This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.

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The electronic properties of graphene

A quantal system in an eigenstate, slowly transported round a circuit C by varying parameters R in its Hamiltonian Ĥ(R), will acquire a geometrical phase factor exp{iγ(C)} in addition to the familiar dynamical phase factor. An explicit general formula for γ(C) is derived in terms of the spectrum and eigenstates of Ĥ(R) over a surface spanning C. If C lies near a degeneracy of Ĥ, γ(C) takes a simple form which includes as a special case the sign change of eigenfunctions of real symmetric matrices round a degeneracy. As an illustration γ(C) is calculated for spinning particles in slowly-changing magnetic fields; although the sign reversal of spinors on rotation is a special case, the effect is predicted to occur for bosons as well as fermions, and a method for observing it is proposed. It is shown that the Aharonov-Bohm effect can be interpreted as a geometrical phase factor.

Quantal phase factors accompanying adiabatic changes

Quantum cryptography could well be the first application of quantum mechanics at the individual quanta level. The very fast progress in both theory and experiments over the recent years are reviewed, with emphasis on open questions and technological issues.

Quantum Cryptography

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

http://www.zenogaburro.it/papers/LightPropagation.pdf

Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction

This paper reviews recent experimental and theoretical progress concerning many-body phenomena in dilute, ultracold gases. It focuses on effects beyond standard weak-coupling descriptions, such as the Mott-Hubbard transition in optical lattices, strongly interacting gases in one and two dimensions, or lowest-Landau-level physics in quasi-two-dimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near-Feshbach resonances in the BCS-BEC crossover.

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Many-Body Physics with Ultracold Gases

For non-Hermitian Hamiltonians with an isolated degeneracy (‘exceptional point’), a model for cycling around loops that enclose or exclude the degeneracy is solved exactly in terms of Bessel functions. Floquet solutions, returning exactly to their initial states (up to a constant) are found, as well as exact expressions for the adiabatic multipliers when the evolving states are represented as a superposition of eigenstates of the instantaneous Hamiltonian. Adiabatically (i.e. for slow cycles), the multipliers of exponentially subdominant eigenstates can vary wildly, unlike those driven by Hermitian operators, which change little. These variations are explained as an example of the Stokes phenomenon of asymptotics. Improved (superadiabatic) approximations tame the variations of the multipliers but do not eliminate them.

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Slow non-Hermitian cycling: exact solutions and the Stokes phenomenon

Wavefront dislocations-i.e. singularities of the phase of a wave psi in the form of moving lines in space where mod psi mod vanishes-are studied for initially plane waves that have passed through a random space and time-dependent phase-changing screen. For transmitted waves that are Gaussian random, incoherent, quasi-monochromatic and paraxial the following quantities are calculated in terms of the statistics of the phase screen: dislocation densities, i.e. the average number of dislocation lines piercing unit area of variously-oriented surfaces, and dislocation fluxes, i.e. the average number of dislocation lines crossing unit length of variously-directed lines in unit time.

/pdf/disruption-of-wavefronts-statistics-of-dislocations-in-2tw373eyd2.pdf

Disruption of wavefronts: statistics of dislocations in incoherent Gaussian random waves

The quantum mechanics of two identical particles with spin S in three dimensions is reformulated by employing not the usual fixed spin basis but a transported spin basis that exchanges the spins al...

https://www.hpl.hp.com/techreports/97/HPL-BRIMS-97-10.pdf

Indistinguishability for quantum particles: spin, statistics and the geometric phase

A magnet in the form of a spinning-top can float stably above a repelling magnetic base. The principal mechanism of stability is static equilibrium in a potential energy field E, arising dynamically from the adiabatic coupling of the spin with the magnetic field B of the base and involving the magnitude B of this field. E is close to a harmonic potential, that is, one whose Laplacian is zero, for which Earnshaw's theorem would forbid stable equilibrium. Therefore its minimum is very shallow, and requires the mass of the top to be adjusted delicately so that it hangs within a small interval of height. The stability interval is increased by a post-adiabatic dynamic coupling of the velocity of the top to B, through an effective `geometric magnetic field' constructed from the spatial derivatives of B; this effect gets stronger as the top is spun faster. The device is analogous to several traps for microscopic particles.

Michael V Berry

Papers

Slow non-Hermitian cycling: exact solutions and the Stokes phenomenon

Disruption of wavefronts: statistics of dislocations in incoherent Gaussian random waves

Indistinguishability for quantum particles: spin, statistics and the geometric phase

The LevitronTM: an adiabatic trap for spins

Topography of random surfaces