SrCu2(BO3)2: A Unique Mott Hubbard Insulator

Shastry, B. Sriram; Kumar, Brijesh

doi:10.1143/ptps.145.1

Cited by 34 publications

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“…In the present work, we perform what seems to be a consistent and simple version of RVB theory, one which yields d-wave order for the square lattice [8] and also for the other recently interesting case of SrCu 2 (BO 3 ) 2 [9]. Earlier calculations have been done by other authors in a similar spirit to ours, Reference [5] is confined to half filling, and a recent preprint by Baskaran [10] makes some qualitative points that are common to our calculations.…”

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confidence: 99%

Superconductivity inCoO2layers and the resonating valence bond mean-field theory of the triangular latticet−Jmodel

2003

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Our recent paper [1] contains an error in the formula for the high frequency Hall constant for the experimentally relevant case of electron doping. Eq(3) giving the high frequency Hall constant for the case of electron filling, i.e. electron density n > 1, should correctly readwhere δ = |1 − n|. This differs essentially from the quoted answer by a minus sign that was missed by us, and by a factor of 2. This equation is obtained from Eq(2) in the paper, calculated for the case of hole doping n < 1, by a particle hole transformation. Performing a canonical transformation c i,σ ↔ c † i,σ in the Kubo formulas for models of lattice fermions (see e.g. [2] Eqn(1,2) ) yields:The crucial overall minus sign arises from the fact that the Hall constant involves a three current propagator, and each current picks up a minus sign under the transformation. Incidentally this identity results in a vanishing at half filling for bipartite lattices of the Hall function R H (1, t, ω, T ) for any value of T, ω and the interaction strength.The changed sign in Eq(3') has a few implications which we would like to state explicitly. The recent experiment of Wang, Rogado, Cava and Ong [3] on the transport Hall constant, actually sees the predicted linear T behaviour over a wide temperature range 200 0 K ≤ T ≤ 400 0 K with a positive slope. From Eq(3') above, we deduce that the appropriate t − J model description of the system has t < 0, i.e. belongs to case B(ii) in our notation, at least at the composition x ∼ .7.We note that with this correction, the sign of the hopping t suggested by our calculation is the same as the one favoured by Refs [4,5], at least for the non superconducting range x ∼ .7. In this composition range, photoemission data [6,7] also suggest the same sign of t, with an unoccupied region around the center of Brillouin zone, namely the Γ point. Motivated by the recent discovery of superconductivity in two dimensional CoO2 layers, we present some possibly useful results of the RVB mean field theory applied to the triangular lattice. An interesting time reversal breaking superconducting state arises from strongly frustrated interactions. Away from half filling, the order parameter is found to be complex, and yields a fully gapped quasiparticle spectrum. The sign of the hopping plays a crucial role in the analysis, and we find that superconductivity is as fragile for one sign as it is robust for the other. NaxCoO2 · yH2O is argued to belong to the robust case, by comparing the LDA fermi surface with an effective tight binding model. The high frequency Hall constant in this system is potentially interesting, since it is pointed out to increase linearly with temperature without saturation for T > T degeneracy .

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Section: Introductionmentioning

confidence: 99%

Superconductivity inCoO2layers and the resonating valence bond mean-field theory of the triangular latticet−Jmodel

2003

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“…It is also the same choice as in Ref. 4. Using exact diagonalization we solve this model on a 32-site SS lattice with periodic boundary conditions as shown in Fig.…”

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confidence: 99%

Absence of hole pairing in a simplet−Jmodel on the Shastry-Sutherland lattice

Leung

Cheng

2004

Phys. Rev. B

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The Shastry-Sutherland model is a two-dimensional frustrated spin model whose ground state is a spin gap state. We study this model doped with one and two holes on a 32-site lattice using exact diagonalization. When tЈϾ0, we find that the diagonal dimer order that exists at half-filling is retained at these moderate doping levels. No other order is found to be favored on doping. The holes are strongly repulsive unless the hopping terms are unrealistically small. Therefore, the existence of a spin gap at half-filling does not guarantee holepairing in the present case. DOI: 10.1103/PhysRevB.69.180403 PACS number͑s͒: 75.40.Mg, 71.10.Fd, 71.27.ϩa The Shastry-Sutherland ͑SS͒ model is an exceptional example of a two-dimensional frustrated spin system that has an exact solution. 1 Remarkably, it is also an excellent theoretical model for the spin gap material Sr Cu 2 (BO 3 ) 2 . 2 The SS model is a Heisenberg model with exchange interaction J on a two-dimensional square lattice frustrated by diagonal bonds JЈ as shown in Fig. 1. When J/JЈ is smaller than a critical value of about 0.677, 3 the ground state is a direct product of orthogonal singlet dimers residing on the JЈ bonds. This is a spin gap state because the lowest spin excitation involves turning a singlet dimer into a triplet. Different experiments have suggested a range of J/JЈ for SrCu 2 (BO 3 ) 2 , out of which 0.635 seems to be the optimal one. A great deal of work has been devoted to studying the excited states of this model as an effort to explain various experimental results. It is found that the almost localized nature of spin excitations leads to superstructures that give rise to plateaux in the magnetization curve. Besides its importance as a theoretical model for Sr Cu 2 (BO 3 ) 2 , the SS model possesses a quantum phase transition. One believes that there is at least one intermediate state between the diagonal dimer state at small J and the Néel state at large J. While the nature of the intermediate state is still controversial, it seems like the plaquette resonating-valence-bond ͑RVB͒ state and the helical state are among the most likely candidates. Although discussions on the quantum phase transition are theoretical, they are not totally irrelevant to SrCu 2 (BO 3 ) 2 . Since its J/JЈ is not too far from the critical value, it may be possible to shift it even closer by applying pressure, substitution, etc. to Sr Cu 2 (BO 3 ) 2 .The relation between disordered spin liquids with a gap in the spin excitation and superconductivity has aroused a lot of interest. It has been suggested that doping a spin gap system may lead to hole-pairing and superconductivity. It is therefore interesting and important to identify spin gap materials that are Mott insulators with no long-range spin order. SrCu 2 (BO 3 ) 2 is one such compound. Although this compound has not been doped, there is no shortage of suggestions that on doping the SS model may exhibit superconductivity. Different types of superconducting states have been suggested at different doping levels...

