Long-range Ordered Ground State Research Articles

Resilience of the Aurivillius structure upon La and Cr doping in a Bi5Ti3FeO15 multiferroic.

Combining the experimental techniques of high-resolution X-ray diffraction, magnetometry, specific heat measurement, and X-ray photoelectron, Raman and dielectric spectroscopy techniques, we have studied the influence of La and Cr doping on the crystal structure and magnetism of the room temperature Aurivillius multiferroic Bi5Ti3FeO15 by investigating the physical properties of (Bi4La)Ti3FeO15 and Bi5Ti3 (Fe0.5Cr0.5)O15. The parent (Bi5Ti3FeO15) and the doped ((Bi4La)Ti3FeO15 and Bi5Ti3(Fe0.5Cr0.5)O15) compounds crystallize in the A21am space group, which is confirmed through our analysis of high-resolution synchrotron X-ray diffraction data obtained on phase-pure polycrystalline powders. We determined the oxidation states of the metal atoms in the studied compounds as Fe3+, Ti4+, Cr3+, and La3+ through the analysis of X-ray photoelectron spectroscopy data. The magnetic susceptibilities of the three compounds are marked by the absence of a long-range ordered ground state, but dominated by superparamagnetic clusters with dominant antiferromagnetic interactions. This signature of short-range magnetism is also seen in specific heat as a low temperature enhancement which is suppressed upon the application of external magnetic fields up to 8 T. Our dielectric spectroscopy experiments showed that the three studied compounds display similar features in the dielectric constant measured as a function of frequency. However, upon doping La at the Bi site, the width of the ferroelectric hysteresis loop increases for (Bi4La)Ti3FeO15 compared to that of the parent compound Bi5Ti3FeO15, and with Cr doping, Bi5Ti3(Fe0.5Cr0.5)O15 becomes a leaky dielectric. The resilience of the Aurivillius crystal structure towards doping of La at the Bi site and Cr at the Fe site is clearly seen in the bulk properties of magnetic susceptibility, specific heat and the average crystal structure. The relevance of changes in the local structure is evident from our Raman spectroscopy and X-ray pair distribution function studies.

Pinch points and half-moons in dipolar-octupolar Nd2Hf2O7

While it is established that the pinch point scattering pattern in spin ice arises from an emergent coulomb phase associated with magnetic moment that is divergence-free, more complex Hamiltonians can introduce a divergence-full part. If these two parts remain decoupled, they give rise to the co-existence of distinct features. Here we show that the moment in ${\rm Nd_2Hf_2O_7}$ forms a static long-range ordered ground state, a flat, gapped pinch point excitation and dispersive excitations. These results confirm recent theories which predict that the dispersive modes, which arise from the divergence-full moment, host a pinch point pattern of their own, observed experimentally as `half-moons'.

Magnetic State Generation using Hamiltonian Guided Variational Autoencoder with Spin Structure Stabilization.

Numerical generation of physical states is essential to all scientific research fields. The role of a numerical generator is not limited to understanding experimental results; it can also be employed to predict or investigate characteristics of uncharted systems. A variational autoencoder model is devised and applied to a magnetic system to generate energetically stable magnetic states with low local deformation. The spin structure stabilization is made possible by taking the explicit magnetic Hamiltonian into account to minimize energy in the training process. A significant advantage of the model is that the generator can create a long‐range ordered ground state of spin configuration by increasing the role of stabilization even if the ground states are not necessarily included in the training process. It is expected that the proposed Hamiltonian‐guided generative model can bring about great advances in numerical approaches used in various scientific research fields.

Reentrant phenomenon and inverse magnetocaloric effect in a generalized spin-(1/2, s) Fisher’s super-exchange antiferromagnet

The thermodynamic and magnetocaloric properties of a generalized spin-(1/2, s) Fisher’s super-exchange antiferromagnet are investigated precisely by using the decoration-iteration mapping transformation. Besides the critical temperature, sublattice magnetization, total magnetization, entropy and specific heat, the isothermal entropy change and adiabatic temperature change are also rigorously calculated in order to examine the cooling efficiency of the model in the vicinity of the first- and second-order phase transitions. It is shown that an enhanced inverse magnetocaloric effect occurs around the temperature interval for any magnetic-field change . The most pronounced inverse magnetocaloric effect can be found nearby the critical field, which corresponds to the zero-temperature phase transition from the long-range ordered ground state to the paramagnetic one. The observed phenomenon increases with an increasing value of decorating spins. Furthermore, sufficiently high values of decorating spins have also been linked to the possibility of observing reentrant phase transitions at finite temperatures.

