Publications

Latest publications & preprints:

Chiral d-wave Superconductivity in a Triangular Surface Lattice Mediated by Long-range Interaction (2017)

Xiaodong Cao, Thomas Ayral, Zhicheng Zhong, Olivier Parcollet, Dirk Manske, and Philipp Hansmann

Correlated ad-atom systems on the Si(111) surface have recently attracted an increased attention as strongly correlated systems with a rich phase diagram. We study these materials by a single band model on the triangular lattice including 1/r long-range interaction. Employing the recently proposed TRILEX method we find an unconventional superconducting phase of chiral d-wave symmetry in hole-doped systems. The superconductivity is driven simultaneously by both charge and spin fluctuations and is strongly enhanced by the long-range tail of the interaction. We provide an analysis of the relevant collective bosonic modes and explain how in triangular symmetry both charge and spin channels contribute to the Cooper-pairing.

arXiv:1710.03467

Origins of bond and spin order in rare-earth nickelate bulk and heterostructures (2017)

Yi Lu, Zhicheng Zhong, Maurits W. Haverkort, Philipp Hansmann

We analyze the charge- and spin response functions of rare-earth nickelates RNiO3 and their heterostructures using random-phase approximation in a two-band Hubbard model. The inter-orbital charge fluctuation is found to be the driving mechanism for the rock-salt type bond order in bulk RNiO3, and good agreement of the ordering temperature with experimental values is achieved for all RNiO3 using realistic crystal structures and interaction parameters. We further show that magnetic ordering in bulk is not driven by the spin fluctuation and should be instead explained as ordering of localized moments. This picture changes for low-dimensional heterostructures, where the charge fluctuation is suppressed and overtaken by the enhanced spin instability, which results in a spin-density-wave ground state observed in recent experiments. Predictions for spectroscopy allow for further experimental testing of our claims.

Phys. Rev. B 95, 195117 (2017)

Band Alignment and Charge Transfer in Complex Oxide Interfaces (2017)

Zhicheng Zhong and Philipp Hansmann

The synthesis of transition metal heterostructures is currently one of the most vivid fields in the design of novel functional materials. In this paper, we propose a simple scheme to predict band alignment and charge transfer in complex oxide interfaces. For semiconductor heterostructures, band-alignment rules like the well-known Anderson or Schottky-Mott rule are based on comparison of the work function or electron affinity of the bulk components. This scheme breaks down for oxides because of the invalidity of a single work-function approximation as recently shown in [Phys. Rev. B 93, 235116 (2016); Adv. Funct. Mater. 26, 5471 (2016)]. Here, we propose a new scheme that is built on a continuity condition of valence states originating in the compounds shared network of oxygen. It allows for the prediction of sign and relative amplitude of the intrinsic charge transfer, taking as input only information about the bulk properties of the components. We support our claims by numerical density functional theory simulations as well as (where available) experimental evidence. Specific applications include (i) controlled doping of SrTiO3 layers with the use of 4d and 5d transition metal oxides and (ii) the  control of magnetic ordering in manganites through tuned charge transfer.

Phys. Rev. X 7, 011023 (2017) [more]

Towards a first-principles determination of effective Coulomb interactions in correlated electron materials: Role of intershell interactions (2017)

Priyanka Seth, Philipp Hansmann, Ambroise van Roekeghem, Loig Vaugier, and Silke Biermann

The determination of the effective Coulomb interactions to be used in low-energy Hamiltonians for materials with strong electronic correlations remains one of the bottlenecks for parameter-free electronic structure calculations. We propose and benchmark a scheme for determining the effective local Coulomb interactions for charge-transfer oxides and related compounds. Intershell interactions between electrons in the correlated shell and ligand orbitals are taken into account in an effective manner, leading to a reduction of the effective local interactions on the correlated shell. Our scheme resolves inconsistencies in the determination of effective interactions as obtained by standard methods for a wide range of materials, and allows for a conceptual understanding of the relation of cluster model and dynamical mean field-based electronic structure calculations.

