Low-temperature Orthorhombic Research Articles

Structural origins of the low-temperature orthorhombic to low-temperature tetragonal phase transition in high- Tc cuprates

We undertake a detailed high-resolution diffraction study of a novel plain band insulator, La$_2$MgO$_4$, which may be viewed as a structural surrogate system of the undoped end-member of the high-T$_c$ superconductors, La$_{2-x-y}$A$^{2+}_x$RE$^{3+}_y$CuO$_{4}$ (A = Ba, Sr, RE= Rare Earth). We find that La$_2$MgO$_4$ exhibits the infamous low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase transition that has been linked to the suppression of superconductivity in a variety of underdoped cuprates, including the well known La$_{2-x}$Ba$_{x}$CuO$_4$ ($x=0.125$). Furthermore, we find that the LTO-to-LTT phase transition in La$_2$MgO$_4$ occurs for an octahedral tilt angle in the 4 $^{\circ}$ to 5 $^{\circ}$ range, similar to that which has previously been identified as a critical tipping point for superconductivity in these systems. We show that this phase transition, occurring in a system lacking spin correlations and competing electronic states such as charge-density waves and superconductivity, can be understood by simply navigating the density-functional theory ground-state energy landscape as a function of the order parameter amplitude. This result calls for a careful re-investigation of the origins of the phase transitions in high-T$_c$ superconductors based on the hole-doped, $n = 1$ Ruddelsden-Popper lanthanum cuprates.

Hidden competing phase revealed by first-principles calculations of phonon instability in the nearly optimally doped cuprate La1.875Sr0.125CuO4

The representative cuprate, La$_{2-x}$M$_x$CuO$_4$, with M = Sr and $x = 1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in La$_{2-x}$Sr$_x$CuO$_4$, is found to be the ground state, where the structural phase transitions, HTT$\rightarrow$LTO$\rightarrow$LTLO, can be understood via phonon instability. While the La-O composition is identified to be responsible for the phonon softening, the superconducting CuO$_2$ layer is dynamically stable. The LTLO phase, which can exhibit a $\sim$20 meV splitting in the density of states, is proposed to have an intimate relationship with the observed pseudogap and the charge density wave giving the stripe. We argue that at low temperatures, the superconducting LTO La$_{1.875}$Sr$_{0.125}$CuO$_4$ competes with the phonon-preferred LTLO phase by spontaneously forming the Cooper pairs, resulting in suppressing the stripe. Therefore, the revealed LTLO phase is indispensable for understanding La$_{2-x}$Sr$_x$CuO$_4$.

Scaling behavior of low-temperature orthorhombic domains in the prototypical high-temperature superconductor La1.875Ba0.125CuO4

Translational/rotational symmetry breaking and recovery in condensed matter systems are closely related to exotic physical properties such as superconductivity (SC), magnetism, spin density waves (SDW) and charge density waves (CDW). The interplay between different order parameters is intricate and often subject to intense debate, as in the case of CDW order and superconductivity. In La1:875Ba0:125CuO4 (LBCO), the locations of CDW domains are found to be pinned on the nanometer size scale. Coherent X-ray diffraction techniques open routes to directly visualize the domain structures associated with these symmetry changes. We have pushed Bragg Coherent Diffractive Imaging (BCDI) into the cryogenic regime where most phase transitions in quantum materials reside. Utilizing BCDI, we image the structural evolution of LBCO microcrystal samples during the high-temperature-tetragonal (HTT) to low-temperature-orthorhombic (LTO) phase transition. Our results show the formation of LTO domains close to the transition temperature and how the domain size varies with temperature. The LTO domain size is shown to decrease with temperature and to be inversely proportional to the magnitude of the orthorhombic distortion. The number of domains follows the secondary order parameter (or orthorhombic strain) measurement with a critical exponent that is consistent with the 3D universality class.

