以高溫高壓方法製作La2-xSrxCu04+-δ(0≦x≦0.3)高溫超導體的超導性及其X光近緣吸收光譜(XANES)研究

The study of the Superconductivity and the X-ray Absorption Near Edge Structure (XANES) of hot-pressed samplesLa2-xSrxCuO4+-δ(0≦x≦0.3)

李泗賓(syh bin lee)

指導教授 ： 張秋男博士

國立臺灣師範大學/理學院/物理學系/碩士(2000年)

摘要

本論文研究的目的在探討高壓及高溫製成的La2-xSrxCuO4+-δ樣品的晶體結構以及電子結構。一系列的鑭鍶銅氧化合物（La2-xSrxCu04+-δ，0≦x≦0.3），以La2O3,SrCO3,CuO粉末混合，於高溫爐中通氧(1 bar)所做成的粉末，稱為AP樣品。上述方法所做成的粉末，再放入白金管囊中進行高溫高壓的製作。依據此方法所做成的粉末，稱為HP樣品（大部份樣品的停留時間為一小時）。另一組樣品則於白金管囊中上下各加入KClO4（過氯酸鉀），以作為充氧劑，其餘製作方法皆與HP樣品相同，稱為OP樣品。
由SQUID所得到的Tc顯示，高溫高壓樣品（HP）的超導溫度Tc都比AP樣品低，應該是因為高溫的環境造成缺氧所致。而OP樣品的臨界超導溫度Tc與AP樣品已經非常接近，顯示KClO4（過氯酸鉀）的充氧效應己經發揮。但HP0.30（Sr=0.3）樣品卻顯示出超導溫度Tc= 36 K，可能因為高溫的環境造成缺氧。但OP＊0.00（Sr=0.0，停留時間為10分鐘，＊表示停留時間與大部份樣品不相同）樣品與其他論文相似，亦顯示出超導性。晶體結構部份，我們使用x光繞射儀與同步輻射中心的Image Plate測量，仔細比較AP樣品與HP樣品的實驗值，繞射峰確有明顯的移動。比較不同測量方法所得到的晶格參數，a值的變動較不規則，而高溫高壓HP樣品的晶格參數c值都較AP樣品小。我們使用應力效應公式計算HP樣品Tc的變化，X光繞射儀及影像板測量的兩組晶格參數，其應力效應所造成Tc的變化結果相似，都朝向Tc減少的方向。將應力效應與我們從x光吸收譜的研究中所觀察到的現象對照比較，若吸收譜中樣品的吸收峰強度相近，則同樣Sr含量的超導溫度並無明顯的改變（例如OP0.15樣品與OP0.20樣品）。若吸收譜中樣品的吸收峰強度相差過多，則應力效應仍無法完全解釋超導溫度改變的原因（例如OP0.10樣品與HP樣品），吸收譜中樣品的吸收峰強度（價電子帶的強度）亦為超導溫度改變的主要原因。從量測樣品磁化率與x光結構分析中，都顯示了樣品的單晶相與均勻性。電子結構部份則使用X光近緣吸收光譜，數據是在同步輻射中心的高能量球形光柵光束線量取。我們分析氧的吸收光譜發現，HP樣品的Ia（528.5eV；表示價電子帶的強度）強度都比AP樣品小，HP樣品的Ib（530.4eV；導電帶的強度）強度變化亦比AP樣品小的多，幾乎在誤差範圍內。顯示HP樣品於價電子帶上的電洞數（Ia）較AP樣品減少。而OP樣品的吸收光譜則顯示OP樣品的Ia（528.5eV）強度比HP樣品更接近AP樣品的強度，OP樣品的Ib（530.4eV）強度變化亦比HP樣品大的多，顯示KClO4（過氯酸鉀）的充氧效應己經發揮。我們發現OP＊0.00（呈現超導性）樣品的吸收譜測量數據與AP0.10樣品的測量數據非常相似，Tc亦同。我們同時測量Cu 2p3/2的吸收邊緣吸收光譜，用來檢驗氧吸收光譜的正確性，結果與樣品的氧吸收光譜是相符的。所以我們的測量數據可做為理論計算的重要參考。O-1s 及Cu-2p3/2 的x光近緣吸收光譜(XNEAS)，都顯示經過高溫高壓處理過的樣品，其價帶的電洞數有明顯的減少，我們猜測電洞數的減少主要是由氧含量的減少所引起。經由添加KClO4當做充氧劑所製作的OP樣品獲得證實。我們將各樣品的Tc 值與價帶的電洞數（取Ia值）畫圖，與一般常見的相關性不相同，測量的結果顯示出是與鍶的含量有關，而各自成一條曲線。

