Charge Ordering Phase Transition Research Articles

Superstructure of Fe5-xGeTe2 Determined by Te K-Edge Extended X-ray Absorption Fine Structure and Te Kα X-ray Fluorescence Holography.

The local structure of the two-dimensional van der Waals material, Fe5-xGeTe2, which exhibits unique structural/magnetic phase transitions, was investigated by Te K-edge extended X-ray absorption fine structure (EXAFS) and Te Kα X-ray fluorescence holography (XFH) over a wide temperature range. The formation of a trimer of Te atoms at low temperatures has been fully explored using these methods. An increase in the Te-Fe distance at approximately 150 K was suggested by EXAFS and presumably indicates the formation of a Te trimer. Moreover, XFH displayed clear atomic images of Te atoms. Additionally, the distance between the Te atoms shortened, as confirmed from the atomic images reconstructed from XFH, indicating the formation of a trimer of Te atoms, i.e., a charge-ordered superstructure. Furthermore, Te Kα XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge Kα XFH that was previously reported because of the low occupancy of Fe and Ge atoms. In this study, EXAFS and XFH clearly showed the local structure around the Te atom; in particular, the formation of Te trimers caused by charge-ordered phase transitions was clearly confirmed. The charge-ordered phase transition is fully discussed based on the structural variation at low temperatures, as established from EXAFS and XFH.

Physical properties and phase diagram of single crystal REBaMn2O6 (RE = Sm, Eu, Gd, Tb, Dy, and Y)

We have carried out a systematic study on antiferromagnetic and charge ordering phase transitions in double-perovskite manganite compounds REBaMn2O6 (RE ​= ​Sm, Eu, Gd, Tb, Dy, and Y) by using single-crystalline samples. In YBaMn2O6, a tiny anomaly near 480 ​K as well as a first-order phase transition near 520 ​K are found in the temperature dependence of resistivity. Synchrotron x-ray single-crystal structure analysis shows that the unit cell is doubled first in the ab plane and further doubled along the c-axis as the temperature decreases. One of the most possible electronic phases between 480 ​K and 520 ​K is Zener polaron ordering. Mn atoms are paired in the ab-plane with seven 3d-electrons localized on each pair. The 3d-electron distribution in each pair begins to become asymmetric below 480 ​K. Similar two-step transitions are also observed for RE ​= ​Tb and Dy. The antiferromagnetic transition at TN ​≃ ​180 ​K is accompanied by a large hysteresis, which is attributable to a simultaneous rearrangement of charge order. The charge rearrangement temperature gradually rises with increasing the RE ionic radius. In SmBaMn2O6, the Néel temperature (TN) is 175 ​K and is lower than charge rearrangement temperature (≃ 200 ​K). These transitions are summarized in a phase diagram with respect to temperature and the ionic radius of RE3+.

Pressure-induced charge ordering transition in CaMn7O12

We use high-pressure resistivity and single crystal x-ray diffraction at ambient and low temperature to investigate the charge ordering phase transition of ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$. We have found that at ambient temperature the Jahn-Teller distortion of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra rapidly decreases above 20 GPa, and vanishes at 28 GPa, when two Mn octahedral sites initially occupied by ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ become regular and equivalent as the result of a charge delocalization. Such a change correlates with a two orders of magnitude drop in the resistivity and a symmetry increase from the low-pressure rhombohedral R$\overline{3}$ phase to the cubic Im$\overline{3}$ structure, the same as one found at ambient pressure above 440 K. This yields the slope of the charge ordering phase boundary of $d{T}_{c}/dp\ensuremath{\approx}\ensuremath{-}6$ K/GPa. This result is further supported by the lack of a structural phase transition up to the maximum measured pressure of 30 GPa when the experiment is performed at 70 K. The satellite reflections of the structural modulation of the multiferroic phase of ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$ observed at 70 K were found to hold up to 25 GPa with the structure keeping a constant modulation vector $k=(0\phantom{\rule{0.28em}{0ex}}0\phantom{\rule{0.28em}{0ex}}0.925)$ with pressure. The average structure at 70 K does not show other indications of further phase transition.

Required nearest-neighbor Coulomb interactions for a charge-ordered phase transition in (TMTTF)2MF6 with inversion symmetry breaking in crystal

We examined the charge-ordered phase transition in (TMTTF)2MF6 systems by exact diagonalization of an extended Hubbard model Hamiltonian whose parameters were referred from quantum chemical calculations with periodic boundary conditions along with inversion symmetry breaking in the crystal. We showed that a nearest-neighbor Coulomb interactions for the charge-ordered phase transition should be twice as much as those previously obtained by fitting for conductivities with a minute inversion symmetry breaking of the crystal (∼0.01 Å) which was confirmed by a recent experimental study.

