水合钴氧化物的超导相图及掺杂EuFe_2As_2体系中超导与铁磁共存研究
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摘要
超导电性是一种宏观量子现象,具有丰富的物理内涵。就超导基础研究而言,一方面,人们致力于揭示产生超导电性的物理机制,特别是那些不同于电声子相互作用的非常规超导机制；另一方面,人们又在努力寻找新超导体,其中那些表现出超导和磁性共存的则格外令人感兴趣。
在本论文中,我们研究了：(1)三角点阵Nax(H3O)：coO2·yH2O超导体的上临界场,电子相图及热稳定性；(2)在三元铁基砷化物EuFe2As2中的Fe位和As位的掺杂效应。本论文的主要结果如下：
(i)超导体的上临界场与超导配对态相关。在Nax(H3O)zCoO2·yH2O超导体中,文献报道磁测量给出的零温时上临界场Hc2(0)远远超过泡利顺磁极限Hp≈8.3T,这与其他测量给出的结果不一致。为了澄清这一矛盾,我们对取向多晶超导样品外磁场平行于ab面的磁化强度进行了详细的测量。通过标度分析,我们发现低温磁化率的上翘来自于顺磁杂质而非以前所认为的自旋涨落。在高磁场下,超导表现为对布里渊函数的偏离,其结果表明(?)(0)值是在泡利顺磁极限Hp≈8.3T之内。我们的结果进一步支持超导Cooper对为自旋单态配对。
(ii)掺杂超导体的电子相图对我们确定其超导机制具有一定的指导作用。国际上不同课题组提出的关于Nax(H3O)zCo02·yH2O的超导相图相去甚远,这主要是由于该超导体不可避免地含有水合氢离子而使得Co价态有很大不确定性。我们设计了一种简单可行的后还原软化学方法来调节和测量Co的价态,所采用的还原剂为NaOH溶液。其中Co价态的变化可以通过测量反应中释放氧气的体积而精确获得。在浓NaOH的条件下,水合氢离子的影响几乎可以消除,使得Co的绝对价态仅由Na含量所决定。以此为基础,我们对超导相图进行了修正,即在Co价态从+3.58到+3.65的区间内超导临界温度随Co价态的升高而单调升高。
(iii)Nax(H30)zCoO2·yH2O超导体在实验室环境中非常不稳定,极易失去部分结晶水而失去超导电性,这也导致文献中经常出现不一致的结果。我们用实验表明在浓NaOH溶液中还原的样品稳定性相比还原前大大提高,从而实现了Nax(H3O)：CoO2·yH2O超导体的稳定化。对稳定化后的超导体进一步研究有助于加深我们对这种奇特材料的认识。
(iv)我们首次对三元铁基砷化物EuFe2As2的输运及热力学性质进行了系统的测量。结果表明EuFe2As2分别在200K和19K经历两个磁相变。其中200K附近的相变可以归结于与Fe磁矩有关的自旋密度波转变。基于此点,我们预期EuFe2As2也是一种可能的超导体母体化合物,随后的实验很快证实了这一点。而在19K时,Eu2+的磁矩形成长程反铁磁序,同时我们也观察到了磁场诱导的从反铁磁有序到铁磁有序的变磁相变。我们认为这与Eu2+磁矩可能的”A型”反铁磁结构紧密相关,后续的实验也支持这一观点。
(v)我们对EuFe2-xNixAs2(0≤x≤0.2)系列多晶样品的磁及输运性质进行研究。其结果发现Ni掺杂同时抑制Fe磁矩的自旋密度波和Eu2+磁矩的反铁磁转变。当x≥0.06时,Eu2+的磁矩从反铁磁有序过渡到铁磁有序,这也是首次在铁基砷化物系统中观察到长程铁磁序。虽然进一步提高Ni含量至x≥0.16使Fe磁矩的自旋密度波转变完全被抑制,但在2K以上却未观察到超导转变。我们给出了关于EuFe2一xNix·As2(0≤x≤0.2)详细的磁相图,并根据RKKY机制对其进行了讨论,表明Eu2+磁性与[Fe2-xNixAS2]2-层的电子态间存在很强的耦合。
(vi)我们发现EuFe2(As0.7P0.3)2在26K发生超导转变,降温到20K时Eu2+磁矩形成长程铁磁序同时在电阻上观察到再入现象,而当温度进一步降低时系统又进入超导态并且与铁磁序是共存的。比热测量的结果表明超导和铁磁均为体性质。这一发现是实验上首次在不改变Fe3d电子数目的前提下引入超导,也是首次通过As位掺杂引入超导,同时提供了一个新的也是很罕见的超导和铁磁共存现象的实例。
最后一章是对全文的总结,同时还对今后的工作进行了展望。
Superconductivity is remarkable in that it is a macroscopic quantum phenomenon, which bears rich physical connotation. As far as the fundamental superconductivity re-search is concerned, on one hand, researchers are committed to unravel the underlying mechanism for superconductivity, especially the exotic ones which are different from the conventional electron-phonon interaction scenario; on the other hand, much efforts are de-voted to search for new superconductors, among which those exhibiting coexistence of superconductivity and ferromagnetism are of particular interest.
