# 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 15:15 - 16:00

Location: E5-103, Realfagbygget NTNU. Natural Science Building

Coordinator: Jacob Linder

# Theory seminars for the Spring 2015

### February 9

Ilya Eremin, Theoretische Physik III, Ruhr-Universität Bochum

«Superconductivity from repulsion in multiband layered systems: novel s-wave symmetry and potential time-reversal symmetry breaking in iron-based superconductors»

# February 9

Ilya Eremin, Theoretische Physik III, Ruhr-Universität Bochum

«Superconductivity from repulsion in multiband layered systems: novel s-wave symmetry and potential time-reversal symmetry breaking in iron-based superconductors»

*Abstract*

### Superconductivity from repulsion in multiband layered systems: novel s-wave symmetry and potential time-reversal symmetry breaking in iron-based superconductors

In my talk we will review the multiband aspects of superconductivity arising from repulsive interaction with weak spin fluctuations.

Most importantly, I will analyze the structure of the pairing interaction and superconducting gaps for LiFeAs, which electronic structure and superconducting gaps are well studied by ARPES. We use the ten-orbital tight-binding model, derived from ab-initio LDA calculations with hopping parameters extracted from the fit to ARPES experiments.

We find that the pairing interaction almost decouples between two subsets, one consists of the outer hole pocket and two electron pockets, which are quasi-2D and are made largely out of dxy orbital, and the other consists of the two inner hole pockets, which are quasi-3D and are made mostly out of dxz and dyz orbitals.

Furthermore, the bare inter-pocket and intra-pocket interactions within each subset are nearly equal. In this situation, small changes in the intra-pocket and inter-pocket interactions due to renormalizations by high-energy fermions give rise to a variety of different gap structures. Different s-wave gap configurations emerge depending on whether the renormalized interactions increase attraction within each subset or increase the coupling between particular components of the two subsets.

We argue that the state with opposite sign of the gaps on the two inner hole pockets has the best overlap with ARPES data.

### February 16

Matthias Eschrig, Royal Holloway, University of London and NTNU (Lars Onsager Professorship 2015)

«Modern theory of polarization in solids - Berry phase approach: part I»

# February 16

Matthias Eschrig, Royal Holloway, University of London and NTNU (Lars Onsager Professorship 2015)

«Modern theory of polarization in solids - Berry phase approach: part I»

*Abstract*

### Modern theory of polarization in solids - Berry phase approach: part I

### February 27

Katherine Freese, University of Michigan and Nordita

«The dark side of the Universe»

# February 27

Katherine Freese, University of Michigan and Nordita

«The dark side of the Universe»

`This Friday colloquium has been cancelled!`

*Abstract*

## The dark side of the Universe

### What is the Universe made of?

This question is the longest outstanding problem in all of modern physics, and it is the most important research topic in cosmology and particle physics today. The reason for the excitement is clear: the bulk of the mass in the Universe consists of a new kind of dark matter particle, and most of us believe its discovery is imminent.

I'll start by discussing the evidence for the existence of dark matter in galaxies, and then show how it fits into a big picture of the Universe containing 5% atoms, 25% dark matter, and 70% dark energy. Probably the best dark matter candidates are WIMPs (Weakly Interacting Massive Particles).

There are three approaches to experimental searches for WIMPS:

- at the Large Hadron Collider at CERN in Geneva
- in underground laboratory experiments
- with astrophysical searches for dark matter annihilation products

Currently there are claimed detections in multiple experiments --- but they cannot possibly all be right. Excitement is building but the answer is still unclear. At the end of the talk I'll turn to dark energy and its effect on the fate of the Universe.

### March 23

«Topological materials and Majorana fermions: part I»

# March 23

«Topological materials and Majorana fermions: part I»

*Abstract*

### Topological materials and Majorana fermions: part I

### April 9

Anna Posazhennikova, Royal Holloway, University of London

«Cold bosons out of equilibrium in multi-well potentials»

# April 9

Anna Posazhennikova, Royal Holloway, University of London

«Cold bosons out of equilibrium in multi-well potentials»

`The seminar will take place on Thursday April 9 at 14:15 in E5-103`

*Abstract*

### Cold bosons out of equilibrium in multi-well potentials

How a finite system approaches thermal equilibrium remains an open question in spite of intense research.

