Effect of Spin-Orbit Interactions on the 0.7 Anomaly in Quantum Point Contacts

Abstract

We study how the conductance of a quantum point contact is affected by spin-orbit interactions, for systems at zero temperature both with and without electron-electron interactions. In the presence of spin-orbit coupling, tuning the strength and direction of an external magnetic field can change the dispersion relation and hence the local density of states in the point contact region. This modifies the effect of electron-electron interactions, implying striking changes in the shape of the 0.7-anomaly and introducing additional distinctive features in the first conductance step.PACS numbers: 71.70. Ej, Spin-orbit interactions (SOI) play an important role in a variety of fields within mesoscopic physics, such as spintronics and topological quantum systems. In this Letter we study the effects of SOI on the conductance of a quantum point contact (QPC), a one-dimensional constriction between two reservoirs [1,2]. The linear conductance G of a QPC is quantized in multiples of G Q = 2e2 /h, showing the famous staircase as a function of gate voltage. In addition, at the onset of the first plateau, measured curves show a shoulderlike structure near 0.7G Q [3]. In this regime QPCs exhibit anomalous behavior in the electrical and thermal conductance, noise, and thermopower [3][4][5][6][7][8][9][10][11]. The microscopic origin of this 0.7-anomaly has been the subject of a long debate [12][13][14][15][16][17][18]. It has recently been attributed to a strong enhancement of the effects of electron-electron interactions (EEI) by a smeared van Hove singularity in the local density of states (LDOS) at the bottom of the lowest QPC subband [15,18]. While this explains the 0.7-anomaly without evoking SOI, the presence of SOI can change the dispersion relation and hence the LDOS, thus strongly affecting the shape of the 0.7-anomaly. Previous studies of SOI in QPCs exist [19][20][21][22][23], but not with the present emphasis on their interplay with the QPC barrier shape and EEI, which are crucial for understanding the effect of SOI on the 0.7-anomaly.Setup. We consider a heterostructure forming a twodimensional electron system (2DES) in the xy-plane. Gate voltages are used to define a smooth, symmetric potential which splits the 2DES into two leads, connected by a short, one-dimensional channel along the x-axis: the QPC [1,2]. The transition between the leads and the QPC is adiabatic. We also assume the confining potential in the transverse direction to be so steep that the subband spacing is much larger than all other energy scales relevant for transport, in particular those related to the magnetic field and SOI, and consider only transport in the first subband, corresponding to the lowest transverse mode. This can be described by a one-dimensional model with a smooth potential barrier and local EEI [18]. The magnetic field B is assumed to be in the xy-plane, acting as a pure Zeeman field, without orbital effects.A moving electron in an electric field can experience an effective magnetic field B SOI proportional to its momentum k. D...

Conductance quantization and shot noise of a double-layer quantum point contact

“…Then, the energy assumes a minimum value of − 2 k 2 R /(2m * ) = −E R at k y = ∓ m * α R 2 = ∓k R . Further, according to analysis 43,70 , the k-directional split subbands are mixed; consequently, the subbands repel and open a gap into the upper and lower branches (see Fig. 11 (b)).…”

Detection and Control of Spin-Orbit Interactions in a GaAs Hole Quantum Point Contact

“…Conductance traces for all three QPCs, exhibiting characteristic 1D quantised plateaus as a function of side gate voltage V SG , are shown in Fig.1 a-c. The first subband shows anomalous features which may be due to the 0.7 structure [18][19][20], which will be studied in detail elsewhere.…”

Transconductance and effective Landé factors for quantum point contacts: Spin-orbit coupling and interaction effects

“…Moreover, the description of the SO effects at the bottom of the subbands and the re-sulting anisotropy of g * factor requires account taken for the subband mixing. The approximation of a single subband [22] is not suitable for evaluation of the g * factors modification by the SO interaction as calculated by the main simulation of this paper, and as observed in the transport experiments [23,24,33,35]. Moreover, for E F between E 1 and E 2 the SO coupling effects involve coupling of a transmitting (n = 1) and evanescent (n = 2) modes, i.e.…”

“…For a narrow channel when the value of ω is high the energy spacings between the subbands are large. Therefore, it seems justified to neglect the subband mixing [22]. We diagonalize the SO Hamiltonian in the basis of the spin-up and spin-down basis of the lowest subband (n = 1),…”

“…The effective g * factors for electrons in quantum wires was studied in a number of papers [29][30][31][32]. The combined effects of SO and electron-electron interactions for 0.7 anomaly in quantum point contacts were recently discussed [22] for neglected mixing of subbands within the QPC constriction.…”