High energy spin dynamics of La2CuO4 and La1.9Sr0.1CuO4

The magnetic excitations in La₂CuO₄ and La_1.9Sr_0.1CuO₄ were studied by inelastic neutron scattering up to energies of 76 meV. For the pure sample, the results forE15 meV can be well described by conventional spin wave theory with a spin wave velocity c=0.89±0.07 eVÅ. For lower energies, the observed intensities were somewhat higher than expected from spin wave theory and did not follow the Bose-Einstein factor. For the doped sample, the linewidth in constant-E scans at smallE shows a very short correlation length of =7.5 Å only, which is considerably below the value expected from the concentration dependence reported by other authors. An increase of the linewidth for largeE indicates a reduced spin-wave stiffness when compared to the undoped material. However, the use of a spin-wave picture may not be appropriate as the standard expression for damped spin waves in a paramagnetic material is in serious conflict with experiment.     

Microhardness of the compounds La2CuO4 and La1.85Sr0.15CuO4 at temperatures from 81 to 292 K

The microhardness of La₂CuO₄ and La_1.85Sr_0.15CuO₄ crystals is measured in the temperature range 81–292 K, and the influence of various factors on it is investigated. The thermal activation parameters of the plastic deformation process in the vicinity of the indenter impression are estimated. The change of the phase state of the compound La_1.85Sr_0.15CuO₄ at the temperature T ₀=180 K and the appearance of domain (twin) boundaries formed in the ferroelastic tetraortho transition are not seen in the temperature dependence of the microhardness. The results of the study are compared with previously published results for YBa₂CuO_7−x crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 493–497 (March 1998)     

μ + SR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sitesSR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sites

We compare the temperature dependence of the zero-field muon spin precession frequency observed in Sr₂CuO₂Cl₂ and La₂CuO₄ below the Néel temperature with a 2-d Heisenberg model with an additional small anisotropy. Good agreement of the observed results with this model reflects the highly 2 dimensional magnetic properties of these cuprates. The measured local fields are compared with a simple calculation of dipolar fields in the crystal to search for muon site locations. Our results provide evidence that the muon sites are related to the oxygen atoms in these compounds.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ∼v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ～v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Electronic structure of pure and doped orthorhombic La2CuO4

The electronic structure of orthorhombic La₂CuO₄ has been investigated by first principles pseudofunction band calculations and group theoretical analysis. We find that pure as well as doped compounds remain metallic at all finite temperatures as a conseqence of the Cmca (D¹⁸_2h) space group symmetry. The experimentally observed rapid rise in resistivity below 30°K suggests a structural transition to a lower symmetry space group that could be driven electronically or magnetically. One possible candidate is monoclinic C2/m (C³_2h) which is a subgroup of Cmca and can be obtained by distorting the CuO bonds or rotating the CuO₆ octahedra. Implications for superconductivity are discussed.     

A study on the dynamical spinsusceptibility of paramagnetic La2CuO4

The one electron wavefunctions and energy bands obtained by the self-consistent LMTO-ASA method in the local density approximation are used to calculate the wavevector and frequency dependent non-interacting spin susceptibility of paramagnetic La₂CuO₄ in thebct-structure. We show that matrix elements effects lead to a substantial depression of the susceptibility, especially for wavevectors near the X-point. Whereas interband transition contributions turn out to be significant, Fermi surface nesting properties are shown to be of less importance. In contradiction to the impression given by spin polarized electronic structure calculations the RPA-LDA approximation for the susceptibility predicts this substance to be far away from a magnetic phase transition, and we shed some light on the cause for this failure.     

Possible superconductivity on the junction surface of dielectric La2CuO4

It is possible to induce excess charge on the surface of dielectric La₂CuO₄ by the field effect of a transistor or in the Schottky junction. Superconductivity can be observed in this two-dimensional system.     

