arXiv subjects

D. A. Bonn

Publications and source records attributed to D. A. Bonn.

At least 91 records · Page 5Linked to original sources

arXiv2013

Lattice dynamical signature of charge density wave formation in underdoped YBa2Cu3O6+x

We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high temperature superconductor YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200 K in underdoped crystals. The temperature dependence of these new features indicates that they are associated with the incommensurate charge density wave state recently discovered using synchrotron x-ray scattering techniques on the same single crystals. Raman scattering has thus the potential to explore the evolution of this state under extreme conditions.

cond-mat.supr-con↗

arXiv2013

Universal inhomogeneous magnetic-field response in the normal state of cuprate high-Tc superconductors

We report the results of a muon spin rotation (muSR) study of the bulk of Bi{2+x}Sr{2-x}CaCu2O{8+δ}, as well as pure and Ca-doped YBa2Cu3Oy, which together with prior measurements reveal a universal inhomogeneous magnetic-field response of hole-doped cuprates extending to temperatures far above the critical temperature (Tc). The primary features of our data are incompatible with the spatially inhomogeneous response being dominated by known charge density wave (CDW) and spin density wave (SDW) orders. Instead the normal-state inhomogeneous line broadening is found to scale with the maximum value Tc^max for each cuprate family, indicating it is controlled by the same energy scale as Tc. Since the degree of chemical disorder varies widely among the cuprates we have measured, the observed scaling constitutes evidence for an intrinsic electronic tendency toward inhomogeneity above Tc.

cond-mat.supr-con↗

arXiv2013

An inelastic x-ray study of phonon broadening and charge density wave formation in ortho-II ordered YBa2Cu3O6.54

Inelastic x-ray scattering is used to investigate charge density wave (CDW) formation and the low-energy lattice dynamics of the underdoped high temperature superconductor ortho-II YBa2Cu3O6.54. We find that, for a temperature ~1/3 of the CDW onset temperature (~155 K), the CDW order is static within the resolution of the experiment, that is the inverse lifetime is less than 0.3 meV. In the same temperature region, low-energy phonons near the ordering wavevector of the CDW show large increases in their linewidths. This contrasts with the usual behavior in CDW systems where the phonon anomalies are strongest near the CDW onset temperature

cond-mat.supr-con↗

arXiv2013

Emergence of charge order from the vortex state of a high temperature superconductor

Evidence is mounting that charge order competes with superconductivity in high Tc cuprates. Whether this has any relationship to the pairing mechanism is unknown since neither the universality of the competition nor its microscopic nature has been established. Here using nuclear magnetic resonance, we show that, similar to La214, charge order in YBCO has maximum strength inside the superconducting dome, at doping levels p = 0.11 - 0.12.We further show that the overlap of halos of incipient charge order around vortex cores, similar to those visualised in Bi2212, can explain the threshold magnetic field at which long-range charge order emerges. These results reveal universal features of a competition in which charge order and superconductivity appear as joint instabilities of the same normal state, whose relative balance can be field-tuned in the vortex state.

cond-mat.supr-con↗

arXiv2013

Bounding the pseudogap with a line of phase transitions in YBCO cuprate superconductors

Close to optimal doping, the copper oxide superconductors show 'strange metal' behavior, suggestive of strong fluctuations associated with a quantum critical point. Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting 'dome'. The underdoped region of the temperature-doping phase diagram from which superconductivity emerges is referred to as the 'pseudogap' because evidence exists for partial gapping of the conduction electrons, but so far there is no compelling thermodynamic evidence as to whether the pseudogap is a distinct phase or a continuous evolution of physical properties on cooling. Here we report that the pseudogap in YBCO cuprate superconductors is a distinct phase, bounded by a line of phase transitions. The doping dependence of this line is such that it terminates at zero temperature inside the superconducting dome. From this we conclude that quantum criticality drives the strange metallic behavior and therefore superconductivity in the cuprates.

