948 research outputs found
Entrapment of magnetic micro-crystals for on-chip electron spin resonance studies
On-chip Electron Spin Resonance (ESR) of magnetic molecules requires the
ability to precisely position nanosized samples in antinodes of the
electro-magnetic field for maximal magnetic interaction. A method is developed
to entrap micro-crystals containing spins in a well defined location on a
substrate's surface. Traditional cavity ESR measurements are then performed on
a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr spins were
entrapped as well and measured while approaching the lower limit of the ESR
sensitivity. This method suggests the feasibility of on-chip ESR measurements
at dilution refrigerator temperatures by enabling the positioning of samples
atop an on-chip superconducting cavity.Comment: to appear in Journal of Applied Physic
Magnetic strong coupling in a spin-photon system and transition to classical regime
We study the energy level structure of the Tavis-Cumming model applied to an
ensemble of independent magnetic spins coupled to a variable number of
photons. Rabi splittings are calculated and their distribution is analyzed as a
functin of photon number and spin system size . A sharp
transition in the distribution of the Rabi frequency is found at . The width of the Rabi frequency spectrum diverges as
at this point. For increased number of photons , the Rabi
frequencies converge to a value proportional to . This
behavior is interpreted as analogous to the classical spin resonance mechanism
where the photon is treated as a classical field and one resonance peak is
expected. We also present experimental data demonstrating cooperative, magnetic
strong coupling between a spin system and photons, measured at room
temperature. This points towards quantum computing implementation with magnetic
spins, using cavity quantum-electrodynamics techniques.Comment: Received 8 April 2010; revised manuscript received 17 June 2010;
published 14 July 201
Tunable multi-photon Rabi oscillations in an electronic spin system
We report on multi-photon Rabi oscillations and controlled tuning of a
multi-level system at room temperature (S=5/2 for Mn2+:MgO) in and out of a
quasi-harmonic level configuration. The anisotropy is much smaller than the
Zeeman splittings, such as the six level scheme shows only a small deviation
from an equidistant diagram. This allows us to tune the spin dynamics by either
compensating the cubic anisotropy with a precise static field orientation, or
by microwave field intensity. Using the rotating frame approximation, the
experiments are very well explained by both an analytical model and a
generalized numerical model. The calculated multi-photon Rabi frequencies are
in excellent agreement with the experimental data
Multi-photon Rabi oscillations in high spin paramagnetic impurity
We report on multiple photon monochromatic quantum oscillations (Rabi
oscillations) observed by pulsed EPR (Electron Paramagnetic Resonance) of
Mn (S=5/2) impurities in MgO. We find that when the microwave magnetic
field is similar or large than the anisotropy splitting, the Rabi oscillations
have a spectrum made of many frequencies not predicted by the S=1/2 Rabi model.
We show that these new frequencies come from multiple photon coherent
manipulation of the multi-level spin impurity. We develop a model based on the
crystal field theory and the rotating frame approximation, describing the
observed phenomenon with a very good agreement.Comment: International Conference: Resonance in Condensed Matter Altshuler 10
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Structural Analysis of Spiro β-Lactone Proteasome Inhibitors
Spiro β-lactone-based proteasome inhibitors were discovered in the context of an asymmetric catalytic total synthesis of the natural product (+)-lactacystin (1). Lactone 4 was found to be a potent inhibitor of the 26S proteasome, while its C-6 epimer (5) displayed weak activity. Crystallographic studies of the two analogues covalently bound to the 20S proteasome permitted characterization of the important stabilizing interactions between each inhibitor and the proteasome’s key catalytic N-terminal threonine residue. This structural data support the hypothesis that the discrepancy in potency between 4 and 5 may be due to differences in the hydrolytic stabilities of the resulting acyl enzyme complexes.Chemistry and Chemical Biolog
Simulated relationships between regional temperatures and large-scale circulation: 125 kyr BP (Eemian) and the preindustrial period
To investigate relationships between large-scale circulation and regional-scale temperatures during the last (Eemian) interglacial, a simulation with a general circulation model (GCM) under orbital forcing conditions of 125 kyr BP is compared with a simulation forced with the Late Holocene preindustrial conditions. Consistent with previous GCM simulations for the Eemian, higher northern summer 2-m temperatures are found, which are directly related to the different insolation. Differences in the mean circulation are evident such as, for instance, stronger northern winter westerlies toward Europe, which are associated with warmer temperatures in central and northeastern Europe in the Eemian simulation, while the circulation variability, analyzed by means of a principal component analysis of the sea level pressure (SLP) field, is very similar in both periods. As