Tetragonal-orthorhombic Structural Phase Transition Research Articles

Electronic Structure, Optical, and Magnetic Properties of Mn100−xGex (x = 20, 25, and 30) Alloys Near Tetragonal–Orthorhombic Structural Phase Transition

Electronic, structural, optical, and magnetic properties of the Mn100−xGex alloys for x = 20, 25, and 30 are investigated near a tetragonal–orthorhombic structural phase transition. In the electronic structure calculations, the ferrimagnetic ground state is found which is in excellent agreement with the experimental observation of magnetic properties. The calculations reveal that the main contribution to the optical absorption is associated with the electronic transitions in the system of 3d manganese bands, these states significantly change near the Fermi energy. The optical conductivity of the alloys is also transformed with an increase of the low‐energy absorption values near structural phase transition due to the enhanced electronic transitions. Among different compositions of the studied Mn–Ge, the highest magnetization is observed for Mn70Ge30 whereas highest coercivity is observed for the Mn75Ge25 alloy. High coercivity can be ascribed to the highest magneto‐crystalline anisotropy of the sample whereas high magnetization of Mn70Ge30 can be due to the presence of a hexagonal ferromagnetic phase with higher magnetization. The effect of Mn–Ge composition on the structural phase transition and its relationship with the electronic structure, optical, and magnetic properties may allow for better designing of new Heusler alloys for various customized applications.

Universality class of the structural phase transition in the normal phase of cuprate superconductors.

The tetragonal-orthorhombic structural phase transition of oxygen atoms in the basal plane of YBa_{2}Cu_{3}O_{6+δ} high-T_{C} cuprate superconductors is studied numerically. By mapping the system onto the asymmetric next-nearest-neighbor Ising model, we characterize this phase transition. Results indicate the degrees of critical behavior. We show that this phase transition occurs at the temperature T_{C}≃0.148eV in the thermodynamic limit. By analyzing the critical exponents, it is found that this universality class displays some common features, with the two-dimensional three-state Potts model universality class, although the possibility of other universality classes cannot be ruled out. Conformal invariance at T=T_{c} is investigated using the Schramm-Loewner evolution (SLE) technique, and it is found that the SLE diffusivity parameter for this system is 3.34±0.01.

Electronic and magnetic properties of orthorhombic iron selenide

Iron orbitals in orthorhombic iron selenide (FeSe) can produce charge-like multipoles that are polar (parity-odd). Orbitals in question include Fe(3d), Fe(4p) and p-type ligands that participate in transport properties and bonding. The polar multipoles may contribute weak, space-group forbidden Bragg spots to diffraction patterns collected with x-rays tuned in energy to a Fe atomic resonance (Templeton & Templeton scattering). Ordering of conventional, axial magnetic dipoles does not accompany the tetragonal-orthorhombic structural phase transition in FeSe, unlike other known iron-based superconductors. We initiate a new line of inquiry for this puzzling property of orthorhombic FeSe, using a hidden magnetic-order that belongs to the m'm'm' magnetic crystal-class. It is epitomized by the absence of ferromagnetism and axial magnetic dipoles, and the appearance of magnetic monopoles and magneto-electric quadrupoles. A similar magnetic order occurs in cuprate superconductors, YBCO & Hg1201, where it was unveiled with the Kerr effect and in Bragg diffraction patterns revealed by polarized neutrons. PACS number 75.25.-j

Tuning the magnetic and structural phase transitions of PrFeAsO via Fe/Ru spin dilution

Neutron diffraction and muon spin relaxation measurements are used to obtain a detailed phase diagram of Pr(Fe,Ru)AsO. The isoelectronic substitution of Ru for Fe acts effectively as spin dilution, suppressing both the structural and magnetic phase transitions. The temperature of the tetragonal-orthorhombic structural phase transition decreases gradually as a function of x. Slightly below the transition temperature coherent precessions of the muon spin are observed corresponding to static magnetism, possibly reflecting a significant magneto-elastic coupling in the FeAs layers. Short range order in both the Fe and Pr moments persists for higher levels of x. The static magnetic moments disappear at a concentration coincident with that expected for percolation of the J1-J2 square lattice model.

NMR Search for the Spin Nematic State in a LaFeAsO Single Crystal

We report a (75)As single crystal NMR investigation of LaFeAsO, the parent phase of a pnictide high T(c) superconductor. We demonstrate that spin dynamics develop a strong twofold anisotropy within each orthorhombic domain below the tetragonal-orthorhombic structural phase transition at T(TO) ≈ 156 K. This intermediate state with a dynamical breaking of the rotational symmetry freezes progressively into a spin density wave below T(SDW) ≈ 142 K. Our findings are consistent with the presence of a spin nematic state below T(TO) with an incipient magnetic order.

