Multiple time scales from hard local constraints: glassiness without disorder

While multiple time scales generally arise in the dynamics of disordered systems, we find multiple time scales in absence of disorder, in a simple model with hard local constraints. The dynamics of the model, which consists of local collective rearrangements of various scales, is not determined by the smallest scale but by a length $l^*$ that grows at low energies. In real space we find a hierarchy of fast and slow regions: each slow region is geometrically insulated from all faster degrees of freedom, which are localized in fast pockets below percolation thresholds. A tentative analogy with structural glasses is given, which attributes the slowing down of the dynamics to the growing size of mobile elementary excitations, rather than to the size of some domains.Comment: 10 pages, 9 figures, v2: pub

Colorings of odd or even chirality on hexagonal lattices

We define two classes of colorings that have odd or even chirality on hexagonal lattices. This parity is an invariant in the dynamics of all loops, and explains why standard Monte-Carlo algorithms are nonergodic. We argue that adding the motion of "stranded" loops allows for parity changes. By implementing this algorithm, we show that the even and odd classes have the same entropy. In general, they do not have the same number of states, except for the special geometry of long strips, where a Z$_2$ symmetry between even and odd states occurs in the thermodynamic limit.Comment: 18 pages, 13 figure

Heterogeneous freezing in a geometrically frustrated spin model without disorder: spontaneous generation of two time-scales

By considering the constrained motion of classical spins in a geometrically frustrated magnet, we find a dynamical freezing temperature below which the system gets trapped in metastable states with a "frozen" moment and dynamical heterogeneities. The residual collective degrees of freedom are strongly correlated, and by spontaneously forming aggregates, they are unable to reorganize the system. The phase space is then fragmented in a macroscopic number of disconnected sectors (broken ergodicity), resulting in self-induced disorder and "thermodynamic" anomalies, measured by the loss of a finite configurational entropy. We discuss these results in the view of experimental results on the kagome compounds, SrCr(9p)Ga(12-9p)O19, (H30)Fe3(SO4)2(OH)6, Cu3V2O7(OH)2.2H2O and Cu3BaV2O8(OH)2.Comment: 17 pages, 14 fi

How to detect weak emergent broken-symmetries of the Kagome antiferromagnet from Raman spectroscopy

We show that the magnetic Raman response of a spin-liquid is independent of the polarizations of the light for triangular symmetries. In contrast, a ground-state that has a broken symmetry shows characteristic oscillations when the polarizations are rotated. This would allow to detect weak broken symmetries and emergent order-parameters. We focus on the Kagome antiferromagnet where no conventional long-range order has been found so far, and present the Raman cross-section of a spin-liquid and a valence bond crystal (VBC) using a random phase approximation.Comment: 4 pages, 2 figures, v2. intro partially rewritte

Instabilities and Insulator-Metal transitions in Half-Doped Manganites induced by Magnetic-Field and Doping

We discuss the phase diagram of the two-orbital model of half-doped manganites by calculating self-consistently the Jahn-Teller (JT) distortion patterns, charge, orbital and magnetic order at zero temperature. We analyse the instabilities of these phases caused by electron or hole doping away from half-doping, or by the application of a magnetic-field. For the CE insulating phase of half-doped manganites, in the intermediate JT coupling regime, we show that there is a competition between canting of spins (which promotes mobile carriers) and polaronic self-trapping of carriers by JT defects. This results in a marked particle-hole asymmetry, with canting winning only on the electron doped side of half-doping. We also show that the CE phase undergoes a first-order transition to a ferromagnetic metallic phase when a magnetic-field is applied, with abrupt changes in the lattice distortion patterns. We discuss the factors that govern the intriguingly small scale of the transition fields. We argue that the ferromagnetic metallic phases involved have two types of charge carriers, localised and band-like, leading to an effective two-fluid model.Comment: 22 pages, 28 figure

Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites

We argue that many properties of the half-doped manganites may be understood in terms of a new two-(eg electron)-fluid description, which is energetically favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a competition between canting of the core spins of Mn promoting mobile carriers and polaronic trapping of carriers by JT defects, in the presence of CE, orbital and charge order. We show that this explains several features of the doping and magnetic field induced insulator-metal transitions, as the particle-hole asymmetry and the smallness of the transition fields.Comment: 4 pages, 4 figure

Quantum phase transition induced by Dzyaloshinskii-Moriya in the kagome antiferromagnet

We argue that the S=1/2 kagome antiferromagnet undergoes a quantum phase transition when the Dzyaloshinskii-Moriya coupling is increased. For $D<D_c$ the system is in a moment-free phase and for $D>D_c$ the system develops antiferromagnetic long-range order. The quantum critical point is found to be $D_c \simeq 0.1J$ using exact diagonalizations and finite-size scaling. This suggests that the kagome compound ZnCu$_3(OH)$_6$Cl$_3$ may be in a quantum critical region controlled by this fixed point.Comment: 5 pages, 4 figures; v2: add. data included, show that D=0.1J is at a quantum critical poin

The incarnation of the Nersesyan-Tsvelik model in (NO)[Cu(NO3)3]

The topology of the magnetic interactions of the copper spins in the nitrosonium nitratocuprate (NO)[Cu(NO3)3] suggests that it could be a realization of the Nersesyan-Tsvelik model, whose ground state was argued to be either a resonating valence bond (RVB) state or a valence bond crystal (VBC). The measurement of thermodynamic and resonant properties reveals a behavior inherent to low dimensional spin S = 1/2 systems and provides indeed no evidence for the formation of long-range magnetic order down to 1.8 K.Comment: 12 pages, 6 figure