1,479 research outputs found
Field Dependent Specific-Heat of Rare Earth Manganites
The low temperature specific heat C(H) of several rare-earth manganites
(La_(0.7)Sr_(0.3)MnO_(3), Nd_(0.5)Sr_(0.5)MnO_(3), Pr_(0.5)Sr_(0.5)MnO_(3),
La_(0.67)Ca_(0.33)MnO$_(3), La_(0.5)Ca_(0.5)MnO_(3), La_(0.45)Ca_(0.55)MnO_(3)
and La_(0.33)Ca_(0.67)MnO_(3)) was measured as a function of magnetic field. We
observed behaviour consistent with thermodynamic expectations, i.e., C(H)
decreases with field for ferromagnetic metallic compounds by an amount which is
in quantitative agreement with spin wave theory. We also find that C(H)
increases with field in most compounds with a charge-ordered antiferromagnetic
ground state. In compounds which show evidence of a coexistence of
ferromagnetic metallic and antiferromagnetic charge-ordered states, C(H)
displays some unusual non-equilibrium effects presumably associated with the
phase-separation of the two states. We also observe a large anomalous low
temperature specific heat at the doping induced metal-insulator transition (at
x = 0.50) in La_(1-x)Ca_(x)MnO_(3).Comment: 13 pages, LATEX, 7 PDF figure
Spin-polarized Tunneling in Hybrid Metal-Semiconductor Magnetic Tunnel Junctions
We demonstrate efficient spin-polarized tunneling between a ferromagnetic
metal and a ferromagnetic semiconductor with highly mismatched conductivities.
This is indicated by a large tunneling magnetoresistance (up to 30%) at low
temperatures in epitaxial magnetic tunnel junctions composed of a ferromagnetic
metal (MnAs) and a ferromagnetic semiconductor (GaMnAs) separated by a
nonmagnetic semiconductor (AlAs). Analysis of the current-voltage
characteristics yields detailed information about the asymmetric tunnel
barrier. The low temperature conductance-voltage characteristics show a zero
bias anomaly and a V^1/2 dependence of the conductance, indicating a
correlation gap in the density of states of GaMnAs. These experiments suggest
that MnAs/AlAs heterostructures offer well characterized tunnel junctions for
high efficiency spin injection into GaAs.Comment: 14 pages, submitted to Phys. Rev.
Complete Break Up of Ortho Positronium (Ps)- Hydrogenic ion System
The dynamics of the complete breakup process in an Ortho Ps - He+ system
including electron loss to the continuum (ELC) is studied where both the
projectile and the target get ionized. The process is essentially a four body
problem and the present model takes account of the two centre effect on the
electron ejected from the Ps atom which is crucial for a proper description of
the ELC phenomena. The calculations are performed in the framework of Coulomb
Distorted Eikonal Approximation. The exchange effect between the target and the
projectile electron is taken into account in a consistent manner. The proper
asymptotic 3-body boundary condition for this ionization process is also
satisfied in the present model. A distinct broad ELC peak is noted in the fully
differential cross sections (5DCS) for the Ps electron corroborating
qualitatively the experiment for the Ps - He system. Both the dynamics of the
ELC from the Ps and the ejected electron from the target He+ in the FDCS are
studied using coplanar geometry. Interesting features are noted in the FDCS for
both the electrons belonging to the target and the projectile.Comment: 14 pages,7 figure
Bell's inequalities for states with positive partial transpose
We study violations of n particle Bell inequalities (as developed by Mermin
and Klyshko) under the assumption that suitable partial transposes of the
density operator are positive. If all transposes with respect to a partition of
the system into p subsystems are positive, the best upper bound on the
violation is 2^((n-p)/2). In particular, if the partial transposes with respect
to all subsystems are positive, the inequalities are satisfied. This is
supporting evidence for a recent conjecture by Peres that positivity of partial
transposes could be equivalent to existence of local classical models.Comment: 4 pages, REVTe
Microwave Surface Waves on Metasurfaces with Planar Discontinuities
The work presented within this thesis details the experimental investigation of the surface waves supported on metasurfaces. Particular attention has been given to the reflection of these surface waves from planar discontinuities associated with these metasurfaces. Various experimental techniques have been developed throughout this work to characterise surface wave supporting metasurfaces. These include a new technique for measuring the dispersion of surface waves supported on metasurfaces, characterisation of the near-field associated with the surface waves, a device for launching planar phase front surface waves and finally a technique for measuring the surface wave reflection coefficient.
The dispersion of surface waves on a square array of square cross-section metal pillars has been fully characterised and compared to FEM modelling. The results show that a family of surface waves may be supported by pillar or crossed slit structures rather than just holes even though there is now no lowest cut-off frequency. A family of TM surface modes have been shown to exist with dispersions which asymptote to frequencies defined by the pillar heights (slit depth) and the refractive index of the material filling the slits.
Primarily this work focussed on the surface wave properties associated with a square array of square metal patches on a dielectric coated ground plane and a Sievenpiper `mushroom' metasurface. The amplitude reflection coefficient of these surface waves has been studied for three distinct systems: Firstly for surface waves incident upon the termination of a these metasurfaces to free space, secondly for surface waves incident upon the interface between a dielectric coated and uncoated metasurface and thirdly for surface waves incident on the boundary between two metaurfaces.
