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G Collin

Publications and source records attributed to G Collin.

At least 19 recordsLinked to original sources
PubMed2006

Direct evidence for a dynamical ground state in the highly frustrated Tb(2)Sn(2)O(7) pyrochlore.

mSR experiments have been performed on a powder sample of the "ordered spin ice" Tb(2)Sn(2)O(7) pyrochlore. At base temperature (T=35 mK), the muon relaxation is found to be of dynamical nature, which demonstrates that strong fluctuations persist below the ferromagnetic transition (T(C)=0.87 K). Hints of long-range ordering appear as oscillations of the muon polarization when an external field is applied and also as a hysteretic behavior below T(C). We propose that dynamics results from fluctuations of clusters of correlated spins with the ordered spin ice structure.

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PubMed2006

Neutron spin-echo investigation of slow spin dynamics in kagomé-bilayer frustrated magnets as evidence for phonon assisted relaxation in SrCr9xGa12-9xO19.

A neutron spin-echo investigation of the low temperature spin dynamics in two well-characterized kagomé bilayer compounds SrCr9xGa12-9xO19 (x=0.95, SCGO) and Ba2Sn2ZnCr7xGa10-7xO22 (x=0.97, BSZCGO) reveals two novel features. One is the slowing down of the relaxation rate without critical behavior at Tg, where a macroscopic spin-glass-like freezing occurs. The second is, in SCGO at 4 K (approximately Tg)<T<7 K, the relaxation rate activation energy Ea=7+/-0.4 meV, equal to the energy of a phonon mode, pointing out the role of spin-lattice coupling.

Journal Article↗
PubMed2006

Impurity-induced local magnetism and density of states in the superconducting state of YBa2Cu3O7.

17O NMR is used to probe the local influence of nonmagnetic Zn and magnetic Ni impurities in the superconducting state of optimally doped high TC YBa2Cu3O7. Zn and Ni induce a staggered paramagnetic polarization, similar to that evidenced above TC, with a typical extension xi=3 cell units for Zn and xi>or=3 for Ni. In addition, Zn is observed to induce a local density of states near the Fermi energy in its neighborhood, which also decays over about 3 cell units. Its magnitude decreases sharply with increasing temperature. This allows direct comparison with the STM observations done in BiSCCO.

Journal Article↗
PubMed2006

NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2: a nesting scenario.

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na(0.5)CoO(2). Above T(N) = 86 K, both sites display a similar T dependence of the spin shift, suggesting that there is no charge segregation into Co(3+) and Co(4+) sites. Below T(N), the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition at T(MIT) = 51 K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive spin-density wave induced by two nestings of the Fermi surface specific to the x = 0.5 Na ordering.

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PubMed2005

Cascade of bulk magnetic phase transitions in NaxCoO2 as studied by muon spin rotation.

Using muon spin rotation, well-defined bulk approximately 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x=0.85 with the highest transition temperature ever observed for x>or=0.75. This stresses the diversity of x>or=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x=0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

Journal Article↗
PubMed2004

muSR study of the quantum dynamics in the frustrated S=3/2 kagomé bilayers.

We present muSR experiments in the S=3/2 kagomé bilayer compound Ba(2)Sn(2)ZnGa(10-7p)Cr(7p)O22 [BSZCGO(p)] and compare it to the isostructural SrCr(9p)Ga(12-9p)O19 [SCGO(p)], including for the latter new results for p > or =0.89. Quantum-dynamical low energy magnetic excitations are evidenced in this novel compound. We study the evolution of the muon relaxation rate with p, T, and field. A phenomenological model for the muon relaxation based on sporadic dynamics due to spin excitations in a singlet sea proposed by Uemura et al. is extended to all fields and T range. Its connection to the RVB picture is discussed, and we argue that such coherent states might mediate the interactions between "impurities" which induce the spin glass freezing.

Journal Article↗
PubMed2004

23Na NMR evidence for charge order and anomalous magnetism in NaxCoO2.

Oriented powder samples of NaxCoO2 are studied by 23Na NMR and SQUID magnetometry. In nominal 0.50<or=x<or=0.70 solid state reacted samples the dominant phase synthesized has a single composition x(0) approximately 0.70. Three Na sites are identified from their single valued quadrupole effects and magnetic shifts, which implies a very definite order of the Na+ ions and of the Co charges in the CoO2 planes. The local susceptibility of the magnetic cobalt sites displays a large enhancement below 100 K with respect to the usually found high T Curie-Weiss law. This contrasts with the Pauli-like magnetism detected for x approximately 0.35, the parent of the superconducting hydrated compound.

Journal Article↗
PubMed2004

Intrinsic susceptibility and bond defects in the novel two dimensional frustrated antiferromagnet Ba2Sn2ZnCr7pGa10-7pO22.

We present microscopic and macroscopic magnetic properties of the highly frustrated antiferromagnet Ba(2)Sn(2)ZnCr(7p)Ga(10-7p)O22, respectively, probed with NMR and SQUID experiments. The T variation of the intrinsic susceptibility of the Cr3+ frustrated Kagomé bilayer, chi(Kag), displays a maximum around 45 K. The dilution of the magnetic lattice has been studied in detail for 0.29</=p</=0.97. Novel dilution independent defects, likely related with magnetic bond disorder, are evidenced and discussed. We compare our results to SrCr(9p)Ga(12-9p)O19. Both bond defects and spin vacancies do not affect the average susceptibility of the Kagomé bilayers.

Journal Article↗
PubMed2004

[Conservation of human embryos in straws: safety in terms of human immunodeficiency virus 1].

