Conventional Long-range Order Research Articles

Eleven competing phases in the Heisenberg-Gamma ( JΓ ) ladder

The spin-orbit generated Γ interaction is known to induce strong frustration and to be significant in realistic models of materials. To gain an understanding of the possible phases that can arise from this interaction, it is of considerable interest to focus on a limited part of parameter space in a quasi one-dimensional model where high precision numerical results can be obtained. Here we study the Heisenberg–Gamma ( JΓ ) ladder, determining the complete zero temperature phase diagram by analyzing the entanglement spectrum (ES) and energy susceptibility. A total of 11 different phases can be identified, among them the well known rung-singlet (RS) phase and 5 other phases, FM, FM-Z, FM-XY, AF and AF-Z, with conventional long-range magnetic order. The 3 ferromagnetic phases, FM, FM-Z and FM-XY simultaneously have non-zero scalar chirality. Two other phases, the antiferromagnetic Gamma (AΓ) and ferromagnetic Gamma (FΓ) phases, have previously been observed in the Kitaev–Gamma ladder, demonstrating that the AΓ-phase is a symmetry protected topological phase (SPT) protected by TR ×Rb symmetry, the product of time-reversal (TR) and π rotation around the b-axis ( Rb ), while the FΓ-phase is related to the RS phase through a local unitary transformation. The 3 remaining phases, ϒ, Ω and δ show no conventional order, a doubling of the ES and for the ϒ and Ω-phases a gap is clearly present. The δ-phase has a significantly smaller gap and displays incommensurate correlations, with a peak in the static structure factor, S(k) continuously shifting from k/π=2/3 to k=π . In the Ω-phase we find pronounced edge-states consistent with a SPT phase protected by the same TR ×Rb symmetry as the AΓ-phase. The precise nature of the ϒ and δ-phases is less clear.

Disordered hyperuniform solid state materials

Disordered hyperuniform (DHU) states are recently discovered exotic states of condensed matter. DHU systems are similar to liquids or glasses in that they are statistically isotropic and lack conventional long-range translational and orientational order. On the other hand, they completely suppress normalized infinite-wavelength density fluctuations like crystals and, in this sense, possess a hidden long-range correlation. Very recently, there have been several exciting discoveries of disordered hyperuniformity in solid-state materials, including amorphous carbon nanotubes, amorphous 2D silica, amorphous graphene, defected transition metal dichalcogenides, defected pentagonal 2D materials, and medium/high-entropy alloys. It has been found that the DHU states of these materials often possess a significantly lower energy than other disorder models and can lead to unique electronic and thermal transport properties, which results from mechanisms distinct from those identified for their crystalline counterparts. For example, DHU states can enhance electronic transport in 2D amorphous silica; DHU medium/high-entropy alloys realize the Vegard's law and possess enhanced electronic bandgaps and thermal transport at low temperatures. These unique properties open up many promising potential device applications in optoelectronics and thermoelectrics. Here, we provide a focused review on these important new developments of hyperuniformity in solid-state materials, taking an applied and “materials” perspective, which complements the existing reviews on hyperuniformity in physical systems and photonic materials. Future directions and outlook are also provided, with a focus on the design and discovery of DHU quantum materials for quantum information science and engineering.

Glassy Dynamics in a heavy ion irradiated NbSe2 crystal

Fascination with glassy states has persisted since Fisher introduced the vortex-glass as a new thermodynamic phase that is a true superconductor that lacks conventional long-range order. Though Fisher’s original model considered point disorder, it was later predicted that columnar defects (CDs) could also induce glassiness — specifically, a Bose-glass phase. In YBa2Cu3O7−x (YBCO), glassy states can cause distinct behavior in the temperature (T ) dependent rate of thermally activated vortex motion (S). The vortex-glass state produces a plateau in S(T ) whereas a Bose-glass can transition into a state hosting vortex excitations called double-kinks that can expand, creating a large peak in S(T ). Although glass phases have been well-studied in YBCO, few studies exist of other materials containing CDs that could contribute to distinguishing universal behavior. Here, we report on the effectiveness of CDs tilted ~30° from the c-axis in reducing S in a NbSe2 crystal. The magnetization is 5 times higher and S is minimized when the field is parallel to the defects versus aligned with the c-axis. We see signatures of glassiness in both field orientations, but do not observe a peak in S(T ) nor a plateau at values observed in YBCO. Finally, we discuss the possibility that competing disorder induces a field-orientation-driven transition from a Bose-glass to an anisotropic glass involving both point and columnar disorder.

