一维关联体系与有机器件中几个实时演化问题的研究：从经典到量子
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摘要
在本论文中,我们利用不同的实时演化方法,研究了在一维强关联体系和有机器件中的多个问题。
首先,我们利用时间相关的密度矩阵重整化群(tDMRG)方法,研究了一维的强关联体系中的自旋和电荷的输运性质。我们考虑的体系主要是一维非半满的Hubbard链,在链中加入一个特殊的键,这个键上电子的跃迁是与自旋相关的。为了方便,我们让下自旋电子的跃迁系数是调。结果发现,由于这个键的加入,自旋流被阻挡了,而相反的,电荷流却几乎不受影响。为了更清楚地认识这一点,我们研究了两种不同的情况, (1)在一个Hubbard链上同时考虑自旋和电荷波包的扩散；(2)在这个Hubbard链的两端加上两个理想的电极,再通过加上相应的自旋相关的偏压来产生自旋和电荷的流。我们的研究充分证明了在这个体系中,自旋-电荷分离对自旋流的阻塞起着至关重要的作用,同时我们认为,这个发现对于研究自旋-电荷分离及其相应的应用,都会有所帮助。
同时,利用tDMRG,我们还研究了一维有机高聚物中的关联效应对极化子运动的影响。我们首先讨论了电子关联效应对极化子在电场下运动的影响。我们所用的模型是基于Su-Schrieffer-Heeger (SSH)模型和Hubbard模型的。我们的结果显示极化子的运动速度被格点内的库伦排斥能抑制了。极化子的饱和速度在小U时是四倍左右声速,而大U时则是声速。此外,我们还讨论了极化子的速度与晶格的结构和极化子的有效质量之间的关系。我们讨论的另一个模型,则是在高聚物链中一个固定格点引入一个磁性杂质,这个杂质可以通过自旋交换相互作用与极化子发生关联。当极化子的自旋与杂质自旋是反平行时,就会发生自旋翻转。我们的模拟结果显示了(1)极化子与杂质之间交换自旋的过程有可能作为一个量子逻辑门(交换门)来应用；(2)在分子层级上,极化子可能是一个很好的信息载体。
利用半经典的器件模型方法,以及与相关的实验组合作,我们研究了有机单层光伏器件中瞬时光电压的极性转换问题。我们的理论研究,不仅很好地与实验符合,也清楚地解释了光电压极性转换的整个过程。同时,我们还从理论上预言了光电压正信号随着光强的增大将达到饱和,这也为相应的实验所证实。此外,我们还研究了体系中不同参数对这个饱和现象的影响,并提出了少子在这个过程中所起的核心作用。因此,我们也建议,利用饱和光电压的现象,我们可以更加准确地在实验上测定少子的迁移率。
有机的电致发光磁效应是有机发光二极管的磁效应中一个非常重要的部分,但是相应的理论和实验研究都显得非常匮乏,以致于其机理一直都与有机磁阻现象混为一谈。我们的工作就是通过建立一个包括了激子生成和自旋混合的双过程模型,成功地解释了我们在混合空穴传输层和电子传输层器件中的实验现象。并且,我们的结果证明,载流子的跃迁率在电致发光磁效应中起着十分重要的作用。通过激子生成机制的研究,我们预期这个磁效应会在较强的陷阱作用的材料中被发现,我们的实验的确证实了这一点。此外,我们还用高能量转移的材料成功的去掉了体系中的单重态激子,从而观察到了与理论定性符合的三重态激子的行为。我们的发现充分证明了,在有机磁效应中,分子间的量子关联效应是不可忽略的重要因素。
通过一个非绝热的动力学方法,我们模拟了在电场作用下,耗散效应对极化子的运动、解体和生成的影响。耗散是通过对晶格原子引入一个阻尼力来实现的。我们发现,不同于理想情况下的阶梯状行为,极化子的饱和速度在耗散存在时呈现出了与电场连续变化的关系。而在大的阻尼下,饱和速度几乎是对电场呈线性依赖。饱和速度对阻尼也呈一种线性关系。此外,我们还发现,有阻尼存在时,使极化子拆分的电场被减小了,而允许其生成的电场则相应的变大。
In this thesis, we use different time-dependent evolution methods to investigate several problems in one-dimensional strongly-correlated systems and organic devices.
First, we investigate the spin/charge transport in a one-dimensional strongly cor-related system by using the adaptive time-dependent density-matrix renormalization group method. The model we consider is a non-half-filled Hubbard chain with a bond of controllable spin-dependent electron hoppings. For convenience we let hopping of spin down electron on this bond adjustable. It is found that this special bond causes a blockade of spin current with little influence on charge current. To understand more clearly, we have considered two cases:(1) the spread of a wave packet of both spin and charge in the Hubbard chain and (2) the spin and charge currents induced by a spin-dependent voltage bias that is applied to the ideal leads attached at the ends of this Hubbard chain. It is found that the spin-charge separation plays a crucial role in the spin-current blockade, and one may utilize this phenomenon to observe the spin-charge separation directly.
