Electronic structure and exchange interactions of the ladder vanadates CaV2O5and MgV2O5

The nuclear spin-lattice relaxation rate at the V sites in CaV2O5 was measured. The relaxation rate (1/T1) in the two-leg ladder was calculated with a strong-coupling expansion (J ⊥ ≫ J ) and the result was compared with the experimental data. The comparison of the temperature dependence shows a good agreement and provides an estimate of both J ⊥ and J as well as the energy gap ∆. The magnetic field dependence of 1/T1 is also reproduced with this approximation in the temperature range T ∆/4. It is found that the magnon-magnon collision, which leads to a diffusive behavior, is negligible in this temperature range. The effect of inter-ladder coupling is investigated and found to be weak in CaV2O5. Thus the strong-coupling expansion calculation agrees satisfactorily with the experimental data, but a discrepancy between the exchange parameters deduced from the magnetic susceptibility and 1/T1 is found.KEYWORDS: CaV 2 O 5 , NMR, spin ladder, spin diffusion, nuclear spin relaxation §1. IntroductionThe discovery of high-T C superconductivity in doped two-dimensional antiferromagnets renewed interest in quantum antiferromagnets. In particular, spin-1 2 two-leg ladder antiferromagnets have been extensively studied, being motivated by a theoretical prediction of the quantum disordered ground state with an energy gap and possible superconductivity.1) Actually, the energy gap was experimentally observed in SrCu 2 O 3 .2) Moreover, superconductivity was discovered in Sr 14−x Ca x Cu 24 O 41 under high pressure.3)The low-energy excitations in such gapful spin systems have not been fully understood yet. One problem is a significant discrepancy between the energy gaps determined from the magnetic susceptibility and the nuclear spinlattice relaxation rate 1/T 1 . For the two-leg ladder antiferromagnet SrCu 2 O 3 , 1/T 1 suggests a much larger gap ∆ ∼ 700 K than ∆ ∼ 420 K by the susceptibility.2, 4) Itoh et al. surveyed the data of the susceptibility and 1/T 1 of many gapful quantum paramagnets and pointed out that this discrepancy is observed widely in one-dimensional gapful systems such as Haldane-gap and two-leg ladder antiferromagnets.5) Kishine and Fukuyama calculated 1/T 1 with the Majorana fermion representation of the two-leg ladders. 6) They found a dominant contribution with three times larger energy gap than the lowest excitation and conclude that this is responsible for the discrepancy. Sachdev and Damle developed a semiclassical continuum theory for one-dimensional gapful systems and obtained diffusive behavior in the long-time spin correlation.7, 8) They showed that 1/T 1 is not proportional to e −∆/T but e −3∆/2T at sufficient but not too low temperatures. The diffusive behavior was actually observed * Present Address: Department of Physics, Faculty of Science, Chiba University, Chiba, 263-8522. in magnetic field dependence of 1/T 1 in the Haldane-gap antiferromagnet AgVP 2 S 6 10) and another gapful system (VO) 2 P 2 O 7 .11, 12) These theoretical arguments suggest that the intrinsic energy gap is unique and that ...

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“…18,19) Their result that J ⊥ ∼ 608 K, J ∼ 122 K, and J ′ ∼ −28 K agrees with the quantum Monte Carlo simulation.…”

Section: )supporting

confidence: 63%

Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV₂O₅

2001

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“…Korotin et al have inferred theoretically that the large differences between the exchange constants in the latter two compounds arise from the stronger tilting of the VO 5 square pyramids in MgV 2 O 5 as compared to CaV 2 O 5 . 49 The conventional empirical rules for estimating the strengths of nearest-neighbor superexchange interactions in oxides are strongly violated in CaV 2 O 5 and also in cuprate spin ladder compounds, as extensively discussed in Ref. 3.…”

