arXiv subjects

Sandra Sajan

Publications and source records attributed to Sandra Sajan.

9 recordsLinked to original sources

arXiv2026

Stacking-tuned superconductivity and competing charge-density-wave states in NbSe$_2$

Layer stacking provides a powerful yet underexplored route for reshaping collective quantum order in van der Waals materials. Here we use high-resolution scanning tunneling microscopy and spectroscopy to show that the stacking sequence alone can qualitatively transform the charge density and superconducting orders in NbSe$_2$, while preserving the same in-plane atomic structure. Comparing the 4Ha and 2H polytypes, we find that, unlike the ubiquitous triangular incommensurate $3Q^\mathrm{I}$ order of 2H-NbSe$_2$, 4Ha-NbSe$_2$ hosts two competing CDW states with no measurable correlation with local strain: a unidirectional commensurate $1Q^\mathrm{C}$ phase and a triangular incommensurate $3Q^\mathrm{I}$ phase, with $Q^\mathrm{I}=Q^\mathrm{C}+\delta$. We introduce a phase-resolved analysis that directly maps the gradient of the CDW phases and reveals vortices bound to the $1Q^\mathrm{C}$ - $3Q^\mathrm{I}$ phase boundaries. These vortices accommodate the momentum mismatch $\delta$ through abrupt $2\pi$ phase slips, providing a mechanism by which distinct charge orders coexist. Superconductivity is also reshaped by stacking, while both polytypes exhibit multiband pairing.

cond-mat.supr-con↗

arXiv2026

Moiré-induced symmetry breaking of charge order in van der Waals heterostructures

Layered materials that stack different lattice symmetries are rare in nature. Misfit layered chalcogenides, which combine square and hexagonal lattices of rocksalt monochalcogenides and transition-metal dichalcogenides, provide a platform to explore how incommensurability and explicit symmetry breaking impact collective electronic phases. Here we use low-temperature scanning tunneling microscopy/spectroscopy to probe the misfit compounds (MS)$_{1+δ}$TaS$_{2}$ with M = Pb, Sn and track how the misfit interface reshapes the electronic ground state of the embedded 1H-TaS$_{2}$ monolayers. High-resolution STM imaging and Fourier analysis reveal that the charge-density wave (CDW) is incommensurate and fragments into nanometer-sized domains. Strikingly, the CDW exhibits a pronounced and anisotropic response to the uniaxial moiré potential imposed by the misfit layer: its coherence lengths and ordering wavevectors become inequivalent, demonstrating a strong nonlinear coupling between the intrinsic CDW instability and the symmetry-breaking moiré field. First-principles-informed multiscale modeling shows that this reorganization arises from the combined effect of interlayer charge transfer and the spatially anisotropic energy landscape introduced by the misfit interface. In contrast, superconductivity is comparatively insensitive to the moiré, revealing a uniform, single full-gap consistent with s-wave pairing. Our results establish heterosymmetry stacking as a route to engineer correlated states in van der Waals materials.

cond-mat.str-el↗

arXiv2025

Layer-selective Cooper pairing in an alternately stacked transition metal dichalcogenide

Multigap superconductivity emerges when superconducting gaps form on distinct Fermi surfaces. Arising from locally overlapping atomic orbitals, multiple superconducting bands introduce a new internal degree of freedom in the material that, however, escapes external control due to their coexistence in real space in the known multigap superconductors. Here, we show that the layered superconductor 4Hb-TaSSe - composed of alternating trigonal (H) and octahedral (T) polymorph layers - is a multigap superconductor, featuring two weakly coupled superconducting condensates with distinct properties, spatially separated in alternating layers. Using high-resolution quasiparticle tunneling and Andreev reflection spectroscopy in the two polymorph layers, we identify two superconducting gaps that vary in size and internal structure. The intrinsic Cooper pairing in each polymorph is corroborated by the temperatures and magnetic fields at which the gaps open up, which differ in each polymorph layer and show opposing resilience to these parameters. This behavior enables selective external actuation upon the condensates. Our theoretical model based on ab-initio calculations reproduces key features of the observed superconducting gaps in the presence of finite interlayer hybridization and explains the unusually high critical field observed in the T-layer. Our results establish TMD polymorphs as platforms for engineering tunable multigap superconductors, offering new opportunities in layered superconducting device architectures.

