arXiv subjects

Maryam Khosravian

Publications and source records attributed to Maryam Khosravian.

6 recordsLinked to original sources

arXiv2024

Moire-enabled topological superconductivity in twisted bilayer graphene

Twisted van der Waals materials have risen as highly tunable platform for realizing unconventional superconductivity. Here we demonstrate how a topological superconducting state can be driven in a twisted graphene multilayer at a twist angle of approximately 1.6 degrees proximitized to other 2D materials. We show that an encapsulated twisted bilayer subject to induced Rashba spin-orbit coupling, s-wave superconductivity and exchange field generates a topological superconducting state enabled by the moire pattern. We demonstrate a variety of topological states with different Chern numbers highly tunable through doping, strain and bias voltage. Our proposal does not depend on a fine tuning of the twist angle, but solely on the emergence of moire minibands and is applicable for twist angles between 1.3 and 3 degrees. Our results establish the potential of twisted graphene bilayers to create artificial topological superconductivity without requiring ultraflat dispersions.

cond-mat.mes-hall↗

arXiv2024

Hamiltonian learning with real-space impurity tomography in topological moire superconductors

Extracting Hamiltonian parameters from available experimental data is a challenge in quantum materials. In particular, real-space spectroscopy methods such as scanning tunneling spectroscopy allow probing electronic states with atomic resolution, yet even in those instances extracting effective Hamiltonian is an open challenge. Here we show that impurity states in modulated systems provide a promising approach to extracting non-trivial Hamiltonian parameters of a quantum material. We show that by combining the real-space spectroscopy of different impurity locations in a moire topological superconductor, modulations of exchange and superconducting parameters can be inferred via machine learning. We demonstrate our strategy with a physically-inspired harmonic expansion combined with a fully-connected neural network that we benchmark against a conventional convolutional architecture. We show that while both approaches allow extracting exchange modulations, only the former approach allows inferring the features of the superconducting order. Our results demonstrate the potential of machine learning methods to extract Hamiltonian parameters by real-space impurity spectroscopy as local probes of a topological state.

cond-mat.mes-hall↗

arXiv2023

Evidence of nodal superconductivity in monolayer 1H-TaS$_2$ with hidden order fluctuations

Unconventional superconductors represent one of the fundamental directions in modern quantum materials research. In particular, nodal superconductors are known to appear naturally in strongly correlated systems, including cuprate superconductors and heavy-fermion systems. Van der Waals materials hosting superconducting states are well known, yet nodal monolayer van der Waals superconductors have remained elusive. Here, using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments, we show that pristine monolayer 1H-TaS$_2$ realizes a nodal superconducting state. By including non-magnetic disorder, we drive the nodal superconducting state to a conventional gapped s-wave state. Furthermore, we observe the emergence of many-body excitations close to the gap edge, signalling a potential unconventional pairing mechanism. Our results demonstrate the emergence of nodal superconductivity in a van der Waals monolayer, providing a building block for van der Waals heterostructures exploiting unconventional superconducting states.

cond-mat.supr-con↗

arXiv2022

Impurity-induced excitations in a topological two-dimensional ferromagnet/superconductor van der Waals moiré heterostructure

The emergence of a topological superconducting state in van der Waals heterostructures provides a new platform for exploring novel strategies to control topological superconductors. In particular, impurities in van der Waals heterostructures, generically featuring a moire pattern, can potentially lead to the unique interplay between atomic and moire length scales, a feature absent in generic topological superconductors. Here we address the impact of non-magnetic impurities on a topological moire superconductor, both in the weak and strong regime, considering both periodic arrays and single impurities in otherwise pristine infinite moire systems. We demonstrate a fine interplay between impurity-induced modes and the moire length, leading to radically different spectral and topological properties depending on the relative impurity location and moire lengths. Our results highlight the key role of impurities in van der Waals heterostructures featuring moire patterns, revealing the key interplay between length and energy scales in artificial moire systems.

cond-mat.supr-con↗

arXiv2021

Designing spin-textured flat bands in twisted graphene multilayers via helimagnet encapsulation

Twisted graphene multilayers provide tunable platforms to engineer flat bands and exploit the associated strongly correlated physics. The two-dimensional nature of these systems makes them suitable for encapsulation by materials that break specific symmetries. In this context, recently discovered two-dimensional helimagnets, such as the multiferroic monolayer NiI$_2$, are specially appealing for breaking time-reversal and inversion symmetries due to their nontrivial spin textures. Here we show that this spin texture can be imprinted on the electronic structure of twisted bilayer graphene by proximity effect. We discuss the dependence of the imprinted spin texture on the wave-vector of the helical structure, and on the strength of the effective local exchange field. Based on these results we discuss the nature of the superconducting instabilities that can take place in helimagnet encapsulated twisted bilayer graphene. Our results put forward helimagnetic encapsulation as a powerful way of designing spin-textured flat band systems, providing a starting point to engineer a new family of correlated moire states.

cond-mat.mes-hall↗

arXiv2021

Quasiperiodic criticality and spin-triplet superconductivity in superconductor-antiferromagnet moire patterns

Quasiperiodicity has long been known to be a potential platform to explore exotic phenomena, realizing an intricate middle point between ordered solids and disordered matter. In particular, quasiperiodic structures are promising playgrounds to engineer critical wavefunctions, a powerful starting point to engineer exotic correlated states. Here we show that systems hosting a quasiperiodic modulation of antiferromagnetism and spin-singlet superconductivity, as realized by atomic chains in twisted van der Waals materials, host a localization-delocalization transition as a function of the coupling strength. Associated with this transition, we demonstrate the emergence of a robust quasiperiodic critical point for arbitrary incommensurate potentials, that appears for generic relative weights of the spin-singlet superconductivity and antiferromagnetism. We show that the inclusion of residual electronic interactions leads to an emergent spin-triplet superconducting state, that gets dramatically enhanced at the vicinity of the quasiperiodic critical point. Our results put forward quasiperiodicity as a powerful knob to engineer robust superconducting states, providing an alternative pathway towards artificially designed unconventional superconductors.

cond-mat.supr-con↗