Analysis of Circularly Polarized Light Irradiation Effects on Double

Analysis of Circularly Polarized Light Irradiation Eﬀects on Double Ferromagnetic-Gate Silicene Junction 17

Analysis of Circularly Polarized Light Irradiation Eﬀects on Double Ferromagnetic-Gate Silicene Junction

1 2 Peerasak Chantngarm and Kou Yamada

ABSTRACT graphene (C), germanene (Ge), Borophene (B), phos- phorene (P), stanene (Sn) and plumbene (Pb) in elec- We present an analytical study of effects that tronics, spintronics, valleytronics and quantum com- off-resonant circularly polarized light irradiation has puting applications. Among these elemental 2D ma- on spin-valley currents in dual ferromagnetic-gate terials, silicene is currently considered to be the most silicene-based junctions. Two identical electric fields promising candidate mainly due to the accumulation are applied to both ferromagnetic (FM) gates. Two of silicon-related technology and knowhow in semi- types of exchange field configurations, parallel (P) conductor industry. Although silicene is a zero-band- and anti-parallel (AP), are applied along with chem- gap semiconductor with a small band gap of 1.55 meV ical potential to the FM gates in this investigation. in theory, the buckled honeycomb lattice structure The results show that application of circularly po- allows Dirac electron mass and its band structure to larized light has an impact on polarized spin and be manipulated easily by electric field [2], which is valley current characteristics, particularly at the off- important in making electronics devices. The honey- resonant frequency region. It also enhances the am- comb lattice structure of silicene results in the exis- plitude of tunnelling magnetoresistance (TMR) sig- tence of two atoms in one unit cell, and gives rise to nificantly. In addition, we found that exchange field two sublattices, A and B. This two-sublattice system configuration has an effect on both spin polarization contributes to a new concept called pseudospin. The and valley polarization. Our study reveals that light two-dimensional buckled honeycomb lattice structure intensity plays the main role on the light irradiation in silicene causes tuneable spin-valley coupled band effects, where the band structure and change elec- structure and gives rise to topological phase transi- tronic properties of the materials are modified by tion, an intriguing transport phenomenon. photon dressing to create a new phase of electronic There have been many theoretical and compu- structure. The change of band structure in each re- tational studies in spin- and valley-polarized trans- gion affects the transmission coefficients and transport in silicene junctions, which helps the progress in mission probability amplitude of electrons, which in this area. The topics of study are, for instance, the turn affects the conductance of each spin-valley cur- mechanism of magnetism that opens different spin- rent component. Our study suggests the potential of dependent band gaps at k and k’ points and results this scheme in applications, such as spin-valleytronic in spin and valley polarized transports [3], the condi- photo-sensing devices under polarized-photo irradia- tions of electric field for the fully valley and spin po- tion. larized transports [4], and ballistic transport through silicene ferromagnetic junctions under the presence Keywords: Silicene, Spin-Valleytronics, Photo- of magnetic exchange field and normal electric field Sensing Devices [5]. Another research area that has been very active is electronic transport of spintronic and valleytronic 1. INTRODUCTION devices based on silicene, such as spin filter and spin- Silicene has recently attracted much attention af- valley filter [6-8], as well as spin-polarized transport ter experimental evidence [1]. It is considered to in a dual-gated silicene system where there is no ex- be a candidate for the post-bulk-silicon era along change field [9]. Double ferromagnetic-gated silicone- with other artificial elemental 2D materials such as based junction was also recently proposed to control lattice-pseudospin current along with pure spin- and Manuscript received on August 10, 2017 ; revised on January valley-polarized current in silicene giving a possibil- 25, 2018. ity for applications in pseudospintronics [10]. The Final manuscript received on January 25, 2018. promising electronic properties of silicene has led to 1 The author is with Department of Electronics and Telecom- more experimental investigations and succeeded in munication Engineering, Faculty of Engineering, Rajamangala University of Technology Krungthep, Bangkok, Thailand., E- making silicene field-effect transistors (FET) operat- mail: [email protected] ing at room temperature [11]. 2 The author is with Domain of Mechanical Science and Tech- nology, Graduate School of Science and Technology, Gunma Another area that attracts attention more recently University, Gunma, Japan., E-mail: [email protected] is photo-induced effects, similarly to the study in 18 ECTI TRANSACTIONS ON COMPUTER AND INFORMATION TECHNOLOGY VOL.12, NO.1 May 2018

