Slow Relaxation Rates Research Articles

Temperature and compression rate independent domain shape in Langmuir monolayers of di- n-dodecyl hydrogen phosphate at the air–water interface

Thermodynamic and morphological properties of Langmuir monolayers of di- n-dodecyl hydrogen phosphate (DDP) have been studied by film balance and Brewster angle microscopy (BAM) over a wide range of temperature between 5 and 40 °C. From π – A isotherms, a generalized phase diagram consisting of gas (G), liquid expanded (LE) and liquid condensed (LC) phases is constructed for the DDP monolayers. The BAM images show the formation of gas bubble in the bright background of LE phase during G–LE phase transitions and fingering LC domains during LE–LC phase transitions. The shapes of these domains are independent of temperature, showing a sharp contrast to the temperature-dependent monolayer morphologies of amphiphilic systems where the shape of the LC domains changes either from compact circular to fingering or from irregular or spiral to compact patterns with increasing temperature. In addition, the domains do not show any change in their shapes with decreasing the compression rate. Since the two-alkyl chains are directly attached by covalent bonds to the phosphate group, the rearrangement of the molecules needs to move the whole molecules including the hydration sphere. The difficulty related to such a movement of the molecules causes the fingering domains, which are independent of external variables. Although the domains are formed in a fingering shape, the equilibrium shape can be attained by about 120 min at 15 °C indicating a rather slow relaxation rate.

Advances on the Nanostructuration of Magnetic Molecules on Surfaces: The Case of Single‐Molecule Magnets (SMM)

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Shielding of N×M projectiles in a collisional, self-gravitating, generalized Lorentzian dusty plasma

The shielding potential of N×M projectiles passing through a collisional, self-gravitating dust contaminated plasma is studied by mean of linearized dielectric theory for a generalized Lorentzian plasma. The correlation and interference effects of projectiles on the shielded potential as well as on the energy loss are presented. The amplitude of the shielded potential is enhanced with the increase of dust Jeans frequency for separation less than the effective Debye length. The dust charge fluctuations produce a potential well for slow charge relaxation rate and the energy is gained by the test charge projectiles. However, for fast charge relaxation rate with a fixed value of Jeans frequency, the energy loss is enhanced. The dust neutral collisions also enhance the energy loss for the test charge velocities greater than the dust acoustic speeds.

Effect of self-gravitation and dust-charge fluctuations on the shielding and energy loss of N×M projectiles in a collisional dusty plasma

A simple derivation of the electrostatic potential and energy loss of N×M test charge projectiles traveling through dusty plasma has been presented. The effect of dust-charge fluctuations, dust neutral collisions, and self-gravitation on the shielded potential and energy loss of charge projectiles has been investigated both analytically as well as numerically. An interference contribution of these projectiles to the shielded potential and energy loss has been observed, which depends upon their relative orientation and separation distance. A comparison has been made for correlated and uncorrelated motion of the two projectiles. The amplitude of the shielded potential is enhanced with the increase of dust Jeans frequency for separation less than the effective Debye length. The dust-charge fluctuations produce a potential well for a slow charge relaxation rate and energy is gained, not lost, by the test charge projectiles. However, a fast charge relaxation rate with a fixed value of Jeans frequency enhances the energy loss. The dust neutral collisions are also found to enhance the energy loss for the test charge velocities greater than the dust acoustic speeds. The present investigation might be useful to explain the coagulation of dust particles such as those in molecular clouds, the interstellar medium, comet tails, planetary rings, etc.

Advances on the nanostructuration of magnetic molecules on surfaces: the case of single-molecule magnets (SMM)

SMMs exhibit slow magnetization relaxation rates characteristic of nanodomain particles whose origin is however on individual molecules. For this reason, they have attracted much interest due to their potential applications in high-density information storage devices and quantum computing applications, where for instance, each molecule can be used as a magnetic bit of information. However, for this to become a reality, several basic studies such as their deposition on surfaces are still highly required. Here we will revise all the experimental approximations that have been so far reported for their addressing, nanostructuration and study on surfaces, from the use of stamps as templates to their anchorage to gold surface through the use of thiol-based ligands. It is also important to emphasize that the results and methodologies described along this review are applicable not only to SMMs but to any molecular material.

