铜氧化物高温超导体的比热容

 基于Nozieres Schmitt Rink的强耦合超导理论和推广的Luttinger理论,文中分别计算了低电子浓度和高电子浓度体系的电子比热容及线性比热系数,描绘出线性比热系数随温度的变化曲线。电子线性比热系数的计算结果表明,在Nozieres Schmitt Rink理论和推广的Luttinger理论框架下,铜氧化物高温超导体中产生赝隙。     

准晶体的电子比热(英)

计算了10次准晶体Al-Cu-Co;;Al-Cu-Li和二十面体准晶Al-Mn-Si在0～1180K温度范围的电子比热;;发现这些准晶体的电子比热随温度增加近似线性地增加;;但其比热曲线的陡度随费米能级附近的态密度变化而发生较大变化.当电子态密度在费米能级附近的值较大时;;电子比热曲线的斜率较大.如:10次准晶Al-Cu-Co;;当温度到达1000K时;;电子比热为0.2209kB/atom;;很明显;;此时电子比热不能忽略;;讨论了这些准晶体中电子的化学势与温度的关系;;发现其化学势随温度增加而增加;;这一点准晶体与晶体材料完全不同.     

Bi2Sr2CaCu2O8+δ正常态霍尔系数的理论分析

以基于高温超导体双成分模型的近局域对理论分析Bi₂Sr₂CaCu₂O_8+δ正常态霍尔系数的温度行为,得到与实验相符的结果。指出赝隙随温度降低的不断加深导致了霍尔系数在比T_c略高的温度出现正的极大值,并且该极大值与赝隙深度随温度变化的谷底附近相对应。     

准晶体的电子比热

计算了10次准晶体Al-Cu-Co，Al-Cu-Li和二十面体准晶Al-Mn-Si在0～1180K温度范围的电子比热，发现这些准晶体的电子比热随温度增加近似线性地增加，但其比热曲线的陡度随费米能级附近的态密度变化而发生较大变化。当电子态密度在费米能级附近的值较大时，电子比热曲线的斜率较大。如：10次准晶Al-Cu-Co，当温度到达1000K时，电子比热为0．2209kB／atom，很明显，此时电子比热不能忽略，讨论了这些准晶体中电子的化学势与温度的关系，发现其化学势随温度增加而增加，这一点准晶体与晶体材料完全不同。     

高T_c氧化物超导体的LO耦合机制

本文利用Eliashberg超导理论,分析了高T_c氧化物超导体的LO耦合机制,给出了此类超导体e-LO耦合参数λ、库仑赝势μ~n、临界温度T_c和0K能隙△_0与T_c比值的计算公式。     

量子棒中强耦合磁极化子基态能量的磁场和温度依赖性

基于Huybrechts-Lee-Low-Pines变分法,研究量子棒中强耦合磁极化子基态能量的磁场和温度依赖性.结果表明:磁极化子基态能量的绝对值随温度参数的增加而增大,随电子-声子耦合强度的增加而增大,随磁场的回旋频率的增加而减小,随量子棒受限强度的增加而减小;磁极化子的基态能量随量子棒纵横比的变化规律呈""型曲线,并受到温度的显著影响.     

电-声机制致高温超导特性随掺杂量演变

利用极性晶体中慢电子的运动规律提出基于BCS机理的分析模型.分析发现,掺杂导致铜氧化物的费米面电子分为低能费米电子和高能电子,并产生快电子效应.欠掺杂区高能电子与晶格振动发生作用形成赝隙,费米电子与晶格作用形成超导隙.赝隙和超导隙的竞争平衡导致欠掺杂区超导隙2Δ0和κBTc比值与高能费米电子速度vF^＊成正比,与费米速度vF成反比.掺杂量（x）与高能费米速度（vF^＊）相图是解释高温超导相图的基础.利用模型和（x-vF^＊）相图能够对欠掺杂区赝隙温度、能隙与κBTc比例随掺杂量增加而线性下降的现象给出简单解释,即最佳掺杂区2Δ0和κBTc比值为7.6、过掺杂区为4.3的常数.显示系统随掺杂量增加由＂非传统＂行为向接近传统行为再到金属行为演变的规律.     

