Superconductivity in the high-transition-temperature (high-T(c)) copper oxides competes with other possible ground states. The physical explanation for superconductivity can be constrained by determining the nature of the closest competing ground state, and establishing if that state is universal among the high-T(c) materials. Antiferromagnetism has been theoretically predicted to be the competing ground state. A competing ground state is revealed when superconductivity is destroyed by the application of a magnetic field, and antiferromagnetism has been observed in hole-doped materials under the influence of modest fields. None of the previous experiments have revealed the quantum phase transition from the superconducting state to the antiferromagnetic state, because they failed to reach the upper critical field B(c2). Here we report the results of transport and neutron-scattering experiments on electron-doped Nd1.85Ce0.15CuO4 (refs 13, 14), where B(c2) can be reached. The applied field reveals a static, commensurate, anomalously conducting long-range ordered antiferromagnetic state, in which the induced moment scales approximately linearly with the field strength until it saturates at B(c2). This and previous experiments on the hole-doped materials therefore establishes antiferromagnetic order as a competing ground state in the high-T(c) copper oxide materials, irrespective of electron or hole doping.

We have used in-field neutron and x-ray single-crystal diffraction to measure the incommensurability ␦ of the crystal and magnetic structures of multiferroic TbMnO 3 . We show that the flop in the electric polarization at the critical field H C , for field H along the a and b axes, coincides with a first-order transition to a commensurate phase with propagation vector = ͑ 0, 1 4 ,0 ͒ . In-field x-ray diffraction measurements show that the quadratic magnetoelastic coupling breaks down with applied field as shown by the observation of the first harmonic lattice reflections above and below H C . This indicates that magnetic field induces a linear magnetoelastic coupling. DOI: 10.1103/PhysRevB.73.020102 PACS number͑s͒: 77.80.Bh, 64.70.Kb, 75.25.ϩz, 77.22.Ej Control of the spontaneous ferroelectric polarization ͑P s ͒ with an external magnetic field ͑H͒ in a material opens the opportunity for new types of magnetoelectric devices. The realization of such devices is based on multiferroic materials in which magnetism and ferroelectricity are strongly coupled. While available multiferroics are limited, it has been shown that frustrated spin materials may offer a unique class of enhanced multiferroics. [1][2][3] In one of these materials, TbMnO 3 , we find that multiferroic behavior arises as a consequence of the release of frustration with H. Here ferroelectricity arises below the Néel temperature ͑T N ͒ from a coupling to the lattice of an incommensurate ͑IC͒ modulation of the magnetic structure ͓Fig. 1͑a͔͒ that is caused from frustration in the ordering of the Mn d orbitals. 1,3 In this communication we show that magnetic field releases this frustration, inducing a linear magnetoelastic coupling, so that ferroelectricity is no longer a secondary effect. The linear magnetoelastic coupling drives a magnetostructural transition from an IC phase, which has P s along the c axis ͑P ʈ c͒, to a commensurate ͑C͒ phase with P s along the a axis ͑P ʈ a͒.In TbMnO 3 , when a magnetic field is applied along the b axis ͑H ʈ b͒ at 2 K, parallel to the direction of the IC magnetic modulation ͓see Fig. 1͑a͔͒, the electric polarization of the lattice flops from P ʈ c to P ʈ a at the critical field H C b ϳ 4.5 T ͓Fig. 2͑d͔͒. When field is applied along the a axis ͑H ʈ a͒, perpendicular to the magnetic modulation, a similar flop is found but at a higher critical field, H C a ϳ 9 T ͓Fig. 2͑a͔͒. Recently there have been a number of examples of magnetoelastic coupling in complex multiferroic oxides such as TbMn 2 O 5 which exhibit a reversable polarization switch with applied field, 4 and hexagonal HoMnO 3 where one magnetic phase is selected over another by applying an electric field.5 However, TbMnO 3 is unique as it is the only known example of a material that exhibits a field-induced flop of its polarization.In TbMnO 3 the staggered ordering of Mn 3+ 3d 3x 2 −r 2 /3d 3y 2 −r 2 orbitals as found in LaMnO 3 is frustrated partly due to the small ionic size of Tb 3+3 . This leads to an IC spin ordering which drives a ferroelectric lattice...

The magnetic excitations in multiferroic TbMnO 3 have been investigated by inelastic scattering of polarized and unpolarized neutrons in the ferroelectric cycloidal and in the paraelectric collinear phase. The polarization analysis of the excitations at the incommensurate magnetic zone center allows one to determine the characters of three distinct modes. In particular we may identify those modes which may directly couple to the ferroelectric polarization. We find a rather complex magnon dispersion with branches split throughout the Brillouin zone, which should be a generic characteristic of elliptical cycloidal order.

Neutron elastic, inelastic and high energy x-ray scattering techniques are used to explore the nature of the polaron order and dynamics in the colossal magnetoresistive (CMR) system La 0.7 Ca 0.3 MnO 3. Polaron correlations are known to develop within a narrow temperature regime as the Curie temperature is approached from low temperatures, with a nanoscale correlation length that is only weakly temperature dependent. The static nature of these short-range polaron correlations indicates the presence of a glass-like state, very similar to the observations for the bilayer manganite in the metallic-ferromagnetic doping region. In addition to this elastic component, inelastic scattering measurements reveal dynamic correlations with a comparable correlation length, and with an energy distribution that is quasielastic. The elastic component disappears at a higher temperature T*, above which the correlations are purely dynamic. These observations are identical to the polaron dynamics found in the bilayer manganite system in the CMR regime, demonstrating that they are a general phenomenon in the manganites.