Intermediate Phase Transition Research Articles

Enhanced electrical and magnetic properties in BZT/NFO multiferroic composites derived by MARH

Multiferroic materials comprising of ferroelectric (FE) and ferromagnetic (FM) composites exhibits added functionalities significant from scientific and technological standouts. But these FE/FM composites are very sensitive to processing parameters, sintering, and chemical modifications. In present work FE/FM composites (1 − x)(Ba Zr0.15, Ti0.85)O3–xNiFe2O4 (BZT–NFO) was sintered by microwave assisted radiant heating (MARH) technique, which has evolved as a hybrid sintering technique, where radiant and microwave sintering methods are coupled together, in such a way that, when conventional radiant heating is applied for sintering, simultaneously different (0%, 15%, 30% and 50%) Microwave power (Mw) percentages are applied in a precise and controlled manner. The present study dwells on the role of different Mw power applied during sintering on the structure, dielectric, ferroelectric and magnetic properties of (1 − x) (Ba Zr0.15, Ti0.85) O3–xNiFe2O4 (BZT–NFO) composites. Structural phase analysis carried out by X-ray diffraction displays Bragg peaks corresponding to both perovskite and ferrite phases. Since the compositional variation of ferrite remains as low as 1% for all the compositions, the overall crystal phase is dominated by perovskite phase which was further confirmed by room temperature Raman spectra. No intermediate phase or structural transitions were observed for all the compounds. Dielectric permittivity increases by 35% for the sample sintered with 15% Mw power, while leakage current decreases by an order of magnitude. At higher temperature, mobility due to oxygen vacancies/or defects present in the samples dominates the conduction mechanism. Magnetic hysteresis curve at room temperature suggests the soft magnetic nature exhibiting ferromagnetic behavior for all the samples. Sample sintered with 15 Mw power exhibits higher saturation magnetization (Ms).

Local Joule Heating Mimicking Electroresistance‐Like Behavior in Antiperovskite Mn3GaC

AbstractElectroresistance has become a stimulating research topic because of its potential applications in next‐generation nonvolatile memory technology. Here, an electroresistance‐like behavior of the antiperovskite Mn3GaC compound around the antiferromagnetic–intermediate phase transition is reported. A high sensing current significantly decreases the fraction of the antiferromagnetic phase of higher resistivity relative to the intermediate phase of lower resistivity upon warming, showing a resistivity change up to 50%. Through extensive studies of the resistivity at different sample locations and of the magnetization under applied current, it is unambiguously shown that this electroresistance‐like behavior originates from the local Joule heating of the sample due to high‐resistive lead contacts and not from an intrinsic effect. These findings open an available avenue to explore electroresistance‐like behavior by achieving phase coexistence of a high‐ and low‐resistivity phase under the driving force of local Joule heating.

Many-body localization of spinless fermions with attractive interactions in one dimension

We study the finite-energy density phase diagram of spinless fermions with attractive interactions in one dimension in the presence of uncorrelated diagonal disorder. Unlike the case of repulsive interactions, a delocalized Luttinger-liquid phase persists at weak disorder in the ground state, which is a well-known result. We revisit the ground-state phase diagram and show that the recently introduced occupation-spectrum discontinuity computed from the eigenspectrum of the one-particle density matrix is noticeably smaller in the Luttinger liquid compared to the localized regions. Moreover, we use the functional renormalization group scheme to study the finite-size dependence of the conductance, which also resolves the existence of the Luttinger liquid and is computationally cheap. Our main results concern the finite-energy density case. Using exact diagonalization and by computing various established measures of the many-body localization-delocalization transition, we argue that the zero-temperature Luttinger liquid smoothly evolves into a finite-energy density ergodic phase without any intermediate phase transition.