CdS_xSe_(1-x)合金结构、电子结构和光学性质的第一性原理研究

用密度泛函理论究了闪锌矿型三元合金体系Cd SxSe1-x的晶体结构、电子结构和光学性质.计算了组分参数在0≤x≤1范围内Cd SxSe1-x的电子结构、态密度和带隙,计算结果表明Cd SxSe1-x为直接带隙半导体材料,其带隙随Se含量的增加而减小.分析了Cd SxSe1-x的复介电函数和吸收系数等光学性质随光子能量变化的关系,随Se元素含量增加,各光学特性曲线向低能方向移动.     

高压下3C SiC的电子结构、弹性与热力学性质

运用密度泛函理论的超软赝势平面波方法对3C SiC晶体结构进行了几何优化,得到与实验值相符的晶格参数,在压强为0～100GPa范围内对3C SiC的电子结构与弹性进行了计算,结果表明晶体结构是稳定的,且带隙随着压强的增大而减小。然后利用准谐德拜模型研究了3C SiC在温度为0～2100K、压强为0～100GPa范围内的热力学性质,结果表明其等容热容、热膨胀系数及熵函数都随温度的升高而增大,随压强的增大而减小,而德拜温度随温度的升高而减小,随压强的增大而增大。     

Ce:KNSBN光折变晶体两波耦合特性研究

采用Ar+ 514.5nm的o光和e光测量了Ce:KNSBN晶体两波耦合增益系数随写入光夹角的变化关系曲线 ,得到了Ce:KNSBN晶体的有效电荷密度、有效电光系数和电子 -空穴对抗率等参数.依据光折变理论对增益系数随写入光夹角变化关系实验数据进行了拟合 ,拟合曲线与实验结果符合较好     

Superconductivity in MgB2

In January of 2001 the superconductivity of the compound MgB2 with a critical temperature Tc of up to 39 K was discovered. This Tc is the highest in all intermetallic compound and alloy superconductors. MgB2 has a simple structure and its manufacturing capital cost is lower, therefore it could become a practical superconductor in the future. The recent progress is reviewed here which covers the progress in electronic structure, high Tc mechanism, superconducting parameters (Debye temperature, specific heat coefficient of electron, critical fields, coherent length, penetration depth, energy gap, critical current and relaxation rate of flux). Moreover the issue on power transmission is discussed.     

Local switching of two-dimensional superconductivity using the ferroelectric field effect

Correlated oxides display a variety of extraordinary physical properties including high-temperature superconductivity and colossal magnetoresistance. In these materials, strong electronic correlations often lead to competing ground states that are sensitive to many parameters--in particular the doping level--so that complex phase diagrams are observed. A flexible way to explore the role of doping is to tune the electron or hole concentration with electric fields, as is done in standard semiconductor field effect transistors. Here we demonstrate a model oxide system based on high-quality heterostructures in which the ferroelectric field effect approach can be studied. We use a single-crystal film of the perovskite superconductor Nb-doped SrTiO3 as the superconducting channel and ferroelectric Pb(Zr,Ti)O3 as the gate oxide. Atomic force microscopy is used to locally reverse the ferroelectric polarization, thus inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature. Field-induced switching from the normal state to the (zero resistance) superconducting state was achieved at a well-defined temperature. This unique system could lead to a field of research in which devices are realized by locally defining in the same material superconducting and normal regions with 'perfect' interfaces, the interface being purely electronic. Using this approach, one could potentially design one-dimensional superconducting wires, superconducting rings and junctions, superconducting quantum interference devices (SQUIDs) or arrays of pinning centres.     

Vortex-like excitations and the onset of superconducting phase fluctuation in underdoped La(2-x)Sr(x)CuO4

Two general features of a superconductor, which appear at the critical temperature, are the formation of an energy gap and the expulsion of magnetic flux (the Meissner effect). In underdoped copper oxides, there is strong evidence that an energy gap (the pseudogap) opens up at a temperature significantly higher than the critical temperature (by 100-220 K). Certain features of the pseudogap suggest that it is closely related to the gap that appears at the critical temperature (for example, the variation of the gap magnitudes around the Fermi surface and their maximum amplitudes are very similar). However, the Meissner effect is absent in the pseudogap state. The nature of the pseudogap state, and its relation (if any) to the superconducting state are central issues in understanding copper oxide superconductivity. Recent evidence suggests that, in the underdoped regime, the Meissner state is destroyed above the critical temperature by strong phase fluctuations (as opposed to a vanishing of the superfluid density). Here we report evidence for vortices (or vortex-like excitations) in La(2-x)Sr(x)CuO4 at temperatures significantly above the critical temperature. A thermal gradient is applied to the sample in a magnetic field. Vortices are detected by the large transverse electric field produced as they diffuse down the gradient (the Nernst effect). We find that the Nernst signal is anomalously enhanced at temperatures as high as 150 K.     

