Lattice Anomalies Research Articles

Spin orders and lattice distortions of geometrically frustrated 6H-perovskites Ba3B′Ru2O9(B′ = La3+, Nd3+, and Y3+)

The magnetic and crystal structures of the title materials have been investigated using high-resolution neutron and synchrotron x-ray powder diffraction. Ferromagnetic Ru dimers are observed in the spin-ordered ground states of all three materials, evidencing an intradimer double exchange interaction. Antiferromagnetic order between dimers breaks the degeneracy of the triangular lattice but no distortion of the ${B}^{\ensuremath{'}}=\mathrm{La}$ and Y structures is observed. The magnetic ordering transition temperature increases with the radius of the ${B}^{\ensuremath{'}}$ cation. An anomalous 120 K structural transition in Ba${}_{3}$NdRu${}_{2}$O${}_{9}$ is attributed to Jahn-Teller distortion, evidenced by large-amplitude ${\ensuremath{\Gamma}}_{6}$ deformations of the NdO${}_{6}$ octahedra. Further lattice anomalies accompany magnetic ordering transitions at 24 and 18 K in Ba${}_{3}$NdRu${}_{2}$O${}_{9}$. Nd spins order ferromagnetically and a substantial magnetic anisotropy leads to large coercivity and $\frac{1}{3}$-magnetization steps in the magnetic hysteresis loop.

Polaron signatures in the phonon dispersion of high-temperature superconducting copper oxides

High-temperature superconducting copper oxides display a variety of both long-range and local lattice anomalies which are related to the onset of the pseudogap phase and / or the onset of superconductivity. Here we show that these anomalies demonstrate polaron formation where specifically the local character of the polarons plays an important role. We predict that unconventional isotope effects will appear in both the long-wavelength and local lattice effects.

Pressure and doping dependent anisotropic compressibility of the SmFeAsO1−xFx (x = 0.0–0.17) system

We have measured by synchrotron powder diffraction the hydrostatic pressure dependence of the structural parameters of the SmFeAsO1−xFx for four doping values (from x=0.0 to 0.17) in order to investigate lattice modifications that might be related to doping. The response of the lattice to the applied hydrostatic pressure is anisotropic with the linear compressibility kc, along the c-axis being a factor of 2.5 greater than the corresponding ka along the a-axis, while bulk modulus increases slightly in the optimally doped compound. The analysis of the data indicates that the anisotropic compressibility of the SmFeAsO1−xFx system is doping dependent, with an increasing deviation from a linear relation between the a- and c-axis with doping, which peaks in the x=0.17 compound with the highest Tc value studied. The width of the superconducting FeAs4 layer in this compound exhibits a non-monotonic pressure dependence, which is not evident in the non-superconducting one. This unequal compression of the subunits of the cell apparently modifies the distribution of the carriers and affects the transition temperature. Our results agree with previous measurements for the highest doping level, and furthermore provide an indication about the connection of doping with lattice anomalies.

Metastable Thin Films for Energy Applications: On Structural Lattice Anomalies and Electrical Transport

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Element-specific and bulk magnetism, electronic, and crystal structures of La0.70Ca0.30Mn1−xCrxO3

The magnetic interactions in La${}_{0.70}$Ca${}_{0.30}$Mn${}_{1\ensuremath{-}x}$Cr${}_{x}$O${}_{3}$ ($x=0.15$, 0.50, and 0.70) are investigated by x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), high-resolution x-ray powder diffraction, and bulk magnetization measurements. XAS in the Mn and Cr ${L}_{2,3}$ edges support stable single valent Cr${}^{3+}$ ions and a varying Mn valence state with $x$, while the O $K$ edge XAS spectrum reveals local maxima in the O $2p$ density of states close to the Fermi level due to mixing with Mn and Cr $3d$ states. A robust antiferromagnetic state is found for $x=0.70$ below ${T}_{N}=258$ K. For $x=0.15$, combined XMCD and bulk magnetization measurements indicate a fully polarized ferrimagnetic state for the Mn and Cr spins below ${T}_{c}=224$ K. For $x=0.50$, a reduced ferrimagnetic component dominated by Mn spins is present below ${T}_{c}=154$ K. No evidence of lattice anomalies due to cooperative charge and orbital orderings is found by x-ray diffraction for all samples. The magnetic properties of this system are rationalized in terms of a competition of ferromagnetic Mn-Mn double exchange and antiferromagnetic Cr-Cr and Cr-Mn superexchange interactions.

