Session A45: Cuprates and Nickelates

Optical evidence for a magnetically driven structural transition in the spin web Cu$_3$TeO$_6$

Cu$_3$TeO$_6$ is a modest frustrated $S=1/2$ spin system, which undergoes an anti-ferromagnetic transition at $T_N\sim61$ $K$. The anti-ferromagnetic spin alignment in Cu$_3$TeO$_6$ below $T_N$ is supposed to induce a magneto-elastic strain of the lattice. The complete absorption spectrum of Cu$_3$TeO$_6$ is obtained through Kramers-Kronig transformation of the optical reflectivity, measured from the far-infrared up to the ultraviolet spectral range as a function of temperature ($T$). Below $T^*\sim 50$ $K$, we find a new mode at 208 $cm^{-1}$. The spectral weight associated to this additional mode increases as $\propto (T^*- T)^{1/2}$ with decreasing $T$ below $T^*$. The implication of the optical findings will be discussed in relation to the magnetic phase transition at $T_N$.   

Transport Anisotropy due to Spiral Spin Order in Underdoped Cuprates

I will discuss the in-plane transport anisotropy in the spin-glass phase of La$_{2-x}$Sr$_x$CuO$_4$ within a theoretical scenario where the physics is purely spin driven (no charge order is present), and a spiral spin density wave is formed in the ground state. Such an approach is well justified for the extended t--J model at low doping. In the low-temperature, variable-range hopping regime, the calculated anisotropy of 50--80 percent (depending on temperature) is in excellent agreement with experiment [1], demonstrating that charge ordering tendencies are not necessary to explain the observed transport anisotropy. This work is part of a series in which we show that the spiral approach provides a consistent description of the low-doping region. \newline [1] V.N. Kotov and O.P. Sushkov, Phys. Rev. B 72, 184519 (2005).   

High Energy Spin Dynamics in the electron-doped high-T$_{c}$ cuprate Pr$_{.88}$LaCe$_{.12}$CuO$_{4}$ (T$_{c}$=21K)

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of electron-doped superconducting Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta }$ (T$_{c}$=21 K) over a wide energy range (4 meV$<$hbar $\omega <$ 260 meV). The effect of electron-doping and superconductivity is to cause a wave vector broadening in the low-energy ($<$50 meV) commensurate spin fluctuations at ($\pi $, $\pi )$ and to suppress the intensity of spin-wave-like excitations at high energies ($>$ 80 meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility $\chi $''($\omega )$, and reveals a new energy scale considerably smaller than that of the hole-doped materials [1]. [1] Stephen D. Wilson et. al., PRL submitted (2005).   

Magnetic and transport properties of lightly doped La$_{2-x}$Sr$_x$CuO$_4$

We address the problem of the static magnetic correlations in La$_2$CuO$_4$ [1] and lightly doped La$_{2-x}$Sr$_x$CuO$_4$ within the framework of a dipolar frustration model for a canted antiferromagnet [2]. We show that the Dzyaloshinskii-Moriya and XY anisotropies are responsible for robustness of the Neel state for $x < 2\%$ while, for higher doping, the antiferromagnetic ground state is unstable towards a helicoidal magnetic phase. The helicoidal spin structure gives rise to incommensurate peaks in elastic neutron scattering, and is consistent with recent Raman and magnetic susceptibility experiments in La$_{2-x}$Sr$_x$CuO$_4$. We propose that the dissipative dynamics of topological defects in a spiral state is responsible for the transport properties in the spin-glass phase of cuprates [3]. The calculated damping matrix is related to the in-plane resistivity, which exhibits an anisotropy and linear temperature dependence in agreement with experimental data. References: [1] M. B. Silva Neto, L. Benfatto, V. Juricic, and C. Morais Smith, cond-mat/0502588. [2] V. Juricic, M. B. Silva Neto, and C. Morais Smith, cond-mat/0510312. [3] V. Juricic, L. Benfatto, A. O. Caldeira, and C. Morais Smith, Phys. Rev. Lett. 92, 137202 (2004).   