Meanwhile, within tDMRG, we also consider the influence of correlated effect on the motion of polaron in one-dimensional conjugated polymers. We first investigate the electron correlation effect on the dynamics of a charged polaron driven by an ex-ternal electric field, based on the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model and the Hubbard model (HM). Our results show that the velocity of the polaron is suppressed by the on-site Coulomb interactions, U. The polaron can move with a supersonic velocity, about four times the sound velocity at the small U limit, and approaching the sound velocity at the large U limit. Furthermore, the dependence of polaron velocity on the lattice structure and the effective mass of polaron is dis-cussed. Another model we consider, it to introduce a magnetic impurity to polymers on a specific site. Then the polaron couples to this magnetic impurity via spin-exchange interactions, and its spin undergoes a spin-flip process if it is antiparallel to the impurity spin. Our numerical simulation shows that (a) spin-exchange between the polaron and the impurity allows the implementation of a swap gate and (b) polarons might be good candidates to be information carriers in molecular scale.
Using device model mothod and cooperating with relative experimentists, we study the polarity transition of transient photovoltage (TPV) in organic single-layer photo-voltaic devices. Our theoretical investigation not only matches the experiments very well and also predicts a significant effect, say, the saturation of positive signal when the strength of light increases, which has been verified by our experiments. We also find that, this effect depends mainly on the mobility of minority carriers. We hereby suggest applying this effect as a new way to measure the corresponding mobility.
Magnetoelectroluminescence (MEL) is an important component in magnetic field effect (MFE) of organic light emitting devices, while few researches on MEL, both theoretical and experimental, make it confusing that MEL shares the same origin with organic magnetoresistance (OMAR). In this work, we establish a two-process model consisting of exciton-generating (EG) and spin-mixing process. Using method of mix-ing hole transporting material and electron transporting layer, we conclude that hopping rates of carriers play an essential role in MEL. As expected by EG, we observe decay of MEL in materials with strong trapping ability. We utilize material with high energy transfer efficiency to remove singlet excitons, and the change of triplet is measured. Our findings imply that the intermolecular quantum correlation should be taken into account in organic MFE.
Within a non-adiabatic dynamics method, we simulate polaron motion and polaron dissociation under an applied electric field in the presence of dissipation. The dissi-pation is introduced through a damping force on molecules. We find that, different from the step-like behavior in the absence of dissipation, the saturation velocity of the polaron exhibits a continuous dependence on the field strength in the presence of dissi-pation. In large damping cases, the saturation velocity almost has a linear dependence on the field strength. Besides, the saturation velocity also shows a linear relationship with damping. In addition, we find that, in the presence of dissipation, the critical field that dissociates a polaron is reduced, but for generation, it is increased.
首先,我们利用时间相关的密度矩阵重整化群(tDMRG)方法,研究了一维的强关联体系中的自旋和电荷的输运性质。我们考虑的体系主要是一维非半满的Hubbard链,在链中加入一个特殊的键,这个键上电子的跃迁是与自旋相关的。为了方便,我们让下自旋电子的跃迁系数是调。结果发现,由于这个键的加入,自旋流被阻挡了,而相反的,电荷流却几乎不受影响。为了更清楚地认识这一点,我们研究了两种不同的情况, (1)在一个Hubbard链上同时考虑自旋和电荷波包的扩散；(2)在这个Hubbard链的两端加上两个理想的电极,再通过加上相应的自旋相关的偏压来产生自旋和电荷的流。我们的研究充分证明了在这个体系中,自旋-电荷分离对自旋流的阻塞起着至关重要的作用,同时我们认为,这个发现对于研究自旋-电荷分离及其相应的应用,都会有所帮助。
同时,利用tDMRG,我们还研究了一维有机高聚物中的关联效应对极化子运动的影响。我们首先讨论了电子关联效应对极化子在电场下运动的影响。我们所用的模型是基于Su-Schrieffer-Heeger (SSH)模型和Hubbard模型的。我们的结果显示极化子的运动速度被格点内的库伦排斥能抑制了。极化子的饱和速度在小U时是四倍左右声速,而大U时则是声速。此外,我们还讨论了极化子的速度与晶格的结构和极化子的有效质量之间的关系。我们讨论的另一个模型,则是在高聚物链中一个固定格点引入一个磁性杂质,这个杂质可以通过自旋交换相互作用与极化子发生关联。当极化子的自旋与杂质自旋是反平行时,就会发生自旋翻转。我们的模拟结果显示了(1)极化子与杂质之间交换自旋的过程有可能作为一个量子逻辑门(交换门)来应用；(2)在分子层级上,极化子可能是一个很好的信息载体。
利用半经典的器件模型方法,以及与相关的实验组合作,我们研究了有机单层光伏器件中瞬时光电压的极性转换问题。我们的理论研究,不仅很好地与实验符合,也清楚地解释了光电压极性转换的整个过程。同时,我们还从理论上预言了光电压正信号随着光强的增大将达到饱和,这也为相应的实验所证实。此外,我们还研究了体系中不同参数对这个饱和现象的影响,并提出了少子在这个过程中所起的核心作用。因此,我们也建议,利用饱和光电压的现象,我们可以更加准确地在实验上测定少子的迁移率。
有机的电致发光磁效应是有机发光二极管的磁效应中一个非常重要的部分,但是相应的理论和实验研究都显得非常匮乏,以致于其机理一直都与有机磁阻现象混为一谈。我们的工作就是通过建立一个包括了激子生成和自旋混合的双过程模型,成功地解释了我们在混合空穴传输层和电子传输层器件中的实验现象。并且,我们的结果证明,载流子的跃迁率在电致发光磁效应中起着十分重要的作用。通过激子生成机制的研究,我们预期这个磁效应会在较强的陷阱作用的材料中被发现,我们的实验的确证实了这一点。此外,我们还用高能量转移的材料成功的去掉了体系中的单重态激子,从而观察到了与理论定性符合的三重态激子的行为。我们的发现充分证明了,在有机磁效应中,分子间的量子关联效应是不可忽略的重要因素。
