168 research outputs found
Nonadiabatic charge pumping across two superconductors connected through a normal metal region by periodically driven potentials
Periodically driven systems exhibit resonance when the difference between an
excited state energy and the ground state energy is an integer multiple of
times the driving frequency. On the other hand, when a superconducting
phase difference is maintained between two superconductors, subgap states
appear which carry a Josephson current. A driven Josephson junction therefore
opens up an interesting avenue where the excitations due to applied driving
affect the current flowing from one superconductor to the other. Motivated by
this, we study charge transport in a superconductor-normal metal-superconductor
(SNS) junction where oscillating potentials are applied to the normal metal
region. We find that for small amplitudes of the oscillating potential, driving
at one site reverses the direction of current at the superconducting phase
differences when difference between the subgap eigenenergies of the undriven
Hamiltonian is integer multiple of times the driving frequency. For
larger amplitudes of oscillating potential, driving at one site exhibits richer
features. We show that even when the two superconductors are maintained at same
superconducting phase, a current can be driven by applying oscillating
potentials to two sites in the normal metal differing by a phase. We find that
when there is a nonzero Josephson current in the undriven system, the local
peaks and valleys in current of the system driven with an amplitude of
oscillating potential smaller than the superconducting gap indicates sharp
excitations in the system. In the adiabatic limit, we find that charge
transferred in one time period diverges as a powerlaw with pumping frequency
when a Josephson current flows in the undriven system. Our calculations are
exact and can be applied to finite systems. We discuss possible experimental
setups where our predictions can be tested.Comment: 9 pages, 9 figures. Published versio
Single Spin Asymmetry in Charmonium Production
We present estimates of Single Spin Asymmetry (SSA) in the electroproduction
of taking into account the transverse momentum dependent (TMD)
evolution of the gluon Sivers function and using Color Evaporation Model of
charmonium production. We estimate SSA for JLab, HERMES, COMPASS and eRHIC
energies using recent parameters for the quark Sivers functions which are
fitted using an evolution kernel in which the perturbative part is resummed up
to next-to-leading logarithms (NLL) accuracy. We find that these SSAs are much
smaller as compared to our first estimates obtained using DGLAP evolution but
are comparable to our estimates obtained using TMD evolution where we had used
approximate analytical solution of the TMD evolution equation for the purpose.Comment: Conference proceedings of Light Cone 2014 at Raleigh, NC, USA. Talk
presented by Prof. Anuradha Misra. arXiv admin note: text overlap with
arXiv:1411.083
Heavy Flavour production as probe of Gluon Sivers Function
Heavy flavour production like and - meson production in
scattering of electrons/unpolarized protons off polarized proton target offer
promising probes to investigate gluon Sivers function. In this talk, I will
summarize our recent work on trasverse single spin asymmetry in
-production and - meson production in scattering using a
generalized parton model approach. We compare predictions obtained using
different models of gluon Sivers function within this approach and then, taking
into account the transverse momentum dependent evolution of the unpolarized
parton distribution functions and gluon Sivers function, we study the effect of
evolution on asymmetry.Comment: Proceedings of Light Cone 2016, September 5-8, 2016, Universidade de
Lisboa, Lisbon, Portuga
Stable Frank-Kasper phases of self-assembled, soft matter spheres
Single molecular species can self-assemble into Frank Kasper (FK) phases,
finite approximants of dodecagonal quasicrystals, defying intuitive notions
that thermodynamic ground states are maximally symmetric. FK phases are
speculated to emerge as the minimal-distortional packings of space-filling
spherical domains, but a precise quantitation of this distortion and how it
affects assembly thermodynamics remains ambiguous. We use two complementary
approaches to demonstrate that the principles driving FK lattice formation in
diblock copolymers emerge directly from the strong-stretching theory of
spherical domains, in which minimal inter-block area competes with minimal
stretching of space-filling chains. The relative stability of FK lattices is
studied first using a diblock foam model with unconstrained particle volumes
and shapes, which correctly predicts not only the equilibrium {\sigma} lattice,
but also the unequal volumes of the equilibrium domains. We then provide a
molecular interpretation for these results via self-consistent field theory,
illuminating how molecular stiffness regulates the coupling between
intra-domain chain configurations and the asymmetry of local packing. These
findings shed new light on the role of volume exchange on the formation of
