Pair correlations, short-range order, and dispersive excitations in the quasi-kagome quantum magnet volborthite

Abstract

We present spatial and dynamic information on the s = 1/2 distorted kagome antiferromagnet volborthite, Cu 3 V 2 O 7 (OD) 2 · 2D 2 O, obtained by polarized and inelastic neutron scattering. The instantaneous structure factor, S(Q), is dominated by nearest-neighbor pair correlations, with short-range order at wave vectors Q 1 = 0.65(3)Å −1 and Q 2 = 1.15(5)Å −1 emerging below 5 K. The excitation spectrum, S(Q,ω), reveals two steep branches dispersing from Q 1 and Q 2 , and a flat mode at ω f = 5.0(2) meV. The results allow us to identify the crossover at T * ∼ 1 K in 51 V NMR and specific-heat measurements as the buildup of correlations at Q 1 . We compare our data to theoretical models proposed for volborthite, and also demonstrate that the excitation spectrum can be explained by spin-wave-like excitations with anisotropic exchange parameters, as suggested by recent local-density calculations. 2 The physics of herbertsmithite is arguably influenced by depletion of the kagome lattice caused by antisite mixing, possibly resulting in a valence bond glass state. 3,4 In volborthite, the kagome planes are slightly distorted, but the lattice coverage is essentially complete. Since both systems deviate from the pure QKHAF model, focus has shifted to the intriguing question of which states arise when the QKHAF is perturbed. Remarkably, a multitude of different states have been proposed theoretically, depending on the nature of the perturbation. These range from ordered states [5][6][7][8][9] to the aforementioned valence bond glass. Experimentally, the ground state of volborthite remains enigmatic, despite over ten years of intensive study.2,10-13 Here we present a neutron-scattering investigation of volborthite, employing both xyz-polarized and inelastic time-of-flight techniques.Volborthite contains distorted kagome planes of edgesharing Cu 2+ octahedra, well separated (∼ 7.2Å) by pyrovanadate columns (V 2 O 7 ), Fig. 1(a). There are two crystallographically distinct Cu 2+ ions: Cu(2) forms chains along the b direction, while Cu(1) populates the interchain sites. Locally, Cu(1) and Cu(2) reside in tetragonally and axially distorted octahedra, respectively, suggesting the d 3z 2 −r 2 orbital being singly occupied on the Cu(1) site, with d x 2 −y 2 the magnetically active orbital for Cu(2). As a consequence, there are two different nearest-neighbor exchange pathways, J 1 (r Cu(2)-Cu(2) = 2.93Å) and J 1 (r Cu(1)-Cu(2) = 3.03Å). J 1 links Cu(2) ions along the b axis, while J 1 connects Cu(1) and Cu(2) ions. Furthermore, the edge sharing of the Cu (2) octahedra along the b direction implies the possibility of a strong next-nearest-neighbor exchange, J 2 .Despite a large estimated average nearest neighbor coupling J avg = (2J 1 + J 1 )/3 = 84 K, magnetic susceptibility and specific-heat measurements on volborthite show no signs of long-range order down to 1.8 K 2 . At yet lower temperatures, muon spin rotation (μSR) and 51 V NMR (Refs. 10-12) indicate slowing down of fluctuations at T * = 1 K, but with dynamics persisting to...

One-Third Magnetization Plateau with a Preceding Novel Phase in Volborthite

“…The structure was first reported to be monoclinic with the space group C2=m, but later a transition into the low-temperature I2=a structure was found near room temperature [20][21][22]. A peculiar magnetic transition is observed in various experiments around 1 K [21,[23][24][25][26][27][28], which is much lower than the Weiss temperature of −115 K; the low-temperature phase is called phase I. In addition, a series of magnetic field induced phase transitions accompanied by stepwise increases in magnetization are observed; phases II, III, and IV appear above 4.5, 25.5, and 45 T, respectively [29][30][31].…”

Vesignieite: AnS=12Kagome Antiferromagnet with Dominant Third-Neighbor Exchange

“…the distortion of the kagome lattice leads to inequivalent NN interactions and different local CuO 6 environments, suppressing long-range order to very low temperatures [19,20] and leading to a thermal Hall effect in the spin liquid state [21].…”

Orbital switching in a frustrated magnet

“…The average of (2J 1 + J 2 )/3 was estimated to be 77 K by a high-temperature series expansion fit of the magnetic susceptibility 9 . The nature of the magnetic ground state of volborthite has not been understood in detail yet, but is suggested to be unusual, with strong fluctuations at low temperature (LT) and short-range spin correlations [10][11][12][13][14][15] . One of the reasons for this ambiguity has been the unavailability of single crystals until now.…”