This is Eric's logbook for Nano Japan project from 2012.6.3-2012.7.27.

#contents

* Daily schedule [#daily]
- 09:00-10:00 Discussion (1)
- 10:00-12:00 Solve the problem and write the progress on pukiwiki
- 12:00-13:30 Lunch
- 13:30-14:30 Discussion (2)
- 14:30-16:30 Solve the problem and write the progress on pukiwiki
- 16:30-17:30 e-mail report and problems

* Goal of the project. [#goal]

- Consider the time evolution of a special vibration (radial breathing mode RBM) of a single wall carbon nanotube after applying the local force at t=0  whose functional shape is a Gaussian in the real space. The time evolution is given by differential equations and solution should be given by animation gif generated by Povray and Giam software. (by R. Saito 2012.5.18)

- Keywords: Coherent phonon, radial breathing mode, carbon nanotubes, Differential equation, Fourier Transform, Povray and Giam, Mathematica

* Schedule for discussion [#discussion]

|Time and Day|Mon|Tue|Wed|Thu|Fri|h
|09:00-10:00|Sato|Nugraha|Tapsanit|Tatsumi|Nugraha|
|13:30-14:30|Saito|Simon|Hasdeo|Simon|Tapsanit|

All appointment should be done by communicating each other on what kind of subjects will you teach.
-- If you do not have any materials to be taught, please give him an easy question to solve.
-- If Eric-san could not answer this question, it means that your question is bad (needs some knowledge).
-- If Eric-san can answer within 2 mins, it means that you gave improper question (the answer is too trivial).

- This section is for posting questions from Eric-san and answers from other group members.
- Please list here with some simple reasons or details.
- For every problem, give a tag double asterisks (**) in the code so that it will appear in the table of contents.
- For the answer, give a tag triple asterisks (***) in the code below the problem in order to make a proper alignment.
- List from new to old.

** Q: About Brillouin Zone [#Q-BZ]
Hey guys! So I believe this is where I should post questions. I don't have any specific questions today, but was just trying to understand Brillouin zones and Bloch's theorem, which appear a lot in Saito-sensei's book on Carbon Nanotubes.

From google I have an ok idea of what Brillouin zones are, but am still trying to figure out how they relate to the energy gap of the cell such as on page 28 of the CN book. I was also wondering how on page 47 the Brillouin zone can be a long line segment instead of a polygon.

If anybody has any quick words of advice for how to understand Bloch's theorem that would be great as well! It's a little bit intimidating and I'm not sure if there's a simple way to begin to understand it.

Thanks so much and see you guys in a couple of weeks!

Eric

Please consider my answers one by one in order to understand the Brillouin zone.
- What is the Bloch'theorem written in page. 17 of Saito's book?
-- A: The electron in solid can behave as wave and its wave function which is denoted by $\Psi(r)$ should satisfy the Bloch's theorem due to the periodicity of the atoms in solid. $\Psi$ is sometimes called the Bloch wave function. It can be written as the phase factor $\exp(ikr)$ times the periodic function in real space $u(r)$, i.e., $u(r+T) = u(r)$; $\Psi(r) = \exp(ikr)u(r)$. It is recommended to  check that $\Psi(r) = \exp(ikr)u(r)$ satisfies the Bloch's theorem.

- What is the Brillouin zone? (solved in the discussion)
-- A: We must understand the reciprocal lattice before answering this question, because the Brillouin zone is defined from the reciprocal lattice.

- What is the reciprocal lattice that define the Brillouin zone?
-- A: As mentioned above, the electron in solid is a kind of wave whose wave function satisfies the Bloch's theorem. This means that it has the wavevector with amplitude and direction just like normal wave. The dimension  of wavevector is 1/[length] which is a reciprocal of length in real lattice. Then, reciprocal lattice is constructed to obtain the wavevector of the electron. Please note that there are many wavevectors possibly obtained from the reciprocal space. The unit vectors of the reciprocal lattice $(\vec{b}_1},\vec{b}_2,\vec{b}_3)$ are obtained from the unit vectors of the real lattice $(\vec{a}_1,\vec{a}_2,\vec{a}_3)$ by following relations ($\cdot$ denotes  dot product of two vectors):
$\vec{b}_i\cdot\vec{a}_j = 2\cdot\pi\delta_{ij}$, where $\delta_{ij} = 1$ if $i = j$ and  $\delta_{ij} = 0$ if $i \neq j$.
It is recommended to try using this relations by deriving the unit vectors of reciprocal lattice, $\vec{b}_1$ and $\vec{b}_2$, of graphene in Eq. (2.23) of Saito-sensei's book using $\vec{a}_1$ and $\vec{a}_2$ in Eq. (2.22).