Raman and XPS characterization of vanadium oxide thin films with temperature

The oxidation mechanisms and the numerous phase transitions undergone by VO2 thin films deposited on SiO2/Si and Al2O3 substrates when heated from room temperature (R.T.) up to 550°C in air are investigated by Raman and X-ray photoelectron spectroscopy. The results show that the films undergo several intermediate phase transitions between the initial VO2 monoclinic phase at R.T. and the final V2O5 phase at 550°C. The information about these intermediate phase transitions is scarce and their identification is important since they are often found during the synthesis of vanadium dioxide films. Significant changes in the film conductivity have also been observed to occur associated to the phase transitions. In this work, current and resistance measurements performed on the surface of the films are implemented in parallel with the Raman measurements to correlate the different phases with the conductivity of the films. A model to explain the oxidation mechanisms and phenomena occurring during the oxidation of the films is proposed. Peak frequencies, full-width half-maxima, binding energies and oxidation states from the Raman and X-ray photoelectron spectroscopy experiments are reported and analyzed for all the phases encountered in VO2 films prepared on SiO2/Si and Al2O3 substrates.

Sodium ion transport in polymorphic scandium NASICON analog Na3Sc2(PO4)3 with new dielectric spectroscopy approach for current-constriction effects

Conductivity, calorimetry, and in-situ XRD can be closely interrelated with each other in terms of the sodium ion ordering and rearrangements in NASICON structure. An intermediate phase transition at 103°C, not associated with heat effects, is evidenced for the first time, in addition to α–β and β–γ phase transitions at 64°C and 166°C. AC characterization of polycrystalline Na3Sc2(PO4)3 has been performed over the wide temperature range from 225 to −100°C. Strongly dispersive spectra of the low temperature monoclinic phase were systematically described by the total sample resistance connected by the several capacitive responses all in parallel: pure dielectric geometric capacitance and two Havriliak–Negami (HN) dielectric functions with opposite skewnesses. The capacitance strengths of HN functions are inversely proportional to the temperature and the relaxation times represent the thermally activated mobile charge carrier transport. While the higher frequency response with a negative skewness as γ=0.388 and β=1 is of a universal character common with many other solid electrolytes, known as K1 model, the lower frequency response with a positive skewness as γ⋅β≈1 where β≈0.59 and γ≈1.67 describes the current-constriction effects occurring in the polycrystalline Na3Sc2(PO4)3. The current-constriction effects are described by a homogeneous polarization throughout the sample with 4 times the hopping distance and 5 times the relaxation time of the K1 response. 14 model parameters competently simulate the AC behavior over a wide temperature and frequency range.

SuperconductingBi2Te: Pressure-induced universality in the(Bi2)m(Bi2Te3)nseries

Using high-pressure magnetotransport techniques we have discovered superconductivity in Bi2Te, a member of the infinitely adaptive (Bi2)m(Bi2Te3)n series, whose end members, Bi and Bi2Te3, can be tuned to display topological surface states or superconductivity. Bi2Te has a maximum Tc= 8.6 K at P= 14.5 GPa and goes through multiple high pressure phase transitions, ultimately collapsing into a bcc structure that suggests a universal behavior across the series. High-pressure magnetoresistance and Hall measurements suggest a semi-metal to metal transition near 5.4 GPa, which accompanies the hexagonal to intermediate phase transition seen via x-ray diffraction measurements. In addition, the linearity of Hc2(T) exceeds the Werthamer-Helfand-Hohenberg limit, even in the extreme spin-orbit scattering limit, yet is consistent with other strong spin-orbit materials. Considering these results in combination with similar reports on strong spin-orbit scattering materials seen in the literature, we suggest the need for a new theory that can address the unconventional nature of their superconducting states.

Enhanced temperature stability in 〈111〉 textured tetragonal Pb(Mg1/3Nb2/3)O3-PbTiO3 piezoelectric ceramics

Two different templates (〈001〉 Ba6Ti17O40 and 〈111〉 BaTiO3) were used for synthesizing 〈111〉 textured tetragonal 0.6Pb(Mg1/3Nb2/3)O3-0.4PbTiO3(PMN-40PT) ceramics. It was found that a texture degree of 95% along 〈111〉 direction can be achieved by using only 1 vol. % 〈111〉 BaTiO3 template due to its high chemical stability in the PMN-40PT matrix. The textured PMN-40PT ceramics with tetragonal structure exhibited an excellent temperature stability of piezoelectric properties due to the absence of intermediate phase transitions between room temperature and the Curie temperature. Unlike the single crystal counterpart, the effect of 〈111〉 grain orientation in the textured PMN-40PT ceramic on enhancing the macroscopic piezoelectric response was not significant in spite of its giant local piezoresponse. We provide detailed discussions on the nature of piezoelectric response in the 〈111〉 textured tetragonal PMN-40PT ceramic with “3T” engineered domain configuration and resultant strategy to realize high performance piezoelectric ceramics.

