Correlation-driven metallic topology

Project Information

CorMeTop

Grant agreement ID: 101055088

DOI

10.3030/101055088

EC signature date 25 May 2022

Start date 1 September 2022

End date 31 August 2027

Funded under

• European Research Council (ERC)

Total cost

€ 3 356 483,00

EU contribution

€ 3 356 483,00

Coordinated by

TECHNISCHE UNIVERSITAET WIEN
Austria

Periodic Reporting for period 1 - CorMeTop (Correlation-driven metallic topology)

Reporting period: 2022-09-01 to 2025-02-28

Summary of the context and overall objectives of the project

Developments in the past decade have shaped the term topological quantum matter. In the solid state, much progress has been made on non- and weakly-interacting systems and correlated insulators, but gapless topological phases governed by strong correlations are a vastly open challenge. They are of great interest because a wealth of new quantum phases with new properties and functionalities are expected.

The PI and her collaborators recently discovered one such phase—the Weyl-Kondo semimetal—and revealed its extreme topological responses. This sets the stage for the present project.

In CorMeTop we aim to discover new correlation-driven gapless topological phases, establish design principles for such phases, reveal their new signatures, and assess their potential for quantum devices. Heavy fermion compounds were chosen as a versatile platform for these studies. Four design principles—symmetry, emergence, engineered platforms, and parameter tuning—will be followed, and a combination of recently established and entirely new experimental probes will be used. The basis for these studies will be high-quality bulk single crystals and thin films grown by molecular beam epitaxy.

Among the questions to be addressed are: To what extent does symmetry dictate the fate of topological states in the limit of strong correlations? What is the connection between quantum criticality or other emergent phenomena, long-range entanglement, and topology? Can new platforms based on heavy fermion systems stabilize robust and even braidable Majorana bound states? Which theoretical parameters control topology and how can one vary them experimentally? Which functionalities bear potential for quantum applications?

We expect the project to establish an emerging field and guide a larger community to boost progress.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During the first two years of the project, activities on all four design principles—symmetry, emergence, engineered platforms, and parameter tuning—have started, with the aim to discover new correlation-driven gapless topological phases, establish design principles for such phases, reveal new signatures of these phases, and assess their potential for quantum devices.

At focus are heavy fermion compounds, as versatile platforms for the investigation of strong correlation phenomena. The compound Ce3Bi4Pd3, recently discovered by the PI and her collaborators as the first material exhibiting a correlation-driven gapless topological phase, serves as the starting point for the project.

The workflow in the project is that once a promising compound is identified (via the above design principles), the first step is the synthesis of pure bulk single crystals, to ensure that the measured properties are intrinsic. Various growth techniques are employed, as well as various structural and analytical measurements to characterize the crystals. In addition, molecular beam epitaxy is used to grow selected compounds as thin films. Then, various physical property measurements are performed, including new ones devised within the project, at need under multiple extreme conditions of temperature, magnetic field, and pressure. Experiments at large facilities (neutrons, muons, x-rays, high fields) complement the laboratory experiments. Another important aspect of the project is close collaboration with different teams of theorists, using complementary approaches to guide the material discovery.

Progress has been made on most project aspects. New equipment was delivered and installed, and new synthesis and measurement techniques were developed. Some promising compounds were identified using the proposed design principles. Some of them were grown and investigated. Several interesting and, in part, unexpected and puzzling observations were already made.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Using parameter tuning we demonstrated that genuine topology control can be achieved in strongly correlated materials. Magnetic field tuning experiments revealed that Weyl nodes can annihilate at a magnetic field where otherwise “nothing happens”. It is thus not the tuning of underlying phases with and without Weyl nodes (such as a magnetic phase with, and a paramagnetic phase without Weyl nodes) but the topological features (here the Weyl nodes) per se that react to the magnetic field drive. This demonstrates a new aspect of topology introduced by strong correlations.

Exploring the design principle of emergence, we discovered that a topological phase can nucleate out of quantum critical fluctuations of beyond-order-parameter type, where Landau quasiparticles are assumed to be absent. New theoretical concepts are needed to rationalize this result. The experimental evidence is a dome of Weyl-Kondo semimetal behavior centered at the material's quantum critical point, and suppressed by tuning parameters that drive the system away from this quantum critical point. This finding may boost the discovery of other correlation-driven topological phases.

To understand the basis of emergent topological phases, the quantum critical state out of which they emerge was studied with a new technique, the quantum Fisher information. It provides a lower bound of multipartite entanglement, a quantity that characterizes the ground state’s wave function. The large detected multipartite entanglement is a new characteristic and may lead to a new level of understanding, both of the quantum critical state itself and of emergent phases nucleating out of it, including ones with nontrivial topology.

Illustration of the annihilation of Weyl nodes. © Raphael Bühler