3,505 research outputs found
Tetrahedratic mesophases, chiral order, and helical domains induced by quadrupolar and octupolar interactions
We present an exhaustive account of phases and phase transitions that can be stabilized in the recently introduced generalized Lebwohl-Lasher model with quadrupolar and octupolar microscopic interactions [ L. Longa, G. PajÄ…k and T. Wydro Phys. Rev. E 79 040701 (2009)]. A complete mean-field analysis of the model, along with Monte Carlo simulations allows us to identify four distinct classes of the phase diagrams with a number of multicritical points where, in addition to the standard uniaxial and biaxial nematic phases, the other nematic like phases are stabilized. These involve, among the others, tetrahedratic (T), nematic tetrahedratic (NT), and chiral nematic tetrahedratic (NT*) phases of global Td, D2d, and D2 symmetry, respectively. Molecular order parameters and correlation functions in these phases are determined. We conclude with generalizations of the model that give a simple molecular interpretation of macroscopic regions with opposite optical activity (ambidextrous chirality), observed, e.g., in bent-core systems. An estimate of the helical pitch in the NT* phase is also given
Intrinsic optical dichroism in the chiral superconducting state of SrRuO
We present an analysis of the Hall conductivity in
time reversal symmetry breaking states of exotic superconductors. We find that
the dichroic signal is non-zero in systems with inter-band order parameters.
This new intrinsic mechanism may explain the Kerr effect observed in strontium
ruthenate and possibly other superconductors. We predict coherence factor
effects in the temperature dependence of the imaginary part of the ac Hall
conductivity , which can be tested experimentally.Comment: 4+ pages, 4 figures, published versio
Electron gun for diffraction experiments off controlled molecules
A dc electron gun, generating picosecond pulses with up to
electrons per pulse, was developed. Its applicability for future
time-resolved-diffraction experiments on state- and conformer-selected
laser-aligned or oriented gaseous samples was characterized. The focusing
electrodes were arranged in a velocity-map imaging spectrometer configuration.
This allowed to directly measure the spatial and velocity distributions of the
electron pulses emitted from the cathode. The coherence length and pulse
duration of the electron beam were characterized by these measurements combined
with electron trajectory simulations. Electron diffraction data off a thin
aluminum foil illustrated the coherence and resolution of the electron-gun
setup
The Kerr rotation in the unconventional superconductor SrRuO
The interpretation of Kerr rotation measurements in the superconducting phase
of SrRuO is a controversial topic. Both intrinsic and extrinsic
mechanisms have been proposed, and it has been argued that the intrinsic
response vanishes by symmetry. We focus on the intrinsic contribution and
clarify several conflicting results in the literature. On the basis of symmetry
considerations and detailed calculations we show that the intrinsic Kerr signal
is not forbidden in a general multi- band system but has a rich structure in
the near infrared regime. We distinguish different optical transitions
determined by the superconducting gap (far infrared) and the inter orbital
coupling of the normal state (near infrared). We argue that the low frequency
transitions do not contribute to the Hall conductivity while only the
inter-orbital transitions in the near infrared regime contribute. Finally, we
discuss the difficulties to connect the calculations for the optical Hall
conductivity to the experimental measurement of the Kerr angle. We will compare
different approximations which might lead to conflicting results.Comment: 9 pages, 8 figures, 1 tabl
Subnormalized states and trace-nonincreasing maps
We investigate the set of completely positive, trace-nonincreasing linear
maps acting on the set M_N of mixed quantum states of size N. Extremal point of
this set of maps are characterized and its volume with respect to the
Hilbert-Schmidt (Euclidean) measure is computed explicitly for an arbitrary N.
The spectra of partially reduced rescaled dynamical matrices associated with
trace-nonincreasing completely positive maps belong to the N-cube inscribed in
the set of subnormalized states of size N. As a by-product we derive the
measure in M_N induced by partial trace of mixed quantum states distributed
uniformly with respect to HS-measure in .Comment: LaTeX, 21 pages, 4 Encapsuled PostScript figures, 1 tabl
Spin-nematic order in the frustrated pyrochlore-lattice quantum rotor model
As an example of ordering due to quantum fluctuations, we examine the
nearest-neighbor antiferromagnetic quantum O(n) rotor model on the pyrochlore
lattice. Classically, this system remains disordered even at zero temperature;
we find that adding quantum fluctuations induces an ordered phase that survives
to positive temperature, and we determine how its phase diagram scales with the
coupling constant and the number of spin components. We demonstrate, using
quantum Monte Carlo simulations, that this phase has long-range spin-nematic
order, and that the phase transition into it appears to be first order.Comment: 10 pages, 8 figure
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