Charge Density Wave States and Structural Transition in Layered Chalcogenide TaSe_(2-x)Te_x

The structural features and three-dimensional nature of the charge density wave （CDW） state of the layered chalcogenide 1T-TaSe2-xTex （0≤x≤2.0） are characterized by Cs-corrected transmission electron microscopy measurements. Notable changes of both average structure and the CDW state arising from Te substitution for Se are clearly demonstrated in samples with x〉0.3. The commensurate CDW state characterized by the known star-of-David clustering in the 1T-TaSe2 crystal becomes visibly unstable with Te substitution and vanishes when x=0.3. The 1T-TaSe2-xTex （0.3≤x≤1.3） samples generally adopt a remarkable incommensurate CDW state with monoclinic distortion, which could be fundamentally in correlation with the strong qq-dependent electron-phonon coupling-induced period-lattice-distortion as identified in TaTe22. Systematic analysis demonstrates that the occurrence of superconductivity is related to the suppression of the commensurate CDW phase and the presence of discommensuration is an evident structural feature observed in the superconducting samples.

Photoinduced phase transitions in two-dimensional charge-density-wave 1T-TaS_2

Charge-density-wave(CDW) materials with strongly correlated electrons have broadband light absorption and ultrafast response to light irradiation, and hence hold great potential in photodetection. 1 T-TaS2 is a typical CDW material with various thermodynamically CDW ground states at different temperatures and fertile out-of-equilibrium intermediate/hidden states. In particular, the light pulses can trigger melting of CDW ordering and also forms hidden states, which exhibits strikingly different electrical conductivity compared to the ground phase. Here, we review the recent research on phase transitions in 1 T-TaS2 and their potential applications in photodetection. We also discuss the ultrafast melting of CDW ordering by ultrafast laser irradiation and the out-of-equilibrium intermediate/hidden states by optical/electrical pulse. For photodetection, demonstrations of photoconductors and bolometers are introduced. Finally, we discuss some of the challenges that remain.

Charge density wave states in phase-engineered monolayer VTe_(2)

Charge density wave(CDW)strongly affects the electronic properties of two-dimensional(2D)materials and can be tuned by phase engineering.Among 2D transitional metal dichalcogenides(TMDs),VTe_(2)was predicted to require small energy for its phase transition and shows unexpected CDW states in its T-phase.However,the CDW state of H-VTe_(2)has been barely reported.Here,we investigate the CDW states in monolayer(ML)H-VTe_(2),induced by phase-engineering from T-phase VTe_(2).The phase transition between T-and H-VTe_(2)is revealed with x-ray photoelectron spectroscopy(XPS)and scanning transmission electron microscopy(STEM)measurements.For H-VTe_(2),scanning tunneling microscope(STM)and low-energy electron diffraction(LEED)results show a robust 2√3×2√3CDW superlattice with a transition temperature above 450 K.Our findings provide a promising way for manipulating the CDWs in 2D materials and show great potential in its application of nanoelectronics.

Tunable charge density wave in TiS3 nanoribbons

Recently, modifications of charge density wave（CDW） in two-dimensional（2D） show intriguing properties in quasi-2D materials such as layered transition metal dichalcogenides（TMDCs）. Optical, electrical transport measurements and scanning tunneling microscopy uncover the enormous difference on the many-body states when the thickness is reduced down to monolayer. However, the CDW in quasi-one-dimensional（1D） materials like transition metal trichalcogenides（TMTCs） is yet to be explored in low dimension whose mechanism is likely distinct from their quasi-2D counterparts.Here, we report a systematic study on the CDW properties of titanium trisulfide（TiS3）. Two phase transition temperatures were observed to decrease from 53 K（103 K） to 46 K（85 K） for the bulk and 〈 15-nm thick nanoribbon, respectively,which arises from the increased fluctuation effect across the chain in the nanoribbon structure, thereby destroying the CDW coherence. It also suggests a strong anisotropy of CDW states in quasi-1D TMTCs which is different from that in TMDCs.Remarkably, by using back gate of-30 V ～ 70 V in 15-nm device, we can tune the second transition temperature from110 K（at-30 V） to 93 K（at 70 V） owing to the altered electron concentration. Finally, the optical approach through the impinging of laser beams on the sample surface is exploited to manipulate the CDW transition, where the melting of the CDW states shows a strong dependence on the excitation energy. Our results demonstrate TiS3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.

