Quantum magneto-transport properties of monolayers MoSi2N4, WSi2N4, and MoSi2As4.

We study the transport properties of monolayers MoSi2N4, WSi2N4, and MoSi2As4in a perpendicular magnetic field. The Landau level (LL) band structures including spin and exchange field effects are derived and discussed using a low-energy effective model. We show that the LLs band structures of these materials are similar to those of phosphorene and transition-metal dichalcogenides rather than graphene or silicene. The combination of strong spin-orbit coupling and exchange fields reduces the degradation of the LLs, leading to new plateaus in the Hall conductivity and Hall resistivity and new peaks in the longitudinal conductivity and longitudinal resistivity. The effect of the exchange field, carrier density, and LLs band structure on the conductivities and resistivities have been investigated. At high temperatures, the steps in Hall conductivity and resistivity plateaus disappear and reduce to their corresponding classical forms.

Phonon-drag thermopower and thermoelectric performance of MoS2monolayer in quantizing magnetic field.

We present a theory of phonon-drag thermopower,Sxxg, in MoS2monolayer at a low-temperature regime in the presence of a quantizing magnetic fieldB. Our calculations forSxxgconsider the electron-acoustic phonon interaction via deformation potential (DP) and piezoelectric (PE) couplings for longitudinal (LA) and transverse (TA) phonon modes. The unscreened TA-DP is found to dominateSxxgover other mechanisms. TheSxxgis found to oscillate with the magnetic field where the lifting effect of the valley and spin degeneracies in MoS2monolayer has been clearly observed. An enhancedSxxgwith a peak value of∼1mV K-1at aboutT = 10 K is predicted, which is closer to the zero field experimental observation. In the Bloch-Grüneisen regime the temperature dependence ofSxxggives the power-lawSxxg∝Tδe, whereδevaries marginally around 3 and 5 for unscreened and screened couplings, respectively. In addition,Sxxgis smaller for larger electron densityne. The power factor PF is found to increase with temperatureT, decrease withne, and oscillate withB. The prediction of an increase of thermal conductivity with temperature and the magnetic field is responsible for the limit of the figure of merit (ZT). At a particular magnetic field and temperature,ZTcan be maximized by optimizing electron density. By fixingne=1012cm-2, the highestZTis found to be 0.57 atT = 5.8 K andB = 12.1 T. Our findings are compared with those in graphene and MoS2for the zero-magnetic field.

Magneto-optical absorption properties of topological insulator thin films.

We theoretically study the magneto-optical absorption coefficients (MOACs) and the refractive index changes (RICs) due to both intra- and inter-band transitions in topological insulator (TI) thin films. The interplay between Zeeman energy and hybridization contribution leads to a transition between the normal insulator phase and the TI phase. The difference in the optical response in these two phases as well as at the phase transition point has been analyzed. The influence of the electron density, magnetic field, and temperature on the MOACs and RICs in both intra- and inter-band transitions is investigated. Our results show that the electron density affects directly the threshold energy. At a finite temperature, the thermal excitation causes the triggering of some new transitions which do not appear atT= 0 K. Evidence of the half-peak feature of the first inter-band transition is also found in TI thin films.

Giant thermopower and power factor in magic angle twisted bilayer graphene at low temperature.

The in-plane phonon-drag thermopowerSg, diffusion thermopowerSdand the power factor PF are theoretically investigated in a twisted bilayer graphene (tBLG) as a function of twist angleθ, temperatureTand electron densitynsin the region of lowT(1-20 K). Asθapproaches magic angleθm, theSgandSdare found to be strongly enhanced, which is manifestation of great suppression of effective Fermi velocityvF*of electrons in moiré flat band nearθm. This enhancement decreases with increasingθandT. In the Bloch-Grüneisen (BG) regime, it is found thatSg∼vF*-2,T3andns-1/2. AsTincreases, the exponentδinSg∼Tδ, changes from 3 to nearly zero and a maximumSgvalue of ∼10 mV K-1at ∼20 K is estimated.Sgis found to be larger (smaller) for smallernsin low (high) temperature region. On the other hand,Sd, taken to be governed by Mott formula withSd∼vF*-1,Tandns-1/2andSd≪SgforT> ∼2 K. The power factor PF is also shown to be stronglyθdependent and is very much enhanced. Consequently, possibility of a giant figure of merit is discussed. In tBLG,θacts as a strong tuning parameter of bothSgandSdand PF in addition toTandns. Our results are qualitatively compared with the measured out-of-plane thermopower in tBLG.