Spin-one bilinear-biquadratic model on a star lattice

We study the ground-state phase diagram of the $S=1$ bilinear-biquadratic model\,(BLBQ) on the star lattice with the state-of-art tensor network algorithms. The system has five phases: the ferromagnetic, anti-ferromagnetic, ferroquadrupolar, and spin-liquid phases. The phases and their phase boundaries are determined by examining various local observables, correlation functions and transfer matrices exhaustively. The spin liquid phase, which is the first quantum disordered phase found in two-dimensional BLBQ model, is gapped and devoid of any conventional long-range order. It is also characterized by fixed-parity virtual bonds in the tensor network formalism, analogous to the Haldane phase, while the parity varies depending on the location of the bond.

Finite-temperature phase transition to a Kitaev spin liquid phase on a hyperoctagon lattice: A large-scale quantum Monte Carlo study

The quantum spin liquid is an enigmatic quantum state in insulating magnets, in which conventional long-range order is suppressed by strong quantum fluctuations. Recently, an unconventional phase transition was reported between the low-temperature quantum spin liquid and the high-temperature paramagnet in the Kitaev model on a three-dimensional hyperhoneycomb lattice. Here, we show that a similar ``liquid-gas'' transition takes place in another three-dimensional lattice, the hyperoctagon lattice. We investigate the critical phenomena by adopting the Green-function based Monte Carlo technique with the kernel polynomial method, which enables systematic analysis of up to 2048 sites. The critical temperature is lower than that in the hyperhoneycomb case, reflecting the smaller flux gap. We also discuss the transition on the basis of an effective model in the anisotropic limit.

Spin liquid and infinitesimal-disorder-driven cluster spin glass in the kagome lattice

The interplay between geometric frustration (GF) and bond disorder is studied in the Ising kagome lattice within a cluster approach. The model considers antiferromagnetic short-range couplings and long-range intercluster disordered interactions. The replica formalism is used to obtain an effective single cluster model from where the thermodynamics is analyzed by exact diagonalization. We found that the presence of GF can introduce cluster freezing at very low levels of disorder. The system exhibits an entropy plateau followed by a large entropy drop close to the freezing temperature. In this scenario, a spin-liquid (SL) behavior prevents conventional long-range order, but an infinitesimal disorder picks out uncompensated cluster states from the multi-degenerate SL regime, potentializing the intercluster-disordered coupling and bringing the cluster spin-glass state. To summarize, our results suggest that the SL state combined with low levels of disorder can activate small clusters, providing hypersensitivity to the freezing process in geometrically frustrated materials and playing a key role in the glassy stabilization. We propose that this physical mechanism could be present in several geometrically frustrated materials. In particular, we discuss our results in connection with the recent experimental investigations of the Ising kagome compound Co3Mg(OH)6Cl2.

Topological order of mixed states in correlated quantum many-body systems

Topological order has become a new paradigm to distinguish ground states of interacting many-body systems without conventional long-range order. Here we discuss possible extensions of this concept to density matrices describing statistical ensembles. For a large class of quasi-thermal states, which can be realized as thermal states of some quasi-local Hamiltonian, we generalize earlier definitions of density matrix topology to generic many-body systems with strong correlations. We point out that the robustness of topological order, defined as a pattern of long-range entanglement, depends crucially on the perturbations under consideration. While it is intrinsically protected against local perturbations of arbitrary strength in an infinite closed quantum system, purely local perturbations can destroy topological order in open systems coupled to baths if the coupling is sufficiently strong. We discuss our classification scheme using the finite-temperature quantum Hall states and point out that the classical Hall effect can be understood as a finite temperature topological phase.

Magnetoelectric Coupling, Ferroelectricity, and Magnetic Memory Effect in Double Perovskite La3Ni2NbO9.

We observe ferroelectricity in an almost unexplored double perovskite La3Ni2NbO9. Ferroelectricity appears below ∼60 K, which is found to be correlated with the significant magnetostriction. A reasonably large value of spontaneous electric polarization is recorded to be ∼260 μC/m(2) at 10 K for E = 5 kV/cm, which decreases signifi- cantly upon application of a magnetic field (H), suggesting considerable magnetoelectric coupling. The dielectric permittivity is also influenced by H below the ferroelectric transition. The magnetodielectric response scales linearly to the squared magnetization, as described by the Ginzburg-Landau theory. Meticulous studies of static and dynamic features of dc magnetization and frequency dependent ac susceptibility results suggest spin-glass state below 29 K. Intrinsic magnetic memory effect is observed from zero-field cooled magnetization and isothermal remanent magnetization studies, also pointing spin-glass state below 29 K. Appearance of ferroelectricity together with a significant magnetoelectric coupling in absence of conventional long-range magnetic order is promising for searching new magnetoelectric materials.