Anisotropic properties, charge ordering, and ferrimagnetic structures in the strongly correlated β−V2PO5 single crystal

A combined study of transport, thermodynamic, neutron diffraction, nuclear magnetic resonance measurements and first principles calculation were performed for $\beta$-V$_2$PO$_5$ single crystal. It was shown to be a semiconductor with a band gap of 0.48 eV, undergoing a charge ordering (unusual V$^{2+}$ and V$^{3+}$) phase transition accompanied by a tetragonal to monoclinic structural distortion at 610 K and a paramagnetic to ferrimagnetic phase transition at 128 K with a propagation vector of $\textbf{k} = 0$. The easy axis is in the monoclinic $ac$ plane pointing 47(9)$^\circ$ away from the monoclinic $a$ axis. This collinear ferrimagnetic structure and anisotropic isothermal magnetization measurements suggest weak magnetic anisotropy in this compound. The first principles calculations indicate that the intra-chain interactions in the face-sharing VO$_6$ chains dominate the magnetic hamiltonian and identify the $\Gamma_5^+$ normal mode of the lattice vibration to be responsible for the charge ordering and thus the structural phase transition.

Charge-Order Phase Transition in the Quasi One-Dimensional Organic Conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the {hbox {(TMTTF)}}_2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3. We found that the {hbox {(TMTTF)}}_2 {hbox {NO}}_3 compound develops a peculiar anisotropy with a doubled periodicity (ab'-plane) of the ESR linewidth below about T_{text {CO}}= ({250pm 10})~hbox {K}. This behavior is similar to observations in the related compounds {hbox {(TMTTF)}}_2X (X = {hbox {PF}}_6, {hbox {SbF}}_6 and {hbox {AsF}}_6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous {hbox {(TMTTF)}}_2 {hbox {ClO}}_4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below T_{text {CO}} approx 250~hbox {K}.

Magnetism and Charge Order in Nanocrystalline Orthorhombic SrFeO3-δ

Orthorhombic SrFeO3-δ is synthesized through solid-state reaction in nanocrystalline form with controlled oxygen flow. Neutron diffraction measurement with Rietveld refinement and selected area electron diffraction (SAED) confirms the crystal structure. Magnetization against temperature shows that orthorhombic SrFeO3-δ is an antiferromagnet with Neel temperature (TN) of 230 K. Field-cooled hysteresis (M-H) measurements show exchange bias present in this system. It undergoes charge ordering near the Neel temperature and large change in resistivity around TN is found. Behavior of transport and magnetotransport properties are understood using the three-dimensional variable range hopping (3DVRH) mechanism and hysteresis is observed around TN in transport data. Ferromagnetic resonance (FMR) study shows that the linewidth is minimum around TN. Charge order and magnetic phase transition are coupled to each other around TN in the orthorhombic SrFeO3-δ system.

First order metamagnetic transition in Vanadium doped charge ordered Pr0.5Ca0·5MnO3

Magnetic properties of 5% non-magnetic V5+ (3 d0) substituted Pr0.5Ca0·5Mn0·95V0·05O3 (PCMVO) polycrystalline manganite have been studied in the temperature range 5–300 K and applied magnetic field up to 3 T. Magnetization study shows that the substitution of V at the Mn site drives the system towards a coexisting antiferromagnetic (AFM)-ferromagnetic (FM) ground state. The charge ordering and ferromagnetic phase transitions are observed at TCO = 240 K and TC = 43 K respectively. Non-saturation of magnetization below TC and a history dependent first order metamagnetic AFM-FM phase transition was observed too. During forward and reverse field sweeping a large irreversibility in magnetization indicates spin memory effect and a potential application of the sample in spintronics. These magnetic properties of the sample are attributed to the interplay between charge, orbital and magnetic ordering.

Charge ordering and successive phase transitions of mixed-valence iron oxide GdBaFe2O5

We investigated the correlations between the crystal structures and electrical properties of the double-perovskite GdBaFe2O5 over a broad range of temperatures. We examined the successive phase transitions and determined the crystal structures of four crystallographic phases of GdBaFe2O5 using electron diffraction and synchrotron radiation X-ray powder diffraction (SRXRD). The valence states of the Fe sites in the respective phases were deduced from the SRXRD data. We observed two types of charge-ordered states caused by charge and orbital orderings at Fe ions. The electrical resistivity displayed a metal-insulator transition, which is compatible with charge-ordered states. The structural phase transitions in GdBaFe2O5 were observed to be related to the charge-, magnetic-, and orbital-ordered states.

