Tuning the S=1/2S = 1/2 square-lattice antiferromagnet Sr2_2Cu(Te1−x_{1-x}Wx_x)O6_6 from N\'eel order to quantum disorder to columnar order

The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ interactions ($J_2/J_1 \approx 0.5$). The double perovskites Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are isostructural spin-1/2 square-lattice antiferromagnets with N\'eel ($J_1$ dominates) and columnar ($J_2$ dominates) magnetic order, respectively. Here we characterize the full isostructural solid solution series Sr$_2$Cu(Te$_{1-x}$W$_x$)O$_6$ ($0 \leq x \leq 1$) tunable from N\'eel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the $x$ = 0.5 phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of $x \approx$ 0.1-0.6. This coincides with significant $T$-linear terms in the low-temperature specific heat. However, density functional theory calculations predict most of the materials are not in the highly frustrated $J_2/J_1 \approx 0.5$ region square-lattice Heisenberg model. Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in $x$ = 0.5.Comment: 20+5 pages, 6+4 figures. Accepted for publication in PR

Muon spin rotation and relaxation study on Nb1-yFe2+y

We present a detailed study of the magnetic properties of weakly ferromagnetic/quantum critical Nb1-yFe2+y using muon spin rotation and relaxation (μSR). By means of an angular dependent study of the muon spin rotation signal in applied magnetic fields on a single crystal in the paramagnetic state we establish the muon stopping site in the crystallographic lattice of NbFe2. With this knowledge we develop models to describe the muon spin rotation and relaxation signals in the weakly ferromagnetic, spin density wave, and quantum critical phases of Nb1-yFe2+y and fit the corresponding experimental data. In particular, we quantify the μSR response for quantum critical behavior in Nb1.0117Fe1.9883 and extract the influence of residual weak structural disorder. From our analysis, Nb1-yFe2+y emerges to be uniquely suited to study quantum criticality close to weak itinerant ferromagnetic order

Spin-liquid-like state in a spin-1/2 square-lattice antiferromagnet perovskite induced by d 10-d 0 cation mixing

A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Néel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor interaction dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use d 10-d 0 cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr2Cu(Te0.5W0.5)O6, where the isostructural end phases Sr2CuTeO6 and Sr2CuWO6 are Néel and columnar type antiferromagnets, respectively. We show that magnetism in Sr2Cu(Te0.5W0.5)O6 is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr2Cu(Te0.5W0.5)O6 - similar to several spin liquid candidates - a plateau in muon spin relaxation rate and a strong T-linear dependence in specific heat. Our observations for Sr2Cu(Te0.5W0.5)O6 highlight the role of disorder in addition to magnetic frustration in spin liquid physics.Peer reviewe

Temperature-driven reorganization of electronic order in CsV3_3Sb5_5

We report x-ray diffraction studies of the electronic ordering instabilities in the kagome material CsV$_3$Sb$_5$ as a function of temperature and applied magnetic field. Our zero-field measurements between 10 and 120 K reveal an unexpected reorganization of the three-dimensional electronic order in the bulk of CsV$_3$Sb$_5$: At low temperatures, a $2×2×2$ superstructure modulation due to electronic order is observed, which upon warming changes to a $2×2×4$ superstructure at 60 K. The electronic order-order transition discovered here involves a change in the stacking of electronically ordered $_3$Sb$_5$ layers, which coincides with anomalies previously observed in magnetotransport measurements. This implies that the temperature-dependent three-dimensional electronic order plays a decisive role for transport properties, which are related to the Berry curvature of the V bands. We also show that the bulk electronic order in CsV$_3$Sb$_5$ breaks the sixfold rotational symmetry of the underlying $P6/mmm$ lattice and perform a crystallographic analysis of the $2×2×2$ phase. The latter yields two possible superlattices, namely a staggered star-of-David and a staggered inverse star-of-David structure. Applied magnetic fields up to 10 T have no effect on the x-ray diffraction signal. This, however, does not rule out time-reversal symmetry breaking in CsV$_3$Sb$_5$

Temperature-driven reorganization of electronic order in CsV₃Sb₅

We report x-ray diffraction studies of the electronic ordering instabilities in the kagome material CsV3Sb5 as a function of temperature and applied magnetic field. Our zero-field measurements between 10 and 120 K reveal an unexpected reorganization of the three-dimensional electronic order in the bulk of CsV3Sb5: At low temperatures, a 2×2×2 superstructure modulation due to electronic order is observed, which upon warming changes to a 2×2×4 superstructure at 60 K. The electronic order-order transition discovered here involves a change in the stacking of electronically ordered V3Sb5 layers, which coincides with anomalies previously observed in magnetotransport measurements. This implies that the temperature-dependent three-dimensional electronic order plays a decisive role for transport properties, which are related to the Berry curvature of the V bands. We also show that the bulk electronic order in CsV3Sb5 breaks the sixfold rotational symmetry of the underlying P6/mmm lattice and perform a crystallographic analysis of the 2×2×2 phase. The latter yields two possible superlattices, namely a staggered star-of-David and a staggered inverse star-of-David structure. Applied magnetic fields up to 10 T have no effect on the x-ray diffraction signal. This, however, does not rule out time-reversal symmetry breaking in CsV3Sb5. © 2022 American Physical Society