arXiv subjects

Jurgen Kohler

Publications and source records attributed to Jurgen Kohler.

3 recordsLinked to original sources

arXiv2026

Diagonal Curvature in Second Order Jahn Teller Theory Can Be Negative: An Analytic Proof with First-Principles Confirmation in NH3

We demonstrate that the diagonal curvature in second-order Jahn-Teller theory (SOJTT) can be negative, invalidating its foundational positivity assumption. Using a frozen-phonon many-body expansion, we show no quantum-mechanical principle enforces its positivity; recasting into the Kohn-Sham framework further reveals that phonon-induced eigen-energy changes arise solely from electron-nuclear, Hartree, and exchange-correlation terms, with kinetic-energy contributions canceling identically. First principles calculations on NH3 place the D3h reference configuration at an A2" saddle point; Schwarz-inequality upper-bound analysis confines the HOMO-LUMO mixing contribution to < 0.2% of the total energy drop,while 99.8% originates from the diagonal electron-nuclear term driving N (2s-2pz) electron redistribution. These findings overturn a core SOJTT tenet and establish that spontaneous structural symmetry breaking demands prior verification of a saddle point, not assumed positivity of the diagonal curvature.

cond-mat.mtrl-sci↗

arXiv2017

The road map toward room temperature superconductivity: manipulating different pairing channels in systems composed of multiple electronic components

While it is known that the amplification of the superconducting critical temperature Tc is possible in a system of multiple electronic components in comparison with a single component system, many different road maps for room temperature superconductivity have been proposed for a variety of multicomponent scenarios. Here we focus on the scenario where the first electronic component is assumed to have a vanishing Fermi velocity corresponding to a case of the intermediate polaronic regime, and the second electronic component is in the weak coupling regime with standard high Fermi velocity using a mean field theory for multiband superconductivity. This roadmap is motivated by compelling experimental evidence for one component in the proximity of a Lifshitz transition in cuprates, diborides and iron based superconductors. By keeping a constant and small exchange interaction between the two electron fluids, we search for the optimum coupling strength in the electronic polaronic component which gives the largest amplification of the superconducting critical temperature in comparison with the case of a single electronic component.

cond-mat.supr-con↗

arXiv2016

High temperature superconductivity in sulfur hydride under ultrahigh pressure: A complex superconducting phase beyond conventional BCS

The recent report of superconductivity under high pressure at the record transition temperature of Tc=203K in sulfur hydride has been identified as conventional in view of the observation of an isotope effect upon deuteration. Here it is demonstrated that conventional theories of superconductivity in the sense of BCS or Eliashberg formalisms can neither account for the observed values of Tc nor the pressure dependence of the isotope coefficient. The only way out of the dilemma is a multi-band approach of superconductivity where already small interband coupling suffices to achieve the high values of Tc together with the anomalous pressure dependent isotope effect. In addition, it is shown that anharmonicity of the hydrogen bonds vanishes under pressure whereas anharmonic phonon modes related to sulfur are still active

cond-mat.supr-con↗