We investigate the non-equilibrium dynamics and thermalisation of a Bose condensed cold gas loaded in a multi-well optical trap (a two-well trap and a 2D optical lattice). The non-equilibrium arises due to a sudden switch of the inter-well hopping. We show a number of interesting and unexpected effects arising in the system, which can be checked experimentally.

### April 28

«Topological materials and Majorana fermions: part II»

# April 28

«Topological materials and Majorana fermions: part II»

`The seminar will take place on Tuesday April 28 at 12:15-14:00 in R10`

*Abstract*

### Topological materials and Majorana fermions: part II

### May 4

William Naylor, Department of Physics (NTNU)

«Towards quantitative comparisons between effective theories and numerical simulation of QCD»

# May 4

William Naylor, Department of Physics (NTNU)

«Towards quantitative comparisons between effective theories and numerical simulation of QCD»

### Towards quantitative comparisons between effective theories and numerical simulation of QCD

I will discuss recent (and ongoing) work using the Polyakov loop coupled Nambu-Jona-Lasinio (PNJL) model aimed at allowing precise comparisons between model calculations and numeric (lattice) calculations within equilibrium non-perturbative QCD. The PNJL calculations are done using 2-color QCD so this will be briefly introduced. Lattice calculations have to deal with a number of unphysical difficulties and we seek to modify the model in a minimal manner that accounts for these. The talk with focus on introducing the model we use and the physics that it describes."

### May 22

Rembert Duine, Utrecht University, The Netherlands

«Néèl skyrmions in constricted geometries and at nonzero temperatures»

# May 22

Rembert Duine, Utrecht University, The Netherlands

«Néèl skyrmions in constricted geometries and at nonzero temperatures»

*Abstract*

### Néèl skyrmions in constricted geometries and at nonzero temperatures

Skyrmions are topological excitations that, in magnetic systems, have recently been attracting a great deal of attention for their potential application in magnetic memories.

In magnets they are stabilized by so-called Dzyaloshinkii-Moriya interactions. The recent discovery of tunable Dzyaloshinskii-Moriya interactions in layered magnetic materials with perpendicular magnetic anisotropy makes them promising candidates for stabilization and manipulation of skyrmions at elevated temperatures. In this talk, I will discuss our results from Monte-Carlo simulations to investigate the robustness of skyrmions in these materials against thermal fluctuations and finite-size effects. We find that even in very constricted geometries at finite temperatures skyrmions are still present in a large part of the phase diagram. Upon tuning the magnetic field through the skyrmion phase, the system undergoes a cascade of first-order phase transitions through states of different number of skyrmions, extended and half-skyrmions, and spiral states. We consider how thermal fluctuations lift the degeneracies that occur at the zero-temperature critical points of these first-order transitions, and find that states with more skyrmions are at moderate temperatures entropically favored over states with less skyrmions.

Finally, I will comment on electrical detection of the various magnetic phases through the topological and anomalous Hall effects, and on the current-driven motion of skyrmions.

### May 26

«Phase diagram of the semimetal graphite in the magnetic ultra-quantum limit»

# May 26

«Phase diagram of the semimetal graphite in the magnetic ultra-quantum limit»

*Abstract*

### Phase diagram of the semimetal graphite in the magnetic ultra-quantum limit

Semimetals like graphite have recently received compelling interest as they not only are able to host topologically non-trivial phases but also can be driven into the ultra-quantum limit by magnetic fields now achievable in modern-day laboratories. Thus, they provide insight into quantum-Hall physics and the physics of massless Dirac fermions in three dimensions. They also represent ideal model systems for studying magnetic-field driven density wave instabilities, as the onset field for such collective excitations is suppressed in semimetals.

Using pulsed high-magnetic fields up to 60 T applied to a single crystal of natural Tanzanian graphite, we find a series of field-induced phase transitions into collinear charge-density wave states resulting from enhanced interactions between the lowest four Landau levels. By analysing magneto-transport data and calculating the renormalized Landau level structure at high fields, we establish the phase diagram of graphite in its ultra-quantum limit.

Our results imply the existence of a topologically-protected chiral edge state at high fields supporting both charge and spin currents.

Irene Aspli

## Contact

Jacob Linder, Coordinator

Email: jacob.linder@ntnu.no

## Open lectures and seminars

Physics Friday Colloquia

Friday 14:15

Physics Theory Seminar

Monday 14:15