Theory of severe slowdown in the relaxation of rings and clusters with antiferromagnetic interactions

We show that in the severe slowing down temperature regime the relaxation of antiferromagnetic rings and similar magnetic nanoclusters is governed by the quasi-continuum portion of their quadrupolar fluctuation spectrum and not by the lowest excitation lines. This is at the heart of the intriguing near-universal power-law temperature dependence of the electronic correlation frequency $\omega_c$ with an exponent close to 4. The onset of this behavior is defined by an energy scale which is fixed by the lowest spin gap $\Delta_0$. This explains why experimental curves of $\omega_c$ for different cluster sizes and spins nearly coincide when $T$ is rescaled by $\Delta_0$.Comment: new slightly extended version (6 pages, 1 fig. added

Chemical and dynamical speciation of mobile ions in the glassy fast ionic conductor Ag2S+B2S3+SiS2: A 109Ag nuclear magnetic resonance study

109Ag NMR in the highly conductive glass 0.525Ag2S+0.475(0.5B2S3+0.5SiS2) was investigated from 230 to 433 K. The 109Ag NMR spectra reveal for the first time three well resolved lines corresponding to three kinds of chemically speciated Ag ions in sites with different chemical shifts in a macroscopically homogeneous glass. This chemical speciation of Ag ions is discussed in relation to the microstructure of the glass. As the temperature is increased, the three lines that originate from three different species of ions are narrowed, but these lines exist independently up to 433 K, the highest temperature measured. Nuclear spin-lattice relaxation rates (NSLR’s), 1/T1, were also measured. Two relaxation processes were found; one is associated with two of the chemically speciated Ag ions and the other is associated with the other Ag ions. The two different NSLR’s gradually approach a common value as the temperature is increased, and finally exhibit a common relaxation rate at and above 373 K. From the results of the NMR spectra and of the NSLR’s, which observe the ion dynamics on different time scales, it is concluded that the silver ions move fast within separate clusters of similar chemical environments (≫kHz), but exchange among the three different clusters at relatively slow rates (⩽100 Hz) above 373 K. From the time the ions reside at any one site, the mean free path of the ions is estimated

Correlation functions for ionic motion from NMR relaxation and electrical conductivity in the glassy fast-ion conductor (Li2S)0.56(SiS2)0.44

The Li7 NMR spin-lattice relaxation and the electrical conductivity in the typical glassy fast-ion conductor (Li2S)0.56(SiS2)0.44 are discussed from models of Li+ionic motion with distributions of activation energies, as well as from stretched-exponential time-correlation functions. The measured correlation times from the two effects differ by two orders of magnitude, and the derived distributions are shifted greatly relative to each other. We relate the great differences to percolation around the high barriers in the distribution. We present a phenomenological theory that yields good quantitative fits to the observed NMR relaxation with a Gaussian distribution, and to the conductivity and related dielectric properties with the continuous-time random-walk model and the same Gaussian truncated at the percolation limit. This correlates the two effects in a simple and effective way; both time-correlation functions can be calculated approximately from the distributions, and even the dc conductivity can be calculated from the NMR results. The present approach is discussed and compared with previously proposed models to explain the anomalies in ac electrical-conductivity and NMR relaxation rates in glassy fast-ion conductors

Reply to ‘‘Comment on ‘Correlation functions for ionic motion from NMR relaxation and electrical conductivity in the glassy fast-ion conductor (Li2S)0.56(SiS2)0.44’ ’’

Hunt’s Comment criticizes our recent article for combining concepts from percolation theory and effective-medium theories to calculate the dc and ac conductivities in ionic conducting glasses. Our approach was an attempt to describe the dc and ac conductivity with input information from our NMR measurements. We used the continuous-time random-walk theory and reasonable assumptions for the glasses which yielded good fits of the dc and ac conductivities at many temperatures

Relaxation and fluctuations in glassy fast-ion conductors: Wide-frequency-range NMR and conductivity measurements

Li7 nuclear spin-lattice relaxation rates (R1) versus the temperature at several resonance frequencies (4 to 40 MHz) are reported together with the conductivity measurements, σ(ω), in the range 1 Hz to 3.76 MHz on 0.56Li2S+0.44Si2S, a glassy fast-ionic conductor. Both R1 and σ(ω) are fitted consistently over the whole temperature and frequency range by using a stretched-exponential, i.e., exp(-t/τ*c)β for the corresponding correlation functions (CF). Formulas that relate R1(ω) and σ(ω) and that give the asymptotic behavior as functions of T and ω of both quantities are tested experimentally. We find significant differences between βσrelated to σ(ω) and βR related to R1, which implies a difference in the corresponding correlation functions of the ionic diffusional motion. An apparent order-of-magnitude difference in τ*0 attempt times was derived from these conductivity and NMR measurements. The implications of these findings are discussed in terms of the microscopic mechanisms which lead to fluctuations and relaxation in fast-ionic conductors

