Chungwei Lin

Optimal boson energy for superconductivity in the Holstein model

We examine the superconducting solution in the Holstein model, where the conduction electrons couple to the dispersionless Boson fields, using the Migdal-Eliashberg theory and Dynamical Mean Field Theory. Although different in numerical values, both methods imply the existence of an optimal Boson energy for superconductivity at a given electron-Boson coupling. This non-monotonous behavior can be understood as an interplay between the polaron and superconducting physics, as the electron-Boson coupling is the origin of the superconductor, but at the same time traps the conduction electrons making the system more insulating. Our calculation provides a simple explanation on the recent experiment on sulfur hydride (H

Near-field enhancement of thermoradiative devices

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Application of coupled mode theory on radiative heat transfer between layered Lorentz materials

S), where an optimal pressure for the superconductivity was observed. The validities of both methods are discussed.

Linear scanning tunneling spectroscopy over a large energy range in black phosphorus

Thermoradiative (TR) device has recently been proposed for noncontact direct photon-electricity energy conversion. We investigate how the near-field effect can boost the performance of a TR device. For a near-field TR device, a heat sink is placed close to the TR cell, with the separation being small compared to the characteristic photon wavelength. It is demonstrated that the TR device, like the thermophotovoltaic device, can be formulated using the transmissivity and the generalized Planck distribution. We quantitatively show that δ-function transmissivity is a very good approximation (capturing up to 90% of total radiative energy transfer) when the radiative energy transfer is governed by resonances. Three practical types of heat sinks are considered, a metallic material described by the Drude model, a polar dielectric material described by the Lorentz oscillator model, and a semiconductor material that is identical to the TR cell. The blackbody heat sink serves as the far-field reference. By properly ...

Nanostructure enhanced near-field radiative heat transfer and designs for energy conversion devices

The coupled mode theory (CMT) provides a simple and clear framework to analyze the radiation energy exchange between reservoirs. We apply CMT to analyze the radiative heat transfer between layered Lorentz materials whose dielectric functions can be approximated by the Lorentz oscillator model. By comparing the transmissivity computed by the exact solution to that computed by CMT, we find that CMT generally gives a good approximation for this class of materials. The biggest advantage of CMT analysis, in our opinion, is that only the (complex) resonant energies are needed to obtain the radiation energy transfer; the knowledge of the spatial profile of resonances is not required. Several issues, including how to choose the resonant modes, what these modes represent, and the limitation of this method, are discussed. Finally, we also apply the CMT method to the electronic systems, demonstrating the generality of this formalism.

Performance comparison between photovoltaic and thermoradiative devices

We reveal the unique electronic characteristics of the conduction band (CB) of black phosphorus (BP) by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), density functional theory calculations, analytic fitting, and model simulations. We discover that the differential conductance spectrum, which represents the local density of states (LDOS) of BP, exhibits a linear character over a large energy range in the unoccupied electronic state region. Combining theoretical calculations, we demonstrate that the linear character right above the conduction band minimum originates from a specific combination of the anisotropic band dispersions of BPs CB. In particular, the wave function of BPs CB possesses a pronounced density between BP layers and extends into the vacuum significantly, which is in sharp contrast to those of adjacent bands. This makes the CB dominate STS signals even when the energy is sufficiently high to involve other bands, and maintains the linearity of the STS spectrum over a wide energy range. The fact that the CB provides linear DOS and possesses pronounced wave function density in BP interlayers provides new insights for engineering the electronic structures and properties of BP and BP based materials.We reveal the unique electronic characteristics of the conduction band (CB) of black phosphorus (BP) by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), density functional theory calculations, analytic fitting, and model simulations. We discover that the differential conductance spectrum, which represents the local density of states (LDOS) of BP, exhibits a linear character over a large energy range in the unoccupied electronic state region. Combining theoretical calculations, we demonstrate that the linear character right above the conduction band minimum originates from a specific combination of the anisotropic band dispersions of BPs CB. In particular, the wave function of BPs CB possesses a pronounced density between BP layers and extends into the vacuum significantly, which is in sharp contrast to those of adjacent bands. This makes the CB dominate STS signals even when the energy is sufficiently high to involve other bands, and maintains the linearity of the STS spec...

Thermoradiative device enhanced by near-field coupled structures

Near-field radiative heat transfer can exceed the blackbody limit, and this property has been explored toward energy transfer and conversion applications, such as thermophtovoltaic (TPV) devices, radiative cooling devices, and thermoradiative (TR) devices. The coupling of resonant modes between two surfaces is important in near- field heat transfer and near-field TPV and TR systems. It was shown that the coupling of resonant modes enhances the transmissivity between two coupled objects, which further determines the radiative heat transfer and energy conversion. Surface plasmon polaritons (SPPs), which are surface resonances existing on metal surfaces, are commonly used for such systems. While the frequency of SPP resonance is fixed for a planar emitter, a nanostructured emitter supports additional resonances such as SPP or cavity modes with lower frequencies that are closer to the bandgap energy of a typical PV cell. We show that the nanostructured designs significantly improves the near-field radiative power transfer, and electric power output for a TR system.

Application of Impedance Matching for Enhanced Transmitted Power in a Thermophotovoltaic System

Photovoltaic (PV) and thermoradiative (TR) devices are power generators that use the radiative energy transfer between a hot and a cold reservoir. For PV devices, the semiconductor at the cold side (PV cell) generates electric power; for TR devices, the semiconductor at the hot side (TR cell) generates electric power. In this work, we compare the performance of the photovoltaic and thermoradiative devices, with and without the non-radiative processes. Without non-radiative processes, PV devices generally produce larger output powers than TR devices. However, when non-radiative processes become important, the TR can outperform the PV devices. This conclusion applies to both far-field and near-field based devices. A key difference in efficiency between PV and TR devices is pointed out.