Keshav N. Shrivastava

Lattice dynamic effects in electron paramagnetic resonance

Abstract The electron-phonon interaction for a paramagnetic impurity in an insulating crystal is derived and from that an electron-electron interaction in a higher order is considered. An attempt is made to provide a unified presentation of the effect of phonons on the parameters measured in an electron-paramagnetic resonance experiment. The phonon-induced corrections to the zero-field splitting, the hyperfine field, the superhyperfine splitting and line intensities is reviewed theoretically along with experimental data. Both the point charge and covalent models are discussed. The effect of local and resonance modes is calculated and discussed in terms of isotope effect. Some related problems of the energy levels, dispersion, screening and phonon force are also included in appendices.

New low-temperature vortex state and flux quantum oscillations at low temperatures in the paramagnetic resonance of high temperature superconductor YBa2Cu3O7−δ

Abstract We report a microwave signal arising upon excitation of YBa2Cu3O7−δ by microwaves of frequency 9.2 GHz which vanishes at a superconducting transition temperature of Tc = 92.5 K. Near the Tc the strength of the signal varies as (Tc − T)2.7. The exponent of the intensity of the microwave signal indicates that different parts of the sample have different transition temperatures and hence it is concluded that there are clusters in the sample. Using the expression for flux quantization, we find the domain length to be ⋍ 0.35 × 10 −4 cm at a temperature of 20 K which varies with temperature. Below a T′ c ⋍ 10 K a noise like signal appears which indicates a transition in the flux state. In a thin chip, well resolved lines at equal magnetic field spacings B0 = nφ/S appear, where n = 0, 1, 2, 3, …, is an integer, φ is the unit flux hc 2e and S is a typical surface area depending on the London penetration depth.

Magnetic resonance in high-temperature superconductors

Abstract Since the discovery of high-temperature superconductors a new microwave absorption has been found which is reviewed here in detail. The microwaves are absorbed in the flux-quantized eigenstates so that the absorption is proportional to the Josephson current and hence it varies as the gap of the superconductor. This absorption is found in the electron paramagnetic resonance, in the parallel pumping and in the ordinary microwave absorption experiments. The flux-quantized fields are found in small domains of the size of 10−4 cm. A giant moment is found to occur. The necessary theory as well as experiments in YBa2Cu3O7−δ type of compounds are described. The Cu2+ electron paramagnetic resonance gives an anisotropic exchange narrowed line with anisotropic g-values with g-shifts proportional to the susceptibility. The symmetry of the g-value also reflects the symmetry of the superconducting gap. The nuclear magnetic resonance of various nuclei has been reviewed. It is pointed out that the NMR line widths are determined by the vortex-lattice spacing and the London penetration depth. Nuclear relaxation rates measured in the superconducting state give the gap energy of the superconductor. The ratio of the gap energy to the transition temperature found from the 63Cu nuclear relaxation rate in La1.83Sr0.17CuO4 is in reasonable agreement with the BCS theory confirming the pair structure of superconductors. The phase diagrams of La2−δBaδCuO4 and YBa2Cu3O7−δ have been described.

Electron Paramagnetic Resonance Determination of Debye Temperature

Electron paramagnetic resonance of ZnSiF6·6H2O:Ni2+, ZnSiF6·6D2O:Ni2+, ZnSiF6·6H2O: Mn2+ and ZnSiF6·6D2O:Mn2+ lattices has been studied. From the temperature dependence of the zero-field splitting, the Debye temperature is found as a function of temperature in all the four systems. We find that the temperature dependence of the g-values arises from the dynamical interaction between the paramagnetic electrons and the phonons. The Debye temperature found from the zero-field splitting agrees with that found from the g-value. We have thus studied the interaction of paramagnetic electrons with lattice vibrations by a new method.

Evaluation of Debye temperatures as a new application of electron-spin resonances

Abstract The phenomenon of splitting of states by the electron-phonon interaction studied in an electron-spin resonance experiment is used to determine the Debye temperature of cubic ZnS.

Rational numbers of the fractionally quantized new states and their observation in fractionally quantized hall effect

Abstract We have found energy states at a fraction of the frequency of the Landau levels. The fraction is in accord with the experimental measurements of fractionally quantized conductance.

Symmetries in the triplet superconducting states

Abstract Depending upon the point-group symmetry of the system several phases occur within the superconducting state. The gap parameter for four different point group symmetries is calculated.

Magnetic superconductors: Model theories and experimental properties of rare-earth compounds

Superconducting and magnetically long-range ordered states were believed to be mutually exclusive phenomena. The discovery of rare-earth compounds in recent years, which exhibit both superconductivity and magnetic ordering (ferromagnetic, antiferromagnetic or sinusoidal), has led to considerable theoretical and experimental work on such systems. In the present article, we give a review of various theoretical models and important experimental results. In the theoretical sections, we start with the Abrikosov-Gorkov pair breaking theory for dilute alloys and discuss its improvement in the work of Muller-Hartmann and Zittartz. Then, in the context of magnetic superconductors, various microscopic theories that have been advanced are presented. These predict re-entrant behaviour in some systems (ferromagnetic superconductors) and coexistence regions in others (particularly antiferromagnetic superconductors). Following this, phenomenological generalized Ginzburg-Landau theories for two kinds of orders (superconducting and magnetic) are presented. A section dealing with renormalization group analysis of phase diagrams in magnetic superconductors is given. In experimental sections, the properties of each rare-earth compounds (ternary as well as some tetranery) are reviewed. These involve susceptibility, heat capacity, resistivity, upper critical field, neutron scattering and magnetic resonance measurements. The anomalous behaviour of the upper critical field of antiferromagnetic superconductors near the Neel temperature is discussed both in theory sections and experimental section for various systems.

Electron Spin Resonance Study of γ‐Irradiated Single Crystals of Semicarbazide Hydrochloride

Electron spin resonance studies of γ‐irradiated single crystals of semicarbazide hydrochloride and the deuterated form, ND2CONHND3+·Cl−, revealed the presence of the stably trapped ·N(α)H(α) − N(β)H(β)3+ radical obtained by C–N bond rupture of the parent molecule. The principal g values were gaa = 2.0041 ± 0.0003, gbb = 2.0023, and gcc = 2.0044. The principal elements of the hyperfine couplings were found to be N(α) = 4.8 ± 4.0, 34.2 ± 1.0, and 11.8 ± 4.0 G; H(α) = − 10.5 ± 4.0, −27.6 ± 1.0, and −36.7 ± 4.0 G; N(β) = − 4 G; H(β) = 15 ± 0.5 G. The N(β) coupling was estimated from the angular dependence of the linewidths. The direction cosines of the N(α)and H(α) couplings were evaluated with reference to the orthorhombic crystallographic axes (a, b, c). The direction cosines of the N–H bond calculated from the ESR spectra were found to agree with those obtained by x‐ray diffraction. The spin densities calculated for this radical indicate that the unpaired electron is highly localized on the α nitrogen, the...

Flux quantized energy oscillations upon microwave excitation of high temperature superconductors

Abstract We find that the imaginary part of the complex susceptibility as a function of magnetic field at a frequency of 9.2 GHz shows equally spaced lines indicating harmonic oscillator type of excitations. The maximum in d ξ″ d H is also found to be proportional to the life time of the electron in the normal state. The predictions are verified by using the microwave absorption in ErBa2Cu3O7−δ and YBa2Cu3O7−δ (δ ≃ 0.2).