Reactive Milling Induced Structure Changes in Phenylphosphonic Acid Functionalized LiMn2O4 Nanocrystals – Synthesis, Rietveld Refinement, and Thermal Stability

Spinel LiMn2O4 formed in a solid‐state reaction was treated with a high energy planetary ball mill. A mechanochemical in situ surface functionalization of the nanocrystallites with a size smaller than 10 nm was achieved by addition of phenylphosphonic acid. The functionalization was proven by infrared spectroscopy and it can be shown that it prevents the formation of Mn2O3 during the milling process. Structural changes of the samples were investigated via XRD and Rietveld refinement. Mild milling conditions induce an anisotropic broadening of the reflections caused by a distribution of lattice parameters. In this first stage, we propose a structure model based on orthorhombic LiMn2O4. DSC and in situ XRD measurements also verify the presence of the orthorhombic low temperature phase. Medium milling conditions induce a change of intensities, correlated to a cation disorder. Harsher milling conditions induce the transformation to a tetragonal phase. The thermal stability of the formed phases was investigated via in situ high temperature XRD. The reformation of cubic spinel is observed in all samples; however, the transition point depends on the previous milling parameters.

Domain Texture of the Orthorhombic Phase of La2\u2212xBaxCuO4

Bragg coherent diffraction imaging (BCDI) experiments have been carried out at the Advanced Photon Source using a new cryostat system developed to achieve high mechanical stability and low vibrations. We measured the (012)LTO Bragg peak which is unique to the low-temperature orthorhombic (LTO) phase of micron-sized crystals of the high-temperature superconductor La2−xBaxCuO4 (LBCO) to study the formation of structural domains. Each time the sample was cooled into the orthorhombic phase, the diffraction pattern of domains was different. This confirms the interpretation of pinning of the lower-temperature charge density Wave domains observed in coherent resonant X-ray speckle correlation analysis experiments.

Influence of Cr doping on magnetocaloric effect and physical properties of slowly cooled NiMn1−xCrxGe

Slowly cooled NiMn1−xCrxGe (x = 0.04, 0.11, 0.18 and 0.25) has been studied by X-ray (90–480 K) and neutron diffraction (1.6–400 K), differential scanning calorimetry (300–480 K) and magnetic (2.0–400 K, magnetic field up to 90 kOe) measurements. The compounds undergo a martensitic phase transition changing their crystal structure between the orthorhombic low temperature phase (space group Pnma) and the hexagonal high temperature one (space group P63/mmc). The temperature of the structural phase transition significantly decreases with increasing x (TS = 464 K for x = 0.04 and 373 K for 0.25 on heating and similarly TS = 445 K for x = 0.04 and 355 K for 0.25 on cooling). Antiferromagnetic helicoidal ordering with the propagation vector k = (kx, 0, 0) for x = 0.04 and 0.11 (TN = 357 and 353 K, respectively) and ferromagnetic one for x = 0.25 (TC = 360 K) has been found. The sample with x = 0.18 shows a coexistence of a helicoidal antiferromagnetic structure and the ferromagnetic one below ∼170 K while at higher temperatures the ferromagnetic ordering remains stable up to 362 K. For all investigated samples the magnetocaloric effect is observed. Maximum entropy change (−ΔS) increases with increasing Cr concentration from about 8 J/(kg K) at 90 kOe, found for x = 0.04 and 0.11 at the Néel temperature, up to 29 J/(kg K) observed for x = 0.25 in cooling regime at the magnetostructural phase transition temperature.

Magnetotransport at the LTO-LTT Structural Transition in La1.48Nd0.4Sr0.12CuO4

Magnetoresistance (MR) was measured in a La1.48Nd0.4Sr0.12CuO4 single crystal in perpendicular magnetic fields H up to 9 T in the region of the structural transition from the low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase. The hysteretic MR exhibits discrete jumps or avalanches only when the transition is approached from the LTT phase, and only during the first field sweep. The properties of the hysteresis are found to be independent of the field driving rate. The results are consistent with the presence of magnetostructural domains coupled with the charge degrees of freedom.

Structure and reactivity with oxygen of Pr2NiO(4+δ): an in situ synchrotron X-ray powder diffraction study.