cond-mat.supr-con↗

arXiv2013

Microwave spectroscopy of vortex dynamics in ortho-II YBa2Cu3O6.52

We present measurements of the vortex-state surface impedance, Z_s = R_s + i X_s, of a high quality, ortho-II-ordered single crystal of the cuprate high temperature superconductor YBa2Cu3O6.52 (T_c = 59K). Measurements have been made at four microwave frequencies (ω/2π= 2.64, 4.51, 9.12 and 13.97 GHz), for magnetic fields ranging from 0 to 7 T. From these data we obtain the field, frequency and temperature dependence of the vortex viscosity, pinning constant, depinning frequency and flux-flow resistivity. The vortex viscosity, η(ω,T), has a surprisingly strong frequency dependence and bears a striking resemblance to the zero-field quasiparticle conductivity, σ_qp(ω,T), suggesting that the dominant dissipative mechanism for the flux lines is induced electric fields coupling to bulk, long-lived d-wave quasiparticles outside the vortex cores. This is in sharp contrast to the conventional Bardeen-Stephen picture, in which dissipation takes place inside quasi-normal vortex cores. The strong frequency dependence of the vortex viscosity in the microwave range requires us to treat it as a complex response function, with an imaginary part that is predicted to contribute to the apparent pinning force on the vortices. Measurements of the frequency dependence of the pinning force confirm that this term is present, and in a form consistent with the requirements of causality. At low temperatures the flux-flow resistivity, ρ_ff \propto 1/η, has the form ρ_ff(T) = ρ_0 + ρ_1 \ln(1/T), reminiscent of the DC resistivity of cuprates in the pseudogap regime.

cond-mat.supr-con↗

arXiv2013

A new collective mode in YBCO observed by time-domain reflectometry

We report the observation of coherent oscillations associated with charge density wave (CDW) order in the underdoped cuprate superconductor YBa2Cu3O6+x by time-resolved optical reflectivity. Oscillations with frequency 1.87 THz onset at approximately 105 K and 130 K for dopings of x = 0.67 (ortho-VIII) and x = 0.75 (ortho-III), respectively. Upon cooling below the superconducting critical temperature (T_c), the oscillation amplitude is enhanced, the phase shifts by π, and the frequency softens by δν/ ν~7%. A bi-quadratically coupled Landau-Ginzburg model qualitatively describes this behavior as arising from competition between superconducting and CDW orders.

cond-mat.supr-con↗

arXiv2013

X-ray diffraction observation of charge density wave order at zero magnetic field in ortho-II YBa$_2$Cu$_3$O$_{6.54}$

X-ray diffraction measurements show that the high-temperature superconductor YBa$_2$Cu$_3$O$_{6.54}$, with ortho-II oxygen order, has charge density wave order (CDW) in the absence of an applied magnetic field. The dominant wavevector of the CDW is $\mathbf{q}_{\mathrm{CDW}} = (0, 0.328(2), 0.5)$, with the in-plane component parallel to the $\mathbf{b}$-axis (chain direction). It has a similar incommensurability to that observed in ortho-VIII and ortho-III samples, which have different dopings and oxygen orderings. Our results for ortho-II contrast with recent high-field NMR measurements, which suggest a commensurate wavevector along the $\mathbf{a}$-axis. We discuss the relationship between spin and charge correlations in YBa$_2$Cu$_3$O$_{y}$, and recent high-field quantum oscillation, NMR and ultrasound experiments.

cond-mat.supr-con↗

arXiv2013

Dispersive spin excitations in highly overdoped cuprates revealed by resonant inelastic x-ray scattering

Using resonant inelastic x-ray scattering (RIXS) at the Cu $L$-absorption edge, we have observed intense, dispersive spin excitations in highly overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$ (superconducting $T_c =6$ K), a compound whose normal-state charge transport and thermodynamic properties have been studied extensively and shown to exhibit canonical Fermi-liquid behavior. Complementary RIXS experiments on slightly overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$ ($T_c =89$ K) and on Y$_{1-x}$Ca$_{x}$Ba$_2$Cu$_3$O$_{6+δ}$ 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. The surprising coexistence of Fermi-liquid-like charge excitations and high-energy spin excitations reminiscent of antiferromagnetic insulators in highly overdoped compounds poses a challenge to current theoretical models of the cuprates.

