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N??el temperature of stoichiometric La_ {2} CuO_ {4}
Abstract
Following doping with 111In in O2 gas at 1273 K, samples of La2CuO4+y were subjected to vacuum anneals of varying lengths to adjust the oxygen content. Susceptibility studies show that vacuum anneals of ∼13 h, starting at 1083 K and terminating at 873 K, consistently produce magnetic ordering at Néel temperature (TN)=317(3) K, the highest value yet published. Perturbed-angular-correlation (PAC) studies of the same samples, using the 171–245-keV γγ cascade of 111Cd populated via the decay of 111In, exhibit a combined magnetic-dipole–electric-quadrupole interaction. Analysis of this yields a magnetic hyperfine field and an electric-field-gradient asymmetry that follow the expected temperature dependence of the local magnetization and orthorhombic distortion, respectively. The samples have uniquely static and homogeneous hyperfine interactions with undetectable PAC line broadening. We argue that this implies a ‘‘defect-free’’ probe environment which we identify as stoichiometric with y≃0. Assuming that any residual defects are randomly distributed on oxygen lattice sites, we find ‖y‖≤0.016.
... Para el La 2 CuO 4+ ha sido reportado por arriba de la temperatura ambiente la transición estructural ortorrómbica a tetragonal [10,11]. En este trabajo reportamos el estudio que hemos realizado del comportamiento de esta fase (ortorrómbica a tetragonal) usando la técnica de van der Pauw, medida de la resistividad por el método de 4 puntas en muestras con y sin deficiencia de oxígeno. ...
... La figura 1 muestra la resistividad versus temperatura para una muestra de La 2 CuO 4+-templada a 1213 K y luego enfriada lentamente en aire. Estos resultados muestran claramente la presencia de una anomalía para barridos de calentamiento y enfriamiento alrededor de 600 K que nosotros atribuimos a la transición ortorrómbica a tetragonal, la cual ya ha sido reportada previamente por otras técnicas [10,11], y a la rápida difusión de oxígeno hacia fuera de la muestra lo que hace aumentar su resistividad. La correlación de con resulta de los portadores de carga (huecos) con el incremento del contenido de oxígeno. ...
In-situ resistivity ρ(T) measurements have been used in air ambient to study the phase behaviorabove room temperature of the La2CuO4+δ system, in samples thermically treated in order to obtain oxygenated and deoxygenated samples. Our results in oxygenated samples have shown an anomaly ataround 600 K corresponding to the orthorhombic to tetragonal structural phase transition. However, this anomaly was observed with higher intensity in the first heating sweep in deoxygenated samples, which we have attributed to the fast oxygen in-diffusion into the sample at around this temperature
... Neither does it appear to be caused by preformed pairs. On the other hand, short range AF spin correlations in Sr 2−x La x IrO 4 closely reflect spin excitations in cuprates [18][19][20][21], although no charge or spin orders were found in iridates. These observations prompted suggestions that the PG in iridates is driven by magnetic fluctuations [1,14]. ...
The fate of the Fermi surface in bulk electron-doped Sr2IrO4 remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr2-xLaxIrO4 up to x=0.2, a factor of two higher than in previous work. Our findings reveal that the Fermi surface evolves smoothly with doping. Notably, the antinodal pseudogap persists up to the highest doping level, while nodal quasiparticle coherence increases monotonously. This demonstrates that the sharp increase in Hall carrier density recently observed above x*=0.16 [Y.-T. Hsu et al., Nature Physics 20, 1596 (2024)] cannot be attributed to the closure of the pseudogap. Further, we determine a temperature boundary of the pseudogap of T*~200 K for x=0.2, comparable to cuprates. Our results suggest that pseudogaps are a generic feature of doped quasi-2D antiferromagnetic Mott insulators, likely related to short range magnetic correlations.
... Isostructural to superconducting cuprates (La 1−x Sr x ) 2 CuO 4 [2,3], layered nickelates (La 1−x Sr x ) 2 NiO 4 have been studied extensively in search of superconductivity [15][16][17][18]. Both the parent compounds are antiferromagnetic insulators, where La 2 CuO 4 is Mott-Hubbard type as it remains insulating in its paramagnetic state [19], while La 2 NiO 4 is Slater-type antiferromagnetic as it becomes metallic above the transition temperature [15][16][17]20]. A small amount of hole doping (by Sr substitution at La site) leads to disappearance of antiferromagnetic order in both the systems and drives cuprates towards superconducting state [2,3] while nickelates remain insulating [15][16][17]21]. ...
We investigate the role of electron correlation and disorder on the electronic structure of layered nickelate (La0.5Sr0.5)2NiO4 using core level and valence band photoemission spectroscopy in conjunction with density functional theory (DFT) and dynamical mean-field theory (DMFT) calculations. Sr 3d and La 4d core level spectra exhibit multiple features associated with photoemission final-state effects. An increase of unscreened features in the Sr 3d and La 4d core level spectra with lowering temperature suggests the reduction in density of states (DOS) at the Fermi level, EF. Valence band spectra collected using different photon energies reveal finite intensity at EF and overall spectra are well captured by DFT+DMFT. Strong renormalization of partially filled eg bands in DFT+DMFT result indicates strong correlation in this system. Mass enhancement factor, m*/mDFT∼ 3, agrees well with values obtained from specific heat measurements. High-resolution spectra in the vicinity of EF show monotonically decreasing spectral intensity with lowering temperature, which evolves to exhibit a Fermi cutoff at low temperatures indicating metallic character in contrast to insulating transport, suggesting Anderson insulating state. |E−EF|1/2 dependence of the spectral DOS and square-root temperature dependence of spectral DOS at EF evidences the role of disorder in the electronic structure of (La0.5Sr0.5)2NiO4.
... Isostructural to superconducting cuprates (La 1−x Sr x ) 2 CuO 4 [2,3], layered nickelates (La 1−x Sr x ) 2 NiO 4 have been studied extensively in search of superconductivity [15][16][17][18]. Both the parent compounds are antiferromagnetic insulators, where La 2 CuO 4 is Mott-Hubbard type as it remains insulating in its paramagnetic state [19], while La 2 NiO 4 is Slater-type antiferromagnetic as it becomes metallic above the transition temperature [15][16][17]20]. A small amount of hole doping (by Sr substitution at La site) leads to disappearance of antiferromagnetic order in both the systems and drives cuprates towards superconducting state [2,3] while nickelates remain insulating [15][16][17]21]. ...
We investigate the role of electron correlation and disorder on the electronic structure of layered nickelate (LaSr)NiO using core level and valence band photoemission spectroscopy in conjunction with density functional theory (DFT) and dynamical mean field theory (DMFT) calculations. Sr 3d and La 4d core level spectra exhibit multiple features associated with photoemission final state effects. An increase of unscreened features in the Sr 3d and La 4d core level spectra with lowering temperature suggests the reduction in density of states (DOS) at the Fermi level, . Valence band spectra collected using different photon energies reveal finite intensity at and overall spectra are well captured by DFT+DMFT. Strong renormalization of partially filled bands in DFT+DMFT result indicates strong correlation in this system. Mass enhancement factor, 3, agrees well with values obtained from specific heat measurements. High resolution spectra in the vicinity of show monotonically decreasing spectral intensity with lowering temperature, which evolves to exhibit a Fermi cut-off at low temperatures indicating metallic character in contrast to insulating transport, suggesting Anderson insulating state. dependence of the spectral DOS and square root temperature dependence of spectral DOS at evidences the role of disorder in the electronic structure of (LaSr)NiO.
... Further investigation demonstrated that this was a consequence of intercalation by excess oxygen [22,23], with phase separation occurring between the antiferromagnetic and superconducting phases [24][25][26][27]. Samples on the insulating edge of the miscibility gap have the low T N , but annealing in Ar gas can raise the transition to 325 K [28]. ...
New surprises continue to be revealed about LaCuO, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the CuO planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in CuO octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the "chiral" phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.
... When x = 0, i.e., undoped, the AF order is slightly higher in La 2 CuO 4 (m = 0.273) than in Ca 2 CuO 2 Cl 2 (m = 0.250). These numbers are smaller than experimental values, but the order is consistent: the Néel temperature for La 2 CuO 4is around 300K, compared to ∼250K for Ca 2 CuO 2 Cl 2[279,280]. The quantitative disagreement may be due to the spin-restricted Fock matrix from DFT calculations, which suppresses the magnetic order. ...
