Search

High resolution resonant inelastic x-ray scattering has been performed to reveal the role of lattice coupling in a family of quasi-1D insulating cuprates, Ca2+5xY2-5xCu5O10. Site-dependent low-energy excitations arising from progressive emissions of a 70 meV lattice vibrational mode are resolved for the first time, providing a direct measurement of electron-lattice coupling strength. We show that such electron-lattice coupling causes doping-dependent distortions of the Cu-O-Cu bond angle, which sets the intrachain spin exchange interactions. Our results indicate that the lattice degrees of freedom are fully integrated into the electronic behavior in low-dimensional systems.

We investigate the order parameter dynamics of the stripe-ordered nickelate, La(1.75)Sr(0.25)NiO(4), using time-resolved resonant x-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters' amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer reorientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetry-broken states does not unbind during the nonequilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom.

The nature of metallicity and the level of electronic correlations in the antiferromagnetically ordered parent compounds are two important open issues for the iron-based superconductivity. We perform a temperature-dependent angle-resolved photoemission spectroscopy study of Fe(1.02)Te, the parent compound for iron chalcogenide superconductors. Deep in the antiferromagnetic state, the spectra exhibit a "peak-dip-hump" line shape associated with two clearly separate branches of dispersion, characteristics of polarons seen in manganites and lightly doped cuprates. As temperature increases towards the Néel temperature (T(N)), we observe a decreasing renormalization of the peak dispersion and a counterintuitive sharpening of the hump linewidth, suggestive of an intimate connection between the weakening electron-phonon (e-ph) coupling and antiferromagnetism. Our finding points to the highly correlated nature of the Fe(1.02)Te ground state featured by strong interactions among the charge, spin, and lattice and a good metallicity plausibly contributed by the coherent polaron motion.

Charge and spin density waves, periodic modulations of the electron, and magnetization densities, respectively, are among the most abundant and nontrivial low-temperature ordered phases in condensed matter. The ordering direction is widely believed to result from the Fermi surface topology. However, several recent studies indicate that this common view needs to be supplemented. Here, we show how an enhanced electron-lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge density wave system ErTe(3). Our joint experimental and theoretical study allows us to establish a relation between the selection rules of the electronic light scattering spectra and the enhanced electron-phonon coupling in the vicinity of band degeneracy points. This alternative proposal for charge density wave formation may be of general relevance for driving phase transitions into other broken-symmetry ground states, particularly in multiband systems, such as the iron-based superconductors.

Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La₀.₇Sr₀.₃MnO₃ (LSMO)- Nb-doped SrTiO₃ (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 ± 0.10 unit cells (u.c.) at -1.9 V, increasing to 2.02 ± 0.16 u.c. at -1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.

We present a determinant quantum Monte Carlo study of the competition between instantaneous on-site Coulomb repulsion and retarded phonon-mediated attraction between electrons, as described by the two-dimensional Hubbard-Holstein model. At half filling, we find a strong competition between antiferromagnetism (AFM) and charge-density-wave (CDW) order. We demonstrate that a simple picture of AFM-CDW competition that incorporates the phonon-mediated attraction into an effective-U Hubbard model requires significant refinement. Specifically, retardation effects slow the onset of charge order so that CDW order remains absent even when the effective U is negative. This delay opens a window where neither AFM nor CDW order is well established and where there are signatures of a possible metallic phase.

High-pressure x-ray emission measurements are used to provide crucial evidence in the longstanding debate over the nature of the isostructural (α, γ) volume collapse in elemental cerium. Extended local atomic model calculations show that the satellite of the Lγ emission line offers direct access to the total angular momentum observable (J(2)). This satellite experiences a 30% steplike decrease across the volume collapse, validating the Kondo model in conjunction with previous measurements. Direct comparisons are made with previous predictions by dynamical mean field theory. A general experimental methodology is demonstrated for analogous work on a wide range of strongly correlated f-electron systems.

A detailed phenomenology of low energy excitations is a crucial starting point for microscopic understanding of complex materials, such as the cuprate high-temperature superconductors. Because of its unique momentum-space discrimination, angle-resolved photoemission spectroscopy (ARPES) is ideally suited for this task in the cuprates, where emergent phases, particularly superconductivity and the pseudogap, have anisotropic gap structure in momentum space. We present a comprehensive doping- and temperature-dependence ARPES study of spectral gaps in Bi(2)Sr(2)CaCu(2)O(8+δ), covering much of the superconducting portion of the phase diagram. In the ground state, abrupt changes in near-nodal gap phenomenology give spectroscopic evidence for two potential quantum critical points, p = 0.19 for the pseudogap phase and p = 0.076 for another competing phase. Temperature dependence reveals that the pseudogap is not static below T(c) and exists p > 0.19 at higher temperatures. Our data imply a revised phase diagram that reconciles conflicting reports about the endpoint of the pseudogap in the literature, incorporates phase competition between the superconducting gap and pseudogap, and highlights distinct physics at the edge of the superconducting dome.

