arXiv subjects

Jan Phillips

Publications and source records attributed to Jan Phillips.

5 recordsLinked to original sources

arXiv2026

Magnetoelectric flat band induced by a $\sqrt{3}\times\sqrt{3}$ charge density wave in monolayer CrSe$_2$

We investigate the electronic and magnetic properties of the polar $\sqrt{3}\times\sqrt{3}$ charge-density-wave (CDW) phase of CrSe$_2$ using ab initio calculations. The CDW introduces a polar distortion out of the van der Waals plane that couples to the spin-polarized Cr d states resulting in a remarkably flat electronic band exactly at the Fermi level. We provide a microscopic understanding of the origin of the flat band by analyzing in detail the structural reconstruction, the effects of orbital hybridization, crystal-field splittings, spin-orbit coupling and electronic correlations. Our calculations show that due to the polar nature of the CDW distortion, an electric field can act as an external switch to induce the CDW phase, providing a way to manipulate strong correlations in the system.

cond-mat.str-el↗

arXiv2025

Two topological phases in exchange alternating spin-1 nanographene chains

Magnetic nanographenes are emerging as versatile building blocks for artificial spin lattices, enabling the exploration of flagship one-dimensional quantum-magnetism models with unprecedented control. The spin-1 Heisenberg model, including bilinear and biquadratic exchange, was first realized using [3]-triangulenes, revealing the Haldane phase. More recently, Clar's goblets enabled the spin-1/2 Heisenberg model with exchange alternation, uncovering additional topological phases. Here we show that spin-1 nanographenes can be used to explore bond-alternating chains both in the Haldane phase and beyond it, in a dimerized phase with emergent edge spin-1. We use density matrix renormalization group (DMRG) to analyze how biquadratic exchange, which is known to be large in spin-1 nanographenes, determines the phase transition boundary. Combining multiconfigurational and first-principles calculations, we identify two realistic candidates to realize these two different phases: the recently synthesized extended Clar's goblet and a passivated [4]-triangulene. We demonstrate how to distinguish these phases experimentally using inelastic electron tunneling spectroscopy, paving the way for their observation.

cond-mat.mes-hall↗

arXiv2024

An ab initio description of the family of Cr selenides: structure, magnetism and electronic structure from bulk to the single-layer limit

Compounds based on Cr have been found to be among the first single-layer magnets. In addition, transition metal dichalcogenides are promising candidates to show long-range ferromagnetic order down to the two-dimensional limit. We use ab initio calculations to provide a description of the various Cr$_x$Se$_{x+1}$ stoichiometries that may occur by analyzing from the bulk materials to the monolayer limit. We study the different structural distortions, including charge density waves that each system can present by analyzing their phonon spectra and dynamic stability. We provide a description of their basic electronic structure and study their magnetic properties, including the magnetocrystalline anisotropy energy. The evolution of all these properties with the dimensionality of the systems is discussed. This intends to be a comprehensive view of the broad family of Cr selenides.

cond-mat.mtrl-sci↗

arXiv2023

Self-doped flat band and spin-triplet superconductivity in monolayer 1T-TaSe$_{2-x}$Te$_{x}$

Two-dimensional van der Waals materials have become an established platform to engineer flat bands which can lead to strongly-correlated emergent phenomena. In particular, the family of Ta dichalcogenides in the 1\textit{T} phase presents a star-of-David charge density wave that creates a flat band at the Fermi level. For TaS$_2$ and TaSe$_2$ this flat band is at half filling leading to a magnetic insulating phase. In this work, we theoretically demonstrate that ligand substitution in the TaSe$_{2-x}$Te$_x$ system produces a transition from the magnetic insulator to a non-magnetic metal in which the flat band gets doped away from half-filling. For $x\in[{0.846},{1.231}]$ the spin-polarized flat band is self-doped and the system becomes a magnetic metal. In this regime, we show that attractive interactions promote three different spin-triplet superconducting phases as a function of $x$, corresponding to a nodal f-wave and two topologically-different chiral p-wave superconducting phases. Our results establish monolayer TaSe$_{2-x}$Te$_{x}$ as a promising platform for correlated flat band physics leading to unconventional superconducting states.

cond-mat.supr-con↗

arXiv2020

Controlled Two-Dimensional Ferromagnetism in 1T-CrTe$_2$. The role of charge density wave and strain

Transition metal dichalcogenides are promising candidates to show long-range ferromagnetic order in the single-layer limit. Based on ab initio calculations, we report the emergence of a charge density wave (CDW) phase in monolayer 1T-CrTe$_2$. We demonstrate that this phase is the ground state in the single-layer limit at any strain value. We obtain an optical phonon mode of $1.96$ THz that connects CDW phase with the undistorted 1T phase. Localization of the $a_{1g}$ orbital of CrTe$_2$ produces an out-of-plane orientation of the magnetic moments, circumventing the restrictions of the Mermin-Wagner theorem and producing ferromagnetic long-range order in the two-dimensional limit. This orbital-localization is enhanced by the CDW phase. Tensile strain also increases the localization of this orbital driving the system to become ordered. CrTe$_2$ becomes an example of a material where the CDW phase produces the stabilization of the long-range ferromagnetic order. Our results show that both strain and phase switching are mechanisms to control the 2D ferromagnetic order of CrTe$_2$.

cond-mat.mtrl-sci↗