Spin-ladder Cuprate Research Articles

Nanosheet fabrication from magnon thermal conductivity cuprates for the advanced thermal management

Spin-chain–spin-ladder cuprates, such as La5Ca9Cu24O41, have notable electronic and thermal properties because of their unique electron spin arrangement. Among them, magnon thermal conductivity, which originates from the excitation of paired electron spins, is promising for the advanced thermal management applications that enable dynamic control of heat flow. This is because of its high, anisotropic thermal conductivity at room temperature and its dynamic controllability. In this study, we report nanosheet fabrications from polycrystalline La5Ca9Cu24O41 to enhance the control width. We obtained that the nanosheets with a thickness of about 3 nm are obtained via immersion of the polycrystals in a NaOH aqueous solution. We propose the exfoliation model based on the chemical reaction between the (La/Ca)2Cu2O3 subsystem and NaOH solution. The nanosheet fabrications can also lead to new research development on spin-ladder system and other strongly correlated cuprates.

Face-on orientation of spin ladders in La5Ca9Cu24O41 films deposited by radio-frequency magnetron sputtering on heated SiO2 and Si substrates

Spin-ladder cuprates such as La5Ca9Cu24O41 are promising materials for advanced thermal management applications owing to their highly anisotropic and dynamically controllable thermal conductivity originating from spin ladders. Therefore, the orientation control of spin ladders is required in film fabrication methods that are extensively used in device engineering, such as sputtering. We found that La5Ca9Cu24O41 films with a columnar structure and the face-on orientation of spin ladders are formed on heated SiO2 (glass) and Si substrates via radio-frequency magnetron sputtering. The structural properties of the films were investigated, and the film formation mechanism was discussed. Our film fabrication method can pave the way for the development of new thermal management devices, such as active heat spreaders.

Effects of Ca substitution on quasiacoustic sliding modes in Sr14−xCaxCu24O41

The low energy lattice dynamics of the quasi-periodic spin-ladder cuprate Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ are investigated using terahertz frequency synchrotron radiation. A high density of low-lying optical excitations are present in the 1-3 THz energy range, while at least two highly absorbing excitations stemming from rigid acoustic oscillations of the incommensurate chain and ladder sublattices, are observed at sub-terahertz frequencies. The effects of Ca substitution on the sub-terahertz quasi-acoustic sliding mode gaps is investigated using coherent synchrotron radiation. Analysis of the results suggest increasing substitution of Sr for Ca is accompanied by a transfer of spectral weight between sliding modes associated with different chain-ladder dynamics. The observation is consistent with a transfer of hole charges from the chains to the ladders and modification of the sublattice dimensions following Ca substitution. The results are discussed in context to the significance of low-lying vibrational dynamics and electron-phonon coupling in the superconducting state of certain quasi-periodic systems.

NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprateSr2Ca12Cu24O41

Pressure-induced superconductivity was studied for a spin-ladder cuprate ${\text{Sr}}_{2}{\text{Ca}}_{12}{\text{Cu}}_{24}{\text{O}}_{41}$ using nuclear magnetic resonance under pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a transitional change from a spin-gapped state to a Fermi-liquid state at temperatures higher than ${T}_{c}$. The relaxation rate $1/{T}_{1}$ shows activated-type behavior at an onset pressure, whereas Korringa-like behavior becomes predominant at the optimal pressure, suggesting that an increase in the density of states at the Fermi energy leads to enhancement of ${T}_{c}$. Nuclear quadrupole resonance spectra suggest that pressure application causes transfer of holes from the chain to the ladder sites. The transfer of holes increases DOS below the optimal pressure. A dome-shaped ${T}_{c}$ versus pressure curve arises from naive balance between the transfer of holes and broadening of the band width.

