Superconductivity-induced transfer of in-plane spectral weight in Bi2Sr2CaCu2O8+delta

Unconventional exciton evolution from the pseudogap to superconducting phases in cuprates

Electron quasiparticles play a crucial role in simplifying the description of many-body physics in solids with surprising success. Conventional Landau's Fermi-liquid and quasiparticle theories for high-temperature superconducting cuprates have, however, received skepticism from various angles. A path-breaking framework of electron fractionalization has been established to replace the Fermi-liquid theory for systems that show the fractional quantum Hall effect and the Mott insulating phenomena; whether it captures the essential physics of the pseudogap and superconducting phases of cuprates is still an open issue. Here, we show that excitonic excitation of optimally doped Bi2Sr2CaCu2O8+δ with energy far above the superconducting-gap energy scale, about 1 eV or even higher, is unusually enhanced by the onset of superconductivity. Our finding proves the involvement of such high-energy excitons in superconductivity. Therefore, the observed enhancement in the spectral weight of excitons imposes a crucial constraint on theories for the pseudogap and superconducting mechanisms. A simple two-component fermion model which embodies electron fractionalization in the pseudogap state provides a possible mechanism of this enhancement, pointing toward a novel route for understanding the electronic structure of superconducting cuprates.

Stranger than metals

In traditional metals, the temperature ( T ) dependence of electrical resistivity vanishes at low or high T , albeit for different reasons. Here, we review a class of materials, known as “strange” metals, that can violate both of these principles. In strange metals, the change in slope of the resistivity as the mean free path drops below the lattice constant, or as T → 0, can be imperceptible, suggesting continuity between the charge carriers at low and high T . We focus on transport and spectroscopic data on candidate strange metals in an effort to isolate and identify a unifying physical principle. Special attention is paid to quantum criticality, Planckian dissipation, Mottness, and whether a new gauge principle is needed to account for the nonlocal transport seen in these materials.

Emergence of quenched disorder as a dominant control for complex phase diagram of rare-earth nickelates

Competing interactions in complex materials tend to induce multiple quantum phases of comparable energetics close to the ground state stability. This requires novel strategies and tools to segregate such phases with desired control to manipulate the properties relevant for contemporary technologies. Here, we show 'quenched disorder (QD)' as a predominant control parameter to realize a broad range of the quantum phases of bulkRNiO₃(R= rare-earth ion) phase diagram in a La_xEu_1-xNiO₃compounds by systematic introduction of QD. Using static and terahertz dynamic transport studies on epitaxial thin films, we demonstrate various phases such as Fermi to non-Fermi liquid crossover, bad metallic behavior, quantum criticality, preservation of orbital and charge order symmetry and increased electronic inhomogeneity responsible for Maxwell-Wagner type of dielectric response, etc. The underlying mechanisms are unveiled by the anomalous responses of microscopic quantities such as scattering rate, plasma frequency, spectral weight, effective mass, and disorder. The results and methodology implemented here can be a generic pursuit of disorder based unified control to extract quantum phases submerged in competing energetics in all complex materials.

Signatures of Enhanced Superconducting Phase Coherence in Optimally Doped Bi_{2}Sr_{2}Y_{0.08}Ca_{0.92}Cu_{2}O_{8+δ} Driven by Midinfrared Pulse Excitations

Optimally doped cuprate are characterized by the presence of superconducting fluctuations in a relatively large temperature region above the critical transition temperature. We reveal here that the effect of thermal disorder, which decreases the condensate phase coherence at equilibrium, can be dynamically contrasted by photoexcitation with ultrashort midinfrared pulses. In particular, our findings reveal that light pulses with photon energy comparable to the amplitude of the superconducting gap and polarized in plane along the copper-copper direction can dynamically enhance the optical response associated with the onset of superconductivity. We propose that this effect can be rationalized by an effective d-wave BCS model, which reveals that midinfrared pulses result in a transient increase of the phase coherence.

Locating the Missing Superconducting Electrons in the Overdoped Cuprates La_{2-x}Sr_{x}CuO_{4}

Overdoped high-temperature cuprate superconductors have often been understood within the standard BCS framework of superconductivity. However, measurements in a variety of overdoped cuprates indicate that the superfluid density is much smaller than expected from BCS theory and decreases smoothly to zero as the doping is increased. Here, we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on the same overdoped La_{2-x}Sr_{x}CuO_{4} films to determine the total, superfluid, and uncondensed spectral weight as a function of doping. A significant fraction of the carriers remains uncondensed in a wide Drude-like peak as T→0, while the superfluid density remains linear in temperature. These observations are seemingly inconsistent with existing, realistic theories of impurity scattering suppressing the superfluid density in a BCS-like d-wave superconductor. Our large measurement frequency range gives us a unique look at the low frequency spectral weight distribution, which may suggest the presence of quantum phase fluctuations as the critical doping is approached.