This thesis describes the development of the density matrix embedding theory (DMET) and its applications to lattice strongly correlated electron problems, including a review of DMET theory and algorithms (Ch 2), investigation of finite size scaling (Ch 3), Applications to high-temperature superconductivity (Ch 4-6), a framework for finite-temperature DMET (Ch 7).
... The details of single crystal growth and the preparation of samples with various N6el temperatures for optical and transport measurements have already been described [ 9 ]. Dependence of TN on the oxygen content x [ 10 ] allows one to evaluate roughly x-0, 0.015 and 0.025 in the 214 single crystals under study with TN=320, 250 and 140 K, respectively. The accuracy of this procedure is not high. ...
Far-infrared reflectivity spectra R(v) in polarization E ⊥ C and DC-conductivity σDC have been studied in a semiconducting phase of La2CuO4+x single crystals with various oxygen contents. The spectrum of infrared-active optical phonons has been obtained for stoichiometric La2CuO4. It has been shown that the phonon spectrum should be analyzed in a framework of the orthorhombic crystal structure at T≤300 K. Extra lines have been found in the phonon spectrum upon doping the crystals with oxygen. The results of studying the temperature dependence of R(v) and σDC allow one to interpret these features as vibrational modes, which are active in the Raman scattering spectrum and displayed in the IR spectrum due to their interaction with self-localized holes.
... Ni 2+ should be the majority charge state, and in fact, the fraction with the smaller EFG is always the higher one. A similar argument was used to describe the two EFGs observed in Cr 2 O 3 [6,7], We therefore propose, that all probes are on the same site of the lattice, probably replacing the lanthanoide, as it was deduced for La 2 CuO 4+δ by Saylor et al. [5]. The two different EFGs are caused by the presence of Ni 2+ or Ni 3+ in the neighborhood of the probes. ...
High temperature measurements on Ln2NiO4 + δ
(Ln=La, Pr, Nd) have been performed indicating a complex behavior in PAC experiments which can be attributed to fluctuating
electric field gradients caused by the high mobility of excess oxygen.
A particle size effect to magnetic properties of the mother material of the La-based high-T superconducting oxides, LaCuO (LCO), was investigated using muon spin relaxation (SR) measurement in the zero-field condition. A decrease in the Nèel temperature was observed in LCO nanoparticles with a decreasing particle size. The magnetic ground state was found to be an antiferromagnetically ordered state as observed in the bulk state. Two muon sites were confirmed, which showed different temperature dependence of internal fields. It is guessed that oxygen molecules are trapped among nanoparticles, forming a domain-like structure [1].
A study on the effects of sample synthesis conditions on the particle size, structure, and magnetic properties of electron-doped cuprate superconductors of Eu1.85Ce0.15CuO4+α-δ (ECCO) nanoparticles has been carried out using transmission electron microscopy (TEM), X-ray diffraction (XRD) and the superconducting quantum interference device magnetometer (SQUID). The ECCO nanoparticles were prepared through the sol-gel method with various sintering and annealing temperatures. From TEM characterization, the average particle sizes are 87 nm and 103 nm for the sintering temperatures of 700 °C and 900 °C, respectively. The XRD results with structural Rietveld refinement reveal that the lattice constants and bond distance Cu-O change considerably compared to the bulk case. Reducing the particle and crystallite size to below 200 nm causes strong suppression in the superconducting state. From SQUID measurements it is found that none of the samples show superconducting behavior. An upturn in magnetic susceptibility below 10 K is observed in the sample when the crystallite size is in the range of 69 nm to 88 nm, indicating the existence of magnetism. The lower the sintering temperature of the sample synthesis, the higher the effective magnetic moment and Curie temperature. It suggests that the magnetic correlation is more developed in the smaller samples.
Electron-doped superconducting cuprate of
Eu2−xCexCuO4+α−δ has been studied in the whole doping regime
from x = 0.10−0.20 with reducing oxygen content to investigate
the relation between the crystal structure and the hopping
conduction in the normal state. Parameter of the crystal structure
has been extracted from the X-ray diffraction (XRD) measurement
while hopping conduction parameters have been obtained from
resistivity measurements. The Eu−O bond length decreases with
the increasing doping concentration, indicating the successful
doping by the partial replacing of Eu3+ with Ce4+. The resistivity
increases with decreasing temperature in all measured samples.
This is an indication of bad metal-like behavior in the whole regime
in the normal state of electron-doped superconducting cuprate of Eu2−xCexCuO4+α−δ. The temperature dependence of resistivity was
analyzed by the Arrhenius law and the variable range hopping model. It is found that the hopping conduction mechanism more likely
follows the variable range hopping rather than the Arrhenius law, indicating that the hopping mechanism occurs in three dimensions.
The Cu−O bond length probably plays an important role in decreasing the activation energy. The decreasing value of the activation
energy correlates with the increase in the localization radius.
New surprises continue to be revealed about La2CuO4, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the CuO2 planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in CuO6 octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the “chiral” phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.
In this chapter, various representative oxides will be discussed in detail to present useful ideas on their electronic transport phenomena. They are representative by virtue of the following characteristic features:
ReO3 (Sect. 4.1): The structure is simple cubic and it shows the highest conductivity in the normal oxides. The conduction band is a simple dε — O2p
type. SnO2 and TiO2 (Sect. 4.2): SnO2 is sometimes called a transparent metal and it is a broad s — p band semiconductor. TiO2 has the same lattice structure but its electron-phonon interaction is large and it is often disputed whether the electrons form large polarons or localized small polarons.
LiTi2O4 and LiV2O4 (Sect. 4.3): LiTi2O4 may be considered as a heavily doped TiO2. When the polarons condence in such a substance with a strong electron-phonon interaction, superconductivity appears, and until the discovery of Cu-oxides, its critical temperature of 13.7 K was the highest among the oxides. In metallic LiV2O4, a localized moment appears, in contrast to LiTi2O4.
WO3 and M
x
W03 (Sect.4.4): The carriers may be large polarons in WO3. They are heavily doped in M
x
WO3 where the M ions distribute randomly and there a metal-insulator transition occurs at certain carrier concentrations. Percolation theory will be useful here.
M
x
V2O5and MMMoO3 (Sect. 4.5): These are low dimensional substances. The former is quasi-one dimensional and the carriers may be small polarons. A bipolaron state has been reported. Mo-bronzes form various low dimensional lattices and charge density waves, CDWs, have been observed.
NiO (Sect. 4.6): NiO is an insulator while the simple Hartree—Fock mean field theory predicts that it should be metallic. In this material, the localized nature of the electrons is strong due to the strong electron correlation and many investigations have been carried out to elucidate a “hopping” conduction. However, the nature of the electrons is not yet clear.
V2O3 (Sect. 4.7): This shows two metal-insulator transitions. The higher temperature one may be the Mott transition with the metallic phase at the lower temperature side. Below the lower transition temperature, the crystal becomes antiferromagnetic and insulating, accompanied by lattice distortion.
Fe3O4 (Sect. 4.8): This is ferrimagnetic below 860 K and shows a transition at 123 K with a jump in the electrical conductivity, which was ascribed to the order-disorder transition of Fe2+ and Fe3+. Many results have been accumulated on the nature of correlated polarons in this fluctuating-valence-material.
EuO (Sect. 4.9): This is a ferromagnetic NaC1 type oxide. The reduced material shows the metallic conductivity below the Curie temperature and the conductivity jump there is order 1013. The MIT is due to the magnetic interaction between the localized 4 f magnetic moments and the propagating electrons.
High T
c Cu-Oxides (Sect. 4.10): These are d
γ conductors whereas most of the metallic oxides are dε conductors. CuO2 planes constitute a multilayer structure and the superconducting transition temperature increases with the layer number at least up to four and is higher than 120 K.
Spin states of antiferromagnetic CuO2 planes in high-T
c
superconductors with small doping (of alcali-earth metals and oxygen) are considered in the framework of microscopic approach. The reasons for hole localization near different types of acceptors are studied and indirect spin interaction between holes via exchange by virtual magnons is calculated. It is shown that within insulating phase the variable sing interaction is present and, though it teraction is present and, though it prevents from long-range spin ordering in the system of chaotically distributed hole spins, is insufficient for effective destroying of antiferromagnetism in CuO2 planes. It is established that the irreversible loss of order can be produced by random deformation fields from impurity ions. The magnetic phase diagram in localized hole concentration vs temperature is obtained for high-T
c
superconductors.
