Spin-gap Behavior Research Articles

Spin Gap State ofS= 1/2 Heisenberg Antiferromagnet YbAl3C3

YbAl 3 C 3 has been investigated in detail, which was previously reported to show an antiferroquadrupolar (AFQ) ordering at an extremely high ordering temperature of 80 K. The present study has revealed that the specific heat, magnetic susceptibility, and electrical resistivity of the single crystalline YbAl 3 C 3 clearly exhibit the characteristic features of the first order transition around 77 K. In addition, we have discovered that LuAl 3 C 3 also exhibits a sharp peak in the specific heat around 110 K, which most likely corresponds to the 77 K transition of YbAl 3 C 3 . Therefore, the 77 K transition of YbAl 3 C 3 is considered to be irrelevant to the nature of the 4 f -electrons such as the AFQ ordering but is ascribed to a structural one. Low temperature physical properties of YbAl 3 C 3 below 77 K is indicative of a spin gap and a nonmagnetic ground state. A new model is proposed, in which the adjacent spins on the triangular lattice of Yb 3+ ions are antiferromagnetically coupled to form a dimer by the structural transition at 77 K. The spin gap behavior at low temperatures can be well explained by an isolated dimer model with a singlet–triplet energy gap of about 15 K.

Spin-Peierls and incommensurate states in layered S=1/2 system TiOCl

We demonstrate that TiOCl is a good model inorganic system to investigate spin-Peierls state. Our 35Cl and 47,49Ti NMR data show that a pseudo spin-gap behavior below T*=135 K precedes successive phase transitions at T c =94 K and T c ′ = 66 K into a singlet spin-Peierls ground state with a large energy gap E g / k B =430 K.

Spin gap behavior inCu2Sc2Ge4O13studied usingSc45nuclear magnetic resonance

We report the results of a $^{45}$Sc nuclear magnetic resonance (NMR) study on the quasi-one-dimensional compound Cu$_2$Sc$_2$Ge$_4$O$_{13}$ at temperatures between 4 and 300 K. This material has been a subject of current interest due to indications of spin gap behavior. The temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a negative broad maximum at $T_{max}$ $\simeq $ 170 K. Below $% T_{max}$, the NMR shifts and spin lattice relaxation rates clearly indicate activated responses, confirming the existence of a spin gap in Cu$_2$Sc$_2$Ge% $_4$O$_{13}$. The experimental NMR data can be well fitted to the spin dimer model, yielding a spin gap value of about 275 K which is close to the 25 meV peak found in the inelastic neutron scattering measurement. A detailed analysis further points out that the nearly isolated dimer picture is proper for the understanding of spin gap nature in Cu$_2$Sc$_2$Ge$_4$O$_{13}$.

Field-induced phase transition in a metalorganic spin-dimer system—a potential model system to study Bose–Einstein condensation of magnons

We report on the results obtained from studying electron spin resonance, magnetic susceptibility, specific heat and thermal expansion experiments on a metalorganic spin-dimer system, C 36 H 48 Cu 2 F 6 N 8 O 12 S 2 (TK91). According to the first principle Density Functional Theory calculations, the compound represents a 3D-coupled dimer system with intradimer coupling J 1 / k B ∼ 10 K and interdimer couplings J 2 / k B ∼ J 3 / k B ∼ 1 K. The measurements have been performed on both pressed powder and single-crystal samples in external magnetic fields up to 12 T and at low temperatures down to ∼ 0.2 K. Susceptibility measurements reveal a spin-gap behavior consistent with the theoretical results. Furthermore, clear indications of a field-induced phase transition have been observed. A similar field-induced phase transition was also detected in an inorganic compound TlCuCl 3 and was interpreted as Bose–Einstein condensation (BEC) of magnons. The possibility of changing both the intradimer and interdimer couplings in TK91 by chemical substitutions makes the system a potentially good system to study BEC of magnons.

Spin-gap behavior of distorted honeycomb lattice system Na 3Cu 2SbO 6

Spin-gap behaviors observed in the temperature dependence of the magnetic susceptibility χ and the specific heat C of Na 3Cu 2SbO 6 with (distorted) honeycomb structure are analyzed in detail. The behavior can be understood by considering both the significant Jahn–Teller distortion in the compound and the characteristics of the shape of the electron orbits, in which the spins exist. We point out that the antiferromagnetic–ferromagnetic alternating chain model successfully describes the observed magnetic behaviors of Na 3Cu 2SbO 6.

