Spin-gap Behavior Research Articles

Pressure effect on the spin gap opening temperature for Ca-doped and oxygen-deficient YBCO superconductors

The spin gap opening temperature in the normal state of bulk Ca-doped and oxygen-deficient YBCO superconductors was investigated under various high hydrostatic pressures. Under the ambient pressure, T * can be expressed in terms of T c as T *( n) = T *(OP) +α(1 - T c ( n)/ T c max) 1 2 . Under various high pressures, this expression is still valid, so, T *( n,P) = T *( OP) +α( P)(1- T c ( n,P)/ T c max) 1 2 . Experiments show that the parameter T *( OP) is pressure-independent, while α(P) decreases linearly with increasing pressure. This indicates that the structural change considerably affects the spin gap behavior.

Experimental evidence for a spin gap in the s=1/2 quantum antiferromagnet Cu 2(OH) 2CO 3

Magnetic properties of the natural mineral Cu 2(OH) 2CO 3 (malachite) were investigated through DC and AC susceptibility measurements. The analysis of the low-temperature data reveals a quantum spin-gap behavior with Δ≈130 K. Consistently with the crystal structure, the magnetic susceptibility can be accurately described by a model of alternating chains. The non-frustrating residual inter-chain magnetic couplings, describing a sort of dimerized square planar lattice, are not strong enough to push the system towards a long-range ordered ground state, in good agreement with recent theoretical studies.

Anomalous Fermi Liquid Effects in Two-Dimensional Hubbard Model near Half-Filling

The Landau parameters F 0 s , F 0 a , F 1 s and F 1 a in the two-dimensional Hubbard model are calculated by the second order perturbation method of Abrikosov-Khalatnikov. The following points are revealed as the half-filling is approached: (1) F 0 a changes its sign from negative to positive, if the Coulomb interaction U is strong enough, i.e., \(U/t\gtrsim 2.0 \). This means that the spin density oscillations, zero spin-sound, can propagate there; (2) F 1 s changes its sign from positive to negative, so that we can understand, on the basis of the anisotropic Fermi liquid theory, the behavior of the Drude weight whose magnitude decreases as the half-filling is approached even if there were no mass enhancement; (3) From the results of F 0 s and F 0 a , it is shown that both the charge and spin susceptibility decrease rapidly close to the half-filling. The latter is regarded as the so-called spin-gap behavior.

Magnetic properties of a Heisenberg spin-ladder Cu 2(C 5H 12N 2) 2Br 4

The compound Cu 2(C 5H 12N 2) 2Br 4 (CHpB) is isostructural with a typical spin-ladder material Cu 2(C 5H 12N 2) 2Cl 4 (CHpC). The susceptibility and magnetization of CHpB show spin-gap behavior at low temperatures as observed in CHpC. The results are in good agreement with a spin-ladder model.

NMR study of the spin-gap behavior in (VO) 2P 2O 7 and [formula omitted

From the detailed analysis of 31P NMR spectra, we have found two inequivalent alternating antiferromagnetic chains with different spin gaps existing in (VO) 2P 2O 7. In VOHPO 4· 1 2 H 2 O , a one-dimensional coupling between V–V dimers has been revealed from the magnetic-field dependence of the nuclear spin-lattice relaxation rate at P sites. All data are compatible with a relatively large V–V exchange via the PO 4 tetrahedra, which has been suggested recently to account for the magnetic properties of these compounds.

Structural and magnetic characterizations of single-crystal (Na 1− xCa x)V 2O 5: charge-ordering and spin-ordering in vanadate systems

We report the X-ray diffraction and magnetic susceptibility measurements on single-crystal samples of the partially filled spin-ladder system (Na 1− x Ca x )V 2O 5. X-ray powder diffraction shows a monotonic expansion of the b- and c-axis lattice parameters with increasing x, consistent with the larger atomic radius of Ca 2+. Small Ca substitution ( x<0.04) into NaV 2O 5 suppresses the charge-ordering transition at 34 K in a manner consistent with a “zig-zag” charge-ordered structure, but the ground state appears to remain a spin-singlet. Further Ca substitution results in a more traditional spin gap behavior with a persistent gapped spin state for x≥0.04. For x<0.04 the spin susceptibility is well described by the same linear chain model as NaV 2O 5, with coupling constant J∼560 K/k B . Analysis of the high-temperature spin susceptibility data suggests that the behavior of the x≥0.04 samples is better explained by a spin-ladder system with coupling J ladder ≈J rung ∼500 K/k B .

Are high-temperature superconductors Kondo lattices?

Physical properties of cuprate-oxide superconductors, not only high- T c superconductivity but also spin-gap behaviors, stripe structure and so on, can be consistently explained with the physical picture of Kondo lattices.

