这是之前的一篇博文,计算的是两端体系的量子输运:非平衡格林函数计算电导(附Python代码)。这里计算六端口体系的量子输运,以方格子为例,给出三份代码:
- 第一个写出全部算法的代码。
- 第二个调用了Guan开源软件包(https://py.guanjihuan.com)。
- 第三个用Kwant进行计算(https://www.kwant-project.org/)。
一、代码1
说明:这里只是一个测试例子。在实际计算中,需要把四个电极往中间移一点,即move>0, 否则相邻电极会相互接触,达不到预期结果。
"""
This code is supported by the website: https://www.guanjihuan.com
The newest version of this code is on the web page: https://www.guanjihuan.com/archives/6075
"""
import numpy as np
import matplotlib.pyplot as plt
import copy
import time
def get_lead_h00(width):
h00 = np.zeros((width, width))
for i0 in range(width-1):
h00[i0, i0+1] = 1
h00[i0+1, i0] = 1
return h00
def get_lead_h01(width):
h01 = np.identity(width)
return h01
def get_center_hamiltonian(Nx, Ny):
h = np.zeros((Nx*Ny, Nx*Ny))
for x0 in range(Nx-1):
for y0 in range(Ny):
h[x0*Ny+y0, (x0+1)*Ny+y0] = 1 # x方向的跃迁
h[(x0+1)*Ny+y0, x0*Ny+y0] = 1
for x0 in range(Nx):
for y0 in range(Ny-1):
h[x0*Ny+y0, x0*Ny+y0+1] = 1 # y方向的跃迁
h[x0*Ny+y0+1, x0*Ny+y0] = 1
return h
def main():
start_time = time.time()
width = 5
length = 50
fermi_energy_array = np.arange(-4, 4, .01)
# 中心区的哈密顿量
H_center = get_center_hamiltonian(Nx=length, Ny=width)
# 电极的h00和h01
lead_h00 = get_lead_h00(width)
lead_h01 = get_lead_h01(width)
transmission_12_array = []
transmission_13_array = []
transmission_14_array = []
transmission_15_array = []
transmission_16_array = []
transmission_1_all_array = []
for fermi_energy in fermi_energy_array:
print(fermi_energy)
# 表面格林函数
right_lead_surface, left_lead_surface = surface_green_function_lead(fermi_energy + 1e-9j, lead_h00, lead_h01, dim=width)
# 由于镜面对称以及xy各项同性,因此这里六个电极的表面格林函数具有相同的形式
lead_1 = copy.deepcopy(right_lead_surface) # 镜面对称使得lead_1=lead_4
lead_2 = copy.deepcopy(right_lead_surface) # xy各向同性使得lead_2=lead_4
lead_3 = copy.deepcopy(right_lead_surface)
lead_4 = copy.deepcopy(right_lead_surface)
lead_5 = copy.deepcopy(right_lead_surface)
lead_6 = copy.deepcopy(right_lead_surface)
# 几何形状如下所示:
# lead2 lead3
# lead1(L) lead4(R)
# lead6 lead5
# 电极到中心区的跃迁矩阵
H_lead_1_to_center = np.zeros((width, width*length), dtype=complex)
H_lead_2_to_center = np.zeros((width, width*length), dtype=complex)
H_lead_3_to_center = np.zeros((width, width*length), dtype=complex)
H_lead_4_to_center = np.zeros((width, width*length), dtype=complex)
H_lead_5_to_center = np.zeros((width, width*length), dtype=complex)
H_lead_6_to_center = np.zeros((width, width*length), dtype=complex)
move = 0 # the step of leads 2,3,6,5 moving to center
for i0 in range(width):
H_lead_1_to_center[i0, i0] = 1
H_lead_2_to_center[i0, width*(move+i0)+(width-1)] = 1
H_lead_3_to_center[i0, width*(length-move-1-i0)+(width-1)] = 1
H_lead_4_to_center[i0, width*(length-1)+i0] = 1
H_lead_5_to_center[i0, width*(length-move-1-i0)+0] = 1
H_lead_6_to_center[i0, width*(move+i0)+0] = 1
# 自能
self_energy_1 = np.dot(np.dot(H_lead_1_to_center.transpose().conj(), lead_1), H_lead_1_to_center)
self_energy_2 = np.dot(np.dot(H_lead_2_to_center.transpose().conj(), lead_2), H_lead_2_to_center)
self_energy_3 = np.dot(np.dot(H_lead_3_to_center.transpose().conj(), lead_3), H_lead_3_to_center)
self_energy_4 = np.dot(np.dot(H_lead_4_to_center.transpose().conj(), lead_4), H_lead_4_to_center)
self_energy_5 = np.dot(np.dot(H_lead_5_to_center.transpose().conj(), lead_5), H_lead_5_to_center)
self_energy_6 = np.dot(np.dot(H_lead_6_to_center.transpose().conj(), lead_6), H_lead_6_to_center)
# 整体格林函数
green = np.linalg.inv(fermi_energy*np.eye(width*length)-H_center-self_energy_1-self_energy_2-self_energy_3-self_energy_4-self_energy_5-self_energy_6)
