1,064 research outputs found
A 3D topological insulator quantum dot for optically controlled quantum memory and quantum computing
We present the model of a quantum dot (QD) consisting of a spherical
core-bulk heterostructure made of 3D topological insulator (TI) materials, such
as PbTe/PbSnTe, with bound massless and helical Weyl states
existing at the interface and being confined in all three dimensions. The
number of bound states can be controlled by tuning the size of the QD and the
magnitude of the core and bulk energy gaps, which determine the confining
potential. We demonstrate that such bound Weyl states can be realized for QD
sizes of few nanometers. We identify the spin locking and the Kramers pairs,
both hallmarks of 3D TIs. In contrast to topologically trivial semiconductor
QDs, the confined massless Weyl states in 3D TI QDs are localized at the
interface of the QD and exhibit a mirror symmetry in the energy spectrum. We
find strict optical selection rules satisfied by both interband and intraband
transitions that depend on the polarization of electron-hole pairs and
therefore give rise to the Faraday effect due to Pauli exclusion principle. We
show that the semi-classical Faraday effect can be used to read out spin
quantum memory. When a 3D TI QD is embedded inside a cavity, the single-photon
Faraday rotation provides the possibility to implement optically mediated
quantum teleportation and quantum information processing with 3D TI QDs, where
the qubit is defined by either an electron-hole pair, a single electron spin,
or a single hole spin in a 3D TI QD. Remarkably, the combination of inter- and
intraband transition gives rise to a large dipole moment of up to 450 Debye.
Therefore, the strong-coupling regime can be reached for a cavity quality
factor of in the infrared wavelength regime of around
m.Comment: 19 pages, 11 figures, RevTe
Dynamics of large anisotropic spin in a sub-ohmic dissipative environment close to a quantum-phase transition
We investigate the dynamics of a large anisotropic spin whose easy-axis
component is coupled to a bosonic bath with a spectral function J(\w)\propto
\omega^s. Such a spin complex might be realized in a single-molecular magnet.
Using the non-perturbative renormalization group, we calculate the line of
quantum-phase transitions in the sub-ohmic regime (). These quantum-phase
transitions only occur for integer spin . For half-integer , the low
temperature fixed-point is identical to the fixed-point of the spin-boson model
without quantum-tunneling between the two levels. Short-time coherent
oscillations in the spin decay prevail even into the localized phase in the
sub-ohmic regime. The influence of the reorganization energy and the recurrence
time on the decoherence in the absence of quantum-tunneling is discussed.Comment: 14 pages,7 figure
Carbon Isotope Constraints on the Deglacial CO2 Rise from Ice Cores
The stable carbon isotope ratio of atmospheric CO2 (d13Catm) is a key parameter in deciphering past carbon cycle changes. Here we present d13Catm data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in d13Catm during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the d13Catm evolution. During the Last Glacial Maximum, d13Catm and atmospheric CO2 concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then
Quantum Computing in Molecular Magnets
Shor and Grover demonstrated that a quantum computer can outperform any
classical computer in factoring numbers and in searching a database by
exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm
requires both superposition and entanglement of a many-particle system, the
superposition of single-particle quantum states is sufficient for Grover's
algorithm. Recently, the latter has been successfully implemented using Rydberg
atoms. Here we propose an implementation of Grover's algorithm that uses
molecular magnets, which are solid-state systems with a large spin; their spin
eigenstates make them natural candidates for single-particle systems. We show
theoretically that molecular magnets can be used to build dense and efficient
memory devices based on the Grover algorithm. In particular, one single crystal
can serve as a storage unit of a dynamic random access memory device. Fast
electron spin resonance pulses can be used to decode and read out stored
numbers of up to 10^5, with access times as short as 10^{-10} seconds. We show
that our proposal should be feasible using the molecular magnets Fe8 and Mn12.Comment: 13 pages, 2 figures, PDF, version published in Nature, typos
correcte
Photoluminescence Quenching in Single-layer MoS2 via Oxygen Plasma Treatment
By creating defects via oxygen plasma treatment, we demonstrate optical
properties variation of single-layer MoS2. We found that, with increasing
plasma exposure time, the photoluminescence (PL) evolves from very high
intensity to complete quenching, accompanied by gradual reduction and
broadening of MoS2 Raman modes, indicative of distortion of the MoS2 lattice
after oxygen bombardment. X-ray photoelectron spectroscopy study shows the
appearance of Mo6+ peak, suggesting the creation of MoO3 disordered regions in
the MoS2 flake. Finally, using band structure calculations, we demonstrate that
the creation of MoO3 disordered domains upon exposure to oxygen plasma leads to
a direct to indirect bandgap transition in single-layer MoS2, which explains
the observed PL quenching.Comment: 12 pages, 7 figure
Electron Paramagnetic Resonance Linewidths and Lineshapes for the Molecular Magnets Fe8 and Mn12
We study theoretically Electron Paramagentic Resonance (EPR) linewidths for
single crystals of the molecular magnets Fe and Mn as functions of
energy eigenstates , frequency, and temperature when a magnetic field
along the easy axis is swept at fixed excitation frequency. This work was
motivated by recent EPR experiments. To calculate the linewidths, we use
density-matrix equations, including dipolar interactions and distributions of
the uniaxial anisotropy parameter and the Land\'{e} factor. Our
calculated linewidths agree well with the experimental data. We also examine
the lineshapes of the EPR spectra due to local rotations of the magnetic
anisotropy axes caused by defects in samples. Our preliminary results predict
that this effect leads to asymmetry in the EPR spectra.Comment: 2001 MMM conferenc
The absence of dyspnoea, cough and wheezing: a reason for undiagnosed airflow obstruction?