Spin and orbital ordering in Y1−xLaxVO3

The spin and orbital ordering in Y${}_{1\ensuremath{-}x}$La${}_{x}$VO${}_{3}$ (0.30 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 1.0) has been studied to map out the phase diagram over the whole doping range 0 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 1. The phase diagram is compared with that for $R$VO${}_{3}$ ($R$ = rare earth or Y) perovskites without $A$-site variance. For $x$ > 0.20, no long-range orbital ordering was observed above the magnetic ordering temperature ${T}_{N}$; the magnetic order is accompanied by a lattice anomaly at a ${T}_{t}$ \ensuremath{\leqslant} ${T}_{N}$ as in LaVO${}_{3}$. The magnetic ordering below ${T}_{t}$ \ensuremath{\leqslant} ${T}_{N}$ is $G$ type in the compositional range 0.20 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 0.40 and $C$ type in the range 0.738 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 1.0. Magnetization and neutron powder diffraction measurements point to the coexistence below ${T}_{N}$ of the two magnetic phases in the compositional range 0.4 $x$ 0.738. Samples in the compositional range 0.20 $x$ \ensuremath{\leqslant} 1.0 are characterized by an additional suppression of a glasslike thermal conductivity in the temperature interval ${T}_{N}$ $T$ ${T}^{*}$ and a change in the slope of 1/\ensuremath{\chi}($T$). We argue that ${T}^{*}$ represents a temperature below which spin and orbital fluctuations couple together via $\ensuremath{\lambda}\mathbf{L}\ifmmode\bullet\else\textbullet\fi{}\mathbf{S}$.

Lattice Anomalies in the Oxygen Deficient NdFeAsO Superconductor

We compare the results of high-resolution synchrotron X-ray powder diffraction with low-temperature spectroscopic studies on the oxygen deficient NdFeAsO0.85 superconductor. IR results indicate that a certain mode related to vibrations of the As ions softens at low temperatures. By employing a dense temperature sampling we have managed to reveal unnoticed structural modifications that start around ∼180 K, as the anomaly of the IR mode, and disappear at the transition temperature. The data show smaller structural changes in the Nd–O charge reservoir layer, while in the superconducting layer the FeAs4 tetrahedron shows gradual modifications below ∼180 K, which peak around 140 K and then suddenly disappear at Tc. The observed lattice anomalies are not connected with a structural phase transition or an AF ordering; they are more evident in the superconducting Fe–As planes, and disappear across Tc, pointing to a connection to superconductivity.

Lattice Anomalies and HTSC in Pnictides and Cuprates Studied by XAS: Polaron Resonance as a Common Clue

Temperature-dependent local lattice structures of 1111-type ReFeAsO1−x F x (Re=La, Sm) are studied by extended X-ray absorption fine structure (EXAFS) and compared with the results of impurity-doped La1.85Sr0.15CuO4. As electron carriers are introduced by doping with F ions, pnictgen coordination around Fe atom shows unusual local lattice response below T ∗, described as an anomalous temperature dependence of mean-squared relative displacement. The observed local lattice distortion (leading to two distinct bond lengths R 1 and R 2 where ΔR=R 1−R 2∼0.1 A) is a signature of localized polaron formation. The effect of magnetic impurities in La1.85Sr0.15CuO4 indicates (bond stretching-type) lattice distortion favours polaron resonance as a common prerequisite for high-temperature superconductivity.

Why the Hidden Order in URu2Si2 Is Still Hidden–One Simple Answer

For more than two decades, the nonmagnetic anomaly observed around 17.5 K in URu2Si2, has been investigated intensively. However, any kind of fingerprint for the lattice anomaly has not been observed. Therefore, the order has been called "the hidden order". One simple answer to why the hidden order is still hidden is presented from the space group analysis. The second order phase transition from I4/mmm (No. 139) to P4_2/mnm (No. 136) does not need any kind of lattice distortion in this system, and allows the NQR frequency at Ru-site unchanged. It is compatible with O_{xy}-type anti-ferro quadrupole ordering with Q=(0, 0, 1). The characteristics of the hidden order are discussed based on the local 5f^2 electron picture.