Anisotropy of 2H-NbSe_(2)in the superconducting and charge density wave states

Anisotropy is an important feature of layered materials,and a large anisotropy is usually related to the two-dimensional charac teristics.We investigated the anisotropy of the layered transition metal dicalcogenide 2H-NbSe_(2)in the superconducting and charge density wave(CDW)states using magnetotransport measurements.In the superconducting state,the normalized H_(c2)^(‖c)/H_(p)is independent of the thickness of 2H-NbSe_(2),while H_(c2)^(‖ab)/H_p increases significantly with decreasing thickness,where H_p is the Pauli limiting magnetic field and H_(c2)^(‖c)anu H_(c2)^(‖ab)are the upper critical fields in the c and ab directions,respectively.It is found that the superconducting anisotropy parameterγH_(c2)=H_(c2)^(‖ab)/H_(c2)^(‖c)increases with reduction in the thickness of 2H-NbSe_(2).In the CDW state,the angular(θ)dependence of magnetoresistance,R(H,θ)scales with H(cos^(2)θ+γ_(CDW)^(-2)sin^(2)θ)^(1/2),which decreases with increasing temperature and disappears at about 40 K.It is found that the CDW anisotropy parameterγ_(CDW)is much larger than the effective mass anisotropy but does not change a lot for ultrathin and bulk samples.Our results suggest the existence of three-dimensional superconductivity and quasi-two dimensional CDWs in bulk 2H-NbSe_(2).

Flat band in hole-doped transition metal dichalcogenide observed by angle-resolved photoemission spectroscopy

Layered transition metal dichalcogenides(TMDCs)gained widespread attention because of their electron-correlationrelated physics,such as charge density wave(CDW),superconductivity,etc.In this paper,we report the high-resolution angle-resolved photoemission spectroscopy(ARPES)studies on the electronic structure of Ti-doped 1T-Ti_(x)Ta_(1-x)S_(2) with different doping levels.We observe a flat band that originates from the formation of the star of David super-cell at the x=5%sample at the low temperature.With the increasing Ti doping levels,the flat band vanishes in the x=8%sample due to the extra hole carrier.We also find the band shift and variation of the CDW gap caused by the Ti-doping.Meanwhile,the band folding positions and the CDW vector g_(CDW)intact.Our ARPES results suggest that the localized flat band and the correlation effect in the 1T-TMDCs could be tuned by changing the filling factor through the doping electron or hole carriers.The Ti-doped 1T-Ti_(x)Ta_(1-x)S_(2) provides a platform to fine-tune the electronic structure evolution and a new insight into the strongly correlated physics in the TMDC materials.

Theoretical investigation on the excited state intramolecular proton transfer in Me_(2)N substituted flavonoid by the time-dependent density functional theory method

Time-dependent density functional theory(TDDFT)method is used to investigate the details of the excited state intramolecular proton transfer(ESIPT)process and the mechanism for temperature effect on the Enol^(*)/Keto^(*)emission ratio for the Me_(2)N-substited flavonoid(MNF)compound.The geometric structures of the S_(0) and S_(1) states are denoted as the Enol,Enol^(*),and Keto*.In addition,the absorption and fluorescence peaks are also calculated.It is noted that the calculated large Stokes shift is in good agreement with the experimental result.Furthermore,our results confirm that the ESIPT process happens upon photoexcitation,which is distinctly monitored by the formation and disappearance of the characteristic peaks of infrared(IR)spectra involved in the proton transfer and in the potential energy curves.Besides,the calculations of highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)reveal that the electronegativity change of proton acceptor due to the intramolecular charge redistribution in the S_(1) state induces the ESIPT.Moreover,the thermodynamic calculation for the MNF shows that the Enol^(*)/Keto^(*)emission ratio decreasing with temperature increasing arises from the barrier lowering of ESIPT.