Diluting the inflationary axion fluctuation by a stronger QCD in the early Universe

We propose a new mechanism to suppress the axion isocurvature perturbation, while producing the right amount of axion dark matter, within the framework of supersymmetric axion models with the axion scale induced by supersymmetry breaking. The mechanism involves an intermediate phase transition to generate the Higgs μ-parameter, before which the weak scale is comparable to the axion scale and the resulting stronger QCD yields an axion mass heavier than the Hubble scale over a certain period. Combined with that the Hubble-induced axion scale during the primordial inflation is well above the intermediate axion scale at present, the stronger QCD in the early Universe suppresses the axion fluctuation to be small enough even when the inflationary Hubble scale saturates the current upper bound, while generating an axion misalignment angle of order unity.

Phase Structure and Piezoelectric Properties of (K<sub>0.5</sub>Nb<sub>0.5</sub>)NbO<sub>3 </sub>- CaZrO<sub>3</sub> Ceramics

Lead-free piezoelectric ceramics of (1-x)(K0.5Na0.5)NbO3-xCaZrO3(abbreviated as KNN-CZ,x= 0 ~ 0.04) were synthesized by the conventional solid state method. Their phase structures and piezoelectric properties were investigated with x-ray diffraction (XRD), Scanning Electron Microscope (SEM), d33meter and Impedance Analyzer. KNN-CZ samples exhibit only pure perovskite phases with a small monoclinic distortion. It was observed that the lattice parameter b has a maximum atx= 0.02 but the monoclinic distortion angle β shows a minimum at the same composition. From the dependence of the lattice parameters (a, b, c, β) and cell volume on the content of CZ, it is considered that the intermediate phase transition exists near atx= 0.02. Piezoelectric properties show the maximum value of d33 = 121 pC/N and kp = 0.35 for the sample withx= 0.02, which seems to be due to the co-existence of the two different phases.

The hcp to fcc transformation path of scandium trihydride under high pressure

We used density functional theory to calculate the phase stability of the hcp (hexagonal close packed) and the fcc (face centered cubic) structures of ScH3. The hcp form is stable up to 22 GPa according to the generalized gradient approximation calculation. Then the fcc form becomes energetically more stable. In order to provide insight into the phase transition mechanism, we modeled the hcp to fcc transition by sliding the hcp basal planes, i.e. (001)h planes, in such a way that the ABABAB sequence of the hcp form is altered into the ABCABC sequence of the fcc form. This sliding was suggested by the experiment. The configurations of these sliding steps are our proposed intermediate configurations, whose symmetry group is the Cm group. By using the Cm crystallography, we can match the d-spacings from the lattice planes of the hcp and fcc forms and the intermediate planes measured from the experiment. We also calculated the enthalpy per step, from which the energy barrier between the two phases at various pressures was derived. The barrier at 35 GPa is 0.370 eV per formula or 0.093 eV/atom. The movements of the hydrogen atoms during the hcp to intermediate phase transition are consistent with the result from the Raman spectra.

Phase diagram of the magnetized planar Gross-Neveu model beyond the large-Napproximation

The phase diagram and thermodynamic properties of the (2+1)-dimensional Gross-Neveu model are studied in the presence of a constant magnetic field. The optimized perturbation theory (OPT) is used to obtain results going beyond the large-N approximation. The free energy and the complete phase diagram of the model, in terms of temperature, chemical potential and magnetic field are obtained and studied in details. By comparing the results from the OPT and the large-N approximation, we conclude that finite N effects favor the phenomenon of inverse magnetic catalysis when the coupling constant is negative. We show that with the OPT the value of the coexistence chemical potential at vanishing temperature always decreases with the magnetic field. This is opposite to what is seen in the large-N approximation, where for large magnetic fields the coexistence chemical potential starts again to increase. Likewise, at finite temperature, the value of the chemical potential at the tricritical point also decreases with the magnetic field in the OPT case. Consequently, the shape of the phase diagrams predicted by the OPT and by the large-N approximation look very different in the presence of high magnetic fields. Finally, for small values of magnetic field and temperature, we identify the presence of possible intermediate nonchiral phase transitions when varying the chemical potential. We show that these phenomena are not an artifact of the large-N approximation and that they also occur within the OPT framework. These intermediate transitions are interpreted to be a consequence of the de Haas--van Alphen oscillations. We also explain why this type of phenomenon can happen in general for negative couplings but not for positive couplings.

