Quantum Destruction of Spiral Order in Two Dimensional Frustrated Magnets

We study the fate of spin-1/2 spiral-ordered two-dimensional quantum antiferromagnets that are disordered by quantum fluctuations. A crucial role is played by the topological point defects of the spiral phase, which are known to have a Z2 character. Previous works established that a nontrivial quantum spin-liquid phase results when the spiral is disordered without proliferating the Z2 vortices. Here, we show that when the spiral is disordered by proliferating and condensing these vortices, valence-bond solid ordering occurs due to quantum Berry phase effects. We develop a general theory for this latter phase transition and apply it to a lattice model. This transition potentially provides a new example of a Landau-forbidden deconfined quantum critical point.Comment: 12 pages (Extended and appendix added

Theory of tunneling conductance of graphene NIS junctions

We calculate the tunneling conductance of a graphene normal metal-insulator-superconductor (NIS) junction with a barrier of thickness $d$ and with an arbitrary voltage $V_0$ applied across the barrier region. We demonstrate that the tunneling conductance of such a NIS junction is an oscillatory function of both $d$ and $V_0$. We also show that the periodicity and amplitude of such oscillations deviate from their universal values in the thin barrier limit as obtained in earlier work [Phys. Rev. Lett. {\bf 97}, 217001 (2006)] and become a function of the applied voltage $V_0$. Our results reproduces the earlier results on tunneling conductance of such junctions in the thin [Phys. Rev. Lett. {\bf 97}, 217001 (2006)] and zero [Phys. Rev. Lett. {\bf 97}, 067007 (2006)] barrier limits as special limiting cases. We discuss experimental relevance of our results.Comment: Revised versio

Z2 topological liquid of hard-core bosons on a kagome lattice at 1/3 filling

We consider hard-core bosons on the kagome lattice in the presence of short range repulsive interactions and focus particularly on the filling factor 1/3. In the strongly interacting limit, the low energy excitations can be described by the quantum fully packed loop coverings on the triangular lattice. Using a combination of tensor-product state based methods and exact diagonalization techniques, we show that the system has an extended Z2 topological liquid phase as well as a lattice nematic phase. The latter breaks lattice rotational symmetry. By tuning appropriate parameters in the model, we study the quantum phase transition between the topological and the symmetry broken phases. We construct the critical theory for this transition using a mapping to an Ising gauge theory that predicts the transition to belong to the O(3) universality class.Comment: 12 pages, 10 figure

Temperature dependence of butterfly effect in a classical many-body system

We study the chaotic dynamics in a classical many-body system of interacting spins on the kagome lattice. We characterise many-body chaos via the butterfly effect as captured by an appropriate out-of-time-ordered correlator. Due to the emergence of a spin liquid phase, the chaotic dynamics extends all the way to zero temperature. We thus determine the full temperature dependence of two complementary aspects of the butterfly effect: the Lyapunov exponent, $\mu$, and the butterfly speed, $v_b$, and study their interrelations with usual measures of spin dynamics such as the spin-diffusion constant, $D$ and spin-autocorrelation time, $\tau$. We find that they all exhibit power law behaviour at low temperature, consistent with scaling of the form $D\sim v_b^2/\mu$ and $\tau^{-1}\sim T$. The vanishing of $\mu\sim T^{0.48}$ is parametrically slower than that of the corresponding quantum bound, $\mu\sim T$, raising interesting questions regarding the semi-classical limit of such spin systems.Comment: 6+4 pages, 4+8 figures, ancillary files include videos of the dynamic

Phases and phase transitions of a perturbed Kekul\'e-Kitaev model

We study the quantum spin liquid phase in a variant of the Kitaev model where the bonds of the honeycomb lattice are distributed in a Kekul\'e pattern. The system supports gapped and gapless Z_2 quantum spin liquids with interesting differences from the original Kitaev model, the most notable being a gapped Z_2 spin liquid on a Kagome lattice. Perturbing the exactly solvable model with antiferromagnetic Heisenberg perturbations, we find a magnetically ordered phase stabilized by a quantum `order by disorder' mechanism, as well as an exotic continuous phase transition between the topological spin liquid and this magnetically ordered phase. Using a combination of field theory and Monte-Carlo simulations, we find that the transition likely belongs to the 3D-XYxZ_2 universality class.Comment: 15 pages, 11 figure

Elastic signatures of a spin-nematic

We study the elastic signatures -- renormalisation of sound velocity and magnetostriction -- of the spin-nematic phase of a spin-$1$ magnet on a triangular lattice described by the bilinear-biquadratic spin Hamiltonian. We show that at low temperatures, the scattering of the acoustic phonons from the Goldstone modes of the nematic phase lead to a powerlaw renormalisation of the fractional change in the sound velocity, $v$, as a function of temperature, $T$, i.e. $\Delta v/v\propto T^3$ as opposed to the same in the high temperature paramagnet where $\Delta v/v\propto T^{-1}$. At the generically discontinuous nematic transition, there is a jump in magnetostriction as well as $\Delta v/v$ along with enhanced $h^4$ dependence on the magnetic field, $h$, near the nematic transition. These signatures can help positively characterise the spin-nematic in general and in particular the realisation of such a phase in the candidate material NiGa$_2$S$_4$

Signatures of spin-triplet excitations in optical conductivity of valence bond solids

We show that the optical responses below the Mott gap can be used to probe the spin-triplet excitations in valence bond solid (VBS) phases in Mott insulators. The optical conductivity in this regime arises due to the electronic polarization mechanism via virtual electron hopping processes. We apply this mechanism to the Hubbard model with spin-orbit couplings and/or the corresponding spin model with significant Dzyaloshinskii-Moriya (DM) interactions, and compute the optical conductivity of VBS states on both ideal and deformed Kagome lattices. In case of the deformed Kagome lattice, we study the antiferromagnet, Rb$_2$Cu$_3$SnF$_{12}$ with the pinwheel VBS state. In case of the ideal Kagome lattice, we explore the optical conductivity signatures of the spin-triplet excitations for three VBS states with (1) a 12-site unit cell, (2) a 36-site unit cell with six-fold rotation symmetry, and (3) a 36-site unit cell with three-fold rotation symmetry, respectively. We find that increasing the DM interactions generally leads to broad and smooth features in the optical conductivity with interesting experimental consequences. The optical conductivity reflects the features of the spin-triplet excitations that can be measured in future experiments.Comment: Updated with the published version. 24 pages and 8 figure