Abstract
Dirac semimetals host threedimensional (3D) Dirac fermion states in the bulk of crystalline solids, which can be viewed as 3D analogs of graphene. Owing to their relativistic spectrum and unique topological character, these materials hold great promise for fundamentalphysics exploration and practical applications. Particularly, they are expected to be ideal parent compounds for engineering various other topological states of matter. In this report, we investigate the possibility to induce and control the topological quantum spin Hall phase in a Dirac semimetal thin film by using a vertical electric field. We show that through the interplay between the quantum confinement effect and the fieldinduced coupling between subbands, the subband gap can be tuned and inverted. During this process, the system undergoes a topological phase transition between a trivial band insulator and a quantum spin Hall insulator. Consequently, one can switch the topological edge channels on and off by purely electrical means, making the system a promising platform for constructing topological field effect transistors.
Introduction
The study of topological insulators (TIs) have been one of the most active research areas in the past ten years^{1,2}, which revolutionized our understanding of the electronic band structure. It is now understood that there could be nontrivial topologies encoded in the electronic wavefunctions, characterized by various topological invariants according to the symmetry class of the system and physically manifested by the appearance of topological boundary states. For example, twodimensional (2D) TIs, also known as the quantum spin Hall (QSH) insulators, are characterized by a topological invariant and have spin helical edge states on sample boundaries^{3}, for which backscattering is suppressed in the presence of time reversal symmetry^{1,2,4}. Hence they hold great promise for applications such as lowdissipation electronics, spintronics and quantum computations. For these TIs, the nontrivial topology as well as the boundary states are protected by the finite energy gap, i.e., they are robust against perturbations as long as the insulating gap does not close.
It was later realized that the topological classification could be pushed beyond insulators to states without a gap^{5,6,7}. In particular, a novel state called Dirac semimetal (DSM) has been proposed and successfully demonstrated in recent experiments for two crystalline materials Na_{3}Bi and Cd_{3}As_{2}^{8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26}. In these materials, the Fermi energy sits at two threedimensional (3D) Dirac points—where the bands touch with a fourfold degeneracy—and the dispersion is linear along all three directions in reciprocal space. Each Dirac point can be viewed as consisting of two Weyl points of opposite chiralities and is protected by the crystalline symmetry^{8,9,10,27,28}. Such unusual electronic structure endows the system with many intriguing properties like the surface Fermi arcs and the quantum magnetoresistance^{18,21,29,30}. Perhaps more importantly, DSMs are expected to be an ideal parent compound for realizing other novel topological states such as Weyl semimetals, TIs and topological superconductors^{11}. Particularly in this regard, DSMs offer a simple alternative to achieve the 2D TI phase through the quantum confinement effect^{10,31}. It has been shown that with increasing thickness, DSM thin films exhibit oscillations in the 2D invariant whenever a quantum well state crosses the Dirac point^{10,31}. Hence a QSH phase can be realized by a proper control of the film thickness. Since the QSH phase has so far been detected in only a few systems, given its fundamental and technological importance, the new approach to realize it using DSMs would be of great interest. Furthermore, the unique properties of DSMs may offer new methods to manipulate the properties of the QSH phase.
Motivated by these recent breakthroughs and by the great interest in utilizing DSMs for topological devices, in this work, we investigate the possibility of electric control of the topological phase transitions in a DSM thin film. We show that by using a vertical electric field, DSM thin films can be switched between a topological QSH phase and a trivial insulator phase. This topological phase transition is enabled by a combined effect of quantum confinement and field induced subband coupling. As a result, one can electrically manipulate the topological edge channels and the charge and spin conduction through a finite sample can be readily switched on and off. This leads to a simple design of a DSMbased topological field effect transistor with advantages of fastspeed, low power consumption and low dissipation, owing to the robust topological edge channels combined with full electric control.
