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Register a new userSpinon confinement in a quasi one dimensional anisotropic Heisenberg magnet
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Confinement is a process by which particles with fractional quantum numbers bind together to form quasiparticles with integer quantum numbers. The constituent particles are confined by an attractive interaction whose strength increases with increasing particle separation and as a consequence, individual particles are not found in isolation. This phenomenon is well known in particle physics where quarks are confined in baryons and mesons. An analogous phenomenon occurs in certain magnetic insulators; weakly coupled chains of spins S=1/2. The collective excitations in these systems is spinons (S=1/2). At low temperatures weak coupling between chains can induce an attractive interaction between pairs of spinons that increases with their separation and thus leads to confinement. In this paper, we employ inelastic neutron scattering to investigate the spinon confinement in the quasi-1D S=1/2 XXZ antiferromagnet SrCo2V2O8. Spinon excitations are observed above TN in quantitative agreement with established theory. Below TN the pairs of spinons are confined and two sequences of meson-like bound states with longitudinal and transverse polarizations are observed. Several theoretical approaches are used to explain the data. A new theoretical technique based on Tangent-space Matrix Product States gives a very complete description of the data and provides good agreement not only with the energies of the bound modes but also with their intensities. We also successfully explained the effect of temperature on the excitations including the experimentally observed thermally induced resonance between longitudinal modes below TN ,and the transitions between thermally excited spinon states above TN. In summary, our work establishes SrCo2V2O8 as a beautiful paradigm for spinon confinement in a quasi-1D quantum magnet and provides a comprehensive picture of this process.
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Magnetic systems composed of weakly coupled spin-1/2 chains are fertile ground for hosting the fractional magnetic excitations that are intrinsic to interacting fermions in one-dimension (1D). However, the exotic physics arising from the quantum many-body interactions beyond 1D are poorly understood in materials of this class. Spinons and psinons are two mutually exclusive low-energy magnetic quasiparticles; the excitation seen depends on the ground state of the spin chain. Here, we present inelastic neutron scattering and neutron diffraction evidence for their coexistence in SrCo$_{2}$V$_{2}$O$_{8}$ at milli-Kelvin temperatures in part of the Neel phase (2.4 T $leq$ $mu_mathrm{{0}}$H $<$ 3.9 T) and possibly also the field-induced spin density wave phase up to the highest field probed ($mu_mathrm{{0}}$H $geq$ 3.9 T, $mu_mathrm{{0}}$H$_mathbf{mathrm{{max}}}$ = 5.5 T). These results unveil a novel spatial phase inhomogeneity for the weakly coupled spin chains in this compound. This quantum dynamical phase separation is a new phenomenon in quasi-1D quantum magnets, highlighting the non-trivial consequences of inter-chain coupling.
We explain how spinons and magnons naturally arise in $mathrm{SU}(2)$ invariant spin chains when describing ground states and elementary excitations using MPS. Within this description, spinons can emerge in a spin-1 chain at a first-order transition between a symmetry-protected topological phase and a trivial phase. We provide MPS simulations for the spinon dispersion relations in a frustrated and dimerized spin-1 chain, and show that these spinons determine the low-lying spectrum in the vicinity of this transition by the formation of spinon/anti-spinon bound states.
We investigated magnetic and thermodynamic properties of $S$ = 1/2 quasi-one-dimensional antiferromagnet KCuMoO$_4$(OH) through single crystalline magnetization and heat capacity measurements. At zero field, it behaves as a uniform $S$ = 1/2 Heisenberg antiferromagnet with $J$ = 238 K, and exhibits a canted antiferromagnetism below $T_mathrm{N}$ = 1.52 K. In addition, a magnetic field $H$ induces the anisotropy in magnetization and opens a gap in the spin excitation spectrum. These properties are understood in terms of an effective staggered field induced by staggered g-tensors and Dzyaloshinsky-Moriya (DM) interactions. Temperature-dependencies of the heat capacity and their field variations are consistent with those expected for quantum sine-Gordon model, indicating that spin excitations consist of soliton, anti-soliton and breather modes. From field-dependencies of the soliton mass, the staggered field normalized by the uniform field $c_mathrm{s}$ is estimated as 0.041, 0.174, and 0.030, for $H parallel a$, $b$, and $c$, respectively. Such a large variation of $c_mathrm{s}$ is understood as the combination of staggered g-tensors and DM interactions which induce the staggered field in the opposite direction for $H parallel a$ and $c$ but almost the same direction for $H parallel b$ at each Cu site.
Conventional crystalline magnets are characterized by symmetry breaking and normal modes of excitation called magnons with quantized angular momentum $hbar$. Neutron scattering correspondingly features extra magnetic Bragg diffraction at low temperatures and dispersive inelastic scattering associated with single magnon creation and annihilation. Exceptions are anticipated in so-called quantum spin liquids as exemplified by the one-dimensional spin-1/2 chain which has no magnetic order and where magnons accordingly fractionalize into spinons with angular momentum $hbar/2$. This is spectacularly revealed by a continuum of inelastic neutron scattering associated with two-spinon processes and the absence of magnetic Bragg diffraction. Here, we report evidence for these same key features of a quantum spin liquid in the three-dimensional Heisenberg antiferromagnet NaCaNi$_2$F$_7$. Through specific heat and neutron scattering measurements, Monte Carlo simulations, and analytic approximations to the equal time correlations, we show that NaCaNi$_2$F$_7$ is an almost ideal realization of the spin-1 antiferromagnetic Heisenberg model on a pyrochlore lattice with weak connectivity and frustrated interactions. Magnetic Bragg diffraction is absent and 90% of the spectral weight forms a continuum of magnetic scattering not dissimilar to that of the spin-1/2 chain but with low energy pinch points indicating NaCaNi$_2$F$_7$ is in a Coulomb phase. The residual entropy and diffuse elastic scattering points to an exotic state of matter driven by frustration, quantum fluctuations and weak exchange disorder.
We study the Kitaev-Heisenberg-$Gamma$ model with antiferromagnetic Kitaev exchanges in the strong anisotropic (toric code) limit to understand the phases and the intervening phase transitions between the gapped $Z_2$ quantum spin liquid and the spin-ordered (in the Heisenberg limit) as well as paramagnetic phases (in the pseudo-dipolar, $Gamma$, limit). We find that the paramagnetic phase obtained in the large $Gamma$ limit has no topological entanglement entropy and is proximate to a gapless critical point of a system described by an equal superposition of differently oriented stacked one-dimensional $Z_2times Z_2$ symmetry protected topological phases. Using a combination of exact diagonalization calculations and field-theoretic analysis we map out the phases and phase transitions to reveal the complete phase diagram as a function of the Heisenberg, the Kitaev, and the pseudo-dipolar interactions. Our work shows a rich plethora of unconventional phases and phase transitions and provides a comprehensive understanding of the physics of anisotropic Kitaev-Heisenberg-$Gamma$ systems along with our recent paper [Phys. Rev. B 102, 235124 (2020)] where the ferromagnetic Kitaev exchange was studied.
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