Spin Ladder Compound Research Articles

Phonon and magnetic dimer excitations in Fe-basedS=2spin-ladder compound BaFe2Se2O

Raman scattering spectra of new Fe-based S=2 spin ladder compound BaFe$_2$Se$_2$O are measured in a temperature range between 15 K and 623 K. All six A$_{1g}$ and two B$_{1g}$ Raman active modes of BaFe$_2$Se$_2$O, predicted by the factor-group analysis, have been experimentally observed at energies that are in a rather good agreement with the lattice dynamics calculation. The antiferromagnetic long-range spin ordering in BaFe$_2$Se$_2$O below $T_N$=240 K leaves a fingerprint both in the A$_{1g}$ and B$_{1g}$ phonon mode linewidth and energy. In the energy range between 400 and 650 cm$^{-1}$ we have observed magnetic excitation related structure in the form of magnon continuum, with the peaks corresponding to the singularities in the one dimensional density of magnon states. The onset value of magnetic continuum (2$\Delta_{S}$) is found at about 437 cm$^{-1}$ at 15 K. The magnetic continuum disappears at about 623 K, which lead us to conclude that the short-range magnetic ordering in BaFe$_2$Se$_2$O exists apparently up to 2.6$T_N$.

Quantum Monte Carlo simulations for stacked spin-ladder systems containing low concentrations of nonmagnetic impurities: Application to the low-temperature broadening of NMR spectra in SrCu2O3

We present a Quantum Monte Carlo study for Heisenberg spin-1/2 two leg ladder systems doped with non magnetic impurities. The simulations are applied to the doped spin ladder compound SrCu2O3 doped with Zn, where a large broadening of the 65Cu NMR lines has been observed in experiment at low temperatures but far above the Neel temperature. We find that interladder-couplings with a sizeable coupling in stacking direction are required to describe the line broadening, which can not be explained by considering a single ladder only. Around a single impurity, spin correlations cause an exponentially decaying antiferromagnetic local magnetization in a magnetic field. We develop an effective model for the local magnetization of systems with many randomly distributed impurities, with few parameters which can be extracted out of QMC calculations with a single impurity. The broadening arises from a drag effect, where the magnetization around an impurity works as an effective field for spins on the neighboring ladders, causing a non-exponentially decaying magnetization cloud around the impurity. Our results show that even for impurity concentrations as small as x = 0.001 and x = 0.0025, the broadening effect is large, in good quantitative agreement with experiment. We also develop a simple model for the effective interaction of two impurity spins.

Honeycomb Heisenberg spin ladder: Unusual ground state and thermodynamic properties

The unusual ground state and thermodynamic properties of spin-(1/2) two-leg honeycomb (HC) spin ladder are systematically studied by jointly utilizing various analytical and numerical methods. The HC spin ladder is found to exhibit behaviors dramatically different from those of the conventional square spin ladder. A strong relevant term and a half saturation magnetization plateau that can be attributed to the formation of diluted dimer states are observed in the HC ladder, both of which are absent in the square ladder. The ground state phase diagram of the HC spin ladder is identified, and the thermodynamic properties of the specific heat and susceptibility for different couplings are thoroughly explored, where two kinds of excitations are unveiled. Our calculated result is well fitted to the experimental data of the two-leg HC spin ladder compound [Cu2L1(N3)4]n.

Coexistence of long-range magnetic ordering and singlet ground state in the spin-ladder superconductor SrCa13Cu24O41

A long-range magnetic order was discovered in the quasi-one-dimensional spin-ladder compound SrCa${}_{13}$Cu${}_{24}$O${}_{41}$ by susceptibility, specific heat, and neutron diffraction experiments. The temperature dependence of the magnetic Bragg peak intensity could be well fitted to the power law with a transition temperature ${T}_{N}$ $=$ 4.23 K and a critical exponent \ensuremath{\beta} $=$ 0.28, indicating a three-dimensional phase transition for a low-dimensional magnet. A computer program was coded and found two possible magnetic structure models fitting best with all the observed magnetic peaks. These models suggest the spin-ladder sublattice is magnetically ordered with Cu moments aligning along the $a$ axis. The spin interactions are primarily antiferromagnetic along rungs and legs, while there are ferromagnetic clusters along legs. Surprisingly, the singlet-triplet spin-gap excitation is observable above and below ${T}_{N}$, indicating a coexistence of the spin-singlet ground state and long-range magnetic ordering state in this compound.

