arXiv subjects

M. Greven

Publications and source records attributed to M. Greven.

At least 91 records · Page 5Linked to original sources

arXiv2005

Charge-Transfer Excitations in the Model Superconductor HgBa$_2$CuO$_{\bf 4+δ}$

We report a Cu $K$-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$_{4+δ}$ at optimal doping ($T_{\rm 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 charge-transfer pseudogap deep in the superconducting phase. Quite generally, our data demonstrate the importance of exploring the incident photon energy dependence of the RIXS cross section.

cond-mat.str-el↗

arXiv2005

Inhomogeneity Induces Resonance Coherence Peaks in Superconducting BSCCO

In this paper we analyze, using scanning tunneling spectroscopy, the density of electronic states in nearly optimally doped BSCCO in zero field. Focusing on the superconducting gap, we find patches of what appear to be two different phases in a background of some average gap, one with a relatively small gap and sharp large coherence peaks and one characterized by a large gap with broad weak coherence peaks. We compare these spectra with calculations of the local density of states for a simple phenomenological model in which a 2 xi_0 * 2 xi_0 patch with an enhanced or supressed d-wave gap amplitude is embedded in a region with a uniform average d-wave gap.

cond-mat.supr-con↗

arXiv2004

Normal State Spectral Lineshapes of Nodal Quasiparticles in Single Layer Bi2201 Superconductor

A detailed study of the normal state photoemission lineshapes and quasiparticle dispersion for the single layer Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (Bi2201) superconductor is presented. We report the first experimental evidence of a double peak structure and a dip of spectral intensity in the energy distribution curves (EDCs) along the nodal direction. The double peak structure is well identified in the normal state, up to ten times the critical temperature. As a result of the same self-energy effect, a strong mass renormalization of the quasiparticle dispersion, i.e. kink, and an increase of the quasiparticle lifetime in the normal state are also observed. Our results provide unambiguous evidence on the existence of bosonic excitation in the normal state, and support a picture where nodal quasiparticles are strongly coupled to the lattice.

cond-mat.supr-con↗

arXiv2004

Spin Correlations and Magnetic Order in Nonsuperconducting Nd_{2-x}Ce_xCuO_{4\pmδ}

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_xCuO_{4\pmδ} ($0 \le x \le 0.18$). The instantaneous correlation length can be effectively described by our quantum Monte Carlo calculations for the randomly site-diluted nearest-neighbor spin-1/2 square-lattice Heisenberg antiferromagnet. However, quantum fluctuations have a stronger effect on the ordered moment, which decreases more rapidly than for the quenched-disorder model.

cond-mat.supr-con↗

arXiv2004

STM Studies of Near-Optimal Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

In this paper we summarize our STM studies of the density of electronic states in nearly optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ in zero field. We report on the inhomogeneity of the gap structure, density of states modulations with four-lattice constant period, and coherence peak modulation.

cond-mat.supr-con↗

arXiv2004

Structural and magnetic properties of the single-layer manganese oxide La{1-x}Sr{1+x}MnO4

Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La{1-x}Sr{1+x}MnO4 (0 < x < 0.7). Single crystals were grown by the traveling-solvent floating-zone method at 18 La/Sr concentrations. The low-temperature phase diagram can be understood by considering the strong coupling of the magnetic and orbital degrees of freedom, and it can be divided into three distinct regions: low (x < 0.12), intermediate (0.12< x < 0.45), and high (x > 0.45) doping. LaSrMnO_4 (x=0) is an antiferromagnetic Mott insulator,and its spin-wave spectrum is well-described by linear spin-wave theory for the spin-2 square-lattice Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the e_g electron concentration (1-x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Neel order disappears above xc = 0.115(10). In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural "nanopatches" begin to form above x ~ 0.25. At high doping (x > 0.45), the ground state of La{1-x}Sr{1+x}MnO4 exhibits long-range superstructural order and a complex (CE-type) antiferromagnetic order which differs from that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites. For x > 0.50, the superstructural order becomes incommensurate with the lattice, with a modulation wavevector that depends linearly on the e_g electron concentration. On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x = 0.50.

cond-mat.str-el↗

arXiv2004

Periodic Coherence Peak Height Modulations in Superconducting BSCCO

In this paper we analyze, using scanning tunneling spectroscopy (STS), the local density of electronic states (LDOS) in nearly optimally doped BSCCO in zero field. We see both dispersive and non-dispersive spatial LDOS modulations as a function of energy in our samples. Moreover, a spatial map of the superconducting coherence peak heights shows the same structure as the low energy LDOS. This suggests that these non-dispersive LDOS modulations originate from an underlying charge-density modulation which interacts with superconductivity.

