Paramagnetic Meissner Effect

The explanation for the paramagnetic Meissner effect (PME) in high-temperature superconductors (HTSC) and niobium disks, distinct from previous discussions, is proposed. It is based on the impurity mechanism of high-temperature superconductivity and the general localization approach to the superconductivity problem. Turns of the local magnetic moments of the two-level systems are considered to be the cause of this effect in HTSC. The estimations of the magnetic moment magnitude and the magnetic moment concentration are given. In niobium disks, this effect is also explained by turns of the local magnetic moments, but of another nature. The role of the layered structure is demonstrated. It is shown that the proposed models can explain all available experimental facts. The connection between this effect and the electric field effect in HTSC is discussed.     

Paramagnetic Meissner Effect

The explanation for the paramagnetic Meissner effect (PME) in high-temperature superconductors (HTSC) and niobium disks, distinct from previous discussions, is proposed. It is based on the impurity mechanism of high-temperature superconductivity and the general localization approach to the superconductivity problem. Turns of the local magnetic moments of the two-level systems are considered to be the cause of this effect in HTSC. The estimations of the magnetic moment magnitude and the magnetic moment concentration are given. In niobium disks, this effect is also explained by turns of the local magnetic moments, but of another nature. The role of the layered structure is demonstrated. It is shown that the proposed models can explain all available experimental facts. The connection between this effect and the electric field effect in HTSC is discussed.     

Wohlleben Effect (Paramagnetic Meissner Effect) in High-Temperature superconductors

Recently a quite unexpected phenomenon was observed during the study of the magnetic properties of High-T_c superconductors: In the field-cooled regime the magnetic response of some HTSC at very low fields ( 1Oe), instead of being diamagnetic, becomes paramagnetic. Such behavior is perfectly reproducible and stable. This effect is now called the Wohlleben Effect. The samples showing the Wohlleben effect also display anomalous behavior in some other properties (microwave absorption, second harmonic of magnetic susceptibility). In this paper a survey is given of the experimental studies of this and related phenomena, carried out in different laboratories. Corresponding theoretical models are also discussed. The effect is attributed to the formation of spontaneous currents (spontaneous orbital magnetic moments) in the ground state of the weak link network in case when Josephson coupling between certain grains is negative (-contacts). Microscopic mechanisms of inverse Josephson coupling are discussed especially in connection with the possible unconventional pairing in HTSC.     

Paramagnetic Meissner Effect in Electrochemically Doped Indium-Tin Oxide Films

Transparent conductive indium-tin oxide (ITO) thin films, electrochemically intercalated with alkali (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), alkali earth (Mg⁺², Ca⁺²), or complex NH\(_{4}^{+}\) ions, show tunable superconducting transitions with dome-shaped behavior of T _c versus electron density around the maximum at ～ 5 K. On field cooling, the transition into the superconducting state is accompanied by a paramagnetic response, i.e., an increase of magnetization, rather than the usual diamagnetic Meissner response. We provide an extensive study of this so-called paramagnetic Meissner effect (PME), using DC SQUID, transport measurements and a variety of sample sizes and growth conditions. We show that the PME in electrochemically doped ITO films results from a higher T _c at the sample edges than at the center of disk-shaped samples, causing flux to be expelled towards the center of the disk, following the flux-compression theory of Koshelev and Larkin. Changing to the opposite spatial T _c profile largely removes the paramagnetic response. The paramagnetic magnetization is strongly influenced by sample geometry and flux pinning conditions. The reduction of pinning defects by thermal annealing removes the paramagnetic response. An alternation of the external magnetic field restores the usual Meissner diamagnetism.     

Calculation of the Higher Order Dipole-Dipole Effect in Paramagnetic Crystals

This report is an attempt to investigate the influence of dipole-dipole coupling in a paramagnetic spin system at low temperatures. It consists of two parts. The first part is a discussion of the use of C_M=C_H=0 for a system with mutual interaction. It is pointed out that only if the external field is large compared to the internal field is this equation correct.The other part consists of a calculation of higher order correction of the dipole-dipole interaction on a system of paramagnetic spins which is subject to a crystalline field which we chose of the Y_2,0 type. The total Hamiltonian consists of a spin Hamiltonian in accordance with this symmetry, a term representing the external magnetic field and the dipole-dipole interaction between the spins. The partition function is calculated by means of the Schwinger trace formula considering a representation in which the first two terms of the Hamiltonian are diagonal. The trace of the density matrix can be expressed as the trace of a product, one factor is the density matrix of the noninteracting spins, the other factor consists of a sum of commutators. These commutators are worked out in detail and the result is given in the form of a finite series over the quantum number m. There seems to be no obvious way to perform these summations.     

