Excitations and transport in quantum liquids

Minoru Yamashita

V. Apaja, E. Krotscheck, Axel Rimnac +1 more

In this work, we study transport currents in excited states. This requires the calculation of particle currents [Formula: see text] to second order in the excitation amplitudes. For that purpose, we take a well-tested microscopic theory of inhomogeneous quantum liquids and extend it to find the mass currents created when atoms scatter off a surface or when excitations evaporate atoms. This is the first theoretical study of transport phenomena in a quantum liquid based on a quantitative microscopic theory.

Kostya Trachenko

Philippe Nozières, David Pines

V. Apaja, E. Krotscheck, Axel Rimnac +1 more

Until now, most of what has been said about excitations in quantum liquids has concerned the dynamic structure function, which is observable by means of neutron scattering. The dynamic structure function can be calculated using standard linear response theory. However, at this level one needs only transition densities 〈0|ρ̂|n〉 or transition currents 〈0|ĵ|n〉, which are oscillatory in time and hence do not describe mass transport. In this work we go a step further and study transport currents in excited states, 〈n|ĵ|n〉, which requires the calculation of 〈n|ĵ|n〉 to second order. For that purpose, we take a well‐tested microscopic theory of inhomogeneous quantum liquids and extend it to find the currents formed when helium atoms scatter off a helium slab or when excitations evaporate atoms (a setup experimented by A. F. G. Wyatt’s group in Exeter). Current conservation was already a major theoretical problem encountered by R. Feynman and led him to introduce backflow corrections. We show that perfect current conservation is expected only for exact solutions of the time‐dependent many‐body Schrödinger equation. This is the first extensive theoretical study of transport phenomena in a quantum liquid based on an accurate microscopic theory.

Tetsuya Minakawa, Yuta Murakami, Akihisa Koga +1 more

We study the spin transport through the quantum spin liquid (QSL) by investigating the real-time and real-space dynamics of the Kitaev spin system with zigzag edges using the time-dependent Majorana mean-field theory. After the magnetic-field pulse is introduced to one of the edges, spin moments are excited in the opposite edge region although spin moments are never induced in the Kitaev QSL region. This unusual spin transport originates from the fact that the S=1/2 spins are fractionalized into the itinerant and localized Majorana fermions in the Kitaev system. Although both Majorana fermions are excited by the magnetic pulse, only the itinerant ones flow through the bulk regime without spin excitations, resulting in the spin transport in the Kitaev system despite the presence of a nonzero spin gap. We also demonstrate that this phenomenon can be observed in the system with small Heisenberg interactions using the exact diagonalization.