Spin-wave contributions to nuclear magnetic relaxation in magnetic metals

The longitudinal and transverse nuclear magnetic relaxation rates 1/T ₁(T) and 1/T ₂(T) are calculated for three- and two-dimensional (3D and 2D) metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic moments in the spin-wave temperature region. The contribution of the one-magnon decay processes is strongly enhanced in comparison with the standard T-linear Korringa term, especially for the FM case. For the 3D AFM case this contribution diverges logarithmically, the divergence being cut at the magnon gap ω due to magnetic anisotropy, and for the 2D AFM case as ω^-1. The electron-magnon scattering processes yield T ²ln(T/ω) and T ²/ω^1/2-terms in 1/T ₁ for the 3D AFM and 2D FM cases, respectively. The two-magnon (“Raman”) contributions are investigated and demonstrated to be large in the 2D FM case. Peculiarities of the isotropic 2D limit (where the correlation length is very large) are analyzed. Received 29 November 1999 and Received in final form 6 June 2000     

Quantum computational gates with radiation free couplings

We examine a generic three level mechanism of quantum computation in which all fundamental single and double qubit quantum logic gates are operating under the effect of adiabatically controllable static (radiation free) bias couplings between the states. Under the time evolution imposed by these bias couplings the quantum state cycles between the two degenerate levels in the ground state and the quantum gates are realized by changing Hamiltonian at certain time intervals when the system collapses to a two state subspace. We propose a physical implementation of the mechanism using Aharonov-Bohm persistent-current loops in crossed electric and magnetic fields, with the output of the loop read out by using a quantum Hall effect aided mechanism. Received 26 March 2002 / Received in final form 8 July 2002 Published online 19 November 2002     

Investigation of nuclear-spin couplings in the lithium fluorides as possible candidates for crystal nuclear magnetic resonance quantum computing devices

We have performed ⁷Li and ¹⁹F nuclear magnetic resonance (NMR) in two lithium fluorides BaLiF₃ and YLiF₄ to explore the possibility of a crystal NMR quantum computing device. We find that (1) both the absolute values and the angular dependences of the line widths can primarily be accounted for by the nuclear dipolar fields, and (2) the spin–lattice relaxation times are long enough for quantum computations. These characteristics indicate that these crystals can be possible candidates for quantum computing devices. We also find that, in the perovskite structures like BaLiF₃, magic angles are quite effective to diminish the nuclear dipole fields, which enables us to treat some nuclei as ‘isolated’. We propose using this feature to create low-dimensional nuclear-spin networks in the crystals. Received: 29 January 2001 / Accepted: 6 February 2001 / Published online: 3 April 2001     

Fast intramolecular dynamics of triphenyl phosphite investigated by <Emphasis Type="Bold">2</Emphasis>H NMR

The first analysis of rapid intramolecular motions of triphenyl phosphite by ²H NMR is presented. The fragile slowing down of the primary relaxation is followed by a solid-echo method. The occurrence of a fast reorientation of the phenyl side groups is demonstrated in the supercooled liquid state, identified as a two-fold flip on the basis of simple lineshape simulations. Coexistence of both static and motionally averaged components in “two phase” spectra indicate a broad distribution of correlation times for this relaxation. This dynamical behavior is shown to persist in the glacial phase. Received 28 May 2002 / Received in final form 1st October 2002 Published online 31 December 2002     

Computational leakage: Grover's algorithm with imperfections

We study the effects of dissipation or leakage on the time evolution of Grover's algorithm for a quantum computer. We introduce an effective two-level model with dissipation and randomness (imperfections), which is based upon the idea that ideal Grover's algorithm operates in a 2-dimensional Hilbert space. The simulation results of this model and Grover's algorithm with imperfections are compared, and it is found that they are in good agreement for appropriately tuned parameters. It turns out that the main features of Grover's algorithm with imperfections can be understood in terms of two basic mechanisms, namely, a diffusion of probability density into the full Hilbert space and a stochastic rotation within the original 2-dimensional Hilbert space. Received 12 August 2002 / Received in final form 14 October 2002 Published online 4 February 2003     

Nuclear magnetic resonance study of ultrananocrystalline diamonds

We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique. Analysis of the ¹³C and ¹H NMR spectra, spin-spin and spin-lattice relaxation times in purified UNCD samples is presented. Our measurements show that UNCD particles consist of a diamond core that is partially covered by a sp ²-carbon fullerene-like shell. The uncovered part of outer diamond surface comprises a number of hydrocarbon groups that saturate the dangling bonds. Our findings are discussed along with recent calculations of the UNCD structure. Significant increase in the spin-lattice relaxation rate (in comparison with that of natural diamond), as well as stretched exponential character of the magnetization recovery, are attributed to the interaction of nuclear spins with paramagnetic centers which are likely fabrication-driven dangling bonds with unpaired electrons. We show that these centers are located mainly at the interface between the diamond core and shell.     