Results
Model and Analytic Analysis
Our analysis is based on a generic lowenergy effective model describing the DSMs A_{3}Bi (A = Na, K, Rb) and Cd_{3}As_{2} as derived in previous works^{9,10}. In these materials, the states around Fermi energy can be expanded using a minimal fourorbital basis of , , and . Around Γpoint in the Brillouin zone, the effective Hamiltonian expanded up to quadratic order in the wavevector k is given by
where with to reproduce the band inversion feature at Γpoint. The materialspecific parameters A, C_{i} and M_{i} are determined by fitting the firstprinciples result or the experimental measurement. It has been shown that this model nicely captures the essential lowenergy physics as compared with experiment^{11,13,16}.
For bulk DSMs, model (1) gives the energy dispersion , where is the 2D wavevector in the k_{x}–k_{y} plane. The spectrum has two Dirac points located along the k_{z}axis at (0, 0, ±k_{D}) with . Each Dirac point is fourfold degenerate and can be viewed as consisting of two Weyl nodes with opposite chiralities (as represented by the two 2 × 2 diagonal subblocks in Hamiltonian (1)). The dispersion around each Dirac point is linear in all three directions, as can be seen by expanding at (τ = ± labels the two Dirac points): . One notes that the lowenergy spectrum is anisotropic as manifested in both the distribution of Dirac points as well as the different Fermi velocities along k_{z} versus that in the k_{x}–k_{y} plane (Fermi velocity along k_{z} is typically much slower), which leads to quite different behaviors when a DSM is confined along different directions^{31}.
DSMs such as Na_{3}Bi and Cd_{3}As_{2} have layered structures along crystal caxis. Hence their thin film structures with confinement along zdirection can be more readily fabricated. Consider a DSM thin film with thickness L confined in the region . For small L, the electron motion along z will be quantized into discrete quantum well levels due to quantum confinement effect. This generally turns the system from a semimetal to a semiconductor. Using quantum well approximation, each quantum well level has a quantized effective wavevector k_{z} such that and with counting the subbands and the angular bracket meaning the average over quantum well states.
One observes that for each subband n, Hamiltonian (1) has a similar form as the lowenergy model describing the 2D QSH systems in HgTe/CdTe quantum wells^{32}, with the term
where is a subband dependent mass which determines the gap of the subband at Γpoint of the 2D Brillouin zone. It’s known that band inversion occurs (around when ^{32,33}, i.e. when and M_{2} have opposite signs, which signals a nontrivial character of the subband n. Given that , this happens when is satisfied. Therefore, for a thinenough film such that , all the subbands are topologically trivial with positive mass terms . With increasing film thickness, the system becomes nontrivial once the first (n = 1) subband has its mass inverted when . The inverted subband contributes a and in the inverted band gap, there appears a pair of spinhelical edge states protected by time reversal symmetry on each edge of the quasi2D system. Further increasing L would invert of the second subband, leading to two pairs of edge states. However, for group: 1 + 1 = 0, hence this state is topologically trivial. Physically, it is because backscattering can occur between edge states from different time reversal pairs. Following this logic, the topological properties as well as the bulk band gap show oscillatory behavior as a function of the film thickness^{10,31}.
Since the subband dependent mass plays the key role in determining the topological properties of the system, we shall focus on the change of by a vertical electric field, aiming to achieve an electric control of the topological phase of DSM thin films. To proceed, one notes that the lower diagonal block of Hamiltonian (1) is formally the time reversal counterpart of the upper block, which share the same energy spectrum and the E field does not mix the two. (This can also be argued by observing that the lowenergy Hamiltonian possesses the unitary symmetry and the antiunitary symmetry , which may be regarded as emergent symmetries for the lowenergy physics. Here σ’s are the Pauli matrices, I is the 2 × 2 identity matrix and K is the complex conjugation operator.) Hence to study the change of , it is enough to consider only the upper block denoted by h(k). Modeling with the hardwall boundary condition for the confinement potential, we have for subband n,
where σ’s are the Pauli matrices, I is the 2 × 2 identity matrix and . The energy eigenstates are given by
with eigenenergies
where S is the area of the thin film, α = ±, are the two eigenspinors along the quantization direction , and
is the quantum well state for the nth subband. The vertical electric field is modeled by adding a diagonal potential energy term where (−e) is the electron charge and E is the effective field strength which may be considered as including the static screening effects.
