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Solid State Machinery of Multiple Dynamic Elements in a Metal-Organic Framework. Angew Chem Int Ed Engl 2024; 63:e202317094. [PMID: 38236628 DOI: 10.1002/anie.202317094] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2023] [Revised: 01/17/2024] [Accepted: 01/18/2024] [Indexed: 01/19/2024]
Abstract
Engineering coordinated rotational motion in porous architectures enables the fabrication of molecular machines in solids. A flexible two-fold interpenetrated pillared Metal-Organic Framework precisely organizes fast mobile elements such as bicyclopentane (BCP) (107 Hz regime at 85 K), two distinct pyridyl rotors and E-azo group involved in pedal-like motion. Reciprocal sliding of the two sub-networks, switched by chemical stimuli, modulated the sizes of the channels and finally the overall dynamical machinery. Actually, iodine-vapor adsorption drives a dramatic structural rearrangement, displacing the two distinct subnets in a concerted piston-like motion. Unconventionally, BCP mobility increases, exploring ultra-fast dynamics (107 Hz) at temperatures as low as 44 K, while the pyridyl rotors diverge into a faster and slower dynamical regime by symmetry lowering. Indeed, one pillar ring gained greater rotary freedom as carried by the azo-group in a crank-like motion. A peculiar behavior was stimulated by pressurized CO2, which regulates BCP dynamics upon incremental site occupation. The rotary dynamics is intrinsically coupled to the framework flexibility as demonstrated by complementary experimental evidence (multinuclear solid-state NMR down to very low temperatures, synchrotron radiation XRD, gas sorption) and computational modelling, which helps elucidate the highly sophisticated rotor-structure interplay.
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Benchmark Dynamics of Dipolar Molecular Rotors in Fluorinated Metal-Organic Frameworks. Angew Chem Int Ed Engl 2023; 62:e202215893. [PMID: 36469012 DOI: 10.1002/anie.202215893] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2022] [Revised: 12/03/2022] [Accepted: 12/05/2022] [Indexed: 12/12/2022]
Abstract
Fluorinated Metal-Organic Frameworks (MOFs), comprising a wheel-shaped ligand with geminal rotating fluorine atoms, produced benchmark mobility of correlated dipolar rotors at 2 K, with practically null activation energy (Ea =17 cal mol-1 ). 1 H T1 NMR revealed multiple relaxation phenomena due to the exchange among correlated dipole-rotor configurations. Synchrotron radiation X-ray diffraction at 4 K, Density Functional Theory, Molecular Dynamics and phonon calculations showed the fluid landscape and pointed out a cascade mechanism converting dipole configurations into each other. Gas accessibility, shown by hyperpolarized-Xe NMR, allowed for chemical stimuli intervention: CO2 triggered dipole reorientation, reducing their collective dynamics and stimulating a dipole configuration change in the crystal. Dynamic materials under limited thermal noise and high responsiveness enable the fabrication of molecular machines with low energy dissipation and controllable dynamics.
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Benchmark Dynamics of Dipolar Molecular Rotors in Fluorinated Metal‐Organic Frameworks. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202215893] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/07/2022]
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Cascade Dynamics of Multiple Molecular Rotors in a MOF: Benchmark Mobility at a Few Kelvins and Dynamics Control by CO 2. J Am Chem Soc 2021; 143:13082-13090. [PMID: 34388339 PMCID: PMC8413000 DOI: 10.1021/jacs.1c03801] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
![]()
Achieving
sophisticated juxtaposition of geared molecular rotors
with negligible energy-requirements in solids enables fast yet controllable
and correlated rotary motion to construct switches and motors. Our
endeavor was to realize multiple rotors operating in a MOF architecture
capable of supporting fast motional regimes, even at extremely cold
temperatures. Two distinct ligands, 4,4′-bipyridine (bipy)
and bicyclo[1.1.1]pentanedicarboxylate (BCP), coordinated to Zn clusters
fabricated a pillar-and-layer 3D array of orthogonal rotors. Variable
temperature XRD, 2H solid-echo, and 1H T1 relaxation NMR, collected down to a temperature of 2 K revealed
the hyperfast mobility of BCP and an unprecedented cascade mechanism
modulated by distinct energy barriers starting from values as low
as 100 J mol–1 (24 cal mol–1),
a real benchmark for complex arrays of rotors. These rotors explored
multiple configurations of conrotary and disrotary relationships,
switched on and off by thermal energy, a scenario supported by DFT
modeling. Furthermore, the collective bipy-ring rotation was concerted
with the framework, which underwent controllable swinging between
two arrangements in a dynamical structure. A second way to manipulate
rotors by external stimuli was the use of CO2, which diffused
through the open pores, dramatically changing the global rotation
mechanism. Collectively, the intriguing gymnastics of multiple rotors,
devised cooperatively and integrated into the same framework, gave
the opportunity to engineer hypermobile rotors (107 Hz
at 4 K) in machine-like double ligand MOF crystals.
