Silicene Superlattice Research Articles

Spin-valley polarization and tunneling magnetoresistance in monomer, dimer, and trimer magnetic silicene superlattices

Monomer, dimer and trimer semiconductor superlattices are an alternative for bandgap engineering due to the possibility of duplicate, triplicate, and in general multiply the number of minibands and minigaps in a specific energy region. Here, we show that monomer, dimer, and trimer magnetic silicene superlattices (MSSLs) can be the basis for tunable magnetoresistive devices due to the multiplication of the peaks of the tunneling magnetoresistance (TMR). In addition, these structures can serve as spin-valleytronic devices due to the formation of two well-defined spin-valley polarization states by appropriately adjusting the superlattice structural parameters. We obtain these conclusions by studying the spin-valley polarization and TMR of monomer, dimer, and trimer MSSLs. The magnetic unit cell is structured with one seed A with positive magnetization, and one, two, or three seeds B with variable magnetization. The number of B seeds defines the monomer, dimer, and trimer superlattice, while its magnetic orientation positive or negative the parallel (PM) or antiparallel magnetization (AM) superlattice configuration. The transfer matrix method and the Landauer–Büttiker formalism are employed to obtain the transmission and transport properties, respectively. We find multiplication of TMR peaks in staircase fashion according to the number of B seeds in the superlattice unit cell. This multiplication is related to the multiplication of the minibands which reflects as multiplication of the descending envelopes of the conductance. We also find well-defined polarization states for both PM and AM by adjusting asymmetrically the width and height of the barrier-well in seeds A and B.

Optimization of the tunneling magnetoresistance and spin-valley polarization in complex magnetic silicene structures

Aperiodic order is ubiquitous in nature and quite relevant in science and technology. There are extensive works in aperiodic structures studying fundamental characteristics in physical properties, such as fractality, self-similarity, and fragmentation. However, there are fewer reports in which aperiodicity signifies an improvement in physical quantities with practical applications. Here, we show that the aperiodicity of fractal or self-similar type optimizes the tunneling magnetoresistance and spin-valley polarization of magnetic silicene structures, raising the prospects of spin-valleytronics. We reach this conclusion by studying the spin-valley-dependent transport properties of complex (Cantor-like) magnetic silicene structures within the lines of the transfer matrix method and the Landauer–Büttiker formalism. We find that the self-similar arrangement of magnetic barriers in conjunction with structural asymmetry reduces the conductance oscillations typical of periodic magnetic silicene superlattices and more importantly makes the K′-spin-down conductance component dominant, resulting in nearly perfect positive and negative spin-valley polarization states accessible by simply reversing the magnetization direction. The tunneling magnetoresistance is not as prominent as in periodic magnetic silicene superlattices; however, it is better than in single magnetic junctions. Furthermore, the optimization of the spin-valley-dependent transport properties caused by the complex structure is superior than the corresponding one reported in typical aperiodic structures, such as Fibonacci and Thue–Morse magnetic silicene superlattices.

Electrical-gate-controlled giant tunneling magnetoresistance and its quasi-periodic oscillation in an interlaced magnetic-electric silicene superlattice.

In this work, we propose a silicene-based lateral resonant tunneling device by placing silicene under the superlattices interlaced, arranged by ferromagnetic gates and electric gates. Its ballistic transport properties are calculated by the transfer matrix method. Combined with the unique electrically tuned energy gap of silicene, its magnetoresistance (MR) can be exaggeratedly modulated over a wide range by applying electrostatic potential and the on-site potential difference. It is interestingly found that there is a quasi-periodic oscillation of the MR in silicene-based superlattice devices from the quantum resonant confinement of the band splitting by the electrostatic field. Moreover, the peak of the MR in a single-period structure can reach more than 104, while the peak of the MR in an interlaced alternating magnetic-electric silicene superlattice can reach more than 1017, which is one of the best-reported values. This may originate from the enhancement effect of the wave vector filtering by the controlled field. Our studies indicate that the silicene superlattices alternately arranged by the ferromagnetic gate and electric gate not only have giant MR (GMR) properties, but also exhibit the periodic oscillation characteristics of MR in which electric gates can be modulated. Therefore, this work provides a more flexible strategy for the construction of silicene-based nanoelectronic devices.