cond-mat.str-el↗

arXiv2012

Vortex Lattice Melting and Hc2 in underdoped YBa2Cu3Oy

Vortices in a type-II superconductor form a lattice structure that melts when the thermal displacement of the vortices is an appreciable fraction of the distance between vortices. In an anisotropic high-Tc superconductor, such as YBa2Cu3Oy, the magnetic field value where this melting occurs can be much lower than the mean-field critical field Hc2. We examine this melting transition in YBa2Cu3Oy with oxygen content y from 6.45 to 6.92, and fit the data to a theory of vortex-lattice melting. The quality of the fits indicates that the transition to a resistive state is indeed the vortex lattice melting transition, with the shape of the melting curves being consistent with the known change in penetration depth anisotropy from underdoped to optimally doped YBa2Cu3Oy. From the fits we extract Hc2(T = 0) as a function of hole doping. The unusual doping dependence of Hc2(T =0) points to some form of electronic order competing with superconductivity around 0.12 hole doping.

cond-mat.supr-con↗

arXiv2012

Thermodynamic phase diagram of static charge order in underdoped YBCO

The interplay between superconductivity and any other competing order is an essential part of the long-standing debate on the origin of high temperature superconductivity in cuprates. Akin to the situation of heavy fermions, organic superconductors and pnictides, it has been proposed that the pairing mechanism in cuprates comes from fluctuations of a nearby quantum phase transition. Recent evidence of charge modulation and the associated fluctuations in the pseudogap phase of YBa_2Cu_3O_y make charge order a likely candidate for a competing order. However, a thermodynamic signature of the charge ordering phase transition is still lacking. Moreover, whether such charge order is one- or two-dimensional is still controversial but pivotal for the understanding the topology of the reconstructed Fermi surface. Here we address both issues by measuring sound velocities in YBCO_6.55 in high magnetic fields, a powerful thermodynamic probe to detect phase transitions. We provide the first thermodynamic signature of the field-induced charge ordering phase transition in YBCO allowing construction of a field-temperature phase diagram, which reveals the competing nature of this charge order. The comparison of different acoustic modes indicates that the charge modulation has a two-dimensional character, thus imposing strong constraints on Fermi surface reconstruction scenarios.

cond-mat.supr-con↗

arXiv2012

Distinct Charge Orders in the Planes and Chains of Ortho-III-Ordered YBa$_2$Cu$_3$O$_{6+δ}$ Superconductors Identified by Resonant Elastic X-ray Scattering

Recently, charge density wave (CDW) order in the CuO$_2$ planes of underdoped YBa$_2$Cu$_3$O$_{6+δ}$ was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned sample of YBa$_2$Cu$_3$O$_{6.75}$ with ortho-III oxygen ordering in the chain layer. We show that the ortho-III CDW order in the chains is distinct from the CDW order in the planes. The ortho-III structure gives rise to a commensurate superlattice reflection at $Q$=[0.33 0 $L$] whose energy and polarization dependence agrees with expectations for oxygen ordering and a spatial modulation of the Cu valence in the chains. Incommensurate peaks at [0.30 0 $L$] and [0 0.30 $L$] from the CDW order in the planes are shown to be distinct in $Q$ as well as their temperature, energy, and polarization dependence, and are thus unrelated to the structure of the chain layer. Moreover, the energy dependence of the CDW order in the planes is shown to result from a spatial modulation of energies of the Cu 2$p$ to 3$d_{x^2-y^2}$ transition, similar to stripe-ordered 214 cuprates.

cond-mat.supr-con↗

arXiv2012

Bound States of Defects in Superconducting LiFeAs Studied by Scanning Tunneling Spectroscopy

Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their position within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multi-gap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair-breaking indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound state patterns, including one with a chiral distribution of the local density of states (LDOS). The multiple bound state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for s$^{\pm}$ gap symmetry proposed for LiFeAs and other iron pnictides.

cond-mat.supr-con↗

arXiv2012

Coherent c-axis transport in the underdoped cuprate superconductor YBCO

The electrical resistivity rho_c of the underdoped cuprate superconductor YBCO was measured perpendicular to the CuO_2 planes on ultra-high quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of rho_c at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature. This crossover coincides with the emergence of the small electron pocket detected in the Fermi surface of YBCO via quantum oscillations, the Hall and Seebeck coefficients and with the detection of a unidirectional modulation of the charge density as seen by high-field NMR measurements. The low coherence temperature is quantitatively consistent with the small hopping integral t_perp inferred from the splitting of the quantum oscillation frequencies. We conclude that the Fermi-surface reconstruction in YBCO at dopings from p = 0.08 to at least p = 0.15, attributed to stripe order, produces a metallic state with 3D coherence deep in the underdoped regime.

