铁基超导体母体角分辨光电子能谱及电荷输运性质的研究
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摘要
铁基超导体是2008年发现的超导体家族中的新成员。目前已经发现的铁基超导体的超导转变温度已经超过了BCS理论预言的极限,被证明是一类新的非常规超导体。FeAs (Te、Se)层是铁基超导体所共有的基本结构,并在此基础上发展出“1111”“122”“111”“11”几个体系大类。在相图上,铁基超导体的超导相在欠掺杂区域近邻反铁磁相,这一现象和铜氧化物超导体的相图非常类似。而在反铁磁相变之上,其特有的结构相变变导致其二维晶体由四方变为正交。为了研究铁基超导体中复杂的相变行为,弄清其超导机制,有大量的实验和理论研究工作在短时间内被相继报道,在国内外形成了一股研究铁基超导体的热潮。我们课题组利用角分辨光电子能谱(ARPES)设备,对不同体系的铁基超导体的能带结构,反铁磁态下的电子结构重构,以及超导体能隙测量等方面做了大量开拓性的重要工作。本文主要介绍了我们组对“111”体系NaFeAs单晶样品的电子结构的细致测量,以及对不同体系铁基超导体母体电荷输运各向异性所进行的研究。
首先,我们对“111”体系铁基超导体母体NaFeAs单晶样品进行了系统的电子结构的测量。我们发现NaFeAs的能带结构和之前报道的“122”体系的类似。但是,因其结构相变和反铁磁相变的相变温度是分离的,这样就便于我们弄清楚在相变时电子结构的演化过程。我们在相变温度附近做了细致的变温实验,发现部分能带在结构相变附近开始向高能移动,并且在温度降低至反铁磁相变时能带移动平滑。这个现象表明,NaFeAs中结构相变和反铁磁相变可能具有相同的驱动机制。同时,我们利用偏振光依赖的ARPES实验,对NaFeAs能带的轨道对称性也进行了分析。在变光子能量的测量过程中,我们发现NaFeAs费米面附近的能带kz方向的色散较弱。以上实验结果,结合最近对detwin样品的变温实验表明,NaFeAs中磁有序的涨落和轨道自由度的耦合显著影响了其相变温度附近的行为。
此外,我们利用Montgomery方法测量了NaFeAs,FeTe等铁基超导体母体的电阻率各向异性行为。我们发现,对于NaFeAs样品,铁磁方向的电阻率大于反铁磁方向的电阻率,这一结果填补了不同类铁基超导体的各向异性电阻率测量的空白,并且表明包含FeAs层的铁基超导体母体中其各向异性电阻率行为是一致的。对于FeTe样品,我们发现它的铁磁方向电阻率小于反铁磁方向,这与NaFeAs和“122”体系中看到的结果完全相反,而这一现象则可能与FeTe所具有的较强的局域磁矩和电子结构的强关联性有关。这有助于我们分析FeTe和具有FeAs层样品的不同相互作用机制。
The iron-based superconductors, as a new member of the superconducting materials, were discovered in the early of the year2008. Later the superconducting transition temperature in the iron-based superconductors broke through the limit that BCS theory predicted, which indicates that they are a new type of unconventional high Tc superconductors. So far, four types of the iron-based superconductors, the "1111","122","111", and "11", were discovered, and the FeAs (FeTe/Se) layers were found to be the structure unit in all these materials. In phase diagram of the iron-based superconductors, the superconducting phase is always close to an anti-ferromagnetic (AFM) order phase in the under-doped region, which is similar with that in the cuprates. Moreover, a distinct structural transition lay upon the anti-ferromagnetic transition. To understand the complex behavior of the transitions, the mechanism of the superconductivity, an abundant of researches on theory and experiment were reported in a short time, which becomes a hot issue in these years. Our research group has been contributing a lot of groundbreaking ARPES experiment results in studying the superconducting gap behavior, the electronic structures and their reconstructions in the iron based superconductors. In this thesis, I introduce the comprehensive study on the electronic structure of NaFeAs single crystal, and the resistivity anisotropy in the various types of iron based superconductors.
We carried out detailed researches on the electronic structures of NaFeAs. In the paramagnetic state, the electronic structure is similar with the "122" iron pnictides. With the advantage of the phase-transition-temperature sepration, a careful temperature dependent experiment was carried out to see the evolution of the electronic structure:some of the bands in NaFeAs start to shift to the high energy around the structural transition temperature (Ts), and envolve smoothly when the temperature decrease though the anti-ferromagnetic transition temperature (Ts). It imples that the structural and anti-ferromagnetic transition may share the same driving force. According to the polarization dependent ARPES experiment, we analyzed the orbital characters of each band in NaFeAs, and the kz dispersions of the bands near the Fermi surface were found to be weak by changing the photon energy. Combined with the recent experiments on the detwined NaFeAs sample, we find that the spin fluctuations at high temperatures and their coupling with the orbital degree of freedom could be the dominant force to drive the nematicity.