Large power dissipation of hot Dirac fermions in twisted bilayer graphene.

We have carried out a theoretical investigation of hot electron power loss P, involving electron-acoustic phonon interaction, as a function of twist angle θ, electron temperature T e and electron density n s in twisted bilayer graphene. It is found that as θ decreases closer to magic angle θ m, P enhances strongly and θ acts as an important tunable parameter, apart from T e and n s. In the range of T e = 1-50 K, this enhancement is ∼250-450 times the P in monolayer graphene (MLG), which is manifestation of the great suppression of Fermi velocity v F * of electrons in moiré flat band. As θ increases away from θ m, the impact of θ on P decreases, tending to that of MLG at θ ∼ 3°. In the Bloch-Grüneisen (BG) regime, P ∼ T e 4, n s -1/2 and v F *-2. In the higher temperature region (∼10-50 K), P ∼ T e δ , with δ ∼ 2.0, and the behavior is still super linear in T e, unlike the phonon limited linear-in-T (lattice temperature) resistivity ρ p. P is weakly, decreasing (increasing) with increasing n s at lower (higher) T e, as found in MLG. The energy relaxation time τ e is also discussed as a function of θ and T e. Expressing the power loss P = F e(T e) - F e(T), in the BG regime, we have obtained a simple and useful relation F e(T)µ p(T) = (ev s 2/2) i.e. F e(T) = (n s e 2 v s 2/2)ρ p, where µ p is the acoustic phonon limited mobility and v s is the acoustic phonon velocity. The ρ p estimated from this relation using our calculated F e(T) is nearly agreeing with the ρ p of Wu et al (2019 Phys. Rev. B 99 165112).

Drift velocity saturation and large current density in intrinsic three-dimensional Dirac semimetal cadmium arsenide.

Transport of electrons at high electric fields is investigated in intrinsic three-dimensional Dirac semimetal cadmium arsenide, considering the scattering of electrons from acoustic and optical phonons. Screening and hot phonon effect are taken in to account. Expressions for the hot electron mobility µ and power loss P are obtained as a function of electron temperature T e. The dependence of drift velocity v d on electric field E and electron density n e has been studied. Hot phonon effect is found to set in the saturation of v d at relatively low E and to significantly degrade its magnitude. The drift velocity is found to saturate at a value v ds ∼ 107 cm s-1 and it is weakly dependent on n e. A large saturation current density ∼ 106 A cm-2 is predicted.

Thermoelectric transport properties in 3D Dirac semimetal Cd3As2.

Thermoelectric transport properties, namely, electrical conductivity, electronic thermal conductivity, and diffusion thermopower are theoretically investigated in 3D Dirac semimetal Cd3As2. We employ Boltzmann transport formalism and consider the electron scattering by charged impurities, short-range disorder, acoustic phonons, and optical phonons. The Boltzmann transport equation is solved using the Ritz iteration technique to obtain the first-order perturbation distribution function for the interaction of electrons with inelastic polar optical phonons scattering. The numerical results are presented in the temperature range 2-300 K with the electron concentration in the range (0.1-10) × 1018 cm-3. It is found that, at low temperature < ~70 K transport coefficients are dominated by charged impurity scattering and at higher temperature the phonon scattering is found to be dominant. The validity of Wiedemann-Franz law is examined. Recently observed experimental results are explained by our theory.

Phonon-drag magnetoquantum oscillations in graphene.

A theory of low-temperature phonon-drag magnetothermopower [Formula: see text] is presented in graphene in a quantizing magnetic field. [Formula: see text] is found to exhibit quantum oscillations as a function of magnetic field B and electron concentration n e . The amplitude of the oscillations is found to increase (decrease) with increasing B (n e ). The behavior of [Formula: see text] is also investigated as a function of temperature. A large value of [Formula: see text] (∼few hundreds of µV K-1) is predicted. Numerical values of [Formula: see text] are compared with the measured magnetothermopower S xx and the diffusion component [Formula: see text] from the modified Girvin-Jonson theory.

Effects of a piezoelectric substrate on phonon-drag thermopower in monolayer graphene.

