Generation of subharmonic shapiro steps in ladder arrays of overdamped superconducting junctions

Summary Subharmonic Shapiro Steps (SSS) are usually observed in 2D arrays of superconducting junctions when their dynamical states are perturbed by factors like disorder, frustration, current inhomogeneities and so on. In order to clarify the conditions under which SSS can be generated we decided to investigate arrays of very simple shape like the ladder ones. In particular we have studied the influence of the frustration (i.e. of an external magnetic field) on the time evolution of the vortex configuration and the commensurability of the latter with the geometry and the dimension of the array. Ladders, parallel or perpendicular to the bias current, turn out to be systems where the properties of the square arrays can be suitably tested. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

The ferroelectric mode in orthorhombic KNbO3

Both Raman scattering and infrared reflectivity measurements have been performed to determine the complete optic mode spectrum of orthorhombic KNbO₃ and characterize the ferroelectric modes. The low frequency, k ～ 0 mode is quasitransverse and possesses a spacial anisotropy similar to the TA branch at finite k. Of the original cubic ferroelectric mode, the component along the remaining soft direction is found at 56 cm^-1 and not overdamped (Γ/ω = 1), while the other two components have stiffened equally so that the mode is essentially isotropic in the orthogonal polarizations.     

Ferromagnetism in epitaxial orthorhombic YMnO3 thin films

Epitaxial orthorhombic YMnO₃ thin films, (0 0 1) oriented, have been grown by pulsed laser deposition on (0 0 1)SrTiO₃ substrates. Their crystal structure and magnetic response have been studied in detail. Although bulk o-YMnO₃ is antiferromagnetic, our magnetic measurements reveal intriguing thermal hysteresis between the zero-field-cooled and field-cooled curves below the onset of the antiferromagnetic ordering temperature, thus signaling a more complex magnetic structure with net ferromagnetic moments. We discuss on the possible origin of this net magnetization and we have found a correlation of the magnetic response with the strain state of the films. We propose that substrate-induced strain modifies the subtle competition of magnetic interactions and leads to a non-collinear magnetic state that can thus be tuned by strain engineering.     

Thermal expansion and structure of orthorhombic CaMnO3

The structure of CaMnO₃ has been studied from room temperature to 800 °C by high-resolution synchrotron X-ray powder diffraction. It is concluded that CaMnO₃ remains orthorhombic in Pbnm over the entire temperature range, with the expansion in the cell volume well fitted to the expression Vol=207.37(2)+0.00563(8)T+1.17(9)×10^-6T². The magnitude of the MnO₆ tilts is estimated from the refined structural parameters and these progressively reduce as the temperate is increased.     

The thermoelectric power of monoclinic TaS3. Comparison with orthorhombic TaS3 and NbSe3.

The thermoelectric power of monoclinic TaS₃ was measured over the temperature range of 90K to 430K. It is positive at high temperatures, negative between the two Peierls transitions and positive at low temperatures. The changes in sign occur at the transitions. The results obtained at high temperatures are compared with those for NbSe₃ and orthorhombic TaS₃.     

Electron paramagnetic resonance of Fe3+ in orthorhombic KNbO3

EPR spectra of KNbO₃:Fe single crystals are obtained in the - 10–110°C temperature range. The angular dependence of resonant lines is well reproduced by spin Hamiltonian parameters relevant to a Fe³⁺ impurity ion substituted to Nb ion in KNbO₃ crystal. The temperature behaviour of resonant lines is explained by a quadratic dependence of axial parameter D vs polarization P_s of the form D = βP²_s in the orthorhombic phase.     

On the nonlinear field-effect of orthorhombic TaS3

The true d.c. measurement of electrical field effect on the resistivity of orthorhombic TaS₃ in the CDW state was performed for the first time with extremely short distances between the contacting leads. Very weak nonlinear effect was observed up to 1300 V cm^?1. A distinct resistivity — increasing effect was found, which cannot be understood within the framework of any currently available theories. We propose a transverse tunneling model to account for this anomaly.     