cond-mat.supr-con↗

arXiv2012

Scanning tunneling spectroscopy of superconducting LiFeAs single crystals: Evidence for two nodeless energy gaps and coupling to a bosonic mode

The superconducting compound, LiFeAs, is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with $Δ_1=5.3\pm0.1$ meV and $Δ_2=2.5\pm0.2$ meV. The gaps close as the temperature is increased to the bulk $T_c$ indicating that the surface accurately represents the bulk. A dip-hump structure is observed below $T_c$ with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering.

cond-mat.supr-con↗

arXiv2012

Direct observation of competition between superconductivity and charge density wave order in YBa2Cu3Oy

Superconductivity often emerges in the proximity of, or in competition with, symmetry breaking ground states such as antiferromagnetism or charge density waves (CDW)1-5. A number of materials in the cuprate family, which includes the high-transition-temperature (high-Tc) superconductors, show spin and charge density wave order5-7. Thus a fundamental question is to what extent these ordered states exist for compositions close to optimal for superconductivity. Here we use high-energy x-ray diffraction to show that a CDW develops at zero field in the normal state of superconducting YBa2Cu3O6.67 (Tc = 67 K). Below Tc, the application of a magnetic field suppresses superconductivity and enhances the CDW. Hence, the CDW and superconductivity are competing orders in this typical high-Tc superconductor, and high-Tc superconductivity can form from a pre-existing CDW state. Our results explain observations of small Fermi surface pockets8, negative Hall and Seebeck effect9,10 and the "Tc plateau"11 in this material when underdoped.

cond-mat.supr-con↗

arXiv2012

Quantum oscillations from nodal bilayer magnetic breakdown in the underdoped high temperature superconductor YBa2Cu3O6+x

We report quantum oscillations in underdoped YBa2Cu3O6.56 over a significantly large range in magnetic field extending from 24 to 101 T, enabling three well-spaced low frequencies at 440 T, 532 T, and 620 T to be clearly resolved. We show that a small nodal bilayer coupling that splits a nodal pocket into bonding and antibonding orbits yields a sequence of frequencies, F0 - ΔF, F0, and F0 + ΔF and accompanying beat pattern similar to that observed experimentally, on invoking magnetic breakdown tunneling at the nodes. The relative amplitudes of the multiple frequencies observed experimentally in quantum oscillation measurements are shown to be reproduced using a value of nodal bilayer gap quantitatively consistent with that measured in photoemission experiments in the underdoped regime.

cond-mat.supr-con↗

arXiv2011

Magnetic-field-induced charge-stripe order in the high temperature superconductor YBa2Cu3Oy

Electronic charges introduced in copper-oxide planes generate high-transition temperature superconductivity but, under special circumstances, they can also order into filaments called stripes. Whether an underlying tendency of charges to order is present in all cuprates and whether this has any relationship with superconductivity are, however, two highly controversial issues. In order to uncover underlying electronic orders, magnetic fields strong enough to destabilise superconductivity can be used. Such experiments, including quantum oscillations in YBa2Cu3Oy (a notoriously clean cuprate where charge order is not observed) have suggested that superconductivity competes with spin, rather than charge, order. Here, using nuclear magnetic resonance, we demonstrate that high magnetic fields actually induce charge order, without spin order, in the CuO2 planes of YBa2Cu3Oy. The observed static, unidirectional, modulation of the charge density breaks translational symmetry, thus explaining quantum oscillation results, and we argue that it is most likely the same 4a-periodic modulation as in stripe-ordered cuprates. The discovery that it develops only when superconductivity fades away and near the same 1/8th hole doping as in La2-xBaxCuO4 suggests that charge order, although visibly pinned by CuO chains in YBa2Cu3Oy, is an intrinsic propensity of the superconducting planes of high Tc cuprates.

cond-mat.supr-con↗