808 research outputs found
Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned
The complex resistivity of the vortex lattice in an
untwinned crystal of 93-K has been measured at frequencies
from 100 kHz to 20 MHz in a 2-Tesla field ,
using a 4-probe RF transmission technique that enables continuous measurements
versus and temperature . As is increased, the inductance increases steeply to a cusp
at the melting temperature , and then undergoes a steep collapse
consistent with vanishing of the shear modulus . We discuss in detail
the separation of the vortex-lattice inductance from the `volume' inductance,
and other skin-depth effects. To analyze the spectra, we consider a weakly
disordered lattice with a low pin density. Close fits are obtained to
over 2 decades in . Values of the pinning parameter
and shear modulus obtained show that collapses by
over 4 decades at , whereas remains finite.Comment: 11 pages, 8 figures, Phys. Rev. B, in pres
Universal Quantum Computation using Exchange Interactions and Teleportation of Single-Qubit Operations
We show how to construct a universal set of quantum logic gates using control
over exchange interactions and single- and two-spin measurements only.
Single-spin unitary operations are teleported instead of being executed
directly, thus eliminating a major difficulty in the construction of several of
the most promising proposals for solid-state quantum computation, such as
spin-coupled quantum dots, donor-atom nuclear spins in silicon, and electrons
on helium. Contrary to previous proposals dealing with this difficulty, our
scheme requires no encoding redundancy. We also discuss an application to
superconducting phase qubits.Comment: 4.5 pages, including 2 figure
Entanglement and Quantum Phase Transitions via Adiabatic Quantum Computation
For a finite XY chain and a finite two-dimensional Ising lattice, it is shown
that the paramagnetic ground state is adiabatically transformed to the GHZ
state in the ferromagnetic phase by slowly turning on the magnetic field. The
fidelity between the GHZ state and an adiabatically evolved state shows a
feature of the quantum phase transition.Comment: Revise
Encoded Recoupling and Decoupling: An Alternative to Quantum Error Correcting Codes, Applied to Trapped Ion Quantum Computation
A recently developed theory for eliminating decoherence and design
constraints in quantum computers, ``encoded recoupling and decoupling'', is
shown to be fully compatible with a promising proposal for an architecture
enabling scalable ion-trap quantum computation [D. Kielpinski et al., Nature
417, 709 (2002)]. Logical qubits are encoded into pairs of ions. Logic gates
are implemented using the Sorensen-Molmer (SM) scheme applied to pairs of ions
at a time. The encoding offers continuous protection against collective
dephasing. Decoupling pulses, that are also implemented using the SM scheme
directly to the encoded qubits, are capable of further reducing various other
sources of qubit decoherence, such as due to differential dephasing and due to
decohered vibrational modes. The feasibility of using the relatively slow SM
pulses in a decoupling scheme quenching the latter source of decoherence
follows from the observed 1/f spectrum of the vibrational bath.Comment: 12 pages, no figure
Neutrino masses and mixing with seesaw mechanism and universal breaking of extended democracy
In the framework of a minimal extension of the SM, where the only additional
fields are three right-handed neutrinos, we suggest that the charged lepton,
the Dirac neutrino and the right-handed Majorana neutrino mass matrices are
all, to leading approximation, proportional to the democratic matrix. With the
further assumption that the breaking of this extended democracy is universal
for all leptonic mass matrices, a large mixing in the 2-3 sector can be
obtained and is linked to the seesaw mechanism, together with the existence of
a strong hierarchy in the masses of right-handed neutrinos. The structure of
the resulting effective mass matrix of light neutrinos is stable against the
RGE evolution, and a good fit to all solar and atmospheric neutrino data is
obtained.Comment: LaTeX, 17 pages, 2 eps figures. A section on RGE evolution and a few
references added; minor typos correcte
Structural Expansion of Cyclohepta[def]fluorene towards Azulene-Embedded Non-Benzenoid Nanographenes
Complete population transfer in a degenerate 3-level atom
We find conditions required to achieve complete population transfer, via
coherent population trapping, from an initial state to a designated final state
at a designated time in a degenerate 3-level atom, where transitions are caused
by an external interaction. Complete population transfer from an initially
occupied state 1 to a designated state 2 occurs under two conditions. First,
there is a constraint on the ratios of the transition matrix elements of the
external interaction. Second, there is a constraint on the action integral over
the interaction, or "area", corresponding to the phase shift induced by the
external interaction. Both conditions may be expressed in terms of simple odd
integers.Comment: 22 pages, 4 figure
Experimental Implementation of the Quantum Random-Walk Algorithm
The quantum random walk is a possible approach to construct new quantum
algorithms. Several groups have investigated the quantum random walk and
experimental schemes were proposed. In this paper we present the experimental
implementation of the quantum random walk algorithm on a nuclear magnetic
resonance quantum computer. We observe that the quantum walk is in sharp
contrast to its classical counterpart. In particular, the properties of the
quantum walk strongly depends on the quantum entanglement.Comment: 5 pages, 4 figures, published versio
Thermopower and thermal conductivity of superconducting perovskite
The thermopower and thermal conductivity of superconducting perovskite
( 8 K) have been studied. The thermopower is negative
from room temperature to 10 K. Combining with the negative Hall coefficient
reported previously, the negative thermopower definetly indicates that the
carrier in is electron-type. The nonlinear temperature dependence of
thermopower below 150 K is explained by the electron-phonon interaction
renormalization effects. The thermal conductivity is of the order for
intermetallics, larger than that of borocarbides and smaller than . In
the normal state, the electronic contribution to the total thermal conductivity
is slightly larger than the lattice contribution. The transverse
magnetoresistance of is also measured. It is found that the classical
Kohler's rule is valid above 50 K. An electronic crossover occures at , resulting in the abnormal behavior of resistivity, thermopower, and
magnetoresistance below 50 K.Comment: Revised on 12 September 2001, Phys. Rev. B in pres
Principles of crop modeling and simulation: I. uses of mathematical models in agricultural science
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