A quantum magnetic RC circuit

We propose a setup that is the spin analog of the charge-based quantum RC circuit. We define and compute the spin capacitance and the spin resistance of the circuit for both ferromagnetic (FM) and antiferromagnetic (AF) systems. We find that the antiferromagnetic setup has universal properties, but the ferromagnetic setup does not. We discuss how to use the proposed setup as a quantum source of spin excitations, and put forward a possible experimental realization using ultracold atoms in optical lattices

Josephson and Persistent Spin Currents in Bose-Einstein Condensates of Magnons

Using the Aharonov-Casher (A-C) phase, we present a microscopic theory of the Josephson and persistent spin currents in quasi-equilibrium Bose-Einstein condensates (BECs) of magnons in ferromagnetic insulators. Starting from a microscopic spin model that we map onto a Gross-Pitaevskii Hamiltonian, we derive a two-state model for the Josephson junction between the weakly coupled magnon-BECs. We then show how to obtain the alternating-current (ac) Josephson effect with magnons as well as macroscopic quantum self-trapping in a magnon-BEC. We next propose how to control the direct-current (dc) Josephson effect electrically using the A-C phase, which is the geometric phase acquired by magnons moving in an electric field. Finally, we introduce a magnon-BEC ring and show that persistent magnon-BEC currents flow due to the A-C phase. Focusing on the feature that the persistent magnon-BEC current is a steady flow of magnetic dipoles that produces an electric field, we propose a method to directly measure it experimentally.Comment: 8 pages, 6 figures, updated into published versio

Frequency dependent transport through a spin chain

Motivated by potential applications in spintronics, we study frequency dependent spin transport in nonitinerant one-dimensional spin chains. We propose a system that behaves as a capacitor for the spin degree of freedom. It consists of a spin chain with two impurities a distance $d$ apart. We find that at low energy (frequency) the impurities flow to strong coupling, thereby effectively cutting the chain into three parts, with the middle island containing a discrete number of spin excitations. At finite frequency spin transport through the system increases. We find a strong dependence of the finite frequency characteristics both on the anisotropy of the spin chain and the applied magnetic field. We propose a method to measure the finite-frequency conductance in this system

Magnetic texture-induced thermal Hall effects

Magnetic excitations in ferromagnetic systems with a noncollinear ground state magnetization experience a fictitious magnetic field due to the equilibrium magnetic texture. Here, we investigate how such fictitious fields lead to thermal Hall effects in two-dimensional insulating magnets in which the magnetic texture is caused by spin-orbit interaction. Besides the well-known geometric texture contribution to the fictitious magnetic field in such systems, there exists also an equally important contribution due to the original spin-orbit term in the free energy. We consider the different possible ground states in the phase diagram of a two-dimensional ferromagnet with spin-orbit interaction: the spiral state and the skyrmion lattice, and find that thermal Hall effects can occur in certain domain walls as well as the skyrmion lattice

Ballistic InSb Nanowires and Networks via Metal-Sown Selective Area Growth

Selective area growth is a promising technique to realize semiconductor-superconductor hybrid nanowire networks, potentially hosting topologically protected Majorana-based qubits. In some cases, however, such as the molecular beam epitaxy of InSb on InP or GaAs substrates, nucleation and selective growth conditions do not necessarily overlap. To overcome this challenge, we propose a metal-sown selective area growth (MS SAG) technique, which allows decoupling selective deposition and nucleation growth conditions by temporarily isolating these stages. It consists of three steps: (i) selective deposition of In droplets only inside the mask openings at relatively high temperatures favoring selectivity, (ii) nucleation of InSb under Sb flux from In droplets, which act as a reservoir of group III adatoms, done at relatively low temperatures, favoring nucleation of InSb, and (iii) homoepitaxy of InSb on top of the formed nucleation layer under a simultaneous supply of In and Sb fluxes at conditions favoring selectivity and high crystal quality. We demonstrate that complex InSb nanowire networks of high crystal and electrical quality can be achieved this way. We extract mobility values of 10※000-25※000 cm V s consistently from field-effect and Hall mobility measurements across single nanowire segments as well as wires with junctions. Moreover, we demonstrate ballistic transport in a 440 nm long channel in a single nanowire under a magnetic field below 1 T. We also extract a phase-coherent length of ∼8 μm at 50 mK in mesoscopic rings