The promising SOFC cathode material Pr2NiO(4.22) has been studied in situ under a pure oxygen atmosphere from 25 to 950 °C by high resolution synchrotron X-ray powder diffraction. At room temperature (RT) δ = 0.22(1), the average crystal structure turns out to be monoclinic. The subtle monoclinic distortion (γ = 90.066(1)° at RT), retained up to 460 °C, is interpreted in terms of specific tilt schemes of the NiO6 octahedra. It is also shown that Pr2NiO(4.22) is incommensurately structurally modulated already at room temperature, in the same manner as the homologous cobaltate La2CoO(4.14). The phase transition to the High Temperature Tetragonal (HTT) phase was completed at 480 °C without any evidence for the Low Temperature Orthorhombic (LTO) phase allowing clarifying the phase diagram of this K2NiF4-type ternary oxide. Moreover, it turns out that above 800 °C, the HTT phase transforms reversibly into two coexisting isomorphous tetragonal phases. The incommensurate modulation subsists up to 950 °C, although modified concomitantly with the two abovementioned phase transformations. In addition, the role of kinetics on the decomposition process is highlighted through thermo-gravimetric analyses.

First-principles study on lattice instabilities and structural phase transitions in Ba doped La2CuO4

We present linear-response density-functional theory calculations for the high-Tc superconductor La2−xBaxCuO4 to study the doping dependence of phonon dispersion. Using the virtual crystal approximation, the doping range up to x=0.20 is investigated. We find unstable phonon modes that soften around high-symmetry points of the Brillouin zone. These branches are analyzed as a function of doping and pressure. The structural distortions related to these phonons are in accordance with the observed phase transitions from the high-temperature tetragonal (HTT) phase to the low-temperature orthorhombic (LTO) and the low-temperature tetragonal (LTT) phases. The calculated tilting angles of the CuO6 octahedra corresponding to these phases are in good agreement with experiment. Our findings confirm that strong electron correlations have only minor impact on the structural properties of high-Tc cuprates.

Solvothermal synthesis and magnetic properties of β-Co2P nanorods

Abstract Hexagonal high temperature phase β-Co2P nanorods with a diameter of around 50 nm were synthesized via a mild solvothermal route. The reaction was carried out at 180 °C using cobalt chloride hexahydrate (CoCl2 · 6H2O) as Co source and yellow phosphorous as P source. The composition, structure as well as morphology were characterized by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and transmission electron microscopy (TEM). The magnetic susceptibility curve indicates that the β-Co2P nanorods show canted antiferromagnetic state, different from the paramagnetic state of orthorhombic low temperature phase β-Co2P.

Reconciliation of local and long-range tilt correlations in underdopedLa2−xBaxCuO4(0≤x≤0.155)

A long standing puzzle regarding the disparity of local and long range CuO6 octahedral tilt correlations in the underdoped regime of La2-xBaxCuO4 is addressed by utilizing complementary neutron powder diffraction and inelastic neutron scattering (INS) approaches. Long-range and static CuO6 tilt order with orthogonally inequivalent Cu-O bonds in the CuO2 planes in the low temperature tetragonal (LTT) phase is succeeded on warming through the low-temperature transition by one with orthogonally equivalent bonds in the low temperature orthorhombic (LTO) phase. In contrast, the signatures of LTT-type tilts in the instantaneous local atomic structure persist on heating throughout the LTO crystallographic phase on the nanoscale, although becoming weaker as temperature increases. Analysis of the INS spectra for the x = 1/8 composition reveals the dynamic nature of the LTT-like tilt fluctuations within the LTO phase and their 3D character. The doping dependence of relevant structural parameters indicates that the magnitude of the Cu-O bond anisotropy has a maximum at x = 1/8 doping where bulk superconductivity is most strongly suppressed, suggesting that the structural anisotropy might be influenced by electron-phonon coupling and the particular stability of the stripe-ordered phase at this composition. The bond-length modulation that pins stripe order is found to be remarkably subtle, with no anomalous bond length disorder at low temperature, placing an upper limit on any in-plane Cu-O bondlength anisotropy. The results further reveal that although appreciable octahedral tilts persist through the high-temperature transition and into the high temperature tetragonal (HTT) phase, there is no significant preference between different tilt directions in the HTT regime.

