Zn掺杂La2CuO4热电势和磁化率研究

测量了La2CuO4掺Zn样品在不同降温速率下(330K保温05h,然后分别以6Kh,02Ks的速度降到8K)的直流磁化率和热电势.实验结果表明,反铁磁温度TN不随降温速率变化而变化,其直流磁化率也未受很大影响.高温热电势弱的温度依赖关系表明为极化子气体的贡献.热电势在转折温度Tdrop之下的快速降低是由于二维反铁磁涨落的贡献.热电势在更低温度的拐点TS与载流子的局域化有关.降温速率变化时,Tdrop和TS都有明显的变化.Zn掺杂对Tdrop和TS没有明显影响,但导致了更强的载流子局域化.讨论了上述现象产生的物理图像 关键词： La2CuO4 直流磁化率 热电势     

电子型超导体Nd1.85Ce0.15CuO4单晶输运性质研究

我们测量了最佳掺杂的Nd1．85Ce0．15CuO4单晶的电阻率和ab面以及c方向热电势．ρ-T曲线在高温下表现为非常好的直线，超导转变比较尖锐Tc,mid=20．6K，说明我们的样品品质很好．低温下的电阻率拟合结果表明频率大约100K的软声子模与载流子有强烈相互作用，这是极化子模型有力的证据．对于热电势结果，令人惊奇的是，无论数值还是曲线形状，包括低温曳引峰的位置，c方向的热电势Sc与ab面内的热电势Sab非常接近．这可能是某种共同的机制导致的结果．比较Nd1．85Ce0．15CuO4和Sm1.85Ce0.15CuO4的热电势曲线可以发现，二的数值和曲线形状基本相同，而与La1．85Sr0．15CuO4差异较大，这进一步说明了顶点氧在这些体系中的作用．     

Nd2CuO4及其微量Zn掺杂单晶样品的传输性质研究

用助溶剂方法高温烧制了电子型超导体母体材料Nd2CuO4及其微量Zn掺杂的单晶样品,并系统地研究了在15～300K范围内样品的电阻率和热电势行为.两者在25～200K之间的温区都表现相同的物理行为高温区为半导体热激活行为,低温区为二维变程跳跃行为.在206K附近发现了反铁磁转变对热电势行为影响的精细峰和弛豫现象.我们的实验结果表明在180K以上温区载流子的行为不满足传统的能带理论.退火对25K以下的温区的热电势的行为有非常显著的影响,其本征的物理性质还需要深入的研究.     

La2-xSrxCuO4单晶膜的热电势与电阻率

测量了高质量的单晶膜La2-xSrxCuO4(x=010,020,025)的电阻率和热电势.La19Sr01CuO4电阻率呈现S型行为,表明存在一个赝能隙,在赝能隙态可以用公式ρ=ρ0+βexp(-ΔT)很好地拟合.热电势的测量表明,在超导转变前样品的残余热电势值非常小,这是膜的高质量引起的,三个样品在200K以上都出现一个宽峰,对其进行了一些理论模型分析,并与电子型超导体热电势结果作了比较. 关键词： 薄膜 输运性质 热电势     

Nd2CuO4及其微量Zn掺杂单晶样品的传输性质研究

用助溶剂方法高温烧制了电子型超导体母体材料Nd2CuO4及其微量Zn掺杂的单晶样品，并系统地研究了在15-300K范围内样品的电阻率和热电势行为．两者在25-200K之间的温区都表现相同的物理行为—高温区为半导体热激活行为，低温区为二维变程跳跃行为．在206K附近发现了反铁磁转变对热电势行为影响的精细峰和弛豫现象．我们的实验结果表明在180K以上温区载流子的行为不满足传统的能带理论．退火对25K以下的温区的热电势的行为有非常显著的影响，其本征的物理性质还需要深入的研究．     

La1.48Nd0.4Sr0.12CuO4的结构相变研究

用高分辨电镜观察了室温下La1.48Nd0.4Sr0.12CuO4和La1.88Sr0.12CuO4样品的原子结构像,观察到了其中的La2O2层和CuO2层沿c轴方向交叉堆砌.这两个样品的微结构非常相似,说明Nd掺杂没有造成周期性的调制结构.用变温X射线研究了La1.48Nd0.4Sr0.12CuO4样品中低温正交(LTO)到低温四方(LTT)结构相变,给出了晶格常数随温度的演化过程.     