Transport phenomena in nonitinerant magnets

Traditionally, the role of information carrier in spin- and electronic devices is taken by respectively the spin or the charge of the conduction electrons in the system. In recent years, however, there has been an increasing awareness that spin excitations in insulating magnets (either magnons or spinons) may offer an interesting alternative to this paradigm. One of the advantageous properties of these excitations is that they are not subject to Joule heating. Hence, the energy associated with the transport of a single unit of information carried by a magnon- or spinon current could be much lower in such insulating magnets. Additionally, the bosonic nature of the magnon quasi-particles may be advantageous. Three crucial requirements for the successful implementation of spintronics in insulating magnets are the ability to create, detect, and control a magnon- or spinon current. The topic of creation and read-out of such currents in insulating magnets has been discussed elsewhere, in this thesis we will mainly focus on the third requirement, that of the ability to control magnon- and spinon currents. In the first part of this thesis is we aim to draw a parallel between spintronics in nonitinerant systems and traditional electronics. We do this by considering the question to which extent it is possible to create the analog of the different elements that are used in electronics for magnetic excitations in insulating magnets. To this end, we consider (in Ch. 2 and 3 respectively) rectification effects and finite-frequency transport in one-dimensional (1D) antiferromagnetic spin chains. We mainly focus our attention on the effects of impurities, which are modeled by local changes in the exchange interaction of the underlying Hamiltonian. Using methods from quantum field theory, which include renormalization group analysis and functional field integration, we determine the effect of such impurities on the transport properties of the spin chains. Our findings allow us to propose systems which behave as a diode and a capacitance for the magnetic excitations. In Ch. 4 we introduce a setup which behaves as a transistor for either magnons or spinons: a triangular molecular magnet, which is weakly exchange-coupled to nonitinerant spin reservoirs. We use the possibility to control the state of triangular molecular magnets by either electric or magnetic fields to affect tunneling of magnons or spinons between the two spin reservoirs. The second part of this thesis is devoted to the study of thermal transport in two-dimensional (2D) nonitinerant ferromagnets with a noncollinear ground state magnetization. More specifically, our interest is in thermal Hall effects. Such effects can be used to control a magnetization current, and arise because the magnons (which carry the thermal current) experience a fictitious magnetic field due to the equilibrium magnetization texture. We consider the different magnetic textures that occur in ferromagnets with spin-orbit interaction, and discuss which of them give rise to a finite thermal Hall conductivity

Ultrafast magnon transistor at room temperature

We study sequential tunneling of magnetic excitations in nonitinerant systems (either magnons or spinons) through triangular molecular magnets. It is known that the quantum state of such molecular magnets can be controlled by application of an electric- or a magnetic field. Here, we use this fact to control the flow of a spin current through the molecular magnet by electric- or magnetic means. This allows us to design a system that behaves as a magnon-transistor. We show how to combine three magnon-transistors to form a NAND-gate, and give several possible realizations of the latter, one of which could function at room temperature using transistors with a 11 ns switching time

Rectification of spin currents in spin chains

We study spin transport in nonitinerant one-dimensional quantum spin chains. Motivated by possible applications in spintronics, we consider rectification effects in both ferromagnetic and antiferromagnetic systems. We find that the crucial ingredients in designing a system that displays a nonzero rectification current are an anisotropy in the exchange interaction of the spin chain combined with an offset magnetic field. For both ferromagnetic and antiferromagnetic systems we can exploit the gap in the excitation spectrum that is created by a bulk anisotropy to obtain a measurable rectification effect at realistic magnetic fields. For antiferromagnetic systems we also find that we can achieve a similar effect by introducing a magnetic impurity, obtained by altering two neighboring bonds in the spin Hamiltonian

Orbital-free approach for large-scale electrostatic simulations of quantum nanoelectronics devices

The route to reliable quantum nanoelectronic devices hinges on precise control of the electrostatic environment. For this reason, accurate methods for electrostatic simulations are essential in the design process. The most widespread methods for this purpose are, respectively: the Thomas-Fermi approximation, which provides quick approximate results, and the Schr\"odinger-Poisson formulation, which better takes into account the quantum mechanical effects. The mentioned methods suffer from relevant shortcomings: the Thomas-Fermi method fails to take into account quantum confinement effects that are crucial in heterostructures, while the Schr\"odinger-Poisson method suffers severe scalability problems. In this paper, we outline the application of an orbital-free approach inspired by density functional theory. By introducing gradient terms in the kinetic energy functional, our proposed method incorporates quantum confinement effects while preserving the scalability of a density functional theory. This method offers a new approach for addressing large-scale electrostatic simulations of nanoelectronic devices.Comment: 9+2 pages, 4 figure