83 research outputs found
Signature of the Overhauser field on the coherent spin dynamics of donor-bound electron in a single CdTe quantum well
We have studied the coherent spin dynamics in an oblique magnetic field of
electrons localized on donors and placed in the middle of a single CdTe quantum
well, by using a time-resolved optical technique: the photo-induced Faraday
rotation. We showed that this dynamics is affected by a weak Overhauser field
created via the hyperfine interaction of optically spin-polarized donor-bound
electrons with the surrounding nuclear isotopes carrying non-zero spins. We
have measured this nuclear field, which is on the order of a few mT and can
reach a maximum experimental value of 9.4 mT. This value represents 13 % of the
maximal nuclear polarization, and corresponds also to 13 % of maximal
electronic polarization.Comment: 15 pages, 4 figure
Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)
The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of
(Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect
(MOKE). A transient MOKE signal followed by low amplitude oscillations was
evidenced, with a strong dependence on applied magnetic field, temperature and
ps-ASP amplitude. Careful interferometric measurement of the layer's thickness
variation induced by the ps-ASP allowed us to model very accurately the
resulting signal, and interpret it as the strain modulated reflectivity
(differing for probe polarizations), independently from dynamic
magnetization effects.Comment: 6 pages, 5 figure
Hole spin dephasing time associated to hyperfine interaction in quantum dots
The spin interaction of a hole confined in a quantum dot with the surrounding
nuclei is described in terms of an effective magnetic field. We show that, in
contrast to the Fermi contact hyperfine interaction for conduction electrons,
the dipole-dipole hyperfine interaction is anisotropic for a hole, for both
pure or mixed hole states. We evaluate the coupling constants of the
hole-nuclear interaction and demonstrate that they are only one order of
magnitude smaller than the coupling constants of the electron-nuclear
interaction. We also study, theoretically, the hole spin dephasing of an
ensemble of quantum dots via the hyperfine interaction in the framework of
frozen fluctuations of the nuclear field, in absence or in presence of an
applied magnetic field. We also discuss experiments which could evidence the
dipole-dipole hyperfine interaction and give information on hole mixing.Comment: 35 pages, 7 figures and 2 table
Enhancement of the electron spin memory by localization on donors in a quantum well
We present easily reproducible experimental conditions giving long electron
spin relaxation and dephasing times at low temperature in a quantum well. The
proposed system consists in an electron localized by a donor potential, and
immerged in a quantum well in order to improve its localization with respect to
donor in bulk. We have measured, by using photoinduced Faraday rotation
technique, the spin relaxation and dephasing times of electrons localized on
donors placed in the middle of a 80A CdTe quantum well, and we have obtained
15ns and 18ns, respectively, which are almost two orders of magnitude longer
than the free electron spin relaxation and dephasing times obtained previously
in a similar CdTe quantum well (J. Tribollet et al. PRB 68, 235316 (2003)).Comment: 15 pages, 4 figure
Structural and magnetic properties of molecular beam epitaxy (MnSb2Te4)x(Sb2Te3)1-x topological materials with exceedingly high Curie temperature
Tuning magnetic properties of magnetic topological materials is of interest
to realize elusive physical phenomena such as quantum anomalous hall effect
(QAHE) at higher temperatures and design topological spintronic devices.
However, current topological materials exhibit Curie temperature (TC) values
far below room temperature. In recent years, significant progress has been made
to control and optimize TC, particularly through defect engineering of these
structures. Most recently we showed evidence of TC values up to 80K for
(MnSb2Te4)x(Sb2Te3)1-x, where x is greater than or equal to 0.7 and less than
or equal to 0.85, by controlling the compositions and Mn content in these
structures. Here we show further enhancement of the TC, as high as 100K, by
maintaining high Mn content and reducing the growth rate from 0.9 nm/min to 0.5
nm/min. Derivative curves reveal the presence of two TC components contributing
to the overall value and propose TC1 and TC2 have distinct origins: excess Mn
in SLs and Mn in Sb2-yMnyTe3QLs alloys, respectively. In pursuit of elucidating
the mechanisms promoting higher Curie temperature values in this system, we
show evidence of structural disorder where Mn is occupying not only Sb sites
but also Te sites, providing evidence of significant excess Mn and a new
crystal structure:(Mn1+ySb2-yTe4)x(Sb2-yMnyTe3)1-x. Our work shows progress in
understanding how to control magnetic defects to enhance desired magnetic
properties and the mechanism promoting these high TC in magnetic topological
materials such as (Mn1+ySb2-yTe4)x(Sb2-yMnyTe3)1-x
Isotope sensitive measurement of the hole-nuclear spin interaction in quantum dots
Decoherence caused by nuclear field fluctuations is a fundamental obstacle to
the realization of quantum information processing using single electron spins.
Alternative proposals have been made to use spin qubits based on valence band
holes having weaker hyperfine coupling. However, it was demonstrated recently
both theoretically and experimentally that the hole hyperfine interaction is
not negligible, although a consistent picture of the mechanism controlling the
magnitude of the hole-nuclear coupling is still lacking. Here we address this
problem by performing isotope selective measurement of the valence band
hyperfine coupling in InGaAs/GaAs, InP/GaInP and GaAs/AlGaAs quantum dots.
