Li NMR in lithium borate glasses

In anticipation of using fluctuations in the nuclear dipolar and quadrupolar interaction as a probe of lithium ion motion in lithium borate glasses, the static values of these interactions were measured using a variety of echo techniques. The static quadrupolar echo spectrum of ⁷Li and a calculation of the dipolar interaction in crystalline Li₂B₄O₇ (same chemical composition as the glass under study) were used to estimate the strength of the two interactions. These indicate that the dipolar and quadrupolar interactions for ⁶Li will be of similar size and the dipolar interaction will be dominated by the unlike spin interaction between the ⁶Li and the ¹⁰B, ¹¹B spins. An appropriate theoretical model is proposed and explicit expressions for the echo amplitude are calculated in terms of the dipolar and quadrupolar second moments. This single spin model takes into account the quadrupolar interaction but treats the dipolar interaction as an effective magnetic field. Experimental results are presented which show the essential validity of the model and measurements lead to reasonable values for the dipolar and quadrupolar second moments. The relative merits of the various echo techniques are discussed.     

6Li NMR in lithium borate glasses

In anticipation of using fluctuations in the nuclear dipolar and quadrupolar interaction as a probe of lithium ion motion in lithium borate glasses, the static values of these interactions were measured using a variety of echo techniques. The static quadrupolar echo spectrum of ⁷Li and a calculation of the dipolar interaction in crystalline Li₂B₄O₇ (same chemical composition as the glass under study) were used to estimate the strength of the two interactions. These indicate that the dipolar and quadrupolar interactions for ⁶Li will be of similar size and the dipolar interaction will be dominated by the unlike spin interaction between the ⁶Li and the ¹⁰B, ¹¹B spins. An appropriate theoretical model is proposed and explicit expressions for the echo amplitude are calculated in terms of the dipolar and quadrupolar second moments. This single spin model takes into account the quadrupolar interaction but treats the dipolar interaction as an effective magnetic field. Experimental results are presented which show the essential validity of the model and measurements lead to reasonable values for the dipolar and quadrupolar second moments. The relative merits of the various echo techniques are discussed.     

Periodic Minimizers in 1D Local Mean Field Theory

There are not many physical systems where it is possible to demonstate rigorously that energy minimizers are periodic. Using reflection positivity techniques we prove, for a class of mesoscopic free-energies representing 1D systems with competing interactions, that all minimizers are either periodic, with zero average, or of constant sign. Examples of both phenomena are given. This extends our previous work where such results were proved for the ground states of lattice systems with ferromagnetic nearest neighbor interactions and dipolar type antiferromagnetic long range interactions.     

Magnetic dipolar ordering and quantum phase transition in an Fe8 molecular magnet

We show that a crystal of mesoscopic Fe(8) single-molecule magnets is an experimental realization of the quantum Ising model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the quantum and classical phase transitions between the paramagnetic and ferromagnetic phases, at thermodynamical equilibrium. The phase diagram and critical exponents that we obtain agree with expectations for the mean-field universality class.     

Mesoscopic Modeling for Continuous Spin Lattice Systems: Model Problems and Micromagnetics Applications

In this paper we derive deterministic mesoscopic theories for model continuous spin lattice systems both at equilibrium and non-equilibrium in the presence of thermal fluctuations. The full magnetic Hamiltonian that includes singular integral (dipolar) interactions is also considered at equilibrium. The non-equilibrium microscopic models we consider are relaxation-type dynamics arising in kinetic Monte Carlo or Langevin-type simulations of lattice systems. In this context we also employ the derived mesoscopic models to study the relaxation of such algorithms to equilibrium     

Cold atoms and molecules in self-assembled dipolar lattices

We study the realization of lattice models, where cold atoms and molecules move as extra particles in a dipolar crystal of trapped polar molecules. The crystal is a self-assembled floating mesoscopic lattice structure with quantum dynamics given by phonons. We show that within an experimentally accessible parameter regime extended Hubbard models with tunable long-range phonon-mediated interactions describe the effective dynamics of dressed particles.     

Observation of quantum coherence in mesoscopic molecular magnets

By applying a transverse magnetic field B( perpendicular) of sufficient strength to the uniaxial molecular magnets Fe8 and Mn12, the tunneling splitting Delta(t) of their S = +/-10 magnetic ground states can be made large compared to perturbations such as hyperfine and dipolar interactions. We present evidence for such a Delta(t) from magnetic specific heat data below 1 K that is consistent with coherent quantum mechanical tunneling in a "mesoscopic" system under such conditions.     

