arXiv subjects

Colin Benjamin

Publications and source records attributed to Colin Benjamin.

At least 55 records · Page 3Linked to original sources

arXiv2022

Probing the topological character of superconductors via non-local Hanbury-Brown and Twiss correlations

Superconductors can be classified as topological or not based on whether time-reversal symmetry (TRS), chiral symmetry, and particle-hole symmetry are preserved or not. Further, topological superconductors can also be classified as chiral or helical. In this paper, using Hanbury-Brown and Twiss (HBT) shot noise correlations and the non-local conductance, we probe metal/2D unconventional superconductor/metal junctions to understand better the pairing topological vs. non-topological or helical vs. chiral or nodal vs. gapful. We see that HBT correlations are asymmetric as a function of bias voltage for non-topological superconductors, whereas they are symmetric for topological superconductors irrespective of the barrier strength. Topological superconductors are associated with Majorana fermions which are important for topological quantum computation. By distinguishing topological superconductors from non-topological superconductors, our study will help search for Majorana fermions, which will aid in designing a topological quantum computer.

cond-mat.mes-hall↗

arXiv2022

Josephson quantum spin thermodynamics

A 1D Josephson junction loop, doped with a spin-flipper and attached to two thermal reservoirs, operates as a heat engine or a refrigerator, a Joule pump, or even a cold pump. When operating as a quantum heat engine, the efficiency of this device exceeds that of some recent Josephson heat engine proposals. Further, as a quantum refrigerator, the coefficient of performance of this device is much higher than previously proposed Josephson junction-based refrigerators. In addition, this device can be tuned from engine mode to refrigerator mode or any other mode, i.e., Joule pump or cold pump, by either tuning the temperature of reservoirs or via the flux enclosed in the Josephson junction loop. In the presence of spin-flip scattering, we can tune our device from engine mode to other operating modes by only changing the enclosed flux in the Josephson junction loop without changing the temperatures of the reservoirs. This is potentially an advantage with respect to other proposals. This makes the proposed device much more versatile as regards possible applications.

cond-mat.stat-mech↗

arXiv2021

Testing quantum speedups in exciton transport through a photosynthetic complex using quantum stochastic walks

Photosynthesis is a highly efficient process, nearly 100 percent of the red photons falling on the surface of leaves reach the reaction center and get transformed into energy. Most theoretical studies on photosynthetic complexes focus mainly on the Fenna-Matthews-Olson complex obtained from green-sulfur bacteria. Quantum coherence was speculated to play a significant role in this very efficient transport process. However, recent reports indicate quantum coherence via exciton transport may not be as relevant as coherence originating via vibronic processes to Photosynthesis. Regardless of the origin, there has been a debate on whether quantum coherence results in any speedup of the exciton transport process. To address this we model exciton transport in FMO using a quantum stochastic walk (QSW) with only incoherence, pure dephasing, and with both dephasing and incoherence. We find that the QSW model with pure dephasing leads to a substantial speedup in exciton transport as compared to a QSW model which includes both dephasing and incoherence and one which includes only incoherence, both of which experience slowdowns.

cond-mat.stat-mech↗

arXiv2021

A thermodynamic probe of the topological phase transition in epitaxial graphene based Floquet topological insulator

One can use light to tune certain materials, from a trivial to a topological phase. A prime example of such materials, classified as Floquet topological insulators (FTI), is epitaxial graphene. In this paper, we probe the topological phase transition of an FTI via the efficiency and work output of quantum Otto and quantum Stirling heat engines. A maximum/minimum in the efficiency or work output invariably signals the phase transition point. Further, both engines' work output and efficiency are markedly robust against the polarization direction of light.

cond-mat.mes-hall↗

arXiv2021

Magic angle twisted bilayer graphene as a highly efficient quantum Otto engine

At a discrete set of magic angles, twisted bilayer graphene has been shown to host extraordinarily flat bands, correlated insulating states, unconventional superconductivity, and distinct Landau level degeneracies. In this work, we design a highly efficient quantum Otto engine using a twisted bilayer graphene sample. Flat bands, which occur at magic angles, make the prospect of extracting useful work from our Otto engine lucrative. We use an eight-band continuum model of twisted bilayer graphene to compute efficiencies and work outputs for magic and non-magic angle twists, and compare the results with an $AB$ stacked bilayer and a monolayer. It is observed that the efficiency varies smoothly with the twist angle and the maximum is attained at the magic angle.

cond-mat.mes-hall↗

arXiv2021

Is the essence of a quantum game captured completely in the original classical game?