Relaxation of strongly coupled electron and phonon fields after photoemission and high-energy part of ARPES spectra of cuprates

An approach to considering systems with a high concentration of correlated carriers and strong long-range electron-phonon interaction and to calculating the high-energy part of the angle-resolved photoemission spectroscopy (ARPES) spectra of such systems is suggested. Joint relaxation of strongly coupled fields-a field of correlated electrons and phonon field-after photoemission is studied to clarify the nature of characteristic features observed in the high-energy part of the ARPES spectra of cuprate superconductors. Such relaxation occurs in systems with strong predominantly long-range electron-phonon interaction at sufficiently high carrier concentration due to the coexistence of autolocalized and delocalized carriers. A simple method to calculate analytically a high-energy part of the ARPES spectrum arising is proposed. It takes advantage of using the coherent states basis for the phonon field in the polaron and bipolaron states. The approach suggested yields all the high-energy spectral features like broad Gaussian band and regions of 'vertical dispersion' being in good quantitative agreement with the experiments on cuprates at any doping with both types of carriers. Demonstrated coexistence of autolocalized and delocalized carriers in superconducting cuprates changes the idea about their ground state above the superconducting transition temperature that is important for understanding transport and magnetic properties. High density of large-radius autolocalized carriers revealed may be a key to the explanation of charge ordering in doped cuprates.

Unconventional superconductivity in generalized Hubbard model: role of electron-hole symmetry breaking terms

We investigate the effect of the electron-hole (e-h) symmetry breaking on d-wave superconductivity induced by non-local effects of correlations in the generalized Hubbard model. The symmetry breaking is introduced in a two-fold manner: by the next-to-nearest neighbor hopping of electrons and by the charge-bond interaction-the off-diagonal term of the Coulomb potential. Both terms lead to a pronounced asymmetry of the superconducting order parameter. The next-to-nearest neighbor hopping enhances superconductivity for h-doping, while diminishes it for e-doping. The charge-bond interaction alone leads to the opposite effect and, additionally, to the kinetic-energy gain upon condensation in the underdoped regime. With both terms included, with similar amplitudes, the height of the superconducting dome and the critical doping remain in favor of h-doping. The influence of the charge-bond interaction on deviations from [Formula: see text] symmetry of the shape of the gap at the Fermi surface in the momentum space is briefly discussed.

Optical studies of high-temperature superconducting cuprates

The optical studies of high-temperature superconducting cuprates (HTSC) are reviewed. From the doping dependence of room temperature spectra, a dramatic change of the electronic state from a Mott (charge transfer) insulator to a Fermi liquid has been revealed. Additionally, the unusual 2D nature of the electronic state has been found. The temperature dependence of the optical spectra provided a rich source of information on the pseudogap, superconducting gap, Josephson plasmon, transverse Josephson plasma mode and precursory superconductivity. Among these issues, Josephson plasmons and transverse Josephson plasma mode were experimentally discovered by optical measurements, and thus are unique to HTSC. The effect of the spin/charge stripe order is also unique to HTSC, reflecting the conducting nature of the stripe order in this system. The pair-breaking due to the stripe order seems stronger in the out-of-plane direction than in the in-plane one.

Unconventional High-Energy-State Contribution to the Cooper Pairing in the Underdoped Copper-Oxide Superconductor HgBa_{2}Ca_{2}Cu_{3}O_{8+δ}

We study the temperature-dependent electronic B_{1g} Raman response of a slightly underdoped single crystal HgBa_{2}Ca_{2}Cu_{3}O_{8+δ} with a superconducting critical temperature T_{c}=122  K. Our main finding is that the superconducting pair-breaking peak is associated with a dip on its higher-energy side, disappearing together at T_{c}. This result reveals a key aspect of the unconventional pairing mechanism: spectral weight lost in the dip is transferred to the pair-breaking peak at lower energies. This conclusion is supported by cellular dynamical mean-field theory on the Hubbard model, which is able to reproduce all the main features of the B_{1g} Raman response and explain the peak-dip behavior in terms of a nontrivial relationship between the superconducting gap and the pseudogap.

