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Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3
In this study we report on the investigation of epitaxially grown Sb2Te3 by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm−1 highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm−1 within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers
Frustration and Melting of Colloidal Molecular Crystals
Using numerical simulations we show that a variety of novel colloidal
crystalline states and multi-step melting phenomena occur on square and
triangular two-dimensional periodic substrates. At half-integer fillings
different kinds of frustration effects can be realized. A two-step melting
transition can occur in which individual colloidal molecules initially rotate,
destroying the overall orientational order, followed by the onset of interwell
colloidal hopping, in good agreement with recent experiments.Comment: 6 pages, 3 postscript figures. Procedings of International Conference
on Strongly Coupled Coulomb Systems, Santa Fe, 200
Geometric frustration and magnetization plateaus in quantum spin and Bose-Hubbard models on tubes
We study XXZ Heisenberg models on frustrated triangular lattices wrapped
around a cylinder. In addition to having interesting magnetic phases, these
models are equivalent to Bose-Hubbard models that describe the physical problem
of adsorption of noble gases on the surface of carbon nanotubes. We find
analytical results for the possible magnetization plateau values as a function
of the wrapping vectors of the cylinder, which in general introduce extra
geometric frustration besides the one due to the underlying triangular lattice.
We show that for particular wrapping vectors , which correspond to the
zig-zag nanotubes, there is a macroscopically degenerate ground state in the
classical Ising limit. The Hilbert space for the degenerate states can be
enumerated by a mapping first into a path in a square lattice wrapped around a
cylinder (a Bratteli diagram), and then to free fermions interacting with a
single degree of freedom. From this model we obtain the spectrum in
the anisotropic Heisenberg limit, showing that it is gapless. The continuum
limit is a conformal field theory with compactification radius set
by the physical tube radius. We show that the compactification radius
quantization is exact in the projective limit, and that
higher order corrections reduce the value of . The particular case of a
tube, which corresponds to a 2-leg ladder with cross links, is
studied separately and shown to be gapped because the fermion mapped problem
contains superconducting pairing terms.Comment: 10 pages, 11 figure
One-dimensional Kondo lattice at partial band filling
An effective Hamiltonian for the localized spins in the one-dimensional Kondo
lattice model is derived via a unitary transformation involving a bosonization
of delocalized conduction electrons. The effective Hamiltonian is shown to
reproduce all the features of the model as identified in various numerical
simulations, and provides much new information on the ferro- to paramagnetic
phase transition and the paramagnetic phase.Comment: 11 pages Revtex, 1 Postscript figure. To appear in Phys. Rev. Let
Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3
In this study we report on the investigation of epitaxially grown Sb2Te3 by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm−1 highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm−1 within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers
Inter-Edge interaction in the Quantum Hall Effect
We consider effects of the interaction between electrons drifting along the
opposite sides of a narrow sample under the conditions of the quantum Hall
effect. A spatial variation of this interaction leads to backward scattering of
collective excitations propagating along the edges. Experiments on propagation
of the edge modes in samples with constrictions may give information about the
strength of the inter-edge electron interaction in the quantum Hall regime.Comment: 12 Pages, Latex, Accepted for publication in PRL
Persistent currents in mesoscopic rings: A numerical and renormalization group study
The persistent current in a lattice model of a one-dimensional interacting
electron system is systematically studied using a complex version of the
density matrix renormalization group algorithm and the functional
renormalization group method. We mainly focus on the situation where a single
impurity is included in the ring penetrated by a magnetic flux. Due to the
interplay of the electron-electron interaction and the impurity the persistent
current in a system of N lattice sites vanishes faster then 1/N. Only for very
large systems and large impurities our results are consistent with the
bosonization prediction obtained for an effective field theory. The results
from the density matrix renormalization group and the functional
renormalization group agree well for interactions as large as the band width,
even though as an approximation in the latter method the flow of the
two-particle vertex is neglected. This confirms that the functional
renormalization group method is a very powerful tool to investigate correlated
electron systems. The method will become very useful for the theoretical
description of the electronic properties of small conducting ring molecules.Comment: 9 pages, 8 figures include
1/f Noise and Extreme Value Statistics
We study the finite-size scaling of the roughness of signals in systems
displaying Gaussian 1/f power spectra. It is found that one of the extreme
value distributions (Gumbel distribution) emerges as the scaling function when
the boundary conditions are periodic. We provide a realistic example of
periodic 1/f noise, and demonstrate by simulations that the Gumbel distribution
is a good approximation for the case of nonperiodic boundary conditions as
well. Experiments on voltage fluctuations in GaAs films are analyzed and
excellent agreement is found with the theory.Comment: 4 pages, 4 postscript figures, RevTe
Sources of Sex Information Used by Young British Women Who Have Sex with Women (WSW) and Women Who Have Sex Exclusively with Men (WSEM): Evidence from the National Survey of Sexual Attitudes and Lifestyles
There is little consideration about the provision of information about sex to women who have sex with women (WSW). This study drew on data from the third National Survey of Sexual Attitudes and Lifestyle, a nationally representative survey of people in Great Britain. Logistic regression was undertaken to examine firstly the relationships between WSW and women who have sex exclusively with men (WSEM) and their main source of information about sex, and secondly between WSW/WSEM and unmet need for information about sex. Each source was included as the binary outcome indicating yes this was the main source, or no this was not the main source of information about sex. The results found that WSW had significantly lower odds of reporting lessons at schools as their main source of information, and significantly higher odds of reporting sources defined as ‘other’ (predominantly first girlfriend/boyfriend or sexual partner) as their main source of information. Reported levels of unmet need for information was also higher amongst young WSW compared with WSEM. This study provides new insights into the sex educational needs of young women and highlights the need for sex education in schools in Great Britain to include information on a full-range of sexual practices, including same-sex sexual relationships
Vibrational spectra of C60C8H8 and C70C8H8 in the rotor-stator and polymer phases
C60-C8H8 and C70-C8H8 are prototypes of rotor-stator cocrystals. We present
infrared and Raman spectra of these materials and show how the rotor-stator
nature is reflected in their vibrational properties. We measured the
vibrational spectra of the polymer phases poly(C60C8H8) and poly(C70C8H8)
resulting from a solid state reaction occurring on heating. Based on the
spectra we propose a connection pattern for the fullerene in poly(C60C8H8),
where the symmetry of the C60 is D2h. On illuminating the C60-C8H8 cocrystal
with green or blue light a photochemical reaction was observed leading to a
similar product to that of the thermal polymerization.Comment: 26 pages, 8 figures, to appear in Journal of Physical Chemistry B 2nd
version: minor changes in wording, accepted version by journa
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