高温超导体中准粒子动力学的超快光谱学研究

发布时间：2018-01-10 18:23

本文关键词：高温超导体中准粒子动力学的超快光谱学研究　出处：《南京大学》2015年博士论文　论文类型：学位论文

【摘要】：高温超导是凝聚态物理的重点难题,过去三十年相关领域的研究极大推动了相关学科特别是强关联基础物理的发展,也推动了低温磁场等领域技术的长足进步。高温超导体具有非常复杂的相图,在低温下,除了超导以外还存在赝隙、电荷密度波、自旋密度波等一系列有序相,这些相产生的机制及其与超导相之间的联系对于揭示高温超导的机制可能有重要关联。光学泵浦探测技术可以从飞秒时间分辨提供有序相相关的准粒子瞬态动力学信息,在本文中,我们采用了光学泵浦探测技术探索了YBa2Cu3O7-δ (YBCO)薄膜,YBa2Cu3O7-δ/La2/3Ca1/3MnO3 (YBCO/LCMO)超晶格材料以及KxFe2-ySe2 (KFS)超导单晶材料准粒子弛豫动力学行为,并得到如下一系列研究的结果：1、我们研究了最优掺杂的YBCO超导体中准粒子动力学随温度的演化。光激发准粒子动力学在90K和160K附近表现出了明显的变化,它们分别对应于超导相和赝隙相的相变温度。在低温下我们得到了赝隙相与超导相共存的实验证据,这是首次在最优掺杂的YBCO样品中得到支持赝隙相存在的瞬态光谱数据。另外我们用宽带光谱检测技术研究超导电性与赝隙相之间的关系。利用奇异值分解的数值分析方法,我们从瞬态反射光谱中得到了超导相和赝隙相频域和时域上的特征。基于金斯伯格-朗道模型,我们建立自超导电性与赝隙相之间弱的竞争作用。2、我们研究了由铁磁层LCMO和超导层YBCO组成的超晶格结构中准粒子动力学,并试图在外延法生长的YBCO/LCMO超晶格中从动力学角度去揭示临近效应。超晶格中的瞬态光谱响应并不能简单地认为是YBCO层和LCMO层信号的叠加。通过数据分析我们认为寿命为3ps的瞬态光学特征与超晶格中的超导电性相关,这个过程随温度演化表现出的相变远高于超晶格样品的超导相变温度Tc,这非常类似于欠掺杂铜基超导体中的行为。3、在强激发下,我们在超晶格中观测到了两种不同模式的相干声学声子。这两种模式的频率对检测光波长显现出明显不同的依赖。其中频率对检测光波长具有线性依赖的模式被认为是由于相干声学声子在体材料中的传播,而另外一个频率与检测波长无关的模式,我们通过改变超晶格样品的周期发现它是由超晶格结构导致的。我们的实验发现表明了超晶格对于相干热调控来说是非常好的结构。4、我们研究了铁基超导体KxFe2-ySe2单晶样品的准粒子动力学。低温下准粒子动力学是一个多指数衰减过程,随着温度的升高或者激发光强的增大,当超导电性被抑制后,由相干声学声子引起的阻尼振荡项将会出现,分析认为这一项来自于与超导竞争的序。随着温度的升高,准粒子弛豫动力学中衰减较慢过程的幅值逐渐增加,在160K附近趋于一个常数,并且在相同的温度范围内相干声学声子信号被明显地抑制了。这个行为与dxy能带中能隙打开的轨道选择Mott相变图像一致,这说明了在铁基超导体当中电子的关联特性扮演了非常重要的角色。
[Abstract]:Is the key problem of high temperature superconducting condensed state physics, in the past thirty years of research in related fields has greatly promoted the development of related disciplines especially strong association of basic physics, but also promoted the progress of low temperature magnetic field technology. High temperature superconductor phase diagram with very complex, at low temperature, but also exist outside the superconducting pseudogap. The charge density wave, spin density wave and a series of ordered phase, the phase and the contact mechanism between the superconducting phase and has important relevance for revealing the possible mechanisms of high-temperature superconductivity. The optical pump probe technique can provide quasi particle from femtosecond time-resolved transient dynamics of the ordered phase correlation, in this paper, we use optical the pump probe technology to explore YBa2Cu3O7- 8 (YBCO) film, YBa2Cu3O7- /La2/3Ca1/3MnO3 (YBCO/LCMO) superlattice and KxFe2-ySe2 (KFS) superconducting quasi single crystal material The particle relaxation behavior, and a series of research results are as follows: 1, we study the optimal doping of YBCO with temperature evolution of quasi particle dynamics in superconductors. Excited quasi particle dynamics in the near 90K and 160K showed obvious changes, which correspond to the superconducting phase and the pseudogap phase transition temperature experimental evidence at low temperature. We obtained the pseudogap and superconducting phase coexistence, this is the first time to get the transient spectrum data support the pseudogap phase exists in the optimal doped YBCO samples. In addition we use broadband spectrum detection technology research of superconductivity and the pseudogap phase. By using singular value decomposition analysis method we got the numerical characteristics of superconducting phase and the pseudogap phase frequency and time domain from transient reflection spectroscopy. The Ginsberg - Landau model based on our established between super conductivity and weak pseudogap phase Competition.2, we study the quasi particle dynamics of superlattice structure composed of ferromagnetic and superconducting layer LCMO layer YBCO, and try to grow in the epitaxy YBCO/LCMO superlattice from the dynamic perspective to reveal the proximity effect in the superlattice. The transient spectral response and is not simply that the YBCO layer and LCMO layer signal superimposed. Through data analysis, we believe that the superconductivity of life for transient optical characteristics of 3ps superlattice and in this process, with the temperature evolution of superconducting transition temperature of Tc phase showed far higher than the superlattice sample, which is very similar to the underdoped copper based superconductor behavior in.3, in the strong excitation we, in the superlattice observed coherent acoustic phonons in two different modes. The frequencies of the two modes of the detection wavelength showed significantly different. The frequency of detection depends on the wavelength is linear in Lai's model is considered to be due to the spread of coherent acoustic phonons in the bulk, while another frequency and detection wavelength independent mode, we changed the periodic superlattice samples found that it is caused by a superlattice structure. We found that the experimental results show that the superlattice structure of.4 is very good for. Dry heat control, quasi particle dynamics we studied iron-based superconductor KxFe2-ySe2 single crystal samples. Under low temperature and quasi particle dynamics is a multi exponential decay process, with the increase of temperature or increasing the excitation intensity, when the superconductivity is suppressed, caused by the coherent acoustic phonon damping oscillation will appear, analysis this one comes from the competition and superconducting order. With the increase of temperature, the quasiparticle relaxation dynamics in the process of the amplitude of slow attenuation increases gradually, in the vicinity of 160K tends to be a constant, and in phase In the same temperature range, the coherent acoustic phonon signal is obviously suppressed. This behavior is consistent with the Mott phase transition image in the DXY energy band opening orbit, which indicates that the electron correlation property plays a very important role in the iron based superconductor.

【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O469

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