# Gamma矩阵
gamma_1 = 1j*(self_energy_1-self_energy_1.transpose().conj())
gamma_2 = 1j*(self_energy_2-self_energy_2.transpose().conj())
gamma_3 = 1j*(self_energy_3-self_energy_3.transpose().conj())
gamma_4 = 1j*(self_energy_4-self_energy_4.transpose().conj())
gamma_5 = 1j*(self_energy_5-self_energy_5.transpose().conj())
gamma_6 = 1j*(self_energy_6-self_energy_6.transpose().conj())
# Transmission
transmission_12 = np.trace(np.dot(np.dot(np.dot(gamma_1, green), gamma_2), green.transpose().conj()))
transmission_13 = np.trace(np.dot(np.dot(np.dot(gamma_1, green), gamma_3), green.transpose().conj()))
transmission_14 = np.trace(np.dot(np.dot(np.dot(gamma_1, green), gamma_4), green.transpose().conj()))
transmission_15 = np.trace(np.dot(np.dot(np.dot(gamma_1, green), gamma_5), green.transpose().conj()))
transmission_16 = np.trace(np.dot(np.dot(np.dot(gamma_1, green), gamma_6), green.transpose().conj()))
transmission_12_array.append(np.real(transmission_12))
transmission_13_array.append(np.real(transmission_13))
transmission_14_array.append(np.real(transmission_14))
transmission_15_array.append(np.real(transmission_15))
transmission_16_array.append(np.real(transmission_16))
transmission_1_all_array.append(np.real(transmission_12+transmission_13+transmission_14+transmission_15+transmission_16))
Plot_Line(fermi_energy_array, transmission_12_array, xlabel='Fermi energy', ylabel='Transmission_12', title='', filename='a')
Plot_Line(fermi_energy_array, transmission_13_array, xlabel='Fermi energy', ylabel='Transmission_13', title='', filename='a')
Plot_Line(fermi_energy_array, transmission_14_array, xlabel='Fermi energy', ylabel='Transmission_14', title='', filename='a')
Plot_Line(fermi_energy_array, transmission_15_array, xlabel='Fermi energy', ylabel='Transmission_15', title='', filename='a')
Plot_Line(fermi_energy_array, transmission_16_array, xlabel='Fermi energy', ylabel='Transmission_16', title='', filename='a')
Plot_Line(fermi_energy_array, transmission_1_all_array, xlabel='Fermi energy', ylabel='Transmission_1_all', title='', filename='a')
end_time = time.time()
print('运行时间=', end_time-start_time)
def Plot_Line(x, y, xlabel='x', ylabel='y', title='', filename='a'):
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
plt.subplots_adjust(bottom=0.20, left=0.18)
ax.plot(x, y, '-')
ax.grid()
ax.set_title(title, fontsize=20, fontfamily='Times New Roman')
ax.set_xlabel(xlabel, fontsize=20, fontfamily='Times New Roman')
ax.set_ylabel(ylabel, fontsize=20, fontfamily='Times New Roman')
ax.tick_params(labelsize=20)
labels = ax.get_xticklabels() + ax.get_yticklabels()
[label.set_fontname('Times New Roman') for label in labels]
# plt.savefig(filename+'.jpg', dpi=300)
plt.show()
plt.close('all')
def transfer_matrix(fermi_energy, h00, h01, dim): # 转移矩阵T
transfer = np.zeros((2*dim, 2*dim))*(0+0j)
transfer[0:dim, 0:dim] = np.dot(np.linalg.inv(h01), fermi_energy*np.identity(dim)-h00)
transfer[0:dim, dim:2*dim] = np.dot(-1*np.linalg.inv(h01), h01.transpose().conj())
transfer[dim:2*dim, 0:dim] = np.identity(dim)
transfer[dim:2*dim, dim:2*dim] = 0
return transfer # 返回转移矩阵
def surface_green_function_lead(fermi_energy, h00, h01, dim): # 电极的表面格林函数
transfer = transfer_matrix(fermi_energy, h00, h01, dim)
eigenvalue, eigenvector = np.linalg.eig(transfer)
ind = np.argsort(np.abs(eigenvalue))
temp = np.zeros((2*dim, 2*dim))*(1+0j)
i0 = 0
for ind0 in ind:
temp[:, i0] = eigenvector[:, ind0]
i0 += 1
s1 = temp[dim:2*dim, 0:dim]
s2 = temp[0:dim, 0:dim]
