Hyperfine interaction in a quantum dot: Non-Markovian electron spin dynamics

We have performed a systematic calculation for the non-Markovian dynamics of a localized electron spin interacting with an environment of nuclear spins via the Fermi contact hyperfine interaction. This work applies to an electron in the s -type orbital ground state of a quantum dot or bound to a donor impurity, and is valid for arbitrary polarization p of the nuclear spin system, and arbitrary nuclear spin I in high magnetic fields. In the limit of p=1 and I=1/2, the Born approximation of our perturbative theory recovers the exact electron spin dynamics. We have found the form of the generalized master equation (GME) for the longitudinal and transverse components of the electron spin to all orders in the electron spin--nuclear spin flip-flop terms. Our perturbative expansion is regular, unlike standard time-dependent perturbation theory, and can be carried-out to higher orders. We show this explicitly with a fourth-order calculation of the longitudinal spin dynamics. In zero magnetic field, the fraction of the electron spin that decays is bounded by the smallness parameter \delta=1/p^{2}N, where N is the number of nuclear spins within the extent of the electron wave function. However, the form of the decay can only be determined in a high magnetic field, much larger than the maximum Overhauser field. In general the electron spin shows rich dynamics, described by a sum of contributions with non-exponential decay, exponential decay, and undamped oscillations. There is an abrupt crossover in the electron spin asymptotics at a critical dimensionality and shape of the electron envelope wave function. We propose a scheme that could be used to measure the non-Markovian dynamics using a standard spin-echo technique, even when the fraction that undergoes non-Markovian dynamics is small.Comment: 22 pages, 8 figure

Giant g factor tuning of long-lived electron spins in Ge

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the highly desirable but contrasting requirements of spin robustness to relaxation mechanisms and sizeable coupling between spin and orbital motion of charge carriers. Here we focus on Ge, which, by matching those criteria, is rapidly emerging as a prominent candidate for shuttling spin quantum bits in the mature framework of Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome such fundamental limitations by investigating a two dimensional electron gas (2DEG) confined in quantum wells of pure Ge grown on SiGe-buffered Si substrates. These epitaxial systems demonstrate exceptionally long spin relaxation and coherence times, eventually unveiling the potential of Ge in bridging the gap between spintronic concepts and semiconductor device physics. In particular, by tuning spin-orbit interaction via quantum confinement we demonstrate that the electron Land\'e g factor and its anisotropy can be engineered in our scalable and CMOS-compatible architectures over a range previously inaccessible for Si spintronics

Persistent Hyperprolactinemia and Bilateral Galactocele in a Male Infant

Galactocele is a benign breast lesion, usually occurring in nursing women. This lesion is a rare cause of breast enlargement in children. In this paper we describe the case of an infant with hyperprolactinemia (which persisted throughout 15 years of clinical observation) and bilateral galactocele. We speculate that a congenital midline defect in our patient might have impaired the normal dopaminergic inhibitory tone on pituitary lactotroph cells, thus leading to an increased prolactin secretion by the pituitary gland; this, in turn, might have favored the development of the galactocele

Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex-situ tapes

Ex-situ Powder-In-Tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H*, the upper critical field Hc2 and the critical current density Jc(H) together with the suppression of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced Jc and H* values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, although at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on Jc is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases Hc2, especially Hc2 perpendicular to the Mg and B planes.Comment: 26 pages, 9 figures, submitted to J. Appl. Phy

Defects and Dopants in Silicon Nanowires Produced by Metal-Assisted Chemical Etching

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Immune Checkpoint Blockade in Lung Carcinoids with Aggressive Behaviour: One More Arrow in Our Quiver?

Lung carcinoids are well-differentiated and low-/intermediate-grade neuroendocrine neoplasms of the lung. Given their relative rarity, and the paucity of data available from prospective studies, no global consensus exists on the systemic treatment of these tumours. In recent years, immune checkpoint inhibitors have revolutionized cancer management and are under evaluation in patients with diverse types of neuroendocrine neoplasms. The aim of this narrative review is to analyse all available data for the use of approved immune checkpoint inhibitors in patients with lung carcinoids. We performed an extensive search for relevant data sources and found five published articles, one meeting abstract, and nine registered clinical trials indicating a growing interest of researchers in this field, and providing preliminary evidence of efficacy for combined nivolumab plus ipilimumab and durvalumab plus tremelimumab regimens in the treatment of advanced and/or metastatic lung carcinoids