Radiocarbon dating of the last glaciation in Peru

In the Cordillera Vilcanota and the vicinity of the Quelccaya ice cap, near lat 14°S in eastern Peru, the last glaciation culminated sometime between about 28,000 and 14,000 B.P. Alpine glaciers were then only about half as long as during an earlier glaciation of unknown date. A rather minor readvance of the Quelccaya ice cap was in progress about 11,500 B.P. and culminated about 11,000 B.P., some 500 to 800 yr before the Younger Dryas Stade in Europe. By 10,000 B.P., the Quelccaya ice cap was little if any larger than it is today, and it was smaller than it is today between about 2700 and 1600 B.P. Glacier fluctuations during the interval 10,000 to 3000 B.P. have not yet been determined. A “Little Ice Age” maximum culminated between 600 and 300 B.P. The relative importance of changes in temperature and precipitation in causing these glacier variations is uncertain

Thermodynamic perturbation theory for dipolar superparamagnets

Thermodynamic perturbation theory is employed to derive analytical expressions for the equilibrium linear susceptibility and specific heat of lattices of anisotropic classical spins weakly coupled by the dipole-dipole interaction. The calculation is carried out to the second order in the coupling constant over the temperature, while the single-spin anisotropy is treated exactly. The temperature range of applicability of the results is, for weak anisotropy (A/kT << 1), similar to that of ordinary high-temperature expansions, but for moderately and strongly anisotropic spins (A/kT > 1) it can extend down to the temperatures where the superparamagnetic blocking takes place (A/kT \sim 25), provided only the interaction strength is weak enough. Besides, taking exactly the anisotropy into account, the results describe as particular cases the effects of the interactions on isotropic (A = 0) as well as strongly anisotropic (A \to \infty) systems (discrete orientation model and plane rotators).Comment: 15 pages, 3 figure

Kondo effect in multielectron quantum dots at high magnetic fields

We present a general description of low temperature transport through a quantum dot with any number of electrons at filling factor $1<\nu <2$. We provide a general description of a novel Kondo effect which is turned on by application of an appropriate magnetic field. The spin-flip scattering of carriers by the quantum dot only involves two states of the scatterer which may have a large spin. This process is described by spin-flip Hubbard operators, which change the angular momentum, leading to a Kondo Hamiltonian. We obtain antiferromagnetic exchange couplings depending on tunneling amplitudes and correlation effects. Since Kondo temperature has an exponential dependence on exchange couplings, quantitative variations of the parameters in different regimes have important experimental consequences. In particular, we discuss the {\it chess board} aspect of the experimental conductance when represented in a grey scale as a function of both the magnetic field and the gate potential affecting the quantum dot

Rotational and vibrational spectra of quantum rings

One can confine the two-dimensional electron gas in semiconductor heterostructures electrostatically or by etching techniques such that a small electron island is formed. These man-made ``artificial atoms'' provide the experimental realization of a text-book example of many-particle physics: a finite number of quantum particles in a trap. Much effort was spent on making such "quantum dots" smaller and going from the mesoscopic to the quantum regime. Far-reaching analogies to the physics of atoms, nuclei or metal clusters were obvious from the very beginning: The concepts of shell structure and Hund's rules were found to apply -- just as in real atoms! In this Letter, we report the discovery that electrons confined in ring-shaped quantum dots form rather rigid molecules with antiferromagnetic order in the ground state. This can be seen best from an analysis of the rotational and vibrational excitations

Renormalization approach for quantum-dot structures under strong alternating fields

We develop a renormalization method for calculating the electronic structure of single and double quantum dots under intense ac fields. The nanostructures are emulated by lattice models with a clear continuum limit of the effective-mass and single-particle approximations. The coupling to the ac field is treated non-perturbatively by means of the Floquet Hamiltonian. The renormalization approach allows the study of dressed states of the nanoscopic system with realistic geometries as well arbitrary strong ac fields. We give examples of a single quantum dot, emphasizing the analysis of the effective-mass limit for lattice models, and double-dot structures, where we discuss the limit of the well used two-level approximation.Comment: 6 pages, 7 figure

Respuesta de la ahuyama (Curcubita maxima L.) a la fertilización con N, P, K y materia orgánica en el Valle del Cauca.

