40 results on '"Vajk, O. P."'
Search Results
2. Spin correlations in the electron-doped high-transition-temperature superconductor Nd{2-x}Ce{x}CuO{4+/-delta}
- Author
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Motoyama, E. M., Yu, G., Vishik, I. M., Vajk, O. P., Mang, P. K., and Greven, M.
- Subjects
Condensed Matter - Superconductivity ,Condensed Matter - Strongly Correlated Electrons - Abstract
High-transition-temperature (high-Tc) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping [1, 2] and appears to overlap with the superconducting phase. The archetypical electron-doped compound Nd{2-x}Ce{x}CuO{4\pm\delta} (NCCO) shows bulk superconductivity above x \approx 0.13 [3, 4], while evidence for antiferromagnetic order has been found up to x \approx 0.17 [2, 5, 6]. Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases [7]. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies [8-11], arises from a build-up of spin correlations, in agreement with recent theoretical proposals [12, 13]., Comment: 5 pages, 4 figures
- Published
- 2006
- Full Text
- View/download PDF
3. Incident-energy and polarization dependent RIXS study of La2CuO4
- Author
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Lu, L., Hancock, J. N., Chabot-Couture, G., Vajk, O. P., Yu, G., Ishii, K., Mizuki, J., Casa, D., Gog, T., and Greven, M.
- Subjects
Condensed Matter - Strongly Correlated Electrons ,Condensed Matter - Superconductivity - Abstract
We present a detailed Cu K-edge resonant inelastic X-ray scattering (RIXS) study of the Mott insulator La2CuO4 in the 1-7 eV energy transfer range. As initially found for the high-temperature superconductor HgBa2CuO4+d, the spectra exhibit a multiplet of weakly-dispersive electron-hole excitations, which are revealed by utilizing the subtle dependence of the cross section on the incident photon energy. The close similarity between the fine structures for in-plane and out-of-plane polarizations is indicative of the central role played by the 1s core hole in inducing charge excitations within the CuO2 planes. On the other hand, we observe a polarization dependence of the spectral weight, and careful analysis reveals two separate features near 2 eV that may be related to different charge-transfer processes. The polarization dependence indicates that the 4p electrons contribute significantly to the RIXS cross section. Third-order perturbation arguments and a shake-up of valence excitations are then applied to account for the final-energy resonance in the spectra. As an alternative scenario, we discuss fluorescence-like emission processes due to 1s -> 4p transitions into a narrow continuum 4p band., Comment: 10 pages, 9 figures Phys. Rev. B, 74, 224509, (2006) http://link.aps.org/abstract/PRB/v74/e224509
- Published
- 2006
- Full Text
- View/download PDF
4. Crystal growth and characterization of the model high-temperature superconductor $HgBa_{2}CuO_{4+\delta}$
- Author
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Zhao, X., Yu, G., Cho, Y., Chabot-Couture, G., Barisic, N., Bourges, P., Kaneko, N., Li, Y., Lu, L., Motoyama, E. M., Vajk, O. P., and Greven, M.
- Subjects
Condensed Matter - Superconductivity ,Condensed Matter - Materials Science ,Condensed Matter - Strongly Correlated Electrons - Abstract
This paper is published in Advanced Materials (available at http://www3.interscience.wiley.com/cgi-bin/abstract/113511105/ABSTRACT). It has been withdrawn from the cond-mat preprint archive in order to avoid a violation of the Journal's policy., Comment: Advanced Materials 18, 3243 (2006)
- Published
- 2006
5. Magnetic field effect on the superconducting magnetic gap of Nd{1.85}Ce{0.15}CuO4
- Author
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Motoyama, E. M., Mang, P. K., Petitgrand, D., Yu, G., Vajk, O. P., Vishik, I. M., and Greven, M.
