Quasiparticle Scattering off Defects and Possible Bound States in Charge-Ordered YBa2Cu3Oy

Abstract

Observing how electronic states in solids react to a local symmetry breaking provides insight into their microscopic nature. A striking example is the formation of bound states when quasiparticles are scattered off defects. This is known to occur, under specific circumstances, in some metals and superconductors but not, in general, in the charge-density-wave (CDW) state. Here, we report the unforeseen observation of bound states when a magnetic field quenches superconductivity and induces long-range CDW order in YBa2Cu3Oy. Bound states indeed produce an inhomogeneous pattern of the local density of states N (EF ) that leads to a skewed distribution of Knight shifts which is detected here through an asymmetric profile of 17 O NMR lines. We argue that the effect arises most likely from scattering off defects in the CDW state, which provides a novel case of disorder-induced bound states in a condensed-matter system and an insightful window into charge ordering in the cuprates.Recent experiments in high-T c copper-oxides at moderate doping levels have led to a consensus regarding the presence of static, but essentially short-range, spatial modulations of the charge density that appear far above the critical temperature T c and compete with superconductivity below T c [1, 3,[3][4][5][6][7][8][9][10][11][12]. As a result, the debate has mainly focused on this "incipient" charge-density-wave (CDW) in zero magnetic field. Somewhat less attention has been paid to another CDW order that also coexists and competes with superconductivity but is present at lower temperatures, only in high magnetic fields, and has been detected only in YBa 2 Cu 3 O y (YBCO) so far, by nuclear magnetic resonance (NMR) [1,13] and more recently by X-ray diffraction [2,15]. This CDW develops long-range three-dimensional (3D) correlations above a critical field (also detected in sound velocity [17] and thermal conductivity measurements [18]). In principle, the long-range coherence of the high-field CDW makes it a unique playground for investigating the nature of CDW order in cuprates and its interplay with superconductivity [19]. However, since its observation requires high fields, experimental characterization of this phase has remained limited. This evidently prompts for further high field experiments in YBCO.Here, we report that a spatial distribution of 17 O Knight shift values develops alongside with the high-field CDW in YBCO. After discarding broadening by the vortex lattice or magnetic order, we explain the effect by an enhancement of N (E F ), the local density-of-states (LDOS) at the Fermi energy E F , due to the formation of quasiparticle bound states as observed around defects in the superconducting state of cuprates [5,[20][21][22][23][25][26][27]. This unforeseen observation raises fundamental questions on the electronic band structure and the nature of defects in the CDW state.17 O-enriched untwinned single crystals of YBa 2 Cu 3 O y were prepared with y nominal = 6.47 (ortho-II oxygen ordering, hereafter O-II), 6.56 (O-II), 6.68 (O...

Structure of the charge density wave in cuprate superconductors: Lessons from NMR

“…6 (a) and (b) demonstrate that disorder is key to the lineshape asymmetry. We emphasize that the mechanism for this asymmetry is different from that proposed by Zhou et al, 36 who suggested that near-unitary impurity resonances generate an asymmetric LDOS distribution. The scattering potential used in this work is far too weak to generate such resonances, and instead we propose below that its main role is to disorder the CDW, which in turn generates skewed lineshapes.…”

“…66 Ref. 36 did indeed find that the left and right linewidths scale with the amount of disorder in the crystal; however, it is also clear that long-range CDW order is prerequisite for this effect, suggesting a different mechanism.…”

“…This depletion is mainly due to the pseudogap, rather than the onset of the CDW. 36 (Note that the data are measured in a magnetic field of 28.5 T, which is believed sufficient to suppress superconductivity. 62,63 ) The onset of long-range order at T CDW ≈ 50-60K is signalled by a splitting of the quadrupole satellites as T is lowered.…”

Locally commensurate charge-density wave with three-unit-cell periodicity in YBa2Cu3Oy

“…Third, depending on the relative magnitude of the two effects and on which line is considered, the changes in K ( r ) and ν quad ( r ) may, partially or entirely, compensate each other. In YBCO, there are five different lines per 17 O site (nuclear spin I = 5/2) and three per 63 Cu (nuclear spin I = 3/2) and our previous work 28 , 41 has shown how each line shape could be quantitatively understood from the combination of magnetic and quadrupole effects. We have established that the effects of the CDW are better seen on the upper frequency quadrupole satellites of 63 Cu and 17 O, hereafter called HF1 and HF2 respectively (see Methods for details).…”

Spin susceptibility of charge-ordered YBa 2 Cu 3 O y across the upper critical field

“…O(2) data are lower than those for O(3), which we attribute to a systematic error in the fitting, due to the uncertainty generated by the absence of splitting of the low-frequency satellites. It should be noted that electronic bound states in high fields make the distribution of K values asymmetric [62]. In low field, the vortex lattice also produces an asymmetric distribution of K. Therefore, the Knight shift extracted from the positions of maximum intensity corresponds, strictly speaking, to the mode of the skewed distribution rather than to its mean value.…”