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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 ] [...]

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]

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]

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]

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]

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]

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]

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., χ''(2.05  T,2.2  K)/χ''(3  T,2.2  K)≈23,500%, comparable to the critical opalescence in water) in the hexagonal insulating antiferromagnet HoMnO3.

Inelastic neutron-scattering measurements on the archetypical electron-doped material Nd1.85Ce0.15CuO4 up to a high relative magnetic-field strength, H/H(c2) approximately 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 H(c2), and the high-field response resembles that of the paramagnetic normal state.

We report a Cu -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(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 remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La(2)CuO(4), demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.

TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T(N)<41 K is accompanied by ferroelectric order for T<28 K. 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.

Hexagonal HoMnO3 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 Mn3+ 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 0.28 meV above the spin-reorientation transition at 40 K and 0.38 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 Ho3+ ions are ordered magnetically.

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

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(2)Cu(1-p)(Zn,Mg)(p)O4 near the percolation threshold appear to be controlled by the proximity to this new quantum critical point.