400 research outputs found
Influence of local fullerene orientation on the electronic properties of A3C60 compounds
We have investigated sodium containing fullerene superconductors Na2AC60, A =
Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T
in the temperature range of 10 to 400 K. Despite the structural differences
from the Rb3C60 class of fullerene superconductors, in these compounds the NMR
line of the tetrahedrally coordinated alkali nuclei also splits into two lines
(T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in
the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80
K. Detailed investigations of the spectrum, spin-spin and spin-lattice
relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show
that these two different tetrahedral sites are mixed on a microscopic scale.
The T and T' sites differ in the orientation of first-neighbor C60 molecules.
We present evidence that the orientations of neighboring molecules are
uncorrelated. Thermally activated molecular reorientations cause an exchange
between the T and T' sites and motional narrowing at high temperature. We infer
the same activation energy, 3300 K, in the temperature range 125 to 300 K. The
spin lattice relaxation rate is the same for T and T' down to 125 K but
different below. Both the spin-lattice relaxation rate and Knight shift are
strongly temperature dependent in the whole range investigated. We interpret
this temperature variation by the effect of phonon excitations involving the
rigid librational motion of the C60 molecules. By extending the understanding
of the structure and molecular dynamics of C60 superconductors, these results
may help in clarifying the effects of the structure on the superconducting
properties.Comment: 13 pages, 10 figures, submitted to PR
Charge Order Driven spin-Peierls Transition in NaV2O5
We conclude from 23Na and 51V NMR measurements in NaxV2O5(x=0.996) a charge
ordering transition starting at T=37 K and preceding the lattice distortion and
the formation of a spin gap Delta=106 K at Tc=34.7 K. Above Tc, only a single
Na site is observed in agreement with the Pmmn space group of this first
1/4-filled ladder system. Below Tc=34.7 K, this line evolves into eight
distinct 23Na quadrupolar split lines, which evidences a lattice distortion
with, at least, a doubling of the unit cell in the (a,b) plane. A model for
this unique transition implying both charge density wave and spin-Peierls order
is discussed.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Entanglement in a Solid State Spin Ensemble
Entanglement is the quintessential quantum phenomenon and a necessary
ingredient in most emerging quantum technologies, including quantum repeaters,
quantum information processing (QIP) and the strongest forms of quantum
cryptography. Spin ensembles, such as those in liquid state nuclear magnetic
resonance, have been powerful in the development of quantum control methods,
however, these demonstrations contained no entanglement and ultimately
constitute classical simulations of quantum algorithms. Here we report the
on-demand generation of entanglement between an ensemble of electron and
nuclear spins in isotopically engineered phosphorus-doped silicon. We combined
high field/low temperature electron spin resonance (3.4 T, 2.9 K) with
hyperpolarisation of the 31P nuclear spin to obtain an initial state of
sufficient purity to create a non-classical, inseparable state. The state was
verified using density matrix tomography based on geometric phase gates, and
had a fidelity of 98% compared with the ideal state at this field and
temperature. The entanglement operation was performed simultaneously, with high
fidelity, to 10^10 spin pairs, and represents an essential requirement of a
silicon-based quantum information processor.Comment: 4 pages, 3 figures plus supporting information of 4 pages, 1 figure
v2: Updated reference
An all silicon quantum computer
A solid-state implementation of a quantum computer composed entirely of
silicon is proposed. Qubits are Si-29 nuclear spins arranged as chains in a
Si-28 (spin-0) matrix with Larmor frequencies separated by a large magnetic
field gradient. No impurity dopants or electrical contacts are needed.
Initialization is accomplished by optical pumping, algorithmic cooling, and
pseudo-pure state techniques. Magnetic resonance force microscopy is used for
readout. This proposal takes advantage of many of the successful aspects of
solution NMR quantum computation, including ensemble measurement, RF control,
and long decoherence times, but it allows for more qubits and improved
initialization.Comment: ReVTeX 4, 5 pages, 2 figure
Spin coherence lifetime extension in Tm:YAG through dynamical decoupling
We report on spin coherence lifetime extension on Tm:YAG obtained
through dynamically decoupling the thulium spins from their magnetic
environment. The coherence lifetime reached with a Carr-Purcell-Meiboom-Gill
sequence revealed a 450-fold extension [ ms] with respect
to previously measured values. Comparison to a simple theoretical model allowed
us to estimate the correlation time of the fluctuations of the ground level
transition frequency to s at 1.7 K. For attaining efficient
decoupling sequences, we developed a strategy inspired by the
\emph{zero-first-order Zeeman} effect to minimize the large inhomogeneous
broadening of the ground level spin transition.Comment: 10 pages, 7 figure
Bang-bang control of fullerene qubits using ultra-fast phase gates
Quantum mechanics permits an entity, such as an atom, to exist in a
superposition of multiple states simultaneously. Quantum information processing
(QIP) harnesses this profound phenomenon to manipulate information in radically
new ways. A fundamental challenge in all QIP technologies is the corruption of
superposition in a quantum bit (qubit) through interaction with its
environment. Quantum bang-bang control provides a solution by repeatedly
applying `kicks' to a qubit, thus disrupting an environmental interaction.