激光调制交流测量高温热电势

本文设计了一种交流法测量室温到1200K的热电势的实验装置。与传统的直流测量方法相比，这一方法有许多优点：方便、适合小样品、能够测量单晶的各向异性，可以连续的测量热电势对温度的依赖关系。为了检验这一新方法的可靠性和准确性，我们测量了纯Pt丝的热电势。     

Sr2RuO4的热电势

Sr2RuO4是第一个无CuO面的层状强关联氧化物超导体.测量了9至260K温度范围内Sr2RuO4的热电势,观测到在此温度范围内其热电势为正值.用两种载流子模型对实验数据进行了拟合,并且与Hall系数的实验结果进行了比较,发现低温下两种载流子对热电势和Hall系数的贡献比较类似,但在高温区空穴对热电势的贡献很大而相应地对Hall系数的贡献不占主要地位. 关键词： Sr2RuO4 热电势     

La1．48Nd0．4Sr0．12CuO4的结构相变研究

用高分辨电镜观察了室温下La1．48Nd0．4Sr0．12CuO4和La1.88Sr0.12CuO4样品的原子结构像，观察到了其中的La2O2层和CuO2层沿c轴方向交叉堆砌．这两个样品的微结构非常相似，说明Nd掺杂没有造成周期性的调制结构．用变温X射线研究了La1．48Nd0．4Sr0．12CuO4样品中低温正交（LTO）到低温四方（LTT）结构相变，给出了晶格常数随温度的演化过程．     

CuO面外Sm掺杂对电子型超导体的影响北大核心CSCD

研究了Nd1.85-x Smx Ce0.15CuO4单晶的热电势S、霍耳系数RH和电阻率.在CuO2平面以外的阳离子格点引入无序,其程度是通过改变Sm含量来控制的.霍耳系数RH的测量证明这种名义掺杂没有改变载流子的密度.热电势S(T)在120K以上可用掺杂的半经验模型来分析,表明有电子的窄能带和宽能带的共存.在这个模型中,无论是态密度的带宽和有效电导率的展宽都随着x的增加而增加,而局域化的趋势随Sm掺杂增加而几乎不变.Sm掺杂对超导转变温度TC的抑制作用很小,这意味着超导TC可能与载流子局域化和巡游电子的能带结构有关.     

La_(1．6-x)Nd_(0．4)Sr_xCuO_4超导电性的反常特性

分析并讨论了La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列单相性样品(x=0．08～0．25)的晶体结构和超导电性。结果表明,晶体结构随着Sr掺杂量的变化规律与La_(2-x)Sr_x CuO_4系列相似。超导电性方面,La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列超导转变温度均远小于相应的La_(2-x)Sr_xCuO_4系列组分。同La_(1．875)Ba_(0．125)CuO_4相似,La_(1．48)Nd_(0．4)Sr_(0．12)CuO_4处出现显著的极小值8K但高于La_(1．875)Ba_(0．125)CuO_4。La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列的超导电性明显与La_(2-x)Sr_x CuO_4系列有着不同的特性,虽然同La_(2-x) Ba_xCuO_4系列相似但仍有区别。我们从其晶体结构的反常变化的角度对La_(1．6-x)Nd_(0．4)Sr_x CuO_4系列反常的超导电性做了初步的解释。     

Contrasting physical properties of the trilayer nickelates Nd_4Ni_3O_(10) and Nd_4Ni_3O_8

We report the crystal structures and physical properties of trilayer nickelates Nd4Ni3O10and Nd4Ni3O8.Measurements of magnetization and electrical resistivity display contrasting behaviors in the two compounds.Nd4Ni3O10shows a paramagnetic metallic behavior with a metal-to-metal phase transition(T^*)at about 162 K,as revealed by both magnetic susceptibility and resistivity.Further magnetoresistance and Hall coefficient results show a negative magnetoresistance at low temperatures and the carrier type of Nd4Ni3O10is dominated by hole-type charge carriers.The significant enhancement of Hall coefficient and resistivity below T*suggests that effective charge carrier density decreases when cooling through the transition temperature.In contrast,Nd4Ni3O8 shows an insulating behavior.In addition,this compound shows a paramagnetic behavior with the similar magnetic moment as that of Nd4Ni3O10derived from the Curie-Weiss fitting.This may suggest that the magnetic moments in both systems are contributed by Nd^3+ ions.By applying pressures up to about 49 GPa,the insulating behavior is still present and becomes even stronger under a high pressure.Our results suggest that the different Ni configurations(Ni^1+/2+ or Ni^2+/3+)and the changes of coordination environment of Ni sites may account for the contrasting behaviors in trilayer nickelates Nd4Ni3O10and Nd4Ni3O8.     

铁基K0.8Fe2-ySe2体系的单晶生长与结构研究

本文报道了采用自助溶剂法对不同组分K0.8Fe2-ySe2单晶生长和结构研究的实验结果.X-射线衍射XRD(Cu靶和同步辐射)结构分析表明样品中存在两套衍射峰(分别对应相1和相2),相1能够用ThCr2Si2结构,空间群I4/mmm(No.139)指标化,对应超导相.而相2能够用单一米勒指数(0 0l)标定,可能对应一个新的物相.相2的存在能够在不同组分的KxFe2-ySe2单晶样品中很好的重复.利用高分辨透射电子显微镜(TEM)研究了K0.8Fe2Se2样品的"结构相分离"特征,结果表明样品中存在明显的位错缺陷,可能与两相结构的竞争有关.对K0.8Fe2Se2单晶样品直流磁化测量表明相2可能为弱铁磁性,从而引起高温Tp～125K的ZFC与FC磁性反常.     

