arXiv subjects

Sayak Biswas

Publications and source records attributed to Sayak Biswas.

7 recordsLinked to original sources

arXiv2026

Probing the pairing symmetry of moir\'e graphene superconductors

The pairing symmetry of magic-angle moir\'e graphene is a fundamental question that remains unresolved. Combining experimental and theoretical inputs, we constrain the superconducting order parameters that can emerge from the incommensurate Kekul\'e spiral (IKS) normal state on the hole-doped side of $\nu = -2$. Imposing the additional experimental constraint of nodal superconductivity, we are left with the task of distinguishing between singlet or triplet pairing, and of determining the d-vector in the latter case. We propose definitive tests to identify the pairing symmetry based on two classes of experiments using the response of the superconducting state to Zeeman field orientation. The first set of predictions is for spectroscopic and thermodynamic measurements sensitive to low-energy excitations near the nodes. The second set is for phase-sensitive measurements of topologically protected Andreev bound states near boundaries, whose spectroscopy is shown to provide a smoking gun signature of the pairing symmetry.

cond-mat.supr-con↗

arXiv2025

Measurement-Induced Dynamical Quantum Thermalization

One of the fundamental problems of quantum statistical physics is how an ideally isolated quantum system can ever reach thermal equilibrium behavior despite the unitary time evolution of quantum-mechanical systems. Here, we study, via explicit time evolution for the generic model system of an interacting, trapped Bose gas with discrete single-particle levels, how the measurement of one or more observables subdivides the system into observed and non-observed Hilbert subspaces and the tracing over the non-measured quantum numbers defines an effective, thermodynamic bath, induces the entanglement of the observed Hilbert subspace with the bath, and leads to a bi-exponential approach of the entanglement entropy and of the measured observables to thermal equilibrium behavior as a function of time. We find this to be more generally fulfilled than in the scenario of the eigenstate thermalization hypothesis (ETH), namely for both local particle occupation numbers and non-local density correlation functions, and independent of the specific initial quantum state of the time evolution.

quant-ph↗

arXiv2025

Andreev versus Tunneling Spectroscopy of Unconventional Flat Band Superconductors

STM experiments in the tunneling and Andreev regimes on graphene-based moire superconductors (SC) show two distinct energy scales whose origin is mysterious. We express the conductance of a normal-SC interface in terms of Green's functions, which allows us to sharpen the issues in two ways. First, we show that the two distinct energy scales cannot be understood in terms of a pseudogap in tunneling and a superconducting gap in the Andreev spectra. Second, the large Fermi velocity vF mismatch between the STM tip and the at band SC renormalizes a transparent interface towards the tunneling regime, and the ballistic Andreev regime cannot be realized in moire SCs. We also discuss self energy corrections to vF that determines the conductance. Finally, we offer a resolution to these problems by modeling the Andreev experiment as a circular metallic disc embedded in an unconventional SC. We show that with strong vF mismatch the low bias conductance is dominated by Andreev bound states induced by the tip at the interface with the unconventional SC. The ABS give rise to the low energy scale seen in Andreev experiments, smaller than the SC gap in tunneling spectroscopy.

cond-mat.supr-con↗

arXiv2023

Pairing Symmetry and Fermion Projective Symmetry Groups

The Ginzburg-Landau (GL) theory is very successful in describing the pairing symmetry, a fundamental characterization of the broken symmetries in a paired superfluid or superconductor. However, GL theory does not describe fermionic excitations such as Bogoliubov quasiparticles or Andreev bound states that are directly related to topological properties of the superconductor. In this work, we show that the symmetries of the fermionic excitations are captured by a Projective Symmetry Group (PSG), which is a group extension of the bosonic symmetry group in the superconducting state. We further establish a correspondence between the pairing symmetry and the fermion PSG. When the normal and superconducting states share the same spin rotational symmetry, there is a simpler correspondence between the pairing symmetry and the fermion PSG, which we enumerate for all 32 crystalline point groups. We also discuss the general framework for computing PSGs when the spin rotational symmetry is spontaneously broken in the superconducting state. This PSG formalism leads to experimental consequences, and as an example, we show how a given pairing symmetry dictates the classification of topological superconductivity.

cond-mat.supr-con↗

arXiv2023

Impurities in quasi-one-dimensional droplets of binary Bose mixtures

Recently created self-bound quantum droplets of binary Bose mixtures open intriguing possibilities for the study of impurity physics. We show that the properties of impurities embedded in quasi-one-dimensional droplets are determined by the interplay between back-action and quantum fluctuations. Due to such back-action, repulsive impurities may form a metastable quasi-bound state inside the droplet. In contrast, attractive impurities remain bound to the droplet, leading to the hybridization of droplet and impurity excitations, as well as to peculiar scattering resonances. Interestingly, impurity trapping may result solely from the effect of quantum fluctuations. These results may readily be probed experimentally by doping the currently available droplets of binary mixtures.

cond-mat.quant-gas↗

arXiv2020

Landau quantization of a circular Quantum Dot using the BenDaniel-Duke boundary condition

We derive the energy levels of a circular Quantum Dot (QD) under a transverse magnetic field, incorporating the Ben-Daniel Duke boundary condition (BDD). The parameters in our model are the confinement barrier height, the size of the QD, the magnetic field strength, and a mass ratio highlighting the effect of using BDD. Charge densities, transition energies, and the dependence of energies on magnetic field has been calculated to show the strong influence of BDD. We find that our numerical calculations agree well with experimental results on the GaAs-InGaAs Quantum Dot and can be used further. We also provide an insightful analytical approximation to our numerical results, which converges well for larger values of size and confinement.

cond-mat.mes-hall↗

arXiv2020

Thermalization in parametrically driven coupled oscillators

We consider a system of two coupled oscillators one of which is driven parametrically and investigate both classical and quantum dynamics within Floquet description. Characteristic changes in the time evolution of the quantum fluctuations are observed for dynamically stable and unstable regions. Dynamical instability generated by the parametrically driven oscillator leads to infinite temperature thermalization of the undriven oscillator which is evident from the equi-partitioning of energy, reduced density matrix and saturation of entanglement entropy. We also confirm that the classical Lyapunov exponent is correctly captured from the growth rate of `unequal time commutator' of dynamical variables which indicates thermalization stems from the instability in quantum system.

quant-ph↗