Chain segment ordering in lamellar sublayers of block copolymers: A NMR study

The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with twice the molecular weight. In the diblock, at room temperature, the PDMS block segments undergo uniaxial reorientations around the normal to the lamellae. In the triblock, the reorientational motions exhibit a lower degree of symmetry: deviations from a uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both PDMS chain ends into the PS glassy layers. Received 27 September 2001 and Received in final form 18 January 2002     

End-chain segment ordering in lamellar sublayers of a diblock copolymer

The segmental ordering within a lamellar polystyrene-polydimethylsiloxane diblock (PS-PDMS) was examined by NMR. A non-uniform stretching of the PDMS chains was evidenced. Although chain segments displaying isotropic reorientational motions are present along the chains, relaxation measurements on the free PDMS extremities demonstrate that these latter are oriented and submitted to a non-zero constraint. This specific result allows to precise some features of the order distribution within the lamellae. Received: 27 March 2003 / Accepted: 9 May 2003 / Published online: 4 June 2003 RID="a" ID="a"e-mail: deloche@lps.u-psud.fr     

Molecular order in bilayer,anticlinic and ferroelectric smectic C mesophases

In this work we present a proton NMR comparative study of the molecular order in the smectic C2 (), smectic C^* () and anticlinic smectic C phases ( and ) of two liquid crystalline compounds. The second moments of the experimental proton NMR spectra are well explained taking into account two different molecular orientations models. From the analysis of our experimental results it was possible to estimate the tilt angles for all the smectic C phases studied in this work. Received 15 September 1999 and Received in final form 18 January 2000     

Measurement of the electric quadrupole moments of 26–29Na

The nuclear electric quadrupole moments of the isotopes ²⁶Na, ²⁷Na, ²⁸Na and ²⁹Na were measured by -NMR spectroscopy in single crystals of LiNbO₃ and NaNO₃. High degrees of nuclear polarization were produced by optical pumping of the sodium atoms in a fast beam with a collinear laser beam. The polarized nuclei were implanted into the crystals and NMR signals were observed in the -decay asymmetries. Preparatory measurements also yielded improved values for the magnetic moments of ^27-31Na and confirmed the spin I=3/2 for ³¹Na. The results are discussed in comparison with large-basis shell model calculations. Received: 1 February 2000 / Accepted: 3 April 2000     

Statistical confidentiality tests for a quantum transmission using continuous variables

We describe a cryptographic protocol consisting of two entangled beams of squeezed light which makes use of statistical tests to deduce the secret key bit. The sender (Alice) encrypts a secret key by modulating the phase of the beam sent in public by the receiver (Bob) who keeps the other beam private. The knowledge of the degree of non classical correlation between the beam quadrature components measured in private and in public allows only Bob to decrypt the secret key. With a view towards absolute security, we formally prove that any external intervention from an eavesdropper (Eve) during the communication process introduces necessarily some modification susceptible to be detected. Statistical confidentiality tests are proposed to detect the presence of Eve. Received 12 July 2001 and Received in final form 11 November 2001     

Electrodeposited cobalt films: hcp versus fcc nanostructuring and magnetic properties

The crystallographic structure and morphology of electrodeposited cobalt films on Au(111) is found to be very sensitive on the electrolyte pH value and on the overpotential applied during deposition. The samples, 2 to 500 nm thick, where characterized by nuclear magnetic resonance (NMR), atomic force microscopy (AFM) and electron diffraction. The latter technique shows that the Co films grow in registry with the gold underlayer, reproducing the Au(111) texture. During the first stage of growth and depending on overpotential and pH value, either continuous hcp Co films or hcp Co islands are formed. Only the latter growth mode leads to an out of plane magnetization with 100% of remanence. Increasing the thickness, fcc Co becomes the prevailing phase. Eventually the fcc to hcp ratio saturates at the same value regardless the overpotential. The thickness for which the equilibrium fcc to hcp ratio is obtained as well as the sample structure and morphology before saturation, depend strongly on the overpotential value. In any case, the predominance of the fcc Co phase leads to an in plane magnetization of the thick samples. This study opens up new opportunities of engineering the properties of electrodeposited cobalt films. Received 29 February 2000 and Received in final form 3 July 2000     

Route to and from the NMR chaos in diamagnets

The route to and from the chaos via period doubling bifurcations in nuclear spin system with dipole-dipole interactions is investigated. The transition points are found. It is shown that route from the chaos proceeds according the Feigenbaum scenario. Received 19 August 1998 and Received in final form 15 December 1998     

