Showing papers on "Relaxation (NMR) published in 2012"
Book•
19 Jan 2012
TL;DR: In this paper, the authors present a survey of the fundamental requirements for persistent spectral hole-burning, including the following: 1.1.1 Background, 2.2.3 Role of local modes, 3.4.3 Inhomogeneous Broadening of the Vibronic Spectrum of an Electron-Vibrational Transition, 4.5.3 Field Effects in Hole-Burning, 5.6.
Abstract: 1. Introduction.- 1.1 Fundamental Requirements for Persistent Spectral Hole-Burning.- 1.2 Significance for Science and Applications.- 1.3 Historical Overview and Survey of Mechanisms.- 1.4 Synopsis of the Book.- References.- 2. Basic Principles and Methods of Persistent Spectral Hole-Burning.- 2.1 Background.- 2.2 Homogeneous Spectrum of an Electron-Vibrational Transition.- 2.2.1 Integrated Intensities of Purely Electronic Lines and Phonon Sidebands, Electron-Phonon Interactions and Temperature Dependence.- 2.2.2 Relative Width (Q-factor) of PEL. Peak Intensities.- 2.2.3 Role of Local Modes.- 2.3 Inhomogeneous Broadening of the Vibronic Spectrum.- 2.3.1 Inhomogeneous Broadening of Purely Electronic Lines Inhomogeneous Distribution Function.- 2.3.2 Selectivity of the Spectral Response of an Inhomogeneous Absorption Band.- 2.3.3 Inhomogeneous Distribution Function Under Monochromatic Laser Excitation. Site-Selection Spectroscopy.- 2.4 Persistent Spectral Hole-Burning.- 2.4.1 Burning of Spectral Holes in the Inhomogeneous Distribution Function.- 2.4.2 Early Observations of Persistent Spectral Hole-Burning.- 2.5 Kinetics of Persistent Spectral Hole-Burning.- 2.6 Spectroscopic Applications.- 2.6.1 Homogeneous Zero-Phonon Line Broadening and Dephasing in Crystals.- 2.6.2 Photochemical Hole-Burning in Glassy Matrices.- 2.6.3 Homogeneous Linewidths of Vibronic Transitions and Relaxation.- 2.6.4 Off-Resonance Hole-Burning and Non-Correlation Effects.- 2.6.5 Hole-Burning in the Spectra of Chlorophyll-like Molecules.- 2.7 Special Methods of Hole-Burning and Detection.- 2.7.1 Detection of Holes by Doppler Scanning.- 2.7.2 Holographic Detection of Spectral Holes.- 2.7.3 Creation of Sharp Antiholes.- 2.8 Hole-Burning Time-and-Space-Domain Holography.- 2.8.1 Hole-Burning by Picosecond Pulses.- 2.8.2 Theory of Time-and-Space-Domain Holographic Recording and Playback.- 2.8.3 Experimental Results and Discussion.- 2.9 Concluding Remarks.- References.- 3. Photochemical Hole-Burning in Electronic Transitions.- 3.1 Photochemical, Photophysical, and Spin Hole-Burning.- 3.1.1 Historic Survey.- 3.1.2 Radiation-Induced Saturation Versus Chemical Depletion.- a) Transient Saturation.- b) Chemical Depletion.- 3.1.3 Photochemical Systems and Mechanisms.- 3.2 Spectroscopic Analysis of Hole-Burning Experiments.- 3.2.1 General Remarks.- 3.2.2 Fast Relaxation Processes and Excited State Dephasing.- a) Lineshape Analysis.- b) Temperature Dependence of the "Homogeneous" Linewidth.- 3.2.3 Spectral Diffusion in Glasses.- a) TLS Parameters and Tunnelling Rates.- b) Spectroscopic Parameters.- 3.3 Field Effects in Hole-Burning Spectroscopy.- 3.3.1 Introduction: The Site Memory Function.- 3.3.2 Electric-Field Effects.- a) Stark Effect for Molecules with Inversion Symmetry.- b) Stark Effect for Molecules Without Inversion Symmetry.- 3.3.3 Strain-Field Effects.- References.- 4. Persistent Spectral Hole-Burning in Inorganic Materials.- 4.1 Introduction.- 4.2 Hole-Burning Mechanisms.- 4.3 Color Centers.- 4.4 Rare Earth Compounds.