High Electrical Conductivity and Conspicuous Phase Transitions in Single Crystals of K-TCNQ

This paper is to report the temperature dependent electrical conductivity of single crystals of radical ion salt (RIS) potassium-TCNQ (K-tetracyanoquino- dimethane) in a wide range of temperatures from 30 to 500 K. This RIS is quasi-one-dimensional in nature. These single crystals of K-TCNQ are grown by different methods like electrochemical, solution growth and diffusion method. Activation energy is determined for the sample in different temperature regions and found different values. More than one semiconductor to metal phase transition is observed in the studied samples during electrical measurements below and above room temperature. All the features observed in the studied samples are analyzed in the framework of their molecular structure as well as under different effects like disorder, impurity, Coulomb interaction, charge density wave (CDW), scattering and 3-D effects etc.

Layer-dependent charge density wave phase transition stiffness in 1T-TaS_(2)nanoflakes evidenced by ultrafast carrier dynamics

Novel physical properties emerge when the thickness of charge density wave(CDW)materials is reduced to the atomic level,owing to the significant modification of the electronic band structure and correlation effects.Here,we investigate the layer-dependent CDW phase transition and evolution of the nonequilibrium state of 1T-TaS_(2)nanoflakes using pump-probe spectroscopy.Both the low-energy single-particle and collective excitation relaxations exhibit sharp changes at〜210 K,indicating a phase transition from commensurate CDW to nearly commensurate CDW state.The single particle process reveals that the phase transition stiffness(PTS)is thickness-dependent.Moreover,a small PTS is observed in thin nanoflakes,which is attributed to the reduced thickness that increases the fluctuation and inhibits the nucleation and growth of discommensurations.In addition,the phase mode vanishes when the discommensuration network appears.Our results suggest that the carrier dynamics could be an efficient operational approach to measuring the quantum phase transition in correlated materials.

苯酚羟基化制备苯二酚反应机理的理论研究

采用密度泛函理论 (DFT)在 UB3 LYP/ 6-3 1 1 G* * / / UB3 LYP/ 6-3 1 G* 水平上研究了水溶液中羟基自由基进攻苯酚的邻位和对位生成邻苯二酚和对苯二酚的反应机理 .结果表明 ,2个反应都存在 3个过渡态 ,3个中间体 ,并通过振动分析对过渡态进行了确认 .电荷密度的拓扑分析发现 ,邻位反应中羟基自由基的氧原子和苯酚环上的 2个氢原子之间形成了氢键 ,并相应地形成了六元环和五元环结构 .经单点能校正后 ,2个反应的主反应活化能十分接近 ,说明邻位和对位产物会同时存在 ,这与实验观测的结果一致 .

单体极性对甲基丙烯酸甲酯/丙烯酸丁酯/甲基丙烯酸钠无皂乳液共聚合的影响

研究了少量甲基丙烯酸钠（ＮａＭＡ）存在下单体极性对甲基丙烯酸甲酯（ＭＭＡ）／丙烯酸丁酯（ＢＡ）无皂乳液共聚合的影响．单体极性降低，使粒径减小、聚合速率提高、乳液表面张力和粘度降低、粒子表面电荷密度增大、聚合物分子量提高．ＭＭＡ／ＮａＭＡ无皂乳液聚合物只呈现一个玻璃化温度Ｔｇ，ＢＡ／ＮａＭＡ无皂乳液聚合物呈现两个Ｔｇ，ＭＭＡ／ＢＡ比为２∶１～１∶２的ＭＭＡ／ＢＡ／ＮａＭＡ无皂乳液聚合物呈现四个Ｔｇ，这是由于粒子外面相对富含ＯＳＯ－３和ＣＯＯ－基聚合物的亚层溶解了较多的水，使ＢＡ向粒子中心扩散，ＭＭＡ向外扩散，造成组成差异和相分离而引起的．