The evolution of octahedral rotations of orthorhombic LaVO3 in superlattices with cubic SrVO3

We have studied the octahedral rotations in LaVO₃/SrVO₃ superlattices, keeping the thickness of the orthorhombic LaVO₃ layers constant and increasing the thickness of cubic SrVO₃ layers. We have found that for a small thickness of SrVO₃, the octahedral rotations in LaVO₃ are maintained, while for an increasing thickness, these rotations are suppressed. This observation cannot be explained by purely elastic effects due to the lattice mismatch between the two materials, but the absence of rotations in SrVO₃ is a crucial ingredient, illustrating the concept of interface engineering of octahedral rotations.     

Precession modes and resonance absorption linewidth in orthorhombic antiferromagnet Fe3BO6

Resonance absorption in Fe₃BO₆ has been investigated for the low-frequency AFMR branch as a function of magnetic RF-field component direction in the bc crystal plane. Anisotropic character of absorption is shown to be due to complex character of weak ferromagnetic moment precession in orthorhombic crystals. Relation between the true resonance line width, and that observed in the quasi-optical experiment is discussed.     

Hydrogen storage: Lattice dynamics of orthorhombic NaMgH3

In this work, we investigate the structural, dynamic and thermodynamic properties of NaMgH₃, devoted for hydrogen storage. Density functional theory using pseudopotential methods and generalized gradient approximation has been used. A good agreement between the calculated structural parameters and the experimental data was found. A linear-response approach for the density functional theory is used in order to derive the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the center of the Brillouin zone, the phonon-dispersion curves and the corresponding density of states for NaMgH₃ material. The obtained phonon frequencies at the zone center (Γ point) for the Raman-active and infrared-active modes are analyzed. Thermodynamic functions using the phonon density of states are also calculated.     

正交多铁性材料DyMnO3的磁性质研究

基于密度泛函理论结合投影缀加平面波方法, 通过VASP软件包执行计算, 在分别考虑电子自旋阻挫共线与非共线的磁性结构基础上, 研究了正交结构下多铁性DyMnO₃材料在不同磁性构型下的晶格参数、总能、磁性、电子态密度和能带结构. 计算过程中选取广义梯度近似赝势, 同时使用局域自旋密度近似+U方法处理强关联作用下3d电子的计算结果. 计算结果表明: Mn离子为A型反铁磁态磁性构型的情况下能量最低结构最为稳定, Dy稀土离子磁性甚微, 可忽略不计; 当考虑电子自旋为非共线排列时, 正交DyMnO₃的总能提高、磁矩增大; 从电子结构图分析可知, 材料为间接能隙绝缘体, 能隙宽度约为0.38 eV, 加U后为1.36 eV, 导致晶格畸变的主要原因为Mn-3d与O-2p电子之间强烈的杂化作用. 关键词： 多铁性 反铁磁 密度泛函理论 非共线     

Zone center frequencies of the orthorhombic NdMnO3 perovskite

A short range force constant model has been applied for the first time to investigate the phonons in NdMnO₃ perovskite in the orthorhombic phase. The calculations with nine stretching and eight bending force constants provide good agreement for the observed Raman frequencies. The infrared frequencies have been assigned for the first time.     

Raman spectra of the orthorhombic semiconductor compound Cu2SnTe3

The optical phonon spectrum of the semiconductor Cu₂SnTe₃, that crystallizes in the orthorhombic structure with space group Imm2 (), have been studied by measuring unpolarized Raman scattering between 10 and 300 K. The experimental frequencies of the phonon modes observed were compared to those calculated by using simplified lattice dynamical models reported in the literature. From combined analysis of these results together with the factor group analysis of the zone-center vibrational modes, valuable information about these modes was obtained and their possible symmetry was assigned. A₁ modes at 71, 123, 167, 176 and 190 cm⁻¹; A₂ modes 115 and 131 cm⁻¹; B₁ modes at 76, 142 and 152 cm⁻¹; B₂ modes at 89, 100 and 206 cm⁻¹; a overtone at 246 cm⁻¹, and combinations at 218, 270 and 292 cm⁻¹; have been observed in this compound.     