- 4.4.1 Trivalent Rare Earth Ions in Glasses.- 4.4.2 Divalent Rare Earth Ions in Crystals.- a) CaF2:Sm2+.- b) SrF2:Sm2+.- c) BaClF:Sm2+.- 4.5 Transition Metal Ions.- 4.5.1 LiGa5 O8:Co2+.- 4.5.2 Y3Al5O12:Ti3+.- 4.6 Conclusion.- References.- 5. Two-Level-System Relaxation in Amorphous Solids as Probed by Nonphotochemical Hole-Burning in Electronic Transitions.- 5.1 Background.- 5.2 Survey of NPHB Systems.- 5.2.1 Hydrogen-Bonded Crystals.- 5.2.2 Molecules in Amorphous Polyacene Films.- 5.2.3 Molecules in Organic Glasses.- 5.2.4 Molecules in Polymers.- 5.2.5 Rare-Earth Ions in Glasses and Polymers.- 5.3 Optical Linewidths and Dephasing in Amorphous Solids.- 5.3.1 Single-Impurity Single-TLS System Hamiltonian.- 5.3.2 Optical Dephasing due to Off-Diagonal Modulation.- 5.3.3 Recent Experiments.- 5.3.4 New Theories.- 5.3.5 Comparison of Theories and Experimental Data.- 5.3.6 Hole Widths and TLS Relaxation Processes in Organic Systems.- 5.4 Density of States Functions for TLS.- 5.5 Laser-Induced Hole Filling.- 5.5.1 Rhodamine 640 in Poly(vinylalcohol).- 5.5.2 Nd3+ and Pr3+ in Poly(vinylalcohol).- 5.5.3 A Tentative Model for LIHF.- 5.6 Recent Developments.- 5.7 Concluding Remarks.- References.- 6. Persistent Infrared Spectral Hole-Burning for Impurity Vibrational Modes in Solids.- 6.1 Introduction.- 6.1.1 Matrix-Isolated Molecules in Van der Waals and Ionic Solids.- 6.1.2 Persistent IR Hole-Burning in Vibrational Modes.- 6.2 Molecules in Van der Waals Matrices.- 6.2.1 1,2-Difluorethane (DFE).- a) Diode Laser Measurements.- b) CO2 Laser Measurements.- 6.2.2 Interpretation of Persistence.- 6.2.3 Molecular Aggregates of Methyl Nitrite or Methanol.- 6.3 ReO4? in Alkali Halide Crystals.- 6.3.1 Background and Spectroscopic Information.- 6.3.2 Measurements of Relaxation Times T1 and T2.- 6.3.3 Persistent Spectral Holes for ReO4? in Alkali Halides.- a) Summary of Characteristics.- b) Model for the PIRSH Process.- 6.3.4 Persistent Spectral Pegs.- 6.3.5 Ultrasonic Studies of Multiple Ground State Configurations.- 6.3.6 Conclusions on the ReO4? System.- 6.4 Persistent Spectral Hole-Burning for CN? Molecules in Alkali Halide Crystals.- 6.4.1 Background Information on Matrix-Isolated CN?.- 6.4.2 High-Resolution FTIR Spectroscopy in the CN? Stretch Region.- 6.4.3 Hole-Burning in the CN? Stretch Mode Region.- 6.4.4 A Study of the CN?:Na+ Center Dynamics.- a) Fluorescence.- b) Hole-Burning and ?l Center Geometry.- 6.4.5 Other CN? Complexes.- 6.5 Conclusion.- 6.5.1 Comparison of the Three Types of Vibrational Hole-Burning Systems.- 6.5.2 Systems Which do not Exhibit PIRSH Formation.- a) Derivatives of the CN? Molecule.- b) Spherical-Top Molecules Which Contain Hydrogen.- 6.5.3 Future Prospects.- a) NO2? in Alkali Halides.- b) Disordered Solids.- References.- 7. Frequency Domain Optical Storage and Other Applications of Persistent Spectral Hole-Burning.- 7.1 Introduction.- 7.2 Systems Issues for Frequency Domain Optical Storage.- 7.2.1 General Remarks.- 7.2.2 Engineering Studies.- 7.3 Materials Research for Frequency Domain Optical Storage.- 7.3.1 General Materials Requirements.- 7.3.2 Limitations of Single-Photon Recording Mechanisms.- 7.3.3 Photon-Gated Mechanisms.- 7.3.4 Limitations on Storage Density.- 7.4 Alternative Data-Storage Configurations.- 7.4.1 Time Domain Storage.- 7.4.2 Electric-Field Readout.- 7.4.3 Holographic Readout.- 7.5 Other Applications of Persistent Spectral Hole-Burning.- 7.5.1 General Remarks.- 7.5.2 Laser Pulse Shaping Based on Fourier Synthesis.- 7.5.3 Laser Pulse Shaping Based on Voltage Modulation.- 7.5.4 Frequency Multiplexed Optical Spatial Filters.- 7.6 Summary and Future Prospects.- References.
