## Physics theory seminar

### - Spring 2014

Division of Theoretical Physics at Department of Physics organizes a series of theory seminars. Physics theory seminars are open.

- Time: Monday 14:15
- Location: E5-103, Realfagbygget NTNU. Natural Science Building

Coordinator: Jacob Linder

### January 20

Stefan Rex, Department of Physics, NTNU

«Majorana fermions at the boundary of topological superconductors»

## January 20

Stefan Rex, Department of Physics, NTNU

«Majorana fermions at the boundary of topological superconductors»

*Abstract*

### Majorana fermions at the boundary of topological superconductors

Majorana fermions at the boundary of topological superconductors In the first part of this talk, I want to give a brief introduction on what the term "topological" means in the context of superconductors, and explain that non-trivial topology causes zero-energy states at the boundary of the system. These states will turn out to be Majorana fermions.

The second part is related to surface states of nodal superconductors. In particular, additional electronic interaction is added to the model Hamiltonian. I will present numerical data indicating that interaction can shift the surface states apart from zero energy.

### February 3

Iver Brevik, Department of Energy and Process Engineering, NTNU

«Viscosity-induced crossing of the phantom divide in the dark cosmic fluid»

## February 3

Iver Brevik, Department of Energy and Process Engineering, NTNU

«Viscosity-induced crossing of the phantom divide in the dark cosmic fluid»

*Abstract*

### Viscosity-induced crossing of the phantom divide in the dark cosmic fluid

A short overview will be given of the application of viscous fluid mechanics to the cosmic fluid. Emphasis will be given on the case when bulk viscosity may play a role for the passage of the dark fluid through the 'phantom divide' w=-1 (w being the equation-of-state parameter), thereby implying a future singularity of the universe, called a Big Rip.

### March 17

Peder Galteland, Department of Physics, NTNU

«Two-component Bose-Einstein condensates»

## March 17

Peder Galteland, Department of Physics, NTNU

«Two-component Bose-Einstein condensates»

### Two-component Bose-Einstein condensates

The topological excitations inherent in Bose-Einstein condensates are important elements of both superconductors and superfluids, and are even relevant in cosmology and high energy physics.

Condensates with multiple components and intercomponent couplings open up new possibilities for novel vortex physics, and have been studied numerically and realized experimentally.

I have studied a uniformly frustrated 2-component Ginzburg-Landau theory with amplitude fluctuations and density-density interactions included, through the use of Metropolis Monte Carlo techniques. I have explored the ground states as a function of rotational frequency, and inter- and intra-component coupling strength. It was found that the model exhibits both hexagonal lattices of co-centered vortices, and square lattices of interpenetrating vortices. These lattices exhibit a first order melting transition. The special case of an SU(2) symmetric potential was also explored. With this additional symmetry, dimer vortex configurations, strong staggering of the amplitude fields and massive degeneracy of the ground states appear.

In this seminar I will explain the model used, and the physical realizations it has, as well as the results obtained. It will also spend some time on explaining the numerical Monte Carlo techniques used in this work.

Thu, 13 Mar 2014 10:04:24 +0100

### May 5

Mathias Boström, Kungliga Tekniska Högskolan (KTH), Stockholm, Sverige

«Dispersion forces in a non-expanded theory»

## May 5

Mathias Boström, Kungliga Tekniska Högskolan (KTH), Stockholm, Sverige

«Dispersion forces in a non-expanded theory»

*Abstract*

### Dispersion forces in a non-expanded theory

The present presentation is based on the studies my PhD student Ms Priyadarshini Thiyam and I have done during this spring on dispersion forces in a vacuum and in a media. It has been done in close collaboration with other researchers at KTH, UiO, NTNU, and ANU. We go beyond the approximate large separation series expansion of dispersion theory by applying the full non-expanded theory. We furthermore introduce finite atomic size corrections. This dramatically alters the van der Waals energy and first order dispersion interaction when two atoms come close together. Similar large effects are observed for the self-energy of atoms. As an extension a theory is also developed for dispersion potentials between atoms and ions dissolved in a medium. Here the dispersion interaction between two atoms in ground or excited state must account for the fact that the two atoms are coupled via the electromagnetic field and must include effects from background media, retardation and finite size.