Dual boson approach with instantaneous interaction

The Dual Boson approach to strongly correlated systems generally involves a dynamic (frequency-dependent) interaction in the auxiliary impurity model. In this work, we explore the consequences of forcing this interaction to be instantaneous (frequency-independent) via the use of a self-consistency condition on the instantaneous susceptibility. The result is a substantial simplification of the impurity model, especially with an eye on realistic multiband implementations, while keeping desireable properties of the Dual Boson approach, such as the charge conservation law, intact. We show and illustrate numerically that this condition enforces the absence of phase transitions in finite systems, as should be expected from general physical considerations, and respects the Mermin-Wagner theorem. In particular, the theory does not allow the metal to insulator phase transition associated with the formation of the magnetic order in the two-dimensional system. At the same time, the metal to charge ordered phase transition is allowed, as it is not associated with the spontaneous breaking of a continuous symmetry, and is accurately captured by the introduced approach.

Charge ordering and nonlocal correlations in the doped extended Hubbard model

We study the extended Hubbard model away from half-filling on a two-dimensional square lattice using cluster dynamical mean field theory on clusters of size $8$. We show that the model exhibits metallic, compressible charge ordered, and insulating charge ordered phases. We determine the location of the charge ordering phase transition line at finite temperature and the properties of the phases as a function of doping, temperature, local interaction, and nearest neighbor interaction. An analysis of the energetics of the charge order transition shows that the charge ordering transition mainly results in a rearrangement of local and non-local potential energy. We show the doping evolution of the spectral function from the isotropic metal via a charge ordered metal to a charge ordered insulator with a big gap, and study finite size effects of the approximation.

Bending and breaking of stripes in a charge ordered manganite

In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal–insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.

Electronic reconstruction on dimerized IrTe2

We investigated the electronic reconstruction in IrTe2 across the charge order phase transition using the angle-resolved photoemission spectroscopy and the ab initio calculation. The complicated shadow bands are attributed to the band folding effect of the charge ordered unit cell, while the valence transition causes the Fermi level shift as well as the size change of the Fermi surface. Implementation of a band unfolding scheme enables us to resolve the complicated folded band features, and successfully provides us information on the electronic reconstruction involving the transition. As a result, we could identify a flattened band and a segmented dispersion near the Γ and A point. We also identified the surface state contributions of additional bands crossing the Fermi level and a Dirac-cone–like band. The observed electronic reconstruction implies that the transition is induced by the strong correlation between the itinerant electrons and the spin-orbit coupled charge ordered states.

Effect of chemical pressure on the electronic phase transition in Ca1−xSrxMn7O12 films

We demonstrate how chemical pressure affects the structural and electronic phase transitions of the quadruple perovskite CaMn7O12 by Sr doping, a compound that exhibits a charge-ordering transition above room temperature making it a candidate for oxide electronics. We have synthesized Ca1−xSrxMn7O12 (0 ≤ x ≤ 0.6) thin films by oxide molecular beam epitaxy on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. The substitution of Sr for Ca results in a linear expansion of the lattice, as revealed by X-ray diffraction. Temperature-dependent resistivity and X-ray diffraction measurements are used to demonstrate that the coupled charge-ordering and structural phase transitions can be tuned with Sr doping. An increase in Sr concentration acts to decrease the phase transition temperature (T*) from 426 K at x = 0 to 385 K at x = 0.6. The presence of a tunable electronic phase transition, above room temperature, points to the potential applicability of Ca1−xSrxMn7O12 in sensors or oxide electronics, for example, via charge doping.

Charge ordering and correlation effects in the extended Hubbard model

We study the half filled extended Hubbard model on a two-dimensional square lattice using cluster dynamical mean field theory on clusters of size 8-20. We show that the model exhibits metallic, Mott insulating, and charge ordered phases, and determine the location of the charge ordering phase transition line and the properties of the ordered and disordered phase as a function of temperature, local interaction, and nearest neighbor interaction. We find strong non-local correlations in the uniform phase and a pronounced screening effect in the vicinity of the phase transition, where non-local interactions push the system towards metallic behavior. In contrast, correlations in the ordered phase are mostly local to the unit cell. Extrapolation to the thermodynamic limit and control of all sources of errors allow us to assess the regime of applicability of simpler approximation schemes for systems with non-local interactions. We also demonstrate how strong non-local electron-electron interactions can increase electron mobility by turning a charge ordered insulator into a metal.

Electronic transition above room temperature in CaMn7O12 films

We report on the electronic phase transition in CaMn7O12 quadruple perovskite films synthesized by oxide molecular beam epitaxy on SrLaAlO4 and La0.3Sr0.7Al0.65Ta0.35O3 substrates. We use x-ray diffraction and transmission electron microscopy to confirm that the CaMn7O12 phase has been realized. Temperature dependent resistivity measurements reveal a signature of a charge ordering phase transition at ≈425 K, consistent with bulk CaMn7O12. The transition temperature is found to be relatively invariant to changes in the cation stoichiometry. Density functional theory calculations reveal the changes in atomic and electronic structure induced by the charge ordering transition.