Tunneling magnetoresistance and spin-valley polarization of aperiodic magnetic silicene superlattices

Magnetic silicene superlattices (MSSLs) are versatile structures with spin-valley polarization and tunneling magnetoresistance (TMR) capabilities. However, the oscillating transport properties related to the superlattice periodicity impede stable spin-valley polarization states reachable by reversing the magnetization direction. Here, we show that aperiodicity can be used to improve the spin-valley polarization and TMR by reducing the characteristic conductance oscillations of periodic MSSLs (P-MSSLs). Using the Landauer–Büttiker formalism and the transfer matrix method, we investigate the spin-valley polarization and the TMR of Fibonacci (F-) and Thue–Morse (TM-) MSSLs as typical aperiodic superlattices. Our findings indicate that aperiodic superlattices with higher disorder provide better spin-valley polarization and TMR values. In particular, TM-MSSLs reduce considerably the conductance oscillations giving rise to two well-defined spin-valley polarization states and a better TMR than F- and P-MSSLs. F-MSSLs also improve the spin-valley polarization and TMR, however they depend strongly on the parity of the superlattice generation.

Spin-valley polarized transport in a field-controllable bilayer silicene superlattice

We have theoretically investigated the spin-valley asymmetric transport of massive Dirac fermions in the field-controllable bilayer silicene superlattices. The spin-valley dependent ballistic transmission, conductance, and polarization have been systematically calculated by formulating the scattering matrix method for the completed four-band low-energy effective Hamiltonian. Our results uncover that for a single valley transport, near-perfect spin polarization and its perfect switching could be efficiently modulated by the gate field engineering. Under the one-dimensional periodic field modulation, two types of flat bands with less dispersion and, importantly, the perfect contrast in the spin-dependent subbands are observed for the bilayer silicene superlattice. Together with its larger spin-orbit coupling and better stability, these spin-valley asymmetric characteristics engineered by the gate field indicate that the field-controllable bilayer silicene could be a potential component candidate to achieve a fully spin-valley polarized beam for quantum logic applications.

Nonlocal transport in Fibonacci superconducting silicene superlattices

The nonlocal transport properties of the silicene-based Fibonacci and periodic superconducting superlattices with two ferromagnets are investigated and compared. It is found that the Fibonacci superconducting superlattice (FSSL) exhibits larger crossed Andreev reflection (CAR) probability than the periodic superconducting superlattice (PSSL) for normal incidence. The CAR differential conductance shows anomalous behavior with the generation number of Fibonacci structure due to disorder effect. Within the subgap regime, the oscillation of the elastic cotunneling differential conductance for the FSSL is more irregular than that for the PSSL. Moreover, the super-Poissonian noise of the FSSL is suppressed compared with that of the PSSL.

Improvement of tunneling magnetoresistance and spin-valley polarization in magnetic silicene superlattices induced by structural disorder

It is well known that the unavoidable variation of the heights and widths of the barriers (structural disorder) in semiconductor superlattices and superlattices based on two-dimensional materials degrades important physical properties such as the electronic transport and the thermoelectric response. Here, we show that the structural disorder contrary to what is expected improves the magnetoresistive and spin-valleytronic properties of magnetic silicene superlattices. We reach this conclusion by studying the impact of the random variations of the width and strength of the magnetic barriers on the tunneling magnetoresistance and spin-valley polarization. The theoretical treatment is based on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer-B\"uttiker formalism to describe silicene electrons, to obtain the transmittance, and to obtain the conductance, respectively. Our results indicate that structural disorder effects improve the magnetoresistance response and the spin-valley polarization by eliminating the conductance oscillations caused by the periodic magnetic modulation as well as by differentiating the response of the conductance for the parallel and antiparallel magnetization configuration. These results could be useful in designing versatile devices with magnetoresistive and spin-valleytronic capabilities. Particularly, magnetic silicene superlattices with moderate structural disorder are more convenient than perfect or nearly perfect ones.