Spin Polarization in Single Spin Experiments on Defects in Diamond

AbstractGeneration of pure spin states is an important step towards coherent control of single spin systems. Especially for nitrogen‐vacancy defects in diamond, where readout of single spins using optical detection is available, fast initialization of spin is significant in the context of quantum computing applications. In this system the spin polarization is caused by an intersystem crossing process via the meta‐stable singlet state 1A. The slowest relaxation rate is the intersystem to the triplet ground‐state 3A on a timescale of 400 ns. The intersystem crossing process mainly populates the ms = 0 spin sublevels of the paramagnetic ground state. Although no direct nuclear spin polarization is seen for low magnetic fields, frequency selective microwave pulses can be used to transfer electron spin polarization to 13C nuclear spin states.

Opportunities for mesoscopics in thermometry and refrigeration: Physics and applications

This review presents an overview of the thermal properties of mesoscopic structures. The discussion is based on the concept of electron energy distribution, and, in particular, on controlling and probing it. The temperature of an electron gas is determined by this distribution: refrigeration is equivalent to narrowing it, and thermometry is probing its convolution with a function characterizing the measuring device. Temperature exists, strictly speaking, only in quasiequilibrium in which the distribution follows the Fermi-Dirac form. Interesting nonequilibrium deviations can occur due to slow relaxation rates of the electrons, e.g., among themselves or with lattice phonons. Observation and applications of nonequilibrium phenomena are also discussed. The focus in this paper is at low temperatures, primarily below $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where physical phenomena on mesoscopic scales and hybrid combinations of various types of materials, e.g., superconductors, normal metals, insulators, and doped semiconductors, open up a rich variety of device concepts. This review starts with an introduction to theoretical concepts and experimental results on thermal properties of mesoscopic structures. Then thermometry and refrigeration are examined with an emphasis on experiments. An immediate application of solid-state refrigeration and thermometry is in ultrasensitive radiation detection, which is discussed in depth. This review concludes with a summary of pertinent fabrication methods of presented devices.

μSR study of spin fluctuations in RMn 4Al 8 compounds ( [formula omitted], Dy, Ho, Er)

The compounds RMn 4Al 8 are characterized by very weak magnetic interactions between the rare earth (R) sites and the Mn sites. The Mn sites form one-dimensional chains and it likely that this inhibits moment formation and leads to low frequency spin fluctuations. ZF-μSR studies show similar behaviour in all four compounds with R = Tb , Dy, Ho and Er. The relaxation spectrum has two components: the slow relaxation rate peaks below the temperature where the ZFC susceptibility has a cusp, associated with ordering of the Mn moments. The second component has a high relaxation rate that increases rapidly at low temperatures. We tentatively attribute this to the slowing down of the rare earth spin dynamics.

Effect of charge fluctuations and collisions on the energy loss of N×M projectiles for a generalized Lorentzian dusty plasma

The shielded potential and the energy loss by N×M projectiles passing through a collisional dust-contaminated plasma with dust-charge fluctuations and grain-size distribution are presented. Analytical general expressions are obtained for the shielded potential and for the energy loss by considering two-body correlation effects. An interference contribution of these projectiles to the potential and energy loss is observed which depends upon their orientation and separation distance. The dust-charge fluctuation produces a potential well instead of Coulomb-type potential for a slowly moving test charge with slow charge relaxation rate and energy is gained by the charged projectiles. However, fast charge relaxation enhances the energy loss and some peaks are observed showing the excitation of some electrostatic modes. On the other hand, the dust neutral collisions also enhance the energy loss for projectile velocities greater than the dust acoustic speed for a Maxwellian plasma (for a large value of the spectral index κ).