En un suelo de la serie Palmeras (PM) clasificado como Vertic ustropept del CNI Palmira, se realizó en 1986 una investigación para evaluar el efecto de la fertilización química y orgánica en el rendimiento del zapallo (Cucurbita maxima L.) variedad peruana. Se estudiaron 27 combinaciones de N-P-K (factorial) más 3 dosis de materia orgánica MO (gallinaza), en un diseño de bloques completos al azar con 3 repeticiones. El P y K se aplicaron 8 días después de la siembra, el N y la MO fraccionadas, en 2 aplicaciones iguales, 8 y 30 días después de la siembra. Los niveles de cada nutriente, en kg/ha, fueron: 0, 25 y 50 de N, 0, 50 y 100 de oxido fosfórico, 0, 25 y 50 kg/ha de oxido de potasio y 2.5, 5.0 y 7.5 t/ha de gallinaza. Hubo diferencias de 19.78 y 19.71 t/ha entre los 2 rendimientos más altos, correspondientes a los tratamientos 50-100-25 y 50-50-25, en su orden y el rendimiento más bajo (25-100-50). Respecto al testigo absoluto se observaron incrementos en rendimiento de 15.03 y 14.83 por ciento, respectivamente. Con las aplicaciones individuales la principal respuesta se obtuvo con 50 kg/ha de oxido fosfórico, para un incremento en rendimiento de 7.4 por ciento con relación al testigo absoluto, el K, en dosis de 25 y 50 kg/ha oxido de potasio, disminuyó el rendimiento en 34 y 22.3 por ciento, respectivamente. Los tratamientos con MO aumentaron el rendimiento en 126, 131 y 105 por ciento, respectivamente, usando dosis de 2.5, 5.0 y 7.5 t/ha de gallinaza. El mayor número de frutos comerciales se obtuvo con aplicaciones de 50-100-25 kg/ha de N, P O y K O, respectivamente. El tratamiento 25-50-25 t/ha de gallinaza, kg P O /ha y kg K O/ha respectivamente, produjo el más alto peso promedio de frutos. El mayor número de flores femeninas (43.3 y 39.3) se consiguió aplicando 5 t/ha de gallinaza, más 50-25 kg/ha de P O y K O y 50-50-25 kg/ha de N, P O y K su orden.;Las relaciones flor femenina-flormasculina más altas (4.76 y 3.88) se obtuvieron con la aplicación de 100 kg/ha de P O y 25-50-0 kg/ha de N, P O y K. Para agricultores con suficiente capital se recomienda el tratamiento 50-50-25 kg/ha de N, P O y K O por presentar: tasa alta de retorno marginal (TRM), mayor beneficio neto, y consistencia a las fluctuaciones de precios de los insumos. Para cultivadores de escasos recursos se recomiendan los tratamientos 25-0-25 y 0-50-0 kg/ha de N, P O y K O, con los cuales se obtiene una TRM alta, menores costos variables y consistencia a fluctuaciones de precios de los insumosAhuyama-Calabaza (cucúrbita), Cucurbita máxim

Competing mechanisms for singlet-triplet transition in artificial molecules

We study the magnetic field induced singlet/triplet transition for two electrons in vertically coupled quantum dots by exact diagonalization of the Coulomb interaction. We identify the different mechanisms occurring in the transition, involving either in-plane correlations or localization in opposite dots, depending on the field direction. Therefore, both spin and orbital degrees of freedom can be manipulated by field strength and direction. The phase diagram of realistic devices is determined.Comment: To appear in Phys. Rev. B - Rapid Comm. - 5 pages, 3 figure

Edge reconstruction in the fractional quantum Hall regime

The interplay of electron-electron interaction and confining potential can lead to the reconstruction of fractional quantum Hall edges. We have performed exact diagonalization studies on microscopic models of fractional quantum Hall liquids, in finite size systems with disk geometry, and found numerical evidence of edge reconstruction under rather general conditions. In the present work we have taken into account effects like layer thickness and Landau level mixing, which are found to be of quantitative importance in edge physics. Due to edge reconstruction, additional nonchiral edge modes arise for both incompressible and compressible states. These additional modes couple to electromagnetic fields and thus can be detected in microwave conductivity measurements. They are also expected to affect the exponent of electron Green's function, which has been measured in tunneling experiments. We have studied in this work the electric dipole spectral function that is directly related to the microwave conductivity measurement. Our results are consistent with the enhanced microwave conductivity observed in experiments performed on samples with an array of antidots at low temperatures, and its suppression at higher temperatures. We also discuss the effects of the edge reconstruction on the single electron spectral function at the edge.Comment: 19 pages, 12 figure

Spin interactions and switching in vertically tunnel-coupled quantum dots

We determine the spin exchange coupling J between two electrons located in two vertically tunnel-coupled quantum dots, and its variation when magnetic (B) and electric (E) fields (both in-plane and perpendicular) are applied. We predict a strong decrease of J as the in-plane B field is increased, mainly due to orbital compression. Combined with the Zeeman splitting, this leads to a singlet-triplet crossing, which can be observed as a pronounced jump in the magnetization at in-plane fields of a few Tesla, and perpendicular fields of the order of 10 Tesla for typical self-assembled dots. We use harmonic potentials to model the confining of electrons, and calculate the exchange J using the Heitler-London and Hund-Mulliken technique, including the long-range Coulomb interaction. With our results we provide experimental criteria for the distinction of singlet and triplet states and therefore for microscopic spin measurements. In the case where dots of different sizes are coupled, we present a simple method to switch on and off the spin coupling with exponential sensitivity using an in-plane electric field. Switching the spin coupling is essential for quantum computation using electronic spins as qubits.Comment: 13 pages, 9 figure