Possible antiferromagnetic ordering in Y2Cu2O5. Paramagnetic behaviour of BaCuO2

The susceptibility anomaly observed at 13 K for Y₂Cu₂O₅ strongly suggests the occurrence of antiferromagnetic Cu spin coupling in this compound. Only paramagnetic behaviour was found for BaCuO₂.     

Ab initio evaluation of the charge ordering in alpha'NaV2O5

We report ab initio calculations of the charge ordering in alpha'NaV2O5 using large configurations interaction methods on embedded fragments. Our major result is that the 2p(y) electrons of the bridging oxygen of the rungs present a very strong magnetic character and should thus be explicitly considered in any relevant effective model. The most striking consequence of this result is that the spin and charge ordering differ substantially, as differ the experimental results depending on whether they are sensitive to the spin or charge density.     

A first-principles study on phase transition induced by charge ordering of Mn3+/Mn4+ in spinel LiMn2O4

A first-order transition at 290 K in LiMn₂O₄ with a cubic spinel-type structure is known to degrade the electrochemical performance of the positive electrode of rechargeable lithium-ion batteries. Using first-principles density functional theory (DFT), we confirm that the phase transition is induced by charge-ordering of Mn³⁺/Mn⁴⁺ accompanied by orbital-ordering due to Jahn–Teller distortion, which is in agreement with the previous experimental results of Rodríguez-Carvajal et al. [J. Rodríguez-Carvajal, G. Rousse, C. Masquelier, M. Hervieu, Phys. Rev. Lett. 81 (1998) 4660]. The optimized structure of the low-temperature (LT) phase has orthorhombic symmetry with five distinct crystallographic sites for Mn. The spin integration at each Mn site shows that Mn³⁺ resides at three Mn sites and the remaining two sites are occupied by Mn⁴⁺ ions. Total energy calculations indicate that the LT phase is about 0.23 eV/ LiMn₂O₄ more stable than cubic LiMn₂O₄ (high-temperature phase). The electrochemical Li extraction reaction from the LT phase is also investigated using DFT calculations. The results indicate that the reaction is initially divided into two voltage regions. Electrostatic interactions in the LT phase are calculated using a point charge model, accounting for the features of the electronic configurations and electrochemical reactions.     

Infrared spectroscopy of the charge ordering transition of Na0.5CoO2

We report infrared reflectivity study of charge ordering in a Na0.5CoO2 single crystal. In comparison with x=0.7 and 0.85 compounds, we found that the effective carrier density increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm(-1). A hump near 800 cm(-1) develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong antiresonance feature of phonon spectra. These observations signify a polaronic characteristic of charge carriers. Below T(co), an optical gap develops at the magnitude of 2Delta approximately 3.5k(B)T(co) (T相似文献   

Weak antiferromagnetic ordering induced by Dzyaloshinskii-Moriya interaction and pure magnetic reflections in MnSi-type crystals

A symmetry analysis of the Dzyaloshinskii-Moriya (DM) interaction in MnSi-type crystals reveals a nontrivial antiferromagnetic pattern of tilted Mn moments remaining even after an unwinding of the ground-state helix by a strong magnetic field. The remaining tilts are caused by that component of the DM vector which is perpendicular to the component responsible for helical spiraling; both components are evaluated and related to the atomic structure using a simple model. It is shown that the tilting should induce pure magnetic reflections 00?(?=2n+1) in neutron or x-ray magnetic scattering. In addition, the DM-induced antiferromagnetic ordering is important for the core structure of intrinsic defects, for the spectra of magnetic resonances, and generally for a better understanding of spin-orbit interaction in this type of magnetics.     

The antiferromagnetic/paramagnetic transition in mixed-spin compounds R2BaNiO5

We present an extensive Quantum Monte Carlo study of the magnetic properties of the mixed-spin quantum systems R2BaNiO5 (R= magnetic rare earth) which show coexistence of 3-dimensional magnetic long-range order with 1-dimensional quantum gap excitations. We discuss the validity of the performed simulations in the critical region and show the excellent agreement with experimental results. We emphasize the importance of quantum fluctuations contained in our study which is absent in previous mean-field-like treatments.     

