M. Hücker

Stripe correlations of spins and holes and phonon heat transport in doped La2CuO4

We present experimental evidence for a dramatic suppression of the phononic thermal conductivity of rare-earth–and Sr–doped La2CuO4. Remarkably, this suppression correlates with the occurrence of superconductivity. Conventional models for the phonon heat transport fail to explain these results. In contrast, a straightforward explanation is possible in terms of static and dynamic stripe correlations of holes and spins.

Consequences of stripe order for the transport properties of rare earth doped La2−xSrxCuO4

Abstract We present a detailed study of the resistivity and the thermopower in the stripe ordered non superconducting low temperature tetragonal phase of rare earth (RE=Nd,Eu) doped La2−xSrxCuO4. The anomalies of the thermopower due to the structural low temperature transition are maximum at x=1/8, whereas the increase of the resistivity is minimum at x≤1/8. We discuss our data within the framework of static stripes of charge and spins. In particular our experimental findings for the resistivity are only compatible with this model if one assumes a rather large conductivity in the quasi one-dimensional charge stripes.

Slow antiferromagnetic dynamics in the low-temperature tetragonal phase of La{sub 2{minus}x}Sr{sub x}CuO{sub 4} as revealed by ESR of Gd spin probes

Measuring the electron spin resonance of Gd spin probes we have studied the magnetic properties of the copper oxide planes in the low-temperature tetragonal (LTT) phase of Eu-doped La{sub 2{minus}x}Sr{sub x}CuO{sub 4}. The data give evidence that at particular levels of Sr and Eu doping the frequency of the antiferromagnetic fluctuations {omega}{sub sf} in the LTT phase dramatically decreases at low temperatures by almost three orders of magnitude. However, no static magnetic order has been found for T{ge}8 K, in contrast to the observation by neutron scattering of stripe ordering of spins below 50 K in a Nd-doped La{sub 2{minus}x}Sr{sub x}CuO{sub 4} single crystal. In our opinion, static order in the Nd-doped compound is induced due to the interaction between the Cu spins with the rare-earth magnetic moments. Therefore, a characteristic property of the magnetism in the LTT structural phase may not be static magnetic order at elevated temperatures but rather extremely slow antiferromagnetic dynamics. {copyright} {ital 1997} {ital The American Physical Society}

Magnetism of the LTT phase of Eu-doped La2-x Sr x CuO4

The ESR signal of Gd spin probes (0.5 at.%) as well as the static normal state susceptibility of Eu (J(Eu3+) = 0) doped La2-x-ySrxEuyCuO4 reveal pronounced changes of the Cu magnetism at the structural transition from the orthorhombic to the low-temperature tetragonal phase for all nonsuperconducting compositions. Both a jumplike decrease ofx as well as the ESR data show an increase of the in-plane magnetic correlation length in the LTT phase. From the Gd3+ ESR linewidth we find that for specific Eu and Sr concentrations in the LTT phase the correlation length increases up to more than 100 lattice constants and the fluctuation frequency of the CuO2 spin system slows down to ~1010-1011 s-1 However, there is no static order above T~8 K in contrast to the LTT phase of Nd-doped La{2-x}SrxCuO4 with pinned stripe correlations.

Strong enhancement of spin fluctuations in the low-temperature tetragonal phase of antiferromagnetically ordered La2-x-yEuySrxCuO4

Measurements of the static magnetization, susceptibility and electron spin resonance (ESR) of Gd spin probes have been performed to study the properties of antiferromagnetically ordered La2-x-yEuySrxCuO4 (x0.02) with the low-temperature tetragonal structure. According to the static magnetic measurements, the CuO2 planes are magnetically decoupled in this structural phase. The ESR study reveals strong magnetic fluctuations at the ESR frequency which are not present in the orthorhombic phase. It is argued that this drastic enhancement of the spin fluctuations is due to a considerable weakening of the interlayer exchange and a pronounced influence of hole motion on the antiferromagnetic properties of lightly hole-doped La2CuO4. No evidence for the stripe phase formation at small hole doping is obtained in the present study.

Transport properties of rare earth doped La2−xSrxCuO4

We present measurements of the resistivity, the thermopower, and the thermal conductivity of rare earth (RE) doped La2−xSrxCuO4. Pronounced anomalies occur at the low temperature structural phase transition showing that the electronic properties as well as the lattice dynamics depend strongly on small structural changes. Our results indicate that a suppressed phonon heat transport is a characteristic feature of La2CuO4 based superconductors. A possible relationship to the presence of stripe correlations of holes and spins in doped La2CuO4 is discussed.

Spin Dynamics in the LTT Phase of ~1/8 Doped Single Crystal La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4

We present {sup 139}La and {sup 63}Cu NMR relaxation measurements in single crystal La{sub 1.67}Eu{sub 0.2}Sr{sub 0.13}CuO{sub 4}. A strong peak in the {sup 139}La spin-lattice relaxation rate observed in the spin ordered state is well described by the BPP mechanism [Bloembergen, Purcell, and Pound, Phys. Rev. 73, 679 (1948)] and arises from continuous slowing of electronic spin fluctuations with decreasing temperature; these spin fluctuations exhibit XY-like anisotropy in the ordered state. The spin pseudogap is significantly enhanced by the static charge-stripe order in the LTT phase. (c) 2000 The American Physical Society.We present La and Cu NMR relaxation measurements in single crystal La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4. A strong peak in the La spin-lattice relaxation rate observed in the spin ordered state is well-described by the BPP mechanism[1] and arises from continuous slowing of electronic spin fluctuations with decreasing temperature; these spin fluctuations exhibit XY-like anisotropy in the ordered state. The spin pseudogap is enhanced by the static charge-stripe order in the LTT phase.

Magnetic properties of the CuO2 planes in the low-temperature tetragonal phase of Eu-doped La 2 − xSrxCuO4

Abstract Measurements of the DC susceptibility and ESR of Gd spin probes in Eu-doped La 2 − x Sr x CuO 4 are reported. The data show that magnetism of the low-temperature tetragonal phase of this compound differs qualitatively from that in the orthorhombic phase but only if the former phase is nonsuperconducting. We discuss the obtained results in connection with stripe correlations and critical buckling of the CuO 2 planes.