Cupric oxide as an induced-multiferroic with high-T C

Physical Review Letters, 2011

The incommensurate-commensurate phases reported in cupric oxide below 230 K are shown theoretically to realize an inverted sequence of symmetry-breaking mechanisms with respect to the usual sequence occurring in low-temperature multiferroic compounds. The sequence inversion results from a strong triggering-coupling mechanism between two antiferromagnetic order parameters inducing a firstorder transition to the multiferroic phase. Such mechanism is favored by the large antiferromagnetic superexchange interactions, responsible of the high-T N temperature, and implies a preeminence of these interactions on the magnetocrystalline anisotropy. The magnetic structures of the equilibrium phases and the microscopic interactions giving rise to the polarization are determined.

physica status solidi (RRL) - Rapid Research Letters, 2013

Scientific Reports, 2017

In copper-oxides that show high-temperature superconductivity (HTS), the critical temperature (T c) has a dome-shaped doping dependence. The cause of demise of both T c and superfluid density n s on the overdoped side is a major puzzle. A recent study of transport and diamagnetism in a large number of overdoped La 2−x Sr x CuO 4 (LSCO) films shows that this cannot be accounted for by disorder within the conventional Bardeen-Cooper-Schrieffer theory. This brings to focus an alternative explanationcompetition of HTS with ferromagnetic order, fluctuating in superconducting samples and static beyond the superconductor-to-metal transition. Here, we examine this proposal by growing singlecrystal LSCO thin films with doping on both sides of the transition by molecular beam epitaxy, and using polarized neutron reflectometry to measure their magnetic moments. In a heavily overdoped, metallic but non-superconducting LSCO (x = 0.35) film, the spin asymmetry of reflectivity shows a very small static magnetic moment (~2 emu/cm 3). Less-doped, superconducting LSCO films show no magnetic moment in neutron reflectivity, both above and below T c. Therefore, the collapse of HTS with overdoping is not caused by competing ferromagnetic order. The superconducting temperature T c in cuprates shows an unusual dome-shaped dependence on the doping level, which presumably originates from an unconventional pairing mechanism. On the low-doping side of the dome in the x-T (doping-temperature) phase diagram, the competition of HTS with antiferromagnetism, charge density waves, and the pseudogap phase has been investigated extensively although some details are still debated 1,2. The overdoped side has been studied less and the situation is even more puzzling. It has been speculated that the demise of T c and the superfluid density n s with overdoping originates from pair breaking caused by impurities, disorder, and phase separation, which could be accounted for within the conventional dirty-BCS (Bardeen-Cooper-Schrieffer) picture 3. However, this hypothesis has been thoroughly scrutinized and refuted in the recently released detailed study of penetration depth and magnetoresistance in a huge number of overdoped LSCO films 4 grown by atomic-layer-by layer molecular beam epitaxy (ALL-MBE). In these high-quality single-crystal films, the superfluid seems to be homogeneous and uniform at every doping. This brings to the forefront an alternative explanation that on the high-doping side HTS competes with ferromagnetic order 5. In fact, ferromagnetism does occur in closely related oxides such as La 1−x Sr x CoO 3 (ref. 6). Electronic band calculations show a tendency towards ferromagnetic ordering 7 at high charge doping in La 2− x Ba x CuO 4. The dramatic evolution of Fermi surface with overdoping in LSCO may provide a chance for magnetic moments to be interacting and correlated in a preferred direction, in some composition window. Several experimental observations provide some support to this scenario 8-16. The temperature dependence of magnetic susceptibility (χ) in overdoped LSCO crystals indicated the absence of local magnetic moments below the critical doping x c ≈ 0.18, and dramatic increase in their density with further overdoping 8-12. Muon spin rotation (μ SR) experiments 15 detected the onset of static magnetic moments at low temperature in heavily overdoped (x = 0.33) LSCO, metallic but non-superconducting (in what follows, we refer to such samples simply as 'metallic'). However, no ferromagnetism has been detected so far in the vicinity of the quantum (T = 0) superconductor-to-metal transition (SMT) that occurs at the overdoped dome edge (x c2 ≈ 0.26).

Nature communications, 2016

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric featu...

Journal of the Physical Society of Japan, 2012

High-temperature superconductivity (HTSC) in copper oxides emerges on a layered CuO2 plane when an antiferromagnetic Mott insulator is doped with mobile hole carriers. We review extensive studies of multilayered copper oxides by site-selective nuclear magnetic resonance (NMR), which have uncovered the intrinsic phase diagram of antiferromagnetism (AFM) and HTSC for a disorder-free CuO2 plane with hole carriers. We present our experimental findings such as the existence of the AFM metallic state in doped Mott insulators, the uniformly mixed phase of AFM and HTSC, and the emergence of d-wave SC with a maximum Tc just outside a critical carrier density, at which the AFM moment on a CuO2 plane disappears. These results can be accounted for by the Mott physics based on the t-J model. The superexchange interaction Jin among spins plays a vital role as a glue for Cooper pairs or mobile spin-singlet pairs, in contrast to the phonon-mediated attractive interaction among electrons established in the Bardeen-Cooper-Schrieffer (BCS) theory. We remark that the attractive interaction for raising the Tc of HTSC up to temperatures as high as 160 K is the large Jin (∼ 0.12 eV), which binds electrons of opposite spins to be on neighboring sites, and that there are no bosonic glues. It is the Coulomb repulsive interaction U (> 6 eV) among Cu-3d electrons that plays a central role in the physics behind high-Tc phenomena. A new paradigm of the SC mechanism opens to strongly correlated electron matter.

