Ab initio Spin-orbitronics
- Silvia Picozzi (Consiglio Nazionale delle Ricerche CNR-SPIN, Chieti, Italy)
- Stefan Blügel (Forschungszentrum Jülich, Germany)
- Ingrid Mertig (Martin Luther University Halle, Germany)
|The main purpose of this Psi-k/CECAM research conference is to to highlight the very recent theoretical and computational developments related to the interplay of spin-orbit interaction with electronic structure, magnetism, transport, its link to strongly correlated materials and ultrafast currents in diverse materials. We aim at discussing spin-orbit coupling as a means of engendering such fundamentally novel physical phenomena in exotic bulk materials, at surfaces and interfaces, in thin films and heterostructures. A brainstorm about concepts and ideas in less understood phenomena, such as orbital magnetism, is also planned.|
The proposed conference will last 5 days (starting Monday Sept. 25th 2017 after lunch and finishing Friday Sept. 29th before lunch), consisting only of invited/plenary speakers and of two poster sessions with lots of time for discussion. While the main focus will be on "ab-initio" simulations, a few leading scientists in experiments will be invited and a strong interface to many-body physics treated on the basis of realistic model Hamiltonians will be included.
Pls check under the tag "Files" the online version of the Booklet, List of Posters for both Monday and Tuesday Session
HOTEL RESERVATION / LUNCH TICKETS / SOCIAL DINNER
If you need any reservation for lodging at Hotel Promenade, buying lunch/dinner tickets or tickets for the social dinner (to be held on Wednesday Sept. 27th), please contact Alessandra Bortone (email: firstname.lastname@example.org or email@example.com ).
LUNCHES during the Conference days (Sept 25th-29th, 2017) will be held at Hotel Promenade: lunch tickets can be bought via Athena Congressi (option available also for Participants not lodging at Hotel Promenade). It is recommended to buy lunch tickets in advance rather than "on-site" at the Conference.
The SOCIAL DINNER is planned for Wednesday Sept 27th, 2017 at Cantina Dora Sarchese , a typical Italian winery located in Ortona (Chieti) on the nice Abruzzo hills.
A short visit to the beautiful Wine Fountain (on the right) and to the Caveau where the wines are stored is planned, just before dinner.
The Cantina Dora Sarchese will be reached by a special bus leaving at 7 pm from Hotel Promenade (also bringing back the participants).
It is recommended to buy tickets for the social dinner in advance rather than "on-site" at the Conference (via Athena Congressi).
Special dietary requirements can be mentioned during reservation.
Dinners are not included in the conference program (except for the Social Dinner).
There is no registration fee for the conference. However, all the local expenses (meals, hotels, local transportation etc) will NOT be supported by the Conference organization and will have to be covered by the participants.
The Submission of an Abstract for the Poster session is HIGHLY RECOMMENDED.
In order to submit your contribution, pls follow the CECAM procedure for Abstract submission:
1) Register on the CECAM database (pls click on the "Apply" page above and follow the guidelines)
2) Send an email to the organisers through the CECAM Website, so that they can add you as a "Participant"
3) Only after you're added as a "Participant", you are allowed to submit your contribution.
4) For contributions to the Poster Session, you should use the tag "Posters".
5) For Abstracts for Invited talks, you should use the tag "Abstracts"
For any question, please contact one of the organisers or the scientific secretariat (e-mail: firstname.lastname@example.org )
- Silvia Picozzi, CNR-SPIN Chieti, Italy
- Stefan Blügel, Forschungszentrum Jülich, Germany
- Ingrid Mertig, M. Luther Univ. Halle, Germany
LOCAL SUPPORTING ORGANIZERS
- Carmine Autieri, CNR-SPIN Chieti, Italy
- Paolo Barone, CNR-SPIN, Italy.
- Domenico Di Sante, Univ. Würzburg, Germany.
- Jagoda Slawinska, CNR SPIN Chieti, Italy
- Alessandro Stroppa, CNR-SPIN, Italy.