Tunneling magnetoresistance and spin-valley polarization in magnetic silicene superlattices

We study the tunneling magnetoresistance and the spin-valley transport in silicene-based magnetic superlattices. The superlattice profile is obtained by the periodic modulation of the exchange field induced by ferromagnetic electrodes. The silicene band gap in the superlattice structure is also modulated through an external perpendicular electric field. The concept of parallel and antiparallel magnetization configurations of single magnetic junctions is extended to the periodic case by switching the magnetization orientation of the adjacent magnetic barriers in parallel and antiparallel fashion. The transfer matrix method and the Landauer-B\"uttiker formalism are used to obtain the transmission and transport properties, respectively. We find an oscillating conductance once the periodic modulation is incorporated. By tuning the external perpendicular electric field a conductance gap is obtained for the antiparallel configuration, which results in an enhancement of the tunneling magnetoresistance with respect to single magnetic junctions. In the case of the spin-valley polarization it is not possible to obtain two well-defined polarization states by simply switching the magnetization orientation, as in single magnetic junctions, due to the equivalence of the spin-valley conductance components. However, by inducing structural asymmetry in the width of barriers-wells, two well-defined polarization states can be reached. Moreover, an additional enhancement of the tunneling magnetoresistance is induced by the structural asymmetry. Our findings indicate that magnetic periodic modulation can be an option to improve the tunneling magnetoresistance and the spin-valley polarization of silicene-based structures.

Spin-valley thermoelectric characteristics of ferromagnetic silicene superlattice

Based on the transfer matrix method valley-spin caloritronic transport properties of ferromagnetic silicene superlattice are investigated theoretically under ac-field effect of various frequencies. The purpose of this work is to describe resolved thermoelectric parameters such as (charge, spin and valley) Seebeck and Peltier coefficients and electronic thermal conductance, by photon-assisted tunneling probability bases. An oscillatory behavior is resulted for all investigated thermoelectric parameters in this investigation. These oscillations might be due to photon assisted tunneling. The improved values of charge, spin, valley power factors might be due to quantum confinement effect, which leads to enhancements of charge, spin, valley figure of merits as indicated from the obtained results. Also, results show that ferromagnetic silicene superlattice is good to use as energy harvesting thermoelectric nanodevice (Seebeck coefficient), and may be used as nanoelectronic devices cooler (Peltier coefficient). So, this present research shows that ferromagnetic silicene superlattice is good promising for flexible thermoelectric power generation.

Ferromagnetic Silicene Superlattice Based Thermoelectric Flexible Renewable Energy Generator Device

The object of the current research manuscript is to analyze the valley-spin thermoelectric properties and Nernst coefficient at two different temperatures for ferromagnetic silicene superlattice. Photon-assisted tunneling probability is used to identify the resolved thermoelectric parameters including (valley, spin, and charge) electronic thermal conductance, Seebeck coefficient, figure of merit, and also electrical conductance and Nernst coefficient. The results show oscillatory behavior to all investigated parameters. The improved data of Seebeck coefficient (valley, spin, and charge) could be because of quantum confinement effect of the present investigated nanodevice. The figure of merit (valley, spin, and charge) attains quite high values with good high thermoelectric efficiency. The enhancement of Nernst coefficient (valley, spin, and charge) might consider Nernst effect is suitable for thermoelectric heat energy conversion system of the present flexible ferromagnetic silicene superlattice. The ferromagnetic silicene superlattice nanodevices are good candidates for flexible renewable energy generation as demonstrated by this analysis.

Enhancement of the Seebeck coefficient and power factor in gated silicene superlattices induced by aperiodicity