An organizing principle for two-dimensional strongly correlated superconductivity

Superconductivity in the cuprates exhibits many unusual features. We study the two-dimensional Hubbard model with plaquette dynamical mean-field theory to address these unusual features and relate them to other normal-state phenomena, such as the pseudogap. Previous studies with this method found that upon doping the Mott insulator at low temperature a pseudogap phase appears. The low-temperature transition between that phase and the correlated metal at higher doping is first-order. A series of crossovers emerge along the Widom line extension of that first-order transition in the supercritical region. Here we show that the highly asymmetric dome of the dynamical mean-field superconducting transition temperature , the maximum of the condensation energy as a function of doping, the correlation between maximum and normal-state scattering rate, the change from potential-energy driven to kinetic-energy driven pairing mechanisms can all be understood as remnants of the normal state first-order transition and its associated crossovers that also act as an organizing principle for the superconducting state.

Electronic correlations and unconventional spectral weight transfer in the high-temperature pnictide BaFe(2-x)Co(x)As(2) superconductor using infrared spectroscopy

We report an infrared optical study of the pnictide high-temperature superconductor BaFe(1.84)Co(0.16)As(2) and its parent compound BaFe(2)As(2). We demonstrate that electronic correlations are moderately strong and do not change across the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hund's coupling J is the primary mechanism that gives rise to correlations.

Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates

In strongly correlated systems the electronic properties at the Fermi energy (E(F)) are intertwined with those at high-energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high-energy scale physics associated with Mott-like excitations (|E-E(F)|>1 eV) is involved in the condensate formation. Here, we report the interplay between the many-body high-energy CuO(2) excitations at 1.5 and 2 eV, and the onset of HTSC. This is revealed by a novel optical pump-supercontinuum-probe technique that provides access to the dynamics of the dielectric function in Bi(2)Sr(2)Ca(0.92)Y(0.08)Cu(2)O(8+δ) over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (T(c)).

Superconductivity-induced optical anomaly in an iron arsenide

One of the central tenets of conventional theories of superconductivity, including most models proposed for the recently discovered iron-pnictide superconductors, is the notion that only electronic excitations with energies comparable to the superconducting energy gap are affected by the transition. Here, we report the results of a comprehensive spectroscopic ellipsometry study of a high-quality crystal of superconducting Ba₀.₆₈K₀.₃₂Fe₂As₂ that challenges this notion. We observe a superconductivity-induced suppression of an absorption band at an energy of 2.5 eV, two orders of magnitude above the superconducting gap energy 2Δ≈20 meV. On the basis of density functional calculations, this band can be assigned to transitions from As-p to Fe-d orbitals crossing the Fermi level. We identify a related effect at the spin-density wave transition in parent compounds of the 122 family. This suggests that As-p states deep below the Fermi level contribute to the formation of the superconducting and spin-density wave states in the iron arsenides.

High-temperature optical spectral weight and fermi-liquid renormalization in bi-based cuprate superconductors

The optical conductivity σ(ω) and the spectral weight W(T) of two superconducting cuprates at optimum doping, Bi2Sr2-xLaxCuO6 and Bi2Sr2CaCu2O8, have been first measured up to 500 K. Above 300 K, W(T) deviates from the usual T2 behavior in both compounds, even though σ(ω→0) remains larger than the Ioffe-Regel limit. The deviation is surprisingly well described by the T4 term of the Sommerfeld expansion, but its coefficients are enhanced by strong correlation, as shown by the good agreement with dynamical mean field calculations.

Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

The mechanism of electron pairing in high-temperature superconductors is still the subject of intense debate. Here, we provide direct evidence of the role of structural dynamics, with selective atomic motions (buckling of copper-oxygen planes), in the anisotropic electron-lattice coupling. The transient structures were determined using time-resolved electron diffraction, following carrier excitation with polarized femtosecond heating pulses, and examined for different dopings and temperatures. The deformation amplitude reaches 0.5% of the c axis value of 30 A when the light polarization is in the direction of the copper-oxygen bond, but its decay slows down at 45 degrees. These findings suggest a selective dynamical lattice involvement with the anisotropic electron-phonon coupling being on a time scale (1-3.5 ps depending on direction) of the same order of magnitude as that of the spin exchange of electron pairing in the high-temperature superconducting phase.

Anomalous criticality in the electrical resistivity of La2-xSrxCuO4

The presence or absence of a quantum critical point and its location in the phase diagram of high-temperature superconductors have been subjects of intense scrutiny. Clear evidence for quantum criticality, particularly in the transport properties, has proved elusive because the important low-temperature region is masked by the onset of superconductivity. We present measurements of the low-temperature in-plane resistivity of several highly doped La2-xSrxCuO4 single crystals in which the superconductivity had been stripped away by using high magnetic fields. In contrast to other quantum critical systems, the resistivity varies linearly with temperature over a wide doping range with a gradient that scales monotonically with the superconducting transition temperature. It is maximal at a critical doping level (pc) approximately 0.19 at which superconductivity is most robust. Moreover, its value at pc corresponds to the onset of quasi-particle incoherence along specific momentum directions, implying that the interaction that first promotes high-temperature superconductivity may ultimately destroy the very quasi-particle states involved in the superconducting pairing.