High binding energy band structure of Bi-2212 as measured by ARPES

The study of the electronic structure of high temperature superconductors by angle resolved photoemission spectroscopy (ARPES) has so far focused on the states near the Fermi level, believed to be fundamental for most of the properties of cuprates. However, it is well known that in doped Mott insulators the low and high energy physics are strongly coupled one to the other. Therefore, to gain insight on the real physics of cuprates a full characterization of the electronic band structure up to energies of the order of the lower Hubbard band and beyond is needed. Here we report a detailed, doping dependent study of the band structure of Bi2212 superconductors at energies of the order of 1-2 eV. The experimental results are interpreted in terms of local density approximation (LDA) based computations, where the presence of the ``spaghetti'' of Cu-O and O-bands is predicted. Comparison between computed and measured bands provides insight into many-body renormalization effects.   

Unidirectional Order and 3D Stacking of Stripes in Orthorhombic $\bf Pr_{1.67}Sr_{0.33}NiO_4$ and $\bf Nd_{1.67}Sr_{0.33}NiO_4$.

The crystal structure and charge stripe order in $\rm Pr_{1. 67}Sr_{0.33}NiO_{4}$ and $\rm Nd_{1. 67}Sr_{0.33}NiO_{4}$ was studied by means of single crystal x-ray diffraction in zero and high electric fields. In contrast to tetragonal $\rm La_ {1.67}Sr_{0.33}NiO_{4}$, these crystals are orthorhombic at room temperature. We find that the distortion of the $\rm NiO_2$ planes associated with the orthorhombic strain dictates the direction of the charge stripes. The critical temperature for charge stripe order is the same as in $\rm La_{1.67}Sr_ {0.33}NiO_{4}$ ($\rm T_{CO}\sim 245$~K), i.e., it does not depend on the crystal symmetry. A second structural transition observed only in $\rm Nd_{1.67}Sr_{0.33}NiO_{4}$ at temperatures $\rm T\sim 100$~K has no noticeable influence on the stripe order. In crystals with a hole content very close to 1/3 we observe a tripling of the charge stripe unit cell along the c-axis for temperatures $\rm T < 225$~K, which indicates a strong tendency towards a well defined three dimensional order. A high electric field applied to $\rm Nd_ {1.67}Sr_{0.33}NiO_{4}$ had no noticeable impact on the charge stripe order, i.e., a sliding of stripes was not observed. {\it The work at Brookhaven was supported by the Office of Science, U.S. Department of Energy under Contract No. DE-AC02-98CH10886.}   

The ground state of the quasi-one-dimensional cuprate PrBa$_{2}$Cu$_{4}$O$_{8}$: field-induced dimensional crossovers and disorder-induced one-dimensionality

PrBa$_{2}$Cu$_{4}$O$_{8}$, the non-superconducting analogue of the high-$T_{c}$ cuprate YBa$_{2}$Cu$_{4}$O$_{8}$, is an extremely anisotropic quasi-one-dimensional (Q1D) metal ($t_{b}^{2}$: $t_{a}^{2}$: $t_{c}^{2} \quad \sim $ 4000: 2: 1). The in-chain ($b$-axis) charge dynamics were investigated as a function of temperature and applied magnetic field for several samples of different disorder content. Measurements in magnetic fields up to 65 T confirm the correspondence between dimensional crossovers due to magnetic field carrier confinement and temperature-induced decoherence, opening the possibility of accessing the Tomonaga-Luttinger liquid state in this material. A metal-insulator transition is observed with increasing disorder concentration for samples with nominal mean free path of $\sim $100$b$. The onset of this remarkable localisation phenomenon reveals a striking correlation between the scattering rate and the interchain hopping rate(s), suggesting a \textit{disorder}-induced crossover to a one-dimensional (1D) ground state. $b$-axis magnetoresistance measurements were performed in order to identify the nature and properties of the Q1D metallic and disorder-induced 1D insulating states.   