graphene, where circularly polarized light is used to open a gap at the Dirac point [12]. By irradiation of circularly polarized light at fixed electric field, the topological class of silicene could be changed from quantum spin-Hall insulator (QSHI) to other phase which has different properties [13]. Light irradiation on silicene has effects on the band structure due to photon dressing effect. It is reported that spin- and valley-polarization depends on the intensity of off- resonant circularly polarized light as well as electric field, and it can be inverted by reversing the direction of electric field or the circular polarization of the Fig.1: Cross-sectional schematic model of double- light [14]. It is found that spin-valley polarizations barrier silicene-based NM1/FM1/NM2/FM2/NM3 and tunnelling magnetoresistance in a ferromagnetic- structure. normal-ferromagnetic (FNF) junction can be significantly enhanced by irradiation of off-resonant cir- the ferromagnetic barriers FMs. Gate potential µ/e cularly polarized light on one of ferromagnetic gates is applied from the top and the bottom of ferro- without electric field or magnetic field [15]. There is magnetic barriers. Circularly polarized light A(t) = also a study in spin-valley filtering and giant mag- A0(sin(Ωt), cos(Ωt)) is irradiated to the NM2 region netoresistance (GMR) under the photo irradiation in between ferromagnetic gates, where Ω is light fre- off-resonant frequency region, with which can be ap- quency, At is time-dependent vector potential, and plied to photo-sensing devices [16]. A0 is the magnitude of light. Circularly polarized In this paper, we study the tight-binding model of light has the planes of the electric field vectors and silicene-based NM/FM/NM/FM/NM junction under magnetic field vectors rotate. It has a constant mag- the effects of off-resonant circularly polarized light at nitude of electric field while rotating at a steady rate the NM region between two FM gates, where NM in a plane perpendicular to the direction of the light. is normal silicene and FM is ferromagnetic silicene. Comparing to linearly polarized light, the circularly Two different exchange field configurations, parallel polarized light has the intensity equivalent in all di- (P) and anti-parallel (AP), are also applied to see rections. The off-resonant light frequency used in the effects. We particularly investigate the effects of this study is the frequency region where the electron photo irradiation on spin- and valley-polarized con- band structures are changed by virtual photon ab- ductance and tunnelling magnetoresistance (TMR) in sorption processes without direct electrons excitation. this junction. It is found that photo irradiation have Light irradiation with off-resonant frequency causes significant impact on spin polarization (SP), valley no depumping on optically pumped specimens, and polarization (VP), as well as TMR. causes no attenuation [18]. In off-resonant frequency scheme, the linear optical process occurs. This means 2. MODEL that the amount of light transmitted through a mat- In this section we explain the structure of double ter is proportional to the irradiated light intensity ferromagnetic-gated silicone-based junction in sub- [19]. section 2.1 and the low-energy effective Hamiltonians based on tight-binding model in subsection 2.2. 2.2 Tight-Binding Hamiltonians In tight-binding model, the off-resonant light fre- 2.1 Device Structure quency can be achieved when h|Ω| ≫ t0, where h is The ballistic transport of electrons in a double the Planck constant and t0 is the nearest hopping en- ferromagnetic-gated silicene-based structure under ergy. The lowest frequency Ω to satisfy this condition external influences as illustrated in Fig. 1 was stud- can be calculated from the bandwidth 3t0 = 4.8 eV ied. The numbers following NM and FM in the figure, = 1015 Hz [13]. The perpendicular distance between as in NM1 and FM1, are designated as the identi- the two sublattices due to the buckling structure is fication for each region. Each of the ferromagnetic 2D = 0.46 A,˚ where D = 0.23 A[20].˚ In this study, gates has length LG, and they are apart from each we investigate two the electronic transport in two ex- other by distance L. The magnetic exchange energies change field configurations, parallel junction (P) and are designated as h1A and h1B for sublattice-A and anti-parallel junction (AP). The direction of the ex- sublattice-B at barrier FM1, while their counterparts change fields are in this study is assumed to be in- are designated as h2A and h2B at barrier FM2. plane with positive sign means right-direction, and Like the proposal for graphene, silicene might be negative sign means left-direction. The parallel junc- induced to FM by magnetic insulators EuO due to tion (P) and anti-parallel junctions (AP) are defined proximity-induced exchange splitting [17]. A con- as the following exchange field configuration. trollable perpendicular electric field Ez is applied to Analysis of Circularly Polarized Light Irradiation Effects on Double Ferromagnetic-Gate Silicene Junction 19