Probing the superconducting energy gap from infrared spectroscopy on a Ba0.6K0.4Fe2As2 single crystal with Tc=37 K

We performed optical spectroscopy measurement on a superconducting Ba0.6K0.4Fe2As2 single crystal with T{c}=37 K. Formation of the superconducting energy gaps in the far-infrared reflectance spectra below T{c} is clearly observed. A flat and close to unity reflectance is observed roughly below 150 cm;{-1} for T<<T{c}, following an s-wave pairing line shape. A more rapid decrease occurs near 200 cm;{-1}, leading to a peak in the ratio of the reflectance at T<<T{c} over that for T>or=T{c}. We determined the absolute value of the penetration depth at 10 K as lambda approximately 2000+/-80 A. A spectral weight analysis shows that the Ferrell-Glover-Tinkham sum rule is satisfied at low energy scale, less than 6Delta.

Sum rules and interlayer infrared response of the high temperature YBa2Cu3Oy superconductor in an external magnetic field

We present infrared magneto-optical measurements of the c-axis conductivity of YBa2Cu3Oy in both the underdoped (y=6.67 and 6.75) and optimally doped (y=6.95) regimes. We show that modest c-axis magnetic fields radically modify the condensate formation and restore conventional BCS-like energetics. Additionally, we demonstrate the pivotal role of interplane coherence in the anomalous high-energy contribution to the superfluid density.

Observation of a 500 meV collective mode in La2-xSrxCuO4 and Nd2CuO4 using resonant inelastic X-ray scattering

Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of La2-xSrxCuO4 and Nd2CuO4 at 500 meV. The mode is peaked around the (pi, 0) point in reciprocal space and is observed to soften, and broaden, away from this point. Samples with x=0, 0.01, 0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed with increasing Sr content and is absent at x=0.17, where it is replaced by a continuum of excitations. This mode is only observed when the incident x-ray polarization is normal to the CuO planes.

Pseudogap dependence of the optical conductivity spectra of Ca3Ru2O7: a possible contribution of the orbital flip excitation

Optical spectra of a double-layered perovskite ruthenate Ca3Ru2O7 show a pseudogap opening around 200 cm(-1) below 50 K, which is attributable to the partial k-space gap opening due to the density wave instability. Unlike most other density wave materials, Ca3Ru2O7 has spectral weight redistributions, not near the energy gap region, but at a much higher energy region around 800 cm(-1). As a possible origin of these intriguing features, we discuss the orbital flip excitation in the density wave ground state.

Abrupt transition in quasiparticle dynamics at optimal doping in a cuprate superconductor system

We report time-resolved measurements of the photoinduced change in reflectivity, DeltaR, in the Bi2Sr2Ca(1-y)Dy(y)Cu2O8+delta (BSCCO) system of cuprate superconductors as a function of hole concentration. We find that the kinetics of quasiparticle decay and the sign of DeltaR both change abruptly where the superconducting transition temperature T(c) is maximal. These coincident changes suggest that a sharp transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system.

Temperature dependence of the optical spectral weight in the cuprates: role of electron correlations

We compare calculations based on the dynamical mean-field theory of the Hubbard model with the infrared spectral weight W(Omega,T) of La(2-x)SrxCuO4 and other cuprates. Without using fitting parameters we show that most of the anomalies found in W(Omega,T) with respect to normal metals, including the existence of two different energy scales for the doping and the T dependence of W(Omega,T), can be ascribed to strong correlation effects.

Gaplike excitations in the superconducting state of Bi2Sr2CaCu2O8 studied by resonant Raman scattering

We report on the response of the electronic continuum from inelastic light-scattering experiments over an extended energy range between 1.970 and 4.504 eV in the superconducting state of Bi2Sr2CaCu2O8. The formation of a substantial Raman feature at shifts below twice the superconducting gap as well as the additional weight above this energy are found to be strongly dependent on the incident photon energy. For excitation wavelengths observed in ultraviolet, we find an enhancement of the integrated spectral weight below T(c). The resulting composite feature shows three distinct resonances at 2.5, 3.3, and 3.8 eV. We strongly suggest that the superconductivity-induced changes are the result of both the opening of a superconducting gap and the appearance of a collective mode.