Investigation of Magnetic and Magnetoelastic Properties of the Unconventional Heavy-fermion Compound CeCu2Ge2

Heavy-fermion compounds, high-Tc cuprates or iron pnictides are characterized by adjacent properties and a rich magnetic phase diagram with unconventional or competing states at low temperatures. CeCu2Ge2, the counterpart of the heavy-fermion superconductor CeCu2Si2, exhibits an incommensurate antiferromagnetically long-range ordered ground state with τ1 = (0.28 0.28 0.54) up to TN = 4.1K which is strongly affected by a screening of the Ce 4f-moments by conduction electrons expressed by a Kondo temperature of TK = (6–10) K.The similar energy scales of the magnetic exchange and the Kondo effect result in a complex magnetic phase diagram with the possibility of quantum criticality at very low temperatures. The magnetic properties of CeCu2Ge2 were studied by magnetization measurements and, using the strong magnetoelastic coupling, by magnetostrictive investigations over a wide temperature range and in fields up to 50 T. Especially, the magnetostriction curves reveal clearly the details of the temperature and field dependence of the phase transitions. Here we present experimental results used for a refinement of the H − T phase diagram in [100] and [110] direction.

Field Dependence of the Magnetic Propagation Vector of the Heavy Fermion Compound CeCu2Ge2 Studied by Neutron Diffraction

CeCu2Ge2, the counterpart of the heavy-fermion superconductor CeCu2Si2, exhibits an in-commensurate antiferromagnetically long-range ordered ground state with τ = (0.28 0.28 0.54) below TN = 4.15K. The magnetism is strongly affected by a Kondo screening of the Ce 4f-moments by conduction electrons. The similar energy scale of both, Kondo and exchange interactions, results in a complex magnetic phase diagram and gives rise to potential quantum critical phenomena at very low temperatures. We present elastic neutron diffraction data obtained on a CeCu2Ge2 single crystal employing the cold triple axis spectrometer PANDA at MLZ and the diffractometer D23 at ILL.The field dependence of the magnetic propagation vector was measured at T ≤ 400 mK in the [110]/[001] plane with vertical magnetic fields applied along [1̄10]. We observe a low-field incommensurate magnetic phase AF1, a first order phase transition around 7.8 T with the coexistence of two phases AF1 and AF2 with slightly different propagation vectors, the disappearance of AF1 at 8 T and the existence of AF2 up to 12 T with a possible modification at 10 T. At 12.6 T, yet still well below the value of 26 T of the saturation for magnetic fields in [110] direction, the AF2-type magnetic order is lost and magnetic intensities are not to be found at incommensurate positions in the [110]/[001] plane any more. These new results contradict a previously suggested scenario with a QCP located at 8 T and contribute new information to the B − T phase diagram of CeCu2Ge2 in [110] direction.

First-order magnetic transition inYb2Ti2O7

The very nature of the ground state of the pyrochlore compound ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ is much debated, because experimental results demonstrate evidence for either a disordered ground state or a long-range ordered ground state. Indeed, the delicate balance of exchange interactions and anisotropy is believed to lead to competing states, such as a quantum spin liquid state or a ferromagnetic state which may originate from an Anderson-Higgs transition. We present a detailed magnetization study demonstrating a first-order ferromagnetic transition at 245 and 150 mK in a powder and a single-crystal sample, respectively. Its first-order character is preserved up to applied fields of $\ensuremath{\sim}200$ Oe. The transition stabilizes a ferromagnetic component and involves slow dynamics in the magnetization. Residual fluctuations are also evidenced, the presence of which might explain some of the discrepancies between previously published data for ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$.

