565 research outputs found
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
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
Experimental study of gradual/abrupt dynamics of HfO<sub>2</sub>-based 1 memristive devices
The resistance switching dynamics of TiN/HfO2/Pt devices is analyzed in this paper. When biased with a voltage ramp of appropriate polarity, the devices experience SET transitions from high to low resistance states in an abrupt manner, which allows identifying a threshold voltage. However, we find that the stimulation with trains of identical pulses at voltages near the threshold results in a gradual SET transition, whereby the resistive state visits a continuum of intermediate levels as it approaches some low resistance state limit. On the contrary, RESET transitions from low to high resistance states proceed in a gradual way under voltage ramp stimulation, while gradual resistance changes driven by trains of identical spikes cover only a limited resistance window. The results are discussed in terms of the relations among the thermo-electrochemical effects of Joule heating, ion mobility, and resistance change, which provide positive and negative closed loop processes in SET and RESET, respectively. Furthermore, the effect of the competition between opposite tendencies of filament dissolution and formation at opposite metal/HfO2 interfaces is discussed as an additional ingredient affecting the switching dynamics
Universal response of the type-II Weyl semimetals phase diagram
The discovery of Weyl semimetals represents a significant advance in
topological band theory. They paradigmatically enlarged the classification of
topological materials to gapless systems while simultaneously providing
experimental evidence for the long-sought Weyl fermions. Beyond fundamental
relevance, their high mobility, strong magnetoresistance, and the possible
existence of even more exotic effects, such as the chiral anomaly, make Weyl
semimetals a promising platform to develop radically new technology. Fully
exploiting their potential requires going beyond the mere identification of
materials and calls for a detailed characterization of their functional
response, which is severely complicated by the coexistence of surface- and
bulk-derived topologically protected quasiparticles, i.e., Fermi arcs and Weyl
points, respectively. Here, we focus on the type-II Weyl semimetal class where
we find a stoichiometry-dependent phase transition from a trivial to a
non-trivial regime. By exploring the two extreme cases of the phase diagram, we
demonstrate the existence of a universal response of both surface and bulk
states to perturbations. We show that quasi-particle interference patterns
originate from scattering events among surface arcs. Analysis reveals that
topologically non-trivial contributions are strongly suppressed by spin
texture. We also show that scattering at localized impurities generate
defect-induced quasiparticles sitting close to the Weyl point energy. These
give rise to strong peaks in the local density of states, which lift the Weyl
node significantly altering the pristine low-energy Weyl spectrum. Visualizing
the microscopic response to scattering has important consequences for
understanding the unusual transport properties of this class of materials.
Overall, our observations provide a unifying picture of the Weyl phase diagram
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