LTT Phase Research Articles

Comparison of stripe modulations in La1.875Ba0.125CuO4and La1.48Nd0.4Sr0.12CuO4

We report combined soft and hard x-ray scattering studies of the electronic and lattice modulations associated with stripe order in La${}_{1.875}$Ba${}_{0.125}$CuO${}_{4}$ and La${}_{1.48}$Nd${}_{0.4}$Sr${}_{0.12}$CuO${}_{4}$. We find that the amplitude of both the electronic modulation of the hole density and the strain modulation of the lattice is significantly larger in La${}_{1.875}$Ba${}_{0.125}$CuO${}_{4}$ than in La${}_{1.48}$Nd${}_{0.4}$Sr${}_{0.12}$CuO${}_{4}$ and is also better correlated. The in-plane correlation lengths are isotropic in each case; for La${}_{1.875}$Ba${}_{0.125}$CuO${}_{4}$, ${\ensuremath{\xi}}^{\mathrm{hole}}=255\ifmmode\pm\else\textpm\fi{}5$ \AA{}, whereas for La${}_{1.48}$Nd${}_{0.4}$Sr${}_{0.12}$CuO${}_{4}$, ${\ensuremath{\xi}}^{\mathrm{hole}}=111\ifmmode\pm\else\textpm\fi{}7$ \AA{}. We find that the modulations are temperature independent in La${}_{1.875}$Ba${}_{0.125}$CuO${}_{4}$ in the low temperature tetragonal phase. In contrast, in La${}_{1.48}$Nd${}_{0.4}$Sr${}_{0.12}$CuO${}_{4}$, the amplitude grows smoothly from zero, beginning 13 K below the LTT phase transition. We speculate that the reduced average tilt angle in La${}_{1.875}$Ba${}_{0.125}$CuO${}_{4}$ results in reduced charge localization and incoherent pinning, leading to the longer correlation length and enhanced periodic modulation amplitude.

Nernst effect of stripe ordering La1.8−xEu0.2SrxCuO4

We investigate the transport properties of La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ ($x=0.04$, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For $x=0.125$ and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except $x=0.2$ exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe order. Moreover, a direct comparison between the Nernst coefficients of stripe ordering La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ and superconducting La$_{2-x}$Sr$_x$CuO$_4$ at the doping levels $x=0.125$ and $x=0.15$ reveals only weak differences. Our findings make high demands on any scenario interpreting the Nernst response in hole-doped cuprates.

Dzyaloshinsky-Moriya spin canting in the low-temperature tetragonal phase ofLa2−x−yEuySrxCuO4

The Cu spin magnetism in La2-x-yEuySrxCuO4 (x<=0.17; y<=0.2) has been studied by means of magnetization measurements up to 14 T. Our results clearly show that in the antiferromagnetic phase Dzyaloshinsky-Moriya (DM)superexchange causes Cu spin canting not only in the LTO phase but also in the LTLO and LTT phases. In La1.8Eu0.2CuO4 the canted DM-moment is about 50% larger than in pure La2CuO4 which we attribute to the larger octahedral tilt angle. We also find clear evidence that the size of the DM-moment does not change significantly at the structural transition at T_LT from LTO to LTLO and LTT. The most important change induced by the transition is a significant reduction of the magnetic coupling between the CuO2 planes. As a consequence, the spin-flip transition of the canted Cu spins which is observed in the LTO phase for magnetic field perpendicular to the CuO2 planes disappears in the LTT phase. The shape of the magnetization curves changes from the well known spin-flip type to a weak-ferromagnet type. However, no spontaneous weak ferromagnetism is observed even at very low temperatures, which seems to indicate that the interlayer decoupling in our samples is not perfect. Nonetheless, a small fraction (<15%) of the DM-moments can be remanently magnetized throughout the entire antiferromagnetically ordered LTT/LTLO phase, i.e. for T<T_LT and x<0.02. It appears that the remanent DM-moment is perpendicular to the CuO2 planes. For magnetic field parallel to the CuO2 planes we find that the critical field of the spin-flop transition decreases in the LTLO phase, which might indicate a competition between different in-plane anisotropies. To study the Cu spin magnetism in La2-x-yEuySrxCuO4, a careful analysis of the Van Vleck paramagnetism of the Eu3+ ions was performed.

