Periodic Reporting for period 1 - TROPIC (Gaining leverage with spin liquids and superconductors)
Periodo di rendicontazione: 2023-05-01 al 2025-10-31
One of the main goals of TROPIC (aim 2) is to further our understanding of unconventional superconductivity. We chose UTe2 – an unconventional superconductor with several high-magnetic-field superconducting phases – as an ideal candidate to study the interplay between magnetism and superconductivity. Unlike most superconductors, which are destroyed in magnetic fields, UTe2 demonstrates superconductivity in magnetic fields as large at 70 tesla. Our initial plan was to study the high-field magnetism in the vicinity of the superconducting phases. We wanted to investigate the magnetic anisotropy near the metamagnetic transition – a transition into a field-polarized phase.
We proposed measurements on RuCl3 (aim 1) in order to confirm the presence or absence of i) oscillations associated with a spinon Fermi surface and ii) Ising topological transitions. While our magnetic measurements were ongoing, much progress has been made by other groups focused on the structural analyses of RuCl3. It was recognized that small crystals of RuCl3 – like those required for our measurements – tend to remain in the monoclinic structure at low temperatures, rather than going through a structural transition into the R3 phase at 150 K. We wanted to perform careful measurements of the magnetic anisotropy on a single crystal free from structural disorder. Our goals were three-fold: i) to search for predicted Ising topological phase transitions that should appear as singularities in our measurements, ii) to study the asymmetries observed in our measurements at higher temperatures to refine the model that describes RuCl3, and iii) to gain clarity on the features that are intrinsic to RuCl3 and those that may arise from stacking defaults or structural disorder.
We proposed measurements on RuCl3 (aim 1) in order to confirm the presence or absence of i) oscillations associated with a spinon Fermi surface and ii) Ising topological transitions. While our magnetic measurements were ongoing, much progress has been made by other groups focused on the structural analyses of RuCl3. It was recognized that small crystals of RuCl3 – like those required for our measurements – tend to remain in the monoclinic structure at low temperatures, rather than going through a structural transition into the R3 phase at 150 K. We wanted to perform careful measurements of the magnetic anisotropy on a single crystal free from structural disorder. Our goals were three-fold: i) to search for predicted Ising topological phase transitions that should appear as singularities in our measurements, ii) to study the asymmetries observed in our measurements at higher temperatures to refine the model that describes RuCl3, and iii) to gain clarity on the features that are intrinsic to RuCl3 and those that may arise from stacking defaults or structural disorder.
Our measurements probe the magnetic anisotropy in UTe2 with fine control over the magnetic field orientation in the sample. While measuring UTe2 in short (<100 ms) 60 tesla pulses, we discovered a large change in the magnetic susceptibility transverse to the external magnetic field. The large change in the transverse magnetic susceptibility is observed for magnetic field orientations near its superconducting phases, suggesting a clear connection to high-field superconductivity. Explanations of unconventional superconductivity often rely on magnetic fluctuations as the mediating source, and evidence for magnetic fluctuations exists in related uranium-based compounds. In these related compounds, reentrant superconductivity exists when magnetic field is applied perpendicular to the direction of their ferromagnetically ordered moments. UTe2 is unique in that it does not exhibit long-range magnetic order; there was no prior evidence for magnetic order, or a reason that moments would be fluctuating. Previous measurements report only a first-order metamagnetic transition. Our high field studies probing the magnetic susceptibility transverse to the magnetic field suggest that large transverse magnetic fluctuations are likely responsible for its remarkable high-field superconducting phase.
Prior measurements suggested that several critical endpoints exist at high magnetic fields in UTe2. Our measurement technique is a second-derivative of the free energy (it’s the angular derivative of magnetic torque), which is ideal for identifying 1st and 2nd order phase transitions. We already have evidence for a couple of the critical endpoints that exist in the vicinity of the magnetic fluctuations, but we want to confirm their presence and fully map out the phase diagram using our measurements of the magnetotropic susceptibility at high magnetic fields. We will also explore the evolution (both in field-orientation and temperature) of observed second-order phase transitions to determine whether their influence stems from a quantum critical point. To extend our measurements on UTe2, we were recently awarded additional magnet time for experiments up to 65 tesla at the National High Magnetic Field Laboratory in the US.
We completed a full angle dependence of the magnetotropic susceptibility in RuCl3, which requires rotation of the crystal in 30 degree increments between each measurement. Our measurements on RuCl3 confirm a TN = 14 K, typical for the monoclinic structure at low temperatures. We mapped out the boundaries of the AFM phase, as well as the boundary of ZZ2 phase for field applied near the a-axis. Our results are consistent with other measurements showing these phases, however, they are much more comprehensive. We show that the Neel temperature is dependent upon in-plane magnetic field angle. We also confirm that there is only one transition (or two for field near the a-axis and symmetric directions due to the ZZ2 phase) when magnetic field is used to suppress the AFM phase for nearly all field orientations. This is not in accordance with longitudinal thermal conductivity measurements that displayed oscillatory behavior under similar conditions. Upon rotating a fixed magnetic field of 14 tesla, oscillatory behavior is observed in our measurements upon crossing the AFM phase boundary, but only when detecting ac-plane anisotropy. We are still working to understand the origin of this behavior, but we believe that the oscillatory behavior may be related to rotation of the ZZ2 structure under a magnetic field.
