CuGe1-xGaxO3的低温磁性质

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新型超导体MgB_2的热电势和电阻率研究

测量了MgB-2的热电势和电阻率与温度的依赖关系.在100K—300K区间,热电势呈近似线性温度依赖关系,其斜率为正,表明载流子为空穴型且与能带贡献的图像相一致.与此对应,在此温区电阻率呈T2依赖关系.在100K以下,热电势和电阻率各自转变了其高温区的温度依赖关系.热电势在超导转变温度Te(零电阻36.6K)到100K间有一宽峰,具有声子曳引峰的特征,表明电子-声子相互作用很强.估算了一些重要的参数,如带米能EF、能带宽度等.     

快速测量单晶样品低温热电势的新方法

对传统的热电势倒相测量方法进行了改进,发展了一种快速的、针对毫米级样品的准连续的热电势测量方法,在不降低测量精度的情况下大幅度提高了热电势的测量速度.我们用高温超导材料Nd1.84Ce0.16CuO4单晶样品对该法进行了不同变温速率的测试,并与传统的倒相法进行比较,一定变温速率(<30K/hr)下和传统方法数据吻合很好,有很高的测量精度.由于此法为准连续测量法,变温速率可精确设定,所以对有明显热滞效应的材料如电子型超导体母体材料Nd2CuO4等的热电势测量有十分重要的意义.     

La2CuO4掺锌样品的低温电阻率与热导率研究

对La2CuO4掺锌样品在不同降温速率下(330K保温30min后,分别以6Kh和02Ks的速率冷却至42K)电阻率(42—330K)和热导率(80—300K)随温度的变化关系进行了研究.实验结果表明,在不同降温速率下,热导率和电阻率都受到很大影响.快速降温过程使得130K以上的热导率减小,而热导率最小值出现在130K,且与降温速率无关.而低温下的热导率不受降温速率变化的影响.样品在高温区(T高于125K)电阻率随降温速率的增大而增大,低温区电阻率的非线性行为可用变程跳跃行为来描述.所有样品的热导率和电阻率在反铁磁相变温度都没有出现反常,这与能带理论框架下预期的结果和Anderson电荷自旋分离理论发生了矛盾,对此进行了讨论,并用极化子理论进行了自洽解释. 关键词： La2CuO4 热导率 电阻率     

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     

Realization of a flux-driven memtranstor at room temperature

The memtranstor has been proposed to be the fourth fundamental circuit memelement in addition to the memristor,memcapacitor, and meminductor. Here, we demonstrate the memtranstor behavior at room temperature in a device made of the magnetoelectric hexaferrite(Ba_(0.5)Sr_(1.5)Co_2Fe_(11) AlO_(22)) where the electric polarization is tunable by external magnetic field. This device shows a nonlinear q–р relationship with a butterfly-shaped hysteresis loop, in agreement with the anticipated memtranstor behavior. The memtranstor, like other memelements, has a great potential in developing more advanced circuit functionalities.     

Long-distance super-exchange and quantum magnetic relaxation in a hybrid metal–organic framework

The hybrid metal–organic framework [(CH3)2NH2]Fe(HCOO)3with a perovskite-like structure exhibits a variety of unusual magnetic behaviors at low temperatures. While the long-distance super-exchange through the Fe-O–CH-O–Fe exchange path leads to a canted antiferromagnetic ordering at TN～ 19 K, a second transition of magnetic blocking develops at TB～ 9 K. The stair-shaped magnetization hysteresis loops below TBresemble the behaviors of resonant quantum tunneling of magnetization in single-molecular quantum magnets. Moreover, the magnetic relaxation also exhibits several features of resonant quantum relaxation, such as the exponential law with a single characteristic relaxation time, and the nonmonotonic dependence of relaxation rate on the applied magnetic field with a much faster relaxation around the resonant fields. The origin of quantum tunneling behaviors in the [(CH3)2NH2]Fe(HCOO)3metal–organic framework is discussed in terms of magnetic phase separation due to the modification of hydrogen bonding on the long-distance super-exchange interaction.     

电子型超导体Sm2-xCexCuO4欠掺杂区域(0.09≤x≤0.15)的异常热电势和电阻率行为

测量了欠掺杂区域Sm2-xCexCuO4(0.09≤x≤0.15）多晶样品的热电势S和电阻率ρ的温度依赖关系，电阻率ρ在低于200K处斜率发生改变，表明低温下发生载流子弱局域化，热电势S高温下满足线性温度依赖关系，随着温度降低，在200－250K发生斜率改变，可以用赝能隙的开放解释，所有的样品在50K观察到一个曳引峰，表明载流子符号发生改变，由电子型转变为空穴型。     

Nd2-xSrxNiO4多晶输运性质的研究

研究了Nd2-xSrxNiO4多晶的输运性质.0< x0.5样品低温下的电阻率很好地满足变程跳跃公式,0.8x1.1样品在室温以上呈现金属性行为,随着温度降低出现金属-绝缘体转变,x=1.4样品的金属性从室温开始一直保持到14K. 用变程跳跃机制很好地解释了x=0.33样品的热电势-温度关系曲线.     

Magnetocaloric effect in the layered organic–inorganic hybrid(CH_3NH_3)_2CuCl_4

We present a study of magnetocaloric effect of the quasi-two-dimensional(2D) ferromagnet(CH_3NH_3)_2CuCl_4 in ab plane(easy-plane). From the measurements of magnetic field dependence of magnetization at various temperatures,we have discovered a large magnetic entropy change associated with the ferromagnetic–paramagnetic transition. The heat capacity measurements reveal an abnormal adiabatic change below the Curie temperature T_c~8.9 K, which is caused by the nature of quasi-2D layered crystal structure. These results suggest that perovskite organic–inorganic hybrids with a layered structure are suitable candidates as working substances in magnetic refrigeration technology.