The following topics will constitute the main focus of the workshop sessions:
- Two-dimensional Spin-Orbit Solids
- Topological Solids
- Chiral Magnets
- Complex Magnets
- Correlations, Cooperative Phenomena and Spin-Orbit Interaction
- Theory of Spin Transport
- Theory of Spin-Orbit Torque
- Dynamical Spin Excitations
- Ultra-Fast Magnetism
- Orbital Magnetism
The conference poster is available on the above page "Files": pls download it and spread!
PLENARY SPEAKER: Stuart Parkin (Halle, DE)
|DETAILED DESCRIPTION OF TOPICS|
TWO-DIMENSIONAL SPIN-ORBIT SOLIDS: Spin-orbit coupling in low dimensional (non-magnetic) materials was found to be crucial in causing k-dependent spin-splitting, of potential appeal for carrying and processing information in future devices. For example, the valley (i.e. band energy extrema in momentum space away from the Brillouin zone center) degrees of freedom  were proposed to be strongly linked to spin degrees-of-freedom in layered transition metal dichalcogenides (i.e. 2H MoS2): a valley contrasting spin-polarization arises at the corner of the hexagonal Brillouin zone, with peculiar symmetry properties and related band structures giving rise to exotic circular dichroism and spin-valley Hall-effects, of interest for the so-called “valleytronics”. Other examples in low-dimensional systems include the Rashba  and Dresselhaus effects, i.e. spin-orbit-induced k-dependent spin-splittings, occurring at surfaces, interfaces, or even in bulk materials lacking inversion symmetry. In most of these situations, ab initio simulations are key in predicting the presence and size of the effects in known materials, in discovering materials where these would be optimized and in guiding experimentalists towards the interpretation of photoemission, transport, optical characterization.
TOPOLOGICAL SOLIDS: Since a few decades, the concept of topology has been increasingly used to classify electronic states in realistic materials according to topological invariance, Chern numbers etc. . Following the first prototypical example of two-dimensional electron gas under external magnetic field, the field of topological materials experienced a hectic growth with the advent of a special class of band insulators, i.e. topological insulators (TI), which can be classified according to Z2 topological indices. In case of odd Z2 indices, TI feature Dirac-like edge or surface states, which are topologically protected as long as the time-reversal symmetry is not broken. Later on, another class of TI was proposed, where the topological protection is granted through certain crystalline symmetries, therefore resulting in the so-called “topological crystalline insulators (TCI)”. More recently, the class of topologically non-trivial materials has branched towards topological semi-metals (TSM), where the Fermi surface consists of isolated band-crossing k-points (or lines), which can be viewed as “topological defects”. Depending on symmetry conditions, TSM can be divided in Weyl metals (where the low-energy physics around the band-crossing points can be described by the Weyl equation and related Weyl points can be thought of as magnetic monopoles in momentum space), Dirac semimetals (where the Fermi surface is constituted by two monopoles at the same k-point) and nodal-line semimetals (where nodal points form a closed ring in k-space). In all the topologically non-trivial materials, spin-orbit coupling (SOC) is absolutely crucial, for example causing the band-gap opening in TI systems showing band-inversion. The role that first-principles calculations have in this field is of paramount importance, as most of the relevant physics is based on independent electrons where DFT is fully adequate. Peculiar surface band-structures, showing Dirac edge-states in TI or Fermi arcs in Weyl semimetals, can be accurately calculated from first-principles, in particular when many-body perturbation theory in the GW approximation or QSGW is combined with SOC, serving as a unique guide for the interpretation of (spin-resolved) ARPES.
