Aharonov-Bohm oscillations in the presence of strong spin-orbit interactions

Superconducting quantum interference effect in NbSe2/NbSe2van der Waals junctions

Layered materials with exotic properties, such as superconducting, ferromagnetic, and so on, have attracted broad interest. The advances in van der Waals (vdW) stacking technology have enabled the fabrication of numerous types of junction structures. The dangling-bond-free interface provides an ideal platform to generate and probe various physics phenomena. Typical progress is the realization of vdW Josephson junctions with high supercurrent transparency constructed of two NbSe₂layers. Here we report the observation of periodic oscillations of the voltage drop across a NbSe₂/NbSe₂vdW junctions under an in-plane magnetic field. The voltage-drop oscillations come from the interface and the magnitude of the oscillations has a non-monotonic temperature dependence which increases first with increasing temperature. These features make the oscillations different from the modulation of the critical current of a Josephson junction by the magnetic field and the Little-Parks effect. The oscillations are determined to be generated by the quantum interference effect between two superconducting junctions formed between the two NbSe₂layers. Our results thus provide a unique way to make an in-plane superconducting quantum interference device that can survive under a high magnetic field utilizing the Ising-paring nature of the NbSe₂.

Spin polarized bound states in the continuum in open Aharonov-Bohm rings with the Rashba spin-orbit interaction

We consider the trapping of electrons with a definite spin polarization by bound states in the continuum (BSC) in the open Aharonov-Bohm rings in the presence of the Rashba spin-orbit interaction (RSOI). Neglecting the Zeeman term we show the existence of BSCs in the one-dimensional ring when the eigenstates of the closed ring are doubly degenerate. With account of the Zeeman term BSCs occur only at the points of threefold degeneracy. The BSCs are found in the parametric space of flux and RSOI strength in close pairs with opposite spin polarization. Thereby the spin polarization of electrons transmitted through the ring can be altered by minor variation of magnetic or electric field at the vicinity of these pairs. Numerical simulations of the two-dimensional open ring show similar results for the BSCs. Encircling the BSC points in the parametric space of the flux and the RSOI constant gives rise to a geometric phase.

Magnetic field-induced helical mode and topological transitions in a topological insulator nanoribbon

The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi2Te3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ∼2 μm. The conductance versus axial magnetic flux Φ exhibits Aharonov-Bohm oscillations with maxima occurring alternately at half-integer or integer flux quanta (Φ0 = h/e, where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector (kF) with period 2π/C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits kF-periodic oscillations, anti-correlated between Φ = 0 and Φ0/2. These oscillations enable us to probe the Bi2Te3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode.

Control of the spin geometric phase in semiconductor quantum rings

Since the formulation of the geometric phase by Berry, its relevance has been demonstrated in a large variety of physical systems. However, a geometric phase of the most fundamental spin-1/2 system, the electron spin, has not been observed directly and controlled independently from dynamical phases. Here we report experimental evidence on the manipulation of an electron spin through a purely geometric effect in an InGaAs-based quantum ring with Rashba spin-orbit coupling. By applying an in-plane magnetic field, a phase shift of the Aharonov-Casher interference pattern towards the small spin-orbit-coupling regions is observed. A perturbation theory for a one-dimensional Rashba ring under small in-plane fields reveals that the phase shift originates exclusively from the modulation of a pure geometric-phase component of the electron spin beyond the adiabatic limit, independently from dynamical phases. The phase shift is well reproduced by implementing two independent approaches, that is, perturbation theory and non-perturbative transport simulations.

The effect of (NH4)2Sx passivation on the (311)A GaAs surface and its use in AlGaAs/GaAs heterostructure devices

We have studied the efficacy of (NH4)2Sx surface passivation on the (311)A GaAs surface. We report XPS studies of simultaneously-grown (311)A and (100) heterostructures showing that the (NH4)2Sx solution removes surface oxide and sulfidizes both surfaces. Passivation is often characterized using photoluminescence measurements; we show that while (NH4)2Sx treatment gives a 40-60 ×  increase in photoluminescence intensity for the (100) surface, an increase of only 2-3 ×  is obtained for the (311)A surface. A corresponding lack of reproducible improvement in the gate hysteresis of (311)A heterostructure transistor devices made with the passivation treatment performed immediately prior to gate deposition is also found. We discuss possible reasons why sulfur passivation is ineffective for (311)A GaAs, and propose alternative strategies for passivation of this surface.

Spin transistor action from hidden Onsager reciprocity

We investigate generic Hamiltonians for confined electrons with weak inhomogeneous spin-orbit coupling. Using a local gauge transformation we show how the SU(2) Hamiltonian structure reduces to a U(1)×U(1) structure for spinless fermions in a fictitious orbital magnetic field, to leading order in the spin-orbit strength. Using an Onsager relation, we further show how the resulting spin conductance vanishes in a two-terminal setup, and how it is turned on by either weakly breaking time-reversal symmetry or opening additional transport terminals, thus allowing one to switch the generated spin current on or off. We numerically check our theory for mesoscopic cavities as well as Aharonov-Bohm rings.

Spin-to-charge conversion of mesoscopic spin currents

Recent theoretical investigations have shown that spin currents can be generated by passing electric currents through spin-orbit coupled mesoscopic systems. Measuring these spin currents has, however, not been achieved to date. We show how mesoscopic spin currents in lateral heterostructures can be measured with a single-channel voltage probe. In the presence of a spin current, the charge current I(qpc) through the quantum point contact connecting the probe is odd in an externally applied Zeeman field B, while it is even in the absence of spin current. Furthermore, the zero-field derivative ∂(B)I(qpc) is proportional to the magnitude of the spin current, with a proportionality coefficient that can be determined in an independent measurement. We confirm these findings numerically.

Weak localization in mesoscopic hole transport: berry phases and classical correlations

We consider phase-coherent transport through ballistic and diffusive two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show that intrinsic heavy-hole-light-hole coupling gives rise to clear-cut signatures of an associated Berry phase in the weak localization which renders the magnetoconductance profile distinctly different from electron transport. Nonuniversal classical correlations determine the strength of these Berry phase effects and the effective symmetry class, leading even to antilocalization-type features for circular quantum dots and Aharonov-Bohm rings in the absence of additional spin-orbit interaction. Our semiclassical predictions are confirmed by numerical calculations.

Magnetic forces and stationary electron flow in a three-terminal semiconductor quantum ring

We study stationary electron flow through a three-terminal quantum ring and describe effects due to deflection of electron trajectories by classical magnetic forces. We demonstrate that generally at high magnetic field (B) the current is guided by magnetic forces to follow a classical path, which for B > 0 leads via the left arm of the ring to the left output terminal. The transport to the left output terminal is blocked for narrow windows of magnetic field for which the interference within the ring leads to formation of wavefunctions that are only weakly coupled to the output channel wavefunctions. These interference conditions are accompanied by injection of the current to the right arm of the ring and by appearance of sharp peaks of the transfer probability to the right output terminal. We find that these peaks at high magnetic field are attenuated by thermal widening of the transport window. We also demonstrate that the interference conditions that lead to their appearance vanish when elastic scattering within the ring is present. The clear effect of magnetic forces on the transfer probabilities disappears along with Aharonov-Bohm oscillations in a chaotic transport regime that is found for rings whose width is larger than the width of the channels.

Spin interference and the Fano effect in electron transport through a mesoscopic ring side-coupled with a quantum dot

We investigate the electron transport through a mesoscopic ring side-coupled with a quantum dot (QD) in the presence of Rashba spin-orbit (SO) interaction. It is shown that both the Fano resonance and the spin interference effects play important roles in the electron transport properties. As the QD level is around the Fermi energy, the total conductance shows a typical Fano resonance line shape. By applying an electrical gate voltage to the QD, the total transmission through the system can be strongly modulated. By threading the mesoscopic ring with a magnetic flux, the time-reversal symmetry of the system is broken, and a spin polarized current can be obtained even though the incident current is unpolarized.

Electrical probing of the spin conductance of mesoscopic cavities

We investigate spin-dependent transport in multiterminal mesoscopic cavities with spin-orbit coupling. Focusing on a three-terminal set-up we show how injecting a pure spin current or a polarized current from one terminal generates additional charge current and/or voltage across the two output terminals. When the injected current is a pure spin current, a single measurement allows us to extract the spin conductance of the cavity. The situation is more complicated for a polarized injected current, and we show in this case how two purely electrical measurements on the output currents give the amount of current that is solely due to spin-orbit interaction. This allows us to extract the spin conductance of the device in this case as well. We use random matrix theory to show that the spin conductance of chaotic ballistic cavities fluctuates universally about zero mesoscopic average and describe experimental implementations of mesoscopic spin to charge current converters.