Jahn-Teller effect and the orbital ground state of Cu2+ ions in Eu2CuO4 and La2CuO4 crystals

Local ion displacements and their anharmonicity in Eu₂CuO₄ and La₂CuO₄ crystals are studied by high-precision x-ray diffractometry. A symmetry analysis and the temperature behavior of the probability density function of Cu²⁺ ions reveal a degenerate ground orbital state in the Eu₂CuO₄ crystal (tetragonal doublet d _xz, d _yz). The pattern and anharmonicity of the local displacements are found to be determined not only by lattice vibrations but by 2D antiferromagnetic spin fluctuations in the CuO₂ sheet as well. By contrast, in the La₂CuO₄ crystal the orbital ground state of the Cu²⁺ ion is a singlet (d _z 2). The pattern of Eu³⁺ local displacements and their effect on the properties of the Cu subsystem in the Eu₂CuO₄ crystal are also studied. Fiz. Tverd. Tela (St. Petersburg) 39, 1600–1608 (September 1997)     

Study of La2CuO4 and related compounds by μSR

Tin exhibits different oxidation kinetics which are composition dependent, when it forms intermetallic compounds with the chalcogenides S and Se. This phenomenon is related to the layer compounds SnS₂ and SnSe₂ crystalline structure. These minerals have anisotropic bonding characteristics, due to Van der Waals bonds presence between chalcogenides adjoining planes. The mentioned weak bonds allow the oxygen diffusion to the bulk, favouring the reaction with the inner tin atoms. In this work we study samples of Sn?S alloy with different thermal treatment by XRD and Mössbauer spectroscopy. Results are discussed and compared with those obtained for Sn?Se alloy in an early work [1].     

Zn掺杂La2CuO4热电势和磁化率研究

测量了La2CuO4掺Zn样品在不同降温速率下(330K保温05h,然后分别以6Kh,02Ks的速度降到8K)的直流磁化率和热电势.实验结果表明,反铁磁温度TN不随降温速率变化而变化,其直流磁化率也未受很大影响.高温热电势弱的温度依赖关系表明为极化子气体的贡献.热电势在转折温度Tdrop之下的快速降低是由于二维反铁磁涨落的贡献.热电势在更低温度的拐点TS与载流子的局域化有关.降温速率变化时,Tdrop和TS都有明显的变化.Zn掺杂对Tdrop和TS没有明显影响,但导致了更强的载流子局域化.讨论了上述现象产生的物理图像 关键词： La2CuO4 直流磁化率 热电势     

La2CuO4掺锌样品的低温电阻率与热导率研究

对La2CuO4掺锌样品在不同降温速率下(330K保温30min后,分别以6Kh和02Ks的速率冷却至42K)电阻率(42—330K)和热导率(80—300K)随温度的变化关系进行了研究.实验结果表明,在不同降温速率下,热导率和电阻率都受到很大影响.快速降温过程使得130K以上的热导率减小,而热导率最小值出现在130K,且与降温速率无关.而低温下的热导率不受降温速率变化的影响.样品在高温区(T高于125K)电阻率随降温速率的增大而增大,低温区电阻率的非线性行为可用变程跳跃行为来描述.所有样品的热导率和电阻率在反铁磁相变温度都没有出现反常,这与能带理论框架下预期的结果和Anderson电荷自旋分离理论发生了矛盾,对此进行了讨论,并用极化子理论进行了自洽解释. 关键词： La2CuO4 热导率 电阻率     

The oxygen isotope effect on the infrared photo-induced absorption spectra of La2CuO4

The infrared photo-induced spectra of the isotope-substituted stoichiometric La₂CuO₄ samples were studied at T=20 K. Measured spectra were analyzed within the small polaron hopping theory. A small, but detectable shift of the effective phonon frequency on the substitution ¹⁶O to ¹⁸O is found for the high-energy surface carrier associated band (~4000 cm^-1) and has a value of α_O = 0.3 ±0.2. The low-energy band (~1500 cm^-1) originating from the bulk carriers doesn't show any difference within experimental error which is fully in agreement with theory predictions. The isotope shifts of the optically active oxygen-related phonon modes are described well by the harmonic approximation.