The Size Factor as Criterion for the Formation of <i>M</i><sub>14</sub>Cu<sub>24</sub>O<sub>41</sub> Phases

The size factor was analyzed as criterion for the formation of M14Cu24O41 phases. Attempts to replace Sr and/or Ca in Sr8Ca6Cu24O41, the best-known spin-ladder compound, by Mg or Ba were carried out. No significant solubility was observed for Mg, whereas the Ca atoms, and up to 4 Sr atoms per formula unit in Sr14Cu24O41, could be replaced by Ba. Based on the present investigation and literature data the minimum “critical” value of the average radius of the cations M above which an M14Cu24O41 phase is formed was found to be close to 1.21 Å and the maximum “critical” value close to 1.40 Å.

Evidence for dimer crystal melting in the frustrated spin-ladder system BiCu2PO6.

In the spin ladder compound BiCu(2)PO(6), there exists a decisive dynamics of spin excitations that we classify and characterize using inelastic light scattering. We observe an interladder singlet bound mode at 24 cm(-1) and two intraladder bound states at 62 and 108 cm(-1) in the leg (bb) and the rung (cc) polarization as well as a broad triplon continuum extending from 36 cm(-1) to 700 cm(-1). Though isolated spin ladder physics can roughly account for the observed excitations at high energies, frustration and interladder interactions need to be considered to fully describe the spectral distribution and scattering selection rules at low and intermediate energies. In addition, we attribute the rich spectrum of singlet bound modes to a melting of a dimer crystal. Our study provides evidence for a Z(2) quantum phase transition from a dimer to a resonating valence bond state driven by singlet fluctuations.

Low temperature Raman study of the spin ladder compound BiCu2PO6

We have investigated the temperature (40–295K) dependence of the Raman active phonons of BiCu2PO6 single crystals oriented along a- and b- axis. The comparison between the two orientations did not show any remarkable effect along the b-axis (two-leg zigzag spin ladder direction). At low temperatures the modes appear shifted and narrowed. The spectral characteristics of the modes below ∼60 K show constant values, indicating strong coupling between the formation of spin-ladder and the crystal lattice.

Anisotropic Cascade of Field-Induced Phase Transitions in the Frustrated Spin-Ladder SystemBiCu2PO6

BiCu(2)PO(6) is a frustrated two-leg spin-ladder compound with a spin gap that can be closed with a magnetic field of approximately 20 T. This quantum phase transition and its related phase diagram as a function of magnetic field and temperature (H, T) are investigated up to 60 T by means of specific heat, magnetocaloric effect, magnetization, and magnetostriction measurements. In contrast to other gapped quantum magnets, BiCu(2)PO(6) undergoes a series of unexpected first- and second-order phase transitions when an external magnetic field is applied along the crystallographic c axis. The application of a magnetic field along the b axis induces two second-order phase transitions. We propose that the anisotropy and complex phase diagram result from the interplay between strong geometrical frustration and spin-orbit interaction necessary for the description of this fascinating magnetic system.

Quantum magnetism of spin-ladder compounds with trapped-ion crystals

The quest for experimental platforms that allow for the exploration, and even control, of the interplay of low dimensionality and frustration is a fundamental challenge in several fields of quantum many-body physics, such as quantum magnetism. Here, we propose the use of cold crystals of trapped ions to study a variety of frustrated quantum spin ladders. By optimizing the trap geometry, we show how to tailor the low dimensionality of the models by changing the number of legs of the ladders. Combined with a method for selectively hiding ions provided by laser addressing, it becomes possible to synthesize stripes of both triangular and Kagome lattices. Besides, the degree of frustration of the phonon-mediated spin interactions can be controlled by shaping the trap frequencies. We support our theoretical considerations by initial experiments with planar ion crystals, where a high and tunable anisotropy of the radial trap frequencies is demonstrated. We take into account an extensive list of possible error sources under typical experimental conditions, and describe explicit regimes that guarantee the validity of our scheme.

Stripelike magnetism in a mixed-valence insulating state of the Fe-based ladder compound CsFe2Se3

Structural and electronic properties of the Fe-based spin-ladder compound CsFe${}_{2}$Se${}_{3}$ was investigated by means of resistivity, susceptibility, specific heat, M\ossbauer, and neutron diffraction measurements. Despite the single-site nature in a mixed-valence state, the ground state is a magnetic insulator characterized by a charge gap \ensuremath{\sim}0.34 eV and an antiferromagnetic transition temperature 175 K. The magnetic structure was stripelike, with magnetic moments of 1.77(6)${\ensuremath{\mu}}_{\mathrm{B}}$ coupled ferromagnetically (antiferromagnetically) along the rung (leg) direction. Both the insulating behavior and stripelike ordering can be understood by assuming extra carriers delocalized on the rung. Our findings reveal that CsFe${}_{2}$Se${}_{3}$ is an appealing compound with the stripelike magnetic structure in an insulating state among Fe-based compounds, and provide significant supplemental insight into the magnetism of Fe-based superconductors.

