Low-temperature Tetragonal Research Articles

Pronounced interplay between intrinsic phase-coexistence and octahedral tilt magnitude in hole-doped lanthanum cuprates

Definitive understanding of superconductivity and its interplay with structural symmetry in the hole-doped lanthanum cuprates remains elusive. The suppression of superconductivity around 1/8th doping maintains particular focus, often attributed to charge-density waves (CDWs) ordering in the low-temperature tetragonal (LTT) phase. Central to many investigations into this interplay is the thesis that La1.875Ba0.125CuO4 and particularly La1.675Eu0.2Sr0.125CuO4 present model systems of purely LTT structure at low temperature. However, combining single-crystal and high-resolution powder X-ray diffraction, we find these to exhibit significant, intrinsic coexistence of LTT and low-temperature orthorhombic domains, typically associated with superconductivity, even at 10 K. Our two-phase models reveal substantially greater tilting of CuO6 octahedra in the LTT phase, markedly buckling the CuO2 planes. This would couple significantly to band narrowing, potentially indicating a picture of electronically driven phase segregation, reminiscent of optimally doped manganites. These results call for reassessment of many experiments seeking to elucidate structural and electronic interplay at 1/8 doping.

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.

Materials preparation, single-crystal growth, and the phase diagram of the cuprate high-temperature superconductor La1.6−xNd0.4SrxCuO4

One branch of the La-214 family of cuprate superconductors, La1.6-xNd0.4SrxCuO4 (Nd-LSCO), has been of significant and sustained interest, in large part because it displays the full complexity of the phase diagram for canonical hole-doped, high Tc superconductivity, while also displaying relatively low superconducting critical temperatures. The low superconducting Tc's imply that experimentally accessible magnetic fields can suppress the superconductivity to zero temperature. In particular, this has enabled various transport and thermodynamic studies of the T = 0 ground state in Nd-LSCO, free of superconductivity, across the critical doping p* = 0.23 where the pseudogap phase ends. The strong dependence of its superconducting properties on its crystal symmetry has itself motivated careful studies of the Nd-LSCO structural phase diagram. This paper provides a systematic study and summary of the materials preparation and characterization of both single crystal and polycrystalline samples of Nd-LSCO. Single-phase polycrystalline samples with x spanning the range from 0.01 to 0.40 have been synthesized, and large single crystals of Nd-LSCO for select x across the region (0.07, 0.12, 0.17, 0.19, 0.225, 0.24, and 0.26) were grown by the optical floating zone method. Systematic neutron and X-ray diffraction studies on these samples were performed at both low and room temperatures, 10 K and 300 K, respectively. These studies allowed us to follow the various structural phase transitions and propose an updated structural phase diagram for Nd-LSCO. In particular, we found that the low-temperature tetragonal (LTT) phase ends at a critical doping pLTT = 0.255(5), clearly separated from p*.

Tunable giant magnetocaloric effect with very low hysteresis in [formula omitted

The structural, magnetic and magnetocaloric properties of the antiperovskite materials Mn3CuN1−xCx have been studied. Substituting N with C increases the temperature of the magnetostructural transition between a paramagnetic cubic high temperature phase and a ferrimagnetic tetragonal low temperature phase. Furthermore, the first order character of the phase transition is retained upon substitution with a hysteresis below 2 K for all compositions. The magnetostructural transition gives rise to giant magnetocaloric effects in a tunable 30 K temperature range with a maximum entropy change of 11.8J/Kkg at a 2 T field change, making these compounds promising for low temperature magnetic refrigeration applications.