For a qualitative analysis, we assume small field and treat V perturbatively. Because V(z) is odd in z, it is easy to see that the first order perturbation in energy vanishes. The leading order perturbation comes at the second order, with
where the summation is over all other states different from and in reality, it has a physical cutoff for which the lowenergy description is no longer valid. One notes that in order to analyze the renormalized , it is sufficient to focus on the change at Γpoint of the 2D Brillouin zone by setting .
We are most interested in the case in which the mass (gap) of the first subband can be inverted by the electric field, because then the two sides of the topological phase transition have the most salient contrast: absence or presence of the topological edge channels, hence leading to the best onoff ratio when considering a topological transistor based on it. For such case, we consider a thickness L such that , i.e. an initially trivial system with in the absence of E field. At , we have and for all n, where and are the two eigenstates of σ_{z}. For the n = 1 subband, we have at ,
where
In the first equality of (8) we used the fact that the state does not mix with the states from the valence bands by the E field because their pesudospin parts χ are orthogonal. Also note that one has in order for the model (1) to describe a semimetal phase, hence for m > 1, hence the perturbation due to the coupling between and (with m > 1) generally pushes down the energy level of , making . In the expression (9) of the constant factor η (with a rapidly converging value , the summation only includes the even integer numbers, because V(z) only couples states with opposite parities in z.
Similarly, the energy shift for state can be calculated,
which is positive, showing that the coupling induced by the E field pushes up the energy of . Therefore, in the Hilbert subspace of the first subband, the E field renormalizes the value of the mass:
with the correction
Using this estimation, one observes that the gap of the first subband would decrease with increasing E field and closes at
which signals a topological phase transition point and beyond which the gap reopens with the system turned into a QSH insulator phase characterized by .
For large E field with eEL being comparable or even larger than , the perturbative calculation is no longer expected to be accurate. Nevertheless, the general physical pictures from the above discussion still applies: the level repulsion due to higher subbands would generally decrease and invert the gap of the first subband, generating a topological phase transition. We shall explicitly demonstrate this in the next section through numerical calculations.
Before proceeding, we mention that our above analysis based on the lowenergy effective model around Γpoint is valid, because for these materials: the band inversion only occurs around Γpoint; while the bands near the Brillouin zone boundary (away from Γpoint) have normal band ordering and are higher in energy, to invert them would require very large applied field if not impossible. Hence for the consideration of topological properties, we can focus on the band evoluation around Γpoint. In addition, in our treatment of confinement we are focusing on the thickness range where the relevant subband gap is small and close to band inversion, which means the corresponding effective wavevector is close to k_{D}, hence its lowenergy behavior and especially the topological property are wellcaptured by this effective model in Eq.(1). Similar treatment based on the lowenergy effective models has been successfully applied in the study of quantum confined structures, e.g. in semiconductor quantum wells^{34}, 3D topological insulator thin films^{35} and also in previous studies of Dirac semimetal thin films^{10,31}. The above analysis can also be applied to higher subbands when a thicker film with the nth (n > 1) subband most close to transition is considered. We will discuss this later in the discussion section.
Numerical Results
For numerical investigation, we discretize the model (1) on a 3D lattice with lattice constants nm (1.264 nm) and with a_{z} = 0.4828 nm (2.543 nm) being set to the interlayer separation pertinent to Na_{3}Bi (Cd_{3}As_{2}). The standard substitutions
are adopted (i = x, y, z) for lattice discretization around Γpoint. Since we require the initial state at E = 0 is of a trivial insulator phase, we need the number of layers , where is the floor function. And in order for the band gap to be inverted by a relatively small E field, one may wish to have the initial gap size not too large.
