Energy gaps in the failed high-Tc superconductor La1.875Ba0.125CuO4

Abstract

A central issue in high-T c superconductivity is the nature of the normal-state gap (pseudogap) 1 in the underdoped regime and its relationship with superconductivity. Despite persistent efforts, theoretical ideas for the pseudogap evolve around fluctuating superconductivity 2 , competing order 3-8 and spectral weight suppression due to many-body effects 9 . Recently, although some experiments in the superconducting state indicate a distinction between the superconducting gap and pseudogap 10-14 , others in the normal state, either by extrapolation from high-temperature data 15 The first high-T c superconductor discovered, La 2−x Ba x CuO 4 (LBCO), holds a unique position in the field because of an anomalously strong bulk T c suppression near x = 1/8. Right around this 'magic' doping level, scattering experiments by neutrons 18,19 and X-rays 20 find a static spin and charge (stripe) order. By itself, this observation raises a series of intriguing questions: whether the stripe order is a competing order that suppresses the superconductivity in LBCO-1/8; if the answer is positive, which aspect, the pairing strength or the phase coherence, is involved in the T c suppression and how this mechanism applies to other dopings or families. For our investigation of the 'ground-state' pseudogap, as defined in ref. 16, which ignores the residual superconductivity, its sufficiently high doping yet extremely low bulk T c (∼4 K) makes LBCO-1/8 an ideal system: especially for the small-gap measurement near the node, difficulties due to either the unscreened disorder potential, a problem for extremely low doping, or trivial thermal broadening, a problem above T c for higher doping, are circumvented.Whereas thermal effects require an extrapolation from high-temperature data to obtain the ground-state physics 15 , a direct measurement on LBCO-1/8 at low temperature has been made 16 . With experimental resolutions compromised to obtain sufficient signal to noise, a simple d-wave gap function is reported with no discernible nodal quasi-particles found. Given the importance of this issue, we have carried out an angle-resolved photoemission 21 study of LBCO-1/8 at T > T c with much improved resolutions in a measurement geometry favourable for the detection of nodal quasi-particles (see Fig. 1 and Supplementary Information, Section SI). Our data provide two important new insights. First, there is a well-defined nodal quasi-particle peak, suggesting nodal quasi-particles can exist in the stripe-ordered state. Second, there is a rich gap structure, suggesting the pseudogap physics is more elaborate than the simple d-wave version. In particular, a new kind of pseudogap, which is not smoothly connected to the usual one tied to the antinodal region, can exist in the nodal region when superconductivity is suppressed owing to the loss of phase coherence.As shown in Fig. 1, there exists a well-defined quasi-particle peak in the energy distribution curves (EDCs) at the Fermi crossing points (k F ) around the node. On dispersion towards the ant...

Stripe order in superconducting La2−xBaxCuOHücker1, Zimmermann2, Gu3 et al. 2011Phys. Rev. B308434112View full textAdd to dashboardCite

“…Previously reported results on some of the compositions, in particular on the x = 1/8 crystal, have demonstrated a very high sample quality. 20,[34][35][36]44,[51][52][53][54][55] Since the compositions of the single crystals can deviate from their nominal stoichiometry (see Ref. 56), it has been vital to measure the structure, stripe order, and SC on pieces of the same crystal.…”

“…Angle resolved photoemission spectroscopy studies show that in the stripe ordered state La 2−x Ba x CuO 4 develops a gapped Fermi surface similar to that in bulk SC La 2−x Sr x CuO 4 , with the antinodal gap energy ∆(x) of both groups of samples describing a dome with a maximum at x ∼ 1/8. 44,55 This motivated the idea that static stripe order does not suppress SC pairing correlations per se, but prevents phase coherence. 35 In our recent work on La 1.875 Ba 0.125 CuO 4 we obtained further evidence for this picture.…”

Theory of the striped superconductor

“…The most extreme illustration of this occurs in the temperature range 10 K < T < 16 K, in which the in-plane direction resistivity is immeasurably small, while the caxis resistivity is in the 1-10 mΩ range, so the resistivity anisotropy ratio is consistent with infinity. 2) Despite the fact that many clear signatures of superconductivity onset at temperatures in excess of 40 K, and that angle resolved photoemission has inferred a "gap" 29,30 of order 20 meV, the fully superconducting state (i.e. the Meissner effect and zero resistance in all directions) only occurs below a critical temperature of 4 K. It is very difficult to imagine a scenario in which a strong conventional superconducting order develops locally on such high scales, but fully orders only at such low temperatures in a system that is three dimensional, non-granular in structure, and not subjected to an external magnetic field.…”

“…In the regime of temperature below the onset of substantial local superconducting coherence but above the transition to a fully superconducting state (if one occurs), the system can be characterized as a "failed superconductor" 30 . Clearly, in this regime, the longitudinal resistivity will be small compared to normal state values, as will the quasiparticle contributions to the thermopower and linear Hall resistance.…”

Effective theory of Fermi pockets in fluctuating antiferromagnets

“…Another issue of interest in experiments has been the angular dependence of the electronic excitation gap energy in the superconducting state. A 'dichotomy' has been noted [10][11][12][13] between the behavior of the gap near the nodal points on the Brillouin zone diagonals, and the antinodal points along the principle axes of the Brillouin zone.…”