Field-dependent resistive transitions in YBa_2Cu_3O_(7-δ) thin films:Influence of the pseudogap on vortex dynamics

The nature of resistive transition of high-quality crystalline thin films of YBa2Cu3O7-δhas been studied under magnetic fields（H） applied along the c direction over a wide range of doped holes, p, in the Cu O2 planes. The field- and temperature-dependent in-plane resistivity, ρab（T, H）, has been analyzed within the thermally assisted flux-flow（TAFF）formalism. The flux activation energy, U（T, H）, has been extracted from this analysis. The low-T part of the ρab（T, H）data can be described by an activation energy having the functional form of U（T, H） =（1- t）m（H-0/H）β, where t = T /Tc（reduced temperature）, and H0 is a field scale that primarily determines the magnitude of U（T, H）. The temperature exponent, m, shows a systematic variation with p, whereas the field exponent, β, is insensitive to the p values and is close to unity. The H0, on the other hand, changes rapidly as p is varied. U（T, H） is linked to the pinning potential and consequently on the superconducting condensation energy. Since the normal state pseudogap directly affects superconducting condensation energy, a clear correspondence between H0 and the PG energy scale, ε g, is found. Possible implications of these results are discussed.

Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface, and these pairs are incoherent and result in the pseudogap state. With the change of doping or temperature, some pairs are formed in the nodal region which locates the Fermi surface, and these pairs are coherent and lead to superconductivity. Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface. It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure, and non-s wave symmetry gap favours the high-temperature superconductivity. Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.

Study of Pseudogap in Underdoped Cuprate

A mean-field spin-density wave (SDW) analysis of pseudogap in the underdoped cuprates is proposed on the dependence of the energy gap which mimics the pseudogap near (π, 0) point at least. It implies that the pseudogap structure near (π, 0) is not sensitive to the long-range order and will survive leading to the pseudogap phenomenon in the underdoped metallic phase. On the other hand, in the long-range ordering antiferromagnetic region, the mean-field SDW theory holds and the pseudogap structure predicated by the theory should be observable experimentallyv. Then one prediction is that the pseudogap would smoothly extrapolate between itinerant antiferromagnetic phase and underdoped metallic phase.

Conservation of the particle–hole symmetry in the pseudogap state in optimally-doped Bi_(2)Sr_(2)CuO_(6+δ) superconductor

The pseudogap state is one of the most enigmatic characteristics in the anomalous normal state properties of the high temperature cuprate superconductors. A central issue is to reveal whether there is a symmetry breaking and which symmetries are broken across the pseudogap transition. By performing high resolution laser-based angle-resolved photoemission measurements on the optimally-doped Bi_(2)Sr_(1.6)La_(0.4)CuO_(6+δ) superconductor, we report the observations of the particle-hole symmetry conservation in both the superconducting state and the pseudogap state along the entire Fermi surface. These results provide key insights in understanding the nature of the pseudogap and its relation with high temperature superconductivity.

Experimental observation of pseudogap in a modulation-doped Mott insulator:Sn/Si(111)-(√3×√3)R30°

Unusual quantum phenomena usually emerge upon doping Mott insulators.Using a molecular beam epitaxy system integrated with cryogenic sc√annin√g tunneling microscope,we investigate the electronic structure of a modulation-doped Mott insulator Sn/Si(111)-(√3×√3)R30°.In underdoped regions,we observe a universal pseudogap opening around the Fermi level,which changes little with the applied magnetic field and the occurrence of Sn vacancies.The pseudogap gets smeared out at elevated temperatures and alters in size with the spatial confinement of the Mott insulating phase.Our findings,along with the previously observed superconductivity at a higher doping level,are highly reminiscent of the electronic phase diagram in the doped copper oxide compounds.

Temperature Evolution of Energy Gap and Band Structure in the Superconducting and Pseudogap States of Bi_2Sr_2CaCu_2O_(8＋δ) Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

We carry out detailed momentum-dependent and temperature-dependent measurements on Bi_2Sr_2CaCu_2O_（8＋δ）（Bi2212） superconductor in the superconducting and pseudogap states by super-high resolution laser-based angleresolved photoemission spectroscopy. The precise determination of the superconducting gap for the nearly optimally doped Bi2212（T_c= 91 K） at low temperature indicates that the momentum-dependence of the superconducting gap deviates from the standard d-wave form（cos（2Φ））. It can be alternatively fitted by including a high-order term（cos（6Φ）） in which the next nearest-neighbor interaction is considered. We find that the band structure near the antinodal region smoothly evolves across the pseudogap temperature without a signature of band reorganization which is distinct from that found in Bi_2Sr_2CuO_（6＋δ） superconductors. This indicates that the band reorganization across the pseudogap temperature is not a universal behavior in cuprate superconductors.These results provide new insights in understanding the nature of the superconducting gap and pseudogap in high-temperature cuprate superconductors.