Inhomogeneous state of the electron system in the Sr14 − x Ca x Cu24O41 superconducting cuprate: 63Cu-17O NMR study

Charge fluctuations in quasi-one-dimensional hole-doped Sr14Cu24O41 (Ca-0) and Sr2Ca12Cu24O41 (Ca-12) spin-ladder cuprates have been studied with the use of 63Cu-17O NMR. Spin-echo decay rates 17(1/T2) and 63(1/T2) have been measured in the temperature range of T = 10–300 K. The variation of 17(1/T2) and 63(1/T2) in the Ca-0 compound is monotonic in the entire temperature range of the NMR study. In the Ca-rich compound, pronounced peaks have been observed in the decay rates 17(1/T2) and 63(1/T2) at temperatures of 25 and 50 K, respectively. This result indicates the presence of collective low-frequency (τc ∼ T2) charge density fluctuations in superconducting Ca-12 with an activation energy of EA = 100(10) K. The fluctuations gradually slow down as the temperature decreases. The amplitude of the charge density fluctuations is only 0.01–0.02 hole per site. 17O-63Cu spin-echo double-resonance (SEDOR) experiments in Sr14 − xCaxCu24O41 oxides with x = 0 and 12 have been performed depending on the temperature and orientation of single crystals in a magnetic field. The constants of an indirect heteronuclear 17O-63Cu interaction of nuclear spins mediated by conduction electrons have been measured. The estimates of the indirect interaction constants for nearest neighbors O1-Cu and O2-Cu, as well as SEDOR experiments with selective excitation of separate sections of 17O and 63Cu NMR spectra, provide convincing evidence of the microscopically inhomogeneous spatial distribution of spin density developing in a crystal.

Terahertz BWO spectroscopy of conductors and superconductors

Methods for direct (without the use of the Kramers–Kronig relations) measuring the dielectric response (dynamic conductivity and permittivity) spectra of dielectrics, conductors, and superconductors in the terahertz frequency range (0.03–1.5 THz) are described. The methods are realised by using BWO (backward-wave oscillator) spectrometers developed at the A.M. Prokhorov General Physics Institute, RAS. Dielectric measurements can be performed at temperatures from 2 to 1000 K in magnetic fields up to 8 T. The dielectric response spectra are investigated for a number of materials with properties determined by electron correlation effects: superconductors, one-dimensional conductors, heavy fermion systems, spin-ladder cuprates, and spin glasses.

Pressure-induced superconductivity in the spin-ladder cuprate

Pressure-induced superconductivity in a spin-ladder cuprateSr2Ca12Cu24O41 was investigated on a microscopic level by using63Cu nuclear magnetic resonance (NMR) under a pressure of 3.5 GPa. We found a fully gappedsuperconducting state, and the superconductivity is stable at magnetic fields close to theconventional Pauli limit. The superconducting gap was observed in addition to a spin gapwhich occurs in a separated temperature range.

Superconductivity and spin gap in the spin-ladder cuprate, [formula omitted

Pressure-induced superconductivity in a spin-ladder cuprate Sr 2 Ca 12 Cu 24 O 41 was investigated on the microscopic level by using 63 Cu nuclear magnetic resonance (NMR) under pressure of 3.5 GPa. We observed both spin and superconducting gaps in separated temperature regions. We found that the superconducting state possesses a full gap and is quite stable even at high fields close to the conventional Pauli limit.

Four-spin cyclic exchange in spin ladder cuprates

The four-spin cyclic exchange term J r i n g of three spin-ladder cuprates (SrCu 2 O 3 . Sr 2 Cu 3 O 5 , and CaCu 2 O 3 ) has been calculated from ab initio quantum chemistry calculations. For the first two compounds, a non-negligible cyclic exchange is found, aproximately 20% of the magnetic coupling across the rungs, J⊥, and always larger than the value obtained for two-dimensional La 2 CuO 4 system. In the case of CaCu 2 O 3 , the J r i n g value is quite small, due to the folding of the Cu-O-Cu rung angle, but the J r i n g /J. ratio is also 0.2 as in the two other systems.