La139 and Cu63 NMR investigation of charge order in La2CuO4+y ( Tc=42 K)

We report $^{139}$La and $^{63}$Cu NMR investigation of the successive charge order, spin order, and superconducting transitions in super-oxygenated La$_2$CuO$_{4+y}$ single crystal with stage-4 excess oxygen order at $T_{stage}\simeq 290$ K. We show that the stage-4 order induces tilting of CuO$_6$ octahedra below $T_{stage}$, which in turn causes $^{139}$La NMR line broadening. The structural distortion continues to develop far below $T_{stage}$, and completes at $T_{charge}\simeq 60$ K, where charge order sets in. This sequence is reminiscent of the the charge order transition in Nd co-doped La$_{1.88}$Sr$_{0.12}$CuO$_4$ that sets in once the low temperature tetragonal (LTT) phase is established. We also show that the paramagnetic $^{63}$Cu NMR signals are progressively wiped out below $T_{charge}$ due to enhanced low frequency spin fluctuations, but the residual $^{63}$Cu NMR signals continue to exhibit the characteristics expected for optimally doped superconducting CuO$_2$ planes. This indicates that charge order in La$_2$CuO$_{4+y}$ does not take place uniformly in space. Low frequency Cu spin fluctuations as probed by $^{139}$La nuclear spin-lattice relaxation rate are mildly glassy, and do not exhibit critical divergence at $T_{spin}$($\simeq T_{c}$)=42 K. These findings, including the spatially inhomogeneous nature of the charge ordered state, are qualitatively similar to the case of La$_{1.885}$Sr$_{0.115}$CuO$_4$ [T. Imai et al., Phys. Rev. B 96 (2017) 224508, and A. Arsenault et al., Phys. Rev. B 97 (2018) 064511], but both charge and spin order take place more sharply in the present case.

Polarized Light Microscopy Study on the Reentrant Phase Transition in a (Ba1 – xKx)Fe2As2 Single Crystal with x = 0.24

A sequence of structural/magnetic transitions on cooling is reported in the literature for hole-doped iron-based superconductor (Ba1 − xKx)Fe2As2 with x = 0.24. By using polarized light microscopy, we directly observe the formation of orthorhombic domains in (Ba1 − xKx)Fe2As2 (x = 0.24) single crystal below a temperature of simultaneous structural/magnetic transition TN ~ 80 K. The structural domains vanish below ~30 K, but reappear below T = 15 K. Our results are consistent with reentrance transformation sequence from high-temperature tetragonal (HTT) to low temperature orthorhombic (LTO1) structure at TN ~ 80 K, LTO1 to low temperature tetragonal (LTT) structure at Tc ~ 25 K, and LTT to low temperature orthorhombic (LTO2) structure at T ~ 15 K.

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.

Growth and characterization of large CeAlO3 perovskite crystals

This study is focused on obtaining large CeAlO3 single crystals. Using Czochralski and Edge-Defined Film Fed Growth techniques, crystals of length up to 130mm were obtained. The crystals belong to the tetragonal low temperature modification of CeAlO3, space group P4/mmm, a=3.7669Å, с=3.7967Å. Density and crystallographic parameters as well as some optical, luminescence and magnetic properties of CeAlO3 were studied. The crystals demonstrate rather bright luminescence under UV-excitation, while no luminescence has been registered under X-rays. Ceramic and single crystalline CeAlO3 samples demonstrate p-type conductivity and promising paramagnetic properties.

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.

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.

Effects of Sr and Zn Doping on the Metallicity and Superconductivity of LSCO and YBCO

Sr doping in La 2CuO 4 is investigated by a Kohn-Sham DFT + U calculation. The Sr charge remains highly localized, confirming earlier calculations. The bulk of the charge in the nodal metal is delocalized in situ in the CuO 2 plane. Standard Kohn-Sham DFT+U reproduces the metal-insulator transition and growth of Fermi-surface arcs, as observed in angle-resolved photoemission spectroscopy (ARPES), when not restricted to the paramagnetic solution space. Substituting planar Cu by isotopically pure 67Zn in yttrium barium copper oxide (YBCO), we measure the nuclear quadrupole resonance (NQR) signal on the Zn site itself for the first time. Zn creates large insulating islands, extending at least to the nearest-neighbor Cu site, thus confirming that doping induces an orbital delocalization transition in the plane, which is locally reversed by Zn substitution. Calculations show that the D4 symmetry-breaking observed in NQR is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the low temperature tetragonal (LTT) transition in lanthanum strontium copper oxide (LSCO).