Mesoscopic linear alignment and thermal-relaxation dynamics of aggregated gold nanorods

Aqueous gold nanorod colloids aggregated by Cl^- are further assembled into linearly aligned structures of linear bundles during evaporation on TEM grids. Gold nanorods in bundles are also oriented in nearly head-to-tail shapes in the microscopic scale. Induced dipolar long-range interactions in the mesoscopic scale are suggested to drive gold nanorods to aggregate. Although surface-plasmon absorption at transverse resonances decreases, that at longitudinal resonances increases with aggregation. The photon-thermalized heat of the dispersed and the aggregated gold nanorods dissipates to immediately surrounding media on the time scales of 100 and 800 ps, respectively.     

Spin-Echo Behavior of Nonintegral-Spin Quadrupolar Nuclei in Inorganic Solids

The response to spin-echo radiofrequency pulse excitation of a variety of nonintegral-spin quadrupolar nuclei (²³Na, ²⁷Al, and ⁹³Nb) in inorganic solids (single-crystal ruby and sapphire, α-Al₂O₃, γ-Al₂O₃, AlN, NaNO_3, KNbO₃, NaNbO₃, LiNbO₃, albite, and the zeolite Linde A), subject to strong quadrupolar interactions and dipolar interactions of varying strength, is reported. It is demonstrated that "soft" RF pulse excitation with a pair of selective π/2 and π pulses yields predictable spin-echo decay behavior as a function of dipolar interaction, the experimental results being in good agreement with the theoretical predictions.     

Functional magnetic resonance imaging with intermolecular multiple-quantum coherences

For the first time, we demonstrate here functional magnetic resonance imaging (fMRI) using intermolecular multiple-quantum coherences (iMQCs). iMQCs are normally not observed in liquid-state NMR because dipolar interactions between spins average to zero. If the magnetic isotropy of the sample is broken through the use of magnetic field gradients, dipolar couplings can reappear, and hence iMQCs can be observed. Conventional (BOLD) fMRI measures susceptibility variations averaged over each voxel. In the experiment performed here, the sensitivity of iMQCs to frequency variations over mesoscopic and well-defined distances is exploited. We show that iMQC contrast is qualitatively and quantitatively different from BOLD contrast in a visual stimulation task. While the number of activated pixels is smaller in iMQC contrast, the intensity change in some pixels exceeds that of BOLD contrast severalfold.     

A neutron diffraction study of the GdA103 magnetic structure

Neutron diffraction experiments have permitted to determine the magnetic order in the GdA10₃ perovskite compound. GdA10₃ is a simple two-sublattice antiferromagnet well described by the Gx-mode. This ordering results from isotropic and negative next neighbours exchange interactions which are more important than dipolar interactions.     

High-field EPR on the cyclic spin cluster Fe6(tea)6

The eigenstates of the antiferromagnetic spin cluster Fe₆(tea)₆ are studied by electron paramagnetic resonance spectroscopy at 9.5 and 94 GHz in order to characterize the interactions which are important for the magnetic properties of the molecule. Experimental results could be obtained for the first excited spin states withS= 1, 2, 3 and 4. This enables a very accurate and detailed analysis of the intramolecular interactions. It turns out that the magnetic properties of Fe₆(tea)₆ are determined by the isotropic exchange (J/k_B = ?31.5 K), the ligand-field interaction of the Fe^III ions and the dipolar interaction. Other kinds of anisotropic interactions are of minor importance.     

Antiferromagnetic linear chains in the crystalline free radical tanol

TANOL (tetramethyl-2,2,6,6, piperidinol-4, oxyl-1), which shows a broad maximum in its specific heat near 4 K was known as an example of the Heisenberg type one-dimensional antiferromagnet. The specific heat has been measured down to 0.35 K. A sharp peak appears at 0.49 K (T_N) and a T³ law is observed in the low temperature region. The interchain interactions which can be deduced are the same order of magnitude as the spin-spin dipolar interactions. The linear chain characteristics of TANOL are compared to those of some other products.     

Information Entropy Dynamics in Spin-1 Dipolar Bose-Einstein Condensates

Information entropy is an important topic due to its relevance to cold atom system. Motivated by the recent work in a scalar dipolar Bose-Einstein condensates (BECs), we extend this issue to the dynamics of information entropy in spin-1 dipolar BECs. Our results show that the periodicity of S_r, S_k and S is broken in the presence of magnetic dipole-dipole interactions (MDDIs). With the increase of dipole strength, the total entropy S and momentum component S_k increase, wile S_r decreases. This is completely opposite with scalar dipolar BECs. In particular, the order parameter δ decay quickly with stronger dipolar interaction, showing that the increase of dipole strength makes the system become more and more disordered.