A Comment on the Letter by Saylor and Hohenemser, Phys. Rev. Lett. 65, 1824 (1990).
High temperature measurements on Ln2NiO4+δ (Ln=La, Pr, Nd) have been performed indicating a complex behavior in PAC experiments which can be attributed to fluctuating electric field gradients caused by the high mobility of excess oxygen.KeywordsPerturbed angular correlation (PAC)La2NiO4
Pr2NiO4
Nd2NiO4
Hight temperature measurements
Recent results of perturbed-angular-correlation experiments on La2−xSrxCuO4+y, for x zero1 or small,2 indicate that as-prepared materials contain an excess of oxygen which may be removed by annealing in vacuum. Inspired by these results, we have explored the magnetic behavior of Sr-doped materials that have received treatment by vacuum annealing, our primary interest being the observation of a bulk transition to the apparent spin-glass phase which occurs at low temperatures (Tf<20 K) for 0.02≤x≤0.07.3 While as-prepared materials do not exhibit a bulk transition at Tf, we observe a clear anomaly at Tf in vacuum-annealed samples, below which the latter also display the magnetic irreversibility characteristic of spin glasses. The values of Tf obtained from bulk magnetization correspond closely to those derived from microscopic probes.3 We will discuss the results of measurements of the bulk magnetization in vacuum-annealed samples with varying Sr content, together with information from recent experiments, utilizing ESR and NMR, designed to improve our understanding of the microscopic nature of the moments which participate in the ordering at Tf and of the nature of the ordering itself.
We have investigated the effect of Sn doping on the magnetic and superconducting properties of La2-xSrx(Cu1-ySny) O4+δ with 0≤x≤0.15 and 0≤y≤0.01. Measurements of the 119Sn Mössbauer isomer shift of samples doped with y=0.005 (119Sn) show that Sn is tetravalent (Sn4+) in the whole concentration range 0≤x≤0.15. On the other hand, the electrical resistivity, the Néel temperature (TN), and the superconducting temperature (Tc) are found to be affected by increasing the Sn content. In the antiferromagnetic state (x=0,0.008), we find that Tc is slightly affected by increasing Sn content. In the superconducting state (x=0.065, 0.10, and 0.15), Tc is strongly suppressed with increasing Sn content. It is shown that the variation of TN upon Sn doping depends on the competition between two mechanisms: (1) magnetic disorder due to the nonmagnetic impurity which induces an uncompensated local magnetic moment on the CuO2 lattice and thereby tends to decrease TN; (2) reduction (compensation) of the number of the free O- holes by excess electrons introduced by Sn4+ which leads to stabilizing the antiferromagnetic order and increasing TN. We further show that the suppression of Tc with increasing Sn content can be explained by the same magnetic mechanism (1) responsible for decreasing TN.
Magnetization data and NMR spectra for both 17O and 139La in La2CuO4 are reported for temperatures T ranging from 285 to 600 K. We find a small, positive paramagnetic state hyperfine coefficient C, corroborating earlier findings that C for the doped phase is not dominated by the Cu-O transferred coupling, contrary to simple models proposed up to now. Near and below TN the planar 17O shift is negative, highly anisotropic, and ~100 times larger in magnitude. We propose a new and more complex magnetic structure in the ordered phase to account for these data.
The perturbed angular correlation (PAC) method was applied to study the electron-doped HTSC Nd2−x
CexCuO4 (NCxCO) forx=0, 0.06, 0.1 and 0.15.111In was implanted at 400 keV into sintered bulk samples. After annealing the radiation damage at 1223 K in air, PAC spectra were taken at measuring temperatures between 15 and 823 K. Point charge model (pcm) calculations support the assumption that111In replaces a Nd-ion. The width of the main frequency depends on the amountx of Ce-doping. The magnitude of the observed electric field gradient (efg) can be explained if the111In probe catches simultaneously an oxygen vacancy at the O(2)-site.
Due to the enormous electronegativity of fluorine, the vast majority of binary and higher inorganic fluorides are high-melting large-band-gap electronic insulators, which are transparent in the visible region of the electromagnetic spectrum. Rare examples of metallic fluorides are known, but valence orbitals of F marginally participate in the electronic transport in these compounds. In this account we describe recent theory-driven attempts to turn unusual fluorides of divalent silver – called fluoroargentates(II) – into a novel class of high-temperature superconductors.
The 119Sn Mössbauer effect technique (300-4.2 K) has been used to study the local magnetic order in La2−xSrx (Cu0.995Sn0.005)O4+δ for three different Sr concentrations corresponding to the antiferromagnetic state (x = 0), the spin-glass-like state (x = .04), and the superconducting state (x = 0.15). Contrary to previous reports we observe no transferred hyperfine (hf) field at the Sn nuclei in the superconducting state down to 1.7 K. On the other hand, we observe small transferred hf fields in both the antiferromagnetic (x = 0) and the spin-glass-like state (x = 0.04).In the spin-glass-like sample (x = 0.04) we find in the temperature range 10 K ⩽ T ⩽ 200 K a significant increase of the Mössbauer linewidth and an anomalous change of the quadrupole splitting which we attribute to a short range structural disorder as recently reported by perturbed angular correlation measurements of 111In on La2−xSrxCuO4 (0 ⩽ x ⩽ 0.07). The magnetic behaviour of this sample (x = 0.04) has been further investigated by 139La-NQR measurements at low temperatures (1.5 K ⩽ T ⩽ 12 K). Here, we observe an enhancement of the relaxation rate (1/T2) below about 7 K.
La2-xCuOy(LCO) after 60kbar O2-HIP treatment was investigated by XRD, susceptibility ( chi ) resistivity(R), and SEM image. More than two superconducting phases were found existing in chi and R measurements, and further, new peaks were observed in XRD.
Superoxygenated La2CuO4+delta, produced at 1800 atm and 550°C, has been investigated by the following means; resistivity, Seebeck coefficient Hall coefficient, magneto-resistance, electron microscopy, AC and DC susceptibility, and specific heat. The results are contrasted with those from ``reduced'' (stoichiometric) La2CuO4. Unlike the latter Mott insulator, the oxygenated material shows mixed valent, delocalized behaviour. That behaviour is rendered more complex because of phase segregation via spinodal decomposition onsetting just below room temperature. The observed microstructure permits detailed interpretation of the resulting modification in properties. The degree to which the observed superconductivity mirrors that of (La/Sr)2CuO4 is discussed. Restricting the phase segregation by fast cooling inhibits the superconductive behaviour, by promoting lower effective carrier counts and mobilities and greater magnetic admixture. It is shown that phase segregation is not triggered by or coupled to the point at which long-range 3D magnetic order develops within the insulating component of the two-phase mixture. The super-conductivity is presented in terms of the negative-U charge fluctuation model developed earlier. To whom all correspondence should be addressed.
We show that the transition to the low temperature (T ≲ 12 K) magnetic phase (LTMP) of La2 - xSrxCuO4 + y (LSCO) for 0.02 ≲ x ≲ 0.03 is clearly observed via bulk magnetometry in vacuum-annealed samples. The transition temperature Tf depends on the strontium concentration and is insensitive to the vacuum treatment. Measurements of an electron spin resonance (ESR) signal are also described. The temperature dependent amplitude of this signal displays a clear anomaly at Tf, suggesting that the observed spins participate in the LTMP. Arguments are presented for the intrinsic nature of the observed ESR signal and the LTMP. We compare temperature dependent effects in both probes to expectations for spin glasses, and relate our results to consideration of the nature of the LTMP, and of the origin of the spin system from which the LTMP is formed.
The phase seperation effect in La2CuO4.015 single crystal has been studied by magnetic susceptibility and EPR experiments. The experiments confirm the percolative phase
separation model based on the concept of magnetic polarons formed by hole doping. It has been shown that the superconducting
subphase has specific spatial structure which is responsible for unconventional low temperature behavior of susceptibility.
The EPR signal as well as the paramagnetic contribution to the lowtemperature susceptibility have been shown to arise from
isolated Cu2+ ions located inside the phase domain walls.