The spectral response and physical features of the 2D Hubbard-Holstein model are calculated both in equilibrium at zero and low chemical dopings, and after an ultrashort powerful light pulse, in undoped systems. At equilibrium and at strong charge-lattice couplings, the optical conductivity reveals a three-peak structure in agreement with experimental observations. After an ultrashort pulse and at nonzero electron-phonon interaction, phonon and spin subsystems oscillate with the phonon period T(ph)≈80 fs. The decay time of the phonon oscillations is about 150-200 fs, similar to the relaxation time of the charge system. We propose a criterion for observing these oscillations in high T(c) compounds: the time span of the pump light pulse τ(pump) has to be shorter than the phonon oscillation period T(ph).

The dynamics of an order parameter's amplitude and phase determines the collective behaviour of novel states emerging in complex materials. Time- and momentum-resolved pump-probe spectroscopy, by virtue of measuring material properties at atomic and electronic time scales out of equilibrium, can decouple entangled degrees of freedom by visualizing their corresponding dynamics in the time domain. Here we combine time-resolved femotosecond optical and resonant X-ray diffraction measurements on charge ordered La(1.75)Sr(0.25)NiO(4) to reveal unforeseen photoinduced phase fluctuations of the charge order parameter. Such fluctuations preserve long-range order without creating topological defects, distinct from thermal phase fluctuations near the critical temperature in equilibrium. Importantly, relaxation of the phase fluctuations is found to be an order of magnitude slower than that of the order parameter's amplitude fluctuations, and thus limits charge order recovery. This new aspect of phase fluctuations provides a more holistic view of the phase's importance in ordering phenomena of quantum matter.

The prevalent view of the high-temperature superconducting cuprates is that their essential low-energy physics is captured by local Coulomb interactions. However, this view been challenged recently by studies indicating the importance of longer-range components. Motivated by this, we demonstrate the importance of these components by examining the electron-phonon (e-ph) interaction with acoustic phonons in connection with the recently discovered renormalization in the near-nodal low-energy (~8-15 meV) dispersion of Bi(2)Sr(2)CaCu(2)O(8+δ). By studying its nontrivial momentum and doping dependence we conclude a predominance of forward scattering arising from the direct interplay between the e-ph and extended Coulomb interactions. Our results thus demonstrate how the low-energy renormalization can provide a pathway to new insights into how these interactions interplay with one another and influence pairing and dynamics in the cuprates.

Knowledge of the gap function is important to understand the pairing mechanism for high-temperature (T(c)) superconductivity. However, Fourier transform scanning tunneling spectroscopy (FT STS) and angle-resolved photoemission spectroscopy (ARPES) in the cuprates have reported contradictory gap functions, with FT-STS results deviating strongly from a canonical d(x2-y2) form. By applying an "octet model" analysis to autocorrelation ARPES, we reveal that a contradiction occurs because the octet model does not consider the effects of matrix elements and the pseudogap. This reaffirms the canonical d(x2-y2) superconducting gap around the node, which can be directly determined from ARPES. Further, our study suggests that the FT-STS reported fluctuating superconductivity around the node at far above T(c) is not necessary to explain the existence of the quasiparticle interference at low energy.

The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T(c)), entangled in an energy-momentum-dependent manner with the preexisting pseudogap features, ushering in a ground state with coexisting orders.

Although the parent iron-based pnictides and chalcogenides are itinerant antiferromagnets, the use of local moment picture to understand their magnetic properties is still widespread. We study magnetic Raman scattering from a local moment perspective for various quantum spin models proposed for this new class of superconductors. These models vary greatly in the level of magnetic frustration and show a vastly different two-magnon Raman response. Light scattering by two-magnon excitations thus provides a robust and independent measure of the underlying spin interactions. In accord with other recent experiments, our results indicate that the amount of magnetic frustration in these systems may be small.

The results of model calculations using exact diagonalization reveal the orbital character of states associated with different Raman loss peaks in Cu K-edge resonant inelastic x-ray scattering (RIXS) from La₂CuO₄. The model includes electronic orbitals necessary to highlight the nonlocal Zhang-Rice singlet, charge transfer, and d-d excitations, as well as states with apical oxygen 2p(z) character. The dispersion of these excitations is discussed with prospects for resonant final state wave-function mapping. A good agreement with experiments emphasizes the substantial multiorbital character of RIXS profiles in the energy transfer range 1-6 eV.