无起爆药雷管内管装药密度对燃烧转爆轰的影响研究

起爆药雷管生产中产生大量有毒废水,且其运输、贮存存在安全隐患。为克服这些困难,笔者自行制备无起爆药雷管。采用圆筒式金属内管中装填超细PETN(季戊四醇四硝酸酯),作为起爆元件,代替起爆药部分,研究装药密度对内管燃烧转爆轰(DDT)的影响。研究结果表明,在内管壁厚1～2 mm,内径Φ4.0 mm,长25 mm,装压密度为0.8～1.14 g/cm3的范围内,内管能可靠实现燃烧转爆轰。

双核Pt(Ⅱ)配合物的光谱结构和激发态性质的密度泛函理论研究

采用基于第一性原理的密度泛函理论对单核和双核三联吡啶Pt(Ⅱ)配合物[Pt(trpy)C≡CH]+(1)和[Pt(trpy)C≡CH]22+(2)的基态和激发态以及光谱性质进行了系统研究.结果揭示了双体配合物中Pt—Pt间距离在激发态时明显短于基态时的距离,而且双体聚合后最低能吸收和发射波长相对单体配合物发生了明显红移,这种激发的本质被指认为是来自于[dσ*(dδ*π*)]的MMLCT(metal-to-metal-to-ligand charge transfer)电荷转移跃迁.另外,对研究的配合物,用VWN(Vosko-Wilk-Nusair)泛函优化得到的几何和用SAOP(轨道势的统计平均)计算的光谱能量和实验值符合得很好,能够准确反映实验现象.

装填密度对乳化粉状炸药燃烧转爆轰敏感性的影响

采用电阻丝加热的点火方式,对不同装填密度的药床进行燃烧转爆轰试验。结果表明,药床装填密度过小或过大时,乳化粉状炸药均不能出现燃烧转爆轰现象。装填密度在0.68～0.89g/cm^3之间的药床,试验中出现了燃烧转爆轰现象,并且爆轰发生时间与装填密度之间呈“U”型曲线,最佳密度区间(0.78～0.84g/cm^3)药床最容易发生DDT现象。热点温度和质量燃耗率分析结果分别解释了爆轰发生时间与装填密度“U”型曲线的左、右两部分。

钝感剂及装药工艺对炸药燃烧转爆轰的影响

分析了高密度成型炸药装药的燃烧转爆轰(DDT)过程及其特性,采用电探针法分别研究了钝感剂以及熔铸与压装两种装药工艺对炸药DDT的影响,对比了不同条件下炸药发生DDT的诱导爆轰距离。结果表明,高密度成型装药的DDT过程,主要是燃烧压缩波汇聚形成冲击波,并诱发爆轰的机制。由于钝感剂具有较大热容和较小导热系数,炸药中加入钝感剂,使炸药的诱导爆轰距离变大。熔铸炸药抗压强度低于压装炸药,且呈一定的脆性,在燃烧过程中易发生破碎,导致燃烧面增大,并进一步提高燃速,因此熔铸工艺可以减小炸药的诱导爆轰距离。