Quantums=12antiferromagnets on Archimedean lattices: The route from semiclassical magnetic order to nonmagnetic quantum states

We investigate ground states of $s$=1/2 Heisenberg antiferromagnets on the eleven two-dimensional (2D) Archimedian lattices by using the coupled cluster method. Magnetic interactions and quantum fluctuations play against each other subtly in 2D quantum magnets and the magnetic ordering is thus sensitive to the features of lattice topology. Archimedean lattices are those lattices that have 2D arrangements of regular polygons and they often build the underlying magnetic lattices of insulating quasi-two-dimensional quantum magnetic materials. Hence they allow a systematic study of the relationship between lattice topology and magnetic ordering. We find that the Archimedian lattices fall into three groups: those with semiclassical magnetic ground-state long-range order, those with a magnetically disordered (cooperative quantum paramagnetic) ground state, and those with a fragile magnetic order. The most relevant parameters affecting the magnetic ordering are the coordination number and the degree of frustration present.

Collective dynamics in the Heisenberg pyrochlore antiferromagnetGd2Sn2O7

${\text{Gd}}_{2}{\text{Sn}}_{2}{\text{O}}_{7}$ is believed to be a good approximation to a Heisenberg antiferromagnet on a pyrochlore lattice with exchange and dipole-dipole interactions. The system is known to enter a long-range ordered ground state (the ``Palmer Chalker'' state) below ${T}_{c}=1\text{ }\text{K}$ with ${\mathbit{k}}_{\text{ord}}=(000)$. However, persistent electronic spin fluctuations have been observed as $T\ensuremath{\rightarrow}0$. Using inelastic neutron scattering, we have studied the buildup of short-range spin-spin correlations as the temperature is lowered, and the eventual formation of a gapped long-range ordered state that is able to sustain spin waves below ${T}_{c}$. As a magnetic field is applied, new magnetic phases develop and the gap widens. These measurements show that ${\text{Gd}}_{2}{\text{Sn}}_{2}{\text{O}}_{7}$ completely relieves itself of frustration, but the self-selected ground state is very delicate.

Ground-state long-range order in quasi-one-dimensional Heisenberg quantum antiferromagnets: high-order coupled-cluster calculations

We investigate the ground-state magnetic long-range order of quasi-one-dimensional quantum Heisenberg antiferromagnets for spin quantum numbers s = 1/2 and s = 1. We use the coupled cluster method to calculate the sublattice magnetization and its dependence on the inter-chain coupling J ⊥ . We find that for the unfrustrated spin-1/2 system, an infinitesimal inter-chain coupling is sufficient to stabilize magnetic long-range order, in agreement with results obtained by other methods. For s = 1, we find that a finite inter-chain coupling is necessary to stabilize magnetic long-range order. Furthermore, we consider a quasi one-dimensional spin-1/2 system, where a frustrating next-nearest neighbor in-chain coupling is included. We find that for stronger frustration as well, a finite inter-chain coupling is necessary to have magnetic long-range order in the ground state, and that the strength of the inter-chain coupling necessary to establish magnetic long-range order is related to the size of the spin gap of the isolated chain.

Experimental evidence for a spin gap in the s=1/2 quantum antiferromagnet Cu 2(OH) 2CO 3

Magnetic properties of the natural mineral Cu 2(OH) 2CO 3 (malachite) were investigated through DC and AC susceptibility measurements. The analysis of the low-temperature data reveals a quantum spin-gap behavior with Δ≈130 K. Consistently with the crystal structure, the magnetic susceptibility can be accurately described by a model of alternating chains. The non-frustrating residual inter-chain magnetic couplings, describing a sort of dimerized square planar lattice, are not strong enough to push the system towards a long-range ordered ground state, in good agreement with recent theoretical studies.

Long-range antiferromagnetic order in quasi-one-dimensionalCa0.83CuO2andSr0.73CuO2

Specific heat, elastic neutron scattering, and torque measurements have been performed on the two quasi-one-dimensional cuprates ${\mathrm{Ca}}_{0.83}{\mathrm{CuO}}_{2}$ and ${\mathrm{Sr}}_{0.73}{\mathrm{CuO}}_{2}.$ These compounds consist of infinite ${\mathrm{CuO}}_{2}$ chains, which are highly hole doped. In both compounds a peak in the specific heat, attributed to a static magnetic transition, has been observed at $T\ensuremath{\sim}10 \mathrm{K}.$ Magnetic Bragg peaks were identified in the neutron diffraction patterns, giving evidence of a three-dimensional long-range ordered ground state.

