Physics Theory Seminar

– Autumn 2016

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

Time: Monday at 13:00-14:00

Location: Realfagbygget E5-103


Theory seminars for the Autumn 2016

December 1. at 14:00

Wolfgang Belzig, University of Konstanz, Germany
«Ground-state cooling a mechanical oscillator by spin-dependent transport and Andreev reflection»

Time and place: Thursday December 1. at 14:00 in Realfagbygget D4-132

December 1. at 14:00

– This Thursday we will have two theory seminars

Wolfgang Belzig, University of Konstanz, Germany
«Ground-state cooling a mechanical oscillator by spin-dependent transport and Andreev reflection»

Time and place: Thursday December 1. at 14:00 in Realfagbygget D4-132

Abstract

Ground-state cooling a mechanical oscillator by spin-dependent transport and Andreev reflection

We study the ground-state cooling of a mechanical oscillator coupled to the charge or the spin of a quantum dot inserted between spin-polarized or a normal metal and a superconducting contact. Such a system can be realized e.g. by a suspended carbon nanotube quantum dot with a suitable coupling between a vibrational mode and the charge or spin.

We show that ground-state cooling of the mechanical oscillator can be achieved for many of the oscillator's modes simultaneously [3] as well as selectively for single modes [1].

We discuss different modes of operation which also include single mode cooling by resonance, which is tunable by a magnetic field [1,2]. We finally discuss how the oscillator’s state can be detected in the current-voltage characteristic [2,3].

  1. P. Stadler, W. Belzig, and G. Rastelli, Phys. Rev. Lett. 113, 047201 (2014)
  2. P. Stadler, W. Belzig, and G. Rastelli, Phys. Rev. B 91, 085432 (2015)
  3. P. Stadler, W. Belzig, and G. Rastelli, Phys. Rev. Lett. 117, 197202 (2016)

December 1. at 11:00

Aurelien Manchon, King Abdullah University of Science and Technology, Saudi Arabia
«Spin Transport in Disordered Antiferromagnetic Devices​»

Time and place: Thursday December 1. at 11:00 in Realfagbygget D4-132

December 1. at 11:00

– This Thursday we will have two theory seminars

Aurelien Manchon, King Abdullah University of Science and Technology, Saudi Arabia
«Spin Transport in Disordered Antiferromagnetic Devices​»

Time and place: Thursday December 1. at 11:00 in Realfagbygget D4-132

Abstract

Spin Transport in Disordered Antiferromagnetic Devices​

Antiferromagnets (AF) have long remained an intriguing and exotic state of matter, whose application has been restricted to enabling interfacial exchange bias in spin-valves. Their role in the expanding field of applied spintronics has been mostly passive and the in-depth investigation of their basic properties considered as fundamental condensed matter physics.

A conceptual breakthrough was achieved ten years ago with the proposal that spin transfer torque could be used to electrically control the direction of the order parameter of AF spin valves, henceforth making these materials potential candidates for low energy spin devices [1,2]. The recent demonstration of current-driven control of the antiferromagnetic order parameter by Wadley et al. [3] opens thrilling avenues for novel spin-based devices.

In this talk, I will first introduce some basics concepts about spin transport in antiferromagnets and current-driven antiferromagnetic dynamics. Then, I will present a drift-diffusion theory in disordered bipartite antiferromagnets, derived from quantum kinetics principles [4]. Based on this equation, I will address the nature of the spin torque in various metallic antiferromagnetic devices (such as spin-valves [5], ultrathin bilayers, and bulk antiferromagnets). Finally, I will discuss possible routes for the development of such devices.

  1. A. S. Núñez, R. A. Duine, Paul Haney, and A. H. MacDonald, Phys. Rev. B 73, 214426 (2006).
  2. Jungwirth et al., Nature Nanotechnology 11, 231 (2016); Baltz et al., arXiv:1606.04284
  3. Wadley, et al. Science 351, 587 (2016).
  4. A. Manchon, submitted
  5. H. B. M. Saidaoui, A. Manchon, and X. Waintal, Phys. Rev. B 89, 174430 (2014); arXiv:1607.01523.
Mon, 28 Nov 2016 19:37:21 +0100

October 31

Mikhail Titov, Radboud university, The Netherlands
«Spin-orbit and spin-transfer torques in two dimensions»

October 31

Mikhail Titov, Radboud university, The Netherlands
«Spin-orbit and spin-transfer torques in two dimensions»

Abstract

Spin-orbit and spin-transfer torques in two dimensions

Spin-orbit and spin-transfer torques are competing forces that drive magnetic domains in the heavy-metal/ferromagnet bi-layers in the presence of electric current.

