Electrical generation of spin polarization in semiconductors is of great interest for the device potential in spintronics. One such mechanism is chirality-induced spin selectivity (CISS), with which structural chirality leads to different electric conductivities for electrons of opposite spins. CISS has been widely reported for chiral molecule assemblies on metal surfaces. However, theoretical understanding of the microscopic mechanism and manifestation of CISS in specific device structures remain controversial. Here, we report direct evidence of CISS in two-terminal devices of heterojunctions of (Ga,Mn)As/AHPA-L molecules/Au. The (Ga,Mn)As layer acts as a spin analyzer for electrons injected from the Au electrode through the AHPA-L monolayer. The CISS of AHPA-L and spin injection into the (Ga,Mn)As are manifested as sharp changes in the junction conductance at the coercive fields of the (Ga,Mn)As. The observations provide a definitive signature of CISS-induced spin valve effect in a two-terminal device. Theoretical implications of the effect and its bias dependence will be discussed.

1. Chirality-induced Spin Selectivity in a
Two-terminal Semiconductor Device
Tianhan Liu, Haoyun Deng, Longqian Hu, Eric Lochner, Pedro Schlottmann ,
Stephan von Molnár, and Peng Xiong
(Department of Physics, Florida State University)
Xiaolei Wang, Hailong Wang and Jianhua Zhao
(Institute of Semiconductors, Chinese Academy of Sciences)
Gang Shi, Fan Gao, Honglei Feng, and Yongqing Li
(Institute of Physics, Chinese Academy of Sciences)
03/02/2019
2020 APS Meeting
Denver, Colorado
Work supported by NSF grant DMR-1905843

2. Outline
• Introduction: Chirality-induced spin selectivity (CISS)
• CISS in a two-terminal semiconductor device
Device fabrication of Au/polyalanines/(Ga,Mn)As junctions
Electrical measurements & Theoretical calculation
• Summary
arXiv:2001.00097 2

3. Molecular Spintronics
• Hybrid structure of organic molecules and solid-state materials
• Applications: bio-sensors, chemical patterning, spin filtering...
Chirality-induced spin selectivity (CISS)
Electron current becomes spin polarized when the charge carriers
have a velocity along the helical axis of the molecules.
G.L.J.A. Rikken, Science. 331, 864 (2011).R. Naaman et al., Annu. Rev. Phys. Chem. 66, 263 (2015).
3

4. Previous experiments on CISS
B. Gohler et al., Science 331, 894 (2011). Z. Xie et al., Nano Lett. 11, 4652 (2011).
Photo-electrons through
dsDNA monolayer
Au NP-dsDNA-Ni complex
from conductive AFM
NOT realized in a
vertical junction device
• Optical and electrical measurements:
spin polarization as high as 60%
• Theory of CISS: spin-orbit coupling
arises as the charge is moving under
an electrostatic potential
4

5. Device structure
• Vertical junctions of Au/polyalanines/(Ga,Mn)As.
• The magnetoconductance (MC) of the junction is measured in a perpendicular field.
H
Gj
Magnetic Field
H
Gj
Magnetic Field
5

6. Previous work in our group
J.G. Braden et al., Phys. Rev. Lett. 91, 56602 (2003).
High spin polarization of (Ga,Mn)As
Self-assembly of thiol molecules on bare GaAs
T. Liu et al., ACS Appl. Mater. Interfaces 9, 43363 (2017).
Ga-S
As-S
6

7. Assembly of α-helix L-polyalanine (AHPA-L) on (Ga,Mn)As
Based on L amino acids (H-CAAAA KAAAA KAAAA KAAAA KAAAA KAAAA KAAAA K-OH)
K (lysine)A (alanine)C (cysteine)
(Ga,Mn)As with perpendicular magnetic anisotropy
L. Thevenard et al., Phys. Rev. B 73, 195331 (2006).
7

8. Device Fabrication
• Define (Ga,Mn)As channel by photolithography
and ion milling
• Make Au contacts and alignment marks for e-
beam lithography by photolithography and
thermal evaporation
• Define effective junction regions on (Ga,Mn)As
channel by e-beam lithography
• Remove the native oxide layer on (Ga,Mn)As by
ion milling and assemble polyalanines on the
junctions
• Deposit Au top electrodes for the junctions by
thermal evaporation under liquid nitrogen
coolingJunction size varies from 1x1 μm2 to 25x25 μm2
8