Peculiarity of magnetoresistance in high pressure annealed Ni43Mn41Co5Sn11 alloy

Magnetic and resistance measurements have been carried out on a Ni43Mn41Co5Sn11 alloy annealed under high-pressure. Due to the existence of the intermediate phase, a change of slope in the temperature dependence of resistivity curves has been observed prior to the martensitic transformation for the high-pressure annealed alloy. As a consequence, two successive magnetoresistance peaks with the same sign are achieved around room-temperature, corresponding to the martensitic transformation and intermediate phase transition, respectively. The origin of the peculiarity of magnetoresistance properties in high-pressure annealed Ni43Mn41Co5Sn11 alloy is discussed.

Temperature dependent phase transition of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric

Phase transformation and electric properties of lead-free piezoceramics (Bi0.5Na0.5)1−xBaxTiO3 with x=0.05, 0.06, and 0.07(BNB5T, BNB6T and BNB7T) were investigated using dielectric, piezoelectric and ferroelectric measurements. Electric field induced strain measurement shows “W” shape bipolar strain characteristics for BNB5T with typical ferroelectric P–E curve, while BNB6T and BNB7T, possessing pinch-off P–E, exhibit “V” shape field-induced strain. All the BNBxT specimens exhibit relaxor characteristic, identified by the Debye Law. Dielectric properties measured at elevated temperatures with the frequency variation (10–500kHz) reveal frequency dispersion below the Td point, but no dispersion between Td and Tm, which may be ascribed to an intermediate phase transition. By adding more Ba2+ ions, the region of intermediate phase, distinguished by frequency dependence dielectric constant, expands to lower temperature. Moreover, the ferroelectric properties measured at elevated temperature were carried out below and at the depolarization temperature to well investigate the existence of this phase. Much less ε–T profile dispersion were observed during the investigation of BNB6T and BNB7T, leading to possible existence of an intermediate phase in the investigated compositions. The results suggest that the linear field-induced-strain of (Bi0.5Na0.5)1−xBaxTiO3 are expected to be attributed to the intermediate phase.

Structural, dielectric, magnetic, and nuclear magnetic resonance studies of multiferroic Y-type hexaferrites

The effect of strontium substitution on structural, magnetic, and dielectric properties of a multiferroic Y-type hexaferrite (chemical formula Ba2−xSrxMg2Fe12O22 with 0 ≤ x ≤ 2) was investigated. Y-type hexaferrite phase formation was not affected by strontium substitution for barium, in the range 0 ≤ x ≤ 1.5, confirmed by x-ray diffraction and Raman spectroscopy measured at room temperature. Two intermediate magnetic spin phase transitions (at tempertures TI and TII) and a ferrimagnetic-paramagnetic transition (at Curie temperature TC) were identified from the temperature dependence of the magnetic susceptibility. Magnetic transition temperatures (TI, TII, and TC) increased with increasing strontium content. Magnetic hysteresis measurements indicated that by increasing strontium concentration, the coercivity increases, while the saturation magnetization decreases. The 57Fe NMR spectrum of the Y-type hexaferrite measured at 5 K and in zero magnetic field showed remarkable differences compared to that of other hexaferrites due to their different number of tetrahedral and octahedral iron sites. The temperature and frequency dependence of the dielectric permittivity evidenced broad peaks with frequency dispersion in correspondence of the Curie temperature.

High pressure structural phase transitions of PbPo

Abstract First-principles calculations have been performed to investigate the high pressure phase transitions and dynamical properties of the less known lead polonium compound. The calculated ground state parameters for the NaCl phase show good agreement with the experimental data. The obtained results show that the intermediate phase transition for this compound is the orthorhombic Pnma phase. The PbPo undergoes from the rocksalt to Pnma phase at 4.20 GPa. Further structural phase transition from intermediate to CsCl phase has been found at 8.5 GPa. In addition, phonon dispersion spectra were derived from linear-response to density functional theory. In particular, we show that the dynamical properties of PbPo exhibit some peculiar features compared to other III–V compounds. Finally, thermodynamics properties have been also addressed from quasiharmonic approximation.