451 citations
TL;DR: An experiment to detect and characterize "invisible" excited protein states in slow exchange with a visible ground-state conformation and far more accurate exchange parameters and chemical shifts were obtained than via analysis of Carr-Purcell-Meiboom-Gill relaxation dispersion data.
Abstract: Ever since its initial development, solution NMR spectroscopy has been used as a tool to study conformational exchange. Although many systems are amenable to relaxation dispersion approaches, cases involving highly skewed populations in slow chemical exchange have, in general, remained recalcitrant to study. Here an experiment to detect and characterize ``invisible'' excited protein states in slow exchange with a visible ground-state conformation (excited-state lifetimes ranging from similar to 5 to 50 ms) is presented. This method, which is an adaptation of the chemical exchange saturation transfer (CEST) magnetic resonance imaging experiment, involves irradiating various regions of the spectrum with a weak B-1 field while monitoring the effect on the visible major-state peaks. The variation in major-state peak intensities as a function of frequency offset and B-1 field strength is quantified to obtain the minor-state population, its lifetime, and excited-state chemical shifts and line widths. The methodology was validated with N-15 CEST experiments recorded on an SH3 domain ligand exchanging system and subsequently used to study the folding transition of the A39G FF domain, where the invisible unfolded state has a lifetime of similar to 20 ms. Far more accurate exchange parameters and chemical shifts were obtained than via analysis of Carr-Purcell-Meiboom-Gill relaxation dispersion data.
395 citations
TL;DR: Ju et al. as discussed by the authors showed that a much richer resonant picture would be observable for higher relaxation times of charge carriers: more resonances appear and transmission can be totally suppressed, and the resonant features of the periodic array are associated with leaky plasmonic modes.
Abstract: Resonance diffraction in the periodic array of graphene microribbons is theoretically studied following a recent experiment [L. Ju et al., Nature Nanotech. 6, 630 (2011)]. Systematic studies over a wide range of parameters are presented. It is shown that a much richer resonant picture would be observable for higher relaxation times of charge carriers: More resonances appear and transmission can be totally suppressed. The comparison with the absorption cross-section of a single ribbon shows that the resonant features of the periodic array are associated with leaky plasmonic modes. The longest-wavelength resonance provides the highest visibility of the transmission dip and has the strongest spectral shift and broadening with respect to the single-ribbon resonance, due to collective effects.
392 citations
TL;DR: The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation.
Abstract: Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1–5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of “green” silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts.
295 citations
TL;DR: The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*(2)Ln)(2)(μ-bpym(•))](+) (Ln = Gd, Tb, Dy), are reported.
