The study of interacting spin systems is of fundamental importance for modern condensed matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states 1 . The frustrated 2D ShastrySutherland lattice 2 realized by SrCu 2 (BO 3 ) 2 3 is an important test to our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated 2-12 . We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu 2 (BO 3 ) 2 . It is observed by inelastic neutron scattering after pressure tuning at 21.5 kbar. This gapped plaquette singlet state with strong 4-spin correlations leads to a transition to an ordered Néel state above 40 kbar, which can realize a deconfined quantum critical point.In the field of quantum magnetism, geometrically frustrated lattices generally imply major difficulties in analytical and numerical studies. For very few particular topologies however, it has been shown that the ground state, at least, can be calculated exactly as for the Majumdar-Gosh model 13 that solves the J 1 -J 2 zigzag chain when J 1 = 2J 2 . In 2D, the Shastry-Sutherland model 2 consisting of an orthogonal dimer network of spin S=1/2 was developed in order to be exactly solvable. For an inter-dimer J to intra-dimer J exchange ratio α ≡ J /J ≤ 0.5 the ground state is a product of singlets on the strong bond J. Numerical calculations have further shown that this remains valid up to α ≤∼ 0.7 and for small values of 3D couplings J between dimer layers. At the other end, for ∼ 0.9 ≤ α ≤ ∞ the system approaches the well known 2D square lattice, which is antiferromagnetically (AFM) ordered, albeit with significant quantum fluctuations that are believed to include resonating singlet correlations resulting in fractional excitations 14 . The phase diagram of the Shastry-Sutherland model, both with and without applied magnetic field, has been intensively studied by numerous theoretical and numerical approaches 3 . In the presence of magnetic field, magnetization plateaus at fractional values of the saturation magnetization corresponding to Mott insulator phases of dimer states, as well as possible superfluid and supersolid phases have been extensively studied 6,15,16 . At zero field, the main unsolved issue is the existence and nature arXiv:1603.02039v1 [cond-mat.str-el]

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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...