s3 = temp[dim:2*dim, dim:2*dim]
s4 = temp[0:dim, dim:2*dim]
right_lead_surface = np.linalg.inv(fermi_energy*np.identity(dim)-h00-np.dot(np.dot(h01, s2), np.linalg.inv(s1)))
left_lead_surface = np.linalg.inv(fermi_energy*np.identity(dim)-h00-np.dot(np.dot(h01.transpose().conj(), s3), np.linalg.inv(s4)))
return right_lead_surface, left_lead_surface # 返回右电极的表面格林函数和左电极的表面格林函数
if __name__ == '__main__':
main()
计算结果为:
二、代码2
如果用Guan开源软件包:https://py.guanjihuan.com,可省去部分代码,程序会更简洁一些,代码如下:
"""
This code is supported by the website: https://www.guanjihuan.com
The newest version of this code is on the web page: https://www.guanjihuan.com/archives/6075
"""
import numpy as np
import time
import guan
def get_lead_h00(width):
h00 = np.zeros((width, width))
for i0 in range(width-1):
h00[i0, i0+1] = 1
h00[i0+1, i0] = 1
return h00
def get_lead_h01(width):
h01 = np.identity(width)
return h01
def get_center_hamiltonian(Nx, Ny):
h = np.zeros((Nx*Ny, Nx*Ny))
for x0 in range(Nx-1):
for y0 in range(Ny):
h[x0*Ny+y0, (x0+1)*Ny+y0] = 1 # x方向的跃迁
h[(x0+1)*Ny+y0, x0*Ny+y0] = 1
for x0 in range(Nx):
for y0 in range(Ny-1):
h[x0*Ny+y0, x0*Ny+y0+1] = 1 # y方向的跃迁
h[x0*Ny+y0+1, x0*Ny+y0] = 1
return h
def main():
start_time = time.time()
width = 5
length = 50
fermi_energy_array = np.arange(-4, 4, .01)
# 中心区的哈密顿量
center_hamiltonian = get_center_hamiltonian(Nx=length, Ny=width)
# 电极的h00和h01
lead_h00 = get_lead_h00(width)
lead_h01 = get_lead_h01(width)
transmission_12_array = []
transmission_13_array = []
transmission_14_array = []
transmission_15_array = []
transmission_16_array = []
transmission_1_all_array = []
for fermi_energy in fermi_energy_array:
print(fermi_energy)
# 几何形状如下所示:
# lead2 lead3
# lead1(L) lead4(R)
# lead6 lead5
transmission_12, transmission_13, transmission_14, transmission_15, transmission_16 = guan.calculate_six_terminal_transmissions_from_lead_1(fermi_energy, h00_for_lead_4=lead_h00, h01_for_lead_4=lead_h01, h00_for_lead_2=lead_h00, h01_for_lead_2=lead_h01, center_hamiltonian=center_hamiltonian, width=width, length=length, internal_degree=1, moving_step_of_leads=0)
transmission_12_array.append(transmission_12)
transmission_13_array.append(transmission_13)
transmission_14_array.append(transmission_14)
transmission_15_array.append(transmission_15)
transmission_16_array.append(transmission_16)
transmission_1_all_array.append(\
transmission_12+transmission_13+transmission_14+transmission_15+transmission_16)
# transmission_matrix = guan.calculate_six_terminal_transmission_matrix(fermi_energy, h00_for_lead_4=lead_h00, h01_for_lead_4=lead_h01, h00_for_lead_2=lead_h00, h01_for_lead_2=lead_h01, center_hamiltonian=center_hamiltonian, width=width, length=length, internal_degree=1, moving_step_of_leads=0)
# transmission_12_array.append(transmission_matrix[0, 1])
# transmission_13_array.append(transmission_matrix[0, 2])
# transmission_14_array.append(transmission_matrix[0, 3])
# transmission_15_array.append(transmission_matrix[0, 4])
# transmission_16_array.append(transmission_matrix[0, 5])
# transmission_1_all_array.append(transmission_matrix[0, 1]+transmission_matrix[0, 2]+transmission_matrix[0, 3]+transmission_matrix[0, 4]+transmission_matrix[0, 5])
guan.plot(fermi_energy_array, transmission_12_array, xlabel='Fermi energy', ylabel='Transmission_12')
guan.plot(fermi_energy_array, transmission_13_array, xlabel='Fermi energy', ylabel='Transmission_13')
guan.plot(fermi_energy_array, transmission_14_array, xlabel='Fermi energy', ylabel='Transmission_14')