Abstract: The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*2Ln)2(μ-bpym•)]+ (Ln = Gd, Tb, Dy), are reported. Strong LnIII-bpym•– exchange coupling is observed for all species, as indicated by the increases in χMT at low temperatures. For the GdIII-containing complex, a fit to the data reveals antiferromagnetic coupling with J = −10 cm–1 to give an S = 13/2 ground state. The TbIII and DyIII congeners show single-molecule magnet behavior with relaxation barriers of Ueff = 44(2) and 87.8(3) cm–1, respectively, a consequence of the large magnetic anisotropies imparted by these ions. Significantly, the latter complex exhibits a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 6.5 K and magnetic hysteresis below this temperature.
289 citations
TL;DR: Two ways to relax: A defect-dicubane Co2Dy2 single-molecule magnet displays slow relaxation of magnetization with a blocking temperature of 22 K (at 1500 Hz), the highest reported for a 3d–4f-based SMM.
Abstract: Two ways to relax: A defect-dicubane Co2Dy2 single-molecule magnet (SMM) displays slow relaxation of magnetization with a blocking temperature of 22 K (at 1500 Hz), the highest reported for a 3d–4f-based SMM. Analysis of the relaxation reveals two distinct blocking regimes, one of which is intraionic, localized on the DyIII ions, while the other is exchange-based.
260 citations
TL;DR: Dy and Ho complexes are determined to be single-ion magnets in addition to the previously reported Er complex and show obvious quantum tunneling magnetization relaxation in the absence of a static field.
Abstract: The synthesis, structures, and magnetic properties of a series of lanthanide organometallic mixed sandwich molecules, (Cp*)Ln(COT), are investigated, where Cp* is the pentamethylcyclopentadiene anion and COT is the cyclooctatetraene dianion and Ln represents TbIII, DyIII, HoIII, ErIII, and TmIII. Among the five complexes, Dy and Ho complexes are determined to be single-ion magnets in addition to the previously reported Er complex. Both Dy and Ho complexes show obvious quantum tunneling magnetization relaxation in the absence of a static field. The diluted Ho complex behaves two sets of thermally activated relaxation as we reported previously in Er due to the COT ring static disorder. A stair-shaped hysteresis for the Er compound can be observed at 1.6 K with Hc = 1 kOe at a sweeping rate over 700 Oe/s. The quantum tunneling decoherence relaxation rate increases from Er to Ho to Dy, which may be caused by the relative increase of transverse anisotropy coming from the larger tilting of the two aromatic ring...
211 citations
TL;DR: A mononuclear Dy(III) complex with a non-Schiff base compartmental ligand has been prepared and characterised by X-ray crystallography and ac magnetic susceptibility measurements.
197 citations
TL;DR: Analytical results based on the Landau-Zener theory of adiabatic transitions, as well as numerical quantum mechanical calculations, provide insight into the dependence of cross effect DNP on various experimental parameters, including MAS frequency, microwave field strength, spin relaxation rates, hyperfine and electron-electron dipole coupling strengths, and the nature of the biradical dopants.
Abstract: We present theoretical calculations of dynamic nuclear polarization (DNP) due to the cross effect in nuclear magnetic resonance under magic-angle spinning (MAS). Using a three-spin model (two electrons and one nucleus), cross effect DNP with MAS for electron spins with a large g-anisotropy can be seen as a series of spin transitions at avoided crossings of the energy levels, with varying degrees of adiabaticity. If the electron spin-lattice relaxation time T1e is large relative to the MAS rotation period, the cross effect can happen as two separate events: (i) partial saturation of one electron spin by the applied microwaves as one electron spin resonance (ESR) frequency crosses the microwave frequency and (ii) flip of all three spins, when the difference of the two ESR frequencies crosses the nuclear frequency, which transfers polarization to the nuclear spin if the two electron spins have different polarizations. In addition, adiabatic level crossings at which the two ESR frequencies become equal serve to maintain non-uniform saturation across the ESR line. We present analytical results based on the Landau-Zener theory of adiabatic transitions, as well as numerical quantum mechanical calculations for the evolution of the time-dependent three-spin system. These calculations provide insight into the dependence of cross effect DNP on various experimental parameters, including MAS frequency, microwave field strength, spin relaxation rates, hyperfine and electron-electron dipole coupling strengths, and the nature of the biradical dopants.
