Quantum magnetism and quantum phase transitions

Abstract:

CuLa2Ge2O8 forms a distorted triangular lattice of quantum spin-1/2 Cu²⁺ ions. A crystal growth method was developed using the traveling-solvent floating zone technique, resulting in the synthesis of a large single crystal (4mm×4mm×10 mm). The crystal was characterized with regard to phase purity and crystallinity using powder x-ray diffraction, energy dispersive x-ray analysis, and Laue diffraction, and found to be of excellent quality. The magnetic properties were characterized using dc-susceptibility, magnetization, and heat capacity measurements, which revealed weak magnetic frustration with long-range magnetic order occurring below TN=1.14(1) K. The magnetic structure determined using neutron powder diffraction is a commensurate, noncollinear antiferromagnetic structure, different from the 120^∘ order of an equilateral triangular antiferromagnet. The ordered moments lie in the bc-plane, with components mb=0.50(3) µB and mc=0.73(5) µB along the b- and c-axes respectively, giving a total ordered moment of Mtotal=0.89(6)µB/Cu²⁺ at 20 mK.

Abstract:

Muon spin rotation (μSR) measurements of internal magnetic field shifts, known as the muon Knight shift, are used for determining pairing symmetries in superconductors. While this technique has been especially effective for f-electron-based heavy-fermion superconductors, it remains challenging in d-electron-based superconductors such as Sr_{2}RuO_{4}, where the Knight shift is intrinsically small. Here, we report high-precision muon Knight shift measurements of superconducting Sr_{2}RuO_{4}. We observe that using multiple pieces of crystals, a common practice in μSR measurements, induces a substantial paramagnetic shift below the superconducting transition temperature, T_{c}, when a weak magnetic field is applied. We attribute such an unresolved paramagnetic shift to stray fields generated by neighboring diamagnetic crystals. To avoid this, one piece of crystal was used in this Letter. We experimentally determine the muon Knight shift of Sr_{2}RuO_{4} in the normal state to be -116±7  ppm. By combining the observed muon Knight shift with independently determined bulk magnetization data from the same crystal used in μSR and carefully separating various contributions to the shift, we confirm a significant reduction in the spin Knight shift below T_{c}, consistent with spin-singlet-like pairing. This result constitutes the precise muon Knight shift measurement in a d-electron-based superconductor. Our results highlight the potential of μSR as a powerful complementary technique to the established method of nuclear magnetic resonance for probing the spin susceptibility in superconductors.

Abstract:

CeTe3 is a van der Waals antiferromagnet composed of magnetic [CeTe]⁺ layers coupled to highly conducting Te^0.5− square nets. Its simple quasi-two-dimensional electronic structure and cleavable nature make it an appealing platform for exploring correlated magnetism in reduced dimensions. To clarify the nature of its low-temperature state, we performed single-crystal neutron diffraction down to 0.3 K, complemented by scanning tunneling microscopy. A magnetic transition near 1.5 K gives rise to incommensurate Bragg peaks at q± ∼ (±0.17, 0, 0.31), consistent with a double-q magnetic order whose moments are predominantly aligned along the c axis. The strongly reduced ordered moment is consistent with enhanced quantum fluctuations driven by c- f hybridization, while the deviation of the propagation vectors from simple nesting suggests a coupling to residual charge-density-wave instabilities of the quasi-one-dimensional Te-derived bands. These results indicate that CeTe3 hosts a correlated magnetic ground state where spin and itinerant charge degrees of freedom are intimately linked in the van der Waals limit.