Interplay of electronic topology and strong correlations

Using high magnetic fields, we tuned the noncentrosymmetric Weyl-Kondo semimetal (WKSM) Ce3Bi4Pd3 through a two-stage transition: first at a critical field Bc1 ~ 9 T, which we associate with the suppression of the WKSM ground state, and second at a critical field Bc2 ~ 14 T, from the underlying, remnant Kondo insulating state to a metallic heavy fermion state. We interpret the magnetic field tuning of electronic topology as motion of Weyl nodes in momentum space driven by a large Zeeman effect acting on local moments in a fully-functioning Kondo background, up to the point where the nodes meet and annihilate in a topological quantum phase transition.

• Control of electronic topology in a strongly correlated electron system
  S. Dzsaber, D. A. Zocco, A. McCollam, F. Weickert, R. McDonald, M. Taupin, X. Yan, A. Prokofiev, L. M. K. Tang, B. Vlaar, L. E. Winter, M. Jaime, Q. Si, and S. Paschen, Nature Commun. 13, 5729 (2022)

Pauli-limited multiband superconductivity and strain-driven quantum criticality in (K,Rb,Cs)Fe2As2

Using low-temperature thermal expansion and magnetostriction experiments, we found compelling evidence for Pauli-limited multiband superconductivity in AFe2As2 (A=K, Rb and Cs), signaled as a crossover from second- to first-order phase transition at the upper critical field for magnetic fields applied parallel to the ab-planes.
Since in (K,Rb,Cs)Fe2As2 the occupation of the iron 3d-orbitals is close to half-filling (N = 5.5), the existence of strong electronic correlations in these compounds has been attributed to their proximity to an orbital-selective Mott transition. Normal-state quantum oscillations observed in magnetostriction revealed band-specific enhancement of the electronic masses as large as 16 times the bare-electron mass. The divergence of the Grüneisen ratio derived from thermal expansion indicates that, with increasing unit cell volume from K to Cs, a strain-driven quantum critical point is approached.

Charge-density waves and superconductivity in rare-earth tritellurides

It has recently been shown that the quasi-two-dimensional rare-earth tritellurides RTe3 (R = La-Nd, Sm, Gd-Tm) enter an unidirectional, incommensurate charge-density-wave (CDW) state when cooled below a temperature TCDW1 ~ 450 - 250 K, which decreases with decreasing unit cell volume due to the lanthanide contraction. For the heavier rare-earths (Dy-Tm), a second CDW, orthogonal to the first, appears at TCDW2 < TCDW1. We have recently found that the application of external pressure induces a superconducting state in GdTe3, TbTe3 and DyTe3 at low temperatures, coexisting or competing with the two CDWs and the magnetism arising from the lanthanide atoms.
DyTe3 orders below TCDW1 = 308 K and TCDW2 = 68 K, with orthogonal ordering wavevectors qCDW1 ~ (0, 0, 0.3) and qCDW2 ~ (0.68, 0, 0), respectively. Using high energy-resolution elastic and inelastic x-ray scattering experiments, we found at TCDW1  a strong softening of phonon modes along both in-plane directions with an energy difference of only 2 meV. However, the phonon modes at qCDW2 that exhibit a strong softening at TCDW1 show no measurable response to the phase transition at TCDW2. This indicates that the low-temperature CDW order is not just the 90°-rotated analogue of the higher-temperature CDW, posing new questions about the observed phase competition of the two structural distortions, e.g., as a function of pressure.

Interplay of superconductivity, spin-density-waves and magnetism in Fe-based superconductors under pressure

Superconductivity, charge- and spin-density waves are collective electronic phenomena that originate from electron-electron and electron-phonon interactions, and the concept of Fermi surface competition between these collective states is one of the most fundamental problems of condensed matter physics. High pressures provide a clean method to tune the electronic properties that determine the superconducting, magnetic or charge-ordered ground states of complex materials. The interplay between the spin-density wave, localized magnetism and superconductivity in the recently discovered Fe-based high-temperature superconductors, have been investigated under extreme conditions of pressure, magnetic field and low temperatures.

Bose-Einstein condensation of spin degrees of freedom in NiCl2-4SC(NH2)2

In traditional Bose-Einstein condensates, such as dilute gases of cold 40K atoms, the bosons form a coherent ground state as the temperature is lowered below the critical temperature TBEC. The Bose-Einstein condensate (BEC) is formed when the lowest energy state of the system has a finite occupation of bosonic particles. A new set of materials is receiving an increasing amount of attention in which Bose-Einstein condensation can be induced by magnetic fields. Here the magnetic field, not the temperature, tunes the number of bosons from zero to nonzero across a critical field. We have investigated single crystals of the organic magnet NiCl2-4SC(NH2)2 for the phenomena of BEC using specific heat and magnetocaloric effect measurements taken in a dilution refrigerator and a 20 T magnet at the NHMFL in LANL. The key prediction of Bose-Einstein condensation that we have been able to verify is the power-law dependence of the critical field line Hc-Hc1 ~ Tα (α = 3/2 for a 3D BEC).

• Bose-Einstein Condensation of S = 1 Ni spin degrees of freedom in NiCl2-4SC(NH2)2
  V. S. Zapf, D. Zocco, B. R. Hansen, M. Jaime, N. Harrison, C. D. Batista, M. Kenzelmann, C. Niedermayer, A. Lacerda, and A. Paduan-Filho, Phys. Rev. Lett. 96, 077204 (2006)