195 citations
TL;DR: This review covers how nuclear singlet states may be generated, observed, and exploited in solution nuclear magnetic resonance (NMR) and potential applications include the study of slow molecular processes, the elucidation of molecular geometry, and the transport of hyperpolarized nuclear spin order.
Abstract: Nuclear singlet states may have lifetimes that are much longer than the conventional relaxation time of nuclear spin magnetization. This review covers how these states may be generated, observed, and exploited in solution nuclear magnetic resonance (NMR). Potential applications include the study of slow molecular processes, the elucidation of molecular geometry, and the transport of hyperpolarized nuclear spin order.
189 citations
TL;DR: A new approach to improve structural and dynamical determination of large metalloproteins using solid-state nuclear magnetic resonance (NMR) with 1H detection under ultrafast magic angle spinning (MAS) is introduced.
Abstract: We introduce a new approach to improve structural and dynamical determination of large metalloproteins using solid-state nuclear magnetic resonance (NMR) with 1H detection under ultrafast magic angle spinning (MAS). The approach is based on the rapid and sensitive acquisition of an extensive set of 15N and 13C nuclear relaxation rates. The system on which we demonstrate these methods is the enzyme Cu, Zn superoxide dismutase (SOD), which coordinates a Cu ion available either in Cu+ (diamagnetic) or Cu2+ (paramagnetic) form. Paramagnetic relaxation enhancements are obtained from the difference in rates measured in the two forms and are employed as structural constraints for the determination of the protein structure. When added to 1H-1H distance restraints, they are shown to yield a twofold improvement of the precision of the structure. Site-specific order parameters and timescales of motion are obtained by a Gaussian axial fluctuation (GAF) analysis of the relaxation rates of the diamagnetic molecule, and interpreted in relation to backbone structure and metal binding. Timescales for motion are found to be in the range of the overall correlation time in solution, where internal motions characterized here would not be observable.
TL;DR: The magnetism of DySc(2)N@C(80) endofullerene was studied with X-ray magnetic circular dichroism and a magnetometer with a superconducting quantum interference device and SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed.
Abstract: The magnetism of DySc2N@C80 endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in DyIII ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc2N@C80 samples with C60 increases the zero-field 4f electron relaxation time at 2 K to several hours.
TL;DR: A new heterodinuclear [Zn(L)Dy](3+) complex, with L being a compartmental Schiff base ligand, exhibits the characteristic Dy(3+) luminescence associated with a single-ion field induced slow relaxation of the magnetisation.
TL;DR: Paramagnetic contrast agents have been used for a long time, but more recently superparamagnetic iron oxide nanoparticles (SPIOs) have been discovered to influence MRI contrast as well, and the potential use of magnetic nanoparticles as diagnostic tracers is explored.
Abstract: Magnetic nanoparticles are useful as contrast agents for magnetic resonance imaging (MRI). Paramagnetic contrast agents have been used for a long time, but more recently superparamagnetic iron oxide nanoparticles (SPIOs) have been discovered to influence MRI contrast as well. In contrast to paramagnetic contrast agents, SPIOs can be functionalized and size-tailored in order to adapt to various kinds of soft tissues. Although both types of contrast agents have a inducible magnetization, their mechanisms of influence on spin-spin and spin-lattice relaxation of protons are different. A special emphasis on the basic magnetism of nanoparticles and their structures as well as on the principle of nuclear magnetic resonance is made. Examples of different contrast-enhanced magnetic resonance images are given. The potential use of magnetic nanoparticles as diagnostic tracers is explored. Additionally, SPIOs can be used in diagnostic magnetic resonance, since the spin relaxation time of water protons differs, whether magnetic nanoparticles are bound to a target or not.