     

Dipolar temperature and multiple-quantum NMR dynamics in dipolar ordered-spin systems

We investigate analytically and numerically the multiple-quantum (MQ) NMR dynamics in systems of nuclear spins 1/2 coupled by dipole-dipole interactions in the case of the dipolar-ordered initial state. We suggest a new method of MQ NMR based on the measurement of the dipolar temperature in the quasi-equilibrium state, which establishes after the time of order ω _loc ⁻¹ (ω_loc is the dipolar local field) after the MQ NMR experiment. Manyspin clusters and correlations are created faster in such an experiment than in usual MQ NMR experiments and can be used for the investigation of the many-spin dynamics of nuclear spins in solids. The text was submitted by the authors in English.     

Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC

We compare several hetero-nuclear dipolar recoupling sequences available for HMQC or HSQC experiments applied to spin-1/2 and quadrupolar nuclei. These sequences, which are applied to a single channel, are based either on the rotary resonance recoupling (R3) irradiation, or on two continuous rotor-synchronized modulations (SFAM1 and SFAM2), or on four symmetry-based sequences (R2(1)1,SR4(1)2,R12(3)5,R20(5)9), or on the REDOR scheme. We analyze systems exhibiting purely hetero-nuclear dipolar interactions as well as systems where homo-nuclear dipolar interactions need to be canceled. A special attention is given to the behavior of these sequences at very fast MAS. It is shown that R3 methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are SR4(1)2 and SFAM (SFAM1 or SFAM2 if homo-nuclear interactions are not negligible). REDOR can also recouple efficiently hetero-nuclear dipolar interactions, provided the sequence is sent on the non-observed channel and homo-nuclear dipolar interactions are negligible. We anticipate that at ultra-fast spinning speed, SFAM1 and SFAM2 will be the most efficient methods.     

Dielectric response of disordered Pb(Sc1/2Ta1/2)O3 single crystal under hydrostatic pressure

The effect of hydrostatic pressure on the dielectric response of Pb(Sc_1/2Ta_1/2)O₃ single crystal with the degree of order S = 0.16 has been studied. At elevated pressures the dielectric anomaly characteristic of ferroelectric relaxors is shifted to lower temperatures, its amplitude decreases and the relaxor peak becomes more diffused. In the pressure range studied up to 0.75 GPa, the temperature dependencies of the characteristic relaxation time obey the Vogel–Fulcher law. It has been found that the Vogel–Fulcher temperature decreases with increasing pressure while the activation energy of the dipolar entities increases. This increase can be ascribed to the pressure-induced changes in the atom–atom and dipolar interactions, resulting in the increased energy barriers for dipolar fluctuations.     

Neutron diffraction study of the magnetic order in the ternary superconductor GdMo6S8

The coexistence of antiferromagnetism and superconductivity has been demonstrated in GdMo₆S₈ by neutron diffraction experiments. Although the magnetic unit cell (2a,a,a) is determined by the RKKY interactions, the direction of magnetic moments is determined by the anisotropic dipolar interactions i.e. in the (100) cubic plane.     

Calculation of Double-Quantum-Coherence Two-dimensional Spectra: Distance Measurements and Orientational Correlations

The double-quantum-coherence (DQC) echo signal for two coupled nitroxides separated by distances ?10 Å, is calculated rigorously for the six-pulse sequence. Successive application of six pulses on the initial density matrix, with appropriate inter-pulse time evolution and coherence pathway selection leaves only the coherent pathways of interest. The amplitude of the echo signal following the last π pulse can be used to obtain a one-dimensional (1D) dipolar spectrum (Pake doublet), and the echo envelope can be used to construct the 2D DQC spectrum. The calculations are carried out using the product space spanned by the two electron-spin magnetic quantum numbers m ₁, m ₂ and the two nuclear-spin magnetic quantum numbers M ₁, M ₂, describing, e.g. two coupled nitroxides in bilabeled proteins. The density matrix is subjected to a cascade of unitary transformations taking into account dipolar and electron exchange interactions during each pulse and during the evolution in the absence of a pulse. The unitary transformations use the eigensystem of the effective spin Hamiltonians obtained by numerical matrix diagonalization. Simulations are carried out for a range of dipolar interactions, D, and microwave magnetic field strength B ₁ for both fixed and random orientations of the two ¹⁴N (and ¹⁵N) nitroxides. Relaxation effects were not included. Several examples of 1D and 2D Fourier transforms of the time-domain signals versus dipolar evolution and spin-echo envelope time variables are shown for illustration. Comparisons are made between 1D rigorous simulations and analytical approximations. The rigorous simulations presented here provide insights into DQC electron spin resonance spectroscopy, they serve as a standard to evaluate the results of approximate theories, and they can be employed to plan future DQC experiments.