Cuprates by Darren C Peets
Science, 2012
The concept that superconductivity competes with other orders in cuprate superconductors has beco... more The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of ~3.2 lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba_2Cu_3O_6+x, with hole concentrations of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature (T_c); further cooling below T_c abruptly reverses the divergence of the charge correlations. In combination with earlier observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge density wave instability that competes with superconductivity.
Phys. Rev. Lett., 2017
Spin excitations in the overdoped high temperature superconductors Tl2Ba2CuO6+δ and (Bi,Pb)2(Sr,L... more Spin excitations in the overdoped high temperature superconductors Tl2Ba2CuO6+δ and (Bi,Pb)2(Sr,La)2CuO6+δ were investigated by resonant inelastic x-ray scattering (RIXS) as functions of doping and detuning of the incoming photon energy above the Cu−L3 absorption peak. The RIXS spectra at optimal doping are dominated by a paramagnon feature with peak energy independent of photon energy, similar to prior results on underdoped cuprates. Beyond optimal doping, the RIXS data indicate a sharp crossover to a regime with a strong contribution from incoherent particle-hole excitations whose maximum shows a fluorescencelike shift upon detuning. The spectra of both compound families are closely similar, and their salient features are reproduced by exact-diagonalization calculations of the single-band Hubbard model on a finite cluster. The results are discussed in the light of recent transport experiments indicating a quantum phase transition near optimal doping.
Phys. Rev. B, 2005
We have investigated the spin-fluctuations at energy transfers up to ∼110meV, well above the reso... more We have investigated the spin-fluctuations at energy transfers up to ∼110meV, well above the resonance energy (33meV) in the YBa_2Cu_3O_6.5 ortho-II superconductor using neutron time-of-flight and triple-axis techniques. The spectrum at high energies differs from the low-energy incommensurate modulations previously reported where the incommensurate wave vector is largely independent of energy. Well above the resonance the peak of the spin response lies at wave vectors that increase with energy. Within error the excitations at all energies above the resonance are best described by a ring around the (π,π) position. The isotropic wave-vector pattern differs from a recently reported square pattern in different but related systems. The spin spectral weight at high energies is similar to that in the insulator but the characteristic velocity is ∼40% lower. We introduce a method of extracting the acoustic and optic weights at all energies from time-of-flight data. We find that the optic spectral weight extends to surprisingly low energies of ∼25meV, and infer that the bilayer spin correlations weaken with increase in hole doping. When the low-energy optic excitations are taken into account we measure the total integrated weight around (π,π), for energies below 120meV, to agree with that expected from the insulator. As a qualitative guide, we compare spin-wave calculations based on models with ordered and disordered stripes and describe the inadequacy of these and other stripe models for the high-energy fluctuations.
Phys. Rev. Lett., 2009
X-ray absorption spectra on the overdoped high-temperature superconductors Tl_2Ba_2CuO_6+δ and La... more X-ray absorption spectra on the overdoped high-temperature superconductors Tl_2Ba_2CuO_6+δ and La_2−xSr_xCuO_4±δ reveal a striking departure in the electronic structure from that of the underdoped regime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygen K edge, while the lowest-energy prepeak gains less intensity than expected above p∼0.21. This suggests a breakdown of the Zhang-Rice singlet approximation and a loss of correlation effects or a significant shift in the most fundamental parameters of the system, rendering single-band Hubbard models inapplicable. Such fundamental changes suggest that the overdoped regime may offer a distinct route to understanding in the cuprates.
Phys. Rev. B, 2007
We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_6+δ throu... more We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_6+δ throughout the overdoped regime. In the T→0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap Δ. We find Δ to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentration of ≃0.1 hole/Cu. As hole concentration is decreased, the gap-to-T_c ratio increases, showing that the suppression of the superconducting transition temperature T_c (relative to the gap) begins in the overdoped regime.
Phys. Rev. Lett., 2010
A Reply to the Comment by Philip Phillips and Mark Jarrell.
A recent article suggested that the saturation of low energy spectral weight observed by X-ray ab... more A recent article suggested that the saturation of low energy spectral weight observed by X-ray absorption spectroscopy in the cuprates at high hole doping could be explained within the single-band Hubbard model. We show that this result is an artifact of inappropriate integration limits.