NMR study of the spin gap in the vanadium bronze [formula omitted

V 51 NMR measurements have been made on a single crystal to study the spin gap in the vanadium bronze η - Na 1.286 V 2 O 5 . We observe four sets of V 51 NMR spectra coming from the nonmagnetic V1 and V2 sites above T 0 ∼ 110 K and eight sets below T 0 due to the doubling of the V1 and V2 sites accompanied by the structural transition. The V 51 Knight shift measurements for the V1 and V2 sites indicate that the magnetic vanadium sites show the spin-gap behavior with the gap energy dependent on the vanadium sites below T 0 .

Magnetic excitation in the spin-gap compoundCu2Sc2Ge4O13: Comparison toCu2Fe2Ge4O13

The compound \CuScGeO is presented as a new member of the family of weakly coupled spin chain and dimer compounds \CuMGeO. Magnetic susceptibility, heat capacity, and neutron inelastic scattering measurements reveal that the compound has the same spin dimer component as \CuFeGeO. The observed narrow band excitation in bulk measurements is consistent with spin gap behavior. The energy scale of the weakly coupled dimers in the Sc compound is perfectly coincident with that in the Fe compound.

Spin-Gap Behavior of Na3Cu2SbO6 with Distorted Honeycomb Structure

The spin-gap behavior observed in the temperature dependences of the magnetic susceptibility χ and the specific heat C of Na 3 Cu 2 SbO 6 with a (distorted) honeycomb structure is analyzed in detail. The behavior is commonly observed in similar systems A 3 Cu 2 SbO 6 and Na 2 Cu 2 TeO 6 (A=Na, Li), and can be understood by considering both the significant Jahn–Teller distortion in the compounds and the characteristics of the shape of the electron orbits, in which spins exist. The behavior is contrasted with the antiferromagnetic transition in systems of Na 3 T 2 SbO 6 and Na 2 T 2 TeO 6 (T=Co, Ni). We point out that the antiferromagnetic–ferromagnetic alternating chain model successfully describes the observed magnetic behavior of Na 3 Cu 2 SbO 6 .

Structural Features of the Modulated BiCu2(P1-xVx)O6 Solid Solution; 4-D Treatment of x = 0.87 Compound and Magnetic Spin-Gap to Gapless Transition in New Cu2+ Two-Leg Ladder Systems

The BiCu(2)(P(1-x)V(x))O(6) system shows the appearance of various phenomena that progressively change as a function of the average (P/V)O(4) groups size. Then, from x = 0 to x approximately 0.7, a solid solution exists with respect to the basic orthorhombic unit cell of BiCu(2)PO(6). For greater x values (0.7 < x <0.96), structural modulations with incommensurate q vector that slightly change versus x appear. The 4-D treatment of single-crystal XRD data of the modulated phase corresponding to x = 0.87 at 100 K (orthorhombic, a = 12.379(3)Angstrom, b = 5.2344(9) Angstrom, c = 7.8270(14) Angstrom, q = 0.268(3) b*, super space group: Xbmm(0gamma0) s00, X stands for the nonprimitive centering vector (1/2,0,1/2,1/2), R(obs)(overall) = 5.27%, R(obs)(fundamental) = 4.48%, R(obs)(satellite) = 6.58%) has evidenced strong positional modulated effects within the [BiCu(2)O(2)](3+) ribbons while three XO(4) configurations compete along the x(4) fourth dimension. There is no P/V segregation along x(4) in good agreement with steric-only origins of the modulation. Finally for 0.96 < x <1, two phases coexist, i.e., BiCu(2)VO(6) (X = 1) and a modulated phase of the previous domain.The BiCu(2)VO(6) crystal structure shows a unit cell tripling associated with monoclinic symmetry lowering. The VO(4) orientations between two ribbons proceed with respect to the interribbon distance. Then the full system shows flexible interactions between modulated Bi/M/O-based ribbons and surrounding tetrahedral groups, depending on the average XO(4) size. Furthermore, between two ribbons the Cu(2+) arrangement forms magnetically isolated zigzag copper two-leg ladders. Our preliminary results show a spin-gap behavior likely due to the existence of true S = (1)/(2) Heisenberg two-leg ladders. Modulated compositions are gapless, in good agreement with band-broadening toward a continuum in the magnetic excitation spectrum. The continuous distribution of Cu-Cu distances along the rungs and legs of the ladders should be mainly responsible for this magnetic change.

