It is inferred from bulk-sensitive muon Knight shift measurement for a Bi$_{1.76}$Pb$_{0.35}$Sr$_{1.89}$CuO$_{6+δ}$ single-layer cuprate that metal-insulator (MI) transition (in the low temperature limit, $T\rightarrow0$) occurs at the critical hole concentration $p=p_{\rm MI}=0.09(1)$, where the electronic density of states (DOS) at the Fermi level is reduced to zero by the pseudogap irrespective of the Néel order or spin glass magnetism. Superconductivity also appears for $p>p_{\rm MI}$, suggesting that this feature is controlled by the MI transition. More interestingly, the magnitude of the DOS reduction induced by the pseudogap remains unchanged over a wide doping range ($0.1\le p\le0.2$), indicating that the pseudogap remains as a hallmark of the MI transition for $p>p_{\rm MI}$.
The low energy magnetic excitation spectrum of the Heisenberg antiferromagnetic $S = 1/2$ chain system Sr$_2$CuO$_3$ with Ni- and Ca-impurities is studied by neutron spectroscopy. In all cases, a defect-induced spectral pseudogap is observed and shown to scale proportionately to the number of scattering centers in the spin chains.
The $S=1/2$ antiferromagnetic Heisenberg spin chain compound Sr$_2$CuO$_3$ doped with $1 \%$ and $2 \%$ of Ni impurities has been studied by means of $^{63}$Cu nuclear magnetic resonance. A strong decrease of the spin-lattice relaxation rate $T_1^{-1}$ at low temperatures points toward a spin gap, while a stretching exponent $λ< 1$ and a frequency dependence of $T_1^{-1}$ indicate that this spin gap varies spatially and should rather be characterized as a spin pseudogap. The magnitude of the spin pseudogap scales with doping level. Our results therefore evidence the finite-size character of th…
While magnetic fields generally compete with superconductivity, a type II superconductor can persist to very high fields by confining the field in topological defects, namely vortices. We propose that a similar physics underlies the pseudogap phase in cuprates, where the relevant topological defects are the antiphase domain walls of an underlying antiferromagnetic (AFM) order. A key consequence of this scenario is that the termination of the pseudogap phase should be quantitatively described by the underlying AFM model. We demonstrate that this picture can explain a number of key experimentall…
Spectral functions are evaluated numerically within the t-t'-J model as relevant for electron-doped cuprates. The Fermi surface develops from a pocket-like into a large one with doping. The corresponding pseudogap in the nodal direction is well resolved at low doping and vanishes at intermediate doping. Its presence is strongly temperature dependent and thus connected to longer-range antiferromagnetic correlations. Features including double effective band and optical conductivity are consistent with experiments on electron-doped cuprates without invoking the closing of the Mott-Hubbard gap.
We have performed a temperature-dependent angle-integrated photoemission study of lightly-doped to heavily-overdoped La$_{2-x}$Sr$_{x}$CuO$_4$ and oxygen-doped La$_2$CuO$_{4.10}$. We found that both the magnitude $Δ$* of the (small) pseudogap and the temperature \textit{T}* at which the pseudogap is opened increases with decreasing hole concentration, consistent with previous studies. On the other hand, the superconducting gap $Δ_{sc}$ was found to remain small for decreasing hole concentration. The results can be explained if the superconducting gap opens only on the Fermi arc around the noda…
We have performed high-resolution angle-resolved photoemission spectroscopy on single-layered cuprate Bi$_2$Sr$_2$CuO$_6$ to clarify the origin of the pseudogap. By using various photon energies, we have succeeded in directly observing two different pseudogaps with two different energy scales which coexist in the antinodal region: one reflects the $d_{x^2-y^2}$-wave pairing strength while the other has a larger energy scale suggesting an origin distinct from superconductivity. The observed two-pseudogap behavior provides a key to fully understand the pseudogap phenomena in cuprates.
Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of compounds, there exists no information for structurally simple HgBa$_2$CuO$_{4+δ}$. Here we report neutron scattering results for HgBa$_2$CuO$_{4+δ}$ (superconducting transition temperature T$_c$ $\sim$ 71 K, pseudogap temperature T* $\sim$ 305 K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of t…
We calculate the magnetic susceptibility in the pseudogap phase of cuprates using the two-dimensional $t-J$ model and the Mori projection operator technique. In this phase, the Fermi arcs lead to a quasi-elastic incommensurate magnetic response for low temperatures. The theory accounts for the reorientation of the susceptibility maxima from the axial to the diagonal direction occurring at small hole concentrations in La$_{2-x}$Sr$_x$CuO$_4$. A small cusp of the hole dispersion near the Fermi level, which is connected with the spin-polaron band, affords the growth of the maxima with decreasing …
We report measurements of the specific heat and the thermal conductivity of the model Heisenberg spin-1/2 chain cuprate Sr$_{2}$CuO$_{3}$ at low temperatures. In addition to a nearly isotropic phonon heat transport, we find a quasi one-dimensional excess thermal conductivity along the chain direction, most likely associated with spin excitations (spinons). The spinon energy current is limited mainly by scattering on defects and phonons. Analyzing the specific heat data, the intrachain magnetic exchange $J/k_{B}$ is estimated to be 2650 K.
We report a doping dependant Electronic Raman Scattering (ERS) study of HgBa$_2$CuO$_{4+δ}$ (Hg-1201) single crystals. We investigate the dynamics of the antinodal and nodal quasiparticles. We show that the dynamical response of the antinodal quasiparticles is strongly reduced towards the underdoped regime in both the normal and superconducting states. When probing the nodal quasiparticles, we are able to distinguish between the energy scale of the pseudogap and that of the superconducting gap. A simple model relating the suppression of the dynamical response of the antinodal quasiparticles to…
The central issue in high-temperature cuprate superconductors is the pseudogap state appearing below the pseudogap temperature $T^*$, which is well above the superconducting transition temperature. In this study, we theoretically investigate the rapid increase of the magnetic anisotropy below the pseudogap temperature detected by the recent torque-magnetometry measurements on YBa$_2$Cu$_3$O$_y$ [Y. Sato et al., Nat. Phys., 13, 1074 (2017)]. Applying the spin Green's function formalism including the Dzyaloshinskii--Moriya interaction arising from the buckling of the CuO$_2$ plane, we obtain res…
The theoretical description of the anomalous properties of the pseudogap phase in the underdoped region of the cuprate phase diagram lags behind the progress in spectroscopic and other experiments. A phenomenological ansatz, based on analogies to the approach to Mott localization at weak coupling in lower dimensional systems, has been proposed by Yang, Rice and Zhang [Phys. Rev. B 73 (2006),174501]. This ansatz has had success in describing a range of experiments. The motivation underlying this ansatz is described and the comparisons to experiment are reviewed. Implications for a more microsco…
The pseudogap state is widely regarded as a precursor to high-temperature superconductivity yet remains poorly understood. Using high field (30 T) NMR, we report two electronically distinct oxygen sites within the copper-oxygen (Cu-O) planes of an underdoped cuprate in the pseudogap state. At temperatures well above the bulk superconducting transition, roughly 25% of the oxygen sites evidence superconducting pair formation, the remaining evidence antiferromagnetic ordering on the nearest neighbor Cu sites. Any superconducting fluctuations are slower than the microsecond timescales of the NMR p…
To examine the so-called small pseudogap and the superconducting (SC) condensation energy U(0), the electronic specific heat Cel was measured on La_{2-x}Sr_{x}CuO_{4} up to ~120K. In samples with doping level p (=x) less than ~0.2, small pseudogap behavior appears in the γ(=Cel/T) vs. T curve around the mean-field critical temperature for a d-wave superconductor Tco (=2*Δ_{0}/(4~5)k_B), where Δ_{0} is the maximum gap at T<<Tc. The condensation energy U(0) is largely reduced in the pseudogap regime (p< ~0.2). The reduction of U(0) can be well reproduced by introducing an effective SC energy sca…
We present the first demonstration of vortices in an electron-type cuprate superconductor, the highest $T_c$ (= 43 K) electron-type cuprate $Sr_{0.9}La_{0.1}CuO_2$. Our spatially resolved quasiparticle tunneling spectra reveal a hidden low-energy pseudogap inside the vortex core and unconventional spectral evolution with temperature and magnetic field. These results cannot be easily explained by the scenario of pure superconductivity in the ground state of high-$T_c$ superconductivity.