The phonon-drag thermopower is studied in a monolayer graphene on a piezoelectric substrate. The phonon-drag contribution [Formula: see text] from the extrinsic potential of piezoelectric surface acoustic (PA) phonons of a piezoelectric substrate (GaAs) is calculated as a function of temperature T and electron concentration n s. At a very low temperature, [Formula: see text] is found to be much greater than [Formula: see text] of the intrinsic deformation potential of acoustic (DA) phonons of the graphene. There is a crossover of [Formula: see text] and [Formula: see text] at around ~5 K. In graphene samples of about >10 µm size, we predict S g ~ 20 µV at 10 K, which is much greater than the diffusion component of the thermopower and can be experimentally observed. In the Bloch-Gruneisen (BG) regime T and n s dependence are, respectively, given by the power laws [Formula: see text] ([Formula: see text]) ~ T 2(T 3) and [Formula: see text], [Formula: see text] ~ [Formula: see text]. The T(n s) dependence is the manifestation of the 2D phonons (Dirac phase of the electrons). The effect of the screening is discussed. Analogous to Herring's law (S g µ p ~ T -1), we predict a new relation S g µ p ~ [Formula: see text], where µ p is the phonon-limited mobility. We suggest that the n s dependent measurements will play a more significant role in identifying the Dirac phase and the effect of screening.

Phonon-drag thermopower in 3D Dirac semimetals.

A theory of low-temperature phonon-drag thermopower S(g) in three-dimensional (3D) Dirac semimetals has been developed considering screened electron-phonon deformation potential coupling. Numerical investigations of S(g), in the boundary scattering regime for phonons, are made in 3D Dirac semimetal Cd3As2, as a function of temperature T and electron concentration n e. S(g) is found to increase rapidly for about T < 1 K and nearly levels off for higher T. It is also seen that S(g) increases (decreases) with decreasing n e at lower (higher) T (<2 K). A screening effect is found to be very significant, strongly affecting T and n e dependence for about <1 K and becoming negligible at higher temperature. In the Bloch-Gruneisen (BG) regime the power laws S(g) ~ T(8) (T(4)) and S(g) ~ n(e)(-5/3)(n(e)(-1/3) with (without) screening are obtained. These laws with respect to T and n e are, respectively, characteristics of 3D phonons and Dirac 3D electrons. Comparison with diffusion thermopower S(d) shows that S (g) dominates (and is much greater than) S(d) for about T > 0.2 K. Herring's law S(g) µ p ~ T (-1), relating phonon limited mobility µ p and S(g) in the BG regime, is shown to be valid in 3D Dirac semimetals. The results obtained here are compared with those in 3D semiconductors, low-dimensional semiconductor heterojunctions and graphene. We conclude that n e-dependent measurements, rather than T-dependent ones, provide a clearer signature of the 3D Dirac semimetal phase.

Phonon-drag thermopower in a monolayer MoS2.

The theory of phonon-drag thermopower S(g) is developed in a monolayer MoS(2), considering the electron­acoustic phonon interaction via deformation potential (DP) and piezoelectric (PE) coupling, as a function of temperature T and electron concentration n(s). DP coupling of TA (LA) phonons is taken to be unscreened (screened) and PE coupling of LA and TA phonons is taken to be screened. S(g) due to DP coupling of TA phonons is found to be dominant over all other mechanisms and in the Bloch­Grüneisen regime it gives power law S(g) ~ T3. All other mechanisms give S(g) ~ T(5). These power laws are characteristic of two-dimensional (2D) phonons with linear dispersion. Screening enhances the exponent of T by 2 and strongly suppresses S(g) due to the large effective mass of the electrons. We find that S(g), due to screened DP and PE couplings is nearly the same in contrast to the results in GaAs heterojunctions. Also, we predict that S(g) ~ n(s)(-3/2), a characteristic of 2D electrons with parabolic relation. With the increasing (decreasing) T(n(s)) its exponent decreases. For comparison, we give diffusion thermopower S(d). At very low T and high n(s), S(d) ~ T and n(2)(-1). S(d) is found to be greater than S(g) for about T < 2­3 K. The results are compared with those in conventional 2D electron gas and graphene.

Phonon-drag thermopower in an armchair graphene nanoribbon.

We calculate the phonon-drag thermopower S(g) of an armchair graphene nanoribbon (AGNR) in the boundary scattering regime of phonons. S(g) is studied as a function of temperature, Fermi energy and width of the AGNR. At very low temperatures T, S(g) is exponentially suppressed and an activated behavior is observed which is characteristic of one-dimensional carriers. This is in contrast to the power law dependence in graphene in the Bloch-Grüneisen regime. However, at higher T, S(g) in the AGNR levels off. S(g) also shows strong dependence on Fermi energy and width of the AGNR. The magnitude of S(g) in the AGNR is compared with that in single-wall carbon nanotube and graphene.