Possible existence of zero spin-sound in two-dimensional Hubbard model

The Landau parameter F a 0 in two-dimensional Hubbard model is calculated by the second-order perturbation method. It is shown that F a 0 changes its sign from negative to positive as the filling is increased to the half-filling for the interaction U/ t≳2.0. This means that the spin density waves propagation, zero spin-sound, is possible in the system described by Hubbard model near half-filling. It is shown that the spin susceptibility decreases for U/ t≳2.0 when the half-filling is approached exhibiting the so-called spin-gap behavior.

Physical Properties of Vanadium Oxide Bronzes β-A0.33V2O5 (A = Ca and Sr)

The vanadium bronzes.β-A0.33V2O5(A= Ca and Sr) have been synthesized, and their electromagnetic properties investigated. They show phase transitions at 150K and 170K, respectively, accompanying with anomalies in the magnetic susceptibility and resistivity curves. These transitions are of charge order type. Below the transition temperature the magnetic susceptibility showed low dimensional behavior in both compounds, especially spin gap behavior in Sr0.33V2O5. Such behavior is closely related to the crystal structure characteristic of the vanadium β-bronzes and a manner of ordering for magnetic V4+ ions at the transition.

Effects of Ca substitution on the phonon anomalies in the Raman scattering of

We report on the temperature dependence of the frequencies and linewidths in the phonon Raman scattering for Y1-yCayBa2Cu4O8 system (y = 0 - 0.15). The phonon anomalies above Tc of the frequencies are observed for the out-of-phase O(2)-O(3) Ag\(( \sim 340c{m^{ - 1}})\) and O(4) Ag\(( \sim 500c{m^{ - 1}})\)modes, and these onset temperatures decrease with increasing Ca content. These features are consistent with the spin-gap behaviors associated with Ca doping reported previously. Furthermore, we find that the smaller gap exists near or just above the Ba phonon frequency at \(\sim 340c{m^{ - 1}}\) in the undoped samples and its energy increases with Ca doping.

AbInitioCalculation of Spin Gap Behavior in CaV4O9

Second neighbor dominated exchange coupling in CaV4O9 has been obtained from ab initio density functional (DF) calculations. A DF-based self-consistent atomic deformation model reveals that the nearest neighbor coupling is small due to strong cancellation among the various superexchange processes. Exact diagonalization of the predicted Heisenberg model yields spin-gap behavior in good agreement with experiment. The model is refined by fitting to the experimental susceptibility. The resulting model agrees very well with the experimental susceptibility and triplet dispersion.

THE STUDY ON THE SPIN GAP PHENOMENON IN OXYGEN DEFICIENT YBa2Cu3O6+δ AND Ca-DOPED Y1-xCaxBa2Cu3O6+δ SUPERCONDUCTORS

The understanding of the spin gap phenomenon in the normal state of the high temperature superconductors can provide the mechanism of the high temperature superconductivity. A specific function of thermal excitation for quasiparticles was introduced based on the experimental relationship between resistivity and temperature. This specific function describes the deviation in resistivity from the T-linear relationship w.r.t. the normal state of the high temperature superconductors. With this function, the spin gap is naturally introduced. Our experimental results indicate that the spin gap behavior is closely related not only to the oxygen deficiency but also to the structural change in the a–b plane. Furthermore, we found that the crossover from anomalous metallic state to spin gap state takes place within a temperature range T S &lt;T&lt;T*, which depends on the oxygen content and the Ca-doping level. Pressure effects on the spin gap phenomenon in bulk samples of oxygen-deficient YBa 2 Cu 3 O 6+δ and Ca-doped Y 1-x Ca x Ba 2 Cu 3 O 6+δ were also investigated. Under high hydrostatic pressure the spin gap behavior is also severely influenced by the contraction of the a–b plane due to the pressure.

Spin-gap behavior and charge ordering inα′−NaV2O5probed by light scattering

We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound $\alpha^\prime$-NaV$_{2}$O$_{5}$ for the temperature range 5 K$\le$T$\le$300 K. This system undergoes a phase transition into a singlet ground state at T=34 K accompanied by the formation of a super structure. For T$\leq$34 K several new modes were detected. Three of these modes are identified as magnetic bound states. Experimental evidence for charge ordering on the V sites is detected as an anomalous shift and splitting of a V-O vibration at 422 cm$^{-1}$ for temperatures above 34 K. The smooth and crossover-like onset of this ordering at T$_{\rm CO}$= 80 K is accompanied by pretransitional fluctuations both in magnetic and phononic Raman scattering. It resembles the effect of stripe order on the super structure intensities in La$_2$NiO$_{4+\delta}$.

Spin-gap like behavior observed in intermediately doped polyacetylene

A notable decrease in ESR spin susceptibility was found near 130 K in intermediately ClO 4 − -doped v -(CH) x . The absence of the corresponding change in thermoelectric power suggests that it is associated with a spin-charge separation. The possible origin is discussed with respect to the structural change in (CH) x .