CHIRAL MAGNETS AND DZYALOSHINSKII-MORIYA INTERACTION: Chiral mag-nets are characterized by the presence of the Dzyaloshinskii-Moriya interaction (DMI), a magnetic interaction that was established in the late 50’s - early 60’s in oxide materials, which results from SOC combined with structure inversion asymmetry. The hallmark of the DMI interaction is (i) it has the form of a vector chirality and clock- and counterclockwise rotating magnetic structures become different in energy; (ii) for any combination of Heisenberg exchange, magnetic anisotropy and DMI, there exist an external magnetic field  that changes the magnetic structure to skyrmions – topologically quantized magnetic whirls, that couple efficiently to electric currents by Berry phases, and can be manipulated by ultra small forces – either as single metastable skyrmions immersed on a ferromagnetic background or as skyrmions lattice. Due to its topological protection, the skyrmion is considered as a novel particle for information technology. In spintronics the focus is on skyrmions in magnetic metals. Very very little is known about the DMI in metals. Currently ab initio investigations have started to shed light on the DMI in magnetic materials, a necessary undertaking for designing materials combinations, such that 5-10 nm skyrmions can be stabilized in thin magnetic films at room temperature. Several methods to extract the DMI have been developed. The regime of validity will be discussed. Currently three types of materials are considered, B20 alloys e,g, MnSi or FeGe, materials with broken bulk inversion asymmetry, films of these materials and the interface stabilized skyrmions in ultrathin magnetic films on substrates of large spin-orbit materials .
COMPLEX MAGNETS: Antiferromagnetic spintronics is a completely new perspective based on antiferromagnetic materials or more complex magnets that have magnetic moments inside, but have a resulting zero net magnetic moment, which makes magnetism in antiferromagnets invisible to the outside. The outstanding question is how to efficiently manipulate and detect the magnetic state of an antiferromagnet. In the focus session we are looking at merits of antiferromagnetic spintronics from a more general perspective of spin-transport, magnetization dynamics, and materials research, and give a brief outlook of research and applications of antiferromagnetic spintronics. We focus also on more complex magnets with a non-trivial vector spin analysis.
CORRELATIONS, COOPERATIVE PHENOMENA AND SPIN-ORBIT INTERACTION: Wherever the energy scales for spin, orbital and lattice degrees of freedom are comparable to that of spin-orbit coupling, a whole class of emergent phenomena and peculiar states can arise:  novel spin-orbital ordered states, correlated topological insulators or semimetals, topological superconductors with Majorana fermions, spin-orbit Mott insulators, novel magnetoelectric effects, materials realization of Kitaev models… The competition between different energy scales and the presence of many active degrees of freedom in the presence of strong SOC paves the way to different cross-coupled phenomena and exotic responses to external probes (i.e. electric and magnetic fields, strain, pressure, chemical doping), such as electrically-controllable Rashba-effects in ferroelectrics, exotic magneto-transport behavior in several iridates, Rashba-effects in oxide-based heterostructures, etc. First-principles calculations, possibly complemented by corrections beyond “standard” density-functional-theory (i.e. DFT+U, non-local exchange-correlation functionals, dynamical mean field theory etc), can uniquely provide a description of all the competing energy scales with a similar degree of accuracy, in complex materials such as 3d-4d-5d transition metal oxides, heterostructures, multiferroics, etc.
THEORY OF SPIN-TRANSPORT. The initial seed for the development of the transport facet of spin-orbitronics was the discovery that the SOC-driven spin-asymmetry in propagation and scattering of electrons subject to an external electric field gives rise to a transverse spin current which results in the spin Hall effect, as experimentally observed some ten years ago . The discovery of the spin Hall effect has triggered a paradigm shift in spintronics by demonstrating the possibility of generating and operating the spin currents in paramagnets. On a broader scale, the spin Hall effect is one of the representatives of the family of effects which group around the SOI-driven entanglement and scattering of electronic states in a solid in response to an applied electric or generally electro-magnetic field . This entanglement / scattering in magnetic systems is the utter source behind the interplay of spin currents and magnetization, and it manifests for example in such central for technology and recently discovered phenomena as current-induced magnetization or Edelstein effect; optically-excited spin currents; magnetization-dynamics driven pumping of charge in metals and insulators; interplay of spin currents with antiferromagnetic magnetization, to name just a few. In systems with broken inversion symmetry the pure spin Hall current can be injected into the ferromagnet brought in contact with the region where spin Hall effect takes place, thus causing a torque on the magnetization.