Phys. Rev. Lett., 2005
The high-T_c superconductor Tl_2Ba_2CuO_6+δ is studied by angle-resolved photoemission spectrosco... more The high-T_c superconductor Tl_2Ba_2CuO_6+δ is studied by angle-resolved photoemission spectroscopy. For a very overdoped T_c=30 K sample, the Fermi surface consists of a single large hole pocket centered at (π,π) and is approaching a topological transition. Although a superconducting gap with d_x²−y² symmetry is tentatively identified, the quasiparticle evolution with momentum and binding energy exhibits a marked departure from the behavior observed in under and optimally doped cuprates. The relevance of these findings to scattering, many-body, and quantum-critical phenomena is discussed.
J. Cryst. Growth, 2010
Highly perfect platelet single crystals of Tl_2Ba_2CuO_6+δ (Tl-2201) were grown by a self-flux te... more Highly perfect platelet single crystals of Tl_2Ba_2CuO_6+δ (Tl-2201) were grown by a self-flux technique. A novel encapsulation scheme allowed the precursors to react prior to the sealing required to contain volatile thallium oxides, and permitted the removal of melt at the conclusion of growth, reproducibly producing high yields of clean crystals. The crystals were annealed under well-controlled oxygen partial pressures, then characterised. They have sharp superconducting transitions, narrow X-ray rocking curves and a low 4% substitution of thallium by copper, all evidence of their high perfection and homogeneity. The crystals are orthorhombic at most dopings, and a previously unreported commensurate superlattice distortion is observed.
Phys. Rev. B, 2004
We describe the relation between spin fluctuations and superconductivity in a highly ordered samp... more We describe the relation between spin fluctuations and superconductivity in a highly ordered sample of YBa_2Cu_3O_6.5 using both polarized and unpolarized neutron inelastic scattering. The spin susceptibility in the superconducting phase exhibits one-dimensional incommensurate modulations at low energies, consistent with hydrodynamic stripes. With increasing energy the susceptibility curves upward to a commensurate, intense, well-defined, and asymmetric resonance at 33 meV with a precipitous high-energy cutoff. In the normal phase, which we show is gapless, the resonance remains surprisingly strong and persists clearly in Q scans and energy scans. Its similar asymmetric spectral form above T_c=59K suggests that incoherent superconducting pairing fluctuations are present in the normal state. On cooling, the resonance and the stripe modulations grow in well above Tc below a temperature that is comparable to the pseudogap temperature where suppression occurs in local and low-momentum properties. The spectral weight that accrues to the resonance is largely acquired by transfer from suppressed low-energy fluctuations. We find the resonance to be isotropically polarized, consistent with a triplet carrying ∼2.6% of the total spectral weight of the Cu spins in the planes.
Scientific Reports, 2015
The spontaneous expulsion of applied magnetic field, the Meissner effect, is a defining feature o... more The spontaneous expulsion of applied magnetic field, the Meissner effect, is a defining feature of superconductors; in Type-II superconductors above the lower critical field, this screening takes the form of a lattice of magnetic flux vortices. Using implanted spin-1/2 positive muons, one can measure the vortex lattice field distribution through the spin precession and deduce key parameters of the superconducting ground state, and thereby fundamental properties of the superconducting pairing. Muon spin rotation/relaxation (µSR) experiments have indeed revealed much interesting physics in the underdoped cuprates, where superconductivity is closely related to, or coexistent with, disordered or fluctuating magnetic and charge excitations. Such complications should be absent in overdoped cuprates, which are believed to exhibit conventional Fermi liquid behaviour. These first transverse field (TF)-µ⁺SR experiments on heavily-overdoped single crystals reveal a superfluid density exhibiting a clear inflection point near 0.5Tc , with a striking doping-independent scaling. This reflects hitherto unrecognized physics intrinsic to d-wave vortices, evidently generic to the cuprates, and may offer fundamentally new insights into their still-mysterious superconductivity.