cond-mat.supr-con↗

arXiv2025

Quantum Geometry in the NbSe$_2$ Family I: Obstructed Compact Wannier Function and New Perturbation Theory

We revisit the electronic structure and band topology of monolayer 1H-NbSe$_2$, which hosts both superconductivity and charge density wave, and its related compounds 1H-MoS$_2$, NbS$_2$, TaS$_2$, TaSe$_2$ and WS$_2$. We construct a 6-band, a 3-band, and - simplest of all - a single-band model for this material family, by directly Wannierizing the ab initio bands. All host obstructed atomic isolated bands away from the atomic positions near the Fermi energy. We find that in the 3-band model, the obstructed atomic Wannier function can be well approximated by an optimally compact Wannier function with more than 90% accuracy for all the compounds, rising to a remarkable 94% accuracy in NbSe$_2$. Interestingly, the simplest single-band model has next nearest-neighboring hopping larger than the nearest-neighboring hopping (by nearly an order of magnitude for MoS$_2$, NbSe$_2$, TaSe$_2$ and WS$_2$), which comes from the cancellation between the atomic onsite terms and the atomic nearest-neighboring hopping after projecting to the obstructed atomic Wannier functions. Furthermore for NbSe$_2$, we employ a novel approximation scheme to obtain an effective Hamiltonian that captures the 3 bands originating mainly from the Nb atom. We also use conventional perturbation theory to derive the ab initio obstructed Wannier function with 95% accuracy. Our results pave the way for future study of the effect of quantum geometry on the correlated phases in this family of materials.

cond-mat.mes-hall↗

arXiv2025

One-dimensional conduction channels in the correlated Mott NiS2 arising from obstructed Wannier charges

NiS2, a compound characterized by its pyrite structure, uniquely bridges the realms of strong correlation physics and topology. While bulk NiS2 is known to be a Mott or charge-transfer insulator, its surface displays anomalous metallic behavior and finite conductivity. Using high-resolution neutron scattering data and symmetry analysis, we propose a refined description of NiS2's magnetic phases by introducing a novel model for its ground state. Combined with high-resolution scanning tunneling microscopy and spectroscopy (STM/STS), we unveil the presence of edge states in both Ni- and S-terminated surfaces, which exhibit remarkable resilience to external magnetic fields. Although both types of edge states exhibit similar properties, only the edge states at the Ni termination populate the vicinity of the Fermi level and, therefore contribute to the surface conductivity. Utilizing ab initio methods combined with a topological quantum chemistry analysis, we attribute these edge states to obstructed atomic charges originating from bulk topology. Overall, this work not only deepens our understanding of NiS2 but also lays a robust experimental and theoretical foundation for further exploration of the interplay between one-dimensional step-edge states and the Wannier obstruction in correlated materials.

cond-mat.str-el↗

arXiv2025

Probing the Quantized Berry Phases in 1H-NbSe$_2$ Using Scanning Tunneling Microscopy

Topologically trivial insulators are classified into two primary categories: unobstructed and obstructed atomic insulators. While both types can be described by exponentially localized Wannier orbitals, a defining feature of obstructed atomic insulators is that the centers of charge of these orbitals are positioned at empty sites within the unit cell, rather than on atoms. Despite extensive theoretical predictions, the unambiguous and quantitative experimental identification of an obstructed atomic phase has remained elusive. In this work, we present the first direct experimental evidence of such a phase in 1H-NbSe$_2$. We develop a novel method to extract the inter-orbital correlation functions from the local spectral function probed by scanning tunneling microscopy (STM), leveraging the orbital wave functions obtained from ab initio calculations. Applying this technique to STM images, we determine the inter-orbital correlation functions for the atomic band of 1H-NbSe$_2$ that crosses the Fermi level. Our results show that this band realizes an optimally compact obstructed atomic phase, providing the first unambiguous experimental identification of such a phase. Our approach of deconvolving the STM signal using ab initio orbital wave functions is broadly applicable to other material platforms, offering a powerful tool for exploring other electronic phases.