We use the Montgomery method to measure the resistivity anisotropies in detwined NaFeAs and FeTe single crystals. In NaFeAs, the resistivity in the ferromagnetic direction is larger than that in the anti-ferromagnetic direction, which is the same as in "122" iron pnictides, indicating that it is a universal character in iron pnictides. However in FeTe, ferromagnetic direction is smaller than that in the anti-ferromagnetic direction, which shows an opposite behavior in the anti-ferromagnetic state comparing with the FeAs-structure iron pnictides. Our result demonstrates that the double exchange interaction in the ferromagnetic direction of FeTe may enhance the conductivity, which is different from the FeAs-structure iron pnictides.
首先,我们对“111”体系铁基超导体母体NaFeAs单晶样品进行了系统的电子结构的测量。我们发现NaFeAs的能带结构和之前报道的“122”体系的类似。但是,因其结构相变和反铁磁相变的相变温度是分离的,这样就便于我们弄清楚在相变时电子结构的演化过程。我们在相变温度附近做了细致的变温实验,发现部分能带在结构相变附近开始向高能移动,并且在温度降低至反铁磁相变时能带移动平滑。这个现象表明,NaFeAs中结构相变和反铁磁相变可能具有相同的驱动机制。同时,我们利用偏振光依赖的ARPES实验,对NaFeAs能带的轨道对称性也进行了分析。在变光子能量的测量过程中,我们发现NaFeAs费米面附近的能带kz方向的色散较弱。以上实验结果,结合最近对detwin样品的变温实验表明,NaFeAs中磁有序的涨落和轨道自由度的耦合显著影响了其相变温度附近的行为。
此外,我们利用Montgomery方法测量了NaFeAs,FeTe等铁基超导体母体的电阻率各向异性行为。我们发现,对于NaFeAs样品,铁磁方向的电阻率大于反铁磁方向的电阻率,这一结果填补了不同类铁基超导体的各向异性电阻率测量的空白,并且表明包含FeAs层的铁基超导体母体中其各向异性电阻率行为是一致的。对于FeTe样品,我们发现它的铁磁方向电阻率小于反铁磁方向,这与NaFeAs和“122”体系中看到的结果完全相反,而这一现象则可能与FeTe所具有的较强的局域磁矩和电子结构的强关联性有关。这有助于我们分析FeTe和具有FeAs层样品的不同相互作用机制。
The iron-based superconductors, as a new member of the superconducting materials, were discovered in the early of the year2008. Later the superconducting transition temperature in the iron-based superconductors broke through the limit that BCS theory predicted, which indicates that they are a new type of unconventional high Tc superconductors. So far, four types of the iron-based superconductors, the "1111","122","111", and "11", were discovered, and the FeAs (FeTe/Se) layers were found to be the structure unit in all these materials. In phase diagram of the iron-based superconductors, the superconducting phase is always close to an anti-ferromagnetic (AFM) order phase in the under-doped region, which is similar with that in the cuprates. Moreover, a distinct structural transition lay upon the anti-ferromagnetic transition. To understand the complex behavior of the transitions, the mechanism of the superconductivity, an abundant of researches on theory and experiment were reported in a short time, which becomes a hot issue in these years. Our research group has been contributing a lot of groundbreaking ARPES experiment results in studying the superconducting gap behavior, the electronic structures and their reconstructions in the iron based superconductors. In this thesis, I introduce the comprehensive study on the electronic structure of NaFeAs single crystal, and the resistivity anisotropy in the various types of iron based superconductors.
We carried out detailed researches on the electronic structures of NaFeAs. In the paramagnetic state, the electronic structure is similar with the "122" iron pnictides. With the advantage of the phase-transition-temperature sepration, a careful temperature dependent experiment was carried out to see the evolution of the electronic structure:some of the bands in NaFeAs start to shift to the high energy around the structural transition temperature (Ts), and envolve smoothly when the temperature decrease though the anti-ferromagnetic transition temperature (Ts). It imples that the structural and anti-ferromagnetic transition may share the same driving force. According to the polarization dependent ARPES experiment, we analyzed the orbital characters of each band in NaFeAs, and the kz dispersions of the bands near the Fermi surface were found to be weak by changing the photon energy. Combined with the recent experiments on the detwined NaFeAs sample, we find that the spin fluctuations at high temperatures and their coupling with the orbital degree of freedom could be the dominant force to drive the nematicity.
We use the Montgomery method to measure the resistivity anisotropies in detwined NaFeAs and FeTe single crystals. In NaFeAs, the resistivity in the ferromagnetic direction is larger than that in the anti-ferromagnetic direction, which is the same as in "122" iron pnictides, indicating that it is a universal character in iron pnictides. However in FeTe, ferromagnetic direction is smaller than that in the anti-ferromagnetic direction, which shows an opposite behavior in the anti-ferromagnetic state comparing with the FeAs-structure iron pnictides. Our result demonstrates that the double exchange interaction in the ferromagnetic direction of FeTe may enhance the conductivity, which is different from the FeAs-structure iron pnictides.
引文
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[2]由维基百科相关条目重新生成：http://en.wikipedia.org/wiki/File:Sc_history.gif
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[1]HUANG Q, QIU Y, BAO W, et al. Neutron-Diffraction Measurements of Magnetic Order and a Structural Transition in the Parent BaFe2As2 Compound of FeAs-Based High-Temperature Superconductors [J]. Physical Review Letters, 2008,101(25):257003.
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[2]由维基百科相关条目重新生成：http://en.wikipedia.org/wiki/File:Sc_history.gif
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