Within a continuum model based on the microscopic magnetic symmetry of Ni2+ ion (spin S = 1) in the metal-oxide compound La2NiO4+delta, a theoretical analysis is made of the stability of antiferromagnetic long-range order under the effect of oxygen interstitials at delta ≪ 1. It is shown that the Néel temperature dicreases significantly with growing delta though does not become zero, in contrast to that in the structurally isomorhic compound La2CuO4+delta (but Cu2+ spin S = 1/2).
Crystal structure and magnetic properties of K2AgF4, related to recently studied Cs2AgF4, have been scrutinized. It crystallizes orthorhombic (Cmca No.64) with a = 6.182(3) Å, b = 12.632(5) Å, c = 6.436(3) Å (Z = 4, V = 502.6(7) Å3). K2AgF4 exhibits slightly puckered [AgF2] sheets and a compressed octahedral coordination of Ag(II) and it is not isostructural to related Cs2AgF4. Violet–coloured K2AgF4 orders ferromagnetically below 26 K. The DFT calculations reproduce semiconducting properties and ferromagnetism of K2AgF4 at the LSDA + U level but only if substantial values of Mott–Hubbard on-site electron–electron repulsion energies for Ag and F are used in calculations. We have also succeeded to solve the crystal structure of a brown KAgF3 (1D antiferromagnet below 64 K; GdFeO3–type, Pnma No. 62, a = 6.2689(2) Å, b = 8.3015(2) Å, c = 6.1844(2) Å, Z = 4, V = 321.84(2) Å3) and to prepare K3Ag2F7, a novel KAgF3/K2AgF4 intergrowth phase and a member of the Ruddlsden–Popper KnAgFn+2 series (n = 1.5). Dark brown K3Ag2F7 crystallizes orthorhombic (K3Cu2Cl7-type, Ccca No. 68, setting 2) with a = 20.8119(14) Å, b = 6.3402(4) Å, c = 6.2134(4) Å (Z = 4, V = 819.87(9) Å3).
We have performed Mössbauer measurements on oriented powders of 57Fe-doped La2-xSrxCuO4 (x=0 and 0.2 ) and 57Co-doped La2CuO4. We find that the main axis OZ of the 57Fe electric field gradient in the series lies at an angle beta=26°+/-5° with respect to the crystal c axis, which is much less than recently published values. Our data were corrected for a fraction of unoriented crystallites, which we find to be systematically present, but which was unsuspected in previous experiments. We describe a convenient X-ray examination procedure for detecting and evaluating the unoriented fraction. We discuss possible origins of the angle beta and we examine the relative disposition of the direction of c, OZ and of the hyperfine field on 57Fe in La2CuO 4 below TN. We also consider the influence of the oxygen stoichiometry on the symmetry at the Fe site.
Nearly all elements can be oxidized and develop oxides, often with different oxygen contents and in different crystalline phases. Applying the classical perturbed angular correlation (PAC)-probes 111In/111Cd or 181Hf/181Ta, the probes are usually found on unperturbed cation lattice sites surrounded by oxygen atoms. Using different oxides of the same structure or comparing different crystal classes the position of the oxygen neighbours near the probe can be varied in a wide range. This allows testing theoretical concepts of electric field gradient (EFG) calculation.In general, the melting point of an oxide is very high, and the PAC experiments span a huge temperature range from Tm=10 to 1700K. Two temperature regions are known, where 111In/111Cd probes show dynamic hyperfine-interactions, which occur when the EFG changes direction or strength during the lifetime of the probe. At low temperatures the electron capture “after-effect” is observed, caused by a low availability of charge carriers in semiconducting or isolating oxides. At very high temperatures intrinsic defects or mobile atoms in ternary oxides move so fast, that undamped perturbation functions arise.Realizing the big impact of STM and AFM to the surface science, a probing technique like PAC for the next neighbours inside a sample seems to be attractive. In the past, numerous discussions asked whether the inserted PAC-probes are really spies—only observers—or if they actually change that neighbourhood, that they are supposed to analyse. Distortions in oxides are discussed.
The compound La2CuO4+δ is known to phase separate for 0.01≲δ≲0.06 below a temperature Tps∼300 K into the nearly stoichiometric antiferromagnetic compound La2CuO4.01–4.02 with Néel temperature TN∼250 K, and a metallic oxygen-rich phase La2CuO≊4.06 with superconducting transition temperature Tc≊34 K. We report studies of the superconducting and normal-state static magnetic susceptibility χ of La2CuO4+δ samples with 0≲δ≤0.11 prepared by electrochemical oxidation or reduction of conventionally synthesized ceramic La2CuO4+δ. The upper limit to the miscibility gap at low T is found be δ≲0.065, in agreement with the previous work. The interstitial oxygen diffusion during the phase-separation process was studied using thermal- and magnetic-field history-dependent χ(T,t) measurements versus temperature T and time t as a probe. Phase separation is found to be suppressed by quenching at ≳100 K/s and favored by slow cooling at ∼0.5 K/min. A large thermal hysteresis of both the normal and superconducting state χ(T) was observed between data obtained after quenching to 5 K and then warming, and data obtained while or after slowly cooling from 300 K, for samples of La2CuO4+δ (δ≊0.030, 0.044) within the miscibility gap. Quenching reduces Tc by ≊5 K relative to the value (34 K) obtained after slow cooling. A similar decrease is found for La2CuO4.065 which does not phase separate, indicating the importance of oxygen-ordering effects within this single phase. A model for the excess oxygen diffusion is presented, from which the data yield a nearly T-independent activation energy for excess oxygen diffusion of (0.24±0.03) eV from 150 to 220 K apart from a possible anomaly near 210 K. © 1996 The American Physical Society.
Samples of La2CuO4+δ in the range 0R and Seeback coefficient α were used to monitor as a function of the compositional parameter δ not only the evolution of the superconductive transition temperature Tc, but also the diffusion-controlled spinodal phase segregation occurring in the temperature range 200T≤300 K. The data also show that the δ interstitial oxygen atoms Oi per formula unit enter as Oi2− ions at least in the range 0Oi-atom segregation over the range 0δo where the Oi atoms become ordered, and 0.066δoTs vs δ that falls to 200 K at δs(max)≊0.05; the diffusion-controlled spinodal phase segregation occurs within the phase field of the Oi-disordered orthorhombic structure.
139La NQR spectra and relaxation measurements in the La2-xSrxCuO4 (LSCO) system are presented for x<0.02. In this compound the introduction of electronic holes in the planar antiferromagnet (AF) La2CuO4 by Sr doping and/or excess oxygen drastically reduces the temperature of ordering TN whereby for x>or=0.02 the long-range AF order is totally suppressed. For 0.02<x<0.08, LSCO systems display 2D short-range AF correlations in regions of mesoscopic size leading to an unconventional spin glass phase at low temperature (T<15 K). For x<0.02 the system is in a doped Neel state, and at low temperature critical effects are observed, probably involving magnetic quasi-particles introduced by holes. Furthermore the authors find for T>200 K a relaxation contribution due to excess O, and this phenomenon is related to phase separation which leads to a superconducting phase for appropriate O doping of La2CuO4.
In the framework of a continuum model based on the microscopic magnetic symmetry properties of the metal oxide compound La2NiO4+ delta , a theoretical analysis is made of the stability of the antiferromagnetic (AFM) long-range order in the presence of impurity centres of excess oxygens at delta <<1. The resulting magnetic phase diagram in the variables delta and T is compared with the recent experimental data for this system. Also the similarities and dissimilarities between the structurally isomorphic systems La2NiO4+ delta and La2CuO4+ delta are traced and their possible connection with superconducting properties is mentioned.
Single crystals of La2CuO4 + δ were exposed to fast-neutron irradiation with fluences of 8.4 1016 n/cm2 and 3.65 1017 n/cm2 (E > 1 MeV). D.c. magnetization measurements of these samples were performed and compared with those of unirradiated samples to study the influence of irradiation-created defects on the magnetic properties of La2CuO4 + δ crystals. It was shown that the defects created by neutron irradiation can effectively limit the diffusion-controlled clustering process, therefore preventing the macroscopic phase separation. The results of our experiments well correspond with the prediction of the percolative phase separation model and give evidence of the existence of magnetic polarons in La2CuO4 + δ oxides.
High temperature (T > 200K) 139La NMR relaxation rates in single crystals La2CuO4+δ at two measuring frequencies are compared with zero field NQR measurements in order to study the diffusion of the interstitial oxygen.
