Quantum spin liquid is a disordered magnetic state with fractional spin excitations. Its clearest example is found in an exactly solved Kitaev honeycomb model where a spin flip fractionalizes into two types of anyons, quasiparticles that are neither fermions nor bosons: a pair of gauge fluxes and a Majorana fermion. Here we demonstrate this kind of fractionalization in the Kitaev paramagnetic state of the honeycomb magnet α-RuCl3. The spin-excitation gap measured by nuclear magnetic resonance consists of the predicted Majorana fermion contribution following the cube of the applied magnetic field, and a finite zero-field contribution matching the predicted size of the gauge-flux gap. The observed fractionalization into gapped anyons survives in a broad range of temperatures and magnetic fields despite inevitable non-Kitaev interactions between the spins, which are predicted to drive the system towards a gapless ground state. The gapped character of both anyons is crucial for their potential application in topological quantum computing.
Employing complementary torque magnetometry and electron spin resonance on single crystals of herbertsmithite, the closest realization to date of a quantum kagome antiferromagnet featuring a spin-liquid ground state, we provide novel insight into different contributions to its magnetism. At low temperatures, two distinct types of defects with different magnetic couplings to the kagome spins are found. Surprisingly, their magnetic response contradicts the threefold symmetry of the ideal kagome lattice, suggesting the presence of a global structural distortion that may be related to the establishment of the spin-liquid ground state.
One of the key questions concerning frustrated lattices that has lately emerged is the role of disorder in inducing spin-liquid-like properties. In this context, the quantum kagome antiferromagnets YCu3(OH)6Cl3, which has been recently reported as the first geometrically perfect realization of the kagome lattice with negligible magnetic/non-magnetic intersite mixing and a possible quantumspin-liquid ground state, is of particular interest. However, contrary to previous conjectures, here we show clear evidence of bulk magnetic ordering in this compound below TN = 15 K by combining bulk magnetization and heat capacity measurements, and local-probe muon spin relaxation measurements. The magnetic ordering in this material is rather unconventional in several respects. Firstly, a crossover regime where the ordered state coexists with the paramagnetic state extends down to TN /3 and, secondly, the fluctuation crossover is shifted far below TN . Moreover, persistent spin dynamics that is observed at temperatures as low as T /TN = 1/300 could be a sign of emergent excitations of correlated spin-loops or, alternatively, a sign of fragmentation of each magnetic moment into an ordered and a fluctuating part. arXiv:1904.02878v2 [cond-mat.str-el] 1 Jul 2019
The emergent behavior of spin liquids that are born out of geometrical frustration makes them an intriguing state of matter. We show that in the quantum kagome antiferromagnet ZnCu3(OH)6SO4 several different correlated, yet fluctuating states exist. By combining complementary local-probe techniques with neutron scattering, we discover a crossover from a critical regime into a gapless spin-liquid phase with decreasing temperature. An additional unconventional instability of the latter phase leads to a second, distinct spin-liquid state that is stabilized at the lowest temperatures. We advance such complex behavior as a feature common to different frustrated quantum magnets.
The magnetic ground state of the ideal quantum kagome antiferromagnet (QKA) has been a longstanding puzzle, mainly because perturbations to the nearest-neighbor isotropic Heisenberg Hamiltonian can lead to various fundamentally different ground states. Here we investigate a recently synthesized QKA representative YCu3(OH)6Cl3, where perturbations commonly present in real materials, like lattice distortion and intersite ion mixing, are absent. Nevertheless, this compound enters a long-range magnetically ordered state below TN = 15 K. Our powder neutron diffraction experiment reveals that its magnetic structure corresponds to a coplanar 120 • state with negative vector spin chirality. The ordered magnetic moments are suppressed to 0.42(2)µB, which is consistent with the previously detected spin dynamics persisting to the lowest experimentally accessible temperatures. This indicates either a coexistence of magnetic order and disorder or the presence of strong quantum fluctuations in the ground state of YCu3(OH)6Cl3. arXiv:1907.07489v1 [cond-mat.str-el]
We present an investigation of the influence of low levels of chemical substitution on the magnetic ground state and Néel skyrmion lattice (SkL) state in GaV 4 S 8-y Se y , where y = 0, 0.1, 7.9, and 8. Muon-spin spectroscopy (μSR) measurements on y = 0 and 0.1 materials reveal the magnetic ground state consists of microscopically coexisting incommensurate cycloidal and ferromagnetic environments, whereas chemical substitution leads to the growth of localized regions of increased spin density. μSR measurements of emergent low-frequency skyrmion dynamics show that the SkL exists under low levels of substitution at both ends of the series. Skyrmionic excitations persist to temperatures below the equilibrium SkL in substituted samples, suggesting the presence of skyrmion precursors over a wide range of temperatures.
We investigate the spin-stripe mechanism responsible for the peculiar nanometer modulation of the incommensurate magnetic order that emerges between the vector-chiral and the spin-density-wave phase in the frustrated zigzag spin-1/2 chain compound β-TeVO4. A combination of magnetictorque, neutron-diffraction and spherical-neutron-polarimetry measurements is employed to determine the complex magnetic structures of all three ordered phases. Based on these results, we develop a simple phenomenological model, which exposes the exchange anisotropy as the key ingredient for the spin-stripe formation in frustrated spin systems.Textured phases, frequently found in nature, typically develop as a result of conflicting interactions that favor rival ground states. In biological systems, alternating patterns have been explained by couplings between competing order parameters [1][2][3], whereas in strongly correlated electron systems stripe phases have been related to the competition between short-(e.g., exchange) and long-range (e.g., dipolar) interactions, leading, for instance, to stripe domains in feromagnetic films [4][5][6][7] or charge patterns in superconductors [8][9][10][11][12]. Moreover, a theoretical study by Edlund et al. [13] showed that a degeneracy of eigenstates in discretized-spin models allows for stripe formation that does not depend on the details of the microscopic interactions. Despite the ubiquity of inhomogeneous phases, the recent discovery of a peculiar antiferromagnetic stripe phase -a nanometer-scale modulation of the underlying incommensurate magnetic order in β-TeVO 4 [14] -exceeds the reach of any known model and thus calls for a new, general explanation.In the monoclinic structure of β-TeVO 4 (space group P 2 1 /c) distorted corner-sharing VO 5 pyramids with magnetic V 4+ ions form zigzag spin-1/2 chains that run along the crystalographic c axis [15,16]. Geometrical frustration stems from the competition between the ferromagnetic nearest-neighbor superexchange interaction J 1 ∼ −38 K and the antiferromagnetic nextnearest-neighboring interaction J 2 ∼ −0.8 J 1 , while the chains are coupled by more than an order of magnitude weaker, also frustrated, interactions. As a result, an incommensurate amplitude-modulated, i.e., a spin-density-wave (SDW), state defined by the magnetic wave vector k = (−0.195, 0, 0.413) is established at T N 1 = 4.65 K, while a vector chiral (VC) spin order develops below T N 3 = 2.28 K. Between these two phases, in the temperature range from T N 2 = 3.28 K to T N 3 , an intriguing spin-stripe phase emerges that is characterized by additional super-satellite reflections appearing in neutron-diffraction profiles at k ± ∆k [∆k(2.5 K) = (−0.030, 0, 0.021)], which have weak inten-. The resulting stripe order exhibits a remarkable long-scale modulation that alters the main SDW ordering in contrast to other known stripe patterns in magnetic systems. The negligible long-range dipolar interactions and the realization of an incommensurate, "nondiscrete", magnetic order lead to t...
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