LiFeAs超导体中磁性与声子软化

运用第一性原理密度泛函理论研究了铁基超导体LiFeAs的电子结构和声子谱.计算得到的LiFeAs基态具有涨落的条型反铁磁构型.通过比较LiFeAs在非磁态与条形反铁磁态下的声子态密度,发现,LiFeAs中各向异性自旋互作用的竞争产生了不稳定的自旋密度波和部分晶格位置弛豫,导致Fe和As原子振动模式的软化,从而提高电声子耦合强度.因此,自旋-声子互作用对非常规超导电性有重要贡献. 关键词： 铁基超导体 反铁磁序 超导电性 电声子耦合     

反铁磁链中Dzyaloshinskii–Moriya机制驱动矢量自旋手征束缚态的研究

Dzyaloshinskii-Moriya(DM)相互作用驱动的矢量自旋手征态,能和声子发生耦合,具备非常丰富的物理现象.本论文以一维反铁磁链中自旋手征-声子耦合模型为基础,研究不同声子环境下,耦合强度对自旋手征动力学演化过程的影响规律.结果表明,对自旋S=1/2的系统,在不同的声子浴(sub-Ohmic(0 1))中,均会在非相干到相干自旋涨落过程中产生无能隙一级相变,其来源是自旋手征束缚态的形成.相变发生的临界自旋-声子耦合强度正比于自旋涨落大小,反比于系统德拜频率.当自旋-声子耦合强度超过其临界值时,自旋手征束缚态的产生将极大地抑制非相干自旋涨落.     

Fr(o)hlich极化子的非经典基态

本文计及波矢q,q'声子间动力学关联效应,采用双模-压缩(声子)相干态作为再一次正则变换方案,基于Huybrechts变分近似,求解Fr(o)hlich大极化子的非经典基态.由于双模一压缩(声子)相干态导致声子相干态-压缩声子态关联效应,相干参量(f)q与双模压缩角φqq'有较大幅度修正,因而显著增强了相干效应和压缩角效应.对极化子基态能量计算与分析说明:在弱耦合区域,位移-声子压缩态效应的修正项ΔEc(1)与Feynman路径积分计算(△Ef)和Huybrechts相干态修正项(ΔE0)相当.但是,声子相干态双模-压缩效应导致相应的修正(ΔEc(2))有大幅度贡献,△Ec(2)＜＜(ΔEf,△E0);在强耦合区域,位移一声子压缩态效应的修正大为减弱而可以忽略,ΔEc(1)≥(ΔEf,△E0).虽然声子相干态双模-压缩效应也会同时减弱,考虑到电子-声子耦合强度(α)较大,仍有ΔEc(2)＜＜(ΔEf,ΔE0).     

Frhlich极化子的非经典基态

本文计及波矢q,q′声子间动力学关联效应,采用双模-压缩(声子)相干态作为再一次正则变换方案,基于Huybrechts变分近似,求解Frhlich大极化子的非经典基态.由于双模-压缩(声子)相干态导致声子相干态-压缩声子态关联效应,相干参量f～q与双模压缩角φqq′有较大幅度修正,因而显著增强了相干效应和压缩角效应.对极化子基态能量计算与分析说明在弱耦合区域,位移-声子压缩态效应的修正项ΔE(c1)与Feynman路径积分计算(ΔEf)和Huybrechts相干态修正项(ΔE0)相当.但是,声子相干态双模-压缩效应导致相应的修正(ΔE(c2))有大幅度贡献,ΔE(c2)(ΔEf,ΔE0);在强耦合区域,位移-声子压缩态效应的修正大为减弱而可以忽略,ΔE(c1)《(ΔEf,ΔE0).虽然声子相干态双模-压缩效应也会同时减弱,考虑到电子-声子耦合强度(α)较大,仍有ΔEc(2)(ΔEf,ΔE0).     

双能态自旋-晶格声子耦合量子隧道系统的非经典能态和量子相干耗散

采用关联表象变分波函数方案, 介入三个非经典关联效应, 求解有限温度双能态自旋-晶格声子耦合量子隧道系统的非经典态, 着重研究化解由于粒子自旋-单声子相互作用引起的量子涨落导致双能态系统的退相干性量子耗散. 这三个非经典关联效应是: 1) 声子位移-粒子自旋 (σ_z)间强非绝热关联; 2) 声子压缩态效应及其伴随发生的单声子相干态-声子压缩态两过程相干效应; 3) 由关联表象导致的声子位移(U_D)与声子压缩(U_S)的表象关联非绝热修正. 结果表明: 由于引入粒子自旋-双声子相互作用, 大幅度地增强了声子场压缩态, 特别是更进一步极大幅度地增强了非经典压缩-相干态效应. 因此, 由粒子自旋-单声子相互作用产生的Debye-Walle相干弹性散射效应导致量子隧道项(-Δ₀σ_x)的强烈指数衰减及其伴随严重的量子相干损失的极大幅度的抑制, 并且自旋-晶格声子耦合量子隧道系统的非经典态能量大幅度降低. 关键词： 非经典能态 量子隧穿相干损失 自旋-双声子相互作用 压缩相干态效应     