Prior measurements suggested that several critical endpoints exist at high magnetic fields in UTe2. Our measurement technique is a second-derivative of the free energy (it’s the angular derivative of magnetic torque), which is ideal for identifying 1st and 2nd order phase transitions. We already have evidence for a couple of the critical endpoints that exist in the vicinity of the magnetic fluctuations, but we want to confirm their presence and fully map out the phase diagram using our measurements of the magnetotropic susceptibility at high magnetic fields. We will also explore the evolution (both in field-orientation and temperature) of observed second-order phase transitions to determine whether their influence stems from a quantum critical point. To extend our measurements on UTe2, we were recently awarded additional magnet time for experiments up to 65 tesla at the National High Magnetic Field Laboratory in the US.
We completed a full angle dependence of the magnetotropic susceptibility in RuCl3, which requires rotation of the crystal in 30 degree increments between each measurement. Our measurements on RuCl3 confirm a TN = 14 K, typical for the monoclinic structure at low temperatures. We mapped out the boundaries of the AFM phase, as well as the boundary of ZZ2 phase for field applied near the a-axis. Our results are consistent with other measurements showing these phases, however, they are much more comprehensive. We show that the Neel temperature is dependent upon in-plane magnetic field angle. We also confirm that there is only one transition (or two for field near the a-axis and symmetric directions due to the ZZ2 phase) when magnetic field is used to suppress the AFM phase for nearly all field orientations. This is not in accordance with longitudinal thermal conductivity measurements that displayed oscillatory behavior under similar conditions. Upon rotating a fixed magnetic field of 14 tesla, oscillatory behavior is observed in our measurements upon crossing the AFM phase boundary, but only when detecting ac-plane anisotropy. We are still working to understand the origin of this behavior, but we believe that the oscillatory behavior may be related to rotation of the ZZ2 structure under a magnetic field.
Our results on UTe2 were unexpected in the sense that we were not aware that our technique probes the transverse magnetic susceptibility prior to this work. This is a highly-relevant quantity to access, as we demonstrated in the case of UTe2. The behavior we observed in UTe2 cannot be uncovered with conventional measurements of the magnetism. Thus, this method should be applied more broadly to other unconventional superconductors.
Our work on RuCl3 demonstrates that not all TN = 14 K crystals have multiple magnetic transitions – transitions that were previously associated with structural inhomogeneities. We mapped out the magnetic anisotropy in a single domain TN = 14 K crystal of RuCl3. Future plans include a better analysis of our high-temperature data in order to better constrain the exchange parameters and the model that is used to describe RuCl3. We also finished the installation of our dilution refrigerator and plan to continue measurements down to 100 mK to search for topological phase transitions in the coming months.
To assist in the characterization of RuCl3 and UTe2 samples and advance our experimental setup in preparation for high-field magnet times, we have successfully installed a 4.5 kV capacitor bank to power our own 35 tesla pulsed field magnet. We are currently constructing the parts needed to perform measurements, and expect the first tests to be performed by the end of 2025.
We just started our first measurements on Sr2RuO4 (aim 3) to try to confirm the presence of superconducting domain walls.
Sidewise, we have been testing new cantilevers with higher frequencies, which are driven by a sputtered dielectric. We are currently exploring new methods for detection of the signal and better characterization of the levers (e.g. scanning the mechanical modes using a laser doppler vibrometer and by using a cryogenic displacement sensor to observe the angular deflections while the levers are exposed to a large magnetic torque).
Our work on RuCl3 demonstrates that not all TN = 14 K crystals have multiple magnetic transitions – transitions that were previously associated with structural inhomogeneities. We mapped out the magnetic anisotropy in a single domain TN = 14 K crystal of RuCl3. Future plans include a better analysis of our high-temperature data in order to better constrain the exchange parameters and the model that is used to describe RuCl3. We also finished the installation of our dilution refrigerator and plan to continue measurements down to 100 mK to search for topological phase transitions in the coming months.
To assist in the characterization of RuCl3 and UTe2 samples and advance our experimental setup in preparation for high-field magnet times, we have successfully installed a 4.5 kV capacitor bank to power our own 35 tesla pulsed field magnet. We are currently constructing the parts needed to perform measurements, and expect the first tests to be performed by the end of 2025.
We just started our first measurements on Sr2RuO4 (aim 3) to try to confirm the presence of superconducting domain walls.
Sidewise, we have been testing new cantilevers with higher frequencies, which are driven by a sputtered dielectric. We are currently exploring new methods for detection of the signal and better characterization of the levers (e.g. scanning the mechanical modes using a laser doppler vibrometer and by using a cryogenic displacement sensor to observe the angular deflections while the levers are exposed to a large magnetic torque).