Pure Spin System Research Articles

Mpemba effect in pure spin systems : A universal picture of the role of spatial correlations at initial states.

The quicker freezing of hotter water, than a colder sample, when quenched to a common lower temperature, is referred to as the Mpemba effect (ME). While this counter-intuitive fact remains a surprize since long, efforts have begun to identify similar effect in other systems. We investigate the ME in a rather general context concerning magnetic phase transitions. From Monte Carlo simulations of model systems, viz., the Ising model and the q-state Potts model, with varying range of interaction and space dimension, we assert that hotter paramagnets undergo ferromagnetic ordering faster than the colder ones. This conclusion we have arrived at following the analyses of the simulation results on decay of energy and growth in ordering following quenches from different starting temperatures, to fixed final temperatures below the Curie points. The general observation, in all the considered models, without any element of frustration, is a crucial and important fact of our study. Furthermore, we have obtained an important scaling picture, on the strength of the effect, with respect to the variation in spatial correlation in the initial states. This behavior appears true irrespective of the nature of order-parameter fluctuation and even order of transition. The observations are expected to be relevant to the understanding of ME in a rather general class of systems.

Octahedral tilting induced isospin reorientation transition in iridate heterostructures

Iridate heterostructures are gaining interest as their magnetic properties are much more sensitive to structural distortion compared to pure spin systems due to spin-orbital entanglement induced by strong spin-orbit coupling. While bulk monolayer and bilayer iridates show $ab$-plane canted and $c$-axis antiferromagnetic (AFM) order, recent experiments on layered iridate superlattices (SL) have revealed striking properties, especially in the bilayer SL. A spin model is presented including the tilting induced Kitaev type interactions, which illustrates the proclivity towards $ab$-plane canted AFM order. A realistic Hubbard model including spin-dependent hopping terms arising from octahedral rotation and tilting is constructed for the bilayer SL in isospin space, and magnetic excitations are investigated in the self-consistently determined magnetic state. The Hubbard model analysis confirms the spin model results and shows strongly reduced magnon energy gap and an isospin reorientation transition from $c$-axis to $ab$-plane canted AFM order with increasing tilting.

Singlet-Triplet Excitations and Long-Range Entanglement in the Spin-Orbital Liquid Candidate FeSc2S4.

Theoretical models of the spin-orbital liquid (SOL) FeSc2S4 have predicted it to be in close proximity to a quantum critical point separating a spin-orbital liquid phase from a long-range ordered magnetic phase. Here, we examine the magnetic excitations of FeSc2S4 through time-domain terahertz spectroscopy under an applied magnetic field. At low temperatures an excitation emerges that we attribute to a singlet-triplet excitation from the SOL ground state. A threefold splitting of this excitation is observed as a function of applied magnetic field. As singlet-triplet excitations are typically not allowed in pure spin systems, our results demonstrate the entangled spin and orbital character of singlet ground and triplet excited states. Using experimentally obtained parameters we compare to existing theoretical models to determine FeSc2S4's proximity to the quantum critical point. In the context of these models, we estimate the characteristic length of the singlet correlations to be ξ/(a/2)≈8.2 (where a/2 is the nearest neighbor lattice constant), which establishes FeSc2S4 as a SOL with long-range entanglement.

Spin-Thermodynamics of Ultra-Cold Spin-1 Atoms

The spin-thermodynamics of a \(N\)-body spin-1 condensate containing only the spin-degrees of freedom is studied via a theory in which \(N\), the total spin \(S\) and its Z-component \(M\) are exactly conserved. The magnetic field \(B\) is considered as zero at first. Then the effect of a residual \(B\) is evaluated. A temperature \(T_3\) is defined as below that all the spatial degrees of freedom can be considered as being frozen and, accordingly, a pure spin-system will emerge. Effort is made to evaluate \(T_3\). When \(T\) goes up from zero, the internal energy \(U\) and the entropy \(S_E\) experience sharp changes in two narrow domains of \(T\) surrounding two turning temperatures \(T_1\) and \(T_2\), the latter is higher. When \(T T_2\), \(U\) and \(S_E\) remain unchanged. Whereas when \(T_1<T<T_2\), \(U\propto T\) and \(S_E\propto \ln T\). It was found that \(T_1\) and \(T_2\) originate from the gap \(E_{\mathrm{gap},1}\) (the energy difference between the ground state (g.s.) and the first excited state) and the width (the energy difference between the g.s. and the highest state without spatial excitation) of the spectra, respectively. Thus their appearance is a common feature in spin-thermodynamics. In fact, \(T_1\) marks the lowest excitation of the spin-modes, while \(T_2\) marks the maximization of the entropy in the spin-space. In particular, the T-dependent population density is defined so that the theory can be checked by experimental data. Two kinds of condensates are notable: (i) the strongly trapped systems with a very small \(N\), they can work as pure spin-systems at relatively higher temperature; (ii) the systems with a high magnetization (say, \(N-|M|\le 4\)), the dimensions of their spin-spaces are very low. Furthermore, a larger \(\omega \) together with a large N (for Rb) or a large \(|M|\) (for Na) will lead to a sufficiently large \(E_{\mathrm{gap},1}\) so that a real g.s. can be experimentally created at a higher temperature. The spin-thermodynamics would remain valid whenever the spatial modes decouple from the spin-modes. This can occur at a higher temperature as demonstrated in Pechkis et al. (Phys Rev Lett 111:025301, 2013).