cond-mat.supr-con↗

arXiv2004

Universal scaling relation in high-temperature superconductors

Scaling laws express a systematic and universal simplicity among complex systems in nature. For example, such laws are of enormous significance in biology. Scaling relations are also important in the physical sciences. The seminal 1986 discovery of high transition-temperature (high-T_c) superconductivity in cuprate materials has sparked an intensive investigation of these and related complex oxides, yet the mechanism for superconductivity is still not agreed upon. In addition, no universal scaling law involving such fundamental properties as T_c and the superfluid density ρ_s, a quantity indicative of the number of charge carriers in the superconducting state, has been discovered. Here we demonstrate that the scaling relation ρ_s \propto σ_{dc} T_c, where the conductivity σ_{dc} characterizes the unidirectional, constant flow of electric charge carriers just above T_c, universally holds for a wide variety of materials and doping levels. This surprising unifying observation is likely to have important consequences for theories of high-T_c superconductivity.

cond-mat.supr-con↗

arXiv2004

Phase Decomposition and Chemical Inhomogeneity in Nd2-xCexCuO4

Extensive X-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd2-xCexCuO4 (NCCO) in a low-oxygen-fugacity environment such as that typically realized during the annealing process required to create a superconducting state. Unlike a typical situation in which a disordered secondary phase results in diffuse powder scattering, a serendipitous match between the in-plane lattice constant of NCCO and the lattice constant of one of the decomposition products, (Nd,Ce)2O3, causes the secondary phase to form an oriented, quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from the secondary phase appear at rational positions (H,K,0) in the reciprocal space of NCCO. Additionally, because of neodymium paramagnetism, the application of a magnetic field increases the low-temperature intensity observed at these positions via neutron scattering. Such effects may mimic the formation of a structural superlattice or the strengthening of antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified through careful and systematic experimentation. For typical reduction conditions, the (Nd,Ce)2O3 volume fraction is ~1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50-100 angstromsthick. The presence of the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements.

cond-mat.supr-con↗

arXiv2004

Susceptibilities and Spin Gaps of Weakly-coupled Spin Ladders

We calculate the uniform and staggered susceptibilities of two-chain spin-1/2 Heisenberg ladders using Monte-Carlo simulations. We show that the gap extracted from the uniform susceptibility and the saturation value of the staggered susceptibility are independent of the sign of the inter-chain coupling J_perp in the asymptotic limit |J_perp|/J -> 0. Furthermore, we examine the existence of logarithmic corrections to the linear scaling of the gap with |J_perp|.

cond-mat.str-el↗

arXiv2003

Effect of chemical inhomogeneity in the bismuth-based copper oxide superconductors

We examine the effect on the superconducting transition temperature (Tc) of chemical inhomogeneities in Bi2201 and Bi2212 single crystals. Cation disorder at the Sr crystallographic site is inherent in these materials and strongly affects the value of Tc. Partial substitution of Sr by Ln (Ln = La, Pr, Nd, Sm, Eu, Gd, and Bi) in Bi_{2}Sr_{1.6}Ln_{0.4}CuO_{6} results in a monotonic decrease of Tc with increasing ionic radius mismatch. By minimizing Sr site disorder at the expense of Ca site disorder, we demonstrate that the Tc of Bi2212 can be increased to 96 K. Based on these results we discuss the effects of chemical inhomogeneity in other bulk high-temperature superconductors.

cond-mat.supr-con↗

arXiv2003

Fully Gapped Single-Particle Excitations in the Lightly Doped Cuprates

The low-energy excitations of the lightly doped cuprates were studied by angle-resolved photoemission spectroscopy. A finite gap was measured over the entire Brillouin zone, including along the d_{x^2 - y^2} nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped La_{2-x}Sr_{x}CuO_{4} and Ca_{2-x}Na_{x}CuO_{2}Cl_{2} and electron-doped Nd_{2-x}Ce_{x}CuO_{4}. In all compounds, the gap appears to close with increasing carrier doping. We consider various scenarios to explain our results, including the possible effects of chemical disorder, electronic inhomogeneity, and a competing phase.

cond-mat.supr-con↗

arXiv2003

Spurious magnetism in high-Tc superconductor

One challenge in condensed-matter physics is to unravel the interplay between magnetism and superconductivity in copper oxides with a high critical temperature (Tc). Kang et al. claim to have revealed a quantum phase transition from the superconducting to an antiferromagnetic state in the electron-doped material Nd2-xCexCuO4 (NCCO) based on the observation of magnetic-field-induced neutron scattering intensity at (1/2,1/2,0), (1/2,0,0), and related reflections. Here we argue that the observed magnetic intensity is due to a secondary phase of (Nd,Ce)2O3. We therefore contend that the effect is spurious and not intrinsic to superconducting NCCO.

cond-mat↗

arXiv2003

Powerlaw optical conductivity with a constant phase angle in high Tc superconductors

