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S. R. Dunsiger,
A. A. Aczel,
C. Arguello, H. Dabkowska,
A. Dabkowski,
M. -H. Du,
T. Goko,
B. Javanparast,
T. Lin,
F. L. Ning,
H. M. L. Noad,
D. J. Singh,
T. J. Williams,
Y. J. Uemura,
M. J. P. Gingras,
G. M. Luke
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ABSTRACT: We respond to the comment of Bramwell et al (arXiv:1111.4168v1) to our
original publication (S. R. Dunsiger et al, Phys. Rev. Lett. 107, 207207
(2011)), detailing muon spin rotation measurements of the Spin Ice compound
Dy2Ti2O7.
05/2012;
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S R Dunsiger,
A A Aczel,
C Arguello, H Dabkowska,
A Dabkowski,
M-H Du,
T Goko,
B Javanparast,
T Lin,
F L Ning,
H M L Noad,
D J Singh,
T J Williams,
Y J Uemura,
M J P Gingras,
G M Luke
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ABSTRACT: Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S. T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.
Physical Review Letters 11/2011; 107(20):207207. · 7.37 Impact Factor
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ABSTRACT: Using neutron scattering techniques, we have studied incommensurate spin ordering as well as low energy spin dynamics in single crystal underdoped \LBCO with x$\sim$0.095 and 0.08; high temperature superconductors with T$_C \sim$ 27 K and 29 K respectively. Static two dimensional incommensurate magnetic order appears below T$_N$=39.5 $\pm$ 0.3 K in \LBCO (x=0.095) and a similar temperature for x=0.08 within the low temperature tetragonal phase. The spin order is unaffected by either the onset of superconductivity or the application of magnetic fields of up to 7 Tesla applied along the c-axis in the x=0.095 sample. Such magnetic field {\it independent} behaviour is in marked contrast with the field induced enhancement of the staggered magnetisation observed in the related \LSCO system, indicating this phenomenon is not a universal property of cuprate superconductors. Surprisingly, we find that incommensurability $\delta $ is only weakly dependent on doping relative to \LSCO. Dispersive excitations in \LBCO (x=0.095) at the same incommensurate wavevector persist up to at least 60 K. The dynamical spin susceptibility of the low energy spin excitations saturates below \tc, in a similar manner to that seen in the superconducting state of La$_2$CuO$_{4+y}$. Comment: 9 pages, 7 figures, submitted to PRB, figures updated
08/2007;
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ABSTRACT: The temperature and field dependences of the magnetization of a single crystal of a new class of layered cobaltites, TbBaCo4O7+x
, with a structure containing a Kagomé lattice and a triangular lattice were measured. The measurements were performed on
a SQUID magnetometer at temperatures in the range 2–300 K and in magnetic fields of up to 55 kOe for two field orientations.
The anisotropy of the magnetization was studied, and the presence of antiferromagnetic ordering in fields H < H
c
and a weak magnetic-field-induced (H > H
c
) ferromagnetic component in the low-temperature range was demonstrated. The magnetic characteristics of the initial TbBaCo4O7+x
single crystal and the single crystal annealed in an O2 atmosphere were compared.
Physics of the Solid State 01/2007; 49(6):1125-1131. · 0.71 Impact Factor
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ABSTRACT: The insulator/metal transition induced by hole-doping due to neodymium vacancies of the Mott- Hubbard antiferromagnetic insulator, Nd1-xTiO3, is studied over the composition range 0.010(6) < x < 0.243(10). Insulating p-types conduction is found for x < 0.071(10). Anderson localization in the presence of a Mott-Hubbard gap, is the dominant localization mechanism for the range of 0.074(10) < x < 0.089(1) samples. For x < 0.089(1), n-type conduction is observed and the activation energy extrapolates to zero by x < 0.1. The 0.095(8) < x < 0.203(10) samples are Fermi-liquid metals and the effects of strong electronic correlations are evident near the metal-to-insulator boundaries in features such as large Fermi liquid T2 coefficients. For 0.074(9) < x < 0.112(4), a weak negative magnetoresistance is found below ~ 15 K and it is attributed to the interaction of conduction electrons with Nd3+ magnetic moments. Combining information from our companion study of the magnetic properties of Nd1-xTiO3 solid solution, a phase diagram is proposed. The main conclusions are that long range antiferromagnetic order disappears before the onset of metallic behavior and that the Anderson-Mott transition occurs over a finite range of doping levels. Our results differ from conclusions drawn from a similar study on the hole doped Nd1-xCaxTiO3 system which found the co-existence of antiferromagnetic order and metallic behavior and that the Mott transition occurs at a discrete doping level.