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Molecular Rotors in a Metal-Organic Framework: Muons on a Hyper-Fast Carousel. NANO LETTERS 2020; 20:7613-7618. [PMID: 32870690 PMCID: PMC8011913 DOI: 10.1021/acs.nanolett.0c03140] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 07/30/2020] [Revised: 08/31/2020] [Indexed: 06/11/2023]
Abstract
Using muon-spin spectroscopy, we study the exceptional dynamical properties of rotating molecular struts engineered within a Zn-based metal-organic framework at cryogenic temperatures, where the internal motions of almost any other organic substance are quenched. Muon-spin spectroscopy is particularly suited for this aim, as the experimental evidence suggests several implantation sites for the muons, among which at least one directly onto the rotating moiety. The dynamics of the molecular rotors are characterized by the exceptionally low activation energy EA ∼ 30 cal mol-1. At the same time, we evidence a highly unusual temperature dependence of the dipolar interaction of muons with nuclear magnetic moments on the rotors, suggesting a complex influence of the rotations on the muon implantation and diffusion.
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Fast motion of molecular rotors in metal–organic framework struts at very low temperatures. Nat Chem 2020; 12:845-851. [DOI: 10.1038/s41557-020-0495-3] [Citation(s) in RCA: 43] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2019] [Accepted: 05/27/2020] [Indexed: 11/09/2022]
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Fast recovery of the pristine magnetic and structural phases in superconducting LaFeAsO 0.89F 0.11 by Mn/Fe substitution. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2019; 31:174002. [PMID: 30690437 DOI: 10.1088/1361-648x/ab0234] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
We report an experimental study on the effect of Mn impurities in the optimally doped [Formula: see text] compound. The results show that a very tiny amount of Mn, of the order of 0.1%, is enough to destroy superconductivity and to recover at low temperatures both the magnetic ground state and the orthorhombic structure of the pristine LaFeAsO parent compound. The results are discussed within a model where electron correlations enhance the Ruderman-Kittel-Kasuya-Yosida interaction among impurities.
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Molecular Dynamics and Hyperpolarization Performance of Deuterated β-Cyclodextrins. J Phys Chem B 2019; 123:3731-3737. [DOI: 10.1021/acs.jpcb.9b01857] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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Abstract
1H and 13C dynamic nuclear polarizations have been studied in 13C-enriched β-cyclodextrins doped with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl free radical. 1H and 13C polarizations raised above 7.5 and 7%, respectively, and for both nuclear species, the transfer of polarization from the electron spins appears to be consistent with a thermal mixing scenario for a concentration of 9 13C nuclei per molecule. When the concentration is increased to 21 13C nuclei per molecule, a decrease in the spin-lattice relaxation and polarization buildup rates is observed. This reduction is associated with the bottleneck effect induced by the decrease in the number of electron spins per nucleus when both the nuclear spin-lattice relaxation and the polarization occur through the electron non-Zeeman reservoir. 13C nuclear spin-lattice relaxation has been studied in the 1.8-340 K range, and the effects of internal molecular motions and of the free radicals on the relaxation are discussed. 13C hyperpolarization performances and room-temperature spin-lattice relaxation times show that these are promising materials for future biomedical applications.