Sign structure and ground-state properties for aspin−St−Jchain

The antiferromagnetic Heisenberg spin chain of odd spin $S$ is in the Haldane phase with several defining physical properties, such as thermodynamical ground-state degeneracy, symmetry-protected edge states, and nonzero string order parameter. If nonzero hole concentration $\ensuremath{\delta}$ and hole hopping energy $t$ are considered, the spin chain is replaced by a $\text{spin}\ensuremath{-}S$ $t\ensuremath{-}J$ chain. The motivation of this paper is to generalize the discussions of the Haldane phase to the doped spin chain. The first result of this paper is that, for the model considered here, the ${\mathbb{Z}}_{2}$ sign structure in the usual Ising basis can be totally removed by two consecutive unitary transformations consisting of a spatially local one and a nonlocal one. Direct from the sign structure, the second result of this paper is that the Marshall theorem and the Lieb-Mattis theorem for pure spin systems are generalized to the $t\ensuremath{-}J$ chain for arbitrary $S$ and $\ensuremath{\delta}$. A corollary of the theorem provides us with the ground-state degeneracy in the thermodynamic limit. The third result of this paper is about the phase diagram. We show that the defining properties of the Haldane phase survive in the small $t/J$ limit. The large $t/J$ phase supports a gapped spin sector with similar properties (ground-state degeneracy, edge state, and string order parameter) of the Haldane chain, although the charge sector is gapless.

Frustration-induced nanometre-scale inhomogeneity in a triangular antiferromagnet

Phase inhomogeneity of otherwise chemically homogenous electronic systems is an essential ingredient leading to fascinating functional properties, such as high-Tc superconductivity in cuprates, colossal magnetoresistance in manganites and giant electrostriction in relaxors. In these materials distinct phases compete and can coexist owing to intertwined ordered parameters. Charge degrees of freedom play a fundamental role, although phase-separated ground states have been envisioned theoretically also for pure spin systems with geometrical frustration that serves as a source of phase competition. Here we report a paradigmatic magnetostructurally inhomogenous ground state of the geometrically frustrated α-NaMnO2 that stems from the system’s aspiration to remove magnetic degeneracy and is possible only due to the existence of near-degenerate crystal structures. Synchrotron X-ray diffraction, nuclear magnetic resonance and muon spin relaxation show that the spin configuration of a monoclinic phase is disrupted by magnetically short-range-ordered nanoscale triclinic regions, thus revealing a novel complex state of matter.

Complex and strongly anisotropic magnetism in the pure spin system EuRh2Si2

In divalent Eu systems, the 4f local moment has a pure spin state J = S = 7/2. Although the absence of orbital moment precludes crystal electric field effects, we report a sizable magnetic anisotropy in single crystals of EuRh2Si2. We observed a surprisingly complex magnetic behavior with three successive phase transitions. The Eu2+ moments order in a probably amplitude-modulated structure below 24.5 K, undergoing a further transition to a structure that is possibly of the equal-moment type, and a first order transition at lower temperatures, presumably into a spin spiral structure. The sharp metamagnetic transition observed at low fields applied perpendicular to the hard axis is consistent with a change from a spiral to a fan structure. These magnetic structures are presumably formed by ferromagnetic planes perpendicular to the c axis, stacked antiferromagnetically along c but not of type I, at least just below the ordering temperature. Since EuRh2Si2 is isoelectronic and isostructural to EuFe2As2 at room temperature, our results are also relevant for the complex Eu-magnetism observed there, especially for the transition from an antiferromagnetic to a ferromagnetic state observed in EuFe2P2 upon substituting As by P.

Incommensurate antiferromagnetism in a pure spin system via cooperative organization of local and itinerant moments.

Materials with strong correlations are prone to spin and charge instabilities, driven by Coulomb, magnetic, and lattice interactions. In materials that have significant localized and itinerant spins, it is not obvious which will induce order. We combine electrical transport, X-ray magnetic diffraction, and photoemission studies with band structure calculations to characterize successive antiferromagnetic transitions in GdSi. GdSi has both sizable local moments and a partially nested Fermi surface, without confounding contributions from orbital effects. We identify a route to incommensurate order where neither type of moment dominates, but is rooted in cooperative feedback between them. The nested Fermi surface of the itinerant electrons induces strong interactions between local moments at the nesting vector, whereas the ordered local moments in turn provide the necessary coupling for a spin-density wave to form among the itinerant electrons. This mechanism echoes the cooperative interactions between electrons and ions in charge-density-wave materials, and should be germane across a spectrum of transition-metal and rare-earth intermetallic compounds.