Contrary to existing models we find that the hole hyperfine constant along the
growth direction of the structure (normalized by the electron hyperfine
constant) has opposite signs for different isotopes and ranges from -15% to
+15%. We attribute such changes in hole hyperfine constants to the competing
positive contributions of p-symmetry atomic orbitals and the negative
contributions of d-orbitals. Furthermore, we find that the d-symmetry
contribution leads to a new mechanism for hole-nuclear spin flips which may
play an important role in hole spin decoherence. In addition the measured
hyperfine constants enable a fundamentally new approach for verification of the
computed Bloch wavefunctions in the vicinity of nuclei in semiconductor
nanostructures
Elective Cancer Surgery in COVID-19-Free Surgical Pathways During the SARS-CoV-2 Pandemic: An International, Multicenter, Comparative Cohort Study.
PURPOSE: As cancer surgery restarts after the first COVID-19 wave, health care providers urgently require data to determine where elective surgery is best performed. This study aimed to determine whether COVID-19-free surgical pathways were associated with lower postoperative pulmonary complication rates compared with hospitals with no defined pathway. PATIENTS AND METHODS: This international, multicenter cohort study included patients who underwent elective surgery for 10 solid cancer types without preoperative suspicion of SARS-CoV-2. Participating hospitals included patients from local emergence of SARS-CoV-2 until April 19, 2020. At the time of surgery, hospitals were defined as having a COVID-19-free surgical pathway (complete segregation of the operating theater, critical care, and inpatient ward areas) or no defined pathway (incomplete or no segregation, areas shared with patients with COVID-19). The primary outcome was 30-day postoperative pulmonary complications (pneumonia, acute respiratory distress syndrome, unexpected ventilation). RESULTS: Of 9,171 patients from 447 hospitals in 55 countries, 2,481 were operated on in COVID-19-free surgical pathways. Patients who underwent surgery within COVID-19-free surgical pathways were younger with fewer comorbidities than those in hospitals with no defined pathway but with similar proportions of major surgery. After adjustment, pulmonary complication rates were lower with COVID-19-free surgical pathways (2.2% v 4.9%; adjusted odds ratio [aOR], 0.62; 95% CI, 0.44 to 0.86). This was consistent in sensitivity analyses for low-risk patients (American Society of Anesthesiologists grade 1/2), propensity score-matched models, and patients with negative SARS-CoV-2 preoperative tests. The postoperative SARS-CoV-2 infection rate was also lower in COVID-19-free surgical pathways (2.1% v 3.6%; aOR, 0.53; 95% CI, 0.36 to 0.76). CONCLUSION: Within available resources, dedicated COVID-19-free surgical pathways should be established to provide safe elective cancer surgery during current and before future SARS-CoV-2 outbreaks
Elective cancer surgery in COVID-19-free surgical pathways during the SARS-CoV-2 pandemic: An international, multicenter, comparative cohort study
PURPOSE As cancer surgery restarts after the first COVID-19 wave, health care providers urgently require data to determine where elective surgery is best performed. This study aimed to determine whether COVID-19–free surgical pathways were associated with lower postoperative pulmonary complication rates compared with hospitals with no defined pathway. PATIENTS AND METHODS This international, multicenter cohort study included patients who underwent elective surgery for 10 solid cancer types without preoperative suspicion of SARS-CoV-2. Participating hospitals included patients from local emergence of SARS-CoV-2 until April 19, 2020. At the time of surgery, hospitals were defined as having a COVID-19–free surgical pathway (complete segregation of the operating theater, critical care, and inpatient ward areas) or no defined pathway (incomplete or no segregation, areas shared with patients with COVID-19). The primary outcome was 30-day postoperative pulmonary complications (pneumonia, acute respiratory distress syndrome, unexpected ventilation). RESULTS Of 9,171 patients from 447 hospitals in 55 countries, 2,481 were operated on in COVID-19–free surgical pathways. Patients who underwent surgery within COVID-19–free surgical pathways were younger with fewer comorbidities than those in hospitals with no defined pathway but with similar proportions of major surgery. After adjustment, pulmonary complication rates were lower with COVID-19–free surgical pathways (2.2% v 4.9%; adjusted odds ratio [aOR], 0.62; 95% CI, 0.44 to 0.86). This was consistent in sensitivity analyses for low-risk patients (American Society of Anesthesiologists grade 1/2), propensity score–matched models, and patients with negative SARS-CoV-2 preoperative tests. The postoperative SARS-CoV-2 infection rate was also lower in COVID-19–free surgical pathways (2.1% v 3.6%; aOR, 0.53; 95% CI, 0.36 to 0.76). CONCLUSION Within available resources, dedicated COVID-19–free surgical pathways should be established to provide safe elective cancer surgery during current and before future SARS-CoV-2 outbreaks
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