The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity. In particular, whether the pseudogap is associated with a crossover or phase transition has been a long-standing controversial issue. The tetragonal cuprate HgBa$_2$CuO$_{4+δ}$, with only one CuO$_2$ layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the anisotropy of magnetic susceptibility within the CuO$_2$ plane with exceptionally high-precision magnetic torque experiments. Our key finding is that a distinct two-fold in-plane anisotropy…
A proposal to test the proposed time-reversal and inversion breaking phase in the Pseudogap region of the Cuprate compounds through the variation of Second-harmonic generation intensity with temperature and polarization and angle of incidence is presented.
A mean field SDW analysis of pseudogap in the underdoped cuprates is proposed on the basis of the $t-t^{\prime}-U$ Hubbard model. The prediction of our theory is consistent with the experiment quite well within the uncertainty of the present experimental measurement. Therefore we argue that the pseudogap phenomenon in the underdoped cuprates can be well explained within the mean field approximation.
The interplay between superconductivity and the pseudogap is an important aspect of cuprate physics. However, the nature of the pseudogap remains controversial, in part because different experiments have suggested different gap functions. Here we present a photon-energy-dependence angle-resolved photoemission spectroscopy (ARPES) study on Bi$_{1.5}$Pb$_{0.55}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$. We find that antinodal ARPES spectra at low photon energies are dominated by background signals which can lead to a misevaluation of the spectral gap size. Once background is properly accounted for, indepe…
The specific heat $C$ of the single-layer cuprate superconductor HgBa$_2$CuO$_{4 + δ}$ was measured in an underdoped crystal with $T_{\rm c} = 72$ K at temperatures down to $2$ K in magnetic fields up to $35$ T, a field large enough to suppress superconductivity at that doping ($p \simeq 0.09$). In the normal state at $H = 35$ T, a residual linear term of magnitude $γ= 12 \pm 2$ mJ/K$^2$mol is observed in $C/T$ as $T \to 0$, a direct measure of the electronic density of states. This high value of $γ$ has two major implications. First, it is significantly larger than the value measured in overd…
We suggest a model for electronic structure of cuprate superconductors that makes it possible to describe evolution of this structure with the doping and provides a new explanation for a number of typical features of cuprates, including the pseudogap and the Fermi arcs. According to this model, the unique electronic structure of cuprates is favorable for the formation of two-atomic negative-U centers (NUCs) and realization of a peculiar mechanism of the electron-electron interaction.
The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La$_2$CuO$_4$ were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders - Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$ - and show that two potential mechanisms can be excluded - the scattering of phonons by rare-earth impurities and by structural domains. Ou…
We study the pseudogap present in cuprate and organic superconductors. We use the dynamical cluster approximation (DCA), treating a cluster embedded in a bath. As the Coulomb interaction is increased, cluster-bath Kondo states are destroyed and bound cluster states formed. We show that this leads to a pseudogap. Due to weaker coupling to the bath for the anti-nodal point, this happens first for this point, explaining the k-dependence of the pseudogap. The pseudogap can be understood in terms of preformed d-wave pairs, but it does not prove the existence of such pairs.
