二维Hubbard模型光电导谱的研究

我们用巨正则量子蒙特卡洛和最大熵方法研究了二维正方格点上Ｈｕｂｂａｒｄ模型的光电导谱，在半满条件下，光电导谱呈现一能隙区，掺杂后，光电导谱在ω＝０处出现Ｄｒｕｄｅ峰，在能隙区出现一新峰，同时谱权重从高频区向能隙区转移，这些特下与高温氧化物超导体的光电导实验相符合。     

关于La_2CuO_(4-y)反铁磁性的研究

本文测量了三个不同制备条件的 La_2CuO_(4-y)样品以及原材料 CuO 样品在液氮温度以上的比热,实验发现四个样品在200K 附近都有明显的比热反常峰,分析表明 La_2CuO_(4-y)的比热反常并非 CuO 杂质相引起,而是样品发生反铁磁有序相变所致.在 Nèel 温度 T_N 附近,磁比热按 C_M∝1n|T-T_N|规律变化,与二维伊辛模型基本吻合,很小的熵变则表明大的零点量子自旋涨落的存在.其磁化率变化服从居里-外斯定律,并计算出 Cu^(2+)的有效磁矩为～0.6μ_B.低温下电阻按 InR∝(T_c/T)^(1/3)规律变化,表现为二维空穴导电机制.     

高温超导体指数损耗的有限元数值分析及相关问题讨论

本文从高温超导体的非线性E～J指数关系式出发,分析了热致磁通蠕动模型下其等效电导率的特性,讨论如何建立针对这种非线性电导率的有限元计算模型,然后分别给出计算交流磁化损耗和传输损耗的二维数值仿真模型,模型针对银基Bi-2223带材,施加相应的边界条件可以计算不同物理条件下的交流损耗.分析此数值模型所得到的带材横截面中电流密度分布特点以及交流损耗热功率的值,并与利用相关解析式得到的结果作对比,证明此二维模型完全适用于分析多种复杂物理条件下超导体的交流损耗问题.     

二维Spin Glass系统——随机格点SO(3)自旋模型的Monte Carlo模拟

本文提出了一种定义在二维随机格点上的Spin Glass模型——具有±J_O Frustration效应的SO(3)自旋系统。利用Monte Carlo方法计算了系统的平均内能,比热和拓扑激发等参量,并考察了系统的热滞过程、有限格点效应以及混合相跑动,以确定系统的相变规律。结果表明,此自旋系统不存在一阶和二阶相变过程。但发生由于拓扑涡旋凝聚引起的高阶相变,从而显示出与Spin Glass低温特性密切相关的Frustration和无序现象。此模型可能为研究Spin Glass磁性系统提出了一条新的途径。     

一类相变的Landau-deGennes模型——高斯近似

本文对一类相变的Landau-deGennes模型(以下简称L-deG模型)进行了高斯近似和平均场近似计算,获得一些“临界指数”的数值。例如,在高斯近似下,α=0.5≠α＇,γ=1≠γ＇,ν=0.5≠ν＇等等。我们计算了比热,并与液晶MBBA的实验进行了比较,讨论了向列相-各向同性液相相变与三重临界点的关系。文中特别着重讨论了一类与二类相变临界行为的异同。我们认为,二类相变的标度律结果不能简单地挪用到一类相变的情况。     

一种可能存在的两分量任意子模型

本文提出了一个可能存在的两分量任意子模型,在玻色表象下给出了多体波函数的一般形式.我们分析了规范矢势引起的相互作用,通过将规范矢势类比于一个二维静电场,发现这个两分量任意子模型近似等价于一个二维两分量库仑气体,在低温下可能发生 Kosterlitz-Thouless 相变.     

二维自旋系统aging现象的数值模拟研究

应用动力学蒙特卡罗模拟方法，对二维Ising模型和二维XY模型的aging现象展开数值研究.系统在零外场条件下从高温无序相淬火到临界温度T_c时，通过测量自关联函数A(t,t′)，观察到二维Ising模型和二维XY模型的aging现象，获得更精确的动力学临界指数λ_c和z的值.特别是对二维XY模型，当初始态为完全无序态时，模拟结果证实存在关于标度行为的对数修正. 关键词： 蒙特卡罗模拟 相变和临界行为 aging现象     

各向异性超导体电阻转变的修正Kosterlitz-Thouless相变模型

从二维系统的Kosterlitz-Thouless (KT) 相变理论出发，在关联长度中引入热激活能和平均钉扎高度，提出了修正的KT相变模型.该模型与库伦气体标度理论和Halperin-Nelson关系具有一致性.应用修正的KT相变模型研究磁场下Tl₂Ba₂CaCu₂O_x(Tl-2212)薄膜电阻转变的标度行为，发现由电阻转变计算得到的平均钉扎高度与温度具有线性依赖关系，实验结果支持修正的KT相变模型. 关键词： 标度行为 各向异性超导体 电阻转变     