P junction : h1A = h1B = h2A = h2B = 5 meV, stant of silicene [12, 15]. The value of is generally less than 1 for the intensity from light sources in the AP junction : −h1A = h1B = −h2A = h2B = 5 meV frequency range of our interest [13]. In the normal regions NM1 and NM3 where there The interaction between each exchange field compo- is neither photo irradiation nor external electric field, nent is not taken into account in this study to sim- the spin-valley dependent energy gap is ∆ησ3 = plify the analysis. The tight-binding Hamiltonians ησ∆SO. Therefore, the Hamiltonian in these two re- and low-energy effective Hamiltonians [21] are used gions could be defined as to describe the electrons transport in this study. In ferromagnetic regions FM1 and FM2, the Hamilto- x y z nian is defined as HNM1 = HNM3 = vF (pxτ −ηpyτ )−ησ∆soτ . (4)

3. TRANSPORT FORMULAE H = v (p τ x − ηp τ y) − ∆ τ z − µ FM1 F x y ησ1 σ1 In this section, we describe the electrons transport x − y − z − HFM2 = vF (pxτ ηpyτ ) ∆ησ2τ µσ2. (1) in the device structure. Using the Hamiltonians from

′ (1), (2) and (4), the wave functions in the NM1, FM1, The k and k valley is represented with η = +1 NM2, FM2, and NM3 regions can be defined respec- − ↑ ↓ and η = 1, while spin and is represented tively as with σ = +1 and σ = 11, respectively. The nota- ∂ ∂ [( ) ( ) ] tionsp ˆx = −i~ , pˆy = −i~ , and the notations ∂x ∂y 1 ikxx 1 −ikxx ik//y x y z ΨNM1= −iηθ e +rησ iηθ e e , τ , τ , τ are elements of Pauli spin-operators used Aησe −Aησe [ ( ) ( ) ] ∼ × 5 to represent pseudospin states. vF = 5.5 10 m/s 1 il x 1 −il x ik y Ψ = a − e x +b e x e // , [22] indicates Fermi velocity near Dirac point. Other FM1 ησ B e iηα1 ησ −B eiηα1 [ ( ησ ) ( ησ ) ] parameters in (1) are explained as follows: 1 1 − Ψ = g eimxx+f e imxx eik//y, NM2 ησ M e−iηβ ησ −M eiηβ [ ( ησ ) ( ησ ) ] ∆ = ησ∆ −∆ +σ∆ : Spin-valley dependent ησ1 SO E M1 1 in x 1 −in x ik y Ψ = p − e x +q e x e // , energy gaps at FM1 gate FM2 ησ N e iηα2 ησ −N eiηα2 [ ( ησ ) ] ησ ∆ησ2 = ησ∆SO −∆E +σ∆M2 :Spin-valley dependent 1 ikxx ik//y ΨNM3= tησ −iηθ e e , energy gaps at FM2 gate Aησe ∆SO = 3.9 meV : Effective spin-orbit interaction (5) ∆E = eDEz : Electric field-induced energy gap (h − h ) where ∆ = 1A 1B : Exchange field-induced energy M1 2 gaps at FM1 gate (h − h ) E + ησ∆SO E + µσ1 + ∆ησ1 2A 2B Aησ = √ ,Bησ = √ . ∆M2 = : Exchange field-induced energy 2 − 2 2 2 E ∆SO (E + µ ) − ∆2 gaps at FM2 gate σ1 ησ1 µσ1 = µ+σuM1 : Spin-dependent chemical potentials at FM1 gate √E + ∆ησ √E + µσ2 + ∆ησ2 µσ2 = µ+σuM2 : Spin-dependent chemical potentials Mησ = ,Nησ = . 2 − 2 2 − 2 at FM2 gate E ∆ησ (E + µσ2) ∆ησ2 Here, the notations uM1 = (h1A + h1B)/2 and uM2 = The coefficients r , a , b , g , f , p , q , t (h2A+h2B)/2, since the chemical potential in the bar- ησ ησ ησ ησ ησ ησ ησ ησ rier is spin-dependent relating to the exchange field. could be obtained by using the following boundary The Hamiltonian in normal regions NM2 with photo conditions to obtain matrix equation. After that, irradiation is defined as Cramer’s rule might be used to solve that matrix equation. x y z HNM2 = vF (pxτ − ηpyτ ) − ∆ηστ , (2)

