EUROPHYSICS LETTERS

1 September 2003

Europhys. Lett., 63 (5), pp. 757–763 (2003)

BiCu2 VO6 : A new narrow-band spin-gap material
T. Masuda 1 , A. Zheludev 1 , H. Kageyama 2 (∗ ) and A. N. Vasiliev 2,3 (∗∗ )1
Condensed Matter Science Division, Oak Ridge National Laboratory
Oak Ridge, TN 37831-6393, USA
2
Institute for Solid State Physics, University of Tokyo
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan3
Department of Low Temperature Physics M. V. Lomonosov
Moscow State University - Moscow 119899, Russia(received 14 April 2003; accepted in ﬁnal form 25 June 2003)
PACS. 75.10.Pq – Spin chain models.
PACS. 75.30.Ds – Spin waves.
PACS. 75.30.Et – Exchange and superexchange interactions.

Abstract. – A new spin-ladder family material BiCu2 VO6 is studied by means of magnetic
susceptibility, heat capacity and neutron inelastic-scattering measurements on a powder sample.
Singlet ground state and ﬁnite spin-gap are conﬁrmed by thermal-activated–type susceptibility
and by a distinct peak at 16 meV in spin excitation. A triple-narrow-band structure in thespin excitation spectrum, probably due to the complex crystal structure, is observed and thepossibility of a weakly interacting spin-cluster system is discussed.

Introduction. – In recent years quantum antiferromagnets with an intrinsically disordered(“spin liquid”) ground state and an energy gap in the spin excitation spectrum have received agreat deal of attention [1–4]. Among the simplest materials of this type are weakly interacting
singlet spin-clusters, such as dimers, spin-rings or plaquettes. The bandwidth of spin excitations from disordered ground state, which we will call “magnons” hereafter, in such materials
are typically smaller or comparable to the gap energy. This feature make them attractive toboth theorists and experimentalists. In many cases, inter-cluster interactions are suﬃciently
weak to be studied perturbatively, yet strong enough to give rise to complex cooperative phenomena that are found in wide-band systems, such as spin alternating-chains and spin-ladders.
Many prototypical coupled-dimer materials turned out to be particularly interesting from thispoint of view. Among the more extensively studied systems are the alternating-chain compound VODPO [5], the spin-bilayer material BaCuSi2 O6 [6], and the Shastry-Sutherland [7]material SrCu2 (BO3 )2 [8]. Neutron scattering experiments on Cu(NO3 )2 2.5D2 O [9] revealed
not only dispersive single-dimer modes, but also a highly structured continuum of multi-dimer
excitations. Another interesting case is that of (Tl, K)CuCl3 [10–14], where antiferromagnetic
long-range order has been induced by a strong external magnetic ﬁeld, pressure, or impurity doping.
(∗ ) Present address: Department of Chemistry, Graduate School of Science, Kyoto University - Japan.
(∗∗ ) Permanent address: Low Temperature Physics Department, Moscow State University - Moscow 119992,
Russia.
c EDP Sciences
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Fig. 1 – (a) Crystal structure of BiCu2 VO6 . S = 1/2 carrying Cu2+ ions form a zig-zag ladder along
the crystallographic c-axis, which is separated by non-magnetic Bi3+ and V5+ ions. (b) Frameworkof Cu2+ ions projected onto the b-c plane. Based on six inequivalent Cu2+ ions and ten inequivalent
Cu-O-Cu bonds, eight diﬀerent exchange interactions are hypothesized.

In our search for new experimental realizations of spin-ladder and related models, wecame across a new interesting family of low-dimensional materials with the general formula
BiCu2 AO6 (A = V [15], P [16], and As [17]). As will be shown in the present paper, at least
one of these compounds, namely BiCu2 VO6 , indeed has a non-magnetic singlet ground state
and a gap in the spin excitation spectrum.
The crystal structure of BiCu2 VO6 is of relatively low symmetry, monoclinic, space group
P 21 /n [15], and is schematically visualized in ﬁg. 1(a). Lattice parameters are a = 13.49 Å,b = 7.822 Å, c = 15.79 Å, and β = 113.113◦ . The most prominent features are zig-zag ladder structures formed by the S = 1/2 carrying Cu2+ , assumed to be responsible for themagnetic properties. The ladders run along the crystallographic c-axis and are separated bynon-magnetic V5+ and Bi3+ ions. The rungs of the ladders are roughly parallel to the crystallographic b-direction. The key role in superexchange interactions in this material is played
by Cu-O-Cu bonds. The spin-ladders incorporate six inequivalent Cu-sites, which result in
ten inequivalent Cu-O-Cu bonds, all with diﬀerent bond angles and distances. Moreover, theCu(1)-Cu(2) and Cu(5)-Cu(6) pairs each participate in two diﬀerent Cu-O-Cu bonds. Under
these circumstances it is diﬃcult to identify all the dominant superexchange pathways. However, it seems reasonable to hypothesize a spin Hamiltonian with up to eight unequal exchange
parameters as shown in ﬁg. 1(b). Even though crystallographic considerations point towards aquasi–one-dimensional distorted ladder-type system, we ﬁnd that BiCu2 VO6 possesses all thefeatures of a narrow-band “cluster” model, possibly composed of weakly interacting dimers.
Experimental procedure and results. – The non-magnetic nature of the ground state andthe energy gap in BiCu2 VO6 were detected in bulk susceptibility measurements. These data