Study of Spin System witht2gOrbital Degeneracy for Ladder and Chain

Effective spin Hamiltonians with t 2g orbital degeneracy have been investigated theoretically. We assume that a d electron occupies one of doubly degenerate d z x and d y z orbitals which have the lowest energy in atomic level. The orbital degrees of freedom are represented as Ising iso-spin operators because of the symmetry of the orbital characters. We apply the Hamiltonians to a ladder and a chain lattices and investigate the models using the exact diagonalization. For the ladder model, a spin gap opens above a finite value of the spin exchange coupling on rung, which is qualitatively different from the ladder model with single orbital. For the chain model, the spin gap behavior is observed in the uniform spin correlation at low temperature. Origin of the spin gap is a spin-dimer triggered by the occurrence of the ferromagnetic orbital ordering, which suggests a new mechanism of the spin gap formation.

51V-NMR Studies on the Spin Dynamics ofS=1/2Zig-Zag Chains of VO5Pyramids

NMR Knight shifts K and nuclear relaxation rates 1/ T 1 have been studied on the V 4+ sites of two compounds [N(CH 3 ) 4 ]V 3 O 7 and (en) 2 ZnV 6 O 14 which are known to exhibit spin-gap behavior from their susceptibility data. The activation-type behavior has been observed in K and 1/ T 1 , confirming the existence of the spin-gap in these systems. The frequency (ω N ) dependence of 1/ T 1 of [N(CH 3 ) 4 ]V 3 O 7 has been found to be \(1/\sqrt {\omega _{\rm N}}\)-type one and 1/ T 1 of (en) 2 ZnV 6 O 14 is linear in -ln (ω N ), indicating the diffusive and ballistic characters of the triplet excitations, respectively. The results are discussed in relation to the theory recently developed by Sachdev and Damle [Phys. Rev. B 57 (1998) 8307].

Pseudogap behavior in single crystals of Bi 2Sr 2CaCu 2O 8+ δ

In under- and optimally doped Bi 2Sr 2CaCu 2O 8 (Bi2212), we found a characteristic temperature T K ∗ , below which the spin part of Knight shift K s starts to decrease steeply and extrapolates to zero at T>0. T K ∗ is in good agreement with the temperatures at which the pseudogap starts to open in the quasiparticle density of states (DOS) at the Fermi level, and T ∗ at which 1/ T 1 T has a broad peak. We propose that the spin-gap behavior observed in NMR measurements has the same origin as the pseudogap in the quasiparticle DOS.

Magnetic ordering in EuB 6, investigated by neutron diffraction

An elastic neutron scattering study of the temperature-dependence of the ordered magnetic moment in EuB 6 is presented. The decrease of the saturation magnetization of ∼7 μ B with increasing temperature is shown to follow the Bloch T 3/2-law, with no indication of a spin–gap behavior. The magnetic coupling constant deduced from the Bloch law fit corresponds to a mean-field ordering temperature of ∼12 K, close to the actually observed ordering temperature and the paramagnetic Curie-temperature deduced from high-temperature magnetic susceptibility.

Pseudogap behavior in single-crystalBi2Sr2CaCu2O8+δprobed by Cu NMR

We report that the nuclear spin-lattice relaxation rate divided by temperature, ${1/T}_{1}T,$ has a broad maximum around ${T}^{*}\ensuremath{\sim}210\mathrm{K}$ and \ensuremath{\sim}100 K in underdoped and overdoped single crystals of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) with ${T}_{c}=79$ and 77.3 K, respectively, showing the normal-state spin-gap behavior. One result is that the steep decrease in the ${}^{63}$Cu Knight shift, $K(T),$ has been observed below ${T}_{K}^{*}\ensuremath{\sim}200\mathrm{K}$ and \ensuremath{\sim}100 K, suggesting the normal-state pseudogap behavior in quasiparticle density of states (DOS), which is consistent with the pseudogap behavior below ${T}_{\mathrm{ARPES}}^{*}\ensuremath{\sim}170\mathrm{K}$ revealed on underdoped Bi2212 from studies of angle-resolved photoemission spectroscopy. We propose that the spin-gap behavior in ${1/T}_{1}T$ has the same origin as the pseudogap in the quasiparticle DOS.

Cu NMR Study of Bilayer HgBa2CaCu2O6+δ–Variation of Pseudo Spin-Gap Spectrum

We observed the pseudo spin-gap behavior in the T -dependences of 63 Cu Knight shift and NMR relaxation rates, as well as electrical resistivity, over a wide doping range for the bilayer high- T c HgBa 2 CaCu 2 O 6+δ using 63 Cu NMR spin-echo technique. We found a difference in the doping dependence of the pseudo spin-gap behavior between HgBa 2 CaCu 2 O 6+δ and the previous monolayer HgBa 2 CuO 4+δ . The results indicate that the pseudo spin-gap behavior strongly depends on the underlying electronic states.