where the spin-valley dependent energy gap could be ΨNM1(0) = ΨFM1(0), expressed with ∆ = ησ∆ + ηλ and the light ησ SO Ω ΨFM1(LG) = ΨNM2(LG), irradiation term λΩ could be described as ΨFM1(LG + L) = ΨNM2(LG + L),

2 ΨFM2(2LG + L) = ΨNM3(2LG + L). (6) λΩ = (eΛVF ) /~Ω. (3)

We use Λ = eA0a/~ which is a dimensionless num- Here, the parameters rησ and tησ represent the ber to characterize the light intensity with e repre- reﬂection and transmission coeﬃcients of electronic sents the elementary charge, A0 represents the mag- waves, respectively. The wave vectors in the x- nitude of light wave, and a represents the lattice con- direction in each region are given by 20 ECTI TRANSACTIONS ON COMPUTER AND INFORMATION TECHNOLOGY VOL.12, NO.1 May 2018

√ √ 2 2 (Gk↑ + Gk↓) − (Gk′↑ + Gk′↓) 2 − 2 (E+µσ1) −∆ cos α1 × E ∆SO cos θ ησ1 VP (%) = 100, kx= , lx= , GT ~vF ~vF

√ √ G − G and TMR(%) = P AP ×100. (9) 2 − 2 2− 2 E ∆ησ cos β (E+µσ2) ∆ησ2 cos α2 GP mx= ~ , nx= ~ . vF vF From (9), spin polarization could be considered as The incident angles on each junction can be ob- the difference between spin-up components and spin- tained through the conservation of components in y- down components divided by total conductance. In direction as given by the same manner, valley polarization could be considered as the difference between k-valley components ′ √ and k -valley components divided by total conduc- √ 2 − 2 tance. On the other hand, tunnelling magnetoresis- E2 − ∆2 sin θ (E+µσ1) ∆ησ1 sin α1 k = SO = tance could be considered as the difference between // ~v ~v √ F √ F conductance in P-junction (GP) and AP-junction 2 − 2 2 − 2 (GAP) normalized by the conductance in P-junction. E ∆ησ sin β (E+µσ2) ∆ησ2 sin α2 = = . Here, the total conductance GT is defined as ~vF ~vF Here, θ is the incident angle of electrons at GT = Gk↑ + Gk↓ + Gk′↑ + Gk′↓. (10) the NM1/FM1 junction, β is the incident angle 4. RESULTS AND DISCUSSION at FM1/NM2 junction, α1 is the incident angle at NM2/FM2 junction, and α2 is the incident angle at In this study, we set the device structure parame- FM2/NM3 junction. The transmission probability ters to be L = 25 nm and LG = 25 nm, and electrons amplitude Tησ is calculated via the formula excited energy to be E = 4 meV. Fig. 2 compares the normalized conductance when there is no electric 2 Tησ = Jt/Jin = |tησ| , (7) fields on the ferromagnetic gates under different exchange field configurations and light irradiation con- where J and J are current densities of transmitted t in ditions. Fig. 2(a) shows conductance in P junction electrons and injected electrons, respectively. when there is no light irradiation. It depicts complete It is known that spin-polarized conductance and spin- and valley-polarization, which