Melting of charge stripes in vibrationally driven La(1.875)Ba(0.125)CuO4: assessing the respective roles of electronic and lattice order in frustrated superconductors.

We report femtosecond resonant soft x-ray diffraction measurements of the dynamics of the charge order and of the crystal lattice in nonsuperconducting, stripe-ordered La1.875Ba0.125CuO4. Excitation of the in-plane Cu-O stretching phonon with a midinfrared pulse has been previously shown to induce a transient superconducting state in the closely related compound La1.675Eu0.2Sr0.125CuO4. In La1.875Ba0.125CuO4, we find that the charge stripe order melts promptly on a subpicosecond time scale. Surprisingly, the low temperature tetragonal (LTT) distortion is only weakly reduced, reacting on significantly longer time scales that do not correlate with light-induced superconductivity. This experiment suggests that charge modulations alone, and not the LTT distortion, prevent superconductivity in equilibrium.

Analysis of the decomposition gases from α and β-Cd(BH4)2 synthesized by temperature controlled mechanical milling

We present a comprehensive study on the controlled phase synthesis and thermal decomposition of Cd(BH2)4, a material for solid state hydrogen storage obtained via the metathesis reaction of LiBH4 with CdCl2. By adjusting the stochiometry of the reactants and controlling the mechanical milling vial temperature, we have isolated the tetragonal (P42mn) low temperature phase and the cubic (Pn3¯m) high temperature phase of the cadmium borohydride. Cd(BH2)4 has a low thermodynamic stability and decomposes with fast kinetic at 348K, when heated at 1Kmin−1 against a backpressure of 1bar H2. A semi-quantitative analysis reveals that the decomposition gases are composed of 1:1 H2+B2H6 and that only Cd remains as solid crystalline phase.

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.

Formula omitted]SR studies of the interplay of magnetic spin stripe order with superconductivity in transition metal oxides

In this work we review muon spin relaxation experiments on the layered La2−xSrxNiO4 nickelate as well as La2−xBaxCuO4 and La2−xSrxCuO4 cuprate systems to examine spin stripe order. In particular, the interplay of stripe order with superconductivity in Nd and Eu doped La2−xSrxCuO4 cuprates is discussed. Detailed studies of the electronic phase diagrams as well as the magnetic and superconducting order parameters for different rare-earth and Sr doping levels in La2−x−yREySrxCuO4 revealed the strong correlation of static spin stripe order with the structural distortion in the low temperature tetragonal (LTT) phase and the competition with the superconducting ground state. High magnetic field studies demonstrate the nearly degenerate ground state energy of the different electronic phases. Slow transverse fluctuations of the charge stripes are found in nickelates and cuprates at low temperatures.

Probing the connections between superconductivity, stripe order, and structure in La1.905Ba0.095Cu1−yZnyO4

The superconducting system La${}_{2\ensuremath{-}x}$Ba${}_{x}$CuO${}_{4}$ is known to show a minimum in the transition temperature ${T}_{c}$ at $x=\frac{1}{8}$ where maximal stripe order is pinned by the anisotropy within the CuO${}_{2}$ planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For $x=0.095$, where ${T}_{c}$ reaches its maximum value of 32 K, there is a roughly coincident structural transition to a phase that is very close to LTT. Here, we present a neutron scattering study of the structural transition, and demonstrate how features of it correlate with anomalies in the magnetic susceptibility, electrical resistivity, thermal conductivity, and thermoelectric power. We also present measurements on a crystal with 1$%$ Zn substituted for Cu, which reduces ${T}_{c}$ to 17 K, enhances the spin stripe order, but has much less effect on the structural transition. We make the case that the structural transition correlates with a reduction of the Josephson coupling between the CuO${}_{2}$ layers, which interrupts the growth of the superconducting order. We also discuss evidence for two-dimensional superconducting fluctuations in the normal state, analyze the effective magnetic moment per Zn impurity, and consider the significance of the anomalous thermopower often reported in the stripe-ordered phase.