Optical sum rules that relate to the potential energy of strongly correlated systems

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).

Frequency-dependent thermal response of the charge system and the restricted sum rules of La2-xSrxCuO4

In La2-xSrxCuO4 (LSCO) the spectral weight W=integralOmega0sigma(ab)1(omega,T)domega [where sigma(ab)1(omega,T) is the ab-plane conductivity] obeys the same law W=W0-BOmegaT2 as in a conventional metal such as gold, for any Omega up to the plasma edge. However, in LSCO BOmega points toward correlation effects and, unlike in gold, is related to an energy scale tT<<t0 approximately W0. The Ferrell-Glover-Tinkham sum rule is fulfilled in LSCO superconductors for Omega greater than or approximately equal to 2000 cm(-1).

Absence of asymptotic freedom in doped mott insulators: breakdown of strong coupling expansions

We show that doped Mott insulators such as the copper-oxide superconductors are asymptotically slaved in that the quasiparticle weight Z near half-filling depends critically on the existence of the high-energy scale set by the upper Hubbard band. In particular, near half-filling, the following dichotomy arises: Z not equal to 0 when the high-energy scale is integrated out but Z=0 in the thermodynamic limit when it is retained. Slavery to the high-energy scale arises from quantum interference between electronic excitations across the Mott gap. Broad spectral features seen in photoemission in the normal state of the cuprates are argued to arise from high-energy slavery.

Strongly correlated superconductivity and pseudogap phase near a multiband Mott insulator

Near a Mott transition, strong electron correlations may enhance Cooper pairing. This is demonstrated in the dynamical mean field theory solution of a twofold-orbital degenerate Hubbard model with an inverted on-site Hund rule exchange, favoring local spin-singlet configurations. Close to the Mott insulator (which here is a local version of a valence bond insulator) a pseudogap non-Fermi-liquid metal, a superconductor, and a normal metal appear, in striking similarity with the physics of cuprates. The strongly correlated s-wave superconducting state has a larger Drude weight than the corresponding normal state. The role of the impurity Kondo problem is underscored.

In-Plane Spectral Weight Shift of Charge Carriers in YBa 2 Cu 3 O 6.9

The temperature-dependent redistribution of the spectral weight of the CuO2 plane-derived conduction band of the YBa2Cu3O6.9 high-temperature superconductor (superconducting transition temperature = 92.7 kelvin) was studied with wide-band (0.01- to 5.6-electron volt) spectroscopic ellipsometry. A superconductivity-induced transfer of the spectral weight involving a high-energy scale in excess of 1 electron volt was observed. Correspondingly, the charge carrier spectral weight was shown to decrease in the superconducting state. The ellipsometric data also provide detailed information about the evolution of the optical self-energy in the normal and superconducting states.

Kinetic energy driven pairing in cuprate superconductors

Pairing occurs in conventional superconductors through a reduction of the electronic potential energy accompanied by an increase in kinetic energy. In the underdoped cuprates, optical experiments show that pairing is driven by a reduction of the electronic kinetic energy. Using the dynamical cluster approximation we study superconductivity in the two-dimensional Hubbard model. We find that pairing is indeed driven by the kinetic energy and that superconductivity evolves from an unconventional state with partial spin-charge separation, to a superconducting state with quasiparticle excitations.

Quantum critical behaviour in a high-Tc superconductor

Quantum criticality is associated with a system composed of a nearly infinite number of interacting quantum degrees of freedom at zero temperature, and it implies that the system looks on average the same regardless of the time- and length scale on which it is observed. Electrons on the atomic scale do not exhibit such symmetry, which can only be generated as a collective phenomenon through the interactions between a large number of electrons. In materials with strong electron correlations a quantum phase transition at zero temperature can occur, and a quantum critical state has been predicted, which manifests itself through universal power-law behaviours of the response functions. Candidates have been found both in heavy-fermion systems and in the high-transition temperature (high-T(c)) copper oxide superconductors, but the reality and the physical nature of such a phase transition are still debated. Here we report a universal behaviour 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 quantum phase transition of an unconventional kind in the high-T(c) superconductors.

ab plane ac conductivity in the cuprates: pseudogap effects below Tc

Building on our understanding of the superfluid density rho(s)(T), we show how the pseudogap enters the in-plane optical conductivity sigma(omega,T) for temperatures T</=T(c). Signatures in sigma(omega,T) associated with competing proposals of a "hidden order" and "superconducting" origin for the pseudogap differ in their qualitative omega,T dependences. For the latter case, as T increases from 0, Re sigma(omega) is enhanced at high and low frequencies. We discuss these theories in light of current experiments.