is useful for ap- valley-polarized conductance in ballistic regime at plications in spin-valley filtering devices. When the zero temperature can be obtained with integration same P junction is irradiated with 2000 THz light, the on all incident angles using the standard Landauer’s spin-valley polarizations particularly the spin-up cur- formalism [23, 24] as shown below rents (Gk↑ and Gk′↑) are affected to become broader as shown in Fig. 2(b). Fig. 2(c) and (d) show con- T = J /J = |t |2 (8) ησ t in ησ ductance when the irradiated light frequency is varied The parameters in (8) are described in details as fol- in P junction and AP junction, respectively. Com- lows: √ plete spin-valley polarization can be seen in both fig- W N(E) = E2 − ∆2 : Density of state at the ures. In term of applications, Fig. 2(d) in particu- ~ SO π vF lar shows filtering of Gk↑ with other spin-valley po- transport channel of normal silicene junction larization components being suppressed, which indi- 4e2 G0 = N0(E) : Unit conductance cates a potential for spin-valley filtering with a cer- h tain polarized-photon frequency. Another notewor- W N0(E) = |E| : Density of state at transport thy point here is the very small normalized conduc- π~vF channel in silicene excluding the spin-orbit interac- tance magnitude of less than 0.03. This is due to tion effect the fact that in this study the energy E = 4 meV approaches ∆ = 3.9 meV, which is in the energy h : Planck’s constant SO range we are interested in due to the intriguing prop- W : The width of silicene film erties near the Dirac cone. As the results, there are For our numerical analysis in this study, the spin fewer density of states available in transport channel polarization (SP), valley polarization (VP) and tun- and causes lower conductance. nelling magnetoresistance (TMR) are defined respec- Fig. 3(a) and (b) show spin polarization (SP) with tively as follows: varied electric field EZ at the ferromagnetic gates when there is no light irradiation in P junction and (G ↑ + G ′↑) − (G ↓ + G ′↓) AP junction, respectively. The comparison of results SP (%) = k k k k × 100, GT indicates the effect of exchange field configuration on Analysis of Circularly Polarized Light Irradiation Effects on Double Ferromagnetic-Gate Silicene Junction 21

(a) (b) (a) (b)

(c) (d) (c) (d) Fig.2: Normalized conductance when there is (a) no Fig.3: Spin polarization when there is (a) no light light in P junction, (b) light irradiation at 2000 THz in P junction, (b) no light in AP junction, (c) light in P junction, (c) light irradiation with varied fre- irradiation in P junction, (d) light irradiation in AP quency in P junction, (d) light irradiation with varied junction. frequency in AP junction.