a consequence of the differences in the mean circulation the simulated Arctic Oscillation (AO) temperature signal in the northern winter, on interannual-to-multidecadal time scales, is weaker during the Eemian than today over large parts of the Northern Hemisphere. Correlations between the AO index and the central European temperature (CET) decrease by about 0.2. The winter and spring SLP anomalies over the North Atlantic/European domain that are most strongly linearly linked to the CET cover a smaller area and are shifted westward over the North Atlantic during the Eemian. However, the strength of the connection between CET and these SLP anomalies is similar in both simulations. The simulated differences in the AO temperature signal and in the SLP anomaly, which is linearly linked to the CET, suggest that during the Eemian the link between the large-scale circulation and temperaturesensitive proxy data from Europe may differ from present-day conditions and that this difference should be taken into account when inferring large-scale climate from temperature-sensitive proxy data
Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity
Large-grain Nb has become a viable alternative to fine-grain Nb for the
fabrication of superconducting radio-frequency cavities. In this contribution
we report the results from a heat treatment study of a large-grain 1.5 GHz
single-cell cavity made of "medium purity" Nb. The baseline surface preparation
prior to heat treatment consisted of standard buffered chemical polishing. The
heat treatment in the range 800 - 1400 C was done in a newly designed vacuum
induction furnace. Q0 values of the order of 2x1010 at 2.0 K and peak surface
magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of
(5+-1)1010 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 C.
This is the highest value ever reported at this temperature, frequency and
field. Samples heat treated with the cavity at 1400 C were analyzed by
secondary ion mass spectrometry, secondary electron microscopy, energy
dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a
complex surface composition which includes titanium oxide, increased carbon and
nitrogen content but reduced hydrogen concentration compared to a non
heat-treated sample
Investigation of the Electric Field and Charge Density Distribution of pristine and defective 2D WSe by Differential Phase Contrast Imaging
Most properties of solid materials are defined by their internal electric
field and charge density distributions which so far have been difficult to
measure with sufficient spatial resolution. For 2D materials, the electric
field at the atomic level in particular influences the optoelectronic
properties. In this study, the atomic-scale electric field and charge density
distribution of 2D WSe are revealed by using an emerging microscopy
technique, differential phase contrast (DPC) imaging in the scanning
transmission electron microscope (STEM). Combined with high-angle annular
dark-field imaging the charge density distribution of bi- and trilayers of
WSe is mapped. A measured higher positive charge density located at the
selenium atomic columns compared to the tungsten atomic columns is reported,
and possible reasons are discussed. Furthermore, the change in the electric
field distribution of a selenium point defect in a trilayer is investigated
exhibiting a characteristic electric field distribution in the vicinity of the
defect: there are characteristic regions with locally enhanced and with locally
reduced electric field magnitudes compared to the pristine lattice.Comment: 20 pages including the supplementary information, 3 figures in the
main part and additional 2 figures in the supplementary informatio
Liposomal amphotericin B twice weekly as antifungal prophylaxis in paediatric haematological malignancy patients
AbstractData on antifungal prophylaxis in paediatric cancer patients at high risk for invasive fungal disease (IFD) are scant. Intermittent administration of liposomal amphotericin B (LAMB) has been shown to be safe and effective in adult patients with haematological malignancies. We prospectively evaluated the safety and efficacy of prophylactic LAMB at a dosage of 2.5 mg/kg twice weekly in children at high risk for IFD. Efficacy was compared with that in a historical control group of patients with similar demographic characteristics not receiving LAMB prophylaxis. A total of 46 high-risk patients (24 boys; mean age, 7.7 years) with 187 episodes of antifungal prophylaxis were analysed. The median duration of neutropenia (<500/µL) was 10 days. LAMB was discontinued in four patients because of acute allergic reactions. Median values for creatinine and liver enzymes at end of treatment did not differ significantly from those at baseline. Hypokalaemia (<3.0 mmol/L) occurred with 13.5% of the prophylactic episodes, but was usually mild and always reversible. No proven/probable IFD occurred in patients receiving LAMB prophylaxis. In comparison, five proven and two probable IFDs were observed in 45 historical controls not receiving LAMB prophylaxis (p 0.01). LAMB prophylaxis had no impact on the use of empirical antifungal therapy. Systemic antifungal prophylaxis with LAMB 2.5 mg/kg twice weekly is feasible and safe, and seems to be an effective approach for antifungal prophylaxis in high-risk paediatric cancer patients
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