In this talk I present a microscopic theory of both effects that is obtained from generalised Kubo formalism. In this formalism the torques are naturally related to the spin-susceptibility tensors that can be evaluated microscopically for a given model. Interestingly this theoretical framework, which is based on local mean-field theory, completely avoids the notion of spin current.

I focus on the torques arising in an effective 2D Bychkov-Rashba model for itinerant electrons. I demonstrate that the field-like spin-orbit torque is always larger than the anti-damping like one. I demonstrate, however, that the anti-damping-like term in the spin-transfer torque is strongly enhanced in the presence of spin-orbit interaction and becomes larger than the field-like terms by the metal parameter E_F tau. These results can be further generalised to investigate torques in the antiferromagnetic bilayers.

Fri, 28 Oct 2016 09:23:16 +0200

October 17

Esben Bork Hansen, University of Copenhagen
«Transport signatures of quasiparticle poisoning in a Majorana island»

October 17

Esben Bork Hansen, University of Copenhagen
«Transport signatures of quasiparticle poisoning in a Majorana island»

Abstract

Transport signatures of quasiparticle poisoning in a Majorana island

Fri, 14 Oct 2016 13:30:25 +0200

October 3

Astrid de Wijn, Department of Engineering Design and Materials, NTNU
«Anisotropy of nanoscale friction»

October 3

Astrid de Wijn, Department of Engineering Design and Materials, NTNU
«Anisotropy of nanoscale friction»

Abstract

Anisotropy of nanoscale friction

(Collaboration with  S. G. Balakrishna and R. Bennewitz, INM - Leibniz Institute for New Materials, Germany)

Mechanical properties of crystalline materials are ultimately determined by their atomic structure. A direct consequence of the symmetry of the atomic surface structure is anisotropy of friction and wear.

We investigate the anisotropy of friction theoretically, as well as experimentally on graphitic surfaces. We find that the anisotropy does not depend on the geometry of the sliding object, only on that of the substrate. Friction vectors can deviate significantly from the pulling directions. For graphitic substrates, the strongest deviations are found for pulling directions which lie almost along one zigzag direction of the honeycomb structure, the preferred sliding directions. Numerical simulation and further theory reveal the role of temperature and of the two-dimensional character of the surface potential for the friction anisotropy. The friction is determined by atomic stick-slip events along and across molecular rows determine direction and magnitude of friction.

Phys. Rev. B 89, 245440 (2014)

Tue, 04 Oct 2016 11:44:57 +0200

September 19

Dariusz Chruściński, Institute of Physics, Nicolaus Copernicus University, Poland
«Non-Markovian quantum evolution»

September 19

Dariusz Chruściński, Institute of Physics, Nicolaus Copernicus University, Poland
«Non-Markovian quantum evolution»

Note! Room E4-107 at 14:15

Abstract

Non-Markovian quantum evolution

 

Tue, 04 Oct 2016 11:45:38 +0200

September 12

Mark Rudner, University of Copenhagen, Denmark
«Chiral plasmons without magnetic field»

September 12

Mark Rudner, University of Copenhagen, Denmark
«Chiral plasmons without magnetic field»

Abstract

Chiral plasmons without magnetic field

In electronic systems, chirality is an interesting and useful property that expresses the system's ability to discriminate between forward and backward propagation along certain directions. Edge states in the quantum Hall effect provide a classic example.

Recently, materials exhibiting chirality in the absence of an applied magnetic field have come into the spotlight. Here, the Berry curvature inherent in the material's band structure crucially alters the motion of free carriers, leading to a variety of interesting phenomena such as the anomalous, spin, and valley Hall effects. In this work we show that the interplay between electron-electron interactions and Bloch band Berry curvature leads to a new type of electronic collective excitation -- the chiral Berry plasmon -- with a variety of properties of both fundamental and potential technological interest.

In this talk I will review the basics of Bloch band Berry curvature and related anomalous transport effects, and then demonstrate how these new chiral plasmon modes emerge at the edges of appropriate 2D systems. Experimental signatures and candidate materials including anomalous Hall and quantum anomalous Hall systems, as well as optically pumped gapped Dirac systems (such as hexagonal boron nitride and transition metal dichalcogenides) will be discussed.

Tue, 04 Oct 2016 11:45:17 +0200

Contact

Jeroen Danon

Jeroen Danon, Coordinator
Email: jeroen.danon@ntnu.no

Open lectures and seminars

Mon, 28 Nov 2016 19:27:33 +0100
Irene Aspli