Strongly correlated fermions
Strongly correlated fermions are elementary particles (like electrons) whose mutual interactions play a crucial role and cannot be neglected. The term is often used in the context of condensed matter physics to describe systems where the collective behavior of particles is highly complex and cannot be explained by simple free electron models.
The study of strongly correlated fermions at LNCMI explores complex systems where particle interactions give rise to astonishing physical properties, often inaccessible without the use of intense magnetic fields
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Research staff
Topics
High-Tc cuprate superconductors have the highest known superconducting critical temperature at ambient pressure. Their phase diagram features several baffling mysteries. The basic questions we are trying to answer are: what is the organizing principle of the phase diagram of high-Tc cuprates? what is the mechanism for high-Tc superconductivity? We use high magnetic fields to suppress superconductivity in order to reach and determine the nature of the electronic interactions at play in the phase diagram and in the pairing mechanism of those systems.
Iron-based superconductors form another family of superconducting materials with relatively high Tcs. Iron-based materials are usually magnets and were not known to be superconducting before the discovery of LaOFeAs in 2008. How the spins on iron contribute to superconductivity is actively studied. The multi-orbital physics of these materials adds another layer of complexity. By applying external forces, such as uniaxial stress, the balance of the orbital occupancy can be modified. Whether oribital fluctuations contribute to the pairing mechanism is an open question.
Techniques
Cantilever magnetization measurement (Toulouse Contact : C. Proust, D. Vignolles)
Torque magnetometry is a powerfull technique to measure quantum oscillations in metals. In pulsed field magnetic torque measurement is performed with a commercial piezoresistive micro-cantilever, as shown in the figure above. The size of the sample studied is approximately 0.1×0.1×0.05 mm3. Variations of the cantilever piezoresistance are measured with a Wheatstone bridge with an ac excitation at a frequency of 63kHz. In static field, the bending of a CuBe cantilever is measured capacitively.
As an example we show quantum oscillations signal measured with torque magnetometry in pulsed field in an organic conductor κ-(BEDT-TTF)2Cu(NCS)2 (left) and in a high-Tc cuprate superconductor YBa2Cu3Oy (right).
Ultrasonic measurement (Grenoble contact : D. Leboeuf, Toulouse contact : C. Proust)
As an example we show results of sound velocity measurements in the cuprate high-Tc YBCO. The field dependence of the sound velocity of a longitudinal mode in YBCO with doping level p = 0.122 measured at temperatures ranging from 9.8 to 45.2 K is shown. The arrows indicate the transition towars charge order onset. Curves are shifted vertically for clarity. Click here for more details about ultrasound in YBCO.
Transport measurement (Grenoble contact : D. Leboeuf, Toulouse contact : C. Proust, D. Vignolles)
Resistivity, Hall effect, magnetoresistance are measured in metals and superconductors in order to investigate the electronic structure (Shubnikov – de Haas oscillations, Hall effect), and the carrier mobility (magnetoresistance, resistivity).
In static field, low noise lock-in measurements are performed. Several samples can be fitted on a single probe, with or without rotation stage. Transport measurements are routinely performed in pulsed magnetic field in sample with resistance as low as 1 mohm. The measurements are performed with a current excitation between 0.1 mA and 10 mA at a frequency in the range 2 – 80 kHz. A high-speed acquisition system is used to digitize the reference signal (current) and the voltage drop across the sample. The data are post-analyzed with software to perform the phase comparison.
Above we show the magnetoresistance measured in an overdoped cuprate high-Tc superconductor, Tl2Ba2CuO6+delta. On top of a monotonic background, a small but clearly resolved oscillatory component appears in high field. Those Shubnikov – de Haas oscillations provide pivotal information about the electronic structure of those systems. Check here for more details on this study.
Non-contact transport measurement (TDO) (Toulouse contact : C. Proust, D. Vignolles)
As an example we show quantum oscillations obtained in the organic conductor κ-(BEDT-TTF)2Cu(NCS)2 measured with torque magnetometry (blue) and with TDO method (green).
More detailed example can be found here : https://iopscience.iop.org/article/10.1209/0295-5075/97/57003
Nuclear Magnetic Resonance (NMR) (Grenoble contacts : M.-H. Julien, I. Vinograd)
In a typical nuclear magnetic resonance experiment (NMR), a given nuclear species is spin-polarized by a static magnetic field and driven off-equilibrium by radiofrequency pulses. The radiofrequency response of the nuclei, recorded during the return back to equilibrium, measures how the environment of the nucleus affects the polarization. Analysis of this response provides us with a wealth of information about the local electronic, magnetic and structural properties, including how they possibly vary in space and how they fluctuate with time (dynamics).
The above example shows a two-component NMR resonance of 17O nuclei that provides evidence of charge-density wave formation in the high temperature cuprate superconductor YBa2Cu3Oy (more details here).
Link to the NMR group at LNCMI
Publications
Selected publications
– Charge order near the antiferromagnetic quantum critical point in the trilayer high Tc cuprate HgBa2Ca2Cu3O8+δ
V. Oliviero, I. Gilmutdinov, D. Vignolles, S. Benhabib, N. Bruyant, A. Forget, D. Colson, W. A. Atkinson & C. Proust
npj Quantum Materials 9, 75 (2024)
– Universal correlation between H-linear magnetoresistance and T-linear resistivity in high-temperature superconductors
J. Ayres, M. Berben, C. Duffy, R. D. H. Hinlopen, Y.-T. Hsu, A. Cuoghi, M. Leroux, I. Gilmutdinov, M. Massoudzadegan, D. Vignolles, Y. Huang, T. Kondo, T. Takeuchi, S. Friedemann, A. Carrington, C. Proust & N. E. Hussey
Nature Communications 15, 8406 (2024)
– Hidden magnetism at the pseudogap critical point of a cuprate superconductor
M. Frachet, I. Vinograd, R. Zhou, S. Benhabib, Sh. Wu, H. Mayaffre, S. Krämer, S. K. Ramakrishna, A. P. Reyes, J. Debray, T. Kurosawa, N. Momono, M. Oda, S. Komiya, Sh. Ono, M. Horio, J. Chang, C. Proust, D. LeBoeuf & M.–H. Julien
Nature Physics 16, 1064 (2020)
– Change of carrier density at the pseudogap critical point of a cuprate superconductor
S. Badoux, W. Tabis, F. Laliberté, G. Grissonnanche, B. Vignolle, D. Vignolles, J. Béard, D. A. Bonn, W. N. Hardy, R. Liang, N. Doiron-Leyraud, Louis Taillefer & Cyril Proust
Nature 531, 210 (2016)
LNCMI publications on HAL