Magnetic Properties of Vanadium Oxide Nanotubes, Nanourchins, and Nanorods.

AbstractA better understanding of magnetic interactions, charge distribution, and redox properties of vanadium oxide nanotubes (VONTs) is necessary for accurate structure and mechanism of formation determination. Magnetic properties have been determined for pristine and lithiated VONTs and for the VONTs arranged in nanourchin morphology. Presence of paramagnetic V4+ ions and V4+ ions coupled in magnetic dimers is found, and their amounts are estimated. Both lithiation and change of morphology to nanourchin destroy the spin-gap behavior, which indicate changes in charge distribution. No ferromagnetic response is observed in lithiated VONTs. Magnetic properties of vanadium oxide nanorods with δ-V4O10 structure are also characterized.

Magnetism in the pseudo-two-leg ladder compound CaCu2O3

Contrary to SrCu2O3, the pseudo-ladder two-leg compound CaCu2O3 does not show a significant spin-gap behavior as expected for typical two-leg ladder systems. Magnetization measurements on single crystals show CaCu2O3 to be a three-dimensional (3D) antiferromagnet (TN=27K), showing relatively strong magnetic anisotropy and metamagnetic transitions. The 3D behavior is due to exchange between the ladders as deduced from electronic structure calculations within the local density approximation.

Synthesis, crystal structures and magnetic properties of organically templated new layered vanadates: [C4H8NH2]V3O7, [(CH3)2NH2]V3O7, [C5H10NH2]V3O7 and [C2H5NH3]V3O7

Four new layered mixed-valance vanadium oxides, which contain the interlamellar organic cations pyrolidinium, dimethylammonium, piperidinium, and ethylammonium, have been prepared under hydrothermal conditions and their structures determined. All four structures contain vanadium oxide layers constructed from V5+O4 tetrahedra and pairs of V4+O5 square pyramids with the protonated organic amines occupying the interlayer space. The materials are further characterized by thermal analysis, FTIR and scanning electron microscopy. Magnetic susceptibility studies of the first three compounds reveal a spin-gap behavior explained by an isolated antiferromagnetic dimers model. Large values of the exchange constants J = 325 − 480 K were found and the dependence of J upon the structural parameters was analyzed assuming a kinetic exchange mechanism.

Sequence of first-order quantum phase transitions in a frustrated spin half dimer-plaquette chain

We study the frustrated dimer-plaquette quantum spin chain for ferromagnetic dimer bonds. This quantum system undergoes a series of first-order ground-state phase transitions driven by frustration or by a magnetic field. We find that the different nature of the ground-state phases has a strong influence on the magnetization curve as well as the low-temperature thermodynamics. In particular, the magnetization curve exhibits plateaus and jumps, the number of which depends on the strength of frustration. The temperature dependence of the susceptibility may either show activated spin-gap behaviour for small frustration or Curie like paramagnetic behaviour for strong frustration.

The spin-gap state and thephase transition in the δ-phase TlxV2O5polaronic bronze

The crystal structure and electronic properties ofδ-phase bronze TlxV2O5, where 0.47⩽x⩽0.496, have been explored by means of x-rayfour-circle diffraction and through measurements of latticeconstants, electrical resistivity, thermoelectric power,magnetization, and electron paramagnetic resonance. The structureof Tl0.479V2O5 redetermined with residual factorsof R = 0.052 and Rw = 0.041 indicates that thetrellis-layer model is applicable to this system as in the caseof δ-Ag2/3V2O5 in the low-temperaturephase. A spin-gap behaviour derived from that model and thehopping conduction of negative small polarons are found. A phasetransition takes place at around 150 K accompanied bysignificant changes of the superexchange coupling constants (orspin-gap energies) and the carrier-creation energies.