cond-mat.mes-hall↗

arXiv2024

Layer thickness and substrate effects on superconductivity in epitaxial FeSe films on BLG/SiC(0001)

The layered nature and simple structure of FeSe reveal this iron-based superconductor as a unique building block for the design of artificial heterostructure materials. While superconductivity develops in ultrathin films of FeSe on SrTiO3 substrates, it remains unclear whether it can be developed on more chemically inert, layered materials such as graphene. Here, we report on the characterization of the structural, chemical and electronic properties of few-layer FeSe on bilayer graphene grown on SiC(0001) using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS). STM imaging of our FeSe films with thicknesses up to three layers exhibit the tetragonal crystal structure of bulk FeSe, which is supported by XPS spectra consistent with the FeSe bulk counterpart. While our STS measurements at 340 mK reveal a metallic character for few-layer FeSe on BLG/SiC(0001), they show an absence of superconductivity, as the low-lying electronic structure exhibits a spatially anisotropic dip-like feature robust against magnetic fields. Superconductivity in FeSe/BLG/SiC(0001), however, emerges for thicker films with a transition temperature of 6 K. Our results underscore the significance of the substrate as key factor driving the suppression superconductivity in FeSe in the 2D limit.

cond-mat.supr-con↗

arXiv2024

Atomic-scale mapping of superconductivity in the incoherent CDW mosaic phase of a transition metal dichalcogenide

The emergence of superconductivity in the octahedrally coordinated (1T) phase of TaS2 is preceded by the intriguing loss of long-range order in the charge density wave (CDW). Such decoherence, attainable by different methods, results in the formation of nm-sized coherent CDW domains bound by a two-dimensional network of domain walls (DW) - mosaic phase -, which has been proposed as the spatial origin of the superconductivity. Here, we report the atomic-scale characterization of the superconducting state of 1T-TaSSe, a model 1T compound exhibiting the CDW mosaic phase. We use high-resolution scanning tunneling spectroscopy and Andreev spectroscopy to probe the microscopic nature of the superconducting state in unambiguous connection with the electronic structure of the mosaic phase. Spatially resolved conductance maps at the Fermi level at the onset of superconductivity reveal that the density of states is mostly localized on the CDW domains compared to the domain walls, which suggests their dominant role in the formation of superconductivity. This scenario is confirmed within the superconducting dome at 340 mK, where superconductivity is fully developed, and the subtle spatial inhomogeneity of the superconducting gap remains unlinked to the domain wall network. Our results provide key new insights into the fundamental interplay between superconductivity and CDW in these relevant strongly correlated systems.

cond-mat.supr-con↗

arXiv2023

Evidence for ground state coherence in a two-dimensional Kondo lattice

Kondo lattices are ideal testbeds for the exploration of heavy-fermion quantum phases of matter. While our understanding of Kondo lattices has traditionally relied on complex bulk f-electron systems, transition metal dichalcogenide heterobilayers have recently emerged as simple, accessible and tunable 2D Kondo lattice platforms where, however, their ground state remains to be established. Here we present evidence of a coherent ground state in the 1T/1H-TaSe2 heterobilayer by means of scanning tunneling microscopy/spectroscopy at 340 mK. Our measurements reveal the existence of two symmetric electronic resonances around the Fermi energy, a hallmark of coherence in the spin lattice. Spectroscopic imaging locates both resonances at the central Ta atom of the charge density wave of the 1T phase, where the localized magnetic moment is held. Furthermore, the evolution of the electronic structure with the magnetic field reveals a non-linear increase of the energy separation between the electronic resonances. Aided by ab initio and auxiliary-fermion mean-field calculations, we demonstrate that this behavior is inconsistent with a fully screened Kondo lattice, and suggests a ground state with magnetic order mediated by conduction electrons. The manifestation of magnetic coherence in TMD-based 2D Kondo lattices enables the exploration of magnetic quantum criticality, Kondo breakdown transitions and unconventional superconductivity in the strict two-dimensional limit.

cond-mat.str-el↗