In the present paper we will review studies of local magnetism by the muon spin rotation technique in two classes of High-TC superconductors, namely La2-xSrxCuO4 and YBa2Cu3Ox. In addition we present some new results for the existence of local magnetism in hydrogen doped YBa2Cu3O7.
We report on the formation of conducting phases in slightly doped La2CuO4 samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.
High-temperature (T>200 K) 139La NMR relaxation rates in single crystals of La2CuO4+δ (δ∼0.015 and δ∼0.002) at two measuring frequencies are presented and compared with zero-field NQR data for a series of La2−x
Sr
x
CuO4+δ powder samples with 0≤x≤0.08 and 0<δ0.013. For δ≠0 an enhancement of the relaxation rate is observed which is related to the time dependence of the
quadrupole interaction. Quantitative information on the oxygen diffusion is obtained from the analysis of the data with a
simple diffusion model. The results are discussed in relation to the problem of macroscopic phase separation occurring for
higher oxygen content.
We consider the antiferromagnetic (AF) state of high-T
c
compounds and assume the existence of the localized magnetic two-level systems (TLS) with the relatively small energyE in the AF copper planes, which is compatible with a series of experimental data implying low-energy scale in these systems. It is shown in our previous paper that these TLS are formed, if one accepts Aharony et al. suggestion that the small doping results in the holes' localization on the oxygen ions in CuO2 planes. Randomly distributed, these TLS cause partial disorder in the average values of AF copper spins. The manifestation of this effect in the
+ SR experiments and the elastic neutron scattering is discussed. Our results are in a qualitative agreement with the temperature dependence and the magnitude of the line shift and the relaxation rate of
+ SR signal. The elastic diffusive neutron scattering at the small wave-vectors near 2D AF Bragg point (1/2, 1/2,l) and near the point (0, 0,l) is predicted. However, the temperature dependence of this scattering, observed in the experiments with YBa2Cu3O6+x atx=0.38, isn't met by our formulae, obtained for a small doping. It is shown, that SR data give stronger confidence to the frustrated bond rather and frustrated plaquette case of the hole localization.
We have determined the phase diagram of (Y1−yCay)Ba2Cu3O6 by measuring TN with 89Y NMR and Tc by AC susceptibility. As in the (La2−xSrx)CuO4 system, the introduction of holes in the CuO2 planes is via heterovalent substitution, giving direct control over the number of holes doped. Similar to (La2−xSrx)CuO4, it is found that the (Y1−yCay)Ba2Cu3O6 phase diagram has, with increasing Ca content, an AF phase (0≤y≤0.07) and a superconducting phase (y≥0.2) separated by an intermediate region. A description of TN versus hole content is proposed where the charge dynamics play a major role. A fit to the data is made using this model resulting in a qualitatively correct curve for low and intermediate doping. An argument for the importance of the planar anisotropy, Jxy, of the CuCu superexchange interaction, J, in the appearance of 3D long range order at finite T is given in the discussion.
The reduction of the Neel point {ital T}{sub {ital N}} of La{sub 2}CuO{sub 4} by substitution of nonmagnetic Mg or Zn for Cu has been studied, with particular emphasis on keeping the oxygen content unaffected. In the low-concentration range ({lt}10%), the rate of {ital T}{sub {ital N}} suppression is found to be the same as in other two-dimensional (2D) Heisenberg magnets such as K{sub 2}CuF{sub 4} ({ital S}=1/2) and K{sub 2}MnF{sub 4} ({ital S}=5/2). While this suppression rate is larger than for 2D Ising magnets, it is much smaller than the rapid {ital T}{sub {ital N}} reduction upon introduction of holes in La{sub 2}CuO{sub 4} through oxygen excess or heterovalent substitution for La. Thus, no unusual quantum-magnetic dilution effects are observed in the {ital S}=1/2 La{sub 2}CuO{sub 4} magnet.
Perturbed-angular-correlation (PAC) spectroscopy was used to measure combined nuclear-magnetic-dipole and nuclear-electric-quadrupole interactions in rare-earth orthoferrite (REO) ceramics, R (R=La,Pr,Nd,...,Lu). The rare-earth orthoferrites are canted antiferromagnets that have orthorhombically distorted perovskite structures. With the \rightarrowCd probe, the PAC measurements were made over a temperature range from laboratory temperature through the antiferromagnetic-to-paramagnetic transitions (740\char21{}620 K) and at or near 800 K. In the heavier REO's, the \rightarrowCd probe substitutes primarily into the rare-earth sites; and in the lighter REO's, it can substitute into both the Fe and the rare-earth sites. At the rare-earth sites, the probe undergoes a high-frequency electric-quadrupole interaction that is shifted in energy by a weak magnetic-dipole interaction.
Using perturbed γγ angular correlations of 111In in the La site, we report a structurally disordered phase in oxygen stoichiometric La2-xSrxCuO4+δ. So far this phase has been defined for 0≤x≤0.07, involves a reversible order-disorder transition at Td=150(20) K, and is observed by changes in the electric-field gradient (EFG). These changes involve a continuous broadening of the EFG distribution and decrease of the average EFG asymmetry parameter as T→0. The new disordered phase is associated with a loss of three dimensional magnetic order in the range 0.015<x<0.018, and may be due to short-range disorder in the tilting of the CuO6 octahedra as proposed by Egami et al.
A Comment on the Letter by P. Kopietz, Phys. Rev. Lett. 68, 3480 (1992).
Zero-field [sup 139]La nuclear quadrupole resonance (NQR) has been used to characterize the antiferromagnetic transition in nearly stoichiometric single crystals of La[sub 2]CuO[sub 4+[delta]] (Neel temperatures [ital T][sub [ital N]] between 275 and 318 K). The onset of the NQR Zeeman splitting at [ital T][sub [ital N]] is abrupt but continuous, indicative of a second-order phase transition with either a crossover in critical behavior just below [ital T][sub [ital N]] or a very small critical exponent [beta][approx lt]0.1. The absence of any anomaly at [ital T][sub [ital N]] in the [sup 139]La electric field gradient indicates no accompanying structural distortion.
New data for the field dependent magnetization M(H) of pure antiferromagnetic La2CuO4 are fitted by a theory which treats the interlayer coupling using mean-field theory (MFT), and includes the Dzyaloshinskii-Moriya coupling. The fits yield a novel measure of the two-dimensional (2D) staggered susceptibility chi°2D, which deviates below 360 K from the 2D Heisenberg model. The fits also imply that the weak-ferromagnetic transition below the Néel point TN is weakly first order, with a tricritical point close to TN. The MFT nicely reproduces the order parameter and the cusp in chi=dM/dH.
The transport and magnetic properties of the (La1-xAx)2CuO4 system are investigated systematically over the composition range from x=0 to 0.1. The change in the resistivity and magnetic susceptibility with alkaline metals and alkali earth metals substitution seems to be consistent with the band picture with nesting Fermi surface. However, the result for the Seebeck coefficient suggests the possibility of a strongly correlated electron system, casting a question about the simple SDW picture. The similar feature in the Seebeck coefficient is also observed in the Ba2YCu3O7-y system.
We have mapped the temperature and composition dependence of the tetragonal-orthorhombic phase-transition temperature Td in the high-temperature superconductor La2-xSrxCuO4-delta using high-resolution powder x-ray diffraction and measurements of the thermal expansion. The maximum volume fraction of ideal diamagnetism occurs in the composition range where Td is approaching the superconducting transition. In the composition range x>0.20 where no orthorhombic distortion is observed above 12 K, no bulk superconductivity is observed in La2-xSrxCuO4-delta.
Two single crystals of undoped are studied by neutron diffraction between T=5 and 300 K. In one crystal oxygen vacancies are created by heat treatment and the magnetic properties are found to depend strongly on this treatment. While the untreated sample is found to order antiferromagnetically at {}50 K, the heat-treated crystal orders at {}150 K with a somewhat larger moment. The magnetic structure is the same for the two crystals and supports previously reported powder results.
Zero- and longitudinal-field muon-spin relaxation μ+SR measurements have been performed on La2-xSrxCuO4 alloys in both single-crystal and sintered powder forms above and below their magnetic transition temperatures, Tf. The μ+ precession frequency v depends only weakly on x and Tf, an observation which together with resistivity data implies classical freezing of magnetic moments in the regime where the carriers are localized. For x=0.05, critical dynamics are observed near Tf. The μ+SR technique is shown to be very sensitive to ferromagnetically aligned pairs of Cu2+ moments; the population of such pairs increases greatly with x.