Physical Review B, 2009

We report 63 Cu-NMR/NQR and 19 F-NMR studies on the multilayered high-Tc copper oxides Ba2Can−1CunO2nF2 with n = 2, 3, 4, where n is the number of CuO2 planes. It is revealed that bilayered Ba2CaCu2O4F2 is an underdoped superconductor with hole carriers, which are introduced into CuO2 planes by an unexpected deviation from the nominal content of apical fluorines. In a previous paper, we proposed a self-doping mechanism as the origin of carrier doping in n = 3 and n = 4; in the mechanism, electrons are transferred from the inner CuO2 plane (IP) to the outer one (OP). However, since it has been found that the bi-layered compound is hole doped, we have reexamined the superconducting and magnetic properties in n = 3 and n = 4 by 63 Cu-NMR/NQR and 19 F-NMR. The extensive NMR studies have confirmed that the apical-fluorine compounds are not self-doped but hole-doped, and that antiferromagnetism (AFM) and superconductivity (SC) coexist in a single CuO2 plane. In n = 4, the AFM ordering occurs at TN = 80 K, well above Tc = 55 K, where the respective AFM moments are MAFM = 0.11 µB and 0.18 µB at the OP and the IP. In n = 3, on the other hand, the underdoped single IP exhibits a spontaneous moment MAFM = 0.12 µB at low temperatures and a peak in the nuclear-spin-lattice relaxation rate 1/T1 of 19 F at TN = 23 K, much lower than Tc = 76 K. We note that the increase in the number of IPs from one to two results in the strengthening of the interlayer coupling; TN increases as the interlayer coupling becomes stronger, although the doping levels for both compounds are comparable. Consequently, we conclude that the uniform mixing of AFM and SC is a general property inherent to a single CuO2 plane in an underdoped regime for hole-doping. This conclusion incorporates the ARPES results on the n=4 compound [Y. Chen et al., Phys. Rev. Lett. 97, 236401 (2006)]; it was found that the two Fermi sheets of the IP and OP are observed, and that the SC gap opens at the IP and OP below Tc = 55 K. *) In particular, the N h s of the OPs are shown for the n = 3 and 4 compounds.

Solid State Communications, 1989

EPR, microwave absorption, and dc magnetization measurements were made on single crystals of the form R2CuO 4, which are the host compounds for the newly discovered series of electron cuprate superconductors. These measurements reveal two characteristic transition temperatures associated with a novel complex magnetic behavior, including weak ferromagnetism, two sharp peaks in the low field DC magnetization, an unusual anisotropy in the EPR resonance field for R = Gd, and two additional anisotropic microwave absorption modes. The higher characteristic transition temperature, at-270K, is attributed to AF ordering of the Cu moments, and the lower, at < 20K, to a spontaneous canted spin reorientation. An understanding of this magnetic behavior is important in order to ascertain its relationship to possible mechanisms of high temperature superconductivity.

Physica B: Condensed Matter, 1990

The experimental evidence for a temperature-dependent build up of antiferromagnctic correlations between Cu' planar spins in the normal state of cuprate oxide superconductors is reviewed, and a phenomenological one-component model. developed in collaboration with A. Millis and H. Monien. which appears capable of providing a quantitative account of existing experiments is described. A scaling law which relates the superconducting transaction temperature to the measurable spin-spin correlation length is proposed. The NMR experimental results in the superconducting state arc shown to be consistent with d-wave pairing in a strong coupling superconductor. Comparison of the results of NMR experiments on the cuprate oxide and heavy electron superconductors reveals striking similarities.

Nature communications, 2014

The so-called proximity effect is the manifestation, across an interface, of the systematic competition between magnetic order and superconductivity. This phenomenon has been well documented and understood for conventional superconductors coupled with metallic ferromagnets; however it is still less known for oxide materials, where much higher critical temperatures are offered by copper oxide-based superconductors. Here we show that, even in the absence of direct Cu-O-Mn covalent bonding, the interfacial CuO2 planes of superconducting La(1.85)Sr(0.15)CuO(4) thin films develop weak ferromagnetism associated to the charge transfer of spin-polarised electrons from the La(0.66)Sr(0.33)MnO(3) ferromagnet. Theoretical modelling confirms that this effect is general to all cuprate/manganite heterostructures and the presence of direct bonding only affects the strength of the coupling. The Dzyaloshinskii-Moriya interaction, also at the origin of the weak ferromagnetism of bulk cuprates, propag...