S. J. van Enk and R. Pike in PRA 66, 024306 (2002) argue that the equilibrium solution to a quantum game isn't unique but is already present in the classical game itself. In this work, we contest this assertion by showing that a random strategy in a particular quantum (Hawk-Dove) game is unique to the quantum game. In other words, one cannot obtain the equilibrium solution of the quantum Hawk-Dove game in the classical Hawk-Dove game. Moreover, we provide an analytical solution to the quantum $2\times2$ strategic form Hawk-Dove game using randomly mixed strategies. The random strategy which we describe is Pareto optimal with their payoff classically unobtainable. We compare quantum strategies to correlated strategies and find that correlated strategies in the quantum Hawk-Dove game or quantum Prisoner's dilemma yield the Nash equilibrium solution.

quant-ph↗

arXiv2021

Exciting odd frequency equal-spin-triplet correlations at metal-superconductor interfaces

We predict the occurrence of odd frequency equal-spin-triplet correlations at a normal metal-superconductor junction. This result is significant because equal-spin-triplet correlations are associated with the presence of dissipation-less pure spin current. Inserting a spin-flipper at the interface of a normal metal-superconductor junction excites equal-spin-triplet correlations. The existence of odd frequency equal-spin-triplet correlations in absence of odd frequency mixed spin-triplet correlations is the main take-home message of this work. It tallies well with the measured local magnetization density of states and spin-polarized local density of states at the interface. The importance of spin-flip scattering to the obtained results is manifest when we compare our normal metal-spin flipper-superconductor junction to other hybrid junctions where either only spin mixing or both spin mixing and spin-flip scattering are present.

cond-mat.supr-con↗

arXiv2021

Order from chaos in quantum walks on cyclic graphs

It has been shown classically that combining two chaotic random walks can yield an ordered(periodic) walk. Our aim in this paper is to find a quantum analog for this rather counter-intuitive result. We study chaotic and periodic nature of cyclic quantum walks and focus on a unique situation wherein a periodic quantum walk on a 3-cycle graph is generated via a deterministic combination of two chaotic quantum walks on the same graph. We extend our results to even-numbered cyclic graphs, specifically a 4-cycle graph too. Our results will be relevant in quantum cryptography and quantum chaos control.

quant-ph↗

arXiv2021

Entanglement and quantum strategies reduce congestion costs in Pigou networks

Pigou's problem has many applications in real life scenarios like traffic networks, graph theory, data transfer in internet networks, etc. The two player classical Pigou's network has an unique Nash equilibrium with the Price of Stability and Price of Anarchy agreeing with each other. The situation changes for the $k-$person classical Pigou's network with $n$ being the total number of people. If we fix the behaviour of $(n-2)$ people and assume that $k-$persons take path $P_2$ where $k<(n-2)$ and the remaining take path $P_1$, the minimum cost of Nash equilibrium becomes $k$ dependent and we find a particular $k$ for which the cost is an absolute minimum. In contrast to the two person classical Pigou's network, the quantum two qubit Pigou's network with maximal entanglement gives a lower cost for the Nash equilibrium, while in contrast to $k-$person classical Pigou's network, it's quantum version gives reduced cost for the Nash equilibrium strategy. This has major implications for information transfer in both classical as well as quantum data networks. By employing entanglement and quantum strategies, one can significantly reduce congestion costs in quantum data networks.