Effect of Isotopic Composition on the Electron Paramagnetic Resonance in Silicon1

The effect of isotopic composition on the electron paramagnetic resonance of dangling bonds was studied in single-crystal and polycrystalline silicon. The results demonstrate that the resonance linewidth in monoisotopic ²⁸Si is smaller than that in Si with the natural isotopic composition because of the smaller contribution of superhyperfine interaction of electrons with the nuclear spins of ²⁹Si. The relaxation contribution to the linewidth is larger in monoisotopic ²⁸Si because of the changes in the scattering of phonons responsible for the spin relaxation of paramagnetic dangling bonds.     

Paramagnetic silica-coated gold nanoparticles

Water soluble gold nanoparticles, obtained by the reduction of the gold (III) chloride with sodium borohydride in the presence of citric acid or thioctic acid, were covered with a paramagnetic silica layer using the Stober method, yielding a hybrid metallic-inorganic nanomaterial (gold nanoparticles, with an average size of 5 nm, embedded into silica nanoparticles, with an average size of 100 nm). The paramagnetic silica layer was formed by copolymerization of a paramagnetic silica precursor (derived from 3-aminopropyltrimethoxysilane) with tetramethyorthosilicate. The paramagnetic silica precursor was obtained by coupling 3-aminopropyltrimethoxysilane with 3-carboxy-proxyl free radical. TEM pictures show that each silica nanoparticle of about 100 nm in size embedded about 10 gold nanoparticles. These hybrid nanoparticles are quite stable and exhibit the expected paramagnetic characteristics, as seen by electron paramagnetic resonance. The accessibility of methanol through the silica layer was also studied. Depending on the capping ligands of the gold nanoparticles (citric or thioctic acid), different silica networks are formed, as seen by the mobility of the spin-label inside the silica layer. The EPR spectra showed that the paramagnetic silica layer is very robust and the mobility of the spin-probe inside the silica layer is very little affected by methanol. However, if spin-labeled thioctic acid protected gold nanoparticles were used in the material synthesis, the mobility of the spins attached to the gold surface is quite high in the presence of methanol, while the spins embedded into the silica layer remains immobilized.     

Paramagnetic meissner effect in conventional Nb superconductors

The paramagnetic Meissner effect (PME), in which the field-cooled-magnetization (FCM) of superconducting samples is positive below the superconducting transition temperature T_c, has been observed in certain ceramic and single-crystal samples of the high-T_c cuprate superconductors and more recently in disk-shaped Nb samples. Through systematic investigations of the conditions for observing the PME in Nb disks, various surface treatments to the Nb disks were found to change both the zero-field-cooled-magnetization (ZFCM) and the FCM, including the appearance of a positive FCM in samples previously not exhibiting the PME as well as the elimination of the PME through surface abrasion. These results suggest that the PME arises from the field distributions created by the flux pinning associated with microstructural defects on the surface layer of the disk.     

X-ray-Induced Paramagnetic Centers in SiO2Xerogels

SiO₂xerogels prepared by a sol–gel process followed by sintering at 350–1100°C were irradiated with 30-keV x-rays at a dose rate of 80 R/s. The room-temperature electron spin resonance (ESR) spectra of the xerogels showed signals arising from radiation-induced intrinsic defects (E" centers and terminal oxygens) and organic radicals. By recording ESR spectra in a vacuum of 10^–2Pa, surface E" centers differing in relaxation time from the volume centers were identified for the first time. The ESR signal from the E" centers was measured as a function of x-ray dose. The possible mechanisms for the formation of E" centers under x-ray irradiation are discussed.     

Paramagnetic Attraction of Impurity-Helium Solids

A small permanent magnet was used to attract impurity-helium solid samples composed of hydrogen, deuterium, and nitrogen radicals. The magnetic field gradient was sufficiently strong to lift each of the impurity-helium solids while submerged in superfluid helium, but only strong enough to lift one of four samples through the liquid surface. This suggests ranges of local atomic radical concentrations that partially agree with previous ESR measurements. The attractive paramagnetic force is strong enough to be useful as a trap for the formation of a pure hydrogen impurity-helium solid, for use in radical concentration measurements and for sorting and moving impurity-helium solids.     