Phys. Rev. B, 2010
We have measured the spin fluctuations in the YBa_2Cu_3O_6.5 (YBCO6.5,T_c=59 K) superconductor at... more We have measured the spin fluctuations in the YBa_2Cu_3O_6.5 (YBCO6.5,T_c=59 K) superconductor at high-energy transfers above ∼100 meV. Within experimental error, the momentum dependence is isotropic at high energies, similar to that measured in the insulator for two-dimensional spin waves, and the dispersion extrapolates back to the incommensurate wave vector at the elastic position. This result contrasts with previous expectations based on measurements around 50 meV which were suggestive of a softening of the spin-wave velocity with increased hole doping. Unlike the insulator, we observe a significant reduction in the intensity of the spin excitations for energy transfers above ∼100 meV similar to that observed above ∼200 meV in the YBCO6.35 (T_c=18 K) superconductor as the spin waves approach the zone boundary. We attribute this high-energy scale with a second gap and find agreement with measurements of the pseudogap in the cuprates associated with electronic anomalies along the antinodal positions. In addition, we observe a sharp peak at around 400 meV whose energy softens with increased hole doping. We discuss possible origins of this excitation including a hydrogen-related molecular excitation and a transition of electronic states between d levels.
Phys. Rev. B, 2011
We performed Raman scattering experiments on superconductivity-induced features in Bi_2Sr_2(Ca_1−... more We performed Raman scattering experiments on superconductivity-induced features in Bi_2Sr_2(Ca_1−xY_x)Cu_2O_8+δ (Bi-2212), YBa_2Cu_3O_6+x (Y-123), and Tl_2Ba_2CuO_6+δ (Tl-2201) single crystals. The results in combination with earlier ones enable us to systematically analyze the spectral features in the doping range 0.07⩽p⩽0.24. In B_2g (xy) symmetry, we find universal spectra and the maximal gap energy Δ_0 to scale with the superconducting transition temperature T_c. The B_1g (x_2−y_2) spectra in all three compounds show an anomalous increase of the intensity toward overdoping. The energy scale of the corresponding peak is neither related to the pairing energy nor to the pseudogap, but possibly stems from a symmetry breaking transition at the onset point of superconductivity at p_sc2≃0.27.
A recent article suggested that the saturation of low energy spectral weight observed by X-ray ab... more A recent article suggested that the saturation of low energy spectral weight observed by X-ray absorption spectroscopy in the cuprates at high hole doping could be explained within the single-band Hubbard model. We show that this result is an artifact of inappropriate integration limits.
In the cuprates, past investigations of the low-temperature ($T < 1$K) thermal conductivity ($... more In the cuprates, past investigations of the low-temperature ($T < 1$K) thermal conductivity ($\kappa$) have focused on the electronic contribution to $\kappa$. Here we examine the phonon contribution to the thermal conductivity in two families of overdoped cuprates: Tl-2201 and ...
Phys. Rev. B, 2002
We present elastic and inelastic neutron-scattering results on highly oxygen-ordered YBa_2Cu_3O_6... more We present elastic and inelastic neutron-scattering results on highly oxygen-ordered YBa_2Cu_3O_6.5 ortho-II. We find no evidence of the presence of long-ranged ordered magnetic moments to a sensitivity of ∼0.003μ_B, an order of magnitude smaller than has been suggested in theories of orbital or d-density-wave (DDW) currents. The absence of sharp elastic peaks rules out the existence of well-correlated static DDW currents in our crystal. We cannot exclude the possibility that a broad peak may exist with extremely short-range DDW correlations. For less ordered or more doped crystals it is possible that disorder may lead to static magnetism. We have also searched for the large normal-state spin gap that is predicted to exist in an ordered DDW phase. Instead of a gap we find that the Q-correlated spin susceptibility persists to the lowest energies studied, ∼6meV. Our results are only compatible with the coexistence of superconductivity and orbital currents if the latter are dynamic and do not participate in a sharp phase transition to a highly ordered DDW state.