- Subjects
Condensed Matter - Superconductivity ,Condensed Matter - Strongly Correlated Electrons - Abstract
Inelastic neutron scattering measurements on the archetypical electron-doped material Nd{1.85}Ce{0.15}CuO4 up to high relative magnetic field strength, H/Hc2 ~ 50%, reveal a simple linear magnetic-field effect on the superconducting magnetic gap and the absence of field-induced in-gap states. The extrapolated gap-closing field value is consistent with the upper critical field Hc2, and the high-field response resembles that of the paramagnetic normal state., Comment: 4 pages, 3 figures; (v2) added reference; published in Phys. Rev. Lett
- Published
- 2006
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6. Magnetic inversion symmetry breaking and ferroelectricity in TbMnO3
- Author
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Kenzelmann, M., Harris, A. B., Jonas, S., Broholm, C., Schefer, J., Kim, S. B., Zhang, C. L., Cheong, S. -W., Vajk, O. P., and Lynn, J. W.
- Subjects
Condensed Matter - Strongly Correlated Electrons ,Condensed Matter - Materials Science - Abstract
TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T_N < 41K is accompanied by ferroelectric order for T < 28K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally-modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3., Comment: 4 pages revtex, accepted by Phys. Rev. Lett. on June 21, 2005
- Published
- 2005
- Full Text
- View/download PDF
7. Magnetic Order and Spin Dynamics in Ferroelectric HoMnO$_{3}$
- Author
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Vajk, O. P., Kenzelmann, M., Lynn, J. W., Kim, S. B., and Cheong, S. -W.
- Subjects
Condensed Matter - Strongly Correlated Electrons - Abstract
Hexagonal HoMnO$_{3}$ is a frustrated antiferromagnet (T$_{N}$=72 K) ferroelectric (T$_{C}$=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin wave dispersion for the S=2 Mn$^{3+}$ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy $D$ that is 0.093 meV above the spin reorientation transition at 40 K, and 0.126 meV below. For $H\parallel c$ the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho$^{3+}$ ions are ordered magnetically., Comment: To be published in Physical Review Letters
- Published
- 2005
- Full Text
- View/download PDF
8. Charge-Transfer Excitations in the Model Superconductor HgBa$_2$CuO$_{\bf 4+\delta}$
- Author
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Lu, L., Zhao, X., Chabot-Couture, G., Hancock, J. N., Kaneko, N., Vajk, O. P., Yu, G., Grenier, S., Kim, Y. J., Casa, D., Gog, T., and Greven, M.
- Subjects
Condensed Matter - Strongly Correlated Electrons ,Condensed Matter - Superconductivity - Abstract
We report a Cu $K$-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa$_2$CuO$_{4+\delta}$ at optimal doping ($T_{\rm c} = 96.5 $ K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly-dispersive ($ < 0.5$ eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a charge-transfer pseudogap deep in the superconducting phase. Quite generally, our data demonstrate the importance of exploring the incident photon energy dependence of the RIXS cross section., Comment: 5 pages, 3 figures
- Published
- 2005
- Full Text
- View/download PDF
9. Structural and magnetic properties of the single-layer manganese oxide La{1-x}Sr{1+x}MnO4
- Author
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Larochelle, S., Mehta, A., Lu, L., Mang, P. K., Vajk, O. P., Kaneko, N., Lynn, J. W., Zhou, L., and Greven, M.
- Subjects
Condensed Matter - Strongly Correlated Electrons - Abstract
Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La{1-x}Sr{1+x}MnO4 (0 < x < 0.7). Single crystals were grown by the traveling-solvent floating-zone method at 18 La/Sr concentrations. The low-temperature phase diagram can be understood by considering the strong coupling of the magnetic and orbital degrees of freedom, and it can be divided into three distinct regions: low (x < 0.12), intermediate (0.12< x < 0.45), and high (x > 0.45) doping. LaSrMnO_4 (x=0) is an antiferromagnetic Mott insulator,and its spin-wave spectrum is well-described by linear spin-wave theory for the spin-2 square-lattice Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the e_g electron concentration (1-x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Neel order disappears above xc = 0.115(10). In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural "nanopatches" begin to form above x ~ 0.25. At high doping (x > 0.45), the ground state of La{1-x}Sr{1+x}MnO4 exhibits long-range superstructural order and a complex (CE-type) antiferromagnetic order which differs from that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites. For x > 0.50, the superstructural order becomes incommensurate with the lattice, with a modulation wavevector that depends linearly on the e_g electron concentration. On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x = 0.50., Comment: 21 pages, 29 figures
- Published
- 2004
- Full Text
- View/download PDF
10. Phase Decomposition and Chemical Inhomogeneity in Nd2-xCexCuO4
- Author
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Mang, P. K., Larochelle, S., Mehta, A., Vajk, O. P., Erickson, A. S., Lu, L., Buyers, W. J. L., Marshall, A. F., Prokes, K., and Greven, M.