In this dissertation, we have studied:(1)the upper critical field, superconducting phase diagram and thermal stability of triangular lattice Naχ(H3O)zCoO2·yH2O superconductor; (2) the doping effect at Fe and As sites in ternary iron arsenide EuFe2As2. The main con-clusions of the dissertation are highlighted as follows.
(i)Resistivity,59Co-NMR, specific heat measurements suggest that the zero tempera-ture upper critical field for Hc2(0) for Naχ(H3O)zCoO2·yH2O superconductor is within the Pauli paramagnetic limit Hp≈8.3T, which is much smaller than that inferred from mag-netic measurements. To resolve this controversy, we present a detailed magnetization study on aligned superconductors with magnetic field parallel to CoO2 layers up to fields of 8T. The temperature-dependent part of the normal-state magnetization is revealed to arise from the paramagnetic impurity rather than the spin fluctuations. The onset of superconductivity is characterized by the deviation of normal-state magnetization governed by Brillouin func-tion. Our result suggests that the H(?) (0) value is just within the Pauli paramagnetic limit, which is much smaller than those of the previous results by magnetic measurements and consistent with the values from other measurements. In addition, our results also support spin-singlet superconductivity in the superconductor.
(ii) The superconducting phase diagrams for Naχ(H3O)zCoO2·yH2O superconductor proposed by different groups are far from consistency, which basically comes from the un- certainty in determining the Co valence(VCo) as a consequence of inevitably hydronium incorporation during the hydration process. Here we present a facile route to tune and mea-sure VCo by a topotactic postreduction using NaOH as reducing agent. The change of VCo can be precisely measured by measuring the volume of released oxygen. The remarkable advantage of this method lies in that the disturbance of hydronium incorporation can be eliminated to a great extent for the sample treated in concentrated NaOH solution, making the absolute Vco determinable. As a result, an updated superconducting phase diagram was obtained, which shows that the superconducting transition temperature increases mono-tonically with increasing Co valence in a narrow range ranging from+3.58 to+3.65.
(iii)The Naχ(H3O)zCoO2·yH2O superconductor is quiet unstable at ambient condi-tion, very easily losing parts of its cyrstalline water and becoming non-superconducting. This sever chemical instability results in frequentely inconsistent experimental results in the literatures. We demonstrate that stabilization of the superconductor is feasible by treat-ing the as-prepared superconductor in concentrated NaOH solution. It is hoped that further study of the stabilized superconductor will resolve the continuing controversy in physical property measurements and enrich our knowledge of this fascinating material.
(iv)We have performed the transport and thermodynamical measurements of the ternary iron arsenide EuFe2As2 for the first time. It is found that EuFe2As2 undergoes two mag-netic transitions at 200K and 19K, respectively. The former is accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribu-tion), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like (SDW) antiferromagnetic transition. The latter is proposed to be associated with an A-type antiferromagnetic ordering of Eu2+ moments, which also undergoes a field-induced metamagnetic transition. Comparing with the physical properties of the iso-structural com-pounds BaFe2As2 and SrFe2As2, we expect that EuFe2As2 is a possible parent compound for superconductors, as has been soon verified by subsequent experiments.