guan.plot(fermi_energy_array, transmission_15_array, xlabel='Fermi energy', ylabel='Transmission_15')
guan.plot(fermi_energy_array, transmission_16_array, xlabel='Fermi energy', ylabel='Transmission_16')
guan.plot(fermi_energy_array, transmission_1_all_array, xlabel='Fermi energy', ylabel='Transmission_1_all')
end_time = time.time()
print('运行时间(分)=', (end_time-start_time)/60)
if __name__ == '__main__':
main()
三、代码3
使用Kwant来计算:https://www.kwant-project.org/,代码如下:
"""
This code is supported by the website: https://www.guanjihuan.com
The newest version of this code is on the web page: https://www.guanjihuan.com/archives/6075
"""
import kwant
import numpy as np
from matplotlib import pyplot
def make_system():
a = 1
lat = kwant.lattice.square(a)
syst = kwant.Builder()
t = 1.0
W = 5
L = 50
syst[(lat(x, y) for x in range(L) for y in range(W))] = 0
syst[lat.neighbors()] = -t
# 几何形状如下所示:
# lead2 lead3
# lead1(L) lead4(R)
# lead6 lead5
move = 0 # the step of leads 2,3,6,5 moving to center
lead1 = kwant.Builder(kwant.TranslationalSymmetry((-a, 0)))
lead1[(lat(0, j) for j in range(W))] = 0
lead1[lat.neighbors()] = -t
syst.attach_lead(lead1)
lead2 = kwant.Builder(kwant.TranslationalSymmetry((0, -a)))
lead2[(lat(move+j, 0) for j in range(W))] = 0
lead2[lat.neighbors()] = -t
syst.attach_lead(lead2)
lead3 = kwant.Builder(kwant.TranslationalSymmetry((0, -a)))
lead3[(lat(j+(L-W-move), 0) for j in range(W))] = 0
lead3[lat.neighbors()] = -t
syst.attach_lead(lead3)
syst.attach_lead(lead1.reversed()) # lead4
syst.attach_lead(lead3.reversed()) # lead5
syst.attach_lead(lead2.reversed()) # lead6
kwant.plot(syst)
syst = syst.finalized()
return syst
def main():
syst = make_system()
energies = np.linspace(-4, 4, 800)
data1 = []
data2 = []
data3 = []
data4 = []
data5 = []
data6 = []
for energy in energies:
smatrix = kwant.smatrix(syst, energy)
data1.append(smatrix.transmission(1, 0)) # compute the transmission probability from lead 0 to lead 1
data2.append(smatrix.transmission(2, 0))
data3.append(smatrix.transmission(3, 0))
data4.append(smatrix.transmission(4, 0))
data5.append(smatrix.transmission(5, 0))
data6.append(smatrix.transmission(1, 0)+smatrix.transmission(2, 0)+smatrix.transmission(3, 0)+smatrix.transmission(4, 0)+smatrix.transmission(5, 0))
pyplot.plot(energies, data1)
pyplot.xlabel("energy [t]")
pyplot.ylabel("Transmission_12 [e^2/h]")
pyplot.show()
pyplot.plot(energies, data2)
pyplot.xlabel("energy [t]")
pyplot.ylabel("Transmission_13 [e^2/h]")
pyplot.show()
pyplot.plot(energies, data3)
pyplot.xlabel("energy [t]")
pyplot.ylabel("Transmission_14 [e^2/h]")
pyplot.show()
pyplot.plot(energies, data4)
pyplot.xlabel("energy [t]")
pyplot.ylabel("Transmission_15 [e^2/h]")
pyplot.show()
pyplot.plot(energies, data5)
pyplot.xlabel("energy [t]")
pyplot.ylabel("Transmission_16 [e^2/h]")
pyplot.show()
pyplot.plot(energies, data6)
pyplot.xlabel("energy [t]")
pyplot.ylabel("Transmission_1_all [e^2/h]")
pyplot.show()
if __name__ == '__main__':
main()
当move=0时,运行结果:
计算得到的透射率和以上程序的结果相同。
当move=5时,运行结果:
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作者您好!我再计算陈绝缘体的六端口体系的时候自能是用Sigma=h10*gr*h01来计算的,在Ef接近0的时候霍尔电阻突然发散,我检查了数据发现Ef接近0时Gamma函数也接近0,但在Ef取其他的值时都正常,想请教您一下这是由什么原因产生的。(ps:我是用matlab来计算的霍尔电阻)
这个问题应该不是bug,是正常数值计算现象。可能的原因是这个地方能带是平的或者有交叉点,可能的解决方法是可以把虚部的小量增大一些,或者哈密顿量上加上极小的对角线,或者直接不考虑E=0的值,只考虑零附近的值。
我画了一下能带,发现霍尔电阻发散时,Ef正处于带隙之中,正常情况应该是当Ef在gap中时,Rxy=1。
感谢作者大大!还有一个问题就是我在这个模型中外加了磁场,参考了您写的方格子的霍夫斯塔特蝴蝶这篇文章,在跳跃能加上了相位因子,然后跑出来的图像还是和不加磁场的是一样的,请问这种情况是对的吗?而且加上磁场后T12=T21,有相位的改变,对称被破坏还会出现这种情况吗?
