The presence of spin-orbit couplings, namely intrinsic and Rashba couplings in graphene, is included via the Kane-Mele Hamiltonian. The transport properties and practical applications of Kane-Mele normal insulators and superconductors are thoroughly investigated.
In the next section we briefly show a more thorough technique to obtain the spin-orbit coupling, starting with relativistic quantum mechanics for completeness. There may be three reasons behind the origin of spin-orbit coupling in material. a) Impurities in the conductive layer, the main source of SOC in metallic systems.
Rashba spin-orbit coupling
If an electron moves along the x direction, the spin order of the eigenvector becomes (1;±i), that is, spin up and spin down are locked in the y direction. Fig.(1.1d) represents the energy spectrum of an electron in the presence of a magnetic field B, the spin degeneracy is canceled by the Zeeman~ splitting, and the gap separating the spin-up and spin-down bands is equal to gµBB.
Fig. (1.1e) presents a one-dimensional view of the energy spectrum for an electron in the presence of RSOC. E0 is the ground state energy and ϸn is the excitation energy of the quasiparticle in the n state.
Normal metal and superconductor junction: Andreev reflection
Here we will specify how the components of the velocity change according to various processes taking place at the interface. iii)Transmission as electron: v⊥→v⊥, vk→ vk. But in a NIS transition, the low energy transmission is strongly suppressed due to the specular reflection.
Normal-insulator-superconductor junction and
Further, if we adjust the distance between the surfaces and change the wavelength of the light, the same oscillations in the intensity spectrum of the transmitted light can be obtained. Quantum interference occurs when such a round trip becomes a multiple of the electron's wavelength.
Basics of Graphene
Each time an electron reaches one of the surfaces, it can bounce back and can also be emitted. So even in this case, if we change the width of the insulating layer or the alternating wavelength of the electron, the conductivity will show fluctuations.
Klein tunneling in graphene
Only two of these six points are unequal and they are usually referred to as the K and K0 points. As in this thesis, the conduction properties of the graphene-based NIS (although the insulating layer is modeled in a different way) junction has been studied, here we will briefly discuss the Klein tunneling phenomenon in graphene.
As we want to study the transport properties of graphene-based junction devices in the presence of spin-orbit interactions, we have considered a Kane-Mele model in the presence of Rashba and intrinsic spin-orbit interactions to calculate the tunneling conductance and properties thermoelectric. of graphene-based coupling devices.
Outline of the thesis
The momenta of the electrons and the holes in the normal region, kN1 and kN2 are given by,. Hieraσ and bσ denote the amplitudes of the Andreev reflection (AR) and normal reflection (NR), respectively.
Normal-insulator-superconductor (NIS) junction
Here aσ and bσ denote the amplitude of pinhole reflection (AR) and normal reflection (NR) respectively in the normal region. Further, pσ and qσ represent the amplitudes of the incoming and reflected holes in the insulating region.
Graphene based NS junction: Kane-Mele model
In the graph near the KandK0 points, the energy dissipation is a linear function of momentum, where the K and K0 points are given by,. The expressions for momenta corresponding to all of them are given by the following compact form, .
Graphene (Kane-Mele) based NIS junction
Since the momentum parallel to the interface is conserved during the tunneling process, we can write: Furthermore, in the tunneling conduction of Sr2RuO4 , the presence of broad subgap peaks can also be described in terms of the surface ABS, which usually occurs in the p-wave type chiral superconductor [125,126].
Results on s-wave pairing symmetry
Thus, we show the variation of the amplitude of AR and NR as the function of the bias energy in Figure (3.2). Therefore, we show the variation of the zero-bias conductance, as a function of the Rashba strength, λRin figure (3.5).
Other pairing symmetries
Again, for an opaque barrier, with the addition of RSOC, the amplitude of AR increases and the amplitude of NR decreases. Furthermore, it is clear that, with the inclusion of RSOC, the surface ABS is not affected, which is the hallmark of the p-wave pair symmetry.
Fig.(3.6a) shows that for transparent case with the inclusion of Rashba spin-orbit coupling there is no change in conductance although the momenta of the electrons and holes are modified. Here we get a maximum value at E =0 due to the fact that the contribution of the AR is maximum at E = ∆0.
Fermi surface mismatch
Therefore, for the above conditions, no transmission will take place to the superconducting region and will therefore not contribute to the conduction. With the addition of RSOC, the conductivity for the transparent case decreases and for the opaque barrier the conductivity increases.
Finite quasiparticle lifetime
With the introduction of finite quasiparticle lifetimes, all expressions containing energy, E are renormalized as mentioned above and then they must be inserted into the corresponding equations to calculate the conductivity. With the introduction of finite quasiparticle lifetimes, the dependence of the conductance spectrum on RSOC does not change.