TL;DR: In this paper, the spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements using non-local Hanle measurements.
TL;DR: In this article, the frequency dependence of dielectric and electric modulus as well as morphological characteristics of poly (∆-caprolactone) (PCL)-ammonium thiocyanate (NH 4 SCN) polymer electrolyte are investigated.
TL;DR: In this article, the dominant nuclear spin-lattice relaxation (T1) mechanism for ZIF-8 in air arises from atmospheric paramagnetic molecular oxygen, which interacts primarily with the imidazolate ring rather than the methyl substituent.
Abstract: NMR relaxation studies and spectroscopic measurements of zeolitic imidazolate framework-8 (ZIF-8) are reported. The dominant nuclear spin–lattice relaxation (T1) mechanism for ZIF-8 in air arises from atmospheric paramagnetic molecular oxygen. The 13C T1 measurements indicate that the oxygen interacts primarily with the imidazolate ring rather than the methyl substituent. Similar relaxation behavior was also observed in a ZIF with an unsubstituted ring, ZIF-4. Single-crystal X-ray diffraction was used to provide data for the study of the thermal ellipsoids of ZIF-8 at variable temperatures from 100 to 298 K, which further confirmed the rigid nature of this ZIF framework. These results highlight a rigid ZIF framework and are in contrast with dynamic metal–organic frameworks based on benzenedicarboxylate linking groups, for which the relaxation reflects the dynamics of the benzenedicarboxylate moiety.
01 Jan 2012
TL;DR: In this article, the magnetism of DySc2N@C80 endoful lerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temper- atures of 2 K and in fields up to 7 T.
Abstract: The magnetism of DySc2N@C80 endoful- lerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temper- atures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy III ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc2N@C80 samples with C60 increases the zero-field 4f electron relaxation time at 2 K to several hours.
TL;DR: The thermal investigation of [Dy(hfac)(3)(PyNO)](2) shows that the dimers can be sublimated intact, suggesting their possible exploit as active materials for surface-confined nanostructures to be investigated by fluorimetry methods.
Abstract: The reaction of [Ln(hfac)(3)]·2H(2)O and pyridine-N-oxide (PyNO) leads to isostructural dimers of the formula [Ln(hfac)(3)(PyNO)](2) (Ln=Eu, Gd, Tb, Dy). The Dy derivative shows a remarkable single-molecule magnet behavior with complex hysteresis at 1.4 K. The dynamics of the magnetization features are two relaxation regimes: a thermally activated one at high temperature (τ(0)=(5.62±0.4)×10(-11) s and Δ=(167±1) K) and a quantum tunneling regime at low temperature with a tunneling frequency of 0.42 Hz. The analysis of the Gd derivative evidences intradimer antiferromagnetic interactions (J=(-0.034±0.001) cm(-1)). Moreover, the Eu, Tb, and Dy derivatives are luminescent with quantum yield of 51, 53, and 0.1%, respectively. The thermal investigation of [Dy(hfac)(3)(PyNO)](2) shows that the dimers can be sublimated intact, suggesting their possible exploit as active materials for surface-confined nanostructures to be investigated by fluorimetry methods.
TL;DR: In this article, the authors investigate how emergent nematic order and nematic fluctuations affect several macroscopic properties of both the normal and superconducting states of the iron pnictides.
