【作者】 杨金虎；

【导师】 方明虎；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2008， 博士

【摘要】 在一些强关联电子系统中(如：铜氧化物超导体、铁基超导体、重费米子以及一些钌氧化物),通过压力、磁场或元素替代等手段,将其磁相变温度抑制0 K,在该临界压力(pc)、磁场(Hc)或者临界替代浓度(xc)附近,系统的诸多物理性质呈现一系列反常。与由温度驱动的传统的相变不同,由外部条件(p,H,x)变化所诱导的这种从磁有序态转变为无序态的T=0 K量子相变,通常伴随着强烈的自旋量子涨落,从而决定了系统在很宽的相空间(T, H;T, p或T,x)的物理性质。由此可见,选择更多的磁性系统,甚至是传统的磁性化合物,采用元素替代、外加磁场或加压力等方法,抑制其长程磁有序,通过观测临界点(pc、Hc或xc)附近样品的物性,在更多的磁性系统中,证实其非Fermi液体行为与零温度下自旋量子涨落存在本质关联,这在凝聚态物理学中是一个非常重要的研究课题,其研究结果无论对高温超导电性的微观机制的理解还是对强关联电子系统物理的建立都具有重要意义。本论文共分为六章,其主要内容如下：第一章我们综述了近年来有关量子相变的理论和实验研究进展,同时对本论文研究的意义及内容给出简单的概括。第二章巡游电子铁磁系统Ni3Al1-x Gax中的量子临界现象研究。我们采用Arc-Melting方法制备了不同Ga替代浓度的Ni3Al1-xGax系列样品,对其结构、磁化强度、电阻率和比热的进行了系统观测,结果发现：随着Ga替代浓度x值的增加,系统的铁磁相变温度逐渐降低,在x=0.34附近被抑制至0 K,系统在该临界浓度附近发生了巡游电子铁磁到顺磁的量子相变。在国际首次获得Ni3Al1-xGax系统的磁相图；该临界浓度附近样品的低温磁化强度-磁场、起始磁化率-温度关系分别呈现M∞H1/3和1/χinitial∝T4/3标度律行为；不同温度下,与自旋量子涨落谱相关的γ值随着磁场变化呈现峰值效应,在国际上,首次通过对不同温度下测得的磁化曲线M(H)分析,结合理论解释,获得了自旋量子涨落谱信息；通过对其不同磁场下低温比热的观测,证实在该系统中自旋量子涨落对比热的贡献满足C/T oc-logT关系,同时发现该系统为中等关联电子系统；在量子临界点附近样品的电阻率-温度关系呈现非费米液体行为。第三章NiS2-xSex中的反铁磁量子临界现象研究。针对NiS2-xSex系统在x=1.0附近发生的反铁磁-顺磁的量子相交,制备了一系列NiS2-xSex(x= 0.96,0.98,1.00,1.05,1.10和1.20)多晶样品,对其结构、磁性质和电阻率进行了系统的观测。结果发现：样品磁化率-温度关系呈现典型的强关联电子系统特征,NiS2-xSex(x>1.0)中存在两类载流子,一类是巡游的,另一类是局域的,甚至可能存在动态的电子相分离；与铜氧化物超导体的正常态具有类似的电阻率-温度关系的线性行为；在反铁磁量子临界点附近的x=0.98和x=1.00样品,由于存在强烈的量子自旋涨落效应,低温下(3-30 K)其电阻率-温度满足p(T)∞T3.2关系,呈现非费米液体行为,而在偏离临界点(如x=1.10和1.20)样品的p-T又呈现费米液体理论所预期的T2关系。第四章有关Co(S1-xSex)2中的铁磁量子相变研究。针对Co(S1-xSex)2系统在x=0.11附近发生的铁磁金属到顺磁金属相变,制备了一系列不同Se替代浓度的样品,对其结构和电阻率-温度p(T)关系进行了系统观测,结果发现：系统铁磁相变温度TC随着Se替代浓度的增加,以x1／2关系单调下降,铁磁相变也由二级转变为一级相变。在临界浓度x=0.11附近,其p(T)关系由Fermi液体行为转变为非Fermi液体行为。第五章重费米子系统Ce1-xYxIn3中Y部分替代对其反铁磁有序抑制的研究。在采用助溶剂方法生长出具有不同Y替代浓度的Ce1-xYxIn3单晶样品的基础上,对其结构和磁性质进行了系统研究。结果发现,随着Y替代浓度的增加,其反铁磁相变温度单调下降,在x=0.38附近,其反铁磁相变温度被抑制至0 K,获得了Ce1-xYxln3系统磁相图。第六章近铁磁超导体MgCNi3的第Ⅱ类超导电性研究。采用高压合成的方法制备出高品质的MgCNi3超导多晶样品,其超导体积比接近100%。通过对其磁通钉扎机制的研究发现：其低场下的磁通钉扎主要起源于晶粒内部的晶格缺陷,与常规的固态反应法制备的多晶样品存在本质区别；同时还观测到高场下磁通格子软化所导致的体钉扎力峰值行为。另外,我们的研究还表明：用高压合成得到MgCNi3超导多晶样品4.2 K的临界电流密度与商业应用的NbTi线相当,为将来MgCNi3超导体材料的应用提供了一个可行的制备方法。更多还原