Section: Discussionmentioning

confidence: 99%

Modeling of the magnetic susceptibilities of the ambient- and high-pressure phases of(VO)2P2
Johnston
¹
,
Saito
²
,
Azuma
³

et al. 2001
Phys. Rev. B
38
4
57
3
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The magnetic susceptibilities χ versus temperature T of powders and single crystals of the ambient-pressure (AP) and high-pressure (HP) phases of (VO)2P2O7 are analyzed using an accurate theoretical prediction of χ(T, J1, J2) for the spin-1/2 antiferromagnetic alternating-exchange (J1, J2) Heisenberg chain. The results are consistent with recent models with two distinct types of alternating-exchange chains in the AP phase and a single type in the HP phase. The spin gap for each type of chain is derived from the respective set of two fitted alternating exchange constants and the one-magnon dispersion relation for each of the two types of chains in the AP phase is predicted. The influences of interchain coupling on the derived intrachain exchange constants, spin gaps, and dispersion relations are estimated using a mean-field approximation for the interchain coupling. The accuracies of the spin gaps obtained using fits to the low-T χ(T ) data by theoretical low-T approximations are determined. The results of these studies are compared with previously reported estimates of the exchange couplings and spin gaps in the AP and HP phases and with the magnon dispersion relations in the AP phase measured previously using inelastic neutron scattering.

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“…2), with oxygen 2p valence bands below −3 eV and vanadium 3d bands at the Fermi level (E F ), is reminiscent of other V +4 oxides. 16,27 The contribution of cadmium is, however, larger than typical for an alkaline-earth (e.g., Ca The band complex between −0.5 eV and 4.2 eV comprises 24 bands (Fig. 3).…”

Section: Band Structure and Exchange Couplingsmentioning

confidence: 93%

Unusual ferromagnetic superexchange in CdVO3: The role of Cd

2011

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A microscopic magnetic model of the low-pressure modification of CdVO3 is established, based on density functional theory (DFT) band-structure calculations, magnetization measurements, and quantum Monte-Carlo simulations. This compound is a rare example of a quasi-one-dimensional spin-1 2 system showing exclusively ferromagnetic exchange. The spin lattice of CdVO3 entails zigzag chains with an effective intrachain coupling J −90 K and interchain couplings of Jc −18 K and Ja −3 K. Quantum fluctuations are partially suppressed by the sizable interchain coupling Jc that leads to an intermediate regime between one-dimensional and two-dimensional ferromagnetic systems. Apart from the peculiar spin model, CdVO3 features an unusual mechanism of ferromagnetic superexchange. The couplings largely originate from Cd 5s states mediating hoppings between half-filled and empty 3d states of V +4 .

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Electronic structure and exchange interactions of the ladder vanadates CaV₂O₅and MgV₂O₅

Cited by 40 publications

References 23 publications

Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV₂O₅

Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV₂O₅

Modeling of the magnetic susceptibilities of the ambient- and high-pressure phases of(VO)2P2
Johnston
¹
,
Saito
²
,
Azuma
³

et al. 2001
Phys. Rev. B
38
4
57
3
View full text Add to dashboard Cite

Unusual ferromagnetic superexchange in CdVO3: The role of Cd

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Electronic structure and exchange interactions of the ladder vanadates CaV2O5and MgV2O5

Cited by 40 publications

References 23 publications

Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV2O5

Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV2O5

Modeling of the magnetic susceptibilities of the ambient- and high-pressure phases of(VO)2P2Johnston1, Saito2, Azuma3 et al. 2001Phys. Rev. B384573View full textAdd to dashboardCite

Unusual ferromagnetic superexchange in CdVO3: The role of Cd

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Electronic structure and exchange interactions of the ladder vanadates CaV₂O₅and MgV₂O₅

Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV₂O₅

Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV₂O₅

Modeling of the magnetic susceptibilities of the ambient- and high-pressure phases of(VO)2P2
Johnston
¹
,
Saito
²
,
Azuma
³

et al. 2001
Phys. Rev. B
38
4
57
3
View full text Add to dashboard Cite