一维介观环中持续电流的电子-声子相互作用非经典效应

基于声子相干态功效和计及声子压缩态非经典效应,研究了电子-磁振子和电子-声子相互作用对一维介观环持续电流的影响. 与自由环比较,由于电子-磁振子相互作用,持续电流的振幅呈现指数减小. 对于正常态电子,电子-声子相互作用导致持续电流以Debye-Waller(D-W)因子衰减.但是计入跳步电子-单声子相干态关联效应导致系统本征态能量大幅度下降,从而持续电流I_n有大幅度增加.另一方面计入双声子相干态行为,由于声子压缩态效应压缩电子-相干(态)声子弹性散射行为,导致电子绕环运 关键词： 持续电流 电子-声子相互作用 声子相干态 声子压缩态效应     

三角阱中考虑Rashba效应的极化子声子平均数（英文）

本文采用改进的线性组合算符和幺正变换相结合的方法,推导出了考虑自旋和轨道相互作用即Rashba效应影响下的三角量子阱中弱耦合极化子的声子平均数,讨论了极化子的声子平均数随电-声子耦合常数和自旋轨道耦合常数的变化.通过取极化子单位后计算结果并绘图表明:电-声子耦合作用和Rashba效应对极化子的声子平均数均有影响,而且发现声子平均数随因为电-声子耦合作用和Rashba效应的增强均会增加.     

Half-Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变简

基于密度泛函理论(DFT),使用局域密度近似(LDA)研究了Heusler合金Cu1-xFex MnSb的电子结构和反铁磁-铁磁相变.研究发现,两种磁状态下的合金晶格常数随掺杂浓度x变化很好地满足Vegard定理.当x0.5时,铁磁态合金的总磁矩很好地符合SP规律,然而当x0.5时,却发生了明显的偏离.由于整个体系存在RKKY和超交换磁耦合的竞争,因而在x=0.25时,我们观察到了独特的反铁磁—铁磁相变.进一步的态密度分析发现,Cu的掺杂浓度可以有效调整铁磁态合金的费米面位置,并且反铁磁态合金由于不同自旋方向的Mn原子的分波态密度相互补偿,总态密度形成了几乎完全对称的自旋向上带和自旋向下带.     

La0.7-xDyxSr0.3MnO3(0.00≤x≤0.30)体系低温磁行为和电阻率的反常

通过测量样品的磁化强度-温度曲线、电阻率-温度曲线及磁电阻-温度曲线.研究了 Dy 掺杂(0.00≤x≤0.30)对 La_(0.7-x)Dy_xSr_(0.3)MnO_3 体系磁电性质的影响.实验发现,随 Dy 掺杂量的增加,体系磁结构从长程铁磁有序向自旋团簇玻璃态、反铁磁状态转变；x=0.20、0.30时的低温磁行为发生异常,电行为存在低温电阻率极小值现象.这些奇特现象不仅来源于掺杂引起的晶格效应,也来源于掺杂引起的额外磁性耦合.     

声子色散对量子点中弱耦合磁极化子声子平均数的影响

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对抛物量子点中弱耦合磁极化子电子周围光学声子平均数的影响．计及纵光学(LO)声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、回旋共振频率以及电子-声子耦合常数之间的关系,电子周围光学声子平均数与声子色散系数以及电子-声子耦合常数的关系．数值计算结果表明在弱耦合情况下抛物量子点中磁极化子的基态能量随声子色散系数的增大而减小;电子周围光学声子平均数随声子色散系数增大而增大,随电子-声子耦合常数的增大而增大．     

(Fe1-xCox)3BO5纳米棒磁性的研究

本文采用高温有机溶剂法制备了(Fe_1-xCo_x)₃BO₅纳米棒, 通过控制反应物中乙酰丙酮钴的含量合成了不同Co含量的(Fe_1-xCo_x)₃BO₅. 利用高分辨透射电子显微镜(HRTEM)、超导量子干涉磁强计(SQUID)对其形貌和磁性能进行了表征. 高分辨透射电子显微镜结果表明制备出的纳米(Fe_1-xCo_x)₃BO₅为多晶棒状, 且具有多折孪晶结构; 磁性测量的结果表明,(Fe_1-xCo_x)₃BO₅纳米棒在室温下表现出铁磁性, 随着Co含量的增加, 纳米棒的铁磁性逐渐增加, 该纳米棒有望用来研究生物大分子的机械性能.     