the spin polarization with full switching from +100% irradiation. Moreover, neither exchange field nor to −100% at electric field around 0 meV in AP junc- light frequency in the off-resonant region which is the tion. Fig. 3(c) and (d) show spin polarization char- range of our investigation has significant impact on acteristics when light with frequency of 1000 THz, the valley polarization characteristics. In both P and 1500 THz and 2000 THz are irradiated to P junc- AP junction, the valley polarization shows sharp full tion and AP junction, respectively. They clearly de- switching from −100% to +100%, particularly in P pict the effect of light irradiation in both exchange junction. These results might be useful for applica- field configurations. Particularly in P junction, the tions in valleytronics. range of spin polarization is enhanced as illustrated Tunnelling magnetoresistance (TMR) is an inter- in Fig. 3(c). The spin polarization shows switching esting phenomenon which is widely applied to many from −100% to +100% in negative electric field, and types of devices such as read heads of hard disk drive, from +100% to −100% in positive electric field when magnetoresistive random-access memory (MRAM), the frequency is 1000 THz. On the other hand, os- and sensing applications. Our study shows that photo cillation can be observed in AP junction as shown in irradiation also affects TMR characteristics as illus- Fig. 3(d), particularly when the frequency is 2000 trated in Fig. 5(b) where light with frequency of 1000 THz. THz, 1500 THz and 2000 THz are irradiated compar- The effects of light irradiation on valley polariza- ing to Fig. 5(a) where there is no light irradiation. tion (VP) characteristics are shown in Fig. 4. The It shows that in all three light frequencies, the am- first two figures, Fig. 4(a) and (b), are the cases with- plitude of TMR increases significantly with photo ir- out light irradiation in P junction and AP junction, radiation. Overall, the changes of electron transport where it is clear that the exchange field configuration characteristics as seen in SP, VP, and TMR after cir- contributes to the characteristics of valley polariza- cularly polarized photo irradiation from Fig. 3 to tion. However, in the opposite to spin polarization Fig. 5 is considered to be the results of photon dress- where the full switching occurred in AP junction as ing effect, which can modify the band structure and shown in Fig. 3(b), the valley polarization indicates change electronic properties of the materials [12, 13]. full switching from +100% to −100% at the electric Photon dressing is the interaction between photons field around 0 meV in P junction as depicted in Fig. and electrons in nanoscale, which causes a new phase 4(a). Fig. 4(c) and (d) show valley polarization when of electronic structure [25]. The interaction results 1000 THz, 1500 THz and 2000 THz light are irradi- in quasiparticles of combined photons and electrons ated to P junction and AP junction, respectively. with a distinct Floquet-Bloch band structure [26]. It is interesting to see that both P junction and When light with off-resonant frequency is irradiated AP junction show similar characteristics with photo to a matter, the light energy would not be absorbed 22 ECTI TRANSACTIONS ON COMPUTER AND INFORMATION TECHNOLOGY VOL.12, NO.1 May 2018

(a) (b)

Fig.5: Tunnelling magnetoresistance when there is (a) no light, (b) circularly polarized light irradiation.

Fig.4: Valley polarization when there is (a) no light in P junction, (b) no light in AP junction, (c) light irradiation in P junction, (d) light irradiation in AP junction. (a) (b) by electronic states of the matter. The photon energy instead dresses the electronic states via virtual photon processes. Because electronic states are affected or dressed by the photon, we call this interaction photon dressing, which gives rise to new electronic states with different properties. We have also investigated the parameters that play an important role on light irradiation effects, where (c) (d) we particularly pay attention to light frequency and light intensity. Fig. 6 compares the normalized con- Fig.6: Normalized conductance when the irradiated ductance in P junction when there is no electric fields light frequency is (a) 500 THz, (b) 1000 THz, (c) under the irradiation of different light frequency. Fig. 1500 THz, (d) 2000 THz. 6(a) shows the insulating states when the light frequency is low at 500 THz comparing to the complete spin- and valley-polarization when there is no light where Λ is 0.03. Furthermore, the tunnelling magne- irradiation in Fig. 2(a). This is considered to be toresistance is even bigger in Fig. 5(b), where Λ is the results from altered band structure due to pho- 0.3, than in Fig. 7(b). When we compare the similar ton dressing effect, which in turn reduces the trans- TMR characteristics in Fig. 5(a) and Fig. 7(a), it mission probability of electrons in the junction. Fig. might be possible to say that when the light intensity 6(b) and (c) show the increasing conductance along is lower, the effect of light irradiation is weaker and with higher light frequency, and Fig. 6(d) illustrates gets closer to the state when there is no light irra- the same conductance characteristics as shown in Fig. diation. The results show that light intensity is the 2(b) when light frequency is 2000 THz. parameter that plays a significant role to trigger light Next we look at the effects of light intensity on irradiation effects. tunnelling magnetoresistance due to its simpler char- The conductance, spin polarization, valley polar- acteristics comparing to those of conductance, which ization, and tunelling magnetoresistance of the struc- makes it is easier to analyse. Fig. 7(a) and (b) illus- ture could be explained with the energy band struc- trate the tunnelling magnetoresistance characteristics ture of each region as shown in Fig. 8. Since with different light frequency when the light intensity the band gap at the ferromagnetic barrier is deter- parameter Λ is 0.03 and 0.1, respectively. It is obvi- mined by spin-valley dependent energy gap ∆ησ = ous to see that the tunnelling magnetoresistance is ησ∆SO − ∆E + σ∆M , the band gap for each spin- bigger in Fig. 7(b) where Λ is 0.1 than in Fig. 7(a) valley component is different among each other. For Analysis of Circularly Polarized Light Irradiation Effects on Double Ferromagnetic-Gate Silicene Junction 23