钼、钨四重键化合物电子光谱的含时密度泛函理论研究

运用含时密度泛函理论(TD-DFT),对具有代表性的钼、钨四重键化合物M2Cl84-,M2Cl4(PMe3)4,M2(O2CCH3)4和M2(PhNCHNPh)4(M=Mo,W)的低占据电子激发能进行了计算,系统地考察了局域(LDA)、离域(GGA)和杂化(Hybrid)泛函对金属四重键化合物中d-d跃迁和电荷转移跃迁(CT)计算的适用性.研究结果表明,LDA和GGA泛函对d-d跃迁激发能计算的较为准确,但对CT跃迁激发能的计算误差较大.出现这种情况的原因在于CT跃迁具有显著的离域特征,采用不含时泛函(LDA或GGA)代替含时泛函后未能对电子在CT跃迁中的长程行为给予正确的描述.包含HF交换项的杂化泛函可以对CT跃迁激发能给予一定的修正,但却降低了d-d跃迁激发能的计算准确性.如果对两种跃迁同时进行严格而准确的描述,则需将准确的含时泛函应用于含时密度泛函理论的计算中.

二维h-BN材料p型可掺杂性研究

掺杂对提高半导体材料的载流子浓度,改善半导体器件性能具有关键意义。理论上,通过计算带电缺陷形成能和电荷转移能级可以预测半导体材料中的p型掺杂是否容易实现。本文基于广义梯度近似理论,利用第一性原理方法,结合二维带电缺陷校正技术,并使用维也纳第一性原理计算软件(VASP)系统计算了二维六方氮化硼(h-BN)中的几种掺杂体系,包括X B体系(X=Be,Mg,Ca,Sr)和Y N体系(Y=C,Si,Ge)共7种潜在的p型掺杂体系的缺陷性质。计算结果表明,SiN缺陷的受主离子化能为0.8 eV,表现为准浅能级受主,而其它6种缺陷均为深能级受主,无法为h-BN提供有效p型载流子。SiN具有较高的缺陷形成能,因而只能靠离子注入等非平衡方式掺入到h-BN中。

微观sdIBM - 2下^(188)Os核的研究

本文在微观sdIBM - 2框架下 ,计算了1 88Os核的能谱、约化E2跃迁矩阵元、E2跃迁玻色子结构函数 (BSF)以及第一个 2 +态到基态的跃迁电荷密度 (TCD)。理论结果与实验值作了比较 ,理论能谱成功地再现了实验能谱 ,低能态间的约化E2跃迁矩阵元以及 2 +1 态到0 +1 态的跃迁电荷密度与相应的实验值符合也较好 ,表明此模型对该核的较低能态描述较为合理。

壳聚糖/环氧氯丙烷对苯丙乳液表面施胶剂改性的研究

采用壳聚糖(CS)、环氧氯丙烷(ECH)作为改性物质对苯丙乳液(SAE)进行改性,控制CS添加量为苯丙乳液表面施胶剂绝干质量的3%~10%,mol(CS)∶mol(ECH)=1∶2~1∶5,对改性后乳液进行电荷强度、表面张力、玻璃化温度等性能表征。研究结果显示:改性后乳液表面张力增大,极性更好;玻璃化温度范围在30~80.2℃之间;改性聚合乳液pH值在4.1~6.1范围内较改性前具有更好的电荷密度,离子性更好,且pH值=4.1时电荷密度最强。研究结果表明添加CS、ECH对SAE表面施胶剂改性改善了其施胶效能。

钨掺杂对铷蓝青铜佩尔斯相变的影响

用电解还原法制备出了铷蓝青铜及其系列钨掺杂样品,实验研究表明:钨掺杂未改变铷蓝青铜单晶的晶体结构,但在佩尔斯相变温度(181 K)附近,纯铷蓝青铜出现电阻-温度曲线宽化行为,并发生金属到半导体的转变.当x=0.001时,钨掺杂铷蓝青铜Rb0.3Mo1-xWxO3呈金属性,出现平缓的佩尔斯相变;当x=0.003和0.020时,呈半导体态,其金属行为和佩尔斯相变消失.因此,引入钨在很大程度上抑制了佩尔斯相变及电荷密度波的形成,并导致佩尔斯相变温度下降和消失.