- Subjects
Condensed Matter - Superconductivity ,Condensed Matter - Materials Science ,Condensed Matter - Strongly Correlated Electrons - Abstract
Extensive X-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd2-xCexCuO4 (NCCO) in a low-oxygen-fugacity environment such as that typically realized during the annealing process required to create a superconducting state. Unlike a typical situation in which a disordered secondary phase results in diffuse powder scattering, a serendipitous match between the in-plane lattice constant of NCCO and the lattice constant of one of the decomposition products, (Nd,Ce)2O3, causes the secondary phase to form an oriented, quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from the secondary phase appear at rational positions (H,K,0) in the reciprocal space of NCCO. Additionally, because of neodymium paramagnetism, the application of a magnetic field increases the low-temperature intensity observed at these positions via neutron scattering. Such effects may mimic the formation of a structural superlattice or the strengthening of antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified through careful and systematic experimentation. For typical reduction conditions, the (Nd,Ce)2O3 volume fraction is ~1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50-100 angstromsthick. The presence of the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements., Comment: 15 pages, 17 figures, submitted to Phys. Rev. B
- Published
- 2004
- Full Text
- View/download PDF
11. Spin Correlations and Magnetic Order in Nonsuperconducting Nd_{2-x}Ce_xCuO_{4\pm\delta}
- Author
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Mang, P. K., Vajk, O. P., Arvanitaki, A., Lynn, J. W., and Greven, M.
- Subjects
Condensed Matter - Superconductivity ,Condensed Matter - Strongly Correlated Electrons - Abstract
We report quantitative neutron scattering measurements of the evolution with doping of the N\'eel temperature, the antiferromagnetic correlations, and the ordered moment of as-grown, nonsuperconducting Nd_{2-x}Ce_xCuO_{4\pm\delta} ($0 \le x \le 0.18$). The instantaneous correlation length can be effectively described by our quantum Monte Carlo calculations for the randomly site-diluted nearest-neighbor spin-1/2 square-lattice Heisenberg antiferromagnet. However, quantum fluctuations have a stronger effect on the ordered moment, which decreases more rapidly than for the quenched-disorder model., Comment: 4 pages, 3 figures, text updated to match published version
- Published
- 2003
- Full Text
- View/download PDF
12. Neutron Scattering, Magnetometry, and Quantum Monte Carlo Study of the Randomly-Diluted Spin-1/2 Square-Lattice Heisenberg Antiferromagnet
- Author
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Vajk, O. P., Greven, M., Mang, P. K., and Lynn, J. W.
- Subjects
Condensed Matter - Strongly Correlated Electrons ,Condensed Matter - Disordered Systems and Neural Networks ,Condensed Matter - Statistical Mechanics - Abstract
We have successfully grown sizable single crystals of La_2Cu_{1-z}(Zn,Mg)_zO_4 with up to nearly half of the magnetic Cu sites replaced by non-magnetic Zn and Mg. Neutron scattering, SQUID magnetometry, and complementary quantum Monte Carlo (QMC) simulations demonstrate that this material is an excellent model system for the study of site percolation of the square-lattice Heisenberg antiferromagnet (SLHAF) in the quantum-spin limit S=1/2. Carefully oxygen-reduced samples exhibit Neel order up to the percolation threshold for site dilution, z_p ~ 40.7%. Up to at least z = 35%, the Neel temperature T_N(z) of the experimental system corresponds to the temperature at which QMC indicates that the spin correlations for the nearest-neighbor S=1/2 SLHAF have grown to approximately 100 lattice constants. Neutron scattering measurements of the static structure factor in the paramagnetic regime allow the determination of the two-dimensional spin correlations, which are found to be in excellent quantitative agreement with QMC over a wide common temperature and doping range. Neutron scattering and QMC results for the temperature dependence of the static structure factor amplitude S(\pi,\pi) are in good agreement as well. The combined experimental and numerical data presented here provide valuable quantitative information for tests of theories of the randomly-diluted S=1/2 SLHAF., Comment: Proceedings of the Superconductivity Workshop, Williamsburg, Virginia (2002); to appear in Solid State Communications
- Published
- 2002
- Full Text
- View/download PDF
13. Magnetic Properties and Phase Diagram of Li-doped La(2)CuO(4): Common Magnetic Response of Hole-doped CuO2 Planes
- Author
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Sasagawa, T., Mang, P. K., Vajk, O. P., Kapitulnik, A., and Greven, M.