Magnetic structure of quasi-one-dimensional antiferromagnetic TaFe1+yTe3

We report the magnetic structure of TaFe${}_{1+y}$Te${}_{3}$ single crystals by means of neutron diffraction measurements. TaFe${}_{1+y}$Te${}_{3}$ possesses a layered structure with a formation of two-leg zigzag ladders along the $b$ axis. We find that TaFe${}_{1+y}$Te${}_{3}$ undergoes an antiferromagnetic transition at 178 K with Fe1 spins of the intraladders ferromagnetically aligned and with Fe1 spins of the interladders antiferromagnetically coupled. Furthermore, spins of the neighboring interstitial Fe2 ($y$) ions align parallel to the Fe1 spins of each ladder. These findings are distinct from the magnetic structure of the recently discovered spin-ladder compound BaFe${}_{2}$Se${}_{3}$. TaFe${}_{1+y}$Te${}_{3}$ may serve as a model system for investigating the interesting physics of quasi-one-dimensional ferromagnetic systems.

Electron Spin Resonance Shift in Spin Ladder Compounds

We analyze the effects of different coupling anisotropies in a spin-1/2 ladder on the electron spin resonance (ESR) shift. Combining a perturbative expression in the anisotropies with density matrix renormalization group computation of the short range correlations at finite temperature, we provide the full temperature and magnetic field evolution of the ESR paramagnetic shift. We show that for well chosen parameters the ESR shift can be in principle used to extract quantitatively the anisotropies and, as an example, discuss the material BPCB.

Iron displacements and magnetoelastic coupling in the antiferromagnetic spin-ladder compound BaFe2Se3

We report long-range ordered antiferromagnetism concomitant with local iron displacements in the spin-ladder compound BaFe$_2$Se$_3$. Short-range magnetic correlations, present at room temperature, develop into long-range antiferromagnetic order below T$_N$ = 256 K, with no superconductivity down to 1.8 K. Built of ferromagnetic Fe$_4$ plaquettes, the magnetic ground state correlates with local displacements of the Fe atoms. These iron displacements imply significant magnetoelastic coupling in FeX$_4$-based materials, an ingredient hypothesized to be important in the emergence of superconductivity. This result also suggests that knowledge of these local displacements is essential for properly understanding the electronic structure of these systems. As with the copper oxide superconductors two decades ago, our results highlight the importance of reduced dimensionality spin ladder compounds in the study of the coupling of spin, charge, and atom positions in superconducting materials.

Disorder instability of the magnon condensate in a frustrated spin ladder

The effect of disorder is studied on the field-induced quantum phase transition in the frustrated spin-ladder compound H${}_{8}$C${}_{4}$SO${}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}$Cu${}_{2}$(Cl${}_{1\ensuremath{-}x}$Br${}_{x}$)${}_{4}$ using bulk magnetic and thermodynamic measurements. The parent material ($x=0$) is a quantum spin liquid, which in applied fields is known to form a magnon condensate with long-range helimagnetic order. We show that bond randomness introduced by a chemical substitution on the nonmagnetic halogen site destroys this phase transition at very low concentrations, already for $x=0.01$. The extreme fragility of the magnon condensate is attributed to random frustration in the incommensurate state.

Single Crystals Sr14-xCaxCu24O41 Grown under Different Oxygen Pressures by Optical Floating-Zone Technique

Abstract Spin ladder compounds Sr 14- x Ca x Cu 24 O 41 (SCCO) single crystals have been grown under different oxygen pressures by an optical floating-zone technique. The effects of oxygen pressure on the crystal growth and properties of SCCO were studied. The SCCO single crystals with x ≤8 can be grown in a flowing oxygen atmosphere. But the growth of the SCCO compounds with higher Ca content ( x >8) needs higher pressure (0.9 MPa). The single crystals were characterized by means of XRD, EDX and resistivity measurement. High pressure is in favor of forming of higher Ca-doped SCCO single crystals. SCCO single crystals grown under different oxygen pressures have almost the same quality.

