https://escholarship.org/content/qt4c38d4sf/qt4c38d4sf.pdf?t=pd3h9r

Embracing equifinality with efficiency : Limits of Acceptability sampling using the DREAM(LOA) algorithm

The theoretical description and the survey of physical properties of superconducting states in the uranium ferromagnetic materials are presented. On the basis of microscopic theory is shown that the coupling between the electrons in these ferromagnetic metals by means of magnetization fluctuations gives rise the triplet pairing superconducting state and the general form of the order parameter dictated by the symmetry is established. 
The theory allows to explain some specific observations including peculiar phenomenon of reentrant superconductivity in URhGe in magnetic field perpendicular to the direction of spontaneous magnetization. 
In addition we describe several particular topics relating to uranium superconducting ferromagnets: (i) critical magnetic relaxation in dual localized-itinerant ferromagnets, (ii) phase transition to ferromagnetic state in Fermi liquid and UGe$_2$, (iii) superconducting ordering in ferromagnetic metals without inversion symmetry.

Superconductivity in Uranium Ferromagnets

Low Temperature Physics

We present a study of magnetic structures with controllable effective exchange energy for Josephson switches and memory applications. As a basis for a weak link we propose to use a periodic structure composed of ferromagnetic (F) layers spaced by thin superconductors (s). Our calculations based on the Usadel equations show that switching from parallel (P) to antiparallel (AP) alignment of neighboring F layers can lead to a significant enhancement of the critical current through the junction. To control the magnetic alignment we propose to use a periodic system whose unit cell is a pseudo spin valve of structure F1/s/F2/s where F1 and F2 are two magnetic layers having different coercive fields. In order to check the feasibility of controllable switching between AP and P states through the whole periodic structure, we prepared a superlattice [Co(1.5 nm)/Nb(8 nm)/Co(2.5 nm)/Nb(8 nm)]6 between two superconducting layers of Nb(25 nm). Neutron scattering and magnetometry data showed that parallel and antiparallel alignment can be controlled with a magnetic field of only several tens of Oersted.

Periodic Co/Nb pseudo spin valve for cryogenic memory.

Carefully controlled interfaces between two materials can give rise to novel physical phenomena and functionalities not exhibited by either of the constituent materials alone. Modern synthesis methods have yielded high-quality heterostructures of oxide materials with competing order parameters. Although magnetic correlations at the interface are expected to be important in determining the macroscopic properties of such systems, a quantitative determination of the interfacial magnetization profile has thus far not been reported. Here we examine superlattices composed of the half-metallic ferromagnet La2/3Ca1/3MnO3 and the high-temperature superconductor YBa2Cu3O7 by absorption spectroscopy with circularly polarized X-rays and by off-specular neutron reflectometry. The resulting data yield microscopic insight into the interplay of spin and orbital degrees of freedom at the interface. The experiments also reveal an extensive rearrangement of the magnetic domain structure at the superconducting transition temperature. This methodology establishes an incisive probe of the interplay between competing electronic order parameters in oxide heterostructures.

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Magnetism at the interface between ferromagnetic and superconducting oxides.

We report on a study of the structural, magnetic, and superconducting properties of $\mathrm{Nb}(25\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Gd}({d}_{f})/\mathrm{Nb}(25\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$ hybrid structures of a superconductor/ ferromagnet (S/F) type. The structural characterization of the samples, including careful determination of the layer thickness, was performed using neutron and x-ray scattering with the aid of depth-sensitive mass spectrometry. The magnetization of the samples was determined by superconducting quantum interference device magnetometry and polarized neutron reflectometry, and the presence of magnetic ordering for all samples down to the thinnest Gd(0.8 nm) layer was shown. The analysis of the neutron spin asymmetry allowed us to prove the absence of magnetically dead layers in junctions with Gd interlayer thickness larger than one monolayer. The measured dependence of the superconducting transition temperature ${T}_{c}({d}_{f})$ has a damped oscillatory behavior with well-defined positions of the minimum at ${d}_{f}=3$ nm and the following maximum at ${d}_{f}=4$ nm, in qualitative agreement with prior work [J. S. Jiang et al., Phys. Rev. B 54, 6119 (1996)]. We use a theoretical approach based on the Usadel equations to analyze the experimental ${T}_{c}({d}_{f})$ dependence. The analysis shows that the observed minimum at ${d}_{f}=3$ nm can be described by the so-called zero to $\ensuremath{\pi}$ phase transitions of highly transparent S/F interfaces with a superconducting correlation length ${\ensuremath{\xi}}_{f}\ensuremath{\approx}4$ nm in Gd. This penetration length is several times higher than for strong ferromagnets like Fe, Co, and Ni, thus simplifying the preparation of S/F structures with ${d}_{f}\ensuremath{\sim}{\ensuremath{\xi}}_{f}$ which are of topical interest in superconducting spintronics.

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Magnetic and superconducting phase diagram of Nb/Gd/Nb trilayers

We have used spin-polarized neutron reflectometry to investigate the magnetization profile of superlattices composed of ferromagnetic Gd and superconducting Nb layers. We have observed a partial suppression of ferromagnetic (F) order of Gd layers in [$\mathrm{Gd}({d}_{F})/\mathrm{Nb}(25 \mathrm{nm}){\mathrm{]}}_{12}$ superlattices below the superconducting (S) transition of the Nb layers. The amplitude of the suppression decreases with increasing ${d}_{F}$. By analyzing the neutron spin asymmetry we conclude that the observed effect has an electromagnetic origin---the proximity-coupled S layers screen out the external magnetic field and thus suppress the F response of the Gd layers inside the structure. Our investigation demonstrates the considerable influence of electromagnetic effects on the magnetic properties of S/F systems.

Magnetic proximity effect in Nb/Gd superlattices seen by neutron reflectometry

Herein, we theoretically suggest one-dimensional photonic crystal composed of polymer doped with quantum dots and porous silicon. The present simulated design is proposed as a refractive index biosensor structure based on parity-time symmetry. Under the parity-time conditions, the transmittance of the resonant peaks is magnified to be 57,843% for refractive index 1.350, 2725% for 1.390, 2117% for 1.392, 1502% for 1.395, 1011% for 1.399, and 847% for 1.401. By magnification, we can distinguish between different refractive indices. The present design can record an efficiency twice the published designs as clear in the comparison table. Results clear that the sensitivities are 635 nm/RIU and 1,000,000%/RIU. So, it can be used for a broader range of detection purposes. 

/pdf/refractive-index-sensor-with-magnified-resonant-signal-1rpxmrb3.pdf

Refractive index sensor with magnified resonant signal

/pdf/refractive-index-sensor-with-magnified-resonant-signal-3vs9wc3k.pdf

Examples of the application of a cryogenic moderator in the REMUR time-of-flight neutron reflectometer of the IBR-2 pulsed reactor (Dubna) are given. The results of two experiments are presented: spatial beam splitting upon reflection from a magnetically noncollinear film and the recording of a microbeam formed by a layered waveguide. A conclusion concerning the efficiency of the cryogenic moderator for experiments of this type is made.

V. D. Zhaketov

Papers

Magnetic and superconducting phase diagram of Nb/Gd/Nb trilayers

Magnetic proximity effect in Nb/Gd superlattices seen by neutron reflectometry

Refractive index sensor with magnified resonant signal

Refractive index sensor with magnified resonant signal

Application of a cryogenic moderator in the REMUR neutron reflectometer