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Exotic atoms at extremely high magnetic fields: the case of neutron star atmosphere. EPJ WEB OF CONFERENCES 2018. [DOI: 10.1051/epjconf/201818101018] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The presence of exotic states of matter in neutron stars (NSs) is currently an open issue in physics. The appearance of muons, kaons, hyperons, and other exotic particles in the inner regions of the NS, favoured by energetic considerations, is considered to be an effective mechanism to soften the equation of state (EoS). In the so-called two-families scenario, the softening of the EoS allows for NSs characterized by very small radii, which become unstable and convert into a quark stars (QSs). In the process of conversion of a NS into a QS material can be ablated by neutrinos from the surface of the star. Not only neutron-rich nuclei, but also more exotic material, such as hypernuclei or deconfined quarks, could be ejected into the atmosphere. In the NS atmosphere, atoms like H, He, and C should exist, and attempts to model the NS thermal emission taking into account their presence, with spectra modified by the extreme magnetic fields, have been done. However, exotic atoms, like muonic hydrogen (p μ−) or the so-called Sigmium (Σ+ e−), could also be present during the conversion process or in its immediate aftermath. At present, analytical expressions of the wave functions and eigenvalues for these atoms have been calculated only for H. In this work, we extend the existing solutions and parametrizations to the exotic atoms (p μ−) and (Σ+ e−), making some predictions on possible transitions. Their detection in the spectra of NS would provide experimental evidence for the existence of hyperons in the interior of these stars.
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Human-brain ferritin studied by muon spin rotation: a pilot study. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:415801. [PMID: 28872048 DOI: 10.1088/1361-648x/aa80b3] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Muon spin rotation is employed to investigate the spin dynamics of ferritin proteins isolated from the brain of an Alzheimer's disease (AD) patient and of a healthy control, using a sample of horse-spleen ferritin as a reference. A model based on the Néel theory of superparamagnetism is developed in order to interpret the spin relaxation rate of the muons stopped by the core of the protein. Using this model, our preliminary observations show that ferritins from the healthy control are filled with a mineral compatible with ferrihydrite, while ferritins from the AD patient contain a crystalline phase with a larger magnetocrystalline anisotropy, possibly compatible with magnetite or maghemite.
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Abstract
1H dynamic nuclear polarization and nuclear spin-lattice relaxation rates have been studied in amorphous complexes of β-cyclodextrins doped with different concentrations of the TEMPO radical. Nuclear polarization increased up to 10% in the optimal case, with a behavior of the buildup rate (1/TPOL) and of the nuclear spin-lattice relaxation rate (1/T1n) consistent with a thermal mixing regime. The temperature dependence of 1/T1n and its increase with the radical concentration indicate a relaxation process arising from the modulation of the electron-nucleus coupling by the glassy dynamics. The high-temperature relaxation is driven by molecular motions, and 1/T1n was studied at room temperature in liquid solutions for dilution levels close to the ones typically used for in vivo studies.
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Superconductivity Emerging from an Electronic Phase Separation in the Charge Ordered Phase of RbFe_{2}As_{2}. PHYSICAL REVIEW LETTERS 2016; 117:217001. [PMID: 27911551 DOI: 10.1103/physrevlett.117.217001] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/11/2016] [Indexed: 06/06/2023]
Abstract
^{75}As, ^{87}Rb, and ^{85}Rb nuclear quadrupole resonance (NQR) and ^{87}Rb nuclear magnetic resonance measurements in a RbFe_{2}As_{2} iron-based superconductor are presented. We observe a marked broadening of the ^{75}As NQR spectrum below T_{0}≃140 K which is associated with the onset of a charge order in the FeAs planes. Below T_{0} we observe a power-law decrease in the ^{75}As nuclear spin-lattice relaxation rate down to T^{*}≃20 K. Below T^{*} the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in light of an orbital selective behavior expected for the electronic correlations.
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Persistence of slow dynamics in Tb(OETAP)2 single molecule magnets embedded in conducting polymers. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2016; 28:386002. [PMID: 27437735 DOI: 10.1088/0953-8984/28/38/386002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
UNLABELLED The spin dynamics of Tb(OETAP)2 single ion magnets was investigated by means of muon spin relaxation (μSR) both in the bulk material as well as when the molecule is embedded into PEDOT PSS polymer conductor. The spin fluctuation time is characterized by a high temperature activated trend, with an energy barrier around 320 K, and by a low temperature tunneling regime. When the single ion magnet is embedded into the polymer the energy barrier only slightly decreases and the fluctuation time remains of the same order of magnitude, even at low temperature. This finding shows that these single molecule magnets preserve their characteristics which, if combined with those of the conducting polymer, result in a hybrid material of potential interest for organic spintronics.
