Colin Benjamin

Quantum spin pumping with adiabatically modulated magnetic barriers

A quantum pump device involving magnetic barriers produced by the deposition of ferromagnetic stripes on heterostructures is investigated. The device for dc transport does not provide spin-polarized currents, but in the adiabatic regime, when one modulates two independent parameters of this device, spin-up and spin-down electrons are driven in opposite directions, with the net result being that a finite net spin current is transported with negligible charge current. We also analyze our proposed device for inelastic scattering and spin-orbit scattering. Strong spin-orbit scattering and more so inelastic scattering have a somewhat detrimental effect on spin/charge ratio especially in the strong pumping regime. Further we show our pump to be almost noiseless, implying an optimal quantum spin pump.

Detecting entangled states in graphene via crossed Andreev reflection

Shot noise cross-correlations across single layer graphene structures are calculated with insulators separating a superconducting region. A new feature of specular crossed Andreev reflection comes into play due to the unique band structure of graphene. This gives rise to a rich structure in the states of the electric current flowing across the graphene sheet. We identified a parametric regime where {\em positive} shot noise cross-correlations of the current appear signifying entanglement. In contrast to previous proposals the sign of the cross-correlations can be easily tuned by the application of a gate voltage.

Detecting Majorana bound states

We propose a set of interferometric methods on how to detect Majorana bound states induced by a topological insulator. The existence of these states can be easily determined by the conductance oscillations as function of magnetic flux and/or electric voltage. We study the system in the presence and absence of Majorana bound states and observe strikingly different behaviors. Importantly, we show that the presence of coupled Majorana bound states can induce a persistent current in absence of any external magnetic field.

Wave attenuation to clock sojourn times.

The subject of time in quantum mechanics is of perennial interest especially because there is no observable for the time taken by a particle to transmit (or reflect) from a particular region. Several methods have been proposed based on scattering phase shifts and using different quantum clocks, where the time taken is clocked by some external input or indirectly from the phase of the scattering amplitudes. In this work we give a general method for calculating conditional sojourn times based on wave attenuation. In this approach clock mechanism does not couple to the Hamiltonian of the system. For simplicity, specific case of a delta dimer is considered in detail. Our analysis re-affirms recent results based on correcting quantum clocks using optical potential methods, albeit in a much simpler way.

Nonlocal pure spin current injection via quantum pumping and crossed Andreev reflection

A pure spin current injector is proposed based on adiabatic pumping and crossed normal and Andreev reflection. The device consists of a three-terminal ferromagnet-superconductor-semiconductor system in which the injection of a pure spin current is into the semiconductor which is coupled to the superconductor within a coherence length away from the ferromagnet enabling the phenomena of crossed normal/Andreev reflection to operate. Quantum pumping is induced by adiabatically modulating two independent parameters of the ferromagnetic lead, namely, the magnetization strength and the strength of coupling between the ferromagnet and the superconductor. The competition between the normal/Andreev reflection and the crossed normal/Andreev reflection, both induced by pumping, leads to nonlocal injection of a pure spin current into the semiconductor. The experimental realization of the proposed device is also discussed.

ROLE OF QUANTUM ENTANGLEMENT DUE TO A MAGNETIC IMPURITY ON CURRENT MAGNIFICATION EFFECT IN MESOSCOPIC OPEN RINGS

We study the current magnification effect in presence of exchange scattering of electron from a magnetic impurity placed in one arm of an open mesoscopic ring. The exchange interaction causes entanglement of electron spin and impurity spin. Earlier studies have shown that such an entanglement causes reduction or loss of interference in the Aharonov–Bohm oscillations leading to decoherence. We find however, that this entanglement, in contradiction to the naive expectation of a reduction in current magnification leads to enhancement as well as suppression of the effect. We also observe additional novel features like new resonances and current reversals.

Features in evanescent Aharonov-Bohm interferometry

In this work we analyze an Aharonov-Bohm interferometer in the tunneling regime. In this regime, a current magnification effect, which arises in presence of transport currents, is absent. A slight modification in the form of a quantum well incorporated in one of the arms leads to the revival of the current magnification. Systematics in magnetoconductance oscillations are observed in this evanescent wave geometry. In this framework we also see absence of Fano line shapes in transmission resonances but once again one can recover these if the direct path supports propagating modes.

Dephasing via stochastic absorption: A case study in Aharonov-Bohm oscillations

The Aharonov-Bohm ring has been the mainstay of mesoscopic physics research since its inception. In this paper we deal with the problem of dephasing of Aharonov-Bohm (AB) oscillations using a phenomenological model based on stochastic absorption. To calculate the conductance in the presence of inelastic scattering we have used the method due to Brouwer and Beenakker. We have shown that conductance is symmetric under flux reversal and visibility of AB oscillations decay to zero as a function of the incoherence parameter thus signaling dephasing in the system. Some comments are made on the relative merits of stochastic absorption with respect to optical potential model, which have been used to mimic dephasing.

Current magnification effect in mesoscopic systems at equilibrium

We study the current magnification effect and associated circulating currents in mesoscopic systems at equilibrium. Earlier studies have revealed that in the presence of transport current (nonequilibrium situation), circulating currents can flow in a ring even in the absence of magnetic field. This was attributed to current magnification that is quantum mechanical in origin. We have shown that the same effect can be obtained in equilibrium systems, however, in the presence of magnetic flux. For this we have considered an one-dimensional open mesoscopic ring connected to a bubble, and the system is in contact with a single reservoir. We have considered a special case where bubble does not enclose magnetic flux, yet circulating currents can flow in it due to current magnification.

Strained-graphene-based highly efficient quantum heat engine operating at maximum power

A strained graphene monolayer is shown to operate as a highly efficient quantum heat engine delivering maximum power. The efficiency and power of the proposed device exceeds that of recent proposals. The reason for these excellent characteristics is that strain enables complete valley separation in transmittance through the device, implying that increasing strain leads to very high Seebeck coefficient as well as lower conductance. In addition, since time-reversal symmetry is unbroken in our system, the proposed strained graphene quantum heat engine can also act as a high-performance refrigerator.