Let’s consider Na_{3}Bi first. The model parameters we use are listed in the caption of Fig. 1, which have been extracted from the firstprinciples calculations and compared well with experiment. For Na_{3}Bi thin films, the critical thickness for which the first subband undergoes band inversion is around ^{31}. Hence we take a film with layers nm) for demonstration. Figure 1(a) shows the variation of the band gap E_{g} as a function of the E field. The result is symmetric between positive and negative values of E, so only the positive part is shown here. Initially, at E = 0, the system has a confinement gap about 71 meV (marked by point A). With increasing E field, the gap decreases and closes at a critical value mV/nm (marked by point B) and then reopens and increases with E. This is consistent with our previous analytic analysis. The value of E_{c} is also not far from our estimation in Eq.(13) which is about 208 mV/nm. We also plot in Fig. 1(b–d) the energy spectrum of the system corresponding to the three representative states marked by A, B and C in Fig. 1(a). It shows that on both sides of the gap closing point, the system is insulating with the gap belong to the first subband. At the critical value E_{c}, the band gap closes at Γpoint, marking the topological phase boundary which separates the topologically trivial and nontrivial phases.
To further demonstrate the topological nature of the transition and to visualize the edge states, we compute the surface local density of states (LDOS) for the side surface. Due to the isotropy in the k_{x}–k_{y} plane of the lowenergy model (1), without loss of generality, we choose the surface perpendicular to ydirection of the quasi2D system. The surface LDOS ρ(k_{x}) can be calculated for each k_{x} from the surface Green’s function , where G_{00} is the retarded Green’s function for the surface layer (labled by index 0) of the lattice^{36}. G_{00} can be evaluated by the transfer matrix through a standard numerical iterative method^{37}. The obtained surface LDOS for states before and after the phase transition (for state A and C) are plotted in Fig. 2. One observes that for both cases, the confinementinduced bulk gap can be clearly identified. Before the topological phase transition (E < E_{c}), there is no states inside the gap. In contrast, after transition (E > E_{c}), there appear two bright lines crossing the gap, corresponding to the spin helical edge states for the nontrivial QSH phase. As long as time reversal symmetry is preserved, these gapless modes are protected and carriers in these channels cannot be backscattered^{1,2}. Therefore transport through these channels is in principle dissipationless. In a twoterminal measurement, this would lead to a quantized conductance, which has been confirmed experimentally^{3}.
Similar analysis applies to Cd_{3}As_{2} as well. In Fig. 3, we plot the variation of its confinementinduced gap versus the film thickness, which clearly shows the oscillation behavior of the gap^{31}. One observes that the critical thickness is at about 37 layers. Here we choose a film thickness of layers (L = 50.86 nm) for demonstration. The variations of the gap with respect to the E field as well as representative energy spectra are shown in Fig. 4. Again the gap closing and reopening process similar to Fig. 1(a) is observed. The critical value of mV/nm also agrees well with the estimation mV/nm from Eq.(13). The energy spectra also coincide with our previous analysis. Figure 5 shows the side surface LDOS plots for states A and C (marked in Fig. 4(a), clearly showing the appearance of topological edge states across the transition. These results show qualitatively the same features as those for Na_{3}Bi.
Our numerical results discussed above thus confirm our analytical analysis. A vertical electric field can be used to control the topological phase transitions and the topological edge channels in a DSM thin film.
Discussion
This work theoretically demonstrates the possibility to electrically control the topological phase transitions in a DSM thin film. Since the bulk topology is tied to the existence of topological edge channels and hence to the charge/spin conductance, this indicates that one can achieve a full electric control of the on/off charge/spin conductance of such a system, making it a suitable candidate for a topological field effect transistor. The electric field can be generated by the standard top and bottom gates setup. Compared with the traditional MOSFET which works by injection and depletion of charge carriers in the channel region and has a response timescale depending on factors such as the charge concentration and the carrier mobility, the operating mechanism for a topological transistor is expected to have a high on/off speed with electronic response timescale and a better power efficiency^{38}. In addition, multiple conducting channels in a transistor can be achieved by designing a multilayer structure with alternating DSM layers and insulating layers, similar to the structure as in Ref. 38.
