arXiv subjects

Samuel Vadnais

Publications and source records attributed to Samuel Vadnais.

4 recordsLinked to original sources

arXiv2026

d-Wave pair density wave superconductivity in a two-orbital model

Motivated by exploring superconductivity in multi-orbital systems, we study two orbital models of spinful fermions representing ($p_x,p_y$) or ($d_{xz}, d_{yz})$ orbitals on the square lattice. For minimal interorbital $t$-$J$ or $t$-$V$ on-site interactions, a random phase approximation uncovers regimes of instability towards incommensurate $d_{xy}$ pair density wave ($d$-PDW) superconductivity with driven by interband pairing. We study the competition of PDW order with uniform nodal $d_{xy}$ pairing states and magnetic and charge density wave (CDW) instabilities. At strong coupling, we derive an effective hard-core Cooper pair Hamiltonian which we study using a bosonic Gutzwiller ansatz to reveal a period-$2$ PDW over a wide range of fillings as well as a checkerboard CDW at quarter-filling. Our results apply to correlated multi-orbital materials with quasi-1D bands, Hubbard models on the square-octagon lattice, and atomic fermions in $p$-orbitals. Our work highlights the role of the orbital content and multiband Fermi surfaces in stabilizing interband PDW states.

cond-mat.str-el↗

arXiv2026

The role of the apical oxygen in cuprate high-temperature superconductors

Scanning tunneling microscopy measurements exploiting the natural superstructure modulation of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212) have revealed a possible correlation between the Cu-apical-O distance $\delta_{\mathrm{api}}$ and the superconducting order parameter $m_{\mathrm{SC}}$, as reported recently by O'Mahony et al. (Proc. Natl. Acad. Sci. 119, e2207449119 (2022)). These observations were interpreted as evidence for a direct link between superconductivity and the charge-transfer gap, and more broadly revived the long-standing question of the role of apical oxygens in cuprate superconductivity. Using a combination of density-functional theory and cluster dynamical mean-field theory, we compute from first principles the variations of $m_{\mathrm{SC}}$ induced solely by apical oxygen displacement in Bi$_2$Sr$_2$CuO$_{6+\delta}$, Bi-2212, and HgBa$_2$CuO$_{4+\delta}$. The quantitative agreement between our calculations and experiments allows us to unambiguously attribute the observed variations of $m_{\mathrm{SC}}$ to changes in $\delta_{\mathrm{api}}$. We demonstrate, however, that these variations of $m_{\mathrm{SC}}$ originate predominantly from changes in the effective hole-doping of the CuO$_2$ planes, with negligible effect on the charge-transfer gap. The modest magnitude of the $m_{\mathrm{SC}}$ modulation induced by apical-oxygen displacement alone therefore warrants caution in interpreting correlations between $T_c$ and $\delta_{\mathrm{api}}$ inferred from comparisons across different cuprate compounds. Our work demonstrates that the present ab initio framework can quantitatively resolve the influence of specific structural degrees of freedom on superconductivity in correlated oxides.

cond-mat.str-el↗

arXiv2025

Altermagnetism and superconductivity in a multiorbital t-J model

Motivated by exploring doped multi-orbital antiferromagnets (AFMs) and altermagnets (ALMs) we explore minimal $t$-$J$ models on the square-octagon lattice which favor such collinear magnetic orders in the regime where spin exchange dominates. While the AFM order breaks translational and time-reversal symmetries, the ALM state (equivalently, a `$d$-wave ferromagnet') features multipolar order which separately breaks time-reversal and crystal rotation symmetries but preserves their product leading to spin-split bands with zero net magnetization. We study the mean field phase diagram of these models as we vary doping and interactions, discovering regimes of weak and strong ALM order, superconductivity including uniform $s$-wave and $d$-wave pairing states, incipient $d$-wave pair density wave order, and phases with coexisting singlet-triplet pairing and AFM/ALM orders which appear unstable to phase separation and could host stripe order with longer-range interactions. We study the mean field phase diagram of these multiorbital models as we vary doping and interactions, discovering two types of ALM order: (i) itinerant weak-coupling ALM metals driven by quasi-1D van Hove singularities, as well as (ii) strong ALM order at half-filling. We also find regimes of superconductivity including uniform $s$-wave and $d$-wave pairing states, incipient $d_{xy}$-wave pair density wave order, and uniform phases with coexisting singlet-triplet pairing and ALM order. Our inhomogeneous mean field theory approach reveals that the coexistence phases are unstable to phase separation, but longer-range interactions could lead to stripe order. Our results may be relevant to doping or pressure studies of multiorbital ALM materials.

cond-mat.str-el↗

arXiv2021

Quantum magnetic oscillations in Weyl semimetals with tilted nodes

A Weyl semimetal (WSM)\ is a three-dimensional topological phase of matter where pairs of nondegenerate bands cross at isolated points in the Brillouin zone called Weyl nodes. Near these points, the electronic dispersion is gapless and linear. A magnetic field $B$ changes this dispersion into a set of Landau levels which are dispersive along the direction of the magnetic field only. The $n=0$ Landau level is special since its dispersion$\ $is linear and unidirectional. The presence of this chiral level distinguishes Weyl from Schrödinger fermions. In this paper, we study the quantum oscillations of the orbital magnetization and magnetic susceptibility in Weyl semimetals. We generalise earlier works% \cite{Mikitik2019} on these De Haas-Van Alphen oscillations by considering the effect of a tilt of the Weyl nodes. We study how the fundamental period of the oscillations in the small $B$ limit and the strength of the magnetic field $B_{1}$ required to reach the quantum limit are modified by the magnitude and orientation of the tilt vector $\mathbf{t}$. We show that the magnetization from a single node is finite in the $B\rightarrow 0$ limit. Its sign depends on the product of the chirality and sign of the tilt component along the magnetic field direction. We also study the magnetic oscillations from a pair of Weyl nodes with opposite chirality and with opposite or identical tilt. Our calculation shows that these two cases lead to a very different behavior of the magnetization in the small and large $B$ limits. We finally consider the effect of an energy shift $\pm Δ_{0}$ of a pair of Weyl nodes on the magnetic oscillations. We assume a constant density of carriers so that both nodes share a common Fermi level. Our calculation can easily be extended to a WSM with an arbitrary number of pairs of Weyl nodes.

cond-mat.mes-hall↗