Stripe orientation in an anisotropict−Jmodel

The tilt pattern of the CuO_6 octahedra in the LTT phase of the cuprate superconductors leads to planar anisotropies for the exchange coupling and hopping integrals. Here, we show that these anisotropies provide a possible structural mechanism for the orientation of stripes. A t_x-t_y-J_x-J_y model thus serves as an effective Hamiltonian to describe stripe formation and orientation in LTT-phase cuprates.

Low temperature X-ray diffraction study of La 2−xM xCuO 4, M [dbnd], Sr

The low temperature phase diagrams for La 2−xM xCuO 4, M Ba and Sr, were redetermined using high resolution synchroton x-ray diffractionm by monitoring the low temperature orthorhombic (LTo1, standard space group Cmca indexed Bmab) 020/200 and low temperature tetragonal (LTT, P4 2/ncm) 200 peaks. For M Ba, the LTT phase exists as a minority phase at the 10–15% level in the nominally LTO1 phase well above the bulk LTT transformation temperature. For M Sr there is no bulk LTT transformation, but up to 10% LTT phase is observed at low temperatures. For unsubstituted La 2CuO 4 no LTT contribution to diffraction is observed, down to an estimated sensitivity of 3%. For x>0.12, both for M Sr and Ba, the LTO1 200/020 diffraction peak splitting at low temperatures is inadequate to allow the observation of a possible minority LTT phase. Based on low temperature transmission electron microscopy studies of La 2−xBa xCuO 4, we propose a two step transformation from the LTO1 to the LTT phase. The LTT phase may be nucleated at relatively high temperatures at the LTO1 twin boundaries. For M Ba, near T=60K, the LTT phase also develops within the LTO1 twins, and the sample essentially completely transforms to LTT.

Transport measurements of La [sbnd]Nd [sbnd]Sr [sbnd]Cu [sbnd]O superconductors under hydrostatic and uniaxial pressure

We investigate the pressure effect on superconducting transition temperature ( T c ) and the Hall coefficeint of La 1.48Nd 0.4Sr 0.12CuO 4 crystal under hydrostatic pressure upto 10 kbar. We found that there was a dramatic change of the Hall coefficient at 2 kbar. The resistivity measurements clearly show the suppression of LTT phase as well as the low temperature upturn at 2 kbar. The uniaxial pressure on the same crystal along a- axis increase T c while T c decreases under the pressure along the c-axis. The suppression of LTT phase with pressure along a-axis with low temperature upturn in resistivity remaining near T c indicates a possible existence of LTT domains in the LTO phase. The uniaxial pressure along the ab-plane turns out to weaken the pinning of stripes (or stripe fluctuations) and in turn to enhance superconductivity in the CuO 2 planes.

Cu-NQR Study for Possible Static Spin-Charge Stripe Order in La2-xBaxCuO4withx=1/8

Cu-NQR spectra were measured in the wide temperature range of 300 K to 1.5 K to examine the possibility of the occurrence of the static spin-charge stripe order in La 2- x Ba x CuO 4 with x =1/8. The usual NQR spectrum is observed above T d2 , the structural transition temperature from the LTO to LTT phase. Below T d2 , the signal intensity decreases rapidly with decreasing temperature, which is accompanied with the abrupt increase of the effective spin echo decay rate, while no significant changes appear in its lineshape down to 10 K. Below 10 K, the spectrum starts to spread and exhibits unusual broadening with a tail to about 80 MHz at 1.5 K. The abnormally broad spectrum at 1.5 K, which is regarded as evidence of the static spin and/or charge order, is simulated satisfactorily in terms of the stripe order proposed for La 2- x - y Nd y Sr x CuO 4 ( x =1/8) with the assumption of the modulation amplitude for Cu moment of about 0.24 µ B .

