arXiv subjects

C. Thomsen

Publications and source records attributed to C. Thomsen.

15 recordsLinked to original sources

arXiv2008

Carbon nanotube as a nanoscale Cherenkov--type light emitter - nanoFEL

A mechanism of stimulated emission of electromagnetic radiation by an electron beam in carbon nanotubes is theoretically considered. Three basic properties of carbon nanotubes, a strong slowing down of surface electromagnetic waves, ballisticity of the electron motion over typical nanotube length, and extremely high electron current density reachable in nanotubes, allow proposing them as candidates for the development of nano-scale Chernekov-type emitters, analogous to traveling wave tube and free electron laser. Dispersion equations of the electron beam instability and the threshold conditions of the stimulated emission have been derived and analyzed, demonstrating realizability of the nanotube-based nanoFEL at realistic parameters of nanotubes and electronic beams.

cond-mat.mes-hall↗

arXiv2008

Analysis of Multiwalled Carbon Nanotubes as Waveguides and Antennas in the Infrared and the Visible Regimes

The propagation of azimuthally symmetric guided waves in multiwalled carbon nanotubes (MWCNTs) was analyzed theoretically in the mid-infrared and the visible regimes. The MWCNTs were modeled as ensembles of concentric, cylindrical, conducting shells. Slightly attenuated guided waves and antenna resonances due to the edge effect exist for not-too-thick MWCNTs in the far- and mid-infrared regimes. Interband transitions hinder the propagation of guided waves and have a deleterious effect on the performance of a finite-length MWCNT as an antenna. Propagation of surface-plasmon waves along an MWCNT with a gold core was also analyzed. In the near-infrared and the visible regimes, the shells behave effectively as lossy dielectrics suppressing surface-plasmon-wave propagation along the gold core.

cond-mat.mtrl-sci↗

arXiv2008

First-principles calculations of the vibrational properties of bulk CdSe and CdSe nanowires

We present first-principles calculations on bulk CdSe and CdSe nanowires with diameters of up to 22 Å. Density functional linear combination of atomic orbitals and plane wave calculations of the electronic and structural properties are presented and discussed. We use an iterative, symmetry-based method to relax the structures into the ground state. We find that the band gap depends on surface termination. Vibrational properties in the whole Brillouin zone of bulk CdSe and the zone-center vibrations of nanowires are calculated and analyzed. We find strongly size-dependent and nearly constant modes, depending on the displacement directions. A comparison with available experimental Raman data is be given.

cond-mat.mtrl-sci↗

arXiv2008

Magnetic excitations in SrCu2O3: a Raman scattering study

We investigated temperature dependent Raman spectra of the one-dimensional spin-ladder compound SrCu2O3. At low temperatures a two-magnon peak is identified at 3160+/-10 cm^(-1) and its temperature dependence analyzed in terms of a thermal expansion model. We find that the two-magnon peak position must include a cyclic ring exchange of J_cycl/J_perp=0.09-0.25 with a coupling constant along the rungs of J_perp approx. 1215 cm^(-1) (1750 K) in order to be consistent with other experiments and theoretical results.

cond-mat.str-el↗

arXiv2007

The phonon dispersion of graphite by inelastic x-ray scattering

We present the full in-plane phonon dispersion of graphite obtained from inelastic x-ray scattering, including the optical and acoustic branches, as well as the mid-frequency range between the $K$ and $M$ points in the Brillouin zone, where experimental data have been unavailable so far. The existence of a Kohn anomaly at the $K$ point is further supported. We fit a fifth-nearest neighbour force-constants model to the experimental data, making improved force-constants calculations of the phonon dispersion in both graphite and carbon nanotubes available.

cond-mat.mtrl-sci↗

arXiv2006

Evidence of breakdown of the spin symmetry in diluted 2D electron gases

Recent claims of an experimental demonstration of spontaneous spin polarisation in dilute electron gases \cite{young99} revived long standing theoretical discussions \cite{ceper99,bloch}. In two dimensions, the stabilisation of a ferromagnetic fluid might be hindered by the occurrence of the metal-insulator transition at low densities \cite{abra79}. To circumvent localisation in the two-dimensional electron gas (2DEG) we investigated the low populated second electron subband, where the disorder potential is mainly screened by the high density of the first subband. This letter reports on the breakdown of the spin symmetry in a 2DEG, revealed by the abrupt enhancement of the exchange and correlation terms of the Coulomb interaction, as determined from the energies of the collective charge and spin excitations. Inelastic light scattering experiments and calculations within the time-dependent local spin-density approximation give strong evidence for the existence of a ferromagnetic ground state in the diluted regime.

