Universal magnetic structure of the half-magnetization phase in Cr-based spinels

Using an elastic neutron scattering technique under a pulsed magnetic field up to 30 T, we determined the magnetic structure in the half-magnetization plateau phase in the spinel CdCr$_2$O$_4$. The magnetic structure has a cubic $P4_3$32 symmetry, which is the same as that observed in HgCr$_2$O$_4$. This suggests that there is a universal field induced spin-lattice coupling mechanism at work in the Cr-based spinels.Comment: 4 pages, 4 figure

Shubnikov-de Haas oscillations in YBa_2Cu_4O_8

We report the observation of Shubnikov-de Haas oscillations in the underdoped cuprate superconductor YBa$_2$Cu$_4$O$_8$ (Y124). For field aligned along the c-axis, the frequency of the oscillations is $660\pm 30$ T, which corresponds to $\sim 2.4$ % of the total area of the first Brillouin zone. The effective mass of the quasiparticles on this orbit is measured to be $2.7\pm0.3$ times the free electron mass. Both the frequency and mass are comparable to those recently observed for ortho-II YBa$_2$Cu$_3$O$_{6.5}$ (Y123-II). We show that although small Fermi surface pockets may be expected from band structure calculations in Y123-II, no such pockets are predicted for Y124. Our results therefore imply that these small pockets are a generic feature of the copper oxide plane in underdoped cuprates.Comment: v2: Version of paper accepted for publication in Physical Review Letters. Only minor changes to the text and reference

Renormalization of Commensurate Magnetic Peak in Ni-doped La1.85_{1.85}Sr0.15_{0.15}CuO4_{4}

We have studied the magnetic excitations in impurity doped La$_{1.85}$Sr$_{0.15}$Cu$_{1-y}$A$_{y}$O$_{4}$ (A=Ni or Zn) by neutron scattering. The dispersion for Zn:$y=0.017$ is similar to that for the impurity free sample: incommensurate peaks with the incommensurability $\delta=0.12\pm0.01$ (rlu) do not change their positions up to 21 meV. On the other hand, for Ni:$y=0.029$, two incommensurate peaks observed at low energies suddenly change into a broad commensurate peak at $E_\mathrm{cross}=15$ meV. Compared to the impurity free sample with a similar Sr-concentration $x=0.16$, [B. Vignolle {\it et al.} Nature Physics {\bf 3} (2007) 163], $E_\mathrm{cross}$ for Ni:$y=0.029$ is decreased by nearly the same factor for the reduction in $T_{c}$. This is very similar to the shift of the resonance energy ($E_\mathrm{res}$) in Ni-doped YBa$_{2}$Cu$_{3}$O$_{7}$.[Y. Sidis {\it et al.}: Phys. Rev. Lett. {\bf 84} (2000) 5900]. These common impurity effects on the shift of $E_\mathrm{cross}$ and $E_\mathrm{res}$ suggest the same magnetic origin for the resonance peak in YBa$_{2}$Cu$_{3}$O$_{\delta}$ and that for a crossing point of upward and downward dispersions in the La$_{2-x}$Sr$_{x}$CuO$_{4}$. We propose that the sudden change in the dispersion is better described by a crossover from incommensurate spin fluctuations to a gapped spin wave rather than a hourglass-like dispersion.Comment: 11 pages, 5 figure

Comments on the d-wave pairing mechanism for cuprate high TcT_c superconductors: Higher is different?

The question of pairing glue for the cuprate superconductors (SC)is revisited and its determination through the angle resolved photo-emission spectroscopy (ARPES) is discussed in detail. There are two schools of thoughts about the pairing glue question: One argues that superconductivity in the cuprates emerges out of doping the spin singlet resonating valence bond (RVB) state. Since singlet pairs are already formed in the RVB state there is no need for additional boson glue to pair the electrons. The other instead suggests that the d-wave pairs are mediated by the collective bosons like the conventional low $T_c$ SC with the alteration that the phonons are replaced by another kind of bosons ranging from the antiferromagnetic (AF) to loop current fluctuations. An approach to resolve this dispute is to determine the frequency and momentum dependences of the diagonal and off-diagonal self-energies directly from experiments like the McMillan-Rowell procedure for the conventional SC. In that a simple d-wave BCS theory describes superconducting properties of the cuprates well, the Eliashberg analysis of well designed high resolution experimental data will yield the crucial frequency and momentum dependences of the self-energies. This line of approach using ARPES are discussed in more detail in this review, and some remaining problems are commented.Comment: Invited review article published in the Journal of Korean Physical Society; several typos corrected and a few comments and references adde

Fermi Surface of the Electron-doped Cuprate Superconductor Nd_{2-x}Ce_xCuO_{4} Probed by High-Field Magnetotransport

We report on the study of the Fermi surface of the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_{4}$ by measuring the interlayer magnetoresistance as a function of the strength and orientation of the applied magnetic field. We performed experiments in both steady and pulsed magnetic fields on high-quality single crystals with Ce concentrations of $x=0.13$ to 0.17. In the overdoped regime of $x > 0.15$ we found both semiclassical angle-dependent magnetoresistance oscillations (AMRO) and Shubnikov-de Haas (SdH) oscillations. The combined AMRO and SdH data clearly show that the appearance of fast SdH oscillations in strongly overdoped samples is caused by magnetic breakdown. This observation provides clear evidence for a reconstructed multiply-connected Fermi surface up to the very end of the overdoped regime at $x\simeq 0.17$. The strength of the superlattice potential responsible for the reconstructed Fermi surface is found to decrease with increasing doping level and likely vanishes at the same carrier concentration as superconductivity, suggesting a close relation between translational symmetry breaking and superconducting pairing. A detailed analysis of the high-resolution SdH data allowed us to determine the effective cyclotron mass and Dingle temperature, as well as to estimate the magnetic breakdown field in the overdoped regime.Comment: 23 pages, 8 figure