Superconductivity of the Sr2Ca12Cu24O41 spin-ladder system: are the superconducting pairing and the spin-gap formation of the same origin?

Pressure-induced superconductivity in a spin-ladder cuprate Sr2Ca12Cu24O41 has not been studied on a microscopic level thus far although the superconductivity was already discovered in 1996. We have improved the high-pressure technique using a large high-quality crystal, and succeeded in studying the superconductivity using 63Cu nuclear magnetic resonance. We found that the anomalous metallic state reflecting the spin-ladder structure is realized and the superconductivity possesses an s-wave-like character in the meaning that a finite gap exists in the quasiparticle excitation: At a pressure of 3.5 GPa, we observed two excitation modes in the normal state from the relaxation rate T-11. One gives rise to an activation-type component in T-11, and the other T-linear component linking directly with the superconductivity. This gapless mode likely arises from free motion of holon-spinon bound states appearing by hole doping, and the pairing of them likely causes the superconductivity.

高圧下ＮＭＲ ４ＧＰａ級クランプセルによる圧力誘起超伝導体Ｓｒ２Ｃａ１２Ｃｕ２４Ｏ４１のＮＭＲ

Pressure-induced superconductivity in a spin-ladder cuprate Sr2Ca12Cu24O41 has not been studied on a microscopic level thus far although the superconductivity was already discovered in 1996. We have improved high-pressure technique with a large high-quality crystal, and succeeded in studying the superconductivity using 63Cu nuclear magnetic resonance (NMR). We found that the superconductivity possesses an s-wave like character in the meaning that a finite gap exists in the quasi-particle excitation.

Superconductivity in a Spin-Ladder Cuprate

Superconductivity in a Spin-Ladder Cuprate

Superconductivity in a spin-ladder cuprate.

Superconductivity in a spin-ladder cuprate.

RETRACTED: Field-induced superconductivity in a spin-ladder cuprate.

We report on the modulation of the transport properties of thin films, grown by molecular beam epitaxy, of the spin-ladder compound [CaCu2O3]4, using the field effect in a gated structure. At high hole-doping levels, superconductivity is induced in the nominally insulating ladder material without the use of high-pressure or chemical substitution. The observation of superconductivity is in agreement with the theoretical prediction that holes doped into spin ladders could pair and possibly superconduct.

Stripes and dimensional crossovers in high- Tc cuprates

The key problem in high T c physics is whether the doping is inhomogeneous and holes are expelled into the 1D domains (“stripes”). Direct comparison of the resistivity vs T curves of the 1D spin-ladder cuprates and underdoped cuprates below the pseudogap temperature T o shows a remarkable similarity which implies that the main scattering mechanism is the same for these two classes of cuprates and therefore in the pseudogap regime the transport in underdoped cuprates is essentially 1D. All the resistivity curves in underdoped cuprates (including high field transport data in fields up to 60 T) can be fitted by the model of the 1D quantum transport with the inelastic length in charge stripes being fully controlled by the magnetic spin correlation length in even-chain spin-ladders formed in the Cu-O plane as a result of inhomogeneous doping. Close to the insulator - metal transition disorder strongly influences the formation of stripes. Knight shift data are also successfully interpreted by taking the same spin-gap value as the one found from resistivity data. The possible dimensional crossovers observed with decreasing temperature at T o(x) (2D-1D) and T c(x) (1D–2D) are discussed and the main unsolved problems of the evolution of the normal state properties with doping are outlined.

Doped CuO 2 Planes in High T c Cuprates: 2D Or Not 2D ?

