Research by our Primary Investigators

Research by our Primary Investigators

 

Spin Insulatronics

Spin Insulatronics

– Arne Brataas
 

 

Theme and goal 

An electron has a spin as well as a charge. Conventional electronics and spintronics are based on the motion of mobile electrons in conductors. Spintronics, the utilization of spin in electronic devices, has revolutionized datastorage and enabled Apple’s iconic devices iPod, iPhone, andiPad.

Spin Insulatronics is profoundly different because there are no moving charges that generate heat. In magnetic insulators, spin information can, nevertheless, propagate.

Controlling electric signals through the deployment of magnetic insulators can facilitate a revolution in information and communication technologies. We aim to determine to what extent spin in antiferromagnetic and ferromagnetic insulators couple to charge currents in adjacent conductors. We will utilize this coupling to control electric signals.

Since spin signals in insulators have extremely low power dissipation, overcoming the limitations can enable low- power technologies such as oscillators, logic devices, non-volatile random-access memories, interconnects, and perhaps even quantum information processing.

 

Key questions

We focus on the fundamental challenges facing Spin Insulatronics. Key questions are how spin can transfer from electrical conductors to insulators, how far and how spin propagates in insulators, and how much power these devices consume.

 

For current activities, see QuSpin Annual Report 2019. 

 

Topological Quantum Matter

Topological Quantum Matter

– Asle Sudbø
 

 

Theme and goal 

Superfluidity and superconductivity are the spectacular phenomena that fluids in the quantum regime may flow without any dissipation. Magnetism is the effect that an astronomical number of tiny magnetic dipoles of electronic, atomic, or molecular origin spontaneously organize themselves in an ordered state.Superconductivity, superfluidity, and magnetism are co-operative phenomena where enormous numbers of degrees of freedom spontaneously self-organize themselves into various ordered states of matter. This involves transitioning from one state of matter into another state of matter, so-called phase transitions.

Topology is a branch of mathematics that investigates global geometric properties of objects. These objects could be physical objects which we could hold in our hand, but also much more abstract objects defined in an abstract space of mathematical functions. In recent years, physics has seen a sharp rise of interest in topological matter and topological phasetransitions.

Topological matter is a term denoting quantum system featuring extremely robust and very useful physical properties which are protected by some deep non-trivial topological properties of the energy spectrum of the system, i.e. a global property of the “geometry” of the energy spectrum. Topological phase transitions are phase transition where the ordered state cannot be characterized by a standard simple local order parameter (like the magnetization of a magnet).

Rather, the ordered state has a lack of topological defects (think of the hole in a doughnut compared to the lack of a hole in an orange), while the disordered states has proliferated large amounts of topological defects “ripping” the ordered state apart” (think of punching a large number of holes in an orange).

We are interested in the interplay between topological matter and various collective effects and broken symmetries such as superfluidity, superconductivity, magnetism, magnons (quantized magnetic fluctuations), phonons (quantized lattice vibrations), and photons (quanta of light). Could their interplay lead to novel and unexpected robust phenomena? Does the interplay lead to novel topological phase transitions? Can different types of topological order and different types of topological defects compete? Tangential to this, could such robust novel phenomena, were they do exist, also lead to interesting quantum-technological applications? 

 

Key Questions 

The overarching goal of our research is to understand how collective effects in quantum systems with topologically protected physical properties, both with and without strong correlation effects, conspire to produce novel and emergent physics. Such effects are of interest from a fundamental physics point of view, and the research is likely to shed light on other areas of physics as well, such as high-energy physics and high-temperature superconductivity. Systems that we study with this in mind are heterostructures of topological insulators and magnetic insulators, topological insulators, and superconductors, and chiral p-wave superconductors, to mention some.

 

For current activities , see QuSpin Annual Report 2019.

 

Superconducting Spintronics

Superconducting Spintronics

– Jacob Linder

 

 

Theme and goal 

In classical physics, matter exists in one of four different states: gas, liquid, solid, or plasma. However, this classification is too crude to capture the wealth of fascinating physics that emerges within each of these states. For instance, not all solid states behave in the same way.