Ferro-orbital order induced by electron-lattice coupling in orthorhombic iron pnictides

In this paper we explore the magnetic and orbital properties closely related to a tetragonal-orthorhombic structural phase transition in iron pnictides based on both two- and five-orbital Hubbard models. The electron-lattice coupling, which interplays with electronic interaction, is self-consistently treated. Our results reveal that the orbital polarization stabilizes the spin density wave (SDW) order in both tetragonal and orthorhombic phases. However, the ferro-orbital density wave (F-ODW) only occurs in the orthorhombic phase rather than in the tetragonal one. Magnetic moments of Fe are small in the intermediate Coulomb interaction region for the striped antiferromangnetic phase in the realistic five orbital model. The anisotropic Fermi surface in the SDW/ODW orthorhombic phase is well in agreement with the recent angle-resolved photoemission spectroscopy experiments. These results suggest a scenario that the magnetic phase transition is driven by the ODW order mainly arising from the electron-lattice coupling.

Elastic anomalies inHoNi2B2Csingle crystals

We have measured temperature and magnetic field dependencies of the sound velocities and the sound attenuation in HoNi2B2C single crystals. The main result is a huge softening the velocity of C66 mode due to a cooperative Jahn-Teller effect, resulting in a tetragonal-orthorhombic structural phase transition. Anomalies in the behavior of the C66 mode through various magnetic phase transitions permit us to revise the low temperature H-T phase diagrams of this compound.

Study of twin formation in NdBa2Cu3O7−δ single crystal by using high temperature polarized optical microscopy

In order to determine an accurate tetragonal–orthorhombic structural transition temperature for NdBa 2 Cu 3 O 7− δ (Nd123) single crystal easily, in-situ observation of twin formation during annealing of the Nd123 single crystal was performed by using high temperature polarized optical microscopy. The Nd123 single crystal was grown under 21% oxygen partial pressure atmosphere by a top-seeded solution-growth (TSSG) method. The cut sample was heated to 700°C in a pure argon gas flow, followed by a step-wise cooling treatment every 10°C from 700°C, in a 100 and a 21% oxygen partial pressure gas flow until the twin was formed. The twin formation, which takes place with a tetragonal–orthorhombic structural phase transition, was observed at 660 and 620°C in a 100 and a 21% oxygen partial pressure gas flow, respectively. Furthermore, the oxygen contents (7− δ ) in the Nd123 single crystal, which are changed by oxygen absorption and desorption in the crystal, were examined with a thermogravimetry (TG) on powder samples and an iodometric titration technique. The relations between the annealing temperatures and the oxygen contents (7− δ ) were determined under 100 and 21% oxygen partial pressure atmospheres. These relations suggested that the critical oxygen content with which the tetragonal–orthorhombic structural phase transition took place was around 6.80 for the Nd123 single crystal.

Structural, magnetic, and transport properties of La2Cu1-xLixO4.

Li substitutes for Cu in La{sub 2}CuO{sub 4} up to the limiting stoichiometry La{sub 2}Cu{sub 0.5}Li{sub 0.5}O{sub 4}, which has superstructure order. The effects of this in-plane hole doping on the structural and magnetic properties of La{sub 2}CuO{sub 4} are very similar to those due to Sr substitution. The tetragonal-orthorhombic structural phase transition occurs, for a given amount of Sr or Li, at nearly the same temperature, and the in-plane lattice constant of La{sub 2{minus}{ital y}}Sr{sub {ital y}}Cu{sub 1{minus}{ital x}}Li{sub {ital x}}O{sub 4} at room temperature depends only on the combined hole count ({ital x}+{ital y}) and not on the individual Sr or Li concentration. Long-range magnetic order is destroyed upon substituting 3{percent} Li for Cu, analogous to the effect of Sr substitution on {ital T}{sub N{acute e}el}. However, the holes introduced by Li substitution are bound. The resistivity as a function of temperature is nonmetallic for all Li concentrations. {copyright} {ital 1996 The American Physical Society.}

Role of structural instability in high-temperature superconductivity and its effect on isotope effect

Abstract The tetragonal-orthorhombic structural phase transition of La2-x Sr x CuO4 is analyzed within an anharmonic electron-phonon interaction model. x-dependence as well as temperature T-dependence of the soft mode are calculated and in quantitative agreement with experimental data. To evaluate the importance of the soft mode for the superconducting properties Eliashberg equations are solved for this specific mode. The resulting electron-phonon coupling Λ is found to be strongly x and T dependent with characteristic anomalies around the structural transition temperature T s . The calculated x-dependent superconducting transition temperature T c follows closely the experimental values but underestimates T c . This discrepancy is attributed to the neglect of higher order electron-phonon interactions. Inclusion of these terms leads to a strong decrease of the isotope effect α from the BCS value of 0·5 to a varied value of 0·1.