Neutron-Scattering Evidence for a Periodically Modulated Superconducting Phase in the Underdoped CuprateLa1.905Ba0.095CuO4

Abstract

The role of antiferromagnetic spin correlations in high-temperature superconductors remains a matter of debate. We present inelastic neutron scattering evidence that gapless spin fluctuations coexist with superconductivity in La1.905Ba0.095CuO4. Furthermore, we observe that both the lowenergy magnetic spectral weight and the spin incommensurability are enhanced with the onset of superconducting correlations. We propose that the coexistence occurs through intertwining of spatial modulations of the pair wave function and the antiferromagnetic correlations. This proposal is also directly relevant to sufficiently underdoped La2−xSrxCuO4 and YBa2Cu3O6+x.It is commonly accepted that cuprate superconductors have a spatially-uniform d-wave pair wave function [1]. It has also become a paradigm that antiferromagnetic spin fluctuations are gapped in the superconducting state, with a pile up of excitations in the magnetic "resonance" peak above the gap [2][3][4][5][6]. A number of neutron scattering studies of underdoped La 2−x Sr x CuO 4 have found evidence for incommensurate spin fluctuations that remain gapless at temperatures far below the superconducting transition temperature, T c [7][8][9][10]. Theoretical analyses have tended to view such spin-density-wave correlations as soft fluctuations of an order that competes with spatially-uniform superconductivity [11] and that may be locally pinned by disorder [12]. As a consequence, researchers have crafted interpretations of the low-energy spin fluctuations that maintain consistency with the spingap paradigm [8,10].In an alternative approach, the superconductivity and antiferromagnetism are both treated as spatially modulated and intimately intertwined [13]. Such a state, which variational calculations indicate to be energetically competitive with uniform superconductivity [14], has been invoked [15,16] to explain the depression of superconducting order in certain stripe-ordered cuprates [17,18]. While the poorly-superconducting phase is fascinating on its own, it leaves open the question of whether a modulated pair wave function might be relevant to the case of a good bulk superconductor.In this paper, we present neutron scattering measurements of the low-energy spin fluctuations in La 1.905 Ba 0.095 CuO 4 , a bulk superconductor with T c = 32 K. Rather than developing a spin gap on cooling below T c , the lowest-energy excitations are actually enhanced. By putting the measurements on an absolute scale, we show that the strength of the spin response is comparable to that of spin waves in antiferromagnetic La 2 CuO 4 . To generate this large a response, we conclude that all parts of the sample must contribute to the signal, ruling out macroscopic phase separation. A previous optical conductivity study has shown that the superfluid density of this sample is consistent with the trend established for bulk superconductivity in all cuprate families in the form of Homes' law [19]. It thus appears that there must be local coexistence of the spin fluctuations and superconductivit...

Magnetic Fluctuations in Pair-Density-Wave Superconductors

“…This absence of the usual fingerprint of a signchanging superconducting gap in the PDW state can be traced back to its gapless single-particle excitation spectrum. These results are in agreement with recent neutron scattering results on x = 0.095 LBCO [31], where neither a spin-gap nor a resonance were observed below the superconducting critical temperature.…”

“…Experimentally, a recent neutron scattering study of the low-energy spin response in stripe ordered La 1.905 Ba 0.095 CuO 4 [31] found a number of remarkable results that were taken as evidence for a PDW state: (1) gapless spin excitations coexisting with superconductivity, and (2) the absence of a neutron resonance in the superconducting state. These results are highly unusual since both a spin-gap and a neutron resonance are expected in unconventional superconductors like the cuprates [1].…”

Multiband Responses in High-T c Cuprate Superconductors

“…This indicates that the transition does not proceed by a Kohn-like anomaly in the paramagnon spectrum, but by a central peak. Such a central peak has just been reported by J. Tranquada at this conference [11], as "gapless magnetic excitations." It appears [11] that such a peak has been noticed in earlier work, where it was interpreted as a gap with unexpected behavior, becoming smaller with underdoping [12].…”

“…The identification of the central peak in the magnetic response [11] has been discussed above. The extension of the SC region to zero doping in PLCCO upon transformation from the T to the T' lattice [21], similar to the one in NCCO thin films [22], is consistent with our understanding of the effects of out-of-plane dopand ions.…”

Exploring intertwined orders in cuprate superconductors

“…We have recently investigated the spin fluctuations in the x ¼0.095 sample by inelastic neutron scattering [67]. This sample exhibits weak spin and charge stripe order in the superconducting state.…”