Quantum Glassiness in Strongly Correlated Clean Systems: An Example of Topological Overprotection

This Letter presents solvable examples of quantum many-body Hamiltonians of systems that are unable to reach their ground states as the environment temperature is lowered to absolute zero. These examples, three-dimensional generalizations of quantum Hamiltonians proposed for topological quantum computing, (1) have no quenched disorder, (2) have solely local interactions, (3) have an exactly solvable spectrum, (4) have topologically ordered ground states, and (5) have slow dynamical relaxation rates akin to those of strong structural glasses.

Accelerated magnetic relaxation of transient disordered vortex states in Bi 2Sr 2CaCu 2O 8+ δ

Time relaxation of the local persistent current in a Bi 2Sr 2CaCu 2O 8+ δ crystal was magneto-optically recorded after suddenly exposing the crystal to a constant magnetic field between 140 and 840 G. A remarkable behavior of the relaxation is revealed for fields below the order–disorder vortex phase transition: The relaxation starts at a slow rate, it then accelerates for a short period of time, after which a slow relaxation rate is resumed. We show that this acceleration marks a local transition of a transient disordered vortex state to a thermodynamically favored quasi-ordered vortex phase. The slow relaxation rates, before and after the transition, reflect ordinary thermally activated flux creep process in the disordered and quasi-ordered vortex phases, respectively.

Strategy for the study of paramagnetic proteins with slow electronic relaxation rates by nmr spectroscopy: application to oxidized human [2Fe-2S] ferredoxin.

NMR studies of paramagnetic proteins are hampered by the rapid relaxation of nuclei near the paramagnetic center, which prevents the application of conventional methods to investigations of the most interesting regions of such molecules. This problem is particularly acute in systems with slow electronic relaxation rates. We present a strategy that can be used with a protein with slow electronic relaxation to identify and assign resonances from nuclei near the paramagnetic center. Oxidized human [2Fe-2S] ferredoxin (adrenodoxin) was used to test the approach. The strategy involves six steps: (1) NMR signals from (1)H, (13)C, and (15)N nuclei unaffected or minimally affected by paramagnetic effects are assigned by standard multinuclear two- and three-dimensional (2D and 3D) spectroscopic methods with protein samples labeled uniformly with (13)C and (15)N. (2) The very broad, hyperfine-shifted signals from carbons in the residues that ligate the metal center are classified by amino acid and atom type by selective (13)C labeling and one-dimensional (1D) (13)C NMR spectroscopy. (3) Spin systems involving carbons near the paramagnetic center that are broadened but not hyperfine-shifted are elucidated by (13)C[(13)C] constant time correlation spectroscopy (CT-COSY). (4) Signals from amide nitrogens affected by the paramagnetic center are assigned to amino acid type by selective (15)N labeling and 1D (15)N NMR spectroscopy. (5) Sequence-specific assignments of these carbon and nitrogen signals are determined by 1D (13)C[(15)N] difference decoupling experiments. (6) Signals from (1)H nuclei in these spin systems are assigned by paramagnetic-optimized 2D and 3D (1)H[(13)C] experiments. For oxidized human ferredoxin, this strategy led to assignments (to amino acid and atom type) for 88% of the carbons in the [2Fe-2S] cluster-binding loops (residues 43-58 and 89-94). These included complete carbon spin-system assignments for eight of the 22 residues and partial assignments for each of the others. Sequence-specific assignments were determined for the backbone (15)N signals from nine of the 22 residues and ambiguous assignments for five of the others.

Synthesis, structure, and magnetic properties of a Mn(21) single-molecule magnet.

