The dynamic effect of Bloch electrons under the action of temporally periodic electric fields is studied. The explicit solutions for the quasienergies and the Floquet states are obtained exactly and generally, from wh...The dynamic effect of Bloch electrons under the action of temporally periodic electric fields is studied. The explicit solutions for the quasienergies and the Floquet states are obtained exactly and generally, from which the localized or extended nature of Floquet states is demonstrated to be controlled by a reduced vector potential in one period of electric fields. When this reduced vector potential is an irrational, all the Floquet states are localized except the cases where the reduced vector potential is extremely well approximated by rationals (i.e., a Liouville number) and simultaneously the instersite hopping does not decay fast enough, for which the Floquet states are found to be more and more extended at large distances. When this reduced vector potential approaches an ordinary rational, all the Floquet states are extended. However, if the reduced vector potential has typical Diophantine properties, and if the intersite hopping decreases fast enough, the Floquet states cannot be extended.展开更多
The Wigner-Seitz unit cell (rhombus) for a honeycomb lattice fails to establish a k-vector in the 2D space, which is required for the Bloch electron dynamics. Phonon motion cannot be discussed in the triangular coordi...The Wigner-Seitz unit cell (rhombus) for a honeycomb lattice fails to establish a k-vector in the 2D space, which is required for the Bloch electron dynamics. Phonon motion cannot be discussed in the triangular coordinates, either. In this paper, we propose a rectangular 4-atom unit cell model, which allows us to discuss the electron and phonon (wave packets) motion in the k-space. The present paper discusses the band structure of graphene based on the rectangular 4-atom unit cell model to establish an appropriate k-vector for the Bloch electron dynamics. To obtain the band energy of a Bloch electron in graphene, we extend the tight-binding calculations for the Wigner-Seitz (2-atom unit cell) model of Reich et al. (Physical Review B, 66, Article ID: 035412 (2002)) to the rectangular 4-atom unit cell model. It is shown that the graphene band structure based on the rectangular 4-atom unit cell model reveals the same band structure of the graphene based on the Wigner-Seitz 2-atom unit cell model;the π-band energy holds a linear dispersion (ε−k ) relations near the Fermi energy (crossing points of the valence and the conduction bands) in the first Brillouin zone of the rectangular reciprocal lattice. We then confirm the suitability of the proposed rectangular (orthogonal) unit cell model for graphene in order to establish a 2D k-vector responsible for the Bloch electron (wave packet) dynamics in graphene.展开更多
We theoretically investigate high-order harmonic generation(HHG)in crystals induced by linearly polarized laser fields.We obtain the HHG spectra by solving the semiconductor Bloch equations and analyze the radiation p...We theoretically investigate high-order harmonic generation(HHG)in crystals induced by linearly polarized laser fields.We obtain the HHG spectra by solving the semiconductor Bloch equations and analyze the radiation process by different models.Here we propose a multiple collision model,in which the electrons and holes are produced at different times and places.It is found that the multiple collision trajectories can help us comprehensively and better explain the results of the quantum calculation.Moreover,we find that the harmonic suppression occurs due to the overlap of multiple collision trajectories.展开更多
The conduction of a single-wall carbon nanotube depends on the pitch. If there are an integral number of carbon hexagons per pitch, then the system is periodic along the tube axis and allows “holes” (not “electrons...The conduction of a single-wall carbon nanotube depends on the pitch. If there are an integral number of carbon hexagons per pitch, then the system is periodic along the tube axis and allows “holes” (not “electrons”) to move inside the tube. This case accounts for a semiconducting behavior with the activation energy of the order of around 3 meV. There is a distribution of the activation energy since the pitch and the circumference can vary. Otherwise nanotubes show metallic behaviors (significantly higher conductivity). “Electrons” and “holes” can move in the graphene wall (two dimensions). The conduction in the wall is the same as in graphene if the finiteness of the circumference is disregarded. Cooper pairs formed by the phonon exchange attraction moving in the wall is shown to generate a temperature-independent conduction at low temperature (3 - 20 K).展开更多
Individual metallic single-wall carbon nanotubes show unusual non-Ohmic transport behaviors at low and high bias fields. For low-resistance contact samples, the differential conductance increases with increasing bias,...Individual metallic single-wall carbon nanotubes show unusual non-Ohmic transport behaviors at low and high bias fields. For low-resistance contact samples, the differential conductance increases with increasing bias, reaching a maximum at ~100 mV. As the bias increases further, drops dramatically [1]. The higher the bias, the system behaves in a more normal (Ohmic) manner. This low-bias anomaly is temperature-dependent (50 - 150 K). We propose a new interpretation. Supercurrents run in the graphene wall below ~150 K. The normal hole currents run on the outer surface of the wall, which are subject to the scattering by phonons and impurities. The currents along the tube length generate circulating magnetic fields and eventually destroy the supercurrent in the wall at high enough bias, and restore the Ohmic behavior. If the prevalent ballistic electron model is adopted, then the temperature-dependent scattering effects cannot be discussed. For the high bias (0.3 - 5 V), (a) the I-V curves are temperature-independent (4 - 150 K), and (b) the currents (magnitudes) saturate. The behavior (a) arises from the fact that the neutral supercurrent below the critical temperature is not accelerated by the electric field. The behavior (b) is caused by the limitation of the number of quantum-states for the “holes” running outside of the tube.展开更多
文摘The dynamic effect of Bloch electrons under the action of temporally periodic electric fields is studied. The explicit solutions for the quasienergies and the Floquet states are obtained exactly and generally, from which the localized or extended nature of Floquet states is demonstrated to be controlled by a reduced vector potential in one period of electric fields. When this reduced vector potential is an irrational, all the Floquet states are localized except the cases where the reduced vector potential is extremely well approximated by rationals (i.e., a Liouville number) and simultaneously the instersite hopping does not decay fast enough, for which the Floquet states are found to be more and more extended at large distances. When this reduced vector potential approaches an ordinary rational, all the Floquet states are extended. However, if the reduced vector potential has typical Diophantine properties, and if the intersite hopping decreases fast enough, the Floquet states cannot be extended.
