Metamagnetic phase transition of the antiferromagnetic Heisenberg icosahedron

The observation of hysteresis effects in single molecule magnets like Mn12-acetate has initiated ideas of future applications in storage technology. The appearance of a hysteresis loop in such compounds is an outcome of their magnetic anisotropy. In this Letter we report that magnetic hysteresis occurs in a spin system without any anisotropy, specifically where spins mounted on the vertices of an icosahedron are coupled by antiferromagnetic isotropic nearest-neighbor Heisenberg interaction giving rise to geometric frustration. At T = 0 this system undergoes a first-order metamagnetic phase transition at a critical field Bc between two distinct families of ground state configurations. The metastable phase of the system is characterized by a temperature and field dependent survival probability distribution.     

Femtosecond transition state dynamics of cis -stilbene

Femtosecond time-resolved photoion and photoelectron spectroscopy were employed to study the transition state dynamics of cis-stilbene in a molecular beam experiment. Two long-lived photoproducts were found with excitation of 267 nm radiation. Received: 4 August 2000 / Revised version: 11 October 2000 / Published online: 6 December 2000     

Quantum phase transition in an antiferromagnetic spinor Bose-Einstein condensate

We have experimentally observed the dynamics of an antiferromagnetic sodium Bose-Einstein condensate quenched through a quantum phase transition. Using an off-resonant microwave field coupling the F = 1 and F = 2 atomic hyperfine levels, we rapidly switched the quadratic energy shift q from positive to negative values. At q = 0, the system undergoes a transition from a polar to antiferromagnetic phase. We measured the dynamical evolution of the population in the F = 1, mF = 0 state in the vicinity of this transition point and observed a mixed state of all 3 hyperfine components for q < 0. We also observed the coarsening dynamics of the instability for q < 0, as it nucleated small domains that grew to the axial size of the cloud.     

Thickness dependence of antiferromagnetic phase transition in Heisenberg-type MnPS3

The behavior of 2-dimensional (2D) van der Waals (vdW) layered magnetic materials in the 2D limit of the few-layer thickness is an important fundamental issue for the understanding of the magnetic ordering in lower dimensions. The antiferromagnetic transition temperature T_N of the Heisenberg-type 2D magnetic vdW material MnPS₃ was estimated as a function of the number of layers. The antiferromagnetic transition was identified by temperature-dependent Raman spectroscopy, from the broadening of a phonon peak at 155 cm⁻¹, accompanied by an abrupt redshift and an increase of its spectral weight. T_N is found to decrease only slightly from ~78 K for bulk to ~66 K for 3L. The small reduction of T_N in thin MnPS₃ approaching the 2D limit implies that the interlayer vdW interaction is playing an important role in stabilizing magnetic ordering in layered magnetic materials.     

The existence of a phase transition in classical antiferromagnetic models

For a wide class of antiferromagnetic models we prove the existence of a phase transition using an extended Peierls argument, taking into account a result of Dobrushin [R. L. Dobrushin,Funct. Anal. Appl. 2:44 (1968); in English,2:302 (1968)] for an antiferromagnetic Ising model and the results of Malyshev [V. Malyshev,Comm. Math. Phys. 40:75–82 (1975)] for ferromagnetic models (such as the anisotropic rotator). In particular we review a result of Fröhlich, Israel, Lieb, and Simon [J. Fröhlichet al., J. Stat. Phys. 22(3):297–347 (198)] obtained when reflection positivity holds.     

Calorimetric investigation on the paramagnetic–antiferromagnetic phase transition in CoO

The paramagnetic–antiferromagnetic phase transition of a single crystal of CoO, whose first- or second-order character is controversial, has been studied using a high sensitive calorimetric technique. Although both specific heat and differential thermal analysis (DTA) trace obtained at very low temperature rate (0.1 K h⁻¹) show strong anomalies at the Néel temperature T_N, the DTA trace and that calculated from the specific heat anomaly coincides indicating a continuous phase transition. In agreement with the theoretical predictions, the specific heat follows the 3D Ising model in a temperature range of 3 K below T_N. Fisher relation for antiferromagnets is also obeyed in the same temperature range.     