OBJECTIVE: The possibility of offering assisted reproductive technologies (ART) to HIV-positive couples has revived questions concerning the safety of the gametes and embryos cryopreservation in liquid nitrogen tanks. PATIENTS AND METHODS: We evaluated the safety of three types of straws - polyvinyl chloride (PVC), polyethylene terephthalate glycol (PETG) and so-called 'high-security' ionomeric resin (IR) - containing HIV-1 under standard conditions of cryopreservation. Potential HIV contamination was assessed by RT-PCR and then nested PCR. RESULTS: Under cryopreservation conditions, the sealed open ends of PVC and PETG straws were not safe. The ultrasound sealing system seems to be the weak link in obtaining total imperviousness of the straws. In contrast, both ends of the IR straws were safe for HIV in the framework of their use for ART. CONCLUSION: Sealing cryopreservation straws ultrasonically could incur the risk of not assuring their impermeability. Under standard cryopreservation conditions thermosealing of IR straws appears to be safe for HIV.

Cryopreservation↗
PubMed2002

Absence of static phase separation in the high T(c) cuprate YBa(2)Cu(3)O(6+y).

We use 89Y NMR in YBa(2)Cu(3)O(6+y) in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Deltap=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates.

Journal Article↗
PubMed2001

HIV type 1 genetic diversity and genotypic drug susceptibility in the Republic of Moldova.

HIV-1 genetic diversity and, for the first time, genotypic drug susceptibility was investigated for strains circulating in the Republic of Moldova (of the former Soviet Union). Eighty-three samples from adults recently infected by intravenous drug use (IDU) (n = 60), heterosexual contact (n = 8), and from blood donors (n = 15) that tested positive from 1997 to 1998, and originating from different regions of Moldova were serotyped. By group-specific and subtype-specific peptide ELISA, patients were infected by serotype A (n = 65), serotype B (n = 1), or were nontypable (n = 17). Heteroduplex mobility assay (HMA) confirmed 11 subtype A and the one subtype B infection. Analyses of pol and env sequences for six of the IDUs confirmed that they were infected with subtype A strain. These strains clustered tightly with subtype A strains isolated from the former Soviet Union in phylogenetic analysis. No mutations associated with drug resistance were detected. The Republic of Moldova is culturally more closely related to Romania (where subtype F dominates the epidemic), but depends economically on Russia (where subtype A is established among IDUs). Thus, our results suggest that the spread of HIV in this region is driven by drug networks rather than being due to cultural similarities.

Adolescent↗
PubMed2001

Persistence of Li induced Kondo moments in the superconducting state of cuprates.

Using 7Li NMR shift data, the anomalous local moment induced by spinless Li impurities persists below T(c) in YBa 2Cu 3O6+y. In the underdoped regime, the moments retain their Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (TK = 135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on first near neighbor Cu) is not drastically modified below Tc, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.

Journal Article↗
PubMed2000

Ga NMR study of the local susceptibility in Kagome-based SrCr8Ga4O19: pseudogap and paramagnetic defects

We present the first Ga(4f ) NMR study of the Cr susceptibility in the archetype of kagome-based frustrated antiferromagnets, SrCr8Ga4O19. Our major finding is that the susceptibility of the frustrated lattice goes through a maximum around 50 K. Our data also support the existence of paramagnetic "clusters" of spins, responsible for the Curie behavior observed in the macroscopic susceptibility at low T. These results set novel features for the constantly debated physics of geometrically frustrated magnets.

Journal Article↗
PubMed2000

In-plane charge modulation below T(c) and charge-density-wave correlations in the chain layer in YBa2Cu3O7

Nuclear quadrupole resonance (NQR) measurements have been performed on Cu(2) plane sites and Cu(1) chain sites in fully doped YBa2Cu3O7 between 300 and 4.2 K. The sharp increase of the Cu(1) NQR linewidth across the superconducting transition and the T dependence of the Cu(1) spin lattice relaxation rate confirm the existence of a charge-density-wave state (CDW) in the chains. The simultaneous broadening of the Cu(2) linewidth below T(c) and the anomalous T dependence of Cu(1) and Cu(2) NQR parameters indicate that these in-chain CDW correlations are strongly involved in the appearance of an in-plane charge modulated structure below T(c).

Journal Article↗
PubMed2000

Dynamics of the local moment induced by nonmagnetic defects in cuprates

We present a study of the spin dynamics of magnetic defects induced by Li substitution of the plane Cu in the normal state of YBa2Cu3O6+x. The fluctuations of the coupled Cu magnetic moments in the vicinity of Li are probed by near-neighbor 89Y and 7Li NMR spin lattice relaxation. The data indicate that the magnetic perturbation fluctuates as a single entity with a correlation time tau which scales with the local static susceptibility. This behavior is reminiscent of the low T Kondo state of magnetic impurities in conventional metals. Surprisingly it extends well above the "Kondo" temperature for the underdoped pseudogapped case.

Journal Article↗
PubMed2000

Quantum impurities and the neutron resonance peak in YBa2Cu3O7: Ni versus Zn.

The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin dynamics of an optimally doped high temperature superconductor is compared in YBa2(Cu0.97Ni0.03)3O7 (Tc=80 K) and YBa2(Cu0.99Zn0.01)3O7 (Tc=78 K). In the Ni-substituted system, the magnetic resonance peak (which is observed at Er approximately 40 meV in the pure system) shifts to lower energy with a preserved Er/Tc ratio while the shift is much smaller upon Zn substitution. By contrast Zn, but not Ni, restores significant spin fluctuations around 40 meV in the normal state. These observations are discussed in the light of models proposed for the magnetic resonance peak.

Journal Article↗
PubMed2000

63Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7

Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.

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