Spin dynamics in the low-temperature tetragonal phase of≅18doped single crystalLa1.67Eu0.2Sr0.13CuO4

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 and structural instabilities in the stripe-phase region of La1.875Ba0.125 - ySryCuO4 (0 y 0.1)

Zero-field positive muon spin relaxation (ZF-µ+SR) experiments were performed to investigate the magnetic properties of the low-temperature structural modifications of the La1.875Ba0.125 - ySryCuO4 ( y = 0.0, 0.025, 0.050, 0.075, 0.100) series, in which the total hole concentration is close to 1 / 8. Together with high-resolution time-of-flight neutron powder diffraction measurements, the results imply that the interplay among lattice distortions, doping and superconductivity is intimately related to the magnetic correlations of the Cu spins, which are interpreted in terms of a stripe-phase model. Materials with y 0.075 exhibit an incomplete structural transition at Td 2, from a low-temperature orthorhombic (LTO, Bmab) to a low-temperature tetragonal (LTT, P42 / ncm) phase. Diffraction patterns collected while approaching Td 2 reveal a rapid reduction of the orthorhombicity in the LTO phase with a residual fraction always surviving to low temperatures. Different behaviour is shown by the composition with y = 0.1. Rietveld analysis shows coexistence of the LTO phase with a less distorted low-temperature orthorhombic phase (LTO-2, Pccn). In the LTT phase, a fraction of muons senses regions with purely static spin correlations due to an incommensurate spin-density wave. The muon spin depolarization suggests that in the spatially separated remnant LTO, hole-rich domains, dynamical correlations of charge density give rise to superconductivity. As the spin fluctuations become static at y 0.075, the London penetration depth, calculated from the transverse-field µ+SR depolarization rate, , reveals a decrease in the superconducting carrier density. The magnetic freezing temperature, Tf is suppressed and the magnetic phase fraction shrinks as y increases, whereas the superconducting correlations persist in a larger sample volume. The disappearance of long-range magnetic order at y = 0.1 and the growth of a quasi-static component correlate well with the presence of the LTO-2 microstructure which behaves as a buffer phase out of which the LTT domains become dominant in the Ba-rich compositions.

Magnetic Phase Transition and Electronic Anomalies in La 1.96−x Y 0.04 Sr x CuO 4 around x∼1/8

La-NMR, Hall coefficient and sound velocity have been measured in order to investigate the correlation between the magnetic ordering, the prominent change of transport properties, the suppression of T c and the structural transformation around x∼1/8 in La 1.96−x Y 0.04 Sr x CuO 4. By the substitution of 0.04/2 Y for La-sites, the structural transformation to the low temperature tetragonal phase (LTT) is caused at 57±14 K for samples around x∼0.115. The. prominent decrease of the Hall coefficient followed by the sign reversal and the magnetic ordering are observed around x∼0.115 below temperatures T s and T N, respectively. T s vs x shows a bell shaped curve with the maximum value of 65 K at x∼00115 where most prominent suppression of T c appears. T N shows similar x dependence to T s with the maximum value of 40 K at x∼0.115. The change of the electronic state below T s and the suppression of T c become more prominent and the magnetic ordering is observed more wide range of x under the LTT phase.

Magnetic order in La Eu Sr CuO studied [2pt] by Fe Mössbauer spectroscopy

57Fe Mossbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4.

High-resolution x-ray diffraction study ofLa1.88−ySr0.12NdyCuO4

The crystallographic transformation in La{sub 1.88{minus}y}Sr{sub 0.12}Nd{sub y}CuO{sub 4}(y=0, 0.1, 0.2, and 0.4) from the orthorhombic Bmab space group (LTO1 phase) to a low-temperature tetragonal (P4{sub 2}/ncm, LTT phase) phase was examined using high-resolution synchrotron x-ray powder diffraction. For Nd contents y=0.2 and 0.4, the phase transformation to LTT takes place over a significant range of temperature, with the transformation essentially complete at the lowest temperatures attained, {approximately}15 K. For Nd y=0.0 and 0.1, the transformation to LTT is incomplete, with the LTT phase corresponding to about 10{percent} and 20{percent} of the LTO1 intensity at lowest temperatures. The evolution of the LTT phase is discussed in the context of earlier transmission electron microscopy studies. Simulated diffraction profiles based on the observed microstructure are used to explore possible mechanisms for development of the LTT phase. The presence of this minority LTT phase in La{sub 1.88}Sr{sub 0.12}CuO{sub 4} (Nd y=0) helps explain the slightly depressed superconducting transition temperature observed for slightly higher and lower Sr contents in terms of the proposed charge stripe phase. {copyright} {ital 1998} {ital The American Physical Society}