Evidence of a field-induced Berezinskii-Kosterlitz-Thouless scenario in a two-dimensional spin-dimer system.

Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex-antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S=½ ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.

Neutron spectroscopic study of crystal field excitations inTb2Ti2O7andTb2Sn2O7

We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. These two materials possess related, but different ground states, with ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ displaying ``soft'' spin ice order below ${T}_{\mathrm{N}}\ensuremath{\sim}0.87$ K, while ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ enters a hybrid, glassy spin ice state below ${T}_{\mathrm{g}}\ensuremath{\sim}0.2$ K. Our neutron measurements, performed at $T=1.5$ and 30 K, probe the crystal field states associated with the $J$ = 6 states of Tb${}^{3+}$ within the appropriate $Fd\overline{3}m$ pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb${}^{3+}$ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of $\ensuremath{\sim}$2 increase in the crystal field bandwidth of the $2J+1=13$ states in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ compared with ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. Our results are consistent with previous measurements on crystal field states in ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$, wherein the ground-state doublet corresponds primarily to ${m}_{J}=|\ifmmode\pm\else\textpm\fi{}5\ensuremath{\rangle}$ and the first excited state doublet to ${m}_{J}=|\ifmmode\pm\else\textpm\fi{}4\ensuremath{\rangle}$. In contrast, our results on ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ differ markedly from earlier studies, showing that the ground-state doublet corresponds to a significant mixture of ${m}_{J}=|\ifmmode\pm\else\textpm\fi{}5\ensuremath{\rangle}$, $|\ensuremath{\mp}4\ensuremath{\rangle}$, and $|\ifmmode\pm\else\textpm\fi{}2\ensuremath{\rangle}$, while the first excited state doublet corresponds to a mixture of ${m}_{J}=|\ifmmode\pm\else\textpm\fi{}4\ensuremath{\rangle}$, $|\ensuremath{\mp}5\ensuremath{\rangle}$, and $|\ifmmode\pm\else\textpm\fi{}1\ensuremath{\rangle}$. We discuss these results in the context of proposed mechanisms for the failure of ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ to develop conventional long-range order down to 50 mK.

Magnetization “Steps” on a Kagome Lattice in Volborthite

Magnetic properties of the spin-1/2 kagome-like compound volborthite are studied using a high-quality polycrystalline sample. It is evidenced from magnetization and specific heat measurements that the spins on the kagome lattice still fluctuate at low temperature, down to T =60 mK that corresponds to 1/1500 of the nearest-neighbor antiferromagnetic interaction, exhibiting neither a conventional long-range order nor a spin gap. In contrast, 51 V NMR experiments revealed a sharp peak at 1 K in relaxation rate, which indicates that a certain exotic order occurs. Surprisingly, we have observed three “steps” in magnetization as a function of magnetic field, suggesting that at least four liquid-like or other quantum states exist under magnetic fields.

Unique Phase Transition on Spin-2 Triangular Lattice of Ag2MnO2

Ag 2 MnO 2 is studied as a possible candidate compound for an antiferromagnetic X Y spin model on a triangular lattice. In spite of the large Curie–Weiss temperature of -430 K found in magnetic susceptibility, Mn 3+ spins with S =2 do not undergo a conventional long-range order down to 2 K probably owing to the geometrical frustration and two dimensionality in the system. Instead, a unique phase transition is found at 80 K, where specific heat exhibits a clear sign of a second-order phase transition, while magnetic susceptibility changes smoothly without a distinct anomaly. We think that this transition is related to the chirality degree of freedom associated with a short-range order, which has been expected for the classical X Y spin model on a triangular lattice. On further cooling, spin-glass-like behavior is observed below 22 K, possibly corresponding to a quasi-long-range order.

Detection of weak emergent broken-symmetries of the kagome antiferromagnet by Raman spectroscopy

We show that the magnetic Raman response of a spin liquid is independent of the polarizations of the light for triangular symmetries. In contrast, a ground state that has a broken-symmetry shows characteristic oscillations when the polarizations are rotated. This would allow one to detect weak broken-symmetries and emergent order parameters. We focus on the kagome antiferromagnet wherein no conventional long-range order has been found so far and present the Raman cross section of a spin liquid and a valence bond crystal by using a random phase approximation.