拓扑激发驱动的热力学相变及其张量网络研究方法

对热力学相及相变的认知构成了我们理解整个物质世界的物理基础,从朗道对称破缺相变范式到拓扑激发驱动的热力学相变,相变理论的研究发展在物质科学进步之路上树立起了一座座丰碑.一个著名的例子就是Berezinskii-Kosterlitz-Thouless相变,它是在从低温到高温的演变过程中, U(1)旋转对称性没有自发破缺情形下,成对涡旋的解耦合所致.近期,人们利用张量网络表示理论和数值计算方法,将统计模型的转移矩阵对应为一维量子模型.再根据量子模型纠缠熵的奇异性,在热力学极限下可以精确确定系统的相图,并准确计算各种物理量,该研究方法为研究具有连续对称性的二维系统的拓扑相变注入了新活力.     

基于投影纠缠对态算法优化的研究

二维强关联电子量子格点系统的投影纠缠对态(PEPS)算法是数值计算领域中研究二维强关联电子量子格点系统最为重要的张量网络算法.基于PEPS算法研究二维量子XYX模型与二维量子Ising模型,本文对PEPS算法进行了一些优化和改进研究,这些优化和改进主要体现在如何进行PEPS张量的更新与如何进行物理观测量的计算这两个方面,从而可以大大提高计算资源的利用.因而优化和改进后的PEPS算法可为研究热力学极限下的二维强关联电子量子格点系统的量子相变和量子临界现象提供一种更有效的强大的工具.     

Growth and dc conductivity studies of tripotassium sodium dichromate single crystal

DC electrical conductivity studies were carried out along the three crystallographic axes for tripotassium sodium dichromate (K₃Na(CrO₄)₂ or KNCr). Earlier studies of phase transition in this crystal show successive phase transitions at 239 K and 853 K. In this paper we report the dc electrical conductivity measurements in the temperature region 303–430 K along the crystallographic axes. An anomaly in conductivity was obtained around 326 K along both the axes. This may be attributed as due to a newly observed phase transition in the crystal. DSC taken for the sample also shows exothermic peak supporting the occurrence of newly observed phase transition.     

Specific features of electrical conductivity of V<Subscript>3</Subscript>O<Subscript>5</Subscript> single crystals

The electrical conductivity of V₃O₅ single crystals has been investigated over a wide temperature range, including the region of existence of the metallic phase and the region of the transition from the metallic phase to the insulating phase. It has been shown that the low electrical conductivity of metallic V₃O₅ is caused, on the one hand, by a lower concentration of electrons and, on the other hand, by a strong electronelectron correlation whose role with decreasing temperature increases as the phase transition temperature is approached. The temperature dependence of the electrical conductivity of the insulating phase of V₃O₅ has been explained in the framework of the theory of hopping conduction, which takes into account the effect of thermal vibrations of atoms on the resonance integral.     

Finite-Temperature Optical Conductivity of the Two-Dimensional Hubbard Model

The finite-temperature optical conductivity of the two-dimensional Hubbard model is studied by means of the grand canonical quantum Monte Carlo method. Results are reported for the half-filled and nonhalf-filled band. The formation and variation of Hubbard gap are observed. For the half-filled band case, our results support that there exists a high-temperature insulator-metal transition.     

Magnetic field dependence of the hexagonal to isotropic transition temperature of a single-walled carbon nanotubes dispersed lyotropic liquid crystal

We have carried out electrical conductivity studies on a single-walled carbon nanotubes dispersed lyotropic liquid crystal consisting of 50 wt.% TX-100 in water as a function of magnetic field and temperature. This system exhibits hexagonal and isotropic phases on heating. For all the applied magnetic fields, the temperature dependence of electrical conductivity of the carbon nanotubes dispersed lyotropic liquid crystal system exhibits a discontinuous change at the hexagonal to isotropic transition temperature. We find that the magnetic field dependence of the hexagonal to isotropic transition temperature is similar to that of the viscosity of the system. Using photo images of the sample, we find that the carbon nanotubes in the lyotropic liquid crystal form magnetic field dependent aggregates. We find spherical, rod and hook-like nanotube aggregates for low and high applied magnetic fields respectively. These nanotube aggregates alter the viscosity of our system which in turn alters the transition temperatures.     

Spingläser — einige typische physikalische eigenschaften und modelle

In this review-article we first present and discuss some typical physical properties of spin-glass-alloys, such as magnetic susceptibility, hysteresis, time-dependent magnetic effects, Mössbauer-spectrum, neutron magnetic scattering, specific heat and electrical conductivity. In the second part we discuss some attempts to understand the empirical properties of these systems on the background of model-calculations. Some of the existing theories explain the low-temperature-properties of spin-glasses on the basis of a magnetic phase transition (of third order), others consider the possibility of a peculiar magnetic freezing phenomenon. More exact measurements of specific heat in the critical temperature region should decide this question.     