Erratum: Thermally activated dynamics of the tilts of theCuO6octahedra, hopping of interstitial O, and possible instability towards the LTT phase inLa2CuO4+δ[Phys. Rev. B57, 8580 (1998)

Erratum: Thermally activated dynamics of the tilts of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>octahedra, hopping of interstitial O, and possible instability towards the LTT phase in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo><mml:mi mathvariant="normal">δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>[Phys. Rev. B<b>57</b>, 8580 (1998)

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.

MAGNETISM AND SUPERCONDUCTIVITY IN LA1.875BA0.125-YSRYCUO4+DELTA AND LA1.6-YND0.4SRYCUO4

μ+SR and neutron diffraction measurements show that at low temperatures in La1.875 Ba0.125 CuO4, there is phase separation into a long range ordered magnetic component of SDW‐like nature, identified with the LTT phase and a superconducting component, identified with the LTO phase. Partial substitution of Ba by Sr at a fixed doping level of 1/8 leads to gradual decrease of the magnetic freezing temperature, accompanied by a decrease in the fraction of the magnetic component. The La1.875 Ba0.025 Sr0.100 CuO_4 composition exhibits freezing of the Cu electronic moments into a disordered arrangement. Excess interstitial oxygen leads to a rapid recovery of superconductivity and a marked depression of the magnetic volume fraction. Study of the LTT phase – after stabilisation with partial substitution of La by Nd – away from the 1/8 doping level, reveals no evidence of long range magnetic ordering with magnetic fluctuations surviving down to 3 K.

High-pressure and low-temperature study of the LTO to LTT phase transition in La1.875Ba0.125CuO4

High-pressure and low-temperature study of the LTO to LTT phase transition in La_1.875Ba_0.125CuO₄

Superconductivity in tetragonal and orthorhombic structural phases of La 2− xM xCuO 4

An effective electron-phonon model for La 2− x M x CuO 4 allowing for structural phase transitions (SPT) from high-temperature tetragonal phase (HTT) to low-temperature orthorhombic phase (LTO) and to low-temperature tetragonal phase (LTT) is proposed. The dependence of the superconducting transition temperature T c on the hole concentration n is calculated for HTT, LTO and LTT phases using the Green function technique. A dip of T c is found at the transition to LTT phase owing to splitting of two oxygen bands (at half filling).

Thermopower of rare earth doped La 2−xSr xCuO 4

We have studied the thermopower S of La 1.4−xNd 0.6Sr xCuO 4 as a function of temperature and the hole concentrations x. At the LTO-LTT phase transition anomalies of S occur, which weaken with increasing x. In the LTT phase S becomes negative in a narrow hole concentration range around x⋍ 1 8 , which gives evidence for a particular role of this hole concentration.

Photoemission study of La-system cuprate oxide superconductors

The electronic structure of La2−xBaxCuO4 (x∼0.125) has beenstudied by ultraviolet photoemission spectroscopy. It is found that the LTO->LTT phase transition has little effect on the electronic density of states (DOS) near the Fermi level. This suggests that the DOS change is not the main cause of the suppression of superconductivity, contrary to previous reports. We also make a comparison between La2−xBaxCuO4 (x≈0.125) and La2−xSrxCuO4 (x≈0.15) in the electronic DOS.

Magnetic order and suppression of superconductivity around x=0.12 in La 2−xSr xCuO 4 and La 2− xBa xCuO 4

Although there exists an antiferromagnetic order of Cu-moments around x=0.115in La 2−xSr xCuO 4 (LSCO) as similar to La 2−xBa xCuO 4, x-ray diffraction and transport measurements reveal no evidences of the structural transition to the LTT phase in LSCO. The results suggest that the structural change is not a primary cause of the suppression of superconductivity at the particular composition around x=0.12, but the freezing of spin fluctuations of Cu-moments may play critical role for the anomalies of the La-systems.