However, the speed and precision required for the kick operations has presented
an obstacle to experimental realization. Here we demonstrate a phase gate of
unprecedented speed on a nuclear spin qubit in a fullerene molecule (N@C60),
and use it to bang-bang decouple the qubit from a strong environmental
interaction. We can thus trap the qubit in closed cycles on the Bloch sphere,
or lock it in a given state for an arbitrary period. Our procedure uses
operations on a second qubit, an electron spin, in order to generate an
arbitrary phase on the nuclear qubit. We anticipate the approach will be vital
for QIP technologies, especially at the molecular scale where other strategies,
such as electrode switching, are unfeasible
Vortex State of TlBaCuO via Tl NMR at 2 Tesla
We report a Tl NMR study of vortex state for an aligned
polycrystalline sample of an overdoped high- superconductor
TlBaCuO (85 K) with magnetic field 2 T along
the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at
=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and
vortex melting between 65 and 85 K. No evidence for local antiferromagnetic
ordering at vortex cores was found for our sample.Comment: 4 pages with 5 figure
Electron Spin Density Distribution in the Polymer Phase of CsC 60 : Assignment of the NMR Spectrum
We present high resolution 133 Cs-13 C double resonance NMR data and 13 C-13 C NMR correlation spectra of 13 C enriched samples of the polymeric phase of CsC 60 . These data lead to a partial assignment of the lines in the 13 C NMR spectrum of CsC 60 to the carbon positions on the C 60 molecule. A plausible completion of the assignment can be made on the basis of an ab initio calculation. The data support the view that the conduction electron density is concentrated at the C 60 "equator," away from the interfullerene bonds. PACS numbers: 71.20.Tx, 76.70.Fz The electronic and magnetic properties of the alkali intercalated fullerides, A n C 60 , are still only partly understood. The case A Rb, Cs, n 1 has attracted particular interest The basic structural features of the polymer phase, such as the dimensions of the unit cell, C 60 center positions, and the 2 1 2 cycloaddition polymerization along the crystallographic a axis are widely supported through x-ray diffraction However the degree of deformation of the C 60 balls [25] and the rotational orientation of the polymer chains are less well characterized. Neutron diffraction NMR has proven a useful probe of structure and electronic properties both for the broader class of alkali intercalated fulleride materials In order to obtain sufficient sensitivity, samples of CsC 60 were synthesized using 13 C enriched fullerenes. These were prepared by packing and sintering 13 C enriched amorphous carbon into graphite tubes to create 13 C enriched carbon rods. The fullerenes were subsequently produced by arcing a 60 A, 25 V dc current between an ordinar
Spin-Gap Proximity Effect Mechanism of High Temperature Superconductivity
When holes are doped into an antiferromagnetic insulator they form a slowly
fluctuating array of ``topological defects'' (metallic stripes) in which the
motion of the holes exhibits a self-organized quasi one-dimensional electronic
character. The accompanying lateral confinement of the intervening
Mott-insulating regions induces a spin gap or pseudogap in the environment of
the stripes. We present a theory of underdoped high temperature superconductors
and show that there is a {\it local} separation of spin and charge, and that
the mobile holes on an individual stripe acquire a spin gap via pair hopping
between the stripe and its environment; i.e. via a magnetic analog of the usual
superconducting proximity effect. In this way a high pairing scale without a
large mass renormalization is established despite the strong Coulomb repulsion
between the holes. Thus the {\it mechanism} of pairing is the generation of a
spin gap in spatially-confined {\it Mott-insulating} regions of the material in
the proximity of the metallic stripes. At non-vanishing stripe densities,
Josephson coupling between stripes produces a dimensional crossover to a state
with long-range superconducting phase coherence. This picture is established by
obtaining exact and well-controlled approximate solutions of a model of a
one-dimensional electron gas in an active environment. An extended discussion
of the experimental evidence supporting the relevance of these results to the
cuprate superconductors is given.Comment: 30 pages, 2 figure
Planar 17O NMR study of Pr_yY_{1-y}Ba_2Cu_3O_{6+x}
We report the planar ^{17}O NMR shift in Pr substituted YBa_{2}Cu_{3}O_{6+x},
which at x=1 exhibits a characteristic pseudogap temperature dependence,
confirming that Pr reduces the concentration of mobile holes in the CuO_{2}
planes. Our estimate of the rate of this counterdoping effect, obtained by
comparison with the shift in pure samples with reduced oxygen content, is found
insufficient to explain the observed reduction of T_c. From the temperature
dependent magnetic broadening of the ^{17}O NMR we conclude that the Pr moment
and the local magnetic defect induced in the CuO_2 planes produce a long range
spin polarization in the planes, which is likely associated with the extra
reduction of T_c. We find a qualitatively different behaviour in the oxygen
depleted Pr_yY_{1-y}Ba_2Cu_3O_{6.6}, i.e. the suppression of T is nearly
the same, but the magnetic broadening of the ^{17}O NMR appears weaker. This
difference may signal a weaker coupling of the Pr to the planes in the
underdoped compound, which might be linked with the larger Pr to CuO_2 plane
distance, and correspondingly weaker hybridization.Comment: 8 pages, 9 figures, accepted in Phys Rev
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