(a) (b) Fig.8: Band structure in each regions of the device structure. Fig.7: Tunnelling magnetoresistance when the irradiated light intensity parameter (a) Λ is 0.03, (b) Λ functional approach [30, 31]. Although it is impos- is 0.1. sible to directly compare the theoretical results with the those from experiments due to the technical chal- instance, the spin-valley index of k ↑ is η = +1, lenges facing the realization of free-standing silicene, σ = +1. In the same manner, the spin-valley in- it has been found that the properties of silicene from dex of k′ ↑ is η = −1, σ = +1, k ↓ is η = +1, tight-binding model is comparable to those from more σ = −1 and k′ ↓ is η = −1, σ = −1. Therefore, the accurate density functional theory (DFT) [32]. There spin-valley dependent energy gap for each spin-valley have been several studies discussing the accuracy and current component is eﬀectiveness of tight-binding approximation elemental 2D materials such as graphene and silicene [33, 34]. The results from our study may be used as a ∆k↑ = ∆SO − ∆E + ∆M , guideline for device applications in the future.

∆k′↑ = −∆SO − ∆E + ∆M , 5. SUMMARY AND CONCLUSION ∆k↓ = −∆SO − ∆E − ∆M ,

∆k′↓ = ∆SO − ∆E − ∆M . (11) We have studied the impact of off-resonant circularly polarized photo irradiation on the spin- On the other hand, the energy band gap at the polarized and valley-polarized transport in a double normal region NM2 with light irradiation is deter- ferromagnetic-gate silicene-based junction. We found mined by another spin-valley dependent energy gap that all components of the polarized spin and valley ′ ′ ∆ησ = ησ∆SO + ηλΩ. This energy gap also has currents (k ↑, k ↓, k ↑, k ↓) are affected by the light different impact to each spin-polarized and valley- irradiation. The irradiation of off-resonant circularly polarized current components. In short, the contri- polarized light on silicene modifies the energy band butions from electric-field-induced energy gap ∆E, structure and changes electronic properties, proba- exchange-field-induced energy gap ∆M , and spin- bly due to the photon dressing effect. Photo irra- orbit coupling ∆SO cause changes on the energy band diation also has strong impact on tunneling magne- gap. Furthermore, the spin-dependent chemical po- toresistances. For the silicene junction in our study, tential µσ = µ + σuM and the middle gate potential we found that the range of spin polarization was en- U/e also affect the low-energy Hamiltonian and wave hanced in P junction, and discovered the oscillation functions as described in Section II Model. Hence, of spin polarization in AP junction. The investigation they also affect the Fermi level and energy band struc- results show that the valley polarization is affected in ture of the junction. Since each spin-valley com- different way from spin polarization. Both P junc- ponent might experience the energy band structure tion and AP junction show similar valley polariza- differently, the transmission probability amplitude of tion characteristics with photo irradiation, and nei- each spin-valley component could be different. As the ther exchange field configuration nor light frequency results, conductance, spin polarization, valley polar- has significant impact on the valley polarization. The ization, and tunelling magnetoresistance of each spin- effect of photo irradiation on tunneling magnetore- valley component might be different. sistances is also non-trivial with the tunneling mag- The results from this theoretical study are based netoresistance amplitude increases significantly com- on tight-binding model, which is a widely accepted paring to when there is no light irradiation. We dis- methodology in the research community. One of the covered that the light intensity plays the major role reasons for the popularity is its simplicity and ef- to trigger the photo irradiation effect, with low light ficiency [27-29]. The accuracy of the tight-binding intensity causes similar characteristics to when there approximation has increased over the time, such as is no light irradiation. On the other hand, the light the inclusion of total-energy calculations and density frequency affects the characteristics in different way 24 ECTI TRANSACTIONS ON COMPUTER AND INFORMATION TECHNOLOGY VOL.12, NO.1 May 2018