07/2006;
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ABSTRACT: Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d) have been studied by means of magnetization measurements in the temperature range between 1.95 K and Tc, in an external magnetic field up to 9 T. Flux jumps were found in the temperature range 1.95 K - 6 K, with the external magnetic field parallel to the c axis of the investigated sample. The effect of sample history on magnetic flux jumping was studied and it was found to be well accounted for by the available theoretical models. The magnetic field sweep rate strongly influences the flux jumping and this effect was interpreted in terms of the influence of both flux creep and the thermal environment of the sample. Strong flux creep was found in the temperature and magnetic field range where flux jumps occur suggesting a relationship between the two. The heat exchange conditions between the sample and the experimental environment also influence the flux jumping behavior. Both these effects stabilize the sample against flux instabilities, and this stabilizing effect increases with decreasing magnetic field sweep rate. Demagnetizing effects are also shown to have a significant influence on flux jumping. Comment: 10 pages, 6 figures, RevTeX4, submitted to Phys. Rev. B
09/2002;
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04/2002;
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ABSTRACT: Lithium manganese oxide crystals with composition (Li(0.91)Mn(0.09))Mn(2)O(4) were synthesized by a flux method. The crystals have a structure closely related to that of the cubic spinel LiMn(2)O(4), but 9% of the lithium ions in the tetrahedral 4a site are substituted by Mn(2+) ions. This substitution lowers the average Mn oxidation state below 3.5+, resulting in a Jahn-Teller distortion of the MnO(6) octahedron.
Acta Crystallographica Section C Crystal Structure Communications 05/2001; 57(Pt 4):331-2. · 0.52 Impact Factor
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ABSTRACT: We report on far-infrared measurements of Zn-, Cd-, and Ni-doped crystals of the spin-Peierls compound CuGeO3. Dopants that replace Cu have the effect of introducing several absorption lines, polarized in the ab plane, between 5 and 55 cm-1. The intensity of the absorption grows with dopant concentration but saturates above 2%. One line near 10 cm-1 loses intensity above 4 K, and a second line near 20 cm-1 is absent at low temperatures but grows to peak at about 40 K in agreement with a three-level model with two excited states 10 and 30 cm-1 above the ground state. As the doping is increased these lines broaden and a temperature-independent absorption develops over the entire range from 5 to 55 cm-1. These features are magnetic-field independent up to 16 T and are absent in samples doped with Si that replaces Ge instead of Cu. We suggest the new absorption is due to localized lattice modes of the dopant ion and the surrounding GeO4 tetrahedra.
Phys. Rev. B. 12/1998; 59(2).
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ABSTRACT: We have examined the critical properties of several lightly doped samples of CuGeO3 through x-ray diffraction measurements of the order parameter. We obtain an exponent β consistent with that obtained for pure CuGeO3, where β=0.345±0.03, for 0.1% Zn-doped, 0.2% Si-doped, and 0.4% Si-doped samples. Measurements on two 0.1% Cd-doped samples produce results clearly inconsistent with those for the pure compound and well described by mean field behavior. This change in critical phenomena from three-dimensional behavior to mean field behavior is interpreted as a consequence of local strain fields induced by the presence of larger dopant ions. Lattice constant measurements on the same doped samples indicate the presence of spontaneous strains which scale with the square of the order parameter, as was the case for pure CuGeO3.
Phys. Rev. B. 10/1998; 58(18).
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ABSTRACT: We report on far infrared measurements on Zn and Si doped crystals of the spin-Peierls compound CuGeO3. Zn doping has the effect of introducing several new absorption lines, polarized in the ab-plane, between 5 and 55 cm-1. The intensity of the absorption grows with Zn concentration but saturates above 2% Zn. One line at 10 cm-1 loses intensity above 4K, and a second line at 20 cm-1 is absent at low temperatures but grows to peak at about 40K in agreement with a three level model with two excited states 10 and 30 cm-1 above the ground state. As the doping is increased these lines broaden, and a temperature independent absorption developes over the entire range from 5 to 55 cm-1. These features are magnetic field independent up to 16T and are absent in Si doped samples. We suggest the new absorption is due to localized lattice modes of the zinc ion and the surrounding GeO4 tetrahedra.