Dimensional crossover in alternating spin chains

The effect of antiferromagnetic interchain coupling in alternating spin (1, 1 2 ) chains is studied by means of spin wave (SW) theory and density matrix renormalization group (DMRG). Two limiting cases are investigated, the two-leg ladder and its two-dimensional (2D) generalization. For the 2D case, SW approximation predicts a smooth-dimensional crossover keeping the ground state ordered, whereas in the ladder case the DMRG results show a gapped ground state for any J ⊥>0. Furthermore, the behavior of the correlation functions closely resemble the uniform spin- 1 2 ladder. However, for small J ⊥, the gap behaves quadratically as Δ∼0.6 J ⊥ 2. Similarly to uniform spin chains, it is conjectured an analogous spin gap behavior for an arbitrary number of mixed spin chains.

Cu NQR study of host and guest spin dynamics in impurity-doped YBa 2Cu 4O 8

We present the planar 63 Cu nuclear spin–lattice relaxation study of impurity-doped YBa 2(Cu 1− x M x ) 4O 8 (M=Ni, Zn) at T=4.2–300 K with Cu nuclear quadrupole resonance (NQR) spin-echo technique. In the light of an impurity-induced NQR relaxation theory, we estimated the host Cu nuclear spin–lattice relaxation time ( T 1) HOST, the impurity-induced Cu nuclear spin–lattice relaxation time ( τ 1), and the wipeout region around the impurity in the superconducting state. The pseudo spin-gap behavior of the host Cu antiferromagnetic spin fluctuation above T c is not readily removed by a small amount of the impurity. The Zn-induced non-Korringa behavior of ( T 1) HOST below T c is in sharp contrast to the doping effect of the magnetic impurity Ni.

NMR study of the magnetic properties of NaV 6O 11: spin-gap behavior of Kagomé lattice

NaV 6O 11 undergoes a metal–semiconductor transition at T t=243 K accompanied by a structural phase transition. Three kinds of 51V NMR lines were observed and each line was assigned to the V(1), V(2) and V(3) site. The temperature dependence of the NMR shift and the spin-lattice relaxation rate of V(1), which forms a Kagomé lattice, shows a spin-gap behavior below T t.

NMR study on magnetism of Ca 0.85CuO 2 with edge sharing CuO 2 chain

We have studied 63Cu-NQR of Ca 0.85CuO 2 with the edge sharing CuO 2 chains. The magnetic susceptibility and nuclear spin-lattice relaxation time, T 1 show spin gap behavior for magnetic excitations at the Cu site. A small hyperfine field at the Cu site is confirmed below 12.5 K by Cu-NQR, indicating a clear evidence for the coexistence of the dimer state and magnetically ordered state in the hole-doped chains of Ca 0.85CuO 2.

Spin gap behavior and electronic phase separation in doped polyacetylene

In ClO 4 −-doped ν-polyacetylene with the dopant concentration of 2–6%, the ESR susceptibility was found to decrease below 200 K. Meanwhile, the thermoelectric power did not exhibit the corresponding anomaly, although the resistivity showed a change suggesting a variation in the carrier dynamics. The ESR line-shape analysis revealed that a phase separation due to droplet nucleation undergoes in a mixed phase of a soliton state associated with spin-charge separation and a polaron state. For the 2.6% doped, the spin-charge separated state appeared with negligible content of the metallic phase, suggesting that the upper bound for a homogeneous soliton state is ∼3% in the dopant concentration. The superconductivity by frustration mechanism is considered to be pre-empted by the soliton phase.

Spin-gap and the superconducting fluctuations in high- Tc cuprates probed by 63Cu NMR

From the 63Cu Knight shift and the nuclear spin–lattice relaxation rate under magnetic fields from zero up to 28 T in the underdoped (U) YBa 2Cu 4O 8 and the slightly overdoped (O) TlSr 2CaCu 2O 6.8, we find that the spin-gap (pseudogap) behaviors show contrasting response to magnetic fields in the U and O compounds. In the O compound, the spin-gap is strongly field dependent and can be accounted for by the superconducting fluctuation effects, but that in the U compound shows no field dependence. The implication is discussed.