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Nuclear magnetic resonance studies of DNP-ready trehalose obtained by solid state mechanochemical amorphization. Phys Chem Chem Phys 2016; 18:16912-20. [PMID: 27282123 DOI: 10.1039/c6cp00914j] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
(1)H nuclear spin-lattice relaxation and Dynamic Nuclear Polarization (DNP) have been studied in amorphous samples of trehalose sugar doped with TEMPO radicals by means of mechanical milling, in the 1.6-4.2 K temperature range. The radical concentration was varied between 0.34 and 0.81%. The highest polarization of 15% at 1.6 K, observed in the sample with concentration 0.50%, is of the same order of magnitude of that reported in standard frozen solutions with TEMPO. The temperature and concentration dependence of the spin-lattice relaxation rate 1/T1, dominated by the coupling with the electron spins, were found to follow power laws with an exponent close to 3 in all samples. The observed proportionality between 1/T1 and the polarization rate 1/Tpol, with a coefficient related to the electron polarization, is consistent with the presence of Thermal Mixing (TM) and a good contact between the nuclear and the electron spins. At high electron concentration additional relaxation channels causing a decrease in the nuclear polarization must be considered. These results provide further support for a more extensive use of amorphous DNP-ready samples, obtained by means of comilling, in dissolution DNP experiments and possibly for in vivo metabolic imaging.
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16
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Evidence of spin-temperature in dynamic nuclear polarization: an exact computation of the EPR spectrum. Phys Chem Chem Phys 2016; 18:25655-25662. [DOI: 10.1039/c6cp05047f] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We show the validity of the spin-temperature approach for typical radical concentration used in dissolution DNP protocols.
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The role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid. Phys Chem Chem Phys 2015; 16:27025-36. [PMID: 25382595 DOI: 10.1039/c4cp02636e] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The temperature dependence of (1)H and (13)C nuclear spin-lattice relaxation rate 1/T1 has been studied in the 1.6-4.2 K temperature range in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of 1/T1 is found to follow a quadratic power law for both nuclei in the two samples. Remarkably the same temperature dependence is displayed also by the electron spin-lattice relaxation rate 1/T1e in the sample containing radicals. These results are explained by considering the effect of the structural dynamics on the relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show that below 4 K the (13)C build up rate scales with 1/T1e, in analogy to (13)C 1/T1 and consistently with a thermal mixing scenario where all the electrons are collectively involved in the dynamic nuclear polarization process and the nuclear spin reservoir is in good thermal contact with the electron spin system.
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Abstract
We report here the evidence of superconductivity in the full series of Sm-doped [n]phenacenes, with n = 3, 4, 5.
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Dynamic nuclear polarization of a glassy matrix prepared by solid state mechanochemical amorphization of crystalline substances. Chem Commun (Camb) 2014; 51:2080-3. [PMID: 25534192 DOI: 10.1039/c4cc08348b] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
A mechanochemical "solvent-free" route is presented for the preparation of solid samples ready to be employed in the Dynamic Nuclear Polarization (DNP). (1)H-DNP build-up curves at 3.46 T as a function of temperature and radical concentration show steady state nuclear polarization of 10% (0.5% TEMPO concentration at 1.75 K).