Electric Polarization, Magnetoelectric Effect, and Orbital State of a Layered Iron Oxide with Frustrated Geometry

A layered iron oxide RFe2O4 (R denotes rare-earth-metal elements) is an exotic dielectric material with charge-order (CO) driven electric polarization and magnetoelectric effect caused by spin-charge coupling. In this paper, a theory of electronic structure and dielectric property in RFe2O4 is presented. Charge frustration in paired-triangular lattices allows a charge imbalance without inversion symmetry. Spin frustration induces reinforcement of this polar CO by a magnetic ordering. We also analyze an orbital model for the Fe ion which does not show a conventional long-range order.

Low-temperature specific heat and possible gap to magnetic excitations in the Heisenberg pyrochlore antiferromagnetGd2Sn2O7

The ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ pyrochlore Heisenberg antiferromagnet displays a phase transition to a four sublattice N\'eel ordered state at a critical temperature ${T}_{c}\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The low-temperature state found via neutron scattering corresponds to that predicted by a classical model that considers nearest-neighbor antiferromagnetic exchange and long-range dipolar interactions. Despite the seemingly conventional nature of the ordered state, the specific heat ${C}_{v}$ has been found to be described in the temperature range $350\phantom{\rule{0.3em}{0ex}}\mathrm{mK}\ensuremath{\leqslant}T\ensuremath{\leqslant}800\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ by an anomalous power law ${C}_{v}\ensuremath{\sim}{T}^{2}$. A similar temperature dependence of ${C}_{v}$ has also been reported for ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, another pyrochlore Heisenberg material. Such behavior is to be contrasted with the typical ${T}^{3}$ behavior expected for a three-dimensional antiferromagnet with conventional long-range order which is then generally accompanied by an $\mathrm{exp}(\ensuremath{-}\ensuremath{\Delta}∕T)$ behavior at lower temperature where anisotropy effects induce a gap $\ensuremath{\Delta}$ to collective spin excitations. Such anomalous ${T}^{2}$ behavior in ${C}_{v}$ has been argued to be correlated to an unusual energy dependence of the density of states which also seemingly manifests itself in low-temperature spin fluctuations found in muon spin relaxation experiments. In this paper, we report calculations of ${C}_{v}$ that consider spin-wave-like excitations out of the N\'eel order observed in ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ via neutron scattering. We argue that the parametric ${C}_{v}\ensuremath{\propto}{T}^{2}$ does not reflect the true low-energy excitations of ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. Rather, we find that the low-energy excitations of this material are antiferromagnetic magnons gapped by single-ion and dipolar anisotropy effects, and that the lowest temperature of $350\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ considered in previous specific heat measurements accidentally happens to coincide with a crossover temperature below which magnons become thermally activated and ${C}_{v}$ takes an exponential form. We argue that further specific heat measurements that extend down to at least $100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ are required in order to ascribe an unconventional description of magnetic excitations out of the ground state of ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ or to invalidate the standard picture of gapped excitations proposed herein.

Berezinskii–Kosterlitz–Thouless crossover in a trapped atomic gas

Any state of matter is classified according to its order, and the type of order that a physical system can possess is profoundly affected by its dimensionality. Conventional long-range order, as in a ferromagnet or a crystal, is common in three-dimensional systems at low temperature. However, in two-dimensional systems with a continuous symmetry, true long-range order is destroyed by thermal fluctuations at any finite temperature. Consequently, for the case of identical bosons, a uniform two-dimensional fluid cannot undergo Bose-Einstein condensation, in contrast to the three-dimensional case. However, the two-dimensional system can form a 'quasi-condensate' and become superfluid below a finite critical temperature. The Berezinskii-Kosterlitz-Thouless (BKT) theory associates this phase transition with the emergence of a topological order, resulting from the pairing of vortices with opposite circulation. Above the critical temperature, proliferation of unbound vortices is expected. Here we report the observation of a BKT-type crossover in a trapped quantum degenerate gas of rubidium atoms. Using a matter wave heterodyning technique, we observe both the long-wavelength fluctuations of the quasi-condensate phase and the free vortices. At low temperatures, the gas is quasi-coherent on the length scale set by the system size. As the temperature is increased, the loss of long-range coherence coincides with the onset of proliferation of free vortices. Our results provide direct experimental evidence for the microscopic mechanism underlying the BKT theory, and raise new questions regarding coherence and superfluidity in mesoscopic systems.

