arXiv subjects

Arijit Kundu

Publications and source records attributed to Arijit Kundu.

49 records · Page 3Linked to original sources

arXiv2017

Transport through Andreev bound states in a Weyl semimetal quantum dot

We study transport through a Weyl semimetal quantum dot sandwiched between an $s$-wave superconductor and a normal lead. The conductance peaks at regular intervals and exhibits double periodicity with respect to two characteristic frequencies of the system, one that originates from Klein tunneling in the system and the other coming from the chiral nature of the excitations. Using a scattering matrix approach as well as a lattice simulation, we demonstrate the universal features of the conductance through the system and discuss the feasibility of observing them in experiments.

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arXiv2017

$0$-$π$ transitions in a Josephson junction of irradiated Weyl semimetal

We propose a setup for the experimental realization of unexpected and anisotropic $0$-$π$ transitions of the Josephson current, in a junction whose link is made of irradiated Weyl semi-metal (WSM), due to the presence of chiral nodes. We show using a Green's function technique that the Josephson current through a time-reversal symmetric WSM has anisotropic periodic oscillations as a function of $k_0L$, where $k_0$ is the (relevant) separation of the chiral nodes and $L$ is the length of the sample. This is unexpected, for time-reversal symmetric systems, although it has been shown earlier for time-reversal symmetry breaking WSMs. We then show that the effective value of $k_0$ can be tuned with precision by irradiating the sample with photons resulting in $0$-$π$ transitions of the critical current. Finally, we discuss the robustness of our setup.

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arXiv2016

Brillouin-Wigner Theory for Floquet Topological Phase Transitions in Spin-orbit Coupled Materials

We develop the high frequency expansion based on the Brillouin-Wigner (B-W) perturbation theory for driven systems with spin-orbit coupling which is applicable to the cases of silicene, germanene and stanene. We compute the effective Hamiltonian in the zero photon subspace not only to order $O(ω^{-1})$, but by keeping all the important terms to order $O(ω^{-2})$, and obtain the photo-assisted correction terms to both the hopping and the spin-orbit terms, as well as new longer ranged hopping terms. We then use the effective static Hamiltonian to compute the phase diagram in the high frequency limit and compare it with the results of direct numerical computation of the Chern numbers of the Floquet bands, and show that at sufficiently large frequencies, the B-W theory high frequency expansion works well even in the presence of spin-orbit coupling terms.

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arXiv2016

Chiral nodes and oscillations in the Josephson current in Weyl semimetals

The separation of the Weyl nodes in a broken time-reversal symmetric Weyl semimetal leads to helical quasi-particle excitations at the Weyl nodes, which, when coupled with overall spin conservation allows only inter-nodal transport at the junction of the Weyl semimetal with a superconductor. This leads to an unusual periodic oscillation in the Josephson current as a function of $k_0L$, where $L$ is the length of the Weyl semimetal and $2k_0$ is the inter-nodal distance. This oscillation is robust and should be experimentally measurable, providing a direct path to confirming the existence of chiral nodes in the Weyl semimetal.

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arXiv2015

Floquet-Engineered Valleytronics in Dirac Systems

Valley degrees of freedom offer a potential resource for quantum information processing if they can be effectively controlled. We discuss an optical approach to this problem in which intense light breaks electronic symmetries of a two-dimensional Dirac material. The resulting quasienergy structures may then differ for different valleys, so that the Floquet physics of the system can be exploited to produce highly polarized valley currents. This physics can be utilized to realize a valley valve whose behavior is determined optically. We propose a concrete way to achieve such valleytronics in graphene as well as in a simple model of an inversion-symmetry broken Dirac material. We study the effect numerically and demonstrate its robustness against moderate disorder and small deviations in optical parameters.

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arXiv2014

Proximity induced superconductivity in Weyl semi-metals

We introduce superconducting proximity effects in Weyl semi-metals (WSM) with broken time reversal symmetry by tunnel coupling one of its surfaces to an s-wave superconductor using the Green's function approach. We find that the band structure develops coherence peaks, but despite the presence of metallic states in the bulk, the coherence peaks do not extend far into the bulk and remain confined to a few layers close to the interface, similar to the proximity effect in the topological insulators (TI) which are gapped in the bulk. The Weyl nodes remain unaffected, and in that sense, no true gap develops. We also study the various induced p and s-wave pairing amplitudes classified by their symmetries, as a function of the various relevant parameters of the theory and note the exponential decay of the induced pairings in the bulk both in the TI and the WSM, even at finite chemical potential.