A method is proposed to test for the nature of the pseudogap phase in cuprates using the recently developed technique of Fourier transform scanning tunneling spectroscopy. We show that the observed quasiparticle interference patterns depend critically on the quasiparticle coherence factors, making it possible to distinguish between the pseudogap dominated by superconducting fluctuations and by various particle-hole condensates.
We argue that incoherent pair tunneling in a cuprate superconductor junction with an optimally doped superconducting and an underdoped normal lead can be used to detect the presence of pairing correlations in the pseudogap phase of the underdoped lead. We estimate that the junction characteristics most suitable for studying the pair tunneling current are close to recently manufactured cuprate tunneling devices.
The ab plane optical spectra of two single crystals of La(2-x)Sr(x)Cu(4), one overdoped and one underdoped, were investigated. We observe a gap-like depression of the effective scattering rate below 700 cm(-1) in both systems. This feature persists up to 300 K in the underdoped sample with the concentration of Sr x=0.14 but loses prominance at temperatures above 300 K in the overdoped regime (x=0.22). Below 700 cm(-1) the scattering rate is temperature dependent and superlinear in frequency for both samples. Above this frequency the effective scattering rate becomes linear in frequency and is …
In the course of seeking the microscopic mechanism of superconductivity in cuprate high temperature superconductors, the pseudogap phase\textemdash the very abnormal 'normal' state on the hole-doped side\textemdash has proven to be as big of a quandary as superconductivity itself. Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for assessing the momentum-dependent phenomenology of the pseudogap, and recent technological developments have permitted a more detailed understanding. This report reviews recent progress in understanding the relationship between superconductivity …
During the last decade, translational and rotational symmetry-breaking phases -- density wave order and electronic nematicity -- have been established as generic and distinct features of many correlated electron systems, including pnictide and cuprate superconductors. However, in cuprates, the relationship between these electronic symmetry-breaking phases and the enigmatic pseudogap phase remains unclear. Here, we employ resonant x-ray scattering in a cuprate high-temperature superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ (Nd-LSCO) to navigate the cuprate phase diagram, probing the rel…
One of the most intriguing aspects of cuprates is a large pseudogap coexisting with a high superconducting transition temperature. Here, we study pairing in the cuprates from electron-electron interactions by constructing the pair vertex using spectral functions derived from angle resolved photoemission data for a near optimal doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ sample that has a pronounced pseudogap. Assuming that that the pseudogap is {\it not} due to pairing, we find that the superconducting instability is strongly suppressed, in stark contrast to what is actually observed. Using an analyti…
Irrespective of the class they belong to, all the hole doped high-Tc cuprate superconductors show an anti-correlation between the superconducting transition temperature and the characteristic pseudogap energy in the underdoped region. The doping dependent pseudogap in the quasiparticle spectral density is believed to remove low-energy electronic states and thereby reduce the superconducting condensate. Impurities within the CuO2 plane, on the other hand, break Cooper pairs in the unitarity limit and diminish superfluid density. Both pseudogap in pure cuprates and impurity scattering in disorde…
We calculate scattering interference patterns for various electronic states proposed for the pseudogap regime of the cuprate superconductors. The scattering interference models all produce patterns whose wavelength changes as a function of energy, in contradiction to the energy-independent wavelength seen by scanning tunneling microscopy (STM) experiments in the pseudogap state. This suggests that the patterns seen in STM local density of states measurements are not due to scattering interference, but are rather the result of some form of ordering.
A new electron-doped superconductor Li_x_Sr_2_CuO_2_Br_2_ with x = 0.15 has successfully been synthesized by an electrochemical Li-intercalation technique. The magnetic susceptibility shows superconductivity of bulk with the superconducting transition temperature Tc = 8 K. This compound is the first electron-doped superconducting cuprate with the K_2_NiF_4_ structure.
The "states-non-conserving" fermion density of states (DOS), deduced from the specific heat of hole-doped cuprates, could arise from a Kondo or heavy fermion-like DOS being suppressed by anti-ferromagnetic spin fluctuations. The large Fermi surface predicted by band theory and observed experimentally, is still expected for zero pseudogap, but with an effective mass corresponding to a Kondo temperature ~ 800 K. A finite pseudogap could divide it into Fermi arcs. Theoretical results for the asymmetric Anderson model can account for the experimental Wilson ratio.
