Networks of Quantum Nanorings: Programmable Spintronic Devices

Abstract

An array of quantum rings with local (ring by ring) modulation of the spin orbit interaction (SOI) can lead to novel effects in spin state transformation of electrons. It is shown that already small (3 × 3, 5 × 5) networks are remarkably versatile from this point of view: Working in a given network geometry, the input current can be directed to any of the output ports, simply by changing the SOI strengths by external gate voltages. Additionally, the same network with different SOI strengths can be completely analogous to the Stern-Gerlach device, exhibiting spatial-spin entanglement. PACS numbers:Using the spin degree of freedom in information processing applications has become an important and rapidly developing field. For most of the spintronic devices, spin is a classical resource, logical states are "up" and "down" with respect to certain quantization directions, but their superpositions play no role. On the other hand, the electron spin degree of freedom can also be considered one of the prospective carriers of qubits, the fundamental units in quantum information processing [1]. To realize this aim, however, requires to perform basic spin operations such as the production of spin-polarized carriers and spin rotation. Here we propose a device that can serve multiple purposes including the delivery of spinpolarized currents and rotating the spin direction, but it can also direct the input current into a given output port in a spin independent way. The significance of these results is related to the flexibility of the device: the same geometry provides qualitatively different transport properties in such a way that parameters are being varied in an experimentally achievable range.We calculate the spin transport properties of twodimensional rectangular arrays of nanoscale quantum rings, which can be fabricated from e.g. InAlAs/InGaAs based heterostructures [2] or HgTe/HgCdTe quantum wells [3], where Rashba-type [4] spin-orbit interaction (SOI) is present. This effect, which is essentially the same as the one which causes the fine structure in atomic spectra, results in spin precession for electrons moving in a semiconductor. It has already been demonstrated in experiments that the strength of this type of SOI can be controlled by external gate voltages [5,6] in the range of a few Volts. We propose devices in which the local manipulation of the SOI strength leads to effects which could be used in various practical spintronic applications. We focus on narrow rings in the ballistic (coherent) regime, where a one dimensional model provides appropriate description.The geometries we are considering (namely 3 × 3 and 5 × 5 ring arrays, see Fig. 1) have already appeared in a recent experiment [7] with rings of 100 nm thickness, and the case of uniform SOI strength has also been investigated theoretically [8,9]. We note that similarly to Ref. [7], the rings we consider here touch each other, thus the lines between them shown in the figures serve only visualization purposes. Considering wires of finite length would not p...

Flux-induced semiconducting behavior of a quantum network

“…A detailed study of the density of states of an infinite network is made together with the conductance of finite sized system to establish the idea.PACS numbers: 63.50.+x, 63.22.+m Low dimensional model quantum systems have been the objects of intense research, both in theory and in experiments, mainly due to the fact that these simple looking systems are prospective candidates for nano devices in electronic as well as spintronic engineering [1,2,3,4,5,6,7,8,9,10,11,12,13]. Apart from this feature, several striking spectral properties are exhibited by such systems owing to the quantum interference which is specially observed in quantum networks containing closed loops.…”

“…Apart from this feature, several striking spectral properties are exhibited by such systems owing to the quantum interference which is specially observed in quantum networks containing closed loops. Examples are, the Aharonov-Bohm (AB) effect [5] in the magnetoconductance of quantum dots [7], electron transport in quantum dot arrays [3,4], Fano effect in a quantum ring-quantum dot system [8], spin filter effects in mesoscopic rings [10,11] and dots [6], to name a few.Recently, Aharony et al [1,2] have proposed a model of a nano spintronic device using a linear chain of diamondlike blocks of atomic sites. Each plaquette of the array is threaded by identical magnetic flux.…”

Aharonov-Casher and spin Hall effects in mesoscopic ring structures with strong spin-orbit interaction

“…Selfinterference effects whether due to particle scattering at the lead-ring interphase or to the backscattering due to AC phase acquired are interesting and can lead to useful effects. For instance, the current in a ring 31 or ring arrays 32 can be manipulated into spin-polarized currents and it is even possible to change the spin direction of the electrons as they exit the device. We would like to remark that analytical studies of the conductance in a multiprobe ring and generalized to ring array have been conducted with the transparent lead approximation and with random scatterers at the junction between FIG.…”

Quantum Transport Phenomena Of Two-Dimensional Mesoscopic Structures

“…Additionally, the spin-polarizing property can be combined with energy filtering, for the 9 × 9 network we discussed earlier (γ = π/2, b/a = 1.01) the degree of polarization at the output can be above 85%. Note that some of these spintronic properties are similar to that of ring arrays [73] and although in the current case the transmission probabilities are lower than unity (but still around 50%), now there is no need for a local modulation of the SOI strength, which is promising from the viewpoint of possible applications.…”

“…The practical importance of Rashba-type SOI is that its strength can be modulated by external gate voltages [69,60]. Finite periodic structures such as quantum ring arrays have already been realized experimentally [21] and have also been described theoretically [70,71,72,73]. The spin-transformation properties of finite networks suggest various possible spintronic applications as well [72,73,74].…”