Singlet-triplet decoherence due to nuclear spins in a double quantum dot

We have evaluated hyperfine-induced electron spin dynamics for two electrons confined to a double quantum dot. Our quantum solution accounts for decay of a singlet-triplet correlator even in the presence of a fully static nuclear spin system, with no ensemble averaging over initial conditions. In contrast to an earlier semiclassical calculation, which neglects the exchange interaction, we find that the singlet-triplet correlator shows a long-time saturation value that differs from 1/2, even in the presence of a strong magnetic field. Furthermore, we find that the form of the long-time decay undergoes a transition from a rapid Gaussian to a slow power law ($\sim 1/t^{3/2}$) when the exchange interaction becomes nonzero and the singlet-triplet correlator acquires a phase shift given by a universal (parameter independent) value of $3\pi/4$ at long times. The oscillation frequency and time-dependent phase shift of the singlet-triplet correlator can be used to perform a precision measurement of the exchange interaction and Overhauser field fluctuations in an experimentally accessible system. We also address the effect of orbital dephasing on singlet-triplet decoherence, and find that there is an optimal operating point where orbital dephasing becomes negligible.Comment: 12 pages, 4 figure

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

Rigorous Born Approximation and beyond for the Spin-Boson Model

Within the lowest-order Born approximation, we present an exact calculation of the time dynamics of the spin-boson model in the ohmic regime. We observe non-Markovian effects at zero temperature that scale with the system-bath coupling strength and cause qualitative changes in the evolution of coherence at intermediate times of order of the oscillation period. These changes could significantly affect the performance of these systems as qubits. In the biased case, we find a prompt loss of coherence at these intermediate times, whose decay rate is set by $\sqrt{\alpha}$, where $\alpha$ is the coupling strength to the environment. We also explore the calculation of the next order Born approximation: we show that, at the expense of very large computational complexity, interesting physical quantities can be rigorously computed at fourth order using computer algebra, presented completely in an accompanying Mathematica file. We compute the $O(\alpha)$ corrections to the long time behavior of the system density matrix; the result is identical to the reduced density matrix of the equilibrium state to the same order in $\alpha$. All these calculations indicate precision experimental tests that could confirm or refute the validity of the spin-boson model in a variety of systems.Comment: Greatly extended version of short paper cond-mat/0304118. Accompanying Mathematica notebook fop5.nb, available in Source, is an essential part of this work; it gives full details of the fourth-order Born calculation summarized in the text. fop5.nb is prepared in arXiv style (available from Wolfram Research

The prevalence of mental disorders among children, adolescents and adults in the Western Cape, South Africa

Objective: To provide estimates of the prevalence of selected mental disorders in the Western Cape, based on the consensus achieved by a working group established for this purpose. Method: An expert working group was established to provide technical expertise for the project. Potential risk factors likely to influence local prevalence rates were identified. Annual prevalence rates for adults and for children and adolescents were derived by consensus, informed by a systematic literature review. Prevalence rates were derived for individual disorders and adjusted for comorbidity. Results: The overall prevalence was 25.0% for adults and 17.0% for children and adolescents. Conclusion: Prevalence rates of child, adolescent and adult mental disorders were derived in a short period of time and with the use of minimal resources. Although of unknown validity, they are useful for policy development and for planning service utilisation estimates, resource costing and targets for service development for local mental health needs. This in the absence of an existing methodologically sound national prevalence study. We recommend that policy and programme developers draw on the expertise of local academics and clinicians to promote research-informed planning and policy development in the public sector. South African Psychiatry Review Vol. 9(3) 2006: 157-16

Effects of Intensive Late-Season Sheep Grazing Following Early-Season Steer Grazing on Population Dynamics of Sericea Lespedeza in the Kansas Flint Hills