The diagnosis of obstructive lung disease (OLD) may be overlooked because of the poor correlation between the intensity of symptoms and the severity of airway obstruction (AO). Undiagnosed airflow obstruction (UDAO) is associated with health impairment and mortality. Questions remain such as the reasons for its occurrence and the underlying diseases. In a pulmonologist's private practice, the objectives were to detect UDAO in the absence of dyspnoea, cough and wheezing, to improve its screening following other anamnestic data, and to separate UDAO patients into "silent asthma" (SA) or "persistent obstruction".
Patients were subjected to a verbal questionnaire for the detection of alternative indication for pulmonary function tests (PFTs), to a physical examination and, in the case of a severe smoking habit, to a chest X-ray. PFTs were performed whenever an OLD history or another lung disease was present and, in the absence of any dyspnoea, cough and wheezing, when other symptoms and conditions occurred (sputum, chest tightness, fatigue, rhinitis, snoring; active/passive smoking, recurrent lower respiratory tract infections, asthma in childhood or in family, atopy).
Of 3762 consecutive patients, 1389 patients with AO were identified. Among them, 147 UDAO patients were detected with no history of dyspnoea, cough and wheezing (3.9% and 10.6%, respectively). All these patients had other suggestive symptoms and AO risk factors which justified PFTs. They presented with mild (65%), moderate (21%) or even severe (16%) AO. SA patients normalized their spirometric values under treatment.
The absence of dyspnoea, cough and wheezing is a fairly frequent finding and a reason for UDAO. PFTs are warranted with any suggestive symptoms and AO risk factors. The favourable follow-up underlines the importance of screening for UDAO
Ultrastructural alterations in capillaries of the diabetic hypertensive rat retina: protective effects of ACE inhibition
Aims/hypothesis: The ACE inhibitor cilazapril was administered to diabetic hypertensive rats to evaluate its ability to influence the development of retinal capillary alterations. Methods: Normotensive (strain: Wistar Kyoto) and genetically hypertensive (strain: spontaneously hypertensive) rats were rendered diabetic by intravenous injections of streptozotocin. Half of the diabetic animals received cilazapril with their daily food. At 20 weeks of diabetes, endothelial cells, pericytes and extracellular matrix were assessed by ultrastructural morphometry. Each experimental group consisted of seven animals. Results: Cilazapril normalised systolic arterial pressure in diabetic hypertensive rats (137±2mmHg compared with 188±16mmHg in non-medicated diabetic hypertensive rats, p<0.001). The number of endothelial intercellular junctions was reduced in untreated diabetic hypertensive rats (0.15±0.05, p<0.02, vs 0.47±0.20 in non-diabetic normotensive rats). In diabetic hypertensive animals treated with cilazapril, this loss was attenuated (0.32±0.16, p<0.05). The significant thickening of the basement membrane observed in the diabetic normotensive (132.8±19.4nm) and diabetic hypertensive (150.3±20.2nm) groups was decreased by cilazapril in the diabetic hypertensive group (116.7±11.0nm, p<0.01), but was unaffected in the normotensive (131.9±17.3nm) group. No protective effect of the drug was observed in either group on pericytes. Conclusions/interpretation: Long-term administration of an effective antihypertensive therapy normalises endothelial alterations and basement membrane thickness in diabetic hypertensive conditions, and thus may account for the well-known improvement of the blood-retinal barrier observed during antihypertensive treatmen
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