For device design, we have seen that a proper film thickness can be chosen such that the starting confinement gap is small hence can be easily inverted by a small applied field. However, there is a tradeoff because if the gap is too small, then the thermally populated carriers in the bulk could strongly contribute to the transport. Therefore, a balance needs to be achieved for the device to have an optimal performance with relatively low power consumption.
In our analysis, we have focused on the phase transition in the first subband. Similar analysis can also be extended to higher subbands if a thicker film with its nth (n > 1) subband close to phase transition is considered. For example, consider a film thickness such that its second subband is just before the gapclosing. In this case, we would have and and the system is in a phase. Perturbation to second order in the field strength gives the energy correction of
for the state of the second subband, with . A similar expression can be obtained for as well. The first term in the parenthesis of (15) is from the coupling with the m > 2 subbands, while the second term is from the coupling with the first subband. The sign of this energy shift (and hence the correction of would depend on the competition between the two terms and is not necessarily negative. Nevertheless, for such higher subband case, even if a fieldinduced topological phase transition can be realized, the topologically trivial phase would in fact still possess edge states. Although these (even number of pairs of) channels are not topologically robust, their presence would make the trivial state not completely ‘off’ hence is detrimental to the performance of a transistor.
Finally, in real systems, there could be other perturbation terms, e.g. Rashba spinorbit couplings from possible structrual inversion asymmetry due to field or substrate effects. Such terms could generate a trivial gap competing with the QSH gap. Since a topological phase is protected by the bulk gap, the QSH phase would survive and is robust as long as the corresponding QSH gap dominates over the trivial gap due to other mechanisms^{39}.
In summary, we have demonstrated that full electric control of topological phase transitions in a DSM thin films can be achieved through the interplay between the quantum confinement effect and the coupling between subbands induced by a vertical electric field. As a result, a topological field effect transistor can be constructed in which carriers are conducted through the topological edge channels. Given that several DSM materials have been experimentally demonstrated and that the progress in material fabrication technology such as molecular beam epitaxy has allowed film growth with atomic precision, it is quite promising for the physical effect and the DSMbased topological transistor proposed here to be realized in the near future.
Methods
Lattice model
To investigate the effect of a vertical electric field, we discretize a generic lowenergy effective model in Eq.(1) on a 3D lattice with lattice constants a_{x}, a_{y} and a_{z} along the three orthogonal directions. The lowenergy effective Hamiltonian for a 3D DSM around the Γpoint in the Brillouin zone is given by
Here,
and
where . Using this lattice Hamiltonian, the low energy spectrum and the wavefunctions for the DSM slab under a perpendicular electric field can be constructed.
Surface LDOS
The surface LDOS can be derived from , where G_{00} is the retarded Green’s function for the surface layer of a semiinfinite 3D lattice model. The surface Green’s function can be obtained through the transfer matrix method, with
where H_{00} and H_{01} are Hamiltonian matrix elements for a single layer and for the interlayer coupling and T is the transfer matrix. Generally, Eq.(22) can be solved by iterative calculations until T converges with the help of a fast iteration algorithm^{37}.
Additional Information
How to cite this article: Pan, H. et al. Electric control of topological phase transitions in Dirac semimetal thin films. Sci. Rep. 5, 14639; doi: 10.1038/srep14639 (2015).
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Acknowledgements
The authors would like to thank D.L. Deng for helpful discussions. This work was supported by NSFC under Grant No. 11174022 and No. 61227902, NCET program of MOE and SUTDSRGEPD2013062.
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S.A.Y. conceived the idea. M.W. and H.P. performed the numerical calculation. Y.L. and S.A.Y. did the analytic calculation and analysis. All authors contribute to the data analysis and interpretation. S.A.Y. and H.P. supervised the project and wrote the manuscript. All authors reviewed the manuscript.
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Pan, H., Wu, M., Liu, Y. et al. Electric control of topological phase transitions in Dirac semimetal thin films. Sci Rep 5, 14639 (2015). https://doi.org/10.1038/srep14639
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