Raman phonon anomalies in Sr(Fe1-xCox)2As2

Phonon anomalies have been reported in iron-pnictide superconductors indicating a diverse interplay between different orders in the materials.Here,we report Raman scattering measurements on Sr(Fe_1-xCo_x)₂As₂(x=0 and x=0.04)single crystals in the B;symmetry with respect to a 1 Fe unit cell.Upon cooling,we observe a larger split(13 cm;)of Eg Raman phonon modes pertaining to in-plane Fe and As displacements as the crystals undergo the tetragonal-toorthorhombic structural phase transition,although a considerable split(9 cm;)has been reported in BaFe_1-xCo_x)₂As₂.Furthermore,the splitting of E;phonon modes is strongly reduced upon doping.We perform an order-parameter analysis revealing a similar doping dependence of E;phonon splitting as reported in other compounds of the 122 family,indicating these phonon anomalies widely exist in 122 iron-based superconductors and might share the same mechanisms.     

Revealing the A_(1g)-type strain effect on superconductivity and nematicity in FeSe thin flake

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate.Recently,a dominant B_(1 g)-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe_(1-x)Co_x)_2 As_2,suggesting a strong interplay between nematicity and superconductivity.Since the long-range spin order is absent in FeSe superconductor,whether a similar strain effect could be also observed or not is an interesting question.Here,by utilizing a flexible film as substrate,we successfully achieve a wide-range-strain tuning of FeSe thin flake,in which both the tensile and compressive strain could reach up to ～0.7%,and systematically study the strain effect on both superconducting and nematic transition(T_c and T_s) in the FeSe thin flake.Our results reveal a predominant A_(1 g)-type strain effect on T_c.Meanwhile,T_s exhibits a monotonic anti-correlation with T_c and the maximum T_c reaches to 12 K when T_s is strongly suppressed under the maximum compressive strain.Finally,in comparison with the results in the underdoped Ba(Fe_(1-x)Co_x)_2 As_2,the absence of B_(1 g)-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity.In addition,our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.     

Dy,Co共掺杂对BiFeO3陶瓷磁特性和磁相变温度Tc的影响

采用快速液相烧结法制备BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0, 0.05, 0.1, 0.15)陶瓷样品. 实验结果表明: 所有样品的主衍射峰与纯相BiFeO₃相符合且具有良好的晶体结构, 随着Co³⁺掺杂量的增大, Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品的主 衍射峰由双峰(104)与(110)逐渐重叠为单峰(110), 当掺杂量x>0.05时, 样品呈现正方晶系结构; SEM形貌分析可知: Dy³⁺, Co³⁺共掺杂使BiFeO₃晶粒尺度由原来的3—5 μ减小到约1 μ. 室温下, BiFeO₃样品表现出较弱的铁磁性, 随着Dy³⁺和Co³⁺掺杂, BiFeO₃样品的铁磁性显著提高. 在外加磁场为30 kOe的作用下, Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0.05, 0.1, 0.15)的M_r分别为0.43, 0.489, 0.973 emu/g; M_S分别为0.77, 1.65, 3.08 emu/g. BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品磁矩M随着温度T的升高而逐渐减小, Dy掺杂使BiFeO₃样品的T_N由644 K升高到648 K, 而T_C基本没有变化. Dy和Co共掺杂导致BiFeO₃样品磁相变温度T_C由870 K降低到780 K, 其T_C变化主要取决于Fe-O-Fe反铁磁超交换作用的强弱和磁结构的相对稳定性. 关键词： 铁磁电材料 磁滞回线 磁相变温度     

Invar effects of laves phase untermetallic compounds

The magnetovolume effects in Laves phase compounds such as AFe₂ and ACo₂ are received. In particular, Invar like behaviour which is found in (Zr_1-xNb_x)Fe₂ and Zr(Fe_1-xCo_x)₂ systems is reported. Comparing the present results with the classical Invar such as Fe-Ni and Fe-Pt alloys, it is proposed that so-called Invar anomalies should be classified into two type, i.e. the giant spontaneous volume magnetostriction, which is the essential characteristic of the Invar effect and the other anomalies which are observed in Fe-Ni and Zr(Fe_1-xCo_x)₂ systems but not in Fe-Pt and (Zr_1-xNb_x)Fe₂ systems.     

