Marc-Henri Julien

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

Electronic charges introduced in copper-oxide (CuO2) planes generate high-transition-temperature (Tc) superconductivity but, under special circumstances, they can also order into filaments called stripes. Whether an underlying tendency towards charge order is present in all copper oxides and whether this has any relationship with superconductivity are, however, two highly controversial issues. To uncover underlying electronic order, magnetic fields strong enough to destabilize superconductivity can be used. Such experiments, including quantum oscillations in YBa2Cu3Oy (an extremely clean copper oxide in which charge order has not until now been observed) have suggested that superconductivity competes with spin, rather than charge, order. Here we report nuclear magnetic resonance measurements showing 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 probably the same 4a-periodic modulation as in stripe-ordered copper oxides. That it develops only when superconductivity fades away and near the same 1/8 hole doping as in La2−xBaxCuO4 (ref. 1) suggests that charge order, although visibly pinned by CuO chains in YBa2Cu3Oy, is an intrinsic propensity of the superconducting planes of high-Tc copper oxides.

63Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7

Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.

Incipient charge order observed by NMR in the normal state of YBa2Cu3Oy

The pseudogap regime of high-temperature cuprates harbours diverse manifestations of electronic ordering whose exact nature and universality remain debated. Here, we show that the short-ranged charge order recently reported in the normal state of YBa2Cu3Oy corresponds to a truly static modulation of the charge density. We also show that this modulation impacts on most electronic properties, that it appears jointly with intra-unit-cell nematic, but not magnetic, order, and that it exhibits differences with the charge density wave observed at lower temperatures in high magnetic fields. These observations prove mostly universal, they place new constraints on the origin of the charge density wave and they reveal that the charge modulation is pinned by native defects. Similarities with results in layered metals such as NbSe2, in which defects nucleate halos of incipient charge density wave at temperatures above the ordering transition, raise the possibility that order–parameter fluctuations, but no static order, would be observed in the normal state of most cuprates if disorder were absent.

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 as neither the universality of the competition nor its microscopic nature has been established. Here, we show using nuclear magnetic resonance that charge order in YBa2Cu3Oy has maximum strength inside the superconducting dome, similar to compounds of the La2-x(Sr,Ba)xCuO4 family. In YBa2Cu3Oy, this occurs at doping levels of 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 Bi2Sr2CaCu2O8+δ, 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.

NUCLEAR MAGNETIC RESONANCE STUDY OF THE S = 1/2 HEISENBERG LADDER CU2(C5HJ2N2)2C14 : QUANTUM PHASE TRANSITION AND CRITICAL DYNAMICS

We present an extensive NMR study of the spin-1/2 antiferromagnetic Heisenberg ladder Cu2(C5H12N2)2Cl4 in a magnetic field range 4.5 - 16.7 T. By measuring the proton NMR relaxation rate 1/T_1 and varying the magnetic field around the critical field H_c1 = Delta / g\mu_B = 7.5 T, we have studied the transition from a gapped spin liquid ground state to a gapless magnetic regime which can be described as a Luttinger liquid. We identify an intermediate regime T > |H-H_c1|, where the spin dynamics is (possibly) only controlled by the T=0 critical point H_c1.

Proton NMR for Measuring Quantum Level Crossing in the Magnetic Molecular Ring Fe10

The proton nuclear spin-lattice relaxation rate 1/T{sub 1} has been measured as a function of temperature and magnetic field (up to 15thinspthinspT) in the molecular magnetic ring Fe{sub 10}( OCH{sub 3}){sub 20}(O{sub 2}CCH{sub 2} Cl){sub 10} (Fe10). Striking enhancement of 1/T{sub 1} is observed around magnetic field values corresponding to a crossing between the ground state and the excited states of the molecule. We propose that this is due to a cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir of the Zeeman levels of the molecule. This effect provides a powerful tool to investigate quantum dynamical phenomena at level crossing. {copyright} {ital 1999} {ital The American Physical Society}

Charge Segregation, Cluster Spin-Glass and Superconductivity in La1.94Sr0.06CuO4

A {sup 63}Cu and {sup 139}La NMR/NQR study of superconducting (T{sub c}=8 K) La{sub 1.94} Sr{sub 0.06} CuO{sub 4} single crystal is reported. Coexistence of spin-glass and superconducting phases is found below {approximately}5 K from {sup 139}La NMR relaxation. {sup 63}Cu and {sup 139}La NMR spectra show that, upon cooling, CuO{sub 2} planes progressively separate into two magnetic phases, one of them having enhanced antiferromagnetic correlations. These results establish the antiferromagnetic-cluster nature of the spin glass. We discuss how this phase can be related to the microsegregation of mobile holes and to the possible pinning of charge stripes. {copyright} {ital 1999} {ital The American Physical Society }

Homogeneous vs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co- and K-doped iron pnictides

In Ba(Fe0.95Co0.05)2As2 all of the 75As NMR intensity at the paramagnetic resonance position vanishes abruptly below TonsetSDW=56 K, indicating that magnetic (spin-density wave) order is present in all of the sample volume, despite bulk superconductivity below Tc=15 K. The two phases thus coexist homogeneously at the microscopic scale. In Ba0.6K0.4Fe2As2, on the other hand, the signal loss below TonsetSDW75 K is not complete, revealing that magnetic order is bound to finite-size areas of the sample, while the remaining NMR signal shows a clear superconducting response below Tc=37 K. Thus, the two phases are not homogeneously mixed, at least for this potassium concentration. For both samples, spatial electronic and/or magnetic inhomogeneity is shown to characterize the NMR properties in the normal state.

Identification of Nuclear Relaxation Processes in a Gapped Quantum Magnet: 1 H NMR in the S = 1 2 Heisenberg Ladder Cu 2 ( C 5 H 12 N 2 ) 2 Cl 4

The proton hyperfine shift K and NMR relaxation rate

Emergence of Orbital Nematicity in the Tetragonal Phase of BaFe2(As1−xPx)2

1/T_1