Magnetic/structural phase diagram and enhanced giant magnetoresistance in Zn-doped antiperovskite compound Ga 1− xZn xCMn 3

The structural, magnetic and electrical transport properties of Zn-doped antiperovskite compounds Ga 1− x Zn x CMn 3 (0≤ x≤0.30) have been investigated. After partial substitution of Zn for Ga, the Curie temperature increases monotonously and the ground antiferromagnetic (AFM)-ferromagnetic intermediate (FI) phase transition is gradually suppressed. With increasing the doping level x, the saturated magnetizations decreases gradually firstly for x≤0.20, then increases with increasing x. The electrical transport properties of Ga 1− x Zn x CMn 3 are studied at different magnetic fields. Enhanced giant magnetoresistance (GMR) was observed around the AFM-FI transition. With increasing x, the maximal values and peak widths of GMR increase. Particularly, for x=0.20, GMR reaches a maximum value of 75%, spanning a temperature range of 80 K at 50 kOe and displays the behavior of strongly depending on the magnetization history. The possible origins are discussed.

BEC–BCS crossover in a ( [formula omitted])-wave pairing Hamiltonian coupled to bosonic molecular pairs

We analyse a ( p + i p )-wave pairing BCS Hamiltonian, coupled to a single bosonic degree of freedom representing a molecular condensate, and investigate the nature of the BEC–BCS crossover for this system. For a suitable restriction on the coupling parameters, we show that the model is integrable and we derive the exact solution by the algebraic Bethe ansatz. In this manner we also obtain explicit formulae for correlation functions and compute these for several cases. We find that the crossover between the BEC state and the strong pairing p + i p phase is smooth for this model, with no intermediate quantum phase transition.

High-pressure phases of lead chalcogenides

Ab initio electronic structures have been carried out to find the pressure-induced structural phase transitions of lead chalcogenides (PbS, PbSe and PbTe) compounds. The zinc-blende, wurtzite, rocksalt, CsCl, GeS, TlI and orthorhombic Pnma phases are considered. Results show that the intermediate phase transition for these compounds is not the GeS nor the TlI type structures, as previously reported, but the orthorhombic Pnma phase. All these compounds are predicted to undergo a structural phase transition from the rocksalt to Pnma phase at about 8.13, 7.45 and 5.40 GPa for PbS, PbSe and PbTe respectively. Moreover, further structural phase transitions from this intermediate phase to the CsCl phase have been predicted at about 25.3, 18.76 and 15.43 GPa for PbS, PbSe and PbTe respectively.

Metastability across the antiferromagnetic–ferromagnetic intermediate phase transition and enhanced giant magnetoresistance in Zn-doped antiperovskite compounds Ga1−xZnxCMn3

We report the detailed investigations of the magnetic properties, electronic transport, and specific heat in Zn-doped antiperovskite compounds Ga1−xZnxCMn3. Magnetic measurements indicate the metastability of the antiferromagnetic (AFM)–ferromagnetic intermediate (FI) phase transition and the coexistence of different magnetic phases at lower temperature. The enhanced giant magnetoresistance (GMR) is observed with a maximum value of 70% spanning a temperature range of 100 K at 50 kOe. The analysis of heat capacity reveals that the metastability of the AFM–FI transition and the enhanced GMR observed in Ga1−xZnxCMn3 may originate from the reconstruction of Fermi surface accompanying AFM–FI transition.

Investigation of the intermediate phase and magnetocaloric properties in high-pressure annealing Ni–Mn–Co–Sn alloy

The Ni–Mn–Co–Sn alloy is prepared by high-pressure annealing method. Besides the enhanced martensitic transformation temperature and the Curie temperature of austenite, an intermediate phase above the martensitic transformation is observed in this alloy. As a result, two successive magnetic entropy changes with the same sign are obtained around room-temperature, corresponding to the martensitic transformation and intermediate phase transition, respectively. The origin of the intermediate phase for high-pressure annealing Ni–Mn–Co–Sn alloy is discussed.