The NQR picture of electronic phase separation in the lanthanum–barium cuprates

We report the results of Cu and La NQR investigations of the spin and charge separation in La_2−xBa_xCuO₄ compounds. For Ba doping inducing the low-temperature tetragonal phase our Cu–NQR measurements revealed three inequivalent Cu sites in CuO₂ planes. In terms of the stripe-phase picture we assign them to the charged stripe, to its neighborhood and to the AF correlated regions free of holes correspondingly. The last site corresponds to the ordered Cu magnetic moment of 0.29μ_B.     

Ordering of oxygen in halogen treated 1-2-3 systems as studied by NQR

We have investigated the low-temperature behavior of Cu NQR for iodinated RBa₂Cu₃O _x (R = Y, Gd) compounds. The different influence of Gd spins on the Cu(1) and Cu(2) sites was used to separate the contributions of both sites in the spectra. The distinct NQR lines associated with the different oxygen-coordinated Cu(1) sites have been observed. The NQR spectra of the Cu(1) sites show that the iodine treatment gives rise to the modifications of the oxygen sublattice. From the analysis of the NQR spectra of the Cu(1) sites follows an increase of the average length of the Cu(1)-O chains at treatment.     

In-plane charge modulation below T(c) and charge-density-wave correlations in the chain layer in YBa2Cu3O7

Nuclear quadrupole resonance (NQR) measurements have been performed on Cu(2) plane sites and Cu(1) chain sites in fully doped YBa2Cu3O7 between 300 and 4.2 K. The sharp increase of the Cu(1) NQR linewidth across the superconducting transition and the T dependence of the Cu(1) spin lattice relaxation rate confirm the existence of a charge-density-wave state (CDW) in the chains. The simultaneous broadening of the Cu(2) linewidth below T(c) and the anomalous T dependence of Cu(1) and Cu(2) NQR parameters indicate that these in-chain CDW correlations are strongly involved in the appearance of an in-plane charge modulated structure below T(c).     

Cu chemistry of YBa2Cu3O6+x investigated by nuclear quadrupole resonance

The nuclear quadrupole resonance (NQR) technique has been utilized to characterize the local oxygen coordination of inequivalent Cu sites in YBa₂Cu₃O_6+x(0 ≤ x ≤ 0.91). Essentially, four distinct NQR lines which correspond to 2, 3,4 oxygen coordinated Cu sites in the Cu-O chains and 5 oxygen coordinated Cu sites in the Cu-O planes have been observed. The zero-field NQR frequencies of these are centered at about 30.1, 24.0, 22.0 and 31.5 MHz for ⁶³Cu, respectively. For the antiferromagnetic ordered state (x ≤ 0.3), antiferromagnetic nuclear resonance (AFNR) has been observed at 90 MHz with quadrupole splittings associated with the moment-bearing Cu sites in the Cu-O planes. The relative intensities of these resonance lines depend on the oxygen content, and this gives us a microscopic understanding of the Cu chemistry of this system.     

NMR and NQR studies in high-T c oxides: YBa2Cu3O y (6.0≤y≤6.91)

The NQR and NMR techniques have been utilized to characterize the local oxygen coordination of inequivalent Cu sites and the electronic properties in both the normal and superconducting states of YBa₂Cu₃O_y (6.0<-y≤6.91). The distinct NQR lines associated with the different oxygen-coordinated Cu sites, hence the locally differentiated charged states, have been observed. The degree of charge differentiation at the Cu(2) plane sites was found to be increased with decreasingy from 6.91, which might be related with the decrease ofT _c. An anomalous temperature dependence of Cu nuclear spin-lattice relaxation timeT ₁ has been observed for both the Cu(1) chain and Cu(2) plane sites fory=6.91 and it is discussed in connection with antiferromagnetic spin fluctuations in the normal state.     