T. Masuda et al.: BiCu2 VO6 : A new narrow-band spin-gap material

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Fig. 2 – (a) Magnetic susceptibility (open circle) and ﬁt by the spin-dimer model (dashed line). The
thermal-activated feature suggests the singlet ground state and a ﬁnite spin-gap. The applied ﬁeld is
0.1 T and the temperature range is 2–300 K. (b) Heat capacity (open circle) and spin-heat capacity
predicted by the spin-dimer model (dashed line). No signature of phase transition is observed. The
applied ﬁeld is 0 T and the temperature range is 6.4–260 K.

were collected using a quantum design SQUID magnetometer on a powder sample in a 1000 Oe
ﬁeld, and are shown in ﬁg. 2(a). Apart from the low-temperature upturn that is most likely
due to impurities, the susceptibility curve has a pronounced thermal-activated character. Asa point of reference, it can be approximated by a S = 1/2 dimer susceptibility curve, with anadditional Curie contribution at low temperatures due to impurities. The result of such a ﬁt
is shown as a dashed line in ﬁg. 2(a). In plotting this curve we used the value g = 2.03 for the
gyromagnetic ratio of Cu2+ , as determined by room-temperature ESR measurements. The
ﬁtted singlet-triplet dimer splitting is ∆ = 16 meV. The amount of impurity was estimated to
be only 0.8% of Cu2+ ions. While this simplest local-singlet model is probably an inappropriate
description of BiCu2 VO6 , the extracted gap energy sets the characteristic magnitude of spin
interactions.
Due to the large magnetic energy scales involved, the magnetic contribution to heat capacity is too weak to be isolated from the phonon part. This fact is borne out in ﬁg. 2(b) thatshows the C(T ) curve measured in a “Termis” calorimeter using the quasi-adiabatic method
on a polycrystalline sample (circles), in comparison with that calculated for the dimer model
described above (dashed line). One useful piece of information that is contained in the speciﬁcheat data, is the absence of any signatures of phase transitions over a temperature range 6.4–

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Intensity (arb units)60

Q=2.4 Å

-1

40

20

0
0

10

20

30

40

50

Energy (meV)Intensity (arb units)60

Q=3.2 Å

-1

40

20

0
0

10

20

30

40

50

Energy (meV)Fig. 3 – Constant-Q scan at 2.4 Å−1 and 3.2 Å−1 . The solid line is a ﬁt by multi-Gaussian, the dashedline is the background from the ﬁt, bars are the resolution in energy. Distinct peaks are observed at
16, 25, and 39 meV. The peak at 16 meV has intrinsic ﬁnite width, while those at 25 and 39 meV are
in resolution limit.

260 K. This supports the notion that the non-magnetic ground state is an intrinsic property ofthe spin network as it is at room temperature, and not a result of some structural transition.
More insight into the nature of the ground state and magnetic gap excitations was obtained
in inelastic neutron scattering experiments on BiCu2 VO6 . The data were collected on a 14.5 g
powder sample using the HB-1 3-axis spectrometer installed at the High Flux Isotope Reactor
at Oak Ridge National Laboratory. Neutrons of a ﬁxed ﬁnal energy Ef = 13.5 meV were usedin combination with a graphite ﬁlter positioned after the sample. Soller collimators deﬁned
the neutron beam divergencies as 48 -60 -60 -120 . Sample environment was a standard closedcycle refrigerator.
Typical constant-Q scans collected at T = 10 K are shown in ﬁg. 3. A number of suchscans were combined to produce the false-color plot shown in ﬁg. 4. Inelastic powder data
are notoriously diﬃcult to interpret due to the spherical averaging that is an intrinsic featureof powder scattering. In our case of BiCu2 VO6 the situation is even more complicated thanusual, largely due to the energy scales involved. The powder-averaged dynamic structure factor is most informative at small momentum transfers, where the averaging occurs across only
a few Brillouin zones, and where the signal often has a lot of “structure”. A good example
of such behavior was witnessed in recent powder experiments on the Haldane-gap antiferromagnet PbNi2 V2 O8 [18–20]. In the present study, however, momentum transfers below 1 Å−1

