Magnetically frustrated spin systems compose a significant proportion of topological quantum spin liquid candidates. Evidence for spin liquids in these materials comes largely from the detection of fractionalised spin-1/2 quasiparticles, known as spinons. However, the one-dimensional Heisenberg chain, which is topologically trivial, also hosts spinons. Thus, observing spinons does not necessarily signify long-range entanglement. Here, we show that spinons arising from one-dimensional physics leave a clear fingerprint in magnetic Raman scattering. We achieve this by calculating the magnetic Raman intensity of coupled Heisenberg chains. Our findings are in excellent agreement with the magnetic Raman scattering measurements on the anisotropic triangular antiferromagnet Ca$_3$ReO$_5$Cl$_2$.
Magnetically frustrated spin systems compose a significant proportion of topological quantum spin liquid candidates. Evidence for spin liquids in these materials comes largely from the detection of fractionalised spin-1/2 quasiparticles, known as spinons. However, the one-dimensional Heisenberg chain, which is topologically trivial, also hosts spinons. Thus, observing spinons does not necessarily signify long-range entanglement. Here, we show that spinons arising from one-dimensional physics leave a clear fingerprint in magnetic Raman scattering. We achieve this by calculating the magnetic Raman intensity of coupled Heisenberg chains. Our findings are in excellent agreement with the magnetic Raman scattering measurements on the anisotropic triangular antiferromagnet Ca3ReO5Cl2.
The spin-1 triangular lattice Heisenberg antiferromagnet Ca3NiNb2O9 and its sister compounds are conjectured to promote the formation of many-body quantum entangled states such as a quantum spin liquid (QSL), an exotic phase which features fractionalized quasiparticle excitations and emergent gauges. We probe the single crystal of Ca3NiNb2O9 using an in-depth Raman spectroscopic technique. Our measurements provide evidence for the fractionalized excitations, suggesting that the current system is in close proximity to the QSL phase. This is also in line with the proposed higher-order fractional magnetization plateau in this system, as these plateaus have an intricate relationship with the spin entanglement. We observed unconventional underlying scattering as a broad continuum with an intensity that shows fermionic statistics. Additionally, phonon modes show Fano asymmetry, also conjectured as a fingerprint of the spin-liquid phase, and above a critical Raman shift also show fermionic statistics in their intensity evolution.
Abstract Quantum spin liquids are outstanding examples of highly quantum entangled phases of matter and serve as a testbed to gauge central concepts of strongly correlated materials. Enormous research efforts in the past few decades have brought an in-depth understanding of these novel phases, although their conundrums have not yet been solved completely. In this review, we give an overview of the three different classes of spin-liquid materials: (i) a one-dimensional spin chain system KCuF 3 , (ii) a kagome antiferromagnet ZnCu 3 (OH) 6 Cl 2 , and (iii) a Kitaev honeycomb material -RuCl 3 . The emphasis is on demonstrating the success of the Raman scattering technique for probing fractionalized excitations in the aforementioned spin-liquid compounds, complementing a well-established neutron scattering method. Irrespective of dimensionality, spin topology, and spin-exchange type, the three materials share several common features in the spectral shape and temperature dependence of magnetic excitations, which can be taken as Raman spectroscopic fingerprints of quantum spin liquids.
In this work diverse results obtained from a number of low-dimensional quantum spin systems using Raman spectroscopy have been presented. The main motivation of this thesis is to hunt for new types of elementary excitations which may be present in low-dimensional spin systems with exotic spin topology. The studied systems can be on the one hand characterized by the formation of spin dimers which originate from a special spin topology like ACuCl3 (A=Tl, K), LiCu2O2, and SrCu(BO3)2. On the other hand, the system such as K2V3O8 and Cu2Te2O5X2 (X=Br,Cl) is dominated by quantum fluctuations due to reduced dimensionalilty. A powerful tool for studying low-dimensional spin systems is Raman spectroscopy which is sensitive to nearly all possible types of collective excitations as well as the interplay among them. In particular, magnetic light scattering can provide information about exchange paths and coupling constants, spinon excitations, spin gaps, and triplet-triplet interactions. The main results are summarized as follows; (i) the observation of a coherent coexistence of long-rang order and dimerized state in the frustrated spin chain system LiCu2O2, (ii) the presence of three-magnon scattering and dynamic spin-phonon coupling in the three-dimensional coupled spin dimer systems ACuCl3 (A=Tl, K), (iii) anomalous low-energy excitations including two-magnon scattering and electronic/orbital excitations in the two-dimensional Heisenberg antiferromagnetic system K2V3O8, (iv) the observation of inter-plane soft mode and a coupling of spin to soft mode in the quasi two-dimensional orthogonal dimer system SrCu(BO3)2, and (v) the existence of longitudinal magnons in the spin tetrahedra system Cu2Te2O5X2 (X=Br,Cl).