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Single-molecule-magnet behavior in the family of [Ln(OETAP)2] double-decker complexes (Ln=lanthanide, OETAP=octa(ethyl)tetraazaporphyrin). Chemistry 2014; 20:12817-25. [PMID: 25137217 DOI: 10.1002/chem.201402869] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2014] [Indexed: 11/09/2022]
Abstract
Double-decker complexes of lanthanide cations can be readily prepared with tetraazaporphyrins (porphyrazines). We have synthesized and characterized a series of neutral double-decker complexes [Ln(OETAP)2 ] (Ln=Tb(3+), Dy(3+), Gd(3+), Y(3+); OETAP=octa(ethyl)tetraazaporphyrin). Some of these complexes show analogous magnetic features to their phthalocyanine (Pc) counterparts. The Tb(3+) and Dy(3+) derivatives exhibit single-molecule magnet (SMM) behavior with high blocking temperatures over 50 and 10 K, respectively. These results confirm that, in double-decker complexes that involve Tb or Dy, the (N4)2 square antiprism coordination mode has an important role in inducing very large activation energies for magnetization reversal. In contrast with their Pc counterparts, the use of tetraazaporphyrin ligands endows the presented [Ln(OETAP)2] complexes with extraordinary chemical versatility. The double-decker complexes that exhibit SMM behavior are highly soluble in common organic solvents, and easily processable even through sublimation.
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Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing. Phys Chem Chem Phys 2014; 16:753-64. [DOI: 10.1039/c3cp52534a] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Correlated trends of coexisting magnetism and superconductivity in optimally electron-doped oxypnictides. PHYSICAL REVIEW LETTERS 2011; 107:227003. [PMID: 22182039 DOI: 10.1103/physrevlett.107.227003] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/12/2011] [Indexed: 05/31/2023]
Abstract
We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe(1-x)Ru(x)AsO(0.85)F(0.15) for 0.1≤x≲0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold x(c)~0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold x(c).
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Spin Dynamics in the Negatively Charged Terbium (III) Bis-phthalocyaninato Complex. J Am Chem Soc 2009. [DOI: 10.1021/ja9025424] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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Spin Dynamics in the Negatively Charged Terbium (III) Bis-phthalocyaninato Complex. J Am Chem Soc 2009; 131:4387-96. [DOI: 10.1021/ja808649g] [Citation(s) in RCA: 149] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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Strong electronic correlations in LixZnPc organic metals. PHYSICAL REVIEW LETTERS 2008; 100:117601. [PMID: 18517827 DOI: 10.1103/physrevlett.100.117601] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/19/2007] [Indexed: 05/26/2023]
Abstract
Nuclear magnetic resonance, electron paramagnetic resonance and magnetization measurements show that bulk LixZnPc are strongly correlated one-dimensional metals. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T_{1} and of the static uniform susceptibility chi_{S} on approaching room temperature are characteristic of a Fermi liquid. Moreover, while for x approximately 2 the electrons are delocalized down to low temperature, for x-->4 a tendency towards localization is noticed upon cooling, yielding an increase both in 1/T_{1} and chi_{s}. The x dependence of the effective density of states at the Fermi level D(E_{F}) displays a sharp enhancement for x approximately 2, at the half filling of the ZnPc lowest unoccupied molecular orbitals. This suggests that LixZnPc is on the edge of a metal-insulator transition where enhanced superconducting fluctuations could develop.
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NMR as a probe of the relaxation of the magnetization in magnetic molecules. PHYSICAL REVIEW LETTERS 2005; 94:077203. [PMID: 15783849 DOI: 10.1103/physrevlett.94.077203] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/13/2004] [Indexed: 05/24/2023]
Abstract
We investigate the time autocorrelation of the molecular magnetization M(t) for three classes of magnetic molecules (antiferromagnetic rings, grids, and nanomagnets), in contact with the phonon heat bath. For all three classes, we find that the exponential decay of the fluctuations of M(t) is characterized by a single characteristic time tau(T,B) for not too high temperature T and field B. This is reflected in a nearly single-Lorentzian shape of the spectral density of the fluctuations. We show that such fluctuations are effectively probed by NMR, and that our theory explains the recent phenomenological observation by Baek et al. [Phys. Rev. B 70, 134434 (2004)] that the Larmor-frequency dependence of 1/T(1) data in a large number of AFM rings fits to a single-Lorentzian form.