Erratum: "Low-lying disordered states when ground-state long-range order exists; large-amplitude spin waves

On montre l'existence de ces etats pour des ferromagnetiques d'Heisenberg, et on en debat dans le cas des antiferromagnetiques d'Heisenberg, independamment de la dimensionnalite. De tels etats ne se rencontrent pas exclusivement dans les materiaux a faible dimensionnalite

Low-lying disordered states when ground-state long-range order exists; large-amplitude spin waves.

Quantum magnets whose ground states show long-range order (LRO) are considered. The existence of disordered states, i.e., without LRO, with zero relative excitation energies, is shown for Heisenberg ferromagnets and argued for in the case of Heisenberg antiferromagnets, independent of dimensionality. The disordered states \ensuremath{\psi} are wave packets of long-wavelength spiral-like states ${\mathrm{\ensuremath{\Phi}}}_{\mathbf{q}}$. For finite systems these states \ensuremath{\psi} exhibit disordered-type correlation-function decay and small relative excitation energy, quite similarly to the low-lying disordered states found recently by Liang, Doucot, and Anderson and given major significance vis \`a vis the resonating-valence-bond theory of superconductivity. Our results show that the existence of such states is not special to low-dimensional antiferromagnets. It is shown that at small wave vectors q the excitation energy of ${\mathrm{\ensuremath{\Phi}}}_{\mathbf{q}}$ is of order ${\mathit{q}}^{2}$ for ferromagnets and for antiferromagnets. The seeming paradox for antiferromagnets, in light of the spin-wave frequencies being linear in q, is resolved, essentially through the observation that ${\mathrm{\ensuremath{\Phi}}}_{\mathbf{q}}$ corresponds to spin waves with large amplitudes that are independent of q.

Long-range order in the XXZ model

The existence of long-range order is proved under certain conditions for the antiferromagnetic quantum spin system with anisotropic interactions (XXZ model) on the simple cubic or the square lattice. In three dimensions (the simple cubic lattice), finite long-range order exists at sufficiently low temperatures for any anisotropyΔ(≥0) ifS≥1, and for 0≤Δ 1.19 (Ising-like) ifS=1/2. In two dimensions (the square lattice), ground-state long-range order exists under the following conditions: for any anisotropy (Δ≥0) ifS≥3/2; 0≤Δ 1.80 (Ising-like) ifS=1;Δ>1.93 (Ising-like) ifS=1/2. We conjecture that the two-dimensional spin-1/2XY model (Δ=0) has finite ground-state long-range order. Numerical evidence supporting this conjecture is given.

Ground-State Long-Range Order in the Two-DimensionalXXZMode

We prove the existence of Néel-type long-range order in the ground state of the spin-1/2 XXZ model with Δ (exchange anisotropy)>1.67 on the square lattice. We further show the existence of long-range order for Δ >1.10 and 0≦ Δ <0.59 by assuming monotonicity of nearest neighbor correlations as functions of the system size. The assumption of monotonicity is supported by numerical calculations.

Long-Range Order in Antiferromagnetic Quantum Spin Systems

The existence of long-range order is proved under certain conditions for the antiferromagnetic XYZ model on the simple cubic or the square lattice. In particular, the spin-1/2 XXZ model on the square lattice is shown to have ground-state long-range order if the exchange anisotropy Δ satisfies 0≦ Δ 1.72, which improves the result of Kubo and Kishi. The existence of long-range order of the z -component of the spin operator is proved for the XXZ model with XY -like anisotropy (0≦ Δ ≦1) under certain conditions. A similar result is shown to hold for the long-range order in the x -direction for the Ising-like model ( Δ ≧1). The XXZ model on the two-dimensional hexagonal lattice is proved to have finite ground-state long-range order for any value of Δ (≧0) if S ≧1 and for Δ >2.55 when S =1/2.