(v)We have systematically studied the transport and magnetic properties on a series of EuFe2-χNiχAs2 polycrystalline samples with 0 (vi)We have studied EuFe2(Aso.7Po.3)2 by the measurements of x-ray diffraction, elec-trical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chem-ical pressure to the system. It is found that EuFe2(Aso.7Po.3)2 becomes superconducting below 26 K, followed by ferromagnetic ordering of Eu2+ moments at 20 K. The observa-tion of sizable anomalies in thermodynamic properties, concomitant with the transitions, indicates that both of them are bulk phenomena. This finding is the first observation of superconductivity without changing the number of Fe 3d electrons, also the realization of superconducting through doping at As site for the first time, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.
在本论文中,我们研究了：(1)三角点阵Nax(H3O)：coO2·yH2O超导体的上临界场,电子相图及热稳定性；(2)在三元铁基砷化物EuFe2As2中的Fe位和As位的掺杂效应。本论文的主要结果如下：
(i)超导体的上临界场与超导配对态相关。在Nax(H3O)zCoO2·yH2O超导体中,文献报道磁测量给出的零温时上临界场Hc2(0)远远超过泡利顺磁极限Hp≈8.3T,这与其他测量给出的结果不一致。为了澄清这一矛盾,我们对取向多晶超导样品外磁场平行于ab面的磁化强度进行了详细的测量。通过标度分析,我们发现低温磁化率的上翘来自于顺磁杂质而非以前所认为的自旋涨落。在高磁场下,超导表现为对布里渊函数的偏离,其结果表明(?)(0)值是在泡利顺磁极限Hp≈8.3T之内。我们的结果进一步支持超导Cooper对为自旋单态配对。
(ii)掺杂超导体的电子相图对我们确定其超导机制具有一定的指导作用。国际上不同课题组提出的关于Nax(H3O)zCo02·yH2O的超导相图相去甚远,这主要是由于该超导体不可避免地含有水合氢离子而使得Co价态有很大不确定性。我们设计了一种简单可行的后还原软化学方法来调节和测量Co的价态,所采用的还原剂为NaOH溶液。其中Co价态的变化可以通过测量反应中释放氧气的体积而精确获得。在浓NaOH的条件下,水合氢离子的影响几乎可以消除,使得Co的绝对价态仅由Na含量所决定。以此为基础,我们对超导相图进行了修正,即在Co价态从+3.58到+3.65的区间内超导临界温度随Co价态的升高而单调升高。
(iii)Nax(H30)zCoO2·yH2O超导体在实验室环境中非常不稳定,极易失去部分结晶水而失去超导电性,这也导致文献中经常出现不一致的结果。我们用实验表明在浓NaOH溶液中还原的样品稳定性相比还原前大大提高,从而实现了Nax(H3O)：CoO2·yH2O超导体的稳定化。对稳定化后的超导体进一步研究有助于加深我们对这种奇特材料的认识。
(iv)我们首次对三元铁基砷化物EuFe2As2的输运及热力学性质进行了系统的测量。结果表明EuFe2As2分别在200K和19K经历两个磁相变。其中200K附近的相变可以归结于与Fe磁矩有关的自旋密度波转变。基于此点,我们预期EuFe2As2也是一种可能的超导体母体化合物,随后的实验很快证实了这一点。而在19K时,Eu2+的磁矩形成长程反铁磁序,同时我们也观察到了磁场诱导的从反铁磁有序到铁磁有序的变磁相变。我们认为这与Eu2+磁矩可能的”A型”反铁磁结构紧密相关,后续的实验也支持这一观点。
(v)我们对EuFe2-xNixAs2(0≤x≤0.2)系列多晶样品的磁及输运性质进行研究。其结果发现Ni掺杂同时抑制Fe磁矩的自旋密度波和Eu2+磁矩的反铁磁转变。当x≥0.06时,Eu2+的磁矩从反铁磁有序过渡到铁磁有序,这也是首次在铁基砷化物系统中观察到长程铁磁序。虽然进一步提高Ni含量至x≥0.16使Fe磁矩的自旋密度波转变完全被抑制,但在2K以上却未观察到超导转变。我们给出了关于EuFe2一xNix·As2(0≤x≤0.2)详细的磁相图,并根据RKKY机制对其进行了讨论,表明Eu2+磁性与[Fe2-xNixAS2]2-层的电子态间存在很强的耦合。
(vi)我们发现EuFe2(As0.7P0.3)2在26K发生超导转变,降温到20K时Eu2+磁矩形成长程铁磁序同时在电阻上观察到再入现象,而当温度进一步降低时系统又进入超导态并且与铁磁序是共存的。比热测量的结果表明超导和铁磁均为体性质。这一发现是实验上首次在不改变Fe3d电子数目的前提下引入超导,也是首次通过As位掺杂引入超导,同时提供了一个新的也是很罕见的超导和铁磁共存现象的实例。
最后一章是对全文的总结,同时还对今后的工作进行了展望。
Superconductivity is remarkable in that it is a macroscopic quantum phenomenon, which bears rich physical connotation. As far as the fundamental superconductivity re-search is concerned, on one hand, researchers are committed to unravel the underlying mechanism for superconductivity, especially the exotic ones which are different from the conventional electron-phonon interaction scenario; on the other hand, much efforts are de-voted to search for new superconductors, among which those exhibiting coexistence of superconductivity and ferromagnetism are of particular interest.