【Abstract】 In some strongly correlated electron systems (for example, cuprate superconductor, iron-based superconductor, heavy fermion compound and ruthenate oxide), the magnetic phase transition temperature can be suppressed to 0 K by adjusting some non-thermal control parameters, such as external pressure, magnetic field and element doping. A series of anomalous physical properties has been discovered in the critical region. In contrast to a classical phase transition driving by thermal fluctuation, a quantum phase transition from an order to a disorder state at 0 K occurs as the result of the variation of external parameter, accompanying by strong quantum spin fluctuations which dominates the behavior of electronic systems over a wide range of the phase diagram. So, it is important in condensed matter physics to research the physical properties close to the critical region and to confirm the relationship between non-Fermi liquid behavior in physical properties and quantum phase transition at 0 K. The result of such a study is a key to understand both high Tc superconductivity and the construction of physics in strongly correlated electron system.The content of the dissertation is divided into six chapters and the main results are summarized as follows:The Chapter 1 reviews the progress of the quantum phase transition both in theory and experiments, Meanwhile, the aims and content of the present work were given.In Chapter 2, we study the quantum critical phenomena in polycrystalline alloys Ni3Al1-xGax synthesized by arc-melting method. We investigated systemically their structure, magnetization, electronic resistivity and specific heat. It is found that the ferromagnetic phase transition temperature is depressed gradually with increasing substitution content x, of Ga for Al, and disappears near x= 0.34, where the system changes from an itinerant ferromagnetic state to a paramagnetic state, a quantum phase transition occurs. The magnetic phase diagram for Ni3Al1-xGax system was obtained. Near this quantum critical point, the field dependence of magnetization satisfies a scaling law M∝H1/3, and the temperature dependence of initial magnetic susceptibility in a way of 1/xinitial∝T4/3. We are the first group to observe the "peak" effect in y-H curves, which reflects the energy spectrum information of spin quantum critical fluctuation by analyzing M(H) curves in different temperatures. The temperature dependence of the part in specific heat due to the spin quantum fluctuation at low temperatures is of C/T oc-logT, which is well satisfied with theory. It is suggested that the strength of electron correlation in Ni3Al1-xGax is medium from the measurements of specific heat. And the non-Fermi liquid behavior in the resistivity near quantum critical point was found.In Chapter 3, we studied the antiferromagnetic quantum critical phenomena in the polycrystalline samples of NiS2-xSex(x= 0.96,0.98,1.00,1.05,1.10 and 1.20) which were prepared by a solid state reaction method. And the measurements on their structure, magnetization and resistivity were carried out. It is found that their temperature dependence of susceptibility shows a typical characteristic of a strongly correlated electron system. There are possible two kind of carrier, one is itinerant one, the other is localized one; or, there even exists dynamic electronic phase separation in NiS2-xSex (x>1.0); Similar to high Tc superconductors, the relationship between resistivity and temperature displays a linear behavior in a wider temperature region from 50 to 300 K. For x= 0.98 and 1.00 samples, which is close to the antiferromagnetic critical point, itsρ(T)∝T3/2 at lower temperatures (3-30K), which takes on a non-Fermi-liquid behavior due to the strong quantum spin fluctuation. The x= 1.10 and 1.20 samples, recovers to the Fermi liquid-behavior at lower temperature:ρ(T)∝T2.In Chapter 4, we studied the ferromagnetic quantum critical phenomena in polycrystalline samples of Co(S1-xSex)2 (0.0≤x≤0.16) which were prepared by a solid state reaction method. The measurements on their structure and resistivity were carried out. It is found that the ferromagnetic transition temperature Tc is suppressed by Se doping in 7C～(1-x)1/2 way. The ferromagnetic phase transition goes from the second to the first order. The temperature dependence of resistivity, p(T), shows a Fermi liquid behavior,ρ(T)=ρo+AT2 in Co(S1-xSex)2 (x< 0.08) samples, while non-Fermi liquid behavior of p(T) occurs in the samples near the critical concentration x= 0.11. It is suggested that the phase transition near x= 0.11 is a quantum phase transition and the quantum critical spin fluctuation at zero temperature results in non-Fermi liquid behavior.We investigated the effect of Y substitution for Ce in Ce1-xYxIn3 system. The single crystal Ce1-xYxIn3 samples were prepared by a self-flux method. We studied its structure and magnetic susceptibility systemically. It is found that the antiferromagnetic phase transition temperature is gradually suppressed with increasing substitution content x of Y, and disappears near x= 0.38. We gained the magnetic phase diagram of Ce1-xYxIn3.In the last Chapter, We investigated the typeⅡsuperconductivity in nearly ferromagnetic materials MgCNi3, which was prepared by high pressure method. The superconducting volume ration is almost 100%. We conclude that the flux pinning at lower fields results from the intra-granular structural defects which is different intrinsically in sample prepared by conventional solid state reaction. We also observed the "peak" effect caused by softening of the flux lattice in high magnetic field. On the other hand, it was found that the critical current density in MgCNi3 samples prepared by high pressure is almost the same value as that in NbTi wire. We put forward a practicable preparing method for superconductor MgCNi3 in application in the future.更多还原