cond-mat.mtrl-sci↗

arXiv2005

Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiral-index assignment

We present a comprehensive study of the chiral-index assignment of carbon nanotubes in aqueous suspensions by resonant Raman scattering of the radial breathing mode. We determine the energies of the first optical transition in metallic tubes and of the second optical transition in semiconducting tubes for more than 50 chiral indices. The assignment is unique and does not depend on empirical parameters. The systematics of the so-called branches in the Kataura plot are discussed; many properties of the tubes are similar for members of the same branch. We show how the radial breathing modes observed in a single Raman spectrum can be easily assigned based on these systematics. In addition, empirical fits provide the energies and radial breathing modes for all metallic and semiconducting nanotubes with diameters between 0.6 and 1.5 nm. We discuss the relation between the frequency of the radial breathing mode and tube diameter. Finally, from the Raman intensities we obtain information on the electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2005

Raman Response of Magnetic Excitations in Cuprate Ladders and Planes

An unified picture for the Raman response of magnetic excitations in cuprate spin-ladder compounds is obtained by comparing calculated two-triplon Raman line-shapes with those of the prototypical compounds SrCu2O3 (Sr123), Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the two-leg ladder contains Heisenberg exchange couplings J_parallel and J_perp plus an additional four-spin interaction J_cyc. Within this model Sr123 and Sr14 can be described by x:=J_parallel/J_perp=1.5, x_cyc:=J_cyc/J_perp=0.2, J_perp^Sr123=1130 cm^-1 and J_perp^Sr14=1080 cm^-1. The couplings found for La6Ca8 are x=1.2, x_cyc=0.2, and J_perp^La6Ca8=1130 cm^-1. The unexpected sharp two-triplon peak in the ladder materials compared to the undoped two-dimensional cuprates can be traced back to the anisotropy of the magnetic exchange in rung and leg direction. With the results obtained for the isotropic ladder we calculate the Raman line-shape of a two-dimensional square lattice using a toy model consisting of a vertical and a horizontal ladder. A direct comparison of these results with Raman experiments for the two-dimensional cuprates R2CuO4 (R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O(6+delta) yields a good agreement for the dominating two-triplon peak. We conclude that short range quantum fluctuations are dominating the magnetic Raman response in both, ladders and planes. We discuss possible scenarios responsible for the high-energy spectral weight of the Raman line-shape, i.e. phonons, the triple-resonance and multi-particle contributions.

cond-mat.str-el↗

arXiv2005

Exciton binding energies in carbon nanotubes from two-photon photoluminescence

One- and two-photon luminescence excitation spectroscopy showed a series of distinct excitonic states in single-walled carbon nanotubes. The energy splitting between one- and two-photon-active exciton states of different wavefunction symmetry is the fingerprint of excitonic interactions in carbon nanotubes. We determine exciton binding energies of 0.3-0.4 eV for different nanotubes with diameters between 0.7 and 0.9 nm. Our results, which are supported by ab-initio calculations of the linear and non-linear optical spectra, prove that the elementary optical excitations of carbon nanotubes are strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole pairs, stable even at room temperature. This alters our microscopic understanding of both the electronic structure and the Coulomb interactions in carbon nanotubes, and has direct impact on the optical and transport properties of novel nanotube devices.

cond-mat.mtrl-sci↗

arXiv2004

Chirality distribution and transition energies of carbon nanotubes

From resonant Raman scattering on isolated nanotubes we obtained the optical transition energies, the radial breathing mode frequency and Raman intensity of both metallic and semiconducting tubes. We unambiguously assigned the chiral index (n_1,n_2) of approximately 50 nanotubes based solely on a third-neighbor tight-binding Kataura plot and find omega_RBM=214.4cm^-1nm/d+18.7cm^-1. In contrast to luminescence experiments we observe all chiralities including zig-zag tubes. The Raman intensities have a systematic chiral-angle dependence confirming recent ab-initio calculations.

