Theory of Quantum Systems at Surfaces


The theory team at QNS has two goals:

1. to be just one door away for every experimentalist and aid in the explanation of new discoveries and
2. to, in turn, guide experimentalists in our search for novel quantum spin systems on surfaces and interfaces with the goal of achieving full quantum control.

We rely on three different approaches to modelling surface spin systems, that are often interconnected: The first is ab initio calculations in the framework of Density Functional Theory (DFT). This method is suitable for treating large surface systems and small molecules at equal footing, but has some drawbacks when describing correlated phenomena such as magnetism at the nanoscale due to approximations made in the implementation of DFT.

To overcome these limitations, QNS and collaborators have developed tools that go beyond the DFT mean field description which we call “DFT+Multiplet”, which allows us to recover the true many-body character of the wavefunction. Using DFT+Multiplet we are able to accurately explore the landscape of magnetic energies and, in the near future, also the coherence times of such systems. 1) 2)

To better understand quantum coherence of our surface spin systems we often perform simulations of open quantum systems in the Lindblad formalism. In such a calculation, the real time evolution of the quantum system in the lab frame including interaction with the environment which leads to decay and decoherence can be accurately described.

Most of our work can be performed in-house on our mid-size high-performance computer cluster. We employ publicly available open-source codes and actively engage in method and code development.
To expand our network, we have hosted a workshop in May 2019 that brought together more than 20 experts in the field of theoretical advances of spins at the nanoscale and through our visitor program we have hosted experts from all over the world, making QNS a global hub for theoretical studies of quantum phenomena on surfaces.

*Fig.1 Spin-polarization isosurfaces of antiferromagnetically coupled nickelocene molecules, a promising candidate as "spin-filter" in STM measurements.

*Fig.2 Low-energy spectrum of the multiplet solution of Fe on MgO under the influence of crystal field, spin-orbit coupling (SO) and magnetic field perpendicular to the sample (Bz)

Longer-term Goals

• Find surface quantum systems that are promising for the realization of full quantum control using only ab initio-based methods
• Develop theoretical understanding of quantum coherence, with emphasis on mitigating decoherence
• Investigate the transition from quantum to classical behavior, including the quantum measurement problem

Near-term Goals

• Expanding our studies from d-electron system to the more complex f-electron systems of the lanthanide series
• Explore more systems for long magnetic lifetime and coherence times
• Thereby guiding experimentalists in their search for such systems
• Expand the QNS global hub with long-term and short-term visitors