Normal-superconductor junction (NS) with interfacial RSOC
It should not be obvious how an inclusion of Γ(orτQP) can interfere with the RSOC present in the metallic lead and thus whether it helps or hinders the low energy conduction properties of an NS junction. Comparison with Fig.
Results and discussions
Now we show the conductance as a function of bias, E, (in units of the superconducting gap, that is, E/∆0) for different values of RSOC for two different ranges of the effective barrier potential, χ1 and χ2 in Fig. .(3.14). So the dependence of the tunneling conductance on the RSOC strength is crucially dependent on the effective barrier potential.
Importantly, the magnitude and sign of the Seebeck coefficient quantifies the asymmetry of the electron distribution near the Fermi level. A first-order expansion of the Fermi-Dirac distribution function in (dI)T and (dI)V gives the following expression for the Seebeck coefficient,S,.
Figure of Merit
We can conclude that δV arises due to the Seebeck effect due to the temperature difference that exists between the junctions. We note that KT and KV indicate the thermal conductivity due to the temperature difference, δT, and the voltage difference, δV.
JNS(EFN, TN;EFS, TS)+INS(EFN, TN;EFS, TS)VB =JSN(EFN, TN;EFS, TS) 4.21 As, in our calculations we shifted energies by the Fermi energies of the respective electrodes, the final form of the thermal currents is given by, . The performance of this junction as a self-cooling device can be measured by the coefficient of performance (COP) where COP is defined as the ratio of the heat removed from the cold reservoir to the electrical power required to power the system drive.
Results and discussions: Thermopower
Figure of Merit
Fig.(4.6) shows the value as a function of the barrier potential for different values of the strength of the Rashba coupling, the temperature being fixed at T =0.3∆0. Further, the variation of the figure of merit as a function of both Rashba spin-orbit coupling and x is shown in Fig. (4.8b).
Results and discussions: Thermoelectric cooling
Coefficient of Performance (COP)
Fig.(4.12) shows the COP as a function of the effective barrier potential (χ) for different values of RSOC, where bias is fixed at VB= 0.15∆0. In fig. (4.13) we present the variation of COPas a function of the bias for two different values of the effective barrier potential, χ (namely some χ1 and χ2).
Results and Discussions on Kane-Mele NS junction
In Fig. (5.2) we have shown the conductivity as a function of the strength of the RSOC for different rotations. Conductivity as a function of ISOC strength (in the absence of RSOC) is shown in Fig. (5.4) where we have assumed a zero bias condition.
Kane-Mele NIS junction
Results and Discussions on Kane-Mele NIS junction
In Fig.(5.7), the spin-polarized conductance, Gσ is shown as a function of the effective barrier potential, χ, while the strength of the RSOC term is varied with the strength of the intrinsic term held to zero. The conduction characteristics of the full Kane-Mele junction as a function of χ are shown in Fig.(5.9).
The population of electrons in the left and right wires is described by the Fermi-Dirac distribution function, fN and fS, respectively, where EFN =EFS at zero external bias. In the next subsection, we will discuss how the thermal current and the thermal conductivity can be calculated.
To get a clear idea of the efficiency, one should calculate the spin-dependent FM, which is given by The performance of this compound as a self-cooling device can be measured by the coefficient of performance (COP), where COP is defined as the ratio of the heat removed from the cold reservoir to the electrical power needed to operate the system.
Results and Discussions
Seebeck Coefficient and Figure of Merit
The variation of the Seebeck coefficient,S as a function of the temperature (in units of superconducting gap, ∆0) for a pristine graphene is shown in Fig.(6.1a). The variation of the Figure of Merit, ZT as a function of the temperature (in units of superconducting gap, ∆0) is shown in Fig.(6.5).
Thermoelectric cooling and coefficient of performance
For higher values of the RSOC parameter, the thermoelectric cooling becomes smaller for both turns. The color plots provide the idea of the range of values of the RSOC, the ISOC parameter and the corresponding coefficient of performance.
The variation of the conductance as a function of the effective barrier potential, χ for generic and graphene-based junction systems, is shown in Figure (7.2a) and Figure (7.2b), respectively. The Fermi energy of the superconducting lead has been considered as EFS = EFN (no Fermi surface mismatch).
Comparison of thermoelectric properties
Both the change in total conductivity as a function of bias energy shows a double peak structure (see Fig. (7.2)). Again, we took into account a certain value of the effective barrier potential, where the heat flow has a maximum value.
We observed a Fabry-Perot-like oscillation for the tunneling conductance spectrum as a function of the effective barrier potential (the product of both V0 and the width, d). We observe the oscillation of the tunneling conductance spectrum as a function of the effective barrier potential.