The key problem in physics of underdoped cuprates is whether the doping is inhomogeneous and holes are expelled into the 1D domains (“stripes”). Direct comparison of the resistivity vs T curves of the recently discovered 1D spin-ladder cuprates and underdoped cuprates below the pseudogap temperature T* shows a remarkable similarity which implies that the main scattering mechanism is the same for these two classes of cuprates and therefore in the pseudogap regime the transport in underdoped cuprates is essentially 1D. All the resistivity curves in underdoped cuprates can then be fitted by the model of the 1D quantum transport with the inelastic length in charge stripes being fully controlled by the magnetic spin correlation length in even-chain spin-ladders formed in the Cu-O plane as a result of inhomogeneous doping. Knight shift data are also successfully interpreted by taking the same spin-gap value as the one found from resistivity data.

Electrical and magnetic anomalies in the spin-ladder cuprate Sr 14−xA xCu 24O 41(A=Ca, Y, La): Possibility of hole pairing in the ladder

We have re-investigated the electrical resistivity ϱ and the magnetic susceptibility χ of the spin-ladder cuprate Sr 14− x A x Cu 24O 41 (A=Ca, Y, La), consisting of two-dimensional Cu 2O 3 ladder planes and one-dimensional CuO 2 chains. The dlnϱ/d( 1 T ) vs. T plot exhibits a broad peak around 200 K for x = 0. The peak temperature T ϱ goes down with increasing hole-concentration p in the ladder for x(Ca)> 0, while it goes up with decreasing p in the ladder for x(Y, La)> 0. The observed T ϱ is regarded as a temperature below which holes in the ladder are localized. On the other hand, the χ −1 vs. T plot exhibits inflection indicating deviation from the Curie-Weiss law around 200 K for x = 0. The inflection temperature T χ also goes down with increasing p in the ladder, and goes up with decreasing p in the ladder. The observed T χ is regarded as a temperature above which Cu 2+ spins in the ladder notably contribute to the susceptibility. Values of T χ and its x-dependence have been found to be almost the same as those of T ϱ . This coincidence strongly suggests that holes in the ladder are paired and localized at low temperatures below T χ (and T ϱ ) where almost all Cu 2+ spins in the ladder complete to form spin-singlet pairs.

Study of apical oxygen atoms in a spin-ladder cuprate compound by X-ray absorption spectroscopy near the Cu K edge

The structure of high- T c superconducting cuprate compounds is based on CuO 2 planes alternating with blocks that behave as charge reservoirs. The apical oxygen atoms which belong to these reservoirs are suspected to play a role in the mechanism of superconductivity. It thus seems necessary to measure the amount of apical oxygen atoms in various compounds, as a function of the superconducting properties. Polarisation dependent X-ray absorption spectroscopy (XAS) measurements were performed near the Cu K-edge on three types of phases. We collected information about the neighbourhood of the copper atom in the cuprate planes and in the direction perpendicular to these planes. Two of these phases have well known structures: Bi2212 in which copper atoms are on a pyramidal site and infinite layer phase, a square planar cuprate without apical oxygen. We used the obtained results as reference data to study a new copper-rich phase related to the spin-ladder series.

Magnetic Behavior of the 2-Leg and 3-Leg Spin Ladder Cuprates Srn-1Cun+1O2n.

We have performed muon spin relaxation measurements of the spin-ladder cuprates ${\mathrm{Sr}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n+1}{\mathrm{O}}_{2n}$ ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3,5$). The 3-leg ladder system ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5$) shows static magnetic order at $T\ensuremath{\sim}52\mathrm{K}$, which is between the ordering temperatures of the one-dimensional chain system ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3}$ ( ${T}_{N}\ensuremath{\sim}5\mathrm{K}$) and of the two-dimensional layer system ${\mathrm{Ca}}_{0.86}{\mathrm{Sr}}_{0.14}{\mathrm{CuO}}_{2}$ ( ${T}_{N}\ensuremath{\sim}540\mathrm{K}$). The 2-leg ladder system ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$) does not show any magnetic order down to 20 mK, in agreement with the theoretical prediction of a nonmagnetic ground state with a spin gap for even-leg-number ladder systems.