Skyrmion lattice in Chiral systems

Abstract:

A skyrmion is a topological spin texture, in which the spins point in all of the directions wrapping a sphere. Skyrmion lattice state has been predicted in theory to appear in Chiral systems and is anticipated to play a vital role in quantum magneto-transport processes such as the quantum Hall and topological Hall effects. In this talk, I review the recent year discoveries of SkX in several Chiral systems such as MnSi, Co doped FeSi and FeGe.

Time: Nov. 27, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.

Nov. 20 Club Meeting: Joseph Prestigiacomo

Topic to be discussed:
Glassy Dynamics of the Superconductor-Insulator Transition of Ultrathin Granular Al Films in a Parallel Magnetic Field

Abstract:
The investigation of the dynamics in glassy systems (magnetic spin glass, electron glass, etc.) have proven to be very useful in understanding the connections between non-equilibrium behavior and their microscopic origins.  In this talk, I review a relatively overlooked system exhibiting slow glassy dynamics, the first-order superconductor-insulator transition of ultrathin granular aluminum films in a parallel magnetic field.  These films may constitute a new class of glassy system, a Josephson glass.

Time: Nov. 20, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.

Nov. 13 Club Meeting: Dr. Ziyang Meng

Topic To be discussed:
Using entanglement entropy to study quantum phase transitions

Abstract:
In recent years, the concept of entanglement entropy has been introduced into the study of condensed matter system. Entanglement entropy can be used to characterize classical and quantum phase transitions, as well as novel quantum states which are not belong to the Landau-Ginzburg symmetry-breaking paradigm. In this talk, the concept of entanglement, entanglement entropy, and area law will be introduced in a pedagogical manner. Furthermore, the numeric implement of the measurement of entanglement entropy will be discussed within the context of quantum Monte Carlo simulations. An concrete example in quantum spin system, where the entanglement entropy has been applied to characterize the quantum phase transition is demonstrated.

References

Roger Melko
http://online.kitp.ucsb.edu/online/fragnets_c12/melko
Tarun Grover
http://online.kitp.ucsb.edu/online/fragnets_c12/grover
Xiao-Gang Wen
http://online.kitp.ucsb.edu/online/fragnets_c12/wen

Time: Nov. 13, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.

Characterizing Localized States

Abstract:

The Anderson model of localization continues to attract much attention, and the role of disorder in the metal insulator transition is still a matter of debate.  In this talk, I will review the result of Anderson for localization in disordered models, and then discuss the role of "rare" fluctuation events.  I will then talk about the parameters that have been proposed to characterize these localized states and the numerical results that have been obtained.

Time: Nov. 6, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.

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.

Oct. 2 Club Meeting: Neel Haldolaarachchige

Interesting physical properties hidden inside an uninteresting correlated electron system.

FeGa3 is a non-magnetic semiconducting compound. For a long period it has been considered only for high temperature thermoelectric properties (TE). However, Yin and Picket (1) last year theoretically suggested that possible magnetic ground state hidden inside this compound. Furthermore, they have closely investigated structure of this compound and found that Fe-Fe dimmers in this compound are in non-magnetic spin singlet state. Immediately following Bitter et al. (2) found that small amount Co doping changes the system into a heavy Fermion like with low temperature paramagnetic (PM) behavior and they suggest PM-state is possibly due to Co-Co dimer formation at very low temperature. Followed by that Storchak at al. (3) suggested spin-polaron state formation below 10K and further suggested spin-polaron as a mechanism for possible magnetic state in this compound. We have seen electronic structure of this compound is very sensitive to pressure when we have investigated this compound for TE properties (4). Further investigation gave us  unbelievable results.

References:

(1) Evidence for a spin singlet state in the intermetallic semiconductor FeGa3, PRB, 82, 155202 (2010)

(2) Correlation effects in the small gap semiconductor FeGa3, Journal of Physics: Conference Series, 200, 012014 (2010)

(3) Thermal destruction of spin-polaron bands in the narrow-gap correlated semiconductors FeGa3 and FeSb2, J.Phys.Cond.Matter, 24, 18 (2012)

(4) Effect of chemical doping on the thermoelectric properties of FeGa(3), J.Applied Physics, 109, 103712 (2011)

Time: Oct 2, Tuesday, noon.

Location: Rm. 435, Nicholson Hall.

Foods and drinks will be provided.

Sept. 25 Club Meeting: Dr. Ziyang Meng

Paper to be discussed:

Antiferromagnetism in the Hubbard Model on the Bernal-Stacked Honeycomb Bilayer

Thomas C. Lang, Zi Yang Meng, Michael M. Scherer, Stefan Uebelacker, Fakher F. Assaad, Alejandro Muramatsu, Carsten Honerkamp, and Stefan Wessel.
Phys. Rev. Lett. 109, 126402 (2012)

Abstract:

Using a combination of quantum Monte Carlo simulations, functional renormalization group calculations and mean-field theory, we study the Hubbard model on the Bernal-stacked honeycomb bilayer at half-filling as a model system for bilayer graphene. The free bands consisting of two Fermi points with quadratic dispersions lead to a finite density of states at the Fermi level, which triggers an antiferromagnetic instability that spontaneously breaks sublattice and spin rotational symmetry once local Coulomb repulsions are introduced. Our results reveal an inhomogeneous participation of the spin moments in the ordered ground state, with enhanced moments at the threefold coordinated sites. Furthermore, we find the antiferromagnetic ground state to be robust with respect to enhanced interlayer couplings and extended Coulomb interactions.

Time: Sep 25, Tuesday, noon.

Location: Rm. 435, Nicholson Hall

Foods and drinks will be provided.

Sep 18 Club Meeting: Dr. Ka Ming Tam

Abstract:

Unlike the commonplace electrically charged particles, magnetic monopoles have not been found despite a long time search for them. However, magnetic monopoles among quasi-particles are not unheard of for physicists. In this talk, Ka Ming will introduce the theoretical grounds and the experimental detection of magnetic monopoles in a class of magnets called spin ice.

Time: Sep 18, Tuesday, noon.
Location: Rm. 435, Nicholson Hall
Food and drink will be provided.

Paper to be discussed:

Magnetic Monopoles in Spin Ice
Claudio Castelnovo, Roderich Moessner, Shivaji L. Sondhi
Nature 451, 42-45 (2008)
Download link through arXiv

Welcome to our first meeting! In case anyone doesn't know Room 435, the stairs and the elevator are located at the east half of Nicholson Hall, accessible conveniently from the gate facing the Student Union. Note that we are only permitted to occupy the half of Rm 435 that has tables.

Our Predecessor: LSU Physics Journal Club

The LA-SIGMA Science Journal Club is an expansion based on last year's physics journal club for graduate students and junior researchers from Department of Physics and Astronomy. The previous physics journal club mainly focused on condensed matter and atomic/molecular/optical physics, while the current LA-SIGMA journal club includes all LA-SIGMA related disciplines. The organization is similar in most parts though. The archives of the physics journal club in 2011 can be accessed at the following address:

http://lsuphysjournalclub.blogspot.com/