文摘The Wigner-Seitz unit cell (rhombus) for a honeycomb lattice fails to establish a k-vector in the 2D space, which is required for the Bloch electron dynamics. Phonon motion cannot be discussed in the triangular coordinates, either. In this paper, we propose a rectangular 4-atom unit cell model, which allows us to discuss the electron and phonon (wave packets) motion in the k-space. The present paper discusses the band structure of graphene based on the rectangular 4-atom unit cell model to establish an appropriate k-vector for the Bloch electron dynamics. To obtain the band energy of a Bloch electron in graphene, we extend the tight-binding calculations for the Wigner-Seitz (2-atom unit cell) model of Reich et al. (Physical Review B, 66, Article ID: 035412 (2002)) to the rectangular 4-atom unit cell model. It is shown that the graphene band structure based on the rectangular 4-atom unit cell model reveals the same band structure of the graphene based on the Wigner-Seitz 2-atom unit cell model;the π-band energy holds a linear dispersion (ε−k ) relations near the Fermi energy (crossing points of the valence and the conduction bands) in the first Brillouin zone of the rectangular reciprocal lattice. We then confirm the suitability of the proposed rectangular (orthogonal) unit cell model for graphene in order to establish a 2D k-vector responsible for the Bloch electron (wave packet) dynamics in graphene.
基金Project supported by the National Natural Science Foundation of China(Grant No.91850121)the K.C.Wong Education Foundation(Grant No.GJTD-2019-15)
文摘We theoretically investigate high-order harmonic generation(HHG)in crystals induced by linearly polarized laser fields.We obtain the HHG spectra by solving the semiconductor Bloch equations and analyze the radiation process by different models.Here we propose a multiple collision model,in which the electrons and holes are produced at different times and places.It is found that the multiple collision trajectories can help us comprehensively and better explain the results of the quantum calculation.Moreover,we find that the harmonic suppression occurs due to the overlap of multiple collision trajectories.
文摘The conduction of a single-wall carbon nanotube depends on the pitch. If there are an integral number of carbon hexagons per pitch, then the system is periodic along the tube axis and allows “holes” (not “electrons”) to move inside the tube. This case accounts for a semiconducting behavior with the activation energy of the order of around 3 meV. There is a distribution of the activation energy since the pitch and the circumference can vary. Otherwise nanotubes show metallic behaviors (significantly higher conductivity). “Electrons” and “holes” can move in the graphene wall (two dimensions). The conduction in the wall is the same as in graphene if the finiteness of the circumference is disregarded. Cooper pairs formed by the phonon exchange attraction moving in the wall is shown to generate a temperature-independent conduction at low temperature (3 - 20 K).
文摘Individual metallic single-wall carbon nanotubes show unusual non-Ohmic transport behaviors at low and high bias fields. For low-resistance contact samples, the differential conductance increases with increasing bias, reaching a maximum at ~100 mV. As the bias increases further, drops dramatically [1]. The higher the bias, the system behaves in a more normal (Ohmic) manner. This low-bias anomaly is temperature-dependent (50 - 150 K). We propose a new interpretation. Supercurrents run in the graphene wall below ~150 K. The normal hole currents run on the outer surface of the wall, which are subject to the scattering by phonons and impurities. The currents along the tube length generate circulating magnetic fields and eventually destroy the supercurrent in the wall at high enough bias, and restore the Ohmic behavior. If the prevalent ballistic electron model is adopted, then the temperature-dependent scattering effects cannot be discussed. For the high bias (0.3 - 5 V), (a) the I-V curves are temperature-independent (4 - 150 K), and (b) the currents (magnitudes) saturate. The behavior (a) arises from the fact that the neutral supercurrent below the critical temperature is not accelerated by the electric field. The behavior (b) is caused by the limitation of the number of quantum-states for the “holes” running outside of the tube.