cond-mat.mtrl-sci↗

arXiv2004

The strength of the radial-breathing mode in single-walled carbon nanotubes

We show by ab initio calculations that the electron-phonon coupling matrix element M of the radial breathing mode in single-walled carbon nanotubes depends strongly on tube chirality. For nanotubes of the same diameter the coupling strength |M|^2 is up to one order of magnitude stronger for zig-zag than for armchair tubes. For (n,m) tubes M depends on the value of (n-m) mod 3, which allows to discriminate semiconducting nano tubes with similar diameter by their Raman scattering intensity. We show measured resonance Raman profiles of the radial breathing mode which support our theoretical predictions.

cond-mat.mtrl-sci↗

arXiv2004

Evidence of spontaneous spin polarization in the two-dimensional electron gas

Density-functional calculations using an exact exchange potential for a two-dimensional electron gas (2DEG) formed in a GaAs single quantum well predict the existence of a spin-polarized phase, when an excited subband becomes slightly populated. Direct experimental evidence is obtained from low temperature and low excitation-power photoluminescence (PL) spectra which display the sudden appearance of a sharp emission peak below the energy of the optical transition from the first excited electron subband upon its occupation. The behavior of this PL feature in magnetic fields applied in-plane as well as perpendicular to the 2DEG indicate the formation of spin-polarized domains in the excited subband with in-plane magnetization. For it speaks also the strong enhancement of exchange-vertex corrections observed in inelastic light scattering spectra by spin-density excitations of a slightly occupied first-excited subband.

cond-mat.mes-hall↗

arXiv2001

Exchange instability of the two-dimensional electron gas in semiconductor quantum wells

A two-dimensional (2D) electron gas formed in a modulation-doped GaAs/AlGaAs single quantum well undergoes a first-order transition when the first excited subband is occupied with electrons, as the Fermi level is tuned into resonance with the excited subband by applying a dc voltage. Direct evidence for this effect is obtained from low-temperature photoluminescence spectra which display the sudden renormalization of the intersubband energy $E_{01}$ upon the abrupt occupation of the first excited subband. Calculations within density-functional theory, which treat the 2D exchange potential {\it exactly}, show that this thermodynamical instability of the electron system is mainly driven by {\it intersubband} terms of the exchange Coulomb interaction. From temperature-dependent measurements the existence of a critical point at $T_c = 35\pm 5$ K is inferred.

cond-mat.mes-hall↗

arXiv2000

Polarized far-infrared and Raman spectra of SrCuO2 single crystals

We measured polarized far-infrared reflectivity and Raman scattering spectra of SrCuO$_2$ single crystals. The frequencies for infrared-active modes were determined using an oscillator-fitting procedure of reflectivity data. The Raman spectra were measured at different temperatures using several laser energies $ω_L$. In addition to eight of twelve Raman active modes, predicted by factor-group analysis, we observed a complex structure in the Raman spectra for polarization parallel to the {\bf c}-axis, which consists of Raman-allowed A$_g$ symmetry modes, and B$_{1u}$ LO infrared-active (Raman-forbidden) modes of the first and higher order as well as their combinations. The Raman-forbidden modes have a stronger intensity at higher $ω_L$ than the Raman-allowed ones. In order to explain this resonance effect, we measured the dielectric function and optical reflection spectra of SrCuO$_2$ in the visible range. We show that the Raman-allowed A$_g$ symmetry modes are resonantly enhanced when a laser energy is close to $E_0$, while Raman-forbidden (IR-active) modes resonate strongly for laser line energies close to the electronic transition of higher energy gaps.

cond-mat.str-el↗

arXiv2000

Enhanced vortex damping by eddy currents in superconductor-semiconduc tor hybrids

An enhancement of vortex-motion damping in thin Pb/In superconducting films is obtained through coupling to an adjacent two-dimensional electron gas formed in a modulation-doped GaAs/AlGaAs heterostructure. This effect is observed by monitoring the power dissipation at the superconductor in the vortex state while increasing the density of the electron gas using a gate voltage. Quantitative agreement is found with calculations based on a viscous model of vortex damping which considers generation of eddy currents in the electron gas by moving flux lines. In the regime of filamentary and channel vortex flow, eddy-current damping leads to striking dissipation breakdown due to stopping of entire vortex channels.

cond-mat.supr-con↗