123 research outputs found
Dynamics of liquid crystalline domains in magnetic field
We study microscopic single domains nucleating and growing within the
coexistence region of the Isotropic (I) and Nematic (N) phases in magnetic
field. By rapidly switching on the magnetic field the time needed to align the
nuclei of sufficiently large size is measured, and is found to decrease with
the square of the magnetic field. When the field is removed the disordering
time is observed to last on a longer time scale. The growth rate of the nematic
domains at constant temperature within the coexistence region is found to
increase when a magnetic field is applied.Comment: 10 pages, 5 figures, unpublishe
Kinetics of indirect excitons in the optically-induced exciton trap
We report on the kinetics of a low-temperature gas of indirect excitons in
the optically-induced exciton trap. The excitons in the region of laser
excitation are found to rapidly -- within 4 ns -- cool to the lattice
temperature T = 1.4 K, while the excitons at the trap center are found to be
cold -- essentially at the lattice temperature -- even during the excitation
pulse. The loading time of excitons to the trap center is found to be about 40
ns, longer than the cooling time yet shorter than the lifetime of the indirect
excitons. The observed time hierarchy is favorable for creating a dense and
cold exciton gas in optically-induced traps and for in situ control of the gas
by varying the excitation profile in space and time before the excitons
recombine.Comment: 4 pages, 3 figure
High Field Anomalies of Equilibrium and Ultrafast Magnetism in Rare-Earth-Transition Metal Ferrimagnets
Magneto-optical spectroscopy in fields up to 30 Tesla reveals anomalies in
the equilibrium and ultrafast magnetic properties of the ferrimagnetic
rare-earth-transition metal alloy TbFeCo. In particular, in the vicinity of the
magnetization compensation temperature, each of the magnetizations of the
antiferromagnetically coupled Tb and FeCo sublattices show triple hysteresis
loops. Contrary to state-of-the-art theory, which explains such loops by sample
inhomogeneities, here we show that they are an intrinsic property of the
rare-earth ferrimagnets. Assuming that the rare-earth ions are paramagnetic and
have a non-zero orbital momentum in the ground state and, therefore, a large
magnetic anisotropy, we are able to reproduce the experimentally observed
behavior in equilibrium. The same theory is also able to describe the
experimentally observed critical slowdown of the spin dynamics in the vicinity
of the magnetization compensation temperature, emphasizing the role played by
the orbital momentum in static and ultrafast magnetism of ferrimagnets
Micropatterned Electrostatic Traps for Indirect Excitons in Coupled GaAs Quantum Wells
We demonstrate an electrostatic trap for indirect excitons in a field-effect
structure based on coupled GaAs quantum wells. Within the plane of a double
quantum well indirect excitons are trapped at the perimeter of a SiO2 area
sandwiched between the surface of the GaAs heterostructure and a
semitransparent metallic top gate. The trapping mechanism is well explained by
a combination of the quantum confined Stark effect and local field enhancement.
We find the one-dimensional trapping potentials in the quantum well plane to be
nearly harmonic with high spring constants exceeding 10 keV/cm^2.Comment: 21 pages, 6 figures, submitted to Phys. Rev.