This paper theoretically investigates the impact of aperiodic sequences in the ballistic transport and thermoelectric effect in silicene gated superlattices. In our analysis, we have implemented the well-known Fibonacci, Thue–Morse, and triadic Cantor type sequences. The transfer matrix technique and the Landauer–Bütikker formalism are used to calculate the transmission probability and the conductance, respectively. The Cutler–Mott formula is employed to estimate the Seebeck coefficient, and the thermoelectric power factor is then obtained. We found that the transmission minibands of aperiodic superlattices exhibit a much more fragmented structure in comparison to that reported in the periodic case. Consequently, the conductance curve presents a more pronounced oscillating shape, which improves the thermoelectric properties. In particular, the Seebeck coefficient has reached values up to 78.2 mV/K for Fibonacci, 233.0 mV/K for Thue–Morse, and 436.3 mV/K for Cantor. In addition, the power factor has been substantially increased, reaching peaks of approximately 8.2, 50.2, and 2.1 nW/K2 for the mentioned sequences, respectively. The best results were obtained for spindown (spinup) charge carriers in the K (K′) valley. Besides, an additional improvement is obtained by considering superior generations of the aperiodic sequences. Finally, our findings are supported through the redistribution of the density of the states, which is induced by the aperiodicity of the nanostructure as well as by the low-dimensionality of the thermoelectric device.

Spin- and valley- dependent energy band and polarization in ferromagnetic silicene superlattice with circularly polarized light

We investigated the spin- and valley-dependent energy band and transport properties in ferromagnetic silicene superlattice with an off-resonant circularly polarized light. It is obviously found that the spin- and valley-dependent energy gap can be well tuned via the off-resonant circularly polarized light. Fully spin and valley-polarized can be realized in certain ranges of light intensity. What is particularly interesting is that by slightly adjusting the energy of the light field, the valley polarization can be suddenly reversed from 100% to -100%. Thus, our results are expected to be informative for switching devices related to valleytronics.

Valley and spin dependent group delay time in silicene superlattice

In this paper, the valley- and spin-dependent transport properties, the tunneling time and the Hartman effect in a silicene superlattice under extrinsic Rashba spin-orbit coupling is theoretically studied. It is found that the transmission probability and the group delay time depend on the degree of freedom of the valley and spin, and that they have an oscillatory behavior with respect to the extrinsic Rashba spin-orbit interaction (ERSOI). Furthermore, the difference of the group delay time between two inequivalent valleys, and also between spin-up and spin-down can be efficiently tuned via the ERSOI strength and the number of superlattice barriers. Thus, valley-spin filtering can be achieved in the time domain in silicene superlattice with ERSOI. Especially, in contrast to the graphene superlattice, the Hartman effect or the superluminal tunneling can be observed in silicene superlattice even for the normal angle of incidence.

Effects of disorder on the transport and thermoelectric properties of silicene superlattices

Abstract Unavoidable structural disorder associated with the variation of the widths of barriers and wells as well as the heights of barriers in superlattices of 2D materials can affect considerably electronic transport and related phenomena. In this work, we study the impact of structural disorder on the electronic transport and thermoelectric properties of gated silicene superlattices (GSSLs). A low-energy effective Hamiltonian has been used to describe electrons in silicene. The transfer matrix approach, the Landauer–Buttiker formalism and the Cutler–Mott formula have been implemented to obtain the transport and thermoelectric properties. In particular, the conductance, Seebeck coefficient and power factor are investigated in ordered and disordered GSSLs. The results show that disorder related to the width of barriers-wells is not equivalent to the one associated to the height of barriers. The former barely changes the transport and thermoelectric coefficients at low energy, while the latter greatly affects them in the whole energy range. We also assess the disorder associated to the on-site potential of silicene layers, finding that its influence is limited. So, our results indicate that in the design and fabrication of GSSLs a precise control of the height of the barriers is required in order to have and preserve good thermoelectric properties.

Spin and valley dependent transport in strained silicene superlattice

Abstract We have theoretically investigated the spin- and valley-dependent transport properties in a silicene superlattice under strain, with extrinsic Rashba spin–orbit coupling. It is demonstrated that due to the extrinsic Rashba spin–orbit coupling, spin-inversion can be achieved. It is shown that the valley and spin polarizations increase with increasing the number of superlattice barriers, and the magnitude and sign of valley and spin polarizations can be tuned by strain and extrinsic Rashba spin–orbit coupling. It is also found that the spin polarization through this superlattice reaches a maximum value at 2% strain. In particular, the fully valley polarized current can be achieved by adjusting the strain strength.