Optical Conductivity Measurement of a Dimer Mott-Insulator to Charge-Order Phase Transition in a Two-Dimensional Quarter-Filled Organic Salt Compound

We report a novel insulator-insulator transition arising from the internal charge degrees of freedom in the two-dimensional quarter-filled organic salt β-(meso-DMBEDT-TTF)2PF6. The optical conductivity spectra above Tc=70 K display a prominent feature of the dimer Mott insulator, characterized by a substantial growth of a dimer peak near 0.6 eV with decreasing temperature. The dimer peak growth is rapidly quenched as soon as a peak of the charge order appears below Tc, indicating a competition between the two insulating phases. Our infrared imaging spectroscopy has further revealed a spatially competitive electronic phase far below Tc, suggesting a nature of quantum phase transition driven by material-parameter variations.

DC magnetization studies of nano- and micro-particles of bilayered manganite LaSr2Mn2O7

Abstract Systematic studies of the magnetic properties of LaSr2Mn2O7 as a function of crystalline grain size provide information on how the crystalline grain size affects the magnetic and charge ordering in this compound. The half-doped bilayered manganite LaSr2Mn2O7 (x = 0.5) in its bulk form has the CE-type anti-ferromagnetic (CE-AFM) charge ordering phase transition. In this work, we prepare LaSr2Mn2O7 ceramic samples using the Pechini sol–gel method in order to produce different grain sizes, and the effects of different crystalline grain sizes between 200 and 1000 nm on the magnetic properties of the compound, obtained by a SQUID magnetometer, are investigated. The DC magnetization (DCM) measurements for all samples indicate that the crystalline grain size has no considerable effect on the charge ordering transition temperature, TCO (just a few degrees shift to lower temperatures as the grain sizes become bigger). The width of the peak observed in these measurements (transition to the charge ordering phase) becomes broader, and susceptibility measurement at the zero field cooling (ZFC) and filed cooling (FC) increases as the grain size becomes systematically smaller. The results obtained from the magnetic hysteresis curves confirm the ground state phase of the system is anti-ferromagnetic and the Arrott plots obtained manifest existence of the first and second order magnetic phase transition in all samples. In addition, in the sample with a grain size of 200 nm, enhancement of the magnetic properties, which is accompanied with the formation of FM phase on the surface of grain or particle, is observed.

In situ control of electronic phase separation in La1/8 Pr4/8Ca3/8MnO3/PNM-PT thin films using ferroelectric-poling-induced strain

The effects of ferroelectric-poling-induced strain on the transport and magnetic properties of the phase separated La1/8Pr4/8Ca3/8MnO3 (LPCMO) thin films epitaxially grown on the ferroelectric 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-PT) single-crystal substrates were investigated. The ferroelectric poling reduces the in-plane tensile strain and enhances the out-of plane tensile strain of LPCMO film, which decreases the resistance and the charge ordering transition temperature but raises the low-field-magnetization of film. These results can be explained by the strain induced change in the volume fraction of coexisting phases, i.e., ferromagnetic, antiferromagnetic, and paramagnetic phases, demonstrating that the charge ordering phase transition of manganites film grown on the ferroelectric PMN-PT substrate can be controlled by modifying the poling state of single crystal substrate.

Thermodynamic phase diagram of static charge order in underdoped YBa2Cu3Oy

The interplay between superconductivity and any other competing order is an essential part of the long-standing debate on the origin of high temperature superconductivity in cuprates. Akin to the situation of heavy fermions, organic superconductors and pnictides, it has been proposed that the pairing mechanism in cuprates comes from fluctuations of a nearby quantum phase transition. Recent evidence of charge modulation and the associated fluctuations in the pseudogap phase of YBa_2Cu_3O_y make charge order a likely candidate for a competing order. However, a thermodynamic signature of the charge ordering phase transition is still lacking. Moreover, whether such charge order is one- or two-dimensional is still controversial but pivotal for the understanding the topology of the reconstructed Fermi surface. Here we address both issues by measuring sound velocities in YBCO_6.55 in high magnetic fields, a powerful thermodynamic probe to detect phase transitions. We provide the first thermodynamic signature of the field-induced charge ordering phase transition in YBCO allowing construction of a field-temperature phase diagram, which reveals the competing nature of this charge order. The comparison of different acoustic modes indicates that the charge modulation has a two-dimensional character, thus imposing strong constraints on Fermi surface reconstruction scenarios.