7,075 research outputs found
Universal linear relations between susceptibility and Tc in cuprates
We developed an experimental method for measuring the intrinsic
susceptibility \chi of powder of cuprate superconductors in the zero field
limit using a DC-magnetometer. The method is tested with lead spheres. Using
this method we determine \chi for a number of cuprate families as a function of
doping. A universal linear (and not proportionality) relation between Tc and
\chi is found. We suggest possible explanations for this phenomenon.Comment: Accepted for publication in PR
The scaling of the decoherence factor of a qubit coupled to a spin chain driven across quantum critical points
We study the scaling of the decoherence factor of a qubit (spin-1/2) using
the central spin model in which the central spin (qubit) is globally coupled to
a transverse XY spin chain. The aim here is to study the non-equilibrium
generation of decoherence when the spin chain is driven across (along) quantum
critical points (lines) and derive the scaling of the decoherence factor in
terms of the driving rate and some of the exponents associated with the quantum
critical points. Our studies show that the scaling of logarithm of decoherence
factor is identical to that of the defect density in the final state of the
spin chain following a quench across isolated quantum critical points for both
linear and non-linear variations of a parameter even if the defect density may
not satisfy the standard Kibble-Zurek scaling. However, one finds an
interesting deviation when the spin chain is driven along a critical line. Our
analytical predictions are in complete agreement with numerical results. Our
study, though limited to integrable two-level systems, points to the existence
of a universality in the scaling of the decoherence factor which is not
necessarily identical to the scaling of the defect density.Comment: 5 pages, 2 figures, Final and accepted versio
String Theory Versus Black Hole Complementarity
It is argued that string theory on the Euclidean version of the Schwarzschild
black hole -- the cigar geometry -- admits a zero mode that is localized at the
tip of the cigar. The presence of this mode implies that in string theory,
unlike in general relativity, the tip of the cigar is a special region. This is
in tension with the Euclidean version of the black hole complementarity
principle. We provide some qualitative arguments that link between this zero
mode and the origin of the black hole entropy and firewall at the horizon.Comment: 8 page
Fabrication of minority-carrier-limited n-Si/insulator/metal diodes
A photoelectrochemical anodization technique has been used to fabricate n-Si/insulator/metal (MIS) diodes with improved electrical properties. MIS structures fabricated with Au have provided the first experimental observation of a solid-state n-Si surface barrier device whose open circuit voltage Voc is controlled by minority-carrier bulk diffusion/recombination processes. For these diodes, variation of the minority-carrier diffusion length and majority-carrier dopant density produced changes in Voc that were in accord with bulk diffusion/recombination theory. Additionally, the variation in Voc in response to changes in the work function of the metal overlayer indicated that these MIS devices were not subject to the Fermi level pinning restrictions observed for n-Si Schottky structures. X-ray photoelectron spectroscopic characterization of the anodically grown insulator indicated 8.2±0.9 Å of a strained SiO2 layer as the interfacial insulator resulting from the photoanodization process
Magnetic quantum tunnelling in Fe8 with excited nuclei
We investigate the effect of dynamic nuclear spin fluctuation on quantum
tunneling of the magnetization (QTM) in the molecular magnet Fe8 by increasing
the nuclei temperature using radio frequency (RF) pulses before the hysteresis
loop measurements. The RF pulses do not change the electrons spin temperature.
Independently we show that the nuclear spin-spin relaxation time T2 has strong
temperature dependence. Nevertheless, we found no effect of the nuclear spin
temperature on the tunneling probability. This suggests that in our
experimental conditions only the hyperfine field strength is relevant for QTM.
We demonstrate theoretically how this can occur.Comment: 4 pages, 4 figure
Long-range and short-range magnetic correlations, and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14
Spin-spin correlations and microscopic origin of net magnetization in the
spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder
neutron diffraction. The present study reveals a 3D long-range magnetic
ordering below 16 K where the magnetic structure consists of ferromagnetic
trimers that are coupled ferromagnetically along the spin-chain. The moment
components along the a and c axes arrange antiferromagnetically. Our study
establishes that the uncompensated moment components along the b axis result in
a net magnetization per unit cell. The magnetic structure, determined in the
present study, is in agreement with the results of recent first principles
calculation; however, it is in contrast to a fascinating experimental
prediction of ferrimagnetic ordering based on the periodicity of the exchange
interactions in CaNi3P4O14. Our study also confirms the presence of broad
diffuse magnetic scattering, due to 1D short-range spin-spin correlations, over
a wide temperature range below ~50 K down to a temperature well below the Tc.
Total neutron scattering analysis by the RMC method reveals that the dominating
spin-spin correlation above Tc is ferromagnetic and along the b axis. The
nearest neighbour spin-spin correlations along the a and c axes are found to be
weakly antiferromagnetic. The nature of the trimer spin structure of the
short-range state is similar to that of the 3D long-range ordered state. The
present investigation of microscopic nature of the magnetic ground state also
explains the condition required for the 1/3 magnetization plateau to be
observed in the trimer spin-chains. In spite of the S=1 trimer chain system,
the present compound CaNi3P4O14 is found to be a good realization of 3D magnet
below the Tc=16 K with full ordered moment values of ~2 mu_B/Ni2+ (1.98 and
1.96 mu_B/Ni2+ for two Ni sites, respectively) at 1.5 K.Comment: 10 pages, 8 figure
Growth pattern of Celastrus paniculatus Willd. in two different habitats of district Tehri Garhwal, Uttarakhand (India)
Tehri Garhwal is one of the most important hilly district of Uttarakhand state, which is located at the foot hills of Himalayan mountain ranges has a rich and diverse form of vegetation along with medicinal and aromatic plants. Traditionally the folk people and the local inhabitants utilize the vegetation from their contiguous in the form of medicine, timber, food, fiber etc. Among these useful plants population of the some useful plants are becoming rare and threatened due to unsustainable utilization, overexploitation, construction of roads, over grazing, pilgrims, construction of large dams, forest encroachment, landslides and natural calamities, modernize agriculture etc. Out of these medicinal plants, a very important and threatened medicinal plant is C. paniculatus (Malkangni). The present study deals with conservation and cultivation practices of C. paniculatus in two altitudinal sites i.e. site 1st - village Budogi lying on the altitude of 2000m asl. and site 2nd - S.R.T. Campus Botanical Garden, Badshahithaul (Tehri Garwal), upto 1600m asl. Measurements of the plant height, number of leaves, size of veins etc. was compared at two altitudinal zones. The samples parameters from site 2nd showed maximum growth rate of C. paniculatus i.e. 8.4± 0.86 cm, 9.01± 2.0 and 2.63± 0.23 cm for plant height, number of leaves and size of veins respectively as compared to site 1st i.e. 5.19±0.52 cm, 4.85±0.85 and 2.27±0.1 cm. Thus, C. paniculatus showed best growth rate at an elevation up to 1600m. The cultivation practice will provide reliable information for cultivation of this species in a particular altitudinal range
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