Imaging phase transitions in quantum materials

Project Information

SEE_QPT

Grant agreement ID: 866236

DOI

10.3030/866236

EC signature date 13 January 2020

Start date 1 April 2021

End date 31 March 2028

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 052 739,00

EU contribution

€ 2 052 739,00

Coordinated by

BAR ILAN UNIVERSITY
Israel

Periodic Reporting for period 3 - SEE_QPT (Imaging phase transitions in quantum materials)

Reporting period: 2024-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

The key goal in SEE_QPT is to provide clear-cut evidence for elusive states that are in the blind spot of global measurements using an ultra-sensitive local probe. Our hypothesis is that close to criticalities, we can access hidden phases and that the spatial distribution of properties can be utilized to resolve the underlying physics. Specifically, we expect disorder-related inhomogeneities to have significant effects on the transition. Our experimental hypothesis is two-fold. First, local, non-invasive, sensitive magnetic imaging is the correct tool for the task. Second, we must focus our efforts right at phase transitions where new phases emerge.
Objectives
1. Use local information to provide a new view of quantum criticality.
2. Detect hidden phases. To search for the signatures of a few predicted electronic phases that have so far evaded experimental observation.
3. Correlation between emergent states at complex oxide interfaces.
4. Detect and quantify topologically protected states.

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

In SEE_QPT we aim to study the emergence of new states of matter in many-electron systems when interactions are turned on. Traditional bulk measurement techniques are not sufficient to reveal these elusive states, so local imaging techniques are necessary. We work to provide clear evidence for hidden phases and utilize the spatial distribution of properties to resolve the underlying physics. We use scanning SQUID microscopy as a local probing tool to detect and understand these states. We investigate quantum materials and test for correlations and interactions between different electronic states. The project also aims to develop new measurement modes for the scanning SQUID microscopy technique to allow in situ tuning of the system under measurement. We aim to explore fundamental questions like the universality of transitions and assist the development of quantum materials.
In order to examine the emergence of new states in many-electron systems, we simultaneously map magnetism, conductivity, and superconductivity. Our main achievements include: (1) Revealing hidden magnetic memory in the van der Waals superconductor 4Hb-TaS2. (2) Developing a method to tune the mixed superconducting state without an applied magnetic field using the vector potential. (3) Exploring the interactions between ferroelectricity, magnetism, and the 2D electron system formed at the interface of LAO/ETO/STO, showcasing its potential as a multiferroic. (4) Investigating the current distribution in amorphous LAO/STO near the metal-insulator transition, while tuning in situ both the donor concentrations through oxygen annealing and the carrier density via electrostatic gating. (5) Visualizing the current flow in superconducting networks, particularly near the breakdown of the network. (6) Resolving how gate-induced spatial variations in the lateral carrier density in STO-based devices depend on device geometry and choice of dielectric materials. (7) Investigating the effect of chiral molecules on superconductivity. In addition to achieving these, we worked on improving the noise characteristics of scanning SQUID measurements in ultra-low T measurements.

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)

The result that was most surprising during this grant period was discovering an unusual magnetic state in a van der Waals superconductor, 4Hb-TaS2, which has several anomalous properties. We investigated the magnetic order and its relationship to superconductivity. The underlying research question is whether the combination of a superconductor and a Mott insulator in 4Hb-TaS2 leads to new phases of matter. We investigated the magnetic landscape of 4Hb-TaS2 in both the superconducting and normal phases and found spontaneous vortices whose density can be trained with external fields in the normal state. While the training indicates that time reversal symmetry is broken already in the normal state, we discovered that it does not generate any detectible magnetic signatures in the normal state. One of the suggested mechanisms is the presence of a chiral spin liquid phase in the system, but it is not confirmed yet, and we are still working to understand the mechanism of this unusual state. These results were published in Nature 607, 692 (2022).

We hope our efforts will make it possible to track and identify elusive states in the blind spot of global measurements. We access hidden phases close to criticalities and work to resolve underlying physics. We utilize the spatial distribution of properties to understand the transition. For us, inhomogeneities and defects are opportunities rather than an obstacle. From a practical point of view, we expect our studies to lead to the development of new and exciting quantum materials devices. We focus our efforts on phase transitions where new phases emerge. In parallel, our improvement of the planar SQUID technology will assist in positioning the SQUID as a highly useful tool for material and device characterization and will lead to new discoveries and applications.

Current injected into a superconducting network near the breakdown of superconductivity