Magnetization “Steps” on a Kagome Lattice in Volborthite

Abstract

Magnetic properties of the spin-1/2 kagome-like compound volborthite are studied using a high-quality polycrystalline sample. It is evidenced from magnetization and specific heat measurements that the spins on the kagome lattice still fluctuate at low temperature, down to T = 60 mK that corresponds to 1/1500 of the nearest-neighbor antiferromagnetic interaction, exhibiting neither a conventional long-range order nor a spin gap. In contrast, 51 V NMR experiments revealed a sharp peak at 1 K in relaxation rate, which indicates that a certain exotic order occurs. Surprisingly, we have observed three "steps" in magnetization as a function of magnetic field, suggesting that at least four liquid-like or other quantum states exist under magnetic fields.KEYWORDS: kagome lattice, spin liquid, magnetization step, volborthite * E-mail address: hiroi@issp.u-tokyo.ac.jpThe magnetic properties of materials arise from the collective interaction of electron spins on atoms in a crystal. The antiferromagnetic interaction often causes long-range order (LRO) in an alternating up-down pattern called the Néel order at low temperatures. However, such LRO can be destroyed completely on trianglular lattices, because the antiferromagnetic interaction is inevitably frustrated on each triangle, suppressing a unique arrangement of spins covering the whole lattice.1, 2) It is expected therefore that the system remains "liquid", called the spin liquid, instead of a Néel order down to the lowest temperature. Since quantum fluctuations should play a crucial role in such a quantum disordered state, one would expect a new state of matter with properties we have never encountered. 3)Moreover, a quantum spin liquid might have exotic excitations or adopt a certain type of subtle order, such as a topological order. 4)Although an extensive study has been carried out to search for this mysterious state, clear experimental evidence has remained elusive until recently. 3,4) One famous candidate for a quantum spin liquid is a resonating-valence-bond (RVB) state on a triangular lattice postulated theoretically by Anderson in 1973. 5)Instead of an ordinary Néel state with LRO, the RVB state consists of paired spins with zero total quantum spin number, such as the configurations on a kagome lattice depicted in Fig.1(a). Since the ground state is defined quantum mechanically as a linear combination of all possible configurations derived from different pairings, a liquid-like behavior is expected even at T = 0. 6)A kagome lattice made of vertex-sharing triangles is one of the typical playgrounds for frustration physics.7) The theoretical ground state for the spin-1/2 kagome antiferromagnet (KAFM) is in fact an RVB state with an energy gap called the spin gap in the excitation spectrum. 1,8,9) This is because in the RVB picture, an excitation results from breaking spin singlet pairs: the larger the magnetic coupling, the larger the spin gap. Since the predicted magnitude of the spin gap Δ is small, J / 4 or J / 20, where J is the magnitude of the nearest-neigh...

Orbital switching in a frustrated magnet

“…In previous studies, polycrystalline samples were obtained by a precipitation, and subsequent heat treatment under hydrothermal conditions 10 . The maximum particle size obtained by this method was on the order of a few micrometers.…”

“…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.…”

High-Field Phase Diagram and Spin Structure of Volborthite Cu3V2O7(OH)2·2H2O

“…The magnetic susceptibility obeys the Curie-Weiss law ¼ C=ðT þ W Þ above 200 K with W ¼ 115 K, exhibits a broad maximum at 20 K, and approaches a finite value at the lowest temperatures. 11,19) An unusual magnetic transition has been observed near 1 K, [20][21][22][23] which is much lower than W , consistent with strong frustration in a kagome lattice. However, a recent density functional calculation 24) proposed that the frustration is attributed to the competition between a ferromagnetic J and an antiferromagnetic J 00 between second neighbors along the chain (Fig.…”

Numerical Study on the Spin Gap of the Kagome Lattice Antiferromagnet

“…The previous theoretical studies indicated that the system is disordered in the ground state. [1][2][3][4][5][6][7][8][9][10][11][12] Experimental studies to observe a novel spin liquid phase have been accelarated since discoveries of several candidate materials; the herbertsmithite, 13,14) the volborthite 15,16) and the vesignieite 17) for the Kagome lattice. However, the spin gap issue is still difficult to solve, because some of numerical methods are not adequate at present.…”