Low-temperature lattice anomaly in LaFeAsO0.93F0.07 probed by X-ray absorption spectroscopy: Evidence for strong electron–lattice interaction

Carrier-induced lattice distortion (signature of strong electron–lattice interaction) in oxypnictide superconductors is found by a local structural study using extended X-ray absorption fine structure (EXAFS). Local lattice instability (polaron formation) is detected as an anomalous upturn of the mean-square relative displacement for the nearest neighbor distances (Fe–As) in LaFeAsO1−xFx (x=0.07). Intimate correlation between local lattice mode and superconductivity is revealed. The results suggest that strong electron–lattice interaction is present as a common ingredient in the microscopic mechanism of superconducting transition.

Continuous metal-insulator transition of the antiferromagnetic perovskiteNaOsO3

Newly synthesized perovskite NaOsO3 shows Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low temperature limit is absent, indicating that the band gap fully opens. In situ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role of the gap opening.

Lattice strain and heat capacity anomalies at the spin reorientation transitions of ErFeO3 orthoferrite

We have investigated the thermal expansivities of an ErFeO3 single crystal along the three crystallographic orientations using a high-resolution capacitance dilatometer and found clear signatures of lattice distortions in the spin rotation region as evidence for strong spin-lattice coupling. The observed lattice strain is consistent with the smooth rotation of the magnetization and it reveals the importance of the magnetoelastic interaction. Heat capacity measurements show a well-defined plateau-like enhancement in the spin re-orientation regime as predicted by the Landau theory of second order phase transitions. The data are used to estimate microscopic anisotropy parameters. Heat capacity is also measured near the low-temperature erbium magnetic ordering transition.

Macroscopic pattern formation preceding martensitic transformation in a ferromagnetic shape memory alloy Ni51Fe22Ga27

This paper reports a macroscopic pattern formation that occurs before the onset of a martensitic transformation in a ferromagnetic shape memory alloy, Ni51Fe22Ga27. Electron holography studies reveal that the magnetic flux pattern in the parent (cubic) phase changes dramatically with cooling. Interestingly, the flux pattern developed in the parent phase is inherited by the martensitic (monoclinic) phase. Observations of the diffuse electron scattering indicate that the change in the magnetic flux pattern is triggered by a lattice anomaly, which appears to produce a long-range strain field in the parent phase.

Spin disorder and magnetic properties of Bi3∕4Ca1∕4MnO3

We present a study on the magnetic and electronic properties of Bi0.75Ca0.25MnO3, investigated by means of magnetometry, diffraction, and muon spin relaxation techniques. The anisotropic changes of cell parameters across the structural transition in Bi0.75Ca0.25MnO3 (275K) differ markedly from the lattice anomalies not only in Bi0.75Sr0.25MnO3 (600K) but also in Bi0.50Ca0.50MnO3 (325K). We have confirmed the absence of a ferromagnetic component under zero field in Bi0.75Ca0.25MnO3. The application of magnetic fields produces a continuous progressive polarization of Mn moments. The ground state of Bi0.75Ca0.25MnO3 is characterized by a high degree of spin disorder and frustrated interactions. A well-developed long-range magnetic order is not observed. Our results indicate a ground state in Bi0.75Ca0.25MnO3 distinct to Bi0.75Sr0.25MnO3.

Triplet dimerization crossover driven by magnetic frustration inIn2VO5

${\mathrm{In}}_{2}\mathrm{V}{\mathrm{O}}_{5}$, containing magnetically frustrated zigzag chains, shows a remarkable crossover at $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ between paramagnetic states with positive $(17\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and negative $(\ensuremath{-}70\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ Weiss temperatures. Magnetic moment and entropy data show that the ${\mathrm{V}}^{4+}$ $S=1∕2$ spins condense into $S=1$ triplet dimers below the crossover. A further freezing of the antiferromagnetically coupled triplet dimers into a global singlet state is observed at $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with no long range magnetic order down to $0.42\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in fields up to $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. No structural V-V dimerization is observed by high-resolution synchrotron x-ray diffraction down to $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but a subtle lattice anomaly evidences a spin-lattice coupling in the triplet dimer state. This is assigned to longitudinal oxygen displacement modes that reduce frustration within the chains and so couple to the spin dimer fluctuations.