Phys. Rev. Lett. 119, 056401 (2017)
Zhicheng Zhong and Philipp HansmannThe development of novel functional materials in experimental labs combined with computer-based compound simulation brings the vision of materials design on a microscopic scale continuously closer to reality. For many applications interface and surface phenomena rather than bulk properties are key. One of the most fundamental qualities of a material-vacuum interface is the energy required to transfer an electron across this boundary, i.e. the work function. It is a crucial parameter for numerous applications, including organic electronics, field electron emitters, and thermionic energy converters. Being generally very resistant to degradation at high temperatures, transition metal oxides present a promising materials class for such devices. We have performed a systematic study for perovskite oxides that provides reference values and, equally important, reports on materials trends and the tunability of work functions. Our results identify and classify dependencies of the work function on several parameters including specific surface termination, surface reconstructions, oxygen vacancies, and heterostructuring.Phys. Rev. B 93, 235116 (2016)

Tuning the work function in transition metal oxides and their heterostructures (2016)

Zhicheng Zhong and Philipp Hansmann

The development of novel functional materials in experimental labs combined with computer-based compound simulation brings the vision of materials design on a microscopic scale continuously closer to reality. For many applications interface and surface phenomena rather than bulk properties are key. One of the most fundamental qualities of a material-vacuum interface is the energy required to transfer an electron across this boundary, i.e. the work function. It is a crucial parameter for numerous applications, including organic electronics, field electron emitters, and thermionic energy converters. Being generally very resistant to degradation at high temperatures, transition metal oxides present a promising materials class for such devices. We have performed a systematic study for perovskite oxides that provides reference values and, equally important, reports on materials trends and the tunability of work functions. Our results identify and classify dependencies of the work function on several parameters including specific surface termination, surface reconstructions, oxygen vacancies, and heterostructuring.

Phys. Rev. B 93, 235116 (2016)

Exploring small energy scales with x-ray absorption and dichroism (2016)

C. Praetorius, M. Zinner, P. Hansmann, M. W. Haverkort, and K. Fauth

Soft x-ray linear and circular dichroism (XLD, XMCD) experiments at the CeM4,5 edges are being used to determine the energy scales characterizing the Ce 4ƒ degrees of freedom in the ultrathin ordered surface intermetallic CeAgx/Ag(111). We find that all relevant interactions, i. e. Kondo scattering, crystal field splitting and magnetic exchange coupling occur on small scales. Our study demonstrates the usefulness of combining x-ray absorption experiments probing linear and circular dichroism owing to their strong sensitivity for anisotropies in both charge distribution and paramagnetic response, respectively.

Phys. Rev. B 93, 165107 (2016)

Uncertainty principle for experimental measurements: Fast versus slow probes (2016)

Philipp Hansmann, Thomas Ayral, Antonio Tejeda, and Silke Biermann

The result of a physical measurement depends on the timescale of the experimental probe. In solid-state systems, this simple quantum mechanical principle has far-reaching consequences: the interplay of several degrees of freedom close to charge, spin or orbital instabilities combined with the disparity of the time scales associated to their fluctuations can lead to seemingly contradictory experimental findings. A particularly striking example is provided by systems of adatoms adsorbed on semiconductor surfaces where different experiments -- angle-resolved photoemission, scanning tunneling microscopy and core-level spectroscopy -- suggest different ordering phenomena. Using most recent first principles many-body techniques, we resolve this puzzle by invoking the time scales of fluctuations when approaching the different instabilities. These findings suggest a re-interpretation of ordering phenomena and their fluctuations in a wide class of solid-state systems ranging from organic materials to high-temperature superconducting cuprates.

Nature Scientific Reports vol. 6, 19728 (2016)

Full lists of publications via Researcher ID:

P. Hansmann Z. Zhong D. Mantadakis X. Cao
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