The pairing state symmetry of the electron-doped cuprate superconductors is thought to be s-wave in nature, in contrast with their hole-doped counterparts which exhibit a d-wave symmetry. We re-examine this issue based on recent improvements in our electron-doped materials and our measurement techniques. We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth of Pr_(2-x)Ce_(x)CuO_(4-y) and Nd_(2-x)Ce_(x)CuO_(4-y) crystals. Our data strongly suggest that the pairing symmetry in these materials is not s-wave.
I review some of the experimental evidence and theoretical arguments that suggest that pseudogap matter is a new form of matter that coexists with coherent electron matter in the normal state and with superconducting matter below the superconducting transition temperature. I describe work in progress on a phenomenological two-fluid description of the evolution of pseudogap behavior that offers an explanation for the unexpectedly simple scaling behavior for the uniform magnetic susceptibility found in the underdoped cuprates and use this to propose a physical picture of the underdoped cuprates …
We reveal the full energy-momentum structure of the pseudogap of underdoped high-Tc cuprate superconductors. Our combined theoretical and experimental analysis explains the spectral-weight suppression observed in the B2g Raman response at finite energies in terms of a pseudogap appearing in the single-electron excitation spectra above the Fermi level in the nodal direction of momentum space. This result suggests an s-wave pseudogap (which never closes in the energy-momentum space), distinct from the d-wave superconducting gap. Recent tunneling and photoemission experiments on underdoped cuprat…
The real-space localized hole pair is constructed in the Cu-O plane of the hole-doped cuprate superconductors. We prove analytically and numerically that two electrons, due to the nearest-neighbor Coulomb repulsive confinement effect, can be in pairing inside a single plaquette with the d-wave symmetry. The scenario supports the `no glue' pairing picture for the cuprates. Based on the scenarios, the physical origin of the Fermi pocket (or Fermi arc) and the two pseudogap behavior are provided. Our framework leads directly to a unified linear relationship between the pseudogap temperature T* an…
We investigate the microscopic origin of the pseudogap in the weakly doped 2D Hubbard model using Quantum Monte Carlo within the dynamical cluster approximation. We compare our results with proposed scenarios for the pseudogap. All our numerical evidence is in favor of spin-charge separation as described in the resonating valence bond picture as the cause of the pseudogap behavior. Scenarios of "preformed pairs", the coupling of quasiparticles to antiferromagnetic spin-fluctuations and stripes are inconsistent with our results.
We consider the scenario where a 4-lattice constant, rotationally symmetric charge density wave (CDW) is present in the underdoped cuprates. We prove a theorem that puts strong constraint on the possible form factor of such a CDW. We demonstrate, within mean-field theory, that a particular form factor within the allowed class describes the angle-resolved photoemission and scan tunneling spectroscopy well. We conjecture that the ``large pseudogap'' in cuprates is the consequence of this type of charge density wave.
We describe the spectral properties of underdoped cuprates as resulting from a momentum-dependent pseudogap in the normal state spectrum. Such a model accounts, within a BCS approach, for the doping dependence of the critical temperature and for the two-parameter leading-edge shift observed in the cuprates. By introducing a phenomenological temperature dependence of the pseudogap, which finds a natural interpretation within the stripe quantum-critical-point scenario for high-T_c superconductors, we reproduce also the T_c-T^* bifurcation near optimum doping. Finally, we briefly discuss the diff…
In this paper we show that local spin-singlet amplitude with d-wave symmetry can be induced by short-range spin correlations even in the absence of pairing interactions. In the present scenario for the pseudogap, the normal state pseudogap is caused by the induced local spin-singlet amplitude due to short-range spin correlations, which compete in the low energy sector with superconducting correlations to make $T_{c}$ go to zero near half-filling.