9. Au/polyalanines/(Ga,Mn)As Junction
Insulating behavior indicates significant
coverage of the junction by the molecules
Junction MR measurements
0 50 100 150 200 250 300
0
1
2
3
4 Sample 1
IDC
= 10 A
GJ(mS)
T (K)
arXiv:2001.00097 9

10. Au/polyalanines/(Ga,Mn)As Junction
Consistent with CISS
-2000 -1000 0 1000 2000
1.05
1.08
1.11
1.14
1.17
Sample 1
IDC
= 100 A
H (Oe)
GJ(mS)
GJ
arXiv:2001.00097 10
Two resistance states of the junction
-150 -100 -50 0 50 100 150
-900
-600
-300
0
300
600
900
02000 Oe
- 2000 Oe
I(A)
V (mV)
Sample 1

11. -150 -100 -50 0 50 100 150
0
1
2
3
GJ(V)
(mS)
V (mV)
Sample 1
0
I
I
V
-1000 -500 0 500 1000
0
200
400
600
GJ(I)
(S)
Sample 1
I (A)
VV
I
0
Bias dependence of CISS
Bias voltage dependence Bias current dependence
arXiv:2001.00097 11

12. 12
Bias dependence of CISS
-2000 -1000 0 1000 2000 3000
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Sample 1
10 A
50 A
100 A
150 A
200 A
300 A
GJ(mS)
H (Oe)
400 A
0 200 400 600 800
0
100
200
300
400
From I-V curves
GJ(I)
(S)
I (A)
0 200 400 600 800
0
100
200
300
400
From I-V curves
From MC
Linear fit
GJ(I)
(S)
I (A)
Sample 1
Bias current dependences
IV curve vs. MC
arXiv:2001.00097

13. Theory on CISS device
S. Dalum and P. Hedegård, Nano Lett. 19, 5253 (2019).
Spin-orbit interaction in chiral molecules
Break Onsager relation
Spin accumulation in nonmagnetic lead.
Magnetic tunnel junction
13
𝛥𝜇 = 𝜇 𝑅 − 𝜇 𝐿 = 𝑒𝑉
Taylor expansion of Fermi function,
𝛼, 𝛽, and 𝛾 are temperature related coefficients, and 𝑇± = 𝑇𝐿𝑅
0
(1 ± 𝐴𝑚 ∙ 𝑎±)
𝐺± =
𝐼±
𝛥𝜇
= 𝛼𝑇±
𝑑𝐸
2π
+ 𝜟𝝁 𝛽𝑇±
𝑑𝐸
2π
+ 𝜟𝝁 𝟐
𝛾𝑇±
𝑑𝐸
2π

14. Summary
• Observed CISS in a two-terminal semiconductor
device manifested in spin-valve effect.
• Consistent with theoretical model predicting
measurable CISS due to an equilibrium state with
spin accumulation.
• The result implies a potential approach to electrical
spin injection and detection on semiconductors
without using any magnetic material.
0 200 400 600 800
0
100
200
300
400
From I-V curves
From MC
Linear fit
Gj
(S)
I (A)
arXiv:2001.00097
-2000 -1000 0 1000 2000
1.05
1.08
1.11
1.14
1.17
Sample 1
IDC
= 100 A
H (Oe)
GJ(mS)
GJ
14

15. (Ga,Mn)As Thin Film
0 50 100 150 200 250 300
11
12
13
14
15
16
R(k)
T (K)
DC 50 nA
TC ~ 130 K Coercive field ~ 500 Oe
Hall resistance ~ 80 Ω
(Ga,Mn)As MR and Hall measurements
-2000 -1000 0 1000 2000
-90
-60
-30
0
30
60
90
DC 1A
4.2 K
RHall
()
H (Oe)
15

16. 16
-2000 -1000 0 1000 2000
40
50
60
70
80
90
100
GJ(mS)
Control Sample
DC 1 A, 4.2 K
H (Oe)
Control sample without polyalanines
• Control junctions consistently have resistances significantly
lower than similar junctions with polyalanine.
• Control junctions show negligible MC without clear jumps at HC.

17. Temperature dependence
0 20 40 60 80 100 120 140
0.0
0.5
1.0
1.5
2.0 Sample 3
RJ(I)
()
T (K)
17

18. 18
-2000 -1000 0 1000 2000
3.3
3.4
3.5
3.6
IDC
= 100 A
IDC
= -100 ASample 2
T = 4.2 KGJ(mS)
H (Oe)
Gj-
=0.101 mS
Gj+
=0.118 mS
Opposite DC bias