The Paramagnetism Carried by Paramagnetic Ions and its Effect on the Superconductivity in High T C Superconductors

Extended Brillouin function with a revision is applied to describe the paramagnetism carried by the rare-earth Gd^{3 +} ions in GdBa₂Cu₃O_6+δ and the Re²⁺ ions in (Hg_0.9Re_0.1)Ba₂Ca₂Cu₃O _g+δ. We believe that the paramagnetism depends on the internal flux density of the sample instead of the applied field. At the field below H _cl the paramagnetism has no contribution to total magnetization; at the field over H _cl the paramagnetic magnetization has different values when in field-increasing and field-decreasing process. The width of the magnetization hysteresis loop Δ M is broadened by the paramagnetism. The effect of the paramagnetism due to paramagnetic ions on the magnetization relaxation rate and the magnetization critical-current density J _c based on the Bean critical-state model is also discussed. The temperature dependence of the paramagnetism is shown in this paper.     

螺旋碳纤维的顺磁响应

采用化学气相沉积法制备了螺旋碳纤维,通过XRD、EDX和SEM对样品进行了表征和分析,采用研磨方法考察了螺旋结构的破坏情况,并对比了研磨前后样品的低温磁性。结果表明,在有效去除催化剂的情况下,螺旋形貌被破坏以后,碳纤维的抗磁性信号增强。基于单电子受缚于螺旋线的物理模型对实验结果进行分析和讨论,认为螺旋形貌具有顺磁响应,并阐释了其产生机理。     

The Positive Magnetic Susceptibility for Rb3C60 Superconductor and the Differential Paramagnetic Effect

AC magnetic susceptibility of Rb₃C₆₀ superconductor as a function of temperature was measured in an applied static magnetic field perpendicular to the AC magnetic field. The peaks in real part of AC susceptibility curves located in the transitional temperature indicate that there may exist the differential paramagnetic effect χ′(T) = dM/dH > 0 in the specimen. The amplitude of the peak and the temperature of diamagnetic onset are proportional to intensity of the applied field, and upon cooling the peaks occur before the transition temperature of zero field. Through discussing and comparing our experimental result with those reported previously, we have put forward a new opinion on producing condition of the differential paramagnetic effect (DPE).     

Tilting Mode Relaxation and Oxygen Isotope Effect in La2−xSrxCuO4 Studied by Electron Paramagnetic Resonance

The electron paramegnetic resonance (EPR) of Mn²⁺ doped into La_2?x Sr _x CuO₄ was used to probe the copper spin relaxation via the bottleneck effect for oxygen isotope (¹⁶O and ¹⁸O)–substituted samples. It was found that the EPR linewidth is larger for the ¹⁸O isotope samples than for the ¹⁶O samples. For x = 0.03, the linewidth for the ¹⁸O sample is larger than for ¹⁶O sample by a factor of 2. The isotope effect is pronounced at low temperatures and decreases with increasing Sr concentration. This effect is quantitatively explained by the Cu²⁺ S = 1/2 spin relaxation to the lattice via Dzyaloshinski terms coupled linearly to the local Q ₄/Q ₅ tilting modes of the CuO₆ octahedra as proposed by Kochelaev et al. [1] The Q ₄/Q ₅ modes are coupled sterically to the Q ₂ Jahn–Teller modes considered to be relevant for the (bi)-polaron formation and thus for the high-temperature superconductivity (HTSC).     

Dedicated Co-deposition System for Metallic Paramagnetic Films

We describe a dedicated co-sputtering/ion-mill system developed to study metallic paramagnetic films for use in magnetic microcalorimetry. Small-diameter sputtering guns allow study of several precious-metal paramagnetic alloy systems within a reasonable budget. We demonstrated safe operation of a 1″ sputtering gun at greater than five times the rated maximum power, achieving deposition rates up to ～ 900 ?/min/gun (Cu) in our co-sputtering geometry. Demonstrated co-sputtering deposition ratios up to 100:1 allow accurate tuning of magnetic dopant concentration and eliminate the difficulty of preparing homogeneous alloy targets of extreme dilution.     

Electron Paramagnetic Resonance Spectra of Carbonate Centers in Natural Apatites

The EPR spectra of carbonate centers in apatites of different compositions are measured in the range 77–290 K. The results indicate that the F^– : OH^– ratio on the 6₃ axis and the state of constitutional water (H₂O molecules) determine vibronic interactions in the CO₂ ^–, CO₃ ^–, and CO₃ ^3– centers and the parameters of their EPR spectra. Depending on the molecular water and OH^– contents, the spectra of the carbonate centers are isotropic or anisotropic, due to the dynamic or static Jahn-Teller effect. Computer simulation is used to estimate the contributions of the centers to the EPR spectrum.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 585–591.Original Russian Text Copyright © 2005 by Gilinskaya.