According to the quantum mechanical description of physics, various solid materials will behave in very different ways. Some materials are magnetic, some do not conduct currents of electric charge, while others can carry currents of not only charge but also a quantum property known as spin. This property is closely related to magnetism and is a fundamental trait of most elementary particles.

It turns out that some materials can conduct electric currents without any energy loss: so-called superconductors.The origin of superconductivity is quantum mechanical, but that does not mean superconductivity only occurs at microscopic length scales invisible to the naked eye. Large chunks of materials can be superconducting, making this phenomenon a macroscopic manifestation of quantum physics. Magnetism is another example of a phenomenon which originates from quantumphysics. When materials such as superconductors and magnets, having very different properties, are combined, things get interesting. This is one of the core motivations behind the field of superconducting spintronics where one studies precisely what happens when magnets are placed in close proximity to superconductors.

We have two main goals guide our research: Firstly, the most important goal is to discover new quantum mechanical phenomena that emerge when combining superconductors with other materials that have fundamentally different properties. A particular emphasis is on spin-dependent quantum effects that arise when magnetic materials are placed in contact with superconductors. Secondly, we focus on discovering specific phenomena that may be relevant to the development of memory technology and information transfer based on superconductors. This is closely related to how transport of charge, spin, and heat occurs in materials.

We use a variety of analytical and numerical tools to address the research questions above, depending on which method is the most appropriate for a particular research project. Some of our theoretical approaches include lattice models, quasiclassical Keldysh theory, Green function techniques, scattering theory, and Landau-Lifshitz-Gilbert phenomenology.

 

Key questions 

The main challenges that we are attempting to solve are related to the functional properties of materials and how they can be controlled or altered by combining different materials.For instance, is it possible to use magnetic materials to control when superconductivity appears and even enhance its properties? Can one use superconductors to generate, control, and detect transport of not only charge but also other quantum degrees of freedom such as spin, without any energy loss? Finally, we are interested in understanding how superconductivity is manifested in unusual solid-state systems, such as atomically thin materials.

 

For current activities , see QuSpin Annual Report 2019.

 

Electronic Structure

Electronic Structure

– Justin Wells

 

 

Theme and goal 

The electronic band structure of a material contains information about all of the electrons in which are relevant for bonding and electronic properties in a solid. It also contains information on the electron spin, and the electrons interactions with each other, impurities, vibrations, spin waves, and more. It is of great interest to try to directly measure the electronic band structure, and hence to gain access to this information. Over the last decade, the instrumentation available has improved dramatically, thus it now possible to measure the electronic band structure with exceptional energy, momentum, and spatial resolutions.

The primary goals are to provide experimental competence to support the theoretical activities within QuSpin, to produce research contributions in important and topical fields, and at an internationally respected level. We also strive to bring interesting and up-to-date research work into the undergraduate classroom, to educate Master students, to contribute to the career development of young scientists and to enable high-quality Ph.D. theses to be produced.

 

Key questions 

Our research predominantly focuses on the interaction of electrons (especially their spin) within a solid. For example, the coupling of electrons with each other (mediated by vibrational waves or spin waves) can give rise to superconductivity, which allows an electrical charge to move without any losses. Unusual forms of coupling between spin and charge allow the efficient conversion of electrical signals into low loss spin signals - and this can also open new avenues for low loss (or lossless) signal transmission, storage, and manipulation. Finally, quantum confinement of charge and spin lies at the heart of the fast developing field of quantum computing.

Most of the methods we use come under the category of “photoelectron spectroscopy” and are based in Einstein’s photoelectric effect, for which he received the Nobel Prize in 1921. Using a refinement of this effect, it is possible to understand chemical bonding within a solid (a discovery which earned Kai Seigbahn the Nobel Prize in 1981), and more recently,it has also been shown that the same method can reveal the spectral function (closely related to the electronic band structure). Furthermore, this information can be resolved by energy, momentum, and spin and the newest instruments also facilitate good spatial resolution. We operate two instruments at NTNU are based on various refinements of this method. We also make significant use of similar instruments at international synchrotron radiation facilities. In 2019 we will purchase and install a state-of-the-art spin-resolved photoelectron microscope to extend the range of possible measurements at NTNU.

 

For current activities, see QuSpin Annual Report 2019.