137 research outputs found
Irreversible magnetization switching using surface acoustic waves
An analytical and numerical approach is developped to pinpoint the optimal
experimental conditions to irreversibly switch magnetization using surface
acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and
two switching mechanisms are considered. In precessional switching, a small
in-plane field initially tilts the magnetization and the passage of the SAW
modifies the magnetic anisotropy parameters through inverse magneto-striction,
which triggers precession, and eventually reversal. Using the micromagnetic
parameters of a fully characterized layer of the magnetic semiconductor
(Ga,Mn)(As,P), we then show that there is a large window of accessible
experimental conditions (SAW amplitude/wave-vector, field
amplitude/orientation) allowing irreversible switching. As this is a resonant
process, the influence of the detuning of the SAW frequency to the magnetic
system's eigenfrequency is also explored. Finally, another - non-resonant -
switching mechanism is briefly contemplated, and found to be applicable to
(Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small
perpendicular field is applied opposite the initial magnetization and the
passage of the SAW lowers the domain nucleation barrier.Comment: 11 pages, 4 figure
Determination of the micromagnetic parameters in (Ga,Mn)As using domain theory
The magnetic domain structure and magnetic properties of a ferromagnetic
(Ga,Mn)As epilayer with perpendicular magnetic easy-axis are investigated. We
show that, despite strong hysteresis, domain theory at thermodynamical
equilibrium can be used to determine the micromagnetic parameters. Combining
magneto-optical Kerr microscopy, magnetometry and ferromagnetic resonance
measurements, we obtain the characteristic parameter for magnetic domains
, the domain wall width and specific energy, and the spin stiffness
constant as a function of temperature. The nucleation barrier for magnetization
reversal and the Walker breakdown velocity for field-driven domain wall
propagation are also estimated
Deep and shallow electronic states associated to doping, contamination and intrinsic defects in ε-Ga2O3 epilayers
Deep and shallow electronic states in undoped and Si-doped ε-Ga2O3 epilayers grown by MOVPE on c-oriented Al2O3 were investigated by cathodoluminescence, optical absorption, photocurrent spectroscopy, transport measurements, and electron-paramagnetic-resonance. Nominally undoped films were highly resistive, with a room temperature resistivity varying in the range 107- 1013 Ωcm depending on the carrier gas used during growth. Films grown with He carrier were generally more resistive than those grown with H2 carrier and exhibited a Fermi level located at about 0.8 eV below the conduction band edge, which tends to shift deeper with temperature. This can tentatively be attributed to the combined action of deep donors (probably carbon impurities and oxygen vacancies) and deep acceptors (Ga vacancies and related complexes), which compensate residual shallow donors. There are strong experimental hints that nitrogen also behaves as deep acceptor. Room temperature resistivity as low as 0.42 Ωcm and electron concentrations around 1018 cm−3 were obtained by silicon doping. Si was confirmed to act as shallow donor with sufficiently high solubility. A variable range hopping conduction was observed in a wide temperature interval in the n-type layers, and compensation by native acceptors also plays a major role on conduction mechanisms. Previous evaluations of curvature and anisotropy of the conduction band are confirmed, which allows for the estimation of the electron effective mass. The present experimental data are discussed considering the theoretical predictions for point defect formation in the ε-polymorph as well as literature data on extrinsic and intrinsic defects in β-Ga2O3
Coherent manipulation of nitrogen vacancy centers in 4H silicon carbide with resonant excitation
Silicon carbide (SiC) has become a key player in realization of scalable
quantum technologies due to its ability to host optically addressable spin
qubits and wafer-size samples. Here, we have demonstrated optically detected
magnetic resonance (ODMR) with resonant excitation, and clearly identified the
ground state energy levels of the NV centers in 4H-SiC. Coherent manipulation
of NV centers in SiC has been achieved with Rabi and Ramsey oscillations.
Finally, we show the successful generation and characterization of single
nitrogen vacancy (NV) center in SiC employing ion implantation. Our results are
highlighting the key role of NV centers in SiC as a potential candidate for
quantum information processing
Crystal structures of self-assembled nanotubes from flexible macrocycles by weak interactions
8 páginas, 7 figuras, 2 tablas, 2 esquemas.Herein we report the crystal structures of tubular self-assemblies of flexible macrooligolides. The assembly is driven by the propensity of the macrocycles to create nearly flat structures displaying a void space within them and the cooperativity of weak directional interactions such as dipole–dipole interactions and CH***Ohydrogen bonds and non-directional interactions such as van der Waals contacts. The significance of the stereochemistry and the size of the cavity in the formation of the nanotubes are also studied.This research was supported by the Spanish MICINN-FEDER
(CTQ2008-03334/BQU, CTQ2008-06806-C02-01/BQU and
CTQ2008-06754-C04-01/PPQ), the MSC (RTICC RD06/0020/
1046) and the Canary Islands FUNCIS (PI 01/06).Peer reviewe
Cathodoluminescence characterization of Ge-doped CdTe crystals
Cathodoluminescence (CL) microscopic techniques have been used to study the spatial distribution of structural defects and the deep levels in CdTe:Ge bulk crystals. The effect of Ge doping with concentrations of 10(17) and 10(19) cm(-3) on the compensation of V-Cd in CdTe has been investigated. Dependence of the intensity distribution of CL emission bands on the dopant concentration has been studied. Ge doping causes a substantial reduction of the generally referred to 1.40 eV luminescence, which is often present in undoped CdTe crystals, and enhances the 0.91 and 0.81 eV emissions
Enhancement of the Electron Spin Resonance of Single-Walled Carbon Nanotubes by Oxygen Removal
We have observed a nearly fourfold increase in the electron spin resonance
(ESR) signal from an ensemble of single-walled carbon nanotubes (SWCNTs) due to
oxygen desorption. By performing temperature-dependent ESR spectroscopy both
before and after thermal annealing, we found that the ESR in SWCNTs can be
reversibly altered via the molecular oxygen content in the samples. Independent
of the presence of adsorbed oxygen, a Curie-law (spin susceptibility ) is seen from 4 K to 300 K, indicating that the probed spins are
finite-level species. For both the pre-annealed and post-annealed sample
conditions, the ESR linewidth decreased as the temperature was increased, a
phenomenon we identify as motional narrowing. From the temperature dependence
of the linewidth, we extracted an estimate of the intertube hopping frequency;
for both sample conditions, we found this hopping frequency to be 100
GHz. Since the spin hopping frequency changes only slightly when oxygen is
desorbed, we conclude that only the spin susceptibility, not spin transport, is
affected by the presence of physisorbed molecular oxygen in SWCNT ensembles.
Surprisingly, no linewidth change is observed when the amount of oxygen in the
SWCNT sample is altered, contrary to other carbonaceous systems and certain 1D
conducting polymers. We hypothesize that physisorbed molecular oxygen acts as
an acceptor (-type), compensating the donor-like (-type) defects that are
responsible for the ESR signal in bulk SWCNTs.Comment: 14 pages, 7 figure
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