Physical realization of a quantum spin liquid based on a complex frustration mechanism

Abstract

Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. Such phenomena is usually sought in simple lattices where antiferromagnetic interactions and/or anisotropies that favor specific alignments of the magnetic moments are frustrated by lattice geometries incompatible with such order e.g. triangular structures. Despite an extensive search among such compounds, experimental realizations remain very few. Here we describe the investigation of a novel, unexplored magnetic system consisting of strong ferromagnetic and weaker antiferromagnetic isotropic interactions as realized by the compound Ca10Cr7O28. Despite its exotic structure we show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid including coherent spin dynamics in the ground state and the complete absence of static magnetism.A quantum spin liquid is a macroscopic lattice of interacting magnetic ions with quantum spin number S=½, whose ground state has no static long-range magnetic order, instead the magnetic moments fluctuate coherently down to the lowest temperatures [1, 2]. It contrasts with the static long-range magnetically ordered ground states usually observed, and also with spin glass states where the spins are frozen into static short-range ordered configurations [3]. The excitations are believed to be spinons which have fractional quantum spin number S=½, and are very different from spin-waves or magnons that possess quantum spin number S=1 and are the characteristic excitations of conventional magnets. Spin liquids exist in one-dimensional magnets and the chain of spin-½ magnetic ions coupled by nearestneighbor, Heisenberg (isotropic), antiferromagnetic interactions is a well-established example [4]. This system has no static long-range magnetic order and the excitations are spinons. There is no energetic reason for the spinons to bind together, indeed if a spin-1 excitation is created e.g. by reversing a spin in the chain, it fractionalizes into two spin-½ spinons [5][6][7][8][9][10][11][12].The existence of spin liquids in dimensions greater than one is much less established. While static order does not occur in one-dimensional magnets, conventional two-and three-dimensional magnets order at temperatures at or above zero Kelvin [13]. This order can be suppressed by introducing competition (known as frustration) between the interactions that couple the magnetic ions. Geometrical frustration is achieved when the magnetic ions are located on lattices constructed from triangular motifs and are coupled by antiferromagnetic interactions. The antiferromagnetic coupling favors antiparallel spin alignment between nearest neighbor spins which can never be fully satisfied since the number of spins around the triangle is odd. This typically leads to h...

Crystal growth, structure and magnetic properties of Ca10Cr7O28

“…It provides the source of highly frustrated magnetism that leads to the quantum spin liquid behavior observed in Ca 10 Cr 7 O 28 as described by C. Balz et al in Ref. [13].…”

“…Our recent experimental investigation has however revealed that this compound has very interesting magnetic properties. It realizes a quantum spin liquid ground state [13] which is a topologically ordered state where the spin moments never become static but remain in collective motion down to the lowest temperatures. Spin liquids are undergoing a lot of theoretical investigation currently however very few physical realizations exist, thus Ca 10 Cr 7 O 28 provides an important addition to our current understanding of these novel states.…”

Topological thermal Hall effect in frustrated kagome antiferromagnets

“…The system can only be driven to a topological phase by an external factor such as an external magnetic field applied perpendicular to the kagomé plane, which can be written as H Z = −B · i S i , where B = gµ B B zẑ , g is the g-factor and µ B is the Bohr 0 0.5 1 magneton. The out-of-plane Zeeman magnetic field has a profound effect on frustrated magnets with QSL phases as it can induce a LRO [36][37][38]. In the ordered regime we find that the Zeeman magnetic field induces a noncoplanar magnetic spin texture with an emergent scalar spin chirality given by (see Ref.…”

“…Hence, the DMI suppresses the QSL phase of frustrated kagomé antiferromagnets up to a critical value [30]. The syntheses of materials have shown that various experimentally accessible frustrated kagomé antiferromagnets show evidence of coplanar/noncollinear q = 0 LRO at specific temperatures [29][30][31][32][33][34] [36] are fragile in the presence of applied magnetic field or pressure and they show evidence of LRO [37,38]. However, the role of DMI and magnetic field in frustrated kagome magnets has not been investigated in the context of thermal Hall effect and topological spin excitations.…”

Hierarchy of Exchange Interactions in the Triangular-Lattice Spin Liquid YbMgGaO4

“…Their detection, however, remains a central challenge for condensed matter physics [6], and relies on the presence of quantum entanglement in their ground state and fractional quasiparticles in their excitation spectra. While the former can be checked by numerics [7], the latter can be experimentally detected by thermodynamic techniques [8][9][10] or spectroscopic probes such as inelastic neutron scattering [11][12][13][14][15][16][17] and electronspin resonance [18][19][20][21].…”