We analyze pseudogap phenomena widely observed in the underdoped cuprates. We assume the existence of a strong d-wave pairing force competing with antiferromagnetic(AFM) fluctuations and the formation of flat and damped dispersion around the $(π,0)$ and $(0,π)$ region as two important elements caused by the proximity from the Mott insulator. Using the mode-mode coupling theory for the d-wave superconducting(dSC) and AFM fluctuations, we reproduce basic properties of the pseudogap seen in the magnetic resonance, neutron scattering, angle resolved photoemission and tunneling measurements in the …
Recent angle resolved photoemission spectrascope (ARPES) experiments on strongly underdoped Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ cuprates have reported an unusual gap in the nodal direction. Transport experiments on these cuprates found variable range hopping behavior observed. These cuprates have both electron and hole doping which has led to proposals that this cuprate is analogous to a partially compensated semiconductors. The nodal gap then corresponds to the Efros-Schklovskii(ES) gap in such semiconductors. We calculate the doping dependence and temperature dependence of a ES gap model and f…
We propose that an extension of the exciton concept to doped Mott insulators offers a fruitful insight into challenging issues of the copper oxide superconductors. In our extension, new fermionic excitations called cofermions emerge in conjunction to generalized excitons. The cofermions hybridize with conventional quasiparticles. Then a hybridization gap opens, and is identified as the pseudogap observed in the underdoped cuprates. The resultant Fermi-surface reconstruction naturally explains a number of unusual properties of the underdoped cuprates, such as the Fermi arc and/or pocket formati…
The thermopower S of the high-Tc superconductor La(1.6-x)Nd(0.4)Sr(x)CuO(4) was measured as a function of temperature T near its pseudogap critical point, the critical hole doping p* where the pseudogap temperature T* goes to zero. Just above p*, S/T varies as ln(1/T) over a decade of temperature. Below p*, S/T undergoes a large increase below T*. As with the temperature dependence of the resistivity, which is linear just above p* and undergoes a large upturn below T*, these are typical signatures of a quantum phase transition. This suggests that p* is a quantum critical point below which some…
In this work zero-temperature phase diagrams of cuprates with Ba$_2$CuO$_3$-type CuO chain structure is investigated. The projective symmetry group analysis is employed in the strong coupling limit, and renormalization group with bosonization analysis is employed in the weak coupling limit. We find that in both of these two limits, large areas of the phase diagrams are filled with nodeless $s$-wave superconducting phases (with weak $d$-wave components), instead of pure $d$-wave phase mostly found in cuprates. This implies that nodeless $s$-wave phase is the dominant superconducting phase in cu…
The temperature evolution of the proximity effect in Au/La$_{2-x}$Sr$_x$CuO$_4$ and La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_{2-x}$Sr$_x$CuO$_4$ bilayers was investigated using scanning tunneling microscopy. Proximity induced gaps, centered at the chemical potential, were found to persist above the superconducting transition temperature, $T_c$, and up to nearly the pseudogap crossover temperature in both systems. Such independence of the spectra on the details of the normal metal cap layer is incompatible with a density-wave order origin. However, our results can be accounted for by a penetration of …
We theoretically investigate the effect of magnetic field on the pseudogap phenomena in High-Tc cuprates. The obtained results well explain the experimental results including their doping dependences. In our previous paper (J. Phys. Soc. Jpn. 68 (1999) 2999.), we have shown that the pseudogap phenomena observed in High-Tc cuprates are naturally understood as a precursor of the strong coupling superconductivity. On the other hand, there is an interpretation for the recent high field NMR measurements to be an evidence denying the pairing scenarios for the pseudogap. In this paper, we investigate…
We develop a theory of normal-metal - superconductor (NS) and superconductor - superconductor (SS) tunnelling in bosonic superconductors with strong attractive correlations taking into account coherence effects in single-particle excitation spectrum and disorder. The theory accounts for the existence of two energy scales, their temperature and doping dependencies, asymmetry and inhomogeneity of tunnelling spectra of underdoped cuprate superconductors.
