Low-temperature Orthorhombic Phase Research Articles

Structural phase transitions in EuFe2As2 superconductor at low temperatures and high pressures

The crystal structure of EuFe2As2 has been studied up to a pressure of 35 GPa and down to a temperature of 8 K using temperature dependent x-ray diffraction in a diamond anvil cell at a synchrotron source. At 4.3 GPa, we have detected a structural phase transition from a high temperature tetragonal phase with I4/mmm space group to a low temperature orthorhombic phase with Fmmm space group around 120 K. With the application of pressure at a low temperature of 10 K, the orthorhombic phase is suppressed and a phase change to a collapsed tetragonal phase with I4/mmm space group is observed at 11 GPa. This collapsed tetragonal phase is similar to the one observed at ambient temperature and pressure above 8.5 GPa. We have shown that the collapsed tetragonal phase of EuFe2As2 has the same pressure–volume (P–V) equation of state at ambient temperature and at 10 K, implying that the high pressure phase of EuFe2As2 has a negligible thermal expansion coefficient.

Nematic spin fluid in the tetragonal phase of BaFe2As2

We use inelastic neutron scattering to study spin waves below and above ${T}_{N}$ in iron-arsenide BaFe${}_{2}$As${}_{2}$. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the antiferromagnetic $a$-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest the presence of a spin nematic fluid in the tetragonal phase of BaFe${}_{2}$As${}_{2}$, which may cause the electronic and orbital anisotropy observed in these materials.

Epitaxial suppression of the metal-insulator transition in CrN

Both single- and polycrystalline CrN layers are grown by reactive sputtering on MgO and quartz substrates, respectively. Temperature-dependent x-ray diffraction indicates a phase transition near 280 K to a low-temperature orthorhombic phase for polycrystalline CrN, while epitaxial constraints cause single-crystal CrN(001) and CrN(111) to remain in the cubic high-temperature phase. Electronic transport measurements indicate variable-range-hopping for the cubic phase below \ensuremath{\sim}120 K, a discontinuity at the phase transition for the polycrystalline layers, strongly and weakly disordered metallic conduction for the orthorhombic phase if deposited at 600 and 800 \ifmmode^\circ\else\textdegree\fi{}C, respectively, and a disorder-induced metal-insulator transition in the cubic phase.

Hyperfine interactions and electronic structures of BaFe 2 As 2 : a first-principles LDA+U study

The hyperfine parameters of hyperfine fields, electric field gradients and isomer shifts at the Fe site are investigated based on the first-principles calculations of the electronic structures using LDA (GGA)+U method in the low-temperature orthorhombic antiferromagnetic phase of undoped BaFe2As2. It is fond that the electric field gradient of Fe nucleus is highly related with the electronic structures close to the Fermi level. Though the addition of negative on-site Coulomb interaction to Fe-3d states improves the calculated magnetic moment of Fe atom and the hyperfine parameters of Fe nucleus when U = −0.1 Ry (−0.08 Ry) for GGA+U (LDA+U) method, a negative U correction does not capture the right physics of this system. The calculations prove the strong coupling between the magnetic, structural and electronic properties in antiferromagnetic BaFe2As2 parent.

Anisotropies in the optical ac and dc conductivities in lightly dopedLa2 − xSrxCuO4: the role of deep and shallow acceptor states

We investigate the origin of the optical ac and dc conductivity anisotropiesobserved in the low temperature orthorhombic phase of lightly doped, untwinnedLa2 − xSrxNiO4 single crystals. We show that these anisotropies can be naturally ascribed to the emergenceof two odd parity, rotational-symmetry-broken, localized impurity acceptor states, onedeeper and one shallower, resulting from the trapping of doped holes by the Coulombpotential provided by the Sr ions. These two lowest-energy, p-wave-like states are split byorthorhombicity and are partially filled with holes. This leaves a unique imprint in theoptical ac conductivity, which shows two distinct far-infrared continuum absorptionenergies corresponding to the photoionization of the deep and shallow acceptor states.Furthermore, we argue that the existence of two independent and orthogonal channels forhopping conductivity, directly associated with the two orthorhombic directions, alsoquantitatively explains the observed low temperature anisotropies in the dc conductivity.