(U1−xPux)Sbsolid solutions. I. Magnetic configurations

Neutron and resonant x-ray magnetic scattering studies have been performed on single crystals of three compositions, $x=0.25,$ 0.50, and 0.75 of the $({\mathrm{U}}_{1\ensuremath{-}x}{\mathrm{Pu}}_{x})\mathrm{Sb}$ solid solution. Neutron diffraction has established the ordering wave vector $(k=1$ for $x=0.25$ and 0.50, as in $x=0,$ USb, and $k=0.25$ for $x=0.75),$ the average magnetic moment, and the temperature dependence of the magnetic order. For the $x=0.75$ sample the presence of a third-order harmonic indicates a substantial squaring of the 4+, 4- magnetic configuration. Experiments with a field applied to the sample have shown that the $x=0.25$ and 0.50 samples are triple k (as in $x=0,$ USb), but that the $x=0.75$ sample is initially (at ${T}_{N})$ triple k, but at a lower temperature of ${T}^{\ensuremath{'}}$ starts to transform to a single-k configuration (as found in $x=1,$ PuSb). The (H, T) phase diagram of this sample is presented. At $T=10\mathrm{K}$ some 30% of the $x=0.75$ sample remains in the (high-temperature) $3\mathbf{k}$ state. The resonant x-ray magnetic scattering experiments have been performed at the U ${M}_{4}$ (3.728 keV) and Pu ${M}_{4}$ (3.963 keV) edges, exploiting the element-specific nature of this probe. For the $x=0.50$ sample, conventional long-range magnetic order is found. However, in the $x=0.75$ sample the better momentum resolution of the x-ray technique (compared to neutrons) shows that the ordering is not truly long range. Both the U and Pu moments have a correlation length of some 400 \AA{} only, even at the lowest temperature. For higher harmonics, especially the third order, the correlation length is even shorter. The observation of a second-order resonance harmonic is linked to the multi-k nature of the configurations, and its unusual temperature dependence is explained by using the phase diagram established for the $x=0.75$ sample by neutron diffraction. We conclude with a discussion of the frustration effects that have been observed in the $x=0.75$ sample and some suggestions for further work.

Superconducting condensate formation in quasi-2D systems with arbitrary carrier density

A phase diagram for a quasi-2D metal with variable carrier density has been derived. The phases present are the normal phase, where the order parameter is zero; the pseudogap phase where the absolute value of the order parameter is non-zero but its phase is random, and a superconducting phase with a crossover quasi-2D Berezinskii–Kosterlitz–Thouless (BKT) region. The crossover region is bounded by the quasi-2D BKT temperature and the temperature for the onset of conventional long-range order (CLRO). The practical observation of these regions, however, critically depends on the carrier density. At high densities both the pseudogap and BKT regions vanish asymptotically, i.e., one obtains the standard BCS picture. At intermediate densities the pseudogap phase is large but the BKT region is negligible. Finally, at very low densities both the pseudogap and BKT regions are sizeable. An attempt is made to explain the behaviour observed in underdoped (intermediate densities) and optimally doped high- T c superconducting compounds above their critical temperature. The transition to the pseudogap phase should also be regarded as a crossover.

Matrix mean-field theory for the paramagnetic susceptibility of disordered magnets.

A matrix-based mean-field theory for the paramagnetic susceptibility of disordered Heisenberg and Ising magnets with conventional long-range order is outlined. The wave-vector-dependent static susceptibility is expressed in terms of the eigenvalues and eigenvectors of a symmetrized exchange matrix. The resulting expression is subsequently written as an integral involving a weighted eigenvalue distribution of the matrix. The conditions under which the distribution can be approximated by a single $\ensuremath{\delta}$ function are investigated, and a criterion is established for the validity of the $\ensuremath{\delta}$-function approximation for the critical temperature. The criterion and the position of the $\ensuremath{\delta}$ function are written in terms of the moments of the distribution which are expressed as products of the elements of the exchange matrix. An alternative approach in which the susceptibility is expressed as the Laplace transform of a linear combination of functions obtained by integrating a set of coupled first-order differential equations is outlined. The theory is applied to a dilute ferromagnet with exponential exchange interactions. The limitations of the theory for strongly disordered systems are discussed, and it is pointed out that when all of the eigenvectors are localized, there is no phase transition in the matrix mean-field approximation.

Magnetic ordering in pyrochlore Ho2Mn2O7

The magnetic susceptibility of Ho2Mn2O7 with a spontaneous rise below about 40 K and a paramagnetic Curie temperature of +39 K suggests a ferromagnetic ordering. Indeed neutron diffraction profiles show strongly enhanced Bragg peaks with a temperature dependence which indicates an apparent Tc≊35 K. Nonetheless, the magnetic diffraction pattern is not consistent with a collinear ferro or ferrimagnetic ordering of the Ho3+ and Mn4+ sublattices. Furthermore, specific heat and small angle neutron scattering (SANS) show features which are also incompatible with conventional long-range order.