Electrostatic Tuning of the Proximity-Induced Exchange Field in EuS/Al Bilayers

Abstract:

The development of a magnetic analog of the ubiquitous field-effect transistor (FET) has been a long term goal of the materials research community. Indeed, the electrical manipulation of magnetism is central to the future development spintronic applications. In contrast to semiconducting FET’s, which use gate-controlled electric fields to modulate a device’s charge carrier concentration, a magnetic FET would use a gate to modulate the magnetism of a thin magnetic film. Recently, gating strategies have been employed to modify the magnetic properties in complex multiferroics and ferromagnetic semiconductors. In these studies a magnetoelectric (ME) effect typically arises from the strain induced by the electric field and/or from the modulation of the carrier density itself. In this Letter we focus on ME effects that arise at the interface between a ferromagnetic insulator (FI) and an elemental paramagnet (PM). In a bilayer configuration in which a PM film is in intimate contact with the FI film, an exchange interaction between local magnetic moments in the FI and PM conduction electrons gives rise to a large effective internal field in the PM layer. This effective field, which only manifests itself through a Zeeman splitting of the PM conduction electrons, is commonly known as the exchange field Hex . Here we show that the exchange field in the Al component of EuS/Al bilayers can be modulated with relatively small perpendicular electric fields. Using a MOSFET-type geometry we demonstrate that an exchange field of the order of several tesla can be modulated by tens of percent with gate voltages ∼ ±10 V. We exploit this effect to electrostatically tune the superconducting transition temperature of the Al layer.

Time: Oct 30, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.

Oct. 23 Club Meeting: Dr. Amar Karki

Paper to be discussed:

High Tc Fe-As based superconductors with skutterudite intermediary layers

1. Ni Ni, 
2. Jared M. Allred, 
3. Benny C. Chan, and 
4. Robert Joseph Cava

PNAS ∣ November 8, 2011 ∣ vol. 108 ∣ no. 45 ∣ E1019–E1026

Abstract:

It has been argued that the very high transition temperatures of the highest T_ccuprate superconductors are facilitated by enhanced CuO₂ plane coupling through heavy metal oxide intermediary layers. Whether enhanced coupling through intermediary layers can also influence T_c in the new high T_c iron arsenide superconductors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca₁₀(Pt₃As₈)(Fe₂As₂)₅ (the “10-3-8 phase”) and Ca₁₀(Pt₄As₈)(Fe₂As₂)₅ (the “10-4-8 phase”). Based on -Ca-(Pt_nAs₈)-Ca-Fe₂As₂- layer stacking, these are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to T_c of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The often-cited empirical rule in the arsenide superconductor literature relating T_c to As-Fe-As bond angles does not explain the observed differences inT_c of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase arising from the two-channel interlayer interactions and the metallic nature of its intermediary Pt₄As₈ layer. The interlayer coupling is thus revealed as important in enhancing T_c in the iron pnictide superconductors.

Time: Oct. 23 2012, Tuesday noon.

Location: Rm. 435, Nicholson Hall

Foods and drinks will be provided.

Simulating Contact Binaries and Their Light Curves

A Contact binary is an interesting stellar system in which the two stars actually touch, as they revolve around each other. It is assumed that ultimately this system will merge, forming a single star. In September 2008 such a merger was observed in the eruption of a "red nova'' called V1309 Sco. This proposal is aimed towards developing an initial model for contact binaries so that we can study their formation, structure, and merger properties using hydrodynamics codes. This model can also be used to generate a light curve from the simulation data to compare to the observed case of V1309 Sco. A comparison between observations and simulations will help us unravel the still mysterious nova-like phenomena of stellar mergers.

Time: Oct 16, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.

Universality and scaling in gravitational collapse of a massless scalar field

Abstract:

I will talk about the seminal work done by Matthew Choptuik in the critical gravitational collapse of scalar fields. I will give a little introduction to what a black hole is and then describe the remarkable results of Choptuik's. I will also discuss the extension of his results to quantum gravity which is what currently I am interested in.

References: http://prl.aps.org/abstract/PRL/v70/i1/p9_1

Time: Oct 9, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.