Anharmonic magnetic deformation of self-assembled molecular nanocapsules
High magnetic fields were used to deform spherical nanocapsules,
self-assembled from bola-amphiphilic sexithiophene molecules. At low fields the
deformation -- measured through linear birefringence -- scales quadratically
with the capsule radius and with the magnetic field strength. These data
confirm a long standing theoretical prediction (W. Helfrich, Phys. Lett. {\bf
43A}, 409 (1973)), and permits the determination of the bending rigidity of the
capsules as (2.60.8) J. At high fields, an enhanced
rigidity is found which cannot be explained within the Helfrich model. We
propose a complete form of the free energy functional that accounts for this
behaviour, and allows discussion of the formation and stability of nanocapsules
in solution.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Let
Definitive observation of the dark triplet ground state of charged excitons in high magnetic fields
The ground state of negatively charged excitons (trions) in high magnetic
fields is shown to be a dark triplet state, confirming long-standing
theoretical predictions. Photoluminescence (PL), reflection, and PL excitation
spectroscopy of CdTe quantum wells reveal that the dark triplet trion has lower
energy than the singlet trion above 24 Tesla. The singlet-triplet crossover is
"hidden" (i.e., the spectral lines themselves do not cross due to different
Zeeman energies), but is confirmed by temperature-dependent PL above and below
24 T. The data also show two bright triplet states.Comment: 4 figure
Distinctive g-factor of moire-confined excitons in van der Waals heterostructures
We investigated experimentally the valley Zeeman splitting of excitonic peaks
in the photoluminescence (PL) spectra of high-quality hBN/WS2/MoSe2/hBN
heterostructures at near-zero twist angles under perpendicular magnetic fields
up to 20 T. We identify two neutral exciton peaks in the PL spectra: the lower
energy one exhibits a reduced g-factor relative to that of the higher energy
peak, and much lower than the recently reported values for interlayer excitons
in other van der Waals (vdW) heterostructures. We provide evidence that such a
discernible g-factor stems from the spatial confinement of the exciton in the
potential landscape created by the moire pattern, due tolattice mismatch and/or
inter-layer twist in heterobilayers. This renders magneto-PL an important tool
to reach deeper understanding of the effect of moire patterns on excitonic
confinement in vdW heterostructures.Comment: 6 pages, 3 figures. Submitte
Unusual spin properties of InP wurtzite nanowires revealed by Zeeman splitting spectroscopy
In this study, we present a complete experimental and theoretical
investigation of the fundamental exciton Zeeman splitting in wurtzite InP
nanowires. We determined the exciton gyromagnetic factor, , by
magneto-photoluminescence spectroscopy using magnetic fields up to 29 T. We
found that is strongly anisotropic with values differing in excess of
50\% between the magnetic field oriented parallel and perpendicular to the
nanowire long axis. Furthermore, for magnetic fields oriented along the
nanowire axis, is nearly three times larger than in bulk zincblende
InP and it shows a marked sublinear dependence on the magnetic field, a common
feature to other non-nitride III-V wurtzite nanowires but not properly
understood. Remarkably, this nonlinearity originates from only one Zeeman
branch characterized by a specific type of light polarization. All the
experimental findings are modeled theoretically by a robust approach combining
the method with the envelope function approximation and including
the electron-hole interaction. We revealed that the nonlinear features arise
due to the coupling between Landau levels pertaining to the A (heavy-hole like)
and B (light-hole like) valence bands of the wurtzite crystal structure. This
general behavior is particularly relevant for the understanding of the spin
properties of several wurtzite nanowires that host the set for the observation
of topological phases potentially at the base of quantum computing platforms
Magnetization of a two-dimensional electron gas with a second filled subband
We have measured the magnetization of a dual-subband two-dimensional electron
gas, confined in a GaAs/AlGaAs heterojunction. In contrast to two-dimensional
electron gases with a single subband, we observe non-1/B-periodic, triangularly
shaped oscillations of the magnetization with an amplitude significantly less
than per electron. All three effects are explained by a
field dependent self-consistent model, demonstrating the shape of the
magnetization is dominated by oscillations in the confining potential.
Additionally, at 1 K, we observe small oscillations at magnetic fields where
Landau-levels of the two different subbands cross.Comment: 4 pages, 4 figure
Magnetic anisotropy of individually addressed spin states
Controlling magnetic anisotropy is a key requirement for the fundamental understanding of molecular magnetism and is a prerequisite for numerous applications in magnetic storage, spintronics, and all-spin logic devices. In order to address the question of molecular magnetic anisotropy experimentally, we have synthesized single crystals of a molecular spin system containing four antiferromagnetically coupled s=5/2 manganese(II) ions. Using low-temperature cantilever magnetometry, we demonstrate the selective population of the
S=0,1,...,10 spin states upon application of magnetic fields up to 33 T and map the magnetic anisotropy of each of these states. We observe a strong dependence of the shape and size of the magnetic anisotropy on the populated spin states, and, in particular, reveal an anisotropy reversal upon going from the lowest to the highest spin state
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