Charge freezing in the zigzag chain PrBa2Cu4O8 cuprate

We report nuclear quadrupole resonance (NQR) studies on the chain Cu sites of PrBa2Cu4O8, a quasi-one-dimensional conductor with a nearly quarter-filled band. The nuclear spin-lattice relaxation rate 1/T1 shows a pronounced peak near 100 K caused by fluctuations of electric field gradient. Similar peak was observed for the spin-echo decay rate 1/T2, however, at a different temperature near 50 K. These results and broadening of the NQR spectrum at low temperatures indicate that slow charge fluctuations of either electronic or ionic origin freeze gradually at low temperatures.     

Copper NQR study of the La 214 system from an aspect of microscopic environment of copper

The nuclear quadrupole resonance (NQR) has been investigated for Cu in La_2–x A _x CuO _y (A=Sr and Ba). Three Cu NQR lines, A, B and C, have been observed, which correspond to three different Cu sites. From the analysis of these NQR intensities, site assignments are possibly as follows. These lines A, B and C are attributed to CuO₆ octahedral, CuO₅ pyramidal and CuO₄ planar sites, respectively. The apical oxygen O(2) is induced to transfer to the interstitial O(3) site between LaO planes when two or more Sr²⁺ ions are located in its neighboring La sites. The excess holes doped into the CuO₂ plane are provided mainly by O(3) as well as the unpaired Sr²⁺ ion.     

c-axis optical response in the static stripe ordered phase of the cuprates

The c-axis far-infrared reflectivity spectra in the superconducting state were investigated for La1.85-yNd(y)Sr0.15CuO4. The Josephson plasma edge rapidly shifts towards lower frequency with increasing y, and almost disappears when y exceeds a critical value ( y(c) approximately 0.12) above which the low-temperature-tetragonal (LTT) deformation switches on, stabilizing the spin/charge ordered stripe phase. In the vicinity of y(c), a "normal state" reentrant behavior was observed with lowering temperature below Tc. This clearly demonstrates that the static stripe order pinned by the LTT distortion suppresses the interlayer phase coherence.     

NQR investigations of the high-Tc superconductor Ba2YCU3O7

NQR measurements of ^63,65Cu in both the normal and superconducting state are reported for the Cu1 and Cu2 sites in Ba₂YCu₃O₇. An anomalous behaviour in the vicinity and below T_c is observed: after a steep decrease at T_c the quadrupole frequency increases with decreasing temperature.     

Novel features of Sr1-xCaxTiO3 phase diagram: evidence for competing antiferroelectric and ferroelectric interactions

Structural and dielectric evidences are advanced to show that the Sr1-xCaxTiO3 (SCT) system undergoes an antiferroelectric (AFE) phase transition in the composition range 0.18相似文献   

Stability of the smectic quantum hall state: a quantitative study

We present an effective elastic theory which quantitatively describes the stripe phase of the two-dimensional electron gas in high Landau levels ( N>/=2). The dynamical matrix is obtained with remarkably high precision using the time-dependent Hartree-Fock approximation. A renormalization group analysis shows that at T = 0, as the partial filling factor Deltanu identical withnu- left floornu right floor moves away from 1/2, the anisotropic conducting state may undergo quantum phase transitions: stripes may get pinned along their conducting direction by disorder, or may lock into one another to form a crystal. The transitions should be reflected in the temperature dependence of the dissipative conductivity.     

Stripe motion in CuO2 planes of Y1−x Pr x Ba2Cu3O7 as observed from the Cu(2) quadrupole resonance

The NQR spectra of copper in the compounds YBa₂Cu₃O₇, TmBa₂Cu₃O₇, and Y_0.9Pr_0.1Ba₂Cu₃O₇ at temperatures of 4.2–200 K are investigated by a pulsed NQR technique at frequencies of 28–33 MHz. Quantitative analysis of the spectra shows that the shape of the “plane” Cu(2) spectra is well described by using a model of 1D correlations of the charge and spin distribution in the CuO₂ planes (stripe correlations). In the undoped superconductors the charge-spin stripe structure moves fast in the CuO₂ planes, but doping the YBa₂Cu₃O₇ lattice with praseodymium slows this motion down. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 782–787 (25 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Dependence of the pseudogap parameter on the wave vector in high-temperature superconductors

The possibilities of obtaining information on the pseudogap parameter dependence on the wave vector in high-temperature superconductors based on layered cuprates by analyzing the temperature behavior of the line shift of the nuclear quadrupole resonance (NQR) on the Cu and O nuclei are discussed. As an example, the Cu NQR data available for YBa₂Cu₄O₈ are analyzed.     