First order antiferromagnetic phase transition in MnS2

The antiferromagnetic structure of MnS₂ has been re-examined by neutron diffraction from a single crystal. The phase transition has been found to be of the first order without any detectable thermal hysteresis. Application of uniaxial stress tends to convert the first order transition into a second order one.     

Charge and spin dynamics in antiferromagnetic metallic phase of iron-based superconductors

The electronic states, charge dynamics, and spin dynamics in the antiferromagnetic metallic phase of iron-arsenide superconductors are investigated by mean-field calculations for a five-band Hubbard model. Taking into account the difference of observed magnetic moments between LaFeAsO (1111 system) and BaFe₂As₂ (122 system), we investigate the effect of the magnitude of the moments on band dispersion, optical conductivity, and dynamical spin susceptibility. We clarify how the magnitude affects on these quantities and predict different behaviors between the 1111 and 122 systems in the antiferromagnetic metallic phase.     

Magnetic phase transition in Heisenberg antiferromagnetic films with easy-axis single-ion anisotropy

The staggered susceptibility of spin-1 and spin-3/2 Heisenberg antiferromagnet with easy-axis single-ion anisotropy on the cubic lattice films consisting of n=2, 3, 4, 5 and 6 interacting square lattice layers is studied by high-temperature series expansions. Sixth order series in J/k_BT have been obtained for free-surface boundary conditions. The dependence of the Néel temperature on film thickness n and easy-axis anisotropy D has been investigated. The shifts of the Néel temperature from the bulk value can be described by a power law n^−λ with a shift exponent λ, where λ is the inverse of the bulk correlation length exponent. The effect of easy-axis single-ion anisotropy on shift exponent of antiferromagnetic films has been studied. A comparison is made with related works. The results obtained are qualitatively consistent with the predictions of finite-size scaling theory.     

Localized ferrons and the Mott transition via the ferromagnetic phase in doped antiferromagnetic semiconductors

The states of single-and double-charge donors in an antiferromagnetic crystal are investigated taking into account the formation of magnetized regions (localized ferrons) around them. Double-charge donors should be in a state of the (1s)(2s) type, which is energetically favored over the (1s)² state. In doped antiferromagnetic semiconductors, the usual Mott transition is impossible because the single-charge donors in them are ferrons. Instead, the donor electrons are delocalized through the transition of the crystal to the single-electron state. It is unlikely that a ferromagnetic-antiferromagnetic (FM-AFM) mixed state will occur in this case. In the case of double-charge donors, transition to an FM-AFM mixed state should occur.     

Dramatic suppression of antiferromagnetic coupling in nanoparticle CuO

Magnetic susceptibility and muon spectroscopy measurements are carried out on antiferromagnetic nanoparticles (AFN) of CuO that are directly prepared by ball-milling a single crystal. The recently reported ferromagnetic features in AFN CuO of sol-gel growth are confirmed. New and significant result of the present work is a direct observation of an unusual dramatic reduction of the antiferromagnetic transition temperature, T_N, from the bulk T_N=230 to 50 K in AFN CuO by the μSR measurement.     

The antiferromagnetic Potts model in two dimensions: Berker-Kadanoff phase, antiferromagnetic transition, and the role of Beraha numbers

Using methods of integrable systems and conformal field theory, we study the Q-state Potts model on the square lattice with e^K real. We discover a surprisingly rich phase diagram that involves, besides the usual ferromagnetic critical line, an antiferromagnetic critical line and a Berker-Kadanoff phase (i.e., a massless low-temperature phase with coupling-independent exponents) that has singularities at the Baraha numbers (including Q integer) Q = 4cos²π/n. Critical properties are derived; we show in particular that the Q = 4cos²π/δ antiferromagnetic critical Potts model is in the “Z_δ−2” universality class with c = 2−6/δ. Extensions to other lattices are considered. We discuss the consequences of our results on the coloring problem and the Beraha conjecture. Three appendices deal with the geometrical interpretation of the Temperley-Lieb algebra and U_qsl(2) symmetry in the Potts and associated loops model, and with the vertex-Potts model correspondence in systems with free boundary conditions.     