** Q: About copy machine [#Q-copymachine]
Can I use a copy machine or printer?
*** A: (not solved) [#A-copymachine]
Yes, and they are across the hall from my room.

I would like to know where the library is and how to use it.
*** A: (not solved) [#A-copymachine]
The library is on the 7th floor. To check out a book I write information on a special wooden card and place it where the book had been. I also give the slip inside of the book to the librarian, and I return the book to the librarian.

* Report [#report]

This part is basically written by Eric-san. Any other people can add this.
Here the information should be from new to old so that we do not need to scroll.

**June 7 [#rep20120607]
In the morning Tatsumi-san and Ominato-san explained to me the reciprocal lattice of graphene. First we derived the unit cell of graphene. We derived the reciprocal lattice and found then found the Brillouin zone of graphene, which is also a hexagon but rotated by 90 degrees.

To somewhat review what Tapsanit-san taught me yesterday we then talked about the wavevector (k) in the reciprocal lattice, and frequency as a function of k. We showed that this satisfied Bloch's theorem and described the periodic boundary conditions. Then, we talked about the acoustic mode vs the optical mode. We showed that the number of modes will increase if we increase the number of atoms in a unit cell. In the problem where two different types of masses are connected with identical springs, the lower valued function is the 'acoustic mode', where all the atoms vibrate in the same direction at the same value of time. The higher valued function is the optical mode, where some of the atoms are pushed towards each other while other atoms are pushed away from each other.

At 10:30 I accompanied Hasdeo-san to Japanese class where I learned the ta form of verbs. For instance: "Kyoto e itta koto ga arimaska?" = "Have you ever been to Kyoto?". Iie, arimasen. Watashi wa Kyoto ni ikitai.

In the afternoon I went to the group meeting where Simon-san presented his animations and some differential equations. His POV-Ray animations were very cool and also really helped to illustrate the acoustic vs. optical vibrational modes. Tomorrow we made plans to discuss some of the differential equations he had been working with, because I need to know them as well.

Later in the afternoon, Ominato-san and Tatsumi-san returned to teach me more physics. Ominato-san had written a long sequence of very good problems some of which we worked through. We defined the Drude Model of electron conduction, and wrote the equation of electron motion including a frictional term:
m\frac{dv}{dt}=e(E+\frac{1}{c}v X H)-(\frac{m}{T})v

We derived v(t) when H=0, and then for the steady state solution (dv/dt=0)
We then Derived $\sigma$ (the conductivity) based on the relation:
j=nev=(conductivity)*E

Next, we derived v(t) for a more general case and put into matrix form the equation:
j=(conductivity)*E

Ominato-san explained the classical Hall effect and we breifly talked about special relativity. The energy of a particle with mass is given by
E=+/- sqrt((cp)^2+(mc^2)^2) were E is a parabolic function of p. However in graphene electron behave as if they have no mass and E=+/- cp.

To Do:
-Finish the problems given by Ominato-san, and rewrite good solutions for the problems we went over this afternoon.
-Consider the differential equation given by Saito-sensei, study his email and study the equation in more detail.
-Finish the graphs for Nugraha-san.
-Finish the graphs for Hasdeo-san and the cascading number Fortran program.

I learned a tremendous amount of theory today. My understanding of the lattice has made good progress over the last several days.

** June 6 [#rep20120606]
- Tapsanit-san explained the Brillouin zone of a 1D lattice, as well as 2D and 3D square lattices. We discussed what k (the wave vector) is and constructed the reciprocal lattice. We then talked about how Bloch's wavefunction has periodic boundary conditions. We have a finite number of k, equal to N, the number of atoms. On Friday we will construct the Brillouin zone of graphene. Later in the morning I fixed the format of the equations I'd posted on PukiWiki and finished the graph of the different modes of phonon frequency as a function of k. I did this graph on SciDAVis and am not entirely happy with the axis format, so I should do it on Xmgrace too.

In the afternoon Hasdeo-san explained further about Fortran programming language. After working through a program he made he gave me several assignments to do. Hasdeo-san also helped me learn to navigate the directories of command prompt.