Large reversible magnetocaloric effect induced by metamagnetic transition in antiferromagnetic HoNiGa compound

The magnetic properties and magnetocaloric effects(MCE) of Ho Ni Ga compound are investigated systematically.The Ho Ni Ga exhibits a weak antiferromagnetic(AFM) ground state below the Neel temperature TNof 10 K, and the AFM ordering could be converted into ferromagnetic(FM) ordering by external magnetic field. Moreover, the field-induced FM phase exhibits a high saturation magnetic moment and a large change of magnetization around the transition temperature,which then result in a large MCE. A large-?S_M of 22.0 J/kg K and a high RC value of 279 J/kg without magnetic hysteresis are obtained for a magnetic field change of 5 T, which are comparable to or even larger than those of some other magnetic refrigerant materials in the same temperature range. Besides, the μ_0H~(2/3)dependence of |?S_M~(pk)| well follows the linear fitting according to the mean-field approximation, suggesting the nature of second-order FM–PM magnetic transition under high magnetic fields. The large reversible MCE induced by metamagnetic transition suggests that Ho Ni Ga compound could be a promising material for magnetic refrigeration in low temperature range.     

Three-dimensional antiferromagnetic ordering in the light rare-earth high-T c superconductor NdBa2Cu3O6.86

Powder neutron diffraction investigations performed in the temperature range from 20 mK to 300K prove that the magnetic Nd moments of the superconductor NdBa₂Cu₃O_6.86 withT _c=88 K order three-dimensionally (3D) antiferromagnetic belowT _N=(551±10) mK. As in similar Gd and Dy compounds the corresponding wave vector isk=[1/2, 1/2, 1/2]. In approximate agreement with crystal field calculations the ordered magnetic moment of Nd amounts at saturation to (1.14±0.06) _B and at 25 mK the magnetic moments are oriented parallel [0,0,1]. The transition to the magnetically ordered state corresponds to the Landau type critical exponent 0.5, in contrast to the predominant 2D character of such heavy rare-earth systems. The crystal structure of NdBa₂Cu₃O_6.86 is orthorhombic similar to the one of YBa₂Cu₃O_7–x.     

Pressure-induced superconductivity in beta- Na (0.33) V(2)O(5) beyond charge ordering

We report the discovery of a new superconducting phase in highly correlated 3d electron systems. The compound is beta-vanadium bronze, beta- Na0.33V 2O5, in which the charge-ordered phase collapses under hydrostatic high pressure and a pressure-induced superconducting phase appears around T(S C)=8 K, P=8 GPa. This report presents the first observation not only of superconductivity in vanadium oxides but also of a phase transition from charge ordered to superconducting on a pressure-temperature (P- T) plane. The phase diagrams seem to have universal aspects across the classes of materials. This indicates a profound physics of superconductivity in highly correlated electron systems.     

Field induced magnetic ordering transition in Kondo insulators

We study the 2D Kondo insulators in a uniform magnetic field using quantum Monte Carlo simulations of the particle-hole symmetric Kondo lattice model and a mean field analysis of the Periodic Anderson model. We find that the field induces a transition to an insulating, antiferromagnetically ordered phase with staggered moment in the plane perpendicular to the field. For fields in excess of the quasi-particle gap, corresponding to a metal in a simple band picture of the periodic Anderson model, we find that the metallic phase is unstable towards the spin density wave type ordering for any finite value of the interaction strength. This can be understood as a consequence of the perfect nesting of the particle and hole Fermi surfaces that emerge as the field closes the gap. We propose a phase diagram and investigate the quasi-particle and charge excitations in the magnetic field. We find good agreement between the mean-field and quantum Monte Carlo results.Received: 17 December 2003, Published online: 8 June 2004PACS: 71.27. + a Strongly correlated electron systems; heavy fermions - 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.30. + h Metal-insulator transitions and other electronic transitions - 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena - 75.30.Fv Spin-density waves     

Heat capacity of MnBr2·4H2O near the antiferromagnetic transition temperature

Heat capacity of MnBr₂·4H₂O has been measured in the critical region around the Neel temperature. The data can be fitted, over a restricted range of |t|⩽10⁻², to the asymptotic power law. The critical exponents and the amplitudesA andA′ are not consistent with any theoretic predictions. However when scaling constraints are imposed, their values agree with the parameters of Ising model. Corrections to scaling are necessary to extend the range of the fit to |t|>10⁻². The correction terms are asymmetric giving −1·15±0·25 as the ratio of the amplitudes of the lowest order correction terms,D andD′ above and belowT _c. This value is in agreement with the recent predictions of the renormalisation group theory.     