The reflection coefficient of surface waves incident upon the termination of the metasurface to free space is found to increase significantly with the confinement of the surface mode. This confinement, and therefore the form of the reflection coefficient, is significantly different for the two metasurfaces considered due to their dispersions. This increase in the reflection coefficient is caused by both the momentum mismatch of the surface wave compared to the freely propagating modes and the different field distributions of the two modes.
The reflection coefficient of surface waves incident upon the boundary between a coated and uncoated metasurface has been experimentally characterised for the metal patch array and Sievenpiper `mushroom' metasurfaces. It is shown that the addition of a thin, significantly subwavelength, dielectric overlayer onto the metasurface vastly perturbs the surface wave dispersion. The reflection coefficient of the surface waves is found to depend on the dispersion of the mode supported on the coated and uncoated metasurface and the overlayer thickness. Most noticeably the thickness of the overlayer, by comparison to the surface wave decay length, has a significant effect on scattering to free space associated with the surface wave reflection.
The final system considered was designed to investigate the impedance approximation, often used to describe metasurfaces, and found it to be an incomplete description of the surface waves supported on the metasurfaces used within this study. In the impedance approximation the two surfaces considered are said to be `impedance matched` at certain frequencies. It is demonstrated that the failure of the impedance approximation to accurately describe this system is due to the behaviour of the electric field within the metasurfaces. These are not analytically described in the impedance approximation and are required for an accurate description of the surface waves supported on these metasurfaces.EPSRCBAE System
Geometry and material effects in Casimir physics - Scattering theory
We give a comprehensive presentation of methods for calculating the Casimir
force to arbitrary accuracy, for any number of objects, arbitrary shapes,
susceptibility functions, and separations. The technique is applicable to
objects immersed in media other than vacuum, to nonzero temperatures, and to
spatial arrangements in which one object is enclosed in another. Our method
combines each object's classical electromagnetic scattering amplitude with
universal translation matrices, which convert between the bases used to
calculate scattering for each object, but are otherwise independent of the
details of the individual objects. This approach, which combines methods of
statistical physics and scattering theory, is well suited to analyze many
diverse phenomena. We illustrate its power and versatility by a number of
examples, which show how the interplay of geometry and material properties
helps to understand and control Casimir forces. We also examine whether
electrodynamic Casimir forces can lead to stable levitation. Neglecting
permeabilities, we prove that any equilibrium position of objects subject to
such forces is unstable if the permittivities of all objects are higher or
lower than that of the enveloping medium; the former being the generic case for
ordinary materials in vacuum.Comment: 44 pages, 11 figures, to appear in upcoming Lecture Notes in Physics
volume in Casimir physic
Slater-Pauling Behavior of the Half-Ferromagnetic Full-Heusler Alloys
Using the full-potential screened Korringa-Kohn-Rostoker method we study the
full-Heusler alloys based on Co, Fe, Rh and Ru. We show that many of these
compounds show a half-metallic behavior, however in contrast to the
half-Heusler alloys the energy gap in the minority band is extremely small.
These full-Heusler compounds show a Slater-Pauling behavior and the total
spin-magnetic moment per unit cell (M_t) scales with the total number of
valence electrons (Z_t) following the rule: M_t=Z_t-24. We explain why the
spin-down band contains exactly 12 electrons using arguments based on the group
theory and show that this rule holds also for compounds with less than 24
valence electrons. Finally we discuss the deviations from this rule and the
differences compared to the half-Heusler alloys.Comment: 10 pages, 8 figures, revised figure 3, new text adde
Fractional Quantum Hall Effect via Holography: Chern-Simons, Edge States, and Hierarchy
We present three holographic constructions of fractional quantum Hall effect
(FQHE) via string theory. The first model studies edge states in FQHE using
supersymmetric domain walls in N=6 Chern-Simons theory. We show that D4-branes
wrapped on CP^1 or D8-branes wrapped on CP^3 create edge states that shift the
rank or the level of the gauge group, respectively. These holographic edge
states correctly reproduce the Hall conductivity. The second model presents a
holographic dual to the pure U(N)_k (Yang-Mills-)Chern-Simons theory based on a
D3-D7 system. Its holography is equivalent to the level-rank duality, which
enables us to compute the Hall conductivity and the topological entanglement
entropy. The third model introduces the first string theory embedding of
hierarchical FQHEs, using IIA string on C^2/Z_n.Comment: 36 pages, 6 figures; v2: with an improved derivation of Hall
conductivity in section 3.2, typo corrections, and additional references; v3:
explanations and comments adde
‘Stray appetites’: a socio-ecological analysis of free-ranging dogs living alongside human communities in Bangalore, India
Across the developing world, humans and free-ranging domestic dogs share common spaces. The relationship between these dogs and humans can range from one of dependence, to apathy, to conflict. Given the high number of humans attacked by dogs every year in India, and the lack of an effective population control strategy, we seek to provide insights into the conflict and propose alternative population management options based on reducing the carrying capacity of the environment. We used a mixed methods approach to understand both ecological and sociological underpinnings of free-ranging dog-human relationships in Bangalore, India. We conducted a photographic capture-recapture survey of free-ranging dogs to estimate population size and linked it to the availability of potential food sources. We also conducted a qualitative sur
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