To Do:
1. Get a correctly formatted w(k) graph to Nugraha-san AND learn about/explain which is the optical mode and which is the acoustic mode.
2. Solve Saito-sensei's differential equation by learning about Fourier transforms.
3. Solve Hasdeo-san's problems:
- plot the cascading list of integers
- graph two fxns (different parabolas) using Fortran
- output fxn data in column form

** June 5 [#rep20120605]
- Nugraha-san explained the problem of an infitie 1D series of masses connected by springs. We solved for the value of w^2, although there was a small error in my calculation. In excel we graphed the value of w^2 as a function of k, the wavevector. In the afternoon we graphed the (incorrect) data generated in excel into an xmgrace graph. Nugraha-san also explained how to use the simpler SciDAVis software.

Also, Florian-san showed me how to use the POV-Ray software. We installed it on the desktop machine and I worked through the basic tutorial.

- The homework is: by friday complete a graph of w(k) for the 1D phonon problem, determine the max/min for each branch, and interpret this physically. Which branch is optical and which is acoustic?

Also, solve the diff eq for a Gaussian.

If I have time consider the infinite 1D series of spring mass problems, but in the case that all masses are the same, but the spring constants are different.

** June 4 [#rep20120604]
- Sato-sensei showed me around the campus and installed Xming on the desktop.
- Hasedo-sensei introduced me to Fortran language and we wrote a very simple program.
- Solved the differential equation for f(t)=(bt)/a=x''+w^2x
Solution is: f(t)=(b/(aw^2))(t-(1/w)sin(wt))
Could look into the eqn and new initial conditions for t>a

to do:
- Solve the same diff eq for f(t)=Ae^(-t^2/a^2)
- Clean up the solution for the reciprocal lattice from last week

+ Check that the $\Psi(\vec{r})= e^{i\vec{k}\cdot\vec{r}} u(\vec{r})$ satisfies the Bloch's theorem.
Bloch's theorem is:
$T_{\vec a_i}\Psi = e^{(i\vec{k}\cdot\vec{a_i})}\Psi$
Here $T_{\vec a_i}$ is a translation operator: $T_{\vec a_i} \Psi(\vec{r})= \Psi(\vec{r}+\vec{a}_i)$.
Then, also note that $u(r)$ is a periodic function. We have $u(\vec{r} + \vec{a}_i) = u({\vec r})$ because $\vec{a}_i$ is a unit lattice vector. Thus:
$T_{\vec{a}_i} \Psi(\vec{r}) = \Psi(\vec{r} + \vec{a}_i)$
$= e^{(i\vec{k} \cdot (\vec{r} + \vec{a}_i))} u(\vec{r} + \vec{a}_i)$
$= e^{i\vec{k} \cdot \vec{a}_i} [e^{i\vec{k}\cdot \vec{r}} u(\vec{r})]$
$=e^{i\vec{k} \cdot \vec{a}_i}\Psi\left(\vec{r}\light)$
The Bloch theorem is satisfied.
+ Derive the unit vectors of reciprocal lattice, $\vec{b}_1$ and $\vec{b}_2$, of graphene in Eq. (2.23) of CN book using $\vec{a}_1$ and $\vec{a}_2$ in Eq. (2.22).
$\vec{a}_i = {\rm unit~vector~of~real~lattice}$
$\vec{b}_i = {\rm unit~vector~of~reciprocal~lattice}$
$\vec{a}_i\cdot\vec{b}_i = 2\pi$
$\vec{a}_i\cdot\vec{b}_j = 0 \quad {\rm if} \quad i \neq j$
From Eq. (2.22) of CN book:
$\vec {a}_1 = \left(\frac{\sqrt{3}}{2}a,\frac{a}{2}\right), \quad \vec {a}_2 = \left(\frac{\sqrt{3}}{2}a,-\frac{a}{2}\right)$

$\vec{a}_1\cdot\vec{b}_1 = a_{1x}b_{1x}+a_{1y}b_{1y}=b_{1x}\frac{a\sqrt{3}}{2}+b_{1y}\frac{a}{2}=2\pi$
$\vec{a}_2\cdot\vec{b}_1 = a_{2x}b_{1x}+a_{2y}b_{1x}=b_{1x}\frac{a\sqrt{3}}{2}-b_{1y}\frac{a}{2}=0$

$b_{1y}\frac{a}{2}=b_{1x}\frac{a\sqrt{3}}{2}$
$b_{1x}\frac{a\sqrt{3}}{2}+b_{1x}\frac{a\sqrt{3}}{2}=2\pi$

$b_{1x}=\frac{2\pi}{a\sqrt{3}}$
$b_{1y}=\frac{2\pi}{a}$

$\vec {b}_1 = \left(\frac{2\pi}{a\sqrt{3}},\frac{2\pi}{a}\right), \quad \vec {b}_2 = \left(\frac{2\pi}{a\sqrt{3}},\frac{-2\pi}{a}\right)$