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 trans- port 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 direc- tion 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 signif- icantly 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 hjΩj ≫ 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].