Influence of the Pseudogap on the Inelastic Neutron Scattering Spectra of the Underdoped Lanthanum Cuprate

The influence of pseudogap on the inelastic neutron scattering spectra of the underdoped lanthanum cuprate is studied on the basis of the model which incorporates both the superconducting state and pscudogap state.It is found that the striking effects of the influence of the pseudogap on the incommensurability of the spin excitation spectrum are that in the superconducting state the pseudogap makes the intensity of the incommensurate peak increase,in the normal state the pseudogap not only makes the intensity of the incommensurate peak increase,but also sharpens the incommensurate peak and increases incommensurability.

Essence of Pseudogap and Oscillation in High-Temperature Cuprate Superconductor Junction of Pseudogap State

这篇论文给方法计算配对的温度T~*，在哪个 apseudogap 被打开，并且在哪个超导性出现，超导的温度 T_c 在 high-T_c 抓着率，并且直接在 T_c 【 T 【 T~表明那*没有远程的阶段连贯，假差距是库珀对的差距，并且在 T 【 T_c ，在库珀对之间有远程的阶段连贯。为交流 Josephson 效果在假差距状态和我们的机制上基于上述清楚的物理图画，这篇论文建议应该在 P-I-P 连接有新奇摆动的水流，在连接上由经常的偏爱导致了。这里，在库珀配对的地方，在假差距的 high-T_c 副牧师说的 Prepresents 没有 long-rangephase 连贯，并且我代表薄绝缘障碍。这篇论文推测那在有可能的高温度的超导性重重地，在做的 high-T_c 下面，杯评价。

Pseudogap phenomena in ultracold atomic Fermi gases

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high Tc superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum sinmlator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high Tc superconductivity. One obstacle to the ultimate understand- ing of high Tc superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics comnnmity, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fuctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high Tc superconductivity.

Microscopic evidence for the intra-unit-cell electronic nematicity inside the pseudogap phase in YBa_(2)Cu_(4)O_(8)

Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-T_(c) superconductors.Here,we report 17O nuclear magnetic resonance(NMR) studies on two planar oxygen sites in stoichio?metric cuprate YBa_(2)Cu_(4)O_(8) to investigate the symmetry breaking inside the pseudogap phase.We observe that the Knight shifts of the two oxygen sites are identical at high temperatures but different below T_(nem) ~185 K,which is close to the pseudogap temperature T^(*).Our result provides a microscopic evidence for intra-unit-cell electronic nematicity.The difference in quadrupole resonance frequency between the two oxygen sites is unchanged below T_(nem),which suggests that the observed nematicity does not directly stem from the local charge density modulation.Furthermore,a short-range charge density wave(CDW) order is observed below T=150 K.The additional broadening in the 17O-NMR spectra because of this CDW order is determined to be inequivalent for the two oxygen sites,which is similar to that observed in case of nematicity.These results suggest a possible connection between nematicity,CDW order,and pseudogap.

Pseudogap,Fermi arc,and Peierls-insulating phase induced by 3D-2D crossover in monolayer VSe2

One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials,as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene.Tran sition-metal dichalcogenides(TMDs)provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change,since they exhibit a variety of electronic/magnetic states owing to quantum confinement.Here we report an anomalous metal-i nsulator transition in duced by three-dimensional(3D)-two-dimensional(2D)crossover in mono layer 1T-VSe2 grown on bilayer graphene.We observed a complete insulating state with a finite energy gap on the entire Fermi surface in monolayer 1T-VSe2 at low temperatures,in sharp contrast to metallic nature of bulk.More surprisingly,monolayer 1T-VSe2 exhibits a pseudogap with Fermi arc at temperatures above the charge-density-wave temperature,showing a close resemblanee to high-temperature cuprates.This similarity suggests a common underlying physics between two apparently different systems,pointing to the importance of charge/spin fluctuations to create the novel electronic states,such as pseudogap and Fermi arc,in these materials.

The normal state of high temperature oxide superconductor——Pseudogapped metallic state

Looking back on the experiments about the normal state of thehigh temperature superconductor (HTS), the authors point out nine important experimental results. On the basis of these results, the authors have argued that the two-dimension, two-subsystem Hamiltonian is the appropriate starting point for describing the normal state of HTS. By this Hamiltonian, using the decoupling approximation of Green's function method by Kaga through numerical calculations, the authors have obtained the temperature dependent pseudogap in the density of states (DOS), which is consistent qualitatively with the experimental results by angle-resolved photoemission spectroscopy (ARPES). Theoretically, this Hamiltonian has the superconducting order parameter of d+s symmetry with d-wave as the main component, which is consistent with experiments. Further, the quantum electronic liquid in HTS is a near Fermi liquid in which there is coexistence of the delocalized states and nearly localized states, and there is finite probability for the nearly localized carriers to form the nearly localized carrier pairs at any finite temperature.