Fractionalized excitations such as spinons and anyons have emerged as a central theme in condensed matter physics with broad implications for superconductivity, quantum statistics, and quantum computation. The nearly ideal one-dimensional S = 1/2 system β-VOSO4 without long-range order down to 85 mK provides a promising platform to experimentally explore such fractionalized excitations. Here, we employ Raman spectroscopy to probe magnetic excitations and the evolution of spin correlations in β-VOSO4. Spinon signatures are found along the chain direction, evidenced by a broad, gapless scattering continuum at low temperatures. The temperature dependence of the spinon spectral weight aligns considerably with numerical density matrix renormalization group calculations. By comparing the experimental spinon spectral weight with calculated results and evaluating the associated quantum Fisher information (QFI) therefrom, we observe a steep increase in QFI upon cooling, indicating rapidly growing correlation lengths. Our study showcases QFI as a probe of spin correlations in quantum magnets. Quantum entanglement plays a key role in contemporary condensed matter research, yet it remains highly elusive from an experimental standpoint. Here, the authors demonstrate that entangled quantum states are accessible through spinon excitations in a one-dimensional spin chain using Raman spectroscopy.
Dynamics and stability of U(1) spin liquids beyond mean-field theory: Triangular-lattice J1-J2 Heisenberg model
Quantum spin liquids (QSLs) are long-range entangled phases of frustrated magnets exhibiting fractionalised spin excitations. In two dimensions, there is limited analytical understanding of their excitation spectra beyond parton mean-field theories, which fail to capture many features of the finite frequency dynamical response. We use a self-consistent mean-field theory combined with the random phase approximation (RPA) for the J1-J2 Heisenberg model on the triangular lattice to describe the strong spinon-spinon interactions of the U(1) Dirac QSL. We obtain quantitative results for the dynamical spin structure factor and phase diagram compatible with comprehensive numerical efforts. We show the continuum response at the Brillouin zone corners is described by a spinon-exciton hybridised with monopole gauge excitations. We further show that the transition from the QSL to 120-degree coplanar order is in the QED3 O(3)-GN universality class. We extend the method to chiral QSLs and XXZ anisotropy, and discuss its broad range of applicability to other models and for describing inelastic neutron scattering experiments in KYbSe2.
The research presented here focuses on the theoretical investigation of spinons in quasi one-dimensional quantum spin chains from the perspective of neutron scattering, with emphasis placed on the calculation of dynamic spin structure factors for both unpolarised and polarised incident neutrons. Two anisotropic versions of the Heisenberg spin chain are considered for such an examination: the spin-1/2 antiferromagnetic XXZ model and the spin-1/2 ferromagnetic XYZ model.The XYZ model supports spinon scattering (modelled at finite temperature) and is related to the so-called Villain mode. The introduction of a perturbation in the form of an external transverse magnetic field or Dzyaloshinskii-Moriya interaction (DMI) leads to the emergence of incommensurability in the system; such a result is a signature of fractional excitations. The presence of these interactions also give rise to a chiral response associated with the spinons in the polarised term of the scattering cross section. The inelastic scattering cross section is computed for both unpolarised and polarised neutrons.In relation to the antiferromagnetic XXZ model, emphasis is on spinon pair creation (modelled at T=0rmK). Motivated by the presence of a chiral response in the case of the XYZ model mentioned above, a new technique is developed in order to incorporate DMI and the transverse magnetic field so that one can determine if there is a chiral property associated with the two-spinon system. This technique is numerical in nature and is based on Green function methods. Using the results produced by the Green function approach, the inelastic cross section in the presence of an external transverse magnetic field is computed for the first time. The result is compared with experimental neutron scattering data; good agreement is demonstrated between theory and experiments on rmCsCoBr3.Finally, the Green function approach is generalised to account for the interactions between spin chains in magnetic compounds described by a staggered field. Various dynamic structure factors are computed for this scenario with results compared to experiment; good agreement between the theory and experiments on rmRbCoCl3 is shown.