NMR spin-lattice relaxation and ionic conductivity in lithium thioborogermanate fast-ion-conducting glasse

Li7 nuclear spin-lattice relaxation (NSLR) and ionic conductivity measurements of LiI-doped Li2S+GeS2+B2S3 glasses were performed to investigate the ion hopping dynamics and the non-Arrhenius conductivity behavior that has also been observed in some silver fast-ion-conducting (FIC) glasses. The NMR NSLR experiments were performed at 4 and 8MHz and at 70kHz in the rotating frame over a temperature range of 183–523K. Conductivity measurements on these glasses were performed over the same temperature range to determine if the commonly observed non-Arrhenius ionic conductivity in silver FIC glasses was also observed in lithium FIC glasses. Our previously developed distribution of activation energies (DAE) model was used to fit both the NSLR and conductivity results. It was found that a bimodel DAE was required to fit the broad NSLR maximum. One DAE was associated with lithium ions residing in anion sites created by tetrahedral boron units in the thioborate structural regions of the glass, and the other was associated with lithium ions residing in the anion sites created by the nonbridging sulfur units in the thiogermanate regions of the glass. The average activation energy for the lithium ions residing in the thioborate and thiogermanate sites in the ternary glasses agreed very well with the average activation energies for lithium ions in pure binary thioborate and thiogermanate glasses, respectively, with the thiogermanate energies being significantly larger (∼45 vs ∼30kJ/mol, respectively) than those for the thioborate sites. This trend is in agreement with the fact that the thiogermanate structures possess nonbridging sulfur units whereas the thioborate structures do not. It was found that some of the non-Arrhenius conductivity behavior could be associated with the bimodal DAE and the conductivity could be fit for the most part with the DAE model. However, the strong deviation from Arrhenius behavior at high temperature could not be accounted for and an extension of the DAE model was therefore included. We consider the effect of a small fraction of mobile ions which are thermally excited above the barriers and are assumed to conduct around many (temperature-dependent) filled sites before reaching a second unoccupied site. This ion-trapping model explains well the high-temperature deviation of the conductivity from Arrhenius behavior

Magnetic susceptibility and spin dynamics of a polyoxovanadate cluster: A proton NMR study of a model spin tetramer

We report susceptibility and nuclear magnetic resonance (NMR) measurements in a polyoxovanadate compound with formula (NHEt)3[VIV8VV4As8O40(H2O)]H2O = (V12). The magnetic properties can be described by considering only the central square of localized V4+ ions and treated by an isotropic Heisenberg Hamiltonian of four intrinsic spins 1/2 coupled by nearest-neighbor antiferromagnetic interaction with J17.6K. In this simplified description the ground state is nonmagnetic with ST = 0. The 1H NMR linewidth (full width at half maximum) data depend on both the magnetic field and temperature, and are explained by the dipolar interaction between proton nuclei and V4+ ion spins. The behavior of the nuclear spin-lattice relaxation rate T-11 in the temperature range (4.2–300 K) is similar to that of χT vs T and it does not show any peak at low temperatures contrary to previous observations in antiferromagnetic rings with larger intrinsic spins. The results are explained by using the general features of the Moriya formula and by introducing a single T-independent broadening parameter for the electronic spin system. From the exponential T dependence of T-11 at low T(2.5K < T < 4.2K) we have obtained a field dependent gap following the linear relation ΔNMR = Δ0 “ gπBH, with the gap Δ0 17.6K in agreement with the susceptibility data. Below 2.5 K the proton T-11 deviates from the exponential decrease indicating the presence of a small, almost temperature independent, but strongly field dependent, nuclear relaxation contribution, which we will investigate in detail in the near future. © 2004 American Physical Society

Fisica con applicazioni in biologia e in medicina

Elementi di fisica classica con cenni di meccanica quantistica. Sono descritti numerose applicazioni biomediche quali per esempio la circolazione del sangue, la statica delle articolazioni, la centrifugazione, la respirazione, il trasporto elettrico nelle cellule e numerose strumentazioni biomedich

Lezioni di fisica con laboratorio

Vengono trattati in modo elementare :la Meccanica, Statica e Dinamica dei Fluidi,Termodinamica,Acustica e Ottica ,Elettromagnetismo , Materia e radiazioni. Contgiene in appendice la descrizione di quattro esperienze di laboratori