Structural modulation and physical properties of cobalt-doped layered La_2M_5As_3O_2(M=Cu,Ni) compounds

We investigate the structural variation and physical properties of layered La_2M_5As_3O_2(M=Cu,Ni) compound upon Co doping. It is found that the substitution of Co ion just induces the monotonous change of lattice constants without observing the anomalous kink in superconducting La_2(Cu~(1-x)Nix)_5 As_3O_2 solid-solutions. Meanwhile, this doping barely changes As–As bond length in [M_5As_3]~(2-) subunit(±2%), being significantly smaller than 7% shrinkage of that in La_2(Cu~(1-x)Nix)_5 As_3 O_2. Therefore, the doping dependence of crystal structure exhibits similar trend with Ba~(1-x)K_xFe_2 As_2 without the interference of As1–As 2 bonding, implying that the Co substitution for Cu/Ni is hole-doped. In terms of physical property, La2(Cu1-xCox)~5As_3O_2 turns into itinerant ferromagnetic metal, while La2(Ni1-x Cox)5 As3 O2 shows paramagnetism and suppressed structural phase transition upon Co-doping. The distinct structural variation and absence of superconductivity provide important clues to understand the effect of As–As bond in [M_5 As_3]~(2-) subunit.     

Co含量对Fe3Si合金磁学性能的影响

采用基于密度泛函理论（DFT）的赝势平面波法,对过渡金属Co掺杂Fe₃Si进行几何结构优化,计算Co含量对Fe₃Si合金磁学性质的影响.研究表明：Fe_3-xCo_xSi的磁性主要来源于过渡金属元素Fe和Co,并且相对于A、C位的Fe和Co原子,B位Fe的原子磁矩较大;在0≤x≤0.75范围内,随着Co含量的增大,Fe_3-xCo_xSi的总磁矩缓慢减小,在0.75≤x≤1.5时,其总磁矩迅速地增大;A、C位Fe原子的磁矩和合金总磁矩的变化趋势相同,Co原子的磁矩随着Co含量的增大缓慢地增大,原子的磁矩变化与自旋向上和向下方向的电荷转移有关.     

Electronic structure of the BaA1-xBxO3 (A, B = Pb, Bi, Sn, Sb) alloy systems Implications for superconductivity

Coherent potential approximation studies of the electronic structure of the BaA_1-xB_xO₃ (where A and B are different combinations of the elements Pb, Bi, Sn and Sb) alloy systems have been performed with accurate tight-binding hamiltonians. The calculations show how semiconducting BaSnO₃ evolves into a metal upon alloying with Sb or Bi. Common characteristics of the densities of states are pointed out. Using the rigid muffin-tin approximation an analysis of the superconducting properties is given.     

纤锌矿结构ZnO/Mg_xZn_(1-x)O量子阱中带间光吸收的尺寸效应和三元混晶效应

对于纤锌矿结构ZnO/MgxZn1-xO有限深单量子阱结构,考虑内建电场、导带弯曲及材料掺杂对实际异质结势的影响,利用有限差分法和自洽法数值求解Schr?dinger方程和Poisson方程,获得电子(空穴)的本征能级和本征波函数.进而,采用费米黄金法则讨论带间光吸收的尺寸效应和三元混晶效应.结果表明:三元混晶材料MgxZn1-xO中Mg组分的增加会增强垒层和阱层的内建电场强度,使得电子(空穴)平均位置靠近左(右)垒,导致带间跃迁吸收峰呈指数减小且发生蓝移;ZnO/MgxZn1-xO量子阱带间跃迁吸收峰随阱宽增大而减小,吸收峰发生红移.所得结果可为改善异质结构材料和器件的光电性能提供理论指导,以期获得实际应用所需的光学吸收频谱和波长.     

Observation of giant magnetocaloric effect under low magnetic fields in EuTi_(1-x)Co_xO_3

The magnetic properties and magnetocaloric effect(MCE) in EuTi_(1-x)Co_xO_3(x = 0, 0.025, 0.05, 0.075, 0.1) compounds have been investigated. When the Ti~(4+) ions were substituted by Co2+ions, the delicate balance was changed between antiferromagnetic(AFM) and ferromagnetic(FM) phases in the EuTiO_3 compound. In EuTi_(1-x)Co_xO_3 system, a giant reversible MCE and large refrigerant capacity(RC) were observed without hysteresis. The values of -?S_M~(max) were evaluated to be around 10 J·kg~(-1)·K~(-1) for EuTi_(0.95)Co_(0.05)O_3 under a magnetic field change of 10 kOe. The giant reversible MCE and large RC suggests that EuTi_(1-x)Co_xO_3 series could be considered as good candidate materials for low-temperature and low-field magnetic refrigerant.