- Subjects
Condensed Matter - Strongly Correlated Electrons ,Condensed Matter - Superconductivity - Abstract
Although La(2)Cu(1-x)Li(x)O(4) [Li-LCO] differs from La(2-x)Sr(x)CuO(4) [Sr-LCO] in many ways (e.g., the absence of metallic transport, high-Tc superconductivity, and incommensurate antiferromagnetic correlations), it has been known that certain magnetic properties are remarkably similar. The present work establishes the detailed bulk magnetic phase diagram of Li-LCO (0 <= x <= 0.07), which is found to be nearly identical to that of lightly-doped Sr-LCO, and therefore extends the universality of the phase diagram to hole-doped but nonsuperconducting cuprates., Comment: 4 pages, 4 figures
- Published
- 2002
- Full Text
- View/download PDF
14. Quantum vs. Geometric Disorder in a Two-Dimensional Heisenberg Antiferromagnet
- Author
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Vajk, O. P. and Greven, M.
- Subjects
Condensed Matter - Disordered Systems and Neural Networks ,Condensed Matter - Strongly Correlated Electrons - Abstract
We present a numerical study of the spin-1/2 bilayer Heisenberg antiferromagnet with random interlayer dimer dilution. From the temperature dependence of the uniform susceptibility and a scaling analysis of the spin correlation length we deduce the ground state phase diagram as a function of nonmagnetic impurity concentration p and bilayer coupling g. At the site percolation threshold, there exists a multicritical point at small but nonzero bilayer coupling g_m = 0.15(3). The magnetic properties of the single-layer material La_2Cu_{1-p}(Zn,Mg)_pO_4 near the percolation threshold appear to be controlled by the proximity to this new quantum critical point., Comment: minor changes, updated figures
- Published
- 2002
- Full Text
- View/download PDF
15. Quantum Impurities in the Two-Dimensional Spin One-Half Heisenberg Antiferromagnet
- Author
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Vajk, O. P., Mang, P. K., Greven, M., Gehring, P. M., and Lynn, J. W.
- Subjects
Condensed Matter - Strongly Correlated Electrons ,Condensed Matter - Disordered Systems and Neural Networks ,Condensed Matter - Materials Science ,Condensed Matter - Statistical Mechanics - Abstract
The study of randomness in low-dimensional quantum antiferromagnets is at the forefront of research in the field of strongly correlated electron systems, yet there have been relatively few experimental model systems. Complementary neutron scattering and numerical experiments demonstrate that the spin-diluted Heisenberg antiferromagnet La2Cu(1-z)(Zn,Mg)zO4 is an excellent model material for square-lattice site percolation in the extreme quantum limit of spin one-half. Measurements of the ordered moment and spin correlations provide important quantitative information for tests of theories for this complex quantum-impurity problem., Comment: 11 pages, 3 figures. NOTE: possible errors in PDF version of Fig. 1. View postscript version of figure if possible
- Published
- 2002
- Full Text
- View/download PDF
16. Spin correlations in the electron-doped high-transition-temperature superconductor Nd2-xCexCuO4±δ
- Author
-
Motoyama, E. M., Yu, G., Vishik, I. M., Vajk, O. P., Mang, P. K., and Greven, M.
- Subjects
Environmental issues ,Science and technology ,Zoology and wildlife conservation - Abstract
Author(s): E. M. Motoyama [1]; G. Yu [1]; I. M. Vishik [1]; O. P. Vajk [2]; P. K. Mang [3]; M. Greven (corresponding author) [3, 4] High-transition-temperature (high-T [sub.c ] [...]
