Condensed Matter Physics

This project investigates theoretically electric control of the magnetic properties of molecular magnets. Two classes of magnetic molecules are considered. The first class consists of molecules that are spin frustrated. As a consequence of the frustration, the ground-state manifold of these molecules is characterized by states of different spin chirality, which can be coupled by an external electric field. Electric control of these spin states can be used to encode and manipulate quantum information. The second class comprises molecules known as single-molecule magnets, which are characterized by a high spin and a large magnetic anisotropy. Here the main goal is to control and manipulate the magnetic properties, such as the anisotropy barriers, by adding and subtracting individual electrons, as achieved in tunneling transport.

Key publications

First-principles study of spin-electric coupling in a Cu3 single molecular magnet, MF Islam, JF Nossa, CM Canali, M Pederson

First-principles studies of spin-orbit and Dzyaloshinskii-Moriya interactions in the Cu3 single-molecule magnet, JF Nossa, MF Islam, CM Canali, MR Pederson

Electric control of a Fe4 single-molecule magnet in a single-electron transistor, JF Nossa, MF Islam, CM Canali, MR Pederson

1) Magnetic solotronics on semiconductor surfaces

Solotronics (solitary dopant optoelectronics) aims at understanding and controlling the properties of individual dopants in a semiconductor host. It bears exciting prospects for novel spintronics devices at the atomic scale. In our work we develop quantum mechanical theories and models to investigate the following problems;
(i) electronic and magnetic properties of subsitutional transition metal impurities, such as Mn and Fe ions, positioned on the surface of GaAs ,
(ii) effect of external magnetic and electric fields on the magnetic anisotropy of single transition-metal acceptors,
(iii) Time-dependent spin dynamics of transition-metal impurities and their bound acceptors and
(iv) Electric local manipulation of transition-metal impurities by means of nearby charged defects on GaAs (110) surface.

Our theoretical tools range from atomistic tight-binding models to first-principles methods based on density-functional theory.

Key publications

Chern number spins of Mn acceptor magnets in GaAs, T.o. Strandberg, C.m. Canali and A.h. Macdonald

Magnetic interactions of substitutional Mn pairs in GaAs, Olof Strandberg, Carlo Canali and Allan Macdonald

Magnetic anisotropy of single Mn acceptors in GaAs in an external magnetic field, M. Bozkurt, M.R. Mahani, P. Studer, J.-M. Tang, S. Schofield, N. Curson, M.e. Flatte, A.yu. Silov, C. F. Hirjibehedin, C.M. Canali and P.m. Koenraad

2) Dilute magnetic semiconductor nanowires

In collaboration with experimentalists of the Nanometer Structure Consortium at Lund University we investigate theoretically magneto-transport properties of Mn-ion implanted semiconductor nanowires.

Key publications

A new route toward semiconductor nanospintronics: highly Mn-doped GaAs nanowires realized by ion-implantation under dynamic annealing conditions, NanoLett 11, 3935 (2011)

Hopping Conduction in Mn Ion-Implanted GaAs Nanowires, NanoLett 2012 (DOI: 10.1021/nl302318f)

Magnetic Polarons and Large Negative Magnetoresistance in GaAs Nanowires Implanted with Mn Ions, NanoLett, Article ASAP DOI: 10.1021/nl402229r

Topological insulator (TI) materials [1] host on their boundaries a new type of topological states of quantum matter, which, unlike the Quantum Hall effect, exist without breaking of time-reversal symmetry. Theoretical prediction and subsequent experimental demonstration of these topological states in both 2D and 3D systems have given rise to what is now one of the most exciting new directions in condensed matter physics. Apart from providing a test platform for some fundamental concepts in mathematics and physics, the study of topological insulators hold promise for novel applications in materials science, chemistry, spintronics and quantum computation. However, to be able to fully explore the potential of TIs, one is required to have a detailed knowledge of the nature and properties of topological surface states in real TI materials as well as a quantitative understanding of how they respond to external perturbations [2]. In laboratory, these questions can be addressed with advanced experimental probes, such as spin-sensitive angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM).

In our work, we investigate the properties of surface states in real 3D TIs theoretically by using advanced computational tools for atomistic modelling of materials. We have implemented a microscopic tight-binding model for bismuth selenide (Bi2Se3) family of 3D TIs, with parameters extracted from ab initio calculations. This constitutes a computationally feasible yet fully atomistic quantitative approach, which enables comparison with experiment. Our studies provide insight into spatial as well spin and orbital character of the wave function associated with the topological surface states, which is crucial for understanding the behaviour of real materials [3]. We also investigate finite-size effects on the surface states in Bi2Se3 films, related to finite thickness and finite (mesoscopic) sample size. Another topic of current interest is the interplay of topological surface states with time-reversal breaking perturbations, such as external magnetic fields and magnetic dopants.

[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010); X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011)
[2] H. Beidenkopf et al., Nature Physics 7, 939 (2011); L. A. Wray et al., Nature Physics 7, 21 (2011)
[3] Cao et al., Nature Physics 9, 499 (2013)

Key publications

Probing the wave function of the surface states in Bi2Se3 topological insulator: a realistic tight-binding approach, Anna Pertsova and Carlo M. Canali

Theoretical studies of surface states in Bi2Se3: effects of finite thickness, finite-cluster boundaries, and surface doping, A. Pertsova, M.R. Mahani, C.M. Canali, F. Islam and A.h. Macdonald

Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators, M. R. Mahani, Anna Pertsova, C. M. Canali, M. F. Islam and A. H. Macdonald

An overall goal of the project is to assess the feasibility of using geometric phases and quantum holonomies to build robust quantum gates, and investigate their behavior when the size of a quantum system grows, thereby gaining insights into large-scale quantum computation.

Key publications

Non-Abelian off-diagonal geometric phases in nano-engineered four-qubit systems, Vahid Azimi Mousolou, Carlo M. Canali and Erik Sjöqvist

Unifying geometric entanglement and geometric phase in a quantum phase transition, Vahid Azimi Mousolou, Carlo M. Canali and Erik Sjöqvist

Universal Non-adiabatic Holonomic Gates in Quantum Dots and Single-Molecule Magnets, Vahid Azimi Mousolou, Carlo M. Canali and Erik Sjöqvist

This project is focused on theoretical description of electron transport in nano-scale junctions.