Long-lived magnons throughout the Brillouin zone of the strong-leg spin ladder (C7H10N)2CuBr4

Inelastic neutron scattering is used to measure spin excitations in fully deuterated single crystal samples of the strong-leg antiferromagnetic S=1/2 spin ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_4$. Sharp resolution-limited magnons are observed across the entire one-dimensional Brillouin zone. The results validate the previously proposed {\it symmetric} spin ladder model and provide a reliable estimate of the relevant exchange interactions.

Synthesis, structural and magnetic properties of spin ladder compound Ca1−xCoxCu2O3

MCu2O3 (M=Ca and Co) system has two-leg spin ladder structure similar to that of the prototype SrCu2O3 system except that the rungs are buckled with an angle of 123° and 105° for CaCu2O3 and CoCu2O3 compounds, respectively. We have synthesized powder samples of (Ca1−xCox)Cu2O3 (x=0.00–1.00) by the solid state reaction method and their structural and magnetic properties have been investigated. All the synthesized compounds crystallize in orthorhombic structure with space group Pmmn. Lattice parameters of (Ca1−xCox)Cu2O3 decrease with the increase in Co content. DC magnetic susceptibility χ(T) results of the end products CaCu2O3 and CoCu2O3 show antiferromagnetic transition (TN) at 27 and 215K, respectively. Co doping into (Ca1−xCox)Cu2O3 enhances its TN systematically with increasing Co concentration. The χ(T) of CoCu2O3 shows a broad transition with the peak temperature around 215K and it was found to be field independent up to 90kOe. The ambiguity concerning the transition was ruled out by recording the temperature dependent X-ray diffraction pattern on CoCu2O3 system, which indicated that there is no structural transition in the investigated temperature range of 115–300K. Further, specific heat measurement on CoCu2O3 confirms the magnetic phase transition by the appearance of a sharp peak at 215K.

Magnetic chirality of the spin triplet in the spin-ladder compound Sr14Cu24O41as seen via polarized inelastic neutron scattering

We report the direct observation by polarized inelastic neutron scattering experiments at low fields (3.5 T) of the splitting of the spin triplet of magnetic excitations in the response associated with the ladders in the composite cuprate Sr${}_{14}$Cu${}_{24}$O${}_{41}$. By conveniently choosing the magnetic field configuration and by making use of the spin chirality of the excitations, the splitting can be observed at relatively low fields. In this way, resonant excitations can be separated from the remainder of magnetic and phonon excitations at high temperatures. In Sr${}_{14}$Cu${}_{24}$O${}_{41}$, the 32-meV gapped mode vanishes above $~$200 K, close to the hole-crystal melting temperature previously observed by resonant x-ray and neutron diffraction.

High oxygen pressure single crystal growth of highly Ca-doped spin ladder compound Sr 14− xCa xCu 24O 41 ( x>12)

Large size high-quality Ca-doped Sr 14− x Ca x Cu 24O 41 ( x=12.2, 12.6 and 13) spin ladder superconducting single crystals were grown using a modified mirror floating zone furnace with oxygen pressure up to 35 bar. The qualities of the as-grown single crystals were confirmed by polarized optical microscopy, neutron diffraction and X-ray diffraction methods. The sample compositions and homogeneity were measured using micro-X-ray fluorescence spectroscopy. The lattice parameters of each sample have been refined using the Rietveld method. The oxygen stoichiometry measurements which were performed by hydrogen reduction/thermogravimetry method show that all the three samples are slightly oxygen deficient even though high oxygen pressure was applied during growth. The high oxygen pressure is indispensible for growing highly Ca-doped Sr 14− x Ca x Cu 24O 41 single crystals. The susceptibility was measured along c-axis for all three compositions, fitting well with the one dimensional dimer model, consistent with the reported values of undoped Sr 14Cu 24O 41 parent compounds.

Challenges in the crystal growth of Li2CuO2 and LiMnPO4

Single crystals of a potential battery material, LiMnPO4, and the spin ladder compound Li2CuO2 are grown under elevated gas pressure in a newly designed vertical floating zone facility with optical heating. Lithium vaporization and its reaction with gases in the growth chamber are principal challenges of the growth process. By applying high argon pressure of 40bar during Travelling Solvent Floating Zone growth of LiMnPO4 the vaporization was reduced by orders of magnitude. The optical radiation, penetrating the transparent melt, affects the crystal/melt interface curvature during the growth. The growth direction was recognized to be decisive for changing the interface curvature from concave to convex and enabling efficient grain selection. By using an Ar/O2 gas mixture in crystal growth of Li2CuO2 the direct exchange between oxygen in the atmosphere and the floating zone, the oxygen content of the melt and finally the quality of Li2CuO2 crystals are controlled.