arXiv subjects

Colin Benjamin

Publications and source records attributed to Colin Benjamin.

At least 73 records · Page 4Linked to original sources

arXiv2019

Designing a highly efficient graphene quantum spin heat engine

We design a quantum spin heat engine using spin polarized ballistic modes generated in a strained graphene monolayer doped with a magnetic impurity. We observe remarkably large efficiency and large thermoelectric figure of merit both for the charge as well as spin variants of the quantum heat engine. This suggests the use of this device as a highly efficient quantum heat engine for charge as well as spin-based transport. Further, a comparison is drawn between the device characteristics of a graphene spin heat engine against a quantum spin Hall heat engine. The reason being edge modes because of their origin should give much better performance. In this respect, we observe our graphene-based spin heat engine can almost match the performance characteristics of a quantum spin Hall heat engine. Finally, we show that a pure spin current can be transported in our device in absence of any charge current.

cond-mat.mes-hall↗

arXiv2019

Quantum Nash equilibrium in the thermodynamic limit

The quantum Nash equilibrium in the thermodynamic limit is studied for games like quantum Prisoner's dilemma and the quantum game of chicken. A phase transition is seen in both games as a function of the entanglement in the game. We observe that for maximal entanglement irrespective of the classical payoffs, a majority of players choose Quantum strategy over Defect in the thermodynamic limit.

quant-ph↗

arXiv2019

Spin flip scattering engendered quantum spin torque in a Josephson junction

We examine a Josephson junction with two Ferromagnets and a spin flipper sandwiched between two superconductors. In such Ferromagnetic Josephson junctions, equilibrium spin torque exists only when Ferromagnets are misaligned. This is explained via the "conventional" mechanism of spin transfer torque, which owes its origin to the misalignment of two Ferromagnets. However, we see surprisingly when the magnetic moments of the Ferromagnets are aligned parallel or antiparallel, there is a finite equilibrium spin torque due to the quantum mechanism of spin-flip scattering. We explore the properties of this unique spin-flip scattering induced equilibrium quantum spin torque, especially its tunability via exchange coupling and phase difference across the superconductors.

cond-mat.supr-con↗

arXiv2019

Entanglement renders free riding redundant in the thermodynamic limit

The free rider problem is one of the most well-studied problems in economics. The solution proposed mainly is punitive in order to deter people from free riding. In this work, we introduce quantum strategies and study the problem in the thermodynamic limit by drawing analogies with the 1D Ising model. We observe that for maximum entanglement, irrespective of the payoffs, quantum strategy is the equilibrium solution, solving the free rider problem.

quant-ph↗

arXiv2018

Are thermal fluctuations the sole reason for finite longitudinal resistance in quantum anomalous Hall experiments?

In some recent experiments [A. J. Bestwick, et. al., Phys. Rev. Lett. 114, 187201 (2015), Cui-Zu Chang, et. al., Nat. Materials. 14, 473-477 (2015)] it has been shown that in observations of the quantum anomalous Hall (QAH) effect the longitudinal resistance $R_L$ increases as temperature $T$ increases, while Hall resistance $R_H$ loses its quantization with increase in $T$. This behavior was explained due to increased thermal fluctuations as $T$ increases. We show that similar effects arise in QAH samples with quasi-helical edge modes as disorder increases in presence of inelastic scattering or otherwise even at temperature $T=0$.

cond-mat.mes-hall↗

arXiv2018

Yu-Shiba-Rusinov bound states induced by a spin flipper in the vicinity of a s-wave superconductor

We theoretically study the formation and characteristics of Yu-Shiba-Rusinov bound states within the superconducting gap using a BTK approach in presence of a spin flipper (high spin magnetic impurity). We focus on the zero energy in the conductance spectra and show how a peak is formed at $E=0$ due to the flipping of the magnetic impurity spin but for no flip case a dip forms at $E=0$ in the conductance spectra. This $E=0$ conductance peak is almost quantized at $2e^2/h$ values, however, it arises due to non-topological reasons in contrast to the $E=0$ peak formed due to Majorana states.

cond-mat.supr-con↗

arXiv2018

Playing a true Parrondo's game with a three state coin on a quantum walk

Playing a Parrondo's game with a qutrit is the subject of this paper. We show that a true quantum Parrondo's game can be played with a 3 state coin(qutrit) in a 1D quantum walk in contrast to the fact that playing a true Parrondo's game with a 2 state coin(qubit) in 1D quantum walk fails in the asymptotic limits.

