is stable in the atmospheric environment. Even if the samples are exposed to the atmospheric environment for nearly six months, the inelastic scatterings are still very weak, and the superconducting properties are unchanged.Such an exceptional performance of Re_3W can be utilized to study the physical properties of its counter electrode in a point contact. As an attempt, we build a point contact between Re_3W and a ferromagnetic Ni tip, and measure its Andreev reflection spectra which are then fitted with a modified BTK model by considering spin polarization. The determined spin polarization of Ni is in good agreement with previously reported result. Moreover, using the Fermi velocities of Re_3W and Ni, we can calculate the effective barrier to be around 0.3in the Re_3W/Ni interface, which coincides with the fitting parameter Z. These results self-consistently demonstrate the validity of the determination of Re_3W's Fermi velocity and the cleanness/transparency of the studied point-contact interface.
引文
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[13]Huang Y,Yan J,Wang Y L,Shan L,Luo Q,Wang WH,Wen H H 2008 Supercond.Sci.Technol.21 075011
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[15]Biswas P K,Lees M R,Hillier A D,Smith R I,Marshall W G,Paul D M 2011 Phys.Rev.B 84 184529
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[20]Soulen R J,Byers J M,Osofsky M S,Nadgorny B,Ambrose T,Cheng S F,Broussard P R,Tanaka C T,Nowak J,Moodera J S,Barry A,Coey J M D 1998 Science 28285
[21]Nadgorny B,Soulen Jr R J,Osofsky M S,Mazin I I,Laprade G,van de Veerdonk R J M,Smits A A,Cheng S F,Skelton E F,Qadri S B 2000 Phys.Rev.B 61 3788
[22]Ji Y,Strijkers G J,Yang F Y,Chien C L,Byers J M,Anguelouch A,Xiao G,Gupta A 2001 Phys.Rev.Lett.86 5585
[23]Panguluri R P,Tsoi G,Nadgorny B,Chun S H,Samarth N,Mazin I I 2003 Phys.Rev.B 68 201307
[24]Clowes S K,Miyoshi Y,Bugoslavsky Y,Branford W R,Grigorescu C,Manea S A,Monnereau O,Cohen L F2004 Phys.Rev.B 69 214425
[25]Biswas P K,Hillier A D,Lees M R,Paul D M 2012 Phys.Rev.B 85 134505
[26]Daghero D,Gonnelli R S 2010 Supercond.Sci.Technol.23 043001
[27]Gall D 2016 J.Appl.Phys.119 85101
[28]Mazin I I,Golubov A A,Nadgorny B 2001 J.Appl.Phys.89 7576
[29]Moodera J S,Mathon G 1999 J.Magn.Magn.Mater.200 248
[30]Strijkers G J,Ji Y,Yang F Y,Chien C L,Byers J M2001 Phys.Rev.B 63 104510
[2]Gor’kov L P,Rashba E I 2001 Phys.Rev.Lett.87 037004
[3]Frigeri P A,Agterberg D F,Koga A,Sigrist M 2004Phys.Rev.Lett.92 097001
[4]Bauer E,Sigrist M 2012 Non-Centrosymmetric Superconductors:Introduction and Overview(Berlin Heidelberg:Springer Verlag)pp4-5
[5]Izawa K,Kasahara Y,Matsuda Y,Behnia K,Yasuda T,Settai R,Onuki Y 2005 Phys.Rev.Lett.94 197002
[6]Bonalde I,Br?mer-Escamilla W,Bauer E 2005 Phys.Rev.Lett.94 207002
[7]Yuan H Q,Agterberg D F,Hayashi N,Badica P,Vandervelde D,Togano K,Sigrist M,Salamon M B 2006Phys.Rev.Lett.97 017006
[8]Sato M,Fujimoto S 2009 Phys.Rev.B 79 094504
[9]Chadov S,Qi X L,Kübler J,Fecher G H,Felser C,Zhang S C 2010 Nat.Mater.9 541
[10]Blaugher R D,Hulm J K 1961 J.Phys.Chem.Solids19 134
[11]Blaugher R D,Taylor A,Hulm J K 1962 IBM J.Res.Dev.6 116
[12]Zuev Y L,Kuznetsova V A,Prozorov R,Vannette M D,Lobanov M V,Christen D K,Thompson J R 2007 Phys.Rev.B 76 132508
[13]Huang Y,Yan J,Wang Y L,Shan L,Luo Q,Wang WH,Wen H H 2008 Supercond.Sci.Technol.21 075011
[14]Blonder G E,Tinkham M,Klapwijk T M 1982 Phys.Rev.B 25 4515
[15]Biswas P K,Lees M R,Hillier A D,Smith R I,Marshall W G,Paul D M 2011 Phys.Rev.B 84 184529
[16]Chu C W,McMillan W L,Luo H L 1971 Phys.Rev.B3 3757
[17]Dynes R C,Narayanamurti V,Garno J P 1978 Phys.Rev.Lett.41 1509
[18]Dynes R C,Garno J P,Hertel G B,Orlando T P 1984Phys.Rev.Lett.53 2437
[19]Plecenik A,Grajcar M,Beňa?ka?,Seidel P,Pfuch A1994 Phys.Rev.B 49 10016
[20]Soulen R J,Byers J M,Osofsky M S,Nadgorny B,Ambrose T,Cheng S F,Broussard P R,Tanaka C T,Nowak J,Moodera J S,Barry A,Coey J M D 1998 Science 28285
[21]Nadgorny B,Soulen Jr R J,Osofsky M S,Mazin I I,Laprade G,van de Veerdonk R J M,Smits A A,Cheng S F,Skelton E F,Qadri S B 2000 Phys.Rev.B 61 3788
[22]Ji Y,Strijkers G J,Yang F Y,Chien C L,Byers J M,Anguelouch A,Xiao G,Gupta A 2001 Phys.Rev.Lett.86 5585
[23]Panguluri R P,Tsoi G,Nadgorny B,Chun S H,Samarth N,Mazin I I 2003 Phys.Rev.B 68 201307
[24]Clowes S K,Miyoshi Y,Bugoslavsky Y,Branford W R,Grigorescu C,Manea S A,Monnereau O,Cohen L F2004 Phys.Rev.B 69 214425
[25]Biswas P K,Hillier A D,Lees M R,Paul D M 2012 Phys.Rev.B 85 134505
[26]Daghero D,Gonnelli R S 2010 Supercond.Sci.Technol.23 043001
[27]Gall D 2016 J.Appl.Phys.119 85101
[28]Mazin I I,Golubov A A,Nadgorny B 2001 J.Appl.Phys.89 7576
[29]Moodera J S,Mathon G 1999 J.Magn.Magn.Mater.200 248
[30]Strijkers G J,Ji Y,Yang F Y,Chien C L,Byers J M2001 Phys.Rev.B 63 104510