RESUMEN
All-inorganic perovskite quantum dots with the usual cubic shape have emerged as a successful and low-cost alternative to electronically functional nanomaterials motivating various fields of applications, including high-efficiency photovoltaics. Here, we present an efficient and almost analytic approach for optical absorption coefficient calculation on self-assembled perovskite quantum dot films with type-II band alignment. The approach takes advantage of the special point technique for integration over the two-dimensional Brillouin zone, which minimizes the computational cost. The set of special wave-vector points is generated using the Monkhorst and Pack method. The optical absorption spectrum for phenyl-C60-butyric acid methyl ester (PCBM)/CsPbI3quantum dot films is computed, in good agreement with the experiment assuming a homogeneous linewidth of 50 meV and considering a ten special-point set. We show that light absorption in these systems is a cooperative optoelectronic property resulting from the quantum-mechanical coupling between perovskite nanocubes, leading to extended system states. The generality of this approach makes it suitable for calculating the optical absorption coefficient in a broad class of perovskite quantum dot systems.
RESUMEN
We present a theoretical investigation of the Goös-Hanchen shift (GHS) experienced by acoustic and optical vibrational modes reflected and transmitted from the surfaces of a semiconductor thin film sandwiched between two semi-infinite media. Our study focuses on the impact of the incident angle on the GHS, considering the coupling between longitudinal and transverse modes. For acoustic vibrations, our findings reveal that the GHS can reach magnitudes up to seven times larger than the thickness of the thin film and up to 20 times larger than the incident wavelength. Besides, it is shown that this significant amplification of the GHS highlights the strong influence of the incident angle and the frequency of the modes involved. In the case of optical vibrations, we observe even more pronounced GHS values, exceeding 30 times the incident wavelength. This demonstrates the potential of GHS in acoustical systems, which opens up possibilities for applications in the design of acoustic devices.
RESUMEN
The transfer matrix method is considered to obtain the fundamental properties of 1D Dirac-like problems. The case of 1D problems in monolayer graphene is addressed. The main characteristics of the transfer matrix are analyzed, contrasting them with the ones corresponding to 1D Schrödinger-like problems. Analytic expressions for the transmission coefficient and bound states are obtained. The continuity between bound states and states of perfect transmission is demonstrated in general, and in particular showed for the case of single electrostatic barriers. These findings in principle can be extended to 2D materials with Hamiltonian similar to monolayer graphene such as silicene and transition metal dichalcogenides.
RESUMEN
Fano resonances of bilayer graphene could be attractive for thermoelectric devices. The special profile presented by such resonances could significantly enhance the thermoelectric properties. In this work, we study the thermoelectric properties of bilayer graphene single and double barrier structures. The barrier structures are typically supported by a substrate and encapsulated by protecting layers, reducing considerably the phonon thermal transport. So, we will focus on the electronic contribution to the thermal transport. The charge carriers are described as massive chiral particles through an effective Dirac-like Hamiltonian. The Hybrid matrix method and the Landauer-Büttiker formalism are implemented to obtain the transmission, transport and thermoelectric properties. The temperature dependence of the Seebeck coefficient, the power factor, the figure of merit and the efficiency is analyzed for gapless single and double barriers. We find that the charge neutrality point and the system resonances shape the thermoelectric response. In the case of single barriers, the low-temperature thermoelectric response is dominated by the charge neutrality point, while the high-temperature response is determined by the Fano resonances. In the case of double barriers, Breit-Wigner resonances dominate the thermoelectric properties at low temperatures, while Fano and hybrid resonances become preponderant as the temperature rises. The values for the figure of merit are close to two for single barriers and above three for double barriers. The system resonances also allows us to optimize the output power and the efficiency at low and high temperatures. By computing the density of states, we also corroborate that the improvement of the thermoelectric properties is related to the accumulation of electron states. Our findings indicate that bilayer graphene barrier structures can be used to improve the response of thermoelectric devices.
RESUMEN
We study the tunneling of optical vibrational modes with transverse horizontal polarization that impinge, at a given angle, on a semiconductor heterostructure. We find a large influence of the Goos-Hänchen shift on tunneling times. In particular, a Goos-Hänchen shift larger than the barrier thickness is reported for the first time. The relation between Goos-Hänchen and Hartman effects is also discussed. The identity that equals the dwell time to the sum of transmission and interference times, previously derived for one-dimensional tunneling problems, is extended to the two-dimensional case. Closed-form expressions are developed for the relevant quantities. Instead of using the standard approach, the interference time is computed from the vibrational energy density. The present study could be useful for the design of semiconductor devices.
RESUMEN
The eigenvalue problem in a cylindrical lens geometry is studied. Using a conformal mapping method, the shape of the boundary and the Hamiltonian for a free particle are reduced to those of a two-dimensional problem with circular symmetry. The wave functions are separated into two independent Hilbert subspaces due to the inherent symmetry of the problem. For small geometry deformations, the solutions are found by a specially designed perturbation approach. Comparisons between exact and perturbative solutions are made for different lens parameters. As the symmetry of the lens is reduced, the characteristics of the spectrum and the corresponding spatial properties of the wave functions are studied. Our results provide a family of billiard geometries in which the electronic level spectrum is well characterized. In analyzing the level spacing distribution of the spectrum, a strong deviation from the Poisson and Wigner limiting distributions is found as the boundary geometry changes. This intermediate distribution is indicative of a mixed phase space, also revealed explicitly in the classical Poincaré maps we present.