關鍵字

高溫超導體 ； 高壓 ； x光吸收譜 ； La2-xSrxCuO4+-δ

並列摘要

The compounds of La2-xSrxCuO4, 0≦x≦0.3, were prepared by the solid reaction of La2O3, SrO2 and CuO powders of high purity. The reagent powders were mixed and ground well in the ambient pressure. This series of powders was designated as ambient-pressure (AP) samples. The AP samples then were pressed up to a high pressure(2.0 Gpa). This series of samples was called hot-pressed (HP) samples. In order to see if depletion of oxygen occurred for the HP samples. We proceeded to prepare some samples by adding KClO4 as an oxygen source. This series of samples was called oxygenated hot-pressed (OP) samples. For instance, OP0.15 is the oxygenated hot-pressed sample with the Sr content, x = 0.15.
The lattice parameters were obtained from the XRD data. The Tc derived from the magnetization measurement, in a magnetic field of 20 Gauss. The superconducting transition temperatures, Tc, are observed to decrease for most of the samples under the hot-press treatment, including the oxygenated hot-pressed ones. The samples with x = 0 and x = 0.30 are the exception. The one with x = 0 becomes superconducting at Tc = 29 K under the oxygenated hot-press treatment for relatively a short period, 10 min. (OP*0.00). And the one with x = 0.30 becomes superconducting at Tc = 36 K under the hot-press treatment (HP0.30). Samples were single phase and homogeneous as examined by x-ray powder diffraction (XRD) as will as by a SQUID.
The O1s x-ray near-edge absorption spectra (XNEAS) which contains two peaks, peak A and peak B. The peak A (528.5 eV) grows from x = 0 to x = 0.30, while the peak B (530.4 eV) decreases. The peak A was interpreted to correspond to the transition to the hole states at the Fermi level in the valence band, and the peak B to correspond to the transition to the conduction band (the upper Hubbard band with mainly Cu 3d character). The hole states at the Fermi level in the valence band are formed upon introduction of extra holes by doping . The O-1s XNEAS of the hot-pressed samples is noticeable, the size of the peak A for most of the OP samples is coming back to what observed in the AP samples. For oxygen depleted samples (e.g. the HP and HP* series), the features of their pre-edge peaks are a little different from those of the fully oxygenated ones. The intensity of the peak A appears to be smaller than that of their counterpart (with same x) of the fully oxygenated samples (the AP series and some of the OP series). This is a reflection of the depletion of holes due to the depletion of oxygen. However, the variation of the intensity of the peak B with x in all the hot-pressed samples is relatively small in comparisons with that in the fully oxygenated samples. The intensities of the peak A and the peak B designated as Ia and Ib. Again, one notes that the Ib of the samples under a hot-press treatment (for both the HP series and the HP* series) varies little with the Sr content, x, different from what occurred in the fully oxygenated samples (the AP series and most of the OP series). This indicates that the electronic structure is different between the oxygen-depleted samples and the fully oxygenated samples. One notes that the correlation between Tc and Ia can not be described by a universal curve. The correlation of Tc and the total number of holes can be grouped into curves according to the Sr content, x, which is not seen before.

並列關鍵字

high-Tc superconductor ； high pressure ； x-ray absorption ； La2-xSrxCuO4+-δ

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