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Conduction,” IBM Journal of Research and De- Graphene in a Magnetic Field,” Physical Review velopment, Vol. 1, pp. 223-231, 1957. B, Vol. 88, pp. 245445, 2013. [25] U. D. Giovannini, H. Hubener, and A. Ru- bio, “Monitoring Electron-Photon Dressing in Peerasak Chantngarm was born in WSe2,” Nano Letters, Vol. 16, pp. 7993-7998, Bangkok, Thailand. He received B.S. degree in Physics from Kyoto Univer- 2016. sity in Japan. He then received M.S. [26] F. H. M. Faisal and J. Z. Kaminski, “Floquet- degree in Computer Engineering from Bloch Theory of High-Harmonic Generation in North Carolina State University in USA and PhD degree from Gunma University Periodic Structures,” Physical Review A, Vol. 56, in Japan. He worked as an IC design en- pp. 748-762, 1997. gineer at Texas Instruments in Dallas, USA. He also worked as a lecturer at [27] J. C. Slater and G. F. Koster, “Simplified LCAO King Mongkut’s Institute of Technology Method for the Periodic Potential Problem,” Ladkrabang before joining Rajamangala University of Technol- Physical Review, Vol. 94, pp. 1498-1524, 1954. ogy Krungthep in Thailand, where he is currently an Associate Professor in Department of Electronics and Telecommunica- [28] W. A. Harrison, Electronic Structure and the tion Engineering. He also worked shortly as a JSPS researcher Properties of Solids: The Physics of the Chemi- at the University of Tokyo and as a visiting scholar at Osaka University in Japan. His current research interests are in spin- cal Bond, Dover Publications, 1989. tronics, density functional theory, and quantum computation. [29] M. Finnis, Interatomic Forces in Condensed Matter, Oxford University Press, 2010. Kou Yamada was born in Akita, [30] D. A. Papaconstantopoulos and M. J. Mehl, Japan, in 1964. He received B.S. and “The Slater-Koster Tight-Binding Method: A M.S. degrees from Yamagata University, Yamagata, Japan, in 1987 and 1989, re- Computationally Efficient and Accurate Ap- spectively, and the Dr. Eng. degree proach,” Journal of Physics: Condensed Matter, from Osaka University, Osaka, Japan in Vol. 15, pp. R413-R440, 2003. 1997. From 1991 to 2000, he was with the Department of Electrical and Infor- [31] M. Elstner, et al., “Self-Consistent-Charge mation Engineering, Yamagata Univer- Density-Functional Tight-Binding Method for sity, Yamagata, Japan, as a research associate. From 2000 to 2008, he was Simulations of Complex Materials Properties,” an associate professor in the Department of Mechanical Sys- Physical Review B, Vol. 58, pp. 7260-7268, 1998. tem Engineering, Gunma University, Gunma, Japan. Since [32] C. C. Liu, H. Jiang, and Y. Yao, “Low-energy 2008, he has been a professor in the Department of Mechan- ical System Engineering, Gunma University, Gunma, Japan. Effective Hamiltonian Involving Spin-Orbit Cou- His research interests include robust control, repetitive con- pling in Silicene and Two-Dimensional Germa- trol, process control and control theory for inverse systems and nium and Tin,” Physical Review B, Vol. 84, pp. infinite-dimensional systems. Dr. Yamada received the 2005 Yokoyama Award in Science and Technology, the 2005 Electri- 195430, 2011. cal Engineering/Electronics, Computer, Telecommunication, [33] L. A. Agapito, et al., “Accurate Tight-Binding and Information Technology International Conference (ECTI- CON 2005) Best Paper Award, the Japanese Ergonomics So- Hamiltonians for Two-Dimensional and Layered ciety Encouragement Award for Academic Paper in 2007, the Materials,” Physical Review B, Vol. 93, pp. 2008 Electrical Engineering/Electronics, Computer, Telecom- 125137, 2016. munication, and Information Technology International Confer- ence (ECTI-CON 2008) Best Paper Award and Fourth Interna- [34] R. Cote and M. Barrette, “Validity of the Two- tional Conference on Innovative Computing, Information and Component Model of Bilayer and Trilayer Control Best Paper Award in 2009.