Variable-range hopping conduction in epitaxial CrN(001)

Epitaxial CrN(001) layers, grown by dc magnetron sputtering on MgO(001) substrates at growth temperatures ${T}_{s}$ $=$ 550--850 \ifmmode^\circ\else\textdegree\fi{}C, exhibit electronic transport that is dominated by variable-range hopping (VRH) at temperatures 120 K. A transition from Efros-Shklovskii to Mott VRH at 30 \ifmmode\pm\else\textpm\fi{} 10 K is well described by a universal scaling relation. The localization length decreases from 1.3 nm at ${T}_{s}$ $=$ 550 \ifmmode^\circ\else\textdegree\fi{}C to 0.9 nm for ${T}_{s}$ $=$ 600--750 \ifmmode^\circ\else\textdegree\fi{}C, but increases again to 1.9 nm for ${T}_{s}$ $=$ 800--850 \ifmmode^\circ\else\textdegree\fi{}C, which is attributed to changes in the density of localized states associated with N vacancies that form due to kinetic barriers for incorporation and enhanced desorption at low and high ${T}_{s}$, respectively. The low-temperature transport data provide lower limits for the CrN effective electron mass of $4.9{m}_{e}$, the donor ionization energy of 24 meV, and the critical vacancy concentration for the metal-insulator transition of 8.4 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{19}$ cm${}^{\ensuremath{-}3}$. The room temperature conductivity is dominated by Hubbard band states near the mobility edge and decreases monotonically from 137 \ensuremath{\Omega}${}^{\ensuremath{-}1}$cm${}^{\ensuremath{-}1}$ for ${T}_{s}$ $=$ 550 \ifmmode^\circ\else\textdegree\fi{}C to 14 \ensuremath{\Omega}${}^{\ensuremath{-}1}$cm${}^{\ensuremath{-}1}$ for ${T}_{s}$ $=$ 850 \ifmmode^\circ\else\textdegree\fi{}C due to a decreasing structural disorder, consistent with the measured x-ray coherence length that increases from 7 to 36 nm for ${T}_{s}$ $=$ 550 to 850 \ifmmode^\circ\else\textdegree\fi{}C, respectively, and a carrier density that decreases from 4 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{20}$ to 0.9 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{20}$ cm${}^{\ensuremath{-}3}$, as estimated from optical reflection and Hall effect measurements. The absence of an expected discontinuity in the conductivity at \ensuremath{\sim}280 K suggests that epitaxial constraints suppress the phase transition to a low-temperature orthorhombic antiferromagnetic phase, such that CrN remains a cubic paramagnetic insulator over the entire measured temperature range of 10--295 K. These results contradict previous experimental studies that report metallic low-temperature conduction for CrN, but support recent computational results suggesting a band gap due to strong electron correlation and a stress-induced phase transition.

Spin-glass-like state in GdCu: Role of phase separation and magnetic frustration

We report investigations on the ground state magnetic properties of intermetallic compound GdCu through dc magnetization measurements. GdCu undergoes first order martensitic type structural transition over a wide temperature window of coexisting phases. The high temperature cubic and the low temperature orthorhombic phases have different magnetic character and they show antiferromagnetic and helimagnetic orderings below 145 K and 45 K respectively. We observe clear signature of a glassy magnetic phase below the helimagnetic ordering temperature, which is marked by thermomagnetic irreversibility, aging and memory effects. The glassy magnetic phase in GdCu is found to be rather intriguing with its origin lies in the interfacial frustration due to distinct magnetic character of the coexisting phases.

Structural, electronic, bonding, and elastic properties of NH3BH3: A density functional study

The structural, electronic, bonding, and elastic properties of the low-temperature orthorhombic phase of NH(3)BH(3) are studied by means of first-principles total energy calculations based on the pseudopotential method. The calculated structural parameters of NH(3)BH(3) are found to be in good agreement with the experimental values. From the band structure calculations, the compound is found to be an indirect bandgap insulator with the bandgap of 5.65 eV (5.90 eV) with LDA(GGA) along the Γ-Z direction. The Mulliken bond population and the charge density distributions are used to analyze the chemical bonding in NH(3)BH(3) . The study reveals that B-H bonds are more covalent than N-H bonds. The elastic constants are predicted for ambient as well as pressures up to 6 GPa, from which theoretical values of all the related mechanical properties such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and anisotropy factors are calculated. It is found that NH(3)BH(3) is mechanically stable at ambient and also external pressures up to 6 GPa. As pressure increases all the calculated elastic moduli of NH(3)BH(3) increase, indicating that the compound becomes more stiffer and hard under pressure. From the ratio of shear modulus to bulk modulus (G/B), we conclude NH(3)BH(3) to be ductile in nature, and the ductility increases with pressure. The present results confirm the experimentally observed less plastic nature of the low-temperature phase of NH(3)BH(3) .

Experimental charge density of LiBD4from maximum entropy method

We report on maximum entropy method study of the experimental atomic and ionic charges of LiBD4 in its low-temperature orthorhombic phase. Synchrotron radiation x-ray powder diffraction data, neutron powder diffraction data, and density functional calculations were used. The atomic and ionic charges were determined for both experimental and theoretical results using the Bader analysis for atoms in molecules. The charge transfer from the Li cation to the BD4 anion is 0.86(+/- 9) e, which is in good agreement with the ab initio calculated value of 0.895 e. The experimental accuracy was determined considering the differences between results obtained for data collected at 10 and 90 K, different experimental setups (high-resolution diffractometer or image plate diffractometer), and different structural models used for the prior density distributions needed for accurate maximum entropy calculations (refined using only synchrotron radiation x-ray powder diffraction data or combined with neutron powder diffraction data).