The influence of Zn ions substitution on the transport properties of Mg-ferrite

The thermoelectric power and electrical conductivity measurements of Zn-substituted Mg-ferrites having the general formula Mg_1−xZn_xFe₂O₄ (where x=0, 0.2, 0.4 and 0.6) were carried out from room temperature to 773 K. The Seebeck coefficient is positive for all the compositions showing that these ferrites behave as p-type semiconductors and the majority charge carriers are holes. The temperature variation of the Seebeck coefficient is also discussed. The Fermi energy (E_F); the density of charge carriers (n) and the carriers mobility (μ) were determined for the studied system. The variation of log σ with reciprocal of temperature shows a discontinuity at Curie temperature. The DC electrical conductivity increases with increasing temperature ensuring the semiconducting nature of the samples. The Curie temperature determined from DC electrical conductivity was found in satisfactory agreement with that determined from initial magnetic permeability measurements. This transition temperature is found to decrease with increasing Zn concentration. The activation energy in the paramagnetic region is found to be lower than that in ferrimagnetic region. The variation of room temperature conductivity with composition indicates that conductivity increases with increasing Zn content. The dependence of the electrical conductivity of Mg-Zn ferrite on Zn content is explained on the basis of the cation distribution.     

Influence of hydrogenation on electrical conductivity of vanadium dioxide thin films

The influence of hydrogenation on electrical conductivity of vanadium dioxide thin films has been investigated. It has been shown using measurements of the electrical conductivity that the hydrogenation of vanadium dioxide thin films leads to a decrease in the temperature of the phase transition from the tetragonal phase (with “metallic” conductivity) to the semiconducting monoclinic phase. It has been found that, upon doping of vanadium dioxide with hydrogen, the electrical conductivity of the monoclinic phase can increase by several orders of magnitude. Nonetheless, the temperature dependence of the electrical conductivity of hydrogenated films exhibits a typical semiconducting behavior in the temperature range where the monoclinic phase is stable.     

Electrical and magnetic properties of pseudo-one-dimensional calcium iridium oxide Ca5Ir3O12

The electrical and magnetic properties of one-dimensional calcium iridium oxide Ca₅Ir₃O₁₂ are investigated. A weak ferromagnetic transition has been found at 7.5 K through magnetic susceptibility measurements. At the same temperature, a λ-type specific heat anomaly has been observed. The effective magnetic moments in the paramagnetic temperature range and the magnetic entropy due to the magnetic transition indicates that the tetravalent and pentavalent Ir ions exist in the ratio of 1:2. Another λ-type anomaly has been observed at 105 K in the temperature dependence of the specific heat. The electrical conductivity shows one-dimensional Mott variable-range hopping conduction behavior.     

Specific features of the electrical conductivity of V<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

The electrical conductivity of V₄O₇ single crystals has been measured over a wide temperature range, including both the region of existence of the metallic phase and the region of the metal-insulator transition. It has been shown that the low conductivity of metallic V₄O₇ is due to the strong electron-electron correlation, whose role increases with decreasing temperature as the phase-transition temperature is approached. The temperature dependence of the conductivity of the insulator phase of V₄O₇ is explained in terms of the theory of hopping conduction taking into account the influence of atomic thermal vibrations on the resonance integral.     

具有本征低晶格热导率的硫化银快离子导体的热电性能

硫化银(Ag_2S)是一种典型的快离子导体材料,前期关于Ag_2S的研究主要集中在光电和生物等领域.最近的研究表明, a-Ag_2S具有和金属一样的良好延展性和变形能力.但是, Ag_2S的热电性能尚无公开报道.本工作合成了单相Ag_2S化合物,系统研究了其在300—600 K范围的物相变化、离子迁移特性和电热输运性质.研究发现, Ag_2S在300—600 K温度区间表现出半导体的电输运性质.由于单斜-体心立方相晶体结构转变, Ag_2S的离子电导率、载流子浓度、迁移率、电导率、泽贝克系数等性质在455 K前后出现急剧变化.在550 K, Ag_2S的功率因子最高可达5μW·cm~(–1)·K~(–2). Ag_2S在300—600 K温度区间均表现出本征的低晶格热导率(低于0.6 W·m~(–1)·K~(–1)). S亚晶格中随机分布的类液态Ag离子是导致b-Ag_2S体心立方相具有低晶格热导率的主要原因.在573 K, Ag_2S的热电优值可达0.55,与Ag_2Se, Ag_2Te, CuAgSe等已报道的Ag基快离子导体热电材料的性能相当.