Quantum supercurrent transistors in carbon nanotubes

Electronic transport through nanostructures is greatly affected by the presence of superconducting leads. If the interface between the nanostructure and the superconductors is sufficiently transparent, a dissipationless current (supercurrent) can flow through the device owing to the Josephson effect. A Josephson coupling, as measured by the zero-resistance supercurrent, has been obtained using tunnel barriers, superconducting constrictions, normal metals and semiconductors. The coupling mechanisms vary from tunnelling to Andreev reflection. The latter process has hitherto been observed only in normal-type systems with a continuous density of electronic states. Here we investigate a supercurrent flowing through a discrete density of states-that is, the quantized single particle energy states of a quantum dot, or 'artificial atom', placed between superconducting electrodes. For this purpose, we exploit the quantum properties of finite-sized carbon nanotubes. By means of a gate electrode, successive discrete energy states are tuned on- and off-resonance with the Fermi energy in the superconducting leads, resulting in a periodic modulation of the critical current and a non-trivial correlation between the conductance in the normal state and the supercurrent. We find, in good agreement with existing theory, that the product of the critical current and the normal state resistance becomes an oscillating function, in contrast to being constant as in previously explored regimes.     

Superinsulator and quantum synchronization

Synchronized oscillators are ubiquitous in nature, and synchronization plays a key part in various classical and quantum phenomena. Several experiments have shown that in thin superconducting films, disorder enforces the droplet-like electronic texture--superconducting islands immersed into a normal matrix--and that tuning disorder drives the system from superconducting to insulating behaviour. In the vicinity of the transition, a distinct state forms: a Cooper-pair insulator, with thermally activated conductivity. It results from synchronization of the phase of the superconducting order parameter at the islands across the whole system. Here we show that at a certain finite temperature, a Cooper--air insulator undergoes a transition to a superinsulating state with infinite resistance. We present experimental evidence of this transition in titanium nitride films and show that the superinsulating state is dual to the superconducting state: it is destroyed by a sufficiently strong critical magnetic field, and breaks down at some critical voltage that is analogous to the critical current in superconductors.     

TR唯象模型中的集体激发对比热的贡献

通过计算非费米液体重费米子体系的ＴＲ唯象模型中的集体激发对比 热的贡献,发现集体激发对比热的贡献与温度的４／３次幂成正比．这说明不能从 集体激发对比热的贡献来解释在实验上观察到的电子比热系数为温度的对数函数的非费来液体行为．     

Spin-orbit-coupled Bose-Einstein condensates

Spin-orbit (SO) coupling--the interaction between a quantum particle's spin and its momentum--is ubiquitous in physical systems. In condensed matter systems, SO coupling is crucial for the spin-Hall effect and topological insulators; it contributes to the electronic properties of materials such as GaAs, and is important for spintronic devices. Quantum many-body systems of ultracold atoms can be precisely controlled experimentally, and would therefore seem to provide an ideal platform on which to study SO coupling. Although an atom's intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the centre-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba and Dresselhaus strengths) in a neutral atomic Bose-Einstein condensate by dressing two atomic spin states with a pair of lasers. Such coupling has not been realized previously for ultracold atomic gases, or indeed any bosonic system. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin-mixed state (lasers off) to a phase-separated state (above a critical laser intensity). We develop a many-body theory that provides quantitative agreement with the observed location of the transition. The engineered SO coupling--equally applicable for bosons and fermions--sets the stage for the realization of topological insulators in fermionic neutral atom systems.     

The origin of the anomalous superconducting properties of MgB(2)

Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB(2) is different. Some early experimental studies have indicated that MgB(2) has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB(2) and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.     

强关联和自旋磁交换作用对高温超导体临界温度的影响

提出了一种可能的超导理论，该理论强调了格点上的强关联 自旋磁综合利用知雪导机制中的重要性。推出了铜氧化合物超导体的临界温度公式。结果表明，临界瀑 仅强烈地依赖于自旋磁相互作用和格点 的强关联相互作用，而且也依赖于掺杂浓度。