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liuyanping105

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[交流] graphene 研究的一个问题（请教前辈） 已有1人参与

graphene 研究的一个问题（请教前辈）：

graphene 在电场的gate的作用下，可以打开bandgap,i.e bilayer----- tunnable bandgap, trilayer -----overlap band gap.

1.那么在perpendicular magnetic field 的作用下，graphene 的情况如何呢？bandgap 能打开？ 还是打开excitonic condensation gap?
2. four-terminal graphene device 在perpendicular magnetic field 的作用下，发现测量的电阻（不是hall effect 磁阻）线性或者非线性的随着磁场增加而增加，也就是： R vs  B， 这个如何解释？

希望前辈指教！
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1楼 2012-12-08 20:56:02
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deane

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1。垂直磁场，是可以直接改变gap的大小的。在一些比较简化的理论中，这些是可以直接算出来的。
而且对于单层和多层，会有不同的规律。但是基本的结论，是随着磁场增强而增大的。磁场较强时，gap大小跟磁场有的是一次关系，有的是二次。
2。电阻也是应该在2x2矩阵里面的吧。你这里说的，是对角的电阻吗？
无论是电阻矩阵里的哪个元素，应该都与gap，继而与磁场有关。
可参考文献：http://arxiv.org/abs/1108.0650，以及其中的一些实验结果。
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2楼2012-12-09 16:50:06
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liuyanping105

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2楼: Originally posted by deane at 2012-12-09 16:50:06
1。垂直磁场，是可以直接改变gap的大小的。在一些比较简化的理论中，这些是可以直接算出来的。
而且对于单层和多层，会有不同的规律。但是基本的结论，是随着磁场增强而增大的。磁场较强时，gap大小跟磁场有的是一 ...

Thanks for your good explains and good comments.


1。垂直磁场，是可以直接改变gap的大小的。在一些比较简化的理论中，这些是可以直接算出来的。
而且对于单层和多层，会有不同的规律。但是基本的结论，是随着磁场增强而增大的。磁场较强时，gap大小跟磁场有的是一次关系，有的是二次。

请问这个GAP 是band gap 还是excitonic gap.

delta gap 大小 ~ square root B  FOR MONOLAYER Graphene
delta gap 大小~  B  FOR bilayer, trilayer  Graphene
所以
Rxx~ exp(delta gap/kT)

这样解释对吗？

有人认为：强磁场下当然是形成朗道能级啦，不过graphene有零能的LL，弱场下应该没有gap

The effect of the applied perpendicular magnetic can induce the splitting of Landau Level and the zero-energy Level is characteristic of half filling.So the bandstucture show discrete dispersion (splitting of Landau Level).

i want to know that the gap is band gap or excitonic gap when the application of the magnetic field on the graphene.

My understanding is that band gap corresponds to the effect of the electric field and the excitonic gap corresponds to the effect of the magnetic field
.
2。电阻也是应该在2x2矩阵里面的吧。你这里说的，是对角的电阻吗？
无论是电阻矩阵里的哪个元素，应该都与gap，继而与磁场有关。
可参考文献：http://arxiv.org/abs/1108.0650，以及其中的一些实验结果。

我这电阻，就是4电极电阻测量法（不同于HALL Effect, Rxy, Rxx)，
也就是4个电极穿过graphene,所以这个电阻可以看做Rxx吗？

The magnetic field range of 0-12T. The negative resistance at low field and low temperature (weak localization effect) originates from the interval scattering but the i want to know the origins of the non-linear magneto resistance increase with the increasing magnetic field ( i think it is originated from the electron-hole puddles).
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3楼2012-12-12 00:48:40
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deane

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Liu,

About the "excitonic gap" and band gap, I believe Delta should be refered as the gap between the conductance and valance band. So it is the gap the enters the expression for conductivity/resistance.

About what your resistence really is, I am not sure how 4 gates only measures 1 resistence. You need to describe your set-up in more details, or can you refer paper for me in order to tell more about this?
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4楼2012-12-12 01:24:32
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liuyanping105

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4楼: Originally posted by deane at 2012-12-12 01:24:32
Liu,

About the "excitonic gap" and band gap, I believe Delta should be refered as the gap between the conductance and valance band. So it is the gap the enters the expression for conduct ...

What is the effect of the applied perpendicular magnetic for graphene?

As we known, graphene is subjected to an electric field with the gate control, resulting in the opening bandgap. For example, the bilayer shows a tunable bandgap and the trilayer displays an overlapping bandgap (energy gap).

So, I want to know the effect of the perpendicular magnetic field. Does it have a similar effect to open the bandgap energy gap)?

Or others, i.e. the opening excitonic condensation gap?



1.jpg


3.jpg

The effect of the applied perpendicular magnetic can induce the splitting of Landau Level and the zero-energy Level is characteristic of half filling. So the bandstucture show discrete dispersion (splitting of Landau Level, as shown in Figure 2.2).

i want to know that the gap is band gap (energy gap) or excitonic gap when the application of the magnetic field on the graphene.

My understanding is that band gap corresponds to the effect of the electric field and the excitonic gap corresponds to the effect of the magnetic field, is correct???




Additionally, I put the four-terminal graphene device on the perpendicular magnetic field (as shown in the Figure 2); the observed resistance (not Hall Effect resistance Rxx and Rxy) shows a nonlinear increase with the increasing magnetic field. What is the origin of the nonlinear magneto resistance?

The magnetic field range of 0-12T. The negative resistance at low field and  low temperature (weak localization effect) originates from the intervalley  scattering( you told me ) but the i want to know the origins of the non-linear magneto  resistance increase with the increasing magnetic field ( i think it is originated from the electron-hole puddles).


2.jpg
Figure 2.(LEFT)schematic drawing of the four-terminal device and Resistance.(Right) experimental results indicate that non-linear magneto  resistance increase with the increasing magnetic field.

[ Last edited by liuyanping105 on 2012-12-12 at 22:22 ]
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5楼2012-12-12 22:00:55
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deane

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I see how you measure the resistance now. Yes it is R_xx if it was in the Hall measurement.

The gap we have been talking about is the exitonic gap.
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6楼2012-12-12 22:25:34
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