distinct FK phases in copolymers, and suggest a paradigm for formation of FK
phases in soft matter systems in which unequal domain volumes are selected by
the thermodynamic competition between distinct measures of shape asymmetry.Comment: 40 pages, 22 figure
Medial packing and elastic asymmetry stabilize the double-gyroid in block copolymers
Triply-periodic networks are among the most complex and functionally valuable self-assembled morphologies, yet they form in nearly every class of biological and synthetic soft matter building blocks. In contrast to simpler assembly motifs – spheres, cylinders, layers – networks require molecules to occupy variable local environments, confounding attempts to understand their formation. Here, we examine the double-gyroid network phase by using a geometric formulation of the strong stretching theory of block copolymer melts, a prototypical soft self-assembly system. The theory establishes the direct link between molecular packing, assembly thermodynamics and the medial map, a generic measure of the geometric center of complex shapes. We show that “medial packing” is essential for stability of double-gyroid in strongly-segregated melts, reconciling a long-standing contradiction between infinite- and finite-segregation theories. Additionally, we find a previously unrecognized non-monotonic dependence of network stability on the relative entropic elastic stiffness of matrix-forming to tubular-network forming blocks. The composition window of stable double-gyroid widens for both large and small elastic asymmetry, contradicting intuitive notions that packing frustration is localized to the tubular domains. This study demonstrates the utility of optimized medial tessellations for understanding soft-molecular assembly and packing frustration via an approach that is readily generalizable far beyond gyroids in neat block copolymers
Convertible staircase lift
The present work investigates the design and analysis of a staircase lift, which can be used as a Material Handling System. A staircase lift is a mechanical device for lifting people and wheelchairs up and down the stairs, who may find difficulty in doing so themselves. For sufficiently wide stairs, a rail is mounted to the treads of the stairs. A chair or lifting platform is attached to the rail. A person on the chair or platform is lifted as the chair or platform moves along the rail, old age and goods are to be carried across the staircase. A staircase lift is a type of lift that can be mounted on the staircase without altering civil structure. Not only altering, but the person need not change the seat from the wheelchair to the staircase lift, there is a foldable supporting rod for the staircase lift which directly attaches to the wheelchair. This lift runs on electric power and consists of a motor, reduction gearbox, rope drive, two rails and a sliding chair. In this system, we use a DC motor for changing the polarity of the power supply which will make the motor run in reverse direction. Advantages over the conventional hydraulic lift are no civil structure and alteration is required, low cost, less bulkiness, less power, less maintenance required. Easy design, easy installations can be of industrial use too. Moreover, considering some drawbacks due to weight carrying capacity completely depends upon the capacity of the motor. There is a lot of scope for further modification in the project as using a monorail instead of two, using a belt drive or chain drive instead of a rope drive. Incorporation and automation/timer unit will ease the use of the device. Rack and carrier arrangement for using the device for a curved staircase and use of work & roller reduction gear assembly
Conductance of Tomonaga-Luttinger liquid wires and junctions with resistances
We study the effect that resistive regions have on the conductance of a
quantum wire with interacting electrons which is connected to Fermi liquid
leads. Using the bosonization formalism and a Rayleigh dissipation function to
model the power dissipation, we use both scattering theory and Green's function
techniques to derive the DC conductance. The resistive regions are generally
found to lead to incoherent transport. For a single wire, we find that the
resistance adds in series to the contact resistance of h/e^2 for spinless
electrons, and the total resistance is independent of the Luttinger parameter
K_W of the wire. We numerically solve the bosonic equations to illustrate what
happens when a charge density pulse is incident on the wire; the results depend
on the parameters of the resistive and interacting regions in interesting ways.
For a junction of Tomonaga-Luttinger liquid wires, we use a dissipationless
current splitting matrix to model the junction. For a junction of three wires
connected to Fermi liquid leads, there are two families of such matrices; we
find that the conductance matrix generally depends on K_W for one family but is
independent of K_W for the other family, regardless of the resistances present
in the system.Comment: 6 pages, 3 figures; added a discussion of time reversal invariance;
this is the published versio
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