Charge delocalisation and hopping in an ultra-cold atomic lattice

利用激光，原子云可以被冷却到绝对零度以上百万分之一度以内。如此冷的原子几乎不会移动，它们可以被激光束捕获，形成一种由规则间隔的原子组成的晶体，被称为光学晶格。该项目的目的是研究当我们使用激光脉冲将这些原子中的电子激发到高能量时会发生什么。通常，原子中的电子的轨道靠近原子核，但是如果我们给最外层的电子越来越多的能量，它的轨道就会越来越大。最终，晶格中相邻原子的电子轨道开始重叠，电子不再属于某个特定原子，可以沿着晶格扩散。这个概念在物理学的许多领域都很重要，例如在解释金属中的电子如何自由形成电流时。用光学晶格中捕获的超冷原子来研究这个问题的好处是，可以非常精确地控制晶格中每个原子的性质。例如，使用激光束可以从晶格中的一个原子中去除电子，留下单个带正电的离子。然后，来自相邻原子的电子可以“跳”过去取代失去的电子，留下一个离子。研究电子如何从一个原子跳到另一个原子，是这项计划的目标之一。

Using lasers, a cloud of atoms can be cooled to within a millionth of a degree above absolute zero. Atoms that are this cold hardly move at all, and they can be trapped using laser beams to make a crystal of regularly spaced atoms held in place by light, known as an optical lattice. The aim of this project is to study what happens when we excite the electrons in these atoms to high energies using a pulse of laser light. Normally the electrons in an atom orbit close tothe nucleus, but if we give the outermost electron more and more energy, its orbit gets larger and larger. Eventually the electron orbits of neighbouring atoms in the lattice begin to overlap, and the electrons no longer belong to a particular atom and can spread out along the lattice. This concept is important in many areas of physics, for example in explaining how the electrons in a metal free themselves to form an electrical current. The advantage of studying this with ultra-cold atoms trapped in an optical lattice is that the properties of each of the atoms in thelattice can be controlled extremely precisely. For example using a laser beam an electron can be removed from just one atom in the lattice, leaving behind a single positively charged ion. An electron from a neighbouring atom can then ``hop'' across to replace the lost electron, leaving behind an ion in its turn. Studying how electrons hop from atom to atom is one of the goals of this proposal.