In certain materials with strong electron correlations a quantum phase transition (QPT) at zero temperature can occur, in the proximity of which a quantum critical state of matter has been anticipated. This possibility has recently attracted much attention because the response of such a state of matter is expected to follow universal patterns defined by the quantum mechanical nature of the fluctuations. Forementioned universality manifests itself through power-law behaviours of the response functions. Candidates are found both in heavy fermion systems and in the cuprate high Tc superconductors. Although there are indications for quantum criticality in the cuprate superconductors, the reality and the physical nature of such a QPT are still under debate. Here we identify a universal behaviour of the phase angle of the frequency dependent conductivity that is characteristic of the quantum critical region. We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of the optical conductivity. This points towards a QPT in the cuprates close to optimal doping, although of an unconventional kind.

cond-mat.str-el↗

arXiv2003

An angle-resolved photoemission spectral function analysis of the electron doped cuprate Nd_1.85Ce_0.15CuO_4

Using methods made possible by recent advances in photoemission technology, we perform an indepth line-shape analysis of the angle-resolved photoemission spectra of the electron doped (n-type) cuprate superconductor Nd_1.85Ce_0.15CuO_4. Unlike for the p-type materials, we only observe weak mass renormalizations near 50-70 meV. This may be indicative of smaller electron-phonon coupling or due to the masking effects of other interactions that make the electron-phonon coupling harder to detect. This latter scenario may suggest limitations of the spectral function analysis in extracting electronic self-energies when some of the interactions are highly momentum dependent.

cond-mat.supr-con↗

arXiv2003

Neutron Scattering, Magnetometry, and Quantum Monte Carlo Study of the Randomly-Diluted Spin-1/2 Square-Lattice Heisenberg Antiferromagnet

We have successfully grown sizable single crystals of La_2Cu_{1-z}(Zn,Mg)_zO_4 with up to nearly half of the magnetic Cu sites replaced by non-magnetic Zn and Mg. Neutron scattering, SQUID magnetometry, and complementary quantum Monte Carlo (QMC) simulations demonstrate that this material is an excellent model system for the study of site percolation of the square-lattice Heisenberg antiferromagnet (SLHAF) in the quantum-spin limit S=1/2. Carefully oxygen-reduced samples exhibit Neel order up to the percolation threshold for site dilution, z_p ~ 40.7%. Up to at least z = 35%, the Neel temperature T_N(z) of the experimental system corresponds to the temperature at which QMC indicates that the spin correlations for the nearest-neighbor S=1/2 SLHAF have grown to approximately 100 lattice constants. Neutron scattering measurements of the static structure factor in the paramagnetic regime allow the determination of the two-dimensional spin correlations, which are found to be in excellent quantitative agreement with QMC over a wide common temperature and doping range. Neutron scattering and QMC results for the temperature dependence of the static structure factor amplitude S(π,π) are in good agreement as well. The combined experimental and numerical data presented here provide valuable quantitative information for tests of theories of the randomly-diluted S=1/2 SLHAF.

cond-mat.str-el↗

arXiv2003

Magnetic Properties and Phase Diagram of Li-doped La(2)CuO(4): Common Magnetic Response of Hole-doped CuO2 Planes

Although La(2)Cu(1-x)Li(x)O(4) [Li-LCO] differs from La(2-x)Sr(x)CuO(4) [Sr-LCO] in many ways (e.g., the absence of metallic transport, high-Tc superconductivity, and incommensurate antiferromagnetic correlations), it has been known that certain magnetic properties are remarkably similar. The present work establishes the detailed bulk magnetic phase diagram of Li-LCO (0 <= x <= 0.07), which is found to be nearly identical to that of lightly-doped Sr-LCO, and therefore extends the universality of the phase diagram to hole-doped but nonsuperconducting cuprates.

cond-mat.str-el↗

arXiv2002

Electron-phonon interaction in n-doped cuprates: an Inelastic X-ray Scattering study

Inelastic x-ray scattering (IXS) with very high (meV) energy resolution has become a valuable spectroscopic tool, complementing the well established coherent inelastic neutron scattering (INS) technique for phonon dispersion investigations. In the study of crystalline systems IXS is a viable alternative to INS, especially in cases where only small samples are available. Using IXS, we have measured the phonon dispersion of Nd_{1.86}Ce_{0.14}CuO_{4+δ} along the [x,0,0] and [x,x,0] in-plane directions. Compared to the undoped parent compound, the two highest longitudinal optical (LO) phonon branches are shifted to lower energies because of Coulomb-screening effects brought about by the doped charge carriers. An additional anomalous softening of the highest branch is observed around q=(0.2,0,0). This anomalous softening, akin to what has been observed in other compounds, provides evidence for a strong electron-phonon coupling in the electron-doped high-temperature superconductors.

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