An explanation of the mechanism for high-temperature superconductivity is given, based upon a strong-coupling analysis of the extended Hubbard model previously introduced by one of us. The basic carriers are oxygen-hole quasiparticles in psigma orbitals, whose spin is strongly correlated with that of adjacent copper holes. These quasiparticles interact through the enhanced superexchange of the associated spins on the Cu sites, and an enhanced zero-point motion of the surrounding Cu holes. These are nonretarded attractive interactions whose strength increases as the oxygen-copper Coulomb repulsion increases and can be strong enough, for realistic parameters, to overcome the direct oxygen-oxygen Coulomb repulsion. The superconducting transition temperature that results is proportional to the Fermi energy of the oxygen holes.
Powder neutron diffraction studies of undoped La2CuO4−y have revealed new superlattice peaks below ∼220 K. The absence of corresponding x-ray superlattice lines and an observed susceptibility anomaly near 220 K suggest the occurrence of antiferromagnetism. From the magnetic peak intensities we deduce a structure consisting of ferromagnetic sheets of Cu spins alternating along the [100] orthorhombic axis, with the spins aligned along the [001] orthorhombic axis, The low-temperature magnetic moment is approximately 0.5μB/(Cu atom). The tetragonal-orthorhombic transition at 505 K has also been studied.
Both the magnetic and superconducting properties of (La1-xBax)2CuO4 systems have been investigated in terms of 139La nuclear quadrupole resonance (NQR). For La2CuO4, it has clearly been demonstrated that the internal field Hn is applied on the La site by magnetic ordering below ˜240 K. The observed Hn is not consistently explained by the spin structure proposed in the neutron diffraction study. For the doped compound with x{=}0.01, the internal field at 1.3 K was almost the same as that for La2CuO4, while for x{=}0.025, it decreased and was significantly distributed. Furthermore, the superconductivity was found to set in around 2 K. From a microscopic point of view, it is concluded that the magnetic ordering and superconductivity compete around x{=}0.025.
Evidence for superconductivity in undoped La2CuO4 obtained from resistivity, thermoelectric power, and susceptibility measurements is reported. The onset temperature is near 40 K, and its pressure and field dependence in resistivity have been determined. The superconducting behavior is of a trace and filamentary nature and is quite sensitive to stoichiometry. Its occurrence is extremely process dependent and can be controlled by oxygen pressure. Several likely sources for the superconducting activity are discussed.
The zero-field nuclear magnetic resonance (NMR) of Cu in the high-Tc related oxide, La2CuO4-delta, has been observed at 93.85± 0.1 MHz for 63Cu and 100.1± 0.1 MHz for 65Cu with well-articulated quadrupole splittings at 1.3 K. The resonance pattern is successfully analyzed as a result of the combined effect of a quadrupole coupling (coupling constant nuQ{=}31.9 MHz for 63Cu and 29.5 MHz for 65Cu with asymmetry parameter eta{=}0.03) and a Zeeman term where the internal field (HN{=}78.78 kOe) is nearly perpendicular to the direction of the maximum electric field gradient (EFG). The quadrupole split zero-field NMR signals have also been observed in CuO. The observed frequencies yield parameter values of 137.1± 0.1 MHz, 146.9± 0.1 MHz, 20.07 MHz, 18.57 MHz, 0.20, 121.5 kOe, and the direction of HN is found to be nearly parallel to the direction of the maximum EFG.
The space group in the orthorhombic phase of La2CuO4-delta was confirmed to be Cmca for both as grown and deoxygenated crystals by using conventional and convergent beam electron diffraction together with high resolution neutron powder diffraction methods. The Rietveld refinements of neutron powder diffraction data determined the structural parameters. Slight differences in lattice constants, positional parameters and occupation factor for oxygen atoms were found between the two crystals.
A reinterpretation of the results of recent 139La nuclear quadrupole resonance (NQR) measurements on La2CuO4 by Kitaoka et al. (Jpn. J. Appl. Phys. 26 (1987) L397) reveals an antiferromagnetic order in this material at low temperature. The magnitude and the direction of the local field at the La sites are calculated. Supposing localized moments on copper atoms as the source of the magnetic field, it is shown that the ordering of magnetic moments on copper sites is not the simplest ``chessboard'' antiferromagnetic type.
The interaction of a static electric field gradient and a static magnetic field with the electromagnetic moments of a nucleus is considered in detail. The eigenvectors and eigenvalues of the interaction Hamiltonian are computed as a function of the angle beta between the electric field gradient and magnetic field directions, and as a function of the electric and magnetic interaction parameters omegaE and omegaH. Numerical calculations of the eigenvectors and eigenvalues have been performed for nuclear spin values I=1, 32, 2, 52, 3, 72, 4, and 92 and for a wide range of the parameters beta, omegaE, and omegaH. The accuracy of the results is 0.001%. Representative numerical results are presented.
Time-differential angular correlation studies of the electric quadrupole interaction of 181Ta in various Hf compounds are presented. Methods for the analysis of the time spectra are discussed. The experiments yield information on the strength of the quadrupole interaction, and on the axial asymmetry and distribution of the field gradient. Electric field gradients Vzz are derived for 181Ta in Hf metal, HfO_2, and (NH_4)_2HfF_6.
Extensions of percolation theory to treat transport are described. Resistor networks, from which resistors are removed at random, provide the natural generalization of the lattice models for which percolation thresholds and percolation probabilities have previously been considered. The normalized conductance, G, of such networks proves to be a sharply defined quantity with a characteristic concentration dependence near threshold which appears sensitive only to dimensionality. Numerical results are presented for several families of 3D and 2D network models. Except close to threshold, the models based on bond percolation are accurately described by a simple effective medium theory, which can also treat continuous media or situations less drastic than the percolation models, for example, materials in which local conductivity has a continuous distribution of values. The present calculations provide the first quantitative test of this theory. A "Green's function" derivation of the effective medium theory, which makes contact with similar treatments of disordered alloys, is presented. Finally, a general expression for the conductance of a percolation model is obtained which relates G to the spin-stiffness coefficient, D, of an appropriately defined model dilute ferromagnet. We use this relationship to argue that the "percolation channels" through which the current flows above threshold must be regarded as three dimensional.
NMR and NQR of 63,65Cu at 1.2–4.2 K and of 139La at 1.2–290 K was observed using magnetic fields up to 6.5 T. The complicated La-spectrum in La2CuO4 shows details of the Cu-antiferromagnetism in this material which is absent in the Sr-doped, superconducting sample. From the Cu-resonance of two different sites in Ba2YCu3O7 we conclude that there is no magnetic order in this superconductor.
The magnetic phase diagram of (La 1-xBax)2CUO4 has been established by 139La nuclear quadrupoIe resonance and the specific heat measurements. The magnetic order changes at x=0.008 from a three dimensional antiferromagnetic (AF) type to an incoherent one due to a short-range coupling among the successive planes and continues to x=0.025 where the superconductivity appears. Since the average staggered moments are found to be almost constant between x=0.0 and 0.01, the static AF-coherence over long distance within the CuO2 planes is demonstrated to survive even in the incoherent ordered state. The phase diagram is considered to be within the same scheme as that proposed by Aharony et al.
Three types of phase transitions in (La1-xBax)2CuO4-y---orthorhombic to tetragonal, spin density wave (SDW) and superconductive---were traced as a function of the Ba concentration, x. The obtained phase diagram indicates that the SDW state does not coexist with superconductivity, whereas samples both in orthorhombic and tetragonal phases can be superconducting.
Measurements of the magnetic moment of antiferromagnetic LaâCuOâ at high fields reveal a new phase boundary originating from a previously undetected canting of the Cu/sup 2+/ spins out of the CuO planes. This canting, together with the exponential temperature dependence of the two-dimensional correlation length, accounts quantitatively for the susceptibility peak at the Neel temperature. Enhancement of the conductivity in the ferromagnetic phase demonstrates a strong connection between the magnetism and charge transport.