- Published
- 2007
- Full Text
- View/download PDF
17. Neutron-scattering studies of magnetism in multiferroic HoMnO3 (invited).
- Author
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Vajk, O. P., Kenzelmann, M., Lynn, J. W., Kim, S. B., and Cheong, S.-W.
- Subjects
- *
MANGANITE , *HOLMIUM , *MAGNETISM , *NEUTRON scattering , *PHASE diagrams , *PHASE transitions , *ANTIFERROMAGNETISM - Abstract
Hexagonal HoMnO3 is a frustrated antiferromagnet (TN=72 K) ferroelectric (TC=875 K) with a rich magnetic phase diagram consisting of multiple temperature- and field-dependent phases. Previously observed anomalies in the dielectric constant at magnetic phase transitions indicate strong coupling between the ferroelectricity and magnetism. Neutron-diffraction measurements in a magnetic field reveal new intermediate-field phases at low temperatures. Inelastic neutron-scattering measurements are used to establish the primary magnetic interactions and demonstrate that the spin dynamics in HoMnO3 are well described by a simple two-dimensional nearest-neighbor Heisenberg antiferromagnetic exchange J=2.44 meV and a temperature-dependent anisotropy D. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
18. Crystal growth and characterization of the model high-temperature superconductor HgBa2CuO4+δ
- Author
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Zhao, X., Yu, G., Cho, Y., Chabot-Couture, G., Barisic, Neven, Bourges, P., Kaneko, N., Li, Y., Lu, L., Motoyama, E. M., Vajk O. P., and Greven, M.
- Subjects
Condensed Matter::Superconductivity ,high-Tc superconductivity ,HgBa2Ca(n-1)Cu(n)O(2n+2+d) - Abstract
Understanding the properties of the mercury-based high-temperature superconductors HgBa2Ca(n-1)Cu(n)O(2n+2+d), the compounds with the highest superconducting transition temperatures (Tc), is one of the major goals in the field of correlated electron materials. Unfortunately, experimental studies have been very limited due to the lack of sizeable single crystals. Single-layer (n=1) (n-1)Cu(n)O(2n+2+d) is of particular interest due to the simplicity of its crystal structure. Employing a significantly improved melt-growth method, we have succeeded in obtaining unprecedentedly large Hg1201 single crystals, exceeding previous crystal sizes by more than two orders of magnitude. The crystals were characterized via ICP-MS, magnetic susceptibility, X-ray and neutron diffraction, and they exhibit an onset Tc of 97 K at optimal doping. The availability of sizable Hg1201 crystals has enabled the first search for magnetic excitations in Hg1201 using inelastic neutron scattering. Importantly, it provides new opportunities for comparative studies directed at the separation of materials-specific properties from those that are essential to high-Tc superconductivity.
- Published
- 2006
19. Spin correlations in the electron-doped high-transition-temperature superconductor Nd2-xCexCuO4±δ.
- Author
-
Motoyama, E. M., Yu, G., Vishik, I. M., Vajk, O. P., Mang, P. K., and Greven, M.
- Subjects
LETTERS to the editor ,SUPERCONDUCTORS - Abstract
High-transition-temperature (high-T
c ) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd2-x Cex CuO4±δ (NCCO) shows bulk superconductivity above x ≈ 0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x ≈ 0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies, arises from a build-up of spin correlations, in agreement with recent theoretical proposals. [ABSTRACT FROM AUTHOR]- Published
- 2007
- Full Text
- View/download PDF
20. Magnetic phase competition in Dy1-xYxMnO3.
- Author
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Vajk, O. P., Wang, Y., Gunasekera, J., Tarwater, K., and Heitmann, T.