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) (1; 3Mn(III)) with [Mn(10)O(4)(OH)(2)(O(2)CMe)(8)(hmp)(8)](ClO(4))(4) (2; 10Mn(III)) in MeCN affords the new mixed-valent complex [Mn(21)O(14)(OH)(2)(O(2)CMe)(16)(hmp)(8)(pic)(2)(py)(H(2)O)](ClO(4))(4) (3; 3Mn(II)-18Mn(III); hmp(-) is the anion of 2-(hydroxymethyl)pyridine), with an average Mn oxidation state of +2.85. Complex 3.7MeCN crystallizes in the triclinic space group P. The structure consists of a low symmetry [Mn(21)(micro(4)-O)(4)(micro(3)-O)(12)(micro-O)(16)] core, with peripheral ligation provided by 16 MeCO(2)(-), 8 hmp(-), and 2 pic(-) groups and one molecule each of water and pyridine. The magnetic properties of 3 were investigated by both dc and ac magnetic susceptibility measurements. Fitting of dc magnetization data collected in the 0.1-0.8 T and 1.8-4.0 K ranges gave S = (17)/(2), D approximately -0.086 cm(-)(1), and g approximately 1.8, where S is the molecular spin of the Mn(21) complex and D is the axial zero-field splitting parameter. ac susceptibility studies in the 10-997 Hz frequency range reveal the presence of a frequency-dependent out-of-phase ac magnetic susceptibility (chi(M)' ') signal consistent with slow magnetization relaxation rates. Fitting of dc magnetization decay versus time data to the Arrhenius equation gave a value of the effective barrier to relaxation (U(eff)) of 13.2 K. Magnetization versus applied dc field sweeps exhibited hysteresis. Thus, complex 3 is a new member of the small but growing family of single-molecule magnets.

Determination of the electron relaxation rates in paramagnetic metal complexes: applicability of available NMR methods

Four different approaches for determining the electron relaxation rates in paramagnetic metallo-proteins are investigated, using a paramagnetic Ni 2+ complex of a protein as an example. All four approaches rely on the determination of the longitudinal paramagnetic relaxation enhancements, R 1p, of the 1H nuclei and the backbone 15N nuclei. Three of the methods utilize the field dependence of the R 1p rates. It is found that the applicability of each of these methods depends on whether the fast-motion condition, ω S 2 τ 2≪1, applies to the electron relaxation, ω S being the Larmor frequency of the electron spin S and τ the correlation time of the electron relaxation. If the fast-motion condition is fulfilled, the electron relaxation rate can be obtained from the ratio of the R 1p rates of one or more protons at two magnetic field strengths (method A). On the other hand, if the fast-motion condition does not apply, more elaborate methods must be used that, in general, require a determination of the R 1p rates over a larger range of magnetic field strengths (method C). However, in the case of paramagnetic metal ions with relatively slow electron relaxation rates only two magnetic field strengths suffice, if the R 1p rates of a hetero nucleus are included in the analysis (method B). In the fourth method (method D), the electron relaxation is estimated as a parameter in a structure calculation, using distance constraints derived from proton R 1p rates at only one magnetic field strength. In general, only methods B and C give unambiguous electron relaxation rates.

Ultrasonic study of antiferro-quadrupole ordering in HoB 2C 2

Elastic properties of the ternary rare earth compound HoB 2C 2 with a tetragonal symmetry have been investigated by means of ultrasonic measurements. The elastic constants of C 44, C 66 and ( C 11− C 12)/2 exhibit the characteristic softening below about 40 K . In order to analyze quadrupole susceptibility, we assume a pseudo-degeneracy model consisting of a ground state E-doublet and an excited state A- or B-singlet at energy Δ∼5.0 K . The considerable softening of the elastic constant and ultrasonic attenuation in phase IV between T C1 =5.9 K and T C2 =5.0 K have been found. The slow relaxation rate of 7×10 −9 s suggests that the fluctuation of quadrupole moments is enhanced in phase IV. We also show a H– T phase diagram of HoB 2C 2 under magnetic fields along [1 0 0] .

Hyperpolarization and lysophosphatidylcholine induce inward currents and ethidium fluorescence in rabbit ventricular myocytes.