通过一个非绝热的动力学方法,我们模拟了在电场作用下,耗散效应对极化子的运动、解体和生成的影响。耗散是通过对晶格原子引入一个阻尼力来实现的。我们发现,不同于理想情况下的阶梯状行为,极化子的饱和速度在耗散存在时呈现出了与电场连续变化的关系。而在大的阻尼下,饱和速度几乎是对电场呈线性依赖。饱和速度对阻尼也呈一种线性关系。此外,我们还发现,有阻尼存在时,使极化子拆分的电场被减小了,而允许其生成的电场则相应的变大。
In this thesis, we use different time-dependent evolution methods to investigate several problems in one-dimensional strongly-correlated systems and organic devices.
First, we investigate the spin/charge transport in a one-dimensional strongly cor-related system by using the adaptive time-dependent density-matrix renormalization group method. The model we consider is a non-half-filled Hubbard chain with a bond of controllable spin-dependent electron hoppings. For convenience we let hopping of spin down electron on this bond adjustable. It is found that this special bond causes a blockade of spin current with little influence on charge current. To understand more clearly, we have considered two cases:(1) the spread of a wave packet of both spin and charge in the Hubbard chain and (2) the spin and charge currents induced by a spin-dependent voltage bias that is applied to the ideal leads attached at the ends of this Hubbard chain. It is found that the spin-charge separation plays a crucial role in the spin-current blockade, and one may utilize this phenomenon to observe the spin-charge separation directly.
Meanwhile, within tDMRG, we also consider the influence of correlated effect on the motion of polaron in one-dimensional conjugated polymers. We first investigate the electron correlation effect on the dynamics of a charged polaron driven by an ex-ternal electric field, based on the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model and the Hubbard model (HM). Our results show that the velocity of the polaron is suppressed by the on-site Coulomb interactions, U. The polaron can move with a supersonic velocity, about four times the sound velocity at the small U limit, and approaching the sound velocity at the large U limit. Furthermore, the dependence of polaron velocity on the lattice structure and the effective mass of polaron is dis-cussed. Another model we consider, it to introduce a magnetic impurity to polymers on a specific site. Then the polaron couples to this magnetic impurity via spin-exchange interactions, and its spin undergoes a spin-flip process if it is antiparallel to the impurity spin. Our numerical simulation shows that (a) spin-exchange between the polaron and the impurity allows the implementation of a swap gate and (b) polarons might be good candidates to be information carriers in molecular scale.
Using device model mothod and cooperating with relative experimentists, we study the polarity transition of transient photovoltage (TPV) in organic single-layer photo-voltaic devices. Our theoretical investigation not only matches the experiments very well and also predicts a significant effect, say, the saturation of positive signal when the strength of light increases, which has been verified by our experiments. We also find that, this effect depends mainly on the mobility of minority carriers. We hereby suggest applying this effect as a new way to measure the corresponding mobility.
Magnetoelectroluminescence (MEL) is an important component in magnetic field effect (MFE) of organic light emitting devices, while few researches on MEL, both theoretical and experimental, make it confusing that MEL shares the same origin with organic magnetoresistance (OMAR). In this work, we establish a two-process model consisting of exciton-generating (EG) and spin-mixing process. Using method of mix-ing hole transporting material and electron transporting layer, we conclude that hopping rates of carriers play an essential role in MEL. As expected by EG, we observe decay of MEL in materials with strong trapping ability. We utilize material with high energy transfer efficiency to remove singlet excitons, and the change of triplet is measured. Our findings imply that the intermolecular quantum correlation should be taken into account in organic MFE.
Within a non-adiabatic dynamics method, we simulate polaron motion and polaron dissociation under an applied electric field in the presence of dissipation. The dissi-pation is introduced through a damping force on molecules. We find that, different from the step-like behavior in the absence of dissipation, the saturation velocity of the polaron exhibits a continuous dependence on the field strength in the presence of dissi-pation. In large damping cases, the saturation velocity almost has a linear dependence on the field strength. Besides, the saturation velocity also shows a linear relationship with damping. In addition, we find that, in the presence of dissipation, the critical field that dissociates a polaron is reduced, but for generation, it is increased.
引文
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