Itinerant chiral ferromagnetism in a trapped Rashba spin-orbit coupled Fermi gas

How ferromagnetic phases emerge in itinerant systems is an outstanding problem in quantum magnetism. Here we consider a repulsive two-component Fermi gas confined in a two dimensional isotropic harmonic potential and subject to a large Rashba spin-orbit (SO) coupling, whose single-particle dispersion can be tailored by adjusting the SO coupling strength. We show that the interplay among SO coupling, correlation effects and mean-field repulsion leads to a competition between ferromagnetic and non-magnetic phases. At intermediate interaction strengths, ferromagnetic phase emerges which can be well described by the mean-field Hartree-Fock theory; whereas at strong interaction strengths, a strongly correlated non-magnetic phase is favored due to the beyond-mean-field quantum correlation effects. Furthermore, the ferromagnetic phase of this system possesses a chiral current density induced by the Rashba spin-orbit coupling, whose experimental signature is investigated.Comment: Main text: 5 pages, 6 figures; Supplement: 4 pages, 2 figure

New mechanisms to engineer magnetic skyrmions and topological superconductors

We propose an alternative route to stabilize magnetic skyrmions which does not require Dzyaloshinkii-Moriya interactions, magnetic anisotropy, or an external Zeeman field. Our so-called magnetic skyrmion catalysis (MSC) solely relies on the emergence of flux in the system's ground state. We review scenarios that allow for a nonzero flux and summarize the magnetic skyrmion phases that it induces. Among these, we focus on the so-called skyrmionic spin-whirl crystal (Sk-SWC$_4$) phase. We discuss aspects of MSC using a concrete model for topological superconductivity, which describes the surface states of a topological crystalline insulator in the presence of proximity induced pairing. By assuming that the surface states can exhibit the Sk-SWC$_4$ phase, we detail how the addition of a pairing gap generates a chiral superconductor. For this purpose, we construct a low-energy model which renders the mechanism for topological superconductivity transparent. Moreover, by employing this model, we perform a self-consistent investigation of the appearance of the Sk-SWC$_4$ phase for different values of the pairing gap and the ground state's flux. Our analysis verifies the catalytic nature of our mechanism in stabilizing the Sk-SWC$_4$ phase, since the magnetization modulus becomes enhanced upon ramping up the flux. The involvement of MSC further shields magnetism against the suppression induced by the pairing gap. Remarkably, even if the pairing gap fully suppresses the Sk-SWC$_4$ phase for a given value of flux, this skyrmion phase can be restored by further increasing the flux. Our findings demonstrate that MSC enables topological superconductivity in a minimal and robust fashion.Comment: 20 pages, 2 figures, Proceedings SPIE 12656, Spintronics XVI (San Diego USA

Features of Rashba-coupled Fermi gases, master equations and memory effects

The first part of this thesis studies interactions in Rashba-coupled Fermi gases. The main objective of this part of the thesis consists of proposing a model to describe dilute Fermi gases. Recent theoretical works propose to use Rashba spin-orbit-coupled systems by means to enhance superconductivity, topologically protected insulators, or quantum computing. The richness and potential of Rashba-coupled Fermi systems has been proved with currently ongoing experiments with cold atoms and synthetic gauge fields, which allow to simulate neutral particles using laser fields. In this part of the thesis, we conclude that it is possible to describe dilute Rashba-coupled Fermi gases with a model that has meaningful predictive power. In the second part of the thesis we analyse the validity of the master equation approach to describe open quantum systems. Using a Jaynes-Cummings Hamiltonian we show that under certain circumstances, the master equation approach does not fully describe the effect of the environment onto the system, hence additional tools are needed

Universal relations for hybridized s- and p-wave interactions from spin-orbital coupling

In this work, we study the universal relations for one-dimensional spin-orbital-coupled fermions near both s- and p-wave resonances using effective field theory. Since the spin-orbital coupling mixes different partial waves, a contact matrix is introduced to capture the nontrivial correlation between dimers. We find the signature of the spin-orbital coupling appears at the leading order for the off-diagonal components of the momentum distribution matrix, which is proportional to 1/q³ (q is the relative momentum). We further derive the large frequency behavior of the Raman spectroscopy, which serves as an independent measurable quantity for contacts. Finally, we give an explicit example of contacts by considering a two-body problem

Topological electronic phases in graphene

Graphene is a two dimensional material made of single layer of carbon atoms arranging into a honeycomb lattice. It can be synthesized by variety of methods as exfoliation, chemical vapor deposition or organic polymerization. Its electronic properties are not the ones of an insulator nor a metal, being usually known as a zero gap semiconductor. Electrons in graphene behave as massless Dirac fermions, having a zero effective mass. The Dirac equation that governs electrons turns graphene into a material that can easily develop topological states due to Berry phase effects of the Dirac points. Such topological states of matter are characterized for having properties which are independent on the defects and imperfections that the material might have. The two dimensional nature of graphene makes it specially suitable to inherit properties from other materials by proximity effect, as superconductivity or magnetism. In this thesis we will explore by means of theoretical techniques how graphene can show topological insulating states by combination of magnetic fields, electron-electron interaction, spin orbit coupling, exchange and superconducting proximity effects

Dissipative dynamics of an impurity with spin-orbit coupling

Brownian motion of a mobile impurity in a bath is affected by spin-orbit coupling (SOC). Here, we discuss a Caldeira-Leggett-type model that can be used to propose and interpret quantum simulators of this problem in cold Bose gases. First, we derive a master equation that describes the model and explore it in a one-dimensional (1D) setting. To validate the standard assumptions needed for our derivation, we analyze available experimental data without SOC; as a byproduct, this analysis suggests that the quench dynamics of the impurity is beyond the 1D Bose-polaron approach at temperatures currently accessible in a cold-atom laboratory -- motion of the impurity is mainly driven by dissipation. For systems with SOC, we demonstrate that 1D spin-orbit coupling can be 'gauged out' even in the presence of dissipation -- the information about SOC is incorporated in the initial conditions. Observables sensitive to this information (such as spin densities) can be used to study formation of steady spin polarization domains during quench dynamics

Spin-polarized supercurrents for spintronics: a review of current progress

During the past 15 years a new field has emerged, which combines superconductivity and spintronics, with the goal to pave a way for new types of devices for applications combining the virtues of both by offering the possibility of long-range spin-polarized supercurrents. Such supercurrents constitute a fruitful basis for the study of fundamental physics as they combine macroscopic quantum coherence with microscopic exchange interactions, spin selectivity, and spin transport. This report follows recent developments in the controlled creation of long-range equal-spin triplet supercurrents in ferromagnets and its contribution to spintronics. The mutual proximity-induced modification of order in superconductor-ferromagnet hybrid structures introduces in a natural way such evasive phenomena as triplet superconductivity, odd-frequency pairing, Fulde-Ferrell-Larkin-Ovchinnikov pairing, long-range equal-spin supercurrents, $\pi$-Josephson junctions, as well as long-range magnetic proximity effects. All these effects were rather exotic before 2000, when improvements in nanofabrication and materials control allowed for a new quality of hybrid structures. Guided by pioneering theoretical studies, experimental progress evolved rapidly, and since 2010 triplet supercurrents are routinely produced and observed. We have entered a new stage of studying new phases of matter previously out of our reach, and of merging the hitherto disparate fields of superconductivity and spintronics to a new research direction: super-spintronics.Comment: 95 pages, 23 Figures; published version with minor typos corrected and few references adde