Direct Observation of Field-Induced Incommensurate Fluctuations in a One-DimensionalS=1/2Antiferromagnet

Abstract

Neutron scattering from copper benzoate, Cu(C 6 D 5 COO) 2 ·3D 2 O, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes occur for wavevectorsq = π ± δq(H) where δq(H) ≈ 2πM (H)/gµ B as predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain. Unexpected was a field-induced energy gap ∆(H) ∝ H α , where α = 0.65(3) as determined from specific heat measurements. At H = 7 T (gµ B H/J = 0.52), the magnitude of the gap varies from 0.06−0.3J depending on the orientation of the applied field. To search for incommensurate spin correlations, we used inelastic neutron scattering to probe the wavevector-dependent spin susceptibility of the model one-dimensional S = 1/2 antiferromagnet (AFM) copper benzoate, Cu(C 6 D 5 COO) 2 · 3D 2 O. In contrast to other systems that have been studied in this context [7,8], in copper benzoate the exchange constant J is small enough to permit the large values of the reduced field, h = gµ B H/J ≈ 1 needed to observe this effect, while still being large enough to allow the interesting low energy part of the spectrum to be explored with a cold neutron triple-axis spectrometer [9]. Our most important result is that a magnetic field induces new low-energy modes in the excitation spectrum of copper benzoate at incommensurate wavevectorsq = π ± δq, where δq(H) ≈ 2πM(H)/gµ B , with M(H) being the magnetization per spin, as predicted by theory [1][2][3]. The modes are not completely soft, however, because the field also induces a gap in the excitation spectrum both at the incommensurate wave vector and atq = π. A, c = 6.30Å, and β = 89.5• [11]. Copper ions within a spin chain are separated by c/2 and coordinated by edge sharing, tetragonally distorted oxygen octahedra. The near-neighbor intrachain exchange interaction is J = 1.57 meV [9]. For this work, deuterated single crystals were prepared as described previously [9]. Specific heat measurements in magnetic fields up to H = 8.8 T were made on individual single crystals of typical mass 0.01 g using the relaxation method. For the neutron scattering measurements, approximately 500 crystals with total mass 3.82 g were mutually aligned to within 5• in the horizontal (h0l) scattering plane. Neutron scattering measurements in fields up to H = 7 T b were performed on the SPINS cold neutron triple-axis spectrometer at NIST. FWHM beam divergences were′ , and the fixed final energy was 2.5 meV, yielding energy and wavevector resolution ∆E = 69 µeV and ∆q/π = 0.02, respectively. We refer to wave vector transfer along the chain asq = Q · c/2 = lπ. The normalized magnetic scattering intensitỹ I(q, ω) [9], was derived from the detector count rate by subtracting T = 25 K data as a background, dividing by the squared magnetic form factor for copper, and normalizing to

Orbital Arrangements and Magnetic Interactions in the Quasi-One-Dimensional Cuprates ACuMoO4(OH) (A = Na, K)

“…In fact, such a gap opening has been extensively studied in Cu-benzoate through heat capacity 29 , inelastic neutron scattering 28,29 and ESR experiments 34 . Thus, the field-induced energy gap observed in the heat capacity data of the K compound also supports the presence of staggered DM interactions.…”

Effects of an external magnetic field on the gaps and quantum corrections in an ordered Heisenberg antiferromagnet with Dzyaloshinskii-Moriya anisotropy

“…This has made possible the observations of condensation of triplet excitations in a variety of chain, ladder, and weakly-coupled dimer compounds, 1,2,3 magnetization plateaux in frustrated magnets, 4 and other new effects. 5,6 It turned out that in many cases experimental data deviate significantly from the theoretical predictions based on the pure isotropic Heisenberg model in external field. 6,7,8 Such deviations are due to anisotropies, most notably the Dzyaloshinskii-Moriya (DM) anisotropy, which are usually small and often neglected from zero-field considerations.…”

“…5,6 It turned out that in many cases experimental data deviate significantly from the theoretical predictions based on the pure isotropic Heisenberg model in external field. 6,7,8 Such deviations are due to anisotropies, most notably the Dzyaloshinskii-Moriya (DM) anisotropy, which are usually small and often neglected from zero-field considerations. Not only such anisotropies can induce qualitatively new effects, but also the strength of such effects seems to be significantly amplified by an external field.…”

“…In particular, in the one-dimensional (1D) spin- 1 2 chains with the DM interaction spin excitation spectrum exhibits a gap ∆ ∝ (DH) 2/3 in contrast with the pure Heisenberg case where the spectrum remains gapless. 6,9 Thus, while usually treated as an insignificant contamination of the problem, the DM interaction can give rise to an interesting phenomena on its own.…”

Magnetic fluctuations above the Néel temperature in κ‐(BEDT‐TTF)2Cu[N(CN)2]Cl, a quasi‐2D Heisenberg antiferromagnet with Dzyaloshinskii–Moriya interaction

“…On the other hand, the excitations and the ESR spectrum of the 1D chain were extensively studied [3][4][5][6]. The DM interaction in an external magnetic field acts approximately as a staggered magnetic field, h = (D 12 /J) × H with an alternating sign for subsequent molecules along the chain [5].…”