- Subjects
- *
MAGNETICS , *MAGNETISM , *MAGNETIC fields , *CRYSTALS , *HIGH temperatures - Abstract
The magnetic phase diagram of Dy1-xYxMnO3 is investigated with neutron diffraction measurements for concentrations ranging from x = 0.6-1. The onset of a transition from YMnO3-like order to DyMnO3-like order of the Mn3+ moments is observed at temperatures as high as 25 K. This transition approaches from low temperatures, as in Er1-xYxMnO3, rather than from high temperatures, as in Ho1-xYxMnO3, but the Y-like phase persists over a broad compositional range, as in Ho1-xYxMnO3 and in contrast to Er1-xYxMnO3. Inelastic scattering measurements in the Y-like phase reveal quasielastic scattering at positions for scattering from the Dy-like phase. These results suggest significant complexity in the magnetic phase competition in RMnO3. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
21. Phase diagram and dynamics in Er1-xYxMnO3.
- Author
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Vajk, O. P., Gunasekera, J., Wang, Y., and Heitmann, T.
- Subjects
- *
PHASE diagrams , *PHASE equilibrium , *ANALYTICAL mechanics , *PARTICLES (Nuclear physics) , *PHYSICAL metallurgy - Abstract
Strong coupling between the ferroelectric and magnetic order has been previously observed in hexagonal HoMnO3 at a spin reorientation transition between a P6′3cm′ and a P6′3c′m magnetic phases. Understanding the transition between these phases may help illuminate the mechanism of multiferroic coupling in hexagonal R MnO3. In order to study this transition, single-crystal samples of the hexagonal multiferroic Er1-x Yx MnO3 were grown at different compositions. YMnO3 orders in a P6′3cm′ phase, whereas ErMnO3 orders in a P6′3c′m phase. Er1-x Yx MnO3 transitions completely from the P6′3cm′ to the P6′3c′m phase over a very narrow composition range on the Y-rich side of the phase diagram, and the phase boundary exhibits reentrant behavior. Inelastic neutron scattering measurements were performed on these samples, quasielastic scattering similar to that found in YMnO3 was observed. In the P6′3cm′ phase, this quasielastic scattering is centered at Bragg positions which correspond to the P6′3c′m and vice versa. These results may provide important information about the competition between these two magnetic phases. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
22. Giant Magnetic Fluctuations at the Critical Endpoint in Insulating HoMnO3.
- Author
-
Choi, Y. J., Lee, N., Sharma, P. A., Kim, S. B., Vajk, O. P., Lynn, J. W., Oh, Y. S., and Cheong, S.-W.
- Subjects
- *
MAGNETIC susceptibility , *FLUCTUATIONS of superconducting magnets , *ANTIFERROMAGNETISM , *CRITICAL opalescence , *INSULATING materials - Abstract
Although abundant research has focused recently on the quantum criticality of itinerant magnets, critical phenomena of insulating magnets in the vicinity of critical endpoints (CEP's) have rarely been revealed. Here we observe an emergent CEP at 2.05 T and 2.2 K with a suppressed thermal conductivity and concomitant strong critical fluctuations evident via a divergent magnetic susceptibility (e.g., X"(2.05 T,2.2 K)/X"(3 T,2.2 K)≈23,500%, comparable to the critical opalescence in water) in the hexagonal insulating antiferromagnet HoMnO3. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
23. Spin correlations in the electron-doped high-transition-temperature superconductor Nd2-xCexCuO4±δ.
- Author
-
Motoyama, E. M., Yu, G., Vishik, I. M., Vajk, O. P., Mang, P. K., and Greven, M.
- Subjects
- *
LETTERS to the editor , *SUPERCONDUCTORS - Abstract
High-transition-temperature (high-Tc) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd2-xCexCuO4±δ (NCCO) shows bulk superconductivity above x ≈ 0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x ≈ 0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies, arises from a build-up of spin correlations, in agreement with recent theoretical proposals. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
24. Spin correlations and magnetic order in nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta).
- Author
-
Mang PK, Vajk OP, Arvanitaki A, Lynn JW, and Greven M
- Abstract
We report quantitative neutron scattering measurements of the evolution with doping of the Néel temperature, the antiferromagnetic correlations, and the ordered moment of as-grown, nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta) (0=x=0.18). The instantaneous correlation length can be effectively described by our quantum Monte Carlo calculations for the randomly site-diluted nearest-neighbor spin-1/2 square-lattice Heisenberg antiferromagnet. However, quantum fluctuations have a stronger effect on the ordered moment, which decreases more rapidly than for the quenched-disorder model.
- Published
- 2004
- Full Text
- View/download PDF
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