315 research outputs found
Nonlinear dynamics of polariton scattering in semiconductor microcavity: bistability vs stimulated scattering
We demonstrate experimentally an unusual behavior of the parametric polariton
scattering in semiconductor microcavity under a strong cw resonant excitation.
The maximum of the scattered signal above the threshold of stimulated
parametric scattering does not shift along the microcavity lower polariton
branch with the change of pump detuning or angle of incidence but is stuck
around the normal direction. We show theoretically that such a behavior can be
modelled numerically by a system of Maxwell and nonlinear Schroedinger
equations for cavity polaritons and explained via the competition between the
bistability of a driven nonlinear MC polariton and the instabilities of
parametric polariton-polariton scattering.Comment: 5 pages, 4 Postscript figures; corrected typo
Spin polarization of the magnetic spiral in NaCu_2O_2, as seen by NMR
The incommensurate (IC) spin ordering in quasi-1D edge-shared cuprate
NaCu_2O_2 has been studied by ^{23}Na nuclear magnetic resonance spectroscopy
in an external magnetic field near 6 Tesla applied along the main
crystallographic axes. The NMR lineshape evolution above and below T_N\approx12
K yields a clear signature of an IC static modulation of the local magnetic
field consistent with a Cu^{2+} spin spiral polarized in the bc-plane rather
than in the ab-plane as reported from earlier neutron diffraction data.Comment: 5 pages, 4 figure
121,123Sb NQR as a microscopic probe in Te doped correlated semimetal FeSb2 : emergence of electronic Griffith phase, magnetism and metallic behavior %
nuclear quadrupole resonance (NQR) was applied to
in the low doping regime (\emph{x = 0, 0.01} and
\emph{0.05}) as a microscopic zero field probe to study the evolution of
\emph{3d} magnetism and the emergence of metallic behavior. Whereas the NQR
spectra itself reflects the degree of local disorder via the width of the
individual NQR lines, the spin lattice relaxation rate (SLRR) probes
the fluctuations at the - site. The fluctuations originate either from
conduction electrons or from magnetic moments. In contrast to the semi metal
with a clear signature of the charge and spin gap formation in
, the 1\% doped system exhibits
almost metallic conductivity and a almost filled gap. A weak divergence of the
SLRR coefficient points towards the
presence of electronic correlations towards low temperatures wheras the
\textit{5\%} doped sample exhibits a much larger divergence in the SLRR
coefficient showing . According to the specific heat
divergence a power law with is expected for the SLRR.
Furthermore -doped as a disordered paramagnetic metal might be a
platform for the electronic Griffith phase scenario. NQR evidences a
substantial asymmetric broadening of the NQR spectrum for the
\emph{5\%} sample. This has purely electronic origin in agreement with the
electronic Griffith phase and stems probably from an enhanced - bond
polarization and electronic density shift towards the atom inside
- dumbbell
Theory of condensation of indirect excitons in a trap
We present theoretical studies of condensation of indirect excitons in a
trap. Our model quantifies the effect of screening of the trap potential by
indirect excitons on exciton condensation. The theoretical studies are applied
to a system of indirect excitons in a GaAs/AlGaAs coupled quantum well
structure in a diamond-shaped electrostatic trap where exciton condensation was
studied in earlier experiments. The estimated condensation temperature of the
indirect excitons in the trap reaches hundreds of milliKelvin
Antiferromagnetic ground state in the MnGa intermetallic compound
Magnetism of the binary intermetallic compound MnGa is re-investigated.
Band-structure calculations predict antiferromagnetic behavior in contrast to
Pauli paramagnetism reported previously. Magnetic susceptibility measurements
on single crystals indeed reveal an antiferromagnetic transition at
K. Neutron powder diffraction and Ga nuclear quadrupole resonance
spectroscopy show collinear antiferromagnetic order with magnetic moments
alligned along the [111] direction of the cubic unit cell. The magnetic moment
of 0.80(3) at 1.5 K extracted from the neutron data is in good agreement
with the band-structure results
BaV3O8: A possible Majumdar-Ghosh system with S=1/2
BaV3O8 contains both magnetic V4+(S=1/2) ions and non-magnetic V5+(S=0) ions.
The V4+ ions are arranged in a coupled Majumdar-Ghosh chain like network. Our
magnetic susceptibility chi(T) data fit well with the Curie-Weiss formula in
the temperature range of 80-300K and it yields a Curie constant
C=0.39cm3K/mole-V4+ and an antiferromagnetic Weiss temperature theta=-26K. The
chi(T) curve shows a broad maximum at T~25K indicative of short-range order
(SRO) and an anomaly corresponding to long-range order (LRO) at TN~6K. The
value of the frustration index (f=mod[theta/TN]~5) suggests that the system is
moderately frustrated. Above the LRO temperature the experimental magnetic
susceptibility data match well with the coupled Majumdar-Ghosh chain model with
the ratio of the nnn (next-nearest neighbor) to nn (nearest neighbor) magnetic
coupling alpha=2 and Jnnn/kB=40K. In a mean-field approach when considering the
inter-chain interactions, we obtain the total inter-chain coupling to be about
16K. The LRO anomaly at TN is also observe in the specific heat Cp(T) data and
is not sensitive to an applied magnetic field up to 90kOe. A 51V NMR signal
corresponding to the non-magnetic vanadium was observed. Anomalies at 6K were
observed in the variation with temperature of the 51V NMR linewidth and in the
spin-lattice relaxation rate 1/T1, indicating that they are sensitive to the
LRO onset and fluctuations at the magnetic V sites. The existence of two
components (one short and another long) is observed in the spin-spin relaxation
rate 1/T2 data in the vicinity of TN. The shorter component seems to be
intimately connected with the magnetically ordered state. We suggest that both
magnetically ordered and non-long range ordered (non-LRO) regions coexist in
this compound below the long range ordering temperature.Comment: Accepted in Phys. Rev.
Comment on "Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2"
The microscopic origin of the recently observed helical structure in LiCu2O2
[1] is considered. It is shown that the frustrated antiferromagnetic double
chain scenario adopted in Ref.1 is unrealistic. It should be replaced by a
frustrated single-chain (CuO2) scenario proposed in Ref. 2 with ferromagnetic
nearest neighbor and antiferromagnetic next nearest neighbor exchange integrals
\.
[1] T. Masuda et al. Phys. Rev. Lett. 92 (2004), 177201. [2] A. Gippius et
al. Phys. Rev. B 70 (2004), R01426; cond-mat/0312576.Comment: one page, one figur
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