quant-ph↗

arXiv2020

Shot Noise as a probe for the pairing symmetry of Iron pnictide superconductors

One of the outstanding problems in Iron pnictide research is the unambiguous detection of its pairing symmetry. The most probable candidates are the two-band s$++$ and sign reversed s$\pm$ wave pairing. In this work, the Andreev conductance and shot noise are used as a probe for the pairing symmetry of Iron pnictide superconductors. Clear differences emerge in both the zero bias differential conductance and the shot noise in the tunneling limit for the two cases enabling an effective distinction between the two.

cond-mat.supr-con↗

arXiv2020

Thermodynamic susceptibility as a measure of cooperative behavior in social dilemmas

The emergence of cooperation in the thermodynamic limit of social dilemmas is an emerging field of research. While numerical approaches (using replicator dynamics) are dime a dozen, analytical approaches are rare. A particularly useful analytical approach is to utilize a mapping between the spin-1/2 Ising model in 1-D and the social dilemma game and calculate the magnetization, which is the net difference between the fraction of cooperators and defectors in a social dilemma. In this paper, we look at the susceptibility, which probes the net change in the fraction of players adopting a certain strategy, for both classical and quantum social dilemmas. The reason being, in statistical mechanics problems, the thermodynamic susceptibility as compared to magnetization is a more sensitive probe for microscopic behavior, e.g., observing small changes in a population adopting a certain strategy. In this paper, we find the thermodynamic susceptibility for reward, sucker's payoff and temptation in classical Prisoner's Dilemma to be positive, implying that the turnover from defect to cooperate is greater than vice-versa, although the Nash Equilibrium for the two-player game is to defect. In classical Hawk-Dove game, the thermodynamic susceptibility for resource suggests that the number of players switching to Hawk from Dove strategy is dominant. Entanglement in Quantum Prisoner's Dilemma (QPD) has a non-trivial role in determining the behavior of thermodynamic susceptibility. At maximal entanglement, we find that sucker's payoff and temptation increase the number of players switching to defect. In the zero-temperature limit, we find that there are two second-order phase transitions in the game, marked by a divergence in the susceptibility. This behavior is similar to that seen in Type-II superconductors wherein also two second-order phase transitions are seen.

physics.soc-ph↗

arXiv2020

Stability of Majorana bound states in the presence of spin-flip scattering

A popular evidence of the existence of Majorana bound states(MBS) is a quantized zero-bias conductance peak(ZBCP) which is robust to scattering by impurities, a consequence of its topological protection. In this work we examine the stability of this MBS induced ZBCP in a metal-superconductor junction in the vicinity of a spin flipper. We analytically calculate the differential charge conductance for metal-spin flipper-superconductor junction with two distinct superconductors: (a) spin less $p$-wave superconductor(pSc) and (b) spin-orbit-coupled s-wave superconducting wire in presence of a Zeeman field(SOCSW). We see that the quantized ZBCP remains stable in presence of spin-flip(SF) scattering for metal-pSc junction, while it loses its stability when pSc is replaced by SOCSW. Further, the scattering matrix(S-Matrix) of the metal-pSc junction satisfies BDI symmetry class regardless of SF scattering. For BDI symmetry class, both Hamiltonian as well as S-Matrix satisfy particle-hole(PH), time reversal(TR) and chiral symmetries. However, in case of metal-SOCSW junction the Hamiltonian as well as S-Matrix belongs to symmetry class D in absence of SF scattering. In symmetry class D both Hamiltonian and S-Matrix satisfy PH symmetry, but do not satisfy TR and chiral symmetry relations. In presence of SF scattering, the S-Matrix for metal-SOCSW junction belongs to symmetry class A for which S-Matrix does not satisfy either PH or TR or chiral symmetry relations. The reason for ZBCP at a metal-pSc junction is perfect Andreev reflection regardless of SF scattering, while for metal-SOCSW junction it is the exact cancellation between normal and Andreev reflection probabilities at zero bias and not perfect Andreev reflection, in absence of SF scattering. However, in presence of SF scattering, there is no exact cancellation at zero bias which leads to loss of quantized ZBCP for a metal-SOCSW junction.

