The nature of the pseudogap phase of cuprate high-temperature superconductors is one of the most important unsolved problems 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 observe the coincident onset at T * of a particle-hole asymmetric antinodal gap, a non-zero Kerr rotation, and a change in the 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 fashion with the pre-existing pseudogap features.As complex oxides, cuprate superconductors belong to a class of materials which exhibit many broken-symmetry states; unravelling the relationship between superconductivity in the cuprates and other possible broken-symmetry states has been a major challenge of condensed matter physics. A possibly related issue concerns the nature of the pseudogap in the cuprates and its relationship with superconductivity. Angle-resolved photoemission spectroscopy (ARPES) studies have shown that the pseudogap develops below a temperature T * near the Brillouin zone boundary while preserving a gapless Fermi arc near the zone diagonal (1). A key issue is the extent to which the pseudogap is a consequence of superconducting fluctuations (2-5), which should exhibit a rough particle-hole symmetry, or another form of (incipient) order (6-12), which typically should induce particle-hole asymmetric spectral changes. Candidate orders include various forms of density wave, nematic or unconventional magnetic orders that break different combinations of lattice translational (6-8, 13-19), rotational (6, 9, 15, 17, 20-22), and time-reversal (7, 9, 23-26) symmetries.We have focused on crystals of nearly optimally-doped (OP) Pb 0.55 Bi 1.5 Sr 1.6 La 0.4 CuO 6+δ (PbBi2201, T c = 38 K, T * = 132 ± 8 K) (27), and combined the ARPES measurements of the evolution of the band structure over a wide range of temperature, momentum and energy, with high-precision measurements of the polar Kerr effect (PKE) and time-resolved reflectivity (TRR).Bi2201 was chosen to avoid the complications resulting from bilayer splitting and strong antinodal bosonic mode coupling inherent to Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) (1). Whereas ARPES is a surface probe, PKE enables us to monitor a bulk, thermodynamic (via the fluctuation-dissipation theorem) 2 property which has proven (28) to be a sensitive probe of the onset of a broken-symmetry state, and TRR gives complementary information on the bulk, near-equilibrium dynamics of the system. We will first analyze our ARPES data collected in different temperature regions. Above T * , PbBi2201 has a simple one-band band structure (right side of Fig. 1). For each cut in momentum space Fig. 1), the only distinct feature in the ...
We present an angle-resolved photoemission doping dependence study of the n-type cuprate superconductor Nd(2-x)Ce(x)CuO(4+/-delta), from the half-filled Mott insulator to the T(c) = 24 K superconductor. In Nd2CuO4, we reveal the charge-transfer band for the first time. As electrons are doped into the system, this feature's intensity decreases with the concomitant formation of near- E(F) spectral weight. At low doping, the Fermi surface is an electron-pocket (with volume approximately x) centered at (pi,0). Further doping leads to the creation of a new holelike Fermi surface (volume approximately 1+x) centered at (pi,pi). These findings shed light on the Mott gap, its doping evolution, as well as the anomalous transport properties of the n-type cuprates.
We report that the doping and temperature dependence of photoemission spectra near the Brillouin zone boundary of Bi 2 Sr 2 CaCu 2 O 8+δ δ exhibit unexpected sensitivity to the superfluid density. In the superconducting state, the photoemission peak intensity as a function of doping scales with the superfluid density and the condensation energy. As a function of temperature, the peak intensity shows an abrupt behavior near the superconducting phase transition temperature where phase coherence sets in, rather than near the temperature where the gap opens. This anomalous manifestation of collective effects in single-particle spectroscopy raises important questions concerning the mechanism of high-temperature superconductivity.The collective nature of superconductivity manifests itself contrastingly in different techniques. Microwave and muon spin relaxation measurements are inherently sensitive to the collective motion of the condensate, whereas single-electron tunneling spectroscopy and photoemission mainly probe single-particle excitations of the condensate. Hence, these two types of spectroscopies can be used to measure two essential but distinct ingredients of superconductivity: the superfluid density, which characterizes the phase coherence of the Cooper pairs, and the superconducting energy gap, which reflects the strength of the pairing. We report a pronounced departure from