Isomorphous Catena Transition Metal Squarates [MII(C4O4)(dmso)2(OH2)2] (M = Co, Mn) and Magnetic Investigation into their Solid Solution M = CoxMn1–x

AbstractThe crystal structures of two new isomorphous transition metal squarato complexes [MII(C4O4)(dmso)2(OH2)2] [MII = CoII (3d7), MnII (3d5); dmso = dimethylsulfoxide] and their magnetic properties are reported. The compounds feature two symmetrically independent chains, in which 1,3‐bridging squarato ligands connect cations in distorted octahedral surroundings of pseudo‐symmetry D4h. From an equimolar solution of CoCl2·6H2O and MnCl2·2H2O a mixed‐metal coordination polymer crystallizes; it represents a solid solution and adopts the same structure as the corresponding monometallic compounds. The results of the diffraction experiment unambiguously proof the presence of both CoII and MnII cations in either independent site albeit no precise ratio between the metal cations involved may be deduced from these findings. The difference in the magnetic properties between CoII and MnII cations in the given ligand field has allowed us to establish their ratio in the solid solution more reliably than by X‐ray diffraction: Accounting for ligand field potential and spin‐orbit coupling of CoII and regarding MnII as a pure spin system, the calculations yielded a fraction of 73 % CoII in the mixed‐metal polymer. With respect to superexchange effects only weak antiferromagnetic interactions have been detected for the three coordination polymers.

Formula omitted] spin systems on frustrated pyrochlore lattice

The effect of magnetic field on S = 3 2 Heisenberg antiferromagnets on pyrochlore lattice was investigated. Chromium spinels CdCr 2 O 4 and HgCr 2 O 4 exhibit first order field-induced transitions to magnetization plateau states with one-half moment of Cr 3 + . One-half moment is reasonably understood as formation of collinear three-up and one-down spin configuration for each tetrahedron, which is a building block of pyrochlore lattice. The effect of lattice distortions to stabilize this spin ordering state makes these magnetization plateau phases appear within wide range of magnetic field. It is quite likely that field-induced collinear three-up and one-down state is universal for antiferromagnetic Heisenberg spin systems on pyrochlore lattice, however, such pure spin system seems to be limited to chromium spinel oxides at present.

Magnetic ordering and ergodicity of the spin system in theCu2Te2O5X2family of quantum magnets

We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{X}_{2}$ family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of (i) isolated tetrahedra of four antiferromagnetically coupled ${\mathrm{Cu}}^{2+}$ spins and (ii) coupled tetrahedra within one-dimensional chains, in both cases involving mean-field coupling to other chains. The results show that ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{X}_{2}$ compounds are true three-dimensional systems of coupled spins. Susceptibility results are consistent with the existence of a singlet-triplet gap, whereas specific heat analysis shows that the singlet-triplet gap is filled with dense singletlike excitations that contribute to finite specific heat at temperatures far below the singlet-triplet gap, but do not contribute to a magnetic response of the system. Furthermore, measured specific heat data show excessive entropy when compared to the numerical results based on a pure spin system, which we attribute to the presence of phonons. Though ${\mathrm{Cu}}^{2+}$ spins are arranged in a geometrically frustrated tetrahedral antiferromagnetic configuration and spin correlation length $\ensuremath{\xi}$ extends beyond the single tetrahedral cluster dimension, ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{X}_{2}$ compounds do not exhibit ergodicity breaking at low temperatures, in contrast to the related geometrically frustrated kagom\'e and pyrochlore antiferromagnets.

Monte Carlo study of low/high spin transitions

We study the finite temperature property of a model on two dimensional square lattices with two Ising spins at each lattice site by Monte Carlo simulations. When those Ising spins at a lattice site are parallel the site is said to be in the high-spin state (HS), while when they are antiparallel the site is said to be in the low-spin state (LS). Throughout the study, the energy of HS is presumed to be higher than that of LS. Two Ising spins at each site are added to form a total spin, which interacts with its nearest neighbour total spins via spin-spin couplings. The spin-phonon coupling also is introduced via harmonic springs between nearest neighbour sites with spring constants and equilibrium distances depending on the spin states of the sites involved. In this system, we investigate the feature of transitions between LS and HS (to be called low/high spin transition (LHST)) by varying the temperature. As for the ferromagnetic interaction between total spins, the second order phase transition: pure HS→mixed state of HS and LS is possible to occur in a pure spin system, as is expected from mean field calculations. The role of lattice distortions by the change of lattice spacings is shown to be essential for LHST: pure LS→(pure)HS. In the model investigated, there appears an indication of the strong first order phase transition which reveals a conspicuous hysteresis.

Large q expansions for q-state gauge-matter Potts models in lagrangian form

We consider the lagrangian form of a q-state generalization of Ising gauge theories with matter fields in d = 3 and 4 dimensions. The theory is exactly soluble in the limit q → ∞ and corrections are easily calculable in power series in 1 q 1 d . Extrapolating the series for the free energies and latent heats by the method of Padé approximants, we have constructed the phase diagrams for all values of q. Our results agree well with known results for pure spin systems and, for the case q = 2, with Ising Monte Carlo data.