Electronic structure, cohesive properties, and magnetism ofSrRuO3

We have performed an extensive test of the ability of density functional theory within several approximations for the exchange-correlation functional, local density approximation+Hubbard $U$ and local density approximation + dynamic mean field theory to describe magnetic and electronic properties of SrRuO$_3$. We focus on the ferromagnetic phase, illustrating differences between the orthorhombic low temperature structure vs the cubic high temperature structure. We assess how magnetism, spectral function, and cohesive properties are affected by methodology, on-site Hubbard $U$ and double counting corrections. Further, we compare the impact of the impurity solver on the quasiparticle weight $Z$, which is in turn compared to experimental results. The spectral functions resulting from the different treatments are also compared to experimental data. The impact of spin-orbit coupling is also studied, allowing us to determine the orbital moments. In the orthorhombic phase the orbital moments are found to be tilted with respect to the spin moments, emphasising the importance of taking into account the distortion of the oxygen octahedra.

Double symmetry breaking in TmFe4Ge2 compared to RFe4Ge2 (R=Y, Lu, Er, Ho, Dy) magnetic behaviour

TmFe4Ge2 undergoes a double magneto-elastic first order transition at TN,Tc where the high temperature (HT) tetragonal phase disproportionate into two distinct orthorhombic low temperature (LT) phases with commensurate and incommensurate magnetic wave vectors respectively:P42/mnm(HT)TN,Tc→Cmmmq1=(0,12,0)+Pnnm(q2=(0,qy,0),qy≈2/11(LT)Neutron diffraction shows the relative portions of the LT Cmmm and Pnnm competing phases change linearly with T. The amount of the majority HT phase Pnnm (54% at 30K) decreases linearly to 30% down to 10K in favour of the Cmmm phase that dominates the range 26–1.5K. The Tm moments point along the c-axis in both phases while the Fe moments have canted arrangements. The μTm=3.54(3)μB/atom at 1.5K is strongly reduced below the Tm3+ free ion value gJJ=7μB for the q1 phase. The q2 phase corresponds to a 3D canted sinusoidal arrangement. The results are summarised on a phase diagram and compared to the findings in RFe4Ge2 (R=Y, Lu, Er, Ho, Dy) that are reviewed. The multitude of transition paths occurring in those systems arise from the competing magnetoelastic mechanisms involving the R-crystal field anisotropy, the exchange interactions R–R, R–Fe, Fe–Fe of the two sublattices and their coupling to the lattice strain. The geometrical frustration emerging from the compact tetrahedral Fe arrangement with antiferromagnetic interactions leads to 2D and 3D canted, incommensurate and non-magnetic states. The Cmmm transition is triggered by dominating R–R and R–Fe interactions becoming stronger at LT while the Pnnm phase is promoted by Fe–Fe and R–Fe interactions that prevail at HT. Included is also the magnetic structure of the ferromagnetic impurity phase Fe3Ge.

Analysis of structural and electronic properties of Pr2NiO4 through first-principles calculations

The structural and electronic properties of bulk Pr2NiO4+δ (δ = 0 and 0.031) were analyzed using first-principles calculations based on the density functional theory (DFT) for application to electrode materials in solid-oxide fuel cells (SOFCs). Two structures of Pr2NiO4 were analyzed: one in space group I4/mmm associated with the high temperature tetragonal (HTT) structure, and the other in Bmab with the low temperature orthorhombic (LTO) structure. The main difference between the two structures is the pronounced tilting of the nickelate octahedra found in the Bmab structure. Here, we will show that the difference in the electronic properties between the two structures, i.e. half-metallic for the I4/mmm structure and metallic for the Bmab structure, is attributed to the tilting of the nickelate octahedra. Furthermore, we found that the presence of interstitial O atoms at the Pr2O2 bilayers is responsible for the tilting of the octahedra and thus is a dominant factor in the transition from the I4/mmm structure to the Bmab structure. These results would be of great significance to materials design related to the enhancement of O diffusivity in this material.

