E L E C T R O N I C S T R U C T U R E , M A G N E T I S M A N D D Y N A M I C S
S C I E N T I F I C H I G H L I G H T S
1 1 2 H I G H L I G H T S 2 0 2 1 I
experiments were carried out at the Cu L3 absorption edge (932 eV) with 60 meV energy resolution, which greatly helps to single out the elastic scattering representative of the CDW. The data were collected on single crystals of YBa2Cu3O6.67, cut down in the form of needles, elongated along either the a- or b-axis, respectively, carefully polished, and mounted in the strain rig (Figure 91). A dedicated loadlock and transfer system with shuttles were developed for a fast entry of the rigs in the UHV chamber of ERIXS, which was prewired and redesigned to allow for instantaneous electrical connectivity and operability of the voltage-controlled strain apparatus upon landing of the shuttle.
Large uniaxial pressures could be applied along both a- and b-axes of YBa2Cu3O6.67 and the response of CDW was monitored in both directions. Figure 92 summarises the main result: the nematic response of the CDW
domains to uniaxial stress. a-axis pressure enhances the b-CDW correlation and intensity, whereas b-axis pressure does not affect it. Conversely, b-axis pressure only enhances the a-CDW. The observed highly anisotropic, nematic response of the 2D CDW to strain provides a definitive experimental answer to the debate about the uniaxial nature of the CDW in the cuprates.
Akin to previous work, it was observed that a 3D CDW appears for large uniaxial stress along the a-axis in the (0,k,l) plane. Its resonant energy profile matches that of the 2D CDW peaking at the resonance of Cu atoms in the CuO2 planes, thus indicating that the two orders involve the same charge carriers. Under b-axis compression, however, no 3D CDW is observed up to 1% strain. These results impose stringent constraints on the theoretical description of the CDW and its competition with high-temperature superconductivity.
PRINCIPAL PUBLICATION AND AUTHORS
Charge density waves in YBa2Cu3O6.67 probed by resonant X-ray scattering under uniaxial compression, H.-H. Kim (a), E. Lefrançois (a), K. Kummer (b), R. Fumagalli (c), N.B. Brookes (b), D. Betto (a,b), S. Nakata (a), M. Tortora (a), J. Porras (a), T. Loew (a), M.E. Barber (d), L. Braicovich (b,c), A.P. Mackenzie (d,e), C.W. Hicks (d), B. Keimer (a), M. Minola (a), M. Le Tacon (f), Phys. Rev. Lett. 126, 037002 (2021); https:/doi.org/10.1103/PhysRevLett.126.037002 (a) Max Planck Institute for Solid State Research, Stuttgart (Germany) (b) ESRF (c) Dipartimento di Fisica, Politecnico di Milano, Milan (Italy) (d) Max Planck Institute for Chemical Physics of Solids, Dresden (Germany) (e) Scottish Universities Physics Alliance, School of Physics and Astronomy, University of St Andrews (UK) (f) Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology (Germany)
 A. Frano et al., J. Phys. Condens. Matter 32, 374005 (2020).  H.-H. Kim et al., Science 362, 1040-1044 (2018)
Quantum order by disorder revealed under magnetic field
Ferromagnetism is suppressed with increasing temperature, usually vanishing at the Curie point. However, in PrPtAl, ferromagnetism transforms into a modulated state. X-ray diffraction shows modulated states also occur when ferromagnetism is weakened by a perpendicular field. This means they can exist at very low temperatures, furthering understanding of how such states arise.
Ferromagnets are ubiquitous in everyday life, present in household items ranging from magnets on a fridge door to the sensors and motors inside it, with dozens present in an average car. As temperature is raised, ferromagnetic order disappears but, more rarely, the uniform magnetic order can first be replaced by a static magnetic wave. One mechanism for this is known as quantum order by disorder (QOBD); the wave (the order) forms because it has more low-energy excited states (the disorder) available to it than
the uniform state and this lowers its energy, offsetting the energy cost of producing the wave.
The first observation of QOBD magnetic waves was reported at the border between paramagnetism and ferromagnetism in the ferromagnet PrPtAl . Two modulated states, SDW1 and SDW2, exist over a small temperature range, forming spiral structures along the c-axis with differing periodicities that are also temperature- dependent. To investigate QOBD in more detail and explore what happens over a broader range of parameters, resonant X-ray diffraction was carried out on PrPtAl under applied magnetic fields at beamline BM28. Resonant X-ray diffraction affords both magnetic site selectivity and high resolution, making it a powerful method for characterising modulated magnetism. The results show that the magnetic field can support the survival of QOBD- driven magnetism extending to lower temperatures and to the absolute zero, vastly increasing the landscape over which these states are known to exist. This could help us understand why other field-induced quantum phenomena,