Sericea lespedeza (Lespedeza cuneata; SL) is a high-tannin, invasive forb in the Tallgrass Prairie ecosystem. In Kansas, sericea lespedeza infests 980 square miles of pasture, primarily in the Flint Hills region. Sericea lespedeza infestations reduce native grass production by up to 92% through a combination of aggressive growth, prolific reproduction, canopy dominance, and chemical inhibition (allelopathy). Herbicides retard the spread of sericea lespedeza, but application is laborious and expensive; moreover, herbicides are lethal to ecologically-important, non-target plant species. Increased grazing pressure on sericea lespedeza by domestic herbivores may slow its spread and facilitate some measure of biological control. Unfortunately, mature plants contain high levels of condensed tannins, which are a strong deterrent to grazing by beef cattle. Small ruminants have greater tolerance for condensed tannins than beef cattle. Sheep, in particular, appear less susceptible to certain plant toxins than beef cattle and may be useful to selectively pressure noxious weeds like sericea lespedeza. The predominant grazing management practice in the Flint Hills region of Kansas involves annual spring burning followed by intensive grazing with yearling beef cattle from April to August. During seasonal grazing, 40 to 60% of annual graminoid production is removed and pastures remain idle for the remainder of the year. Under this prevailing management practice, invasion by sericea lespedeza into the Tallgrass Prairie biome has steadily increased. Sericea lespedeza flowers and produces seed in late summer from August to September. The absence of grazing pressure during this interval strongly promotes seed production, seed distribution, and continued invasion of the Flint Hills ecoregion by this noxious weed. Therefore, the objective of our study was to evaluate effects of late-season sheep grazing following locally-conventional steer grazing on vigor and reproductive capabilities of sericea lespedeza

Effects of Growing-Season Prescribed Burning on Vigor of the Noxious Weed Sericea Lespedeza (Lespedeza cuneata) in the Kansas Flint Hills

Sericea lespedeza (SL) was introduced into the United States from Asia in the late 19th century. Early land managers recognized that SL was adaptable; tolerant of shallow, acidic or low-fertility soils; and resistant to insects and disease. This combination of traits made SL a widely-used plant for reseeding strip-mined lands, highway right-ofways, dams, and waterways in the US for nearly a century. Regrettably, SL is highly fecund. Individual plants are capable of producing up to 850 lb of seed per acre annually. Vigorous seed production allows SL to rapidly infiltrate native grasslands that are adjacent to reseeding projects; seed can be transported great distances via the alimentary canal and hair of wild and domestic herbivores. In Kansas alone, SL has infested approximately 980 square miles of pasture, primarily in the Flint Hills region. The resulting damage to native habitats for wildlife and pasture quality for domestic herbivores has been devastating. The predominant grazing management practice in the Kansas Flint Hills involves annual spring burning in March or April, followed by intensive grazing with yearling beef cattle for a relatively short period from April to August. During seasonal grazing, 40 to 60% of annual graminoid production is removed and grazing lands then remain idle for the remainder of the year. Under this prevailing management practice, invasion by SL into the Tallgrass Prairie biome has steadily increased. Oklahoma State University researchers speculated that dormant-season, spring fires may stimulate SL growth by scarifying seeds lying on the surface of the soil. In contrast, plants with robust canopies respond more strongly to growing-season prescribed burns than to dormant-season prescribed burns. Previous research reported that application of growing season fire at 3-yr intervals decreased the rate of SL invasion. Therefore, the objective of our study was to evaluate the effects of growing-season prescribed burning of native tallgrass range on vigor of sericea lespedeza

Effects of Growing-Season Prescribed Burning on Vigor of Sericea Lespedeza in the Kansas Flint Hills: I. Suppression of Seed Production and Canopy Dominance

Sericea lespedeza (Lespedeza cuneata) is a highly fecund noxious weed in Kansas and surrounding states. Individual plants are capable of producing greater than 1,000 seeds annually. Vigorous seed production allows sericea lespedeza to rapidly infiltrate native and cultivated grasslands; seed can be transported great distances via farm machinery and the alimentary canal of wild and domestic herbivores. In Kansas alone, sericea lespedeza infests more than 700 square miles of pasture, primarily in the Flint Hills region. The resulting damage to native habitats for wildlife and pasture quality for domestic herbivores has been devastating.The predominant grazing management practice in the Kansas Flint Hills involves annual spring burning in April followed by intensive grazing with yearling beef cattle for a relatively short period from late April to August. During seasonal grazing, 40 to 60% of annual graminoid production is removed and grazing lands then remain idle for the remainder of the year. Under this prevailing management practice, invasion by sericea lespedeza into the tallgrass prairie biome has steadily increased. Oklahoma State University researchers speculated that dormant-season, spring fires may stimulate sericea lespedeza seed germination by scarifying seeds cast the previous fall. Previous research reported that application of growing season fire at 3 year intervals decreased the rate of sericea lespedeza invasion. Therefore, the objective of our study was to evaluate the effects of annual prescribed burning applied during the growing season on vigor of sericea lespedeza infesting native tallgrass range