08/1997;
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ABSTRACT: The effects of growth conditions and composition on the crystal growth of (La1-xCax)(2)CaCu2O6+delta (x=0.05-0.15) superconducting materials have been studied by using the floating zone method. A smooth solid-liquid growth interface of (La1-xCax)(2)CaCu2O6+delta (x=0.05-0.06) tended to break down into a cellular interface when the growth velocity was higher than 0.5 mm/h. When the solid-liquid inter-face became cellular, the single crystal size in an as-grown rod decreased abruptly and the as-grown rod was not single phase. Various sized CaO inclusions were found inside the as-grown single crystals of (La1-xCax)(2)CaCu2O6+delta (x >= 0.075). Cubic centimeter size single crystals were successfully grown for both (La0.95Ca0.05)(2)CaCu2O6+delta and (La0.9Ca0.06)(2)CaCu2O6+delta. The as-grown single crystals of (La1-xCax)(2)CaCu2O6+delta under 1 bar pressure of oxygen were not superconducting. The superconducting temperature of the (La1-xCax)(2)CaCu2O6+y crystals grown at 11 bar oxygen is 28 K. The application of high pressure oxygen during crystal growth is an effective method to get bulk superconductivity in the crystals. (c) 2006 Elsevier Ltd. All rights reserved.
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ABSTRACT: We have performed x-ray-diffraction measurements of the critical phenomena associated with the spin-Peierls phase transition in CuGeO3. Measurements of the order parameter indicate a value of the critical exponent β of 0.345±0.03 consistent with conventional three-dimensional (3D) behavior and in closest agreement with 3D XY universality, as expected on theoretical grounds. This work also indicates a rather narrow asymptotic critical region which explains much of the inconsistency reported in previous critical exponent estimates. Measurements of the relative changes in lattice constants have also been carried out to allow for detailed comparison between the spontaneous strains present below the transition temperature and the order parameter. The order parameter and lattice constant measurements were performed in identical thermal environments on the same sample to allow for direct comparison with no adjustment of transition temperature. This comparison suggests good agreement between the two except near the transition temperature where fluctuation effects are observed in the spontaneous strain which are less evident in the order parameter.
Phys. Rev. B. 57(22).
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Acta Phys. Pol. A 117(2010)2,319-322.
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ABSTRACT: Single crystals of the lightly doped spin-Peierls system Cu1[?]xCdxGeO3 have been studied using bulk susceptibility, x-ray diffraction, and inelastic neutron scattering techniques. We investigate the triplet gap in the magnetic excitation spectrum of this quasi-one-dimensional quantum antiferromagnet, and its relation to the spin-Peierls dimerization order parameter. We employ two different theoretical forms to model the inelastic neutron scattering cross section and kh''(Q,o), and show the sensitivity of the gap energy to the choice of \chi ''(\mathbf {Q}, \hbar \omega ). We find that a finite gap exists at the spin-Peierls phase transition.
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ABSTRACT: We measured the magnetization versus field of SrCu2(BO3)2 using a 45 T, 400 ms pulsed magnet. By achieving good thermal coupling of the sample to the 3He and 4He baths, we obtained a realistic temperature-dependence for the 1/8, 1/4 and 1/3 magnetization plateaux. In addition, we measured the low temperature specific heat in continuous magnetic fields up to 18 T. We found that a contribution attributed to singlet spin–dimer excitations is suppressed with increasing magnetic fields. At fields H=12 T a second anomaly is evident that shifts to lower temperatures as the field increases. The second anomaly is attributed to triplet spin excitations that come closer to the ground state in presence of an external field. A linear extrapolation of this anomaly plotted versus field indicate that the gap associated to these excitations could be suppressed at H≈25 T, just before the 1/8 magnetization plateau.
Journal of Alloys and Compounds 369:90-92. · 2.29 Impact Factor