Abstract: We investigate how emergent nematic order and nematic fluctuations affect several macroscopic properties of both the normal and superconducting states of the iron pnictides. Due to its magnetic origin, long-range nematic order enhances magnetic fluctuations, leaving distinctive signatures in the spin?lattice relaxation rate, the spin?spin correlation function, and the uniform magnetic susceptibility. This enhancement of magnetic excitations is also manifested in the electronic spectral function, where a pseudogap can open at the hot spots of the Fermi surface. In the nematic phase, electrons are scattered by magnetic fluctuations that are anisotropic in momentum space, giving rise to a non-zero resistivity anisotropy whose sign changes between electron-doped and hole-doped compounds. We also show that due to the magneto-elastic coupling, nematic fluctuations soften the shear modulus in the normal state, but harden it in the superconducting state. The latter effect is an indirect consequence of the competition between magnetism and superconductivity, and also causes a suppression of the orthorhombic distortion below Tc. We also demonstrate that ferro-orbital fluctuations enhance the nematic susceptibility, cooperatively promoting an electronic tetragonal symmetry-breaking. Finally, we argue that Tc in the iron pnictides might be enhanced due to nematic fluctuations of magnetic origin.
TL;DR: The effect of electron tunneling on the mechanical resonance of an as-grown suspended carbon nanotube is found to depend on frequency as the tunneling time becomes comparable to the vibration period.
Abstract: We measure the mechanical resonances of an as-grown suspended carbon nanotube, detected via electrical mixing in the device. A sequence of modes extending to 39 GHz is observed with a quality factor of 35 000 in the highest mode. This unprecedentedly high combination corresponds to a thermal excited state probability below 10–8 and a relaxation time of 140 ns with microsecond relaxation times for lower modes. The effect of electron tunneling on the mechanical resonance is found to depend on frequency as the tunneling time becomes comparable to the vibration period.
TL;DR: These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior.
Abstract: The model compounds (NBu4)2[ReCl4(CN)2] (1), (DMF)4ZnReCl4(CN)2 (2), and [(PY5Me2)2Mn2ReCl4(CN)2](PF6)2 (3) have been synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)4MnReCl4(CN)2 (4). High-field electron paramagnetic resonance spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in compounds 1–3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the ReIV centers that determines the easy axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with...
Journal Article•
TL;DR: In this paper, the effects of finite relaxation time on nuclear magnetic resonance in flowing liquids have been investigated experimentally, and a method of measuring relaxation times directly was proposed to measure the effect of finite relaxations.
Abstract: Nuclear magnetic resonance in flowing liquids has been investigated experimentally. The experiments afford a striking demonstration of the effects of finite relaxation time and afford a method of measuring relaxation times directly.
TL;DR: In this article, the crystallization characteristics of Zr55Al10Ni5Cu30 bulk metallic glasses BMGs during pulsed laser surface melting (PLSM) were examined, and the crystallisation behavior during laser solid forming (LSF) with the pre-laid powder method on amorphous substrates was further investigated.
TL;DR: Two recent variable-temperature NMR spin-lattice relaxation studies focussing on Li(+) dynamics in crystalline ion conductors which are of relevance for battery applications are exemplarily presented.
TL;DR: It is proposed that the relaxation length of the spin signal shows a nearly linear increase with magnetic field, hinting at a freeze-out of relaxation by the Zeeman splitting, rather than a renormalized spin-diffusion length.
Abstract: We report on nonlocal transport in multiterminal superconductor-ferromagnet structures, which were fabricated by means of $e$-beam lithography and shadow evaporation techniques. In the presence of a significant Zeeman splitting of the quasiparticle states, we find signatures of spin transport over distances of several $\ensuremath{\mu}\mathrm{m}$, exceeding other length scales such as the coherence length, the normal-state spin-diffusion length, and the charge-imbalance length. The relaxation length of the spin signal shows a nearly linear increase with magnetic field, hinting at a freeze-out of relaxation by the Zeeman splitting. We propose that the relaxation length is given by the recombination length of the quasiparticles rather than a renormalized spin-diffusion length.
TL;DR: A significant temperature dependence of the dipole moment of the protein, indicating conformational changes is found, and a breakdown of the Debye-Stokes-Einstein relation is found in this protein solution, i.e., the dc conductivity is not completely governed by the mobility of the solvent molecules, instead it seems that the dc Conductivity is closely connected to the hydration shell dynamics.