Effects of Growing-Season Prescribed Burning on Vigor of Sericea Lespedeza in the Kansas Flint Hills: II. Plant-Species Composition

Fire has, for centuries, been a key force for sustainability of native ecosystems in the Kansas Flint Hills. Prior to the arrival of European settlers, prescribed and wild fires occurred at less than 3-year intervals in the tallgrass prairie region. As a result, native tallgrass plant communities adapted to fire at regular intervals and plant-species composition became stable on a geologic time scale.Currently, prescribed fire is used in the Kansas Flint Hills as a treatment for control of woody-stemmed invasive species such as eastern red cedar, honey locust, and roughleaf dogwood. These fires are generally applied in March and April and have become an integral part of the most common grazing management practice in the Kansas Flint Hills: annual spring burning in April followed by intensive grazing with yearling beef cattle for a relatively short period of time from late April to early August. Annual burning reportedly results in 0.2 to 0.3 lb of additional daily weight gain for yearling cattle when used in that way. In contrast, prescribed and wild fires during the presettlement era were not concentrated during any particular season of the year.Use of prescribed burning that is limited to a short interval during the spring has coincided with a steady increase of an invasive, non-woody, perennial legume known as sericea lespedeza (Lespedeza cuneata). Introduced into North America during the late 19th century, sericea lespedeza has proven highly adaptable to Flint Hills soils and climate. Prolific seed production appears to be the primary means of invasion. Seeds of sericea lespedeza are not wind-borne but are easily transported via the digestive tract of tannin-resistant herbivores and via machinery.Until recently, control of sericea lespedeza has relied heavily on costly, repeated application of herbicides, which has not checked the spread of the plant. We previously reported that prescribed burning during the months of August and September had strong suppressive effects on stand vigor and seed production of sericea lespedeza at a greatly reduced cost compared with herbicide. Questions remain, however, about the effects of growing-season prescribed burning on non-target plant species and soil cover. Therefore, the objective of this study was to evaluate the effects of growing-season prescribed fire on soil cover and populations of native grasses, forbs, and shrubs

Magnetization reversal driven by spin-injection : a mesoscopic spin-transfer effect

A mesoscopic description of spin-transfer effect is proposed, based on the spin-injection mechanism occurring at the junction with a ferromagnet. The effect of spin-injection is to modify locally, in the ferromagnetic configuration space, the density of magnetic moments. The corresponding gradient leads to a current-dependent diffusion process of the magnetization. In order to describe this effect, the dynamics of the magnetization of a ferromagnetic single domain is reconsidered in the framework of the thermokinetic theory of mesoscopic systems. Assuming an Onsager cross-coefficient that couples the currents, it is shown that spin-dependent electric transport leads to a correction of the Landau-Lifshitz-Gilbert equation of the ferromagnetic order parameter with supplementary diffusion terms. The consequence of spin-injection in terms of activation process of the ferromagnet is deduced, and the expressions of the effective energy barrier and of the critical current are derived. Magnetic fluctuations are calculated: the correction to the fluctuations is similar to that predicted for the activation. These predictions are consistent with the measurements of spin-transfer obtained in the activation regime and for ferromagnetic resonance under spin-injection.Comment: 20 pages, 2 figure

Constraints on Gamma-ray Emission from the Galactic Plane at 300 TeV

We describe a new search for diffuse ultrahigh energy gamma-ray emission associated with molecular clouds in the galactic disk. The Chicago Air Shower Array (CASA), operating in coincidence with the Michigan muon array (MIA), has recorded over 2.2 x 10^{9} air showers from April 4, 1990 to October 7, 1995. We search for gamma rays based upon the muon content of air showers arriving from the direction of the galactic plane. We find no significant evidence for diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma rays to normal hadronic cosmic rays at less than 2.4 x 10^{-5} at 310 TeV (90% confidence limit) from the galactic plane region: (50 degrees < l < 200 degrees); -5 degrees < b < 5 degrees). This limit places a strong constraint on models for emission from molecular clouds in the galaxy. We rule out significant spectral hardening in the outer galaxy, and conclude that emission from the plane at these energies is likely to be dominated by the decay of neutral pions resulting from cosmic rays interactions with passive target gas molecules.Comment: Astrophysical Journal, submitted, 11 pages, AASTeX Latex, 3 Postscript figure