13 research outputs found
Spin correlations and phase diagram of the perturbed Kitaev model
We present a general classification of the perturbations to the Kitaev model
on the basis of their effect on it's spin correlation functions. We derive a
necessary and sufficient condition for the spin correlators to exhibit a long
ranged power-law behavior in the presence of such perturbations. We
substantiate our result by a study of the phase diagram of the Kitaev model
augmented by a loop term and perturbed by an Ising term, within a RVB
mean-field theory. We estimate the stability of the spin-liquid phase against
such perturbations and show that this model exhibits both
confinement-deconfinement transitions from spin liquid to
antiferromagnetic/spin-chain/ferromagnetic phases as well as topological
quantum phase transitions between gapped and gapless spin liquid phases.Comment: 4 pgs., 3 figs, v
Interplay of Spin and Lattice Degrees of Freedom in the Frustrated Antiferromagnet CdCr_2O_4: High-field and Temperature Induced Anomalies of the Elastic Constants
Temperature and magnetic field studies of the elastic constants of the
chromium spinel CdCr_2O_4 show pronounced anomalies related to strong
spin-phonon coupling in this frustrated antiferromagnet. A detailed comparison
of the longitudinal acoustic mode propagating along the [111] direction with
theory based on an exchange-striction mechanism leads to an estimate of the
strength of the magneto-elastic interaction. The derived spin-phonon coupling
constant is in good agreement with previous determinations based on infrared
absorption. Further insight is gained from intermediate and high magnetic field
experiments in the field regime of the magnetization plateau. The role of the
antisymmetric Dzyaloshinskii-Moriya interaction discussed and we compare the
spin-phonon coupling in CdCr_2O_4 in both the ordered and disordered states.Comment: 12 pages, 8 figures; Appendix added,To appear in Phys Rev.
Signatures of a spin-1/2 cooperative paramagnet in the diluted triangular lattice of YCuTiO
We present a combination of thermodynamic and dynamic experimental signatures
of a disorder driven dynamic cooperative paramagnet in a 50% site diluted
triangular lattice spin-1/2 system, YCuTiO. Magnetic ordering and spin
freezing are absent down to 50 mK, far below the Curie Weiss scale of ~-134 K.
We observe scaling collapses of the magnetic field- and temperature-dependent
magnetic heat capacity and magnetisation data, respectively, in conformity with
expectations from the random singlet physics. Our experiments establish the
suppression of any freezing scale, if at all present, by more than three orders
of magnitude, opening a plethora of interesting possibilities such as
disorder-stabilized long range quantum entangled ground states.Comment: 18 pages, 9 figures, published in Physical Review Letter
Proximity Induced Superconductivity and Multiple Andreev Reflections in Few-Layer-Graphene
We have investigated electronic transport of few-layer-graphene (FLG)
connected to superconducting electrodes. The device is prepared by mechanical
exfoliation of graphite. A small mesa of FLG is placed on the surface of an
insulating Alumina layer over silicon substrate, and is connected with two
tungsten electrodes, separated by 2.5 microns, grown by focused ion beam. While
tungsten electrodes are superconducting below 4 K, proximity induced
superconductivity in FLG is observed below 1K with a large differential
resistance drop at low bias. Signatures of multiple Andreev reflections are
observed as peaks located at voltages corresponding to sub-multiple values of
the superconducting gap of the electrodes
Confinement-deconfinement transition and spin correlations in a generalized Kitaev model
We present a spin model, namely, the Kitaev model augmented by a loop term and perturbed by an Ising Hamiltonian, and show that it exhibits both confinement-deconfinement transitions from spin liquid to antiferromagnetic/spin-chain/ferromagnetic phases and topological quantum phase transitions between gapped and gapless spin-liquid phases. We develop a fermionic resonating-valence-bonds (RVB) mean-field theory to chart out the phase diagram of the model and estimate the stability of its spin-liquid phases, which might be relevant for attempts to realize the model in optical lattices and other spin systems. We present an analytical mean-field theory to study the confinement-deconfinement transition for large coefficient of the loop term and show that this transition is first order within such mean-field analysis in this limit. We also conjecture that in some other regimes, the confinement-deconfinement transitions in the model, predicted to be first order within the mean-field theory, may become second order via a defect condensation mechanism. Finally, we present a general classification of the perturbations to the Kitaev model on the basis of their effect on it's spin correlation functions and derive a necessary and sufficient condition, within the regime of validity of perturbation theory, for the spin correlators to exhibit a long-ranged power-law behavior in the presence of such perturbations. Our results reproduce those of Tikhonov et al. [Phys. Rev. Lett. 106, 067203 (2011)] as a special case
Interplay of spin and lattice degrees of freedom in the frustrated antiferromagnet CdCr2O4: High-field and temperature-induced anomalies of the elastic constants
Temperature and magnetic field studies of the elastic constants of the chromium spinel CdCr2O4 show pronounced anomalies related to strong spin-phonon coupling in this frustrated antiferromagnet. A detailed comparison of the longitudinal acoustic mode propagating along the 111] direction with a theory based on an exchange-striction mechanism leads to an estimate of the strength of the magnetoelastic interaction. The derived spin-phonon coupling constant is in good agreement with previous determinations based on infrared absorption. Further insight is gained from intermediate and high magnetic field experiments in the field regime of the magnetization plateau. The role of the antisymmetric Dzyaloshinskii-Moriya interaction is discussed