cond-mat.mes-hall↗

arXiv2020

Emergence of Cooperation in the thermodynamic limit

Predicting how cooperative behavior arises in the thermodynamic limit is one of the outstanding problems in evolutionary game theory. For two player games, cooperation is seldom the Nash equilibrium. However, in the thermodynamic limit cooperation is the natural recourse regardless of whether we are dealing with humans or animals. In this work, we use the analogy with the Ising model to predict how cooperation arises in the thermodynamic limit.

physics.soc-ph↗

arXiv2019

Disordered contacts can localize chiral edge electrons

Chiral integer quantum Hall (QH) edge modes are immune to backscattering and therefore are non-localized and show a vanishing longitudinal as well as non-local resistance along with quantized 2-terminal and Hall resistance even in the presence of sample disorder. However, this is not the case for contact disorder, which refers to the possibility that a contact can reflect edge modes either partially or fully. This paper shows that when all contacts are disordered in a N-terminal quantum Hall bar, then transport via chiral QH edge modes can have a significant localization correction. The Hall and 2-terminal resistance in an N-terminal quantum Hall sample deviate from their values derived while neglecting the phase acquired at disordered contacts, and this deviation is called the quantum localization correction. This correction term increases with the increase of disorderedness of contacts but decreases with the increase in the number of contacts in an N-terminal Hall bar. The presence of inelastic scattering, however, can completely destroy the quantum localization correction.

cond-mat.mes-hall↗

arXiv2019

Optimal quantum refrigeration in strained graphene

Refrigerators soak up heat energy from a low-temperature region and dump it into a higher temperature region using external work done on the system. Refrigerators are useful in cooling down a system to very low temperatures. In this letter, we show that a monolayer of strained graphene can be used in designing a quantum refrigerator with an excellent coefficient of performance and large cooling power. The operating point at which the strained graphene device works best as a quantum refrigerator is derived and the effects of strain and temperature on cooling are discussed.

cond-mat.mes-hall↗

arXiv2019

Quantized Josephson phase battery

A ferromagnetic Josephson junction with a spin-flipper (magnetic impurity) sandwiched in-between acts as a phase battery that can store quantized amounts of superconducting phase difference $Φ_0$ in the ground state of the junction. Moreover, for such $Φ_0$-Josephson junction anomalous Josephson current appears at zero phase difference. We study the properties of this quantum spin-flip scattering induced anomalous Josephson current, especially its tun-ability via misorientation angle between two Ferromagnets.

cond-mat.supr-con↗

arXiv2019

Disordered contacts can localize helical edge electrons

It is well known that quantum spin Hall (QSH) edge modes being helical are immune to backscattering due to non-magnetic disorder within the sample. Thus, quantum spin Hall edge modes are non-localized and show a vanishing Hall resistance along with quantized 2-terminal, longitudinal and non-local resistances even in presence of sample disorder. However, this is not the case for contact disorder. This paper shows that when all contacts are disordered in an N-terminal quantum spin Hall sample, then transport via these helical QSH edge modes can have a significant localization correction. All the resistances in an N-terminal quantum spin Hall sample deviate from their values derived while neglecting the phase acquired at disordered contacts, and this deviation is called the quantum localization correction. This correction term increases with the increase of disorderedness of contacts but decreases with the increase in the number of contacts in an N terminal sample. The presence of inelastic scattering, however, can completely destroy the quantum localization correction.

cond-mat.mes-hall↗

arXiv2019

Triggers for cooperative behavior in the thermodynamic limit: a case study in Public goods game

In this work, we aim to answer the question: what triggers cooperative behavior in the thermodynamic limit by taking recourse to the Public goods game. Using the idea of mapping the 1D Ising model Hamiltonian with nearest neighbor coupling to payoffs in the game theory we calculate the Magnetisation of the game in the thermodynamic limit. We see a phase transition in the thermodynamic limit of the two player Public goods game. We observe that punishment acts as an external field for the two player Public goods game triggering cooperation or provide strategy, while cost can be a trigger for suppressing cooperation or free riding. Finally, reward also acts as a trigger for providing while the role of inverse temperature (fluctuations in choices) is to introduce randomness in strategic choices.

physics.soc-ph↗