quant-ph↗

arXiv2018

Characterizing a high spin magnetic impurity via Andreev reflection spectroscopy

The ground state properties of a high spin magnetic impurity and its interaction with an electronic spin are probed via Andreev reflection. We see that through the charge and spin conductance one can effectively estimate the interaction strength, the ground state spin and magnetic moment of any high spin magnetic impurity. We show how a high spin magnetic impurity at the junction between a normal metal and superconductor can contribute to superconducting spintronics applications. Particularly, while spin conductance is absent below the gap for Ferromagnet-Insulator-Superconductor junctions we show that in the case of a Normal metal-High spin magnetic impurity-Normal Metal-Insulator-Superconductor (NMNIS) junction it is present. Further, it is seen that pure spin conduction can exist without any accompanying charge conduction in the NMNIS junction.

cond-mat.mes-hall↗

arXiv2018

Tuning the 0$-π$ Josephson junction with a magnetic impurity: Role of tunnel contacts, exchange coupling, $e-e$ interactions and high-spin states

We propose Josephson junction with a high-spin magnetic impurity sandwiched between two superconductors. This system shows a $π$ junction behavior as a function of the spin magnetic moment state of the impurity, the interface transparency, exchange coupling and electron-electron interactions in the system. The system is theoretically analyzed for possible reason behind the $π$ shift. The crucial role of spin flip scattering is highlighted. Possible applications in quantum computation of our proposed tunable high spin magnetic impurity $π$ junction is underscored.

cond-mat.supr-con↗

arXiv2018

Helical thermoelectrics and refrigeration

The thermoelectric properties of a three terminal quantum spin Hall (QSH) sample are examined. Inherent helicity of the QSH sample helps to generate a large charge power efficiently. Along with charge the system can be designed to work as a highly efficient spin heat engine too. The advantage of a helical over a chiral sample is that, while a multiterminal quantum Hall sample can only work as a quantum heat engine due to broken time reversal(TR) symmetry, a multiterminal QSH system can work effectively both as a charge/spin heat engine as well as a charge/spin refrigerator as TR symmetry is preserved.

cond-mat.mes-hall↗

arXiv2018

Implementing Parrondo's paradox with two coin quantum walks

Parrondo's paradox is ubiquitous in games, ratchets and random walks.The apparent paradox, devised by J.~M.~R.~Parrondo, that two losing games $A$ and $B$ can produce an winning outcome has been adapted in many physical and biological systems to explain their working. However, proposals on demonstrating Parrondo's paradox using quantum walks failed {for large number of steps}. In this work, we show that instead of a single coin if we consider a two coin initial state which may or may not be entangled, we can observe a genuine Parrondo's paradox with quantum walks. Further we focus on reasons for this and pin down the asymmetry in initial two-coin state or asymmetry in shift operator, either of which are necessary for observing a genuine Parrondo's paradox. We extend our work to a 3-coin initial state too with similar results. The implications of our work for observing quantum ratchet like behavior using quantum walks is also discussed.

quant-ph↗

arXiv2017

Role of helical edge modes in the chiral quantum anomalous Hall state

Although indications are that a single chiral quantum anomalous Hall(QAH) edge mode might have been experimentally detected. There have been very many recent experiments which conjecture that a single chiral QAH edge mode always materializes along with a pair of quasi-helical quantum spin Hall (QSH) edge modes. The reason for this seems to lie in the origin of QAH edge modes. These evolve from QSH edge modes via suppression of one of the spin edge modes by application of a ferromagnet or magnetic impurity. In this work we deal with a substantial 'What If ?' question- in case the QSH edge modes, from which these QAH edge modes evolve, are not topologically protected then the QAH edge modes wont be topologically protected too and thus unfit for use in any applications. Further, as a corollary one can also ask if the topological protection of QSH edge modes does not carry over during the evolution process to QAH edge modes then again our 'What if?' scenario becomes apparent. The "how" of the resolution of this 'What if?' conundrum is the main objective of our work. We show in similar set-ups affected by disorder and inelastic scattering, transport via trivial QAH edge mode leads to quantization of Hall resistance and not that via topological QAH edge modes. This perhaps begs a substantial reinterpretation of those experiments which purported to find signatures of chiral(topological) QAH edge modes albeit in conjunction with quasi helical QSH edge modes.

cond-mat.mes-hall↗

arXiv2017

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.

cond-mat.mes-hall↗

arXiv2017

Probing helicity and the topological origins of helicity via non-local Hanbury-Brown and Twiss correlations