Director dynamics near the nematic-smectic-A phase transition

The Landau-de Gennes model for the free energy of a nematic liquid crystal near the phase transition to the smectic A-phase is used to determine the frequency dependence of the fluctuation corrections to the Frank elastic constants. It is shown that the interaction of the fluctuations of the smectic order parameter and the director results in corrections to all the Frank elastic constants. In the low-frequency limit (ω→0), the corrections to the Frank elastic constants K ₂₂ and K ₃₃ are the largest, and decrease to zero in the infinite-frequency limit. The correction to K ₁₁ is negative, and vanishes in both limits. The absolute value of the correction to K ₁₁ is the largest at frequencies in the megahertz range. It is shown that in oriented nematics the interaction of the smectic fluctuations and the director limits deviations of the director from the direction of preferred orientation, as a result of which relaxation of both inhomogeneous and homogeneous distortions of the director field can be observed. It is also shown that this gives rise to a frequency interval in the megahertz range in which shear waves begin to propagate in the nematic. The propagation speed of these waves is roughly a hundred times smaller than that of sound and strongly depends on the direction of propagation. Zh. éksp. Teor. Fiz. 114, 2022–2033 (December 1998)     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

PAC study on111Cd in antiferromagnetic CuO

400 keV¹¹¹In⁺ ions were implanted into CuO powder. After annealing at 570 K, more than 50% of the implants were found on substitutional sites in the monoclinic CuO lattice. PAC-spectra taken below the Néel temperatureT _N≈230 K revealed a broadening of the quadrupole spectrum and additional satellite frequencies caused by combined electric and magnetic hyperfine interaction. Assuming that the orientation of the electric field gradient is given by the point charge model and taking the 〈010〉 direction of the supertransferred fieldB _tr in CuO from neutron diffraction, we find a Larmor frequency of ω_L≈40 MHz corresponding toB _tr≈2.7 T at 60 K. Supported in part by BMFT under contract FK213N54930.     

Absence of phase transition for antiferromagnetic Potts models via the Dobrushin uniqueness theorem

We prove that theq-state Potts antiferromagnet on a lattice of maximum coordination numberr exhibits exponential decay of correlations uniformly at all temperatures (including zero temperature) wheneverq>2r. We also prove slightly better bounds for several two-dimensional lattices: square lattice (exponential decay forq7), triangular lattice (q11), hexagonal lattice (q4), and Kagomé lattice (q6). The proofs are based on the Dobrushin uniqueness theorem.     

Structural phase transition in substituted La2CuO4 and Pr2CuO4−y

The structural phase transition from orthorhombic (T) phase to tetragonal (T′) phase in substituted La_2−x R _x CuO₄ (R = Pr, Nd, Sm, Eu and Gd) and T′ to T-phase in Pr_2−x M _x CuO_4−y (M = Sr, Ca) has been studied by X-ray diffraction technique. The T-phase of La₂CuO₄ is transferred to T′ phase abruptly atx=0.8, 0.4, 0.4, 0.3 and 0.4 respectively for substitution of Pr, Nd, Sm, Eu and Gd for La in La₂CuO₄ without evidence of the T* phase. The T′ structure of Pr₂CuO₄ (x = 0.0) gets transformed to the T* structure at 30% Ca doping (x=0.6) and then to the T structure at 50% Ca doping (x=1.0), while for Sr-contentx=0.0, 0.4 and 1.0 it shows T′, T* and T structure respectively.