By formally diagonalizing with accuracy the Hamiltonian of electrons in a crystal subject to electromagnetic perturbations, we resolve the debate on the Hamiltonian nature of semiclassical equations of motion with Berry-phase corrections, and therefore confirm the validity of the Liouville theorem. We show that both the position and momentum operators acquire a Berry-phase dependence, leading to a non-canonical Hamiltonian dynamics. The equations of motion turn out to be identical to the ones previously derived in the context of electron wave-packets dynamics.
PACS numbers:The notion of Berry phase has found many applications in several branches of quantum physics, such as atomic and molecular physics, optic and gauge theories, and more recently, in spintronics, to cite just a few. Most studies focused on the geometric phase a wave function acquires when a quantum mechanical system has an adiabatic evolution. It is only recently that a possible influence of the Berry phase on semiclassical dynamics of several physical systems has been investigated. It was then shown that Berry phases modify semiclassical dynamics of spinning particles in electric [1] and magnetic fields [2], as well as in semiconductors [3]. In the above cited examples, a noncommutative geometry, originating from the presence of a Berry phase, which turns out to be a spin-orbit coupling, underlies the semiclassical dynamics. Also, spin-orbit contributions to the propagation of light have been the focus of several other works [1,4,5], and have led to a generalization of geometric optics called geometric spinoptics [6].Semiclassical methods in solid-state physics have also played an important role in studying the dynamics of electrons to account for the various properties of metals, semiconductors, and insulators [7]. In a series of papers [8,9] (see also [10]), the following new set of semiclassical equations with a Berry-phase correction was proposed to account for the semiclassical dynamic of electrons in magnetic Bloch bands (in the usual one-band approximation)ṙwhere E and B are the electric and magnetic fields respectively and E(k) = E 0 (k)−m(k).B is the energy of the nth band with a correction due to the orbital magnetic moment [9]. The correction term to the velocity −k × Θ with Θ(k) the Berry curvature of electronic Bloch state in the nth band is known as the anomalous velocity predicted to give rise to a spontaneous Hall conductivity in ferromagnets [11]. For crystals with broken time-reversal symmetry or spatial inversion symmetry, the Berry curvature is nonzero [9]. Eqs.1 were derived by considering a wave packet in a band and using a time-dependent variational principle in a Lagrangian formulation. The derivation of a semiclassical Hamiltonian was shown to lead to difficulties in the presence of Berry-phase terms [9]. The apparent non-Hamiltonian character of Eqs.1 led the authors of [12] to conclude that the naive phase space volume is not conserved in the presence of a Berry phase, thus violating Liouville's theorem. To remedy this...