Quantum Hall edge modes are chiral while quantum spin Hall edge modes are helical. However, unlike chiral edge modes which always occur in topological systems, quasi-helical edge modes may arise in a trivial insulator too. These trivial quasi-helical edge modes are not topologically protected and therefore need to be distinguished from helical edge modes arising due to topological reasons. Earlier conductance measurements were used to identify these helical states, in this work we report on the advantage of using the non local shot noise as a probe for the helical nature of these states as also their topological or otherwise origin and compare them with chiral quantum Hall states. We see that in similar set-ups affected by same degree of disorder and inelastic scattering, non local shot noise "HBT" correlations can be positive for helical edge modes but are always negative for the chiral quantum Hall edge modes. Further, while trivial quasi-helical edge modes exhibit negative non-local "HBT" charge correlations, topological helical edge modes can show positive non-local "HBT" charge correlation. We also study the non-local spin correlations and Fano factor for clues as regards both the distinction between chirality/helicity as well as the topological/trivial dichotomy for helical edge modes.

cond-mat.mes-hall↗

arXiv2016

A scheme to realize the quantum spin-valley Hall effect in monolayer graphene

Quantum spin Hall effect was first predicted in graphene. However, the weak spin orbit interaction in graphene meant that the search for quantum spin Hall effect in graphene never fructified. In this work we show how to generate the quantum spin-valley Hall effect in graphene via quantum pumping by adiabatically modulating a magnetic impurity and an electrostatic potential in a monolayer of strained graphene. We see that not only exclusive spin polarized currents can be pumped in the two valleys in exactly opposite directions but one can have pure spin currents flowing in opposite directions in the two valleys, we call this novel phenomena the quantum spin-valley Hall effect. This means that the twin effects of quantum valley Hall and quantum spin Hall can both be probed simultaneously in the proposed device. This work will significantly advance the field of graphene spintronics, hitherto hobbled by the lack of spin-orbit interaction. We obviate the need for any spin orbit interaction and show how graphene can be manipulated to posses features exclusive to topological insulators.

cond-mat.mes-hall↗

arXiv2016

Topologically induced fractional Hall steps in the integer quantum Hall regime of $MoS_2$

The quantum magnetotransport properties of a monolayer of molybdenum disulfide are derived using linear response theory. Especially, the effect of topological terms on longitudinal and Hall conductivity is analyzed. The Hall conductivity exhibits fractional steps in the integer quantum Hall regime. Further complete spin and valley polarization of the longitudinal conductivity is seen in presence of these topological terms. Finally, the Shubnikov-de Hass oscillations are suppressed or enhanced contingent on the sign of these topological terms.

cond-mat.mes-hall↗

arXiv2016

Fragility of non-local edge mode transport in the quantum spin Hall state

Non-local currents and voltages are better able at withstanding the deleterious effects of dephasing than local currents and voltages in nanoscale systems. This hypothesis is known to be true in quantum Hall set-ups. We test this hypothesis in a four terminal quantum spin Hall set up wherein we compare the local resistance measurement with the non-local one. In addition to inelastic scattering induced dephasing we also test resilience of the resistance measurements in the aforesaid set-ups to disorder and spin-flip scattering. We find the axiom that non-local resistance is less affected by the detrimental effects of disorder and dephasing to be in general untrue for quantum spin Hall case. This has important consequences since it has been widely communicated that non-local transport through edge channels in topological insulators will have potential applications in low power information processing.

cond-mat.mes-hall↗

arXiv2016

Are quantum spin Hall edge modes more resilient to disorder, sample geometry and inelastic scattering than quantum Hall edge modes?

On the surface of 2D Topological insulators occur 1D quantum spin Hall(QSH) edge modes with Dirac like dispersion. Unlike quantum Hall(QH) edge modes which occur at high magnetic fields in 2DEGs, the occurrence of QSH edge modes is because of spin-orbit scattering in the bulk of the material. These QSH edge modes are spin dependent and chiral- opposite spins move in opposing directions. Electronic spin has larger decoherence and relaxation time than charge- in view of this its expected that QSH edge modes will be more robust to disorder and inelastic scattering than QH edge modes which are charge dependent and spin unpolarized. However, we notice no such advantage accrues to QSH edge modes when subjected to same degree of contact disorder and/or inelastic scattering in similar setups as QH edge modes. In fact we observe that QSH edge modes are more susceptible to inelastic scattering and contact disorder than QH edge modes. Further, while a single disordered contact has no effect on QH edge modes it leads to a finite charge Hall current in case of quantum spin Hall edge modes and thus vanishing of pure quantum spin Hall effect. For more than a single disordered contact while quantum Hall states continue to remain immune to disorder, quantum spin Hall edge modes become more susceptible- the Hall resistance for quantum spin Hall effect changes sign with increasing disorder. In case of many disordered contacts with inelastic scattering included while quantization of Hall edge modes holds, for quantum spin Hall edge modes- a finite charge Hall current still flows. For quantum spin Hall edge modes in the inelastic scattering regime we distinguish between two cases: with spin-flip and without spin-flip scattering. Finally, while asymmetry in sample geometry can have a deleterious effect on quantum spin Hall case it has no impact in quantum Hall case.

cond-mat.mes-hall↗