Observation of pseudogap in SnSe2 atomic layers grown on graphite

Superconducting metal dichalcogenides(MDCs)present several similarities to the other layered SI1-perconductors like cuprates.The superconductivity in atomically thin MDCs has been demonstrated by recent experiments,however,the investigation of the superconductivity intertwined with other or-ders are scarce.Investigating the pseudogap in atomic layers of MDCs may help to understand the superconducting mechanism for these true two dimensional(2D)superconducting systemns.Herein we report a pseudogap opening in the tunneling spectra of thin layers of SnSe2 epitaxially grown on highly oriented pyrolytic graphite(HOPG)with scanning tunneling microscopy/spectroscopy(STM/STS).A significant V-shaped pseudogap was observed to open near the Fermi level(Er)in the sTS.And at elevated temperatures,the gap gradually evolves to a shallow dip.Our experimental observations provide direct evidence of a pseudogap state in the electron-doped SnSe2 atomic layers on the HOPG surface,which may stimulate further exploration of the mechanism of superconductivity at 2D limit in MDCs.

Unveiling the charge density wave inhomogeneity and pseudogap state in 1T-TiSe_2

By using scanning tunneling microscopy(STM)/spectroscopy(STS), we systematically characterize the electronic structure of lightly doped 1 T-TiSe_2, and demonstrate the existence of the electronic inhomogeneity and the pseudogap state. It is found that the intercalation induced lattice distortion impacts the local band structure and reduce the size of the charge density wave(CDW) gap with the persisted 2 × 2 spatial modulation. On the other hand, the delocalized doping electrons promote the formation of pseudogap. Domination by either of the two effects results in the separation of two characteristic regions in real space, exhibiting rather different electronic structures. Further doping electrons to the surface confirms that the pseudogap may be the precursor for the superconducting gap. This study suggests that the competition of local lattice distortion and the delocalized doping effect contribute to the complicated relationship between charge density wave and superconductivity for intercalated 1 T-TiSe_2.

YBa_2Cu_3O_7和YBa_2Cu_3O_6能隙结构的计算

用电子密度泛函方法(DFT),系统计算了高温超导体YBa2Cu3O7单胞的能隙随温度变化(5K^300K)的规律。计算结果表明,其超导能隙值2?(T)在101meV量级;并在超导临界温度点(Tc=93K)附近,有超导能隙不为零的极小值;当温度超过Tc时,YBa2Cu3O7有赝能隙存在,且赝能隙随温度升高至T*=120K时,达到极大值;当温度超过T*时,赝能隙又有明显减小趋势。用DFT方法对YBa2Cu3O6单胞的能隙进行了计算,发现其能隙值是102meV量级,表明它具有半导体能级特征,因而是非超导体。

现代汉语假空缺句及其限制研究

本文就现代汉语假空缺句及其相关的各种省略限制条件进行了初步的综合分析。本文首先讨论了以往假空缺句研究中涉及到的一些难点、疑点问题,确认了以往文献中所谓的"汉语空缺句"的性质,即这类句子大多是一种类似于英语的假空缺结构。而后,本文就汉语假空缺句的特点、研究路径和句法生成机制等问题展开探讨,验证了本文所提出的系列省略限制假设对汉语假空缺句的解释力。

YBa2Cu3O7能隙及低温电子比热容的第一性原理计算

用电子密度泛函方法(DFT)对YBa2Cu3O7晶胞39个原子模型系统计算了其能隙随温度的变化规律,并在能隙计算基础上,应用电子比热的双极化和热激发的极化子模型计算了电子比热及其系数随温度的变化关系。计算结果表明:超导能隙在超导临界温度点93K附近有不为零的极小值;能隙曲线中的几个转折温度点依次分别与Tc,T*和T0相对应;在Tc^T*温度范围,超导态消失,但有反铁磁自旋相干引起的自旋能隙的存在;在T*~T0温度范围,有周期性条纹相产生的电子密度波引起的能隙的存在;在T=T0,电子比热系数发生了从增大到减小的转折变化。

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v*F成正比,与费米速度vF成反比.掺杂量(x)与高能费米速度(v*F)相图是解释高温超导相图的基础.利用模型和(x-v*F)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.

高温超导体中的能斯特效应与类涡旋激发

简要地介绍了通常金属的能斯特(Nernst)效应,指出 类超导体在混合态的磁通运动产生的能斯特效应远大于正常载流子的贡献.接着介绍了高温超导的La2-xSrxCuO4(LSCO)单晶中涡旋运动产生的大的能斯特信号从Tc一直延续到远高于Tc的温度T0,并报道在Zn掺杂的YBa2Cu3O7-δ(YBCO)外延薄膜中观测到的Tc和T0随Zn质量分数的变化.这些体系的能斯特效应结果充分地显示出高温超导体中在Tc以上可能存在着类磁通涡旋.根据预超导配对的理论图像对实验结果进行了讨论.

高温超导体中超导位相涨落:Nernst效应研究

高温超导体赝隙态具有许多反常的现象,与高温超导机理之间有密切联系,一直是研究的焦点。有理论提出在赝隙态存在预超导配对。能斯特(Nernst)效应测量探测到了超导转变温度TC0以上温区一定范围内存在磁通涡旋激发,支持了赝隙态中存在有限的超导序参量振幅和强烈的位相涨落的图象,说明TC0处的相变是由Cooper对之间长程位相关联的消失所驱动的。