Thu, 15 May 2014 13:49:26 +0200

### May 19

Michael Kachelriess, Department of Physics, NTNU

«Gravitational Waves in the Microwave»

## May 19

Michael Kachelriess, Department of Physics, NTNU

«Gravitational Waves in the Microwave»

*Abstract*

### Gravitational Waves in the Microwave

The BICEPS-2 collaboration has recently announced evidence for the detection of gravitational waves analyzing the cosmic microwave background. If confirmed, this result would provide strong support that the early universe went through an inflationary phase, that our naive treatment of this process is not too wrong, and that gravity is quantised as other fields.

I will try to present the physics underlying this result on a level accessible for non-experts.

Thu, 15 May 2014 13:49:06 +0200

### May 26

Peng Yan, Postdoc, TU Delft.

«Dynamics of Ferromagnetic Bilayers»

## May 26

Peng Yan, Postdoc, TU Delft.

«Dynamics of Ferromagnetic Bilayers»

*Abstract*

### Dynamics of Ferromagnetic Bilayers

Motivated by recent experiments [1] we study the dynamics of exchange-coupled ferromagnetic bilayers consisting of a ferromagnetic insulator (FI) and a ferromagnetic metal (FM). A microscopic spin-wave theory is formulated, implementing an interface-rescaling approach. The propagating spin waves are found to lead to both electrical and spin currents in the bilayers. Spin scattering at the interface is formulated by a time-dependent potential restricted in the half-plane by an infinite wall. The corresponding spin-pumping near the interface involves spin-flip scattering as well as transitions into other energy bands.

- P. Hyde, Lihui Bai, D.M.J. Kumar, B.W. Southern, C.-M. Hu, S.Y. Huang, B.F. Miao, and C.L. Chien, Phys. Rev. B 89, 180404(R) (2014).

Mon, 26 May 2014 12:02:01 +0200

### June 2

Asle Sudbø, Department of Physics, NTNU

«Chiral metallic states in iron-pnictide superconductors»

## June 2

Asle Sudbø, Department of Physics, NTNU

«Chiral metallic states in iron-pnictide superconductors»

*Abstract*

### Chiral metallic states in iron-pnictide superconductors

### June 16

Cecilia Holmqvist, Department of Physics, NTNU

«Non-equilibrium effects in nanoscale superconducting hybrid junctions»

## June 16

Cecilia Holmqvist, Department of Physics, NTNU

«Non-equilibrium effects in nanoscale superconducting hybrid junctions»

*Abstract*

### Non-equilibrium effects in nanoscale superconducting hybrid junctions

Recently, superconducting-ferromagnetic hybrid devices have received increased attention due to their potential as spintronics devices. Information can typically be stored in the magnetization direction of a small ferromagnet whose state can be read out or manipulated by currents through a nanoscale contact coupled to the ferromagnet. In the case of superconducting junctions, the interplay between ferromagnetism and superconductivity taking place at the interface regions is crucial in the understanding of such nanoscale devices.

Here, we theoretically investigate the charge and spin transport of a superconducting point contact coupled to a nanomagnet both in the phase-bias and the voltage-bias regimes. If an external magnetic field is applied, the spin of the nanomagnet starts to precess with the Larmor frequency. This magnetization dynamics constitutes a time-dependent spin-active boundary condition which is solved using non-equilibrium Green's functions in the quasiclassical approximation. The coupling between tunneling quasiparticles and the precessing magnetization leads to the appearance of superconducting spin-triplet correlations as well as ac spin Josephson currents, spin-transfer torques and other non-equilibrium effects.

### References

- S. Teber, C. Holmqvist, and M. Fogelström, Phys. Rev. B 81, 174503 (2010)
- C. Holmqvist, S. Teber, and M. Fogelström, Phys. Rev. B 83, 104521 (2011)
- C. Holmqvist, W. Belzig, and M. Fogelström, Phys. Rev. B 86, 054519 (2012)
- C. Holmqvist, M. Fogelström, and W. Belzig, arXiv:1302.7107 (2013)

Thu, 12 Jun 2014 09:54:07 +0200

Mon, 26 May 2014 11:58:37 +0200

## Contact information

Coordinator Physics Theory Seminar

Jacob Linder

E-mail: jacob.linder@ntnu.no

## Physics Theory Seminar

## Open lectures & seminars

# Physics Friday Colloquia

Friday 14:15

# Physics theory seminar

Monday 14:15

# Biophysics seminar series

Tuesday 14:15