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Fig. 4 – Energy vs. Q false-color map. Triple narrow bands are indicated by arrows. The absence ofintensity below the lowest-energy band at 16 meV means the existence of a bound state there and an
intrinsic spin-gap in this material. The triple-band structure is discussed in text.

are unreachable in the interesting energy transfer range (15 to 40 meV) due to kinematicconstraints [21]. At higher wave vectors, any characteristic features of the cross-section areprogressively “smeared” by spherical averaging over several Brillouin zones. Magnetic scattering then becomes similar to the magnon density-of-states function. Moreover, the background,largely due to inelastic incoherent scattering by phonons that involve the V-nuclei, becomes
progressively stronger at higher momentum transfers, while magnetic scattering is suppressed
due to the magnetic form factor.
Given these intrinsic limitations of the present powder experiment and the complexity of
the crystal structure and geometry of magnetic interactions, a quantitative analysis of thedata, similar to the one described in refs. [18–20] does not appear feasible. The data obtainedin our neutron experiments can nevertheless provide some qualitative clues to the natureof the singlet ground state and energy gap in BiCu2 VO6 . The most prominent features of
the inelastic scattering, clearly visible in ﬁg. 4, are three intensity bands at energy transfers
16 meV, 25 meV, and 39 meV, respectively. The central energies were determined in Gaussianﬁts as those shown in solid lines in ﬁg. 3. Overall, in the admittedly narrow q-range sampled,
there is very little wave vector dependence to scattering. As shown in ﬁg. 3, only the lower
peak is somewhat broader than the experimental energy resolution (intrinsic width 3.2 meV
FWHM), and the two higher-energy features are resolution limited. The 16 meV peak isclearly to be associated with the energy gap in the system.
Discussion. – There are at least two alternative explanations for the observed intensity
bands. One possibility is that the scattering is due to a single dominant magnon branch witha rather wide dispersion bandwidth between the gap energy ∆ ≈ 16 meV and a zone-boundary
energy of 39 meV. The three peaks are then associated with Van Hove singularities in themagnon density of states. The location and intensity of the peaks primarily depends on thedetails of the magnon dispersion relation. Such is the case for powder experiments on several
materials including PbNi2 V2 O8 [18–20, 22, 23]. A diﬀerent explanation attributes the three

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observed peaks to three distinct branches of narrow-band singlet-triplet excitations in one ormore local spin-clusters, such as dimers. For BiCu2 VO6 this explanation seems to be themore likely one. Indeed, the measured powder spectrum (ﬁg. 3) has many similarities withthat seen in the well-characterized isolated-dimer material [24] (see ﬁg. 4 in ref. [24]). The
observed q-dependence is a slowly varying function that may represent smooth oscillationsof the single-cluster structure factor, rather than the sharp features expected from powderaveraging of magnon cross-sections with a steep dispersion [18–20, 22, 23]. In the local-cluster
interpretation the intrinsic width of the 16 meV peak in BiCu2 VO6 is attributed to weakinteractions between the clusters, the magnon bandwidth being directly related to the apparent
energy width of the intensity band in the powder data. The symmetry of BiCu2 VO6 is low
enough to allow up to three diﬀerent dimer types, based on the 6 inequivalent Cu2+ positions.
Other possibilities may include 4- and even 6-spin-clusters. The simplest case might be thetriple-dimer model, where J1 , J2 , and J3 are dominant. (See ﬁg. 1(b).) However, lacking a
reliable model of magnetic interactions in BiCu2 VO6 , a further exploration of this avenue will
have to be postponed until single-crystal samples become available.
Conclusion. – There are two lessons to be learned from our experiments on BiCu2 VO6
powder samples. First, we have shown that at least one member of the BiCu2 AO6 family isa singlet ground-state material, and identiﬁed the underlying ladder-type spin arrangement.
Second, it became obvious that the main obstacle to fully understand the physics of BiCu2 VO6lies in its distorted and complicated crystal structure. Other members of the family, forexample BiCu2 PO6 may be more valuable as model quantum magnets, since their structurefeatures undistorted spin-ladders.
∗∗∗
We thank Dr. E. Popova for assistance in speciﬁc-heat measurements and Dr. Y. Petrusevich and Dr. Y. Koksharov for taking ESR spectra at room temperature. We also
appreciate Prof. Y. Ueda for useful discussions. Work at ORNL was carried out under contracts No. DE-AC05-00OR22725, US Department of Energy. Work at ISSP was supportedby the MECSST (Japan) through Grant-in-Aid for Scientiﬁc Research No. 40302640. ANV
acknowledges support through RFBR Grant 03-02-16108 and NWO Grant 008-012-047.
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