# Physics theory seminar

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

Time: Monday 14:15 - 15:00

Location: Realfagbygget E5-103

## Theory Seminars for the Spring 2016

### June 13

Akashdeep Kamra, University of Konstanz, Germany

«Super-Poissonian shot noise of squeezed-magnon mediated spin transport»

# June 13

Akashdeep Kamra, University of Konstanz, Germany

«Super-Poissonian shot noise of squeezed-magnon mediated spin transport»

*Abstract*

### Super-Poissonian shot noise of squeezed-magnon mediated spin transport

The magnetization of a ferromagnet (F) driven out of equilibrium injects pure spin current into an adjacent conductor (N). Such F|N bilayers have become basic building blocks in a wide variety of spin based devices. We evaluate the shot noise of the spin current traversing the F|N interface when F is subjected to a coherent microwave drive. We find that the noise spectrum is frequency independent up to the drive frequency, and increases linearly with frequency thereafter. The low frequency noise indicates super-Poissonian spin transfer, which results from quasi-particles with effective spin (h/2p)* = (h/2p) (1 + d). For typical ferromagnetic thin films, d ~ 1 is related to the dipolar interaction-mediated squeezing of F eigenmodes.

Super-Poissonian Shot Noise of Squeezed-Magnon Mediated Spin Transport (Physical Review Letters)

Magnon-mediated spin current noise in ferromagnet|non-magnetic conductor hybrids

### February 15

Yukio Tanaka, Nagoya University, Japan

«Surface Andreev-Bound States and Unconventional Superconductivity»

# February 15

Yukio Tanaka, Nagoya University, Japan

«Surface Andreev-Bound States and Unconventional Superconductivity»

*Abstract*

## Surface Andreev-Bound States and Unconventional Superconductivity

It is known that surface Andreev bound state (SABS) is an important ingredient to identify unconventional superconductors. Up to now, there have been several types of SABSs stemming from their topological origins. They can be classified into:

- dispersionless flat band type realized in cuprate
- linear dispersion type realized in chiral superconductor like Sr2RuO4
- helical dispersion type realized in non-centrosymmetric superconductor in quasi two-dimensional superconductors

In three-dimensional superconductor, we can consider chiral superconductor where both line and point nodes exist.

Based on the topological nature of nodes, it is shown that the surface flat bands are fragile against (i) the surface misorientation and (ii) the surface Rashba spin-orbit interaction. We demonstrate that these flat-band instabilities vanish or suppress a zero-bias conductance peak in a tunnelling spectroscopy, which behavior is clearly different from high-Tc cuprates and noncentrosymmetric superconductors.

We have predicted new types of SABS in doped topological insulators (TI) and Weyl semimetals which will be discussed in this talk. We have also studied charge conductance in anomalous Rashba metal (ARM) /superconductor junctions for various types of pairing symmetries in a superconductor.

### March 17

«Can Gravitational Waves provide»

# March 17

«Can Gravitational Waves provide»

*Abstract*

## Can Gravitational Waves provide

### April 8

Hans Skarsvåg, Eirik Fjærbu, Stefan Rex, Vetle Risinggård, Jabir Ali Ouassou (PhD candidates at the Department of Physics)

«Highlights of the APS March Meeting»

# April 8

Hans Skarsvåg, Eirik Fjærbu, Stefan Rex, Vetle Risinggård, Jabir Ali Ouassou (PhD candidates at the Department of Physics)

«Highlights of the APS March Meeting»

`Extraordinary seminar on `

**Friday **8. April, 14:15-15:00 in D4-132

## Highlights of the APS March Meeting

The seminar will be presented by the PhD students from the section who attended the meeting: Hans Skarsvåg, Eirik Fjærbu, Stefan Rex, Vetle Risinggård and Jabir Ali Ouassou

### April 11

Jeroen Danon - Onsager Fellowship in Theoretical Condensed Matter Physics at the Department of Physics

«Multi-level interference resonances in strongly-driven three-level systems»

# April 11

Jeroen Danon - Onsager Fellowship in Theoretical Condensed Matter Physics at the Department of Physics

«Multi-level interference resonances in strongly-driven three-level systems»

*Abstract*

## Multi-level interference resonances in strongly-driven three-level systems

I will talk about multi-photon resonances in a strongly-driven three-level quantum system, where one level is periodically swept through a pair of levels with a constant energy separation. Near the n-photon resonance condition (where the splitting of the pair matches n times the driving frequency) we find qualitatively different behavior for n being even or odd. We can explain this phenomenon in terms of families of interfering trajectories of the multi-level system. Remarkably, the behavior is insensitive to fluctuations of the energy of the driven level, and also survives deep into the strong dephasing regime. The setup can be relevant for a variety of solid state and atomic or molecular systems. In particular, I will show that it provides a clear mechanism to explain recent puzzling experimental observations in strongly-driven double quantum dots.

### April 18

Øyvind Almelid, Higgs Centre for Theoretical Physics, The University of Edinburgh

«Soft singularities in general field theories»

# April 18

Øyvind Almelid, Higgs Centre for Theoretical Physics, The University of Edinburgh

«Soft singularities in general field theories»

*Abstract*

## Soft singularities in general field theories

Infrared singularities are a salient feature of any field theory containing massless particles, including QED, QCD and theories beyond the standard model. Their proper description and treatment involve an intricate cancellation between virtual and real emissions, which yield logarithmic corrections to scattering amplitudes. At energies relevant to scattering experiments, these corrections can grow large and threaten perturbativity. Thankfully, IR singularities also have a number of properties which enable their computation and treatment to all orders in perturbation theory. Chiefly: they factorise from the high-energy scattering, they are process-independent, and they do not depend on the spin of the emitting particle.

In this talk I will show how we may categorise and understand soft singularities (IR singularities which arise due to a small loop momentum) in a simplified, general gauge theory. I will then show how we may obtain a direct, diagrammatic all-order description of exponentiated soft singularities, ultimately allowing us to resum soft singularities to all orders. I will then discuss current results and the general understanding of the structure of soft singularities at two and three loops.

### May 30

«Neutron stars -and a special black hole- in our neighbourhood»

# May 30

«Neutron stars -and a special black hole- in our neighbourhood»

*Abstract*

## Neutron stars -and a special black hole- in our neighbourhood

In the last five years we have uncovered a nearby population of compact binary millisecond pulsars: fast-spinning neutron stars and light or ultra-light companion stars in orbits with periods shorter than a day.

We have also witnessed "neutron star metamorphosis" for the first time: transitions between accretion- and rotation-powered phenomena. I will summarize recent results on the X-ray and optical properties of this new pulsar population and discuss ongoing work on their potential neutrino production. As a bonus, I will present the recent discovery of a fast and strong wind from a nearby accreting black hole.

## Contact

Jan Myrheim, Coordinator

Email: jan.myrheim@ntnu.no

## Open lectures and seminars

Physics Friday Colloquia

Friday 14:15

Physics Theory Seminar

Monday 14:15