Hydrogen isotope effects on the structural phase transition of NH 3BH 3

A systematic study of the structural phase transition of NH 3BH 3 and of its fully deuterated analogue was performed combining DSC and anelastic spectroscopy measurements. The transition is accompanied by a latent heat, and therefore is of the 1st order. On the deuterated sample the enthalpy variation is reduced of more than 20%, from 1.29 to 1.01 kJ/mol and the transition is shifted by ∼1.5 K toward higher temperatures. Both NH 3BH 3 and ND 3BD 3 display a temperature hysteresis between cooling and heating, thus denoting that the phase transition is of first-order. In addition, this hysteresis is extremely small (∼0.5 K) indicating that the coexistence region between the two phases is very narrow. During isothermal ageing, the transformation of the low-temperature orthorhombic phase into the high-temperature tetragonal one occurs with a time constant of ∼16 min in NH 3BH 3 and ∼64 min in ND 3BD 3, evidencing a drastic slowing down of kinetics in the deuterated compound.

Low Temperature Phase Diagram of NH3BH3

ABSTRACTThe pressure-temperature (p-T) phase diagram of NH3BH3 has been investigated by thermal conductivity measurements up to 1.5 GPa at temperatures between 100 and 300 K, and the phase boundaries between the three known structural phases have been identified. The transformation between the room temperature tetragonal I4mm phase and the low temperature orthorhombic Pmn21 phase (Tc = 218 K at p = 0) shows only a small hysteresis. The transformation into the high pressure orthorhombic Cmc21 phase (at 1.0 GPa near 292 K) has a very strong hysteresis, up to Δp = 0.5 GPa, and below 230 K a fraction of this phase is metastable even at atmospheric pressure.

Quasiparticle band structure and optical properties of NH3 BH3

The quasiparticle band structure of the low temperature orthorhombic phase of NH3BH3 is studied by using the GW approximation. It is found that NH3BH3 is an insulator with a value of the band gap of 5.90 eV with GGA and of 9.60 eV with the GW approximation. Then, the optical properties of NH3BH3 are obtained by the calculation of the dielectric function, corrected by a scissor shift operation corresponding to the GW correction on the band gap. Also, the optical anisotropy in NH3BH3 is analyzed through the refractive index and static dielectric constants along the different crystallographic directions. Finally, it is found that the energy loss function has a prominent peak at 22.26 eV; at these frequencies (above 22.26 eV) NH3BH3 becomes transparent. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Density functional calculations of the electronic structure and magnetism of the different phases of BaFe2As2

Abstract We have carried out noncollinear electronic structure calculations to investigate the structural, electronic and magnetic properties of the low-temperature orthorhombic and high-temperature tetragonal phases of BaFe 2 As 2 . We find that the GGA optimized orthorhombic phase has an LDA magnetic moment of 0.46 μ B in excellent agreement with the Mossbauer spectroscopy value of approximately 0.4 – 0.5 μ B per Fe atom. The very low LDA magnetic moment of 0.02 μ B for the Fe atoms in the GGA optimized tetragonal phase is consistent with the experimentally observed Pauli paramagnetism of the tetragonal phase. Thus, these calculations offer an improvement in the description of the Fe–As interaction in the 122 pnictide compound family.

Spin-orbit coupling in the superconducting phase and DDW states of high-Tc cuprates

The effects of the spin-orbit coupling are considered for the high Tc cuprates with asymmetric superconducting gap (SC) and the d-density wave (DDW) phase due to its vital role in the experimental determination of the DDW state. Experiments predict an anisotropy in the DSC gap where |Δ(0,π)|>|Δ(π,0)| and the gap node deviates from the diagonal direction towards the kx axis. Measurements also demonstrate DDW to be a possible candidate for the pseudogap in the underdoped phase. Due to the spin-orbit (SO) coupling in the low temperature orthorhombic (LTO) phase, the phase diagram of the cuprates suffers a change due to the modification of the T* value, the temperature characteristic of pseudogap, although Tc remains unaltered. Moreover, for a more generalized SO coupling, the DDW gap decreases with the angle but has no effect on the SC gap. We calculate the density of states in the various regimes of doping for the mixed SC+DDW states in the underdoped (UD) phase, SC state in the overdoped phase and also the DDW state in the UD phase and compare them with various theoretical and experimental works. The temperature dependence of the specific heat does not exhibit any qualitative change due to the SO coupling.