Structural phase transition induced by the Jahn-Teller effect. Antiferrodistortive ordering

We study the phase transitions induced by the Jahn-Teller effect ofE-doublet ions in a cubic crystal with antiferrodistortive interactions. AnS=1 pseudospin model is constructed which takes the three lowest vibronic levels of the Jahn-Teller complexes into account. We find a second-order phase transition to a tetragonal phase with two inequivalent sublattices. The transitions between the vibronic levels give rise to bands of collective vibronic excitations with strongly temperature-dependent frequencies. The nature of the various modes is analyzed in detail. We also study the coupling to the elastic displacement field of the crystal. For a sufficiently large coupling constant, this coupling stabilizes a different low-temperature tetragonal phase with two equivalent sublattices. In a certain region of coupling constants, a transition occurs between the two tetragonal phases by second-order transitions to an intermediate phase of lower symmetry. The influence of the coupling on the dynamic behaviour is discussed.Supported by Schweizerischer Nationalfonds     

Structural phase transition induced by the Jahn-Teller effect. Antiferrodistortive ordering

We study the phase transitions induced by the Jahn-Teller effect ofE-doublet ions in a cubic crystal with antiferrodistortive interactions. AnS=1 pseudospin model is constructed which takes the three lowest vibronic levels of the Jahn-Teller complexes into account. We find a second-order phase transition to a tetragonal phase with two inequivalent sublattices. The transitions between the vibronic levels give rise to bands of collective vibronic excitations with strongly temperature-dependent frequencies. The nature of the various modes is analyzed in detail. We also study the coupling to the elastic displacement field of the crystal. For a sufficiently large coupling constant, this coupling stabilizes a different low-temperature tetragonal phase with two equivalent sublattices. In a certain region of coupling constants, a transition occurs between the two tetragonal phases by second-order transitions to an intermediate phase of lower symmetry. The influence of the coupling on the dynamic behaviour is discussed.     

Magnetic properties and antiferromagnetic Cu ordering in Pr2CuO4

Bulk-magnetisation measurements and neutron-scattering experiments were performed both on a polycrystalline sample and on a single-crystal of Pr₂CuO₄ in the temperature range from 1.5 to 300 K. Pr₂CuO₄ crystallizes with theT (Nd₂CuO₄)-type structure. We observed antiferromagnetic ordering of the Cu moments belowT _N =(190±2) K in a single crystal and belowT _N =(250±10)K in powder material. The magnetic unit cell dimensions area _m =2a _o ,c _m =c _o ;the Cu moments are oriented in the basal plane with a magnetic saturation moment of _Cu=(0.45±0.12) _B . Pr³⁺ does not order magnetically above 1.5K due to its crystal-field induced singlet ground state as verified by inelastic neutron scattering.     

"Devil's staircase"-type phase transition in NaV2O5 under high pressure

The "devil's staircase"-type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a x 2b x zc type superstructures. The observed temperature and pressure dependence of modulation wave number q(c), defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q(c) is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.     

Damping of antiferromagnetic spin waves by valence fluctuations in the double layer perovskite YBaFe2O5

Inelastic neutron scattering experiments show that spin dynamics in the charge-ordered insulating ground state of the double layer perovskite YBaFe(2)O(5) is well described in terms of e(g) superexchange interactions. Above the Verwey transition at T(V)=308 K, t(2g) double exchange-type conduction proceeds within antiferromagnetic FeO(2)-BaO-FeO(2) double layers by an electron hopping process that requires a spin flip of the five-coordinated Fe ions, costing an energy of 5S(2) approximately 0.1 eV. The hopping process disrupts near-neighbor spin correlations, leading to massive damping of zone-boundary spin waves.     

Ferroelectricity induced by acentric spin-density waves in YMn2O5

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition.