16,764 research outputs found
Magnetic and orbital ordering in cuprates and manganites
The mechanisms of magnetic and orbital interactions due to double exchange
(DE) and superexchange (SE) in transition metal oxides with degenerate e_g
orbitals are presented. Specifically, we study the effective spin-orbital
models derived for the d^9 ions as in KCuF_3, and for the d^4 ions as in
LaMnO_3, for spins S=1/2 and S=2, respectively. Such models are characterized
by three types of elementary excitations: spin waves, orbital waves, and
spin-and-orbital waves. The SE interactions between Cu^{2+} (d^9) ions are
inherently frustrated, which leads to a new mechanism of spin liquid which
operates in three dimensions. The SE between Mn^{3+} (d^4) ions explains the
A-type antiferromagnetic order in LaMnO_3 which coexists with the orbital
order. In contrast, the ferromagnetic metallic phase and isotropic spin waves
observed in doped manganites are explained by DE for degenerate e_g orbitals.
It is shown that although a hole does not couple to spin excitations in
ferromagnetic planes of LaMnO_3, the orbital excitations change the energy
scale for the coherent hole propagation and cause a large redistribution of
spectral weight. Finally, we point out some open problems in the present
understanding of doped manganites.Comment: 155 pages, 66 figure
High Temperature Behavior and Long-term Stability of Lithium Drifted Silicon Surface-barrier Detectors
High temperature behavior and long-term stability of lithium drifted silicon surface barrier charged particle detector
Virtual RTCP: A Case Study of Monitoring and Repair for UDP-based IPTV Systems
IPTV systems have seen widespread deployment, but often lack robust mechanisms for monitoring the quality of experience. This makes it difficult for network operators to ensure that their services match the quality of traditional broadcast TV systems, leading to consumer dissatisfaction. We present a case study of virtual RTCP, a new framework for reception quality monitoring and reporting for UDP-encapsulated MPEG video delivered over IP multicast. We show that this allows incremental deployment of reporting infrastructure, coupled with effective retransmission-based packet loss repair
Understanding the Transition between High School and College Mathematics and Science
Mathematics and science education is gaining increasing recognition as key for the well-being of individuals and society. Accordingly, the transition from high school to college is particularly important to ensure that students are prepared for college mathematics and science. The goal of this study was to understand how high school mathematics and science course-taking related to performance in college. Specifically, the study employed a nonparametric regression method to examine the relationship between high school mathematics and science courses, and academic performance in college mathematics and science courses. The results provide some evidence pertaining to the positive benefits from high school course-taking. Namely, students who completed high school trigonometry and lab-based chemistry tended to earn higher grades in college algebra and general chemistry, respectively. However, there was also evidence that high school coursework in biology and physics did not improve course performance in general biology and college physics beyond standardized test scores. Interestingly, students who completed high school calculus earned better grades in general biology. The implications of the findings are discussed for high school curriculum and alignment in standards between high schools and colleges
Direct comparison of the performance of CZT detectors contacted with various metals
Cadmium Zinc Telluride (CZT) achieves excellent spatial resolution and good
energy resolution over the broad energy range from several keV into the MeV
energy range. In this paper we present the results of a systematic study of the
performance of CZT detectors manufacturered by Orbotech (before IMARAD)
depending on surface preparation, contact materials and contact deposition. The
standard Orbotech detectors have the dimension of 2.0 x 2.0 x 0.5 cm. They have
a pixellated In anode with 8 x 8 pixels and a monolithic In cathode. Using the
same CZT substrates several times, we have made a direct comparison of the
performance of different contact materials by replacing the cathode and/or the
anode contacts with several high-workfunction metals. We present the
performance of the detectors and conclude with an overview over our ongoing
detector optimization.Comment: 8 pages, 5 figures, to appear in the proceedings of the conference
5922, "Hard X-Ray and Gamma-Ray Detector Physics VII" on the "Optics &
Photonics 2005" SPIE Symposium, July 31- August 4, 2005, San Diego, C
Octet baryon magnetic moments from QCD sum rules
A comprehensive study is made for the magnetic moments of octet baryons in
the method of QCD sum rules. A complete set of QCD sum rules is derived using
the external field method and generalized interpolating fields. For each
member, three sum rules are constructed from three independent tensor
structures. They are analyzed in conjunction with the corresponding mass sum
rules. The performance of each of the sum rules is examined using the criteria
of OPE convergence and ground-state dominance, along with the role of the
transitions in intermediate states. Individual contributions from the u, d and
s quarks are isolated and their implications in the underlying dynamics are
explored. Valid sum rules are identified and their predictions are obtained.
The results are compared with experiment and previous calculations.Comment: 21 pages, 11 figures, 6 figures; added a reference, minor change in
tex
Mapping the dynamic interactions between vortex species in highly anisotropic superconductors
Here we use highly sensitive magnetisation measurements performed using a
Hall probe sensor on single crystals of highly anisotropic high temperature
superconductors to study the dynamic interactions
between the two species of vortices that exist in such superconductors. We
observe a remarkable and clearly delineated high temperature regime that
mirrors the underlying vortex phase diagram. Our results map out the parameter
space over which these dynamic interaction processes can be used to create
vortex ratchets, pumps and other fluxonic devices.Comment: 7 pages, 3 figures, to be published in Supercond. Sci. Techno
The role of spatial and temporal radiation deposition in inertial fusion chambers: the case of HiPER¿
The first wall armour for the reactor chamber of HiPER will have to face short energy pulses of 5 to 20 MJ mostly in the form of x-rays and charged particles at a repetition rate of 5–10 Hz. Armour material and chamber dimensions have to be chosen to avoid/minimize damage to the chamber, ensuring the proper functioning of the facility during its planned lifetime. The maximum energy fluence that the armour can withstand without risk of failure, is determined by temporal and spatial deposition of the radiation energy inside the material. In this paper, simulations on the thermal effect of the radiation–armour interaction are carried out with an increasing definition of the temporal and spatial deposition of energy to prove their influence on the final results. These calculations will lead us to present the first values of the thermo-mechanical behaviour of the tungsten armour designed for the HiPER project under a shock ignition target of 48 MJ. The results will show that only the crossing of the plasticity limit in the first few micrometres might be a threat after thousands of shots for the survivability of the armour
Viable 3C-SiC-on-Si MOSFET design disrupting current Material Technology Limitations
The cubic polytype (3C-) of Silicon Carbide (SiC) is an emerging semiconductor technology for power devices. The featured isotropic material properties along with the Wide Band Gap (WBG) characteristics make it an excellent choice for power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). Nonetheless, material related limitations originate from the advantageous fact that 3C-SiC can be grown on Silicon (Si) wafers. One of these major limitations is an almost negligible activation of the p-type dopants after ion implantation because the annealing has to take place at relatively low temperatures. In this paper, a novel process flow for a vertical 3C-SiC-on-Si MOSFET is presented to overcome the difficulties that currently exist in obtaining a p-body region through implantation. The proposed design has been accurately simulated with Technology Computer Aided Design (TCAD) process and device software and a comparison is performed with the conventional SiC MOSFET design. The simulated output characteristics demonstrated a reduced on-resistance and at the same time it is shown that the blocking capability can be maintained to the same level. The promising performance of the novel design discussed in this paper is potentially the solution needed and a huge step towards the realisation of 3C-SiC-on-Si MOSFETs with commercially grated characteristics
The Fluctuations of the Quark Number and of the Chiral Condensate
The distributions of the quark number and chiral condensate over the gauge
fields are computed for QCD in Euclidean space at nonzero quark chemical
potential. As both operators are non-hermitian the distributions are in the
complex plane. Moreover, because of the sign problem, the distributions are not
real and positive. The computations are carried out within leading order chiral
perturbation theory and give a direct insight into the delicate cancellations
that take place in contributions to the total baryon number and the chiral
condensate.Comment: 19 pages, 2 figure
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