Physical Review B, 2014

CuCrO 2 offers insights into the different types of spiral magnetic orderings that can form spontaneously due to frustration in triangular-lattice antiferromagnets. We explore the magnetic phase diagram up to 65 T along all the principal axes, and also use electric polarization to probe changes in the spiral order at high magnetic fields. It is known that at zero magnetic field a proper-screw spiral of the Cr S = 3 2 spins forms that in turn induces electric polarization with six possible orientations in the ab plane. Applied magnetic fields in the (hard) ab plane have been shown to induce a transition to cycloidal-spiral magnetic order above 5.3 T in those domains that have spins perpendicular to the applied magnetic field. We show that the cycloidal order remains unchanged all the way up to 65 T, which is one quarter of the extrapolated saturation magnetization. On the other hand, for magnetic fields along the (easy) c axis, we observe a transition in the electric polarization near 45 T, and it is followed by a series of steps and/or oscillations in the electric polarization. The data are consistent with a proper-screw-to-cycloidal transition that is pushed from 5.3 to 45 T by easy-axis anisotropy, and is in turn followed by stretching of the magnetic spiral through commensurate and incommensurate wave vectors. This work also highlights the ability of the magnetically induced electric polarization to probe complex magnetic orders in regimes of phase space that are difficult to reach with neutron diffraction.

Several findings in the area of copper oxide high-Tc superconductivity are shown; from the first high-Tc cuprate superconductor at 30 k, to a Barium-Itrium-doped cuprate superconducting at around 90 K. The main structure of said compound and the systematic analysis for each one are explained and the preparation methods for some of them are exposed. Finally, the theories attempting to describe the superconducting behavior at said temperatures are examined and briefly explained.

Nature Communications, 2014

spin and charge excitations in electron-doped materials. Copper L 3 resonant inelastic x-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone center and deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the Γ-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin-excitations (paramagnons) recently observed in hole-doped compounds even at high doping-levels. The differences and similarities between the electron-and hole-doped classes of superconductors have been one of the central subjects in the studies of the copper oxides [1]. They are crucially important for understanding the emergence of high-T c superconductivity-and likewise of Mott insulator behavior-in strongly correlated electron systems. Extensive works for this purpose have been done since the discovery of the high-T c superconductivity and it is now established that carrier doping to the parent antiferromagnetic Mott insulator induces a metallic state, where the superconductivity in the cuprates emerges. Electrons in the metallic state often reveal both itinerant and localized characters, and the dual nature is a difficulty of the doped Mott insulators. It is generally believed that superconductivity in cuprates is intimately related to the antiferromagnetic spin fluctuation and inelastic neutron scattering (INS) has been widely used for studying the spin dynamics in the reciprocal lattice space. However momentum-resolved excitation spectra of the charge degrees of freedom have been less explored because of the limitations of this experimental technique, even though both degrees of freedom should be clarified for the comprehensive understanding of cuprates.

Proceedings of the National Academy of Sciences, 2019

Significance Superconductivity is one of the most mysterious phenomena in nature in that the materials can conduct electrical current without any resistance. The cuprates hold the record high superconducting temperature at room pressure so far, but understanding their superconducting mechanism remains one of the big challenges. Here, we report high- T c superconductivity in Ba 2 CuO 4- y with two unique features: an exceptionally compressed local octahedron and heavily overdoped hole carriers. These two features are in sharp contrast to the favorable criteria for all previously known cuprate superconductors. Thus, the discovery of high- T c superconductivity in Ba 2 CuO 4- y calls into question the widely accepted scenario of superconductivity in the cuprates. This discovery provides a direction to search for additional high- T c superconductors.

Physical Review B, 2002

Using elastic neutron scattering, we evidence a commensurate antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-$\rm T_c$ cuprate $\rm YBa_2(Cu_{1-y}Co_y)_3O_{7+\delta}$ (y=0.013, $\rm T_c$=93 K). As in the Co-free system, the spin excitation spectrum is dominated by a magnetic resonance peak at 41 meV but with a reduced spectral weight. The substitution of Co thus leads to a state where

Physical Review B, 1992

We show that the retarded interaction between quasiparticles on a two dimensional square lattice induced by the exchange of antiferromagnetic paramagnons leads uniquely to a transition to a superconducting state with d» symmetry. We find that the effective quasiparticle interaction responsible for superconductivity possesses considerable structure in both momentum and frequency space, and show, by explicit calculations, that if one wishes to obtain quantitatively meaningful results it is essentia1 to allow for that structure in solving the full integra1 equations that determine the superconducting transition temperature and the superconducting properties. With a spin-excitation spectrum. and a quasiparticleparamagnon coupling determined by fits to normal-state experiments, we obtain high transition temperatures and energy-gap behaviors comparable to those measured for YBa2Cu307, YBa2Cu30663 and Lai. ssSro. isCu04