In physics, Berry connection and Berry curvature are related concepts which can be viewed, respectively, as a local gauge potential and gauge field associated with the Berry phase or geometric phase. These concepts were introduced by Michael Berry in a paper published in 1984 emphasizing how geometric phases provide a powerful unifying concept in several branches of classical and quantum physics.
In quantum mechanics, the Berry phase arises in a cyclic adiabatic evolution. The quantum adiabatic theorem applies to a system whose Hamiltonian depends on a parameter that varies with time. If the 'th eigenvalue remains non-degenerate everywhere along the path and the variation with time t is sufficiently slow, then a system initially in the eigenstate will remain in an instantaneous eigenstate of the Hamiltonian, up to a phase, throughout the process. Regarding the phase, the state at time t can be written as where the second exponential term is the "dynamic phase factor." The first exponential term is the geometric term, with being the Berry phase. From the requirement that satisfy the time-dependent Schrödinger equation, it can be shown that indicating that the Berry phase only depends on the path in the parameter space, not on the rate at which the path is traversed. In the case of a cyclic evolution around a closed path such that, the closed-path Berry phase is An example of physical system where an electron moves along a closed path is cyclotron motion. Berry phase must be considered to obtain the correct quantization condition.
Gauge transformation
A gauge transformation can be performed to a new set of states that differ from the original ones only by an -dependent phase factor. This modifies the open-path Berry phase to be. For a closed path, continuity requires that , and it follows that is invariant, modulo, under an arbitrary gauge transformation.
Berry connection
The closed-path Berry phase defined above can be expressed as where is a vector-valued function known as the Berry connection. The Berry connection is gauge-dependent, transforming as . Hence the local Berry connection can never be physically observable. However, its integral along a closed path, the Berry phase, is gauge-invariant up to an integer multiple of. Thus, is absolutely gauge-invariant, and may be related to physical observables.
Berry curvature
The Berry curvature is an anti-symmetric second-rank tensor derived from the Berry connection via In a three-dimensional parameter space the Berry curvature can be written in the pseudovector form The tensor and pseudovector forms of the Berry curvature are related to each other through the Levi-Civitaantisymmetric tensor as . In contrast to the Berry connection, which is physical only after integrating around a closed path, the Berry curvature is a gauge-invariant local manifestation of the geometric properties of the wavefunctions in the parameter space, and has proven to be an essential physical ingredient for understanding a variety of electronic properties. For a closed path that forms the boundary of a surface, the closed-path Berry phase can be rewritten using Stokes' theorem as If the surface is a closed manifold, the boundary term vanishes, but the indeterminacy of the boundary term modulo manifests itself in the Chern theorem, which states that the integral of the Berry curvature over a closed manifold is quantized in units of. This number is the so-called Chern number, and is essential for understanding various quantization effects. Finally, note that the Berry curvature can also be written as a sum over all other eigenstates in the form
The Hamiltonian of a spin-1/2 particle in a magnetic field can be written as where denote the Pauli matrices, is the magnetic moment, and B is the magnetic field. In three dimensions, the eigenstates have energies and their eigenvectors are Now consider the state. Its Berry connection can be computed as and the Berry curvature is If we choose a new gauge by multiplying by, the Berry connections are and , while the Berry curvature remains the same. This is consistent with the conclusion that the Berry connection is gauge-dependent while the Berry curvature is not. The Berry curvature per solid angle is given by. In this case, the Berry phase corresponding to any given path on the unit sphere in magnetic-field space is just half the solid angle subtended by the path. The integral of the Berry curvature over the whole sphere is therefore exactly, so that the Chern number is unity, consistent with the Chern theorem.
Applications in crystals
The Berry phase plays an important role in modern investigations of electronic properties in crystalline solids and in the theory of the quantum Hall effect. The periodicity of the crystalline potential allows the application of the Bloch theorem, which states that the Hamiltonian eigenstates take the form where is a band index, is a wavevector in the reciprocal-space, and is a periodic function of. Then, letting play the role of the parameter, one can define Berry phases, connections, and curvatures in the reciprocal space. For example, the Berry connection in reciprocal space is Because the Bloch theorem also implies that the reciprocal space itself is closed, with the Brillouin zone having the topology of a 3-torus in three dimensions, the requirements of integrating over a closed loop or manifold can easily be satisfied. In this way, such properties as the electric polarization, orbital magnetization, anomalous Hall conductivity, and orbital magnetoelectric coupling can be expressed in terms of Berry phases, connections, and curvatures.
