arXiv subjects

Boris Yakobson

Publications and source records attributed to Boris Yakobson.

4 recordsLinked to original sources

arXiv2026

Quaternionic superconductivity links spinful pairing, topology, and charge-$4e$ order

We recast spinful superconductivity as a \textit{quaternion field theory}, where a quaternion is a four-component hypercomplex number with units $(\boldsymbol{e}_x,\boldsymbol{e}_y,\boldsymbol{e}_z)$, that encodes the spin-singlet/triplet gap in a single field $q(\mathbf{k})$. This yields a compact Bogoliubov-de Gennes (BdG) Hamiltonian $H_{\rm BdG}=\xi_{\mathbf{k}}\tau_z+\tau_+q+\tau_-\,q^\ddagger$ and keeps time-reversal symmetry, Altland-Zirnbauer classification, and topological diagnostics in the same variables. In general the mixed singlet--triplet spectrum is branch-split, while the familiar perfect-square form is recovered only for unitary pairing. We introduce a quarteting field $Q\!\propto\!\mathrm{Sc}(q^2)$ and a minimal Ginzburg-Landau (GL) functional with covariant derivatives $(\nabla-2ie\mathbf A)q$ and $(\nabla-4ie\mathbf A)Q$. Analytically, a one-loop evaluation of the fluctuation bubble $\Pi(0)$ gives a quantitative vestigial charge-$4e$ criterion $\mu_{\rm eff}=\mu-\frac{g^2}{2}\Pi(0)<0$. Numerically, we verified: (i) a two-dimensional (2D) class-DIII lattice model whose $\mathbb{Z}_2$ index, computed from the occupied BdG eigenvectors via the standard sewing-matrix (Pfaffian) construction at time-reversal-invariant momenta, matches helical edge spectra; (ii) a GL simulation of a pure-$Q$ vortex carrying $h/4e$ flux within $\sim2\%$ and exhibiting $\xi_Q\!\propto\!\sqrt{\eta/|\mu_{\rm eff}|}$; and (iii) a short-junction current-phase relation with a controlled window where the second harmonic dominates ($I_2\!\gg\!I_1$), together with doubled alternating-current Josephson emission and a Shapiro response consistent with $4e$-dominated transport. The framework provides a compact, symmetry-faithful route from microscopic pairing to device-level charge-$4e$ signatures.

cond-mat.supr-con↗

arXiv2025

Gauge-Invariant Long-Wavelength TDDFT Without Empty States: From Polarizability to Kubo Conductivity Across Heterogeneous Materials

Electromagnetic response is commonly computed in two languages: length-gauge molecular polarizabilities and velocity-gauge (Kubo) conductivities for periodic solids. We introduce a compact, gauge-invariant bridge that carries the same microscopic inputs-transition dipoles and interaction kernels-from molecules to crystals and heterogeneous media, with explicit SI prefactors and fine-structure scaling via $(\alpha_{\rm fs})$. The long-wavelength limit is handled through a reduced dielectric matrix that retains local-field mixing, interfaces and 2D layers are treated with sheet boundary conditions (rather than na\"ive ultrathin films), and length-velocity equivalence is enforced in practice by including the equal-time (diamagnetic/contact) term alongside the paramagnetic current. Finite temperature is addressed on the Matsubara axis with numerically stable real-axis evaluation (complex polarization propagator), preserving unit consistency end-to-end. The framework enables predictive, unit-faithful observables from radio frequency to ultraviolet-RF/microwave heating and penetration depth, dielectric-logging contrast, interfacial optics of thin films and 2D sheets, and adsorption metrics via imaginary-axis polarizabilities. Numerical checks (gauge overlay and optical $(f)$-sum saturation) validate the implementation. Immediate priorities include compact, temperature- and salinity-aware kernels with quantified uncertainties and \emph{operando} interfacial diagnostics for integration into multiphysics digital twins.

cond-mat.mtrl-sci↗

arXiv2024

Electron correlations in the kagome flat band metal $\rm CsCr_3Sb_5$

Kagome metals offer a unique platform for investigating robust electron-correlation effects because of their lattice geometry, flat bands and multi-orbital nature. In the cases with active flat bands, recent theoretical studies have pointed to a rich phase diagram that contains not only electronic orders but also quantum criticality. Very recently, $\rm CsCr_3Sb_5$ has emerged as a strong candidate for exploring such new physics. Here, using effective tight-binding models obtained from ab initio calculations, we study the effects of electronic correlations and symmetries on the electronic structure of $\rm CsCr_3 Sb_5$. The effective tight-binding model and Fermi surface comprise multiple Cr-$d$ orbitals and Sb-$p$ orbitals. The introduction of Hubbard-Kanamori interactions leads to orbital-selective band renormalization dominated by the $d_{xz}$ band, concurrently producing emergent flat bands very close to the Fermi level. Our analysis sets the stage for further investigations into the electronic properties of $\rm CsCr_3Sb_5$, including electronic orders, quantum criticality and unconventional superconductivity, which promise to shed much new light into the electronic materials with frustrated lattices and bring about new connections with the correlation physics of a variety of strongly correlated systems.

cond-mat.str-el↗

arXiv2023

Bottom-up Integration of TMDCs with Pre-Patterned Device Architectures via Transfer-free Chemical Vapor Deposition

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) remain a topic of immense interest. Specifically, given their low operational switching costs, they find many niche applications in new computing architectures with the promise of continued miniaturization. However, challenges lie in Back End of Line (BEOL) integration temperature and time compliance regarding current requirements for crystal growth. Additionally, deleterious and time-consuming transfer processes and multiple steps involved in channel/contact engineering can cripple device performance. This work demonstrates kinetics-governed in-situ growth regimes (surface or edge growth from gold) of WSe2 and provides a mechanistic understanding of these regimes via energetics across various material interfaces. As a proof-of-concept, field effect transistors (FET) with an in-situ grown WSe2 channel across Au contacts are fabricated, demonstrating a 2D semiconductor transistor via a transfer-free method within the 450-600 C 2h-time window requirement BEOL integration. We leverage directional edge growth to fabricate contacts with robust thickness-dependent Schottky-to-Ohmic behavior. By transitioning between Au and SiO2 growth substrates in situ, this work achieves strain-induced subthreshold swing of 140 mV/decade, relatively high mobility of 107 +- 19 cm2V-1s-1, and robust ON/OFF ratios 10^6 in the fabricated FETs.

cond-mat.mtrl-sci↗