Lattice fermion models with spontaneous orbital currents in strong-interaction limit

A suggestion that the cuprate pseudogap state has a hidden order, in the form of staggered currents [1], led to toy spinfull fermion models on ladders and bilayers which exhibit such a symmetry breaking [2]. This invites the question, which features of the model are conducive to such phases? Taking (for maximal simplicity) spinless fermions, with large or infinite repulsion, I find a ground state with spontaneous-current order in two models: each consists of rings of sites, coupled by weak hopping to form a ladder or a $d=2$ lattice. So far, all my examples require lattices modulated with alternately strong and weak hopping. I also argue that spontaneous currents depend on the presence of Berry-like phases as one takes the system around loops in the abstract graph of all configurations coupled by hops: it would follow that spontaneous currents are impossible in lattice boson models. \newline \newline [1] S. Chakravarty et al, Phys. Rev. B 63, 094503 (2001). \newline [2] M. Tsuchiizu and A. Furusaki, \%\%\% spontaneous currents Phys. Rev. B 66, 245106 (2002); U. Schollw\"{o}ck et al, Phys. Rev. Lett. 90, 186401 (2003); S. Capponi, C.~Wu, and S.-C. Zhang, Phys. Rev. B 70, 220505 (2004).   

A proposal for giant spin-orbital resonance in AFM/SDW conductors.

Essential dependence of the electron $g$-factor on the quasiparticle momentum is a fundamental property of antiferromagnetic conductors (AFM), which so far has been largely overlooked. It leads to an anomalously strong spin-orbit interaction, of which a giant combined spin-orbital resonance may be a striking manifestation. We advance a theory of this combined resonance (excitation of electron spin transitions by AC {\it electric} field) in a weakly doped antiferromagnetic insulator. The combined resonance intensity exceeds that of the electron spin resonance (ESR) by orders of magnitude. We study transitions in the continuous spectrum, as well as in a quantizing magnetic field, and calculate the resonance lineshape, and the angular dependence of the resonance intensity. Our predictions may be relevant for various magnetically ordered conductors, including electron- and hole-doped cuprates, and organic metals with a spin density wave (SDW).   

Dual vortex theory of doped antiferromagnets

We present a general framework for describing the quantum phases obtained by doping paramagnetic Mott insulators on the square lattice. The undoped insulators are efficiently characterized by the projective transformations of various fields under the square lattice space group (the PSG). We show that the PSG also imposes powerful constraints on the doped system, and enables derivation of an effective action for the vortex and Bogoliubov quasiparticle excitations of superconducting states. This action also describes transitions to supersolid or insulating states at nonzero doping. For the case of a valence bond solid (VBS) insulator, we show that the doped system has the same PSG as that of elementary bosons with density equal to the density of electron pairs. We also obtain the action for a d-wave superconductor obtained by doping a ``staggered-flux'' spin liquid state.   

Strong coupling theory of phonons in the Hubbard Model

The role of phonons in strongly correlated electronic systems including the high Tc cuprates has not been completely well understood. Recent experimental results, notably those of Lanzara et. al reveal the existence of a kink in the quasiparticle dispersion data for various classes of cuprate superconductors including BiSCO , LSCO and NCCO at an energy scale of around 50 meV to 70 meV using ARPES techniques. Direct comparison of ARPES and neutron scattering data has also been done for LSCO. One of the possible explanations for this kink is believed to be coupling of electrons with phonon modes of the above energy scale. We obtain the electron spectral function as a function of electron- phonon coupling strength using Cluster Dynamic Mean Field Theory (CDMFT on the 2-D Hubbard Model with phonons and compare it with the experimental data.   

Spin Correlations and Magnetic Susceptibilities of Lightly Doped Antiferromagnets

We calculate the spin correlation function and the magnetic longitudinal and transverse susceptibilities of a two-dimensional antiferromagnet doped with a small concentration of holes, in the t-J model. We find that the motion of holes generates spin fluctuations which add to the quantum fluctuations, the spin correlations decaying with the inverse of the spin distance, while increasing with doping as the critical hole concentration, where the long-range order disappears, is approached. Moreover, the longitudinal susceptibility becomes finite in the presence of doping, due to the strong damping effects induced by the hole motion, while the transverse susceptibility is renormalized by softening effects. Both the longitudinal and the transverse susceptibilities increase with doping, the former more significantly than the latter. Our results imply that doping destroys the long-range order while local antiferromagnetic spin correlations persist. This is consistent with experiments on the doped copper oxide superconductors.   

