arXiv subjects

S. S. Kubakaddi

Publications and source records attributed to S. S. Kubakaddi.

8 recordsLinked to original sources

arXiv2021

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

The in-plane phonon-drag thermopower $S^g$, diffusion thermopower $S^d$ and the power factor $PF$ are theoretically investigated in twisted bilayer graphene (tBLG) as a function of twist angle $θ$, temperature $T$ and electron density $n_s$ in the region of low $T$ (1-20 K). As $θ$ approaches magic angle $θ_m$, the $S^g$ and $S^d$ are found to be strongly enhanced, which is manifestation of great suppression of Fermi velocity ${ν_F}^*$ of electrons in moire flat band near $θ_m$. This enhancement decreases with increasing $θ$ and $T$. In the Bloch- Gruneisen (BG) regime, it is found that $S^g \sim {ν_F}^{* -2}$, $T^3$ and ${n_s}^{-1/2}$. As $T$ increases, the exponent $δ$ in $S^g \sim T^δ$, changes from 3 to nearly zero and a maximum $S^g$ value of $\sim$ 10 mV/K at $\sim$ 20 K is estimated. $S^g$ is larger (smaller) for smaller $n_s$ in low (high) temperature region. On the other hand, $S^d$, taken to be governed by Mott formula, $\sim {ν_F}^{* -1}$, $T$ and ${n_s}^{-1/2}$ and $S^d< \sim$ 2 K. The power factor $PF$ is also found to be strongly $θ$ dependent and very much enhanced. Consequently, possibility of a giant figure of merit is discussed.In tBLG, $θ$ acts as a strong tuning parameter of both $S^g$ and $S^d$ and $PF$ in addition to $T$ and $n_s$. Our results are qualitatively compared with the measured out-of-plane thermopower in tBLG.

cond-mat.mes-hall↗

arXiv2020

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 (tBLG). 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 $\sim$ 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 $θ$ $\sim$ 3$^{\circ}$. In the Bloch-Grüneisen (BG) regime, $P$ $\sim$ ${T_e}^4$, ${n_s}^{-1/2}$ and ${v_F}^{*-2}$. In the higher temperature region ($\sim$10- 50 K), $P$ $\sim$ ${T_e}^δ$, with $δ\sim$ 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 $F_e(T) μ_p (T) = (e{v_s}^2$/2) i.e. $Fe(T) = (n_se^2 {v_s}^2/2)ρ_p$, 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 (Phys. Rev. B 99, 165112 (2019)).

cond-mat.mtrl-sci↗

arXiv2020

Power loss of hot Dirac fermions in silicene and its near equivalence with graphene

The power loss $P$ of hot Dirac fermions through the coupling to the intrinsic intravalley and intervalley acoustic and optical phonons is analytically investigated in silicene as a function of electron temperature $T_e$ and density $n_s$. At very low $T_e$, the power dissipation is found to follow the Bloch-Grüneisen power-law $\propto T_e^4$ and $n_s^{-0.5}$, as in graphene, and for $T_e \lesssim20-30$ K, the power loss is predominantly due to the intravalley acoustic phonon scattering. On the other hand, dispersionless low energy intervalley acoustic phonons begin to dominate the power transfer at temperatures as low as $\sim$$30$ K, and optical phonons dominate at $T_e \gtrsim200$ K, unlike the graphene. The total power loss increases with $T_e$ with a value of $\sim$$10^{10}$ eV/s at $300$ K, which is the same order of magnitude as in graphene. The power loss due to intravalley acoustic phonons increases with $n_s$ at higher $T_e$, whereas due to the intervalley acoustic and optical phonons is found to be independent of $n_s$. Interestingly, the energy relaxation time in silicene is about $4$ times higher than that in graphene. For this reason, silicene may be superior over graphene for its applications in bolometers and calorimeters. Power transfer to the surface optical phonons $P_{\text{SO}}$ is also studied as a function of $T_e$ and $n_s$ for silicene on Al$_2$O$_3$ substrate and it is found to be greater than the intrinsic phonon contribution at higher $T_e$. Substrate engineering is discussed to reduce $P_{\text{SO}}$.

cond-mat.mtrl-sci↗

arXiv2019

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

Transport of electrons at high electric fields is investigated in an 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 significantly degrades its magnitude. The drift velocity is found to saturate at a value $v_{ds} \sim 10^7$ cm/s and it is weakly dependent on $n_e$. A large saturation current density $\sim 10^6$ A/cm$^2$ is predicted.

cond-mat.mtrl-sci↗

arXiv2017

The role of vector potential coupling in hot electron cooling power in bilayer graphene at low temperature

