Paper

{
  "raw_xml": "<entry>\n    <id>http://arxiv.org/abs/2603.09609v1</id>\n    <title>Analytic treatment of a polaron in a nonparabolic conduction band</title>\n    <updated>2026-03-10T12:52:57Z</updated>\n    <link href='https://arxiv.org/abs/2603.09609v1' rel='alternate' type='text/html'/>\n    <link href='https://arxiv.org/pdf/2603.09609v1' rel='related' title='pdf' type='application/pdf'/>\n    <summary>We develop and compare several analytical approximations for the polaron problem in finite-width, non-parabolic conduction bands. The main focus of the work is an extension of the Feynman variational method to a tight-binding lattice, where the effective-mass approximation is no longer applicable. The resulting variational formulation is not restricted to a specific phonon dispersion or electron-phonon interaction and provides a uniform description across weak-, intermediate-, and strong-coupling regimes. We revisit and generalize other analytical approaches traditionally formulated for continuum polarons, including canonical transformations and self-consistent Wigner-Brillouin-type approximations. For lattice polarons, these methods exhibit qualitative features absent in the continuum case, such as a nontrivial connection between weak- and strong-coupling limits. We show that an improved Wigner-Brillouin scheme yields a momentum-dependent polaron self-energy free of resonances and in good agreement with numerically exact results over the whole range of momenta within the Brillouin zone. All methods are applied to the Holstein model and are benchmarked against numerically exact calculations, including Diagrammatic Monte Carlo (both our calculations and preceding works), exact diagonalization, and density-matrix renormalization-group results. The analytical approaches are extended to polarons with Rashba-type spin-orbit coupling, providing a stringent test of their applicability in systems with nontrivial band structure. Our results demonstrate that the modified Feynman variational method yields ground-state energies and dispersions with accuracy comparable to, and in many cases exceeding, that of other established analytical approaches. The developed framework offers a versatile and reliable analytical description of lattice polarons beyond the continuum approximation.</summary>\n    <category scheme='http://arxiv.org/schemas/atom' term='cond-mat.str-el'/>\n    <published>2026-03-10T12:52:57Z</published>\n    <arxiv:comment>41 pages, 9 figures</arxiv:comment>\n    <arxiv:primary_category term='cond-mat.str-el'/>\n    <author>\n      <name>S. N. Klimin</name>\n      <arxiv:affiliation>TQC, Departement Fysica, Universiteit Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium</arxiv:affiliation>\n    </author>\n    <author>\n      <name>J. Tempere</name>\n      <arxiv:affiliation>TQC, Departement Fysica, Universiteit Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium</arxiv:affiliation>\n      <arxiv:affiliation>Lyman Laboratory of Physics, Harvard University, Cambridge, MA 02138, USA</arxiv:affiliation>\n    </author>\n    <author>\n      <name>M. Houtput</name>\n      <arxiv:affiliation>TQC, Departement Fysica, Universiteit Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium</arxiv:affiliation>\n    </author>\n    <author>\n      <name>I. Zappacosta</name>\n      <arxiv:affiliation>TQC, Departement Fysica, Universiteit Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium</arxiv:affiliation>\n    </author>\n    <author>\n      <name>S. Ragni</name>\n      <arxiv:affiliation>Department for Research of Materials under Extreme Conditions, Institute of Physics, 10000 Zagreb, Croatia</arxiv:affiliation>\n      <arxiv:affiliation>Faculty of Physics, Computational Materials Physics, University of Vienna, Kolingasse 14-16, Vienna A-1090, Austria</arxiv:affiliation>\n    </author>\n    <author>\n      <name>T. Hahn</name>\n      <arxiv:affiliation>Center for Computational Quantum Physics, Flatiron Institute, 162 5th Avenue, New York, New York 10010, USA</arxiv:affiliation>\n    </author>\n    <author>\n      <name>L. Celiberti</name>\n      <arxiv:affiliation>Faculty of Physics, Computational Materials Physics, University of Vienna, Kolingasse 14-16, Vienna A-1090, Austria</arxiv:affiliation>\n    </author>\n    <author>\n      <name>C. Franchini</name>\n      <arxiv:affiliation>Faculty of Physics, Computational Materials Physics, University of Vienna, Kolingasse 14-16, Vienna A-1090, Austria</arxiv:affiliation>\n      <arxiv:affiliation>Department of Physics and Astronomy \"Augusto Righi\", Alma Mater Studiorum - Università di Bologna, Bologna, 40127 Italy</arxiv:affiliation>\n    </author>\n    <author>\n      <name>A. S. Mishchenko</name>\n      <arxiv:affiliation>Department for Research of Materials under Extreme Conditions, Institute of Physics, 10000 Zagreb, Croatia</arxiv:affiliation>\n      <arxiv:affiliation>RIKEN Center for Emergent Matter Science</arxiv:affiliation>\n    </author>\n  </entry>"
}