La-site substitution effect by Ce ion on carrier concentration and structural phase transition in La2−xSrxCuO4

We have studied an effect of the partial substitution of Ce4+ ion for La-site on the superconductivity and the structural phase transition between the high-temperature tetragonal (HTT) and the low-temperature orthorhombic (LTO) phases in La2−xSrxCuO4. For La2−xSrxCuO4 without substitution by Ce ion, x dependence of the critical temperature Tc showed a dip at x=0.115 and a maximum at x=0.15. The dip structure shifts to x=0.125 and the Tc maximum shifts to x=0.16 for La1.99−xSrxCe0.01CuO4. On the other hand, x dependence of the structural phase transition temperature Td1 was hardly affected by the Ce substitution. These results indicate that the Ce substitution for the La-site gives a possibility to realize the flatter CuO2 planes without the change of the carrier concentration.

Detection of structural and magnetic transitions in La 0.67Ba 0.23Ca 0.1MnO 3 using the rf resonance technique

We report radio-frequency ( rf) electrodynamics in polycrystalline La 0.67Ba 0.23Ca 0.1MnO 3 as a function of temperature and magnetic field using a home-built LC resonant circuit powered by an integrated chip oscillator. The resonance frequency ( f r ) of the oscillator and the power ( P) absorbed by the sample are measured simultaneously. The paramagnetic to ferromagnetic phase transition in the absence of an external magnetic field is accompanied by a rapid decrease in both P and f r around the Curie temperature T C =300 K. However, much below T C , the f r shows a step-like anomaly around 165 K (195 K) while cooling (warming), which we attribute to a structural phase transition from high temperature rhombohedral ( R 3 ¯ c ) to low temperature orthorhombic ( Imma) phase. The step-like anomaly in f r versus T disappears in a field of 300 G. Fractional changes as large as 19% in Δ f r /f r and 10% in Δ P/ P are observed under H=1 kG around T C . Our study suggests that the rf resonance technique is a versatile tool to study the magnetization dynamics as well as to investigate the structural phase transition in manganites.

Pulsed High-Frequency EPR Investigation of Gadolinium-Doped PbZrO3 and PbTiO3

Using a resonator-free setup, pulsed high-frequency (240 GHz) electron paramagnetic resonance (EPR) experiments on gadolinium-doped PbTiO3 and PbZrO3 samples have been performed. It could be demonstrated that echo-detected EPR spectra can be recorded routinely from these materials. These compounds are highly absorptive at microwave frequencies, thus preventing the use of microwave resonators at very high frequencies. As echo-detected EPR allows us to record the EPR absorption directly, the effect of relative suppression of broad unstructured spectral components in conventional field-derivative EPR is avoided. The analysis of the high-frequency EPR spectra indicated that Gd3+ ions are additionally also positioned at highly distorted sites. This might indicate that charge compensation leads to the formation of closely correlated Gd3+-VPb′′-Gd3+ defects under high doping conditions in addition to Gd3+ inserted substitutionally at Pb2+ sites with undistorted oxygen and lead neighboring shells. For the orthorhombic low-temperature phase of PbZrO3 two crystallographically inequivalent Pb2+ sites of equal abundance are present. The contribution of Gd3+ inserted substitutionally at these sites could be confirmed.

Synthesis and transport properties of La2NiO4

La2NiO4 compounds were prepared by a modified sol–gel auto-combustion method, which is a low-temperature combustion synthesis procedure using microwave-assisted sol–gel as precursors. The high-temperature transport properties of the samples were investigated. The band structure, total density of states (DOS), and partial density of states (PDOS) of low-temperature orthorhombic (Bmab) phase and high-temperature tetragonal (I4/mmm) phase for La2NiO4 were calculated in order to study the transport properties of the as-obtained samples.

Influence of magnetism on phonons inCaFe2As2as seen via inelastic x-ray scattering

In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high temperature tetragonal phase of CaFe2As2 using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab initio calculations of the phonons. Spin polarized calculations imposing the antiferromagnetic order present in the low temperature orthorhombic phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase.

Temperature stability of ([Na0.5K0.5NbO3]0.93–[LiTaO3]0.07) lead-free piezoelectric ceramics

A polymorphic phase transition (PPT) is often engineered into lead-free materials to generate high piezoelectric activity at room temperature, limiting their temperature stability. We report [Na0.5K0.5NbO3]0.93–[LiTaO3]0.07 tetragonal ceramics with favorable properties over a broad temperature range due to a high Curie temperature at 447 °C and PPT at −15 °C involving a transition to a monoclinic rather than low temperature orthorhombic phase. Piezoelectric k31 and d31 coefficients varied from 0.19 to 0.14 and −53 to −33 pC/N, respectively, over the range of −15 to 300 °C. Strain-electric field loops provided strains of ∼0.2% and a high-field d33 of 205 pm/V.