高压对高温超导体La1.85Sr0.15CuO4电子结构的影响

 采用Recursion方法计算了常压和1 GPa、4 GPa、8 GPa高压下La_1.85Sr_0.15CuO₄的电子结构，给出了费米能、费米能处态密度以及各晶位原子价的定量变化。结果表明：高压下T_c的变化可能源于CuO₂面上氧空穴和铜空穴数目的改变，且氧空穴数目的增加有利于超导性。     

用触针下分布电阻的光电导相位法测量半导体中非平衡载流子寿命

本文提出一种测量半导体中非平衡载流子寿命的新方法。这方法是测量触针下分布电阻的光电导相位移,在不同的表面情况和不同的激发光光谱成分下,导得各种结果表达式,文中对通常所使用的测量条件即粗磨表面和长波激发条件给出数值计算的结果。同时,对这些结果进行较为详细的分析和讨论,这个方法具有一系列的优点。例如:1)可以在锭状晶体上测量;2)表面处理十分简单;3)在样品上不需制作固定电极;4)测量仪器简单,操作方便;5)有一定准确度。这个方法可以检验不均匀材料,可供科研机关作研究用,更适合于工厂检验单晶材料用。用这方法在锗和硅样品上进行测量,测得寿命值基本上与其它方法的结果一致。     

Hydrophones' effective diameter measurements as a quasi-continuous function of frequency

The spatial averaging effect is strongly dependent on the active aperture of the hydrophone probes used to measure ultrasound fields. An experimental method was developed to determine the effective diameter of the probes as a quasi-continuous function of frequency. The implementation of the method utilizes the time delay spectrometry (TDS) technique and a set of focused acoustic sources. The use of focused sources ensured plane wave conditions for the whole frequency range and TDS eliminated all the reflections from the water tank boundaries. This approach allows effective diameter of circular aperture hydrophones to be determined as a quasi-continuous function of frequency up to 40 MHz. The measurements were performed for both needles and membrane designs having nominal diameters ranging from 50 to 500 microm. The results were successfully employed in the development of spatial averaging correction algorithms. Current efforts are being focused on extension of the frequency range up to 60 MHz by using a novel measurement technique termed time gating frequency analysis.     

Planar and Channel Optical Waveguide Loss Measurement Using Optical Phase Conjugation in BaTiO 3

A new methodology of the propagation loss coefficient measurement with an accuracy of at least 10 -3 cm -1 is demonstrated using a single-pass transmission measurement method and beam coupling based on a self-pumped phase conjugation process in a single BaTiO 3 crystal. A new, high-precision optical "bridge system" for routine measurement of the planar waveguide propagation loss coefficient is designed.     

Planar and Channel Optical Waveguide Loss Measurement Using Optical Phase Conjugation in BaTiO 3

A new methodology of the propagation loss coefficient measurement with an accuracy of at least 10 -3 cm -1 is demonstrated using a single-pass transmission measurement method and beam coupling based on a self-pumped phase conjugation process in a single BaTiO 3 crystal. A new, high-precision optical "bridge system" for routine measurement of the planar waveguide propagation loss coefficient is designed.     

Preparation and Raman Spectrum of Rutile Single Crystals Using Floating Zone Method

With anatase-type titanium dioxide as the raw materials, the futile type titanium dioxide single crystal is prepared using the floating zone method. The results of XRD measurement show that the grown crystal is highly crystalline with a futile structure, which has orientation to the c-axis. The four Raman vibration characteristic peaks （143, 240, 450 and 610 cm^-1） at room temperature show that the crystalline structure of the single crystal is a typical futile phase, meanwhile a new Raman peak at around 690 cm^-1 is found. The results of the Raman measurement at various temperatures for the single crystal show that the Raman frequency shifts are different.     

In∶Nd∶LiNbO3晶体倍频性能的研究

在 LiNbO₃中掺进In₂O₃和Nd₂O₃, 以Czochralski技术生长了In∶Nd∶LiNO₃晶体. 通过光斑畸变法测得In∶Nd∶LiNbO₃晶体的光损伤阈值为1.98×104 W/cm², 比Nd∶LiNbO₃晶体的1.6×102 W/cm²高两个数量级以上;晶体吸收光谱的测试表明, In∶Nd∶LiNbO₃晶体的吸收边相对Nd∶LiNbO₃晶体发生紫移. 研究了In∶Nd∶LiNbO₃晶体的倍频性能, 结果表明, In∶Nd∶LiNbO₃晶体的相位匹配温度在室温附近, 倍频转换效率比Nd∶LiNbO₃晶体提高二倍.