The improved resolution of laser-based angle-resolved photoemission spectroscopy (ARPES) allows reliable access to fine structures in the spectrum. We present a systematic, doping-dependent study of a recently discovered low-energy kink in the nodal dispersion of Bi2Sr2CaCu2O(8+δ) (Bi-2212), which demonstrates the ubiquity and robustness of this kink in underdoped Bi-2212. The renormalization of the nodal velocity due to this kink becomes stronger with underdoping, revealing that the nodal Fermi velocity is nonuniversal, in contrast with assumed phenomenology. This is used together with laser ARPES measurements of the gap velocity (v2) to resolve discrepancies with thermal conductivity measurements.

Here we report the first angle-resolved photoemission measurement on nearly optimally doped multilayer Tl-based superconducting cuprates (Tl-2212 and Tl-1223) and a comparison study to single-layer (Tl-2201) compound. A "kink" in the band dispersion is found in all three compounds but exhibits different momentum dependence for the single-layer and multilayer compounds, reminiscent to that of Bi-based cuprates. This layer number dependent renormalization effect strongly implies that the spin-resonance mode is unlikely to be responsible for the dramatic renormalization effect near the antinodal region.

Low energy ultrahigh momentum resolution angle resolved photoemission spectroscopy study on four-layer self-doped high Tc superconductor Ba2Ca3Cu4O8F2 (F0234) revealed fine structure in the band dispersion, identifying the unconventional association of hole and electron doping with the inner and outer CuO2 layers, respectively. For the states originating from two inequivalent CuO2 layers, different energy scales are observed in dispersion kinks associated with the collective mode coupling, with the larger energy scale found in the electron (n-) doped state which also has stronger coupling strength. Given the earlier finding that the superconducting gap is substantially larger along the n-type Fermi surface, our observations connect the mode coupling energy and strength with magnitude of the pairing gap.

We report pressure-tuned Raman and x-ray diffraction data of Bi(1.98.)Sr(2.06)Y(0.68)Cu(2)O(8+delta) revealing a critical pressure at 21 GPa with anomalies in electronic Raman background, electron-phonon coupling lambda, spectral weight transfer, density dependent behavior of phonons and magnons, and a compressibility change in the c axis. For the first time in a cuprate, mobile charge carriers, lattice, and magnetism all show anomalies at a distinct critical pressure in the same experimental setting. Furthermore, the spectral changes suggest that the critical pressure at 21 GPa is related to the critical point at optimal doping.

The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay between the electron correlation and electron-phonon interaction resolving the long standing mystery of the midinfrared band.

The superconducting gap--an energy scale tied to the superconducting phenomena--opens on the Fermi surface at the superconducting transition temperature (T(c)) in conventional BCS superconductors. In underdoped high-T(c) superconducting copper oxides, a pseudogap (whose relation to the superconducting gap remains a mystery) develops well above T(c) (refs 1, 2). Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above T(c) is one of the central questions in high-T(c) research. Although some experimental evidence suggests that the two gaps are distinct, this issue is still under intense debate. A crucial piece of evidence to firmly establish this two-gap picture is still missing: a direct and unambiguous observation of a single-particle gap tied to the superconducting transition as function of temperature. Here we report the discovery of such an energy gap in underdoped Bi2Sr2CaCu2O8+delta in the momentum space region overlooked in previous measurements. Near the diagonal of Cu-O bond direction (nodal direction), we found a gap that opens at T(c) and has a canonical (BCS-like) temperature dependence accompanied by the appearance of the so-called Bogoliubov quasi-particles, a classical signature of superconductivity. This is in sharp contrast to the pseudogap near the Cu-O bond direction (antinodal region) measured in earlier experiments.

We used angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2Ca(1-x)YxCu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increased with underdoping, a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near-nodal regime where a coherent peak in the spectrum can be observed, did not increase with less doping, a behavior not previously observed in the single particle spectra. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high-transition temperature superconductivity.

We report the discovery of a self-doped multilayer high Tc superconductor Ba2Ca3Cu4O8F2 (F0234) which contains distinctly different superconducting gap magnitudes along its two Fermi-surface sheets. While formal valence counting would imply this material to be an undoped insulator, it is a self-doped superconductor with a Tc of 60 K, possessing simultaneously both electron- and hole-doped Fermi-surface sheets. Intriguingly, the Fermi-surface sheet characterized by the much larger gap is the electron-doped one, which has a shape disfavoring two electronic features considered to be important for the pairing mechanism: the van Hove singularity and the antiferromagnetic (pi/a, pi/a) scattering.