Progress in Neutron Scattering Studies of Spin Excitations in High-Tc Cuprates

Neutron scattering experiments continue to improve our knowledge of spin fluctuations in layered cuprates, excitations that are symptomatic of the electronic correlations underlying high-temperature superconductivity. Time-of-flight spectrometers, together with new and varied single crystal samples, have provided a more complete characterization of the magnetic energy spectrum and its variation with carrier concentration. While the spin excitations appear anomalous in comparison with simple model systems, there is clear consistency among a variety of cuprate families. Focusing initially on hole-doped systems, we review the nature of the magnetic spectrum, and variations in magnetic spectral weight with doping. We consider connections with the phenomena of charge and spin stripe order, and the potential generality of such correlations as suggested by studies of magnetic-field and impurity induced order. We contrast the behavior of the hole-doped systems with the trends found in the electron-doped superconductors. Returning to hole-doped cuprates, studies of translation-symmetry-preserving magnetic order are discussed, along with efforts to explore new systems. We conclude with a discussion of future challenges.Comment: revised version, to be published in JPSJ, 20 pages, 21 figure

Theory of quantum magneto-oscillations in underdoped cuprate superconductors

Magneto-oscillations in kinetic and magnetic response functions of a few underdoped cuprates are perhaps one of the most striking observations since many probes of underdoped cuprates clearly point to a non Fermi-liquid normal state. Their observation in the vortex state well below the upper critical field raises a doubt concerning their normal state origin. Here I propose an explanation of the magneto-oscillations as emerging from the quantum interference of the vortex lattice and checkerboard modulations of the electron density of states revealed by STM with atomic resolution in some cuprate superconductors. The checkerboard effectively pins the vortex lattice, when the period of the latter is commensurate with the period of the checkerboard. This condition yields 1/\sqrt{B} periodicity of the response functions versus magnetic field B, rather than 1/B periodicity of conventional normal state oscillations. Our solution of the Gross-Pitaevskii-type equation for composed charged bosons accounting for the d-wave symmetry of the order-parameter and its checkerboard modulations describes well changes in resonant frequency of the tunnel-diode oscillator circuit with YBa2Cu4O8 and the oscillatory part of the Hall resistance and magnetic susceptibility in the mixed state of YBa2Cu3O6.5.Comment: 4 pages, 3 figures, experimental conditions allowing for a resolution of conventional normal-state and unconventional vortex-state magneto-oscillations are outline

Hidden magnetic excitation in the pseudogap phase of a model cuprate superconductor

The elucidation of the pseudogap phenomenon of the cuprates, a set of anomalous physical properties below the characteristic temperature T* and above the superconducting transition temperature Tc, has been a major challenge in condensed matter physics for the past two decades. Following initial indications of broken time-reversal symmetry in photoemission experiments, recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T*. These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point. Here we report inelastic neutron scattering results for HgBa2CuO4+x (Hg1201) that reveal a fundamental collective magnetic mode associated with the unusual order, and that further support this picture. The mode's intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter. Its energy of 52-56 meV and its enormous integrated spectral weight render it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies.Comment: 8 pages, 3 figures, with supplementary information and figure

Fermi-surface reconstruction by stripe order in cuprate superconductors

Quantum oscillations have revealed the presence of a small pocket in the Fermi surface of the cuprate superconductor YBCO, whose nature and origin are the subject of much debate. Interpretations include electron and hole pockets; scenarios include Fermi-surface reconstruction by antiferromagnetism, d-density-wave order, and stripe order. Here we report quantum oscillations in the Seebeck and Nernst coefficients of YBCO and show, from the magnitude and sign of the Seebeck coefficient, that they come from an electron pocket. Using measurements of the Seebeck coefficient as a function of hole doping p, we show that the evolution of the Fermi surface in YBCO is the same as in Eu-LSCO, a cuprate where stripe order (a modulation of spin and charge densities) is well established. The electron pocket is most prominent where stripe order is strongest, at p = 1/8. This shows that Fermi-surface reconstruction is a generic mechanism of underdoped cuprates, intimately related to stripe order.Comment: 15 pages, 5 figures, Supplementary information now integrated into articl

Intense paramagnon excitations in a large family of high-temperature superconductors

In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum upon doping from the antiferromagnetic insulating to the superconducting states of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic x-ray scattering to show that a large family of superconductors, encompassing underdoped YBa$_2$Cu$_4$O$_8$ and overdoped YBa$_2$Cu$_3$O$_{7}$, exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. %The results are in excellent agreement with the spin excitations obtained by exact diagonalization of the $\bf t-J$ Hamiltonian on finite-sized clusters. The comprehensive experimental description of this surprisingly simple spectrum permits quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa$_2$Cu$_3$O$_{7}$ reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to Eliashberg theories of conventional superconductors.Comment: Main text (11 pages, 4 figures) + supplementary information (4 pages, 4 figures, 1 table). An updated version will appear in Nature Physic