Thermal expansion and pressure effect in [formula omitted

MnWO 4 has attracted attention because of its ferroelectric property induced by frustrated helical spin order. Strong spin–lattice interaction is necessary to explain ferroelectricity associated with this type of magnetic order. We have conducted thermal expansion measurements along the a, b, c axes revealing the existence of strong anisotropic lattice anomalies at T 1 = 7.8 K , the temperature of the magnetic lock-in transition into a commensurate low-temperature (reentrant paraelectric) phase. The effect of hydrostatic pressure up to 1.8 GPa on the FE phase is investigated by measuring the dielectric constant and the FE polarization. The low-temperature commensurate and paraelectric phase is stabilized and the stability range of the ferroelectric phase is diminished under pressure.

Lattice distortion and elasticity change ins-wave andd-wave superconductors

The lattice anomaly, i.e., the lattice distortion and the elasticity change, caused by the appearance of superconductivity (SC) is studied by use of the BCS theory. The BCS Hamiltonian with any attractive interaction between electrons is introduced for crystals with the symmetry ${D}_{4h}$, so that the $s$-wave, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave, and ${d}_{xy}$-wave SC's may appear. The microscopic parameters in the Hamiltonian are expanded in powers of distortions $u$ up to their first order, so as to make the Hamiltonian satisfy the crystal symmetry. The free energy of the system as a function of SC order parameters (SOP) and $u$'s is obtained to reveal the properties of the lattice anomaly. The temperature dependence of the equilibrium distortion and the elastic constants in the SC phases with the different SOP's is calculated. The calculated distortion is shown to give its temperature dependence consistent with the observation. The elastic constants obtained can exhibit not only softenings but also hardenings below the transition temperature ${T}_{c}$, depending on the microscopic parameters. This behavior of elastic constant is just that observed in some high-${T}_{c}$ cuprates.

Magnetic and electronic properties ofBi0.75Ca0.25MnO3

The magnetic, structural and electronic properties ofBi0.75Ca0.25MnO3 have been investigated in comparison with those ofBi0.75Sr0.25MnO3. Magnetometry, diffraction and muon spin relaxation(μSR) data confirm different structural, magnetic and electronic transitions in the twocompounds. The anisotropic changes of cell parameters across the structural transition inBi0.75Ca0.25MnO3 (275 K) differ markedly from the lattice anomalies inBi0.75Sr0.25MnO3 (600 K) and also fromthose in Bi0.50Ca0.50MnO3 (325 K).The ground state of Bi0.75Ca0.25MnO3 is characterized by a high degree of spin disorder and frustrated interactions.There is no evidence of a ferromagnetic component in the ground state ofBi0.75Ca0.25MnO3. However, the application of a magnetic field (even of a few gauss)produces a continuous progressive polarization of the Mn moments(≈2 μB/Mn at 5 T, ZFC, 5 K). Differences between Ca and Sr perovskites withx = 1/4 are greaterthan for the x = 1/2 counterparts, and point to distinct ground states and charge/orbital configurations.

The effect of magnetic order and thickness in the Raman spectra of oriented thin films ofLaMnO3

Low-temperature Raman studies on (100)-orientedLaMnO3 thin films of different thicknesses deposited onLaAlO3(100) substrate by using the pulsed laser deposition technique are presented. The phonon modefrequencies depend strongly on the thickness of the film and in comparison to theLaMnO3 single crystal shift to higher values with decreasing thickness. The broadening of Ramanline widths was also observed with decreasing film thickness. Anomalous softening of theB2g mode (Jahn–Teller distortion activated) was observed for all films below the magnetic orderingtemperature. A signature of lattice anomalies in the form of changes in line width of theB2g mode correlated with the magnetic ordering temperature was also seen. The results wereinterpreted by considering strong spin–phonon coupling and reduction in Jahn–Tellerdistortion driven polarons due to magnetic order.

Lattice distortion and elasticity change caused by the d-wave superconductivity

The lattice distortion and the elasticity change caused by the d-wave superconductivity are studied on the basis of the BCS Hamiltonian, which is constructed for a 2D electron system so as to satisfy its assumed crystal symmetry D 4 h . The superconducting order parameters (SOP) considered are those with the symmetries of ( k x 2 - k y 2 ) / 2 , 2 k x k y and also ( k x 2 + k y 2 ) / 2 , k being a 2D wave vector. The obtained free energy as a function of the SOPs and appropriate distortions is used to connect the lattice anomaly and the microscopic parameters in the Hamiltonian. The softening and even the stiffening of the lattice are derived within the present theory to identify their origins.