Structural Modulation and Phase Transitions in La2CoO4.14 Investigated by Synchrotron X-ray and Neutron Single-Crystal Diffraction

We report a combined synchrotron X-ray and neutron diffraction study on as-grown La(2)CoO(4.14) single-crystal from 10 to 470 K. Unprecedented structural features in terms of a (3 + 2)D incommensurate modulation have been detected and characterized in the Low Temperature Orthorhombic (LTO) phase already at room temperature despite the complex twinning that was unravelled. A new intermediate phase between the LTO and High Temperature Tetragonal (HTT) phases has been observed for the first time (in the range of 413-433 K). The transformation from LTO to this so-called HTLO (High Temperature Less Orthorhombic) phase is associated to a lowering of orthorhombicity and a loss of one modulation vector, yielding a (3 + 1)D incommensurate modulation. Conversely, above 433 K the HTT phase appears as nonmodulated but exhibits a strong dynamic disorder of CoO(6) octahedra, which has been characterized in detail by reconstruction of nuclear densities via the Maximum Entropy Method (MEM).

Pinning of Stripes by Local Structural Distortions in Cuprate High-T c Superconductors

We study the spin-density wave (stripe) instability in lattices with mixed low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) crystal symmetry. Within an explicit mean-field model, it is shown how local LTT regions act as pinning centers for static stripe formation. We calculate the modulations in the local density of states near these local stripe regions and find that mainly the coherence peaks and the van Hove singularity (VHS) are spatially modulated. Lastly, we use the real-space approach to simulate recent tunneling data in the overdoped regime where the VHS has been detected by utilizing local normal state regions.

An x-ray scattering study of the structural phase transition in La2−xSrxCu0.99Fe0.01O4 (x = 0.20)

We report a comprehensive x-ray scattering study of the low-temperature orthorhombic (LTO)–high-temperature tetragonal (HTT) structural phase transition in 1% iron-doped La2−xSrxCuO4 (x = 0.2). The superlattice (032) peak intensity and the width are investigated in detail for a wide temperature range. We found that the structural phase transition is not sharp and the tilt ordering of the CuO6 octahedra persists above the transition temperature TS (≈77 K). Even at room temperature, the superlattice peak is still observable. The structural phase transition is identified as an order–disorder type phase transition. We found that the tilt ordering in our iron-doped material is always short-ranged, and in the HTT phase the correlation between the tilts along the b axis is better preserved than that along the a axis. Moreover, we identify the role of the Fe as the nucleation centers of the LTO domains in the structural phase transition.

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.

Phonon softening and anomalous mode near thexc=0.5quantum critical point inCa2−xSrxRuO4

Inelastic neutron scattering is used to measure the temperature-dependent phonon dispersion in ${\text{Ca}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{RuO}}_{4}$ $(x=0.4,0.6)$. The in-plane ${\ensuremath{\Sigma}}_{4}$ octahedral tilt mode softens significantly at the zone boundary of the high-temperature tetragonal (HTT) $I{4}_{1}/acd$ structure as the temperature approaches the transition to a low-temperature orthorhombic (LTO) $Pbca$ phase. This behavior is similar to that in ${\text{La}}_{2}{\text{CuO}}_{4}$, but an inelastic feature that is not found in the cuprate is present. An anomalous phonon mode is observed at energy transfers greater than the ${\ensuremath{\Sigma}}_{4}$, albeit with similar dispersion. This anomalous phonon mode never softens below $\ensuremath{\sim}5\text{ }\text{meV}$, even for temperatures below the HTT-LTO transition. This mode is attributed to the presence of intrinsic structural disorder within the $I{4}_{1}/acd$ tetragonal structure of the doped ruthenate.