We present a model for the superconductivity in CuO2 planar systems based on the magnetic frustration mechanism introduced recently by Aharony et al.; our model incorporates explicitly the concentration dependence of the Cu++ spin-spin correlation function. We argue that the transport is via holes in the non-bonding in-plane oxygen orbitals. These two features lead tod-state pairing with, in the BCS approximation, a predictedTc vsx dependence which agrees well experiments on La2-xSrxCuO4.
Powder neutron diffraction studies of undoped La2CuO4-y have revealed new superlattice peaks below ∼220 K. The absence of corresponding x-ray superlattice lines and an observed susceptibility anomaly near 220 K suggest the occurrence of antiferromagnetism. From the magnetic peak intensities we deduce a structure consisting of ferromagnetic sheets of Cu spins alternating along the [100] orthorhombic axis, with the spins aligned along the [001] orthorhombic axis, The low-temperature magnetic moment is approximately 0.5μB/(Cu atom). The tetragonal-orthorhombic transition at 505 K has also been studied.
In this paper we report an extensive study using neutron scattering techniques of the spin fluctuations in the two-dimensional diluted near-Heisenberg antiferromagnet Rb2MncMg1-cF4. The concentrations studied are c=0.54, c=0.57, and c=0.60; the site-percolation concentration for the nearest-neighbor square lattice is cp=0.593 so that these experiments span the percolation threshold. The point c=cp, T=0 represents the percolation multicritical point which terminates the line of second-order transition of the infinite network. We give a detailed description of the theory of the magnetic behavior around the percolation point; as the temperaturelike scaling field we suggest μ(T)=κ1(T) where κ1(T) is the inverse correlation length for the associated one-dimensional chain; for the static structure factor we propose the formula S(|cp-c|,μ,Q⃗)∝κη(κ2+Q2)-1, where κ=κ(|cp-c|,0)+κ(0,μ); that is, we assume that the geometrical and thermal inverse correlation lengths are simply additive. The c=0.60 sample is found to have a smeared second-order phase transition at about 8 K to a state with two-dimensional long-range order but only weak correlations in the third direction. The spin fluctuations in the precritical region are essentially identical to those in the concentrated systems, thus demonstrating that the phase transition in the c=0.60 infinite network is little affected by the proximity to the percolation threshold. For the c=0.54 and c=0.57 samples the correlation lengths and susceptibility increase with decreasing temperature down to about 3 K, below which temperature they saturate; for both samples the inverse correlation length is well described by the simple formula κ=κ(ΔC,0)+[κ1(T)]0.9±0.05; the susceptibility follows the law S(0)∼κ1.5±0.15; both results are consistent with our scaling crossover formula and the assumption of the importance of the underlying one-dimensional links in the percolation clusters. The corresponding thermal critical exponents are νT=0.9±0.1 and γT=1.5±0.15 compared with the percolation exponents νp=1.36 and γp=2.43, so that the experimental crossover exponent is φ=1.56±0.15. There is currently no first-principle theory which properly accounts for this value of the crossover exponent, although the experiment includes within the errors the value φ=1.7, appropriate to a self-avoiding walk ansatz for the principal paths along which the correlations spread.
We present the results of magnetic-susceptibility, heat-capacity, electrical-resistivity, and thermoelectric-power measurements on La2CuO4-y. The crystallographic, magnetic, and electronic transport properties are found to be extremely sensitive to the oxygen-defect concentration y. Coupled with x-ray and neutron-diffraction data presented in a companion paper, we find that long-range antiferromagnetic order develops with increasing y out of a nonmagnetic ground state at y≃0 into a state with a maximum Néel temperature of 290 K at y≃0.03.
In the past six years perturbed angular correlations (PAC) and perturbed angular distribution (PAD) of γ rays have been widely applied to study the migration, trapping, and clustering of lattice defects in metals. Essential features of these experiments are distinctive precessional signals arising in the nuclear hyperfine interaction. These signals serve as convenient labels for specific lattice defects bound to γ-ray-emitting probe atoms, and under favorable circumstances provide information on the lattice symmetry of the defects. Because we believe that the maximum information is obtained from such experiments if systematic comparisons are made between similar metals, we describe here our survey and interpretation of PAC and PAD data for seven fcc metals (Ag, Al, Au, Cu, Ni, Pd, Pt) using 111Cd probe. We consider defect production by irradiation, quenching, and ion-implantation, and note that the same defect types recur in all three methods. We include a total of 19 distinctive states in our discussion, and through a series of independent, comparative observations we argue that these states involve vacancy defects bound to the 111Cd probe. On the basis of the observed frequencies, annealing behavior, and electric-field-gradient symmetry we divide vacancy states into four classes. For three of these classes structural assignments can be made. These include the nearest-neighbor monovacancy observed in five metals, divacancies or faulted loops in the {111} plane observed in six metals, and a tetrahedral cluster seen only in Ni. For the nearest-neighbor monovacancy the data permit estimation of migration enthalpies. For the monovacancy the data also permit an interpretation of the observed electric field gradients. Though some of the observed states remain with undetermined structure, we believe our comparative analysis provides an excellent foundation for further detailed study of small vacancy clusters under a variety of metallurgical conditions.
The critical exponent β for magnetically ordered materials has been obtained from a variety of hyperfine experiments, such as nuclear magnetic resonance, perturbed angular correlations, and Mössbauer effect. In this paper we discuss probe disturbance effects on hyperfine measurements of β. We consider both chemically pure substances and materials into which the hyperfine probe has been introduced as a dilute impurity, and emphasize the latter. From a theoretical point of view, we investigate several molecular-field models and present results of new calculations for an isolated nonmagnetic impurity in a three-dimensional Ising model. It is found that the disturbance produced by the impurity is substantially smaller in the latter case than for the corresponding molecular-field model. From an experimental point of view we present a survey of cases for which bulk and hyperfine measurements have been made on the same substance. We also report on a reanalysis of data in Ni, and present the results of power-law fits made for various ranges of reduced temperature. The cases studied include Ni 100Rh, Ni 111Cd, Ni 57Fe, and Ni 181Ta, and are restricted to samples produced by diffusion or melting. On the available theoretical and experimental evidence, we conclude that values of β determined from hyperfine measurements involve probe-disturbance errors that are certainly smaller than 2% if the reduced temperature is restricted to t<10-2 and care is taken to avoid source inhomogeneities and temperature gradients.
The time-differential perturbed angular correlation of 111Cdm substituted as a dilute impurity into antiferromagnetic NiO, CoO, and MnO has been observed. The following magnetic fields are found at the Cd nucleus (4°K): NiO (191.1 ± 2.5 kOe), CoO (170.8 ± 3.0 kOe), MnO (194.7 ± 2.5 kOe). They are compared with the Cd hyperfine fields in the antiferromagnetic perovskites KNiF3:Cd, KCoF3:Cd, and RbMnF3:Cd reported earlier. The oxides are found to be more covalent than the fluorides. From the ratio of the hyperfine fields in MnO:Cd and NiO:Cd a new value of the spin density parameter fσ(Mn-O)=8.1% has been estimated.
This paper reports a study using neutron scattering techniques of the spin fluctuations in the two-dimensional diluted Ising system, Rb2CocMg1-cF4. The explicit concentrations studied, c=0.55,0.575,0.585,and 0.595 are all close to the percolation concentration for nearest-neighbor interactions cp=0.594, so that these experiments span the percolation threshold. The experiments were performed at the Brookhaven high-flux beam reactor. A detailed study was made of the dynamics for c=0.575. The results showed four dispersionless bands corresponding to spin deviations on Co ions surrounded by one, two, three, or four Co ions. Calculations of the spectra using the equation-of-motion method for computer simulations gave a good account of the results. The few minor discrepancies probably arise from the effects of those crystal-field levels of the Co ion which have energies above the ground-state doublet. The c=0.595 crystal has a transition at 30 K to a state of two-dimensional order with negligible order in the third direction. The inverse correlation length in this system is zero to within the errors at 30 K. In the more dilute systems the inverse correlation length is found to be well described as an additive sum of a geometrical and a thermal part as observed previously in Rb2MncMg1-cF4. The thermal part is given by [κ1(T)]1.32±0.05 where κ1(T) is the inverse correlation length of a one-dimensional chain of Co ions. This exponent is consistent with the percolation exponent νp=1.35 and the theoretically calculated crossover exponent, φ=1. The intensity of the scattering is more difficult to analyze in the absence of a knowledge of the form of the scaling function, but the behavior is consistent with the multicritical picture with φ=1.