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Direct determination of the magnetic ground state in the square lattice S = 1/2 antiferromagnet Li2VOSiO4. PHYSICAL REVIEW LETTERS 2004; 93:027202. [PMID: 15323947 DOI: 10.1103/physrevlett.93.027202] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/15/2003] [Indexed: 05/24/2023]
Abstract
Powder neutron diffraction and resonant x-ray scattering measurements from a single crystal have been performed to study the low-temperature state of the 2D frustrated, quantum-Heisenberg system Li2VOSiO4. Both techniques indicate a collinear antiferromagnetic ground state, with propagation vector k=(1 / 2 1 / 2 0), and magnetic moments in the a-b plane. Contrary to previous reports, the ordered moment at 1.44 K, m=0.63(3)micro(B), is very close to the value expected for the square lattice Heisenberg model ( approximately 0.6micro(B)). The magnetic order is three dimensional, with antiferromagnetic a-b layers stacked ferromagnetically along the c axis. Neither x-ray nor neutron diffraction shows evidence for a structural distortion between 1.6 and 10 K.
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Very-low-frequency excitations in frustrated two-dimensional S = 1/2 Heisenberg antiferromagnets. PHYSICAL REVIEW LETTERS 2002; 88:047601. [PMID: 11801167 DOI: 10.1103/physrevlett.88.047601] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/20/2001] [Indexed: 05/23/2023]
Abstract
Muon spin resonance and 7Li NMR relaxation measurements in frustrated two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice are presented. It is found that, in both Li2VOSiO4 and Li2VOGeO4, spin dynamics at frequencies orders of magnitude below the Heisenberg exchange frequency are present. These dynamics are associated with the motions of walls separating coexisting collinear domains with a magnetic wave vector rotated by 90 degrees.
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Successful use of central venous catheter as permanent hemodialysis access: 84-month follow-Up in lucania. Blood Purif 2001; 19:39-43. [PMID: 11114576 DOI: 10.1159/000014477] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Cuffed tunneled venous access catheters are commonly used for temporary and permanent access in hemodialysis (HD) patients. These catheters serve an essential role in providing permanent access in subjects in whom all other access options have been exhausted. The predominant complications are catheter thrombosis, catheter fibrin sheating and infection. The aim of this study was to evaluate long-term survival and complications of permanent venous catheters (PVC) placed for the purpose of HD during the period from January 1992 to December 1998, at the Dialysis Units of Lucania (a southern Italian region). A total of 98 PVC were placed in 88 patients during this period. The catheters used were of three types: (a) 72 VasCath Soft Cell catheters (Bard Instrument Company, Toronto, Ont., Canada); (b) 22 PermCath catheters (Quinton Instrument Company, Seattle, Wash., USA), and (c) 4 Tesio catheters (Bellco SpA, Mirandola, Italy). Survival curves of catheters were calculated using the Kaplan-Meier product-limit estimator. The patient survival was 60% at the 78th month. Actually, 52 patients (27 males, 25 females) are still alive: 15 (26.9%) of these patients have diabetes mellitus and 1 has been transplanted. The actuarial survival rate of PVC was 89% in the whole population studied and 82% in subjects alive after 84 months. Twenty-five patients (28.4%) had PVC as the first reliable vascular access. Long-term complications occurred 27 times (1 episode every 44.81 month/patient) as: breakage (3.1%); thrombosis (10.2%); displacement (2.0%); subcutaneous tunnel bleeding (3.1%); inadequate blood flow (7.1%), and infection (10.2%). In conclusion, our data confirm that PVC might represent an effective long-term blood access route for HD. Again, PVC are getting the access of choice for selected patients (i.e., older subjects with cardiovascular diseases and cancer patients) and are enjoying a dramatic increase in use for subjects who are terrified of repetitive venopuncture.
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Li2VO(Si,Ge)O4, a prototype of a two-dimensional frustrated quantum heisenberg antiferromagnet. PHYSICAL REVIEW LETTERS 2000; 85:1318-1321. [PMID: 10991541 DOI: 10.1103/physrevlett.85.1318] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/05/2000] [Indexed: 05/23/2023]
Abstract
NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.