Strong electric pulses produce reversible or irreversible membrane breakdown (electroporation). We analysed the permeation properties of minute pores caused by hyperpolarization or lysophosphatidylcholine (LPC) by comparing the amount of charge carried by irregular inward currents (I(hi)) with changes in ethidium bromide (EB) fluorescence in isolated rabbit ventricular myocytes. Forty-second negative pulses from a holding potential of -20 mV induced I(hi) whose conductance increased with hyperpolarization; the mean conductance (G(hi)) was 63.6 +/- 9.9 pS pF(-1) (mean +/- S.E.M., n = 9) at -160 mV. EB fluorescence increased during voltage pulses in parallel with the time integral of I(hi) (Q(hi)), with the magnitude of the increases in nuclear EB fluorescence being 5.3 times greater than in the cytoplasm at -160 mV. Similar hyperpolarization-induced parallel increases in I(hi) and EB fluorescence were also obtained in Na(+)-free, N-methyl-D-glucamine (NMDG) solution. LPC (10 microM) induced large (101.2 +/- 21.2 pS pF(-1), n = 16), rapid (rise times, 1-10 ms) I(hi) with slow relaxation rates at -80 mV that reflected increases in G(hi) to 94.3 +/- 24.8 pS pF(-1) (n = 8) at 6 min. Plots of EB fluorescence vs. Q(hi) were well fitted by a common Hill's equation with a Hill coefficient of 0.97. Taken together, our findings indicate that hyperpolarization and LPC produced pores having the same filter properties for the permeation of small ions, including ethidium(+), and that I(hi) (carried in part by Ca(2+)) generated by membrane breakdown are capable of supplying sufficient ions to evoke abnormal excitation and contraction in cardiac myocytes.

(13)C[(13)C] 2D NMR: a novel strategy for the study of paramagnetic proteins with slow electronic relaxation rates.

Oxidized human [2Fe-2S] ferredoxin has a notably slow electronic relaxation rate, which precludes the observation of signals from nuclei near the iron-sulfur cluster by conventional 2D or 3D methods that utilize proton detection. We have demonstrated the utility of (13)C[(13)C]CT-COSY in identifying connectivity information from fast relaxing carbon nuclei near the paramagnetic center, including those from residues that ligate the metal center.

Coexistence of domains with distinct order and polarity in fluid bacterial membranes.

In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan-labeled Escherichia coli and temperature dependencies of both laurdan's GP and fluorescence anisotropy of 1,3-diphenyl-1,3,5-hexatriene (DPH) (rDPH) affirmed that at physiological temperatures, the E. coli membrane is in a liquid-crystalline phase. However, the strong excitation wavelength dependence of rlaurdan at 37 degrees C reflects membrane heterogeneity. Time-resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdan's emission. In the proteo-lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple-Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid-crystalline bacterial membrane: a lipid-enriched and a proteolipid domain. In bacteria with chloramphenicol (Cam)-inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature-dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam-induced domain dissipation.

Excited-state lifetimes of far-infrared collective modes in proteins.

Vibrational excitations of low frequency collective modes are essential for functionally important conformational transitions in proteins. Here we report the first direct measurement on the lifetime of vibrational excitations of the collective modes at 87 microm (115 cm(-1)) in bacteriorhodopsin, a transmembrane protein. The data show that these modes have extremely long lifetime of vibrational excitations, over 500 ps, accommodating 1500 vibrations. We suggest that there is a connection between this relatively slow anharmonic relaxation rate of approximately 10(9) sec(-1) and the similar observed rate of conformational transitions in proteins, which require multilevel vibrational excitations.

Diffusion of point defects in two-dimensional colloidal crystals.

Uniform colloidal microspheres dispersed in a solvent will, under appropriate conditions, self-assemble into ordered crystalline structures. Using these colloidal crystals as a model system, a great variety of problems of interest to materials science, physical chemistry, and condensed-matter physics have been investigated during the past two decades. Recently, it has been demonstrated that point defects can be created in two-dimensional colloidal crystals by manipulating individual particles with optical tweezers. Direct imaging of these defects verified that their stable configurations have lower symmetry than the underlying triangular lattice, as predicted by numerical simulations for a number of two-dimensional systems. It was also observed that point defects can dissociate into pairs of well-separated dislocations, a topological excitation especially important in two dimensions. Here we use a similar experimental system to study the dynamics of mono- and di-vacancies in two-dimensional colloidal crystals. We see evidence that the excitation of point defects into dislocation pairs enhances the diffusion of di-vacancies. Moreover, the hopping of the defects does not follow a pure random walk, but exhibits surprising memory effects. We expect the results presented in this work to be relevant for explaining the dynamics of other two-dimensional systems.