S. Sergeenkov

Dynamical reentrance and geometry imposed quantization effects in Nb–AlOx–Nb Josephson junction arrays

In this paper, we report on different phenomena related to the magnetic properties of artificially prepared highly ordered (periodic) two-dimensional Josephson junction arrays (2D-JJAs) of both shunted and unshunted Nb–AlOx–Nb tunnel junctions. By employing mutual-inductance measurements and using a high-sensitivity home-made bridge, we have thoroughly investigated (both experimentally and theoretically) the temperature and magnetic field dependence of complex AC susceptibility of 2D-JJAs. After a brief description of the measurement technique and the numerical simulations method, we proceed to demonstrate that the observed dynamic reentrance (DR) phenomenon is directly linked to the value of the Stewart–McCumber parameter βC. By simultaneously varying the inductance related parameter βL, we obtain a phase diagram βC–βL (which demarcates the border between the reentrant and non-reentrant behavior) and show that only arrays with sufficiently large value of βC will exhibit DR behavior. The second topic of this paper is related to the step-like structure (with the number of steps n = 4 corresponding to the number of flux quanta that can be screened by the maximum critical current of the junctions) which has been observed in the temperature dependence of AC susceptibility in our unshunted 2D-JJA with βL (4.2xa0K) =xa030 and attributed to the geometric properties of the array. The steps are predicted to manifest themselves in arrays with βL(T) matching a quantization condition βL(0) = 2π(n+1). In conclusion, we demonstrate the use of the scanning SQUID microscope for imaging the local flux distribution within our unshunted arrays.

Influence of zeeman splitting and thermally excited polaron states on magnetoelectrical and magnetothermal properties of magnetoresistive polycrystalline manganite La0.8Sr0.2MnO3

Some possible connection between spin and charge degrees of freedom in magnetoresistive manganites is investigated through a thorough experimental study of the magnetic [alternating current susceptibility and direct current (dc) magnetization] and transport (resistivity and thermal conductivity) properties. Measurements are reported in the case of well characterized polycrystalline La0.8Sr0.2MnO3 samples. The experimental results suggest rather strong field-induced polarization effects in our material, clearly indicating the presence of ordered ferromagnetic regions inside the semiconducting phase. Using an analytical expression which fits the spontaneous dc magnetization, the temperature and magnetic field dependences of both electrical resistivity and thermal conductivity data are found to be well reproduced through a universal scenario based on two mechanisms: (i) a magnetization dependent spin polaron hopping influenced by a Zeeman splitting effect and (ii) properly defined thermally excited polaron s...

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1-xPrxBa2Cu3O7-δ

Abstract We present our recent results on the temperature dependence of current–voltage characteristics for polycrystalline Y1−xPrxBa2Cu3O7−δ superconductors with x = 0.0 , 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V = R ( I − I c ) a ( T ) . Here, we assume that a ( T ) = 1 + Φ 0 I C ( T ) / 2 π k B T and I C ( T ) = I C ( 0 ) ( 1 − T / T C ) 3 / 2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current–voltage characteristics curves from Ohmic behavior (with a ( T C ) = 1 ) below T C is attributed to the manifestation of dissipation processes. They have a characteristic temperature T p defined via the power exponent as a ( T p ) = 2 and are related to the current induced depinning of Abrikosov vortices. Both T C ( x ) and T p ( x ) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.

Scaling like behaviour of resistivity observed in LaNiO3 thin films grown on SrTiO3 substrate by pulsed laser deposition

We discuss the origin of the temperature dependence of resistivity ρ observed in highly oriented LaNiO3 thin films grown on SrTiO3 substrate by a pulsed laser deposition technique. All the experimental data are found to collapse into a single universal curve ρ(T, d) ∝ [T/T(sf)(d)]3/2 for the entire temperature interval (20K<T<300 K) with T(sf)(d) being the onset temperature for triggering a resonant scattering of conduction electrons by spin fluctuations in LaNiO3/SrTiO3 heterostructure.

Evidence for resonant scattering of electrons by spin fluctuations in LaNiO3/LaAlO3 heterostructures grown by pulsed laser deposition

We present measurements of resistivity ρ in highly oriented LaNiO3 films grown on LaAlO3 substrates by using a pulsed laser deposition technique. The experimental data are found to follow a universal ρ(T) ∝ T3/2 dependence for the entire temperature interval (20 K T 300 K). The observed behavior has been attributed to a resonant scattering of electrons on antiferromagnetic fluctuations (with a characteristic energy hωsf ≃ 2.1 meV) triggered by spin-density wave propagating through the interface boundary of LaNiO3/LaAlO3 sandwich.

Analog of the susceptibility spectrum for levitation forces between a superconductor and a permanent magnet

We report on the measurements of both vertical and lateral levitation forces between a permanent magnet NdFeB and a polycrystalline YBa4Cu6O7−δ superconductor. The analysis of the obtained results revealed an interesting correlation between the behavior of the forces in the field-cooled and zero-field-cooled regimes, resembling the structure of the so-called susceptibility spectrum χ″(χ′). Such force-force diagrams can be useful for identifying flux distribution structure inside a superconducting material.

Novel magnetoinductance effects in Josephson junction arrays : A single-plaquette approximation

Using a single-plaquette approximation, novel magnetoinductance effects in Josephson junction arrays (JJAs) are predicted, including the appearance of steps in the temperature behavior of magnetic susceptibility. The number of steps (as well as their size) is controlled by the kinetic inductance of the plaquette whose field dependence is governed by the Abrikosov vortices penetrating superconducting regions of the array. The experimental conditions under which the predicted effects should manifest themselves in artificially prepared JJAs are discussed.

Origin of a nonlinear contribution to the shift of the critical temperature in atomic Bose-Einstein condensates

We discuss a possible origin of the experimentally observed nonlinear contribution to the shift ΔTc = Tc − Tc0 of the critical temperature Tc in an atomic Bose-Einstein condensate (BEC) with respect to the critical temperature Tc0 of an ideal gas. We found that accounting for a nonlinear (quadratic) Zeeman effect (with applied magnetic field closely matching a Feshbach resonance field B0) in the mean-field approximation results in a rather significant renormalization of the field-free nonlinear contribution b2, namely, Tc/Tc0 ≃ b2* (a/λT)2 (where a is the s-wave scattering length, λT is the thermal wavelength at Tc0) with b2* = γ2b2 and γ = γ(B0). In particular, we predict b2* ≃ 42.3 for the B0 ≃ 403 G resonance observed in the 39K BEC.

On the electronic viscosity of a Dirac fluid in deformed graphene

We discuss the properties of the electronic viscosity of a Dirac fluid in deformed graphene by introducing a strain and a velocity gradient , as equivalent to a pseudo-magnetic and a pseudo-electric field respectively into the Dirac equation. It is thereby analytically established that the dynamic shear viscosity coefficient substantially decreases with the applied strain as , reaching as much as for .

Analogue of the quantum Hall effect for neutral particles with magnetic dipole moment

Abstract In this paper we investigate a possibility for the existence of an analog of the Quantum Hall Effect for neutral particles with a permanent magnetic moment μ in the presence of crossed inhomogeneous magnetic and electric fields. We predict the appearance of Hall conductivity σ H = ( e 2 / h ) ν ( μ ) with the Landau filling factor ν ( μ ) ∝ μ 2 . The estimates of the model parameters suggest quite an optimistic possibility to experimentally verify this prediction in optically trapped clouds of atomic BEC.