Tunable spin transport and quantum phase transitions in silicene materials and superlattices

In this paper, we study spin transport properties of silicene structures such as ribbons and superlattices with the Kane–Mele model. We investigate the effects of ferromagnetic and antiferromagnetic exchange fields, vertical and transverse electric fields and defects on the band structure, density of states, as well as conductance of the system. Our calculated results indicate that by applying a vertical electric field and/or an antiferromagnetic exchange field, metal–semimetal and also metal–semiconductor quantum phase transitions occur. We show that a zigzag silicene ribbon, when exposed to a transverse electric field in combination with an antiferromagnetic exchange field, behaves as a half-metal that allows one spin state electrons to move. Furthermore, under a vertical electric field and in the simultaneous presence of ferromagnetic and antiferromagnetic exchange fields applied to two half-widths of the ribbon, we observe the half-metallic phase. Our results help to control spin currents and to have new applications in silicene spintronics.

Negative-Differential-Resistance Devices Achieved by Band-Structure Engineering in Silicene under Periodic Potentials

In terms of band-gap engineering for devices, despite silicene's great promise, it has received considerably less attention than its carbon cousin graphene. Taking advantage of its tunable band gap, the authors use an effective Hamiltonian formalism to examine the formation of silicene superlattices. They theoretically consider engineering of the local band structure of silicene through application of an out-of-plane electric field, together with control over the spin and valley degrees of freedom, and finally show how to optimize negative differential resistance. This design principle could be extended to stacks of other two-dimensional materials as well.

Transport properties and thermoelectric effects in gated silicene superlattices

Low-dimensional thermoelectricity opens the possibility of improving the performance and the efficiency of thermoelectric devices by redistributing the electron density of states through the reduction of dimensionality. In this work, we explore this possibility in silicene by reducing its dimensionality through the periodic arrangement of gated electrodes, the so-called gated silicene superlattices. Silicene electrons were described quantum relativistically. The transmission, conductance, and thermoelectric properties were obtained with the transfer matrix method, the Landauer-Büttiker formalism, and the Cutler-Mott formula, respectively. We find that the redistribution of the density of states together with the intrinsic characteristics of silicene, the local bandgap and the large spin-orbit coupling, contribute to the enhancement of the thermoelectric properties. In particular, the Seebeck coefficient and the power factor reach values of a few mV/K and nW/K2. These findings in conjunction with the low thermal conductivity of silicene indicate that silicene-based nanostructures could be the basis of more efficient thermoelectric devices.

Controllable spin-dependent transport in silicene superlattice

Based on the transfer-matrix method, we theoretically investigate the spin-dependent transport properties in magnetic silicene superlattice in the presence of extrinsic Rashba spin–orbit interaction (RSOI). It is found that the spin transmission probability and spin conductivities can be efficiently controlled by the number of magnetic barriers. As the number of magnetic barriers increases, spin conductivities strongly decrease, and reduce to zero in the large on-site potential difference between A and B sublattices (Δz) region. The results indicate that a magnetic silicene superlattice exhibits a remarkable wavevector-dependent spin filtering effect. Also, the magnetoresistance (MR) ratio exhibits an oscillatory behavior with the Fermi energy. The MR ratio can be tuned by the Fermi energy, number of magnetic barriers and extrinsic RSOI. Specifically, in the presence of magnetic field the spin polarization can be observed, and increases by increasing the magnetic field.

Multi-peak negative differential resistance in silicene superlattice: Toward multi-valued silicene logic devices

Negative differential resistance (NDR) in two dimensional materials has been the subject of strong interest for ultra-low power nanoelectronic applications. Here, we report NDR characteristics of silicene superlattice (SL) at low bias voltages. Transport process manipulated by miniband regime for low bias region and Wannier-Stark (WS) ladders regime with multi-peak NDR for higher bias windows. Local tunneling peaks rise from hybridization of Wannier-Stark rungs in certain bias voltages. The bias position of WS states crossings down shifted with increasing device dimension leading to red shifted NDR window and increases peak to valley ratio (PVR) values. The multiple NDR windows are represented by changing the size and the number of well/barriers in silicene SL. Maximum PVR of 8 obtained for device with low height barriers. Multi-peak NDR with same PVR values in very low bias regime can find key applications in multi-valued memories with low static power dissipation.