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“…In zero field, the intensity of the triplet excitation measured by inelastic neutron scattering (INS) decays faster than expected for a dimer system with a gap Δ ¼ 35 K. This implies that very small amounts of thermal energy dramatically modify the spin correlations of the system. SrCu 2 ðBO 3 Þ 2 has been proposed in the context of valence bond crystals [29], resonating valence bond states [16,30,31], and superfluid, supersolid [18,32], and (upon doping) superconducting phases [33][34][35]. The understanding of its finite temperature properties is therefore of crucial importance.…”

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Correlated Decay of Triplet Excitations in the Shastry-Sutherland CompoundSrCu2(BO3)2
Zayed
¹
,
Rüegg
²
,
Strässle
³

et al. 2014
Phys. Rev. Lett.
19
2
19
0
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The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu 2 ðBO 3 Þ 2 is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature, while the integrated spectral weight can be explained by an isolated dimer model up to 10 K. Analyzing this anomalous spectral line shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long lived if no thermally populated triplons are nearby but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry-Sutherland lattice. Quantum spin systems display a wide range of intriguing many-body quantum effects. A particularly active field is the study of interacting dimer systems. Two antiferromagnetically coupled spins forming a dimer have a singlet ground state with an energy gap to excited triplet states. In extended systems where dimers couple to each other, the ground state often remains a spin singlet and gapped. The excitations are known as triplons and can be described in terms of quasiparticles as hard-core bosons [1][2][3][4]. Because of coupling between dimers, triplons usually become mobile and can hop to neighboring dimer sites. For particular cases, the thermodynamic finite temperature behavior of the mobility of these hard-core bosons has been treated using statistical models reproducing triplon band renormalization and damping observed in experiments [5][6][7][8][9]. In general, the increase of the thermal population of bosons produces an increased repulsion and a reduced mobility, which is observed as a reduction of the dispersion bandwidth. The theoretical treatment of finite temperature damping, reflecting the lifetime of the boson and the related spectral functions, remains an ongoing challenge [10][11][12][13].The compound SrCu 2 ðBO 3 Þ 2 constitutes an important example for testing our understanding of quantum spin systems, as it is a close realization of the frustrated but "exactly solvable" 2D Shastry-Sutherland model [14,15] (see Refs. [16,17] for a review). In SrCu 2 ðBO 3 Þ 2 , triplons are prevented from hopping already at zero temperature by frustrated interdimer interactions. Despite the strong (frustrated) coupling, the triplon dispersion is very shallow. Theoretical studies show that hopping is allowed only from the sixth order in the inter-to intradimer coupling ratio J 0 =J or in the presence of Dzyaloshinskii-Moriya (DM) terms. It has, however, been shown that multiple triplons forming a bound state are more mobile than a single triplon due to correlated hopping processes, appearing already in second order in J 0 =J [16,18]. In high magnetic fields, triplons in SrCu 2 ðBO 3 Þ 2 crystalliz...

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SrCu₂(BO₃)₂: A Unique Mott Hubbard Insulator

Cited by 34 publications

References 0 publications

Superconductivity inCoO2layers and the resonating valence bond mean-field theory of the triangular latticet−Jmodel

Superconductivity inCoO2layers and the resonating valence bond mean-field theory of the triangular latticet−Jmodel

Absence of hole pairing in a simplet−Jmodel on the Shastry-Sutherland lattice

Correlated Decay of Triplet Excitations in the Shastry-Sutherland CompoundSrCu2(BO3)2
Zayed
¹
,
Rüegg
²
,
Strässle
³

et al. 2014
Phys. Rev. Lett.
19
2
19
0
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SrCu2(BO3)2: A Unique Mott Hubbard Insulator

Cited by 34 publications

References 0 publications

Superconductivity inCoO2layers and the resonating valence bond mean-field theory of the triangular latticet−Jmodel

Superconductivity inCoO2layers and the resonating valence bond mean-field theory of the triangular latticet−Jmodel

Absence of hole pairing in a simplet−Jmodel on the Shastry-Sutherland lattice

Correlated Decay of Triplet Excitations in the Shastry-Sutherland CompoundSrCu2(BO3)2Zayed1, Rüegg2, Strässle3 et al. 2014Phys. Rev. Lett.192190View full textAdd to dashboardCite

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SrCu₂(BO₃)₂: A Unique Mott Hubbard Insulator

Correlated Decay of Triplet Excitations in the Shastry-Sutherland CompoundSrCu2(BO3)2
Zayed
¹
,
Rüegg
²
,
Strässle
³

et al. 2014
Phys. Rev. Lett.
19
2
19
0
View full text Add to dashboard Cite