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In mathematics, the family of Debye functions is defined by

The functions are named in honor of Peter Debye, who came across this function (with n = 3) in 1912 when he analytically computed the heat capacity of what is now called the Debye model.


Mathematical propertiesEdit

Relation to other functionsEdit

The Debye functions are closely related to the polylogarithm.

Series ExpansionEdit

They have the series expansion[1]


where   is the n-th Bernoulli number.

Limiting valuesEdit


If   is the gamma function and   is the Riemann zeta function, then, for  ,



The derivative obeys the relation


where   is the Bernoulli function.

Applications in solid-state physicsEdit

The Debye modelEdit

The Debye model has a density of vibrational states


with the Debye frequency ωD.

Internal energy and heat capacityEdit

Inserting g into the internal energy


with the Bose–Einstein distribution


one obtains


The heat capacity is the derivative thereof.

Mean squared displacementEdit

The intensity of X-ray diffraction or neutron diffraction at wavenumber q is given by the Debye-Waller factor or the Lamb-Mössbauer factor. For isotropic systems it takes the form


In this expression, the mean squared displacement refers to just once Cartesian component ux of the vector u that describes the displacement of atoms from their equilibrium positions. Assuming harmonicity and developing into normal modes,[3] one obtains


Inserting the density of states from the Debye model, one obtains



  1. ^ Abramowitz, Milton; Stegun, Irene Ann, eds. (1983) [June 1964]. "Chapter 27". Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. Applied Mathematics Series. 55 (Ninth reprint with additional corrections of tenth original printing with corrections (December 1972); first ed.). Washington D.C.; New York: United States Department of Commerce, National Bureau of Standards; Dover Publications. p. 998. ISBN 978-0-486-61272-0. LCCN 64-60036. MR 0167642. LCCN 65-12253.
  2. ^ Gradshteyn, Izrail Solomonovich; Ryzhik, Iosif Moiseevich; Geronimus, Yuri Veniaminovich; Tseytlin, Michail Yulyevich; Jeffrey, Alan (2015) [October 2014]. "3.411.". In Zwillinger, Daniel; Moll, Victor Hugo. Table of Integrals, Series, and Products. Translated by Scripta Technica, Inc. (8 ed.). Academic Press, Inc. pp. 355ff. ISBN 0-12-384933-0. LCCN 2014010276. ISBN 978-0-12-384933-5.
  3. ^ Ashcroft & Mermin 1976, App. L,

Further readingEdit