Linear and nonlinear optical susceptibilities of orthorhombic rare earth molybdates RE2(MoO4)3

The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu.     

Native defects and Pr impurities in orthorhombic CaTiO3 by first-principles calculations

Formation energies of native defects and Pr impurities in orthorhombic CaTiO₃ are explored using the first-principles calculations. The Ca vacancy (V_Ca), Ti vacancy (V_Ti) and Ca antisite (Ca_Ti) are found to be energetically preferable. The Ti antisite (Ti_Ca) and O vacancy (V_O) are not energetically favorable in the wide range of Fermi level. In Pr-doped CaTiO₃, Pr substituting for Ca (Pr_Ca) is likely to form under condition A in which CaTiO₃ is in equilibrium with CaO and O₂. Under condition B (TiO₂, CaTiO₃ and O₂ are in equilibrium), Pr_Ti defect is energetically preferable depending on the Fermi levels. Several native defects and the two sites of Pr impurities in CaTiO₃ are coincided with several different defects in Pr-doped CaTiO₃ reported in the literature. Based on the present calculations, we can elucidate that the Ca deficiency design of the traditional formula Ca_1−xV_Ca(x/2)Pr_xTiO₃ is not the best for efficient red photoluminescence, which is realized via the experimental measurements.     

Light scattering by polariton modes of KNbO3 in orthorhombic phase

The polariton modes of A₁, B₁ and B₂ species in biaxial KNbO₃ are observed by near forward Raman scattering. The oscillator strengths and dielectric constants along the principal axes are determined from the polariton dispersion. The mode assignments of each species are also discussed.     

Strandlike microstructures in the charge-density wave states of orthorhombic TaS3

Strandlike domains are observed in the commensurate charge-density wave states of orthorhombic TaS₃ using dark field electron microscopy. The images of the strandlike domains are found to be time-dependent and sensitive to the radiation damage induced by the incident high energy electrons. Our observation suggests that the appearance of the strandlike structure may be common to this class of materials regardless of the commensurability of the charge-density waves.     

Current induced deformation of charge density waves in orthorhombic TaS3

The low electric field ohmic resistance R of orthorhombic TaS₃ measured at 90 and 120 K well below the Peierls transition temperature depends on the product of a temperature difference ΔT applied along the sample and the sign of a previously applied current pulse if this pulse is larger than threshold for non-ohmic conductivity. This resistance change is about ΔR/RΔT ∽ 1×10^-3 K^-1 for a pure sample and ΔR/RΔT ∽ 6×10^-3 K^-1 for a slightly electron irradiated one at 90 K. The relative resistance change is insensitive to the sample length. We deduce that the CDW current changes inhomogeneously the Peierls gap E_g. ΔE_g < O at the contact where the CDW current enters and ΔE_g > O at the exit. The effect is attributed to a CDW current induced inhomogeneous deformation of the CDW itself.     

EPR of orthorhombic Gd3+-M+ complexes in SrF2

In this paper we report new results of EPR experiments on orthorhombic Gd³⁺-M⁺ complexes (M = Na, K, Rb and Ag) in SrF₂. Special attention is payed to the second degree crystal field parameters B⁰₂ and B²₂ and a comparison with earlier results on corresponding complexes in CaF₂ and BaF₂ is made. We conclude that the main contributions to these crystal field parameters are of electrostatic nature.     

Subharmonic shapiro steps and assisted tunneling in superconducting point contacts

We analyze the current in a superconducting point contact of arbitrary transmission in the presence of a microwave radiation. The interplay between the ac Josephson current and the microwave signal gives rise to Shapiro steps at voltages V = (m/n)h(omega)(r)/2e, where n,m are integer numbers and omega(r) is the radiation frequency. The subharmonic steps (n not equal 1) are a consequence of multiple Andreev reflections (MAR) and provide a signature of the peculiar ac Josephson effect at high transmission. Moreover, the dc current exhibits a rich subgap structure due to photon-assisted MARs.