The momentum and temperature dependence of the superconducting gap and pseudogap in optimally-doped Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_6$ superconductor is investigated by super-high resolution laser-based angle-resolved photoemission spectroscopy. The measured energy gap in the superconducting state exhibits a standard {\it d}-wave form. Pseudogap opens above T$_c$ over a large portion of the Fermi surface with a "Fermi arc" formed near the nodal region. In the region outside of the "Fermi arc", the pseudogap has the similar magnitude and momentum dependence as the gap in the superconducting stat…
The unclear relationship between cuprate superconductivity and the pseudogap state remains an impediment to understanding the high transition temperature (Tc) superconducting mechanism. Here we employ magnetic-field-dependent scanning tunneling microscopy to provide phase-sensitive proof that d-wave superconductivity coexists with the pseudogap on the antinodal Fermi surface of an overdoped cuprate. Furthermore, by tracking the hole doping (p) dependence of the quasiparticle interference pattern within a single Bi-based cuprate family, we observe a Fermi surface reconstruction slightly below o…
The angle-resolved photoemission spectroscopy studies on cuprates in the pseudogap region reveal an extraordinary topological transition in which the ground state changes from one with a normal Fermi surface to one with four Fermi points. Such a state is not possible without some symmetry breaking, which allows interference between one-particle basis states that is normally forbidden. We also show that the experimental results are quantitatively given without any free parameters by a theory and discuss the implications of the results.
Using the two-particle self-consistent approach and cluster perturbation theory for the two-dimensional t-t'-t''-U Hubbard model, we discuss weak- and strong-coupling mechanisms for the pseudogap observed in recent angle resolved photoemission spectroscopy on electron-doped cuprates. In the case of the strong-coupling mechanism, which is more relevant near half-filling, the pseudogap can be mainly driven by short range correlations near the Mott insulator. In the vicinity of optimal doping, where weak-coupling physics is more relevant, large antiferromagnetic correlation lengths, seen in neutr…
The specific heat $C$ of the cuprate superconductors La$_{2-x}$Sr$_x$CuO$_4$ and Bi$_{2+y}$Sr$_{2-x-y}$La$_x$CuO$_{6+δ}$ was measured at low temperature (down to $0.5~{\rm K}$), for dopings $p$ close to $p^\star$, the critical doping for the onset of the pseudogap phase. A magnetic field up to $35~{\rm T}$ was applied to suppress superconductivity, giving direct access to the normal state at low temperature, and enabling a determination of $C_e$, the electronic contribution to the normal-state specific heat, at $T \to 0$. In La$_{2-x}$Sr$_x$CuO$_4$ at $x=p = 0.22$, $0.24$ and $0.25$, $C_e / T …
We report on the magnetic phase diagram of a hole-doped cuprate Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$, which is free from buckling of CuO$_2$ planes, determined by muon spin rotation and relaxation. It is characterized by a quasi-static spin glass-like phase over a range of sodium concentration ($0.05\leq x\leq 0.12$), which is held between long range antiferromagnetic (AF) phase ($x\leq 0.02$) and superconducting phase where the system is non-magnetic for $x\geq 0.15$. The obtained phase diagram qualitatively agrees well with that commonly found for hole-doped high-\tc cuprates, strongly sugges…
Recent angle resolved photoemission \cite{yang-nature-08} and scanning tunneling microscopy \cite{kohsaka-nature-08} measurements on underdoped cuprates have yielded new spectroscopic information on quasiparticles in the pseudogap phase. New features of the normal state such as particle-hole asymmetry, maxima in the energy dispersion and accompanying drops in the spectral weight of quasiparticles agree with the ansatz of Yang \textit{et al.} for the single particle propagator in the pseudogap phase. The coherent quasiparticle dispersion and reduced asymmetry in the tunneling density of states …