Phys. Rev. B, 2013
Using resonant inelastic x-ray scattering (RIXS) at the Cu L-absorption edge, we have observed in... more Using resonant inelastic x-ray scattering (RIXS) at the Cu L-absorption edge, we have observed intense, dispersive spin excitations in highly overdoped Tl_2Ba_2CuO_6+δ (superconducting T_c=6 K), a model compound whose normal-state charge transport and thermodynamic properties have been shown to exhibit canonical Fermi-liquid behavior. Complementary RIXS experiments on slightly overdoped Tl_2Ba_2CuO_6+δ (T_c=89 K) and on Y_1−xCa_xBa_2Cu_3O_6+δ compounds spanning a wide range of doping levels indicate that these excitations exhibit energies and energy-integrated spectral weights closely similar to those of antiferromagnetic magnons in undoped cuprates, indicating the persistence of substantial antiferromagnetic spin correlations over a wide doping range. The surprising coexistence of such correlations with Fermi-liquid-like charge excitations in highly overdoped cuprates poses a challenge to current theoretical models of correlated-electron metals.
Phys. Rev. B, 2013
We present measurements of the microwave surface impedance of the single-layer cuprate Tl_2Ba_2Cu... more We present measurements of the microwave surface impedance of the single-layer cuprate Tl_2Ba_2CuO_6+δ, deep in the overdoped regime, with T_c≈25 K. Measurements have been made using cavity perturbation of a dielectric resonator at 17 discrete frequencies ranging from 2.50 to 19.16 GHz, and at temperatures from 0.12 to 27.5 K. From the surface impedance we obtain the microwave conductivity, penetration depth, and superfluid density. The superfluid density displays a strong linear temperature dependence from 2 to 14 K, indicative of line nodes in the energy gap. The microwave data are compared with theoretical predictions for a d-wave superconductor with pointlike impurities, with the conclusion that disorder in Tl_2Ba_2CuO_6+δ acts predominantly in the weak-to-intermediate-strength scattering regime, and that small-angle scattering is important.
New J. Phys., 2007
Single-particle spectroscopic probes, such as scanning tunnelling and angle-resolved photoemissio... more Single-particle spectroscopic probes, such as scanning tunnelling and angle-resolved photoemission spectroscopy (ARPES), have provided us with crucial insights into the complex electronic structure of the high-T_c cuprates, in particular for the under- and optimally-doped regimes where high-quality crystals suitable for surface-sensitive experiments are available. Conversely, the elementary excitations on the heavily overdoped side of the phase diagram remain largely unexplored. Important breakthroughs could come from the study of Tl_2Ba_2CuO_6+δ (Tl2201), a structurally simple system whose doping level can be tuned from optimal to extreme overdoping by varying the oxygen content. Using a self-flux method and encapsulation, we have grown single crystals of Tl2201, which were then carefully annealed under controlled oxygen partial pressures. Their high quality and homogeneity are demonstrated by narrow rocking curves and superconducting transition widths. For higher dopings, the crystals are orthorhombic, a lattice distortion stabilized by O interstitials in the TlO layer. These crystals have enabled the first successful ARPES study of both normal and superconducting-state electronic structure in Tl2201, allowing a direct comparison with the Fermi surface from magnetoresistance and the gap from thermal conductivity experiments. This establishes Tl2201 as the first high-T_c cuprate for which a surface-sensitive single-particle spectroscopy and a comparable bulk transport technique have arrived at quantitative agreement on a major feature such as the normal state Fermi surface. The momentum dependence of the ARPES lineshape reveals, however, an unexpected phenomenology: in contrast to the case of under- and optimally-doped cuprates, quasiparticles are sharp near (π,0), the antinodal region where the gap is maximum, and broad at (π/2, π/2), the nodal region where the gap vanishes. This reversed quasiparticle anisotropy past optimal doping, and its relevance to scattering, many-body, and quantum-critical phenomena in the high-T_c cuprates, is discussed.
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Cuprates by Darren C Peets
EuFe2As2 family is unique among these materials for having magnetic order which onsets within the superconducting state, just below the superconducting transition. Superconductivity and magnetic order are normally antagonistic and often vie for the same unpaired electrons, but in this family the magnetism arises from largely localized Eu moments and they coexist, with the competition between these evenly matched opponents leading to reentrant superconducting behavior. To help elucidate the physics in this family and the interactions between the magnetic order and superconductivity, we investigate the H−T phase diagram near optimal Rh doping through specific heat, resistivity, and magnetization measurements, and study the electronic structure by angular-resolved photoemission spectroscopy. The competition between the Eu and FeAs layers may offer a route to directly access the electronic structure under effective magnetic fields via angle-resolved photoemission spectroscopy, which is ordinarily a strictly zero-field technique.