In this dissertation, we have studied:(1)the upper critical field, superconducting phase diagram and thermal stability of triangular lattice Naχ(H3O)zCoO2·yH2O superconductor; (2) the doping effect at Fe and As sites in ternary iron arsenide EuFe2As2. The main con-clusions of the dissertation are highlighted as follows.
(i)Resistivity,59Co-NMR, specific heat measurements suggest that the zero tempera-ture upper critical field for Hc2(0) for Naχ(H3O)zCoO2·yH2O superconductor is within the Pauli paramagnetic limit Hp≈8.3T, which is much smaller than that inferred from mag-netic measurements. To resolve this controversy, we present a detailed magnetization study on aligned superconductors with magnetic field parallel to CoO2 layers up to fields of 8T. The temperature-dependent part of the normal-state magnetization is revealed to arise from the paramagnetic impurity rather than the spin fluctuations. The onset of superconductivity is characterized by the deviation of normal-state magnetization governed by Brillouin func-tion. Our result suggests that the H(?) (0) value is just within the Pauli paramagnetic limit, which is much smaller than those of the previous results by magnetic measurements and consistent with the values from other measurements. In addition, our results also support spin-singlet superconductivity in the superconductor.
(ii) The superconducting phase diagrams for Naχ(H3O)zCoO2·yH2O superconductor proposed by different groups are far from consistency, which basically comes from the un- certainty in determining the Co valence(VCo) as a consequence of inevitably hydronium incorporation during the hydration process. Here we present a facile route to tune and mea-sure VCo by a topotactic postreduction using NaOH as reducing agent. The change of VCo can be precisely measured by measuring the volume of released oxygen. The remarkable advantage of this method lies in that the disturbance of hydronium incorporation can be eliminated to a great extent for the sample treated in concentrated NaOH solution, making the absolute Vco determinable. As a result, an updated superconducting phase diagram was obtained, which shows that the superconducting transition temperature increases mono-tonically with increasing Co valence in a narrow range ranging from+3.58 to+3.65.
(iii)The Naχ(H3O)zCoO2·yH2O superconductor is quiet unstable at ambient condi-tion, very easily losing parts of its cyrstalline water and becoming non-superconducting. This sever chemical instability results in frequentely inconsistent experimental results in the literatures. We demonstrate that stabilization of the superconductor is feasible by treat-ing the as-prepared superconductor in concentrated NaOH solution. It is hoped that further study of the stabilized superconductor will resolve the continuing controversy in physical property measurements and enrich our knowledge of this fascinating material.
(iv)We have performed the transport and thermodynamical measurements of the ternary iron arsenide EuFe2As2 for the first time. It is found that EuFe2As2 undergoes two mag-netic transitions at 200K and 19K, respectively. The former is accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribu-tion), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like (SDW) antiferromagnetic transition. The latter is proposed to be associated with an A-type antiferromagnetic ordering of Eu2+ moments, which also undergoes a field-induced metamagnetic transition. Comparing with the physical properties of the iso-structural com-pounds BaFe2As2 and SrFe2As2, we expect that EuFe2As2 is a possible parent compound for superconductors, as has been soon verified by subsequent experiments.
(v)We have systematically studied the transport and magnetic properties on a series of EuFe2-χNiχAs2 polycrystalline samples with 0
引文
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