A critical review is given of measurements of the critical exponent β in three dimensional magnetic systems. Many claims of discrepancies between experimental data sets and between experiments and theory are either corrected or are found to be groundless on the basis of our reanalysis. A large percentage of the experiments are found to lie too far from the critical point to allow deduction of exponent values with uncertainties smaller than the differences between model calculation predictions. Those data which do allow deduction of exponent values appear to show a slight deviation from recent theoretical calculations.
The 48.7 m111m Cd activity was implanted in Zn and Be single crystals which were soldered to the cold finger of a dilution refrigerator and kept below 0.2 K during implantation. Subsequent nuclear orientation experiments allowed the determination of the quadrupole interaction frequencyv Q of the 11/2− isomeric state of111Cd in Zn and Be as −139 (15) MHz and +43(16) MHz respectively. With these results we derive the quadrupole moment of the 5/2+ 245 keV level of111Cd including sign asQ = +0.83(13) b and the sign of the electric field gradient for Cd in Be. The half-life of111m Cd was redetermined as 48.67 (6) m.
BaF2 scintillation counters, coupled to constant fraction differential discriminatiors, permit simultaneous energy and timing
decisions in recording delayed coincidence spectra. We explore the advantages and limitations of this innovation in time differential
perturbed angular correlation experiments, and illustrate it for111CdNi and100RhNi.
In this paper we present perturbed angular correlation (PAC) measurements on100Rh and111Cd in a number of dilute alloys of Cu, Ag and Au. We discuss the electric field gradient (EFG) values derived from these and other experiments in the framework of a theory of the EFG at nearest neighbours of point defects in metals, essentially based on the screening of a point charge in a free electron gas. We include in our discussion the EFG due to a neighbour monovacancy for a number of fcc metals.
The Knight shift and its temperature dependence for a Cd impurity in palladium metal were measured by means of DPAD- and DPAC-methods utilizing the well known 5/2+, 247-keV state in111Cd. The shift at 80 K was found to be KS (CdPd, 80 K)=–0.8(2)%. The observed variation of the KS in the temperature range from 80 K up to 1400 K is 0.5%. For calibration purposes an accurate remeasurement of the magnetic moment of the 5/2+ state in111Cd was necessary and yielded(111Cd, 5/2+, 247 keV)=–0.7697(20) n.m.
Since pioneering NMR studies of magnetic critical phenomena in MnF2 by Heller, a range of hyperfine techniques, particularly Mössbauer effect and perturbed angular correlations, have been successfully
applied to the problem. When optimally designed, these methods provide experimental approaches that are in some ways more
successful than magnetic neutron scattering. Although the number of hyperfine results now number in the hundreds, only a few
are sufficiently asymptotic to provide clearcut tests of theory. In this paper we outline the theoretical framework used to
describe the modern theory of critical phenomena, discuss the strengths and weakness of various experimental methods, and
provide a selection of the best experiments that test the theory. Whereas we emphasize hyperfine interaction studies, we also
describe the principles and results of other methods where appropriate. Particular topics include: (1) static critical phenomena
in chemically ordered systems studied as a function ofn andd, the spin and lattice dimensionality; (2) static critical phenomena in magnetically disordered systems studied as a function
of the type of disorder; and (3) dynamic critical phenomena in chemically ordered systems.
Neutron diffraction and magnetic susceptibility measurements revealed antiferromagnetic order in single crystals of La2CuO4 where both sublattice moments and Néel temperatures (TN) strongly depend on heat treatments. Deoxidization enhances MS and TN (MS=0.6±0.05μB:TN=298±5K) whereas oxidization reduces these values (MS=0.22±0.04μB:TN=135±20K) from those of as-grown crystal (MS=0.38±0.05μB:TN=230±10K). A broad peak in the temperature dependence of magnetic susceptibility near TN with the field parallel to the c-axis (a<b<c) is also enhanced in deoxidized crystal.
The occurrence of long- and/or short-range magnetic ordering in La2−zCuO4−y, La2−xSrxCuO4−y and YBa2Cu3O7−x is reviewed and discussed with respect to the occurrence of high Tc superconductivity in these systems.
We report a study of the magnetic susceptibility of La2−xSrxCuO4for x=0, 0.10, 0.15, 0.20 from 4–350K. Our data suggest that La2CuO4 has a spin-density wave or antiferromagnetic transition near 250K. The Sr doped superconductors have a Pauli susceptibility above Tc=35−40K that is enhanced by electron-electron correlations. The variation in Tc with Sr doping is not directly correlated with the change in electron density of states.
Positive muon spin rotation (μSR) experiments were performed on a polycrystalline sample of La2CuO4 in zero applied magnetic field. Below 250 K a clear μSR frequency is observed which unambiguously demonstrates magnetic ordering in this sample. The temperature dependence of the frequency shows a continuous decrease with increasing temperature and a sharp drop to zero at 250 K. In the same temperature range the depolarization rate increases continuously and rises strongly close to the transition temperature. The extrapolated frequency at T = 0 K is 5.6 MHz which corresponds to a local magnetic field of 41 mT.
Experiments measuring the dynamical exponent z in the isotropic ferromagnets EuS, EuO, Ni, Fe, and Co are reviewed. Our recent hyperfine interaction experiments demonstrating crossover in z are described in detail. In contrast to early neutron experiments on isotropic ferromagnetis it is found that with the exception of Co, pure Heisenberg behavior, i.e., z=2.5, is not observed in any of the materials surveyed. Instead, when either the wave number or the reduced temperature is sufficiently small, isotropic ferromagnets exhibit asymptotic behavior characterized by z=2.0. The most likely theoretical explanation is that significant spin-nonconserving forces are perturbing the Heisenberg exchange interaction. For ESR and neutron studies of EuS and EuO it has been shown that the observed behavior can be explained by dipolar forces. For hyperfine interaction experiments on Ni and Fe, the observed crossover to z=2.0 must be attributed to other, stronger spin-nonconserving forces. In the exceptional case of Co, where crossover to z=2.0 has not yet been observed, it is expected that future, more nearly asymptotic experiments, will detect crossover.
Data reduction schemes currently used for time differential perturbed angular correlation measurements are evaluated in terms of (i) their relative effectiveness in eliminating irrelevant variables, including single counter efficiencies and spectrum time shifts; and (ii) their effectiveness in putting data in a form that can easily be fitted by theoretical correlation functions. It is pointed out that erroneous conclusions may be derived from improperly reduced data, but that properly analyzed experiments performed at four angles allow a good determination of both the time-dependent and time-independent parts of the correlation function. Correction of data for accidentals, source self-absorption and spectrum livetime differences are discussed.
The perturbed angular correlation (PAC) method has been employed to study the hyperfine interaction of the 245-keV state in 111Cd, following 111In implantation into the fcc metals Ag, Al, Cu, Ni, Pd, Au, and Pt and subsequent oxidation. Broad quadrupole frequency distributions around a peak frequency =160 MHz were found in all systems and attributed to oxygen trapping at 111In. The fraction of oxidized 111In increases with the oxidation temperature in such a way that about 50% of the implanted In ions have trapped oxygen at 60% of the melting temperature of the host. An interpretation of these frequency distributions has been attempted on the basis of PAC measurements for 111In implanted into the metal oxides AgO, Ag2O, and In2O3. In these systems, well-defined electric field gradients have been found which can be related to the respective crystallographic structures. In addition, the magnetic hyperfine interaction of 111In in antiferromagnetic NiO proves substitutional implantation.
Superconducting samples of {\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4+\ensuremath{\delta}} are shown by neutron powder diffraction to consist of two nearly identical orthorhombic phases. The primary phase has a stoichiometry near Cu. The second phase is an oxygen-rich phase that is superconducting. The abundance of the second phase increases with the oxygen pressure at which the samples are annealed. Neutron-diffraction measurements as a function of temperature show that the phase separation occurs reversibly near 320 K.
The temperature-concentration phase diagram of doped La2CuO4. The addition of holes introduces a local ferromagnetic exchange coupling between Cu spins. The resulting frustration destroys the three-dimensional Neel state characterizing pure La2CuO4, and generates a new three-dimensional spin-glass phase. In the paramagnetic phase, the strongly correlated Cu spins in the planes are canted by the holes, yielding an oscillating dipole-dipole attraction between holes. The possible relevance to superconductivity is discussed.