TL;DR: The contraction and stiffening of the H-O polar-covalent bond enlarges and stiffens those H2O molecules having fewer than four neighbors such as molecular clusters, hydration shells, and the surface skins of water and ice, which gives rise to their elastic, hydrophobic, highly-polarized, ice-like, and low-density behavior at room temperature.
Abstract: The interplay between intra-molecular H-O covalent bond contraction, due to molecular under-coordination, and inter-molecular O:H expansion, due to inter-electron pair Coulomb repulsion, has been shown to be the source of the anomalous behavior of under-coordinated water molecules in nanoclusters and in the surfaces of water. The shortening of the H-O bond raises the local density of bonding electrons, which in turn polarizes the lone pairs of electrons on oxygen. The stiffening of the H-O bond increases the magnitude of O1s binding energy shift, causes the blueshift of the H-O phonon frequencies, and furthermore, elevates the melting point of molecular clusters and ultrathin films of water, which gives rise to their ice-like behavior at room temperature. At the same time, the elongation of the entire O:H-O bond polarizes and enlarges the under-coordinated H$_2$O molecules.
TL;DR: The results indicate that the magnetic relaxation properties of dinuclear Ln(III)-Pc multiple-decker complexes are affected by the local molecular symmetry and are extremely sensitive to tiny distortions in the coordination geometry.
Abstract: The SMM behaviour of dinuclear Ln(III)–Pc multiple-decker complexes (Ln = Tb3+ and Dy3+) with energy barriers and slow-relaxation behaviour were explained by using X-ray crystallography and static and dynamic susceptibility measurements. In particular, interactions among the 4f electrons of several dinuclear Ln(III)–Pc type SMMs have never been discussed on the basis of the crystal structure. For dinuclear Tb(III)–Pc complexes, a dual magnetic relaxation process was observed. The relaxation processes are due to the anisotropic centres. Our results clearly show that the two Tb3+ ion sites are equivalent and are consistent with the crystal structure. On the other hand, the mononuclear Tb(III)–Pc complex exhibited only a single magnetic relaxation process. This is clear evidence that the magnetic relaxation mechanism depends heavily on the dipole–dipole (f–f) interactions between the Tb3+ ions in the dinuclear systems. Furthermore, the SMM behaviour of dinuclear Dy(III)–Pc type SMMs with smaller energy barriers compared with that of Tb(III)–Pc and slow-relaxation behaviour was explained. Dinuclear Dy(III)–Pc SMMs exhibited single-component magnetic relaxation behaviour. The results indicate that the magnetic relaxation properties of dinuclear Ln(III)–Pc multiple-decker complexes are affected by the local molecular symmetry and are extremely sensitive to tiny distortions in the coordination geometry. In other words, the spatial arrangement of the Ln3+ ions (f–f interactions) in the crystal is important. Our work shows that the SMM properties can be fine-tuned by introducing weak intermolecular magnetic interactions in a controlled SMM spatial arrangement.
TL;DR: A systematic study on the magnetic field induced dielectrics properties, dc transport and dc bias effect on the dielectric permittivity has revealed the extrinsic origin of the MD effect in the bulk sample of La(2)NiMnO(6).
Abstract: A La2NiMnO6 polycrystalline sample prepared by the sol–gel method showed monoclinic crystal structure with the P21/n space group and a saturation magnetization of 4.63 μB/f.u. at 5 K. Impedance spectroscopy results in the temperature range of 10 K < T < 300 K have revealed a distinct conduction process at grains and grain boundaries, where the grains followed the variable range hopping mechanism and the grain boundaries obeyed Arrhenius thermal activation. A negative magnetoresistance of 2.5% was observed at the paramagnetic to ferromagnetic transition, and this became temperature independent below the magnetic ordering. A marginal positive magnetodielectric (MD) effect that followed the dielectric relaxation was observed and its magnitude was found to decrease with increase of the frequency. A systematic study on the magnetic field induced dielectric properties, dc transport and dc bias effect on the dielectric permittivity has revealed the extrinsic origin of the MD effect in the bulk sample of La2NiMnO6.