Stripe order in superconducting La2−xBaxCuO4(0.095⩽x⩽0.155)

The correlations between stripe order, superconductivity, and crystal structure in La(2-x)Ba(x)CuO(4) single crystals have been studied by means of x-ray and neutron diffraction as well as static magnetization measurements. The derived phase diagram shows that charge stripe order (CO) coexists with bulk superconductivity in a broad range of doping around x=1/8, although the CO order parameter falls off quickly for x<>1/8. Except for x=0.155, the onset of CO always coincides with the transition between the orthorhombic and the tetragonal low temperature structures. The CO transition evolves from a sharp drop at low x to a more gradual transition at higher x, eventually falling below the structural phase boundary for optimum doping. With respect to the interlayer CO correlations, we find no qualitative change of the stripe stacking order as a function of doping, and in-plane and out-of-plane correlations disappear simultaneously at the transition. Similarly to the CO, the spin stripe order (SO) is also most pronounced at x=1/8. Truly static SO sets in below the CO and coincides with the first appearance of in-plane superconducting correlations at temperatures significantly above the bulk transition to superconductivity (SC). Indications that bulk SC causes a reduction of the spin or charge stripe order could not be identified. We argue that CO is the dominant order that is compatible with SC pairing but competes with SC phase coherence. Comparing our results with data from the literature, we find good agreement if all results are plotted as a function of x' instead of the nominal x, where x' represents an estimate of the actual Ba content, extracted from the doping dependence of the structural transition between the orthorhombic phase and the tetragonal high-temperature phase.

Interlayer Josephson coupling in stripe-ordered superconducting cuprates

Motivated by experiments on ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ which suggest that stripe order coexists with two-dimensional pairing without interlayer phase coherence, we determine the interlayer Josephson coupling in the presence of stripe order. We employ a mean-field description of bond-centered stripes, with a zero-momentum superconducting condensate and alternating stripe directions pinned by the low-temperature tetragonal (LTT) lattice structure. We show that the Fermi-surface reconstruction arising from strong stripe order can suppress the Josephson coupling between adjacent layers by more than an order of magnitude.

Neutron-Scattering Study of Impurity Effect on Stripe Correlations in La-Based 214 High-Tc Cuprate

We present results of neutron-scattering study of impurity effects on the static stripe correlations in La-based 214 high-Tc cuprate. We found that the superlattice peaks from the charge-density-wave (CDW) order was induced by doping magnetic Fe ions in the low-temperature-orthorhombic (LTO) phase of La2−xSrxCuO4 (LSCO). The spin-density-wave (SDW) order is also stabilized by Fe-doping, suggesting the appearance of stripe ordered state in the LTO phase, as is seen it the low-temperature-tetragonal (LTT) phase of La1.875Ba0.125Cu4 (LBCO). Therefore, the stripe order can be potentially realized in the La-214 system irrespective of crystal structure. On the other hand, the onset temperature for the appearance of SDW and CDW orders is lowered by Zn-doping in La1.875Ba0.125CuO4 system. This result suggests that the existing well-stabilized stripe order is degraded by substituting non-magnetic Zn ions.

Spin- and charge-density-wave orders in La 1.87Sr 0.13Cu 0.99Fe 0.01O 4

We have performed elastic neutron-scattering measurements on La 1.87Sr 0.13Cu 0.99Fe 0.01O 4 to study the magnetic impurity effects on the stripe correlations in high- T c cuprates. Both charge-density-wave (CDW) and spin-density-wave (SDW) orders are observed in the low-temperature-orthorhombic (LTO) phase for the first time. With decreasing the temperature, CDW order first appears at ∼ 60 K followed by the SDW order at lower temperature of ∼ 50 K . The incommensurability for the CDW order ( ε ) was found to be 0.224 ± 0.002 r.l.u., which is approximately twice of that for SDW order ( δ ) of 0.115 ± 0.003 r.l.u. Both ordering sequence and relation of ε ≈ 2 δ are the same for SDW and CDW orders observed in the low-temperature-tetragonal (LTT) phase of La 1.60 - x Nd 0.4 Sr x CuO 4 (LNSCO), suggesting that the similar stripe correlations exist in the cuprate oxides, irrespective of crystal structure.