Jing-Tao Lü

Quantum thermal transport in nanostructures

Abstract.In this colloquia review we discuss methods for thermal transport calculations for nanojunctions connected to two semi-infinite leads served as heat-baths. Our emphases are on fundamental quantum theory and atomistic models. We begin with an introduction of the Landauer formula for ballistic thermal transport and give its derivation from scattering wave point of view. Several methods (scattering boundary condition, mode-matching, Piccard and Caroli formulas) of calculating the phonon transmission coefficients are given. The nonequilibrium Greens function (NEGF) method is reviewed and the Caroli formula is derived. We also give iterative methods and an algorithm based on a generalized eigenvalue problem for the calculation of surface Greens functions, which are starting point for an NEGF calculation. A systematic exposition for the NEGF method is presented, starting from the fundamental definitions of the Greens functions, and ending with equations of motion for the contour ordered Greens functions and Feynman diagrammatic expansion. In the later part, we discuss the treatments of nonlinear effects in heat conduction, including a phenomenological expression for the transmission, NEGF for phonon-phonon interactions, molecular dynamics (generalized Langevin) with quantum heat-baths, and electron-phonon interactions. Some new results are also shown. We briefly review the experimental status of the thermal transport measurements in nanostructures.

Monte Carlo simulation of hot phonon effects in resonant-phonon-assisted terahertz quantum-cascade lasers

We study the influence of nonequilibrium optical phonons on the electron transport in resonant-phonon-assisted terahertz (THz) quantum-cascade lasers (QCLs). The hot phonon effect is included in the Monte Carlo simulation by introducing a time constant accounting for the decay of polar optical phonons into other phonon modes. We find that nonequilibrium polar optical phonons may modify the electron-phonon interaction, affect the electron distribution in different subbands, and consequently change the device current. We conclude that the hot phonon effect should be taken into account in the study of the transport properties of resonant-phonon-assisted THz QCLs.

Coulomb scattering in the Monte Carlo simulation of terahertz quantum-cascade lasers

The authors compare different Coulomb screening models in the Monte Carlo simulation of terahertz quantum-cascade lasers: the static multisubband screening model and two single subband models. In quantum-cascade structures, electrons are located in many different subbands. Coulomb screening from all these subbands influences the intra- and intersubband scattering processes. The simulation results show that one of the two single subband models overestimates the screening effect, while the other underestimates it. The authors show the reasons for this and propose simple modification to the single subband models.

Nuclear quantum effects of hydrogen bonds probed by tip-enhanced inelastic electron tunneling

Quantum effects in single hydrogen bonds Hydrogen bonds are a combination of electrostatics with a nuclear quantum contribution arising from the light mass of hydrogen nuclei. However, the quantum states of hydrogen nuclei are extremely sensitive to coupling with local environments, and these effects are broadened and averaged with conventional spectroscopic or diffraction techniques. Guo et al. show that quantum effects change the strength of individual hydrogen bonds in water layers adsorbed on a salt surface. These effects are revealed in inelastic tunneling spectra obtained with a chlorine-terminated scanning tunneling microscope tip. Science, this issue p. 321 Quantum effects in water hydrogen bonding are revealed with a chlorine-terminated scanning tunneling microscope tip. We report the quantitative assessment of nuclear quantum effects on the strength of a single hydrogen bond formed at a water-salt interface, using tip-enhanced inelastic electron tunneling spectroscopy based on a scanning tunneling microscope. The inelastic scattering cross section was resonantly enhanced by “gating” the frontier orbitals of water via a chlorine-terminated tip, so the hydrogen-bonding strength can be determined with high accuracy from the red shift in the oxygen-hydrogen stretching frequency of water. Isotopic substitution experiments combined with quantum simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the relatively strong ones. However, this trend can be completely reversed when a hydrogen bond is strongly coupled to the polar atomic sites of the surface.

Terahertz generation and chaotic dynamics in GaN NDR diode

Inflection of carriers in the ? valley, prior to the usual intervalley scattering when the electric field is approximately less than 300 kV cm?1, can cause a negative differential velocity in zinc-blende (Zb) GaN. GaN n+nn+ oscillators based on this mechanism have a self-oscillating frequency in the terahertz (THz) range. The situation is completely different from the case of traditional Gunn-effect devices. We have carefully studied the electric domain dynamics and the self-oscillation frequency dependence on the applied dc field. When the diode is driven by a dc and an ac bias, a typical nonlinear dynamic system is constructed with the dc bias, the ac amplitude and the ac frequency as the control parameters. Different mode locking and chaotic modes show up when tuning the control parameters.

Monte Carlo simulation of extraction barrier width effects on terahertz quantum cascade lasers

The effect of extraction barrier width on the performance of a resonant-phonon terahertz quantum cascade laser is studied using the ensemble Monte Carlo method. The width of extraction barrier is varied from 48to33A with a stepsize of −3A. The calculated threshold current density increases monotonously from 436to1054A∕cm2 with decreasing extraction barrier width, while the peak gain shows nontrivial behavior. These findings agree well with an experimental study. The simulation results suggest an optimal extraction barrier width of 36A for peak gain and for the best temperature performance.

Coupled electron and phonon transport in one-dimensional atomic junctions

Employing the nonequilibrium Greens function method, we develop a fully quantum mechanical model to study the coupled electron-phonon transport in one-dimensional atomic junctions connecting to one-dimensional leads. This model enables us to study the electronic and phononic transport on an equal footing. We derive the electrical and energy currents of the coupled electron-phonon system and present a self-consistent picture of energy exchange between them. As an application, we study the heat dissipation in current-carrying atomic junctions with metallic and semiconductor leads. We find that the inclusion of phonon transport is important in determining the heat dissipation and temperature change of the atomic junctions.

Monte Carlo simulation of carrier transport and output characteristics of terahertz quantum cascade lasers

An ensemble Monte Carlo method, including electron-phonon, electron-electron, electron-impurity scatterings and the hot phonon effect, is used to simulate the carrier transport and output characteristics of a lasing terahertz quantum cascade laser. A simulated I-V curve fits well with the measurement at the lasing domain. Extracted output characteristics, e.g., the gain, threshold current density, and threshold bias, are in good agreement with the experimental results. All the above indicate that the proposed Monte Carlo model is a useful tool for investigating the physical characteristics of terahertz quantum cascade lasers, as well as for analyzing and optimizing device performances.

Tunneling Diode Based on WSe2/SnS2 Heterostructure Incorporating High Detectivity and Responsivity

van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next-generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe2 /SnS2 photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe2 /SnS2 vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe2 /SnS2 vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 1013 Jones (Iph /Idark ratio of ≈106 ) and photoresponsivity of 244 A W-1 at a reverse bias under the illumination of 550 nm light (3.77 mW cm-2 ).

Spin Manipulation by Creation of Single-Molecule Radical Cations.

All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising only C, H, and O atoms, is investigated on a Au(111) substrate using scanning tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are switched to a number of states, three of which carry a localized spin as evidenced by conductance spectroscopy in high magnetic fields. The spin of a single molecule may be reversibly switched on and off without affecting its neighbors. We suggest that ReA on Au is readily converted to a radical by the abstraction of an electron.