Unconventional antiferromagnetism in the presence of Dzyaloshinskii-Moriya interactions: the case of La$_{2}$CuO$_{4}$

Between the several anomalous properties of cuprates superconductors, the attention has been put recently on the magnetic properties of the undoped compounds, which display antiferromagnetism. In particular, La$_{2}$CuO$_{4}$ has been extensively investigated, and several unusal magnetic properties have been observed, as for example the temperature dependence of the uniform magnetic susceptibility and the magnetic-field dependence of the magnon gaps. Using a long-wavelength non-linear sigma model approach, we study effect of Dzyaloshinskii-Moriya interactions in the quantum Heisenberg antiferromagnet. With this technique one can easily recognize that the Dzyaloshinskii-Moriya interaction mediates an anomalous coupling between the uniform magnetic field and the antiferromagnetic order parameter. We can then provide a simple and clear explanation for all the recently reported anomalies, and we demonstrate explicitly why La$_{2}$CuO$_{4}$ can not be classified as an ordinary easy-axis antiferromagnet.   

$^{63,65}$Cu NMR Spectrum and Spin Lattice Relaxation in the Two-dimensional Antiferromagnet Pr$_{2}$CuO$_{4-y}$

The $^{63,65}$Cu NMR spectra and spin lattice relaxation rate (1/$T_{1})$ are reported for a single crystal of the two-dimensional antiferromagnet Pr$_{2}$CuO$_{4-y}$ as a function of the applied magnetic field (\textbf{\textit{B}}$_{0})$ over the temperature ($T)$ range 3~-~20 K. When \textbf{\textit{B}}$_{0}$ =~0, the NMR spectrum has six lines, which correspond to the quadrupolar spectrum (central transition and two satellites) of both Cu isotopes in an antiferromagnetic (AF) internal field of 9.626 T. This value is 0.75 T less than that of the related compound Nd$_{2}$CuO$_{4-y }$[1]. The spectra as a function of \textbf{\textit{B}}$_{0}$ are consistent with the noncollinear structure of the AF ordered Cu$^{2+}$ spins seen in neutron diffraction studies [2]. The values of 1/$T_{1}$ over the range $5\;\mbox{K}\le T\le 20\;\mbox{K}$ follow the power law $1/T_1 \propto T^2$, which may indicate 1/$T_{1}$ is dominated by two-dimensional AF spin waves [3]. [1] Y. Yosinari \textit{et al.}, J. Phys. Soc. Jpn. \textbf{59}, 36 (1990). [2] I. Sumarlin \textit{et al.}, Phys Rev B, \textbf{51}, 5824 (1995). [3] S. Chakravarty \textit{et al.}, Phys Rev B\textbf{ 43}, 2796 (1991).   

Spectroscopy of stripe order in La$_{1.8}$Sr$_{0.2}$NiO$_{4}$ using resonant soft x-ray diffraction

We studied the electronic structure and temperature dependence of the stripe phase in Sr-doped La$_{2}$NiO$_{4}$ using resonant diffraction at the Ni $L_{2,3}$ and La $M_{4,5}$ edge in combination with a realistic microscopic theory. Making use of the very high sensitivity of the method to ordered modulations of the electronic state, we found the doped holes to be mainly located on the oxygen ligands in the NiO$_{2}$ planes, each centered around one Ni ion. The Ni ions in the hole-rich stripes are essentially in a high-spin d$^{8}$ state and the hole spin is coupled antiparallel to the Ni spin in close analogy to the Zhang-Rice singlet state in cuprates. Such an understanding of the charge-ordered phase could not be obtained before, neither by conventional x-ray or neutron diffraction techniques, nor by x-ray absorption spectroscopy.