Recent scanning tunneling microscopy (STM) experimentally observed strong gap inhomogeneity in Bi2Sr2CaCu2O(8+delta) (BSCCO). We argue that disorder in the pair potential underlies the gap inhomogeneity, and investigate its role in the Fourier-transformed inelastic tunneling spectra as revealed in the STM. We find that the random pair potential induces unique q-space patterns in the local density of states (LDOS) of a d-wave superconductor. We consider the effects of electron coupling to various bosonic modes and find the pattern of LDOS modulation due to coupling to the B(1g) phonon mode to be consistent with the one observed in the inelastic electron tunnneling STM experiment in BSCCO. These results suggest strong electron-lattice coupling as an essential part of the superconducting state in high-Tc materials.

A recent highlight in the study of high-T(c) superconductors is the observation of band renormalization or self-energy effects on the quasiparticles. This is seen in the form of kinks in the quasiparticle dispersions as measured by photoemission and interpreted as signatures of collective bosonic modes coupling to the electrons. Here we compare for the first time the self-energies in an optimally doped and strongly overdoped, nonsuperconducting single-layer Bi-cuprate (Bi2Sr2CuO6). In addition to the appearance of a strong overall weakening, we also find that the weight of the self-energy in the overdoped system shifts to higher energies. We present evidence that this is related to a change in the coupling to c-axis phonons due to the rapid change of the c-axis screening in this doping range.

A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the 'd-wave' symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction. Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite 'Fermi arcs'. This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity or by invoking orders which are natural competitors of d-wave superconductors. Here we report experimental evidence that a very similar pseudogap state with a nodal-antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La1.2Sr1.8Mn2O7. Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state.

A class of sum rules for inelastic light scattering is developed. We show that the first moment of the nonresonant response provides information about the potential energy in strongly correlated systems. The polarization dependence of the sum rules provides information about the electronic excitations in different regions of the Brillouin zone. We determine the sum rule for the Falicov-Kimball model, which possesses a metal-insulator transition, and compare our results to the light scattering experiments in SmB(6).

We use dynamical mean field theory to find an exact solution for inelastic light scattering in strongly correlated materials such as those near a quantum-critical metal-insulator transition. We evaluate the results for q=0 (Raman) scattering and find that resonant effects can be quite large, and yield a double resonance, a significant enhancement of nonresonant scattering peaks, a joint resonance of both peaks when the incident photon frequency is on the order of U, and the appearance of an isosbestic point in all symmetry channels for an intermediate range of incident photon frequencies.

Angle-resolved photoemission spectroscopy on optimally doped Bi(2)Sr(2)Ca(0.92)Y(0.08)Cu(2)O(8+delta) uncovers a coupling of the electronic bands to a 40 meV mode in an extended k-space region away from the nodal direction, leading to a new interpretation of the strong renormalization of the electronic structure seen in Bi2212. Phenomenological agreements with neutron and Raman experiments suggest that this mode is the B(1g) oxygen bond-buckling phonon. A theoretical calculation based on this assignment reproduces the electronic renormalization seen in the data.

We explore manifestations of electron-phonon coupling on the electron spectral function for two phonon modes in the cuprates exhibiting strong renormalizations with temperature and doping. Applying simple symmetry considerations and kinematic constraints, we find that the out-of-plane, out-of-phase O buckling mode (B(1g)) involves small momentum transfers and couples strongly to electronic states near the antinode while the in-plane Cu-O breathing modes involve large momentum transfers and couples strongly to nodal electronic states. Band renormalization effects are found to be strongest in the superconducting state near the antinode, in full agreement with angle-resolved photoemission spectroscopy data.

The role of commensurability and the interplay of correlated disorder and interactions on vortex dynamics in the presence of columnar pins is studied via molecular dynamics simulations. Simulations of dynamics reveal substantial caging effects and a nonmonotonic dependence of the critical current with enhancements near integer values of the matching field Bphi and 3Bphi in agreement with experiments on the cuprates. We find qualitative differences in the phase diagram for small and large values of the matching field.

We calculate the inelastic light scattering from x rays, which allows the photon to transfer both energy and momentum to the strongly correlated charge excitations. We find that the charge-transfer peak and the low-energy peak both broaden and disperse through the Brillouin zone similar to what is seen in experiments in materials such as Ca2CuO2Cl2.

The electron dynamics in the normal state of Bi(2)Sr(2)CaCu(2)O(8+delta) is studied by inelastic light scattering over a wide range of doping. A strong anisotropy of the electron relaxation is found which cannot be explained by single-particle properties alone. The results strongly indicate the presence of an unconventional quantum-critical metal-insulator transition where "hot" (antinodal) quasiparticles become insulating while "cold" (nodal) quasiparticles remain metallic. A phenomenology is developed which allows a quantitative understanding of the Raman results and provides a scenario which links single- and many-particle properties.