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arXiv2014

Effective Theory of Floquet Topological Transitions

We develop a theory of topological transitions in a Floquet topological insulator, using graphene irradiated by circularly polarized light as a concrete realization. We demonstrate that a hallmark signature of such transitions in a static system, i.e. metallic bulk transport with conductivity of order $e^2/h$, is substantially suppressed at some Floquet topological transitions in the clean system. We determine the conditions for this suppression analytically and confirm our results in numerical simulations. Remarkably, introducing disorder dramatically enhances this transport by several orders of magnitude.

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arXiv2014

Tunable Floquet Majorana Fermions in Driven Coupled Quantum Dots

We propose a system of coupled quantum dots in proximity to a superconductor and driven by separate ac potentials to realize and detect Floquet Majorana fermions. We show that the appearance of Floquet Majorana fermions can be finely controlled in the expanded parameter space of the drive frequency, amplitude, and phase difference across the two dots. While these Majorana fermions are not topologically protected, the highly tunable setup provides a realistic system for observing the exotic physics associated with Majorana fermions as well as their dynamical generation and manipulation.

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arXiv2013

Transport signatures of Floquet Majorana fermions in driven topological superconductors

Floquet Majorana fermions are steady states of equal superposition of electrons and holes in a periodically driven superconductor. We study the experimental signatures of Floquet Majorana fermions in transport measurements and show, both analytically and numerically, that their presence is signaled by a quantized conductance sum rule over discrete values of lead bias differing by multiple absorption or emission energies at drive frequency. We also study the effects of static disorder and find that the quantized sum rule is robust against weak disorder. Thus, we offer a unique way to identify the topological signatures of Floquet Majorana fermions.

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arXiv2011

Electron-phonon scattering in topological insulator thin films

We present a theoretical study of electron-phonon scattering effects in thin films made of a strong topological insulator. Phonons are modelled by isotropic elastic continuum theory with stress-free boundary conditions, and the interaction with the helical surface Dirac fermions is mediated by the deformation potential. We determine the temperature-dependent electrical resistivity $ρ(T)$ and the quasiparticle decay rate $Γ(T)$ observable in photoemission. The low- and high-temperature power laws for both quantities are obtained analytically. Detailed estimates covering the full temperature range are provided for Bi$_2$Se$_3$.

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arXiv2011

Quantum charge pumping through a superconducting double barrier structure in graphene

We consider the phenomenon of quantum charge pumping of electrons across a superconducting double barrier structure in graphene in the adiabatic limit. In this geometry, quantum charge pumping can be achieved by modulating the amplitudes (Delta_1 and Delta_2) of the gaps associated with the two superconducting strips. We show that the superconducting gaps give rise to a transmission resonance in the Delta_1-Delta_2 plane, resulting in a large value of pumped charge, when the pumping contour encloses the resonance. This is in sharp contrast to the case of charge pumping in a normal double barrier structure in graphene, where the pumped charge is very small, due to the phenomenon of Klein tunneling. We analyse the behaviour of the pumped charge through the superconducting double barrier geometry as a function of the pumping strength and the phase difference between the two pumping parameters, for various angles of the incident electron.

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arXiv2010

Resonant tunneling through superconducting double barrier structures in graphene

We study resonant tunneling through a superconducting double barrier structure in graphene as a function of the system parameters. At each barrier, due to the proximity effect, an incident electron can either reflect as an electron or a hole (specular as well as retro Andreev reflection in graphene). Similarly, transport across the barriers can occur via electrons as well as via the crossed (specular and/or retro) Andreev channel, where a hole is transmitted nonlocally to the other lead. In this geometry, in the subgap regime, we find resonant suppression of Andreev reflection at certain energies, due to the formation of Andreev bound levels between the two superconducting barriers, where the transmission probability T for electrons incident on the double barrier structure becomes unity. The evolution of the transport through the superconducting double barrier geometry as a function of the incident energy for various angles of incidence shows the damping of the resonance as normal reflection between the barriers increases.

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arXiv2009

Resonant spin transport through a superconducting double barrier structure

We study resonant transport through a superconducting double barrier structure. At each barrier, due to the proximity effect, an incident electron can either reflect as an electron or a hole (Andreev reflection). Similarly, transport across the barrier can occur via direct tunneling as electrons as well as via the crossed Andreev channel, where a hole is transmitted. In the subgap regime, for a symmetric double barrier system (with low transparency for each barrier), we find a new T=1/4 resonance (T is the transmission probability for electrons incident on the double barrier structure) due to interference between electron and hole wave-functions between the two barriers, in contrast to a normal double barrier system which has the standard transmission resonance at T=1. We also point out as an application that the resonant value of T=1/4 can produce pure spin current through the superconducting double barrier structure.

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