Periodic Reporting for period 1 - Distort-to-Grasp (Distorting unconventional superconductivity - A grasp of electronic phases with multiple broken symmetries)
Reporting period: 2023-01-01 to 2025-06-30
When electrons in a solid interact strongly, they can form novel states of matter with fascinating technological possibilities and intriguing intellectual challenges. They might realize the macroscopic quantum state with zero electrical resistivity – superconductivity – or spontaneously lose the spherical symmetry of their properties, such that the electrical resistivity becomes different when current flows in different directions – an electronic nematic state. Surprisingly, more and more nematic superconductors, combining both, have recently been discovered. Such observations suggest a fundamental link between nematicity and superconductivity that is not yet understood. It may be that the emergence of superconductivity is favoured or strengthened if a material is already nematic, or superconductivity itself might cause nematicity.
Answering these questions is hindered by the lack of systematic data on the nematicity-superconductivity interaction, due to the absence of routine high-resolution probes of nematicity that are applicable in the superconducting state. To investigate this interaction, we are working on establishing a novel “distortiometry” approach based on measuring a material’s elastic response to anisotropic stress, taking full advantage of established capacitance dilatometry and recent advances in strain-tuning techniques.
We first investigate materials that show superconductivity with a “background” of nematic order. One established example for this is FeSe. Second, we investigate materials whose superconductivity seems to cause nematicity. A much-studied example for this is Sr-Bi2Se3. Our goals are (1) to obtain an exhaustive picture of the relation between unconventional superconductivity and competing nematic phases, (2) to solve the question whether nematicity is really induced by superconductivity in materials such as Sr-Bi2Se3, and (3) provide a broad experimental based of materials and results to determine whether nematicity is an unknown natural stability of superconductors. Thus, we aim to establish whether nematicity and unconventional superconductivity are linked by a universal principle, possibly uncovering a dramatic twist in the long-standing superconductivity research.
Answering these questions is hindered by the lack of systematic data on the nematicity-superconductivity interaction, due to the absence of routine high-resolution probes of nematicity that are applicable in the superconducting state. To investigate this interaction, we are working on establishing a novel “distortiometry” approach based on measuring a material’s elastic response to anisotropic stress, taking full advantage of established capacitance dilatometry and recent advances in strain-tuning techniques.
We first investigate materials that show superconductivity with a “background” of nematic order. One established example for this is FeSe. Second, we investigate materials whose superconductivity seems to cause nematicity. A much-studied example for this is Sr-Bi2Se3. Our goals are (1) to obtain an exhaustive picture of the relation between unconventional superconductivity and competing nematic phases, (2) to solve the question whether nematicity is really induced by superconductivity in materials such as Sr-Bi2Se3, and (3) provide a broad experimental based of materials and results to determine whether nematicity is an unknown natural stability of superconductors. Thus, we aim to establish whether nematicity and unconventional superconductivity are linked by a universal principle, possibly uncovering a dramatic twist in the long-standing superconductivity research.
For the ambitious objectives, we need high-quality single crystals of the materials under study. During the first reporting period, we focused on the synthesis of well-known candidate materials for nematic superconductivity. Single crystals of Fe(Se,S) were grown using chemical vapor transport. An effort was undertaken to maximize the dimensions of the grown crystals so that they are now suitable for the subsequent measurements.
To investigate the materials whose superconductivity might be nematic by itself, we need single crystals of Sr and/or Nb intercalated Bi2Se3 with appropriate dimensions. As the materials are relatively soft, a growth procedure that yields free-standing single crystals (which do not need any mechanical extraction) is preferable, so that crystals do not get damaged before the measurement. Therefore, in a first step, we developed successfully a solution growth of Bi2Se3 out of a non-stoichiometric melt. In a second step, we introduced Sr and Nb into the melt. The samples were characterized with x-ray diffraction, electron microscopy and low-temperature electrical transport measurements in magnetic fields up to 9 T. We identified and investigated secondary phases and their solidification temperatures.
To investigate the materials whose superconductivity might be nematic by itself, we need single crystals of Sr and/or Nb intercalated Bi2Se3 with appropriate dimensions. As the materials are relatively soft, a growth procedure that yields free-standing single crystals (which do not need any mechanical extraction) is preferable, so that crystals do not get damaged before the measurement. Therefore, in a first step, we developed successfully a solution growth of Bi2Se3 out of a non-stoichiometric melt. In a second step, we introduced Sr and Nb into the melt. The samples were characterized with x-ray diffraction, electron microscopy and low-temperature electrical transport measurements in magnetic fields up to 9 T. We identified and investigated secondary phases and their solidification temperatures.
In this first reporting period, we were first to apply the solution growth out of non-stoichiometry melts (“self-flux technique”) to the Bi2Se3 system. This has the significant advantage to produce free-standing single crystals that are much more suitable for the planned measurements. In addition, this growth method allows to identify secondary phases and their solidification temperatures much better than traditional growth techniques. In this way, we worked on identifying possible material issues that might be the cause of the reported nematic superconductivity in Sr-Bi2Se3. Future experiments will reveal, as planned, the structural and elastic response of nematic superconductivity.