arXiv subjects

Eddwi H. Hasdeo

Publications and source records attributed to Eddwi H. Hasdeo.

28 records · Page 2Linked to original sources

arXiv2018

Intra-cell dynamics and cyclotron motion without magnetic field

Intra-cell motion endows rich non-trivial phenomena to a wide variety of quantum materials. The most prominent example is a transverse current in the absence of a magnetic field (i.e. the anomalous Hall effect). Here we show that, in addition to a dc Hall effect, anomalous Hall materials possess circulating currents and cyclotron motion without magnetic field. These are generated from the intricate wavefunction dynamics within the unit cell, and correspond to interband transitions (coherences) in much the same way that cyclotron resonances arise from inter-Landau level transitions in magneto-optics. Curiously, anomalous cyclotron motion exhibits an intrinsic decay in time (even in pristine materials) displaying a characteristic power law decay. This reveals an intrinsic dephasing similar to that of inhomogeneous broadening of spinors. Circulating currents can manifest as the emission of circularly polarized light pulses in response to incident linearly polarized (pulsed) electric field, and provide a direct means of interrogating the intra-unit-cell dynamics of quantum materials.

cond-mat.mes-hall↗

arXiv2017

Long-lived domain wall plasmons in gapped bilayer graphene

Topological domain walls in dual-gated gapped bilayer graphene host edge states that are gate- tunable and valley polarized. Here we predict that plasmonic collective modes can propagate along these topological domain walls even at zero bulk density, and possess a markedly different character from that of bulk plasmons. Strikingly, domain wall plasmons are extremely long-lived, with plasmon lifetimes that can be orders of magnitude larger than the transport scattering time in the bulk. While most pronounced at low temperatures, long domain wall plasmon lifetimes persist even at room temperature with values up to a few picoseconds. Domain wall plasmons possess a rich phenomenology including a wide range of frequencies (up to the mid-infrared), tunable sub-wavelength electro-magnetic confinement lengths, as well as a valley polarization for forward/backward propagating modes. Its unusual features render them a new tool for realizing low-dissipation plasmonics that transcend the restrictions of the bulk.

cond-mat.mes-hall↗

arXiv2016

Selective coherent phonon mode generation in single wall carbon nanotubes

The pulse-train technique within ultrafast pump-probe spectroscopy is theoretically investigated to enhance a specific coherent phonon mode while suppressing the other phonon modes generated in single wall carbon nanotubes (SWNTs). In particular, we focus on the selectivity of the radial breathing mode (RBM) and the G-band for a given SWNT. We find that if the repetition period of the pulse train matches with integer multiple of the RBM phonon period, the RBM amplitude could be kept while the amplitudes of the other modes are suppressed. As for the G-band, when we apply a repetition period of half-integer multiple of the RBM period, the RBM could be suppressed because of destructive interference, while the G-band still survives. It is also possible to keep the G-band and suppress the RBM by applying a repetition period that matches with integer multiple of the G-band phonon period. However, in this case we have to use a large number of laser pulses having a property of "magic ratio" of the G-band and RBM periods.

cond-mat.mes-hall↗

arXiv2016

Broadband Transverse Electric Surface Wave in Silicene

Transverse electric (TE) surface wave in silicine is theoretically investigated. The TE surface wave in silicene is found to exhibit better characteristics compared with that in graphene, in terms of a broader frequency range and more confinement to the surface which originate from the buckled structure of silicene. We found that even undoped silicene can support the TE surface wave. We expect to obtain the similar characteristics of the TE surface wave in other two-dimensional materials that have slightly buckled honeycomb lattice.

cond-mat.mes-hall↗

arXiv2016

Quantum effects in the thermoelectric power factor of low-dimensional semiconductors

We theoretically investigate the interplay between the confinement length $L$ and the thermal de Broglie wavelength $Λ$ to optimize the thermoelectric power factor of semiconducting materials. An analytical formula for the power factor is derived based on the one-band model assuming nondegenerate semiconductors to describe quantum effects on the power factor of the low dimensional semiconductors. The power factor is enhanced for one- and two-dimensional semiconductors when $L$ is smaller than $Λ$ of the semiconductors. In this case, the low-dimensional semiconductors having $L$ smaller than their $Λ$ will give a better thermoelectric performance compared to their bulk counterpart. On the other hand, when $L$ is larger than $Λ$, bulk semiconductors may give a higher power factor compared to the lower dimensional ones.