THEORY OF SPIN-ORBIT TORQUE. The effect, called the spin-orbit torque effect, is one of the technologically most promising SOI-driven phenomena which was discovered very recently, and which is rapidly moving to the center of attention both in theoretical as well as applied spintronics, owing to its complex physics, multiple practical advantages and its efficiency in switching ferromagnetic as well as antiferromagnetic magnetization [9,10]. Manifestly, it is by realizing the topological nature of spin, anomalous and quantum Hall effects that the geometric Berry phase concepts entered the field of “conventional” solid state physics, thus triggering a discovery of novel states of matter and a revolution in our understanding of nature and expectations from future technology. The utter fragility of the aforementioned phenomena to the microscopics of SOC, magnetic interactions, symmetry breaking and disorder details, poses a challenge addressing not only proper full quantum-mechanical description of the underlying electronic structure of materials, but higher up at the level of corresponding response characteristics. Only an ability to reliably test our general understanding and the importance of various origins of novel effects down to precise numbers, which can be directly compared to experiments on complex systems, can advance this part of spinorbitronics to the new level. A hope for possibility of such a description came rather recently after realizing that combining general quantum-mechanical linear response formalism with advanced ab initio techniques can explain and shed light onto such spinorbitronics phenomena as spin Hall effects and spin-orbit torque in disordered systems, as well their thermoelectric analogues, correctly reproduce their behavior and magnitude, as well as justify the relevance of Berry phase topological concepts when applied to dynamics of electrons in spin-orbit coupled bands. Currently, the area of first principles studies of the transport spinorbitronics effects and spin-orbit torque is probably one of the most turbulent and exciting in modern solid state physics.
DYNAMICAL SPIN EXCITATIONS: The time-dependent aspect of spinorbitronics is just being explored from an ab initio perspective. Access to magnetic excitation spectra is of high value due to the role of spin excitations as a dynamical route to control magnetic elements and spin-currents, the building blocks of spinorbitronics. In the context of complex magnetic structures, chiral or not, the dynamics and the impact of SOC is still a formidable task from first-principles. Indeed the excitation of the magnetization involves interaction with different degrees of freedom: e.g. electron-hole excitations, phonons and SOC, which are sources of decay and energy consumption that not only affect the dynamics of a material but many of its usual properties. It is only recently that major advancements have been made in the study of collective, localized, and discrete excitations in low-dimensional systems made of itinerant nanomagnets and atomic quantum magnets. This progress has been triggered by the development of spin-polarized electron energy loss spectroscopy (SPEELS)  and by low temperature scanning probe techniques (STM) . These tools probe inelastic magnetic effects. Challenged by these experiments, new and different theoretical approaches have been developed, i.e. based on time-dependent density functional theory (TD-DFT), many-body perturbation theory or on multi-scale modeling approaches. A reasonable understanding has then been collected from ab initio in the Terahertz and Gigahertz frequencies in thin films [13,14] and small ad-nanostructures . It was found that electronic details of the material, symmetry, shape and size of the magnets are important in defining the main properties of the excitations (lifetimes and energies), which can leave non-trivial signatures in transport properties. Presently, the goal is to simulate and understand from first-principles the dynamics of not only collinear magnets but also complex magnetic textures considering the coupling of different excitation channels and to unveil the intimate links to the generated (spin) current.
ULTRA-FAST MAGNETISM: What if we were able to manipulate magnetic bits on a timescale of femtoseconds? Devices would operate at speeds several orders of magnitude fasten than at present. The first experiment demonstrating ultrafast demagnetization with laser pulses  triggered a new field, opening the vast and largely unexplored physical landscape of ultrashort time scales. However a key component to successful transfer such a process to technology is the controllability aspect, i.e. that it can be tuned in order to overcome the practical and physical limitations imposed on the system. For that a large effort has been made on the underlying physical causes of laser-induced demagnetization but still many open questions remaining both theoretically and experimentally . There are phenomenological based models, e.g. the three temperature model , that are used to explain the experiments but an ab-initio description is still lacking behind. Different underlying physical mechanisms are discussed in the literature. For example, the superdiffusive spin transport  has been proposed and other mechanisms suggest ultrafast thermal heating of electrons, direct coupling between spins and carriers, combined action of spin-orbit coupling, interactions between spins and laser photons, magnons generation, phonons, Eliott-Yafet-like mechanism and so on . Only recently, a breakthrough has been achieved by developing and applying methods based on TD-DFT applied to ultrafast magnetism of bulk materials, thin films  down to free standing clusters.
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