We have studied, in bilayer graphene (BLG), the hot electron cooling power $F_{VP} (T, n_s)$ due to acoustic phonons via vector potential (VP) coupling. It is calculated as a function of electron concentration $ns$ and temperature $T$ and compared with $F_{DP} (T, n_s)$ that due to deformation potential coupling. For the $n_s$ around $1\times10^{12} cm^{-2}$, $F_{VP} (T, n_s)$ is much smaller than $F_{DP} (T, n_s)$ . With increase of $n_s$, $F_{DP} (T, n_s)$ decreases faster than $F_{VP} (T, n_s)$ does. A cross over is predicted and dominant contribution of $F_{VP} (T, n_s)$ can be observed at large $n_s$. In the Bloch- Grüneisen (BG) regime $F_{VP} (T, n_s)~ n_s^{-1/2}$ and $F_{DP} (T, n_s)~ n_s^{-3/2}$. Both $F_{VP} (T, n_s)$ and $F_{DP} (T, n_s)$ have the same $T$ dependence with $T^4$ power law in BG regime. Behaviour of $F_{DP} (T, n_s)~ n_s^{-3/2}$ and $T^4$ is in agreement with the experimental results at moderate $n_s$. Besides, in BG regime, we have predicted, for both the VP and DP coupling, a relation between $F(T, n_s)$ and the acoustic phonon limited mobility $μ_p$, opening a new door to determine $μ_p$ from the measurements of $F(T, n_s)$.

cond-mat.mes-hall↗

arXiv2017

A new relation between the hot electron power loss and acoustic phonon limited mobility in Bloch-Grüneisen regime

Expressions for the electron power loss F(T) and mobility μ_p due to acoustic phonon scattering are given in the Bloch-Grüneisen (BG) regime for three- and two- dimensional electron gas in semiconductors and Dirac-fermions. We obtain a simple relation F(T) μ_p = ηev_s^2, where η (~1) is a constant, e is the electron charge and v_s is acoustic phonon velocity. It is found to be independent of temperature and electron concentration. This relation is applied to GaAs heterojucntions and graphene, to obtain μ_p from the measured F(T). We propose that, using this relation, the measurements of F(T), in BG regime, which depends exclusively upon acoustic phonons coupling, could serve as a tool to determine the low temperature μ_p, which is otherwise difficult to measure due to the contributions from disorders

cond-mat.mes-hall↗

arXiv2017

Phonon-drag magnetoquantum oscillations in graphene

A theory of low-temperature phonon-drag magnetothermopower $S_{xx}^g$ is presented in graphene in a quantizing magnetic field. $S_{xx}^g$ is found to exhibit quantum oscillations as a function of magnetic field $B$ and electron concentration $n_e$. Amplitude of the oscillations is found to increase (decrease) with increasing $B$ ($n_e$). The behavior of $S_{xx}^g$ is also investigated as a function of temperature. A large value of $S_{xx}^g $($\sim$ few hundreds of $μ$V/K) is predicted. Numerical values of $S_{xx}^g $ are compared with the measured magnetothermopower $S_{xx}$ and the diffusion component $S_{xx}^d$ from the modified Girvin-Jonson theory.

cond-mat.mes-hall↗

arXiv2014

Hot-electron cooling by acoustic and optical phonons in monolayers of MoS$_2$ and other transition-metal dichalcogenides

We study hot-electron cooling by acoustic and optical phonons in monolayer MoS$_2$. The cooling power $P$ ($P_e = P/n$) is investigated as a function of electron temperature $T_e$ (0-500 $\mathrm{K}$) and carrier density $n$ ($10^{10}$-$10^{13}$ $\mathrm{cm}^{-2}$) taking into account all relevant electron-phonon (el-ph) couplings. We find that the cross over from acoustic phonon dominated cooling at low $T_e$ to optical phonon dominated cooling at higher $T_e$ takes place at $T_e \sim 50$-$75$ $\mathrm{K}$. The unscreened deformation potential (DP) coupling to the TA phonon is shown to dominate $P$ due to acoustic phonon scattering over the entire temperature and density range considered. The cooling power due to screened DP coupling to the LA phonon and screened piezoelectric (PE) coupling to the TA and LA phonons is orders of magnitude lower. In the Bloch-Grüneisen (BG) regime, $P\sim T_e^4$ ($T_e^6$) and $P\sim n^{-1/2}$ ($P_e\sim n^{-3/2}$) are predicted for unscreened (screened) el-ph interaction. The cooling power due to optical phonons is dominated by zero-order DP couplings and the Fröhlich interaction, and is found to be significantly reduced by the hot-phonon effect when the phonon relaxation time due to phonon-phonon scattering is large compared to the relaxation time due to el-ph scattering. The $T_e$ and $n$ dependence of the hot-phonon distribution function is also studied. Our results for monolayer MoS$_2$ are compared with those in conventional two-dimensional electron gases (2DEGs) as well as monolayer and bilayer graphene.

cond-mat.mes-hall↗