Mechanical detection of nuclear spin relaxation in a micron-size crystal

A room temperature nuclear magnetic resonance force microscope (MRFM), fitted in a 1 tesla electromagnet, has been used to measure the nuclear spin relaxation of ¹H in a micron-size (70 ng) crystal of ammonium sulfate. NMR sequences, combining both pulsed and continuous wave radio-frequency fields, have allowed us to measure mechanically T₂ and T₁, the transverse and longitudinal spin relaxation times. Because two spin species with different T₁ values are measured in our 7 μm thick crystal, magnetic resonance imaging of their spatial distribution inside the sample section have been performed. To understand quantitatively the measured signal, we carefully study the influence of spin-lattice relaxation and non-adiabaticity of the continuous-wave sequence on the intensity and time dependence of the detected signal. Received 23 February 2000     

Interferometry with entangled atoms

A quantum gravity-gradiometer consists of two spatially separated ensembles of atoms interrogated by pulses of a common laser beam. The laser pulses cause the probability amplitudes of atomic ground-state hyperfine levels to interfere, producing two, motion-sensitive, phase shifts, which allow the measurement of the average acceleration of each ensemble, and, via simple differencing, of the acceleration gradient. Here we propose entangling the quantum states of atoms from the two ensembles prior to the pulse sequence, and show that entanglement encodes their relative acceleration in a single interference phase which can be measured directly, with no need for differencing. Received 6 June 2002 / Received in final form 25 October 2002 Published online 28 January 2003     

Competing polarization mechanisms in the heavy-fermion paramagnet CeCu<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Nuclear magnetic resonance (NMR) and relaxation of ⁶³Cu and ⁶⁵Cu in a powder sample of the heavy-fermion paramagnet CeCu₆ is measured and analysed quantitatively. Five different Cu sites are accessible to a detailed analysis. We derive quadrupolar splitting frequencies, Ce to Cu transferred hyperfine field coupling constants, and transversal as well as longitudinal relaxation behaviour. Only small relaxation anomalies are observed at the orthorhombic to monoclinic structural phase transition of CeCu₆. We point to the different importance of transferred hyperfine interaction and local conduction electron density for static or dynamic part, respectively, of Cu hyperfine interaction. The different sign of the transferred hyperfine interaction from Ce³⁺ to different Cu neighbours reveals the different competing interaction mechanisms, giving rise to the heavy-fermion paramagnetic behavior of CeCu₆. Received 20 November 2001     

Quantum structures in traveling-wave spontaneous parametric down-conversion

We analyze theoretically spatial structures appearing in the far diffraction zone of the electromagnetic field emitted in the cavityless parametric down-conversion. We investigate in detail the spatial correlation functions of intensity and demonstrate the existence of strong quantum correlations between the regions of the far field symmetrical with respect to the optical axis. Our simplified model allows us to obtain analytical results for some limiting cases. We demonstrate that in the limit of small diffraction and ideal quantum efficiency of photodetection the noise reduction in the photocurrent difference between symmetrical regions in the far diffraction field becomes complete at zero frequency of photocurrent fluctuations. Received 5 February 2001 and Received in final form 11 April 2001     

NMR study of the dissolution of laser-polarized xenon

NMR of laser-polarized xenon is used to probe the dissolution behaviour of the noble gas in different liquids. The dissolution and self-relaxation rates are extracted via a macroscopic model, and comparison of the decay rate of the xenon magnetization in deuterated and non-deuterated solvent pairs allows the determination of the pure dipole-dipole contribution to relaxation. A transient convective effect, tentatively assigned to the xenon concentration gradient, is observed and characterized by diffusion encoding MRI experiments. The flow of xenon penetrates inside the solvent near the walls of the NMR tube, the longitudinal images showing a “” shape, the transverse ones a “O” shape. This convection effect has implications for delivery conditions of laser-polarized xenon in continuous flow experiments and magnetic resonance imaging. Received 29 April 2002 / Received in final form 26 July 2002 Published online 22 October 2002 RID="a" ID="a"e-mail: hdesvaux@cea.fr RID="b" ID="b"URA CNRS/CEA 331     

Single-crystal like NMR spectra of CeCu 5 Au powder samples

Single-crystal like nuclear magnetic resonance spin-echo spectra are obtained for powder samples of the antiferromagnet CeCu₅Au in the paramagnetic phase. Line shifts and quadrupolar splittings are analyzed for ⁶³Cu and ⁶⁵Cu. The influence of the extreme magnetic anisotropy of the CeCu_6-xAu_x compounds on nuclear spin relaxation is discussed. Received 19 June 2000     

Modelling of dynamical processes in a molecular crystal by NMR

In this contribution we report on the plastic crystal 1-chloroadamantane dynamics via conventional frequency dependent (¹H and ¹³C) and field cycling NMR measurements. A suitable microscopic dynamical model, worked out from from X-ray analysis is developed and the molecular motions are interpreted in terms of: self diffusion and dipolar molecular axis combined with uniaxial rotation. In the rotator phase the molecules execute a bimodal reorientation process whereas the uniaxial rotation solely persists in the low temperature phase. In both phases, the residence times exhibit an Arrhenius temperature dependence. The results confirm the existence of a dynamic crossover transition predicted by molecular dynamics simulation.