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Spin dynamics and magnetic correlation length in two-dimensional quantum heisenberg antiferromagnets. PHYSICAL REVIEW LETTERS 2000; 84:366-369. [PMID: 11015912 DOI: 10.1103/physrevlett.84.366] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/10/1999] [Indexed: 05/23/2023]
Abstract
The correlated spin dynamics and temperature dependence of the correlation length xi(T) in two-dimensional quantum (S = 1/2) Heisenberg antiferromagnets (2DQHAF) on a square lattice are discussed in light of experimental results of proton spin lattice relaxation in copper formiate tetradeuterate. In this compound the exchange constant is much smaller than the one in recently studied 2DQHAF, such as La2CuO4 and Sr2CuO2Cl2. Thus the spin dynamics can be probed in detail over a wider temperature range. The NMR relaxation rates turn out to be in excellent agreement with a theoretical mode-coupling calculation. The deduced temperature behavior of xi(T) is in agreement with high-temperature expansions, quantum Monte Carlo simulations, and the pure quantum self-consistent harmonic approximation. Contrary to the predictions of the theories based on the nonlinear sigma model, no evidence of crossover between different quantum regimes is observed.
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Superconducting fluctuations and 63Cu NQR-NMR relaxation in YBa2Cu3O7- delta : Effect of magnetic field and a test for the pairing-state symmetry. PHYSICAL REVIEW. B, CONDENSED MATTER 1996; 54:R9682-R9685. [PMID: 9984791 DOI: 10.1103/physrevb.54.r9682] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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34
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Spin gap in HgBa2Ca2Cu3O8+ delta single crystals from 63Cu NMR. PHYSICAL REVIEW LETTERS 1996; 76:4238-4241. [PMID: 10061236 DOI: 10.1103/physrevlett.76.4238] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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35
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Erratum: 139La NQR relaxation and microSR study of Zn-doping effects in La2CuO4. PHYSICAL REVIEW. B, CONDENSED MATTER 1996; 53:2893. [PMID: 9983805 DOI: 10.1103/physrevb.53.2893] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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36
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139La NQR relaxation and microSR study of Zn-doping effects in La2CuO4. PHYSICAL REVIEW. B, CONDENSED MATTER 1995; 52:4226-4236. [PMID: 9981552 DOI: 10.1103/physrevb.52.4226] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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37
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Confinement in Bechgaard salts: Anomalous magnetoresistance and nuclear relaxation. PHYSICAL REVIEW LETTERS 1995; 74:5272-5275. [PMID: 10058726 DOI: 10.1103/physrevlett.74.5272] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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38
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Spin fluctuations in doped antiferromagnets from 7Li NMR relaxation in CuO:Li. PHYSICAL REVIEW. B, CONDENSED MATTER 1994; 49:7044-7047. [PMID: 10009435 DOI: 10.1103/physrevb.49.7044] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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39
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Temperature dependence of the sublattice spontaneous magnetization of YBa2Cu3O6. PHYSICAL REVIEW. B, CONDENSED MATTER 1993; 48:16769-16774. [PMID: 10008261 DOI: 10.1103/physrevb.48.16769] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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40
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Spin dynamics in CuO and Cu1-xLixO from 63Cu and 7Li nuclear relaxation. PHYSICAL REVIEW. B, CONDENSED MATTER 1993; 48:3433-3444. [PMID: 10008773 DOI: 10.1103/physrevb.48.3433] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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41
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Motion of flux lines under an applied Lorentz force and rf penetration from 89Y and 63Cu NMR in YBa2Cu3O7- delta. PHYSICAL REVIEW. B, CONDENSED MATTER 1993; 48:528-531. [PMID: 10006804 DOI: 10.1103/physrevb.48.528] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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Erratum: Vortex lattice and vortex dynamics in YBa2Cu4O8 from 89Y NMR. PHYSICAL REVIEW. B, CONDENSED MATTER 1992; 46:5885. [PMID: 10004408 DOI: 10.1103/physrevb.46.5885] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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43
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Vortex lattice and vortex dynamics in YBa2Cu4O8 from 89Y NMR. PHYSICAL REVIEW. B, CONDENSED MATTER 1992; 45:5760-5763. [PMID: 10000311 DOI: 10.1103/physrevb.45.5760] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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44
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89Y NMR observations of Knight shift anisotropy and motional line narrowing in YBa2Cu3O7- delta. PHYSICAL REVIEW LETTERS 1992; 68:1236-1239. [PMID: 10046114 DOI: 10.1103/physrevlett.68.1236] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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