139La and63Cu-NQR of high-T c oxide superconductors

Nuclear quadrupole resonances (NQR) of¹³⁹La in La_2−xM_xCuO₄ (M=Ba, Sr) and⁶³Cu in YBa₂CU₃O₇ have been investigated with substitution of Cu by magnetic impurities. For La-system, the strong enhancement of the nuclear relaxation rate, 1/T ₁ atT _c ^*≈10 K (suggesting the occurrence of magnetic instability with hole-doping), is suppressed by 3D-antiferromagnetism induced by magnetic impurities. In the superconducting region, fluctuations of Cu moments at low temperatures remain extremely fast as at high temperatures. For Y-system, Fe-ions are substituted for both Cu(1) and Cu(2) sites, But Co-ions are preferentially substituted to Cu1 sites. The peaks in the relaxation rate of Cu indicate the appearance of magnetic ordering without destroying superconductivity for the 0.5%-Fe doped sample.     

Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance

We report detailed ¹⁷O, ¹³⁹La, and ^63,65Cu Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La_1.875Ba_0.125CuO₄ single crystal and in oriented powder samples of La_1.8−x Eu_0.2Sr _x CuO₄. We observe a partial wipeout of the ¹⁷O NMR intensity and a simultaneous drop of the ¹⁷O electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the ^63,65Cu intensity is completely wiped out at the same temperature. The drop of the ¹⁷O quadrupole frequency is compatible with a charge stripe order. The ¹⁷O spin lattice relaxation rate shows a peak similar to that of the ¹³⁹La, which is of magnetic origin. This peak is doping dependent and is maximal at x ≈ 1/8.     

Ag induced restructuring of the oxygen precovered Cu(1 1 0) surface

The interplay between chemisorbed oxygen and deposited Ag on the Cu(1 1 0) surface has been studied by scanning tunneling microscopy (STM) and photoelectron emission microscopy (PEEM). The Cu-CuO stripe phase formed on the clean Cu(1 1 0) surface upon oxygen chemisorption at 660 K is partly dissolved by Ag deposition at 300 K. Upon annealing, however, a phase separation is observed, where the Cu-O compounds agglomerate into large CuO islands and the Ag is located in between. Also a strong preference for the Ag to attach to step bunches is observed. Especially on the fully (2×1)O reconstructed Cu(1 1 0) surface, all the deposited Ag is found at the step bunches giving rise to a contrast in PEEM.     

63Cu NQR in copper (II) compounds

We report observations of ⁶³Cu NQR in CuF₂, KCuF₃, and RbCuF₃ in the paramagnetic state, NQR line widths of ⁶³Cu in CuF₂ and CuBr₂ and of ⁸¹Br in CuBr₂, SnBr₂ and ZnBr₂. The NQR resonances of certain Cu (II) paramagnetic compounds are exchange-narrowed to values commensurate with linewidths of the diamagnetic infinite-lattice compounds.     

Absence of static phase separation in the high T(c) cuprate YBa(2)Cu(3)O(6+y)

We use 89Y NMR in YBa(2)Cu(3)O(6+y) in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Deltap=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates.     

Evidence for low-temperature internal dynamics in Cu12As4S13 according to copper NQR and nuclear relaxation

^63,65Cu nuclear quadrupole resonance (NQR) was applied to study the natural mineral Cu₁₂As₄S₁₃ (tennantite) in the temperature range 4.2–210 K. The obtained results point to the presence of field fluctuations caused by internal motions in tennantite. Consistently with the crystal structure, the experimental data can be described by an occurrence of a magnetic phase transition, which takes place near 65 K. The low-temperature phase is characterized by Cu(II) electron magnetic moments freezing in the form of a spin-glass-like constitution.