加了磁场,有朗道能级和对应的边缘态,结果应该是相邻的电极之间有传输,可以参考:介观体系中的Landauer–Büttiker公式、方格子紧束缚模型中朗道能级的陈数/霍尔电导(附Python代码)。
十分感谢!!!!!!!
作者您好!请问一下这个透射系数T是可能大于1的嘛?因为在您的结果中T12已经到达2了。
一个电极中有多个通道模式,例如,这里宽度取为5,能带在费米能为零的位置有5个通道模式,因此T_{1->other leads}之和最高可以达到5,如果包括了反射系数,那么值就是5。
作者您好,我想问一下如果只链接一个通道是需要修改电极这个部分吗?怎么修改呀?
def get_lead_h00(width):
h00 = np.zeros((width, width))
for i0 in range(width-1):
h00[i0, i0+1] = 1
h00[i0+1, i0] = 1
return h00
def get_lead_h01(width):
h01 = np.identity(width)
return h01
如果只是修改电极的通道数,那么把电极的 width 取为1,而中心区的 width 为其他值。这时候主要修改 H_lead_1_to_center 这个电极和中心区的耦合矩阵。
博主你好,为什么transmission fuction 有震荡呢?
电极对电子也会有散射的,以“自能”的形式考虑进去了
博主您好,我想问一下有的时候H01不存在逆,这时候该咋办?
对角线上加无穷小量,或者换一种编号方式。
博主您好,首先感谢您分享那么多的知识和代码。然后我有一个问题想请教您。我目前在画外尔体系的霍尔电导,我是利用非平衡格林函数方法,根据Landauer-Buttiker公式来获得体系霍尔电导。然后我的电极选择的是利用金属电极。我想问一下,就是我们在计算霍尔电导的时候(也就是计算表面格林函数的时候),四个端口是不是各自要设置一个费米能啊?如果要,请问该如何设置啊?
看你考虑的是什么情况了。可以看下Datta的这本书《Electronic Transport In Mesoscopic Systems》。这是摘抄于第42页:For linear response we conceptually let the bias tend to zero; that is, .
如果考虑大偏压,费米能可以取为固定,改变电极的on-site值,参照这篇文章的做法:Quantized four-terminal resistances in a ferromagnetic graphene p–n junction。另外也可以直接改变费米能来计算。我做了简单的数值验证,两种方法好像是等价的,因为求出的电极自能是相同的。
好的,非常感谢,我仔细看看这篇文章!!!
博主您好,首先感谢您之前的回答,我下去也进行了验证。然后我想再请教一个问题。就是根据Landaur-Buttiker公式来计算霍尔电导,那编代码的时候该如何考虑每个端口的电压啊?我是通过调节端口的费米能,然后端口的费米能调来调去,但还是无法完全和文献相对应,总是差一截。
我也不清楚,没算过这方面的。具体得看器件的设计吧,文献中应该会说明。也可以找其他文献看看。
好的,谢谢!
博主您好!我想请教个问题。您在求表面格林函数的时候,根据镜面对称以及xy对称得到所有电极的表面格林函数是一样的,这里是不是因为左右的hopping以及xy方向的hopping都相同(您的代码参数设的是1)才可以,如果Tx≠Tx' Ty≠Ty'的话,就不能这样做了,而是需要每个电极求一次表面格林函数。
是的。左右电极之所以用相同的表面格林函数,是因为h01等于h01^{dagger}。
好的好的,非常感谢!