cond-mat.mes-hall↗

arXiv2016

Fermi energy dependence of first- and second-order Raman spectra in graphene: Kohn anomaly and quantum interference effect

Intensity of the first- and the second-order Raman spectra are calculated as a function of the Fermi energy. We show that the Kohn anomaly effect, i.e., phonon frequency renormalization, in the first-order Raman spectra originates from the phonon renormalization by the interband electron-hole excitation, whereas in the second-order Raman spectra, a competition between the interband and intraband electron-hole excitations takes place. By this calculation, we confirm the presence of different dispersive behaviors of the Raman peak frequency as a function of the Fermi energy for the first- and the second-order Raman spectra, as observed in experiments. Moreover, the calculated results of the Raman intensity sensitively depend on the Fermi energy for both the first- and the second-order Raman spectra. These results thus also show the importance of quantum interference effect phenomena.

cond-mat.mes-hall↗

arXiv2015

Diameter dependence of thermoelectric power of semiconducting carbon nanotubes

We calculate the thermoelectric power (or thermopower) of many semiconducting single wall carbon nanotubes (s-SWNTs) within a diameter range 0.5-1.5 nm by using the Boltzmann transport formalism combined with an extended tight-binding model. We find that the thermopower of s-SWNTs increases as the tube diameter decreases. For some s-SWNTs with diameters less than 0.6 nm, the thermopower can reach a value larger than 2000 μV/K at room temperature, which is about 6 to 10 times larger than that found in commonly used thermoelectric materials. The large thermopower values may be attributed to the one-dimensionality of the nanotubes and to the presence of large band gaps of the small-diameter s-SWNTs. We derive an analytical formula to reproduce the numerical calculation of the thermopower and we find that the thermopower of a given s-SWNT is directly related with its band gap. The formula also explains the shape of the thermopower as a function of tube diameter, which looks similar to the shape of the so-called Kataura plot of the band gap dependence on tube diameter.

cond-mat.mes-hall↗

arXiv2014

Breit-Wigner-Fano lineshapes in Raman spectra of graphene

Excitation of electron-hole pairs in the vicinity of the Dirac cone by the Coulomb interaction gives rise to an asymmetric Breit-Wigner-Fano lineshape in the phonon Raman spectra in graphene. This asymmetric lineshape appears due to the interference effect between the phonon spectra and the electron-hole pair excitation spectra. The calculated Breit-Wigner-Fano asymmetric factor 1/qBWF as a function of the Fermi energy shows a V-shaped curve with a minimum value at the charge neutrality point and gives good agreement with the experimental result.

cond-mat.mes-hall↗

arXiv2013

Excitonic effects on coherent phonon dynamics in single wall carbon nanotubes

We discuss how excitons can affect the generation of coherent radial breathing modes in ultrafast spectroscopy of single wall carbon nanotubes. Photoexcited excitons can be localized spatially and give rise to a spatially distributed driving force in real space which involves many phonon wavevectors of the exciton-phonon interaction. The equation of motion for the coherent phonons is modeled phenomenologically by the Klein-Gordon equation, which we solve for the oscillation amplitudes as a function of space and time. By averaging the calculated amplitudes per nanotube length, we obtain time-dependent coherent phonon amplitudes that resemble the homogeneous oscillations that are observed in some pump-probe experiments. We interpret this result to mean that the experiments are only able to see a spatial average of coherent phonon oscillations over the wavelength of light in carbon nanotubes and the microscopic details are averaged out. Our interpretation is justified by calculating the time-dependent absorption spectra resulting from the macroscopic atomic displacements induced by the coherent phonon oscillations. The calculated coherent phonon spectra including excitonic effects show the experimentally observed symmetric peaks at the nanotube transition energies in contrast to the asymmetric peaks that would be obtained if excitonic effects were not included.

cond-mat.mtrl-sci↗

arXiv2013

Origin of electronic Raman scattering and the Fano resonance in metallic carbon nanotubes

Fano resonance spectra for the G band in metallic carbon nanotubes are calculated as a function of laser excitation energy in which the origin of the resonance is given by an interference between the continuous electronic Raman spectra and the discrete phonon spectra. We found that the second-order scattering process of the non-zero q electron-electron interaction is more relevant to the continuous spectra rather than the q = 0 first-order process because the q = 0 direct Coulomb interaction vanishes due to the symmetry of the two sublattices of a nanotube. We also show that the RBM spectra of metallic carbon nanotubes have an asymmetric line shape which previously had been overlooked.

cond-mat.mtrl-sci↗