With significantly improved sample quality and instrumental resolution, we clearly identify in the (π,0) ARPES spectra from YBa2Cu3O6.993, in the superconducting state, the long-sought 'peakdip-hump' structure. This advance allows us to investigate the large a-b anisotropy of the in-plane electronic structure including, in particular, a 50% difference in the magnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the CuO chains, raises serious questions about attempts to quantitatively explain the YBa2Cu3O 7−δ data from various experiments using models based on a perfectly square lattice.PACS numbers: 74.25.Jb, 74.72.Bk, 79.60.Bm High-temperature superconductivity (HTSC) is intimately related to the CuO 2 plane, which is the only common structural feature in all cuprates. This fact has led most of the proposed microscopic theories to assume a CuO 2 square planar structure. However, for the practical reason of sample quality, some of the most important and defining experiments have been performed on YBa 2 Cu 3 O 7−δ (Y123), which does not have a square lattice, but rather an orthorhombic structure (b/a ≈ 1.015), caused by the presence of a CuO chain layer [1]. This orthorhombicity, according to LDA calculation [2], should result in significant anisotropy in the in-plane electronic structure (this term will be used throughout this paper to refer to the electronic states associated with the CuO 2 plane), making it problematic to compare theories based on a square lattice with experimental data from Y123. Therefore, it is crucial to quantify the effect of orthorhombicity, if any, on the in-plane electronic structure in Y123. The problem is that angle-resolved photoemission spectroscopy (ARPES), being a uniquely powerful tool for this important task, has, until now, not been particularly effective for the study of Y123 [3]. The important 'peak-dip-hump' structure, which is seen routinely in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) [4] has never been observed in Y123. This absence, together with the presence of a surface state [5], raises questions about ARPES data from Y123, and the universality of the superconducting peak in the cuprates.This paper reports a breakthrough in this important issue, made possible by significantly improved sample quality and instrumental resolution. By isolating a surface state peak near the Fermi energy (E F ), we can clearly resolve a 'peak-dip-hump' structure in the ARPES spectra around (π, 0) in Y123 that resembles the superconducting peak observed in Bi2212 [4]. More significantly, we find a strong a-b asymmetry of the in-plane electronic structure, such as the superconducting gap magnitude, which differs by about 50%. We argue that such a strong inplane a-b anisotropy should be taken into account when interpreting experiments performed on Y123.ARPES experiments were carried out at beamline 5-4 at SSRL, which is equipped with a normal-incidencemonochromator and a SCIENTA SES-200 analyzer. Untwinned Y...
We use high-resolution angle-resolved photoemission to study the electronic structure of the BaFe 2 As 2 pnictides. We observe two electron bands and two hole bands near the X point, ͑ , ͒ of the Brillouin zone, in the paramagnetic state for electron-doped Ba͑Co 0.06 Fe 0.94 ͒ 2 As 2 , undoped BaFe 2 As 2 , and hole-doped Ba 0.6 K 0.4 Fe 2 As 2 . Among these bands, only the electron bands cross the Fermi level, forming two electron pockets around X while the hole bands approach but never reach the Fermi level. We show that the band structure of the BaFe 2 As 2 family matches reasonably well with the prediction of local-density approximation calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.
The low-energy electronic structure of the nearly optimally doped trilayer cuprate superconductor Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta) is investigated by angle-resolved photoemission spectroscopy. The normal state quasiparticle dispersion and Fermi surface and the superconducting d-wave gap and coherence peak are observed and compared with those of single- and bilayer systems. We find that both the superconducting gap magnitude and the relative coherence-peak intensity scale linearly with T(c) for various optimally doped materials.
In normal metals, macroscopic properties are understood using the concept of quasiparticles. In the cuprate high-temperature superconductors, the metallic state above the highest transition temperature is anomalous and is known as the “strange metal.” We studied this state using angle-resolved photoemission spectroscopy. With increasing doping across a temperature-independent critical value pc ~ 0.19, we observed that near the Brillouin zone boundary, the strange metal, characterized by an incoherent spectral function, abruptly reconstructs into a more conventional metal with quasiparticles. Above the temperature of superconducting fluctuations, we found that the pseudogap also discontinuously collapses at the very same value of pc. These observations suggest that the incoherent strange metal is a distinct state and a prerequisite for the pseudogap; such findings are incompatible with existing pseudogap quantum critical point scenarios.
Exploring cuprate chains Superconductivity in cuprates takes place in their two-dimensional (2D) layers but solving even the simplest model of interacting fermions in 2D is a challenge. The theory problem simplifies in 1D, with experiment becoming the tricky part. Chen et al . synthesized a cuprate that consists of parallel chains and behaves like a 1D system. Crucially, the material could be doped over a wide range of hole concentrations. The researchers showed that including a near-neighbor attractive interaction in a 1D model of interacting fermions was necessary to explain their photoemission measurements. —JS
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